Internet Engineering Task Force (IETF)                          A. Begen
Request for Comments: 5725                                        D. Hsu
Category: Standards Track                                       M. Lague
ISSN: 2070-1721                                                    Cisco
                                                          February 2010



              Post-Repair Loss RLE Report Block Type for
          RTP Control Protocol (RTCP) Extended Reports (XRs)

Abstract

  This document defines a new report block type within the framework of
  RTP Control Protocol (RTCP) Extended Reports (XRs).  One of the
  initial XR report block types is the Loss Run Length Encoding (RLE)
  Report Block.  This report conveys information regarding the
  individual Real-time Transport Protocol (RTP) packet receipt and loss
  events experienced during the RTCP interval preceding the
  transmission of the report.  The new report, which is referred to as
  the Post-repair Loss RLE report, carries information regarding the
  packets that remain lost after all loss-repair methods are applied.
  By comparing the RTP packet receipts/losses before and after the loss
  repair is completed, one can determine the effectiveness of the loss-
  repair methods in an aggregated fashion.  This document also defines
  the signaling of the Post-repair Loss RLE report in the Session
  Description Protocol (SDP).

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/rfc5725.










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RFC 5725         Post-Repair Loss RLE Report Block Type    February 2010


Copyright Notice

  Copyright (c) 2010 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
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  publication of this document.  Please review these documents
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  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.

  This document may contain material from IETF Documents or IETF
  Contributions published or made publicly available before November
  10, 2008.  The person(s) controlling the copyright in some of this
  material may not have granted the IETF Trust the right to allow
  modifications of such material outside the IETF Standards Process.
  Without obtaining an adequate license from the person(s) controlling
  the copyright in such materials, this document may not be modified
  outside the IETF Standards Process, and derivative works of it may
  not be created outside the IETF Standards Process, except to format
  it for publication as an RFC or to translate it into languages other
  than English.

Table of Contents

  1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
  2.  Requirements Notation . . . . . . . . . . . . . . . . . . . . . 4
  3.  Post-Repair Loss RLE Report Block . . . . . . . . . . . . . . . 4
  4.  Session Description Protocol Signaling  . . . . . . . . . . . . 6
  5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
  6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
  7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
  8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
    8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
    8.2.  Informative References  . . . . . . . . . . . . . . . . . . 8












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RFC 5725         Post-Repair Loss RLE Report Block Type    February 2010


1.  Introduction

  The RTP Control Protocol (RTCP) is the out-of-band control protocol
  for applications that are using the Real-time Transport Protocol
  (RTP) for media delivery and communications [RFC3550].  RTCP allows
  RTP entities to monitor data delivery and provides them minimal
  control functionality via sender and receiver reports as well as
  other control packets.  [RFC3611] expands the RTCP functionality
  further by introducing the RTCP Extended Reports (XRs).

  One of the initial XR report block types defined in [RFC3611] is the
  Loss Run Length Encoding (RLE) Report Block.  This report conveys
  information regarding the individual RTP packet receipt and loss
  events experienced during the RTCP interval preceding the
  transmission of the report.  However, the Loss RLE in an RTCP XR
  report is usually collected only on the primary source stream before
  any loss-repair method is applied.  Once one or more loss-repair
  methods, e.g., Forward Error Correction (FEC) [RFC5109] and/or
  retransmission [RFC4588], are applied, some or all of the lost
  packets on the primary source stream may be recovered.  However, the
  pre-repair Loss RLE cannot indicate which source packets were
  recovered and which are still missing.  Thus, the pre-repair Loss RLE
  cannot specify how well the loss repair performed.

  This issue can be addressed by generating an additional report block
  (within the same or a different RTCP XR report), which reflects the
  packet receipt/loss events after all loss-repair methods are applied.
  This report block, which we refer to as the post-repair Loss RLE,
  indicates the remaining missing, i.e., unrepairable, source packets.
  When the pre-repair and post-repair Loss RLEs are compared, the RTP
  sender or another third-party entity can evaluate the effectiveness
  of the loss-repair methods in an aggregated fashion.  To avoid any
  ambiguity in the evaluation, it is RECOMMENDED that the post-repair
  Loss RLE be generated for the source packets that have no further
  chance of being repaired.  If the loss-repair method(s) may still
  recover one or more missing source packets, the post-repair Loss RLE
  SHOULD NOT be sent until the loss-recovery process has been
  completed.  However, a potential ambiguity may result from sequence-
  number wrapping in the primary source stream.  Thus, the Post-repair
  Loss RLE reports may not be delayed arbitrarily.  In case of an
  ambiguity in the incoming reports, it is the sender's or the
  monitoring entity's responsibility to understand which packets the
  Post-repair Loss RLE report is related to.

  Similar to the pre-repair Loss RLE, the post-repair Loss RLE conveys
  the receipt/loss events at the packet level and considers partially
  repaired packets as unrepaired.  Thus, the methods that can partially
  recover the missing data SHOULD NOT be evaluated based on the



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RFC 5725         Post-Repair Loss RLE Report Block Type    February 2010


  information provided by the Post-repair Loss RLE reports since such
  information may underestimate the effectiveness of such methods.

  Note that the idea of using pre-repair and post-repair Loss RLEs can
  be further extended when multiple sequential loss-repair methods are
  applied to the primary source stream.  Reporting the Loss RLEs before
  and after each loss-repair method can provide specific information
  about the individual performances of these methods.  However, it can
  be a difficult task to quantify the specific contribution made by
  each loss-repair method in hybrid systems, where different methods
  collectively work together to repair the lost source packets.  Thus,
  in this specification we only consider reporting the Loss RLE after
  all loss-repair methods have been applied.

  This document registers a new report block type to cover the post-
  repair Loss RLE within the framework of RTCP XR.  Applications that
  are employing one or more loss-repair methods MAY use Post-repair
  Loss RLE reports for every packet they receive or for a set of
  specific packets they have received.  In other words, the coverage of
  the post-repair Loss RLEs may or may not be contiguous.

2.  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 [RFC2119].

3.  Post-Repair Loss RLE Report Block

  The Post-repair Loss RLE Report Block is similar to the existing Loss
  RLE Report Block defined in [RFC3611].  The report format is shown in
  Figure 1.  Using the same structure for reporting both pre-repair and
  post-repair Loss RLEs allows the implementations to compare the Loss
  RLEs very efficiently.

















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RFC 5725         Post-Repair Loss RLE Report Block Type    February 2010


     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     BT=10     | rsvd. |   T   |         block length          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        SSRC of source                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          begin_seq            |             end_seq           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          chunk 1              |             chunk 2           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :                              ...                              :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          chunk n-1            |             chunk n           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 1: Format for the Post-repair Loss RLE Report Block

  o  block type (BT): 8 bits
     A Post-repair Loss RLE Report Block is identified by the constant
     10.

  o  rsvd.: 4 bits
     This field is reserved for future definition.  In the absence of
     such definition, the bits in this field MUST be set to zero and
     MUST be ignored by the receiver.

  o  thinning (T): 4 bits
     The amount of thinning performed on the sequence-number space.
     Only those packets with sequence numbers 0 mod 2^T are reported by
     this block.  A value of 0 indicates that there is no thinning and
     all packets are reported.  The maximum thinning is one packet in
     every 32,768 (amounting to two packets within each 16-bit sequence
     space).

     If thinning is desired, it is RECOMMENDED to use the same thinning
     value in the Pre-repair and Post-repair Loss RLE reports.  This
     will allow easier report processing and correlation.  However,
     based on the specific needs of the application or the monitoring
     entity, different values of thinning MAY be used for Pre-repair
     and Post-repair Loss RLE reports.

  o  block length: 16 bits
     The length of this report block, including the header, in 32-bit
     words minus one.






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RFC 5725         Post-Repair Loss RLE Report Block Type    February 2010


  o  SSRC of source: 32 bits
     The SSRC of the RTP data packet source being reported upon by this
     report block.

  o  begin_seq: 16 bits
     The first sequence number that this block reports on.

  o  end_seq: 16 bits
     The last sequence number that this block reports on plus one.

  o  chunk i: 16 bits
     There are three chunk types: run length, bit vector, and
     terminating null.  These are defined in Section 4 of [RFC3611].
     If the chunk is all zeroes, then it is a terminating null chunk.
     Otherwise, the left-most bit of the chunk determines its type: 0
     for run length and 1 for bit vector.

  Note that the sequence numbers that are included in the report refer
  to the primary source stream.

  When using Post-repair Loss RLE reports, the amount of bandwidth
  consumed by the detailed reports should be considered carefully.  The
  bandwidth usage rules, as they are described in [RFC3611], apply to
  Post-repair Loss RLE reports as well.

4.  Session Description Protocol Signaling

  A new parameter is defined for the Post-repair Loss RLE Report Block
  to be used with Session Description Protocol (SDP) [RFC4566] using
  the Augmented Backus-Naur Form (ABNF) [RFC5234].  It has the
  following syntax within the "rtcp-xr" attribute [RFC3611]:

        pkt-loss-rle-post = "post-repair-loss-rle" ["=" max-size]

                 max-size = 1*DIGIT ; maximum block size in octets

  Refer to Section 5.1 of [RFC3611] for a detailed description and the
  full syntax of the "rtcp-xr" attribute.  The "pkt-loss-rle-post"
  parameter is compatible with the definition of "format-ext" in the
  "rtcp-xr" attribute.

5.  Security Considerations

  The security considerations of [RFC3611] apply in this document as
  well.  Additional security considerations are briefly mentioned
  below.





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RFC 5725         Post-Repair Loss RLE Report Block Type    February 2010


  An attacker who monitors the regular Pre-repair Loss RLE reports sent
  by a group of receivers in the same multicast distribution network
  may infer the network characteristics (Multicast Inference of Network
  Characteristics).  However, monitoring the Post-repair Loss RLE
  reports will not reveal any further information about the network.
  Without the regular Pre-repair Loss RLE reports, the Post-repair ones
  will not be any use to attackers.  Even when used with the regular
  Pre-repair Loss RLE reports, the Post-repair Loss RLE reports only
  reveal the effectiveness of the repair process.  However, this does
  not enable any new attacks, nor does it provide information to an
  attacker that could not be similarly obtained by watching the RTP
  packets fly by himself, performing the repair algorithms and
  computing the desired output.

  An attacker may interfere with the repair process for an RTP stream.
  In that case, if the attacker is able to see the post-repair Loss
  RLEs, the attacker may infer whether or not the attack is effective.
  If not, the attacker may continue attacking or alter the attack.  In
  practice, however, this does not pose a security risk.

  An attacker may put incorrect information in the regular Pre-repair
  and Post-repair Loss RLE reports such that it impacts the proactive
  decisions made by the sender in the repair process or the reactive
  decisions when responding to the feedback messages coming from the
  receiver.  A sender application should be aware of such risks and
  should take the necessary precautions to minimize the chances for
  (or, better, eliminate) such attacks.

  Similar to other RTCP XR reports, the Post-repair Loss RLE reports
  MAY be protected by using the Secure RTP (SRTP) and Secure RTP
  Control Protocol (SRTCP) [RFC3711].

6.  IANA Considerations

  New block types for RTCP XR are subject to IANA registration.  For
  general guidelines on IANA considerations for RTCP XR, refer to
  [RFC3611].

  This document assigns the block type value 10 in the RTCP XR Block
  Type Registry to "Post-repair Loss RLE Report Block".  This document
  also registers the SDP [RFC4566] parameter "post-repair-loss-rle" for
  the "rtcp-xr" attribute in the RTCP XR SDP Parameters Registry.









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RFC 5725         Post-Repair Loss RLE Report Block Type    February 2010


  The contact information for the registrations is:

  Ali Begen
  [email protected]

  170 West Tasman Drive
  San Jose, CA 95134 USA

7.  Acknowledgments

  The authors would like to thank the members of the VQE Team at Cisco
  and Colin Perkins for their inputs and suggestions.

8.  References

8.1.  Normative References

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

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

  [RFC3611]  Friedman, T., Caceres, R., and A. Clark, "RTP Control
             Protocol Extended Reports (RTCP XR)", RFC 3611,
             November 2003.

  [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.

8.2.  Informative References

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

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

  [RFC5109]  Li, A., "RTP Payload Format for Generic Forward Error
             Correction", RFC 5109, December 2007.





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RFC 5725         Post-Repair Loss RLE Report Block Type    February 2010


Authors' Addresses

  Ali Begen
  Cisco
  170 West Tasman Drive
  San Jose, CA  95134
  USA

  EMail: [email protected]


  Dong Hsu
  Cisco
  1414 Massachusetts Ave.
  Boxborough, MA  01719
  USA

  EMail: [email protected]


  Michael Lague
  Cisco
  1414 Massachusetts Ave.
  Boxborough, MA  01719
  USA

  EMail: [email protected]
























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