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IETF RFC 6843
Last modified on Tuesday, January 15th, 2013
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Internet Engineering Task Force (IETF) A. Clark
Request for Comments: 6843 Telchemy
Category: Standards Track K. Gross
ISSN: 2070-1721 AVA Networks
Q. Wu
Huawei
January 2013
RTP Control Protocol (RTCP) Extended Report (XR)
Block for Delay Metric Reporting
 Abstract
This document defines an RTP Control Protocol (RTCP) Extended Report
(XR) block that allows the reporting of delay metrics for use in a
range of Real-time Transport Protocol (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 6843.
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 
RFC 6843 RTCP XR Delay January 2013
Table of Contents
1. Introduction ....................................................2
1.1. Packet Delay Metrics Block .................................2
1.2. RTCP and RTCP XR Reports ...................................2
1.3. Performance Metrics Framework ..............................3
1.4. Applicability ..............................................3
2. Terminology .....................................................3
2.1. Standards Language .........................................3
3. Delay Block .....................................................3
3.1. Report Block Structure .....................................4
3.2. Definition of Fields in Delay Metrics Report Block .........4
4. SDP Signaling ...................................................6
4.1. SDP rtcp-xr-attrib Attribute Extension .....................7
4.2. Offer/Answer Usage .........................................7
5. IANA Considerations .............................................7
5.1. New RTCP XR Block Type Value ...............................7
5.2. New RTCP XR SDP Parameter ..................................7
5.3. Contact Information for Registrations ......................7
6. Security Considerations .........................................8
7. Contributors ....................................................8
8. Acknowledgments .................................................8
9. References ......................................................8
9.1. Normative References .......................................8
9.2. Informative References .....................................9
1. Introduction
1.1. Packet Delay 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 mean, minimum, and maximum values of
the network round-trip delay between RTP interfaces in peer RTP end
systems as measured, for example, using the RTCP method described in
[RFC 3550]. It also supports reporting of the component of the round-
trip delay internal to the local RTP system.
The network metrics belong to the class of transport metrics defined
in [RFC 6792].
1.2. RTCP and RTCP XR 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].
Clark, et al. Standards Track PAGE 2
RFC 6843 RTCP XR Delay January 2013
1.3. Performance Metrics Framework
The Performance Metrics Framework [RFC 6390] provides guidance on the
definition and specification of performance metrics. The RTP
Monitoring Architectures [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 in which
this report block would be useful, such as multimedia conferencing
and streaming audio and video. Knowledge of the round-trip delay and
delay characteristics can aid other receivers in sizing their receive
buffers and selecting a playout delay. The same information is also
valuable to network managers in troubleshooting network and user
experience issues.
2. Terminology
2.1. Standards Language
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].
3. Delay Block
Metrics in this block report on packet delay in the stream arriving
at the RTP system. The measurement of these metrics is made either
at the receiving end of the RTP stream or at the sending end of the
RTP stream. Instances of this metrics block refer by synchronization
source (SSRC) to the separate auxiliary Measurement Information block
[RFC 6776], which contains measurement periods (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 and
SHOULD be sent in the same compound RTCP packet as the Measurement
Information block. If the measurement period is not received in the
same compound RTCP packet as this metrics block, this metrics block
MUST be discarded.
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RFC 6843 RTCP XR Delay January 2013
3.1. Report Block Structure
Delay metrics block
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=16 | I | resv. | block length = 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of Source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mean Network Round-Trip Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Min Network Round-Trip Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max Network Round-Trip Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| End System Delay - Seconds (bit 0-31) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| End System Delay - Fraction (bit 0-31) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Report Block Structure
3.2. Definition of Fields in Delay Metrics Report Block
Block type (BT): 8 bits
A Delay Report Block is identified by the constant 16.
Interval Metric flag (I): 2 bit
This field is used to indicate whether the delay metrics are
Sampled, Interval or Cumulative metrics:
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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RFC 6843 RTCP XR Delay January 2013
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 delay block, the block length is equal to 6.
SSRC of source: 32 bits
As defined in Section 4.1 of [RFC 3611].
Mean Network Round-Trip Delay: 32 bits
The Mean Network Round-Trip Delay is the mean value of the RTP-to-
RTP interface round-trip delay over the measurement period,
expressed in units of 1/65536 seconds. This value is typically
determined using "the NTP timestamp field" in the RTCP sender
report (SR) and "the last SR (LSR) field","delay since last SR
(DLSR) field" in the RTCP receiver report (RR) (see [RFC 3550],
Section 6.4.1 and Figure 2). It also can be determined using "the
NTP timestamp field" in the RTCP Receiver Reference Time Report
Block and "last RR (LRR) field", "delay since last RR (DLRR)
field" in the DLRR Report Block (see [RFC 3611], Section 4.5).
If only one measurement of Round-Trip Delay is available for the
time span of the report (i.e., the measurement period) (whether
Interval or Cumulative), this single value SHOULD be reported as
the mean value.
If the measurement is unavailable, the value of this field with
all bits set to 1 MUST be reported.
Min Network Round-Trip Delay: 32 bits
The Min Network Round Trip Delay is the minimum value of the RTP-
to-RTP interface round-trip delay over the measurement period,
expressed in units of 1/65536 seconds. This value is typically
determined using the NTP timestamp field in the RTCP SR and LSR
field and DLSR field in the RTCP RR. It also can be determined
using the NTP timestamp field in the RTCP Receiver Reference Time
Report Block and LRR field and DLRR field in the DLRR Report
Block.
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RFC 6843 RTCP XR Delay January 2013
If only one measurement of Round Trip Delay is available for the
time span of the report (i.e., the measurement period) (whether
Interval or Cumulative), this single value SHOULD be reported as
the minimum value.
If the measurement is unavailable, the value of this field with
all bits set to 1 MUST be reported.
Max Network Round-Trip Delay: 32 bits
The Max Network Round-Trip Delay is the maximum value of the RTP-
to-RTP interface round-trip delay over the measurement period,
expressed in units of 1/65536 seconds. This value is typically
determined using the NTP timestamp field in the RTCP SR and LSR
field and DLSR field in the RTCP RR. It also can be determined
using the NTP timestamp field in the RTCP Receiver Reference Time
Report Block and LRR field and DLRR field in the DLRR Report
Block.
If only one measurement of Round-Trip Delay is available for the
time span of the report (i.e.,the measurement period) (whether
Interval or Cumulative), this single value SHOULD be reported as
the maximum value.
If the measurement is unavailable, the value of this field with
all bits set to 1 MUST be reported.
End System Delay: 64 bits
The End System Delay is the internal round-trip delay within the
reporting endpoint, calculated using the nominal value of the
jitter buffer delay plus the accumulation/encoding and decoding/
playout delay associated with the codec being used. The value of
this field is represented using a 64-bit NTP-format timestamp as
defined in [RFC 5905], which is a 64-bit unsigned fixed-point
number with the integer part in the first 32 bits and the
fractional part in the last 32 bits.
If the measurement is unavailable, the value of this field with
all bits set to 1 MUST be reported.
4. 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.
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RFC 6843 RTCP XR Delay January 2013
4.1. SDP rtcp-xr-attrib 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.
xr-format =/ xr-delay-block
xr-delay-block ="delay"
4.2. Offer/Answer Usage
When SDP is used in offer/answer context, the SDP Offer/Answer usage
defined in [RFC 3611] applies.
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].
5.1. New RTCP XR Block Type Value
This document assigns the block type value 16 in the IANA "RTP
Control Protocol Extended Reports (RTCP XR) Block Type Registry" to
the "Delay Metrics Block".
5.2. New RTCP XR SDP Parameter
This document also registers a new parameter "delay" in the "RTP
Control Protocol Extended Reports (RTCP XR) Session Description
Protocol (SDP) Parameters" registry.
5.3. Contact Information for Registrations
The contact information for the registrations is:
Qin Wu (sunseawq@huawei.com)
Huawei
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
Clark, et al. Standards Track PAGE 7
RFC 6843 RTCP XR Delay January 2013
6. Security Considerations
It is believed that this proposed RTCP XR report 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 version of this document.
8. Acknowledgments
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, Jing
Zhao, Kevin Gross, Colin Perkins, Charles Eckel, Glen Zorn, Shida
Schubert, Barry Leiba, Sean Turner, Robert Sparks, Benoit Claise, and
Stephen Farrell.
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 4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC 5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
Clark, et al. Standards Track PAGE 8
RFC 6843 RTCP XR Delay January 2013
[RFC 6709] Carpenter, B., Aboba, B., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709,
September 2012.
9.2. Informative References
[RFC 6390] Clark, A. and B. Claise, "Guidelines for Considering New
Performance Metric Development", BCP 170, RFC 6390,
October 2011.
[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 6792] Wu, Q., Hunt, G., and P. Arden, "Guidelines for Use of the
RTP Monitoring Framework", RFC 6792, November 2012.
Authors' Addresses
Alan Clark
Telchemy Incorporated
2905 Premiere Parkway, Suite 280
Duluth, GA 30097
USA
EMail: alan.d.clark@telchemy.com
Kevin Gross
AVA Networks
EMail: kevin.gross@avanw.com
Qin Wu
Huawei
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
EMail: sunseawq@huawei.com
Clark, et al. Standards Track PAGE 9
RFC TOTAL SIZE: 18354 bytes
PUBLICATION DATE: Tuesday, January 15th, 2013
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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