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RFC 4585

 Errata 
Proposed STD
Pages: 51
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Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)

Part 1 of 2, p. 1 to 23
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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.

Top      ToC       Page 15 
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.

Top      ToC       Page 16 
          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.

Top      ToC       Page 17 
   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.

Top      ToC       Page 18 
          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.

Top      ToC       Page 19 
   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.

Top      ToC       Page 20 
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

Top      ToC       Page 21 
   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

Top      ToC       Page 22 
   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.

Top      ToC       Page 23 
   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.



(page 23 continued on part 2)

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