Network Working Group G. Camarillo
Request for Comments: 4117 Ericsson
Category: Informational E. Burger
Brooktrout
H. Schulzrinne
Columbia University
A. van Wijk
Viataal
June 2005
Transcoding Services Invocation in
the Session Initiation Protocol (SIP)
Using Third Party Call Control (3pcc)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes how to invoke transcoding services using
Session Initiation Protocol (SIP) and third party call control. This
way of invocation meets the requirements for SIP regarding
transcoding services invocation to support deaf, hard of hearing and
speech-impaired individuals.
Table of Contents
1. Introduction ....................................................2
2. General Overview ................................................2
3. Third Party Call Control Flows ..................................2
3.1. Terminology ................................................3
3.2. Callee's Invocation ........................................3
3.3. Caller's Invocation ........................................8
3.4. Receiving the Original Stream ..............................8
3.5. Transcoding Services in Parallel ..........................10
3.6. Multiple Transcoding Services in Series ...................14
4. Security Considerations ........................................16
5. Normative References ...........................................17
6. Informative References .........................................17
Camarillo, et al. Informational [Page 1]RFC 4117 3pcc Transcoding in SIP June 20051. Introduction
The framework for transcoding with SIP [4] describes how two SIP [1]
UAs (User Agents) can discover incompatibilities that prevent them
from establishing a session (e.g., lack of support for a common codec
or common media type). When such incompatibilities are found, the
UAs need to invoke transcoding services to successfully establish the
session. 3pcc (third party call control) [2] is one way to perform
such invocation.
2. General Overview
In the 3pcc model for transcoding invocation, a transcoding server
that provides a particular transcoding service (e.g., speech-to-text)
is identified by a URI. A UA that wishes to invoke that service
sends an INVITE request to that URI establishing a number of media
streams. The way the transcoder manipulates and manages the contents
of those media streams (e.g., the text received over the text stream
is transformed into speech and sent over the audio stream) is service
specific.
All the call flows in this document use SDP. The same call flows
could be used with another session description protocol that provides
similar session description capabilities.
3. Third Party Call Control Flows
Given two UAs (A and B) and a transcoding server (T), the invocation
of a transcoding service consists of establishing two sessions; A-T
and T-B. How these sessions are established depends on which party,
the caller (A) or the callee (B), invokes the transcoding services.
Section 3.2 deals with callee invocation and Section 3.3 deals with
caller invocation.
In all our 3pcc flows we have followed the general principle that a
200 (OK) response from the transcoding service has to be received
before contacting the callee. This tries to ensure that the
transcoding service will be available when the callee accepts the
session.
Still, the transcoding service does not know the exact type of
transcoding it will be performing until the callee accepts the
session. So, there is always the chance of failing to provide
transcoding services after the callee has accepted the session. A
system with more stringent requirements could use preconditions to
avoid this situation. When preconditions are used, the callee is not
alerted until everything is ready for the session.
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