draft-ietf-mmusic-msid-00.txt   draft-ietf-mmusic-msid-01.txt 
Network Working Group H. Alvestrand Network Working Group H. Alvestrand
Internet-Draft Google Internet-Draft Google
Intended status: Standards Track February 10, 2013 Intended status: Standards Track August 13, 2013
Expires: August 14, 2013 Expires: February 14, 2014
Cross Session Stream Identification in the Session Description Protocol Cross Session Stream Identification in the Session Description Protocol
draft-ietf-mmusic-msid-00 draft-ietf-mmusic-msid-01
Abstract Abstract
This document specifies a grouping mechanism for RTP media streams This document specifies a grouping mechanism for RTP media streams
that can be used to specify relations between media streams within that can be used to specify relations between media streams.
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 This mechanism is used to signal the association between the SDP
concept of SSRC and the WebRTC concept of "MediaStream" / concept of "m-line" and the WebRTC concept of "MediaStream" /
"MediaStreamTrack" using SDP signaling. "MediaStreamTrack" using SDP signaling.
This document is a work item of the MMUSIC WG, whose discussion list This document is a work item of the MMUSIC WG, whose discussion list
is mmusic@ietf.org. is mmusic@ietf.org.
Requirements Language Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
skipping to change at page 1, line 47 skipping to change at page 1, line 44
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 14, 2013. This Internet-Draft will expire on February 14, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
skipping to change at page 2, line 23 skipping to change at page 2, line 21
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Structure Of This Document . . . . . . . . . . . . . . . . 3 1.1. Structure Of This Document . . . . . . . . . . . . . . . . 3
1.2. Why A New Mechanism Is Needed . . . . . . . . . . . . . . 3 1.2. Why A New Mechanism Is Needed . . . . . . . . . . . . . . 3
1.3. Application to the WEBRTC MediaStream . . . . . . . . . . 4 1.3. Application to the WEBRTC MediaStream . . . . . . . . . . 4
2. The Msid Mechanism . . . . . . . . . . . . . . . . . . . . . . 5 2. The Msid Mechanism . . . . . . . . . . . . . . . . . . . . . . 4
3. The Msid-Semantic Attribute . . . . . . . . . . . . . . . . . 6 3. The Msid-Semantic Attribute . . . . . . . . . . . . . . . . . 5
4. Applying Msid to WebRTC MediaStreams . . . . . . . . . . . . . 6 4. Applying Msid to WebRTC MediaStreams . . . . . . . . . . . . . 6
4.1. Handling of non-signalled tracks . . . . . . . . . . . . . 7 4.1. Handling of non-signalled tracks . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 10 8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 10 8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Design considerations, open questions and and Appendix A. Design considerations, open questions and and
alternatives . . . . . . . . . . . . . . . . . . . . 11 alternatives . . . . . . . . . . . . . . . . . . . . 11
Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 11 Appendix B. Usage with multiple MediaStreams per M-line . . . . . 11
B.1. Changes from rtcweb-msid-00 to -01 . . . . . . . . . . . . 12 B.1. Mechanism design with multiple SSRCs . . . . . . . . . . . 11
B.2. Changes from alvestrand-rtcweb-msid-01 to -02 . . . . . . 12 B.2. Usage with the SSRC attribute . . . . . . . . . . . . . . 12
B.3. Changes from alvestrand-rtcweb-msid-02 to Appendix C. Change log . . . . . . . . . . . . . . . . . . . . . 13
mmusic-msid-00 . . . . . . . . . . . . . . . . . . . . . . 12 C.1. Changes from alvestrand-rtcweb-msid-00 to -01 . . . . . . 13
B.4. Changes from alvestrand-mmusic-msid-00 to -01 . . . . . . 12 C.2. Changes from alvestrand-rtcweb-msid-01 to -02 . . . . . . 13
B.5. Changes from alvestrand-mmusic-msid-01 to -02 . . . . . . 12 C.3. Changes from alvestrand-rtcweb-msid-02 to
B.6. Changes from alvestrand-mmusic-msid-02 to mmusic-msid-00 . . . . . . . . . . . . . . . . . . . . . . 13
ietf-mmusic-00 . . . . . . . . . . . . . . . . . . . . . . 13 C.4. Changes from alvestrand-mmusic-msid-00 to -01 . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13 C.5. Changes from alvestrand-mmusic-msid-01 to -02 . . . . . . 14
C.6. Changes from alvestrand-mmusic-msid-02 to
ietf-mmusic-00 . . . . . . . . . . . . . . . . . . . . . . 14
C.7. Changes from mmusic-msid-00 to -01 . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction 1. Introduction
1.1. Structure Of This Document 1.1. Structure Of This Document
This document extends the SSRC grouping framework [RFC5888] by adding This document extends the SSRC grouping framework [RFC5888] by adding
a new grouping relation that can cross RTP session boundaries if a new grouping relation that can cross RTP session boundaries if
needed. needed.
Section 1.2 gives the background on why a new mechanism is needed. Section 1.2 gives the background on why a new mechanism is needed.
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1.2. Why A New Mechanism Is Needed 1.2. Why A New Mechanism Is Needed
When media is carried by RTP [RFC3550], each RTP media stream is 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 inside an RTP session by its SSRC; each RTP session is
distinguished from all other RTP sessions by being on a different distinguished from all other RTP sessions by being on a different
transport association (strictly speaking, 2 transport associations, transport association (strictly speaking, 2 transport associations,
one used for RTP and one used for RTCP, unless RTCP multiplexing one used for RTP and one used for RTCP, unless RTCP multiplexing
[RFC5761] is used). [RFC5761] is used).
SDP gives a description based on m-lines. According to the model
used in [I-D.roach-mmusic-unified-plan], each m-line describes
exactly one media source, and if mulitple media sources are carried
in an RTP session, this is signalled using BUNDLE
[I-D.ietf-mmusic-sdp-bundle-negotiation]; if BUNDLE is not used, each
media source is carried in its own RTP session.
There exist cases where an application using RTP and SDP needs to There exist cases where an application using RTP and SDP needs to
signal some relationship between RTP media streams that may be signal some relationship between RTP media streams that may be
carried in either the same RTP session or different RTP sessions. carried in either the same RTP session or different RTP sessions.
For instance, there may be a need to signal a relationship between a 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 video track and an audio track, and where the generator of the SDP
session. In traditional SDP, it is not possible to signal that these does not yet know if they will be carried in the same RTP session or
two tracks should be carried in one session, so they are carried in
different RTP sessions. 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. 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 However, there is sometimes the need for an application to specify
possible to associate RTP media streams across different m-lines. some application-level information about the association between the
However, if an m-line has multiple RTP media streams, using multiple SSRC and the group. This is not possible using the SDP grouping
SSRCs, the SDP grouping framework cannot be used for this purpose. framework.
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 1.3. Application to the WEBRTC MediaStream
The W3C WebRTC API specification [W3C.WD-webrtc-20120209] specifies The W3C WebRTC API specification [W3C.WD-webrtc-20120209] specifies
that communication between WebRTC entities is done via MediaStreams, that communication between WebRTC entities is done via MediaStreams,
which contain MediaStreamTracks. A MediaStreamTrack is generally which contain MediaStreamTracks. A MediaStreamTrack is generally
carried using a single SSRC in an RTP session (forming an RTP media carried using a single SSRC in an RTP session (forming an RTP media
stream. The collision of terminology is unfortunate.) There might stream. The collision of terminology is unfortunate.) There might
possibly be additional SSRCs, possibly within additional RTP possibly be additional SSRCs, possibly within additional RTP
sessions, in order to support functionality like forward error sessions, in order to support functionality like forward error
correction or simulcast. This complication is ignored below. correction or simulcast. This complication is ignored below.
In the RTP specification, media streams are identified using the SSRC In the RTP specification, media streams are identified using the SSRC
field. Streams are grouped into RTP Sessions, and also carry a field. Streams are grouped into RTP Sessions, and also carry a
CNAME. Neither CNAME nor RTP session correspond to a MediaStream. CNAME. Neither CNAME nor RTP session correspond to a MediaStream.
Therefore, the association of an RTP media stream to MediaStreams Therefore, the association of an RTP media stream to MediaStreams
need to be explicitly signaled. need to be explicitly signaled.
The marking needs to be on a per-SSRC basis, since one RTP session The WebRTC work has come to agreement (documented in
can carry media from multiple MediaStreams, and one MediaStream can [I-D.roach-mmusic-unified-plan]) that one M-line is used to describe
have media in multiple RTP sessions. This means that the [RFC4574] each MediaStreamTrack, and that the BUNDLE mechanism
"label" attribute, which is used to label m-lines, is not usable for [I-D.ietf-mmusic-sdp-bundle-negotiation] is used to group
this purpose. MediaStreamTracks into RTP sessions. Therefore, the need is to
specify the ID of a MediaStreamTrack and its containing MediaStream
The marking needs to also carry the unique identifier of the RTP for each M-line, which can be accomplished with a media-level
media stream as a MediaStreamTrack within the media stream; this is attribute.
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. This usage is described in Section 4.
2. The Msid Mechanism 2. The Msid Mechanism
This document extends the Source-Specific Media Attributes framework This document registers a new SDP [RFC4566] media-level "msid"
[RFC5576] by adding a new "msid" attribute that can be used with the attribute. This new attribute allows endpoints to associate RTP
"a=ssrc" SDP attribute. This new attribute allows endpoints to media streams that are carried in the same or different m-lines, as
associate RTP media streams that are carried in the same or different well as allowing application-specific information to the association.
m-lines, as well as allowing application-specific information to the
association.
The value of the "msid" attribute consists of an identifier and The value of the "msid" attribute consists of an identifier and
optional application-specific data, according to the following ABNF optional application-specific data, according to the following ABNF
[RFC5234] grammar: [RFC5234] grammar:
; "attribute" is defined in RFC 4566. ; "attribute" is defined in RFC 4566.
; This attribute should be used with the ssrc-attr from RFC 5576.
attribute =/ msid-attr attribute =/ msid-attr
msid-attr = "msid:" identifier [ " " appdata ] msid-attr = "msid:" identifier [ " " appdata ]
identifier = token identifier = token
appdata = token appdata = token
An example MSID value for the SSRC 1234 might look like this: An example MSID value for a group with the identifier "examplefoo"
a=ssrc:1234 msid:examplefoo v1 and application data "examplebar" might look like this:
msid:examplefoo examplebar
The identifier is a string of ASCII characters chosen from 0-9, a-z, 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 A-Z and - (hyphen), consisting of between 1 and 64 characters. It
MUST be unique among the identifier values used in the same SDP MUST be unique among the identifier values used in the same SDP
session. It is RECOMMENDED that is generated using a random-number session. It is RECOMMENDED that is generated using a random-number
generator. generator.
Application data is carried on the same line as the identifier, Application data is carried on the same line as the identifier,
separated from the identifier by a space. separated from the identifier by a space.
The identifier uniquely identifies a group within the scope of an SDP The identifier uniquely identifies a group within the scope of an SDP
description. description.
There may be multiple msid attributes on a single SSRC. There may There may be multiple msid attributes on a single m-line. There may
also be multiple SSRCs that have the same value for identifier and also be multiple m-lines that have the same value for identifier and
application data. application data.
Endpoints can update the associations between SSRCs as expressed by Endpoints can update the associations between SSRCs as expressed by
msid attributes at any time; the semantics and restrictions of such msid attributes at any time; the semantics and restrictions of such
grouping and ungrouping are application dependent. grouping and ungrouping are application dependent.
3. The Msid-Semantic Attribute 3. The Msid-Semantic Attribute
In order to fully reproduce the semantics of the SDP and SSRC In order to fully reproduce the semantics of the SDP and SSRC
grouping frameworks, a session-level attribute is defined for grouping frameworks, a session-level attribute is defined for
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appdata, it means that these two SSRCs are both intended for the same appdata, it means that these two SSRCs are both intended for the same
MediaStreamTrack. This may occur if the sender wishes to use MediaStreamTrack. This may occur if the sender wishes to use
simulcast or forward error correction, or if the sender intends to simulcast or forward error correction, or if the sender intends to
switch between multiple codecs on the same MediaStreamTrack. switch between multiple codecs on the same MediaStreamTrack.
When an SDP description is updated, a specific msid continues to When an SDP description is updated, a specific msid continues to
refer to the same MediaStream. Once negotiation has completed on a refer to the same MediaStream. Once negotiation has completed on a
session, there is no memory; an msid value that appears in a later session, there is no memory; an msid value that appears in a later
negotiation will be taken to refer to a new MediaStream. negotiation will be taken to refer to a new MediaStream.
The following are the rules for handling updates of the list of SSRCs The following are the rules for handling updates of the list of
and their msid values. m-lines and their msid values.
o When a new msid value occurs in the description, the recipient can o When a new msid value occurs in the description, the recipient can
signal to its application that a new MediaStream has been added. signal to its application that a new MediaStream has been added.
o When a description is updated to have more SSRCs with the same o When a description is updated to have more SSRCs with the same
msid value, but different appdata values, the recipient can signal msid value, but different appdata values, the recipient can signal
to its application that new media stream tracks have been added to to its application that new MediaStreamTracks have been added to
the media stream. the media stream.
o When a description is updated to no longer list the msid value on 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 a specific ssrc, the recipient can signal to its application that
the corresponding media stream track has been closed. the corresponding media stream track has been closed.
o When a description is updated to no longer list the msid value on 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 any ssrc, the recipient can signal to its application that the
media stream has been closed. media stream has been closed.
In addition to signaling that the track is closed when it disappears 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 from the SDP, the track will also be signaled as being closed when
the SSRC disappears by the rules of [RFC3550] section 6.3.4 (BYE all associated SSRCs have disappeared by the rules of [RFC3550]
packet received) and 6.3.5 (timeout). section 6.3.4 (BYE packet received) and 6.3.5 (timeout).
4.1. Handling of non-signalled tracks 4.1. Handling of non-signalled tracks
Pre-WebRTC entities will not send msid. This means that there will Pre-WebRTC entities will not send msid. This means that there will
be some incoming RTP packets with SSRCs where the recipient does not be some incoming RTP packets with SSRCs where the recipient does not
know about a corresponding MediaStream id. know about a corresponding MediaStream id.
Handling will depend on whether or not any SSRCs are signaled in the Handling will depend on whether or not any MSIDs are signaled in the
relevant m-line(s). There are two cases: relevant m-line(s). There are two cases:
o No SSRC is signaled with an msid attribute. The SDP session is o No msid-semantic:WMS attribute is present. The SDP session is
assumed to be a backwards-compatible session. All incoming SSRCs, assumed to be a backwards-compatible session. All incoming SSRCs,
on all m-lines that are part of the SDP session, are assumed to on all m-lines that are part of the SDP session, are assumed to
belong to independent media streams, each with one track. The belong to independent media streams, each with one track. The
identifier of this media stream and of the media stream track is a identifier of this media stream and of the media stream track is a
randomly generated string; the label of this media stream will be randomly generated string; the label of this media stream will be
set to "Non-WMS stream". set to "Non-WMS stream".
o Some SSRCs are signaled with an msid attribute. In this case, the o An msid-semantic:WMS attribute is present. In this case, the
session is WebRTC compatible, and the newly arrived SSRCs are session is WebRTC compatible, and the newly arrived SSRCs are
either caused by a bug or by timing skew between the arrival of either caused by a bug or by timing skew between the arrival of
the media packets and the SDP description. These packets MAY be the media packets and the SDP description. These packets MAY be
discarded, or they MAY be buffered for a while in order to allow discarded, or they MAY be buffered for a while in order to allow
immediate startup of the media stream when the SDP description is immediate startup of the media stream when the SDP description is
updated. The arrival of media packets MUST NOT cause a new updated. The arrival of media packets MUST NOT cause a new
MediaStreamTrack to be signaled. MediaStreamTrack to be signaled.
If a WebRTC entity sends a description, it MUST include the msid- If a WebRTC entity sends a description, it MUST include the msid-
semantic:WMS attribute, even if no media streams are sent. This semantic:WMS attribute, even if no media streams are sent. This
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we do not know if the SSRCs are synchronized or not before we learn 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 the CNAME of the SSRCs, which only happens when an RTCP packet
arrives. How to identify a non-WMS stream is still open for arrives. How to identify a non-WMS stream is still open for
discussion - including whether it's necessary to do so. Using the discussion - including whether it's necessary to do so. Using the
stream label seems like an easy thing to do for debuggability - it's stream label seems like an easy thing to do for debuggability - it's
not signalled, and is intended for human consumption anyway. not signalled, and is intended for human consumption anyway.
5. IANA Considerations 5. IANA Considerations
This document requests IANA to register the "msid" attribute in the This document requests IANA to register the "msid" attribute in the
"att-field (source level)" registry within the SDP parameters "att-field (media level only)" registry within the SDP parameters
registry, according to the procedures of [RFC5576] registry, according to the procedures of [RFC4566]
The required information is: The required information is:
o Contact name, email: IETF, contacted via mmusic@ietf.org, or a o Contact name, email: IETF, contacted via mmusic@ietf.org, or a
successor address designated by IESG successor address designated by IESG
o Attribute name: msid o Attribute name: msid
o Long-form attribute name: Media stream group Identifier o Long-form attribute name: Media stream group Identifier
o The attribute value contains only ASCII characters, and is o The attribute value contains only ASCII characters, and is
therefore not subject to the charset attribute. therefore not subject to the charset attribute.
o The attribute gives an association over a set of SSRCs, o The attribute gives an association over a set of m-lines. It can
potentially in different m-lines. It can be used to signal the be used to signal the relationship between a WebRTC MediaStream
relationship between a WebRTC MediaStream and a set of SSRCs. and a set of SSRCs.
o The details of appropriate values are given in RFC XXXX. o The details of appropriate values are given in RFC XXXX.
This document requests IANA to create a new registry called This document requests IANA to create a new registry called
"Semantics for the msid-semantic SDP attribute", which should have "Semantics for the msid-semantic SDP attribute", which should have
exactly the same rules as for the "Semantics for the ssrc-group SDP exactly the same rules as for the "Semantics for the ssrc-group SDP
attribute" registry (Expert Review), and to register the "WMS" attribute" registry (Expert Review), and to register the "WMS"
semantic within this new registry. semantic within this new registry.
The required information is: The required information is:
skipping to change at page 10, line 24 skipping to change at page 10, line 4
reviewing this draft, with many specific language suggestions. reviewing this draft, with many specific language suggestions.
8. References 8. References
8.1. Normative References 8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 3550, July 2003. Applications", RFC 3550, July 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 5234, January 2008. Specifications: ABNF", RFC 5234, January 2008.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009. (SDP)", RFC 5576, June 2009.
[W3C.WD-webrtc-20120209] [W3C.WD-webrtc-20120209]
Bergkvist, A., Burnett, D., Narayanan, A., and C. Bergkvist, A., Burnett, D., Narayanan, A., and C.
Jennings, "WebRTC 1.0: Real-time Communication Between Jennings, "WebRTC 1.0: Real-time Communication Between
Browsers", World Wide Web Consortium WD WD-webrtc- Browsers", World Wide Web Consortium WD WD-webrtc-
20120209, February 2012, 20120209, February 2012,
<http://www.w3.org/TR/2012/WD-webrtc-20120209>. <http://www.w3.org/TR/2012/WD-webrtc-20120209>.
8.2. Informative References 8.2. Informative References
[I-D.ietf-mmusic-sdp-bundle-negotiation] [I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C. and H. Alvestrand, "Multiplexing Negotiation Holmberg, C., Alvestrand, H., and C. Jennings,
Using Session Description Protocol (SDP) Port Numbers", "Multiplexing Negotiation Using Session Description
draft-ietf-mmusic-sdp-bundle-negotiation-01 (work in Protocol (SDP) Port Numbers",
progress), August 2012. draft-ietf-mmusic-sdp-bundle-negotiation-03 (work in
progress), February 2013.
[I-D.roach-mmusic-unified-plan]
Roach, A., Uberti, J., and M. Thomson, "A Unified Plan for
Using SDP with Large Numbers of Media Flows",
draft-roach-mmusic-unified-plan-00 (work in progress),
July 2013.
[I-D.westerlund-avtcore-multiplex-architecture] [I-D.westerlund-avtcore-multiplex-architecture]
Westerlund, M., Burman, B., Perkins, C., and H. Westerlund, M., Burman, B., Perkins, C., and H.
Alvestrand, "Guidelines for using the Multiplexing Alvestrand, "Guidelines for using the Multiplexing
Features of RTP", Features of RTP",
draft-westerlund-avtcore-multiplex-architecture-02 (work draft-westerlund-avtcore-multiplex-architecture-02 (work
in progress), July 2012. 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 [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010. Control Packets on a Single Port", RFC 5761, April 2010.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, June 2010. Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
Appendix A. Design considerations, open questions and and alternatives Appendix A. Design considerations, open questions and and alternatives
This appendix should be deleted before publication as an RFC. This appendix should be deleted before publication as an RFC.
skipping to change at page 11, line 48 skipping to change at page 11, line 34
Discarding of incoming data when the SDP description isn't updated Discarding of incoming data when the SDP description isn't updated
yet (section 3) may cause clipping. However, the same issue exists yet (section 3) may cause clipping. However, the same issue exists
when crypto keys aren't available. Input sought. when crypto keys aren't available. Input sought.
There's been a suggestion that acceptable SSRCs should be signaled in There's been a suggestion that acceptable SSRCs should be signaled in
a response, giving a recipient the ability to say "no" to certain a response, giving a recipient the ability to say "no" to certain
SSRCs. This is not supported in the current version of this SSRCs. This is not supported in the current version of this
document. document.
Appendix B. Change log Appendix B. Usage with multiple MediaStreams per M-line
This appendix is included to document the usage of msid as a source-
specific attribute. Prior to the acceptance of the Unified Plan
document, some implementations used this mechanism to distinguish
between multiple MediaStreamTracks that were carried in the same
M-line.
It reproduces some of the original justification text for this
mechanism that is not relevant when Unified Plan is used.
B.1. Mechanism design with multiple SSRCs
When media is carried by RTP [RFC3550], each RTP media stream is
distinguished inside an RTP session by its SSRC; each RTP session is
distinguished from all other RTP sessions by being on a different
transport association (strictly speaking, 2 transport associations,
one used for RTP and one used for RTCP, unless RTCP multiplexing
[RFC5761] is used).
There exist cases where an application using RTP and SDP needs to
signal some relationship between RTP media streams that may be
carried in either the same RTP session or different RTP sessions.
For instance, there may be a need to signal a relationship between a
video track in one RTP session and an audio track in another RTP
session. In traditional SDP, it is not possible to signal that these
two tracks should be carried in one session, so they are carried in
different RTP sessions.
Traditionally, SDP was used to describe the RTP sessions, with one
m-line being used to describe each RTP session. With the advent of
extensions like BUNDLE [I-D.ietf-mmusic-sdp-bundle-negotiation], this
association may be more complex, with multiple m-lines being used to
describe one RTP session; the rest of this document therefore talks
about m-lines, not RTP sessions, when describing the signalling
mechanism.
The SSRC grouping mechanism ("a=ssrc-group") [RFC5576] can be used to
associate RTP media streams when those RTP media streams are
described by the same m-line. The semantics of this mechanism
prevent the association of RTP media streams that are spread across
different m-lines.
The SDP grouping framework [RFC5888] can be used to group m-lines.
When an m-line describes one and only one RTP media stream, it is
possible to associate RTP media streams across different m-lines.
However, if an m-line has multiple RTP media streams, using multiple
SSRCs, the SDP grouping framework cannot be used for this purpose.
There are use cases (some of which are discussed in
[I-D.westerlund-avtcore-multiplex-architecture] ) where neither of
these approaches is appropriate; In those cases, a new mechanism is
needed.
In addition, there is sometimes the need for an application to
specify some application-level information about the association
between the SSRC and the group. This is not possible using either of
the frameworks above.
B.2. Usage with the SSRC attribute
When the MSID attribute was used with the SSRC attribute, it had to
be registered in the "Attribute names (source level)" registry rather
than the "Attribute names (media level only)" registry, and the msid
line was prefixed with "a=ssrc:<ssrc> ". Apart from that, usage of
the attribute with SSRC-bound flows was identical with the current
proposal.
Appendix C. Change log
This appendix should be deleted before publication as an RFC. This appendix should be deleted before publication as an RFC.
B.1. Changes from rtcweb-msid-00 to -01 C.1. Changes from alvestrand-rtcweb-msid-00 to -01
Added track identifier. Added track identifier.
Added inclusion-by-reference of draft-lennox-mmusic-source-selection Added inclusion-by-reference of draft-lennox-mmusic-source-selection
for track muting. for track muting.
Some rewording. Some rewording.
B.2. Changes from alvestrand-rtcweb-msid-01 to -02 C.2. Changes from alvestrand-rtcweb-msid-01 to -02
Split document into sections describing a generic grouping mechanism Split document into sections describing a generic grouping mechanism
and sections describing the application of this grouping mechanism to and sections describing the application of this grouping mechanism to
the WebRTC MediaStream concept. the WebRTC MediaStream concept.
Removed the mechanism for muting tracks, since this is not central to Removed the mechanism for muting tracks, since this is not central to
the MSID mechanism. the MSID mechanism.
B.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00 C.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00
Changed the draft name according to the wishes of the MMUSIC group Changed the draft name according to the wishes of the MMUSIC group
chairs. chairs.
Added text indicting cases where it's appropriate to have the same Added text indicting cases where it's appropriate to have the same
appdata for multiple SSRCs. appdata for multiple SSRCs.
Minor textual updates. Minor textual updates.
B.4. Changes from alvestrand-mmusic-msid-00 to -01 C.4. Changes from alvestrand-mmusic-msid-00 to -01
Increased the amount of explanatory text, much based on a review by Increased the amount of explanatory text, much based on a review by
Miguel Garcia. Miguel Garcia.
Removed references to BUNDLE, since that spec is under active Removed references to BUNDLE, since that spec is under active
discussion. discussion.
Removed distinguished values of the MSID identifier. Removed distinguished values of the MSID identifier.
B.5. Changes from alvestrand-mmusic-msid-01 to -02 C.5. Changes from alvestrand-mmusic-msid-01 to -02
Changed the order of the "msid-semantic: " attribute's value fields Changed the order of the "msid-semantic: " attribute's value fields
and allowed multiple identifiers. This makes the attribute useful as and allowed multiple identifiers. This makes the attribute useful as
a marker for "I understand this semantic". a marker for "I understand this semantic".
Changed the syntax for "identifier" and "appdata" to be "token". Changed the syntax for "identifier" and "appdata" to be "token".
Changed the registry for the "msid-semantic" attribute values to be a Changed the registry for the "msid-semantic" attribute values to be a
new registry, based on advice given in Atlanta. new registry, based on advice given in Atlanta.
B.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00 C.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00
Updated terminology to refer to m-lines rather than RTP sessions when Updated terminology to refer to m-lines rather than RTP sessions when
discussing SDP formats and the ability of other linking mechanisms to discussing SDP formats and the ability of other linking mechanisms to
refer to SSRCs. refer to SSRCs.
Changed the "default" mechanism to return independent streams after Changed the "default" mechanism to return independent streams after
considering the synchronization problem. considering the synchronization problem.
Removed the space from between "msid-semantic" and its value, to be Removed the space from between "msid-semantic" and its value, to be
consistent with RFC 5576. consistent with RFC 5576.
C.7. Changes from mmusic-msid-00 to -01
Reworked msid mechanism to be a per-m-line attribute, to align with
[I-D.roach-mmusic-unified-plan]
Author's Address Author's Address
Harald Alvestrand Harald Alvestrand
Google Google
Kungsbron 2 Kungsbron 2
Stockholm, 11122 Stockholm, 11122
Sweden Sweden
Email: harald@alvestrand.no Email: harald@alvestrand.no
 End of changes. 39 change blocks. 
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