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RFC 4585

 
 
 

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

Part 2 of 2, p. 23 to 51
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4.  SDP Definitions

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

4.1.  Profile Identification

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

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

4.2.  RTCP Feedback Capability Attribute

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

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

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   specified for a subset of the payload types, several "a=rtcp-fb"
   lines MUST be used.

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

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

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

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

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

   When used in conjunction with the offer/answer model [8], the offerer
   MAY present a set of these AVPF attributes to its peer.  The answerer
   MUST remove all attributes it does not understand as well as those it
   does not support in general or does not wish to use in this
   particular media session.  The answerer MUST NOT add feedback
   parameters to the media description and MUST NOT alter values of such
   parameters.  The answer is binding for the media session, and both
   offerer and answerer MUST only use feedback mechanisms negotiated in
   this way.  Both offerer and answerer MAY independently decide to send
   RTCP FB messages of only a subset of the negotiated feedback
   mechanisms, but they SHOULD react properly to all types of the
   negotiated FB messages when received.

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

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

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

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

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

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

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

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

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

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

   The literals of the above grammar have the following semantics:

   Feedback type "ack":

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

      The feedback type "ack" MUST only be used if the media session is
      allowed to operate in ACK mode as defined in Section 3.6.1.

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

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

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

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

   Feedback type "nack":

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

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

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

      o "pli" indicates the use of Picture Loss Indication feedback as
        defined in Section 6.3.1.

      o "sli" indicates the use of Slice Loss Indication feedback as
        defined in Section 6.3.2.

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

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

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

   Other feedback types <rtcp-fb-id>:

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

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   Regular RTCP minimum interval "trr-int":

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

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

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

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

4.3.  RTCP Bandwidth Modifiers

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

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

4.4.  Examples

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

      v=0
      o=alice 3203093520 3203093520 IN IP4 host.example.com
      s=Media with feedback
      t=0 0

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      c=IN IP4 host.example.com
      m=audio 49170 RTP/AVPF 0 96
      a=rtpmap:0 PCMU/8000
      a=rtpmap:96 telephone-event/8000
      a=fmtp:96 0-16
      a=rtcp-fb:96 nack

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

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

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

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

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

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

   Note that these two m= lines SHOULD be grouped by some appropriate
   mechanism to indicate that both are alternatives actually conveying
   the same contents.  A sample framework by which this can be
   achieved is defined in [10].

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

5.  Interworking and Coexistence of AVP and AVPF Entities

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

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

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

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   o  AVP entities (senders and receivers)

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

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

   o  AVPF entities (senders and receivers)

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

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

   o  AVPF senders

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

   o  AVPF receivers

      AVPF receivers SHOULD send Early RTCP feedback packets only if
      all sending entities in the media session support AVPF.  AVPF
      receivers MAY send feedback information as part of regularly
      scheduled compound RTCP packets following the timing rules of

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      [1] and [2] also in media sessions operating in mixed mode.
      However, the receiver providing feedback MUST NOT rely on the
      sender reacting to the feedback at all.

6.  Format of RTCP Feedback Messages

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

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

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

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

   Application layer FB messages provide a means to transparently convey
   feedback from the receiver's to the sender's application.  The
   information contained in such a message is not expected to be acted
   upon at the transport/RTP or the codec layer.  The data to be
   exchanged between two application instances is usually defined in the
   application protocol specification and thus can be identified by the
   application so that there is no need for additional external
   information.  Hence, this document defines only a common header to be
   used along with all application layer FB messages.  From a protocol
   point of view, an application layer FB message is treated as a
   special case of a payload-specific FB message.

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

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      payload-specific as well as application layer FB messages.  It is
      up to the specification of an FB message to define how payload
      type information is transmitted.

   This document defines two transport layer and three (video) payload-
   specific FB messages as well as a single container for application
   layer FB messages.  Additional transport layer and payload-specific
   FB messages MAY be defined in other documents and MUST be registered
   through IANA (see Section 9, "IANA Considerations").

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

6.1.   Common Packet Format for Feedback Messages

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

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

           Figure 3: Common Packet Format for Feedback Messages

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

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

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

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

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

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

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

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

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

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

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

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6.2.   Transport Layer Feedback Messages

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

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

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

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

6.2.1.  Generic NACK

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

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

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

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

   The Feedback Control Information (FCI) field has the following Syntax
   (Figure 4):

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

               Figure 4: Syntax for the Generic NACK message

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

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

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

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

6.3.  Payload-Specific Feedback Messages

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

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

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

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

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6.3.1.  Picture Loss Indication (PLI)

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

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

6.3.1.1.  Semantics

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

   Other RTP payload specifications such as RFC 2032 [6] already define
   a feedback mechanism for some for certain codecs.  An application
   supporting both schemes MUST use the feedback mechanism defined in
   this specification when sending feedback.  For backward compatibility
   reasons, such an application SHOULD also be capable to receive and
   react to the feedback scheme defined in the respective RTP payload
   format, if this is required by that payload format.

6.3.1.2.  Message Format

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

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

6.3.1.3.  Timing Rules

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

6.3.1.4.  Remarks

   PLI messages typically trigger the sending of full intra-pictures.
   Intra-pictures are several times larger then predicted (inter-)
   pictures.  Their size is independent of the time they are generated.
   In most environments, especially when employing bandwidth-limited
   links, the use of an intra-picture implies an allowed delay that is a

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

6.3.2.  Slice Loss Indication (SLI)

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

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

6.3.2.1.  Semantics

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

6.3.2.2.  Format

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            First        |        Number           | PictureID |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

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

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

6.3.2.3.  Timing Rules

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

6.3.2.4.  Remarks

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

   The first field of the FCI defines the first macroblock of a picture
   as 1 and not, as one could suspect, as 0.  This was done to align
   this specification with the comparable mechanism available in ITU-T
   Rec. H.245 [24].  The maximum number of macroblocks in a picture
   (2**13 or 8192) corresponds to the maximum picture sizes of most of
   the ITU-T and ISO/IEC video codecs.  If future video codecs offer
   larger picture sizes and/or smaller macroblock sizes, then an
   additional FB message has to be defined.  The six least significant
   bits of the Temporal Reference field are deemed to be sufficient to
   indicate the picture in which the loss occurred.

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

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   Algorithms were reported that keep track of the regions affected by
   motion compensation, in order to allow for a transmission of Intra
   macroblocks to all those areas, regardless of the timing of the FB
   (see H.263 (2000) Appendix I [17] and [15]).  Although the timing of
   the FB is less critical when those algorithms are used than if they
   are not, it has to be observed that those algorithms correct large
   parts of the picture and, therefore, have to transmit much higher
   data volume in case of delayed FBs.

6.3.3.  Reference Picture Selection Indication (RPSI)

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

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

6.3.3.1.  Semantics

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

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

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

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6.3.3.2.  Format

   The FCI for the RPSI message follows the format depicted in Figure 7:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      PB       |0| Payload Type|    Native RPSI bit string     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   defined per codec          ...                | Padding (0) |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

   Payload Type: 7 bits
      Indicates the RTP payload type in the context of which the native
      RPSI bit string MUST be interpreted.

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

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

6.3.3.3.  Timing Rules

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

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

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6.4.  Application Layer Feedback Messages

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

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

   Usually, applications define their own set of messages, e.g., NEWPRED
   messages in MPEG-4 [16] (carried in RTP packets according to RFC 3016
   [23]) or FB messages in H.263/Annex N, U [17] (packetized as per RFC
   2429 [14]).  These messages do not need any additional information
   from the RTCP message.  Thus, the application message is simply
   placed into the FCI field as follows and the length field is set
   accordingly.

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

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

7.  Early Feedback and Congestion Control

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

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

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

   A congestion control algorithm that shares the available bandwidth
   reasonably fairly with competing TCP connections, e.g., TFRC [7],
   MUST be used to determine the data rate for the media stream within
   the bounds of the RTP sender's and the media session's capabilities
   if the RTP/AVPF session is transmitted in a best-effort environment.

8.  Security Considerations

   RTP packets transporting information with the proposed payload format
   are subject to the security considerations discussed in the RTP
   specification [1] and in the RTP/AVP profile specification [2].  This
   profile does not specify any additional security services.

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

   Feedback information may be suppressed if unknown RTCP feedback
   packets are received.  This introduces the risk of a malicious group
   member reducing Early feedback by simply transmitting payload-
   specific RTCP feedback packets with random contents that are not
   recognized by any receiver (so they will suppress feedback) or by the
   sender (so no repair actions will be taken).

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

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

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

   Attacks using false RTCP packets (Regular as well as Early ones) can
   be avoided by authenticating all RTCP messages.  This can be achieved
   by using the AVPF profile together with the Secure RTP profile as
   defined in [22]; as a prerequisite, an appropriate combination of
   those two profiles (an "SAVPF") is being specified [21].  Note that,
   when employing group authentication (as opposed to source
   authentication), the aforementioned attacks may be carried out by
   malicious or malfunctioning group members in possession of the right
   keying material.

9.  IANA Considerations

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

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

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

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   SDP Protocol ("proto"):

     Name:               RTP/AVPF
     Long form:          Extended RTP Profile with RTCP-based Feedback
     Type of name:       proto
     Type of attribute:  Media level only
     Purpose:            RFC 4585
     Reference:          RFC 4585

   SDP Attribute ("att-field"):

     Attribute name:     rtcp-fb
     Long form:          RTCP Feedback parameter
     Type of name:       att-field
     Type of attribute:  Media level only
     Subject to charset: No
     Purpose:            RFC 4585
     Reference:          RFC 4585
     Values:             See this document and registrations below

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

   Initial value registration for the attribute "rtcp-fb"

     Value name:     ack
     Long name:      Positive acknowledgement
     Reference:      RFC 4585.

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

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

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

   Further entries may be registered on a first-come first-serve basis.
   Each new registration needs to indicate the parameter name and the
   syntax of possible additional arguments.  For each new registration,
   it is mandatory that a permanent, stable, and publicly accessible
   document exists that specifies the semantics of the registered
   parameter, the syntax and semantics of its parameters as well as

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

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

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

     Value name:     sli
     Long name:      Slice Loss Indication
     Usable with:    nack
     Reference:      RFC 4585.

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

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

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

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

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

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   RTP RTCP Control Packet types (PT):

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

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

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

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

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

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

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

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

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

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

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

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

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

   Further entries may be registered following the "Specification
   Required" rules as defined in RFC 2434 [9].  Each registration needs
   to indicate the FMT value, if there is a specific FB message to go
   into the FCI field, and whether or not multiple FB messages may be
   stacked in a single FCI field.  For each new registration, it is
   mandatory that a permanent, stable, and publicly accessible document
   exists that specifies the semantics of the registered parameter as
   well as the syntax and semantics of the associated FB message (if
   any).  The general registration procedures of [3] apply.

10.  Acknowledgements

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

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

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11.  References

11.1.  Normative References

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

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

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

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

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

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

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

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

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

11.2.  Informative References

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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Authors' Addresses

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

   EMail: jo@acm.org


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

   EMail: stewe@stewe.org


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

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


   Carsten Burmeister
   Panasonic R&D Center Germany GmbH

   EMail: carsten.burmeister@eu.panasonic.com


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

   EMail: jose.rey@eu.panasonic.com

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Full Copyright Statement

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