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rfc:rfc7244

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.

Asaeda, et al. Standards Track [Page 1] RFC 7244 SDO Report Blocks May 2014

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].

Asaeda, et al. Standards Track [Page 2] RFC 7244 SDO Report Blocks May 2014

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

Asaeda, et al. Standards Track [Page 3] RFC 7244 SDO Report Blocks May 2014

 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.

Asaeda, et al. Standards Track [Page 4] RFC 7244 SDO Report Blocks May 2014

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

Asaeda, et al. Standards Track [Page 5] RFC 7244 SDO Report Blocks May 2014

    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.

Asaeda, et al. Standards Track [Page 6] RFC 7244 SDO Report Blocks May 2014

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.

Asaeda, et al. Standards Track [Page 7] RFC 7244 SDO Report Blocks May 2014

 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.

Asaeda, et al. Standards Track [Page 8] RFC 7244 SDO Report Blocks May 2014

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

Asaeda, et al. Standards Track [Page 9] RFC 7244 SDO Report Blocks May 2014

 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 <rai-ads@tools.ietf.org>

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.

Asaeda, et al. Standards Track [Page 10] RFC 7244 SDO Report Blocks May 2014

 [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.

Asaeda, et al. Standards Track [Page 11] RFC 7244 SDO Report Blocks May 2014

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.

Asaeda, et al. Standards Track [Page 12] RFC 7244 SDO Report Blocks May 2014

Authors' Addresses

 Hitoshi Asaeda
 National Institute of Information and Communications Technology
 4-2-1 Nukui-Kitamachi
 Koganei, Tokyo  184-8795
 Japan
 EMail: asaeda@nict.go.jp
 Qin Wu
 Huawei Technologies Co., Ltd.
 101 Software Avenue, Yuhua District
 Nanjing, Jiangsu  210012
 China
 EMail: bill.wu@huawei.com
 Rachel Huang
 Huawei Technologies Co., Ltd.
 101 Software Avenue, Yuhua District
 Nanjing, Jiangsu  210012
 China
 EMail: Rachel@huawei.com

Asaeda, et al. Standards Track [Page 13]

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