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Internet Engineering Task Force                              Biggs/Dean
Internet Draft                                                     3Com
draft-biggs-sip-replaces-00.txt
November 2000
Expires May 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 [1].

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

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

   This document is an individual submission to the IETF. Comments
   should be directed to the authors.


Abstract

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


1 Overview

   This document defines a SIP [2] extension to help provide attended
   transfer or call handoff capabilities.  This header has its roots in
   [3], however both the syntax and semantics are changed.

   Call handoff 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.
   While the transfer functionality provided by REFER [4] allows the
   conference initiator to connect the two remote parties, there is no
   standard way for the target of the transfer to associate the incoming
   call with the active one.

   The header defined here is intended to provide a simple and backwards
   compatible method to allow this association to occur.  We also define
   a signature scheme which allows the replacement request to be
   authenticated by the target user agent.


2 Rationale

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

  2. A unique call-id may be given to the replacement call, which helps
     call leg matching.

  3. No adverse effects if the header is unsupported.


3 The Replaces Header

   The Replaces header is used to indicate that the call leg identified
   in 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 INVITEs. 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 )


3.1 Examples

   Replaces: 98732@sip.billybiggs.com

   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 related 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 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.


      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              |                     |
           |<----------------------->|                     |
           |                         |                     |


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.


      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
   3Com
   3800 Golf Rd
   Rolling Meadows, IL
   USA

   Phone: sip:Billy_Biggs@sip.3com.com
   Email: Billy_Biggs@3com.com

   Rick Dean
   3Com
   3800 Golf Rd
   Rolling Meadows, IL
   USA

   Phone: sip:Rick_Dean@sip.3com.com
   Email: Rick_Dean@3com.com

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-02.txt, Internet Engineering Task
       Force, November 2000, Work in Progress.


Internet Draft     draft-biggs-sip-replaces-00.txt         November 2000


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