Internet DRAFT - draft-biggs-sip-replaces
draft-biggs-sip-replaces
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
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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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