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
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
用于同步延迟和偏移度量报告的RTP控制协议（RTCP）扩展报告（XR）块
Abstract
摘要
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.
本文档定义了两个RTP控制协议（RTCP）扩展报告（XR）块，允许在一系列RTP应用程序中报告初始同步延迟和同步偏移度量。
Status of This Memo
关于下段备忘
This is an Internet Standards Track document.
这是一份互联网标准跟踪文件。
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.
本文件是互联网工程任务组（IETF）的产品。它代表了IETF社区的共识。它已经接受了公众审查，并已被互联网工程指导小组（IESG）批准出版。有关互联网标准的更多信息，请参见RFC 5741第2节。
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.
有关本文件当前状态、任何勘误表以及如何提供反馈的信息，请访问http://www.rfc-editor.org/info/rfc7244.
Copyright Notice
版权公告
Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.
版权所有（c）2014 IETF信托基金和确定为文件作者的人员。版权所有。
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.
本文件受BCP 78和IETF信托有关IETF文件的法律规定的约束(http://trustee.ietf.org/license-info)自本文件出版之日起生效。请仔细阅读这些文件，因为它们描述了您对本文件的权利和限制。从本文件中提取的代码组件必须包括信托法律条款第4.e节中所述的简化BSD许可证文本，并提供简化BSD许可证中所述的无担保。
Table of Contents
目录
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 ....................................................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
This document defines two new block types to augment those defined in [RFC3611], for use in a range of RTP applications.
本文件定义了两种新的块类型，以扩充[RFC3611]中定义的块类型，用于一系列RTP应用。
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].
第一种新的块类型支持报告初始同步延迟以建立多媒体会话。记录RTP会话开始与RTP接收器在多媒体会话中获取RTP会话的所有组件之间的时间差的信息[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].
第二种新的块类型支持报告两个任意流（例如，音频和视频流之间）的相对同步偏移时间，RTCP源描述项（SDE）数据包中包含相同的RTCP CNAME[RFC3550]。
These metrics belong to the class of transport-level metrics defined in [RFC6792].
这些指标属于[RFC6792]中定义的传输级指标类别。
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].
[RFC3550]中定义了使用RTCP进行报告。[RFC3611]为报告定义了一个可扩展结构——RTCP扩展报告（XR）。本文档定义了一个新的扩展报告块，用于[RFC3550]和[RFC3611]。
"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].
“考虑制定新绩效指标的指南”[RFC6390]提供了关于绩效指标定义和规范的指南。“RTP监控框架使用指南”[RFC6792]提供了使用RTCP XR报告块格式的指南。本文件中描述的度量块符合[RFC6390]和[RFC6792]中的指南。
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.
当在分层视频会话[RFC6190]或多媒体会话中加入每个会话时，在RTP会话的所有组件上接收到RTCP发送方报告（SR）数据包之前，接收器可能不会在多媒体会话或分层视频会话中同步播放。RTP会话的组件是用于多媒体会话的每媒体类型RTP会话或用于分层视频会话的每层RTP会话。对于多播会话，初始同步延迟度量随会话带宽、成员数和会话中的发送方数而变化。本文档中定义的RTP流初始同步延迟度量块可用于报告此类度量，即接收属于同一多媒体会话或分层视频会话的所有RTP流的初始同步延迟。在没有数据包丢失的情况下，初始同步延迟等于在RTP会话中以最长RTCP报告间隔接收第一个RTCP数据包所用的平均时间。在存在数据包丢失的情况下，媒体同步应依赖RTP和NTP格式时间戳[RFC6051]的带内映射，或等待报告间隔过去，然后发送下一个RTCP SR数据包。
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.
RTP流初始同步延迟度量块的接收器可以使用该度量与目标（即，系统的服务级别协议或阈值）进行比较，以帮助确保实时应用程序性能的质量。
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
在RTP多媒体会话中，不同RTP会话中可以携带任意数量的流，并且具有相同的RTCP CNAME。这些流可能彼此不同步。例如，一个音频流和一个视频流属于同一会话，并且音频流的传输滞后于视频流，以便进行会话
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.
数十毫秒[TR-126]。RTP流同步偏移块可用于报告视频和音频流之间的这种同步偏移。此块还适用于RTP会话可以包含具有多种媒体类型的媒体的媒体流的情况。网络管理器可以使用RTP流同步偏移度量块中定义的度量进行故障排除和处理用户体验问题。
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].
本文件中的关键词“必须”、“不得”、“要求”、“应”、“不应”、“应”、“不应”、“建议”、“可”和“可选”应按照RFC 2119[RFC2119]中所述进行解释。
In addition, the following terms are defined:
此外，定义了以下术语：
Initial Synchronization Delay:
初始同步延迟：
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].
多媒体会话包括一组公共参与者之间的并发RTP会话，每个媒体类型使用一个RTP会话。初始同步延迟是接收器同步多媒体会话所有组件的平均时间[RFC6051]。
Synchronization Offset:
同步偏移量：
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.
必须保持两个媒体流之间的同步，以确保令人满意的体验质量（QoE）。两个媒体流可以是属于一个RTP会话的相同或不同媒体类型，也可以是属于一个多媒体会话的不同媒体类型。同步偏移量是需要同步的两个媒体流的相对时间差。
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.
此块由RTP接收器发送，并报告超出标准RTCP数据包格式所载信息的初始同步延迟。记录关于多媒体会话的开始和RTP接收器获取RTP会话的所有组件[RFC6051]的时间之间的时间差的信息，该时间差在RTP流的接收端测量。
This block needs to be exchanged only occasionally, for example, sent once at the start of the RTP session.
该块只需要偶尔交换，例如，在RTP会话开始时发送一次。
The RTP Flow Initial Synchronization Delay Metrics Block has the following format:
RTP流初始同步延迟度量块具有以下格式：
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
图1：报告块结构
3.2. Definition of Fields in RTP Flow Initial Synchronization Delay Metrics Block
3.2. RTP流初始同步延迟度量块中字段的定义
Block type (BT): 8 bits
块类型（BT）：8位
The RTP Flow Initial Synchronization Delay Metrics Block is identified by the constant 27.
RTP流初始同步延迟度量块由常数27标识。
Reserved: 8 bits
保留：8位
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.
此字段保留供将来定义。在没有这种定义的情况下，此字段中的位必须设置为零，并被接收器忽略。
Block length: 16 bits
块长度：16位
The constant 2, in accordance with the definition of this field in Section 3 of RFC 3611 [RFC3611].
常数2，符合RFC 3611[RFC3611]第3节中该字段的定义。
SSRC of source: 32 bits
源的SSRC：32位
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.
媒体源的SSRC应设置为属于同一多媒体会话的RTP会话的任意组件中携带的SSRC标识符的值。
Initial Synchronization Delay: 32 bits
初始同步延迟：32位
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
从多媒体会话[RFC6051]开始到在所有组件RTP会话上接收RTCP数据包的时间的平均延迟，以1/65536秒为单位表示。建议在多媒体会话开始时
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.
选择作为接收器加入多媒体会话的第一个RTP会话的时间。初始同步延迟的值根据接收到的RTCP SR数据包或包含RTP和NTP格式时间戳带内映射的RTP报头扩展来计算[RFC6051]。如果没有数据包丢失，则初始同步延迟预计等于在RTP会话中以最长RTCP报告间隔接收第一个RTCP数据包所用的平均时间，或等于接收包含RTP和NTP格式时间戳带内映射的第一个RTP报头扩展所用的平均时间。
If the measurement is unavailable, the value of this field with all bits set to 1 MUST be reported.
如果测量不可用，则必须报告所有位设置为1的该字段值。
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].
在RTP多媒体会话或一个RTP会话中，可以存在任意数量的媒体流，并且每个媒体流（例如，音频流或视频流）在单独的RTP流中发送。在一个RTP会话的情况下，每个媒体流或每个媒体使用不同的SSRC。接收器通过RTCP源描述（SDES）数据包[RFC3550]中包含的RTCP CNAME将这些需要同步的媒体流关联起来。
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.
此块由RTP接收器发送，并报告需要在RTP多媒体会话中同步的两个任意RTP流的同步偏移量。记录关于具有相同CNAME的两个任意RTP流（报告流和参考流）之间的相对平均时间差的信息，并在RTP流的接收端测量。为了说明报告流的偏移量相对于什么，应该报告同步偏移量为零的参考流的块。
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.
该块的实例通过同步源（SSRC）指向单独的辅助测量信息块[RFC6776]，该信息块描述了使用中的测量周期（见[RFC6776]第4.2节）。此度量块依赖于度量信息块中指示报告范围的度量周期，并且应在与度量信息块相同的复合RTCP数据包中发送。如果在与此块相同的复合RTCP数据包中未接收到测量周期，则必须丢弃此块。
The RTP Flow General Synchronization Offset Metrics Block has the following format:
RTP流常规同步偏移度量块具有以下格式：
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
图2：报告块结构
4.2. Definition of Fields in RTP Flow General Synchronization Offset Metrics Block
4.2. RTP流通用同步偏移度量块中字段的定义
Block type (BT): 8 bits
块类型（BT）：8位
The RTP Flow General Synchronization Offset Metrics Block is identified by the constant 28.
RTP流通用同步偏移度量块由常量28标识。
Interval Metric Flag (I): 2 bits
间隔度量标志（I）：2位
This field is used to indicate whether the Burst/Gap Discard Summary Statistics metrics are Sampled, Interval, or Cumulative metrics:
此字段用于指示突发/间隔丢弃汇总统计指标是采样的、间隔的还是累积的：
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.
I=10：间隔持续时间-报告值适用于连续度量报告之间的最新度量间隔持续时间。I=11：累积持续时间-报告值适用于累积测量的累积周期特征。I=01：采样值-报告值为采样瞬时值。
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.
在本文件中，值I=00为保留值，不得使用。如果接收到值I=00，则接收器必须忽略XR块。
Reserved: 6 bits
保留：6位
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.
此字段保留供将来定义。在没有这种定义的情况下，此字段中的位必须设置为零，并且必须被接收器忽略。
Block length: 16 bits
块长度：16位
The constant 3, in accordance with the definition of this field in Section 3 of RFC 3611 [RFC3611].
常数3，符合RFC 3611[RFC3611]第3节中该字段的定义。
SSRC of Source: 32 bits
源的SSRC：32位
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.
媒体源的SSRC应设置为XR相关的报告RTP流的SSRC标识符的值。
Synchronization Offset: 64 bits
同步偏移量：64位
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
报告RTP流相对于具有相同CNAME的参考流的同步偏移量。同步偏移量的计算类似于RFC 3550中的差分D计算。也就是说，如果Si是来自报告RTP分组i的NTP时间戳，Ri是以用于报告RTP分组i的NTP时间戳单位表示的到达时间，Sj是来自参考RTP分组j的NTP时间戳，Rj是以用于参考RTP分组j的NTP时间戳单位表示的到达时间，然后，同步偏移量D的值可以表示为
D(i,j) = (Rj - Ri) - (Sj - Si) = (Rj - Sj) - (Ri - Si)
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.
如果支持带内传递NTP格式时间戳[RFC6051]，则应直接从RTP数据包中获取Si和Sj，其中NTP时间戳可用。如果不是，则应根据相应的RTP时间戳计算Si和Sj。同步偏移量的值使用[RFC5905]中定义的64位有符号NTP格式时间戳表示，该时间戳是一个64位有符号定点数字，整数部分在前32位，小数部分在后32位。同步偏移量的正值表示报告流领先于参考流，而负值表示报告流落后于参考流。同步偏移量为零意味着该流是参考流。
If the measurement is unavailable, the value of this field with all bits set to 1 MUST be reported.
如果测量不可用，则必须报告所有位设置为1的该字段值。
[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.
[RFC3611]定义了SDP（会话描述协议）[RFC4566]的使用，用于发送使用XR块的信号。XR块可以在没有事先信令的情况下使用。
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]:
使用扩展的Backus Naur表单（ABNF）[RFC5234]，为本文档中定义的两个报告块定义了两个新参数，用于SDP[RFC4566]。它们在“rtcp xr”属性[RFC3611]中具有以下语法：
xr-format =/ xr-rfisd-block / xr-rfso-block
xr format=/xr rfisd块/xr rfso块
xr-rfisd-block = "rtp-flow-init-syn-delay" xr-rfso-block = "rtp-flow-syn-offset"
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.
有关“rtcp xr”属性的详细说明和完整语法，请参阅RFC 3611[RFC3611]第5.1节。
When SDP is used in the offer/answer context, the SDP Offer/Answer usage defined in [RFC3611] applies.
在提供/应答上下文中使用SDP时，[RFC3611]中定义的SDP提供/应答用法适用。
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].
RTCP XR的新报告块类型需要IANA注册。有关RTCP XR IANA分配的一般指南，请参阅[RFC3611]第6.2节。
This document assigns two new block type values in the RTCP XR Block Type Registry:
本文档在RTCP XR块类型注册表中分配两个新的块类型值：
Name: RFISD Long Name: RTP Flow Initial Synchronization Delay Value 27 Reference: Section 3
名称：RFID长名称：RTP流初始同步延迟值27参考：第3节
Name: RFSO Long Name: RTP Flow Synchronization Offset Value 28 Reference: Section 4
名称：RFSO长名称：RTP流同步偏移值28参考：第4节
This document also registers two new SDP [RFC4566] parameters for the "rtcp-xr" attribute in the RTCP XR SDP Parameters Registry:
本文档还为rtcp xr SDP参数注册表中的“rtcp xr”属性注册了两个新的SDP[RFC4566]参数：
* "rtp-flow-init-syn-delay " * "rtp-flow-syn-offset"
* “rtp流初始化同步延迟”*“rtp流同步偏移”
The contact information for the registrations is:
RAI Area Directors <rai-ads@tools.ietf.org>
The contact information for the registrations is:
RAI Area Directors <rai-ads@tools.ietf.org>
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.
当使用安全RTP[RFC3711]或其他媒体层安全性时，报告准确的同步偏移量信息可以公开有关用于保护媒体的加密操作时间的一些详细信息。此定时信息可能会对加密发起侧通道攻击。对于关注此攻击的环境，实现需要注意确保加密处理和媒体压缩花费相同的时间，而不管媒体内容如何，以避免潜在的攻击。
Besides this, it is believed that this RTCP XR block introduces no new security considerations beyond those described in [RFC3611].
除此之外，人们认为，除了[RFC3611]中所述的安全注意事项外，该RTCP XR块没有引入任何新的安全注意事项。
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.
作者感谢Bill Ver Steeg、David R.Oran、Ali Begen、Colin Perkins、Roni Even、Kevin Gross、Jing Zhao、Fernando Boronat Segui、Mario Montagud Climent、Youqing Yang、Wennao Yu、Yinliang Hu、Jonathan Lennox和Stephen Farrel对本文件提出的宝贵意见和建议。
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2119]Bradner，S.，“RFC中用于表示需求水平的关键词”，BCP 14，RFC 2119，1997年3月。
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003.
[RFC3550]Schulzrinne，H.，Casner，S.，Frederick，R.，和V.Jacobson，“RTP：实时应用的传输协议”，STD 64，RFC 35502003年7月。
[RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control Protocol Extended Reports (RTCP XR)", RFC 3611, November 2003.
[RFC3611]Friedman，T.，Caceres，R.，和A.Clark，“RTP控制协议扩展报告（RTCP XR）”，RFC 36112003年11月。
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004.
[RFC3711]Baugher，M.，McGrew，D.，Naslund，M.，Carrara，E.，和K.Norrman，“安全实时传输协议（SRTP）”，RFC 37112004年3月。
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.
[RFC4566]Handley，M.，Jacobson，V.，和C.Perkins，“SDP：会话描述协议”，RFC4566，2006年7月。
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5234]Crocker，D.和P.Overell，“语法规范的扩充BNF:ABNF”，STD 68，RFC 5234，2008年1月。
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, June 2010.
[RFC5905]Mills，D.，Martin，J.，Burbank，J.，和W.Kasch，“网络时间协议版本4：协议和算法规范”，RFC 59052010年6月。
[RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP Flows", RFC 6051, November 2010.
[RFC6051]Perkins，C.和T.Schierl，“RTP流的快速同步”，RFC 60512010年11月。
[RFC6190] Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis, "RTP Payload Format for Scalable Video Coding", RFC 6190, May 2011.
[RFC6190]Wenger，S.，Wang，Y.，Schierl，T.，和A.Eleftheriadis，“可伸缩视频编码的RTP有效载荷格式”，RFC 61902011年5月。
[RFC6776] Wu, Q., "Measurement Identity and information Reporting using SDES item and XR Block", RFC 6776, August 2012.
[RFC6776]Wu，Q.“使用SDES项目和XR块的测量标识和信息报告”，RFC 67762012年8月。
[RFC6390] Clark, A. and B. Claise, "Guidelines for Considering New Performance Metric Development", RFC 6390, October 2011.
[RFC6390]Clark，A.和B.Claise，“考虑新性能指标开发的指南”，RFC 63902011年10月。
[RFC6792] Wu, Q., "Guidelines for Use of the RTP Monitoring Framework", RFC 6792, November 2012.
[RFC6792]Wu，Q.“RTP监控框架的使用指南”，RFC 67922012年11月。
[TR-126] Broadband Forum, "Triple-play Services Quality of Experience (QoE) Requirements", Technical Report TR-126, December 2006.
[TR-126]宽带论坛，“三网融合服务体验质量（QoE）要求”，技术报告TR-126，2006年12月。
[Y.1540] ITU-T, "IP packet transfer and availability performance parameters", ITU-T Recommendation Y.1540, November 2007.
[Y.1540]ITU-T，“IP数据包传输和可用性性能参数”，ITU-T建议Y.1540，2007年11月。
a. Initial Synchronization Delay Metric
a. 初始同步延迟度量
* Metric Name: RTP Initial Synchronization Delay
* 度量名称：RTP初始同步延迟
* Metric Description: See the definition of "Initial Synchronization Delay" in Section 2.1.
* 指标说明：见第2.1节“初始同步延迟”的定义。
* Method of Measurement or Calculation: See the definition of the "Initial Synchronization Delay" field in Section 3.2.
* 测量或计算方法：见第3.2节“初始同步延迟”字段的定义。
* Units of Measurement: See the definition of the "Initial Synchronization Delay" field in Section 3.2.
* 测量单位：见第3.2节“初始同步延迟”字段的定义。
* Measurement Point(s) with Potential Measurement Domain: See the first paragraph of Section 3.
* 具有潜在测量域的测量点：见第3节第一段。
* Measurement Timing: See the second paragraph of Section 3.
* 测量时间：见第3节第二段。
* Use and applications: See Section 1.4.
* 用途和应用：见第1.4节。
* Reporting model: See RFC 3611.
* 报告模式：见RFC 3611。
b. Synchronization Offset Metric
b. 同步偏移度量
* Metric Name: RTP Synchronization Offset Delay
* 度量名称：RTP同步偏移延迟
* Metric Description: See the definition of "Synchronization Offset" in Section 1.2.
* 指标说明：参见第1.2节中“同步偏移”的定义。
* Method of Measurement or Calculation: See the definition of the "Synchronization Offset" field in Section 4.2.
* 测量或计算方法：参见第4.2节中“同步偏移”字段的定义。
* Units of Measurement: See the definition of the "Synchronization Offset" field in Section 4.2.
* 测量单位：参见第4.2节中“同步偏移”字段的定义。
* Measurement Point(s) with Potential Measurement Domain: See the second paragraph of Section 4.
* 具有潜在测量域的测量点：见第4节第二段。
* Measurement Timing: See the third paragraph of Section 4.2 for measurement timing and the Interval Metric flag.
* 测量时间：测量时间和间隔度量标志见第4.2节第三段。
* Use and applications: See Section 1.4.
* 用途和应用：见第1.4节。
* Reporting model: See RFC 3611.
* 报告模式：见RFC 3611。
Authors' Addresses
作者地址
Hitoshi Asaeda National Institute of Information and Communications Technology 4-2-1 Nukui-Kitamachi Koganei, Tokyo 184-8795 Japan
浅田仁国立信息与通信技术研究所4-2-1日本东京北天町小江内Nukui Kitamachi Koganei 184-8795
EMail: asaeda@nict.go.jp
EMail: asaeda@nict.go.jp
Qin Wu Huawei Technologies Co., Ltd. 101 Software Avenue, Yuhua District Nanjing, Jiangsu 210012 China
中国江苏省南京市雨花区软件大道101号秦武华为技术有限公司210012
EMail: bill.wu@huawei.com
EMail: bill.wu@huawei.com
Rachel Huang Huawei Technologies Co., Ltd. 101 Software Avenue, Yuhua District Nanjing, Jiangsu 210012 China
中国江苏省南京市雨花区软件大道101号瑞秋黄华为技术有限公司210012
EMail: Rachel@huawei.com
EMail: Rachel@huawei.com