Network Working Group M. Westerlund
Internet-Draft B. Burman
Intended status: Standards Track P. Sandgren
Expires: January 17, 2013 Ericsson
July 16, 2012
RTCP SDES Item SRCNAME to Label Individual Sources
draft-westerlund-avtext-rtcp-sdes-srcname-01
Abstract
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.
Status of this Memo
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This Internet-Draft will expire on January 17, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Problem Description . . . . . . . . . . . . . . . . . . . . . 3
4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. RTP SSRC . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. RTCP SDES CNAME . . . . . . . . . . . . . . . . . . . . . 4
4.3. SDP . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.4. Implicit Methods . . . . . . . . . . . . . . . . . . . . . 5
5. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . 6
5.1. SRCNAME Contents . . . . . . . . . . . . . . . . . . . . . 6
5.2. SRCNAME in SDES . . . . . . . . . . . . . . . . . . . . . 7
5.3. SRCNAME in SDP . . . . . . . . . . . . . . . . . . . . . . 7
5.4. SRCNAME in RTP Header Extension . . . . . . . . . . . . . 7
6. SRCNAME Format . . . . . . . . . . . . . . . . . . . . . . . . 8
7. SDES Item SRCNAME . . . . . . . . . . . . . . . . . . . . . . 8
8. SRCNAME in SDP . . . . . . . . . . . . . . . . . . . . . . . . 9
9. SRCNAME as RTP Header Extension . . . . . . . . . . . . . . . 10
10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Simulcast . . . . . . . . . . . . . . . . . . . . . . . . 10
10.2. SVC with multi-session transmission . . . . . . . . . . . 12
10.3. Retransmission . . . . . . . . . . . . . . . . . . . . . . 14
10.4. Forward Error Correction . . . . . . . . . . . . . . . . . 15
11. Usage with the Offer/Answer Model . . . . . . . . . . . . . . 16
12. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 16
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
14. Security Considerations . . . . . . . . . . . . . . . . . . . 17
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
15.1. Normative References . . . . . . . . . . . . . . . . . . . 17
15.2. Informative References . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
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 SVC multi-
session transmission [RFC6190] 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.
2. 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].
3. Problem Description
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.
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.
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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 SDP
group attribute [RFC5888], as well as the media decoding dependency
attribute [RFC5583] can be used as an indication of the different
roles that may need to be described.
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.
4. Motivation
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.
4.1. RTP SSRC
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.
4.2. RTCP SDES CNAME
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.
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4.3. SDP
A common solution is to use SDP attributes to convey the relation
between streams. Session-multiplexed streams can be associated with
an attribute that groups different RTP sessions [RFC5888], and SSRC-
multiplexed streams can be grouped at the media level for each RTP
session [RFC5576]. For example, Forward Error Correction Grouping
Semantics in the Session Description Protocol [RFC5956] 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.
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.
4.4. Implicit Methods
RTP Retransmission Payload Format [RFC4588] 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
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in the future.
Note: This document does not update RFC 4588 to use this solution,
but it may be done in the future.
5. Proposed Solution
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.
5.1. SRCNAME Contents
The basic idea is that streams with matching SRCNAME are related,
similar to the idea with RTCP SDES CNAME.
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.
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".
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.
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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.
5.2. SRCNAME in SDES
RTP [RFC3550] 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.
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.
5.3. SRCNAME in SDP
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. "Source-Specific Media
Attributes in the Session Description Protocol (SDP)" [RFC5576]
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.
5.4. SRCNAME in RTP Header Extension
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 RTP header extension [RFC5285] could be
used, containing a combination of CNAME and SRCNAME information.
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This type of rapid information synchronization through RTP header
extension is similar to what is described in [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.
6. SRCNAME Format
The SRCNAME MUST fulfill the requirements Section 6.5 in RTP
[RFC3550] puts on SDES item values in general. These requirements is
that it is a UTF-8 [RFC3629] string that have a maximum length of 255
bytes.
In addition, there are format restrictions to accommodate the
relation hierarchy and multiple roles, as described by the following
ABNF [RFC5234]:
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)
Figure 1: SRCNAME Format ABNF
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.
7. SDES Item SRCNAME
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.
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
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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.
The SDES SRCNAME item follows the same format as the other SDES items
defined in RTP [RFC3550]:
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 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: SDES SRCNAME Format
The source name field MUST follow the above srcname-content
definition. Multiple SDES SRCNAME describing different relation
roles MAY be included.
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.
8. SRCNAME in SDP
"Source-Specific Media Attributes in the Session Description Protocol
(SDP)" [RFC5576] 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.
Hence, we propose a new SDP source attribute called srcname with the
following structure:
a=ssrc:<ssrc-id> srcname:<srcname>
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.
FormalABNF syntax [RFC5234] for the "srcname" attribute:
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srcname-attr = "srcname:" srcname
srcname = srcname-content
attribute =/ srcname-attr
; The definition of "attribute" is in RFC 4566.
Figure 3: SRCNAME Attribute ABNF
9. SRCNAME as RTP Header Extension
The RTP Header Extension [RFC5285] MUST contain both CNAME and
SRCNAME information, since SRCNAME is scoped by CNAME.
Editor's note: To be amended with more explicit information.
10. Examples
This section shows SDP examples of declaring the SRCNAME in SDP.
10.1. Simulcast
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 Using
Simulcast in RTP sessions [I-D.westerlund-avtcore-rtp-simulcast].
The following SDP describes this.
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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
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.
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.
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10.2. SVC with multi-session transmission
Here an example is shown of a client that uses SVC with multi-session
transmission as described in RTP Payload Format for Scalable Video
Coding [RFC6190]. RTP Payload Format for Scalable Video Coding
[RFC6190] 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 Signaling
Media Decoding Dependency in the Session Description Protocol (SDP)
[RFC5583] and implicitly CNAME.
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.
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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
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
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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.
10.3. Retransmission
This use case shows how SRCNAME can be used to connect retransmission
streams to the original streams in the case of SSRC multiplexed RTP
retransmission [RFC4588]. 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.
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
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
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understanding of the example and will not affect the ability to match
related streams.
10.4. Forward Error Correction
Forward Error Correction Grouping Semantics in the Session
Description Protocol [RFC5956] 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.
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
In this example the client proposes to send two video streams in one
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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.
11. Usage with the Offer/Answer Model
The SDP offer/answer procedures for the a=ssrc is specified in
Source-Specific Media Attributes in the Session Description Protocol
(SDP) [RFC5576].
12. Backward Compatibility
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 RTP [RFC3550].
According to the definition of SDP attributes in SDP: Session
Description Protocol [RFC4566], 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.
Source-Specific Media Attributes in the Session Description Protocol
(SDP) [RFC5576] 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.
13. IANA Considerations
Following the guidelines in SDP [RFC4566], in The Session Description
Protocol (SDP) Grouping Framework [RFC5888], and in RTP [RFC3550],
the IANA is requested to register:
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1. A new SDES item named SRCNAME, as defined in Section 7. This
item needs to be assigned an identifier TBA1.
2. A new SDP source attribute named srcname, as defined in
Section 8.
14. Security Considerations
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 Guidelines for Choosing RTP Control Protocol (RTCP)
Canonical Names (CNAMEs) [RFC6222] for more discussion.
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 SRTP
[RFC3711], DTLS [RFC6347], or IPsec [RFC4301]. For protecting the
SDP, the signalling channel needs to provide protection. For SIP
S/MIME [RFC3261] are the ideal, and hop by hopTLS [RFC5246] provides
at least some protection, although not perfect. For SDPs retrieved
using RTSP DESCRIBE [RFC2326], TLS would be the RECOMMENDED solution.
15. References
15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
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[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
[RFC6222] Begen, A., Perkins, C., and D. Wing, "Guidelines for
Choosing RTP Control Protocol (RTCP) Canonical Names
(CNAMEs)", RFC 6222, April 2011.
15.2. Informative References
[I-D.westerlund-avtcore-rtp-simulcast]
Westerlund, M., Burman, B., Lindqvist, M., and F. Jansson,
"Using Simulcast in RTP sessions",
draft-westerlund-avtcore-rtp-simulcast (work in progress),
October 2011.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, July 2008.
[RFC5583] Schierl, T. and S. Wenger, "Signaling Media Decoding
Dependency in the Session Description Protocol (SDP)",
RFC 5583, July 2009.
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[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
[RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in
the Session Description Protocol", RFC 5956,
September 2010.
[RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
Flows", RFC 6051, November 2010.
[RFC6190] Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis,
"RTP Payload Format for Scalable Video Coding", RFC 6190,
May 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
Authors' Addresses
Magnus Westerlund
Ericsson
Farogatan 6
SE-164 80 Kista
Sweden
Phone: +46 10 714 82 87
Email: magnus.westerlund@ericsson.com
Bo Burman
Ericsson
Farogatan 6
SE-164 80 Kista
Sweden
Phone: +46 10 714 13 11
Email: bo.burman@ericsson.com
Westerlund, et al. Expires January 17, 2013 [Page 19]
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Patrik Sandgren
Ericsson
Farogatan 6
SE-164 80 Kista
Sweden
Phone: +46 10 717 97 41
Email: patrik.sandgren@ericsson.com
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