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/RFC 7003.
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.
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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
[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 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 [RFC 7002], which provides the
total packets discarded and on which this block therefore depends.
However, the metric in [RFC 7002] 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.
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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 the definitions in [RFC 3611]. 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 [RFC 3611]. 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 [RFC 3550]. [RFC 3611]
defined an extensible structure for reporting using an RTCP Extended
Report (XR). This document defines a new Extended Report block for
use with [RFC 3550] and [RFC 3611].
1.3. Performance Metrics Framework
The Performance Metrics Framework [RFC 6390] provides guidance on the
definition and specification of performance metrics. The RTP
Monitoring Framework [RFC 6792] provides guidelines for reporting
block format using RTCP XR. The metrics block described in this
document is in accordance with the guidelines in [RFC 6390] and
[RFC 6792].
1.4. Applicability
These metrics are applicable to a range of RTP applications that
contain de-jitter buffers [RFC 5481] at the receiving end to smooth
variation in packet-arrival time and don't use stream repair means,
e.g., Forward Error Correction (FEC) [RFC 5109] and/or retransmission
[RFC 4588].
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 [RFC 2119].
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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 [RFC 3550]
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
[RFC 7002] 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 [RFC 5725], 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 [RFC 3611] but extends the
concept to take into account packet duplication and reports
different types of discard counts [RFC 7002].
Bursts and Gaps
The terms "burst" and "gap" are used in a manner consistent with
that of RTCP XR [RFC 3611]. 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 [RFC 6776], which describes
measurement periods in use (see [RFC 6776], 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
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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 [RFC 6792]:
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.
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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 [RFC 6709], 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 [RFC 3611].
Threshold: 8 bits
The Threshold is equivalent to Gmin in [RFC 3611], 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
[RFC 3611].
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).
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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 [RFC 6709], 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 [RFC 6776] and
also with the metric "number of packets discarded" provided in the
RTCP XR Discard Count Metrics Block [RFC 7002].
These metrics provide the following information relevant to
statistical parameters, including:
o The fraction of packets discarded during bursts (Burst Discard
Rate in [RFC 7004]), 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
[RFC 7004]), 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 [RFC 7004].
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.
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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 [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 (PCM)
severely errored second.
5. SDP Signaling
[RFC 3611] defines the use of SDP (Session Description Protocol)
[RFC 4566] 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 [RFC 4566] attribute "rtcp-xr" defined
in [RFC 3611] by providing an additional value of "xr-format" to
signal the use of the report block defined in this document. The
ABNF [RFC 5234] 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 [RFC 3611] for unilateral "rtcp-xr" attribute parameters
applies. For detailed usage in Offer/Answer for unilateral
parameters, refer to Section 5.2 of [RFC 3611].
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
[RFC 3611].
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".
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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 [RFC 3611]. This block does
not provide per-packet statistics, so the risk to confidentiality
documented in Section 7, paragraph 3 of [RFC 3611] 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.
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10. References
10.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC 3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control
Protocol Extended Reports (RTCP XR)", RFC 3611, November
2003.
[RFC 4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC 5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC 5725] 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.
[RFC 6776] 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.
[RFC 7002] Clark, A., Zorn, G., and Q. Wu, "RTP Control Protocol
(RTCP) Extended Report (XR) for Discard Count Metric
Reporting", RFC 7002, September 2013.
[RFC 7004] 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
[RFC 4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[RFC 5109] Li, A., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, December 2007.
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[RFC 5481] Morton, A. and B. Claise, "Packet Delay Variation
Applicability Statement", RFC 5481, March 2009.
[RFC 6390] Clark, A. and B. Claise, "Guidelines for Considering New
Performance Metric Development", BCP 170, RFC 6390,
October 2011.
[RFC 6709] Carpenter, B., Aboba, B., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709,
September 2012.
[RFC 6792] Wu, Q., Hunt, G., and P. Arden, "Guidelines for Use of the
RTP Monitoring Framework", RFC 6792, November 2012.
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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
[RFC 3611], 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.
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* 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.
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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
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