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Versions: (draft-wenger-avt-rtcp-feedback) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 4585

INTERNET-DRAFT                               Joerg Ott/Uni Bremen TZI
draft-ietf-avt-rtcp-feedback-02.txt          Stephan Wenger/TU Berlin
                                                 Shigeru Fukunaga/Oki
                                                    Noriyuki Sato/Oki
                                    Koichi Yano/Fast Forward Networks
                                          Akihiro Miyazaki/Matsushita
                                               Koichi Hata/Matsushita
                                            Rolf Hakenberg/Matsushita
                                        Carsten Burmeister/Matsushita

                                                         1 March 2002
                                               Expires September 2002


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


Status of this Memo

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

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

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

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


   Abstract

   Real-time media streams that use RTP are not resilient against
   packet losses.  RTP [1] provides all the necessary mechanisms to
   restore ordering and timing to properly reproduce a media stream at
   the recipient.  RTP also provides continuous feedback about the
   overall reception quality from all receivers -- thereby allowing
   the sender(s) in the mid-term (in the order of several seconds to
   minutes) to adapt their coding scheme and transmission behavior to
   the observed network QoS.  However, except for a few payload
   specific mechanisms [10], RTP makes no provision for timely
   feedback that would allow a sender to repair the media stream
   immediately: through retransmissions, retro-active FEC, or media-

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   specific mechanisms such as reference picture selection for some
   video codecs.

   Generally, real-time transport of media streams across IP
   networks follows RTP[1] in conjunction with the RTP Profile for
   Audio and Video Conferences with Minimal Control [2].  This
   document modifies the profile defined in [2] in two ways:

   o by providing additional RTCP messages that enable a receiver to
      convey more precise feedback to a sender and

   o by adapting the timing algorithm for scheduling RTCP packets in
      order to allow for occasional timely feedback about events
      observed by a receiver (such as lost packets).

   The result is an RTP Profile for Audio and Video Conferences with
   Minimal Control that allows for more explicit and more immediate
   receiver feedback but shares all other properties (including all
   other message types and formats, all code points for codecs,
   payload formats, scaling capabilities, etc. of [2]).  Therefore,
   this document only specifies the additions and modifications to [2]
   rather than the repeating the entire specification.


1. Introduction

   Real-time media streams that use RTP are not resilient against
   packet losses.  RTP [1] provides all the necessary mechanisms to
   restore ordering and timing present at the sender to properly
   reproduce a media stream at a recipient.  RTP also provides
   continuous feedback about the overall reception quality from all
   receivers -- thereby allowing the sender(s) in the mid-term (in the
   order of several seconds to minutes) to adapt their coding scheme
   and transmission behavior to the observed network QoS.  However,
   except for a few payload specific mechanisms [10], RTP makes no
   provision for timely feedback that would allow a sender to repair
   the media stream immediately: through retransmissions, retro-active
   FEC, or media-specific mechanisms such as reference picture
   selection for some video codecs.

   Current mechanisms available with RTP to improve error resilience
   include audio redundancy coding [7], video redundancy coding [11],
   RTP-level FEC [5], and general considerations on more robust media
   streams transmission [6].  These mechanisms may be applied pro-
   actively (thereby increasing the bandwidth of a given media
   stream).  Alternatively, in sufficiently small groups with small
   RTTs, the senders may perform repair on-demand, using the above
   mechanisms and/or media-encoding-specific approaches.  Note that
   "small group" and "sufficiently small RTT" are both highly
   application dependent.

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   This document specifies a modified RTP Profile for audio and video
   conferences with minimal control based upon [1] and [2] by means of
   two modifications/additions:  To achieve timely feedback, the
   concepts of Immediate Feedback messages and 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.  And a small number of general-purpose feedback messages
   as well as a format for codec and application-specific feedback
   information are defined as specific RTCP payloads.


   1.1 Definitions

   The definitions from [1] and [2] apply.  In addition, the following
   definitions are used in this document:

   Early RTCP mode:
           The mode of operation in which a receiver of a media stream
           is, statistically, often (but not always) capable of
           reporting events of interest back to the sender close to
           their occurrence.  In Early RTCP mode, RTCP feedback
           messages 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 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.

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

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

   Feedback (FB) threshold:
           The FB threshold indicates the transition between Immediate
           Feedback and Early RTCP mode.  For a multicast scenario,

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           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 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.

   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 feedback messages are
           transmitted according to the timing rules defined in this
           document.

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

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



   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
    [8]


2. RTP and RTCP Packet Formats and Protocol Behavior

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

   RTCP packet types:
           Three additional RTCP packet types to convey feedback
           information are defined in section 4.


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   RTCP report intervals:
           This memo describes three modes of operation which
           influence the RTCP report intervals (see section 3.2).  In
           regular RTCP mode, all rules from [1] apply.  In both
           Immediate Feedback and Early RTCP modes the minimal
           interval of 5 seconds between 2 RTCP reports is dropped and
           the rules specified in section 3 apply if RTCP packets
           containing feedback messages (defined in section 4) 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 5, further consideration is given to the
           impact of feedback and a sender's reaction to feedback
           messages.


3. Rules for RTCP Feedback

   3.1 Compound RTCP Feedback Packets

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

   o Status reports are contained in SR/RR messages and are
      transmitted at regular intervals as part of compound RTCP
      packets (which also include SDES and possibly other messages);
      these status reports provide an overall indication for the
      recent reception quality of a media stream.

   o Feedback 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 feedback messages are
      newly introduced in this memo.

   RTCP Feedback (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.

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


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   o OPTIONAL encryption prefix that MUST be present if the RTCP
      message is to be encrypted.
   o MANDATORY SR or RR.
   o MANDATORY SDES which MUST contain the CNAME item; all other SDES
      items are OPTIONAL.
   o One or more FB messages.

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

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

   a)  Minimal compound RTCP feedback packet

       A minimal compound RTCP feedback packet MUST contain only the
       mandatory information as listed above: encryption prefix if
       necessary, exactly one RR or SR, exactly one SDES with only the
       CNAME item present, and the feedback 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 feedback
       message is sent as part of an Early RTCP packet.

   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 feedback
       message is sent as part of a regularly scheduled RTCP packet or
       in Regular RTCP mode.  It MAY also be used to send RTCP
       feedback messages in Immediate Feedback or Early RTCP mode.

   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 are thus
   subject to the same bandwidth constraints as other RTCP traffic.
   This means in particular that it may not be possible to report an
   event observed at a receiver immediately back to the sender.

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   However, the value of feedback given to a sender typically
   decreases over time -- in terms of the media quality as perceived
   by the user at the receiving end and/or the cost required to
   achieve media stream repair.

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

   The modified RTCP transmission algorithm can be outlined as
   follows: Normally, when no FB messages have to be conveyed,
   compound RTCP packets are sent following the rules of RTP [1] --
   except that the 5s minimum interval between RTCP reports is not
   enforced and 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.  If a receiver detects the need to send an
   FB message, the receiver waits for a short, random dithering
   interval (in case of multicast) and then checks whether it has
   already seen a corresponding FB message from any other receiver
   (which it can do with all FB messages that are transmitted via
   multicast; for unicast sessions, there is no such delay).  If this
   is the case then the receiver refrains from sending the FB message
   and continues to follow the regular RTCP sending schedule.  If the
   receiver has not yet seen a similar FB message from any other
   receiver, it checks whether it has recently exceeded its RTCP bit
   rate budget to transmit another FB message (without waiting for its
   regularly scheduled RTCP transmission time).  Only if this is not
   the case, it sends the FB message as part of a (minimal) compound
   RTCP packet.

   FB messages may also be sent as part of full compound RTCP packets
   which are interspersed 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 is a hint whether or not a receiver
   should send early feedback at all and, if so, whether,
   statistically, all events observed at the receiver can be reported
   back to the sender in a timely fashion.  The current mode of
   operation is continuously derived independently at each receiver
   and the receivers do not have to agree on a common mode.

   a) Immediate feedback mode: the group size is below the FB
       threshold which gives each receiving party sufficient bandwidth
       to transmit the RTCP feedback packets for the intended purpose.

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       This means that, for each receiver, there is enough bandwidth
       to report each event it is supposed/expected to 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 -- again depending on the type of FB used -- the (worst
       case or observed) frequency of events to report (e.g. frame
       received, packet lost).

       A special case of this is the ACK mode (where positive
       acknowledgements are used to confirm reception of data) which
       is restricted to point-to-point communications.

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

   b) Early RTCP mode: In this mode, the group size and other
       parameters no longer allow each receiver to react to each event
       that would be worth (or needed) to report.  But feedback can
       still be given sufficiently often so that it allows the sender
       to adapt the media stream transmission accordingly and thereby
       increase the overall reproduced media 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) From some group size upwards, it is no longer useful to provide
       feedback from individual 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 threshold can be specified when this occurs.

   As the feedback algorithm described in this memo scales smoothly,
   there is no need for an agreement among the participants on the
   precise values of the respective "thresholds" within the group.
   Hence the borders between all these modes are allowed to be
   fluent.

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     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 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

   The following pieces of state information need to be maintained per
   receiver (largely taken from [1]).  Note that all variables (except
   for g) are calculated independently at each receiver and so their
   local values may differ at a given point in time.

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

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

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

   d) Let T_rr be the interval after which, having just sent a
      regularly scheduled RTCP packet, a receiver would schedule the
      transmission of its next RTCP packet following the rules of [1]:
      T_rr = tn - tp.  Note that the 5s minimum interval between two
      report as defined in [1] SHOULD NOT be enforced.

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


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   f) Let T_dither_max be the maximum interval for which an RTCP
      feedback packet may be additionally delayed (to prevent
      implosions).

   g) 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.  Note that this value is application-specific.

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

   i) Let T_fd be the actual (randomized) delay for the transmission
      of feedback message in response to an event that a certain
      packet P caused.

   j) Let allow_early be a Boolean variable that indicates whether the
      receiver currently may transmit feedback 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 adjusted (set to FALSE) after early feedback
      transmission and is reset to TRUE as soon as the next regular
      RTCP transmission is scheduled.

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

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

   To avoid an implosion of immediate feedback packets, the receiver
   MUST delay the transmission of the RTCP feedback packet by a random
   amount 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 (which, in turn, is based upon the group size).

   For a certain application scenario, a receiver may determine an
   upper bound for the acceptable local delay of feedback 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.



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   If an RTCP feedback packet is scheduled, the time slot for the next
   scheduled (full) compound RTCP packet MUST be updated accordingly
   to a new tn (which will then be in the order of tn=tp+2*T_rr).
   This is to ensure that the short term average bandwidth used for
   RTCP with feedback does not exceed the bandwidth limit that would
   be used without 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 Early RTCP Algorithm

   Assume an active sender S0 (out of S senders) and a number N of
   receivers with R being one of these receivers.

   Assume further 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 memo).

   Then, receiver R MUST use the following rules for transmitting one
   or more Feedback messages as minimal or full compound RTCP packet:

   Initially, R MUST set allow_early = TRUE.

   Assume that R has transmitted the last RTCP RR packet at tp and has
   scheduled the next transmission (prior to reconsideration) for tn.

   At time t0, R detects the need to transmit one or more RTCP
   feedback messages (e.g. because media "units" needs to be ACKed or
   NACKed) and finds that sending the feedback information is useful
   for the sender.

   R first checks whether there is still a compound RTCP feedback
   packet waiting for transmission (scheduled as early or regular RTCP
   packet).  If so, the new feedback message MUST be appended to the

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   packet; the schedule for the waiting RTCP feedback packet MUST
   remain unchanged.  When appending, the feedback information of
   several RTCP feedback packets SHOULD be merged to produce as few
   packets as possible.


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

   i)   If the session is a unicast session (group size = 2) then
        T_dither_max = 0.

   ii)  If the session is a multicast session with potentially more
        than two group members then

            T_dither_max = l * T_rr

        with l=0.5.

   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.

   Then, R MUST check whether its next regularly scheduled RTCP packet
   is within the time bounds for the RTCP FB (t0 + T_dither_max > tn).
   If so, an Early RTCP packet MUST NOT be scheduled; instead the FB
   message(s) MUST be stored to be appended to the regular RTCP packet
   scheduled for tn.

   Otherwise, R MUST check whether it is allowed to transmit an Early
   RTCP packet (allow_early == TRUE).

      If so, 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.

      If, while waiting for te, R receives RTCP feedback packets
      contained in one or more (minimal) compound RTCP packets, R MUST
      act as follows for each of the RTCP feedback packets in the one
      or more compound RTCP packets received:

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



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      2.  If R understands the received feedback message's semantics
           and the message contents is not a superset of the feedback R
           wanted to send then R SHOULD transmit its own feedback
           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 send its feedback information unchanged, R MAY as well
           eliminate any redundancy between its own feedback and the
           feedback received so far.

      3.  If R does not understand the received feedback message's
           semantics, R MAY send its own feedback message as Early RTCP
           packet, or R MAY re-schedule the next regular RTCP message
           transmission for tn (as calculated before) and MAY append
           the feedback message to the now regularly scheduled RTCP
           message.

           Note: With rule #3, receiving unknown feedback packets may
           not lead to feedback suppression at a particular receiver.
           As a consequence, a given event may cause M different types
           of feedback packets (which are all appropriate but not the
           same and mutually not understood) to be scheduled, and a
           "large" receiver group may be partitioned into at most M
           groups.  Among members of each of these M groups, feedback
           suppression will occur following the rules #1 and #2 but no
           suppression will happen across groups.  As a result, O(M)
           RTCP feedback messages may be received by the sender.  Given
           that these M groups consist of receivers for the same
           application using the same (set of) codecs in the same RTP
           session, M is assumed to be small in the general case.
           Given further that the O(M) feedback packets are randomly
           distributed over a time interval of T_dither_max, the
           resulting limited number of extra feedback packets (a) is
           assumed not to overwhelm the sender and (b) should be
           conveyed as all contain complementary pieces of information.

      Refer to section 4 on the comparison of feedback messages and
      for which feedback messages MUST be understood by a receiver.

      Otherwise, when te is reached, R MUST transmit the RTCP packet
      containing the FB message.  R then MUST set allow_early = FALSE
      and MUST recalculate tn = tp + 2*T_rr.  The value from the last
      calculation of T_rr SHOULD be used.  As soon as R sends its next
      regularly scheduled RTCP RR (at the new tn), it MUST set
      allow_early = TRUE again.

   If allow_early == FALSE then R MUST check the time for the next
   scheduled RR:



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   1.  If tn - t0 < T_max_fb_delay (i.e. if, despite late reception,
       the feedback could still be useful for the sender) then R MAY
       create an RTCP FB message for transmission along with the RTCP
       packet at tn.

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

   In regular RTCP intervals as specified by [1] (except for the five
   second minimum), a full compound RTCP packet MUST be sent (which
   MAY also contain a feedback message if one has been created
   according to the above rules and scheduled for transmission along
   the full compound RTCP message).

   Whenever an RTCP packet is sent or received -- minimal or full
   compound, early or regularly scheduled -- the avg_rtcp_size
   variable MUST be updated accordingly (see [1]) and the tn MUST be
   calculated using the new avg_rtcp_size.


   3.6 Considerations on the Group Size

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


   3.6.1 ACK mode

   The group size MUST be exactly two participants, i.e. point-to-
   point communications.  Unicast addresses MUST be used in the
   session description.

   For unidirectional as well as bi-directional communication between
   two parties, 2.5% of the RTP session bandwidth are available for
   RTCP traffic from the receivers including feedback.  For a 64
   kbit/s stream this yields 1600 bit/s for RTCP.  As every other RTCP
   packet needs to be a full compound packet, we assume an average of
   96 bytes (=768 bits) per RTCP packet so that a receiver can report
   2 events per second back to the sender.  If acknowledgments for 10
   events are collected in each feedback 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 only three ACKs per RTCP feedback 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 accordingly 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


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   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 (or similar types of feedback) MUST be
   used for all groups larger than two.  Of course, NACKs MAY be used
   for point-to-point communications as well.

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

   The crucial parameters -- to which virtually all of the above can
   be reduced -- is the allowed minimal interval between two RTCP
   reports and the (average) number of events that presumably need
   reporting per time interval (plus their distribution over time, of
   course).  The minimum interval can be derived from the available
   RTCP bandwidth and the expected average size of an RTCP packet.
   The number of events to report e.g. per second may be derived from
   the packet loss rate and sender's rate of transmitting packets.
   From these two values, the allowable group size for the Immediate
   feedback mode can be calculated.

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

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

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

   Example: If a 256kbit/s video with 30 fps is transmitted through a
   network with an MTU size of some 1500 bytes, then, in most cases,
   each frame would fit in its own 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 have to report 3 packets lost each two seconds.

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   Assuming a single sender and more than three receivers, this yields
   3.75% of the RTCP bandwidth allocated to the receivers and thus
   9.6kbit/s.  Assuming further a size of 120 bytes for the average
   compound RTCP packet allows 10 RTCP packets to be sent per second
   or 20 in two seconds.  If every receiver needs to report three
   packets, this yields a maximum group size of 6-7 receivers if all
   loss events shall be reported.  The rules for transmission of
   immediate RTCP packets should provide sufficient flexibility for
   most of this reporting to occur in a timely fashion.

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

   3) If these tests pass, the application has to follow the rules for
      transmitting Early RTCP packets or regularly scheduled RTCP
      packets with piggybacked feedback.


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   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 mechanisms consists of a format
   used to describe a media session and another mechanism for
   transporting this description.

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

   A media session description format MAY include parameters to
   indicate that RTCP feedback mechanisms MAY be used (=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 profile 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 (for use with
   "a=fmtp:") is defined to indicate the capability of using RTCP

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   feedback as specified in this document: "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
   feedback messages MAY be used in this media session for the
   indicated payload type.  If several types of feedback are
   supported, several "a=rtcp-fb:" lines MUST be used.

   If no "rtcp-fb" attribute is specified the RTP receivers SHOULD
   assume that the RTP senders only support generic NACKs.  In
   addition, 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 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=fmtp: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 feedback messages than those listed in one of
      the "rtcp-fb" attribute lines.

   RTP senders MUST be prepared to receive any kind of RTCP feedback
   messages and MUST silently discard all those RTCP feedback 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):


   rtcp-fb-syntax = "a=fmtp:" <format> WS "rtcp-fb" WS rtcp-fb-val

   rtcp-fb-val        = "ack" rtcp-fb-ack-param
                      | "nack" rtcp-fb-nack-param
                      | rtcp-fb-id rtcp-fb-param

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

   rtcp-fb-param      = "app"
                      | byte-string

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                      | ; empty

   rtcp-fb-ack-param  = "rpsi"
                      | "app"
                      | byte-string
                      | ; empty

   rtcp-fb-nack-param = "pli"
                      | "sli"
                      | "rpsi"
                      | "app"
                      | byte-string
                      | ; empty


   The literals of the above grammar have the following semantics:

   Feedback type "ack":

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

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

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

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

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

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

   Feedback type "nack":

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

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

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        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.

   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) to make use of feedback
   provided.


   4.3 Unicasting vs. Multicasting

   If a media session description indicates unicast addresses for a
   particular media type (and does not operate in multi-unicast mode
   with all recipients listed explicitly but still addressed via
   unicast), the RTCP feedback MAY operate in ACK feedback mode.


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   If a media session description indicates multicast addresses for a
   particular media type or a multi-unicast session, ACK feedback mode
   MUST NOT be used.

   4.4 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).


   4.5 Examples

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

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

   Example 2: The following session description indicates a multicast
   video-only session (using H.263+) with the video source accepting
   Generic NACKs and Reference Picture Selection.  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

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      m=audio 49170 RTP/AVP 0
      c=IN IP4 224.2.1.183
      a=rtpmap:0 PCMU/8000
      m=video 51372 RTP/AVPF 98
      c=IN IP4 224.2.1.184
      a=rtpmap:98 H263-1998/90000
      a=fmtp:98 rtcp-fb nack
      a=fmtp:98 rtcp-fb nack rpsi

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

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

   Note that these two m= lines SHOULD be grouped by some appropriate
   mechanisms to indicate that both are alternatives actually
   conveying the same contents.  A sample mechanism by which this can
   be achieved is defined in [14].


5. Interworking and Co-Existence of AVP and AVPF Entities

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

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


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   functioning.  However, the quality of the media presented may
   suffer.

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

   o AVP entities (senders and receivers)

      AVP senders will receive RTCP feedback packets from AVPF
      receivers and ignore these packets.  They will see occasional
      closer spacing of RTCP messages (e.g. violating the 5s 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 5s 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).

   o AVPF senders

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

   o AVPF receivers

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


6. Format of RTCP Feedback Messages

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

   - Transport layer feedback messages
   - Payload-specific feedback messages
   - Application layer feedback messages

   Transport layer feedback messages are intended to transmit general
   purpose feedback information, i.e. information independent of the

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   particular codec or the application in use.  The information is
   expected to be generated and processed at the transport/RTP layer.
   Currently, only a general positive acknowledgement (ACK) and
   negative acknowledgement (NACK) message are defined.

   Payload-specific feedback 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 feedback
   messages.  The definition of specific messages is left to either
   RTP Payload Format specifications or to additional feedback format
   documents.

   Application layer feedback 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's 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
   feedback messages.  From a protocol point of view, an application
   layer feedback message is treated as a special case of a payload-
   specific feedback message.

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

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


   6.1 Common Packet Format for Feedback Message

   All feedback message MUST use a common packet format that is
   depicted in figure 3:










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

   Figure 3: Common Packet Format for Feedback Messages


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

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

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

   Feedback message type (FMT): 4 bits
       This field identifies the type of the feedback message and is
       interpreted relative to the RTCP message type (transport,
       payload-specific, or application layer feedback).  The values
       for each of the three feedback types are defined in the
       respective sections below.

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

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


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   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 feedback message for each
       type of feedback (further FCI contents MAY be specified in
       further documents).  Each RTCP feedback packet MUST contain
       exactly one FCI field of the types defined in sections 6.2 and
       6.3.  If multiple FCI fields (even of the same type) need to be
       conveyed, then several RTCP feedback packets MUST be generated
       and concatenated in the same compound RTCP packet.


   6.2 Transport Layer Feedback Messages

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

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

         0:    forbidden
         1:    Generic NACK
         2:    Generic ACK
         3-15: reserved

   The following two subsections define the packet formats for these
   messages.


   6.2.1 Generic NACK

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

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

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




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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            PID                |             BLP               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 4: Syntax for the Generic NACK message


   Packet ID (PID): 16 bits
       The PID field is used to specify a lost packet.  Typically, the
       RTP sequence number is used for PID as the default format, but
       RTP Payload Formats may decide to identify a packet
       differently.

   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 [10].
       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 sender has not received RTP packet number PID+i
       (modulo 2^16) and the receiver decides 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.


   6.2.2 Generic ACK

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

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

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






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

   Figure 5: Syntax for the Generic ACK message


   Packet ID (1st PID): 16 bits
       This PID field is used to specify a correctly received packet.
       Typically, the RTP sequence number is used for PID as the
       default format, but RTP Payload Formats may decide to identify
       a packet differently.

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

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

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

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

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

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

       If R=0, this field carries a bit mask. The BLP allows for
       reporting reception of any of the 15 RTP packets immediately
       following the RTP packet indicated by the PID.  The BLP's
       definition is identical to that given in [10] except that,
       here, BLP is only 15 bits wide.  Denoting the BLP's least
       significant bit as bit 1, and its most significant bit as bit
       15, then bit i of the bitmask is set to 1 if the sender has
       received RTP packet number PID+i (modulo 2^16) and the receiver

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       decides to ACK this packet; bit i is set to 0 otherwise.  If
       only the packet indicated by PID is to be ACKed and R=0 then
       BLP MUST be set to 0x0000.


   6.3 Payload Specific Feedback Messages

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

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

         0:    forbidden
         1:    Picture Loss Indication (PLI)
         2:    Slice Lost Indication (SLI)
         3:    Reference Picture Selection Indication (RPSI)
         4-14: reserved
         15:   Application layer feedback message

   The following subsections define the packet formats for the
   payload-specific messages, section 6.4 defines the application
   layer feedback message.


   6.3.1 Picture Loss Indication (PLI)

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


   6.3.1.1 Semantics

   With the Picture Loss Indication message, a decoder informs the
   encoder about the loss of an undefined amount of coded video data
   belonging to one or more pictures.  When used in conjunction with
   any video coding scheme that is based on inter-picture prediction,
   an encoder that receives a PLI becomes aware that the prediction
   chain may be broken.  The sender MAY react to a PLI by transmitting
   an intra-picture to achieve resynchronization (making effectively
   similar to the FIR as defined in [10]); 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 [10] 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

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   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.


   6.3.1.3 Timing Rules

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


   6.3.1.4 Remarks

   PLI messages typically trigger the sending of full intra pictures.
   Intra pictures are several times larger then predicted (inter)
   pictures.  Their size is independent of the time they are
   generated.  In most environments, especially when employing
   bandwidth-limited links, the use of an intra picture implies an
   allowed delay that is a significant multitude of the typical frame
   duration.  An example: If the sending frame rate is 10 fps, and an
   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 feedback message.  Hence waiting for the next possible
   time slot allowed by RTCP timing rules as per [2] does not have a
   negative impact on the system performance.


   6.3.2 Slice Lost Indication (SLI)

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


   6.3.2.1 Semantics

   With the Slice Lost Indication a decoder can inform an encoder that
   it has detected the loss or corruption of one or several
   consecutive macroblock(s) in scan order (see below).  This feedback
   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.


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   6.3.2.2 Format

   The  Slice Lost Indication uses one additional PCI field the
   content of which is depicted in figure 6.  The length of the
   feedback message MUST be set to 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            First        |  Number                 | PictureID |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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


   First: 13 bits
       The macroblock (MB) address of the first lost macroblock.  The
       MB numbering is done such that the macroblock in the upper left
       corner of the picture is considered macroblock number 1 and the
       number for each macroblock increases from left to right and
       then from top to bottom in raster-scan order (such that if
       there is a total of N macroblocks in a picture, the bottom
       right macroblock is considered macroblock number N).

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

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


   6.3.2.3 Timing Rules

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


   6.3.2.4 Remarks

   The term Slice is defined and used here in the sense of MPEG-1 -- a
   consecutive number of macroblocks in scan order.  More recent video

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   coding standards sometimes have a different understanding of the
   term Slice.  In H.263 (1998), for example, a concept known as
   "rectangular Slice" exist.  The loss of one Rectangular Slice may
   lead to the necessity of sending more than one SLI in order to
   precisely identify the region of lost/damaged MBs.

   The first field of the FCI defines the first macroblock of a
   picture as 1 and not, as one could suspect, as 0.  This was done to
   align this specification with the comparable mechanism available in
   H.245.  The maximum number of macroblocks in a picture (2**13 or
   8192) corresponds to the maximum picture sizes of most of the ITU-T
   and ISO/IEC video codecs.  If future video codecs offer larger
   picture sizes and/or smaller macroblock sizes, then an additional
   feedback 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 a SLI is not part of this specification.  One
   typical way of reacting to a SLI is to use intra refresh for the
   affected spatial region.

   Algorithms were reported that keep track of the regions affected by
   motion compensation, in order to allow for a transmission of Intra
   macroblocks to all those areas, regardless of the timing of the FB
   (see H.263 (2000) Appendix I [13] and [15]).  While, when those
   algorithms are used, the timing of the FB is less critical then
   without, it has to be observed that those algorithms correct large
   parts of the picture and, therefore, have to transmit much higher
   data volume in case of delayed FBs.


   6.3.3 Reference Picture Selection Indication (RPSI)

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


   6.3.3.1 Semantics

   Modern video coding standards such as MPEG-4 visual version 2 [12]
   or H.263 version 2 [13] allow to use older reference pictures than
   the most recent one for predictive coding.  Typically, a first-in-
   first-out queue of reference pictures is maintained.  If an encoder
   has learned about a loss of encoder-decoder synchronicity, a known-
   as-correct reference picture can be used. As this reference picture
   is temporally further away then usual, the resulting predictively
   coded picture will use more bits.

   Both MPEG-4 and H.263 define a binary format for the "payload" of
   an RPSI message that includes information such as the temporal ID
   of the damaged picture and the size of the damaged region.  This

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   bit string is typically small -- a couple of dozen bits --, of
   variable length, and self-contained, i.e. contains all information
   that is necessary to perform reference picture selection.

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


   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       |   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.

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

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


   6.3.3.3 Timing Rules

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


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


   6.4 Application Layer Feedback Messages

   Payload-Specific Feedback Messages are a special case of payload-
   specific messages and identified by PT=PSFB and FMT=15.

   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 feedback message.
   Therefore, the application MUST be able to identify the messages
   payload.

   Usually, applications define their own set of messages, e.g.
   NEWPRED  messages in MPEG-4 or feedback messages in H.263/Annex N,
   U.  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 byte aligned, padding
       bits must be added.  Identification of padding bits is up to
       the application layer and not defined in this specification.



7. Early Feedback and Congestion Control

   In the previous sections, the feedback 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.

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

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

   o The potentially more frequent RTCP messages allow the sender to
      monitor the network state more closely than with regular RTCP
      and therefore enable reacting to upcoming congestion in a more
      timely fashion.

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

   A congestion control algorithm that shares the available bandwidth
   fair with competing TCP connections, e.g. TFRC [16], SHOULD be used
   to determine the data rate for the media stream (if the low delay
   RTP session is transmitted in a best effort environment).

   RTCP feedback messages or RTCP SR/RR packets that indicate recent
   packet loss MUST NOT lead to a (mid-term) increase in the
   transmission data rate and SHOULD lead to a (short-term) decrease
   of the transmission data rate.  Such messages SHOULD cause the
   sender to adjust the transmission data rate to the order of the
   throughput TCP would achieve under similar conditions (e.g. using
   TFRC).

   RTCP feedback messages or RTCP SR/RR packets that indicate no
   recent packet loss MAY cause the sender to increase the
   transmission data rate to roughly the throughput TCP would achieve
   under similar conditions (e.g. using TFRC).


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 5 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 feedback
   messages.  (Note that an entity need not be member of a multicast
   group to cause these effects.)

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

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

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

   Senders as well as receivers SHOULD behave conservative 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 [17].


9. IANA Considerations

   The feedback profile as an extension to the profile for audio-
   visual conferences with minimal control needs to be registered:
   "RTP/AVPF".

   For the Session Description Protocol, the following "fmtp:"
   attribute needs to be registered: "rtcp-fb".

   Along with "rtcp-fb", the feedback types "ack" and "nack" need to
   be registered.

   Along with "nack", the feedback type parameters "sli" and "pli"
   need to be registered.

   Along with "ack" and "nack", the feedback type parameters "rpsi"
   and "app" need to be registered.

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

   Within the RTPFB range, three format (FMT) values need to be
   registered:

       0:    forbidden
       1:    General NACK
       2:    General ACK

   Within the PSFB range, five format (FMT) values need to be
   registered:

       0:    forbidden
       1:    Picture Loss Indication (PLI)
       2:    Slice Loss Indication (SLI)
       3:    Reference Picture Selection Indication (SLI)
      15:    Application layer feedback (AFB)


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.


11. Full Copyright Statement

   Copyright (C) The Internet Society (2001). All Rights Reserved.
   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain
   it or assist in its implementation may be prepared, copied,
   published and distributed, in whole or in part, without restriction
   of any kind, provided that the above copyright notice and this
   paragraph are included on all such copies and derivative works.

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



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

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


12. Authors' Addresses

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

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

   Shigeru Fukunaga
   Oki Electric Industry Co., Ltd.
   1-2-27 Shiromi, Chuo-ku, Osaka 540-6025 Japan
   Tel.  +81 6 6949 5101
   Fax.  +81 6 6949 5108
   Mail  fukunaga444@oki.com

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

   Koichi Yano
   FastForward Networks,
   75 Hawthorne St. #601
   San Francisco, CA 94105
   Tel.  +1.415.430.2500





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   Akihiro Miyazaki
   Matsushita Electric Industrial Co., Ltd
   1006, Kadoma, Kadoma City, Osaka, Japan
   Tel.  +81-6-6900-9192
   Fax.  +81-6-6900-9193
   Mail  akihiro@isl.mei.co.jp

   Koichi Hata
   Matsushita Electric Industrial Co., Ltd
   1006, Kadoma, Kadoma City, Osaka, Japan
   Tel.  +81-6-6900-9192
   Fax.  +81-6-6900-9193
   Mail  hata@isl.mei.co.jp

   Rolf Hakenberg
   Panasonic European Laboratories GmbH
   Monzastr. 4c, 63225 Langen, Germany
   Tel.  +49-(0)6103-766-162
   Fax.  +49-(0)6103-766-166
   Mail  hakenberg@panasonic.de

   Carsten Burmeister
   Panasonic European Laboratories GmbH
   Monzastr. 4c, 63225 Langen, Germany
   Tel.  +49-(0)6103-766-263
   Fax.  +49-(0)6103-766-166
   Mail  burmeister@panasonic.de


11. Bibliography

   [1]  H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP
        - A Transport Protocol for Real-time Applications," Internet
        Draft, draft-ietf-avt-rtp-new-11.txt, Work in Progress,
        November 2001.

   [2]  H. Schulzrinne and S. Casner, "RTP Profile for Audio and Video
        Conferences with Minimal Control," Internet Draft draft-ietf-
        avt-profile-new-12.txt, November 2001.

   [3]  M. Handley and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

   [4]  S. Casner, "SDP Bandwidth Modifiers for RTCP Bandwidth",
        Internet Draft draft-ietf-avt-rtcp-bw-05.txt, November 2001.

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



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

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

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

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

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

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

   [12] ISO/IEC 14496-2:1999/Amd.1:2000, "Information technology -
        Coding of audio-visual objects - Part2: Visual", July 2000.

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

   [14] G. Camarillo, J. Holler, G. Eriksson, H. Schulzrinne,
        "Grouping of media lines in SDP," Internet Draft, draft-ietf-
        mmusic-fid-05.txt, Work in Progress, September 2001.

   [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] M. Handley, J. Padhye, S. Floyd, J. Widmer, "TCP friendly Rate
        Control (TFRC): Protocol Specification," Internet Draft,
        draft-ietf-tsvwg-tfrc-03.txt, Work in Progress, July 2001.

   [17] M. Baugher, R. Blom, E. Carrarra, D. McGrew, M. Naslund, K.
        Norrman, D. Oran, "The Secure Real-Time Transport Protocol,"
        Internet Draft, draft-ietf-avt-srtp-02.txt, Work in Progress,
        November 2001.








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