RTCP XR Discard Count: RTP Control Protocol (RTCP) Extended Report (XR) Block for Discard Count Metric Reporting
A small RTCP report block that tells a sender how many packets arrived fine but were never played.
Clark, A., Zorn, G., and Q. Wu, "RTP Control Protocol (RTCP) Extended Report (XR) Block for Discard Count Metric Reporting", RFC 7002, DOI 10.17487/RFC7002, September 2013, <https://www.rfc-editor.org/info/rfc7002>.
@misc{rfc7002,
series = {Request for Comments},
number = 7002,
howpublished = {RFC 7002},
publisher = {RFC Editor},
doi = {10.17487/RFC7002},
url = {https://www.rfc-editor.org/info/rfc7002},
author = {A. Clark and G. Zorn and Q. Wu},
title = {{RTP Control Protocol (RTCP) Extended Report (XR) Block for Discard Count Metric Reporting}},
pagetotal = 12,
year = 2013,
month = sep,
} RFC text
Contents
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/rfc7002.
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. 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
[RFC3611] 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 [RFC5109] or
retransmission [RFC4588]) 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 [RFC7003], which provides
additional information on the pattern of discarded packets. However,
the metric in [RFC7003] may be used independently of the metrics in
this block.
When a Discard Count Metrics Block is sent together with a Burst/Gap
Discard Metrics Block (defined in [RFC7003]) 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 [RFC6792].
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
"Guidelines for Considering New Performance Metric Development"
[RFC6390] provides guidance on the definition and specification of
performance metrics. "Guidelines for Use of the RTP Monitoring
Framework" [RFC6792] provides guidance 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
This metric is believed to be applicable to a large class of RTP
applications that use a de-jitter buffer [RFC5481].
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 [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 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 [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" 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.
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 [RFC6776], which describes measurement
periods in use (see [RFC6776], 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.
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 [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, 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.
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 [RFC6709], 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 [RFC3611]. 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 [RFC3611].
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 [RFC6776], so it need not be
repeated in this block.
4. SDP Signaling
[RFC3611] defines the use of the Session Description Protocol (SDP)
[RFC4566] 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 [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-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 [RFC3611] for unilateral "rtcp-xr" attribute parameters
applies. For detailed usage of Offer/Answer for unilateral
parameters, refer to Section 5.2 of [RFC3611].
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
[RFC3611].
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
[RFC3711]; an appropriate combination of those two profiles ("SAVPF")
is specified in [RFC5124].
Besides this, 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.
7. Contributors
Geoff Hunt wrote the initial draft of this document.
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
[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.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, February 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC6709] Carpenter, B., Aboba, B., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709,
September 2012.
[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.
9.2. 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.
[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.
[RFC6390] Clark, A. and B. Claise, "Guidelines for Considering New
Performance Metric Development", BCP 170, RFC 6390,
October 2011.
[RFC6792] Wu, Q., Hunt, G., and P. Arden, "Guidelines for Use of the
RTP Monitoring Framework", RFC 6792, November 2012.
[RFC7003] 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.
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.
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
AI ELI5
On this page
§1Counting Packets That Were Never Played
RFC 7002 adds a new block type to the RTCP Extended Report (XR) framework from RFC 3611. The block reports the number of packets that were received correctly but never played out, typically because they arrived too late (buffer underflow) or too early (buffer overflow).
The metric applies both to systems that repair lost packets, with forward error correction or retransmission, and to those that do not. It helps identify, and characterize the severity of, transport problems that may affect how users perceive a service delivered over RTP.
The block can be used together with the Burst/Gap Discard block from RFC 7003. Sent together, the two let a receiver of the reports calculate burst/gap summary statistics such as the gap discard rate. The block belongs to the class of transport-related end-system metrics defined in RFC 6792.
Sometimes a mail truck brings your letter but you cannot use it because it came at the wrong time. This RFC lets the receiver tell the sender how many letters ended up unused.
§2Received, Lost and Discarded
The RFC sorts every packet into exactly one of three classes. 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 or too late to be played out, or is thrown away before playout because of duplication or redundancy, is discarded. Everything else is received (or OK).
A packet that arrives in the window but is dropped later by a higher-layer decoder, for example because of a decoding error, is not counted as discarded. The discard count metric counts only discarded packets, so it complements the loss count in RFC 3550 and the post-repair loss metric in RFC 5725.
A letter is either delivered and read, never shows up, or shows up but gets binned. Only the binned ones are counted here.
§3Where and How It Is Measured
The measurement is made at the receiving end of the RTP stream and covers one stream, identified by its SSRC. The block does not carry its own timing. It relies on the separate Measurement Information Block from RFC 6776, which describes the measurement period in use.
Because of that, the block MUST be sent in the same compound RTCP packet as the Measurement Information Block. If a receiver gets the block without the measurement interval in the same compound packet, it MUST discard the block.
The number of packets expected in the period is not repeated in this block. It can be worked out from a pair of extended sequence numbers in the Measurement Information Block.
The count only makes sense with a calendar saying which days it covers, so the count and the calendar must arrive in the same envelope.
§3.1The Discard Count Metrics Block
The block is fixed-size. It starts with BT=24 in an 8-bit Block Type field, then the 2-bit Interval Metric flag (I), the 2-bit Discard Type (DT), 4 reserved bits and a 16-bit Block Length that MUST be 2. Next come the 32-bit SSRC of Source and the 32-bit Discard Count.
The I flag says what the count covers: I=10 is the most recent interval between reports, and I=11 is a cumulative total. The value I=01 (sampled) MUST NOT be sent because discards can only be measured over definite intervals, and I=00 is reserved. Receivers MUST discard blocks with either value.
The Discard Type is 00 for duplicates or packets thrown away before playout, 01 for too early and 10 for too late. DT=11 is reserved. An endpoint MAY report any combination of types by putting several blocks in one RTCP XR packet.
The Discard Count is an unsigned value. If it exceeds 0xFFFFFFFD, the value 0xFFFFFFFE MUST be reported as an over-range measurement, and 0xFFFFFFFF MUST be reported if the measurement is unavailable. Reserved bits MUST be zero when sent and ignored on receipt. A block with any Block Length other than 2 MUST be discarded.
Some systems send duplicate RTP packets for robustness. The RFC says this is NOT RECOMMENDED because it breaks RTCP packet statistics, and points to a stream duplication mechanism that does not.
The block is a short form with a few tick boxes: whether the count is for the last stretch or the whole time, and why the packets were binned. The last line is the number itself.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
BT | BT | 1 byte | 24 | Block type, the constant 24. |
I | I | 2 bits | Interval (10) or cumulative (11); 01 and 00 must not be sent. | |
DT | DT | 2 bits | Discard type: 00 duplicate, 01 too early, 10 too late. | |
RESV | resv | 4 bits | Reserved, set to zero. | |
LEN | Block Length | 2 bytes | 2 | Must be 2. |
SSRC | SSRC of Source | 4 bytes | The stream the report is about. | |
COUNT | Discard Count | 4 bytes | Packets discarded over the interval or cumulative period. |
§4Signaling With SDP
Use of XR blocks can be announced in the Session Description Protocol through the rtcp-xr attribute from RFC 3611. This RFC adds one new xr-format value, pkt-discard-count, to signal this block.
In an offer/answer exchange, the existing rules for unilateral rtcp-xr parameters from RFC 3611 apply. Signaling is optional, since XR blocks MAY be used without prior signaling.
Before a game you can tell the other team you will keep score of a new thing. You do not have to, but it is polite.
§5Security Considerations
Very detailed error information, such as the over-range or unavailable values, could give an attacker insight into the security processing. Where that matters, the implementation should apply encryption and authentication to this block, for example by using the AVPF profile together with Secure RTP, as in the combined SAVPF profile of RFC 5124 and RFC 3711.
Apart from that, the RFC believes the block adds no new security considerations beyond those in RFC 3611. Because it carries no per-packet statistics, the confidentiality risk noted in RFC 3611 does not apply.
If a report says too much, a snoop could learn how your system works, so secret reports should be sent in a locked box.
§6IANA Registrations
The RFC assigns block type value 24 to the Discard Count Metrics Block in the IANA RTCP XR Block Type Registry. It also registers the new SDP parameter pkt-discard-count in the RTCP XR SDP Parameters Registry.
Appendix A restates the metric using the template from RFC 6390, mostly by pointing back to the sections above.
The RFC puts its new block's name and number in the official phone book so nobody else uses them.