One document matched: draft-ietf-mmusic-msid-04.xml
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<rfc category="std" docName="draft-ietf-mmusic-msid-04" 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></region>
<code>11122</code>
<country>Sweden</country>
</postal>
<email>harald@alvestrand.no</email>
</address>
</author>
<date day="13" month="February" year="2014" />
<abstract>
<t>This document specifies a 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></t>
<section title="Structure Of This Document">
<t>This document adds a new grouping relation between M-lines 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"></xref> gives the background on why a new
mechanism is needed.</t>
<t><xref target="sect-definition"></xref> gives the definition of the
new mechanism.</t>
<t><xref target="sect-media-stream"></xref> gives the application of
the new mechanism for providing necessary semantic information for the
association of MediaStreamTracks to MediaStreams in the WebRTC API
.</t>
</section>
<section anchor="sect-why" title="Why A New Mechanism Is Needed">
<t>When media is carried by RTP <xref target="RFC3550"></xref>, 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"></xref> is used).</t>
<t>SDP gives a description based on m-lines. According to the model
used in <xref target="I-D.roach-mmusic-unified-plan"></xref>, 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"></xref>; 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"></xref> 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"></xref> 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>The WebRTC work has come to agreement (documented in <xref
target="I-D.roach-mmusic-unified-plan"></xref>) that one M-line is
used to describe each MediaStreamTrack, and that the BUNDLE mechanism
<xref target="I-D.ietf-mmusic-sdp-bundle-negotiation"></xref> is used
to group MediaStreamTracks into RTP sessions. Therefore, the need is
to specify the ID of a MediaStreamTrack and its containing MediaStream
for each M-line, which can be accomplished with a media-level
attribute.</t>
<t>This usage is described in <xref
target="sect-media-stream"></xref>.</t>
</section>
</section>
<section anchor="sect-definition" title="The Msid Mechanism">
<t>This document registers a new SDP <xref target="RFC4566"></xref>
media-level "msid" attribute. This new attribute allows endpoints to
associate RTP media streams that are carried in the same or different
m-lines, as well as allowing 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"></xref> grammar:</t>
<t></t>
<figure>
<artwork><![CDATA[
; "attribute" is defined in RFC 4566.
attribute =/ msid-attr
msid-attr = "msid:" identifier [ " " appdata ]
identifier = token
appdata = token
]]></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 chosen from 0-9, a-z,
A-Z and - (hyphen), 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 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 on a single m-line. There may
also be multiple m-lines 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 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:" token identifier-list
identifier-list = (" " identifier)* / " *"
token = <as defined in RFC 4566>
]]></artwork>
</figure>
<t>The semantic field may hold values from the IANA registriy "Semantics
for the msid-semantic SDP attribute" (which is defined by this
memo).</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"></xref>, to be used for
associating m-lines representing MediaStreamTracks within MediaStreams
as defined in <xref target="W3C.WD-webrtc-20120209"></xref>.</t>
<t>The semantic token for this semantic is "WMS" (short for WebRTC Media
Stream).</t>
<t>The value of the msid corresponds to the "id" attribute of a
MediaStream.</t>
<t>The appdata for a WebRTC MediaStreamTrack consists of the "id"
attribute of a MediaStreamTrack.</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 continues to
refer to the same MediaStream. Once negotiation has completed on a
session, there is no memory; an msid 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 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 m-lines with the same
msid value, but different appdata values, the recipient can signal
to its application that new MediaStreamTracks have been added to the
media stream.</t>
<t>When a description is updated to no longer list the msid value on
a specific m-line, the recipient can signal to its application that
the corresponding media stream track has been closed.</t>
<t>When a description is updated to no longer list the msid value on
any m-line, the recipient can signal to its application that the
media stream has been closed.</t>
</list>In addition to signaling that the track is closed when it
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"></xref> section 6.3.4 (BYE packet received) and 6.3.5
(timeout).</t>
<t>The association between SSRCs and m-lines is specified in <xref
target="I-D.roach-mmusic-unified-plan"></xref>.</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>Handling will depend on whether or not any MSIDs are signaled in
the relevant m-line(s). 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 label 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 whether or not a
"default" MediaStream should be created. 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 signaling; 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.</t>
<t>NOTE IN DRAFT: Previous versions of this memo suggested adding all
incoming SSRCs to a single MediaStream. This is problematic because we
do not know if the SSRCs are synchronized or not before we learn the
CNAME of the SSRCs, which only happens when an RTCP packet arrives.
How to identify a non-WMS stream is still open for discussion -
including whether it's necessary to do so. Using the stream label
seems like an easy thing to do for debuggability - it's not signalled,
and is intended for human consumption anyway.</t>
<t>Another alternative is to group the incoming media streams based on
CNAME; this preseerves the synchronization semantics of CNAME, but
means that one cannot signal the MediaStreamTrack before the CNAME of
the SSRC is known (which will happen only on arrival of the relevant
RTCP packet).</t>
</section>
</section>
<section title="WebRTC MediaStream Control">
<t>A need has been identified for signalling from a receiver to a sender
some information about the receiver's desires for the sender to take
action on a MediaStream track.</t>
<t>This section describes such a mechanism. It is intended to be used
for streams that are signalled using the semantics in section <xref
target="sect-media-stream"></xref>.</t>
<t>This mechanism consists of a single new field, "msid-control", which
can have the following values:</t>
<t><list style="symbols">
<t>msid-control: enable - the receiver positively acknowledges that
it wants the content of this media stream track. This has the same
semantics as leaving out the field altogether, but is specified for
completeness.</t>
<t>msid-control: disable - the receiver desires that the sender
stops sending media on this track, but allows for a later round of
negotiation to resume transmission. The sender and receiver are
expected to continue including the corresponding SSRCs in RTCP
reports, keeping the information on the SSRCs from timing out.</t>
<t>msid-control: stop - the receiver desires that the sender stops
sending media on this track, and guarantees that it will never ask
for data to be sent on this track again. The sender is expected to
stop reporting on the corresponding SSRCs, and MAY send a BYE
message when it stops sending.</t>
<t>msid-control: reject - the exact same semantics as for
msid-control: stop apply, but this form is only used if the stream
has never been enabled. The intended use is for support of rejecting
a MediaStream, rather than stopping it (such a function has not been
specified so far).</t>
</list>The msid-control field is significant only for the direction
from the receiver to the sender; if a single m-line is used for
MediaStreams in both directions, only the streams sent by the receiver
of the SDP message will be affected.</t>
<t>If the MediaStream in both directions is cancelled by msid-control:
stop or msid-control: reject, the m-line MAY be disabled by setting its
port number to 0. If there is a MediaStream in use in either direction,
whether it's enabled or disabled, the m-line port number MUST NOT be set
to 0.</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>This document requests IANA to register the "msid" attribute and the
"msid-control" attribute in the "att-field (media level only)" registry
within the SDP parameters registry, according to the procedures of <xref
target="RFC4566"></xref></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>The attribute value contains only ASCII characters, and is
therefore not subject to the charset attribute.</t>
<t>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>The details of appropriate values are given in RFC XXXX.</t>
</list></t>
<t>The required information for "msid-control" is:<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-control</t>
<t>Long-form attribute name: Media stream control</t>
<t>The attribute value contains only ASCII characters, and is
therefore not subject to the charset attribute.</t>
<t>The attribute states the desires of a recipient with regards to a
WebRTC MediaStream in the context of an m-line.</t>
<t>The details of appropriate values 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 section <xref
target="s-nonsignal"></xref>, the amount of buffering should be limited
to avoid memory exhaustion attacks.</t>
<t>No other attacks that are relevant to the browser's security 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 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'?>
</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, open questions and and 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>
<t>There has been a suggestion that this mechanism could be used to mute
tracks too. This is not done at the moment.</t>
<t>Discarding of incoming data when the SDP description isn't updated
yet (section 3) may cause clipping. However, the same issue exists when
crypto keys aren't available. Input sought.</t>
<t>There's been a suggestion that acceptable SSRCs should be signaled in
a response, giving a recipient the ability to say "no" to certain SSRCs.
This is not supported in the current version of this document.</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"></xref>, 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"></xref> 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"></xref>, 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"></xref> 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"></xref> 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"></xref> ) 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></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"></xref></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>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-24 01:20:10 |