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 Burst/Gap Discard metric Reporting
draft-ietf-avt-rtcp-xr-burst-gap-discard-01.txt
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Abstract
This document defines an RTCP XR Report Block that allows the
reporting of Burst and Gap Discard metrics for use in a range of RTP
applications.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Burst and Gap 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. Burst/Gap Discard Block . . . . . . . . . . . . . . . . . . . 6
3.1. Report Block Structure . . . . . . . . . . . . . . . . . . 6
3.2. Definition of Fields in Burst/Gap Discard 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 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 proportion of packets discarded by the
receiver due to jitter. The discards during discard 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. This block is
intended to be used in conjunction with [DISCARD] which provides the
total packets discarded, and on which this block therefore depends.
However the metric in [DISCARD] 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 MAY be used
in Interval reports. The burstiness of packet discard 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].
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
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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 the large class of RTP
applications which contain jitter buffers.
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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].
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3. Burst/Gap Discard 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=NBGD |I| tag | resv | block length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold | Sum of Burst Durations (ms) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packets Discarded 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 Discard Report Block
block type (BT): 8 bits
A Burst/Gap Discard Report Block is identified by the constant
NBGD.
[Note to RFC Editor: please replace NBGD 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 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 Burst/
Gap Discard 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 Discard 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 not be discarded prior to and
following a discarded frame in order for this discarded frame to
be regarded as part of a gap.
Sum of Burst Durations (ms): 24 bits
The total duration of bursts of discarded 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 discarded in bursts: 24 bits
The total number of packets discarded during discard 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 discard 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 Discard block) and also with the metric "number of packets
discarded" provided in the RTCP XR Discard Block [DISCARD]. The RTCP
XR Discard Block SHOULD be sent if the Burst/Gap Discard block is
sent, but the converse does not apply.
The fraction of packets discarded during bursts is the quotient:
Packets Discarded in Bursts / Total Packets expected in Bursts
The fraction of packets discarded during gaps is the quotient:
(number of packets discarded - Packets Discarded in Bursts) /
(Packets Expected - Total Packets expected in Bursts)
where "number of packets discarded" is obtained from the RTCP XR
Discard Block [DISCARD] 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
Discard block.
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:
var = ( Sum of Squares of Burst Durations - Number of Bursts * mean^2
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) / (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 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.
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-format / xr-bgd-block
xr-bgd-block = "brst-gap-dscrd"
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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 NBGD in the IANA "RTCP XR
Block Type Registry" to the "Concealed Seconds Metrics Block".
[Note to RFC Editor: please replace NBGD 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-dscrd" 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
Extended and clarified IANA considerations section
Changed SDP tag for block to "brst-gap-dscrd"
Changed short title to "RTCP XR Burst/Gap Discard" to avoid confusion
with Burst/Gap Loss
Moved VoIP considerations to a separate section to emphasise the more
general applicability of the metrics.
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10. References
10.1. 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.
10.2. Informative References
[DISCARD] Hunt, G., "RTCP XR Report Block for Discard metric
Reporting", ID draft-ietf-rtcp-xr-discard-01,
February 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-00, July 2008.
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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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