





Internet Engineering Task Force (IETF)                          A. Clark
Request for Comments: 7003                                      Telchemy
Category: Standards Track                                       R. Huang
ISSN: 2070-1721                                               Q. Wu, Ed.
                                                                  Huawei
                                                          September 2013


         RTP Control Protocol (RTCP) Extended Report (XR) Block
                 for Burst/Gap Discard Metric Reporting

Abstract

   This document defines an RTP Control Protocol (RTCP) Extended Report
   (XR) block that allows the reporting of burst and gap discard metrics
   for use in a range of RTP applications.

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/rfc7003.

Copyright Notice

   Copyright (c) 2013 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.






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Table of Contents
   1. Introduction ....................................................2
      1.1. Burst/Gap Discard Metrics Block ............................2
      1.2. RTCP and RTCP Extended Reports .............................3
      1.3. Performance Metrics Framework ..............................3
      1.4. Applicability ..............................................3
   2. Terminology .....................................................3
   3. Burst/Gap Discard Metrics Block .................................4
      3.1. Report Block Structure .....................................5
      3.2. Definition of Fields in Burst/Gap Discard Metrics Block ....5
      3.3. Derived Metrics Based on Reported Metrics ..................7
   4. Considerations for Voice-over-IP Applications ...................7
   5. SDP Signaling ...................................................8
      5.1. SDP rtcp-xr Attribute Extension ............................8
      5.2. Offer/Answer Usage .........................................8
   6. IANA Considerations .............................................8
      6.1. New RTCP XR Block Type Value ...............................8
      6.2. New RTCP XR SDP Parameter ..................................9
      6.3. Contact Information for Registrations ......................9
   7. Security Considerations .........................................9
   8. Contributors ....................................................9
   9. Acknowledgments .................................................9
   10. References ....................................................10
      10.1. Normative References .....................................10
      10.2. Informative References ...................................10
   Appendix A.  Metrics Represented Using the Template from RFC 6390..12

1.  Introduction

1.1.  Burst/Gap Discard Metrics Block

   This document defines a new block type to augment those defined in
   [RFC3611] for use in a range of RTP applications.  The new block type
   supports the reporting of the proportion of packets discarded by the
   jitter buffer at the receiver, using packet discard logic according
   to the jitter buffer algorithms.  The discards during discard bursts
   are reported, together with the number of bursts.  This block is
   intended to be used in conjunction with [RFC7002], which provides the
   total packets discarded and on which this block therefore depends.
   However, the metric in [RFC7002] may be used independently of the
   metrics in this block.

   This block provides information on transient IP problems.  Burst/gap
   metrics are typically used in cumulative reports; however, they also
   may be used in interval reports (see the Interval Metric flag in
   Section 3.2).  The burstiness of packet discard affects user
   experience, may influence any sender strategies to mitigate the
   problem, and may also have diagnostic value.



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   The metric belongs to the class of transport-related end-system
   metrics defined in [RFC6792].

   The definitions of "burst", "gap", "loss", and "discard" are
   consistent with the definitions in [RFC3611].  To accommodate the
   range of jitter buffer algorithms and packet discard logic that may
   be used by implementors, the method used to distinguish between
   bursts and gaps shall use an equivalent method to that defined in
   Section 4.7.2 of [RFC3611].  Note that reporting the specific jitter
   buffer algorithms and/or packet discard logic is out of the scope of
   this document.

1.2.  RTCP and RTCP Extended Reports

   The use of RTCP for reporting is defined in [RFC3550].  [RFC3611]
   defined an extensible structure for reporting using an RTCP Extended
   Report (XR).  This document defines a new Extended Report block for
   use with [RFC3550] and [RFC3611].

1.3.  Performance Metrics Framework

   The Performance Metrics Framework [RFC6390] provides guidance on the
   definition and specification of performance metrics.  The RTP
   Monitoring Framework [RFC6792] provides guidelines for reporting
   block format using RTCP XR.  The metrics block described in this
   document is in accordance with the guidelines in [RFC6390] and
   [RFC6792].

1.4.  Applicability

   These metrics are applicable to a range of RTP applications that
   contain de-jitter buffers [RFC5481] at the receiving end to smooth
   variation in packet-arrival time and don't use stream repair means,
   e.g., Forward Error Correction (FEC) [RFC5109] and/or retransmission
   [RFC4588].

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 RFC 2119 [RFC2119].










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   In addition, the following terms are defined:

   Received, Lost, and Discarded

      A packet shall be regarded as "lost" if it fails to arrive within
      an implementation-specific time window.  A packet that arrives
      within this time window but is too early to be played out, too
      late to be played out, or thrown away before playout due to packet
      duplication or redundancy shall be regarded as discarded.  A
      packet shall not be regarded as discarded if it arrives within
      this time window but is dropped during decoding by some higher-
      layer decoder, e.g., due to a decoding error.  A packet shall be
      classified as one of received (or OK), discarded, or lost.  The
      metric "cumulative number of packets lost" defined in [RFC3550]
      reports a count of packets lost from the media stream (single
      synchronization source (SSRC) within a single RTP session).
      Similarly, the metric "number of packets discarded" defined in
      [RFC7002] reports a count of packets discarded from the media
      stream (single SSRC within a single RTP session) arriving at the
      receiver.  Another metric, defined in [RFC5725], is available to
      report on packets that are not recovered by any repair techniques
      that may be in use.  Note that the term "discard" defined here
      builds on the "discard" definition in [RFC3611] but extends the
      concept to take into account packet duplication and reports
      different types of discard counts [RFC7002].

   Bursts and Gaps

      The terms "burst" and "gap" are used in a manner consistent with
      that of RTCP XR [RFC3611].  RTCP XR views an RTP stream as being
      divided into bursts, which are periods during which the discard
      rate is high enough to cause noticeable quality degradation
      (generally over 5 percent discard rate), and gaps, which are
      periods during which discarded packets are infrequent and hence
      quality is generally acceptable.

3.  Burst/Gap Discard Metrics Block

   Metrics in this block report on burst/gap discard in the stream
   arriving at the RTP system.  Measurements of these metrics are made
   at the receiving end of the RTP stream.  Instances of this metrics
   block use the synchronization source (SSRC) to refer to the separate
   auxiliary Measurement Information Block [RFC6776], which describes
   measurement periods in use (see [RFC6776], Section 4.2).

   This metrics block relies on the measurement period in the
   Measurement Information Block indicating the span of the report.
   Senders MUST send this block in the same compound RTCP packet as the



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   Measurement Information Block.  Receivers MUST verify that the
   measurement period is received in the same compound RTCP packet as
   this metrics block.  If not, this metrics block MUST be discarded.

3.1.  Report Block Structure

   The structure of the Burst/Gap Discard Metrics Block is as follows.

       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=20     | I |   resv    |      Block Length = 3         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        SSRC of Source                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Threshold   |         Packets Discarded in Bursts           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       Total Packets Expected in Bursts        |   Reserved    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 1: Report Block Structure

3.2.  Definition of Fields in Burst/Gap Discard Metrics Block

   Block Type (BT): 8 bits

      A Burst/Gap Discard Metrics Block is identified by the constant
      20.

   Interval Metric flag (I): 2 bits

      This field is used to indicate whether the burst/gap discard
      metrics are Sampled, Interval, or Cumulative metrics [RFC6792]:

         I=10: Interval Duration - the reported value applies to the
         most recent measurement interval duration between successive
         metrics reports.

         I=11: Cumulative Duration - the reported value applies to the
         accumulation period characteristic of cumulative measurements.

         I=01: Sampled Value - the reported value is a sampled
         instantaneous value.








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      In this document, burst/gap discard metrics can only be measured
      over definite intervals and cannot be sampled.  Also, the value
      I=00 is reserved for future use.  Senders MUST NOT use the values
      I=00 or I=01.  If a block is received with I=00 or I=01, the
      receiver MUST discard the block.

   Reserved (resv): 6 bits

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

   Block Length: 16 bits

      The length of this report block in 32-bit words, minus one.  For
      the Burst/Gap Discard Metrics Block, the block length is equal to
      3.  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].

   Threshold: 8 bits

      The Threshold is equivalent to Gmin in [RFC3611], i.e., the number
      of successive packets that must not be discarded prior to and
      following a discard packet in order for this discarded packet to
      be regarded as part of a gap.  Note that the Threshold is set in
      accordance with the Gmin calculation defined in Section 4.7.2 of
      [RFC3611].

   Packets Discarded in Bursts: 24 bits

      The total number of packets discarded during discard bursts.

      The measured value is unsigned value.  If the measured value
      exceeds 0xFFFFFD, the value 0xFFFFFE MUST be reported to indicate
      an over-range measurement.  If the measurement is unavailable, the
      value 0xFFFFFF MUST be reported.

   Total Packets Expected in Bursts: 24 bits

      The total number of packets expected during discard bursts (that
      is, the sum of received packets and lost packets).







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      The measured value is unsigned value.  If the measured value
      exceeds 0xFFFFFD, the value 0xFFFFFE MUST be reported to indicate
      an over-range measurement.  If the measurement is unavailable, the
      value 0xFFFFFF MUST be reported.

   Reserved: 8 bits

      These bits are reserved.  They MUST be set to zero by senders and
      ignored by receivers (See [RFC6709], Section 4.2).

3.3.  Derived Metrics Based on Reported Metrics

   The metrics described here are intended to be used in conjunction
   with information from the Measurement Information Block [RFC6776] and
   also with the metric "number of packets discarded" provided in the
   RTCP XR Discard Count Metrics Block [RFC7002].

   These metrics provide the following information relevant to
   statistical parameters, including:

   o  The fraction of packets discarded during bursts (Burst Discard
      Rate in [RFC7004]), which can be calculated using the metric
      "Packets Discarded in Bursts" and the metric "Total Packets
      Expected in Bursts" provided in the Burst/Gap Discard Metrics
      Block.

   o  The fraction of packets discarded during gaps (Gap Discard Rate in
      [RFC7004]), which can be calculated using the metric "Packets
      Discarded in Bursts" and the metric "Total Packets Expected in
      Bursts" provided in the Burst/Gap Discard Metrics Block.

   The details on calculation these parameters in the metrics are
   described in [RFC7004].

4.  Considerations for Voice-over-IP Applications

   This metrics block is applicable to a broad range of RTP
   applications.  Where the metric is used with a Voice-over-IP (VoIP)
   application and the stream repair means is not available, the
   following considerations apply.

   RTCP XR views a call as being divided into bursts, which are periods
   during which the discard rate is high enough to cause noticeable call
   quality degradation (generally over 5 percent discard rate) and gaps,
   which are periods during which discarded packets are infrequent and
   hence call quality is generally acceptable.





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   If voice activity detection is used, the burst and gap duration shall
   be determined as if silence packets had been sent, i.e., a period of
   silence in excess of Gmin packets will terminate a burst condition.

   The recommended value for the threshold Gmin in [RFC3611] results in
   a burst being a period of time during which the call quality is
   degraded to a similar extent to a typical pulse code modulation (PCM)
   severely errored second.

5.  SDP Signaling

   [RFC3611] defines the use of SDP (Session Description Protocol)
   [RFC4566] for signaling the use of XR blocks.  XR blocks MAY be used
   without prior signaling.

5.1.  SDP rtcp-xr Attribute Extension

   This section augments the SDP [RFC4566] attribute "rtcp-xr" defined
   in [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-bgd-block

   xr-bgd-block = "burst-gap-discard"

5.2.  Offer/Answer Usage

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

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 20 in the IANA "RTP
   Control Protocol Extended Reports (RTCP XR) Block Type Registry" to
   the "Burst/Gap Discard Metrics Block".







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6.2.  New RTCP XR SDP Parameter

   This document also registers a new parameter "burst-gap-discard" 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:

   Qin Wu (sunseawq@huawei.com)
   101 Software Avenue, Yuhua District
   Nanjing, Jiangsu  210012
   China

7.  Security Considerations

   It is believed that this 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.

8.  Contributors

   Geoff Hunt wrote the initial draft of this document.

9.  Acknowledgments

   The authors gratefully acknowledge reviews and feedback provided by
   Bruce Adams, Philip Arden, Amit Arora, Claire Bi, Bob Biskner, Benoit
   Claise, Kevin Connor, Claus Dahm, Randy Ethier, Roni Even, Stephen
   Farrell, Jim Frauenthal, Albert Higashi, Tom Hock, Shane Holthaus,
   Paul Jones, Rajesh Kumar, Paul Kyzivat, Keith Lantz, Mohamed Mostafa,
   Amy Pendleton, Colin Perkins, Mike Ramalho, Ravi Raviraj, Dan
   Romascanu, Albrecht Schwarz, Tom Taylor, and Hideaki Yamada.
















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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.

   [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.

   [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, February 2010.

   [RFC6776]  Clark, A. and Q. Wu, "Measurement Identity and Information
              Reporting Using a Source Description (SDES) Item and an
              RTCP Extended Report (XR) Block", RFC 6776, October 2012.

   [RFC7002]  Clark, A., Zorn, G., and Q. Wu, "RTP Control Protocol
              (RTCP) Extended Report (XR) for Discard Count Metric
              Reporting", RFC 7002, September 2013.

   [RFC7004]  Zorn, G., Schott, R., Wu, Q., Ed., and R. Huang, "RTP
              Control Protocol (RTCP) Extended Report (XR) Blocks for
              Summary Statistics Metrics Reporting", RFC 7004, September
              2013.

10.2.  Informative References

   [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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   [RFC5481]  Morton, A. and B. Claise, "Packet Delay Variation
              Applicability Statement", RFC 5481, March 2009.

   [RFC6390]  Clark, A. and B. Claise, "Guidelines for Considering New
              Performance Metric Development", BCP 170, RFC 6390,
              October 2011.

   [RFC6709]  Carpenter, B., Aboba, B., and S. Cheshire, "Design
              Considerations for Protocol Extensions", RFC 6709,
              September 2012.

   [RFC6792]  Wu, Q., Hunt, G., and P. Arden, "Guidelines for Use of the
              RTP Monitoring Framework", RFC 6792, November 2012.






































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Appendix A.  Metrics Represented Using the Template from RFC 6390

   a.  Burst/Gap Discard Threshold in RTP Metric

       *  Metric Name: Burst/gap discard threshold in RTP

       *  Metric Description: The Threshold is equivalent to Gmin in
          [RFC3611], i.e., the number of successive packets that must
          not be discarded prior to and following a discard packet in
          order for this discarded packet to be regarded as part of a
          gap.

       *  Method of Measurement or Calculation: See Section 3.2,
          Threshold definition.

       *  Units of Measurement: See Section 3.2, Threshold definition.

       *  Measurement Point(s) with Potential Measurement Domain: See
          Section 3, 1st paragraph.

       *  Measurement Timing: See Section 3, 2nd paragraph for
          measurement timing and Section 3.2 for Interval Metric flag.

       *  Use and Applications: See Section 1.4.

       *  Reporting Model: See RFC 3611.

   b.  Packets Discarded in Bursts Metric

       *  Metric Name: RTP packets discarded in bursts

       *  Metric Description: The total number of RTP packets discarded
          during discard bursts.

       *  Method of Measurement or Calculation: See Section 3.2, Packets
          Discarded in Bursts definition.

       *  Units of Measurement: See Section 3.2, Packets Discarded in
          Bursts definition.

       *  Measurement Point(s) with Potential Measurement Domain: See
          Section 3, 1st paragraph.

       *  Measurement Timing: See Section 3, 2nd paragraph for
          measurement timing and Section 3.2 for Interval Metric flag.






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       *  Use and Applications: See Section 1.4.

       *  Reporting Model: See RFC 3611.

   c.  Total Packets Expected in Discard Bursts Metric

       *  Metric Name: Total RTP packets expected in discard bursts

       *  Metric Description: The total number of packets expected
          during discard bursts (that is, the sum of received packets
          and lost packets).

       *  Method of Measurement or Calculation: See Section 3.2, Total
          Packets Expected in Bursts definition.

       *  Units of Measurement: See Section 3.2, Total Packets Expected
          in Bursts definition.

       *  Measurement Point(s) with Potential Measurement Domain: See
          Section 3, 1st paragraph.

       *  Measurement Timing: See Section 3, 2nd paragraph for
          measurement timing and Section 3.2 for Interval Metric flag.

       *  Use and Applications: See Section 1.4.

       *  Reporting Model: See RFC 3611.
























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Authors' Addresses

   Alan Clark
   Telchemy Incorporated
   2905 Premiere Parkway, Suite 280
   Duluth, GA  30097
   USA

   EMail: alan.d.clark@telchemy.com


   Rachel Huang
   Huawei Technologies Co., Ltd.
   101 Software Avenue, Yuhua District
   Nanjing, Jiangsu  210012
   China

   EMail: Rachel@huawei.com


   Qin Wu (editor)
   Huawei
   101 Software Avenue, Yuhua District
   Nanjing, Jiangsu  210012
   China

   EMail: sunseawq@huawei.com
























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