wdiff rfc8830.original rfc8830.txt

Network Working Group

Internet Engineering Task Force (IETF) H. Alvestrand
Internet-Draft
Request for Comments: 8830 Google
Intended status:
Category: Standards Track December 13, 2018
Expires: June 16, 2019 January 2021
ISSN: 2070-1721

WebRTC MediaStream Identification in the Session Description Protocol
draft-ietf-mmusic-msid-17

Abstract

This document specifies a Session Description Protocol (SDP) Grouping 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 "media description" and the WebRTC Web Real-Time Communication
(WebRTC) concept of
"MediaStream" / "MediaStreamTrack" 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 an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained 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 June 16, 2019.
https://www.rfc-editor.org/info/rfc8830.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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(https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Structure Of of This Document . . . . . . . . . . . . . . . 3
1.3. Why A a New Mechanism Is Needed . . . . . . . . . . . . . . 4
1.4. The WEBRTC WebRTC MediaStream . . . . . . . . . . . . . . . . . 4
2. The Msid MSID Mechanism . . . . . . . . . . . . . . . . . . . . . 5
3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Handling of non-signalled tracks . . . . . . . . . . . . 7 Nonsignaled Tracks
3.2. Detailed Offer/Answer Procedures . . . . . . . . . . . . 9
3.2.1. Generating the initial offer . . . . . . . . . . . . 9 Initial Offer
3.2.2. Answerer processing Processing of the Offer . . . . . . . . . . 9
3.2.3. Generating the answer . . . . . . . . . . . . . . . . 10 Answer
3.2.4. Offerer processing Processing of the answer . . . . . . . . . . 10 Answer
3.2.5. Modifying the session . . . . . . . . . . . . . . . . 10 Session
3.3. Example SDP description . . . . . . . . . . . . . . . . . 10 Description
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
4.1. Attribute registration Registration in existing registries . . . . . . 11 Existing Registries
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1.
6.1. Normative References . . . . . . . . . . . . . . . . . . 12
7.2.
6.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Design considerations, rejected alternatives . . . . 14
Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 14
B.1. Changes from alvestrand-rtcweb-msid-00 to -01 . . . . . . 14
B.2. Changes from alvestrand-rtcweb-msid-01 to -02 . . . . . . 15
B.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00 15
B.4. Changes from alvestrand-mmusic-msid-00 to -01 . . . . . . 15
B.5. Changes from alvestrand-mmusic-msid-01 to -02 . . . . . . 15
B.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00 15
B.7. Changes from mmusic-msid-00 to -01 . . . . . . . . . . . 16
B.8. Changes from mmusic-msid-01 to -02 . . . . . . . . . . . 16
B.9. Changes from mmusic-msid-02 to -03 . . . . . . . . . . . 16
B.10. Changes from mmusic-msid-03 to -04 . . . . . . . . . . . 16
B.11. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 16
B.12. Changes from -05 to -06 . . . . . . . . . . . . . . . . . 16
B.13. Changes from -06 to -07 . . . . . . . . . . . . . . . . . 17
B.14. Changes from -07 to -08 . . . . . . . . . . . . . . . . . 17
B.15. Changes from -08 to -09 . . . . . . . . . . . . . . . . . 17
B.16. Changes from -09 to -10 . . . . . . . . . . . . . . . . . 18
B.17. Changes from -10 to -11 . . . . . . . . . . . . . . . . . 18
B.18. Changes from -11 to -12 . . . . . . . . . . . . . . . . . 18
B.19. Changes from -12 to -13 . . . . . . . . . . . . . . . . . 18
B.20. Changes from -13 to -14 . . . . . . . . . . . . . . . . . 18
B.21. Changes from -14 to -15 . . . . . . . . . . . . . . . . . 18
B.22. Changes from -15 to -16 . . . . . . . . . . . . . . . . . 19
B.23. Changes from -16 to -17 . . . . . . . . . . . . . . . . . 19 Considerations, Rejected Alternatives
Acknowledgements
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19

1. Introduction

1.1. Terminology

This document uses terminology from [I-D.ietf-rtcweb-overview]. [RFC8825]. In addition, the
following terms are used as described below:

RTP stream Defined in [RFC7656] as a stream: A stream of RTP packets containing media data.

MediaStream Defined in [W3C.CR-mediacapture-streams-20160519]as an data [RFC7656].

MediaStream: An assembly of MediaStreamTracks. MediaStreamTracks
[W3C.CR-mediacapture-streams]. One MediaStream can contain
multiple MediaStreamTracks, of the same or different types.

MediaStreamTrack

MediaStreamTrack: Defined in [W3C.CR-mediacapture-streams-20160519]as
an [W3C.CR-mediacapture-streams] as a
unidirectional flow of media data (either audio or video, but not
both). Corresponds to the [RFC7656] term "Source Stream". "source stream". One
MediaStreamTrack can be present in zero, one one, or multiple
MediaStreams.

Media description description: Defined in [RFC4566] as a set of fields starting
with an "m=" field and terminated by eitehr either 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)

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

1.2. Structure of This Document

This document adds a new Session Description Protocol (SDP) [RFC4566]
mechanism that can attach identifiers to the RTP streams and
attaching attach
identifiers to the groupings they form. It is designed for use with WebRTC[I-D.ietf-rtcweb-overview] .
WebRTC [RFC8825].

Section 1.3 gives the background on why a new mechanism is needed.

Section 2 gives the definition of the new mechanism.

Section 3 gives the necessary semantic information and procedures for
using the msid "msid" attribute to signal the association of
MediaStreamTracks to MediaStreams in support of the WebRTC API
[W3C.WD-webrtc-20160531].
[W3C-WebRTC].

1.3. Why A a New Mechanism Is Needed

When media is carried by RTP [RFC3550], each RTP stream is
distinguished inside an RTP session by its SSRC; Synchronization Source
(SSRC); each RTP session is distinguished from all other RTP sessions
by being on a different transport association (strictly speaking, 2 two
transport associations, one used for RTP and one used for RTCP, the RTP
Control Protocol (RTCP), unless RTP/RTCP multiplexing [RFC5761] is
used).

SDP [RFC4566] gives a format for describing an SDP session that can
contain multiple media descriptions. According to the model used in
[I-D.ietf-rtcweb-jsep],
[RFC8829], each media description describes exactly one media source, and if source.
If multiple media sources are carried in an RTP session, this is signalled
signaled using BUNDLE
[I-D.ietf-mmusic-sdp-bundle-negotiation]; [RFC8843]; if BUNDLE is not used, each media
source is carried in its own RTP session.

The SDP grouping framework Grouping Framework [RFC5888] can be used to group media
descriptions. However, for the use case of WebRTC, there is the need
for an application to specify some application-level information
about the association between the media description and the group.
This is not possible using the SDP grouping framework. Grouping Framework.

1.4. The WEBRTC WebRTC MediaStream

The W3C WebRTC API specification [W3C.WD-webrtc-20160531] [W3C-WebRTC] 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 session, forming an RTP stream. The
collision of terminology is unfortunate.) unfortunate. There might possibly be
additional SSRCs, possibly within additional RTP sessions, in order
to support functionality like forward error correction or simulcast.
These additional SSRCs are not affected by this specification.

MediaStreamTracks are unidirectional; they carry media on in one
direction only.

In the RTP specification, RTP streams are identified using the SSRC
field. Streams are grouped into RTP Sessions, sessions and also carry a CNAME.
Neither CNAME nor RTP session correspond corresponds to a MediaStream.
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]) [RFC8829]) where one SDP
media description is used to describe each MediaStreamTrack, and the
BUNDLE mechanism [I-D.ietf-mmusic-sdp-bundle-negotiation] [RFC8843] is used to group MediaStreamTracks into
RTP sessions. Therefore, the need is to specify the ID identifier (ID)
of a the MediaStreamTrack and its associated MediaStream for each media
description, which can be accomplished with a media-level SDP
attribute.

This usage is described in Section 3.

2. The Msid MSID Mechanism

This document defines a new SDP [RFC4566] media-level "msid"
attribute. This new attribute allows endpoints to associate RTP
streams that are described in different separate media descriptions with the
same MediaStreams
right MediaStreams, as defined in [W3C.WD-webrtc-20160531], and [W3C-WebRTC]. It also allows
endpoints to carry an identifier for each MediaStreamTrack in its
"appdata" field.

The value of the "msid" attribute consists of an identifier and an
optional "appdata" field.

The name of the attribute is "msid".

The value of the attribute is specified by the following ABNF
[RFC5234] grammar:

msid-value = msid-id [ SP msid-appdata ]
msid-id = 1*64token-char ; see RFC 4566
msid-appdata = 1*64token-char ; see RFC 4566

An example msid "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.

Application data (msid-appdata) is carried on the same line as the
identifier, separated from the identifier by a space.

The identifier (msid-id) ("msid-id") uniquely identifies a group within the
scope of an SDP description.

There may be multiple msid "msid" attributes in a single media
description. This represents the case where a single
MediaStreamTrack is present in multiple MediaStreams; the value of
"msid-appdata" MUST be identical for all occurences. occurrences.

Multiple media descriptions with the same value for msid-id "msid-id" and msid-
appdata
"msid-appdata" are not permitted.

Endpoints can update the associations between RTP streams as
expressed by msid "msid" attributes at any time.

The msid "msid" attributes depend on the association of RTP streams with
media descriptions, descriptions but does do not depend on the association of RTP
streams with RTP transports; therefore, its mux category transports. Therefore, their Mux Category (as
defined in [I-D.ietf-mmusic-sdp-mux-attributes]) [RFC8859]) is NORMAL - NORMAL; the process of deciding on MSID "msid"
attributes doesn't have to take into consideration whether or not the
RTP streams are bundled or not. bundled.

3. Procedures

This section describes the procedures for associating media
descriptions representing MediaStreamTracks within MediaStreams MediaStreams, as
defined in [W3C.WD-webrtc-20160531]. [W3C-WebRTC].

In the Javascript API described in that specification, each
MediaStream and MediaStreamTrack has an "id" attribute, which is a
DOMString.

The value of the "msid-id" field in the msid MSID consists of the "id"
attribute of a MediaStream, as defined in the MediaStream's WebIDL
specification.
specification [WEBIDL]. The special value "-" indicates "no
MediaStream".

The value of the "msid-appdata" field in the msid, MSID, if present,
consists of the "id" attribute of a MediaStreamTrack, as defined in
the MediaStreamTrack's WebIDL specification.

When an SDP session description is updated, a specific "msid-id"
value continues to refer to the same MediaStream, and a specific
"msid-appdata" to the same MediaStreamTrack. There is no memory
apart from the currently valid SDP descriptions; if an msid MSID
"identifier" value disappears from the SDP and appears in a later
negotiation, it will be taken to refer to a new MediaStream.

If the MSID "msid" attribute does not conform to the ABNF given here, it
SHOULD be ignored.

The following is a high level high-level description of the rules for handling
SDP updates. Detailed procedures are located in Section 3.2.

* When a new msid MSID "identifier" value occurs in a session
description, and it is not "-", the recipient can signal to its
application that a new MediaStream has been added.

* When a session description is updated to have media descriptions
with an msid MSID "identifier" value, with one or more different
"appdata" values, the recipient can signal to its application that
new MediaStreamTracks have been added, added and note to which
MediaStream it
has they have been added to. added. This is done for each different msid
MSID "identifier" value, including the special value "-", which
indicates that a MediaStreamTrack has been added with no
corresponding MediaStream.

* If an msid MSID "identifier" value with no "appdata" value appears, it
means that the sender did not inform the recipient of the desired
identifier of the MediaStreamTrack, and the recipient will assign
the "id" value of the created MediaStreamTrack on its own. All
msid
MSIDs in a media section that do not have an "appdata" value are
assumed to refer to the same MediaStreamTrack.

* When a session description is updated to no longer list any msid "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 ended when its msid "msid"
attribute disappears from the SDP, the track will also be signaled as
being ended when all associated SSRCs have disappeared by the rules
of [RFC3550] section [RFC3550], Sections 6.3.4 (BYE packet received) and 6.3.5
(timeout), or when the corresponding media description is disabled by
setting the port number to zero. Changing the direction of the media
description (by setting "sendonly", "recvonly" "recvonly", or "inactive"
attributes) will not end the MediaStreamTrack.

The association between SSRCs and media descriptions is specified in
[I-D.ietf-rtcweb-jsep].
[RFC8829].

3.1. Handling of non-signalled tracks Nonsignaled Tracks

Entities that do not use msid the mechanism described in this document
will not send the "msid" attribute and thus will not send msid. information
allowing the mapping of RTP packets to MediaStreams. This means that
there will be some incoming RTP packets that for which the recipient has
no predefined MediaStream id value for. ID value.

Note that this the handling described below is triggered by incoming RTP
packets, not by SDP negotiation.

When communicating with entities that use the MSID is used, mechanism, the
only time this incoming RTP packets can happen be received without an associated
MediaStream ID value is when, after the initial negotiation, a
negotiation is performed where the answerer adds a MediaStreamTrack
to an already established connection and starts sending data before
the answer is received by the offerer. For initial negotiation,
packets won't flow until the ICE Interactive Connectivity Establishment
(ICE) candidates and fingerprints have been exchanged, so this is not
an issue.

The recipient of those packets will perform the following steps:

* When RTP packets are initially received, it will create an
appropriate MediaStreamTrack based on the type of the media
(carried in PayloadType), PayloadType) and use the MID RTP header extension
[I-D.ietf-mmusic-sdp-bundle-negotiation]
[RFC8843] (if present) to associate the RTP packets with a
specific media section.

* If the connection is not in the RTCSignalingState "stable", it
will wait at this point.

* When the connection is in the RTCSignalingState "stable", it will
assign ID values.

The following steps are performed to assign ID values:

* If there is an msid "msid" attribute, it will use that attribute to
populate the "id" field of the MediaStreamTrack and associated
MediaStreams, as described above.

* If there is no msid "msid" attribute, the identifier of the
MediaStreamTrack will be set to a randomly generated string, and
it will be signalled signaled as being part of a MediaStream with the WebIDL
"label" attribute set to "Non-WebRTC stream".

* After deciding on the "id" field to be applied to the
MediaStreamTrack, the track will be signalled signaled to the user.

The process above may involve a considerable amount of buffering
before the stable "stable" state is entered. If the implementation wishes
to limit this buffering, it MUST signal to the user that media has
been discarded.

It follows from the above that MediaStreamTracks in the "default"
MediaStream cannot be closed by removing the msid "msid" attribute; the
application must instead signal these as closed when the SSRC
disappears
disappears, either according to the rules of RFC 3550 section Sections 6.3.4 and 6.3.5
of [RFC3550] or by disabling the media description by setting its
port to zero.

3.2. Detailed Offer/Answer Procedures

These procedures are given in terms of RFC 3264-recommended sections. sections recommended by
[RFC3264]. They describe the actions to be taken in terms of
MediaStreams and MediaStreamTracks; they do not include event signalling
signaling inside the application, which is described in JSEP. the
JavaScript Session Establishment Protocol (JSEP) [RFC8829].

3.2.1. Generating the initial offer Initial Offer

For each media description in the offer, if there is an associated
outgoing 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. If the sender wishes to
signal identifiers for the MediaStreamTracks, the "appdata" field is
set to the WebIDL "id" attribute of the MediaStreamTrack; otherwise otherwise,
it is omitted.

3.2.2. Answerer processing Processing of the Offer

For each media description in the offer, offer and for each "a=msid" attribute
in the media description, the receiver of the offer will perform the
following steps:

* Extract the "appdata" field of the "a=msid" attribute, if present.

* If the "appdata" field exists: Check if a MediaStreamTrack with
the same WebIDL "id" attribute as the "appdata" field already
exists,
exists and is not in the "ended" state. If it such a
MediaStreamTrack is not found, create it.

* If the "appdata" field does not exist, and a MediaStreamTrack is
not associated with this media section, create one a MediaStreamTrack
and associate it with this media section for future use.

* Extract the "identifier" field of the "a=msid" attribte.

o attribute.

* Check if a MediaStream with the same WebIDL "id" attribute already
exists. If not, create it.

* Add the MediaStreamTrack to the MediaStream

o MediaStream.

* Signal to the user that a new MediaStreamTrack is available.

3.2.3. Generating the answer Answer

The answer is generated in exactly the same manner as the offer.
"a=msid" values in the offer do not influence the answer.

3.2.4. Offerer processing Processing of the answer Answer

The answer is processed in exactly the same manner as the offer.

3.2.5. Modifying the session 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:

* 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.

* If no such attribute is found, stop the MediaStreamTrack. This
will set its state to "ended".

3.3. Example SDP description Description

The following SDP description shows the representation of a WebRTC
PeerConnection with two MediaStreams, each of which has one audio and
one video track. Only the parts relevant to the MSID are shown.

Line wrapping, empty lines lines, and comments are added for clarity. They
are not part of the SDP.

# First MediaStream - id is 4701...
m=audio 56500 UDP/TLS/RTP/SAVPF 96 0 8 97 98
a=msid:47017fee-b6c1-4162-929c-a25110252400
f83006c5-a0ff-4e0a-9ed9-d3e6747be7d9

m=video 56502 UDP/TLS/RTP/SAVPF 100 101
a=msid:47017fee-b6c1-4162-929c-a25110252400
b47bdb4a-5db8-49b5-bcdc-e0c9a23172e0

# Second MediaStream - id is 6131....
m=audio 56503 UDP/TLS/RTP/SAVPF 96 0 8 97 98
a=msid:61317484-2ed4-49d7-9eb7-1414322a7aae
b94006c5-cade-4e0a-9ed9-d3e6747be7d9

m=video 56504 UDP/TLS/RTP/SAVPF 100 101
a=msid:61317484-2ed4-49d7-9eb7-1414322a7aae
f30bdb4a-1497-49b5-3198-e0c9a23172e0

4. IANA Considerations

4.1. Attribute registration Registration in existing registries

This document requests Existing Registries

IANA to register has registered the "msid" attribute in the
"att-field "att-field" (media
level only)" only) registry within the SDP parameters "Session Description Protocol (SDP)
Parameters" registry, according to the procedures of [RFC4566] [RFC4566].

The required information for "msid" is:

o registration information is as follows:

Contact name, email: IETF, contacted via mmusic@ietf.org, or a
successor address designated by IESG

Attribute name: msid

Attribute syntax:

msid-value = msid-id [ SP msid-appdata ]
msid-id = 1*64token-char ; see RFC 4566
msid-appdata = 1*64token-char ; see RFC 4566

Attribute semantics: Described in RFC 8830

Attribute value: msid-value

Long-form attribute name: MediaStream group Identifier

Usage level: media

Subject to charset: The attribute value contains only ASCII
characters,
characters and is therefore not subject to the charset attribute.

Purpose: The attribute can be used to signal the relationship
between a WebRTC MediaStream and a set of media descriptions.

O/A Procedures: Described in RFC 8830

Appropriate values: The details of appropriate values are given in
RFC XXXX.

o MUX category: 8830 (this document).

Mux Category: NORMAL

The MUX category Mux Category is defined in [I-D.ietf-mmusic-sdp-mux-attributes]. [RFC8859].

5. Security Considerations

An adversary with the ability to modify SDP descriptions has the
ability to switch around tracks between MediaStreams. 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 3.1, the amount of
buffering should be limited to avoid memory exhaustion attacks.

Careless generation of identifiers can leak privacy-sensitive
information. [W3C.CR-mediacapture-streams-20160519] [W3C.CR-mediacapture-streams] recommends that
identifiers are be generated using UUID a Universally Unique IDentifier (UUID)
class 3 or 4 as a basis, which avoids such leakage.

No other attacks have been identified that depend on this mechanism.

6. Acknowledgements

This note is based on sketches from, among others, Justin Uberti and
Cullen Jennings.

Special thanks to Flemming Andreassen, Ben Campbell, Miguel Garcia,
Martin Thomson, Ted Hardie, Adam Roach, Magnus Westerlund, Alissa
Cooper, Sue Hares and Paul Kyzivat for their work in reviewing this
draft, with many specific language suggestions.

7. References

7.1.

6.1. Normative References

[I-D.ietf-mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-12
(work in progress), January 2016.

[I-D.ietf-rtcweb-jsep]
Uberti, J., Jennings, C., and E. Rescorla, "Javascript
Session Establishment Protocol", draft-ietf-rtcweb-jsep-14
(work in progress), March 2016.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/
RFC2119, 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
<https://www.rfc-editor.org/info/rfc2119>.

[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>. <https://www.rfc-editor.org/info/rfc3550>.

[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>. <https://www.rfc-editor.org/info/rfc4566>.

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/
RFC5234, 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.

[W3C.CR-mediacapture-streams-20160519]
Burnett, D., Bergkvist, A.,
<https://www.rfc-editor.org/info/rfc5234>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8829] Uberti, J., 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]
Wilson, C. E. Rescorla, Ed.,
"JavaScript Session Establishment Protocol (JSEP)",
RFC 8829, DOI 10.17487/RFC8829, January 2021,
<https://www.rfc-editor.org/info/rfc8829>.

[RFC8859] Nandakumar, S., "A Framework for Session Description
Protocol (SDP) Attributes When Multiplexing", RFC 8859,
DOI 10.17487/RFC8859, January 2021,
<https://www.rfc-editor.org/info/rfc8859>.

[W3C-WebRTC]
Jennings, C., Boström, H., and J. Kalliokoski, J-I. Bruaroey, "WebRTC 1.0:
Real-time Communication Between Browsers", World Wide Web Consortium
WD WD-webrtc-20160531, May 2016,
<http://www.w3.org/TR/2016/WD-webrtc-20160531>.

7.2. W3C Proposed
Recommendation, <https://www.w3.org/TR/webrtc/>.

[W3C.CR-mediacapture-streams]
Jennings, C., Aboba, B., Bruaroey, J.-I., and H. Boström,
"Media Capture and Streams", W3C Candidate Recommendation,
<https://www.w3.org/TR/mediacapture-streams/>.

6.2. Informative References

[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H.,

[RFC3264] Rosenberg, J. and C. Jennings,
"Negotiating Media Multiplexing Using the H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-29 (work in progress), April 2016.

[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", draft-ietf-rtcweb-overview-15
(work in progress), January 2016. RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<https://www.rfc-editor.org/info/rfc3264>.

[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761,
DOI 10.17487/
RFC5761, 10.17487/RFC5761, April 2010,
<http://www.rfc-editor.org/info/rfc5761>.
<https://www.rfc-editor.org/info/rfc5761>.

[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888,
DOI 10.17487/RFC5888, June 2010,
<http://www.rfc-editor.org/info/rfc5888>.
<https://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>.
<https://www.rfc-editor.org/info/rfc7656>.

[RFC8825] Alvestrand, H., "Overview: Real-Time Protocols for
Browser-Based Applications", RFC 8825,
DOI 10.17487/RFC8825, January 2021,
<https://www.rfc-editor.org/info/rfc8825>.

[RFC8843] Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", RFC 8843,
DOI 10.17487/RFC8843, January 2021,
<https://www.rfc-editor.org/info/rfc8843>.

[WEBIDL] Chen, E. and T. Gu, "Web IDL", W3C Editor's Draft, August
2020, <https://heycam.github.io/webidl/>.

Appendix A. Design considerations, rejected alternatives Considerations, Rejected Alternatives

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 "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 MediaStream.

A suggestion that survived for a number of drafts of this document
was to define
"msid" MSID as a generic mechanism, where the particular
semantics of this usage of the mechanism would be defined by an
"a=wms-semantic" attribute. This was removed in April 2015.

Appendix B. Change log

Acknowledgements

This appendix should be deleted before publication as an RFC.

B.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.

B.2. Changes from alvestrand-rtcweb-msid-01 to -02

Split document into sections describing a generic grouping mechanism
and sections describing the application of this grouping mechanism to
the WebRTC MediaStream concept.

Removed the mechanism for muting tracks, since this is not central to
the MSID mechanism.

B.3. Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00

Changed the draft name according to the wishes of the MMUSIC group
chairs.

Added text indicting cases where it's appropriate to have the same
appdata for multiple SSRCs.

Minor textual updates.

B.4. Changes from alvestrand-mmusic-msid-00 to -01

Increased the amount of explanatory text, much based on a review by
Miguel Garcia.

Removed references to BUNDLE, since that spec is under active
discussion.

Removed distinguished values of the MSID identifier.

B.5. Changes from alvestrand-mmusic-msid-01 to -02

Changed the order of the "msid-semantic: " attribute's value fields
and allowed multiple identifiers. This makes the attribute useful as
a marker for "I understand this semantic".

Changed the syntax for "identifier" and "appdata" to be "token".

Changed the registry for the "msid-semantic" attribute values to be a
new registry, based on advice given in Atlanta.

B.6. Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00

Updated terminology to refer to m-lines rather than RTP sessions when
discussing SDP formats and the ability of other linking mechanisms to
refer to SSRCs.

Changed the "default" mechanism to return independent streams after
considering the synchronization problem.

Removed the space from between "msid-semantic" and its value, to be
consistent with RFC 5576.

B.7. Changes from mmusic-msid-00 to -01

Reworked msid mechanism to be a per-m-line attribute, to align with
draft-roach-mmusic-unified-plan.

B.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.

B.9. Changes from mmusic-msid-02 to -03

Corrected even more cases where the word "ssrc" was not changed to
"M-line".

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.

B.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.

B.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.

B.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.

B.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 note is based on a proposal by Paul
Kyzivat.

Included a section on generic offer/answer semantics.

B.14. Changes from -07 sketches from, among others, Justin Uberti and
Cullen Jennings.

Special thanks 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 Flemming Andreasen, Ben Campbell, Miguel Garcia,
Martin Thomson.

A number of text and ABNF clarifications based on suggestions from Thomson, 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

B.15. Changes from -08 to -09

Removed "wms-semantic" and all mention of multiple semantics for
msid, as agreed at the Dallas IETF, March 2015.

Addressed a number of review comments from Fleming Andresen Roach, Magnus Westerlund, Alissa
Cooper, Sue Hares, and
others.

Changed the term "m-line" to "media description", since that is the
term used in RFC 4566.

Tried to make sure this document does not describe the API to the
application.

B.16. Changes from -09 to -10

Addressed review comments from Paul Kyzivat.

B.17. Changes from -10 to -11

Defined the semantics of multiple MSIDs in a media section to be a
MediaStreamTrack present in multiple MediaStreams.

Made an explicit note that MediaStreamTracks are unidirectional.

Disallowed the option of sending multiple media sections with the
same msid (id and appdata identical).

B.18. Changes from -11 to -12

Added mux-category to the IANA considerations section.

B.19. Changes from -12 to -13

Modified registration description to delete dependency on -4566-bis

B.20. Changes from -13 to -14

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 Kyzivat 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.

B.22. Changes from -15 to -16

Added the special "-" value that means "no MediaStream".

Changed instances of a MediaStreamTrack being "closed" to saying it's
"ended", their work in accordance reviewing this
document, with WebRTC terminology.

B.23. Changes from -16 to -17

Added text to allow omitting track identifiers, per JSEP PR #850 many specific language suggestions.

Author's Address

Harald Alvestrand
Google
Kungsbron 2
SE-11122 Stockholm 11122
Sweden

Email: harald@alvestrand.no