Post-Repair Loss RLE Report Block Type for RTP Control Protocol (RTCP) Extended Reports (XRs)
RFC 5725

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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Copyright Notice

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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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   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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   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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   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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   The contact information for the registrations is:

   Ali Begen
   abegen@cisco.com

   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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Authors' Addresses

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

   EMail: abegen@cisco.com

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

   EMail: dohsu@cisco.com

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

   EMail: mlague@cisco.com

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