Internet Engineering Task Force (IETF) H. Asaeda

Internet Engineering Task Force (IETF) H. Asaeda
Request for Comments: 7244 NICT
Category: Standards Track Q. Wu
ISSN: 2070-1721 R. Huang
Huawei
May 2014

RTP Control Protocol (RTCP) Extended Report (XR) Blocks
for Synchronization Delay and Offset Metrics Reporting

Abstract

This document defines two RTP Control Protocol (RTCP) Extended Report
(XR) blocks that allow the reporting of initial synchronization delay
and synchronization offset 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 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/rfc7244.

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.

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Table of Contents

1. Introduction ....................................................2
1.1. Synchronization Delay and Offset Metrics Reporting Blocks ..2
1.2. RTCP and RTCP XR Reports ...................................3
1.3. Performance Metrics Framework ..............................3
1.4. Applicability ..............................................3
2. Terminology .....................................................4
2.1. Standards Language .........................................4
3. RTP Flow Initial Synchronization Delay Report Block .............4
3.1. Metric Block Structure .....................................5
3.2. Definition of Fields in RTP Flow Initial
Synchronization Delay Metrics Block ........................5
4. RTP Flow Synchronization Offset Metrics Block ...................6
4.1. Metric Block Structure .....................................7
4.2. Definition of Fields in RTP Flow General
Synchronization Offset Metrics Block .......................7
5. SDP Signaling ...................................................9
5.1. SDP rtcp-xr-attrib Attribute Extension .....................9
5.2. Offer/Answer Usage .........................................9
6. IANA Considerations .............................................9
7. Security Considerations ........................................10
8. Acknowledgements ...............................................10
9. References .....................................................10
9.1. Normative References ......................................10
9.2. Informative References ....................................11
Appendix A. Metrics Represented Using the Template from RFC 6390 ..12

1. Introduction

1.1. Synchronization Delay and Offset Metrics Reporting Blocks

This document defines two new block types to augment those defined in
[RFC3611], for use in a range of RTP applications.

The first new block type supports reporting of the Initial
Synchronization Delay to establish a multimedia session. Information
is recorded about the time difference between the start of RTP
sessions and the time the RTP receiver acquires all components of RTP
sessions in the multimedia session [RFC6051].

The second new block type supports reporting of the relative
synchronization offset time of two arbitrary streams (e.g., between
audio and video streams), with the same RTCP CNAME included in RTCP
Source description items (SDES) packets [RFC3550].

These metrics belong to the class of transport-level metrics defined
in [RFC6792].

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1.2. RTCP and RTCP XR Reports

The use of RTCP for reporting is defined in [RFC3550]. [RFC3611]
defined an extensible structure for reporting -- the RTCP Extended
Report (XR). This document defines a new Extended Report block for
use with [RFC3550] and [RFC3611].

1.3. Performance Metrics Framework

"Guidelines for Considering New Performance Metric Development"
[RFC6390] provides guidance on the definition and specification of
performance metrics. "Guidelines for Use of the RTP Monitoring
Framework" [RFC6792] provides guidance 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

When joining each session in layered video sessions [RFC6190] or the
multimedia session, a receiver may not synchronize playout across the
multimedia session or layered video session until RTCP Sender Report
(SR) packets have been received on all components of RTP sessions.
The components of RTP sessions are per-media-type RTP sessions for
the multimedia sessions or per-layer RTP sessions for the layered
video sessions. For multicast sessions, the Initial Synchronization
Delay metric varies with the session bandwidth, the number of
members, and the number of senders in the session. The RTP Flow
Initial Synchronization Delay Metrics Block defined in this document
can be used to report such a metric, i.e., the Initial
Synchronization Delay to receive all the RTP streams belonging to the
same multimedia session or layered video session. In the absence of
packet loss, the Initial Synchronization Delay is equal to the
average time taken to receive the first RTCP packet in the RTP
session with the longest RTCP reporting interval. In the presence of
packet loss, the media synchronization should rely on the in-band
mapping of RTP and NTP-format timestamps [RFC6051] or wait until the
reporting interval has passed, and the next RTCP SR packet is sent.

Receivers of the RTP Flow Initial Synchronization Delay Metrics Block
could use this metric to compare with targets (i.e., Service Level
Agreement or thresholds of the system) to help ensure the quality of
real-time application performance.

In an RTP multimedia session, there can be an arbitrary number of
streams carried in different RTP sessions, with the same RTCP CNAME.
These streams may be not synchronized with each other. For example,
one audio stream and one video stream belong to the same session, and
the audio stream is transmitted lagging behind the video stream for

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multiple tens of milliseconds [TR-126]. The RTP Flow Synchronization
Offset block can be used to report such synchronization offset
between video and audio streams. This block is also applied to the
case where an RTP session can contain media streams with media from
multiple media types. The metrics defined in the RTP Flow
Synchronization Offset Metrics Block can be used by the network
manager for troubleshooting and dealing with user-experience issues.

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:

Initial Synchronization Delay:

A multimedia session comprises a set of concurrent RTP sessions
among a common group of participants, using one RTP session for
each media type. The Initial Synchronization Delay is the average
time for the receiver to synchronize all components of a
multimedia session [RFC6051].

Synchronization Offset:

Synchronization between two media streams must be maintained to
ensure satisfactory Quality of Experience (QoE). Two media
streams can be of the same or different media types belonging to
one RTP session, or of different media types belonging to one
multimedia session. The Synchronization Offset is the relative
time difference of the two media streams that need to be
synchronized.

3. RTP Flow Initial Synchronization Delay Metrics Block

This block is sent by RTP receivers and reports the Initial
Synchronization Delay beyond the information carried in the standard
RTCP packet format. Information is recorded about the time
difference between the start of the multimedia session and the time
when the RTP receiver acquires all components of RTP sessions
[RFC6051] measured at the receiving end of the RTP stream.

This block needs to be exchanged only occasionally, for example, sent
once at the start of the RTP session.

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3.1. Metric Block Structure

The RTP Flow Initial Synchronization Delay Metrics Block has the
following format:

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=27 | Reserved | Block length=2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of Source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial Synchronization Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: Report Block Structure

3.2. Definition of Fields in RTP Flow Initial Synchronization Delay
Metrics Block

Block type (BT): 8 bits

The RTP Flow Initial Synchronization Delay Metrics Block is
identified by the constant 27.

Reserved: 8 bits

This field is reserved for future definition. In the absence of
such a definition, the bits in this field MUST be set to zero and
ignored by the receiver.

Block length: 16 bits

The constant 2, in accordance with the definition of this field in
Section 3 of RFC 3611 [RFC3611].

SSRC of source: 32 bits

The SSRC of the media source SHALL be set to the value of the SSRC
identifier carried in any arbitrary component of RTP sessions
belonging to the same multimedia session.

Initial Synchronization Delay: 32 bits

The average delay, expressed in units of 1/65536 seconds, from the
beginning of the multimedia session [RFC6051] to the time when
RTCP packets are received on all of the component RTP sessions.
It is recommended that the beginning of the multimedia session is

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chosen as the time when the receiver has joined the first RTP
session of the multimedia session. The value of the Initial
Synchronization Delay is calculated based on received RTCP SR
packets or the RTP header extension containing the in-band mapping
of RTP and NTP-format timestamps [RFC6051]. If there is no packet
loss, the Initial Synchronization Delay is expected to be equal to
the average time taken to receive the first RTCP packet in the RTP
session with the longest RTCP reporting interval or to the average
time taken to receive the first RTP header extension containing
the in-band mapping of RTP and NTP-format timestamps.

If the measurement is unavailable, the value of this field with
all bits set to 1 MUST be reported.

4. RTP Flow Synchronization Offset Metrics Block

In the RTP multimedia sessions or one RTP session, there can be an
arbitrary number of media streams and each media stream (e.g., audio
stream or video stream) is sent in a separate RTP stream. In case of
one RTP session, each media stream or each medium uses a different
SSRC. The receiver correlates these media streams that need to be
synchronized by means of the RTCP CNAME contained in the RTCP Source
Description (SDES) packets [RFC3550].

This block is sent by RTP receivers and reports the synchronization
offset of two arbitrary RTP streams that need to be synchronized in
the RTP multimedia session. Information is recorded about the
relative average time difference between two arbitrary RTP streams
(the reporting stream and the reference stream) with the same CNAME
and measured at the receiving end of the RTP stream. In order to
tell what the offset of the reporting stream is relative to, the
block for the reference stream with synchronization offset of zero
should be reported.

Instances of this block refer by synchronization source (SSRC) to the
separate auxiliary Measurement Information block [RFC6776], which
describes measurement periods in use (see Section 4.2 of [RFC6776]).
This metrics block relies on the measurement period in the
Measurement Information block indicating the span of the report and
SHOULD be sent in the same compound RTCP packet as the Measurement
Information Block. If the measurement period is not received in the
same compound RTCP packet as this block, this block MUST be
discarded.

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4.1. Metric Block Structure

The RTP Flow General Synchronization Offset Metrics Block has the
following format:

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=28 | I | Reserved | Block length=3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Synchronization Offset, most significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Synchronization Offset, least significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: Report Block Structure

4.2. Definition of Fields in RTP Flow General Synchronization Offset
Metrics Block

Block type (BT): 8 bits

The RTP Flow General Synchronization Offset Metrics Block is
identified by the constant 28.

Interval Metric Flag (I): 2 bits

This field is used to indicate whether the Burst/Gap Discard
Summary Statistics metrics are Sampled, Interval, or Cumulative
metrics:

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, the value I=00 is the reserved value and MUST
NOT be used. If the value I=00 is received, then the XR block
MUST be ignored by the receiver.

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Reserved: 6 bits

This field is reserved for future definition. In the absence of
such a definition, the bits in this field MUST be set to zero and
MUST be ignored by the receiver.

Block length: 16 bits

The constant 3, in accordance with the definition of this field in
Section 3 of RFC 3611 [RFC3611].

SSRC of Source: 32 bits

The SSRC of the media source SHALL be set to the value of the SSRC
identifier of the reporting RTP stream to which the XR relates.

Synchronization Offset: 64 bits

The synchronization offset of the reporting RTP stream relative to
the reference stream with the same CNAME. The calculation of
Synchronization Offset is similar to the Difference D calculation
in the RFC 3550. That is to say, if Si is the NTP timestamp from
the reporting RTP packet i, Ri is the time of arrival in NTP
timestamp units for reporting RTP packet i, Sj is the NTP
timestamp from the reference RTP packet j, and Rj is the time of
arrival in NTP timestamp units for reference RTP packet j, then
the value of the Synchronization Offset D may be expressed as

D(i,j) = (Rj - Ri) - (Sj - Si) = (Rj - Sj) - (Ri - Si)

If in-band delivery of NTP-format timestamps is supported
[RFC6051], Si and Sj should be obtained directly from the RTP
packets where NTP timestamps are available. If not, Si and Sj
should be calculated from their corresponding RTP timestamps. The
value of the Synchronization Offset is represented using a 64-bit
signed NTP-format timestamp as defined in [RFC5905], which is a
64-bit signed fixed-point number with the integer part in the
first 32 bits and the fractional part in the last 32 bits. A
positive value of the Synchronization Offset means that the
reporting stream leads before the reference stream, while a
negative one means the reporting stream lags behind the reference
stream. The Synchronization Offset of zero means the stream is
the reference stream.

If the measurement is unavailable, the value of this field with
all bits set to 1 MUST be reported.

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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-attrib Attribute Extension

Using the Augmented Backus-Naur Form (ABNF) [RFC5234], two new
parameters are defined for the two report blocks defined in this
document to be used with SDP [RFC4566]. They have the following
syntax within the "rtcp-xr" attribute [RFC3611]:

xr-format =/ xr-rfisd-block
/ xr-rfso-block

xr-rfisd-block = "rtp-flow-init-syn-delay"
xr-rfso-block = "rtp-flow-syn-offset"

Refer to Section 5.1 of RFC 3611 [RFC3611] for a detailed description
and the full syntax of the "rtcp-xr" attribute.

5.2. Offer/Answer Usage

When SDP is used in the offer/answer context, the SDP Offer/Answer
usage defined in [RFC3611] applies.

6. IANA Considerations

New report block types for RTCP XR are subject to IANA registration.
For general guidelines on IANA allocations for RTCP XR, refer to
Section 6.2 of [RFC3611].

This document assigns two new block type values in the RTCP XR Block
Type Registry:

Name: RFISD
Long Name: RTP Flow Initial Synchronization Delay
Value 27
Reference: Section 3

Name: RFSO
Long Name: RTP Flow Synchronization Offset
Value 28
Reference: Section 4

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This document also registers two new SDP [RFC4566] parameters for the
"rtcp-xr" attribute in the RTCP XR SDP Parameters Registry:

* "rtp-flow-init-syn-delay "
* "rtp-flow-syn-offset"

The contact information for the registrations is:
RAI Area Directors <[email protected]>

7. Security Considerations

When using Secure RTP [RFC3711], or other media-layer security,
reporting accurate synchronization offset information can expose some
details about the timing of the cryptographic operations that are
used to protect the media. There is a possibility that this timing
information might enable a side-channel attack on the encryption. For
environments where this attack is a concern, implementations need to
take care to ensure cryptographic processing and media compression
take the same amount of time irrespective of the media content, to
avoid the potential attack.

Besides this, it is believed that this RTCP XR block introduces no
new security considerations beyond those described in [RFC3611].

8. Acknowledgements

The authors would like to thank Bill Ver Steeg, David R. Oran, Ali
Begen, Colin Perkins, Roni Even, Kevin Gross, Jing Zhao, Fernando
Boronat Segui, Mario Montagud Climent, Youqing Yang, Wenxiao Yu,
Yinliang Hu, Jonathan Lennox, and Stephen Farrel for their valuable
comments and suggestions on 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.

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[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.

[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.

[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.

[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.

[RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
Flows", RFC 6051, November 2010.

[RFC6190] Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis,
"RTP Payload Format for Scalable Video Coding", RFC 6190,
May 2011.

[RFC6776] Wu, Q., "Measurement Identity and information Reporting
using SDES item and XR Block", RFC 6776, August 2012.

9.2. Informative References

[RFC6390] Clark, A. and B. Claise, "Guidelines for Considering New
Performance Metric Development", RFC 6390, October 2011.

[RFC6792] Wu, Q., "Guidelines for Use of the RTP Monitoring
Framework", RFC 6792, November 2012.

[TR-126] Broadband Forum, "Triple-play Services Quality of
Experience (QoE) Requirements", Technical Report TR-126,
December 2006.

[Y.1540] ITU-T, "IP packet transfer and availability performance
parameters", ITU-T Recommendation Y.1540, November 2007.

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Appendix A. Metrics Represented Using the Template from RFC 6390

a. Initial Synchronization Delay Metric

* Metric Name: RTP Initial Synchronization Delay

* Metric Description: See the definition of "Initial
Synchronization Delay" in Section 2.1.

* Method of Measurement or Calculation: See the definition of
the "Initial Synchronization Delay" field in Section 3.2.

* Units of Measurement: See the definition of the "Initial
Synchronization Delay" field in Section 3.2.

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

* Measurement Timing: See the second paragraph of Section 3.

* Use and applications: See Section 1.4.

* Reporting model: See RFC 3611.

b. Synchronization Offset Metric

* Metric Name: RTP Synchronization Offset Delay

* Metric Description: See the definition of "Synchronization
Offset" in Section 1.2.

* Method of Measurement or Calculation: See the definition of
the "Synchronization Offset" field in Section 4.2.

* Units of Measurement: See the definition of the
"Synchronization Offset" field in Section 4.2.

* Measurement Point(s) with Potential Measurement Domain: See
the second paragraph of Section 4.

* Measurement Timing: See the third paragraph of Section 4.2 for
measurement timing and the Interval Metric flag.

* Use and applications: See Section 1.4.

* Reporting model: See RFC 3611.

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Authors' Addresses

Hitoshi Asaeda
National Institute of Information and Communications Technology
4-2-1 Nukui-Kitamachi
Koganei, Tokyo 184-8795
Japan

EMail: [email protected]

Qin Wu
Huawei Technologies Co., Ltd.
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China

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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