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Internet Engineering Task Force                        B. Biggs, R. Dean
Internet-Draft                                              12 July 2001
draft-biggs-sip-replaces-01.txt
Expires: December 2001


                        The SIP Replaces Header

Status of this Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six
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   The list of current Internet-Drafts can be accessed at
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   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire December 2001.


Copyright Notice

   Copyright (C) The Internet Society (2001). All Rights Reserved.


Abstract

   This document describes an extension to the Session Initiation
   Protocol (SIP) [2].  The purpose of this extension is to allow an
   active call-leg to be replaced by an incoming call-leg for the
   purposes of call transfer.


1 Overview

   This document defines a SIP [2] extension to help provide attended



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   transfer capabilities.  This header has its roots in [3], however
   both the syntax and semantics are changed.

   An attended call transfer occurs when a SIP user agent which is
   involved in a 3-way call conference wishes to silently leave the call
   by requesting the two remote parties to establish a direct signalling
   relationship.  The Replaces header augments the transfer
   functionality provided by REFER [4] to allow the transfer target to
   associate the incoming call with the active one.

   Replaces is intended to provide a simple and backwards compatible
   method of allowing this association to occur.  We also define a
   simple signature scheme which allows the replacement request to be
   authenticated by the target user agent.


2 Rationale

   Use of a new header for attended transfer leg matching was chosen
   over making associations based on call-id or other fields for the
   following reasons:

   1. Using a Replaces header in the request makes the intent obvious.

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

   3. No adverse effects if the header is unsupported.


3 The Replaces Header

   The Replaces header indicates that the call leg 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 syntax is:


   Replaces         = "Replaces" ":" 1#replaces-values
   replaces-values  =  callid *( ";" replaces-param )
   callid           = token [ "@" token ]
   replaces-param   = to-tag | from-tag | rep-signature
                      | extension-param
   to-tag           = "to-tag=" UUID
   from-tag         = "from-tag=" UUID
   rep-signature    = signature-scheme *( ";" sig-scheme-params )
   signature-scheme = "scheme" "=" token
   sig-scheme-parms = token "=" ( token | quoted-string )




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   The to-tag and from-tag parameters MUST be included in the Replaces
   header, as they are required for unique call-leg matching.  Since we
   rely on the tags for matching purposes, implementations which support
   Replaces MUST support at least the SIP bis [5] standard which
   requires tags.


3.1 Examples

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

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


3.2 A PGP based signature-scheme

   One signature-scheme for Replaces headers uses PGP as follows:

     signature-scheme = "scheme" "=" "pgp"
     sig-scheme-parms = pgp-version | signed-by | pgp-signature

   pgp-version, signed-by and pgp-signature are defined in section 15.1
   of RFC2543, with the modification that the signature is computed
   across the concatenation of the callid, to-tag, and from-tag in that
   order.


4 Behavior of SIP User Agents

   The Replaces header contains matching information for a call-leg,
   identified by the call-id and tags.  Upon receiving an INVITE with a
   Replaces header, the user agent will attempt to match the information
   with any active call legs.  If no match is found, the Replaces header
   MUST be ignored.  The to tag and from tag should be matched as if
   they were present in an incoming request, that is, the to tag should
   match the local tag for the call leg, and the from tag should match
   the remote tag.

   If the Replaces header matches more than one call-leg, the user agent
   MAY use the Referred-By header if present to attempt to match the
   call with an appropriate call-leg, but should otherwise ignore the
   header.

   If the Replaces header matches an unestablished call leg, that it,
   the INVITE request for the call has not received or been sent a final
   response, the Replaces header MUST be ignored.



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   If a matching call-leg is found and the signature checked, the user
   agent SHOULD silently accept the new call, replacing the old call in
   the user interface.  The user agent SHOULD also shut down the
   replaced call leg by sending a BYE.


5 Attended Call Transfer to a Supporting Target

   In these examples, the names of the actors are taken from [4].

   In this scenario, the transferor is in a 3-way call with both the
   transferee and the transfer target and wishes to leave the call.  The
   transferor REFERs the transferee to the transfer target, using the
   Replaces header to signify to the target that the incoming call
   should replace its current call to the transferor.  The Replaces
   header is placed as a header parameter in the SIP URI used in the
   Refer-To header.

   After accepting the call, the target's BYE is received by the
   transferor before it has a chance to send its own BYE.

   Tags are omitted in this example for brevity.


      Transferor                Transferee              Transfer
           |                         |                   Target
           |                         |                     |
 Call-ID:1 | INVITE/200 OK/ACK       |                     |
           |<----------------------->|                     |
           |                         |                     |
 Call-ID:2 | INVITE/200 OK/ACK       |                     |
           |<--------------------------------------------->|
           |                         |                     |
           | REFER target?Replaces=1 |                     |
           |------------------------>|                     |
           | 100 Trying              |                     |
           |<------------------------|                     |
           |                         | INVITE Replaces:1   |
           |                         |-------------------->| Immediate
           |                         | 200 OK/ACK          | Acceptance
           |                         |<------------------->|
           |                         |          BYE/200 OK |
           |<--------------------------------------------->|
           | 200 OK (to the REFER)   |                     |
           |<------------------------|                     |
           | BYE/200 OK              |                     |
           |<----------------------->|                     |
           |                         |                     |



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6 Attended Call Transfer to an Unsupporting Target

   In this scenario, the transfer target ignores the Replaces header.
   In this case, the transferor sends a BYE to confirm shutdown of the
   replaced call.

   Tags are omitted in this example for brevity.


      Transferor                Transferee              Transfer
           |                         |                   Target
           |                         |                     |
 Call-ID:1 | INVITE/200 OK/ACK       |                     |
           |<----------------------->|                     |
           |                         |                     |
 Call-ID:2 | INVITE/200 OK/ACK       |                     |
           |<--------------------------------------------->|
           |                         |                     |
           | REFER target?Replaces=1 |                     |
           |------------------------>|                     |
           | 100 Trying              |                     |
           |<------------------------|                     |
           |                         | INVITE Replaces:1   |
           |                         |-------------------->| User
           |                         | 200 OK/ACK          | Accepts
           |                         |<------------------->| Call
           | 200 OK (to the REFER)   |                     |
           |<------------------------|                     |
           | BYE/200 OK              |                     |
           |<--------------------------------------------->|
           | BYE/200 OK              |                     |
           |<----------------------->|                     |
           |                         |                     |


7 Author's Addresses

   Billy Biggs
   bbiggs@div8.net

   Rick Dean
   rfc@fdd.com


8 Open Issues

   1. There is currently a proposal to make call leg matching based



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      solely on the call-id, to-tag and from-tag.  If this is not
      accepted, Replaces may be updated to include the full to and from
      as parameters.


8 References

   [1] S. Bradner, "The Internet Standards Process -- Revision 3",
       BCP9, RFC2026, October 1996.

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

   [3] H. Schulzrinne, J. Rosenberg, "SIP Call Control Services",
       Internet Draft draft-ietf-sip-cc-01, Internet Engineering Task
       Force, June 17, 1999 Work in Progress (expired).

   [4] R. Sparks, "SIP Call Control: Transfer", Internet Draft
       draft-ietf-sip-cc-transfer-04.txt, Internet Engineering Task
       Force, February 2001, Work in Progress.

   [5] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg,
       "SIP: Session Initiation Protocol", Internet Draft
       draft-ietf-sip-rfc2543bis-03.txt, Internet Engineering Task
       Force, May 29, 2001, Work in Progress.



























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