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MMUSIC                                                          O. Levin
Internet-Draft                                     Microsoft Corporation
Expires: December 28, 2004                                  G. Camarillo
                                                                Ericsson
                                                           June 29, 2004


         The SDP (Session Description Protocol) Label Attribute
                 draft-levin-mmmusic-sdp-media-label-00

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This document defines a new Session Description Protocol (SDP)
   media-level attribute: "label".  The "label" attribute carries a
   pointer to an application layer media stream identifier in the
   context of an arbitrary network application that uses SDP.  The
   sender of the SDP document can attach the "label" attribute to a
   particular media stream or media streams.  The application receiving
   the SDP document can then associate the particular media stream with
   its application semantics or role.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  The Label Attribute  . . . . . . . . . . . . . . . . . . . . .  4
   4.  The Label Attribute in the Offer/Answer Model  . . . . . . . .  4
   5.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  5
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  5
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   8.1   Normative References . . . . . . . . . . . . . . . . . . . .  5
   8.2   Informative References . . . . . . . . . . . . . . . . . . .  6
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  6
       Intellectual Property and Copyright Statements . . . . . . . .  7





































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1.  Introduction

   SDP is being used by a variety of distributed over the network
   applications.  These applications deal with multiple SDP sessions
   serving multiple users or services in the context of a single
   application instance.  As such, applications of this kind have their
   means to identify a particular media stream across multiple SDP
   sessions.

   XCON framework [6] is an example of a centralized conference
   architecture that uses SDP according to the Offer/Answer mechanism
   defined in [3] to establish media streams with each of the conference
   participants.  Additionally, XCON defines the means to uniquely
   identify a media stream in terms of its role in a conference
   regardless its media type, transport protocol, and media format.  It
   is necessary to convey a pointer to this application layer identifier
   in SDP.  As a result, the application can choose from the media
   streams offered in the SDP based not only on their media
   characteristics, but also according to their roles in the
   application.

   This specification defines the SDP [2] "label" media-level attribute,
   which carries the pointer to the application layer media stream
   identifier in the SDP document across the network.

   Note that the "i" SDP attribute, defined in RFC 2327 [2], can be used
   to label media streams as well.  Nevertheless, the audiences for the
   "i" and the "label" attributes are different.  While "i" values are
   presented to the users as is, "label" values are processed by
   automata.

   Note that the "mid" SDP attribute, defined in RFC 3388 [4], can be
   used to identify media streams as well.  Nevertheless, the scopes of
   the "mid" and the "label" attributes are different.  While "mid"
   values are meaningful in the context of a single SDP session, "label"
   values are meaningful in the context of an application (e.g., a
   multiparty application).

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [1] and indicate requirement levels for
   compliant implementations.






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3.  The Label Attribute

   This specification defines a new media-level value attribute:
   "label".  Its formatting in SDP is described by the following BNF:


       label-attribute    = "a=label:" pointer
       pointer            = token

   The semantics of the "label" attribute MUST be defined by
   applications that use SDP with "label".  This value is the
   application layer identifier for a media stream regardless of its
   media type, transport protocol, or media format.

4.  The Label Attribute in the Offer/Answer Model

   This specification does not define means to discover whether or not
   the remote endpoint understands the "label" attribute.  We have
   chosen not to provide such a mechanism within this specification
   because "label" values are only informative at the Offer/Answer model
   level.

   To the Offer/Answer mechanism, it means that the fact that an offer
   does not contain label attributes does not imply that the answer
   should not have them.  It also means that the fact that an offer
   contains label attributes does not imply that the answer should have
   them too.

   In addition to the basic Offer/Answer rule above, applications that
   use "label" as a pointer to media streams MUST define both the
   "label" application semantics and its usage constraints.  For
   example, the specifications of such applications MAY mandate support
   for "label".  In this case, the application MUST define means for
   negotiation of the "label" attribute support as a part of the
   application.

5.  Example

   The following is an example of an SDP session description that uses
   the "label" attribute:


         v=0
         o=bob 280744730 28977631 IN IP4 host.example.com
         s=
         c=IN IP4 192.0.2.2
         t=0 0
         m=audio 6886 RTP/AVP 0



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         a=label:1
         m=audio 22334 RTP/AVP 0
         a=label:2


6.  IANA Considerations

   Contact name:          Orit Levin oritl@microsoft.com.

   Attribute name:        "label".

   Type of attribute      Media level.

   Subject to charset:    Not.

   Purpose of attribute:  "Label" attribute associates the media stream
      with specific application semantics.  During the SDP Offer-Answer
      mechanism, it allows application to make smarter choices
      especially among m-lines of the same media type.  In a context of
      a multiparty application, it allows the application to pick out an
      offered media stream based on its role in the application as the
      only available meaningful identifier beyond a single SDP session
      (i.e., across the application participants).

   Allowed attribute values:  Any octet string.

7.  Security Considerations

   An attacker may attempt to add, modify, or remove "label" attributes
   from a session description.  This could result in an application
   behaving in a non-desirable way.  So, it is STRONGLY RECOMMENDED that
   integrity protection be applied to the SDP session descriptions.  For
   session descriptions carried in SIP [5], S/MIME is the natural choice
   to provide such end-to-end integrity protection, as described in RFC
   3261 [5].  Other applications MAY use a different form of integrity
   protection.

8.  References

8.1  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

   [3]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with



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        Session Description Protocol (SDP)", RFC 3264, June 2002.

   [4]  Camarillo, G., Eriksson, G., Holler, J. and H. Schulzrinne,
        "Grouping of Media Lines in the Session Description Protocol
        (SDP)", RFC 3388, December 2002.

8.2  Informative References

   [5]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
        Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
        Session Initiation Protocol", RFC 3261, June 2002.

   [6]  Koskelainen, P. and H. Khartabil, "Requirements for Conference
        Policy Control Protocol", draft-ietf-xcon-cpcp-reqs-03 (work in
        progress), April 2004.


Authors' Addresses

   Orit Levin
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   USA

   EMail: oritl@microsoft.com


   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   EMail: Gonzalo.Camarillo@ericsson.com
















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Acknowledgment

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   Internet Society.




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