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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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 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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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


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













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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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