One document matched: draft-westerlund-avtext-rtcp-sdes-srcname-01.xml
<?xml version="1.0" encoding="US-ASCII"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<?rfc toc="yes"?>
<?rfc tocompact="yes"?>
<?rfc tocdepth="3"?>
<?rfc tocindent="yes"?>
<?rfc symrefs="yes"?>
<?rfc sortrefs="yes"?>
<?rfc comments="yes"?>
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<?rfc compact="yes"?>
<?rfc subcompact="no"?>
<rfc category="std" docName="draft-westerlund-avtext-rtcp-sdes-srcname-01"
ipr="trust200902">
<front>
<title abbrev="RTCP SDES SRCNAME">RTCP SDES Item SRCNAME to Label
Individual Sources</title>
<author fullname="Magnus Westerlund" initials="M." surname="Westerlund">
<organization>Ericsson</organization>
<address>
<postal>
<street>Farogatan 6</street>
<city>SE-164 80 Kista</city>
<country>Sweden</country>
</postal>
<phone>+46 10 714 82 87</phone>
<email>magnus.westerlund@ericsson.com</email>
</address>
</author>
<author fullname="Bo Burman" initials="B." surname="Burman">
<organization>Ericsson</organization>
<address>
<postal>
<street>Farogatan 6</street>
<city>SE-164 80 Kista</city>
<country>Sweden</country>
</postal>
<phone>+46 10 714 13 11</phone>
<email>bo.burman@ericsson.com</email>
</address>
</author>
<author fullname="Patrik Sandgren" initials="P." surname="Sandgren">
<organization>Ericsson</organization>
<address>
<postal>
<street>Farogatan 6</street>
<city>SE-164 80 Kista</city>
<country>Sweden</country>
</postal>
<phone>+46 10 717 97 41</phone>
<email>patrik.sandgren@ericsson.com</email>
</address>
</author>
<date day="16" month="July" year="2012"/>
<abstract>
<t>This document defines a new SDES item called SRCNAME which uniquely
identifies a single media source, like a camera or a microphone. That
way anyone receiving the SDES information from a set of interlinked RTP
sessions can determine which SSRCs are related to the same source. It
can equally be used to label SSRC multiplexed related streams, such as
FEC or Retransmission streams related to the original source stream in
the same session. In addition the new SDES item is also defined for
usage with the SDP source specific media attribute ("a=ssrc") enabling
an end-point to declare and learn the source bindings ahead of receiving
RTP/RTCP packets through signalling.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>RTP has always been a protocol that supports multiple participants,
each sending their own media streams in RTP sessions. Previously, many
implementations have aimed only at point to point voice over IP with a
single source in each end-point. Even client implementations aimed at
video conferences have often been built with the assumption around
central mixers that only deliver a single media stream per media type.
However, more advanced client implementations may transmit multiple
streams in the same RTP session and there may be tight relations between
different streams and their SSRCs. For example, a client with several
cameras that uses simulcast to send streams with different encodings of
the video from each camera have the need of conveying the relation of
the streams to the receiver. A similar example is a client with several
cameras that uses <xref target="RFC6190">SVC multi-session
transmission</xref> and also here the receiver needs to know which
streams relate to which video source. Other examples of tight RTP
relations are a retransmission stream and its original stream, and cases
of forward error correction (FEC), where a client needs to associate a
number of source streams with, in general, a different number of repair
streams.</t>
</section>
<section 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>
</section>
<section title="Problem Description">
<t>In a scenario where an endpoint needs to send several RTP media
streams, in a single RTP session or spread across several RTP sessions,
and where two or more of those streams are somehow related, that
relation information is today not always possible to convey in a timely
manner to entities (endpoints and middle nodes) that need it.</t>
<t>An RTP Mixer, on the other hand, must have all the SDP information
available and can provide it to any number of participants, since there
must be a mapping from the original sources to the Mixer's own streams,
which is in turn distributed to all other participants. That is also
true for a source projecting mixer, since there is a projection
algorithm that must be made to work. It is even likely that the Mixer is
allowed to provide the stream relation and impose that onto all of the
clients, rather than trying to map a wide variety of different relations
onto what it provides.</t>
<t>A single relation between two or more streams means that each stream
has a certain "role" in that specific relation. A "role" is related to a
specific reason to group a set of streams. The number of different
grouping tags defined in various RFC for use with the <xref
target="RFC5888">SDP group attribute</xref>, as well as the <xref
target="RFC5583">media decoding dependency attribute</xref> can be used
as an indication of the different roles that may need to be
described.</t>
<t>Those stream relational roles are typically application-specific, can
sometimes be complex, and a single stream can even take on several
roles. The major difference between roles is that they commonly do not
share the same hierarchy root node and sometimes also middle nodes
differ between roles. All roles however use the same hierarchy leaves,
being the RTP media streams, but different roles may want to name leaves
differently. It should be possible to express such relation structure
and allow a single stream to hold several roles. It is believed to be
sufficient if a single stream role can be described as being part of a
relation hierarchy.</t>
</section>
<section title="Motivation">
<t>This section contains a brief description of existing techniques that
conceivably could be used to provide information on RTP stream
relations, and a motivation why those are not always sufficient.</t>
<section title="RTP SSRC">
<t>To rely on using the same RTP Synchronization SouRCe (SSRC) for all
streams related to a particular media source is many times not
possible when the related streams are part of the same RTP session,
since the SSRC itself is the identifier to tell the streams apart.
This method is not robust against SSRC collision and potentially
forces cascading SSRC changes between sessions. It does also not
provide any information in how the streams are related.</t>
</section>
<section title="RTCP SDES CNAME">
<t>CNAME is not sufficient to express the necessary type of relation,
although that is commonly inferred from end-points that have only one
media stream per media type. The primary use of CNAME in multi-source
usages is instead to indicate which end-point and what synchronization
context a particular media stream relates to, and that usually means
that all streams sent from a client have the same CNAME.</t>
</section>
<section title="SDP">
<t>A common solution is to use SDP attributes to convey the relation
between streams. Session-multiplexed streams can be associated with an
attribute that <xref target="RFC5888">groups different RTP
sessions</xref>, and SSRC-multiplexed streams can be <xref
target="RFC5576">grouped at the media level for each RTP
session</xref>. For example, <xref target="RFC5956">Forward Error
Correction Grouping Semantics in the Session Description
Protocol</xref> uses that media level grouping with the "FEC-FR" tag
to group FEC associations when the different streams from a source are
SSRC-multiplexed in the same RTP session.</t>
<t>Using SDP attributes may work fine in the case when the receivers
of the streams also get an SDP describing the bindings of all the
streams, but that is not always the case. One such example is a highly
dynamic conference session where a large amount of clients are
communicating with each other via an RTP Translator. The RTP
Translator forwards all RTP and RTCP traffic from a client to all
other clients and the clients can be prepared to receive any number of
streams of certain specified media. When a new client joins the
session, the other clients may not be notified via explicit signalling
before starting to receive media streams from this new client. Such
notification could for example be made through a SIP Update with a new
SDP containing an explicit list of the new streams, but there are also
other possibilities. The clients will instead detect the new client's
streams directly via RTP and RTCP. Similar situations typically arise
in multicast scenarios. In those cases, there is no way for a client
or middle node to identify if and how certain streams are related to
each other, since that information was only included in the SDP, if at
all.</t>
</section>
<section title="Implicit Methods">
<t><xref target="RFC4588">RTP Retransmission Payload Format</xref>
describes a solution for finding the association between original
streams and retransmission streams when SSRC-multiplexing is used. The
association can be resolved when the receiver receives a
retransmission packet matching a retransmission request sent earlier.
However, the RFC continues with describing that this mechanism might
fail if there are two outstanding requests for the same packet
sequence number in two different original streams of a session.
Therefore, to avoid ambiguity in unicast a receiver MUST NOT have two
outstanding requests for the same packet sequence number in two
different original streams before the association is resolved. For
multicast, however, this ambiguity cannot be avoided and
SSRC-multiplexing of original and retransmission streams is therefore
prohibited in multicast. By defining a solution for one to one mapping
between an original stream and any supporting streams, this issue can
be avoided in the future. <list style="empty">
<t>Note: This document does not update RFC 4588 to use this
solution, but it may be done in the future.</t>
</list></t>
</section>
</section>
<section title="Proposed Solution">
<t>To enable an RTP session participant to determine the close relation
of different streams without the above mentioned problems, a new method
for identifying such sources is needed. This identification is called
Source Name, or SRCNAME and is a unique identifier identifying a single
media source, like a camera, a microphone, a particular media mix, or
conceptual stream.</t>
<section title="SRCNAME Contents">
<t>The basic idea is that streams with matching SRCNAME are related,
similar to the idea with RTCP SDES CNAME.</t>
<t>It is assumed that related streams will share the same
synchronization context, meaning that the SRCNAME is scoped by CNAME
and need not duplicate any CNAME information.</t>
<t>The SRCNAME format includes "." (%x2E) as a hierarchy separator,
allowing a stream to relate to another stream at a certain hierarchy
level. Each hierarchy level is then a node in a hierarchy tree. For
example, assume a video stream being provided in two different
resolutions, "lowres" and "hires", each being protected by a Forward
Error Correction stream, with another additive FEC stream covering
both resolutions. The low resolution video could have a SRCNAME being
"program1.video.lowres", and its FEC stream
"program1.video.lowres.fec". The SRCNAME for the additive FEC stream,
covering both resolutions and their per-stream FEC, could be
"program1.video.fec". Building on the same example, the high fidelity
audio stream belonging to the above video could be
"program1.audio.hifi".</t>
<t>Note that the hierarchy structure can be chosen entirely by the
media sender, but it is anyway possible to decide stream relations, at
what level the streams relate, and which other streams that are
included in the relation at that level by matching SRCNAME
hierarchically left-to-right between "." hierarchy separators. The
specific type of relation is not encoded into SRCNAME in any mandated
way, but need to be stringently described by other means, for example
SDP, and is out of scope for this specification. SRCNAME needs only
express that streams are related, not exactly how the related streams
should be processed together.</t>
<t>Note that SRCNAME need not be particularly human-readable as long
as each node in the hierarchy has a tag that is unique for that CNAME
context, which makes it possible to limit the SRCNAME size.</t>
</section>
<section title="SRCNAME in SDES">
<t><xref target="RFC3550">RTP</xref> defines the Source Description
RTCP Packet (SDES), which contains one or more chunks, each of which
is composed of SDES items describing the SSRC identified in that
chunk. None of the present SDES items is, however, suitable for
uniquely identifying a media source.</t>
<t>Therefore, we propose to define a new SDES item called the SRCNAME,
which uses a unique label to identify a single media source, like a
camera or a microphone. The source may also be a particular media mix
or conceptual stream, such as the "most active speaker" output by a
RTP mixer performing stream switching. That way, anyone receiving the
SDES information from a set of interlinked RTP sessions or multiple
SSRCs in the same session can determine which SSRCs are the same
source. Connecting streams with SRCNAME can be done irrespective of
which multiplexing type is used and it solves the problems with the
current solutions described above.</t>
</section>
<section title="SRCNAME in SDP">
<t>It is, however, possible that a receiver will receive the RTP
streams before receiving SDES packets with all SRCNAME items and that
would mean that the receiver cannot make the connections between SSRCs
and SRCNAMEs when starting to receive the media. <xref
target="RFC5576">"Source-Specific Media Attributes in the Session
Description Protocol (SDP)"</xref> defines a way of declaring
different attributes for SSRCs in each session in SDP, and if a new
source attribute is added to this framework, it would be suitable for
conveying the connections between SSRCs and SRCNAMEs before the media
communication starts. Thus, in addition to the new SDES item we also
define a new SDP source-specific media attribute called srcname, which
enables an end-point to declare and learn the source bindings ahead of
receiving RTP/RTCP packets. Of course, this new SDP source attribute
will not be useful for the case described above when clients did not
get updates with new client's stream bindings, but it will be useful
in most other cases.</t>
</section>
<section title="SRCNAME in RTP Header Extension">
<t>There is a risk that neither RTCP SDES nor SDP attributes are
timely enough in cases where RTP streams are received before the SDES
has arrived, in which case an <xref target="RFC5285">RTP header
extension</xref> could be used, containing a combination of CNAME and
SRCNAME information. This type of rapid information synchronization
through RTP header extension is similar to what is described in <xref
target="RFC6051"/>. The RTP header extension need not be present in
every RTP packet, for example only in the beginning of the stream, at
key points, or periodically, according to the application's needs and
as chosen by the media sender.</t>
</section>
</section>
<section title="SRCNAME Format">
<t>The SRCNAME MUST fulfill the requirements Section 6.5 in <xref
target="RFC3550">RTP</xref> puts on SDES item values in general. These
requirements is that it is a <xref target="RFC3629">UTF-8</xref> string
that have a maximum length of 255 bytes.</t>
<t>In addition, there are format restrictions to accommodate the
relation hierarchy and multiple roles, as described by the following
<xref target="RFC5234">ABNF</xref>:</t>
<figure anchor="fig-format-abnf" title="SRCNAME Format ABNF">
<artwork><![CDATA[
srcname-node = 1*(%x01-09 / %x0B-0C / %x0E-2D / %x2F-FF)
; Same as RFC 4566 "byte-string"
; except for the hierarchy separator
srcname-content = srcname-node *(%x2E srcname-node)
]]></artwork>
</figure>
<t>It is RECOMMENDED to use per communication session unique random
identifiers, applying srcname-node restrictions, as srcname-node. The
length of such srcname-node identifiers MAY be limited down to a single
character, especially when the resulting SRCNAME has several nodes.</t>
</section>
<section anchor="sec-2" title="SDES Item SRCNAME">
<t>Source Descriptions are a method that should work with all RTP
topologies (assuming that any intermediary node is supporting this item)
and existing RTP extensions. We propose to define a new SDES item called
SRCNAME. That way, anyone receiving the SDES information from a set of
interlinked RTP sessions or SSRCs in a single session can determine
which SSRCs are related to the same source.</t>
<t>This SRCNAME's relation to CNAME is the following. CNAME represents
an end-point and a synchronization context. If the different sources
identified by SRCNAMEs should be played out synchronized when receiving
them in a multi-stream context, then the sources need to be in the same
synchronization context. Thus in all cases, all SSRCs with the same
SRCNAME will have the same CNAME. A given CNAME may contain multiple
sets of sources using different SRCNAMEs.</t>
<t>The SDES SRCNAME item follows the same format as the other SDES items
defined in <xref target="RFC3550">RTP</xref>:</t>
<figure anchor="fig-sdes-format" title="SDES SRCNAME Format">
<artwork><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRCNAME=TBA1 | length | source name ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
<!--TBA1 will replaced with a number assigned by IANA. -->
</figure>
<t>The source name field MUST follow the above srcname-content
definition. Multiple SDES SRCNAME describing different relation roles
MAY be included.</t>
<t>When using the SRCNAME SDES item, it is equally important as CNAME.
Thus SRCNAME is RECOMMENDED to be included in all full compound RTCP
packets being sent. It MAY also be included in non-compound packets in
cases where the implementation believes that there might be new
receivers needing the information.</t>
</section>
<section anchor="sec-3" title="SRCNAME in SDP">
<t><xref target="RFC5576">"Source-Specific Media Attributes in the
Session Description Protocol (SDP)"</xref> defines a way of declaring
attributes for SSRC in each session in SDP. With a new SDES item, it is
possible to use this framework to define how SRCNAME can also be
provided in the SDP for each SSRC in each RTP session, thus enabling an
end-point to declare and learn the source bindings ahead of receiving
RTP/RTCP packets.</t>
<t>Hence, we propose a new SDP source attribute called srcname with the
following structure:</t>
<figure>
<artwork><![CDATA[
a=ssrc:<ssrc-id> srcname:<srcname>
]]></artwork>
</figure>
<t>The srcname value MUST be identical to the SRCNAME value the media
sender will send in the SDES SRCNAME item in the SDES RTCP packets.
Multiple srcname attributes MAY be used to describe multiple relation
roles.</t>
<t>Formal<xref target="RFC5234"> ABNF syntax</xref> for the "srcname"
attribute:</t>
<figure anchor="fig-attribute-abnf" title="SRCNAME Attribute ABNF">
<artwork><![CDATA[
srcname-attr = "srcname:" srcname
srcname = srcname-content
attribute =/ srcname-attr
; The definition of "attribute" is in RFC 4566.
]]></artwork>
</figure>
<t/>
</section>
<section title="SRCNAME as RTP Header Extension">
<t>The <xref target="RFC5285">RTP Header Extension</xref> MUST contain
both CNAME and SRCNAME information, since SRCNAME is scoped by
CNAME.<list style="empty">
<t>Editor's note: To be amended with more explicit information.</t>
</list></t>
</section>
<section anchor="sec-4" title="Examples">
<t>This section shows SDP examples of declaring the SRCNAME in SDP.</t>
<section title="Simulcast">
<t>In this use case the end-point is a client with a single audio
source and two video sources, and it uses simulcast for sending
different encodings of the same video source. This example is based on
<xref target="I-D.westerlund-avtcore-rtp-simulcast">Using Simulcast in
RTP sessions</xref>. The following SDP describes this.</t>
<figure>
<artwork><![CDATA[
v=0
o=alice 3203093520 3203093520 IN IP4 foo.example.com
s=Simulcast enabled client
t=0 0
c=IN IP4 foo.example.com
m=audio 49200 RTP/AVP 96
a=rtpmap:96 G719/48000/2
a=ssrc:521923924 cname:alice@foo.example.com
a=ssrc:521923924 srcname:a1
a=mid:1
m=video 49300 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=42c01e
a=imageattr:96 send [x=640,y=360] recv [x=640,y=360] [x=320,y=180]
a=ssrc:192392452 cname:alice@foo.example.com
a=ssrc:192392452 srcname:v1
a=ssrc:834753488 cname:alice@foo.example.com
a=ssrc:834753488 srcname:v2
a=mid:2
a=content:main
m=video 49400 RTP/AVP 97
a=rtpmap:97 H264/90000
a=fmtp:97 profile-level-id=42c00d
a=imageattr:97 send [x=320,y=180]
a=ssrc:239245219 cname:alice@foo.example.com
a=ssrc:239245219 srcname:v1
a=ssrc:734623563 cname:alice@foo.example.com
a=ssrc:734623563 srcname:v2
a=mid:3
a=sendonly
]]></artwork>
</figure>
<t>The audio session is proposing to use one stereo stream of G.719
and the video sessions are proposing to send two different encodings
of each video source, one with the resolution 640x360 and one with
320x180. The end-point also declares the SSRCs it intends to use with
bindings to CNAME and SRCNAME, enabling the receiver of the SDP to
bind together the video streams that originate from the same video
camera. For example, the two streams having an SRCNAME of "v1"
originate from the same video camera and belong together.</t>
<t>The use of the srcname attribute in the SDP is optional and the
information can be retrieved from RTCP reporting, but it will then not
be possible to correctly relate the video sources until the first RTCP
report is received.</t>
</section>
<section title="SVC with multi-session transmission">
<t>Here an example is shown of a client that uses SVC with
multi-session transmission as described in <xref target="RFC6190">RTP
Payload Format for Scalable Video Coding</xref>. <xref
target="RFC6190">RTP Payload Format for Scalable Video Coding</xref>
only describes examples for a client with one video source and the
decoder dependencies of the different sessions are grouped using the
Session grouping DDP attribute as defined in <xref
target="RFC5583">Signaling Media Decoding Dependency in the Session
Description Protocol (SDP)</xref> and implicitly CNAME.</t>
<t>However, if a client has two video sources and wishes to use
multi-session transmission and send streams from both sources in each
session, an additional grouping mechanism is needed to group the
different streams in the different sessions. SRCNAME is suitable for
this and here we show an example where the DDP attribute groups the
different sessions and the SRCNAME is used to relate the different
SSRCs in each RTP session to one of the two video sources.</t>
<figure>
<artwork><![CDATA[
v=0
o=bob 8473948250 8473948250 IN IP4 foo.example.com
s=SVC MST client
t=0 0
c=IN IP4 foo.example.com
a=group:DDP L1 L2 L3
m=audio 49500 RTP/AVP 96
a=rtpmap:96 G719/48000/2
a=ssrc:293848928 cname:bob@foo.example.com
a=mid:A1
m=video 20000 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 profile-level-id=4de00a; packetization-mode=1;
mst-mode=NI-TC; sprop-parameter-sets={sps0},{pps0};
a=ssrc:743947584 cname:bob@foo.example.com
a=ssrc:743947584 srcname:V1.L1
a=ssrc:283894947 cname:bob@foo.example.com
a=ssrc:283894947 srcname:V2.L1
a=mid:L1
m=video 20002 RTP/AVP 97
a=rtpmap:97 H264-SVC/90000
a=fmtp:97 profile-level-id=53000c; packetization-mode=1;
mst-mode=NI-T; sprop-parameter-sets={sps1},{pps1};
a=ssrc:492784823 cname:bob@foo.example.com
a=ssrc:492784823 srcname:V1.L2
a=ssrc:892362397 cname:bob@foo.example.com
a=ssrc:892362397 srcname:V2.L2
a=mid:L2
a=depend:97 lay L1:96
m=video 20004 RTP/AVP 98
a=rtpmap:98 H264-SVC/90000
a=fmtp:98 profile-level-id=53001F; packetization-mode=1;
mst-mode=NI-T; sprop-parameter-sets={sps2},{pps2};
a=ssrc:184562894 cname:bob@foo.example.com
a=ssrc:184562894 srcname:V1.L3
a=ssrc:305605682 cname:bob@foo.example.com
a=ssrc:305605682 srcname:V2.L3
a=mid:L3
a=depend:98 lay L1:96 L2:97
]]></artwork>
</figure>
<t>Thus, the client declares that it will send two video streams in
each RTP session and the receiver is then able to relate the streams
in the different sessions by using the SRCNAME binding, with matching
(first parts of the) SRCNAME value. Without the SRCNAME binding it
would not be possible for the receiver to know which streams belong to
the same source. Note that the audio stream does not have an explicit
srcname attribute in this example, but only relate to the video
streams through the same CNAME. Note that the last part of the
SRCNAMEs in the example, ".L1", ".L2" and ".L3" are not necessary but
allowed and will not impact the ability to tell that the streams
belong together, since related streams have the first part in
common.</t>
</section>
<section title="Retransmission">
<t>This use case shows how SRCNAME can be used to connect
retransmission streams to the original streams in the case of SSRC
multiplexed <xref target="RFC4588">RTP retransmission</xref>. This is
included to exemplify how RTP retransmission could be updated to
provide explicit bindings between the source and the repair stream,
but just an example and not a specification.</t>
<figure>
<artwork><![CDATA[
v=0
o=carol 3462534872 3462534872 IN IP4 foo.example.com
s=SSRC-multiplexed retransmission client
t=0 0
c=IN IP4 foo.example.com
m=audio 49800 RTP/AVP 96
a=rtpmap:96 G719/48000/2
a=ssrc:8372496978 cname:carol@foo.example.com
a=mid:1
m=video 49300 RTP/AVP 96 97
a=rtpmap:96 H264/90000
a=rtcp-fb:96 nack
a=fmtp:96 profile-level-id=42c01e
a=rtpmap:97 rtx/90000
a=fmtp:97 apt=96;rtx-time=200
a=ssrc:192392452 cname:carol@foo.example.com
a=ssrc:192392452 srcname:v1.o
a=ssrc:834753488 cname:carol@foo.example.com
a=ssrc:834753488 srcname:v1.r
a=ssrc:682394013 cname:carol@foo.example.com
a=ssrc:682394013 srcname:v2.o
a=ssrc:284576129 cname:carol@foo.example.com
a=ssrc:284576129 srcname:v2.r
a=mid:2
]]></artwork>
</figure>
<t>The client proposes to send two original video streams in the video
session and a retransmission stream for each one of them. The
retransmission streams are associated with the respective original
stream by using matching SRCNAME and a receiver would then know which
original stream a certain retransmission stream is associated with.
This solves the ambiguity problem when SSRC-multiplexing is used for
retransmission and it enables SSRC-multiplexing of original and
retransmission streams to be used also in multicast sessions. Note
that ".o" and ".r" parts of SRCNAME are not needed, but may improve
understanding of the example and will not affect the ability to match
related streams.</t>
</section>
<section title="Forward Error Correction">
<t><xref target="RFC5956">Forward Error Correction Grouping Semantics
in the Session Description Protocol</xref> defines two SDP attributes
for grouping the associated source and FEC-based repair streams. One
can be used for grouping different RTP sessions and the other can be
used for grouping SSRCs in the same RTP session, i.e. when session-
respective SSRC-multiplexing is used. However, it may be advantageous
to SSRC-multiplex the source streams in one RTP session and the repair
streams in another since that gives a receiver the possibility to
reject the repair session in case it does not support the proposed
FEC. In this case, the above mentioned grouping attributes cannot be
used to associate the repair streams with the respective source stream
since grouping of SSRCs cannot be made across RTP sessions. The
following example shows how SRCNAME can be used for that.</t>
<figure>
<artwork><![CDATA[
v=0
o=dave 7352395826 7352395826 IN IP4 foo.example.com
s=FEC client
t=0 0
c=IN IP4 foo.example.com
a=group:FEC-FR 2 3
m=audio 49300 RTP/AVP 96
a=rtpmap:96 G719/48000/2
a=ssrc:237847298 cname:dave@foo.example.com
a=mid:1
m=video 49200 RTP/AVP 100
a=rtpmap:100 MP2T/90000
a=ssrc:847612849 cname:dave@foo.example.com
a=ssrc:847612849 srcname:v1.o
a=ssrc:558237845 cname:dave@foo.example.com
a=ssrc:558237845 srcname:v2.o
a=mid:2
m=application 49300 RTP/AVP 101
a=rtpmap:101 1d-interleaved-parityfec/90000
a=fmtp:101 L=5; D=10; repair-window=200000
a=ssrc:389572053 cname:dave@foo.example.com
a=ssrc:389572053 srcname:v1.r
a=ssrc:185729479 cname:dave@foo.example.com
a=ssrc:185729479 srcname:v2.r
a=mid:3
]]></artwork>
</figure>
<t>In this example the client proposes to send two video streams in
one session and two repair streams in the other session. The repair
streams are associated with the respective video stream by using a
matching SRCNAME. When receiving either this SDP or the SDES SRCNAME
packets, a receiver can make the connection between the source streams
and the repair streams. Even a client not receiving the SDP will be
able to do the association, if it has established one RTP session for
receiving source streams and another for receiving repair streams.
Note that ".o" and ".r" parts of SRCNAME are not needed, but may
improve understanding of the example and will not affect the ability
to match related streams.</t>
</section>
</section>
<section title="Usage with the Offer/Answer Model">
<t>The SDP offer/answer procedures for the a=ssrc is specified in <xref
target="RFC5576">Source-Specific Media Attributes in the Session
Description Protocol (SDP)</xref>.</t>
</section>
<section title="Backward Compatibility">
<t>Clients not supporting SRCNAME will not have the possibility to bind
different streams to a specific media source, since they will not
understand the SRCNAME SDES item. However, sending SRCNAME SDES items to
a client not supporting it should not impose any problems since all
clients should be prepared that new SDES items may be specified
according to <xref target="RFC3550">RTP</xref>.</t>
<t>According to the definition of SDP attributes in <xref
target="RFC4566">SDP: Session Description Protocol</xref>, if an
attribute is received that is not understood, it MUST be ignored by the
receiver. So a receiver not supporting the ssrc attribute will simply
ignore it.</t>
<t><xref target="RFC5576">Source-Specific Media Attributes in the
Session Description Protocol (SDP)</xref> defines rules of how new
source attributes should be registered, which means that a receiver
supporting RFC 5576 should be prepared that new source attributes may be
defined. This means that a user supporting some of the source attributes
should not have any problems when the user receives an SDP with unknown
source attributes.</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>Following the guidelines in <xref target="RFC4566">SDP</xref>, in
<xref target="RFC5888">The Session Description Protocol (SDP) Grouping
Framework</xref>, and in <xref target="RFC3550">RTP</xref>, the IANA is
requested to register:</t>
<t><list style="numbers">
<t>A new SDES item named SRCNAME, as defined in <xref
target="sec-2"/>. This item needs to be assigned an identifier
TBA1.</t>
<t>A new SDP source attribute named srcname, as defined in <xref
target="sec-3"/>.</t>
</list></t>
</section>
<section anchor="Security" title="Security Considerations">
<t>The SDES SRCNAMEs being close to opaque identifiers could potentially
carry additional meanings or function as overt channel. If the SRCNAME
would be permanent between sessions, they have the potential for
compromising the users’ privacy as they can be tracked between
sessions. See <xref target="RFC6222">Guidelines for Choosing RTP Control
Protocol (RTCP) Canonical Names (CNAMEs)</xref> for more discussion.</t>
<t>A third party modification of the srcname labels either in the RTCP
SDES items or in the SDP a=ssrc attribute can cause service disruption.
By modifying labels the wrong streams could be associated, with
potentially serious effects including media disruptions. If streams that
are to be associated aren't associated, then another type of failures
occur. To prevent modification, insertion or deletion of the srcname
labels, the carrying channel needs to be protected by integrity
protection and source authentication. For RTCP various solutions exist,
such as <xref target="RFC3711">SRTP</xref>, <xref
target="RFC6347">DTLS</xref>, or <xref target="RFC4301">IPsec</xref>.
For protecting the SDP, the signalling channel needs to provide
protection. For <xref target="RFC3261">SIP S/MIME</xref> are the ideal,
and hop by hop<xref target="RFC5246"> TLS</xref> provides at least some
protection, although not perfect. For SDPs retrieved using <xref
target="RFC2326">RTSP DESCRIBE</xref>, TLS would be the RECOMMENDED
solution.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
<?rfc include='reference.RFC.3629'?>
<?rfc include='reference.RFC.3550'?>
<?rfc include='reference.RFC.5234'?>
<?rfc include='reference.RFC.5576'?>
<?rfc include='reference.RFC.6222'?>
</references>
<references title="Informative References">
<?rfc include='reference.I-D.westerlund-avtcore-rtp-simulcast'?>
<?rfc include='reference.RFC.4566'?>
<?rfc include='reference.RFC.4301'?>
<?rfc include='reference.RFC.2326'?>
<?rfc include='reference.RFC.3261'?>
<?rfc include='reference.RFC.3711'?>
<?rfc include='reference.RFC.4588'?>
<?rfc include='reference.RFC.5246'?>
<?rfc include='reference.RFC.5285'?>
<?rfc include='reference.RFC.5583'?>
<?rfc include='reference.RFC.5888'?>
<?rfc include='reference.RFC.5956'?>
<?rfc include='reference.RFC.6051'?>
<?rfc include='reference.RFC.6190'?>
<?rfc include='reference.RFC.6347'?>
</references>
</back>
</rfc>
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