Network Working Group                                      H. Alvestrand
Internet-Draft                                                    Google
Intended status: Standards Track                            May 29, 2012
Expires: November 30, 2012

Cross Session Stream Identification in the Session Description Protocol


   This document specifies a grouping mechanism for RTP media streams
   that can be used to specify relations betweeen media streams within
   different RTP sessions.

   This mechanism is used to signal the association between the RTP
   concept of SSRC and the WebRTC concept of "media stream" / "media
   stream track" using SDP signalling.

   This document is an input document for discussion.  It should be
   discussed in the RTCWEB WG 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   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 November 30, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the

Alvestrand              Expires November 30, 2012               [Page 1]

Internet-Draft                 MSID in SDP                      May 2012

   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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.  Why A New Mechanism Is Needed  . . . . . . . . . . . . . .  3
     1.2.  Application to the WEBRTC MediaStream  . . . . . . . . . .  3
   2.  The Msid Mechanism . . . . . . . . . . . . . . . . . . . . . .  4
   3.  The Msid-Semantic Attribute  . . . . . . . . . . . . . . . . .  5
   4.  Applying Msid to WebRTC Media Streams  . . . . . . . . . . . .  5
     4.1.  Handling of non-signalled tracks . . . . . . . . . . . . .  6
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  8
   Appendix A.  Design considerations, open questions and and
                alternatives  . . . . . . . . . . . . . . . . . . . .  9
   Appendix B.  Change log  . . . . . . . . . . . . . . . . . . . . . 10
     B.1.  Changes from -00 to -01  . . . . . . . . . . . . . . . . . 10
     B.2.  Changes from -01 to -02  . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

Alvestrand              Expires November 30, 2012               [Page 2]

Internet-Draft                 MSID in SDP                      May 2012

1.  Introduction

1.1.  Why A New Mechanism Is Needed

   There exist cases where an application using RTP and SDP needs to
   signal some relationship between RTP media streams (packets carried
   using a single SSRC) that may be carried in either the same RTP
   session or different RTP sessions.

   When all SSRCs are carried in a single RTP session, the "a=ssrc-
   group" mechanism [RFC5576] can be used.

   When each RTP session carries one and only one SSRC, the SDP grouping
   framework [RFC5888] can be used.

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

   (Note: When the bundle mechanism,
   [I-D.ietf-mmusic-sdp-bundle-negotiation], is used, the extension is
   still needed to link SSRCs under different m= lines, even when they
   are in the same RTP session).

   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.

1.2.  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 with 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 signalled.

Alvestrand              Expires November 30, 2012               [Page 3]

Internet-Draft                 MSID in SDP                      May 2012

   The marking needs to be on a per-SSRC basis, since one RTP session
   can carry media from multiple MediaStreams, and one MediaStream can
   have media in multiple RTP sessions.  This means that the [RFC4574]
   "label" attribute, which is used to label RTP sessions, is not usable
   for this purpose.

   The marking needs to also carry the unique identifier of the RTP
   media stream as a MediaStreamTrack within the media stream; this is
   done using a single letter to identify whether it belongs in the
   video or audio track list, and the MediaStreamTrack's position within
   that array.

   This usage is described in Section 4.

2.  The Msid Mechanism

   Grouping of SSRCs is done via an "msid" attribute attached to the
   SSRC in the SDP description, using the "Source Specific Media
   Attribute" mechanism [RFC5576]:
     a=ssrc:1234 msid:examplefoo v1

   The ID is a randomly-generated string of ASCII characters chosen from
   0-9, a-z, A-Z and - (hyphen), consisting of between 1 and 64
   characters.  It MUST be unique among the ID values used in the same
   SDP session.

   The value "default" (all lower case) has special meaning, and MUST
   NOT be generated.  Values starting with "example" (all lower case)
   are reserved for documentation, and MUST NOT be generated by an

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

   ABNF[RFC5234] grammar:

     msidattribute = "msid:" identifier [ " " appdata ]
     identifier = 1*64 ("0".."9" / "a".."z" / "-")
     appdata = 1*64 ("0".."9" / "a".."z" / "-")

   (Note: one possible generation algorithm is to generate 6 random
   bytes, base64 encode them (giving 8 bytes), and prefixing with a
   letter that is neither "d" nor "e".  Another possibility is using
   some form of UUID.)

Alvestrand              Expires November 30, 2012               [Page 4]

Internet-Draft                 MSID in SDP                      May 2012

   The ID uniquely identifies a group within the scope of an SDP

   There may be multiple msid attributes on a single SSRC.

3.  The Msid-Semantic Attribute

   In order to fully reproduce the semantics of the SDP and SSRC
   grouping frameworks, a session-level attribute is defined for
   signalling the semantics associated with an msid grouping.

   This OPTIONAL attribute gives the message ID and its group semantic.
     a=msid-semantic: examplefoo LS

   The ABNF of msid-semantic is:

     msid-semantic-attr = "msid-semantic:" " " msid token
     token = <as defined in RFC 4566>

   The semantic field may hold values from the IANA registries
   "Semantics for the "ssrc-group" SDP Attribute" and "Semantics for the
   "group" SDP Attribute".

4.  Applying Msid to WebRTC Media Streams

   The semantic for WebRTC Media Streams is "WMS".

   The value of the msid corresponds to the "id" attribute of a
   MediaStream. (note: as of Jan 11, 2012, this is called "label".  The
   word "label" means many other things, so the same word should not be

   In a WebRTC-compatible SDP description, all SSRCs intending to be
   sent from one peer will be identified in the SDP generated by that

   The appdata for a WebRTC MediaStreamTrack consists of the track type
   and the track number; the track type is encoded as the single letter
   "a" (audio) or "v" (video), and the track number is encoded as a
   decimal integer with no leading zeroes.  The first track is track
   zero, and is identified as "a0" for audio, and "v0" for video.

   When an SDP description is updated, a specific msid continues to
   refer to the same media stream; an msid value MUST NOT be reused for
   another media stream within a PeerConnection's lifetime.

Alvestrand              Expires November 30, 2012               [Page 5]

Internet-Draft                 MSID in SDP                      May 2012

   The following are the rules for handling updates of the list of SSRCs
   and their msid values.

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

   o  When a description is updated to have more SSRCs with the same
      msid value, the recipient can signal to its application that new
      media stream tracks have been added to the media stream.

   o  When a description is updated to no longer list the msid value on
      a specific ssrc, the recipient can signal to its application that
      the corresponding media stream track has been closed.

   o  When a description is updated to no longer list the msid value on
      any ssrc, the recipient can signal to its application that the
      media stream has been closed.

   OPEN ISSUE: Exactly when should the recipient signal that the track
   is closed?  When the msid value disappears from the description, when
   the SSRC disappears by the rules of RFC 3550 section 6.3.4 (BYE
   packet received) and 6.3.5 (timeout), any of the above, or some
   combination of the above?

4.1.  Handling of non-signalled tracks

   Pre-WebRTC entities will not send msid.  This means that there will
   be some incoming RTP packets with SSRCs where the recipient does not
   know about a corresponding MediaStream id.

   Handling will depend on whether or not any SSRCs are signalled in the
   relevant RTP session.  There are two cases:

   o  No SSRC is signalled with an msid attribute.  The SDP session is
      assumed to be a backwards-compatible session.  All incoming SSRCs,
      on all RTP sessions that are part of the SDP session, are assumed
      to belong to a single media stream.  The ID of this media stream
      is "default".

   o  Some SSRCs are signalled with an msid attribute.  In this case,
      the session is WebRTC compatible, and the newly arrived SSRCs 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 created.

Alvestrand              Expires November 30, 2012               [Page 6]

Internet-Draft                 MSID in SDP                      May 2012

   Note: This means that it is wise to include at least one a=ssrc: line
   with an msid attribute, even when no media streams are yet attached
   to the session.  (Alternative: Mark the RTP session explicitly as "I
   will signal the media stream tracks explicitly").

   It follows from the above that media stream tracks in the "default"
   media stream cannot be closed by signalling; the application must
   instead signal these as closed when either an RTCP BYE packet or the
   absence of media for a defined interval <what interval?> indicates
   that the stream is gone.

5.  IANA Considerations

   This document requests IANA to register the "msid" attribute in the
   "att-field (source level)" registry within the SDP parameters
   registry, according to the procedures of [RFC5576]

   The required information is:

   o  Contact name, email: IETF, contacted via, or a
      successor address designated by IESG

   o  Attribute name: msid

   o  Long-form attribute name: Media stream group Identifier

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

   o  The attribute gives an association over a set of SSRCs,
      potentially in different RTP sessions.  It can be used to signal
      the relationship between a WebRTC MediaStream and a set of SSRCs.

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

   This document requests IANA to register the "WMS" semantic within the
   "Semantics for the "ssrc-group" SDP Attribute" registry within the
   SDP parameters registry.

   The required information is:

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

   o  Token: WMS

   o  Standards track reference: RFC XXXX

Alvestrand              Expires November 30, 2012               [Page 7]

Internet-Draft                 MSID in SDP                      May 2012

   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

   No attacks that are relevant to the browser's security have been
   identified that depend on this mechanism.

7.  Acknowledgements

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

8.  References

8.1.  Normative References

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

   [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.

              Bergkvist, A., Burnett, D., Narayanan, A., and C.
              Jennings, "WebRTC 1.0: Real-time Communication Between
              Browsers", World Wide Web Consortium WD WD-webrtc-
              20120209, February 2012,

8.2.  Informative References

              Holmberg, C. and H. Alvestrand, "Multiplexing Negotiation
              Using Session Description Protocol (SDP) Port Numbers",

Alvestrand              Expires November 30, 2012               [Page 8]

Internet-Draft                 MSID in SDP                      May 2012

              draft-ietf-mmusic-sdp-bundle-negotiation-00 (work in
              progress), February 2012.

              Westerlund, M., Burman, B., and C. Perkins, "RTP
              Multiplexing Architecture",
              draft-westerlund-avtcore-multiplex-architecture-00 (work
              in progress), October 2011.

   [RFC4574]  Levin, O. and G. Camarillo, "The Session Description
              Protocol (SDP) Label Attribute", RFC 4574, August 2006.

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

Appendix A.  Design considerations, open questions and and 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 media streams and
   wanting to have tracks from multiple synchronization contexts within
   one media stream.

   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.

   There has been a suggestion that this mechanism could be used to mute
   tracks too.  This is not done at the moment.

   The special value "default" and the reservation of "example*" seems
   bothersome; apart from that, it's a random string.  It's uncertain
   whether "example" has any benefit.

   An alternative to the "default" media stream is to let each new media
   stream track without a msid attribute create its own media stream.
   Input on this question is sought.

   Discarding of incoming data when the SDP description isn't updated
   yet (section 3) may cause clipping.  However, the same issue exists
   when crypto keys aren't available.  Input sought.

   There's been a suggestion that acceptable SSRCs should be signalled

Alvestrand              Expires November 30, 2012               [Page 9]

Internet-Draft                 MSID in SDP                      May 2012

   in a response, giving a recipient the ability to say "no" to certain
   SSRCs.  This is not supported in the current version of this

   This specification reuses the ssrc-group semantics registry for this
   semantic, on the argument that the WMS purpose is more similar to an
   SSRC grouping than a session-level grouping, and allows values from
   both registries, on the argument that some semantics (like LS) are
   well defined for MSID.  Input sought.

Appendix B.  Change log

   This appendix should be deleted before publication as an RFC.

B.1.  Changes from -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 -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.

Author's Address

   Harald Alvestrand
   Kungsbron 2
   Stockholm,   11122


Alvestrand              Expires November 30, 2012              [Page 10]