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IETF RFC 7509
Last modified on Wednesday, May 13th, 2015
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Internet Engineering Task Force (IETF) R. Huang
Request for Comments: 7509 Huawei
Category: Standards Track V. Singh
ISSN: 2070-1721 Aalto University
May 2015
RTP Control Protocol (RTCP) Extended Report (XR)
for Post-Repair Loss Count Metrics
 Abstract
This document defines an RTP Control Protocol (RTCP) Extended Report
(XR) block that allows reporting of a post-repair loss count metric
for a range of RTP applications. In addition, another metric,
repaired loss count, is also introduced in this report block for
calculating the pre-repair loss count when needed, so that the RTP
sender or a third-party entity is able to evaluate the effectiveness
of the repair methods used by the system.
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 7509.
Copyright Notice
Copyright (c) 2015 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.
Singh & Huang Standards Track PAGE 1 
RFC 7509 Post-Repair Non-RLE Loss Count May 2015
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................3
3. Post-Repair Loss Count Metrics Report Block .....................3
3.1. Report Block Structure .....................................4
3.2. Example Usage ..............................................5
4. SDP Signaling ...................................................6
4.1. SDP rtcp-xr-attrib Attribute Extension .....................6
4.2. Offer/Answer Usage .........................................7
5. Security Considerations .........................................7
6. IANA Considerations .............................................7
6.1. New RTCP XR Block Type Value ...............................7
6.2. New RTCP XR SDP Parameter ..................................7
6.3. Contact Information for Registrations ......................7
7. References ......................................................8
7.1. Normative References .......................................8
7.2. Informative References .....................................9
Appendix A. Metrics Represented Using the Template from RFC 6390 ..10
Acknowledgments ...................................................11
Authors' Addresses ................................................11
1. Introduction
RTCP Sender Reports (SRs) / Receiver Reports (RRs) [RFC 3550] contain
some rough statistics about the data received from the particular
source indicated in that block. One of them is the cumulative number
of packets lost, which is called the pre-repair loss metric in this
document. This metric conveys information regarding the total number
of RTP data packets that have been lost since the beginning of the
RTP session.
However, this metric is measured on the media stream before any loss-
repair mechanism, e.g., retransmission [RFC 4588] or Forward Error
Correction (FEC) [RFC 5109], is applied. Using a repair mechanism
usually results in recovering some or all of the lost packets. The
recovery process does not reduce the values reported by the two loss
metrics in RTCP RR [RFC 3550] -- namely, the fraction lost and the
cumulative loss. Hence, the sending endpoint cannot infer the
performance of the repair mechanism based on the aforementioned
metrics in [RFC 3550].
Consequently, [RFC 5725] specifies a post-repair loss Run-Length
Encoding (RLE) XR report block to address this issue. The sending
endpoint is able to infer which packets were repaired from the RLE
report block, but the reporting overhead for the packet-by-packet
report block is higher compared to other report blocks.
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RFC 7509 Post-Repair Non-RLE Loss Count May 2015
When applications use multiple XR blocks, the endpoints may require
more concise reporting to save bandwidth. This document defines a
new XR block type to augment those defined in [RFC 3611] and
complement the report block defined in [RFC 5725] for use in a range
of RTP applications. This new block type reports the post-repair
loss count metric, which records the number of primary source RTP
packets that are still lost after applying one or more loss-repair
mechanisms. In addition, another metric, repaired loss count, is
also introduced in this report block for calculating the pre-repair
loss count during this range, so that the RTP sender or a third-party
entity is able to evaluate the effectiveness of the repair methods
used by the system. The metrics defined in this document are packet
level rather than slice/picture level; this means the partial
recovery of a packet will not be regarded as a repaired packet.
The metrics defined in this document belong to the class of
transport-related metrics defined in [RFC 6792] and are specified in
accordance with the guidelines in [RFC 6390] and [RFC 6792]. These
metrics are applicable to any RTP application, especially those that
use loss-repair mechanisms.
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 [KEYWORDS].
primary source RTP packet: The original RTP packet sent from the RTP
sender for the first time. A lost primary source RTP packet may
be repaired by some other RTP packets used in repair mechanisms
like FEC or retransmission.
3. Post-Repair Loss Count Metrics Report Block
This block reports the number of packets lost after applying repair
mechanisms (e.g., FEC). It complements the RTCP XR metrics defined
in [RFC 5725]. As noted in [RFC 5725], ambiguity may occur when
comparing this metric with a pre-repair loss metric reported in an
RTCP SR/RR, i.e., some packets were not repaired in the current RTCP
interval, but they may be repaired later. Therefore, this block uses
a begin sequence number and an end sequence number to explicitly
indicate the actual sequence number range reported by this RTCP XR.
Accordingly, only packets that have no further chance of being
repaired and that have been repaired are included in this report
block.
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RFC 7509 Post-Repair Non-RLE Loss Count May 2015
3.1. Report Block Structure
The Post-Repair Loss Count Metrics Report Block has the following
format:
0 1 2 3 4
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 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=33 | Reserved | Block length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of Source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| begin_seq | end_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Post-repair loss count | Repaired loss count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Block Type (BT): 8 bits
A Post-Repair Loss Count Metrics Report Block is identified by the
constant 33.
Reserved: 8 bits
These bits are reserved for future use. They MUST be set to zero
by senders and ignored by receivers (see Section 4.2 of
[RFC 6709]).
Block length: 16 bits
This field is in accordance with the definition in [RFC 3611]. In
this report block, it MUST be set to 4. 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].
begin_seq: 16 bits
The first sequence number that this block reports on. It can
remain fixed when calculating metrics over several RTCP reporting
intervals.
end_seq: 16 bits
The last sequence number that this block reports on plus one.
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RFC 7509 Post-Repair Non-RLE Loss Count May 2015
Post-repair loss count: 16 bits
Total number of packets finally lost after applying one or more
loss-repair methods, e.g., FEC and/or retransmission, during the
actual sequence number range indicated by begin_seq and end_seq.
This metric MUST NOT count the lost packets for which repair might
still be possible. Note that this metric MUST measure only
primary source RTP packets.
Repaired loss count: 16 bits
Total number of packets fully repaired after applying one or more
loss-repair methods, e.g., FEC and/or retransmission, during the
actual sequence number range indicated by begin_seq and end_seq.
Note that this metric MUST measure only primary source RTP
packets.
3.2 Example Usage
The metrics defined in this report block are all measured at the RTP
receiver. However, the receiving endpoint can report the metrics in
two different ways:
1) Cumulative report
In this case, implementations may set begin_seq to the first packet
in the RTP session, and it will remain fixed across all reports.
Hence, the "Post-repair loss count" and "Repaired loss count",
respectively, will correspond to "Cumulative post-repair loss count"
and "Cumulative repaired loss count" in this case. These cumulative
metrics when combined with the cumulative loss metrics reported in an
RTCP RR (pre-repair) assist in calculating the "Still-to-be-repaired
lost packets":
Still-to-be-repaired lost packets =
Cumulative number of packets lost -
Cumulative post-repair loss count -
Cumulative repaired loss count
2) Interval report
Some implementations may align the begin_seq and end_seq number with
the highest sequence numbers of consecutive RTCP RRs (RTCP interval).
This is NOT RECOMMENDED as packets that are not yet repaired in this
current RTCP interval and may be repaired in the subsequent intervals
will not be reported. An interval report is illustrated in the
following example:
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RFC 7509 Post-Repair Non-RLE Loss Count May 2015
Interval A: The extended highest sequence number received in RTCP
RR is 20. Begin_seq is 10 and end_seq is 20.
Interval B: The extended highest sequence number received in RTCP
RR is 30. Begin_seq is 20 and end_seq is 30.
If packets 17 and 19 are lost and not yet repaired in interval A and
subsequently repaired in interval B, they will not be reported
because their sequence numbers do not belong in interval B.
Therefore, if implementations want these packets to be reported as
repaired, they MUST NOT align the begin_seq and end_seq to the RTCP
intervals.
Alternatively, implementations may choose the begin_seq and end_seq
numbers that cover several RTCP intervals. Additionally, the
reported range of sequence numbers may overlap with the previous
report blocks, so that the packets that were not yet repaired in one
interval, but were subsequently repaired or deemed unrepairable, were
reported in subsequent intervals.
In this case, the "Cumulative number of packets lost" cannot be
easily compared with the post-repair metrics. However, the sending
endpoint can calculate the efficiency of the error resilience
algorithm using the post-repair and repaired loss count,
respectively.
4. SDP Signaling
[RFC 3611] defines the use of SDP (Session Description Protocol) for
signaling the use of RTCP XR blocks. However, XR blocks MAY be used
without prior signaling (see Section 5 of [RFC 3611]).
4.1. SDP rtcp-xr-attrib Attribute Extension
This session augments the SDP attribute "rtcp-xr" defined in Section
5.1 of [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-prlr-block
xr-prlr-block = "post-repair-loss-count"
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RFC 7509 Post-Repair Non-RLE Loss Count May 2015
4.2. Offer/Answer Usage
When SDP is used in offer/answer context, the SDP Offer/Answer usage
defined in [RFC 3611] for the unilateral "rtcp-xr" attribute
parameters applies. For detailed usage of Offer/Answer for
unilateral parameters, refer to Section 5.2 of [RFC 3611].
5. Security Considerations
This proposed 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.
An attacker may put incorrect information in the Post-Repair Loss
Count reports, which will affect the performance of loss-repair
mechanisms. Implementers should consider the guidance in [RFC 7202]
for using appropriate security mechanisms, i.e., where security is a
concern, the implementation should apply encryption and
authentication to the report block. For example, this can be
achieved by using the AVPF profile together with the Secure RTP
profile as defined in [RFC 3711]; an appropriate combination of the
two profiles (an "SAVPF") is specified in [RFC 5124]. However, other
mechanisms also exist (documented in [RFC 7201]) and might be more
suitable.
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 33 in the IANA "RTP
Control Protocol Extended Reports (RTCP XR) Block Type Registry" to
the "Post-Repair Loss Count Metrics Report Block".
6.2. New RTCP XR SDP Parameter
This document also registers a new parameter "post-repair-loss-count"
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:
RAI Area Directors <rai-ads@ietf.org>
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RFC 7509 Post-Repair Non-RLE Loss Count May 2015
7. References
7.1. Normative References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC 3550,
July 2003, <http://www.rfc-editor.org/info/RFC 3550>.
[RFC 3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
"RTP Control Protocol Extended Reports (RTCP XR)",
RFC 3611, DOI 10.17487/RFC 3611, November 2003,
<http://www.rfc-editor.org/info/RFC 3611>.
[RFC 3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC 3711, March 2004,
<http://www.rfc-editor.org/info/RFC 3711>.
[RFC 5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC 5124, February
2008, <http://www.rfc-editor.org/info/RFC 5124>.
[RFC 5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC 5234, January 2008,
<http://www.rfc-editor.org/info/RFC 5234>.
[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, DOI 10.17487/RFC 5725, February
2010, <http://www.rfc-editor.org/info/RFC 5725>.
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RFC 7509 Post-Repair Non-RLE Loss Count May 2015
7.2. Informative References
[RFC 4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
DOI 10.17487/RFC 4588, July 2006,
<http://www.rfc-editor.org/info/RFC 4588>.
[RFC 5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, DOI 10.17487/RFC 5109, December
2007, <http://www.rfc-editor.org/info/RFC 5109>.
[RFC 6390] Clark, A. and B. Claise, "Guidelines for Considering New
Performance Metric Development", BCP 170, RFC 6390,
DOI 10.17487/RFC 6390, October 2011,
<http://www.rfc-editor.org/info/RFC 6390>.
[RFC 6709] Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709, DOI
10.17487/RFC 6709, September 2012,
<http://www.rfc-editor.org/info/RFC 6709>.
[RFC 6792] Wu, Q., Ed., Hunt, G., and P. Arden, "Guidelines for Use
of the RTP Monitoring Framework", RFC 6792,
DOI 10.17487/RFC 6792, November 2012,
<http://www.rfc-editor.org/info/RFC 6792>.
[RFC 7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC 7201, April 2014,
<http://www.rfc-editor.org/info/RFC 7201>.
[RFC 7202] Perkins, C. and M. Westerlund, "Securing the RTP
Framework: Why RTP Does Not Mandate a Single Media
Security Solution", RFC 7202, DOI 10.17487/RFC 7202, April
2014, <http://www.rfc-editor.org/info/RFC 7202>.
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RFC 7509 Post-Repair Non-RLE Loss Count May 2015
Appendix A. Metrics Represented Using the Template from RFC 6390
a. Post-Repair RTP Packet Loss Count Metric
* Metric Name: Post-Repair RTP Packet Loss Count Metric.
* Metric Description: Total number of RTP packets still lost
after loss-repair methods are applied.
* Method of Measurement or Calculation: See the "Post-repair
loss count" definition in Section 3.1. It is directly
measured and must be measured for the primary source RTP
packets with no further chance of repair.
* Units of Measurement: This metric is expressed as a 16-bit
unsigned integer value giving the number of RTP packets.
* Measurement Point(s) with Potential Measurement Domain: It is
measured at the receiving end of the RTP stream.
* Measurement Timing: This metric relies on the sequence number
interval to determine measurement timing. See the Cumulative
and Interval reports defined in Section 3.2.
* Use and Applications: These metrics are applicable to any RTP
application, especially those that use loss-repair mechanisms.
See Section 1 for details.
* Reporting Model: See RFC 3611.
b. Repaired RTP Packet Loss Count Metric
* Metric Name: Repaired RTP Packet Count Metric.
* Metric Description: The number of RTP packets lost but
repaired after applying loss-repair methods.
* Method of Measurement or Calculation: See the "Repaired loss
count" in Section 3.1. It is directly measured and must be
measured for the primary source RTP packets with no further
chance of repair.
* Units of Measurement: This metric is expressed as a 16-bit
unsigned integer value giving the number of RTP packets.
* Measurement Point(s) with Potential Measurement Domain: It is
measured at the receiving end of the RTP stream.
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RFC 7509 Post-Repair Non-RLE Loss Count May 2015
* Measurement Timing: This metric relies on the sequence number
interval to determine measurement timing. See the Cumulative
and Interval reports defined in Section 3.2.
* Use and Applications: These metrics are applicable to any RTP
application, especially those that use loss-repair mechanisms.
See Section 1 for details.
* Reporting Model: See RFC 3611.
Acknowledgments
The authors would like to thank Roni Even, Colin Perkins, and Qin Wu
for giving valuable comments and suggestions.
Authors' Addresses
Rachel Huang
Huawei
101 Software Avenue, Yuhua District
Nanjing 210012
China
EMail: rachel.huang@huawei.com
Varun Singh
Aalto University
School of Electrical Engineering
Otakaari 5 A
Espoo, FIN 02150
Finland
EMail: varun@comnet.tkk.fi
URI: http://www.netlab.tkk.fi/~varun/
Singh & Huang Standards Track PAGE 11
RFC TOTAL SIZE: 23350 bytes
PUBLICATION DATE: Wednesday, May 13th, 2015
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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