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Audio/Video Transport Working                                    G. Hunt
Group                                                                 BT
Internet-Draft                                                  A. Clark
Intended status: Standards Track                                Telchemy
Expires: April 28, 2009                                 October 25, 2008


           RTCP XR Report Block for Discard metric Reporting
                 draft-ietf-avt-rtcp-xr-discard-00.txt

Status of this Memo

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Abstract

   This document defines an RTCP XR Report Block that allows the
   reporting of a simple discard count metric for use in a range of RTP
   applications.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Discard Report Block . . . . . . . . . . . . . . . . . . .  3
     1.2.  RTCP and RTCP XR Reports . . . . . . . . . . . . . . . . .  3
     1.3.  Performance Metrics Framework  . . . . . . . . . . . . . .  3
     1.4.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Discard Metric Report Block  . . . . . . . . . . . . . . . . .  6
     3.1.  Report Block Structure . . . . . . . . . . . . . . . . . .  6
     3.2.  Definition of Fields in Discard Metric Report Block  . . .  6
   4.  SDP Signaling  . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 14
























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1.  Introduction

1.1.  Discard 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 number of packets which are received
   correctly but are never played out, typically because they arrive too
   late to be played out (buffer underflow) or too early (buffer
   overflow).  The metric is applicable both to systems which use packet
   loss repair techniques (such as forward error correction [RFC5109] or
   retransmission [RFC4588]) and to those which do not.

   This metric is useful for identifying the existence, and
   characterising the severity, of a packet transport problem which may
   affect users' perception of a service delivered over RTP.

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

   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.

   The Measurement Identity block provides extended sequence numbers for
   the beginning and end of both Interval and Cumulative reporting
   periods.  This allows systems receiving the report to calculate the
   number of packets expected.

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].

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].







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1.4.  Applicability

   This metric is believed to be applicable to a large class of RTP
   applications which use a jitter buffer.















































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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.  The Discard Metric counts
      only discarded packets.  The metric "cumulative number of packets
      lost" defined in [RFC3550] reports a count of lost packets in the
      primary media stream.  Another metric defined in [POSTREPAIRLOSS]
      is available to report on packets which are not recovered by any
      repair techniques which may be in use.  If Voice Activity
      Detection is used then the proportion of packets lost and
      discarded shall be determined based on transmitted packets, i.e.
      packets that contained silence and were not transmitted shall not
      be considered, because they do not form part of the RTP sequence.

































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3.  Discard Metric Report 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=ND     |I| tag | resv  |       block length=1          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   number of packets discarded                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 1: Report Block Structure

3.2.  Definition of Fields in Discard Metric Report Block

   block type (BT): 8 bits

      A Discard Metric Report Block is identified by the constant ND.

   [Note to RFC Editor: please replace ND 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 Discard 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 Discard
      Metric block.  Note that there may be more than one Measurement
      Identifier block per RTCP packet.

   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



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      The length of this report block in 32-bit words minus one.  For
      the Discard Metric block, the block length is equal to 1.

   Number of packets lost: 32 bits

      Number of packets discarded over the period (Interval or
      Cumulative) covered by this report.

      If the measured value exceeds 0xFFFFFFFD, the value 0xFFFFFFFE
      SHOULD be reported to indicate an over-range measurement.  If the
      measurement is unavailable, the value 0xFFFFFFFF SHOULD be
      reported.

      Note that the number of packets expected in the period covered by
      the metric (whether interval or cumulative) is available from the
      difference between a pair of extended sequence numbers in the
      Measurement Identity block, so need not be repeated in this block.


































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4.  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-format / xr-pd-block

   xr-pd-block = "xr-pd"


































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5.  IANA Considerations

   This document creates a new block type within the IANA "RTCP XR Block
   Type Registry" called the Discard Metric Block, and a new parameter
   xr-pd within the "RTCP XR SDP Parameters Registry".














































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6.  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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7.  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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8.  References

8.1.  Normative References

   [MEASIDENT]
              Hunt, G., "RTCP XR Measurement Identifier Block",
              ID draft-ietf-avt-rtcp-xr-measid-00, August 2008.

   [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.

8.2.  Informative References

   [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-00, July 2008.

   [POSTREPAIRLOSS]
              Hunt, G., "RTCP XR Report Block for Post-Repair Loss
              metric Reporting",
              ID draft-ietf-rtcp-xr-postrepair-loss-00, October 2008.

   [RFC4588]  Rey, J., "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

   Geoff Hunt
   BT
   Orion 1 PP9
   Adastral Park
   Martlesham Heath
   Ipswich, Suffolk  IP4 2TH
   United Kingdom

   Phone: +44 1473 608325
   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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