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Versions: (draft-alvestrand-rtcweb-msid) 00 01 02 draft-ietf-mmusic-msid

Network Working Group                                      H. Alvestrand
Internet-Draft                                                    Google
Intended status: Standards Track                        October 15, 2012
Expires: April 18, 2013


Cross Session Stream Identification in the Session Description Protocol
                    draft-alvestrand-mmusic-msid-01

Abstract

   This document specifies a grouping mechanism for RTP media streams
   that can be used to specify relations between 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 MMUSIC WG list, 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 April 18, 2013.

Copyright Notice

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



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   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 Media Streams  . . . . . . . . . . . .  6
     4.1.  Handling of non-signalled tracks . . . . . . . . . . . . .  7
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Design considerations, open questions and and
                alternatives  . . . . . . . . . . . . . . . . . . . . 10
   Appendix B.  Change log  . . . . . . . . . . . . . . . . . . . . . 11
     B.1.  Changes from rtcweb-msid-00 to -01 . . . . . . . . . . . . 11
     B.2.  Changes from rtcweb-msid-01 to -02 . . . . . . . . . . . . 11
     B.3.  Changes from rtcweb-msid-02 to mmusic-msid-00  . . . . . . 11
     B.4.  Changes from mmusic-msid-00 to -01 . . . . . . . . . . . . 12
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12














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1.  Introduction

1.1.  Structure Of This Document

   This document extends the SSRC grouping framework [RFC5888] by adding
   a new grouping relation that can cross RTP session boundaries.

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

   Section 2 gives the definition of the new mechanism.

   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 .

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 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.

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

   The SDP grouping framework [RFC5888] can be used to group RTP
   sessions.  When an RTP session carries one and only one RTP media
   stream, it is possible to associate RTP media streams across
   different RTP sessions.  However, if an RTP session 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



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   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.

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 signalled.

   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

   This document extends the Source-Specific Media Attributes framework
   [RFC5576] by adding a new "msid" attribute that can be used with the
   "a=ssrc" SDP attribute.  This new attribute allows endpoints to
   associate RTP media streams that are carried in different RTP
   sessions, as well as allowing 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.
     ; This attribute should be used with the ssrc-attr from RFC 5576.
     attribute =/ msid-attr
     msid-attr = "msid:" identifier [ " " appdata ]
     identifier = 1*64 ("0".."9" / "a".."z" / "-")
     appdata = 1*64 ("0".."9" / "a".."z" / "-")


   An example MSID value for the SSRC 1234 might look like this:
     a=ssrc:1234 msid:examplefoo v1


   The identifier is a 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 identifier values used in the same SDP
   session.  It is RECOMMENDED that 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 on a single SSRC.  There may
   also be multiple SSRCs that have the same value for identifier and
   application data.

   Endpoints can update the associations between SSRCs 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

   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 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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   The ABNF of msid-semantic is:

     attribute =/ msid-semantic-attr
     msid-semantic-attr = "msid-semantic:" " " identifier 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".

   An example msid-semantic might look like this:
     a=msid-semantic: examplefoo LS



4.  Applying Msid to WebRTC Media Streams

   This section creates a new semantic for use with the framework
   defined in Section 2, to be used for associating SSRCs representing
   media stream tracks with media streams as defined in
   [W3C.WD-webrtc-20120209].

   The semantic token 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
   used.)

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

   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 zeros.  The first track is track
   zero, and is identified as "a0" for audio, and "v0" for video.

   If two different SSRCs have the same value for identifier and
   appdata, it means that these two SSRCs are both intended for the same
   MediaStreamTrack.  This may occur if the sender wishes to use
   simulcast or forward error correction, or if the sender intends to
   switch between multiple codecs on the same MediaStreamTrack.

   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.



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   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 [RFC3550] 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 identifier 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 signalled.




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   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 the SSRC disappears according to
   the rules of RFC 3550 section 6.3.4 and 6.3.5.


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 rtcweb@ietf.org, 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

   IANA is requested to replace "RFC XXXX" with the RFC number of this



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   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.

   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.

   Special thanks to Miguel Garcia for his work in reviewing this draft,
   with many specific language suggestions.


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.

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

   [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., Narayanan, A., and C.
              Jennings, "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>.



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8.2.  Informative References

   [I-D.westerlund-avtcore-multiplex-architecture]
              Westerlund, M., Burman, B., Perkins, C., and H.
              Alvestrand, "Guidelines for using the Multiplexing
              Features of RTP",
              draft-westerlund-avtcore-multiplex-architecture-02 (work
              in progress), July 2012.

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

   [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.


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



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   when crypto keys aren't available.  Input sought.

   There's been a suggestion that acceptable SSRCs should be signalled
   in a response, giving a recipient the ability to say "no" to certain
   SSRCs.  This is not supported in the current version of this
   document.

   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 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 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 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.






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B.4.  Changes from 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.


Author's Address

   Harald Alvestrand
   Google
   Kungsbron 2
   Stockholm,   11122
   Sweden

   Email: harald@alvestrand.no































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