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Versions: 00 01 02                                                      
Audio/Video Transport Working                                    G. Hunt
Group                                                                 BT
Internet-Draft                                                  A. Clark
Intended status: Standards Track                                Telchemy
Expires: November 16, 2009                                  May 15, 2009


        RTCP XR Report Block for Burst/Gap Loss metric Reporting
              draft-ietf-avt-rtcp-xr-burst-gap-loss-02.txt

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   This Internet-Draft is submitted to IETF in full conformance with the
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Abstract

   This document defines an RTCP XR Report Block that allows the
   reporting of Burst and Gap Loss metrics for use in a range of RTP
   applications.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Burst and Gap Loss Report Block  . . . . . . . . . . . . .  3
     1.2.  RTCP and RTCP XR Reports . . . . . . . . . . . . . . . . .  3
     1.3.  Performance Metrics Framework  . . . . . . . . . . . . . .  4
     1.4.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Burst/Gap Loss Block . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Report Block Structure . . . . . . . . . . . . . . . . . .  6
     3.2.  Definition of Fields in Burst/Gap Loss Report Block  . . .  6
     3.3.  Derived metrics based on reported metrics  . . . . . . . .  8
   4.  Considerations for Voice-over-IP applications  . . . . . . . . 10
   5.  SDP Signaling  . . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
     6.1.  New RTCP XR Block Type value . . . . . . . . . . . . . . . 12
     6.2.  New RTCP XR SDP Parameter  . . . . . . . . . . . . . . . . 12
     6.3.  Contact information for registrations  . . . . . . . . . . 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 14
   9.  Changes from previous version  . . . . . . . . . . . . . . . . 15
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     10.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17



















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1.  Introduction

1.1.  Burst and Gap Loss Report Block

   This draft 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 lost by the
   network.  The losses during loss bursts are reported, together with
   the number of bursts and additional data allowing the calculation of
   statistical parameters (mean and variance) of the distribution of
   burst lengths.  Some uses of these metrics depend on the availability
   of the metric "cumulative number of packets lost" from RTCP
   [RFC3550].

   This block provides information on transient IP problems.  Burst/Gap
   metrics are typically used in Cumulative reports however MAY be used
   in Interval reports.  The burstiness of packet loss affects user
   experience, may influence any sender strategies to mitigate the
   problem, and may also have diagnostic value.

   The metric belongs to the class of transport-related terminal metrics
   defined in [MONARCH] (work in progress).

   The definitions of Burst, Gap, Loss and Discard are consistent with
   definitions in [RFC3611], with the clarification that Loss and
   Discard are defined in terms of frames.  To accomodate 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
   may be an equivalent method to that defined in [RFC3611].  The method
   used SHOULD produce the same result as that defined in [RFC3611] for
   conditions of burst packet loss, but MAY produce different results
   for conditions of time varying jitter.

   Instances of this Metrics Block refer by tag to the separate
   auxiliary Measurement Identity block [MEASIDENT] which contains
   information such as the SSRC of the measured stream, and RTP sequence
   numbers and time intervals indicating the span of the report.

1.2.  RTCP and RTCP XR 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 draft defines a new Extended Report block that
   MUST be used as defined in [RFC3550] and [RFC3611].







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1.3.  Performance Metrics Framework

   The Performance Metrics Framework [PMOLFRAME] provides guidance on
   the definition and specification of performance metrics.  Metrics
   described in this draft either reference external definitions or
   define metrics generally in accordance with the guidelines in
   [PMOLFRAME].

1.4.  Applicability

   This metric is believed to be applicable to all RTP applications.








































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2.  Definitions

   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 or late to be played out shall
      be regarded as discarded.  A packet shall be classified as one of
      received (or OK), discarded or lost.

   Bursts and Gaps

      The terms Burst and Gap are used in a manner consistent with that
      of RTCP XR [RFC3611].  RTCP XR views a call as being divided into
      bursts, which are periods during which the loss rate is high
      enough to cause noticeable call quality degradation (generally
      over 5 percent loss rate), and gaps, which are periods during
      which lost packets are infrequent and hence call quality is
      generally acceptable.

      In the application of the metric to Voice over IP, if Voice
      Activity Detection is used the Burst and Gap Duration shall be
      determined as if silence frames had been sent, i.e. a period of
      silence in excess of Gmin frames MUST terminate a burst condition.



























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3.  Burst/Gap Loss Block

3.1.  Report Block Structure

       0               1               2               3
       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    BT=NBGL    |I| tag | resv  |      block length = 4         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Threshold     |         Sum of Burst Durations (ms)           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Packets Lost in Bursts             |    Total...   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | ...Packets expected in bursts |    Number of bursts   | Sum of|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                ...Squares of Burst Durations (ms-squared)     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 1: Report Block Structure

3.2.  Definition of Fields in Burst/Gap Loss Report Block

   block type (BT): 8 bits

      A Burst/Gap Loss Report Block is identified by the constant NBGL.

   [Note to RFC Editor: please replace NBGL with the IANA provided RTCP
   XR block type for this block.]

   Interval Metric flag (I): 1 bit

      This field is used to indicate whether the Burst/Gap Loss metric
      is an Interval or a Cumulative metric, that is, whether the
      reported value applies to the most recent measurement interval
      duration between successive metrics reports (I=1) (the Interval
      Duration) or to the accumulation period characteristic of
      cumulative measurements (I=0) (the Cumulative Duration).
      Numerical values for both these intervals are provided in the
      Measurement Identifier block referenced by the tag field below.

   Measurement Identifier association (tag): 3 bits

      This field is used to identify the Measurement Identifier block
      [MEASIDENT] which describes this measurement.  The relevant
      Measurement Identifier block has the same tag value as the Burst/
      Gap Loss block.  Note that there may be more than one Measurement
      Identifier block per RTCP packet.




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   Reserved (resv): 4 bits

      These bits are reserved.  They SHOULD be set to zero by senders
      and MUST be ignored by receivers.

   block length: 16 bits

      The length of this report block in 32-bit words, minus one.  For
      the Burst/Gap Loss block, the block length is equal to 4.

   Threshold: 8 bits

      The Threshold is equivalent to Gmin in [RFC3611], i.e. the number
      of successive frames that must be received prior to and following
      a lost frame in order for this lost frame to be regarded as part
      of a gap.

   Sum of Burst Durations (ms): 24 bits

      The total duration of bursts of lost frames in the period of the
      report (Interval or Cumulative).

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

   Packets lost in bursts: 24 bits

      The total number of packets lost during loss bursts.

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

   Total packets expected in bursts: 24 bits

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

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

   Number of bursts: 16 bits

      The number of bursts in the period of the report (Interval or
      Cumulative).




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      If the measured value exceeds 0xFFFD, the value 0xFFFE SHOULD be
      reported to indicate an over-range measurement.  If the
      measurement is unavailable, the value 0xFFFF SHOULD be reported.

   Sum of Squares of Burst Durations (ms-squared): 36 bits

      The sum of the squares of burst durations (where individual burst
      durations are expressed in ms) over in the period of the report
      (Interval or Cumulative).  The units for this quantity are
      milliseconds-squared.

      If the measured value exceeds 0xFFFFFFFFD, the value 0xFFFFFFFFE
      SHOULD be reported to indicate an over-range measurement.  If the
      measurement is unavailable, the value 0xFFFFFFFFF SHOULD be
      reported.

3.3.  Derived metrics based on reported metrics

   The metrics described here are intended to be used as described in
   this section, in conjunction with information from the Measurement
   Identity block (which MUST be present in the same RTCP packet as the
   Burst/Gap Loss block) and also with the metric "cumulative number of
   packets lost" provided in standard RTCP [RFC3550].

   The fraction of packets lost during bursts is the quotient: Packets
   Lost in Bursts / Total Packets expected in Bursts

   The fraction of packets lost during gaps is the quotient: (number of
   packets lost - Packets Lost in Bursts) / (Packets Expected - Total
   Packets expected in Bursts)

   where "number of packets lost" is obtained from standard RTCP
   [RFC3550] and Packets Expected is calculated as the difference
   between "extended last sequence number" and "extended first sequence
   number" (Interval or Cumulative) provided in the Measurement Identity
   block [MEASIDENT] associated with this Burst/Gap Loss block.

   Note that if the metric is to be calculated on an Interval basis, a
   difference must be taken between the current and preceding values of
   "cumulative number of packets lost" in RTCP, to obtain the "number of
   packets lost" for the reporting interval.

   The mean burst duration is obtained as the quotient:

   mean = Sum of Burst Durations / Number of Bursts

   The variance of the burst duration is obtained using the standard
   result:



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   var = ( Sum of Squares of Burst Durations - Number of Bursts * mean^2
   ) / (Number of Bursts - 1)

















































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4.  Considerations for Voice-over-IP applications

   This metric block is applicable to a broad range of RTP applications.
   Where the metric is used with a Voice-overIP (VoIP) application, the
   following considerations apply.

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

   If Voice Activity Detection is used the Burst and Gap Duration shall
   be determined as if silence frames had been sent, i.e. a period of
   silence in excess of Gmin frames MUST 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 PCM Severely Errored
   Second.































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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.

   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.

   rtcp-xr-attrib = "a=" "rtcp-xr" ":" [xr-format *(SP xr-format)] CRLF

   (defined in [RFC3611])

   xr-format =/ xr-bgl-block

   xr-bgl-block = "brst-gap-loss"


































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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 NBGL in the IANA "RTCP XR
   Block Type Registry" to the "Concealed Seconds Metrics Block".

   [Note to RFC Editor: please replace NBGL with the IANA provided RTCP
   XR block type for this block.]

6.2.  New RTCP XR SDP Parameter

   This document also registers a new parameter "brst-gap-loss" in the
   "RTCP XR SDP Parameters Registry".

6.3.  Contact information for registrations

   The contact information for the registrations is:

   Geoff Hunt (geoff.hunt@bt.com)

   Orion 2 PP3, Adastral Park, Martlesham Heath, Ipswich IP5 3RE, United
   Kingdom
























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7.  Security Considerations

   It is believed that this proposed RTCP XR report 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.












































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8.  Contributors

   The authors gratefully acknowledge the comments and contributions
   made by Bruce Adams, Philip Arden, Amit Arora, Bob Biskner, Kevin
   Connor, Claus Dahm, Randy Ethier, Roni Even, Jim Frauenthal, Albert
   Higashi, Tom Hock, Shane Holthaus, Paul Jones, Rajesh Kumar, Keith
   Lantz, Mohamed Mostafa, Amy Pendleton, Colin Perkins, Mike Ramalho,
   Ravi Raviraj, Albrecht Schwarz, Tom Taylor, and Hideaki Yamada.











































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9.  Changes from previous version

   Changed BNF for SDP following Christian Groves' and Tom Taylor's
   comments (4th and 5th May 2009), now aligned with RFC 5234 section 
   3.3 "Incremental Alternatives".

   Updated references.












































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10.  References

10.1.  Normative References

   [MEASIDENT]
              Hunt, G., "RTCP XR Measurement Identifier Block",
              ID draft-ietf-avt-rtcp-xr-meas-identity-02, May 2009.

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

   [RFC3550]  Schulzrinne, H., "RTP: A Transport Protocol for Real-Time
              Applications", RFC 3550, July 2003.

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

   [RFC4566]  Handley, M., "SDP: Session Description Protocol",
              RFC 4566, July 2006.

10.2.  Informative References

   [DISCARD]  Hunt, G., "RTCP XR Report Block for Discard metric
              Reporting", ID draft-ietf-rtcp-xr-discard-02, May 2009.

   [MONARCH]  Hunt, G., "Monitoring Architectures for RTP",
              ID draft-hunt-avt-monarch-01, August 2008.

   [PMOLFRAME]
              Clark, A., "Framework for Performance Metric Development",
              ID draft-ietf-pmol-metrics-framework-02, March 2009.




















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Authors' Addresses

   Geoff Hunt
   BT
   Orion 2 PP3
   Adastral Park
   Martlesham Heath
   Ipswich, Suffolk  IP5 3RE
   United Kingdom

   Phone: +44 1473 651704
   Email: geoff.hunt@bt.com


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

   Email: alan.d.clark@telchemy.com






























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