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Versions: (RFC 2543) 00 01 02 03 04 05 06 07 08 RFC 3261

Internet Engineering Task Force                                   SIP WG
Internet Draft                                        Jonathan Rosenberg
                                                             dynamicsoft
                                                     Henning Schulzrinne
                                                             Columbia U.
                                                       Gonzalo Camarillo
                                                                Ericsson
                                                           Alan Johnston
                                                                Worldcom
                                                            Jon Peterson
                                                                 Neustar
                                                           Robert Sparks
                                                             dynamicsoft
                                                            Mark Handley
                                                                   ACIRI
                                                            Eve Schooler
                                                                    AT&T

draft-ietf-sip-rfc2543bis-07.txt
February 4, 2002
Expires: Aug 2002


                    SIP: Session Initiation Protocol

STATUS OF THIS MEMO

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   To view the list Internet-Draft Shadow Directories, see
   http://www.ietf.org/shadow.html.

Abstract

   The Session Initiation Protocol (SIP) is an application-layer control
   (signaling) protocol for creating, modifying and terminating sessions
   with one or more participants. These sessions include Internet
   telephone calls, multimedia distribution and multimedia conferences.

   SIP invitations used to create sessions carry session descriptions
   which allow participants to agree on a set of compatible media types.
   SIP makes use of elements called proxy servers to help route requests
   to the users current location, authenticate and authorize users for
   services, implement provider call routing policies, and provide
   features to users. SIP also provides a registration function that
   allows them to upload their current location for use by proxy
   servers.  SIP runs ontop of several different transport protocols.




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                           Table of Contents



   1          Introduction ........................................    2
   2          Overview of SIP Functionality .......................    2
   3          Terminology .........................................    3
   4          Overview of Operation ...............................    4
   5          Structure of the Protocol ...........................   11
   6          Definitions .........................................   13
   7          SIP Messages ........................................   19
   7.1        Requests ............................................   20
   7.2        Responses ...........................................   21
   7.3        Header Fields .......................................   22
   7.3.1      Header Field Format .................................   22
   7.3.2      Header Field Classification .........................   25
   7.3.3      Compact Form ........................................   25
   7.4        Bodies ..............................................   26
   7.4.1      Message Body Type ...................................   26
   7.4.2      Message Body Length .................................   26
   7.5        Framing SIP messages ................................   27
   8          General User Agent Behavior .........................   27
   8.1        UAC Behavior ........................................   27
   8.1.1      Generating the Request ..............................   27
   8.1.1.1    Request-URI .........................................   28
   8.1.1.2    To ..................................................   28
   8.1.1.3    From ................................................   29
   8.1.1.4    Call-ID .............................................   30
   8.1.1.5    CSeq ................................................   31
   8.1.1.6    Max-Forwards ........................................   31
   8.1.1.7    Via .................................................   32
   8.1.1.8    Contact .............................................   32
   8.1.1.9    Supported and Require ...............................   33
   8.1.1.10   Additional Message Components .......................   33
   8.1.2      Sending the Request .................................   34
   8.1.3      Processing Responses ................................   34
   8.1.3.1    Transaction Layer Errors ............................   34
   8.1.3.2    Unrecognized Responses ..............................   35
   8.1.3.3    Vias ................................................   35
   8.1.3.4    Processing Reliable 1xx Responses ...................   35
   8.1.3.5    Processing 3xx responses ............................   35
   8.1.3.6    Processing 4xx responses ............................   37
   8.2        UAS Behavior ........................................   38
   8.2.1      Method Inspection ...................................   38
   8.2.2      Header Inspection ...................................   38



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   8.2.2.1    To and Request-URI ..................................   38
   8.2.2.2    Merged Requests .....................................   39
   8.2.2.3    Require .............................................   39
   8.2.3      Content Processing ..................................   40
   8.2.4      Applying Extensions .................................   41
   8.2.5      Processing the Request ..............................   41
   8.2.6      Generating the Response .............................   41
   8.2.6.1    Sending a Provisional Response ......................   41
   8.2.6.2    Headers and Tags ....................................   42
   8.2.7      Stateless UAS Behavior ..............................   42
   8.3        Redirect Servers ....................................   43
   9          Canceling a Request .................................   45
   9.1        Client Behavior .....................................   45
   9.2        Server Behavior .....................................   46
   10         Registrations .......................................   47
   10.1       Overview ............................................   47
   10.2       Constructing the REGISTER Request ...................   48
   10.2.1     Adding Bindings .....................................   51
   10.2.1.1   Setting the Expiration Interval of Contact
   Addresses ......................................................   51
   10.2.1.2   Preferences among Contact Addresses .................   52
   10.2.2     Removing Bindings ...................................   52
   10.2.3     Fetching Bindings ...................................   52
   10.2.4     Refreshing Bindings .................................   53
   10.2.5     Setting the Internal Clock ..........................   53
   10.2.6     Discovering a Registrar .............................   53
   10.2.7     Transmitting a Request ..............................   54
   10.2.8     Error Responses .....................................   54
   10.3       Processing REGISTER Requests ........................   54
   11         Querying for Capabilities ...........................   57
   11.1       Construction of OPTIONS Request .....................   58
   11.2       Processing of OPTIONS Request .......................   59
   12         Dialogs .............................................   60
   12.1       Creation of a Dialog ................................   61
   12.1.1     UAS behavior ........................................   61
   12.1.2     UAC behavior ........................................   62
   12.2       Requests within a Dialog ............................   63
   12.2.1     UAC Behavior ........................................   63
   12.2.1.1   Generating the Request ..............................   63
   12.2.1.2   Processing the Responses ............................   65
   12.2.2     UAS behavior ........................................   66
   12.3       Termination of a Dialog .............................   67
   13         Initiating a Session ................................   68
   13.1       Overview ............................................   68
   13.2       Caller Processing ...................................   68
   13.2.1     Creating the Initial INVITE .........................   68
   13.2.2     Processing INVITE Responses .........................   71
   13.2.2.1   1xx responses .......................................   71



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   13.2.2.2   3xx responses .......................................   72
   13.2.2.3   4xx, 5xx and 6xx responses ..........................   72
   13.2.2.4   2xx responses .......................................   72
   13.3       Callee Processing ...................................   73
   13.3.1     Processing of the INVITE ............................   73
   13.3.1.1   Progress ............................................   74
   13.3.1.2   The INVITE is redirected ............................   75
   13.3.1.3   The INVITE is rejected ..............................   75
   13.3.1.4   The INVITE is accepted ..............................   76
   14         Modifying an Existing Session .......................   77
   14.1       UAC Behavior ........................................   77
   14.2       UAS Behavior ........................................   78
   15         Terminating a Session ...............................   80
   15.1       Terminating a Dialog with a BYE Request .............   81
   15.1.1     UAC Behavior ........................................   81
   15.1.2     UAS Behavior ........................................   82
   16         Proxy Behavior ......................................   82
   16.1       Overview ............................................   82
   16.2       Stateful Proxy ......................................   83
   16.3       Request Validation ..................................   84
   16.4       Making a Routing Decision ...........................   87
   16.5       Request Processing ..................................   90
   16.6       Response Processing .................................   97
   16.7       Processing Timer C ..................................  105
   16.8       Handling Transport Errors ...........................  105
   16.9       CANCEL Processing ...................................  105
   16.10      Stateless Proxy .....................................  106
   16.11      Summary of Proxy Route Processing ...................  108
   16.11.1    Examples ............................................  108
   16.11.1.1  Basic SIP Trapezoid .................................  108
   16.11.1.2  Traversing a strict-routing proxy ...................  110
   16.11.1.3  Rewriting Record-Route header field values ..........  112
   17         Transactions ........................................  113
   17.1       Client Transaction ..................................  116
   17.1.1     INVITE Client Transaction ...........................  116
   17.1.1.1   Overview of INVITE Transaction ......................  116
   17.1.1.2   Formal Description ..................................  117
   17.1.1.3   Construction of the ACK Request .....................  120
   17.1.2     non-INVITE Client Transaction .......................  121
   17.1.2.1   Overview of the non-INVITE Transaction ..............  121
   17.1.2.2   Formal Description ..................................  122
   17.1.3     Matching Responses to Client Transactions ...........  123
   17.1.4     Handling Transport Errors ...........................  123
   17.2       Server Transaction ..................................  123
   17.2.1     INVITE Server Transaction ...........................  125
   17.2.2     non-INVITE Server Transaction .......................  126
   17.2.3     Matching Requests to Server Transactions ............  129
   17.2.4     Handling Transport Errors ...........................  131



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   17.3       RTT Estimation ......................................  131
   18         Reliability of Provisional Responses ................  132
   18.1       UAS Behavior ........................................  132
   18.2       UAC Behavior ........................................  135
   19         Transport ...........................................  136
   19.1       Clients .............................................  137
   19.1.1     Sending Requests ....................................  137
   19.1.2     Receiving Responses .................................  138
   19.2       Servers .............................................  139
   19.2.1     Receiving Requests ..................................  139
   19.2.2     Sending Responses ...................................  140
   19.3       Framing .............................................  141
   19.4       Error Handling ......................................  141
   20         Usage of HTTP Authentication ........................  141
   20.1       Framework ...........................................  142
   20.2       User-to-User Authentication .........................  144
   20.3       Proxy-to-User Authentication ........................  145
   20.4       The Digest Authentication Scheme ....................  148
   20.4.1     HTTP Digest .........................................  148
   21         S/MIME ..............................................  150
   21.1       S/MIME Certificates .................................  150
   21.2       S/MIME Key Exchange .................................  151
   21.3       Securing MIME bodies ................................  153
   21.4       Tunneling SIP in MIME ...............................  154
   21.4.1     Integrity and Confidentiality Properties of SIP
   Headers ........................................................  155
   21.4.1.1   Integrity ...........................................  155
   21.4.1.2   Confidentiality .....................................  155
   21.4.2     Tunneling Integrity and Authentication ..............  156
   21.4.3     Tunneling Encryption ................................  158
   22         Security Considerations .............................  159
   22.1       Attacks and Threat Models ...........................  159
   22.1.1     Registration Hijacking ..............................  160
   22.1.2     Impersonating a Server ..............................  160
   22.1.3     Tampering with Message Bodies .......................  161
   22.1.4     Tearing Down Sessions ...............................  162
   22.1.5     Denial of Service and Amplification .................  162
   22.2       Security Mechanisms .................................  163
   22.2.1     Transport and Network Layer Security ................  164
   22.2.2     HTTP Authentication .................................  165
   22.2.3     S/MIME ..............................................  165
   22.3       Implementing Security Mechanisms ....................  166
   22.3.1     Requirements for Implementers of SIP ................  166
   22.3.2     Security Solutions ..................................  167
   22.3.2.1   Registration ........................................  167
   22.3.2.2   Requests and Transitive Trust .......................  168
   22.3.2.3   Peer to Peer Requests ...............................  170
   22.3.2.4   DoS Protection ......................................  171



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   22.4       Limitations .........................................  172
   22.4.1     HTTP Digest .........................................  172
   22.4.2     S/MIME ..............................................  173
   22.4.3     TLS .................................................  174
   22.5       Privacy .............................................  174
   23         Common Message Components ...........................  175
   23.1       SIP Uniform Resource Indicators .....................  175
   23.1.1     SIP URI Components ..................................  175
   23.1.2     Character Escaping Requirements .....................  179
   23.1.3     Example SIP URIs ....................................  180
   23.1.4     SIP URI Comparison ..................................  180
   23.1.5     Forming Requests from a SIP URI .....................  183
   23.1.6     Relating SIP URIs and tel URLs ......................  184
   23.2       Option Tags .........................................  186
   23.3       Tags ................................................  186
   24         Header Fields .......................................  187
   24.1       Accept ..............................................  189
   24.2       Accept-Encoding .....................................  189
   24.3       Accept-Language .....................................  192
   24.4       Alert-Info ..........................................  192
   24.5       Allow ...............................................  192
   24.6       Authentication-Info .................................  193
   24.7       Authorization .......................................  193
   24.8       Call-ID .............................................  194
   24.9       Call-Info ...........................................  194
   24.10      Contact .............................................  195
   24.11      Content-Disposition .................................  196
   24.12      Content-Encoding ....................................  196
   24.13      Content-Language ....................................  197
   24.14      Content-Length ......................................  197
   24.15      Content-Type ........................................  198
   24.16      CSeq ................................................  198
   24.17      Date ................................................  198
   24.18      Error-Info ..........................................  199
   24.19      Expires .............................................  199
   24.20      From ................................................  200
   24.21      In-Reply-To .........................................  200
   24.22      Max-Forwards ........................................  201
   24.23      Min-Expires .........................................  201
   24.24      MIME-Version ........................................  201
   24.25      Organization ........................................  202
   24.26      Priority ............................................  202
   24.27      Proxy-Authenticate ..................................  203
   24.28      Proxy-Authorization .................................  203
   24.29      Proxy-Require .......................................  204
   24.30      RAck ................................................  204
   24.31      Record-Route ........................................  204
   24.32      Reply-To ............................................  204



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   24.33      Require .............................................  205
   24.34      Retry-After .........................................  205
   24.35      Route ...............................................  206
   24.36      RSeq ................................................  206
   24.37      Server ..............................................  206
   24.38      Subject .............................................  207
   24.39      Supported ...........................................  207
   24.40      Timestamp ...........................................  207
   24.41      To ..................................................  208
   24.42      Unsupported .........................................  208
   24.43      User-Agent ..........................................  208
   24.44      Via .................................................  209
   24.45      Warning .............................................  210
   24.46      WWW-Authenticate ....................................  211
   25         Response Codes ......................................  212
   25.1       Provisional 1xx .....................................  212
   25.1.1     100 Trying ..........................................  212
   25.1.2     180 Ringing .........................................  212
   25.1.3     181 Call Is Being Forwarded .........................  212
   25.1.4     182 Queued ..........................................  212
   25.1.5     183 Session Progress ................................  213
   25.2       Successful 2xx ......................................  213
   25.2.1     200 OK ..............................................  213
   25.3       Redirection 3xx .....................................  213
   25.3.1     300 Multiple Choices ................................  213
   25.3.2     301 Moved Permanently ...............................  214
   25.3.3     302 Moved Temporarily ...............................  214
   25.3.4     305 Use Proxy .......................................  214
   25.3.5     380 Alternative Service .............................  214
   25.4       Request Failure 4xx .................................  215
   25.4.1     400 Bad Request .....................................  215
   25.4.2     401 Unauthorized ....................................  215
   25.4.3     402 Payment Required ................................  215
   25.4.4     403 Forbidden .......................................  215
   25.4.5     404 Not Found .......................................  215
   25.4.6     405 Method Not Allowed ..............................  215
   25.4.7     406 Not Acceptable ..................................  215
   25.4.8     407 Proxy Authentication Required ...................  216
   25.4.9     408 Request Timeout .................................  216
   25.4.10    410 Gone ............................................  216
   25.4.11    413 Request Entity Too Large ........................  216
   25.4.12    414 Request-URI Too Long ............................  216
   25.4.13    415 Unsupported Media Type ..........................  216
   25.4.14    416 Unsupported URI Scheme ..........................  217
   25.4.15    420 Bad Extension ...................................  217
   25.4.16    421 Extension Required ..............................  217
   25.4.17    423 Registration Too Brief ..........................  217
   25.4.18    480 Temporarily Unavailable .........................  217



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   25.4.19    481 Call/Transaction Does Not Exist .................  218
   25.4.20    482 Loop Detected ...................................  218
   25.4.21    483 Too Many Hops ...................................  218
   25.4.22    484 Address Incomplete ..............................  218
   25.4.23    485 Ambiguous .......................................  218
   25.4.24    486 Busy Here .......................................  219
   25.4.25    487 Request Terminated ..............................  219
   25.4.26    488 Not Acceptable Here .............................  219
   25.4.27    491 Request Pending .................................  219
   25.4.28    493 Undecipherable ..................................  219
   25.5       Server Failure 5xx ..................................  220
   25.5.1     500 Server Internal Error ...........................  220
   25.5.2     501 Not Implemented .................................  220
   25.5.3     502 Bad Gateway .....................................  220
   25.5.4     503 Service Unavailable .............................  220
   25.5.5     504 Server Time-out .................................  220
   25.5.6     505 Version Not Supported ...........................  221
   25.5.7     513 Message Too Large ...............................  221
   25.6       Global Failures 6xx .................................  221
   25.6.1     600 Busy Everywhere .................................  221
   25.6.2     603 Decline .........................................  221
   25.6.3     604 Does Not Exist Anywhere .........................  221
   25.6.4     606 Not Acceptable ..................................  222
   26         Examples ............................................  222
   26.1       Registration ........................................  222
   26.2       Session Setup .......................................  223
   27          Augmented BNF for the SIP Protocol .................  228
   27.1       Basic Rules .........................................  229
   28         IANA Considerations .................................  246
   28.1       Option Tags .........................................  246
   28.1.1     Registration of 100rel ..............................  247
   28.2       Warn-Codes ..........................................  248
   28.3       Header Field Names ..................................  248
   28.4       Method and Response Codes ...........................  249
   29         Changes From RFC 2543 ...............................  249
   29.1       Major Functional Changes ............................  249
   29.2       Minor Functional Changes ............................  253
   30         Acknowledgments .....................................  254
   31         Authors' Addresses ..................................  255
   32         Normative References ................................  256
   33         Non-Normative References ............................  258

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1 Introduction

   There are many applications of the Internet that require the creation
   and management of a session, where a session is considered an
   exchange of data between an association of participants. The
   implementation of these services is complicated by the practices of
   participants; users may move between endpoints, they may be
   addressable by multiple names, and they may communicate in several
   different media - sometimes simultaneously. Numerous protocols have
   been authored that carry various forms of real-time multimedia
   session data such as voice, video, or text messages. SIP works in
   concert with these protocols by enabling Internet endpoints (called
   "user agents") to discover one another and to agree on a
   characterization of a session they would like to share.  For locating
   prospective session participants, and for other functions, SIP
   enables creation of an infrastructure of network hosts (called "proxy
   servers") to which user agents can send registrations, invitations to
   sessions and other requests. SIP is an agile, general-purpose tool
   for creating, modifying and terminating sessions that works
   independently of underlying transport protocols and without
   dependency on the type of session that is being established.

2 Overview of SIP Functionality

   The Session Initiation Protocol (SIP) is an application-layer control
   protocol that can establish, modify, and terminate multimedia
   sessions (conferences) such as Internet telephony calls. SIP can also
   invite participants to already existing sessions, such as multicast
   conferences. Media can be added to (and removed from) an existing
   session. SIP transparently supports name mapping and redirection
   services, which supports personal mobility [29] - users can maintain
   a single externally visible identifier (SIP URI) regardless of their
   network location.

   SIP supports five facets of establishing and terminating multimedia
   communications:

        User location: determination of the end system to be used for
             communication;

        User availability: determination of the willingness of the
             called party to engage in communications;

        User capabilities: determination of the media and media
             parameters to be used;

        Session setup: "ringing", establishment of session parameters at
             both called and calling party;



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        Session management: including transfer and termination of
             sessions, modifying session parameters, and invoking
             services.

   SIP is not a vertically integrated communications system. SIP is
   rather a component that can be used with other IETF protocols to
   build a complete multimedia architecture. Typically, these
   architectures will include protocols such as the real-time transport
   protocol (RTP) (RFC 1889 [32]) for transporting real-time data and
   providing QoS feedback, the real-time streaming protocol (RTSP) (RFC
   2326 [35]) for controlling delivery of streaming media, the Media
   Gateway Control Protocol (MEGACO) (RFC 3015 [43]) for controlling
   gateways to the Public Switched Telephone Network (PSTN), and the
   session description protocol (SDP) (RFC 2327 [11]) for describing
   multimedia sessions. Therefore, SIP should be used in conjunction
   with other protocols in order to provide complete services to the
   users. However, the basic functionality and operation of SIP does not
   depend on any of these protocols.

   SIP does not provide services. SIP rather provides primitives that
   can be used to implement different services. For example, SIP can
   locate a user and deliver an opaque object to his current location.
   If this primitive is used to deliver a session description written in
   SDP, for instance, the parameters of a session can be agreed between
   endpoints.  If the same primitive is used to deliver a photo of the
   caller as well as the session description, a "caller ID" service can
   be easily implemented.  As this example shows, a single primitive is
   typically used to provide several different services.

   SIP does not offer conference control services such as floor control
   or voting and does not prescribe how a conference is to be managed.
   SIP can be used to initiate a session that uses some other conference
   control protocol. Since SIP messages and the sessions they establish
   can pass through entirely different networks, SIP cannot, and does
   not, provide any kind of network resource reservation capabilities.

   The nature of the services provided by SIP make security particularly
   important. To that end, SIP provides a suite of security services,
   which include denial-of-service prevention, authentication (both user
   to user and proxy to user), integrity protection, and encryption and
   privacy services.

   SIP works with both IPv4 and IPv6.

3 Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",



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   and "OPTIONAL" are to be interpreted as described in RFC 2119 [24]
   and indicate requirement levels for compliant SIP implementations.

4 Overview of Operation

   This section introduces the basic operations of SIP using simple
   examples. This section is tutorial in nature and does not contain any
   normative statements.

   The first example shows the basic functions of SIP: location of an
   end point, signal of a desire to communicate, negotiation of session
   parameters to establish the session, and teardown of the session once
   established.

   Figure 1 shows a typical example of a SIP message exchange between
   two users, Alice and Bob. (Each message is labeled with the letter
   "F" and a number for reference by the text.) In this example, Alice
   uses a SIP application on her PC (referred to as a softphone) to call
   Bob on his SIP phone over the Internet. Also shown are two SIP proxy
   servers that act on behalf of Alice and Bob to facilitate the session
   establishment. This typical arrangement is often referred to as the
   "SIP trapezoid" as shown by the geometric shape of the dashed lines
   in Figure 1.


   Alice "calls" Bob using his SIP identity, a type of Uniform Resource
   Identifier (URI) called a SIP URI and defined in Section 23.1. It has
   a similar form to an email address, typically containing a username
   and a host name. In this case, it is sip:bob@biloxi.com, where
   biloxi.com is the domain of Bob's SIP service provider (which can be
   an enterprise, retail provider, etc). Alice also has a SIP URI of
   sip:alice@atlanta.com. Alice might have typed in Bob's URI or perhaps
   clicked on a hyperlink or an entry in an address book.

   SIP is based on an HTTP-like request/response transacton model. Each
   transaction consists of a request that invokes a particular "Method",
   or function, on the server, and at least one response. In this
   example, the transaction begins with Alice's softphone sending an
   INVITE request addressed to Bob's SIP URI. INVITE is an example of a
   SIP method which specifies the action that the requestor (Alice)
   wants the server (Bob) to take. The INVITE request contains a number
   of header fields. Header fields are named attributes that provide
   additional information about a message. The ones present in an INVITE
   include a unique identifier for the call, the destination address,
   Alice's address, and information about the type of session that Alice
   wishes to establish with Bob. The INVITE (message F1 in Figure 1)
   might look like this:




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                 atlanta.com  . . . biloxi.com
             .      proxy              proxy     .
           .                                       .
   Alice's  . . . . . . . . . . . . . . . . . . . .  Bob's
  softphone                                        SIP Phone
     |                |                |                |
     |    INVITE F1   |                |                |
     |--------------->|    INVITE F2   |                |
     |  100 Trying F3 |--------------->|    INVITE F4   |
     |<---------------|  100 Trying F5 |--------------->|
     |                |<-------------- | 180 Ringing F6 |
     |                | 180 Ringing F7 |<---------------|
     | 180 Ringing F8 |<---------------|     200 OK F9  |
     |<---------------|    200 OK F10  |<---------------|
     |    200 OK F11  |<---------------|                |
     |<---------------|                |                |
     |                       ACK F12                    |
     |------------------------------------------------->|
     |                   Media Session                  |
     |<================================================>|
     |                       BYE F13                    |
     |<-------------------------------------------------|
     |                     200 OK F14                   |
     |------------------------------------------------->|
     |                                                  |




   Figure 1: SIP session setup example with SIP trapezoid


     INVITE sip:bob@biloxi.com SIP/2.0
     Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bK776asdhds
     To: Bob <sip:bob@biloxi.com>
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 314159 INVITE
     Contact: <sip:alice@pc33.atlanta.com>
     Max-Forwards: 70
     Content-Type: application/sdp
     Content-Length: 142

     (Alice's SDP not shown)





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   The first line of the text-encoded message contains the method name
   (INVITE). The lines that follow are a list of header fields.  This
   example contains a minimum required set. The headers are briefly
   described below:

   Via contains the address (pc33.atlanta.com) on which Alice is
   expecting to receive responses to this request. It also contains a
   branch parameter that contains an identifier for this transaction.

   To contains a display name (Bob) and a SIP URI (sip:bob@biloxi.com)
   towards which the request was originally directed. Display names are
   described in RFC 2822 [20].

   From also contains a display name (Alice) and a SIP URI
   (sip:alice@atlanta.com) that indicate the originator of the request.
   This header field also has a tag parameter containing a pseudorandom
   string (1928301774) that was added to the URI by the softphone. It is
   used for identification purposes.

   Call-ID contains a globally unique identifier for this call,
   generated by the combination of a pseudorandom string and the
   softphone's IP address. The combination of the To, From, and Call-ID
   completely define a peer-to-peer SIP relationship betwee Alice and
   Bob, and is referred to as a "dialog".

   CSeq or Command Sequence contains an integer and a method name. The
   CSeq number is incremented for each new request, and is a traditional
   sequence number.

   Contact contains a SIP URI that represents a direct route to reach or
   contact Alice, usually composed of a username at an FQDN.  While an
   FQDN is preferred, many end systems do not have registered domain
   names, so IP addresses are permitted. While the Via header field
   tells other elements where to send the response, the Contact header
   field tells other elements where to send future requests for this
   dialog.

   Content-Type contains a description of the message body (not shown).

   Content-Length contains an octet (byte) count of the message body.

   The complete set of SIP header fields is defined in Section 24.

   The details of the session, type of media, codec, sampling rate, etc.
   are not described using SIP. Rather, the body of a SIP message
   contains a description of the session, encoded in some other protocol
   format.  One such format is Session Description Protocol (SDP) [11].
   This SDP message (not shown in the example) is carried by the SIP



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   message in a way that is analogous to a document attachment being
   carried by an email message, or a web page being carried in an HTTP
   message.

   Since the softphone does not know the location of Bob or the SIP
   server in the biloxi.com domain, the softphone sends the INVITE to
   the SIP server that serves Alice's domain, atlanta.com.  The IP
   address of the atlanta.com SIP server could have been configured in
   Alice's softphone, or it could have been discovered by DHCP, for
   example.

   The atlanta.com SIP server is a type of SIP server known as a proxy
   server. A proxy server receives SIP requests and forwards them on
   behalf of the requestor. In this example, the proxy server receives
   the INVITE request and sends a 100 (Trying) response back to Alice's
   softphone. The 100 (Trying) response indicates that the INVITE has
   been received and that the proxy is working on her behalf to route
   the INVITE to the destination. Responses in SIP use a three-digit
   code followed by a descriptive phrase. This response contains the
   same To, From, Call-ID, and CSeq as the INVITE, which allows Alice's
   softphone to correlate this response to the sent INVITE. The
   atlanta.com proxy server locates the proxy server at biloxi.com,
   possibly by performing a particular type of DNS (Domain Name Service)
   lookup to find the SIP server that serves the biloxi.com domain. This
   is described in [2]. As a result, it obtains the IP address of the
   biloxi.com proxy server and forwards, or proxies, the INVITE request
   there. Before forwarding the request, the atlanta.com proxy server
   adds an additional Via header field that contains its own IP address
   (the INVITE already contains Alice's IP address in the first Via).
   The biloxi.com proxy server receives the INVITE and responds with a
   100 (Trying) response back to the Atlanta.com proxy server to
   indicate that it has received the INVITE and is processing the
   request. The proxy server consults a database, generically called a
   location service, that contains the current IP address of Bob. (We
   shall see in the next section how this database can be populated.)
   The biloxi.com proxy server adds another Via header with its own IP
   address to the INVITE and proxies it to Bob's SIP phone.

   Bob's SIP phone receives the INVITE and alerts Bob to the incoming
   call from Alice so that Bob can decide whether or not to answer the
   call, i.e., Bob's phone rings. Bob's SIP phone sends an indication of
   this in a 180 (Ringing) response, which is routed back through the
   two proxies in the reverse direction. Each proxy uses the Via header
   to determine where to send the response and removes its own address
   from the top. As a result, although DNS and location service lookups
   were required to route the initial INVITE, the 180 (Ringing) response
   can be returned to the caller without lookups or without state being
   maintained in the proxies. This also has the desirable property that



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   each proxy that sees the INVITE will also see all responses to the
   INVITE.

   When Alice's softphone receives the 180 (Ringing) response, it passes
   this information to Alice, perhaps using an audio ringback tone or by
   displaying a message on Alice's screen.

   In this example, Bob decides to answer the call. When he picks up the
   handset, his SIP phone sends a 200 (OK) response to indicate that the
   call has been answered. The 200 (OK) contains a message body with the
   SDP media description of the type of session that Bob is willing to
   establish with Alice. As a result, there is a two-phase exchange of
   SDP messages; Alice sent one to Bob, and Bob sent one back to Alice.
   This two-phase exchange provides basic negotiation capabilities and
   is based on a simple offer/answer model of SDP exchange. If Bob did
   not wish to answer the call or was busy on another call, an error
   response would have been sent instead of the 200 (OK), which would
   have resulted in no media session being established. The complete
   list of SIP response codes is in Section 25. The 200 (OK) (message F9
   in Figure 1) might look like this as Bob sends it out:


     SIP/2.0 200 OK
     Via: SIP/2.0/UDP server10.biloxi.com;branch=z9hG4bKnashds8
     Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branch=z9hG4bK77ef4c2312983.1
     Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bK776asdhds
     To: Bob <sip:bob@biloxi.com>;tag=a6c85cf
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 314159 INVITE
     Contact: <sip:bob@192.0.2.8>
     Content-Type: application/sdp
     Content-Length: 131

     (Bob's SDP not shown)



   The first line of the response contains the response code (200) and
   the reason phrase (OK). The remaining lines contain header fields.
   The Via header fields, To, From, Call- ID, and CSeq are all copied
   from the INVITE request.  (There are three Via headers - one added by
   Alice's SIP phone, one added by the atlanta.com proxy, and one added
   by the biloxi.com proxy.) Bob's SIP phone has added a tag parameter
   to the To header field. This tag will be incorporated by both User
   Agents into the dialog and will be included in all future requests
   and responses in this call. The Contact header field contains a URI
   at which Bob can be directly reached at his SIP phone. The Content-



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   Type and Content-Length refer to the message body (not shown) that
   contains Bob's SDP media information.

   In additon to DNS and location service lookups shown in this example,
   proxy servers can make flexible "routing decisions" to decide where
   to send a request. For example, if Bob's SIP phone returned a 486
   (Busy Here) response, the biloxi.com proxy server could proxy the
   INVITE to Bob's voicemail server. A proxy server can also send an
   INVITE to a number of locations at the same time.  This type of
   parallel search is known as "forking".

   In this case, the 200 (OK) is routed back through the two proxies and
   is received by Alice's softphone which then stops the ringback tone
   and indicates that the call has been answered. Finally, an
   acknowledgement message, ACK, is sent by Alice to Bob to confirm the
   reception of the final response (200 (OK)). In this example, the ACK
   is sent directly from Alice to Bob, bypassing the two proxies. This
   is because, through the INVITE/200 (OK) exchange, the two SIP user
   agents have learned each other's IP address through the Contact
   header fields, which was not known when the initial INVITE was sent.
   The lookups performed by the two proxies are no longer needed, so
   they drop out of the call flow. This completes the INVITE/200/ACK
   three-way handshake used to establish SIP sessions and is the end of
   the transaction. Full details on session setup are in Section 13.

   Alice and Bob's media session has now begun, and they send media
   packets using the format agreed to in the exchange of SDP. In
   general, the end-to-end media packets take a different path from the
   SIP signaling messages.

   During the session, either Alice or Bob may decide to change the
   characteristics of the media session. This is accomplished by sending
   a re-INVITE containing a new media description. If the change is
   accepted by the other party, a 200 (OK) is sent, which is itself
   responded to with an ACK. This re-INVITE references the existing
   dialog so the other party knows that it is to modify an existing
   session instead of establishing a new session. If the change is not
   accepted, an error response, such as a 406 (Not Acceptable), is sent,
   which also receives an ACK. However, the failure of the re-INVITE
   does not cause the existing call to fail - the session continues
   using the previously negotiated characteristics.  Full details on
   session modification are in Section 14.

   At the end of the call, Bob disconnects (hangs up) first, and
   generates a BYE message. This BYE is routed directly to Alice's
   softphone, again bypassing the proxies. Alice confirms receipt of the
   BYE with a 200 (OK) response, which terminates the session and the
   BYE transaction. No ACK is sent - an ACK is only sent in response to



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   a response to an INVITE request. The reasons for this special
   handling for INVITE will be discussed later, but relate to the
   reliability mechanisms in SIP, the length of time it can take for a
   ringing phone to be answered, and forking. For this reason, request
   handling in SIP is often classified as either INVITE or non- INVITE,
   referring to all other methods besides INVITE. Full details on
   session termination are in Section 15.

   Full details of all the messages shown in the example of Figure 1 are
   shown in Section 26.2.

   In some cases, it may be useful for proxies in the SIP signaling path
   to see all the messaging between the endpoints for the duration of
   the session. For example, if the biloxi.com proxy server wished to
   remain in the SIP messaging path beyond the initial INVITE, it would
   add to the INVITE a required routing header field known as Record-
   Route that contained a URI resolving to the proxy.  This information
   would be received by both Bob's SIP phone and (due to the Record-
   Route header field being passed back in the 200 (OK)) Alice's
   softphone and stored for the duration of the dialog.  The biloxi.com
   proxy server would then receive and proxy the ACK, BYE, and 200 (OK)
   to the BYE. Each proxy can independently decide to receive subsequent
   messaging, and that messaging will go through all proxies that elect
   to receive it.  This capability is frequently used for proxies that
   are providing mid-call features.

   Registration is another common operation in SIP. Registration is one
   way that the biloxi.com server can learn the current location of Bob.
   Upon initialization, and at periodic intervals, Bob's SIP phone sends
   REGISTER messages to a server in the biloxi.com domain known as a SIP
   registrar. The REGISTER messages associate Bob's SIP URI
   (sip:bob@biloxi.com) with the machine he is currently logged in at
   (conveyed as a SIP URI in the Contact header). The registrar writes
   this association, also called a binding, to a database, called the
   location service , where it can be used by the proxy in the
   biloxi.com domain. Often, a registrar server for a domain is co-
   located with the proxy for that domain. It is an important concept
   that the distinction between types of SIP servers is logical, not
   physical.

   Bob is not limited to registering from a single device. For example,
   both his SIP phone at home and the one in the office could send
   registrations. This information is stored together in the location
   service and allows a proxy to perform various types of searches to
   locate Bob. Similarly, more than one user can be registered on a
   single device at the same time.

   The location service is just an abstract concept. It generally



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   contains information that allows a proxy to input a URI and get back
   a translated URI that tells the proxy where to send the request.
   Registrations are one way to create this information, but not the
   only way. Arbitrary mapping functions can be programmed, at the
   discretion of the administrator.

   Finally, it is important to note that in SIP, registration is used
   for routing incoming SIP requests and has no role in authorizing
   outgoing requests. Authorization and authentication are handled in
   SIP either on a request-by-request, challenge/response mechanism, or
   using a lower layer scheme as discussed in Section 22.

   The complete set of SIP message details for this registration example
   is in Section 26.1.

   Additional operations in SIP, such as querying for the capabilities
   of a SIP server or client using OPTIONS, canceling a pending request
   using CANCEL, or supporting reliability of provisional responses
   using PRACK will be introduced in later sections.

5 Structure of the Protocol

   SIP is structured as a layered protocol, which means that its
   behavior is described in terms of a set of fairly independent
   processing stages with only a loose coupling between each stage. The
   protocol is structured into layers for the purpose of presentation
   and conciseness; it allows the grouping of functions common across
   elements into a single place. It does not dictate an implementation
   in any way. When we say that an element "contains" a layer, we mean
   it is compliant to the set of rules defined by that layer.

   Not every element specified by the protocol contains every layer.
   Furthermore, the elements specified by SIP are logical elements, not
   physical ones. A physical realization can choose to act as different
   logical elements, perhaps even on a transaction-by-transaction basis.

   The lowest layer of SIP is its syntax and encoding. Its encoding is
   specified using a BNF. The complete BNF is specified in Section 27.
   However, a basic overview of the structure of a SIP message can be
   found in Section 7. This section provides enough understanding of the
   format of a SIP message to facilitate understanding the remainder of
   the protocol.

   The next higher layer is the transport layer. This layer defines how
   a client takes a request and physically sends it over the network,
   and how a response is sent by a server and then received by a client.
   All SIP elements contain a transport layer. The transport layer is
   described in Section 19.



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   The next higher layer is the transaction layer. Transactions are a
   fundamental component of SIP. A transaction is a request, sent by a
   client transaction (using the transport layer), to a server
   transaction, along with all responses to that request sent from the
   server transaction back to the client. The transaction layer handles
   application layer retransmissions, matching of responses to requests,
   and application layer timeouts. Any task that a UAC accomplishes
   takes place using a series of transactions. Discussion of
   transactions can be found in Section 17. User agents contain a
   transaction layer, as do stateful proxies. Stateless proxies do not
   contain a transaction layer.

   The transaction layer has a client component (referred to as a client
   transaction), and a server component (referred to as a server
   transaction), each of which are represented by an FSM that is
   constructed to process a particular request. The layer on top of the
   transaction layer is called the transaction user (TU), of which there
   are several types. When a TU wishes to send a request, it creates a
   client transaction instance and passes it the request along with the
   destination IP address, port, and transport to which to send the
   request.

   A TU which creates a client transaction can also cancel it. When a
   client cancels a transaction, it requests that the server stop
   further processing, revert to the state that existed before the
   transaction was initiated, and generate a specific error response to
   that transaction.  This is done with a CANCEL request, which
   constitutes its own transaction, but references the transaction to be
   cancelled.  Cancellation is described in Section 9.

   There are several different types of transaction users. A UAC
   contains a UAC core, a UAS contains a UAS core, and a proxy contains
   a proxy core. The behavior of the UAC and UAS cores depend largely on
   the method. However, there are some common rules for all methods.
   These rules are captured in Section 8. They primarily deal with
   construction of a request, in the case of a UAC, and processing of
   that request and generation of a response, in the case of a UAS.

   UAC and UAS core behavior for the REGISTER method is described in
   Section 10. Registrations play an important role in SIP. In fact, a
   UAS that handles a REGISTER is given a special name - a registrar -
   and it is described in that section.

   UAC and UAS core behavior for the OPTIONS method, used for
   determining the capabilities of a UA, are described in Section 11.

   Certain other requests are sent within a dialog.  A dialog is a
   peer-to-peer SIP relationship between two user agents that persists



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   for some time. The dialog facilitates sequencing of messages and
   proper routing of requests between the user agents. The INVITE method
   is the only way defined in this specification to establish a dialog.
   When a UAC sends a request that is within the context of a dialog, it
   follows the common UAC rules as discussed in Section 8, but also the
   rules for mid-dialog requests. Section 12 discusses dialogs and
   presents the procedures for their construction, and maintenance, in
   addition to construction of requests within a dialog.

   The UAS core can generate provisional responses to requests, which
   are responses that provide additional information about the request
   processing but do not indicate completion. Normally, provisional
   responses are not transmitted reliably. However, an optional
   mechanism exists for them to be transmitted reliably. This mechanism
   makes use of a method called PRACK, sent as a separate transaction
   within the dialog between the UAC and UAS, which is used to
   acknowledge a reliable provisional response.

   The most important method in SIP is the INVITE method, which is used
   to establish a session between participants. A session is a
   collection of participants, and streams of media between them, for
   the purposes of communication. Section 13 discusses how sessions are
   initiated, resulting in one or more SIP dialogs. Section 14 discusses
   how characteristics of that session are modified through the use of
   an INVITE request within a dialog.  Finally, section 15 discusses how
   a session is terminated.

   The procedures of Sections 8, 10, 11, 12, 13, 14, and 15 deal
   entirely with the UA core (Section 9 describes cancellation, which
   applies to both UA core and proxy core). Section 16 discusses the
   proxy element, which facilitates routing of messages between user
   agents.

6 Definitions

   This specification uses a number of terms to refer to the roles
   played by participants in SIP communications. The terms and generic
   syntax of URI and URL are defined in RFC 2396 [13]. The following
   terms have special significance for SIP.

        Back-to-Back user agent: A back-to-back user agent (B2BUA) is a
             logical entity that receives a request and processes it as
             an user agent server (UAS). In order to determine how the
             request should be answered, it acts as an user agent client
             (UAC) and generates requests. Unlike a proxy server, it
             maintains dialog state and must participate in all requests
             sent on the dialogs it has established. Since it is a
             concatenation of a UAC and UAS, no explicit definitions are



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             needed for its behavior.

        Call: A call is an informal term that refers to a dialog between
             peers generally set up for the purposes of a multimedia
             conversation.

        Call leg: Another name for a dialog.

        Call stateful: A proxy is call stateful if it retains state for
             a dialog from the initiating INVITE to the terminating BYE
             request. A call stateful proxy is always stateful, but the
             converse is not true.

        Client: A client is any network element that sends SIP requests
             and receives SIP responses. Clients may or may not interact
             directly with a human user. User agent clients and proxies
             are clients.

        Conference: A multimedia session (see below) that contains
             multiple participants.

        Dialog: A dialog is a peer-to-peer SIP relationship between a
             UAC and UAS that persists for some time. A dialog is
             established by SIP messages, such as a 2xx response to an
             INVITE request. A dialog is identified by a call
             identifier, local address, and remote address.  A dialog
             was formerly known as a call leg in RFC 2543.

        Downstream: A direction of message forwarding within a
             transaction that refers to the direction that requests flow
             from the user agent client to user agent server.

        Final response: A response that terminates a SIP transaction, as
             opposed to a provisional response that does not. All 2xx,
             3xx, 4xx, 5xx and 6xx responses are final.

        Header: A header is a component of a sip message that conveys
             information about the message. It is structured as a header
             name, followed by a colon, followed by its value.

        Home Domain: The domain providing service to a SIP user.
             Typically, this is the domain present in the URI in the
             address-of-record of a registration.

        Informational Response: Same as a provisional response.

        Initiator, calling party, caller: The party initiating a session
             (and dialog) with an INVITE request. A caller retains this



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             role from the time it sends the initial INVITE which
             established a dialog, until the termination of that dialog.

        Invitation: An INVITE request.

        Invitee, invited user, called party, callee: The party that
             receives an INVITE request for the purposes of establishing
             a new session. A callee retains this role from the time it
             receives the INVITE until the termination of the dialog
             established by that INVITE.

        Location service: A location service is used by a SIP redirect
             or proxy server to obtain information about a callee's
             possible location(s). It contains a list of bindings of
             adress-of-record keys to zero or more contact addresses.
             The bindings can be created and removed in many ways; this
             specification defines a REGISTER method that updates the
             bindings.

        Loop: A request that arrives at a proxy, is forwarded, and later
             arrives back at the same proxy. When it arrives the second
             time, its Request-URI is identical to the first time, and
             other headers that affect proxy operation are unchanged, so
             that the proxy would make the same processing decision on
             the request it made the first time around. Looped requests
             are errors, and the procedures for detecting them and
             handling them are described by the protocol.

        Loose Routing: A proxy is said to be loose routing if it follows
             the procedures defined in this specification for processing
             of the Route header field. These procedures separate the
             destination of the request (present in the Request-URI)
             from the set of proxies that need to be visited along the
             way (present in the Route header field). A proxy compliant
             to these mechanisms is also known as a loose router.

        Message: Data sent between SIP elements as part of the the
             protocol. SIP messages are either requests or responses.

        Method: The method is the primary function that a request is
             meant to invoke on a server. The method is carried in the
             request message itself. Example methods are INVITE and BYE.

        Outbound proxy: A proxy that receives all requests from a
             client, even though it is not the server resolved by the
             Request-URI. The outbound proxy sends these requests, after
             any local processing, to the address indicated in the
             Request-URI, or to another outbound proxy. Typically, a UA



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             is manually configured with its outbound proxy, or can
             learn it through auto-configuration protocols.

        Parallel search: In a parallel search, a proxy issues several
             requests to possible user locations upon receiving an
             incoming request.  Rather than issuing one request and then
             waiting for the final response before issuing the next
             request as in a sequential search , a parallel search
             issues requests without waiting for the result of previous
             requests.

        Provisional response: A response used by the server to indicate
             progress, but that does not terminate a SIP transaction.
             1xx responses are provisional, other responses are
             considered final.  Normally, provisional responses are not
             sent reliably. A provisional response that is sent reliably
             is referred to as a reliable provisional response

        Proxy, proxy server: An intermediary entity that acts as both a
             server and a client for the purpose of making requests on
             behalf of other clients. A proxy server primarily plays the
             role of routing, which means its job is to ensure that a
             request is passed on to another entity "closer" to the
             targeted user. Proxies are also useful for enforcing policy
             (for example, making sure a user is allowed to make a
             call). A proxy interprets, and, if necessary, rewrites
             specific parts of a request message before forwarding it.

        Recursion: A client recurses on a 3xx response when it generates
             a new request to the URIs in the Contact headers in the
             response.

        Redirect Server: A redirect server is a server that generates
             3xx responses to requests it receives, directing the client
             to contact an alternate URI.

        Registrar: A registrar is a server that accepts REGISTER
             requests, and places the information it receives in those
             requests into the location service for the domain it
             handles.

        Regular Transaction: A regular transaction is any transaction
             with a method other than INVITE, ACK, or CANCEL.

        Reliable Provisional Response: A provisional response that is
             sent reliably from the UAS to UAC.

        Request: A SIP message sent from a client to a server, for the



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             purpose of invoking a particular operation.

        Response: A SIP message sent from a server to a client, for
             indicating the status of a request sent from the client to
             the server.

        Ringback: Ringback is the signaling tone produced by the calling
             party's application indicating that a called party is being
             alerted (ringing).

        Route Refresh Request: A route refresh request sent within a
             dialog is defined as a request that can modify the route
             set of the dialog.

        Server: A server is a network element that receives requests in
             order to service them and sends back responses to those
             requests.  Examples of servers are proxies, user agent
             servers, redirect servers, and registrars.

        Sequential search: In a sequential search, a proxy server
             attempts each contact address in sequence, proceeding to
             the next one only after the previous has generated a non-
             2xx final response.

        Session: From the SDP specification: "A multimedia session is a
             set of multimedia senders and receivers and the data
             streams flowing from senders to receivers. A multimedia
             conference is an example of a multimedia session." (RFC
             2327 [11]) (A session as defined for SDP can comprise one
             or more RTP sessions.) As defined, a callee can be invited
             several times, by different calls, to the same session. If
             SDP is used, a session is defined by the concatenation of
             the user name , session id , network type , address type ,
             and address elements in the origin field.

        (SIP) transaction: A SIP transaction occurs between a client and
             a server and comprises all messages from the first request
             sent from the client to the server up to a final (non-1xx)
             response sent from the server to the client, and the ACK
             for the response in the case the response was a non-2xx.
             The ACK for a 2xx response is a separate transaction.

        Spiral: A spiral is a SIP request that is routed to a proxy,
             forwarded onwards, and arrives once again at that proxy,
             but this time, differs in a way that will result in a
             different processing decision than the original request.
             Typically, this means that the request's Request-URI
             differs from its previous arrival. A spiral is not an error



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             condition, unlike a loop. A typical cause for this is call
             forwarding. A user calls joe@example.com. The example.com
             proxy forwards it to Joe's PC, which in turn, forwards it
             to bob@example.com. This request is proxied back to the
             example.com proxy. However, this is not a loop. Since the
             request is targeted at a different user, it is considered a
             spiral, and is a valid condition.

        Stateful proxy: A logical entity that maintains the client and
             server transaction state machines defined by this
             specification during the processing of a request. Also
             known as a transaction stateful proxy. The behavior of a
             stateful proxy is further defined in Section 16. A stateful
             proxy is not the same as a call stateful proxy.

        Stateless proxy: A logical entity that does not maintain the
             client or server transaction state machines defined in this
             specification when it processes requests. A stateless proxy
             forwards every request it receives downstream and every
             response it receives upstream.

        Strict Routing: A proxy is is said to be strict routing if it
             follows the Route processing rules of RFC 2543 and many
             prior Internet Draft versions of this RFC. That rule caused
             proxies to destroy the contents of the Request-URI when a
             Route header field was present. Strict routing behavior is
             not used in this specification, in favor of a loose routing
             behavior. Proxies that perform strict routing are also
             known as strict routers.

        Transaction User (TU): The layer of protocol processing that
             resides above the transaction layer. Transaction users
             include the UAC core, UAS core, and proxy core.

        Upstream: A direction of message forwarding within a transaction
             that refers to the direction that responses flow from the
             user agent server to user agent client.

        URL-encoded: A character string encoded according to RFC 1738,
             Section 2.2 [4].

        User agent client (UAC): A user agent client is a logical entity
             that creates a new request, and then uses the client
             transaction state machinery to send it. The role of UAC
             lasts only for the duration of that transaction. In other
             words, if a piece of software initiates a request, it acts
             as a UAC for the duration of that transaction. If it
             receives a request later on, it assumes the role of a user



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             agent server for the processing of that transaction.

        UAC Core: The set of processing functions required of a UAC that
             reside above the transaction and transport layers.

        User agent server (UAS): A user agent server is a logical entity
             that generates a response to a SIP request.  The response
             accepts, rejects or redirects the request. This role lasts
             only for the duration of that transaction. In other words,
             if a piece of software responds to a request, it acts as a
             UAS for the duration of that transaction. If it generates a
             request later on, it assumes the role of a user agent
             client for the processing of that transaction.

        UAS Core: The set of processing functions required at a UAS that
             reside above the transaction and transport layers.

        User agent (UA): A logical entity that can act as both a user
             agent client and user agent server for the duration of a
             dialog.

   The role of UAC and UAS as well as proxy and redirect servers are
   defined on a transaction-by-transaction basis. For example, the user
   agent initiating a call acts as a UAC when sending the initial INVITE
   request and as a UAS when receiving a BYE request from the callee.
   Similarly, the same software can act as a proxy server for one
   request and as a redirect server for the next request.

   Proxy, location, and registrar servers defined above are logical
   entities; implementations MAY combine them into a single application.

7 SIP Messages

   SIP is a text-based protocol and uses the ISO 10646 character set in
   UTF-8 encoding (RFC 2279 [25]).

   A SIP message is either a request from a client to a server, or a
   response from a server to a client.

   Both Request (section 7.1) and Response (section 7.2) messages use
   the basic format of RFC 2822 [20], even though the syntax differs in
   character set and syntax specifics. (SIP allows header fields that
   would not be valid RFC 2822 header fields, for example.)

   Both types of messages consist of a start-line, one or more header
   fields (also known as "headers"), an empty line indicating the end of
   the header fields, and an optional message-body.




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        generic-message  =  start-line
                            *message-header
                            CRLF
                            [ message-body ]


   The start-line, each message-header line, and the empty line MUST be
   terminated by a carriage-return line-feed sequence (CRLF).  Note that
   the empty line MUST be present even if the message-body is not.

   Except for the above difference in character sets, much of SIP's
   message and header field syntax is identical to HTTP/1.1. Rather than
   repeating the syntax and semantics here, we use [HX.Y] to refer to
   Section X.Y of the current HTTP/1.1 specification (RFC 2616 [15]).

   However, SIP is not an extension of HTTP.

7.1 Requests

   SIP requests are distinguished by having a Request-Line for a start-
   line. A Request-Line contains a method name, a Request-URI, and the
   protocol version separated by a single space (SP) character.

   The Request-Line ends with CRLF. No CR or LF are allowed except in
   the end-of-line CRLF sequence. No LWS is allowed in any of the
   elements.

                      Method Request-URI SIP-Version

        Method:

             This specification defines seven methods: REGISTER for
             registering contact information, INVITE, ACK, PRACK and
             CANCEL for setting up sessions, BYE for terminating
             sessions and OPTIONS for querying servers about their
             capabilities. SIP extensions, documented in standards track
             RFCs, may define additional methods.

        Request-URI: The Request-URI is a SIP URI as described in
             Section 23.1 or a general URI (RFC 2396 [13]).  It
             indicates the user or service to which this request is
             being addressed. The Request-URI MUST NOT contain unescaped
             spaces or control characters and MUST NOT be enclosed in
             "<>".

             SIP elements MAY support Request-URIs with schemes other
             than "sip", for example the "tel" URI scheme of RFC 2806
             [19]. SIP elements MAY translate non-SIP URIs using any



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             mechanism at their disposal, resulting in either a SIP URI
             or some other scheme.

        SIP-Version: Both request and response messages include the
             version of SIP in use, and follow [H3.1] (with HTTP
             replaced by SIP, and HTTP/1.1 replaced by SIP/2.0)
             regarding version ordering, compliance requirements, and
             upgrading of version numbers. To be compliant with this
             specification, applications sending SIP messages MUST
             include a SIP-Version of "SIP/2.0". The SIP-Version string
             is case-insensitive, but implementations MUST send upper-
             case.


             Unlike HTTP/1.1, SIP treats the version number as a
             literal string. In practice, this should make no
             difference.

7.2 Responses

   SIP responses are distinguished from requests by having a Status-Line
   as their start-line. A Status-Line consists of the protocol version
   followed by a numeric Status-Code and its associated textual phrase,
   with each element separated by a single SP character.

   No CR or LF is allowed except in the final CRLF sequence.

                   SIP-version Status-Code Reason-Phrase

   The Status-Code is a 3-digit integer result code that indicates the
   outcome of an attempt to understand and satisfy a request. The
   Reason-Phrase is intended to give a short textual description of the
   Status-Code. The Status-Code is intended for use by automata, whereas
   the Reason-Phrase is intended for the human user. A client is not
   required to examine or display the Reason-Phrase.

   While this specification suggests specific wording for the reason
   phrase, implementations MAY choose other text, e.g., in the language
   indicated in the Accept-Language header field of the request.

   The first digit of the Status-Code defines the class of response. The
   last two digits do not have any categorization role. For this reason,
   any response with a status code between 100 and 199 is referred to as
   a "1xx response", any response with a status code between 200 and 299
   as a "2xx response", and so on. SIP/2.0 allows six values for the
   first digit:

        1xx: Provisional -- request received, continuing to process the



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             request;

        2xx: Success -- the action was successfully received,
             understood, and accepted;

        3xx: Redirection -- further action needs to be taken in order to
             complete the request;

        4xx: Client Error -- the request contains bad syntax or cannot
             be fulfilled at this server;

        5xx: Server Error -- the server failed to fulfill an apparently
             valid request;

        6xx: Global Failure -- the request cannot be fulfilled at any
             server.

   Section 25 defines these classes and describes the individual codes.

7.3 Header Fields

   SIP header fields are similar to HTTP header fields in both syntax
   and semantics. In particular, SIP header fields follow the [H4.2]
   definitions of syntax for message-header, and the rules for extending
   header fields over multiple lines. However, the latter is specified
   in HTTP with implicit white space and folding. This specification
   conforms with RFC 2234 [28] and uses only explicit white space and
   folding as an integral part of the grammar.

   [H4.2] also specifies that multiple header fields of the same field
   name whose value is a comma separated list can be combined into one
   header field. That applies to SIP as well, but the specific rule is
   different because of the different grammars. Specifically, any SIP
   header whose grammar is of the form:



        header  =  "header-name" HCOLON header-value *(COMMA header-value)


   allows for combining header fields of the same name into a comma
   separated list. This is also true for the Contact header, as long as
   none of the header instances have a value of "*".

7.3.1 Header Field Format

   Header fields follow the same generic header format as that given in
   Section 2.2 of RFC 2822 [20]. Each header field consists of a field



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   name followed by a colon (":") and the field value.
                          field-name: field-value
   The formal grammar for a message-header specified in Section 27
   allows for an arbitrary amount of whitespace on either side of the
   colon; however, implementations should avoid spaces between the field
   name and the colon and use a single space (SP) between the colon and
   the field-value. Thus,

   Subject:            lunch
   Subject      :      lunch
   Subject            :lunch
   Subject: lunch


   are all valid and equivalent, but the last is the preferred form.

   Header fields can be extended over multiple lines by preceding each
   extra line with at least one SP or horizontal tab (HT). The line
   break and the whitespace at the beginning of the next line are
   treated as a single SP character. Thus, the following are equivalent:


   Subject: I know you're there, pick up the phone and talk to me!
   Subject: I know you're there,
            pick up the phone
            and talk to me!



   The relative order of header fields with different field names is not
   significant. However, it is RECOMMENDED that headers which are needed
   for proxy processing (Via, Route, Record-Route, Proxy-Require, Max-
   Forwards, and Proxy-Authorization, for example) appear towards the
   top of the message, to facilitate rapid parsing. The relative order
   of header fields with the same field name is important.  Multiple
   header fields with the same field-name MAY be present in a message if
   and only if the entire field-value for that header field is defined
   as a comma-separated list (that is, if follows the grammar defined in
   Section 7.3). It MUST be possible to combine the multiple header
   fields into one "field-name:  field-value" pair, without changing the
   semantics of the message, by appending each subsequent field-value to
   the first, each separated by a comma. The exception to this rule are
   the Authorization, Proxy-Authorization, Proxy-Authenticate and
   Proxy-Authorization headers. Multiple header fields with these names
   MAY be present in a message, but since their grammar does not follow
   the general form listed in Section 7.3, they MUST NOT be combined
   into a single header field.




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   Implementations MUST be able to process multiple header fields with
   the same name in any combination of the single-value-per-line or
   comma-separated value forms.

   The following groups of header fields are valid and equivalent:

   Route: <sip:alice@atlanta.com>
   Subject: Lunch
   Route: <sip:bob@biloxi.com>
   Route: <sip:carol@chicago.com>

   Route: <sip:alice@atlanta.com>, <sip:bob@biloxi.com>
   Route: <sip:carol@chicago.com>
   Subject: Lunch

   Subject: Lunch
   Route: <sip:alice@atlanta.com>, <sip:bob@biloxi.com>, <sip:carol@chicago.com>



   Each of the following blocks is valid but not equivalent to the
   others:

   Route: <sip:alice@atlanta.com>
   Route: <sip:bob@biloxi.com>
   Route: <sip:carol@chicago.com>

   Route: <sip:bob@biloxi.com>
   Route: <sip:alice@atlanta.com>
   Route: <sip:carol@chicago.com>

   Route: <sip:alice@atlanta.com>,<sip:carol@chicago.com>,<sip:bob@biloxi.com>



   The format of a header field-value is defined per header-name. It
   will always be either an opaque sequence of TEXT-UTF8 octets, or a
   combination of whitespace, tokens, separators, and quoted strings.
   Many existing headers will adhere to the general form of a value
   followed by a semi-colon separated sequence of parameter-name,
   parameter-value pairs:
        field-name: field-value *(;parameter-name=parameter-value)

   Even though an arbitrary number of parameter pairs may be attached to
   a header field value, any given parameter-name MUST NOT appear more
   than once.

   All new header fields MUST follow this generic format unless they



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   have been inherited from other RFC 2822-like specifications.

   When comparing header fields, field names are always case-
   insensitive.  Unless otherwise stated in the definition of a
   particular header field, field values, parameter names, and parameter
   values are case-insensitive. Tokens are always case-insensitive.
   Unless specified otherwise, values expressed as quoted strings are
   case-sensitive.

   For example,

   Contact: <sip:alice@atlanta.com>;expires=3600


   is equivalent to

   CONTACT: <sip:alice@atlanta.com>;ExPiReS=3600


   and

   Content-Disposition: session;handling=optional


   is equivalent to

   content-disposition: Session;HANDLING=OPTIONAL



   The following two header fields are not equivalent:

   Warning: 370 devnull "Choose a bigger pipe"
   Warning: 370 devnull "CHOOSE A BIGGER PIPE"



7.3.2 Header Field Classification

   Some header fields only make sense in requests or responses. These
   are called request header fields and response header fields,
   respectively.  If a header appears in a message not matching its
   category (such as a request header field in a response), it MUST be
   ignored.  Section 24 defines the classification of each header field.

7.3.3 Compact Form

   SIP provides a mechanism to represent common header fields in an



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   abbreviated form. This may be useful when messages would otherwise
   become too large to be carried on the transport available to it
   (exceeding the maximum transmission unit (MTU) when using UDP, for
   example). These compact forms are defined in Section 24. A compact
   form MAY be substituted for the longer form of a header name at any
   time without changing the semantics of the message. The same type of
   header field MAY appear in both long and short forms within the same
   message. Implementations MUST accept both the long and short forms of
   each header name.

7.4 Bodies

   Requests, including new requests defined in extensions to this
   specification, MAY contain message bodies unless otherwise noted.
   The interpretation of the body depends on the request method.

   For response messages, the request method and the response status
   code determine the type and interpretation of any message body. All
   responses MAY include a body.

7.4.1 Message Body Type

   The Internet media type of the message body MUST be given by the
   Content-Type header field. If the body has undergone any encoding
   such as compression, then this MUST be indicated by the Content-
   Encoding header field; otherwise, Content-Encoding MUST be omitted.
   If applicable, the character set of the message body is indicated as
   part of the Content-Type header-field value.

   The "multipart" MIME type defined in RFC 2046 [8] MAY be used within
   the body of the message. Implementations that send requests
   containing multipart message bodies MUST send a session description
   as a non-multipart message body if the remote implementation requests
   this through an Accept header field that does not contain multipart.

   Note that SIP messages MAY contain binary bodies or body parts.

7.4.2 Message Body Length

   The body length in bytes is provided by the Content-Length header
   field. Section 24.14 describes the necessary contents of this header
   in detail.

   The "chunked" transfer encoding of HTTP/1.1 MUST NOT be used for SIP.
   (Note: The chunked encoding modifies the body of a message in order
   to transfer it as a series of chunks, each with its own size
   indicator.)




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7.5 Framing SIP messages

   Unlike HTTP, SIP implementations can use UDP or other unreliable
   datagram protocols. Each such datagram carries one request or
   response.  See Section 19 on constraints on usage of unreliable
   transports.

   Likewise, implementations processing SIP messages over stream-
   oriented transports MUST ignore any CRLF appearing before the start-
   line [H4.1]

8 General User Agent Behavior

   A user agent represents an end system. It contains a User Agent
   Client (UAC), which generates requests, and a User Agent Server (UAS)
   which responds to them. A UAC is capable of generating a request
   based on some external stimulus (the user clicking a button, or a
   signal on a PSTN line), and processing a response. A UAS is capable
   of receiving a request, and generating a response, based on user
   input, external stimulus, the result of a program execution, or some
   other mechanism.

   When a UAC sends a request, it will pass through some number of proxy
   servers, which forward the request towards the UAS. When the UAS
   generates a response, the response is forwarded towards the UAC.

   UAC and UAS procedures depend strongly on two factors. First, whether
   the request or response is inside or outside of a dialog, and second,
   based on the method of a request. Dialogs are discussed thoroughly in
   Section 12; they represent a peer-to-peer relationship between user
   agents, and are established by specific SIP methods, such as INVITE.

   In this section, we discuss the method independent rules for UAC and
   UAS behavior when processing requests that are outside of a dialog.
   This includes, of course, the requests which themselves establish a
   dialog.

   Security procedures for requests and responses outside of a dialog
   are described in Section 22. Specifically, mechanisms exist for the
   UAS and UAC to mutually authenticate. A limited set of privacy
   features are also supported through encryption of bodies using
   S/MIME.

8.1 UAC Behavior

   This section covers UAC behavior outside of a dialog.

8.1.1 Generating the Request



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   A valid SIP request formulated by a UAC MUST at a minimum contain the
   following headers: To, From, CSeq, Call-ID, Max-Forwards, and Via;
   all of these headers are mandatory in all SIP messages. These six
   headers are the fundamental building blocks of a SIP message, as they
   jointly provide for most of the critical message routing services
   including the addressing of messages, the routing of responses,
   limiting message propagation, ordering of messages, and the unique
   identification of transactions. These headers are in addition to the
   mandatory request line, which contains the method, Request-URI and
   SIP version.

   Examples of requests sent outside of a dialog include an INVITE to
   establish a session (Section 13) and an OPTIONS to query for
   capabilities (Section 11).

8.1.1.1 Request-URI

   The initial Request-URI of the message SHOULD be set to the value of
   the URI in the To field. One notable exception is the REGISTER
   method; behavior for setting the Request-URI of register is given in
   Section 10.

   In some special circumstances, the presence of a pre-existing route
   set can affect the Request-URI of the message. A pre-existing route
   set is an ordered set of URIs that identify a chain of servers, to
   which a UAC will send outgoing requests that are outside of a dialog.
   Commonly, they are configured on the user agent by a user or service
   provider manually, or through some non-SIP mechanism. When a provider
   wishes to configure a UA with an outbound proxy, it is RECOMMENDED
   that this by done by providing it with a pre-existing route set with
   a single URI, that of the outbound proxy.

   When a pre-existing route set is present, the procedures for
   populating the Request-URI and Route header field detailed in Section
   12.2.1.1 MUST be followed, even though there is no dialog.

8.1.1.2 To

   The To field first and foremost specifies the desired "logical"
   recipient of the request, or the address-of-record of the user or
   resource that is the target of this request. This may or may not be
   the ultimate recipient of the request. The To header MAY contain a
   SIP URI, but it may also make use of other URI schemes (the tel URL
   [19], for example) when appropriate. All SIP implementations MUST
   support the SIP URI. The To header field allows for a display name.

   A UAC may learn how to populate the To header field for a particular
   request in a number of ways. Usually the user will suggest the To



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   header field through a human interface, perhaps inputting the URI
   manually or selecting it from some sort of address book. Frequently,
   the user will not enter a complete URI, but rather, a string of
   digits or letters (i.e., "bob"). It is at the discretion of the UA to
   choose how to interpret this input. Using it to form the user part of
   a SIP URL implies that the UA wishes the name to be resolved in the
   domain the right hand side (RHS) of the at-sign in the SIP URI (i.e.,
   sip:bob@example.com). The RHS will frequently be the home domain of
   the user, which allows for the home domain to process the outgoing
   request. This is useful for features like "speed dial" which require
   interpretation of the user part in the home domain. The tel URL is
   used when the UA does not wish to specify the domain that should
   interpret the user input. Rather, each domain that the request passes
   through would be given that opportunity. As an example, a user in an
   airport might log in, and send requests through an outbound proxy in
   the airport. If they enter "411" (this is the phone number for local
   directory assistance in the United States), that needs to be
   interpreted and processed by the outbound proxy in the airport, not
   the user's home domain. In this case, tel:411 would be the right
   choice.

   A request outside of a dialog MUST NOT contain a tag; the tag in the
   To field of a request identifies the peer of the dialog. Since no
   dialog is established, no tag is present.

   For further information on the To header field, see Section 24.41.
   The following is an example of valid To header:

     To: Carol <sip:carol@chicago.com>



8.1.1.3 From

   The From general-header field indicates the logical identity of the
   initiator of the request, possibly the user's address of record.
   Like the To field, it contains a URI and optionally a display name.
   It is used by SIP elements to determine processing rules to apply to
   a request (for example, automatic call rejection). As such, it is
   very important that the From URI not contain IP addresses or the FQDN
   of the host the UA is running on, since these are not logical names.

   The From header field allows for a display name. A UAC SHOULD use the
   display name "Anonymous", along with a syntactically correct, but
   otherwise meaningless URI (like sip:988776a@ahhs.aa), if the identity
   of the client is to remain hidden.

   Usually the value that populates the From header field in requests



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   generated by a particular user agent is pre-provisioned by the user
   or by the administrators of the user's local domain. If a particular
   user agent is used by multiple users, it might have switchable
   profiles that include a URI corresponding to the identity of the
   profiled user. Recipients of requests can authenticate the originator
   of a request in order to ascertain that they are who their From
   header field claims they are (see Section 20 for more on
   authentication).

   The From field MUST contain a new "tag" parameter, chosen by the UAC.
   See Section 23.3 for details on choosing a tag.

   For further information on the From header see Section 24.20.
   Examples:


     From: "Bob" <sip:bob@biloxi.com> ;tag=a48s
     From: sip:+12125551212@server.phone2net.com;tag=887s
     From: Anonymous <sip:c8oqz84zk7z@privacy.org>;tag=hyh8



8.1.1.4 Call-ID

   The Call-ID general-header field acts as a unique identifier to group
   together a series of messages. It MUST be the same for all requests
   and responses sent by either UA in a dialog. It SHOULD be the same in
   each registration from a UA.

   In a new request created by a UAC outside of any dialog, the Call-ID
   header MUST be selected by the UAC as a globally unique identifier
   over space and time unless overridden by method specific behavior.
   All SIP user agents must have a means to guarantee that the Call-ID
   headers they produce will not be inadvertently generated by any other
   user agent. Note that when requests are retried after certain failure
   responses that solicit an amendment to a request (for example, a
   challenge for authentication), these retried requests are not
   considered new requests, and therefore do not need new Call-ID
   headers; see Section 8.1.3.6.

   Use of cryptographically random identifiers [5] in the generation of
   Call-IDs is RECOMMENDED. Implementations MAY use the form
   "localid@host". Call-IDs are case-sensitive and are simply compared
   byte-by-byte.

        Using cryptographically random identifiers provides some
        protection against session hijacking and reduces the
        likelihood of unintentional Call-ID collisions.



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   No provisioning or human interface is required for the selection of
   the Call-ID header field value for a request.

   For further information on the Call-ID header see Section 24.8.

   Example:


     Call-ID: f81d4fae-7dec-11d0-a765-00a0c91e6bf6@foo.bar.com



8.1.1.5 CSeq

   The Cseq header serves as a way to identify and order transactions.
   It consists of a sequence number and a method. The method MUST match
   that of the request. For requests outside of a dialog, the sequence
   number value is arbitrary, but MUST be expressible as a 32-bit
   unsigned integer and MUST be less than 2**31. As long as it follows
   the above guidelines, a client may use any mechanism it would like to
   select CSeq header field values.

   Section 12.2.1.1 discusses construction of the CSeq for requests
   within a dialog.

   Example:


     CSeq: 4711 INVITE



8.1.1.6 Max-Forwards

   The Max-Forwards header serves to limit the number of hops a request
   can transit on the way to its destination. It consists of an integer
   that is decremented by one at each hop. If the Max-Forwards value
   reaches 0 before the request reaches its destination, it will be
   rejected with a 483 Too Many Hops error response.

   A UAC MUST insert a Max-Forwards header field into each request it
   originates with a value which SHOULD be 70. This number was chosen to
   be sufficiently large to guarantee that a request would not be
   dropped in any SIP network when there were no loops, but not so large
   as to consume proxy resources when a loop does occur. Lower values
   should be used with caution, only in networks where topologies are
   known by the UA.




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8.1.1.7 Via

   The Via header is used to indicate the transport used for the
   transaction, and to identify the location where the response is to be
   sent.

   When the UAC creates a request, it MUST insert a Via into that
   request. The protocol and version in the header MUST be SIP and 2.0,
   respectively. The Via header it inserts MUST contain a branch
   parameter. This parameter is used to uniquely identify the
   transaction created by that request. This parameter is used by both
   the client, and the server.

   The branch parameter value MUST be unique across time for all
   requests sent by the UA. The exception to this rule is CANCEL.  As
   discussed below, a CANCEL request will have the same value of the
   branch parameter as the request it cancels.


        The uniqueness property of the branch ID parameter, to
        facilitate its use as a transaction ID, was not part of RFC
        2543

   The branch ID inserted by an element compliant with this
   specification MUST always begin with the characters "z9hG4bK". These
   7 characters are used as a magic cookie (7 is deemed sufficient to
   ensure that an older RFC 2543 implementation would not pick such a
   value), so that servers receiving the request can determine that the
   branch ID was constructed in the fashion described by this
   specification (i.e., globally unique). Beyond this requirement, the
   precise format of the branch token is implementation-defined.

   The Via header maddr, ttl, and sent-by components will be set when
   the request is processed by the transport layer (Section 19).

   Via processing for proxies is described in Sections 3 and sec:proxy-
   response-processing-via.

8.1.1.8 Contact

   The Contact header provides a SIP URI that can be used to contact
   that specific instance of the user agent for subsequent requests. The
   Contact header MUST be present in any request that can result in the
   establishment of a dialog. For the methods defined in this
   specification, that includes only the INVITE request.  For these
   requests, the scope of the Contact is global.  That is, the Contact
   header refers to the URI at which the UA would like to receive
   requests, and this URI MUST be valid even if used in subsequent



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   requests outside of any dialogs.  Only a single URI MUST be present.

   For further information on the Contact header, see Section 24.10.

8.1.1.9 Supported and Require

   If the UAC supports extensions to SIP that can be applied by the
   server to the response, the UAC SHOULD include a Supported header in
   the request listing the option tags (Section 23.2) for those
   extensions. This includes support for reliability for provisional
   responses, which is an extension even though it is defined within
   this specification. The option tag for reliability of provisional
   responses is 100rel

   The option-tags listed MUST only refer to extensions defined in
   standards-track RFCs. This is to prevent servers from insisting that
   clients implement non-standard, vendor-defined features in order to
   receive service. Extensions defined by experimental and informational
   RFCs are explicitly excluded from usage with the Supported header in
   a request, since they too are often used to document vendor-defined
   extensions.

   If the UAC wishes to insist that a UAS understand an extension that
   the UAC will apply to the request in order to process the request, it
   MUST insert a Require header into the request listing the option tag
   for that extension. If the UAC wishes to apply an extension to the
   request and insist that any proxies that are traversed understand
   that extension, it MUST insert a Proxy-Require header into the
   request listing the option tag for that extension.

   As with the Supported header, the option-tags in the Require header
   MUST only refer to extensions defined in standards-track RFCs.

   A Require header in a request with the option tag 100rel means that
   the UAC wishes for all provisional responses to this request to be
   transmitted reliably. This header MUST NOT be present in any requests
   excepting INVITE, although extensions to SIP may allow its usage with
   other request methods.

8.1.1.10 Additional Message Components

   After a new request has been created, and the headers described above
   have been properly constructed, any additional optional headers are
   added, as are any headers specific to the method.

   SIP requests MAY contain a MIME-encoded message-body. Regardless of
   the type of body that a request contains, certain headers must be
   formulated to characterize the contents of the body. For further



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   information on these headers see Sections 24.14, 24.15 and 24.12.

8.1.2 Sending the Request

   The destination for the request is then computed. Unless there is
   local policy specifying otherwise, then the destination MUST be
   determined by applying the DNS proceedures described in [2] as
   follows.  If the first element in the route set indicated a strict
   router (resulting in forming the request as described in Section
   12.2.1.1), the proceedures MUST be applied to the Request-URI of the
   request.  Otherwise, the proceedures are applied to the first Route
   header field value in the request (if one exists), or to the
   request's Request-URI if there is no Route header field present.
   These procedures yield an ordered set of address, port, and
   transports to attempt.

   Local policy MAY specify an alternate set of destinations to attempt.
   There are no restrictions on the alternate destinations if the
   request contains no Route headers. This provides a simple alternative
   to a pre-existing route set as way to specify an outbound proxy.
   However, that approach for configuring outbound proxy is NOT
   RECOMMENDED; a pre-existing route set with a single URI SHOULD be
   used instead. If the request contains Route headers, the request MAY
   be sent to any server that the UA is certain will honor the Route and
   Request-URI policies specified in this document (as opposed to those
   in RFC 2543).

   The UAC SHOULD follow the procedures defined in [2] for stateful
   elements, trying each address until a server is contacted. Each try
   constitutes a new transaction, and therefore each carries a different
   Via header with a new branch parameter. Furthermore, the transport
   value in the Via header is set to whatever transport was determined
   for the target server.

8.1.3 Processing Responses

   Responses are first processed by the transport layer and then passed
   up to the transaction layer. The transaction layer performs its
   processing and then passes it up to the TU. The majority of response
   processing in the TU is method specific. However, there are some
   general behaviors independent of the method.

8.1.3.1 Transaction Layer Errors

   In some cases, the response returned by the transaction layer will
   not be a SIP message, but rather a transaction layer event. The only
   event that the TU will encounter is the timeout event. When the
   timeout event is received from the transaction layer, it MUST be



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   treated as if a 408 (Request Timeout) status code has been received.

8.1.3.2 Unrecognized Responses

   A UAC MUST treat any response it does not recognize as being
   equivalent to the x00 response code of that class, and MUST be able
   to process the x00 response code for all classes. For example, if a
   UAC receives an unrecognized response code of 431, it can safely
   assume that there was something wrong with its request and treat the
   response as if it had received a 400 (Bad Request) response code.

8.1.3.3 Vias

   If more than one Via header field is present in a response, the UAC
   SHOULD discard the message.

        The presence of additional Via header fields that precede
        the originator of the request suggests that the message was
        misrouted or possibly corrupted.

8.1.3.4 Processing Reliable 1xx Responses

   A 1xx response that contains a Require header with the option tag
   100rel is a reliable provisional response. The UA core follows the
   procedures in Section 18.2 to process the response, which will result
   in the generation of a PRACK request to acknowledge the reliable
   provisional response.

8.1.3.5 Processing 3xx responses

   Upon receipt of a redirection response (for example, a 3xx response
   status code), clients SHOULD use the URI(s) in the Contact header
   field to formulate one or more new requests based on the redirected
   request.

   If more than one URI is present in Contact header fields within the
   3xx response, the UA MUST determine an order in which these contact
   addresses should be processed. UAs MUST consult the "q" parameter
   value of the Contact header fields (see Section 24.10) if available.
   Contact addresses MUST be ordered from highest qvalue to lowest. If
   no qvalue is present, a contact address is considered to have a
   qvalue of 1.0. Note that two or more contact addresses might have an
   equal qvalue - these URIs are eligible to be tried in parallel.

   Once an ordered list has been established, UACs MUST try to contact
   each URI in the ordered list in turn until a server responds. If
   there are contact addresses with an equal qvalue, the UAC MAY decide
   randomly on an order in which to process these addresses, or it MAY



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   attempt to process contact addresses of equal qvalue in parallel.

   Note that for example, the UAC may effectively divide the ordered
   list into groups, processing the groups serially and processing the
   destinations in each group in parallel.

   If contacting an address in the list results in a failure, as defined
   in the next paragraph, the element moves to the next address in the
   list, until the list is exhausted. If the list is exhausted, then the
   request has failed.

   Failures SHOULD be detected through failure response codes (codes
   greater than 399) or network timeouts. Client transaction will report
   any transport layer failures to the transaction user.

   When a failure for a particular contact address is received, the
   client SHOULD try the next contact address. This will involve
   creating a new client transaction to deliver a new request.

   In order to create a request based on a contact address in a 3xx
   response, a UAC MUST copy the entire URI from the Contact header into
   the Request-URI, except for the "method-param" and "header" URI
   parameters (see Section 23.1.1 for a definition of these parameters).
   It uses the "header" parameters to create headers for the new
   request, overwriting headers associated with the redirected request
   in accordance with the guidelines in Section 23.1.5.

   Note that in some instances, headers that have been communicated in
   the contact address may instead append to existing request headers in
   the original redirected request. As a general rule, if the header can
   accept a comma-separated list of values, then the new header value
   MAY be appended to any existing values in the original redirected
   request. If the header does not accept multiple values, the value in
   the original redirected request MAY be overwritten by the header
   value communicated in the contact address. For example, if a contact
   address is returned with the following value:


   sip:user@host?Subject=foo&Call-Info=<http://www.foo.com>



   Then any Subject header in the original redirected request is
   overwritten, but the HTTP URL is merely appended to any existing
   Call-Info header field values.

   It is RECOMMENDED that the UAC reuse the same To, From, and Call-ID
   used in the original redirected request, but the UAC MAY also choose



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   to update for example the Call-ID header field value for new
   requests.

   Finally, once the new request has been constructed, it is sent using
   a new client transaction, and therefore MUST have a new branch ID in
   the top Via field as discussed in Section 8.1.1.7.

   In all other respects, requests sent upon receipt of a redirect
   response SHOULD re-use the headers and bodies of the original
   request.

   In some instances, Contact header values may be cached at UAC
   temporarily or permanently depending on the status code received and
   the presence of an expiration interval; see Sections 25.3.2 and
   25.3.3.

8.1.3.6 Processing 4xx responses

   Certain 4xx response codes require specific UA processing,
   independent of the method.

   If a 401 (Unauthorized) or 407 (Proxy Authentication Required)
   response is received, the UAC SHOULD follow the authorization
   procedures of Section 20.2 and Section 20.3 to retry the request with
   credentials.

   If a 413 (Request Entity Too Large) response is received (Section
   25.4.11), the request contained a body that was longer than the UAS
   was willing to accept. If possible, the UAC SHOULD retry the request,
   either omitting the body or using one of a smaller length.

   If a 415 (Unsupported Media Type) response is received (Section
   25.4.13), the request contained media types not supported by the UAS.
   The UAC SHOULD retry sending the request, this time only using
   content with types listed in the Accept header in the response, with
   encodings listed in the Accept-Encoding header in the response, and
   with languages listed in the Accept-Language in the response.

   If a 416 (Unsupported URI Scheme) response is received (Section
   25.4.14, the Request-URI used a URI scheme not supported by the
   server. The client SHOULD retry the request, this time, using a SIP
   URI.

   If a 420 (Bad Extension) response is received (Section 25.4.15), the
   request contained a Require or Proxy-Require header listing an
   option-tag for a feature not supported by a proxy or UAS. The UAC
   SHOULD retry the request, this time omitting any extensions listed in
   the Unsupported header in the response.



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   In all of the above cases, the request is retried by creating a new
   request with the appropriate modifications. This new request SHOULD
   have the same value of the Call-ID, To, and From of the previous
   request, but the CSeq should contain a new sequence number that is
   one higher than the previous.

   With other 4xx responses, including those yet to be defined, a retry
   may or may not be possible depending on the method and the use case.

8.2 UAS Behavior

   When a request outside of a dialog is processed by a UAS, there is a
   set of processing rules which are followed, independent of the
   method.  Section 12 gives guidance on how a UAS can tell whether a
   request is inside or outside of a dialog.

   Note that request processing is atomic. If a request is accepted, all
   state changes associated with it MUST be performed. If it is
   rejected, all state changes MUST NOT be performed.

8.2.1 Method Inspection

   Once a request is authenticated (or no authentication was desired),
   the UAS MUST inspect the method of the request. If the UAS does not
   support the method of a request it MUST generate a 405 (Method Not
   Allowed) response. Procedures for generation of responses are
   described in Section 8.2.6. The UAS MUST also add an Allow header to
   the 405 (Method Not Allowed) response. The Allow header field MUST
   list the set of methods supported by the UAS generating the message.
   The Allow header field is presented in Section 24.5.

   If the method is one supported by the server, processing continues.

8.2.2 Header Inspection

   If a UAS does not understand a header field in a request (that is,
   the header is not defined in this specification or in any supported
   extension), the server MUST ignore that header and continue
   processing the message. A UAS SHOULD ignore any malformed headers
   that are not necessary for processing requests.

8.2.2.1 To and Request-URI

   The To header field identifies the original recipient of the request
   designated by the user identified in the From field.  The original
   recipient may or may not be the UAS processing the request, due to
   call forwarding or other proxy operations. A UAS MAY apply any policy
   it wishes in determination of whether to accept requests when the To



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   field is not the identity of the UAS. However, it is RECOMMENDED that
   a UAS accept requests even if they do not recognize the URI scheme
   (for example, a tel: URI) in the To header, or if the To header field
   does not address a known or current user of this UAS. If, on the
   other hand, the UAS decides to reject the request, it SHOULD generate
   a response with a 403 (Forbidden) status code and pass it to the
   server transaction layer for transmission.

   However, the Request-URI identifies the UAS that is to process the
   request. If the Request-URI uses a scheme not supported by the UAS,
   it SHOULD reject the request with a 416 (Unsupported URI Scheme)
   response. If the Request-URI does not identify an address that the
   UAS is willing to accept requests for, it SHOULD reject the request
   with a 404 (Not Found) response. Typically, a UA that uses the
   REGISTER method to bind its address of record to a specific contact
   address will see requests whose Request-URI equals those contact
   addressess. Other potential sources of received Request-URIs include
   the Contact headers of requests and responses sent by the UA that
   establish or refresh dialogs.

8.2.2.2 Merged Requests

   If the request has no tag in the To, the TU checks ongoing
   transactions. If the To, From, Call-ID, CSeq exactly match (including
   tags) those of any request received previously, but the branch-ID in
   the topmost Via is different from those received previously, the TU
   SHOULD generate a 482 (Loop Detected) response and pass it to the
   server transaction.

        The same request has arrived at the UAS more than once,
        following different paths, most likely due to forking. The
        UAS processes the first such request received and responds
        with a 482 (Loop Detected) to the rest of them.

8.2.2.3 Require

   Assuming the UAS decides that it is the proper element to process the
   request, it examines the Require header field, if present.

   The Require general-header field is used by a UAC to tell a UAS about
   SIP extensions that the UAC expects the UAS to support in order to
   process the request properly. Its format is described in Section
   24.33. If a UAS does not understand an option-tag listed in a Require
   header field, it MUST respond by generating a response with status
   code 420 (Bad Extension). The UAS MUST add an Unsupported header
   field, and list in it those options it does not understand amongst
   those in the Require header of the request. Upon receipt of the 420
   (Bad Extension) the client SHOULD retry the request, this time



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   without using those extensions listed in the Unsupported header field
   in the response.

   Note that Require and Proxy-Require MUST NOT be used in a SIP CANCEL
   request, or in an ACK request sent for a non-2xx response. These
   headers should be ignored if they are present in these requests.

   An ACK request for a 2xx response MUST contain only those Require and
   Proxy-Require values that were present in the initial request.

   Example:

   UAC->UAS:   INVITE sip:watson@bell-telephone.com SIP/2.0
               Require: 100rel


   UAS->UAC:   SIP/2.0 420 Bad Extension
               Unsupported: 100rel




        This behavior ensures that the client-server interaction
        will proceed without delay when all options are understood
        by both sides, and only slow down if options are not
        understood (as in the example above). For a well-matched
        client-server pair, the interaction proceeds quickly,
        saving a round-trip often required by negotiation
        mechanisms. In addition, it also removes ambiguity when the
        client requires features that the server does not
        understand. Some features, such as call handling fields,
        are only of interest to end systems.

8.2.3 Content Processing

   Assuming the UAS understands any extensions required by the client,
   the UAS examines the body of the message, and the headers that
   describe it.  If there are any bodies whose type (indicated by the
   Content-Type), language (indicated by the Content-Language) or
   encoding (indicated by the Content-Encoding) are not understood, and
   that body part is not optional (as indicated by the Content-
   Disposition header), the UAS MUST reject the request with a 415
   (Unsupported Media Type) response. The response MUST contain an
   Accept header listing the types of all bodies it understands, in the
   event the request contained bodies of types not supported by the UAS.
   If the request contained content encodings not understood by the UAS,
   the response MUST contain an Accept-Encoding header listing the
   encodings understood by the UAS. If the request contained content



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   with languages not understood by the UAS, the response MUST contain
   an Accept-Language header indicating the languages understood by the
   UAS. Beyond these checks, body handling depends on the method and
   type. For further information on the processing of content-specific
   headers see Section 7.4 as well as Section 24.11 through 24.15.

8.2.4 Applying Extensions

   A UAS that wishes to apply some extension when generating the
   response MUST only do so if support for that extension is indicated
   in the Supported header in the request. If the desired extension is
   not supported, the server SHOULD rely only on baseline SIP and any
   other extensions supported by the client. To ensure that the SHOULD
   can be fulfilled, any specification of a new extension MUST include
   discussion of how to return gracefully to baseline SIP when the
   extension is not present. In rare circumstances, where the server
   cannot process the request without the extension, the server MAY send
   a 421 (Extension Required) response. This response indicates that the
   proper response cannot be generated without support of a specific
   extension. The needed extension(s) MUST be included in a Require
   header in the response. This behavior is NOT RECOMMENDED, as it will
   generally break interoperability.

   Any extensions applied to a non-421 response MUST be listed in a
   Require header included in the response. Of course, the server MUST
   NOT apply extensions not listed in the Supported header in the
   request. As a result of this, the Require header in a response will
   only ever contain option tags defined in standards-track RFCs.

8.2.5 Processing the Request

   Assuming all of the checks in the previous subsections are passed,
   the UAS processing becomes method-specific. Section 10 covers the
   REGISTER request, section 11 covers the OPTIONS request, section 13
   covers the INVITE request, and section 15 covers the BYE request.

8.2.6 Generating the Response

   When a UAS wishes to construct a response to a request, it follows
   these procedures. Additional procedures may be needed depending on
   the status code of the response and the circumstances of its
   construction. These additional procedures are documented elsewhere.

8.2.6.1 Sending a Provisional Response

   One largely non-method-specific guideline for the generation of
   responses is that UASs SHOULD NOT issue a provisional response for a
   non-INVITE request. Rather, UASs SHOULD generate a final response to



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   a non-INVITE request as soon as possible.

   When a 100 (Trying) response is generated, any Timestamp header
   present in the request MUST be copied into this 100 (Trying)
   response.  If there is a delay in generating the response, the UAS
   SHOULD add a delay value into the Timestamp value in the response.
   This value MUST contain the difference between time of sending of the
   response and receipt of the request, measured in seconds.

8.2.6.2 Headers and Tags

   The From field of the response MUST equal the From field of the
   request. The Call-ID field of the response MUST equal the Call-ID
   field of the request. The Cseq field of the response MUST equal the
   Cseq field of the request. The Via headers in the response MUST equal
   the Via headers in the request and MUST maintain the same ordering.

   If a request contained a To tag in the request, the To field in the
   response MUST equal that of the request. However, if the To field in
   the request did not contain a tag, the URI in the To field in the
   response MUST equal the URI in the To field in the request;
   additionally, the UAS MUST add a tag to the To field in the response
   (with the exception of the 100 (Trying) response, in which a tag MAY
   be present). This serves to identify the UAS that is responding,
   possibly resulting in a component of a dialog ID. The same tag MUST
   be used for all responses to that request, both final and provisional
   (again excepting the 100 (Trying)). Procedures for generation of tags
   are defined in Section 23.3.

8.2.7 Stateless UAS Behavior

   A stateless UAS is a UAS that does not maintain transaction state. It
   replies to requests normally, but discards any state that would
   ordinarily be retained by a UAS after a response has been sent. If a
   stateless UAS receives a retransmission of a request, it regenerates
   the response and resends it, just as if it were replying to the first
   instance of the request. Stateless UASs do not use a transaction
   layer; they receive requests directly from the transport layer and
   send responses directly to the transport layer.

   The stateless UAS role is needed primarily to handle unauthenticated
   requests for which a challenge response is issued. If unauthenticated
   requests were handled statefully, then malicious floods of
   unauthenticated requests could create massive amounts of transaction
   state that might slow or completely halt call processing in a UAS,
   effectively creating a denial of service condition; for more
   information see Section 22.1.5.




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   The most important behaviors of a stateless UAS are the following:

        o A stateless UAS MUST NOT send provisional (1xx) responses.

        o A stateless UAS MUST NOT retransmit responses.

        o A stateless UAS MUST ignore ACK requests.

        o A stateless UAS MUST ignore CANCEL requests.

        o To header tags MUST be generated for responses in a stateless
          manner - in a manner that will generate the same tag for the
          same request consistently.  For information on tag
          construction see Section 23.3.

   In all other respects, a stateless UAS behaves in the same manner as
   a stateful UAS. A UAS can operate in either a stateful or stateless
   mode for each new request.

8.3 Redirect Servers

   In some architectures it may be desirable to reduce the processing
   load on proxy servers that are responsible for routing requests, and
   improve signaling path robustness, by relying on redirection.
   Redirection allows servers to push routing information for a request
   back in a response to the client, thereby taking themselves out of
   the loop of further messaging for this transaction while still aiding
   in locating the target of the request. When the originator of the
   request receives the redirection, it will send a new request based on
   the URI it has received. By propagating URIs from the core of the
   network to its edges, redirection allows for considerable network
   scalability.

   A redirect server is logically constituted of a server transaction
   layer and a transaction user that has access to a location service of
   some kind (see Section 10 for more on registrars and location
   services). This location service is effectively a database containing
   mappings between a single URI and a set of one or more alternative
   locations at which the target of that URI can be found.

   A redirect server does not issue any SIP requests of its own. After
   receiving a request other than CANCEL, the server gathers the list of
   alternative locations from the location service and either returns a
   final response of class 3xx or it refuses the request. For well-
   formed CANCEL requests, it SHOULD return a 2xx response. This
   response ends the SIP transaction. The redirect server maintains
   transaction state for an entire SIP transaction. It is the
   responsibility of clients to detect forwarding loops between redirect



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   servers.

   When a redirect server returns a 3xx response to a request, it
   populates the list of (one or more) alternative locations into
   Contact headers. An "expires" parameter to the Contact header may
   also be supplied to indicate the lifetime of the Contact data.

   The Contact header field contains URIs giving the new locations or
   user names to try, or may simply specify additional transport
   parameters. A 301 (Moved Permanently) or 302 (Moved Temporarily)
   response may also give the same location and username that was
   targeted by the initial request but specify additional transport
   parameters such as a different server or multicast address to try, or
   a change of SIP transport from UDP to TCP or vice versa.

   However, redirect servers MUST NOT redirect a request to a URI equal
   to the one in the Request-URI; instead, provided that the URI does
   not point to itself, the redirect server SHOULD proxy the request to
   the destination URI.

        If a client is using an outbound proxy, and that proxy
        actually redirects requests, a potential arises for
        infinite redirection loops.

   Note that the Contact header field MAY also refer to a different
   entity than the one originally called. For example, a SIP call
   connected to GSTN gateway may need to deliver a special informational
   announcement such as "The number you have dialed has been changed."

   A Contact response header field can contain any suitable URI
   indicating where the called party can be reached, not limited to SIP
   URIs. For example, it could contain URIs for phones, fax, or irc (if
   they were defined) or a mailto: (RFC 2368, [36]) URL.

   The "expires" parameter of the Contact header field indicates how
   long the URI is valid. The value of the parameter is a number
   indicating seconds. If this parameter is not provided, the value of
   the Expires header field determines how long the URI is valid.
   Implementations MAY treat values larger than 2**32-1 (4294967295
   seconds or 136 years) as equivalent to 2**32-1. Malformed values
   should be treated as equivalent to 3600.

   Redirect servers MUST ignore features that are not understood
   (including unrecognized headers, Required extensions, or even method
   names) and proceed with the redirection of the session in question.
   If a particular extension requires that intermediate devices support
   it, the extension MUST be tagged in the Proxy-Require field as well
   (see Section 24.29).



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9 Canceling a Request

   The previous section has discussed general UA behavior for generating
   requests, and processing responses, for requests of all methods. In
   this section, we discuss a general purpose method, called CANCEL.

   The CANCEL request, as the name implies, is used to cancel a previous
   request sent by a client. Specifically, it asks the UAS to cease
   processing the request and to generate an error response to that
   request. CANCEL has no effect on a request to which a UAS has already
   responded. Because of this, it is most useful to CANCEL requests to
   which can take a long time to respond. For this reason, CANCEL is
   most useful for INVITE requests, which can take a long time to
   generate a response. In that usage, a UAS that receives a CANCEL
   request for an INVITE, but has not yet sent a response, would "stop
   ringing", and then respond to the INVITE with a specific error
   response (a 487).

   CANCEL requests can be constructed and sent by any type of client,
   including both proxies and user agent clients. Section 15 discusses
   under what conditions a UAC would CANCEL an INVITE request, and
   Section 16.9 discusses proxy usage of CANCEL.

   Because a stateful proxy can generate its own CANCEL, a stateful
   proxy also responds to a CANCEL, rather than simply forwarding a
   response it would receive from a downstream element. For that reason,
   CANCEL is referred to as a "hop-by-hop" request, since it is
   responded to at each stateful proxy hop.

9.1 Client Behavior

   A CANCEL request SHOULD NOT be sent to cancel a request other than
   INVITE.

        Since requests other than INVITE are responded to
        immediately, sending a CANCEL for a non-INVITE request
        would always create a race condition.

   The following procedures are used to construct a CANCEL request. The
   Request-URI, Call-ID, To, the numeric part of CSeq and From header
   fields in the CANCEL request MUST be identical to those in the
   request being cancelled, including tags. A CANCEL constructed by a
   client MUST have only a single Via header, whose value matches the
   top Via in the request being cancelled. Using the same values for
   these headers allows the CANCEL to be matched with the request it
   cancels (Section 9.2 indicates how such matching occurs). However,
   the method part of the CSeq header MUST have a value of CANCEL. This
   allows it to be identified and processed as a transaction in its own



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   right (See Section 17).

   If the request being cancelled contains Route header fields, the
   CANCEL request MUST include these Route header fields.

        This is needed so that stateless proxies are able to route
        CANCEL requests properly.

   The CANCEL request MUST NOT contain any Require or Proxy-Require
   header fields.

   Once the CANCEL is constructed, the client SHOULD check whether any
   response (provisional or final) has been received for the request
   being cancelled (herein referred to as the "original request"). The
   CANCEL request MUST NOT be sent if no provisional response has been
   received, rather, the client MUST wait for the arrival of a
   provisional response before sending the request. If the original
   request has generated a final response, the CANCEL SHOULD NOT be
   sent, as it is an effective no-op, since CANCEL has no effect on
   requests that have already generated a final response. When the
   client decides to send the CANCEL, it creates a client transaction
   for the CANCEL and passes it the CANCEL request along with the
   destination address, port, and transport. The destination address,
   port, and transport for the CANCEL MUST be identical to those used to
   send the original request.


        If it was allowed to send the CANCEL before receiving a
        response for the previous request, the server could receive
        the CANCEL before the original request.

   Note that both the transaction corresponding to the original request
   and the CANCEL transaction will complete independently. However, a
   UAC canceling a request cannot rely on receiving a 487 (Request
   Terminated) response for the original request, as an RFC 2543-
   compliant UAS will not generate such a response. If there is no final
   response for the original request in 64*T1 seconds (T1 is defined in
   Section 17.1.1.1), the client SHOULD then consider the original
   transaction cancelled and SHOULD destroy the client transaction
   handling the original request.

9.2 Server Behavior

   The CANCEL method requests that the TU at the server side cancel a
   pending transaction. The transaction to be canceled is determined by
   taking the CANCEL request, and then assuming that the request method
   were anything but CANCEL, apply the transaction matching procedures
   of Section 17.2.3. The matching transaction is the one to be



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   canceled.

   The processing of a CANCEL request at a server depends on the type of
   server. A stateless proxy will forward it, a stateful proxy might
   respond to it and generate some CANCEL requests of its own, and a UAS
   will respond to it. See Section 16.9 for proxy treatment of CANCEL.

   A UAS first processes the CANCEL request according to the general UAS
   processing described in Section 8.2. However, since CANCEL requests
   are hop-by-hop and cannot be resubmitted, they cannot be challenged
   by the server in order to get proper credentials in an Authorization
   header field. Note also that CANCEL requests do not contain Require
   header fields.

   If the CANCEL did not find a matching transaction according to the
   procedure above, the CANCEL SHOULD be responded to with a 481 (Call
   Leg/Transaction Does Not Exist). If the transaction for the original
   request still exists, the behavior of the UAS on receiving a CANCEL
   request depends on whether it has already sent a final response for
   the original request. If it has, the CANCEL request has no effect on
   the processing of the original request, no effect on any session
   state, and no effect on the responses generated for the original
   request. If the UAS has not issued a final response for the original
   request, its behavior depends on the method of the original request.
   If the original request was an INVITE, the UAS SHOULD immediately
   respond to the INVITE with a 487 (Request Terminated). The behavior
   upon reception of a CANCEL request for any other method defined in
   this specification is effectively no-op. Extensions to this
   specification that define new methods MUST define the behavior of a
   UAS upon reception of a CANCEL for those methods.

   Regardless of the method of the original request, as long as the
   CANCEL matched an existing transaction, the CANCEL request itself is
   answered with a 200 (OK) response. This response is constructed
   following the procedures described in Section 8.2.6 noting that the
   To tag of the response to the CANCEL and the To tag in the response
   to the original request SHOULD be the same. The response to CANCEL is
   passed to the server transaction for transmission.

10 Registrations

10.1 Overview

   SIP offers a discovery capability. If a user wants to initiate a
   session with another user, SIP must discover the current host(s) at
   which the destination user is reachable. This discovery process is
   accomplished by SIP proxy servers, which are responsible for
   receiving a request, determining where to send it based on knowledge



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   of the location of the user, and then sending it there. To do this,
   proxies consult an abstract service known as a location service ,
   which provides address bindings for a particular domain. These
   address bindings map an incoming SIP URI, sip:bob@Biloxi.com , for
   example, to one or more SIP URIs that are somehow "closer" to the
   desired user, sip:bob@engineering.Biloxi.com , for example.
   Ultimately, a proxy will consult a location service that maps a
   received URI to the current host(s) into which a user is logged.

   Registration creates bindings in a location service for a particular
   domain that associate an address-of-record URI with one or more
   contact addresses. Thus, when a proxy for that domain receives a
   request whose Request-URI matches the address-of-record, the proxy
   will forward the request to the contact addresses registered to that
   address-of-record. Generally, it only makes sense to register an
   address-of-record at a domain's location service when requests for
   that address-of-record would be routed to that domain. In most cases,
   this means that the domain of the registration will need to match the
   domain in the URI of the address-of-record.

   There are many ways by which the contents of the location service can
   be established. One way is administratively. In the above example,
   Bob is known to be a member of the engineering department through
   access to a corporate database. However, SIP provides a mechanism for
   a UA to create a binding explicitly. This mechanism is known as
   registration.

   Registration entails sending a REGISTER request to a special type of
   UAS known as a registrar. The registrar acts as a front end to the
   location service for a domain, reading and writing mappings based on
   the contents of the REGISTER requests. This location service will
   then be consulted by a proxy server that is responsible for routing
   requests for that domain.

   SIP does not mandate a particular mechanism for implementing the
   location service. The only requirement is that a registrar for some
   domain MUST be able to read and write data to the location service,
   and a proxy for that domain MUST be capable of reading that same
   data. A registrar MAY be co-located with a particular SIP proxy
   server for the same domain.


10.2 Constructing the REGISTER Request

   REGISTER requests add, remove, and query bindings. A REGISTER request
   may add a new binding between an address-of-record and one or more
   contact addresses. Registration on behalf of a particular address-
   of-record may be performed by a suitably authorized third party.  A



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   client may also remove previous bindings or query to determine which
   bindings are currently in place for an address-of-record.

   Except as noted, the construction of the REGISTER request and the
   behavior of clients sending a REGISTER request is identical to the
   general UAC behavior described in Section 8.1 and Section 17.1. The
   following header fields MUST be included:

        Request-URI: The Request-URI names the domain of the location
             service for which the registration is meant (for example,
             "sip:chicago.com"). The "userinfo" and "@" components of
             the SIP URI MUST NOT be present.

        To: The To header field contains the address of record whose
             registration is to be created, queried, or modified. The To
             header field and the Request-URI field typically differ, as
             the former contains a user name. This address-of-record
             MUST be a SIP URI.

        From: The From header field contains the address-of-record of
             the person responsible for the registration.  The value is
             the same as the To header field unless the request is a
             third-party registration.

        Call-ID: All registrations from a UAC SHOULD use the same Call-
             ID header value for registrations sent to a particular
             registrar.


             If the same client were to use different Call-ID
             values, a registrar could not detect whether a delayed
             REGISTER request might have arrived out of order.

        CSeq: The CSeq value guarantees proper ordering of REGISTER
             requests. A UA MUST increment the CSeq value by one for
             each REGISTER request with the same Call-ID.

        Contact: REGISTER requests contain zero or more Contact header
             fields, containing address bindings.

   UAs MUST NOT send a new registration (that is, containing new Contact
   header fields, as opposed to a retransmission) until they have
   received a final response from the registrar for the previous one or
   the previous REGISTER request has timed out.

   The following Contact header parameters have a special meaning in
   REGISTER requests:




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                                                   bob
                                                 +----+
                                                 | UA |
                                                 |    |
                                                 +----+
                                                    |
                                                    |3)INVITE
                                                    |   carol@chicago.com
           chicago.com        +--------+            V
           +---------+ 2)Store|Location|4)Query +-----+
           |Registrar|=======>| Service|<=======|Proxy|sip.chicago.com
           +---------+        +--------+=======>+-----+
                 A                      5)Resp      |
                 |                                  |
                 |                                  |
       1)REGISTER|                                  |
                 |                                  |
              +----+                                |
              | UA |<-------------------------------+
     cube2214a|    |                            6)INVITE
              +----+                    carol@cube2214a.chicago.com
               carol
























   Figure 2: REGISTER example

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        action: The "action" parameter from RFC 2543 has been
             deprecated. UACs SHOULD NOT use the "action" parameter.

        expires: The "expires" parameter indicates how long the UA would
             like the binding to be valid.  The value is a number
             indicating seconds. If this parameter is not provided, the
             value of the Expires header field is used instead.
             Implementations MAY treat values larger than 2**32-1
             (4294967295 seconds or 136 years) as equivalent to 2**32-1.
             Malformed values should be treated as equivalent to 3600.

10.2.1 Adding Bindings

   The REGISTER request sent to a registrar includes contact addresses
   to which SIP requests for the address-of-record should be forwarded.
   The address-of-record is included in the To header field of the
   REGISTER request.

   The Contact header fields of the request typically contain SIP URIs
   that identify particular SIP endpoints (for example,
   "sip:carol@cube2214a.chicago.com"), but they MAY use any URI scheme.
   A SIP UA can choose to register telephone numbers (with the tel URL,
   [19]) or email addresses (with a mailto URL, [36]) as Contacts for an
   address-of-record.

   For example, Carol, with address-of-record "sip:carol@chicago.com",
   would register with the SIP registrar of the domain chicago.com. Her
   registrations would then be used by a proxy server in the chicago.com
   domain to route requests for Carol's address-of-record to her SIP
   endpoint.

   Once a client has established bindings at a registrar, it MAY send
   subsequent registrations containing new bindings or modifications to
   existing bindings as necessary. The 2xx response to the REGISTER
   request will contain, in Contact header fields, a complete list of
   bindings that have been registered for this address-of-record at this
   registrar.

   Registrations do not need to update all bindings. Typically, a UA
   only updates its own SIP URI as well as any non-SIP URIs.

10.2.1.1 Setting the Expiration Interval of Contact Addresses

   When a client sends a REGISTER request, it MAY suggest an expiration
   interval that indicates how long the client would like the
   registration to be valid. (As described in Section 10.3, the
   registrar selects the actual time interval based on its local
   policy.)



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   There are two ways in which a client can suggest an expiration
   interval for a binding: through an Expires header field or an
   "expires" Contact header parameter. The latter allows expiration
   intervals to be suggested on a per-binding basis when more than one
   binding is given in a single REGISTER request, whereas the former
   suggests an expiration interval for all Contact header fields that do
   not contain the "expires" parameter.

   If neither mechanism for expressing a suggested expiration time is
   present in a REGISTER, a default suggestion of one hour is assumed.

10.2.1.2 Preferences among Contact Addresses

   If more than one Contact is sent in a REGISTER request, the
   registering UA intends to associate all of the URIs given in these
   Contact header fields with the address-of-record present in the To
   field. This list can be prioritized with the "q" parameter in the
   Contact header fields. The "q" parameter indicates a relative
   preference for the particular Contact header field compared to other
   bindings present in this REGISTER message or existing within the
   location service of the registrar. Section 16.5 describes how a proxy
   server uses this preference indication.

10.2.2 Removing Bindings

   Registrations are soft state and expire unless refreshed, but can
   also be explicitly removed. A client can attempt to influence the
   expiration interval selected by the registrar as described in Section
   10.2.1. A UA requests the immediate removal of a binding by
   specifying an expiration interval of "0" for that contact address in
   a REGISTER request. UAs SHOULD support this mechanism so that
   bindings can be removed before their expiration interval has passed.

   The REGISTER-specific Contact header field value of "*" applies to
   all registrations, but it MUST only be used when the Expires header
   field is present with a value of "0".


        Use of the "*" Contact header field value allows a
        registering UA to remove all of its bindings without
        knowing their precise values.

   If no Contact header fields are present in a REGISTER request, the
   list of bindings is left unchanged.

10.2.3 Fetching Bindings

   A success response to any REGISTER request contains the complete list



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   of existing bindings, regardless of whether the request contained a
   Contact header field.

10.2.4 Refreshing Bindings

   Each UA is responsible to refresh the bindings that it has previously
   established. A UA SHOULD NOT refresh bindings set up by other UAs.

   The 200 (OK) response from the registrar contains a list of Contact
   fields enumerating all current bindings. The UA compares each contact
   address to see if it created the contact address, using comparison
   rules in Section 23.1.4. If so, it updates the expiration time
   interval according to the expires parameter or, if absent, the
   Expires field value. The UA then issues a REGISTER request for each
   of its bindings before the expiration interval has elapsed. It MAY
   combine several updates into one REGISTER request.

   A UA SHOULD use the same Call-ID for all registrations during a
   single boot cycle. Registration refreshes SHOULD be sent to the same
   network address as the original registration, unless redirected.

10.2.5 Setting the Internal Clock

   If the response for REGISTER request contains a Date header field,
   the client MAY use this header field to learn the current time in
   order to set any internal clocks.

10.2.6 Discovering a Registrar

   UAs can use three ways to determine the address to which to send
   registrations:  by configuration, using the address-of-record, and
   multicast. A UA can be configured, in ways beyond the scope of this
   specification, with a registrar address. If there is no configured
   registrar address, the UA SHOULD use the host part of the address-
   of-record as the Request-URI and address the request there, using the
   normal SIP server location mechanisms [2]. For example, the UA for
   the user "sip:carol@chicago.com" addresses the REGISTER request to
   "chicago.com".

   Finally, a UA can be configured to use multicast. Multicast
   registrations are addressed to the well-known "all SIP servers"
   multicast address "sip.mcast.net" (224.0.1.75 for IPv4). No well-
   known IPv6 multicast address has been allocated; such an allocation
   will be documented separately when needed. This request MUST be
   scoped to ensure it is not forwarded beyond the boundaries of the
   administrative system.  This MAY be done with either TTL or
   administrative scopes (see [12]), depending on what is implemented in
   the network. SIP UAs MAY listen to that address and use it to become



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   aware of the location of other local users (see [40]); however, they
   do not respond to the request.


        Multicast registration may be inappropriate in some
        environments, for example, if multiple businesses share the
        same local area network.

10.2.7 Transmitting a Request

   Once the REGISTER method has been constructed, and the destination of
   the message identified, UACs should follow the procedures described
   in Section 8.1.2 to hand off the REGISTER to the transaction layer.

   If the transaction layer returns a timeout error because the REGISTER
   yielded no response, the UAC SHOULD wait some reasonable time
   interval before re-attempting a registration to the same registrar;
   no specific interval is mandated.

10.2.8 Error Responses

   If a UA receives a 423 (Registration Too Brief) response, it MAY
   retry the registration after making the expiration interval of all
   contact addresses in the REGISTER request equal to or greater than
   the expiration interval within the Min-Expires header field of the
   423 (Registration Too Brief) response.

10.3 Processing REGISTER Requests

   A registrar is a UAS that responds to REGISTER requests and maintains
   a list of bindings that are accessible to proxy servers within its
   administrative domain. A registrar handles requests according to
   Section 8.2 and Section 17.2, but it accepts only REGISTER requests.
   A registrar does not generate 6xx responses.

   If a registrar listens at a multicast interface, it MAY redirect
   multicast REGISTER requests to its own unicast interface with a 302
   (Moved Temporarily) response.

   A REGISTER request MUST NOT contain Record-Route or Route header
   fields; registrars MUST ignore them if they appear.

   A registrar must know (for example, through configuration) the set of
   domain(s) for which it maintains bindings. REGISTER requests MUST be
   processed by a registrar in the order that they are received.
   REGISTER requests MUST also be processed atomically, meaning that
   REGISTER requests are either processed completely or not at all.
   Each REGISTER message must be processed independently of any other



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   registration or binding changes.

   When receiving a REGISTER request, a registrar follows these steps:

        1.   The registrar inspects the Request-URI to determine whether
             it has access to bindings for the domain identified in the
             Request-URI. If not, and if the server also acts as a proxy
             server, the server SHOULD forward the request to the
             addressed domain, following the general behavior for
             proxying messages described in Section 16.

        2.   To guarantee that the registrar supports any necessary
             extensions, the registrar processes Require header fields
             as described for UASs in Section 8.2.2.

        3.   A registrar SHOULD authenticate the UAC. Mechanisms for the
             authentication of SIP user agents are described in Section
             20; registration behavior in no way overrides the generic
             authentication framework for SIP. If no authentication
             mechanism is available, the registrar MAY take the From
             address as the asserted identity of the originator of the
             request.

        4.   The registrar SHOULD determine if the authenticated user is
             authorized to modify registrations for this address-of-
             record. For example, a registrar might consult a
             authorization database that maps user names to a list of
             addresses-of-record for which this identity is authorized
             to modify bindings. If not, the registrar returns 403
             (Forbidden) and skips the remaining steps.


             In architectures that support third-party
             registration, one entity may be responsible for
             updating the registrations associated with multiple
             addresses-of-record.

        5.   The registrar extracts the address-of-record from the To
             header field of request. If the address-of-record is not
             valid for the domain in the Request-URI, the registrar
             sends a 404 (Not Found) response and skips the remaining
             steps. The URI MUST then be converted to a canonical form.
             To do that, all URI parameters are removed (including the
             user-param), and any escaped characters are converted to
             their unescaped form. The result serves as an index into
             the list of bindings.

        6.   The registrar checks whether the request contains any



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             Contact header fields. If not, it skips to the last step.

             Next, the registrar checks if there is one Contact field
             that contains the special value "*" and a Expires field. If
             the request has additional Contact fields or an expiration
             time other than zero, the request is invalid, and the
             server returns 400 (Invalid Request) and skips the
             remaining steps. If not, the registrar checks whether the
             Call-ID agrees with the value stored for each binding. If
             not, it removes the binding. If it does agree, it only
             removes the binding if the CSeq in the request is higher
             than the value stored for that binding and leaves the
             binding as is otherwise.  It then skips to the last step.

        7.   The registrar now processes each contact address in the
             Contact header field in turn. For each address, it
             determines the expiration interval as follows:

             - If the field value has an "expires" parameter, that value
               is used.

             - If there is no such parameter, but the request has an
               Expires header field, that value is used.

             - If there is neither, a locally-configured default value
               is used.

             The registrar MAY shorten the expiration interval. If and
             only if the expiration interval is greater than zero AND
             smaller than one hour AND less than a registrar-configured
             minimum, the registrar MAY reject the registration with a
             response of 423 (Registration Too Brief).  This response
             MUST contain a Min-Expires header field that states the
             minimum expiration interval the registrar is willing to
             honor. It then skips the remaining steps.


             Allowing the registrar to set the registration
             interval protects it against excessively frequent
             registration refreshes while limiting the state that
             it needs to maintain and decreasing the likelihood of
             registrations going stale. The expiration interval of
             a registration is frequently used in the creation of
             services. An example is a follow-me service, where the
             user may only be available at a terminal for a brief
             period. Therefore, registrars should accept brief
             registrations; a request should only be rejected if
             the interval is so short that the refreshes would



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             degrade registrar performance.

             For each address, the registrar then searches the list of
             current bindings using the URI comparison rules. If the
             binding does not exist, it is tentatively added. If the
             binding does exist, the registrar checks the Call-ID value.
             If the Call-ID value in the existing binding differs from
             the Call-ID value in the request, the binding is removed if
             the expiration time is zero and updated otherwise.  If they
             are the same, the registrar compares the CSeq value. If the
             value is higher than that of the existing binding, it
             updates or removes the binding as above. If not, the update
             is aborted and the request fails.


             This algorithm ensures that out-of-order requests from
             the same UA are ignored.

             Each binding record records the Call-ID and CSeq values
             from the request.

             The binding updates are committed (that is, made visible to
             the proxy) if and only if all binding updates and additions
             succeed. If any one of them fails, the request fails with
             500 (Server Error) response and all tentative binding
             updates are removed.

        8.   The registrar returns a 200 (OK) response. The response
             MUST contain Contact header fields enumerating all current
             bindings.  Each Contact value MUST feature an "expires"
             parameter indicating its expiration interval chosen by the
             registrar.  The response SHOULD include a Date header
             field.

11 Querying for Capabilities

   The SIP method OPTIONS allows a UA to query another UA or a proxy
   server as to its capabilities. This allows a client to discover
   information about the supported methods, content types, extensions,
   codecs, etc.  without "ringing" the other party. For example, before
   a client inserts a Require header field into an INVITE listing an
   option that it is not certain the destination UAS supports, the
   client can query the destination UAS with an OPTIONS to see if this
   option is returned in a Supported header field.

   The target of the OPTIONS request is identified by the Request-URI,
   which could identify another UA or a SIP server. If the OPTIONS is
   addressed to a proxy server, the Request-URI is set without a user



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   part, similar to the way a Request-URI is set for a REGISTER request.

   Alternatively, a server receiving an OPTIONS request with a Max-
   Forwards header value of 0 MAY respond to the request regardless of
   the Request-URI.


        This behavior is common with HTTP/1.1. This behavior can be
        used as a "traceroute" functionality to check the
        capabilities of individual hop servers by sending a series
        of OPTIONS requests with incremented Max-Forwards values.

   As is the case for general UA behavior, the transaction layer can
   return a timeout error if the OPTIONS yields no response. This may
   indicate that the target is unreachable and hence unavailable.

   An OPTIONS request MAY be sent as part of an established dialog to
   query the peer on capabilities that may be utilized later in the
   dialog.

11.1 Construction of OPTIONS Request

   An OPTIONS request is constructed using the standard rules for a SIP
   request as discussed Section 8.1.1.

   A Contact header field MAY be present in an OPTIONS.

   An Accept header field SHOULD be included to indicate the type of
   message body the UAC wishes to receive in the response. Typically,
   this is set to a format that is used to describe the media
   capabilities of a UA, such as SDP (application/sdp).

   The response to an OPTIONS request is assumed to be scoped to the
   Request-URI in the original request. However, only when an OPTIONS is
   sent as part of an established dialog is it guaranteed that future
   requests will be received by the server which generated the OPTIONS
   response.

   Example OPTIONS request:



     OPTIONS sip:carol@chicago.com SIP/2.0
     Via: SIP/2.0/UDP 192.0.2.4;branch=z9hG4bKhjhs8ass877
     To: <sip:carol@chicago.com>
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 63104 OPTIONS



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     Contact: <sip:alice@192.0.2.4>
     Max-Forwards: 70
     Accept: application/sdp
     Content-Length: 0



11.2 Processing of OPTIONS Request

   The response to an OPTIONS is constructed using the standard rules
   for a SIP response as discussed in Section 8.2.6.  The response code
   chosen is the same that would have been chosen had the request been
   an INVITE. That is, a 200 (OK) would be returned if the UAS is ready
   to accept a call, a 486 (Busy Here) would be returned if the UAS is
   busy, etc. This allows an OPTIONS request to be used to determine the
   basic state of a UAS, which can be an indication of whether the UAC
   will accept an INVITE request.

   An OPTIONS request received within a dialog generates a 200 (OK)
   response that is identical to one constructed outside a dialog and
   does not have any impact on the dialog.

   This use of OPTIONS has limitations due the differences in proxy
   handling of OPTIONS and INVITE requests. While a forked INVITE can
   result in multiple 200 (OK) responses being returned, a forked
   OPTIONS will only result in a single 200 (OK) response, since it is
   treated by proxies using the non-INVITE handling. See Section 13.2.1
   for the normative details.

   If the response to an OPTIONS is generated by a proxy server, the
   proxy returns a 200 (OK) listing the capabilities of the server. The
   response does not contain a message body.

   Allow, Accept, Accept-Encoding, Accept-Language, and Supported header
   fields SHOULD be present in a 200 (OK) response to an OPTIONS
   request. If the response is generated by a proxy, the Allow header
   field SHOULD be omitted as it is ambiguous since a proxy is method
   agnostic. Contact header fields MAY be present in a 200 (OK) response
   and have the same semantics as in a redirect. That is, they may list
   a set of alternative names and methods of reaching the user. A
   Warning header field MAY be present.

   A message body MAY be sent, the type of which is determined by the
   Accept header in the OPTIONS request (application/sdp if the Accept
   header was not present). If the types include one that can describe
   media capabilities, the UA SHOULD include a body in the response for
   that purpose. Details on construction of such a body in the case of
   application/sdp are described in [1].



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   Example OPTIONS response generated by a UAS (corresponding to the
   request in Section 11.1):



     SIP/2.0 200 OK
     Via: SIP/2.0/UDP 192.0.2.4;branch=z9hG4bKhjhs8ass877
     To: <sip:carol@chicago.com>;tag=93810874
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710@100.1.3.3
     CSeq: 63104 OPTIONS
     Contact: <sip:carol@chicago.com>
     Contact: <mailto:carol@chicago.com>
     Allow: INVITE, ACK, CANCEL, OPTIONS, BYE
     Accept: application/sdp
     Accept-Encoding: gzip
     Accept-Language: en
     Supported: foo
     Content-Type: application/sdp
     Content-Length: 274

     (SDP not shown)



12 Dialogs

   A key concept for a user agent is that of a dialog. A dialog
   represents a peer-to-peer SIP relationship between a two user agents
   that persists for some time. The dialog facilitates sequencing of
   messages between the user agents and proper routing of requests
   between both of them.  The dialog represents a context in which to
   interpret SIP messages.  Section 8 discussed method independent UA
   processing for requests and responses outside of a dialog. This
   section discusses how those requests and responses are used to
   construct a dialog, and then how subsequent requests and responses
   are sent within a dialog.

   A dialog is identified at each UA with a dialog ID, which consists of
   a Call-ID value, a local URI and local tag (together called the local
   address), and a remote URI and remote tag (together called the remote
   address). The dialog ID at each UA involved in the dialog is not the
   same. Specifically, the local URI and local tag at one UA are
   identical to the remote URI and remote tag at the peer UA. The tags
   are opaque tokens that facilitate the generation of unique dialog
   IDs.

   A dialog ID is also associated with all responses and with any



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   request that contains a tag in the To field. The rules for computing
   the dialog ID of a message depend on whether the entity is a UAC or
   UAS.  For a UAC, the Call-ID value of the dialog ID is set to the
   Call-ID of the message, the remote address is set to the To field of
   the message, and the local address is set to the From field of the
   message (these rules apply to both requests and responses). As one
   would expect, for a UAS, the Call-ID value of the dialog ID is set to
   the Call-ID of the message, the remote address is set to the From
   field of the message, and the local address is set to the To field of
   the message.

   A dialog contains certain pieces of state needed for further message
   transmissions within the dialog. This state consists of the dialog
   ID, a local sequence number (used to order requests from the UA to
   its peer), a remote sequence number (used to order requests from its
   peer to the UA), the URI of the remote target, and a route set, which
   is an ordered list of URIs. The route set is the set of servers that
   need to be traversed to send a request to the peer. A dialog can also
   be in the "early" state, which occurs when it is created with a
   provisional response, and then transition to the "confirmed" state
   when the final response comes.

12.1 Creation of a Dialog

   Dialogs are created through the generation of non-failure responses
   to requests with specific methods. Within this specification, only
   2xx and 101-199 responses with a To tag to INVITE establish a dialog.
   A dialog established by a non-final response to a request is in the
   "early" state and it is called an early dialog. Extensions MAY define
   other means for creating dialogs. Section 13 gives more details that
   are specific to the INVITE method. Here, we describe the process for
   creation of dialog state that is not dependent on the method.

   A dialog is identified by a dialog ID. A dialog ID consists of three
   components, namely a call identifier component, a local address
   component and a remote address component. UAs MUST assign values to
   these components as described below.

12.1.1 UAS behavior

   When a UAS responds to a request with a response that establishes a
   dialog (such as a 2xx to INVITE), the UAS MUST copy all Record-Route
   headers from the request into the response (including the URIs, URI
   parameters, and any Record-Route header parameters, whether they are
   known or unknown to the UAS) and MUST maintain the order of those
   headers. The UAS MUST add a Contact header field to the response. The
   Contact header field contains an address where the UAS would like to
   be contacted for subsequent requests in the dialog (which includes



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   the ACK for a 2xx response in the case of an INVITE).  Generally, the
   host portion of this URI is the IP address or FQDN of the host. The
   URI provided in the Contact header field MUST be a SIP URI and have
   global scope (i.e., the same SIP URI can be used outside this dialog
   to contact the UAS). The same way, the scope of the SIP URI in the
   Contact header field of the INVITE is not limited to this dialog
   either. It can therefore be used to contact the UAC even outside this
   dialog.

   The UAS then constructs the state of the dialog. This state MUST be
   maintained for the duration of the dialog.

   The route set MUST be set to the list of URIs in the Record-Route
   header field from the request, taken in order and preserving all URI
   parameters. If no Record-Route header field is present in the
   request, the route set MUST be set to the empty set. This route set,
   even if empty, overrides any pre-existing route set for future
   requests in this dialog. The remote target MUST be set to the URI
   from the Contact header field of the request.

   The remote sequence number MUST be set to the value of the sequence
   number in the Cseq header field of the request. The local sequence
   number MUST be empty. The call identifier component of the dialog ID
   MUST be set to the value of the Call-ID in the request. The local
   address component of the dialog ID MUST be set to the To field in the
   response to the request (which therefore includes the tag), and the
   remote address component of the dialog ID MUST be set to the From
   field in the request. A UAS MUST be prepared to receive a request
   without a tag in the From field, in which case the tag is considered
   to have a value of null.

        This is to maintain backwards compatibility with RFC 2543,
        which did not mandate From tags.

12.1.2 UAC behavior

   When a UAC receives a response that establishes a dialog, it
   constructs the state of the dialog. This state MUST be maintained for
   the duration of the dialog.

   The route set MUST be set to the list of URIs in the Record-Route
   header field from the response, taken in reverse order and preserving
   all URI parameters. If no Record-Route header field is present in the
   response, the route set MUST be set to the empty set. This route set,
   even if empty, overrides any pre-existing route set for future
   requests in this dialog. The remote target MUST be set to the URI
   from the Contact header field of the response.  The local sequence
   number MUST be set to the value of the sequence number in the Cseq



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   header field of the request. The remote sequence number MUST be empty
   (it is established when the UA sends a request within the dialog).
   The call identifier component of the dialog ID MUST be set to the
   value of the Call-ID in the request. The local address component of
   the dialog ID MUST be set to the From field in the request, and the
   remote address component of the dialog ID MUST be set to the To field
   of the response.  A UAC MUST be prepared to receive a response
   without a tag in the To field, in which case the tag is considered to
   have a value of null.

        This is to maintain backwards compatibility with RFC 2543,
        which did not mandate To tags.

12.2 Requests within a Dialog

   Once a dialog has been established between two UAs, either of them
   MAY initiate new transactions as needed within the dialog. However, a
   dialog imposes some restrictions on the use of simultaneous
   transactions.

   A TU MUST NOT initiate a new regular transaction within a dialog
   while a regular transaction is in progress (in either direction)
   within that dialog. If there is a non-INVITE client or server
   transaction in progress the TU MUST wait until this transaction
   enters the completed or the terminated state to initiate the new
   transaction.


        OPEN ISSUE #113: Should we relax the constraint on non-
        overlapping regular transactions?

   A route refresh request sent within a dialog is defined as a request
   that can modify the route set of the dialog. For dialogs that have
   been established with an INVITE, the only route refresh request
   defined is re-INVITE (see Section  14). Other extensions may define
   different route refresh requests for dialogs established in other
   ways.

        Note that an ACK is NOT a route refresh request.

12.2.1 UAC Behavior

12.2.1.1 Generating the Request

   A request within a dialog is constructed by using many of the
   components of the state stored as part of the dialog.

   The To header field of the request MUST be set to the remote address,



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   and the From header field MUST be set to the local address (both
   including tags, assuming the tags are not null).

   The Call-ID of the request MUST be set to the Call-ID of the dialog.
   Requests within a dialog MUST contain strictly monotonically
   increasing and contiguous CSeq sequence numbers (increasing-by-one)
   in each direction. Therefore, if the local sequence number is not
   empty, the value of the local sequence number MUST be incremented by
   one, and this value MUST placed into the Cseq header. If the local
   sequence number is empty, an initial value MUST be chosen using the
   guidelines of Section 8.1.1.5. The method field in the Cseq header
   MUST match the method of the request.


        With a length of 32 bits, a client could generate, within a
        single call, one request a second for about 136 years
        before needing to wrap around. The initial value of the
        sequence number is chosen so that subsequent requests
        within the same call will not wrap around. A non-zero
        initial value allows clients to use a time-based initial
        sequence number. A client could, for example, choose the 31
        most significant bits of a 32-bit second clock as an
        initial sequence number.

   The UAC uses the remote target and route set to build the Request-URI
   and Route header field of the request.

   If the route set is empty, the UAC MUST place the remote target URI
   into the Request-URI. The UAC MUST NOT add a Route header field to
   the request.

   If the route set is not empty, and the first URI in the route set
   contains the lr parameter (see Section 23.1.1), the UAC MUST place
   the remote target URI into the Request-URI and MUST a Route header
   field containing the route set values in order, including all
   parameters.

   If the route set is not empty and its first URI does not contain the
   lr parameter, the UAC MUST place the first URI from the route set
   into the Request-URI, stripping any parameters that are not allowed
   in a Request-URI. The UAC MUST add a Route header field containing
   the remainder of the route set values in order, including all
   parameters. The UAC MUST then place the the remote target URI into
   the Route header field as the last value.

   For example, if the remote target is sip:user@remoteua and the route
   set contains




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   <sip:proxy1>,<sip:proxy2>,<sip:proxy3;lr>,<sip:proxy4>


   The request will be formed with the following Request-URI and Route
   header field:

   METHOD sip:proxy1
   Route: <sip:proxy2>,<sip:proxy3;lr>,<sip:proxy4>,<sip:user@remoteua>




        If the first URI of the route set does not contain the lr
        parameter, the proxy indicated does not understand the
        routing mechanisms described in this document and will act
        as specified in RFC 2543, replacing the Request-URI with
        the first Route header field value it receives while
        forwarding the message. Placing the Request-URI at the end
        of the Route header field preserves the information in that
        Request-URI across the strict router (it will be returned
        to the Request-URI when the request reaches a loose-
        router).

   A UAC SHOULD include a Contact header in any route refresh requests
   within a dialog, and unless there is a need to change it, the URI
   SHOULD be the same as used in previous requests within the dialog. As
   discussed in Section 12.2.2, a Contact header in a route refresh
   request updates the remote target URI. This allows a UA to provide a
   new contact address, should its address change during the duration of
   the dialog.

   However, requests that are not route refresh requests do not affect
   the remote target URI for the dialog.

   Once the request has been constructed, the address of the server is
   computed and the request is sent, using the same procedures for
   requests outside of a dialog (Section 8.1.1).

12.2.1.2 Processing the Responses

   The UAC will receive responses to the request from the transaction
   layer. If the client transaction returns a timeout this is treated as
   a 408 (Request Timeout) response.

   The behavior of a UAC that receives a 3xx response for a request sent
   within a dialog is the same as if the request had been sent outside a
   dialog. This behavior is described in Section 13.2.2.




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        Note, however, that when the UAC tries alternative
        locations, it still uses the route set for the dialog to
        build the Route header of the request.

   When a UAC recieves a 2xx response to a route refresh resquest, it
   MUST replace the dialog's remote target URI with the URI from the
   Contact header field in that response, if present.

   If the response for the a request within a dialog is a 481
   (Call/Transaction Does Not Exist) or a 408 (Request Timeout), the UAC
   SHOULD terminate the dialog. A UAC SHOULD also terminate a dialog if
   no response at all is received for the request (the client
   transaction would inform the TU about the timeout.)

        For INVITE initiated dialogs, terminating the dialog
        consists of sending a BYE.

12.2.2 UAS behavior

   Requests sent within a dialog, as any other requests, are atomic. If
   a particular request is accepted by the UAS, all the state changes
   associated with it are performed. If the request is rejected, none of
   the state changes is performed.

        Note that some requests such as INVITEs affect several
        pieces of state.

   The UAS will receive the request from the transaction layer. If the
   request has a tag in the To header field, the UAS core computes the
   dialog identifier corresponding to the request and compares it with
   existing dialogs. If there is a match, this is a mid-dialog request.
   In that case, the UAS applies the same processing rules for requests
   outside of a dialog, discussed in Section 8.2.

   If the request has a tag in the To header field, but the dialog
   identifier does not match any existing dialogs, the UAS may have
   crashed and restarted, or it may have received a request for a
   different (possibly failed) UAS (the UASs can construct the To tags
   so that a UAS can identify that the tag was for a UAS for which it is
   providing recovery). Another possibility is that the incoming request
   has been simply missrouted. Based on the To tag, the UAS MAY either
   accept or reject the request. Accepting the request for acceptable To
   tags provides robustness, so that dialogs can persist even through
   crashes. UAs wishing to support this capability must take into
   consideration some issues such as choosing monotonically increasing
   CSeq sequence numbers even across reboots, reconstructing the route
   set, and accepting out-of-range RTP timestamps and sequence numbers.




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   If the UAS wishes to reject the request, because it does not wish to
   recreate the dialog, it MUST respond to the request with a 481
   (Call/Transaction Does Not Exist) status code and pass that to the
   server transaction.

   Requests that do not change in any way the state of a dialog may be
   received within a dialog (for example, an OPTIONS request). They are
   processed as if they had been received outside the dialog.

   Requests within a dialog MAY contain Record-Route and Contact header
   fields.  However, these requests do not cause the dialog's route set
   to be modified, although they may modify the remote target URI.
   Specifically, requests which are not refresh requests do not modify
   the dialog's remote target URI, and requests which are route refresh
   requests do.  This specification only defines one route refresh
   request:  re-INVITE (see Section  14).


        Route refresh requests only update the dialog's remote
        target URI, and not the route set formed from Record-Route.
        Updating the latter would introduce severe backwards
        compatibility problems with RFC 2543-compliant systems.

   If the remote sequence number is empty, it MUST be set to the value
   of the sequence number in the Cseq header in the request. If the
   remote sequence number was not empty, but the sequence number of the
   request is lower than the remote sequence number, the request is out
   of order and MUST be rejected with a 500 (Server Internal Error)
   response. If the remote sequence number was not empty, and the
   sequence number of the request is greater than the remote sequence
   number, the request is in order. It is possible for the CSeq header
   to be higher than the remote sequence number by more than one. This
   is not an error condition, and a UAS SHOULD be prepared to receive
   and process requests with CSeq values more than one higher than the
   previous received request. The UAS MUST then set the remote sequence
   number to the value of the sequence number in the Cseq header in the
   request.

        If a proxy challenges a request generated by the UAC, the
        UAC has to resubmit the request with credentials. The
        resubmitted request will have a new Cseq number. The UAS
        will never see the first request, and thus, it will notice
        a gap in the Cseq number space. Such a gap does not
        represent any error condition.

12.3 Termination of a Dialog

   Dialogs can end in several different ways, depending on the method.



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   When a dialog is established with INVITE, it is terminated with a
   BYE. No other means to terminate a dialog are described in this
   specification, but extensions can define other ways.

13 Initiating a Session

13.1 Overview

   When a user agent client desires to initiate a session (for example,
   audio, video, or a game), it formulates an INVITE request. The INVITE
   request asks a server to establish a session. This request is
   forwarded by proxies, eventually arriving at one or more UAS that can
   potentially accept the invitation. These UASs will frequently need to
   query the user about whether to accept the invitation. After some
   time, those UAS can accept the invitation (meaning the session is to
   be established) by sending a 2xx response. If the invitation is not
   accepted, a 3xx, 4xx, 5xx or 6xx response is sent, depending on the
   reason for the rejection. Before sending a final response, the UAS
   can also send a provisional response (1xx), either reliably or
   unreliably, to advise the UAC of progress in contacting the called
   user.

   After possibly receiving one or more provisional responses, the UA
   will get one or more 2xx responses or one non-2xx final response.
   Because of the protracted amount of time it can take to receive final
   responses to INVITE, the reliability mechanisms for INVITE
   transactions differ from those of other requests (like OPTIONS). Once
   it receives a final response, the UAC needs to send an ACK for every
   final response it receives. The procedure for sending this ACK
   depends on the type of response. For final responses between 300 and
   699, the ACK processing is done in the transaction layer and follows
   one set of rules (See Section 17). For 2xx responses, the ACK is
   generated by the UAC core.

   A 2xx response to an INVITE establishes a session, and it also
   creates a dialog between the UA that issued the INVITE and the UA
   that generated the 2xx response. Therefore, when multiple 2xx
   responses are received from different remote UAs (because the INVITE
   forked), each 2xx establishes a different dialog. All these dialogs
   are part of the same call.

   This section provides details on the establishment of a session using
   INVITE.

13.2 Caller Processing

13.2.1 Creating the Initial INVITE




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   Since the initial INVITE represents a request outside of a dialog,
   its construction follows the procedures of Section 8.1.1. Additional
   processing is required for the specific case of INVITE.

   An Allow header field (Section  24.5) SHOULD be present in the
   INVITE. It indicates what methods can be invoked within a dialog, on
   the UA sending the INVITE, for the duration of the dialog. For
   example, a UA capable of receiving INFO requests within a dialog [39]
   SHOULD include an Allow header listing the INFO method.

   A Supported header field (Section  24.39) SHOULD be present in the
   INVITE. It enumerates all the extensions understood by the UAC.

   An Accept (Section  24.1) header field MAY be present in the INVITE.
   It indicates which content-types are acceptable to the UA, in both
   the response received by it, and in any subsequent requests sent to
   it within dialogs established by the INVITE. The Accept header is
   especially useful for indicating support of various session
   description formats.

   The UA MAY add an Expires header field (Section 24.19) to limit the
   validity of the invitation. If the time indicated in the Expires
   header field is reached and no final answer for the INVITE has been
   received the UAC core SHOULD generate a CANCEL request for the
   original INVITE.

   A UAC MAY also find useful to add, among others, Subject (Section
   24.38), Organization (Section 24.25) and User-Agent (Section 24.43)
   header fields. They all contain information related to the INVITE.

   The UAC MAY choose to add a message body to the INVITE.  Section
   8.1.1.10 deals with how to construct the header fields -- Content-
   Type among others -- needed to describe the message body.

   There are special rules for message bodies that contain a session
   description - their corresponding Content-Disposition is "session".
   SIP uses an offer/answer model where one UA sends a session
   description, called the offer, which contains a proposed description
   of the session. The offer indicates the desired communications means
   (audio, video, games), parameters of those means (such as codec
   types) and addresses for receiving media from the answerer. The other
   UA responds with another session description, called the answer,
   which indicates which communications means are accepted, the
   parameters which apply to those means, and addresses for receiving
   media from the offerer. The offer/answer model defines restrictions
   on when offers and answers can be made. This results in restrictions
   on where the offers and answers can appear in SIP messages. In this
   specification, offers and answers can only appear in INVITE and PRACK



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   requests and responses. The usage of offers and answers is further
   restricted. For the initial INVITE transaction, the rules are:

        o The initial offer MUST be in either an INVITE or, if not
          there, in the first reliable message from the callee back to
          the caller. In this specification, that is either the first
          reliable provisional response or the final 2xx response.

        o If the initial offer is in an INVITE, the answer MUST be in a
          reliable message from callee back to caller which is
          correlated to that INVITE. For this specification, that is
          either a reliable provisional response or the final 2xx
          response to that INVITE.

        o If the initial offer is in the first reliable message from the
          callee back to caller, the answer MUST be in the
          acknowledgement for that message (PRACK for a reliable
          provisional response or ACK for a 2xx response).

        o After having sent or received an answer to the first offer,
          the UAC MAY generate subsequent offers in requests (PRACK
          alone for this specification), but only if it has received
          answers to any previous offers, and has not send any offers to
          which it hasn't gotten an answer.

        o Once the UAS has sent or received an answer to the initial
          offer, it MUST NOT generate subsequent offers in any responses
          to the INVITE. Since only the UAC can send PRACK, this means
          the a UAS based on this specification alone can never generate
          subsequent offers.

   Extensions to SIP which define new methods MAY specify whether offers
   and answers can appear in requests of that method or its responses.
   However, those extensions MUST adhere to the protocol rules specified
   in [2], and MUST adhere to the additional constraints in the list
   above.

   Concretely, the above rules specify two exchanges for UAs which don't
   support reliable provisional responses - the offer is in the INVITE,
   and the answer in the 2xx, or the offer is in the 2xx, and the answer
   is in the ACK. When reliable provisional responses is supported,
   several more flows are possible. One possibility is to have the offer
   in the INVITE, and the answer in a reliable provisional response,
   with no further SDP exchanges.

   All user agents that support INVITE and/or PRACK MUST support all
   exchanges that are possible based on the above rules and on their
   support for PRACK.



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   The Session Description Protocol (SDP) [11] MUST be supported by all
   user agents as a means to describe sessions, and its usage for
   constructing offers and answers MUST follow the procedures defined in
   [1].

   The restrictions of the offer-answer model just described only apply
   to bodies whose Content-Disposition header field is "session".
   Therefore, it is possible that both the INVITE and the ACK contain a
   body message (e.g., the INVITE carries a photo (Content-Disposition:
   render) and the ACK a session description (Content-Disposition:
   session) ).

        If the Content-Disposition header field is missing, bodies
        of Content-Type application/sdp imply the disposition
        "session", while other content types imply "render".

   Once the INVITE has been created, the UAC follows the procedures
   defined for sending requests outside of a dialog (Section 8).  This
   results in the construction of a client transaction that will
   ultimately send the request and deliver responses to the UAC.

13.2.2 Processing INVITE Responses

   Once the INVITE has been passed to the INVITE client transaction, the
   UAC waits for responses for the INVITE. Responses are matched to
   their corresponding INVITE because they have the same Call-ID, the
   same From header field, the same To header field, excluding the tag,
   and the same CSeq.  Rules for comparisons of these headers are
   described in Section 24. If the INVITE client transaction returns a
   timeout rather than a response the TU acts as if a 408 (Request
   Timeout) response had been received.

13.2.2.1 1xx responses

   Zero, one or multiple provisional responses may arrive before one or
   more final responses are received. Provisional responses for an
   INVITE request can create "early dialogs". If a provisional response
   has a tag in the To field, and if the dialog ID of the response does
   not match an existing dialog, one is constructed using the procedures
   defined in Section 12.1.2.

   The early dialog will only be needed if the UAC needs to send a
   request to its peer within the dialog before the initial INVITE
   transaction completes.  This will be the case for all reliable
   provisional responses, which require transmission of PRACK.  Header
   fields present in a provisional response are applicable as long as
   the dialog is in the early state (e.g., an Allow header field in a
   provisional response contains the methods that can be used in the



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   dialog while this is in the early state).

   If the 1xx is reliable and contains a session description, the UAC
   MUST generate an answer if the description is an offer. If the
   description is an answer, the session SHOULD be established based on
   the parameters of the offer and answer.

13.2.2.2 3xx responses

   A 3xx response may contain a Contact header field providing new
   addresses where the callee might be reachable. Depending on the
   status code of the 3xx response (see Section  25.3) the UAC MAY
   choose to try those new addresses.

13.2.2.3 4xx, 5xx and 6xx responses

   A single non-2xx final response may be received for the INVITE. 4xx,
   5xx and 6xx responses may contain a Contact header field indicating
   the location where additional information about the error can be
   found.

   All early dialogs are considered terminated upon reception of the
   non-2xx final response.

   After having received the non-2xx final response the UAC core
   considers the INVITE transaction completed. The INVITE client
   transaction handles generation of ACKs for the response (see Section
   17).

13.2.2.4 2xx responses

   Multiple 2xx responses may arrive at the UAC for a single INVITE
   request due to a forking proxy. Each response is distinguished by the
   tag parameter in the To header field, and each represents a distinct
   dialog, with a distinct dialog identifier.

   If the dialog identifier in the 2xx response matches the dialog
   identifier of an existing dialog, the dialog MUST be transitioned to
   the "confirmed" state, and the route set for the dialog MUST be
   recomputed based on the 2xx response using the procedures of Section
   12.1.2. Otherwise, a new dialog in the "confirmed" state is
   constructed in the same fashion.


        The route set only is recomputed for backwards
        compatibility. RFC 2543 did not mandate mirroring of
        Record-Route headers in a 1xx, only 2xx. However, we cannot
        update the entire state of the dialog, since mid-dialog



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        requests may have been sent within the early call leg,
        modifying the sequence numbers, for example.

   The UAC core MUST generate an ACK request for each 2xx received from
   the transaction layer. The header fields of the ACK are constructed
   in the same way as for any request sent within a dialog (see Section
   12) with the exception of the CSeq and the header fields related to
   authentication. The sequence number of the CSeq header field MUST be
   the same as the INVITE being acknowledged, but the CSeq method MUST
   be ACK. The ACK MUST contain the same credentials as the INVITE.  If
   the 2xx contains an offer (based on the rules above), the ACK MUST
   carry an answer in its body.  If the offer in the 2xx response is not
   acceptable, the UAC core MUST generate a valid answer in the ACK and
   then send a BYE immediately.

   Once the ACK has been constructed, the procedures of [2] are used to
   determine the destination address, port and transport. However, the
   request is passed to the transport layer directly for transmission,
   rather than a client transaction. This is because the UAC core
   handles retransmissions of the ACK, not the transaction layer. The
   ACK MUST be passed to the client transport every time a
   retransmission of the 2xx final response that triggered the ACK
   arrives.

   The UAC core considers the INVITE transaction completed 64*T1 seconds
   after the reception of the first 2xx response. At this point all the
   early dialogs that have not transitioned to established dialogs are
   terminated. Once the INVITE transaction is considered completed by
   the UAC core, no more new 2xx responses are expected to arrive.

   If, after acknowledging any 2xx response to an INVITE, the caller
   does not want to continue with that dialog, then the caller MUST
   terminate the dialog by sending a BYE request as described in Section
   15.

13.3 Callee Processing

13.3.1 Processing of the INVITE

   The UAS core will receive INVITE requests from the transaction layer.
   It first performs the request processing procedures of Section 8.2,
   which are applied for both requests inside and outside of a dialog.

   Assuming these processing states complete without generating a
   response, the UAS core performs the additional processing steps:

        1.   If the request is an INVITE that contains an Expires header
             field the UAS core inspects this header field. If the



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             INVITE has already expired a 487 (Request Terminated)
             response SHOULD be generated. In any case, if the INVITE
             expires before the UAS has generated a final response a 487
             (Request Terminated) response SHOULD be generated.

        2.   If the request is a mid-dialog request, the method-
             independent processing described in Section 12.2.2 is first
             applied.  It might also modify the session; Section 14
             provides details.

        3.   If the request has a tag in the To header field but the
             dialog identifier does not match any of the existing
             dialogs, the UAS may have crashed and restarted, or may
             have received a request for a different (possibly failed)
             UAS. Section 12.2.2 provides guidelines to achieve a robust
             behaviour under such a situation.

   Processing from here forward assumes that the INVITE is outside of a
   dialog, and is thus for the purposes of establishing a new session.

   The INVITE may contain a session description, in which case the UAS
   is being presented with an offer for that session. It is possible
   that the user is already a participant in that session, even though
   the INVITE is outside of a dialog. This can happen when a user is
   invited to the same multicast conference by multiple other
   participants.  If desired, the UAS MAY use identifiers within the
   session description to detect this duplication. For example, SDP
   contains a session id and version number in the origin (o) field. If
   the user is already a member of the session, and the session
   parameters contained in the session description have not changed, the
   UAS MAY silently accept the INVITE (that is, send a 2xx response
   without prompting the user).

   The INVITE may not contain a session description at all, in which
   case the UAS is being asked to participate in a session, but the UAC
   has asked that the UAS provide the offer of the session. It MUST
   provide the offer in its first reliable message back to the UAC.

   The callee can indicate progress, accept, redirect, or reject the
   invitation. In all of these cases, it formulates a response using the
   procedures described in Section  8.2.6.

13.3.1.1 Progress

   The UAS may not be able to answer the invitation immediately, and
   might choose to indicate some kind of progress to the caller (for
   example, an indication that a phone is ringing). This is accomplished
   with a provisional response between 101 and 199. These provisional



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   responses establish early dialogs and therefore follow the procedures
   of Section 12.1.1 in addition to those of Section 8.2.6. A UAS MAY
   send as many provisional responses as it likes. Each of these MUST
   indicate the same dialog ID. However, these will not be delivered
   reliably unless reliable provisional responses are used.

   If the INVITE contained an offer, the UAS MAY generate an answer in a
   reliable provisional response (assuming these are supported by the
   UAC). That results in the establishment of the session before
   completion of the call. Similarly, if a reliable provisional response
   is the first reliable message sent back to the caller, and the INVITE
   did not contain an offer, one MUST appear in that reliable
   provisional response.

   If the UAS will require an extended period of time to answer the
   INVITE, it will need to ask for an "extension" in order to prevent
   proxies from cancelling the transaction. A proxy has the option of
   canceling a transaction when there is a gap of 3 minutes between
   messages in a transaction. To prevent cancellation, the UAS MUST send
   a non-100 provisional response at least that often. This response
   SHOULD be sent reliably, if supported by the UAC. If not, the UAS
   SHOULD send provisional responses every minute, to handle the
   possibility of lost provisional responses.


        An INVITE transaction can go on for extended durations when
        the user is placed on hold, or when interworking with PSTN
        systems which allow communications to take place without
        answering the call. The latter is common in Interactive
        Voice Response (IVR) systems.

13.3.1.2 The INVITE is redirected

   If the UAS decides to redirect the call, a 3xx response is sent. A
   300 (Multiple Choices), 301 (Moved Permanently) or 302 (Moved
   Temporarily) response SHOULD contain a Contact header field
   containing URIs of new addresses to be tried. The response is passed
   to the INVITE server transaction, which will deal with its
   retransmissions.

13.3.1.3 The INVITE is rejected

   A common scenario occurs when the callee is currently not willing or
   able to take additional calls at this end system. A 486 (Busy Here)
   SHOULD be returned in such scenario. If the UAS knows that no other
   end system will be able to accept this call a 600 (Busy Everywhere)
   response SHOULD be sent instead. However, it is unlikely that a UAS
   will be able to know this in general, and thus this response will not



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   usually be used. The response is passed to the INVITE server
   transaction, which will deal with its retransmissions.

   A UAS rejecting an offer contained in an INVITE SHOULD return a 488
   (Not Acceptable Here) response. Such a response SHOULD include a
   Warning header field explaining why the offer was rejected.

13.3.1.4 The INVITE is accepted

   The UAS core generates a 2xx response. This response establishes a
   dialog, and therefore follows the procedures of Section 12.1.1 in
   addition to those of Section 8.2.6.

   If the UAS had placed a session description in any reliable
   provisional response that is unacknowledged when the INVITE is
   accepted, the UAS MUST delay sending the 2xx until the provisional
   response is acknowledged. Otherwise, the reliability of the 1xx
   cannot be guaranteed.

   A 2xx response to an INVITE SHOULD contain the Allow header field and
   the Supported header field, and MAY contain the Accept header field.
   Including these header fields allows the UAC to determine the
   features and extensions supported by the UAS for the duration of the
   call, without probing.

   If the INVITE request contained an offer, and the UAS had not yet
   sent an answer, the 2xx MUST contain an answer. If the INVITE did not
   contain an offer, the 2xx MUST contain an offer if the UAS had not
   yet sent an offer.

   Once the response has been constructed it is passed to the INVITE
   server transaction. Note, however, that the INVITE server transaction
   will be destroyed as soon as it receives this final response.
   Therefore, it is necessary to pass periodically the response to the
   transport until the ACK arrives. The 2xx response is passed to the
   transport with an interval that starts at T1 seconds and doubles for
   each retransmission until it reaches T2 seconds (T1 and T2 are
   defined in Section 17). Response retransmissions cease when an ACK
   request is received with the same dialog ID as the response. This is
   independent of whatever transport protocols are used to send the
   response.


        Since 2xx is retransmitted end-to-end, there may be hops
        between UAS and UAC which are UDP. To ensure reliable
        delivery across these hops, the response is retransmitted
        periodically even if the transport at the UAS is reliable.




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   If the server retransmits the 2xx response for 64*T1 seconds without
   receiving an ACK, it considers the dialog completed, the session
   terminated, and therefore it SHOULD send a BYE.

14 Modifying an Existing Session

   A successful INVITE request (see Section 13) establishes both a
   dialog between two user agents and a session (using the offer/answer
   model). Section 12 explains how to modify an existing dialog using a
   route refresh request (for example, changing the remote target URI of
   the dialog).  This section describes how to modify the actual
   session. This modification can involve changing addresses or ports,
   adding a media stream, deleting a media stream, and so on. This is
   accomplished by sending a new INVITE request within the same dialog
   that established the session. An INVITE request sent within an
   existing dialog is known as a re-INVITE.


        Note that a single re-INVITE can modify the dialog and the
        parameters of the session at the same time.

   Either the caller or callee can modify an existing session.

   The behavior of a UA on detection of media failure is a matter of
   local policy. However, automated generation of re-INVITE or BYE is
   NOT RECOMMENDED to avoid flooding the network with traffic when there
   is congestion. In any case, if these messages are sent automatically,
   they SHOULD be sent after some randomized interval.

        Note that the paragraph above refers to automatically
        generated BYEs and re-INVITEs. If the user hangs up upon
        media failure the UA would send a BYE request as usual.

14.1 UAC Behavior

   The same offer-answer model that applies to session descriptions in
   INVITEs (Section 13.2.1) applies to re-INVITEs.  As a result, a UAC
   that wants to add a media stream, for example, will create a new
   offer that contains this media stream, and send that in an INVITE
   request to its peer. It is important to note that the full
   description of the session, not just the change, is sent. This
   supports stateless session processing in various elements, and
   supports failover and recovery capabilities.  Of course, a UAC MAY
   send a re-INVITE with no session description, in which case the first
   reliable response to the re-INVITE will contain the offer.

   If the session description format has the capability for version
   numbers, the offerer SHOULD indicate that the version of the session



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   description has changed.

   The To, From, Call-ID, CSeq, and Request-URI of a re-INVITE are set
   following the same rules as for regular requests within an existing
   dialog, described in Section 12.

   A UAC MAY choose not to add Alert-Info header fields or bodies with
   Content-Disposition "alert" to re-INVITEs because UASs do not
   typically alert the user upon reception of a re-INVITE.

   Note that, as opposed to initial INVITEs (see Section 13), re-INVITEs
   contain tags in the To header field and are sent using the route set
   for the dialog.  Therefore, a single final (2xx or non-2xx) response
   is received for re-INVITEs.

   Note that a UAC MUST NOT initiate a new INVITE transaction within a
   dialog while another transaction (INVITE or non-INVITE) is in
   progress in either direction.

        1.   If there is an ongoing INVITE client transaction, the TU
             MUST wait until the transaction reaches the completed or
             terminated state before initiating the new INVITE.

        2.   If there is an ongoing INVITE server transaction, the TU
             MUST wait until the transaction reaches the confirmed or
             terminated state before initiating the new INVITE.

        3.   If there is an ongoing non-INVITE client or server
             transaction, the TU MUST wait until the transaction reaches
             the completed or terminated state before initiating the new
             INVITE.

   However, a UA MAY initiate a regular transaction while an INVITE
   transaction is in progress.

   If a UA receives a non-2xx final response to a re-INVITE, the session
   parameters MUST remain unchanged, as if no re-INVITE had been issued.
   Note that, as stated in Section 12.2.1.2, if the non-2xx final
   response is a 481 (Call/Transaction Does Not Exist), or a 408
   (Request Timeout), or no response at all is received for the re-
   INVITE (that is, a timeout is returned by the INVITE client
   transaction), the UAC will terminate the dialog.

   The rules for transmitting a re-INVITE and for generating an ACK for
   a 2xx response to re-INVITE are the same as for an INVITE (Section
   13.2.1).

14.2 UAS Behavior



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   Section 13.3.1 describes the steps to follow in order to distinguish
   incoming re-INVITEs from incoming initial INVITEs.  This section
   describes the procedures to follow upon reception of a re-INVITE for
   an existing dialog.

   A UAS that receives a second INVITE before it sends the final
   response to a first INVITE with a lower CSeq sequence number on the
   same dialog MUST return a 500 (Server Internal Error) response to the
   second INVITE and MUST include a Retry-After header field with a
   randomly chosen value of between 0 and 10 seconds.

   A UAS that receives an INVITE on a dialog while an INVITE it had sent
   on that dialog is in progress MUST return a 491 (Request Pending)
   response to the received INVITE and MUST include a Retry-After header
   field with a value chosen as follows:

        1.   If the UAS is the owner of the Call-ID of the dialog ID,
             the Retry-After header field has a randomly chosen value of
             between 2.1 and 4 seconds in units of 10 ms.

        2.   If the UAS is not the owner of the Call-ID of the dialog
             ID, the Retry-After header field has a randomly chosen
             value of between 0 and 2 seconds in units of 10 ms.

   If a UA receives a re-INVITE for an existing dialog, it MUST check
   any version identifiers in the session description or, if there are
   no version identifiers, the content of the session description to see
   if it has changed. If the session description has changed, the UAS
   MUST adjust the session parameters accordingly, possibly after asking
   the user for confirmation.

        Versioning of the session description can be used to
        accommodate the capabilities of new arrivals to a
        conference, add or delete media or change from a unicast to
        a multicast conference.  If the new session description is
        not acceptable, the UAS can reject it by returning a 488
        (Not Acceptable Here) response for the re-INVITE. This
        response SHOULD include a Warning header field.

   If a UAS generates a 2xx response and never receives an ACK, it
   SHOULD generate a BYE to terminate the dialog.

   A UAS MAY choose not to generate 180 (Ringing) responses for a re-
   INVITE because UACs do not typically render this information to the
   user. For the same reason, UASs MAY choose not to use Alert-Info
   header fields or bodies with Content-Disposition "alert" in responses
   to a re-INVITE.




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   A UAS providing an offer in a 2xx (because the INVITE did not contain
   an offer) SHOULD construct the offer as if the UAS were making a
   brand new call, subject to the constraints of sending an offer which
   updates an existing session, as described in [1] in the case of SDP.
   Specifically, this means that it SHOULD include as many media formats
   and media types that the UA is willing to support. The UAS MUST
   ensure that the session description overlaps with its previous
   session description in media formats, transports, or other parameters
   that require support from the peer. This is to avoid the need for the
   peer to reject the session description. If, however, it is
   unacceptable to the UAC, the UAC SHOULD generate an answer with a
   valid session description, and then send a BYE to terminate the
   session.

15 Terminating a Session

   This section describes the procedures for terminating a SIP dialog.
   For two-party sessions that are otherwise unbound in time, the
   termination of the dialog implies the termination of the session.
   Other types of sessions, such as multicast sessions, are not
   terminated when a participant terminates the SIP dialog that he used
   to join the session. However, the SIP dialog SHOULD be terminated
   even though its termination does not imply the termination of the
   session. A UA joining a multicast session MAY terminate the SIP
   dialog immediately after the INVITE transaction used to join the
   session has completed.

   Either the caller or callee may terminate a dialog for any reason. A
   caller terminates a dialog either with BYE or CANCEL depending on the
   state of the dialog. A callee uses BYE to terminate a confirmed
   dialog.

        If the callee wants to terminate an early dialog, it just
        returns a non-2xx final response for the INVITE.  Sections
        13 and 12 document some cases where dialog termination is
        normative behavior. If a UA decides to terminate the
        dialog, it MUST follow the procedures here to initiate
        signaling action to convey that.

   When a UAC sends an INVITE request to create a session, if a 1xx
   response with a tag in the To field is received, an early dialog is
   created. When a 2xx response is received, the dialog becomes
   confirmed. For a confirmed dialog, if the UAC desires to terminate
   the session, the UAC SHOULD follow the procedures described in
   Section 15.1.1 to terminate the session. If the callee for a new
   session wishes to terminate the dialog, it uses the procedures of
   Section 15.1.1, but MUST NOT do so until it has received an ACK or
   until the server transaction times out.



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        This does not mean a user cannot hang up right away; it
        just means that the software in his phone needs to maintain
        state for a short while in order to clean up properly.

   If the UAC desires to end the session before a confirmed dialog has
   been created, it SHOULD send a CANCEL for the INVITE request that
   requested establishment of the session that is to be terminated. The
   UAC constructs and sends the CANCEL following the procedures
   described in Section 9. This CANCEL will normally result in a 487
   (Request Terminated) response to be returned to the INVITE,
   indicating successful cancellation. However, it is possible that the
   CANCEL and a 2xx response to the INVITE "pass on the wire". In this
   case, the UAC will receive a 2xx to the INVITE. It SHOULD then
   terminate the call by following the procedures described in Section
   15.1.1.

   A UAC can terminate a specific early dialog by following the
   procedures described in Section 15.1.1. This would only terminate one
   particular early dialog.

15.1 Terminating a Dialog with a BYE Request

15.1.1 UAC Behavior

   A user agent client uses the BYE request, sent within a dialog, to
   indicate to the server that it wishes to terminate the session. This
   will also terminate the dialog. A BYE request MAY be issued by either
   caller or callee. A BYE request SHOULD NOT be sent before the
   creation of a dialog (either early or confirmed). In that case the
   UAC SHOULD follow the procedures described in Section 9 instead.

        Proxies ensure that a CANCEL request is routed in the same
        way as the INVITE was.  However, a proxy performing load
        balancing may route a BYE without a Route header field in a
        different way than the INVITE, since both requests have
        different CSeq sequence numbers.

   The To, From, Call-ID, CSeq, and Request-URI of a BYE are set
   following the same rules as for regular requests sent within a
   dialog, described in Section 12.

   Once the BYE is constructed, it creates a new non-INVITE client
   transaction, and passes it the BYE request. The UA SHOULD stop
   sending media as soon as the BYE request is passed to the client
   transaction. If the response for the BYE is a 481 (Call/Transaction
   Does Not Exist) or a 408 (Request Timeout) or no response at all is
   received for the BYE (that is, a timeout is returned by the client
   transaction), the UAC considers the dialog down.



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15.1.2 UAS Behavior

   A UAS first processes the BYE request according to the general UAS
   processing described in Section 8.2. A UAS core receiving a BYE
   request checks if it matches an existing dialog. If the BYE does not
   match an existing dialog, the UAS core SHOULD generate a 481
   (Call/Transaction Does Not Exist) response and pass that to the
   server transaction.


        This rule means that a BYE sent without tags by a UAC will
        be rejected. This is a change from RFC 2543, which allowed
        BYE without tags.

   A UAS core receiving a BYE request for an existing dialog MUST follow
   the procedures of Section 12.2.2 to process the request. Once done,
   the UAS MUST cease transmitting media streams for the session being
   terminated. The UAS core MUST generate a 2xx response to the BYE, and
   MUST pass that to the server transaction for transmission.

   The UAS MUST still respond to any pending requests received for that
   dialog, (which can only be an INVITE). It is RECOMMENDED that a 487
   (Request Terminated) response is generated to those pending requests.

16 Proxy Behavior

16.1 Overview

   SIP proxies are elements that route SIP requests to user agent
   servers and SIP responses to user agent clients. A request may
   traverse several proxies on its way to a UAS. Each will make routing
   decisions, modifying the request before forwarding it to the next
   element.  Responses will route through the same set of proxies
   traversed by the request in the reverse order.

   Being a proxy is a logical role for a SIP element. When a request
   arrives, an element that can play the role of a proxy must first
   decide if it needs to respond to the request on its own. For
   instance, the request could be malformed or the element may need
   credentials from the client before acting as a proxy. The element MAY
   respond with any appropriate error code. When responding directly to
   a request, the element is playing the role of a UAS and MUST behave
   as described in Section 8.2.

   A proxy can operate in either a stateful or stateless mode for each
   new request. When stateless, a proxy acts as a simple forwarding
   element.  It forwards each request downstream to a single element
   determined by making a routing decision based on the request. It



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   simply forwards every response it receives upstream. A stateless
   proxy discards information about a message once it has been
   forwarded.

   On the other hand, a stateful proxy remembers information
   (specifically, transaction state) about each incoming request and any
   requests it sends as a result of processing the incoming request. It
   uses this information to affect the processing of future messages
   associated with that request. A stateful proxy MAY chose to "fork" a
   request, routing it to multiple destinations. Any request that is
   forwarded to more than one location MUST be handled statefully.

   In some circumstances, a proxy MAY forward requests using stateful
   transports (such as TCP) without being transaction stateful.  For
   instance, a proxy MAY forward a request from one TCP connection to
   another transaction statelessly as long as it places enough
   information in the message to be able to forward the response down
   the same connection the request arrived on.  Requests forwarded
   between different types of transports where the proxy's TU must take
   an active role in ensuring reliable delivery on one of the transports
   MUST be forwarded transaction statefully.

   A stateful proxy MAY transition to stateless operation at any time
   during the processing of a request, so long as it did not do anything
   that would otherwise prevent it from being stateless initially
   (forking, for example, or generation of a 100 response). When
   performing such a transition, all state is simply discarded. The
   proxy SHOULD NOT send a CANCEL.

   Much of the processing involved when acting statelessly or statefully
   for a request is identical. The next several subsections are written
   from the point of view of a stateful proxy. The last section calls
   out those places where a stateless proxy behaves differently.

16.2 Stateful Proxy

   When stateful, a proxy is purely a SIP transaction processing engine.
   Its behavior is modeled here in terms of the Server and Client
   Transactions defined in Section 17. A stateful proxy has a server
   transaction associated with one or more client transactions by a
   higher layer proxy processing component (see figure 3), known as a
   proxy core. An incoming request is processed by a server transaction.
   Requests from the server transaction are passed to a proxy core. The
   proxy core determines where to route the request, choosing one or
   more next-hop locations. An outgoing request for each next-hop
   location is processed by its own associated client transaction. The
   proxy core collects the responses from the client transactions and
   uses them to send responses to the server transaction.



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   A stateful proxy creates a new server transaction for each new
   request received. Any retransmissions of the request will then be
   handled by that server transaction per Section 17.

   This is a model of proxy behavior, not of software. An implementation
   is free to take any approach that replicates the external behavior
   this model defines.


   For all new requests, including any with unknown methods, an element
   intending to proxy the request MUST:

        1.   Validate the request (Section 16.3) .IP 2.  Make a routing
             decision (Section 16.4) .IP 3.  Forward the request to each
             chosen destination (Section 16.5) .IP 4.  Process all
             responses (Section 16.6)

16.3 Request Validation

   Before an element can proxy a request, it MUST verify the message's
   validity. A valid message must pass the following checks:

        1.   Reasonable Syntax

        2.   Max-Forwards

        3.   (Optional) Loop Detection

        4.   Proxy-Require

        5.   Proxy-Authorization

   If any of these checks fail, the element MUST behave as a user agent
   server (see Section 8.2) and respond with an error code.

   Notice that a proxy is not required to detect merged requests and
   MUST NOT treat merged requests as an error condition.  The endpoints
   receiving the requests will resolve the merge as described in Section
   8.2.2.2.

        1.   Reasonable Syntax check

             The request MUST be well-formed enough to be handled with a
             server transaction. Any components involved in the
             remainder of these Request Validation steps or the Request
             Processing section MUST be well-formed. Any other
             components, well-formed or not, SHOULD be ignored and
             remain unchanged when the message is forwarded. For



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           +------------------------------+
           |                              |     +---+
           |                              |     |  T|
           |                              |     |  r|
           |                              |     |C a|
           |                              |     |l n|
           |                              |     |i s|
           |                              |     |e a|
           |                              |     |n c|
           |                              |     |t t|
           |                              |     |  i|
           |                              |     |  o|
           |                              |     |  n|
           |                              |     +---+
  +---+    |                              |     +---+
  |  T|    |                              |     |  T|
  |  r|    |                              |     |  r|
  |S a|    |                              |     |C a|
  |e n|    |             Proxy            |     |l n|
  |r s|    |         "Higher" Layer       |     |i s|
  |v a|    |                              |     |e a|
  |e c|    |                              |     |n c|
  |r t|    |                              |     |t t|
  |  i|    |                              |     |  i|
  |  o|    |                              |     |  o|
  |  n|    |                              |     |  n|
  +---+    |                              |     +---+
           |                              |     +---+
           |                              |     |  T|
           |                              |     |  r|
           |                              |     |C a|
           |                              |     |l n|
           |                              |     |i s|
           |                              |     |e a|
           |                              |     |n c|
           |                              |     |t t|
           |                              |     |  i|
           |                              |     |  o|
           |                              |     |  n|
           |                              |     +---+
           +------------------------------+



   Figure 3: Stateful Proxy Model


             instance, an element SHOULD NOT reject a request because of
             a malformed Date header field.  Likewise, a proxy SHOULD
             NOT remove a malformed Date header field before forwarding
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             This protocol is designed to be extended. Future extensions
             may define new methods and header fields at any time. An
             element MUST NOT refuse to proxy a request because it
             contains a method or header field it does not know about.

        2.   Max-Forwards check

             The Max-Forwards header field (Section 24.22) is used to
             limit the number of elements a SIP request can traverse.

             If the request does not contain a Max-Forwards header
             field, this check is passed.

             If the request contains a Max-Forwards header field with a
             field value greater than zero, the check is passed.

             If the request contains a Max-Forwards header field with a
             field value of zero (0), the element MUST NOT forward the
             request. If the request was for OPTIONS, the element MAY
             act as the final recipient and respond per Section 11.
             Otherwise, the element MUST return a 483 (Too many hops)
             response.

        3.   Optional Loop Detection check

             An element MAY check for forwarding loops before forwarding
             a request. If the request contains a Via header field with
             a sent-by value that equals a value placed into previous
             requests by the proxy, the request has been forwarded by
             this element before. The request has either looped or is
             legitimately spiraling through the element. To determine if
             the request has looped, the element MAY perform the branch
             parameter calculation described in Step 3 of Section 16.5
             on this message and compare it to the parameter received in
             that Via header field. If the parameters match, the request
             has looped. If they differ, the request is spiraling, and
             processing continues. If a loop is detected, the element
             MAY return a 482 (Loop Detected) response.


             In earlier versions of this memo, loop detection was
             REQUIRED. This requirement has been relaxed in favor
             of the Max-Forwards mechanism.

        4.   Proxy-Require check

             Future extensions to this protocol may introduce features
             that require special handling by proxies. Endpoints will



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             include a Proxy-Require header field in requests that use
             these features, telling the proxy it should not process the
             request unless the feature is understood.

             If the request contains a Proxy-Require header field
             (Section 24.29) with one or more option-tags this element
             does not understand, the element MUST return a 420 (Bad
             Extension) response. The response MUST include an
             Unsupported (Section 24.42) header field listing those
             option-tags the element did not understand.

        5.   Proxy-Authorization check

             If an element requires credentials before forwarding a
             request, the request MUST be inspected as described in
             Section 20.3. That section also defines what the element
             must do if the inspection fails.

16.4 Making a Routing Decision

   At this point, the proxy must decide where to forward the request.
   This can be modeled as computing a set of destinations for the
   request. This set will either be predetermined by the contents of the
   request or will be obtained from an abstract location service.  Each
   destination is represented as a URI, and is is referred to as a
   "next-hop location".

   First, the proxy MUST inspect the Request-URI of the request.  If the
   Request-URI of the request contains a value this proxy previously
   placed into a Record-Route header field (see Section 16.5 item 6),
   the proxy MUST replace the Request-URI in the request with the last
   value from the Route header field, and remove that value from the
   Route header field. The proxy MUST then proceed as if it received
   this modified request.


        This will only happen when the element sending the request
        to the proxy (which may have been an endpoint) is a strict
        router. This rewrite on receive is necessary to enable
        backwards compatibility with those elements. It also allows
        elements following this specification to preserve the
        Request-URI through strict-routing proxies (see Section
        refsec:dialog:uac:generate).


        This requirement does not obligate a proxy to keep state in
        order to detect URIs it previously placed in Record-Route
        header fields. Instead, a proxy need only place enough



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        information in those URIs to recognize them as values it
        provided when they later appear.

   If the Request-URI has a URI whose scheme is not understood by the
   proxy, the proxy SHOULD reject the request with a 416 (Unsupported
   URI Scheme) response. If the Request-URI contains an maddr parameter,
   the proxy MUST check to see if its value is in the set of addresses
   or domains the proxy is configured to be responsible for.  If the
   Request-URI has an maddr parameter with a value the proxy is
   responsible for, and the request was received using the port and
   transport indicated (explicitly or by default) in the Request-URI,
   the proxy MUST strip the maddr and any non-default port or transport
   parameter and continue processing as if those values had not been
   present in the request.  Otherwise, if the Request-URI contains an
   maddr parameter, the Request-URI MUST be placed into the destination
   set as the only next hop URI, and the proxy MUST proceed to Section
   16.5.


        A request may arrive with an maddr matching the proxy, but
        on a port or transport different from that indicated in the
        URI. Such a request needs to be forwarded to the proxy
        using the indicated port and transport.

   If the domain of the Request-URI indicates a domain this element is
   not responsible for, it SHOULD set the next hop URI to the Request-
   URI.  That next hop MUST be placed into the destination set as the
   only next hop, and the element MUST proceed to the task of Request
   Processing (Section 16.5).


        There are many circumstances in which a proxy might receive
        a request for a domain it is not responsible for. A
        firewall proxy handling outgoing calls (the way HTTP
        proxies handle outgoing requests) is an example of where
        this is likely to occur.

   If the destination set for the request has not been predetermined as
   described above, this implies that the element is responsible for the
   domain in the Request-URI, and the element MAY use whatever mechanism
   it desires to determine where to send the request.  However, if the
   request contains a Route header, the proxy MUST only choose a single
   destination for the request.  Any of these mechanisms can be modeled
   as accessing an abstract Location Service. This may consist of
   obtaining information from a location service created by a SIP
   Registrar, reading a database, consulting a presence server,
   utilizing other protocols, or simply performing an algorithmic
   substitution on the Request-URI.  When accessing the location service



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   constructed by the registrar, the Request-URI MUST first be
   canonicalized as described in Section 10.3 before being used as an
   index.  The output of these mechanisms is used to construct the
   destination set.

   If the Request-URI does not provide sufficient information for the
   proxy to determine the destination set, it SHOULD return a 485
   (Ambiguous) response. This response SHOULD contain a Contact header
   field containing URIs of new addresses to be tried. For example, an
   INVITE to sip:John.Smith@company.com may be ambiguous at a proxy
   whose location service has multiple John Smiths listed. See Section
   25.4.23 for details.

   Any information in or about the request or the current environment of
   the element MAY be used in the construction of the destination set.
   For instance, different sets may be constructed depending on contents
   or the presence of header fields and bodies, the time of day of the
   request's arrival, the interface on which the request arrived,
   failure of previous requests, or even the element's current level of
   utilization.

   As potential destinations are located through these services, their
   next hops are added to the destination set  (although, as pointed out
   above, the destination set MUST NOT ever contain more than one
   destination if the request contains a Route header).  Next-hop
   locations may only be placed in the destination set once. If a next-
   hop location is already present in the set (based on the definition
   of equality for the URI type), it MUST NOT be added again.

   If the received request contained no Route header fields, a proxy MAY
   continue to add destinations to the set after beginning Request
   Processing. It MAY use any information obtained during that
   processing to determine new locations. For instance, a proxy may
   choose to incorporate contacts obtained in a redirect response (3xx)
   into the destination set. If a proxy uses a dynamic source of
   information while building the destination set (for instance, if it
   consults a SIP Registrar), it SHOULD monitor that source for the
   duration of processing the request. New locations SHOULD be added to
   the destination set as they become available. As above, any given URI
   MUST NOT be added to the set more than once.


        Allowing a URI to be added to the set only once reduces
        unnecessary network traffic, and in the case of
        incorporating contacts from redirect requests prevents
        infinite recursion.

   For example, a trivial location service is a "no-op", where the



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   destination URI is equal to the incoming request URI. The request is
   sent to a specific next hop proxy for further processing.  During
   request processing of Section 16.5, Item 5, the identity of that next
   hop, expressed as a SIP URI, is inserted as the top most Route header
   into the request.

   If the Request-URI indicates a resource at this proxy that does not
   exist, the proxy MUST return a 404 (Not Found) response.

   If the destination set remains empty after applying all of the above,
   the proxy MUST return an error response, which SHOULD be the 480
   (Temporarily Unavailable) response.

16.5 Request Processing

   As soon as the destination set is non-empty, a proxy MAY begin
   forwarding the request. A stateful proxy MAY process the set in any
   order. It MAY process multiple destinations serially, allowing each
   client transaction to complete before starting the next. It MAY start
   client transactions with every destination in parallel. It also MAY
   arbitrarily divide the set into groups, processing the groups
   serially and processing the destinations in each group in parallel.

   A common ordering mechanism is to use the qvalue parameter of
   destinations obtained from Contact header fields (see Section 24.10).
   Destinations are processed from highest qvalue to lowest.
   Destinations with equal qvalues may be processed in parallel.

   A stateful proxy must have a mechanism to maintain the destination
   set as responses are received and associate the responses to each
   forwarded request with the original request. For the purposes of this
   model, this mechanism is a "response context" created by the proxy
   layer before forwarding the first request.

   For each destination, the proxy forwards the request following these
   steps:

        1.   Make a copy of the received request

        2.   Update the Request-URI

        3.   Add a Via header field

        4.   Update the Max-Forwards header field

        5.   Update the Route header field if present

        6.   Optionally add a Record-route header field value



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        7.   Optionally add additional header fields

        8.   send the new request

        9.   Set timer C

   Each of these steps is detailed below:

        1.   Copy request

             The proxy starts with a copy of the received request. The
             copy MUST initially contain all of the header fields from
             the received request.  Only those fields detailed in the
             processing described below may be removed. The copy SHOULD
             maintain the ordering of the header fields as in the
             received request. The proxy MUST NOT reorder field values
             with a common field name (See Section 7.3.1).


             An actual implementation need not perform a copy; the
             primary requirement is that the processing of each
             next hop begin with the same request.

        2.   Request-URI

             The Request-URI in the copy's start line MUST be replaced
             with the URI for this destination. If the URI contains any
             parameters not allowed in a Request-URI, they MUST be
             removed.

             This is the essence of a proxy's role. This is the
             mechanism through which a proxy routes a request toward its
             destination.

             In some circumstances, the received Request-URI is placed
             into the destination set without being modified. For that
             destination, the replacement above is effectively a no-op.

        3.   Via

             The proxy MUST insert a Via header field into the copy
             before the existing Via header fields. The construction of
             this header field follows the same guidelines of Section
             8.1.1.7. This implies that the proxy will compute its own
             branch parameter, which will be globally unique for that
             branch, and contain the requisite magic cookie.

             Proxies choosing to detect loops have an additional



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             constraint in the value they use for construction of the
             branch parameter. A proxy choosing to detect loops SHOULD
             create a branch parameter separable into two parts by the
             implementation. The first part MUST satisfy the constraints
             of Section 8.1.1.7 as described above. The second is used
             to perform loop detection and distinguish loops from
             spirals.

             Loop detection is performed by verifying that, when a
             request returns to a proxy, those fields having an impact
             on the processing of the request have not changed. The
             value placed in this part of the branch parameter SHOULD
             reflect all of those fields (including any Route,  Proxy-
             Require and Proxy-Authorization header fields). This is to
             ensure that if the request is routed back to the proxy and
             one of those fields changes, it is treated as a spiral and
             not a loop (Section 16.3 item  3) A common way to create
             this value is to compute a cryptographic hash of the To,
             From, Call-ID header fields, the Request-URI of the request
             received (before translation) and the sequence number from
             the CSeq header field, in addition to any Proxy-Require and
             Proxy-Authorization header fields that may be present. The
             algorithm used to compute the hash is implementation-
             dependent, but MD5 [31], expressed in hexadecimal, is a
             reasonable choice. (Base64 is not permissible for a token.)


             If a proxy wishes to detect loops, the "branch"
             parameter it supplies MUST depend on all information
             affecting processing of a request, including the
             incoming Request-URI and any header fields affecting
             the request's admission or routing. This is necessary
             to distinguish looped requests from requests whose
             routing parameters have changed before returning to
             this server.

             The request method MUST NOT be included in the calculation
             of the branch parameter. In particular, CANCEL and ACK
             requests (for non-2xx responses) MUST have the same branch
             value as the corresponding request they cancel or
             acknowledge. The branch parameter is used in correlating
             those requests at the server handling them (see Section
             17.2.3 and 9.2).

        4.   Max-Forwards

             If the copy does not contain a Max-Forwards header field,
             the proxy MUST add one with a field value which SHOULD be



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             70.


             Some existing UAs will not provide a Max-Forwards
             header field in a request.

             If the copy contains a Max-Forwards header field, the proxy
             must decrement its value by one (1).

        5.   Route

             A proxy MAY have a local policy that mandates that a
             request visit a specific set of proxies before being
             delivered to the destination. A proxy MUST ensure that all
             such proxies are loose routers. Generally, this can only be
             known with certainty if the proxies are within the same
             administrative domain. This set of proxies is represented
             by a set of URIs (each of which contains the lr parameter).
             This set MUST be pushed into the Route header field ahead
             of any existing values, if present. If the Route header
             field is empty, it MUST be added, containing that list of
             URIs.

             If the proxy has a local policy that mandates that the
             request visit one specific proxy, an alternative to pushing
             a Route value into the Route header field is to bypass the
             forwarding logic of item 8 below, and instead just send the
             request to the address, port and transport for that
             specific proxy. If the request has Route headers, this
             alternative MUST NOT be used unless it known that next hop
             proxy is a loose router. Otherwise, this approach MAY be
             used, but the Route insertion mechanism above is preferred
             for its robustness, flexibility, generality and consistency
             of operation.

             In absence of a policy for forwarding a request through
             specific next hops, the proxy MUST inspect the topmost
             Route header field value. If that value indicates this
             proxy, the proxy MUST remove the value from the copy
             (removing the Route header field if that was the only
             value).

             If a Route header field remains after the previous step,
             the proxy MUST inspect the URI in its first value. If that
             URI does not contain a lr parameter, the proxy MUST modify
             the request as follows:

             - The proxy MUST place the Request-URI into the Route



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               header field as the last value.

             - The proxy MUST then place the first Route header field
               value into the Request-URI and remove that value from the
               Route header field.


             Appending the Request-URI to the Route header field is
             part of a mechanism used to pass the information in
             that Request-URI through strict-routing elements.
             "Popping" the first Route header field value into the
             Request-URI formats the message the way a strict-
             routing element expects to receive it (with its own
             URI in the Request-URI and the next location to visit
             in the first Route header field value).

        6.   Record-Route

             If this proxy wishes to remain on the path of future
             requests in a dialog created by this request, it MUST
             insert a Record-Route header field into the copy before any
             existing Record-Route header field, even if a Route header
             field is already present.


             Requests establishing a dialog may contain preloaded
             Route header fields.

             If this request is already part of a dialog, the proxy
             SHOULD insert a Record-Route header field value if it
             wishes to remain on the path of future requests in the
             dialog. In normal endpoint operation as described in
             Section 12 these Record-Route header field values will not
             have any effect on the route sets used by the endpoints.


             The proxy will remain on the path if it choses to not
             insert a Record-Route header field value into requests
             that are already part of a dialog. However, it would
             be removed from the path when an endpoint that has
             failed reconstitutes the dialog.

             A proxy MAY insert a Record-Route header field into any
             request. If the request does not initiate a dialog, the
             endpoints will ignore the value. See Section 12 for details
             on how endpoints use the Record-Route header field values
             to construct Route header fields.




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             Each proxy in the path of a request chooses whether to add
             a Record-Route header field independently - the presence of
             a Record-Route header field in a request does not obligate
             this proxy to add a value.

             The URI placed in the Record-Route header field value MUST
             be a SIP URI.  This URI MUST contain an lr parameter (see
             Section 23.1.1).  This URI MAY be different for each
             destination the request is forwarded to. The URI SHOULD NOT
             contain the transport parameter unless the proxy has
             knowledge (such as in a private network) that the next
             downstream element that will be in the path of subsequent
             requests supports that transport.


             The URI this proxy provides will be used by some other
             element to make a routing decision. This proxy, in
             general, has no way to know what the capabilities of
             that element are, so it must restrict itself to the
             mandatory elements of a SIP implementation: SIP URIs
             and either the TCP or UDP transports.

             The URI placed in the Record-Route header field MUST
             resolve to this element when the server location procedures
             of [2] are applied to it. This ensures subsequent requests
             are routed back to this element.

             If the URI placed in the Record-Route header field needs to
             be be rewritten when it passes back through in a response,
             the URI MUST be distinct enough to locate at that time.
             (The request may spiral through this proxy, resulting in
             more than one Record-Route header field value being added).
             Item 8 of Section 16.6 recommends a mechanism to make the
             URI sufficiently distinct.

             The proxy MAY include Record-Route header field parameters
             in the value it provides. These will be returned in some
             responses to the request (200 (OK) responses to INVITE for
             example) and may be useful for pushing state into the
             message.

             If a proxy needs to be in the path of any type of dialog
             (such as one straddling a firewall), it SHOULD add a
             Record-Route header field to every request with a method it
             does not understand since that method may have dialog
             semantics.

             The URI a proxy places into a Record-Route header field is



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             only valid for the lifetime of any dialog created by the
             transaction in which it occurs. A dialog-stateful proxy,
             for example, MAY refuse to accept future requests with that
             value in the Request-URI after the dialog has terminated.
             Non-dialog-stateful proxies, of course, have no concept of
             when the dialog has terminated, but they MAY encode enough
             information in the value to compare it against the dialog
             identifier of future requests and MAY reject requests not
             matching that information. Endpoints MUST NOT use a URI
             obtained from a Record-Route header field outside the
             dialog in which it was provided. See Section 12 for more
             information on an endpoint's use of Record-Route header
             fields.

             Generally, the choice about whether to record-route or not
             is a tradeoff of features vs. performance. Faster request
             processing and higher scalability is achieved when proxies
             do not record route. However, provision of certain services
             may require a proxy to observe all messages in a dialog. It
             is RECOMMENDED that proxies do not automatically record
             route. They should do so only if specifically required.

             The Record-Route process is designed to work for any SIP
             request that initiates a dialog. The only such request in
             this specification is INVITE. Extensions to the protocol
             MAY define others, and the mechanisms described here will
             apply.

        7.   Adding Additional Header Fields

             The proxy MAY add any other appropriate header fields to
             the copy at this point.

        8.   Forward Request

             A stateful proxy creates a new client transaction for this
             request as described in Section 17.1. The proxy MAY have a
             local policy to send the request to a specific IP address,
             port, and transport, independent of the values of the Route
             and Request-URI. Such a policy MUST NOT be used if the
             proxy is not certain that the IP address, port, and
             transport correspond to a server that is a loose router.
             However, this mechanism for sending the request through a
             specific next hop is NOT RECOMMENDED; instead a Route
             header field should be used for that purpose as described
             above.

             In the absence of such an overriding mechanism, the proxy



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             applies the procedures listed in [2] as follows to
             determine where to send the request.  If the proxy has
             reformatted the request to send to a strict-routing element
             as described in Section 5, the proxy MUST apply those
             proceedures to the Request-URI of the request. Otherwise,
             the proxy MUST apply the proceedures to the first value in
             the Route header field, if present, else the Request-URI.
             The proceedures will produce an ordered set of addresses.
             As described in [2], the proxy MUST attempt to contact the
             first address by instructing the client transaction to send
             the request there.  If the client transaction reports
             failure to send the request or a timeout from its state
             machine, the stateful proxy continues to the next address
             in that ordered set. Each attempt is a new client
             transaction, and therefore represents a new branch, so that
             the processing described above for each branch would need
             to be repeated. This results in a requirement to use a
             different branch ID parameter for each attempt. If the
             ordered set is exhausted, the request cannot be forwarded
             to this element in the destination set. The proxy does not
             need to place anything in the response context, but
             otherwise acts as if this element of the destination set
             returned a 408 (Request Timeout) final response.

        9.   Set timer C

             In order to handle the case where an INVITE request never
             generates a final response, a transaction timeout value is
             used. This is accomplished through a timer, called timer C,
             which MUST be set for each client transaction when an
             INVITE request is proxied. The timer MUST be larger than 3
             minutes. Section 16.6 bullet 2 discusses how this timer is
             updated with provisional responses, and Section 16.7
             discusses processing when it fires.

16.6 Response Processing

   When a response is received by an element, it first tries to locate a
   client transaction (Section 17.1.3) matching the response. If none is
   found, the element MUST process the response (even if it is an
   informational response) as a stateless proxy (described below). If a
   match is found, the response is handed to the client transaction.


        Forwarding responses for which a client transaction (or
        more generally any knowledge of having sent an associated
        request) is not found improves robustness. In particular,
        it ensures that "late" 2xx responses to INVITE requests are



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        forwarded properly.

   As client transactions pass responses to the proxy layer, the
   following processing MUST take place:

        1.   Find the appropriate response context

        2.   Update timer C for provisional responses

        3.   Remove the topmost Via

        4.   Add the response to the response context

        5.   Check to see if this response should be forwarded

   The following processing MUST be performed on each response that is
   forwarded. It is likely that more than one response to each request
   will be forwarded: at least each provisional and one final response.

        1.   Aggregate authorization header fields if necessary;

        2.   forward the response;

        3.   generate any necessary CANCEL requests.

   If no final response has been forwarded after every client
   transaction associated with the response context has been terminated,
   the proxy must choose and forward the "best" response from those it
   has seen so far.

   Each of the above steps are detailed below:

        1.   Find Context

             The proxy locates the "response context" it created before
             forwarding the original request using the key described in
             Section 16.5. The remaining processing steps take place in
             this context.

        2.   Update timer C for provisional responses

             For an INVITE transaction, if the response is a provisional
             response with status codes 101 to 199 inclusive (i.e.,
             anything but 100), the proxy MUST reset timer C for that
             client transaction. The timer MAY be reset to a different
             value, but this value MUST be greater than 3 minutes.

        3.   Via



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             The proxy removes the topmost Via header field from the
             response.

             If no Via header fields remain in the response, the
             response was meant for this element and MUST NOT be
             forwarded. The remainder of the processing described in
             this section is not performed on this message, the UAC
             processing rules described in Section 8.1.3 are followed
             instead (transport layer processing has already occurred).

             This will happen, for instance, when the element generates
             CANCEL requests as described in Section 10.

        4.   Add response to context ;

             Final responses received are stored in the response context
             until a final response is generated on the server
             transaction associated with this context. The response may
             be a candidate for the best final response to be returned
             on that server transaction. Information from this response
             may be needed in forming the best response even if this
             response is not chosen.

             If the proxy chooses to recurse on any contacts in a 3xx
             response by adding them to the destination set, it MUST
             remove them from the response before adding the response to
             the response context. If the proxy recurses on all of the
             contacts in a 3xx response, the proxy SHOULD NOT add the
             resulting contactless response to the response context.


             Removing the contact before adding the response to the
             response contact prevents the next element upstream
             from retrying a location this proxy has already
             attempted.

             3xx responses may contain a mixture of SIP and non-SIP
             URIs. A proxy may choose to recurse on the SIP URIs and
             place the remainder into the response context to be
             returned potentially in the final response.

             If a proxy receives a 416 (Unsupported URI Scheme) response
             to a request whose Request-URI scheme was not SIP, but the
             scheme in the original received request was SIP (that is,
             the proxy changed the scheme from SIP to something else
             when it proxied a request), the proxy SHOULD add a new URI
             to the destination set. This URI SHOULD be a SIP URI
             version of the non-SIP URI that was just tried. In the case



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             of the tel URL, this is accomplished by placing the
             telephone-subscriber part of the tel URL into the user part
             of the SIP URI, and setting the hostpart to the domain
             where the prior request was sent.

             As with a 3xx response, if a proxy "recurses" on the 416 by
             trying a SIP URI instead, the 416 response SHOULD NOT be
             added to the response context.

        5.   Check response for forwarding

             Until a final response has been sent on the server
             transaction, the following responses MUST be forwarded
             immediately:

             - Any provisional response other than 100 (Trying)

             - Any 2xx response

             If a 6xx response is received, it is not immediately
             forwarded, but the stateful proxy SHOULD cancel all pending
             transactions as described in Section 10.


             This is a change from RFC 2543, which mandated that
             the proxy was to forward the 6xx response immediately.
             For an INVITE transaction, this approach had the
             problem that a 2xx response could arrive on another
             branch, in which case the proxy would have to forward
             the 2xx. The result was that the UAC could receive a
             6xx response followed by a 2xx response, which should
             never be allowed to happen.  Under the new rules, upon
             receiving a 6xx, a proxy will issue a CANCEL request,
             which will generally result in 487 responses from all
             outstanding client transactions, and then at that
             point the 6xx is forwarded upstream.

             After a final response has been sent on the server
             transaction, the following responses MUST be forwarded
             immediately:

             - Any 2xx response to an INVITE request

             A stateful proxy MUST NOT immediately forward any other
             responses. In particular, a stateful proxy MUST NOT forward
             any 100 (Trying) response. Those responses that are
             candidates for forwarding later as the "best" response have
             been gathered as described in step "Add Response to



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             Context".

             Any response chosen for immediate forwarding MUST be
             processed as described in steps "Aggregate Authorization
             Header Fields" through "Record-Route".

             This step, combined with the next, ensures that a stateful
             proxy will forward exactly one final response to a non-
             INVITE request, and either exactly one non-2xx response or
             one or more 2xx responses to an INVITE request.

        6.   Choosing the best response

             A stateful proxy MUST send a final response to a response
             context's server transaction if no final responses have
             been immediately forwarded by the above rules and all
             client transactions in this response context have been
             terminated.

             The stateful proxy MUST choose the "best" final response
             among those received and stored in the response context.

             If there are no final responses in the context, the proxy
             MUST send a 408 (Request Timeout) response to the server
             transaction.

             Otherwise, the proxy MUST forward one of the responses from
             the lowest response class stored in the response context.
             The proxy MAY select any response within that lowest class.
             The proxy SHOULD give preference to responses that provide
             information affecting resubmission of this request, such as
             401, 407, 415, 420, and 484.

             A proxy which receives a 503 (Service Unavailable) response
             SHOULD NOT forward it upstream unless it can determine that
             any subsequent requests it might proxy will also generate a
             503. In other words, forwarding a 503 means that the proxy
             knows it cannot service any requests, not just the one for
             the Request-URI in the request which generated the 503.

             The forwarded response MUST be processed as described in
             steps "Aggregate authorization Header Fields" through
             "Record-Route".

             For example, if a proxy forwarded a request to 4 locations,
             and received 503, 407, 501, and 404 responses, it may
             choose to forward the 407 (Proxy Authentication Required)
             response.



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             1xx and 2xx responses may be involved in the establishment
             dialogs. When a request does not contain a To tag, the To
             tag in the response is used by the UAC to distinguish
             multiple responses to a dialog creating request. A proxy
             MUST NOT insert a tag into the To header field of a 1xx or
             2xx response if the request did not contain one. A proxy
             MUST NOT modify the tag in the To header field of a 1xx or
             2xx response.

             Since a proxy may not insert a tag into the To header field
             of a 1xx response to a request that did not contain one, it
             cannot issue non-100 provisional responses on its own.
             However, it can branch the request to a UAS sharing the
             same element as the proxy. This UAS can return its own
             provisional responses, entering into an early dialog with
             the initator of the request. The UAS does not have to be a
             discreet process from the proxy. It could be a virtual UAS
             implemented in the same code space as the proxy.

             3-6xx responses are delivered hop-hop. When issuing a 3-6xx
             response, the element is effectivly acting as a UAS,
             issuing its own response, usually based on the responses
             received from downstream elements. An element SHOULD
             preserve the To tag when simply forwarding a 3-6xx response
             to a request that did not contain a To tag.

             A proxy MUST NOT modify the To tag in any forwarded
             response to a request that contains a To tag.


             While it makes no difference to the upstream elements
             if the proxy replaced the To tag in a forwarded 3-6xx
             response, preserving the original tag may assist with
             debugging.

             When the proxy is aggregating information from several
             responses, choosing a To tag from among them is arbitrary,
             and generating a new To tag may make debugging easier. This
             happens, for instance, when combining 401 (Unauthorized)
             and 407 (Proxy Authentication Required) challenges, or
             combining Contact values from unencrypted and
             unauthenticated 3xx responses.

        7.   Aggregate Authorization Header Fields

             If the selected response is a 401 (Unauthorized) or 407
             (Proxy Authentication Required), the proxy MUST collect any
             WWW-Authenticate and Proxy-Authenticate header fields from



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             all other 401 (Unauthorized) and 407 (Proxy Authentication
             Required) responses received so far in this response
             context and add them to this response before forwarding.
             Each WWW-Authenticate and Proxy-Authenticate header field
             added to the response MUST preserve that header field
             value. The resulting 401 (Unauthorized) or 407 (Proxy
             Authenication Required) response may have several WWW-
             Authenticate AND Proxy-Authenticate header fields.

             This is necessary because any or all of the destinations
             the request was forwarded to may have requested
             credentials. The client must receive all of those
             challenges and supply credentials for each of them when it
             retries the request. Motivation for this behavior is
             provided in Section 22.

        8.   Record-Route

             If the selected response contains a Record-Route header
             field value originally provided by this proxy, the proxy
             MAY chose to rewrite the value before forwarding the
             response. This allows the proxy to provide different URIs
             for itself to the next upstream and downstream elements. A
             proxy may choose to use this mechanism for any reason. For
             instance, it is useful for multi-homed hosts.

             The new URI provided by the proxy MUST satisfy the same
             constraints on URIs placed in Record-Route header fields in
             requests (see Step 6 of Section 16.5) with the following
             modifications:

             The URI SHOULD NOT contain the transport parameter unless
             the proxy has knowledge that the next upstream (as opposed
             to downstream) element that will be in the path of
             subsequent requests supports that transport.

             When a proxy does decide to modify the Record-Route header
             field in the response, one of the operations it must
             perform is to locate the Record-Route that it had inserted.
             If the request spiraled, and the proxy inserted a Record-
             Route in each iteration of the spiral, locating the correct
             header field in the response (which must be the proper
             iteration in the reverse direction) is tricky. The rules
             above recommend that a proxy wishing to rewrite Record-
             Route header field values insert sufficiently distinct URIs
             into the Record-Route header field so that the right one
             may be selected for rewriting.  A RECOMMENDED mechanism to
             achieve this is for the proxy to append a unique identifier



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             for the proxy instance to to the user portion of the URI.
             When the response arrives, the proxy modifies the first
             Record-Route whose identifier matches the proxy instance.
             The modification results in a URI without this piece of
             data appended to the user portion of the URI. Upon the next
             iteration, the same algorithm (find the topmost Record-
             Route header field with the parameter) will correctly
             extract the next Record-Route header field inserted by that
             proxy.

        9.   Forward response

             After performing the processing described in steps
             "Aggregate Authorization Header Fields" through "Record-
             Route", the proxy may perform any feature specific
             manipulations on the selected response. Unless otherwise
             specified, the proxy MUST NOT remove the message body or
             any header fields other than the Via header field discussed
             in Section 3. In particular, the proxy MUST NOT remove any
             "received" parameter it may have added to the next Via
             header field while processing the request associated with
             this response. The proxy MUST pass the response to the
             server transaction associated with the response context.
             This will result in the response being sent to the location
             now indicated in the topmost Via header field value. If the
             server transaction is no longer available to handle the
             transmission, the element MUST forward the response
             statelessly by sending it to the server transport. The
             server transaction may indicate failure to send the
             response or signal a timeout in its state machine. These
             errors should be logged for diagnostic purposes as
             appropriate, but the protocol requires no remedial action
             from the proxy.

             The proxy MUST maintain the response context until all of
             its associated transactions have been terminated, even
             after forwarding a final response.

        10.  Generate CANCELs

             If the forwarded response was a final response, the proxy
             MUST generate a CANCEL request for all pending client
             transactions associated with this response context. A proxy
             SHOULD also generate a CANCEL request for all pending
             client transactions associated with this response context
             when it receives a 6xx response. A pending client
             transaction is one that has received a provisional
             response, but no final response and has not had an



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             associated CANCEL generated for it.  Generating CANCEL
             requests is described in Section 9.1.

             The requirement to CANCEL pending client transactions upon
             forwarding a final response does not guarantee that an
             endpoint will not receive multiple 200 (OK) responses to an
             INVITE. 200 (OK) responses on more than one branch may be
             generated before the CANCEL requests can be sent and
             processed. Further, it is reasonable to expect that a
             future extension may override this requirement to issue
             CANCEL requests.

16.7 Processing Timer C

   If timer C should fire, the proxy MUST either reset the timer with
   any value it chooses, or generate a CANCEL for that particular
   request.

16.8 Handling Transport Errors

   If the transport layer notifies a proxy of an error when it tries to
   forward a request (see Section 19.4), the proxy MUST behave as if the
   forwarded request received a 400 (Bad Request) response.

   If the proxy is notified of an error when forwarding a response, it
   drops the response. The proxy SHOULD NOT cancel any outstanding
   client transactions associated with this response context due to this
   notification.


        If a proxy cancels its outstanding client transactions, a
        single malicious or misbehaving client can cause all
        transactions to fail through its Via header field.

16.9 CANCEL Processing

   A stateful proxy may generate a CANCEL to any other request it has
   generated at any time (subject to receiving a provisional response to
   that request as described in section 9.1). A proxy MUST cancel any
   pending client transactions associated with a response context when
   it receives a matching CANCEL request.

   A stateful proxy MAY generate CANCEL requests for pending INVITE
   client transactions based on the period specified in the INVITE's
   Expires header field elapsing. However, this is generally unnecessary
   since the endpoints involved will take care of signaling the end of
   the transaction.




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   While a CANCEL request is handled in a stateful proxy by its own
   server transaction, a new response context is not created for it.
   Instead, the proxy layer searches its existing response contexts for
   the server transaction handling the request associated with this
   CANCEL.  If a matching response context is found, the element MUST
   immediately return a 200 (OK) response to the CANCEL request. In this
   case, the element is acting as a user agent server as defined in
   Section 8.2. Furthermore, the element MUST generate CANCEL requests
   for all pending client transactions in the context as described in
   Section 10.

   If a response context is not found, the element does not have any
   knowledge of the request to apply the CANCEL to. It MUST forward the
   CANCEL request (it may have statelessly forwarded the associated
   request previously).

16.10 Stateless Proxy

   When acting statelessly, a proxy is a simple message forwarder. Much
   of the processing performed when acting statelessly is the same as
   when behaving statefully. The differences are detailed here.

   A stateless proxy does not have any notion of a transaction, or of
   the response context used to describe stateful proxy behavior.
   Instead, the stateless proxy takes messages, both requests and
   responses, directly from the transport layer (See section 19). As a
   result, stateless proxies do not retransmit messages on their own.
   They do, however, forward all retransmission they receive (they do
   not have the ability to distinguish a retransmission from the
   original message).  Furthermore, when handling a request statelessly,
   an element MUST NOT generate its own 100 (Trying) or any other
   provisional response.

   A stateless proxy must validate a request as described in Section
   16.3

   A stateless proxy must make a routing decision as described in
   Section 16.4 with the following exception:

        o A stateless proxy MUST choose one and only one destination
          from the destination set. This choice MUST only rely on fields
          in the message and time-invariant properties of the server. In
          particular, a retransmitted request MUST be forwarded to the
          same destination each time it is processed. Furthermore,
          CANCEL and non-Routed ACK requests MUST generate the same
          choice as their associated INVITE.

   A stateless proxy must process the request before forwarding as



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   described in Section 16.5 with the following exceptions:

        o The requirement for unique branch IDs across time applies to
          stateless proxies as well. However, a stateless proxy cannot
          simply use a random number generator to compute the first
          component of the branch ID, as described in Section 16.5
          bullet 3. This is because retransmissions of a request need to
          have the same value, and a stateless proxy cannot tell a
          retransmission from the original request. Therefore, the
          component of the branch parameter that makes it unique MUST be
          the same each time a retransmitted request is forwarded. Thus
          for a stateless proxy, the branch parameter MUST be computed
          as a combinatoric function of message parameters which are
          invariant on retransmission.

        o The stateless proxy MAY use any technique it likes to
          guarantee uniqueness of its branch IDs across transactions.
          However, the following procedure is RECOMMENDED. The proxy
          examines the branch ID of the received request. If it begins
          with the magic cookie, the first component of the branch ID of
          the outgoing request is computed as a hash of the received
          branch ID. Otherwise, the first component of the branch ID is
          computed as a hash of the topmost Via, the To header field,
          the From header field, the Call-ID header field, the CSeq
          number (but not method), and the Request-URI from the received
          request. One of these fields will always vary across two
          different transactions.

        o The request is sent directly to the transport layer instead of
          through a client transaction. If the next-hop destination
          parameters don't provide an explicit destination, the element
          applies the procedures of [2] to the Request-URI to determine
          where to send the request.


        Since a stateless proxy must forward retransmitted requests
        to the same destination and add identical branch parameters
        to each of them, it can only use information from the
        message itself and time-invariant configuration data for
        those calculations. If the configuration state is not
        time-invariant (for example, if a routing table is updated)
        any requests that could be affected by the change may not
        be forwarded statelessly during an interval equal to the
        transaction timeout window before or after the change. The
        method of processing the affected requests in that interval
        is an implementation decision. A common solution is to
        forward them transaction statefully.




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   Stateless proxies MUST NOT perform special processing for CANCEL
   requests. They are processed by the above rules as any other
   requests.  In particular, a stateless proxy applies the same Route
   header field processing to CANCEL requests that it applies to any
   other request.

   Response processing as described in Section 16.6 does not apply to a
   proxy behaving statelessly. When a response arrives at a stateless
   proxy, the proxy inspects the sent-by value in the first (topmost)
   Via header field. If that address matches the proxy (it equals a
   value this proxy has inserted into previous requests) the proxy MUST
   remove that value from the response and forward the result to the
   location indicated in the next Via header field. Unless specified
   otherwise, the proxy MUST NOT remove any other header fields or the
   message body. If the address does not match the proxy, the message
   MUST be silently discarded.

16.11 Summary of Proxy Route Processing

   In the absence of local policy to the contrary, the processing a
   proxy performs on a request containing a route header can be
   summarized in the following steps.

        o 1 The proxy will inspect the Request-URI. If it indicates a
          resource owned by this proxy, the proxy will replace it with
          the results of running a location service. Otherwise, the
          proxy will not change the Request-URI.

        o 2 The proxy will inspect the URI in the topmost Route header
          field value. If it indicates this proxy, the proxy removes it
          from the Route header field (this route node has been
          reached).

        o 3 The proxy will forward the request to the resource indicated
          by the URI in the topmost Route header field value or in the
          Request-URI if no Route header field is present. The proxy
          determines the address, port and transport to use when
          forwarding the request by applying the proceedures in [2] to
          that URI.

   If no strict-routing elements are encountered on the path of the
   request, the Request-URI will always indicate the target of the
   request.

16.11.1 Examples

16.11.1.1 Basic SIP Trapezoid




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   This scenario is the basic sip trapeziod, U1 -> P1 -> P2 -> U2, with
   both proxies record-routing. Here is the flow.

   U1 sends:


   INVITE sip:callee@domain.com SIP/2.0
   Contact: sip:caller@u1.example.com



   to P1. P1 is an outbound proxy. P1 is not responsible for domain.com,
   so it looks it up in DNS and sends it there. It also adds a Record-
   Route header field value:


   INVITE sip:callee@domain.com SIP/2.0
   Contact: sip:caller@u1.example.com
   Record-Route: <sip:p1.example.com;lr>



   P2 gets this. It is responsible for domain.com so it runs a location
   service and rewrites the Request-URI.  There are no Route headers, so
   it sends to the result of the location lookup. It also adds a
   Record-Route header field value:


   INVITE sip:callee@u2.domain.com SIP/2.0
   Contact: sip:caller@u1.example.com
   Record-Route: <sip:p2.domain.com;lr>
   Record-Route: <sip:p1.example.com;lr>



   The callee at u2.domain.com gets this and responds with a 200 OK:


   SIP/2.0 200 OK
   Contact: sip:callee@u2.domain.com
   Record-Route: <sip:p2.domain.com;lr>
   Record-Route: <sip:p1.example.com;lr>



   The callee at u2 also sets its dialog state's remote target URI to
   sip:caller@u1.example.com and its route set to




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   (<sip:p2.domain.com;lr>,<sip:p1.example.com;lr>)



   This is forwarded by P2 to P1 to U1 as normal. Now, U1 sets its
   dialog state's remote target URI to sip:callee@u2.domain.com and its
   route set to

   (<sip:p1.example.com;lr>,<sip:p2.domain.com;lr>)



   Since all the route set elements contain the lr parameter, U1
   constructs the following for the BYE:


   BYE sip:callee@u2.domain.com SIP/2.0
   Route: <sip:p1.example.com;lr>,<sip:p2.domain.com;lr>



   As any other element (including proxies) would do, it sends this
   request to the location obtained by looking up the topmost Route
   header field value in DNS. This goes to P1.  P1 notices that it is
   not responsible for the resource indicated in the Request-URI so it
   doesn't change it.  It does see that it is the first value in the
   Route header field, so it removes that value, and forwards the
   request to P2:


   BYE sip:callee@u2.domain.com SIP/2.0
   Route: <sip:p2.domain.com;lr>



   P2 also notices it is not responsible for the resource indicated by
   the Request-URI (it is responsible for domain.com, not
   u2.domain.com), so it doesn't change it. It does see itself in the
   first Route header field value, so it removes it and forwards the
   following to u2.domain.com based on a DNS lookup against the
   Request-URI:


   BYE sip:callee@u2.domain.com SIP/2.0



16.11.1.2 Traversing a strict-routing proxy



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   In this scanario, a dialog is established across three proxies, each
   of which adds Record-Route header field values.  The second proxy
   implements the strict-routing proceedures specified in RFC2543 and
   the bis drafts up to bis-05.


   U1->P1->P2->P3->U2



   The INVITE arriving at U2 contains

   INVITE sip:callee@u2.domain.com SIP/2.0
   Contact: sip:caller@u1.example.com
   Record-Route: <sip:p3.domain.com;lr>
   Record-Route: <sip:p2.middle.com>
   Record-Route: <sip:p1.example.com;lr>



   Which U2 responds to with a 200 OK. Later, U2 sends the following BYE
   to P3 based on the first Route header field value.


   BYE sip:caller@u1.example.com SIP/2.0
   Route: <sip:p3.domain.com;lr>
   Route: <sip:p2.middle.com>
   Route: <sip:p1.example.com;lr>



   P3 is not responsible for the resource indicated in the Request-URI
   so it will leave it alone.  It notices that it is the element in the
   first Route header field value so it removes it.  It then prepares to
   send the request based on the now first Route header field value of
   sip:p2.middle.com, but it notices that this URI does not contain the
   lr parameter, so before sending, it reformats the request to be:


   BYE sip:p2.middle.com SIP/2.0
   Route: <sip:p1.example.com;lr>
   Route: <sip:caller@u1.example.com>



   P2 is a strict router, so it forwards the following to P1:





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   BYE sip:p1.example.com;lr SIP/2.0
   Route: <sip:caller@u1.example.com>



   P1 sees the request-URI is a value it placed into a Record-Route
   header field, so before further processing, it rewrites the request
   to be


   BYE sip:caller@u1.example.com SIP/2.0



   Since P1 is not responsible for u1.example.com and there is no Route
   header field, P1 will forward the request to u1.example.com based on
   the Request-URI:


   BYE sip:caller@u1.example.com SIP/2.0



16.11.1.3 Rewriting Record-Route header field values

   In this scenario, U1 and U2 are in different private namespaces and
   they enter a dialog through a proxy P1 which acts as a gateway
   between the namespaces.


   U1->P1->U2



   U1 receives:


   INVITE sip:callee@gateway.leftprivatespace.com SIP/2.0
   Contact: <sip:caller@u1.leftprivatespace.com>



   P1 its location service and sends the following to U2:


   INVITE sip:callee@rightprivatespace.com SIP/2.0
   Contact: <sip:caller@u1.leftprivatespace.com>
   Record-Route: <sip:gateway.rightprivatespace.com;lr>



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   U2 sends this 200 OK back to the gateway:


   SIP/2.0 200 OK
   Contact: <sip:callee@u2.rightprivatespace.com>
   Record-Route: <sip:gateway.rightprivatespace.com;lr>



   P1 rewrites its Record-Route header parameter to provide a value that
   U1 will find useful, and sends the following to U1:


   SIP/2.0 200 OK
   Contact: <sip:callee@u2.rightprivatespace.com>
   Record-Route: <sip:gateway.leftprivatespace.com;lr>



   Later, U1 sends the following BYE to P1:


   BYE sip:callee@u2.rightprivatespace.com SIP/2.0
   Route: <sip:gateway.leftprivatespace.com;lr>



   which P1 forwards to U2 as


   BYE sip:callee@u2.rightprivatespace.com SIP/2.0



17 Transactions

   SIP is a transactional protocol: interactions between components take
   place in a series of independent message exchanges. Specifically, a
   SIP transaction consists of a single request, and any responses to
   that request (which include zero or more provisional responses and
   one or more final responses). In the case of a transaction where the
   request was an INVITE (known as an INVITE transaction), the
   transaction also includes the ACK only if the final response was not
   a 2xx response. If the response was a 2xx, the ACK is not considered
   part of the transaction.

        The reason for this separation is rooted in the importance
        of delivering all 200 (OK) responses to an INVITE to the



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        UAC. To deliver them all to the UAC, the UAS alone takes
        responsibility for retransmitting them (see Section
        13.3.1.4) , and the UAC alone takes responsibility for
        acknowledging them with ACK (see Section 13.2.2.4). Since
        this ACK is retransmitted only by the UAC, it is
        effectively considered its own transaction.

   Transactions have a client side and a server side. The client side is
   known as a client transaction, and the server side, as a server
   transaction. The client transaction sends the request, and the server
   transaction sends the response. The client and server transactions
   are logical functions that are embedded in any number of elements.
   Specifically, they exist within user agents and stateful proxy
   servers.  Consider the example of Section 4. In this example, the UAC
   executes the client transaction, and its outbound proxy executes the
   server transaction. The outbound proxy also executes a client
   transaction, which sends the request to a server transaction in the
   inbound proxy. That proxy also executes a client transaction, which
   in turn, sends the request to a server transaction in the UAS. This
   is shown pictorially in Figure 4.


   A stateless proxy does not contain a client or server transaction.
   The transaction exists between the UA or stateful proxy on one side
   of the stateless proxy, and the UA or stateful proxy on the other
   side. As far as SIP transactions are concerned, stateless proxies are
   effectively transparent. The purpose of the client transaction is to
   receive a request from the element the client is embedded in (call
   this element the "Transaction User" or TU; it can be a UA or a
   stateful proxy), and reliably deliver the request to that server
   transaction. The client transaction is also responsible for receiving
   responses, and delivering them to the TU, filtering out any
   retransmissions or disallowed responses (such as a response to ACK).
   In the case of an INVITE transaction, that includes generation of the
   ACK request for any final response excepting a 2xx response.

   Similarly, the purpose of the server transaction is to receive
   requests from the transport layer, and deliver them to the TU. The
   server transaction filters any request retransmissions from the
   network. The server transaction accepts responses from the TU, and
   delivers them to the transport layer for transmission over the
   network. In the case of an INVITE transaction, it absorbs the ACK
   request for any final response excepting a 2xx response.

   The 2xx response, and the ACK for it, have special treatment. This
   response is retransmitted only by a UAS, and its ACK generated only
   by the UAC. This end-to-end treatment is needed so that a caller
   knows the entire set of users that have accepted the call. Because of



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 +---------+        +---------+        +---------+        +---------+
 |      +-+|Request |+-+   +-+|Request |+-+   +-+|Request |+-+      |
 |      |C||------->||S|   |C||------->||S|   |C||------->||S|      |
 |      |l||        ||e|   |l||        ||e|   |l||        ||e|      |
 |      |i||        ||r|   |i||        ||r|   |i||        ||r|      |
 |      |e||        ||v|   |e||        ||v|   |e||        ||v|      |
 |      |n||        ||e|   |n||        ||e|   |n||        ||e|      |
 |      |t||        ||r|   |t||        ||r|   |t||        ||r|      |
 |      | ||        || |   | ||        || |   | ||        || |      |
 |      |T||        ||T|   |T||        ||T|   |T||        ||T|      |
 |      |r||        ||r|   |r||        ||r|   |r||        ||r|      |
 |      |a||        ||a|   |a||        ||a|   |a||        ||a|      |
 |      |n||        ||n|   |n||        ||n|   |n||        ||n|      |
 |      |s||Response||s|   |s||Response||s|   |s||Response||s|      |
 |      +-+|<-------|+-+   +-+|<-------|+-+   +-+|<-------|+-+      |
 +---------+        +---------+        +---------+        +---------+
    UAC               Outbound           Inbound              UAS
                      Proxy               Proxy








   Figure 4: Transaction relationships


   this special handling, retransmissions of the 2xx response are
   handled by the UA core, not the transaction layer. Similarly,
   generation of the ACK for the 2xx is handled by the UA core. Each
   proxy along the path merely forwards each 2xx response to INVITE, and
   its corresponding ACK.

   A reliable provisional response, and the PRACK for it, also have
   special treatment. Reliable provisional responses are also only
   retransmitted by the UAS core, and the PRACK generated by the UAC
   core. Unlike ACK, however, PRACK is a normal non-INVITE transaction,
   which means that it will generate its own final response.  The reason
   for this seemingly inexplicable difference between PRACK and ACK is
   that reliability of provisional responses was added on later as an
   extra feature, and therefore needed to be done within the confines of
   SIP extensibility. SIP extensibility only allowed the additions of
   new methods which behaved like any other non-INVITE method.



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17.1 Client Transaction

   The client transaction provides its functionality through the
   maintenance of a state machine.

   The TU communicates with the client transaction through a simple
   interface. When the TU wishes to initiate a new transaction, it
   creates a client transaction, and passes it the SIP request to send,
   and an IP address, port, and transport to send it to. The client
   transaction begins execution of its state machine. Valid responses
   are passed up to the TU from the client transaction.

   There are two types of client transaction state machines, depending
   on the method of the request passed by the TU. One handles client
   transactions for INVITE request. This type of machine is referred to
   as an INVITE client transaction. Another type handles client
   transactions for all requests except INVITE and ACK. This is referred
   to as a non-INVITE client transaction. There is no client transaction
   for ACK. If the TU wishes to send an ACK, it passes one directly to
   the transport layer for transmission.

   The INVITE transaction is different from those of other methods
   because of its extended duration. Normally, human input is required
   in order to respond to an INVITE. The long delays expected for
   sending a response argue for a three way handshake. Requests of other
   methods, on the other hand, are expected to complete rapidly. In
   fact, because of its reliance on just a two way handshake, TUs SHOULD
   respond immediately to non-INVITE requests. Protocol extensions which
   require longer durations for generation of a response (such as a new
   method that does require human interaction) SHOULD instead use two
   transactions - one to send the request, and another in the reverse
   direction to convey the result of the request.

17.1.1 INVITE Client Transaction

17.1.1.1 Overview of INVITE Transaction

   The INVITE transaction consists of a three-way handshake. The client
   transaction sends an INVITE, the server transaction sends responses,
   and the client transaction sends an ACK. For unreliable transports
   (such as UDP), the client transaction will retransmit requests at an
   interval that starts at T1 seconds and doubles after every
   retransmission. T1 is an estimate of the RTT, and it defaults to 500
   ms. Nearly all of the transaction timers described here scale with
   T1, and changing T1 is how their values are adjusted. The request is
   not retransmitted over reliable transports. After receiving a 1xx
   response, any retransmissions cease altogether, and the client waits
   for further responses. The server transaction can send additional 1xx



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   responses, which are not transmitted reliably by the server
   transaction.  If the provisional response needs to be sent reliably,
   this is handled by the TU. Eventually, the server transaction decides
   to send a final response. For unreliable transports, that response is
   retransmitted periodically, and for reliable transports, it is sent
   once. For each final response that is received at the client
   transaction, the client transaction sends an ACK, the purpose of
   which is to quench retransmissions of the response.

17.1.1.2 Formal Description


   The state machine for the INVITE client transaction is shown in
   Figure 5. The initial state, "calling", MUST be entered when the TU
   initiates a new client transaction with an INVITE request. The client
   transaction MUST pass the request to the transport layer for
   transmission (see Section 19).  If an unreliable transport is being
   used, the client transaction SHOULD start timer A with a value of T1,
   and SHOULD NOT start timer A when a reliable transport is being used
   (Timer A controls request retransmissions). For any transport, the
   client transaction MUST start timer B with a value of 64*T1 seconds
   (Timer B controls transaction timeouts).

   When timer A fires, the client transaction SHOULD retransmit the
   request by passing it to the transport layer, and SHOULD reset the
   timer with a value of 2*T1. The formal definition of retransmit
   within the context of the transaction layer, is to take the message
   previously sent to the transport layer, and pass it to the transport
   layer once more.

   When timer A fires 2*T1 seconds later, the request SHOULD be
   retransmitted again (assuming the client transaction is still in this
   state). This process SHOULD continue, so that the request is
   retransmitted with intervals that double after each transmission.
   These retransmissions SHOULD only be done while the client
   transaction is in the "calling" state.

   The default value for T1 is 500 ms. T1 is an estimate of the RTT
   between the client and server transactions. The optional RTT
   estimation procedure of Section 17.3 MAY be followed, in which case
   the resulting estimate MAY be used instead of 500 ms. If no RTT
   estimation is used, other values MAY be used in private networks
   where it is known that RTT has a different value. On the public
   Internet, T1 MAY be chosen larger, but SHOULD NOT be smaller.

   If the client transaction is still in the "calling"state when timer B
   fires, the client transaction SHOULD inform the TU that a timeout has
   occurred. The client transaction MUST NOT generate an ACK.  The value



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   of 64*T1 is equal to the amount of time required to send seven
   requests in the case of an unreliable transport.

   If the client transaction receives a provisional response while in
   the "calling" state, it transitions to the "proceeding" state. In the
   "proceeding" state, the client transaction SHOULD NOT retransmit the
   request any longer. Furthermore, the provisional response MUST be
   passed to the TU. Any further provisional responses MUST be passed up
   to the TU while in the "proceeding" state. Passing of all provisional
   responses is necessary since the TU will handle reliability of these
   messages, and therefore even retransmissions of a provisional
   response must be passed upwards.

   When in either the "calling" or "proceeding" states, reception of a
   response with status code from 300-699 MUST cause the client
   transaction to transition to "completed". The client transaction MUST
   pass the received response up to the TU, and the client transaction
   MUST generate an ACK request, even if the transport is reliable
   (guidelines for constructing the ACK from the response are given in
   Section 17.1.1.3) and then pass the ACK to the transport layer for
   transmission. The ACK MUST be sent to the same address, port and
   transport that the original request was sent to. The client
   transaction SHOULD start timer D when it enters the "completed"
   state, with a value of at least 32 seconds for unreliable transports,
   and a value of zero seconds for reliable transports. Timer D is a
   reflection of the amount of time that the server transaction can
   remain in the "completed" state when unreliable transports are used.
   This is equal to Timer H in the INVITE server transaction, whose
   default is 64*T1. However, the client transaction does not know the
   value of T1 in use by the server transaction, so an absolute minimum
   of 32s is used instead of basing Timer D on T1.

   Any retransmissions of the final response that are received while in
   the "completed" state SHOULD cause the ACK to be re-passed to the
   transport layer for retransmission, but the newly received response
   MUST NOT be passed up to the TU. A retransmission of the response is
   defined as any response which would match the same client
   transaction, based on the rules of Section 17.1.3.

   If timer D fires while the client transaction is in the "completed"
   state, the client transaction MUST move to the terminated state, and
   it MUST inform the TU of the timeout.

   When in either the "calling" or "proceeding" states, reception of a
   2xx response MUST cause the client transaction to enter the
   terminated state, and the response MUST be passed up to the TU. The
   handling of this response depends on whether the TU is a proxy core
   or a UAC core. A UAC core will handle generation of the ACK for this



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                               |INVITE from TU
             Timer A fires     |INVITE sent
             Reset A,          V                      Timer B fires
             INVITE sent +-----------+                or Transport Err.
               +---------|           |---------------+inform TU
               |         |  Calling  |               |
               +-------->|           |-------------->|
                         +-----------+ 2xx           |
                            |  |       2xx to TU     |
                            |  |1xx                  |
    300-699 +---------------+  |1xx to TU            |
   ACK sent |                  |                     |
resp. to TU |  1xx             V                     |
            |  1xx to TU  -----------+               |
            |  +---------|           |               |
            |  |         |Proceeding |-------------->|
            |  +-------->|           | 2xx           |
            |            +-----------+ 2xx to TU     |
            |       300-699    |                     |
            |       ACK sent,  |                     |
            |       resp. to TU|                     |
            |                  |                     |      NOTE:
            |  300-699         V                     |
            |  ACK sent  +-----------+Transport Err. |  transitions
            |  +---------|           |Inform TU      |  labeled with
            |  |         | Completed |-------------->|  the event
            |  +-------->|           |               |  over the action
            |            +-----------+               |  to take
            |              ^   |                     |
            |              |   | Timer D fires       |
            +--------------+   | -                   |
                               |                     |
                               V                     |
                         +-----------+               |
                         |           |               |
                         | Terminated|<--------------+
                         |           |
                         +-----------+
















   Figure 5: INVITE client transaction

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   response, while a proxy core will always forward the 200 (OK)
   upstream.  The differing treatment of 200 (OK) between proxy and UAC
   is the reason that handling of it does not take place in the
   transaction layer.

   The client transaction MUST be destroyed the instant it enters the
   terminated state. This is actually necessary to guarantee correct
   operation. The reason is that 2xx responses to an INVITE are treated
   differently; each one is forwarded by proxies, and the ACK handling
   in a UAC is different. Thus, each 2xx needs to be passed to a proxy
   core (so that it can be forwarded) and to a UAC core (so it can be
   acknowledged). No transaction layer processing takes place. Whenever
   a response is received by the transport, if the transport layer finds
   no matching client transaction (using the rules of Section 17.1.3),
   the response is passed directly to the core. Since the matching
   client transaction is destroyed by the first 2xx, subsequent 2xx will
   find no match and therefore be passed to the core.

17.1.1.3 Construction of the ACK Request

   The ACK request constructed by the client transaction MUST contain
   values for the Call-ID, From, and Request-URI which are equal to the
   values of those header fields in the request passed to the transport
   by the client transaction (call this the "original request"). The To
   header field in the ACK MUST equal the To header field in the
   response being acknowledged, and will therefore usually differ from
   the To header field in the original request by the addition of the
   tag parameter. The ACK MUST contain a single Via header field, and
   this MUST be equal to the top Via header field of the original
   request. The ACK request MUST contain the same Route header fields as
   the request whose response it is acknowledging. The CSeq header field
   in the ACK MUST contain the same value for the sequence number as was
   present in the original request, but the method parameter MUST be
   equal to "ACK".

   If the INVITE request whose response is being acknowledged had Route
   header fields, those header fields MUST appear in the ACK.  This is
   to ensure that the ACK can be routed properly through any downstream
   stateless proxies.

   Although any request MAY contain a body, a body in an ACK is special
   since the request cannot be rejected if the body is not understood.
   Therefore, placement of bodies in ACK for non-2xx is NOT RECOMMENDED,
   but if done, the body types are restricted to any that appeared in
   the INVITE, assuming that that the response to the INVITE was not
   415. If it was, the body in the ACK MAY be any type listed in the
   Accept header field in the 415.




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   These rules for construction of ACK only apply to the client
   transaction. A UAC core which generates an ACK for 2xx MUST instead
   follow the rules described in Section 13. For example, consider the
   following request:


   INVITE sip:bob@biloxi.com SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKkjshdyff
   To: Bob <sip:bob@biloxi.com>
   From: Alice <sip:alice@atlanta.com>;tag=88sja8x
   Max-Forwards: 70
   Call-ID: 987asjd97y7atg
   CSeq: 986759 INVITE



   The ACK request for a non-2xx final response to this request would
   look like this:


   ACK sip:bob@biloxi.com SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKkjshdyff
   To: Bob <sip:bob@biloxi.com>;tag=99sa0xk
   From: Alice <sip:alice@atlanta.com>;tag=88sja8x
   Max-Forwards: 70
   Call-ID: 987asjd97y7atg
   CSeq: 986759 ACK



17.1.2 non-INVITE Client Transaction

17.1.2.1 Overview of the non-INVITE Transaction

   Non-INVITE transactions do not make use of ACK. They are a simple
   request-response interaction. For unreliable transports, requests are
   retransmitted at an interval which starts at T1, and doubles until it
   hits T2. If a provisional response is received, retransmissions
   continue for unreliable transports, but at an interval of T2. The
   server transaction retransmits the last response it sent (which can
   be a provisional or final response) only when a retransmission of the
   request is received. This is why request retransmissions need to
   continue even after a provisional response, they are what ensure
   reliable delivery of the final response.

   Unlike an INVITE transaction, a non-INVITE transaction has no special
   handling for the 2xx response. The result is that only a single 2xx
   response to a non-INVITE is ever delivered to a UAC.



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17.1.2.2 Formal Description


   The state machine for the non-INVITE client transaction is shown in
   Figure 6. It is very similar to the state machine for INVITE.

   The "Trying" state is entered when the TU initiates a new client
   transaction with a request. When entering this state, the client
   transaction SHOULD set timer F to fire in 64*T1 seconds. The request
   MUST be passed to the transport layer for transmission. If an
   unreliable transport is in use, the client transaction MUST set timer
   E to fire in T1 seconds. If timer E fires while still in this state,
   the timer is reset, but this time with a value of MIN(2*T1, T2). When
   the timer fires again, it is reset to a MIN(4*T1, T2). This process
   continues, so that retransmissions occur with an exponentially
   increasing inverval that caps at T2. The default value of T2 is 4s,
   and it represents the amount of time a non-INVITE server transaction
   will take to respond to a request, if it does not respond
   immediately. For the default values of T1 and T2, this results in
   intervals of 500 ms, 1 s, 2 s, 4 s, 4 s, 4 s, etc.

   If Timer F fires while the client transaction is still in the
   "Trying" state, the client transaction SHOULD inform the TU about the
   timeout, and then it SHOULD enter the "Terminated" state. If a
   provisional response is received while in the "Trying" state, the
   response MUST be passed to the TU, and then the client transaction
   SHOULD move to the "Proceeding" state. If a final response (status
   codes 200-699) is received while in the "Trying" state, the response
   MUST be passed to the TU, and the client transaction MUST transition
   to the "Completed" state.

   If Timer E fires while in the "Proceeding" state, the request MUST be
   passed to the transport layer for retransmission, and Timer E MUST be
   reset with a value of T2 seconds. If timer F fires while in the
   "Proceeding" state, the TU MUST be informed of a timeout, and the
   client transaction MUST transition to the terminated state. If a
   final response (status codes 200-699) is received while in the
   "Proceeding" state, the response MUST be passed to the TU, and the
   client transaction MUST transition to the "Completed" state.

   Once the client transaction enters the "Completed" state, it MUST set
   Timer K to fire in T4 seconds for unreliable transports, and zero
   seconds for reliable transports. The "Completed" state exists to
   buffer any additional response retransmissions that may be received
   (which is why the client transaction remains there only for
   unreliable transports). T4 represents the amount of time the network
   will take to clear messages between client and server transactions.
   The default value of T4 is 5s. A response is a retransmission when it



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   matches the same transaction, using the rules specified in Section
   17.1.3. If Timer K fires while in this state, the client transaction
   MUST transition to the "Terminated" state.

   Once the transaction is in the terminated state, it MUST be
   destroyed. As with client transactions, this is needed to ensure
   reliability of the 2xx responses to INVITE.

17.1.3 Matching Responses to Client Transactions

   When the transport layer in the client receives a response, it has to
   figure out which client transaction will handle the response, so that
   the processing of Sections 17.1.1 and 17.1.2 can take place.

   The branch parameter in the top Via header field is used for this
   purpose. A response matches a client transaction under two
   conditions.  First, if the response has the same value of the branch
   parameter in the top Via header field as the branch parameter in the
   top Via header field of the request that created the transaction.
   Second, if the method parameter in the CSeq header field matches the
   method of the request that created the transaction. The method is
   needed since a CANCEL request constitutes a different transaction,
   but shares the same value of the branch parameter.

   A response which matches a transaction matched by a previous response
   is considered a retransmission of that response.

17.1.4 Handling Transport Errors

   When the client transaction sends a request to the transport layer to
   be sent, the following procedures are followed if the transport layer
   indicates a failure.

   The client transaction SHOULD inform the TU that a transport failure
   has occurred, and the client transaction SHOULD transition directly
   to the terminated state.

17.2 Server Transaction

   The server transaction is responsible for the delivery of requests to
   the TU, and the reliable transmission of responses. It accomplishes
   this through a state machine. Server transactions are created by the
   core when a request is received, and transaction handling is desired
   for that request (this won't always be the case).

   As with the client transactions, the state machine depends on whether
   the received request is an INVITE request or not.




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                              |Request from app
                              |send request
          Timer E             V
          send request  +-----------+
              +---------|           |-------------------+
              |         |  Trying   |  Timer F          |
              +-------->|           |  or Transport Err.|
                        +-----------+  inform TU        |
           200-699         |  |                         |
           resp. to TU     |  |1xx                      |
           +---------------+  |resp. to TU              |
           |                  |                         |
           |   Timer E        V       Timer F           |
           |   send req +-----------+ or Transport Err. |
           |  +---------|           | inform TU         |
           |  |         |Proceeding |------------------>|
           |  +-------->|           |-----+             |
           |            +-----------+     |1xx          |
           |              |      ^        |resp to TU   |
           | 200-699      |      +--------+             |
           | resp. to TU  |                             |
           |              |                             |
           |              V                             |
           |            +-----------+                   |
           |            |           |                   |
           |            | Completed |                   |
           |            |           |                   |
           |            +-----------+                   |
           |              ^   |                         |
           |              |   | Timer K                 |
           +--------------+   | -                       |
                              |                         |
                              V                         |
        NOTE:           +-----------+                   |
                        |           |                   |
    transitions         | Terminated|<------------------+
    labeled with        |           |
    the event           +-----------+
    over the action
    to take












   Figure 6: non-INVITE client transaction

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17.2.1 INVITE Server Transaction


   The state diagram for the INVITE server transaction is shown in
   Figure 7.

   When a server transaction is constructed with a request, it enters
   the "Proceeding" state. The server transaction MUST generate a 100
   response (not any status code -- the specific value of 100) unless it
   knows that the TU will generate a provisional or final response
   within 200 ms, in which case it MAY generate a 100 (Trying) response.
   This provisional response is needed to rapidly quench request
   retransmissions in order to avoid network congestion. The 100
   response is constructed according to the procedures in Section 8.2.6,
   except that insertion of tags in the To header field of the response
   (when none was present in the request), is downgraded from MAY to
   SHOULD NOT. The request MUST be passed to the TU.

   The TU passes any number of provisional responses to the server
   transaction. So long as the server transaction is in the "Proceeding"
   state, each of these MUST be passed to the transport layer for
   transmission. They are not sent reliably by the transaction layer
   (they are not retransmitted by it), and do not cause a change in the
   state of the server transaction. When provisional responses need to
   be delivered reliably, it is handled by the TU, which will retransmit
   the provisional responses itself, and pass downwards each
   retransmission to the server transaction. If a request retransmission
   is received while in the "Proceeding" state, the most recent
   provisional response that was received from the TU MUST be passed to
   the transport layer for retransmission. A request is a retransmission
   if it matches the same server transaction based on the rules of
   Section 17.2.3.

   If, while in the "proceeding" state, the TU passes a 2xx Response to
   the server transaction, the server transaction MUST pass this
   response to the transport layer for transmission. It is not
   retransmitted by the server transaction; retransmissions of 2xx
   responses are handled by the TU. The server transaction MUST then
   transition to the "terminated" state.

   While in the "Proceeding" state, if the TU passes a response with
   status code from 300 to 699 to the server transaction, the response
   MUST be passed to the transport layer for transmission, and the state
   machine MUST enter the "Completed" state. For unreliable transports,
   timer G is set to fire in T1 seconds, and is not set to fire for
   reliable transports.





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        This is a change from RFC 2543, where responses were always
        retransmitted, even over reliable transports.

   When the "Completed" state is entered, timer H MUST be set to fire in
   64*T1 seconds, for all transports. Timer H determines when the server
   transaction gives up retransmitting the response. Its value is chosen
   to equal Timer B, the amount of time a client transaction will
   continue to retry sending a request. If timer G fires, the response
   is passed to the transport layer once more for retransmission, and
   timer G is set to fire in MIN(2*T1, T2) seconds. From then on, when
   timer G fires, the response is passed to the transport again for
   transmission, and timer G is reset with a value that doubles, unless
   that value exceeds T2, in which case it is reset with the value of
   T2. This is identical to the retransmit behavior for requests in the
   "Trying" state of the non- INVITE client transaction. Furthermore,
   while in the "completed" state, if a request retransmission is
   received, the server SHOULD pass the response to the transport for
   retransmission.

   If an ACK is received while the server transaction is in the
   "Completed" state, the server transaction MUST transition to the
   "confirmed" state. As Timer G is ignored in this state, any
   retransmissions of the response will cease.

   If timer H fires while in the "Completed" state, it implies that the
   ACK was never received. In this case, the server transaction MUST
   transition to the terminated state, and MUST indicate to the TU that
   a transaction failure has occurred.

   The purpose of the "confirmed" state is to absorb any additional ACK
   messages that arrive, triggered from retransmissions of the final
   response. When this state is entered, timer I is set to fire in T4
   seconds for unreliable transports, and zero seconds for reliable
   transports. Once timer I fires, the server MUST transition to the
   "Terminated" state.

   Once the transaction is in the terminated state, it MUST be
   destroyed. As with client transactions, this is needed to ensure
   reliability of the 2xx responses to INVITE.

17.2.2 non-INVITE Server Transaction


   The state machine for the non-INVITE server transaction is shown in
   Figure 8.

   The state machine is initialized in the "Trying" state, and is passed
   a request other than INVITE or ACK when initialized. This request is



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                                  |INVITE
                                  |pass INV to TU
               INVITE             V send 100 if TU won't in 200ms
               send response+-----------+
                   +--------|           |--------+101-199 from TU
                   |        | Proceeding|        |send response
                   +------->|           |<-------+
                            |           |          Transport Err.
                            |           |          Inform TU
                            |           |--------------->+
                            +-----------+                |
               300-699 from TU |     |2xx from TU        |
               send response   |     |send response      |
                               |     +------------------>+
                               |                         |
               INVITE          V          Timer G fires  |
               send response+-----------+ send response  |
                   +--------|           |--------+       |
                   |        | Completed |        |       |
                   +------->|           |<-------+       |
                            +-----------+                |
                               |     |                   |
                           ACK |     |                   |
                           -   |     +------------------>+
                               |        Timer H fires    |
                               V        or Transport Err.|
                            +-----------+  Inform TU     |
                            |           |                |
                            | Confirmed |                |
                            |           |                |
                            +-----------+                |
                                  |                      |
                                  |Timer I fires         |
                                  |-                     |
                                  |                      |
                                  V                      |
                            +-----------+                |
                            |           |                |
                            | Terminated|<---------------+
                            |           |
                            +-----------+











   Figure 7: INVITE server transaction

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                                  |Request received
                                  |pass to TU
                                  V
                            +-----------+
                            |           |
                            | Trying    |-------------+
                            |           |             |
                            +-----------+             |200-699 from TU
                                  |                   |send response
                                  |1xx from TU        |
                                  |send response      |
                                  |                   |
               Request            V      1xx from TU  |
               send response+-----------+send response|
                   +--------|           |--------+    |
                   |        | Proceeding|        |    |
                   +------->|           |<-------+    |
            +<--------------|           |             |
            |Trnsprt Err    +-----------+             |
            |Inform TU            |                   |
            |                     |                   |
            |                     |200-699 from TU    |
            |                     |send response      |
            |  Request            V                   |
            |  send response+-----------+             |
            |      +--------|           |             |
            |      |        | Completed |-------------+
            |      +------->|           |
            +<--------------|           |
            |Trnsprt Err    +-----------+
            |Inform TU            |
            |                     |Timer J fires
            |                     |-
            |                     |
            |                     V
            |               +-----------+
            |               |           |
            +-------------->| Terminated|
                            |           |
                            +-----------+












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   Figure 8: non-INVITE server transaction


   passed up to the TU. Once in the "Trying" state, any further request
   retransmissions are discarded. A request is a retransmission if it
   matches the same server transaction, using the rules specified in
   Section 17.2.3.

   While in the "Trying" state, if the TU passes a provisional response
   to the server transaction, the server transaction MUST enter the
   "Proceeding" state. The response MUST be passed to the transport
   layer for transmission. Any further provisional responses that are
   received from the TU while in the "Proceeding" state MUST be passed
   to the transport layer for transmission. If a retransmission of the
   request is received while in the "Proceeding" state, the most
   recently sent provisional response MUST be passed to the transport
   layer for retransmission. If the TU passes a final response (status
   codes 200-699) to the server while in the "Proceeding" state, the
   transaction MUST enter the "Completed" state, and the response MUST
   be passed to the transport layer for transmission.

   When the server transaction enters the "Completed" state, it MUST set
   Timer J to fire in 64*T1 seconds for unreliable transports, and zero
   seconds for reliable transports. While in the "Completed" state, the
   server transaction MUST pass the final response to the transport
   layer for retransmission whenever a retransmission of the request is
   received. Any other final responses passed by the TU to the server
   transaction MUST be discarded while in the "Completed" state. The
   server transaction remains in this state until Timer J fires, at
   which point it MUST transition to the "Terminated" state.

   The server transaction MUST be destroyed the instant it enters the
   "Terminated" state.

17.2.3 Matching Requests to Server Transactions

   When a request is received from the network by the server, it has to
   be matched to an existing transaction. This is accomplished in the
   following manner.

   The branch parameter in the topmost Via header field the request is
   examined. If it is present, and begins with the magic cookie
   "z9hG4bK", the request was generated by a client transaction
   compliant to this specification. Therefore, the branch parameter will
   be unique across all transactions sent by that client. The request
   matches a transaction if the branch parameter in the request is equal
   to the one in the top Via header field of the request that created
   the transaction, the source address and port of the request are the



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   same as the source address and port of the the request that created
   the transaction, and in the case of a CANCEL request, the method of
   the request that created the transaction was also CANCEL. This
   matching rule applies to both INVITE and non-INVITE transactions
   alike.


        Source address and port are used as part of the matching
        process because there could be duplication of branch
        parameters from different clients; uniqueness in time is
        mandated for construction of the parameter, but not
        uniqueness in space.

   If the branch parameter in the top Via header field is not present,
   or does not contain the magic cookie, the following procedures are
   used. These exist to handle backwards compatibility with RFC 2543
   compliant implementations.

   The INVITE request matches a transaction if the Request-URI, To,
   From, Call-ID, CSeq, and top Via header field match those of the
   INVITE request which created the transaction. In this case, the
   INVITE is a retransmission of the original one that created the
   transaction. The ACK request matches a transaction if the Request-
   URI, From, Call-ID, CSeq number (not the method), and top Via header
   field match those of the INVITE request which created the
   transaction, and the To header field of the ACK matches the To header
   field of the response sent by the server transaction (which then
   includes the tag). Matching is done based on the matching rules
   defined for each of those header fields. The usage of the tag in the
   To header field helps disambiguate ACK for 2xx from ACK for other
   responses at a proxy which may have forwarded both responses (which
   can occur in unusual conditions). An ACK request that matches an
   INVITE transaction matched by a previous ACK is considered a
   retransmission of that previous ACK.

   For all other request methods, a request is matched to a transaction
   if the Request-URI, To, From, Call-ID and Cseq (including the method)
   and top Via header field match those of the request which created the
   transaction. Matching is done based on the matching rules defined for
   each of those header fields.  When a non-INVITE request matches an
   existing transaction, it is a retransmission of the request which
   created that transaction.

   Because the matching rules include the Request-URI, the server cannot
   match a response to a transaction. When the TU passes a response to
   the server transaction, it must pass it to the specific server
   transaction for which the response is targeted.




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17.2.4 Handling Transport Errors

   When the server transaction sends a response to the transport layer
   to be sent, the following procedures are followed if the transport
   layer indicates a failure.

   First, the procedures in [2] are followed, which attempt to deliver
   the response to a backup. If those should all fail, such that all
   elements generate ICMP errors, or no SRV records are present, the
   server transaction SHOULD inform the TU that a failure has occurred,
   and SHOULD transition to the terminated state.

17.3 RTT Estimation

   Most of the timeouts used in the transaction state machines derive
   from T1, which is an estimate of the RTT between the client and
   server transactions. This subsection defines optional procedures that
   a client can use to build up estimates of the RTT to a particular IP
   address. To perform this procedure, the client MUST maintain a table
   of variables for each destination IP address to which an RTT estimate
   is being made.

   If a client wishes to measure RTT for a particular IP address, it
   MUST include a Timestamp header field into a request containing the
   time when the request is initially created and passed to a new client
   transaction, which transmits the request. If a 100 (Trying) response
   (not any 1xx, only the 100 (Trying) response) is received before the
   client transaction generates a retransmission, an RTT estimate is
   made. This is consistent with the RFC 2988 requirements on TCP for
   using Karn's algorithm in RTT estimation.

   The estimate, called R, is made by computing the difference between
   the current time and the value of Timestamp header field in the 100
   response, and then subtracting the value of the delay field of the
   Timestamp header in the response, if present.  The value of R is
   applied to the estimation of RTO as described in Section 2 of RFC
   2988 [26], with the following differences. First, the initial value
   of RTO is 500 ms for SIP, not 3 s as is used for TCP. Second, there
   is no minimum value for the RTO, as there is for TCP, if SIP is being
   run on a private network. When run on the public Internet, the
   minimum is 500 ms, as opposed to 1 s for TCP.  This difference is
   because of the expected usage of SIP in private networks where rapid
   call setup times are service critical. Once RTO is computed, the
   timer T1 is set to the value of RTO, and all other timers scale
   proportionally as described above.

   This value of T1 would be used for scaling all of the client and
   server transaction timers described above, when a request or



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   response, respectively, is sent to that IP address.

   If the IP address is that of a stateless proxy, the actual round trip
   time that is measured will be the average to all transaction stateful
   proxies or UAs that are reached through the stateless proxy. This
   estimate may therefore be too low or too high for a specific
   transactional element being communicated with through the stateless
   proxy.

18 Reliability of Provisional Responses

   Normally, provisional responses are not transmitted reliably. The TU
   generates a single provisional response and passes it to the server
   transaction, which sends it once. RFC 2543 provided no means for
   reliable transmission of these messages.

   It was later observed that reliability was important in several
   cases, including interoperability scenarios with the PSTN. Therefore,
   an optional capability was added in this specification to support
   reliable transmission of provisional responses.

   The reliability mechanism works by mirroring the current reliability
   mechanisms for 2xx final responses to INVITE. Those requests are
   transmitted periodically by the TU until a separate transaction, ACK,
   is received that indicates reception of the 2xx by the UAC. The
   reliability for the 2xx responses to INVITE and ACK messages are
   end-to-end. In order to achieve reliability for provisional
   responses, we do nearly the same thing. Reliable provisional
   responses are retransmitted by the TU with an exponential backoff.
   Those retransmissions cease when a PRACK message is received. The
   PRACK request plays the same role as ACK, but for provisional
   responses. There is an important difference, however. PRACK is a
   normal SIP message, like BYE. As such, its own reliability is ensured
   hop-by-hop through each stateful proxy. Similarly, PRACK has its own
   response. If this were not the case, the PRACK message could not
   traverse existing proxy servers.

   Each provisional response is given a sequence number, carried in the
   RSeq header field in the response. The PRACK messages contain an RAck
   header field, which indicates the sequence number of the provisional
   response that is being acknowledged. The acknowledgements are not
   cumulative, and the specifications recommend a single outstanding
   provisional response at a time, for purposes of congestion control.

18.1 UAS Behavior

   A UAS MAY send any non-100 provisional response to INVITE reliably,
   so long as the initial INVITE request (the request whose provisional



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   response is being sent reliably) contained a Supported header field
   with the option tag 100rel specification does not allow reliable
   provisional responses for any method but INVITE, extensions that
   define new methods that can establish dialogs may make use of the
   mechanism.

   The UAS MUST send any non-100 provisional response reliably if the
   initial request contained a Require header field with the option tag
   100rel initial request with a 420 (Bad Extension) and include a
   Unsupported header field containing the option tag 100rel

   A UAS MUST NOT attempt to send a 100 (Trying) response reliably. Only
   provisional responses numbered 101 to 199 may be sent reliably. If
   the request did not include either a Supported or Require header
   field indicating this feature, the UAS MUST NOT send the provisional
   response reliably.


        100 (Trying) responses are hop-by-hop only. For this
        reason, the reliability mechanisms described here, which
        are end-to-end, cannot be used.

   An element that can act as a proxy can also send reliable provisional
   Responses. In this case, it acts as a UAS for purposes of that
   transaction. However, it MUST NOT attempt to do so for any request
   that contains a tag in the To field. That is, a proxy cannot generate
   reliable provisional responses to requests sent within the context of
   a dialog. Of course, unlike a UAS, when the proxy element receives a
   PRACK that does not match any outstanding reliable provisional
   response, the PRACK MUST be proxied.

   The rest of this discussion assumes that the initial request
   contained a Supported or Require header field listing 100rel , and
   that there is a provisional response to be sent reliably.

   The provisional response to be sent reliably is constructed by the
   UAS core according to the procedures of Section 8.2.6 and Section 12.
   Specifically, the provisional response MUST establish a dialog if one
   is not yet created. In addition, it MUST contain a Require header
   field containing the option tag 100rel , and MUST include an RSeq
   header field. The value of the header field for the first reliable
   provisional response in a transaction MUST be between 1 and 2**31 -
   1. It is RECOMMENDED that it be chosen uniformly in this range. The
   RSeq numbering space is within a single transaction. This means that
   provisional responses for different requests MAY use the same values
   for the RSeq number.

   The reliable provisional response is passed to the transaction layer



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   periodically with an interval that starts at T1 seconds and doubles
   for each retransmission (T1 is defined in Section 17). Once passed to
   the server transaction, it is added to an internal list of
   unacknowledged reliable provisional responses.


        This differs from retransmissions of 2xx responses, which
        cap at T2 seconds. This is because retransmissions of ACK
        are triggered on receipt of a 2xx, but retransmissions of
        PRACK take place independently of reception of 1xx.

   Retransmissions cease when a matching PRACK is received. PRACK is
   like any other request within a dialog, and the UAS core processes it
   according to the procedures of Sections 8.2 and 12.2.2. A matching
   PRACK is defined as one within the same dialog as the response, and
   whose method, CSeq-num, and response-num in the RAck header field
   match, respectively, the method and sequence number from the CSeq and
   sequence number from the RSeq of the reliable provisional response.

   If a PRACK request is received that does not match any unacknowledged
   reliable provisional response, the UAS MUST respond to the PRACK with
   a 481 response. If the PRACK does match an unacknowledged reliable
   provisional response, it MUST be responded to with a 2xx response.
   The UAS can be certain at this point that the provisional response
   has been received in order. It SHOULD cease retransmissions of the
   reliable provisional response, and MUST remove it from the list of
   unacknowledged provisional responses.

   If a reliable provisional response is retransmitted for 64*T1 seconds
   without reception of a corresponding PRACK, the UAS SHOULD reject the
   original request with a 5xx response.

   If the PRACK contained a body, the body is treated in the same way a
   body in an ACK is treated.

   After the first reliable provisional response for a request has been
   acknowledged, the UAS MAY send additional reliable provisional
   responses. The UAS MUST NOT send a second reliable provisional
   response until the first is acknowledged. After the first, it is
   RECOMMENDED that the UAS not send an additional reliable provisional
   response until the previous is acknowledged. The first reliable
   provisional response receives special treatment because it conveys
   the initial sequence number. If additional reliable provisional
   responses were sent before the first was acknowledged, the UAS could
   not be certain these were received in order.

   The value of the RSeq in each subsequent reliable provisional
   response for the same request MUST be greater by exactly one.  RSeq



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   numbers MUST NOT wrap around. Because the initial one is chosen to be
   less than 2**31 - 1, but the maximum is 2**32 - 1, there can be up to
   2**31 reliable provisional responses per request, which is more than
   sufficient.

   Note that the UAS MAY send a final response to the initial request
   before having received PRACKs for all unacknowledged reliable
   provisional responses. In that case, it SHOULD NOT continue to
   retransmit the unacknowledged reliable provisional responses, but it
   MUST be prepared to process PRACK requests for those outstanding
   responses. A UAS MUST NOT send new reliable provisional responses (as
   opposed to retransmissions of unacknowledged ones) after sending a
   final response to a request.

18.2 UAC Behavior

   If a provisional response is received for the initial request, and
   that response contains a Require header field containing the option
   tag 100rel , the response is to be sent reliably. If the response is
   a 100 (Trying) (as opposed to 101 to 199), this option tag MUST be
   ignored, and the procedures below MUST NOT be used.

   Assuming the response is to be transmitted reliably, the UAC MUST
   create a new request with method PRACK. This request is sent within
   the dialog associated with the provisional response (indeed, the
   provisional response may have created the dialog). PRACK requests MAY
   contain bodies, which are interpreted according to their type and
   disposition.

   Note that the PRACK is like any other non-INVITE request within a
   dialog. In particular, a UAC SHOULD NOT retransmit the PRACK request
   when it receives a retransmission of the provisional response being
   acknowledged, although doing so does not create a protocol error.

   Once a reliable provisional response is received, retransmissions of
   that response MUST be discarded. A response is a retransmission when
   its dialog ID, CSeq, and RSeq match the original response. The UAC
   MUST maintain a sequence number that indicates the most recently
   received in-order reliable provisional response for the initial
   request. This sequence number MUST be maintained until a final
   response is received for the initial request. Its value MUST be
   initialized to the RSeq header field in the first reliable
   provisional response received for the initial request.

   Handling of subsequent reliable provisional responses for the same
   initial request follows the same rules as above, with the following
   difference: reliable provisional responses are guaranteed to be in
   order. As a result, if the UAC receives another reliable provisional



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   response to the same request, and its RSeq value is not one higher
   than the value of the sequence number, that response MUST NOT be
   acknowledged with a PRACK, and MUST NOT be processed further by the
   TU. An implementation MAY discard the response, or MAY cache the
   response in the hopes of receiving the missing responses.

   The UAC MAY acknowledge reliable provisional responses received after
   the final response or MAY discard them.

19 Transport

   The transport layer is responsible for the actual transmission of
   requests and responses over network transports. This includes
   determination of the connection to use for a request or response, in
   the case of connection oriented transports.

   The transport layer is responsible for managing any persistent
   connections (for transports like TCP, TLS and SCTP) including ones it
   opened, as well as ones opened to it. This includes connections
   opened by the client or server transports, so that connections are
   shared between client and server transport functions. These
   connections are indexed by the [address, port, transport] at the far
   end of the connection. When a connection is opened by the transport
   layer, this index is set to the destination IP, port and transport.
   When the connection is accepted by the transport layer, this index is
   set to the source IP, port and transport. Note that, because the
   source port is often ephemeral, connections accepted by the transport
   layer will frequently not be reused. The result is that two proxies
   in a "peering" relationship using a connection oriented transport
   will frequently have two connections in use, one for transactions
   initiated in each direction.

   It is RECOMMENDED that connections be kept open for some
   implementation defined duration after the last message was sent or
   received over that connection. This duration SHOULD at least equal
   the longest amount of time the element would need in order to bring a
   transaction from instantiation to the terminated state. This is to
   insure that transactions complete over the same connection they are
   initiated on (i.e., request, response, and in the case of INVITE, ACK
   for non-2xx responses)). This usually means at least the maximum of
   T3 and 64*T1. However, it could be larger in an element that has a TU
   that is using a large value for timer C, for example.

   All SIP elements MUST implement UDP and TCP. Other transports MAY be
   implemented by any entity.


        Making TCP mandatory for UA is a substantial change from



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        RFC 2543. It has arisen out of the need to handle larger
        messages, which MUST use TCP, as discussed below. Thus,
        even if an element never sends large messages, it may
        receive one, and needs to be able to do that.

19.1 Clients

19.1.1 Sending Requests

   The client side of the transport layer is responsible for sending the
   request and receiving responses. The user of the transport layer
   passes the client transport the request, an IP address, port,
   transport, and possibly TTL for multicast destinations.

   If a request is within 500 bytes of the path MTU, or if it is larger
   than 1000 bytes when the path MTU is unknown, it MUST be sent using
   TCP. This is to prevent fragmentation of messages over UDP, and to
   provide congestion control for larger messages. However,
   implementations MUST be able to handle messages up to the maximum
   datagram packet size. For UDP, this size is 65,535 bytes, including
   header fields.


        The 500 byte "buffer" between the message size and the MTU
        accomodates the fact that the response in SIP can be larger
        than the request. This happens due to the addition of
        Record-Route header fields to the responses to INVITE, for
        example. With the extra buffer, the response can be 500
        bytes larger than the request, and still not be fragmented.
        1000 is chosen when path MTU is not known, based on the
        assumption of a 1500 byte ethernet MTU.

   A client that sends a request to a multicast address MUST add the
   "maddr" parameter to its Via header field, and SHOULD add the "ttl"
   parameter. (In that case, the maddr parameter SHOULD contain the
   destination multicast address, although under exceptional
   circumstances it MAY contain a unicast address.) Requests sent to
   multicast groups SHOULD be scoped to ensure that they are not
   forwarded beyond the administrative domain to which they were
   targeted. This scoping MAY be done with either TTL or administrative
   scopes [12], depending on what is implemented in the network.

   It is important to note that the layers above the transport layer do
   not operate differently for multicast as opposed to unicast requests.
   This means that SIP treats multicast more like anycast, assuming that
   there is a single recipient generating responses to requests. If this
   is not the case, the first response will end up "winning", based on
   the client transaction rules. Any other responses from different UA



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   will appear as retransmissions and be discarded. This limits the
   utility of multicast to cases where an anycast type of function is
   desired, such as registrations.

   Before a request is sent, the client transport MUST insert a value of
   the sent-by field into the Via header field. This field contains an
   IP address or host name, and port. The usage of an FQDN is
   RECOMMENDED. This field is used for sending responses under certain
   conditions.

   For reliable transports, the response is normally sent on the
   connection the request was received on. Therefore, the client
   transport MUST be prepared to receive the response on the same
   connection used to send the request. Under error conditions, the
   server may attempt to open a new connection to send the response. To
   handle this case, the transport layer MUST also be prepared to
   receive an incoming connection on the source IP address that the
   request was sent from, and port number in the sent-by field. It also
   MUST be prepared to receiving incoming connections on any address and
   port which would be selected by a server based on the procedures
   described in Section 5 of [2].

   For unreliable unicast transports, the client transport MUST be
   prepared to receive responses on the source IP address that the
   request is sent from (as responses are sent back to the source
   address), but the port number in the sent-by field. Furthermore, as
   with reliable transports, in certain cases the response will be sent
   elsewhere. The client MUST be prepared to receive responses on any
   address and port which would be selected by a server based on the
   procedures described in Section 5 of [2].

   For multicast, the client transport MUST be prepared to receive
   responses on the same multicast group and port that the request is
   sent to (e.g., it needs to be a member of the multicast group it sent
   the request to.)

   If a request is destined to an IP address, port, and transport to
   which an existing connection is open, it is RECOMMENDED that this
   connection be used to send the request, but another connection MAY be
   opened and used.

   If a request is sent using multicast, it is sent to the group
   address, port, and TTL provided by the transport user. If a request
   is sent using unicast unreliable transports, it is sent to the IP
   address and port provided by the transport user.

19.1.2 Receiving Responses




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   When a response is received, the client transport examines the top
   Via header field. If the value of the sent-by parameter in that
   header field does not correspond to a value that the client transport
   is configured to insert into requests, the response MUST be rejected.

   If there are any client transactions in existence, the client
   transport uses the matching procedures of Section 17.1.3 to attempt
   to match the response to an existing transaction. If there is a
   match, the response MUST be passed to that transaction. Otherwise,
   the response MUST be passed to the core (whether it be stateless
   proxy, stateful proxy, or UA) for further processing. Handling of
   these "stray" responses is dependent on the core (a stateless proxy
   will forward all responses, for example).

19.2 Servers

19.2.1 Receiving Requests

   When the server transport receives a request over any transport, it
   MUST examine the value of the sent-by parameter in the top Via header
   field. If the host portion of the sent-by parameter contains a domain
   name, or if it contains an IP address that differs from the packet
   source address, the server MUST add a "received" attribute to that
   Via header field. This attribute MUST contain the source address that
   the packet was received from. This is to assist the server transport
   layer in sending the response, since it must be sent to the source IP
   address that the request came from.

   Consider a request received by the server transport which looks like,
   in part:


     INVITE sip:bob@Biloxi.com SIP/2.0
     Via: SIP/2.0/UDP bobspc.biloxi.com:5060



   The request is received with a source IP address of 1.2.3.4. Before
   passing the request up, the transport would add a received parameter,
   so that the request would look like, in part:


     INVITE sip:bob@Biloxi.com SIP/2.0
     Via: SIP/2.0/UDP bobspc.biloxi.com:5060;received=1.2.3.4



   Next, the server transport attempts to match the request to the



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   server transaction. It does so using the matching rules described in
   Section 17.2.3. If a matching server transaction is found, the
   request is passed to that transaction for processing. If no match is
   found, the request is passed to the core, which may decide to
   construct a new server transaction for that request. Note that when a
   UAS core sends a 2xx response to INVITE, the server transaction is
   destroyed. This means that when the ACK arrives, there will be no
   matching server transaction, and based on this rule, the ACK is
   passed to the UAS core, where it is processed.

19.2.2 Sending Responses

   The server transport uses the value of the top Via header field in
   order to determine where to send a response. It MUST follow the
   following process:

        o If the "sent-protocol" is a reliable transport protocol such
          as TCP, TLS or SCTP, the response MUST be sent using the
          existing connection to the source of the original request that
          created the transaction, if that connection is still open.
          This does require the server transport to maintain an
          association between server transactions and transport
          connections. If that connection is no longer open, the server
          MAY open a connection to the IP address in the received
          parameter, if present, using the port in the sent-by value, or
          the default port for that transport, if no port is specified
          (5060 for UDP and TCP, 5061 for TLS and SSL). If that
          connection attempt fails, the server SHOULD use the procedures
          in [2] for servers in order to determine the IP address and
          port to open the connection and send the response to.

        o Otherwise, if the Via header field contains a "maddr"
          parameter, forward the response to the address listed there,
          using the port indicated in "sent-by", or port 5060 if none is
          present. If the address is a multicast address, the response
          SHOULD be sent using the TTL indicated in the "ttl" parameter,
          or with a TTL of 1 if that parameter is not present.

        o Otherwise (for unreliable unicast transports), if the top Via
          has a received parameter, send the response to the address in
          the "received" parameter, using the port indicated in the
          "sent-by" value, or using port 5060 if none is specified
          explicitly. If this fails, e.g., elicits an ICMP "port
          unreachable" response, send the response to the address in the
          "sent-by" parameter. The address to send to is determined by
          following the procedures defined in Section 5 of [2].

        o Otherwise, if it is not receiver-tagged, send the response to



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          the address indicated by the "sent-by" value, using the
          procedures in Section 5 of [2].

19.3 Framing

   In the case of message oriented transports (such as UDP), if the
   message has a Content-Length header field, the message body is
   assumed to contain that many bytes. If there are additional bytes in
   the transport packet below the end of the body, they MUST be
   discarded. If the transport packet ends before the end of the message
   body, this is considered an error. If the message is a response, it
   MUST be discarded. If its a request, the element SHOULD generate a
   400 class response. If the message has no Content-Length header
   field, the message body is assumed to end at the end of the transport
   packet.

   In the case of stream oriented transports (such as TCP), the
   Content-Length header field indicates the size of the body. The
   Content-Length header field MUST be used with stream oriented
   transports.

19.4 Error Handling

   Error handling is independent of whether the message was a request or
   response.

   If the transport user asks for a message to be sent over an
   unreliable transport, and the result is an ICMP error, the behavior
   depends on the type of ICMP error. A host, network, port or protocol
   unreachable errors, or parameter problem errors SHOULD cause the
   transport layer to inform the transport user of a failure in sending.
   Source quench and TTL exceeded ICMP errors SHOULD be ignored.

   If the transport user asks for a request to be sent over a reliable
   transport, and the result is a connection failure, the transport
   layer SHOULD inform the transport user of a failure in sending.

20 Usage of HTTP Authentication

   SIP provides a stateless, challenge-based mechanism for
   authentication that is based on authentication in HTTP. Any time that
   a proxy server or UA receives a request (with the exceptions given in
   Section 20.1), it MAY challenge the initiator of the request to
   provide assurance of its identity. Once the originator has been
   identified, the recipient of the request SHOULD ascertain whether or
   not this user is authorized to make the request in question. No
   authorization systems are recommended or discussed in this document.




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   The "Digest" authentication mechanism described in this section
   provides message authentication and replay protection only, without
   message integrity or confidentiality.  Protective measures above and
   beyond those provided by Digest need to be taken to prevent active
   attackers from modifying SIP requests and responses.

   Note that due to its weak security, the usage of "Basic"
   authentication has been deprecated. Servers MUST NOT accept
   credentials using the "Basic" authorization scheme, and servers also
   MUST NOT challenge with "Basic". This is a change from RFC 2543.

20.1 Framework

   The framework for SIP authentication closely parallels that of HTTP
   (RFC 2617 [16]). In particular, the BNF for auth-scheme, auth-param,
   challenge, realm, realm-value, and credentials is identical (although
   the usage of "Basic" as a scheme is not permitted). In SIP, a UAS
   uses the 401 (Unauthorized) response to challenge the identity of a
   UAC. Additionally, registrars and redirect servers MAY make use of
   401 (Unauthorized) responses for authentication, but proxies MUST
   NOT, and instead MAY use the 407 (Proxy Authentication Required)
   response. The requirements for inclusion of the Proxy-Authenticate,
   Proxy-Authorization, WWW-Authenticate, and Authorization in the
   various messages are identical to those described in RFC 2617 [16].

   Since SIP does not have the concept of a canonical root URL, the
   notion of protection spaces is interpreted differently in SIP. The
   realm string alone defines the protection domain. This is a change
   from RFC 2543, in which the Request-URI and the realm together
   defined the protection domain.


        This previous definition of protection domain caused some
        amount of confusion since the Request-URI sent by the UAC
        and the Request-URI received by the challenging server
        might be different, and indeed the final form of the
        Request-URI might not be known to the UAC. Also, the
        previous definition depended on the presence of a SIP URI
        in the Request-URI and seemed to rule out alternative URI
        schemes (for example, the tel URL).

   Operators of user agents or proxy servers that will authenticate
   received requests MUST adhere to the following guidelines for
   creation of a realm string for their server:

        o Realm strings MUST be globally unique. It is RECOMMENDED that
          a realm string contain a hostname or domain name, following
          the recommendation in Section 3.2.1 of RFC 2617 [16].



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        o Realm strings SHOULD present a human-readable identifier that
          can be rendered to a user.

   For example:



      INVITE sip:bob@biloxi.com SIP/2.0
      WWW-Authenticate:  Digest realm="biloxi.com", <...>



   Generally, SIP authentication is meaningful for a specific realm, a
   protection domain. Thus, for Digest authentication, each such
   protection domain has its own set of usernames and passwords. If a
   server does not require authentication for a particular request, it
   MAY accept a default username, "anonymous", which has no password
   (password of ""). Similarly, UACs representing many users, such as
   PSTN gateways, MAY have their own device-specific username and
   password, rather than accounts for particular users, for their realm.

   While a server can legitimately challenge most SIP requests, there
   are two requests defined by the SIP standard today that require
   special handling for authentication: ACK and CANCEL.

   Under an authentication scheme that uses responses to carry values
   used to compute nonces (such as Digest), some problems come up for
   any requests that take no response, including ACK. For this reason,
   any credentials in the INVITE that were accepted by a server MUST be
   accepted by that server for the ACK. UACs creating an ACK message
   should duplicate all of the Authorization and Proxy-Authorization
   header fields that appeared in the INVITE to which the ACK
   corresponds. Servers MUST NOT attempt to challenge an ACK.

   Although the CANCEL method does take a response (a 2xx), servers MUST
   NOT attempt to challenge CANCEL requests since these requests cannot
   be resubmitted. Generally, a CANCEL request SHOULD be accepted by a
   server if it comes from the same host that sent the request being
   canceled (provided that some sort of transport or network layer
   security association, as described in Section 22.2.1, is in place).

   When a UAC receives a challenge, it SHOULD render to the user the
   contents of the "realm" parameter in the challenge (which appears in
   either a WWW-Authenticate header field or Proxy-Authenticate header
   field) if the UAC device does not already know of a credential for
   the realm in question. A service provider that pre-configures UAs
   with credentials for its realm should be aware that users will not
   have the opportunity to present their own credentials for this realm



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   when challenged at a pre-configured device.

   Finally, note that even if a UAC can locate credentials that are
   associated with the proper realm, the potential exists that these
   credentials may no longer be valid or that the challenging server
   will not accept these credentials for whatever reason (especially
   when "anonymous" with no password is submitted).  In this instance a
   server may repeat its challenge, or it may respond with a 403
   Forbidden. A UAC MUST NOT re-attempt requests with the credentials
   that have just been rejected (unless the request was rejected because
   of a stale nonce).

20.2 User-to-User Authentication

   When a UAS receives a request from a UAC, the UAS MAY authenticate
   the originator before the request is processed. If no credentials (in
   the Authorization header field) are provided in the request, the UAS
   can challenge the originator to provide credentials by rejecting the
   request with a 401 (Unauthorized) status code.

   The WWW-Authenticate response-header field MUST be included in 401
   (Unauthorized) response messages. The field value consists of at
   least one challenge that indicates the authentication scheme(s) and
   parameters applicable to the Request-URI. See [H14.47] for a
   definition of the syntax.

   An example of the WWW-Authenticate header field in a 401 challenge
   is:



            WWW-Authenticate: Digest
                    realm="biloxi.com",
                    qop="auth,auth-int",
                    nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
                    opaque="5ccc069c403ebaf9f0171e9517f40e41"



   When the originating UAC receives the 401 (Unauthorized), it SHOULD,
   if it is able, re-originate the request with the proper credentials.
   The UAC may require input from the originating user before
   proceeding.  Once authentication credentials have been supplied
   (either directly by the user, or discovered in an internal keyring),
   UAs SHOULD cache the credentials for a given value of the To header
   field and "realm" and attempt to re-use these values on the next
   request for that destination. UAs MAY cache credentials in any way
   they would like.



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   If no credentials for a realm can be located, UACs MAY attempt to
   retry the request with a username of "anonymous" and no password (a
   password of "").

   Once credentials have been located, any UA that wishes to
   authenticate itself with a UAS or registrar -- usually, but not
   necessarily, after receiving a 401 (Unauthorized) response -- MAY do
   so by including an Authorization header field with the request. The
   Authorization field value consists of credentials containing the
   authentication information of the UA for the realm of the resource
   being requested as well as parameters required in support of
   authentication and replay protection.

   An example of the Authorization header field is:



      Authorization: Digest username="bob",
              realm="biloxi.com",
              nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
              uri=sip:alice@atlanta.com,
              qop=auth,
              nc=00000001,
              cnonce="0a4f113b",
              response="6629fae49393a05397450978507c4ef1",
              opaque="5ccc069c403ebaf9f0171e9517f40e41"




   When a UAC resubmits a request with its credentials after receiving a
   401 (Unauthorized) or 407 (Proxy Authentication Required) response,
   it MUST increment the CSeq header field as it would normally when
   sending an updated request.

20.3 Proxy-to-User Authentication

   Similarly, when a UAC sends a request to a proxy server, the proxy
   server MAY authenticate the originator before the request is
   processed. If no credentials (in the Proxy-Authorization header
   field) are provided in the request, the UAS can challenge the
   originator to provide credentials by rejecting the request with a 407
   (Proxy Authentication Required) status code. The proxy MUST populate
   the 407 (Proxy Authentication Required) message with a Proxy-
   Authenticate header field applicable to the proxy for the requested
   resource.

   The use of Proxy-Authentication and Proxy-Authorization parallel that



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   described in [16], with one difference. Proxies MUST NOT add the
   Proxy-Authorization header field. 407 (Proxy Authentication Required)
   responses MUST be forwarded upstream toward the UAC following the
   procedures for any other response. It is the UAC's responsibility to
   add the Proxy-Authorization header field containing credentials for
   the realm of the proxy that has asked for authentication.


        If a proxy were to resubmit a request with a Proxy-
        Authorization header field, it would need to increment the
        CSeq in the new request. However, this would cause the UAC
        that submitted the original request to discard a response
        from the UAS, as the CSeq value would be different.

   When the originating UAC receives the 407 (Proxy Authentication
   Required) it SHOULD, if it is able, re-originate the request with the
   proper credentials. It should follow the same procedures for the
   display of the "realm" parameter that are given above for responding
   to 401.  If no credentials for a realm can be located, UACs MAY
   attempt to retry the request with a username of "anonymous" and no
   password (a password of "").  The UAC SHOULD also cache the
   credentials used in the re-originated request.

   The following rule is RECOMMENDED for proxy credential caching:

   If a UA receives a Proxy-Authenticate header field in a 401/407
   response to a request with a particular Call-ID, it should
   incorporate credentials for that realm in all subsequent requests
   that contain the same Call-ID. These credentials MUST NOT be cached
   across dialogs; however, if a UA is configured with the realm of its
   local outbound proxy, when one exists, then the UA MAY cache
   credentials for that realm across dialogs. Note that this does mean a
   future request in a dialog could contain credentials that are not
   needed by any proxy along the Route header path.

   Any UA that wishes to authenticate itself to a proxy server --
   usually, but not necessarily, after receiving a 407 (Proxy
   Authentication Required) response -- MAY do so by including a Proxy-
   Authorization header field with the request. The Proxy-Authorization
   request-header field allows the client to identify itself (or its
   user) to a proxy that requires authentication.  The Proxy-
   Authorization header field value consists of credentials containing
   the authentication information of the UA for the proxy and/or realm
   of the resource being requested.

   A Proxy-Authorization header field applies only to the proxy whose
   realm is identified in the "realm" parameter (this proxy may
   previously have demanded authentication using the Proxy-Authenticate



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   field). When multiple proxies are used in a chain, the Proxy-
   Authorization header field MUST NOT be consumed by any proxy whose
   realm does not match the "realm" parameter specified in the Proxy-
   Authorization header field.

   Note that if an authentication scheme that does not support realms is
   used in the Proxy-Authorization header field, a proxy server MUST
   attempt to parse all Proxy-Authorization header fields to determine
   whether one of them has what the proxy server considers to be valid
   credentials. Because this is potentially very time-consuming in large
   networks, proxy servers SHOULD use an authentication scheme that
   supports realms in the Proxy-Authorization header field.

   If a request is forked (as described in Section 16.6), various proxy
   servers and/or UAs may wish to challenge the UAC. In this case, the
   forking proxy server is responsible for aggregating these challenges
   into a single response. Each WWW-Authenticate and Proxy-Authenticate
   received in responses to the forked request MUST be placed into the
   single response that is sent by the forking proxy to the UA; the
   ordering of these header fields is not significant.


        When a proxy server issues a challenge in response to a
        request, it will not proxy the request until the UAC has
        provided valid credentials. A forking proxy may forward a
        request simultaneously to multiple proxy servers that
        require authentication, each of which in turn will not
        forward the request until the originating UAC has
        authenticated itself in their respective realm. If the UAC
        does not provide credentials for each challenge, then the
        proxy servers that issued the challenges will not forward
        requests to the UA where the destination user might be
        located, and therefore, the virtues of forking are largely
        lost.

   If at least one UAS responds to a forked request with a challenge,
   then a 401 (Unauthorized) MUST be sent as the aggregated response by
   the forking proxy to the UAC; otherwise, if only proxy servers
   respond, a 407 MUST be used.

   When resubmitting its request in response to a 401 (Unauthorized) or
   407 (Proxy Authentication Required) that contains multiple
   challenges, a UAC MAY include an Authorization for each WWW-
   Authenticate and Proxy-Authorization for each Proxy-Authenticate for
   which the UAC wishes to supply a credential. As noted above, multiple
   credentials in a request SHOULD be differentiated by the "realm"
   parameter.




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   It is possible for multiple challenges associated with the same realm
   to appear in the same 401 (Unauthorized) or 407 (Proxy Authentication
   Required). This can occur, for example, when multiple proxies within
   the same administrative domain, which use a common realm, are reached
   by a forking request.

   See [H14.34] for a definition of the syntax of Proxy-Authentication
   and Proxy-Authorization.

20.4 The Digest Authentication Scheme

   This section describes the modifications and clarifications required
   to apply the HTTP Digest authentication scheme to SIP. The SIP scheme
   usage is almost completely identical to that for HTTP [16].

   Since RFC 2543 is based on HTTP Digest as defined in RFC 2069 [27],
   SIP servers supporting RFC 2617 MUST ensure they are backwards
   compatible with RFC 2069. Procedures for this backwards compatibility
   are specified in RFC 2617.  Note, however, that servers MUST NOT
   accept or request Basic authentication.

20.4.1 HTTP Digest

   The rules for Digest authentication follow those defined in [16],
   with "HTTP 1.1" replaced by "SIP/2.0" in addition to the following
   differences:

        1.   The URI included in the challenge has the following BNF:


             URI  =  SIP-URI


        2.   The BNF in RFC 2617 has an error in that the 'uri'
             parameter of the Authorization header field for HTTP Digest
             authentication is not enclosed in quotation marks. (The
             example in Section 3.5 of RFC 2617 is correct.) For SIP,
             the 'uri' MUST be enclosed in quotation marks.

        3.   The BNF for digest-uri-value is:


             digest-uri-value  =  Request-URI ; as defined in
             Section 27


        4.   The example procedure for choosing a nonce based on Etag
             does not work for SIP.



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        5.   The text in RFC 2617 [16] regarding cache operation does
             not apply to SIP.

        6.   RFC 2617 [16] requires that a server check that the URI in
             the request line and the URI included in the Authorization
             header field point to the same resource. In a SIP context,
             these two URIs may refer to different users, due to
             forwarding at some proxy.  Therefore, in SIP, a server MAY
             check that the Request-URI in the Authorization header
             field corresponds to a user for whom that the server is
             willing to accept forwarded or direct requests.

        7.   As a clarification to the calculation of the A2 value for
             message integrity assurance in the Digest authentication
             scheme, implementers should assume, when the entity-body is
             empty (that is, when SIP messages have no body) that the
             hash of the entity-body resolves to the MD5 hash of an
             empty string, or:



             H(entity-body) = MD5("") = "d41d8cd98f00b204e9800998ecf8427e"


        8.   RFC 2617 notes that a cnonce value MUST NOT be sent in an
             Authorization (and by extension Proxy-Authorization) header
             field if no qop directive has been sent. Therefore, any
             algorithms that have a dependency on the cnonce (including
             "MD5-Sess") require that the qop directive be sent. Use of
             the "qop" parameter is optional in RFC 2617 for the
             purposes of backwards compatibility with RFC 2069; since
             RFC 2543 was based on RFC 2069, the "qop" parameter must
             unfortunately remain optional for clients and servers to
             receive.  However, servers MUST always send a "qop"
             parameter in WWW-Authenticate and Proxy-Authenticate header
             fields. If a client receives a "qop" parameter in a
             challenge header field, it MUST send the "qop" parameter in
             any resulting authorization header field.

   RFC 2543 did not allow usage of the Authentication-Info header field
   (it effectively used RFC 2069). However, we now allow usage of this
   header field, since it provides integrity checks over the bodies and
   provides mutual authentication. RFC 2617 [16] defines mechanisms for
   backwards compatibility using the qop attribute in the request. These
   mechanisms MUST be used by a server to determine if the client
   supports the new mechanisms in RFC 2617 that were not specified in
   RFC 2069.




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21 S/MIME

   SIP messages carry MIME bodies and the MIME standard includes
   mechanisms for securing MIME contents to ensure both integrity and
   confidentiality (including the 'multipart/signed' and
   'application/pkcs7-mime' MIME types, see RFC 1847 [7], RFC 2630 [17]
   and RFC 2633 [18]). Implementers should note, however, that there may
   be rare network intermediaries (not typical proxy servers) that rely
   on viewing or modifying the bodies of SIP messages (especially SDP),
   and that secure MIME may prevent these sorts of intermediaries from
   functioning.

        This applies particularly to certain types of firewalls.


        The PGP mechanism for encrypting the headers and bodies of
        SIP messages described in RFC 2543 has been deprecated.

21.1 S/MIME Certificates

   The certificates that are used to identify an end-user for the
   purposes of S/MIME differ from those used by servers in one important
   respect - rather than asserting that the identity of the holder
   corresponds to a particular hostname, these certificates assert that
   the holder is identified by an end-user address. This address is
   composed of the concatenation of the "userinfo" "@" and "domainname"
   portions of a SIP URI (in other words, an email address of the form
   "bob@biloxi.com"), most commonly corresponding to a user's address of
   record.

   These certificates are used to sign or encrypt bodies of SIP
   messages.  Bodies are signed with the private key of the sender (who
   may include their public key with the message as appropriate), but
   bodies are encrypted with the public key of the intended recipient.
   Obviously, senders must have foreknowledge of the public key of
   recipients in order to encrypt message bodies. Public keys can be
   stored within a UA on a virtual keyring.

   Each user agent that supports S/MIME MUST contain a keyring
   specifically for end-users' certificates. This keyring should map
   between addresses of record and corresponding certificates, including
   any associated with the owner or operator of the UA, when
   appropriate. Over time, users SHOULD use the same certificate when
   they populate the originating URI of signaling (the From header
   field) with the same address of record.

   Any mechanisms depending on the existence of end-user certificates,
   is seriously limitated in that there is virtually no consolidated



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   authority today that provides certificates for end-user applications.
   However, users SHOULD acquire certificates from known public
   certificate authorities. As an alternative, users MAY create self-
   signed certificates. The implications of self-signed certificates are
   explored further in Section 22.4.2.

   Above and beyond the problem of acquiring an end-user certificate,
   there are few well-known centralized directories that distribute
   end-user certificates. However, the holder of a certificate SHOULD
   publish their certificate in any public directories as appropriate.
   Similarly, UACs SHOULD support a mechanism for importing (manually or
   automatically) certificates discovered in public directories
   corresponding to the target URIs of SIP requests.

21.2 S/MIME Key Exchange

   SIP itself can also be used as a means to distribute public keys in
   the following manner.

   Whenever the CMS SignedData message is used in S/MIME for SIP, it
   MUST contain the certificate bearing the public key necessary to
   verify the signature.

   When a UAC sends a request containing an S/MIME body that initiates a
   dialog, or sends a non-INVITE request outside the context of a
   dialog, the UAC SHOULD structure the body as an S/MIME EnvelopedData,
   the UAC SHOULD send the EnvelopedData message encapsulated within a
   SignedData message.

   When a UAS receives a request containing an S/MIME CMS body that
   includes a certificate, the UAS SHOULD first verify the certificate,
   if possible, with any available certificate authority. The UAS SHOULD
   also determine the subject of the certificate and compare this value
   to the From field of the request. If the certificate cannot be
   verified, because it is self-signed, or signed by no known authority,
   the UAS MUST notify the user of the status of the certificate
   (including the subject of the certificate, its signer, and any key
   fingerprint information) and request explicit permission before
   proceeding. If the certificate was successfully verified and the
   subject of the certificate corresponds to the From header field of
   the SIP request, or if the user (after notification) explicitly
   authorizes the use of the certificate, the UAS SHOULD add this
   certificate to a local keyring, indexed by the address of record of
   the holder of the certificate.

   When a UAS sends a response containing an S/MIME body that answers
   the first request in a dialog, or a response to a non-INVITE request
   outside the context of a dialog, the UAS SHOULD structure the body as



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   a S/MIME 'multipart/signed' CMS SignedData body. If the desired CMS
   service is EnvelopedData, the UAS SHOULD send the EnvelopedData
   message encapsulated within a SignedData message. If the S/MIME body
   received by the UAS was encrypted with a public key recognized by the
   UAS, it MAY opt not to sign its response when appropriate.

   When a UAC receives a response containing an S/MIME CMS body which
   includes a certificate, the UAC SHOULD first verify the certificate,
   if possible, with any available certificate authority. The UAC SHOULD
   also determine the subject of the certificate and compare this value
   to the To field of the response; although the two may very well be
   different, and this is not necessarily indicative of a security
   breach. If the certificate cannot be verified because it is self-
   signed, or signed by no known authority, the UAC MUST notify the user
   of the status of the certificate (including the subject of the
   certificate, its signator, and any key fingerprint information) and
   request explicit permission before proceeding. If the certificate was
   successfully verified, and the subject of the certificate corresponds
   to the To header in the response, or if the user (after notification)
   explicitly authorizes the use of the certificate, the UAC SHOULD add
   this certificate to a local keyring, indexed by the address of record
   of the holder of the certificate. If the UAC had not transmitted its
   own certificate to the UAS in any previous transaction, it SHOULD use
   a CMS SignedData body for its next request or response.

   On future occasions, when the UA receives requests or responses that
   contain a From header field corresponding to a value in its keyring,
   the UA SHOULD compare the certificate offered in these messages with
   the existing certificate in its keyring. If there is a discrepancy,
   the UA MUST notify the user of a change of the certificate
   (preferably in terms that indicate that this is a potential security
   breach) and acquire the user's permission before continuing to
   process the signaling. If the user authorizes this certificate, it
   MUST be added to the keyring alongside any previous value(s) for this
   address of record.

   Note well however, that this key exchange mechanism does not
   guarantee the secure exchange of keys when self-signed certificates,
   or certificates signed by an obscure authority, are used - it is
   vulnerable to well-known attacks. In the opinion of the authors,
   however, the security it provides is proverbially better than
   nothing; it is in fact comparable to the widely used SSH application.
   These limitations are explored in greater detail in Section 22.4.2.

   If a UA receives an S/MIME body that has been encrypted with a public
   key unknown to the recipient, it MUST reject the request with a 493
   (Undecipherable) response. This response SHOULD contain a valid
   certificate for the respondent (corresponding, if possible, to any



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   address of record given in the To header of the rejected request)
   within a MIME body with a `certs-only' "smime-type" parameter.  A 493
   (Undecipherable) sent without any certificate indicates that the
   respondent cannot or will not utilize S/MIME encrypted messages,
   though they may still support S/MIME signatures

   Note that a user agent that receives a request containing an S/MIME
   body that is not optional (with a Content-Disposition header
   "handling" parameter of "required") MUST reject the request with a
   415 Unsupported Media Type response if the MIME type is not
   understood. A user agent that receives such a response when S/MIME is
   sent SHOULD notify its user that the remote device does not support
   S/MIME, and it MAY subsequently resend the request without S/MIME, if
   appropriate.

   If a user agent sends an S/MIME body in a request, but receives a
   response that contains a MIME body that is not secured, the user
   agent SHOULD notify the end user that the session could not be
   secured. However, if a user agent that supports S/MIME receives a
   request with an unsecured body, it SHOULD NOT respond with a secured
   body.

   Finally, if during the course of a dialog a UA receives a certificate
   in a CMS SignedData message that does not correspond with the
   certificates previously exchanged during a dialog, the UA MUST notify
   its user of the change, preferably in terms that indicate that this
   is a potential security breach.

21.3 Securing MIME bodies

   There are two types of secure MIME bodies that are of interest to
   SIP:  use of these bodies should follow the S/MIME specification
   ([18]) with a few variations.

        o UAs that support S/MIME MUST support the `signed-data' and
          `certs-only' "smime-types". UAs MAY support the `enveloped-
          data' "smime-type".

        o "multipart/signed" MUST be used only with CMS detached
          signatures.


             This allows backwards compatibility with non-S/MIME-
             compliant recipients.

        o S/MIME bodies SHOULD have a Content-Disposition header field,
          and the value of the "handling" parameter SHOULD be
          "required."



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        o If a UAC has no certificate on its keyring associated with the
          address of record to which it wants to send a request, it
          cannot send an encrypted 'application/pkcs7-mime' MIME
          message. UACs MAY send an initial request such as an OPTIONS
          message with a CMS detached signature in order to solicit the
          certificate of the remote side (the signature SHOULD be over a
          'message/sip' body of the type described in Section 21.4).

        o Senders of S/MIME bodies SHOULD use the 'SMIMECapabilities'
          (see Section 2.5.2 of [18]) attribute to express their
          capabilities and preferences for further communications. Note
          especially that senders MAY use the 'preferSignedData'
          capability to encourage receivers to respond with CMS
          SignedData messages (for example, when sending an OPTIONS
          request as described above).

        o S/MIME implementations MUST at a minimum support SHA1 as a
          digital signature algorithm, and 3DES as an encryption
          algorithm. All other signature and encryption algorithms MAY
          be supported.  Implementations can negotiate support for these
          algorithms with the

        o Each S/MIME body in a SIP message SHOULD be signed with only
          one certificate. If a UA receives a message with multiple
          signatures, the outermost signature should be treated as the
          single certificate for this body.

21.4 Tunneling SIP in MIME

   As a means of providing some degree of end-to-end authentication,
   integrity or confidentiality for SIP headers, S/MIME can encapsulate
   entire SIP messages within MIME bodies of type "message/sip" and then
   apply MIME security to these bodies in the same manner as typical SIP
   bodies. These encapsulated SIP requests and responses do not
   constitute a separate dialog or transaction, they are a copy of the
   "outer" message that is used to verify integrity or to supply
   additional information.

   If a UAS receives a request that contains a tunneled "message/sip"
   S/MIME body, it SHOULD include a tunneled "message/sip" body in the
   response with the same smime-type.

   Any traditional MIME bodies (such as SDP) SHOULD be attached to the
   `inner" message so that they can also benefit from S/MIME security.
   Note that "message/sip" bodies can be sent as a part of a MIME
   "multipart/mixed" body if any unsecured MIME types should also be
   transmitted in a request.




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21.4.1 Integrity and Confidentiality Properties of SIP Headers

   When the S/MIME integrity or confidentiality mechanisms are used,
   there may be discrepancies between the values in the "inner" message
   and values in the "outer" message. The rules for handling any such
   differences for all of the headers described in this document are
   given in this section.

21.4.1.1 Integrity

   Headers that can be legitimately modified by proxy servers are:
   Request-URI, Via, Record-Route, Route, Max-Forwards, and Proxy-
   Authorization. If these headers are not intact end-to-end,
   implementations SHOULD NOT consider this a breach of security.
   Changes to any other headers constitute an integrity violation; users
   MUST be notified of a discrepancy.

21.4.1.2 Confidentiality

   When messages are encrypted, headers may be included in the encrypted
   body that are not present in the "outer" message.

   Some headers must always have a plaintext version because they are
   required headers in requests and responses - these include: To, From,
   Call-ID, CSeq, Contact. While it is probably not useful to provide an
   encrypted alternative for the Call-ID, Cseq, or Contact, providing an
   alternative to the information in the "outer" To or From is
   permitted. Note that the values in an encrypted body are not used for
   the purposes of identifying transactions or dialogs - they are merely
   informational. If the From header in an encrypted body differs from
   the value in the "outer" message, the value within the encrypted body
   SHOULD be displayed to the user, but MUST NOT be used in the "outer"
   headers of any future messages.

   Primarily, a user agent will want to encrypt headers that have an
   end-to-end semantic, including: Subject, Reply-To, Organization,
   Accept, Accept-Encoding, Accept-Language, Alert-Info, Error-Info,
   Authentication-Info, Expires, In-Reply-To, Require, Supported,
   Unsupported, Retry-After, User-Agent, Server, and Warning. If any of
   these headers are present in an encrypted body, they should be used
   instead of any "outer" headers, whether this entails displaying the
   header field values to users or setting internal states in the UA.

   Since MIME bodies are attached to the "inner" message,
   implementations will usually encrypt MIME-specific headers,
   including: MIME-Version, Content-Type, Content-Length, Content-
   Language, Content-Encoding and Content-Disposition. The "outer"
   message will have the proper MIME headers for S/MIME bodies.  These



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   headers (and any MIME bodies they preface) should be treated as
   normal MIME headers and bodies received in a SIP message.

   It is not particularly useful to encrypt the following headers:
   Date, Min-Expires, RAck, RSeq, Timestamp, Authorization, Priority,
   and WWW-Authenticate. This category also includes those headers that
   can be changed by proxy servers (described in the preceding section).
   UAs SHOULD never include these in an "inner" message if they are not
   included in the "outer" message. UAs that receive any of these
   headers in an encrypted body SHOULD ignore the encrypted values.

   Note that extensions to SIP may define additional headers; the
   authors of these extensions should describe the integrity and
   confidentiality properties of such headers. If a SIP UA encounters an
   unknown header with an integrity violation, it MUST ignore the
   header.

21.4.2 Tunneling Integrity and Authentication

   Tunneling SIP messages within S/MIME bodies can provide integrity for
   SIP headers if the headers which the sender wishes to secure are
   replicated in a "message/sip" MIME body signed with a CMS detached
   signature.

   Provided that the "message/sip" body contains at least the
   fundamental dialog identifiers (To, From, Call-ID, CSeq), then a
   signed MIME body can provide limited authentication. At the very
   least, if the certificate used to sign the body is unknown to the
   recipient and cannot be verified, the signature can be used to
   ascertain that a later request in a dialog was transmitted by the
   same certificate-holder that initiated the dialog.  If the recipient
   of the signed MIME body has some stronger incentive to trust the
   certificate (they were able to verify it, acquire it from a trusted
   repository, or they have used it frequently) then the signature can
   be taken as a stronger assertion of the identity of the subject of
   the certificate.

   In order to eliminate possible confusions about the addition or
   subtraction of entire headers, senders SHOULD replicate all headers
   from the request within the signed body. Any message bodies that
   require integrity protection SHOULD be attached to the "inner"
   message.

   If an integrity violation in a message is detected by its recipient,
   the message MAY be rejected with a 403 (Forbidden) response if it is
   a request, or any existing dialog MAY be terminated. UAs SHOULD
   notify users of this circumstance and request explicit guidance on
   how to proceed.



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   The following is an example of the use of a tunneled "message/sip"
   body:


        INVITE sip:bob@biloxi.com SIP/2.0
        Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
        To: Bob <bob@biloxi.com>
        From: Alice <alice@atlanta.com>;tag=1928301774
        Call-ID: a84b4c76e66710
        CSeq: 314159 INVITE
        Max-Forwards: 70
        Contact: <sip:alice@pc33.atlanta.com>
        Content-Type: multipart/signed;
          protocol="application/pkcs7-signature";
          micalg=sha1; boundary=boundary42

        --boundary42
        Content-Type: message/sip

        INVITE sip:bob@biloxi.com SIP/2.0
        Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
        To: Bob <bob@biloxi.com>
        From: Alice <alice@atlanta.com>;tag=1928301774
        Call-ID: a84b4c76e66710
        CSeq: 314159 INVITE
        Max-Forwards: 70
        Contact: <sip:alice@pc33.atlanta.com>
        Content-Type: application/sdp
        Content-Length: 147

        v=0
        o=UserA 2890844526 2890844526 IN IP4 here.com
        s=Session SDP
        c=IN IP4 pc33.atlanta.com
        t=0 0
        m=audio 49172 RTP/AVP 0
        a=rtpmap:0 PCMU/8000

        --boundary42
        Content-Type: application/pkcs7-signature; name=smime.p7s
        Content-Transfer-Encoding: base64
        Content-Disposition: attachment; filename=smime.p7s;
           handling=required

        ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
        4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
        n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
        7GhIGfHfYT64VQbnj756



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        --boundary42-



21.4.3 Tunneling Encryption

   It may also be desirable to use this mechanism to encrypt a
   "message/sip" MIME body within a CMS EnvelopedData message S/MIME
   body, but in practice, most headers are of at least some use to the
   network; the general use of encryption with S/MIME is to secure
   message bodies like SDP rather than message headers. Some
   informational headers, such as the Subject or Organization could
   perhaps warrant end-to-end security. Headers defined by future SIP
   applications might also require obfuscation.

   Another possible application of encrypting headers is selective
   anonymity. A request could be constructed with a From header field
   that contains no personal information (for example,
   sip:anonymous@anonymizer.com). However, a second From header field
   containing the genuine address of record of the originator could be
   encrypted within a "message/sip" MIME body where it will only be
   visible to the endpoints of a dialog.

   In order to guarantee end-to-end integrity, encrypted "message/sip"
   MIME bodies SHOULD be signed by the sender.

   In the following example, the text boxed in asterisks ("*") is
   encrypted (note that this example is unsigned):


        INVITE sip:bob@biloxi.com SIP/2.0
        Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
        To: Bob <bob@biloxi.com>
        From: Alice <alice@atlanta.com>;tag=1928301774
        Call-ID: a84b4c76e66710
        CSeq: 314159 INVITE
        Max-Forwards: 70
        Contact: <sip:alice@pc33.atlanta.com>
        Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
             name=smime.p7m
        Content-Transfer-Encoding: base64
        Content-Disposition: attachment; filename=smime.p7m
           handling=required

      *******************************************************
      * Content-Type: application/sdp                       *
      *                                                     *
      * v=0                                                 *



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      * o=alice 53655765 2353687637 IN IP4 pc33.atlanta.com *
      * s=-                                                 *
      * t=0 0                                               *
      * c=IN IP4 pc33.atlanta.com                           *
      * m=audio 3456 RTP/AVP 0 1 3 99                       *
      * a=rtpmap:0 PCMU/8000                                *
      *******************************************************




22 Security Considerations

   SIP is not an easy protocol to secure. Its use of intermediaries, its
   multi-faceted trust relationships, its expected usage between
   elements with no trust at all, and its user-to-user operation make
   security far from trivial. Security solutions are needed that are
   deployable today, without extensive coordination, in a wide variety
   of environments and usages. In order to meet these diverse needs,
   several distinct mechanisms applicable to different aspects and
   usages of SIP will be required.

   Note that the security of SIP signaling itself has no bearing on the
   security of protocols used in concert with SIP such as RTP, or with
   the security implications of any specific bodies SIP might carry
   (although MIME security plays a substantial role in securing SIP).
   Any media associated with a session can be encrypted end-to-end
   independently of any associated SIP signaling. Media encryption is
   outside the scope of this document.

   The considerations that follow first examine a set of classic threat
   models which broadly identify the security needs of SIP.  The set of
   security services required to address these threats is then detailed,
   followed by an explanation of several security mechanisms that can be
   used to provide these services. Next, the requirements for
   implementers of SIP are enumerated, along with exemplary deployments
   in which these security mechanisms could be used to improve the
   security of SIP. Some notes on privacy conclude this section.

22.1 Attacks and Threat Models

   This section details some threats that should be common to most
   deployments of SIP. These threats have been chosen specifically to
   illustrate each of the security services that SIP requires.

   The following examples by no means provide an exhaustive list of the
   threats against SIP; rather, these are "classic" threats that
   demonstrate the need for particular security services which can



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   potentially prevent whole categories of threats.

   These attacks assume an environment in which attackers can
   potentially read any packet on the network - it is anticipated that
   SIP will frequently be used on the public Internet. Attackers on the
   network may be able to modify packets (perhaps at some compromised
   intermediary).  Attackers may wish to steal services, eavesdrop on
   communications, or disrupt sessions.

22.1.1 Registration Hijacking

   The SIP registration mechanism allows a user agent to identify itself
   to a registrar as a device at which a user (designated by an address
   of record) is located. A registrar assesses the identity asserted in
   the From header field of a REGISTER message to determine whether this
   request can modify the contact addresses associated with the address
   of record in the To header field. While these two fields are
   frequently the same, there are many valid deployments in which a
   third-party may register contacts on a user's behalf.

   The From header field of a SIP request, however, can be modified
   arbitrarily by the owner of a UA, and this opens the door to
   malicious registrations. An attacker that successfully impersonates a
   party authorized to change contacts associated with an address of
   record could, for example, de-register all existing contacts for a
   URI and then register their own device as the appropriate contact
   address, thereby directing all requests for the affected user to the
   attacker's device.

   This threat belongs to a family of threats that rely on the absence
   of cryptographic assurance of a request's originator. Any SIP UAS
   that represents a valuable service (a gateway that interworks SIP
   requests with traditional telephone calls, for example) might want to
   control access to its resources by authenticating requests that it
   receives.  Even end-user UAs, for example SIP phones, have an
   interest in ascertaining the identities of originators of requests.

   This threat demonstrates the need for security services that enable
   SIP entities to authenticate the originators of requests.

22.1.2 Impersonating a Server

   The domain to which a request is destined is generally specified in
   the Request-URI. UAs commonly contact a server in this domain
   directly in order to deliver a request. However, there is always a
   possibility that an attacker could impersonate the remote server, and
   that the UA's request could be intercepted by some other party.




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   For example, consider a case in which a redirect server at one
   domain, chicago.com, impersonates a redirect server at another
   domain, biloxi.com. A user agent sends a request to biloxi.com, but
   the redirect server at chicago.com answers with a forged response
   that has appropriate SIP headers for a response from biloxi.com. The
   forged contact addresses in the redirection response could direct the
   originating UA to inappropriate or insecure resources, or simply
   prevent requests for biloxi.com from succeeding.

   This family of threats has a vast membership, many of which are
   critical. As a converse to the registration hijacking threat,
   consider the case in which a registration sent to biloxi.com is
   intercepted by chicago.com, which replies to the intercepted
   registration with a forged 301 (Moved Permanently) response. This
   response might seem to come from biloxi.com yet designate chicago.com
   as the appropriate registrar. All future REGISTER requests from the
   originating UA would then go to chicago.com.

   Prevention of this threat requires a means by which UAs can
   authenticate the servers to whom they send requests.

22.1.3 Tampering with Message Bodies

   As a matter of course, SIP UAs route requests through trusted proxy
   servers. Regardless of how that trust is established (authentication
   of proxies is discussed elsewhere in this section), a UA may trust a
   proxy server to route a request, but not to inspect or possibly
   modify the bodies contained in that request.

   Consider a UA that is using SIP message bodies to communicate session
   encryption keys for a media session. Although it trusts the proxy
   server of the domain it is contacting to deliver signaling properly,
   it may not want the administrators of that domain to be capable of
   decrypting any subsequent media session. Worse yet, if the proxy
   server were actively malicious, it could modify the session key,
   either acting as a man-in-the-middle, or perhaps changing the
   security characteristics requested by the originating UA.

   This family of threats applies not only to session keys, but to most
   conceivable forms of content carried end-to-end in SIP. These might
   include MIME bodies that should be rendered to the user, SDP, or
   encapsulated telephony signals, among others. Attackers might attempt
   to modify SDP bodies, for example, in order to point RTP media
   streams to a wiretapping device in order to eavesdrop on subsequent
   voice communications.

   Also note that some header fields in SIP are meaningful end-to-end,
   for example, Subject. UAs might be protective of these headers as



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   well as bodies (a malicious intermediary changing the Subject header
   field might make an important request appear to be spam, for
   example). However, since many header fields are legitimately
   inspected or altered by proxy servers as a request is routed, not all
   headers should be secured end-to-end.

   For these reasons, the UA might want to secure SIP message bodies,
   and in some limited cases headers, end-to-end. The security services
   required for bodies include confidentiality, integrity, and
   authentication. These end-to-end services should be independent of
   the means used to secure interactions with intermediaries such as
   proxy servers.

22.1.4 Tearing Down Sessions

   Once a dialog has been established by initial messaging, subsequent
   requests can be sent that modify the state of the dialog and/or
   session.  It is critical that principals in a session can be certain
   that such requests are not forged by attackers.

   Consider a case in which a third-party attacker captures some initial
   messages in a dialog shared by two parties in order to learn the
   parameters of the session (To, From, and so forth) and then inserts a
   BYE request into the session. The attacker could opt to forge the
   request such that it seemed to come from either participant. Once the
   BYE is received by its target, the session will be torn down
   prematurely.

   Similar mid-session threats include the transmission of forged re-
   INVITEs that alter the session (possibly to reduce session security
   or redirect media streams as part of a wiretapping attack).

   The most effective countermeasure to this threat is the
   authentication of the sender of the BYE. In this instance, the
   recipient needs only know that the BYE came from the same party with
   whom the corresponding dialog was established (as opposed to
   ascertaining the absolute identity of the sender). Also, if the
   attacker is unable to learn the parameters of the session due to
   confidentiality, it would not be possible to forge the BYE. However,
   some intermediaries (like proxy servers) will need to inspect those
   parameters as the session is established.

22.1.5 Denial of Service and Amplification

   Denial-of-service attacks focus on rendering a particular network
   element unavailable, usually by directing an excessive amount of
   network traffic at its interfaces. A distributed denial-of-service
   attack allows one network user to cause multiple network hosts to



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   flood a target host with a large amount of network traffic.

   In many architectures, SIP proxy servers face the public Internet in
   order to accept requests from worldwide IP endpoints. SIP creates a
   number of potential opportunities for distributed denial-of-service
   attacks that must be recognized and addressed by the implementers and
   operators of SIP systems.

   Attackers can create bogus requests that contain a falsified source
   IP address and a corresponding Via header field that identify a
   targeted host as the originator of the request and then send this
   request to a large number of SIP network elements, thereby using
   hapless SIP UAs or proxies to generate denial-of-service traffic
   aimed at the target.

   Similarly, attackers might use falsified Route headers in a request
   that identify the target host and then send such messages to forking
   proxies that will amplify messaging sent to the target.  Record-Route
   could be used to similar effect when the attacker is certain that the
   SIP dialog initiated by the request will result in numerous
   transactions originating in the backwards direction.

   A number of denial-of-service attacks open up if REGISTER requests
   are not properly authenticated and authorized by registrars.
   Attackers could de-register some or all users in an administrative
   domain, thereby preventing these users from being invited to new
   sessions. An attacker could also register a large number of contacts
   designating the same host for a given address of record in order to
   use the registrar and any associated proxy servers as amplifiers in a
   denial-of-service attack.  Attackers might also attempt to deplete
   available memory and disk resources of a registrar by registering
   huge numbers of bindings.

   The use of multicast to transmit SIP requests can greatly increase
   the potential for denial-of-service attacks.

   These problems demonstrate a general need to define architectures
   that minimize the risks of denial-of-service, and the need to be
   mindful in recommendations for security mechanisms of this class of
   attacks.

22.2 Security Mechanisms

   From the threats described above, we gather that the fundamental
   security services required for the SIP protocol are: preserving the
   confidentiality and integrity of messaging, preventing replay attacks
   or message spoofing, providing for the authentication and privacy of
   the participants in a session, and preventing denial-of-service



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   attacks. Bodies within SIP messages separately require the security
   services of confidentiality, integrity, and authentication.

   Rather than defining new security mechanisms specific to SIP, SIP
   reuses wherever possible existing security models derived from the
   HTTP and SMTP space.

   Full encryption of messages provides the best means to preserve the
   confidentiality of signaling - it can also guarantee that messages
   are not modified by any malicious intermediaries. However, SIP
   requests and responses cannot be naively encrypted end-to-end in
   their entirety because message fields such as the Request-URI, Route,
   and Via need to be visible to proxies in most network architectures
   so that SIP requests are routed correctly. Note that proxy servers
   need to modify some features of messages as well (such as adding Via
   headers) in order for SIP to function. Proxy servers must therefore
   be trusted, to some degree, by SIP UAs. To this purpose, low-layer
   security mechanisms for SIP are recommended, which encrypt the entire
   SIP requests or responses on the wire on a hop-by-hop basis, and
   which allow endpoints to verify the identity of proxy servers to whom
   they send requests.

   SIP entities also have a need to identify one another in a secure
   fashion. When a SIP endpoint asserts the identity of its user to a
   peer UA or to a proxy server, that identity should in some way be
   verifiable. A cryptographic authentication mechanism is provided in
   SIP to address this requirement.

   An independent security mechanism for SIP message bodies supplies an
   alternative means of end-to-end mutual authentication, as well as
   providing a limit on the degree to which user agents must trust
   intermediaries.

22.2.1 Transport and Network Layer Security

   Transport or network layer security encrypts signaling traffic,
   guaranteeing message confidentiality and integrity.  Oftentimes,
   certificates are used in the establishment of lower-layer security,
   and these certificates can also be used to provide a means of
   authentication in many architectures.

   Two popular alternatives for providing security at the transport and
   network layer are, respectively, TLS [9] and IPSec [14].

   IPSec is a set of network-layer protocol tools that collectively can
   be used as a secure replacement for traditional IP (Internet
   Protocol).  IPSec is most commonly used in architectures in which a
   set of hosts or administrative domains have an existing trust



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   relationship with one another. IPSec is usually implemented at the
   operating system level in a host, or on a security gateway that
   provides confidentiality and integrity for all traffic it receives
   from a particular interface (as in a VPN architecture). IPSec can
   also be used on a hop-by-hop basis.

   In many architectures IPSec does not require integration with SIP
   applications; IPSec is perhaps best suited to deployments in which
   adding security directly to SIP hosts would be arduous. UAs which
   have a pre-shared keying relationship with their first-hop proxy
   server are also good candidates to use IPSec. Any deployment of IPSec
   for SIP would require an IPSec profile describing the protocol tools
   that would be required to secure SIP. No such profile is given in
   this document.

   TLS provides transport-layer security over connection-oriented
   protocols (for the purposes of this document, TCP); "tls" (signifying
   TLS over TCP) can be specified as the desired transport protocol
   within a Via header field or a SIP-URI. TLS is most suited to
   architectures in which hop-by-hop security is required between hosts
   with no pre-existing trust association.  For example, Alice trusts
   her local proxy server, which after a certificate exchange decides to
   trust Bob's local proxy server, which Bob trusts, hence Bob and Alice
   can communicate securely.

   TLS must be tightly coupled with a SIP application. Note that
   transport mechanisms are specified on a hop-by-hop basis in SIP, and
   that thus a UA that sends requests over TLS to a proxy server has no
   assurance that TLS will be used end-to-end.

   The TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite MUST be supported at a
   minimum by implementors when TLS is used in a SIP application. For
   purposes of backwards compatibility, proxy servers, redirect servers,
   and registrars SHOULD support TLS_RSA_WITH_3DES_EDE_CBC_SHA.
   Implementers MAY also support any other ciphersuite.

22.2.2 HTTP Authentication

   SIP provides a challenge capability, based on HTTP authentication,
   that relies on the 401 and 407 response codes as well as headers for
   carrying challenges and credentials. Without significant
   modification, the reuse of the HTTP Digest authentication scheme in
   SIP allows for replay protection and one-way authentication.

   The usage of Digest authentication in SIP is detailed in Section 20.

22.2.3 S/MIME




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   As is discussed above, encrypting entire SIP messages end-to-end for
   the purpose of confidentiality is not appropriate because network
   intermediaries (like proxy servers) need to view certain headers in
   order to route messages correctly, and if these intermediaries are
   excluded from security associations, then SIP messages will
   essentially be non-routable.

   However, S/MIME allows SIP UAs to encrypt MIME bodies within SIP,
   securing these bodies end-to-end without affecting message headers.
   S/MIME can provide end-to-end confidentiality and integrity for
   message bodies, as well as mutual authentication. It is also possible
   to use S/MIME to provide a form of integrity and confidentiality for
   SIP headers through SIP message tunneling.

   The usage of S/MIME in SIP is detailed in Section 21.

22.3 Implementing Security Mechanisms

22.3.1 Requirements for Implementers of SIP

   Proxy servers, redirect servers, and registrars MUST implement TLS,
   and MUST support both mutual and one-way authentication. It is
   strongly RECOMMENDED that UAs be capable initiating TLS; UAs MAY also
   be capable of acting as a TLS server. Proxy servers, redirect
   servers, and registrars SHOULD possess a site certificate whose
   subject corresponds to their hostname. UAs MAY have certificates of
   their own for mutual authentication with TLS, but no provisions are
   set forth in this document for their use. UAs MUST support a
   mechanism for verifying certificates they receive during TLS
   negotiation.

   Proxy servers, redirect servers, registrars, and UAs MAY also
   implement IPSec or other lower-layer security protocols.

   When a UA attempts to contact a proxy server, redirect server, or
   registrar, the UAC SHOULD initiate a TLS connection over which it
   will send SIP messages. In some architectures, UACs MAY receive
   requests over such TLS connections as well.

   Proxy servers, redirect servers, registrars, and UAs MUST implement
   Digest Authorization. Proxy servers, redirect servers, and registrars
   SHOULD be configured with at least one Digest realm, and at least one
   "realm" string supported by a given server SHOULD correspond to the
   server's hostname or domainname.

   Proxy servers, redirect servers, registrars, and UAs MAY also
   implement enhancements to Digest or alternate header-level security
   mechanisms.



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   UAs SHOULD support S/MIME encryption and signing of SIP message MIME
   bodies. If a UA holds one or more root certificates of certificate
   authorities in order to verify certificates for TLS or IPSec, it
   SHOULD be capable of reusing these to verify an S/MIME certificates,
   as appropriate. A UA MAY hold root certificates specifically for
   verifying S/MIME certifices.

22.3.2 Security Solutions

   The operation of these security mechanisms in concert can follow the
   existing web and email security models to some degree. At a high
   level, UAs authenticate themselves to servers (proxy servers,
   redirect servers, and registrars) with a Digest username and
   password; servers authenticate themselves to UAs, and to one another,
   with a site certificate delivered by TLS.

   On a peer-to-peer level, UAs transitively trust the network to
   authenticate one another ordinarily; however, S/MIME can also be used
   to provide direct authentication when the network does not, or if the
   network itself is not trusted.

   The following is an illustrative example in which these security
   mechanisms are used by various UAs and servers to prevent the sorts
   of threats described in Section 22. While implementers and network
   administrators MAY follow the normative guidelines given in the
   remainder of this section, these are provided only as example
   implementations.

22.3.2.1 Registration

   When a UA comes online and registers with its local administrative
   domain, it SHOULD establish a TLS connection with its registrar
   (Section 10 describes how the UA reaches its registrar).  The
   registrar SHOULD offer a certificate to the UA, and the site
   identified by the certificate MUST correspond with the domain in
   which the UA intends to register; for example, if the UA intends to
   register the address of record 'alice@atlanta.com', the site
   certificate must identify a host within the atlanta.com domain (such
   as UA SHOULD verify the certificate and inspect the site identified
   by the certificate. If the certificate is invalid, revoked, or if it
   does not identify the appropriate party, the UA MUST NOT send the
   REGISTER message and otherwise proceed with the registration.


        When a valid certificate has been provided by the
        registrar, the UA knows that the registrar is not an
        attacker who might redirect the UA, steal passwords, or
        attempt any similar attacks.



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   The UA then creates a REGISTER request that SHOULD be addressed to a
   Request-URI corresponding to the site certificate received from the
   registrar. When the UA sends the REGISTER request over the existing
   TLS connection, the registrar SHOULD challenge the request with a 407
   (Proxy Authentication Required) response. The "realm" parameter
   within the Proxy-Authenticate header field of the response SHOULD
   correspond to the domain previously given by the site certificate.
   When the UAC receives the challenge, it SHOULD either prompt the user
   for credentials or take an appropriate credential from a keyring
   corresponding to the "realm" parameter in the challenge. The username
   of this credential SHOULD correspond with the "userinfo" portion of
   the URI in the To header field of the REGISTER request. Once the
   Digest credentials have been inserted into an appropriate Proxy-
   Authorization header field, the REGISTER should be resubmitted to the
   registrar.


        Since the registrar requires the user agent to authenticate
        itself, it would be difficult for an attacker to forge
        REGISTER requests for the user's address of record. Also
        note that since the REGISTER is sent over a confidential
        TLS connection, attackers will not be able to intercept the
        REGISTER to record credentials for any possible replay
        attack.

   Once the registration has been accepted by the registrar, the UA
   SHOULD leave this TLS connection open provided that the registrar
   also acts as the proxy server to which requests are sent for users in
   this administrative domain. The existing TLS connection will be
   reused to deliver incoming requests to the UA that has just completed
   registration.


        Because the UA has already authenticated the server on the
        other side of the TLS connection, all requests that come
        over this connection are known to have passed through the
        proxy server - attackers cannot create spoofed requests
        that appear to have been sent through that proxy server.

22.3.2.2 Requests and Transitive Trust

   Now let's say that Alice's UA would like to initiate a session with a
   user in a remote administrative domain, namely 'bob@biloxi.com'.  We
   will also say that the local administrative domain ('atlanta.com')
   has a local outbound proxy.

   The proxy server that handles inbound requests for an administrative
   domain MAY also act as a local outbound proxy; for simplicity's sake



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   we'll assume this to be the case for 'atlanta.com' (otherwise the
   user agent would initiate a new TLS connection to a separate server
   at this point). Assuming that the client has completed the
   registration process described in the preceding section, it SHOULD
   reuse the TLS connection to the local proxy server when it sends an
   INVITE request to another user. The UA SHOULD reuse cached
   credentials in the INVITE to avoid prompting the user unnecessarily.

   When the local outbound proxy server has validated the credentials
   presented by the UA in the INVITE, it SHOULD inspect the Request-URI
   to determine how the message should be routed (see [2]). If the
   "domainname" portion of the Request-URI had corresponded to the local
   domain ('atlanta.com') rather than "biloxi.com", then the proxy
   server would have consulted its location service to determine how
   best to reach the requested user.


        Had 'alice@atlanta.com' been attempting to contact, say,
        the TLS connection Alex had established with the registrar
        when he registered. Since Alex would receive this request
        over his authenticated channel, he would be assured that
        Alice's request had been authorized by the proxy server of
        the local administrative domain.

   However, in this instance the Request-URI designates a remote domain.
   The local outbound proxy server at 'atlanta.com' SHOULD therefore
   establish a TLS connection with the remote proxy server at servers
   that possess site certificates, mutual TLS authentication SHOULD
   occur. Each side of the connection SHOULD verify and inspect the
   certificate of the other, noting the domain name that appears in the
   certificate for comparison with the headers of SIP messages. The
   'atlanta.com' proxy server, for example, SHOULD verify at this stage
   that the certificate received from the remote side corresponds with
   the 'biloxi.com' domain. Once it has done so, and TLS negotiation has
   completed, resulting in a secure channel between the two proxies, the
   'atlanta.com' proxy can forward the INVITE request to

   The proxy server at 'biloxi.com' SHOULD inspect the certificate of
   the proxy server at 'atlanta.com' in turn and compare the domain
   asserted by the certificate with the "domainname" portion of the From
   header field in the INVITE request. The biloxi proxy can thereby
   ascertain whether it should consider Alice to be authenticated
   transitively. The biloxi proxy MAY have a strict security policy that
   requires it to reject requests that do not match the administrative
   domain from which they have been proxied, or perhaps even more
   strictly, requests that originate from administrative domains that do
   not have some policy agreement with biloxi.




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        Such security policies could be instituted to prevent the
        SIP equivalent of SMTP 'open relays' which are frequently
        exploited to generate spam.

   Once the INVITE has been approved by the biloxi proxy, the proxy
   server SHOULD identify the existing TLS channel, if any, associated
   with the user targeted by this request (in this case
   'bob@biloxi.com').  The INVITE should be proxied through this channel
   to Bob. Since the request is received over a TLS connection that had
   previously been authenticated as the biloxi proxy, Bob transitively
   trusts the identity asserted in the From header.

   Before they forward the request, both proxy servers SHOULD add
   Record-Route header fields to the request so that all future requests
   in this dialog will pass through the proxy servers. The proxy servers
   can thereby continue to provide transitive authentication,
   confidentiality, replay protection, and so forth for lifetime of this
   dialog. If the proxy servers do not add themselves to the Record-
   Route, future messages will pass directly end-to-end between Alice
   and Bob without any security services (unless the two parties agree
   on some independent end-to-end security).


        An attacker preying on this architecture would, for
        example, be unable to forge a BYE request and insert it
        into the signaling stream between Bob and Alice because the
        attacker has no way of ascertaining the parameters of the
        session and also because the integrity mechanism
        transitively protects the traffic between Alice and Bob.

22.3.2.3 Peer to Peer Requests

   Alternatively, consider a UA asserting the identity to send an INVITE
   to 'bob@biloxi.com', her UA SHOULD initiate a TLS connection with the
   biloxi proxy directly (using the mechanism described in [2] to
   determine how to best to reach the given Request-URI). When her UA
   receives a certificate from the biloxi proxy, it SHOULD be verified
   normally before she passes her INVITE across the TLS connection.
   However, proxy, but she does have a CMS-detached signature over a
   "message/sip" body in the INVITE. It is unlikely in this instance
   that Carol would have any credentials in the 'biloxi.com' realm,
   since she has no formal association with biloxi.com. The biloxi proxy
   MAY also have a strict policy that precludes it from even bothering
   to challenge requests that do not have 'biloxi.com' in the
   "domainname" portion of the From header - it treats these users as
   unauthenticated.

   The biloxi proxy has a policy for Bob that all non-authenticated



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   requests should be redirected to the appropriate contact address
   registered against 'bob@biloxi.com', namely <sip:bob@192.0.2.4>.
   Carol receives the redirection response over the TLS connection she
   established with the biloxi proxy, so she trusts the veracity of the
   contact address.

   Carol SHOULD then establish a TCP connection with the designated
   address and send a new INVITE with a Request-URI containing the
   received contact address (recomputing the signature in the body as
   the request is readied). Bob receives this INVITE on an insecure
   interface, but his UA inspects and, in this instance, recognizes the
   From header field of the request and subsequently matches a locally
   cached certificate with the one presented in the signature of the
   body of the INVITE. He replies in similar fashion, authenticating
   himself to Carol, and a secure dialog begins.


        Sometimes firewalls or NATs in an administrative domain
        could preclude the establishment of a direct TCP connection
        to a UA.  In these cases, proxy servers could also
        potentially relay requests to UAs in a way that has no
        trust implications (for example, forgoing an existing TLS
        connection and forwarding the request over cleartext TCP)
        as local policy dictates.

22.3.2.4 DoS Protection

   In order to minimize the risk of a denial-of-service attack against
   architectures using these security solutions, implementers should
   take note of the following guidelines.

   When the host on which a SIP proxy server is operating is routable
   from the public Internet, it SHOULD be deployed in an administrative
   domain with secure routing policies (blocking source-routed traffic,
   preferably filtering ping traffic). Both TLS and IPSec can also make
   use of bastion hosts at the edges of administrative domains that
   participate in the security associations to aggregate secure tunnels
   and sockets. These bastion hosts can also take the brunt of denial-
   of-service attacks, ensuring that SIP hosts within the administrative
   domain are not encumbered with superfluous messaging.

   No matter what security solutions are deployed, floods of messages
   directed at proxy servers can lock up proxy server resources and
   prevent desirable traffic from reaching its destination. There is a
   computational expense associated with processing a SIP transaction at
   a proxy server, and that expense is greater for stateful proxy
   servers than it is for stateless proxy servers. Therefore, stateful
   proxies are more susceptible to flooding than stateless proxy



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   servers.

   UAs and proxy servers SHOULD challenge questionable requests with
   only a single 401 (Unauthorized) or 407 (Proxy Authentication
   Required), forgoing the normal response retransmission algorithm, and
   behaving statelessly towards unauthenticated requests.

        Retransmitting the 401 (Unauthorized) or 407 (Proxy
        Authentication Required) status response amplifies the
        problem of an attacker using a falsified header (such as
        Via) to direct traffic to a third party.

   With either TCP or UDP, a denial-of-service attack exists by a rogue
   proxy sending 6xx responses. Although a client SHOULD choose to
   ignore such responses if it requested authentication, a proxy cannot
   do so. It is obliged to forward the 6xx response back to the client.
   The client can then ignore the response, but if it repeats the
   request, it will probably reach the same rogue proxy again, and the
   process will repeat.

22.4 Limitations

   Although these security mechanisms, when applied in a judicious
   manner, can thwart many threats, there are limitations in the scope
   of the mechanisms that must be understood by implementers and network
   operators.

22.4.1 HTTP Digest

   One of the primary limitations of using HTTP Digest in SIP is that
   the integrity mechanisms in Digest do not work very well for SIP.
   Specifically, they offer protection of the Request-URI and the method
   of a message, but not for any of the headers that UAs would most
   likely wish to secure.

   The existing replay protection mechanisms described in RFC 2617 also
   have some limitations for SIP. The next-nonce mechanism, for example,
   does not support pipelined requests. The nonce-count mechanism should
   be used for replay protection.

   Another limitation of HTTP Digest is the scope of realms. Digest is
   valuable when a user wants to authenticate themselves to a resource
   with which they have a pre-existing association, like a service
   provider of which the user is a customer. Consider that, by contrast,
   the scope of TLS is global, since certificates are globally
   verifiable regardless of any pre-existing association between the UA
   and the server.




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   Future enhancements to HTTP Digest could conceivably resolve some or
   all of these limitations.

22.4.2 S/MIME

   The largest outstanding defect with the S/MIME mechanism is the lack
   of prevalent public key infrastructure for end users. If self-signed
   certificates (or certificates that cannot be verified by one of the
   participants in a dialog) are used, the SIP-based key exchange
   mechanism described in Section 21.2 is susceptible to a man-in-the-
   middle attack with which an attacker can potentially inspect and
   modify S/MIME bodies. The attacker needs to intercept the first
   exchange of keys between the two parties in a dialog, remove the
   existing CMS-detached signatures from the request and response, and
   insert a different CMS-detached signature containing a certificate
   supplied by the attacker (but which seems to be a certificate for the
   proper address of record). Each party will think they have exchanged
   keys with the other, when in fact each has the public key of the
   attacker.

   It is important to note that the attacker can only leverage this
   vulnerability on the first exchange of keys between two parties - on
   subsequent occasions, the alteration of the key would be noticeable
   to the UAs. It would also be difficult for the attacker to remain in
   the path of all future dialogs between the two parties over time (as
   potentially days, weeks, or years pass).

   SSH is susceptible to the same man-in-the-middle attack on the first
   exchange of keys; however, it is widely acknowledged that while SSH
   is not perfect, it does improve the security of connections. The use
   of key fingerprints could provide some assistance to SIP, just as it
   does for SSH. For example, if two parties use SIP to establish a
   voice communications session, each could read off the fingerprint of
   the key they received from the other, which could be compared against
   the original. It would certainly be more difficult for the man-in-
   the-middle to emulate the voices of the participants than their
   signaling.

   The S/MIME mechanism allows UAs to send encrypted requests without
   preamble if they possess a certificate for the destination address of
   record on their keyring. However, it is also possible that a device
   that does not hold certificates, or at least not that particular
   certificate, will be currently registered as the sole contact address
   for that address of record, and it will therefore be unable to
   process the encrypted request properly, which could lead to some
   avoidable error signaling. This is especially likely when an
   encrypted request is forked.




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   The keys associated with S/MIME are most useful when associated with
   a particular user (an address of record) rather than a device (a UA).
   When users move between devices, it may be difficult to transport
   private keys securely between UAs; how such keys might be acquired by
   a device is outside the scope of this document.

   Another, more prosaic difficulty with the S/MIME mechanism is that it
   can result in very large messages, especially when the SIP tunneling
   mechanism described in Section 21.4 is used.  For that reason, it is
   RECOMMENDED that TCP should be used as a transport protocol when
   S/MIME tunneling is employed.

22.4.3 TLS

   The most commonly voiced concern about TLS is that it cannot run over
   UDP; TLS requires a connection-oriented underlying transport
   protocol, which for the purposes of this document means TCP. Even
   running TCP, regardless of any additional overhead incurred by TLS,
   is argued to be too intensive for some embedded devices.

   It may also be arduous for a local outbound proxy server and/or
   registrar to maintain many simultaneous long-lived TLS connections
   with numerous UAs. This introduces some valid scalability concerns,
   especially for intensive ciphersuites. Maintaining redundancy of
   long-lived TLS connections, especially when a UA is solely
   responsible for their establishment, could also be cumbersome.

   TLS only allows SIP entities to authenticate servers to which they
   are adjacent; TLS offers strictly hop-by-hop security. Neither TLS,
   nor any other mechanism specified in this document, allows clients to
   authenticate proxy servers to whom they cannot form a direct TCP
   connection.

   Note, however, when any lower-layer network security is employed the
   originator and recipient of a session may be deducible by observers
   performing a network traffic analysis.

22.5 Privacy

   SIP messages frequently contain sensitive information about their
   senders - not just what they have to say, but with whom they
   communicate, when they communicate and for how long, and from where
   they participate in sessions. Many applications and their users
   require that this sort of private information be hidden from any
   parties that do not need to know it.

   Note that there are also less direct ways in which private
   information can be divulged. If a user or service chooses to be



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   reachable at an address that is guessable from the person's name and
   organizational affiliation (which describes most addresses of
   record), the traditional method of ensuring privacy by having an
   unlisted "phone number" is compromised. A user location service can
   infringe on the privacy of the recipient of a session invitation by
   divulging their specific whereabouts to the caller; an implementation
   consequently SHOULD be able to restrict, on a per-user basis, what
   kind of location and availability information is given out to certain
   classes of callers.

23 Common Message Components

   There are certain components of SIP messages that appear in various
   places within SIP messages (and sometimes, outside of them) that
   merit separate discussion.

23.1 SIP Uniform Resource Indicators

   A SIP URI identifies a communications resource. Like all URIs, SIP
   URIs may be placed in web pages, email messages, or printed
   literature. They contain sufficient information to initiate and
   maintain a communication session with the resource.

   Examples of communications resources include the following:

        o a user of an online service

        o an appearance on a multi-line phone

        o a mailbox on a messaging system

        o a PSTN number at a gateway service

        o a group (such as "sales" or "helpdesk") in an organization

23.1.1 SIP URI Components

   The "sip:" scheme follows the guidelines in RFC 2396 [13].  It uses a
   form similar to the mailto URL, allowing the specification of SIP
   request-header fields and the SIP message-body. This makes it
   possible to specify the subject, media type, or urgency of sessions
   initiated by using a URI on a web page or in an email message. The
   formal syntax for a SIP URI is presented in Section 27. Its general
   form is
            sip:user:password@host:port;url-parameters?headers
   have the following meanings:

        user: The identifier of a particular resource at the host being



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             addressed. The term "host" in this context frequently
             refers to a domain. The "userpart" of a URI consists of
             this user field, the password field, and the @ sign
             following them.  The userpart of a URI is optional and MAY
             be absent when the destination host does not have a notion
             of users or when the host itself is the resource being
             identified. If the @ sign is present in a SIP URI, the user
             field MUST NOT be empty.

             If the host being addressed can process telephone numbers,
             for instance, an Internet telephony gateway, a telephone-
             subscriber field defined in RFC 2806 [19] MAY be used to
             populate the user field. There are special escaping rules
             for encoding telephone-subscriber fields in SIP URIs
             described in Section 23.1.2.

        password: A password associated with the user.  While the SIP
             URI syntax allows this field to be present, its use is NOT
             RECOMMENDED, because the passing of authentication
             information in clear text (such as URIs) has proven to be a
             security risk in almost every case where it has been used.
             For instance, transporting a PIN number in this field
             exposes the PIN.

             Note that the password field is just an extension of user
             portion. Implementations not wishing to give special
             significance to the password portion of the field MAY
             simply treat "user:password" as a single string.

        host: The entity hosting the SIP resource. The host part
             contains either a fully-qualified domain name or numeric
             IPv4 or IPv6 address. Using the fully-qualified domain name
             form is RECOMMENDED whenever possible.

        port: The port number where the request is to be sent.

        URI parameters: Parameters affecting a request constructed from
             the URI.

             URI parameters are added after the hostport component and
             are separated by semi-colons.

             URI parameters take the form:
                         parameter-name "=" parameter-value
             Even though an arbitrary number of URI parameters may be
             included in a URI, any given parameter-name MUST NOT appear
             more than once.




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             This extensible mechanism includes the transport, maddr,
             ttl, user, method and lr parameters.

             The transport parameter determines the transport mechanism
             to be used for sending SIP messages, as specified in [2].
             SIP can use any network transport protocol.  Parameter
             names are defined for UDP [23], TCP [22], TLS [9] (note
             that this is specifically TLS over TCP), and SCTP [21].

             The maddr parameter indicates the server address to be
             contacted for this user, overriding any address derived
             from the host field. When an maddr parameter is present,
             the port and transport components of the URI apply to the
             address indicated in the maddr parameter value. [2]
             describes the proper interpretation of the transport,
             maddr, and hostport in order to obtain the destination
             address, port, and transport for sending a request.


             The maddr field has been used as a simple form of
             loose source routing. It allows a URI to specify a
             proxy that must be traversed en-route to the
             destination. Continuing to use the maddr parameter
             this way is strongly discouraged (the mechanisms that
             enable it are deprecated). Implementations should
             instead use the Route mechanism described in this
             document, establishing a pre-existing route set if
             necessary (see item 8.1.1.1 in section 8.1.1). This
             provides a full URI to describe the node to be
             traversed.

             The ttl parameter determines the time-to-live value of the
             UDP multicast packet and MUST only be used if maddr is a
             multicast address and the transport protocol is UDP. For
             example, to specify to call alice@atlanta.com using
             multicast to 239.255.255.1 with a ttl of 15, the following
             URI would be used:



               sip:alice@atlanta.com;maddr=239.255.255.1;ttl=15



             The set of valid telephone-subscriber strings is a subset
             of valid user strings. The user URI parameter exists to
             distinguish telephone numbers from user names that happen
             to look like telephone numbers.  If the user string



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             contains a telephone number formatted as a telephone-
             subscriber, the user parameter value "phone" SHOULD be
             present. Even without this parameter, recipients of SIP
             URIs MAY interpret the pre-@ part as a telephone number if
             local restrictions on the name space for user name allow
             it.

             The method of the SIP request constructed from the URI can
             be specified with the method parameter.

             The lr parameter, when present, indicates that the element
             responsible for this resource implements the routing
             mechanisms specified in this document. This parameter will
             be used in the URIs proxies place into Record-Route header
             field values, and may appear in the URIs in a pre-existing
             route set.

             This parameter is used to achieve backwards
             compatibility with systems implementing the strict-
             routing mechanisms of RFC2543 and the rfc2543bis
             drafts up to bis-05. An element preparing to send a
             request based on a URI not containing this parameter
             can assume the receiving element implements strict-
             routing and reformat the message to preserve the
             information in the Request-URI.

             Since the url-parameter mechanism is extensible, SIP
             elements MUST silently ignore any url-parameters that they
             do not understand.

        Headers: Headers to be included in a request constructed from
             the URI. Headers fields in the SIP request can be specified
             with the "?" mechanism within a SIP URI. The header names
             and values are encoded in ampersand separated hname =
             hvalue pairs. The special hname "body" indicates that the
             associated hvalue is the message-body of the SIP request.

   Table 1 summarizes the use of SIP URI components based on the context
   in which the URI appears. The external column describes URIs
   appearing anywhere outside of a SIP message, for instance on a web
   page or business card. Entries marked "m" are mandatory, those marked
   "o" are optional, and those marked "-" are not allowed. Elements
   processing URIs SHOULD ignore any disallowed components if they are
   present. The second column indicates the default value of an optional
   element if it is not present. "--" indicates that the element is
   either not optional, or has no default value.

   SIP URIs in Contact header fields have different restrictions



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   depending on the context in which the header field appears. One set
   applies to messages that establish and maintain dialogs (INVITE and
   its 200 (OK) response). The other applies to registration and
   redirection messages (REGISTER, its 200 (OK) response, and 3xx class
   responses to any method).


                                                             dialog
                                               reg./redir.  Contact/
                  default  Req.-URI  To  From    Contact    R-R/Route  external
   user           --          o      o    o         o           o         o
   password       --          o      o    o         o           o         o
   host           --          m      m    m         m           m         m
   port           5060        o      -    -         o           o         o
   user-param     ip          o      o    o         o           o         o
   method         INVITE      -      -    -         -           -         o
   maddr-param    --          o      -    -         o           o         o
   ttl-param      1           o      -    -         o           -         o
   transp.-param  udp         o      -    -         o           o         o
   lr-param       --          o      -    -         -           o         o
   other-param    --          o      o    o         o           o         o
   headers        --          -      -    -         o           -         o


   Table 1: Use and default values of URI components  for  SIP  headers,
   Request-URI and references


23.1.2 Character Escaping Requirements

   SIP follows the requirements and guidelines of RFC 2396 [13] when
   defining the set of characters that must be escaped in a SIP URI, and
   uses its ""%" HEX HEX" mechanism for escaping. From RFC 2396:


        The set of characters actually reserved within any given
        URI component is defined by that component. In general, a
        character is reserved if the semantics of the URI changes
        if the character is replaced with its escaped US-ASCII
        encoding. [13].  Excluded US-ASCII characters [13], such as
        space and control characters and characters used as URI
        delimiters, also MUST be escaped. URIs MUST NOT contain
        unescaped space and control characters.

   For each component, the set of valid BNF expansions defines exactly
   which characters may appear unescaped. All other characters MUST be
   escaped.




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   For example, "@" is not in the set of characters in the user
   component, so the user "j@s0n" must have at least the @ sign encoded,
   as in "j%40s0n".

   Expanding the hname and hvalue tokens in Section 27 show that all URI
   reserved characters in header names and values MUST be escaped.

   The telephone-subscriber subset of the user component has special
   escaping considerations. The set of characters not reserved in the
   RFC 2806 [19] description of telephone-subscriber contains a number
   of characters in various syntax elements that need to be escaped when
   used in SIP URIs. Any characters occurring in a telephone-subscriber
   that do not appear in an expansion of the BNF for the user rule MUST
   be escaped.

   Note that character escaping is not allowed in the host component of
   a SIP URI (the % character is not valid in its expansion). This is
   likely to change in the future as requirements for Internationalized
   Domain Names are finalized. Current implementations MUST NOT attempt
   to improve robustness by treating received escaped characters in the
   host component as literally equivalent to their unescaped
   counterpart.  The behavior required to meet the requirements of IDN
   may be significantly different.

23.1.3 Example SIP URIs



     sip:alice@atlanta.com
     sip:alice:secretword@atlanta.com;transport=tcp
     sip:alice@atlanta.com?subject=project
     sip:+1-212-555-1212:1234@gateway.com;user=phone
     sip:1212@gateway.com
     sip:alice@192.0.2.4
     sip:atlanta.com;method=REGISTER?to=alice
     sip:alice;day=tuesday@atlanta.com



   The last example URI above has a user field value of
   "alice;day=tuesday". The escaping rules defined above allow a
   semicolon to appear unescaped in this field. Note, however, that for
   the purposes of this protocol, the field is opaque. The apparent
   structure in that value is only useful to the entity responsible for
   the resource.

23.1.4 SIP URI Comparison




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   SIP URIs are compared for equality according to the following rules:

        o Comparison of the userpart of sip URIs is case-sensitive. This
          includes userparts containing passwords or formatted as
          telephone-subscribers. Comparison of all other components of
          the URI is case-insensitive unless explicitly defined
          otherwise.

        o The ordering of parameters and headers is not significant in
          comparing SIP URIs.

        o Characters other than those in the "reserved" and "unsafe"
          sets (see RFC 2396 [13]) are equivalent to their ""%" HEX HEX"
          encoding.

        o An IP address that is the result of a DNS lookup of a host
          name does not match that host name.

        o For two URIs to be equal, the user, password, host, and port
          components must match. A URI omitting the optional port
          component will match a URI explicitly declaring port 5060. A
          URI omitting the user component will not match a URI that
          includes one. A URI omitting the password component will not
          match a URI that includes one.

        o URI uri-parameter components are compared as follows

          - Any uri-parameter appearing in both URIs must match.

          - A user, transport, ttl, or method url-parameter appearing in
            only one URI must contain its default value or the URIs do
            not match.

            A URI that includes an maddr parameter will not match a URI
            that contains no maddr parameter.

          - All other url-parameters appearing in only one URI are
            ignored when comparing the URIs.

        o URI header components are never ignored. Any present header
          component MUST be present in both URIs and match for the URIs
          to match. The matching rules are defined for each header in
          Section sec:header-fields.

   The URIs within each of the following sets are equivalent:






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   sip:%61lice@atlanta.com:5060
   sip:alice@AtLanTa.CoM;Transport=udp





   sip:carol@chicago.com
   sip:carol@chicago.com;newparam=5
   sip:carol@chicago.com;security=on





   sip:biloxi.com;transport=tcp;method=REGISTER?to=sip:bob
   sip:biloxi.com;method=REGISTER;transport=tcp?to=sip:bob





   sip:alice@atlanta.com?subject=project
   sip:alice@atlanta.com?priority=urgent&subject=project



   The URIs within each of the following sets are not equivalent:



   SIP:ALICE@AtLanTa.CoM;Transport=udp               (different usernames)
   sip:alice@AtLanTa.CoM;Transport=UDP





   sip:bob@biloxi.com                       (different port and transport)
   sip:bob@biloxi.com:6000;transport=tcp





   sip:carol@chicago.com                      (different header component)
   sip:carol@chicago.com?Subject=next




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   sip:bob@phone21.boxesbybob.com     (even though that's what
   sip:bob@192.0.2.4                    phone21.boxesbybob.com resolves to)



   Note that equality is not transitive:

        o sip:carol@chicago.com and sip:carol@chicago.com;security=on
          are equivalent

        o sip:carol@chicago.com and sip:carol@chicago.com;security=off
          are equivalent

        o sip:carol@chicago.com;security=on and
          sip:carol@chicago.com;security=off are not equivalent

   Comparing URIs is a major part of comparing several SIP headers (see
   Section 24).

23.1.5 Forming Requests from a SIP URI

   An implementation must take care when forming requests directly from
   a URI. URIs from business cards, web pages, and even from sources
   inside the protocol such as registered contacts may contain
   inappropriate header fields or body parts.

   An implementation MUST include any provided transport, maddr, ttl, or
   user parameter in the Request-URI of the formed request. If the URI
   contains a method parameter, its value MUST be used as the method of
   the request. The method parameter MUST NOT be placed in the Request-
   URI. Unknown URI parameters MUST be placed in the message's Request-
   URI.

   An implementation SHOULD treat the presence of any headers or body
   parts in the URI as a request to include them in the message, and
   choose to honor the request on an per-component basis.

   An implementation SHOULD NOT honor these obviously dangerous header
   fields: From, Call-ID, CSeq, Via, and Record-Route.

   An implementation SHOULD honor any requested Route header field
   values in order to not be used as an unwitting agent in malicious
   attacks.

   An implementation SHOULD NOT honor requests to include headers that
   may cause it to falsely advertise its location or capabilities. These
   include: Accept, Accept-Encoding, Accept-Language, Allow, Contact (in
   its dialog usage), Organization, Supported, and User-Agent.



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   An implementation SHOULD verify the accuracy of any requested
   descriptive headers, including: Content-Disposition, Content-
   Encoding, Content-Language, Content-Length, Content-Type, Date,
   Mime-Version, and Timestamp.

   If the request formed from constructing a message from a given URI is
   not a valid SIP request, the URI is invalid. An implementation MUST
   NOT proceed with transmitting the request. It should instead pursue
   the course of action due an invalid URI in the context it occurs.


        The constructed request can be invalid in many ways. These
        include, but are not limited to, syntax error in header
        fields, invalid combinations of URI parameters, or an
        incorrect description of the message body.

   Sending a request formed from a given URI may require capabilities
   unavailable to the implementation. The URI might indicate use of an
   unimplemented transport or extension, for example. An implementation
   SHOULD refuse to send these requests rather than modifying them to
   match their capabilities. An implementation MUST NOT send a request
   requiring an extension that it does not support.


        For example, such a request can be formed through the
        presence of a headerRequire header parameter or a method
        URI parameter with an unknown or explicitly unsupported
        value.

23.1.6 Relating SIP URIs and tel URLs

   When a tel URL [19] is converted to a SIP URI, the entire telephone-
   subscriber portion of the tel URL, including any parameters, is
   placed into the userpart of the SIP URI.

   Thus, tel:+358-555-1234567;postd=pp22 becomes


     sip:+358-555-1234567;postd=pp22@foo.com


   not


     sip:+358-555-1234567@foo.com;postd=pp22






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   In general, equivalent "tel" URLs converted to SIP URIs in this
   fashion may not produce equivalent SIP URIs. The userpart of SIP URIs
   is compared as a case-sensitive string. Variance in case-insensitive
   portions of tel URLs and reordering of tel URL parameters does not
   affect tel URL equivalence, but does affect the equivalence of SIP
   URIs formed from them.

   For example,


     tel:+358-555-1234567;postd=pp22
     tel:+358-555-1234567;POSTD=PP22


   are equivalent, while


     sip:+358-555-1234567;postd=pp22@foo.com
     sip:+358-555-1234567;POSTD=PP22@foo.com


   are not.

   Likewise,


     tel:+358-555-1234567;postd=pp22;isub=1411
     tel:+358-555-1234567;isub=1411;postd=pp22


   are equivalent, while


     sip:+358-555-1234567;postd=pp22;isub=1411@foo.com
     sip:+358-555-1234567;isub=1411;postd=pp22@foo.com


   are not.

   To mitigate this problem, elements constructing telephone-subscriber
   fields to place in the userpart of a SIP URI SHOULD fold any case-
   insensitive portion of telephone-subscriber to lower case, and order
   the telephone-subscriber parameters lexically by parameter name.
   (All components of a tel URL except for future-extension parameters
   are defined to be compared case-insensitive.)

   Following this suggestion, both




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     tel:+358-555-1234567;postd=pp22
     tel:+358-555-1234567;POSTD=PP22


   become


     sip:+358-555-1234567;postd=pp22@foo.com


   and both


     tel:+358-555-1234567;postd=pp22;isub=1411
     tel:+358-555-1234567;isub=1411;postd=pp22


   become


     sip:+358-555-1234567;isub=1411;postd=pp22



23.2 Option Tags

   Option tags are unique identifiers used to designate new options
   (extensions) in SIP. These tags are used in Require (Section 24.33),
   Proxy-Require (Section 24.29, Supported (Section 24.39) and
   Unsupported (Section 24.42) header fields. Note that these options
   appear as parameters in those headers in an  option-tag = token  form
   (see Section 27 for the definition of token).

   The creator of a new SIP option MUST either prefix the option with
   their reverse domain name or register the new option with the
   Internet Assigned Numbers Authority (IANA) (See Section 28).

   An example of a reverse-domain-name option is "com.foo.mynewfeature",
   whose inventor can be reached at "foo.com". For these features,
   individual organizations are responsible for ensuring that option
   names do not collide within the same domain. The host name part of
   the option MUST use lower-case; the option name is case-insensitive.

   Options registered with IANA do not contain periods and are globally
   unique. IANA option tags are case-insensitive.

23.3 Tags




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   The "tag" parameter is used in the To and From fields of SIP
   messages. It serves as a general mechanism to identify a particular
   instance of a user agent for a particular SIP URI.

   As proxies can fork requests, the same request can reach multiple
   instances of a user (mobile and home phones, for example). Since each
   can respond, there needs to be a means for the originator of a
   session to distinguish the responses. Tag fields in the To and From
   disambiguate these multiple instances of the same user.

   This situation also arises with multicast requests.

   When a tag is generated by a UA for insertion into a request or
   response, it MUST be globally unique and cryptographically random
   with at least 32 bits of randomness. A property of this selection
   requirement is that a UA will place a different tag into the From
   header of an INVITE as it would place into the To header of the
   response to the same INVITE. This is needed in order for a UA to
   invite itself to a session, a common case for "hairpinning" of calls
   in PSTN gateways.  Similarly, two INVITEs for different calls will
   have different From tags.

   Besides the requirement for global uniqueness, the algorithm for
   generating a tag is implementation specific. Tags are helpful in
   fault tolerant systems, where a dialog is to be recovered on an
   alternate server after a failure. A UAS can select the tag in such a
   way that a backup can recognize a request as part of a dialog on the
   failed server, and therefore determine that it should attempt to
   recover the dialog and any other state associated with it.

24 Header Fields

   The general syntax for header fields is covered in Section 7.3. This
   section lists the full set of header fields along with notes on
   syntax, meaning, and usage.  Throughout this section, we use [HX.Y]
   to refer to Section X.Y of the current HTTP/1.1 specification RFC
   2616 [15]. Examples of each header field are given.

   Information about header fields in relation to methods and proxy
   processing is summarized in Tables 2 and 3.

   The "where" column describes the request and response types in which
   the header field can be used. Values in this column are:

        R: header fields may only appear in requests;

        r: header field may only appear in responses;




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        2xx, 4xx, etc.: A numerical value or range indicates response
             codes with which the header field can be used;

        c: header field is copied from the request to the response.

        An empty entry in the "where" column indicates that the header
             may be present in all requests and responses.

   The "proxy" column describes the operations a proxy may perform on a
   header:

        c: A proxy can add (concatenate) comma-separated elements to the
             header.

        m: A proxy can modify the header.

        a: A proxy can add the header if not present.

        r: A proxy must be be able to read the header and thus this
             header cannot be encrypted.

   The next six columns relate to the presence of a header field in a
   method:

        o: The header field is optional.

        m: The header field is mandatory.

        m*: The header field SHOULD be sent, but servers need to be
             prepared to receive messages without that header field.

        t: The header field SHOULD be sent, but servers need to be
             prepared to receive messages without that header field. If
             TCP is used as transport, then the header field MUST be
             sent.

        *: The header field is required if the message body is not
             empty. See sections 24.14, 24.15 and 7.4 for details.

        -: The header field is ignored.

        c: Conditional; the header field is either mandatory or
             optional, depending on the presence of a route set or the
             response code.

   "Optional" means that a UA MAY include the header field in a request
   or response, and a UA MAY ignore the header field if present in the
   request or response (The exception to this rule is the Require header



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   field discussed in 24.33). A "mandatory" header field MUST be present
   in a request, and MUST be understood by the UAS receiving the
   request. A mandatory response header field MUST be present in the
   response, and the header field MUST be understood by the UAC
   processing the response. "Not applicable" means that the header field
   MUST NOT be present in a request. If one is placed in a request by
   mistake, it MUST be ignored by the UAS receiving the request.
   Similarly, a header field labeled "not applicable" for a response
   means that the UAS MUST NOT place the header in the response, and the
   UAC MUST ignore the header in the response.

   A UA SHOULD ignore extension header parameters that are not
   understood.



   A compact form of some common header fields is also defined for use
   when overall message size is an issue.

   The Contact, From, and To header fields contain a URI. If the URI
   contains a comma, question mark or semicolon, the URI MUST be
   enclosed in angle brackets (< and >). Any URI parameters are
   contained within these brackets. If the URI is not enclosed in angle
   brackets, any semicolon-delimited parameters are header-parameters,
   not URI parameters.

24.1 Accept

   The Accept header follows the syntax defined in [H14.1]. The
   semantics are also identical, with the exception that if no Accept
   header is present, the server SHOULD assume a default value of
   application/sdp

   An empty Accept header field means that no formats are acceptable.

   Example:


     Accept: application/sdp;level=1, application/x-private, text/html



24.2 Accept-Encoding

   The Accept-Encoding header field is similar to Accept, but restricts
   the content-codings [H3.5] that are acceptable in the response. See
   [H14.3]. The syntax of this header is defined in [H14.3]. The
   semantics in SIP are identical to those defined in [H14.3].



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      Header field          where   proxy ACK BYE CAN INV OPT REG PRA
      _______________________________________________________________
      Accept                  R            -   o   -   m*  m*  o   o
      Accept                 2xx           -   -   -   m*  m*  o   -
      Accept                 415           -   o   -   o   o   o   o
      Accept-Encoding         R            -   o   -   m*  o   o   o
      Accept-Encoding        2xx           -   -   -   m*  m*  o   -
      Accept-Encoding        415           -   o   -   o   o   o   o
      Accept-Language         R            -   o   -   m*  o   o   o
      Accept-Language        2xx           -   -   -   m*  m*  o   -
      Accept-Language        415           -   o   -   o   o   o   o
      Alert-Info              R      am    -   -   -   o   -   -   -
      Alert-Info             180     am    -   -   -   o   -   -   -
      Allow                   R            o   o   o   o   o   o   o
      Allow                  2xx           -   o   o   m*  m*  o   o
      Allow                   r            -   o   o   o   o   o   o
      Allow                  405           -   m   m   m   m   m   m
      Authentication-Info    2xx           -   o   -   o   o   o   o
      Authorization           R            o   o   o   o   o   o   o
      Call-ID                 c       r    m   m   m   m   m   m   m
      Call-Info                      am    -   -   -   o   o   o   -
      Contact                 R            o   -   -   m   o   o   -
      Contact                1xx           -   -   -   o   o   -   -
      Contact                2xx           -   -   -   m   o   o   -
      Contact                3xx           -   o   -   o   o   o   o
      Contact                485           -   o   -   o   o   o   o
      Content-Disposition                  o   o   -   o   o   o   o
      Content-Encoding                     o   o   -   o   o   o   o
      Content-Language                     o   o   -   o   o   o   o
      Content-Length                  r    t   t   t   t   t   t   t
      Content-Type                         *   *   -   *   *   *   *
      CSeq                    c       r    m   m   m   m   m   m   m
      Date                            a    o   o   o   o   o   o   o
      Error-Info           300-699         -   o   o   o   o   o   o
      Expires                              -   -   -   o   -   o   -
      From                    c       r    m   m   m   m   m   m   m
      In-Reply-To             R            -   -   -   o   -   -   -
      Max-Forwards            R      amr   m   m   m   m   m   m   m
      Min-Expires            423           -   -   -   -   -   m   -
      MIME-Version                         o   o   o   o   o   o   o
      Organization                   am    -   -   -   o   o   o   -


   Table 2: Summary of header fields, A--O






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   Header field              where       proxy ACK BYE CAN INV OPT REG PRA
   _______________________________________________________________________
   Priority                    R           a    -   -   -   o   -   -   -
   Proxy-Authenticate         407               -   m   m   m   m   m   m
   Proxy-Authorization         R           r    o   o   o   o   o   o   o
   Proxy-Require               R           r    -   o   -   o   o   o   o
   RAck                        R                -   -   -   -   -   -   m
   Record-Route                R          amr   o   o   o   o   o   -   o
   Record-Route           2xx,401,484           -   o   o   o   o   -   o
   Reply-To                                     -   -   -   o   -   -   -
   Require                                acr   -   o   -   o   o   o   o
   Retry-After          404,413,480,486         -   o   o   o   o   o   o
                            500,503             -   o   o   o   o   o   o
                            600,603             -   o   o   o   o   o   o
   Route                       R           r    c   c   c   c   c   -   c
   RSeq                       1xx               -   o   -   o   o   o   -
   Server                      r                -   o   o   o   o   o   o
   Subject                     R                -   -   -   o   -   -   -
   Supported                   R                -   o   o   o   o   o   o
   Supported                  2xx               -   o   o   o   m*  o   o
   Timestamp                                    o   o   o   o   o   o   o
   To                        c(1)          r    m   m   m   m   m   m   m
   Unsupported                420               -   o   o   o   o   o   o
   User-Agent                                   o   o   o   o   o   o   o
   Via                         c         acmr   m   m   m   m   m   m   m
   Warning                     r                -   o   o   o   o   o   o
   WWW-Authenticate           401               -   m   m   m   m   m   m


   Table 3: Summary of header fields, P--Z; (1):  copied  with  possible
   addition of tag

   An empty Accept-Encoding header field is permissible, even though the
   syntax in [H14.3] does not provide for it. It is equivalent to
   Accept-Encoding: identity, that is, only the identity encoding,
   meaning no encoding, is permissible.

   If no Accept-Encoding header is present, the server SHOULD assume a
   default value of identity.

   This differs slightly from the HTTP definition, which indicates that
   when not present, any encoding can be used, but the identity encoding
   is preferred.

   Example:


     Accept-Encoding: gzip


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24.3 Accept-Language

   The Accept-Language header is used in requests to indicate the
   preferred languages for reason phrases, session descriptions, or
   status responses carried as message bodies in the response. If no
   Accept-Language header is present, the server SHOULD assume all
   languages are acceptable to the client.

   The Accept-Language header follows the syntax defined in [H14.4].
   The rules for ordering the languages based on the "q" parameter apply
   to SIP as well.

   Example:


     Accept-Language: da, en-gb;q=0.8, en;q=0.7



24.4 Alert-Info

   When present in an INVITE request, the Alert-Info header field
   specifies an alternative ring tone to the UAS. When present in a 180
   (Ringing) response, the Alert-Info header field specifies an
   alternative ringback tone to the UAC. A typical usage is for a proxy
   to insert this header to provide a distinctive ring feature.

   The Alert-Info header can introduce security risks. These risks and
   the ways to handle them are discussed in Section 24.9, which
   discusses the Call-Info header since the risks are identical.

   In addition, a user SHOULD be able to disable this feature
   selectively.


        This helps prevent disruptions that could result from the
        use of this header by untrusted elements.

   Example:

   Alert-Info: <http://wwww.example.com/sounds/moo.wav>



24.5 Allow

   The Allow header field lists the set of methods supported by the UA
   generating the message.



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   All methods, including ACK and CANCEL, understood by the UA MUST be
   included in the list of methods in the Allow header, when present.
   The absence of an Allow header MUST NOT be interpreted to mean that
   the UA sending the message supports no methods.  Rather, it implies
   that the UA is not providing any information on what methods it
   supports.

   Supplying an Allow header in responses to methods other than OPTIONS
   reduces the number of messages needed.

   Example:

     Allow: INVITE, ACK, OPTIONS, CANCEL, BYE



24.6 Authentication-Info

   The Authentication-Info header provides for mutual authentication
   with HTTP Digest. A UAS MAY include this header in a 2xx response to
   a request that was successfully authenticated using digest based on
   the Authorization header.

   Syntax and semantics follow those specified in RFC 2617 [16].

   Example:

     Authentication-Info: nextnonce="47364c23432d2e131a5fb210812c"



24.7 Authorization

   The Authorization header field contains authentication credentials of
   a UA. Section 20.2 overviews the use of the Authorization header
   field, and Section 20.4 describes the syntax and

   semantics when used with HTTP authentication.

   This header field, along with Proxy-Authorization, breaks the general
   rules about multiple header fields. Although not a comma-separated
   list, this header field may be present multiple times, and MUST NOT
   be combined into a single header using the usual rules described in
   Section 7.3.

   In the example below, there are no quotes around the Digest
   parameter:




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     Authorization: Digest username="Alice", realm="Bob's Friends",
      nonce="84a4cc6f3082121f32b42a2187831a9e",
      response="7587245234b3434cc3412213e5f113a5432"



24.8 Call-ID

   The Call-ID header field uniquely identifies a particular invitation
   or all registrations of a particular client. A single multimedia
   conference can give rise to several calls with different Call-IDs,
   for example, if a user invites a single individual several times to
   the same (long-running) conference. Call-IDs are case- sensitive and
   are simply compared byte-by-byte.

   The compact form of the Call-ID header field is i.

   Examples:

     Call-ID: f81d4fae-7dec-11d0-a765-00a0c91e6bf6@biloxi.com
     i:f81d4fae-7dec-11d0-a765-00a0c91e6bf6@192.0.2.4



24.9 Call-Info

   The Call-Info header field provides additional information about the
   caller or callee, depending on whether it is found in a request or
   response. The purpose of the URI is described by the "purpose"
   parameter. The "icon" parameter designates an image suitable as an
   iconic representation of the caller or callee. The "info" parameter
   describes the caller or callee in general, for example, through a web
   page. The "card" parameter provides a business card, for example, in
   vCard [37] or LDIF [38] formats. Additonal tokens can be registered
   using IANA and the procedures in Section 28.

   Use of the Call-Info header field can pose a security risk. If a
   callee fetches the URIs provided by a malicious caller, the callee
   may be at risk for displaying inappropriate or offensive content,
   dangerous or illegal content, and so on. Therefore, it is RECOMMENDED
   that a UA only render the information in the Call-Info header if it
   can verify the authenticity of the element that originated the header
   and trusts that element. This need not be the peer UA; a proxy can
   insert this header into requests.

   Example:

   Call-Info: <http://wwww.example.com/alice/photo.jpg> ;purpose=icon,



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     <http://www.example.com/alice/> ;purpose=info



24.10 Contact

   The Contact header field provides a URI whose meaning depends on the
   the type of request or response it is in.

   A Contact header field can contain a display name, a URI with URI
   parameters, and header parameters.

   This document defines the Contact parameters "q" and "expires". These
   parameters are only used when the Contact is present in a REGISTER
   request or response, or in a 3xx response. Additional parameters may
   be defined in other specifications.

   When the header field contains a display name, the URI including all
   URI parameters is enclosed in "<" and ">". If no "<" and ">" are
   present, all parameters after the URI are header parameters, not URI
   parameters. The display name can be tokens, or a quoted string, if a
   larger character set is desired.

   Even if the "display-name" is empty, the "name-addr" form MUST be
   used if the "addr-spec" contains a comma, semicolon, or question
   mark.  There may or may not be LWS between the display-name and the
   "<".

   These rules for parsing a display name, URI and URI parameters, and
   header parameters also apply for the header fields To and From.


        The Contact header has a role similar to the Location
        header field in HTTP. However, the HTTP header field only
        allows one address, unquoted. Since URIs can contain commas
        and semicolons as reserved characters, they can be mistaken
        for header or parameter delimiters, respectively.

   The compact form of the Contact header field is m (for "moved").

   The second example below shows a Contact header field containing both
   a URI parameter (transport) and a header parameter (expires).


     Contact: "Mr. Watson" <sip:watson@worcester.bell-telephone.com>
        ;q=0.7; expires=3600,
        "Mr. Watson" <mailto:watson@bell-telephone.com> ;q=0.1
     m: <sip:bob@192.0.2.4;transport=tcp>;expires=60



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24.11 Content-Disposition

   The Content-Disposition header field describes how the message body
   or, for multipart messages, a message body part is to be interpreted
   by the UAC or UAS. This SIP header field extends the MIME Content-
   Type (RFC 1806 [6]).

   The value "session" indicates that the body part describes a session,
   for either calls or early (pre-call) media. The value "render"
   indicates that the body part should be displayed or otherwise
   rendered to the user. For backward-compatibility, if the Content-
   Disposition header is missing,

   the server SHOULD assume bodies of Content-Type application/sdp are
   the disposition "session", while other content types are "render".

   The disposition type "icon" indicates that the body part contains an
   image suitable as an iconic representation of the caller or callee.
   The value "alert" indicates that the body part contains information,
   such as an audio clip, that should be rendered instead of ring tone.

   The handling parameter, handling-parm, describes how the UAS should
   react if it receives a message body whose content type or disposition
   type it does not understand. The parameter has defined values of
   "optional" and "required". If the handling parameter is missing, the
   value "required" SHOULD be assumed.

   If this header field is missing, the MIME type determines the default
   content disposition. If there is none, "render" is assumed.

   Example:

     Content-Disposition: session



24.12 Content-Encoding

   The Content-Encoding header field is used as a modifier to the
   "media-type". When present, its value indicates what additional
   content codings have been applied to the entity-body, and thus what
   decoding mechanisms MUST be applied in order to obtain the media-type
   referenced by the Content-Type header field.  Content-Encoding is
   primarily used to allow a body to be compressed without losing the
   identity of its underlying media type.

   If multiple encodings have been applied to an entity, the content
   codings MUST be listed in the order in which they were applied.



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   All content-coding values are case-insensitive. IANA acts as a
   registry for content-coding value tokens. See [H3.5] for a definition
   of the syntax for content-coding.

   Clients MAY apply content encodings to the body in requests. A server
   MAY apply content encodings to the bodies in responses. The server
   MUST only use encodings listed in the Accept-Encoding header in the
   request.

   The compact form of the Content-Encoding header field is e. Examples:

     Content-Encoding: gzip
     e: tar



24.13 Content-Language

   See [H14.12]. Example:

     Content-Language: fr



24.14 Content-Length

   The Content-Length header field indicates the size of the message-
   body, in decimal number of octets, sent to the recipient.
   Applications SHOULD use this field to indicate the size of the
   message-body to be transferred, regardless of the media type of the
   entity. If TCP is used as transport, the header field MUST be used.

   The size of the message-body does not include the CRLF separating
   headers and body. Any Content-Length greater than or equal to zero is
   a valid value. If no body is present in a message, then the Content-
   Length header field MUST be set to zero.

        The ability to omit Content-Length simplifies the creation
        of cgi-like scripts that dynamically generate responses.

   The compact form of the header is l.

   Examples:

     Content-Length: 349
     l: 173





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24.15 Content-Type

   The Content-Type header field indicates the media type of the
   message-body sent to the recipient. The "media-type" element is
   defined in [H3.7]. The Content-Type header MUST be present if the
   body is not empty. If the body is empty, and a Content-Type header is
   present, it indicates that the body of the specific type has zero
   length (for example, an empty audio file).

   The compact form of the header is c.

   Examples:

     Content-Type: application/sdp
     c: text/html; charset=ISO-8859-4



24.16 CSeq

   A CSeq header field in a request contains a single decimal sequence
   number and the request method. The sequence number MUST be
   expressible as a 32-bit unsigned integer. The CSeq header serves to
   order transactions within a dialog, to provide a means to uniquely
   identify transactions, and to differentiate between new requests and
   request retransmissions.

   Example:


     CSeq: 4711 INVITE



24.17 Date

   The Date header field contains an RFC 1123 date (see [H14.18]).
   Unlike HTTP/1.1, SIP only supports the most recent RFC 1123 [3]
   format for dates. As in [H3.3], SIP restricts the timezone in SIP-
   date to "GMT", while RFC 1123 allows any timezone. rfc1123-date is
   case-sensitive.

   The Date header field reflects the time when the request or response
   is first sent.


        The Date header field can be used by simple end systems
        without a battery-backed clock to acquire a notion of



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        current time. However, in its GMT form, it requires clients
        to know their offset from GMT.

   Example:

     Date: Sat, 13 Nov 2010 23:29:00 GMT



24.18 Error-Info

   The Error-Info header field provides a pointer to additional
   information about the error status response.


        SIP UACs have user interface capabilities ranging from
        pop-up windows and audio on PC softclients to audio-only on
        "black" phones or endpoints connected via gateways. Rather
        than forcing a server generating an error to choose between
        sending an error status code with a detailed reason phrase
        and playing an audio recording, the Error-Info header field
        allows both to be sent. The UAC then has the choice of
        which error indicator to render to the caller.

   A UAC MAY treat a SIP URI in an Error-Info header field as if it were
   a Contact in a redirect and generate a new INVITE, resulting in a
   recorded announcement session being established. A non-SIP URI MAY be
   rendered to the user.

   Examples:

     SIP/2.0 404 The number you have dialed is not in service
     Error-Info: <sip:not-in-service-recording@atlanta.com>



24.19 Expires

   The Expires header field gives the relative time after which the
   message (or content) expires.

   The precise meaning of this is method dependent.

   The expiration time in an INVITE does not affect the duration of the
   actual session that may result from the invitation. Session
   description protocols may offer the ability to express time limits on
   the session duration, however.




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   The value of this field is an integer number of seconds (in decimal),
   measured from the receipt of the request.

   Example:

     Expires: 5



24.20 From

   The From header field indicates the initiator of the request.  This
   may be different from the initiator of the dialog. Requests sent by
   the callee to the caller use the callee's address in the From header
   field.

   The optional "display-name" is meant to be rendered by a human user
   interface. A system SHOULD use the display name "Anonymous" if the
   identity of the client is to remain hidden. Even if the "display-
   name" is empty, the "name-addr" form MUST be used if the "addr-spec"
   contains a comma, question mark, or semicolon. Syntax issues are
   discussed in Section 7.3.1.

   Section 12 describes how From header fields are compared for the
   purpose of matching requests to dialogs. See Section 24.10 for the
   rules for parsing a display name, URI and URI parameters, and header
   parameters.

   The compact form of the header is f.

   Examples:

     From: "A. G. Bell" <sip:agb@bell-telephone.com> ;tag=a48s
     From: sip:+12125551212@server.phone2net.com;tag=887s
     f: Anonymous <sip:c8oqz84zk7z@privacy.org>;tag=hyh8



24.21 In-Reply-To

   The In-Reply-To header field enumerates the Call-IDs that this call
   references or returns. These Call-IDs may have been cached by the
   client then included in this header in a return call.


        This allows automatic call distribution systems to route
        return calls to the originator of the first call. This also
        allows callees to filter calls, so that only return calls



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        for calls they originated will be accepted. This field is
        not a substitute for request authentication.

   Example:

     In-Reply-To: 70710@saturn.bell-tel.com, 17320@saturn.bell-tel.com



24.22 Max-Forwards

   The Max-Forwards header field must be used with any SIP method to
   limit the number of proxies or gateways that can forward the request
   to the next downstream server. This can also be useful when the
   client is attempting to trace a request chain that appears to be
   failing or looping in mid-chain.

   The Max-Forwards value is an integer in the range 0-255 indicating
   the remaining number of times this request message is allowed to be
   forwarded. This count is decremented by each server that forwards the
   request.

   This header field should be inserted by elements that can not
   otherwise guarantee loop detection. For example, a B2BUA should
   insert a Max-Forwards header field.

   Example:

     Max-Forwards: 6



24.23 Min-Expires

   The Min-Expires header field conveys the minimum registration
   expiration interval to a registrar. The header field contains a
   decimal integer number of seconds. The use of the header field in a
   423 (Registration Too Brief) response is described in Sections
   10.2.8, 10.3, and 25.4.17.

   Example:

     Min-Expires: 60



24.24 MIME-Version




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   See [H19.4.1].

   Example:

     MIME-Version: 1.0



24.25 Organization

   The Organization header field conveys the name of the organization to
   which the entity issuing the request or response belongs.


        The field MAY be used by client software to filter calls.

   Example:

     Organization: Boxes by Bob



24.26 Priority

   The Priority header field indicates the urgency of the request as
   perceived by the client. The Priority header field describes the
   priority that the SIP request should have to the receiving human or
   its agent. For example, it may be factored into decisions about call
   routing and acceptance. It does not influence the use of
   communications resources such as packet forwarding priority in
   routers or access to circuits in PSTN gateways. The header field can
   have the values "non-urgent", "normal", "urgent", and "emergency",
   but additional values can be defined elsewhere.  It is RECOMMENDED
   that the value of "emergency" only be used when life, limb, or
   property are in imminent danger.  Otherwise, there are no semantics
   defined for this header field.


        These are the values of RFC 2076 [34], with the addition of
        "emergency".

   Examples:


     Subject: A tornado is heading our way!
     Priority: emergency





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   or

     Subject: Weekend plans
     Priority: non-urgent



24.27 Proxy-Authenticate

   The Proxy-Authenticate header field contains an authentication
   challenge.

   The syntax for this header and its use is defined in [H14.33]. See
   20.3 for further details on its usage.

   Example:

      Proxy-Authenticate: Digest realm="Carrier SIP",
       domain="sip:ss1.carrier.com",
       nonce="f84f1cec41e6cbe5aea9c8e88d359",
       opaque="", stale=FALSE, algorithm=MD5



24.28 Proxy-Authorization

   The Proxy-Authorization header field allows the client to identify
   itself (or its user) to a proxy that requires authentication.  The
   Proxy-Authorization field value consists of credentials containing
   the authentication information of the user agent for the proxy and/or
   realm of the resource being requested.

   See [H14.34] for a definition of the syntax, and section 20.3 for a
   discussion of its usage.

   This header field, along with Authorization, breaks the general rules
   about multiple header fields. Although not a comma-separated list,
   this header field may be present multiple times, and MUST NOT be
   combined into a single header using the usual rules described in
   Section 7.3.1.

   Example:

   Proxy-Authorization: Digest username="Alice", realm="Atlanta ISP",
      nonce="c60f3082ee1212b402a21831ae",
      response="245f23415f11432b3434341c022"





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24.29 Proxy-Require

   The Proxy-Require header field is used to indicate proxy-sensitive
   features that must be supported by the proxy. See Section 24.33 for
   more details on the mechanics of this message and a usage example.

   Example:

     Proxy-Require: foo



24.30 RAck

   The RAck header is sent in a PRACK request to support reliability of
   provisional responses. It contains two numbers and a method tag. The
   first number is the value from the RSeq header in the provisional
   response that is being acknowledged. The next number, and the method,
   are copied from the CSeq in the response that is being acknowledged.
   The method name in the RAck header is case sensitive.

   Example:

     RAck: 776656 1 INVITE



24.31 Record-Route

   The Record-Route is inserted by proxies in a request to force future
   requests in the session to be routed through the proxy.

   Details of its use with the Route header field are described in
   Section 16.4.

   Example:

     Record-Route: <sip:bob@biloxi.com;maddr=192.0.2.4>,
      <sip:bob@biloxi.com;maddr=192.0.6.1>



24.32 Reply-To

   The Reply-To header field contains a logical return URI which may be
   different from the From header field. For example, the URI MAY be
   used to return missed calls or unestablished sessions. If the user
   wished to remain anonymous, the header field SHOULD either be omitted



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   from the request or populated in such as way that does not reveal any
   private information.

   Even if the "display-name" is empty, the "name-addr" form MUST be
   used if the "addr-spec" contains a comma, question mark, or
   semicolon.  Syntax issues are discussed in Section 7.3.1.

   Example:

     Reply-To: Bob <sip:bob@biloxi.com>



24.33 Require

   The Require header field is used by UACs to tell UASs about options
   that the UAC expects the UAS to support in order to process the
   request. Although an optional header, the Require MUST NOT be ignored
   if it is present.

   The Require header contains a list of option tags, described in
   Section 23.2. Each option tag defines a SIP extension that MUST be
   understood to process the request. Frequently, this is used to
   indicate that a specific set of extension headers need to be
   understood. A UAC compliant to this specification MUST only include
   option tags corresponding to standards-track RFCs.

   Example:

     Require: 100rel



24.34 Retry-After

   The Retry-After header field can be used with a 503 (Service
   Unavailable) response to indicate how long the service is expected to
   be unavailable to the requesting client and with a 404 (Not Found),
   600 (Busy), or 603 (Decline) response to indicate when the called
   party anticipates being available again. The value of this field is a
   positive integer number of seconds (in decimal) after the time of the
   response.

   An optional comment can be used to indicate additional information
   about the time of callback. An optional "duration" parameter
   indicates how long the called party will be reachable starting at the
   initial time of availability. If no duration parameter is given, the
   service is assumed to be available indefinitely.



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   Examples:

     Retry-After: 18000;duration=3600
     Retry-After: 120 (I'm in a meeting)



24.35 Route

   The Route is used to force routing for a request through the listed
   set of proxies. Details of its use with the Record-Route header field
   are described in Section 13.

   Example:

     Route: <sip:bob@biloxi.com;maddr=192.0.2.4>, <sip:bob@pc33.atlanta.com>



24.36 RSeq

   The RSeq header is used in provisional responses in order to transmit
   them reliably. It contains a single numeric value from 1 to 2**32 -
   1. For details on its usage, see Section 18.1.

   Example:

     RSeq: 988789



24.37 Server

   The Server header field contains information about the software used
   by the UAS to handle the request. The syntax for this field is
   defined in [H14.38].

   Revealing the specific software version of the server might allow the
   server to become more vulnerable to attacks against software that is
   known to contain security holes. Implementors SHOULD make the Server
   header field a configurable option.

   Example:

     Server: HomeProxy v2






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24.38 Subject

   The Subject header field provides a summary or indicates the nature
   of the call, allowing call filtering without having to parse the
   session description. The session description does not have to use the
   same subject indication as the invitation.

   The compact form of the header is s.

   Example:

     Subject: Need more boxes
     s: Tech Support



24.39 Supported

   The Supported header field enumerates all the extensions supported by
   the UAC or UAS.

   The Supported header contains a list of option tags, described in
   Section 23.2, that are understood by the UAC or UAS.  A UA compliant
   to this specification MUST only include option tags corresponding to
   standards-track RFCs. If empty, it means that no extensions are
   supported.

   Example:

     Supported: 100rel



24.40 Timestamp

   The Timestamp header field describes when the UAC sent the request to
   the UAS.

   See Section 8.2.6 for details on how to generate a response to a
   request that contains the header field, and Section 17.3 for usage in
   RTT estimation.

   Example:

     Timestamp: 54






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24.41 To

   The To header field specifies the logical recipient of the request.

   The optional "display-name" is meant to be rendered by a human-user
   interface. The "tag" parameter serves as a general mechanism to
   distinguish multiple instances of a user identified by a single SIP
   URI.

   See Section 13 for details of the "tag" parameter.

   Section 12 describes how To and From header fields are compared for
   the purpose of matching requests to dialogs. See Section 24.10 for
   the rules for parsing a display name, URI and URI parameters, and
   header parameters.

   The compact form of the header is t.

   The following are examples of valid To headers:

     To: The Operator <sip:operator@cs.columbia.edu>;tag=287447
     t: sip:+12125551212@server.phone2net.com



24.42 Unsupported

   The Unsupported header field lists the features not supported by the
   UAS. See Section 24.33 for motivation.

   Example:

     Unsupported: foo



24.43 User-Agent

   The User-Agent header field contains information about the UAC
   originating the request. The syntax and semantics are defined in
   [H14.43].

   Revealing the specific software version of the user agent might allow
   the user agent to become more vulnerable to attacks against software
   that is known to contain security holes. Implementors SHOULD make the
   User-Agent header field a configurable option.

   Example:



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     User-Agent: Softphone Beta1.5



24.44 Via

   The Via field indicates the path taken by the request so far and
   indicates the path that should be followed in routing responses.  The
   branch ID parameter in the Via header serves as a transaction
   identifier, and is used by proxies to detect loops.

   The Via header field contains the transport protocol used to send the
   message, the client's host name or network address and, if not the
   default port number, the port number at which it wishes to receive
   responses. The Via header field can also contain parameters such as
   "maddr", "ttl", "received", and "branch", whose meaning and use are
   described in other sections.

   Transport protocols defined here are "UDP", "TCP", "TLS", and "SCTP".
   "TLS" means TLS over TCP.

   The host or network address and port number are not required to
   follow the SIP URI syntax. Specifically, LWS on either side of the
   ":" or "/" is allowed, as shown in the second example below.


     Via: SIP/2.0/UDP erlang.bell-telephone.com:5060;branch=z9hG4bK87asdks7
     Via: SIP/2.0/UDP 128.59.16.1:5060 ;received=128.59.19.3;branch=z9hG4bK77asjd



   The compact form of the header is v.

   In this example, the message originated from a multi-homed host with
   two addresses, 128.59.16.1 and 128.59.19.3. The sender guessed wrong
   as to which network interface would be used. Erlang.bell-
   telephone.com noticed the mismatch and added a parameter to the
   previous hop's Via header field, containing the address that the
   packet actually came from.

   Another example:

     Via: SIP / 2.0 / UDP first.example.com: 4000;ttl=16
       ;maddr=224.2.0.1 ;branch=z9hG4bKa7c6a8dlze.1



   Even though this specification mandates that the branch parameter be



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   present in all requests, the BNF for the header indicates that it is
   optional. This allows interoperation with RFC 2543 elements, which
   did not have to insert the branch parameter.

24.45 Warning

   The Warning header field is used to carry additional information
   about the status of a response. Warning headers are sent with
   responses and contain a three-digit warning code, host name, and
   warning text.

   The "warn-text" should be in a natural language that is most likely
   to be intelligible to the human user receiving the response.  This
   decision can be based on any available knowledge, such as the
   location of the user, the Accept-Language field in a request, or the
   Content-Language field in a response. The default language is i-
   default [10].

   The currently-defined "warn-code"s are listed below, with a
   recommended warn-text in English and a description of their meaning.
   These warnings describe failures induced by the session description.
   The first digit of warning codes beginning with "3" indicates
   warnings specific to SIP. Warnings 300 through 329 are reserved for
   indicating problems with keywords in the session description, 330
   through 339 are warnings related to basic network services requested
   in the session description, 370 through 379 are warnings related to
   quantitative QoS parameters requested in the session description, and
   390 through 399 are miscellaneous warnings that do not fall into one
   of the above categories.

        300 Incompatible network protocol: One or more network protocols
             contained in the session description are not available.

        301 Incompatible network address formats: One or more network
             address formats contained in the session description are
             not available.

        302 Incompatible transport protocol: One or more transport
             protocols described in the session description are not
             available.

        303 Incompatible bandwidth units: One or more bandwidth
             measurement units contained in the session description were
             not understood.

        304 Media type not available: One or more media types contained
             in the session description are not available.




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        305 Incompatible media format: One or more media formats
             contained in the session description are not available.

        306 Attribute not understood: One or more of the media
             attributes in the session description are not supported.

        307 Session description parameter not understood: A parameter
             other than those listed above was not understood.

        330 Multicast not available: The site where the user is located
             does not support multicast.

        331 Unicast not available: The site where the user is located
             does not support unicast communication (usually due to the
             presence of a firewall).

        370 Insufficient bandwidth: The bandwidth specified in the
             session description or defined by the media exceeds that
             known to be available.

        399 Miscellaneous warning: The warning text can include
             arbitrary information to be presented to a human user or
             logged. A system receiving this warning MUST NOT take any
             automated action.


        1xx and 2xx have been taken by HTTP/1.1.

   Additional "warn-code"s, as in the example below, can be defined
   through IANA.

   Examples:

     Warning: 307 isi.edu "Session parameter 'foo' not understood"
     Warning: 301 isi.edu "Incompatible network address type 'E.164'"



24.46 WWW-Authenticate

   The WWW-Authenticate header field contains an authentication
   challenge. The syntax for this header field and use is defined in
   [H14.47]. See 20.2 for further details on its usage.

   Example:

     WWW-Authenticate: Digest realm="Bob's Friends",
       domain="sip:boxesbybob.com",



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       nonce="f84f1cec41e6cbe5aea9c8e88d359",
       opaque="", stale=FALSE, algorithm=MD5



25 Response Codes

   The response codes are consistent with, and extend, HTTP/1.1 response
   codes. Not all HTTP/1.1 response codes are appropriate, and only
   those that are appropriate are given here. Other HTTP/1.1 response
   codes SHOULD NOT be used. Response codes not defined by HTTP/1.1 have
   codes x80 upwards to avoid clashes with future HTTP response codes.
   Also, SIP defines a new class, 6xx.

25.1 Provisional 1xx

   Provisional responses, also known as informational responses,
   indicate that the server or proxy contacted is performing some
   further action and does not yet have a definitive response. A server
   typically sends a 1xx response if it expects to take more than 200 ms
   to obtain a final response. Note that 1xx responses are not
   transmitted reliably. That is, they do not cause the client to send
   an ACK. Provisional (1xx) responses MAY contain message bodies,
   including session descriptions.

25.1.1 100 Trying

   This response indicates that the request has been received by the
   next-hop server and that some unspecified action is being taken on
   behalf of this call (for example, a database is being consulted).
   This response, like all other provisional responses, stops
   retransmissions of an INVITE by a UAC. The 100 (Trying) response is
   different from other provisional responses, in that it is never
   forwarded upstream by a stateful proxy.

25.1.2 180 Ringing

   The UA receiving the INVITE is trying to alert the user. This
   response MAY be used to initiate local ringback.

25.1.3 181 Call Is Being Forwarded

   A proxy server MAY use this status code to indicate that the call is
   being forwarded to a different set of destinations.

25.1.4 182 Queued

   The called party is temporarily unavailable, but the callee has



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   decided to queue the call rather than reject it. When the callee
   becomes available, it will return the appropriate final status
   response. The reason phrase MAY give further details about the status
   of the call, for example, "5 calls queued; expected waiting time is
   15 minutes". The server MAY issue several 182 (Queued) responses to
   update the caller about the status of the queued call.

25.1.5 183 Session Progress

   The 183 (Session Progress) response is used to convey information
   about the progress of the call which is not otherwise classified. The
   Reason-Phrase, header fields, or message body MAY be used to convey
   more details about the call progress.

25.2 Successful 2xx

   The request was successful.

25.2.1 200 OK

   The request has succeeded. The information returned with the response
   depends on the method used in the request.

25.3 Redirection 3xx

   3xx responses give information about the user's new location, or
   about alternative services that might be able to satisfy the call.

25.3.1 300 Multiple Choices

   The address in the request resolved to several choices, each with its
   own specific location, and the user (or UA) can select a preferred
   communication end point and redirect its request to that location.

   The response MAY include a message body containing a list of resource
   characteristics and location(s) from which the user or UA can choose
   the one most appropriate, if allowed by the Accept request header.
   However, no MIME types have been defined for this message body.

   The choices SHOULD also be listed as Contact fields (Section 24.10).
   Unlike HTTP, the SIP response MAY contain several Contact fields or a
   list of addresses in a Contact field. UAs MAY use the Contact header
   field value for automatic redirection or MAY ask the user to confirm
   a choice. However, this specification does not define any standard
   for such automatic selection.


        This status response is appropriate if the callee can be



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        reached at several different locations and the server
        cannot or prefers not to proxy the request.

25.3.2 301 Moved Permanently

   The user can no longer be found at the address in the Request-URI,
   and the requesting client SHOULD retry at the new address given by
   the Contact header field (Section 24.10). The requestor SHOULD update
   any local directories, address books, and user location caches with
   this new value and redirect future requests to the address(es)
   listed.

25.3.3 302 Moved Temporarily

   The requesting client SHOULD retry the request at the new address(es)
   given by the Contact header field (Section 24.10).  The Request-URI
   of the new request uses the value of the Contact header in the
   response.

   The duration of the validity of the Contact URI can be indicated
   through an Expires (Section 24.19) header field or an expires
   parameter in the Contact header field. Both proxies and UAs MAY cache
   this URI for the duration of the expiration time. If there is no
   explicit expiration time, the address is only valid once for
   recursing, and MUST NOT be cached for future transactions.

   If the URI cached from the Contact header field fails, the Request-
   URI from the redirected request MAY be tried again a single time.


        The temporary URI may have become out-of-date sooner than
        the expiration time, and a new temporary URI may be
        available.

25.3.4 305 Use Proxy

   The requested resource MUST be accessed through the proxy given by
   the Contact field. The Contact field gives the URI of the proxy. The
   recipient is expected to repeat this single request via the proxy.
   305 (Use Proxy) responses MUST only be generated by UASs.

25.3.5 380 Alternative Service

   The call was not successful, but alternative services are possible.
   The alternative services are described in the message body of the
   response.  Formats for such bodies are not defined here, and may be
   the subject of future standardization.




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25.4 Request Failure 4xx

   4xx responses are definite failure responses from a particular
   server.  The client SHOULD NOT retry the same request without
   modification (for example, adding appropriate authorization).
   However, the same request to a different server might be successful.

25.4.1 400 Bad Request

   The request could not be understood due to malformed syntax. The
   Reason-Phrase SHOULD identify the syntax problem in more detail, for
   example, "Missing Call-ID header".

25.4.2 401 Unauthorized

   The request requires user authentication. This response is issued by
   UASs and registrars, while 407 (Proxy Authentication Required) is
   used by proxy servers.

25.4.3 402 Payment Required

   Reserved for future use.

25.4.4 403 Forbidden

   The server understood the request, but is refusing to fulfill it.
   Authorization will not help, and the request SHOULD NOT be repeated.

25.4.5 404 Not Found

   The server has definitive information that the user does not exist at
   the domain specified in the Request-URI. This status is also returned
   if the domain in the Request-URI does not match any of the domains
   handled by the recipient of the request.

25.4.6 405 Method Not Allowed

   The method specified in the Request-Line is understood, but not
   allowed for the address identified by the Request-URI.

   The response MUST include an Allow header field containing a list of
   valid methods for the indicated address.

25.4.7 406 Not Acceptable

   The resource identified by the request is only capable of generating
   response entities that have content characteristics not acceptable
   according to the Accept header fields sent in the request.



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25.4.8 407 Proxy Authentication Required

   This code is similar to 401 (Unauthorized), but indicates that the
   client MUST first authenticate itself with the proxy. SIP access
   authentication is explained in section 22 and 20.3.

   This status code can be used for applications where access to the
   communication channel (for example, a telephony gateway) rather than
   the callee requires authentication.

25.4.9 408 Request Timeout

   The server could not produce a response within a suitable amount of
   time, for example, if it could not determine the location of the user
   in time. The client MAY repeat the request without modifications at
   any later time.

25.4.10 410 Gone

   The requested resource is no longer available at the server and no
   forwarding address is known. This condition is expected to be
   considered permanent. If the server does not know, or has no facility
   to determine, whether or not the condition is permanent, the status
   code 404 (Not Found) SHOULD be used instead.

25.4.11 413 Request Entity Too Large

   The server is refusing to process a request because the request
   entity is larger than the server is willing or able to process. The
   server MAY close the connection to prevent the client from continuing
   the request.

   If the condition is temporary, the server SHOULD include a Retry-
   After header field to indicate that it is temporary and after what
   time the client MAY try again.

25.4.12 414 Request-URI Too Long

   The server is refusing to service the request because the Request-URI
   is longer than the server is willing to interpret.

25.4.13 415 Unsupported Media Type

   The server is refusing to service the request because the message
   body of the request is in a format not supported by the server for
   the requested method. The server SHOULD return a list of acceptable
   formats using the Accept, Accept-Encoding and Accept-Language header
   fields. UAC processing of this response is described in Section



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   8.1.3.6.

25.4.14 416 Unsupported URI Scheme

   The server cannot process the request because the scheme of the URI
   in the Request-URI is unknown to the server. Client processing of
   this response is described in Section 8.1.3.6.

25.4.15 420 Bad Extension

   The server did not understand the protocol extension specified in a
   Proxy-Require (Section 24.29) or Require (Section 24.33) header
   field. The server SHOULD include a list of the unsupported extensions
   in an Unsupported header in the response. UAC processing of this
   response is described in Section 8.1.3.6.

25.4.16 421 Extension Required

   The UAS needs a particular extension to process the request, but this
   extension is not listed in a Supported header in the request.
   Responses with this status code MUST contain a Require header field
   listing the required extensions.

   A UAS SHOULD NOT use this response unless it truly cannot provide any
   useful service to the client. Instead, if a desirable extension is
   not listed in the Supported header field, servers SHOULD process the
   request using baseline SIP capabilities and any extensions supported
   by the client.

25.4.17 423 Registration Too Brief

   The registrar is rejecting a registration request because a Contact
   header field expiration time was too small. The use of this response
   and the related Min-Expires header field are described in Sections
   10.2.8, 10.3, and 24.23.

25.4.18 480 Temporarily Unavailable

   The callee's end system was contacted successfully but the callee is
   currently unavailable (for example, is not logged in, logged in such
   a manner as to preclude communication with the callee, or has
   activated the "do not disturb" feature). The response MAY indicate a
   better time to call in the Retry-After header. The user could also be
   available elsewhere (unbeknownst to this host). The reason phrase
   SHOULD indicate a more precise cause as to why the callee is
   unavailable. This value SHOULD be settable by the UA. Status 486
   (Busy Here) MAY be used to more precisely indicate a particular
   reason for the call failure.



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   This status is also returned by a redirect or proxy server that
   recognizes the user identified by the Request-URI, but does not
   currently have a valid forwarding location for that user.

25.4.19 481 Call/Transaction Does Not Exist

   This status indicates that the UAS received a request that does not
   match any existing dialog or transaction.

25.4.20 482 Loop Detected

   The server has detected a loop (Section 3).

25.4.21 483 Too Many Hops

   The server received a request that contains a Max-Forwards (Section
   24.22) header with the value zero.

25.4.22 484 Address Incomplete

   The server received a request with a Request-URI that was incomplete.
   Additional information SHOULD be provided in the reason phrase.


        This status code allows overlapped dialing. With overlapped
        dialing, the client does not know the length of the dialing
        string. It sends strings of increasing lengths, prompting
        the user for more input, until it no longer receives a 484
        (Address Incomplete) status response.

25.4.23 485 Ambiguous

   The Request-URI was ambiguous. The response MAY contain a listing of
   possible unambiguous addresses in Contact header fields. Revealing
   alternatives can infringe on privacy of the user or the organization.
   It MUST be possible to configure a server to respond with status 404
   (Not Found) or to suppress the listing of possible choices for
   ambiguous Request-URIs.

   Example response to a request with the Request-URI
   sip:lee@example.com :


   485 Ambiguous SIP/2.0
   Contact: Carol Lee <sip:carol.lee@example.com>
   Contact: Ping Lee <sip:p.lee@example.com>
   Contact: Lee M. Foote <sip:lee.foote@example.com>




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        Some email and voice mail systems provide this
        functionality. A status code separate from 3xx is used
        since the semantics are different: for 300, it is assumed
        that the same person or service will be reached by the
        choices provided. While an automated choice or sequential
        search makes sense for a 3xx response, user intervention is
        required for a 485 (Ambiguous) response.

25.4.24 486 Busy Here

   The callee's end system was contacted successfully, but the callee is
   currently not willing or able to take additional calls at this end
   system. The response MAY indicate a better time to call in the
   Retry-After header. The user could also be available elsewhere, such
   as through a voice mail service. Status 600 (Busy Everywhere) SHOULD
   be used if the client knows that no other end system will be able to
   accept this call.

25.4.25 487 Request Terminated

   The request was terminated by a BYE or CANCEL request. This response
   is never returned for a CANCEL request itself.

25.4.26 488 Not Acceptable Here

   The response has the same meaning as 606 (Not Acceptable), but only
   applies to the specific entity addressed by the Request-URI and the
   request may succeed elsewhere.

   A message body containing a description of media capabilities MAY be
   present in the response, which is formatted according to the Accept
   header field in the INVITE (or application/sdp if not present), the
   same as a message body in a 200 (OK) response to an OPTIONS request.

25.4.27 491 Request Pending

   The request was received by a UAS which had a pending request within
   the same dialog. Section 14.2 describes how such "glare" situations
   are resolved.

25.4.28 493 Undecipherable

   The request was received by a UAS that contained an encrypted MIME
   body for which the recipient does not possess or will not provide an
   appropriate decryption key. This response MAY have a single body
   containing an appropriate public key that should be used to encrypt
   MIME bodies sent to this UA. Details of the usage of this response
   code can be found in Section 21.2.



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25.5 Server Failure 5xx

   5xx responses are failure responses given when a server itself has
   erred.

25.5.1 500 Server Internal Error

   The server encountered an unexpected condition that prevented it from
   fulfilling the request. The client MAY display the specific error
   condition and MAY retry the request after several seconds.

   If the condition is temporary, the server MAY indicate when the
   client may retry the request using the Retry-After header.

25.5.2 501 Not Implemented

   The server does not support the functionality required to fulfill the
   request. This is the appropriate response when a UAS does not
   recognize the request method and is not capable of supporting it for
   any user. (Proxies forward all requests regardless of method.)

   Note that a 405 (Method Not Allowed) is sent when the server
   recognizes the request method, but that method is not allowed or
   supported.

25.5.3 502 Bad Gateway

   The server, while acting as a gateway or proxy, received an invalid
   response from the downstream server it accessed in attempting to
   fulfill the request.

25.5.4 503 Service Unavailable

   The server is temporarily unable to process the request due to a
   temporary overloading or maintenance of the server. The server MAY
   indicate when the client should retry the request in a Retry-After
   header field. If no Retry-After is given, the client MUST act as if
   it had received a 500 (Server Internal Error) response.

   A client (proxy or UAC) receiving a 503 (Service Unavailable) SHOULD
   attempt to forward the request to an alternate server. It SHOULD NOT
   forward any other requests to that server for the duration specified
   in the Retry-After header field, if present.

   Servers MAY refuse the connection or drop the request instead of
   responding with 503 (Service Unavailable).

25.5.5 504 Server Time-out



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   The server did not receive a timely response from an external server
   it accessed in attempting to process the request. 408 (Request
   Timeout) should be used instead if there was no response within the
   period specified in the Expires header field from the upstream
   server.

25.5.6 505 Version Not Supported

   The server does not support, or refuses to support, the SIP protocol
   version that was used in the request. The server is indicating that
   it is unable or unwilling to complete the request using the same
   major version as the client, other than with this error message.

25.5.7 513 Message Too Large

   The server was unable to process the request since the message length
   exceeded its capabilities.

25.6 Global Failures 6xx

   6xx responses indicate that a server has definitive information about
   a particular user, not just the particular instance indicated in the
   Request-URI.

25.6.1 600 Busy Everywhere

   The callee's end system was contacted successfully but the callee is
   busy and does not wish to take the call at this time. The response
   MAY indicate a better time to call in the Retry-After header. If the
   callee does not wish to reveal the reason for declining the call, the
   callee uses status code 603 (Decline) instead. This status response
   is returned only if the client knows that no other end point (such as
   a voice mail system) will answer the request. Otherwise, 486 (Busy
   Here) should be returned.

25.6.2 603 Decline

   The callee's machine was successfully contacted but the user
   explicitly does not wish to or cannot participate. The response MAY
   indicate a better time to call in the Retry-After header. This status
   response is returned only if the client knows that no other end point
   will answer the request.

25.6.3 604 Does Not Exist Anywhere

   The server has authoritative information that the user indicated in
   the Request-URI does not exist anywhere.




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25.6.4 606 Not Acceptable

   The user's agent was contacted successfully but some aspects of the
   session description such as the requested media, bandwidth, or
   addressing style were not acceptable.

   A 606 (Not Acceptable) response means that the user wishes to
   communicate, but cannot adequately support the session described. The
   606 (Not Acceptable) response MAY contain a list of reasons in a
   Warning header field describing why the session described cannot be
   supported.

   A message body containing a description of media capabilities MAY be
   present in the response, which is formatted according to the Accept
   header field in the INVITE (or application/sdp if not present), the
   same as a message body in a 200 (OK) response to an OPTIONS request.

   Reasons are listed in Section 24.45. It is hoped that negotiation
   will not frequently be needed, and when a new user is being invited
   to join an already existing conference, negotiation may not be
   possible. It is up to the invitation initiator to decide whether or
   not to act on a 606 (Not Acceptable) response.

   This status response is returned only if the client knows that no
   other end point will answer the request.

26 Examples

   In the following examples, we often omit the message body and the
   corresponding Content-Length and Content-Type headers for brevity.

26.1 Registration

   Bob registers on start-up. The message flow is shown in Figure 9.




   F1 REGISTER Bob -> Registrar

     REGISTER sip:registrar.biloxi.com SIP/2.0
     Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
     To: Bob <sip:bob@biloxi.com>
     From: Bob <sip:bob@biloxi.com>;tag=456248
     Call-ID: 843817637684230@998sdasdh09
     CSeq: 1826 REGISTER
     Contact: <sip:bob@192.0.2.4>
     Max-Forwards: 70



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biloxi.com         Bob's
 registrar       softphone
    |                |
    |   REGISTER F1  |
    |<---------------|
    |    200 OK F2   |
    |--------------->|




   Figure 9: SIP Registration Example


     Expires: 7200
     Content-Length: 0



   The registration expires after two hours. The registrar responds with
   a 200 OK:



   F2 200 OK Registrar -> Bob

     SIP/2.0 200 OK
     Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
     To: Bob <sip:bob@biloxi.com>
     From: Bob <sip:bob@biloxi.com>;tag=456248
     Call-ID: 843817637684230@998sdasdh09
     CSeq: 1826 REGISTER
     Contact: <sip:bob@192.0.2.4>
     Expires: 7200
     Content-Length: 0




26.2 Session Setup

   This example contains the full details of the example session setup
   in Section 4. The message flow is shown in Figure 1.






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   F1 INVITE Alice -> atlanta.com proxy

     INVITE sip:bob@biloxi.com SIP/2.0
     Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
     To: Bob <sip:bob@biloxi.com>
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 314159 INVITE
     Contact: <sip:alice@pc33.atlanta.com>
     Max-Forwards: 70
     Content-Type: application/sdp
     Content-Length: 142

     (Alice's SDP not shown)





   F2 100 Trying atlanta.com proxy -> Alice

     SIP/2.0 100 Trying
     Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
     To: Bob <sip:bob@biloxi.com>
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 314159 INVITE
     Content-Length: 0





   F3 INVITE atlanta.com proxy -> biloxi.com proxy

     INVITE sip:bob@biloxi.com SIP/2.0
     Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branch=z9hG4bK77ef4c2312983.1
     Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
     To: Bob <sip:bob@biloxi.com>
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 314159 INVITE
     Contact: <sip:alice@pc33.atlanta.com>
     Max-Forwards: 69
     Content-Type: application/sdp
     Content-Length: 142

     (Alice's SDP not shown)



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   F4 100 Trying biloxi.com proxy -> atlanta.com proxy

     SIP/2.0 100 Trying
     Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branch=z9hG4bK77ef4c2312983.1
     Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
     To: Bob <sip:bob@biloxi.com>
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 314159 INVITE
     Content-Length: 0





   F5 INVITE biloxi.com proxy -> Bob

     INVITE sip:bob@192.0.2.4 SIP/2.0
     Via: SIP/2.0/UDP server10.biloxi.com;branch=z9hG4bK4b43c2ff8.1
     Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branch=z9hG4bK77ef4c2312983.1
     Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
     To: Bob <sip:bob@biloxi.com>
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 314159 INVITE
     Contact: <sip:alice@pc33.atlanta.com>
     Max-Forwards: 68
     Content-Type: application/sdp
     Content-Length: 142

     (Alice's SDP not shown)





   F6 180 Ringing Bob -> biloxi.com proxy

     SIP/2.0 180 Ringing
     Via: SIP/2.0/UDP server10.biloxi.com;branch=z9hG4bK4b43c2ff8.1
     Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branch=z9hG4bK77ef4c2312983.1
     Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
     To: Bob <sip:bob@biloxi.com>;tag=a6c85cf
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 314159 INVITE
     Content-Length: 0




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   F7 180 Ringing biloxi.com proxy -> atlanta.com proxy

     SIP/2.0 180 Ringing
     Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branch=z9hG4bK77ef4c2312983.1
     Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
     To: Bob <sip:bob@biloxi.com>;tag=a6c85cf
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 314159 INVITE
     Content-Length: 0





   F8 180 Ringing atlanta.com proxy -> Alice

     SIP/2.0 180 Ringing
     Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
     To: Bob <sip:bob@biloxi.com>;tag=a6c85cf
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 314159 INVITE
     Content-Length: 0





   F9 200 OK Bob -> biloxi.com proxy

     SIP/2.0 200 OK
     Via: SIP/2.0/UDP server10.biloxi.com;branch=z9hG4bK4b43c2ff8.1
     Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branch=z9hG4bK77ef4c2312983.1
     Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
     To: Bob <sip:bob@biloxi.com>;tag=a6c85cf
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 314159 INVITE
     Contact: <sip:bob@192.0.2.4>
     Content-Type: application/sdp
     Content-Length: 131

     (Bob's SDP not shown)







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   F10 200 OK biloxi.com proxy -> atlanta.com proxy

     SIP/2.0 200 OK
     Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branch=z9hG4bK77ef4c2312983.1
     Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
     To: Bob <sip:bob@biloxi.com>;tag=a6c85cf
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 314159 INVITE
     Contact: <sip:bob@192.0.2.4>
     Content-Type: application/sdp
     Content-Length: 131

     (Bob's SDP not shown)





   F11 200 OK atlanta.com proxy -> Alice

     SIP/2.0 200 OK
     Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
     To: Bob <sip:bob@biloxi.com>;tag=a6c85cf
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 314159 INVITE
     Contact: <sip:bob@192.0.2.4>
     Content-Type: application/sdp
     Content-Length: 131

     (Bob's SDP not shown)





   F12 ACK Alice -> Bob

     ACK sip:bob@192.0.2.4 SIP/2.0
     Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds9
     To: Bob <sip:bob@biloxi.com>;tag=a6c85cf
     From: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 314159 ACK
     Max-Forwards: 70
     Content-Length: 0




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   The media session between Alice and Bob is now established.

   Bob hangs up first. Note that Bob's SIP phone maintains its own CSeq
   numbering space, which, in this example, begins with 231. Since Bob
   is making the request, the To and From URIs and tags have been
   swapped.



   F13 BYE Bob -> Alice

     BYE sip:alice@pc33.atlanta.com SIP/2.0
     Via: SIP/2.0/UDP 192.0.2.4;branch=z9hG4bKnashds10
     From: Bob <sip:bob@biloxi.com>;tag=a6c85cf
     To: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 231 BYE
     Max-Forwards: 70
     Content-Length: 0





   F14 200 OK Alice -> Bob

     SIP/2.0 200 OK
     Via: SIP/2.0/UDP 192.0.2.4;branch=z9hG4bKnashds10
     From: Bob <sip:bob@biloxi.com>;tag=a6c85cf
     To: Alice <sip:alice@atlanta.com>;tag=1928301774
     Call-ID: a84b4c76e66710
     CSeq: 231 BYE
     Content-Length: 0



   The SIP Call Flows document [30] contains further examples of SIP
   messages.

27  Augmented BNF for the SIP Protocol

   All of the mechanisms specified in this document are described in
   both prose and an augmented Backus-Naur Form (BNF) defined in RFC
   2234 [28]. Section 6.1 of RFC 2234 defines a set of core rules which
   are used by this specification, and not repeated here. Implementors
   need to be familiar with the notation and content of RFC 2234 in
   order to understand this specification. Certain basic rules are in
   uppercase, such as SP, LWS, HTAB, CRLF, DIGIT, ALPHA, etc. Angle



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   brackets are used within definitions to clarify the use of rule
   names.

27.1 Basic Rules

   The following rules are used throughout this specification to
   describe basic parsing constructs. The US-ASCII coded character set
   is defined by ANSI X3.4-1986.



        alphanum  =  ALPHA / DIGIT


   Several rules are incorporated from RFC 2396 [13] but are updated to
   make them compliant with RFC 2234 [28]. These include:


        reserved    =  ";" / "/" / "?" / ":" / "@" / "&" / "=" / "+"
                       / "$" / ","
        unreserved  =  alphanum / mark
        mark        =  "-" / "_" / "." / "!" / "~" / "*" / "'"
                       / "(" / ")"
        escaped     =  "%" HEXDIG HEXDIG


   SIP header field values can be folded onto multiple lines if the
   continuation line begins with a space or horizontal tab. All linear
   white space, including folding, has the same semantics as SP. A
   recipient MAY replace any linear white space with a single SP before
   interpreting the field value or forwarding the message downstream.
   This is intended to behave exactly as HTTP 1.1 as described in RFC
   2616 [15]. The SWS construct is used when linear white space is
   optional, generally between tokens and separators.



        LWS  =  [*WSP CRLF] 1*WSP ; linear whitespace
        SWS  =  [LWS] ; sep whitespace


   To separate the header name from the rest of value, a colon is used,
   which, by the above rule, allows whitespace before, but no line
   break, and whitespace after, including a linebreak. The HCOLON
   defines this construct.






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        HCOLON  =  *( SP / HTAB ) ":" SWS


   The TEXT-UTF8 rule is only used for descriptive field contents and
   values that are not intended to be interpreted by the message parser.
   Words of *TEXT-UTF8 contain characters from the UTF-8 character set
   (RFC 2279 [25]). The TEXT-UTF8-TRIM rule is used for descriptive
   field contents that are not quoted strings, where leading and
   trailing LWS is not meaningful. In this regard, SIP differs from
   HTTP, which uses the ISO 8859-1 character set.



        TEXT-UTF8       =  *(TEXT-UTF8char / LWS)
        TEXT-UTF8-TRIM  =  *TEXT-UTF8char *(*LWS TEXT-UTF8char)
        TEXT-UTF8char   =  %x21-7E / UTF8-NONASCII
        UTF8-NONASCII   =  %xC0-DF 1UTF8-CONT
                        /  %xE0-EF 2UTF8-CONT
                        /  %xF0-F7 3UTF8-CONT
                        /  %xF8-Fb 4UTF8-CONT
                        /  %xFC-FD 5UTF8-CONT
        UTF8-CONT       =  %x80-BF


   A CRLF is allowed in the definition of TEXT-UTF8 only as part of a
   header field continuation. It is expected that the folding LWS will
   be replaced with a single SP before interpretation of the TEXT-UTF8
   value.

   Hexadecimal numeric characters are used in several protocol elements.
   Some elements (authentication) force hex alphas to be lower case.


        LHEX  =  DIGIT / %x61-66 ;lowercase a-f


   Many SIP header field values consist of words separated by LWS or
   special characters. Unless otherwise stated, tokens are case-
   insensitive. These special characters MUST be in a quoted string to
   be used within a parameter value. The word construct is used in
   Call-ID to allow most separators to be used.



        token       =  1*(alphanum / "-" / "." / "!" / "%" / "*"
                       / "_" / "+" / "`" / "'" / "~" )
        separators  =  "(" / ")" / "<" / ">" / "@" /
                       "," / ";" / ":" / "\" / <"> /



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                       "/" / "[" / "]" / "?" / "=" /
                       "{" / "}" / SP / HTAB
        word        =  1*(alphanum / "-" / "." / "!" / "%" / "*"
                       / "_" / "+" / "`" / "'" / "~"
                       "(" / ")" / "<" / ">"
                       ":" / "\" / <"> /
                       "/" / "[" / "]" / "?" /
                       "{" / "}" )


   When tokens are used or separators are used between elements,
   whitespace is often allowed before or after these characters:



        MINUS    =  SWS "-" SWS ; minus
        DOT      =  SWS "." SWS ; period
        PERCENT  =  SWS "%" SWS ; percent
        BANG     =  SWS "!" SWS ; exclamation
        PLUS     =  SWS "+" SWS ; plus
        STAR     =  SWS "*" SWS ; asterisk
        SLASH    =  SWS "/" SWS ; slash
        TILDE    =  SWS "~" SWS ; tilde
        EQUAL    =  SWS "=" SWS ; equal
        LPAREN   =  SWS "(" SWS ; left parenthesis
        RPAREN   =  SWS ")" SWS ; right parenthesis
        LANGLE   =  SWS "<" SWS ; left angle bracket
        RAQUOT   =  ">" SWS ; right angle quote
        LAQUOT   =  SWS "<"; left angle quote
        RANGLE   =  SWS ">" SWS ; right angle bracket
        BAR      =  SWS "|" SWS ; vertical bar
        ATSIGN   =  SWS "@" SWS ; atsign
        COMMA    =  SWS "," SWS ; comma
        SEMI     =  SWS ";" SWS ; semicolon
        COLON    =  SWS ":" SWS ; colon
        DQUOT    =  SWS <"> SWS ; double quotation mark
        LDQUOT   =  SWS <">; open double quotation mark
        RDQUOT   =  <"> SWS ; close double quotation mark
        LBRACK   =  SWS "{" SWS ; left square bracket
        RBRACK   =  SWS "}" SWS ; right square bracket


   Comments can be included in some SIP header fields by surrounding the
   comment text with parentheses. Comments are only allowed in fields
   containing "comment" as part of their field value definition. In all
   other fields, parentheses are considered part of the field value.





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        comment  =  LPAREN *(ctext / quoted-pair / comment) RPAREN
        ctext    =  %x21-27 / %x2A-5B / %x5D-7E / UTF8-NONASCII
                    / LWS


   ctext includes all chars except left and right parens and backslash.
   A string of text is parsed as a single word if it is quoted using
   double-quote marks. In quoted strings, quotation marks (") and
   backslashes (\) need to be escaped.



        quoted-string  =  ( SWS <"> *(qdtext / quoted-pair ) <"> )
        qdtext         =  LWS / %x21 / %x23-5B / %x5D-7E
                          / UTF8-NONASCII


   The backslash character ("\") MAY be used as a single-character
   quoting mechanism only within quoted-string and comment constructs.
   Unlike HTTP/1.1, the characters CR and LF cannot be escaped by this
   mechanism to avoid conflict with line folding and header separation.



        quoted-pair  =  "\" (%x00-09 / %x0A / %x0C
                        / %x0E-7F)




        SIP-URI          =  "sip:" [ userinfo "@" ] hostport
                            url-parameters [ headers ]
        userinfo         =  [ user / telephone-subscriber [ ":" password ]]
        user             =  *( unreserved / escaped / user-unreserved )
        user-unreserved  =  "&" / "=" / "+" / "$" / "," / ";" / "?" / "/"
        password         =  *( unreserved / escaped /
                            "&" / "=" / "+" / "$" / "," )
        hostport         =  host [ ":" port ]
        host             =  hostname / IPv4address / IPv6reference
        hostname         =  *( domainlabel "." ) toplabel [ "." ]
        domainlabel      =  alphanum
                            / alphanum *( alphanum / "-" ) alphanum
        toplabel         =  ALPHA / ALPHA *( alphanum / "-" ) alphanum




        IPv4address    =  1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT



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        IPv6reference  =  "[" IPv6address "]"
        IPv6address    =  hexpart [ ":" IPv4address ]
        hexpart        =  hexseq / hexseq "::" [ hexseq ] / "::" [ hexseq ]
        hexseq         =  hex4 *( ":" hex4)
        hex4           =  1*4HEXDIG
        port           =  1*DIGIT


   The BNF for telephone-subscriber can be found in RFC 2806 [19].
   Note, however, that any characters allowed there which are not
   allowed in the user part of the SIP URI MUST be escaped.



        url-parameters    =  *( ";" url-parameter)
        url-parameter     =  transport-param / user-param / method-param
                             |ttl-param / maddr-param / lr-param / other-param
        transport-param   =  "transport="
                             ( "udp" / "tcp" / "sctp" / "tls"
                             / other-transport)
        other-transport   =  token
        user-param        =  "user=" ( "phone" / "ip" / other-user)
        other-user        =  token
        method-param      =  "method=" Method
        ttl-param         =  "ttl=" ttl
        maddr-param       =  "maddr=" host
        lr-param          =  "lr"
        other-param       =  pname [ "=" pvalue ]
        pname             =  1*paramchar
        pvalue            =  1*paramchar
        paramchar         =  param-unreserved / unreserved / escaped
        param-unreserved  =  "[" / "]" / "/" / ":" / "&" / "+" / "$"




        headers         =  "?" header *( "&" header )
        header          =  hname "=" hvalue
        hname           =  1*( hnv-unreserved / unreserved / escaped )
        hvalue          =  *( hnv-unreserved / unreserved / escaped )
        hnv-unreserved  =  "[" / "]" / "/" / "?" / ":" / "+" / "$"




        SIP-message    =  Request / Response
        Request        =  Request-Line
                          *( message-header )



Various Authors                                             [Page 233]

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                          CRLF
                          [ message-body ]
        Request-Line   =  Method SP Request-URI SP SIP-Version CRLF
        Request-URI    =  SIP-URI / absoluteURI
        absoluteURI    =  scheme ":" ( hier-part / opaque-part )
        hier-part      =  ( net-path / abs-path ) [ "?" query ]
        net-path       =  "//" authority [ abs-path ]
        abs-path       =  "/" path-segments
        opaque-part    =  uric-no-slash *uric
        uric           =  reserved / unreserved / escaped
        uric-no-slash  =  unreserved / escaped / ";" / "?" / ":" / "@"
                          / "&" / "=" / "+" / "$" / ","
        path-segments  =  segment *( "/" segment )
        segment        =  *pchar *( ";" param )
        param          =  *pchar
        pchar          =  unreserved / escaped /
                          ":" / "@" / "&" / "=" / "+" / "$" / ","
        scheme         =  ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
        authority      =  srvr / reg-name
        srvr           =  [ [ userinfo "@" ] hostport ]
        reg-name       =  1*( unreserved / escaped / "$" / ","
                          / ";" / ":" / "@" / "&" / "=" / "+" )
        query          =  *uric
        SIP-Version    =  "SIP/2.0"




        message-header  =  (Accept
                        /  Accept-Encoding
                        /  Accept-Language
                        /  Alert-Info
                        /  Allow
                        /  Authentication-Info
                        /  Authorization
                        /  Call-ID
                        /  Call-Info
                        /  Contact
                        /  Content-Disposition
                        /  Content-Encoding
                        /  Content-Language
                        /  Content-Length
                        /  Content-Type
                        /  CSeq
                        /  Date
                        /  Error-Info
                        /  Expires
                        /  From



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                        /  In-Reply-To
                        /  Max-Forwards
                        /  MIME-Version
                        /  Min-Expires
                        /  Organization
                        /  Priority
                        /  Proxy-Authenticate
                        /  Proxy-Authorization
                        /  Proxy-Require
                        /  RAck
                        /  Record-Route
                        /  Reply-To
                        /  Require
                        /  Retry-After
                        /  Route
                        /  RSeq
                        /  Server
                        /  Subject
                        /  Supported
                        /  Timestamp
                        /  To
                        /  Unsupported
                        /  User-Agent
                        /  Via
                        /  Warning
                        /  WWW-Authenticate
                        /  extension-header) CRLF




        INVITEm           =  %x49.4E.56.49.54.45 ; INVITE in caps
        ACKm              =  %x41.43.4B ; ACK in caps
        OPTIONSm          =  %x4F.50.54.49.4F.4E.53 ; OPTIONS in caps
        BYEm              =  %x42.59.45 ; BYE in caps
        CANCELm           =  %x43.41.4E.43.45.4C ; CANCEL in caps
        REGISTERm         =  %x52.45.47.49.53.54.45.52 ; REGISTER in caps
        PRACKm            =  %x50.52.41.43.4B ; PRACK in caps
        Method            =  INVITEm / ACKm / OPTIONSm / BYEm
                             / CANCELm / REGISTERm / PRACKm
                             / extension-method
        extension-method  =  token
        Response          =  Status-Line
                             *( message-header )
                             CRLF
                             [ message-body ]





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        Status-Line     =  SIP-Version SP Status-Code SP Reason-Phrase CRLF
        Status-Code     =  Informational
                       /   Redirection
                       /   Success
                       /   Client-Error
                       /   Server-Error
                       /   Global-Failure
                       /   extension-code
        extension-code  =  3DIGIT
        Reason-Phrase   =  *(reserved / unreserved / escaped
                           / UTF8-NONASCII / UTF8-CONT / SP / HTAB)




        Informational  =  "100"  ;  Trying
                      /   "180"  ;  Ringing
                      /   "181"  ;  Call Is Being Forwarded
                      /   "182"  ;  Queued
                      /   "183"  ;  Session Progress




        Success  =  "200"  ;  OK




        Redirection  =  "300"  ;  Multiple Choices
                    /   "301"  ;  Moved Permanently
                    /   "302"  ;  Moved Temporarily
                    /   "305"  ;  Use Proxy
                    /   "380"  ;  Alternative Service




        Client-Error  =  "400"  ;  Bad Request
                     /   "401"  ;  Unauthorized
                     /   "402"  ;  Payment Required
                     /   "403"  ;  Forbidden
                     /   "404"  ;  Not Found
                     /   "405"  ;  Method Not Allowed
                     /   "406"  ;  Not Acceptable
                     /   "407"  ;  Proxy Authentication Required
                     /   "408"  ;  Request Timeout
                     /   "409"  ;  Conflict



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                     /   "410"  ;  Gone
                     /   "413"  ;  Request Entity Too Large
                     /   "414"  ;  Request-URI Too Large
                     /   "415"  ;  Unsupported Media Type
                     /   "416"  ;  Unsupported URI Scheme
                     /   "420"  ;  Bad Extension
                     /   "423"  ;  Registration Too Brief
                     /   "480"  ;  Temporarily not available
                     /   "481"  ;  Call Leg/Transaction Does Not Exist
                     /   "482"  ;  Loop Detected
                     /   "483"  ;  Too Many Hops
                     /   "484"  ;  Address Incomplete
                     /   "485"  ;  Ambiguous
                     /   "486"  ;  Busy Here
                     /   "487"  ;  Request Terminated
                     /   "488"  ;  Not Acceptable Here
                     /   "491"  ;  Request Pending
                     /   "493"  ;  Undecipherable




        Server-Error  =  "500"  ;  Internal Server Error
                     /   "501"  ;  Not Implemented
                     /   "502"  ;  Bad Gateway
                     /   "503"  ;  Service Unavailable
                     /   "504"  ;  Server Time-out
                     /   "505"  ;  SIP Version not supported




        Global-Failure  =  "600"  ;  Busy Everywhere
                       /   "603"  ;  Decline
                       /   "604"  ;  Does not exist anywhere
                       /   "606"  ;  Not Acceptable




        Accept            =  "Accept" HCOLON
                             ( accept-range *(COMMA accept-range) )
        accept-range      =  media-range [ accept-params ]
        media-range       =  ( "*/*"
                             / ( m-type SWS "/" "*" SWS )
                             / ( m-type SLASH m-subtype )
                             ) *( SEMI m-parameter )
        accept-params     =  SEMI "q" EQUAL qvalue *( accept-extension )



Various Authors                                             [Page 237]

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        accept-extension  =  SEMI ae-name [ EQUAL ae-value ]
        ae-name           =  token
        ae-value          =  token / quoted-string




        Accept-Encoding  =  "Accept-Encoding" HCOLON
                            ( encoding *(COMMA encoding) )
        encoding         =  codings [ SEMI "q" EQUAL qvalue ]
        codings          =  content-coding / "*"
        content-coding   =  token
        qvalue           =  ( "0" [ "." 0*3DIGIT ] )
                            / ( "1" [ "." 0*3("0") ] )




        Accept-Language  =  "Accept-Language" HCOLON
                            ( language *(COMMA language) )
        language         =  language-range [ SEMI "q" EQUAL qvalue ]
        language-range   =  ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) / "*" )




        Alert-Info     =  "Alert-Info" HCOLON alert-param *(COMMA alert-param)
        alert-param    =  LAQUOT URI RAQUOT *( SEMI generic-param )
        generic-param  =  token [ EQUAL gen-value ]
        gen-value      =  token / host / quoted-string




        Allow  =  "Allow" HCOLON Method *(COMMA Method)




        Authorization     =  "Authorization" HCOLON credentials
        credentials       =  ("Digest" LWS digest-response)
                             / other-response
        digest-response   =  dig-resp *(COMMA dig-resp)
        dig-resp          =  username / realm / nonce / digest-uri
                             / dresponse / [ algorithm ] / [cnonce]
                             / [opaque] / [message-qop]
                             / [nonce-count] / [auth-param]
        username          =  "username" EQUAL username-value



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        username-value    =  quoted-string
        digest-uri        =  "uri" EQUAL digest-uri-value
        digest-uri-value  =  rquest-uri ; Equal to request-uri as specified by HTTP/1.1
        message-qop       =  "qop" EQUAL qop-value
        cnonce            =  "cnonce" EQUAL cnonce-value
        cnonce-value      =  nonce-value
        nonce-count       =  "nc" EQUAL nc-value
        nc-value          =  8LHEX
        dresponse         =  "response" EQUAL request-digest
        request-digest    =  LDQUOT 32LHEX RDQUOT
        auth-param        =  auth-param-name EQUAL
                             ( token / quoted-string )
        auth-param-name   =  token
        other-response    =  auth-scheme LWS auth-param
                             *(COMMA auth-param)
        auth-scheme       =  token




        Authentication-Info  =  "Authentication-Info" HCOLON ainfo
                                *(COMMA ainfo)
        ainfo                =  [nextnonce] / [ message-qop ]
                                / [ response-auth ] / [ cnonce ]
                                / [nonce-count]
        nextnonce               "nextnonce" EQUAL nonce-value
        response-auth        =  "rspauth" EQUAL response-digest
        response-digest      =  LDQUOT *LHEX RDQUOT




        Call-ID  =  ( "Call-ID" / "i" ) HCOLON callid
        callid   =  word [ "@" word ]




        Call-Info   =  "Call-Info" HCOLON info *(COMMA info)
        info        =  LAQUOT URI RAQUOT *( SEMI info-param)
        info-param  =  ( "purpose" EQUAL ( "icon" / "info"
                       / "card" / token ) ) / generic-param




        Contact        =  ("Contact" / "m" ) HCOLON
                          STAR / (contact-param *(COMMA contact-param))



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        contact-param  =  (name-addr / addr-spec) *(SEMI contact-params)
        name-addr      =  [ display-name ] LAQUOT addr-spec RAQUOT
        addr-spec      =  SIP-URI / URI
        display-name   =  *(token LWS)/ quoted-string




        contact-params     =  c-p-q / c-p-expires
                              / contact-extension
        c-p-q              =  "q" EQUAL qvalue
        c-p-expires        =  "expires" EQUAL delta-seconds
        contact-extension  =  generic-param
        delta-seconds      =  1*DIGIT




        Content-Disposition   =  "Content-Disposition" HCOLON
                                 disp-type *( SEMI disp-param )
        disp-type             =  "render" / "session" / "icon" / "alert"
                                 / disp-extension-token
        disp-param            =  handling-param / generic-param
        handling-param        =  "handling" EQUAL
                                 ( "optional" / "required"
                                 / other-handling )
        other-handling        =  token
        disp-extension-token  =  token




        Content-Encoding  =  ( "Content-Encoding" / "e" ) HCOLON
                             content-coding *(COMMA content-coding)




        Content-Language  =  "Content-Language" HCOLON
                             language-tag *(COMMA language-tag)
        language-tag      =  primary-tag *( "-" subtag )
        primary-tag       =  1*8ALPHA
        subtag            =  1*8ALPHA




        Content-Length  =  ( "Content-Length" / "l" ) HCOLON 1*DIGIT



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        Content-Type     =  ( "Content-Type" / "c" ) HCOLON media-type
        media-type       =  m-type SLASH m-subtype *(SEMI m-parameter)
        m-type           =  discrete-type / composite-type
        discrete-type    =  "text" / "image" / "audio" / "video"
                            / "application" / extension-token
        composite-type      "message" / "multipart" / extension-token
        extension-token  =  ietf-token / x-token
        ietf-token       =  token
        x-token          =  "x-" token
        m-subtype        =  extension-token / iana-token
        iana-token       =  token
        m-parameter      =  m-attribute EQUAL m-value
        m-attribute      =  token
        m-value          =  token / quoted-string




        CSeq  =  "CSeq" HCOLON 1*DIGIT LWS Method




        Date          =  "Date" HCOLON SIP-date
        SIP-date      =  rfc1123-date
        rfc1123-date  =  wkday "," date1 SP time SP "GMT"
        date1         =  2DIGIT SP month SP 4DIGIT
                         ; day month year (e.g., 02 Jun 1982)
        time          =  2DIGIT ":" 2DIGIT ":" 2DIGIT
                         ; 00:00:00 - 23:59:59
        wkday         =  "Mon" / "Tue" / "Wed"
                         / "Thu" / "Fri" / "Sat" / "Sun"
        month         =  "Jan" / "Feb" / "Mar" / "Apr"
                         / "May" / "Jun" / "Jul" / "Aug"
                         / "Sep" / "Oct" / "Nov" / "Dec"




        Error-Info  =  "Error-Info" HCOLON error-uri *(COMMA error-uri)
        error-uri   =  LAQUOT URI RAQUOT *( SEMI generic-param )




        Expires     =  "Expires" HCOLON delta-seconds
        From        =  ( "From" / "f" ) HCOLON from-spec
        from-spec   =  ( name-addr / addr-spec )



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                       *( SEMI from-param )
        from-param  =  tag-param / generic-param
        tag-param   =  "tag" EQUAL token




        In-Reply-To  =  "In-Reply-To" HCOLON callid *(COMMA callid)




        Max-Forwards  =  "Max-Forwards" HCOLON 1*DIGIT




        MIME-Version  =  "MIME-Version" HCOLON 1*DIGIT "." 1*DIGIT




        Min-Expires  =  "Min-Expires" HCOLON delta-seconds




        Organization  =  "Organization" HCOLON TEXT-UTF8-TRIM




        Priority        =  "Priority" HCOLON priority-value
        priority-value  =  "emergency" / "urgent" / "normal"
                           / "non-urgent" / other-priority
        other-priority  =  token




        Proxy-Authenticate  =  "Proxy-Authenticate" HCOLON challenge
        challenge           =  ("Digest" LWS digest-cln *(COMMA digest-cln))
                               / other-challenge
        other-challenge     =  auth-scheme LWS auth-param
                               *(COMMA auth-param)
        digest-cln          =  realm / [ domain ] / nonce
                               / [ opaque ] / [ stale ] / [ algorithm ]
                               / [ qop-options ] / [auth-param]



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        realm               =  "realm" EQUAL realm-value
        realm-value         =  quoted-string
        domain              =  "domain" EQUAL LDQUOT URI
                               *( 1*SP URI ) RDQUOT
        URI                 =  absoluteURI / abs-path
        nonce               =  "nonce" EQUAL nonce-value
        nonce-value         =  quoted-string
        opaque              =  "opaque" EQUAL quoted-string
        stale               =  "stale" EQUAL ( "true" / "false" )
        algorithm           =  "algorithm" EQUAL ( "MD5" / "MD5-sess"
                               / token )
        qop-options         =  "qop" EQUAL LDQUOT qop-value
                               *("," qop-value) RDQUOT
        qop-value           =  "auth" / "auth-int" / token




        Proxy-Authorization  =  "Proxy-Authorization" HCOLON credentials




        Proxy-Require  =  "Proxy-Require" HCOLON option-tag
                          *(COMMA option-tag)
        option-tag     =  token




        RAck          =  "RAck" HCOLON response-num LWS CSeq-num LWS Method
        response-num  =  1*DIGIT
        CSeq-num      =  1*DIGIT




        Record-Route  =  "Record-Route" HCOLON rec-route *(COMMA rec-route)
        rec-route     =  name-addr *( SEMI rr-param )
        rr-param      =  generic-param




        Reply-To      =  "Reply-To" HCOLON rplyto-spec
        rplyto-spec   =  ( name-addr / addr-spec )
                         *( SEMI rplyto-param )
        rplyto-param  =  generic-param



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        Require       =  "Require" HCOLON option-tag *(COMMA option-tag)




        Retry-After  =  "Retry-After" HCOLON delta-seconds
                        [ comment ] *( SEMI retry-param )
        retry-param  =  ("duration" EQUAL delta-seconds)
                        / generic-param




        Route        =  "Route" HCOLON route-param *(COMMA route-param)
        route-param  =  name-addr *( SEMI rr-param )



        RSeq  =  "RSeq" HCOLON response-num




        Server           =  "Server" HCOLON 1*( product / comment )
        product          =  token [SLASH product-version]
        product-version  =  token




        Subject  =  ( "Subject" / "s" ) HCOLON TEXT-UTF8-TRIM




        Supported  =  ( "Supported" / "k" ) HCOLON
                      option-tag *(COMMA option-tag)




        Timestamp  =  "Timestamp" HCOLON 1*(DIGIT)
                      [ "." *(DIGIT) ] [ delay ]
        delay      =  *(DIGIT) [ "." *(DIGIT) ]







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        To        =  ( "To" / "t" ) HCOLON ( name-addr
                     / addr-spec ) *( SEMI to-param )
        to-param  =  tag-param / generic-param




        Unsupported  =  "Unsupported" HCOLON option-tag *(COMMA option-tag)




        User-Agent  =  "User-Agent" HCOLON 1*( product / comment )




        Via               =  ( "Via" / "v" ) HCOLON via-parm *(COMMA via-parm)
        via-parm          =  sent-protocol LWS sent-by *( SEMI via-params )
        via-params        =  via-ttl / via-maddr
                             / via-received / via-branch
                             / via-extension
        via-ttl           =  "ttl" EQUAL ttl
        via-maddr         =  "maddr" EQUAL host
        via-received      =  "received" EQUAL (IPv4address / IPv6address)
        via-branch        =  "branch" EQUAL token
        via-extension     =  generic-param
        sent-protocol     =  protocol-name SLASH protocol-version
                             SLASH transport
        protocol-name     =  "SIP" / token
        protocol-version  =  token
        transport         =  "UDP" / "TCP" / "TLS" / "SCTP"
                             / other-transport
        sent-by           =  host [ COLON port ]
        ttl               =  1*3DIGIT ; 0 to 255




        Warning        =  "Warning" HCOLON warning-value *(COMMA warning-value)
        warning-value  =  warn-code SP warn-agent SP warn-text
        warn-code      =  3DIGIT
        warn-agent     =  hostport / pseudonym
                          ;  the name or pseudonym of the server adding
                          ;  the Warning header, for use in debugging
        warn-text      =  quoted-string
        pseudonym      =  token




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        WWW-Authenticate  =  "WWW-Authenticate" HCOLON challenge




        extension-header  =  header-name HCOLON header-value
        header-name       =  token
        header-value      =  *(TEXT-UTF8CHAR / UTF8-CONT / LWS)




        message-body  =  *OCTET


28 IANA Considerations

   All new or experimental method names, header field names, and status
   codes used in SIP applications SHOULD be registered with IANA in
   order to prevent potential naming conflicts. It is RECOMMENDED that
   new "option- tag"s and "warn-code"s also be registered. Before IANA
   registration, new protcol elements SHOULD be described in an
   Internet-Draft or, preferably, an RFC.

   For Internet-Drafts, IANA is requested to make the draft available as
   part of the registration database.

        By the time an RFC is published, colliding names may have
        already been implemented.

   When a registration for either a new header field, new method, or new
   status code is created based on an Internet-Draft, and that
   Internet-Draft becomes an RFC, the person that performed the
   registration MUST notify IANA to change the registration to point to
   the RFC instead of the Internet-Draft.

   Registrations should be sent to iana@iana.org

28.1 Option Tags

   Option tags are used in header fields such as Require, Supported,
   Proxy-Require, and Unsupported in support of SIP compatibility
   mechanisms for extensions ( Section 23.2). The option tag itself is a
   string that is associated with a particular SIP option (that is, an
   extension). It identifies the option to SIP endpoints.

   When registering a new SIP option with IANA, the following
   information MUST be provided:



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        o Name and description of option. The name MAY be of any length,
          but SHOULD be no more than twenty characters long. The name
          MUST consist of alphanum (Section 27) characters only.

        o A listing of any new SIP header fields, header parameter
          fields, or parameter values defined by this option. A SIP
          option MUST NOT redefine header fields or parameters defined
          in either RFC 2543, any standards-track extensions to RFC
          2543, or other extensions registered through IANA.

        o Indication of who has change control over the option (for
          example, IETF, ISO, ITU-T, other international standardization
          bodies, a consortium, or a particular company or group of
          companies).

        o A reference to a further description if available, for example
          (in order of preference) an RFC, a published paper, a patent
          filing, a technical report, documented source code, or a
          computer manual.

        o Contact information (postal and email address).


        This procedure has been borrowed from RTSP [35] and the RTP
        AVP [33].

28.1.1 Registration of 100rel

   This specification registers a single option tag, "100rel". The
   required information is:

        Name: "100rel"

        Description: This option tag is for reliability of provisional
             responses. When present in a Supported header, it indicates
             that the UA can send or receive reliable provisional
             responses. When present in a Require header in a request,
             it indicates that the UAS MUST send all provisional
             responses reliably. When present in a Require header in a
             reliable provisional response, it indicates that the
             response is to be sent reliably.

        New Headers: The RSeq and RAck header fieds are defined by this
             optio.

        Change Control: IETF.

        Reference: RFCXXXX [Note to IANA: Fill in with the RFC number of



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             this specification.

        Contact Information: Jonathan Rosenberg, jdrosen@jdrosen.net. 72
             Eagle Rock Avenue, First Floor, East Hanover, NJ, 07936,
             USA.

28.2 Warn-Codes

   Warning codes provide information supplemental to the status code in
   SIP response messages when the failure of the transaction results
   from a Session Description Protocol (SDP, [11]). New "warn-code"
   values can be registered with IANA as they arise.

   The "warn-code" consists of three digits. A first digit of "3"
   indicates warnings specific to SIP.

   Warnings 300 through 329 are reserved for indicating problems with
   keywords in the session description, 330 through 339 are warnings
   related to basic network services requested in the session
   description, 370 through 379 are warnings related to quantitative QoS
   parameters requested in the session description, and 390 through 399
   are miscellaneous warnings that do not fall into one of the above
   categories.


        1xx and 2xx have been taken by HTTP/1.1.

28.3 Header Field Names

   Header field names do not require working group or working group
   chair review prior to IANA registration, but SHOULD be documented in
   an RFC or Internet-Draft before IANA is consulted.

   The following information needs to be provided to IANA in order to
   register a new header field name:

        o The name and email address of the individual performing the
          registration;

        o the name of the header field being registered;

        o a compact form version for that header field, if one is
          defined;

        o the name of the draft or RFC where the header field is
          defined;

        o a copy of the draft or RFC where the header field is defined.



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   Header fields SHOULD NOT use the X prefix notation and MUST NOT
   duplicate the names of header fields used by SMTP or HTTP unless the
   syntax is a compatible superset and the semantics are similar. Some
   common and widely used header fields MAY be assigned one-letter
   compact forms (Section 7.3.3). Compact forms can only be assigned
   after SIP working group review. In the absence of this working group,
   a designated expert reviews the request.

28.4 Method and Response Codes

   Because the status code space is limited, they do require working
   group or working group chair review, and MUST be documented in an RFC
   or Internet draft. The same procedures apply to new method names.

   The following information needs to be provided to IANA in order to
   register a new response code or method:

        o The name and email address of the individual performing the
          registration;

        o the number of the response code or name of the method being
          registered;

        o the default reason phrase for that status code, if applicable;

        o the name of the draft or RFC where the method or status code
          is defined;

        o a copy of the draft or RFC where the method or status code is
          defined.

29 Changes From RFC 2543

   This RFC revises RFC 2543. It is mostly backwards compatible with RFC
   2543. The changes described here fix many errors discovered in RFC
   2543 and provide information on scenarios not detailed in RFC 2543.
   The protocol has been presented in a more cleanly layered model here.

   We break the differences into functional behavior that is a
   substantial change from RFC 2543, which has impact on
   interoperability or correct operation in some cases, and functional
   behavior that is different from RFC 2543 but not a potential source
   of interoperability problems. There have been countless
   clarifications as well, which are not documented here.

29.1 Major Functional Changes

        o When a UAC wishes to terminate a call before it has been



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          answered, it sends CANCEL. If the original INVITE still
          returns a 2xx, the UAC then sends BYE. BYE can only be sent on
          an existing call leg (now called a dialog in this RFC),
          whereas it could be sent at any time in RFC 2543.

        o The SIP BNF was converted to be RFC 2234 compliant.

        o SIP URL BNF was made more general, allowing a greater set of
          characters in the user part. Furthermore, comparison rules
          were simplified to be primarily case insensitive, and detailed
          handling of comparison in the presence of parameters was
          described.

        o Removed Via hiding. It had serious trust issues, since it
          relied on the next hop to perform the obfuscation process.
          Instead, Via hiding can be done as a local implementation
          choice in stateful proxies, and thus is no longer documented.

        o In RFC 2543, CANCEL and INVITE transactions were intermingled.
          THey are separated now. When a user sends an INVITE, and then
          a CANCEL, the INVITE transaction still terminates normally. A
          UAS needs to respond to the original INVITE request with a 487
          response.

        o Similarly, CANCEL and BYE transactions were intermingled; RFC
          2543 allowed the UAS not to send a response to INVITE when a
          BYE was received. That is disallowed here. The original INVITE
          needs to be responded to.

        o In RFC 2543, UAs needed to only support UDP. In this RFC, UAs
          need to support both UDP and TCP.

        o In RFC 2543, a forking proxy only passed up one challenge from
          downstream elements in the event of multiple challenges. In
          this RFC, proxies are supposed to collect all challenges and
          place them into the forwarded response.

        o In Digest credentials the URI needs to be quoted; this is
          unclear from RFC 2617 and RFC 2069 which are both inconsistent
          on it.

        o SDP processing has been split off into a separate
          specification [1], and more fully specified as a formal
          offer/answer exchange process that is effectively tunnelled
          through SIP. SDP is allowed in INVITE/200 or 200/ACK for
          baseline SIP implementations; RFC 2543 alluded to the ability
          to use it in INVITE, 200 and ACK in a single transaction, but
          this was not well specified. More complex SDP usages are



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          allowed in extensions.

        o Added full support for IPv6 in URIs and in the Via header.

        o DNS SRV procedure is now documented in a separate
          specification [2]. This procedure uses both SRV and NAPTR
          resource records, and no longer combines data from across SRV
          records as described in RFC 2543.

        o Loop detection has been made optional, supplanted by a
          mandatory usage of Max-Forwards. The loop detection procedure
          in RFC 2543 had a serious bug which would report "spirals" as
          an error condition when it was not. The optional loop
          detection procedure is more fully and correctly specified
          here.

        o Usage of tags is now mandatory (they were optional in RFC
          2543), as they are now the fundamental building blocks of
          dialog identification.

        o Added the Supported header, allowing for clients to indicate
          what extensions are supported to a server, which can apply
          those extensions to the response, and indicate their usage
          with a Require in the response.

        o Extension parameters were missing from the BNF for several
          headers, and they have been added.

        o Handling of Route and Record-Route construction was very
          underspecified in RFC 2543, and also not the right approach.
          It has been substantially reworked in this specification (and
          vastly simpler), and this is arguably the largest change.
          Backwards compatibility is still provided for deployments that
          do not use "pre-loaded routes", where the initial request has
          a set of Route headers obtained in some way outside of
          Record-Route. In those situations, the new mechanism is not
          interoperable.

        o In RFC 2543, lines in a message could be terminated with CR,
          LF, or CRLF. This specification only allows CRLF.

        o Comments (expressed with rounded brackets) have been removed
          from the grammar of SIP.

        o Usage of Route in CANCEL and ACK was not well defined in RFC
          2543. It is now well specified; if a request had Route
          headers, its CANCEL or ACK for a non-2xx response to the
          request need to carry the same Route headers. ACK for 2xx



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          responses use the Route headers learned from the Record-Route
          of the 2xx responses.

        o RFC 2543 allowed multiple requests in a single UDP packet.
          This usage has been removed.

        o Usage of absolute time in the Expires header and parameter has
          been removed. It caused interoperability problems in elements
          that were not time synchronized, a common occurence. Relative
          times are used instead.

        o The branch parameter of the Via header is now mandatory for
          all elements to use. It now plays the role of a unique
          transaction identifier. This avoids the complex and bug-laden
          transaction identification rules from RFC 2543. A magic cookie
          is used in the Via header to determine if the previous hop has
          made the parameter globally unique, and comparison falls back
          to the old rules when it is not present. Thus,
          interoperability is assured.

        o In RFC 2543, closure of a TCP connection was made equivalent
          to a CANCEL. This was nearly impossible to implement (and
          wrong) for TCP connections between proxies. This has been
          eliminated, so that there is no coupling between TCP
          connection state and SIP processing.

        o RFC 2543 was silent on whether a UA could initiate a new
          transaction to a peer while another was in progress. That is
          now specified here. It is allowed for non-INVITE requests,
          disallowed for INVITE.

        o PGP was removed. It was not sufficiently specified, and not
          compatible with the more complete PGP MIME. It was replaced
          with S/MIME.

        o Additional security features were added with TLS, and these
          are described in a much larger and complete security
          considerations section.

        o In RFC 2543, a proxy was not required to forward provisional
          responses from 101 to 199 upstream. This was changed to MUST.
          This is important, since many subsequent features depend on
          delivery of all provisional responses from 101 to 199.

        o Little was said about the 503 response code in RFC 2543. It
          has since found substantial use in indicating failure or
          overload conditions in proxies. This requires somewhat special
          treatment. Specifically, receipt of a 503 should trigger an



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          attempt to contact the next element in the result of a DNS SRV
          lookup. Also, 503 response is only forwarded upstream by a
          proxy under certain conditions.

        o RFC 2543 defined, but did no sufficiently specify, a mechanism
          for UA authentication of a server. That has been removed.
          Instead, the mutual authentication procedures of RFC 2617 are
          allowed.

        o A UA cannot send a BYE for a call until its gotten an ACK for
          the initital INVITE. This was allowed in RFC 2543 but leads to
          a potential race condition.

        o A UA or proxy cannot send CANCEL for a transaction until it
          gets a provisional response for the request. This was allowed
          in RFC 2543 but leads to potential race conditions.

        o The action parameter in registrations has been deprecated. It
          was insufficent for any useful services, and caused conflicts
          when application processing was applied in proxies.

        o RFC 2543 had a number of special cases for multicast. For
          example, certain responses were supressed, timers were
          adjusted, and so on. Multicast now plays a more limited role,
          and the protocol operation is unaffected by usage of multicast
          as opposed to unicast. The limitations as a result of that are
          documented.

        o Basic authentication has been removed entirely and its usage
          forbidden.

        o Proxies no longer forward a 6xx immediately on receiving it.
          Instead, they CANCEL pending branches immediately. This avoids
          a potential race condition that would result in a UAC getting
          a 6xx followed by a 2xx. In all cases except this race
          condition, the result will be the same - the 6xx is forwarded
          upstream.

        o Reliability of provisional responses was developed as an
          extension so SIP, and has been folded into this specification.

        o RFC 2543 did not address the problem of request merging. This
          occurs when a request forks at a proxy, and later rejoins at
          an element. Handling of merging is done only at a UA, and
          procedures are defined for rejecting all but the first
          request.

29.2 Minor Functional Changes



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        o Added the Alert-Info, Error-Info and Call-Info headers for
          optional content presentation to users.

        o Added the Content-Language, Content-Disposition and MIME-
          Version headers.

        o Added a "glare handling" mechanism to deal with the case where
          both parties send each other a re-INVITE simultaneously. It
          uses the new 491 (Request Pending) error code.

        o Added the In-Reply-To and Reply-To headers for supporting the
          return of missed calls or messages at a later time.

        o Added TLS and SCTP as valid SIP transports.

        o There were a variety of mechanisms described for handling
          failures at any time during a call; those are now generally
          unified. BYE is sent to terminate.

        o RFC 2543 mandating retransmission of INVITE responses over
          TCP, but noted it was really only needed for 2xx. That was an
          artifact of insufficient protocol layering. With a more
          coherent transaction layer defined here, that is no longer
          needed. Only the 2xx response to INVITE is transmitted over
          TCP.

        o Formally specified an RTT estimation procedure using
          Timestamp. Its usage was mentioned in RFC 2543, but no details
          provided.

        o Client and server transaction machines are now driven based on
          timeouts rather than retransmit counts. This allows the state
          machines to be properly specified for TCP and UDP.

        o The Date header is used in REGISTER responses to provide a
          simple means for auto-configuration of dates in user agents.

        o Allowed a registrar to reject registrations with expirations
          that are too short in duration. Defined the 423 response code
          and the Min-Expires for this purpose.

30 Acknowledgments

   We wish to thank the members of the IETF MMUSIC and SIP WGs for their
   comments and suggestions. Detailed comments were provided by Brian
   Bidulock, Jim Buller, Neil Deason, Dave Devanathan, Keith Drage,
   Cdric Fluckiger, Yaron Goland, John Hearty, Bernie Honeisen, Jo
   Hornsby, Phil Hoffer, Christian Huitema, Jean Jervis, Gadi Karmi,



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   Peter Kjellerstedt, Anders Kristensen, Jonathan Lennox, Gethin
   Liddell, Allison Mankin, William Marshall, Keith Moore, Vern Paxson,
   Moshe J. Sambol, Chip Sharp, Igor Slepchin, Eric Tremblay., and Rick
   Workman.

   Brian Rosen provided the compiled BNF.

   This work is based, inter alia, on [41,42].

31 Authors' Addresses

   Authors addresses are listed alphabetically for the editors, the
   writers, and then the original authors of RFC 2543. All listed
   authors actively contributed large amounts of text to this document.

   Jonathan Rosenberg
   dynamicsoft
   72 Eagle Rock Ave
   East Hanover, NJ 07936
   USA
   electronic mail:  jdrosen@dynamicsoft.com

   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue
   New York, NY 10027
   USA
   electronic mail:  schulzrinne@cs.columbia.edu

   Gonzalo Camarillo
   Ericsson
   Advanced Signalling Research Lab.
   FIN-02420 Jorvas
   Finland
   electronic mail:  Gonzalo.Camarillo@ericsson.com

   Alan Johnston
   WorldCom
   100 South 4th Street
   St. Louis, MO 63102
   USA
   electronic mail:  alan.johnston@wcom.com

   Jon Peterson
   NeuStar, Inc
   1800 Sutter Street, Suite 570
   Concord, CA 94520



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   USA
   electronic mail:  jon.peterson@neustar.com

   Robert Sparks
   dynamicsoft, Inc.
   5100 Tennyson Parkway
   Suite 1200
   Plano, Texas 75024
   USA
   electronic mail:  rsparks@dynamicsoft.com

   Mark Handley
   ACIRI
   electronic mail:  mjh@aciri.org

   Eve Schooler
   Computer Science Department 256-80
   California Institute of Technology
   Pasadena, CA 91125
   USA
   electronic mail:  schooler@cs.caltech.edu

32 Normative References

   [1] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
   SDP," Internet Draft, Internet Engineering Task Force, Jan. 2002.
   Work in progress.

   [2] H. Schulzrinne and J. Rosenberg, "SIP: Session initiation
   protocol -- locating SIP servers," Internet Draft, Internet
   Engineering Task Force, Mar. 2001.  Work in progress.

   [3] R. Braden, "Requirements for internet hosts - application and
   support," Request for Comments 1123, Internet Engineering Task Force,
   Oct. 1989.

   [4] T. Berners-Lee, L. Masinter, and M. McCahill, "Uniform resource
   locators (URL)," Request for Comments 1738, Internet Engineering Task
   Force, Dec.  1994.

   [5] D. Eastlake, S. Crocker, and J. Schiller, "Randomness
   recommendations for security," Request for Comments 1750, Internet
   Engineering Task Force, Dec.  1994.

   [6] R. Troost and S. Dorner, "Communicating presentation information
   in internet messages: The content-disposition header," Request for
   Comments 1806, Internet Engineering Task Force, June 1995.




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   [7] J. Galvin, S. Murphy, S. Crocker, and N. Freed, "Security
   multiparts for MIME: multipart/signed and multipart/encrypted,"
   Request for Comments 1847, Internet Engineering Task Force, Oct.
   1995.

   [8] N. Freed and N. Borenstein, "Multipurpose internet mail
   extensions (MIME) part two: Media types," Request for Comments 2046,
   Internet Engineering Task Force, Nov. 1996.

   [9] T. Dierks and C. Allen, "The TLS protocol version 1.0," Request
   for Comments 2246, Internet Engineering Task Force, Jan. 1999.

   [10] H. Alvestrand, "IETF policy on character sets and languages,"
   Request for Comments 2277, Internet Engineering Task Force, Jan.
   1998.

   [11] M. Handley and V. Jacobson, "SDP: session description protocol,"
   Request for Comments 2327, Internet Engineering Task Force, Apr.
   1998.

   [12] D. Meyer, "Administratively scoped IP multicast," Request for
   Comments 2365, Internet Engineering Task Force, July 1998.

   [13] T. Berners-Lee, R. Fielding, and L. Masinter, "Uniform resource
   identifiers (URI): generic syntax," Request for Comments 2396,
   Internet Engineering Task Force, Aug. 1998.

   [14] S. Kent and R. Atkinson, "Security architecture for the internet
   protocol," Request for Comments 2401, Internet Engineering Task
   Force, Nov. 1998.

   [15] R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P.
   Leach, and T. Berners-Lee, "Hypertext transfer protocol -- HTTP/1.1,"
   Request for Comments 2616, Internet Engineering Task Force, June
   1999.

   [16] J. Franks, P. Hallam-Baker, J. Hostetler, S. Lawrence, P. Leach,
   A. Luotonen, and L. Stewart, "HTTP authentication: Basic and digest
   access authentication," Request for Comments 2617, Internet
   Engineering Task Force, June 1999.

   [17] R. Housley, "Cryptographic message syntax," Request for Comments
   2630, Internet Engineering Task Force, June 1999.

   [18] B. Ramsdell and Ed, "S/MIME version 3 message specification,"
   Request for Comments 2633, Internet Engineering Task Force, June
   1999.




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   [19] M. Day, S. Aggarwal, G. Mohr, and J. Vincent, "Instant messaging
   / presence protocol requirements," Request for Comments 2779,
   Internet Engineering Task Force, Feb. 2000.

   [20] A. Vaha-Sipila, "URLs for telephone calls," Request for Comments
   2806, Internet Engineering Task Force, Apr. 2000.

   [21] P. Resnick and Editor, "Internet message format," Request for
   Comments 2822, Internet Engineering Task Force, Apr. 2001.

   [22] R. Stewart, Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer, T.
   Taylor, I. Rytina, M. Kalla, L. Zhang, and V. Paxson, "Stream control
   transmission protocol," Request for Comments 2960, Internet
   Engineering Task Force, Oct.  2000.

   [23] J. Postel, "DoD standard transmission control protocol," Request
   for Comments 761, Internet Engineering Task Force, Jan. 1980.

   [24] J. Postel, "User datagram protocol," Request for Comments 768,
   Internet Engineering Task Force, Aug. 1980.

   [25] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," Request for Comments 2119, Internet Engineering Task Force,
   Mar. 1997.

   [26] F. Yergeau, "UTF-8, a transformation format of ISO 10646,"
   Request for Comments 2279, Internet Engineering Task Force, Jan.
   1998.

   [27] V. Paxson and M. Allman, "Computing TCP's retransmission timer,"
   Request for Comments 2988, Internet Engineering Task Force, Nov.
   2000.

   [28] J. Franks, P. Hallam-Baker, J. Hostetler, P. Leach, A. Luotonen,
   E. Sink, and L. Stewart, "An extension to HTTP : Digest access
   authentication," Request for Comments 2069, Internet Engineering Task
   Force, Jan. 1997.

   [29] D. Crocker, Ed., and P. Overell, "Augmented BNF for syntax
   specifications:  ABNF," Request for Comments 2234, Internet
   Engineering Task Force, Nov.  1997.

33 Non-Normative References

   [30] W. R. Stevens, TCP/IP illustrated: the protocols , Vol. 1.
   Reading, Massachusetts: Addison-Wesley, 1994.

   [31] R. Pandya, "Emerging mobile and personal communication systems,"



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   Vol. 33, pp. 44--52, June 1995.

   [32] A. Johnston, S. Donovan, R. Sparks, C. Cunningham, D. Willis, J.
   Rosenberg, K. Summers, and H. Schulzrinne, "SIP telephony call flow
   examples," Internet Draft, Internet Engineering Task Force, Apr.
   2001.  Work in progress.

   [33] J. C. Mogul and S. E. Deering, "Path MTU discovery," Request for
   Comments 1191, Internet Engineering Task Force, Nov. 1990.

   [34] R. Rivest, "The MD5 message-digest algorithm," Request for
   Comments 1321, Internet Engineering Task Force, Apr. 1992.

   [35] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP:
   a transport protocol for real-time applications," Request for
   Comments 1889, Internet Engineering Task Force, Jan. 1996.

   [36] H. Schulzrinne, "RTP profile for audio and video conferences
   with minimal control," Request for Comments 1890, Internet
   Engineering Task Force, Jan.  1996.

   [37] J. Palme, "Common internet message headers," Request for
   Comments 2076, Internet Engineering Task Force, Feb. 1997.

   [38] R. Braden, Ed., L. Zhang, S. Berson, S. Herzog, and S. Jamin,
   "Resource ReSerVation protocol (RSVP) -- version 1 functional
   specification," Request for Comments 2205, Internet Engineering Task
   Force, Sept. 1997.

   [39] H. Schulzrinne, A. Rao, and R. Lanphier, "Real time streaming
   protocol (RTSP)," Request for Comments 2326, Internet Engineering
   Task Force, Apr.  1998.

   [40] P. Hoffman, L. Masinter, and J. Zawinski, "The mailto URL
   scheme," Request for Comments 2368, Internet Engineering Task Force,
   July 1998.

   [41] F. Dawson and T. Howes, "vcard MIME directory profile," Request
   for Comments 2426, Internet Engineering Task Force, Sept. 1998.

   [42] G. Good, "The LDAP data interchange format (LDIF) - technical
   specification," Request for Comments 2849, Internet Engineering Task
   Force, June 2000.

   [43] M. Handley, C. Perkins, and E. Whelan, "Session announcement
   protocol," Request for Comments 2974, Internet Engineering Task
   Force, Oct. 2000.




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   [44] S. Donovan, "The SIP INFO method," Request for Comments 2976,
   Internet Engineering Task Force, Oct. 2000.

   [45] E. M. Schooler, "A multicast user directory service for
   synchronous rendezvous," Master's Thesis CS-TR-96-18, Department of
   Computer Science, California Institute of Technology, Pasadena,
   California, Aug. 1996.

   [46] E. M. Schooler, "Case study: multimedia conference control in a
   packet-switched teleconferencing system," Journal of Internetworking:
   Research and Experience , Vol. 4, pp. 99--120, June 1993.  ISI
   reprint series ISI/RS-93-359.

   [47] H. Schulzrinne, "Personal mobility for multimedia services in
   the Internet," in European Workshop on Interactive Distributed
   Multimedia Systems and Services (IDMS) , (Berlin, Germany), Mar.
   1996.

   [48] F. Cuervo, N. Greene, A. Rayhan, C. Huitema, B. Rosen, and J.
   Segers, "Megaco protocol version 1.0," Request for Comments 3015,
   Internet Engineering Task Force, Nov. 2000.

   [49] R. Hinden, B. Carpenter, and L. Masinter, "Format for literal
   IPv6 addresses in URL's," Request for Comments 2732, Internet
   Engineering Task Force, Dec. 1999.


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   This document and the information contained herein is provided on an
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