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Keywords: RTP, RTCP, discard metrics
Internet Engineering Task Force (IETF) J. Ott
Request for Comments: 7097 V. Singh, Ed.
Category: Standards Track Aalto University
ISSN: 2070-1721 I. Curcio
Nokia Research Center
January 2014
RTP Control Protocol (RTCP) Extended Report (XR)
for RLE of Discarded Packets
Abstract
The RTP Control Protocol (RTCP) is used in conjunction with the Real-
time Transport Protocol (RTP) in order to provide a variety of short-
term and long-term reception statistics. The available reporting may
include aggregate information across longer periods of time as well
as individual packet reporting. This document specifies a per-packet
report metric capturing individual packets discarded from the de-
jitter buffer after successful reception.
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/rfc7097.
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Copyright Notice
Copyright (c) 2014 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 ....................................................3
2. Terminology .....................................................4
3. RTCP XR Discard RLE Report Block ................................4
4. Protocol Operation ..............................................6
4.1. Reporting Node (Receiver) ..................................6
4.2. Media Sender ...............................................6
5. SDP Signaling ...................................................6
6. Security Considerations .........................................7
7. IANA Considerations .............................................7
7.1. XR Report Block Registration ...............................7
7.2. SDP Parameter Registration .................................8
7.3. Contact Information for IANA Registrations .................8
8. Acknowledgments .................................................8
9. References ......................................................8
9.1. Normative References .......................................8
9.2. Informative References .....................................9
Appendix A. Metrics Represented Using the Template from RFC 6390 ..10
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1. Introduction
RTP [RFC3550] provides a transport for real-time media flows such as
audio and video together with the RTP Control Protocol (RTCP), which
provides periodic feedback about the media streams received in a
specific duration. In addition, RTCP can be used for timely feedback
about individual events to report (e.g., packet loss) [RFC4585].
Both long-term and short-term feedback enable a media sender to adapt
its media transmission and/or encoding dynamically to the observed
path characteristics.
RFC 3611 [RFC3611] defines RTCP Extended Reports as a detailed
reporting framework to provide more than just the coarse Receiver
Report (RR) statistics. The detailed reporting may enable a media
sender to react more appropriately to the observed networking
conditions as these can be characterized better, although at the
expense of extra overhead.
Among many other report blocks, RFC 3611 specifies the Loss Run
Length Encoding (RLE) block, which reports runs of packets received
and lost with the granularity of individual packets. This can help
both error recovery and path loss characterization. In addition to
lost packets, RFC 3611 defines the notion of "discarded" packets:
packets that were received but dropped from the de-jitter buffer
because they were either too early (for buffering) or too late (for
playout). The "discard rate" metric is part of the Voice over IP
(VoIP) metrics report block even though it is not just applicable to
audio: it is specified as the fraction of discarded packets since the
beginning of the session (see Section 4.7.1 of RFC 3611 [RFC3611]).
The discard metric is believed to be applicable to a large class of
RTP applications that use a de-jitter buffer [RFC5481].
Recently proposed extensions to the Extended Reports (XRs) reporting
suggest enhancing this discard metric:
o Reporting the number of discarded packets in a measurement
interval, i.e., either during the last reporting interval or since
the beginning of the session, as indicated by a flag in the
suggested XR [RFC7002]. If an endpoint needs to report packet
discard due to reasons other than early and late arrival (for
example, discard due to duplication, redundancy, etc.), then it
should consider using the Discarded Packets report block
[RFC7002].
o Reporting gaps and bursts of discarded packets during a
measurement interval, i.e., the last reporting interval or the
duration of the session [RFC7003].
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o Reporting the sum of payload bytes discarded during a measurement
interval, i.e., the last reporting interval or the duration of the
session [DISCARD-METRIC].
However, none of these metrics allow a receiver to report precisely
which packets were discarded. While this information could in theory
be derived from high-frequency reporting on the number of discarded
packets [RFC7002] or from the gap/burst report [RFC7003], these two
mechanisms do not appear feasible: the former would require an unduly
high amount of reporting, which still might not be sufficient due to
the non-deterministic scheduling of RTCP packets. The latter incurs
significant complexity and reporting overhead and might still not
deliver the desired accuracy.
This document defines a discard report block following the idea of
the run-length encoding applied for lost and received packets in
[RFC3611].
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 BCP 14, RFC 2119
[RFC2119].
The terminology defined in RTP [RFC3550] and in the extensions for XR
reporting [RFC3611] applies.
3. RTCP XR Discard RLE Report Block
The RTCP XR Discard RLE report block uses the same format as
specified for the loss and duplicate report blocks in RFC 3611
[RFC3611]. Figure 1 describes the packet format. The fields "BT",
"T", "block length", "SSRC of source", "begin_seq", and "end_seq"
have the same semantics and representation as defined in [RFC3611],
with the addition of the "E" flag to indicate the reason for discard.
The "chunks" encoding the run length have the same representation as
in RFC 3611, but encode discarded packets. A definition of a
discarded packet is given in RFC 7002 [RFC7002].
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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=25 |rsvd |E| T | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| begin_seq | end_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| chunk 1 | chunk 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| chunk n-1 | chunk n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RTCP XR Discard RLE Report Block
Block Type (BT, 8 bits): A Discard RLE report block is identified by
the constant 25.
rsvd (3 bits): This field is reserved for future definition. In the
absence of such definition, the bits in this field MUST be set to
zero and MUST be ignored by the receiver.
The 'E' bit is introduced to distinguish between packets discarded
due to early arrival and those discarded due to late arrival. The
'E' bit is set to '1' if the chunks represent packets discarded due
to arriving too early and is set to '0' otherwise.
In case both early and late discarded packets shall be reported, two
Discard RLE report blocks MUST be included; their sequence number
range MAY overlap, but individual packets MUST only be reported as
either early or late and not appear marked in both. If packets
appear in both report blocks, the conflicting packets will be
ignored. Packets not reported in either block are considered to be
properly received and not discarded.
Discard RLE report blocks SHOULD be sent in conjunction with an RTCP
RR as a compound RTCP packet.
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4. Protocol Operation
This section describes the behavior of the reporting node (= media
receiver) and the media sender.
4.1. Reporting Node (Receiver)
Transmission of RTCP XR Discard RLE report blocks is up to the
discretion of the media receiver, as is the reporting granularity.
However, it is RECOMMENDED that the media receiver signal all
discarded packets using the method defined in this document. If all
packets over a reporting period are discarded, the media receiver MAY
use the Discard Report Block [RFC7002] instead. In case of limited
available reporting bandwidth, it is up to the receiver whether or
not to include RTCP XR Discard RLE report blocks.
The media receiver MAY send the Discard RLE report blocks as part of
the regularly scheduled RTCP packets, as per RFC 3550. It MAY also
include Discard RLE report blocks in immediate or early feedback
packets, as per RFC 4585.
4.2. Media Sender
The media sender MUST be prepared to operate without receiving any
Discard RLE report blocks. If Discard RLE report blocks are
generated by the media receiver, the media sender cannot rely on all
these reports being received, nor can the media sender rely on a
regular generation pattern from the media receiver.
However, if the media sender receives RTCP XR reports but the reports
contain no Discard RLE report blocks and is aware that the media
receiver supports Discard RLE report blocks, it MAY assume that no
packets were discarded at the media receiver.
5. SDP Signaling
A participant of a media session MAY use SDP to signal its support
for the report block specified in this document or use them without
any prior signaling (see Section 5 of RFC 3611 [RFC3611]).
For signaling in SDP, the RTCP XR attribute as defined in RFC 3611
[RFC3611] MUST be used. The SDP [RFC4566] attribute 'xr-format'
defined in RFC 3611 is augmented as described in the following to
indicate the discard RLE metric.
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rtcp-xr-attrib = "a=" "rtcp-xr" ":" [xr-format *(SP xr-format)]
CRLF ; defined in [RFC3611]
xr-format =/ xr-discard-rle
xr-discard-rle = "discard-rle"
The parameter 'discard-rle' is used to indicate support for the
Discard RLE report block defined in Section 3.
When SDP is used in Offer/Answer context, the mechanism defined in
RFC 3611 [RFC3611] for unilateral "rtcp-xr" attribute parameters
applies (see Section 5.2 of RFC 3611 [RFC3611]).
6. Security Considerations
The Discard RLE report block provides per-packet statistics so the
risk to confidentiality documented in Section 7, Paragraph 3, of RFC
3611 [RFC3611] applies. 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. Implementers should consider
the guidance in [NO-SRTP] 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 [RFC3711]; an appropriate
combination of the two profiles (an "SAVPF") is specified in RFC 5124
[RFC5124]. However, other mechanisms also exist [SRTP-OPTIONS] and
might be more suitable.
Additionally, The security considerations of RFC 3550 [RFC3550], RFC
3611 [RFC3611], and RFC 4585 [RFC4585] apply.
7. 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.
7.1. XR Report Block Registration
This document extends the IANA "RTP Control Protocol Extended Reports
(RTCP XR) Block Type Registry" by assigning value 25 to DRLE (Discard
RLE Report).
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7.2. SDP Parameter Registration
This document registers 'discard-rle' in the "RTCP XR SDP
Parameters".
7.3. Contact Information for IANA Registrations
Joerg Ott (jo@comnet.tkk.fi)
Aalto University Comnet, Otakaari 5A, 02150 Espoo, Finland.
8. Acknowledgments
The authors would like to thank Alan Clark, Roni Even, Sam Hartman,
Colin Perkins, Dan Romascanu, Dan Wing, and Qin Wu for providing
valuable feedback on earlier draft versions of this document.
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.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July
2006.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC7002] Clark, A., Zorn, G., and Q. Wu, "RTP Control Protocol
(RTCP) Extended Report (XR) Block for Discard Count Metric
Reporting", RFC 7002, September 2013.
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9.2. Informative References
[RFC7003] Clark, A., Huang, R., and Q. Wu, "RTP Control Protocol
(RTCP) Extended Report (XR) Block for Burst/Gap Discard
Metric Reporting", RFC 7003, September 2013.
[RFC5481] Morton, A. and B. Claise, "Packet Delay Variation
Applicability Statement", RFC 5481, March 2009.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[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.
[NO-SRTP] Perkins, C. and M. Westerlund, "Securing the RTP Protocol
Framework: Why RTP Does Not Mandate a Single Media
Security Solution", Work in Progress, October 2013.
[SRTP-OPTIONS]
Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", Work in Progress, November 2013.
[DISCARD-METRIC]
Singh, V., Ed., Ott, J., and I. Curcio, "RTP Control
Protocol (RTCP) Extended Report (XR) for Bytes Discarded
Metric", Work in Progress, November 2013.
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Appendix A. Metrics Represented Using the Template from RFC 6390
a. RLE of Discarded RTP Packets Metric
* Metric Name: RLE - Run-length encoding of Discarded RTP
Packets Metric.
* Metric Description: Instances of RTP packets discarded over
the period covered by this report.
* Method of Measurement or Calculation: See Section 3 for the
definition of Discard RLE, and Section 4.1 of RFC 3611 for
RLE.
* Units of Measurement: Every RTP packet in the interval is
reported as discarded or not. See Section 3 for the
definition.
* Measurement Point(s) with Potential Measurement Domain: The
measurement of these metrics is made at the receiving end of
the RTP stream.
* Measurement Timing: Each RTP packet between a beginning
sequence number (begin_seq) and ending sequence number
(end_seq) is reported as discarded or not. See Section 3 for
the definition of Discard RLE.
* Use and applications: See Section 1, paragraph 1.
* Reporting model: See RFC 3611.
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Authors' Addresses
Joerg Ott
Aalto University
School of Electrical Engineering
Otakaari 5 A
Espoo, FIN 02150
Finland
EMail: jo@comnet.tkk.fi
Varun Singh (editor)
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/
Igor D.D. Curcio
Nokia Research Center
P.O. Box 1000 (Visiokatu 3)
Tampere, FIN 33721
Finland
EMail: igor.curcio@nokia.com
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