Internet Engineering Task Force SIP WG
Internet Draft G. Camarillo
Ericsson
H. Schulzrinne
Columbia University
E. Burger
SnowShore Networks
draft-camarillo-mmusic-source-sink-00.txt
September 10, 2002
Expires: March 2003
The source and sink attributes for the Session Description Protocol
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
To view the list Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html.
Abstract
This document defines two media level SDP attributes, namely source
and sink. They are intended to be used to invoke services that
involve media manipulation, such as transcoding services.
G. Camarillo et. al. [Page 1]
Internet Draft SIP September 10, 2002
Table of Contents
1 Introduction ........................................ 3
1.1 Terminology ......................................... 3
2 Applicability ....................................... 3
3 Syntax of source and sink ........................... 3
4 SDP example ......................................... 4
5 Use of Source and Sink with SIP ..................... 4
6 Open Issue .......................................... 5
7 IANA considerations ................................. 5
8 Acknowledgements .................................... 5
9 Authors' Addresses .................................. 5
10 Normative References ................................ 5
11 Informative References .............................. 6
G. Camarillo et. al. [Page 2]
Internet Draft SIP September 10, 2002
1 Introduction
Servers performing media manipulations, such as transcoding or
mixing, take the contents of one or several media streams as input
and send their output over another media stream. A client requesting
this type of service from a server needs to identify which media
streams are to be used as input and which ones will be used to send
the output of the media manipulation process. This document defines
two SDP media level attributes, namely source and sink, that can be
used to explicitly convey this information in an SDP session
description.
1.1 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and
indicate requirement levels for compliant SIP implementations.
2 Applicability
A server that provides simple media manipulation services between a
single unidirectional input (recvonly) stream and a single
unidirectional output (sendonly) stream, such as a text-to-speech
server, does not need to specify source and sink attributes in the
SDP. However, a server that needs to correlate more than the simple
media manipulation service needs a mechanism to specify which media
descriptions refer to which directionality of the input streams.
Thus, servers that use SDP [2] to provide more complex services that
involve more media streams (like some of the ones described by [5])
SHOULD make use of the source and sink attributes.
The source and sink attributes MUST NOT be used to perform media
alignment between SIP [3] user agents. The nth matching rules defined
by the offer/answer model [4] must be used regardless of the presence
or absence of the sink and source attributes.
3 Syntax of source and sink
We define the following media level SDP attributes:
source-attribute = "a=source:" identification-tag
sink-attribute = "a=sink:" identification-tag
identification-tag = token
G. Camarillo et. al. [Page 3]
Internet Draft SIP September 10, 2002
An SDP session description that contains a media stream with a
particular identification tag in a source attribute MUST have the
same identification tag in, at least, one sink attribute. An SDP
session description that contains a media stream with a particular
identification tag in a sink attribute MUST have the same
identification tag in, at least, one source attribute.
If an entity receives a session description that breaks the rules
stated above, it MUST act as if it had received a malformed session
description.
4 SDP example
The SDP session description below sent to a server indicates that
incoming audio from the first stream has to be sent over the second
audio stream and over the text stream. Incoming text has to be sent
over the first audio stream (but not over the second one). The exact
media manipulations to be applied are typically identified by the URI
that identifies the service [6] (e.g., sip:texttospeech@domain.com).
m=audio 40000 RTP/AVP 0
c=IN IP4 B.domain.com
a=source:1
a=sink:2
m=audio 20000 RTP/AVP 0
c=IN IP4 A.domain.com
a=recvonly
a=sink:1
m=text 20002 RTP/AVP t140
c=IN IP4 A.domain.com
a=source:2
a=sink:1
5 Use of Source and Sink with SIP
A user agent that wishes to use the source and sink attributes adds
them to an offer. The answerer SHOULD copy the same source and sink
attributes in its answer for all the streams that were accepted
(i.e., their port number is different than zero).
An answerer that understand the source and sink attributes MUST NOT
add or remove any of them from a stream that was accepted. The
offerer knows whether the answerer understands these attributes
because the answer will contain source and sink attributes. If the
G. Camarillo et. al. [Page 4]
Internet Draft SIP September 10, 2002
answerer does not understand them, the answer will not contain source
and sink attributes.
6 Open Issue
Right now, an offerer discovers whether or not the answerer supports
source and sink by checking if there are source and sink attributes
in the answer. If the answerer did not support source and sink, the
offerer can send a BYE right away.
If such a behavior is not acceptable, we could define a SIP option
tag to be used in the Require header field associated with source and
sink.
7 IANA considerations
This document defines two media level SDP attributes: "source" and
"sink".
8 Acknowledgements
Jeff Van Dyke provided useful comments on this document.
9 Authors' Addresses
Gonzalo Camarillo
Ericsson
Advanced Signalling Research Lab.
FIN-02420 Jorvas
Finland
electronic mail: Gonzalo.Camarillo@ericsson.com
Henning Schulzrinne
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue
New York, NY 10027
USA
electronic mail: schulzrinne@cs.columbia.edu
Eric W. Burger
SnowShore Networks, Inc.
Chelmsford, MA
USA
electronic mail: eburger@snowshore.com
10 Normative References
G. Camarillo et. al. [Page 5]
Internet Draft SIP September 10, 2002
[1] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[2] M. Handley and V. Jacobson, "SDP: session description protocol,"
RFC 2327, Internet Engineering Task Force, Apr. 1998.
[3] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
initiation protocol," RFC 3261, Internet Engineering Task Force, June
2002.
[4] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
session description protocol (SDP)," RFC 3264, Internet Engineering
Task Force, June 2002.
11 Informative References
[5] N. Charlton et al. , "User requirements for the session
initiation protocol (SIP) in support of deaf, hard of hearing and
speech-impaired individuals," Internet Draft, Internet Engineering
Task Force, Apr. 2002. Work in progress.
[6] J. V. Dyke, E. Burger, A. Spitzer, and W. O'Connor, "Basic
network media services with SIP," Internet Draft, Internet
Engineering Task Force, July 2002. Work in progress.
Full Copyright Statement
Copyright (c) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
G. Camarillo et. al. [Page 6]
Internet Draft SIP September 10, 2002
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
G. Camarillo et. al. [Page 7]