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IETF RFC 7002
Last modified on Thursday, September 5th, 2013
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Internet Engineering Task Force (IETF) A. Clark
Request for Comments: 7002 Telchemy
Category: Standards Track G. Zorn
ISSN: 2070-1721 Network Zen
Q. Wu
Huawei
September 2013
RTP Control Protocol (RTCP) Extended Report (XR) Block
for Discard Count Metric Reporting
 Abstract
This document defines an RTP Control Protocol (RTCP) Extended Report
(XR) block that allows the reporting of a simple discard count metric
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 7002.
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.
Clark, et al. Standards Track PAGE 1 
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Discard Count Report Block . . . . . . . . . . . . . . . 2
1.2. RTCP and RTCP Extended Reports . . . . . . . . . . . . . 3
1.3. Performance Metrics Framework . . . . . . . . . . . . . . 3
1.4. Applicability . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Discard Count Metrics Block . . . . . . . . . . . . . . . . . 4
3.1. Report Block Structure . . . . . . . . . . . . . . . . . 5
3.2. Definition of Fields in the Discard Count Metrics Block . 5
4. SDP Signaling . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. SDP rtcp-xr Attribute Extension . . . . . . . . . . . . . 7
4.2. Offer/Answer Usage . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5.1. New RTCP XR Block Type Value . . . . . . . . . . . . . . 8
5.2. New RTCP XR SDP Parameter . . . . . . . . . . . . . . . . 8
5.3. Contact Information for Registrations . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Metrics Represented Using the Template from RFC 6390 11
1. Introduction
1.1. Discard Count Report 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 number of packets that are received
correctly but are never played out, typically because they arrive too
late (buffer underflow) or too early (buffer overflow) to be played
out. The metric is applicable both to systems that use packet loss
repair techniques (such as forward error correction [RFC 5109] or
retransmission [RFC 4588]) and to those that do not.
This metric is useful for identifying the existence, and
characterizing the severity, of packet transport problems that may
affect users' perceptions of a service delivered over RTP.
This block may be used in conjunction with [RFC 7003], which provides
additional information on the pattern of discarded packets. However,
the metric in [RFC 7003] may be used independently of the metrics in
this block.
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When a Discard Count Metrics Block is sent together with a Burst/Gap
Discard Metrics Block (defined in [RFC 7003]) to the media sender or
RTP-based network management system, the information carried in the
Discard Count Metrics Block and the Burst/Gap Discard Metrics Block
allows systems receiving the blocks to calculate burst/gap summary
statistics (e.g., the gap discard rate).
The metric belongs to the class of transport-related end-system
metrics defined in [RFC 6792].
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
"Guidelines for Considering New Performance Metric Development"
[RFC 6390] provides guidance on the definition and specification of
performance metrics. "Guidelines for Use of the RTP Monitoring
Framework" [RFC 6792] provides guidance 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
This metric is believed to be applicable to a large class of RTP
applications that use a de-jitter buffer [RFC 5481].
Discards due to late or early arriving packets affect user
experience. The reporting of discards alerts senders and other
receivers to the need to adjust their transmission or reception
strategies. The reports allow network managers to diagnose these
user experience problems.
The ability to detect duplicate packets can be used by managers to
detect network layer or sender behavior, which may indicate network
or device issues. Based on the reports, these issues may be
addressed prior to any impact on user experience.
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 either too early or too late to be played
out or is 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 the
following: received (or OK), discarded, or lost. The discard
count metric counts only discarded packets. 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" 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.
3. Discard Count Metrics Block
Metrics in this block report on the number of packets discarded in
the stream arriving at the RTP end system. The measurement of these
metrics is made at the receiving end of the RTP stream. Instances of
this metrics block use the 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 interval in the Measurement Information
Block indicating the span of the report and MUST be sent in the same
compound RTCP packet as the Measurement Information Block. If the
measurement interval is not received in the same compound RTCP packet
as this metrics block, this metrics block MUST be discarded.
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3.1. Report Block Structure
The structure of the Discard Count 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=24 | I |DT | resv | Block Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of Source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Discard Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Report Block Structure
3.2. Definition of Fields in the Discard Count Metrics Block
Block Type (BT): 8 bits
A Discard Count Metrics Block is identified by the constant 24.
Interval Metric flag (I): 2 bits
This field indicates whether the reported metric is an Interval,
Cumulative, or Sampled metric [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.
In this document, the discard count metric can only be measured
over definite intervals and cannot be sampled. Accordingly, the
value I=01, indicating a sampled value, MUST NOT be sent, and MUST
be discarded when received. In addition, the value I=00 is
reserved and also MUST NOT be sent, and MUST be discarded when
received.
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Discard Type (DT): 2 bits
This field is used to identify the discard type used in this
report block. The discard type is defined as follows:
00: Report packet discarded or being thrown away before playout
due to packet duplication.
01: Report packet discarded due to too early to be played out.
10: Report packet discarded due to too late to be played out.
The value DT=11 is reserved for future definition and MUST NOT be
sent, and MUST be discarded when received.
An endpoint MAY report any combination of discard types in each
reporting interval by including several Discard Count Metrics
Blocks in a single RTCP XR packet.
Some systems send duplicate RTP packets for robustness or error
resilience. This is NOT RECOMMENDED since it breaks RTCP packet
statistics. If duplication is desired for error resilience, the
mechanism described in [RTPDUP] can be used, since this will not
cause breakage of RTP streams or RTCP statistics.
Reserved (resv): 4 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, in
accordance with the definition in [RFC 3611]. This field MUST be
set to 2 to match the fixed length of the report block. 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].
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Discard Count
Number of packets discarded over the period (Interval or
Cumulative) covered by this report.
The measured value is an unsigned value. If the measured value
exceeds 0xFFFFFFFD, the value 0xFFFFFFFE MUST be reported to
indicate an over-range measurement. If the measurement is
unavailable, the value 0xFFFFFFFF MUST be reported.
Note that the number of packets expected in the period associated
with this metric (whether Interval or Cumulative) is available
from the difference between a pair of extended sequence numbers in
the Measurement Information Block [RFC 6776], so it need not be
repeated in this block.
4. SDP Signaling
[RFC 3611] defines the use of the Session Description Protocol (SDP)
[RFC 4566] for signaling the use of XR blocks. However, XR blocks MAY
be used without prior signaling (see Section 5 of RFC 3611).
4.1. 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-pdc-block
xr-pdc-block = "pkt-discard-count"
4.2. 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 of Offer/Answer for unilateral
parameters, refer to Section 5.2 of [RFC 3611].
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
[RFC 3611].
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5.1. New RTCP XR Block Type Value
This document assigns the block type value 24 in the IANA "RTP
Control Protocol Extended Reports (RTCP XR) Block Type Registry" to
the "Discard Count Metrics Block".
5.2. New RTCP XR SDP Parameter
This document also registers a new parameter "pkt-discard-count" in
the "RTP Control Protocol Extended Reports (RTCP XR) Session
Description Protocol (SDP) Parameters Registry".
5.3. Contact Information for Registrations
The following contact information is provided for all registrations
in this document:
Qin Wu (sunseawq@huawei.com)
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
6. Security Considerations
In some situations, returning very detailed error information (e.g.,
over-range measurement or measurement unavailable) using this report
block can provide an attacker with insight into the security
processing. Where this is a concern, the implementation should apply
encryption and authentication to this report block. For example,
this can be achieved by using the Audio-Visual Profile with Feedback
(AVPF) profile together with the Secure RTP profile, as defined in
[RFC 3711]; an appropriate combination of those two profiles ("SAVPF")
is specified in [RFC 5124].
Besides this, 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.
7. Contributors
Geoff Hunt wrote the initial draft of this document.
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8. Acknowledgments
The authors gratefully acknowledge reviews and feedback provided by
Bruce Adams, Philip Arden, Amit Arora, Claire Bi, Bob Biskner,
Gonzalo Camarillo, Benoit Claise, Kevin Connor, Claus Dahm, Spencer
Dawkins, Randy Ethier, Roni Even, Stephen Farrel, Jim Frauenthal,
Kevin Gross, Albert Higashi, Tom Hock, Shane Holthaus, Paul Jones,
Rajesh Kumar, Keith Lantz, Jonathan Lennox, Mohamed Mostafa, Amy
Pendleton, Colin Perkins, Mike Ramalho, Ravi Raviraj, Dan Romascanu,
Albrecht Schwarz, Varun Singh, Tom Taylor, Dan Wing, and Hideaki
Yamada.
9. References
9.1. 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 3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC 4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC 5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, February 2008.
[RFC 5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC 6709] Carpenter, B., Aboba, B., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709,
September 2012.
[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.
Clark, et al. Standards Track PAGE 9
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9.2. 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.
[RFC 5481] Morton, A. and B. Claise, "Packet Delay Variation
Applicability Statement", RFC 5481, March 2009.
[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 6390] Clark, A. and B. Claise, "Guidelines for Considering New
Performance Metric Development", BCP 170, RFC 6390,
October 2011.
[RFC 6792] Wu, Q., Hunt, G., and P. Arden, "Guidelines for Use of the
RTP Monitoring Framework", RFC 6792, November 2012.
[RFC 7003] Clark, A., Huang, R., and Q. Wu, Ed., "RTP Control
Protocol(RTCP) Extended Report (XR) Block for Burst/Gap
Discard Metric Reporting", RFC 7003, September 2013.
[RTPDUP] Begen, A. and C. Perkins, "Duplicating RTP Streams", Work
in Progress, March 2013.
Clark, et al. Standards Track PAGE 10
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Appendix A. Metrics Represented Using the Template from RFC 6390
a. Number of Packets Discarded Metric
* Metric Name: Number of RTP packets discarded.
* Metric Description: Number of RTP packets discarded over the
period covered by this report.
* Method of Measurement or Calculation: See Section 3.2, Discard
Count definition.
* Units of Measurement: See Section 3.2, Discard Count
definition.
* Measurement Point(s) with Potential Measurement Domain: See
Section 3, 1st paragraph.
* Measurement Timing: See Section 3, 1st paragraph for
measurement timing and Section 3.2 for Interval Metric flag.
* Use and Applications: See Section 1.4.
* Reporting Model: See RFC 3611.
Clark, et al. Standards Track PAGE 11
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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
Glen Zorn
Network Zen
227/358 Thanon Sanphawut
Bang Na, Bangkok 10260
Thailand
Phone: +66 (0) 8-1000-4155
EMail: glenzorn@gmail.com
Qin Wu
Huawei
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
EMail: sunseawq@huawei.com
Clark, et al. Standards Track PAGE 12
RFC TOTAL SIZE: 22523 bytes
PUBLICATION DATE: Thursday, September 5th, 2013
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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