draft-ietf-mmusic-msid-07.txt   draft-ietf-mmusic-msid-08.txt 
Network Working Group H. Alvestrand Network Working Group H. Alvestrand
Internet-Draft Google Internet-Draft Google
Intended status: Standards Track October 14, 2014 Intended status: Standards Track February 11, 2015
Expires: April 17, 2015 Expires: August 15, 2015
WebRTC MediaStream Identification in the Session Description Protocol WebRTC MediaStream Identification in the Session Description Protocol
draft-ietf-mmusic-msid-07 draft-ietf-mmusic-msid-08
Abstract Abstract
This document specifies a Session Description Protocol (SDP) Grouping This document specifies a Session Description Protocol (SDP) Grouping
mechanism for RTP media streams that can be used to specify relations mechanism for RTP media streams that can be used to specify relations
between media streams. between media streams.
This mechanism is used to signal the association between the SDP This mechanism is used to signal the association between the SDP
concept of "m-line" and the WebRTC concept of "MediaStream" / concept of "m-line" and the WebRTC concept of "MediaStream" /
"MediaStreamTrack" using SDP signaling. "MediaStreamTrack" using SDP signaling.
skipping to change at page 1, line 45 skipping to change at page 1, line 45
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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 . . . . . . . . . . . . . . . . . . . . . 5
3. The Msid-Semantic Attribute . . . . . . . . . . . . . . . . . 6 3. The Msid-Semantic Attribute . . . . . . . . . . . . . . . . . 6
4. Generic SDP Offer/Answer Procedures . . . . . . . . . . . . . 6 4. Generic SDP Offer/Answer Procedures . . . . . . . . . . . . . 7
4.1. Generating the Initial Offer . . . . . . . . . . . . . . 7 4.1. Generating the Initial Offer . . . . . . . . . . . . . . 7
4.2. Answerer Processing of the Offer . . . . . . . . . . . . 7 4.2. Answerer Processing of the Offer . . . . . . . . . . . . 7
4.3. Generating the Answer . . . . . . . . . . . . . . . . . . 7 4.3. Generating the Answer . . . . . . . . . . . . . . . . . . 7
4.4. Offerer Processing of the Answer . . . . . . . . . . . . 7 4.4. Offerer Processing of the Answer . . . . . . . . . . . . 7
5. Applying Msid to WebRTC MediaStreams . . . . . . . . . . . . 7 5. Applying Msid to WebRTC MediaStreams . . . . . . . . . . . . 7
5.1. Handling of non-signalled tracks . . . . . . . . . . . . 8 5.1. Handling of non-signalled tracks . . . . . . . . . . . . 9
5.2. Detailed Offer/Answer Procedures . . . . . . . . . . . . 9 5.2. Detailed Offer/Answer Procedures . . . . . . . . . . . . 10
5.2.1. Generating the initial offer . . . . . . . . . . . . 10 5.2.1. Generating the initial offer . . . . . . . . . . . . 10
5.2.2. Parsing the initial offer . . . . . . . . . . . . . . 10 5.2.2. Parsing the initial offer . . . . . . . . . . . . . . 10
5.2.3. Generating the answer . . . . . . . . . . . . . . . . 10 5.2.3. Generating the answer . . . . . . . . . . . . . . . . 11
5.2.4. Offerer processing of the answer . . . . . . . . . . 10 5.2.4. Offerer processing of the answer . . . . . . . . . . 11
5.2.5. Modifying the session . . . . . . . . . . . . . . . . 11 5.2.5. Modifying the session . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 6.1. Attribute registration in existing registries . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 6.2. New registry creation . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . 13 9.1. Normative References . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . 13 9.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. Design considerations, rejected alternatives . . . . 14 Appendix A. Design considerations, rejected alternatives . . . . 14
Appendix B. Usage with multiple MediaStreams per M-line . . . . 14 Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 14
B.1. Mechanism design with multiple SSRCs . . . . . . . . . . 15 B.1. Changes from alvestrand-rtcweb-msid-00 to -01 . . . . . . 15
B.2. Usage with the SSRC attribute . . . . . . . . . . . . . . 16 B.2. Changes from alvestrand-rtcweb-msid-01 to -02 . . . . . . 15
Appendix C. Change log . . . . . . . . . . . . . . . . . . . . . 16 B.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00 15
C.1. Changes from alvestrand-rtcweb-msid-00 to -01 . . . . . . 16 B.4. Changes from alvestrand-mmusic-msid-00 to -01 . . . . . . 15
C.2. Changes from alvestrand-rtcweb-msid-01 to -02 . . . . . . 16 B.5. Changes from alvestrand-mmusic-msid-01 to -02 . . . . . . 15
C.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00 16 B.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00 16
C.4. Changes from alvestrand-mmusic-msid-00 to -01 . . . . . . 16 B.7. Changes from mmusic-msid-00 to -01 . . . . . . . . . . . 16
C.5. Changes from alvestrand-mmusic-msid-01 to -02 . . . . . . 17 B.8. Changes from mmusic-msid-01 to -02 . . . . . . . . . . . 16
C.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00 17 B.9. Changes from mmusic-msid-02 to -03 . . . . . . . . . . . 16
C.7. Changes from mmusic-msid-00 to -01 . . . . . . . . . . . 17 B.10. Changes from mmusic-msid-03 to -04 . . . . . . . . . . . 16
C.8. Changes from mmusic-msid-01 to -02 . . . . . . . . . . . 17 B.11. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 17
C.9. Changes from mmusic-msid-02 to -03 . . . . . . . . . . . 17 B.12. Changes from -05 to -06 . . . . . . . . . . . . . . . . . 17
C.10. Changes from mmusic-msid-03 to -04 . . . . . . . . . . . 18 B.13. Changes from -06 to -07 . . . . . . . . . . . . . . . . . 17
C.11. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 18 B.14. Changes from -07 to -08 . . . . . . . . . . . . . . . . . 17
C.12. Changes from -05 to -06 . . . . . . . . . . . . . . . . . 18 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18
C.13. Changes from -06 to -07 . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction 1. Introduction
1.1. Structure Of This Document 1.1. Structure Of This Document
This document adds a new Session Description Protocol (SDP) This document adds a new Session Description Protocol (SDP) [RFC4566]
Grouping[RFC5888] relation between SDP m-lines [RFC4566] that can mechanism that can associate application layer identifiers with the
associate application layer identifiers with the binding between binding between media streams, attaching identifiers to the media
media streams, attaching identifiers to the media streams and streams and attaching identifiers to the groupings they form.
attaching identifiers to the groupings they form.
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.
Section 2 gives the definition of the new mechanism. Section 2 gives the definition of the new mechanism.
Section 3 gives the definition of the msid-semantic field, which Section 3 gives the definition of the msid-semantic field, which
gives the possibility of using MSIDs with different semantics in the gives the possibility of using MSIDs with different semantics in the
same SDP message. same SDP message.
Section 5 gives the application of the new mechanism for providing Section 5 gives the application of the new mechanism for providing
necessary semantic information for the association of necessary semantic information for the association of
MediaStreamTracks to MediaStreams in the WebRTC MediaStreamTracks to MediaStreams in the WebRTC API
API[W3C.WD-webrtc-20120209]. [W3C.WD-webrtc-20120209].
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 RTP/RTCP multiplexing one used for RTP and one used for RTCP, unless RTP/RTCP multiplexing
[RFC5761] is used). [RFC5761] is used).
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2. The Msid Mechanism 2. The Msid Mechanism
This document defines a new SDP [RFC4566] media-level "msid" This document defines a new SDP [RFC4566] media-level "msid"
attribute. This new attribute allows endpoints to associate RTP attribute. This new attribute allows endpoints to associate RTP
media streams that are carried in the same or different m-lines. The media streams that are carried in the same or different m-lines. The
attribute also allows application-specific information to the attribute also allows application-specific information to the
association. 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.
The name of the attribute is "msid".
The value of the attribute is specified by the following ABNF
[RFC5234] grammar: [RFC5234] grammar:
; "attribute" is defined in RFC 4566. msid-value = msid-id [ SP msid-appdata ]
attribute =/ msid-attr
msid-attr = "msid:" msid-id [ SP msid-appdata ]
msid-id = 1*64token-char ; see RFC 4566 msid-id = 1*64token-char ; see RFC 4566
msid-appdata = 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" An example msid value for a group with the identifier "examplefoo"
and application data "examplebar" might look like this: and application data "examplebar" might look like this:
msid:examplefoo examplebar msid:examplefoo examplebar
The identifier is a string of ASCII characters that are legal in a The identifier is a string of ASCII characters that are legal in a
"token", consisting of between 1 and 64 characters. It MUST be "token", consisting of between 1 and 64 characters. It MUST be
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5576 section 4.2, but uses the identifier of the group rather than a 5576 section 4.2, but uses the identifier of the group rather than a
list of SSRC values. list of SSRC values.
This attribute MUST be present if "a=msid" is used. This attribute MUST be present if "a=msid" is used.
An empty list of identifiers is an indication that the sender An empty list of identifiers is an indication that the sender
supports the indicated semantic, but has no msid groupings of the supports the indicated semantic, but has no msid groupings of the
given type in the present SDP. given type in the present SDP.
An identifier of "*" is an indication that all "a=msid" lines in the An identifier of "*" is an indication that all "a=msid" lines in the
SDP have this specific semantic. SDP have this specific semantic. If "*" is not used, each msid-id in
the SDP MUST appear in one and only one "msid-semantic" line.
The ABNF of msid-semantic is: The name of the attribute is "msid-semantic".
attribute =/ msid-semantic-attr The value of the attribute is given by the following ABNF:
msid-semantic-attr = "msid-semantic:" msid-semantic msid-list
msid-semantic-value = msid-semantic msid-list
msid-semantic = token ; see RFC 4566 msid-semantic = token ; see RFC 4566
msid-list = *(" " msid-id) / " *" msid-list = *(" " msid-id) / " *"
The semantic field holds values from the IANA registriy "Semantics The semantic field holds values from the IANA registriy "Semantics
for the msid-semantic SDP attribute" (which is defined in Section 6). 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 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 registered by IANA for the same purpose as the existing LS grouping
semantic: semantic:
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The msid-semantic attribute can occur more than once, but MUST NOT The msid-semantic attribute can occur more than once, but MUST NOT
occur more than once with the same msid-semantic value. occur more than once with the same msid-semantic value.
4. Generic SDP Offer/Answer Procedures 4. Generic SDP Offer/Answer Procedures
In accordance with guidance on definitions of SDP extensions, this In accordance with guidance on definitions of SDP extensions, this
section gives the generic procedures that have to be followed by all section gives the generic procedures that have to be followed by all
implementations of Msid, independent of which semantics they support. implementations of Msid, independent of which semantics they support.
Note that the use of msid is not negotiated; each side declares what Note that the use of msid is not negotiated; each side declares what
it uses. semantics it uses. This means that an offerer has to be willing and
able to take appropriate action if the other side does not wish to
use the semantic, and an answerer adding new semantics to an answer
has to be willing and able to deal with the offerer not wishing to
use that semantic.
4.1. Generating the Initial Offer 4.1. Generating the Initial Offer
An entity implmementing an MSID semantic MUST add one or more "msid- An entity wishing to use an MSID semantic MUST add one or more "msid-
semantic" attributes to its session level attributes, indicating the semantic" attributes to its session level attributes, indicating the
MSID semantic it supports. MSID semantic it wishes to have available..
4.2. Answerer Processing of the Offer 4.2. Answerer Processing of the Offer
If an "msid-semantic" attribute is present in the answer, and the If an "msid-semantic" attribute is present in the offer, and the
offerer supports the indicated semantic, the offerer MUST follow the answerer wishes to use the indicated semantic, the offerer MUST
procedures described for that semantic. follow the procedures described for that semantic.
4.3. Generating the Answer 4.3. Generating the Answer
An entity implmementing an MSID semantic MUST add a "msid-semantic" An entity wishing to use an MSID semantic MUST add one or more "msid-
attribute to its session level attributes, indicating the MSID semantic" attributes to its session level attributes, indicating the
semantic it supports. MSID semantic it wishes to have available. If the answerer does not
wish to use one or more of the semantics indicated in the offer, the
answerer MUST NOT include "msid-semantic" lines indicating these
semantics in the answer.
4.4. Offerer Processing of the Answer 4.4. Offerer Processing of the Answer
If an "msid-semantic" attribute is present in the answer, and the If an "msid-semantic" attribute is present in the answer, and the
offerer supports the indicated semantic, the offerer MUST follow the offerer wishes to use the indicated semantic, the offerer MUST follow
procedures described for that semantic. the procedures described for that semantic. The offerer MUST follow
the procedures for all semantics that were indicated in its offer and
were also present in the answer.
5. Applying Msid to WebRTC MediaStreams 5. Applying Msid to WebRTC MediaStreams
This section creates a new semantic for use with the framework This section creates a new semantic for use with the framework
defined in Section 2, to be used for associating m-lines representing defined in Section 2, to be used for associating m-lines representing
MediaStreamTracks within MediaStreams as defined in MediaStreamTracks within MediaStreams as defined in
[W3C.WD-webrtc-20120209]. [W3C.WD-webrtc-20120209].
In the Javascript API, each MediaStream and MediaStreamTrack has an In the Javascript API, each MediaStream and MediaStreamTrack has an
"id" attribute, which is a DOMString. "id" attribute, which is a DOMString.
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If two different m-lines have MSID attributes with the same value for 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 identifier and appdata, it means that these two m-lines are both
intended for the same MediaStreamTrack. So far, no semantic for such intended for the same MediaStreamTrack. So far, no semantic for such
a mixture have been defined, but this specification does not forbid a mixture have been defined, but this specification does not forbid
the practice. the practice.
When an SDP description is updated, a specific msid "identifier" When an SDP description is updated, a specific msid "identifier"
continues to refer to the same MediaStream, and a specific "appdata" continues to refer to the same MediaStream, and a specific "appdata"
to the same MediaStreamTrack. Once negotiation has completed on a to the same MediaStreamTrack. Once negotiation has completed on a
session, there is no memory apart from the currently valid SDP session, there is no memory apart from the currently valid SDP
descriptions; an msid "identifier" value that appears in a later descriptions; if an msid "identifier" value disappears from the SDP
negotiation will be taken to refer to a new MediaStream. and appears in a later negotiation, it will be taken to refer to a
new MediaStream.
The following are the rules for handling updates of the list of The following are the rules for handling updates of the list of
m-lines and their msid values. m-lines and their msid values.
o When a new msid "identifier" value occurs in the description, the o When a new msid "identifier" value occurs in the description, the
recipient can signal to its application that a new MediaStream has recipient can signal to its application that a new MediaStream has
been added. been added.
o When a description is updated to have more media sections with the o When a description is updated to have more media sections with the
same msid "identifier" value, but different "appdata" values, the same msid "identifier" value, but different "appdata" values, the
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o When a description is updated to no longer list the msid attribute o When a description is updated to no longer list the msid attribute
on a specific media description, the recipient can signal to its on a specific media description, the recipient can signal to its
application that the corresponding MediaStreamTrack has ended. application that the corresponding MediaStreamTrack has ended.
In addition to signaling that the track is closed when its msid In addition to signaling that the track is closed when its msid
attribute disappears from the SDP, the track will also be signaled as attribute disappears from the SDP, the track will also be signaled as
being closed when all associated SSRCs have disappeared by the rules 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), 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 and when the corresponding media section is disabled by setting the
port number to zero. Changing the direction of the media section to port number to zero. Changing the direction of the media section (by
"recvonly" will not close the MediaStreamTrack. setting "sendonly", "recvonly" or "inactive" attributes) will not
close the MediaStreamTrack.
The association between SSRCs and m-lines is specified in The association between SSRCs and m-lines is specified in
[I-D.ietf-rtcweb-jsep]. [I-D.ietf-rtcweb-jsep].
5.1. Handling of non-signalled tracks 5.1. Handling of non-signalled tracks
Entities that do not implement the WMS semantic will not send "msid- Entities that do not use the WMS semantic will not send "msid-
semantic:WMS". This means that there will be some incoming RTP semantic:WMS". This means that there will be some incoming RTP
packets that the recipient has no predefined MediaStream id value packets that the recipient has no predefined MediaStream id value
for. for.
Note that this handling is triggered by incoming RTP packets, not by Note that this handling is triggered by incoming RTP packets, not by
SDP negotiation. SDP negotiation.
Handling will depend on whether or not the msid-semantic:WMS Handling will depend on whether or not the msid-semantic:WMS
attribute is present. There are two cases: attribute is present. There are two cases:
o No "msid-semantic:WMS" attribute is present. 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 media, assumed to be a backwards-compatible session. All incoming media,
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 tracks of the same media stream (the "default media
identifier of this media stream and of the media stream track is a stream"). The identifier of this media stream and of the media
randomly generated string; the WebIDL "label" attribute of this stream track is a randomly generated string; the WebIDL "label"
media stream will be set to "Non-WMS stream". attribute of this media stream will be set to "Non-WMS stream".
o An "msid-semantic:WMS" attribute is present. In this case, the o An "msid-semantic:WMS" attribute is present. In this case, the
sender implements the WMS semantic, and the packets are either sender implements the WMS semantic, and the packets are either
caused by a bug or by timing skew between the arrival of the media caused by a bug or by timing skew between the arrival of the media
packets and the SDP description. These packets MAY be discarded, packets and the SDP description. These packets MAY be discarded,
or they MAY be buffered for a while in order to allow immediate or they MAY be buffered for a while in order to allow immediate
startup of the media stream when the SDP description is updated. startup of the media stream when the SDP description is updated.
The arrival of media packets MUST NOT cause a new MediaStreamTrack The arrival of media packets MUST NOT cause a new MediaStreamTrack
to be signaled. to be signaled.
If an entity implementing the WMS semantic sends a description, it If an entity wishing to use the WMS semantic sends a description, it
MUST include the msid-semantic:WMS attribute, even if no media MUST include the msid-semantic:WMS attribute, even if no media
streams are sent. This allows us to distinguish between the case of 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. no media streams at the moment and the case of SDP generated by an
entity that wishes to use the backwards-compatible mechanism.
It follows from the above that the media receiver implmementing the It follows from the above that the media receiver implmementing the
WMS semantic must have the SDP of the other party before it can WMS semantic must have the SDP of the other party before it can
decide correctly which of the two cases described above applies. RTP decide correctly which of the two cases described above applies. RTP
media packets that arrive before the remote party's SDP MUST be media packets that arrive before the remote party's SDP MUST be
buffered or discarded, and MUST NOT cause a new MediaStreamTrack to buffered or discarded, and MUST NOT cause a new MediaStreamTrack to
be signalled. be signalled.
It follows from the above that media stream tracks in the "default" It follows from the above that media stream tracks in the "default"
media stream cannot be closed by removing the msid attribute; the media stream cannot be closed by removing the msid attribute; the
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WebIDL "id" attribute of the MediaStream, and the "appdata" field is WebIDL "id" attribute of the MediaStream, and the "appdata" field is
set to the WebIDL "id" attribute of the MediaStreamTrack. set to the WebIDL "id" attribute of the MediaStreamTrack.
The offerer adds an "msid-semantic:WMS" field to the session-level 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 headers, and appends to it either a list of all the identifiers used
in the offer, or the single character "*". in the offer, or the single character "*".
5.2.2. Parsing the initial offer 5.2.2. Parsing the initial offer
For each media section in the offer, and for each "a=msid" attribute 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 in the media section where the "msid-id" is associated with the "WMS"
following steps: semantic, the receiver of the offer will perform the following steps:
o Extract the "appdata" field of the "a=msid" attribute o Extract the "appdata" field of the "a=msid" attribute
o Check if a MediaStreamTrack with the same WebIDL "id" attribute as o Check if a MediaStreamTrack with the same WebIDL "id" attribute as
the "appdata" field already exists, and is not in the "ended" the "appdata" field already exists, and is not in the "ended"
state. If it is not found, create it. state. If it is not found, create it.
o Extract the "identifier" field of the "a=msid" attribte. o Extract the "identifier" field of the "a=msid" attribte.
o Check if a MediaStream with the same WebIDL "id" attribute already o Check if a MediaStream with the same WebIDL "id" attribute already
skipping to change at page 11, line 22 skipping to change at page 11, line 36
previous offer/answer exchanges, and is not in the "ended" state, previous offer/answer exchanges, and is not in the "ended" state,
check to see if there is still an "a=msid" attribute in the 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" present SDP whose "appdata" field is the same as the WebIDL "id"
attribute of the track. attribute of the track.
o If no such attribute is found, close the MediaStreamTrack. This o If no such attribute is found, close the MediaStreamTrack. This
will set its state to "ended". will set its state to "ended".
6. IANA Considerations 6. IANA Considerations
6.1. Attribute registration in existing registries
This document requests IANA to register the "msid" attribute in the This document requests IANA to register the "msid" attribute in the
"att-field (media level only)" registry within the SDP parameters "att-field (media level only)" registry within the SDP parameters
registry, according to the procedures of [RFC4566] registry, according to the procedures of [RFC4566]
The required information for "msid" is: The required information for "msid" 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 Subject to charset: The attribute value contains only ASCII o Subject to charset: The attribute value contains only ASCII
characters, and is therefore not subject to the charset attribute. characters, and is therefore not subject to the charset attribute.
o Purpose: The attribute gives an association over a set of m-lines. o Purpose: The attribute gives an association over a set of m-lines.
It can be used to signal the relationship between a WebRTC For example, it can be used to signal the relationship between a
MediaStream and a set of m-lines. WebRTC MediaStream and a set of m-lines.
o Appropriate values: The details of appropriate values are given in o Appropriate values: The details of appropriate values are given in
RFC XXXX. RFC XXXX.
This document requests IANA to register the "msid-semantic" attribute This document requests IANA to register the "msid-semantic" attribute
in the "att-field (session level) registry within the SDP parameters in the "att-field (session level) registry within the SDP parameters
registry, according to the same procedures. registry, according to the same procedures.
The required information is: The required information is:
skipping to change at page 12, line 17 skipping to change at page 12, line 33
o Long-form attribute name: Msid group semantic identifier o Long-form attribute name: Msid group semantic identifier
o Subject to charset: The attribute value contains only ASCII o Subject to charset: The attribute value contains only ASCII
characters, and is therefore not subject to the charset attribute. characters, and is therefore not subject to the charset attribute.
o Purpose: The attribute gives the semantics of an association over o Purpose: The attribute gives the semantics of an association over
a set of m-lines. a set of m-lines.
o Appropriate values: The details are given in RFC XXXX. o Appropriate values: The details are given in RFC XXXX.
6.2. New registry creation
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" in the "Session
exactly the same rules as for the "Semantics for the ssrc-group SDP Description Protocol (SDP) Parameters" group. This registry operates
attribute" registry (Expert Review), and to register the "WMS" on the Expert Review policy [RFC5226]. Usage of the registry is
semantic within this new registry. expected to be low, so the expert should feel free to consult widely
if a new request ever comes in.
This document requests IANA to register the "WMS" semantic within
this new registry.
The required information is: The required information is:
o Description: WebRTC Media Stream, as given in RFC XXXX. o Description: WebRTC Media Stream, as given in RFC XXXX.
o Token: WMS o Token: WMS
o Standards track reference: RFC XXXX o Standards track reference: RFC XXXX
IANA is requested to replace "RFC XXXX" with the RFC number of this IANA is requested to replace "RFC XXXX" with the RFC number of this
document upon publication. document upon publication.
7. Security Considerations 7. Security Considerations
An adversary with the ability to modify SDP descriptions has the An adversary with the ability to modify SDP descriptions has the
ability to switch around tracks between media streams. This is a ability to switch around tracks between media streams. This is a
special case of the general security consideration that modification special case of the general security consideration that modification
of SDP descriptions needs to be confined to entities trusted by the of SDP descriptions needs to be confined to entities trusted by the
application. application.
skipping to change at page 13, line 5 skipping to change at page 13, line 25
If implementing buffering as mentioned in Section 5.1, the amount of If implementing buffering as mentioned in Section 5.1, the amount of
buffering should be limited to avoid memory exhaustion attacks. buffering should be limited to avoid memory exhaustion attacks.
No other attacks have been identified that depend on this mechanism. No other attacks have been identified that depend on this mechanism.
8. Acknowledgements 8. Acknowledgements
This note is based on sketches from, among others, Justin Uberti and This note is based on sketches from, among others, Justin Uberti and
Cullen Jennings. Cullen Jennings.
Special thanks to Flemming Andreassen, Miguel Garcia and Paul Kyzivat Special thanks to Flemming Andreassen, Miguel Garcia, Martin Thomson,
for their work in reviewing this draft, with many specific language Ted Hardie, Adam Roach and Paul Kyzivat for their work in reviewing
suggestions. this draft, with many specific language suggestions.
9. References 9. References
9.1. Normative References 9.1. Normative References
[I-D.ietf-rtcweb-jsep] [I-D.ietf-rtcweb-jsep]
Uberti, J., Jennings, C., and E. Rescorla, "Javascript Uberti, J., Jennings, C., and E. Rescorla, "Javascript
Session Establishment Protocol", draft-ietf-rtcweb-jsep-07 Session Establishment Protocol", draft-ietf-rtcweb-jsep-08
(work in progress), July 2014. (work in progress), October 2014.
[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", STD 64, RFC 3550, July 2003. Applications", STD 64, RFC 3550, July 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006. Description Protocol", RFC 4566, July 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008. 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] [W3C.WD-webrtc-20120209]
Bergkvist, A., Burnett, D., Jennings, C., and A. Bergkvist, A., Burnett, D., Jennings, C., and A.
Narayanan, "WebRTC 1.0: Real-time Communication Between Narayanan, "WebRTC 1.0: Real-time Communication Between
Browsers", World Wide Web Consortium WD WD- Browsers", World Wide Web Consortium WD WD-
webrtc-20120209, February 2012, webrtc-20120209, February 2012,
<http://www.w3.org/TR/2012/WD-webrtc-20120209>. <http://www.w3.org/TR/2012/WD-webrtc-20120209>.
9.2. Informative References 9.2. Informative References
[I-D.ietf-mmusic-sdp-bundle-negotiation] [I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings, Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session "Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle- Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-12 (work in progress), October 2014. negotiation-07 (work in progress), April 2014.
[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 [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, rejected alternatives Appendix A. Design considerations, rejected 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 14, line 39 skipping to change at page 14, line 46
from the same synchronization context in multiple MediaStreams and from the same synchronization context in multiple MediaStreams and
wanting to have tracks from multiple synchronization contexts within wanting to have tracks from multiple synchronization contexts within
one MediaStream (but the latter is impossible, since a MediaStream is one MediaStream (but the latter is impossible, since a MediaStream is
defined to impose synchronization on its members). defined to impose synchronization on its members).
Another suggestion has been to put the msid value within an attribute Another suggestion has been to put the msid value within an attribute
of RTCP SR (sender report) packets. This doesn't offer the ability 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 to know that you have seen all the tracks currently configured for a
media stream. media stream.
Appendix B. Usage with multiple MediaStreams per M-line Appendix B. Change log
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.
C.1. Changes from alvestrand-rtcweb-msid-00 to -01 B.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.
C.2. Changes from alvestrand-rtcweb-msid-01 to -02 B.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.
C.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00 B.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.
C.4. Changes from alvestrand-mmusic-msid-00 to -01 B.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.
C.5. Changes from alvestrand-mmusic-msid-01 to -02 B.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.
C.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00 B.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 B.7. Changes from mmusic-msid-00 to -01
Reworked msid mechanism to be a per-m-line attribute, to align with Reworked msid mechanism to be a per-m-line attribute, to align with
[I-D.roach-mmusic-unified-plan] draft-roach-mmusic-unified-plan.
C.8. Changes from mmusic-msid-01 to -02 B.8. Changes from mmusic-msid-01 to -02
Corrected several missed cases where the word "ssrc" was not changed Corrected several missed cases where the word "ssrc" was not changed
to "M-line". to "M-line".
Added pointer to unified-plan (which should be moved to point to Added pointer to unified-plan (which should be moved to point to
-jsep) -jsep)
Removed suggestion that ssrc-group attributes can be used with "msid- Removed suggestion that ssrc-group attributes can be used with "msid-
semantic", it is now only the msid-semantic registry. semantic", it is now only the msid-semantic registry.
C.9. Changes from mmusic-msid-02 to -03 B.9. Changes from mmusic-msid-02 to -03
Corrected even more cases where the word "ssrc" was not changed to Corrected even more cases where the word "ssrc" was not changed to
"M-line". "M-line".
Added the functionality of using an asterisk (*) in the msid-semantic 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- line, in order to remove the need for listing all msids in the msid-
semantic line whne only one msid-semantic is in use. semantic line whne only one msid-semantic is in use.
Removed some now-unnecessary text. Removed some now-unnecessary text.
C.10. Changes from mmusic-msid-03 to -04 B.10. Changes from mmusic-msid-03 to -04
Changed title to reflect focus on WebRTC MediaStreams Changed title to reflect focus on WebRTC MediaStreams
Added a section on receiver-side media stream control, using the Added a section on receiver-side media stream control, using the
"msid-control" attribute. "msid-control" attribute.
C.11. Changes from -04 to -05 B.11. Changes from -04 to -05
Removed the msid-control section after WG discussion. Removed the msid-control section after WG discussion.
Removed some text that seemed only to pertain to resolved issues. Removed some text that seemed only to pertain to resolved issues.
C.12. Changes from -05 to -06 B.12. Changes from -05 to -06
Addressed issues found in Fleming Andreassen's review Addressed issues found in Fleming Andreassen's review
Referenced JSEP rather than unified-plan for the M-line mapping model Referenced JSEP rather than unified-plan for the M-line mapping model
Relaxed MSID definition to allow "token-char" in values rather than Relaxed MSID definition to allow "token-char" in values rather than
a-z 0-9 hyphen; tightened ABNF by adding length description to it. a-z 0-9 hyphen; tightened ABNF by adding length description to it.
Deleted discussion of abandoned alternatives, as part of preparing Deleted discussion of abandoned alternatives, as part of preparing
for publication. for publication.
Added a "detailed procedures" section to the WMS semantics Added a "detailed procedures" section to the WMS semantics
description. description.
Added IANA registration of the "msid-semantic" attribute. Added IANA registration of the "msid-semantic" attribute.
C.13. Changes from -06 to -07 B.13. Changes from -06 to -07
Changed terminology from referring to "WebRTC device" to referring to Changed terminology from referring to "WebRTC device" to referring to
"entities that implement the WMS semantic". "entities that implement the WMS semantic".
Changed names for ABNF constructions based on a proposal by Paul Changed names for ABNF constructions based on a proposal by Paul
Kyzivat. Kyzivat.
Included a section on generic offer/answer semantics. Included a section on generic offer/answer semantics.
B.14. Changes from -07 to -08
Removed Appendix B that described the (now obsolete) ssrc-specific
usage of MSID.
Adopted a restructuring of the IANA section based on a suggestion
from Martin Thomson.
A number of text and ABNF clarifications based on suggestions from
Ted Hardie, Paul Kyzivat and Adam Roach.
Changed the "non-signalled track handling" to create a single stream
with multiple tracks again, according to discussions at TPAC in
November 2014
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
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