draft-ietf-mmusic-msid-14.txt   draft-ietf-mmusic-msid-15.txt 
Network Working Group H. Alvestrand Network Working Group H. Alvestrand
Internet-Draft Google Internet-Draft Google
Intended status: Standards Track June 6, 2016 Intended status: Standards Track July 7, 2016
Expires: December 8, 2016 Expires: January 8, 2017
WebRTC MediaStream Identification in the Session Description Protocol WebRTC MediaStream Identification in the Session Description Protocol
draft-ietf-mmusic-msid-14 draft-ietf-mmusic-msid-15
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 "media description" and the WebRTC concept of concept of "media description" and the WebRTC concept of
"MediaStream" / "MediaStreamTrack" using SDP signaling. "MediaStream" / "MediaStreamTrack" using SDP signaling.
skipping to change at page 1, line 45 skipping to change at page 1, line 45
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Copyright Notice Copyright Notice
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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. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Why A New Mechanism Is Needed . . . . . . . . . . . . . . 3 1.2. Structure Of This Document . . . . . . . . . . . . . . . 3
1.3. The WEBRTC MediaStream . . . . . . . . . . . . . . . . . 4 1.3. Why A New Mechanism Is Needed . . . . . . . . . . . . . . 4
2. The Msid Mechanism . . . . . . . . . . . . . . . . . . . . . 4 1.4. The WEBRTC MediaStream . . . . . . . . . . . . . . . . . 4
3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. The Msid Mechanism . . . . . . . . . . . . . . . . . . . . . 5
3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Handling of non-signalled tracks . . . . . . . . . . . . 7 3.1. Handling of non-signalled tracks . . . . . . . . . . . . 7
3.2. Detailed Offer/Answer Procedures . . . . . . . . . . . . 8 3.2. Detailed Offer/Answer Procedures . . . . . . . . . . . . 8
3.2.1. Generating the initial offer . . . . . . . . . . . . 8 3.2.1. Generating the initial offer . . . . . . . . . . . . 8
3.2.2. Answerer processing of the Offer . . . . . . . . . . 8 3.2.2. Answerer processing of the Offer . . . . . . . . . . 9
3.2.3. Generating the answer . . . . . . . . . . . . . . . . 8 3.2.3. Generating the answer . . . . . . . . . . . . . . . . 9
3.2.4. Offerer processing of the answer . . . . . . . . . . 9 3.2.4. Offerer processing of the answer . . . . . . . . . . 9
3.2.5. Modifying the session . . . . . . . . . . . . . . . . 9 3.2.5. Modifying the session . . . . . . . . . . . . . . . . 9
3.3. Example SDP description . . . . . . . . . . . . . . . . . 9 3.3. Example SDP description . . . . . . . . . . . . . . . . . 10
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
4.1. Attribute registration in existing registries . . . . . . 10 4.1. Attribute registration in existing registries . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10 5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . 11 7.1. Normative References . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . 12 7.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Design considerations, rejected alternatives . . . . 12 Appendix A. Design considerations, rejected alternatives . . . . 13
Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 12 Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 13
B.1. Changes from alvestrand-rtcweb-msid-00 to -01 . . . . . . 13 B.1. Changes from alvestrand-rtcweb-msid-00 to -01 . . . . . . 13
B.2. Changes from alvestrand-rtcweb-msid-01 to -02 . . . . . . 13 B.2. Changes from alvestrand-rtcweb-msid-01 to -02 . . . . . . 14
B.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00 13 B.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00 14
B.4. Changes from alvestrand-mmusic-msid-00 to -01 . . . . . . 13 B.4. Changes from alvestrand-mmusic-msid-00 to -01 . . . . . . 14
B.5. Changes from alvestrand-mmusic-msid-01 to -02 . . . . . . 13 B.5. Changes from alvestrand-mmusic-msid-01 to -02 . . . . . . 14
B.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00 14 B.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00 14
B.7. Changes from mmusic-msid-00 to -01 . . . . . . . . . . . 14 B.7. Changes from mmusic-msid-00 to -01 . . . . . . . . . . . 15
B.8. Changes from mmusic-msid-01 to -02 . . . . . . . . . . . 14 B.8. Changes from mmusic-msid-01 to -02 . . . . . . . . . . . 15
B.9. Changes from mmusic-msid-02 to -03 . . . . . . . . . . . 14 B.9. Changes from mmusic-msid-02 to -03 . . . . . . . . . . . 15
B.10. Changes from mmusic-msid-03 to -04 . . . . . . . . . . . 14 B.10. Changes from mmusic-msid-03 to -04 . . . . . . . . . . . 15
B.11. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 15 B.11. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 15
B.12. Changes from -05 to -06 . . . . . . . . . . . . . . . . . 15 B.12. Changes from -05 to -06 . . . . . . . . . . . . . . . . . 15
B.13. Changes from -06 to -07 . . . . . . . . . . . . . . . . . 15 B.13. Changes from -06 to -07 . . . . . . . . . . . . . . . . . 16
B.14. Changes from -07 to -08 . . . . . . . . . . . . . . . . . 15 B.14. Changes from -07 to -08 . . . . . . . . . . . . . . . . . 16
B.15. Changes from -08 to -09 . . . . . . . . . . . . . . . . . 16 B.15. Changes from -08 to -09 . . . . . . . . . . . . . . . . . 16
B.16. Changes from -09 to -10 . . . . . . . . . . . . . . . . . 16 B.16. Changes from -09 to -10 . . . . . . . . . . . . . . . . . 17
B.17. Changes from -10 to -11 . . . . . . . . . . . . . . . . . 16 B.17. Changes from -10 to -11 . . . . . . . . . . . . . . . . . 17
B.18. Changes from -11 to -12 . . . . . . . . . . . . . . . . . 16 B.18. Changes from -11 to -12 . . . . . . . . . . . . . . . . . 17
B.19. Changes from -12 to -13 . . . . . . . . . . . . . . . . . 16 B.19. Changes from -12 to -13 . . . . . . . . . . . . . . . . . 17
B.20. Changes from -13 to -14 . . . . . . . . . . . . . . . . . 16 B.20. Changes from -13 to -14 . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16 B.21. Changes from -14 to -15 . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction 1. Introduction
1.1. Structure Of This Document 1.1. Terminology
This document uses terminology from [I-D.ietf-rtcweb-overview]. In
addition, the following terms are used as described below:
RTP stream Defined in [RFC7656] as a stream of RTP packets
containing media data.
MediaStream Defined in [W3C.CR-mediacapture-streams-20160519]as an
assembly of MediaStreamTracks. One MediaStream can contain
multiple MediaStreamTracks, of the same or different types.
MediaStreamTrack Defined in [W3C.CR-mediacapture-streams-20160519]as
an unidirectional flow of media data (either audio or video, but
not both). Corresponds to the [RFC7656] term "Source Stream".
One MediaStreamTrack can be present in zero, one or multiple
MediaStreams.
Media description Defined in [RFC4566] as a set of fields starting
with an "m=" field and terminated by eitehr the next "m=" field or
by the end of the session description.
1.2. Structure Of This Document
This document adds a new Session Description Protocol (SDP) [RFC4566] This document adds a new Session Description Protocol (SDP) [RFC4566]
mechanism that can associate application layer identifiers with the mechanism that can attach identifiers to the RTP streams and
binding between media streams, attaching identifiers to the media attaching identifiers to the groupings they form. It is designed for
streams and attaching identifiers to the groupings they form. It is use with WebRTC[I-D.ietf-rtcweb-overview] .
designed for use with WebRTC[I-D.ietf-rtcweb-overview] .
Section 1.2 gives the background on why a new mechanism is needed. Section 1.3 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 necessary semantic information and procedures for Section 3 gives the necessary semantic information and procedures for
using the msid attribute to signal the association of using the msid attribute to signal the association of
MediaStreamTracks to MediaStreams in support of the WebRTC API MediaStreamTracks to MediaStreams in support of the WebRTC API
[W3C.WD-webrtc-20160531]. [W3C.WD-webrtc-20160531].
1.2. Why A New Mechanism Is Needed 1.3. 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 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).
SDP gives a description based on media descriptions. According to SDP [RFC4566] gives a format for describing an SDP session that can
the model used in [I-D.ietf-rtcweb-jsep], each media description contain multiple media descriptions. According to the model used in
describes exactly one media source, and if mulitple media sources are [I-D.ietf-rtcweb-jsep], each media description describes exactly one
carried in an RTP session, this is signalled using BUNDLE media source, and if multiple 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 [I-D.ietf-mmusic-sdp-bundle-negotiation]; if BUNDLE is not used, each
media source is carried in its own RTP session. media source is carried in its own RTP session.
The SDP grouping framework [RFC5888] can be used to group media The SDP grouping framework [RFC5888] can be used to group media
descriptions. However, for the use case of WebRTC, there is the need descriptions. However, for the use case of WebRTC, there is the need
for an application to specify some application-level information for an application to specify some application-level information
about the association between the media description and the group. about the association between the media description and the group.
This is not possible using the SDP grouping framework. This is not possible using the SDP grouping framework.
1.3. The WEBRTC MediaStream 1.4. The WEBRTC MediaStream
The W3C WebRTC API specification [W3C.WD-webrtc-20160531] specifies The W3C WebRTC API specification [W3C.WD-webrtc-20160531] 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 stream.
stream. The collision of terminology is unfortunate.) There might The collision of terminology is unfortunate.) There might possibly
possibly be additional SSRCs, possibly within additional RTP be additional SSRCs, possibly within additional RTP sessions, in
sessions, in order to support functionality like forward error order to support functionality like forward error correction or
correction or simulcast. This complication is ignored below. simulcast. These additional SSRCs are not affected by this
specification.
MediaStreamTracks are unidirectional; they carry media on one MediaStreamTracks are unidirectional; they carry media on one
direction only. direction only.
In the RTP specification, media streams are identified using the SSRC In the RTP specification, RTP 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
need to be explicitly signaled. Therefore, the association of an RTP stream to MediaStreams need to
be explicitly signaled.
WebRTC defines a mapping (documented in [I-D.ietf-rtcweb-jsep]) where WebRTC defines a mapping (documented in [I-D.ietf-rtcweb-jsep]) where
one SDP media description is used to describe each MediaStreamTrack, one SDP media description is used to describe each MediaStreamTrack,
and the BUNDLE mechanism [I-D.ietf-mmusic-sdp-bundle-negotiation] is and the BUNDLE mechanism [I-D.ietf-mmusic-sdp-bundle-negotiation] is
used to group MediaStreamTracks into RTP sessions. Therefore, the used to group MediaStreamTracks into RTP sessions. Therefore, the
need is to specify the ID of a MediaStreamTrack and its associated need is to specify the ID of a MediaStreamTrack and its associated
MediaStream for each media description, which can be accomplished MediaStream for each media description, which can be accomplished
with a media-level SDP attribute. with a media-level SDP attribute.
This usage is described in Section 3. This usage is described in Section 3.
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 described in different media descriptions with streams that are described in different media descriptions with the
the same MediaStreams as defined in [W3C.WD-webrtc-20160531], and to same MediaStreams as defined in [W3C.WD-webrtc-20160531], and to
carry an identifier for each MediaStreamTrack in its "appdata" field. carry an identifier for each MediaStreamTrack in its "appdata" field.
The value of the "msid" attribute consists of an identifier and an The value of the "msid" attribute consists of an identifier and an
optional "appdata" field. optional "appdata" field.
The name of the attribute is "msid". The name of the attribute is "msid".
The value of the attribute is specified by the following ABNF The value of the attribute is specified by the following ABNF
[RFC5234] grammar: [RFC5234] grammar:
skipping to change at page 5, line 32 skipping to change at page 6, line 11
The identifier (msid-id) uniquely identifies a group within the scope The identifier (msid-id) uniquely identifies a group within the scope
of an SDP description. of an SDP description.
There may be multiple msid attributes in a single media description. There may be multiple msid attributes in a single media description.
This represents the case where a single MediaStreamTrack is present This represents the case where a single MediaStreamTrack is present
in multiple MediaStreams; the value of "msid-appdata" MUST be in multiple MediaStreams; the value of "msid-appdata" MUST be
identical for all occurences. identical for all occurences.
Multiple media descriptions with the same value for msid-id and msid- Multiple media descriptions with the same value for msid-id and msid-
appdata is not permitted. appdata are not permitted.
Endpoints can update the associations between RTP media streams as Endpoints can update the associations between RTP streams as
expressed by msid attributes at any time. expressed by msid attributes at any time.
The msid attributes depend on the association of RTP media streams The msid attributes depend on the association of RTP streams with
with media descriptions, but does not depend on the association of media descriptions, but does not depend on the association of RTP
RTP media streams with RTP transports; therefore, its mux category is streams with RTP transports; therefore, its mux category (as defined
NORMAL - the process of deciding on MSID attributes doesn't have to in [I-D.ietf-mmusic-sdp-mux-attributes]) is NORMAL - the process of
take into consideration whether the media streams are bundled or not. deciding on MSID attributes doesn't have to take into consideration
whether the RTP streams are bundled or not.
3. Procedures 3. Procedures
This section describes the procedures for associating media This section describes the procedures for associating media
descriptions representing MediaStreamTracks within MediaStreams as descriptions representing MediaStreamTracks within MediaStreams as
defined in [W3C.WD-webrtc-20160531]. defined in [W3C.WD-webrtc-20160531].
In the Javascript API, each MediaStream and MediaStreamTrack has an In the Javascript API described in that specification, each
"id" attribute, which is a DOMString. MediaStream and MediaStreamTrack has an "id" attribute, which is a
DOMString.
The value of the "msid-id" field in the msid consists of the "id" The value of the "msid-id" field in the msid consists of the "id"
attribute of a MediaStream, as defined in its WebIDL specification. attribute of a MediaStream, as defined in the MediaStream's WebIDL
specification.
The value of the "msid-appdata" field in the msid consists of the The value of the "msid-appdata" field in the msid consists of the
"id" attribute of a MediaStreamTrack, as defined in its WebIDL "id" attribute of a MediaStreamTrack, as defined in the
specification. MediaStreamTrack's WebIDL specification.
When an SDP session description is updated, a specific "msid-id" When an SDP session description is updated, a specific "msid-id"
continues to refer to the same MediaStream, and a specific "msid- value continues to refer to the same MediaStream, and a specific
appdata" to the same MediaStreamTrack. There is no memory apart from "msid-appdata" to the same MediaStreamTrack. There is no memory
the currently valid SDP descriptions; if an msid "identifier" value apart from the currently valid SDP descriptions; if an msid
disappears from the SDP and appears in a later negotiation, it will "identifier" value disappears from the SDP and appears in a later
be taken to refer to a new MediaStream. negotiation, it will be taken to refer to a new MediaStream.
If the MSID attribute does not conform to the ABNF given here, it
SHOULD be ignored.
The following is a high level description of the rules for handling The following is a high level description of the rules for handling
SDP updates. Detailed procedures are in Section 3.2. SDP updates. Detailed procedures are in Section 3.2.
o When a new msid "identifier" value occurs in a session o When a new msid "identifier" value occurs in a session
description, the recipient can signal to its application that a description, the recipient can signal to its application that a
new MediaStream has been added. new MediaStream has been added.
o When a session description is updated to have media descriptions o When a session description is updated to have media descriptions
with an msid "identifier" value, with one or more different with an msid "identifier" value, with one or more different
"appdata" values, the recipient can si2gnal to its application "appdata" values, the recipient can siggnal to its application
that new MediaStreamTracks have been added to the MediaStream. that new MediaStreamTracks have been added to the MediaStream.
This is done for each different msid "identifier" value. This is done for each different msid "identifier" value.
o When a session description is updated to no longer list any msid o When a session description is updated to no longer list any msid
attribute on a specific media description, the recipient can attribute on a specific media description, the recipient can
signal to its application that the corresponding MediaStreamTrack signal to its application that the corresponding MediaStreamTrack
has ended. 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 description is disabled by setting or when the corresponding media description is disabled by setting
the port number to zero. Changing the direction of the media the port number to zero. Changing the direction of the media
description (by setting "sendonly", "recvonly" or "inactive" description (by setting "sendonly", "recvonly" or "inactive"
attributes) will not close the MediaStreamTrack. attributes) will not close the MediaStreamTrack.
The association between SSRCs and media descriptions is specified in The association between SSRCs and media descriptions is specified in
[I-D.ietf-rtcweb-jsep]. [I-D.ietf-rtcweb-jsep].
3.1. Handling of non-signalled tracks 3.1. Handling of non-signalled tracks
Entities that do not use msid will not send msid. This means that Entities that do not use msid will not send msid. This means that
skipping to change at page 7, line 26 skipping to change at page 8, line 4
adds a MediaStreamTrack to an already established connection and adds a MediaStreamTrack to an already established connection and
starts sending data before the answer is received by the offerer. starts sending data before the answer is received by the offerer.
For initial negotiation, packets won't flow until the ICE candidates For initial negotiation, packets won't flow until the ICE candidates
and fingerprints have been exchanged, so this is not an issue. and fingerprints have been exchanged, so this is not an issue.
The recipient of those packets will perform the following steps: The recipient of those packets will perform the following steps:
o When RTP packets are initially received, it will create an o When RTP packets are initially received, it will create an
appropriate MediaStreamTrack based on the type of the media appropriate MediaStreamTrack based on the type of the media
(carried in PayloadType), and use the MID RTP header extension (carried in PayloadType), and use the MID RTP header extension
[I-D.ietf-mmusic-sdp-bundle-negotiation] (if present) to associate [I-D.ietf-mmusic-sdp-bundle-negotiation] (if present) to associate
the RTP packets with a specific media section. If the connection the RTP packets with a specific media section.
is not in the RTCSignalingState "stable", it will wait at this
point. o If the connection is not in the RTCSignalingState "stable", it
will wait at this point.
o When the connection is in the RTCSignalingState "stable", it will o When the connection is in the RTCSignalingState "stable", it will
look at the relevant media section to find the msid attribute. assign ID values.
The following steps are performed to assign ID values:
o If there is an msid attribute, it will use that attribute to o If there is an msid attribute, it will use that attribute to
populate the "id" field of the MediaStreamTrack and associated populate the "id" field of the MediaStreamTrack and associated
MediaStreams, as described above. MediaStreams, as described above.
o If there is no msid attribute, the identifier of the o If there is no msid attribute, the identifier of the
MediaStreamTrack will be set to a randomly generated string, and MediaStreamTrack will be set to a randomly generated string, and
it will be signalled as being part of a MediaStream with the it will be signalled as being part of a MediaStream with the
WebIDL "label" attribute set to "Non-WebRTC stream". WebIDL "label" attribute set to "Non-WebRTC stream".
o After deciding on the "id" field to be applied to the o After deciding on the "id" field to be applied to the
MediaStreamTrack, the track will be signalled to the user. MediaStreamTrack, the track will be signalled to the user.
The process above may involve a considerable amount of buffering The process above may involve a considerable amount of buffering
before the stable state is entered. If the implementation wishes to before the stable state is entered. If the implementation wishes to
limit this buffering, it MUST signal to the user that media has been limit this buffering, it MUST signal to the user that media has been
discarded. discarded.
It follows from the above that media stream tracks in the "default" It follows from the above that MediaStreamTracks in the "default"
media stream cannot be closed by removing the msid attribute; the MediaStream cannot be closed by removing the msid attribute; the
application must instead signal these as closed when the SSRC 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 disappears according to the rules of RFC 3550 section 6.3.4 and 6.3.5
or by disabling the media description by setting its port to zero. or by disabling the media description by setting its port to zero.
3.2. Detailed Offer/Answer Procedures 3.2. Detailed Offer/Answer Procedures
These procedures are given in terms of RFC 3264-recommended sections. These procedures are given in terms of RFC 3264-recommended sections.
They describe the actions to be taken in terms of MediaStreams and They describe the actions to be taken in terms of MediaStreams and
MediaStreamTracks; they do not include event signalling inside the MediaStreamTracks; they do not include event signalling inside the
application, which is described in JSEP. application, which is described in JSEP.
skipping to change at page 10, line 20 skipping to change at page 10, line 47
"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: MediaStream 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 can be used to signal the relationship o Purpose: The attribute can be used to signal the relationship
between a WebRTC MediaStream and a set of media descriptions. between a WebRTC MediaStream and a set of media descriptions.
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.
o MUX category: NORMAL o MUX category: NORMAL
The MUX category is defined in [I-D.ietf-mmusic-sdp-mux-attributes]. The MUX category is defined in [I-D.ietf-mmusic-sdp-mux-attributes].
5. Security Considerations 5. 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 MediaStreams. 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.
If implementing buffering as mentioned in Section 3.1, the amount of If implementing buffering as mentioned in Section 3.1, the amount of
buffering should be limited to avoid memory exhaustion attacks. buffering should be limited to avoid memory exhaustion attacks.
Careless generation of identifiers can leak privacy-sensitive
information. [W3C.CR-mediacapture-streams-20160519] recommends that
identifiers are generated using UUID class 3 or 4 as a basis, which
avoids such leakage.
No other attacks have been identified that depend on this mechanism. No other attacks have been identified that depend on this mechanism.
6. Acknowledgements 6. 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, Martin Thomson, Special thanks to Flemming Andreassen, Ben Campbell, Miguel Garcia,
Ted Hardie, Adam Roach, Magnus Westerlund and Paul Kyzivat for their Martin Thomson, Ted Hardie, Adam Roach, Magnus Westerlund, Alissa
work in reviewing this draft, with many specific language Cooper, Sue Hares and Paul Kyzivat for their work in reviewing this
suggestions. draft, with many specific language suggestions.
7. References 7. References
7.1. Normative References 7.1. Normative References
[I-D.ietf-mmusic-sdp-mux-attributes] [I-D.ietf-mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-12 Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-02
(work in progress), January 2016. (work in progress), July 2014.
[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-14 Session Establishment Protocol", draft-ietf-rtcweb-jsep-07
(work in progress), March 2016. (work in progress), July 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, DOI 10.17487/ Requirement Levels", BCP 14, RFC 2119, March 1997.
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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, DOI 10.17487/RFC3550, Applications", STD 64, RFC 3550, July 2003.
July 2003, <http://www.rfc-editor.org/info/rfc3550>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566, Description Protocol", RFC 4566, July 2006.
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/ Specifications: ABNF", STD 68, RFC 5234, January 2008.
RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>. [W3C.CR-mediacapture-streams-20160519]
Burnett, D., Bergkvist, A., Jennings, C., Narayanan, A.,
and B. Aboba, "Media Capture and Streams", World Wide Web
Consortium CR CR-mediacapture-streams-20160519, May 2016,
<http://www.w3.org/TR/2016/
CR-mediacapture-streams-20160519>.
[W3C.WD-webrtc-20160531] [W3C.WD-webrtc-20160531]
Wilson, C. and J. Kalliokoski, "WebRTC 1.0: Real-time Wilson, C. and J. Kalliokoski, "WebRTC 1.0: Real-time
Communication Between Browsers", World Wide Web Consortium Communication Between Browsers", World Wide Web Consortium
WD WD-webrtc-20160531, May 2016, WD WD-webrtc-20160531, May 2016,
<http://www.w3.org/TR/2016/WD-webrtc-20160531>. <http://www.w3.org/TR/2016/WD-webrtc-20160531>.
7.2. Informative References 7.2. 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-29 (work in progress), April 2016. negotiation-07 (work in progress), April 2014.
[I-D.ietf-rtcweb-overview] [I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", draft-ietf-rtcweb-overview-15 Browser-based Applications", draft-ietf-rtcweb-overview-10
(work in progress), January 2016. (work in progress), June 2014.
[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, DOI 10.17487/ Control Packets on a Single Port", RFC 5761, April 2010.
RFC5761, April 2010,
<http://www.rfc-editor.org/info/rfc5761>.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, DOI Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
10.17487/RFC5888, June 2010,
<http://www.rfc-editor.org/info/rfc5888>. [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015,
<http://www.rfc-editor.org/info/rfc7656>.
Appendix A. Design considerations, rejected alternatives Appendix A. Design considerations, rejected alternatives
One suggested mechanism has been to use CNAME instead of a new One suggested mechanism has been to use CNAME instead of a new
attribute. This was abandoned because CNAME identifies a attribute. This was abandoned because CNAME identifies a
synchronization context; one can imagine both wanting to have tracks synchronization context; one can imagine both wanting to have tracks
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. MediaStream.
A suggestion that survived for a number of drafts was to define A suggestion that survived for a number of drafts was to define
"msid" as a generic mechanism, where the particular semantics of this "msid" as a generic mechanism, where the particular semantics of this
usage of the mechanism would be defined by an "a=wms-semantic" usage of the mechanism would be defined by an "a=wms-semantic"
attribute. This was removed in April 2015. attribute. This was removed in April 2015.
Appendix B. Change log Appendix B. Change log
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 16, line 45 skipping to change at page 17, line 34
Added mux-category to the IANA considerations section. Added mux-category to the IANA considerations section.
B.19. Changes from -12 to -13 B.19. Changes from -12 to -13
Modified registration description to delete dependency on -4566-bis Modified registration description to delete dependency on -4566-bis
B.20. Changes from -13 to -14 B.20. Changes from -13 to -14
Addressed nits found in Gen-ART review Addressed nits found in Gen-ART review
B.21. Changes from -14 to -15
Added the terminology section. Switched from "(RTP) media stream" to
"RTP stream" per RFC 7656.
Added a mention of random ID generation to the security
considerations section.
Moved definition pointers for MediaStream and MediaStreamTrack to the
"mediacapture-streams" document.
Added note that syntactically invalid MSID fields SHOULD be ignored.
Various small changes based on review feedback during IESG
processing.
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. 53 change blocks. 
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