draft-ietf-sipping-transc-framework-01.txt   draft-ietf-sipping-transc-framework-02.txt 
SIPPING Working Group G. Camarillo SIPPING Working Group G. Camarillo
Internet-Draft Ericsson Internet-Draft Ericsson
Expires: August 22, 2005 February 21, 2005 Expires: December 3, 2005 June 1, 2005
Conference Establishment Using Request-Contained Lists in the Session Framework for Transcoding with the Session Initiation Protocol (SIP)
Initiation Protocol (SIP) draft-ietf-sipping-transc-framework-02.txt
draft-ietf-sipping-transc-framework-01.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft and is subject to all provisions By submitting this Internet-Draft, each author represents that any
of section 3 of RFC 3667. By submitting this Internet-Draft, each applicable patent or other IPR claims of which he or she is aware
author represents that any applicable patent or other IPR claims of have been or will be disclosed, and any of which he or she becomes
which he or she is aware have been or will be disclosed, and any of aware will be disclosed, in accordance with Section 6 of BCP 79.
which he or she become aware will be disclosed, in accordance with
RFC 3668.
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
other groups may also distribute working documents as other groups may also distribute working documents as Internet-
Internet-Drafts. Drafts.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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 August 22, 2005. This Internet-Draft will expire on December 3, 2005.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2005).
Abstract Abstract
This document defines a framework for transcoding with SIP. This This document defines a framework for transcoding with SIP. This
framework includes how to discover the need of transcoding services framework includes how to discover the need of transcoding services
in a session and how to invoke those transcoding services. Two in a session and how to invoke those transcoding services. Two
skipping to change at page 2, line 17 skipping to change at page 2, line 16
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Discovery of the Need for Transcoding Services . . . . . . . . 3 2. Discovery of the Need for Transcoding Services . . . . . . . . 3
3. Transcoding Services Invocation . . . . . . . . . . . . . . . 4 3. Transcoding Services Invocation . . . . . . . . . . . . . . . 4
3.1 Third Party Call Control Transcoding Model . . . . . . . . 5 3.1 Third Party Call Control Transcoding Model . . . . . . . . 5
3.2 Conference Bridge Transcoding Model . . . . . . . . . . . 6 3.2 Conference Bridge Transcoding Model . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7 4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 8 6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1 Normative References . . . . . . . . . . . . . . . . . . . . 8 7.1 Normative References . . . . . . . . . . . . . . . . . . . 8
7.2 Informational References . . . . . . . . . . . . . . . . . . 9 7.2 Informational References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . . 10
1. Introduction 1. Introduction
Two user agents involved in a SIP [2] dialog may find it impossible Two user agents involved in a SIP [3] dialog may find it impossible
to establish a media session due to a variety of incompatibilities. to establish a media session due to a variety of incompatibilities.
Assuming that both user agents understand the same session Assuming that both user agents understand the same session
description format (e.g., SDP [9]), incompatibilities can be found at description format (e.g., SDP [11]), incompatibilities can be found
the user agent level and at the user level. At the user agent level, at the user agent level and at the user level. At the user agent
both terminals may not support any common codec or may not support level, both terminals may not support any common codec or may not
common media types (e.g., a text-only terminal and an audio-only support common media types (e.g., a text-only terminal and an audio-
terminal). At the user level, a deaf person will not understand only terminal). At the user level, a deaf person will not understand
anything said over an audio stream. anything said over an audio stream.
In order to make communications possible in the presence of In order to make communications possible in the presence of
incompatibilities, user agents need to introduce intermediaries that incompatibilities, user agents need to introduce intermediaries that
provide transcoding services to a session. From the SIP point of provide transcoding services to a session. From the SIP point of
view, the introduction of a transcoder is done in the same way to view, the introduction of a transcoder is done in the same way to
resolve both user level and user agent level incompatibilities. So, resolve both user level and user agent level incompatibilities. So,
the invocation mechanisms described in this document are generally the invocation mechanisms described in this document are generally
applicable to any type of incompatibility related to how the applicable to any type of incompatibility related to how the
information that needs to be communicated is encoded. information that needs to be communicated is encoded.
skipping to change at page 3, line 41 skipping to change at page 3, line 41
This document does not describe media server discovery. That is an This document does not describe media server discovery. That is an
orthogonal problem that one can address using user agent provisioning orthogonal problem that one can address using user agent provisioning
or other methods. or other methods.
The remainder of this document is organized as follows. Section 2 The remainder of this document is organized as follows. Section 2
deals with the discovery of the need of transcoding services for a deals with the discovery of the need of transcoding services for a
particular session. Section 3 introduces the third party call particular session. Section 3 introduces the third party call
control and conference bridge transcoding invocation models, which control and conference bridge transcoding invocation models, which
are further described in Section 3.1 and Section 3.2 respectively. are further described in Section 3.1 and Section 3.2 respectively.
Both models meet the requirements regarding transcoding services Both models meet the requirements regarding transcoding services
invocation in RFC3351 [4] to support deaf, hard of hearing and invocation in RFC3351 [6] to support deaf, hard of hearing and
speech-impaired individuals. speech-impaired individuals.
2. Discovery of the Need for Transcoding Services 2. Discovery of the Need for Transcoding Services
According to the one-party consent model defined in RFC 3238 [1] , According to the one-party consent model defined in RFC 3238 [2],
services that involve media manipulation invocation are best invoked services that involve media manipulation invocation are best invoked
by one of the end-points involved in the communication, as opposed to by one of the end-points involved in the communication, as opposed to
being invoked by an intermediary in the network. Following this being invoked by an intermediary in the network. Following this
principle, one of the end-points should be the one detecting that principle, one of the end-points should be the one detecting that
transcoding is needed for a particular session. transcoding is needed for a particular session.
In order to decide whether or not transcoding is needed, a user agent In order to decide whether or not transcoding is needed, a user agent
needs to know the capabilities of the remote user agent. A user needs to know the capabilities of the remote user agent. A user
agent acting as an offerer typically obtains this knowledge by agent acting as an offerer typically obtains this knowledge by
downloading a presence document that includes media capabilities downloading a presence document that includes media capabilities
(e.g., Bob is available on a terminal that only supports audio) or by (e.g., Bob is available on a terminal that only supports audio) or by
getting an SDP description of media capabilities as defined in RFC getting an SDP description of media capabilities as defined in RFC
3264 [3]. 3264 [4].
Presence documents are typically received in a NOTIFY request as a Presence documents are typically received in a NOTIFY [5] request as
result of a subscription. SDP media capabilities descriptions are a result of a subscription. SDP media capabilities descriptions are
typically received in a 200 (OK) response to an OPTIONS request or in typically received in a 200 (OK) response to an OPTIONS request or in
a 488 (Not Acceptable Here) response to an INVITE. a 488 (Not Acceptable Here) response to an INVITE.
It is recommended that an offerer does not invoke transcoding It is recommended that an offerer does not invoke transcoding
services before making sure that the answerer does not support the services before making sure that the answerer does not support the
capabilities needed for the session. Making wrong assumptions about capabilities needed for the session. Making wrong assumptions about
the answerer's capabilities can lead to situations where two the answerer's capabilities can lead to situations where two
transcoders are introduced (one by the offerer and one by the transcoders are introduced (one by the offerer and one by the
answerer) in a session that would not need any transcoding services answerer) in a session that would not need any transcoding services
at all. at all.
skipping to change at page 4, line 51 skipping to change at page 4, line 51
As we said earlier, transcoder location is outside the scope of this As we said earlier, transcoder location is outside the scope of this
document. So, we assume that the user agent invoking transcoding document. So, we assume that the user agent invoking transcoding
services knows the URI of a server that provides them. services knows the URI of a server that provides them.
Invoking transcoding services from a server (T) for a session between Invoking transcoding services from a server (T) for a session between
two user agents (A and B) involves establishing two media sessions; two user agents (A and B) involves establishing two media sessions;
one between A and T and another between T and B. How to invoke T's one between A and T and another between T and B. How to invoke T's
services (i.e., how to establish both A-T and T-B sessions) depends services (i.e., how to establish both A-T and T-B sessions) depends
on how we model the transcoding service. We have considered two on how we model the transcoding service. We have considered two
models for invoking a transcoding service. The first is to use third models for invoking a transcoding service. The first is to use third
party call control [5], also referred to as 3pcc. The second is to party call control [7], also referred to as 3pcc. The second is to
use a (dial-in and dial-out) conference bridge that negotiates the use a (dial-in and dial-out) conference bridge that negotiates the
appropriate media parameters on each individual leg (i.e., A-T and appropriate media parameters on each individual leg (i.e., A-T and
T-B). T-B).
Section 3.1 analyzes the applicability of the third party call Section 3.1 analyzes the applicability of the third party call
control model and Section 3.2 analyzes the applicability of the control model and Section 3.2 analyzes the applicability of the
conference bridge transcoding invocation model. conference bridge transcoding invocation model.
3.1 Third Party Call Control Transcoding Model 3.1 Third Party Call Control Transcoding Model
In the 3pcc transcoding model, defined in [7], the user agent In the 3pcc transcoding model, defined in [10], the user agent
invoking the transcoding service has a signalling relationship with invoking the transcoding service has a signalling relationship with
the transcoder and another signalling relationship with the remote the transcoder and another signalling relationship with the remote
user agent. There is no signalling relationship between the user agent. There is no signalling relationship between the
transcoder and the remote user agent, as shown in Figure 1. transcoder and the remote user agent, as shown in Figure 1.
+-------+ +-------+
| | | |
| T |** | T |**
| | ** | | **
+-------+ ** +-------+ **
skipping to change at page 6, line 18 skipping to change at page 6, line 18
points cannot cope with the changes (e.g., they had common audio points cannot cope with the changes (e.g., they had common audio
codecs but no common video codecs). codecs but no common video codecs).
The privacy level that is achieved using 3pcc is high, since the The privacy level that is achieved using 3pcc is high, since the
transcoder does no see the signalling between both end-points. In transcoder does no see the signalling between both end-points. In
this model, the transcoder only has access to the information that is this model, the transcoder only has access to the information that is
strictly needed to perform its function. strictly needed to perform its function.
3.2 Conference Bridge Transcoding Model 3.2 Conference Bridge Transcoding Model
OPEN ISSUE: this section outlines how to use the URI-list
mechanism for INVITEs specified in [8] to invoke a transcoder.
Some people think that having an even simpler mechanism to perform
transcoding invocation would be useful. We need to decide whether
we are happy with the current solution or we want to use a
different mechanism.
In a centralized conference, there are a number of media streams In a centralized conference, there are a number of media streams
between the conference server and each participant of a conference. between the conference server and each participant of a conference.
For a given media type (e.g., audio) the conference server sends, For a given media type (e.g., audio) the conference server sends,
over each individual stream, the media received over the rest of the over each individual stream, the media received over the rest of the
streams, typically performing some mixing. If the capabilities of streams, typically performing some mixing. If the capabilities of
all the end-points participating in the conference are not the same, all the end points participating in the conference are not the same,
the conference server may have to send audio to different the conference server may have to send audio to different
participants using different audio codecs. participants using different audio codecs.
Consequently, we can model a transcoding service as a two-party Consequently, we can model a transcoding service as a two-party
conference server that may change not only the codec in use, but also conference server that may change not only the codec in use, but also
the format of the media (e.g., audio to text). the format of the media (e.g., audio to text).
Using this model, T behaves as a B2BUA and the whole A-T-B session is Using this model, T behaves as a B2BUA (Back-to-Back User Agent) and
established as described in [draft-camarillo-sipping-transc-b2bua]. the whole A-T-B session is established as described in
Figure 2 shows the signalling relationships between the end-points [draft-ietf-sipping-transc-conf]. Figure 2 shows the signalling
and the transcoder. relationships between the end-points and the transcoder.
+-------+ +-------+
| |** | |**
| T | ** | T | **
| |\ ** | |\ **
+-------+ \\ ** +-------+ \\ **
^ * \\ ** ^ * \\ **
| * \\ ** | * \\ **
| * SIP ** | * SIP **
SIP * \\ ** SIP * \\ **
skipping to change at page 7, line 40 skipping to change at page 7, line 40
the 3pcc model. This may be an important feature for end-poing using the 3pcc model. This may be an important feature for end-poing using
low bandwidth or high-delay access links (e.g., some wireless low bandwidth or high-delay access links (e.g., some wireless
accesses). accesses).
On the other hand, this model is less flexible than the 3pcc model. On the other hand, this model is less flexible than the 3pcc model.
It is not possible to use different transcoders for different streams It is not possible to use different transcoders for different streams
or for different directions of a stream. or for different directions of a stream.
Invoking a transcoder in the middle of an ongoing session or changing Invoking a transcoder in the middle of an ongoing session or changing
from one transcoder to another requires the remote end-point to from one transcoder to another requires the remote end-point to
support the Replaces [6] extension. At present, not many user agents support the Replaces [9] extension. At present, not many user agents
support it. support it.
Simple end-points that cannot perform 3pcc and thus cannot use the Simple end-points that cannot perform 3pcc and thus cannot use the
3pcc model, of course, need to use the conference bridge model. 3pcc model, of course, need to use the conference bridge model.
4. Security Considerations 4. Security Considerations
TBD. The specifications of the 3pcc and the conferencing transcoding
models discuss security issues directly related to the implementation
of those models. Additionally, there are some considerations that
apply to transcoding in general.
In a session, a transcoder has access to at least some of the media
exchanged between the end points. In order to avoid rogue
transcoders getting access to those media, it is recommended that end
points authenticate the transcoder. TLS [1] and S/MIME [8] can be
used for this purpose.
To achieve a higher degree of privacy, end points following the 3pcc
transcoding model can use one transcoder in one direction and a
different one in the other direction. This way, no single transcoder
has access to all the media exchanged between the end points.
5. IANA Considerations 5. IANA Considerations
This document does not contain any IANA actions. This document does not contain any IANA actions.
6. Contributors 6. 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.
7. References 7. References
7.1 Normative References 7.1 Normative References
[1] Floyd, S. and L. Daigle, "IAB Architectural and Policy [1] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[2] Floyd, S. and L. Daigle, "IAB Architectural and Policy
Considerations for Open Pluggable Edge Services", RFC 3238, Considerations for Open Pluggable Edge Services", RFC 3238,
January 2002. January 2002.
[2] 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.
[3] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with [4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002. Session Description Protocol (SDP)", RFC 3264, June 2002.
[4] Charlton, N., Gasson, M., Gybels, G., Spanner, M. and A. van [5] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[6] Charlton, N., Gasson, M., Gybels, G., Spanner, M., and A. van
Wijk, "User Requirements for the Session Initiation Protocol Wijk, "User Requirements for the Session Initiation Protocol
(SIP) in Support of Deaf, Hard of Hearing and Speech-impaired (SIP) in Support of Deaf, Hard of Hearing and Speech-impaired
Individuals", RFC 3351, August 2002. Individuals", RFC 3351, August 2002.
[5] Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo, [7] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
"Best Current Practices for Third Party Call Control (3pcc) in "Best Current Practices for Third Party Call Control (3pcc) in
the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April the Session Initiation Protocol (SIP)", BCP 85, RFC 3725,
2004. April 2004.
[6] Mahy, R., Biggs, B. and R. Dean, "The Session Initiation [8] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Certificate Handling", RFC 3850,
July 2004.
[9] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
Protocol (SIP) "Replaces" Header", RFC 3891, September 2004. Protocol (SIP) "Replaces" Header", RFC 3891, September 2004.
[7] Camarillo, G., "Transcoding Services Invocation in the Session [10] Camarillo, G., "Transcoding Services Invocation in the Session
Initiation Protocol (SIP) Using Third Party Call Control Initiation Protocol (SIP) Using Third Party Call Control
(3pcc)", draft-ietf-sipping-transc-3pcc-02 (work in progress), (3pcc)", draft-ietf-sipping-transc-3pcc-02 (work in progress),
September 2004. September 2004.
[8] Camarillo, G. and A. Johnston, "Conference Establishment Using
Request-Contained Lists in the Session Initiation Protocol
(SIP)", draft-ietf-sipping-uri-list-conferencing-01 (work in
progress), October 2004.
7.2 Informational References 7.2 Informational References
[9] Handley, M. and V. Jacobson, "SDP: Session Description [11] Handley, M., "SDP: Session Description Protocol",
Protocol", RFC 2327, April 1998. draft-ietf-mmusic-sdp-new-24 (work in progress), February 2005.
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
Intellectual Property Statement Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be on the procedures with respect to rights in RFC documents can be
 End of changes. 

This html diff was produced by rfcdiff 1.23, available from http://www.levkowetz.com/ietf/tools/rfcdiff/