Network Working Group H. T. Alvestrand
Internet-Draft Google
Intended status: Standards Track February 10, 2013
Expires: August 14, 2013

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 between media streams within different RTP sessions as well as within a single RTP session, and independently of whether these media streams are described within one SDP m-line or in multiple m-lines.

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

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

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

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This Internet-Draft will expire on August 14, 2013.

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Table of Contents

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 if needed.

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.

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

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.

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 m-lines, 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 the same or different m-lines, as well as allowing application-specific information to the association.

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 = token
  appdata = token

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 signaling 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.

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.

The ABNF of msid-semantic is:

  attribute =/ msid-semantic-attr
  msid-semantic-attr = "msid-semantic:" token (" " identifier)*
  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: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 SSRCs representing MediaStreamTracks within MediaStreams as defined in [W3C.WD-webrtc-20120209].

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

The value of the msid corresponds to the "id" attribute of a MediaStream.

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 "id" attribute of a MediaStreamTrack.

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 MediaStream. Once negotiation has completed on a session, there is no memory; an msid 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 SSRCs and their msid values.

[RFC3550] section 6.3.4 (BYE packet received) and 6.3.5 (timeout).

In addition to signaling that the track is closed when it disappears from the SDP, the track will also be signaled as being closed when the SSRC disappears by the rules of

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 signaled in the relevant m-line(s). There are two cases:

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 whether or not a "default" MediaStream should be created. 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 signaling; 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.

NOTE IN DRAFT: Previous versions of this memo suggested adding all incoming SSRCs to a single MediaStream. This is problematic because we do not know if the SSRCs are synchronized or not before we learn the CNAME of the SSRCs, which only happens when an RTCP packet arrives. How to identify a non-WMS stream is still open for discussion - including whether it's necessary to do so. Using the stream label seems like an easy thing to do for debuggability - it's not signalled, and is intended for human consumption anyway.

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:

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:

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 Section 4.1, the amount of buffering should be limited to avoid memory exhaustion attacks.

No other 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 and Paul Kyzivat for their 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.
[RFC5576] Lennox, J., Ott, J. and T. Schierl, "Source-Specific Media Attributes in the Session Description Protocol (SDP)", RFC 5576, June 2009.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.
[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.

8.2. Informative References

[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.
[I-D.ietf-mmusic-sdp-bundle-negotiation] Holmberg, C., Alvestrand, H. and C. Jennings, "Multiplexing Negotiation Using Session Description Protocol (SDP) Port Numbers", Internet-Draft draft-ietf-mmusic-sdp-bundle-negotiation-03, February 2013.
[I-D.westerlund-avtcore-multiplex-architecture] Westerlund, M., Burman, B., Perkins, C. and H. Alvestrand, "Guidelines for using the Multiplexing Features of RTP", Internet-Draft draft-westerlund-avtcore-multiplex-architecture-02, July 2012.

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 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.

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

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 signaled 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.

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 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 chairs.

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 discussion.

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.

Author's Address

Harald Alvestrand Google Kungsbron 2 Stockholm, 11122 Sweden EMail: