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Internet Engineering Task Force                                   SIP WG
Internet Draft                                              G. Camarillo
                                                                Ericsson
                                                          H. Schulzrinne
                                                     Columbia University
                                                               E. Burger
                                                      SnowShore Networks
draft-camarillo-mmusic-source-sink-01.txt
August 28, 2003
Expires: February 2004


  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.

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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.










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                           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          IANA Considerations .................................    5
   7          Security Considerations .............................    5
   8          Acknowledgements ....................................    5
   9          Authors' Addresses ..................................    5
   10         Normative References ................................    5
   11         Informative References ..............................    6
































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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




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   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 (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



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   answerer does not understand them, the answer will not contain source
   and sink attributes.

6 IANA Considerations

   This document defines two media level SDP attributes: "source" and
   "sink". They should be registered in SDP parameters registry.

   http://www.iana.org/assignments/sdp-parameters

7 Security Considerations

   An attacker adding, removing or modifying source or sink a= lines
   could change the expected behavior from a media manipulation service.
   It is thus STRONGLY RECOMMENDED that integrity protection be applied
   to the SDP session descriptions. For session descriptions carried in
   SIP [3], S/MIME is the natural choice to provide such end-to-end
   integrity protection, as described in RFC 3261. Other applications
   MAY use a different form of integrity protection.

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



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   [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. R. 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, M. Gasson, G. Gybels, M. Spanner, and A. van Wijk,
   "User requirements for the session initiation protocol (SIP) in
   support of deaf, hard of hearing and speech-impaired individuals,"
   RFC 3351, Internet Engineering Task Force, Aug. 2002.



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