Internet Engineering Task Force (IETF) V. Singh

Internet Engineering Task Force (IETF) V. Singh
Request for Comments: 8015 callstats.io
Category: Standards Track C. Perkins
ISSN: 2070-1721 University of Glasgow
A. Clark
Telchemy
R. Huang
Huawei
November 2016

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

Abstract

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

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 7841.

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/rfc8015.

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Copyright Notice

Copyright (c) 2016 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
1.1. Independent Burst/Gap Discard Metrics Block . . . . . . . 3
1.2. RTCP and RTCP Extended Reports . . . . . . . . . . . . . 4
1.3. Performance Metrics Framework . . . . . . . . . . . . . . 4
1.4. Applicability . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Independent Burst/Gap Discard Metrics Block . . . . . . . . . 5
3.1. Report Block Structure . . . . . . . . . . . . . . . . . 6
3.2. Definition of Fields in the Independent Burst/Gap Discard
Metrics Block . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Derived Metrics Based on the Reported Metrics . . . . . . 8
4. Considerations for Voice-over-IP Applications . . . . . . . . 9
5. SDP Signaling . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. SDP rtcp-xr Attribute Extension . . . . . . . . . . . . . 9
5.2. Offer/Answer Usage . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6.1. New RTCP XR Block Type Value . . . . . . . . . . . . . . 10
6.2. New RTCP XR SDP Parameter . . . . . . . . . . . . . . . . 10
6.3. Contact Information for Registrations . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Metrics Represented Using the Template from RFC 6390 13
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 14
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15

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1. Introduction

1.1. Independent Burst/Gap Discard Metrics Block

This document defines a new block type that extends the metrics
defined in [RFC7003]. The new block type reports the proportion of
packets discarded in a burst by the de-jitter buffer at the receiver.
The number of packets discarded depends on the de-jitter 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] depend on the
metrics in the burst/gap loss metric defined in [RFC6958].
Consequently, an endpoint that sends a Burst/Gap Discard Metrics
Block [RFC7003] also needs to send a Burst/Gap Loss Metrics Block
[RFC6958]. 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 could prefer to send a concise report block that just
reports the burst/gap 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
can also be used in interval reports (see the Interval Metric flag in
Section 3.2). The variation in the number of packet discards in a
burst affects the user experience. Based on the metrics reported in
the block, the sending endpoint can change the packetization
interval, vary the bitrate, etc. The report can 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 a range
of de-jitter buffer algorithms and packet discard logic that can be
used by implementers, the method used to distinguish between bursts
and gaps uses an equivalent method to that defined in Section 4.7.2
of [RFC3611]. Note that reporting the specific de-jitter buffer
algorithm and/or the packet discard logic is out of the scope of this
document.

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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 the
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 at the receiver to smooth variation in
packet-arrival time and don't use stream repair means, e.g., Forward
Error Correction (FEC) [FLEX_FEC] 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 is 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 is be recorded 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. Each packet is 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

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receiver. Another metric, defined in [RFC5725], is available to
report on packets that are not recovered by any repair techniques
that are 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 a discard rate over 5 percent), and gaps, which are
periods during which discarded packets are infrequent, and hence
quality is generally acceptable.

3. Independent 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.

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3.1. Report Block Structure

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

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=35 | 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

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

Block Type (BT): 8 bits

An Independent Burst/Gap Discard Metrics Block is identified by
the constant 35.

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.

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.

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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 Independent 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 of [RFC3611].

Threshold: 8 bits

The Threshold is equivalent to Gmin in [RFC3611], i.e., the number
of successive packets that have to be received prior to, and
following, a discarded packet in order for that 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].

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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 the discard 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:

o The average discarded burst size, which can be calculated by
dividing the metric "Packets Discarded in Bursts" by the "Number
of Bursts".

o The average burst duration, which can be calculated by dividing
the metric "Sum of Burst Durations (ms)" by the "Number of
Bursts".

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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 a discard rate over 5 percent) 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/gap duration is
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 can be used
without prior signaling.

5.1. SDP rtcp-xr Attribute Extension

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.

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

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

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].

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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 35 in the IANA "RTP
Control Protocol Extended Reports (RTCP XR) Block Type Registry" to
the "Independent Burst/Gap Discard Metrics 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

The contact information for the registrations is:

ART Area Directors <[email protected]>

7. Security Considerations

This block does not provide per-packet statistics, so the risk to
confidentiality documented in Section 7, paragraph 3 of [RFC3611]
does not apply. However, the gap indicated within this block could
be used to detect the timing of other events on the path between the
sender and receiver. For example, a competing multimedia stream
might cause a discard burst for the duration of the stream, allowing
the receiver of this block to know when the competing stream was
active. This risk is not a significant threat since the only
information leaked is the timing of the discard, not the cause.

Where this is a concern, the implementation SHOULD apply encryption
and authentication to this report block. For example, this can be
achieved by using the Audio-Visual Profile with Feedback (AVPF)
profile together with the Secure RTP profile, as defined in
[RFC3711]; an appropriate combination of those two profiles ("SAVPF")
is specified in [RFC5124]. Besides this, it is believed that this
RTCP XR block introduces no new security considerations beyond those
described in [RFC3611].

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8. References

8.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.

[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, <http://www.rfc-editor.org/info/rfc3550>.

[RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
"RTP Control Protocol Extended Reports (RTCP XR)",
RFC 3611, DOI 10.17487/RFC3611, November 2003,
<http://www.rfc-editor.org/info/rfc3611>.

[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<http://www.rfc-editor.org/info/rfc3711>.

[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.

[RFC5124] 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/RFC5124, February
2008, <http://www.rfc-editor.org/info/rfc5124>.

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.

[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, <http://www.rfc-editor.org/info/rfc5725>.

[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,
<http://www.rfc-editor.org/info/rfc6776>.

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[RFC7003] Clark, A., Huang, R., and Q. Wu, Ed., "RTP Control
Protocol (RTCP) Extended Report (XR) Block for Burst/Gap
Discard Metric Reporting", RFC 7003, DOI 10.17487/RFC7003,
September 2013, <http://www.rfc-editor.org/info/rfc7003>.

8.2. Informative References

[FLEX_FEC]
Singh, V., Begen, A., Zanaty, M., and G. Mandyam, "RTP
Payload Format for Flexible Forward Error Correction
(FEC)", Work in Progress, draft-ietf-payload-flexible-fec-
scheme-03, October 2016.

[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
DOI 10.17487/RFC4588, July 2006,
<http://www.rfc-editor.org/info/rfc4588>.

[RFC6390] Clark, A. and B. Claise, "Guidelines for Considering New
Performance Metric Development", BCP 170, RFC 6390,
DOI 10.17487/RFC6390, October 2011,
<http://www.rfc-editor.org/info/rfc6390>.

[RFC6709] Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709,
DOI 10.17487/RFC6709, September 2012,
<http://www.rfc-editor.org/info/rfc6709>.

[RFC6792] Wu, Q., Ed., Hunt, G., and P. Arden, "Guidelines for Use
of the RTP Monitoring Framework", RFC 6792,
DOI 10.17487/RFC6792, November 2012,
<http://www.rfc-editor.org/info/rfc6792>.

[RFC6958] Clark, A., Zhang, S., Zhao, J., and Q. Wu, Ed., "RTP
Control Protocol (RTCP) Extended Report (XR) Block for
Burst/Gap Loss Metric Reporting", RFC 6958,
DOI 10.17487/RFC6958, May 2013,
<http://www.rfc-editor.org/info/rfc6958>.

[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, <http://www.rfc-editor.org/info/rfc7002>.

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Appendix A. Metrics Represented Using the Template from RFC 6390

a. Threshold Metric

* Defined in item a of Appendix A of [RFC7003].

b. Sum of Burst Durations (ms)

* Metric Name: Sum of Burst Durations with Discarded RTP
Packets.

* Metric Description: The total duration of bursts of discarded
RTP packets in the period of the report.

* Method of Measurement or Calculation: See Section 3.2, Sum of
Burst Durations definition.

* Units of Measurement: See Section 3.2, Sum of Burst Durations
definition.

* Measurement Point(s) with Potential Measurement Domain: See
Section 3, first paragraph.

* Measurement Timing: See Section 3, second paragraph for
measurement timing and Section 3.2 for Interval Metric flag.

* Use and Applications: See Section 1.4.

* Reporting Model: See [RFC3611].

c. Packets Discarded in Bursts Metric

* Defined in item b of Appendix A of [RFC7003].

d. Number of Bursts

* Metric Name: Number of discard bursts in RTP.

* Metric Description: The total number of bursts with discarded
RTP packets in the period of the report.

* Method of Measurement or Calculation: See Section 3.2, Number
of Bursts definition.

* Units of Measurement: See Section 3.2 for the Number of Bursts
definition.

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* Measurement Point(s) with Potential Measurement Domain: See
Section 3, first paragraph.

* Measurement Timing: See Section 3, second paragraph for
measurement timing and Section 3.2 for Interval Metric flag.

* Use and Applications: See Section 1.4.

* Reporting Model: See [RFC3611].

e. Total Packets Expected in Bursts Metric

* Defined in item c of Appendix A of [RFC7003].

f. Discard Count

* Defined in Appendix A of [RFC7002].

Acknowledgments

The authors would like to thank Ben Campbell, Stephen Farrell, Paul
Kyzivat, Shucheng LIU, Jan Novak, and Dan Romascanu for providing
valuable feedback on this document.

Contributors

Qin Wu, Rachel Huang, and Alan Clark wrote RFC 7003, which this
document extends.

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Authors' Addresses

Varun Singh
CALLSTATS I/O Oy
Runeberginkatu 4c A 4
Helsinki 00100
Finland

Email: [email protected]
URI: https://www.callstats.io/about

Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
United Kingdom

Email: [email protected]

Alan Clark
Telchemy Incorporated
2905 Premiere Parkway, Suite 280
Duluth, GA 30097
United States of America

Email: [email protected]

Rachel Huang
Huawei Technologies Co., Ltd.
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China

Email: [email protected]

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