RTCP XR Report Block for Burst/Gap Discard metric Reporting
draft-ietf-xrblock-rtcp-xr-burst-gap-discard-05.txt

Abstract

This document defines an RTCP XR Report Block that allows the reporting of Burst and Gap Discard metrics for use in a range of RTP applications.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on January 29, 2013.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Burst and Gap Discard Report Block
• 1.2. RTCP and RTCP XR Reports
• 1.3. Performance Metrics Framework
• 1.4. Applicability
• 2. Terminology
• 2.1. Standards Language
• 3. Burst/Gap Discard Block
• 3.1. Report Block Structure
• 3.2. Definition of Fields in Burst/Gap Discard Report Block
• 3.3. Derived metrics based on reported metrics
• 4. Considerations for Voice-over-IP applications
• 5. SDP Signaling
• 6. IANA Considerations
• 6.1. New RTCP XR Block Type value
• 6.2. New RTCP XR SDP Parameter
• 6.3. Contact information for registrations
• 7. Security Considerations
• 8. Acknowledgments
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Authors' Addresses

1. Introduction

1.1. Burst and Gap Discard 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 proportion of packets discarded by the receiver due to jitter. The discards during discard bursts are reported, together with the number of bursts. This block is intended to be used in conjunction with [DISCARD] which provides the total packets discarded, and on which this block therefore depends. However the metric in [DISCARD] may be used independently of the metrics in this block.

This block provides information on transient IP problems. Burst/Gap metrics are typically used in Cumulative reports however MAY be used in Interval reports. The burstiness of packet discard affects user experience, may influence any sender strategies to mitigate the problem, and may also have diagnostic value.

The metric belongs to the class of transport-related terminal metrics defined in [MONARCH].

The definitions of Burst, Gap, Loss and Discard are consistent with 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 may be an equivalent method to that defined in[RFC3611].

1.2. RTCP and RTCP XR 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. The use of Extended Report blocks is defined by [RFC3611].

1.3. Performance Metrics Framework

The Performance Metrics Framework [RFC6390] provides guidance on the definition and specification of performance metrics. Metrics described in this draft either reference external definitions or define metrics generally in accordance with the guidelines in [RFC6390].

1.4. Applicability

These metrics are applicable to a range of RTP applications which don't use stream repair means.

2. Terminology

2.1. Standards Language

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 or late to be played out or thrown away before playout due to packet duplication or redundancy shall be regarded as discarded. 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 SSRC within single RTP session). Similarly the metric "number of packets discarded" defined in [DISCARD] reports a count of packets discarded from the media stream (single SSRC within single RTP session) arriving at the receiver. Another metric defined in [RFC5725] is available to report on packets which are not recovered by any repair techniques which may be in use.

Bursts and Gaps

The terms Burst and Gap are used in a manner consistent with that of RTCP XR [RFC3611]. RTCP XR views a 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 Block

Metrics in this block report on Burst/Gap Discard in the stream arriving at the RTP system.

3.1. Report Block Structure

       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=NBGD    | I |   resv.   |      block length = 3         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        SSRC of Source                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Threshold   |         Packets Discarded in Bursts           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       Total Packets expected in bursts        |   Reserved.   | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Burst/Gap Discard metrics block

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

Block type (BT): 8 bits

A Burst/Gap Discard Report Block is identified by the constant NBGD.

[Note to RFC Editor: please replace NBGD 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, that is, whether the reported values applies to the most recent measurement interval duration between successive metrics reports (I=10) (the Interval Duration) or to the accumulation period characteristic of cumulative measurements (I=11) (the Cumulative Duration) or is a sampled instantaneous value (I=01) (Sampled Value). In this document, Burst/Gap Discard Metric is not measured at a particular time instant but over one or several reporting intervals. Therefore Burst/Gap Discard Metric MUST NOT be chosen as Sampled Metric.

Reserved (resv): 6 bits

These bits are reserved. They SHOULD be set to zero by senders and MUST be ignored by receivers.

block length: 16 bits

The length of this report block in 32-bit words, minus one. For the Burst/Gap discard block, the block length is equal to 3.

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.

Packets discarded in bursts: 24 bits

The total number of packets discarded during discard bursts.

If the measured value exceeds 0xFFFFFD, the value 0xFFFFFE SHOULD be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFFFF SHOULD 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).

If the measured value exceeds 0xFFFFFD, the value 0xFFFFFE SHOULD be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFFFF SHOULD be reported.

Reserved (resv): 8 bits

These bits are reserved. They SHOULD be set to zero by senders and MUST be ignored by receivers.

3.3. Derived metrics based on reported metrics

The metrics described here are intended to be used in conjunction with information from the Measurement Information block [MEASID], discard block [DISCARD] (which MUST be present in the same RTCP packet as the Burst/Gap Discard block).

These metrics provides the following information relevant to statistical parameters, including:

• The fraction of packets discarded during bursts (burst discard rate in [SUMSTAT])
• The fraction of packets discarded during gaps (gap discard rate in [SUMSTAT])

The details on calculation these parameters in the metrics are described in [SUMSTAT].

4. Considerations for Voice-over-IP applications

This metric block is applicable to a broad range of RTP applications. Where the metric is used with a Voice-overIP (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 MUST 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 PCM Severely Errored Second [PSES].

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.

rtcp-xr-attrib = "a=" "rtcp-xr" ":" [xr-format *(SP xr-format)] CRLF

(defined in [RFC3611])

xr-format =/ xr-bgd-block

xr-bgd-block = "brst-gap-dscrd"

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.

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 NBGD in the IANA "RTCP XR Block Type Registry" to the "Burst/Gap Discard Metrics Block".

[Note to RFC Editor: please replace NBGD 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 "brst-gap-dscrd" in the "RTCP XR SDP Parameters Registry".

6.3. Contact information for registrations

The contact information for the registrations is:

   Qin Wu (sunseawq@huawei.com)

   101 Software Avenue, Yuhua District
   Nanjing, Jiangsu  210012
   China

7. Security Considerations

It is believed that this proposed RTCP XR report 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. Acknowledgments

The authors gratefully acknowledge the comments and contributions made by Bruce Adams, Philip Arden, Amit Arora, Bob Biskner, Kevin Connor, Claus Dahm, Randy Ethier, Roni Even, Jim Frauenthal, Albert Higashi, Tom Hock, Shane Holthaus, Paul Jones, Rajesh Kumar, Keith Lantz, Mohamed Mostafa, Amy Pendleton, Colin Perkins, Mike Ramalho, Ravi Raviraj, Albrecht Schwarz, Tom Taylor, and Hideaki Yamada.

9. References

9.1. Normative References

9.2. Informative References

Authors' Addresses