Internet Engineering Task Force                                MMUSIC WG
Internet Draft                    Handley/Schulzrinne/Schooler/Rosenberg
ietf-mmusic-sip-08.txt
ietf-mmusic-sip-09.txt                ISI/Columbia U./Caltech/Bell Labs.
August 7,
September 18, 1998
Expires: February 1999

                    SIP: Session Initiation     Protocol

STATUS OF THIS MEMO

   This document is an Internet-Draft. 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
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   Distribution of this document is unlimited.

                                 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 multimedia
         conferences, Internet telephone calls and multimedia
         distribution. Members in a session can communicate via
         multicast or via a mesh of unicast relations, or a
         combination of these.

         SIP invitations used to create sessions carry session
         descriptions which allow participants to agree on a set
         of compatible media types. It supports user mobility by
         proxying and redirecting requests to the user's current
         location. Users can register their current location.  SIP
         is not tied to any particular conference control
         protocol. SIP is designed to be independent of the
         lower-layer transport protocol and can be extended with
         additional capabilities.

         This document is a product of the Multi-party Multimedia
         Session Control (MMUSIC) working group of the Internet
         Engineering Task Force.  Comments are solicited and
         should be addressed to the working group's mailing list
         at confctrl@isi.edu and/or the authors.

1 Introduction

1.1 Overview of SIP Functionality

   The Session Initiation Protocol (SIP) is an application-layer control
   protocol that can establish, modify and terminate multimedia sessions
   or calls. These multimedia sessions include multimedia conferences,
   distance learning, Internet telephony and similar applications. SIP
   can invite both persons and "robots", such as a media storage
   service.  SIP can invite parties to both unicast and multicast
   sessions; the initiator does not necessarily have to be a member of
   the session to which it is inviting. Media and participants can be
   added to an existing session.

   SIP can be used to initiate sessions as well as invite members to
   sessions that have been advertised and established by other means.
   Sessions may can be advertised using multicast protocols such as SAP,
   electronic mail, news groups, web pages or directories (LDAP), among
   others.

   SIP transparently supports name mapping and redirection services,
   allowing the implementation of ISDN and Intelligent Network telephony
   subscriber services. These facilities also enable personal mobility
   services, this is defined as: "Personal mobility is the ability of
   end users to originate and receive calls and access subscribed
   telecommunication services on any terminal in any location, and the
   ability of the network to identify end users as they move. Personal
   mobility is based on the use of a unique personal identity (i.e.,
   mobility complements terminal mobility, i.e., the ability to maintain
   communications when moving a single end system from one subnet to
   another.

   SIP supports five facets of establishing and terminating multimedia
   communications:

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

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

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

   Call setup: "ringing", establishment of call parameters at both
        called and calling party;

   Call handling: including transfer and termination of calls.

   SIP can also initiate multi-party calls using a multipoint control
   unit (MCU) or fully-meshed interconnection instead of multicast.
   Internet telephony gateways that connect PSTN parties may can also use
   SIP to set up calls between them.

   SIP is designed as part of the overall IETF multimedia data and
   control architecture currently incorporating protocols such as RSVP
   (RFC 2205 [2]) for reserving network resources, the real-time
   transport protocol (RTP) (RFC 1889 [3]) for transporting real-time
   data and providing QOS feedback, the real-time streaming protocol
   (RTSP) (RFC 2326 [4]) for controlling delivery of streaming media,
   the session announcement protocol (SAP) for advertising multimedia
   sessions via multicast and the session description protocol (SDP)
   (RFC 2327 [5]) for describing multimedia sessions. However, the
   functionality and operation of SIP does not depend on any of these
   protocols.

   SIP may can also be used in conjunction with other call setup and
   signaling protocols. In that mode, an end system uses SIP exchanges
   to determine the appropriate end system address and protocol from a
   given address that is protocol-independent. For example, SIP could be
   used to determine that the party may can be reached via H.323, obtain the
   H.245 gateway and user address and then use H.225.0 to establish the
   call.

   In another example, it may SIP might be used to determine that the callee is
   reachable via the public switched telephone network (PSTN) and
   indicate the phone number to be called, possibly suggesting an
   Internet-to-PSTN gateway to be used.

   SIP does not offer conference control services such as floor control
   or voting and does not prescribe how a conference is to be managed,
   but SIP can be used to introduce conference control protocols. SIP
   does not allocate multicast addresses.

   SIP can invite users to sessions with and without resource
   reservation.  SIP does not reserve resources, but may can convey to the
   invited system the information necessary to do this. Quality-of-
   service guarantees, if required, may depend on knowing the full
   membership of the session; this information may or may not be known
   to the agent performing session invitation.

1.2 Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [6] and
   indicate requirement levels for compliant SIP implementations.

1.3 Definitions

   This specification uses a number of terms to refer to the roles
   played by participants in SIP communications. The definitions of
   client, server and proxy are similar to those used by the Hypertext
   Transport Protocol (HTTP) (RFC 2068 [7]).  The terms URI and URL are
   defined in [8].  The following terms have special significance for
   SIP.

   Call: A call consists of all participants in a conference invited by
        a common source. A SIP call is identified by a globally unique
        call-id (Section 6.12). Thus, if a user is, for example, invited
        to the same multicast session by several people, each of these
        invitations will be a unique call. A point-to-point Internet
        telephony conversation maps into a single SIP call. In a MCU-
        based call-in conference, each participant uses a separate call
        to invite himself to the MCU.

   Call leg: A call leg is identified by the combination of Call-ID, To
        and From.

   Client: An application program that establishes connections for the
        purpose of sending requests. Clients may or may not interact
        directly with a human user. User agents and proxies contain
        clients (and servers).

   Conference: A multimedia session (see below), identified by a common
        session description. A conference may can have zero or more members
        and includes the cases of a multicast conference, a full-mesh
        conference and a two-party "telephone call", as well as
        combinations of these.  Any number of calls may can be used to
        create a conference.

   Downstream: Requests sent in the direction from the caller to the
        callee.

   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.

   Initiator, calling party, caller: The party initiating a conference
        invitation. Note that the calling party does not have to be the
        same as the one creating the conference.

   Invitation: A request sent to a user (or service) requesting
        participation in a session. A successful SIP invitation consists
        of two transactions: an INVITE request followed by an ACK
        request.

   Invitee, invited user, called party, callee: The person or service
        that the calling party is trying to invite to a conference.

   Isomorphic request or response: Two requests or responses are defined
        to be isomorphic for the purposes of this document if   they have
        the same values for the Call-ID, To, From and CSeq header
        fields. In addition, requests have to have the same Request-URI.

   Location server: See location service

   Location service: A location service is used by a SIP redirect or
        proxy server to obtain information about a callee's possible
        location(s). Location services are offered by location servers.
        Location servers may MAY be co-located with a SIP server, but the
        manner in which a SIP server requests location services is
        beyond the scope of this document.

   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

   Proxy, proxy server: An intermediary program that acts as both a
        server and a client for the purpose of making requests on behalf
        of other clients. Requests are serviced internally or by passing
        them on, possibly after translation, to other servers. A proxy
        must interpret,
        interprets, and, if necessary, rewrite rewrites a request message before
        forwarding it.

   Redirect server: A redirect server is a server that accepts a SIP
        request, maps the address into zero or more new addresses and
        returns these addresses to the client. Unlike a proxy server    ,
        it does not initiate its own SIP request. Unlike a user agent
        server , it does not accept calls.

   Registrar: A registrar is server that accepts REGISTER requests. A
        registrar is typically co-located with a proxy or redirect
        server and may MAY offer location services.

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

   Server: A server is an application program that accepts requests in
        order to service requests and sends back responses to those
        requests.  Servers are either proxy, redirect or user agent
        servers or registrars.

   Session: "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 [5]) (A session as defined for SDP may can
        comprise one or more RTP sessions.) As defined, a callee may 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. A transaction is identified
        by the CSeq sequence number (Section 6.16) 6.17) within a single call
        leg The ACK request has the same CSeq number    as the
        corresponding INVITE request, but comprises a transaction of its
        own.

   Upstream: Responses sent in the direction from the called client to
        the caller.

   URL-encoded: A character string encoded according to RFC 1738,
        Section 2.2 [8]. [9].

   User agent client (UAC), calling user agent: A user agent client is a
        client application that initiates the SIP request.

   User agent server (UAS), called user agent: A user agent server is a
        server application that contacts the user when a SIP request is
        received and that returns a response on behalf of the user. The
        response may accept, reject accepts, rejects or redirect redirects the request.

   An application program may MAY be capable of acting both as a client and
   a server. For example, a typical multimedia conference control
   application would act as a user agent client to initiate calls or to
   invite others to conferences and as a user agent server to accept
   invitations. The properties of the different SIP server types are
   summarized in Table 1.

   property                     redirect    proxy     user agent    registrar
                                 server     server      server
   __________________________________________________________________________
   also acts as a SIP client       no        yes          no           no
   returns 1xx status             yes        yes         yes           yes
   returns 2xx status              no        yes         yes           yes
   returns 3xx status             yes        yes         yes           yes
   returns 4xx status             yes        yes         yes           yes
   returns 5xx status             yes        yes         yes           yes
   returns 6xx status              no        yes         yes           no
   inserts Via header              no        yes          no           no
   accepts ACK                    yes        yes         yes           no

   Table 1: Properties of the different SIP server types

1.4 Summary of SIP Operation

   This section explains the basic protocol functionality and operation.
   Callers and callees are identified by SIP addresses, described in
   Section 1.4.1. When making a SIP call, a caller first locates the
   appropriate server (Section 1.4.2) and then sends a SIP request
   (Section 1.4.3). The most common SIP operation is the invitation
   (Section 1.4.4). Instead of directly reaching the intended callee, a
   SIP request may be redirected or may trigger a chain of new SIP
   requests by proxies (Section 1.4.5). Users can register their
   location(s) with SIP servers (Section 4.2.6).

1.4.1 SIP Addressing

   The "objects" addressed by SIP are users at hosts, identified by a
   SIP URL. The SIP URL takes the form similar to a mailto or telnet
   URL, i.e., user@host user part is a user     name, a civil name or a
   telephone number. The host part is either a domain name having a     DNS
   SRV (RFC 2052 [9]), [10]), MX (RFC 974 [10], [11], CNAME or A record (RFC 1035
   [11]),
   [12]), or a numeric network address.

   A user's SIP address can be obtained out-of-band, can be learned via
   existing media agents, can be included in some mailers' message
   headers, or can be recorded during previous invitation interactions.
   In many cases, a user's SIP URL can be guessed from his email
   address.

   Examples of SIP URLs include:

     sip:mjh@metro.isi.edu
     sip:watson@bell-telephone.com
     sip:root@193.175.132.42
     sip:info@ietf.org

   A SIP URL address can designate an individual (possibly located at
   one of several end systems), the first available person from a group
   of individuals or a whole group. The form of the address, e.g.,
   sip:sales@example.com , is not sufficient, in general, to determine
   the intent of the caller.

   If a user or service chooses to be reachable at an address that is
   guessable from the person's name and organizational affiliation, the
   traditional method of ensuring privacy by having an unlisted "phone"
   number is compromised. However, unlike traditional telephony, SIP
   offers authentication and access control mechanisms and can avail
   itself of lower-layer security mechanisms, so that client software
   can reject unauthorized or undesired call attempts.

1.4.2 Locating a SIP Server

   When a client wishes to send a request, the client either sends it to
   a locally configured SIP proxy server (as in HTTP), independent of
   the Request-URI, or sends it to the IP address and port corresponding
   to the Request-URI. For the latter case, the client performs the
   following steps to obtain the server's IP address.

   A SIP client MUST follow the following steps to resolve the host     part
   of the Request-URI. If a client supports only TCP or UDP, but not
   both, the client omits the respective address type. If the SIP
   address contains a port number, that number is to be used, otherwise,
   the default port number 5060 is to be used. The default port number
   is the same for UDP and TCP. In all cases, the client first attempts
   to contact the server using UDP, then TCP.

   A client SHOULD rely on ICMP "Port Unreachable" messages rather than
   time-outs to determine that a server is not reachable at a particular
   address. (For socket-based programs: For TCP, connect() returns
   ECONNREFUSED if there is no server at the designated address; for
   UDP, the socket should needs to be bound to the destination address using
   connect() rather than sendto() or similar so that a second write()
   fails with ECONNREFUSED.  )

   If the SIP address contains a numeric IP address, the client contacts
   the SIP server at that address. Otherwise, the client follows the
   steps below.

        1.   If there is a SRV DNS resource record (RFC 2052 [9]) [10]) of
             type sip.udp, sip.udp or type sip.tcp, order all such records by
             their priority value and attempt to contact the listed SIP servers in the order
             of the preference values contained in those resource
             records, using UDP as
             that order. If a transport protocol at the port number given is explicitly specified in the URL or, if none provided,
             SIP URL, it overrides the one
             listed port number in the DNS resource SRV record.

        2.   If there It
             is a SRV RECOMMENDED that DNS resource record (RFC 2052 [9]) of
             type sip.tcp, contact the listed SIP zone files give higher weight to
             servers in the order
             of the preference value contained in running UDP than those resource
             records, using TCP as running TCP. If a transport protocol at the port
             number given in the URL or, if none provided, the one
             listed in server
             responds, skip the DNS resource record.

        3. remaining steps below.

        2.   If there is a DNS MX record (RFC 974 [10]), [11]), contact the
             hosts listed in their order of preference at the port
             number listed in the URL or the default SIP port number if
             none. For each host listed, first try to contact the SIP
             server using UDP, then TCP.

        4. If a server responds, skip the
             remaining steps.

        3.   Finally, check if there is a DNS CNAME or A record for the
             given host and     try to contact a SIP server at the one or
             more addresses listed, again trying first UDP, then TCP. If
             a server responds, skip the remaining step.

        4.   If all of the above methods fail to locate a server, the
             caller MAY contact an SMTP server at the user's host and
             use the SMTP EXPN command to obtain an alternate address
             and repeat the steps above. As a last resort, a client MAY
             choose to deliver the session description to the callee
             using electronic mail.

   A client MAY cache the result of the reachability steps for a
   particular address and retry that host address for the next request.
   If the client does not find a SIP server at the cached address, it
   MUST start the search at the beginning of the sequence.

        This sequence is modeled after that described for SMTP,
        where MX records are to be checked before A records (RFC
        1123 [12]). [13]).

1.4.3 SIP Transaction

   Once the host part has been resolved to a SIP server, the client
   sends one or more SIP requests to that server and receives one or
   more responses from the server. A request (and its retransmissions)
   together with the responses triggered by that request make up a SIP
   transaction.  The ACK request following an INVITE is not part of the
   transaction since it may traverse a different set of hosts.

   If TCP is used, request and responses within a single SIP transaction
   are carried over the same TCP connection (see Section 10). Several
   SIP requests from the same client to the same server may MAY use the same
   TCP connection or may MAY open a new connection for each request.

   If the client sent the request via unicast UDP, the response is sent
   to the address contained in the next Via header field (Section 6.40)
   of the response. If the request is sent via multicast UDP, the
   response is directed to the same multicast address and destination
   port. For UDP, reliability is achieved using retransmission (Section
   10).

   The SIP message format and operation is independent of the transport
   protocol.

1.4.4 SIP Invitation

   A successful SIP invitation consists of two requests, INVITE followed
   by ACK. The INVITE (Section 4.2.1) request asks the callee to join a
   particular conference or establish a two-party conversation. After
   the callee has agreed to participate in the call, the caller confirms
   that it has received that response by sending an ACK (Section 4.2.2)
   request. If the caller no longer wants to participate in the call, it
   sends a BYE request instead of an ACK.

   The INVITE request typically contains a session description, for
   example written in SDP (RFC 2327 [5]) format, that provides the
   called party with enough information to join the session. For
   multicast sessions, the session description enumerates the media
   types and formats that may are allowed to be distributed to that session.
   For a unicast session, the session description enumerates the media
   types and formats that the caller is willing to receive and where it
   wishes the media data to be sent. In either case, if the callee
   wishes to accept the call, it responds to the invitation by returning
   a similar description listing the media it wishes to receive. For a
   multicast session, the callee should SHOULD only return a session
   description if it is unable to receive the media indicated in the
   caller's description or wants to receive data via unicast.

   The protocol exchanges for the INVITE method are shown in Fig. 1 for
   a proxy server and in Fig. 2 for a redirect server. (Note that the
   messages shown in the figures have been abbreviated slightly.) In
   Fig. 1, the proxy server accepts the INVITE request (step 1),
   contacts the location service with all or parts of the address (step
   2) and obtains a more precise location (step 3). The proxy server
   then issues a SIP INVITE request to the address(es) returned by the
   location service (step 4). The user agent server alerts the user
   (step 5) and returns a success indication to the proxy server (step
   6). The proxy server then returns the success result to the original
   caller (step 7). The receipt of this message is confirmed by the
   caller using an ACK request, which is forwarded to the callee (steps
   8 and 9). Note that an ACK can also be sent directly to the callee,
   bypassing the proxy. All requests and responses have the same Call-
   ID.

                                            +....... cs.columbia.edu .......+
                                            :                               :
                                            : (~~~~~~~~~~)                  :
                                            : ( location )                  :
                                            : ( service  )                  :
                                            : (~~~~~~~~~~)                  :
                                            :     ^    |                    :
                                            :     | hgs@play                :
                                            :    2|   3|                    :
                                            :     |    |                    :
                                            : henning  |                    :
   +.. cs.tu-berlin.de ..+ 1: INVITE        :     |    |                    :
   :                     :    henning@cs.col:     |    | 4: INVITE  5: ring :
   : cz@cs.tu-berlin.de ========================>(~~~~~~)=========>(~~~~~~) :
   :                    <........................(      )<.........(      ) :
   :                     : 7: 200 OK        :    (      )6: 200 OK (      ) :
   :                     :                  :    ( tune )          ( play ) :
   :                     : 8: ACK           :    (      )9: ACK    (      ) :
   :                    ========================>(~~~~~~)=========>(~~~~~~) :
   +.....................+                  +...............................+

     ====> SIP request
     ....> SIP response
     ----> non-SIP protocols

   Figure 1: Example of SIP proxy server

   The redirect server shown in Fig. 2 accepts the INVITE request (step
   1), contacts the location service as before (steps 2 and 3) and,
   instead of contacting the newly found address itself, returns the
   address to the caller (step 4), which is then acknowledged via an ACK
   request (step 5). The caller issues a new request, with the same
   call-ID but a higher CSeq, to the address returned by the first
   server (step 6). In the example, the call succeeds (step 7). The
   caller and callee complete the handshake with an ACK (step 8).

   The next section discusses what happens if the location service
   returns more than one possible alternative.

1.4.5 Locating a User

   A callee may move between a number of different end systems over
   time.  These locations can be dynamically registered with the SIP
   server (Sections 1.4.7, 4.2.6). A location server may MAY also use one or
   more other protocols, such as finger (RFC 1288 [13]), [14]), rwhois (RFC
   2167 [14]), [15]), LDAP (RFC 1777 [15]), [16]), multicast-based protocols [16] [17] or
   operating-system dependent mechanisms to actively determine the end
   system where a user might be reachable. A location server may MAY return
   several locations because the user is logged in at several hosts
   simultaneously or because the location server has (temporarily)
   inaccurate information. The SIP server combines the results to yield
   a list of a zero or more locations. It is recommended that each
   location server sorts results according to the likelihood of success.

   The action taken on receiving a list of locations varies with the
   type of SIP server. A SIP redirect server returns the list to the
   client as Location Contact headers (Section 6.22). 6.13). A SIP proxy server can
   sequentially or in parallel try the addresses until the call is
   successful (2xx response) or the callee has declined the call (6xx
   response). With sequential attempts, a proxy server can implement an
   "anycast" service.

   If a proxy server forwards a SIP request, it MUST add itself to the
   end of the list of forwarders noted in the Via (Section 6.40)
   headers. The Via trace ensures that replies can take the same path
   back, ensuring correct operation through compliant firewalls and
   avoiding request loops. On the response path, each host MUST remove
   its Via, so that routing internal information is hidden from the
   callee and outside networks. A proxy server MUST check that it does
   not generate a request to a host listed in the Via sent-by, via-
   received or via-maddr parameters (Section 6.40). (Note: If a host has
   several names or network addresses, this may does not always work.  Thus,
   each host also checks if it is part of the Via list.)

   A SIP invitation may traverse more than one SIP proxy server. If one
   of these "forks" the request, i.e., issues more than one request in
   response to receiving the invitation request, it is possible that a
   client is reached, independently, by more than one copy of the
   invitation request. Each of these copies bears the same Call-ID. The
   user agent MUST return the appropriate status response. Duplicate
                                            +....... cs.columbia.edu .......+
                                            :                               :
                                            : (~~~~~~~~~~)                  :
                                            : ( location )                  :
                                            : ( service  )                  :
                                            : (~~~~~~~~~~)                  :
                                            :    ^   |                      :
                                            :    | hgs@play                 :
                                            :   2|  3|                      :
                                            :    |   |                      :
                                            : henning|                      :
   +.. cs.tu-berlin.de ..+ 1: INVITE        :    |   |                      :
   :                     :    henning@cs.col:    |   |                      :
   : cz@cs.tu-berlin.de =======================>(~~~~~~)                    :
   :       | ^ |        <.......................(      )                    :
   :       | . |         : 4: 302 Moved     :   (      )                    :
   :       | . |         :    hgs@play      :   ( tune )                    :
   :       | . |         :                  :   (      )                    :
   :       | . |         : 5: ACK           :   (      )                    :
   :       | . |        =======================>(~~~~~~)                    :
   :       | . |         :                  :                               :
   +.......|...|.........+                  :                               :
           | . |                            :                               :
           | . |                            :                               :
           | . |                            :                               :
           | . |                            :                               :
           | . | 6: INVITE hgs@play.cs.columbia.edu                (~~~~~~) :
           | . ==================================================> (      ) :
           | ..................................................... (      ) :
           |     7: 200 OK                  :                      ( play ) :
           |                                :                      (      ) :
           |     8: ACK                     :                      (      ) :
           ======================================================> (~~~~~~) :
                                            +...............................+

     ====> SIP request
     ....> SIP response
     ----> non-SIP protocols

   Figure 2: Example of SIP redirect server
   A SIP invitation may traverse more than one SIP proxy server. If one
   of these "forks" the request, i.e., issues more than one request in
   response to receiving the invitation request, it is possible that a
   client is reached, independently, by more than one copy of the
   invitation request. Each of these copies bears the same Call-ID. The
   user agent MUST return the appropriate status response. Duplicate
   requests are not an error.

1.4.6 Changing an Existing Session

   In some circumstances, it may be necessary is desirable to change the parameters of an
   existing session. For example, two parties may have been conversing
   and then want to add a third party, switching to multicast for
   efficiency.  One of the participants invites the third party with the
   new multicast address and simultaneously sends an INVITE to the
   second party, with the new multicast session description, but with
   the old call identifier.

1.4.7 Registration Services

   The REGISTER request allows a client to let a proxy or redirect
   server know at which address(es) it may can be reached. A client may MAY also
   use it to install call handling features at the server.

1.5 Protocol Properties

1.5.1 Minimal State

   A single conference session or call may involve involves one or more SIP
   request-response transactions. Proxy servers do not have to keep
   state for a particular call, however, they MAY maintain state for a
   single SIP transaction, as discussed in Section 12. For efficiency, a
   server may MAY cache the results of location service requests.

1.5.2 Lower-Layer-Protocol Neutral

   SIP makes minimal assumptions about the underlying transport and
   network-layer protocols. The lower-layer may can provide either a packet
   or a byte stream service, with reliable or unreliable service.

   In an Internet context, SIP is able to utilize both UDP and TCP as
   transport protocols, among others. UDP allows the application to more
   carefully control the timing of messages and their retransmission, to
   perform parallel searches without requiring TCP connection state for
   each outstanding request, and to use multicast. Routers can more
   readily snoop SIP UDP packets. TCP allows easier passage through
   existing firewalls, and given the similar protocol design, allows
   common servers for SIP, HTTP and the Real Time Streaming Protocol
   (RTSP) (RFC 2326 [4]).

   When TCP is used, SIP can use one or more connections to attempt to
   contact a user or to modify parameters of an existing conference.
   Different SIP requests for the same SIP call may MAY use different TCP
   connections or a single persistent connection, as appropriate.

   For concreteness, this document will only refer to Internet
   protocols.  However, SIP may MAY also be used directly with protocols
   such as ATM AAL5, IPX, frame relay or X.25. The necessary naming
   conventions are beyond the scope of this document. User agents SHOULD
   implement both UDP and TCP transport, proxy and redirect servers
   MUST.

1.5.3 Text-Based

   SIP is text-based, using ISO 10646 in UTF-8 encoding throughout. This
   allows easy implementation in languages such as Java, Tcl and Perl,
   allows easy debugging, and most importantly, makes SIP flexible and
   extensible. As SIP is used for initiating multimedia conferences
   rather than delivering media data, it is believed that the additional
   overhead of using a text-based protocol is not significant.

2 SIP Uniform Resource Locators

   SIP URLs are used within SIP messages to indicate the originator
   (From), current destination (Request-URI) and final recipient (To) of
   a SIP request, and to specify redirection addresses (Location). (Contact). A SIP
   URL can also be embedded in web pages or other hyperlinks to indicate
   that a particular user or service may be called.

   Because interaction with some resources may require message headers
   or message bodies to can be specified as well called via SIP.  When used
   as a hyperlink, the SIP address, URL indicates the use of the INVITE method.

   The SIP URL scheme is defined to allow setting SIP request-header
   fields and the SIP message-body.

        This corresponds to the use of mailto: URLs. It makes it
        possible, for example, to specify the subject, urgency or
        media types of calls initiated through a web page or as
        part of an email message.

   A SIP URL follows the guidelines of RFC 1630 [17], as revised [18], 2396 [18] and has the syntax
   shown in Fig. 3. Note that reserved characters have to be escaped.

   The URI character classes referenced above are described in Section
   C.

   userinfo: The URI specification is currently being revised. It is
   anticipated that future versions of this specification will reference SIP scheme MAY use the format "user:password" in the revised edition. Note that all URL reserved characters MUST be
     SIP-URL         = "sip:" [
        userinfo "@" ] hostport
                       url-parameters [ headers ] field. The use of passwords in the userinfo        = user [ ":" password 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.

     SIP-URL         = "sip:" [ userinfo "@" ] hostport
                       url-parameters [ headers ]
     userinfo        = user [ ":" password ]
     user            = *( unreserved | escaped
                     | "&" | "=" | "+" | "$" | "," )
     password        = *( unreserved | escaped
                     | "&" | "=" | "+" | "$" | "," )
     hostport        = host [ ":" port ]
     host            = hostname | IPv4address
     hostname        = *( domainlabel "." ) toplabel [ "." ]
     domainlabel     = alphanum | alphanum *( alphanum | "-" ) alphanum
     toplabel        = alpha | alpha *( alphanum | "-" ) alphanum
     IPv4address     = 1*digit "." 1*digit "." 1*digit "." 1*digit
     port            = *digit
     url-parameters  = *( ";" url-parameter )
     url-parameter   = transport-param | user-param | method-param
                     | ttl-param | maddr-param | tag-param | other-param
     transport-param = "transport=" ( "udp" | "tcp" )
     user-param      = "user=" ( "phone" | "ip" )
     method-param    = "method=" Method
     ttl-param       = "ttl=" ttl
     ttl             = 1*3DIGIT       ; 0 to 255
     maddr-param     = "maddr=" host
     user-param      = "user=" ( "phone" $|$ "ip" )
     tag-param       = "tag=" UUID
     UUID            = 1*( hex | "-" )
     other-param     = *uric
     headers         = "?" header *( "&" header )
     header          = hname "=" hvalue
     hname           = *uric
     hvalue          = *uric
     uric            = reserved | unreserved | escaped
     reserved        = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" |
                       "$" | ","
     digits          = 1*DIGIT

   Figure 3: SIP URL syntax

   Figure 4: SIP URL syntax; telephone subscriber

   encoded.

   host: The mailto: URL and RFC 822 email addresses require that
        numeric host addresses ("host numbers") are enclosed in square
        brackets (presumably, since host names might be numeric), while
        host numbers without brackets are used for all other URLs. The
        SIP URL requires the latter form, without brackets.

   userinfo: The SIP scheme MAY use the format "user:password" in the
        userinfo field. The use of passwords in the userinfo 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.

   If the host is an Internet telephony gateway, the userinfo user field can MAY also
   encode a telephone number using the notation of telephone-
   subscriber telephone-subscriber
   (Fig. 4). The telephone number is a special case of a user name and
   cannot be distinguished by a BNF. Thus, a URL parameter, user, is
   added to distinguish telephone numbers from user names. The phone
   identifier is to be used when connecting to a telephony gateway. Even
   without this parameter, recipients of SIP URLs MAY interpret the
   pre-@ part as a phone number if local restrictions on the name space
   for user name allow it.

     telephone-subscriber  = global-phone-number | local-phone-number
     global-phone-number   = "+" 1*phonedigit [isdn-subaddress]
                               [post-dial]
     local-phone-number    = 1*(phonedigit | dtmf-digit |
                               pause-character) [isdn-subaddress]
                               [post-dial]
     isdn-subaddress       = ";isub=" 1*phonedigit
     post-dial             = ";postd=" 1*(phonedigit | dtmf-digit
                           |  pause-character)
     phonedigit            = DIGIT | visual-separator
     visual-separator      = "-" | "."
     pause-character       = one-second-pause | wait-for-dial-tone
     one-second-pause      = "p"
     wait-for-dial-tone    = "w"
     dtmf-digit            = "*" | "#" | "A" | "B" | "C" | "D"

   Figure 4: SIP URL syntax; telephone subscriber

   If a server handles SIP addresses for another domain, it MUST URL-
   encode the "@" character (%40). The ";" character MUST be URL-
   encoded, as otherwise it is not possible to distinguish, in one
   parsing pass, the case host;parameter and user;moreuser@host

   host: The mailto: URL and RFC 822 email addresses require that
        numeric host addresses ("host numbers") are enclosed in square
        brackets (presumably, since host names might be numeric), while
        host numbers without brackets are used for all other URLs. The
        SIP URL requires the latter form, without brackets.

   port: If missing, the port number is assumed to be the SIP default
        port, 5060.

   URL parameters: SIP URLs can define specific parameters of the
        request:
        request. URL parameters are added after the host component and
        are separated by semi-colons. The transport parameter determines
        the the transport mechanism (UDP or TCP). UDP is to be assumed
        when no explicit transport parameter is included. The maddr
        parameter provides the server address to be contacted for this
        user, overriding the address supplied in the host field.  This
        address is typically a multicast address, but could also be the
        address of a backup server. 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. The user parameter was described above, the tag parameter is described in
        its own section below.  URL parameters are added after the host
        component and are separated by semi-colons. above. For example, to
        specify to call j.doe@big.com using multicast to 239.255.255.1
        with a ttl of 15, the following URL would be used:

     sip:j.doe@big.com;maddr=239.255.255.1;ttl=15

   The transport, maddr, and ttl parameters MUST NOT be used in the From
   and To header fields and the Request-URI; they are ignored if
   present.

   Headers: Headers of the SIP request can be defined with the "?"
        mechanism within a SIP URL. The special hname "body" indicates
        that the associated hvalue is the message-body of the SIP INVITE
        request. Headers MUST NOT be used in the From and To header
        fields and the Request-URI; they are ignored if present.

   Tag:

   Method: The tag parameter serves as a general mechanism to distinguish
        multiple instances method of a user identified by a single SIP URL.
        Such distinction is needed in two cases. First, as proxies can
        fork requests, the same SIP request can reach multiple instances of
        a user (mobile and home phones, for example). As each can
        respond, there needs to be a means to distinguish the responses
        from each at the caller. The situation also arises specified with
        multicast requests. The tag serves to distinguish responses at the UAC. It
        method parameter.  This parameter MUST NOT be placed used in the From
        and To field of the response by
        each instance when there is a possibility that header fields and the request was
        forked at an intermediate proxy. This, in general, means that Request-URI; they are ignored if
        present.

   Table 2 summarizes where the Tag MUST be inserted when components of the SIP URL in the To does not refer
        to a fully qualified hostname. The tag MUST can be added by UAS,
        registrars used
   and redirect servers, but MUST NOT be inserted into
        responses forwarded upstream by proxies. The Tag is added for
        all responses for all methods. All subsequent transactions
        between two entities MUST include the Tag parameter, as
        described in Section 11.

   Secondly, the tag MAY appear in the From field of a call invitation.
   This is needed when it is anticipated that two instances of a user
   sharing a SIP address may make call invitations with the same Call-
   ID. In this case, it MUST be present.

   The use of version-1 (time based) or version-4 (random) UUID [19] is
   OPTIONAL. The tag value is designed to be globally unique and
   cryptographically random with at least 32 bits of randomness. It
   SHOULD NOT be included in long-lived SIP URLs, e.g., those found on
   web pages or user databases.  A single user maintains the same tag
   throughout the call identified by the Call-ID. The tag parameter in
   To headers is ignored when matching responses to requests that did what default values they assume if not contain a tag in their To header. (See Section 6.37.)

   Table 2 summarizes where the components of the SIP URL can be used.

   Examples of SIP URLs are:

     sip:j.doe@big.com
     sip:j.doe:secret@big.com;transport=tcp
     sip:j.doe@big.com?subject=project
     sip:+1-212-555-1212:1234@gateway.com;user=phone present.

                      default      Request-URI    To    From    Location    Contact    external
   user               --           x              x     x       x          x
   password           --           x              x             x          x
   host               mandatory    x              x     x       x          x
   user-param
   port               5060         x              x     x       x          x
   tag-param
   user-param         ip           x              x     x       x          x
   method             INVITE                                    x          x
   maddr-param        --                                        x          x
   ttl-param          1                                         x          x
   transport-param    --                                        x          x
   headers            --                                        x          x

   Table 2: Use and default values of URL components  for  SIP  headers,
   Request-URI and references

   Examples of SIP URLs are:

     sip:j.doe@big.com
     sip:j.doe:secret@big.com;transport=tcp
     sip:j.doe@big.com?subject=project
     sip:+1-212-555-1212:1234@gateway.com;user=phone
     sip:1212@gateway.com
     sip:alice@10.1.2.3
     sip:alice@example.com;tag=f81d4fae-7dec-11d0-a765-00a0c91e6bf6
     sip:alice
     sip:alice@registrar.com;method=REGISTER

   Within a SIP message, URLs are used to indicate the source and
   intended destination of a request, redirection addresses and the
   current destination of a request. Normally all these fields will
   contain SIP URLs.

   SIP URLs are case-insensitive, so that for example the two URLs
   sip:j.doe@example.com and SIP:J.Doe@Example.com are equivalent.  All
   URL parameters are included when comparing SIP URLs for equality.

   In some circumstances a non-SIP URL may be used in a

   SIP message. An
   example might be making header fields MAY contain non-SIP URLs. As an example, if a call
   from a telephone which is relayed by a
   gateway onto the internet as a SIP request. In such a case, the
   source of the call is really the telephone number of the caller, and
   so a SIP URL is inappropriate and a phone URL might be used instead.
   To allow for this flexibility, SIP headers that specify user
   addresses allow these addresses to be SIP and non-SIP URLs.

   Clearly not all URLs are appropriate to be used in a SIP message as a
   user address. The correct behavior when an unknown scheme is
   encountered by a SIP server is defined in the context of each of Internet via SIP, the SIP From
   header fields that use field might contain a SIP phone URL.

3 SIP Message Overview

   SIP is a text-based protocol and uses the ISO 10646 character set in
   UTF-8 encoding (RFC 2279 [20]). Lines are terminated by CRLF, but
   receivers should be prepared to MUST also interpret CR and LF by themselves as line
   terminators.

   Except for the above difference in character sets, much of the
   message syntax is identical to HTTP/1.1; rather than repeating it
   here we use [HX.Y] to refer to Section X.Y of the current HTTP/1.1
   specification (RFC 2068 [7]). In addition, we describe SIP in both
   prose and an augmented Backus-Naur form (BNF) [H2.1] described in
   detail in RFC 2234 [21].

   Unlike HTTP, SIP MAY use UDP. When sent over TCP or UDP, multiple SIP
   transactions can be carried in a single TCP connection or UDP
   datagram. UDP datagrams, including all headers, should not normally SHOULD NOT be larger
   than the path maximum transmission unit (MTU) if the MTU is known, or
   1400 bytes if the MTU is unknown.

        The 1400 bytes accommodates lower-layer packet headers
        within the "typical" MTU of around 1500 bytes. Recent
        studies [22] indicate that an MTU of 1500 bytes is a
        reasonable assumption. The next lower common MTU values are
        1006 bytes for SLIP and 296 for low-delay PPP (RFC 1191
        [23]). Thus, another reasonable value would be a message
        size of 950 bytes, to accommodate packet headers within the
        SLIP MTU without fragmentation.

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

        SIP-message  ___   Request |    Response

   Both Request (section 4) and Response (section 5) messages use the
   generic-message format of RFC 822 [24] for transferring entities (the
   body of the message). Both types of messages consist of a start-line,
   one or more header fields (also known as "headers"), an empty line
   (i.e., a line with nothing preceding the carriage-return line-feed
   (CRLF)) indicating the end of the header fields, and an optional
   message-body. To avoid confusion with similar-named headers in HTTP,
   we refer to the header headers describing the message body as entity
   headers. These components are described in detail in the upcoming
   sections.

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

        start-line         =    Request-Line |       Section 4.1
                                Status-Line          Section 5.1

        message-header    =    ( general-header
                               | request-header
                               | response-header
                               | entity-header )

   In the interest of robustness, any leading empty line(s) MUST be
   ignored. In
   other words, if the Request or Response message begins with a CRLF,
   CR, or LF, these characters should MUST be ignored.

4 Request

   The Request message format is shown below:

        Request    =    Request-Line         ;  Section 4.1
                        *( general-header
                        | request-header
                        | entity-header )
                        CRLF
                        [ message-body ]     ;  Section 8

4.1 Request-Line

   The Request-Line begins with a method token, followed by the
   Request-URI and the protocol version, and ending with CRLF. The
   elements are separated by SP characters.  No CR or LF are allowed
   except in the final CRLF sequence.

        Request-Line    =    Method SP Request-URI SP SIP-Version CRLF

4.2 Methods
        general-header     =    Call-ID                ; Section 6.12
                           |    Contact                ; Section 6.13
                           |    CSeq                   ; Section 6.16 6.17
                           |    Date                   ; Section 6.17 6.18
                           |    Encryption             ; Section 6.18 6.19
                           |    Expires                ; Section 6.19 6.20
                           |    From                   ; Section 6.20 6.21
                           |    Record-Route           ; Section 6.29
                           |    Timestamp              ; Section 6.36
                           |    To                     ; Section 6.37
                           |    Via                    ; Section 6.40
        entity-header      =    Content-Encoding       ; Section 6.13 6.14
                           |    Content-Length         ; Section 6.14 6.15
                           |    Content-Type           ; Section 6.15 6.16
        request-header     =    Accept                 ; Section 6.7
                           |    Accept-Encoding        ; Section 6.8
                           |    Accept-Language        ; Section 6.9
                           |    Authorization          ; Section 6.11
                           |    Hide    Contact                ; Section 6.21 6.13
                           |    Location    Hide                   ; Section 6.22
                           |    Max-Forwards           ; Section 6.23
                           |    Organization           ; Section 6.24
                           |    Priority               ; Section 6.25
                           |    Proxy-Authorization    ; Section 6.27
                           |    Proxy-Require          ; Section 6.28
                           |    Route                  ; Section 6.33
                           |    Require                ; Section 6.30
                           |    Response-Key           ; Section 6.31
                           |    Subject                ; Section 6.35
                           |    User-Agent             ; Section 6.39
        response-header    =    Allow                  ; Section 6.10
                           |    Location               ; Section 6.22
                           |    Proxy-Authenticate     ; Section 6.26
                           |    Retry-After            ; Section 6.32
                           |    Server                 ; Section 6.34
                           |    Unsupported            ; Section 6.38
                           |    Warning                ; Section 6.41
                           |    WWW-Authenticate       ; Section 6.42

   Table 3: SIP headers

   The methods are defined below. Methods that are not supported by Request message format is shown below:

        Request    =    Request-Line         ;  Section 4.1
                        *( general-header
                        | request-header
                        | entity-header )
                        CRLF
                        [ message-body ]     ;  Section 8

4.1 Request-Line

   The Request-Line begins with a
   proxy or redirect server are treated by that server as if they were
   an OPTIONS method token, followed by the
   Request-URI and the protocol version, and ending with CRLF. The
   elements are separated by SP characters.  No CR or LF are allowed
   except in the final CRLF sequence.

        Request-Line    =    Method SP Request-URI SP SIP-Version CRLF

4.2 Methods

   The methods are defined below. Methods that are not supported by a
   proxy or redirect server are treated by that server as if they were
   an OPTIONS method and forwarded accordingly.  Methods that are not
   supported by a user agent server or registrar cause a 501 (Not
   Implemented) response to be returned (Section 7).

        Method    =    "ACK" | "BYE" | "CANCEL" | "INVITE"
                 |     "OPTIONS" | "REGISTER"

4.2.1 INVITE

   The INVITE method indicates that the user or service is being invited
   to participate in a session. The message body contains a description
   of the session to which the callee is being invited. For two-party
   calls, the caller indicates the type of media it is able to receive
   as well as their parameters such as network destination. If the
   session description format allows this, it may also indicate "send-
   only" media. A success
   response indicates in its message body which media the callee wishes
   to receive.

   A server MAY automatically respond to an invitation for a conference
   the user is already participating in, identified either by the SIP
   Call-ID or a globally unique identifier within the session
   description, with a 200 (OK) response.

   If a user agent receives an INVITE request for an existing Call-ID
   with a higher CSeq sequence number than any previous INVITE for the
   same Call-ID, 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. It MUST also
   inspect any other header fields for changes and act accordingly. If
   the session description has changed, the user agent server MUST
   adjust the session parameters accordingly, possibly after asking the
   user for confirmation. (Versioning of the session description may 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.)

   This method MUST be supported by SIP proxy, redirect and user agent
   servers as well as clients.

4.2.2 ACK

   The ACK request confirms that the client has received a final
   response to an INVITE request. (ACK is used only     with INVITE
   requests.) 2xx responses are acknowledged by client user agents, all
   other final responses by the first proxy or client user agent to
   receive the response. The Via is always initialized to the host that
   originates the ACK request, i.e., the client user agent after a 2xx
   response or the first proxy to receive a non-2xx final response. The
   ACK request is forwarded as the corresponding INVITE request, based
   on its Request-URI. See Section 10 for details.

   The ACK request MAY contain a message body with the final session
   description to be used by the callee. If the ACK message body is
   empty, the callee uses the session description in the INVITE request.

   A proxy server receiving an ACK request after having sent a 3xx, 4xx,
   5xx, or 6xx response must make a determination about whether the ACK
   is for it, or for some user agent or proxy server further downstream.
   This determination is made by examining the tag in the To field. If
   the tag in the ACK To header field matches the tag in the To header
   field of the response, the ACK is meant for the proxy server.
   Otherwise, the ACK SHOULD be proxied downstream as any other request.

        It is possible for a user agent client or proxy server to
        receive multiple 3xx, 4xx, 5xx, and 6xx responses to a
        request along a single branch. This can happen under
        various error conditions, typically when a forking proxy
        transitions from stateful to stateless before receiving all
        responses. The various responses will all be identical,
        except for the tag in the To field, which is different for
        each one. It can therefore be used as a means to
        disambiguate them.

   This method MUST be supported by SIP proxy, redirect and user agent
   servers as well as clients.

4.2.3 OPTIONS

   The server is being queried as to its capabilities. A server that
   believes it can contact the user, such as a user agent where the user
   is logged in and has been recently active, MAY respond to this
   request with a capability set. A called user agent MAY return a
   status reflecting how it would have responded to an invitation, e.g.,
   600 (Busy). Such a server SHOULD return an Allow header field
   indicating the methods that it supports. Proxy and redirect servers
   simply forward the request without indicating their capabilities.

   This method MUST be supported by SIP proxy, redirect and user agent
   servers, registrars and clients.

4.2.4 BYE

   The user agent client uses BYE to indicate to the server that it
   wishes to abort release the call. A BYE request is forwarded like an INVITE
   request. A
   request and MAY be issued by either caller or callee. A party to a
   call SHOULD issue a BYE request before aborting releasing a call ("hanging
   up"). Note that  A party receiving a BYE request may also be issued by MUST cease transmitting media
   streams specifically directed at the
   callee. party issuing the BYE request.

   If the INVITE request contained a Location Contact header, the callee MAY send
   a BYE request to that address rather than the From address.

   This method MUST be supported by proxy servers and SHOULD be
   supported by redirect and user agent SIP servers.

4.2.5 CANCEL

   The CANCEL request cancels a pending request with the same Call-ID,
   To, From and CSeq (sequence number only) header field values, but
   does not affect a completed request. (A request is considered
   completed if the server has returned a final status response.)

   A user agent client or proxy client MAY issue a CANCEL request at any
   time. A proxy, in particular, MAY choose to send a CANCEL to
   destinations that have not yet returned a final response after it has
   received a 2xx or 6xx response for one or more of the parallel-search
   requests. A proxy that receives a CANCEL request forwards the request
   to all destinations with pending requests.

   The Call-ID, To, the numeric part of CSeq and From headers in the
   CANCEL request are identical to those in the original request. This
   allows a CANCEL request to be matched with the request it cancels.
   However, to allow the client to distinguish responses to the CANCEL
   from those to the original request, the CSeq Method component is set
   to CANCEL. The Via header field is initialized to the proxy issuing
   the CANCEL request. (Thus, responses to this CANCEL request only
   reach the issuing proxy.)

   Once a user agent server has received a CANCEL, it MUST NOT issue a
   2xx response for the cancelled original request.

   A redirect or user agent server receiving a CANCEL request responds
   with a status of 200 (OK) if the Call-ID transaction exists and a status of
   481
   (Invalid Call-ID) (Transaction Does Not Exist) if not, but takes no further action.
   In particular, any existing call is unaffected.

        The BYE request cannot be used to cancel branches of a
        parallel search, since several branches may, through
        intermediate proxies, find the same user agent server and
        then terminate the call.  To terminate a call instead of
        just pending searches, the UAC must use BYE instead of or
        in addition to CANCEL. While CANCEL can terminate any
        pending request other than ACK or CANCEL, it is typically
        useful only for INVITE. 200 responses to INVITE and 200
        responses to CANCEL are distinguished by the method in the
        Cseq header field, so there is no ambiguity.

   This method MUST be supported by proxy servers and SHOULD be
   supported by all other SIP server types.

4.2.6 REGISTER

   A client uses the REGISTER method to register the address listed in
   the To header field with a SIP server.

   A user agent MAY register with a local server on startup by sending a
   REGISTER request to the well-known "all SIP servers" multicast
   address "sip.mcast.net" (224.0.1.75), with a time-to-live value of 1.
   SIP user agents on the same subnet MAY listen to that address and use
   it to become aware of the location of other local users [16]; [17];
   however, they do not respond to the request.  A user agent MAY also
   be configured with the address of a registrar server to which it
   sends a REGISTER request upon startup.

   The meaning of the REGISTER request-header fields is defined as
   follows. We define "address-of-record" as the SIP address that the
   registry knows the registrand, typically of the form "user@domain"
   rather than "user@host". In third-party registration, the entity
   issuing the request is different from the entity being registered.

   To: The To header field contains the address-of-record whose
        registration is to be created or updated.

   From: The From header field contains the address-of-record of the
        person responsible for the registration. For first-party
        registration, it is identical to the To header field value.

   Request-URI: The Request-URI names the destination of the
        registration request, i.e., the domain of the registrar. The
        user name MUST be empty. Generally, the domains in the Request-
        URI and the To header field have the same value; however, it is
        possible to register as a "visitor", while maintaining one's
        name. For example, a traveller sip:alice@acme.com (To) may might
        register under the Request-URI sip:@atlanta.ayh.org , with the
        former as the To header field and the latter as the Request-URI.
        The request is no longer forwarded once it reached the server
        whose authoritative domain is the one listed in the Request-URI.

   Location:

   Contact: The request MAY contain a Location Contact header field; future
        non-REGISTER non-
        REGISTER requests for the URI given in the To header field will
        be directed to the address(es) given in the Location Contact header. It is
        RECOMMENDED that user agents include SIP URLs with both UDP and
        TCP transport parameters in their registration. If the
        registration contains a Location field whose URL includes a
        transport parameter, future requests will use that protocol.
        Otherwise, requests use the same transport protocol as used by
        the registration. However, a multicast REGISTER request still
        causes future requests to be unicast unless the maddr URL
        parameter explicitly requests otherwise. If the Location header
        does not contain a port number, the default SIP port number is
        used for future requests.

        We cannot require that registration and subsequent INVITE
        requests use the same transport protocol, as multicast
        registrations may be quite useful.

   If the request does not contain a Location Contact header, the registration
   remains unchanged. Registrations using SIP URIs that differ in one or
   more of host, port, transport transport-param or maddr maddr-param from an existing
   registration are added to the list of registrations. If these parameters Other URI types
   are compared according to the same as standard URI equivalency rules for the
   URI schema.  If the URIs are equivalent to that of an existing
   registration, the new registration replaces the old one if it has a
   higher q value or, for the same value of q, if the ttl value is
   higher. All current locations registrations MUST share the same action value.
   Registrations that have a different action than current registrations
   for the same user are rejected with status of 409 (Conflict).

   A proxy server ignores the q parameter in when processing non-REGISTER
   requests, while a redirect server simply returns the that parameter in
   its
   Location header. Contact response header field.

        Having the proxy server interpret the q parameter is not
        sufficient to guide proxy behavior, as it is not clear, for
        example, how long it should is supposed to wait between trying
        addresses.

   If the registration is changed while a user agent or proxy server
   processes an invitation, the new information should SHOULD be used.

        This allows a service known as "directed pick-up".

   A server SHOULD silently drop the registration after one hour, unless
   refreshed by the client. A client may MAY request a lower or higher
   refresh interval through the Expires header (Section 6.19). 6.20). Based on
   this request and its configuration, the server chooses the expiration
   interval and indicates it through the Expires header field in the
   response. A single address (if host-independent) may MAY be registered
   from several different clients.

   A client cancels an existing registration by sending a REGISTER
   request with an expiration time (Expires) of zero seconds for a
   particular Location Contact or the wildcard Location Contact designated by a "*" for
   all registrations. Registrations are matched based on the user, host,
   port and maddr parameters.

   The server SHOULD return the current list of registrations in the 200
   response as Location Contact header fields.

   It is particularly important that REGISTER requests are authenticated
   since they allow to redirect future requests.

        Beyond its use as a simple location service, this method is
        needed if there are several SIP servers on a single host.
        In that case, only one of the servers can use the default
        port number. Each server that cannot would register registers with a
        server for the administrative domain.  Since a client may clients do not
        always have easy access to the host address or port number,
        using the source address and port from the request itself
        seems simpler.

   Support of this method is RECOMMENDED.

4.3 Request-URI

   The Request-URI field is a SIP URL as described in Section 2 or a general
   URI. It indicates the user or service to which this request is being
   addressed. Unlike the To field, the Request-URI field may MAY be re-written by
   proxies.

   When used as a Request-URI, a SIP-URL MUST NOT contain the
   transport-param, maddr-param, ttl-param, or headers elements. A
   server that receives a SIP-URL with these elements removes them
   before further processing.

        Typically, the UAC sets the Request-URI and To to the same
        SIP URL, presumed to remain unchanged over long time
        periods. However, if the UAC has cached a more direct path
        to the callee, e.g., from the Location Contact header field of a
        response to a previous request, the To would still contain
        the long-term, "public" address, while the Request-URI
        would be set to the cached address.

   Proxy and redirect servers may MAY use the information in the Request-URI
   and request header fields to handle the request and possibly rewrite
   the Request-URI. For example, a request addressed to the generic
   address sip:sales@acme.com might be is proxied to the particular person, e.g.,
   sip:bob@ny.acme.com , with the To remaining as sales@acme.com
   ny.acme.com , Bob may have designated then designates Alice as the temporary substitute.

   The host part of the Request-URI typically agrees with one of the
   host names of the server. If it does not, the server SHOULD proxy the
   request to the address indicated or return a 404 (Not Found) response
   if it is unwilling or unable to do so. For example, the Request-URI
   and server host name may can disagree in the case of a firewall proxy
   that handles outgoing calls. This mode of operation similar to that
   of HTTP proxies.

   If a SIP server receives a request with a URI indicating a scheme
   other than SIP which that server does not understand, the server MUST
   return a 400 (Bad Request) response. It MUST do this even if the To
   header field contains a scheme it does understand.

4.3.1 SIP Version

   Both request and response messages include the version of SIP in use,
   and basically follow [H3.1], with HTTP replaced by SIP. To be
   conditionally compliant with this specification, applications sending
   SIP messages MUST include a SIP-Version of "SIP/2.0".

4.4 Option Tags

   Option tags are unique identifiers used to designate new options in
   SIP.  These tags are used in Require (Section 6.30) and Unsupported
   (Section 6.38) fields.

   Syntax:

        option-tag  ___   1*uric
   The creator of a new SIP option should MUST either prefix the option with a
   reverse domain name or register the new option with the Internet
   Assigned Numbers Authority (IANA). For example,
   "com.foo.mynewfeature" is an apt name for a feature whose inventor
   can be reached at "foo.com". Options registered with IANA have the
   prefix "org.ietf.sip.", options described in RFCs have the prefix
   "org.ietf.rfc.N", where N is the RFC number. Option tags are case-
   insensitive.

4.4.1 Registering New Option Tags with IANA

   When registering a new SIP option, the following information should MUST be
   provided:

        o Name  and description of option. The name may MAY be of any length,
          but SHOULD be no more than twenty characters long. The name
          MUST NOT contain any spaces, control characters or periods.

        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 For proprietary options, contact information  (postal and email
          address);

        Borrowed from RTSP and the RTP AVP.

5 Response

   After receiving and interpreting a request message, the recipient
   responds with a SIP response message. The response message format is
   shown below:

        Response    =    Status-Line          ;  Section 5.1
                         *( general-header
                         | response-header
                         | entity-header )
                         CRLF
                         [ message-body ]     ;  Section 8
   [H6] applies except that HTTP-Version is replaced by SIP-Version.
   Also, SIP defines additional response codes and does not use some
   HTTP codes.

5.1 Status-Line

   The first line of a Response message is the Status-Line, consisting
   of the protocol version (Section 4.3.1) followed by a numeric
   Status-Code and its associated textual phrase, with each element
   separated by SP characters. No CR or LF is allowed except in the
   final CRLF sequence.

        Status-Line    =    SIP-version SP Status-Code SP Reason-Phrase CRLF

5.1.1 Status Codes and Reason Phrases

   The Status-Code is a 3-digit integer result code that indicates the
   outcome of the attempt to understand and satisfy the 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. The client is not
   required to examine or display the Reason-Phrase.

        Status-Code       =    Informational                  Fig. 5
                         |     Success                        Fig. 5
                         |     Redirection                    Fig. 6
                         |     Client-Error                   Fig. 7
                         |     Server-Error                   Fig. 8
                         |     Global-Failure                 Fig. 9
                         |     extension-code
        extension-code    =    3DIGIT
        Reason-Phrase     =    *<TEXT,  excluding CR, LF>

   We provide an overview of the Status-Code below, and provide full
   definitions in Section 7. The first digit of the Status-Code defines
   the class of response. The last two digits do not have any
   categorization role. SIP/2.0 allows 6 values for the first digit:

   1xx: Informational -- request received, continuing to process the
        request;

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

   3xx: Redirection -- further action must 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 is invalid at any server.

   Figures 5 through 9 present the individual values of the numeric
   response codes, and an example set of corresponding reason phrases
   for SIP/2.0. These reason phrases are only recommended; they may be
   replaced by local equivalents without affecting the protocol. Note
   that SIP adopts many HTTP/1.1 response codes. SIP/2.0 adds response
   codes in the range starting at x80 to avoid conflicts with newly
   defined HTTP response codes, and adds a new class, 6xx, of response
   codes.

   SIP response codes are extensible. SIP applications are not required
   to understand the meaning of all registered response codes, though
   such understanding is obviously desirable. However, applications MUST
   understand the class of any response code, as indicated by the first
   digit, and treat any unrecognized response as being equivalent to the
   x00 response code of that class, with the exception that an
   unrecognized response MUST NOT be cached. For example, if a client
   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. In
   such cases, user agents SHOULD present to the user the message body
   returned with the response, since that message body is likely to
   include human-readable information which will explain the unusual
   status.

        Informational    =    "100"    ;  Trying
                        |     "180"    ;  Ringing
                        |     "181"    ;

6 Header Field Definitions

   SIP header fields are similar to HTTP header fields in both syntax
   and semantics [H4.2, H14]. In general, the ordering of the header
   fields is not of importance (with the exception of Via fields, see
   below). The only requirement is that header fields which are hop-by-
        Informational    =    "100"    ;  Trying
                        |     "180"    ;  Ringing
                        |     "181"    ;  Call Is Being Forwarded
                        |     "182"    ;  Queued
        Success          =    "200"    ;  OK

   Figure 5: Informational and success status codes

        Redirection    =    "300"    ;  Multiple Choices
                      |     "301"    ;  Moved Permanently
                      |     "302"    ;  Moved Temporarily
                      |     "303"    ;  See Other
                      |     "305"    ;  Use Proxy
                      |     "380"    ;  Alternative Service

   Figure 6: Redirection status codes

6 Header Field Definitions

   SIP header fields are similar to HTTP header fields in both syntax
   and semantics [H4.2, H14]. In general, the ordering of the header
   fields is not of importance (with the exception of Via fields, see
   below). The only requirement is that header fields which are hop-by-

   hop MUST appear before any header fields which are end-to-end.
   Proxies MUST NOT reorder or otherwise modify header fields other than
   by adding a new Via or other hop-by-hop field. Proxies MUST NOT, for
   example, change how header fields are broken across lines. This
   allows an authentication field to be added after the Via header
   fields that will not be invalidated by proxies.

   The header fields required, optional and not applicable for each
   method are listed in Table 4 and Table 5.  The table uses "o" to
   indicate optional, "m" mandatory and "-" for not applicable. A "*"
   indicates that the header fields are needed only if message body is
   not empty: The Content-Type and Content-Length header fields are
   required when there is a valid message body (of non-zero length)
   associated with the message (Section 8).

   The "where" column describes the request and response types with
   which the header field can be used. "R" refers to header fields that
   can be used in requests (that is, request and general header fields).
   "r" designates a response or general-header field as applicable to
   all responses, while a list of numeric values indicates the status
   codes with which the header field can be used. "g" and "e" designate
   general (Section 6.1) and entity header (Section 6.2) fields,
   respectively. If a header field is marked "c", it is copied from the
        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
                       |     "410"    ;  Gone
                       |     "411"    ;  Length Required
                       |     "413"    ;  Request Message Body Too Large
                       |     "414"    ;  Request-URI Too Large
                       |     "415"    ;  Unsupported Media Type
                       |     "420"    ;  Bad Extension
                       |     "480"    ;  Temporarily not available
                       |     "481"    ;  Invalid Call-ID  Call Leg/Transaction Does Not Exist
                       |     "482"    ;  Loop Detected
                       |     "483"    ;  Too Many Hops
                       |     "484"    ;  Address Incomplete
                       |     "485"    ;  Ambiguous
                       |     "486"    ;  Busy Here

   Figure 7: Client error status codes

        Server-Error    =    "500"    ;  Internal Server Error
                       |     "501"    ;  Not Implemented
                       |     "502"    ;  Bad Gateway
                       |     "503"    ;  Service Unavailable
                       |     "504"    ;  Gateway Timeout
                       |     "505"    ;  SIP Version not supported

   Figure 8: Server error status codes

   request to the response.

   The "enc." column describes whether this message header fields required, optional and not applicable for each
   method are listed in Table 4. The table uses "o" to indicate
   optional, "m" mandatory and "-" for not applicable. field MAY be
   encrypted end-to-end. A "*" indicates "n" designates fields that the header MUST NOT be
   encrypted, while "c" designates fields are needed only that SHOULD be encrypted if message body is not empty:
   The Content-Type and Content-Length headers are required when there
   is a valid message body (of non-zero length) associated with the
        Global-Failure   |    "600"    ;  Busy Everywhere
                         |    "603"    ;  Decline
                         |    "604"    ;  Does not exist anywhere
                         |    "606"    ;  Not Acceptable

   Figure 9: Global failure status codes

   message (Section 8).

   The "type" column describes the request and response types for which
   the header field may be used. A numeric value indicates the status
   code for a response, while "R" refers to any request header, "r" to
   any response header. "g" and "e" designate general (Section 6.1) and
   entity header (Section 6.2) fields, respectively.

   The "enc." column describes whether this message header may be
   encrypted end-to-end. A "n" designates fields that MUST NOT be
   encrypted, while "c" designates fields that SHOULD be encrypted if

   encryption is used.

   The "e-e" column has a value of "e" for end-to-end and a value of "h"
   for hop-by-hop headers.

   Other headers may be added as required; a server MAY ignore optional
   headers that it does not understand. A compact form of these header
   fields is also defined in Section 9 for use over UDP when the request
   has to fit into a single packet and size is an issue.

   Table 5 in Appendix A indicates which system components should be
   capable of parsing which header fields.

6.1 General Header Fields

   General header fields apply to both request and response messages.
   The general-header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields may be given the semantics of general
   header fields if all parties in the communication recognize them to
   be general-header fields. Unrecognized header fields are treated as
   entity-header fields.

6.2 Entity Header Fields
   The entity-header fields define meta-information about the message-
   body or, if no body is present, about the resource identified by the
   request. The term "entity header" is an HTTP 1.1 term

                       where the
   response body may contain a transformed version of the message body.
   The original message body is referred to as the "entity". We retain
   the same terminology for header fields but usually refer to the
   "message body" rather then the entity as the two are the same in SIP.

6.3 Request Header Fields

   The request-header fields allow the client to pass additional
   information about the request, and about the client itself, to the
   server. These fields act as request modifiers, with semantics
   equivalent to the parameters of a programming language method
   invocation.

   The request-header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields MAY be given the semantics of request-
   header fields if all parties in the communication recognize them to
   be request-header fields. Unrecognized header fields are treated as
   entity-header fields.

6.4 Response Header Fields

   The response-header fields allow the server to pass additional
   information about the response which cannot be placed in the Status-
   Line. These header fields give information about the server and about
   further access to the resource identified by the Request-URI.

   Response-header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields MAY be given the semantics of response-
   header fields if all parties in the communication recognize them to
   be response-header fields. Unrecognized header fields are treated as
   entity-header fields.

6.5 End-to-end and Hop-by-hop Headers

   End-to-end headers must be transmitted unmodified across all proxies,
   while hop-by-hop headers may be modified or added by proxies.

6.6 Header Field Format

   Header fields (general-header, request-header, response-header, and
   entity-header) follow the same generic header format as that given in
   Section 3.1 of RFC 822 [24]. Each header field consists of a name
   followed by a colon (":") and the field value. Field names are case-
                                 type    enc.    e-e   ACK   BYE   CAN   INV   OPT   REG
   _____________________________________________________________________________________________
   ____________________________________________________________________________
   Accept                R               e     -     -     -     o     o     o
   Accept-Encoding       R               e     -     -     -     o     o     o
   Accept-Language       R       n       e     -     o     o     o     o     o
   Allow                200              e     -     -     -     -     m     -
   Allow                405              e     o     o     o     o     o     o
   Authorization         R               e     o     o     o     o     o     o
   Call-ID                         g              gc       n       e     m     m     m     m     m     m
   Contact               R               e     o     -     -     o     o     o
   Contact              1xx              e     -     -     -     o     o     -
   Contact              2xx              e     -     -     -     o     o     o
   Contact              3xx              e     -     o     -     o     o     o
   Contact              485              e     -     o     -     o     o     o
   Content-Encoding      e               e     *     -     -     *     *     *
   Content-Length        e               e     m     o     -     -     m     m     m     o     o     o
   Content-Type          e               e     *     -     -     *     *     *
   CSeq                            g                 gc       n       e     m     m     m     m     m     m
   Date                  g               e     o     o     o     o     o     o
   Encryption            g       n       e     o     o     o     o     o     o
   Expires               g               e     -     -     -     o     -     o
   From                            g                 gc       n       e     m     m     m     m     m     m
   Hide                  R       n       h     o     o     o     o     o     o
   Location
   Max-Forwards          R       n       e     o     -     -     o     o     o
   Location                       1xx                     e     -     -     -     o     o     -
   Location                       2xx                     e     -     -     -     o     o     o
   Location                       3xx                     e     -     o     -     o     o     o
   Location                       485                     e     -     o     -     o     o     o
   Max-Forwards                    R              n       e     o     o     o     o     o     o
   Organization                    g              c       h
   Organization          g       c       h     -     -     -     o     o     o

   Table 4: Summary of header fields, A--O
                             where       enc.    e-e   ACK   BYE   CAN   INV   OPT   REG
   _____________________________________________________________________________________
   Proxy-Authenticate         407         n       h     o     o     o     o     o     o
   Proxy-Authorization         R          n       h     o     o     o     o     o     o
   Proxy-Require               R          n       h     o     o     o     o     o     o
   Priority                    R          c       e     -     -     -     o     -     -
   Require                     R                  e     o     o     o     o     o     o
   Retry-After                 R          c       e     -     -     -     -     -     o
   Retry-After            404,480,503,600,603            404,480,486     c       e     o     o     o     o     o     o
                              503         c       e     o     o     o     o     o     o
                            600,603       c       e     o     o     o     o     o     o
   Response-Key                R          c       e     -     o     o     o     o     o
   Record-Route                R                  h     o     o     o     o     o     o
   Record-Route               2xx                 h     o     o     o     o     o     o
   Route                       R                  h     -     o     o     o     o     o
   Server                      r          c       e     o     o     o     o     o     o
   Subject                     R          c       e     -     -     -     o     -     -
   Timestamp                   g                  e     o     o     o     o     o     o
   To                              g                        gc(1)        n       e     m     m     m     m     m     m
   Unsupported                420                 e     o     o     o     o     o     o
   User-Agent                  g          c       e     o     o     o     o     o     o
   Via                             g                       gc(2)        n       e     m     m     m     m     m     m
   Warning                     r                  e     o     o     o     o     o     o
   WWW-Authenticate           401         c       e     o     o     o     o     o     o

   Table 4: 5: Summary of header fields
   insensitive. The fields, P--Z; (1):  copied  with  possible
   addition of tag; (2): UAS removes first Via header field value may

   Other header fields can be preceded by any amount added as required; a server MAY ignore
   optional header fields that it does not understand. A compact form of leading
   white space (LWS), though
   these header fields is also defined in Section 9 for use over UDP
   when the request has to fit into a single space (SP) packet and size is preferred. an
   issue.

   Table 6 in Appendix A lists those header fields that different client
   and server types MUST be able to parse.

6.1 General Header Fields

   General header fields apply to both request and response messages.
   The general-header field names can be extended over multiple lines by preceding each extra
   line reliably only in
   combination with at least one SP or horizontal tab (HT). Applications SHOULD
   follow HTTP "common form" when generating these constructs, since
   there might exist some implementations that fail to accept anything
   beyond the common forms.

        message-header    =    field-name ":" [ field-value ] CRLF
        field-name        =    token
        field-value       =    *( field-content | LWS )
        field-content     =    < the OCTETs  making up a change in the field-value
                                and consisting of either *TEXT protocol version. However, new or combinations
   experimental header fields may be given the semantics of token,
                                tspecials, and quoted-string>

   The order in which general
   header fields are received is not significant if all parties in the communication recognize them to
   be general-header fields. Unrecognized header fields have different field names. Multiple header are treated as
   entity-header fields.

6.2 Entity Header Fields

   The entity-header fields
   with define meta-information about the same field-name may be present in a message if and only message-
   body or, if no body is present, about the entire field-value for that header field resource identified by the
   request. The term "entity header" is defined as an HTTP 1.1 term where the
   response body can contain a comma-
   separated list (i.e., #(values)). It MUST be possible transformed version of the message body.
   The original message body is referred to combine as the
   multiple "entity". We retain
   the same terminology for header fields into one "field-name: field-value" pair,
   without changing but usually refer to the semantics of
   "message body" rather then the message, by appending each
   subsequent field-value to entity as the first, each separated by a comma. The
   order two are the same in which header SIP.

6.3 Request Header Fields

   The request-header fields with allow the same field-name are received is
   therefore significant client to pass additional
   information about the interpretation of the combined field
   value, request, and thus a proxy MUST NOT change about the order client itself, to the
   server. These fields act as request modifiers, with semantics
   equivalent to the parameters of these field
   values when a message is forwarded.

   Field names are not case-sensitive, although their values may be.

6.7 Accept

   See [H14.1] for syntax. This programming language method
   invocation.

   The request-header field is used names can be extended reliably only in
   combination with
   the a change in the protocol version. However, new or
   experimental header fields MAY be given the semantics of request-
   header fields if all parties in the communication recognize them to
   be request-header fields. Unrecognized header fields are treated as
   entity-header fields.

6.4 Response Header Fields

   The response-header fields allow the server to pass additional
   information about the response which cannot be placed in the Status-
   Line. These header fields give information about the server and about
   further access to the resource identified by the Request-URI.

   Response-header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields MAY be given the semantics of response-
   header fields if all parties in the communication recognize them to
   be response-header fields. Unrecognized header fields are treated as
   entity-header fields.

6.5 End-to-end and Hop-by-hop Headers

   End-to-end headers MUST be transmitted unmodified across all proxies,
   while hop-by-hop headers MAY be modified or added by proxies.

6.6 Header Field Format

   Header fields (general-header, request-header, response-header, and
   entity-header) follow the same generic header format as that given in
   Section 3.1 of RFC 822 [24]. Each header field consists of a name
   followed by a colon (":") and the field value. Field names are case-
   insensitive. The field value MAY be preceded by any amount of leading
   white space (LWS), though a single space (SP) is preferred. Header
   fields can be extended over multiple lines by preceding each extra
   line with at least one SP or horizontal tab (HT). Applications SHOULD
   follow HTTP "common form" when generating these constructs, since
   there might exist some implementations that fail to accept anything
   beyond the common forms.

        message-header    =    field-name ":" [ field-value ] CRLF
        field-name        =    token
        field-value       =    *( field-content | LWS )
        field-content     =    < the OCTETs  making up the field-value
                                and consisting of either *TEXT
                                or combinations of token,
                                tspecials, and quoted-string>

   The relative order of header fields with different field names is not
   significant. 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 (i.e., #(values)).
   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 order in which header fields with the same
   field-name are received is therefore significant to the
   interpretation of the combined field value, and thus a proxy MUST NOT
   change the order of these field values when a message is forwarded.

   Field names are not case-sensitive, although their values may be.

6.7 Accept

   See [H14.1] for syntax. This request-header field is used only with
   the INVITE, OPTIONS and REGISTER request methods to indicate what
   media types are acceptable in the response.

   Example:

     Accept: application/sdp;level=1, application/x-private, text/html

6.8 Accept-Encoding

   The Accept-Encoding request-header field is similar to Accept, but
   restricts the content-codings [H3.4.1] that are acceptable in the
   response. See [H14.3].

6.9 Accept-Language

   See [H14.4] for syntax. The Accept-Language request header request-header field can
   be used to allow the client to indicate to the server in which
   language it would prefer to receive reason phrases, session
   descriptions or status responses carried as message bodies. A proxy may
   MAY use this field to help select the destination for the call, for
   example, a human operator conversant in a language spoken by the
   caller.

   Example:

     Accept-Language: da, en-gb;q=0.8, en;q=0.7

6.10 Allow

   See [H14.7]. The Allow entity-header field lists the set of methods
   supported by the resource identified by the Request-URI. The purpose
   of this field is strictly to inform the recipient of valid methods
   associated with the resource. An Allow header field MUST be present
   in a 405 (Method Not Allowed) response and SHOULD be present in an
   OPTIONS response.

6.11 Authorization

   See [H14.8].

   A user agent that wishes to authenticate itself with a server --
   usually, but not necessarily, after receiving a 401 response -- MAY
   do so by including an Authorization request-header field with the
   request. The Authorization field value consists of credentials
   containing the authentication information of the user agent for the
   realm of the resource being requested.

6.12 Call-ID

   The Call-ID general header general-header field uniquely identifies a particular
   invitation or all registrations of a particular client. Note that a
   single multimedia conference may can give rise to several calls with
   different Call-IDs, e.g., if a user invites a single individual
   several times to the same (long-running) conference.

   For an INVITE request, a callee user agent server SHOULD NOT alert
   the user if the user has responded previously to the Call-ID in the
   INVITE request. If the user is already a member of the conference and
   the conference parameters contained in the session description have
   not changed, a callee user agent server MAY silently accept the call,
   regardless of the Call-ID. An invitation for an existing Call-ID or
   session may can change the parameters of the conference. A client
   application MAY decide to simply indicate to the user that the
   conference parameters have been changed and accept the invitation
   automatically or it MAY require user confirmation.

   A user may be invited to the same conference or call using several
   different Call-IDs. If desired, the client may 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.

   The REGISTER and OPTIONS methods use the Call-ID value to
   unambiguously match requests and responses. All REGISTER requests
   issued by a single client MUST use the same Call-ID.

        Since the Call-ID is generated by and for SIP, there is no
        reason to deal with the complexity of URL-encoding and
        case-ignoring string comparison.

        Call-ID     =    ( "Call-ID" | "i" ) ":" local-id "@" host
        local-id    =    *uric

   host MUST be either a fully qualified domain name or a globally
   routable IP address, while the local-id is a random identifier
   consisting of URI characters that is unique within host. It MUST NOT
   be reused for a different call.  Call-IDs are case-sensitive. The use
   of a UUID as local-id is OPTIONAL. The UUID is a version-4 (random)
   UUID [19].

        Using cryptographically random identifiers provides some
        protection against session hijacking. Call-ID, To and From
        are needed to identify a call leg call leg matters in calls
        with third-party control.

   Example:

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

6.13 Content-Encoding Contact

   The Content-Encoding entity-header Contact general-header field can appear in requests, 1xx, 2xx
   responses and 3xx responses.

   INVITE and ACK requests: INVITE and ACK requests MAY contain Contact
        headers indicating from which location the request is used as a modifier to
        originating.

        This allows the
   media-type. When present, its value indicates what additional content
   codings have been applied callee to the entity-body, and thus what decoding
   mechanisms MUST be applied in order send a BYE directly to obtain the media-type
   referenced by the Content-Type caller
        instead of through a series of proxies. The Via header field.  Content-Encoding is
   primarily used to allow a document to be compressed without losing
        not sufficient since the identity desired address may be that of its underlying media type.  See [H14.12].

6.14 Content-Length a
        proxy.

   INVITE 2xx responses: A user agent server sending a definitive,
        positive response (2xx) MAY insert a Contact response header
        field indicating the SIP address under which it is reachable
        most directly for future SIP requests, such as ACK, within the
        same Call-ID. The Content-Length entity-header Contact header field indicates contains the size address of
        the
   message-body, in decimal number server itself or that of octets, sent to a proxy, e.g., if the recipient.

        Content-Length    =    "Content-Length" ":" 1*DIGIT

   An example host is

     Content-Length: 3495

   Applications SHOULD use
        behind a firewall. The value of this field to indicate Contact header is copied
        into the size Request-URI of the
   message-body to subsequent requests for this call.

        The Contact value SHOULD NOT be transferred, regardless of the media type of cached across calls, as it
        may not represent the
   entity. Any Content-Length greater than or equal to zero is most desirable location for a valid
   value. If no body is present in
        particular destination address.

   INVITE 1xx responses: A UAS sending a message, then provisional response (1xx) MAY
        insert a Contact response header. It has the Content-Length
   header MUST be set to zero. If same semantics in a server receives
        1xx response as a UDP request
   without Content-Length, it 2xx INVITE response.  Note that CANCEL
        requests MUST assume NOT be sent to that address, but rather follow the
        same path as the original request.

   REGISTER requests: REGISTER requests MAY contain a Contact header
        field indicating at which locations the user is reachable. The
        REGISTER request encompasses defines a wildcard Contact field, "*", which
        MUST only be used with Expires: 0 to remove all registrations
        for a particular user.  An optional expires parameter indicates
        the remainder desired expiration time of the packet. registration. If a response Contact
        entry does not contain a
   Content-Length, the client assumes that it encompasses the remainder
   of the UDP packet or the data until have an expires parameter, the TCP connection Expires header
        field is closed, used as
   applicable.  Section 8 describes how to determine the length default value. If neither of these
        mechanisms is used, SIP URIs are assumed to expire after one
        hour. Other URI schemes have no expiration times.

   REGISTER 2xx responses: A REGISTER response MAY return all locations
        at which the
   message body.

6.15 Content-Type

   The Content-Type entity-header field user is currently reachable.  An optional expires
        parameter indicates the media type expiration time of the
   message-body sent to registration. If
        a Contact entry does not have an expires parameter, the recipient. The media-type element is defined
   in [H3.7].

        Content-Type    =    ( "Content-Type" ":" media-type
   Examples value of this header field are

     Content-Type: application/sdp
     Content-Type: text/html; charset=ISO-8859-4

6.16 CSeq

   Clients MUST add
        the CSeq (command sequence) general-header field to
   every request. A CSeq request Expires header field contains a single decimal
   sequence number chosen by indicates the requesting client, unique within a
   single value of Call-ID. The sequence number MUST be expressible as a
   32-bit unsigned integer. The initial value of expiration time. If
        neither mechanism is used, the sequence number expiration time specified in the
        request, explicitly or by default, is
   arbitrary, but MUST used.

   3xx and 485 responses: The Contact response-header field can be less than 2**31.  Consecutive requests that
   differ in request method, headers used
        with a 3xx or body, but have 485 (Ambiguous) response codes to indicate one or
        more alternate addresses to try. It can appear in responses to
        BYE, INVITE and OPTIONS methods. The Contact header field
        contains URIs giving the same Call-ID
   MUST new locations or user names to try, or
        may simply specify additional transport parameters. A 300
        (Multiple Choices), 301 (Moved Permanently), 302 (Moved
        Temporarily) or 485 (Ambiguous) response SHOULD contain strictly monotonically increasing and contiguous
   sequence numbers; sequence numbers do not wrap around.
   Retransmissions a
        Contact field containing URIs of new addresses to be tried. A
        301 or 302 response may also give the same request carry the same sequence number, location and username
        that was being tried but an INVITE with specify additional transport parameters
        such as a different message body server or multicast address to try or different header
   fields (a "re-invitation") acquires a new, higher sequence number. A
   server MUST echo the CSeq value
        change of SIP transport from the request in its response. If
   the Method value is missing, the server fills it in appropriately. UDP to TCP or vice versa. The ACK
        client copies the user, password, host, port and CANCEL requests MUST contain user-param
        elements of the same CSeq value as Contact URI into the
   INVITE request that it refers to, while a BYE request cancelling an
   invitation MUST have a higher sequence number.

   A user agent server MUST remember Request-URI of the highest sequence number for any
   INVITE
        redirected request with and directs the same Call-ID value. The server MUST respond
   to, but ignore any INVITE request with a lower sequence number.

   All requests spawned to the address
        specified by the maddr and port parameters, using the transport
        protocol given in the transport parameter. If maddr is a parallel search have
        multicast address, the same CSeq value of ttl is used as the request triggering time-to-live
        value.

   Note that the parallel search.

        CSeq    =    "CSeq" ":" 1*DIGIT Method

        Strictly speaking, CSeq Contact header fields are needed for any field MAY also refer to a different
   entity than the one originally called. For example, a SIP request that call
   connected to GSTN gateway may need to deliver a special information
   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 cancelled by reached, not limited to SIP
   URLs. For example, it can contain a BYE phone or CANCEL
        request fax,

   mailto: (RFC 2368, [25]) or where a client can issue several requests for
        the same Call-ID irc: URL.

   The following parameters are defined. Additional parameters may be
   defined in close succession. Without a sequence
        number, other specifications.

   q: The qvalue indicates the response to an INVITE could be mistaken for relative preference among the
        response locations
        given. qvalue values are decimal numbers from 0.0 to 1.0, with
        higher values indicating higher preference.

   action: The action parameter is only used when registering with the cancellation (BYE or CANCEL). Also, if
        REGISTER request. It indicates whether the
        network duplicates packets client wishes that
        the server proxy or if an ACK redirect future requests intended for the
        client. If this parameter is delayed until not specified the action taken
        depends on server has sent an additional configuration. In its response, the client
        could interpret an old response as registrar
        SHOULD indicate the response to a re-
        invitation issued shortly thereafter. Using CSeq also makes
        it easy mode used. This parameter is ignored for
        other requests.

   expires: The expires parameter indicates how long the server to distinguish different versions of URI is valid.
        The parameter is either a number indicating seconds or a quoted
        string containing an invitation, without comparing HTTP-date. If this parameter is not
        provided, the message body.

   The Method value allows of the client to distinguish Expires header field determines how
        long the response to an
   INVITE request from that of a CANCEL response. CANCEL requests can be
   generated by proxies; if they were URI is valid.

   Contact = ( "Contact" | "m" ) ":" ("*" | (1# ( address-spec
             [ *( ";" contact-params ) ] [ comment ] ))

   contact-params = "q"       "=" qvalue
                  | "action"  "=" "proxy" | "redirect"
                  | "expires" "=" delta-seconds | <"> HTTP-date <">
                  | extension-attribute

   extension-attribute = extension-name [ "=" & extension-value ]

        The Contact header field fulfills functionality similar to increase the sequence number,
   it might conflict with a later request issued by
        the user agent for Location header field in HTTP. However, the same call.

   With a length of 32 bits, a server could generate, within a single
   call, HTTP header
        only allows one request a second address, unquoted. Since URIs can contain
        commas and semicolons as reserved characters, they can be
        mistaken for about 136 years before needing to wrap
   around. header or parameter delimiters, respectively.
        The initial value of the sequence number is chosen so current syntax corresponds to that
   subsequent requests within for the same call will not wrap around. A
   non-zero initial value To and From
        header, which also allows to the use of display names.

   Example:

     Contact: "Mr. Watson" <sip:watson@worcester.bell-telephone.com>
        ;q=0.7; expires=3600,
        "Mr. Watson" <mailto:watson@bell-telephone.com> ;q=0.1

6.14 Content-Encoding

   The Content-Encoding entity-header field is used as a time-based initial sequence
   number, which protects against ambiguities when clients are re-
   invited modifier to the same call after rebooting. A client could, for
   example, choose the 31 most significant bits of a 32-bit second clock
   as an initial sequence number.

   Forked requests must
   media-type. When present, its value indicates what additional content
   codings have been applied to the same CSeq as there would be ambiguity
   otherwise between these forked requests entity-body, and later BYE issued thus what decoding
   mechanisms MUST be applied in order to obtain the media-type
   referenced by the
   client user agent.

   Example:

     CSeq: 4711 INVITE

6.17 Date

   General Content-Type header field. See [H14.19].

        The Date header field can be  Content-Encoding is
   primarily used by simple end systems
        without a battery-backed clock to acquire allow a notion document to be compressed without losing
   the identity of
        current time.

6.18 Encryption its underlying media type.  See [H14.12].

6.15 Content-Length

   The Encryption general-header Content-Length entity-header field specifies that the content has
   been encrypted. Section 13 describes indicates the overall SIP security
   architecture and algorithms. This header field is intended for end-
   to-end encryption size of requests and responses. Requests are encrypted
   with a public key belonging to the entity named
   message-body, in decimal number of octets, sent to the To header
   field. Responses are encrypted with the public key conveyed in the
   Response-Key header field.

        SIP chose not recipient.

        Content-Length    =    "Content-Length" ":" 1*DIGIT

   An example is

     Content-Length: 3495

   Applications SHOULD use this field to adopt HTTP's Content-Transfer-Encoding
        header because indicate the encrypted body may contain additional
        SIP header fields as well as size of the body
   message-body to be transferred, regardless of the message.

   For any encrypted message, at least media type of the message body and possibly
   other message header fields are encrypted. An application receiving a
   request
   entity. Any Content-Length greater than or response containing an Encryption header field decrypts
   the equal to zero is a valid
   value. If no body and is present in a message, then concatenates the plaintext Content-Length
   header field MUST be set to zero. If a server receives a UDP request
   without Content-Length, it MUST assume that the request line and
   headers encompasses
   the remainder of the original message. Message headers in packet. If a response does not contain a
   Content-Length, the decrypted
   part completely replace those with client assumes that it encompasses the same field name in remainder
   of the
   plaintext part.  (Note: If only UDP packet or the body of data until the message TCP connection is closed, as
   applicable.  Section 8 describes how to be
   encrypted, determine the body has length of the
   message body.

6.16 Content-Type

   The Content-Type entity-header field indicates the media type of the
   message-body sent to be prefixed with CRLF the recipient. The media-type element is defined
   in [H3.7].

        Content-Type    =    ( "Content-Type" ":" media-type

   Examples of this header field are

     Content-Type: application/sdp
     Content-Type: text/html; charset=ISO-8859-4

6.17 CSeq

   Clients MUST add the CSeq (command sequence) general-header field to allow proper
   concatenation.) Note that
   every request. A CSeq header field in a request contains the request
   method and Request-URI cannot a single decimal sequence number chosen by the requesting
   client, unique within a single value of Call-ID. The sequence number
   MUST be encrypted.

        Encryption only provides privacy; expressible as a 32-bit unsigned integer. The initial value
   of the recipient has no
        guarantee sequence number is arbitrary, but MUST be less than 2**31.
   Consecutive requests that the differ in request method, headers or response came from body,
   but have the party
        listed in same Call-ID MUST contain strictly monotonically
   increasing and contiguous sequence numbers; sequence numbers do not
   wrap around.  Retransmissions of the From message header, only that same request carry the sender
        used same
   sequence number, but an INVITE with a different message body or
   different header fields (a "re-invitation") acquires a new, higher
   sequence number. A server MUST echo the recipients public key. However, proxies will not
        be able to modify CSeq value from the request or
   in its response.

        Encryption           =    "Encryption" ":" encryption-scheme 1*SP
                                  #encryption-params
        encryption-scheme    =    token
        encryption-params    =    token "=" ( token | quoted-string )

        The token indicates  If the form of encryption used; it Method value is
        described missing, the server fills it
   in section 13. appropriately.

   The following example for ACK and CANCEL requests MUST contain the same CSeq value as the
   INVITE request that it refers to, while a message encrypted with ASCII-armored PGP
   was generated by applying "pgp -ea" to BYE request cancelling an
   invitation MUST have a higher sequence number.

   A user agent server MUST remember the payload to be encrypted. highest sequence number for any
   INVITE sip:watson@boston.bell-telephone.com SIP/2.0
   Via: SIP/2.0/UDP 169.130.12.5
   From: <sip:a.g.bell@bell-telephone.com>
   To: T. A. Watson <sip:watson@bell-telephone.com>
   Call-ID: 187602141351@worcester.bell-telephone.com
   Content-Length: 885
   Encryption: PGP version=2.6.2,encoding=ascii
   hQEMAxkp5GPd+j5xAQf/ZDIfGD/PDOM1wayvwdQAKgGgjmZWe+MTy9NEX8O25Red
   h0/pyrd/+DV5C2BYs7yzSOSXaj1C/tTK/4do6rtjhP8QA3vbDdVdaFciwEVAcuXs
   ODxlNAVqyDi1RqFC28BJIvQ5KfEkPuACKTK7WlRSBc7vNPEA3nyqZGBTwhxRSbIR
   RuFEsHSVojdCam4htcqxGnFwD9sksqs6LIyCFaiTAhWtwcCaN437G7mUYzy2KLcA
   zPVGq1VQg83b99zPzIxRdlZ+K7+bAnu8Rtu+ohOCMLV3TPXbyp+err1YiThCZHIu
   X9dOVj3CMjCP66RSHa/ea0wYTRRNYA/G+kdP8DSUcqYAAAE/hZPX6nFIqk7AVnf6
   IpWHUPTelNUJpzUp5Ou+q/5P7ZAsn+cSAuF2YWtVjCf+SQmBR13p2EYYWHoxlA2/
   GgKADYe4M3JSwOtqwU8zUJF3FIfk7vsxmSqtUQrRQaiIhqNyG7KxJt4YjWnEjF5E
   WUIPhvyGFMJaeQXIyGRYZAYvKKklyAJcm29zLACxU5alX4M25lHQd9FR9Zmq6Jed
   wbWvia6cAIfsvlZ9JGocmQYF7pcuz5pnczqP+/yvRqFJtDGD/v3s++G2R+ViVYJO
   z/lxGUZaM4IWBCf+4DUjNanZM0oxAE28NjaIZ0rrldDQmO8V9FtPKdHxkqA5iJP+
   6vGOFti1Ak4kmEz0vM/Nsv7kkubTFhRl05OiJIGr9S1UhenlZv9l6RuXsOY/EwH2
   z8X9N4MhMyXEVuC9rt8/AUhmVQ==
   =bOW+

   Since proxies may base their forwarding decision on request with the same Call-ID value. The server MUST respond
   to, but ignore any combination
   of SIP header fields, there is no guarantee that an encrypted INVITE request
   "hiding" header fields will reach with a lower sequence number.

   All requests spawned in a parallel search have the same destination CSeq value as an
   otherwise identical un-encrypted request.

6.19 Expires

   The Expires entity-header field gives
   the date and time after which request triggering the message content expires.

   This parallel search.

        CSeq    =    "CSeq" ":" 1*DIGIT Method

        Strictly speaking, CSeq header field is currently defined only fields are needed for the REGISTER and
   INVITE methods. For REGISTER, it is any
        SIP request that can be cancelled by a BYE or CANCEL
        request and response-header
   field. In or where a REGISTER request, the client indicates how long it wishes can issue several requests for
        the registration same Call-ID in close succession. Without a sequence
        number, the response to an INVITE could be valid. In the response, mistaken for the server indicates
        response to the earliest expiration time of all registrations. The server MAY
   choose a shorter time interval than that requested by cancellation (BYE or CANCEL). Also, if the client, but
   SHOULD NOT choose a longer one.

   For INVITE, it
        network duplicates packets or if an ACK is a request and response-header field. In a request,
   the callee can limit delayed until
        the validity of server has sent an invitation. For example, if a additional response, the client wants to limit how long a search should take at most or when a
   conference invitation is time-limited. A user interface may take this
        could interpret an old response as a hint to leave the response to a re-
        invitation window on issued shortly thereafter. Using CSeq also makes
        it easy for the screen even if server to distinguish different versions of
        an invitation, without comparing the
   user is not currently at message body.

   The Method value allows the workstation. This also limits client to distinguish the
   duration response to an
   INVITE request from that of a search. If CANCEL response. CANCEL requests can be
   generated by proxies; if they were to increase the sequence number,
   it might conflict with a later request expires before issued by the search
   completes, user agent for
   the proxy returns a 408 (Request Timeout) status. In same call.

   With a 302
   (Moved Temporarily) response, length of 32 bits, a server can advise the client of the
   maximal duration of the redirection. could generate, within a single
   call, one request a second for about 136 years before needing to wrap
   around.  The initial value of this field can be either an HTTP-date or an integer
   number of seconds (in decimal), measured from the receipt of the
   request. The latter approach sequence number is preferable for short durations, as it
   does chosen so that
   subsequent requests within the same call will not depend on clients and servers sharing a synchronized clock.

        Expires    =    "Expires" ":" ( HTTP-date | delta-seconds )

   Two examples of its wrap around. A
   non-zero initial value allows to use are

     Expires: Thu, 01 Dec 1994 16:00:00 GMT
     Expires: 5

6.20 From

   Requests and responses MUST contain a From general-header field,
   indicating time-based initial sequence
   number, which protects against ambiguities when clients are re-
   invited to the initiator same call after rebooting. A client could, for
   example, choose the 31 most significant bits of a 32-bit second clock
   as an initial sequence number.

   Forked requests MUST have the request. The From field MAY contain same CSeq as there would be ambiguity
   otherwise between these forked requests and later BYE issued by the Tag parameter.
   client user agent.

   Example:

     CSeq: 4711 INVITE

6.18 Date

   General-header field. See [H14.19].

   The server copies the To and From Date header fields
   from the request to the response  and MUST add the tag parameter to
   the To field in the response if the URL in reflects the To field is not a
   fully qualified hostname . The optional display-name is meant to be
   rendered by a human-user interface.

   The SIP-URL MUST NOT contain time when the transport-param, maddr-param, ttl-
   param, request or headers elements.  A server that receives a SIP-URL with
   these elements removes them before further processing.

        From            =    ( "From" | "f" ) ":" ( name-addr | addr-spec )
        name-addr       =    [ display-name ] "<" addr-spec ">"
        addr-spec       =    SIP-URL | URI
        display-name    =    *token | quoted-string

   Examples:

     From: A. G. Bell <sip:agb@bell-telephone.com>
     From: sip:+12125551212@server.phone2net.com
     From: Anonymous <sip:c8oqz84zk7z@privacy.org>

        Call-ID, To and From are needed to identify a call leg
        matters in calls with third-party control. response
   is first sent. Thus, retransmissions have the same Date header field
   value as the original.

        The format is
        similar Date header field can be used by simple end systems
        without a battery-backed clock to the equivalent RFC 822 [24] header, but with acquire a
        URI instead notion of just an email address.

6.21 Hide
        current time.

6.19 Encryption

   The Hide request header Encryption general-header field indicates specifies that the path comprised of content has
   been encrypted. Section 13 describes the Via header fields (Section 6.40) should be hidden from subsequent
   proxies overall SIP security
   architecture and user agents. It can take two forms:  Hide: route algorithms. This header field is intended for end-
   to-end encryption of requests and
   Hide: hop. Hide responses. Requests are encrypted
   with a public key belonging to the entity named in the To header fields
   field. Responses are typically added by encrypted with the client user
   agent, but MAY be added by any proxy along public key conveyed in the path.

   If a request contains
   Response-Key header field.

        SIP chose not to adopt HTTP's Content-Transfer-Encoding
        header field because the "Hide: route" encrypted body may contain
        additional SIP header field, all following
   proxies SHOULD hide their previous hop. If fields as well as the body of the
        message.

   For any encrypted message, at least the message body and possibly
   other message header fields are encrypted. An application receiving a
   request contains the
   "Hide: hop" or response containing an Encryption header field, only the next proxy SHOULD hide field decrypts
   the
   previous hop body and then remove concatenates the Hide option unless it also wants plaintext to
   remain anonymous.

   A server hides the previous hop by encrypting the host request line and port parts
   headers of the top-most Via header original message. Message headers in the decrypted
   part completely replace those with an algorithm the same field name in the
   plaintext part.  (Note: If only the body of its choice. Servers
   SHOULD add additional "salt" the message is to be
   encrypted, the host and port information prior body has to encryption be prefixed with CRLF to prevent malicious downstream proxies from guessing
   earlier parts of allow proper
   concatenation.) Note that the path based on seeing identical encrypted Via
   headers. Hidden Via fields are marked with request method and Request-URI cannot
   be encrypted.

        Encryption only provides privacy; the hidden Via option, as
   described recipient has no
        guarantee that the request or response came from the party
        listed in Section 6.40.

   A server the From message header, only that is capable the sender
        used the recipients public key. However, proxies will not
        be able to modify the request or response.

        Encryption           =    "Encryption" ":" encryption-scheme 1*SP
                                  #encryption-params
        encryption-scheme    =    token
        encryption-params    =    token "=" ( token | quoted-string )

        The token indicates the form of hiding Via headers MUST attempt encryption used; it is
        described in section 13.

   The following example for a message encrypted with ASCII-armored PGP
   was generated by applying "pgp -ea" to
   decrypt all Via headers marked as "hidden" the payload to perform loop detection.
   Servers that are not capable be encrypted.

   INVITE sip:watson@boston.bell-telephone.com SIP/2.0
   Via: SIP/2.0/UDP 169.130.12.5
   From: <sip:a.g.bell@bell-telephone.com>
   To: T. A. Watson <sip:watson@bell-telephone.com>
   Call-ID: 187602141351@worcester.bell-telephone.com
   Content-Length: 885
   Encryption: PGP version=2.6.2,encoding=ascii
   hQEMAxkp5GPd+j5xAQf/ZDIfGD/PDOM1wayvwdQAKgGgjmZWe+MTy9NEX8O25Red
   h0/pyrd/+DV5C2BYs7yzSOSXaj1C/tTK/4do6rtjhP8QA3vbDdVdaFciwEVAcuXs
   ODxlNAVqyDi1RqFC28BJIvQ5KfEkPuACKTK7WlRSBc7vNPEA3nyqZGBTwhxRSbIR
   RuFEsHSVojdCam4htcqxGnFwD9sksqs6LIyCFaiTAhWtwcCaN437G7mUYzy2KLcA
   zPVGq1VQg83b99zPzIxRdlZ+K7+bAnu8Rtu+ohOCMLV3TPXbyp+err1YiThCZHIu
   X9dOVj3CMjCP66RSHa/ea0wYTRRNYA/G+kdP8DSUcqYAAAE/hZPX6nFIqk7AVnf6
   IpWHUPTelNUJpzUp5Ou+q/5P7ZAsn+cSAuF2YWtVjCf+SQmBR13p2EYYWHoxlA2/
   GgKADYe4M3JSwOtqwU8zUJF3FIfk7vsxmSqtUQrRQaiIhqNyG7KxJt4YjWnEjF5E
   WUIPhvyGFMJaeQXIyGRYZAYvKKklyAJcm29zLACxU5alX4M25lHQd9FR9Zmq6Jed
   wbWvia6cAIfsvlZ9JGocmQYF7pcuz5pnczqP+/yvRqFJtDGD/v3s++G2R+ViVYJO
   z/lxGUZaM4IWBCf+4DUjNanZM0oxAE28NjaIZ0rrldDQmO8V9FtPKdHxkqA5iJP+
   6vGOFti1Ak4kmEz0vM/Nsv7kkubTFhRl05OiJIGr9S1UhenlZv9l6RuXsOY/EwH2
   z8X9N4MhMyXEVuC9rt8/AUhmVQ==
   =bOW+

   Since proxies can base their forwarding decision on any combination
   of hiding can ignore hidden Via SIP header fields, there is no guarantee that an encrypted request
   "hiding" header fields
   in their loop detection algorithm.

        If hidden headers were not marked, a proxy would have to
        decrypt all headers to detect loops, just in case one was
        encrypted, will reach the same destination as an
   otherwise identical un-encrypted request.

6.20 Expires

   The Expires entity-header field gives the Hide: Hop option may have been removed
        along date and time after which
   the way.

   A host MUST NOT add such a "Hide: hop" message content expires.

   This header field unless it can
   guarantee it will is currently defined only send for the REGISTER and
   INVITE methods. For REGISTER, it is a request for this destination and response-header
   field. In a REGISTER request, the client indicates how long it wishes
   the registration to be valid. In the
   same next hop. response, the server indicates
   the earliest expiration time of all registrations. The reason for this is server MAY
   choose a shorter time interval than that requested by the client, but
   SHOULD NOT choose a longer one.

   For INVITE requests, it is possible that the a request will loop back through this same hop from and response-header field. In a downstream proxy.
   The loop will be detected by
   request, the next hop if callee can limit the choice validity of next hop
   is fixed, but could loop an arbitrary number invitation, for
   example, if a client wants to limit the time duration of times otherwise. a search or
   a conference invitation. A client requesting "Hide: route" can only rely user interface MAY take this as a hint to
   leave the invitation window on keeping the
   request path private screen even if it sends the request to a trusted proxy.
   Hiding user is not
   currently at the route workstation. This also limits the duration of a SIP
   search. If the request may be expires before the search completes, the proxy
   returns a 408 (Request Timeout) status. In a 302 (Moved Temporarily)
   response, a server can advise the client of limited value if the
   request results in data packets being exchanged directly between maximal duration of
   the
   calling and called user agent. redirection.

   The use value of Hide header fields is discouraged unless path privacy is
   truly needed; Hide fields impose extra processing costs and
   restrictions for proxies and this field can cause requests to generate 482 (Loop
   Detected) responses that could otherwise be avoided.

   The encryption either an HTTP-date or an integer
   number of Via header fields is described in more detail in
   Section 13.

   The Hide header field has seconds (in decimal), measured from the following syntax:

        Hide receipt of the
   request. The latter approach is preferable for short durations, as it
   does not depend on clients and servers sharing a synchronized clock.

        Expires    =    "Hide"    "Expires" ":" ( "route" HTTP-date | "hop" delta-seconds )

6.22 Location

   The Location general-header field can appear in requests, 1xx, 2xx
   responses and 3xx responses.

   REGISTER requests: REGISTER requests MAY contain a Location header
        field indicating at which locations the user may be reachable.
        The REGISTER request defines a wildcard Location field, "*",
        which MUST only be used with

   Two examples of its use are

     Expires: 0 to remove all
        registrations for a particular user.

   INVITE and ACK requests: INVITE Thu, 01 Dec 1994 16:00:00 GMT
     Expires: 5

6.21 From

   Requests and ACK requests SHOULD responses MUST contain
        Location headers a From general-header field,
   indicating the initiator of the request. The From field MAY contain
   the tag parameter. The server copies the From header field from which location the
   request is
        originating.

        This allows the callee to send a BYE directly to the caller
        instead of through a series of proxies. response. The Via header optional display-name is
        not sufficient since the desired address may meant to be that of
   rendered by a
        proxy.

   INVITE 2xx responses: human-user interface.

   The SIP-URL MUST NOT contain the transport-param, maddr-param, ttl-
   param, or headers elements.  A user agent server sending a definitive,
        positive response (2xx) MAY insert that receives a Location response header
        indicating SIP-URL with
   these elements removes them before further processing.

   Even if the SIP address under which it display-name is reachable most
        directly for future SIP requests, such as ACK, within empty, the same
        Call-ID. This may name-addr form MUST be the address of the server itself or that of
        a proxy, e.g., used if
   the host is behind addr-spec contains a firewall. comma or semicolon.

        From            =    ( "From" | "f" ) ":" ( name-addr | addr-spec )
                             *( ";" addr-params )
        name-addr       =    [ display-name ] "<" addr-spec ">"
        addr-spec       =    SIP-URL | URI
        display-name    =    *token | quoted-string
        addr-params     =    tag-param
        tag-param       =    "tag=" UUID
        UUID            =    1*( hex | "-" )

   Examples:

     From: "A. G. Bell" <sip:agb@bell-telephone.com>
     From: sip:+12125551212@server.phone2net.com
     From: Anonymous <sip:c8oqz84zk7z@privacy.org>
   The value of
        this Location header is copied into tag MAY appear in the Request-URI From field of
        subsequent requests for this call.

        The Location value a request. It MUST NOT be cached across calls, as present
   when it
        may not represent the most desirable location for a
        particular destination address.

   INVITE 1xx responses: A UAS sending is possible that two instances of a provisional response (1xx) MAY
        insert user sharing a Location response header. It has SIP
   address can make call invitations with the same semantics in
        a 1xx response as a 2xx INVITE response.

   REGISTER 2xx responses: Similarly, a REGISTER response MAY return all
        locations at which the user Call-ID.

   The use of version-1 (time based) or version-4 (random) UUID [19] is currently reachable.

   3xx and 485 responses:
   OPTIONAL. The Location response-header field can tag value is designed to be used globally unique and
   cryptographically random with a 3xx or 485 response codes to indicate one or more
        alternate addresses at least 32 bits of randomness. A
   single user maintains the same tag throughout the call identified by
   the Call-ID.

        Call-ID, To and From are needed to try. It can appear identify a call leg leg
        matters in calls with multiple responses to BYE,
        INVITE and OPTIONS methods. a forked
        request. The Location header field contains
        URIs giving the new locations or user names format is similar to try, or may
        simply specify additional transport parameters. A 300 (Multiple
        Choices), 301 (Moved Permanently), 302 (Moved Temporarily) or
        485 (Ambiguous) response SHOULD contain the equivalent RFC 822
        [24] header, but with a Location field
        containing URIs URI instead of new addressed to be tried. just an email
        address.

6.22 Hide

   A 301 or 302
        response may also give client uses the same location and username that was
        being tried but specify additional transport parameters such as
        a different server or multicast address Hide request header field to try or a change indicate that it wants
   the path comprised of
        SIP transport from UDP to TCP or vice versa. The client copies the host, Via header fields (Section 6.40) to be
   hidden from subsequent proxies and user agents. It can take two
   forms: Hide: route and tag elements of Hide:  hop. Hide header fields are typically
   added by the Location URI into client user agent, but MAY be added by any proxy along
   the
        Request-URI of path.

   If a request contains the redirected "Hide: route" header field, all following
   proxies SHOULD hide their previous hop. If a request contains the
   "Hide: hop" header field, only the next proxy SHOULD hide the
   previous hop and directs then remove the request Hide option unless it also wants to
   remain anonymous.

   A server hides the address specified in previous hop by encrypting the maddr host and port parameter, using the
        transport protocol given in parts
   of the transport parameter. If maddr is
        a multicast address, top-most Via header field with an algorithm of its choice.
   Servers SHOULD add additional "salt" to the value host and port information
   prior to encryption to prevent malicious downstream proxies from
   guessing earlier parts of ttl is used as the time-to-
        live value.

   Note that path based on seeing identical
   encrypted Via headers. Hidden Via fields are marked with the Location header may also refer hidden
   Via option, as described in Section 6.40.

   A server that is capable of hiding Via headers MUST attempt to
   decrypt all Via headers marked as "hidden" to perform loop detection.
   Servers that are not capable of hiding can ignore hidden Via fields
   in their loop detection algorithm.

        If hidden headers were not marked, a different entity
   than the one originally called. For example, a SIP call connected proxy would have to
   GSTN gateway may need
        decrypt all headers to deliver a special information announcement
   such detect loops, just in case one was
        encrypted, as "The number you the Hide: Hop option may have dialed has been changed." removed
        along the way.

   A Location response host MUST NOT add such a "Hide: hop" header may contain any suitable URI indicating
   where the called party may be reached, not limited to SIP URLs. For
   example, field unless it may contain can
   guarantee it will only send a phone or fax request for this destination to the
   same next hop. The reason for this is that it is possible that the
   request will loop back through this same hop from a mailto: (RFC 2368, [25]) or irc: URL. downstream proxy.
   The following parameters are defined. Additional parameters may loop will be
   defined in other specifications.

   q: The qvalue indicates detected by the relative preference among next hop if the locations
        given. qvalue values are decimal numbers from 0.0 to 1.0, with
        higher values indicating higher preference.

   action: The action choice of next hop
   is fixed, but could loop an arbitrary number of times otherwise.

   A client requesting "Hide: route" can only used when registering with rely on keeping the REGISTER
        request. It indicates whether
   request path private if it sends the client wishes that request to a trusted proxy.
   Hiding the server
        proxies or redirects future requests intended for route of a SIP request is of limited value if the request
   results in data packets being exchanged directly between the client. calling
   and called user agent.

   The action taken if this parameter use of Hide header fields is not specified depends on
        server configuration. In its response, the registrar SHOULD
        indicate the mode used. This parameter discouraged unless path privacy is ignored
   truly needed; Hide fields impose extra processing costs and
   restrictions for other
        requests.

        Location proxies and can cause requests to generate 482 (Loop
   Detected) responses that could otherwise be avoided.

   The encryption of Via header fields is described in more detail in
   Section 13.

   The Hide header field has the following syntax:

        Hide    =    ( "Location" | "m" )    "Hide" ":"
                         ("*" | (1# (( SIP-URL ( "route" | URI )
                         [ LWS *( ";" location-params "hop" ) ] [ comment ] ))

        location-params       =    "q"                     "="     qvalue
                              |    "action"                "="     "proxy" | "redirect"
                              |    extension-attribute
        extension-attribute   =    extension-name         [ "="    extension-value ]

   Example:

     Location: sip:watson@worcester.bell-telephone.com ;q=0.7,
        mailto:watson@bell-telephone.com ;q=0.1

6.23 Max-Forwards

   The Max-Forwards request-header field may be used with any SIP method
   to limit the number of proxies or gateways that can forward the
   request to the next inbound downstream server. This can also be useful when
   the client is attempting to trace a request chain which appears to be
   failing or looping in mid-chain. [H14.31]

        Max-Forwards    =    "Max-Forwards" ":" 1*DIGIT

   The Max-Forwards value is a decimal integer indicating the remaining
   number of times this request message may is allowed to be forwarded.

   Each proxy or gateway recipient of a request containing a Max-
   Forwards header field MUST check and update its value prior to
   forwarding the request. If the received value is zero (0), the
   recipient MUST NOT forward the request. Instead, for the OPTIONS and
   REGISTER methods, it MUST respond as the final recipient. For all
   other methods, the server returns 483 (Too many hops).

   If the received Max-Forwards value is greater than zero, then the
   forwarded message MUST contain an updated Max-Forwards field with a
   value decremented by one (1).

   Example:

     Max-Forwards: 6

6.24 Organization

   The Organization general-header field conveys the name of the
   organization to which the entity issuing the request or response
   belongs. It may MAY also be inserted by proxies at the boundary of an
   organization and may
   organization.

        The field MAY be used by client software to filter calls.

        Organization    =    "Organization" ":" *text

6.25 Priority

   The Priority request header signals request-header field indicates the urgency of the call to
   request as perceived by the
   callee. client.

        Priority          =    "Priority" ":" priority-value
        priority-value    =    "emergency" | "urgent" | "normal"
                          |    "non-urgent"

   The value of "emergency" should MUST only be used when life, limb or
   property are in imminent danger.

   Examples:

     Subject: A tornado is heading our way!
     Priority: emergency

     Subject: Weekend plans
     Priority: non-urgent

        These are the values of RFC 2076 [26], with the addition of
        "emergency".

6.26 Proxy-Authenticate

   The Proxy-Authenticate response-header field MUST be included as part
   of a 407 (Proxy Authentication Required) response. The field value
   consists of a challenge that indicates the authentication scheme and
   parameters applicable to the proxy for this Request-URI. See [H14.33]
   for further details.

   A client SHOULD cache the credentials used for a particular proxy
   server and realm for the next request to that server. Credentials
   are, in general, valid for a specific value of the Request-URI at a
   particular proxy server. If a client contacts a proxy server that has
   required authentication in the past, but the client does not have
   credentials for the particular Request-URI, it MAY attempt to use the
   most-recently used credential. The server responds with 401
   (Unauthorized) if the client guessed wrong.

        This suggested caching behavior is motivated by proxies
        restricting phone calls to authenticated users. It seems
        likely that in most cases, all destinations require the
        same password. Note that end-to-end authentication is
        likely to be destination-specific.

6.27 Proxy-Authorization

   The Proxy-Authorization request-header field allows the client to
   identify itself (or its user) to a proxy which 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 further details.

6.28 Proxy-Require

   The Proxy-Require header field is used to indicate proxy-sensitive
   features that MUST be supported by the proxy. Any Proxy-Require
   header field features that are not supported by the proxy MUST be
   negatively acknowledged by the proxy to the client if not supported.
   Servers treat this field identically to the Require field.

   See Section 6.30 for more details on the mechanics of this message
   and a usage example.

6.29 Record-Route

   The Record-Route request and response header field is added to an
   INVITE a
   request by any proxy that insists on being in the path of subsequent ACK and BYE
   requests for the same call. call leg. It contains a globally reachable
   Request-URI that identifies the proxy server. Each proxy server adds
   its Request-URI to the beginning of the list.

   The server copies the Record-Route header field unchanged into the
   response. (Record-Route is only relevant for 2xx responses.)

   The calling user agent client copies the Record-Route header into a
   Route header field of subsequent requests, requests within the same call leg,
   reversing the order  of requests, so that the first entry is closest
   to the caller. user agent client. If the response contained a Location Contact header
   field, the calling user agent adds its content as the last Route
   header. Unless this would cause a loop, any client MUST send any
   subsequent requests for this Call-ID call leg to the first Request-URI in the
   Route request header field and remove that entry.

   The calling user agent MUST NOT use the Record-Route header field in
   requests that contain Route header fields.

        Some proxies, such as those controlling firewalls or in an
        automatic call distribution (ACD) system, need to maintain
        call state and thus need to receive any BYE and ACK packets
        for the call.

   The Record-Route header field has the following syntax:

        Record-Route    =    "Record-Route" ":" 1# SIP-URL name-addr

   Proxy servers SHOULD use the maddr URL parameter containing their
   address to ensure that subsequent requests are guaranteed to reach
   exactly the same server.

   Example for a request that has traversed the hosts ieee.org and
   bell-telephone.com , in that order:

     Record-Route: sip:a.g.bell@bell-telephone.com, sip:a.bell@ieee.org

6.30 Require

   The Require request header request-header field is used by clients to tell user
   agent servers about options that the client expects the server to
   support in order to properly process the request. If a server does
   not understand the option, it MUST respond by returning status code
   420 (Bad Extension) and list those options it does not understand in
   the Unsupported header.

        Require    =    "Require" ":" 1#option-tag

   Example:

   C->S:   INVITE sip:watson@bell-telephone.com SIP/2.0
           Require: com.example.billing
           Payment: sheep_skins, conch_shells

   S->C:   SIP/2.0 420 Bad Extension
           Unsupported: com.example.billing

        This is to make sure 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.

   Proxy and redirect servers MUST ignore features that are not
   understood. If a particular extension requires that intermediate
   devices support it, the extension should MUST be tagged in the Proxy-
   Require Proxy-Require
   field instead (see Section 6.28).

6.31 Response-Key

   The Response-Key request header request-header field can be used by a client to
   request the key that the called user agent SHOULD use to encrypt the
   response with. The syntax is:

        Response-Key    =    "Response-Key" ":" key-scheme 1*SP #key-param
        key-scheme      =    token
        key-param       =    token "=" ( token | quoted-string )

   The key-scheme gives the type of encryption to be used for the
   response. Section 13 describes security schemes.

   If the client insists that the server return an encrypted response,
   it includes a
                  Require: org.ietf.sip.encrypt-response
   header field in its request. If the client cannot encrypt for
   whatever reason, it MUST follow normal Require header field
   procedures and return a 420 (Bad Extension) response. If this Require
   header field is not present, a client SHOULD still encrypt, but MAY
   return an unencrypted response if unable to.

6.32 Retry-After

   The Retry-After response header general-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 may be anticipates being available again. The value of this field can
   be either an HTTP-date or an integer number of seconds (in decimal)
   after the time of the response.

   A REGISTER request may MAY include this header field when deleting
   registrations with Location: *; Expires: Contact: * ;expires: 0. The Retry-After value then
   indicates when the user might again be reachable. The registrar MAY
   then include this information in responses to future calls.

   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.

        Retry-After    =    "Retry-After" ":" ( HTTP-date | delta-seconds )
                            [ comment ] [ ";duration" "=" delta-seconds ]

   Examples of its use are

     Retry-After: Mon, 21 Jul 1997 18:48:34 GMT (I'm in a meeting)
     Retry-After: Mon,  1 Jan 9999 00:00:00 GMT
       (Dear John: Don't call me back, ever)
     Retry-After: Fri, 26 Sep 1997 21:00:00 GMT;duration=3600
     Retry-After: 120

   In the third example, the callee is reachable for one hour starting
   at 21:00 GMT. In the last example, the delay is 2 minutes.

6.33 Route

   The Route request header request-header field determines the route taken by a
   request. Each host removes the first entry and then proxies the
   request to the host listed in that entry, also using it as the
   Request-URI. The operation is further described in Section 6.29.

   The Route header field has the following syntax:

        Route    =    "Route" ":" 1# request-uri addr-spec

6.34 Server

   The Server response-header field contains information about the
   software used by the user agent server to handle the request. See
   [H14.39].

6.35 Subject

   This is intended to provide a summary, or to indicate the nature, of
   the call, allowing call filtering without having to parse the session
   description. (Also, the session description may does not necessarily have to use the
   same subject indication as the invitation.)

        Subject    =    ( "Subject" | "s" ) ":" *text
   Example:

     Subject: Tune in - they are talking about your work!

6.36 Timestamp

   The timestamp general header general-header field describes when the client sent the
   request to the server. The value of the timestamp is of significance
   only to the client and may MAY use any timescale. The server MUST echo
   the exact same value and MAY, if it has accurate information about
   this, add a floating point number indicating floating point number indicating the number of seconds
   that have elapsed since it has received the request. The timestamp is
   used by the client to compute the round-trip time to the server so
   that it can adjust the timeout value for retransmissions.

        Timestamp    =    "Timestamp" ":" *(DIGIT) [ "." *(DIGIT) ] [ delay ]
        delay        =    *(DIGIT) [ "." *(DIGIT) ]

6.37 To

   The To general-header field specifies recipient of the request, with
   the same SIP URL syntax as the From field.

        To    =    ( "To" | "t" ) ":" ( name-addr | addr-spec ) addr-params

   Requests and responses MUST contain a To general-header field,
   indicating the desired recipient of the request. The optional
   display-name is meant to be rendered by a human-user interface.  The
   UAS or redirect server copies the To header field into its response,
   and MUST add a tag parameter if the URL in the To header field is not
   a full qualified hostname.

   The SIP-URL MUST NOT contain the transport-param, maddr-param, ttl-
   param, or headers elements.  A server that receives a SIP-URL with
   these elements removes them before further processing.

   The tag parameter serves as a general mechanism to distinguish
   multiple instances of a user identified by a single SIP URL. As
   proxies can fork requests, the same request can reach multiple
   instances of a user (mobile and home phones, for example). As each
   can respond, there needs to be a means to distinguish the number of seconds
   that have elapsed since it has received responses
   from each at the request. caller. The timestamp is
   used by the client to compute situation also arises with multicast
   requests. The tag in the round-trip time To header field serves to distinguish
   responses at the server so
   that it can adjust UAC. It MUST be placed in the timeout value for retransmissions.

        Timestamp    =    "Timestamp" ":" *(DIGIT) [ "." *(DIGIT) ] [ delay ]
        delay        =    *(DIGIT) [ "." *(DIGIT) ]

6.37 To

   The To general-header field specifies recipient of the request, with
   response by each instance when there is a possibility that the
   request was forked at an intermediate proxy. This, in general, means
   that the tag MUST be inserted when the same SIP URL syntax as in the From field. To    =    ( "To" | "t" ) ":" ( name-addr | addr-spec ) does not refer
   to a fully qualified hostname. The UAS copies the To header into its response, tag MUST be added by UAS,
   registrars and redirect servers, but MUST add NOT be inserted into
   responses forwarded upstream by proxies. The tag is added for all
   definitive responses for all methods, and MAY be added for
   informational responses from a UAS or redirect server. All subsequent
   transactions between two entities MUST include the tag
   parameter if parameter, as
   described in Section 11.

   The use of version-1 (time based) or version-4 (random) UUID [19] is
   OPTIONAL. The tag value is designed to be globally unique and
   cryptographically random with at least 32 bits of randomness. A
   single user maintains the URL same tag throughout the call identified by
   the Call-ID.

   The tag parameter in To headers is ignored when matching responses to
   requests that did not contain a full qualified hostname. tag in their To header.

   A SIP server returns a 400 (Bad Request) response if it receives a
   request with a To header field containing a URI with a scheme it does
   not recognize.

   Example:

     To: The Operator <sip:operator@cs.columbia.edu> <sip:operator@cs.columbia.edu>;tag=287447
     To: sip:+12125551212@server.phone2net.com

        Call-ID, To and From are needed to identify a call leg leg
        matters in calls with third-party control. multiple responses from a forked
        request.  The tag is added to the To header field in the
        response to allow forking of future requests for the same
        call by proxies, while addressing only one of the possibly
        several responding user agent servers. It also allows
        several instances of the callee to send requests that can
        be distinguished.

6.38 Unsupported
   The Unsupported response header response-header field lists the features not
   supported by the server. See Section 6.30 for a usage example and
   motivation.

6.39 User-Agent

   The User-Agent general-header field contains information about the
   client user agent originating the request. See [H14.42].

6.40 Via

   The Via field indicates the path taken by the request so far.  This
   prevents request looping and ensures replies take the same path as
   the requests, which assists in firewall traversal and other unusual
   routing situations.

6.40.1 Requests

   The client originating the request MUST insert into the request a Via
   field containing its host name or network address and, if not the
   default port number, the port number at which it wishes to receive
   responses. (Note that this port number may can differ from the UDP source
   port number of the request.) A fully-qualified domain name is
   RECOMMENDED. Each subsequent proxy server that sends the request
   onwards MUST add its own additional Via field before any existing Via
   fields. A proxy that receives a redirection (3xx) response and then
   searches recursively, MUST use the same Via headers as on the
   original request.

   A proxy SHOULD check the top-most Via header field to ensure that it
   contains the sender's correct network address, as seen from that
   proxy. If the sender's address is incorrect, the proxy should MUST add an
   additional received attribute, as described 6.40.2.

        A host behind a network address translator (NAT) or
        firewall may not be able to insert a network address into
        the Via header that can be reached by the next hop beyond
        the NAT. Hosts behind NATs or NAPTs should MUST insert the local
        port number of the outgoing socket, rather than the port
        number for incoming requests, as NAPTs assume that
        responses return with reversed source and destination
        ports.

   A proxy sending a request to a multicast address MUST add the maddr
   parameter to its Via header field, and SHOULD add the ttl parameter.
   If a server receives a request which contained an maddr parameter in
   the topmost Via field, it should SHOULD send the response to the multicast
   address listed in the maddr parameter.

   If a proxy server receives a request which contains its own address,
   it MUST respond with a 482 (Loop Detected) status code.

        This prevents a malfunctioning proxy server from causing
        loops. Also, it cannot be guaranteed that a proxy server
        can always detect that the address returned by a location
        service refers to a host listed in the Via list, as a
        single host may have aliases or several network interfaces.

6.40.2 Receiver-tagged Via Fields

   Normally, every host that sends or forwards a SIP message adds a Via
   field indicating the path traversed. However, it is possible that
   Network Address Translators (NAT) may change changes the source address of the
   request (e.g., from net-10 to a globally routable address), in which
   case the Via field cannot be relied on to route replies. To prevent
   this, a proxy SHOULD check the top-most Via header field to ensure
   that it contains the sender's correct network address, as seen from
   that proxy. If the sender's address is incorrect, the proxy should MUST add
   a received tag to the Via field inserted by the previous hop. Such a
   modified Via field is known as a receiver-tagged Via field. An
   example is:

     Via: SIP/2.0/UDP erlang.bell-telephone.com:5060
     Via: SIP/2.0/UDP 10.0.0.1:5060 ;received=199.172.136.3

   In this example, the message originated from 10.0.0.1 and traversed a
   NAT with the external address border.ieee.org (199.172.136.3) to
   reach erlang.bell-telephone.com and tagged the previous hop's Via
   field with the address that it actually came from.

6.40.3 Responses

   In the return path, Via fields are processed by a proxy or client
   according to the following rules:

        1.   The first Via field should indicate the proxy or client
             processing this response. If it does not, discard the
             message.  Otherwise, remove this Via field.

        2.   If the second Via field contains a maddr parameter, the
             response is sent to the address listed there, using the
             port indicated in sent-by, or 5060 if none is present. The
             response SHOULD be sent using the TTL indicated in the ttl
             parameter, or with a TTL of 1 if none is present.

        3.   Otherwise, if the second Via field is a receiver-tagged
             field (Section 6.40.2), send the message to the address in
             the received tag, using the port present in sent-by, or
             port 5060 if none is present.

        4.   Otherwise, send the message to the address indicated by
             sent-by in the second Via field.

        5.   If there is no second Via field, this response is destined
             for this client.

   A user agent server or redirect server should send sends a response by pretending
   to insert the received tag into the topmost Via header field in the
   request, and treating this header field as the second Via in the
   above procedure.

   These rules ensure that a client only has to check the first Via
   field in a response to see if it needs processing.

6.40.4 Syntax

   The format for a Via header field is shown in Fig. 10.

     Via              = ( "Via" $|$ "v") ":" 1#( sent-protocol sent-by
                        *( ";" via-params ) [ comment ] )
     via-params       = via-hidden | via-ttl | via-maddr
                      | via-received | via-branch
     via-hidden       = "hidden"
     via-ttl          = "ttl" "=" ttl
     via-maddr        = "maddr" "=" maddr
     via-received     = "received" "=" host
     via-branch       = "branch" "=" token
     sent-protocol    = [ protocol-name "/" ] protocol-version
                        [ "/" transport ]
     protocol-name    = "SIP" $|$ token
     protocol-version = token
     transport        = "UDP" $|$ "TCP" $|$ token
     sent-by          = ( host [ ":" port ] ) $|$ ( concealed-host )
     concealed-host   = token
     ttl              = 1*3DIGIT     ; 0 to 255

   Figure 10: Syntax of Via header field

   The defaults for "protocol-name" and "transport" are "SIP" and "UDP",
   respectively. The "maddr" parameter, designating the multicast
   address, and the "ttl" parameter, designating the time-to-live (TTL)
   value, are included only if the request was sent via multicast. The
   "received" parameter is added only for receiver-added Via fields
   (Section 6.40.2). For reasons of privacy, a client or proxy may wish
   to hide its Via information by encrypting it (see Section 6.21). 6.22). The
   "hidden" parameter is included if this header field was hidden by the
   upstream proxy (see 6.21). 6.22).  Note that privacy of the proxy relies on
   the cooperation of the next hop, as the next-hop proxy will, by
   necessity, know the IP address and port number of the source host.

   The "branch" parameter is included by every forking proxy.  The token
   MUST be unique for each distinct request generated when a proxy
   forks. When a response arrives at the proxy it can use the branch
   value to figure out which branch the response corresponds to. A proxy
   which generates a single request (non-forking) MAY also insert the
   "branch" parameter. The identifier has to be unique only within a set be unique only within a set
   of isomorphic requests.

     Via              = ( "Via" $|$ "v") ":" 1#( sent-protocol sent-by
                        *( ";" via-params ) [ comment ] )
     via-params       = via-hidden | via-ttl | via-maddr
                      | via-received | via-branch
     via-hidden       = "hidden"
     via-ttl          = "ttl" "=" ttl
     via-maddr        = "maddr" "=" maddr
     via-received     = "received" "=" host
     via-branch       = "branch" "=" token
     sent-protocol    = protocol-name "/" protocol-version "/" transport
     protocol-name    = "SIP" $|$ token
     protocol-version = token
     transport        = "UDP" $|$ "TCP" $|$ token
     sent-by          = ( host [ ":" port ] ) $|$ ( concealed-host )
     concealed-host   = token
     ttl              = 1*3DIGIT     ; 0 to 255

   Figure 10: Syntax of isomorphic requests. Via header field

     Via: SIP/2.0/UDP first.example.com:4000;ttl=16
       ;maddr=224.2.0.1 (Example)
     Via: SIP/2.0/UDP adk8

6.41 Warning

   The Warning response-header field is used to carry additional
   information about the status of a response. Warning headers are sent
   with responses and have the following format:

        Warning          =    "Warning" ":" 1#warning-value
        warning-value    =    warn-code SP warn-agent SP warn-text
        warn-code        =    3DIGIT
        warn-agent       =    ( host [ ":" port ] ) | pseudonym
                              ;  the name or pseudonym of the server adding
                              ;  the Warning header, for use in debugging
        warn-text        =    quoted-string

   A response may MAY carry more than one Warning header.

   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 may can be based on any available knowledge, such as the
   location of the cache or user, the Accept-Language field in a
   request, or the Content-Language field in a response, etc. response. The default
   language is English.

   Any server may MAY add Warning headers to a response. Proxy servers MUST
   place additional Warning headers before any Authorization headers.
   Within that constraint, Warning headers MUST be added after any
   existing Warning headers not covered by a signature. A proxy server
   MUST NOT delete any Warning header field that it received with a
   response.

   When multiple Warning headers are attached to a response, the user
   agent SHOULD display as many of them as possible, in the order that
   they appear in the response. If it is not possible to display all of
   the warnings, the user agent first displays warnings that appear
   early in the response. Systems that generate multiple Warning headers
   should order them with this user agent behavior in mind.

   The warn-code consists of three digits. The A first digit indicates the
   significance of the warning, with 3xx indicating a warning that did
   not cause the request to fail and 4xx indicating a fatal error
   condition that contributed "3"
   indicates warnings specific to the failure of the request. SIP.

   This is a list of the currently-defined warn-codes, each with a
   recommended warn-text in English, and a description of its meaning.
   Additional warn-codes may be defined through IANA.
   Note that these warnings describe failures induced by the session
   description.

   x01 Insufficient bandwidth: The bandwidth specified

   Warnings 300 through 329 are reserved for indicating problems with
   keywords in the session
        description or defined by description, 330 through 339 are warnings
   related to basic network services requested in the media exceeds that known session
   description, 370 through 379 are warnings related to be
        available.

   x02 Incompatible transport protocol: One or more transport protocols
        described quantitative QoS
   parameters requested in the request session description, and 390 through 399
   are miscellaneous warnings that do not available.

   x03 fall into one of the above
   categories.

   300 Incompatible network protocol: One or more network protocols
        described
        contained in the request session description are not available.

   x04

   301 Incompatible network address formats: One or more network address
        formats described contained in the request session description are not available.

   x05

   302 Incompatible media format: transport protocol: One or more media formats transport protocols
        described in the request session description are not available.

   x06

   303 Incompatible bandwidth description: units: One or more bandwidth
        descriptions measurement
        units contained in the request session description were not understood.

   x07 Multicast not available: The site where the user is located does
        not support multicast.

   x08 Unicast not available: The site where the user is located does
        not support unicast communication (usually due to the presence
        of a firewall).

   x09

   304 Media type not available: One or more media types contained in
        the request session description are not available.

   x10

   305 Incompatible media format: One or more media formats contained in
        the session description available.

   306 Attribute not understood: One or more of the media attributes in
        the request session description are not supported.

   x09

   307 Session description parameter not understood: A parameter other
        than those listed above was not understood.

   x99

   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 may 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-codes, as in the example below, can be defined
   through IANA.

   Examples:

     Warning: 309 307 isi.edu "Session parameter 'foo' not understood"
     Warning: 404 301 isi.edu "Incompatible network address type 'E.164'"

6.42 WWW-Authenticate

   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 [H14.46] and [27].

   The content of the realm parameter SHOULD be displayed to the user. A
   user agent SHOULD cache the authorization credentials for a given
   value of the destination (To header) and realm and attempt to re-use
   these values on the next request for that destination.

   In addition to the "basic" and "digest" authentication schemes
   defined in the specifications cited above, SIP defines a new scheme,
   PGP (RFC 2015, [28]), Section 14. Other schemes, such as S-MIME, are
   for further study.

7 Status Code Definitions

   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 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. The default behavior for unknown
   response codes is given for each category of codes.

7.1 Informational 1xx

   Informational responses indicate that the server or proxy contacted
   is performing some further action and does not yet have a definitive
   response. The client SHOULD wait for a further response from the
   server, and the server SHOULD send such a response without further
   prompting. Typically a A server should SHOULD send a 1xx response if it expects to take
   more than 200 ms to obtain a final response. A server can MAY issue zero
   or more 1xx responses, with no restriction on their ordering or
   uniqueness. Note that 1xx responses are not transmitted reliably,
   that is, they do not cause the client to send an ACK. Servers are
   free to retransmit informational responses and clients can inquire
   about the current state of call processing by re-sending the request.

7.1.1 100 Trying

   Some unspecified action is being taken on behalf of this call (e.g.,
   a database is being consulted), but the user has not yet been
   located.

7.1.2 180 Ringing

   The called user agent has located a possible location where the user
   has registered recently and is trying to alert the user.

7.1.3 181 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. The new
   destinations are listed in Location Contact headers. Proxies SHOULD be
   configurable not to reveal this information.

7.1.4 182 Queued

   The called party is temporarily unavailable, but the callee has
   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, e.g., "5 calls queued; expected waiting time is 15
   minutes". The server MAY issue several 182 responses to update the
   caller about the status of the queued call.

7.2 Successful 2xx

   The request was successful and MUST terminate a search.

7.2.1 200 OK

   The request has succeeded. The information returned with the response
   depends on the method used in the request, for example:

   BYE: The call has been terminated. The message body is empty.

   CANCEL: The search has been cancelled. The message body is empty.

   INVITE: The callee has agreed to participate; the message body
        indicates the callee's capabilities.

   OPTIONS: The callee has agreed to share its capabilities, included in
        the message body.

   REGISTER: The registration has succeeded. The client treats the
        message body according to its Content-Type.

7.3 Redirection 3xx

   3xx responses give information about the user's new location, or
   about alternative services that may might be able to satisfy the call.
   They SHOULD terminate an existing search, and MAY cause the initiator
   to begin a new search if appropriate.

   Any redirection (3xx) response MUST NOT suggest any of the addresses
   in the Via (Section 6.40) path of the request in the Location Contact header
   field. (Addresses match if their host and port number match.)

   To avoid forwarding loops, a user agent client or proxy MUST check
   whether the address returned by a redirect server equals an address
   tried earlier.

7.3.1 300 Multiple Choices

   The address in the request resolved to several choices, each with its
   own specific location, and the user (or user agent) can select a
   preferred communication end point and redirect its request to that
   location.

   The response SHOULD include an entity containing a list of resource
   characteristics and location(s) from which the user or user agent can
   choose the one most appropriate, if allowed by the Accept request
   header. The entity format is specified by the media type given in the
   Content-Type header field. The choices SHOULD also be listed as
   Location
   Contact fields (Section 6.22). 6.13).  Unlike HTTP, the SIP response may MAY
   contain several Location Contact fields or a list of addresses in a Location Contact
   field. User agents MAY use the Location 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 header status response is appropriate if the callee can be
        reached at several different locations and the server
        cannot or prefers not to proxy the request.

7.3.2 301 Moved Permanently

   The user can no longer be found at the address in the Request-URI and
   the requesting client should SHOULD retry at the new address given by the
   Location
   Contact header field (Section 6.22). 6.13). The caller SHOULD update any
   local directories, address books and user location caches with this
   new value and redirect future requests to the address(es) listed.

7.3.3 302 Moved Temporarily

   The requesting client should SHOULD retry the request at the new address(es)
   given by the Location Contact header field (Section 6.22). 6.13). The duration of the
   redirection can be indicated through an Expires (Section 6.19) 6.20)
   header.

7.3.4 380 Alternative Service

   The call was not successful, but alternative services are possible.
   The alternative services are described in the message body of the
   response.

7.4 Request Failure 4xx
   4xx responses are definite failure responses from a particular
   server.  The client SHOULD NOT retry the same request without
   modification (e.g., adding appropriate authorization). However, the
   same request to a different server may might be successful.

7.4.1 400 Bad Request

   The request could not be understood due to malformed syntax.

7.4.2 401 Unauthorized

   The request requires user authentication.

7.4.3 402 Payment Required

   Reserved for future use.

7.4.4 403 Forbidden

   The server understood the request, but is refusing to fulfill it.
   Authorization will not help, and the request should SHOULD not be repeated.

7.4.5 404 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.

7.4.6 405 Method Not Allowed

   The method specified in the Request-Line is 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.

7.4.7 406 Not Acceptable

   The resource identified by the request is only capable of generating
   response entities which have content characteristics not acceptable
   according to the accept headers sent in the request.

7.4.8 407 Proxy Authentication Required

   This code is similar to 401 (Unauthorized), but indicates that the
   client MUST first authenticate itself with the proxy. The proxy MUST
   return a Proxy-Authenticate header field (section 6.26) containing a
   challenge applicable to the proxy for the requested resource. The
   client MAY repeat the request with a suitable Proxy-Authorization
   header field (section 6.27). SIP access authentication is explained
   in section 13.2 and [H11].

   This status code should be is used for applications where access to the
   communication channel (e.g., a telephony gateway) rather than the
   callee herself requires authentication.

7.4.9 408 Request Timeout

   The server could not produce a response, e.g., a user location,
   within the time indicated in the Expires request-header field. The
   client MAY repeat the request without modifications at any later
   time.

7.4.10 409 Conflict

   The request could not be completed due to a conflict with a conflict with the current
   state of the resource. This response is returned is the action
   parameter in a REGISTER request conflicts with existing
   registrations.

7.4.11 410 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.

7.4.12 411 Length Required

   The server refuses to accept the request without a defined Content-
   Length. The client MAY repeat the request if it adds a valid
   Content-Length header field containing the length of the message-body
   in the request message.

7.4.13 413 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 current
   state of client from continuing
   the resource. This response is returned request.

   If the condition is temporary, the action
   parameter in server SHOULD include a REGISTER request conflicts with existing
   registrations.

7.4.11 Retry-
   After header field to indicate that it is temporary and after what
   time the client MAY try again.

7.4.14 414 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.

7.4.12

7.4.15 415 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 requested
   resource for the requested method.

7.4.13

7.4.16 420 Bad Extension

   The server did not understand the protocol extension specified in a
   Require (Section 6.30) header field.

7.4.14

7.4.17 480 Temporarily Unavailable

   The callee's end system was contacted successfully but the callee is
   currently unavailable (e.g., not logged in or logged in in such a
   manner as to preclude communication with the callee). The response
   may
   MAY indicate a better time to call in the Retry-After header. The
   user may could also be available elsewhere (unbeknownst to this host),
   thus, this response does not terminate any searches. The reason
   phrase SHOULD indicate a more precise cause as to why the callee is
   unavailable. This value SHOULD be setable by the user agent.

7.4.15  Status
   486 (Busy Here) MAY be used to more precisely indicate a particular
   reason for the call failure.

7.4.18 481 Invalid Call-ID Call Leg/Transaction Does Not Exist

   This status is returned under two conditions: The server received a
   BYE request that does not match any existing call leg or the server
   received a CANCEL request with a Call-ID (Section
   6.12) value it that does not recognize. match any existing
   transaction. (A server simply discards an ACK
   with referring to an invalid Call-ID.)

7.4.16 unknown
   transaction.)

7.4.19 482 Loop Detected

   The server received a request with a Via (Section 6.40) path
   containing itself.

7.4.17

7.4.20 483 Too Many Hops

   The server received a request that contains more Via entries (hops)
   (Section 6.40) than allowed by the Max-Forwards (Section 6.23) header
   field.

7.4.18

7.4.21 484 Address Incomplete

   The server received a request with a To (Section 6.37) address or
   Request-URI that was incomplete. Additional information should SHOULD be
   provided.

        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
        status response.

7.4.19

7.4.22 485 Ambiguous

   The callee address provided in the request was ambiguous. The
   response MAY contain a listing of possible unambiguous addresses in
   Location
   Contact headers.

   Revealing alternatives may can infringe on privacy concerns 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 if the request address was ambiguous.

   Example response to a request with the URL lee@example.com :

   485 Ambiguous SIP/2.0
   Location: sip:carol.lee@example.com (Carol Lee)
   Location: sip:p.lee@example.com (Ping Lee)
   Location: sip:lee.foote@example.com (Lee
   Contact: Carol Lee <sip:carol.lee@example.com>
   Contact: Ping Lee <sip:p.lee@example.com>
   Contact: Lee M. Foote) Foote <sip:lee.foote@example.com>

        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 response.

7.4.23 486 Busy Here

   The callee's end system was contacted successfully but the callee is
   currently not willing or able to take additional calls. 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, thus, this response does not terminate any searches.  Status
   600 (Busy Everywhere) SHOULD be used if the client knows that no
   other end system will be able to accept this call.

7.5 Server Failure 5xx

   5xx responses are failure responses given when a server itself has
   erred. They are not definitive failures, and MUST NOT terminate a
   search if other possible locations remain untried.

7.5.1 500 Server Internal Error

   The server encountered an unexpected condition that prevented it from
   fulfilling the request.

7.5.2 501 Not Implemented

   The server does not support the functionality required to fulfill the
   request. This is the appropriate response when the server does not
   recognize the request method and is not capable of supporting it for
   any user.

7.5.3 502 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.

7.5.4 503 Service Unavailable

   The server is currently unable to handle the request due to a
   temporary overloading or maintenance of the server. The implication
   is that this is a temporary condition which will be alleviated after
   some delay. If known, the length of the delay may MAY be indicated in a
   Retry-After header. If no Retry-After is given, the client MUST
   handle the response as it would for a 500 response.

   Note: The existence of the 503 status code does not imply that a
   server has to use it when becoming overloaded. Some servers may MAY wish
   to simply refuse the connection.

7.5.5 504 Gateway Timeout

   The server, while acting as a gateway, did not receive a timely
   response from the server (e.g., a location server) it accessed in
   attempting to complete the request.

7.5.6 505 Version Not Supported
   The server does not support, or refuses to support, the SIP protocol
   version that was used in the request message. 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. The response SHOULD contain an entity describing why
   that version is not supported and what other protocols are supported
   by that server.

7.6 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. All further searches for this user are doomed to failure
   and pending searches SHOULD be terminated.

7.6.1 600 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
   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 should use 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.

7.6.2 603 Decline

   The callee's machine was successfully contacted but the user
   explicitly does not wish to or cannot participate. The response may MAY
   indicate a better time to call in the Retry-After header.

7.6.3 604 Does Not Exist Anywhere

   The server has authoritative information that the user indicated in
   the To request field does not exist anywhere. Searching for the user
   elsewhere will not yield any results.

7.6.4 606 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 or headers field describing why the session described cannot be
   supported. Reasons are listed in Section 6.41.  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.

8 SIP Message Body

8.1 Body Inclusion

   For a request message, the presence of a body is signaled by

   Requests MAY contain message bodies unless otherwise noted. Within
   this specification, the
   inclusion of a Content-Length header. Only ACK, INVITE, OPTIONS and
   REGISTER requests may BYE request MUST NOT contain a message bodies. body.
   For ACK, INVITE and OPTIONS, the message body is always a session
   description. The use of message bodies for REGISTER requests is for
   further study.

   For response messages, whether or not a body is included is dependent
   on both the request method and the response message's response code. status
   code determine the type and interpretation of any message body. All
   responses MAY include a body, although it may be of zero length. body. Message bodies for 1xx responses
   contain advisory information about the progress of the request. 2xx
   responses to INVITE requests contain session descriptions. In 3xx
   respones, the message body MAY contain the description of alternative
   destinations or services, as described in Section 7.3. For responses
   with status 400 or greater, the message body MAY contain additional,
   human-readable information about the reasons for failure.  It is
   RECOMMENDED that information in 1xx and 300 and greater responses be
   of type text/plain or text/html

8.2 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.

8.3 Message Body Length

   The body length in bytes SHOULD be given by the Content-Length header
   field. Section 6.22 6.15 describes the behavior 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.)

9 Compact Form
   When SIP is carried over UDP with authentication and a complex
   session description, it may be possible that the size of a request or
   response is larger than the MTU. To reduce this problem, a more
   compact form of SIP is also defined by using alternative names for
   common header fields.  These short forms are NOT abbreviations, they
   are field names. No other header field abbreviations are allowed.

   short field name    long field name     note
   c                   Content-Type
   e                   Content-Encoding
   f                   From
   i                   Call-ID
   l                   Content-Length
   m                   Location                   Contact             from "moved"
   l                   Content-Length
   s                   Subject
   t                   To
   v                   Via

   Thus, the header in section 15.2 could also be written:

     INVITE sip:schooler@vlsi.caltech.edu SIP/2.0
     v:SIP/2.0/UDP 131.215.131.131;maddr=239.128.16.254;ttl=16
     v:SIP/2.0/UDP 128.16.64.19
     f:sip:mjh@isi.edu
     t:sip:schooler@cs.caltech.edu
     i:62729-27@128.16.64.19
     c:application/sdp
     CSeq: 4711 INVITE
     l:187

     v=0
     o=user1 53655765 2353687637 IN IP4 128.3.4.5
     s=Mbone Audio
     i=Discussion of Mbone Engineering Issues
     e=mbone@somewhere.com
     c=IN IP4 224.2.0.1/127
     t=0 0
     m=audio 3456 RTP/AVP 0

   Mixing short field names and long field names is allowed, but not
   recommended. Servers MUST accept both short and long field names for
   requests. Proxies MUST NOT translate a request between short and long
   forms if authentication fields are present.
   forms.

10 Behavior of SIP Clients and Servers

10.1 General Remarks

   SIP is defined so it can use either UDP (unicast or multicast) or TCP
   as a transport protocol; it provides its own reliability mechanism.

10.1.1 Requests

   Servers ignore isomorphic requests, but retransmit the appropriate
   response. (SIP requests are said to be idempotent , i.e., receiving
   more than one copy of a request does not change the server state.)

   After receiving a CANCEL request from an upstream client, a stateful
   proxy server MAY send a CANCEL on all branches where it has not yet
   received a final response.

   When a user agent receives a request, it checks the Call-ID against
   those of in-progress calls. If the Call-ID was found, it compares the
   tag value of To with the user's tag and rejects the request if the
   two do not match. If the From header, including any tag value,
   matches the value for an existing call leg, the server compares the
   CSeq header field value. If less than or equal to the current
   sequence number, the request is a retransmission.  Otherwise, it is a
   new request. If the From header does not match an existing call leg,
   a new call leg is created.

   If the Call-ID was not found, a new call leg is created, with entries
   for the To, From and Call-ID headers.  In this case, the To header
   field should not have contained a tag. The server returns a response
   containing the same To value, but with a unique tag added. The tag
   MAY be omitted if the To refers to a fully qualified host name.

10.1.2 Responses

   A server MAY issue one or more provisional responses at any time
   before sending a final response. If a stateful proxy, user agent
   server, redirect server or registrar cannot respond to a request with
   a final response within 200 ms, it MUST issue a provisional (1xx)
   response as soon as possible. Stateless proxies MUST NOT issue
   provisional responses on their own.

   Responses are mapped to requests by the matching To, From, Call-ID,
   CSeq headers and the branch parameter of the first Via header.
   Responses terminate request retransmissions even if they have Via
   headers that cause them to be delivered to an upstream client.

   A stateful proxy may receive a response that it does not have state
   for, that is, where it has no a record of an isomorphic request. If
   the Via header field indicates that the upstream server used TCP, the
   proxy actively opens a TCP connection to that address. Thus, proxies
   have to be prepared to receive responses on the incoming side of
   passive TCP connections, even though most responses will arrive on
   the incoming side of an active connection. (An active connection is a
   TCP connection initiated by the proxy, a passive connection is one
   accepted by the proxy, but initiated by another entity.)

   100 responses are not forwarded, other 1xx responses MAY be
   forwarded, possibly after the server eliminates responses with status
   codes that had already been sent earlier. 2xx responses are forwarded
   according to the Via header. Once a stateful proxy has received a 2xx
   response, it MUST NOT forward non-2xx final responses.  Responses
   with status 300 and higher are retransmitted by each stateful proxy
   until the next upstream proxy sends an ACK (see below for timing
   details) or CANCEL.

   A stateful proxy can remove state for a call attempt and close any
   connections 20 seconds after receiving the first final response.

        The 20 second window is given by the maximum retransmission
        duration of 200 responses (10 times T4), in case the ACK is
        lost somewhere on the way to the called user agent or the
        next stateful proxy.

10.2 Source Addresses, Destination Addresses and Connections

10.2.1 Unicast UDP

   Responses are returned to the address listed in the Via header field
   (Section 6.40), not the source address of    the request.

10.2.2 Multicast UDP

   Requests may MAY be multicast; multicast requests likely feature a host-
   independent Request-URI. Multicast requests SHOULD have a time-to-
   live value of no greater than one, i.e., be restricted to the local
   network.

   A client receiving a multicast query does not have to check whether
   the host     part of the Request-URI matches its own host or domain name.
   If the request was received via multicast, the response is also
   returned via multicast. Responses to multicast requests are multicast
   with the same TTL as the request, where the TTL is derived from the
   ttl parameter in the Via header (Section 6.40).

   To avoid response implosion, servers MUST NOT answer multicast
   requests with a status code other than 2xx or 6xx. Servers only
   return 6xx responses if the To represents a single individual rather
   than a group of people. The server delays its response by a random
   interval between zero and one second. Servers MAY suppress responses
   if they hear a lower-numbered or 6xx response from another group
   member prior to sending. Servers do not respond to CANCEL requests
   received via multicast to avoid request implosion. A proxy or UAC
   SHOULD send a CANCEL on receiving the first 2xx or 6xx response to a
   multicast request.

        Server response suppression is a MAY since it requires a
        server to violate some basic message processing rules. Lets
        say A sends a multicast request, and it is received by B,C,
        and D. B sends a 200 response. The topmost Via field in the
        response will contain the address of A. C will also receive
        this response, and could use it to suppress its own
        response. However, C would normally not examine this
        response, as the topmost Via is not its own. Normally, a
        response received with an incorrect topmost Via MUST be
        dropped, but not in this case. To distinguish this packet
        from a misrouted or multicast looped packet is fairly
        complex, and for this reason the procedure is a MAY. The
        CANCEL, instead, provides a simpler and more standard way
        to perform response suppression. It is for this reason that
        the use of CANCEL here is a SHOULD

10.3 TCP

   A single TCP connection can serve one or more SIP transactions. A
   transaction contains zero or more provisional responses followed by
   one or more final responses. (Typically, transactions contain exactly
   one final response, but there are exceptional circumstances, where,
   for example, multiple 200 responses may can be generated.)

   The client MAY close the connection at any time, but SHOULD keep the
   connection open at least until the first final response arrives.  The
   server SHOULD NOT close the TCP connection until it has sent its
   final response, at which point it MAY close the TCP connection if it
   wishes to. However, normally it is the client's responsibility to
   close the connection.

   If the server leaves the connection open, and if the client so
   desires it may MAY re-use the connection for further SIP requests or for
   requests from the same family of protocols (such as HTTP or stream
   control commands).

   If a client closes a connection or the connection is reset (e.g.,
   because the client has crashed and rebooted), the server treats this
   as equivalent to having received a CANCEL request. request  for all pending
   transactions.

   If a server needs to return a response to a client and no longer has
   a connection open to that client, it MAY open a connection to the
   address listed in the Via header. Thus, a proxy or user agent MUST be
   prepared to receive both requests and responses on a "passive"
   connection.

10.4 Reliability for BYE, CANCEL, OPTIONS, REGISTER Requests

10.4.1 UDP

   A SIP client using UDP SHOULD retransmit a BYE, CANCEL, OPTIONS, or
   REGISTER request periodically with timer T1 until it receives a
   response, or until it has reached a set limit on the number of
   retransmissions. If the response is provisional, the client continues
   to retransmit the request, albeit less frequently, using timer T2.
   The default values of timer T1 and T2 are 1 and 5 seconds,
   respectively. The default retransmit limit is 20 times. After the
   server sends a final response, it cannot be sure the client has
   received the response, and thus SHOULD cache the results for at least
   100 seconds to avoid having to, for example, contact the user or
   location server again upon receiving a retransmission.

   Each server in a proxy chain generates its own final response to a
   CANCEL request. The server responds immediately upon receipt of the
   CANCEL request rather than not waiting until it has received final
   responses from the CANCEL requests it generates.

   BYE and OPTIONS final responses are generated by redirect and user
   agent servers; REGISTER final responses are generated by registrars.
   Note that in contrast to the reliability mechanism described in
   Section 10.5, responses to these commands requests are not    retransmitted and
   not acknowledged via ACK.

        The value of the initial retransmission timer is smaller
        than that that for TCP since it is expected that network
        paths suitable for interactive communications have round-
        trip times smaller than 1 second. To avoid flooding the
        network with packets every second even if the destination
        network is unreachable, the retransmission count has to be
        bounded. Given that most transactions should are expected to
        consist of one request and a few responses, round-trip time
        estimation is not likely to be very useful. If RTT
        estimation is desired to more quickly discover a missing
        final response, each request retransmission needs to be
        labeled with its own Timestamp (Section 6.36), returned in
        the response. The server caches the result until it can be
        sure that the client will not retransmit the same request
        again.

10.4.2 TCP

   Clients using TCP do not need to retransmit requests.

10.5 Reliability for ACK Requests
   The ACK request does not generate responses. It is only retransmitted
   when a response to an INVITE request arrives. This behavior is
   independent of the transport protocol. Note that the ACK request MAY
   take a different path than the original INVITE request, and even may
   cause a new TCP connection to be opened in order to send it.

10.6 Reliability for INVITE Requests

   Special considerations apply for the INVITE method.

        1.   After receiving an invitation, considerable time may can elapse
             before the server can determine the outcome. For example,
             if the called party may be is "rung" or extensive searches may be are
             performed, so delays between the request and a definitive
             response can reach several tens of seconds. If either
             caller or callee are automated servers not directly
             controlled by a human being, a call attempt may could be
             unbounded in time.

        2.   If a telephony user interface is modeled or if we need to
             interface to the PSTN, the caller's user interface will
             provide "ringback", a signal that the callee is being
             alerted. (The status response 180 (Ringing) may MAY be used to
             initiate ringback.) Once the callee picks up, the caller
             needs to know so that it can enable the voice path and stop
             ringback. The callee's response to the invitation could get
             lost. Unless the response is transmitted reliably, the
             caller will continue to hear ringback while the callee
             assumes that the call exists.

        3.   The client has to be able to terminate an on-going request,
             e.g., because it is no longer willing to wait for the
             connection or search to succeed. The server will have to
             wait several round-trip times to interpret the lack of
             request retransmissions as the end of a call. If the call
             succeeds shortly after the caller has given up, the callee
             will "pick up the phone" and not be "connected".

10.6.1

10.5.1 UDP

   For UDP, A SIP client SHOULD retransmit a SIP INVITE request
   periodically with timer T1 until it receives a response. If the
   client receives no response, it ceases retransmission after 20
   attempts.  If the response is provisional, the client continues to
   retransmit the request, albeit less frequently, using timer T3. The
   default values of timer T1 and T3 are 1 and 30 seconds, respectively.

        The value of T3 was chosen so that for most normal phone
        calls, only one INVITE request will be issued. Typically, a
        phone switches to an answering machine or voice mail after
        about 20--22 seconds. The number of retransmissions after
        receiving a provisional response is unlimited to allow call
        queueing. Clients may MAY limit the number of invitations sent
        for each call attempt.

   For 2xx final responses, only

   Only the user agent client generates an ACK. ACK for 2xx final responses,
   If the response contained a Location header, Contact header field, the ACK MAY be sent
   to the address listed in that Location Contact header field. If the response
   did not contain a Location Contact header, the client uses the same To header
   field and Request-URI as for the INVITE request and sends the ACK to
   the same destination as the original INVITE request. ACKs for final
   responses other than 2xx are sent to the same server that the
   original request was sent to, using the same Request-URI as the
   original request. Note, however, that the To header field in the ACK
   is copied from the response being acknowledged, not the request, and
   thus may MAY additionally contain the tag parameter. Also note than
   unlike 2xx final responses, a proxy generates an ACK for non-2xx
   final responses.

   The server retransmits the final response at intervals of T4 (default
   value of T4 = 2 seconds) until it receives an one of the following conditions is
   true:

        1.   An ACK request for the same Call-ID and CSeq from transaction is received;

        2.   a BYE request for the same From source call leg is received;

        3.   a CANCEL request for the same call leg is received and the
             final response status was equal or until it has
   retransmitted greater to 300;

        4.   the final response has been retransmitted 10 times.

   The ACK request MUST NOT be acknowledged to prevent a response-ACK
   feedback loop. Fig. 11 and 12 show the client and server state
   diagram for invitations.

        The mechanism in Sec. 10.4 would not work well for INVITE
        because of the long delays between INVITE and a final
                 +===========+
                 *           *
     ...........>*  Initial  *<;;;;;;;;;;
     :  20*T1    *           *          ;
     :           +===========+          ;
     :                 |                ;
     :                 |    -           ;
     :                 |  INVITE        ;
     :                 |                ;
     :                 v                ;
     :           *************          ;
     :   T1   <--*           *          ;
     : INVITE -->*  Calling  *--------+ ;
     :           *           *        | ;
     :           *************        | ;
     :             :   |              | ;
     :.............:   | 1xx      xxx | ;
                       |  -       ACK | ;
                       |              | ;
                       v              | ;
                 *************        | ;
         T3   <--*           *        | ;
       INVITE -->*  Ringing  *<->1xx  | ;
                 *           *        | ;
                 *************        | ;
                       |              | ;
                       |<-------------+ ;
                       |                ;
                       v                ;
                 *************          ;
         xxx  <--*           *          ;
         ACK  -->* Completed *          ;
                 *           *          ;
                 *************          ;
                       ; 10*T4          ;
                       ;;;;;;;;;;;;;;;;;;

    event (xxx=status)
        message

   Figure 11: State transition diagram of client for INVITE method

        response. If the 200 response were to get lost, the callee
        would believe the call to exist, but the voice path would
        be dead since the caller does not know that the callee has
      10*T4        +===============+
   +-------------->*               *
   |               *   Initial     *<...............
   |;;;;;;;;;;;;;;>*               *
   |;              +===============+               :
   |; CANCEL               !                       :
   |;  200                 !                       :
   |;                      !  INVITE               :
   |;                      !   1xx                 :
   |;                      !                       :
   |;                      v                       :
   |;              *****************          BYE  :
   |;    INVITE -->*               *          200  :
   |;      1xx  <--* Call proceed. *..............>:
   |;              *               *               :
   |;;;;;;;;;;;;;;;*****************               :
   |;                    !   !                     :
   |:                    !   !                     :
   |;         failure    !   !  picks up           :
   |;         >= 300     !   !    200              :
   |;            +-------+   +-------+             :
   |;            v                   v             :
   |;       ***********         ***********        :
   |;INVITE<*         *<  T4  ->*         *>INVITE :
   |;status>* failure *>status<-* success *<status :
   |;       *         *         *         *        :
   |;;;;;;;;***********         ***********        :
   |             ! : |            |  !  :          :
   |             ! : |            |  !  :          :
   +-------------!-:-+------------+  !  :          :
                 ! :.................!..:.........>:
                 !                   !         BYE :
                 +---------+---------+         200 :
                           ! ACK                   :
                           !                       :
                           v                       :
                   *****************               :
               V---*               *               :
              ACK  *   Confirmed   *               :
               |-->*               *               :
                   *****************               .
                           :                       :
                           :......................>:
        event
     message sent

   Figure 12: State transition diagram of server for INVITE method
        picked up. Thus, the INVITE retransmission interval would
        have to be on the order of a second or two to limit the
        duration of this state confusion. Retransmitting the
        response a fixed number of times increases the probability
        of success, but at the cost of significantly higher
        processing and network load.

10.6.2

10.5.2 TCP
                 +===========+
                 |  Initial  |
                 +===========+
                       |
                       |
                       |    -
                       |  ------
                       |  INVITE
           +------v    v
          T1     +-----------+
        ------   |  Calling  |--------+
        INVITE   +-----------+        |
           +------| |  |              |
   +----------------+  |              |
   |                   | 1xx          |  >= 200
   |                   | ---          |  ------
   |                   |  -           |   ACK
   |                   |              |
   |       +------v    v    v-----|   |
   |      T3     +-----------+   1xx  |
   |    ------   |  Ringing  |   ---  |
   |    INVITE   +-----------+    -   |
   |       +------|    |    |-----+   |
   |                   |              |
   |     2xx           |              |
   |     ---           | 2xx          |
   |     ACK           | ---          |
   |                   | ACK          |
   +----------------+  |              |
           +------v |  v              |
          xxx    +-----------+        |
          ---    | Completed |<-------+
          ACK    +-----------+
           +------|

    event
   -------
   message

   Figure 11: State transition diagram of client for INVITE method

   A client using TCP MUST NOT retransmit requests, but uses the same
   algorithm as for UDP (Section 10.6.1) 10.5.1) to retransmit responses until
   it receives an ACK. (An implementation can simply set T1 and T3 to
   infinity and otherwise maintain the same state diagram.)
                 +===========+
                 |  Initial  |<-------------+
                 +===========+              |
                       |                    |
                       |                    |
                       |  INVITE            |
                       |  ------            |
                       |   1xx              |
           +------v    v                    |
        INVITE   +-----------+              |
        ------   | Searching |              |
          1xx    +-----------+              |
           +------| |  |  +---------------->+
                    |  |                    |
          failure   |  |  callee picks up   |
          -------   |  |  ---------------   |
          >= 300    |  |       200          |
                    |  |                    | BYE
           +------v v  v    v-----|         | ---
        INVITE   +-----------+    T4        | 200
        ------   | Answered  |  ------      |
        status   +-----------+  status      |
           +------|    |  | |-----+         |
                       |  +---------------->+
                       |                    |
                       | ACK                |
                       | ---                |
                       |  -                 |
                       |                    |
           +------v    v                    |
          ACK    +-----------+              |
          ---    | Connected |              |
           -     +-----------+              |
           +------|       |                 |
                          +-----------------+

    event
   -------
   message

   Figure 12: State transition diagram of server for INVITE method

        It is necessary to retransmit 2xx responses as their
        reliability is assured end-to-end only. If the chain of
        proxies has a UDP link UDP link in the middle, it could lose the
        response, with no possibility of recovery. For simplicity,
        we also retransmit non-2xx responses, although that is not
        strictly necessary.

10.6 Reliability for ACK Requests

   The ACK request does not generate responses. It is only generated
   when a response to an INVITE request arrives (see Section 10.5). This
   behavior is independent of the transport protocol. Note that the ACK
   request MAY take a different path than the original INVITE request,
   and MAY even cause a new TCP connection to be opened in the middle, it could lose the
        response, with no possibility of recovery. For simplicity,
        we also retransmit non-2xx responses, although that is not
        strictly necessary. order to send
   it.

11 Behavior of SIP User Agents

   This section describes the rules for user agent client and servers
   for generating and processing requests and responses.

11.1 Caller Issues Initial INVITE Request

   When a user agent client desires to initiate a call, it formulates an
   INVITE request. The To field in the request contains the address of
   the callee. The Request-URI contains the same address. The From field
   contains the address of the caller.  If the From address can appear
   in requests generated by other user agent clients for the same call,
   the caller should MUST insert the tag parameter in the From field. A UAC MAY
   optionally add a Location Contact header containing an address where it would
   like to be contacted for transactions from the callee back to the
   caller.

11.2 Callee Issues Response

   When the initial INVITE request is received at the callee, the callee
   may
   can accept, redirect, or reject the call. In all of these cases, it
   formulates a response. The response MUST copy the To, From, Call-ID,
   CSeq and Via fields from the request. Additionally, the responding
   UAS MUST add the tag parameter to the To field in the response if the
   To field in the request was not the fully-qualified hostname of the
   UAS. Since a request from a UAC may fork and arrive at multiple
   hosts, the tag parameter serves to distinguish, at the UAC, multiple
   responses from different UAS's. The UAS MAY add a Location Contact header
   field in the response. It contains an address where the callee would
   like to be contacted for subsequent transactions, including the ACK
   for the current INVITE. The UAS stores the values of the To and From
   field, including any tags. These become the local and remote
   addresses of the call leg, respectively.

11.3 Caller Receives Response to Initial Request

   Multiple 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 call
   leg. The caller may MAY choose to acknowledge or terminate the call with
   each responding UAS. To acknowledge, it sends an ACK request, and to
   terminate it sends a BYE request.  The To header field in the ACK or
   BYE MUST be the same as the To field in the 200 response, including
   any tag. The From header field MUST be the same as the From header
   field in the 200 (OK) response, including any tag. The Request-URI of
   the ACK or BYE request MAY be set to whatever address was found in
   the Location Contact header field in the 200 (OK) response, if present.
   Alternately, a UAC may copy the address from the To header field into
   the Request-URI. The UAC also notes the value of the To and From field
   header fields in each response. For each call leg, the To header
   field becomes the remote address, and the From header field becomes
   the local address.

11.4 Caller or Callee Generate Subsequent Requests

   Once the call has been established, either the caller or callee may MAY
   generate either INVITE or BYE requests to change or terminate the call.
   Regardless of whether the caller or callee is generating the new
   request, the header fields in the request are set as follows. For the
   desired call leg, the To header field is set to the remote address,
   and the From header field is set to the local address (both including
   any tags). The Location Contact header field MAY be different than the
   Location Contact
   header field sent in a previous response or request. The Request-URI
   MAY be set to the value of the Location Contact header field received in a
   previous request or response from the remote party, or to the value
   of the remote address.

11.5 Receiving Subsequent Requests

   When a request is received subsequently, the following checks are
   made:

        1.   If the Call-ID is new, the request is for a new call,
             regardless of the values of the To and From header fields.

        2.   If the Call-ID exists, the request is for an existing call.
             If the To, From, Call-ID, and CSeq values exactly match
             (including tags) those of any requests received previously,
             the request is a retransmission.

        3.   If there was no match to the previous step, the To and From
             fields are compared against existing call leg local and
             remote addresses. If there is a match, and the CSeq in the
             request is higher than the last CSeq received on that leg,
             the request is a new transaction for an existing call leg.

12 Behavior of SIP Proxy and Redirect Servers

   This section describes behavior of SIP redirect and proxy servers in
   detail. Proxy servers can "fork" connections, i.e., a single incoming
   request spawns several outgoing (client) requests.

12.1 Redirect Server

   A redirect server does not issue any SIP requests of its own. After
   receiving a request, the server gathers the list of alternative
   locations and returns a final response of class 3xx or it refuses the
   request. For CANCEL requests, it SHOULD also return a 2xx response.
   This response ends the SIP transaction. The redirect server maintains
   transaction state for the whole SIP transaction. It is up to the
   client to detect forwarding loops between redirect servers.

12.2 User Agent Server

   User agent servers behave similarly to redirect servers, except that
   they may also accept requests and can return a response of class 2xx.

12.3 Proxy Server

   This section outlines processing rules for proxy servers. A proxy
   server can either be stateful or stateless. When stateful, a proxy
   remembers the incoming request which generated outgoing requests, and
   the outgoing requests. A stateless proxy forgets all information once
   an outgoing request is generated. A forking proxy SHOULD be stateful.
   A stateful proxy MAY become stateless at any time, but SHOULD remain
   stateful until it sends a definitive response upstream.

   A stateful proxy acts as a virtual UAS/UAC. It implements the server
   state machine when receiving requests, and the client state machine
   for generating outgoing requests, with the exception of receiving a
   2xx response to an INVITE. Instead of generating an ACK, the 2xx
   response is always forwarded upstream towards the caller.
   Furthermore, ACK's for 200 responses to INVITE's are always proxied
   downstream towards the UAS, as they would be for a stateless proxy.

   A stateless proxy does not act as a virtual UAS/UAC (as this would
   require state). Rather, a stateless proxy forwards every request it
   receives downstream, and every response it receives upstream.

12.3.1 Proxying Requests

   To prevent loops, a server MUST check if its own address is already
   contained in the Via header field of the incoming request.

   The To, From, Call-ID, and Location Contact tags are copied exactly from the
   original request. The proxy SHOULD change the Request-URI to indicate
   the server where it intends to send the request.

   A proxy server always inserts a Via header field containing its own
   address into those requests that are caused by an incoming request.
   Each proxy MUST insert a "branch" parameter (Section 6.40).

12.3.2 Proxying Responses

   A proxy only processes a response if the topmost Via field matches
   one of its addresses. A response with a non-matching top Via field
   MUST be dropped.

12.3.3 Stateless Proxy: Proxying Responses

   A stateless proxy removes its own Via field, and checks the address
   in the next Via field. If the field indicates TCP as the transport
   protocol, the proxy checks to see if it has a connection currently
   open to that address. If so, the response is sent on that connection.
   Otherwise, a new TCP connection is opened to the address and port in
   the Via field, and the response is sent there. In the case of UDP,
   the response is sent to the address listed in the "maddr" tag if
   present, otherwise to the "received" tag if present, and finally to
   the address in the "sent-by" field. Note that this implies that a UAC
   or proxy must MUST be prepared to receive responses on the incoming side
   of a TCP connection.

   A stateless proxy MUST NOT generate its own provisional responses.

12.3.4 Stateful Proxy: Receiving Requests

   When a stateful proxy receives a request, it checks the To, From
   (including tags), Call-ID and CSeq against existing request records.
   If the tuple exists, the request is a retransmission. The provisional
   or final response sent previously is retransmitted, as per the server
   state machine. If the tuple does not exist, the request corresponds
   to a new transaction, and the request should be proxied.

   A stateful proxy server MAY generate its own provisional (1xx)
   responses.

12.3.5 Stateful Proxy: Receiving ACKs

   When an ACK request is received, it must is either be processed locally or
   proxied. To make this determination, the To, From, CSeq and Call-
   ID Call-ID
   fields are compared against those in previous requests. If there is
   no match, the ACK request is proxied as if it were an INVITE request.
   If there is a match, and if the server had ever sent a 200 response
   upstream, the ACK is proxied.  If the server had never sent any
   responses upstream, the ACK is also proxied. If the server had sent a
   3xx, 4xx, 5xx or 6xx response, but no 2xx response, the ACK is
   processed locally, as it is acknowledging the response generated by
   the proxy.

12.3.6 Stateful Proxy: Receiving Responses

   When a proxy server receives a response that has passed the Via
   checks, the proxy server checks the To (without the tag), From
   (including the tag), Call-ID and CSeq against values seen in previous
   requests. If there is no match, the response is forwarded upstream to
   the address listed in the Via field. If there is a match, the
   "branch" tag in the Via field is examined. If it matches a known
   branch identifier, the response is for the given branch, and
   processed by the virtual client for the given branch. Otherwise, the
   response is dropped.

   A stateful proxy should obey the rules in Section 12.4 to determine
   if the response should be proxied upstream. If it is to be proxied,
   the same rules for stateless proxies above are followed.

12.3.7 Stateless, Non-Forking Proxy

   Proxies in this category issue at most a single unicast request for
   each incoming SIP request, that is, they do not "fork" requests.
   However, servers may MAY choose to always operate in a mode that allows
   issuing of several requests, as described in Section 12.4.

   The server can forward the request and any responses. It does not
   have to maintain any state for the SIP transaction. Reliability is
   assured by the next redirect or stateful proxy server in the server
   chain.

   A proxy server SHOULD cache the result of any address translations
   and the response to speed forwarding of retransmissions. After the
   cache entry has been expired, the server cannot tell whether an
   incoming request is actually a retransmission of an older request.
   The server will treat it as a new request and commence another
   search.

12.4 Forking Proxy

   The server MUST respond to the request immediately with a 100
   (Trying) response.

   Successful responses to an INVITE request SHOULD contain a Location Contact
   header field so that the following ACK or BYE bypasses the proxy
   search mechanism. If the proxy requires future requests to be routed
   through it, it adds a Record-Route header to the request (Section
   6.29).

   The following pseudo-code describes the behavior of a proxy server
   issuing several requests in response to an incoming INVITE request.
   The function request(r, a, b) sends a SIP request of type r to
   address a, with branch id b. await_response() waits until a response
   is received and returns the response. close(a) closes the TCP
   connection to client with address a. response(r) sends a response to
   the client. ismulticast() returns 1 if the location is a multicast
   address and zero otherwise.  The variable timeleft indicates the
   amount of time left until the maximum response time has expired. The
   variable recurse indicates whether the server will recursively try
   addresses returned through a 3xx response. A server MAY decide to
   recursively try only certain addresses, e.g., those which are within
   the same domain as the proxy server. Thus, an initial multicast
   request may can trigger additional unicast requests.

     /* request type */
     typedef enum {INVITE, ACK, BYE, OPTIONS, CANCEL, REGISTER} Method;

     process_request(Method R, int N, address_t address[])
     {
       struct {
         address_t address;  /* address */
         int branch;         /* branch id */
         int done;           /* has responded */
       } outgoing[];
       int done[];           /* address has responded */
       char *location[];     /* list of locations */
       int heard = 0;        /* number of sites heard from */
       int class;            /* class of status code */
       int timeleft = 120;   /* sample timeout value */
       int loc = 0;          /* number of locations */
       struct {              /* response */
         int status;         /* response: CANCEL=-1 */
         int locations;      /* number of redirect locations */
         char *location[];   /* redirect locations */
         address_t a;        /* address of respondent */
         int branch;         /* branch identifier */
       } r, best;            /* response, best response */
       int i;

       best.status = 1000;
       for (i = 0; i < N; i++) {
         request(R, address[i], i);
         outgoing[i].done = 0;
         outgoing[i].branch = i;
       }

       while (timeleft > 0 && heard < N) {
         r = await_response();
         class = r.status / 100;

         /* If final response, mark branch as done. */
         if (class >= 2) {
           heard++;
           for (i = 0; i < N; i++) {
             if (r.branch == outgoing[i].branch) {
               outgoing[i].done = 1;
               break;
             }
           }
         }
         /* CANCEL: respond, fork and wait for responses */
         else if (class < 0) {
           best.status = 200;
           response(best);
           for (i = 0; i < N; i++) {
             request(CANCEL, address[i], outgoing[i].branch);
           }
           best.status = -1;
         }
         if (class == 2) {
           if (r.status < best) best = r;
           break;
         }
         else if (class == 3) {
                   /* A server may MAY optionally recurse.  The server MUST check
                    * whether it has tried this location before and whether the
                    * location is part of the Via path of the incoming request.
                    * This check is omitted here for brevity.  Multicast locations
                    * MUST NOT be returned to the client if the server is not
                    * recursing.
            */
           if (recurse) {
             multicast = 0;
             N += r.locations;
             for (i = 0; i < r.locations; i++) {
               request(R, r.location[i]);
             }
           } else if (!ismulticast(r.location)) {
             best = r;
           }
           if (R == INVITE) request(ACK, r.a, r.branch);
         }
         else if (class == 4) {
           if (R == INVITE) request(ACK, r.a, r.branch);
           if (best.status >= 400) best = r;
         }
         else if (class == 5) {
           if (R == INVITE) request(ACK, r.a, r.branch);
           if (best.status >= 500) best = r;
         }
         else if (class == 6) {
           if (R == INVITE) request(ACK, r.a, r.branch);
           best = r;
           break;
         }
       }

       /* We haven't heard anything useful from anybody. */
       if (best.status == 1000) {
         best.status = 404;
       }
       if (best.status/100 != 3) loc = 0;
       response(best);
     }
   Responses are processed as follows. The process completes (and state
   can be freed) when all requests have been answered by final status
   responses (for unicast) or 60 seconds have elapsed (for multicast). A
   proxy MAY send a CANCEL to all branches and return a 408 (Timeout) to
   the client after 60 seconds or more.

   1xx: The proxy MAY forward the response upstream towards the client.

   2xx: The proxy MUST forward the response upstream towards the client,
        without sending an ACK downstream. After receiving a 2xx, the
        server MAY terminate all other pending requests by sending a
        CANCEL request and closing the TCP connection, if applicable.
        (Terminating pending requests is advisable as searches consume
        resources. Also, INVITE requests may could "ring" on a number of
        workstations if the callee is currently logged in more than
        once.)

   3xx: The proxy MUST send an ACK and MAY recurse on the listed
        Location Contact
        addresses. Otherwise, the lowest-numbered response is returned
        if there were no 2xx responses.

        Location lists are not merged as that would prevent
        forwarding of authenticated responses. Also, some responses
        may can
        have message bodies, so that merging is not feasible.

   4xx, 5xx: The proxy MUST send an ACK and remember the response if it
        has a lower status code than any previous 4xx and 5xx responses.
        On completion, the lowest-numbered response is returned if there
        were no 2xx or 3xx responses.

   6xx: The proxy MUST forward the response to the client and send an
        ACK. Other pending requests MAY be terminated with CANCEL as
        described for 2xx responses.

   A proxy server forwards any response for Call-IDs for which it does
   not have a pending transaction according to the response's Via
   header. User agent servers respond to BYE requests with an for unknown
   Call-ID call
   legs with status code 481 (Invalid Call-ID). (Transaction Does Not Exist); they drop ACK
   requests are
   silently dropped. with unknown call legs silently.

   Special considerations apply for choosing forwarding destinations for
   ACK and BYE requests. In most cases, these requests will bypass
   proxies and reach the desired party directly, keeping proxies from
   having to make forwarding decisions.

   A proxy MAY maintain call state for a period of its choosing. If a
   proxy still has list of destinations that it forwarded the last
   INVITE to, it SHOULD direct ACK requests only to those downstream
   servers.

13 Security Considerations

13.1 Confidentiality and Privacy: Encryption

13.1.1 End-to-End Encryption

   SIP requests and responses can contain sensitive information about
   the communication patterns and communication content of individuals
   and thus should be protected against eavesdropping. individuals.
   The SIP message body may MAY also contain encryption keys for the session
   itself. SIP supports three complementary forms of encryption to
   protect privacy:

        o End-to-end encryption of the  SIP message body and certain
          sensitive header fields;

        o hop-by-hop encryption to prevent eavesdropping that tracks who
          is calling whom;

        o hop-by-hop encryption of Via  fields to hide the route a
          request has taken.

   Not all of the SIP request or response can be encrypted end-to-end
   because header fields such as To and Via need to be visible to
   proxies so that the SIP request can be routed correctly.  Hop-by-hop
   encryption encrypts the entire SIP request or response on the wire
   (the request may already have been end-to-end encrypted) so
   that packet sniffers or other eavesdroppers cannot see who is calling
   whom. Hop-by-hop encryption can also encrypt requests and responses
   that have been end-to-end encrypted. Note that proxies can still see
   who is calling whom, and this information may is also be deducible by
   performing a network traffic analysis, so this provides a very
   limited but still worthwhile degree of protection.

   SIP Via fields are used to route a response back along the path taken
   by the request and to prevent infinite request loops. However, the
   information given by them may can also provide useful information to an
   attacker. Section 6.21 6.22 describes how a sender can request that Via
   fields be encrypted by cooperating proxies without compromising the
   purpose of the Via field.

   End-to-end encryption relies on keys shared by the two user agents
   involved in the request. Typically, the message is sent encrypted
   with the public key of the recipient, so that only that recipient can
   read the message. SIP does not limit the security mechanisms that may
   be used, but all All implementations SHOULD support PGP-based encryption.
   encryption and MAY implement other schemes.

   A SIP request (or response) is end-to-end encrypted by splitting the
   message to be sent into a part to be encrypted and a short header
   that will remain in the clear. Some parts of the SIP message, namely
   the request line, the response line and certain header fields marked
   with "n" in the "enc." column in Table 4 and 5 need to be read and
   returned by proxies and thus MUST NOT be encrypted end-to-end.
   Possibly sensitive information that needs to be made available as
   plaintext include destination address (To) and the forwarding path
   (Via) of the call. The Authorization header field MUST remain in the
   clear if it contains a digital signature as the signature is
   generated after encryption, but MAY be encrypted if it contains
   "basic" or "digest" authentication. The From header field SHOULD
   normally remain in the clear, but MAY be encrypted if required, in
   which case some proxies MAY return a 401 (Unauthorized) status if
   they require a From field.

   Other header fields MAY be encrypted or MAY travel in the clear as
   desired by the sender. The Subject, Allow, Call-ID, and Content-Type
   header fields will typically be encrypted. The Accept, Accept-
   Language, Date, Expires, Priority, Require, Cseq, and Timestamp
   header fields will remain in the clear.

   All fields that will remain in the clear MUST precede those that will
   be encrypted. The message is encrypted starting with the first
   character of the first header field that will be encrypted and
   continuing through to the end of the message body. If no header
   fields are to be encrypted, encrypting starts with the second CRLF
   pair after the last header field, as shown below. Carriage return and
   line feed characters have been made visible as "$", and the encrypted
   part of the message is outlined.

     INVITE sip:watson@boston.bell-telephone.com SIP/2.0$
     Via: SIP/2.0/UDP 169.130.12.5$
     To: T. A. Watson <sip:watson@bell-telephone.com>$
     From: A. Bell <sip:a.g.bell@bell-telephone.com>$
     Encryption: PGP version=5.0$
     Content-Length: 224$
     CSeq: 488$
     $
   *******************************************************
   * Call-ID: 187602141351@worcester.bell-telephone.com$ *
   * Subject: Mr. Watson, come here.$                    *
   * Content-Type: application/sdp$                      *
   * $                                                   *
   * v=0$                                                *
   * o=bell 53655765 2353687637 IN IP4 128.3.4.5$        *
   * c=IN IP4 135.180.144.94$                            *
   * m=audio 3456 RTP/AVP 0 3 4 5$                       *
   *******************************************************

   An Encryption header field MUST be added to indicate the encryption
   mechanism used. A Content-Length field is added that indicates the
   length of the encrypted body. The encrypted body is preceded by a
   blank line as a normal SIP message body would be.

   Upon receipt by the called user agent possessing the correct
   decryption key, the message body as indicated by the Content-Length
   field is decrypted, and the now-decrypted body is appended to the
   clear-text header fields. There is no need for an additional
   Content-Length header field within the encrypted body because the
   length of the actual message body is unambiguous after decryption.

   Had no SIP header fields required encryption, the message would have
   been as below. Note that the encrypted body must MUST then include a blank
   line (start with CRLF) to disambiguate between any possible SIP
   header fields that might have been present and the SIP message body.

     INVITE sip:watson@boston.bell-telephone.com SIP/2.0$
     Via: SIP/2.0/UDP 169.130.12.5$
     To: T. A. Watson <sip:watson@bell-telephone.com>$
     From: A. Bell <a.g.bell@bell-telephone.com>$
     Encryption: PGP version=5.0$
     Content-Type: application/sdp$
     Content-Length: 107$
     $
   *************************************************
   * $                                             *
   * v=0$                                          *
   * o=bell 53655765 2353687637 IN IP4 128.3.4.5$  *
   * c=IN IP4 135.180.144.94$                      *
   * m=audio 3456 RTP/AVP 0 3 4 5$                 *
   *************************************************

13.1.2 Privacy of SIP Responses

   SIP requests may can be sent securely using end-to-end encryption and
   authentication to a called user agent that sends an insecure
   response.  This is allowed by the SIP security model, but is not a
   good idea.  However, unless the correct behaviour is explicit, it
   would not always be possible for the called user agent to infer what
   a reasonable behaviour was. Thus when end-to-end encryption is used
   by the request originator, the encryption key to be used for the
   response SHOULD be specified in the request. If this were not done,
   it might be possible for the called user agent to incorrectly infer
   an appropriate key to use in the response. Thus, to prevent key-
   guessing becoming an acceptable strategy, we specify that a called
   user agent receiving a request that does not specify a key to be used
   for the response SHOULD send that response unencrypted.

   Any SIP header fields that were encrypted in a request should SHOULD also be
   encrypted in an encrypted response. Location Contact response fields MAY be
   encrypted if the information they contain is sensitive, or MAY be
   left in the clear to permit proxies more scope for localized
   searches.

13.1.3 Encryption by Proxies

   Normally, proxies are not allowed to alter end-to-end header fields
   and message bodies. Proxies MAY, however, encrypt an unsigned request
   or response with the key of the call recipient.

        Proxies may need to encrypt a SIP request if the end system
        cannot perform encryption or to enforce organizational
        security policies.

13.1.4 Hop-by-Hop Encryption

   It is RECOMMENDED that SIP requests and responses are also protected
   by security mechanisms at the transport and network layer.

13.1.5 Via field encryption

   When Via fields are to be hidden, a proxy that receives a request
   containing an appropriate "Hide: hop" header field (as specified in
   section 6.21) 6.22) SHOULD encrypt the header field. As only the proxy that
   encrypts the field will decrypt it, the algorithm chosen is entirely
   up to the proxy implementor. Two methods satisfy these requirements:

        o The server keeps a cache of Via fields and the associated To
          field, and replaces the Via field with an index into the
          cache. On the reverse path, take the Via field from the cache
          rather than the message.

        This is insufficient to prevent message looping, and so an
        additional ID must MUST be added so that the proxy can detect loops.
        This should not SHOULD NOT normally be the address of the proxy as the goal
        is to hide the route, so instead a sufficiently large random
        number should SHOULD be used by the proxy and maintained in the cache.
        Obtaining sufficiently much randomness to give sufficient
        protection against clashes may be hard.

        It may also be is possible for replies to get directed to the wrong
        originator if the cache entry gets reused, so great care must needs
        to be taken to ensure this does not happen.

        o The server may MAY use a  secret key to encrypt the Via field, a
          timestamp and an appropriate checksum in any such message with
          the same secret key. The checksum is needed to detect whether
          successful decoding has occurred, and the timestamp is
          required to prevent possible response attacks and to ensure
          that no two requests from the same previous hop have the same
          encrypted Via field.

   The latter  This is the preferred solution, although proxy developers may
   devise other methods that might also satisfy the requirements. solution.

13.2 Message Integrity and Access Control: Authentication

   An active attacker may

   Protective measures need to be able taken to modify prevent an active attacker
   from modifying and replay replaying SIP requests and
   responses unless protective measures are taken. responses. The same
   cryptographic measures that are used to ensure the authenticity of
   the SIP message also serve to authenticate the originator of the
   message.  However, the "basic" and "digest" authentication mechanism
   offer authentication only, without message integrity.

   Transport-layer or network-layer authentication may MAY be used for hop-
   by-hop authentication. SIP also extends the HTTP WWW-Authenticate
   (Section 6.42) and Authorization (Section 6.11) header field and
   their Proxy counterparts to include cryptographically strong
   signatures. SIP also supports the HTTP "basic" and "digest" schemes
   and other HTTP authentication schemes to be defined that offer a
   rudimentary mechanism of ascertaining the identity of the caller.

        Since SIP requests are often sent to parties with which no
        prior communication relationship has existed, we do not
        specify authentication based on shared secrets.

   SIP requests may MAY be authenticated using the Authorization header
   field to include a digital signature of certain header fields, the
   request method and version number and the payload, none of which are
   modified between client and called user agent. The Authorization
   header field may be is used in requests to authenticate the request
   originator end-to-end to proxies and the called user agent, and in
   responses to authenticate the called user agent or proxies returning
   their own failure codes. It does not provide If required, hop-by-hop
   authentication, which may authentication can
   be provided if required using provided, for example, by the IPSEC Authentication Header.

   SIP does not dictate which digital signature scheme is used for
   authentication, but does define how to provide authentication using
   PGP in Section 14. As indicated above, SIP may implementations MAY also
   use "basic" and "digest" authentication and other authentication
   mechanisms defined for HTTP. Note that "basic" authentication has
   severe security limitations. The following does not apply to these
   schemes.

   To cryptographically sign a SIP request, the order of the SIP header
   fields is important. When an Authorization header field is present,
   it indicates that all header fields following the Authorization
   header field have been included in the signature.  Therefore, hop-by-
   hop hop-
   by-hop header fields which MUST or SHOULD be modified by proxies MUST
   precede the Authorization header field as they will generally be
   modified or added-to by proxy servers.  Hop-by-hop header fields
   which MAY be modified by a proxy MAY appear before or after the
   Authorization header. When the appear before, the MAY be modified by
   a proxy. When they appear after, they MUST NOT be modified by a
   proxy. To sign a request, a client constructs a message from the
   request method (in upper case) followed, without LWS, by the SIP
   version number, followed, again without LWS, by the request headers
   to be signed and the message body.  The message thus constructed is
   then signed.

   For example, if the SIP request is to be:

   INVITE sip:watson@boston.bell-telephone.com SIP/2.0
   Via: SIP/2.0/UDP 169.130.12.5
   Authorization: PGP version=5.0, signature=...
   From: A. Bell <sip:a.g.bell@bell-telephone.com>
   To: T. A. Watson <sip:watson@bell-telephone.com>
   Call-ID: 187602141351@worcester.bell-telephone.com
   Subject: Mr. Watson, come here.
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=bell 53655765 2353687637 IN IP4 128.3.4.5
   c=IN IP4 135.180.144.94
   m=audio 3456 RTP/AVP 0 3 4 5

   Then the data block that is signed is:

   INVITESIP/2.0From: A. Bell <sip:a.g.bell@bell-telephone.com>
   To: T. A. Watson <sip:watson@bell-telephone.com>
   Call-ID: 187602141351@worcester.bell-telephone.com
   Subject: Mr. Watson, come here.
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=bell 53655765 2353687637 IN IP4 128.3.4.5
   c=IN IP4 135.180.144.94
   m=audio 3456 RTP/AVP 0 3 4 5

   Note that if a message is encrypted and authenticated using a digital
   signature, when the message is generated encryption is performed
   before the digital signature is generated. On receipt, the digital
   signature is checked before decryption.

   A client MAY require that a server sign its response by including a
   Require: org.ietf.sip.signed-response request header field. The
   client indicates the desired authentication method via the WWW-
   Authenticate header.

   The correct behaviour in handling unauthenticated responses to a
   request that requires authenticated responses is described in section
   13.2.1.

13.2.1 Trusting responses

   It may be possible for

   There is the possibility that an eavesdropper to listen listens to requests and to
   inject
   then injects unauthenticated responses that would terminate, redirect or
   otherwise interfere with a call. (Even encrypted requests contain
   enough information to fake a response.)

   Client should need to be particularly careful with 3xx redirection
   responses.  Thus a client receiving, for example, a 301 (Moved
   Permanently) which was not authenticated when the public key of the
   called user agent is known to the client, and authentication was
   requested in the request SHOULD be treated as suspicious. The correct
   behaviour in such a case would be for the called-user to form a dated
   response containing the
   Location Contact field to be used, to sign it, and
   give this signed stub response to the proxy that will provide the
   redirection. Thus the response can be authenticated correctly. There may be circumstances
   where such unauthenticated responses are unavoidable, but a A
   client SHOULD NOT automatically redirect such a request to the new
   location without alerting the user to the authentication failure
   before doing so.

   Another problem might be responses such as 6xx failure responses
   which would simply terminate a search, or "4xx" and "5xx" response
   failures.

   If TCP is being used, a proxy SHOULD treat 4xx and 5xx responses as
   valid, as they will not terminate a search. However, fake 6xx
   responses from a rogue proxy may terminate a search incorrectly. 6xx
   responses SHOULD be authenticated if requested by the client, and
   failure to do so SHOULD cause such a client to ignore the 6xx
   response and continue a search.

   With UDP, the same problem with 6xx responses exists, but also an
   active eavesdropper can generate 4xx and 5xx responses that might
   cause a proxy or client to believe a failure occurred when in fact it
   did not. Typically 4xx and 5xx responses will not be signed by the
   called user agent, and so there is no simple way to detect these
   rogue responses. This problem is best prevented by using hop-by-hop
   encryption of the SIP request, which removes any additional problems
   that UDP might have over TCP.

   These attacks are prevented by having the client require response
   authentication and dropping unauthenticated responses. A server user
   agent that cannot perform response authentication responds using the
   normal Require response of 420 (Bad Extension).

13.3 Callee Privacy

   User location and SIP-initiated calls may can violate a callee's privacy.
   An implementation 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.

13.4 Known Security Problems

   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.

14 SIP Security Using PGP

14.1 PGP Authentication Scheme

   The "pgp" authentication scheme is based on the model that the client
   must authenticate
   authenticates itself with a request signed with the client's private
   key. The server can then ascertain the origin of the request if it
   has access to the public key, preferably signed by a trusted third
   party.

14.1.1 The WWW-Authenticate Response Header
        WWW-Authenticate    =    "WWW-Authenticate" ":" "pgp" pgp-challenge
        pgp-challenge       =    * (";" pgp-params )
        pgp-params          =    realm | pgp-version | pgp-algorithm
        realm               =    "realm" "=" realm-value
        realm-value         =    quoted-string
        pgp-version         =    "version" "=" digit *( "." digit ) *letter
        pgp-algorithm       =    "algorithm" "=" ( "md5" | "sha1" | token )

   The meanings of the values of the parameters used above are as
   follows:

   realm: A string to be displayed to users so they know which identity
        to use. This string should SHOULD contain at least the name of the host
        performing the authentication and might MAY additionally indicate the
        collection of users who might have access. An example might be "
        Users with call-out privileges ".

   pgp-algorithm: A string indicating the PGP message integrity check
        (MIC) to be used to produce the signature. If this not present
        it is assumed to be "md5". The currently defined values are
        "md5" for the MD5 checksum, and "sha1" for the SHA.1 algorithm.

   pgp-version: The version of PGP that the client MUST use. Common
        values are "2.6.2" and "5.0". The default is 5.0.

   Example:

   WWW-Authenticate: pgp ;version="5.0"
     ;realm="Your Startrek identity, please" ;algorithm="md5"

14.1.2 The Authorization Request Header

   The client is expected to retry the request, passing an Authorization
   header line, which is defined as follows.

        Authorization  ___      "Authorization" ":" "pgp" *( ";" pgp-response )
        pgp-response   ___      realm | pgp-version | pgp-signature | signed-by
        pgp-signature  ___      "signature" "=" quoted-string
        signed-by      ___      "signed-by" "=" URI

   The signature MUST correspond to the From header of the request
   unless the signed-by parameter is provided.

   pgp-signature: The PGP ASCII-armored signature, as it appears between
        the "BEGIN PGP MESSAGE" and "END PGP MESSAGE" delimiters,
        without the version indication. The signature is included
        without any linebreaks.

   The signature is computed across the request method, request version
   and header fields following the Authorization header and the message
   body, in the same order as they appear in the message. The request
   method and version are prepended to the header fields without any
   white space. The signature is computed across the headers as sent,
   including any folding and the terminating CRLF. The CRLF following
   the Authorization header is NOT included in the signature.

        Using the ASCII-armored version is about 25% less space-
        efficient than including the binary signature, but it is
        significantly easier for the receiver to piece together.
        Versions of the PGP program always include the full
        (compressed) signed text in their output unless ASCII-
        armored mode ( -sta ) is specified.  Typical signatures are
        about 200 bytes long. -- The PGP signature mechanism allows
        the client to simply pass the request to an external PGP
        program. This relies on the requirement that proxy servers
        are not allowed to reorder or change header fields.

   realm: The realm is copied from the corresponding WWW-Authenticate
        header field parameter.

   signed-by: If and only if the request was not signed by the entity
        listed in the From header, the signed-by header indicates the
        name of the signing entity, expressed as a URI.

   Receivers of signed SIP messages SHOULD discard any end-to-end header
   fields above the Authorization header, as they may have been
   maliciously added en route by a proxy.

   Example:

   Authorization: pgp version="5.0"
     ;realm="Your Startrek identity, please"
     ;signature="iQB1AwUBNNJiUaYBnHmiiQh1AQFYsgL/Wt3dk6TWK81/b0gcNDf
     VAUGU4rhEBW972IPxFSOZ94L1qhCLInTPaqhHFw1cb3lB01rA0RhpV4t5yCdUt
     SRYBSkOK29o5e1KlFeW23EzYPVUm2TlDAhbcjbMdfC+KLFX
     =aIrx"

14.2 PGP Encryption Scheme
   The PGP encryption scheme uses the following syntax:

        Encryption    ___   "Encryption" ":"    "pgp" pgp-eparams
        pgp-eparams   ___   1# ( pgp-version    | pgp-encoding )
        pgp-encoding  ___   "encoding" "=" "ascii" |    token

   encoding: Describes the encoding or "armor" used by PGP. The value
        "ascii" refers to the standard PGP ASCII armor, without the
        lines containing "BEGIN PGP MESSAGE" and "END PGP MESSAGE" and
        without the version identifier. By default, the encrypted part
        is included as binary.

   Example:

   Encryption: pgp version="2.6.2", encoding="ascii"

14.3 Response-Key Header Field for PGP

        Response-Key  ___   "Response-Key" ":" "pgp"    pgp-eparams
        pgp-eparams   ___   1# ( pgp-version    | pgp-encoding | pgp-key)
        pgp-key       ___   "key" "=" quoted-string

   If ASCII encoding has been requested via the encoding parameter, the
   key parameter contains the user's public key as extracted with the
   "pgp -kxa user ".

   Example:

   Response-Key: pgp version="2.6.2", encoding="ascii",
     key="mQBtAzNWHNYAAAEDAL7QvAdK2utY05wuUG+ItYK5tCF8HNJM60sU4rLaV+eUnkMk
     mOmJWtc2wXcZx1XaXb2lkydTQOesrUR75IwNXBuZXPEIMThEa5WLsT7VLme7njnx
     sE86SgWmAZx5ookIdQAFEbQxSGVubmluZyBTY2h1bHpyaW5uZSA8c2NodWx6cmlu
     bmVAY3MuY29sdW1iaWEuZWR1Pg==
     =+y19"

15 Examples

   In the following examples, we often omit the message body and the
   corresponding Content-Length and Content-Type headers for brevity.

15.1 Registration

   A user at host saturn.bell-tel.com registers on start-up, via
   multicast, with the local SIP server named sip.bell-tel.com the
   example, the user agent on saturn expects to receive SIP requests on
   UDP port 3890.

   C->S: REGISTER sip:sip.bell-tel.com SIP/2.0
         Via: SIP/2.0/UDP 128.16.64.19
         From: sip:watson@bell-tel.com
         To: sip:watson@bell-tel.com
         Call-ID: 4236500900@saturn.bell-tel.com
         CSeq: 1 REGISTER
         Location: sip:saturn.bell-tel.com:3890;transport=udp
         Contact: <sip:saturn.bell-tel.com:3890;transport=udp>
         Expires: 7200

   The registration expires after two hours. Any future invitations for
   watson@bell-tel.com arriving at sip.bell-tel.com will now be
   redirected to watson@saturn.bell-tel.com , UDP port 3890.

   If Watson wants to be reached elsewhere, say, an on-line service he
   uses while traveling, he updates his reservation after first
   cancelling any existing locations:

   C->S: REGISTER sip:bell-tel.com SIP/2.0
         Via: SIP/2.0/UDP 128.16.64.19
         From: sip:watson@bell-tel.com
         To: sip:watson@bell-tel.com
         Call-ID: 1345441868@saturn.bell-tel.com
         CSeq: 1 REGISTER
         Location:
         Contact: *
         Expires: 0

   C->S: REGISTER sip:bell-tel.com SIP/2.0
         Via: SIP/2.0/UDP 128.16.64.19
         From: sip:watson@bell-tel.com
         To: sip:watson@bell-tel.com
         Call-ID: 81791800@saturn.bell-tel.com
         CSeq: 1 REGISTER
         Location:
         Contact: sip:tawatson@example.com
   Now, the server will forward any request for Watson to the server at
   example.com , using the Request-URI tawatson@example.com

   It is possible to use third-party registration. Here, the secretary
   jon.diligent registers his boss:

   C->S: REGISTER sip:bell-tel.com SIP/2.0
         Via: SIP/2.0/UDP 128.16.64.19
         From: sip:jon.diligent@bell-tel.com
         To: sip:watson@bell-tel.com
         Call-ID: 1212759220@saturn.bell-tel.com
         CSeq: 1 REGISTER
         Location:
         Contact: sip:tawatson@example.com

   The request could be send to either the registrar at bell-tel.com or
   the server at example.com example.com would proxy the request to the
   address indicated in the Request-URI. Then, Max-Forwards header could
   be used to restrict the registration to that server.

15.2 Invitation to a Multicast Conference

   The first example invites schooler@vlsi.cs.caltech.edu to a multicast
   session. All examples use the Session Description Protocol (SDP) (RFC
   2327 [5]) as the session description format.

15.2.1 Request

   C->S: INVITE sip:schooler@vlsi.cs.caltech.edu SIP/2.0
         Via: SIP/2.0/UDP csvax.cs.caltech.edu;branch=8348
           ;maddr=239.128.16.254;ttl=16
         Via: SIP/2.0/UDP north.east.isi.edu
         From: Mark Handley <sip:mjh@isi.edu>
         To: Eve Schooler <sip:schooler@caltech.edu>
         Call-ID: 2963313058@oregon.isi.edu
         CSeq: 1 INVITE
         Subject: SIP will be discussed, too
         Content-Type: application/sdp
         Content-Length: 187

         v=0
         o=user1 53655765 2353687637 IN IP4 128.3.4.5
         s=Mbone Audio
         i=Discussion of Mbone Engineering Issues
         e=mbone@somewhere.com
         c=IN IP4 224.2.0.1/127
         t=0 0
         m=audio 3456 RTP/AVP 0

   The Via fields list the hosts along the path from invitation
   initiator (the last element of the list) towards the invitee. In the
   example above, the message was last multicast to the administratively
   scoped group 239.128.16.254 with a ttl of 16 from the host
   131.215.131.131

   The request header above states that the request was initiated by
   mjh@isi.edu from the host 128.16.64.19 schooler@caltech.edu is being
   invited; the message is currently being routed to
   schooler@vlsi.cs.caltech.edu

   In this case, the session description is using the Session
   Description Protocol (SDP), as stated in the Content-Type header.

   The header is terminated by an empty line and is followed by a
   message body containing the session description.

15.2.2 Response

   The called user agent, directly or indirectly through proxy servers,
   indicates that it is alerting ("ringing") the called party:

   S->C: SIP/2.0 180 Ringing
         Via: SIP/2.0/UDP csvax.cs.caltech.edu;branch=8348
           ;maddr=239.128.16.254;ttl=16
         Via: SIP/2.0/UDP north.east.isi.edu
         From: Mark Handley <sip:mjh@isi.edu>
         To: Eve Schooler <sip:schooler@caltech.edu;tag=9883472> <sip:schooler@caltech.edu> ;tag=9883472
         Call-ID: 2963313058@oregon.isi.edu
         CSeq: 1 INVITE

   A sample response to the invitation is given below. The first line of
   the response states the SIP version number, that it is a 200 (OK)
   response, which means the request was successful. The Via headers are
   taken from the request, and entries are removed hop by hop as the
   response retraces the path of the request. A new authentication field
   MAY be added by the invited user's agent if required. The Call-ID is
   taken directly from the original request, along with the remaining
   fields of the request message. The original sense of From field is
   preserved (i.e., it is the session initiator).

   In addition, the Location Contact header gives details of the host where the
   user was located, or alternatively the relevant proxy contact point
   which should be reachable from the caller's host.

   S->C: SIP/2.0 200 OK
         Via: SIP/2.0/UDP csvax.cs.caltech.edu;branch=8348
           ;maddr=239.128.16.254 16;ttl=16
           ;maddr=239.128.16.254;ttl=16
         Via: SIP/2.0/UDP north.east.isi.edu
         From: Mark Handley <sip:mjh@isi.edu>
         To: Eve Schooler <sip:schooler@caltech.edu;tag=9883472> <sip:schooler@caltech.edu> ;tag=9883472
         Call-ID: 2963313058@oregon.isi.edu
         CSeq: 1 INVITE
         Location:
         Contact: sip:es@jove.cs.caltech.edu

   The caller confirms the invitation by sending a request to the
   location named in the Location Contact header:

   C->S: ACK sip:es@jove.cs.caltech.edu SIP/2.0
         Via: SIP/2.0/UDP csvax.cs.caltech.edu;branch=8348
           ;maddr=239.128.16.254;ttl=16
         From: Mark Handley <sip:mjh@isi.edu>
         To: Eve Schooler <sip:schooler@caltech.edu;tag=9883472> <sip:schooler@caltech.edu> ;tag=9883472
         Call-ID: 2963313058@oregon.isi.edu
         CSeq: 1 ACK

15.3 Two-party Call

   For two-party Internet phone calls, the response must contain a
   description of where to send the data. In the example below, Bell
   calls Watson. Bell indicates that he can receive RTP audio codings 0
   (PCMU), 3 (GSM), 4 (G.723) and 5 (DVI4).

   C->S: INVITE sip:watson@boston.bell-tel.com SIP/2.0
         Via: SIP/2.0/UDP 169.130.12.5
         From: A. Bell <sip:a.g.bell@bell-tel.com>
         To: T. Watson <sip:watson@bell-tel.com>
         Call-ID: 3298420296@kton.bell-tel.com
         CSeq: 1 INVITE
         Subject: Mr. Watson, come here.
         Content-Type: application/sdp
         Content-Length: ...

         v=0
         o=bell 53655765 2353687637 IN IP4 128.3.4.5
         s=Mr. Watson, come here.
         c=IN IP4 135.180.144.94
         m=audio 3456 RTP/AVP 0 3 4 5

   S->C: SIP/2.0 100 Trying
         Via: SIP/2.0/UDP 169.130.12.5
         From: A. Bell <sip:a.g.bell@bell-tel.com>
         To: T. Watson <sip:watson@bell-tel.com;tag=37462311> <sip:watson@bell-tel.com> ;tag=37462311
         Call-ID: 3298420296@kton.bell-tel.com
         CSeq: 1 INVITE
         Content-Length: 0

   S->C: SIP/2.0 180 Ringing
         Via: SIP/2.0/UDP 169.130.12.5
         From: A. Bell <sip:a.g.bell@bell-tel.com>
         To: T. Watson <sip:watson@bell-tel.com;tag=37462311> <sip:watson@bell-tel.com> ;tag=37462311
         Call-ID: 3298420296@kton.bell-tel.com
         CSeq: 1 INVITE
         Content-Length: 0

   S->C: SIP/2.0 182 Queued, 2 callers ahead
         Via: SIP/2.0/UDP 169.130.12.5
         From: A. Bell <sip:a.g.bell@bell-tel.com>
         To: T. Watson <sip:watson@bell-tel.com;tag=37462311> <sip:watson@bell-tel.com> ;tag=37462311
         Call-ID: 3298420296@kton.bell-tel.com
         CSeq: 1 INVITE
         Content-Length: 0

   S->C: SIP/2.0 182 Queued, 1 caller ahead
         Via: SIP/2.0/UDP 169.130.12.5
         From: A. Bell <sip:a.g.bell@bell-tel.com>
         To: T. Watson <sip:watson@bell-tel.com;tag=37462311> <sip:watson@bell-tel.com> ;tag=37462311
         Call-ID: 3298420296@kton.bell-tel.com
         CSeq: 1 INVITE
         Content-Length: 0

   S->C: SIP/2.0 200 OK
         Via: SIP/2.0/UDP 169.130.12.5
         From: A. Bell <sip:a.g.bell@bell-tel.com>
         To: sip:watson@bell-tel.com;tag=37462311 <sip:watson@bell-tel.com> ;tag=37462311
         Call-ID: 3298420296@kton.bell-tel.com
         CSeq: 1 INVITE
         Location:
         Contact: sip:watson@boston.bell-tel.com
         Content-Length: ...

         v=0
         o=watson 4858949 4858949 IN IP4 192.1.2.3
         s=I'm on my way
         c=IN IP4 135.180.161.25
         m=audio 5004 RTP/AVP 0 3

   The example illustrates the use of informational status responses.
   Here, the reception of the call is confirmed immediately (100), then,
   possibly after some database mapping delay, the call rings (180) and
   is then queued, with periodic status updates.

   Watson can only receive PCMU and GSM. Note that Watson's list of
   codecs may or may not be a subset of the one offered by Bell, as each
   party indicates the data types it is willing to receive. Watson will
   send audio data to port 3456 at 135.180.144.94, Bell will send to
   port 5004 at 135.180.161.25.

   By default, the media session is one RTP session. Watson will receive
   RTCP packets on port 5005, while Bell will receive them on port 3457.

   Since the two sides have agreed on the set of media, Watson confirms
   the call without enclosing another session description:

   C->S: ACK sip:watson@boston.bell-tel.com SIP/2.0
         Via: SIP/2.0/UDP 169.130.12.5
         From: A. Bell <sip:a.g.bell@bell-tel.com>
         To: T. Watson <sip:watson@bell-tel.com;tag=37462311> <sip:watson@bell-tel.com> ;tag=37462311
         Call-ID: 3298420296@kton.bell-tel.com
         CSeq: 1 ACK

15.4 Terminating a Call

   To terminate a call, caller or callee can send a BYE request:

   C->S: BYE sip:watson@boston.bell-tel.com SIP/2.0
         Via: SIP/2.0/UDP 169.130.12.5
         From: A. Bell <sip:a.g.bell@bell-tel.com>
         To: T. A. Watson <sip:watson@bell-tel.com;tag=37462311> <sip:watson@bell-tel.com> ;tag=37462311
         Call-ID: 3298420296@kton.bell-tel.com
         CSeq: 2 BYE
   If the callee wants to abort the call, it simply reverses the To and
   From fields. Note that it is unlikely that an BYE from the callee
   will traverse the same proxies as the original INVITE.

15.5 Forking Proxy

   In this example, Bell ( a.g.bell@bell-tel.com ) (C), currently seated
   at host c.bell-tel.com wants to call Watson ( t.watson@ieee.org ). At
   the time of the call, Watson is logged in at two workstations,
   watson@x.bell-tel.com (X) and watson@y.bell-tel.com (Y), and has
   registered with the IEEE proxy server (P) called sip.ieee.org
   registration for the home machine of Watson, at watson@h.bell-tel.com
   (H), as well as a permanent registration at watson@acm.org (A). For
   brevity, the examples omit the session description.

   Watson's user agent sends the invitation to the SIP server for the
   ieee.org domain:

   C->P: INVITE sip:watson@ieee.org SIP/2.0
         Via:     SIP/2.0/UDP c.bell-tel.com
         From:    A. Bell <sip:a.g.bell@bell-tel.com>
         To:      T. Watson <sip:t.watson@ieee.org>
         Call-ID: 31415@kton.bell-tel.com
         CSeq:    1 INVITE

   The SIP server at ieee.org tries the four addresses in parallel.  It
   sends the following message to the home machine:

   P->H: INVITE sip:watson@h.bell-tel.com SIP/2.0
         Via:     SIP/2.0/UDP sip.ieee.org ;branch=1
         Via:     SIP/2.0/UDP c.bell-tel.com
         From:    A. Bell <sip:a.g.bell@bell-tel.com>
         To:      T. Watson <sip:t.watson@ieee.org>
         Call-ID: 31415@kton.bell-tel.com
         CSeq:    1 INVITE

   This request immediately yields a 404 (Not Found) response, since
   Watson is not currently logged in at home:

   H->P: SIP/2.0 404 Not Found
         Via:     SIP/2.0/UDP sip.ieee.org ;branch=1
         Via:     SIP/2.0/UDP c.bell-tel.com
         From:    A. Bell <sip:a.g.bell@bell-tel.com>
         To:      T. Watson <sip:t.watson@ieee.org;tag=87454273> <sip:t.watson@ieee.org>;tag=87454273
         Call-ID: 31415@kton.bell-tel.com
         CSeq:    1 INVITE

   The proxy ACKs the response so that host H can stop retransmitting
   it:

   P->H: ACK sip:watson@h.bell-tel.com SIP/2.0
         Via:     SIP/2.0/UDP sip.ieee.org ;branch=1
         From:    A. Bell <sip:a.g.bell@bell-tel.com>
         To:      T. Watson <sip:t.watson@ieee.org;tag=37462311> <sip:t.watson@ieee.org>;tag=37462311
         Call-ID: 31415@kton.bell-tel.com
         CSeq:    1 ACK

   Also, P attempts to reach Watson through the ACM server:

   P->A: INVITE sip:watson@acm.org SIP/2.0
         Via:     SIP/2.0/UDP sip.ieee.org ;branch=2
         Via:     SIP/2.0/UDP c.bell-tel.com
         From:    A. Bell <sip:a.g.bell@bell-tel.com>
         To:      T. Watson <sip:t.watson@ieee.org>
         Call-ID: 31415@kton.bell-tel.com
         CSeq:    1 INVITE

   In parallel, the next attempt proceeds, with an INVITE to X and Y:

   P->X: INVITE sip:watson@x.bell-tel.com SIP/2.0
         Via:     SIP/2.0/UDP sip.ieee.org ;branch=3
         Via:     SIP/2.0/UDP c.bell-tel.com
         From:    A. Bell <sip:a.g.bell@bell-tel.com>
         To:      T. Watson <sip:t.watson@ieee.org>
         Call-ID: 31415@kton.bell-tel.com
         CSeq:    1 INVITE

   P->Y: INVITE sip:watson@y.bell-tel.com SIP/2.0
         Via:     SIP/2.0/UDP sip.ieee.org ;branch=4
         Via:     SIP/2.0/UDP c.bell-tel.com
         From:    A. Bell <sip:a.g.bell@bell-tel.com>
         To:      T. Watson <sip:t.watson@ieee.org>
         Call-ID: 31415@kton.bell-tel.com
         CSeq:    1 INVITE

   As it happens, both Watson at X and a colleague in the other lab at
   host Y hear the phones ringing and pick up. Both X and Y return 200s
   via the proxy to Bell.

   X->P: SIP/2.0 200 OK
         Via:      SIP/2.0/UDP sip.ieee.org ;branch=3
         Via:      SIP/2.0/UDP c.bell-tel.com
         From:     A. Bell <sip:a.g.bell@bell-tel.com>
         To:       T. Watson <sip:t.watson@ieee.org;tag=192137601> <sip:t.watson@ieee.org> ;tag=192137601
         Call-ID:  31415@kton.bell-tel.com
         CSeq:     1 INVITE
         Location:
         Contact:  sip:t.watson@x.bell-tel.com

   Y->P: SIP/2.0 200 OK
         Via:      SIP/2.0/UDP sip.ieee.org ;branch=4
         Via:      SIP/2.0/UDP c.bell-tel.com
         Location:
         Contact:  sip:t.watson@y.bell-tel.com
         From:     A. Bell <sip:a.g.bell@bell-tel.com>
         To:       T. Watson <sip:t.watson@ieee.org;tag=35253448> <sip:t.watson@ieee.org> ;tag=35253448
         Call-ID:  31415@kton.bell-tel.com
         CSeq:     1 INVITE

   Both responses are forwarded to Bell, using the Via information.  At
   this point, the ACM server is still searching its database. P can now
   cancel this attempt:

   P->A: CANCEL sip:watson@acm.org SIP/2.0
         Via:     SIP/2.0/UDP sip.ieee.org ;branch=2
         From:    A. Bell <sip:a.g.bell@bell-tel.com>
         To:      T. Watson <sip:t.watson@ieee.org>
         Call-ID: 31415@kton.bell-tel.com
         CSeq:    1 CANCEL

   The ACM server gladly stops its neural-network database search and
   responds with a 200. The 200 will not travel any further, since P is
   the last Via stop.

   A->P: SIP/2.0 200 OK
         Via:     SIP/2.0/UDP sip.ieee.org ;branch=2
         From:    A. Bell <sip:a.g.bell@bell-tel.com>
         To:      T. Watson <sip:t.watson@ieee.org>
         Call-ID: 31415@kton.bell-tel.com
         CSeq:    1 CANCEL

   Bell gets the two 200 responses from X and Y in short order. Bell's
   reaction now depends on his software. He can either send an ACK to
   both if human intelligence is needed to determine who he wants to
   talk to or he can automatically reject one of the two calls. Here, he
   acknowledges both, separately and directly to the final destination:

   C->X: ACK sip:watson@x.bell-tel.com SIP/2.0
         Via:      SIP/2.0/UDP c.bell-tel.com
         From:     A. Bell <sip:a.g.bell@bell-tel.com>
         To:       T. Watson <sip:t.watson@ieee.org;tag=192137601> <sip:t.watson@ieee.org>;tag=192137601
         Call-ID:  31415@c.bell-tel.com
         CSeq:     1 ACK

   C->Y: ACK sip:watson@y.bell-tel.com SIP/2.0
         Via:      SIP/2.0/UDP c.bell-tel.com
         From:     A. Bell <sip:a.g.bell@bell-tel.com>
         To:       T. Watson <sip:t.watson@ieee.org;tag=35253448> <sip:t.watson@ieee.org>;tag=35253448
         Call-ID:  31415@c.bell-tel.com
         CSeq:     1 ACK

   After a brief discussion between the three, it becomes clear that
   Watson is at X, thus Bell sends a BYE to Y, which is replied to:

   C->Y: BYE sip:watson@y.bell-tel.com SIP/2.0
         Via:      SIP/2.0/UDP c.bell-tel.com
         From:     A. Bell <sip:a.g.bell@bell-tel.com>
         To:       T. Watson <sip:t.watson@ieee.org;tag=35253448> <sip:t.watson@ieee.org>;tag=35253448
         Call-ID:  31415@c.bell-tel.com
         CSeq:     2 BYE

   Y->C: SIP/2.0 200 OK
         Via:      SIP/2.0/UDP c.bell-tel.com
         From:     A. Bell <sip:a.g.bell@bell-tel.com>
         To:       T. Watson <sip:t.watson@ieee.org;tag=35253448> <sip:t.watson@ieee.org>;tag=35253448
         Call-ID:  31415@c.bell-tel.com
         CSeq:     2 BYE

15.6 Redirects

   Replies with status codes 301 (Moved Permanently) or 302 (Moved
   Temporarily) specify another location using the Location Contact field.
   Continuing our earlier example, the server at ieee.org decides to
   redirect rather than proxy the request:

   S->C: SIP/2.0 302 Moved temporarily
         Via:     SIP/2.0/UDP sip.ieee.org
         From:    A. Bell <sip:a.g.bell@bell-tel.com>
         To:      T. Watson <sip:t.watson@ieee.org;tag=72538263> <sip:t.watson@ieee.org>;tag=72538263
         Call-ID: 31415@kton.bell-tel.com
         CSeq:    1 INVITE
         Location:
         Contact: sip:watson@h.bell-tel.com,
                   sip:watson@acm.org, sip:watson@x.bell-tel.com,
                   sip:watson@y.bell-tel.com
         CSeq: 1 INVITE

   As another example, assume Alice wants to delegate her calls to Bob
   while she is on vacation until July 29th. Any calls meant for her
   will reach Bob with Alice's To field, indicating to him what role he
   is to play. In the example below, Charlie calls Alice.

   S->C: SIP/2.0 302 Moved temporarily
         From: sip:charlie@caller.com Charlie <sip:charlie@caller.com>
         To: sip:alice@anywhere.com;tag=2332462 Alice <sip:alice@anywhere.com> ;tag=2332462
         Call-ID: 27182@caller.com
         Location:
         Contact: sip:bob@anywhere.com
         Expires: Wed, 29 Jul 1998 9:00:00 GMT
         CSeq: 1 INVITE

   Charlie then sends the following request to the SIP server of the
   anywhere.com domain.

   C->S: INVITE sip:bob@anywhere.com SIP/2.0
         From: sip:charlie@caller.com
         To: sip:alice@anywhere.com
         Call-ID: 27182@caller.com
         CSeq: 2 INVITE

   In the third redirection example, we assume that all requests are
   directed through a local firewall at caller.com firewall happens to
   be overloaded and thus redirects the call from Charlie to a secondary
   server.

   S->C: SIP/2.0 302 Moved temporarily
         From: sip:charlie@caller.com
         To: sip:alice@anywhere.com;tag=273462236 Alice <sip:alice@anywhere.com> ;tag=273462236
         Call-ID: 27182@caller.com
         CSeq: 2 INVITE
         Location: sip:alice@anywhere.com:5080 ;maddr=secondary.caller.com
         Contact: <sip:alice@anywhere.com:5080;maddr=secondary.caller.com>

   Charlie directs the invitation to the secondary server at port 5080,
   but maintains the same Request-URI as before.

   C->S: INVITE sip:alice@anywhere.com SIP/2.0
         From: sip:charlie@caller.com
         To: sip:alice@anywhere.com
         Call-ID: 27182@caller.com
         CSeq: 3 INVITE

15.7 Alternative Services

   An example of a 380 (Alternative Service) response is:

   S->C: SIP/2.0 380 Alternative Service
         Via: SIP/2.0/UDP 131.215.131.131
         Via: SIP/2.0/UDP 128.16.64.19
         From: sip:mjh@isi.edu
         To: sip:schooler@cs.caltech.edu;tag=11223647 <sip:schooler@cs.caltech.edu> ;tag=11223647
         Call-ID: 14142@oregon.isi.edu
         CSeq: 1 INVITE
         Location:
         Contact: sip:recorder@131.215.131.131
         Content-Type: application/sdp
         Content-Length: 146
         v=0
         o=mm-server 2523535 0 IN IP4 131.215.131.131
         s=Answering Machine
         i=Leave an audio message
         c=IN IP4 131.215.131.131
         t=0 0
         m=audio 12345 RTP/AVP 0

   In this case, the answering server provides a session description
   that describes an "answering machine". If the invitation initiator
   decides to take advantage of this service, it should send an
   invitation request to the answering machine at 131.215.131.131 with
   the session description provided (modified as appropriate for a
   unicast session to contain the appropriate local address and port for
   the invitation initiator). This request SHOULD contain a different
   Call-ID from the one in the original request. An example would be:

   C->S: INVITE sip:recorder@131.215.131.131 SIP/2.0
         Via: SIP/2.0/UDP 128.16.64.19
         From: sip:mjh@isi.edu
         To: sip:schooler@cs.caltech.edu
         Call-ID: 1732@oregon.isi.edu
         CSeq: 1 INVITE
         Content-Type: application/sdp
         Content-Length: 146

         v=0
         o=mm-server 2523535 0 IN IP4 131.215.131.131
         s=Answering Machine
         i=Leave an audio message
         c=IN IP4 128.16.64.19
         t=0 0
         m=audio 26472 RTP/AVP 0

   Invitation initiators MAY choose to treat a 350 (Alternative Service)
   response as a failure if they wish to do so.

15.8 Negotiation

   An example of a 606 (Not Acceptable) response is:

   S->C: SIP/2.0 606 Not Acceptable
         From: sip:mjh@isi.edu
         To: sip:schooler@cs.caltech.edu;tag=7434264 <sip:schooler@cs.caltech.edu> ;tag=7434264
         Call-ID: 14142@oregon.isi.edu
         CSeq: 1 INVITE
         Location:
         Contact: sip:mjh@131.215.131.131
         Warning: 606.1 Insufficient 370 "Insufficient bandwidth (only have ISDN),
           606.3 Incompatible format,
           606.4 Multicast ISDN)",
           305 "Incompatible media format",
           330 "Multicast not available available"
         Content-Type: application/sdp
         Content-Length: 50

         v=0
         s=Lets talk
         b=CT:128
         c=IN IP4 131.215.131.131
         m=audio 3456 RTP/AVP 7 0 13
         m=video 2232 RTP/AVP 31

   In this example, the original request specified 256 kb/s total
   bandwidth, and the response states that only 128 kb/s is available.
   The original request specified GSM audio, H.261 video, and WB
   whiteboard.  The audio coding and whiteboard are not available, but
   the response states that DVI, PCM or LPC audio could be supported in
   order of preference. The response also states that multicast is not
   available.  In such a case, it might be appropriate to set up a
   transcoding gateway and re-invite the user.

15.9 OPTIONS Request

   A caller Alice can use an OPTIONS request to find out the
   capabilities of a potential callee Bob, without "ringing" the
   designated address. Bob returns a description indicating that he is
   capable of receiving audio and video, with a list of supported
   encodings.

   C->S: OPTIONS sip:bob@example.com SIP/2.0
         From: Alice <sip:alice@anywhere.org>
         To: Bob <sip:bob@example.com>
         Call-ID: 6378@host.anywhere.org
         CSeq: 1 OPTIONS
         Accept: application/sdp

   S->C: SIP/2.0 200 OK
         From: Alice <sip:alice@anywhere.org>
         To: Bob <sip:bob@example.com;tag=376364382> <sip:bob@example.com> ;tag=376364382
         Call-ID: 6378@host.anywhere.org
         Content-Length: 81
         Content-Type: application/sdp

         v=0
         m=audio 0 RTP/AVP 0 1 3 99
         m=video 0 RTP/AVP 29 30
         a=rtpmap:99 SX7300/8000

A Minimal Implementation

A.1 Client

   All clients MUST be able to generate the INVITE and ACK requests.
   Clients MUST generate and parse the Call-ID, Content-Length,
   Content-Type, CSeq, From and To headers. Clients MUST also parse the
   Require header. A minimal implementation MUST understand SDP (RFC
   2327, [5]). It MUST be able to recognize the status code classes 1
   through 6 and act accordingly.

   The following capability sets build on top of the minimal
   implementation described in the previous paragraph:

   Basic: A basic implementation adds support for the BYE method to
        allow the interruption of a pending call attempt. It includes a
        User-Agent header in its requests and indicate its preferred
        language in the Accept-Language header.

   Redirection: To support call forwarding, a client needs to be able to
        understand the Location Contact header, but only the SIP-URL part, not
        the parameters.

   Negotiation: A client MUST be able to request the OPTIONS method and
        understand the 380 (Alternative Service) status and the Location Contact
        parameters to participate in terminal and media negotiation. It
        SHOULD be able to parse the Warning response header to provide
        useful feedback to the caller.

   Authentication: If a client wishes to invite callees that require
        caller authentication, it must MUST be able to recognize the 401
        (Unauthorized) status code, must MUST be able to generate the
        Authorization request header and MUST understand the WWW-
        Authenticate response header.

   If a client wishes to use proxies that require caller authentication,
   it MUST be able to recognize the 407 (Proxy Authentication Required)
   status code, MUST be able to generate the Proxy-Authorization request
   header and understand the Proxy-Authenticate response header.

A.2 Server

   A minimally compliant server implementation MUST understand the
   INVITE, ACK, OPTIONS and BYE requests. A proxy server MUST also
   understand CANCEL. It MUST parse and generate, as appropriate, the
   Call-ID, Content-Length, Content-Type, CSeq, Expires, From, Max-
   Forwards, Require, To and Via headers. It MUST echo the CSeq and
   Timestamp headers in the response. It SHOULD include the Server
   header in its responses.

A.3 Header Processing

   Table 5 6 lists the headers that different implementations support. UAC
   refers to a user-agent client (calling user agent), UAS to a user-
   agent server (called user-agent).

B Usage of SDP

   By default, the nth nth media session in a unicast INVITE request will
   become a single RTP session with the nth media session in the
   response. Thus, the callee should be careful to order media
   descriptions appropriately.

   It is assumed that if caller or callee include a particular media
   type, they want to both send and receive media data. If the callee
   does not want to send a particular media     type, it marks the media
   entry as recvonly receive a particular media type, it        may mark it as
   sendonly wants to neither receive nor send a particular media type,
   it sets the port to zero. (RTCP ports are not needed in this case.)

   The caller includes all media types that it is willing to send so
   that the receiver can provide matching media descriptions.

   The callee sets the port to zero if callee and caller only want to
   receive a media type.

   Either party can set the "c" destination address to zero (0.0.0.0) if
   it wants to signal to the other party to stop sending media session in a unicast INVITE request will
   become data.
   This implements a single RTP session with (far-side) "mute" or "hold" functionality.

        The SDP fields "s" and the nth media session in SIP Subject header have
        different meanings when inviting to a multicast session.
        The SDP field describes the
   response. Thus, subject of the callee should be careful to order media multicast
                                   type    UAC    proxy    UAS
            __________________________________________________
            Accept                  R       -       o       o
            Accept-Language         R       -       b       b
            Allow                  405      o       -       -
            Authorization           R       a       o       a
            Call-ID                 g       m       m       m
            Content-Length          g       m       m       m
            Content-Type            g       m       -       m
            CSeq                    g       m       m       m
            Encryption              g       e       -       e
            Expires                 g       -       o       o
            From                    g       m       o       m
            Location
            Contact                 R       -       -       -
            Location
            Contact                 r       r       r       -
            Max-Forwards            R       -       b       -
            Proxy-Authenticate     407      a       -       -
            Proxy-Authorization     R       -       a       -
            Proxy-Require           R       -       m       -
            Require                 R       m       -       m
            Response-Key            R       -       -       e
            Timestamp               g       o       o       m
            To                      g       m       m       m
            Unsupported             r       b       b       -
            Via                     g       m       m       m
            WWW-Authenticate       401      a       -       -

   Table 5: 6: This  table  indicates  which  systems should be able  to  parse  which  header
   fields.  Type  is as in Table 4. 4 and 5. "-" indicates the field is not
   meaningful to this system (although it might be generated by it). "m"
   indicates  the  field  MUST  be  understood.  "b" indicates the field
   SHOULD be understood by a Basic  implementation.  "r"  indicates  the
   field  SHOULD  be  understood  if  the  system  claims  to understand
   redirection.  "a" indicates the field SHOULD  be  understood  if  the
   system  claims  to  support  authentication.  "e" indicates the field
   SHOULD be understood if the system claims to support encryption.  "o"
   indicates  support  of  the  field  is purely optional. Headers whose
   support is optional for all implementations are not shown.

   descriptions appropriately.

   It is assumed that if caller or callee include a particular media
   type, they want to both send and receive media data. If

        session, while the callee
   does not want to send a particular media type, it should mark SIP Subject header describes the
   media entry as recvonly receive a particular media type, it may mark
   it as sendonly wants to neither receive nor send a particular media
   type, it should set reason
        for the port to zero. (RTCP ports are not needed invitation. The example in Section 15.2 illustrates
        this case.)

   The caller should include all media types that it is willing point. For invitations to send
   so that two-party sessions, the receiver can provide matching media descriptions. SDP
        "s" field MAY be left empty. The callee should set the port to zero if callee and caller only want "o" field is not strictly
        necessary for two-party sessions, but MUST be present to receive a media type.
        allow re-use of SDP-based tools.

C Summary of Augmented BNF

   In this specification we use the Augmented Backus-Naur Form notation
   described in RFC 2234 [21]. For quick reference, the following is a
   brief summary of the main features of this ABNF.

   "abc"
        The case-insensitive string of characters "abc" (or "Abc",
        "aBC", etc.);

   %d32
        The character with ASCII code decimal 32 (space);

   *term
        zero of more instances of  term;

   3*term
        three or more instances of  term;

   2*4term
        two, three or four instances of  term;

   [ term ]
        term is optional;

   term1 term2 term3
        set notation:  term1,  term2 and  term3 must all appear but
        their order is unimportant;

   term1 | term2
        either  term1 or  term2 may appear but not both;

   #term
        a comma separated list of  term;

   2#term
        a comma separated list of  term containing at least 2 items;

   2#4term
        a comma separated list of  term containing 2 to 4 items.

   Common       Tokens

   Certain tokens are used frequently in the BNF of this document, and
   not defined elsewhere. Their meaning is well understood but we
   include it here for completeness.

        CR       =    %d13 ; US-ASCII CR, carriage return character
        LF       =    %d10 ; US-ASCII LF, line feed character
        CRLF     =    CR LF ; typically the end of a line
        SP       =    %d32 ; US-ASCII SP, space character
        HT       =    %d09 ; US-ASCII HT, horizontal tab character
        LWS      =    [CRLF] 1*( SP | HT ) ; linear whitespace
        DIGIT    =    "0" .. "9" ; a single decimal digit
        CHAR     =    <any US-ASCII character (octets 0 - 127)>
        CTL      =    <any US-ASCII control character
                      (octets 0 - -- 31) and DEL (127)>
        OCTET    =    <any 8-bit sequence of data>
        TEXT     =    <any OCTET  except CTLs, but including LWS>

        unreserved    =    alphanum | mark
        mark          =    "-" | "_" | "." | "!" | "~" | "*" | "'"
                     |     "(" | ")"
        separators    =    "(" | ")" | "<" | ">" | "@" |
                           "," | ";" | ":" | "/" | <"> |
                           "/" | "[" | "]" | "?" | "=" |
                           "" | "" | SP | HT
        escaped       =    "%" hex hex
        hex           =    digit | "A" | "B" | "C" | "D" | "E" | "F" |
                           "a" | "b" | "c" | "d" | "e" | "f"
        alphanum      =    alpha | digit
        alpha         =    lowalpha | upalpha
        lowalpha      =    "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" |
                           "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" |
                           "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z"
        upalpha       =    "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" |
                           "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" |
                           "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"
        digit         =    "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
                           "8" | "9"
        token         =    1*<any    1*< any CHAR  except CTL's  or separators>
        comment       =    "(" *(ctext | quoted-pair | comment) ")"
        ctext         =    <any    < any TEXT  excluding "("  and ")">

D IANA Considerations

   Section 4.4 describes a name space and mechanism for registering SIP
   options.

   Section 6.41 describes the name space for registering SIP warn-codes.

E Changes in Version -08 -09

   Since version -07, -08, the following changes have been made.

        o Allow maddr to contain host names, not just IP multicast
          addresses. This is useful to allow multicast addresses Checked and capitalized or rewrote "may", "should" and "must".

        o Consistent use of "header field" throughout the
          form mcast.net as well as for redirecting requests to a
          different server without changing the Request-URI. spec.

        o Warning messages updated.

        o Added note
          to method  URL parameter that effect to description indicates request method.  Now
          allows complete construction of 302.

        o Added references to symbolic name sip.mcast.net

        o REGISTER requests without SIP request from within a web
          page and a Location are quite header. The latter is particularly useful
          for third-party call control.

        o Clarified use of SRV  DNS records. TCP and UDP may have equal
          priority, but different weight.

        o Added 486 (Busy here) status  hen there might be other
          branches, such as answering machines, ready to obtain take the current lists of registrations. call.

        o Added "comment" expires parameter to Location header field to allow responses like
          Location: alice@example.com (Alice) solution conforming
          reporting of expiration times for each location.

        o Clarified 481 as referring to a mismatch in describing a call
          leg (BYE) or transaction (CANCEL).

        o Clarified that the
          format of From and CSeq method in the request is always the
          same as the request method, even for ACK.

        o Made  the tag parameter a To is, unfortunately, or From parameter, not backward
          compatible a URL
          parameter. Tags may be needed for some non-SIP URIs.

        o Location header field renamed to Contact, with HTTP. syntax
          according to To and From.

        o Allow Content-Length Rules for defaults in Contact headers now simply point to be optional, URL
          rules.

        o Added wording to ACK  section on how to process requests.

        o Made  all elements in line with HTTP.  This
          does not seem sent-protocol mandatory to complicate matters, but makes the processing avoid
          ambiguity of cgi-style output much easier, since the server doesn't have whether "SIP/2.0" referes to store the output to compute default protocol
          of version "SIP" with transport "2.0" or the Content-Length.

        o Fixed table 4: CSeq is now mandatory for all requests. correct
          interpretation.

        o Since To, From, and Call-ID are used as a callleg identifier
          even Filled in proxies, encryption descriptions of From needs to be disallowed. status codes 410, 411 and 413,
          basically unchanged from HTTP.

        o Organization Made  Record-Route and Route header fields consistent with
          From, To, Contact. As long as the URI doesn't contain
          parameters or commas, this is now a general header; no reason not change, but it avoids the
          same problems that lead to allow the callee change in Contact syntax.

        o Added remark  to insert its organization. SDP section to use a zero address to
          temporarily disable media transmission ("put call on hold").

        o Changed wording for Record-Route BNF changed to SIP-URL.

        o Via operation clarified.

        o Additional BNF added apply to appendix.

        o Tag usage descriptions made consistent.  any subsequent
          request.

F Acknowledgments

   We wish to thank the members of the IETF MMUSIC WG for their comments
   and suggestions. Detailed comments were provided by Jim Buller, Dave
   Devanathan, Yaron Goland, Christian Huitema, Gadi Karmi, Jonathan
   Lennox, Moshe J. Sambol, and Eric Tremblay.

   This work is based, inter alia, on [29,30].

G Authors' Addresses

   Mark Handley
   USC Information Sciences Institute
   c/o MIT Laboratory for Computer Science
   545 Technology Square
   Cambridge, MA 02139
   USA
   electronic mail:  mjh@isi.edu

   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue
   New York, NY 10027
   USA
   electronic mail:  schulzrinne@cs.columbia.edu

   Eve Schooler
   Computer Science Department 256-80
   California Institute of Technology
   Pasadena, CA 91125
   USA
   electronic mail:  schooler@cs.caltech.edu

   Jonathan Rosenberg
   Lucent Technologies, Bell Laboratories
   Rm. 4C-526
   101 Crawfords Corner Road
   Holmdel, NJ 07733
   USA
   electronic mail:  jdrosen@bell-labs.com

H Bibliography

   [1] R. Pandya, "Emerging mobile and personal communication systems,"
   IEEE Communications Magazine , vol. 33, pp. 44--52, June 1995.

   [2] B. Braden, L. Zhang, S. Berson, S. Herzog, and S. Jamin,
   "Resource ReSerVation protocol (RSVP) -- version 1 functional
   specification," RFC 2205, Internet Engineering Task Force, Oct. 1997.

   [3] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: a
   transport protocol for real-time applications," RFC 1889, Internet
   Engineering Task Force, Jan. 1996.

   [4] H. Schulzrinne, R. Lanphier, and A. Rao, "Real time streaming
   protocol (RTSP)," RFC 2326, Internet Engineering Task Force, Apr.
   1998.

   [5] M. Handley and V. Jacobson, "SDP: session description protocol,"
   RFC 2327, Internet Engineering Task Force, Apr. 1998.

   [6] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.

   [7] R. Fielding, J. Gettys, J. Mogul, H. Nielsen, and T. Berners-Lee,
   "Hypertext transfer protocol -- HTTP/1.1," RFC 2068, Internet
   Engineering Task Force, Jan. 1997.

   [8] T. Berners-Lee, L. Masinter, and R. Fielding, "Uniform resource
   identifiers (URI): generic syntax," Internet Draft, Internet
   Engineering Task Force, Mar. 1998.  Work in progress.

   [9] T. Berners-Lee, L. Masinter, and M. McCahill, "Uniform resource
   locators (URL)," RFC 1738, Internet Engineering Task Force, Dec.
   1994.

   [9]

   [10] A. Gulbrandsen and P. Vixie, "A DNS RR for specifying the
   location of services (DNS SRV)," RFC 2052, Internet Engineering Task
   Force, Oct.  1996.

   [10]

   [11] C. Partridge, "Mail routing and the domain system," RFC STD 14,
   974, Internet Engineering Task Force, Jan. 1986.

   [11]

   [12] P. Mockapetris, "Domain names - implementation and
   specification," RFC STD 13, 1035, Internet Engineering Task Force,
   Nov. 1987.

   [12]

   [13] B. Braden, "Requirements for internet hosts - application and
   support," RFC STD 3, 1123, Internet Engineering Task Force, Oct.
   1989.

   [13]

   [14] D. Zimmerman, "The finger user information protocol," RFC 1288,
   Internet Engineering Task Force, Dec. 1991.

   [14]

   [15] S. Williamson, M. Kosters, D. Blacka, J. Singh, and K. Zeilstra,
   "Referral whois (rwhois) protocol V1.5," RFC 2167, Internet
   Engineering Task Force, June 1997.

   [15]

   [16] W. Yeong, T. Howes, and S. Kille, "Lightweight directory access
   protocol," RFC 1777, Internet Engineering Task Force, Mar. 1995.

   [16]

   [17] 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.

   [17] T. Berners-Lee, "Universal resource identifiers in WWW: a
   unifying syntax for the expression of names and addresses of objects
   on the network as used in the world-wide web," RFC 1630, Internet
   Engineering Task Force, June 1994.

   [18] T. Berners-Lee, L. Masinter, and R. Fielding, and L. Masinter, "Uniform resource
   identifiers (URI): generic syntax," Internet Draft, RFC 2396, Internet Engineering
   Task Force, Mar. Aug.  1998.  Work in progress.

   [19] P. Leach and R. Salz, "UUIDs and GUIDs," Internet Draft,
   Internet Engineering Task Force, Feb. 1998.  Work in progress.

   [20] F. Yergeau, "UTF-8, a transformation format of ISO 10646," RFC
   2279, Internet Engineering Task Force, Jan. 1998.

   [21] D. Crocker and P. Overell, "Augmented BNF for syntax
   specifications:  ABNF," RFC 2234, Internet Engineering Task Force,
   Nov. 1997.

   [22] W. R. Stevens, TCP/IP illustrated: the protocols , vol. 1.
   Reading, Massachusetts: Addison-Wesley, 1994.

   [23] J. Mogul and S. Deering, "Path MTU discovery," RFC 1191,
   Internet Engineering Task Force, Nov. 1990.

   [24] D. Crocker, "Standard for the format of ARPA internet text
   messages," RFC STD 11, 822, Internet Engineering Task Force, Aug.
   1982.

   [25] P. Hoffman, L. Masinter, and J. Zawinski, "The mailto URL
   scheme," RFC 2368, Internet Engineering Task Force, July 1998.

   [26] J. Palme, "Common internet message headers," RFC 2076, Internet
   Engineering Task Force, Feb. 1997.

   [27] J. Mogul, T. Berners-Lee, L. Masinter, P. Leach, R. Fielding, H.
   Nielsen, and J. Gettys, "Hypertext transfer protocol -- HTTP/1.1,"
   Internet Draft, Internet Engineering Task Force, Mar. 1998.  Work in
   progress.

   [28] M. Elkins, "MIME security with pretty good privacy (PGP)," RFC
   2015, Internet Engineering Task Force, Oct. 1996.

   [29] 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.

   [30] 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.

   Full Copyright Statement

   Copyright (c) The Internet Society (1998). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
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   English.

   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

                           Table        of Contents

   1          Introduction ........................................    2
   1.1        Overview of SIP Functionality .......................    2
   1.2        Terminology .........................................    4
   1.3        Definitions .........................................    4
   1.4        Summary of SIP Operation ............................    7
   1.4.1      SIP Addressing ......................................    7
   1.4.2      Locating a SIP Server ...............................    8
   1.4.3      SIP Transaction .....................................   10    9
   1.4.4      SIP Invitation ......................................   10
   1.4.5      Locating a User .....................................   12
   1.4.6      Changing an Existing Session ........................   14
   1.4.7      Registration Services ...............................   14
   1.5        Protocol Properties .................................   14
   1.5.1      Minimal State .......................................   14
   1.5.2      Lower-Layer-Protocol Neutral ........................   14
   1.5.3      Text-Based ..........................................   15
   2          SIP Uniform Resource Locators .......................   15
   3          SIP Message Overview ................................   19
   4          Request .............................................   21   20
   4.1        Request-Line ........................................   21   22
   4.2        Methods .............................................   21   22
   4.2.1      INVITE ..............................................   23   22
   4.2.2      ACK .................................................   23
   4.2.3      OPTIONS .............................................   24
   4.2.4      BYE .................................................   24
   4.2.5      CANCEL ..............................................   24
   4.2.6      REGISTER ............................................   25
   4.3        Request-URI .........................................   28   27
   4.3.1      SIP Version .........................................   28
   4.4        Option Tags .........................................   29   28
   4.4.1      Registering New Option Tags with IANA ...............   29
   5          Response ............................................   30   29
   5.1        Status-Line .........................................   30
   5.1.1      Status Codes and Reason Phrases .....................   30
   6          Header Field Definitions ............................   32   31
   6.1        General Header Fields ...............................   34   35
   6.2        Entity Header Fields ................................   34   36
   6.3        Request Header Fields ...............................   35   36
   6.4        Response Header Fields ..............................   35   36
   6.5        End-to-end and Hop-by-hop Headers ...................   35   36
   6.6        Header Field Format .................................   35   36
   6.7        Accept ..............................................   37
   6.8        Accept-Encoding .....................................   37   38
   6.9        Accept-Language .....................................   38
   6.10       Allow ...............................................   38
   6.11       Authorization .......................................   38
   6.12       Call-ID .............................................   38
   6.13       Content-Encoding ....................................       Contact .............................................   40
   6.14       Content-Encoding ....................................   42
   6.15       Content-Length ......................................   40
   6.15   43
   6.16       Content-Type ........................................   40
   6.16   43
   6.17       CSeq ................................................   41
   6.17   44
   6.18       Date ................................................   42
   6.18   45
   6.19       Encryption ..........................................   42
   6.19   45
   6.20       Expires .............................................   44
   6.20   47
   6.21       From ................................................   45
   6.21   48
   6.22       Hide ................................................   46
   6.22       Location ............................................   47   49
   6.23       Max-Forwards ........................................   49   50
   6.24       Organization ........................................   50   51
   6.25       Priority ............................................   50   51
   6.26       Proxy-Authenticate ..................................   51   52
   6.27       Proxy-Authorization .................................   51   52
   6.28       Proxy-Require .......................................   51   52
   6.29       Record-Route ........................................   52   53
   6.30       Require .............................................   52   54
   6.31       Response-Key ........................................   53   55
   6.32       Retry-After .........................................   54   55
   6.33       Route ...............................................   55   56
   6.34       Server ..............................................   55   56
   6.35       Subject .............................................   55   56
   6.36       Timestamp ...........................................   55   57
   6.37       To ..................................................   56   57
   6.38       Unsupported .........................................   56   58
   6.39       User-Agent ..........................................   57   59
   6.40       Via .................................................   57   59
   6.40.1     Requests ............................................   57   59
   6.40.2     Receiver-tagged Via Fields ..........................   58   60
   6.40.3     Responses ...........................................   58   60
   6.40.4     Syntax ..............................................   59   61
   6.41       Warning .............................................   60   62
   6.42       WWW-Authenticate ....................................   62   64
   7          Status Code Definitions .............................   62   65
   7.1        Informational 1xx ...................................   63   65
   7.1.1      100 Trying ..........................................   63   65
   7.1.2      180 Ringing .........................................   63   65
   7.1.3      181 Call Is Being Forwarded .........................   63   65
   7.1.4      182 Queued ..........................................   63   66
   7.2        Successful 2xx ......................................   64   66
   7.2.1      200 OK ..............................................   64   66
   7.3        Redirection 3xx .....................................   64   66
   7.3.1      300 Multiple Choices ................................   64   67
   7.3.2      301 Moved Permanently ...............................   65   67
   7.3.3      302 Moved Temporarily ...............................   65   67
   7.3.4      380 Alternative Service .............................   65   67
   7.4        Request Failure 4xx .................................   65   67
   7.4.1      400 Bad Request .....................................   65   68
   7.4.2      401 Unauthorized ....................................   65   68
   7.4.3      402 Payment Required ................................   66   68
   7.4.4      403 Forbidden .......................................   66   68
   7.4.5      404 Not Found .......................................   66   68
   7.4.6      405 Method Not Allowed ..............................   66   68
   7.4.7      406 Not Acceptable ..................................   66   68
   7.4.8      407 Proxy Authentication Required ...................   66   68
   7.4.9      408 Request Timeout .................................   66   69
   7.4.10     409 Conflict ........................................   67   69
   7.4.11     410 Gone ............................................   69
   7.4.12     411 Length Required .................................   69
   7.4.13     413 Request Entity Too Large ........................   69
   7.4.14     414 Request-URI Too Long ............................   67
   7.4.12   70
   7.4.15     415 Unsupported Media Type ..........................   67
   7.4.13   70
   7.4.16     420 Bad Extension ...................................   67
   7.4.14   70
   7.4.17     480 Temporarily Unavailable .........................   67
   7.4.15   70
   7.4.18     481 Invalid Call-ID .................................   67
   7.4.16 Call Leg/Transaction Does Not Exist .............   70
   7.4.19     482 Loop Detected ...................................   67
   7.4.17   70
   7.4.20     483 Too Many Hops ...................................   67
   7.4.18   70
   7.4.21     484 Address Incomplete ..............................   68
   7.4.19   71
   7.4.22     485 Ambiguous .......................................   68   71
   7.4.23     486 Busy Here .......................................   71
   7.5        Server Failure 5xx ..................................   68   72
   7.5.1      500 Server Internal Error ...........................   69   72
   7.5.2      501 Not Implemented .................................   69   72
   7.5.3      502 Bad Gateway .....................................   69   72
   7.5.4      503 Service Unavailable .............................   69   72
   7.5.5      504 Gateway Timeout .................................   69   72
   7.5.6      505 Version Not Supported ...........................   69   72
   7.6        Global Failures 6xx .................................   70   73
   7.6.1      600 Busy ............................................   70 Everywhere .................................   73
   7.6.2      603 Decline .........................................   70   73
   7.6.3      604 Does Not Exist Anywhere .........................   70   73
   7.6.4      606 Not Acceptable ..................................   70   73
   8          SIP Message Body ....................................   70   74
   8.1        Body Inclusion ......................................   71   74
   8.2        Message Body Type ...................................   71   74
   8.3        Message Body Length .................................   71   74
   9          Compact Form ........................................   71   74
   10         Behavior of SIP Clients and Servers .................   72   76
   10.1       General Remarks .....................................   72   76
   10.1.1     Requests ............................................   73   76
   10.1.2     Responses ...........................................   73   76
   10.2       Source Addresses, Destination Addresses and
   Connections ....................................................   74   77
   10.2.1     Unicast UDP .........................................   74   77
   10.2.2     Multicast UDP .......................................   74   77
   10.3       TCP .................................................   75   78
   10.4       Reliability for BYE, CANCEL, OPTIONS, REGISTER
   Requests .......................................................   76   79
   10.4.1     UDP .................................................   76   79
   10.4.2     TCP .................................................   76   80
   10.5       Reliability for ACK Requests ........................   76
   10.6       Reliability for INVITE Requests .....................   77
   10.6.1   80
   10.5.1     UDP .................................................   77
   10.6.2   80
   10.5.2     TCP .................................................   78   84
   10.6       Reliability for ACK Requests ........................   84
   11         Behavior of SIP User Agents .........................   81   84
   11.1       Caller Issues Initial INVITE Request ................   81   84
   11.2       Callee Issues Response ..............................   81   85
   11.3       Caller Receives Response to Initial Request .........   81   85
   11.4       Caller or Callee Generate Subsequent Requests .......   82   85
   11.5       Receiving Subsequent Requests .......................   82   86
   12         Behavior of SIP Proxy and Redirect Servers ..........   82   86
   12.1       Redirect Server .....................................   82   86
   12.2       User Agent Server ...................................   83   86
   12.3       Proxy Server ........................................   83   86
   12.3.1     Proxying Requests ...................................   83   87
   12.3.2     Proxying Responses ..................................   84   87
   12.3.3     Stateless Proxy: Proxying Responses .................   84   87
   12.3.4     Stateful Proxy: Receiving Requests ..................   84   88
   12.3.5     Stateful Proxy: Receiving ACKs ......................   84   88
   12.3.6     Stateful Proxy: Receiving Responses .................   84   88
   12.3.7     Stateless, Non-Forking Proxy ........................   85   88
   12.4       Forking Proxy .......................................   85   89
   13         Security Considerations .............................   89   93
   13.1       Confidentiality and Privacy: Encryption .............   89   93
   13.1.1     End-to-End Encryption ...............................   89   93
   13.1.2     Privacy of SIP Responses ............................   92   95
   13.1.3     Encryption by Proxies ...............................   92   96
   13.1.4     Hop-by-Hop Encryption ...............................   92   96
   13.1.5     Via field encryption ................................   92   96
   13.2       Message Integrity and Access Control:
   Authentication .................................................   93   97
   13.2.1     Trusting responses ..................................   95   99
   13.3       Callee Privacy ......................................   96  100
   13.4       Known Security Problems .............................   96  100
   14         SIP Security Using PGP ..............................   97  100
   14.1       PGP Authentication Scheme ...........................   97  100
   14.1.1     The WWW-Authenticate Response Header ................   97  100
   14.1.2     The Authorization Request Header ....................   98  101
   14.2       PGP Encryption Scheme ...............................   99  102
   14.3       Response-Key Header Field for PGP ...................   99  103
   15         Examples ............................................  100  103
   15.1       Registration ........................................  100  104
   15.2       Invitation to a Multicast Conference ................  101  105
   15.2.1     Request .............................................  102  105
   15.2.2     Response ............................................  102  106
   15.3       Two-party Call ......................................  104  107
   15.4       Terminating a Call ..................................  106  109
   15.5       Forking Proxy .......................................  106  110
   15.6       Redirects ...........................................  110  114
   15.7       Alternative Services ................................  112  115
   15.8       Negotiation .........................................  113  116
   15.9       OPTIONS Request .....................................  114  117
   A          Minimal Implementation ..............................  114  118
   A.1        Client ..............................................  114  118
   A.2        Server ..............................................  115  119
   A.3        Header Processing ...................................  115  119
   B          Usage of SDP ........................................  115  119
   C          Summary of Augmented BNF ............................  117  121
   D          IANA Considerations .................................  119  122
   E          Changes in Version -08 -09 ..............................  119  123
   F          Acknowledgments .....................................  120  124
   G          Authors' Addresses ..................................  120  124
   H          Bibliography ........................................  120  125