Audio/Video Transport Working Group V. Singh, Ed.
Intended status: Standards Track C. Perkins
Expires: April 21, 2016 University of Glasgow
A. Clark
R. Huang
October 19, 2015

RTP Control Protocol (RTCP) Extended Report (XR) Block for Independent Reporting of Burst/Gap Discard Metric


This document defines an RTP Control Protocol (RTCP) Extended Report (XR) block that allows the reporting of burst and gap discard metrics independently of the burst and gap loss metrics for use in a range of RTP applications.

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Table of Contents

1. Introduction

1.1. Burst-Gap Discard Metrics Block

This document defines a new block type that extends the metrics defined in [RFC7003], the metrics in this report block can be used in a range of RTP applications. The new block type reports the proportion of packets discarded by the jitter buffer at the receiver in a burst and number of packets discarded depends on the dejitter buffer algorithm implemented by the endpoint.

The new report block defined in this document is different from the one defined in [RFC7003]. The metrics in [RFC7003] depends on the metrics in the burst⁠gap loss metric defined in [RFC6958]. Consequently, an endpoint using [RFC7003] MUST report it along with [RFC6958] for it to be useful. The combined usage is useful when an endpoint observes correlated packet losses and discard. However, when the burst of packet losses and discards do not occur simultaneously, the application may prefer a concise report block that just reports the burst⁠gap of discarded packets. The report block in this document provides the complete information and does not require additional report blocks. That is, this block reports: the total number of packets discarded, the total burst duration, and the total number of bursts, all of these metrics are missing in [RFC7003].

This block provides information on transient network issues. Burst/gap metrics are typically used in cumulative reports; however, they may also be used in interval reports (see the Interval Metric flag in Section 3.2). The variation in the number of packet discard in a burst affects the user experience. Based on the metrics reported in the block, the sending endpoint may change the packetisation interval, vary the bitrate, etc. The report may additionally be used for diagnostics [RFC6792]. The metric belongs to the class of transport-related end-system metrics defined in [RFC6792].

The definitions of "burst", "gap", "loss", and "discard" are consistent with the definitions in [RFC3611]. To accommodate the range of jitter buffer algorithms and packet discard logic that may be used by implementors, the method used to distinguish between bursts and gaps shall use an equivalent method to that defined in Section 4.7.2 of [RFC3611]. Note that reporting the specific jitter buffer algorithms and/or packet discard logic is out of the scope of this document.

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

The Performance Metrics Framework [RFC6390] provides guidance on the definition and specification of performance metrics. The RTP Monitoring Framework [RFC6792] provides guidelines 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

These metrics are applicable to a range of RTP applications that contain de-jitter buffers [RFC5481] at the receiving end to smooth variation in packet-arrival time and don't use stream repair means, e.g., Forward Error Correction (FEC) [I-D.ietf-payload-flexible-fec-scheme] and/or retransmission [RFC4588].

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 too early to be played out, too late to be played out, or 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 received (or OK), discarded, or lost. 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" defined in [RFC7002] 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. Note that the term "discard" defined here builds on the "discard" definition in [RFC3611] but extends the concept to take into account packet duplication and reports different types of discard counts [RFC7002].
Bursts and Gaps

The terms "burst" and "gap" are used in a manner consistent with that of RTCP XR [RFC3611]. RTCP XR views an RTP stream as being divided into bursts, which are periods during which the discard rate is high enough to cause noticeable quality degradation (generally over 5 percent discard rate), and gaps, which are periods during which discarded packets are infrequent and hence quality is generally acceptable.

3. Burst/Gap Discard Metrics Block

Metrics in this block report on burst/gap discard in the stream arriving at the RTP system. Measurements of these metrics are made at the receiving end of the RTP stream. Instances of this metrics block use the synchronization source (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 period in the Measurement Information Block indicating the span of the report. Senders MUST send this block in the same compound RTCP packet as the Measurement Information Block. Receivers MUST verify that the measurement period is received in the same compound RTCP packet as this metrics block. If not, this metrics block MUST be discarded.

3.1. Report Block Structure

     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=IBGD    | I |   resv    |      Block Length = 5         |
    |                        SSRC of Source                         |
    |   Threshold   |         Sum of Burst Durations (ms)           |
    |          Packets Discarded in Bursts          |    Number of  |
    |    Bursts     |           Total Packets Expected in Bursts    |
    |                        Discard Count                          |

Figure 1: Report Block Structure

The structure of the Burst/Gap Discard Metrics Block is as follows.

3.2. Definition of Fields in Burst/Gap Discard Metrics Block

Block Type (BT): 8 bits

A Burst/Gap Discard Metrics Block is identified by the constant IBGD.
[Note to RFC Editor: Please replace IBGD with the IANA provided RTCP XR block type for this block.]
Interval Metric flag (I): 2 bits

This field is used to indicate whether the burst/gap discard metrics are Sampled, Interval, or Cumulative metrics [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, burst/gap discard metrics can only be measured over definite intervals and cannot be sampled. Also, the value I=00 is reserved for future use. Senders MUST NOT use the values I=00 or I=01. If a block is received with I=00 or I=01, the receiver MUST discard the block.

Reserved (resv): 6 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. For the Burst/Gap Discard Metrics Block, the block length is equal to 5. 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].
Threshold: 8 bits

The Threshold is equivalent to Gmin in [RFC3611], i.e., the number of successive packets that must not be discarded prior to and following a discard packet in order for this discarded packet to be regarded as part of a gap. Note that the Threshold is set in accordance with the Gmin calculation defined in Section 4.7.2 of [RFC3611].
Sum of Burst Durations (ms): 24 bits

The total duration of bursts of discarded packets in the period of the report (Interval or Cumulative).

The measured value is an unsigned value. If the measured value exceeds 0xFFFFFD, the value 0xFFFFFE MUST be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFFFF MUST be reported.
Packets Discarded in Bursts: 24 bits

The total number of packets discarded during discard bursts, as defined in Section 3.2 of [RFC7002].
Number of Bursts: 16 bits

The number of discard bursts in the period of the report (Interval or Cumulative).

The measured value is an unsigned value. If the measured value exceeds 0xFFFD, the value 0xFFFE MUST be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFF MUST be reported.
Total Packets Expected in Bursts: 24 bits

The total number of packets expected during discarded bursts (that is, the sum of received packets and lost packets). The metric is defined in [RFC7003].
Discard Count: 32 bits

Number of packets discarded over the period (Interval or Cumulative) covered by this report, as defined in Section 3.2 of [RFC7002].

3.3. Derived Metrics Based on the Reported Metrics

The metrics described here are intended to be used in conjunction with information from the Measurement Information Block [RFC6776].

These metrics provide the following information relevant to statistical parameters (depending on cumulative of interval measures), for example:

4. Considerations for Voice-over-IP Applications

This metrics block is applicable to a broad range of RTP applications. Where the metric is used with a Voice-over-IP (VoIP) application and the stream repair means is not available, the following considerations apply.

RTCP XR views a call as being divided into bursts, which are periods during which the discard rate is high enough to cause noticeable call quality degradation (generally over 5 percent discard rate) and gaps, which are periods during which discarded packets are infrequent and hence call quality is generally acceptable.

If voice activity detection is used, the burst and gap duration shall be determined as if silence packets had been sent, i.e., a period of silence in excess of Gmin packets will terminate a burst condition.

The recommended value for the threshold Gmin in [RFC3611] results in a burst being a period of time during which the call quality is degraded to a similar extent to a typical pulse code modulation (PCM) severely errored second.

5. SDP Signaling

[RFC3611] defines the use of SDP (Session Description Protocol) [RFC4566] for signaling the use of XR blocks. XR blocks MAY be used without prior signaling.

5.1. SDP rtcp-xr Attribute Extension

xr-format =/ xr-ind-bgd-block

xr-ind-bgd-block = "ind-burst-gap-discard"

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.

5.2. 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 in Offer/Answer for unilateral parameters, refer to Section 5.2 of [RFC3611].

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 [RFC3611].

6.1. New RTCP XR Block Type Value

This document assigns the block type value IBGD in the IANA "RTP Control Protocol Extended Reports (RTCP XR) Block Type Registry" to the "Burst/Gap Discard Metrics Block".

[Note to RFC Editor: Please replace IBGD with the IANA provided RTCP XR block type for this block.]

6.2. New RTCP XR SDP Parameter

This document also registers a new parameter "ind-burst-gap-discard" in the "RTP Control Protocol Extended Reports (RTCP XR) Session Description Protocol (SDP) Parameters Registry".

6.3. Contact Information for Registrations

   RAI Area Directors <>

The contact information for the registrations is:

7. Security Considerations

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.

8. Contributors

Qin Wu, Rachel Huang, and Alan Clark wrote RFC7003, which this document extends.

9. Acknowledgments

The authors acknowledge the reviews and feedback provided by various people.

10. References

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, July 2003.
[RFC3611] Friedman, T., Caceres, R. and A. Clark, "RTP Control Protocol Extended Reports (RTCP XR)", RFC 3611, DOI 10.17487/RFC3611, November 2003.
[RFC4566] Handley, M., Jacobson, V. and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[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, DOI 10.17487/RFC5725, February 2010.
[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, DOI 10.17487/RFC6776, October 2012.

10.2. Informative References

[I-D.ietf-payload-flexible-fec-scheme] Singh, V., Begen, A., Zanaty, M. and G. Mandyam, "RTP Payload Format for Flexible Forward Error Correction (FEC)", Internet-Draft draft-ietf-payload-flexible-fec-scheme-01, October 2015.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V. and R. Hakenberg, "RTP Retransmission Payload Format", RFC 4588, DOI 10.17487/RFC4588, July 2006.
[RFC5481] Morton, A. and B. Claise, "Packet Delay Variation Applicability Statement", RFC 5481, DOI 10.17487/RFC5481, March 2009.
[RFC6390] Clark, A. and B. Claise, "Guidelines for Considering New Performance Metric Development", BCP 170, RFC 6390, DOI 10.17487/RFC6390, October 2011.
[RFC6709] Carpenter, B., Aboba, B. and S. Cheshire, "Design Considerations for Protocol Extensions", RFC 6709, DOI 10.17487/RFC6709, September 2012.
[RFC6792] Wu, Q., Hunt, G. and P. Arden, "Guidelines for Use of the RTP Monitoring Framework", RFC 6792, DOI 10.17487/RFC6792, November 2012.
[RFC6958] Clark, A., Zhang, S., Zhao, J. and Q. Wu, "RTP Control Protocol (RTCP) Extended Report (XR) Block for Burst/Gap Loss Metric Reporting", RFC 6958, DOI 10.17487/RFC6958, May 2013.
[RFC7002] 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.
[RFC7003] Clark, A., Huang, R. and Q. Wu, "RTP Control Protocol (RTCP) Extended Report (XR) Block for Burst/Gap Discard Metric Reporting", RFC 7003, DOI 10.17487/RFC7003, September 2013.

Appendix A. Metrics Represented Using the Template from RFC 6390

  1. Threshold Metric
  2. Sum of burst durations (ms)
  3. Packets Discarded in Bursts Metric
  4. Number of bursts
  5. Total Packets Expected in Bursts Metric
  6. Discard Count

Authors' Addresses

Varun Singh (editor) Nemu Dialogue Systems Oy Runeberginkatu 4c A 4 Helsinki, 00100 Finland EMail: URI:
Colin Perkins University of Glasgow School of Computing Science Glasgow, G12 8QQ United Kingdom EMail:
Alan Clark Telchemy Incorporated 2905 Premiere Parkway, Suite 280 Duluth, GA 30097 USA EMail:
Rachel Huang Huawei Technologies Co., Ltd. 101 Software Avenue, Yuhua District Nanjing, Jiangsu 210012 China EMail: