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Internet Engineering Task Force (IETF)                          A. BegenRequest for Comments: 5725                                        D. HsuCategory: Standards Track                                       M. LagueISSN: 2070-1721                                                    Cisco                                                           February 2010Post-Repair Loss RLE Report Block Type forRTP 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5725.Begen, et al.                Standards Track                    [Page 1]

RFC 5725         Post-Repair Loss RLE Report Block Type    February 2010Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.   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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .32.  Requirements Notation . . . . . . . . . . . . . . . . . . . . .43.  Post-Repair Loss RLE Report Block . . . . . . . . . . . . . . .44.  Session Description Protocol Signaling  . . . . . . . . . . . .65.  Security Considerations . . . . . . . . . . . . . . . . . . . .66.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .77.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . .88.  References  . . . . . . . . . . . . . . . . . . . . . . . . . .88.1.  Normative References  . . . . . . . . . . . . . . . . . . .88.2.  Informative References  . . . . . . . . . . . . . . . . . .8Begen, et al.                Standards Track                    [Page 2]

RFC 5725         Post-Repair Loss RLE Report Block Type    February 20101.  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 theBegen, et al.                Standards Track                    [Page 3]

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.Begen, et al.                Standards Track                    [Page 4]

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.Begen, et al.                Standards Track                    [Page 5]

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 inSection 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 toSection 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.Begen, et al.                Standards Track                    [Page 6]

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.Begen, et al.                Standards Track                    [Page 7]

RFC 5725         Post-Repair Loss RLE Report Block Type    February 2010   The contact information for the registrations is:   Ali Begen   abegen@cisco.com   170 West Tasman Drive   San Jose, CA 95134 USA7.  Acknowledgments   The authors would like to thank the members of the VQE Team at Cisco   and Colin Perkins for their inputs and suggestions.8.  References8.1.  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.Begen, et al.                Standards Track                    [Page 8]

RFC 5725         Post-Repair Loss RLE Report Block Type    February 2010Authors' 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.comBegen, et al.                Standards Track                    [Page 9]