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Versions: (draft-wu-avt-retransmission-supression-rtp) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 RFC 6642

Network Working Group                                              Q. Wu
Internet-Draft                                                    F. Xia
Intended status: Standards Track                                 R. Even
Expires: August 19, 2011                                          Huawei
                                                       February 15, 2011


           RTCP Extension for Feedback Suppression Indication
             draft-ietf-avtcore-feedback-supression-rtp-00

Abstract

   In a large RTP session using the RTCP feedback mechanism defined in
   RFC 4585, a media source or middlebox may experience transient
   overload if some event causes a large number of receivers to send
   feedback at once.  This feedback implosion can be mitigated if the
   device suffering from overload can send a third party loss report
   message to the receivers to inhibit further feedback.  This memo
   defines RTCP extensions for third party loss report, to suppress NACK
   and FIR feedback requests.  It also defines associated SDP
   signalling.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on August 19, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
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   it for publication as an RFC or to translate it into languages other
   than English.


































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  5
   4.  RTCP Feedback Report Extension . . . . . . . . . . . . . . . .  7
     4.1.  Transport Layer Feedback:  Third-party Loss Report . . . .  7
     4.2.  Payload Specific Feedback: Third-party Loss Report . . . .  8
   5.  SDP Signaling  . . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Example Use Cases  . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  Source Specific Multicast (SSM) use case . . . . . . . . .  9
       6.1.1.  Simple Feedback Model  . . . . . . . . . . . . . . . . 10
       6.1.2.  Distribution Source Feedback Summary Model . . . . . . 11
     6.2.  Unicast based Rapid Acquisition of Multicast Stream
           (RAMS) use case  . . . . . . . . . . . . . . . . . . . . . 12
     6.3.  RTP transport translator use case  . . . . . . . . . . . . 13
     6.4.  Multipoint Control Unit (MCU) use case . . . . . . . . . . 13
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  IANA Consideration . . . . . . . . . . . . . . . . . . . . . . 14
   9.  Acknowledgement  . . . . . . . . . . . . . . . . . . . . . . . 15
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     10.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17



























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1.  Introduction

   RTCP feedback messages [RFC4585] allow the receivers in an RTP
   session to report events and ask for action from the media source (or
   a delegated feedback target defined in SSM [RFC5760]).  There are
   cases where multiple receivers may initiate the same, or an
   equivalent message towards the same media source.  When the receiver
   count is large, this behavior may cause transient overload of the
   media source, the network or both.  This is known as a "feedback
   storm" or a "NACK storm".  One common cause of such a feedback storm
   is receivers utilizing RTP retransmission [RFC4588] as a packet loss
   recovery technique based, sending feedback using RTCP NACK messages
   [RFC4585] without proper dithering of the retransmission requests.

   Another use case involves video Fast Update requests.  A storm of
   these feedback messages can occur in conversational multimedia
   scenarios like Topo-Video-switch-MCU [RFC5117].  In this scenario,
   packet loss may happen on an upstream link of an intermediate network
   element such as a Multipoint Control Unit(MCU).  Poorly designed
   receivers that blindly issue fast update requests (i.e., Full Intra
   Request (FIR) described in [RFC5104]), can cause an implosion of FIR
   requests from receivers to the same media source.

   RTCP feedback storms may cause short term overload and, and in
   extreme cases to pose a possible risk of increasing network
   congestion on the control channel (e.g.  RTCP feedback), the data
   channel, or both.  It is therefore desirable to provide a way of
   suppressing unneeded feedback.

   One approach to this, suggested in [DVB-IPTV], involves sending a
   NACK message to the other clients (or receiver) in the same group as
   the sender of NACK.  However sending multicast NACK to the group can
   not prevent large amount of unicast NACK addressed to the same media
   source or middlebox, for example when the NACK is used as a
   retransmission request [RFC4588].  Also NACK is defined as a receiver
   report sent from a receiver observing a packet loss, therefore it
   only inform others that sender of NACK detected loss while the case
   the sender of the feedback has received reports that the indicated
   packets were lost is not covered.  This document specifies a new
   message for this function.  It further is more precise in the
   intended uses and less likely to be confusing to receivers.  It tells
   receivers explicitly that feedback for a particular packet or frame
   loss is not needed for a period of time and can provide an early
   indication before the receiver reacts to the loss and invokes its
   packet loss repair machinery.






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2.  Terminology

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


3.  Protocol Overview

   This document extends the RTCP feedback messages defined in the
   Audio-Visual Profile with Feedback (AVPF) and define the Third Party
   Loss Report message.  The Third Party Loss Report message informs the
   receiver in the downstream path of the middlebox that the sender of
   the Third Party Loss Report has received reports that the indicated
   packets were lost and asks a receiver to not send feedback messages
   for particular packets (indicated by their RTP sequence numbers)
   independent of whether the receiver detected the packet loss or
   detected a need for a decoder refresh point.

   In order to observe packet loss before the receivers perceive it, one
   or more intermediate nodes may be placed between the media source and
   the receivers.  These intermediates are variously referred to as
   Distribution servers, MCUs, RTP translator, or RTP mixers, depending
   on the precise use case.  These intermediaries monitor for packet
   loss upstream of themselves by checking RTP sequence numbers, just as
   receivers do.  Upon observing (or suspecting) an upstream loss, the
   intermediary may send Loss Party Loss Report message towards the
   receivers as defined in this specification.

   These intermediate nodes need to take into account such factors as
   the tolerable application delay, the network dynamics, and the media
   type.  When the packet loss is detected upstream of the intermediary
   and additional latency is tolerable, the intermediate node may itself
   send a feedback message asking for the suspected lost packet or ask
   for the correct decoder refresh point.  Because it has already
   provided the necessary feedback toward the source, the intermediate
   node can be reasonably certain that it will help the situation by
   sending a Third Party Loss Report message to all the relevant
   receivers, thereby indicating to the receivers that they should not
   transmit feedback messages for a period of time.

   Alternatively, the media source may directly monitor the amount of
   feedback requests it receives, and send Third Party Loss Report
   messages to the receivers.

   When a receiver gets such a Third Party Loss Report message, it
   should refrain from sending a feedback request (e.g., NACK or FIR)
   for the missing packets reported in the message for a period of time.



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   A receiver may still have sent a Feedback message according to the
   AVPF scheduling algorithm of [RFC4585]before receiving a Third Party
   Loss Report message, but further feedback messages for those sequence
   numbers will be suppressed by this technique for a period of time.
   Nodes that do not understand the Third Party Loss Report message will
   ignore it, and might therefore still send feedback according to the
   AVPF scheduling algorithm of [RFC4585].  The media source or
   intermediate nodes cannot assume that the use of a Third Party Loss
   Report message actually reduces the amount of feedback it receives.

   RTCP Third Party Loss Report follows the similar format of message
   type as RTCP NACK.  But unlike RTCP NACK, the third party loss report
   is defined as an indication that the sender of the feedback has
   received reports that the indicated packets were lost and conveys the
   packet receipt/loss events at the sequence number level from the
   middlebox to the receivers in the downstream path of middlebox while
   NACK [RFC4585]just indicates that the sender of the NACK observed
   that these packets were lost.  The Third Party Loss Report message
   can also be generated by RTP middleboxs that has not seen the actual
   packet loss and sent to the corresponding receivers.  Intermediaries
   downstream of an intermediary detecting loss obviously SHOULD NOT
   initiate their own additional Third Party Loss Report messages for
   the same packet sequence numbers.  They may either simply forward the
   Third Party Loss Report message received from upstream, or replace it
   with a Third Party Loss Report message that reflects the loss pattern
   they have themselves seen.  The Third Party Loss Report does not have
   the retransmission request [rfc4588] semantics.

   Since Third Party Loss Report interacts strongly with repair timing,
   it has to work together with feedback to not adversely impact the
   repair of lost source packets.  One example is the middle box gets
   the retransmitted packet by sending a NACK upstream and sent it
   downstream.  This retransmitted packet was lost on the downstream
   link.  In order to deal with this, the downstream receiver can start
   a timeout in which it expected to get a retransmission packet.  When
   this timeout expires and there is no retransmitted packet or a new
   third party loss report message, it can take its normal behavior as
   if there is no current retransmission suppression.  In some cases
   where the loss was detected and repair initiated much closer to the
   source, the delay for the receiver to recover from packet loss can be
   reduced through the combination of intermediary feedback to the
   source and Third Party Loss Report downstream.  In all (properly
   operating) cases, the risk of increasing network congestion is
   decreased.







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4.  RTCP Feedback Report Extension

   This document registers two new RTCP Feedback messages for Third
   Party Loss Report.  Applications that are employing one or more loss-
   repair methods MAY use Third Party Loss Report together with their
   existing loss-repair methods either for every packet they expect to
   receive, or for an application-specific subset of the RTP packets in
   a session.  In other words, receivers MAY ignore Third Party Loss
   Report messages, but SHOULD react to them unless they have good
   reason to still send feedback messages despite having been requested
   to suppress them.

4.1.  Transport Layer Feedback:  Third-party Loss Report

   This Third Party Loss Report message is an extension to the RTCP
   Transport Layer Feedback Report and identified by RTCP packet type
   value PT=RTPFB and FMT=TBD.

   The FCI field MUST contain one or more entries of transport layer
   third party loss Early Indication (TLLEI).  Each entry applies to a
   different media source, identified by its SSRC.

   The Feedback Control Information (FCI) for TLLEI uses the similar
   format of message Types defined in the section 4.3.1.1 of [RFC5104].
   The format is shown in Figure 1.

         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 1: Message Format for the Third Party Loss Report

   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.

   bitmask of proceeding 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 [RFC4585].







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4.2.  Payload Specific Feedback: Third-party Loss Report

   This message is an extension to the RTCP Payload Specific Feedback
   report and identified by RTCP packet type value PT=PSFB and FMT=TBD.

   The FCI field MUST contain a Payload Specific Third Party Loss Early
   Indication (PSLEI) entry.  Each entry applies to a different media
   source, identified by its SSRC.

   The Feedback Control Information (FCI) for PSLEI uses the similar
   format of message Types defined in the section 4.3.1.1 of [RFC5104].
   The format is shown in Figure 2.

         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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              SSRC                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Seq nr.   |                   Reserved                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 2: Message Format for the Third Party Loss Report

   SSRC (32 bits):

      The SSRC value of the media source that is requested to send a
      decoder refresh point.

   Seq nr:8bits  Command sequence number.  The sequence number space is
      unique for each pairing of the SSRC of command source and the SSRC
      of the command target.  The sequence number SHALL be increased by
      1 modulo 256 for each new request.

   Reserved: 24 bits

      All bits SHALL be set to 0 by the media source and SHALL be
      ignored on reception.



5.  SDP Signaling

   A new feedback value "tplr" needs to be defined for the Third Party
   Loss Report message to be used with Session Description Protocol
   (SDP) [RFC4566] using the Augmented Backus-Naur Form (ABNF)
   [RFC4585].

   The "tplr" feedback value SHOULD be used with parameters that



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   indicate the third party loss supported.  In this document, we define
   two such parameter, namely:

   o  "tllei" denotes support of transport layer third party loss early
      indication (fsei).

   o  "pslei" denotes support of payload specific third party loss early
      indication.

   In the ABNF for rtcp-fb-val defined in [RFC4585], there is a
   placeholder called rtcp-fb-id to define new feedback types. "tplr" is
   defined as a new feedback type in this document, and the ABNF for the
   parameters for tplr is defined here (please refer to section 4.2 of
   [RFC4585] for complete ABNF syntax).

         rtcp-fb-val        =/ "tplr" rtcp-fb-tplr-param
         rtcp-fb-tplr-param  = SP "tllei";transport layer third party loss early indication
                             / SP "pslei";payload specific third party loss early indication
                             / SP token [SP byte-string]
                                       ; for future commands/indications
      byte-string = <as defined in section 4.2 of [RFC4585] >

   Refer to Section 4.2 of [RFC4585] for a detailed description and the
   full syntax of the "rtcp-fb" attribute.


6.  Example Use Cases

   The operation of feedback suppression is similar for all types of RTP
   sessions and topologies [RFC5117], however the exact messages used
   and the scenarios in which suppression is employed differ for various
   use cases.  The following sections outline the intended use cases of
   using Third Party Loss Report for feedback suppression and give an
   overview of the particular mechanisms.

6.1.  Source Specific Multicast (SSM) use case

   In SSM RTP sessions as described in [RFC5760], one or more Media
   Sources send RTP packets to a Distribution Source.  The Distribution
   Source relays the RTP packets to the receivers using a source-
   specific multicast group.

   In order to avoid the forms of Feedback implosion described in
   section 1,the distribution source should be told that the indicated
   packets were lost.  How the distribution source know the indicated
   packets were lost is beyond of scope of this document.  When upstream
   link or downstream aggregate link packet loss occurs, the
   distribution source creates a Third Party Loss Report and sent it to



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   all the RTP receivers, over the multicast channel.  Another
   possibility is when there may be multiple distribution sources placed
   between the media source and the receivers, the upstream distribution
   source may inform downstream distribution sources of the detected
   packet loss using Third Party Loss Report messages.  In response, the
   downstream distribution sources forward Third Party Loss Report
   received from upstream to all the RTP receivers, over the multicast
   channel.  This Third Party Loss Report message tells the receivers
   that the sender of the third party loss report has received reports
   that the indicated packets were lost.  The distribution source then
   can (optionally) ask for the lost packets from the media source on
   behalf of all the RTP receivers.  The lost packets will either be
   forthcoming from distribution source, or it irretrievably lost such
   that there is nothing to be gained by the receiver sending a NACK to
   the media source.

   The distribution source must be able to communicate with all group
   members in order for either mechanism to be effective at suppressing
   feedback.

   As outlined in the [RFC5760], there are two Unicast Feedback models
   that may be used for reporting, - the Simple Feedback model and the
   Distribution Source Feedback Summary Model.  The RTCP Feedback
   extension for Third Party Loss Report specified in the Section 4 of
   this document will work in both Feedback models.  Details of
   operation in each are specified below.

6.1.1.  Simple Feedback Model

   In the simple Feedback Model, NACKs from the receiver observing the
   loss will be reflected to the other receivers, and there's no need
   for distribution source to create the third-party loss report.  The
   distribution source that has not seen the actual packet loss should
   pass through any Third Party Loss Report message it receives from the
   upstream direction.

   This RTCP Third Party Loss Report message lets the receivers know
   that the sender of the Third party Loss Report has received reports
   that the indicated packets were lost and feedback for this packet
   loss is not needed and should not be sent to the media source(s).  If
   the media source(s) are part of the SSM group for RTCP packet
   reflection, the Distribution Source must filter this packet out.  If
   the media source(s) are not part of the SSM group for RTCP packets,
   the Distribution Source must not forward this RTCP Third Party Loss
   Report message to the media source(s).






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6.1.2.  Distribution Source Feedback Summary Model

   In the distribution source feedback summary model, there may be
   multiple distribution sources and the Loss Detection instances are
   distributed into different distribution sources.  In some cases,
   these Loss Detection instances for the same session can exist at the
   same time, e.g., one Loss Detection instance is implemented in the
   upstream distribution source A, a second Loss Detection instance for
   the same session is part of feedback target A and feedback target B
   respectively within the distribution source B. The distribution
   source B is placed in the path between distribution A and downstream
   receivers.  In this section, we focus on this generic case to discuss
   the distribution Source Feedback Summary Model.

   The distribution source A must listen on the RTP channel for data.
   When the distribution source A observes RTP packets from a media
   source are not consecutive by checking the sequence number of
   packets, the distribution source A generates the new RTCP Third Party
   Loss Report message described in the Section 4, and then send it to
   receivers in the downstream path via the multicast channel.  Note
   that the distribution source A must use its own SSRC value as packet
   sender SSRC for transmitting the new RTCP Third Party Loss Report
   message.

   a second detection instance within the Distribution Source B must
   also listen for RTCP data sent to the RTCP port.  Upon receiving the
   RTCP Third Party Loss Report from the Distribution Source A, the
   distribution source B needs to check whether it sees upstream third
   party loss report from distribution source A reporting the same
   event.  If the upstream Third Party Loss Report reports the different
   event, the distribution source B passes through any Third Party Loss
   Report message it receives from the upstream direction.  If the same
   event is reported from distribution source A, the distribution source
   B replaces it with the summary Third Party Loss Report with the
   information summarization received from two loss detection instances
   within the Distribution Source B. In order to reduce the processing
   load at the distribution source, each loss detection instance may
   provide preliminary summarization report.

   During the summary third party loss report creating, the Distribution
   Source B must use its own SSRC value as packet sender SSRC for
   transmitting summarization information and MUST perform proper SSRC
   collision detection and resolution.

   The distribution source B may send this new RTCP summary third party
   loss report described in the Section 4to the group on the multicast
   RTCP channel and meanwhile send a packet loss request to the media
   source.



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   In some case, the distribution source B may receive RTCP NACK
   messages from the receivers behind the Distribution Source before the
   distribution source detects the packet loss which may cause potential
   Feedback implosion.  In such case, the distribution source B may
   filter them out if it already detected the same loss or sent a packet
   loss request for the missing packet to the media source.

   When the host receives the RTCP Third Party Loss Report message, if
   the host understands this message it will not send packet loss
   request (e.g., NACK) for the missing packets reported in the message.
   If it did not understand this new message, the host MAY send packet
   loss request(e.g., NACK messages) to the specified media source.

6.2.  Unicast based Rapid Acquisition of Multicast Stream (RAMS) use
      case

   The typical RAMS architecture
   [I-D.ietf-avt-rapid-acquisition-for-rtp]may have several Burst/
   Retransmission Sources(BRS) behind the multicast source (MS) These
   BRSes will receive the multicast SSM stream from the media source.
   If one of the BRSes detects packet loss (i.e., First loss in
   Figure 3) on its upstream link between the MS and BRS, but the others
   BRSes have not, as the packet loss took place on SSM tree branch that
   does not impact the other BRSes.  In such case, the BRSes with loss
   detection functionality support cannot detect packet loss at their
   upstream link, therefore these BRSes will not create new Third Party
   Loss Report message and send it to receivers in their downstream
   path.  If the BRS impacted by packet loss has loss detection support,
   the BRS MAY choose to create new Third Party Loss Report message and
   send it to the receivers in the downstream link.  Note that BRS must
   use its own SSRC as packet sender SSRC for transmitting the feedback
   suppress message.

   The BRS may also send a NACK upstream to request the retransmitted
   packet.  Upon receiving the retransmitted packet, the BRS sent it
   downstream.  Note that this retransmitted packet may get lost (i.e.,
   second loss in the Figure 3) on the downstream link.  In order to
   deal with this issue, the downstream receiver can start a timeout
   clock in which it expected to get a retransmission packet.  When this
   timeout expires and there is no retransmitted packet or a new Third
   Party Loss Report message, it can take its normal behavior as if
   there is no current retransmission suppression in place.









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                            First    +------------+            +----------+
                            loss     |Burst and   |Second Loss |          |
                         +-----X-----| Retrans.   |----X------>|          |
                         | Upstream  |Source1(BRS)| Downstream |          |
           Link close    | link 1    +------------+ link 1     |          |
           to multicast  |                                     |          |
           source        |                                     |          |
                |        |                                     |          |
                |        |           +------------+            |   RTP    |
   +---------+  |  +-----++          |Burst and   |            | Receiver |
   |Multicast|  V| |      +----------| Retrans.   |----------->|          |
   | Source  +-----|Router|Upstream  |Source2(BRS)| Downstream |  RTP_Rx  |
   +---------+     |      |link 2    +------------+ link 2     |          |
                   +-----++                                    |          |
                         |                                     |          |
                         |                                     |          |
                         |                                     |          |
                         |           +------------+            |          |
                         |           |Burst and   |            |          |
                         +-----------+ Retrans.   |----------->|          |
                           Upstream  |Source k(BRS| Downstream |          |
                           link k    +------------+ link k     +----------+

                          Figure 3: RAMS Use Case

6.3.  RTP transport translator use case

   A Transport Translator (Topo-Trn-Translator), as defined in [RFC5117]
   is typically forwarding the RTP and RTCP traffic between RTP clients,
   for example converting between multicast and unicast for domains that
   do not support multicast.  The translator can identify packet loss
   from the upstream and send the Third Party Loss Report message to the
   unicast receivers.  Note that the translator must be a participant in
   the session and can then use it's own SSRC as packet sender SSRC for
   transmitting the Third Party Loss Report message

6.4.  Multipoint Control Unit (MCU) use case

   In point to multipoint topologies using video switching MCU (Topo-
   Video-switch-MCU) [RFC5117], the MCU typically forwards a single
   media stream to each participant, selected from the available input
   streams.  The selection of the input stream is often based on voice
   activity in the audio-visual conference, but other conference
   management mechanisms (like presentation mode or explicit floor
   control) exist as well.

   In this case the MCU may detect packet loss from the sender or may
   decide to switch to a new source.  In both cases the receiver may



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   lose synchronization with the video stream and may send a FIR
   request.  If the MCU itself can detect the mis-synchronization of the
   video, the MCU can send the FIR suppression message to the receivers
   and send a FIR request to the video source.  As suggested in RFC
   5117, this topology is better implemented as an Topo-mixer, in which
   case the mixer's SSRC is used as packet sender SSRC for transmitting
   Third Party Loss Report message.


7.  Security Considerations

   The defined messages have certain properties that have security
   implications.  These must be addressed and taken into account by
   users of this protocol.

   Spoofed or maliciously created feedback messages of the type defined
   in this specification can have the following implications:

   Sending Third Party Loss Report with wrong sequence number of lost
   packet that makes missing RTP packets can not be compensated.

   To prevent these attacks, there is a need to apply authentication and
   integrity protection of the feedback messages.  This can be
   accomplished against threats external to the current RTP session
   using the RTP profile that combines Secure RTP [RFC3711] and AVPF
   into SAVPF [RFC5124].

   Note that middleboxes that are not visible at the RTP layer that wish
   to send Third Party Loss Reports on behalf of the media source can
   only do so if they spoof the SSRC of the media source.  This is
   difficult in case SRTP is in use.  If the middlebox is visible at the
   RTP layer, this is not an issue, provided the middlebox is part of
   the security context for the session.

   Also note that endpoints that receive a Third Party Loss Report would
   be well-advised to ignore it, unless it is authenticated via SRTCP or
   similar.  Accepting un-authenticated Third Party Loss Report can lead
   to a denial of service attack, where the endpoint accepts poor
   quality media that could be repaired.


8.  IANA Consideration

   New feedback type and New parameters for RTCP Third Party Loss Report
   are subject to IANA registration.  For general guidelines on IANA
   considerations for RTCP feedback, refer to [RFC4585].

   This document assigns one new feedback type value x in the RTCP



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   feedback report registry to "Third Party Loss Report" with the
   following registrations format:

                    Name:            TPLR
                    Long Name:       Third Party Loss Report
                    Value:           TBD
                    Reference:       This document.

   This document also assigns the parameter value y in the RTCP TPLR
   feedback report Registry to " Transport Layer Third Party Loss Early
   Indication ", with the following registrations format:

        Name:           TLLEI
        Long name:      Transport Layer Third Party Loss Early Indication
        Value:          TBD
        Reference:      this document.

   This document also assigns the parameter value z in the RTCP TPLR
   feedback report Registry to "Payload Specific Third Party Loss Early
   Indication ", with the following registrations format:

        Name:           PSLEI
        Long name:      Payload Specific Third Party Loss Early Indication
        Value:          TBD
        Reference:      this document.

   The contact information for the registrations is:

     Qin Wu
     sunseawq@huawei.com
     101 Software Avenue, Yuhua District
     Nanjing, Jiangsu  210012, China


9.  Acknowledgement

   The authors would like to thank David R Oran, Ali C. Begen, Colin
   Perkins,Tom VAN CAENEGEM, Ingemar Johansson S, Bill Ver Steeg,
   Jonathan Lennox, WeeSan Lee for their valuable comments and
   suggestions on this document.


10.  References

10.1.  Normative References

   [RFC5760]  Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
              Protocol (RTCP) Extensions for Single-Source Multicast



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              Sessions with Unicast Feedback", RFC 5760, February 2010.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              July 2006.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC5117]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
              January 2008.

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              July 2006.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5104]  Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
              "Codec Control Messages in the RTP Audio-Visual Profile
              with Feedback (AVPF)", RFC 5104, February 2008.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for
              Real-time Transport Control Protocol (RTCP)-Based Feedback
              (RTP/SAVPF)", RFC 5124, February 2008.

10.2.  Informative References

   [RFC5740]  Adamson, B., Bormann, C., Handley, M., and J. Macker,
              "NACK-Oriented Reliable Multicast (NORM) Transport
              Protocol", November 2009.

   [DVB-IPTV]
              ETSI Standard, "Digital Video Broadcasting(DVB); Transport
              of MPEG-2 TS Based DVB Services over IP Based Networks",



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              ETSI TS 102 034, V1.4.1 , August 2009.

   [I-D.ietf-avt-rapid-acquisition-for-rtp]
              Steeg, B., Begen, A., Caenegem, T., and Z. Vax, "Unicast-
              Based Rapid Acquisition of Multicast RTP Sessions",
              November 2010.

   [I-D.hunt-avt-monarch-01]
              Hunt, G. and P. Arden, "Monitoring Architectures for RTP",
              August 2008.

   [I-D.ietf-pmol-metrics-framework-02]
              Clark, A., "Framework for Performance Metric Development".


Authors' Addresses

   Qin Wu
   Huawei
   101 Software Avenue, Yuhua District
   Nanjing, Jiangsu  210012
   China

   Email: sunseawq@huawei.com


   Frank Xia
   Huawei
   1700 Alma Dr. Suite 500
   Plano, TX 75075
   USA

   Phone: +1 972-509-5599
   Email: xiayangsong@huawei.com


   Roni Even
   Huawei
   14 David Hamelech
   Tel Aviv 64953
   Israel

   Email: even.roni@huawei.com








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