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Versions: (draft-wenger-avt-avpf-ccm) 00 01 02 03 04 05 06 07 08 09 10 RFC 5104

Network Working Group                                   Stephan Wenger
INTERNET-DRAFT                                           Umesh Chandra
Expires: October 2007                                            Nokia
                                                     Magnus Westerlund
                                                             Bo Burman
                                                              Ericsson
                                                          May 14, 2007

                        Codec Control Messages in the
                RTP Audio-Visual Profile with Feedback (AVPF)
                       draft-ietf-avt-avpf-ccm-05.txt>


Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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



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   extensions discussed are messages related to the ITU-T H.271 Video
   Back Channel, Full Intra Request, Temporary Maximum Media Stream Bit
   Rate and Temporal Spatial Trade-off.
















































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TABLE OF CONTENTS

1. Introduction....................................................5
2. Definitions.....................................................6
   2.1. Glossary...................................................6
   2.2. Terminology................................................6
   2.3. Topologies.................................................9
3. Motivation (Informative).......................................10
   3.1. Use Cases.................................................10
   3.2. Using the Media Path......................................12
   3.3. Using AVPF................................................13
      3.3.1. Reliability..........................................13
   3.4. 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 Notification..15
         3.5.2.1. Point-to-Point..................................16
         3.5.2.2. Point-to-Multipoint Using Multicast or
                  Translators.....................................16
         3.5.2.3. Point-to-Multipoint Using RTP Mixer.............17
         3.5.2.4. Reliability.....................................17
      3.5.3. H.271 Video Back Channel Message.....................17
         3.5.3.1. Reliability.....................................20
      3.5.4. Temporary Maximum Media Stream Bit Rate Request and
      Notification................................................20
         3.5.4.1. Behavior for media receivers using TMMBR........22
         3.5.4.2. Algorithm for establishing current limitations..24
         3.5.4.3. Use of TMMBR in a Mixer Based Multipoint
                  Operation.......................................30
         3.5.4.4. Use of TMMBR in Point-to-Multipoint Using
                  Multicast or Translators........................32
         3.5.4.5. Use of TMMBR in Point-to-point operation........32
         3.5.4.6. Reliability.....................................32
4. RTCP Receiver Report Extensions................................34
   4.1. Design Principles of the Extension Mechanism..............34
   4.2. Transport Layer Feedback Messages.........................35
      4.2.1. Temporary Maximum Media Stream Bit Rate Request
             (TMMBR)..............................................36
         4.2.1.1. Message Format..................................36
         4.2.1.2. Semantics.......................................37
         4.2.1.3. Timing Rules....................................40
         4.2.1.4. Handling in Translator and Mixers...............40
      4.2.2. Temporary Maximum Media Stream Bit Rate Notification
             (TMMBN)..............................................41
         4.2.2.1. Message Format..................................41
         4.2.2.2. Semantics.......................................41
         4.2.2.3. Timing Rules....................................43
         4.2.2.4. Handling by Translators and Mixers..............43
   4.3. Payload Specific Feedback Messages........................43


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      4.3.1. Full Intra Request (FIR).............................44
         4.3.1.1. Message Format..................................44
         4.3.1.2. Semantics.......................................45
         4.3.1.3. Timing Rules....................................47
         4.3.1.4. Handling of FIR Message in Mixer and
                  Translators.................................... 47
         4.3.1.5. Remarks.........................................47
      4.3.2. Temporal-Spatial Trade-off Request (TSTR)............47
         4.3.2.1. Message Format..................................47
         4.3.2.2. Semantics.......................................48
         4.3.2.3. Timing Rules....................................49
         4.3.2.4. Handling of message in Mixers and Translators...49
         4.3.2.5. Remarks.........................................49
      4.3.3. Temporal-Spatial Trade-off Notification (TSTN).......50
         4.3.3.1. Message Format..................................50
         4.3.3.2. Semantics.......................................50
         4.3.3.3. Timing Rules....................................51
         4.3.3.4. Handling of TSTN in Mixer and Translators.......51
         4.3.3.5. Remarks.........................................51
      4.3.4. H.271 Video Back Channel Message (VBCM)..............51
         4.3.4.1. Message Format..................................52
         4.3.4.2. Semantics.......................................52
         4.3.4.3. Timing Rules....................................54
         4.3.4.4. Handling of message in Mixer or Translator......54
         4.3.4.5. Remarks.........................................54
5. Congestion Control.............................................54
6. Security Considerations........................................55
7. SDP Definitions................................................56
   7.1. Extension of the rtcp-fb Attribute........................56
   7.2. Offer-Answer..............................................58
   7.3. Examples..................................................58
8. IANA Considerations............................................61
9. Acknowledgements...............................................62
10. References....................................................63
   10.1. Normative references.....................................63
   10.2. Informative references...................................63
11. Authors' Addresses............................................64














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1.1. Introduction

   When the Audio-Visual Profile with Feedback (AVPF) [RFC4585] was
   developed, the main emphasis lay 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 support centralized multipoint
   conferencing efficiently.  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] bit strings for Video Back
   Channel messages.

   In Real-time Transport Protocol (RTP) [RFC3550] terminology, MCUs
   comprise mixers and translators.  Most MCUs also include signaling
   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 are not documented in
   sufficient detail in [RFC3550].  These topologies are documented in
   [Topologies], and understanding this memo requires previous or
   parallel study of [Topologies].

   Some of the messages defined here are forward only, in that they do
   not require an explicit notification to the message emitter that they
   have been received and/or indicating the message receiver's actions.
   Other messages require a response, leading to a two way communication
   model that one could view as useful for control purposes.  However,
   it is not the intention of this memo to open up RTP Control Protocol
   (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 Session Initiation
   Protocol (SIP) re-INVITEs [RFC3261], undesirable when used for the
   same purpose.  Furthermore, all messages are of a very simple format
   that can be easily processed by an RTP/RTCP sender/receiver.
   Finally, all messages relate only to the RTP stream with which they
   are associated, and not to any other property of a communication
   system.  In particular, none of them relate to the properties of the
   access links traversed by the session.




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2. Definitions

2.1. Glossary

   AMID   - Additive Increase Multiplicative Decrease
   AVPF   - The extended RTP profile for RTCP-based feedback
   FEC    - Forward Error Correction
   FCI    - Feedback Control Information [RFC4585]
   FIR    - Full Intra Request
   MCU    - Multipoint Control Unit
   MPEG   - Moving Picture Experts Group
   TMMBN  - Temporary Maximum Media Stream Bit Rate Notification
   TMMBR  - Temporary Maximum Media Stream Bit Rate Request
   PLI    - Picture Loss Indication
   PR     - Packet rate
   QP     - Quantizer Parameter
   RTT    - Round trip time
   SSRC   - Synchronization Source
   TSTN   - Temporal Spatial Trade-off Notification
   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:
          An RTCP feedback message [RFC4585] defined by this
          specification, of one of the following types:

          Request:
              Message that requires acknowledgement

          Command:
              Message that forces the receiver to an action

          Indication:
              Message that reports a situation

          Notification:



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             Message that provides a notification that an event has
              occurred. Notifications are commonly generated in response
              to a Request.

          Note that, with the exception of "Notification", this
          terminology is in alignment with ITU-T Rec. H.245 [H245].

     Decoder Refresh Point:
          A bit string, packetized in one or more RTP packets, which
          completely resets the decoder to a known state.

          Examples for "hard" decoder refresh points are Intra pictures
          in H.261, H.263, MPEG-1, MPEG-2, and MPEG-4 part 2, and
          Instantaneous Decoder Refresh (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
          Network Adaptation Layer (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).

   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 media-aware, implying
          that media packets are removed in the order of increasing
          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.  In contrast to transcoding, stream
          thinning is typically seen as a computationally lightweight
          operation.

   Media:



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          Often used (sometimes in conjunction with terms like bit rate,
          stream, sender ...) to identify the content of the forward RTP
          packet stream (carrying the codec data), to which the codec
          control message applies.

   Media Stream:
          The stream of RTP packets labeled with a single
          Synchronization Source (SSRC) carrying the media (and also in
          some cases repair information such as retransmission or
          Forward Error Correction (FEC) information).

   Total media bit rate:
          The total bits per second transferred in a media stream,
          measured at an observer-selected protocol layer and averaged
          over a reasonable timescale, the length of which depends on
          the application.  In general, a media sender and a media
          receiver will observe different total media bit rates for the
          same stream, first because they may have selected different
          reference protocol layers, and second, because of changes in
          per-packet overhead along the transmission path.  The goal
          with bit rate averaging is to be able to ignore any burstiness
          on very short timescales, below for example 100 ms, introduced
          by scheduling or link layer packetization effects.

   Maximum total media bit rate:
          The upper limit on total media bit rate for a given media
          stream at a particular receiver and for its selected protocol
          layer. Note that this value cannot be measured on the received
          media stream, instead it needs to be calculated or determined
          through other means, such as QoS negotiations or local
          resource limitations. Also note that this value is an average
          (on a timescale that is reasonable for the application) and
          that it may be different from the instantaneous bit-rate seen
          by packets in the media stream.

   Overhead:
          All protocol header information required to convey a packet
          with media data from sender to receiver, from the application
          layer down to a pre-defined protocol level (for example down
          to, and including, the IP header).  Overhead may include, for
          example, IP, UDP, and RTP headers, any layer 2 headers, any
          Contributing Sources (CSRCs), RTP-Padding, and RTP header
          extensions.  Overhead excludes any RTP payload headers and the
          payload itself.

   Net media bit rate:
          The bit rate carried by a media stream, net of overhead.  That
          is, the bits per second accounted for by encoded media, any



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          applicable payload headers, and any directly associated meta
          payload information placed in the RTP packet.  A typical
          example of the latter is redundancy data provided by the use
          of RFC 2198 [RFC2198].  Note that, unlike the total media bit
          rate, the net media bit rate will have the same value at the
          media sender and at the media receiver unless any mixing or
          translating of the media has occurred.

          For a given observer, the total media bit rate for a media
          stream is equal to the sum of the net media bit rate and the
          per-packet overhead as defined above multiplied by the packet
          rate.

   Feasible region:
          The set of all combinations of packet rate and net media bit
          rate that do not exceed the restrictions in maximum media bit
          rate placed on a given media sender by the Temporary Maximum
          Media Stream Bit-rate Request (TMMBR)  messages it has
          received.  The feasible region will change as new TMMBR
          messages are received.

   Bounding set:
          The set of TMMBR tuples, selected from all those received at a
          given media sender, that define the feasible region for that
          media sender.  The media sender uses an algorithm such as that
          in section 3.5.4.2 to determine or iteratively approximate the
          current bounding set, and reports that set back to the media
          receivers in a Temporary Maximum Media Stream Bit-rate
          Notification (TMMBN) message.

2.3. Topologies

   Please refer to [Topologies] for an in depth discussion.  The
   topologies referred to throughout this memo are labeled (consistently
   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









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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 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 us begin by looking through the use cases Basso et al.
   [Basso] proposed.  Some of the use cases have been reformulated and
   comments have been added.

   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 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 that the
      desired trade-off between temporal and spatial resolution has
      changed.  For example, one user may prefer a higher frame rate and
      a lower spatial quality, and another user 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.






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   4. Use case 4 of the Basso draft applies only to Picture Loss
      Indication (PLI) as defined in AVPF [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 trans-rating.  One way of achieving this is to set up
      sessions with endpoints using the maximum bit rate accepted by
      each endpoint, and accepted by the call admission method used by
      the mixer.  By means of commands that reduce the maximum media
      stream bit rate below what has been negotiated during session set
      up, the mixer can reduce the maximum bit rate sent by endpoints to
      the lowest of all the accepted bit rates.  As the lowest accepted
      bit rate changes due to endpoints joining and leaving or due to
      network congestion, the mixer can adjust the limits at which
      endpoints can send their streams to match the new value.  The
      mixer then requests a new maximum bit rate, which is equal to 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. Currently deployed congestion control algorithms (AMID and TFRC
      [RFC3448]) probe for additional available capacity as long as
      there is something to send.  With congestion control algorithms
      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.  A media receiver behind this link can, in most (if not
      all) cases, calculate at least an upper bound for the bit rate
      available for each media stream it presently receives.  How this



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      is done is an implementation detail and not discussed herein.
      Indicating the maximum available bit rate to the transmitting
      party for the various media streams can be beneficial to prevent
      that party from probing for bandwidth for this stream in excess of
      a known hard limit.  For cellular or other mobile devices, the
      known available bit rate for each stream (deduced from the link
      bit rate) can change quickly, due to handover to another
      transmission technology, QoS renegotiation due to congestion, etc.
      To enable minimal disruption of service, quick convergence is
      necessary, and therefore media path signaling is desirable.

    8. The use of reference picture selection (RPS) as an error
       resilience tool has been introduced in 1997 as NEWPRED [NEWPRED],
       and is now widely deployed.  When RPS is in use, simplistically
       put, the receiver can send a feedback message to the sender,
       indicating a reference picture that should be used for future
       prediction. ([NEWPRED] mentions other forms of feedback as well.)
       AVPF contains a mechanism for conveying such a message, but did
       not specify for which codec and according to which syntax the
       message should conform.  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 fairly
       quickly enjoy wide support.  Therefore, a 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 use the media path for the codec
   control messages.

   First, systems employing MCUs often separate the control and media
   processing parts.  As these messages are intended for or generated by
   the media part rather than the signaling part of the MCU, having them
   on the media path avoids transmission across interfaces and
   unnecessary control traffic between signaling and processing.  If the
   MCU is physically decomposed, the use of the media path avoids the
   need for media control protocol extensions (e.g. in MEGACO
   [RFC3525]).

   Secondly, the signaling path quite commonly contains several
   signaling entities, e.g. SIP proxies and application servers.
   Avoiding going through signaling 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 signaling entities are also commonly not optimized for minimal
   delay, but rather towards other architectural goals.  Thus the




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   signaling path can be significantly longer in both geographical and
   delay sense.


3.3. Using AVPF

   The AVPF feedback message framework [RFC4585] provides the
   appropriate framework to implement the new messages.  AVPF implements
   rules controlling the timing of feedback messages to avoid congestion
   through network flooding by RTCP traffic.  We re-use these rules by
   referencing AVPF.

   The signaling setup for AVPF allows each individual type of function
   to be configured or negotiated on an 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 codec control messages might be used with multicast.  The RTCP
   timing rules specified in [RFC3550] and [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.



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   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 bit rate 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, plus any AVPF-induced
   RTCP packet sending delays, if those are 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 the specified delay
   is to avoid sending unnecessary decoder refresh points.  A session
   participant may have sent its own request while another participant's
   request was in-flight to them.  Suppressing those requests that may
   have been sent without knowledge about the other request avoids this
   issue.

   Using the FIR command to recover from errors is explicitly
   disallowed, and instead the PLI message defined in AVPF [RFC4585]
   should be used.  The PLI message reports lost pictures and has been
   included in AVPF for precisely that purpose.

   Full Intra Request is applicable in use-cases 1 and 2.

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 notification, 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.



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   To ensure the best possible reliability, a sender of FIR may repeat
   the FIR request until the desired content has been received.  The
   repetition interval is determined by the RTCP timing rules applicable
   to the session.  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.  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.

   The receiver of FIR (i.e. the media sender) behaves in complementary
   fashion to ensure delivery of a decoder refresh point.  If it
   receives repetitions of the FIR more than 2*RTT after it has sent a
   decoder refresh point, it shall send a new decoder refresh point.
   Two round trip times allow time for the decoder refresh point to
   arrive back to the requestor and for the end of repetitions of FIR to
   reach and be detected by the media sender.

   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 for the mixer to
   generate FIR commands.  From a reliability perspective, the two legs
   (FIR-requesting endpoint to mixer, and mixer to decoder refresh point
   generating endpoint) are handled independently from each other.


3.5.2. Temporal Spatial Trade-off Request and Notification

   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.  That is, a requester asking for an
   index of 0 prefers a high quality and is willing to accept a low
   frame rate, whereas a requester asking for 31 wishes a high frame
   rate, potentially at the cost of low spatial quality.

   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 whether and to what extent the request results in a
   change of the trade-off.  It returns a Temporal Spatial Trade-Off
   Notification (TSTN) message to indicate the trade-off that it will
   use henceforth.

   TSTR and TSTN 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.



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   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 [RFC3261] would require at least two round-trips more
   (compared to the TSTR/TSTN mechanism) and may involve proxies and
   other complex mechanisms.  Even in a well-designed system, it could
   take a second or so until finally the new trade-off is selected.
   Furthermore the use of RTCP solves the multicast use case very
   efficiently.

   The use of TSTR and TSTN in multipoint scenarios is a non-trivial
   subject, and can be achieved in many implementation-specific ways.
   Problems stem 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
   translator, mixer 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 TSTN, carrying
   the new trade-off chosen by its own criteria (which may or may not be
   based on the trade-off conveyed by the TSTR).  In other words, the
   trade-off sent in TSTR is a non-binding recommendation, nothing more.

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


3.5.2.1. Point-to-Point

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


3.5.2.2. Point-to-Multipoint Using Multicast or Translators

   RTCP Multicast is used either with media multicast according to Topo-
   Multicast, or following RFC 3550's translator model according to
   Topo-Translator.  In these cases, unsynchronized TSTR messages from
   different receivers may be received, 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, giving priority to certain participants,



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   or continuing to use 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 TSTN, 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 mixer 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).  As in the previous scenario, the strategy for forming
   this "consensus" is up to the implementation, and can, for example,
   encompass averaging the participants' request values, giving priority
   to certain participants, or using session default values.

   Even if a mixer or translator performs transcoding, it is very
   difficult to deliver media with the requested trade-off, unless the
   content the mixer or translator receives is already close to that
   trade-off.  Thus if the mixer changes its trade-off, it needs to
   request the media sender(s) to use 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 to the downstream
   requestors.  Only in cases where the original source has
   substantially higher quality (and bit rate), is it likely that
   transcoding alone 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 Notification (TSTN) 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 knowing 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




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   ITU-T Rec. H.271 defines syntax, semantics, and suggested encoder
   reaction to a video back channel message.  The structure defined in
   this memo is used to transparently convey such a message from media
   receiver to media sender.  In this memo, we refrain from an in-depth
   discussion of the available code points within H.271 and refer to the
   specification text [H.271] instead.

   However, we note that some H.271 messages bear similarities with
   native messages of AVPF and this memo.  Furthermore, we note that
   some H.271 message are known to require caution in multicast
   environments -- or are plainly not usable in multicast or multipoint
   scenarios.  Table 1 provides a brief, oversimplifying overview of the
   messages currently defined in H.271, their roughly corresponding AVPF
   or CCM messages (the latter as specified in this memo), and an
   indication of our current knowledge of 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 picture loss       AVPF PLI         Yes
   2 partial loss       AVPF SLI         Yes
   3 one parameter CRC  N/A              Yes (no required sender action)
   4 all parameter CRC  N/A              Yes (no required sender action)
   5 refresh point      CCM FIR          Yes

   Table 1: H.271 messages and their AVPF/CCM equivalents


          Note: H.271 message type 0 is not a strict equivalent to
          AVPF's Reference Picture Selection Indication (RPSI); it is an
          indication of known-as-correct reference picture(s) at the
          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 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.




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   2. An implementation SHOULD utilize the IETF-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 1 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 table indicates.

   3. It has been observed that some of the H.271 code points currently
      in existence are not multicast-safe.  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 documented below, the implementation of such a
      mixer and cooperative endpoints is a very difficult and tedious
      task.  Therefore, H.271 messages 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 to be taken into account when considering the use of H.271 in
   multipoint environments:

   1. Different state on different receivers.  In many environments it
      cannot be guaranteed that the decoder state of all media receivers
      is identical at any given point in time.  The most obvious reason
      for such a possible misalignment of state is a loss that occurs on
      the path to only one of many media receivers.  However, there are
      other not so obvious reasons, such as recent joins to the
      multipoint conference (be it by joining the multicast group or
      through additional mixer output).  Different states can lead the
      media receivers to issue potentially contradicting H.271 messages
      (or one media receiver issuing an H.271 message that, when
      observed by the media sender, is not helpful for the other media
      receivers).  A naive reaction of the media sender to these
      contradicting messages can lead to unpredictable and annoying
      results.

   2. Combining messages from different media receivers in a media
      sender is a non-trivial task.  As reasons, we note that these
      messages may be contradicting each other, and that their transport



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      is unreliable (there may well be other reasons).  In case of many
      H.271 messages (i.e. types 0, 2, 3, and 4), the algorithm for
      combining must be aware both of the network/protocol environment
      (i.e. with respect to congestion) and of the media codec employed,
      as H.271 messages of a given type can have different semantics for
      different media codecs.

   3. The suppression of requests may need to go beyond the basic
      mechanisms described in AVPF (which are driven exclusively by
      timing and transport considerations on the protocol level).  For
      example, a receiver is often required to refrain from (or delay)
      generating requests, based on information it receives from the
      media stream.  For instance, it makes no sense for a receiver to
      issue a FIR when a transmission of an Intra/IDR picture is
      ongoing.

   4. When using the non-multicast-safe messages (e.g. H.271 type 0
      positive ACK of received pictures/slices) in larger multicast
      groups, the media receiver will likely be forced to delay or even
      omit sending these messages.  For the media sender this looks like
      data has not been properly received (although it was received
      properly), and a naively implemented media sender reacts to these
      perceived problems where it should not.

3.5.3.1. Reliability

   H.271 Video Back Channel messages do not require reliable
   transmission, and confirmation of 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 Stream Bit Rate Request and Notification

   A receiver, translator or mixer uses the Temporary Maximum Media
   Stream Bit Rate Request (TMMBR, "timber") to request a sender to
   limit the maximum bit rate for a media stream (see 2.2) to, or below,
   the provided value.  The Temporary Maximum Media Stream Bit Rate
   Notification (TMMBN) contains the media sender's current view of the
   most limiting subset of the TMMBR-defined limits it has received, to
   help the participants to suppress TMMBR requests that would not
   further restrict the media sender.  The primary usage for the
   TMMBR/TMMBN messages is in a scenario with an MCU or mixer (use case
   6), corresponding to Topo-Translator or Topo-Mixer, but also to Topo-
   Point-to-Point.




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   Each temporary limitation on the media stream is expressed as a
   tuple.  The first component of the tuple is the maximum total media
   bit rate (as defined in section 2.2) that the media receiver is
   currently prepared to accept for this media stream.  The second
   component is the per-packet overhead that the media receiver has
   observed for this media stream at its chosen reference protocol
   layer.

   As indicated in section 2.2, the overhead as observed by the sender
   of the TMMBR (i.e. the media receiver) may differ from the overhead
   observed at the receiver of the TMMBR (i.e. the media sender) due to
   use of a different reference protocol layer at the other end or due
   to the intervention of translators or mixers that affect the amount
   of per packet overhead.  For example, a gateway in between the two
   that converts between IPv4 and IPv6 affects the per-packet overhead
   by 20 bytes.  Other mechanisms that change the overhead include
   tunnels.  The problem with varying overhead is also discussed in
   [RFC3890].  As will be seen in the description of the algorithm for
   use of TMMBR, the difference in perceived overhead between the
   sending and receiving ends presents no difficulty because
   calculations are carried out in terms of variables (packet rate, net
   media bit rate) that have the same value at the sender as at the
   receiver.

   Reporting both maximum total media bit rate and per-packet overhead
   allows different receivers to provide bit rate and overhead values
   for different protocol layers, for example at the IP level, at the
   outer part of a tunnel protocol, or at the link layer.  The protocol
   level a peer reports on depends on the level of integration the peer
   has, as it needs to be able to extract the information from that
   protocol level.  For example, an application with no knowledge of the
   IP version it is running over can not meaningfully determine the
   overhead of the IP header, and hence will not want to include IP
   overhead in the overhead or maximum total media bit rate calculation.

   It is expected that most peers will be able to report values at least
   for the IP layer.  In certain implementations it may be advantageous
   to also include information pertaining to the link layer, which in
   turn allows for a more precise overhead calculation and a better
   optimization of connectivity resources.

   The Temporary Maximum Media Stream Bit Rate messages are generic
   messages that can be applied to any RTP packet stream.  This
   separates them from the other codec control messages defined in this
   specification, which apply only to specific media types or payload
   formats.  The TMMBR functionality applies to the transport, and the
   requirements the transport places on the media encoding.




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   The reasoning below assumes that the participants have negotiated a
   session maximum bit rate, using a signaling 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 either case, the bit rate negotiated in signaling
   is the one that the participant guarantees to be able to handle
   (depacketize and decode).  In practice, the connectivity of the
   participant also influences the negotiated value -- it does not make
   much sense to negotiate a total media bit rate that one's network
   interface does not support.

   It is also beneficial to have negotiated a maximum packet rate for
   the session or sender.  RFC 3890 provides an SDP [RFC4566] attribute
   that can be used for this purpose; however, that attribute is not
   usable in RTP sessions established using offer/answer [RFC3264].
   Therefore an optional maximum packet rate signaling parameter is
   specified in this memo.

   An already established maximum total media bit rate may be changed at
   any time, subject to the timing rules governing the sending of
   feedback messages. The limit may change to any value between zero and
   the session maximum, as negotiated during session establishment
   signaling.  However, even if a sender has received a TMMBR message
   allowing an increase in the bit rate, all increases must be governed
   by a congestion control mechanism.  TMMBR indicates known limitations
   only, usually in the local environment, and does not provide any
   guarantees about the full path.  Furthermore, any increases in TMMBR-
   established bit rate limits are to be executed only after a certain
   delay from the sending of the TMMBN message that notifies the world
   about the increase in limit.  The delay is specified as at least
   twice the longest RTT as known by the media sender, plus the media
   sender's calculation of the required wait time for the sending of
   another TMMBR message for this session based on AVPF timing rules.
   This delay is introduced to allow other session participants to make
   known their bit rate limit requirements, which may be lower.

   If it is likely that the new value indicated by TMMBR will be valid
   for the remainder of the session, the TMMBR sender is expected to
   perform a renegotiation of the session upper limit using the session
   signaling protocol.

3.5.4.1. Behavior for media receivers using TMMBR

   This section is an informal description of behaviour described more
   precisely in section 4.2.

   A media sender begins the session limited by the maximum media bit
   rate and maximum packet rate negotiated in session signaling, if any.



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   Note that this value may be negotiated for another protocol layer
   than the one the participant uses in its TMMBR messages.  Each media
   receiver selects a reference protocol layer, forms an estimate of the
   overhead it is observing (or estimating it if no packets has been
   seen yet) at that reference level, and determines the maximum total
   media bit rate it can accept, taking into account its own limitations
   and any transport path limitations of which it may be aware.  In case
   the current limitations are more restricting then what was agreed on
   in the session signaling, the media receiver reports its initial
   estimate of these two quantities to the media sender using a TMMBR
   message.  Overall message traffic is reduced by the possibility of
   including tuples for multiple media senders in the same TMMBR
   message.

   The media sender applies an algorithm such as that specified in
   section 3.5.4.2 to select which of the tuples it has received are
   most limiting (i.e. the bounding set as defined in section 2.2).  It
   modifies its operation to stay within the feasible region (as defined
   in section 2.2), and also sends out a TMMBN notification to the media
   receivers indicating the selected bounding set.

   If a media receiver does not own one of the tuples in the bounding
   set reported by the TMMBN, it applies the same algorithm as the media
   sender to determine if its current estimated (maximum total media bit
   rate, overhead) tuple would enter the bounding set if known to the
   media sender.  If so, it issues a TMMBR request reporting the tuple
   value to the sender.  Otherwise it takes no action for the moment.
   Periodically, its estimated tuple values may change or it may receive
   a new TMMBN.  If so, it reapplies the algorithm to decide whether it
   needs to issue a TMMBR request.

   If, alternatively, a media receiver owns one of the tuples in the
   reported bounding set, it takes no action until such time as its
   estimate of its own tuple values changes.  At that time it sends a
   TMMBR request to the media sender to report the changed values.

   A media receiver may change status between owner and non-owner of a
   bounding tuple between one TMMBN message and the next.  Thus it must
   check the contents of each TMMBN to determine its subsequent actions.

   Implementations may use other algorithms of their choosing, as long
   as the bit rate limitations resulting from the exchange of TMMBR and
   TMMBN messages are at least as strict (at least as low, in the bit
   rate dimension) as the ones resulting from the use of the
   aforementioned algorithm.

   Obviously, in point-to-point cases, when there is only one media
   receiver, this receiver becomes "owner" once it receives the first



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   TMMBN in response to its own TMMBR, and stays "owner" for the rest of
   the session.  Therefore, when it is known that there will always be
   only a single media receiver, the above algorithm is not required.
   Media receivers that are aware they are the only ones in a session
   can send TMMBR messages with bit rate limits both higher and lower
   than the previously notified limit, at any time (subject to the AVPF
   [RFC4585] RTCP RR send timing rules).  However, it may be difficult
   for a session participant to determine if it is the only receiver in
   the session.  Because of this any implementation of TMMBR is required
   to include the algorithm described in the next section or a stricter
   equivalent.

3.5.4.2. Algorithm for establishing current limitations

   This section introduces an example algorithm for the calculation of a
   session limit.  Other algorithms can be employed, as long as the
   result of the calculation is at least as restrictive as the result
   that is obtained by this algorithm.

   First it is important to consider the implications of using a tuple
   for limiting the media sender's behavior.  The bit rate and the
   overhead value result in a two-dimensional solution space for the
   calculation of the bit rate of media streams.  Fortunately the two
   variables are linked. Specifically, the bit rate available for RTP
   payloads is equal to the TMMBR reported bit rate minus the packet
   rate used, multiplied by the TMMBR reported overhead converted to
   bits.  As a result, when different bit rate/overhead combinations
   need to be considered, the packet rate determines the correct
   limitation.  This is perhaps best explained by an example:

   Example:

   Receiver A: TMMBR_max total BR = 35 kbps, TMMBR_OH = 40 bytes
   Receiver B: TMMBR_max total BR = 40 kbps, TMMBR_OH = 60 bytes

   For a given packet rate (PR) the bit rate available for media
   payloads in RTP will be:

   Max_net media_BR_A = TMMBR_max total BR_A - PR * TMMBR_OH_A * 8 ...
   (1)
   Max_net media_BR_B = TMMBR_max total BR_B - PR * TMMBR_OH_B * 8 ...
   (2)

   For a PR = 20 these calculations will yield a Max_net media_BR_A =
   28600 bps and Max_net media_BR_B = 30400 bps, which suggests that
   receiver A is the limiting one for this packet rate.  However at a
   certain PR there is a switchover point at which receiver B becomes




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   the limiting one.  The switchover point can be identified by setting
   Max_media_BR_A equal to Max_media_BR_B and breaking out PR:


         TMMBR_max total BR_A - TMMBR_max total BR_B
   PR =  ------------------------------------------- ... (3)
                8*(TMMBR_OH_A - TMMBR_OH_B)

   which, for the numbers above yields 31.25 as the switchover point
   between the two limits.  That is, for packet rates below 31.25 per
   second, receiver A is the limiting receiver, and for higher packet
   rates, receiver B is more limiting.  The implications of this
   behavior have to be considered by implementations that are going to
   control media encoding and its packetization.  As exemplified above,
   multiple TMMBR limits may apply to the trade-off between net media
   bit rate and packet rate.  Which limitation applies depends on the
   packet rate being considered.

   This also has implications for how the TMMBR mechanism needs to work.
   First, there is the possibility that multiple TMMBR tuples are
   providing limitations on the media sender.  Secondly there is a need
   for any session participant (media sender and receivers) to be able
   to determine if a given tuple will become a limitation upon the media
   sender, or if the set of already given limitations is stricter than
   the given values.  In the absence of the ability to make this
   determination the suppression of TMMBR requests would not work.

   The basic idea of the algorithm is as follows.  Each TMMBR tuple can
   be viewed as the equation of a straight line (cf. equations (1) and
   (2)) in a space where packet rate lies along the X-axis and maximum
   bit rate lies along the Y-axis. The lower envelope of the set of
   lines corresponding to the complete set of TMMBR tuples defines a
   polygon. Points lying along or below this polygon are combinations of
   packet rate and bit rate that meet all of the TMMBR constraints. The
   highest feasible packet rate within this region is the minimum of the
   rate at which the bounding polygon meets the X-axis or the session
   maximum packet rate (SMAXPR) provided by signaling, if any. Typically
   a media sender will prefer to operate at a lower rate than this
   theoretical maximum, so as to increase the rate at which actual media
   content reaches the receivers.  The purpose of the algorithm is to
   distinguish the TMMBR tuples constituting the bounding set and thus
   delineate the feasible region, so that the media sender can select
   its preferred operating point within that region

   Figure 1 below shows a bounding polygon formed by TMMBR tuples A and
   B. A third tuple C lies outside the bounding polygon and is therefore
   irrelevant in determining feasible tradeoffs between media rate and
   packet rate.  The line labeled ss..s represents the limit on packet



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   rate imposed by the session maximum packet rate (SMAXPR) obtained by
   signaling during session setup.  In Figure 1 the limit determined by
   tuple B happens to be more restrictive than SMAXPR.  The situation
   could easily be the reverse, meaning that the bounding polygon is
   terminated on the right by the vertical line representing the SMAXPR
   constraint.



        ^
        |a   c   b             s
   Bit  |  a   c  b            s
   Rate |    a   c b           s
        |      a   cb          s
        |        a   c         s
        |          a  bc       s
        |            a b c     s
        |              ab  c   s
        |  Feasible      b   c s
        |   region        ba   s
        |                  b a s c
        |                   b  s   c
        |                    b s a
        |_____________________bs________
        +------------------------------>____________

              Packet rate

    Figure 1 - Geometric Interpretation of TMMBR Tuples

   Note that the slopes of the lines making up the bounding polygon are
   increasingly negative as one moves in the direction of increasing
   packet rate.  Note also that with slight rearrangement, equations (1)
   and (2) have the canonical form:

          y = mx + b

   where
     m is the slope and has value equal to the negative of the tuple
     overhead (in bits),
   and
     b is the y-intercept and has value equal to the tuple maximum total
     media bit rate.

   These observations lead to the conclusion that when processing the
   TMMBR tuples to select the initial bounding set, one should sort and
   process the tuples by order of increasing overhead. Once a particular
   tuple has been added to the bounding set, all tuples not already



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   selected and having lower overhead can be eliminated, because the
   next side of the bounding polygon has to be steeper (i.e. the
   corresponding TMMBR must have higher overhead) than the latest added
   tuple.

   Line cc..c in Figure 1 illustrates another principle. This line is
   parallel to line aa..a, but has a higher Y-intercept.  That is, the
   corresponding TMMBR tuple contains a higher maximum total media bit
   rate value.  Since line cc..c is outside the bounding polygon, it
   illustrates the conclusion that if two TMMBR tuples have the same
   overhead value, the one with higher maximum total media bit rate
   value cannot be part of the bounding set and can be set aside.

   Two further observations complete the algorithm.  Obviously, moving
   from the left, the successive corners of the bounding polygon (i.e.
   the intersection points between successive pairs of sides) lie at
   successively higher packet rates.  On the other hand, again moving
   from the left, each successive line making up the bounding set
   crosses the X-axis at a lower packet rate.

   The complete algorithm can now be specified.  The algorithm works
   with two lists of TMMBR tuples, the candidate list X and the selected
   list Y, both ordered by increasing overhead value.  The algorithm
   terminates when all members of X have been discarded or removed for
   processing.  Membership of the selected list Y is probationary until
   the algorithm is complete.  Each member of the selected list is
   associated with an intersection value, which is the packet rate at
   which the line corresponding to that TMMBR tuple intersects with the
   line corresponding to the previous TMMBR tuple in the selected list.
   Each member of the selected list is also associated with a maximum
   packet rate value, which is the lesser of the session maximum packet
   rate SMAXPR (if any) and the packet rate at which the line
   corresponding to that tuple crosses the X-axis.

   When the algorithm terminates, the selected list is equal to the
   bounding set as defined in section 2.2.

Initial Algorithm

   This algorithm is used by the media sender when it has received one
   or more TMMBR requests and before it has determined a bounding set
   for the first time.

   1. Sort the TMMBR tuples by order of increasing overhead.  This is
      the initial candidate list X.

   2. When multiple tuples in the candidate list have the same
      overhead value, discard all but the one with the lowest maximum



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      total media bit rate value.

   3. Select and remove from the candidate list the TMMBR tuple with the
      lowest maximum total media bit rate value.  If there is more than
      one tuple with that value, choose the one with the highest
      overhead value.  This is the first member of the selected list Y.
      Set its intersection value equal to zero.  Calculate its maximum
      packet rate as the minimum of SMAXPR (if available) and the value
      obtained from the following formula, which is the packet rate at
      which the corresponding line crosses the X-axis.

          Max PR = TMMBR max total BR / (8 * TMMBR OH) ... (4)

   4. Discard from the candidate list all tuples with a lower overhead
      value than the selected tuple.

   5. Remove the first remaining tuple from the candidate list for
      processing.  Call this the current candidate.

   6. Calculate the packet rate PR at the intersection of the line
      generated by the current candidate with the line generated by the
      last tuple in the selected list Y, using equation (3).

   7. If the calculated value PR is equal to or lower than the
      intersection value stored for the last tuple of the selected list,
      discard the last tuple of the selected list and go back to step 6
      (retaining the same current candidate).

      Note that the choice of the initial member of the selected list Y
      in step 3 guarantees that the selected list will never be emptied
      by this process, meaning that the algorithm must eventually (if
      not immediately) fall through to the step 8.

   8. (This step is reached when the calculated PR value of the current
      candidate is greater than the intersection value of the current
      last member of the selected list Y.)  If the calculated value PR
      of the current candidate is lower than the maximum packet rate
      associated with the last tuple in the selected list, add the
      current candidate tuple to the end of the selected list.  Store
      PR as its intersection value.  Calculate its maximum packet rate
      as the lesser of SMAXPR (if available) and the maximum packet
      rate calculated using equation (4).

   9. If any tuples remain in the candidate list, go back to step 5.

Incremental Algorithm





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   The previous algorithm covered the initial case, where no selected
   list had previously been created.  It also applied only to the media
   sender.  When a previously-created selected list is available at
   either the media sender or media receiver, two other cases can be
   considered:

        o when a TMMBR tuple not currently in the selected list is a
          candidate for addition;

        o when the values change in a TMMBR tuple currently in the
          selected list.

   At the media receiver these cases correspond respectively to those
   of the non-owner and owner of a tuple in the TMMBN-reported bounding
   set.

   In either case, the process of updating the selected list to take
   account of the new/changed tuple can use the basic algorithm
   described above, with the modification that the initial candidate
   set consists only of the existing selected list and the new or
   changed tuple.  Some further optimization is possible (beyond
   starting with a reduced candidate set) by taking advantage of the
   following observations.

   The first observation is that if the new/changed candidate becomes
   part of the new selected list, the result may be to cause zero or
   more other tuples to be dropped from the list.  However, if more than
   one other tuple is dropped, the dropped tuples will be consecutive.
   This can be confirmed geometrically by visualizing a new line that
   cuts off a series of segments from the previously-existing bounding
   polygon.  The cut-off segments are connected one to the next, the
   geometric equivalent of consecutive tuples in a list ordered by
   overhead value.  Beyond the dropped set in either direction all of
   the tuples that were in the earlier selected list will be in the
   updated one.  The second observation is that, leaving aside the new
   candidate, the order of tuples remaining in the updated selected list
   is unchanged because their overhead values have not changed.

   The consequence of these two observations is that, once the placement
   of the new candidate and the extent of the dropped set of tuples (if
   any) has been determined, the remaining tuples can be copied directly
   from the candidate list into the selected list, preserving their
   order.  This conclusion suggests the following modified algorithm:

       o Run steps 1-4 of the basic algorithm.

       o If the new candidate has survived steps 2 and 4 and has become
          the new first member of the selected list, run steps 5-9 on



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          subsequent candidates until another candidate is added to the
          selected list.  Then move all remaining candidates to the
          selected list, preserving their order.

       o If the new candidate has survived steps 2 and 4 and has not
          become the new first member of the selected list, start by
          moving all tuples in the candidate list with lower overhead
          values than that of the new candidate to the selected list,
          preserving their order.  Run steps 5 through 9 for the new
          candidate, with the modification that the intersection values
          and maximum packet rates for the tuples on the selected list
          have to be calculated on the fly because they were not
          previously stored.  Continue processing only until a
          subsequent tuple has been added to the selected list, then
          move all remaining candidates to the selected list, preserving
          their order.

          Note that the new candidate could be added to the selected
          list only to be dropped again when the next tuple is
          processed.  It can easily be seen that in this case the new
          candidate does not displace any of the earlier tuples in the
          selected list.  The limitations of ASCII art make this
          difficult to show in a figure.  Line cc..c in Figure 1 would
          be an example if it had a steeper slope (tuple C had a higher
          overhead value), but still intersected line aa..a beyond where
          line aa..a intersects line bb..b.

   The algorithm just described is approximate, because it does not take
   account of tuples outside the selected list.  To see how such tuples
   can become relevant, consider Figure 1 and suppose that the maximum
   total media bit rate in tuple A increases to the point that line
   aa..a moves outside line cc..c.  Tuple A will remain in the bounding
   set calculated by the media sender.  However, once it issues a new
   TMMBN, media receiver C will apply the algorithm and discover that
   its tuple C should now enter the bounding set.  It will issue a TMMBR
   request to the media sender, which will repeat its calculation and
   come to the appropriate conclusion.

   The rules of section 4.2 require that the media sender refrain from
   raising its sending rate until media receivers have had a chance to
   respond to the TMMBN.  In the example just given, this delay ensures
   that the relaxation of tuple A does not actually result in an attempt
   to send media at a rate exceeding the capacity at C.

3.5.4.3. Use of TMMBR in a Mixer Based Multipoint Operation

   Assume a small mixer-based multiparty conference is ongoing, as
   depicted in Topo-Mixer of [Topologies].  All participants have



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   negotiated a common maximum bit rate that this session can use.  The
   conference operates over a number of unicast paths between the
   participants and the mixer.  The congestion situation on each of
   these paths can be monitored by the participant in question and by
   the mixer, utilizing, for example, RTCP receiver reports (RR) or the
   transport protocol, e.g. DCCP [RFC4340].  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 mixer (which is
   aware of the congestion situation on all connections it manages) has
   no standardized means to inform media senders to slow down, short of
   forging its own receiver reports (which is undesirable).  In
   principle, a mixer confronted with such a situation is obliged to
   thin or transcode streams intended for connections that detected
   congestion.

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

   To allow the mixer to throttle traffic on the individual links,
   without performing transcoding, there is a need for a mechanism that
   enables the mixer to ask a participant's media encoders to limit the
   media stream bit rate they are currently generating.  TMMBR provides
   the required mechanism.  When the mixer detects congestion between
   itself and a given participant, it executes the following procedure:

   1. It starts thinning the media traffic to the congested participant
      to the supported bit rate.

   2. It uses TMMBR to request the media sender(s) to reduce the total
      media bit rate sent by them to the mixer, to a value that is in
      compliance with congestion control principles for the slowest
      link.  Slow refers here to the available bandwidth / bit rate /
      capacity and packet rate after congestion control.

   3. As soon as the bit rate has been reduced by the sending part, the
      mixer stops stream thinning implicitly, because there is no need
      for it once the stream is in compliance with congestion control.

   This use of stream thinning as an immediate reaction tool followed up
   by a quick control mechanism appears to be a reasonable compromise
   between media quality and the need to combat congestion.






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3.5.4.4. Use of TMMBR in Point-to-Multipoint Using Multicast or
   Translators

   In these topologies, corresponding to Topo-Multicast or Topo-
   Translator, RTCP RRs are transmitted globally.  This allows all
   participants to detect 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 for the use of TMMBR
   in the previous section does not apply.  However, even in this case
   the congestion control response can be improved when the unicast
   links are using congestion controlled transport protocols (such as
   TCP or DCCP).  A peer may also report local limitations to the media
   sender.


3.5.4.5. Use of TMMBR in Point-to-point operation

   In use case 7 it is possible to use TMMBR to improve the performance
   when the known upper limit of the bit rate changes.  In this use case
   the signaling protocol has established an upper limit for the session
   and total media bit rates.  However, at the time of transport link
   bit rate reduction, a receiver can avoid serious congestion by
   sending a TMMBR to the sending side.  Thus TMMBR is useful for
   putting restrictions on the application and thus placing the
   congestion control mechanism in the right ballpark.  However TMMBR is
   usually unable to provide the continuously quick feedback loop
   required for real congestion control.  Nor do its semantics match
   those of congestion control given its different purpose.  For these
   reasons TMMBR SHALL NOT be used as a substitute for congestion
   control.


3.5.4.6. 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 would
   not allow for easy suppression of other participants' requests.  For
   these reasons, a mechanism based on explicit notification is used.

   Upon the reception of a request a media sender sends a TMMBN
   notification containing the current bounding set, and indicating
   which session participants own that limit.  In multicast scenarios,
   that allows all other participants to suppress any request they may



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   have, if their limitations are less strict than the current ones
   (i.e. define lines lying outside the feasible region as defined in
   section 2.2).  Keeping and notifying only the bounding set of tuples
   allows for small message sizes and media sender states.  A media
   sender only keeps state for the SSRCs of the current owners of the
   bounding set of tuples; all other requests and their sources are not
   saved.  Once the bounding set has been established, new TMMBR
   messages should be generated only by owners of the bounding tuples
   and by other entities that determine (by applying the algorithm of
   section 3.5.4.2 or its equivalent) that their limitations should now
   be part of the bounding set.








































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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 Notification (TSTN), and Video Back Channel Message
   (VBCM) are "Payload Specific Feedback Messages" as defined in Section
   6.3 of AVPF [RFC4585].  The Temporary Maximum Media Stream Bit Rate
   Request (TMMBR) and Temporary Maximum Media Stream Bit Rate
   Notification (TMMBN) are "Transport Layer Feedback Messages" as
   defined in Section 6.2 of AVPF.

   The new feedback messages are defined in the following subsections,
   following a similar structure to that in sections 6.2 and 6.3 of the
   AVPF specification [RFC4585].


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 were introduced in early
   RTP payload formats for video formats, for example in RFC 4587
   [RFC4587].  However, this specification, for the first time, suggests
   a two-way handshake for some of its messages.  There is danger that
   this introduction could be misunderstood as a precedent for the use
   of RTCP as an RTP session control protocol.  To prevent such a
   misunderstanding, 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 [RFC4585], only such messages have been
   included as:

   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; and

   c) are directly related to activities of a certain media codec, class
      of media codecs (e.g. video codecs), or a given RTP packet stream.






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   In this memo, a two-way handshake is introduced only for messages for
   which:

   a) a notification or acknowledgement is required due to their nature.
      An analysis to determine whether this requirement exists has been
      performed separately for each message.

   b) the notification or acknowledgement cannot be easily derived from
      the media bit stream.

   All messages in AVPF [RFC4585] and in this memo present their
   contents in a simple, fixed binary format.  This accommodates media
   receivers which have not implemented higher control protocol
   functionalities (SDP, XML parsers and such) in their media path.

4.2. Transport Layer Feedback Messages

   As specified in section 6.1 of RFC 4585 [RFC4585], Transport Layer
   Feedback messages are identified by the RTCP packet type value RTPFB
   (205).

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

   Assigned in AVPF [RFC4585]:

      1:    Generic NACK
      31:   reserved for future expansion of the identifier number space

   Assigned in this memo:

      2:    reserved (see note below)
      3:    Temporary Maximum Media Stream Bit Rate Request (TMMBR)
      4:    Temporary Maximum Media Stream Bit Rate Notification (TMMBN)

          Note: early drafts of AVPF [RFC4585] reserved FMT=2 for a code
          point that has later been removed.  It has been pointed out
          that there may be implementations in the field using this
          value in accordance with the expired draft.  As there is
          sufficient numbering space available, we mark FMT=2 as
          reserved so to avoid possible interoperability problems with
          any such early implementations.

   Available for assignment:

      0:    unassigned
      5-30: unassigned



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   The following subsection defines the formats of the FCI entries for
   the TMMBR and TMMBN messages respectively and specify the associated
   behaviour at the media sender and receiver.


4.2.1. Temporary Maximum Media Stream Bit Rate Request (TMMBR)

   The FCI field of a Temporary Maximum Media Stream Bit-Rate Request
   (TMMBR) message SHALL contain one or more FCI entries.

4.2.1.1. 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                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | MxTBR Exp |  MxTBR Mantissa                 |Measured Overhead|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 2 - Syntax of an FCI entry in the TMMBR message


     SSRC (32 bits): The SSRC value of the media sender that is
              requested to obey the new maximum bit rate.

     MxTBR Exp (6 bits): The exponential scaling of the mantissa for the
              maximum total media bit rate value.  The value is an
              unsigned integer [0..63].

     MxTBR Mantissa (17 bits): The mantissa of the maximum total media
              bit rate value as an unsigned integer.

     Measured Overhead (9 bits): The measured average packet overhead
              value in bytes.  The measurement SHALL be done according
              to description in section 4.2.1.2. The value is an
              unsigned integer [0..512].


   The maximum total media bit rate (MxTBR) value in bits per second is
   calculated from the MxTBR exponent (exp) and mantissa in the
   following way:




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      MxTBR = mantissa * 2^exp

   This allows for 17 bits of resolution in the range 0 to 131072*2^63
   (approximately 1.2*10^24).

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

4.2.1.2. Semantics

Behaviour at the Media Receiver (Sender of the TMMBR)

   TMMBR is used to indicate a transport related limitation at the
   reporting entity acting as a media receiver.  TMMBR has the form of a
   tuple containing two components.  The first value is the highest bit
   rate per sender of a media stream, observed at a receiver-chosen
   protocol layer, which the receiver currently supports in this RTP
   session.  The second value is the measured header overhead in bytes
   as defined in section 2.2 and measured at the chosen protocol layer
   in the packets received for the stream.  The measurement of the
   overhead is a running average that is updated for each packet
   received for this particular media source (SSRC), using the following
   formula:

       avg_OH (new) = 15/16*avg_OH (old) + 1/16*pckt_OH,

   where avg_OH is the running (exponentially smoothed) average and
   pckt_OH is the overhead observed in the latest packet.

   If a maximum bit rate has been negotiated through signaling, the
   maximum total media bit rate that the receiver reports in a TMMBR
   message MUST NOT exceed the negotiated value converted to a common
   basis (i.e. with overheads adjusted to bring it to the same reference
   protocol layer).

   Within the common packet header for feedback messages (as defined in
   section 6.1 of [RFC4585]), 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.  Within a particular TMMBR FCI
   entry, the "SSRC of media sender" in the FCI field denotes the media
   sender the tuple applies to.  This is useful in the multicast or
   translator topologies where the reporting entity may address all of
   the media senders in a single TMMBR message using multiple FCI
   entries.

   The media receiver SHALL save the contents of the latest TMMBN
   message received from each media sender.




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   The media receiver MAY send a TMMBR FCI entry to a particular media
   sender under the following circumstances:

     o   before any TMMBN message has been received from that media
          sender;

     o   when the media receiver has been identified as the source of a
          bounding tuple within the latest TMMBN message received from
          that media sender, and the value of the maximum total media
          bit rate or the overhead relating to that media sender has
          changed;

     o   when the media receiver has not been identified as the source
          of a bounding tuple within the latest TMMBN message received
          from that media sender, and, after the media receiver applies
          the incremental algorithm from section 3.5.4.2 or a stricter
          equivalent, the media receiver's tuple relating to that media
          sender is determined to belong to the bounding set.

   A TMMBR FCI entry MAY be repeated in subsequent TMMBR messages if no
   Temporary Maximum Media Stream Bit-Rate Notification (TMMBN) FCI has
   been received from the media sender at the time of transmission of
   the next RTCP packet.  The bit rate value of a TMMBR FCI entry MAY be
   changed from one TMMBR message to the next.  The overhead measurement
   SHALL be updated to the current value of avg_OH each time the entry
   is sent.

   If the value set by a TMMBR message is expected to be permanent, the
   TMMBR setting party SHOULD renegotiate the session parameters to
   reflect that using session setup signaling, e.g. a SIP re-invite.

Behaviour at the Media Sender (Receiver of the TMMBR)

   When it receives a TMMBR message containing an FCI entry relating to
   it, the media sender SHALL use an initial or incremental algorithm as
   applicable to determine the bounding set of tuples based on the new
   information.  The algorithm used SHALL be at least as strict as the
   corresponding algorithm defined in section 3.5.4.2.  The media sender
   MAY accumulate TMMBR requests over a small interval (relative to the
   RTCP sending interval) before making this calculation.

   Once it has determined the bounding set of tuples, the media sender
   MAY use any combination of packet rate and net media bit rate within
   the feasible region that these tuples describe to produce a lower
   total media stream bit rate, as it may need to address a congestion
   situation or other limiting factors.  See section 5
 (congestion
   control) for more discussion.




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   If the media sender concludes that it can increase the maximum total
   media bit rate value, it SHALL wait before actually doing so, for a
   period long enough to allow a media receiver to respond to the TMMBN
   if it determines that its tuple belongs in the bounding set.  This
   delay period is estimated by the formula:

      2 * RTT + T_Dither_Max,

   where RTT is the longest round trip time known to the media sender
   and T_Dither_Max is defined in section 3.4 of [RFC4585].

   A TMMBN message SHALL be sent by the media sender at the earliest
   possible point in time, in response to any TMMBR messages received
   since the last sending of TMMBN.  The TMMBN message indicates the
   calculated set of bounding tuples and the owners of those tuples at
   the time of the transmission of the message.

   An SSRC may time out according to the default rules for RTP session
   participants, i.e. the media sender has not received any RTP or RTCP
   packets from the owner for the last five regular reporting intervals.
   An SSRC may also explicitly leave the session, with the participant
   indicating this through the transmission of an RTCP BYE packet or
   using an external signaling channel.  If the media sender determines
   that the owner of a tuple in the bounding set has left the session,
   the media sender shall transmit a new TMMBN containing the
   previously-determined set of bounding tuples but with the tuple
   belonging to the departed owner removed.

Discussion

   Due to the unreliable nature of transport of TMMBR and TMMBN, the
   above rules may lead to the sending of TMMBR messages which appear to
   disobey those rules.  Furthermore, in multicast scenarios it can
   happen that more than one "non-owning" session participant may
   determine, rightly or wrongly, that its tuple belongs in the bounding
   set.  This is not critical for a number of reasons:

   a) If a TMMBR message is lost in transmission, either the media
      sender sends a new TMMBN message in response to some other media
      receiver or it does not send a new TMMBN message at all.  In the
      first case, the media receiver applies the incremental algorithm
      and, if it determines that its tuple should be part of the
      bounding set, sends out another TMMBR.  In the second case, it
      repeats the sending of a TMMBR unconditionally.  Either way, the
      media sender eventually gets the information it needs.

   b) Similarly, if a TMMBN message gets lost, the media receiver that
      has sent the corresponding TMMBR request does not receive the



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      notification and is expected to re-send the request and trigger
      the transmission of another TMMBN.

   c) If multiple competing TMMBR messages are sent by different session
      participants, then the algorithm can be applied taking all of
      these messages into account, and the resulting TMMBN provides the
      participants with an updated view of how their tuples compare with
      the bounded set.

   d) If more than one session participant happens to send TMMBR
      messages at the same time and with the same tuple component
      values, it does not matter which if either tuple is taken into the
      bounding set.  The losing session participant will determine after
      applying the algorithm that its tuple does not enter the bounding
      set, and will therefore stop sending its TMMBR request.

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

   As indicated already, the feedback messages may be used in both
   multicast and unicast sessions in any of the specified topologies.
   However, for sessions with a large number of participants, using the
   lowest common denominator, as required by this mechanism, may not be
   the most suitable course of action.  Large sessions may need to
   consider other ways to adapt the bit rate to participants'
   capabilities, such as partitioning the session into different quality
   tiers, or using some other method of achieving bit rate scalability.

4.2.1.3. Timing Rules

   The first transmission of the TMMBR 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.


4.2.1.4. Handling in Translator and Mixers

   Media translators and mixers will need to receive and respond to
   TMMBR messages as they are part of the chain that provides a certain
   media stream to the receiver.  The mixer or translator may act
   locally on the TMMBR request and thus generate a TMMBN to indicate
   that it has done so.  Alternatively, in the case of a media
   translator it can forward the request, or in the case of a mixer
   generate one of its own and pass it forward.  In the latter case, the
   mixer will need to send a TMMBN back to the original requestor to
   indicate that it is handling the request.




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4.2.2. Temporary Maximum Media Stream Bit Rate Notification (TMMBN)

   The FCI field of the TMMBN Feedback message may contain zero, one or
   more TMMBN FCI entries.

4.2.2.1. Message Format

   The Feedback Control Information (FCI) consists of zero, one or more
   TMMBN 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                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | MxTBR Exp |  MxTBR Mantissa                 |Measured Overhead|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 3 - Syntax of an FCI entry in the TMMBN message


     SSRC (32 bits): The SSRC value of the "owner" of this tuple.

     MxTBR Exp (6 bits): The exponential scaling of the mantissa for the
              maximum total media bit rate value.  The value is an
              unsigned integer [0..63].

     MxTBR Mantissa (17 bits): The mantissa of the maximum total media
              bit rate value as an unsigned integer.

     Measured Overhead (9 bits): The measured average packet overhead
              value in bytes represented as an unsigned integer.

   Thus the FCI within the TMMBN message contains entries indicating the
   bounding tuples.  For each tuple, the entry gives the owner by the
   SSRC, followed by the applicable maximum total media bit rate and
   overhead value.

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



4.2.2.2. Semantics

   This feedback message is used to notify the senders of any TMMBR
   message that one or more TMMBR messages have been received or that an



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   owner has left the session.  It indicates to all participants the
   current set of bounding tuples and the "owners" of those tuples.

   Within the common packet header for feedback messages (as defined in
   section 6.1 of [RFC4585]), the "SSRC of the packet sender" field
   indicates the source of the notification.  The "SSRC of media source"
   is not used and SHALL be set to 0.

   A TMMBN message SHALL be scheduled for transmission after the
   reception of a TMMBR message with an FCI entry identifying this media
   sender.  Only a single TMMBN SHALL be sent, even if more than one
   TMMBR message is received between the scheduling of the transmission
   and the actual transmission of the TMMBN message.  The TMMBN message
   indicates the bounding tuples and their owners at the time of
   transmitting the message.  The bounding tuples included SHALL be the
   set arrived at through application of the applicable algorithm of
   section 3.5.4.2 or an equivalent, applied to the previous bounding
   set if any and tuples received in TMMBR messages since the last TMMBN
   was transmitted.

   The reception of a TMMBR message SHALL still result in the
   transmission of a TMMBN message even if, after application of the
   algorithm, the newly reported TMMBR tuple is not accepted into the
   bounding set.  In such a case the bounding tuples and their owners
   are not changed, unless the TMMBR was from an owner of a tuple within
   the previously calculated bounding set.  This procedure allows
   session participants that did not see the last TMMBN message to get a
   correct view of this media sender's state.

   As indicated in section Error! Reference source not found., when a
   media sender determines that an "owner" of a bounding tuple has left
   the session, then that tuple is removed from the bounding set, and
   the media sender SHALL send a TMMBN message indicating the remaining
   bounding tuples.  If there are no remaining bounding tuples a TMMBN
   without any FCI SHALL be sent to indicate this.

     Note: if any media receivers remain in the session, this last will
     be a temporary situation.  The empty TMMBN will cause every
     remaining media receiver to determine that its limitation belongs
     in the bounding set and send a TMMBR in consequence.

   In unicast scenarios (i.e. where a single sender talks to a single
   receiver), the aforementioned algorithm to determine ownership
   degenerates to the media receiver becoming the "owner" of the one
   bounding tuple as soon as the media receiver has issued the first
   TMMBR message.





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4.2.2.3.
         Timing Rules

   The TMMBN 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.

4.2.2.4. Handling by Translators and Mixers

   As discussed in Section 4.2.1.4 mixers or translators may need to
   issue TMMBN messages as responses to TMMBR messages for SSRC's
   handled by them.


4.3. Payload Specific Feedback Messages

   As specified by section 6.1 of RFC 4585 [RFC4585], Payload-Specific
   FB messages are identified by the RTCP packet type value PT=PSFB
   (206).

   AVPF [RFC4585] defines three payload-specific feedback messages and
   one application layer feedback message.  This memo specifies four
   additional payload-specific feedback messages.  All are identified by
   means of the FMT parameter as follows:

   Assigned in [RFC4585]:

     1:     Picture Loss Indication (PLI)
     2:     Slice Lost Indication (SLI)
     3:     Reference Picture Selection Indication (RPSI)
     15:    Application layer FB message
     31:    reserved for future expansion of the number space

   Assigned in this memo:

     4:     Full Intra Request Command (FIR)
     5:     Temporal-Spatial Trade-off Request (TSTR)
     6:     Temporal-Spatial Trade-off Notification (TSTN)
      7:     Video Back Channel Message (VBCM)

   Unassigned:

     0:     unassigned
      8-14:  unassigned
     16-30: unassigned

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




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4.3.1. Full Intra Request (FIR)

   The FIR message is identified by RTCP packet type value PT=PSFB and
   FMT=4.

   The FCI field MUST contain one or more FIR entries.  Each entry
   applies to a different media sender, identified by its SSRC.

4.3.1.1. Message Format

   The Feedback Control Information (FCI) for the Full Intra Request
   consists of one or more FCI entries, the content of which is depicted
   in Figure 4.  The length of the FIR feedback 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 4 - Syntax of an FCI entry in the FIR message


     SSRC (32 bits): The SSRC value of the media sender which is
              requested to send a decoder refresh point.

     Seq. nr (8 bits): Command sequence number.  The sequence number
              space is unique for each pairing 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.  The initial value is arbitrary.

     Reserved (24 bits): All bits SHALL be set to 0 by the sender and
              SHALL be ignored on reception.


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






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4.3.1.2. Semantics

   Upon reception of FIR, the 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 payload 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 [RFC4585], 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 herein.

     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 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 so on.  However a
     session which predominately handles pre-coded content is not
     expected to use 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 [RFC4585], a
     Picture Loss Indication informs the decoder about the loss of a
     picture and hence the likelihood of misalignment of the reference
     pictures between the 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 for an encoder consists in
     sending a decoder refresh point.  However, there are other options.
     One example is that the media sender ignores the PLI, because the



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     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 does not allow 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 is 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
   situations where not sending a decoder refresh point would render the
   video unusable for the users.

     Note: A typical example where sending FIR is appropriate 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 FIR to the new sender so to force it to
     emit a decoder refresh point.  The decoder refresh point normally
     includes 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 4587 [RFC4587] 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.

   Within the common packet header for feedback messages (as defined in
   section 6.1 of [RFC4585]), 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 SSRCs of the media senders to
   which the FIR command applies are in the corresponding FCI entries.
   A TSTR message MAY contain requests to multiple media senders, using
   one FCI entry per target media sender.








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4.3.1.3. Timing Rules

   The timing follows the rules outlined in section 3 of [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 one RTT before being sent.  In
   early or regular RTCP mode the repetition is sent in the next regular
   RTCP packet.


4.3.1.4. Handling of FIR Message in Mixer and Translators

   A media translator or a mixer performing media encoding of the
   content for which the session participant has issued a FIR is
   responsible for acting upon it.  A mixer acting upon a FIR SHOULD NOT
   forward the message unaltered; instead it SHOULD issue a FIR itself.


4.3.1.5. Remarks

   In conjunction with video codecs, 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 multiple 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 particularly short delay
   in sending the FIR message.  Hence waiting for the next possible time
   slot allowed by RTCP timing rules as per [RFC4585] should not have an
   overly negative impact on the system performance.


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

   The TSTR feedback message is identified by RTCP packet type value
   PT=PSFB and FMT=5.

   The FCI field MUST contain one or more TSTR FCI entries.

4.3.2.1. Message Format

   The content of the FCI entry for the Temporal-Spatial Trade-off
   Request is depicted in Figure 5.  The length of the feedback message
   MUST be set to 2+2*N, where N is the number of FCI entries included.




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    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 an FCI Entry in the TSTR Message


     SSRC (32 bits): The SSRC of the media sender which is requested to
              apply the tradeoff value given in Index.

     Seq. nr (8 bits): Request sequence number.  The sequence number
              space is unique for pairing 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.  The
              initial value is arbitrary.

     Reserved (19 bits): All bits SHALL be set to 0 by the sender and
              SHALL be ignored on reception.

     Index (5 bits): 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.



4.3.2.2. Semantics

   A decoder can suggest a temporal-spatial trade-off level 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 progression of values from 0 to 31 indicate
   monotonically a desire for higher frame rate.  The index 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 TSTN message.




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   Within the common packet header for feedback messages (as defined in
   section 6.1 of [RFC4585]), 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 SSRCs of the media senders to
   which the TSTR applies to are in the corresponding FCI entries.

   A TSTR message MAY contain requests to multiple media senders, using
   one FCI entry per target media sender.

4.3.2.3. Timing Rules

   The timing follows the rules outlined in section 3 of [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. Handling of message in Mixers and Translators

   A mixer or media translator that encodes content sent to the session
   participant issuing the TSTR SHALL consider the request to determine
   if it can fulfill it by changing its own encoding parameters.  A
   media translator unable to fulfill the request MAY forward the
   request unaltered towards the media sender.  A mixer encoding for
   multiple session participants will need to consider the joint needs
   of these participants before generating a TSTR on its own behalf
   towards the media sender.  See also the discussion in Section 3.5.2.


4.3.2.5. 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 value to
   frame rate and QP is intentionally left open here, as it depends on
   factors such as the compression standard employed, spatial
   resolution, content, bit rate, and so on.





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4.3.3. Temporal-Spatial Trade-off Notification (TSTN)

   The TSTN message is identified by RTCP packet type value PT=PSFB and
   FMT=6.

   The FCI field SHALL contain one or more TSTN FCI entries.

4.3.3.1. Message Format

   The content of an FCI entry for the Temporal-Spatial Trade-off
   Notification is depicted in Figure 6.  The length of the TSTN 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 6 - Syntax of the TSTN


     SSRC (32 bits): The SSRC of the source of the TSTR request which
              resulted in this Notification.

     Seq. nr (8 bits): The sequence number value from the TSTN request
              that is being acknowledged.

     Reserved (19 bits): All bits SHALL be set to 0 by the sender and
              SHALL be ignored on reception.

     Index (5 bits): The trade-off value the media sender is using
              henceforth.


      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.

4.3.3.2. Semantics

   This feedback message is used to acknowledge the reception of a TSTR.
   One TSTN entry in a TSTN 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.



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   A single TSTN message MAY acknowledge multiple requests using
   multiple FCI entries.  The index value included SHALL be the same in
   all FCI entries of the TSTN message.  Including a FCI for each
   requestor allows each requesting entity to determine that the media
   sender received the request.  The Notification SHALL also be sent in
   response to TSTR 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 TSTN 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 the 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.

   Within the common packet header for feedback messages (as defined in
   section 6.1 of [RFC4585]), the "SSRC of the packet sender" field
   indicates the source of the Notification, and the "SSRC of media
   source" is not used and SHALL be set to 0.  The SSRCs of the
   requesting entities to which the Notification applies are in the
   corresponding FCI entries.

4.3.3.3. Timing Rules

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

4.3.3.4. Handling of TSTN in Mixer and Translators

   A mixer or translator that acts upon a TSTR SHALL also send the
   corresponding TSTN.  In cases where it needs to forward a TSTR itself
   the notification message MAY need to be delayed until the TSTR has
   been responded to.

4.3.3.5. Remarks

   None


4.3.4. H.271 Video Back Channel Message (VBCM)

   The VBCM is identified by RTCP packet type value PT=PSFB and FMT=7.

   The FCI field MUST contain one or more VBCM FCI entries.





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4.3.4.1.
         Message Format

   The syntax of an FCI entry within the VBCM indication is depicted in
   Figure 7.

   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 Octet String....      |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 7 - Syntax of an FCI Entry in the VBCM Message

   SSRC (32 bits): The SSRC value of the media sender that is requested
          to instruct its encoder to react to the VBCM message

   Seq. nr (8 bits): Command sequence number.  The sequence number space
          is unique for pairing 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.  The initial value is
          arbitrary.

   0: Must be set to 0 by the sender and should not be acted upon by the
          message receiver.

   Payload Type (7 bits): The RTP payload type for which the VBCM bit
          stream must be interpreted.

   Length (16 bits): The length of the VBCM octet string in octets
          exclusive of any padding octets

   VBCM Octet String (Variable length): This is the octet string
          generated by the decoder carrying a specific feedback sub-
          message.

   Padding (Variable length): Bits set to 0 to make up a 32 bit
          boundary.

4.3.4.2. Semantics




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   The "payload" of the VBCM indication carries different types of
   codec-specific, feedback information.  The type of feedback
   information can be classified as a 'status report' (such as an
   indication that a bit stream was received without errors, or that a
   partial or complete picture or block was lost) or 'update requests'
   (such as complete refresh of the bit stream).

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

   The different types of feedback sub-messages carried in the VBCM are
   indicated by the "payloadType" as defined in [VBCM].  These sub-
   message types are reproduced 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          Message Content
   Type
   ---------------------------------------------------------------------
   0      One or more pictures without detected bit stream 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 bit stream as if no prior bit stream data
          had been received
   > 5    Reserved for future use by ITU-T


   Table 2: H.271 message types ("payloadTypes")


   The bit string or the "payload" of a 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) may have different semantics depending on the codec
   used.

   Within the common packet header for feedback messages (as defined in
   section 6.1 of [RFC4585]), the "SSRC of the packet sender" field
   indicates the source of the request, and the "SSRC of media source"



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   is not used and SHALL be set to 0.  The SSRCs of the media senders to
   which the VBCM message applies to are in the corresponding FCI
   entries.  The sender of the VBCM message MAY send H.271 messages to
   multiple media senders and MAY send more than one H.271 message to
   the same media sender within the same VBCM message.


4.3.4.3. Timing Rules

   The timing follows the rules outlined in section 3 of [RFC4585].  The
   different sub-message types may have different properties in regards
   to the timing of messages that should be used.  If several different
   types are included in the same feedback packet then the requirements
   for the sub-message type with the most stringent requirements should
   be followed.

4.3.4.4. Handling of message in Mixer or Translator

   The handling of VBCM in a mixer or translator is sub-message type
   dependent.


4.3.4.5. Remarks

   Please see section 3.5.3 for a discussion of the usage of H.271
   messages and messages defined in AVPF [RFC4585] and this memo with
   similar functionality.

     Note: There has been some discussion whether the payload type field
     in this message is needed.  It will be needed if there is
     potentially more than one VBCM-capable RTP payload type in the same
     session, and the semantics of a given VBCM message changes between
     payload types.  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).  Therefore,
     the payload field is justified here.  There was a further comment
     that for TSTS and FIR such a need does not exist, 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.


5. Congestion Control

   The correct application of the AVPF [RFC4585] timing rules prevents
   the network from being flooded by feedback messages.  Hence, assuming
   a correct implementation and configuration, the RTCP channel cannot
   break its bit rate commitment and introduce congestion.



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   The reception of some of the feedback messages modifies the behaviour
   of the media senders or, more specifically, the media encoders.  Thus
   modified behaviour MUST respect the bandwidth limits that the
   application of congestion control provides.  For example, when a
   media sender is 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 Stream Bit Rate value can
   only mitigate congestion, but not cause 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 signaling 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 signaling 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. assignment of the ownership of a bounding tuple to the wrong
           participant within a TMMBN message, potentially causing
           unnecessary oscillation in the bounding set as the mistakenly
           identified owner reports a change in its tuple and the true
           owner possibly holds back on changes until a correct TMMBN
           message reaches the participants;

        c. sending TSTR requests that result in a video quality
           different from the user's desire, rendering the session less
           useful.




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


7. SDP Definitions

   Section 4 of [RFC4585] defines a new SDP [RFC4566] attribute, rtcp-
   fb, that may be used to negotiate the capability to handle specific
   AVPF commands and indications, such as Reference Picture Selection,
   Picture Loss Indication etc.  The ABNF for rtcp-fb is described in
   section 4.2 of [RFC4585].  In this section we extend the rtcp-fb
   attribute to include the commands and indications that are described
   for codec control protocol in the present document.  We also discuss
   the Offer/Answer implications for the codec control commands and
   indications.


7.1. Extension of the rtcp-fb Attribute

   As described in AVPF [RFC4585], the rtcp-fb attribute indicates the
   capability of using RTCP feedback.  AVPF specifies that 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 [RFC4585]
   for rtcp-fb attribute relating to payload type and to multiple rtcp-
   fb attributes in a session description also apply to the new feedback
   messages defined in this memo.

   The ABNF [RFC4234] for rtcp-fb as defined in [RFC4585] is

     "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



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         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 "ccm" which indicates
   the support of codec control using RTCP feedback messages.  The "ccm"
   feedback value SHOULD be used with parameters, which indicate the
   specific codec control commands supported.  In this draft we define
   four parameters, which can be used with the ccm feedback value type.

      o  "fir" indicates the support of the Full Intra Request (FIR).
      o  "tmmbr" indicates the support of the Temporary Maximum Media
         Stream Bit Rate Request/Notification (TMMBR/TMMBN).  It has an
         optional sub parameter to indicate the session maximum packet
         rate to be used.  If not included this defaults to infinity.
      o  "tstr" indicates the support of the Temporal-Spatial Trade-off
         Request/Notification (TSTR/TSTN).
      O  "vbcm" indicates the support of H.271 video back channel
         messages (VBCM).  It has zero or more subparameters identifying
         the supported H.271 "payloadType" values.

   In the ABNF for rtcp-fb-val defined in [RFC4585], there is a
   placeholder called rtcp-fb-id to define new feedback types.  "ccm" is
   defined as a new feedback type in this document and the ABNF for the
   parameters for ccm are defined here (please refer to section 4.2 of
   [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" [SP "smaxpr=" MaxPacketRateValue]
                        ; Temporary max media bit rate
              / "tstr"  ; Temporal Spatial Trade Off
              / "vbcm" *(SP subMessageType) ; H.271 VBCM messages
              / token [SP byte-string]
                         ; for future commands/indications
   subMessageType = 1*8DIGIT
   byte-string = <as defined in section 4.2 of [RFC4585] >
   MaxPacketRateValue = 1*15DIGIT




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7.2. Offer-Answer

   The Offer/Answer [RFC3264] implications for codec control protocol
   feedback messages are similar those described in [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.

   The session maximum packet rate parameter part of the TMMBR
   indication is declarative and everyone shall use the highest value
   indicated in a response.  If the session maximum packet rate
   parameter is not present in an offer it SHALL NOT be included by the
   answerer.


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 the FIR and TSTR/TSTN codec control messages.  The SDP is
   carried in a high level signaling protocol like SIP.

         v=0
         o=alice 3203093520 3203093520 IN IP4 host.example.com
         s=Point-to-Point call
         c=IN IP4 192.0.2.124
         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, when the sender receives a TSTR message from
   the remote party it is capable of adjusting the trade off as
   indicated in the RTCP TSTN feedback message.

   Example 2: The following SDP describes a SIP end point joining a
   video mixer 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.




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         v=0
         o=alice 3203093520 3203093520 IN IP4 host.example.com
         s=Multiparty Video Call
         c=IN IP4 192.0.2.124
         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 required 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 "tstr", "fir" and "tmmbr".  The offered SDP is

   -------------> Offer
         v=0
         o=alice 3203093520 3203093520 IN IP4 host.example.com
         s=Offer/Answer
         c=IN IP4 192.0.2.124
         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:* ccm tmmbr smaxpr=120


   The answerer wishes to support only the FIR and TSTR/TSTN messages
   and the answerer SDP is

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

         v=0
         o=alice 3203093520 3203093524 IN IP4 otherhost.example.com
         s=Offer/Answer
         c=IN IP4 192.0.2.37
         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




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         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 sub-messages of payloadType 1
   (one or more pictures that are entirely or partially lost) and 2 (a
   set of blocks of one picture that are entirely or partially lost).

   -------------> Offer
         v=0
         o=alice 3203093520 3203093520 IN IP4 host.example.com
         s=Offer/Answer
         c=IN IP4 192.0.2.124
         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 1 2



   The answerer only wishes to support sub-messages of type 1 only

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

         v=0
         o=alice 3203093520 3203093524 IN IP4 otherhost.example.com
         s=Offer/Answer
         c=IN IP4 192.0.2.37
         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 1

   So in the above example only VBCM indications comprised of
   "payloadType" 1 will be supported.














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8. IANA Considerations

   The new value "ccm" needs to be registered with IANA in the "rtcp-fb"
   Attribute Values registry located at the time of publication at:
   http://www.iana.org/assignments/sdp-parameters

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

   A new registry "Codec Control Messages" needs to be created to hold
   "ccm" parameters located at time of publication at:
   http://www.iana.org/assignments/sdp-parameters

   New registration in this registry follows the "Specification
   required" policy as defined by [RFC2434]. In addition they are
   required to indicate which, if any additional RTCP feedback types,
   such as "nack", "ack".

   The initial content of the registry is the following values:

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

   Value name:       tmmbr
   Long name:        Temporary Maximum Media Stream 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

   The following values need to be registered as FMT values in the "FMT
   Values for RTPFB Payload Types" registry located at the time of
   publication at: http://www.iana.org/assignments/rtp-parameters

   RTPFB range
   Name           Long Name                         Value  Reference
   -------------- --------------------------------- -----  ---------



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                  Reserved                             2   [RFCxxxx]
   TMMBR          Temporary Maximum Media Stream Bit   3   [RFCxxxx]
                  Rate Request
   TMMBN          Temporary Maximum Media Stream Bit   4   [RFCxxxx]
                  Rate Notification

   The following values need to be registered as FMT values in the "FMT
   Values for PSFB Payload Types" registry located at the time of
   publication at: http://www.iana.org/assignments/rtp-parameters

   PSFB range
   Name           Long Name                             Value  Reference
   -------------- ---------------------------------     -----  ---------
   FIR            Full Intra Request Command              4    [RFCxxxx]
   TSTR           Temporal-Spatial Trade-off Request      5    [RFCxxxx]
   TSTN           Temporal-Spatial Trade-off Notification 6    [RFCxxxx]
   VBCM           Video Back Channel Message              7    [RFCxxxx]



9. Contributors

   Tom Taylor has made a very significant contribution, for which the
   authors are very grateful, to this specification by helping rewrite
   the specification. Especially the parts regarding the algorithm for
   determining bounding sets for TMMBR have benefited.


10.  Acknowledgements

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

   Drafts of this memo were reviewed and extensively commented by Roni
   Even, Colin Perkins, Randell Jesup, Keith Lantz, Harikishan Desineni,
   Guido Franceschini and others.  The authors appreciate these reviews.

   Funding for the RFC Editor function is currently provided by the
   Internet Society.











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11.  References

11.1. Normative references

   [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.
   [RFC4566]    Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
                Description Protocol", RFC 4566, July 2006.
   [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, "RTP Topologies", draft-
                ietf-avt-topologies-04, work in progress, Feb 2007.
   [RFC2434]    Narten, T. and H. Alvestrand, "Guidelines for Writing an
                IANA Considerations Section in RFCs", BCP 26, RFC 2434,
                October 1998.
   [RFC4234]    Crocker, D. and P. Overell, "Augmented BNF for Syntax
                Specifications: ABNF", RFC 4234, October 2005.

11.2. 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.
   [H245]       ITU-T Rec. HG.245, "Control protocol for multimedia
                communication", MAY 2006
   [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.
   [RFC4587]    Even, R., "RTP Payload Format for H.261 Video Streams",
                RFC 4587, August 2006.

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   [SAVPF]      J. Ott, E. Carrara, "Extended Secure RTP Profile for
                RTCP-based Feedback (RTP/SAVPF),"
                draft-ietf-avt-profile-savpf-10.txt, Feb, 2007.
   [RFC3525]    Groves, C., Pantaleo, M., Anderson, T., and T. Taylor,
                "Gateway Control Protocol Version 1", RFC 3525, June
                2003.
   [RFC3448]    M. Handley, S. Floyd, J. Padhye, J. Widmer, "TCP
                Friendly Rate Control (TFRC): Protocol Specification",
                RFC 3448, Jan 2003
   [VBCM]       ITU-T Rec. H.271, "Video Back Channel Messages", June
                2006
   [RFC3890]    Westerlund, M., "A Transport Independent Bandwidth
                Modifier for the Session Description Protocol (SDP)",
                RFC 3890, September 2004.
   [RFC4340]    Kohler, E., Handley, M., and S. Floyd, "Datagram
                Congestion Control Protocol (DCCP)", RFC 4340, March
                2006.
   [RFC3261]    Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
                A., Peterson, J., Sparks, R., Handley, M., and E.
                Schooler, "SIP: Session Initiation Protocol", RFC 3261,
                June 2002.
   [RFC2198]    Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
                Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
                Parisis, "RTP Payload for Redundant Audio Data", RFC
                2198, September 1997.





12.  Authors' Addresses

   Stephan Wenger
   Nokia Corporation
   975, Page Mill Road,
   Palo Alto,CA 94304
   USA

   Phone: +1-650-862-7368
   EMail: stewe@stewe.org

   Umesh Chandra
   Nokia Research Center
   975, Page Mill Road,
   Palo Alto,CA 94304
   USA

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


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   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






Full Copyright Statement

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   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).



RFC Editor Considerations

   The RFC editor is requested to replace all occurrences of XXXX with
   the RFC number this document receives.


























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