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rfc:rfc4585

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.

Ott, et al. Standards Track [Page 1] RFC 4585 RTP/AVPF July 2006

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

Ott, et al. Standards Track [Page 2] RFC 4585 RTP/AVPF July 2006

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:

Ott, et al. Standards Track [Page 3] RFC 4585 RTP/AVPF July 2006

 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.

Ott, et al. Standards Track [Page 4] RFC 4585 RTP/AVPF July 2006

 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].

Ott, et al. Standards Track [Page 5] RFC 4585 RTP/AVPF July 2006

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),

Ott, et al. Standards Track [Page 6] RFC 4585 RTP/AVPF July 2006

    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.

Ott, et al. Standards Track [Page 7] RFC 4585 RTP/AVPF July 2006

 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

Ott, et al. Standards Track [Page 8] RFC 4585 RTP/AVPF July 2006

 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.

Ott, et al. Standards Track [Page 9] RFC 4585 RTP/AVPF July 2006

 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.

Ott, et al. Standards Track [Page 10] RFC 4585 RTP/AVPF July 2006

 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.

Ott, et al. Standards Track [Page 11] RFC 4585 RTP/AVPF July 2006

 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

Ott, et al. Standards Track [Page 12] RFC 4585 RTP/AVPF July 2006

    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.

Ott, et al. Standards Track [Page 13] RFC 4585 RTP/AVPF July 2006

 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.

Ott, et al. Standards Track [Page 14] RFC 4585 RTP/AVPF July 2006

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.

Ott, et al. Standards Track [Page 15] RFC 4585 RTP/AVPF July 2006

        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.

Ott, et al. Standards Track [Page 16] RFC 4585 RTP/AVPF July 2006

 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.

Ott, et al. Standards Track [Page 17] RFC 4585 RTP/AVPF July 2006

        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.

Ott, et al. Standards Track [Page 18] RFC 4585 RTP/AVPF July 2006

 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.

Ott, et al. Standards Track [Page 19] RFC 4585 RTP/AVPF July 2006

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

Ott, et al. Standards Track [Page 20] RFC 4585 RTP/AVPF July 2006

 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

Ott, et al. Standards Track [Page 21] RFC 4585 RTP/AVPF July 2006

 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.

Ott, et al. Standards Track [Page 22] RFC 4585 RTP/AVPF July 2006

 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

Ott, et al. Standards Track [Page 23] RFC 4585 RTP/AVPF July 2006

 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].)

Ott, et al. Standards Track [Page 24] RFC 4585 RTP/AVPF July 2006

    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.

Ott, et al. Standards Track [Page 25] RFC 4585 RTP/AVPF July 2006

    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.

Ott, et al. Standards Track [Page 26] RFC 4585 RTP/AVPF July 2006

 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

Ott, et al. Standards Track [Page 27] RFC 4585 RTP/AVPF July 2006

    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.

Ott, et al. Standards Track [Page 28] RFC 4585 RTP/AVPF July 2006

    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.

Ott, et al. Standards Track [Page 29] RFC 4585 RTP/AVPF July 2006

 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

Ott, et al. Standards Track [Page 30] RFC 4585 RTP/AVPF July 2006

    [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:
  1. Transport layer FB messages
  2. Payload-specific FB messages
  3. 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

Ott, et al. Standards Track [Page 31] RFC 4585 RTP/AVPF July 2006

    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.

Ott, et al. Standards Track [Page 32] RFC 4585 RTP/AVPF July 2006

 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.

Ott, et al. Standards Track [Page 33] RFC 4585 RTP/AVPF July 2006

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.

Ott, et al. Standards Track [Page 34] RFC 4585 RTP/AVPF July 2006

 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.

Ott, et al. Standards Track [Page 35] RFC 4585 RTP/AVPF July 2006

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

Ott, et al. Standards Track [Page 36] RFC 4585 RTP/AVPF July 2006

 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).

Ott, et al. Standards Track [Page 37] RFC 4585 RTP/AVPF July 2006

 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.

Ott, et al. Standards Track [Page 38] RFC 4585 RTP/AVPF July 2006

 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).

Ott, et al. Standards Track [Page 39] RFC 4585 RTP/AVPF July 2006

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.

Ott, et al. Standards Track [Page 40] RFC 4585 RTP/AVPF July 2006

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.

Ott, et al. Standards Track [Page 41] RFC 4585 RTP/AVPF July 2006

 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.

Ott, et al. Standards Track [Page 42] RFC 4585 RTP/AVPF July 2006

 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".

Ott, et al. Standards Track [Page 43] RFC 4585 RTP/AVPF July 2006

 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

Ott, et al. Standards Track [Page 44] RFC 4585 RTP/AVPF July 2006

 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.

Ott, et al. Standards Track [Page 45] RFC 4585 RTP/AVPF July 2006

 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.

Ott, et al. Standards Track [Page 46] RFC 4585 RTP/AVPF July 2006

   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.

Ott, et al. Standards Track [Page 47] RFC 4585 RTP/AVPF July 2006

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.

Ott, et al. Standards Track [Page 48] RFC 4585 RTP/AVPF July 2006

 [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.

Ott, et al. Standards Track [Page 49] RFC 4585 RTP/AVPF July 2006

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

Ott, et al. Standards Track [Page 50] RFC 4585 RTP/AVPF July 2006

Full Copyright Statement

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Ott, et al. Standards Track [Page 51]

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