Audio/Video Transport Working G. Hunt
Group BT
Internet-Draft A. Clark
Intended status: Standards Track Telchemy
Expires: August 29, 2009 February 25, 2009
RTCP XR Report Block for Discard metric Reporting
draft-ietf-avt-rtcp-xr-discard-01.txt
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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
5.1. New RTCP XR Block Type value . . . . . . . . . . . . . . . 9
5.2. New RTCP XR SDP Parameter . . . . . . . . . . . . . . . . 9
5.3. Contact information for registrations . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Changes from previous version . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 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 packets lost from
the media stream (single SSRC within single RTP session).
Similarly the metric "number of packets discarded" reports a count
of packets discarded from the media stream (single SSRC within
single RTP session) arriving at the receiver. Another metric
defined in [POSTREPAIRLOSS] is available to report on packets
which are not recovered by any repair techniques which may be in
use.
For Voice-over-IP applications of the metric, 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 discarded: 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 = "pkt-dscrd"
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5. 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].
5.1. New RTCP XR Block Type value
This document assigns the block type value ND in the IANA "RTCP XR
Block Type Registry" to the "Concealed Seconds Metrics Block".
[Note to RFC Editor: please replace ND with the IANA provided RTCP XR
block type for this block.]
5.2. New RTCP XR SDP Parameter
This document also registers a new parameter "pkt-dscrd" in the "RTCP
XR SDP Parameters Registry".
5.3. 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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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. Changes from previous version
Expanded and clarified IANA Considerations section
Changed SDP tag for block to "pkt-dscrd"
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9. References
9.1. Normative References
[MEASIDENT]
Hunt, G., "RTCP XR Measurement Identifier Block",
ID draft-ietf-avt-rtcp-xr-measid-01, February 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.
9.2. 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-01, February 2009.
[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 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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