Internet Engineering Task Force SIP WG
Internet Draft G. Camarillo
Ericsson
draft-camarillo-sipping-transc-b2bua-00.txt
August 28, 2003
Expires: February, 2004
The Session Initiation Protocol Conference Bridge Transcoding Model
STATUS OF THIS MEMO
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Abstract
This document describes how to invoke transcoding services using the
conference bridge model. This way of invocation meets the
requirements for SIP regarding transcoding services invocation to
support deaf, hard of hearing and speech-impaired individuals.
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Table of Contents
1 Introduction ........................................ 3
2 Caller's Invocation ................................. 3
3 Callee's Invocation ................................. 3
4 Security Considerations ............................. 4
5 Contributors ........................................ 5
6 OPEN ISSUES ......................................... 5
7 Authors' Addresses .................................. 5
8 Bibliography ........................................ 5
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1 Introduction
The framework for transcoding with SIP (draft-camarillo-sipping-
transc-framework) describes how two SIP UAs can discover
imcompatibilities that prevent them from establishing a session
(e.g., lack of support for a common codec or for a common media
type). When such incompatibilities are found, the UAs need to invoke
transcoding services to successfully establish the session. Using the
conference bridge model is one way to perform such invocation.
In the conference bridge model for transcoding invocation, a
transcoding server that provides a particular transcoding service
(e.g., speech-to-text) behaves as a B2BUA between both UAs and is
identified by a URI.
2 Caller's Invocation
Figure 1 shows the message flow for the caller's invocation of a
transcoder T. The caller (A) sends an INVITE (1) to the URI of the
transcoder (T) to establish the session A-T.
Once this session is established, A sends a REFER (4) to T intructing
T to INVITE B. The session description that T will use in this INVITE
to B (6) depends on the service provided by T. A speech-to-text
text-to-speech transcoder, for instance, would generate a session
description with an audio stream or with a text stream depending on
the incoming session description in the first INVITE (1).
This INVITE from T to B establishes the session T-B. During session
establishment, following standard REFER procedures, A receives
NOTIFIES from T informing A about the progress of the establishment
of this session (T-B).
3 Callee's Invocation
If a UA receives an INVITE with an offer that is not acceptable it
can only invoke a transcoder if the caller supports the Replaces [1]
extension. This support is indicated by the Supported header field in
the INVITE.
If the caller (A) does not support Replaces, the callee (B) can
always reject the session and attempt to establish a new session with
A following the procedures in Section 2. This way, B would act as a
caller and, consequently, it would follow the procedures for caller's
invocation of transcoders.
Assuming that the caller (A) supports Replaces, the callee (B)
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A T B
| | |
|------(1) INVITE SDP A----->| |
| | |
|<----(2) 200 OK SDP TA------| |
| | |
|----------(3) ACK---------->| |
| | |
| | |
|---------(4) REFER--------->| |
| | |
|<--------(5) 200 OK---------| |
| | |
| |-----(6) INVITE SDP TB----->|
| | |
| |<-----(7) 200 OK SDP B------|
| | |
| |----------(8) ACK---------->|
| | |
|<--------(9) NOTIFY---------| |
| | |
|---------(10) 200 OK------->| |
| | |
| ************************** | ************************** |
|* MEDIA *|* MEDIA *|
| ************************** | ************************** |
|
Figure 1: Caller's Invocation of a Transcoder
follows the steps shown in Figure 2 to invoke a transcoder. B sends
an INVITE to the transcoder (T). Once the session B-T is established,
B sends a REFER to T instructing T to send an INVITE to A with a
Replaces header field. When A receives this INVITE, it will replace
the original dialog (1) with this new dialog.
4 Security Considerations
This document describes how to use the REFER method invoke
transcoding services. It does not introduce new security
considerations besides the ones discussed in [2].
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5 Contributors
This document is the result of discussions amongst the conferencing
design team. The members of this team include Eric Burger, Henning
Schulzrinne and Arnoud van Wijk.
6 OPEN ISSUES
Now we have A send an INVITE and then a REFER to T. If we combined
those two transactions in one somehow, we would be saving one RTT and
some traffic over A's access.
It is an open issue to decide whether we want to optimize it this way
or not. In case we decide to do it, this is a similar problem to the
1:N MESSAGE, where A asks a B2BUA in the network to send a MESSAGE to
a set of N receivers.
It is also similar to presence lists, where a B2BUA in the network
sends a bunch of SUBSCRIBEs on behalf of the UA. There, a URI at the
B2BUA identifies a set of destinations. In the transcoding case, the
URI at the B2BUA would identify only one destination, and the binding
between that URI and the URI of the destination would need to be
created on the fly (the binding for presence lists is created in
advance).
Session policies is something we may want to look at as well,
although, in principle, it deals with proxies rather than with
B2BUAs.
In SIP, the Route header field is used to traverse proxies, but is
seems that using it for traversing B2BUAs would be stretching its
semantics too much.
7 Authors' Addresses
Gonzalo Camarillo
Ericsson
Advanced Signalling Research Lab.
FIN-02420 Jorvas
Finland
electronic mail: Gonzalo.Camarillo@ericsson.com
8 Bibliography
[1] B. Biggs, R. W. Dean, and R. Mahy, "The session inititation
protocol (SIP) Engineering Task Force, Aug. 2003. Work in progress.
[2] R. Sparks, "The SIP refer method," internet draft, Internet
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A T B
| | |
|-------------------(1) INVITE SDP A--------------------->|
| | |
| |<-----(2) INVITE SDP B------|
| | |
| |------(3) 200 OK SDP TB---->|
| | |
| |<--------- (4) ACK ---------|
| | |
| | |
| |<--------(5) REFER----------|
| | |
| |---------(6) 200 OK-------->|
| | |
|<-----(7) INVITE SDP TA-----| |
| | |
|------(8) 200 OK SDP A----->| |
| | |
|<----------(9) ACK----------| |
| | |
| |---------(10) NOTIFY------->|
| | |
| |<--------(11) 200 OK--------|
| | |
|---------------------(12) CANCEL------------------------>|
| | |
|<--------------------(13) 200 OK-------------------------|
| | |
|<-------------(14) 487 Request Terminated----------------|
| | |
|-----------------------(15) ACK------------------------->|
| | |
| ************************** | ************************** |
|* MEDIA *|* MEDIA *|
| ************************** | ************************** |
| | |
Figure 2: Caller's Invocation of a Transcoder
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