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Versions: (draft-biggs-sip-replaces) 00 01 02 03 04 05 RFC 3891

SIP WG                                                           R. Mahy
Internet-Draft                                       Cisco Systems, Inc.
Expires: October 28, 2002                                       B. Biggs
                                                                 R. Dean
                                                          April 29, 2002


                        The SIP Replaces Header
                     draft-ietf-sip-replaces-01.txt

Status of this Memo

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

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

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   The list of current Internet-Drafts can be accessed at http://
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on October 28, 2002.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document proposes a new header for use with the SIP call control
   architecture.  The Replaces header is used in peer-to-peer call
   control to logically replace an existing SIP dialog with a new SIP
   dialog.  This primitive can be used to enable a variety of features,
   for example: "Attended Transfer" and "Retrieve from Call Park".  Note
   that definition of these example features is non-normative.







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Table of Contents

   1.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.1 The Replaces Header  . . . . . . . . . . . . . . . . . . . . .  4
   3.2 Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.3 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.4 New option tag for Require and Supported headers . . . . . . .  5
   3.5 687 Response Code: "Dialog Terminated" . . . . . . . . . . . .  5
   4.  User Agent Behavior: Receiving a Replaces Header . . . . . . .  5
   4.1 Matching Dialogs . . . . . . . . . . . . . . . . . . . . . . .  5
   4.2 Matching with Explicit Local Tags  . . . . . . . . . . . . . .  6
   4.3 Matching with the * to-tag . . . . . . . . . . . . . . . . . .  6
   4.4 Replaces Semantics . . . . . . . . . . . . . . . . . . . . . .  7
   4.5 Proxy behavior . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.6 Consequences of a forking proxy not supporting this extension   8
   5.  Usage Examples . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.1 Replacing an Active Dialog . . . . . . . . . . . . . . . . . .  9
   5.2 Replacing an Early Dialog initiated by someone else  . . . . . 10
   5.3 Replacing an Early Dialog you initiated  . . . . . . . . . . . 12
   5.4 Handling Replaces for a Terminated Dialog  . . . . . . . . . . 14
   5.5 An Error Case  . . . . . . . . . . . . . . . . . . . . . . . . 15
   5.6 Backwards compatibility with RFC2543 User Agents . . . . . . . 16
   5.7 Replacing with to-tag set to '*' . . . . . . . . . . . . . . . 18
   5.8 A non-compliant forking proxy  . . . . . . . . . . . . . . . . 19
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   7.1 Registration of "Replaces" SIP header  . . . . . . . . . . . . 21
   7.2 Registration of "replaces" SIP Option-tag  . . . . . . . . . . 22
   7.3 Registration of "687" SIP Response code  . . . . . . . . . . . 22
   8.  To Do and Open Issues  . . . . . . . . . . . . . . . . . . . . 22
   8.1 Open Issues: . . . . . . . . . . . . . . . . . . . . . . . . . 22
   8.2 To Do: . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   8.3 Changes Since -00  . . . . . . . . . . . . . . . . . . . . . . 22
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23
       Normative References . . . . . . . . . . . . . . . . . . . . . 23
       Informational References . . . . . . . . . . . . . . . . . . . 23
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 24
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 25











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1. Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC-2119 [3].

   Throughout this document, an "established dialog" means an active SIP
   dialog in which the request that created the dialog has received a
   successful (2xx-class) final response (ex: 200 OK).  An "early
   dialog" means transaction and dialog state that exists after a
   request is sent which would create a new dialog, but before a final
   response is received for the initial request.

2. Overview

   This document describes a SIP [1] extension for distributed call
   control as part of the SIP multiparty applications architecture
   framework [6].  The Replaces header is used in peer-to-peer call
   control to logically replace an existing SIP dialog with a new SIP
   dialog.

   INVITEs are requests which can be accepted, rejected or declined.  A
   User Agent that accepts a request with call-control semantics agrees
   to take responsibility for setting up the appropriate requested media
   relationships.

   In the parlance of the SIP multiparty architecture, the "Replaces"
   header is used to replace one participant with another in a
   conversation space.  This functionality is already available using
   3rd party call control [8] style call control.  The 3pcc model
   requires a central point of control which may not be desirable in
   many environments.  As such, a method of performing these same call
   control primitives in a distributed, peer-to-peer fashion is very
   desirable.

   Use of a new INVITE with a new header for dialog matching was chosen
   over making implicit associations in an incoming INVITE based on
   call-id or other fields for the following reasons:

   o  An INVITE already has the correct semantics for a new call

   o  Using an explicit Replaces header in a new request makes the
      intent of the request obvious.

   o  A unique call-id may be given to the replacement call.  This
      avoids call-leg matching problems in any of the clients.

   o  There are no adverse effects if the header is unsupported.



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   The Replaces header enables services such as attended call transfer,
   retrieve from park, and transition from locally mixed conferences to
   two party calls in a distributed peer-to-peer way.  This list of
   services is not exhaustive.  Although the Replaces header is
   frequently used in combination with the REFER [2] method as used in
   cc-transfer [7], they may be used independently.

3. Syntax

3.1 The Replaces Header

   The Replaces header indicates that the dialog identified by the
   header is to be shut down and logically replaced by the incoming
   INVITE in which it is contained.  It is a request header only, and
   defined here only for INVITE requests.  The Replaces header MAY be
   encrypted as part of end-to-end encryption.

   This document adds the following entry to Table 3 of [1]:

        Header field    where   proxy   ACK  BYE  CAN  INV  OPT  REG
        ------------    -----   -----   ---  ---  ---  ---  ---  ---
        Replaces          R              -    -    -    o    -    -


                                        SUB  NOT  REF  INF  UPD  PRA
                                        ---  ---  ---  ---  ---  ---
        Replaces          R              -    -    -    -    -    -

   Note that the Replaces header has specific call control semantics.
   If both a Replaces header and another header with contradictory
   semantics are present in a request, the request MUST be rejected with
   a 400 "Bad Request" response.

3.2 Formal Syntax

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in RFC-2234 [4].

      Replaces         = "Replaces" HCOLON replaces-values
                          *(COMMA replaces-values)

      replaces-values  = callid *( SEMI replaces-param )
      callid           = token [ "@" token ]
      replaces-param   = to-tag | from-tag | extension-param
      to-tag           = "to-tag" EQUAL ( UUID | "*" )
      from-tag         = "from-tag" EQUAL UUID
      extension-param  = token [ EQUAL ( token | quoted-string ) ]




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   A Replaces header MUST contain exactly one to-tag and exactly one
   from-tag, as they are required for unique dialog matching.  Since we
   rely on the tags for matching purposes, implementations which support
   Replaces MUST support the SIP specification, which requires tags.
   For compatibility with early dialogs and dialogs initiated by RFC2543
   [5] compliant UAs, a tag of zero must match both tags of zero and
   null tags.

3.3 Examples

      Replaces: 98732@sip.billybiggs.com
                ;from-tag=r33th4x0r
                ;to-tag=ff87ff

      Replaces: 12345@149.112.118.3;to-tag=12345;from-tag=54321

      Replaces: 87134@171.161.34.23;to-tag=24796;from-tag=0

      Replaces: 12345@149.112.118.3;to-tag=*;from-tag=24583


3.4 New option tag for Require and Supported headers

   This specification defines a new Require/Supported header option tag
   "replaces".  UAs which support the Replaces header MUST include the
   "replaces" option in the Supported header.  UAs that want explicit
   failure notification if Replaces is not supported MAY include the
   "replaces" option in the Require header.

   Example:

      Require: replaces, 100rel


3.5 687 Response Code: "Dialog Terminated"

   This specification defines a new SIP response code.  The 687 "Dialog
   Terminated" response code indicates that an early dialog has been
   completely replaced by a new dialog.  A new response code was chosen
   from the 6xx class to prevent intervening proxies from attempting to
   fork additional branches of the replaced dialog.

4. User Agent Behavior: Receiving a Replaces Header

4.1 Matching Dialogs

   The Replaces header contains information used to match an existing
   SIP dialog (call-id, to-tag, and from-tag).  Upon receiving an INVITE



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   with the Replaces header, the UA MUST attempt to match this
   information with an established or early dialog.  The to-tag and
   from-tag are matched as if they were present in an incoming request.
   In other words the to-tag is compared to the local tag, and the from-
   tag is compared to the remote tag.  The to-tag of "*" is a special
   token which matches all local tags.  When this special "*" token is
   present, the matching semantics are slightly different.

4.2 Matching with Explicit Local Tags

   If the Replaces header matches more than one dialog, the UA MAY use
   other headers if present (ex: the Referred-By header) to attempt to
   match a single dialog.  If a single matching dialog is not found, the
   UA MUST act as if no match is found.

   If no match is found, the UAS rejects the INVITE and returns a 481
   Call/Transaction Does Not Exist response.

   If the Replaces header matches a dialog which was not created with an
   INVITE, the UAS MUST reject the request with an appropriate response.

   If the Replaces header matches a dialog which has already terminated,
   the UA SHOULD decline the request with a 603 Declined response.  This
   prevents phantom ringing in cases like example 6.4.

   Once a matching call-leg is found, the UAS MAY authenticate the
   INVITE request.  If the request is successfully authenticated or
   already preauthorized, the UAS SHOULD proceed with processing.  The
   UAS MAY prompt the user to accept or reject unauthenticated requests.
   The UAS MAY reject the request with any appropriate response (for
   example: 603 "Decline", 403 "Forbidden", or 488 "Not Acceptable
   Here")

4.3 Matching with the * to-tag

   If the Replaces header matches more than one dialog, the UA MAY use
   other headers if present (ex: the Referred-By header) to attempt to
   match a single dialog.  If a single matching dialog is not found, the
   UA MUST act as if no match is found.

   If the Replaces header matches a dialog which was not created with an
   INVITE, the UAS MUST reject the request with an appropriate response.

   If the Replaces header matches a dialog which has already been
   terminated with a CANCEL or BYE, the UAS MUST reject the INVITE and
   return a 481 Call/Transaction Does Not Exist response.

   If no match is found, the UAS MUST ignore the Replaces header and



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   continue processing the INVITE as if it was not present.

   Once a matching call-leg is found, the UAS MAY authenticate the
   INVITE request.  If the request is successfully authenticated or
   already preauthorized, the UAS SHOULD proceed with processing.  The
   UAS MAY prompt the user to accept or reject unauthenticated requests.
   The UAS MAY reject the request with any appropriate response (for
   example: 603 "Decline", 403 "Forbidden", or 488 "Not Acceptable
   Here")

4.4 Replaces Semantics

   If the Replaces header matches an established active dialog, the UA
   SHOULD attempt to accept the new INVITE, reassign the user interface
   and other resources of the matched dialog to the new INVITE, and shut
   down the replaced dialog by sending a BYE.  If the UA cannot accept
   the new INVITE (for example: it cannot establish required QoS or
   keying, or it has incompatible media), the UA MUST return an
   appropriate response and leave the matched dialog unchanged.

   If the Replaces header matches an early dialog that was initiated by
   the UA, the UA SHOULD attempt to accept the new INVITE.  If the UA
   cannot accept the new INVITE, the UA MUST return an appropriate
   response and leave the matched dialog unchanged.  If the UA
   successfully accepts the new INVITE, the UA MUST reassign the
   resources of the early dialog to the new INVITE, and CANCEL the
   replaced early dialog.

   If the Replaces header matches an early dialog that was not initiated
   by the UA, the UA SHOULD attempt to provisionally accept the new
   INVITE.  In other words, the UA should attempt whatever steps are
   necessary to return a provisional or final response suitable for the
   state of the resources used by the matched dialog.

   If this is successful, the UA MUST reassign the resources of the
   early dialog to the new INVITE, and respond to the replaced early
   dialog with a 687 "Transaction Terminated" response (defined earlier
   in this document).

4.5 Proxy behavior

   A proxy which does not fork merely needs to pass the Replaces header
   transparently as described in SIP.

   A forking proxy which supports this specification that would normally
   fork a request for the Request-URI in the request MUST examine that
   INVITE request for a Replaces header.




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   The proxy MUST attempt to match the Call-ID, to-tag, and from-tag
   with a transaction that that proxy (or cluster of proxies) previously
   forked.  If the Replaces header includes a "*" in the to-tag, then
   the request will match all branches that match the Call-ID, and from-
   tag.

   If the proxy cannot locate a matching transaction, it continues
   processing the request as if the Replaces header did not exist.

   If the proxy locates a matching transaction, if SHOULD treat the new
   request exactly as if it were the matching request.  In other words,
   all the branching logic and state (including timers such as call
   forward timers) of the matched request should be copied and used for
   the new request.  The proxy will skip contacts which have already
   failed and forward the new INVITE request to all the Contacts for
   which the replaced request has active matching branches.  The proxy
   MUST NOT forward the original INVITE on to new branches.
   Furthermore, if the proxy forwards the new request to more than one
   active branch, it MUST wait until it receives a non-"100 Trying"
   response from all active branches of the new request, before sending
   CANCEL requests to any of the replaced branches.  This prevents a
   race condition which is described in Section 7.8.

   If necessary, the proxy MUST continue to forward the new INVITE
   request to new branches as if it were the replaced INVITE.

        OPEN ISSE: This is a lot of work.  Do we really want to do
        this or just punt on early attended transfer?


4.6 Consequences of a forking proxy not supporting this extension

   Forking proxies which do not support this extension may inadvertently
   forward an INVITE request with a Replaces header to a different set
   of Contacts than the original request it was intended to replace.
   This may occur as a result of many policy-based Contact selection
   algorithms, including time-of-day, load-sharing, presence-based, or
   caller-based policies.

   A parallel forking proxy may cause a race condition whereby only one
   branch of many caused by the initial INVITE is replaced.  This race
   condition is illustrated in the example in Section 7.8.

   Call forwarding timers may be reset for active branches.  For
   example, take a forking proxy which implements a call forwarding
   service after 12 seconds of inactivity.  The original INVITE may have
   been tried already for 10 seconds.  The replacement INVITE is likely
   to begin this timer again at 12 seconds.



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5. Usage Examples

   The following non-normative examples are not intended to enumerate
   all the possibilities for the usage of these extensions, but rather
   to provide examples or ideas only.  For more examples, please see
   service-examples [9].

5.1 Replacing an Active Dialog

   In this example, Alice is talking to Bob from phone1.  She transfers
   Bob to a Parking Place while she goes to the lab.  When she gets
   there she retrieves the "parked" call from phone2 by sending an
   INVITE with Replaces to Bob with the dialog information Bob shared
   with the Parking Place.  How did Alice get this information?  Maybe
   she subscribed to this information from the Parking Place, or went to
   a website and clicked on a URL.

        Alice          Alice                             Parking
        phone1         phone2            Bob               Place
        |               |                 |                   |
        |<===============================>|                   |
        |               |                 |                   |
        |        Alice transfers Bob to Parking Place         |
        |               |                 |                   |
        |------------REFER/200----------->|    *1    *2       |
        |               |                 |--INVITE/200/ACK-->|
        |<-----------NOTIFY/200-----------|<=================>|
        |------------BYE/200------------->|                   |
        |               |                 |                   |
        |               |                 |                   |
        |  Alice later retrieves call from another phone      |
        |               |                 |                   |
        |            *3 |-INV w/Replaces->|                   |
        |            *4 |<--200-----------|    *5             |
        |               |---ACK---------->|----BYE/200------->|
        |               |<===============>|                   |
        |               |                 |                   |


   Message *1: Bob-> Parking Place

   INVITE sip:parkingplace@sip.org SIP/2.0
   To: <sip:parkingplace@sip.org>
   From: <sip:bob@sip.org>;tag=7743
   Call-ID: 425928@bobster.sip.org
   CSeq: 1 INVITE
   Contact: <sip:bob@bobster.sip.org>
   Referred-By: <sip:alice@phone1.sip.org>



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   Message *2: Parking Place -> Bob

   SIP/2.0 200 OK
   To: <sip:parkingplace@sip.org>;tag=6472
   From: <sip:bob@sip.org>;tag=7743
   Call-ID: 425928@bobster.sip.org
   CSeq: 1 INVITE
   Contact: <sip:parkplace@monopoly.sip.org>

   Message *3: Alice@phone2 -> Bob

   INVITE sip:bob@bobster.sip.org
   To: <sip:bob@sip.org>
   From: <sip:alice@phone2.sip.org>;tag=8983
   Call-ID: 09870@phone2.sip.org
   CSeq: 1 INVITE
   Contact: <sip:alice@phone2.sip.org>
   Require: replaces
   Replaces: 425928@bobster.sip.org;to-tag=7743;from-tag=6472

   Message *4: Bob -> Alice@phone2

   SIP/2.0 200 OK
   To: <sip:bob@sip.org>;tag=9343
   From: <sip:alice@phone2.sip.org>;tag=8983
   Call-ID: 09870@phone2.sip.org
   CSeq: 1 INVITE
   Contact: <sip:alice@phone2.sip.org>

   Message *5: Bob -> Parking Place

   BYE sip:parkingplace@sip.org SIP/2.0
   To: <sip:parkingplace@sip.org>;tag=6472
   From: <sip:bob@sip.org>;tag=7743
   Call-ID: 425928@bobster.sip.org
   CSeq: 2 BYE
   Contact: <sip:bob@bobster.sip.org>


5.2 Replacing an Early Dialog initiated by someone else

   In this example, a Customer tries calling a call center and for some
   reason cannot get through properly.  The customer calls an Operator
   and asks for help.  The operator calls the contact center, and upon
   receiving a provisional response, assumes that everything is OK and
   transfers the Customer to the Call Center, replacing the operator's
   place in the queue.




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                                                       Call
      Operator               Customer                  Center
        |                       |                        |
        |<--INVITE/180/200/ACK--|                        |
        |<=====================>|  "Hello, I'm having    |
        |                       |   trouble calling ..." |
        |"OK, I'll try it and   |                        |
        | transfer you if it    |                        |
        | works for me"         |                        |
        |                       |                        |
     *1 |-----INVITE ----------------------------------->|
     *2 |<----182: You are caller number 7---------------|
        |                       |                        |
        |  completes transfer   |                        |
        |                       |                        |
        |---REFER/200---------->|                        |
        |                       |--INVITE with Replaces->| *3
        |                       |<----182: caller #7-----| *4
        |<----687 Dialog Terminated----------------------| *5
        |-----ACK--------------------------------------->|
        |<--NOTIFY/200----------|                        |
        |---BYE/200------------>|                        |
        |                       |  ...time passes..      |
        |                       |                        |
        |                       |                        |
        |                       |                        |
        |                       |<---200 OK--------------|
        |<--NOTIFY/200----------|----ACK---------------->|
        |                       |                        |
        |                       |                        |

   Message *1: Operator -> Call Center

   INVITE sip:helpdesk@clueless.org SIP/2.0
   To: <sip:helpdesk@clueless.org>
   From: <sip:operator@acme.com>;tag=7743
   Call-ID: 425928@dhcp23311.acme.com
   CSeq: 1 INVITE
   Contact: <sip:jdoe@dhcp2311.acme.com>
   Accept-Language: en

   Message *2: Call Center -> Operator

   SIP/2.0 182 You are 7th in Queue
   To: <sip:helpdesk@clueless.org>;tag=6472
   From: <sip:operator@acme.com>;tag=7743
   Call-ID: 425928@dhcp23311.acme.com
   CSeq: 1 INVITE



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   Contact: <sip:helpdesk@frontline.clueless.org>

   Message *3: Customer -> Call Center

   INVITE sip:helpdesk@frontline.clueless.org
   To: <sip:helpdesk@clueless.org>
   From: <sip:customer@acme.com>;tag=8983
   Call-ID: 09870@lobby12.acme.com
   CSeq: 1 INVITE
   Contact: <sip:customer@lobby12.acme.com>
   Replaces: 425928@dhcp23311.acme.com;to-tag=7743;from-tag=6472
   Accept-Language: en
   Referred-By: <sip:jdoe@dhcp2311.acme.com>

   Message *4: Call Center -> Customer

   SIP/2.0 182 You are 7th in Queue
   To: <sip:helpdesk@clueless.org>
   From: <sip:customer@acme.com>;tag=8983
   Call-ID: 09870@lobby12.acme.com
   CSeq: 1 INVITE
   Contact: <sip:helpdesk@frontline.clueless.org>

   Message *5: Call Center -> Operator

   SIP/2.0 687 Dialog Terminated
   To: <sip:helpdesk@clueless.org>;tag=6472
   From: <sip:operator@acme.com>;tag=7743
   Call-ID: 425928@dhcp23311.acme.com
   CSeq: 1 INVITE
   Contact: <sip:helpdesk@frontline.clueless.org>


5.3 Replacing an Early Dialog you initiated

   In this example, Bob just arrived in the lab and hasn't registered
   there yet.  He hears his desk phone ring.  He quickly logs into a
   software UA on a nearby computer.  Among other things, the software
   UA subscribes to the call-state of his desk phone.  When it notices
   that his phone is ringing it offers him the choice to take the call
   there.  The software UA sends an INVITE with Replaces to Alice.  When
   Alice's UA receives this new INVITE, it CANCELs her original INVITE
   and connects Alice to Bob.

                              Bob                      Bob
       Alice                  desk                     lab
        |                       |                        |
    *1  |-----INVITE----------->|                        |



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    *2  |<----180---------------|  Bob hears desk phone  |
        |                       |  ringing from lab but  |
        |                       |  isn't REGISTERed yet  |
        |                       |                        |
        |                       |<--SUB callpackage/200--|
        |                       |---NOTIFY/200---------->|
   *3/4 |<-----INVITE with Replaces/200/ACK--------------|
   *5/6 |------CANCEL/200------>|                        |
   *7   |<-----487--------------|                        |
        |------ACK------------->|                        |
        |                       |                        |
        |                       |                        |

   Message *1: Alice -> Bob's desk phone

   INVITE sip:bob@sip.org SIP/2.0
   To: <sip:bob@sip.org>
   From: <sip:alice@sip.org>;tag=7743
   Call-ID: 425928@phone.sip.org
   CSeq: 1 INVITE
   Contact: <sip:alice@phone.sip.org>

   Message *2: Bob's desk phone -> Alice

   SIP/2.0 180 Ringing
   To: <sip:bob@sip.org>;tag=6472
   From: <sip:alice@sip.org>;tag=7743
   Call-ID: 425928@phone.sip.org
   CSeq: 1 INVITE
   Contact: <sip:bob@bobster.sip.org>

   Message *3: Bob in lab -> Alice

   INVITE sip:alice@phone.sip.org
   To: <sip:alice@sip.org>
   From: <sip:bob@sip.org>;tag=8983
   Call-ID: 09870@labpc.sip.org
   CSeq: 1 INVITE
   Contact: <sip:bob@labpc.sip.org>
   Replaces: 425928@phone.sip.org;to-tag=7743;from-tag=6472

   Message *4: Alice -> Bob in lab

   SIP/2.0 200 OK
   To: <sip:alice@sip.org>;tag=9232
   From: <sip:bob@sip.org>;tag=8983
   Call-ID: 09870@labpc.sip.org
   CSeq: 1 INVITE



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   Contact: <sip:alice@phone.sip.org>

   Message *5: Alice -> Bob's desk

   CANCEL sip:bob@sip.org SIP/2.0
   To: <sip:bob@sip.org>
   From: <sip:alice@sip.org>;tag=7743
   Call-ID: 425928@phone.sip.org
   CSeq: 1 CANCEL
   Contact: <sip:alice@phone.sip.org>

   Message *6: Bob's desk -> Alice

   SIP/2.0 200 OK
   To: <sip:bob@sip.org>
   From: <sip:alice@sip.org>;tag=7743
   Call-ID: 425928@phone.sip.org
   CSeq: 1 CANCEL
   Contact: <sip:bob@bobster.sip.org>

   Message *7: Bob's desk -> Alice

   SIP/2.0 487 Request Terminated
   To: <sip:bob@sip.org>;tag=6472
   From: <sip:alice@sip.org>;tag=7743
   Call-ID: 425928@phone.sip.org
   CSeq: 1 INVITE
   Contact: <sip:bob@bobster.sip.org>


5.4 Handling Replaces for a Terminated Dialog

   In this example, Alice, Bob, and Cathy participate in a 3-way call
   mixed locally by Bob's UA.  Bob's UA is programmed to revert to a
   simple 2-party call when any party hangs up (including Bob).
   Ordinarily this would be a very polite feature--Cathy and Alice could
   continue to talk after Bob hangsup.  If all three hang up at about
   the same time, but Bob hangs up first (this will happen about one-
   third of the time), an INVITE with Replaces header can arrive at
   Cathy's UA shortly after she has hung up.  Because Cathy's UA needs
   to keep transaction state around for a while anyway (typically 32
   seconds), the dialog information in the Replaces header should match
   a terminated dialog.  Cathy declines the INVITE, and cleanup proceeds
   normally.







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    Alice                   Bob                     Cathy
     |                                                |
     |   Alice, Bob, and Cathy are participants in    |
     |   a 3-way call mixed by Bob                    |
     |                                                |
     |<=====================>#<======================>|
     |                       |                        |
     | All three hang up at  |                        |
     | about the same time   |                        |
     |                       |                        |
     | Bob's UA tries to     |                        |
     | setup a 2-way call    |                        |
     | btwn Alice and Cathy  |                        |
     |                       |                        |
     |<---REFER--------------|                        |
     |----INVITE with Replaces--->XX (lost or late)   |
     |                       |                        |
     |                       |<-----BYE/200-----------| the dialog is
     |                       |                        | already dead
     |----INVITE with Replaces----------------------->|    so
     |<---603 Declined--------------------------------| Cathy Declines
     |----ACK---------------------------------------->|
     |----NOTIFY/200-------->|                        |
     |                       |                        |
     |<-----BYE/200--------->|                        |
     |  (either side sends)  |                        |
     |                       |                        |
     |                       |                        |


5.5 An Error Case

   The following example illustrates one reason an INVITE with Replaces
   may fail.  In this example, both Bob and Cathy have a common audio
   codec with Alice, but Bob and Cathy do not share a common codec.
   When Cathy receives an INVITE from Bob with the Replaces header,
   Cathy determines she cannot communicate, sends a 488 response to Bob,
   and maintains her session with Alice.













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       Alice                   Bob                        Cathy
        |                       |                           |
        |--INVITE/200/ACK------>|                           |
        |                       |                           |
        |<=audio w/GSM codec===>|                           |
        |                       |                           |
        |----INVITE/200/ACK-------------------------------->|
        |                       |                           |
        |<===audio with G.729 codec========================>|
        |                       |                           |
        |                       |                           |
        |--REFER/200----------->|                           |
        |                       |--INVITE w/Replaces------->|
        |                       |                           |
        |                       |  no codec in common!      |
        |                       |                           |
        |                       |<-488 Not Acceptable Here--|
        |<--NOTIFY/200----------|--ACK--------------------->|
        |                       |                           |
        |<=====================>|                           |
        |<=================================================>|
        |                       |                           |


5.6 Backwards compatibility with RFC2543 User Agents

   In this example, both Alice and Bob use tags, but Alice wishes to
   replace a dialog at Bob that was initiated by a User Agent that does
   not support tags.


                                                         RFC2543
                                                         User Agent
       Alice                   Bob                       (no tags)
        |                       |                           |
        |                       |<---------INVITE-----------| *1
        |                       |----------200--------------| *2
        |                       |<---------ACK--------------|
        |                       |                           |
        |                       |<=========================>|
        |                       |                           |
        |                       |                           |
     *3 |--INVITE w/Replaces--->|                           |
     *4 |<----200 OK------------|----------BYE------------->| *5
        |-----ACK-------------->|<---------200--------------|
        |                       |                           |
        |<=====================>|                           |
        |                       |                           |



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   Message *1: Oldtimer (RFC 2543 User Agent)-> Bob

   INVITE sip:bob@sip.org SIP/2.0
   To: <sip:bob@sip.org>
   From: <sip:oldtimer@sip.org>
   Call-ID: 425928@test-ua.sip.org
   CSeq: 1 INVITE
   Contact: <sip:oldtimer@test-ua.sip.org>

   Message *2: Bob -> Oldtimer

   SIP/2.0 200 OK
   To: <sip:bob@sip.org>;tag=3245
   From: <sip:oldtimer@sip.org>
   Call-ID: 425928@test-ua.sip.org
   CSeq: 1 INVITE
   Contact: <sip:bob@bobster.sip.org>

   Message *3: Alice -> Bob

   INVITE sip:bob@bobster.sip.org
   To: <sip:bob@sip.org>
   From: <sip:alice@phone2.sip.org>;tag=8983
   Call-ID: 09870@phone2.sip.org
   CSeq: 1 INVITE
   Contact: <sip:alice@phone2.sip.org>
   Replaces: 425928@test-ua.sip.org;to-tag=3245;from-tag=0

   Message *4: Bob -> Alice

   SIP/2.0 200 OK
   To: <sip:bob@sip.org>;tag=9343
   From: <sip:alice@phone2.sip.org>;tag=8983
   Call-ID: 09870@phone2.sip.org
   CSeq: 1 INVITE
   Contact: <sip:bob@bobster.sip.org>

   Message *5: Bob -> Oldtimer

   BYE sip:oldtimer@test-ua.sip.org SIP/2.0
   To: <sip:oldtimer@sip.org>
   From: <sip:bob@sip.org>;tag=3245
   Call-ID: 425928@test-ua.sip.org
   CSeq: 2 BYE
   Contact: <sip:bob@bobster.sip.org>






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5.7 Replacing with to-tag set to '*'

   In this example, Alice and Bob have a stable call.  Bob call Cathy,
   who is available at a multiplicity of Contacts.  Cathy's proxy first
   forwards INVITEs to her desk and lab for 6 seconds, and then tries
   here home and car for 6 seconds, finally arriving at her voicemail
   server if none of these contacts answer.  Bob intends to talk with
   Cathy, and then transfer Alice to Cathy, but for whatever reason,
   becomes impatient and transfers Alice's call before an established
   dialog is created.  Bob's intent is for Alice to reach Cathy at any
   of her contacts as opposed to at a single contact, so he asks her to
   send an INVITE replacing his entire call with the wildcard to-tag
   parameter.  Cathy's proxy replaces Bob's entire chain of proxy
   forwarding logic and call forwarding timers with the new INVITE from
   Alice.

                                                   Cathy's Contacts
                                               desk  lab  home  car  VM
   Alice          Bob            Proxy           C1   C2   C3   C4   C5
    |              |               |              |    |    |    |    |
    | stable call  |               |              |    |    |    |    |
    |<============>|               |              |    |    |    |    |
    |              |--INVITE-(1)-->|              |    |    |    |    |
    |              |               |-INVITE-(1a)->|    |    |    |    |
    |              |               |-INVITE-(1b)------>|    |    |    |
    |              |               |<---18x-(1a)--|    |    |    |    |
    |              |<----18x-(1a)--|<---18x-(1b)-------|    |    |    |
    |              |<----18x-(1b)--|              |    |    |    |    |
    |              |               |  proxy waits |    |    |    |    |
    |              |               |  6 seconds   |    |    |    |    |
    |              |               |              |    |    |    |    |
    |              |               |-CANCEL/200-->|    |    |    |    |
    |              |               |-CANCEL/200------->|    |    |    |
    |              |               |<-487/ACK (1a)|    |    |    |    |
    |              |               |<-487/ACK (1b)-----|    |    |    |
    |              |               |-INVITE-(1c)----------->|    |    |
    |              |               |-INVITE-(1d)---------------->|    |
    |              |               |<---18x-(1c)------------|    |    |
    |              |<----18x-(1c)--|<---18x-(1d)-----------------|    |
    |              |<----18x-(1d)--|              |    |    |    |    |
    |              |               | proxy sets   |    |    |    |    |
    |              | Bob gets      | timer for    |    |    |    |    |
    |              | impatient     | 6 more secs  |    |    |    |    |
    |<-REFER/202---|               |              |    |    |    |    |
    |--INVITE w/Replace to-tag=* ->|              |    |    |    |    |
    |              |               |-INVITE w/Replaces(2c)->|    |    |
    |              |               |-INVITE w/Replaces(2d)------>|    |
    |              |               |<---18x-(2c)------------|    |    |



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    |<---18x (2c)------------------|<-687/ACK (1c)----------|    |    |
    |              |               |<---18x-(2d)-----------------|    |
    |<---18x (2d)------------------|<-687/ACK (1d)---------------|    |
    |              |<----687 (1)---|              |    |    |    |    |
    |              |-----ACK (1)-->|              |    |    |    |    |
    |<---BYE/200---|               | call forward |    |    |    |    |
    |              |               | timer expires|    |    |    |    |
    |              |               |              |    |    |    |    |
    |              |               |-CANCEL/200------------>|    |    |
    |              |               |-CANCEL/200----------------->|    |
    |              |               |<-487/ACK (2c)----------|    |    |
    |              |               |<-487/ACK (2d)----------|    |    |
    |              |               |-INVITE-(2e)--------------------->|
    |              |               |<---200-(2e)----------------------|
    |<---200 (2e)------------------|              |    |    |    |    |
    |----ACK--------------------------------------------------------->|
    |--NOTIFY/200->|               |              |    |    |    |    |


5.8 A non-compliant forking proxy

   The following example illustrates two of the consequences of a
   forking proxy not supporting the Replaces extension.  An annoying but
   less serious consequence is that the proxy is likely to start call
   forwarding timers from scratch (see the "#" sign in the call flow),
   resulting in longer delays than in the example in Section 5.7.  A
   more serious consequence is that if an INVITE with a Replaces header
   is lost, as in INVITE 2d, a race condition can arise whereby a
   parallel branch (2c) of the same INVITE with Replaces (2) request
   will trigger a 687 to be sent to terminate the replaced branch (1c).
   Since the proxy does not understand the linkage between the matching
   INVITE and the replacement INVITE, the proxy will CANCEL remaining
   branches (1d) for INVITE 1.  When the proxy later retransmits INVITE
   2d, the request will fail, because the matching dialog was previously
   cancelled.

   Note however that despite these two problems (the race condition
   being unlikely, and the timing problem being merely annoying), the
   proxy will still work well enough that Alice's INVITE will continue
   to follow the proxy's chain of contact logic past the set of contacts
   originally visited by Bob.  This is the intent of an INVITE request
   with a Replaces header with the wildcard to-tag parameter.

   The most serious consequence of a forking proxy not supporting
   Replaces is that it may decide to fork the replacement INVITE request
   to a completely orthogonal set of Contacts.  In this case, neither a
   specific-dialog replacement, nor a wildcard replacement will work.




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                                                     Cathy's Contacts
                                               desk  lab  home  car  VM
   Alice          Bob            Proxy           C1   C2   C3   C4   C5
    |              |               |              |    |    |    |    |
    | stable call  |               |              |    |    |    |    |
    |<============>|               |              |    |    |    |    |
    |              |--INVITE-(1)-->|              |    |    |    |    |
    |              |               |-INVITE-(1a)->|    |    |    |    |
    |              |               |-INVITE-(1b)------>|    |    |    |
    |              |               |<---18x-(1a)--|    |    |    |    |
    |              |<----18x-(1a)--|<---18x-(1b)-------|    |    |    |
    |              |<----18x-(1b)--|              |    |    |    |    |
    |              |               |  proxy waits |    |    |    |    |
    |              |               |  6 seconds   |    |    |    |    |
    |              |               |              |    |    |    |    |
    |              |               |-CANCEL/200-->|    |    |    |    |
    |              |               |-CANCEL/200------->|    |    |    |
    |              |               |<-487/ACK (1a)|    |    |    |    |
    |              |               |<-487/ACK (1b)-----|    |    |    |
    |              |               |-INVITE-(1c)----------->|    |    |
    |              |               |-INVITE-(1d)---------------->|    |
    |              |               |<---18x-(1c)------------|    |    |
    |              |<----18x-(1c)--|<---18x-(1d)---------------->|    |
    |              |<----18x-(1d)--|              |    |    |    |    |
    |              |               |              |    |    |    |    |
    |              | Bob gets      |              |    |    |    |    |
    |              | impatient     |              |    |    |    |    |
    |<-REFER/202---|               |              |    |    |    |    |
    |--INVITE w/Replace to-tag=* ->|              |    |    |    |    |
    |              |               |-INVITE (2a)->|    |    |    |    |
    |              |               | Replaces *   |    |    |    |    |
    |              |               |-INVITE (2b)->|    |    |    |    |
    |              |               | Replaces *   |    |    |    |    |
    |              |               |<---481-(2a)--|    |    |    |    |
    |              |               |<---420-(2b)--|    |    |    |    |
    |              |  # proxy sets |              |    |    |    |    |
    |              |    timer for  |-INVITE w/Replaces(2c)->|    |    |
    |              |    6 more secs|-INVITE w/Replaces(2d)-LOST..|*   |
    |              |               |<---18x-(2c)------------|    |    |
    |<---18x (2c)------------------|<-687/ACK (1c)----------|    |*   |
    |              |<----687 (1)---|-CANCEL/200-(1d)------------>|*   |
    |              |-----ACK (1)-->|<-487/ACK (1d)---------------|    |
    |<---BYE/200---|               |              |    |    |    |    |
    |              |               |-INVITE (2d) retransmitted-->|*   |
    |              |               |<-481/ACK (2d)---------------|*   |
    |              |               |              |    |    |    |    |
    |              |               | call forward |    |    |    |    |
    |              |               | timer expires|    |    |    |    |



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    |              |               |              |    |    |    |    |
    |              |               |-CANCEL/200------------>|    |    |
    |              |               |<-487/ACK (2c)----------|    |    |
    |              |               |-INVITE-(2e)--------------------->|
    |              |               |<---200-(2e)----------------------|
    |<---200 (2e)------------------|              |    |    |    |    |
    |----ACK--------------------------------------------------------->|
    |--NOTIFY/200->|               |              |    |    |    |    |


6. Security Considerations

   This extension can be used to disconnect or replace participants of a
   multimedia conversation with an attacker.  As such, invitations with
   the Replaces header SHOULD only be accepted in a dialog in which the
   peer has been properly authenticated using a standard SIP mechanism,
   and for which message integrity is checked so that the header cannot
   be added or modified in transit.

   The extensions proposed in this document do not significantly change
   the relative security of SIP devices.  Currently in SIP, an
   eavesdropper who learns the Call-ID, To, and From headers can easily
   modify or destroy a dialog using a reINVITE.  In practice, dialog
   information (Call-ID, to-tag, and from-tag) for most uses of Replaces
   is obtained via subscription to a "call-package" event package or via
   transitivity using the REFER method.  Encryption of SIP signaling to
   insure confidentiality of this information is RECOMMENDED.

   This extension was designed to take advantage of future signature or
   authorization schemes defined by the SIP Working Group.  In general,
   call control features would benefit considerably from such work.

7. IANA Considerations

7.1 Registration of "Replaces" SIP header

   Name of Header:          Replaces

   Short form:              none

   Registrant:              Rohan Mahy
                            rohan@cisco.com

   Normative description:   section 4.1 of this document







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7.2 Registration of "replaces" SIP Option-tag

   Name of option:          replaces

   Description:             Support for the SIP Replaces header

   SIP headers defined:     Replaces

   Normative description:   This document

   Registrant:              Rohan Mahy
                            rohan@cisco.com


7.3 Registration of "687" SIP Response code

   Number of response code: 687

   Default reason phrase:   Dialog Terminated

   Registrant:              Rohan Mahy
                            rohan@cisco.com

   Normative description:   section 4.3 of this document


8. To Do and Open Issues

8.1 Open Issues:

   - Are the proposals for early dialog and terminated dialog matching
   acceptable? - Is the proposed tag matching scheme for pre-bis UAs
   acceptable?

8.2 To Do:

   - Update references

8.3 Changes Since -00

   o  When no dialog matches the Call-ID and tags in a Replaces header,
      the UAS now returns a 481 instead of silently accepting the
      INVITE.

   o  Changed the BNF to match the explicit whitespace BNF now used by
      SIP.

   o  Added the to-tag=* matching mechanism.



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   o  Added requirements for forking proxies and a discussion of the
      consequences if forking proxies do not support Replaces.

   o  Added last two examples.

   o  Split normative and non-normative references


9. Acknowledgments

   Thanks to Robert Sparks, Alan Johnston, and Ben Campbell and many
   other members of the SIP WG for their continued support of the cause
   of distributed call control in SIP.

Normative References

   [1]  Rosenberg, J. and H. Schulzrinne, "SIP: Session Initiation
        Protocol", draft-ietf-sip-rfc2543bis-09 (work in progress),
        February 2002.

   [2]  Sparks, R., "The Refer Method", draft-ietf-sip-refer-02 (work in
        progress), October 2001.

   [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [4]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

Informational References

   [5]  Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
        "SIP: Session Initiation Protocol", RFC 2543, March 1999.

   [6]  Mahy, R., "A Multi-party Application Framework for SIP", draft-
        ietf-sipping-cc-framework-00 (work in progress), March 2002.

   [7]  Sparks, R., "SIP Call Control - Transfer", draft-ietf-sip-cc-
        transfer-05.txt (work in progress), July 2001.

   [8]  Rosenberg, J., Schulzrinne, H., Camarillo, G. and J. Peterson,
        "Third Party Call Control in SIP", draft-rosenberg-sip-3pcc-03
        (work in progress), November 2001.

   [9]  Johnston, A., "SIP Service Examples", draft-ietf-sipping-
        service-examples-01 (work in progress), April 2002.





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Authors' Addresses

   Rohan Mahy
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   EMail: rohan@cisco.com


   Billy Biggs

   EMail: bbiggs@dumbterm.net


   Rick Dean

   EMail: rfc@fdd.com
































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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   or assist in its implementation may be prepared, copied, published
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   kind, provided that the above copyright notice and this paragraph are
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   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
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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