Network Working Group                                      H. Alvestrand
Internet-Draft                                                    Google
Intended status: Standards Track                        February 7, 2017                       December 13, 2018
Expires: August 11, 2017 June 16, 2019

 WebRTC MediaStream Identification in the Session Description Protocol


   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 "media description" and the WebRTC concept of
   "MediaStream" / "MediaStreamTrack" using SDP signaling.

   This document is a work item of the MMUSIC WG, whose discussion list

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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
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   This Internet-Draft will expire on August 11, 2017. June 16, 2019.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Structure Of This Document  . . . . . . . . . . . . . . .   3
     1.3.  Why A New Mechanism Is Needed . . . . . . . . . . . . . .   4
     1.4.  The WEBRTC MediaStream  . . . . . . . . . . . . . . . . .   4
   2.  The Msid Mechanism  . . . . . . . . . . . . . . . . . . . . .   5
   3.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Handling of non-signalled tracks  . . . . . . . . . . . .   7
     3.2.  Detailed Offer/Answer Procedures  . . . . . . . . . . . .   8   9
       3.2.1.  Generating the initial offer  . . . . . . . . . . . .   9
       3.2.2.  Answerer processing of the Offer  . . . . . . . . . .   9
       3.2.3.  Generating the answer . . . . . . . . . . . . . . . .   9  10
       3.2.4.  Offerer processing of the answer  . . . . . . . . . .   9  10
       3.2.5.  Modifying the session . . . . . . . . . . . . . . . .   9  10
     3.3.  Example SDP description . . . . . . . . . . . . . . . . .  10
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10  11
     4.1.  Attribute registration in existing registries . . . . . .  10  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11  12
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Design considerations, rejected alternatives . . . .  13  14
   Appendix B.  Change log . . . . . . . . . . . . . . . . . . . . .  14
     B.1.  Changes from alvestrand-rtcweb-msid-00 to -01 . . . . . .  14
     B.2.  Changes from alvestrand-rtcweb-msid-01 to -02 . . . . . .  14  15
     B.3.  Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00   14   15
     B.4.  Changes from alvestrand-mmusic-msid-00 to -01 . . . . . .  14  15
     B.5.  Changes from alvestrand-mmusic-msid-01 to -02 . . . . . .  14  15
     B.6.  Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00   15
     B.7.  Changes from mmusic-msid-00 to -01  . . . . . . . . . . .  15  16
     B.8.  Changes from mmusic-msid-01 to -02  . . . . . . . . . . .  15  16
     B.9.  Changes from mmusic-msid-02 to -03  . . . . . . . . . . .  15  16
     B.10. Changes from mmusic-msid-03 to -04  . . . . . . . . . . .  16
     B.11. Changes from -04 to -05 . . . . . . . . . . . . . . . . .  16
     B.12. Changes from -05 to -06 . . . . . . . . . . . . . . . . .  16
     B.13. Changes from -06 to -07 . . . . . . . . . . . . . . . . .  16  17
     B.14. Changes from -07 to -08 . . . . . . . . . . . . . . . . .  16  17
     B.15. Changes from -08 to -09 . . . . . . . . . . . . . . . . .  17
     B.16. Changes from -09 to -10 . . . . . . . . . . . . . . . . .  17  18
     B.17. Changes from -10 to -11 . . . . . . . . . . . . . . . . .  17  18
     B.18. Changes from -11 to -12 . . . . . . . . . . . . . . . . .  17  18
     B.19. Changes from -12 to -13 . . . . . . . . . . . . . . . . .  17  18
     B.20. Changes from -13 to -14 . . . . . . . . . . . . . . . . .  17  18
     B.21. Changes from -14 to -15 . . . . . . . . . . . . . . . . .  18
     B.22. Changes from -15 to -16 . . . . . . . . . . . . . . . . .  18  19
     B.23. Changes from -16 to -17 . . . . . . . . . . . . . . . . .  19
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18  19

1.  Introduction

1.1.  Terminology

   This document uses terminology from [I-D.ietf-rtcweb-overview].  In
   addition, the following terms are used as described below:

   RTP stream  Defined in [RFC7656] as a stream of RTP packets
      containing media data.

   MediaStream  Defined in [W3C.CR-mediacapture-streams-20160519]as an
      assembly of MediaStreamTracks.  One MediaStream can contain
      multiple MediaStreamTracks, of the same or different types.

   MediaStreamTrack  Defined in [W3C.CR-mediacapture-streams-20160519]as
      an unidirectional flow of media data (either audio or video, but
      not both).  Corresponds to the [RFC7656] term "Source Stream".
      One MediaStreamTrack can be present in zero, one or multiple

   Media description  Defined in [RFC4566] as a set of fields starting
      with an "m=" field and terminated by eitehr the next "m=" field or
      by the end of the session description.

1.2.  Structure Of This Document

   This document adds a new Session Description Protocol (SDP) [RFC4566]
   mechanism that can attach identifiers to the RTP streams and
   attaching identifiers to the groupings they form.  It is designed for
   use with WebRTC[I-D.ietf-rtcweb-overview] .

   Section 1.3 gives the background on why a new mechanism is needed.

   Section 2 gives the definition of the new mechanism.

   Section 3 gives the necessary semantic information and procedures for
   using the msid attribute to signal the association of
   MediaStreamTracks to MediaStreams in support of the WebRTC API

1.3.  Why A New Mechanism Is Needed

   When media is carried by RTP [RFC3550], each RTP 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 [RFC4566] gives a format for describing an SDP session that can
   contain multiple media descriptions.  According to the model used in
   [I-D.ietf-rtcweb-jsep], each media description describes exactly one
   media source, and if multiple 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.

   The SDP grouping framework [RFC5888] can be used to group media
   descriptions.  However, for the use case of WebRTC, there is the need
   for an application to specify some application-level information
   about the association between the media description and the group.
   This is not possible using the SDP grouping framework.

1.4.  The WEBRTC MediaStream

   The W3C WebRTC API specification [W3C.WD-webrtc-20160531] 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 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.  These additional SSRCs are not affected by this

   MediaStreamTracks are unidirectional; they carry media on one
   direction only.

   In the RTP specification, RTP 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 stream to MediaStreams need to
   be explicitly signaled.

   WebRTC defines a mapping (documented in [I-D.ietf-rtcweb-jsep]) where
   one SDP media description is used to describe each MediaStreamTrack,
   and 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 media description, which can be accomplished
   with a media-level SDP attribute.

   This usage is described in Section 3.

2.  The Msid Mechanism

   This document defines a new SDP [RFC4566] media-level "msid"
   attribute.  This new attribute allows endpoints to associate RTP
   streams that are described in different media descriptions with the
   same MediaStreams as defined in [W3C.WD-webrtc-20160531], and to
   carry an identifier for each MediaStreamTrack in its "appdata" field.

   The value of the "msid" attribute consists of an identifier and an
   optional "appdata" field.

   The name of the attribute is "msid".

   The value of the attribute is specified by the following ABNF
   [RFC5234] grammar:

     msid-value = msid-id [ SP msid-appdata ]
     msid-id = 1*64token-char ; see RFC 4566
     msid-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.

   Application data (msid-appdata) is carried on the same line as the
   identifier, separated from the identifier by a space.

   The identifier (msid-id) uniquely identifies a group within the scope
   of an SDP description.

   There may be multiple msid attributes in a single media description.
   This represents the case where a single MediaStreamTrack is present
   in multiple MediaStreams; the value of "msid-appdata" MUST be
   identical for all occurences.

   Multiple media descriptions with the same value for msid-id and msid-
   appdata are not permitted.

   Endpoints can update the associations between RTP streams as
   expressed by msid attributes at any time.

   The msid attributes depend on the association of RTP streams with
   media descriptions, but does not depend on the association of RTP
   streams with RTP transports; therefore, its mux category (as defined
   in [I-D.ietf-mmusic-sdp-mux-attributes]) is NORMAL - the process of
   deciding on MSID attributes doesn't have to take into consideration
   whether the RTP streams are bundled or not.

3.  Procedures

   This section describes the procedures for associating media
   descriptions representing MediaStreamTracks within MediaStreams as
   defined in [W3C.WD-webrtc-20160531].

   In the Javascript API described in that specification, each
   MediaStream and MediaStreamTrack has an "id" attribute, which is a

   The value of the "msid-id" field in the msid consists of the "id"
   attribute of a MediaStream, as defined in the MediaStream's WebIDL
   specification.  The special value "-" indicates "no MediaStream".

   The value of the "msid-appdata" field in the msid msid, if present,
   consists of the "id" attribute of a MediaStreamTrack, as defined in
   the MediaStreamTrack's WebIDL specification.

   When an SDP session description is updated, a specific "msid-id"
   value continues to refer to the same MediaStream, and a specific
   "msid-appdata" to the same MediaStreamTrack.  There is no memory
   apart from the currently valid SDP descriptions; if an msid
   "identifier" value disappears from the SDP and appears in a later
   negotiation, it will be taken to refer to a new MediaStream.

   If the MSID attribute does not conform to the ABNF given here, it
   SHOULD be ignored.

   The following is a high level description of the rules for handling
   SDP updates.  Detailed procedures are in Section 3.2.

   o  When a new msid "identifier" value occurs in a session
      description, and it is not "-", the recipient can signal to its
      application that a new MediaStream has been added.

   o  When a session description is updated to have media descriptions
      with an msid "identifier" value, with one or more different
      "appdata" values, the recipient can signal to its application that
      new MediaStreamTracks have been added, and which MediaStream it
      has been added to.  This is done for each different msid
      "identifier" value, including the special value "-", which
      indicates that a MediaStreamTrack has been added with no
      corresponding MediaStream.

   o  If an msid "identifier" value with no "appdata" value appears, it
      means that the sender did not inform the recipient of the desired
      identifier of the MediaStreamTrack, and the recipient will assign
      the "id" value of the created MediaStreamTrack on its own.  All
      msid in a media section that do not have an "appdata" value are
      assumed to refer to the same MediaStreamTrack.

   o  When a session description is updated to no longer list any 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 ended when its msid
   attribute disappears from the SDP, the track will also be signaled as
   being ended when all associated SSRCs have disappeared by the rules
   of [RFC3550] section 6.3.4 (BYE packet received) and 6.3.5 (timeout),
   or when the corresponding media description is disabled by setting
   the port number to zero.  Changing the direction of the media
   description (by setting "sendonly", "recvonly" or "inactive"
   attributes) will not end the MediaStreamTrack.

   The association between SSRCs and media descriptions is specified in

3.1.  Handling of non-signalled tracks

   Entities that do not use msid 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.

   When MSID is used, the only time this can happen is when, after the
   initial negotiation, a negotiation is performed where the answerer
   adds a MediaStreamTrack to an already established connection and
   starts sending data before the answer is received by the offerer.
   For initial negotiation, packets won't flow until the ICE candidates
   and fingerprints have been exchanged, so this is not an issue.

   The recipient of those packets will perform the following steps:

   o  When RTP packets are initially received, it will create an
      appropriate MediaStreamTrack based on the type of the media
      (carried in PayloadType), and use the MID RTP header extension
      [I-D.ietf-mmusic-sdp-bundle-negotiation] (if present) to associate
      the RTP packets with a specific media section.

   o  If the connection is not in the RTCSignalingState "stable", it
      will wait at this point.

   o  When the connection is in the RTCSignalingState "stable", it will
      assign ID values.

   The following steps are performed to assign ID values:

   o  If there is an msid attribute, it will use that attribute to
      populate the "id" field of the MediaStreamTrack and associated
      MediaStreams, as described above.

   o  If there is no msid attribute, the identifier of the
      MediaStreamTrack will be set to a randomly generated string, and
      it will be signalled as being part of a MediaStream with the
      WebIDL "label" attribute set to "Non-WebRTC stream".

   o  After deciding on the "id" field to be applied to the
      MediaStreamTrack, the track will be signalled to the user.

   The process above may involve a considerable amount of buffering
   before the stable state is entered.  If the implementation wishes to
   limit this buffering, it MUST signal to the user that media has been

   It follows from the above that MediaStreamTracks in the "default"
   MediaStream 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 media description by setting its port to zero.

3.2.  Detailed Offer/Answer 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.

3.2.1.  Generating the initial offer

   For each media description in the offer, if there is an associated
   outgoing 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 MediaStream.  If the sender wishes to
   signal identifiers for the MediaStreamTracks, the "appdata" field is
   set to the WebIDL "id" attribute of the MediaStreamTrack. MediaStreamTrack; otherwise
   it is omitted.

3.2.2.  Answerer processing of the Offer

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

   o  Extract the "appdata" field of the "a=msid" attribute attribute, if present.

   o  If the "appdata" field exists: 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  If the "appdata" field does not exist, and a MediaStreamTrack is
      not associated with this media section, create one and associate
      it with this media section for future use.

   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

   o  Signal to the user that a new MediaStreamTrack is available.

3.2.3.  Generating the answer

   The answer is generated in exactly the same manner as the offer.
   "a=msid" values in the offer do not influence the answer.

3.2.4.  Offerer processing of the answer

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

3.2.5.  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.

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

3.3.  Example SDP description

   The following SDP description shows the representation of a WebRTC
   PeerConnection with two MediaStreams, each of which has one audio and
   one video track.  Only the parts relevant to the MSID are shown.

   Line wrapping, empty lines and comments are added for clarity.  They
   are not part of the SDP.

   # First MediaStream - id is 4701...
   m=audio 56500 UDP/TLS/RTP/SAVPF 96 0 8 97 98

   m=video 56502 UDP/TLS/RTP/SAVPF 100 101

   # Second MediaStream - id is 6131....
   m=audio 56503 UDP/TLS/RTP/SAVPF 96 0 8 97 98

   m=video 56504 UDP/TLS/RTP/SAVPF 100 101

4.  IANA Considerations

4.1.  Attribute registration in existing registries

   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, or a
      successor address designated by IESG

   o  Attribute name: msid

   o  Long-form attribute name: MediaStream 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 can be used to signal the relationship
      between a WebRTC MediaStream and a set of media descriptions.

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

   o  MUX category: NORMAL
   The MUX category is defined in [I-D.ietf-mmusic-sdp-mux-attributes].

5.  Security Considerations

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

   If implementing buffering as mentioned in Section 3.1, the amount of
   buffering should be limited to avoid memory exhaustion attacks.

   Careless generation of identifiers can leak privacy-sensitive
   information.  [W3C.CR-mediacapture-streams-20160519] recommends that
   identifiers are generated using UUID class 3 or 4 as a basis, which
   avoids such leakage.

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

6.  Acknowledgements

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

   Special thanks to Flemming Andreassen, Ben Campbell, Miguel Garcia,
   Martin Thomson, Ted Hardie, Adam Roach, Magnus Westerlund, Alissa
   Cooper, Sue Hares and Paul Kyzivat for their work in reviewing this
   draft, with many specific language suggestions.

7.  References

7.1.  Normative References

              Nandakumar, S., "A Framework for SDP Attributes when
              Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-12
              (work in progress), January 2016.

              Uberti, J., Jennings, C., and E. Rescorla, "Javascript
              Session Establishment Protocol", draft-ietf-rtcweb-jsep-14
              (work in progress), March 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
              RFC5234, January 2008,

              Burnett, D., Bergkvist, A., Jennings, C., Narayanan, A.,
              and B. Aboba, "Media Capture and Streams", World Wide Web
              Consortium CR CR-mediacapture-streams-20160519, May 2016,

              Wilson, C. and J. Kalliokoski, "WebRTC 1.0: Real-time
              Communication Between Browsers", World Wide Web Consortium
              WD WD-webrtc-20160531, May 2016,

7.2.  Informative References

              Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
              negotiation-29 (work in progress), April 2016.

              Alvestrand, H., "Overview: Real Time Protocols for
              Browser-based Applications", draft-ietf-rtcweb-overview-15
              (work in progress), January 2016.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761, DOI 10.17487/
              RFC5761, April 2010,

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888, DOI
              10.17487/RFC5888, June 2010,

   [RFC7656]  Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
              for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
              DOI 10.17487/RFC7656, November 2015,

Appendix A.  Design considerations, rejected alternatives

   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

   A suggestion that survived for a number of drafts was to define
   "msid" as a generic mechanism, where the particular semantics of this
   usage of the mechanism would be defined by an "a=wms-semantic"
   attribute.  This was removed in April 2015.

Appendix B.  Change log

   This appendix should be deleted before publication as an RFC.

B.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.

   Some rewording.

B.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.

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

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

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

   Minor textual updates.

B.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

   Removed distinguished values of the MSID identifier.

B.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.

B.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.

   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.

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

   Reworked msid mechanism to be a per-m-line attribute, to align with

B.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

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

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

   Corrected even more cases where the word "ssrc" was not changed to

   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.

B.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.

B.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.

B.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

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

B.13.  Changes from -06 to -07

   Changed terminology from referring to "WebRTC device" to referring to
   "entities that implement the WMS semantic".

   Changed names for ABNF constructions based on a proposal by Paul

   Included a section on generic offer/answer semantics.

B.14.  Changes from -07 to -08

   Removed Appendix B that described the (now obsolete) ssrc-specific
   usage of MSID.

   Adopted a restructuring of the IANA section based on a suggestion
   from Martin Thomson.

   A number of text and ABNF clarifications based on suggestions from
   Ted Hardie, Paul Kyzivat and Adam Roach.

   Changed the "non-signalled track handling" to create a single stream
   with multiple tracks again, according to discussions at TPAC in
   November 2014

B.15.  Changes from -08 to -09

   Removed "wms-semantic" and all mention of multiple semantics for
   msid, as agreed at the Dallas IETF, March 2015.

   Addressed a number of review comments from Fleming Andresen and

   Changed the term "m-line" to "media description", since that is the
   term used in RFC 4566.

   Tried to make sure this document does not describe the API to the

B.16.  Changes from -09 to -10

   Addressed review comments from Paul Kyzivat.

B.17.  Changes from -10 to -11

   Defined the semantics of multiple MSIDs in a media section to be a
   MediaStreamTrack present in multiple MediaStreams.

   Made an explicit note that MediaStreamTracks are unidirectional.

   Disallowed the option of sending multiple media sections with the
   same msid (id and appdata identical).

B.18.  Changes from -11 to -12

   Added mux-category to the IANA considerations section.

B.19.  Changes from -12 to -13

   Modified registration description to delete dependency on -4566-bis

B.20.  Changes from -13 to -14

   Addressed nits found in Gen-ART review

B.21.  Changes from -14 to -15

   Added the terminology section.  Switched from "(RTP) media stream" to
   "RTP stream" per RFC 7656.

   Added a mention of random ID generation to the security
   considerations section.

   Moved definition pointers for MediaStream and MediaStreamTrack to the
   "mediacapture-streams" document.

   Added note that syntactically invalid MSID fields SHOULD be ignored.

   Various small changes based on review feedback during IESG

B.22.  Changes from -15 to -16

   Added the special "-" value that means "no MediaStream".

   Changed instances of a MediaStreamTrack being "closed" to saying it's
   "ended", in accordance with WebRTC terminology.

B.23.  Changes from -16 to -17

   Added text to allow omitting track identifiers, per JSEP PR #850

Author's Address

   Harald Alvestrand
   Kungsbron 2
   Stockholm  11122