





Internet Engineering Task Force (IETF)                          R. Huang
Request for Comments: 7509                                        Huawei
Category: Standards Track                                       V. Singh
ISSN: 2070-1721                                         Aalto University
                                                                May 2015


            RTP Control Protocol (RTCP) Extended Report (XR)
                   for Post-Repair Loss Count Metrics

Abstract

   This document defines an RTP Control Protocol (RTCP) Extended Report
   (XR) block that allows reporting of a post-repair loss count metric
   for a range of RTP applications.  In addition, another metric,
   repaired loss count, is also introduced in this report block for
   calculating the pre-repair loss count when needed, so that the RTP
   sender or a third-party entity is able to evaluate the effectiveness
   of the repair methods used by the system.

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/rfc7509.

Copyright Notice

   Copyright (c) 2015 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.



Singh & Huang                Standards Track                    [Page 1]

RFC 7509             Post-Repair Non-RLE Loss Count             May 2015


Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. Post-Repair Loss Count Metrics Report Block .....................3
      3.1. Report Block Structure .....................................4
      3.2. Example Usage ..............................................5
   4. SDP Signaling ...................................................6
      4.1. SDP rtcp-xr-attrib Attribute Extension .....................6
      4.2. Offer/Answer Usage .........................................7
   5. Security Considerations .........................................7
   6. IANA Considerations .............................................7
      6.1. New RTCP XR Block Type Value ...............................7
      6.2. New RTCP XR SDP Parameter ..................................7
      6.3. Contact Information for Registrations ......................7
   7. References ......................................................8
      7.1. Normative References .......................................8
      7.2. Informative References .....................................9
   Appendix A. Metrics Represented Using the Template from RFC 6390 ..10
   Acknowledgments ...................................................11
   Authors' Addresses ................................................11

1.  Introduction

   RTCP Sender Reports (SRs) / Receiver Reports (RRs) [RFC3550] contain
   some rough statistics about the data received from the particular
   source indicated in that block.  One of them is the cumulative number
   of packets lost, which is called the pre-repair loss metric in this
   document.  This metric conveys information regarding the total number
   of RTP data packets that have been lost since the beginning of the
   RTP session.

   However, this metric is measured on the media stream before any loss-
   repair mechanism, e.g., retransmission [RFC4588] or Forward Error
   Correction (FEC) [RFC5109], is applied.  Using a repair mechanism
   usually results in recovering some or all of the lost packets.  The
   recovery process does not reduce the values reported by the two loss
   metrics in RTCP RR [RFC3550] -- namely, the fraction lost and the
   cumulative loss.  Hence, the sending endpoint cannot infer the
   performance of the repair mechanism based on the aforementioned
   metrics in [RFC3550].

   Consequently, [RFC5725] specifies a post-repair loss Run-Length
   Encoding (RLE) XR report block to address this issue.  The sending
   endpoint is able to infer which packets were repaired from the RLE
   report block, but the reporting overhead for the packet-by-packet
   report block is higher compared to other report blocks.




Singh & Huang                Standards Track                    [Page 2]

RFC 7509             Post-Repair Non-RLE Loss Count             May 2015


   When applications use multiple XR blocks, the endpoints may require
   more concise reporting to save bandwidth.  This document defines a
   new XR block type to augment those defined in [RFC3611] and
   complement the report block defined in [RFC5725] for use in a range
   of RTP applications.  This new block type reports the post-repair
   loss count metric, which records the number of primary source RTP
   packets that are still lost after applying one or more loss-repair
   mechanisms.  In addition, another metric, repaired loss count, is
   also introduced in this report block for calculating the pre-repair
   loss count during this range, so that the RTP sender or a third-party
   entity is able to evaluate the effectiveness of the repair methods
   used by the system.  The metrics defined in this document are packet
   level rather than slice/picture level; this means the partial
   recovery of a packet will not be regarded as a repaired packet.

   The metrics defined in this document belong to the class of
   transport-related metrics defined in [RFC6792] and are specified in
   accordance with the guidelines in [RFC6390] and [RFC6792].  These
   metrics are applicable to any RTP application, especially those that
   use loss-repair mechanisms.

2.  Terminology

   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 [KEYWORDS].

   primary source RTP packet: The original RTP packet sent from the RTP
      sender for the first time.  A lost primary source RTP packet may
      be repaired by some other RTP packets used in repair mechanisms
      like FEC or retransmission.

3.  Post-Repair Loss Count Metrics Report Block

   This block reports the number of packets lost after applying repair
   mechanisms (e.g., FEC).  It complements the RTCP XR metrics defined
   in [RFC5725].  As noted in [RFC5725], ambiguity may occur when
   comparing this metric with a pre-repair loss metric reported in an
   RTCP SR/RR, i.e., some packets were not repaired in the current RTCP
   interval, but they may be repaired later.  Therefore, this block uses
   a begin sequence number and an end sequence number to explicitly
   indicate the actual sequence number range reported by this RTCP XR.
   Accordingly, only packets that have no further chance of being
   repaired and that have been repaired are included in this report
   block.






Singh & Huang                Standards Track                    [Page 3]

RFC 7509             Post-Repair Non-RLE Loss Count             May 2015


3.1.  Report Block Structure

   The Post-Repair Loss Count Metrics Report Block has the following
   format:

      0               1               2               3               4
      0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     BT=33     |   Reserved    |      Block length = 4         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       SSRC of Source                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       begin_seq               |          end_seq              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Post-repair loss count       |     Repaired loss count       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Block Type (BT): 8 bits

      A Post-Repair Loss Count Metrics Report Block is identified by the
      constant 33.

   Reserved: 8 bits

      These bits are reserved for future use.  They MUST be set to zero
      by senders and ignored by receivers (see Section 4.2 of
      [RFC6709]).

   Block length: 16 bits

      This field is in accordance with the definition in [RFC3611].  In
      this report block, it MUST be set to 4.  The block MUST be
      discarded if the block length is set to a different value.

   SSRC of source: 32 bits

      As defined in Section 4.1 of [RFC3611].

   begin_seq: 16 bits

      The first sequence number that this block reports on.  It can
      remain fixed when calculating metrics over several RTCP reporting
      intervals.

   end_seq: 16 bits

      The last sequence number that this block reports on plus one.




Singh & Huang                Standards Track                    [Page 4]

RFC 7509             Post-Repair Non-RLE Loss Count             May 2015


   Post-repair loss count: 16 bits

      Total number of packets finally lost after applying one or more
      loss-repair methods, e.g., FEC and/or retransmission, during the
      actual sequence number range indicated by begin_seq and end_seq.
      This metric MUST NOT count the lost packets for which repair might
      still be possible.  Note that this metric MUST measure only
      primary source RTP packets.

   Repaired loss count: 16 bits

      Total number of packets fully repaired after applying one or more
      loss-repair methods, e.g., FEC and/or retransmission, during the
      actual sequence number range indicated by begin_seq and end_seq.
      Note that this metric MUST measure only primary source RTP
      packets.

3.2  Example Usage

   The metrics defined in this report block are all measured at the RTP
   receiver.  However, the receiving endpoint can report the metrics in
   two different ways:

   1) Cumulative report

   In this case, implementations may set begin_seq to the first packet
   in the RTP session, and it will remain fixed across all reports.
   Hence, the "Post-repair loss count" and "Repaired loss count",
   respectively, will correspond to "Cumulative post-repair loss count"
   and "Cumulative repaired loss count" in this case.  These cumulative
   metrics when combined with the cumulative loss metrics reported in an
   RTCP RR (pre-repair) assist in calculating the "Still-to-be-repaired
   lost packets":

      Still-to-be-repaired lost packets =
            Cumulative number of packets lost -
            Cumulative post-repair loss count -
            Cumulative repaired loss count

   2) Interval report

   Some implementations may align the begin_seq and end_seq number with
   the highest sequence numbers of consecutive RTCP RRs (RTCP interval).
   This is NOT RECOMMENDED as packets that are not yet repaired in this
   current RTCP interval and may be repaired in the subsequent intervals
   will not be reported.  An interval report is illustrated in the
   following example:




Singh & Huang                Standards Track                    [Page 5]

RFC 7509             Post-Repair Non-RLE Loss Count             May 2015


      Interval A: The extended highest sequence number received in RTCP
                  RR is 20.  Begin_seq is 10 and end_seq is 20.

      Interval B: The extended highest sequence number received in RTCP
                  RR is 30.  Begin_seq is 20 and end_seq is 30.

   If packets 17 and 19 are lost and not yet repaired in interval A and
   subsequently repaired in interval B, they will not be reported
   because their sequence numbers do not belong in interval B.
   Therefore, if implementations want these packets to be reported as
   repaired, they MUST NOT align the begin_seq and end_seq to the RTCP
   intervals.

   Alternatively, implementations may choose the begin_seq and end_seq
   numbers that cover several RTCP intervals.  Additionally, the
   reported range of sequence numbers may overlap with the previous
   report blocks, so that the packets that were not yet repaired in one
   interval, but were subsequently repaired or deemed unrepairable, were
   reported in subsequent intervals.

   In this case, the "Cumulative number of packets lost" cannot be
   easily compared with the post-repair metrics.  However, the sending
   endpoint can calculate the efficiency of the error resilience
   algorithm using the post-repair and repaired loss count,
   respectively.

4.  SDP Signaling

   [RFC3611] defines the use of SDP (Session Description Protocol) for
   signaling the use of RTCP XR blocks.  However, XR blocks MAY be used
   without prior signaling (see Section 5 of [RFC3611]).

4.1.  SDP rtcp-xr-attrib Attribute Extension

   This session augments the SDP attribute "rtcp-xr" defined in Section
   5.1 of [RFC3611] by providing an additional value of "xr-format" to
   signal the use of the report block defined in this document.  The
   ABNF [RFC5234] syntax is as follows.

   xr-format =/ xr-prlr-block

   xr-prlr-block = "post-repair-loss-count"









Singh & Huang                Standards Track                    [Page 6]

RFC 7509             Post-Repair Non-RLE Loss Count             May 2015


4.2.  Offer/Answer Usage

   When SDP is used in offer/answer context, the SDP Offer/Answer usage
   defined in [RFC3611] for the unilateral "rtcp-xr" attribute
   parameters applies.  For detailed usage of Offer/Answer for
   unilateral parameters, refer to Section 5.2 of [RFC3611].

5.  Security Considerations

   This proposed RTCP XR block introduces no new security considerations
   beyond those described in [RFC3611].  This block does not provide
   per-packet statistics, so the risk to confidentiality documented in
   Section 7, paragraph 3 of [RFC3611] does not apply.

   An attacker may put incorrect information in the Post-Repair Loss
   Count reports, which will affect the performance of loss-repair
   mechanisms.  Implementers should consider the guidance in [RFC7202]
   for using appropriate security mechanisms, i.e., where security is a
   concern, the implementation should apply encryption and
   authentication to the report block.  For example, this can be
   achieved by using the AVPF profile together with the Secure RTP
   profile as defined in [RFC3711]; an appropriate combination of the
   two profiles (an "SAVPF") is specified in [RFC5124].  However, other
   mechanisms also exist (documented in [RFC7201]) and might be more
   suitable.

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].

6.1.  New RTCP XR Block Type Value

   This document assigns the block type value 33 in the IANA "RTP
   Control Protocol Extended Reports (RTCP XR) Block Type Registry" to
   the "Post-Repair Loss Count Metrics Report Block".

6.2.  New RTCP XR SDP Parameter

   This document also registers a new parameter "post-repair-loss-count"
   in the "RTP Control Protocol Extended Reports (RTCP XR) Session
   Description Protocol (SDP) Parameters Registry".

6.3.  Contact Information for Registrations

   The contact information for the registrations is:
      RAI Area Directors <rai-ads@ietf.org>



Singh & Huang                Standards Track                    [Page 7]

RFC 7509             Post-Repair Non-RLE Loss Count             May 2015


7.  References

7.1.  Normative References

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <http://www.rfc-editor.org/info/rfc3550>.

   [RFC3611]  Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
              "RTP Control Protocol Extended Reports (RTCP XR)",
              RFC 3611, DOI 10.17487/RFC3611, November 2003,
              <http://www.rfc-editor.org/info/rfc3611>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <http://www.rfc-editor.org/info/rfc3711>.

   [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for
              Real-time Transport Control Protocol (RTCP)-Based Feedback
              (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
              2008, <http://www.rfc-editor.org/info/rfc5124>.

   [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC5725]  Begen, A., Hsu, D., and M. Lague, "Post-Repair Loss RLE
              Report Block Type for RTP Control Protocol (RTCP) Extended
              Reports (XRs)", RFC 5725, DOI 10.17487/RFC5725, February
              2010, <http://www.rfc-editor.org/info/rfc5725>.













Singh & Huang                Standards Track                    [Page 8]

RFC 7509             Post-Repair Non-RLE Loss Count             May 2015


7.2.  Informative References

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              DOI 10.17487/RFC4588, July 2006,
              <http://www.rfc-editor.org/info/rfc4588>.

   [RFC5109]  Li, A., Ed., "RTP Payload Format for Generic Forward Error
              Correction", RFC 5109, DOI 10.17487/RFC5109, December
              2007, <http://www.rfc-editor.org/info/rfc5109>.

   [RFC6390]  Clark, A. and B. Claise, "Guidelines for Considering New
              Performance Metric Development", BCP 170, RFC 6390,
              DOI 10.17487/RFC6390, October 2011,
              <http://www.rfc-editor.org/info/rfc6390>.

   [RFC6709]  Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
              Considerations for Protocol Extensions", RFC 6709, DOI
              10.17487/RFC6709, September 2012,
              <http://www.rfc-editor.org/info/rfc6709>.

   [RFC6792]  Wu, Q., Ed., Hunt, G., and P. Arden, "Guidelines for Use
              of the RTP Monitoring Framework", RFC 6792,
              DOI 10.17487/RFC6792, November 2012,
              <http://www.rfc-editor.org/info/rfc6792>.

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
              <http://www.rfc-editor.org/info/rfc7201>.

   [RFC7202]  Perkins, C. and M. Westerlund, "Securing the RTP
              Framework: Why RTP Does Not Mandate a Single Media
              Security Solution", RFC 7202, DOI 10.17487/RFC7202, April
              2014, <http://www.rfc-editor.org/info/rfc7202>.

















Singh & Huang                Standards Track                    [Page 9]

RFC 7509             Post-Repair Non-RLE Loss Count             May 2015


Appendix A.  Metrics Represented Using the Template from RFC 6390

   a.  Post-Repair RTP Packet Loss Count Metric

       *  Metric Name: Post-Repair RTP Packet Loss Count Metric.

       *  Metric Description: Total number of RTP packets still lost
          after loss-repair methods are applied.

       *  Method of Measurement or Calculation: See the "Post-repair
          loss count" definition in Section 3.1.  It is directly
          measured and must be measured for the primary source RTP
          packets with no further chance of repair.

       *  Units of Measurement: This metric is expressed as a 16-bit
          unsigned integer value giving the number of RTP packets.

       *  Measurement Point(s) with Potential Measurement Domain: It is
          measured at the receiving end of the RTP stream.

       *  Measurement Timing: This metric relies on the sequence number
          interval to determine measurement timing.  See the Cumulative
          and Interval reports defined in Section 3.2.

       *  Use and Applications: These metrics are applicable to any RTP
          application, especially those that use loss-repair mechanisms.
          See Section 1 for details.

       *  Reporting Model: See RFC 3611.

   b. Repaired RTP Packet Loss Count Metric

       *  Metric Name: Repaired RTP Packet Count Metric.

       *  Metric Description: The number of RTP packets lost but
          repaired after applying loss-repair methods.

       *  Method of Measurement or Calculation: See the "Repaired loss
          count" in Section 3.1.  It is directly measured and must be
          measured for the primary source RTP packets with no further
          chance of repair.

       *  Units of Measurement: This metric is expressed as a 16-bit
          unsigned integer value giving the number of RTP packets.

       *  Measurement Point(s) with Potential Measurement Domain: It is
          measured at the receiving end of the RTP stream.




Singh & Huang                Standards Track                   [Page 10]

RFC 7509             Post-Repair Non-RLE Loss Count             May 2015


       *  Measurement Timing: This metric relies on the sequence number
          interval to determine measurement timing.  See the Cumulative
          and Interval reports defined in Section 3.2.

       *  Use and Applications: These metrics are applicable to any RTP
          application, especially those that use loss-repair mechanisms.
          See Section 1 for details.

       *  Reporting Model: See RFC 3611.

Acknowledgments

   The authors would like to thank Roni Even, Colin Perkins, and Qin Wu
   for giving valuable comments and suggestions.

Authors' Addresses

   Rachel Huang
   Huawei
   101 Software Avenue, Yuhua District
   Nanjing 210012
   China

   EMail: rachel.huang@huawei.com


   Varun Singh
   Aalto University
   School of Electrical Engineering
   Otakaari 5 A
   Espoo, FIN  02150
   Finland

   EMail: varun@comnet.tkk.fi
   URI:   http://www.netlab.tkk.fi/~varun/
















Singh & Huang                Standards Track                   [Page 11]

