





Network Working Group                                             J. Ott
Request for Comments: 4585             Helsinki University of Technology
Category: Standards Track                                      S. Wenger
                                                                   Nokia
                                                                 N. Sato
                                                                     Oki
                                                           C. Burmeister
                                                                  J. Rey
                                                              Matsushita
                                                               July 2006


                        Extended RTP Profile for
 Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   Real-time media streams that use RTP are, to some degree, resilient
   against packet losses.  Receivers may use the base mechanisms of the
   Real-time Transport Control Protocol (RTCP) to report packet
   reception statistics and thus allow a sender to adapt its
   transmission behavior in the mid-term.  This is the 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 allows
   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 ................................................3
      1.2. Terminology ................................................5
   2. RTP and RTCP Packet Formats and Protocol Behavior ...............6
      2.1. RTP ........................................................6
      2.2. Underlying Transport Protocols .............................6
   3. Rules for RTCP Feedback .........................................7
      3.1. Compound RTCP Feedback Packets .............................7
      3.2. Algorithm Outline ..........................................8
      3.3. Modes of Operation .........................................9
      3.4. Definitions and Algorithm Overview ........................11
      3.5. AVPF RTCP Scheduling Algorithm ............................14
           3.5.1. Initialization .....................................15
           3.5.2. Early Feedback Transmission ........................15
           3.5.3. Regular RTCP Transmission ..........................18
           3.5.4. Other Considerations ...............................19
      3.6. Considerations on the Group Size ..........................20
           3.6.1. ACK Mode ...........................................20
           3.6.2. NACK Mode ..........................................20
      3.7. Summary of Decision Steps .................................22
           3.7.1. General Hints ......................................22
           3.7.2. Media Session Attributes ...........................22
   4. SDP Definitions ................................................23
      4.1. Profile Identification ....................................23
      4.2. RTCP Feedback Capability Attribute ........................23
      4.3. RTCP Bandwidth Modifiers ..................................27
      4.4. Examples ..................................................27
   5. Interworking and Coexistence of AVP and AVPF Entities ..........29
   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
      6.3. Payload-Specific Feedback Messages ........................35
           6.3.1. Picture Loss Indication (PLI) ......................36
           6.3.2. Slice Loss Indication (SLI) ........................37
           6.3.3. Reference Picture Selection Indication (RPSI) ......39
      6.4. Application Layer Feedback Messages .......................41
   7. Early Feedback and Congestion Control ..........................41
   8. Security Considerations ........................................42
   9. IANA Considerations ............................................43
   10. Acknowledgements ..............................................47
   11. References ....................................................48
      11.1. Normative References .....................................48
      11.2. Informative References ...................................48





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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 quality of service
   (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,
   retroactive Forward Error Correction (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
   proactively (thereby increasing the bandwidth of a given media
   stream).  Alternatively, in sufficiently small groups with small
   round-trip times (RTTs), the senders may perform repair on-demand,
   using the above 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: Firstly, 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.  Secondly, 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:







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   Early RTCP mode:
      The mode of operation in that 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 that is carried in RTCP
      receiver reports (RRs) -- back to the sender of the media 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.






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   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.

   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].













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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 that 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.

   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 the Datagram Congestion Control Protocol (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 limited error repair.  As RTCP feedback is not guaranteed to
      operate on sufficiently small timescales (in the order of RTT),



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      RTCP feedback is not suitable to support congestion control.  This
      memo addresses both unicast and multicast operation.

   DCCP: DCCP [19] provides for congestion-controlled but unreliable
      datagram flows for unicast communications.  With TCP Friendly Rate
      Control (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).

      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
      refrain 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 sender report (SR)/received report
      (RR) packets and are transmitted at regular intervals as part of
      compound RTCP packets (which also include source description
      (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.



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   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

      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





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   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 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.



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   And, depending on the reception quality and the locally monitored
   state of the RTP session, individual receivers may not (and do not
   have to) agree on a common perception on the current mode of
   operation.

   a) Immediate Feedback mode: In this 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 reporting (or that needed reporting).  But feedback can
      still be given sufficiently often so that it allows the sender 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) above.



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   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.

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 timer 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.





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   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 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



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      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.

   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 an 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.




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   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 have a new tn
   (tn=tp+2*T_rr) and a new 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.

             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.




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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 for 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.

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).

      2a) 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.

      2b) 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.






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          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
          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.

      3a) 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.

      3b) 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.

      4a) 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.

      4b) 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.





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   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 a 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:

      5a) 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).

      5b) 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.

      5c) 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 5c), 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 5a and 5b 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.



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          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

   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 Sections 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 Sections 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.






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   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 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 for timing out RTP
   entities.

   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.








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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 acknowledgements 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 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 the 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, and 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



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   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.

   As stated in Section 3.3:

      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.

   As stated in Section 3.3:

      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.

   Example: If a 256-kbit/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 9.6 kbit/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 are 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 and increases the group size to 10.  Assuming
   further that some number of packet losses are correlated, feedback



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   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.

   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 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.




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   In the IETF, the Session Description Protocol (SDP) is currently used
   to describe media sessions while protocols such as SIP, Session
   Announcement Protocol (SAP), Real Time Streaming Protocol (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.

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



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   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

   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 [8], 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].)





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      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"
                         / 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 Section 3.6.1.

      Parameters MUST be provided to further distinguish different types
      of positive acknowledgement feedback.

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








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      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
      Generic 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.

      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.








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   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 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 NACKs.  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



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      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 nack

   This allows sender and receiver to provide reliable transmission of
   DTMF events in an audio session.  Assuming a 64-kbit/s audio stream
   with one receiver, the receiver has 2.5% RTCP bandwidth available for
   the negative acknowledgement stream, i.e., 250 bytes per second or
   some 2 RTCP feedback messages every second.  Hence, the receiver can
   individually communicate up to two missing DTMF audio packets per
   second.

   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
      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 a Reference Picture Selection Indication (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.




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      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 framework by which this can be
   achieved is defined in [10].

   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.

5.  Interworking and Coexistence 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 using either of the two profiles can be
   mixed in a single session (see 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.




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   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 underestimate 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 underestimate 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.

      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 suboptimal.

   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





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      [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 generic negative acknowledgement (NACK) message is 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 either to 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
   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




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      payload-specific as well as application layer FB messages.  It is
      up to the specification of an 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 9, "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

   The 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 that are not part of the
      control information but are included in the length field.









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   Feedback message type (FMT): 5 bits
      This field identifies the type of the FB message and is
      interpreted relative to the type (transport layer, 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 that identifies the packet as being
      an RTCP FB message.  Two values are defined by the 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.

   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.








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6.2.   Transport Layer Feedback Messages

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

   A single general purpose transport layer FB message is defined in
   this document: Generic NACK.  It is identified by means of the FMT
   parameter as follows:

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

   The following subsection defines the formats of the FCI field for
   this type of FB message.  Further generic feedback messages MAY be
   defined in the future.

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 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):

    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.




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   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).

6.3.  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 Loss Indication (SLI)
      3:     Reference Picture Selection Indication (RPSI)
      4-14:  unassigned
      15:    Application layer FB (AFB) 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.







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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.

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 this message
   effectively similar to the FIR message 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



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   significant multitude of the typical frame duration.  An example: If
   the sending frame rate is 10 fps, and an intra-picture is assumed to
   be 10 times as big as an inter-picture, then a full second of latency
   has to be accepted.  In such an environment, there is no need for a
   particular short delay in sending the FB message.  Hence, waiting for
   the next possible time slot allowed by RTCP timing rules as per [2]
   with Tmin=0 does not have a negative impact on the system
   performance.

6.3.2.  Slice Loss 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 Loss 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 Loss Indication uses one additional FCI 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 Loss 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 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).



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

   PictureID: 6 bits
      The six least significant bits of the 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; therefore, no explicit payload type information is
   provided.

6.3.2.3.  Timing Rules

   The efficiency of algorithms using the Slice Loss 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" exists.  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 ITU-T
   Rec. H.245 [24].  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.

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






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   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]).  Although the timing of
   the FB is less critical when those algorithms are used than if they
   are not, 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 using 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.

   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: 7 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

   RPSI is even more critical to delay than algorithms using SLI.  This
   is because the older the RPSI 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 RPSI for certain bit
   rate/frame rate/loss rate scenarios.

   Therefore, RPSI 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 are 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 message payload.

   Usually, applications define their own set of messages, e.g., NEWPRED
   messages in MPEG-4 [16] (carried in RTP packets according to RFC 3016
   [23]) or FB messages in H.263/Annex N, U [17] (packetized as per RFC
   2429 [14]).  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 to achieve 32-bit alignment.  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.

   It should be noted that RTCP feedback itself is insufficient for
   congestion control purposes as it is likely to operate at much slower
   timescales than other transport layer feedback mechanisms (that
   usually operate in the order of RTT).  Therefore, additional
   mechanisms are required to perform proper congestion control.

   A congestion control algorithm that shares the available bandwidth
   reasonably fairly with competing TCP connections, e.g., TFRC [7],
   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.

8.  Security Considerations

   RTP packets transporting information with the proposed payload format
   are subject to the security considerations discussed in the 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 a 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 not
   recognized by any receiver (so they will suppress feedback) or 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.



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   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 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].  Note that,
   when employing group authentication (as opposed to source
   authentication), the aforementioned attacks may be carried out by
   malicious or malfunctioning group members in possession of the right
   keying material.

9.  IANA Considerations

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

     Contact:      Joerg Ott
                   mailto:jo@acm.org
                   tel:+358-9-451-2460

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










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   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 4585
     Reference:          RFC 4585

   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 4585
     Reference:          RFC 4585
     Values:             See this document and registrations below

   A new registry has been 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 4585.

     Value name:     nack
     Long name:      Negative Acknowledgement
     Reference:      RFC 4585.

     Value name:     trr-int
     Long name:      Minimal receiver report interval
     Reference:      RFC 4585.

     Value name:     app
     Long name:      Application-defined parameter
     Reference:      RFC 4585.

   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



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   corresponding feedback packet formats (if needed).  The general
   registration procedures of [3] apply.

   For use with both "ack" and "nack", a joint sub-registry has been 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 4585.

     Value name:     pli
     Long name:      Picture Loss Indication
     Usable with:    nack
     Reference:      RFC 4585.

     Value name:     rpsi
     Long name:      Reference Picture Selection Indication
     Usable with:    ack, nack
     Reference:      RFC 4585.

     Value name:     app
     Long name:      Application layer feedback
     Usable with:    ack, nack
     Reference:      RFC 4585.

   Further entries may be registered on a first-come first-serve basis.
   Each registration needs to indicate the parameter name, 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").  Per Section 6, RTPFB=205 and PSFB=206 have been added to
   the RTCP registry.








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   RTP RTCP Control Packet types (PT):

     Name:          RTPFB
     Long name:     Generic RTP Feedback
     Value:         205
     Reference:     RFC 4585.

     Name:          PSFB
     Long name:     Payload-specific
     Value:         206
     Reference:     RFC 4585.

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

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

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

     Name:           Generic NACK
     Long name:      Generic negative acknowledgement
     Value:          1
     Reference:      RFC 4585.

     Name:           Extension
     Long name:      Reserved for future extensions
     Value:          31
     Reference:      RFC 4585.

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

     Name:           PLI
     Long name:      Picture Loss Indication
     Value:          1
     Reference:      RFC 4585.

     Name:           SLI
     Long name:      Slice Loss Indication
     Value:          2
     Reference:      RFC 4585.






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     Name:           RPSI
     Long name:      Reference Picture Selection Indication
     Value:          3
     Reference:      RFC 4585.

     Name:           AFB
     Long name:      Application Layer Feedback
     Value:          15
     Reference:      RFC 4585.

     Name:           Extension
     Long name:      Reserved for future extensions.
     Value:          31
     Reference:      RFC 4585.

   Further entries may be registered following the "Specification
   Required" rules as defined in RFC 2434 [9].  Each registration 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.

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 and Shigeru Fukunaga for the contributions
   on 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.













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11.  References

11.1.  Normative References

   [1]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
        "RTP: A Transport Protocol for Real-Time Applications", STD 64,
        RFC 3550, July 2003.

   [2]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
        Conferences with Minimal Control", STD 65, RFC 3551, July 2003.

   [3]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
        Description Protocol", RFC 4566, July 2006.

   [4]  Casner, S., "Session Description Protocol (SDP) Bandwidth
        Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556,
        July 2003.

   [5]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

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

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

   [8]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
        Session Description Protocol (SDP)", RFC 3264, June 2002.

   [9]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

11.2.  Informative References

   [10] Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne,
        "Grouping of Media Lines in the Session Description Protocol
        (SDP)", RFC 3388, December 2002.

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

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






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

   [14] Bormann, C., Cline, L., Deisher, G., Gardos, T., Maciocco, C.,
        Newell, D., Ott, J., Sullivan, G., Wenger, S., 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] Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF Digits,
        Telephony Tones and Telephony Signals", RFC 2833, May 2000.

   [19] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion
        Control Protocol (DCCP)", RFC 4340, March 2006.

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

   [21] Ott, J. and E. Carrara, "Extended Secure RTP Profile for RTCP-
        based Feedback (RTP/SAVPF)", Work in Progress, December 2005.

   [22] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
        Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
        3711, March 2004.

   [23] Kikuchi, Y., Nomura, T., Fukunaga, S., Matsui, Y., and H.
        Kimata, "RTP Payload Format for MPEG-4 Audio/Visual Streams",
        RFC 3016, November 2000.

   [24] ITU-T Recommendation H.245, "Control protocol for multimedia
        communication", May 2006.









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Authors' Addresses

   Joerg Ott
   Helsinki University of Technology (TKK)
   Networking Laboratory
   PO Box 3000
   FIN-02015 TKK
   Finland

   EMail: jo@acm.org


   Stephan Wenger
   Nokia Research Center
   P.O. Box 100
   33721 Tampere
   Finland

   EMail: stewe@stewe.org


   Noriyuki Sato
   Oki Electric Industry Co., Ltd.
   1-16-8 Chuo, Warabi-city, Saitama 335-8510
   Japan

   Phone: +81 48 431 5932
   Fax:   +81 48 431 9115
   EMail: sato652@oki.com


   Carsten Burmeister
   Panasonic R&D Center Germany GmbH

   EMail: carsten.burmeister@eu.panasonic.com


   Jose Rey
   Panasonic R&D Center Germany GmbH
   Monzastr. 4c
   D-63225 Langen, Germany

   EMail: jose.rey@eu.panasonic.com








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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
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Acknowledgement

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