draft-ietf-sipping-transc-framework-02.txt   draft-ietf-sipping-transc-framework-03.txt 
SIPPING Working Group G. Camarillo SIPPING Working Group G. Camarillo
Internet-Draft Ericsson Internet-Draft Ericsson
Expires: December 3, 2005 June 1, 2005 Expires: May 30, 2006 November 26, 2005
Framework for Transcoding with the Session Initiation Protocol (SIP) Framework for Transcoding with the Session Initiation Protocol (SIP)
draft-ietf-sipping-transc-framework-02.txt draft-ietf-sipping-transc-framework-03.txt
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Copyright Notice Copyright Notice
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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
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conference bridge model and the third party call control model. Both conference bridge model and the third party call control model. Both
models meet the requirements for SIP regarding transcoding services models meet the requirements for SIP regarding transcoding services
invocation to support deaf, hard of hearing, and speech-impaired invocation to support deaf, hard of hearing, and speech-impaired
individuals. individuals.
Table of Contents Table of Contents
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 . . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . . . . 11
1. Introduction 1. Introduction
Two user agents involved in a SIP [3] 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 [11]), incompatibilities can be found description format (e.g., SDP [12]), incompatibilities can be found
at the user agent level and at the user level. At the user agent at the user agent level and at the user level. At the user agent
level, both terminals may not support any common codec or may not level, both terminals may not support any common codec or may not
support common media types (e.g., a text-only terminal and an audio- support common media types (e.g., a text-only terminal and an audio-
only 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
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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 [6] 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 [2], 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 [4] 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 [4]. 3264 [4].
Presence documents are typically received in a NOTIFY [5] request as Presence documents are typically received in a NOTIFY [5] request as
a 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.
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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.
An example of the situation above is a call between two GSM phones An example of the situation above is a call between two GSM phones
(without using transcoding-free operation). Both phones use a GSM (without using transcoding-free operation). Both phones use a GSM
codec, but the speech is converted from GSM to PCM by the codec, but the speech is converted from GSM to PCM by the
originating MSC and from PCM back to GSM by the terminating MSC. originating MSC and from PCM back to GSM by the terminating MSC.
Note that transcoding services can be symmetric (e.g., speech-to-text Note that transcoding services can be symmetric (e.g., speech-to-text
plus text-to-speech) or asymmetric (e.g., a one-way speech-to-text plus text-to-speech) or asymmetric (e.g., a one-way speech-to-text
transcoding for a hearing impaired user that can talk). transcoding for a hearing-impaired user that can talk).
3. Transcoding Services Invocation 3. Transcoding Services Invocation
Once the need for transcoding for a particular session has been Once the need for transcoding for a particular session has been
identified as described in Section 2, one of the user agents needs to identified as described in Section 2, one of the user agents needs to
invoke transcoding services. invoke transcoding services.
As we said earlier, transcoder location is outside the scope of this As stated 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 [7], 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 [10], 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 |**
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quite simple. This is useful when session changes occur (e.g., an quite simple. This is useful when session changes occur (e.g., an
audio session is upgraded to an audio/video session) and the end- audio session is upgraded to an audio/video session) and the end-
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
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 (Back-to-Back User Agent) and Using this model, T behaves as a B2BUA (Back-to-Back User Agent) and
the whole A-T-B session is established as described in the whole A-T-B session is established as described in [11].
[draft-ietf-sipping-transc-conf]. Figure 2 shows the signalling Figure 2 shows the signalling relationships between the end-points
relationships between the end-points and the transcoder. and the transcoder.
+-------+ +-------+
| |** | |**
| T | ** | T | **
| |\ ** | |\ **
+-------+ \\ ** +-------+ \\ **
^ * \\ ** ^ * \\ **
| * \\ ** | * \\ **
| * SIP ** | * SIP **
SIP * \\ ** SIP * \\ **
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| | | | | | | |
+-------+ +-------+ +-------+ +-------+
<-SIP-> Signalling <-SIP-> Signalling
******* Media ******* Media
Figure 2: Conference bridge model Figure 2: Conference bridge model
In the conferencing bridge model, the end-point invoking the In the conferencing bridge model, the end-point invoking the
transcoder is generally involved in less signalling exchanges than in transcoder is generally involved in less signalling exchanges than in
the 3pcc model. This may be an important feature for end-poing using the 3pcc model. This may be an important feature for end-points
low bandwidth or high-delay access links (e.g., some wireless using 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 [9] extension. At present, not many user agents support the Replaces [9] extension. At present, not many user agents
support it. support it.
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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] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", [1] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999. RFC 2246, January 1999.
[2] Floyd, S. and L. Daigle, "IAB Architectural and Policy [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.
[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:
skipping to change at page 9, line 15 skipping to change at page 9, line 20
the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, the Session Initiation Protocol (SIP)", BCP 85, RFC 3725,
April 2004. April 2004.
[8] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions [8] 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.
[9] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation [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.
[10] Camarillo, G., "Transcoding Services Invocation in the Session [10] Camarillo, G., Burger, E., Schulzrinne, H., and A. van Wijk,
Initiation Protocol (SIP) Using Third Party Call Control "Transcoding Services Invocation in the Session Initiation
(3pcc)", draft-ietf-sipping-transc-3pcc-02 (work in progress), Protocol (SIP) Using Third Party Call Control (3pcc)",
September 2004. RFC 4117, June 2005.
7.2 Informational References [11] Camarillo, G., "The Session Initiation Protocol (SIP)
Conference Bridge Transcoding Model",
draft-ietf-sipping-transc-conf-00 (work in progress),
June 2005.
[11] Handley, M., "SDP: Session Description Protocol", 7.2. Informational References
draft-ietf-mmusic-sdp-new-24 (work in progress), February 2005.
[12] Handley, M., "SDP: Session Description Protocol",
draft-ietf-mmusic-sdp-new-25 (work in progress), July 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
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