draft-ietf-sipping-transc-conf-02.txt   draft-ietf-sipping-transc-conf-03.txt 
SIPPING Working Group G. Camarillo SIPPING Working Group G. Camarillo
Internet-Draft Ericsson Internet-Draft Ericsson
Expires: July 20, 2006 January 16, 2006 Expires: November 23, 2006 May 22, 2006
The Session Initiation Protocol (SIP) Conference Bridge Transcoding The Session Initiation Protocol (SIP) Conference Bridge Transcoding
Model Model
draft-ietf-sipping-transc-conf-02.txt draft-ietf-sipping-transc-conf-03.txt
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that any By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79. aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
skipping to change at page 1, line 34 skipping to change at page 1, line 33
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on July 20, 2006. This Internet-Draft will expire on November 23, 2006.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2006). Copyright (C) The Internet Society (2006).
Abstract Abstract
This document describes how to invoke transcoding services using the This document describes how to invoke transcoding services using the
conference bridge model. This way of invocation meets the conference bridge model. This way of invocation meets the
requirements for SIP regarding transcoding services invocation to requirements for SIP regarding transcoding services invocation to
skipping to change at page 2, line 15 skipping to change at page 2, line 15
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Caller's Invocation . . . . . . . . . . . . . . . . . . . . . 4 3. Caller's Invocation . . . . . . . . . . . . . . . . . . . . . 4
3.1. Procedures at the User Agent . . . . . . . . . . . . . . . 4 3.1. Procedures at the User Agent . . . . . . . . . . . . . . . 4
3.2. Procedures at the Transcoder . . . . . . . . . . . . . . . 4 3.2. Procedures at the Transcoder . . . . . . . . . . . . . . . 4
3.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.4. Unsuccessful Session Establishment . . . . . . . . . . . . 7 3.4. Unsuccessful Session Establishment . . . . . . . . . . . . 7
4. Callee's Invocation . . . . . . . . . . . . . . . . . . . . . 8 4. Callee's Invocation . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 9 7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 10 8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informational References . . . . . . . . . . . . . . . . . 11 8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 13 Intellectual Property and Copyright Statements . . . . . . . . . . 13
1. Introduction 1. Introduction
The Framework for Transcoding with SIP [9] describes how two SIP [3] The Framework for Transcoding with SIP [8] describes how two SIP [3]
UAs (User Agents) can discover imcompatibilities that prevent them UAs (User Agents) can discover imcompatibilities that prevent them
from establishing a session (e.g., lack of support for a common codec from establishing a session (e.g., lack of support for a common codec
or for a common media type). When such incompatibilities are found, or for a common media type). When such incompatibilities are found,
the UAs need to invoke transcoding services to successfully establish the UAs need to invoke transcoding services to successfully establish
the session. The transcoding framework introduces two models to the session. The transcoding framework introduces two models to
invoke transcoding services: the 3pcc (third-party call control) invoke transcoding services: the 3pcc (third-party call control)
model [8] and the conference bridge model. This document specifies model [7] and the conference bridge model. This document specifies
the conference bridge model. the conference bridge model.
In the conference bridge model for transcoding invocation, a In the conference bridge model for transcoding invocation, a
transcoding server that provides a particular transcoding service transcoding server that provides a particular transcoding service
(e.g., speech-to-text) behaves as a B2BUA (Back-to-Back User Agent) (e.g., speech-to-text) behaves as a B2BUA (Back-to-Back User Agent)
between both UAs and is identified by a URI. As shown in Figure 1, between both UAs and is identified by a URI. As shown in Figure 1,
both UAs, A and B, exchange signalling and media with the transcoder both UAs, A and B, exchange signalling and media with the transcoder
T. The UAs do not exchange any traffic (signalling or media) directly T. The UAs do not exchange any traffic (signalling or media) directly
between them. between them.
skipping to change at page 4, line 24 skipping to change at page 4, line 24
User agent A needs to perform two operations to invoke transcoding User agent A needs to perform two operations to invoke transcoding
services from T for a session between user agent A and user agent B. services from T for a session between user agent A and user agent B.
User agent A needs to establish a session with T and provide T with User agent A needs to establish a session with T and provide T with
user agent B's URI so that T can generate an INVITE towards user user agent B's URI so that T can generate an INVITE towards user
agent B. agent B.
3.1. Procedures at the User Agent 3.1. Procedures at the User Agent
User agent A uses the procedures for Conference Establishment Using User agent A uses the procedures for Conference Establishment Using
Request-Contained Lists in SIP [11] to provide T with B's URI using Request-Contained Lists in SIP [10] to provide T with B's URI using
the same INVITE that establishes the session between A and T. That the same INVITE that establishes the session between A and T. That
is, user agent A adds to the INVITE a body part whose disposition is, user agent A adds to the INVITE a body part whose disposition
type is recipient-list [10]. This body part consists of a URI-list type is recipient-list [9]. This body part consists of a URI-list
that MUST contain a single URI: user agent B's URI. that contains a single URI: user agent B's URI.
Note that, as described in the transcoding framework [8], the
transcoding model described in this document is modeled as a two-
party conference server. Consequently, this document focuses on
two-party sessions that need transcoding. Multi-party sessions
can be established using INVITE requests with multiple URIs in
their bodies, as specified in [10].
3.2. Procedures at the Transcoder 3.2. Procedures at the Transcoder
On receiving an INVITE with a URI-list body, the transcoder follows On receiving an INVITE with a URI-list body, the transcoder follows
the procedures in [11] to generate an INVITE request towards the URI the procedures in [10] to generate an INVITE request towards the URI
contained in the URI-list body. Note that the transcoder acts as a contained in the URI-list body. Note that the transcoder acts as a
B2BUA, not as a proxy. B2BUA, not as a proxy.
Additionally, the transcoder MUST generate the From header field of Additionally, the transcoder MUST generate the From header field of
the outgoing INVITE request using the same value as the From header the outgoing INVITE request using the same value as the From header
field included in the incoming INVITE request, subject to the privacy field included in the incoming INVITE request, subject to the privacy
requirements (see [5] and [6]) expressed in the incoming INVITE requirements (see [4] and [5]) expressed in the incoming INVITE
request. Note that this does not apply to the "tag" parameter. request. Note that this does not apply to the "tag" parameter.
The session description the transcoder includes in the outgoing The session description the transcoder includes in the outgoing
INVITE request depends on the type of transcoding service that INVITE request depends on the type of transcoding service that
particular transcoder provides. For example, a transcoder resolving particular transcoder provides. For example, a transcoder resolving
audio codec incompatibilities would generate a session description audio codec incompatibilities would generate a session description
listing the audio codecs the transcoder supports. listing the audio codecs the transcoder supports.
When the transcoder receives a final response for the outgoing INVITE When the transcoder receives a final response for the outgoing INVITE
requests, it generates a new final response for the incoming INVITE requests, it generates a new final response for the incoming INVITE
skipping to change at page 5, line 14 skipping to change at page 5, line 21
the one received in the response for the outgoing INVITE request. the one received in the response for the outgoing INVITE request.
If a trancoder receives an INVITE request with a URI-list with more If a trancoder receives an INVITE request with a URI-list with more
than one URI, it SHOULD return a 488 (Max 1 URI allowed in URI-list) than one URI, it SHOULD return a 488 (Max 1 URI allowed in URI-list)
response. response.
3.3. Example 3.3. Example
Figure 2 shows the message flow for the caller's invocation of a Figure 2 shows the message flow for the caller's invocation of a
transcoder T. The caller A sends an INVITE (1) to the transcoder (T) transcoder T. The caller A sends an INVITE (1) to the transcoder (T)
to establish the session A-T. Following the procedures in [11], the to establish the session A-T. Following the procedures in [10], the
caller A adds a body part whose disposition type is recipient-list caller A adds a body part whose disposition type is recipient-list
[10]. [9].
A T B A T B
| | | | | |
|-----(1) INVITE SDP A----->| | |-----(1) INVITE SDP A----->| |
| | | | | |
|<-(2) 183 Session Progress-| | |<-(2) 183 Session Progress-| |
| |-----(3) INVITE SDP TB---->| | |-----(3) INVITE SDP TB---->|
| | | | | |
| |<-----(4) 200 OK SDP B-----| | |<-----(4) 200 OK SDP B-----|
| | | | | |
skipping to change at page 5, line 40 skipping to change at page 5, line 47
|---------(7) ACK---------->| | |---------(7) ACK---------->| |
| | | | | |
| ************************* | ************************* | | ************************* | ************************* |
|** Media **|** Media **| |** Media **|** Media **|
| ************************* | ************************* | | ************************* | ************************* |
| | | | | |
Figure 2: Successful invocation of a transcoder by the caller Figure 2: Successful invocation of a transcoder by the caller
The following example shows an INVITE with two body parts: an SDP The following example shows an INVITE with two body parts: an SDP
[14] session description and a URI-list. [13] session description and a URI-list.
INVITE sip:transcoder@example.com SIP/2.0 INVITE sip:transcoder@example.com SIP/2.0
Via: SIP/2.0/TCP client.chicago.example.com Via: SIP/2.0/TCP client.chicago.example.com
;branch=z9hG4bKhjhs8ass83 ;branch=z9hG4bKhjhs8ass83
Max-Forwards: 70 Max-Forwards: 70
To: Transcoder <sip:transcoder@example.org> To: Transcoder <sip:transcoder@example.org>
From: A <sip:A@chicago.example.com>;tag=32331 From: A <sip:A@chicago.example.com>;tag=32331
Call-ID: d432fa84b4c76e66710 Call-ID: d432fa84b4c76e66710
CSeq: 1 INVITE CSeq: 1 INVITE
Contact: <sip:A@client.chicago.example.com> Contact: <sip:A@client.chicago.example.com>
skipping to change at page 7, line 36 skipping to change at page 7, line 36
|---------(7) ACK---------->| | |---------(7) ACK---------->| |
| | | | | |
Figure 3: Unsuccessful session establishment Figure 3: Unsuccessful session establishment
The ambiguity in this flow is that, if the provisional response (2) The ambiguity in this flow is that, if the provisional response (2)
gets lost, the caller does not know whether the 603 (Decline) gets lost, the caller does not know whether the 603 (Decline)
response means that the initial INVITE (1) was rejected by the response means that the initial INVITE (1) was rejected by the
transcoder or that the INVITE generated by the transcoder (4) was transcoder or that the INVITE generated by the transcoder (4) was
rejected by the callee. The use of the "History-Info" header field rejected by the callee. The use of the "History-Info" header field
[12] between the transcoder and the caller resolves the previous [11] between the transcoder and the caller resolves the previous
ambiguity. ambiguity.
Callers that do not support the "History-Info" header field can,
alternatively, require the use of the reliable provisional responses
[4] SIP extension. If the caller receives a response reporting a
reachability problem, the caller can also send an OPTIONS request to
the transcoder to check whether or not the transcoder is reachable.
If the transcoder is reachable, the party that could not be reached
was the callee.
Note that this ambiguity problem could also have been resolved by Note that this ambiguity problem could also have been resolved by
having transcoders act as a pure conference bridge. The transcoder having transcoders act as a pure conference bridge. The transcoder
would respond with a 200 (OK) the INVITE request from the caller and would respond with a 200 (OK) the INVITE request from the caller and
generate an outgoing INVITE request towards the callee. The caller generate an outgoing INVITE request towards the callee. The caller
would get information about the result of the latter INVITE request would get information about the result of the latter INVITE request
by subscribing to the conference event package [15] at the by subscribing to the conference event package [14] at the
transcoder. Nevertheless, while this flow would have resolved the transcoder. Nevertheless, while this flow would have resolved the
ambiguity problem without requiring support for the "History-Info" ambiguity problem without requiring support for the "History-Info"
header field, it is more complex, requires a higher number on header field, it is more complex, requires a higher number on
messages, and introduces higher session setup delays. That is why it messages, and introduces higher session setup delays. That is why it
was not chosen to implement transcoding services. was not chosen to implement transcoding services.
4. Callee's Invocation 4. Callee's Invocation
If a UA receives an INVITE with a session description that is not If a UA receives an INVITE with a session description that is not
acceptable, it can redirect it to the transcoder by using a 302 acceptable, it can redirect it to the transcoder by using a 302
skipping to change at page 8, line 49 skipping to change at page 8, line 42
|<----(9) 200 OK SDP TA-----| | |<----(9) 200 OK SDP TA-----| |
| | | | | |
|--------(10) ACK---------->| | |--------(10) ACK---------->| |
| | | | | |
| ************************* | ************************* | | ************************* | ************************* |
|** Media **|** Media **| |** Media **|** Media **|
| ************************* | ************************* | | ************************* | ************************* |
Figure 4: Callee's invocation of a transcoder Figure 4: Callee's invocation of a transcoder
Note that A does not necessarily need to be the one performing the Note that the syntax resulting from encoding a body into a URI as
recursion on the 302 (Moved Temporarily) response. Any proxy in the described earlier is quite complex. It is actually simpler for
path between A and B may perform such a recursion. callees to invoke transcoding services using the 3pcc transcoding
model [7] instead.
5. Security Considerations 5. Security Considerations
Transcoders implementing this specification behave as a URI-list Transcoders implementing this specification behave as a URI-list
service as described in [11]. Therefore, the security considerations service as described in [10]. Therefore, the security considerations
for URI-list services discussed in [10] apply here as well. for URI-list services discussed in [9] apply here as well.
In particular, the requirements related to list integrity and In particular, the requirements related to list integrity and
unsolicited requests are important for transcoding services. User unsolicited requests are important for transcoding services. User
agents SHOULD integrity protect URI-lists using mechanisms such as agents SHOULD integrity protect URI-lists using mechanisms such as
S/MIME [7] or TLS [2], which can also provide URI-list S/MIME [6] or TLS [2], which can also provide URI-list
confidentiality if needed. Additionally, transcoders MUST confidentiality if needed. Additionally, transcoders MUST
authenticate and authorize users and MAY provide information about authenticate and authorize users and MAY provide information about
the identity of the original sender of the request in their outgoing the identity of the original sender of the request in their outgoing
requests by using the SIP identity mechanism [13]. requests by using the SIP identity mechanism [12].
The requirement in [10] to use opt-in lists (e.g., using the The requirement in [9] to use opt-in lists (e.g., using the Framework
Framework for Consent-Based Communications in SIP [16]) deserves for Consent-Based Communications in SIP [15]) deserves special
special discussion. The type of URI-list service implemented by discussion. The type of URI-list service implemented by transcoders
transcoders following this specification does not produce following this specification does not produce amplification (only one
amplification (only one INVITE request is generated by the transcoder INVITE request is generated by the transcoder on receiving an INVITE
on receiving an INVITE request from a user agent) and does not request from a user agent) and does not involve a translation to a
involve a translation to a URI that may be otherwise unknown to the URI that may be otherwise unknown to the caller (the caller places
caller (the caller places the callee's URI in the body of its initial the callee's URI in the body of its initial INVITE request).
INVITE request). Additionally, the identity of the caller is present Additionally, the identity of the caller is present in the INVITE
in the INVITE request generated by the transcoder. Therefore, there request generated by the transcoder. Therefore, there is no
is no requirement for transcoders implementing this specification to requirement for transcoders implementing this specification to use
use opt-in lists. opt-in lists.
6. IANA Considerations 6. IANA Considerations
This document does not contain any IANA actions. This document does not contain any IANA actions.
7. Contributors 7. Contributors
This document is the result of discussions amongst the conferencing This document is the result of discussions amongst the conferencing
design team. The members of this team include Eric Burger, Henning design team. The members of this team include Eric Burger, Henning
Schulzrinne, and Arnoud van Wijk. Schulzrinne, and Arnoud van Wijk.
skipping to change at page 10, line 17 skipping to change at page 10, line 7
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[2] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", [2] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999. RFC 2246, January 1999.
[3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002. Session Initiation Protocol", RFC 3261, June 2002.
[4] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional [4] Peterson, J., "A Privacy Mechanism for the Session Initiation
Responses in Session Initiation Protocol (SIP)", RFC 3262,
June 2002.
[5] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323, November 2002. Protocol (SIP)", RFC 3323, November 2002.
[6] Jennings, C., Peterson, J., and M. Watson, "Private Extensions [5] Jennings, C., Peterson, J., and M. Watson, "Private Extensions
to the Session Initiation Protocol (SIP) for Asserted Identity to the Session Initiation Protocol (SIP) for Asserted Identity
within Trusted Networks", RFC 3325, November 2002. within Trusted Networks", RFC 3325, November 2002.
[7] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions [6] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Certificate Handling", RFC 3850, (S/MIME) Version 3.1 Certificate Handling", RFC 3850,
July 2004. July 2004.
[8] Camarillo, G., Burger, E., Schulzrinne, H., and A. van Wijk, [7] Camarillo, G., Burger, E., Schulzrinne, H., and A. van Wijk,
"Transcoding Services Invocation in the Session Initiation "Transcoding Services Invocation in the Session Initiation
Protocol (SIP) Using Third Party Call Control (3pcc)", Protocol (SIP) Using Third Party Call Control (3pcc)",
RFC 4117, June 2005. RFC 4117, June 2005.
[9] Camarillo, G., "Framework for Transcoding with the Session [8] Camarillo, G., "Framework for Transcoding with the Session
Initiation Protocol", Initiation Protocol",
draft-camarillo-sipping-transc-framework-00 (work in progress), draft-camarillo-sipping-transc-framework-00 (work in progress),
August 2003. August 2003.
[10] Camarillo, G. and A. Roach, "Framework and Security [9] Camarillo, G. and A. Roach, "Framework and Security
Considerations for Session Initiation Protocol (SIP) Uniform Considerations for Session Initiation Protocol (SIP) Uniform
Resource Identifier (URI)-List Services", Resource Identifier (URI)-List Services",
draft-ietf-sipping-uri-services-04 (work in progress), draft-ietf-sipping-uri-services-05 (work in progress),
October 2005. January 2006.
[11] Camarillo, G. and A. Johnston, "Conference Establishment Using [10] Camarillo, G. and A. Johnston, "Conference Establishment Using
Request-Contained Lists in the Session Initiation Protocol Request-Contained Lists in the Session Initiation Protocol
(SIP)", draft-ietf-sipping-uri-list-conferencing-04 (work in (SIP)", draft-ietf-sipping-uri-list-conferencing-05 (work in
progress), October 2005. progress), February 2006.
[12] Barnes, M., "An Extension to the Session Initiation Protocol [11] Barnes, M., "An Extension to the Session Initiation Protocol
for Request History Information", for Request History Information",
draft-ietf-sip-history-info-06 (work in progress), draft-ietf-sip-history-info-06 (work in progress),
January 2005. January 2005.
[13] Peterson, J. and C. Jennings, "Enhancements for Authenticated [12] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)", Identity Management in the Session Initiation Protocol (SIP)",
draft-ietf-sip-identity-06 (work in progress), October 2005. draft-ietf-sip-identity-06 (work in progress), October 2005.
8.2. Informational References 8.2. Informative References
[14] Handley, M., "SDP: Session Description Protocol", [13] Handley, M., "SDP: Session Description Protocol",
draft-ietf-mmusic-sdp-new-25 (work in progress), July 2005. draft-ietf-mmusic-sdp-new-26 (work in progress), January 2006.
[15] Rosenberg, J., "A Session Initiation Protocol (SIP) Event [14] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Conference State", Package for Conference State",
draft-ietf-sipping-conference-package-12 (work in progress), draft-ietf-sipping-conference-package-12 (work in progress),
July 2005. July 2005.
[16] Rosenberg, J., "A Framework for Consent-Based Communications in [15] Rosenberg, J., "A Framework for Consent-Based Communications in
the Session Initiation Protocol (SIP)", the Session Initiation Protocol (SIP)",
draft-ietf-sipping-consent-framework-03 (work in progress), draft-ietf-sipping-consent-framework-04 (work in progress),
October 2005. March 2006.
Author's Address Author's Address
Gonzalo Camarillo Gonzalo Camarillo
Ericsson Ericsson
Hirsalantie 11 Hirsalantie 11
Jorvas 02420 Jorvas 02420
Finland Finland
Email: Gonzalo.Camarillo@ericsson.com Email: Gonzalo.Camarillo@ericsson.com
 End of changes. 39 change blocks. 
71 lines changed or deleted 66 lines changed or added

This html diff was produced by rfcdiff 1.32. The latest version is available from http://www.levkowetz.com/ietf/tools/rfcdiff/