Network Working Group H. Alvestrand
Internet-Draft Google
Intended status: Standards Track October 14, 2014
Expires: April 17, 2015
WebRTC MediaStream Identification in the Session Description Protocol
draft-ietf-mmusic-msid-07
Abstract
This document specifies a Session Description Protocol (SDP) Grouping
mechanism for RTP media streams that can be used to specify relations
between media streams.
This mechanism is used to signal the association between the SDP
concept of "m-line" and the WebRTC concept of "MediaStream" /
"MediaStreamTrack" using SDP signaling.
This document is a work item of the MMUSIC WG, whose discussion list
is 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 17, 2015.
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Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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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 . . . . . . . . . . . . . . . . . . . . . 5
3. The Msid-Semantic Attribute . . . . . . . . . . . . . . . . . 6
4. Generic SDP Offer/Answer Procedures . . . . . . . . . . . . . 6
4.1. Generating the Initial Offer . . . . . . . . . . . . . . 7
4.2. Answerer Processing of the Offer . . . . . . . . . . . . 7
4.3. Generating the Answer . . . . . . . . . . . . . . . . . . 7
4.4. Offerer Processing of the Answer . . . . . . . . . . . . 7
5. Applying Msid to WebRTC MediaStreams . . . . . . . . . . . . 7
5.1. Handling of non-signalled tracks . . . . . . . . . . . . 8
5.2. Detailed Offer/Answer Procedures . . . . . . . . . . . . 9
5.2.1. Generating the initial offer . . . . . . . . . . . . 10
5.2.2. Parsing the initial offer . . . . . . . . . . . . . . 10
5.2.3. Generating the answer . . . . . . . . . . . . . . . . 10
5.2.4. Offerer processing of the answer . . . . . . . . . . 10
5.2.5. Modifying the session . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Design considerations, rejected alternatives . . . . 14
Appendix B. Usage with multiple MediaStreams per M-line . . . . 14
B.1. Mechanism design with multiple SSRCs . . . . . . . . . . 15
B.2. Usage with the SSRC attribute . . . . . . . . . . . . . . 16
Appendix C. Change log . . . . . . . . . . . . . . . . . . . . . 16
C.1. Changes from alvestrand-rtcweb-msid-00 to -01 . . . . . . 16
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C.2. Changes from alvestrand-rtcweb-msid-01 to -02 . . . . . . 16
C.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00 16
C.4. Changes from alvestrand-mmusic-msid-00 to -01 . . . . . . 16
C.5. Changes from alvestrand-mmusic-msid-01 to -02 . . . . . . 17
C.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00 17
C.7. Changes from mmusic-msid-00 to -01 . . . . . . . . . . . 17
C.8. Changes from mmusic-msid-01 to -02 . . . . . . . . . . . 17
C.9. Changes from mmusic-msid-02 to -03 . . . . . . . . . . . 17
C.10. Changes from mmusic-msid-03 to -04 . . . . . . . . . . . 18
C.11. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 18
C.12. Changes from -05 to -06 . . . . . . . . . . . . . . . . . 18
C.13. Changes from -06 to -07 . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
1.1. Structure Of This Document
This document adds a new Session Description Protocol (SDP)
Grouping[RFC5888] relation between SDP m-lines [RFC4566] that can
associate application layer identifiers with the binding between
media streams, attaching identifiers to the media streams and
attaching identifiers to the groupings they form.
Section 1.2 gives the background on why a new mechanism is needed.
Section 2 gives the definition of the new mechanism.
Section 3 gives the definition of the msid-semantic field, which
gives the possibility of using MSIDs with different semantics in the
same SDP message.
Section 5 gives the application of the new mechanism for providing
necessary semantic information for the association of
MediaStreamTracks to MediaStreams in the WebRTC
API[W3C.WD-webrtc-20120209].
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 RTP/RTCP multiplexing
[RFC5761] is used).
SDP gives a description based on m-lines. According to the model
used in [I-D.ietf-rtcweb-jsep], each m-line describes exactly one
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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
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 and an audio track, and where the generator of the SDP
does not yet know if they will be carried in the same RTP session or
different RTP sessions.
The SDP grouping framework [RFC5888] can be used to group m-lines.
However, there is sometimes the need for an application to specify
some application-level information about the association between the
m-line and the group. This is not possible using the SDP grouping
framework.
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.
WebRTC defines a mapping (documented in [I-D.ietf-rtcweb-jsep]) where
one SDP m-line is used to describe each MediaStreamTrack, and that
the BUNDLE mechanism [I-D.ietf-mmusic-sdp-bundle-negotiation] is used
to group MediaStreamTracks into RTP sessions. Therefore, the need is
to specify the ID of a MediaStreamTrack and its associated
MediaStream for each m-line, which can be accomplished with a media-
level SDP attribute.
This usage is described in Section 5.
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2. The Msid Mechanism
This document defines a new SDP [RFC4566] media-level "msid"
attribute. This new attribute allows endpoints to associate RTP
media streams that are carried in the same or different m-lines. The
attribute also allows 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.
attribute =/ msid-attr
msid-attr = "msid:" msid-id [ SP msid-appdata ]
msid-id = 1*64token-char ; see RFC 4566
msid-appdata = 1*64token-char ; see RFC 4566
An example msid value for a group with the identifier "examplefoo"
and application data "examplebar" might look like this:
msid:examplefoo examplebar
The identifier is a string of ASCII characters that are legal in a
"token", consisting of between 1 and 64 characters. It MUST be
unique among the identifier values used in the same SDP session. It
is RECOMMENDED that it 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 in a single media description.
There may also be multiple media descriptions that have the same
value for identifier and application data.
Endpoints can update the associations between RTP media streams as
expressed by msid attributes at any time; the semantics and
restrictions of such grouping and ungrouping are application
dependent.
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3. The Msid-Semantic Attribute
A session-level attribute is defined for signaling the semantics
associated with an msid grouping. This allows msid groupings with
different semantics to coexist.
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.
This attribute MUST be present if "a=msid" is used.
An empty list of identifiers is an indication that the sender
supports the indicated semantic, but has no msid groupings of the
given type in the present SDP.
An identifier of "*" is an indication that all "a=msid" lines in the
SDP have this specific semantic.
The ABNF of msid-semantic is:
attribute =/ msid-semantic-attr
msid-semantic-attr = "msid-semantic:" msid-semantic msid-list
msid-semantic = token ; see RFC 4566
msid-list = *(" " msid-id) / " *"
The semantic field holds values from the IANA registriy "Semantics
for the msid-semantic SDP attribute" (which is defined in Section 6).
An example msid-semantic might look like this, if a semantic LS was
registered by IANA for the same purpose as the existing LS grouping
semantic:
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.
The msid-semantic attribute can occur more than once, but MUST NOT
occur more than once with the same msid-semantic value.
4. Generic SDP Offer/Answer Procedures
In accordance with guidance on definitions of SDP extensions, this
section gives the generic procedures that have to be followed by all
implementations of Msid, independent of which semantics they support.
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Note that the use of msid is not negotiated; each side declares what
it uses.
4.1. Generating the Initial Offer
An entity implmementing an MSID semantic MUST add one or more "msid-
semantic" attributes to its session level attributes, indicating the
MSID semantic it supports.
4.2. Answerer Processing of the Offer
If an "msid-semantic" attribute is present in the answer, and the
offerer supports the indicated semantic, the offerer MUST follow the
procedures described for that semantic.
4.3. Generating the Answer
An entity implmementing an MSID semantic MUST add a "msid-semantic"
attribute to its session level attributes, indicating the MSID
semantic it supports.
4.4. Offerer Processing of the Answer
If an "msid-semantic" attribute is present in the answer, and the
offerer supports the indicated semantic, the offerer MUST follow the
procedures described for that semantic.
5. 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 m-lines representing
MediaStreamTracks within MediaStreams as defined in
[W3C.WD-webrtc-20120209].
In the Javascript API, each MediaStream and MediaStreamTrack has an
"id" attribute, which is a DOMString.
The semantic token for this semantic is "WMS" (short for WebRTC Media
Stream).
The value of the "identifier" field in the msid consists of the "id"
attribute of a MediaStream, as defined in its WebIDL specification.
The value of the "appdata" field in the msid consists of the "id"
attribute of a MediaStreamTrack, as defined in its WebIDL
specification.
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If two different m-lines have MSID attributes with the same value for
identifier and appdata, it means that these two m-lines are both
intended for the same MediaStreamTrack. So far, no semantic for such
a mixture have been defined, but this specification does not forbid
the practice.
When an SDP description is updated, a specific msid "identifier"
continues to refer to the same MediaStream, and a specific "appdata"
to the same MediaStreamTrack. Once negotiation has completed on a
session, there is no memory apart from the currently valid SDP
descriptions; an msid "identifier" 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
m-lines and their msid values.
o When a new msid "identifier" value occurs in the description, the
recipient can signal to its application that a new MediaStream has
been added.
o When a description is updated to have more media sections with the
same msid "identifier" value, but different "appdata" values, the
recipient can signal to its application that new MediaStreamTracks
have been added to the MediaStream.
o When a description is updated to no longer list the msid attribute
on a specific media description, the recipient can signal to its
application that the corresponding MediaStreamTrack has ended.
In addition to signaling that the track is closed when its msid
attribute disappears from the SDP, the track will also be signaled as
being closed when all associated SSRCs have disappeared by the rules
of [RFC3550] section 6.3.4 (BYE packet received) and 6.3.5 (timeout),
and when the corresponding media section is disabled by setting the
port number to zero. Changing the direction of the media section to
"recvonly" will not close the MediaStreamTrack.
The association between SSRCs and m-lines is specified in
[I-D.ietf-rtcweb-jsep].
5.1. Handling of non-signalled tracks
Entities that do not implement the WMS semantic will not send "msid-
semantic:WMS". This means that there will be some incoming RTP
packets that the recipient has no predefined MediaStream id value
for.
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Note that this handling is triggered by incoming RTP packets, not by
SDP negotiation.
Handling will depend on whether or not the msid-semantic:WMS
attribute is present. There are two cases:
o No "msid-semantic:WMS" attribute is present. The SDP session is
assumed to be a backwards-compatible session. All incoming media,
on all m-lines that are part of the SDP session, are assumed to
belong to independent media streams, each with one track. The
identifier of this media stream and of the media stream track is a
randomly generated string; the WebIDL "label" attribute of this
media stream will be set to "Non-WMS stream".
o An "msid-semantic:WMS" attribute is present. In this case, the
sender implements the WMS semantic, and the packets 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 signaled.
If an entity implementing the WMS semantic 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 media receiver implmementing the
WMS semantic must have the SDP of the other party before it can
decide correctly which of the two cases described above applies. 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 removing the msid attribute; the
application must instead signal these as closed when the SSRC
disappears according to the rules of RFC 3550 section 6.3.4 and 6.3.5
or by disabling the m-line by setting its port to zero.
5.2. Detailed Offer/Answer Procedures
These procedures are given in terms of RFC 3264-recommended sections.
They describe the actions to be taken in terms of MediaStreams and
MediaStreamTracks; they do not include event signalling inside the
application, which is described in JSEP.
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They are specifically applicable to the WMS semantic; other semantics
will have their own consideration.
5.2.1. Generating the initial offer
For each media section in the offer, if there is an associated
MediaStreamTrack, the offerer adds one "a=msid" attribute to the
section for each MediaStream with which the MediaStreamTrack is
associated. The "identifier" field of the attribute is set to the
WebIDL "id" attribute of the MediaStream, and the "appdata" field is
set to the WebIDL "id" attribute of the MediaStreamTrack.
The offerer adds an "msid-semantic:WMS" field to the session-level
headers, and appends to it either a list of all the identifiers used
in the offer, or the single character "*".
5.2.2. Parsing the initial offer
For each media section in the offer, and for each "a=msid" attribute
in the media section, the receiver of the offer will perform the
following steps:
o Extract the "appdata" field of the "a=msid" attribute
o Check if a MediaStreamTrack with the same WebIDL "id" attribute as
the "appdata" field already exists, and is not in the "ended"
state. If it is not found, create it.
o Extract the "identifier" field of the "a=msid" attribte.
o Check if a MediaStream with the same WebIDL "id" attribute already
exists. If not, create it.
o Add the MediaStreamTrack to the MediaStream
5.2.3. Generating the answer
The answer is generated in exactly the same manner as the offer.
This includes adding a "msid-semantic:WMS" attribute in the session-
level headers, independent of whether or not such a header was
present in the offer.
5.2.4. Offerer processing of the answer
The answer is processed in exactly the same manner as the offer.
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5.2.5. Modifying the session
On subsequent exchanges, precisely the same procedure as for the
initial offer/answer is followed, but with one additional step in the
parsing of the offer and answer:
o For each MediaStreamTrack that has been created as a result of
previous offer/answer exchanges, and is not in the "ended" state,
check to see if there is still an "a=msid" attribute in the
present SDP whose "appdata" field is the same as the WebIDL "id"
attribute of the track.
o If no such attribute is found, close the MediaStreamTrack. This
will set its state to "ended".
6. IANA Considerations
This document requests IANA to register the "msid" attribute in the
"att-field (media level only)" registry within the SDP parameters
registry, according to the procedures of [RFC4566]
The required information for "msid" is:
o Contact name, email: IETF, contacted via mmusic@ietf.org, or a
successor address designated by IESG
o Attribute name: msid
o Long-form attribute name: Media stream group Identifier
o Subject to charset: The attribute value contains only ASCII
characters, and is therefore not subject to the charset attribute.
o Purpose: The attribute gives an association over a set of m-lines.
It can be used to signal the relationship between a WebRTC
MediaStream and a set of m-lines.
o Appropriate values: The details of appropriate values are given in
RFC XXXX.
This document requests IANA to register the "msid-semantic" attribute
in the "att-field (session level) registry within the SDP parameters
registry, according to the same procedures.
The required information is:
o Contact name, email: IETF, contacted via mmusic@ietf.org, or a
successor address designated by IESG
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o Attribute name: msid-semantic
o Long-form attribute name: Msid group semantic identifier
o Subject to charset: The attribute value contains only ASCII
characters, and is therefore not subject to the charset attribute.
o Purpose: The attribute gives the semantics of an association over
a set of m-lines.
o Appropriate values: The details are given in RFC XXXX.
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:
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
document upon publication.
7. 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 5.1, the amount of
buffering should be limited to avoid memory exhaustion attacks.
No other attacks have been identified that depend on this mechanism.
8. Acknowledgements
This note is based on sketches from, among others, Justin Uberti and
Cullen Jennings.
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Special thanks to Flemming Andreassen, Miguel Garcia and Paul Kyzivat
for their work in reviewing this draft, with many specific language
suggestions.
9. References
9.1. Normative References
[I-D.ietf-rtcweb-jsep]
Uberti, J., Jennings, C., and E. Rescorla, "Javascript
Session Establishment Protocol", draft-ietf-rtcweb-jsep-07
(work in progress), July 2014.
[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.
[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
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., Jennings, C., and A.
Narayanan, "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>.
9.2. Informative References
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-12 (work in progress), October 2014.
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[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]
Westerlund, M., Perkins, C., and H. Alvestrand,
"Guidelines for using the Multiplexing Features of RTP",
draft-westerlund-avtcore-multiplex-architecture-03 (work
in progress), February 2013.
[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, rejected 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.
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.
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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.
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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.
C.1. Changes from alvestrand-rtcweb-msid-00 to -01
Added track identifier.
Added inclusion-by-reference of draft-lennox-mmusic-source-selection
for track muting.
Some rewording.
C.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.
C.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.
C.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.
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Removed distinguished values of the MSID identifier.
C.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.
C.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.
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]
C.8. Changes from mmusic-msid-01 to -02
Corrected several missed cases where the word "ssrc" was not changed
to "M-line".
Added pointer to unified-plan (which should be moved to point to
-jsep)
Removed suggestion that ssrc-group attributes can be used with "msid-
semantic", it is now only the msid-semantic registry.
C.9. Changes from mmusic-msid-02 to -03
Corrected even more cases where the word "ssrc" was not changed to
"M-line".
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Added the functionality of using an asterisk (*) in the msid-semantic
line, in order to remove the need for listing all msids in the msid-
semantic line whne only one msid-semantic is in use.
Removed some now-unnecessary text.
C.10. Changes from mmusic-msid-03 to -04
Changed title to reflect focus on WebRTC MediaStreams
Added a section on receiver-side media stream control, using the
"msid-control" attribute.
C.11. Changes from -04 to -05
Removed the msid-control section after WG discussion.
Removed some text that seemed only to pertain to resolved issues.
C.12. Changes from -05 to -06
Addressed issues found in Fleming Andreassen's review
Referenced JSEP rather than unified-plan for the M-line mapping model
Relaxed MSID definition to allow "token-char" in values rather than
a-z 0-9 hyphen; tightened ABNF by adding length description to it.
Deleted discussion of abandoned alternatives, as part of preparing
for publication.
Added a "detailed procedures" section to the WMS semantics
description.
Added IANA registration of the "msid-semantic" attribute.
C.13. Changes from -06 to -07
Changed terminology from referring to "WebRTC device" to referring to
"entities that implement the WMS semantic".
Changed names for ABNF constructions based on a proposal by Paul
Kyzivat.
Included a section on generic offer/answer semantics.
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Author's Address
Harald Alvestrand
Google
Kungsbron 2
Stockholm 11122
Sweden
Email: harald@alvestrand.no
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