RTCP XR Burst/Gap Discard: RTP Control Protocol (RTCP) Extended Report (XR) Block for Burst/Gap Discard Metric Reporting
An RTCP report block that shows whether discarded packets came in clumps or were spread out.
Clark, A., Huang, R., and Q. Wu, Ed., "RTP Control Protocol (RTCP) Extended Report (XR) Block for Burst/Gap Discard Metric Reporting", RFC 7003, DOI 10.17487/RFC7003, September 2013, <https://www.rfc-editor.org/info/rfc7003>.
@misc{rfc7003,
series = {Request for Comments},
number = 7003,
howpublished = {RFC 7003},
publisher = {RFC Editor},
doi = {10.17487/RFC7003},
url = {https://www.rfc-editor.org/info/rfc7003},
author = {A. Clark and R. Huang and Q. Wu},
title = {{RTP Control Protocol (RTCP) Extended Report (XR) Block for Burst/Gap Discard Metric Reporting}},
pagetotal = 14,
year = 2013,
month = sep,
} RFC text
Contents
Internet Engineering Task Force (IETF) A. Clark
Request for Comments: 7003 Telchemy
Category: Standards Track R. Huang
ISSN: 2070-1721 Q. Wu, Ed.
Huawei
September 2013
RTP Control Protocol (RTCP) Extended Report (XR) Block
for Burst/Gap Discard Metric Reporting
Abstract
This document defines an RTP Control Protocol (RTCP) Extended Report
(XR) block that allows the reporting of burst and gap discard metrics
for use in a range of RTP applications.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7003.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
1.1. Burst/Gap Discard Metrics Block ............................2
1.2. RTCP and RTCP Extended Reports .............................3
1.3. Performance Metrics Framework ..............................3
1.4. Applicability ..............................................3
2. Terminology .....................................................3
3. Burst/Gap Discard Metrics Block .................................4
3.1. Report Block Structure .....................................5
3.2. Definition of Fields in Burst/Gap Discard Metrics Block ....5
3.3. Derived Metrics Based on Reported Metrics ..................7
4. Considerations for Voice-over-IP Applications ...................7
5. SDP Signaling ...................................................8
5.1. SDP rtcp-xr Attribute Extension ............................8
5.2. Offer/Answer Usage .........................................8
6. IANA Considerations .............................................8
6.1. New RTCP XR Block Type Value ...............................8
6.2. New RTCP XR SDP Parameter ..................................9
6.3. Contact Information for Registrations ......................9
7. Security Considerations .........................................9
8. Contributors ....................................................9
9. Acknowledgments .................................................9
10. References ....................................................10
10.1. Normative References .....................................10
10.2. Informative References ...................................10
Appendix A. Metrics Represented Using the Template from RFC 6390..12
1. Introduction
1.1. Burst/Gap Discard Metrics Block
This document 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
jitter buffer at the receiver, using packet discard logic according
to the jitter buffer algorithms. The discards during discard bursts
are reported, together with the number of bursts. This block is
intended to be used in conjunction with [RFC7002], which provides the
total packets discarded and on which this block therefore depends.
However, the metric in [RFC7002] 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, they also
may be used in interval reports (see the Interval Metric flag in
Section 3.2). 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 end-system
metrics defined in [RFC6792].
The definitions of "burst", "gap", "loss", and "discard" are
consistent with the definitions in [RFC3611]. To accommodate 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 shall use an equivalent method to that defined in
Section 4.7.2 of [RFC3611]. Note that reporting the specific jitter
buffer algorithms and/or packet discard logic is out of the scope of
this document.
1.2. RTCP and RTCP Extended 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 document defines a new Extended Report block for
use with [RFC3550] and [RFC3611].
1.3. Performance Metrics Framework
The Performance Metrics Framework [RFC6390] provides guidance on the
definition and specification of performance metrics. The RTP
Monitoring Framework [RFC6792] provides guidelines for reporting
block format using RTCP XR. The metrics block described in this
document is in accordance with the guidelines in [RFC6390] and
[RFC6792].
1.4. Applicability
These metrics are applicable to a range of RTP applications that
contain de-jitter buffers [RFC5481] at the receiving end to smooth
variation in packet-arrival time and don't use stream repair means,
e.g., Forward Error Correction (FEC) [RFC5109] and/or retransmission
[RFC4588].
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
In addition, the following terms are defined:
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 to be played out, too
late to be played out, or thrown away before playout due to packet
duplication or redundancy shall be regarded as discarded. A
packet shall not be regarded as discarded if it arrives within
this time window but is dropped during decoding by some higher-
layer decoder, e.g., due to a decoding error. A packet shall be
classified as one of received (or OK), discarded, or lost. The
metric "cumulative number of packets lost" defined in [RFC3550]
reports a count of packets lost from the media stream (single
synchronization source (SSRC) within a single RTP session).
Similarly, the metric "number of packets discarded" defined in
[RFC7002] reports a count of packets discarded from the media
stream (single SSRC within a single RTP session) arriving at the
receiver. Another metric, defined in [RFC5725], is available to
report on packets that are not recovered by any repair techniques
that may be in use. Note that the term "discard" defined here
builds on the "discard" definition in [RFC3611] but extends the
concept to take into account packet duplication and reports
different types of discard counts [RFC7002].
Bursts and Gaps
The terms "burst" and "gap" are used in a manner consistent with
that of RTCP XR [RFC3611]. RTCP XR views an RTP stream as being
divided into bursts, which are periods during which the discard
rate is high enough to cause noticeable quality degradation
(generally over 5 percent discard rate), and gaps, which are
periods during which discarded packets are infrequent and hence
quality is generally acceptable.
3. Burst/Gap Discard Metrics Block
Metrics in this block report on burst/gap discard in the stream
arriving at the RTP system. Measurements of these metrics are made
at the receiving end of the RTP stream. Instances of this metrics
block use the synchronization source (SSRC) to refer to the separate
auxiliary Measurement Information Block [RFC6776], which describes
measurement periods in use (see [RFC6776], Section 4.2).
This metrics block relies on the measurement period in the
Measurement Information Block indicating the span of the report.
Senders MUST send this block in the same compound RTCP packet as the
Measurement Information Block. Receivers MUST verify that the
measurement period is received in the same compound RTCP packet as
this metrics block. If not, this metrics block MUST be discarded.
3.1. Report Block Structure
The structure of the Burst/Gap Discard Metrics Block is as follows.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=20 | I | resv | Block Length = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of Source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold | Packets Discarded in Bursts |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Total Packets Expected in Bursts | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Report Block Structure
3.2. Definition of Fields in Burst/Gap Discard Metrics Block
Block Type (BT): 8 bits
A Burst/Gap Discard Metrics Block is identified by the constant
20.
Interval Metric flag (I): 2 bits
This field is used to indicate whether the burst/gap discard
metrics are Sampled, Interval, or Cumulative metrics [RFC6792]:
I=10: Interval Duration - the reported value applies to the
most recent measurement interval duration between successive
metrics reports.
I=11: Cumulative Duration - the reported value applies to the
accumulation period characteristic of cumulative measurements.
I=01: Sampled Value - the reported value is a sampled
instantaneous value.
In this document, burst/gap discard metrics can only be measured
over definite intervals and cannot be sampled. Also, the value
I=00 is reserved for future use. Senders MUST NOT use the values
I=00 or I=01. If a block is received with I=00 or I=01, the
receiver MUST discard the block.
Reserved (resv): 6 bits
These bits are reserved. They MUST be set to zero by senders and
ignored by receivers (see [RFC6709], Section 4.2).
Block Length: 16 bits
The length of this report block in 32-bit words, minus one. For
the Burst/Gap Discard Metrics Block, the block length is equal to
3. The block MUST be discarded if the block length is set to a
different value.
SSRC of Source: 32 bits
As defined in Section 4.1 of [RFC3611].
Threshold: 8 bits
The Threshold is equivalent to Gmin in [RFC3611], i.e., the number
of successive packets that must not be discarded prior to and
following a discard packet in order for this discarded packet to
be regarded as part of a gap. Note that the Threshold is set in
accordance with the Gmin calculation defined in Section 4.7.2 of
[RFC3611].
Packets Discarded in Bursts: 24 bits
The total number of packets discarded during discard bursts.
The measured value is unsigned value. If the measured value
exceeds 0xFFFFFD, the value 0xFFFFFE MUST be reported to indicate
an over-range measurement. If the measurement is unavailable, the
value 0xFFFFFF MUST 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).
The measured value is unsigned value. If the measured value
exceeds 0xFFFFFD, the value 0xFFFFFE MUST be reported to indicate
an over-range measurement. If the measurement is unavailable, the
value 0xFFFFFF MUST be reported.
Reserved: 8 bits
These bits are reserved. They MUST be set to zero by senders and
ignored by receivers (See [RFC6709], Section 4.2).
3.3. Derived Metrics Based on Reported Metrics
The metrics described here are intended to be used in conjunction
with information from the Measurement Information Block [RFC6776] and
also with the metric "number of packets discarded" provided in the
RTCP XR Discard Count Metrics Block [RFC7002].
These metrics provide the following information relevant to
statistical parameters, including:
o The fraction of packets discarded during bursts (Burst Discard
Rate in [RFC7004]), which can be calculated using the metric
"Packets Discarded in Bursts" and the metric "Total Packets
Expected in Bursts" provided in the Burst/Gap Discard Metrics
Block.
o The fraction of packets discarded during gaps (Gap Discard Rate in
[RFC7004]), which can be calculated using the metric "Packets
Discarded in Bursts" and the metric "Total Packets Expected in
Bursts" provided in the Burst/Gap Discard Metrics Block.
The details on calculation these parameters in the metrics are
described in [RFC7004].
4. Considerations for Voice-over-IP Applications
This metrics block is applicable to a broad range of RTP
applications. Where the metric is used with a Voice-over-IP (VoIP)
application and the stream repair means is not available, 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 packets had been sent, i.e., a period of
silence in excess of Gmin packets will 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 pulse code modulation (PCM)
severely errored second.
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.
5.1. SDP rtcp-xr Attribute Extension
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
ABNF [RFC5234] syntax is as follows.
xr-format =/ xr-bgd-block
xr-bgd-block = "burst-gap-discard"
5.2. Offer/Answer Usage
When SDP is used in Offer/Answer context, the SDP Offer/Answer usage
defined in [RFC3611] for unilateral "rtcp-xr" attribute parameters
applies. For detailed usage in Offer/Answer for unilateral
parameters, refer to Section 5.2 of [RFC3611].
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 20 in the IANA "RTP
Control Protocol Extended Reports (RTCP XR) Block Type Registry" to
the "Burst/Gap Discard Metrics Block".
6.2. New RTCP XR SDP Parameter
This document also registers a new parameter "burst-gap-discard" in
the "RTP Control Protocol Extended Reports (RTCP XR) Session
Description Protocol (SDP) Parameters Registry".
6.3. Contact Information for Registrations
The contact information for the registrations is:
Qin Wu (sunseawq@huawei.com)
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
7. Security Considerations
It is believed that this RTCP XR 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.
8. Contributors
Geoff Hunt wrote the initial draft of this document.
9. Acknowledgments
The authors gratefully acknowledge reviews and feedback provided by
Bruce Adams, Philip Arden, Amit Arora, Claire Bi, Bob Biskner, Benoit
Claise, Kevin Connor, Claus Dahm, Randy Ethier, Roni Even, Stephen
Farrell, Jim Frauenthal, Albert Higashi, Tom Hock, Shane Holthaus,
Paul Jones, Rajesh Kumar, Paul Kyzivat, Keith Lantz, Mohamed Mostafa,
Amy Pendleton, Colin Perkins, Mike Ramalho, Ravi Raviraj, Dan
Romascanu, Albrecht Schwarz, Tom Taylor, and Hideaki Yamada.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control
Protocol Extended Reports (RTCP XR)", RFC 3611, November
2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5725] Begen, A., Hsu, D., and M. Lague, "Post-Repair Loss RLE
Report Block Type for RTP Control Protocol (RTCP) Extended
Reports (XRs)", RFC 5725, February 2010.
[RFC6776] Clark, A. and Q. Wu, "Measurement Identity and Information
Reporting Using a Source Description (SDES) Item and an
RTCP Extended Report (XR) Block", RFC 6776, October 2012.
[RFC7002] Clark, A., Zorn, G., and Q. Wu, "RTP Control Protocol
(RTCP) Extended Report (XR) for Discard Count Metric
Reporting", RFC 7002, September 2013.
[RFC7004] Zorn, G., Schott, R., Wu, Q., Ed., and R. Huang, "RTP
Control Protocol (RTCP) Extended Report (XR) Blocks for
Summary Statistics Metrics Reporting", RFC 7004, September
2013.
10.2. Informative References
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[RFC5109] Li, A., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, December 2007.
[RFC5481] Morton, A. and B. Claise, "Packet Delay Variation
Applicability Statement", RFC 5481, March 2009.
[RFC6390] Clark, A. and B. Claise, "Guidelines for Considering New
Performance Metric Development", BCP 170, RFC 6390,
October 2011.
[RFC6709] Carpenter, B., Aboba, B., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709,
September 2012.
[RFC6792] Wu, Q., Hunt, G., and P. Arden, "Guidelines for Use of the
RTP Monitoring Framework", RFC 6792, November 2012.
Appendix A. Metrics Represented Using the Template from RFC 6390
a. Burst/Gap Discard Threshold in RTP Metric
* Metric Name: Burst/gap discard threshold in RTP
* Metric Description: The Threshold is equivalent to Gmin in
[RFC3611], i.e., the number of successive packets that must
not be discarded prior to and following a discard packet in
order for this discarded packet to be regarded as part of a
gap.
* Method of Measurement or Calculation: See Section 3.2,
Threshold definition.
* Units of Measurement: See Section 3.2, Threshold definition.
* Measurement Point(s) with Potential Measurement Domain: See
Section 3, 1st paragraph.
* Measurement Timing: See Section 3, 2nd paragraph for
measurement timing and Section 3.2 for Interval Metric flag.
* Use and Applications: See Section 1.4.
* Reporting Model: See RFC 3611.
b. Packets Discarded in Bursts Metric
* Metric Name: RTP packets discarded in bursts
* Metric Description: The total number of RTP packets discarded
during discard bursts.
* Method of Measurement or Calculation: See Section 3.2, Packets
Discarded in Bursts definition.
* Units of Measurement: See Section 3.2, Packets Discarded in
Bursts definition.
* Measurement Point(s) with Potential Measurement Domain: See
Section 3, 1st paragraph.
* Measurement Timing: See Section 3, 2nd paragraph for
measurement timing and Section 3.2 for Interval Metric flag.
* Use and Applications: See Section 1.4.
* Reporting Model: See RFC 3611.
c. Total Packets Expected in Discard Bursts Metric
* Metric Name: Total RTP packets expected in discard bursts
* Metric Description: The total number of packets expected
during discard bursts (that is, the sum of received packets
and lost packets).
* Method of Measurement or Calculation: See Section 3.2, Total
Packets Expected in Bursts definition.
* Units of Measurement: See Section 3.2, Total Packets Expected
in Bursts definition.
* Measurement Point(s) with Potential Measurement Domain: See
Section 3, 1st paragraph.
* Measurement Timing: See Section 3, 2nd paragraph for
measurement timing and Section 3.2 for Interval Metric flag.
* Use and Applications: See Section 1.4.
* Reporting Model: See RFC 3611.
Authors' Addresses
Alan Clark
Telchemy Incorporated
2905 Premiere Parkway, Suite 280
Duluth, GA 30097
USA
EMail: alan.d.clark@telchemy.com
Rachel Huang
Huawei Technologies Co., Ltd.
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
EMail: Rachel@huawei.com
Qin Wu (editor)
Huawei
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
EMail: sunseawq@huawei.com
ISO Plain Language
On this page
§1What the Block Adds
The RFC defines a new block type to augment those defined in RFC 3611, for use in a range of RTP applications. The new block type supports the reporting of the proportion of packets discarded by the jitter buffer at the receiver, using the packet discard logic of the jitter buffer algorithms. The discards during discard bursts are reported, together with the number of bursts. The block is intended to be used together with RFC 7002, which provides the total number of packets discarded and on which this block therefore depends, although the metric in RFC 7002 can be used on its own.
The block gives information on transient IP problems. Burst and gap metrics are typically used in cumulative reports, but they can also be used in interval reports. The burstiness of packet discard affects user experience, may influence any sender strategies used to mitigate the problem, and also has diagnostic value. The metric belongs to the class of transport-related end-system metrics defined in RFC 6792.
The definitions of burst, gap, loss and discard are consistent with those in RFC 3611. To accommodate the range of jitter buffer algorithms and packet discard logic that implementors may use, the method used to tell bursts from gaps shall use an equivalent method to the one defined in Section 4.7.2 of RFC 3611. Reporting which jitter buffer algorithm or discard logic was used is out of the scope of the document.
The metrics apply to a range of RTP applications that contain a de-jitter buffer (RFC 5481) at the receiving end to smooth the variation in packet arrival time, and that do not use stream repair means such as Forward Error Correction (RFC 5109) or retransmission (RFC 4588).
This block counts the packets the receiver threw away while things were going badly, and says how many packets it expected to receive during those bad periods. The two numbers together describe how bad the bad periods were.
It is meant to be used with the discard count block of RFC 7002, which reports the total number of packets thrown away. The two blocks answer different questions: that one asks how many were dropped, this one asks when.
§2Bursts and Gaps
The terms burst and gap are used in the same manner as in RTCP XR. RTCP XR views an RTP stream as being divided into bursts, which are periods during which the discard rate is high enough to cause noticeable quality degradation, generally over 5 percent, and gaps, which are periods during which discarded packets are infrequent and hence quality is generally acceptable.
The RFC also restates the three fates of a packet. A packet is lost if it fails to arrive within an implementation-specific time window. A packet that arrives within that window but is too early to be played out, too late to be played out, or thrown away before playout because of packet duplication or redundancy is discarded. A packet that arrives within the window but is dropped during decoding by a higher layer decoder, for example because of a decoding error, is not discarded. Every packet is classified as received, which the RFC also calls OK, discarded, or lost.
The discard count metric defined in RFC 7002 reports the packets discarded from the media stream, a single synchronization source within a single RTP session, arriving at the receiver. The term discard here builds on the definition in RFC 3611 but extends it to take packet duplication into account, and reports different types of discard counts.
The document divides the stream into two kinds of period. In a burst, the receiver is dropping so many packets that people notice, generally more than one in twenty. In a gap, drops are rare and the call still sounds acceptable.
The threshold between the two is set by the receiver, and it counts the number of good packets that have to go by, before and after a dropped packet, for that packet to count as part of a quiet period rather than a burst.
| Period | What happens in it |
|---|---|
| Burst | The discard rate is high enough to cause noticeable quality degradation, generally over 5 percent |
| Gap | Discarded packets are infrequent and hence quality is generally acceptable |
§3.1The Block Layout
The metrics in the block report on burst and gap discard in the stream arriving at the RTP system, and the measurements are made at the receiving end. Instances of the block use the synchronization source to refer to the separate auxiliary Measurement Information Block of RFC 6776, which describes the measurement periods in use.
The block relies on the measurement period in that block to indicate the span of the report. Senders MUST send this block in the same compound RTCP packet as the Measurement Information Block. Receivers MUST verify that the measurement period was received in the same compound RTCP packet as this block, and if it was not, this block MUST be discarded.
The block has a fixed layout of four 32-bit words. The first word holds the block type, the interval metric flag and the reserved bits, followed by the block length. The second word holds the SSRC of the source. The third word holds the threshold and the number of packets discarded in bursts. The fourth holds the total number of packets expected during bursts and a final reserved field.
The count has to be sent with the block that says which time period it covers, in the same packet. If that block is missing, the receiver is told to throw the count away, because a count without a period cannot be interpreted.
The block itself is four words long: a header word, a word naming the stream, and two words carrying the threshold and the two counts.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
BT | BT | 1 byte | 20 | Block type, the constant 20. |
INTERVAL-FLAG | I | 2 bits | Interval metric flag: interval or cumulative only. | |
RESERVED | resv | 6 bits | Reserved bits; senders set them to zero and receivers ignore them. | |
BLOCK-LENGTH | Block Length | 2 bytes | 3 | Length of the block in 32-bit words minus one. |
SSRC | SSRC of Source | 4 bytes | The synchronization source the counts belong to. | |
THRESHOLD | Threshold | 1 byte | The number of successive packets that must not be discarded for a discard to count as part of a gap. | |
DISCARDED-IN-BURSTS | Packets Discarded in Bursts | 3 bytes | The total number of packets discarded during the discard bursts. | |
EXPECTED-IN-BURSTS | Total Packets Expected in Bursts | 3 bytes | The total number of packets expected during the discard bursts, received plus lost. | |
RESERVED-2 | Reserved | 1 byte | Reserved bits; senders set them to zero and receivers ignore them. |
§3.2The Fields and Their Values
The block type is 8 bits and identifies the block with the constant 20. The interval metric flag is 2 bits and says whether the metrics are sampled, interval or cumulative: I=10 for the most recent measurement interval between successive metrics reports, I=11 for the accumulation period characteristic of cumulative measurements, and I=01 for a sampled instantaneous value. The burst and gap discard metrics can only be measured over definite intervals and cannot be sampled, and the value I=00 is reserved for future use. Senders MUST NOT use I=00 or I=01, and a receiver that gets a block with either value MUST discard it.
The reserved field after the flag is 6 bits. Senders MUST set them to zero and receivers ignore them. The block length is 16 bits and gives the length of the report block in 32-bit words minus one. For this block it is 3, and the block MUST be discarded if the length is set to any other value. The SSRC of source is 32 bits, as defined in Section 4.1 of RFC 3611.
The threshold is 8 bits and is equivalent to Gmin in RFC 3611: the number of successive packets that must not be discarded before and after a discarded packet for that packet to be regarded as part of a gap. It is set in accordance with the Gmin calculation of Section 4.7.2 of that document.
Packets discarded in bursts is 24 bits and is the total number of packets discarded during the discard bursts. The measured value is unsigned. If it exceeds 0xFFFFFD, the value 0xFFFFFE MUST be reported to indicate an over-range measurement, and if the measurement is unavailable the value 0xFFFFFF MUST be reported. Total packets expected in bursts is 24 bits and is the total number of packets expected during the discard bursts, that is the sum of received packets and lost packets, with the same over-range and unavailable values. A final reserved field of 8 bits is set to zero by senders and ignored by receivers.
The header says how the counts were measured and how long the block is, and a receiver that sees the wrong values for either is told to ignore the block. Only two of the three possible measurement styles are allowed, because these counts cannot be taken at a single instant.
The threshold is the number of good packets that have to go by, before and after a dropped packet, for that packet to count as part of a gap rather than a burst. The two counts are then how many packets were dropped during the bursts and how many were expected during them.
§3.3What the Counts Let You Work Out
The metrics are intended to be used with the information from the Measurement Information Block of RFC 6776 and with the number of packets discarded provided by the Discard Count Metrics Block of RFC 7002.
Together they provide the information needed for statistical parameters. The first is the fraction of packets discarded during bursts, which the RFC names the Burst Discard Rate in RFC 7004, calculated from the packets discarded in bursts and the total packets expected in bursts. The second is the fraction of packets discarded during gaps, which the RFC names the Gap Discard Rate in RFC 7004, which the RFC also calculates from those same two metrics. The details of how these parameters are calculated are given in RFC 7004.
The two counts are not the end of the story. Combined with the total discard count from RFC 7002, they let the reader work out two rates: the proportion of packets lost during the bad periods, and the proportion lost during the quiet ones.
A rate that is high during bursts and low during gaps says the trouble is concentrated. A rate that is the same in both says the trouble is steady. The exact arithmetic is set out in RFC 7004.
- Burst discard rateThe fraction of packets discarded during bursts, calculated from the packets discarded in bursts and the packets expected in bursts.
- Gap discard rateThe fraction of packets discarded during gaps, calculated by RFC 7004 from the same two metrics.
- Total discardedThe number of packets discarded in the period, taken from the Discard Count Metrics Block of RFC 7002.
- The period itselfThe span of the report, taken from the Measurement Information Block of RFC 6776, which has to arrive in the same packet.
§4Voice-over-IP Considerations
The metrics block is applicable to a broad range of RTP applications. Where it is used with a Voice-over-IP application and no stream repair means is available, the RFC sets out further considerations.
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, and gaps, during which discarded packets are infrequent and the call quality is generally acceptable.
If voice activity detection is used, the burst and gap duration shall be determined as if silence packets had been sent, so a period of silence in excess of Gmin packets will terminate a burst condition. The recommended value for the threshold Gmin in RFC 3611 results in a burst being a period of time during which the call quality is degraded to a similar extent to a typical pulse code modulation severely errored second.
For a voice call, the same two kinds of period apply, with the same threshold of about one dropped packet in twenty.
One adjustment is needed for calls that go quiet when nobody is speaking. The length of a burst or a gap is measured as if the silence packets had been sent, so that a quiet spell ends the burst rather than extending it.
§5Signalling the Block with SDP
RFC 3611 defines the use of SDP (RFC 4566) for signalling the use of XR blocks, although XR blocks MAY be used without prior signalling.
This RFC extends the rtcp-xr SDP attribute of RFC 3611 with an additional value of xr-format to signal the use of the report block defined here. The ABNF is xr-format =/ xr-bgd-block and xr-bgd-block = "burst-gap-discard". Where SDP is used in an offer and answer context, the unilateral rtcp-xr attribute parameter usage defined in Section 5.2 of RFC 3611 applies.
IANA has assigned the block type value 20 in the RTCP XR Block Type Registry to the Burst/Gap Discard Metrics Block, and the new parameter burst-gap-discard in the RTCP XR SDP Parameters Registry.
The block can be used without any negotiation. If the two ends want to agree on it first, it is announced in the session description, in the same attribute used for the other extended report blocks, with the value burst-gap-discard.
§7Security Considerations
It is believed that this RTCP XR block introduces no new security considerations beyond those described in RFC 3611. The block does not provide per-packet statistics, so the risk to confidentiality documented in Section 7, paragraph 3 of RFC 3611 does not apply.
The block adds no new security risk, because it reports summary counts rather than anything about individual packets.