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INTERNET-DRAFT                               Joerg Ott/Uni Bremen TZI
draft-ietf-avt-rtcp-feedback-08.txt          Stephan Wenger/TU Berlin
                                                    Noriyuki Sato/Oki
                                        Carsten Burmeister/Matsushita
                                                  Jose Rey/Matsushita

                                                      31 January 2004
                                                    Expires July 2004


         Extended RTP Profile for RTCP-based Feedback (RTP/AVPF)


   Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.  Internet-Drafts are
   working documents of the Internet Engineering Task Force (IETF),
   its areas, and its working groups.  Note that other groups may also
   distribute working documents as Internet-Drafts.

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

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   "Copyright (C) The Internet Society (date). All Rights Reserved.


   Abstract

   Real-time media streams that use RTP are, to some degree, resilient
   against packet losses.  Receivers may use the base mechanisms of
   RTCP to report packet reception statistics and thus allow a sender
   to adapt its transmission behavior in the mid-term as sole means
   for feedback and feedback-based error repair (besides a few codec-
   specific mechanisms).  This document defines an extension to the
   Audio-visual Profile (AVP) that enables receivers to provide,
   statistically, more immediate feedback to the senders and thus
   allow for short-term adaptation and efficient feedback-based repair
   mechanisms to be implemented.  This early feedback profile (AVPF)
   maintains the AVP bandwidth constraints for RTCP and preserves
   scalability to large groups.

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   Table of Contents

   1  Introduction.................................................3
      1.1  Definitions.............................................4
      1.2  Terminology.............................................6
   2  RTP and RTCP Packet Formats and Protocol Behavior............6
      2.1  RTP.....................................................6
      2.2  Underlying Transport Protocols..........................7
   3  Rules for RTCP Feedback......................................7
      3.1  Compound RTCP Feedback Packets..........................7
      3.2  Algorithm Outline.......................................9
      3.3  Modes of Operation.....................................10
      3.4  Definitions and Algorithm Overview.....................12
      3.5  AVPF RTCP Scheduling Algorithm.........................15
           3.5.1 Initialization...................................15
           3.5.2 Early Feedback Transmission......................16
           3.5.3 Regular RTCP Transmission........................18
           3.5.4 Other Considerations.............................20
      3.6  Considerations on the Group Size.......................20
           3.6.1 ACK mode.........................................20
           3.6.2 NACK mode........................................21
      3.7  Summary of decision steps..............................22
           3.7.1 General Hints....................................22
           3.7.2 Media Session Attributes.........................23
   4  SDP Definitions.............................................23
      4.1  Profile identification.................................24
      4.2  RTCP Feedback Capability Attribute.....................24
      4.3  RTCP Bandwidth Modifiers...............................27
      4.4  Examples...............................................28
   5  Interworking and Co-Existence of AVP and AVPF Entities......30
   6  Format of RTCP Feedback Messages............................31
      6.1  Common Packet Format for Feedback Messages.............32
      6.2  Transport Layer Feedback Messages......................34
           6.2.1 Generic NACK.....................................34
           6.2.2 Generic ACK......................................35
      6.3  Payload Specific Feedback Messages.....................37
           6.3.1 Picture Loss Indication (PLI)....................37
                 6.3.1.1 Semantics................................38
                 6.3.1.2 Message Format...........................38
                 6.3.1.3 Timing Rules.............................38
                 6.3.1.4 Remarks..................................38







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           6.3.2 Slice Lost Indication (SLI)......................39
                 6.3.2.1 Semantics................................39
                 6.3.2.2 Format...................................39
                 6.3.2.3 Timing Rules.............................40
                 6.3.2.4 Remarks..................................40
           6.3.3 Reference Picture Selection Indication (RPSI)....41
                 6.3.3.1 Semantics................................41
                 6.3.3.2 Format...................................42
                 6.3.3.3 Timing Rules.............................42
      6.4  Application Layer Feedback Messages....................43
   7  Early Feedback and Congestion Control.......................43
   8  Security Considerations.....................................44
   9  IANA Considerations.........................................46
   10 Acknowledgements.............................................50
   11 Authors' Addresses...........................................50
   12 Bibliography.................................................51
      12.1 Normative references...................................51
      12.2 Informative References.................................52
   13 IPR Notice...................................................53
   14 Full Copyright Statement.....................................53






1  Introduction

   Real-time media streams that use RTP are, to some degree, resilient
   against packet losses.  RTP [1] provides all the necessary
   mechanisms to restore ordering and timing present at the sender to
   properly reproduce a media stream at a recipient.  RTP also
   provides continuous feedback about the overall reception quality
   from all receivers -- thereby allowing the sender(s) in the mid-
   term (in the order of several seconds to minutes) to adapt their
   coding scheme and transmission behavior to the observed network
   QoS.  However, except for a few payload specific mechanisms [6],
   RTP makes no provision for timely feedback that would allow a
   sender to repair the media stream immediately: through
   retransmissions, retro-active FEC control, or media-specific
   mechanisms for some video codecs, such as reference picture
   selection.

   Current mechanisms available with RTP to improve error resilience
   include audio redundancy coding [13], video redundancy coding [14],
   RTP-level FEC [11], and general considerations on more robust media
   streams transmission [12].  These mechanisms may be applied pro-
   actively (thereby increasing the bandwidth of a given media
   stream).  Alternatively, in sufficiently small groups with small
   RTTs, the senders may perform repair on-demand, using the above

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   mechanisms and/or media-encoding-specific approaches.  Note that
   "small group" and "sufficiently small RTT" are both highly
   application dependent.

   This document specifies a modified RTP Profile for audio and video
   conferences with minimal control based upon [1] and [2] by means of
   two modifications/additions: to achieve timely feedback, the
   concept of Early RTCP messages as well as algorithms allowing for
   low delay feedback in small multicast groups (and preventing
   feedback implosion in large ones) are introduced.  Special
   consideration is given to point-to-point scenarios.  A small number
   of general-purpose feedback messages as well as a format for codec
   and application-specific feedback information are defined for
   transmission in the RTCP payloads.


   1.1  Definitions

   The definitions from RTP/RTCP [1] and the RTP Profile for Audio and
   Video Conferences with Minimal Control [2] apply.  In addition, the
   following definitions are used in this document:

   Early RTCP mode:
           The mode of operation in which a receiver of a media stream
           is often (but not always) capable of reporting events of
           interest back to the sender close to their occurrence.  In
           Early RTCP mode, RTCP packets are transmitted according to
           the timing rules defined in this document.

   Early RTCP packet:
           An Early RTCP packet is a packet which is transmitted
           earlier than would be allowed if following the scheduling
           algorithm of [1], the reason being an "event" observed by a
           receiver.  Early RTCP packets may be sent in Immediate
           Feedback and in Early RTCP mode.  Sending an Early RTCP
           packet is also referred to as sending Early Feedback in
           this document.

   Event:
           An observation made by the receiver of a media stream that
           is (potentially) of interest to the sender -- such as a
           packet loss or packet reception, frame loss, etc. -- and
           thus useful to be reported back to the sender by means of a
           Feedback message.

   Feedback (FB) message:
           An RTCP message as defined in this document is used to
           convey information about events observed at a receiver --
           in addition to long-term receiver status information which
           is carried in RTCP RRs -- back to the sender of the media

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           stream.  For the sake of clarity, feedback message is
           referred to as FB message throughout this document.

   Feedback (FB) threshold:
           The FB threshold indicates the transition between Immediate
           Feedback and Early RTCP mode.  For a multiparty scenario,
           the FB threshold indicates the maximum group size at which,
           on average, each receiver is able to report each event back
           to the sender(s) immediately, i.e. by means of an Early
           RTCP packet without having to wait for its regularly
           scheduled RTCP interval.  This threshold is highly
           dependent on the type of feedback to be provided, network
           QoS (e.g. packet loss probability and distribution), codec
           and packetization scheme in use, the session bandwidth, and
           application requirements.  Note that the algorithms do not
           depend on all senders and receivers agreeing on the same
           value for this threshold.  It is merely intended to provide
           conceptual guidance to application designers and is not
           used in any calculations. For the sake of clarity, the term
           feedback threshold is referred to as FB threshold
           throughout this document.

   Immediate Feedback mode:
           A mode of operation in which each receiver of a media
           stream is, statistically, capable of reporting each event
           of interest immediately back to the media stream sender.
           In Immediate Feedback mode, RTCP FB messages are
           transmitted according to the timing rules defined in this
           document.

   Media packet:
           A media packet is an RTP packet.

   Regular RTCP mode:
           Mode of operation in which no preferred transmission of FB
           messages is allowed.  Instead, RTCP messages are sent
           following the rules of [1].  Nevertheless, such RTCP
           messages may contain feedback information as defined in
           this document.

   Regular RTCP packet:
           An RTCP packet that is not sent as an Early RTCP packet.

   RTP sender:
           An RTP sender is an RTP entity that transmits media packets
           as well as RTCP packets and receives Regular as well as
           Early RTCP (i.e. feedback) packets.  Note that the RTP
           sender is a logical role and that the same RTP entity may
           at the same time act as an RTP receiver.


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   RTP receiver:
           An RTP receiver is an RTP entity that receives media
           packets as well as RTCP packets and transmits Regular as
           well as Early RTCP (i.e. feedback) packets.  Note that the
           RTP receiver is a logical role and that the same RTP entity
           may at the same time act as an RTP sender.


   1.2  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
    [5].


2  RTP and RTCP Packet Formats and Protocol Behavior

   2.1  RTP

   The rules defined in [2] also apply to this profile except for
   those rules mentioned in the following:

   RTCP packet types:
           Two additional RTCP packet types are registered and the
           corresponding FB messages to convey feedback information
           are defined in section 6 of this memo.

   RTCP report intervals:
           This document describes three modes of operation which
           influence the RTCP report intervals (see section 3.2 of
           this memo).  In Regular RTCP mode, all rules from [1] apply
           except for the recommended minimal interval of five seconds
           between two RTCP reports from the same RTP entity.  In both
           Immediate Feedback and Early RTCP modes, the minimal
           interval of five seconds between two RTCP reports is
           dropped and, additionally, the rules specified in section 3
           of this memo apply if RTCP packets containing FB messages
           (defined in section 4 of this memo) are to be transmitted.

           The rules set forth in [1] may be overridden by session
           descriptions specifying different parameters (e.g. for the
           bandwidth share assigned to RTCP for senders and receivers,
           respectively).  For sessions defined using the Session
           Description Protocol (SDP) [3], the rules of [4] apply.






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   Congestion control:
           The same basic rules as detailed in [2] apply.  Beyond
           this, in section 7, further consideration is given to the
           impact of feedback and a sender's reaction to FB messages.


   2.2  Underlying Transport Protocols

   RTP is intended to be used on top of unreliable transport
   protocols, including UDP and DCCP.  This section briefly describes
   the specifics beyond plain RTP operation introduced by RTCP
   feedback as specified in this memo.
   UDP:   UDP provides best effort delivery of datagrams for point-
           to-point as well as for multicast communications.  UDP does
           not support congestion control or error repair.  The RTCP-
           based feedback defined in this memo is able to provide
           minimal support for congestion control for RTP-over-UDP
           traffic as well as for limited error repair.  It should be
           noted, however, that RTCP feedback does not operate on RTT
           timescales.  This memo addresses both unicast and multicast
           operation.

   DCCP:  DCCP [19] provides for congestion-controlled but unreliable
           datagram flows for unicast communications.  With TFRC-based
           [20] congestion control (CCID 3), DCCP is particularly
           suitable for audio and video communications.  DCCP's
           acknowledgement messages may provide detailed feedback
           reporting about received and missed datagrams (and thus
           about congestion).  RTCP-based feedback using the Generic
           Feedback messages (section 6.2) may be used to provide
           similar information, albeit at a much lower rate.

           When running RTP over DCCP, congestion control is performed
           at the DCCP layer and no additional mechanisms are required
           at the RTP layer.  Furthermore, an RTCP-feedback capable
           sender may leverage the more frequent DCCP-based feedback
           and thus a receiver may abstain from using (additional)
           Generic Feedback messages where appropriate.


3  Rules for RTCP Feedback

   3.1  Compound RTCP Feedback Packets

   Two components constitute RTCP-based feedback as described in this
   document:

   o Status reports are contained in SR/RR packets and are
      transmitted at regular intervals as part of compound RTCP


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      packets (which also include SDES and possibly other messages);
      these status reports provide an overall indication for the
      recent reception quality of a media stream.

   o FB messages as defined in this document that indicate loss or
      reception of particular pieces of a media stream (or provide
      some other form of rather immediate feedback on the data
      received).  Rules for the transmission of FB messages are newly
      introduced in this document.

   RTCP FB messages are just another RTCP packet type (see section
   4).  Therefore, multiple FB messages MAY be combined in a single
   compound RTCP packet and they MAY also be sent combined with other
   RTCP packets.

   Compound RTCP packets containing FB messages as defined in this
   document MUST contain RTCP packets in the order defined in [1]:

   o OPTIONAL encryption prefix that MUST be present if the RTCP
      packet(s) is to be encrypted according to section 9.1 of [1].
   o MANDATORY SR or RR.
   o MANDATORY SDES which MUST contain the CNAME item; all other SDES
      items are OPTIONAL.
   o One or more FB messages.

   The FB message(s) MUST be placed in the compound packet after RR
   and SDES RTCP packets defined in [1].  The ordering with respect to
   other RTCP extensions is not defined.

   Two types of compound RTCP packets carrying feedback packets are
   used in this document:

   a)  Minimal compound RTCP feedback packet

       A minimal compound RTCP feedback packet MUST contain only the
       mandatory information as listed above: encryption prefix if
       necessary, exactly one RR or SR, exactly one SDES with only the
       CNAME item present, and the FB message(s).  This is to minimize
       the size of the RTCP packet transmitted to convey feedback and
       thus to maximize the frequency at which feedback can be
       provided while still adhering to the RTCP bandwidth
       limitations.

       This packet format SHOULD be used whenever an RTCP FB message
       is sent as part of an Early RTCP packet. This packet type is
       referred to as minimal compound RTCP packet in this document.

   b)  (Full) compound RTCP feedback packet



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       A (full) compound RTCP feedback packet MAY contain any
       additional number of RTCP packets (additional RRs, further SDES
       items, etc.).  The above ordering rules MUST be adhered to.

       This packet format MUST be used whenever an RTCP FB message is
       sent as part of a Regular RTCP packet or in Regular RTCP mode.
       It MAY also be used to send RTCP FB messages in Immediate
       Feedback or Early RTCP mode. This packet type is referred to as
       full compound RTCP packet in this document.

   RTCP packets that do not contain FB messages are referred to as
   non-FB RTCP packets.  Such packets MUST follow the format rules in
   [1].


   3.2  Algorithm Outline

   FB messages are part of the RTCP control streams and thus subject
   to the RTCP bandwidth constraints.  This means, in particular, that
   it may not be possible to report an event observed at a receiver
   immediately back to the sender.  However, the value of feedback
   given to a sender typically decreases over time -- in terms of the
   media quality as perceived by the user at the receiving end and/or
   the cost required to achieve media stream repair.

   RTP [1] and the commonly used RTP profile [2] specify rules when
   compound RTCP packets should be sent.  This document modifies those
   rules in order to allow applications to timely report events (e.g.
   loss or reception of RTP packets) and to accommodate algorithms
   that use FB messages.

   The modified RTCP transmission algorithm can be outlined as
   follows: As long as no FB messages have to be conveyed, compound
   RTCP packets are sent following the rules of RTP [1] -- except that
   the five second minimum interval between RTCP reports is not
   enforced.  Hence, the interval between RTCP reports is only derived
   from the average RTCP packet size and the RTCP bandwidth share
   available to the RTP/RTCP entity.  Optionally, a minimum interval
   between Regular RTCP packets may be enforced.

   If a receiver detects the need to send an FB message, it may do so
   earlier than the next regular RTCP reporting interval (for which it
   would be scheduled following the above regular RTCP algorithm).
   Feedback suppression is used to avoid feedback implosion in
   multiparty sessions:  The receiver waits for a (short) random
   dithering interval to check whether it sees a corresponding FB
   message from any other receiver reporting the same event.  Note
   that for point-to-point sessions there is no such delay.  If a
   corresponding FB message from another member is received, this
   receiver refrains from sending the FB message and continues to

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   follow the Regular RTCP transmission schedule.  In case the
   receiver has not yet seen a corresponding FB message from any other
   member, it checks whether it is allowed to send Early feedback.  If
   sending Early feedback is permissible , the receiver sends the FB
   message as part of a minimal compound RTCP packet.  The permission
   to send Early feedback depends on the type of the previous RTCP
   packet sent by this receiver and the time the previous Early
   feedback message was sent.

   FB messages may also be sent as part of full compound RTCP packets
   which are transmitted as per [1] (except for the five second lower
   bound) in regular intervals.


   3.3  Modes of Operation

   RTCP-based feedback may operate in one of three modes (figure 1) as
   described below.   The mode of operation is just an indication of
   whether or not the receiver will, on average, be able to report all
   events to the sender in a timely fashion; the mode does not
   influence the algorithm used for scheduling the transmission of FB
   messages.  And, depending on the reception quality and the locally
   monitored state of the RTP session, individual receivers may not
   (and not have to) agree on a common perception on the current mode
   of operation.

   a) Immediate Feedback mode: the group size is below the FB
       threshold which gives each receiving party sufficient bandwidth
       to transmit the RTCP feedback packets for the intended purpose.
       This means that, for each receiver, there is enough bandwidth
       to report each event by means of a virtually "immediate" RTCP
       feedback packet.

       The group size threshold is a function of a number of
       parameters including (but not necessarily limited to): the type
       of feedback used (e.g. ACK vs. NACK), bandwidth, packet rate,
       packet loss probability and distribution, media type, codec,
       and the (worst case or observed) frequency of events to report
       (e.g. frame received, packet lost).

       As a rough estimate, let N be the average number of events to
       be reported per interval T by a receiver, B the RTCP bandwidth
       fraction for this particular receiver and R the average RTCP
       packet size, then the receiver operates in Immediate Feedback
       mode as long as N<=B*T/R.

   b) Early RTCP mode: In this mode, the group size and other
       parameters no longer allow each receiver to react to each event
       that would be worth (or needed) to report.  But feedback can
       still be given sufficiently often so that it allows the sender

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       to adapt the media stream transmission accordingly and thereby
       increase the overall media playback quality.

       Using the above notation, Early RTCP mode can be roughly
       characterized by N > B*T/R as "lower bound".  An estimate for
       an upper bound is more difficult.  Setting N=1, we obtain for a
       given R and B the interval T = R/B as average interval between
       events to be reported.  This information can be used as a hint
       to determine whether or not early transmission of RTCP packets
       is useful.

   c) Regular RTCP Mode: From some group size upwards, it is no
       longer useful to provide feedback for individual events from
       receivers at all -- because of the time scale in which the
       feedback could be provided and/or because in large groups the
       sender(s) have no chance to react to individual feedback
       anymore.

       No precise group size threshold can be specified at which this
       mode starts but, obviously, this boundary matches the upper
       bound of the Early RTCP mode as specified in item b).

   As the feedback algorithm described in this document scales
   smoothly, there is no need for an agreement among the participants
   on the precise values of the respective FB thresholds within the
   group.  Hence the borders between all these modes are soft.


     ACK
   feedback
     V
     :<- - - -  NACK feedback - - - ->//
     :
     :   Immediate   ||
     : Feedback mode ||Early RTCP mode   Regular RTCP mode
     :<=============>||<=============>//<=================>
     :               ||
    -+---------------||---------------//------------------> group size
     2               ||
      Application-specific FB Threshold
         = f(data rate, packet loss, codec, ...)

   Figure 1: Modes of operation


   As stated before, the respective FB thresholds depend on a number
   of technical parameters (of the codec, the transport, the type of
   feedback used, etc.) but also on the respective application
   scenarios.  Section 3.6 provides some useful hints (but no precise
   calculations) on estimating these thresholds.

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   3.4  Definitions and Algorithm Overview

   The following pieces of state information need to be maintained per
   receiver (largely taken from [1]).  Note that all variables (except
   in item h) below) are calculated independently at each receiver.
   Therefore, their local values may differ at any given point in
   time.

   a) Let "senders" be the number of active senders in the RTP
      session.

   b) Let "members" be the current estimate of the number of receivers
      in the RTP session.

   c) Let tn and tp be the time for the next (last) scheduled
      RTCP RR transmission calculated prior to reconsideration.

   d) Let Tmin be the minimal interval between RTCP packets as per
      [1].  Unlike in [1], the initial Tmin is set to 1 second to
      allow for some group size sampling before sending the first RTCP
      packet. After the first RTCP packet is sent, Tmin is set to 0.

   e) Let T_rr be the interval after which, having just sent a
      regularly scheduled RTCP packet, a receiver would schedule the
      transmission of its next Regular RTCP packet. This value is
      obtained following the rules of [1] but with Tmin as defined in
      this document: T_rr = T (the "calculated interval" as defined in
      [1]) with tn = tp + T.  T_rr always refers to the last value of
      T that has been computed (because of reconsideration or to
      determine tn). T_rr is also referred to as Regular RTCP interval
      in this document.

   f) Let t0 be the time at which an event that is to be reported is
      detected by a receiver.

   g) Let T_dither_max be the maximum interval for which an RTCP
      feedback packet MAY be additionally delayed to prevent
      implosions in multiparty sessions; the value for T_dither_max is
      dynamically calculated based upon T_rr (or may be derived by
      means of another mechanism common across all RTP receivers to be
      specified in the future).  For point-to-point sessions (i.e.
      sessions with exactly two members with no change in the group
      size expected, e.g. unicast streaming sessions), T_dither_max is
      set to 0.

   h) Let T_max_fb_delay be the upper bound within which feedback to
      an event needs to be reported back to the sender to be useful at


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      all.  This value is application-specific; and no values are
      defined in this document.

   i) Let te be the time for which a feedback packet is scheduled.

   j) Let T_fd be the actual (randomized) delay for the transmission
      of FB message in response to an event at time t0.

   k) Let allow_early be a Boolean variable that indicates whether the
      receiver currently may transmit FB messages prior to its next
      regularly scheduled RTCP interval tn.  This variable is used to
      throttle the feedback sent by a single receiver.  allow_early is
      set to FALSE after Early feedback transmission and is set to
      TRUE as soon as the next Regular RTCP transmission takes place.

   l) Let avg_rtcp_size be the moving average on the RTCP packet size
      as defined in [1].

   m) Let T_rr_interval be an OPTIONAL minimal interval to be used
      between Regular RTCP packets.  If T_rr_interval == 0, then this
      variable does not have any impact on the overall operation of
      the RTCP feedback algorithm.  If T_rr_interval != 0 then the
      next Regular RTCP packet will not be scheduled T_rr after the
      last Regular RTCP transmission (i.e. at tp+T_rr). Instead, the
      next Regular RTCP packet will be delayed until at least
      T_rr_interval after the last Regular RTCP transmission, i.e. it
      will be scheduled at or later than tp+T_rr_interval.  Note that
      T_rr_interval does not affect the calculation of T_rr and tp;
      instead, Regular RTCP packets scheduled for transmission before
      tp+T_rr_interval will be suppressed if, for example, they do not
      contain any FB messages.  The T_rr_interval does not affect
      transmission scheduling of Early RTCP packets.

   NOTE: Providing T_rr_interval as an independent variable is meant
   to minimize Regular RTCP feedback (and thus bandwidth consumption)
   as needed by the application while additionally allowing the use of
   more frequent Early RTCP packets to provide timely feedback.  This
   goal could not be achieved by reducing the overall RTCP bandwidth
   as RTCP bandwidth reduction would also impact the frequency of
   Early feedback.

   n) Let t_rr_last be the point in time at which the last Regular
      RTCP packet has been scheduled and sent, i.e. has not been
      suppressed due to T_rr_interval.

   o) Let T_retention be the time window for which past FB messages
      are stored by an AVPF entity.  This is to ensure that feedback
      suppression also works for entities that have received FB
      messages from other entities prior to noticing the feedback
      event itself.  T_retention MUST be set to at least 2 seconds.

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   p) Let M*Td be the timeout value for a receiver to be considered
      inactive (as defined in [1]).

   The feedback situation for an event to report at a receiver is
   depicted in figure 2 below.  At time t0, such an event (e.g. a
   packet loss) is detected at the receiver.  The receiver decides --
   based upon current bandwidth, group size, and other application-
   specific parameters -- that a FB message needs to be sent back to
   the sender.

   To avoid an implosion of feedback packets in multiparty sessions,
   the receiver MUST delay the transmission of the RTCP feedback
   packet by a random amount of time T_fd (with the random number
   evenly distributed in the interval [0, T_dither_max]).
   Transmission of the compound RTCP packet MUST then be scheduled for
   te = t0 + T_fd.

   The T_dither_max parameter is derived from the Regular RTCP
   interval, T_rr, which, in turn, is based upon the group size.  A
   future document may also specify other calculations for
   T_dither_max (e.g. based upon RTT) if it can be assured that all
   RTP receivers will use the same mechanism for calculating
   T_dither_max.

   For a certain application scenario, a receiver may determine an
   upper bound for the acceptable local delay of FB messages:
   T_max_fb_delay.  If an a priori estimation or the actual
   calculation of T_dither_max indicates that this upper bound MAY be
   violated (e.g. because T_dither_max > T_max_fb_delay), the receiver
   MAY decide not to send any feedback at all because the achievable
   gain is considered insufficient.

   If an Early RTCP packet is scheduled, the time slot for the next
   Regular RTCP packet MUST be updated accordingly to a new tn:
   tn=tp+2*T_rr and so MUST tp: tp=tp+T_rr afterwards.  This is to
   ensure that the short-term average RTCP bandwidth used with Early
   feedback does not exceed the bandwidth  used without Early
   feedback.












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             event to
             report
             detected
                |
                |  RTCP feedback range
                |   (T_max_fb_delay)
                vXXXXXXXXXXXXXXXXXXXXXXXXXXX     ) )
   |---+--------+-------------+-----+------------| |--------+--->
       |        |             |     |            ( (        |
       |       t0            te                             |
       tp                                                   tn
                 \_______  ________/
                         \/
                   T_dither_max

   Figure 2: Event report and parameters for Early RTCP scheduling


   3.5  AVPF RTCP Scheduling Algorithm

   Let S0 be an active sender (out of S senders) and let N be the
   number of receivers with R being one of these receivers.

   Assume that R has verified that using feedback mechanisms is
   reasonable at the current constellation (which is highly
   application specific and hence not specified in this document).

   Assume further that T_rr_interval is 0, if no minimal interval
   between Regular RTCP packets is to be enforced, or T_rr_interval is
   set to some meaningful value, as given by the application.  This
   value then denotes the minimal interval between Regular RTCP
   packets.

   With this, a receiver R MUST use the following rules for
   transmitting one or more FB messages as minimal or full compound
   RTCP packet:


   3.5.1 Initialization

   Initially, R MUST set allow_early = TRUE and t_rr_last = NaN (Not-
   a-Number, i.e. some invalid value that can be distinguished from a
   valid time).

   Furthermore, the initialization of the RTCP variables as per [1]
   applies except that the initial value for Tmin.  For a point-to-
   point session, the initial Tmin is set to 0.  For a multiparty
   session, Tmin is initialized to 1.0 seconds.



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   3.5.2 Early Feedback Transmission

   Assume that R had scheduled the last Regular RTCP RR packet for
   transmission at tp (and sent or suppressed this packet at tp) and
   has scheduled the next transmission (including possible
   reconsideration as per [1]) for tn = tp + T_rr.  Assume also that
   the last Regular RTCP packet transmission has occurred at
   t_rr_last.

   The Early Feedback algorithm then comprises the following steps:

   1. At time t0, R detects the need to transmit one or more  FB
   messages, e.g. because media "units" need to be ACKed or NACKed,
   and finds that providing the feedback information is potentially
   useful for the sender.

   2. R first checks whether there is already a compound RTCP packet
   containing one or more FB messages scheduled for transmission
   (either as Early or as Regular RTCP packet).

        2.a) If so, the new FB message MUST be included in the
        scheduled packet; the scheduling of the waiting compound RTCP
        packet MUST remain unchanged.  When doing so, the available
        feedback information SHOULD be merged to produce as few FB
        messages as possible.  This completes the course of immediate
        actions to be taken.

        2.b) If no compound RTCP packet is already scheduled for
        transmission, a new (minimal or full) compound RTCP packet
        MUST be created and the minimal interval for T_dither_max MUST
        be chosen as follows:

        i)   If the session is a point-to-point session then
             T_dither_max = 0.

        ii)  If the session is a multiparty session then

                 T_dither_max = l * T_rr

             with l=0.5.

        The value for T_dither_max MAY be calculated differently (e.g.
        based upon RTT) which MUST then be specified in a future
        document.  Such a future specification MUST ensure that all
        RTP receivers use the same mechanism to calculate
        T_dither_max.

        The values given above for T_dither_max are minimal values.
        Application-specific feedback considerations may make it


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        worthwhile to increase T_dither_max beyond this value.  This
        is up to the discretion of the implementer.

   3. Then, R MUST check whether its next Regular RTCP packet would be
   within the time bounds for the Early RTCP packet triggered at t0,
   i.e. if t0 + T_dither_max > tn.

        3.a) If so, an Early RTCP packet MUST NOT be scheduled;
        instead the FB message(s) MUST be stored to be included in the
        Regular RTCP packet scheduled for tn.  This completes the
        course of immediate actions to be taken.

       3.b) Otherwise, the following steps are carried out.

   4. R MUST check whether it is allowed to transmit an Early RTCP
   packet, i.e. allow_early == TRUE, or not.

       4.a) If allow_early == FALSE then R MUST check the time for the
       next scheduled Regular RTCP packet:

            1.  If tn - t0 < T_max_fb_delay then the feedback could
                 still be useful for the sender, despite the late
                 reporting. Hence, R MAY create an RTCP FB message to
                 be included in the Regular RTCP packet for
                 transmission at tn.

            2.  Otherwise, R MUST discard the RTCP FB message.

       This completes the immediate course of actions to be taken.

       4.b) If allow_early == TRUE then R MUST schedule an Early RTCP
       packet for te = t0 + RND * T_dither_max with RND being a pseudo
       random function evenly distributed between 0 and 1.

   5. R MUST detect overlaps in FB messages received from other
   members of the RTP session and the FB messages R wants to send.
   Therefore, while member of the RTP session, R MUST continuously
   monitor the arrival of (minimal) compound RTCP packets and store
   each FB message contained in these RTCP packets for at least
   T_retention.  When scheduling the transmission of its own FB
   message following steps 1. through 4. above, R MUST check each of
   the stored and newly received FB messages from the RTCP packets
   received during the interval [t0 - T_retention ; te] and act as
   follows:

        5.a) If R understands the received FB message's semantics and
        the message contents is a superset of the feedback R wanted to
        send then R MUST discard its own FB message and MUST re-
        schedule the next Regular RTCP packet transmission for tn (as
        calculated before).

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        5.b) If R understands the received FB message's semantics and
        the message contents is not a superset of the feedback R wanted
        to send then R SHOULD transmit its own FB message as scheduled.
        If there is an overlap between the feedback information to send
        and the feedback information received, the amount of feedback
        transmitted is up to R: R MAY leave its feedback information to
        be sent unchanged, R MAY as well eliminate any redundancy
        between its own feedback and the feedback received so far from
        other session members.

        5.c) If R does not understand the received FB message's
        semantics, R MAY keep its own FB message scheduled as an Early
        RTCP packet, or R MAY re-schedule the next Regular RTCP packet
        transmission for tn (as calculated before) and MAY append the
        FB message to the now regularly scheduled RTCP message.

        Note: With 5.c), receiving unknown FB messages may not lead to
        feedback suppression at a particular receiver.  As a
        consequence, a given event may cause M different types of FB
        messages (which are all appropriate but not mutually
        understood) to be scheduled, so that a "large" receiver group
        may effectively be partitioned into at most M groups.  Among
        members of each of these M groups, feedback suppression will
        occur following 5.a and 5.b but no suppression will happen
        across groups.  As a result, O(M) RTCP FB messages may be
        received by the sender.  Hence, there is a chance for a very
        limited feedback implosion.  However, as sender(s) and all
        receivers make up the same application using the same (set of)
        codecs in the same RTP session, only little divergence in
        semantics for FB messages can safely be assumed and, therefore,
        M is assumed to be small in the general case.  Given further
        that the O(M) FB messages are randomly distributed over a time
        interval of T_dither_max we find that the resulting limited
        number of extra compound RTCP packets (a) is assumed not to
        overwhelm the sender and (b) should be conveyed as all contain
        complementary pieces of information.

   6. If R's FB message(s) was not suppressed by other receiver FB
   messages as per 5., when te is reached, R MUST transmit the
   (minimal) compound RTCP packet containing its FB message(s).  R
   then MUST set allow_early = FALSE, MUST recalculate tn = tp +
   2*T_rr, and MUST set tp to the previous tn.   As soon as the newly
   calculated tn is reached, regardless whether R sends its next
   Regular RTCP packet or suppresses it because of T_rr_interval, it
   MUST set allow_early = TRUE again.


   3.5.3 Regular RTCP Transmission


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   Full compound RTCP packets MUST be sent in regular intervals.
   These packets MAY also contain one or more FB messages.
   Transmission of Regular RTCP packets is scheduled as follows:


   If T_rr_interval == 0 then the transmission MUST follow the rules
   as specified in section 3.2 and 3.4 of this document and MUST
   adhere to the adjustments of tn specified in section 3.5.2, i.e.
   skip one regular transmission if an Early RTCP packet transmission
   has occurred.  Timer reconsideration takes place when tn is reached
   as per [1]. The Regular RTCP packet is transmitted after timer
   reconsideration.  Whenever a Regular RTCP packet is sent or
   suppressed, allow_early MUST be set to TRUE and tp, tn MUST be
   updated as per [1].  After the first transmission of a Regular RTCP
   packet, Tmin MUST be set to 0.

   If T_rr_interval != 0 then the calculation for the transmission
   times MUST follow the rules as specified in section 3.2 and 3.4 of
   this document and MUST adhere to the adjustments of tn specified in
   section 3.5.2 (i.e. skip one regular transmission if an Early RTCP
   transmission has occurred).  Timer reconsideration takes place when
   tn is reached as per [1].  After timer reconsideration, the
   following actions are taken:

        1. If no Regular RTCP packet has been sent before (i.e. if
           t_rr_last == NaN) then a Regular RTCP packet MUST be
           scheduled.  Stored FB messages MAY be included in the
           Regular RTCP packet.  After the scheduled packet has been
           sent, t_rr_last MUST be set to tn.  Tmin MUST be set to 0.

        2. Otherwise, a temporary value T_rr_current_interval is
           calculated as follows:

              T_rr_current_interval = RND*T_rr_interval

           with RND being a pseudo random function evenly distributed
           between 0.5 and 1.5.  This dithered value is used to
           determine one of the following alternatives:

           2a) If t_rr_last + T_rr_current_interval <= tn then a
               Regular RTCP packet MUST be scheduled.  Stored RTCP FB
               messages MAY be included in the Regular RTCP packet.
               After the scheduled packet has been sent, t_rr_last
               MUST be set to tn.

           2b) If t_rr_last + T_rr_current_interval > tn and RTCP FB
               messages have been stored and are awaiting
               transmission, an RTCP packet MUST be scheduled for
               transmission at tn.  This RTCP packet MAY be a minimal
               or a Regular RTCP packet (at the discretion of the

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               implementer) and the compound RTCP packet MUST include
               the stored RTCP FB message(s).  t_rr_last MUST remain
               unchanged.

           2c) Otherwise (if t_rr_last + T_rr_current_interval > tn
               but no stored RTCP FB messages are awaiting
               transmission), the compound RTCP packet MUST be
               suppressed (i.e. it MUST NOT be scheduled).  t_rr_last
               MUST remain unchanged.

   In all the four cases above (1, 2a, 2b, and 2c), allow_early MUST
   be set to TRUE (possibly after sending the Regular RTCP packet) and
   tp and tn MUST be updated following the rules of [1] except for the
   five second minimum.


   3.5.4 Other Considerations

   If T_rr_interval != 0 then the timeout calculation for RTP/AVPF
   entities (section 6.3.5 of [1]) MUST be modified to use
   T_rr_interval instead of Tmin for computing Td and thus M*Td.

   Whenever a compound RTCP packet is sent or received -- minimal or
   full compound, Early or Regular -- the avg_rtcp_size variable MUST
   be updated accordingly (see [1]) and subsequent computations of tn
   MUST use the new avg_rtcp_size.


   3.6  Considerations on the Group Size

   This section provides some guidelines to the group sizes at which
   the various feedback modes may be used.


   3.6.1 ACK mode

   The RTP session MUST have exactly two members and this group size
   MUST NOT grow, i.e. it MUST be point-to-point communications.
   Unicast addresses SHOULD be used in the session description.

   For unidirectional as well as bi-directional communication between
   two parties, 2.5% of the RTP session bandwidth is available for
   RTCP traffic from the receivers including feedback.  For a 64
   kbit/s stream this yields 1,600 bit/s for RTCP.  If we assume an
   average of 96 bytes (=768 bits) per RTCP packet a receiver can
   report 2 events per second back to the sender.  If acknowledgments
   for 10 events are collected in each FB message then 20 events can
   be acknowledged per second.  At 256 kbit/s 8 events could be



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   reported per second; thus the ACKs may be sent in a finer
   granularity (e.g. only combining three ACKs per FB message).

   From 1 Mbit/s upwards, a receiver would be able to acknowledge each
   individual frame (not packet!) in a 30 fps video stream.

   ACK strategies MUST be defined to work properly with these
   bandwidth limitations.  An indication whether or not ACKs are
   allowed for a session and, if so, which ACK strategy should be
   used, MAY be conveyed by out-of-band mechanisms, e.g. media-
   specific attributes in a session description using SDP.


   3.6.2 NACK mode

   Negative acknowledgements (and other types of feedback exhibiting
   similar reporting characteristics) MUST be used for all sessions
   with a group size that may grow larger than two.  Of course, NACKs
   MAY be used for point-to-point communications as well.

   Whether or not the use of Early RTCP packets should be considered
   depends upon a number of parameters including session bandwidth,
   codec, special type of feedback, number of senders and receivers.

   The most important parameters when determining the mode of
   operation are the allowed minimal interval between two compound
   RTCP packets (T_rr) and the average number of events that
   presumably need reporting per time interval (plus their
   distribution over time, of course).  The minimum interval can be
   derived from the available RTCP bandwidth and the expected average
   size of an RTCP packet.  The number of events to report e.g. per
   second may be derived from the packet loss rate and sender's rate
   of transmitting packets.  From these two values, the allowable
   group size for the Immediate Feedback mode can be calculated.

       Let N be the average number of events to be reported per
       interval T by a receiver, B the RTCP bandwidth fraction for
       this particular receiver and R the average RTCP packet size,
       then the receiver operates in Immediate Feedback mode as long
       as N<=B*T/R.

   The upper bound for the Early RTCP mode then solely depends on the
   acceptable quality degradation, i.e. how many events per time
   interval may go unreported.

       Using the above notation, Early RTCP mode can be roughly
       characterized by N > B*T/R as "lower bound".  An estimate for
       an upper bound is more difficult.  Setting N=1, we obtain for a
       given R and B the interval T = R/B as average interval between
       events to be reported.  This information can be used as a hint

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       to determine whether or not early transmission of RTCP packets
       is useful.

   Example: If a 256kbit/s video with 30 fps is transmitted through a
   network with an MTU size of some 1,500 bytes, then, in most cases,
   each frame would fit in into one packet leading to a packet rate of
   30 packets per second.  If 5% packet loss occurs in the network
   (equally distributed, no inter-dependence between receivers), then
   each receiver will, on average, have to report 3 packets lost each
   two seconds. Assuming a single sender and more than three
   receivers, this yields 3.75% of the RTCP bandwidth allocated to the
   receivers and thus 9.6kbit/s.  Assuming further a size of 120 bytes
   for the average compound RTCP packet allows 10 RTCP packets to be
   sent per second or 20 in two seconds.  If every receiver needs to
   report three lost packets per two seconds, this yields a maximum
   group size of 6-7 receivers if all loss events shall be reported.
   The rules for transmission of Early RTCP packets should provide
   sufficient flexibility for most of this reporting to occur in a
   timely fashion.

   Extending this example to determine the upper bound for Early RTCP
   mode could lead to the following considerations: assume that the
   underlying coding scheme and the application (as well as the
   tolerant users) allow on the order of one loss without repair per
   two seconds.  Thus the number of packets to be reported by each
   receiver decreases to two per two seconds second and increases the
   group size to 10.  Assuming further that some number of packet
   losses are correlated, feedback traffic is further reduced and
   group sizes of some 12 to 16 (maybe even 20) can be reasonably well
   supported using Early RTCP mode.  Note that all these
   considerations are based upon statistics and will fail to hold in
   some cases.

   3.7  Summary of decision steps

   3.7.1 General Hints

   Before even considering whether or not to send RTCP feedback
   information an application has to determine whether this mechanism
   is applicable:

   1) An application has to decide whether -- for the current ratio of
      packet rate with the associated (application-specific) maximum
      feedback delay and the currently observed round-trip time (if
      available) -- feedback mechanisms can be applied at all.

      This decision may be based upon (and dynamically revised
      following)  RTCP reception statistics as well as out-of-band
      mechanisms.


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   2) The application has to decide -- for a certain observed error
      rate, assigned bandwidth, frame/packet rate, and group size --
      whether (and which) feedback mechanisms can be applied.

      Regular RTCP reception statistics provide valuable input to this
      step, too.

   3) If the application decides to send feedback, the application has
      to follow the rules for transmitting Early RTCP packets or
      Regular RTCP packets containing FB messages.

   4) The type of RTCP feedback sent should not duplicate information
      available to the sender from a lower layer transport protocol.
      That is, if the transport protocol provides negative or positive
      acknowledgements about packet reception (such as DCCP), the
      receiver should avoid repeating the same information at the RTCP
      layer (i.e. abstain from sending Generic ACKs or Generic NACKs).


   3.7.2 Media Session Attributes

   Media sessions are typically described using out-of-band mechanisms
   to convey transport addresses, codec information, etc. between
   sender(s) and receiver(s).  Such a mechanism is two-fold:  a format
   used to describe a media session and another mechanism for
   transporting this description.

   In the IETF, the Session Description Protocol (SDP) is currently
   used to describe media sessions while protocols such as SIP, SAP,
   RTSP, and HTTP (among others) are used to convey the descriptions.

   A media session description format MAY include parameters to
   indicate that RTCP feedback mechanisms are supported in this
   session and which of the feedback mechanisms MAY be applied.

   To do so, the profile "AVPF" MUST be indicated instead of "AVP".
   Further attributes may be defined to show which type(s) of feedback
   are supported.

   Section 4 contains the syntax specification to support RTCP
   feedback with SDP.  Similar specifications for other media session
   description formats are outside the scope of this document.


4  SDP Definitions

   This section defines a number of additional SDP parameters that are
   used to describe a session.  All of these are defined as media
   level attributes.


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   4.1  Profile identification

   The AV profile defined in [4] is referred to as "AVP" in the
   context of e.g. the Session Description Protocol (SDP) [3].  The
   profile specified in this document is referred to as "AVPF".

   Feedback information following the modified timing rules as
   specified in this document MUST NOT be sent for a particular media
   session unless the description for this session indicates the use
   of the "AVPF" profile (exclusively or jointly with other AV
   profiles).


   4.2  RTCP Feedback Capability Attribute

   A new payload format-specific SDP attribute is defined to indicate
   the capability of using RTCP feedback as specified in this
   document: "a=rtcp-fb".  The "rtcp-fb" attribute MUST only be used
   as an SDP media attribute and MUST NOT be provided at the session
   level.  The "rtcp-fb" attribute MUST only be used in media sessions
   for which the "AVPF" is specified.

   The "rtcp-fb" attribute SHOULD be used to indicate which RTCP FB
   messages MAY be used in this media session for the indicated
   payload type.  A wildcard payload type ("*") MAY be used to
   indicate that the RTCP feedback attribute applies to all payload
   types.  If several types of feedback are supported and/or the same
   feedback shall be specified for a subset of the payload types,
   several "a=rtcp-fb" lines MUST be used.

   If no "rtcp-fb" attribute is specified the RTP receivers MAY send
   feedback using other suitable RTCP feedback packets as defined for
   the respective media type.  The RTP receivers MUST NOT rely on the
   RTP senders reacting to any of the FB messages.  The RTP sender MAY
   choose to ignore some feedback messages.

   If one or more "rtcp-fb" attributes are present in a media session
   description, the RTCP receivers for the media session(s) containing
   the "rtcp-fb"

   o MUST ignore all "rtcp-fb" attributes of which they do not fully
      understand the semantics (i.e. where they do not understand the
      meaning of all values in the "a=rtcp-fb" line);

   o SHOULD provide feedback information as specified in this
      document using any of the RTCP feedback packets as specified in
      one of the "rtcp-fb" attributes for this media session; and



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   o MUST NOT use other FB messages than those listed in one of the
      "rtcp-fb" attribute lines.

   When used in conjunction with the offer/answer model [9], the
   offerer MAY present a set of these AVPF attributes to its peer.
   The answerer MUST remove all attributes it does not understand as
   well as those it does not support in general or does not wish to
   use in this particular media session.  The answerer MUST NOT add
   feedback parameters to the media description and MUST NOT alter
   values of such parameters.  The answer is binding for the media
   session and both offerer and answerer MUST only use feedback
   mechanisms negotiated in this way.  Both offerer and answerer MAY
   independently decide to send RTCP FB messages of only a subset of
   the negotiated feedback mechanisms; but they SHOULD react properly
   to all types of the negotiated FB messages when received.

   RTP senders MUST be prepared to receive any kind of RTCP FB
   messages and MUST silently discard all those RTCP FB messages that
   they do not understand.

   The syntax of the "rtcp-fb" attribute is as follows (the feedback
   types and optional parameters are all case sensitive):

   (In the following ABNF, fmt, SP and CRLF are used as defined in
   [3].)

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

   rtcp-fb-pt         = "*"   ; wildcard: applies to all formats
                      / fmt   ; as defined in SDP spec

   rtcp-fb-val        = "ack" rtcp-fb-ack-param
                      / "nack" rtcp-fb-nack-param
                      / "trr-int" SP 1*DIGIT
                      / rtcp-fb-id rtcp-fb-param

   rtcp-fb-id         = 1*(alpha-numeric / "-" / "_")

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

   rtcp-fb-ack-param  = SP "rpsi"
                      / SP "app" [SP byte-string]
                      / SP token [SP byte-string]
                      / ; empty

   rtcp-fb-nack-param = SP "pli"
                      / SP "sli"
                      / SP "rpsi"

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                      / SP "app" [SP byte-string]
                      / SP token [SP byte-string]
                      / ; empty


   The literals of the above grammar have the following semantics:

   Feedback type "ack":

        This feedback type indicates that positive acknowledgements
        for feedback are supported.

        The feedback type "ack" MUST only be used if the media session
        is allowed to operate in ACK mode as defined in 3.6.1.2.

        Parameters MAY be provided to further distinguish different
        types of positive acknowledgement feedback.  If no parameters
        are present, the Generic ACK as specified in section 6.2.2 is
        implied.

        The parameter "rpsi" indicates the use of Reference Picture
        Selection Indication feedback as defined in section 6.3.3.

        If the parameter "app" is specified, this indicates the use of
        application layer feedback.  In this case, additional
        parameters following "app" MAY be used to further
        differentiate various types of application layer feedback.
        This document does not define any parameters specific to
        "app".

        Further parameters for "ack" MAY be defined in other
        documents.

   Feedback type "nack":

        This feedback type indicates that negative acknowledgements
        for feedback are supported.

        The feedback type "nack", without parameters, indicates use of
        the General NACK feedback format as defined in section 6.2.1.

        The following three parameters are defined in this document
        for use with "nack" in conjunction with the media type
        "video":

        o "pli" indicates the use of Picture Loss Indication feedback
           as defined in section 6.3.1.
        o "sli" indicates the use of Slice Loss Indication feedback
           as defined in section 6.3.2.


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        o "rpsi" indicates the use of Reference Picture Selection
           Indication feedback as defined in section 6.3.3.

        "app" indicates the use of application layer feedback.
        Additional parameters after "app" MAY be provided to
        differentiate different types of application layer feedback.
        No parameters specific to "app" are defined in this document.

        Further parameters for "nack" MAY be defined in other
        documents.

   Other feedback types <rtcp-fb-id>:

        Other documents MAY define additional types of feedback; to
        keep the grammar extensible for those cases, the rtcp-fb-id is
        introduced as a placeholder.  A new feedback scheme name MUST
        to be unique (and thus MUST be registered with IANA).  Along
        with a new name, its semantics, packet formats (if necessary),
        and rules for its operation MUST be specified.

   Regular RTCP minimum interval "trr-int":

        The attribute "trr-int" is used to specify the minimum
        interval T_rr_interval between two Regular (full compound)
        RTCP packets in milliseconds for this media session.  If "trr-
        int" is not specified, a default value of 0 is assumed.

   Note that it is assumed that more specific information about
   application layer feedback (as defined in section 6.4) will be
   conveyed as feedback types and parameters defined elsewhere.
   Hence, no further provision for any types and parameters is made in
   this document.

   Further types of feedback as well as further parameters may be
   defined in other documents.

   It is up to the recipients whether or not they send feedback
   information and up to the sender(s) (how) to make use of feedback
   provided.



   4.3  RTCP Bandwidth Modifiers

   The standard RTCP bandwidth assignments as defined in [1] and [2]
   may be overridden by bandwidth modifiers that explicitly define the
   maximum RTCP bandwidth.  For use with SDP, such modifiers are
   specified in [4]: "b=RS:<bw>" and "b=RR:<bw>" MAY be used to assign
   a different bandwidth (measured in bits per second) to RTP senders


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   and receivers, respectively.  The precedence rules of [4] apply to
   determine the actual bandwidth to be used by senders and receivers.

   Applications operating knowingly over highly asymmetric links (such
   as satellite links) SHOULD use this mechanism to reduce the
   feedback rate for high bandwidth streams to prevent deterministic
   congestion of the feedback path(s).


   4.4  Examples

   Example 1: The following session description indicates a session
   made up from audio and DTMF [18] for point-to-point communication
   in which the DTMF stream uses Generic ACKs.  This session
   description could be contained in a SIP INVITE, 200 OK, or ACK
   message to indicate that its sender is capable of and willing to
   receive feedback for the DTMF stream it transmits.

      v=0
      o=alice 3203093520 3203093520 IN IP4 host.example.com
      s=Media with feedback
      t=0 0
      c=IN IP4 host.example.com
      m=audio 49170 RTP/AVPF 0 96
      a=rtpmap:0 PCMU/8000
      a=rtpmap:96 telephone-event/8000
      a=fmtp:96 0-16
      a=rtcp-fb:96 ack

   This allows sender and receiver to provide reliable transmission of
   DTMF events in an audio session.  Assuming a 64kbit/s audio stream
   with one receiver, the receiver has 2.5% RTCP bandwidth available
   for the acknowledgment stream, i.e. 250 bytes per second or some 2
   RTCP feedback message.  Hence, the receiver can individually
   confirm receipt of two DTMF digits per seconds.


   Example 2: The following session description indicates a multicast
   video-only session (using either H.261 or H.263+) with the video
   source accepting Generic NACKs for both codecs and Reference
   Picture Selection for H.263.  Such a description may have been
   conveyed using the Session Announcement Protocol (SAP).

      v=0
      o=alice 3203093520 3203093520 IN IP4 host.example.com
      s=Multicast video with feedback
      t=3203130148 3203137348
      m=audio 49170 RTP/AVP 0
      c=IN IP4 224.2.1.183
      a=rtpmap:0 PCMU/8000

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      m=video 51372 RTP/AVPF 98 99
      c=IN IP4 224.2.1.184
      a=rtpmap:98 H263-1998/90000
      a=rtpmap:99 H261/90000
      a=rtcp-fb:* nack
      a=rtcp-fb:98 nack rpsi

   The sender may use an incoming Generic NACK as a hint to send a new
   intra-frame as soon as possible (congestion control permitting).
   Receipt of an RPSI message allows the sender to avoid sending a
   large intra-frame; instead it may continue to send inter-frames,
   however, choosing the indicated frame as new encoding reference.


   Example 3: The following session description defines the same media
   session as example 2 but allows for mixed mode operation of AVP and
   AVPF RTP entities (see also next section).  Note that both media
   descriptions use the same addresses; however, two m= lines are
   needed to convey information about both applicable RTP profiles.

      v=0
      o=alice 3203093520 3203093520 IN IP4 host.example.com
      s=Multicast video with feedback
      t=3203130148 3203137348
      m=audio 49170 RTP/AVP 0
      c=IN IP4 224.2.1.183
      a=rtpmap:0 PCMU/8000
      m=video 51372 RTP/AVP 98 99
      c=IN IP4 224.2.1.184
      a=rtpmap:98 H263-1998/90000
      a=rtpmap:99 H261/90000
      m=video 51372 RTP/AVPF 98 99
      c=IN IP4 224.2.1.184
      a=rtpmap:98 H263-1998/90000
      a=rtpmap:99 H261/90000
      a=rtcp-fb:* nack
      a=rtcp-fb:98 nack rpsi

   Note that these two m= lines SHOULD be grouped by some appropriate
   mechanism to indicate that both are alternatives actually conveying
   the same contents.  A sample mechanism by which this can be
   achieved is defined in [7].

   In this example, the RTCP feedback-enabled receivers will gain an
   occasional advantage to report events earlier back to the sender
   (which may benefit the entire group).  On average, however, all RTP
   receivers will provide the same amount of feedback.  The
   interworking between AVP and AVPF entities is discussed in depth in
   the next section.


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5  Interworking and Co-Existence of AVP and AVPF Entities

   The AVPF profile defined in this document is an extension of the
   AVP profile as defined in [2].  Both profiles follow the same basic
   rules (including the upper bandwidth limit for RTCP and the
   bandwidth assignments to senders and receivers).  Therefore,
   senders and receivers of using either of the two profiles can be
   mixed in a single session (see e.g. example 3 in section 4.5).

   AVP and AVPF are defined in a way that, from a robustness point of
   view, the RTP entities do not need to be aware of entities of the
   respective other profile: they will not disturb each other's
   functioning.  However, the quality of the media presented may
   suffer.

   The following considerations apply to senders and receivers when
   used in a combined session.

   o AVP entities (senders and receivers)

      AVP senders will receive RTCP feedback packets from AVPF
      receivers and ignore these packets.  They will see occasional
      closer spacing of RTCP messages (e.g. violating the five second
      rule) by AVPF entities.  As the overall bandwidth constraints
      are adhered to by both types of entities, they will still get
      their share of the RTCP bandwidth.  However, while AVP entities
      are bound by the five second rule, depending on the group size
      and session bandwidth, AVPF entities may provide more frequent
      RTCP reports than AVP ones will.  Also, the overall reporting
      may decrease slightly as AVPF entities may send bigger compound
      RTCP packets (due to the extra RTCP packets).

      If T_rr_interval is used as lower bound between Regular RTCP
      packets, T_rr_interval is sufficiently large (e.g. T_rr_interval
      > M*Td as per section 6.3.5 of [1]), and no Early RTCP packets
      are sent by AVPF entities, AVP entities may accidentally time
      out those AVPF group members and hence under-estimate the group
      size.  Therefore, if AVP entities may be involved in a media
      session, T_rr_interval SHOULD NOT be larger than five seconds.

   o AVPF entities (senders and receivers)

      If the dynamically calculated T_rr is sufficiently small (e.g.
      less than one second), AVPF entities may accidentally time out
      AVP group members and hence under-estimate the group size.
      Therefore, if AVP entities may be involved in a media session,
      T_rr_interval SHOULD be used and SHOULD be set to five seconds.



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      In conclusion, if AVP entities may be involved in a media
      session and T_rr_interval is to be used, T_rr_interval SHOULD be
      set to five seconds.

   o AVPF senders

      AVPF senders will receive feedback information only from AVPF
      receivers.  If they rely on feedback to provide the target media
      quality, the quality achieved for AVP receivers may be sub-
      optimal.

   o AVPF receivers

      AVPF receivers SHOULD send Early RTCP feedback packets only if
      all sending entities in the media session support AVPF.  AVPF
      receivers MAY send feedback information as part of regularly
      scheduled compound RTCP packets following the timing rules of
      [1] and [2] also in media sessions operating in mixed mode.
      However, the receiver providing feedback MUST NOT rely on the
      sender reacting to the feedback at all.


6  Format of RTCP Feedback Messages

   This section defines the format of the low delay RTCP feedback
   messages.  These messages are classified into three categories as
   follows:

   - Transport layer FB messages
   - Payload-specific FB messages
   - Application layer FB messages

   Transport layer FB messages are intended to transmit general
   purpose feedback information, i.e. information independent of the
   particular codec or the application in use.  The information is
   expected to be generated and processed at the transport/RTP layer.
   Currently, only a general positive acknowledgement (ACK) and a
   negative acknowledgement (NACK) message are defined.

   Payload-specific FB messages transport information that is specific
   to a certain payload type and will be generated and acted upon at
   the codec "layer".  This document defines a common header to be
   used in conjunction with all payload-specific FB messages.  The
   definition of specific messages is left to either RTP payload
   format specifications or to additional feedback format documents.

   Application layer FB messages provide a means to transparently
   convey feedback from the receiver's to the sender's application.
   The information contained in such a message is not expected to be
   acted upon at the transport/RTP or the codec layer.  The data to be

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   exchanged between two application instances is usually defined in
   the application protocol specification and thus can be identified
   by the application so that there is no need for additional external
   information.  Hence, this document defines only a common header to
   be used along with all application layer FB messages.  From a
   protocol point of view, an application layer FB message is treated
   as a special case of a payload-specific FB message.

        NOTE: Proper processing of some FB messages at the media
        sender side may require the sender to know which payload type
        the FB message refers to.  Most of the time, this knowledge
        can likely be derived from a media stream using only a single
        payload type.  However, if several codecs are used
        simultaneously (e.g. with audio and DTMF) or when codec
        changes occur, the payload type information may need to be
        conveyed explicitly as part of the FB message.  This applies
        to all payload-specific as well as application layer FB
        messages.  It is up to the specification of a FB message to
        define how payload type information is transmitted.

   This document defines two transport layer and three (video)
   payload-specific FB messages as well as a single container for
   application layer FB messages.  Additional transport layer and
   payload specific FB messages MAY be defined in other documents and
   MUST be registered through IANA (see section IANA considerations).

   The general syntax and semantics for the above RTCP FB message
   types are described in the following subsections.


   6.1  Common Packet Format for Feedback Messages

   All FB messages MUST use a common packet format that is depicted in
   figure 3:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V=2|P|   FMT   |       PT      |          length               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  SSRC of packet sender                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  SSRC of media source                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :            Feedback Control Information (FCI)                 :
   :                                                               :

   Figure 3: Common Packet Format for Feedback Messages



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   The various fields V, P, SSRC and length are defined in the RTP
   specification [2], the respective meaning being summarized below:

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

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

   Feedback message type (FMT): 5 bits
       This field identifies the type of the FB message and is
       interpreted relative to the   type (transport, payload-
       specific, or application layer feedback).  The values for each
       of the three feedback types are defined in the respective
       sections below.

   Payload type (PT): 8 bits
       This is the RTCP packet type which identifies the packet as
       being an RTCP FB message.  Two values are defined (TBA. by
       IANA):

             Name   | Value | Brief Description
          ----------+-------+------------------------------------
             RTPFB  |  205  | Transport layer FB message
             PSFB   |  206  | Payload-specific FB message

   Length: 16 bits
       The length of this packet in 32-bit words minus one, including
       the header and any padding.  This is in line with the
       definition of the length field used in RTCP sender and receiver
       reports [3].

   SSRC of packet sender: 32 bits
       The synchronization source identifier for the originator of
       this packet.

   SSRC of media source: 32 bits
       The synchronization source identifier of the media source that
       this piece of feedback information is related to.

   Feedback Control Information (FCI): variable length
       The following three sections define which additional
       information MAY be included in the FB message for each type of
       feedback: transport layer, payload-specific or application
       layer feedback. Note that further FCI contents MAY be specified
       in further documents.


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   Each RTCP feedback packet MUST contain at least one FB message in
   the FCI field.  Sections 6.2 and 6.3 define for each FCI type,
   whether or not multiple FB messages MAY be compressed into a single
   FCI field.  If this is the case, they MUST be of the same type,
   i.e. same FMT.  If multiple types of feedback messages, i.e.
   several FMTs, need to be conveyed, then several RTCP FB messages
   MUST be generated and SHOULD be concatenated in the same compound
   RTCP packet.

   6.2  Transport Layer Feedback Messages

   Transport Layer FB messages are identified by the value RTPFB as
   RTCP message type.

   Two general purpose transport layer FB messages are defined so far:
   General ACK and General NACK.  They are identified by means of the
   FMT parameter as follows:

       0:   unassigned
       1:   Generic NACK
       2:   Generic ACK
       3-30: unassigned
       31:  reserved for future expansion of the identifier number
   space

   The following two subsections define the formats of the FCI field
   for these types of FB messages:


   6.2.1 Generic NACK

   The Generic NACK message is identified by PT=RTPFB and FMT=1.

   The FCI field MUST contain at least one and MAY contain more than
   one Generic NACK.

   The Generic NACK packet is used to indicate the loss of one or more
   RTP packets.  The lost packet(s) are identified by the means of a
   packet identifier and a bit mask.

   Generic NACK feedback SHOULD NOT be used if the underlying
   transport protocol is capable of providing similar feedback
   information to the sender (as may be the case e.g. with DCCP).

   The Feedback control information (FCI) field has the following
   Syntax (figure 4):





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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            PID                |             BLP               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 4: Syntax for the Generic NACK message


   Packet ID (PID): 16 bits
       The PID field is used to specify a lost packet.  The PID field
       refers to the RTP sequence number of the lost packet.

   bitmask of following lost packets (BLP): 16 bits
       The BLP allows for reporting losses of any of the 16 RTP
       packets immediately following the RTP packet indicated by the
       PID.  The BLP's definition is identical to that given in [6].
       Denoting the BLP's least significant bit as bit 1, and its most
       significant bit as bit 16, then bit i of the bit mask is set to
       1 if the receiver has not received RTP packet number (PID+i)
       (modulo 2^16) and indicates this packet is lost; bit i is set
       to 0 otherwise.  Note that the sender MUST NOT assume that a
       receiver has received a packet because its bit mask was set to
       0.   For example, the least significant bit of the BLP would be
       set to 1 if the packet corresponding to the PID and the
       following packet have been lost.  However, the sender cannot
       infer that packets PID+2 through PID+16 have been received
       simply because bits 2 through 15 of the BLP are 0; all the
       sender knows is that the receiver has not reported them as lost
       at this time.

   The length of the FB message MUST be set to 2+n, with n being the
   number of Generic NACKs contained in the FCI field.

   The Generic NACK message implicitly references the payload type
   through the sequence number(s).


   6.2.2 Generic ACK

   The Generic ACK message is identified by PT=RTPFB and FMT=2.

   The FCI field MUST contain at least one and MAY contain more than
   one Generic ACK.

   The Generic ACK packet is used to indicate that one or several RTP
   packets were received correctly.  The received packet(s) are
   identified by the means of a packet identifier and a bit mask.
   ACKing of a range of consecutive packets is also possible.


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   Generic ACK feedback SHOULD NOT be used if the underlying transport
   protocol is capable of providing similar feedback information to
   the sender (as may be the case e.g. with DCCP).

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              PID              |R|       BLP/#packets          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 5: Syntax for the Generic ACK message


   Packet ID (1st PID): 16 bits
       This PID field is used to specify a correctly received packet.
       The PID field refers to the RTP sequence number of the lost
       packet.

   Range of ACKs (R): 1 bit
       The R-bit indicates that a range of consecutive packets are
       received correctly.  If R=1 then the PID field specifies the
       first packet of that range and the next field (BLP/#packets)
       will carry the number of packets being acknowledged.  If R=0
       then PID specifies the first packet to be acknowledged and
       BLP/#packets provides a bit mask to selectively indicate
       individual packets that are acknowledged.

   Bit mask of lost packets (BLP)/#packets (PID): 15 bits
       The semantics of this field depends on the value of the R-bit.

       If R=1, this field is used to identify the number of additional
       packets of to be acknowledged:

            #packets = <highest seq# to be ACKed> - <PID>

       That is, #packets MUST indicate the number of packet to be
       ACKed minus one.  In particular, if only a single packet is to
       be ACKed and R=1 then #packets MUST be set to 0x0000.

       Example: If all packets between and including PIDx = 380 and
       PIDy = 422 have been received, the Generic ACK would contain
       PID = PIDx = 380 and #packets = PIDy - PID = 42.  In case the
       PID wraps around, modulo arithmetic is used to calculate the
       number of packets.

       If R=0, this field carries a bit mask. The BLP allows for
       reporting reception of any of the 15 RTP packets immediately

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       following the RTP packet indicated by the PID.  The BLP's
       definition is identical to that given in [6] except that, here,
       BLP is only 15 bits wide.  Denoting the BLP's least significant
       bit as bit 1, and its most significant bit as bit 15, then bit
       i of the bitmask is set to 1 if the receiver has received RTP
       packet number (PID+i) (modulo 2^16) and decides to ACK this
       packet; bit i is set to 0 otherwise.  If only the packet
       indicated by PID is to be ACKed and R=0 then BLP MUST be set to
       0x0000.

   The length of the FB message MUST be set to 2+n, with n being the
   number of Generic ACKs contained in the FCI field.

   The Generic ACK message implicitly references the payload type
   through the sequence number(s).


   6.3  Payload Specific Feedback Messages

   Payload-Specific FB messages are identified by the value PT=PSFB as
   RTCP message type.

   Three payload-specific FB messages are defined so far plus an
   application layer FB message.  They are identified by means of the
   FMT parameter as follows:

     0:     unassigned
     1:     Picture Loss Indication (PLI)
     2:     Slice Lost Indication (SLI)
     3:     Reference Picture Selection Indication (RPSI)
     4-14:  unassigned
     15:    Application layer FB message
     16-30: unassigned
     31:    reserved for future expansion of the sequence number space

   The following subsections define the  FCI formats for the payload-
   specific FB messages, section 6.4 defines FCI format for the
   application layer FB message.


   6.3.1 Picture Loss Indication (PLI)

   The PLI FB message is identified by PT=PSFB and FMT=1.

   There MUST be exactly one PLI contained in the FCI field.






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   6.3.1.1 Semantics

   With the Picture Loss Indication message, a decoder informs the
   encoder about the loss of an undefined amount of coded video data
   belonging to one or more pictures.  When used in conjunction with
   any video coding scheme that is based on inter-picture prediction,
   an encoder that receives a PLI becomes aware that the prediction
   chain may be broken.  The sender MAY react to a PLI by transmitting
   an intra-picture to achieve resynchronization (making effectively
   similar to the FIR as defined in [6]); however, the sender MUST
   consider congestion control as outlined in section 7 which MAY
   restrict its ability to send an intra frame.

   Other RTP payload specifications such as RFC 2032 [6] already
   define a feedback mechanism for some 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.


   6.3.1.2 Message Format

   PLI does not require parameters.  Therefore, the length field MUST
   be 2, and there MUST NOT be any Feedback Control Information.

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


   6.3.1.3 Timing Rules

   The timing follows the rules outlined in section 3.  In systems
   that employ both PLI and other types of feedback it may be
   advisable to follow the Regular RTCP RR timing rules for PLI, since
   PLI is not as delay critical as other FB types.


   6.3.1.4 Remarks

   PLI messages typically trigger the sending of full intra pictures.
   Intra pictures are several times larger then predicted (inter)
   pictures.  Their size is independent of the time they are
   generated.  In most environments, especially when employing
   bandwidth-limited links, the use of an intra picture implies an
   allowed delay that is a significant multitude of the typical frame
   duration.  An example: If the sending frame rate is 10 fps, and an


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   intra picture is assumed to be 10 times as big as an inter picture,
   then a full second of latency has to be accepted.  In such an
   environment there is no need for a particular short delay in
   sending the FB message.  Hence waiting for the next possible time
   slot allowed by RTCP timing rules as per [2] does not have a
   negative impact on the system performance.


   6.3.2 Slice Lost Indication (SLI)

   The SLI FB message is identified by PT=PSFB and FMT=2.

   The FCI field MUST contain at least one and MAY contain more than
   one SLI.


   6.3.2.1 Semantics

   With the Slice Lost Indication a decoder can inform an encoder that
   it has detected the loss or corruption of one or several
   consecutive macroblock(s) in scan order (see below).  This FB
   message MUST NOT be used for video codecs with non-uniform,
   dynamically changeable macroblock sizes such as H.263 with enabled
   Annex Q.  In such a case, an encoder cannot always identify the
   corrupted spatial region.


   6.3.2.2 Format

   The  Slice Lost Indication uses one additional PCI field the
   content of which is depicted in figure 6.  The length of the FB
   message MUST be set to 2+n, with n being the number of SLIs
   contained in the FCI field.


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            First        |  Number                 | PictureID |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 6: Syntax of the Slice Lost Indication (SLI)


   First: 13 bits
       The macroblock (MB) address of the first lost macroblock.  The
       MB numbering is done such that the macroblock in the upper left
       corner of the picture is considered macroblock number 1 and the
       number for each macroblock increases from left to right and


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       then from top to bottom in raster-scan order (such that if
       there is a total of N macroblocks in a picture, the bottom
       right macroblock is considered macroblock number N).

   Number: 13 bits
       The number of lost macroblocks, in scan order as discussed
   above.

   PictureID: 6 bits
       The six least significant bits of the a codec-specific
       identifier that is used to reference the picture in which the
       loss of the macroblock (s) has occurred.  For many video
       codecs, the PictureID is identical to the Temporal Reference.

   The applicability of this FB message is limited to a small set of
   video codecs and therefore, no explicit payload type information is
   provided.


   6.3.2.3 Timing Rules

   The efficiency of algorithms using the Slice Lost Indication is
   reduced greatly when the Indication is not transmitted in a timely
   fashion.  Motion compensation propagates corrupted pixels that are
   not reported as being corrupted.  Therefore, the use of the
   algorithm discussed in section 3 is highly recommended.


   6.3.2.4 Remarks

   The term Slice is defined and used here in the sense of MPEG-1 -- a
   consecutive number of macroblocks in scan order.  More recent video
   coding standards sometimes have a different understanding of the
   term Slice.  In H.263 (1998), for example, a concept known as
   "rectangular Slice" exist.  The loss of one Rectangular Slice may
   lead to the necessity of sending more than one SLI in order to
   precisely identify the region of lost/damaged MBs.

   The first field of the FCI defines the first macroblock of a
   picture as 1 and not, as one could suspect, as 0.  This was done to
   align this specification with the comparable mechanism available in
   H.245.  The maximum number of macroblocks in a picture (2**13 or
   8192) corresponds to the maximum picture sizes of most of the ITU-T
   and ISO/IEC video codecs.  If future video codecs offer larger
   picture sizes and/or smaller macroblock sizes, then an additional
   FB message has to be defined.  The six least significant bits of
   the Temporal Reference field are deemed to be sufficient to
   indicate the picture in which the loss occurred.



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   The reaction to a SLI is not part of this specification.  One
   typical way of reacting to a SLI is to use intra refresh for the
   affected spatial region.

   Algorithms were reported that keep track of the regions affected by
   motion compensation, in order to allow for a transmission of Intra
   macroblocks to all those areas, regardless of the timing of the FB
   (see H.263 (2000) Appendix I [17] and [15]).  While, when those
   algorithms are used, the timing of the FB is less critical then
   without, it has to be observed that those algorithms correct large
   parts of the picture and, therefore, have to transmit much higher
   data volume in case of delayed FBs.


   6.3.3 Reference Picture Selection Indication (RPSI)

   The RPSI FB message is identified by PT=PSFB and FMT=3.

   There MUST be exactly one RPSI contained in the FCI field.


   6.3.3.1 Semantics

   Modern video coding standards such as MPEG-4 visual version 2 [16]
   or H.263 version 2 [17] allow to use older reference pictures than
   the most recent one for predictive coding.  Typically, a first-in-
   first-out queue of reference pictures is maintained.  If an encoder
   has learned about a loss of encoder-decoder synchronicity, a known-
   as-correct reference picture can be used. As this reference picture
   is temporally further away then usual, the resulting predictively
   coded picture will use more bits.

   Both MPEG-4 and H.263 define a binary format for the "payload" of
   an RPSI message that includes information such as the temporal ID
   of the damaged picture and the size of the damaged region.  This
   bit string is typically small -- a couple of dozen bits --, of
   variable length, and self-contained, i.e. contains all information
   that is necessary to perform reference picture selection.

   Note that both MPEG-4 and H.263 allow the use of RPSI with positive
   feedback information as well.  That is, pictures (or Slices) are
   reported that were decoded without error.  Note that any form of
   positive feedback MUST NOT be used when in a multiparty session
   (reporting positive feedback about individual reference pictures at
   RTCP intervals is not expected to be of much use anyway).






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   6.3.3.2 Format

   The FCI for the RPSI message follows the format 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      PB       |0| Payload Type|  Native RPSI bit string       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   defined per codec          ...                |  Padding (0)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 7: Syntax of the Reference Picture Selection Indication
   (RPSI)


   PB: 8 bits
       The number of unused bits required to pad the length of the
       RPSI message to a multiple of 32 bits.

   0:  1 bit
        MUST be set to zero upon transmission and ignored upon
        reception.

   Payload Type: 8 bits
        Indicates the RTP payload type in the context of which the
        native RPSI bit string MUST be interpreted.

   Native RPSI bit string: variable length
       The RPSI information as natively defined by the video codec.

   Padding: #PB bits
       A number of bits set to zero to fill up the contents of the
       RPSI message to the next 32 bit boundary.  The number of
       padding bits MUST be indicated by the PB field.


   6.3.3.3 Timing Rules

   RPS is even more critical to delay then algorithms using SLI.  This
   is due to the fact that the older the RPS message is, the more bits
   the encoder has to spend to re-establish encoder-decoder
   synchronicity.  See [15] for some information about the overhead of
   RPS for certain bit rate/frame rate/loss rate scenarios.

   Therefore, RPS messages should typically be sent as soon as
   possible, employing the algorithm of section 3.



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   6.4  Application Layer Feedback Messages

   Application Layer FB messages are a special case of payload-
   specific messages and identified by PT=PSFB and FMT=15.
   There MUST be exactly one Application Layer FB message contained in
   the FCI field, unless the Application Layer FB message structure
   itself allows for stacking (e.g. by means of a fixed size or
   explicit length indicator).

   These messages are used to transport application defined data
   directly from the receiver's to the sender's application. The data
   that is transported is not identified by the FB message.
   Therefore, the application MUST be able to identify the messages
   payload.

   Usually, applications define their own set of messages, e.g.
   NEWPRED  messages in MPEG-4 [16] or FB messages in H.263/Annex N, U
   [17].  These  messages do not need any additional information from
   the RTCP  message.  Thus the application message is simply placed
   into the FCI field as follows and the length field is set
   accordingly.

   Application Message (FCI): variable length
       This field contains the original application message that
       should be transported from the receiver to the source. The
       format is application dependent. The length of this field is
       variable. If the application data is not 32-bit-aligned,
       padding bits and bytes must be added.  Identification of
       padding is up to the application layer and not defined in this
       specification.

   The application layer FB message specification MUST define whether
   or not the message needs to be interpreted specifically in the
   context of a certain codec (identified by the RTP payload type).
   If a reference to the payload type is required for proper
   processing, the application layer FB message specification MUST
   define a way to communicate the payload type information as part of
   the application layer FB message itself.


7  Early Feedback and Congestion Control

   In the previous sections, the FB messages were defined as well as
   the timing rules according to which to send these messages.  The
   way to react to the feedback received depends on the application
   using the feedback mechanisms and hence is beyond the scope of this
   document.




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   However, across all applications, there is a common requirement for
   (TCP-friendly) congestion control on the media stream as defined in
   [1] and [2] when operating in a best-effort network environment.

   Low delay feedback supports the use of congestion control
   algorithms in two ways:

   o The potentially more frequent RTCP packets allow the sender to
      monitor the network state more closely than with Regular RTCP
      packets and therefore enable a more timely reaction to upcoming
      congestion.

   o The FB messages themselves may convey additional information as
      input to congestion control algorithms and thus improve reaction
      over conventional RTCP. (For example, ACK-based feedback may
      even allow to construct closed loop algorithms and  NACK-based
      systems may provide further information on the packet loss
      distribution.)

   It should be noted, however, that RTCP feedback may operate at much
   slower timescales than other transport layer feedback mechanisms
   (that usually operate in the order of RTT).  Therefore, any rate
   adaptation may occur slower than TCP.

   A congestion control algorithm that shares the available bandwidth
   reasonably fairly with competing TCP connections, e.g. TFRC [8],
   MUST be used to determine the data rate for the media stream within
   the bounds of the RTP sender's and the media session's capabilities
   if the RTP/AVPF session is transmitted in a best effort
   environment.

   That is, RTCP FB messages or RTCP SR/RR packets that indicate
   recent packet loss SHOULD cause the sender to adjust the
   transmission data rate to the order of the throughput TCP would
   achieve under similar conditions (e.g. using TFRC).  RTCP FB
   messages or RTCP SR/RR packets that indicate no recent packet loss
   MAY cause the sender to increase the transmission data rate to
   roughly the throughput TCP would achieve under similar conditions
   (e.g. using TFRC).  Such messages MUST NOT cause a larger increase
   in the sender's transmission rate than a comparable TCP flow would
   achieve.  The procedural details MUST be specified by the document
   defining the sender behavior for a certain codec and/or
   application.


8  Security Considerations

   RTP packets transporting information with the proposed payload
   format are subject to the security considerations discussed in the


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   RTP specification [1] and in the RTP/AVP profile specification [2].
   This profile does not specify any additional security services.

   This profile modifies the timing behavior of RTCP and eliminates
   the minimum RTCP interval of five seconds and allows for earlier
   feedback to be provided by receivers.  Group members of the
   associated RTP session (possibly pretending to represent a large
   number of entities) may disturb the operation of RTCP by sending
   large numbers of RTCP packets thereby reducing the RTCP bandwidth
   available for Regular RTCP reporting as well as for Early FB
   messages.  (Note that an entity need not be member of a multicast
   group to cause these effects.)  Similarly, malicious members may
   send very large RTCP messages, thereby increasing the avg_rtcp_size
   variable and reducing the effectively available RTCP bandwidth.

   Feedback information may be suppressed if unknown RTCP feedback
   packets are received.  This introduces the risk of a malicious
   group member reducing Early feedback by simply transmitting
   payload-specific RTCP feedback packets with random contents that
   are neither recognized by any receiver (so they will suppress
   feedback) nor by the sender (so no repair actions will be taken).

   A malicious group member can also report arbitrary high loss rates
   in the feedback information to make the sender throttle the data
   transmission and increase the amount of redundancy information or
   take other action to deal with the pretended packet loss (e.g. send
   fewer frames or decrease audio/video quality).  This may result in
   a degradation of the quality of the reproduced media stream.

   Finally, a malicious group member can act as a large number of
   group members and thereby obtain an artificially large share of the
   Early feedback bandwidth and reduce the reactivity of the other
   group members -- possibly even causing them to no longer operate in
   Immediate or Early feedback mode and thus undermining the whole
   purpose of this profile.

   Senders as well as receivers SHOULD behave conservatively when
   observing strange reporting behavior.  For excessive failure
   reporting from one or a few receivers, the sender MAY decide to no
   longer consider this feedback when adapting its transmission
   behavior for the media stream.  In any case, senders and receivers
   SHOULD still adhere to the maximum RTCP bandwidth but make sure
   that they are capable of transmitting at least regularly scheduled
   RTCP packets.  Senders SHOULD carefully consider how to adjust
   their transmission bandwidth when encountering strange reporting
   behavior; they MUST NOT increase their transmission bandwidth even
   if ignoring suspicious feedback.

   Attacks using false RTCP packets (Regular as well as Early ones)
   can be avoided by authenticating all RTCP messages.  This can be

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   achieved by using the AVPF profile together with the Secure RTP
   profile as defined in [22]; as a prerequisite, an appropriate
   combination of those two profiles (an "SAVPF") is being specified
   [21].


9  IANA Considerations

   The following contact information shall be used for all
   registrations included here:

     Contact:      Joerg Ott
                   mailto:jo@acm.org
                   tel:+49-421-201-7028

   The feedback profile as an extension to the profile for audio-
   visual conferences with minimal control needs to be registered for
   the Session Description Protocol (specifically the type "proto"):
   "RTP/AVPF".

   SDP Protocol ("proto"):

     Name:               RTP/AVPF
     Long form:          Extended RTP Profile with RTCP-based Feedback
     Type of name:       proto
     Type of attribute:  Media level only
     Purpose:            RFC XXXX
     Reference:          RFC XXXX

   SDP Attribute ("att-field"):

     Attribute name:     rtcp-fb
     Long form:          RTCP Feedback parameter
     Type of name:       att-field
     Type of attribute:  Media level only
     Subject to charset: No
     Purpose:            RFC XXXX
     Reference:          RFC XXXX
     Values:             See this document and registrations below

   A new registry needs to be set up for the "rtcp-fb" attribute, with
   the following registrations created initially: "ack", "nack", "trr-
   int", and "app" as defined in this document.

   Initial value registration for the attribute "rtcp-fb"

     Value name:     ack
     Long name:      Positive acknowledgement
     Reference:      RFC XXXX.


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     Value name:     nack
     Long name:      Negative Acknowledgement
     Reference:      RFC XXXX.

     Value name:     trr-int
     Long name:      Minimal receiver report interval
     Reference:      RFC XXXX.

     Value name:     app
     Long name:      Application-defined paramater
     Reference:      RFC XXXX.

   Further entries may be registered on a first-come first-serve
   basis.  Each new registration needs to indicate the parameter name
   and the syntax of possible additional arguments.  For each new
   registration, it is mandatory that a permanent, stable, and
   publicly accessible document exists that specifies the semantics of
   the registered parameter, the syntax and semantics of its
   parameters as well as corresponding feedback packet formats (if
   needed).  The general registration procedures of [3] apply.


   For use with both "ack" and "nack", a joint sub-registry needs to
   be set up that initially registers the following values:

   Initial value registration for the attribute values "ack" and
   "nack":

     Value name:     sli
     Long name:      Slice Loss Indication
     Usable with:    nack
     Reference:      RFC XXXX.

     Value name:     pli
     Long name:      Picture Loss Indication
     Usable with:    nack
     Reference:      RFC XXXX.

     Value name:     rpsi
     Long name:      Reference Picture Selection Indication
     Usable with:    ack, nack
     Reference:      RFC XXXX.

     Value name:     app
     Long name:      Application layer feedback
     Usable with:    ack, nack
     Reference:      RFC XXXX.

   Further entries may be registered on a first-come first-serve
   basis.  Each registrations needs to indicate the parameter name,

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   the syntax of possible additional arguments, and whether the
   parameter is applicable to "ack" or "nack" feedback or both or some
   different "rtcp-fb" attribute parameter.  For each new
   registration, it is mandatory that a permanent, stable, and
   publicly accessible document exists that specifies the semantics of
   the registered parameter, the syntax and semantics of its
   parameters as well as corresponding feedback packet formats (if
   needed).  The general registration procedures of [3] apply.

   Two RTCP Control Packet Types: for the class of transport layer FB
   messages ("RTPFB") and for the class of payload-specific FB
   messages ("PSFB").  Section 6 suggests RTPFB=205 and PSFB=206 to be
   added to the RTCP registry.

   RTP RTCP Control Packet types (PT):

     Name:          RTPFB
     Long name:     Generic RTP Feedback
     Value:         205
     Reference:     RFC XXXX.

     Name:          PSFB
     Long name:     Payload-specific
     Value:         206
     Reference:     RFC XXXX.

   As AVPF defines additional RTCP payload types, the corresponding
   "reserved" RTP payload type space (72--76, as defined in [2]),
   needs to be expanded accordingly.

   A new sub-registry needs to be set up for the FMT values for both
   the RTPFB payload type and the PSFB payload type, with the
   following registrations created initially:


















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   Within the RTPFB range, the following three format (FMT) values are
   initially registered:

     Name:           Generic NACK
     Long name:      Generic negative acknowledgement
     Value:          1
     Reference:      RFC XXXX.

     Name:           Generic ACK
     Long name:      Generic positive acknowledgement
     Value:          2
     Reference:      RFC XXXX.

     Name:           Extension
     Long name:      Reserved for future extensions
     Value:          31
     Reference:      RFC XXXX.


   Within the PSFB range, the following five format (FMT) values are
   initially registered:

     Name:           PLI
     Long name:      Picture Loss Indication
     Value:          1
     Reference:      RFC XXXX.

     Name:           SLI
     Long name:      Slice Loss Indication
     Value:          2
     Reference:      RFC XXXX.

     Name:           RPSI
     Long name:      Reference Picture Selection Indication
     Value:          3
     Reference:      RFC XXXX.

     Name:           AFB
     Long name:      Application Layer Feedback
     Value:          15
     Reference:      RFC XXXX.

     Name:           Extension
     Long name:      Reserved for future extensions.
     Value:          31
     Reference:      RFC XXXX.

   Further entries may be registered following the "Specification
   Required" rules as defined in RFC 2434 [10].  Each registration

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   needs to indicate the FMT value, if there is a specific FB message
   to go into the FCI field, and whether or not multiple FB messages
   may be stacked in a single FCI field.  For each new registration,
   it is mandatory that a permanent, stable, and publicly accessible
   document exists that specifies the semantics of the registered
   parameter as well as the syntax and semantics of the associated FB
   message (if any).  The general registration procedures of [3]
   apply.

NOTE TO THE RFC EDITOR: Please replace all occurrences of RFC XXXX by
the RFC number assigned to this document.


10 Acknowledgements

   This document is a product of the Audio-Visual Transport (AVT)
   Working Group of the IETF.  The authors would like to thank Steve
   Casner and Colin Perkins for their comments and suggestions as well
   as for their responsiveness to numerous questions.  The authors
   would also like to particularly thank Magnus Westerlund for his
   review and his valuable suggestions, Shigeru Fukunaga for the
   contributions on for FB message formats and semantics.

   We would also like to thank Andreas Buesching and people at
   Panasonic for their simulations and the first independent
   implementations of the feedback profile.


11 Authors' Addresses

   Joerg Ott            {sip,mailto}:jo@tzi.org
   Uni Bremen TZI
   MZH 5180
   Bibliothekstr. 1
   D-28359 Bremen
   Germany

   Stephan Wenger       stewe@cs.tu-berlin.de
   TU Berlin
   Sekr. FR 6-3
   Franklinstr. 28-29
   D-10587 Berlin
   Germany

   Noriyuki Sato
   Oki Electric Industry Co., Ltd.
   1-2-27 Shiromi, Chuo-ku, Osaka 540-6025 Japan
   Tel.  +81 6 6949 5101
   Fax.  +81 6 6949 5108
   Mail  sato652@oki.com

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   Carsten Burmeister
   Panasonic European Laboratories GmbH
   Mail  burmeister@panasonic.de

   Jose Rey
   Panasonic European Laboratories GmbH
   Monzastr. 4c, 63225 Langen, Germany
   Tel.  +49-(0)6103-766-134
   Fax.  +49-(0)6103-766-166
   Mail  rey@panasonic.de


12 Bibliography

   12.1 Normative references

   [1]  H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP
        - A Transport Protocol for Real-time Applications," RFC 3550,
        July 2003.

   [2]  H. Schulzrinne and S. Casner, "RTP Profile for Audio and Video
        Conferences with Minimal Control," RFC 3551, July 2003.

   [3]  M. Handley, V. Jacobson, and Colin Perkins, "SDP: Session
        Description Protocol", Internet Draft draft-ietf-mmusic-sdp-
        new-15.txt, October 2003.

   [4]  S. Casner, "SDP Bandwidth Modifiers for RTCP Bandwidth", RFC
        3556, July 2003.

   [5]  S. Bradner, "Key words for use in RFCs to Indicate Requirement
        Levels," RFC 2119, March 1997.

   [6]  T. Turletti and C. Huitema, "RTP Payload Format for H.261
        Video Streams, RFC 2032, October 1996.

   [7]  G. Camarillo, J. Holler, G. Eriksson, H. Schulzrinne,
        "Grouping of media lines in SDP," RFC 3388, December 2002.

   [8]  M. Handley, J. Padhye, S. Floyd, J. Widmer, "TCP friendly Rate
        Control (TFRC): Protocol Specification," RFC 3448, January
        2003.

   [9] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
        SDP," RFC 3264, June 2002.

   [10] T. Narten and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs," RFC 2434, October 1998.


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   12.2 Informative References

    [11]        C. Perkins and O. Hodson, "Options for Repair of
        Streaming Media," RFC 2354, June 1998.

   [12] J. Rosenberg and H. Schulzrinne, "An RTP Payload Format for
        Generic Forward Error Correction,", RFC 2733, December 1999.

   [13] C. Perkins, I. Kouvelas, O. Hodson, V. Hardman, M. Handley,
        J.C. Bolot, A. Vega-Garcia, and S. Fosse-Parisis, "RTP Payload
        for Redundant Audio Data," RFC 2198, September 1997.

   [14] C. Bormann, L. Cline, G. Deisher, T. Gardos, C. Maciocco, D.
        Newell, J. Ott, G. Sullivan, S. Wenger, and C. Zhu, "RTP
        Payload Format for the 1998 Version of ITU-T Rec. H.263 Video
        (H.263+)," RFC 2429, October 1998.

   [15] B. Girod, N. Faerber, "Feedback-based error control for mobile
        video transmission," Proceedings IEEE, Vol. 87, No. 10, pp.
        1707 - 1723, October, 1999.

   [16] ISO/IEC 14496-2:2001/Amd.1:2002, "Information technology -
        Coding of audio-visual objects - Part2: Visual", 2001.

   [17] ITU-T Recommendation H.263, "Video Coding for Low Bit Rate
        Communication," November 2000.

   [18] H. Schulzrinne and S. Petrack, "RTP Payload for DTMF Digits,
        Telephony Tones and Telephony Signals," RFC 2833, May 2000.

   [19] E. Kohler, M. Handley, S. Floyd, and J. Padhye, "Datagram
        Congestion Control Protocol (DCCP)," Internet Draft draft-
        ietf-dccp-spec-05.txt, Work in Progress, October 2003.

   [20] M. Handley, S. Floyd, J. Padhye, and J. Widmer, "TCP Friendly
        Rate Control (TFRC): Protocol Specification," RFC 3448,
        January 2003.

   [21] J. Ott and E. Carrara, "Extended Secure RTP Profile for RTCP-
        based Feedback (RTP/SAVPF)," Internet Draft draft-ietf-avt-
        profile-savpf-00.txt, Work in Progress, October 2003.

   [22] M. Baugher, R. Blom, E. Carrarra, D. McGrew, M. Naslund, K.
        Norrman, D. Oran, "The Secure Real-Time Transport Protocol,"
        Internet Draft, draft-ietf-avt-srtp-09.txt, Work in Progress,
        June 2003.




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13 IPR Notice

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on
   the IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication 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 implementors or users of this specification
   can be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF
   Executive Director.


14 Full Copyright Statement

   Copyright (C) The Internet Society (2003). All Rights Reserved.
   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain
   it or assist in its implementation may be prepared, copied,
   published and distributed, in whole or in part, without restriction
   of any kind, provided that the above copyright notice and this
   paragraph are included on all such copies and derivative works.

   However, this document itself may not be modified in any way, such
   as by removing the copyright notice or references to the Internet
   Society or other Internet organizations, except as needed for the
   purpose of developing Internet standards in which case the
   procedures for copyrights defined in the Internet Standards process
   must be followed, or as required to translate it into languages
   other than English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on
   an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
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   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."

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Internet Draft   draft-ietf-avt-rtcp-feedback-08.txt  31 January 2004




















































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