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Versions: (draft-alvestrand-mmusic-msid) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17

Network Working Group                                      H. Alvestrand
Internet-Draft                                                    Google
Intended status: Standards Track                            June 9, 2014
Expires: December 11, 2014


 WebRTC MediaStream Identification in the Session Description Protocol
                       draft-ietf-mmusic-msid-06

Abstract

   This document specifies a Session Description Protocol (SDP) Grouping
   mechanism for RTP media streams that can be used to specify relations
   between media streams.

   This mechanism is used to signal the association between the SDP
   concept of "m-line" and the WebRTC concept of "MediaStream" /
   "MediaStreamTrack" using SDP signaling.

   This document is a work item of the MMUSIC WG, whose discussion list
   is mmusic@ietf.org.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on December 11, 2014.







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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Structure Of This Document  . . . . . . . . . . . . . . .   3
     1.2.  Why A New Mechanism Is Needed . . . . . . . . . . . . . .   3
     1.3.  Application to the WEBRTC MediaStream . . . . . . . . . .   4
   2.  The Msid Mechanism  . . . . . . . . . . . . . . . . . . . . .   4
   3.  The Msid-Semantic Attribute . . . . . . . . . . . . . . . . .   5
   4.  Applying Msid to WebRTC MediaStreams  . . . . . . . . . . . .   6
     4.1.  Handling of non-signalled tracks  . . . . . . . . . . . .   7
     4.2.  Detailed procedures . . . . . . . . . . . . . . . . . . .   8
       4.2.1.  Generating the initial offer  . . . . . . . . . . . .   9
       4.2.2.  Parsing the initial offer . . . . . . . . . . . . . .   9
       4.2.3.  Generating the answer . . . . . . . . . . . . . . . .   9
       4.2.4.  Offerer processing of the answer  . . . . . . . . . .   9
       4.2.5.  Modifying the session . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Design considerations, rejected alternatives . . . .  13
   Appendix B.  Usage with multiple MediaStreams per M-line  . . . .  13
     B.1.  Mechanism design with multiple SSRCs  . . . . . . . . . .  13
     B.2.  Usage with the SSRC attribute . . . . . . . . . . . . . .  14
   Appendix C.  Change log . . . . . . . . . . . . . . . . . . . . .  14
     C.1.  Changes from alvestrand-rtcweb-msid-00 to -01 . . . . . .  14
     C.2.  Changes from alvestrand-rtcweb-msid-01 to -02 . . . . . .  15
     C.3.  Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00   15
     C.4.  Changes from alvestrand-mmusic-msid-00 to -01 . . . . . .  15
     C.5.  Changes from alvestrand-mmusic-msid-01 to -02 . . . . . .  15
     C.6.  Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00   15



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     C.7.  Changes from mmusic-msid-00 to -01  . . . . . . . . . . .  16
     C.8.  Changes from mmusic-msid-01 to -02  . . . . . . . . . . .  16
     C.9.  Changes from mmusic-msid-02 to -03  . . . . . . . . . . .  16
     C.10. Changes from mmusic-msid-03 to -04  . . . . . . . . . . .  16
     C.11. Changes from -04 to -05 . . . . . . . . . . . . . . . . .  16
     C.12. Changes from -05 to -06 . . . . . . . . . . . . . . . . .  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

1.1.  Structure Of This Document

   This document adds a new Session Description Protocol (SDP)
   Grouping[RFC5888] relation between SDP m-lines [RFC4566] that can
   associate application layer identifiers with the binding between
   media streams, attaching identifiers to the media streams and
   attaching identifiers to the groupings they form.

   Section 1.2 gives the background on why a new mechanism is needed.

   Section 2 gives the definition of the new mechanism.

   Section 3 gives the definition of the msid-semantic field, which
   gives the possibility of using MSIDs with different semantics in the
   same SDP message.

   Section 4 gives the application of the new mechanism for providing
   necessary semantic information for the association of
   MediaStreamTracks to MediaStreams in the WebRTC
   API[W3C.WD-webrtc-20120209].

1.2.  Why A New Mechanism Is Needed

   When media is carried by RTP [RFC3550], each RTP media stream is
   distinguished inside an RTP session by its SSRC; each RTP session is
   distinguished from all other RTP sessions by being on a different
   transport association (strictly speaking, 2 transport associations,
   one used for RTP and one used for RTCP, unless RTP/RTCP multiplexing
   [RFC5761] is used).

   SDP gives a description based on m-lines.  According to the model
   used in [I-D.ietf-rtcweb-jsep], each m-line describes exactly one
   media source, and if mulitple media sources are carried in an RTP
   session, this is signalled using BUNDLE
   [I-D.ietf-mmusic-sdp-bundle-negotiation]; if BUNDLE is not used, each
   media source is carried in its own RTP session.





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   There exist cases where an application using RTP and SDP needs to
   signal some relationship between RTP media streams that may be
   carried in either the same RTP session or different RTP sessions.
   For instance, there may be a need to signal a relationship between a
   video track and an audio track, and where the generator of the SDP
   does not yet know if they will be carried in the same RTP session or
   different RTP sessions.

   The SDP grouping framework [RFC5888] can be used to group m-lines.
   However, there is sometimes the need for an application to specify
   some application-level information about the association between the
   m-line and the group.  This is not possible using the SDP grouping
   framework.

1.3.  Application to the WEBRTC MediaStream

   The W3C WebRTC API specification [W3C.WD-webrtc-20120209] specifies
   that communication between WebRTC entities is done via MediaStreams,
   which contain MediaStreamTracks.  A MediaStreamTrack is generally
   carried using a single SSRC in an RTP session (forming an RTP media
   stream.  The collision of terminology is unfortunate.)  There might
   possibly be additional SSRCs, possibly within additional RTP
   sessions, in order to support functionality like forward error
   correction or simulcast.  This complication is ignored below.

   In the RTP specification, media streams are identified using the SSRC
   field.  Streams are grouped into RTP Sessions, and also carry a
   CNAME.  Neither CNAME nor RTP session correspond to a MediaStream.
   Therefore, the association of an RTP media stream to MediaStreams
   need to be explicitly signaled.

   WebRTC defines a mapping (documented in [I-D.ietf-rtcweb-jsep]) where
   one SDP m-line is used to describe each MediaStreamTrack, and that
   the BUNDLE mechanism [I-D.ietf-mmusic-sdp-bundle-negotiation] is used
   to group MediaStreamTracks into RTP sessions.  Therefore, the need is
   to specify the ID of a MediaStreamTrack and its associated
   MediaStream for each m-line, which can be accomplished with a media-
   level SDP attribute.

   This usage is described in Section 4.

2.  The Msid Mechanism

   This document defines a new SDP [RFC4566] media-level "msid"
   attribute.  This new attribute allows endpoints to associate RTP
   media streams that are carried in the same or different m-lines.  The
   attribute also allows application-specific information to the
   association.



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   The value of the "msid" attribute consists of an identifier and
   optional application-specific data, according to the following ABNF
   [RFC5234] grammar:

     ; "attribute" is defined in RFC 4566.
     attribute =/ msid-attr
     msid-attr = "msid:" identifier [ SP appdata ]
     identifier = 1*64token-char ; see RFC 4566
     appdata = 1*64token-char  ; see RFC 4566


   An example msid value for a group with the identifier "examplefoo"
   and application data "examplebar" might look like this:

     msid:examplefoo examplebar


   The identifier is a string of ASCII characters that are legal in a
   "token", consisting of between 1 and 64 characters.  It MUST be
   unique among the identifier values used in the same SDP session.  It
   is RECOMMENDED that it is generated using a random-number generator.

   Application data is carried on the same line as the identifier,
   separated from the identifier by a space.

   The identifier uniquely identifies a group within the scope of an SDP
   description.

   There may be multiple msid attributes in a single media description.
   There may also be multiple media descriptions that have the same
   value for identifier and application data.

   Endpoints can update the associations between RTP media streams as
   expressed by msid attributes at any time; the semantics and
   restrictions of such grouping and ungrouping are application
   dependent.

3.  The Msid-Semantic Attribute

   A session-level attribute is defined for signaling the semantics
   associated with an msid grouping.  This allows msid groupings with
   different semantics to coexist.

   This OPTIONAL attribute gives the group identifier and its group
   semantic; it carries the same meaning as the ssrc-group-attr of RFC
   5576 section 4.2, but uses the identifier of the group rather than a
   list of SSRC values.




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   This attribute MUST be present if "a=msid" is used.

   An empty list of identifiers is an indication that the sender
   understands the indicated semantic, but has no msid groupings of the
   given type in the present SDP.

   An identifier of "*" is an indication that all "a=msid" lines in the
   SDP have this specific semantic.

   The ABNF of msid-semantic is:

     attribute =/ msid-semantic-attr
     msid-semantic-attr = "msid-semantic:" semantic identifier-list
     semantic = token ; see RFC 4566
     identifier-list = (" " identifier)* / " *"

   The semantic field holds values from the IANA registriy "Semantics
   for the msid-semantic SDP attribute" (which is defined in Section 5).

   An example msid-semantic might look like this, if a semantic LS was
   registered by IANA for the same purpose as the existing LS grouping
   semantic:

     a=msid-semantic:LS xyzzy forolow


   This means that the SDP description has two lip sync groups, with the
   group identifiers xyzzy and forolow, respectively.

4.  Applying Msid to WebRTC MediaStreams

   This section creates a new semantic for use with the framework
   defined in Section 2, to be used for associating m-lines representing
   MediaStreamTracks within MediaStreams as defined in
   [W3C.WD-webrtc-20120209].

   In the Javascript API, each MediaStream and MediaStreamTrack has an
   "id" attribute, which is a DOMString.

   The semantic token for this semantic is "WMS" (short for WebRTC Media
   Stream).

   The value of the "identifier" field in the msid consists of the "id"
   attribute of a MediaStream, as defined in its WebIDL specification.

   The value of the "appdata" field in the msid consists of the "id"
   attribute of a MediaStreamTrack, as defined in its WebIDL
   specification.



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   If two different m-lines have MSID attributes with the same value for
   identifier and appdata, it means that these two m-lines are both
   intended for the same MediaStreamTrack.  So far, no semantic for such
   a mixture have been defined, but this specification does not forbid
   the practice.

   When an SDP description is updated, a specific msid "identifier"
   continues to refer to the same MediaStream, and a specific "appdata"
   to the same MediaStreamTrack.  Once negotiation has completed on a
   session, there is no memory apart from the currently valid SDP
   descriptions; an msid "identifier" value that appears in a later
   negotiation will be taken to refer to a new MediaStream.

   The following are the rules for handling updates of the list of
   m-lines and their msid values.

   o  When a new msid "identifier" value occurs in the description, the
      recipient can signal to its application that a new MediaStream has
      been added.

   o  When a description is updated to have more media sections with the
      same msid "identifier" value, but different "appdata" values, the
      recipient can signal to its application that new MediaStreamTracks
      have been added to the MediaStream.

   o  When a description is updated to no longer list the msid attribute
      on a specific media description, the recipient can signal to its
      application that the corresponding MediaStreamTrack has ended.

   In addition to signaling that the track is closed when its msid
   attribute disappears from the SDP, the track will also be signaled as
   being closed when all associated SSRCs have disappeared by the rules
   of [RFC3550] section 6.3.4 (BYE packet received) and 6.3.5 (timeout),
   and when the corresponding media section is disabled by setting the
   port number to zero.  Changing the direction of the media section to
   "recvonly" will not close the MediaStreamTrack.

   The association between SSRCs and m-lines is specified in
   [I-D.ietf-rtcweb-jsep].

4.1.  Handling of non-signalled tracks

   Non-WebRTC entities will not send msid.  This means that there will
   be some incoming RTP packets that the recipient has no predefined
   MediaStream id value for.

   Note that this handling is triggered by incoming RTP packets, not by
   SDP negotiation.



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   Handling will depend on whether or not the msid-semantic:WMS
   attribute is present.  There are two cases:

   o  No "msid-semantic:WMS" attribute is present.  The SDP session is
      assumed to be a backwards-compatible session.  All incoming media,
      on all m-lines that are part of the SDP session, are assumed to
      belong to independent media streams, each with one track.  The
      identifier of this media stream and of the media stream track is a
      randomly generated string; the WebIDL "label" attribute of this
      media stream will be set to "Non-WMS stream".

   o  An "msid-semantic:WMS" attribute is present.  In this case, the
      session is WebRTC compatible, and the packets are either caused by
      a bug or by timing skew between the arrival of the media packets
      and the SDP description.  These packets MAY be discarded, or they
      MAY be buffered for a while in order to allow immediate startup of
      the media stream when the SDP description is updated.  The arrival
      of media packets MUST NOT cause a new MediaStreamTrack to be
      signaled.

   If a WebRTC entity sends a description, it MUST include the msid-
   semantic:WMS attribute, even if no media streams are sent.  This
   allows us to distinguish between the case of no media streams at the
   moment and the case of legacy SDP generation.

   It follows from the above that the WebRTC entity must have the SDP of
   the other party before it can decide correctly which of the two cases
   described above applies.  RTP media packets that arrive before the
   remote party's SDP MUST be buffered or discarded, and MUST NOT cause
   a new MediaStreamTrack to be signalled.

   It follows from the above that media stream tracks in the "default"
   media stream cannot be closed by removing the msid attribute; the
   application must instead signal these as closed when the SSRC
   disappears according to the rules of RFC 3550 section 6.3.4 and 6.3.5
   or by disabling the m-line by setting its port to zero.

4.2.  Detailed procedures

   These procedures are given in terms of RFC 3264-recommended sections.
   They describe the actions to be taken in terms of MediaStreams and
   MediaStreamTracks; they do not include event signalling inside the
   application, which is described in JSEP.

   They are specifically applicable to the WMS semantic; other semantics
   will have their own consideration.





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4.2.1.  Generating the initial offer

   For each media section in the offer, if there is an associated
   MediaStreamTrack, the offerer adds one "a=msid" attribute to the
   section for each MediaStream with which the MediaStreamTrack is
   associated.  The "identifier" field of the attribute is set to the
   WebIDL "id" attribute of the MediaStream, and the "appdata" field is
   set to the WebIDL "id" attribute of the MediaStreamTrack.

4.2.2.  Parsing the initial offer

   For each media section in the offer, and for each "a=msid" attribute
   in the media section, the receiver of the offer will perform the
   following steps:

   o  Extract the "appdata" field of the "a=msid" attribute

   o  Check if a MediaStreamTrack with the same WebIDL "id" attribute as
      the "appdata" field already exists, and is not in the "ended"
      state.  If it is not found, create it.

   o  Extract the "identifier" field of the "a=msid" attribte.

   o  Check if a MediaStream with the same WebIDL "id" attribute already
      exists.  If not, create it.

   o  Add the MediaStreamTrack to the MediaStream

4.2.3.  Generating the answer

   The answer is generated in exactly the same manner as the offer.

4.2.4.  Offerer processing of the answer

   The answer is processed in exactly the same manner as the offer.

4.2.5.  Modifying the session

   On subsequent exchanges, precisely the same procedure as for the
   initial offer/answer is followed, but with one additional step in the
   parsing of the offer and answer:

   o  For each MediaStreamTrack that has been created as a result of
      previous offer/answer exchanges, and is not in the "ended" state,
      check to see if there is still an "a=msid" attribute in the
      present SDP whose "appdata" field is the same as the WebIDL "id"
      attribute of the track.




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   o  If no such attribute is found, close the MediaStreamTrack.  This
      will set its state to "ended".

5.  IANA Considerations

   This document requests IANA to register the "msid" attribute in the
   "att-field (media level only)" registry within the SDP parameters
   registry, according to the procedures of [RFC4566]

   The required information for "msid" is:

   o  Contact name, email: IETF, contacted via mmusic@ietf.org, or a
      successor address designated by IESG

   o  Attribute name: msid

   o  Long-form attribute name: Media stream group Identifier

   o  Subject to charset: The attribute value contains only ASCII
      characters, and is therefore not subject to the charset attribute.

   o  Purpose: The attribute gives an association over a set of m-lines.
      It can be used to signal the relationship between a WebRTC
      MediaStream and a set of m-lines.

   o  Appropriate values: The details of appropriate values are given in
      RFC XXXX.

   This document requests IANA to register the "msid-semantic" attribute
   in the "att-field (session level) registry within the SDP parameters
   registry, according to the same procedures.

   The required information is:

   o  Contact name, email: IETF, contacted via mmusic@ietf.org, or a
      successor address designated by IESG

   o  Attribute name: msid-semantic

   o  Long-form attribute name: Msid group semantic identifier

   o  Subject to charset: The attribute value contains only ASCII
      characters, and is therefore not subject to the charset attribute.

   o  Purpose: The attribute gives the semantics of an association over
      a set of m-lines.

   o  Appropriate values: The details are given in RFC XXXX.



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   This document requests IANA to create a new registry called
   "Semantics for the msid-semantic SDP attribute", which should have
   exactly the same rules as for the "Semantics for the ssrc-group SDP
   attribute" registry (Expert Review), and to register the "WMS"
   semantic within this new registry.

   The required information is:

   o  Description: WebRTC Media Stream, as given in RFC XXXX.

   o  Token: WMS

   o  Standards track reference: RFC XXXX

   IANA is requested to replace "RFC XXXX" with the RFC number of this
   document upon publication.

6.  Security Considerations

   An adversary with the ability to modify SDP descriptions has the
   ability to switch around tracks between media streams.  This is a
   special case of the general security consideration that modification
   of SDP descriptions needs to be confined to entities trusted by the
   application.

   If implementing buffering as mentioned in Section 4.1, the amount of
   buffering should be limited to avoid memory exhaustion attacks.

   No other attacks have been identified that depend on this mechanism.

7.  Acknowledgements

   This note is based on sketches from, among others, Justin Uberti and
   Cullen Jennings.

   Special thanks to Flemming Andreassen, Miguel Garcia and Paul Kyzivat
   for their work in reviewing this draft, with many specific language
   suggestions.

8.  References

8.1.  Normative References

   [I-D.ietf-rtcweb-jsep]
              Uberti, J. and C. Jennings, "Javascript Session
              Establishment Protocol", draft-ietf-rtcweb-jsep-06 (work
              in progress), February 2014.




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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5576]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", RFC 5576, June 2009.

   [W3C.WD-webrtc-20120209]
              Bergkvist, A., Burnett, D., Jennings, C., and A.
              Narayanan, "WebRTC 1.0: Real-time Communication Between
              Browsers", World Wide Web Consortium WD WD-
              webrtc-20120209, February 2012,
              <http://www.w3.org/TR/2012/WD-webrtc-20120209>.

8.2.  Informative References

   [I-D.ietf-mmusic-sdp-bundle-negotiation]
              Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
              negotiation-07 (work in progress), April 2014.

   [I-D.roach-mmusic-unified-plan]
              Roach, A., Uberti, J., and M. Thomson, "A Unified Plan for
              Using SDP with Large Numbers of Media Flows", draft-roach-
              mmusic-unified-plan-00 (work in progress), July 2013.

   [I-D.westerlund-avtcore-multiplex-architecture]
              Westerlund, M., Perkins, C., and H. Alvestrand,
              "Guidelines for using the Multiplexing Features of RTP",
              draft-westerlund-avtcore-multiplex-architecture-03 (work
              in progress), February 2013.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761, April 2010.

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888, June 2010.



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Appendix A.  Design considerations, rejected alternatives

   This appendix should be deleted before publication as an RFC.

   One suggested mechanism has been to use CNAME instead of a new
   attribute.  This was abandoned because CNAME identifies a
   synchronization context; one can imagine both wanting to have tracks
   from the same synchronization context in multiple MediaStreams and
   wanting to have tracks from multiple synchronization contexts within
   one MediaStream (but the latter is impossible, since a MediaStream is
   defined to impose synchronization on its members).

   Another suggestion has been to put the msid value within an attribute
   of RTCP SR (sender report) packets.  This doesn't offer the ability
   to know that you have seen all the tracks currently configured for a
   media stream.

Appendix B.  Usage with multiple MediaStreams per M-line

   This appendix is included to document the usage of msid as a source-
   specific attribute.  Prior to the acceptance of the Unified Plan
   document, some implementations used this mechanism to distinguish
   between multiple MediaStreamTracks that were carried in the same
   M-line.

   It reproduces some of the original justification text for this
   mechanism that is not relevant when Unified Plan is used.

B.1.  Mechanism design with multiple SSRCs

   When media is carried by RTP [RFC3550], each RTP media stream is
   distinguished inside an RTP session by its SSRC; each RTP session is
   distinguished from all other RTP sessions by being on a different
   transport association (strictly speaking, 2 transport associations,
   one used for RTP and one used for RTCP, unless RTCP multiplexing
   [RFC5761] is used).

   There exist cases where an application using RTP and SDP needs to
   signal some relationship between RTP media streams that may be
   carried in either the same RTP session or different RTP sessions.
   For instance, there may be a need to signal a relationship between a
   video track in one RTP session and an audio track in another RTP
   session.  In traditional SDP, it is not possible to signal that these
   two tracks should be carried in one session, so they are carried in
   different RTP sessions.

   Traditionally, SDP was used to describe the RTP sessions, with one
   m-line being used to describe each RTP session.  With the advent of



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   extensions like BUNDLE [I-D.ietf-mmusic-sdp-bundle-negotiation], this
   association may be more complex, with multiple m-lines being used to
   describe one RTP session; the rest of this document therefore talks
   about m-lines, not RTP sessions, when describing the signalling
   mechanism.

   The SSRC grouping mechanism ("a=ssrc-group") [RFC5576] can be used to
   associate RTP media streams when those RTP media streams are
   described by the same m-line.  The semantics of this mechanism
   prevent the association of RTP media streams that are spread across
   different m-lines.

   The SDP grouping framework [RFC5888] can be used to group m-lines.
   When an m-line describes one and only one RTP media stream, it is
   possible to associate RTP media streams across different m-lines.
   However, if an m-line has multiple RTP media streams, using multiple
   SSRCs, the SDP grouping framework cannot be used for this purpose.

   There are use cases (some of which are discussed in
   [I-D.westerlund-avtcore-multiplex-architecture] ) where neither of
   these approaches is appropriate; In those cases, a new mechanism is
   needed.

   In addition, there is sometimes the need for an application to
   specify some application-level information about the association
   between the SSRC and the group.  This is not possible using either of
   the frameworks above.

B.2.  Usage with the SSRC attribute

   When the MSID attribute was used with the SSRC attribute, it had to
   be registered in the "Attribute names (source level)" registry rather
   than the "Attribute names (media level only)" registry, and the msid
   line was prefixed with "a=ssrc:<ssrc> ".  Apart from that, usage of
   the attribute with SSRC-bound flows was identical with the current
   proposal.

Appendix C.  Change log

   This appendix should be deleted before publication as an RFC.

C.1.  Changes from alvestrand-rtcweb-msid-00 to -01

   Added track identifier.

   Added inclusion-by-reference of draft-lennox-mmusic-source-selection
   for track muting.




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   Some rewording.

C.2.  Changes from alvestrand-rtcweb-msid-01 to -02

   Split document into sections describing a generic grouping mechanism
   and sections describing the application of this grouping mechanism to
   the WebRTC MediaStream concept.

   Removed the mechanism for muting tracks, since this is not central to
   the MSID mechanism.

C.3.  Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00

   Changed the draft name according to the wishes of the MMUSIC group
   chairs.

   Added text indicting cases where it's appropriate to have the same
   appdata for multiple SSRCs.

   Minor textual updates.

C.4.  Changes from alvestrand-mmusic-msid-00 to -01

   Increased the amount of explanatory text, much based on a review by
   Miguel Garcia.

   Removed references to BUNDLE, since that spec is under active
   discussion.

   Removed distinguished values of the MSID identifier.

C.5.  Changes from alvestrand-mmusic-msid-01 to -02

   Changed the order of the "msid-semantic: " attribute's value fields
   and allowed multiple identifiers.  This makes the attribute useful as
   a marker for "I understand this semantic".

   Changed the syntax for "identifier" and "appdata" to be "token".

   Changed the registry for the "msid-semantic" attribute values to be a
   new registry, based on advice given in Atlanta.

C.6.  Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00

   Updated terminology to refer to m-lines rather than RTP sessions when
   discussing SDP formats and the ability of other linking mechanisms to
   refer to SSRCs.




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   Changed the "default" mechanism to return independent streams after
   considering the synchronization problem.

   Removed the space from between "msid-semantic" and its value, to be
   consistent with RFC 5576.

C.7.  Changes from mmusic-msid-00 to -01

   Reworked msid mechanism to be a per-m-line attribute, to align with
   [I-D.roach-mmusic-unified-plan]

C.8.  Changes from mmusic-msid-01 to -02

   Corrected several missed cases where the word "ssrc" was not changed
   to "M-line".

   Added pointer to unified-plan (which should be moved to point to
   -jsep)

   Removed suggestion that ssrc-group attributes can be used with "msid-
   semantic", it is now only the msid-semantic registry.

C.9.  Changes from mmusic-msid-02 to -03

   Corrected even more cases where the word "ssrc" was not changed to
   "M-line".

   Added the functionality of using an asterisk (*) in the msid-semantic
   line, in order to remove the need for listing all msids in the msid-
   semantic line whne only one msid-semantic is in use.

   Removed some now-unnecessary text.

C.10.  Changes from mmusic-msid-03 to -04

   Changed title to reflect focus on WebRTC MediaStreams

   Added a section on receiver-side media stream control, using the
   "msid-control" attribute.

C.11.  Changes from -04 to -05

   Removed the msid-control section after WG discussion.

   Removed some text that seemed only to pertain to resolved issues.






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C.12.  Changes from -05 to -06

   Addressed issues found in Fleming Andreassen's review

   Referenced JSEP rather than unified-plan for the M-line mapping model

   Relaxed MSID definition to allow "token-char" in values rather than
   a-z 0-9 hyphen; tightened ABNF by adding length description to it.

   Deleted discussion of abandoned alternatives, as part of preparing
   for publication.

   Added a "detailed procedures" section to the WMS semantics
   description.

   Added IANA registration of the "msid-semantic" attribute.

Author's Address

   Harald Alvestrand
   Google
   Kungsbron 2
   Stockholm  11122
   Sweden

   Email: harald@alvestrand.no

























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