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 Concealed Seconds metric Reporting
                 draft-ietf-avt-rtcp-xr-concsec-02.txt

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Abstract

   This document defines an RTCP XR Report Block that allows the
   reporting of Concealed Seconds metrics primarily for audio
   applications of RTP.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Concealed Seconds Block  . . . . . . . . . . . . . . . . .  3
     1.2.  RTCP and RTCP XR Reports . . . . . . . . . . . . . . . . .  3
     1.3.  Performance Metrics Framework  . . . . . . . . . . . . . .  3
     1.4.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Concealed Seconds Metrics Block  . . . . . . . . . . . . . . .  5
     2.1.  Report Block Structure . . . . . . . . . . . . . . . . . .  5
     2.2.  Definition of Fields in Concealed Seconds Metrics Block  .  5
   3.  SDP Signaling  . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
     4.1.  New RTCP XR Block Type value . . . . . . . . . . . . . . . 11
     4.2.  New RTCP XR SDP Parameter  . . . . . . . . . . . . . . . . 11
     4.3.  Contact information for registrations  . . . . . . . . . . 11
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   6.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 13
   7.  Changes from previous version  . . . . . . . . . . . . . . . . 14
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16






















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1.  Introduction

1.1.  Concealed Seconds Block

   This draft defines a new block type to augment those defined in
   [RFC3611], for use primarily in audio applications of RTP.

   At any instant, the audio output at a receiver may be classified as
   either 'normal' or 'concealed'.  'Normal' refers to playout of audio
   payload received from the remote end, and also includes locally
   generated signals such as announcements, tones and comfort noise.
   Concealment refers to playout of locally-generated signals used to
   mask the impact of network impairments such as lost packets or to
   reduce the audibility of jitter buffer adaptations.

   The new block type provides metrics for concealment.  Specifically,
   the first metric (Unimpaired Seconds) reports the number of whole
   seconds occupied only with normal playout of data which the receiver
   obtained from the sender's stream.  The second metric (Concealed
   Seconds) reports the number of whole seconds during which the
   receiver played out any locally-generated media data.  A third metric
   (Severely Concealed Seconds) reports the number of whole seconds
   during which the receiver played out locally-generated data for more
   than SCS Threshold (ms).

   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.

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 primarily applicable to audio applications of RTP.
   The reason for this restriction is that, for many video codecs,
   packet data may contain occasional complete reference pictures, and
   otherwise consists of data specifying picture changes relative to a
   complete reference picture.  Loss of an RTP video media packet could
   degrade the user experience for a variable amount of time between the
   time of loss and the next complete reference picture.  In contrast,
   in the audio case the degradation almost always persists for a
   predictable period of time from the loss of the packet, which might
   be simply the duration of the audio data encoded in the lost packet.
   However if a useful Concealed Seconds metric can be defined for an
   RTP video application, either in general or for a specific type of
   video codec, the Concealed Seconds and Severely Concealed Seconds
   metrics and the metric block type defined here MAY be used.



































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2.  Concealed Seconds Metrics Block

   This sub-block provides a description of potentially audible
   impairments due to lost and discarded packets at the endpoint,
   expressed on a time basis analogous to a traditional PSTN T1/E1
   errored seconds metric.

   The following metrics are based on successive one second intervals as
   declared by a local clock.  This local clock does NOT need to be
   synchronized to any external time reference.  The starting time of
   this clock is unspecified.  Note that this implies that the same loss
   pattern could result in slightly different count values, depending on
   where the losses occur relative to the particular one-second
   demarcation points.  For example, two loss events occurring 50ms
   apart could result in either one concealed second or two, depending
   on the particular 1000 ms boundaries used.

   The seconds in this sub-block are not necessarily calendar seconds.
   At the tail end of a call, periods of time of less than 1000ms shall
   be incorporated into these counts if they exceed 500ms and shall be
   disregarded if they are less than 500ms.

2.1.  Report Block Structure

      Concealed seconds metrics block
       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=NCS    |I| tag |plc|rsv|       block length = 3        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Unimpaired Seconds                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Concealed Seconds                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Severely Concealed Seconds    | RESERVED      | SCS Threshold |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 1: Report Block Structure

2.2.  Definition of Fields in Concealed Seconds Metrics Block

   block type (BT): 8 bits

      A Concealed Seconds Metrics Report Block is identified by the
      constant NCS.

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



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   Interval Metric flag (I): 1 bit

      This field is used to indicate whether the Concealed Seconds
      metric block is an Interval or a Cumulative report, that is,
      whether the reported values apply 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
      Concealed Seconds Metrics block.  Note that there may be more than
      one Measurement Identifier block per RTCP packet.

   Packet Loss Concealment Method (plc): 2 bits

      This field is used to identify the packet loss concealment method
      in use at the receiver, according to the following code:


           bits 0-3
                 0 = silence insertion
                 1 = simple replay, no attenuation
                 2 = simple replay, with attenuation
                 3 = enhanced
                 Other values reserved

   Reserved (rsv): 2 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 Concealed Seconds block, the block length is equal to 3.

   Unimpaired Seconds: 32 bits

      A count of the number of unimpaired Seconds that have occurred.

      An unimpaired Second is defined as a continuous period of 1000ms
      during which no frame loss or discard due to late arrival has



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      occurred.  Every second in a call must be classified as either OK
      or Concealed.

      Normal playout of comfort noise or other silence concealment
      signal during periods of talker silence, if VAD is used, shall be
      counted as unimpaired seconds.

      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.



   Concealed Seconds: 32 bits

      A count of the number of Concealed Seconds that have occurred.

      A Concealed Second is defined as a continuous period of 1000ms
      during which any frame loss or discard due to late arrival has
      occurred.

      Equivalently, a concealed second is one in which some Loss-type
      concealment has occurred.  Buffer adjustment-type concealment
      SHALL not cause Concealed Seconds to be incremented, with the
      following exception.  An implementation MAY cause Concealed
      Seconds to be incremented for 'emergency' buffer adjustments made
      during talkspurts.

      Loss-type concealment is reactive insertion or deletion of samples
      in the audio playout stream due to effective frame loss at the
      audio decoder.  "Effective frame loss" is the event in which a
      frame of coded audio is simply not present at the audio decoder
      when required.  In this case, substitute audio samples are
      generally formed, at the decoder or elsewhere, to reduce audible
      impairment.

      Buffer Adjustment-type concealment is proactive or controlled
      insertion or deletion of samples in the audio playout stream due
      to jitter buffer adaptation, re-sizing or re-centering decisions
      within the endpoint.

      Because this insertion is controlled, rather than occurring
      randomly in response to losses, it is typically less audible than
      loss-type concealment.  For example, jitter buffer adaptation
      events may be constrained to occur during periods of talker
      silence, in which case only silence duration is affected, or
      sophisticated time-stretching methods for insertion/deletion



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      during favorable periods in active speech may be employed.  For
      these reasons, buffer adjustment-type concealment MAY be exempted
      from inclusion in calculations of Concealed Seconds and Severely
      Concealed Seconds.

      However, an implementation SHOULD include buffer-type concealment
      in counts of Concealed Seconds and Severely Concealed Seconds if
      the event occurs at an 'inopportune' moment, with an emergency or
      large, immediate adaptation during active speech, or for
      unsophisticated adaptation during speech without regard for the
      underlying signal, in which cases the assumption of low-audibility
      cannot hold.  In other words, jitter buffer adaptation events
      which may be presumed to be audible SHOULD be included in
      Concealed Seconds and Severely Concealed Seconds counts.

      Concealment events which cannot be classified as Buffer
      Adjustment- type MUST be classified as Loss-type.

      For clarification, the count of Concealed Seconds MUST include the
      count of Severely Concealed Seconds.

      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.

   Severely Concealed Seconds: 16 bits

      A count of the number of Severely Concealed Seconds.

      A Severely Concealed Second is defined as a non-overlapping period
      of 1000 ms during which the cumulative amount of time that has
      been subject to frame loss or discard due to late arrival, exceeds
      the SCS Threshold.

      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.

   RESERVED: 8 bits

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

   SCS Threshold: 8 bits






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      The SCS Threshold defines the amount of time corresponding to lost
      or discarded frames that must occur within a one second period in
      order for the second to be classified as a Severely Concealed
      Second.  This is expressed in milliseconds and hence can represent
      a range of 0.1 to 25.5 percent loss or discard.

      A default threshold of 50ms (5% effective frame loss per second)
      is suggested.











































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3.  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.

   The SDP attribute for the block has an additional optional paremeter,
   "thresh", used to supply a value for the SCS Threshold parameter.  If
   this parameter is present, the RTP system receiving the SDP SHOULD
   use this value for the current session.  If the parameter is not
   present, the RTP system SHOULD use a locally configured value.

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

   (defined in [RFC3611])

   xr-format =/ xr-conc-sec-block

   xr-conc-sec-block = "conc-sec" ["=" thresh]

        thresh      = 1*DIGIT          ; threshold for SCS (ms)
        DIGIT          = %x30-39

























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4.  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].

4.1.  New RTCP XR Block Type value

   This document assigns the block type value NCS in the IANA "RTCP XR
   Block Type Registry" to the "Concealed Seconds Metrics Block".

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

4.2.  New RTCP XR SDP Parameter

   This document also registers a new parameter "conc-sec" in the "RTCP
   XR SDP Parameters Registry".

4.3.  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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5.  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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6.  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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7.  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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8.  References

8.1.  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.

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-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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