Network Working Group                                   Stephan Wenger
INTERNET-DRAFT                                           Umesh Chandra
Expires: February March 2007                                              Nokia
                                                     Magnus Westerlund
                                                             Bo Burman
                                                              Ericsson
                                                       August 28,
                                                    September 17, 2006

                       Codec Control Messages in the
                 Audio-Visual Profile with Feedback (AVPF)
                      draft-ietf-avt-avpf-ccm-00.txt>
                      draft-ietf-avt-avpf-ccm-01.txt>

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document specifies a few extensions to the messages defined in
   the Audio-Visual Profile with Feedback (AVPF).  They are helpful
   primarily in conversational multimedia scenarios where centralized
   multipoint functionalities are in use. However some are also usable
   in smaller multicast environments and point-to-point calls. The
   extensions discussed are H.271 video back channel, Full Intra
   Request, Temporary Maximum Media Bit-rate and Temporal Spatial Trade-off. Trade-
   off.

TABLE OF CONTENTS

1. Introduction....................................................5
2. Definitions.....................................................7
  2.1. Glossary...................................................7
  2.2. Terminology................................................7 Terminology................................................8
  2.3. Topologies.................................................9
3. Motivation (Informative)........................................9
  3.1. Use Cases..................................................9 Cases.................................................10
  3.2. Using the Media Path......................................11 Path......................................12
  3.3. Using AVPF................................................12
     3.3.1. Reliability..........................................12
  3.4. Multicast.................................................12 Multicast.................................................13
  3.5. Feedback Messages.........................................13
     3.5.1. Full Intra Request Command...........................13
        3.5.1.1. Reliability.....................................14
     3.5.2. Temporal Spatial Trade-off Request and Announcement..14 Announcement..15
        3.5.2.1. Point-to-point..................................15
        3.5.2.2. Point-to-Multipoint using Multicast or Translators15 Translators16
        3.5.2.3. Point-to-Multipoint using RTP Mixer.............16
        3.5.2.4. Reliability.....................................16
     3.5.3. H.271 Video Back Channel Message conforming to ITU-T Rec.
     H.271.......................................................17
        3.5.3.1. Reliability.....................................17 Reliability.....................................19
     3.5.4. Temporary Maximum Media Bit-rate Request.............17 Request.............19
        3.5.4.1. MCU based Multi-point operation.................18 operation.................20
        3.5.4.2. Point-to-Multipoint using Multicast or Translators20 Translators21
        3.5.4.3. Point-to-point operation........................20 operation........................22
        3.5.4.4. Reliability.....................................20 Reliability.....................................22
4. RTCP Receiver Report Extensions................................22 Extensions................................23
  4.1. Design Principles of the Extension Mechanism..............22 Mechanism..............23
  4.2. Transport Layer Feedback Messages.........................23 Messages.........................24
     4.2.1. Temporary Maximum Media Bit-rate Request (TMMBR).....23 (TMMBR).....24
        4.2.1.1. Semantics.......................................23 Semantics.......................................24
        4.2.1.2. Message Format..................................25 Format..................................26
        4.2.1.3. Timing Rules....................................26 Rules....................................27
     4.2.2. Temporary Maximum Media Bit-rate Notification (TMMBN) 26 27
        4.2.2.1. Semantics.......................................26 Semantics.......................................27
        4.2.2.2. Message Format..................................27 Format..................................28
        4.2.2.3. Timing Rules....................................27 Rules....................................29
  4.3. Payload Specific Feedback Messages........................27 Messages........................29
     4.3.1. Full Intra Request (FIR) command.....................28 command.....................29
        4.3.1.1. Semantics.......................................28 Semantics.......................................29
        4.3.1.2. Message Format..................................30 Format..................................31
        4.3.1.3. Timing Rules....................................31 Rules....................................32
        4.3.1.4. Remarks.........................................31 Remarks.........................................32
     4.3.2. Temporal-Spatial Trade-off Request (TSTR)............31 (TSTR)............33
        4.3.2.1. Semantics.......................................32 Semantics.......................................33
        4.3.2.2. Message Format..................................32 Format..................................33
        4.3.2.3. Timing Rules....................................33 Rules....................................34
        4.3.2.4. Remarks.........................................33 Remarks.........................................34
     4.3.3. Temporal-Spatial Trade-off Announcement (TSTA).......33 (TSTA).......35
        4.3.3.1. Semantics.......................................34 Semantics.......................................35
        4.3.3.2. Message Format..................................34 Format..................................35
        4.3.3.3. Timing Rules....................................35 Rules....................................36
        4.3.3.4. Remarks.........................................35 Remarks.........................................36
     4.3.4. H.271 VideoBackChannelMessage (VBCM).................35 (VBCM).................36
5. Congestion Control.............................................37 Control.............................................39
6. Security Considerations........................................38 Considerations........................................39
7. SDP Definitions................................................38 Definitions................................................40
  7.1. Extension of rtcp-fb attribute............................39 attribute............................40
  7.2. Offer-Answer..............................................40 Offer-Answer..............................................42
  7.3. Examples..................................................40 Examples..................................................42
8. IANA Considerations............................................43 Considerations............................................45
9. Acknowledgements...............................................44 Acknowledgements...............................................45
10. References....................................................45 References....................................................46
  10.1. Normative references.....................................45 references.....................................46
  10.2. Informative references...................................45 references...................................46
11. Authors' Addresses............................................46 Addresses............................................47
12. List of Changes relative to previous drafts...................46 drafts...................47

1.  Introduction

   When the Audio-Visual Profile with Feedback (AVPF) [RFC4548] [RFC4585] was
   developed, the main emphasis lied in the efficient support of point-
   to-point and small multipoint scenarios without centralized
   multipoint control.  However, in practice, many small multipoint
   conferences operate utilizing devices known as Multipoint Control
   Units (MCUs).  Long standing experience of the conversational video
   conferencing industry suggests that there is a need for a few
   additional feedback messages, to efficiently support MCU-based
   multipoint conferencing.  Some of the messages have applications
   beyond centralized multipoint, and this is indicated in the
   description of the message.  This is especially true for the message
   intended to carry ITU-T Rec. H.271 [H.271] bitstrings for video back
   channel messages.

   In RTP [RFC3550] terminology, MCUs comprise mixers and translators.
   Most MCUs also include signalling support.  During the development of
   this memo, it was noticed that there is considerable confusion in the
   community related to the use of terms such as "mixer",
   "translator", and "MCU".  In response to these concerns, a number of
   topologies have been identified that are of practical relevance to
   the industry, but were not envisioned (or at least not documented in
   sufficient detail) in RTP.  These topologies are documented in
   [Topologies], and understanding this memo frequently refers to sections in that document. requires previous or
   parallel study of [Topologies].

   Some of the messages defined here are forward only, in that they do
   not require an explicit acknowledgement.  Other messages require
   acknowledgement, leading to a two way communication model that could
   suggest to some to be useful for control purposes.  It is not the
   intention of this memo to open up RTCP to a generalized control
   protocol.  All mentioned messages have relatively strict real-time
   constraints - -- in the sense that their value diminishes with
   increased delay.  This makes the use of more traditional control
   protocol means, such as SIP re-invites, undesirable.  Furthermore,
   all messages are of a very simple format that can be easily processed
   by an RTP/RTCP sender/receiver.  Finally, all messages infer only to
   the RTP stream they are related to, and not to any other property of
   a communication system.

   The Full Intra Request (FIR) Command requires the receiver of the
   message (and sender of the stream) to immediately insert a decoder
   refresh point.  In video coding, one commonly used form of a decoder
   refresh point is an IDR or Intra picture.  Other codecs may have
   other forms of decoder refresh points.  In order to fulfil congestion
   control constraints, sending a decoder refresh point may imply a
   significant drop in frame rate, as they are commonly much larger than
   regular predicted content.  The use of this message is restricted to
   cases where no other means of decoder refresh can be employed, e.g.
   during the join-phase of a new participant in a multipoint
   conference.  It is explicitly disallowed to use the FIR command for
   error resilience purposes, and instead it is referred to AVPF's
   [RFC4585] PLI message, which reports lost pictures and has been
   included in AVPF for precisely that purpose.  The message does not
   require an acknowledgement, as the presence of a decoder refresh
   point can be easily derived from the media bit stream.  Today, the
   FIR message appears to be useful primarily with video streams, but in
   the future it may become helpful also in conjunction with other media
   codecs that support prediction across RTP packets.

   The Temporary Maximum Media Bandwidth Request (TMMBR) Message allows
   to signal, from media receiver to media sender, the current maximum
   supported media bit-rate for a given media stream.  Once a bandwidth
   limitation is established by the media sender, that sender notifies
   the initiator of the request, and all other session participants, by
   sending a TMMBN notification message.  One usage scenarios can be
   seen as limiting media senders in multiparty conferencing to the
   slowest receiver's maximum media bandwidth reception/handling
   capability.  Such a use is helpful, for example, because the
   receiver's situation may have changed due to computational load, or
   because the receiver has just joined the conference and it is helpful
   to inform media sender(s) about its constraints, without waiting for
   congestion induced bandwidth reduction.  Another application involves
   graceful bandwidth adaptation in scenarios where the upper limit
   connection bandwidth to a receiver changes, but is known in the
   interval between these dynamic changes.  The TMMBR message is useful
   for all media types that are not inherently of constant bit rate.

   The Video back channel message (VBCM) allows conveying bit streams
   conforming to ITU-T Rec. H.271 [H.271], from a video receiver to
   video sender.  This ITU-T Recommendation defines codepoints for a
   number of video-specific feedback messages.  Examples include
   messages to signal signal:
   - the corruption of reference pictures or parts thereof,
   - the corruption of decoder state information, e.g. parameter sets,
   - the suggestion of using a reference picture other than the one
     typically used, e.g. to support the NEWPRED algorithm [NEWPRED].
   The ITU-T plans to add codepoints to H.271 every time a need arises,
   e.g. with the introduction of new video codecs or new tools into
   existing video codecs.

   There exists some overlap between H.271 messages and "native"
   messages specified in this memo and in AVPF.  Examples include the
   PLI message of [RFC4548] [RFC4585] and the FIR message specified herein.  As a
   general rule, the "native" messages should be prefered over the
   sending of VBCM messages when all senders and receivers implement
   this memo.  However, if gateways are in the picture, it may be more
   advisable to utilize VBCM.  Similarly, for feedback message types
   that exist in H.271 but do not exist in this memo or AVPF, there is
   no other choice but using VBCM.
   Video feedback channel messages according to H.271 do not require
   acknowledgements on a protocol level, because the appropriate
   reaction of the video encoder and sender can be derived from the
   forward video bit stream.

   Finally, the Temporal-Spatial Trade-off Request (TSTR) Message
   enables a video receiver to signal to the video sender its preference
   for spatial quality or high temporal resolution (frame rate).  The
   receiver of the video stream generates this signal typically based on
   input from its user interface, so to react to explicit requests of
   the user.  However, some implicit use forms are also known.  For
   example, the trade-offs commonly used for live video and document
   camera content are different.  Obviously, this indication is relevant
   only with respect to video transmission.  The message is acknowledged
   by an announcement message indicating the newly chosen tradeoff, so
   to allow immediate user feedback.

2.  Definitions

2.1.    Glossary

   ASM    - Asynchronous Multicast
   AVPF   - The Extended RTP Profile for RTCP-based Feedback
   FEC    - Forward Error Correction
   FIR    - Full Intra Request
   MCU    - Multipoint Control Unit
   MPEG   - Moving Picture Experts Group
   PtM    - Point to Multipoint
   PtP    - Point to Point
   TMMBN  - Temporary Maximum Media Bit-rate Notification
   TMMBR  - Temporary Maximum Media Bit-rate Request
   PLI    - Picture Loss Indication
   TSTA   - Temporal Spatial Trade-off Announcement
   TSTR   - Temporal Spatial Trade-off Request
   VBCM   - Video Back Channel Message indication.

2.2.    Terminology

   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].

     Message:
          Codepoint defined by this specification, of one of the
          following types:

     Request:
             Message that requires Acknowledgement

     Acknowledgment:
             Message that answers a Request

     Command:
             Message that forces the receiver to an action

     Indication:
             Message that reports a situation

     Notification:
             See Indication.

    Note that, with the exception of "Notification", this terminology
    is in alignment with ITU-T Rec. H.245.

  Decoder Refresh Point:
           A bit string, packetised in one or more RTP packets, which
           completely resets the decoder to a known state. Typical
           examples of Decoder Refresh Points are H.261 Intra pictures
           and H.264 IDR pictures. However, there are also much more
           complex decoder refresh points.

           Typical examples for "hard" decoder refresh points are Intra
           pictures in H.261, H.263, MPEG 1, MPEG 2, and MPEG-4 part 2,
           and IDR pictures in H.264.  "Gradual" decoder refresh points
           may also be used; see for example [AVC].  While both "hard"
           and "gradual" decoder refresh points are acceptable in the
           scope of this specification, in most cases the user
           experience will benefit from using a "hard" decoder refresh
           point.

           A decoder refresh point also contains all header information
           above the picture layer (or equivalent, depending on the
           video compression standard) that is conveyed in-band.  In
           H.264, for example, a decoder refresh point contains
           parameter set NAL units that generate parameter sets
           necessary for the decoding of the following slice/data
           partition NAL units (and that are not conveyed out of band).
           To the best of the author's knowledge, the term "Decoder
           Refresh Point" has been formally defined only in H.264; hence
           we are referring here to this video compression standard.

  Decoding:
           The operation of reconstructing the media stream.

  Rendering:
           The operation of presenting (parts of) the reconstructed
           media stream to the user.

  Stream thinning:
          The operation of removing some of the packets from a media
           stream.  Stream thinning, preferably, is performed media
           aware, implying that media packets are removed in the order
           of their relevance to the reproductive quality. However even
           when employing media-aware stream thinning, most media
           streams quickly lose quality when subject to increasing
           levels of thinning.  Media-unaware stream thinning leads to
           even worse quality degradation.

2.3.    Topologies

   Please refer to [Topologies] for an in depth discussion.  the
   topologies referred to throughout this memo are labeled (consistent
   with [Topologies] as follows:

   Topo-Point-to-Point . . . . . point-to-point communication
   > Topo-Multicast  . . . . . . multicast communication as in RFC 3550
   > Topo-Translator . . . . . . translator based as in RFC 3550
   > Topo-Mixer  . . . . . . . . mixer based as in RFC 3550
   > Topo-Video-switch-MCU . . . video switching MCU,
   > Topo-RTCP-terminating-MCU . mixer but terminating RTCP

3.  Motivation (Informative)

   This section discusses the motivation and usage of the different
   video and media control messages. The video control messages have
   been under discussion for a long time, and a requirement draft was
   drawn up [Basso]. This draft has expired; however we do quote
   relevant sections of it to provide motivation and requirements.

3.1.    Use Cases

   There are a number of possible usages for the proposed feedback
   messages. Let's begin with looking through the use cases Basso et al.
   [Basso] proposed. Some of the use cases have been reformulated and
   commented:

   1. An RTP video mixer composes multiple encoded video sources into a
      single encoded video stream. Each time a video source is added,
      the RTP mixer needs to request a decoder refresh point from the
      video source, so as to start an uncorrupted prediction chain on
      the spatial area of the mixed picture occupied by the data from
      the new video source.

   2. An RTP video mixer that receives multiple encoded RTP video
      streams from conference participants, and dynamically selects one
      of the streams to be included in its output RTP stream.  At the
      time of a bit stream change (determined through means such as
      voice activation or the user interface), the mixer requests a
      decoder refresh point from the remote source, in order to avoid
      using unrelated content as reference data for inter picture
      prediction.  After requesting the decoder refresh point, the video
      mixer stops the delivery of the current RTP stream and monitors
      the RTP stream from the new source until it detects data belonging
      to the decoder refresh point.  At that time, the RTP mixer starts
      forwarding the newly selected stream to the receiver(s).

   3. An application needs to signal to the remote encoder a request of
      change of the desired trade-off in temporal/spatial resolution.
      For example, one user may prefer a higher frame rate and a lower
      spatial quality, and another use may prefer the opposite.  This
      choice is also highly content dependent.  Many current video
      conferencing systems offer in the user interface a mechanism to
      make this selection, usually in the form of a slider.  The
      mechanism is helpful in point-to-point, centralized multipoint and
      non-centralized multipoint uses.

   4. Use case 4 of the Basso draft applies only to AVPF's PLI [RFC4585]
      and is not reproduced here.

   5. Use case 5 of the Basso draft relates to a mechanism known as
      "freeze picture request".  Sending freeze picture requests
      over a non-reliable forward RTCP channel has been identified as
      problematic.  Therefore, no freeze picture request has been
      included in this memo, and the use case discussion is not
      reproduced here.

   6. A video mixer dynamically selects one of the received video
      streams to be sent out to participants and tries to provide the
      highest bit rate possible to all participants, while minimizing
      stream transrating. One way of achieving this is to setup sessions
      with endpoints using the maximum bit rate accepted by that
      endpoint, and by the call admission method used by the mixer. By
      means of commands that allow reducing the maximum media bitrate
      beyond what has been negotiated during session setup, the mixer
      can then reduce the maximum bit rate sent by endpoints to the
      lowest common denominator of all received streams. As the lowest
      common denominator changes due to endpoints joining, leaving, or
      network congestion, the mixer can adjust the limits to which
      endpoints can send their streams to match the new limit. The mixer
      then would request a new maximum bit rate, which is equal or less
      than the maximum bit-rate negotiated at session setup, for a
      specific media stream, and the remote endpoint can respond with
      the actual bit-rate that it can support.

   The picture Basso, et al draws up covers most applications we
   foresee. However we would like to extend the list with two additional
   use cases:

   7. The used congestion control algorithms (AMID and TFRC) probe for
      more bandwidth as long as there is something to send. With
      congestion control using packet-loss as the indication for
      congestion, this probing does generally result in reduced media
      quality (often to a point where the distortion is large enough to
      make the media unusable), due to packet loss and increased delay.
      In a number of deployment scenarios, especially cellular ones, the
      bottleneck link is often the last hop link. That cellular link
      also commonly has some type of QoS negotiation enabling the
      cellular device to learn the maximal bit-rate available over this
      last hop. Thus indicating the maximum available bit-rate to the
      transmitting part can be beneficial to prevent it from even trying
      to exceed the known hard limit that exists. For cellular or other
      mobile devices the available known bit-rate can also quickly
      change due to handover to another transmission technology, QoS
      renegotiation due to congestion, etc. To enable minimal disruption
      of service a possibility for quick convergence, especially in
      cases of reduced bandwidth, a media path signalling method is
      desired.

   8. The use of reference picture selection as an error resilience tool
      has been introduced in 1997 as NEWPRED [NEWPRED], and is now
      widely deployed.  It operates the receiver sending a feedback
      message to the sender, indicating a reference picture that should
      be used for future prediction.  AVPF contains a mechanism for
      conveying such a message, but did not specify for which codec and
      according to which syntax the message conforms to.  Recently, the
      ITU-T finalized Rec. H.271 which (among other message types) also
      includes a feedback message.  It is expected that this feedback
      message will enjoy wide support and fairly quickly.  Therefore, a
      mechanisms
      mechanism to convey feedback messages according to H.271 appears
      to be desirable.

3.2.    Using the Media Path

   There are multiple reasons why we propose to use the media path for
   the codec control messages. First, systems employing MCUs are often
   separating the control and media processing parts. As these messages
   are intended or generated by the media part rather than the
   signalling part of the MCU, having them on the media path avoids
   interfaces and unnecessary control traffic between signalling and
   processing.  If the MCU is physically decomposite, the use of the
   media path avoids the need for media control protocol extensions
   (e.g. in MEGACO [RFC3525]).

   Secondly, the signalling path quite commonly contains several
   signalling entities, e.g. SIP-proxies and application servers.
   Avoiding signalling entities avoids delay for several reasons.
   Proxies have less stringent delay requirements than media processing
   and due to their complex and more generic nature may result in
   significant processing delay. The topological locations of the
   signalling entities are also commonly not optimized for minimal
   delay, rather other architectural goals. Thus the signalling path can
   be significantly longer in both geographical and delay sense.

3.3.    Using AVPF

   The AVPF feedback message framework [RFC4585] provides a simple way
   of implementing the new messages.  Furthermore, AVPF implements rules
   controlling the timing of feedback messages so to avoid congestion
   through network flooding.  We re-use these rules by referencing to
   AVPF.

   The signalling setup for AVPF allows each individual type of function
   to be configured or negotiated on a RTP session basis.

3.3.1.      Reliability

   The use of RTCP messages implies that each message transfer is
   unreliable, unless the lower layer transport provides reliability.

   The different messages proposed in this specification have different
   requirements in terms of reliability. However, in all cases, the
   reaction to an (occasional) loss of a feedback message is specified.

3.4.    Multicast

   The media related requests might be used with multicast. The RTCP
   timing rules specified in [RFC3550] and [RFC4548] [RFC4585] ensure that the
   messages do not cause overload of the RTCP connection.  The use of
   multicast may result in the reception of messages with inconsistent
   semantics.   The reaction to inconsistencies depends on the message
   type, and is discussed for each message type separately.

3.5.    Feedback Messages

   This section describes the semantics of the different feedback
   messages and how they apply to the different use cases.

3.5.1.      Full Intra Request Command

   A Full Intra Request (FIR) command, when received by the designated
   media sender, requires that the media sender sends a "decoder refresh
   point" (see 2.2) at the earliest opportunity. The evaluation of such
   opportunity includes the current encoder coding strategy and the
   current available network resources.

   FIR is also known as an "instantaneous decoder refresh request" or
   "video fast update request".

   Using a decoder refresh point implies refraining from using any
   picture sent prior to that point as a reference for the encoding
   process of any subsequent picture sent in the stream.  For predictive
   media types that are not video, the analogue applies.  For example,
   if in MPEG-4 systems scene updates are used, the decoder refresh
   point consists of the full representation of the scene and is not
   delta-coded relative to previous updates.

   Decoder Refresh points, especially Intra or IDR pictures, are in
   general several times larger in size than predicted pictures.  Thus,
   in scenarios in which the available bandwidth is small, the use of a
   decoder refresh point implies a delay that is significantly longer
   than the typical picture duration.

   Usage in multicast is possible; however aggregation of the commands
   is recommended. A receiver that receives a request closely (within 2
   times the longest Round Trip Time (RTT) known) after sending a
   decoder refresh point should await a second request message to ensure
   that the media receiver has not been served by the previously
   delivered decoder refresh point. The reason for delaying 2 times the
   longest known RTT is to avoid sending unnecessary decoder refresh
   points. A session participant may have sent its own request while
   another participants request was in-flight to them. Thus suppressing
   those requests that may have been sent without knowledge about the
   other request avoids this issue.

   Full Intra Request is applicable in use-case 1, 2, and 5.

3.5.1.1.        Reliability

   The FIR message results in the delivery of a decoder refresh point,
   unless the message is lost. Decoder refresh points are easily
   identifiable from the bit stream. Therefore, there is no need for
   protocol-level acknowledgement, and a simple command repetition
   mechanism is sufficient for ensuring the level of reliability
   required. However, the potential use of repetition does require a
   mechanism to prevent the recipient from responding to messages
   already received and responded to.

   To ensure the best possible reliability, a sender of FIR may repeat
   the FIR request until a response has been received. The repetition
   interval is determined by the RTCP timing rules the session operates
   under. Upon reception of a complete decoder refresh point or the
   detection of an attempt to send a decoder refresh point (which got
   damaged due to a packet loss) the repetition of the FIR must stop. If
   another FIR is necessary, the request sequence number must be
   increased. To combat loss of the decoder refresh points sent, the
   sender that receives repetitions of the FIR 2*RTT after the
   transmission of the decoder refresh point shall send a new decoder
   refresh point. Two round trip times allow time for the request to
   arrive at the media sender and the decoder refresh point to arrive
   back to the requestor.  A FIR sender shall not have more than one FIR
   request (different request sequence number) outstanding at any time
   per media sender in the session.

   An RTP Mixer that receives an FIR from a media receiver is
   responsible to ensure that a decoder refresh point is delivered to
   the requesting receiver. It may be necessary to generate FIR commands
   by the MCU. The two legs (FIR-requesting endpoint to MCU, and MCU to
   decoder refresh point generating MCU) are handled independently from
   each other from a reliability perspective.

3.5.2.      Temporal Spatial Trade-off Request and Announcement

   The Temporal Spatial Trade-off Request (TSTR) instructs the video
   encoder to change its trade-off between temporal and spatial
   resolution.  Index values from 0 to 31 indicate monotonically a
   desire for higher frame rate.  In general the encoder reaction time
   may be significantly longer than the typical picture duration.  See
   use case 3 for an example.  The encoder decides if the request
   results in a change of the trade off.  An acknowledgement process has
   been defined to provide feedback of the trade-off that is used
   henceforth.

     Informative note: TSTR and TSTA have been introduced primarily
     because it is believed that control protocol mechanisms, e.g. a SIP
     re-invite, are too heavyweight, and too slow to allow for a
     reasonable user experience.  Consider, for example, a user
     interface where the remote user selects the temporal/spatial trade-
     off with a slider (as it is common in state-of-the-art video
     conferencing systems).  An immediate feedback to any slider
     movement is required for a reasonable user experience.  A SIP re-
     invite would require at least 2 round-trips more (compared to the
     TSTR/TSTA mechanism) and may involve proxies and other complex
     mechanisms.  Even in a well-designed system, it may take a second
     or so until finally the new trade-off is selected.
     Furthermore the use of RTCP solves very efficiently the multicast
     use case.

   The use of TSTR and TSTA in multipoint scenarios is a non-trivial
   subject, and can be solved in many implementation specific ways.
   Problems are stemming from the fact that TSTRs will typically arrive
   unsynchronized, and may request different trade-off values for the
   same stream and/or endpoint encoder.  This memo does not specify a
   MCU's or endpoint's reaction to the reception of a suggested trade-
   off as conveyed in the TSTR -- we only require the receiver of a TSTR
   message to reply to it by sending a TSTA, carrying the new trade-off
   chosen by its own criteria (which may or may not be based on the
   trade-off conveyed by TSTR).  In other words, the trade-off sent in
   TSTR is a non-binding recommendation; nothing more.

   With respect to TSTR/TSTA, four scenarios based on the topologies
   described in [Topologies] need to be distinguished. The scenarios are
   described in the following sub-clauses.

3.5.2.1.        Point-to-point

   In this most trivial case, case (Topo-Point-to-Point), the media sender
   typically adjusts its temporal/spatial trade-off based on the
   requested value in TSTR, and within its capabilities.  The TSTA
   message conveys back the new trade-off value (which may be identical
   to the old one if, for example, the sender is not capable to adjust
   its trade-off).

3.5.2.2.        Point-to-Multipoint using Multicast or Translators

   RTCP Multicast is used either with media multicast according to
   Section 2.3.2 of [Topologies], Topo-
   Multicast, or following RFC 3550's translator model according to Section 2.3.3 of [Topologies].
   Topo-Translator.  In these cases, TSTR messages from different
   receivers may be received unsynchronized, and possibly with different
   requested trade-offs (because of different user preferences).  This
   memo does not specify how the media sender tunes its trade-off.
   Possible strategies include selecting the mean, or median, of all
   trade-off requests received, prioritize certain participants, or
   continue using the previously selected trade-off (e.g. when the
   sender is not capable of adjusting it).  Again, all TSTR messages
   need to be acknowledged by TSTA, and the value conveyed back has to
   reflect the decision made.

3.5.2.3.        Point-to-Multipoint using RTP Mixer

   In this scenario (Topo-Mixer) the RTP Mixer receives all TSTR
   messages, and has the opportunity to act on them based on its own
   criteria.  In most cases, the MCU should form a "consensus" of
   potentially conflicting TSTR messages arriving from different
   participants, and initiate its own TSTR message(s) to the media
   sender(s).  The strategy of forming this "consensus" is open for the
   implementation, and can, for example, encompass averaging the
   participant's request values, prioritizing certain participants, or
   use session default values. If the Mixer changes its trade-off, it
   needs to request from the media sender(s) the use of the new value,
   by creating a TSTR of its own. Upon reaching a decision on the used
   trade-off it includes that value in the acknowledgement.

   Even if a Mixer or Translator performs transcoding, it is very
   difficult to deliver media with the requested trade-off, unless the
   content the MCU receives is already close to that trade-off. Only in
   cases where the original source has substantially higher quality (and
   bit-rate), it is likely that transcoding can result in the requested
   trade-off.

3.5.2.4.        Reliability
   A request and reception acknowledgement mechanism is specified. The
   Temporal Spatial Trade-off Announcement (TSTA) message informs the
   request-sender that its request has been received, and what trade-off
   is used henceforth. This acknowledgment mechanism is desirable for at
   least the following reasons:

   o A change in the trade-off cannot be directly identified from the
    media bit stream,
   o User feedback cannot be implemented without information of the
    chosen trade-off value, according to the media sender's
    constraints,
   o Repetitive sending of messages requesting an unimplementable trade-
    off can be avoided.

3.5.3.      H.271 Video Back Channel Message

   ITU-T Rec. H.271 defines syntax, semantics, and suggested encoder
   reaction to a video back channel message.  The codepoint defined in
   this memo is used to transparently convey such a message from media
   receiver to media sender.

   We refrain from an in-depth discussion of the available codepoints
   within H.271 in this memo for a number of reason. reasons.  The perhaps most
   important reason is that we expect backward-compatible additions of
   codepoints to H.271 outside the update/maturity cycle of this memo.
   The situation
   Another reason lies in the complexity of the H.271 specification: it
   is similar a dense document with currently 16 pages of content.   It does not
   make any sense to try to summarize its content in a few sentences of
   IETF lingo -- oversimplification and misguidance would be inevitable.
   Finally, please note that H.271 contains many statements of
   applicability and interpretation of its various messages in
   conjunction with specific video compression standards.  This type of
   discussion would overload the present memo.

   In so far, this memo follows the guidance of a decade of RTP payload
   format specs - specification work -- the data
   carried within details of the spec media format carried
   is normally not described in any significant detail.

   However, we note that some H.271 messages bear similarities with
   "native"
   native messages of AVPF and this memo, which memo.  Furthermore, we note that
   some H.271 message are known to require caution in multicast environments.  One example is the reference
   picture feedback message, which appears to be critical to
   contradicting information.  While it would perhaps be possible to
   specify an algorithm to resolve eventual contradictions, this would
   require an amount of awareness to the details
   environments -- or are plainly not usable in multicast or multipoint
   scenarios.  Table xxx provides a brief, oversimplifying overview of H.271 and
   the video
   codec employed which we would like to avoid messages currenty defined in H.271, their similar AVPF or CCM
   messages (the latter as specified in this memo.  Therefore,
   we err on the side of caution memo), and discourage the use an indication of VBCM in
   topologies other than point-to-point (section 2.3.1
   our current knowledge of  [Topologies])
   and point-to-multipoint utilizing their multicast safety.

   H.271 msg type   AVPF/CCM msg type   multicast-safe
   0 (when used for reference
       picture selection)     AVPF RPSI No (positive ACK of pictures)
   1                          AVPF PLI  Yes
   2                          AVPF SLI  Yes
   3                          N/A       Yes (no required sender action)
   4                          N/A       Yes (no required sender action)

          Note: H.271 message type 0 is not a mixer (section 2.3.4. strict equivalent to
          AVPF's RPSI; it is an indication of
   [Topologies]).   In known-as-correct reference
          picture(s) at the former case, obviously, no inconsistency
   problem exists. decoder.  It does not command an encoder to
          use a defined reference picture (the form of control
          information envisioned to be carried in RPSI).  However, it is
          believed and intended that H.271 message type 0 will be used
          for the same purpose as AVPF's RPSI -- although other use
          forms are also possible.

   In response to the latter case, opaqueness of the H.271 messages especially with
   respect to the multicast safety, the following guidelines MUST be
   followed when an implementation wishes to employ the H.271 video back
   channel message:

   1. Implementations utilizing the H.271 feedback message MUST stay in
      compliance with congestion control principles, as outlined in
      section 5.
   2. An implementation SHOULD utilize the native messages as defined in
      [RFC4585] and in this memo instead of similar messages defined in
      [H.271].  Our current understanding of similar messages is
      documented in table xxx above.  One good reason to divert from the
      SHOULD statement above would be if it is clearly understood that,
      for a given application and video compression standard, the
      aforementioned "similarity" is not given, in contrast to what
      the mixer's responsibility table indicates.
   3. It has been observed that some of the H.271 codepoints currently
      in existence are not multicast-save.  Therefore, the sensible
      thing to do is not to use the H.271 feedback message type in
      multicast environments.  It MAY be used only when all the issues
      mentioned later are fully understood by the implementer, and
      properly taken into account by all endpoints.  In all other cases,
      the H.271 message type MUST NOT be used in conjunction with
      multicast.
   4. It has been observed that even in centralized multipoint
      environments, where the mixer should theoretically be able to
      resolve issues as deocumented below, the inconsistencies, implementation of such a
      mixer and cooperative endpoints is a very difficult and tedious
      task.  Therefore, H.271 message MUST NOT be used in centralized
      multipoint scenarios, unless all the issues mentioned below are
      fully understood by the implementer, and properly taken into
      account by both mixer and endpoints.

   Issues with point to Multi-point:

   1. Different state established on different receivers. One example is
   the reference picture feedback message, which, when sent to receivers
   in which the video codecs are at different state due to previous
   losses or stream switches, the results can be unpredictable and
   annoying.
   2. Combination of multiple messages/requests by a media aware sender into
   an action and can
   do so. or response.
   3. Suppression of requests may need to go beyond the basic mechanism
   described in AVPF. For example forward messages may be need to
   suppress the generation of requests.

   Issues with translators and mixers
   1. Combination of multiple message or requests into an action or
      response.
   2.

3.5.3.1.        Reliability

   H.271 video back channel messages do not require reliable
   transmission, and the reception of a message can be derived from the
   forward video bit stream.  Therefore, no specific reception
   acknowledgement is specified.

   With respect to re-sending rules, clause 3.5.1.1. applies.

3.5.4.      Temporary Maximum Media Bit-rate Request

   A receiver, translator or mixer uses the Temporary Maximum Media Bit-
   rate Request (TMMBR, "timber") to request a sender to limit the
   maximum bit-rate for a media stream to, or below, the provided value.
   The primary usage for this is a scenario with MCU (use case 6),
   corresponding to topologies in 2.3.3 of [Topologies] (translator) and
   2.3.4 of [Topologies] (mixer), Topo-Translator or Topo-Mixer, but also .2.3.1 of Topologies (point-
   to-point). Topo-Point-
   to-Point.

   The temporary maximum media bit-rate messages are generic messages
   that can be applied to any media.

   The reasoning below assumes that the participants have negotiated a
   session maximum bit-rate, using the signalling protocol. This value
   can be global, for example in case of point-to-point, multicast, or
   translators.  It may also be local between the participant and the
   peer or mixer. In both cases, the bit-rate negotiated in signalling
   is the one that the participant guarantees to be able to handle
   (encode and decode).  In practice, the connectivity of the
   participant also bears an influence to the negotiated value -- it
   does not necessarily make much sense to negotiate a media bit rate
   that one's network interface does not support.

   An already established temporary bit-rate value may be changed at any
   time (subject to the timing rules of the feedback message sending),
   and to any value between zero and the session maximum, as negotiated
   during signalling. Even if a sender has received a TMMBR message
   increasing the bit-rate, all increases must be governed by a
   congestion control algorithm. TMMBR only indicates known limitations,
   usually in the local environment, and does not provide any
   guarantees.

   If it is likely that the new bit-rate indicated by TMMBR will be
   valid for the remainder of the session, the TMMBR sender can perform
   a renegotiation of the session upper limit using the session
   signalling protocol.

3.5.4.1.        MCU based Multi-point operation

   Assume a small mixer-based multiparty conference is ongoing, as
   depicted in
   Section 2.3.4 Topo-Mixer of [Topologies]. All participants (A-D) have
   negotiated a common maximum bit-rate that this session can use. The
   conference operates over a number of unicast links between the
   participants and the MCU.  The congestion situation on each of these
   links can easily be monitored by the participant in question and by
   the MCU, utilizing, for example, RTCP Receiver Reports.  However, any
   given participant has no knowledge of the congestion situation of the
   connections to the other participants.  Worse, without mechanisms
   similar to the ones discussed in this draft, the MCU (who is aware of
   the congestion situation on all connections it manages) has no
   standardized means to inform participants to slow down, short of
   forging its own receiver reports (which is undesirable).  In
   principle, an MCU confronted with such a situation is obliged to thin
   or transcode streams intended for connections that detected
   congestion.

   In practice, stream thinning - if performed media aware - is
   unfortunately a very difficult and cumbersome operation and adds
   undesirable delay.  If done media unaware, it leads very quickly to
   unacceptable reproduced media quality.  Hence, means to slow down
   senders even in the absence of congestion on their connections to the
   MCU are desirable.

   To allow the MCU to perform congestion control on the individual
   links, without performing transcoding, there is a need for a
   mechanism that enables the MCU to request the participant's media
   encoders to limit their maximum media bit-rate currently used. The
   MCU handles the detection of a congestion state between itself and a
   participant as follows:
   1. Start thinning the media traffic to the supported bit-rate.
   2. Use the TMMBR to request the media sender(s) to reduce the media
      bit-rate sent by them to the MCU, to a value that is in compliance
      with congestion control principles for the slowest link.  Slow
      refers here to the available bandwidth and packet rate after
      congestion control.
   3. As soon as the bit-rate has been reduced by the sending part, the
      MCU stops stream thinning implicitly, because there is no need for
      it any more as the stream is in compliance with congestion
      control.

   Above algorithms may suggest to some that there is no need for the
   TMMBR - it should be sufficient to solely rely on stream thinning.
   As much as this is desirable from a network protocol designer's
   viewpoint, it has the disadvantage that it doesn't work very
   well - the reproduced media quality quickly becomes unusable.

   It appears to be a reasonable compromise to rely on stream thinning
   as an immediate reaction tool to combat congestions, and have a quick
   control mechanism that instructs the original sender to reduce its
   bitrate.

   Note also that the standard RTCP receiver report cannot serve for the
   purpose mentioned.  In an environment with RTP Mixers, the RTCP RR is
   being sent between the RTP receiver in the endpoint and the RTP
   sender in the Mixer only - as there is no multicast transmission.
   The stream that needs to be bandwidth-reduced, however, is the one
   between the original sending endpoint and the Mixer.  This endpoint
   doesn't see the aforementioned RTCP RRs, and hence needs explicitly
   informed about desired bandwidth adjustments.

   In this topology it is the Mixer's responsibility to collect, and
   consider jointly, the different bit-rates which the different links
   may support, into the bit rate requested. This aggregation may also
   take into account that the Mixer may contain certain transcoding
   capabilities (as discussed in section 2.3.4 of [Topologies]), which
   can be employed for those few of the session participants that have
   the lowest available bit-rates.

3.5.4.2.        Point-to-Multipoint using Multicast or Translators

   In this topology, these topologies, corresponding to Topo-Multicast or Topo-
   Translator RTCP RRs are transmitted globally which allows for the
   detection of transmission problems such as congestion, on a medium
   timescale.  As all media senders are aware of the congestion
   situation of all media receivers, the rationale of the use of TMMBR
   of section 3.5.4.1 does not apply.  However, even in this case the
   congestion control response can be improved when the unicast links
   are employing congestion controlled transport protocols (such as TCP
   or DCCP). A peer may also report local limitation to the media
   sender.

3.5.4.3.        Point-to-point operation

   In use case 7 it is possible to use TMMBR to improve the performance
   at times of changes in the known upper limit of the bit-rate.  In
   this use case the signalling protocol has established an upper limit
   for the session and media bit-rates.  However at the time of
   transport link bit-rate reduction, a receiver could avoid serious
   congestion by sending a TMMBR to the sending side.

3.5.4.4.        Reliability

   The reaction of a media sender to the reception of a TMMBR message is
   not immediately identifiable through inspection of the media stream.
   Therefore a more explicit mechanism is needed to avoid unnecessary
   re-sending of TMMBR messages. Using a statistically based
   retransmission scheme would only provide statistical guarantees of
   the request being received. It would also not avoid the
   retransmission of already received messages. In addition it does not
   allow for easy suppression of other participants requests. For the
   reasons mentioned, a mechanism based on explicit notification is
   used.

   Upon the reception of a request a media sender sends a notification
   containing the current applicable limitation of the bit-rate, and
   which session participants that own that limit. That allows all other
   participants to suppress any request they may have, with limitation
   value equal or higher to the current one. The identity of the owner
   allows for small message sizes and media sender states. A media
   sender only keeps state for the SSRC of the current owner of the
   limitation; all other requests and their sources are not saved. Only
   the participant with the lowest value is allowed to remove or change
   its limitation. Otherwise anyone that ever set a limitation would
   need to remove it to allow the maximum bit-rate to be raised beyond
   that value.

4.  RTCP Receiver Report Extensions

   This memo specifies six new feedback messages. The Full Intra Request
   (FIR), Temporal-Spatial Trade-off Request (TSTR), Temporal-Spatial
   Trade-off Announcement (TSTA), and Video Back Channel Message (VBCM)
   are "Payload Specific Feedback Messages" in the sense of section 6.3
   of AVPF [RFC4548]. [RFC4585].  The Temporary Maximum Media Bit-rate Request
   (TMMBR) and Temporary Maximum Media Bit-rate Notification (TMMBN) are
   "Transport Layer Feedback Messages" in the sense of section 6.2 of
   AVPF.

   In the following subsections, the new feedback messages are defined,
   following a similar structure as in the AVPF specification's sections
   6.2 and 6.3, respectively.

4.1.    Design Principles of the Extension Mechanism

   RTCP was originally introduced as a channel to convey presence,
   reception quality statistics and hints on the desired media coding.
   A limited set of media control mechanisms have been introduced in
   early RTP payload formats for video formats, for example in RFC 2032
   [RFC2032].  However, this specification, for the first time, suggests
   a two-way handshake for one of its messages.  There is danger that
   this introduction could be misunderstood as the precedence for the
   use of RTCP as an RTP session control protocol. In order to prevent
   these misunderstandings, this subsection attempts to clarify the
   scope of the extensions specified in this memo, and strongly suggests
   that future extensions follow the rationale spelled out here, or
   compellingly explain why they divert from the rationale.

   In this memo, and in AVPF [RFC4548], [RFC4585], only such messages have been
   included which

   a) have comparatively strict real-time constraints, which prevent the
      use of mechanisms such as a SIP re-invite in most application
      scenarios.  The real-time constraints are explained separately for
      each message where necessary
   b) are multicast-safe in that the reaction to potentially
      contradicting feedback messages is specified, as necessary for
      each message
   c) are directly related to activities of a certain media codec, class
      of media codecs (e.g. video codecs), or the given media stream.

   In this memo, a two-way handshake is only introduced for such
   messages that
   a) require a notification or acknowledgement due to their nature,
      which is motivated separately for each message
   b) the notification or acknowledgement cannot be easily derived from
      the media bit stream.

   All messages in AVPF [RFC4548] [RFC4585] and in this memo follow a number of
   common design principles.  In particular:

   a) Media receivers are not always implementing higher control
      protocol functionalities (SDP, XML parsers and such) in their
      media path.  Therefore, simple binary representations are used in
      the feedback messages and not an (otherwise desirable) flexible
      format such as, for example, XML.

4.2.    Transport Layer Feedback Messages

   Transport Layer FB messages are identified by the value RTPFB (205)
   as RTCP packet type.

   In AVPF, one message of this category had been defined.  This memo
   specifies two more messages for a total of three messages of this
   type.  They are identified by means of the FMT parameter as follows:

      0:    unassigned
      1:    Generic NACK (as per AVPF)
      2:    Temporary Maximum Media Bit-rate Request
      3:    Temporary Maximum Media Bit-rate Notification
      4-30: unassigned
      31:   reserved for future expansion of the identifier number space

   The following subsection defines the formats of the FCI field for
   this type of FB message.

4.2.1.      Temporary Maximum Media Bit-rate Request (TMMBR)

   The FCI field of a TMMBR Feedback message SHALL contain one or more
   FCI entries.

4.2.1.1.        Semantics

   The TMMBR is used to indicate the highest bit-rate per sender of a
   media, which the receiver currently supports in this RTP session.
   The media sender MAY use any lower bit-rate, as it may need to
   address a congestion situation or other limiting factors.  See
   section 5 (congestion control) for more discussion.

   The "SSRC of the packet sender" field indicates the source of the
   request, and the "SSRC of media source" is not used and SHALL be set
   to 0. The SSRC of media sender in the FCI field denotes the media
   sender the message applies to. This is useful in the multicast or
   translator topologies where each media sender may be addressed in a
   single TMMBR message using multiple FCIs.

   A TMMBR FCI MAY be repeated in subsequent TMMBR messages if no
   applicable TMMBN FCI has been received at the time of transmission of
   the next RTCP packet. The bit-rate value of a TMMBR FCI MAY be
   changed from a previous TMMBR message and the next, regardless of the
   eventual reception of an applicable TMMBN FCI.

   Please note that a TMMBN message is SHALL be sent by the media sender at
   the earliest possible point in time, as a result of any TMMBR
   messages received since the last sending of TMMBN.  The TMMBN message
   indicates the limit and the owner of that limit at the time of the
   transmission of the message. The limit is the lowest of the previous
   value and all values received in TMMBR FCI's since the last TMMBN was
   transmitted.

   A media receiver who is not the owner of the bandwidth limit when
   sending
   planning to send a TMMBR, MUST SHOULD request a bandwidth lower than their
   knowledge of currently established bandwidth limit for this media sender.
   Therefore, all
   sender, or suppres their transmission for TMMBR.  The exception to
   the above rule is when a receiver either doesn't know the limit or
   are certain that their local representation of the value is in error.
   All received requests for bandwidth limits greater or equal to the
   one currently established are ignored. ignored, with the exception of them
   resulting in the transmission of a TMMBN.  A media receiver who is
   the owner of the current bandwidth limit, MAY lower the value
   further, raise the value or remove the restriction completely by
   setting the bandwidth limit equal to the session limit.

   Once a session participant receives the TMMBN in response to its
   TMMBR, with its own SSRC, it knows that it "owns" the bandwidth
   limitation.  Only the "owner" of a bandwidth limitation can raise it
   or reset it to the session limit.

   Note that, due to the unreliable nature of transport of TMMBR and
   TMMBN, the above rules may lead to the sending of TMMBR messages
   disobeying the rules above.  Furthermore, in multicast scenarios it
   can happen that more than one session participants believes it "owns"
   the current bandwidth limitation.  This is not critical for a number
   of reasons:
   a) If a TMMBR message is lost in transmission, the media sender does
   not learn about the restrictions imposed on it.  However, it also
   does not send a TMMBN message notifying reception of a request it has
   never received.  Therefore, no new limit is established, the media
   receiver sending the more restrictive TMMBR is not the owner.  Since
   this media receiver has not seen a notification corresponding to its
   request, it is free to re-send it.
   b) Similarly, if a TMMBN message gets lost, the media receiver that
   has sent the corresponding TMMBR request does not receive
   acknowledgement.  In that case, it is also not the "owner" of the
   restriction and is free to re-send the request.
   c) If multiple competing TMMBR messages are sent by different session
   participants, then the resulting TMMBN indicates the lowest bandwidth
   requested; the owner is set to the sender of the TMMBR with the
   lowest requested bandwidth value.

   TMMBR feedback SHOULD NOT be used if the underlying transport
   protocol is capable of providing similar feedback information from
   the receiver to the sender.

   It also important to consider the security risks involved with faked
   TMMBRs. See security considerations in Section 6.

   The feedback messages may be used in both multicast and unicast
   sessions of any of the specified topologies.

   For sessions with a larger number of participants using the lowest
   common denominator, as required by this mechanism, may not be the
   most suitable course of action. Larger session may need to consider
   other ways to support adapted bit-rate to participants, such as
   partitioning the session in different quality tiers, or use some
   other method of achieving bit-rate scalability.

   If the value set by a TMMBR message is expected to be permanent the
   TMMBR setting party is RECOMMENDED to renegotiate the session
   parameters to reflect that using the setup signalling.

   An SSRC may time out according to the default rules for RTP session
   participants, i.e. the media sender has not received any RTCP packet
   from the owner for the last five regular reporting intervals. An SSRC
   may also leave the session, indicating this through the transmission
   of an RTCP BYE packet or an external signalling channel. In all of
   these cases the entity is considered to have left the session. In the
   case the "owner" leaves the session, the value SHALL be set to the
   session maximum and the transmission of a TMMBN is scheduled.

4.2.1.2.        Message Format
   The Feedback control information (FCI) consists of one or more TMMBR
   FCI entries with the following syntax:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              SSRC                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Maximum bit-rate in units of 128 bits/s            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 1 - Syntax for the TMMBR message

     SSRC:   The SSRC value of the target of this specific maximum bit-
             rate request.

     Maximum bit-rate: The temporary maximum media bit-rate value in
             units of 128 bit/s. This provides range from 0 to
             549755813888 bits/s (~550 Tbit/s) with a granularity of 128
             bits/s.

   The length of the FB message is be set to 2+2*N where N is the number
   of TMMBR FCI entries.

4.2.1.3.        Timing Rules

   The first transmission of the request message MAY use early or
   immediate feedback in cases when timeliness is desirable. Any
   repetition of a request message SHOULD use regular RTCP mode for its
   transmission timing.

4.2.2.      Temporary Maximum Media Bit-rate Notification (TMMBN)

   The FCI field of the TMMBN Feedback message SHALL contain one TMMBN
   FCI entry.

4.2.2.1.        Semantics

   This feedback message is used to notify the senders of any TMMBR
   message that one or more TMMBR messages have been received.  It
   indicates to all participants the currently employed maximum bit-rate
   value and the "owner" of the current limitation.  The "owner" of a
   limitation is the sender of the last (most restrictive) TMMBR message
   received by the media sender.

   The "SSRC of the packet sender" field indicates the source of the
   notification.  The "SSRC of media source" SHALL be set to the SSRC of
   the media receiver that currently owns the bit-rate limitation.

   A TMMBN message SHALL be scheduled for transmission after the
   reception of a TMMBR message with a FCI including the session
   participant's SSRC.  Only a single TMMBN SHALL be sent, even if more
   than one TMMBR messages are received between the scheduling of the
   transmission and the actual transmission of the TMMBN message. The
   TMMBN message indicates the limit and the owner of that limit at the
   time of transmitting the message. The limit SHALL be the lowest of
   the existing and all values received in TMMBR messages since the last
   TMMBN was transmitted. The one sending that request SHALL become the
   owner of the limit.

   The reception of a TMMBR message with a transmission limit greater or
   equal than the current limit SHALL still result in the transmission
   of a TMMBN message. However the limit and owner is not changed,
   unless it was from the same owner, and the current limit and owner is
   indicated in the TMMBN message. This procedure allows session
   participants that haven't seen the last TMMBN message to get a
   correct view of this media sender's state.

   When a media sender determines an "owner" of a limitation has left
   the session, then the current limitation is removed, and the media
   sender SHALL send a TMMBN message indicating the maximum session
   bandwidth.

4.2.2.2.        Message Format

   The TMMBN Feedback control information (FCI) entry has the following
   syntax:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Maximum bit-rate in units of 128 bits/s            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 2 - Syntax for the TMMBN message

     Maximum bit-rate: The current temporary maximum media bit-rate
             value in units of 128 bit/s.

     The length field value of the FB message SHALL be 3.

4.2.2.3.        Timing Rules

   The acknowledgement SHOULD be sent as soon as allowed by the applied
   timing rules for the session. Immediate or early feedback mode SHOULD
   be used for these messages.

4.3.    Payload Specific Feedback Messages

   Payload-Specific FB messages are identified by the value PT=PSFB
   (206) as RTCP packet type.

   AVPF defines three payload-specific FB messages and one application
   layer FB message.  This memo specifies four additional payload
   specific feedback messages.  All are identified by means of the FMT
   parameter as follows:

     0:     unassigned
     1:     Picture Loss Indication (PLI)
     2:     Slice Lost Indication (SLI)
     3:     Reference Picture Selection Indication (RPSI)
     4:     Full Intra Request Command (FIR)
     5:     Temporal-Spatial Trade-off Request (TSTR)
     6:     Temporal-Spatial Trade-off Announcement (TSTA)
     7:     Video Back Channel Message (VBCM)
     8-14:  unassigned
     15:    Application layer FB message
     16-30: unassigned
     31:    reserved for future expansion of the sequence number space

   The following subsections define the new FCI formats for the payload-
   specific FB messages.

4.3.1.      Full Intra Request (FIR) command

   The FIR command FB message is identified by PT=PSFB and FMT=4.

   There MUST be one or more FIR entry contained in the FCI field.

4.3.1.1.        Semantics

   Upon reception of a FIR message, an encoder MUST send a decoder
   refresh point (see Section 2.2) as soon as possible.

     Note: Currently, video appears to be the only useful application
     for FIR, as it appears to be the only RTP payloads widely deployed
     that relies heavily on media prediction across RTP packet
     boundaries.  However, use of FIR could also reasonably be
     envisioned for other media types that share essential properties
     with compressed video, namely cross-frame prediction (whatever a
     frame may be for that media type).  One possible example may be the
     dynamic updates of MPEG-4 scene descriptions.  It is suggested that
     payload formats for such media types refer to FIR and other message
     types defined in this specification and in AVPF, instead of
     creating similar mechanisms in the payload specifications.  The
     payload specifications may have to explain how the payload specific
     terminologies map to the video-centric terminology used here.

     Note: In environments where the sender has no control over the
     codec (e.g. when streaming pre-recorded and pre-coded content), the
     reaction to this command cannot be specified.  One suitable
     reaction of a sender would be to skip forward in the video bit
     stream to the next decoder refresh point.  In other scenarios, it
     may be preferable not to react to the command at all, e.g. when
     streaming to a large multicast group.  Other reactions may also be
     possible.  When deciding on a strategy, a sender could take into
     account factors such as the size of the receiving multicast group, the
     "importance" of the sender of the FIR message (however "importance"
     may be defined in this specific application), the frequency of
     decoder refresh points in the content, and others. However the usage of FIR in a
     session which predominately handles pre-coded content shouldn't use
     the FIR at all.

   The sender MUST consider congestion control as outlined in section 5,
   which MAY restrict its ability to send a decoder refresh point
   quickly.

     Note: The relationship between the Picture Loss Indication and FIR
     is as follows. As discussed in section 6.3.1 of AVPF, a Picture
     Loss Indication informs the decoder about the loss of a picture and
     hence the likeliness of misalignment of the reference pictures in
     encoder and decoder.  Such a scenario is normally related to losses
     in an ongoing connection.  In point-to-point scenarios, and without
     the presence of advanced error resilience tools, one possible
     option an encoder has is to send a decoder refresh point.  However,
     there are other options including ignoring the PLI, for example if
     only one receiver of many has sent a PLI or when the embedded
     stream redundancy is likely to clean up the reproduced picture
     within a reasonable amount of time.  The FIR, in contrast, leaves a
     real-time encoder no choice but to send a decoder refresh point.
     It disallows the encoder to take into account any considerations
     such as the ones mentioned above.

     Note: Mandating a maximum delay for completing the sending of a
     decoder refresh point would be desirable from an application
     viewpoint, but may be problematic from a congestion control point
     of view.  "As soon as possible" as mentioned above appears to be a
     reasonable compromise.

   FIR SHALL NOT be sent as a reaction to picture losses - it is
   RECOMMENDED to use PLI instead.  FIR SHOULD be used only in such
   situations where not sending a decoder refresh point would render the
   video unusable for the users.

     Note: a typical example where sending FIR is adequate is when, in a
     multipoint conference, a new user joins the session and no regular
     decoder refresh point interval is established.  Another example
     would be a video switching MCU that changes streams.  Here,
     normally, the MCU issues a freeze picture request (through protocol
     means outside this specification) to the receiver(s), switches the
     streams, and issues a FIR to the new sender so to force it to
     emit a decoder refresh point.  The decoder refresh point includes
     normally a Freeze Picture Release (defined outside this
     specification), which re-starts the rendering process of the
     receivers.  Both techniques mentioned are commonly used in MCU-
     based multipoint conferences.

   Other RTP payload specifications such as RFC 2032 [RFC2032] already
   define a feedback mechanism for certain codecs.  An application
   supporting both schemes MUST use the feedback mechanism defined in
   this specification when sending feedback.  For backward compatibility
   reasons, such an application SHOULD also be capable to receive and
   react to the feedback scheme defined in the respective RTP payload
   format, if this is required by that payload format.

   The "SSRC of the packet sender" field indicates the source of the
   request, and the "SSRC of media source" is not used and SHALL be set
   to 0. The SSRC of media sender to which the FIR command applies to is
   in the FCI.

4.3.1.2.           Message Format

   Full Intra Request uses one additional FCI field, the content of
   which is depicted in Figure 3  The length of the FB message MUST be
   set to 2+2*N, where N is the number of FCI entries.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              SSRC                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Seq. nr       |    Reserved                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 3 - Syntax for the FIR message

     SSRC:   The SSRC value of the media sender of this specific FIR
             command.

     Seq. nr: Command sequence number. The sequence number space is
             unique for each tuple consisting of the SSRC of command
             source and the SSRC of the command target. The sequence
             number SHALL be increased by 1 modulo 256 for each new
             command. A repetition SHALL NOT increase the sequence
             number. Initial value is arbitrary.

     Reserved: All bits SHALL be set to 0 and SHALL be ignored on
             reception.

   The semantics of this FB message is independent of the RTP payload
   type.

4.3.1.3.        Timing Rules

   The timing follows the rules outlined in section 3 of [RFC4548]. [RFC4585].  FIR
   commands MAY be used with early or immediate feedback.  The FIR
   feedback message MAY be repeated. If using immediate feedback mode
   the repetition SHOULD wait at least on onee RTT before being sent. In
   early or regular RTCP mode the repetition is sent in the next regular
   RTCP packet.

4.3.1.4.        Remarks

   FIR messages typically trigger the sending of full intra or IDR
   pictures. Both are several times larger then predicted (inter)
   pictures.  Their size is independent of the time they are generated.
   In most environments, especially when employing bandwidth-limited
   links, the use of an intra picture implies an allowed delay that is a
   significant multitude of the typical frame duration.  An example: If
   the sending frame rate is 10 fps, and an intra picture is assumed to
   be 10 times as big as an inter picture, then a full second of latency
   has to be accepted.  In such an environment there is no need for a
   particular short delay in sending the FIR message.  Hence waiting for
   the next possible time slot allowed by RTCP timing rules as per
   [RFC4548]
   [RFC4585] may not have an overly negative impact on the system
   performance.

4.3.2.      Temporal-Spatial Trade-off Request (TSTR)

   The TSTR FB message is identified by PT=PSFB and FMT=5.

   There MUST be one or more TSTR entry contained in the FCI field.

4.3.2.1.        Semantics

   A decoder can suggest the use of a temporal-spatial trade-off by
   sending a TSTR message to an encoder.  If the encoder is capable of
   adjusting its temporal-spatial trade-off, it SHOULD take into account
   the received TSTR message for future coding of pictures.  A value of
   0 suggests a high spatial quality and a value of 31 suggests a high
   frame rate. The values from 0 to 31 indicate monotonically a desire
   for higher frame rate. Actual values do not correspond to precise
   values of spatial quality or frame rate.

   The reaction to the reception of more than one TSTR message by a
   media sender from different media receivers is left open to the
   implementation.  The selected trade-off SHALL be communicated to the
   media receivers by the means of the TSTA message.

   The "SSRC of the packet sender" field indicates the source of the
   request, and the "SSRC of media source" is not used and SHALL be set
   to 0. The SSRC of media sender to which the TSTR applies to is in the
   FCI entries.

   A TSTR message may contain multiple requests to different media
   senders, using multiple FCI entries.

4.3.2.2.           Message Format

   The Temporal-Spatial Trade-off Request uses one FCI field, the
   content of which is depicted in Figure 4.  The length of the FB
   message MUST be set to 2+2*N, where N is the number of FCI entries
   included.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              SSRC                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Seq nr.      |  Reserved                           | Index   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 4 - Syntax of the TSTR

     SSRC:   The SSRC value of the target (or the media sender) of this
             specific TSTR request.

     Seq. nr: Request sequence number. The sequence number space is
             unique for each tuple consisting of the SSRC of request
             source and the SSRC of the request target. The sequence
             number SHALL be increased by 1 modulo 256 for each new
             command. A repetition SHALL NOT increase the sequence
             number. Initial value is arbitrary.

     Index:  An integer value between 0 and 31 that indicates the
             relative trade off that is requested. An index value of 0
             index highest possible spatial quality, while 31 indicates
             highest possible temporal resolution.

     Reserved: All bits SHALL be set to 0 and SHALL be ignored on
             reception.

4.3.2.3.        Timing Rules

   The timing follows the rules outlined in section 3 of [RFC4548]. [RFC4585].
   This request message is not time critical and SHOULD be sent using
   regular RTCP timing. Only if it is known that the user interface
   requires a quick feedback, the message MAY be sent with early or
   immediate feedback timing.

4.3.2.4.        Remarks

   The term "spatial quality" does not necessarily refer to the
   resolution, measured by the number of pixels the reconstructed video
   is using.  In fact, in most scenarios the video resolution stays
   constant during the lifetime of a session.  However, all video
   compression standards have means to adjust the spatial quality at a
   given resolution, often influenced by the Quantizer Parameter or QP.
   A numerically low QP results in a good reconstructed picture quality,
   whereas a numerically high QP yields a coarse picture.  The typical
   reaction of an encoder to this request is to change its rate control
   parameters to use a lower frame rate and a numerically lower (on
   average) QP, or vice versa.  The precise mapping of Index, frame
   rate, and QP is intentionally left open here, as it depends on
   factors such as compression standard employed, spatial resolution,
   content, bit rate, and many more.

4.3.3.      Temporal-Spatial Trade-off Announcement (TSTA)

   The TSTA FB message is identified by PT=PSFB and FMT=6.

   There SHALL be one or more TSTA contained in the FCI field.

4.3.3.1.        Semantics

   This feedback message is used to acknowledge the reception of a TSTR.
   A TSTA entry in a TSTA feedback message SHALL be sent for each TSTR
   entry targeted to this session participant, i.e. each TSTR received
   that in the SSRC field in the entry has the receiving entities SSRC.
   A single TSTA message MAY acknowledge multiple requests using
   multiple FCI entries.  The index value included SHALL be the same in
   all FCI's part of the TSTA message.  Including a FCI for each
   requestor allows each requesting entity to determine that the media
   sender targeted have received the request.  The acknowledgement SHALL
   be sent also for repetitions received. If the request receiver has
   received TSTR with several different sequence numbers from a single
   requestor it SHALL only respond to the request with the highest
   (modulo 256) sequence number.

   The TSTA SHALL include the Temporal-Spatial Trade-off index that will
   be used as a result of the request. This is not necessarily the same
   index as requested, as media sender may need to aggregate requests
   from several requesting session participants. It may also have some
   other policies or rules that limit the selection.

   A single TSTA message MAY acknowledge multiple requests using
   multiple FCI entries.

4.3.3.2.           Message Format

   The Temporal-Spatial Trade-off Announcement uses one additional FCI
   field, the content of which is depicted in Figure 5.  The length of
   the FB message MUST be set to 2+2*N, where N is the number of FCI
   entries.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              SSRC                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Seq nr.      |  Reserved                           | Index   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 5 - Syntax of the TSTA

     SSRC:   The SSRC of the source of the TSTA request that is
             acknowledged.

     Seq. nr: The sequence number value from the TSTA request that is
             being acknowledged.

     Index:  The trade-off value the media sender is using henceforth.

     Reserved: All bits SHALL be set to 0 and SHALL be ignored on
             reception.

     Informative note: The returned trade-off value (Index) may differ
     from the requested one, for example in cases where a media encoder
     cannot tune its trade-off, or when pre-recorded content is used.

4.3.3.3.        Timing Rules

   The timing follows the rules outlined in section 3 of [RFC4548]. [RFC4585].
   This acknowledgement message is not extremely time critical and
   SHOULD be sent using regular RTCP timing.

   Edt. Note: a comment from Magnus: We might like to expand on this in
   relation to certain applications

4.3.3.4.        Remarks

   None

4.3.4.       H.271 VideoBackChannelMessage (VBCM)

   The VBCM FB message is identified by PT=PSFB and FMT=7.

   There MUST be one or more VBCM entry contained in the FCI field.

4.3.4.1.        Semantics

   The "payload" of VBCM indication carries codec specific, different
   types of feedback information.  The type of feedback information can
   be classified as "status report" such as receiving bit stream
   without errors, loss of partial or complete picture or block or
   "update requests" such as complete refresh of the bit stream.

         Note: There are possible overlap between the VBCM sub-messages
         and CCM/AVPF feedback messages, such FIR.  Please see section
         3.5.3 for further discussions.

   The different types of feedback sub-messages carried in the VBCM are
   indicated by the "payloadType" as defined in [VBCM]. The different
   sub-message types as defined in [VBCM] are re-produced below for
   convenience.  "payloadType", in ITU-T Rec. H.271 terminology,
   refers to the sub-type of the H.271 message and should not be
   confused with an RTP payload type.

   Payload Type     Message Content

   0      One or more pictures without detected bitstream error mismatch
   1      One or more pictures that are entirely or partially lost
   2      A set of blocks of one picture that is entirely or partially
   lost
   3      CRC for one parameter set
   4      CRC for all parameter sets of a certain type
   5      A "reset" request indicating that the sender should completely
   refresh the video bitstream as if no prior bitstream data had been
   received
   >      5    Reserved for future use by ITU-T

   The bit string or the "payload" of VBCM message is of variable
   length and is self-contained and coded in a variable length, binary
   format. The media sender necessarily has to be able to parse this
   optimized binary format to make use of VBCM messages

   Each of the different types of sub-messages (indicated by
   payloadType)e may have different semantic based on the codec used.

4.3.4.2.        Message Format

   The VBCM indication uses one FCI field and the syntax is depicted in
   Figure 6.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              SSRC                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Seq. nr       |0| Payload Type| Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    VBCM Bit String....             | Padding  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  Figure 6 - Syntax for VBCM Message

  SSRC: The SSRC value of the media media sender that is target of the
          message, i.e. the sender of this specific whose encoder should to react to the
          VBCM
          indication message. message

  Seq. nr : Command sequence number. The sequence number space is
          unique for each tuple consisting of the SSRC of command source
          and the SSRC of the command target. The sequence number SHALL
          be increased by 1 modulo 256 for each new command. A
          repetition SHALL NOT increase the sequence number. Initial
          value is arbitrary.

  0: Must be set to 0 and should not be acted upon receiving.

  Payload: The RTP payload type for which the VBCM bit stream must be
          interpreted.

               NOTE : Stephan I think this payload type is redundant,
          since during session set up phase you do lock down

  Length: The length of the payload
          type that you are going to use for this session. But I am
          keeping it since it is there VBCM bit string in RPSI of AVPF. Is there any
          reason for this ? From an implementation point of view I can
          see this being helpful (and fast) since you don't have to go
          back to your session information. octets.

  VBCM Bit String : This is the bit string generated by the decoder
          carrying a specific feedback sub-message. It is of variable
          length.

  Padding: Bits set to 0 to make up a 32 bit boundry

   Timing Rules

     The timing follows the rules outlined in section 3 of [RFC4548] [RFC4585]

   Remarks
     Please see section 3.5.3 for the applicability of the VBCM message
     in relation to messages in both AVPF and this memo with similar
     functionality.

     Edt. note: Between the authors there is an ongoing discussion
     whether we need the payload type field in this message.  It would
     be needed if there were potentially more than one VBCM-capable
     payload types in the same session, && that the semantics of a given
     VBCM message changes from PT to PT.  This appears to be the case.
     For example, the picture identification mechanism in messages of
     H.271 type 0 is fundamentally different between H.263 and H.264
     (although both use the same syntax.  So the payload field appears
     to be justified.  It was further commented that for TSTS and FIR
     such a need may not exist, simply because the semantics of TSTS and
     FIR are either loosely enough defined, or generic enough, to apply
     to all video payloads currently in existence/envisioned.  So that
     part of the draft seems ok.

     Edt. note: (related to SSRC field): Magnus commented [...]. There
     is also need to define what the meaning of the fixed header SSRC
     values are.

5.  Congestion Control

   The correct application of the AVPF timing rules prevents the network
   flooding by feedback messages. Hence, assuming a correct
   implementation, the RTCP channel cannot break its bit-rate commitment
   and introduce congestion.

   The reception of some of the feedback messages modifies the behaviour
   of the media senders or, more specifically, the media encoders.  All
   of these modifications MUST only be performed within the bandwidth
   limits the applied congestion control provides. For example, when
   reacting to a FIR, the unusually high number of packets that form the
   decoder refresh point have to be paced in compliance with the
   congestion control algorithm, even if the user experience suffers
   from a slowly transmitted decoder refresh point.

   A change of the Temporary Maximum Media Bit-rate value can only
   mitigate congestion, but not cause congestion. congestion as long as congestion
   control is also employed. An increase of the value by a request
   REQUIRES the media sender to use congestion control when increasing
   its transmission rate to that value. A reduction of the value results
   in a reduced transmission bit-rate thus reducing the risk for
   congestion.

6.  Security Considerations

   The defined messages have certain properties that have security
   implications. These must be addressed and taken into account by users
   of this protocol.

   The defined setup signalling mechanism is sensitive to modification
   attacks that can result in session creation with sub-optimal
   configuration, and, in the worst case, session rejection. To prevent
   this type of attack, authentication and integrity protection of the
   setup signalling is required.

   Spoofed or maliciously created feedback messages of the type defined
   in this specification can have the following implications:
        a. Severely reduced media bit-rate due to false TMMBR messages
           that sets the maximum to a very low value.
        b. The assignment of the ownership of a bit-rate limit with a
          TMMBN message to the wrong participant. Thus potentially
          freezing the mechanism until a correct TMMBN message reached
          the participants.
        c. Sending TSTR that result in a video quality different from
           the user's desire, rendering the session less useful.
        d. Frequent FIR commands will potentially reduce the frame-rate
           making the video jerky due to the frequent usage of decoder
           refresh points.

   To prevent these attacks there is need to apply authentication and
   integrity protection of the feedback messages. This can be
   accomplished against group external threats using the RTP profile
   that combines SRTP [SRTP] and AVPF into SAVPF [SAVPF]. In the MCU
   cases, separate security contexts and filtering can be applied
   between the MCU and the participants thus protecting other MCU users
   from a misbehaving participant.

7.  SDP Definitions

   Section 4 of [RFC4548] [RFC4585] defines new SDP [RFC2327] attributes that are
   used for the capability exchange of the AVPF commands and
   indications, such as Reference Picture selection, Picture loss
   indication etc. The defined SDP attribute is known as rtcp-fb and its
   ABNF is described in section 4.2 of [RFC4548]. [RFC4585]. In this section we
   extend the rtcp-fb attribute to include the commands and indications
   that are described in this document for codec control protocol. We
   also discuss the Offer/Answer implications for the codec control
   commands and indications.

7.1.    Extension of rtcp-fb attribute

   As described in [RFC4548], [RFC4585], the rtcp-fb attribute is defined to
   indicate the capability of using RTCP feedback. As defined in AVPF
   the rtcp-fb attribute must only be used as a media level attribute
   and must not be provided at session level.

   All the rules described in [RFC4548] [RFC4585] for rtcp-fb attribute relating
   to payload type, multiple rtcp-fb attributes in a session description
   hold for the new feedback messages for codec control defined in this
   document.

   The ABNF for rtcp-fb attributed as defined in [RFC4548] [RFC4585] is

   Rtcp-fb-syntax = "a=rtcp-fb: " rtcp-fb-pt SP rtcp-fb-val CRLF

   Where rtcp-fb-pt is the payload type and rtcp-fb-val defines the type
   of the feedback message such as ack, nack, trr-int and rtcp-fb-id.
   For example to indicate the support of feedback of picture loss
   indication, the sender declares the following in SDP

         v=0
         o=alice 3203093520 3203093520 IN IP4 host.example.com
         s=Media with feedback
         t=0 0
         c=IN IP4 host.example.com
         m=audio 49170 RTP/AVPF 98
         a=rtpmap:98 H263-1998/90000
         a=rtcp-fb:98 nack pli

   In this document we define a new feedback value type called "ccm"
   which indicates the support of codec control using RTCP feedback
   messages. The "ccm" feedback value should be used with parameters,
   which indicates the support of which codec commands the session may
   use. In this draft we define four parameters, which can be used with
   the ccm feedback value type.

     o  "fir" indicates the support of Full Intra Request
     o  "tmmbr" indicates the support of Temporal Maximum Media Bit-rate
     o  "tstr" indicates the support of temporal spatial trade-off
        request.
     O  "bbcm"  "vbcm" indicates the support of H.271 video back channel
        messages.

   In ABNF for rtcp-fb-val defined in [RFC4548], [RFC4585], there is a placeholder
   called rtcp-fb-id to define new feedback types. The ccm is defined as
   a new feedback type in this document and the ABNF for the parameters
   for ccm are defined here (please refer section 4.2 of [RFC4548] [RFC4585] for
   complete ABNF syntax).

   Rtcp-fb-param = SP "app" [SP byte-string]
                 / SP rtcp-fb-ccm-param
                 /       ; empty
   rtcp-fb-ccm-param = "ccm" SP ccm-param

   ccm-param  = "fir"   ; Full Intra Request
               / "tmmbr" ; Temporary max media bit rate
               / "tstr"  ; Temporal Spatial Trade Off
               / "vbcm" 1*[SP subMessageType] ; H.271 VBCM messages
               / token [SP byte-string]
                         ; for future commands/indications
   subMessageType = 1*[integer];
   byte-string = <as defined in section 4.2 of [RFC4548] [RFC4585] >

7.2.    Offer-Answer

   The Offer/Answer [RFC3264] implications to codec control protocol
   feedback messages are similar to as described in [RFC4548]. [RFC4585]. The
   offerer MAY indicate the capability to support selected codec
   commands and indications. The answerer MUST remove all ccm
   parameters, which it does not understand or does not wish to use in
   this particular media session. The answerer MUST NOT add new ccm
   parameters in addition to what has been offered. The answer is
   binding for the media session and both offerer and answerer MUST only
   use feedback messages negotiated in this way.

7.3.    Examples

   Example 1: The following SDP describes a point-to-point video call
   with H.263 with the originator of the call declaring its capability
   to support codec control messages - fir, tstr. The SDP is carried in
   a high level signalling protocol like SIP

         v=0
         o=alice 3203093520 3203093520 IN IP4 host.example.com
         s=Point-to-Point call
         c=IN IP4 172.11.1.124
         m=audio 49170 RTP/AVP 0
         a=rtpmap:0 PCMU/8000
         m=video 51372 RTP/AVPF 98
         a=rtpmap:98 H263-1998/90000
         a=rtcp-fb:98 ccm tstr
         a=rtcp-fb:98 ccm fir
   In the above example the sender when it receives a TSTR message from
   the remote party can adjust the trade off as indicated in the RTCP
   TSTA feedback message.

   Example 2: The following SDP describes a SIP end point joining a
   video MCU that is hosting a multiparty video conferencing session.
   The participant supports only the FIR (Full Intra Request) codec
   control command and it declares it in its session description. The
   video MCU can send an FIR RTCP feedback message to this end point
   when it needs to send this participants video to other participants
   of the conference.

         v=0
         o=alice 3203093520 3203093520 IN IP4 host.example.com
         s=Multiparty Video Call
         c=IN IP4 172.11.1.124
         m=audio 49170 RTP/AVP 0
         a=rtpmap:0 PCMU/8000
         m=video 51372 RTP/AVPF 98
         a=rtpmap:98 H263-1998/90000
         a=rtcp-fb:98 ccm fir

   When the video MCU decides to route the video of this participant it
   sends an RTCP FIR feedback message. Upon receiving this feedback
   message the end point is mandated to generate a full intra request.

   Example 3: The following example describes the Offer/Answer
   implications for the codec control messages. The Offerer wishes to
   support all the commands "tstr", "fir" and indications of codec control "tmmbr" messages. The offered SDP is

   -------------> Offer
         v=0
         o=alice 3203093520 3203093520 IN IP4 host.example.com
         s=Offer/Answer
         c=IN IP4 172.11.1.124
         m=audio 49170 RTP/AVP 0
         a=rtpmap:0 PCMU/8000
         m=video 51372 RTP/AVPF 98
         a=rtpmap:98 H263-1998/90000
         a=rtcp-fb:98 ccm tstr
         a=rtcp-fb:98 ccm fir
         a=rtcp-fb:98 ccm tmmbr

   The answerer only wishes to support FIR and TSTR message as the codec
   control messages and the answerer SDP is
   <---------------- Answer

         v=0
         o=alice 3203093520 3203093524 IN IP4 otherhost.example.com
         s=Offer/Answer
         c=IN IP4 189.13.1.37
         m=audio 47190 RTP/AVP 0
         a=rtpmap:0 PCMU/8000
         m=video 53273 RTP/AVPF 98
         a=rtpmap:98 H263-1998/90000
         a=rtcp-fb:98 ccm tstr
         a=rtcp-fb:98 ccm fir

   Example 4: The following example describes the Offer/Answer
   implications for H.271 Video back channel messages (VBCM). The
   Offerer wishes to support VBCM and the submessages of payloadType 2(
   A set of blocks of one picture that is entirely or partially lost, 3
   (CRC for one parameter set) and 4 (CRC for all parameter sets of a
   certain type).

   -------------> Offer
         v=0
         o=alice 3203093520 3203093520 IN IP4 host.example.com
         s=Offer/Answer
         c=IN IP4 172.11.1.124
         m=audio 49170 RTP/AVP 0
         a=rtpmap:0 PCMU/8000
         m=video 51372 RTP/AVPF 98
         a=rtpmap:98 H263-1998/90000
         a=rtcp-fb:98 ccm vbcm 2 3 4

   The answerer only wishes to support sub-messages 3 and 4 only

   <---------------- Answer

         v=0
         o=alice 3203093520 3203093524 IN IP4 otherhost.example.com
         s=Offer/Answer
         c=IN IP4 189.13.1.37
         m=audio 47190 RTP/AVP 0
         a=rtpmap:0 PCMU/8000
         m=video 53273 RTP/AVPF 98
         a=rtpmap:98 H263-1998/90000
         a=rtcp-fb:98 ccm vbcm 3 4
   So in the above example only VBCM indication comprising of only
   "payloadType" 3 and 4 will be supported.

8.  IANA Considerations

   The new value of ccm for the rtcp-fb attribute needs to be registered
   with IANA.

   Value name:       ccm
   Long Name:        Codec Control Commands and Indications
   Reference:        RFC XXXX

   For use with "ccm" the following values also needs to be
   registered.

   Value name:       fir
   Long name:        Full Intra Request Command
   Usable with:      ccm
   Reference:        RFC XXXX

   Value name:       tmmbr
   Long name:        Temporary Maximum Media Bit-rate
   Usable with:      ccm
   Reference:        RFC XXXX

   Value name:       tstr
   Long name:        temporal Spatial Trade Off
   Usable with:      ccm
   Reference:        RFC XXXX

   Value name:       vbcm
   Long name:        H.271 video back channel messages
   Usable with:      ccm
   Reference:        RFC XXXX

9.  Acknowledgements

   The authors would like to thank Andrea Basso, Orit Levin, Nermeen
   Ismail for their work on the requirement and discussion draft
   [Basso].

10.   References

10.1.     Normative references

   [RFC4548]

   [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., Rey, J.,
            "Extended RTP Profile for Real-Time Transport Control
            Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July
            2006
   [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.
   [RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description
            Protocol", RFC 2327, April 1998.
   [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
            with Session Description Protocol (SDP)", RFC 3264, June
            2002.
   [Topologies] M. Westerlund, and S. Wenger, "Topologies", RFC xxxx, x "RTP Topologies", draft-
            ietf-avt-topologies-00, work in progress, August 2006

10.2.     Informative references

   [Basso]  A. Basso, et. al., "Requirements for transport of video
            control commands", draft-basso-avt-videoconreq-02.txt,
            expired Internet Draft, October 2004.
   [AVC]    Joint Video Team of ITU-T and ISO/IEC JTC 1, Draft ITU-T
            Recommendation and Final Draft International Standard of
            Joint Video Specification (ITU-T Rec. H.264 | ISO/IEC
            14496-10 AVC), Joint Video Team (JVT) of ISO/IEC MPEG and
            ITU-T VCEG, JVT-G050, March 2003.
   [NEWPRED] S. Fukunaga, T. Nakai, and H. Inoue, "Error Resilient Video
            Coding by Dynamic Replacing of Reference Pictures," in
            Proc. Globcom'96, vol. 3, pp. 1503 - 1508, 1996.
   [SRTP]   Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
            Norrman, "The Secure Real-time Transport Protocol (SRTP)",
            RFC 3711, March 2004.
   [RFC2032] Turletti, T. and C. Huitema, "RTP Payload Format for H.261
            Video Streams", RFC 2032, October 1996.
   [SAVPF]  J. Ott, E. Carrara, "Extended Secure RTP Profile for RTCP-
            based Feedback (RTP/SAVPF)," draft-ietf-avt-profile-savpf-
            02.txt, July, 2005.
   [RFC3525] Groves, C., Pantaleo, M., Anderson, T., and T. Taylor,
            "Gateway Control Protocol Version 1", RFC 3525, June 2003.
   [VBCM]   ITU-T Rec. H.271, "Video Bach Channel Messages", pre-
   Any 3GPP document can be downloaded from the 3GPP web server,
   "http://www.3gpp.org/", see specifications.

11.   Authors' Addresses

   Stephan Wenger
   Nokia Corporation
   P.O. Box 100
   FIN-33721 Tampere
   FINLAND

   Phone: +358-50-486-0637
   EMail: stewe@stewe.org

   Umesh Chandra
   Nokia Research Center
   6000 Connection Drive
   Irving, Texas 75063
   975, Page Mill Road,
   Palo Alto,CA 94304
   USA

   Phone: +1-972-894-6017 +1-650-796-7502
   Email: Umesh.Chandra@nokia.com

   Magnus Westerlund
   Ericsson Research
   Ericsson AB
   SE-164 80 Stockholm, SWEDEN

   Phone: +46 8 7190000
   EMail: magnus.westerlund@ericsson.com

   Bo Burman
   Ericsson Research
   Ericsson AB
   SE-164 80 Stockholm, SWEDEN

   Phone: +46 8 7190000
   EMail: bo.burman@ericsson.com

12.   List of Changes relative to previous drafts

   The following changes since draft-wenger-avt-avpf-ccm-01 have been
   made:

   - The topologies have been rewritten and clarified.
   - The TMMBR mechanism has been completely revised to use notification
     and suppress messages in deployments with large common SSRC spaces.

   The following changes since draft-wenger-avt-avpf-ccm-02 have been
   made:

   - Update of section 4.2.2.1 (TMMBN) as per discussions between
     Harikishan Desineni and Magnus Westerlund on the AVT list around
     Feb 21, 2006
   - Section 2.3.4 clarified as per email exchange between Colin Perkins
     and Magnus Westerlund around Feb 24
   - Section 3.5.2 and other occurrences throughout the draft,
     Temporal/Spatial Acknowledgement renamed to Temporal/Spatial
     Annoucement

   Changes relative to draft-wenger-avt-avpf-ccm-03

   - Moved "topologies" out to another draft
   - Editorial improvements
   - Added new code point VBCM for H.271 Video back channel messages.
     Several sections -
     Sections 3,4 and 7 were modified for this new CCM
     message. in response to H.271 introduction.
   - Removed Basso use case referring to forward Freeze command, added
     justification.

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   The RFC editor is requested to replace all occurrences of XXXX with
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