draft-ietf-sipping-transc-framework-04.txt   draft-ietf-sipping-transc-framework-05.txt 
SIPPING Working Group G. Camarillo SIPPING Working Group G. Camarillo
Internet-Draft Ericsson Internet-Draft Ericsson
Expires: November 19, 2006 May 18, 2006 Expires: June 3, 2007 November 30, 2006
Framework for Transcoding with the Session Initiation Protocol (SIP) Framework for Transcoding with the Session Initiation Protocol (SIP)
draft-ietf-sipping-transc-framework-04.txt draft-ietf-sipping-transc-framework-05.txt
Status of this Memo Status of this Memo
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This Internet-Draft will expire on November 19, 2006. This Internet-Draft will expire on June 3, 2007.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2006). Copyright (C) The Internet Society (2006).
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 for transcoding services framework includes how to discover the need for 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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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 . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 8 6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . . 8 7.1. Normative References . . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . . 9 7.2. Informative References . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 11 Intellectual Property and Copyright Statements . . . . . . . . . . 12
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 [12]), 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-
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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.
In the absence of presence information, routing logic that involves
parallel forking to several user agents may make it difficult (or
impossible) for the caller to know which user agent will answer the
next call attempt. For example, a call attempt may reach the user's
voice mail while the next one may reach a SIP phone where the user is
available. If both terminating user agents have different
capabilities, the caller cannot know, even after the first call
attempt, whether or not transcoding will be necessary for the
session. This is a well-known SIP problem that is referred to as
HERFP (Heterogeneous Error Response Forking Problem). Resolving
HERFP is outside the scope of this document.
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.
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
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[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., Burger, E., Schulzrinne, H., and A. van Wijk, [10] 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.
[11] Camarillo, G., "The Session Initiation Protocol (SIP) [11] Camarillo, G., "The Session Initiation Protocol (SIP)
Conference Bridge Transcoding Model", Conference Bridge Transcoding Model",
draft-ietf-sipping-transc-conf-02 (work in progress), draft-ietf-sipping-transc-conf-03 (work in progress),
January 2006. June 2006.
7.2. Informative References 7.2. Informative References
[12] Handley, M., "SDP: Session Description Protocol", [12] Handley, M., "SDP: Session Description Protocol",
draft-ietf-mmusic-sdp-new-26 (work in progress), January 2006. draft-ietf-mmusic-sdp-new-26 (work in progress), January 2006.
Author's Address Author's Address
Gonzalo Camarillo Gonzalo Camarillo
Ericsson Ericsson
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