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
1. 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.
3.1. Terminology
All the flows in this document follow the naming convention below:
SDP A: A session description generated by A. It contains, among
other things, the transport address/es (IP address and
port number) where A wants to receive media for each
particular stream.
SDP B: A session description generated by B. It contains, among
other things, the transport address/es where B wants to
receive media for each particular stream.
SDP A+B: A session description that contains, among other things,
the transport address/es where A wants to receive media
and the transport address/es where B wants to receive
media.
SDP TA: A session description generated by T and intended for A.
It contains, among other things, the transport address/es
where T wants to receive media from A.
SDP TB: A session description generated by T and intended for B.
It contains, among other things, the transport address/es
where T wants to receive media from B.
SDP TA+TB: A session description generated by T that contains, among
other things, the transport address/es where T wants to
receive media from A and the transport address/es where T
wants to receive media from B.
3.2. Callee’s Invocation
In this scenario, B receives an INVITE from A, and B decides to
introduce T in the session. Figure 1 shows the call flow for this
scenario.
In Figure 1, A can both hear and speak, and B is a deaf user with a
speech impairment. A proposes to establish a session that consists
of an audio stream (1). B wants to send and receive only text, so it
invokes a transcoding service T that will perform both speech-to-text
and text-to-speech conversions (2). The session descriptions of
Figure 1 are partially shown below.
A T B
| | |
|--------------------(1) INVITE SDP A-------------------->|
| | |
| |<---(2) INVITE SDP A+B------|
| | |
| |---(3) 200 OK SDP TA+TB---->|
| | |
| |<---------(4) ACK-----------|
| | |
|<-------------------(5) 200 OK SDP TA--------------------|
| | |
|------------------------(6) ACK------------------------->|
| | |
| ************************** | ************************** |
|* MEDIA *|* MEDIA *|
| ************************** | ************************** |
| | |
Figure 1: Callee’s Invocation of a Transcoding Service
(1) INVITE SDP A
m=audio 20000 RTP/AVP 0
c=IN IP4 A.example.com
(2) INVITE SDP A+B
m=audio 20000 RTP/AVP 0
c=IN IP4 A.example.com
m=text 40000 RTP/AVP 96
c=IN IP4 B.example.com
a=rtpmap:96 t140/1000
(3) 200 OK SDP TA+TB
m=audio 30000 RTP/AVP 0
c=IN IP4 T.example.com
m=text 30002 RTP/AVP 96
c=IN IP4 T.example.com
a=rtpmap:96 t140/1000
(5) 200 OK SDP TA
m=audio 30000 RTP/AVP 0
c=IN IP4 T.example.com
Four media streams (i.e., two bi-directional streams) have been
established at this point:
1. Audio from A to T.example.com:30000
2. Text from T to B.example.com:40000
3. Text from B to T.example.com:30002
4. Audio from T to A.example.com:20000
When either A or B decides to terminate the session, it sends a BYE
indicating that the session is over.
If the first INVITE (1) received by B is empty (no session
description), the call flow is slightly different. Figure 2 shows
the messages involved.
B may have different reasons for invoking T before knowing A’s
session description. B may want to hide its lack of native
capabilities, and therefore wants to return a session description
with all the codecs that B supports, plus all the codecs that T
supports. Or T may provide recording services (besides transcoding),
and B wants T to record the conversation, regardless of whether
transcoding is needed.
This scenario (Figure 2) is a bit more complex than the previous one.
In INVITE (2), B still does not have SDP A, so it cannot provide T
with that information. When B finally receives SDP A in (6), it has
to send it to T. B sends an empty INVITE to T (7) and gets a 200 OK
with SDP TA+TB (8). In general, this SDP TA+TB can be different than
the one sent in (3). That is why B needs to send the updated SDP TA
to A in (9). A then sends a possibly updated SDP A (10) and B sends
it to T in (12). On the other hand, if T happens to return the same
SDP TA+TB in (8) as in (3), B can skip messages (9), (10), and (11).
So, implementors of transcoding services are encouraged to return the
same session description in (8) as in (3) in this type of scenario.
The session descriptions of this flow are shown below:
A T B
| | |
|----------------------(1) INVITE------------------------>|
| | |
| |<-----(2) INVITE SDP B------|
| | |
| |---(3) 200 OK SDP TA+TB---->|
| | |
| |<---------(4) ACK-----------|
| | |
|<-------------------(5) 200 OK SDP TA--------------------|
| | |
|-----------------------(6) ACK SDP A-------------------->|
| | |
| |<-------(7) INVITE----------|
| | |
| |---(8) 200 OK SDP TA+TB---->|
| | |
|<-----------------(9) INVITE SDP TA----------------------|
| | |
|------------------(10) 200 OK SDP A--------------------->|
| | |
|<-----------------------(11) ACK-------------------------|
| | |
| |<-----(12) ACK SDP A+B------|
| | |
| ************************** | ************************** |
|* MEDIA *|* MEDIA *|
| ************************** | ************************** |
Figure 2: Callee’s invocation after initial INVITE without SDP
(2) INVITE SDP A+B
m=audio 20000 RTP/AVP 0
c=IN IP4 0.0.0.0
m=text 40000 RTP/AVP 96
c=IN IP4 B.example.com
a=rtpmap:96 t140/1000
(3) 200 OK SDP TA+TB
m=audio 30000 RTP/AVP 0
c=IN IP4 T.example.com
m=text 30002 RTP/AVP 96
c=IN IP4 T.example.com
a=rtpmap:96 t140/1000
(5) 200 OK SDP TA
m=audio 30000 RTP/AVP 0
c=IN IP4 T.example.com
(6) ACK SDP A
m=audio 20000 RTP/AVP 0
c=IN IP4 A.example.com
(8) 200 OK SDP TA+TB
m=audio 30004 RTP/AVP 0
c=IN IP4 T.example.com
m=text 30006 RTP/AVP 96
c=IN IP4 T.example.com
a=rtpmap:96 t140/1000
(9) INVITE SDP TA
m=audio 30004 RTP/AVP 0
c=IN IP4 T.example.com
(10) 200 OK SDP A
m=audio 20002 RTP/AVP 0
c=IN IP4 A.example.com
(12) ACK SDP A+B
m=audio 20002 RTP/AVP 0
c=IN IP4 A.example.com
m=text 40000 RTP/AVP 96
c=IN IP4 B.example.com
a=rtpmap:96 t140/1000
Four media streams (i.e., two bi-directional streams) have been
established at this point:
1. Audio from A to T.example.com:30004
2. Text from T to B.example.com:40000
3. Text from B to T.example.com:30006
4. Audio from T to A.example.com:20002
3.3. Caller’s Invocation
In this scenario, A wishes to establish a session with B using a
transcoding service. A uses 3pcc to set up the session between T and
B. The call flow we provide here is slightly different than the ones
in [2]. In [2], the controller establishes a session between two
user agents, which are the ones deciding the characteristics of the
streams. Here, A wants to establish a session between T and B, but A
wants to decide how many and which types of streams are established.
That is why A sends its session description in the first INVITE (1)
to T, as opposed to the media-less initial INVITE recommended by [2].
Figure 3 shows the call flow for this scenario.
We do not include the session descriptions of this flow, since they
are very similar to those in Figure 2. In this flow, if T returns
the same SDP TA+TB in (8) as in (2), messages (9), (10), and (11) can
be skipped.
3.4. Receiving the Original Stream
Sometimes, as pointed out in the requirements for SIP in support of
deaf, hard of hearing, and speech-impaired individuals [5], a user
wants to receive both the original stream (e.g., audio) and the
transcoded stream (e.g., the output of the speech-to-text
conversion). There are various possible solutions for this problem.
One solution consists of using the SDP group attribute with Flow
Identification (FID) semantics [3]. FID allows requesting that a
stream is sent to two different transport addresses in parallel, as
shown below:
A T B
| | |
|-------(1) INVITE SDP A---->| |
| | |
|<----(2) 200 OK SDP TA+TB---| |
| | |
|----------(3) ACK---------->| |
| | |
|--------------------(4) INVITE SDP TA------------------->|
| | |
|<--------------------(5) 200 OK SDP B--------------------|
| | |
|-------------------------(6) ACK------------------------>|
| | |
|--------(7) INVITE--------->| |
| | |
|<---(8) 200 OK SDP TA+TB --| |
| | |
|--------------------(9) INVITE SDP TA------------------->|
| | |
|<-------------------(10) 200 OK SDP B--------------------|
| | |
|-------------------------(11) ACK----------------------->|
| | |
|------(12) ACK SDP A+B----->| |
| | |
| ************************** | ************************** |
|* MEDIA *|* MEDIA *|
| ************************** | ************************** |
| | |