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rfc:rfc7243

Internet Engineering Task Force (IETF) V. Singh, Ed. Request for Comments: 7243 J. Ott Category: Standards Track Aalto University ISSN: 2070-1721 I. Curcio

                                                 Nokia Research Center
                                                              May 2014
       RTP Control Protocol (RTCP) Extended Report (XR) Block
                   for the Bytes Discarded Metric

Abstract

 The RTP Control Protocol (RTCP) is used in conjunction with the Real-
 time Transport Protocol (RTP) 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 report
 computing the bytes 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/rfc7243.

Singh, et al. Standards Track [Page 1] RFC 7243 RTCP XR Bytes Discarded May 2014

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. Bytes Discarded Report Block ....................................4
 4. Protocol Operation ..............................................6
    4.1. Reporting Node (Receiver) ..................................6
    4.2. Media Sender ...............................................6
 5. SDP Signaling ...................................................7
 6. Security Considerations .........................................7
 7. IANA Considerations .............................................8
    7.1. XR Report Block Registration ...............................8
    7.2. SDP Parameter Registration .................................8
    7.3. Contact Information for IANA Registrations .................8
 8. Acknowledgments .................................................8
 9. References ......................................................9
    9.1. Normative References .......................................9
    9.2. Informative References .....................................9
 Appendix A. Metrics Represented Using the Template from RFC 6390 ..11

Singh, et al. Standards Track [Page 2] RFC 7243 RTCP XR Bytes Discarded May 2014

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.
 [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.
 In addition to lost packets, [RFC3611] 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
 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 [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 (XR) reporting
 suggest enhancing the discard metric:
 o  Reporting the number of discarded packets in a measurement
    interval, i.e., during either the last reporting interval or since
    the beginning of the session, as indicated by a flag in the
    suggested XR report [RFC7002].  If an endpoint needs to report
    packet discard due to other reasons 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].
 o  Reporting run-length encoding of a discarded packet during a
    measurement interval, i.e., between a set of sequence numbers
    [RFC7097].

Singh, et al. Standards Track [Page 3] RFC 7243 RTCP XR Bytes Discarded May 2014

 However, none of these metrics allow a receiver to report precisely
 the number of RTP payload bytes that were discarded.  While this
 information could in theory be derived from high-frequency reporting
 on the number of discarded packets [RFC7002] or from the Discard RLE
 (Run Length Encoding) report [RFC7097], these two mechanisms do not
 appear feasible.  The former would require an unduly high amount of
 reporting that still might not be sufficient due to the non-
 deterministic scheduling of RTCP packets.  The latter incurs
 significant complexity (by storing a map of sequence numbers and
 packet sizes) and reporting overhead.
 An XR block is defined in this document to indicate the number of RTP
 payload bytes discarded, per interval or for the duration of the
 session, similar to the other XR blocks.

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, [RFC2119].
 The terminology defined in RTP [RFC3550] and in the extensions for XR
 reporting [RFC3611] applies.

3. Bytes Discarded Report Block

 The Bytes Discarded Report Block uses the following format, which
 follows the model of the framework for performance metric development
 [RFC6390].
  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=26     | I |E|Reserved |       Block length=2          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        SSRC of source                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             Number of RTP payload bytes discarded             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 1: XR Bytes Discarded Report Block
 Block Type (BT): 8 bits. A Bytes Discarded Packets Report Block is
 identified by the constant 26.
 Interval Metric flag (I): 2 bits. It is used to indicate whether the
 discard metric is an Interval or a Cumulative metric, that is,
 whether the reported value applies to the most recent measurement

Singh, et al. Standards Track [Page 4] RFC 7243 RTCP XR Bytes Discarded May 2014

 interval duration between successive reports (I=10, the Interval
 Duration) or to the accumulation period characteristic of cumulative
 measurements (I=11, the Cumulative Duration).  Since the bytes
 discarded are not measured at a particular time instance but over one
 or several reporting intervals, the metric MUST NOT be reported as a
 Sampled Metric (I=01).  In addition, the value I=00 is reserved and
 MUST NOT be sent, and it MUST be discarded when received.
 Early bit (E): It 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 it reports bytes discarded due
 to early arrival and is set to '0' if it reports bytes discarded due
 to late arrival.  If a duplicate packet is received and discarded,
 these duplicate packets are ignored and not reported.  In case both
 early and late discarded packets shall be reported, two Bytes
 Discarded report blocks MUST be included.
 Reserved: 5 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.
 Block length: 16 bits.  It MUST be set to 2, in accordance with the
 definition of this field in [RFC3611].  The block MUST be discarded
 if the block length is set to a different value.
 Number of RTP payload bytes discarded: It is a 32-bit unsigned
 integer value indicating the total number of bytes discarded.  The
 'bytes discarded' corresponds to the RTP payload size of every RTP
 packet that is discarded (due to early or late arrival).  Hence, the
 'bytes discarded' ignores the size of any RTP header extensions and
 the size of the padding bits.  Also the discarded packet is
 associated to the interval in which it was discarded, not when it was
 expected.
 If the Interval Metric flag is set as I=11, the value in the field
 indicates the number of RTP payload bytes discarded from the start of
 the session; if the Interval Metric flag is set as I=10, it indicates
 the number of bytes discarded in the most recent reporting interval.
 If the XR block follows a Measurement Information Block [RFC6776] in
 the same RTCP compound packet, then the cumulative (I=11) or the
 interval (I=10) for this report block corresponds to the values of
 the "measurement duration" in the Measurement Information Block.
 If the receiver sends the Bytes Discarded Report Block without the
 Measurement Information Block, then the Bytes Discarded Report Block
 MUST be sent in conjunction with an RTCP Receiver Report (RR) as a
 compound RTCP packet.

Singh, et al. Standards Track [Page 5] RFC 7243 RTCP XR Bytes Discarded May 2014

4. Protocol Operation

 This section describes the behavior of the reporting node (i.e., the
 media receiver) and the media sender.

4.1. Reporting Node (Receiver)

 The media receiver MAY send the Bytes Discarded Reports as part of
 the regularly scheduled RTCP packets as per RFC 3550.  It MAY also
 include Bytes Discarded Reports in immediate or early feedback
 packets as per [RFC4585].
 Transmission of the RTCP XR Bytes Discarded Report is up to the
 discretion of the media receiver, as is the reporting granularity.
 However, it is RECOMMENDED that the media receiver signals the bytes
 discarded packets using the method defined in this document.  When
 reporting several metrics in a single RTCP packet, the reporting
 intervals for the report blocks are synchronized, therefore the media
 receiver may choose to additionally send the Discarded Packets
 [RFC7002] or Discard RLE [RFC7097] Report Block to assist the media
 sender in correlating the bytes discarded to the packets discarded in
 that particular interval.
 If all packets over a reporting period were discarded, the media
 receiver MAY use the Discarded Packets Report Block [RFC7002]
 instead.

4.2. Media Sender

 The media sender MUST be prepared to operate without receiving any
 Bytes Discarded reports.  If Bytes Discarded reports 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 any RTCP reports but no Bytes
 Discarded report blocks and is aware that the media receiver supports
 Bytes Discarded report blocks, it MAY assume that no packets were
 discarded by the media receiver.
 The media sender SHOULD accept the Bytes Discarded Report Block only
 if it is received in a compound RTCP receiver report or if it is
 preceded by a Measurement Information Block [RFC6776].  Under all
 other circumstances, it MUST ignore the block.

Singh, et al. Standards Track [Page 6] RFC 7243 RTCP XR Bytes Discarded May 2014

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 [RFC3611]).
 For signaling in SDP, the RTCP XR attribute as defined in [RFC3611]
 MUST be used.  The SDP [RFC4566] attribute 'xr-format' defined in RFC
 3611 is augmented to indicate the Bytes Discarded metric.  This is
 described in the following ABNF [RFC5234]:
 rtcp-xr-attrib = "a=" "rtcp-xr" ":" [xr-format *(SP xr-format)]
                  CRLF   ; defined in [RFC3611]
 xr-format       =/ xr-discard-bytes
 xr-discard-bytes = "discard-bytes"
 The parameter 'discard-bytes' to indicate support for the Bytes
 Discarded Report Block is defined in Section 3.
 When SDP is used in the offer/answer context, the mechanism defined
 in [RFC3611] for unilateral "rtcp-xr" attribute parameters applies
 (see Section 5.2 of [RFC3611]).

6. Security Considerations

 The Bytes Discarded block does not provide per-packet statistics,
 hence the risk to confidentiality documented in Section 7, paragraph
 3 of [RFC3611] does not apply.  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.  For example,
 assume that the attacker sends a packet with a stale timestamp (i.e.,
 time in the past) to the receiver.  If the receiver now sends a
 discard report with the same number of bytes as the payload of the
 injected packet, the attacker can infer that no security processing
 was performed.  If, on the other hand, the attacker does not receive
 a discard report, it can equivalently assume that the security
 procedures were performed on the packet.
 Implementers should therefore consider the guidance in [RFC7202] 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

Singh, et al. Standards Track [Page 7] RFC 7243 RTCP XR Bytes Discarded May 2014

 profile as defined in [RFC3711]; an appropriate combination of the
 two profiles (an "SAVPF") is specified in [RFC5124].  However, other
 mechanisms also exist (documented in [RFC7201]) and might be more
 suitable.
 The Bytes Discarded report is employed by the sender to perform
 congestion control, typically, for calculating goodput (i.e.,
 throughput that is useful).  In these cases, an attacker MAY drive
 the endpoint to lower its sending rate and under-utilize the link;
 therefore, media senders should choose appropriate security measures
 to mitigate such attacks.
 Lastly, the security considerations of [RFC3550], [RFC3611], and
 [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
 [RFC3611].

7.1. XR Block Registration

 This document registers a new value in the IANA "RTP Control Protocol
 Extended Reports (RTCP XR) Block Type Registry": 26 for BDR (Bytes
 Discarded Report).

7.2. SDP Parameter Registration

 This document registers a new parameter for the Session Description
 Protocol (SDP), "discard-bytes" in the "RTP Control Protocol Extended
 Reports (RTCP XR) Session Description Protocol (SDP) Parameters
 Registry".

7.3. Contact Information for IANA Registrations

 RAI Area Directors <rai-ads@tools.ietf.org>

8. Acknowledgments

 The authors would like to thank Benoit Claise, Alan Clark, Roni Even,
 Vijay Gurbani, Sam Hartman, Vinayak Hegde, Jeffrey Hutzelman, Barry
 Leiba, Colin Perkins, Dan Romascanu, Dan Wing, and Qin Wu for
 providing valuable feedback on this document during its development.

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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.
 [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
            Description Protocol", RFC 4566, July 2006.
 [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.
 [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
            Syntax Specifications: ABNF", STD 68, RFC 5234, January
            2008.
 [RFC6390]  Clark, A. and B. Claise, "Guidelines for Considering New
            Performance Metric Development", BCP 170, RFC 6390,
            October 2011.
 [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.
 [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.

9.2. Informative References

 [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.

Singh, et al. Standards Track [Page 9] RFC 7243 RTCP XR Bytes Discarded May 2014

 [RFC5481]  Morton, A. and B. Claise, "Packet Delay Variation
            Applicability Statement", RFC 5481, March 2009.
 [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.
 [RFC7097]  Ott, J., Singh, V., and I. Curcio, "RTP Control Protocol
            (RTCP) Extended Report (XR) for RLE of Discarded Packets",
            RFC 7097, January 2014.
 [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
            Sessions", RFC 7201, April 2014.
 [RFC7202]  Perkins, C. and M. Westerlund, "Securing the RTP
            Framework: Why RTP Does Not Mandate a Single Media
            Security Solution", RFC 7202, April 2014.

Singh, et al. Standards Track [Page 10] RFC 7243 RTCP XR Bytes Discarded May 2014

Appendix A. Metrics Represented Using the Template from RFC 6390

 a.  RTP Payload Bytes Discarded Metric
  • Metric Name: RTP Payload Bytes Discarded Metric
  • Metric Description: Total number of RTP payload bytes

discarded over the period covered by this report.

  • Method of Measurement or Calculation: See the definition of

"Number of RTP payload bytes discarded" in Section 3.

  • Units of Measurement: See the definition of "Number of RTP

payload bytes discarded" in Section 3.

  • Measurement Point(s) with Potential Measurement Domain: See

the first paragraph of Section 3.

  • Measurement Timing: See the last three paragraphs of Section 3

for measurement timing and for the Interval Metric flag.

  • Use and applications: See the third paragraph of Section 1.
  • Reporting model: See RFC 3611.

Singh, et al. Standards Track [Page 11] RFC 7243 RTCP XR Bytes Discarded May 2014

Authors' Addresses

 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/
 Joerg Ott
 Aalto University
 School of Electrical Engineering
 Otakaari 5 A
 Espoo, FIN  02150
 Finland
 EMail: jo@comnet.tkk.fi
 Igor D.D. Curcio
 Nokia Research Center
 P.O. Box 1000 (Visiokatu 3)
 Tampere, FIN  33721
 Finland
 EMail: igor.curcio@nokia.com

Singh, et al. Standards Track [Page 12]

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