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rfc:rfc6642

Internet Engineering Task Force (IETF) Q. Wu, Ed. Request for Comments: 6642 F. Xia Category: Standards Track R. Even ISSN: 2070-1721 Huawei

                                                             June 2012
RTP Control Protocol (RTCP) Extension for a Third-Party Loss Report

Abstract

 In a large RTP session using the RTP Control Protocol (RTCP) feedback
 mechanism defined in RFC 4585, a feedback target may experience
 transient overload if some event causes a large number of receivers
 to send feedback at once.  This overload is usually avoided by
 ensuring that feedback reports are forwarded to all receivers,
 allowing them to avoid sending duplicate feedback reports.  However,
 there are cases where it is not recommended to forward feedback
 reports, and this may allow feedback implosion.  This memo discusses
 these cases and defines a new RTCP Third-Party Loss Report that can
 be used to inform receivers that the feedback target is aware of some
 loss event, allowing them to suppress feedback.  Associated Session
 Description Protocol (SDP) signaling is also defined.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6642.

Wu, et al. Standards Track [Page 1] RFC 6642 Third-Party Loss Report June 2012

Copyright Notice

 Copyright (c) 2012 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
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Introduction ....................................................3
 2. Terminology .....................................................3
    2.1. Requirements Notation ......................................3
    2.2. Glossary ...................................................4
 3. Example Use Cases ...............................................4
    3.1. Source-Specific Multicast (SSM) Use Case ...................4
    3.2. Unicast-Based Rapid Acquisition of Multicast Stream
         (RAMS) Use Case ............................................5
    3.3. RTP Transport Translator Use Case ..........................5
    3.4. Multipoint Control Unit (MCU) Use Case .....................6
    3.5. Mixer Use Case .............................................6
 4. Protocol Overview ...............................................6
 5. Format of RTCP Feedback Messages ................................7
    5.1. Transport-Layer Feedback: Third-Party Loss Report (TPLR) ...8
    5.2. Payload-Specific Feedback:  Third-Party Loss Report (TPLR) .8
 6. SDP Signaling ...................................................9
 7. Security Considerations ........................................10
 8. IANA Considerations ............................................11
 9. Acknowledgments ................................................11
 10. References ....................................................12
    10.1. Normative References .....................................12
    10.2. Informative References ...................................12

Wu, et al. Standards Track [Page 2] RFC 6642 Third-Party Loss Report June 2012

1. Introduction

 The RTP Control Protocol (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 when using unicast
 RTCP feedback with Source-Specific Multicast (SSM) [RFC5760]).  There
 are cases where multiple receivers may initiate the same, or an
 equivalent, message towards the same media source or the same
 feedback target.  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 scenario that can cause such feedback storms involves video Fast
 Update requests.  A storm of these feedback messages can occur in
 conversational multimedia scenarios like multipoint video switching
 conference [RFC4587], where many receivers may simultaneously lose
 synchronization with the video stream when the speaker is changed in
 the middle of a session.  Receivers that issue Fast Update requests
 (i.e., Full Intra Request (FIR) described in RFC 5104 [RFC5104]), can
 cause an implosion of FIR requests from receivers to the same media
 source since these requests must currently be made blind, without
 knowledge of requests made by other receivers.
 RTCP feedback storms may cause short-term overload and, in extreme
 cases, 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.  This document specifies a new Third-Party Loss Report for
 this function.  It supplements the existing use of RTCP NACK packets
 and is also more precise in the uses where the network is active to
 suppress feedback.  It tells receivers explicitly that feedback for a
 particular packet or frame loss is not needed and can provide an
 early indication before the receiver reacts to the loss and invokes
 its packet loss repair machinery.  Section 3 provides some example
 use cases of when to send the Third-Party Loss Report message.

2. Terminology

2.1. Requirements Notation

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].

Wu, et al. Standards Track [Page 3] RFC 6642 Third-Party Loss Report June 2012

2.2. Glossary

 TPLR  - Third-Party Loss Report
 TLLEI - Transport-Layer Third-Party Loss Early Indication
 PSLEI - Payload-Specific Third-Party Loss Early Indication
 PT    - Payload Type
 FMT   - Feedback Message Type
 FCI   - Feedback Control Information [RFC4585]
 AVPF  - Audio-Visual Profile with RTCP-based feedback [RFC4585]
 SSRC  - Synchronization Source
 BRS   - Burst/Retransmission Source [RFC6285]
 FIR   - Full Intra Request [RFC5104]
 PLI   - Picture Loss Indication [RFC4585]
 SSM   - Source-Specific Multicast [RFC5760]
 RAMS  - Unicast-based Rapid Acquisition of Multicast Stream [RFC6285]
 MCU   - Multipoint Control Unit [RFC5117]

3. 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 some of the intended use
 cases for using the Third-Party Loss Report for feedback suppression
 and give an overview of each.

3.1. Source-Specific Multicast (SSM) Use Case

 In SSM RTP sessions as described in "RTP Control Protocol (RTCP)
 Extensions for Single-Source Multicast Sessions with Unicast
 Feedback" [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.

Wu, et al. Standards Track [Page 4] RFC 6642 Third-Party Loss Report June 2012

 As outlined in RFC 5760 [RFC5760], there are two Unicast Feedback
 models that may be used for reporting: the Simple Feedback Model and
 the Distribution Source Feedback Summary Model.  In the Simple
 Feedback Model, there's no need for the distribution source to create
 the RTCP TPLRs; instead, RTCP NACKs are reflected by the distribution
 source to the other receivers.  However, in the Distribution Source
 Feedback Summary Model, the distribution source will not redistribute
 the NACK for some reason (e.g., to prevent revealing the identity or
 existence of a system sending NACK) and may send an RTCP TPLR message
 to the systems that were unable to receive the NACK and won't receive
 the NACK via other means.  The RTCP TPLR can be generated at the
 distribution source when downstream loss is reported (e.g.,
 downstream loss report is received), which indicates to the receivers
 that they should not transmit feedback messages for the same loss
 event for a certain time.  Therefore, the distribution source in the
 Distribution Source Feedback Summary Model can be reasonably certain
 that it will help the situation (i.e., the distribution source is
 unable receive the NACK) by sending this RTCP TPLR message to all the
 relevant receivers impacted by the packet loss.

3.2. Unicast-Based Rapid Acquisition of Multicast Stream (RAMS) Use

    Case
 The typical RAMS architecture [RFC6285] may have several Burst/
 Retransmission Sources (BRSs) behind the multicast source placed at
 the same level.  These BRSs will receive the primary multicast RTP
 stream from the media source and cache the most recent packets after
 joining the multicast session.  If packet loss happens at the
 upstream of all the BRSs or the downstream of BRSs, one or all of the
 BRSs may send an RTCP NACK or RTCP TPLR message to the distribution
 source, where the SSRC in this RTCP NACK or RTCP TPLR message is the
 BRS that is sending the message.  The distribution source forwards/
 reflects this message down on the primary SSM.  The details on how
 the distribution source deals with this message are specified in
 [RETRANS-FOR-SSM].

3.3. RTP Transport Translator Use Case

 A Transport Translator (Topo-Trn-Translator), as defined in RFC 5117
 [RFC5117], is typically forwarding the RTP and RTCP traffic between
 RTP clients, for example, converting from multicast to unicast for
 domains that do not support multicast.  The translator may suffer a
 loss of important video packets.  In this case, the translator may
 forward an RTCP TPLR message received from upstream in the same way
 it forwards other RTCP traffic.  If the translator acting as the
 monitor [MONARCH] is aware of packet loss, it may use the SSRC of the
 monitor as the SSRC of the packet sender to create a NACK message and
 send it to the receivers that are not aware of packet loss.

Wu, et al. Standards Track [Page 5] RFC 6642 Third-Party Loss Report June 2012

3.4. Multipoint Control Unit (MCU) Use Case

 When the speaker is changed in a voice-activated multipoint video
 switching conference [RFC4587], an RTP mixer can be used to select
 the available input streams and forward them to each participant.  If
 the MCU is doing a blind switch without waiting for a synchronization
 point on the new stream, it can send a FIR to the new video source.
 In this case, the MCU should send a FIR suppression message to the
 new receivers.  For example, when the RTP mixer starts to receive FIR
 from some participants, it can suppress the remaining session
 participants from sending FIR by sending out an RTCP TPLR message.

3.5. Mixer Use Case

 A mixer, in accordance with RFC 5117 [RFC5117], aggregates multiple
 RTP streams from other session participants and generates a new RTP
 stream sent to the session participants.  In some cases, the delivery
 of video frames delivery may get damaged, for example, due to packet
 loss or delayed delivery, between the media source and the mixer.  In
 such cases, the mixer needs to check if the packet loss will result
 in PLI or FIR transmissions from most of the group by analyzing the
 received video.  If so, the mixer may initiate FIR or PLI towards the
 media source on behalf of all the session participants and send out
 an RTCP TPLR message to the session participants that may or are
 expected to send a PLI or FIR.  Alternatively, when the mixer starts
 to receive FIR or PLI from some participants and would like to
 suppress the remaining session participants from sending FIR or PLI,
 it can just forward the FIR/PLI from one session participant to
 others.

4. Protocol Overview

 This document extends the RTCP feedback messages defined in the RTP/
 AVPF [RFC4585] by defining an RTCP Third-Party Loss Report (TPLR)
 message.  The RTCP TPLR message can be used by the intermediaries to
 inform the receiver that the sender of the RTCP TPLR has received
 reports that the indicated packets were lost and ask the receiver not
 to send feedback to it regarding these packets.  Intermediaries are
 variously referred to as distribution sources, Burst/Retransmission
 Sources, MCUs, RTP translators, or RTP mixers, depending on the
 precise use case described Section 3.
 RTCP TPLR follows a similar message type format as RTCP NACK or Full
 Intra Request Command.  However, RTCP TPLR is defined as an
 indication that the sender of the feedback has received reports that
 the indicated packets were lost, while NACK [RFC4585] just indicates
 that the sender of the NACK observed that these packets were lost.
 The RTCP TPLR message is generated by an intermediary that may not

Wu, et al. Standards Track [Page 6] RFC 6642 Third-Party Loss Report June 2012

 have seen the actual packet loss.  It is sent following the same
 timing rule as sending NACK, defined in RFC 4585 [RFC4585].  The RTCP
 TPLR message may be sent in a regular full compound RTCP packet or in
 an early RTCP packet, as per the RTP/AVPF rules.  Intermediaries in
 the network that receive an RTCP TPLR MUST NOT send their own
 additional Third-Party Loss Report messages for the same packet
 sequence numbers.  They SHOULD simply forward the RTCP TPLR message
 received from upstream to the receiver(s).  Additionally, they may
 generate their own RTCP TPLR that reports a set of the losses they
 see, which are different from ones reported in the RTCP TPLR they
 received.  The RTCP TPLR does not have retransmission request
 [RFC4588] semantics.
 When a receiver gets an RTCP TPLR message, it MUST follow the rules
 for NACK suppression in RFC 4585 [RFC4585] and refrain from sending a
 feedback request (e.g., NACK or FIR) for the missing packets reported
 in the message, which is dealt with in the same way as receiving a
 NACK.
 To increase the robustness to the loss of a TPLR, the RTCP TPLR may
 be retransmitted.  If the additional TPLR arrives at the receiver,
 the receiver SHOULD deal with the additional TPLR in the same way as
 receiving the first TPLR for the same packet, and no additional
 behavior for receiver is required.
 A receiver may have sent a feedback message according to the RTP/AVPF
 scheduling algorithm of RFC 4585 [RFC4585] before receiving an RTCP
 TPLR message, but further feedback messages for those sequence
 numbers SHOULD be suppressed after receiving the RTCP TPLR.  Nodes
 that do not understand the RTCP TPLR message will ignore it and might
 therefore still send feedback according to the AVPF scheduling
 algorithm of RFC 4585 [RFC4585].  The media source or intermediate
 nodes cannot be certain that the use of an RTCP TPLR message actually
 reduces the amount of feedback they receive.

5. Format of RTCP Feedback Messages

 This document introduces two new RTCP feedback messages for Third-
 Party Loss Report.  Applications that are employing one or more loss-
 repair methods MAY use the RTCP TPLR 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.
 The following two sections each define an RTCP TPLR message.  Both
 messages are feedback messages as defined in Section 6.1 of RFC 4585
 [RFC4585] and use the header format defined there.  Each section
 defines how to populate the PT, FMT, length, SSRC of packet sender,
 SSRC of media source, and FCI fields in that header.

Wu, et al. Standards Track [Page 7] RFC 6642 Third-Party Loss Report June 2012

5.1. Transport-Layer Feedback: Third-Party Loss Report (TPLR)

 This TPLR message is identified by RTCP packet type values PT=RTPFB
 and FMT=7.
 Within the common packet header for feedback messages (as defined in
 Section 6.1 of RFC 4585 [RFC4585]), the "SSRC of packet sender" field
 indicates the source of the request, and the "SSRC of media source"
 field denotes the media sender of the flow for which the indicated
 losses are being suppressed.
 The FCI field MUST contain one or more entries of Transport-Layer
 Third-Party Loss Early Indication (TLLEI).  Each entry applies to the
 same media source identified by the SSRC contained in the "SSRC of
 media source" field of the Feedback header.  The length field in the
 TLLEI feedback message MUST be set to N+2, where N is the number of
 FCI entries.
 The FCI field for TLLEI uses a similar message type format to that
 defined in the Section 6.2.1 of RFC 4585 [RFC4585].  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: Syntax of an FCI Entry in the TLLEI Feedback 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.
 bitmask of 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 Section 6.2.1 of
    [RFC4585].

5.2. Payload-Specific Feedback: Third-Party Loss Report (TPLR)

 This TPLR message is identified by RTCP packet type values PT=PSFB
 and FMT=8, which are used to suppress FIR [RFC5104] and PLI
 [RFC4585].

Wu, et al. Standards Track [Page 8] RFC 6642 Third-Party Loss Report June 2012

 Within the common packet header for feedback messages (as defined in
 Section 6.1 of RFC 4585 [RFC4585]), the "SSRC of packet sender" field
 indicates the source of the request, and the "SSRC of media source"
 is not used and SHALL be set to 0.  The SSRCs of the media senders to
 which this message apply are in the corresponding FCI entries.
 The FCI field for a Payload-Specific Third-Party Loss Early
 Indication (PSLEI) consists one or more FCI entries.  Each entry
 applies to a different media source, identified by its SSRC, the
 content of which is depicted in Figure 2.  The length field in the
 PSLEI feedback message MUST be set to N+2, where N is the number of
 FCI entries.
 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                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Figure 2: Syntax of an FCI Entry in the PSLEI Feedback Message
 Synchronization source (SSRC): 32 bits
    The SSRC value of the media source that is already aware, or in
    the process of being made aware, that some receiver lost
    synchronization with the media stream and for which the PSLEI
    receiver's own response to any such error is suppressed.

6. SDP Signaling

 The Session Description Protocol (SDP) [RFC4566] attribute, rtcp-fb,
 is defined in Section 4 of RFC 4585 [RFC4585] and may be used to
 negotiate the capability to handle specific AVPF commands and
 indications.  The ABNF for rtcp-fb is described in Section 4.2 of RFC
 4585 [RFC4585].  In this section, we extend the rtcp-fb attribute to
 include the commands and indications that are described for Third-
 Party Loss Reports in the present document.
 In the ABNF [RFC5234] for rtcp-fb-val defined in RFC 4585 [RFC4585],
 the feedback type "nack", without parameters, indicates use of the
 Generic NACK feedback format as defined in Section 6.2.1 of RFC 4585
 [RFC4585].  In this document, we define two parameters that indicate
 the third-party loss supported for use with "nack", namely:
 o  "tllei" denotes support of Transport-Layer Third-Party Loss Early
    Indication.

Wu, et al. Standards Track [Page 9] RFC 6642 Third-Party Loss Report June 2012

 o  "pslei" denotes support of Payload-Specific Third-Party Loss Early
    Indication.
 The ABNF for these two parameters for use with "nack" is defined here
 (please refer to Section 4.2 of RFC4585 [RFC4585] for complete ABNF
 syntax).
         rtcp-fb-val        =/ "nack" rtcp-fb-nack-param
         rtcp-fb-nack-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
         token =     <as defined in Section 9 of [RFC4566]>
         byte-string = <as defined in Section 9 of [RFC4566]>

7. Security Considerations

 The security considerations documented in [RFC4585] are also
 applicable for the TPLR messages defined in this document.
 More specifically, spoofed or maliciously created TPLR feedback
 messages cause missing RTP packets to not be repaired in a timely
 fashion and add risk of (undesired) feedback suppression at RTCP
 receivers that accept such TPLR messages.  Any packet loss detected
 by a receiver that also receives a TPLR message for the same missing
 packet(s) will negatively impact the application that relies on the
 (timely) RTP retransmission capabilities.
 A solution to prevent such attack with maliciously sent TPLR messages
 is to apply an authentication and integrity protection framework for
 the feedback messages.  This can be accomplished using the RTP
 profile that combines Secure RTP [RFC3711] and AVPF into SAVPF
 [RFC5124].
 Note that intermediaries that are not visible at the RTP layer that
 wish to send the 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 if SRTP is in use.  If the intermediary is visible
 at the RTP layer, this is not an issue, provided the intermediary is
 part of the security context for the session.

Wu, et al. Standards Track [Page 10] RFC 6642 Third-Party Loss Report June 2012

8. IANA Considerations

 Per this document, IANA has added two values to the '"ack" and "nack"
 Attribute Values' sub-registry [RFC4585] of the 'Session Description
 Protocol (SDP) Parameters' registry.
 The value registration for the attribute value "nack":
    Value name:     tllei
    Long name:      Transport-Layer Third-Party Loss Early Indication
    Usable with:    nack
    Reference:      RFC 6642
    Value name:     pslei
    Long name:      Payload-Specific Third-Party Loss Early Indication
    Usable with:    nack
    Reference:      RFC 6642
 The following value has been registered as one FMT value in the "FMT
 Values for RTPFB Payload Types" registry
 (http://www.iana.org/assignments/rtp-parameters).
   RTPFB range
   Name           Long Name                         Value  Reference
   -------------- --------------------------------- -----  ---------
   TLLEI          Transport-Layer Third-Party         7    [RFC6642]
                  Loss Early Indication
 The following value has been registered as one FMT value in the "FMT
 Values for PSFB Payload Types" registry
 (http://www.iana.org/assignments/rtp-parameters).
   PSFB range
   Name            Long Name                        Value  Reference
   -------------- --------------------------------- -----  ---------
   PSLEI          Payload-Specific Third-Party        8    [RFC6642]
                  Loss Early Indication

9. Acknowledgments

 The authors would like to thank David R. Oran, Magnus Westerlund,
 Colin Perkins, Ali C. Begen, Tom Van Caenegem, Francis Dupont,
 Ingemar Johansson, Bill Ver Steeg, Jonathan Lennox, and WeeSan Lee
 for their valuable comments and suggestions on this document.

Wu, et al. Standards Track [Page 11] RFC 6642 Third-Party Loss Report June 2012

10. References

10.1. Normative References

 [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.
 [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

 [RFC6285]  Ver Steeg, B., Begen, A., Van Caenegem, T., and Z. Vax,
            "Unicast-Based Rapid Acquisition of Multicast RTP
            Sessions", RFC 6285, June 2011.
 [MONARCH]  Wu, Q., Hunt, G., and P. Arden, "Monitoring Architectures
            for RTP", Work in Progress, May 2012.
 [RETRANS-FOR-SSM]
            Van Caenegem, T., Ver Steeg, B., and A. Begen,
            "Retransmission for Source-Specific Multicast (SSM)
            Sessions", Work in Progress, May 2011.

Wu, et al. Standards Track [Page 12] RFC 6642 Third-Party Loss Report June 2012

 [RFC5117]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
            January 2008.
 [RFC4587]  Even, R., "RTP Payload Format for H.261 Video Streams",
            RFC 4587, August 2006.
 [RFC5760]  Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
            Protocol (RTCP) Extensions for Single-Source Multicast
            Sessions with Unicast Feedback", RFC 5760, February 2010.

Authors' Addresses

 Qin Wu (editor)
 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

Wu, et al. Standards Track [Page 13]

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