AVTCORE WG                                                     J. Lennox
Internet-Draft                                                     Vidyo
Updates: 3550 (if approved)                                M. Westerlund
Intended status: Standards Track                                Ericsson
Expires: January 12, July 17, 2014                                             Q. Wu
                                                              C. Perkins
                                                   University of Glasgow
                                                           July 11, 2013
                                                        January 13, 2014

 Sending Multiple Media Streams in a Single RTP Session: Grouping RTCP
                Reception Statistics and Other Feedback


   RTP allows multiple media streams to be sent in a single session, but
   requires each Synchronisation Source (SSRC) to send RTCP reception
   quality reports for every other SSRC visible in the session.  This
   causes the number of RTCP reception reports to grow with the number
   of SSRCs, rather than the number of endpoints.  In many cases most of
   these RTCP reception reports are unnecessary, since all SSRCs of an
   endpoint are co-located and see the same reception quality.  This
   memo defines a Reporting Group extension to RTCP to reduce the
   reporting overhead in such scenarios.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on January 12, July 17, 2014.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Grouping of RTCP Reception Statistics and Other Feedback  . .   3
     3.1.  Semantics and Behavior of Reporting Groups  . . . . . . .   3
     3.2.  Determine the Report Group  . . . . . . . . . . . . . . .   4
     3.3.  RTCP Packet Reporting Group's Reporting Sources . . . . .   5
     3.4.  RTCP Source Description (SDES) item for Reporting Groups    6
     3.5.  Middlebox Considerations  . . . . . . . . . . . . . . . .   6
     3.6.  SDP signaling for Reporting Groups  . . . . . . . . . . .   6
     3.7.  Bandwidth Benefits of RTCP Reporting Groups . . . . . . .   6
     3.8.  Consequences of RTCP Reporting Groups . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The Real-time Transport Protocol (RTP) [RFC3550] is a protocol for
   group communication, supporting multiparty multimedia sessions.  A
   single RTP session can support multiple participants sending at once,
   and can also support participants sending multiple simultaneous media
   streams.  Examples of the latter might include a participant with
   multiple cameras who chooses to send multiple views of a scene, or a
   participant that sends audio and video flows multiplexed in a single
   RTP session.  Rules for handling RTP sessions containing multiple
   media streams are described in [RFC3550] with some clarifications in

   An RTP endpoint will have one or more synchronisation sources (SSRCs)
   that send and receive media streams (it will have one SSRC for each
   media stream it sends).  Each SSRC has to send RTCP sender reports
   corresponding to the RTP packets it sends, and receiver reports for
   traffic it receives.  That is, every SSRC will send RTCP packets to
   report on every other SSRC.  This rule is simple, but can be quite
   inefficient for endpoints that send large numbers of media streams in
   a single RTP session.  Consider a session comprising ten
   participants, each sending three media streams with their own SSRC.
   There will be 30 SSRCs in such an RTP session, and 30 RTCP reception
   reports will be sent per reporting interval as each SSRC reports on
   all the others.  However, the three SSRCs comprising each participant
   will almost certainly see identical reception quality, since they are
   co-located.  If there was a way to indicate that several SSRCs are
   co-located, and see the same reception quality, then two-thirds of
   those RTCP reports could be suppressed.

   This memo defines such an RTCP extension, Reporting Groups.  This
   extension is used to indicate the SSRCs that originate from the same
   endpoint, and therefore have identical reception quality, allowing
   the endpoint to suppress unnecessary RTCP reception reports.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Grouping of RTCP Reception Statistics and Other Feedback

3.1.  Semantics and Behavior of Reporting Groups

   An RTCP Reporting Group indicates that a set of sources (SSRCs) that
   originate from a single entity (endpoint or middlebox) in an RTP
   session, and therefore all the sources in the group have an identical
   view of the network.  If reporting groups are in use, two sources
   SHOULD be put into the same reporting group if their view of the
   network is identical; i.e., if they report on traffic received at the
   same interface of an RTP endpoint.  Sources with different views of
   the network MUST NOT be put into the same reporting group.

   If reporting groups are in use, an endpoint MUST NOT send reception
   reports from one source in a reporting group about another one in the
   same group ("self-reports").  Similarly, an endpoint MUST NOT send
   reception reports about a remote media source from more than one
   source in a reporting group ("cross-reports").  Instead, it MUST pick
   one of its local media sources as the "reporting" source for each
   remote media source, and use it to send reception reports about that
   remote source; all the other media sources in the reporting group
   MUST NOT send any reception reports about that remote media source.

   This reporting source MUST also be the source for any RTP/AVPF
   Feedback [RFC4585] or Extended Report (XR) [RFC3611] packets about
   the corresponding remote sources as well.  If a reporting source
   leaves the session (i.e., if it sends a BYE, or leaves the group
   without sending BYE under the rules of [RFC3550] section 6.3.7),
   another reporting source MUST be chosen if any members of the group
   still exist.

   An endpoint or middlebox MAY use multiple sources as reporting
   sources; however, each reporting source MUST have non-overlapping
   sets of remote SSRCs it reports on.  This is primarily to be done
   when the reporting source's number of reception report blocks is so
   large that it would be forced to round-robin around the sources.
   Thus, by splitting the reports among several reporting SSRCs, more
   consistent reporting can be achieved.

   If RTP/AVPF feedback is in use, a reporting source MAY send immediate
   or early feedback at any point when any member of the reporting group
   could validly do so.

   An endpoint SHOULD NOT create single-source reporting groups, unless
   it is anticipated that the group might have additional sources added
   to it in the future.

3.2.  Determine the Report Group

   A remote RTP entity, such as an endpoint or a middlebox needs to be
   able to determine the report group used by another RTP entity.  To
   achieve this goal two RTCP extensions have been defined.  For the
   SSRCs that are reporting on behalf of the reporting group, an SDES
   item RGRP has been defined for providing the report group with an
   identifier.  For SSRCs that aren't reporting on any peer SSRC a new
   RTCP packet type is defined.  This RTCP packet type "Reporting
   Sources" lists the SSRC that are reporting on this SSRC's behalf.

   This divided approach has been selected for the following reasons:

   1.  To enable an explicit indication of who reports on this SSRC's
       behalf.  Being explicit prevents the remote entity from detecting
       that is missing the reports if there issues with the reporting
       SSRC's RTCP packets.

   2.  To enable explicit identification of the SSRCs that are actively
       reporting as one entity.

3.3.  RTCP Packet Reporting Group's Reporting Sources

   This section defines a new RTCP packet type called "Reporting Group's
   Reporting Sources" (RGRS).  It identifies the SSRC(s) that report on
   behalf of the SSRC that is the sender of the RGRS packet.

   This packet consists of the fixed RTCP packet header which indicates
   the packet type, the number of reporting sources included and the
   SSRC which behalf the reporting SSRCs report on.  This is followed by
   the list of reporting SSRCs.

    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
   |V=2|P|    SC   | PT=RGRS(TBA)  |             length            |
   |                     SSRC of packet sender                     |
   :                     SSRC for Reporting Source                 :

   The RTCP Packets field has the following definition.

   version (V):  This field identifies the RTP version.  The current
      version is 2.

   padding (P):  1 bit If set, the padding bit indicates that the packet
      contains additional padding octets at the end that are not part of
      the control information but are included in the length field.  See

   Source Count (SC):  5 bits Indicating the number of reporting SSRCs
      (1-31) that are included in this RTCP packet type.

   Payload type (PT):  8 bits This is the RTCP packet type that
      identifies the packet as being an RTCP FB message.  The RGRS RTCP
      packet has the value [TBA].

   Length:  16 bits The length of this packet in 32-bit words minus one,
      including the header and any padding.  This is in line with the
      definition of the length field used in RTCP sender and receiver
      reports [RFC3550].

   SSRC of packet sender:  32 bits.  The SSRC of the sender of this
      packet which indicates which SSRCs that reports on its behalf,
      instead of reporting itself.

   SSRC for Reporting Source:  A variable number (as indicated by Source
      Count) of 32-bit SSRC values.  Each SSRC is an reporting SSRC
      belonging to the same Report Group.

   Each RGRS packet MUST contain at least one reporting SSRC.  In case
   the reporting SSRC field is insufficient to list all the SSRCs that
   are reporting in this report group, the SSRC SHALL round robin around
   the reporting sources.

   Any RTP mixer or translator which forwards SR or RR packets from
   members of a reporting group MUST forward the corresponding RGRS RTCP
   packet as well.

3.4.  RTCP Source Description (SDES) item for Reporting Groups

   A new RTCP Source Description (SDES) item is defined for the purpose
   of identifying reporting groups.

   The Source Description (SDES) item "RGRP" is sent by a reporting
   group's reporting SSRC.  Syntactically, its format is the same as the
   RTCP SDES CNAME item [RFC6222], and MUST be chosen with the same
   global-uniqueness and privacy considerations as CNAME.  This name
   MUST be stable across the lifetime of the reporting group, even if
   the SSRC of a reporting source changes.

   Every source which belongs to a reporting group MUST either include
   an RGRP SDES item in an SDES packet (if it is a reporting source), or
   an RGRS packet (if it is not), in every compound RTCP packet in which
   it sends an RR or SR packet (i.e., in every RTCP packet it sends,
   unless Reduced-Size RTCP [RFC5506] is in use).

   Any RTP mixer or translator which forwards SR or RR packets from
   members of a reporting group MUST forward the corresponding RGRP SDES
   items as well, even if it otherwise strips SDES items other than

3.5.  Middlebox Considerations

   This section discusses middlebox considerations for Reporting groups.

   To be expanded.

3.6.  SDP signaling for Reporting Groups


3.7.  Bandwidth Benefits of RTCP Reporting Groups
   To understand the benefits of RTCP reporting groups, consider a
   scenario in which the two endpoints in a session each have a hundred
   sources, of which eight each are sending within any given reporting

   For ease of analysis, we can make the simplifying approximation that
   the duration of the RTCP reporting interval is equal to the total
   size of the RTCP packets sent during an RTCP interval, divided by the
   RTCP bandwidth.  (This will be approximately true in scenarios where
   the bandwidth is not so high that the minimum RTCP interval is
   reached.)  For further simplification, we can assume RTCP senders are
   following the recommendations regarding Compound RTCP Packets in
   [I-D.ietf-avtcore-rtp-multi-stream]; thus, the per-packet transport-
   layer overhead will be small relative to the RTCP data.  Thus, only
   the actual RTCP data itself need be considered.

   In a report interval in this scenario, there will, as a baseline, be
   200 SDES packets, 184 RR packets, and 16 SR packets.  This amounts to
   approximately 6.5 kB of RTCP per report interval, assuming 16-byte
   CNAMEs and no other SDES information.

   Using the original [RFC3550] everyone-reports-on-every-sender
   feedback rules, each of the 184 receivers will send 16 report blocks,
   and each of the 16 senders will send 15.  This amounts to
   approximately 76 kB of report block traffic per interval; 92% of RTCP
   traffic consists of report blocks.

   If reporting groups are used, however, there is only 0.4 kB of
   reports per interval, with no loss of useful information.
   Additionally, there will be (assuming 16-byte RGRPs, and a single
   reporting source per reporting group) an additional 2.4 kB per cycle
   of RGRP SDES items and RGRS packets.  Put another way, the unmodified
   [RFC3550] reporting interval is approximately 8.9 times longer than
   if reporting groups are in use.

3.8.  Consequences of RTCP Reporting Groups

   The RTCP traffic generated by receivers using RTCP Reporting Groups
   might appear, to observers unaware of these semantics, to be
   generated by receivers who are experiencing a network disconnection,
   as the non-reporting sources appear not to be receiving a given
   sender at all.

   This could be a potentially critical problem for such a sender using
   RTCP for congestion control, as such a sender might think that it is
   sending so much traffic that it is causing complete congestion

   However, such an interpretation of the session statistics would
   require a fairly sophisticated RTCP analysis.  Any receiver of RTCP
   statistics which is just interested in information about itself needs
   to be prepared that any given reception report might not contain
   information about a specific media source, because reception reports
   in large conferences can be round-robined.

   Thus, it is unclear to what extent such backward compatibility issues
   would actually cause trouble in practice.

4.  Security Considerations

   The security considerations of [RFC3550] and
   [I-D.ietf-avtcore-rtp-multi-stream] apply.

   (tbd: any security considerations due to these extensions?)

5.  IANA Considerations

   (Note to the RFC-Editor: please replace "TBA" with the IANA-assigned
   value, and "XXXX" with the number of this document, prior to
   publication as an RFC.)

   The IANA is requested to register one new RTCP SDES items in the
   "RTCP SDES Item Types" registry, as follows:

   Value    Abbrev      Name              Reference
   TBA      RGRP        Reporting Group   [RFCXXXX]

           Figure 1: Item for the IANA Source Attribute Registry

   The IANA is also requested to register one new RTCP packet type as

   Value    Abbrev      Name                                Reference
   TBA      RGRR        Reporting Group Reporting Sources   [RFCXXXX]

    Figure 2: Item for the IANA RTCP Control Packet Types (PT) Registry

6.  References

6.1.  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.

   [RFC6222]  Begen, A., Perkins, C., and D. Wing, "Guidelines for
              Choosing RTP Control Protocol (RTCP) Canonical Names
              (CNAMEs)", RFC 6222, April 2011.

6.2.  Informative References

              Lennox, J., Westerlund, M., Wu, W., and C. Perkins, "RTP
              Considerations for Endpoints Sending
              "Sending Multiple Media
              Streams", draft-ietf-avtcore-rtp-multi-stream-00 Streams in a Single RTP Session",
              draft-ietf-avtcore-rtp-multi-stream-01 (work in progress), April
              July 2013.

   [RFC3611]  Friedman, T., Caceres, R., and A. Clark, "RTP Control
              Protocol Extended Reports (RTCP XR)", RFC 3611, November

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July

   [RFC5506]  Johansson, I. and M. Westerlund, "Support for Reduced-Size
              Real-Time Transport Control Protocol (RTCP): Opportunities
              and Consequences", RFC 5506, April 2009.

Authors' Addresses

   Jonathan Lennox
   Vidyo, Inc.
   433 Hackensack Avenue
   Seventh Floor
   Hackensack, NJ  07601

   Email: jonathan@vidyo.com
   Magnus Westerlund
   Farogatan 6
   SE-164 80 Kista

   Phone: +46 10 714 82 87
   Email: magnus.westerlund@ericsson.com

   Qin Wu
   101 Software Avenue, Yuhua District
   Nanjing, Jiangsu 210012

   Email: sunseawq@huawei.com

   Colin Perkins
   University of Glasgow
   School of Computing Science
   Glasgow  G12 8QQ
   United Kingdom

   Email: csp@csperkins.org