One document matched: draft-ietf-mmusic-msid-06.xml
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<?rfc tocdepth="3"?>
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<rfc category="std" docName="draft-ietf-mmusic-msid-06" ipr="trust200902">
<front>
<title abbrev="MSID in SDP">WebRTC MediaStream Identification in the
Session Description Protocol</title>
<author fullname="Harald Alvestrand" initials="H. T." surname="Alvestrand">
<organization>Google</organization>
<address>
<postal>
<street>Kungsbron 2</street>
<city>Stockholm</city>
<region/>
<code>11122</code>
<country>Sweden</country>
</postal>
<email>harald@alvestrand.no</email>
</address>
</author>
<date day="9" month="June" year="2014"/>
<abstract>
<t>This document specifies a Session Description Protocol (SDP) Grouping
mechanism for RTP media streams that can be used to specify relations
between media streams.</t>
<t>This mechanism is used to signal the association between the SDP
concept of "m-line" and the WebRTC concept of "MediaStream" /
"MediaStreamTrack" using SDP signaling.</t>
<t>This document is a work item of the MMUSIC WG, whose discussion list
is mmusic@ietf.org.</t>
</abstract>
<note title="Requirements Language">
<t>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 <xref
target="RFC2119">RFC 2119</xref>.</t>
</note>
</front>
<middle>
<section title="Introduction">
<t/>
<section title="Structure Of This Document">
<t>This document adds a new Session Description Protocol (SDP)
Grouping<xref target="RFC5888"/> relation between SDP m-lines <xref
target="RFC4566"/> that can associate application layer identifiers
with the binding between media streams, attaching identifiers to the
media streams and attaching identifiers to the groupings they
form.</t>
<t><xref target="sect-why"/> gives the background on why a new
mechanism is needed.</t>
<t><xref target="sect-definition"/> gives the definition of the new
mechanism.</t>
<t><xref target="sect-semantic"/> gives the definition of the
msid-semantic field, which gives the possibility of using MSIDs with
different semantics in the same SDP message.</t>
<t><xref target="sect-media-stream"/> gives the application of the new
mechanism for providing necessary semantic information for the
association of MediaStreamTracks to MediaStreams in the WebRTC
API<xref target="W3C.WD-webrtc-20120209"> </xref>.</t>
</section>
<section anchor="sect-why" title="Why A New Mechanism Is Needed">
<t>When media is carried by RTP <xref target="RFC3550"/>, each RTP
media stream is distinguished inside an RTP session by its SSRC; each
RTP session is distinguished from all other RTP sessions by being on a
different transport association (strictly speaking, 2 transport
associations, one used for RTP and one used for RTCP, unless RTP/RTCP
multiplexing <xref target="RFC5761"/> is used).</t>
<t>SDP gives a description based on m-lines. According to the model
used in <xref target="I-D.ietf-rtcweb-jsep"/>, each m-line describes
exactly one media source, and if mulitple media sources are carried in
an RTP session, this is signalled using BUNDLE <xref
target="I-D.ietf-mmusic-sdp-bundle-negotiation"/>; if BUNDLE is not
used, each media source is carried in its own RTP session.</t>
<t>There exist cases where an application using RTP and SDP needs to
signal some relationship between RTP media streams that may be carried
in either the same RTP session or different RTP sessions. For
instance, there may be a need to signal a relationship between a video
track and an audio track, and where the generator of the SDP does not
yet know if they will be carried in the same RTP session or different
RTP sessions.</t>
<t>The SDP grouping framework <xref target="RFC5888"/> can be used to
group m-lines. However, there is sometimes the need for an application
to specify some application-level information about the association
between the m-line and the group. This is not possible using the SDP
grouping framework.</t>
</section>
<section title="Application to the WEBRTC MediaStream">
<t>The W3C WebRTC API specification <xref
target="W3C.WD-webrtc-20120209"/> specifies that communication between
WebRTC entities is done via MediaStreams, which contain
MediaStreamTracks. A MediaStreamTrack is generally carried using a
single SSRC in an RTP session (forming an RTP media stream. The
collision of terminology is unfortunate.) There might possibly be
additional SSRCs, possibly within additional RTP sessions, in order to
support functionality like forward error correction or simulcast. This
complication is ignored below.</t>
<t>In the RTP specification, media streams are identified using the
SSRC field. Streams are grouped into RTP Sessions, and also carry a
CNAME. Neither CNAME nor RTP session correspond to a MediaStream.
Therefore, the association of an RTP media stream to MediaStreams need
to be explicitly signaled.</t>
<t>WebRTC defines a mapping (documented in <xref
target="I-D.ietf-rtcweb-jsep"/>) where one SDP m-line is used to
describe each MediaStreamTrack, and that the BUNDLE mechanism <xref
target="I-D.ietf-mmusic-sdp-bundle-negotiation"/> is used to group
MediaStreamTracks into RTP sessions. Therefore, the need is to specify
the ID of a MediaStreamTrack and its associated MediaStream for each
m-line, which can be accomplished with a media-level SDP
attribute.</t>
<t>This usage is described in <xref target="sect-media-stream"/>.</t>
</section>
</section>
<section anchor="sect-definition" title="The Msid Mechanism">
<t>This document defines a new SDP <xref target="RFC4566"/> media-level
"msid" attribute. This new attribute allows endpoints to associate RTP
media streams that are carried in the same or different m-lines. The
attribute also allows application-specific information to the
association.</t>
<t>The value of the "msid" attribute consists of an identifier and
optional application-specific data, according to the following ABNF
<xref target="RFC5234"/> grammar:</t>
<t/>
<figure>
<artwork><![CDATA[
; "attribute" is defined in RFC 4566.
attribute =/ msid-attr
msid-attr = "msid:" identifier [ SP appdata ]
identifier = 1*64token-char ; see RFC 4566
appdata = 1*64token-char ; see RFC 4566
]]></artwork>
</figure>
<t>An example msid value for a group with the identifier "examplefoo"
and application data "examplebar" might look like this:</t>
<figure>
<artwork><![CDATA[
msid:examplefoo examplebar
]]></artwork>
</figure>
<t>The identifier is a string of ASCII characters that are legal in a
"token", consisting of between 1 and 64 characters. It MUST be unique
among the identifier values used in the same SDP session. It is
RECOMMENDED that it is generated using a random-number generator.</t>
<t>Application data is carried on the same line as the identifier,
separated from the identifier by a space.</t>
<t>The identifier uniquely identifies a group within the scope of an SDP
description.</t>
<t>There may be multiple msid attributes in a single media description.
There may also be multiple media descriptions that have the same value
for identifier and application data.</t>
<t>Endpoints can update the associations between RTP media streams as
expressed by msid attributes at any time; the semantics and restrictions
of such grouping and ungrouping are application dependent.</t>
</section>
<section anchor="sect-semantic" title="The Msid-Semantic Attribute">
<t>A session-level attribute is defined for signaling the semantics
associated with an msid grouping. This allows msid groupings with
different semantics to coexist.</t>
<t>This OPTIONAL attribute gives the group identifier and its group
semantic; it carries the same meaning as the ssrc-group-attr of RFC 5576
section 4.2, but uses the identifier of the group rather than a list of
SSRC values.</t>
<t>This attribute MUST be present if "a=msid" is used.</t>
<t>An empty list of identifiers is an indication that the sender
understands the indicated semantic, but has no msid groupings of the
given type in the present SDP.</t>
<t>An identifier of "*" is an indication that all "a=msid" lines in the
SDP have this specific semantic.</t>
<t>The ABNF of msid-semantic is:</t>
<figure>
<artwork><![CDATA[
attribute =/ msid-semantic-attr
msid-semantic-attr = "msid-semantic:" semantic identifier-list
semantic = token ; see RFC 4566
identifier-list = (" " identifier)* / " *"
]]></artwork>
</figure>
<t>The semantic field holds values from the IANA registriy "Semantics
for the msid-semantic SDP attribute" (which is defined in <xref
target="IANA"/>).</t>
<t>An example msid-semantic might look like this, if a semantic LS was
registered by IANA for the same purpose as the existing LS grouping
semantic:</t>
<figure>
<artwork><![CDATA[
a=msid-semantic:LS xyzzy forolow
]]></artwork>
</figure>
<t>This means that the SDP description has two lip sync groups, with the
group identifiers xyzzy and forolow, respectively.</t>
</section>
<section anchor="sect-media-stream"
title="Applying Msid to WebRTC MediaStreams">
<t>This section creates a new semantic for use with the framework
defined in <xref target="sect-definition"/>, to be used for associating
m-lines representing MediaStreamTracks within MediaStreams as defined in
<xref target="W3C.WD-webrtc-20120209"/>.</t>
<t>In the Javascript API, each MediaStream and MediaStreamTrack has an
"id" attribute, which is a DOMString.</t>
<t>The semantic token for this semantic is "WMS" (short for WebRTC Media
Stream).</t>
<t>The value of the "identifier" field in the msid consists of the "id"
attribute of a MediaStream, as defined in its WebIDL specification.</t>
<t>The value of the "appdata" field in the msid consists of the "id"
attribute of a MediaStreamTrack, as defined in its WebIDL
specification.</t>
<t>If two different m-lines have MSID attributes with the same value for
identifier and appdata, it means that these two m-lines are both
intended for the same MediaStreamTrack. So far, no semantic for such a
mixture have been defined, but this specification does not forbid the
practice.</t>
<t>When an SDP description is updated, a specific msid "identifier"
continues to refer to the same MediaStream, and a specific "appdata" to
the same MediaStreamTrack. Once negotiation has completed on a session,
there is no memory apart from the currently valid SDP descriptions; an
msid "identifier" value that appears in a later negotiation will be
taken to refer to a new MediaStream.</t>
<t>The following are the rules for handling updates of the list of
m-lines and their msid values.</t>
<t><list style="symbols">
<t>When a new msid "identifier" value occurs in the description, the
recipient can signal to its application that a new MediaStream has
been added.</t>
<t>When a description is updated to have more media sections with
the same msid "identifier" value, but different "appdata" values,
the recipient can signal to its application that new
MediaStreamTracks have been added to the MediaStream.</t>
<t>When a description is updated to no longer list the msid
attribute on a specific media description, the recipient can signal
to its application that the corresponding MediaStreamTrack has
ended.</t>
</list>In addition to signaling that the track is closed when its msid
attribute disappears from the SDP, the track will also be signaled as
being closed when all associated SSRCs have disappeared by the rules of
<xref target="RFC3550"/> section 6.3.4 (BYE packet received) and 6.3.5
(timeout), and when the corresponding media section is disabled by
setting the port number to zero. Changing the direction of the media
section to "recvonly" will not close the MediaStreamTrack.</t>
<t>The association between SSRCs and m-lines is specified in <xref
target="I-D.ietf-rtcweb-jsep"/>.</t>
<section anchor="s-nonsignal" title="Handling of non-signalled tracks">
<t>Non-WebRTC entities will not send msid. This means that there will
be some incoming RTP packets that the recipient has no predefined
MediaStream id value for.</t>
<t>Note that this handling is triggered by incoming RTP packets, not
by SDP negotiation.</t>
<t>Handling will depend on whether or not the msid-semantic:WMS
attribute is present. There are two cases:</t>
<t><list style="symbols">
<t>No "msid-semantic:WMS" attribute is present. The SDP session is
assumed to be a backwards-compatible session. All incoming media,
on all m-lines that are part of the SDP session, are assumed to
belong to independent media streams, each with one track. The
identifier of this media stream and of the media stream track is a
randomly generated string; the WebIDL "label" attribute of this
media stream will be set to "Non-WMS stream".</t>
<t>An "msid-semantic:WMS" attribute is present. In this case, the
session is WebRTC compatible, and the packets are either caused by
a bug or by timing skew between the arrival of the media packets
and the SDP description. These packets MAY be discarded, or they
MAY be buffered for a while in order to allow immediate startup of
the media stream when the SDP description is updated. The arrival
of media packets MUST NOT cause a new MediaStreamTrack to be
signaled.</t>
</list></t>
<t>If a WebRTC entity sends a description, it MUST include the
msid-semantic:WMS attribute, even if no media streams are sent. This
allows us to distinguish between the case of no media streams at the
moment and the case of legacy SDP generation.</t>
<t>It follows from the above that the WebRTC entity must have the SDP
of the other party before it can decide correctly which of the two
cases described above applies. RTP media packets that arrive before
the remote party's SDP MUST be buffered or discarded, and MUST NOT
cause a new MediaStreamTrack to be signalled.</t>
<t>It follows from the above that media stream tracks in the "default"
media stream cannot be closed by removing the msid attribute; the
application must instead signal these as closed when the SSRC
disappears according to the rules of RFC 3550 section 6.3.4 and 6.3.5
or by disabling the m-line by setting its port to zero.</t>
</section>
<section title="Detailed procedures">
<t>These procedures are given in terms of RFC 3264-recommended
sections. They describe the actions to be taken in terms of
MediaStreams and MediaStreamTracks; they do not include event
signalling inside the application, which is described in JSEP.</t>
<t>They are specifically applicable to the WMS semantic; other
semantics will have their own consideration.</t>
<section title="Generating the initial offer">
<t>For each media section in the offer, if there is an associated
MediaStreamTrack, the offerer adds one "a=msid" attribute to the
section for each MediaStream with which the MediaStreamTrack is
associated. The "identifier" field of the attribute is set to the
WebIDL "id" attribute of the MediaStream, and the "appdata" field is
set to the WebIDL "id" attribute of the MediaStreamTrack.</t>
</section>
<section title="Parsing the initial offer">
<t>For each media section in the offer, and for each "a=msid"
attribute in the media section, the receiver of the offer will
perform the following steps:</t>
<t><list style="symbols">
<t>Extract the "appdata" field of the "a=msid" attribute</t>
<t>Check if a MediaStreamTrack with the same WebIDL "id"
attribute as the "appdata" field already exists, and is not in
the "ended" state. If it is not found, create it.</t>
<t>Extract the "identifier" field of the "a=msid" attribte.</t>
<t>Check if a MediaStream with the same WebIDL "id" attribute
already exists. If not, create it.</t>
<t>Add the MediaStreamTrack to the MediaStream</t>
</list></t>
</section>
<section title="Generating the answer">
<t>The answer is generated in exactly the same manner as the
offer.</t>
</section>
<section title="Offerer processing of the answer">
<t>The answer is processed in exactly the same manner as the
offer.</t>
</section>
<section title="Modifying the session">
<t>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:</t>
<t><list style="symbols">
<t>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.</t>
<t>If no such attribute is found, close the MediaStreamTrack.
This will set its state to "ended".</t>
</list></t>
</section>
</section>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>This document requests IANA to register the "msid" attribute in the
"att-field (media level only)" registry within the SDP parameters
registry, according to the procedures of <xref target="RFC4566"/></t>
<t>The required information for "msid" is:</t>
<t><list style="symbols">
<t>Contact name, email: IETF, contacted via mmusic@ietf.org, or a
successor address designated by IESG</t>
<t>Attribute name: msid</t>
<t>Long-form attribute name: Media stream group Identifier</t>
<t>Subject to charset: The attribute value contains only ASCII
characters, and is therefore not subject to the charset
attribute.</t>
<t>Purpose: The attribute gives an association over a set of
m-lines. It can be used to signal the relationship between a WebRTC
MediaStream and a set of m-lines.</t>
<t>Appropriate values: The details of appropriate values are given
in RFC XXXX.</t>
</list></t>
<t>This document requests IANA to register the "msid-semantic" attribute
in the "att-field (session level) registry within the SDP parameters
registry, according to the same procedures.</t>
<t>The required information is:</t>
<t><list style="symbols">
<t>Contact name, email: IETF, contacted via mmusic@ietf.org, or a
successor address designated by IESG</t>
<t>Attribute name: msid-semantic</t>
<t>Long-form attribute name: Msid group semantic identifier</t>
<t>Subject to charset: The attribute value contains only ASCII
characters, and is therefore not subject to the charset
attribute.</t>
<t>Purpose: The attribute gives the semantics of an association over
a set of m-lines.</t>
<t>Appropriate values: The details are given in RFC XXXX.</t>
</list></t>
<t>This document requests IANA to create a new registry called
"Semantics for the msid-semantic SDP attribute", which should have
exactly the same rules as for the "Semantics for the ssrc-group SDP
attribute" registry (Expert Review), and to register the "WMS" semantic
within this new registry.</t>
<t>The required information is:</t>
<t><list style="symbols">
<t>Description: WebRTC Media Stream, as given in RFC XXXX.</t>
<t>Token: WMS</t>
<t>Standards track reference: RFC XXXX</t>
</list>IANA is requested to replace "RFC XXXX" with the RFC number of
this document upon publication.</t>
</section>
<section anchor="Security" title="Security Considerations">
<t>An adversary with the ability to modify SDP descriptions has the
ability to switch around tracks between media streams. This is a special
case of the general security consideration that modification of SDP
descriptions needs to be confined to entities trusted by the
application.</t>
<t>If implementing buffering as mentioned in <xref
target="s-nonsignal"/>, the amount of buffering should be limited to
avoid memory exhaustion attacks.</t>
<t>No other attacks have been identified that depend on this
mechanism.</t>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>This note is based on sketches from, among others, Justin Uberti and
Cullen Jennings.</t>
<t>Special thanks to Flemming Andreassen, Miguel Garcia and Paul Kyzivat
for their work in reviewing this draft, with many specific language
suggestions.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
<?rfc include='reference.RFC.3550'?>
<?rfc include='reference.RFC.4566'?>
<?rfc include='reference.RFC.5576'?>
<?rfc include='reference.RFC.5234'?>
<?rfc include='reference.W3C.WD-webrtc-20120209'?>
<?rfc include='reference.I-D.ietf-rtcweb-jsep'?>
</references>
<references title="Informative References">
<?rfc include='reference.RFC.5761'?>
<?rfc include='reference.RFC.5888'?>
<?rfc include='reference.I-D.ietf-mmusic-sdp-bundle-negotiation'?>
<?rfc include='reference.I-D.westerlund-avtcore-multiplex-architecture'?>
<?rfc include='reference.I-D.roach-mmusic-unified-plan'?>
</references>
<section title="Design considerations, rejected alternatives">
<t>This appendix should be deleted before publication as an RFC.</t>
<t>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).</t>
<t>Another suggestion has been to put the msid value within an attribute
of RTCP SR (sender report) packets. This doesn't offer the ability to
know that you have seen all the tracks currently configured for a media
stream.</t>
</section>
<section title="Usage with multiple MediaStreams per M-line">
<t>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.</t>
<t>It reproduces some of the original justification text for this
mechanism that is not relevant when Unified Plan is used.</t>
<section anchor="sect-why-old"
title="Mechanism design with multiple SSRCs">
<t>When media is carried by RTP <xref target="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 <xref target="RFC5761"/> is used).</t>
<t>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.</t>
<t>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 <xref
target="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.</t>
<t>The SSRC grouping mechanism ("a=ssrc-group") <xref
target="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.</t>
<t>The SDP grouping framework <xref target="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.</t>
<t>There are use cases (some of which are discussed in <xref
target="I-D.westerlund-avtcore-multiplex-architecture"/> ) where
neither of these approaches is appropriate; In those cases, a new
mechanism is needed.</t>
<t>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.</t>
</section>
<section title="Usage with the SSRC attribute">
<t>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.</t>
</section>
</section>
<section title="Change log">
<t>This appendix should be deleted before publication as an RFC.</t>
<section title="Changes from alvestrand-rtcweb-msid-00 to -01">
<t>Added track identifier.</t>
<t>Added inclusion-by-reference of
draft-lennox-mmusic-source-selection for track muting.</t>
<t>Some rewording.</t>
</section>
<section title="Changes from alvestrand-rtcweb-msid-01 to -02">
<t>Split document into sections describing a generic grouping
mechanism and sections describing the application of this grouping
mechanism to the WebRTC MediaStream concept.</t>
<t>Removed the mechanism for muting tracks, since this is not central
to the MSID mechanism.</t>
</section>
<section title="Changes from alvestrand-rtcweb-msid-02 to mmusic-msid-00">
<t>Changed the draft name according to the wishes of the MMUSIC group
chairs.</t>
<t>Added text indicting cases where it's appropriate to have the same
appdata for multiple SSRCs.</t>
<t>Minor textual updates.</t>
</section>
<section title="Changes from alvestrand-mmusic-msid-00 to -01">
<t>Increased the amount of explanatory text, much based on a review by
Miguel Garcia.</t>
<t>Removed references to BUNDLE, since that spec is under active
discussion.</t>
<t>Removed distinguished values of the MSID identifier.</t>
<t/>
</section>
<section title="Changes from alvestrand-mmusic-msid-01 to -02">
<t>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".</t>
<t>Changed the syntax for "identifier" and "appdata" to be
"token".</t>
<t>Changed the registry for the "msid-semantic" attribute values to be
a new registry, based on advice given in Atlanta.</t>
</section>
<section title="Changes from alvestrand-mmusic-msid-02 to ietf-mmusic-00">
<t>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.</t>
<t>Changed the "default" mechanism to return independent streams after
considering the synchronization problem.</t>
<t>Removed the space from between "msid-semantic" and its value, to be
consistent with RFC 5576.</t>
</section>
<section title="Changes from mmusic-msid-00 to -01">
<t>Reworked msid mechanism to be a per-m-line attribute, to align with
<xref target="I-D.roach-mmusic-unified-plan"/></t>
</section>
<section title="Changes from mmusic-msid-01 to -02">
<t>Corrected several missed cases where the word "ssrc" was not
changed to "M-line".</t>
<t>Added pointer to unified-plan (which should be moved to point to
-jsep)</t>
<t>Removed suggestion that ssrc-group attributes can be used with
"msid-semantic", it is now only the msid-semantic registry.</t>
</section>
<section title="Changes from mmusic-msid-02 to -03">
<t>Corrected even more cases where the word "ssrc" was not changed to
"M-line".</t>
<t>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.</t>
<t>Removed some now-unnecessary text.</t>
</section>
<section title="Changes from mmusic-msid-03 to -04">
<t>Changed title to reflect focus on WebRTC MediaStreams</t>
<t>Added a section on receiver-side media stream control, using the
"msid-control" attribute.</t>
</section>
<section title="Changes from -04 to -05">
<t>Removed the msid-control section after WG discussion.</t>
<t>Removed some text that seemed only to pertain to resolved
issues.</t>
</section>
<section title="Changes from -05 to -06">
<t>Addressed issues found in Fleming Andreassen's review</t>
<t>Referenced JSEP rather than unified-plan for the M-line mapping
model</t>
<t>Relaxed MSID definition to allow "token-char" in values rather than
a-z 0-9 hyphen; tightened ABNF by adding length description to it.</t>
<t>Deleted discussion of abandoned alternatives, as part of preparing
for publication.</t>
<t>Added a "detailed procedures" section to the WMS semantics
description.</t>
<t>Added IANA registration of the "msid-semantic" attribute.</t>
</section>
</section>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 20:36:04 |