Network Working Group A. Morton
Request for Comments: 5481 AT&T Labs
Category: Informational B. Claise
Cisco Systems, Inc.
March 2009
Network Working Group A. Morton
Request for Comments: 5481 AT&T Labs
Category: Informational B. Claise
Cisco Systems, Inc.
March 2009
Packet Delay Variation Applicability Statement
包延迟变化适用性声明
Status of This Memo
关于下段备忘
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
本备忘录为互联网社区提供信息。它没有规定任何类型的互联网标准。本备忘录的分发不受限制。
Copyright Notice
版权公告
Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved.
版权所有（c）2009 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 in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document.
本文件受BCP 78和IETF信托在本文件出版之日生效的与IETF文件有关的法律规定的约束(http://trustee.ietf.org/license-info). 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。
This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.
本文件可能包含2008年11月10日之前发布或公开的IETF文件或IETF贡献中的材料。控制某些材料版权的人员可能未授予IETF信托允许在IETF标准流程之外修改此类材料的权利。在未从控制此类材料版权的人员处获得充分许可的情况下，不得在IETF标准流程之外修改本文件，也不得在IETF标准流程之外创建其衍生作品，除了将其格式化以RFC形式发布或将其翻译成英语以外的其他语言。
Abstract
摘要
Packet delay variation metrics appear in many different standards documents. The metric definition in RFC 3393 has considerable flexibility, and it allows multiple formulations of delay variation through the specification of different packet selection functions.
包延迟变化度量出现在许多不同的标准文档中。RFC3393中的度量定义具有相当大的灵活性，它允许通过指定不同的分组选择函数来对延迟变化进行多种表述。
Although flexibility provides wide coverage and room for new ideas, it can make comparisons of independent implementations more difficult. Two different formulations of delay variation have come into wide use in the context of active measurements. This memo examines a range of circumstances for active measurements of delay variation and their uses, and recommends which of the two forms is best matched to particular conditions and tasks.
尽管灵活性提供了广泛的覆盖范围和新想法的空间，但它会使独立实现的比较更加困难。两种不同的延迟变化公式在主动测量中得到了广泛的应用。本备忘录审查了一系列主动测量延迟变化的情况及其用途，并建议两种形式中哪一种最适合特定条件和任务。
Table of Contents
目录
1. Introduction ....................................................4
1.1. Requirements Language ......................................5
1.2. Background Literature in IPPM and Elsewhere ................5
1.3. Organization of the Memo ...................................6
2. Purpose and Scope ...............................................7
3. Brief Descriptions of Delay Variation Uses ......................7
3.1. Inferring Queue Occupation on a Path .......................7
3.2. Determining De-Jitter Buffer Size ..........................8
3.3. Spatial Composition .......................................10
3.4. Service-Level Comparison ..................................10
3.5. Application-Layer FEC Design ..............................10
4. Formulations of IPDV and PDV ...................................10
4.1. IPDV: Inter-Packet Delay Variation ........................11
4.2. PDV: Packet Delay Variation ...............................11
4.3. A "Point" about Measurement Points ........................12
4.4. Examples and Initial Comparisons ..........................12
5. Survey of Earlier Comparisons ..................................13
5.1. Demichelis' Comparison ....................................13
5.2. Ciavattone et al. .........................................15
5.3. IPPM List Discussion from 2000 ............................16
5.4. Y.1540 Appendix II ........................................18
5.5. Clark's ITU-T SG 12 Contribution ..........................18
6. Additional Properties and Comparisons ..........................18
6.1. Packet Loss ...............................................18
6.2. Path Changes ..............................................19
6.2.1. Lossless Path Change ...............................20
6.2.2. Path Change with Loss ..............................21
6.3. Clock Stability and Error .................................22
6.4. Spatial Composition .......................................24
6.5. Reporting a Single Number (SLA) ...........................24
6.6. Jitter in RTCP Reports ....................................25
1. Introduction ....................................................4
1.1. Requirements Language ......................................5
1.2. Background Literature in IPPM and Elsewhere ................5
1.3. Organization of the Memo ...................................6
2. Purpose and Scope ...............................................7
3. Brief Descriptions of Delay Variation Uses ......................7
3.1. Inferring Queue Occupation on a Path .......................7
3.2. Determining De-Jitter Buffer Size ..........................8
3.3. Spatial Composition .......................................10
3.4. Service-Level Comparison ..................................10
3.5. Application-Layer FEC Design ..............................10
4. Formulations of IPDV and PDV ...................................10
4.1. IPDV: Inter-Packet Delay Variation ........................11
4.2. PDV: Packet Delay Variation ...............................11
4.3. A "Point" about Measurement Points ........................12
4.4. Examples and Initial Comparisons ..........................12
5. Survey of Earlier Comparisons ..................................13
5.1. Demichelis' Comparison ....................................13
5.2. Ciavattone et al. .........................................15
5.3. IPPM List Discussion from 2000 ............................16
5.4. Y.1540 Appendix II ........................................18
5.5. Clark's ITU-T SG 12 Contribution ..........................18
6. Additional Properties and Comparisons ..........................18
6.1. Packet Loss ...............................................18
6.2. Path Changes ..............................................19
6.2.1. Lossless Path Change ...............................20
6.2.2. Path Change with Loss ..............................21
6.3. Clock Stability and Error .................................22
6.4. Spatial Composition .......................................24
6.5. Reporting a Single Number (SLA) ...........................24
6.6. Jitter in RTCP Reports ....................................25
6.7. MAPDV2 ....................................................25
6.8. Load Balancing ............................................26
7. Applicability of the Delay Variation Forms and
Recommendations ................................................27
7.1. Uses ......................................................27
7.1.1. Inferring Queue Occupancy ..........................27
7.1.2. Determining De-Jitter Buffer Size (and FEC
Design) ............................................27
7.1.3. Spatial Composition ................................28
7.1.4. Service-Level Specification: Reporting a
Single Number ......................................28
7.2. Challenging Circumstances .................................28
7.2.1. Clock and Storage Issues ...........................28
7.2.2. Frequent Path Changes ..............................29
7.2.3. Frequent Loss ......................................29
7.2.4. Load Balancing .....................................29
7.3. Summary ...................................................30
8. Measurement Considerations .....................................31
8.1. Measurement Stream Characteristics ........................31
8.2. Measurement Devices .......................................32
8.3. Units of Measurement ......................................33
8.4. Test Duration .............................................33
8.5. Clock Sync Options ........................................33
8.6. Distinguishing Long Delay from Loss .......................34
8.7. Accounting for Packet Reordering ..........................34
8.8. Results Representation and Reporting ......................35
9. Security Considerations ........................................35
10. Acknowledgments ...............................................35
11. Appendix on Calculating the D(min) in PDV .....................35
12. References ....................................................36
12.1. Normative References .....................................36
12.2. Informative References ...................................37
6.7. MAPDV2 ....................................................25
6.8. Load Balancing ............................................26
7. Applicability of the Delay Variation Forms and
Recommendations ................................................27
7.1. Uses ......................................................27
7.1.1. Inferring Queue Occupancy ..........................27
7.1.2. Determining De-Jitter Buffer Size (and FEC
Design) ............................................27
7.1.3. Spatial Composition ................................28
7.1.4. Service-Level Specification: Reporting a
Single Number ......................................28
7.2. Challenging Circumstances .................................28
7.2.1. Clock and Storage Issues ...........................28
7.2.2. Frequent Path Changes ..............................29
7.2.3. Frequent Loss ......................................29
7.2.4. Load Balancing .....................................29
7.3. Summary ...................................................30
8. Measurement Considerations .....................................31
8.1. Measurement Stream Characteristics ........................31
8.2. Measurement Devices .......................................32
8.3. Units of Measurement ......................................33
8.4. Test Duration .............................................33
8.5. Clock Sync Options ........................................33
8.6. Distinguishing Long Delay from Loss .......................34
8.7. Accounting for Packet Reordering ..........................34
8.8. Results Representation and Reporting ......................35
9. Security Considerations ........................................35
10. Acknowledgments ...............................................35
11. Appendix on Calculating the D(min) in PDV .....................35
12. References ....................................................36
12.1. Normative References .....................................36
12.2. Informative References ...................................37
There are many ways to formulate packet delay variation metrics for the Internet and other packet-based networks. The IETF itself has several specifications for delay variation [RFC3393], sometimes called jitter [RFC3550] or even inter-arrival jitter [RFC3550], and these have achieved wide adoption. The International Telecommunication Union - Telecommunication Standardization Sector (ITU-T) has also recommended several delay variation metrics (called parameters in their terminology) [Y.1540] [G.1020], and some of these are widely cited and used. Most of the standards above specify more than one way to quantify delay variation, so one can conclude that standardization efforts have tended to be inclusive rather than selective.
有许多方法可以为Internet和其他基于数据包的网络制定数据包延迟变化度量。IETF本身有几个关于延迟变化[RFC3393]的规范，有时称为抖动[RFC3550]，甚至是到达间抖动[RFC3550]，这些规范已被广泛采用。国际电信联盟-电信标准化部门（ITU-T）也推荐了几种延迟变化指标（术语中称为参数）[Y.1540][G.1020]，其中一些指标被广泛引用和使用。上述大多数标准规定了不止一种量化延迟变化的方法，因此可以得出结论，标准化工作往往是包容性的，而不是选择性的。
This memo uses the term "delay variation" for metrics that quantify a path's ability to transfer packets with consistent delay. [RFC3393] and [Y.1540] both prefer this term. Some refer to this phenomenon as "jitter" (and the buffers that attempt to smooth the variations as de-jitter buffers). Applications of the term "jitter" are much broader than packet transfer performance, with "unwanted signal variation" as a general definition. "Jitter" has been used to describe frequency or phase variations, such as data stream rate variations or carrier signal phase noise. The phrase "delay variation" is almost self-defining and more precise, so it is preferred in this memo.
本备忘录使用术语“延迟变化”来衡量路径传输具有一致延迟的数据包的能力。[RFC3393]和[Y.1540]都喜欢这个术语。有些人将这种现象称为“抖动”（试图平滑变化的缓冲器称为去抖动缓冲器）。术语“抖动”的应用比数据包传输性能更广泛，一般定义为“不需要的信号变化”。“抖动”已用于描述频率或相位变化，例如数据流速率变化或载波信号相位噪声。“延迟变化”一词几乎是自我定义的，更为精确，因此在本备忘录中更为可取。
Most (if not all) delay variation metrics are derived metrics, in that their definitions rely on another fundamental metric. In this case, the fundamental metric is one-way delay, and variation is assessed by computing the difference between two individual one-way-delay measurements, or a pair of singletons. One of the delay singletons is taken as a reference, and the result is the variation with respect to the reference. The variation is usually summarized for all packets in a stream using statistics.
大多数（如果不是全部的话）延迟变化度量都是派生度量，因为它们的定义依赖于另一个基本度量。在这种情况下，基本度量是单向延迟，通过计算两个单独的单向延迟测量值或一对单态之间的差值来评估变化。其中一个延迟单态作为参考，结果是相对于参考的变化。通常使用统计信息汇总流中所有数据包的变化。
The industry has predominantly implemented two specific formulations of delay variation (for one survey of the situation, see [Krzanowski]):
该行业主要实施了两种延迟变化的具体公式（关于一项情况调查，请参见[Krzanowski]）：
1. Inter-Packet Delay Variation, IPDV, where the reference is the previous packet in the stream (according to sending sequence), and the reference changes for each packet in the stream. Properties of variation are coupled with packet sequence in this formulation. This form was called Instantaneous Packet Delay Variation in early IETF contributions, and is similar to the packet spacing difference metric used for interarrival jitter calculations in [RFC3550].
1. 分组间延迟变化，IPDV，其中参考是流中的前一个分组（根据发送序列），并且参考针对流中的每个分组改变。在这个公式中，变量的性质与数据包序列相耦合。这种形式在早期IETF贡献中称为瞬时数据包延迟变化，类似于[RFC3550]中用于到达间抖动计算的数据包间隔差度量。
2. Packet Delay Variation, PDV, where a single reference is chosen from the stream based on specific criteria. The most common criterion for the reference is the packet with the minimum delay in the sample. This term derives its name from a similar definition for Cell Delay Variation, an ATM performance metric [I.356].
2. 分组延迟变化，PDV，其中基于特定标准从流中选择单个参考。最常见的参考标准是样本中延迟最小的数据包。该术语的名称来源于信元延迟变化的类似定义，即ATM性能指标[I.356]。
It is important to note that the authors of relevant standards for delay variation recognized there are many different users with varying needs, and allowed sufficient flexibility to formulate several metrics with different properties. Therefore, the comparison is not so much between standards bodies or their specifications as it is between specific formulations of delay variation. Both Inter-Packet Delay Variation and Packet Delay Variation are compliant with [RFC3393], because different packet selection functions will produce either form.
值得注意的是，延迟变化相关标准的作者认识到有许多不同的用户具有不同的需求，并允许足够的灵活性来制定具有不同属性的若干指标。因此，与其说是标准机构或其规范之间的比较，不如说是延迟变化的具体公式之间的比较。数据包间延迟变化和数据包延迟变化均符合[RFC3393]，因为不同的数据包选择函数将产生任何一种形式。
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。
With more people joining the measurement community every day, it is possible this memo is the first from the IP Performance Metrics (IPPM) Working Group that the reader has consulted. This section provides a brief road map and background on the IPPM literature, and the published specifications of other relevant standards organizations.
随着越来越多的人每天加入度量社区，本备忘录可能是读者咨询的IP性能度量（IPPM）工作组的第一份备忘录。本节提供了IPPM文献的简要路线图和背景，以及其他相关标准组织发布的规范。
The IPPM framework [RFC2330] provides a background for this memo and other IPPM RFCs. Key terms such as singleton, sample, and statistic are defined there, along with methods of collecting samples (Poisson streams), time-related issues, and the "packet of Type-P" convention.
IPPM框架[RFC2330]为本备忘录和其他IPPM RFC提供了背景。这里定义了单例、样本和统计等关键术语，以及收集样本的方法（泊松流）、与时间相关的问题和“P型数据包”约定。
There are two fundamental and related metrics that can be applied to every packet transfer attempt: one-way loss [RFC2680] and one-way delay [RFC2679]. The metrics use a waiting time threshold to distinguish between lost and delayed packets. Packets that arrive at the measurement destination within their waiting time have finite delay and are not lost. Otherwise, packets are designated lost and their delay is undefined. Guidance on setting the waiting time threshold may be found in [RFC2680] and [IPPM-Reporting].
有两个基本的和相关的指标可以应用于每个数据包传输尝试：单向丢失[RFC2680]和单向延迟[RFC2679]。这些度量使用等待时间阈值来区分丢失和延迟的数据包。在等待时间内到达测量目的地的数据包具有有限延迟且不会丢失。否则，数据包被指定为丢失，其延迟未定义。有关设置等待时间阈值的指导，请参见[RFC2680]和[IPPM报告]。
Another fundamental metric is packet reordering as specified in [RFC4737]. The reordering metric was defined to be "orthogonal" to packet loss. In other words, the gap in a packet sequence caused by loss does not result in reordered packets, but a rearrangement of packet arrivals from their sending order constitutes reordering.
另一个基本指标是[RFC4737]中规定的数据包重新排序。重新排序度量被定义为与数据包丢失“正交”。换言之，由丢失引起的分组序列中的间隔不会导致分组重新排序，但是分组从其发送顺序到达的重新排列构成了重新排序。
Derived metrics are based on the fundamental metrics. The metric of primary interest here is delay variation [RFC3393], a metric that is derived from one-way delay [RFC2680]. Another derived metric is the loss patterns metric [RFC3357], which is derived from loss.
衍生指标基于基本指标。这里主要关注的指标是延迟变化[RFC3393]，这是一个从单向延迟[RFC2680]衍生出来的指标。另一个衍生指标是损失模式指标[RFC3357]，它源自损失。
The measured values of all metrics (both fundamental and derived) depend to great extent on the stream characteristics used to collect them. Both Poisson streams [RFC3393] and Periodic streams [RFC3432] have been used with the IPDV and PDV metrics. The choice of stream specification for active measurement will depend on the purpose of the characterization and the constraints of the testing environment. Periodic streams are frequently chosen for use with IPDV and PDV, because the application streams that are most sensitive to delay variation exhibit periodicity. Additional details that are method-specific are discussed in Section 8 on "Measurement Considerations".
所有度量（基本度量和派生度量）的测量值在很大程度上取决于用于收集它们的流特征。泊松流[RFC3393]和周期流[RFC3432]都已用于IPDV和PDV度量。主动测量流规格的选择将取决于表征的目的和测试环境的约束。周期流经常被选择用于IPDV和PDV，因为对延迟变化最敏感的应用程序流表现出周期性。第8节“测量注意事项”中讨论了特定于方法的其他细节。
In the ITU-T, the framework, fundamental metrics, and derived metrics for IP performance are specified in Recommendation Y.1540 [Y.1540]. [G.1020] defines additional delay variation metrics, analyzes the operation of fixed and adaptive de-jitter buffers, and describes an example adaptive de-jitter buffer emulator. Appendix II of [G.1050] describes the models for network impairments (including delay variation) that are part of standardized IP network emulator that may be useful when evaluating measurement techniques.
在ITU-T中，建议Y.1540[Y.1540]规定了IP性能的框架、基本指标和衍生指标。[G.1020]定义了额外的延迟变化度量，分析了固定和自适应去抖动缓冲区的操作，并描述了一个示例自适应去抖动缓冲区模拟器。[G.1050]的附录II描述了作为标准化IP网络仿真器一部分的网络损伤（包括延迟变化）模型，该模型在评估测量技术时可能有用。
The Purpose and Scope follows in Section 2. We then give a summary of the main tasks for delay variation metrics in Section 3. Section 4 defines the two primary forms of delay variation, and Section 5 presents summaries of four earlier comparisons. Section 6 adds new comparisons to the analysis, and Section 7 reviews the applicability and recommendations for each form of delay variation. Section 8 then looks at many important delay variation measurement considerations. Following the Security Considerations, there is an appendix on the calculation of the minimum delay for the PDV form.
目的和范围见第2节。然后，我们在第3节中总结了延迟变化度量的主要任务。第4节定义了延迟变化的两种主要形式，第5节总结了之前的四种比较。第6节为分析增加了新的比较，第7节审查了每种形式延迟变化的适用性和建议。第8节接着介绍了许多重要的延迟变化测量注意事项。出于安全考虑，有一个关于PDV表格最小延迟计算的附录。
The IPDV and PDV formulations have certain features that make them more suitable for one circumstance and less so for another. The purpose of this memo is to compare two forms of delay variation, so that it will be evident which of the two is better suited for each of many possible uses and their related circumstances.
IPDV和PDV配方具有某些特征，使其更适合一种情况，而不适合另一种情况。本备忘录的目的是比较两种形式的延迟变化，以便明确这两种形式中的哪一种更适合各种可能的用途及其相关情况。
The scope of this memo is limited to the two forms of delay variation briefly described above (Inter-Packet Delay Variation and Packet Delay Variation), circumstances related to active measurement, and uses that are deemed relevant and worthy of inclusion here through IPPM Working Group consensus.
本备忘录的范围仅限于上述两种形式的延迟变化（数据包间延迟变化和数据包延迟变化）、与主动测量相关的情况以及通过IPPM工作组协商一致认为相关且值得包含的用途。
It is entirely possible that the analysis and conclusions drawn here are applicable beyond the intended scope, but the reader is cautioned to fully appreciate the circumstances of active measurement on IP networks before doing so.
此处得出的分析和结论完全有可能超出预期范围，但读者应注意，在这样做之前，充分了解IP网络上主动测量的情况。
The scope excludes assessment of delay variation for packets with undefined delay. This is accomplished by conditioning the delay distribution on arrival within a reasonable waiting time based on an understanding of the path under test and packet lifetimes. The waiting time is sometimes called the loss threshold [RFC2680]: if a packet arrives beyond this threshold, it may as well have been lost because it is no longer useful. This is consistent with [RFC3393], where the Type-P-One-way-ipdv is undefined when the destination fails to receive one or both packets in the selected pair. Furthermore, it is consistent with application performance analysis to consider only arriving packets, because a finite waiting time-out is a feature of many protocols.
该范围不包括对具有未定义延迟的数据包的延迟变化的评估。这是通过基于对测试路径和数据包寿命的理解，在合理的等待时间内调节到达时的延迟分布来实现的。等待时间有时称为丢失阈值[RFC2680]：如果数据包到达的时间超过该阈值，它也可能因为不再有用而丢失。这与[RFC3393]一致，其中，当目的地无法接收所选对中的一个或两个数据包时，类型-P-One-way-ipdv未定义。此外，仅考虑到达分组与应用性能分析是一致的，因为有限等待超时是许多协议的特征。
This section presents a set of tasks that call for delay variation measurements. Here, the memo provides several answers to the question, "How will the results be used?" for the delay variation metric.
本节介绍了一组需要进行延迟变化测量的任务。在这里，备忘录为延迟变化度量的“结果将如何使用？”问题提供了几个答案。
As packets travel along the path from source to destination, they pass through many network elements, including a series of router queues. Some types of the delay sources along the path are constant, such as links between two locations. But the latency encountered in each queue varies, depending on the number of packets in the queue when a particular packet arrives. If one assumes that at least one of the packets in a test stream encounters virtually empty queues all
当数据包沿着从源到目的地的路径传输时，它们会通过许多网络元素，包括一系列路由器队列。沿路径的某些类型的延迟源是恒定的，例如两个位置之间的链路。但是，每个队列中遇到的延迟会有所不同，这取决于特定数据包到达时队列中的数据包数量。如果假设测试流中至少有一个数据包遇到几乎为空的队列，则所有
along the path (and the path is stable), then the additional delay observed on other packets can be attributed to the time spent in one or more queues. Otherwise, the delay variation observed is the variation in queue time experienced by the test stream.
沿着路径（并且路径是稳定的），在其他数据包上观察到的额外延迟可以归因于在一个或多个队列中花费的时间。否则，观察到的延迟变化是测试流经历的队列时间变化。
It is worth noting that delay variation can occur beyond IP router queues, in other communication components. Examples include media contention: DOCSIS, IEEE 802.11, and some mobile radio technologies.
值得注意的是，延迟变化可能发生在IP路由器队列之外的其他通信组件中。示例包括媒体争用：DOCSIS、IEEE 802.11和一些移动无线电技术。
However, delay variation from all sources at the IP layer and below will be quantified using the two formulations discussed here.
然而，IP层及以下所有源的延迟变化将使用此处讨论的两个公式进行量化。
Note -- while this memo and other IPPM literature prefer the term "delay variation", the terms "jitter buffer" and the more accurate "de-jitter buffer" are widely adopted names for a component of packet communication systems, and they will be used here to designate that system component.
注——虽然本备忘录和其他IPPM文献更喜欢术语“延迟变化”，但术语“抖动缓冲区”和更准确的“去抖动缓冲区”是分组通信系统组件的广泛采用名称，在这里它们将用于指定该系统组件。
Most isochronous applications (a.k.a. real-time applications) employ a buffer to smooth out delay variation encountered on the path from source to destination. The buffer must be big enough to accommodate the expected variation of delay, or packet loss will result. However, if the buffer is too large, then some of the desired spontaneity of communication will be lost and conversational dynamics will be affected. Therefore, application designers need to know the range of delay variation they must accommodate, whether they are designing fixed or adaptive buffer systems.
大多数等时应用程序（也称为实时应用程序）使用缓冲区来消除从源到目标的路径上遇到的延迟变化。缓冲区必须足够大，以适应预期的延迟变化，否则将导致数据包丢失。然而，如果缓冲区太大，那么一些期望的交流自发性就会丢失，会话动态也会受到影响。因此，无论是设计固定缓冲系统还是自适应缓冲系统，应用程序设计者都需要知道他们必须适应的延迟变化范围。
Network service providers also attempt to constrain delay variation to ensure the quality of real-time applications, and monitor this metric (possibly to compare with a numerical objective or Service Level Agreement).
网络服务提供商还试图限制延迟变化，以确保实时应用程序的质量，并监控该指标（可能与数字目标或服务级别协议进行比较）。
De-jitter buffer size can be expressed in units of octets of storage space for the packet stream, or in units of time that the packets are stored. It is relatively simple to convert between octets and time when the buffer read rate (in octets per second) is constant:
去抖动缓冲器大小可以用分组流的存储空间的八位字节单位表示，或者用分组存储的时间单位表示。当缓冲区读取速率（以每秒八位字节为单位）恒定时，在八位字节和时间之间转换相对简单：
read_rate * storage_time = storage_octets
读取速率*存储时间=存储八位字节
Units of time are used in the discussion below.
在下面的讨论中使用时间单位。
The objective of a de-jitter buffer is to compensate for all prior sources of delay variation and produce a packet stream with constant delay. Thus, a packet experiencing the minimum transit delay from source to destination, D_min, should spend the maximum time in a
去抖动缓冲器的目标是补偿所有先前的延迟变化源，并产生具有恒定延迟的分组流。因此，经历从源到目的地的最小传输延迟D_min的数据包应在一个时间段内花费最大的时间
de-jitter buffer, B_max. The sum of D_min and B_max should equal the sum of the maximum transit delay (D_max) and the minimum buffer time (B_min). We have
去抖动缓冲区，B_max。D_min和B_max之和应等于最大传输延迟（D_max）和最小缓冲时间（B_min）之和。我们有
Constant = D_min + B_max = D_max + B_min,
常数=D_最小值+B_最大值=D_最大值+B_最小值，
after rearranging terms,
在重新安排条款之后，
B_max - B_min = D_max - D_min = range(B) = range(D)
B_max - B_min = D_max - D_min = range(B) = range(D)
where range(B) is the range of packet buffering times, and range(D) is the range of packet transit delays from source to destination.
其中，范围（B）是分组缓冲时间的范围，范围（D）是从源到目的地的分组传输延迟的范围。
Packets with transit delay between the max and min spend a complementary time in the buffer and also see the constant delay.
传输延迟在最大值和最小值之间的数据包在缓冲区中花费一段互补的时间，也可以看到恒定的延迟。
In practice, the minimum buffer time, B_min, may not be zero, and the maximum transit delay, D_max, may be a high percentile (99.9th percentile) instead of the maximum.
实际上，最小缓冲时间B_min可能不是零，最大传输延迟D_max可能是高百分位（99.9%）而不是最大值。
Note that B_max - B_min = range(B) is the range of buffering times needed to compensate for delay variation. The actual size of the buffer may be larger (where B_min > 0) or smaller than range(B).
请注意，B_max-B_min=范围（B）是补偿延迟变化所需的缓冲时间范围。缓冲区的实际大小可能大于（其中B_min>0）或小于范围（B）。
There must be a process to align the de-jitter buffer time with packet transit delay. This is a process to identify the packets with minimum delay and schedule their play-out time so that they spend the maximum time in the buffer. The error in the alignment process can be accounted for by a variable, A. In the equation below, the range of buffering times *available* to the packet stream, range(b), depends on buffer alignment with the actual arrival times of D_min and D_max.
必须有一个过程将去抖动缓冲时间与数据包传输延迟对齐。这是一个识别具有最小延迟的数据包并安排其播放时间以使其在缓冲区中花费最大时间的过程。对齐过程中的错误可以通过变量a来解释。在下面的等式中，分组流的缓冲时间*可用*的范围，范围（b），取决于缓冲区与实际到达时间D_min和D_max的对齐。
range(b) = b_max - b_min = D_max - D_min + A
range(b) = b_max - b_min = D_max - D_min + A
where variable b represents the *available* buffer in a system with a specific alignment, A, and b_max and b_min represent the limits of the available buffer.
其中变量b表示具有特定对齐的系统中的*可用*缓冲区，a和b_max和b_min表示可用缓冲区的限制。
When A is positive, the de-jitter buffer applies more delay than necessary (where Constant = D_max + b_min + A represents one possible alignment). When A is negative, there is insufficient buffer time available to compensate for range(D) because of misalignment. Packets with D_min may be arriving too early and encountering a full buffer, or packets with D_max may be arriving too late, and in either case, the packets would be discarded.
当A为正时，去抖动缓冲区应用的延迟大于必要的延迟（其中常数=D_max+b_min+A表示一种可能的对齐）。当A为负值时，由于未对准，没有足够的缓冲时间来补偿范围（D）。具有D_min的数据包可能到达得太早并且遇到满缓冲区，或者具有D_max的数据包可能到达得太迟，并且在任何一种情况下，数据包都将被丢弃。
In summary, the range of transit delay variation is a critical factor in the determination of de-jitter buffer size.
总之，传输延迟变化范围是确定去抖动缓冲区大小的关键因素。
In Spatial Composition, the tasks are similar to those described above, but with the additional complexity of a multiple network path where several sub-paths are measured separately and no source-to-destination measurements are available. In this case, the source-to-destination performance must be estimated, using Composed Metrics as described in [IPPM-Framework] and [Y.1541]. Note that determining the composite delay variation is not trivial: simply summing the sub-path variations is not accurate.
在空间构成中，任务类似于上述任务，但是具有多个网络路径的额外复杂性，其中多个子路径被单独测量并且没有可用的源到目的地测量。在这种情况下，必须使用[IPPM框架]和[Y.1541]中所述的组合指标来估计源到目标的性能。请注意，确定复合延迟变化并非易事：简单地将子路径变化求和是不准确的。
IP performance measurements are often used as the basis for agreements (or contracts) between service providers and their customers. The measurement results must compare favorably with the performance levels specified in the agreement.
IP性能度量通常用作服务提供商与其客户之间协议（或合同）的基础。测量结果必须与协议中规定的性能水平相比较。
Packet delay variation is usually one of the metrics specified in these agreements. In principle, any formulation could be specified in the Service Level Agreement (SLA). However, the SLA is most useful when the measured quantities can be related to ways in which the communication service will be utilized by the customer, and this can usually be derived from one of the tasks described above.
数据包延迟变化通常是这些协议中指定的指标之一。原则上，任何表述都可以在服务水平协议（SLA）中指定。然而，当测量的数量可以与客户将使用通信服务的方式相关时，SLA是最有用的，并且这通常可以从上述任务之一得到。
The design of application-layer Forward Error Correction (FEC) components is closely related to the design of a de-jitter buffer in several ways. The FEC designer must choose a protection interval (time to send/receive a block of packets in a constant packet rate system) consistent with the packet-loss characteristics, but also mindful of the extent of delay variation expected. Further, the system designer must decide how long to wait for "late" packets to arrive. Again, the range of delay variation is the relevant expression delay variation for these tasks.
应用层前向纠错（FEC）组件的设计在多个方面与去抖动缓冲器的设计密切相关。FEC设计者必须选择与分组丢失特征一致的保护间隔（在恒定分组速率系统中发送/接收分组块的时间），但也要注意预期的延迟变化程度。此外，系统设计者必须决定等待“迟到”数据包到达的时间。同样，延迟变化的范围是这些任务的相关表达式延迟变化。
This section presents the formulations of IPDV and PDV, and provides some illustrative examples. We use the basic singleton definition in [RFC3393] (which itself is based on [RFC2679]):
本节介绍IPDV和PDV的公式，并提供一些示例。我们使用[RFC3393]中的基本单例定义（其本身基于[RFC2679]）：
"Type-P-One-way-ipdv is defined for two packets from Src to Dst selected by the selection function F, as the difference between the value of the Type-P-One-way-delay from Src to Dst at T2 and the value of the Type-P-One-Way-Delay from Src to Dst at T1".
“类型-P-单向-ipdv定义为选择函数F选择的从Src到Dst的两个数据包，作为T2处从Src到Dst的类型-P-单向延迟值与T1处从Src到Dst的类型-P-单向延迟值之间的差值”。
If we have packets in a stream consecutively numbered i = 1,2,3,... falling within the test interval, then IPDV(i) = D(i)-D(i-1) where D(i) denotes the one-way delay of the ith packet of a stream.
如果我们在一个连续编号为i=1,2,3的流中有数据包，。。。在测试间隔内，则IPDV（i）=D（i）-D（i-1），其中D（i）表示流的第i个分组的单向延迟。
One-way delays are the difference between timestamps applied at the ends of the path, or the receiver time minus the transmission time.
单向延迟是应用于路径末端的时间戳之间的差，或接收器时间减去传输时间。
So D(2) = R2-T2. With this timestamp notation, it can be shown that IPDV also represents the change in inter-packet spacing between transmission and reception:
所以D（2）=R2-T2。使用此时间戳表示法，可以表明IPDV还表示传输和接收之间的包间间隔的变化：
IPDV(2) = D(2) - D(1) = (R2-T2) - (R1-T1) = (R2-R1) - (T2-T1)
IPDV(2) = D(2) - D(1) = (R2-T2) - (R1-T1) = (R2-R1) - (T2-T1)
An example selection function given in [RFC3393] is "Consecutive Type-P packets within the specified interval". This is exactly the function needed for IPDV. The reference packet in the pair is the previous packet in the sending sequence.
[RFC3393]中给出的示例选择函数是“指定间隔内的连续P型数据包”。这正是IPDV所需的功能。该对中的参考分组是发送序列中的前一个分组。
Note that IPDV can take on positive and negative values (and zero). One way to analyze the IPDV results is to concentrate on the positive excursions. However, this approach has limitations that are discussed in more detail below (see Section 5.3).
请注意，IPDV可以具有正值和负值（以及零）。分析IPDV结果的一种方法是集中于正向偏移。然而，这种方法有一些限制，下面将详细讨论（见第5.3节）。
The mean of all IPDV(i) for a stream is usually zero. However, a slow delay change over the life of the stream, or a frequency error between the measurement system clocks, can result in a non-zero mean.
流的所有IPDV（i）的平均值通常为零。然而，流寿命期间的缓慢延迟变化，或测量系统时钟之间的频率误差，可能导致非零平均值。
The name Packet Delay Variation is used in [Y.1540] and its predecessors, and refers to a performance parameter equivalent to the metric described below.
[Y.1540]及其前身中使用了“数据包延迟变化”这一名称，它指的是与下文所述指标等效的性能参数。
The Selection Function for PDV requires two specific roles for the packets in the pair. The first packet is any Type-P packet within the specified interval. The second, or reference packet is the Type-P packet within the specified interval with the minimum one-way delay.
PDV的选择功能要求对中的数据包具有两个特定的角色。第一个数据包是指定间隔内的任何P型数据包。第二个或参考分组是指定间隔内具有最小单向延迟的P型分组。
Therefore, PDV(i) = D(i)-D(min) (using the nomenclature introduced in the IPDV section). D(min) is the delay of the packet with the lowest value for delay (minimum) over the current test interval. Values of PDV may be zero or positive, and quantiles of the PDV distribution are direct indications of delay variation.
因此，PDV（i）=D（i）-D（min）（使用IPDV部分介绍的术语）。D（min）是在当前测试间隔内具有最小延迟值（minimum）的数据包的延迟。PDV的值可以是零或正，并且PDV分布的分位数是延迟变化的直接指示。
PDV is a version of the one-way-delay distribution, shifted to the origin by normalizing to the minimum delay.
PDV是单向延迟分布的一个版本，通过标准化为最小延迟将其移到原点。
Both IPDV and PDV are derived from the one-way-delay metric. One-way delay requires knowledge of time at two points, e.g., the source and destination of an IP network path in end-to-end measurement. Therefore, both IPDV and PDV can be categorized as 2-point metrics because they are derived from one-way delay. Specific methods of measurement may make assumptions or have a priori knowledge about one of the measurement points, but the metric definitions themselves are based on information collected at two measurement points.
IPDV和PDV均源自单向延迟度量。单向延迟需要了解两点的时间，例如端到端测量中IP网络路径的源和目的地。因此，IPDV和PDV都可以被归类为两点度量，因为它们是从单向延迟派生的。具体的测量方法可能会对其中一个测量点做出假设或具有先验知识，但度量定义本身是基于在两个测量点收集的信息。
Note: This material originally presented in Slides 2 and 3 of [Morton06].
注：本材料最初在[Morton06]的第2和第3张幻灯片中介绍。
The Figure below gives a sample of packet delays, calculates IPDV and PDV values, and depicts a histogram for each one.
下图给出了数据包延迟的示例，计算了IPDV和PDV值，并描绘了每个值的直方图。
Packet # 1 2 3 4 5
-------------------------------
Delay, ms 20 10 20 25 20
Packet # 1 2 3 4 5
-------------------------------
Delay, ms 20 10 20 25 20
IPDV U -10 10 5 -5
IPDV U-10105-5
PDV 10 0 10 15 10
PDV 10 0 10 15 10
| |
4| 4|
| |
3| 3| H
| | H
2| 2| H
| | H
H H 1| H H 1|H H H
H H | H H |H H H
---------+-------- +---------------
-10 -5 0 5 10 0 5 10 15
| |
4| 4|
| |
3| 3| H
| | H
2| 2| H
| | H
H H 1| H H 1|H H H
H H | H H |H H H
---------+-------- +---------------
-10 -5 0 5 10 0 5 10 15
IPDV Histogram PDV Histogram
IPDV直方图PDV直方图
Figure 1: IPDV and PDV Comparison
图1:IPDV和PDV比较
The sample of packets contains three packets with "typical" delays of 20 ms, one packet with a low delay of 10 ms (the minimum of the sample) and one packet with 25 ms delay.
数据包样本包含三个“典型”延迟为20ms的数据包、一个低延迟为10ms的数据包（样本的最小值）和一个延迟为25ms的数据包。
As noted above, this example illustrates that IPDV may take on positive and negative values, while the PDV values are greater than or equal to zero. The histograms of IPDV and PDV are quite different in general shape, and the ranges are different, too (IPDV range = 20ms, PDV range = 15 ms). Note that the IPDV histogram will change if the sequence of delays is modified, but the PDV histogram will stay the same. PDV normalizes the one-way-delay distribution to the minimum delay and emphasizes the variation independent from the sequence of delays.
如上所述，该示例说明IPDV可以具有正值和负值，而PDV值大于或等于零。IPDV和PDV的直方图在总体形状上有很大不同，范围也不同（IPDV范围=20ms，PDV范围=15ms）。请注意，如果修改延迟序列，IPDV直方图将发生变化，但PDV直方图将保持不变。PDV将单向延迟分布标准化为最小延迟，并强调独立于延迟序列的变化。
This section summarizes previous work to compare these two forms of delay variation.
本节总结了以前的工作，以比较这两种形式的延迟变化。
In [Demichelis], Demichelis compared the early versions of two forms of delay variation. Although the IPDV form would eventually see widespread use, the ITU-T work-in-progress he cited did not utilize
在[Demichelis]中，Demichelis比较了两种形式的延迟变化的早期版本。虽然IPDV形式最终将得到广泛使用，但他所引用的ITU-T正在进行的工作并未得到利用
the same reference packets as PDV. Demichelis compared IPDV with the alternatives of using the delay of the first packet in the stream and the mean delay of the stream as the PDV reference packet. Neither of these alternative references were used in practice, and they are now deprecated in favor of the minimum delay of the stream [Y.1540].
与PDV相同的参考数据包。Demichelis将IPDV与使用流中第一个分组的延迟和流的平均延迟作为PDV参考分组的备选方案进行比较。这两种替代参考都没有在实践中使用，现在它们被弃用，取而代之的是流的最小延迟[Y.1540]。
Active measurements of a transcontinental path (Torino to Tokyo) provided the data for the comparison. The Poisson test stream had 0.764 second average inter-packet interval, with more than 58 thousand packets over 13.5 hours. Among Demichelis' observations about IPDV are the following:
对横贯大陆路径（都灵到东京）的主动测量为比较提供了数据。泊松测试流的平均数据包间隔为0.764秒，在13.5小时内超过5.8万个数据包。Demichelis对IPDV的观察如下：
1. IPDV is a measure of the network's ability to preserve the spacing between packets.
1. IPDV是网络保持数据包间距能力的一种度量。
2. The distribution of IPDV is usually symmetrical about the origin, having a balance of negative and positive values (for the most part). The mean is usually zero, unless some long-term delay trend is present.
2. IPDV的分布通常与原点对称，具有正负值的平衡（大部分情况下）。平均值通常为零，除非存在某些长期延迟趋势。
3. IPDV singletons distinguish quick-delay variations (short-term, on the order of the interval between packets) from longer-term variations.
3. IPDV单例将快速延迟变化（短期，按数据包之间的间隔顺序）与长期变化区分开来。
4. IPDV places reduced demands on the stability and skew of measurement clocks.
4. IPDV降低了对测量时钟稳定性和偏差的要求。
He also notes these features of PDV:
他还指出了PDV的以下特点：
1. The PDV distribution does not distinguish short-term variation from variation over the complete test interval. (Comment: PDV can be determined over any sub-intervals when the singletons are stored.)
1. PDV分布不区分整个试验间隔内的短期变化和变化。（注释：当存储单例时，可以在任何子间隔上确定PDV。）
2. The location of the distribution is very sensitive to the delay of the first packet, IF this packet is used as the reference. This would be a new formulation that differs from the PDV definition in this memo (PDV references the packet with minimum delay, so it does not have this drawback).
2. 如果将第一个数据包用作参考，则分布的位置对该数据包的延迟非常敏感。这将是一个不同于本备忘录中PDV定义的新公式（PDV以最小延迟引用数据包，因此它没有这个缺点）。
3. The shape of the PDV distribution is identical to the delay distribution, but shifted by the reference delay.
3. PDV分布的形状与延迟分布相同，但因参考延迟而改变。
4. Use of a common reference over measurement intervals that are longer than a typical session length may indicate more PDV than would be experienced by streams that support such sessions.
4. 在超过典型会话长度的测量间隔上使用公共参考可能表明比支持此类会话的流所经历的PDV更多。
(Ideally, the measurement interval should be aligned with the session length of interest, and this influences determination of the reference delay, D(min).)
（理想情况下，测量间隔应与感兴趣的会话长度一致，这会影响参考延迟D（min）的确定。）
5. The PDV distribution characterizes the range of queue occupancies along the measurement path (assuming the path is fixed), but the range says nothing about how the variation took place.
5. PDV分布描述了沿测量路径的队列占用范围（假设路径是固定的），但该范围没有说明变化是如何发生的。
The summary metrics used in this comparison were the number of values exceeding a +/-50ms range around the mean, the Inverse Percentiles, and the Inter-Quartile Range.
本次比较中使用的汇总指标是平均值、反百分位和四分位间距周围超过+/-50ms范围的数值数量。
5.2. Ciavattone et al.
5.2. Ciavattone等人。
In [Cia03], the authors compared IPDV and PDV (referred to as delta) using a periodic packet stream conforming to [RFC3432] with inter-packet interval of 20 ms.
在[Cia03]中，作者使用符合[RFC3432]的周期性数据包流（数据包间隔为20 ms）比较了IPDV和PDV（称为增量）。
One of the comparisons between IPDV and PDV involves a laboratory setup where a queue was temporarily congested by a competing packet burst. The additional queuing delay was 85 ms to 95 ms, much larger than the inter-packet interval. The first packet in the stream that follows the competing burst spends the longest time queued, and others experience less and less queuing time until the queue is drained.
IPDV和PDV之间的一个比较涉及一个实验室设置，其中一个队列因竞争性数据包突发而暂时拥塞。额外的排队延迟为85 ms到95 ms，远大于包间间隔。竞争突发之后的流中的第一个数据包花费的排队时间最长，而其他数据包经历的排队时间越来越少，直到队列耗尽为止。
The authors observed that PDV reflects the additional queuing time of the packets affected by the burst, with values of 85, 65, 45, 25, and 5 ms. Also, it is easy to determine (by looking at the PDV range) that a de-jitter buffer of >85 ms would have been sufficient to accommodate the delay variation. Again, the measurement interval is a key factor in the validity of such observations (it should have similar length to the session interval of interest).
作者观察到，PDV反映了受突发影响的数据包的额外排队时间，其值为85、65、45、25和5毫秒。此外，很容易确定（通过查看PDV范围）大于85毫秒的去抖动缓冲区足以适应延迟变化。同样，测量间隔是此类观察有效性的关键因素（其长度应与感兴趣的会话间隔相似）。
The IPDV values in the congested queue example are very different: 85, -20, -20, -20, -20, -5 ms. Only the positive excursion of IPDV gives an indication of the de-jitter buffer size needed. Although the variation exceeds the inter-packet interval, the extent of negative IPDV values is limited by that sending interval. This preference for information from the positive IPDV values has prompted some to ignore the negative values, or to take the absolute value of each IPDV measurement (sacrificing key properties of IPDV in the process, such as its ability to distinguish delay trends).
拥塞队列示例中的IPDV值非常不同：85、-20、-20、-20、-20、-5毫秒。只有IPDV的正偏移指示所需的去抖动缓冲区大小。尽管变化超过了包间间隔，但负IPDV值的范围受到该发送间隔的限制。这种对正IPDV值信息的偏好促使一些人忽略负值，或采用每个IPDV测量值的绝对值（牺牲过程中IPDV的关键特性，例如其区分延迟趋势的能力）。
Note that this example illustrates a case where the IPDV distribution is asymmetrical, because the delay variation range (85 ms) exceeds the inter-packet spacing (20 ms). We see that the IPDV values 85, -20, -20, -20, -20, -5 ms have zero mean, but the left side of the distribution is truncated at -20 ms.
注意，该示例说明了IPDV分布不对称的情况，因为延迟变化范围（85ms）超过了分组间间隔（20ms）。我们看到IPDV值85、-20、-20、-20、-20、-5 ms的平均值为零，但分布的左侧在-20 ms处被截断。
Elsewhere in the article, the authors considered the range as a summary statistic for IPDV, and the 99.9th percentile minus the minimum delay as a summary statistic for delay variation, or PDV.
在文章的其他部分，作者将范围视为IPDV的汇总统计，将99.9%减去最小延迟作为延迟变化或PDV的汇总统计。
Mike Pierce made many comments in the context of a working version of [RFC3393]. One of his main points was that a delay histogram is a useful approach to quantifying variation. Another point was that the time duration of evaluation is a critical aspect.
Mike Pierce在[RFC3393]的工作版本中发表了许多评论。他的主要观点之一是延迟直方图是量化变化的有用方法。另一点是，评价的持续时间是一个关键方面。
Carlo Demichelis then mailed his comparison paper [Demichelis] to the IPPM list, as discussed in more detail above.
Carlo Demichelis随后将其对比文件[Demichelis]邮寄至IPPM列表，如上文所述。
Ruediger Geib observed that both IPDV and the delay histogram (PDV) are useful, and suggested that they might be applied to different variation time scales. He pointed out that loss has a significant effect on IPDV, and encouraged that the loss information be retained in the arrival sequence.
Ruediger Geib观察到IPDV和延迟直方图（PDV）都是有用的，并建议它们可能适用于不同的变化时间尺度。他指出，丢失对IPDV有重大影响，并鼓励在到达序列中保留丢失信息。
Several example delay variation scenarios were discussed, including:
讨论了几个示例延迟变化场景，包括：
Packet # 1 2 3 4 5 6 7 8 9 10 11
-------------------------------------------------------
Ex. A
Lost
Packet # 1 2 3 4 5 6 7 8 9 10 11
-------------------------------------------------------
Ex. A
Lost
Delay, ms 100 110 120 130 140 150 140 130 120 110 100
延迟，毫秒100 110 130 140 140 130 110 100
IPDV U 10 10 10 10 10 -10 -10 -10 -10 -10
IPDV U 10 10 10 10 10 -10 -10 -10 -10 -10
PDV 0 10 20 30 40 50 40 30 20 10 0
PDV 0 10 20 40 50 40 30 10 0
-------------------------------------------------------
Ex. B
Lost L
-------------------------------------------------------
Ex. B
Lost L
Delay, ms 100 110 150 U 120 100 110 150 130 120 100
延迟，毫秒100 110 150 U 120 100 110 130 120 100
IPDV U 10 40 U U -10 10 40 -20 -10 -20
IPDV U 10 40 U U -10 10 40 -20 -10 -20
PDV 0 10 50 U 20 0 10 50 30 20 0
PDV 0 10 50 U 20 0 10 50 30 20 0
Figure 2: Delay Examples
图2：延迟示例
Clearly, the range of PDV values is 50 ms in both cases above, and this is the statistic that determines the size of a de-jitter buffer. The IPDV range is minimal in response to the smooth variation in Example A (20 ms). However, IPDV responds to the faster variations in Example B (60 ms range from 40 to -20). Here the IPDV range is larger than the PDV range, and overestimates the buffer size requirements.
显然，在上述两种情况下，PDV值的范围都是50 ms，这是确定去抖动缓冲区大小的统计数据。IPDV范围最小，以响应示例A中的平滑变化（20 ms）。然而，IPDV响应于示例B中更快的变化（60 ms范围从40到-20）。这里IPDV范围大于PDV范围，并且高估了缓冲区大小要求。
A heuristic method to estimate buffer size using IPDV is to sum the consecutive positive or zero values as an estimate of PDV range. However, this is more complicated to assess than the PDV range, and has strong dependence on the actual sequence of IPDV values (any negative IPDV value stops the summation, and again causes an underestimate).
使用IPDV估计缓冲区大小的一种启发式方法是将连续的正值或零值相加作为PDV范围的估计值。然而，这比PDV范围更难评估，并且强烈依赖于IPDV值的实际序列（任何负IPDV值都会停止求和，并再次导致低估）。
IPDV values can be viewed as the adjustments that an adaptive de-jitter buffer would make, if it could make adjustments on a packet-by-packet basis. However, adaptive de-jitter buffers don't make adjustments this frequently, so the value of this information is unknown. The short-term variations may be useful to know in some other cases.
IPDV值可被视为自适应去抖动缓冲区将进行的调整，前提是它可以逐包进行调整。但是，自适应去抖动缓冲区不会如此频繁地进行调整，因此该信息的值未知。在其他一些情况下，了解短期变化可能有用。
Appendix II of [Y.1540] describes a secondary terminology for delay variation. It compares IPDV, PDV (referred to as 2-point PDV), and 1-point packet delay variation (which assumes a periodic stream and assesses variation against an ideal arrival schedule constructed at a single measurement point). This early comparison discusses some of the same considerations raised in Section 6 below.
[Y.1540]的附录II描述了延迟变化的次要术语。它比较了IPDV、PDV（称为2点PDV）和1点数据包延迟变化（假设周期流，并根据在单个测量点构建的理想到达计划评估变化）。这一早期比较讨论了下文第6节中提出的一些相同的考虑因素。
Alan Clark's contribution to ITU-T Study Group 12 in January 2003 provided an analysis of the root causes of delay variation and investigated different techniques for measurement and modeling of "jitter" [COM12.D98]. Clark compared a metric closely related to IPDV, Mean Packet-to-Packet Delay Variation, MPPDV = mean(abs(D(i)- D(i-1))) to the newly proposed Mean Absolute Packet Delay Variation (MAPDV2, see [G.1020]). One of the tasks for this study was to estimate the number of packet discards in a de-jitter buffer. Clark concluded that MPPDV did not track the ramp delay variation he associated access link congestion (similar to Figure 2, Example A above), but MAPDV2 did.
Alan Clark在2003年1月对ITU-T研究小组12的贡献中分析了延迟变化的根本原因，并研究了测量和建模“抖动”的不同技术[COM12.D98]。Clark比较了与IPDV密切相关的指标，即平均包到包延迟变化，MPPDV=平均值（abs（D（i）-D（i-1）），以及新提出的平均绝对包延迟变化（MAPDV2，见[G.1020]）。本研究的任务之一是估计去抖动缓冲区中的数据包丢弃数。Clark得出结论，MPPDV没有跟踪与接入链路拥塞相关的匝道延迟变化（类似于上面的图2示例A），但MAPDV2跟踪了。
Clark also briefly looked at PDV (as described in the 2002 version of [Y.1541]). He concluded that if PDV was applied to a series of very short measurement intervals (e.g., 200 ms), it could be used to determine the fraction of intervals with high packet discard rates.
Clark还简要介绍了PDV（如2002年版[Y.1541]所述）。他得出结论，如果将PDV应用于一系列非常短的测量间隔（例如，200 ms），则可以使用PDV确定具有高数据包丢弃率的间隔分数。
This section treats some of the earlier comparison areas in more detail and introduces new areas for comparison.
本节将更详细地讨论一些早期的比较区域，并介绍新的比较区域。
The measurement of packet loss is of great influence for the delay variation results, as displayed in the Figures 3 and 4 (L means Lost and U means Undefined). Figure 3 shows that in the extreme case of every other packet loss, the IPDV metric doesn't produce any results, while the PDV produces results for all arriving packets.
如图3和图4所示（L表示丢失，U表示未定义），分组丢失的测量对延迟变化结果有很大影响。图3显示，在极端情况下，每隔一个数据包丢失，IPDV度量不会产生任何结果，而PDV会为所有到达的数据包产生结果。
Packet # 1 2 3 4 5 6 7 8 9 10
Lost L L L L L
---------------------------------------
Delay, ms 3 U 5 U 4 U 3 U 4 U
Packet # 1 2 3 4 5 6 7 8 9 10
Lost L L L L L
---------------------------------------
Delay, ms 3 U 5 U 4 U 3 U 4 U
IPDV U U U U U U U U U U
IPDVU
PDV 0 U 2 U 1 U 0 U 1 U
PDV0U2U1U0U1U
Figure 3: Path Loss Every Other Packet
图3：每隔一个数据包的路径丢失
In case of a burst of packet loss, as displayed in Figure 4, both the IPDV and PDV metrics produce some results. Note that PDV still produces more values than IPDV.
在突发数据包丢失的情况下，如图4所示，IPDV和PDV度量都会产生一些结果。请注意，PDV仍然比IPDV生成更多的值。
Packet # 1 2 3 4 5 6 7 8 9 10
Lost L L L L L
---------------------------------------
Delay, ms 3 4 U U U U U 5 4 3
Packet # 1 2 3 4 5 6 7 8 9 10
Lost L L L L L
---------------------------------------
Delay, ms 3 4 U U U U U 5 4 3
IPDV U 1 U U U U U U -1 -1
IPDVU-1-1
PDV 0 1 U U U U U 2 1 0
PDV 0 1 U U 2 1 0
Figure 4: Burst of Packet Loss
图4：突发数据包丢失
In conclusion, the PDV results are affected by the packet-loss ratio. The IPDV results are affected by both the packet-loss ratio and the packet-loss distribution. In the extreme case of loss of every other packet, IPDV doesn't provide any results.
总之，PDV结果受丢包率的影响。IPDV结果受丢包率和丢包分布的影响。在其他数据包丢失的极端情况下，IPDV不会提供任何结果。
When there is little or no stability in the network under test, then the devices that attempt to characterize the network are equally stressed, especially if the results displayed are used to make inferences that may not be valid.
当被测网络的稳定性很低或没有稳定性时，则尝试表征网络的设备也会受到同样的压力，尤其是当显示的结果用于做出可能无效的推断时。
Sometimes the path characteristics change during a measurement interval. The change may be due to link or router failure, administrative changes prior to maintenance (e.g., link-cost change), or re-optimization of routing using new information. All these causes are usually infrequent, and network providers take appropriate measures to ensure this. Automatic restoration to a back-up path is seen as a desirable feature of IP networks.
有时，路径特性在测量间隔期间发生变化。更改可能是由于链路或路由器故障、维护前的管理更改（例如，链路成本更改）或使用新信息重新优化路由。所有这些原因通常很少发生，网络提供商会采取适当的措施来确保这一点。自动恢复到备份路径被视为IP网络的理想功能。
Frequent path changes and prolonged congestion with substantial packet loss clearly make delay variation measurements challenging.
频繁的路径变化和长时间的拥塞以及大量的数据包丢失显然使时延变化测量具有挑战性。
Path changes are usually accompanied by a sudden, persistent increase or decrease in one-way delay. [Cia03] gives one such example. We assume that a restoration path either accepts a stream of packets or is not used for that particular stream (e.g., no multi-path for flows).
路径变化通常伴随着单向延迟的突然、持续增加或减少。[Cia03]给出了一个这样的例子。我们假设恢复路径要么接受数据包流，要么不用于该特定流（例如，流没有多路径）。
In any case, a change in the Time to Live (TTL) (or Hop Limit) of the received packets indicates that the path is no longer the same. Transient packet reordering may also be observed with path changes, due to use of non-optimal routing while updates propagate through the network (see [Casner] and [Cia03] )
在任何情况下，所接收分组的生存时间（TTL）（或跳数限制）的改变指示路径不再相同。当更新通过网络传播时，由于使用非最佳路由，也可能在路径更改时观察到瞬态数据包重新排序（请参见[Casner]和[Cia03]）
Many, if not all, packet streams experience packet loss in conjunction with a path change. However, it is certainly possible that the active measurement stream does not experience loss. This may be due to use of a long inter-packet sending interval with respect to the restoration time, and it becomes more likely as "fast restoration" techniques see wider deployment (e.g., [RFC4090]).
许多（如果不是全部的话）数据包流都会在路径改变的同时经历数据包丢失。然而，肯定有可能主动测量流不经历损失。这可能是由于使用了关于恢复时间的长分组间发送间隔，并且随着“快速恢复”技术看到更广泛的部署（例如，[RFC4090]），这变得更可能。
Thus, there are two main cases to consider, path changes accompanied by loss, and those that are lossless from the point of view of the active measurement stream. The subsections below examine each of these cases.
因此，有两个主要的情况要考虑，路径变化伴随着损失，以及那些从活动测量流的角度来看是无损的。下面的小节将对每种情况进行分析。
In the lossless case, a path change will typically affect only one IPDV singleton. For example, the delay sequence in the Figure below always produces IPDV=0 except in the one case where the value is 5 (U, 0, 0, 0, 5, 0, 0, 0, 0).
在无损情况下，路径更改通常只影响一个IPDV单例。例如，下图中的延迟序列总是产生IPDV=0，除非在一种情况下该值为5（U，0，0，0，5，0，0，0）。
Packet # 1 2 3 4 5 6 7 8 9
Lost
------------------------------------
Delay, ms 4 4 4 4 9 9 9 9 9
Packet # 1 2 3 4 5 6 7 8 9
Lost
------------------------------------
Delay, ms 4 4 4 4 9 9 9 9 9
IPDV U 0 0 0 5 0 0 0 0
IPDVU 0 0 0 5 0 0 0 0 0 0
PDV 0 0 0 0 5 5 5 5 5
PDV 0 0 0 5 5 5 5 5
Figure 5: Lossless Path Change
图5：无损路径更改
However, if the change in delay is negative and larger than the inter-packet sending interval, then more than one IPDV singleton may be affected because packet reordering is also likely to occur.
然而，如果延迟的变化为负且大于分组间发送间隔，则可能会影响多个IPDV单例，因为分组重新排序也可能发生。
The use of the new path and its delay variation can be quantified by treating the PDV distribution as bi-modal, and characterizing each mode separately. This would involve declaring a new path within the sample, and using a new local minimum delay as the PDV reference delay for the sub-sample (or time interval) where the new path is present.
新路径的使用及其延迟变化可以通过将PDV分布视为双模，并分别描述每个模式来量化。这将涉及在样本内声明新路径，并使用新的本地最小延迟作为存在新路径的子样本（或时间间隔）的PDV参考延迟。
The process of detecting a bi-modal delay distribution is made difficult if the typical delay variation is larger than the delay change associated with the new path. However, information on a TTL (or Hop Limit) change or the presence of transient reordering can assist in an automated decision.
如果典型延迟变化大于与新路径相关的延迟变化，则检测双模延迟分布的过程将变得困难。但是，有关TTL（或跃点限制）更改或暂时重新排序的信息可以帮助进行自动决策。
The effect of path changes may also be reduced by making PDV measurements over short intervals (minutes, as opposed to hours). This way, a path change will affect one sample and its PDV values. Assuming that the mean or median one-way delay changes appreciably on the new path, then subsequent measurements can confirm a path change and trigger special processing on the interval to revise the PDV result.
路径变化的影响也可以通过在短时间间隔（分钟，而不是小时）内进行PDV测量来降低。这样，路径更改将影响一个样本及其PDV值。假设平均或中值单向延迟在新路径上发生明显变化，则后续测量可确认路径变化，并触发间隔上的特殊处理以修正PDV结果。
Alternatively, if the path change is detected, by monitoring the test packets TTL or Hop Limit, or monitoring the change in the IGP link-state database, the results of measurement before and after the path change could be kept separated, presenting two different distributions. This avoids the difficult task of determining the different modes of a multi-modal distribution.
或者，如果通过监测测试包TTL或Hop Limit或监测IGP链路状态数据库中的变化来检测到路径变化，则路径变化前后的测量结果可以保持分离，呈现两种不同的分布。这避免了确定多模态分布的不同模态的困难任务。
If the path change is accompanied by loss, such that there are no consecutive packet pairs that span the change, then no IPDV singletons will reflect the change. This may or may not be desirable, depending on the ultimate use of the delay variation measurement. Figure 6, in which L means Lost and U means Undefined, illustrates this case.
如果路径变化伴随着丢失，因此没有跨越该变化的连续数据包对，则没有IPDV单例将反映该变化。根据延迟变化测量的最终用途，这可能是可取的，也可能不是可取的。图6说明了这种情况，其中L表示丢失，U表示未定义。
Packet # 1 2 3 4 5 6 7 8 9
Lost L L
------------------------------------
Delay, ms 3 4 3 3 U U 8 9 8
Packet # 1 2 3 4 5 6 7 8 9
Lost L L
------------------------------------
Delay, ms 3 4 3 3 U U 8 9 8
IPDV U 1 -1 0 U U U 1 -1
IPDVU1-10U1-1
PDV 0 1 0 0 U U 5 6 5
PDV 01 0 U 5 6 5
Figure 6: Path Change with Loss
图6：有损耗的路径变化
PDV will again produce a bi-modal distribution. But here, the decision process to define sub-intervals associated with each path is further assisted by the presence of loss, in addition to TTL, reordering information, and use of short measurement intervals consistent with the duration of user sessions. It is reasonable to assume that at least loss and delay will be measured simultaneously with PDV and/or IPDV.
PDV将再次产生双峰分布。但是在这里，除了TTL、重新排序信息和使用与用户会话持续时间一致的短测量间隔之外，还通过丢失的存在来进一步辅助定义与每个路径相关联的子间隔的决策过程。合理的假设是，至少损失和延迟将与PDV和/或IPDV同时测量。
IPDV does not help to detect path changes when accompanied by loss, and this is a disadvantage for those who rely solely on IPDV measurements.
IPDV在伴随丢失时无法帮助检测路径变化，这对于仅依赖IPDV测量的人是不利的。
Low cost or low complexity measurement systems may be embedded in communication devices that do not have access to high stability clocks, and time errors will almost certainly be present. However, larger time-related errors (~1 ms) may offer an acceptable trade-off for monitoring performance over a large population (the accuracy needed to detect problems may be much less than required for a scientific study, ~0.01 ms for example).
低成本或低复杂度的测量系统可以嵌入到无法访问高稳定性时钟的通信设备中，并且几乎肯定会出现时间误差。然而，较大的时间相关误差（~1 ms）可能为监测大量人群的性能提供可接受的折衷方案（检测问题所需的准确度可能远低于科学研究所需的准确度，例如~0.01 ms）。
Maintaining time accuracy <<1 ms has typically required access to dedicated time receivers at all measurement points. Global positioning system (GPS) receivers have often been installed to support measurements. The GPS installation conditions are fairly restrictive, and many prospective measurement efforts have found the deployment complexity and system maintenance too difficult.
保持时间精度<1 ms通常需要在所有测量点访问专用时间接收器。通常安装全球定位系统（GPS）接收器以支持测量。GPS安装条件相当严格，许多预期测量工作发现部署复杂性和系统维护过于困难。
As mentioned above, [Demichelis] observed that PDV places greater demands on clock synchronization than for IPDV. This observation deserves more discussion. Synchronization errors have two components: time-of-day errors and clock-frequency errors (resulting in skew).
如上所述，[Demichelis]观察到PDV对时钟同步的要求比IPDV更高。这一观察值得更多讨论。同步错误有两个组成部分：时间错误和时钟频率错误（导致偏差）。
Both IPDV and PDV are sensitive to time-of-day errors when attempting to align measurement intervals at the source and destination. Gross misalignment of the measurement intervals can lead to lost packets, for example, if the receiver is not ready when the first test packet arrives. However, both IPDV and PDV assess delay differences, so the error present in any two one-way-delay singletons will cancel as long as the error is constant. So, the demand for NTP or GPS synchronization comes primarily from one-way-delay measurement time-of-day accuracy requirements. Delay variation and measurement interval alignment are relatively less demanding.
IPDV和PDV在尝试在源和目标处对齐测量间隔时，对一天中的时间错误都很敏感。测量间隔的严重偏差可能导致数据包丢失，例如，如果第一个测试数据包到达时接收器未准备好。然而，IPDV和PDV都评估延迟差异，因此只要误差保持不变，任何两个单向延迟单例中存在的误差都将被抵消。因此，对NTP或GPS同步的需求主要来自单向延迟测量一天中的时间精度要求。延迟变化和测量间隔校准要求相对较低。
Skew is a measure of the change in clock time over an interval with respect to a reference clock. Both IPDV and PDV are affected by skew, but the error sensitivity in IPDV singletons is less because the intervals between consecutive packets are rather small, especially when compared to the overall measurement interval. Since PDV computes the difference between a single reference delay (the sample minimum) and all other delays in the measurement interval, the constraint on skew error is greater to attain the same accuracy as IPDV. Again, use of short PDV measurement intervals (on the order of minutes, not hours) provides some relief from the effects of skew error. Thus, the additional accuracy demand of PDV can be expressed as a ratio of the measurement interval to the inter-packet spacing.
歪斜是相对于参考时钟的时钟时间间隔变化的度量。IPDV和PDV都受到歪斜的影响，但IPDV单例中的错误敏感性较小，因为连续数据包之间的间隔相当小，尤其是与总体测量间隔相比。由于PDV计算单个参考延迟（样本最小值）与测量间隔内所有其他延迟之间的差值，因此对倾斜误差的约束更大，以达到与IPDV相同的精度。同样，使用较短的PDV测量间隔（以分钟为单位，而不是以小时为单位）可以减轻偏斜误差的影响。因此，PDV的额外精度需求可以表示为测量间隔与分组间间隔的比率。
A practical example is a measurement between two hosts, one with a synchronized clock and the other with a free-running clock having 50 parts per million (ppm) long term accuracy.
一个实际示例是两台主机之间的测量，一台具有同步时钟，另一台具有50 ppm长期精度的自由运行时钟。
o If IPDV measurements are made on packets with a 1 second spacing, the maximum singleton error will be 1 x 5 x 10^-5 seconds, or 0.05 ms.
o 如果对间隔为1秒的数据包进行IPDV测量，则最大单例误差将为1x5x10^-5秒，或0.05毫秒。
o If PDV measurements are made on the same packets over a 60 second measurement interval, then the delay variation due to the max free-running clock error will be 60 x 5 x 10-5 seconds, or 3 ms delay variation error from the first packet to the last.
o 如果在60秒测量间隔内对相同数据包进行PDV测量，则由于最大自由运行时钟错误导致的延迟变化将为60 x 5 x 10-5秒，或从第一个数据包到最后一个数据包的3 ms延迟变化错误。
Therefore, the additional accuracy required for equivalent PDV error under these conditions is a factor of 60 more than for IPDV. This is a rather extreme scenario, because time-of-day error of 1 second would accumulate in ~5.5 hours, potentially causing the measurement interval alignment issue described above.
因此，在这些条件下，等效PDV误差所需的额外精度比IPDV高60倍。这是一种非常极端的情况，因为1秒的时间误差将在约5.5小时内累积，可能导致上述测量间隔校准问题。
If skew is present in a sample of one-way delays, its symptom is typically a nearly linear growth or decline over all the one-way-delay values. As a practical matter, if the same slope appears consistently in the measurements, then it may be possible to fit the slope and compensate for the skew in the one-way-delay measurements, thereby avoiding the issue in the PDV calculations that follow. See [RFC3393] for additional information on compensating for skew.
如果单向延迟样本中存在偏差，则其症状通常是所有单向延迟值的近似线性增长或下降。实际上，如果测量中始终出现相同的斜率，则可以拟合斜率并补偿单向延迟测量中的倾斜，从而避免随后PDV计算中的问题。有关补偿倾斜的更多信息，请参见[RFC3393]。
Values for IPDV may have non-zero mean over a sample when clock skew is present. This tends to complicate IPDV analysis when using the assumptions of a zero mean and a symmetric distribution.
当存在时钟偏移时，IPDV的值在样本上可能具有非零平均值。当使用零均值和对称分布的假设时，这往往使IPDV分析复杂化。
There is a third factor related to clock error and stability: this is the presence of a clock-synchronization protocol (e.g., NTP) and the time-adjustment operations that result. When a time error is detected (typically on the order of a few milliseconds), the host
第三个因素与时钟错误和稳定性有关：这是存在时钟同步协议（例如NTP）以及由此产生的时间调整操作。当检测到时间错误时（通常为几毫秒），主机
clock frequency is continuously adjusted to reduce the time error. If these adjustments take place during a measurement interval, they may appear as delay variation when none was present, and therefore are a source of error (regardless of the form of delay variation considered).
时钟频率不断调整，以减少时间误差。如果这些调整发生在测量间隔期间，则在不存在延迟变化时，它们可能显示为延迟变化，因此是误差源（无论考虑延迟变化的形式如何）。
ITU-T Recommendation [Y.1541] gives a provisional method to compose a PDV metric using PDV measurement results from two or more sub-paths. Additional methods are considered in [IPPM-Spatial].
ITU-T建议[Y.1541]给出了使用来自两个或多个子路径的PDV测量结果组成PDV度量的临时方法。[IPPM Spatial]中考虑了其他方法。
PDV has a clear advantage at this time, since there is no validated method to compose an IPDV metric. In addition, IPDV results depend greatly on the exact sequence of packets and may not lend themselves easily to the composition problem, where segments must be assumed to have independent delay distributions.
目前，PDV具有明显的优势，因为没有经过验证的方法来组成IPDV度量。此外，IPDV结果在很大程度上取决于数据包的精确序列，并且可能不容易用于组合问题，其中必须假设数据段具有独立的延迟分布。
Despite the risk of over-summarization, measurements must often be displayed for easy consumption. If the right summary report is prepared, then the "dashboard" view correctly indicates whether there is something different and worth investigating further, or that the status has not changed. The dashboard model restricts every instrument display to a single number. The packet network dashboard could have different instruments for loss, delay, delay variation, reordering, etc., and each must be summarized as a single number for each measurement interval. The single number summary statistic is a key component of SLAs, where a threshold on that number must be met x% of the time.
尽管存在过度汇总的风险，但为了便于使用，必须经常显示度量值。如果准备了正确的摘要报告，“dashboard”视图将正确指示是否存在不同的、值得进一步调查的内容，或者状态没有改变。仪表板型号将每个仪表显示限制为一个数字。数据包网络仪表板可以有不同的丢失、延迟、延迟变化、重新排序等仪表，每个仪表必须汇总为每个测量间隔的单个数字。单个数字摘要统计信息是SLA的一个关键组成部分，其中该数字的阈值必须在x%的时间内达到。
The simplicity of the PDV distribution lends itself to this summarization process (including use of the percentiles, median or mean). An SLA of the form "no more than x% of packets in a measurement interval shall have PDV >= y ms, for no less than z% of time" is relatively straightforward to specify and implement. [Y.1541] introduced the notion of a pseudo-range when setting an objective for the 99.9th percentile of PDV. The conventional range (max-min) was avoided for several reasons, including stability of the maximum delay. The 99.9th percentile of PDV is helpful to performance planners (seeking to meet some user-to-user objective for delay) and in design of de-jitter buffer sizes, even those with adaptive capabilities.
PDV分布的简单性有助于总结过程（包括使用百分位数、中位数或平均值）。“测量间隔内不超过x%的数据包的PDV>=y ms，时间不少于z%”形式的SLA相对容易指定和实现。[Y.1541]在为PDV的99.9%设定目标时引入了伪范围的概念。由于几个原因，包括最大延迟的稳定性，避免了常规范围（最大-最小）。PDV的99.9%有助于性能规划人员（寻求满足某些用户对用户的延迟目标）和去抖动缓冲区大小的设计，即使是那些具有自适应能力的缓冲区。
IPDV does not lend itself to summarization so easily. The mean IPDV is typically zero. As the IPDV distribution will have two tails (positive and negative), the range or pseudo-range would not match
IPDV不太容易总结。平均IPDV通常为零。由于IPDV分布将有两个尾部（正和负），因此范围或伪范围将不匹配
the needed de-jitter buffer size. Additional complexity may be introduced when the variation exceeds the inter-packet sending interval, as discussed above (in Sections 5.2 and 6.2.1). Should the Inter-Quartile Range be used? Should the singletons beyond some threshold be counted (e.g., mean +/- 50 ms)? A strong rationale for one of these summary statistics has yet to emerge.
所需的去抖动缓冲区大小。如上文（第5.2节和第6.2.1节）所述，当变化超过包间发送间隔时，可能会引入额外的复杂性。是否应使用四分位间距？是否应计算超出某个阈值的单重态（例如，平均+/-50 ms）？这些汇总统计数据中有一个强有力的理由尚未出现。
When summarizing IPDV, some prefer the simplicity of the single-sided distribution created by taking the absolute value of each singleton result, abs(D(i)-D(i-1)). This approach sacrifices the two-sided inter-arrival spread information in the distribution. It also makes the evaluation using percentiles more confusing, because a single late packet that exceeds the variation threshold will cause two pairs of singletons to fail the criteria (one positive, the other negative converted to positive). The single-sided PDV distribution is an advantage in this category.
在总结IPDV时，有些人更喜欢通过取每个单例结果的绝对值abs（D（i）-D（i-1））创建的单边分布的简单性。这种方法牺牲了分布中的双边到达间传播信息。这也使得使用百分位数的评估更加混乱，因为超过变化阈值的单个延迟数据包将导致两对单例不符合标准（一个为正，另一个为负转换为正）。单边PDV分布是这一类别的优势。
Section 6.4.1 of [RFC3550] gives the calculation of the "inter-arrival jitter" field for the RTP Control Protocol (RTCP) report, with a sample implementation in an Appendix.
[RFC3550]第6.4.1节给出了RTP控制协议（RTCP）报告的“到达间抖动”字段的计算，附录中给出了示例实现。
The RTCP "interarrival jitter" value can be calculated using IPDV singletons. If there is packet reordering, as defined in [RFC4737], then estimates of Jitter based on IPDV may vary slightly, because [RFC3550] specifies the use of receive-packet order.
RTCP“到达间抖动”值可以使用IPDV单态来计算。如果存在[RFC4737]中定义的分组重新排序，则基于IPDV的抖动估计值可能略有不同，因为[RFC3550]指定使用接收分组顺序。
Just as there is no simple way to convert PDV singletons to IPDV singletons without returning to the original sample of delay singletons, there is no clear relationship between PDV and [RFC3550] "interarrival jitter".
正如在不返回延迟单例的原始样本的情况下，没有简单的方法将PDV单例转换为IPDV单例一样，PDV和[RFC3550]“到达间抖动”之间也没有明确的关系。
MAPDV2 stands for Mean Absolute Packet Delay Variation (version) 2, and is specified in [G.1020]. The MAPDV2 algorithm computes a smoothed running estimate of the mean delay using the one-way delays of 16 previous packets. It compares the current one-way delay to the estimated mean, separately computes the means of positive and negative deviations, and sums these deviation means to produce MAPVDV2. In effect, there is a MAPDV2 singleton for every arriving packet, so further summarization is usually warranted.
MAPDV2代表平均绝对数据包延迟变化（版本）2，并在[G.1020]中规定。MAPDV2算法使用前16个数据包的单向延迟计算平均延迟的平滑运行估计。它将当前单向延迟与估计平均值进行比较，分别计算正偏差和负偏差的平均值，并将这些偏差平均值相加以生成MAPVDV2。实际上，每个到达的数据包都有一个MAPDV2单例，因此通常需要进一步的摘要。
Neither IPDV or PDV forms assist in the computation of MAPDV2.
IPDV或PDV表单都不能帮助计算MAPDV2。
Network traffic load balancing is a process to divide packet traffic in order to provide a more even distribution over two or more equally viable paths. The paths chosen are based on the IGP cost metrics, while the delay depends on the path's physical layout. Usually, the balancing process is performed on a per-flow basis to avoid delay variation experienced when packets traverse different physical paths.
网络流量负载平衡是一个划分数据包流量的过程，以便在两条或多条同样可行的路径上提供更均匀的分布。选择的路径基于IGP成本指标，而延迟取决于路径的物理布局。通常，在每个流的基础上执行平衡过程，以避免当数据包穿过不同的物理路径时所经历的延迟变化。
If the sample includes test packets with different characteristics such as IP addresses/ports, there could be multi-modal delay distributions present. The PDV form makes the identification of multiple modes possible. IPDV may also reveal that multiple paths are in use with a mixed-flow sample, but the different delay modes are not easily divided and analyzed separately.
如果样本包括具有不同特征（如IP地址/端口）的测试数据包，则可能存在多模式延迟分布。PDV表格使多种模式的识别成为可能。IPDV还可能揭示混合流样本中使用了多条路径，但不同的延迟模式不容易单独划分和分析。
Should the delay singletons using multiple addresses/ports be combined in the same sample? Should we characterize each mode separately? (This question also applies to the Path Change case.) It depends on the task to be addressed by the measurement.
使用多个地址/端口的延迟单例是否应该组合在同一个示例中？我们应该分别描述每种模式吗？（这个问题也适用于路径更改情况。）这取决于测量要解决的任务。
For the task of de-jitter buffer sizing or assessing queue occupation, the modes should be characterized separately because flows will experience only one mode on a stable path. Use of a single flow description (address/port combination) in each sample simplifies this analysis. Multiple modes may be identified by collecting samples with different flow attributes, and characterization of multiple paths can proceed with comparison of the delay distributions from each sample.
对于消除抖动缓冲区大小或评估队列占用的任务，应分别描述这些模式，因为流在稳定路径上只会经历一种模式。在每个示例中使用单个流描述（地址/端口组合）简化了此分析。可通过收集具有不同流量属性的样本来识别多个模式，并且可通过比较每个样本的延迟分布来对多个路径进行表征。
For the task of capacity planning and routing optimization, characterizing the modes separately could offer an advantage. Network-wide capacity planning (as opposed to link capacity planning) takes as input the core traffic matrix, which corresponds to a matrix of traffic transferred from every source to every destination in the network. Applying the core traffic matrix along with the routing information (typically the link state database of a routing protocol) in a capacity planning tool offers the possibility to visualize the paths where the traffic flows and to optimize the routing based on the link utilization. In the case where equal cost multiple paths (ECMPs) are used, the traffic will be load balanced onto multiple paths. If each mode of the IP delay multi-modal distribution can be associated with a specific path, the delay performance offers an extra optimization parameter, i.e., the routing optimization based on the IP delay variation metric. As an example, the load balancing across ECMPs could be suppressed so that the Voice over IP (VoIP) calls would only be routed via the path with the lower IP delay
对于容量规划和路由优化任务，单独描述模式可能提供优势。网络范围的容量规划（与链路容量规划相反）将核心流量矩阵作为输入，该矩阵对应于网络中从每个源传输到每个目的地的流量矩阵。在容量规划工具中应用核心流量矩阵和路由信息（通常是路由协议的链路状态数据库），可以可视化流量流动的路径，并基于链路利用率优化路由。在使用等成本多路径（ECMP）的情况下，流量将在多条路径上进行负载平衡。如果IP延迟多模分布的每个模式都可以与特定路径相关联，则延迟性能提供了一个额外的优化参数，即基于IP延迟变化度量的路由优化。例如，可以抑制ECMP之间的负载平衡，以便IP语音（VoIP）呼叫只能通过IP延迟较低的路径路由
variation. Clearly, any modifications can result in new delay performance measurements, so there must be a verification step to ensure the desired outcome.
变异显然，任何修改都可能导致新的延迟性能测量，因此必须有一个验证步骤来确保预期结果。
Based on the comparisons of IPDV and PDV presented above, this section matches the attributes of each form with the tasks described earlier. We discuss the more general circumstances first.
基于上述IPDV和PDV的比较，本节将每个表单的属性与前面描述的任务相匹配。我们先讨论更一般的情况。
The PDV distribution is anchored at the minimum delay observed in the measurement interval. When the sample minimum coincides with the true minimum delay of the path, then the PDV distribution is equivalent to the queuing time distribution experienced by the test stream. If the minimum delay is not the true minimum, then the PDV distribution captures the variation in queuing time and some additional amount of queuing time is experienced, but unknown. One can summarize the PDV distribution with the mean, median, and other statistics.
PDV分布固定在测量间隔内观察到的最小延迟处。当样本最小值与路径的真实最小延迟一致时，则PDV分布等效于测试流所经历的排队时间分布。如果最小延迟不是真正的最小值，则PDV分布会捕获排队时间的变化，并且会经历一些额外的排队时间，但这是未知的。可以用均值、中位数和其他统计数据总结PDV分布。
IPDV can capture the difference in queuing time from one packet to the next, but this is a different distribution from the queue occupancy revealed by PDV.
IPDV可以捕获从一个数据包到下一个数据包的排队时间差异，但这与PDV显示的队列占用情况不同。
This task is complimentary to the problem of inferring queue occupancy through measurement. Again, use of the sample minimum as the reference delay for PDV yields a distribution that is very relevant to de-jitter buffer size. This is because the minimum delay is an alignment point for the smoothing operation of de-jitter buffers. A de-jitter buffer that is ideally aligned with the delay variation adds zero buffer time to packets with the longest accommodated network delay (any packets with longer delays are discarded). Thus, a packet experiencing minimum network delay should be aligned to wait the maximum length of the de-jitter buffer. With this alignment, the stream is smoothed with no unnecessary delay added. Figure 5 of [G.1020] illustrates the ideal relationship between network delay variation and buffer time.
此任务与通过测量推断队列占用率的问题是互补的。同样，使用样本最小值作为PDV的参考延迟会产生与去抖动缓冲区大小非常相关的分布。这是因为最小延迟是去抖动缓冲器平滑操作的对齐点。理想地与延迟变化对齐的去抖动缓冲器将零缓冲时间添加到具有最长适应网络延迟的分组（具有较长延迟的任何分组被丢弃）。因此，经历最小网络延迟的数据包应对齐以等待去抖动缓冲区的最大长度。通过这种对齐，流被平滑，没有添加不必要的延迟。[G.1020]的图5说明了网络延迟变化和缓冲时间之间的理想关系。
The PDV distribution is also useful for this task, but different statistics are preferred. The range (max-min) or the 99.9th percentile of PDV (pseudo-range) are closely related to the buffer size needed to accommodate the observed network delay variation.
PDV分布对于此任务也很有用，但最好使用不同的统计信息。范围（max-min）或PDV的99.9%（伪范围）与容纳观察到的网络延迟变化所需的缓冲区大小密切相关。
The PDV distribution directly addresses the FEC waiting time question. When the PDV distribution has a 99th percentile of 10 ms, then waiting 10 ms longer than the FEC protection interval will allow 99% of late packets to arrive and be used in the FEC block.
PDV分布直接解决了FEC等待时间问题。当PDV分布具有10 ms的第99百分位时，比FEC保护间隔长10 ms的等待将允许99%的延迟数据包到达并在FEC块中使用。
In some cases, the positive excursions (or series of positive excursions) of IPDV may help to approximate the de-jitter buffer size, but there is no guarantee that a good buffer estimate will emerge, especially when the delay varies as a positive trend over several test packets.
在某些情况下，IPDV的正偏移（或一系列正偏移）可能有助于近似解抖动缓冲区大小，但不能保证会出现良好的缓冲区估计，尤其是当延迟在多个测试数据包上以正趋势变化时。
PDV has a clear advantage at this time, since there is no validated method to compose an IPDV metric.
目前，PDV具有明显的优势，因为没有经过验证的方法来组成IPDV度量。
The one-sided PDV distribution can be constrained with a single statistic, such as an upper percentile, so it is preferred. The IPDV distribution is two-sided, usually has zero mean, and no universal summary statistic that relates to a physical quantity has emerged in years of experience.
单侧PDV分布可以用单个统计数据（如上百分位）进行约束，因此它是首选。IPDV分布是双边的，通常平均值为零，多年的经验中没有出现与物理量相关的通用汇总统计。
Note that measurement of delay variation may not be the primary concern under unstable and unreliable circumstances.
注意，在不稳定和不可靠的情况下，延迟变化的测量可能不是主要问题。
When appreciable skew is present between measurement system clocks, IPDV has an advantage because PDV would require processing over the entire sample to remove the skew error. However, significant skew can invalidate IPDV analysis assumptions, such as the zero-mean and symmetric-distribution characteristics. Small skew may well be within the error tolerance, and both PDV and IPDV results will be usable. There may be a portion of the skew, measurement interval, and required accuracy 3-D space where IPDV has an advantage, depending on the specific measurement specifications.
当测量系统时钟之间存在明显偏差时，IPDV具有优势，因为PDV需要对整个样本进行处理以消除偏差误差。然而，显著的偏斜会使IPDV分析假设失效，如零均值和对称分布特征。小偏差可能在误差容许范围内，PDV和IPDV结果都可用。根据具体的测量规范，可能存在一部分倾斜、测量间隔和所需精度的三维空间，其中IPDV具有优势。
Neither form of delay variation is more suited than the other to on-the-fly summarization without memory, and this may be one of the reasons that [RFC3550] RTCP Jitter and MAPDV2 in [G.1020] have attained deployment in low-cost systems.
这两种延迟变化形式都不比另一种更适合无内存的即时摘要，这可能是[G.1020]中[RFC3550]RTCP抖动和MAPDV2在低成本系统中实现部署的原因之一。
If the network under test exhibits frequent path changes, on the order of several new routes per minute, then IPDV appears to isolate the delay variation on each path from the transient effect of path change (especially if there is packet loss at the time of path change). However, if one intends to use IPDV to indicate path changes, it cannot do this when the change is accompanied by loss.
如果被测网络以每分钟几个新路由的顺序频繁改变路径，则IPDV似乎将每条路径上的延迟变化与路径改变的瞬态效应隔离开来（特别是在路径改变时存在数据包丢失的情况下）。但是，如果打算使用IPDV来指示路径更改，则当更改伴随丢失时，它不能这样做。
It is possible to make meaningful PDV measurements when paths are unstable, but great importance would be placed on the algorithms that infer path change and attempt to divide the sample on path change boundaries.
当路径不稳定时，可以进行有意义的PDV测量，但对于推断路径变化并尝试在路径变化边界上划分样本的算法，将给予高度重视。
When path changes are frequent and cause packet loss, delay variation is probably less important than the loss episodes and attention should be turned to the loss metric instead.
当路径变化频繁并导致数据包丢失时，延迟变化可能不如丢失事件重要，应将注意力转向丢失度量。
If the network under test exhibits frequent loss, then PDV may produce a larger set of singletons for the sample than IPDV. This is due to IPDV requiring consecutive packet arrivals to assess delay variation, compared to PDV where any packet arrival is useful. The worst case is when no consecutive packets arrive and the entire IPDV sample would be undefined, yet PDV would successfully produce a sample based on the arriving packets.
如果被测网络频繁丢失，则PDV可能会产生比IPDV更大的样本单态集。这是因为IPDV需要连续的数据包到达来评估延迟变化，而PDV中的任何数据包到达都是有用的。最坏的情况是，没有连续的数据包到达，并且整个IPDV样本未定义，但PDV将基于到达的数据包成功地生成样本。
PDV distributions offer the most straightforward way to identify that a sample of packets have traversed multiple paths. The tasks of de-jitter buffer sizing or assessing queue occupation with PDV should be use a sample with a single flow because flows will experience only one mode on a stable path, and it simplifies the analysis.
PDV发行版提供了最直接的方法来识别数据包样本是否已穿越多条路径。消除抖动缓冲区大小或使用PDV评估队列占用的任务应使用具有单个流的样本，因为流在稳定路径上只会经历一种模式，并且简化了分析。
+---------------+----------------------+----------------------------+
| Comparison | PDV = D(i)-D(min) | IPDV = D(i)-D(i-1) |
| Area | | |
+---------------+----------------------+----------------------------+
| Challenging | Less sensitive to | Preferred when path |
| Circumstances | packet loss, and | changes are frequent or |
| | simplifies analysis | when measurement clocks |
| | when load balancing | exhibit some skew |
| | or multiple paths | |
| | are present | |
|---------------|----------------------|----------------------------|
| Spatial | All validated | Has sensitivity to |
| Composition | methods use this | sequence and spacing |
| of DV metric | form | changes, which tends to |
| | | break the requirement for |
| | | independent distributions |
| | | between path segments |
|---------------|----------------------|----------------------------|
| Determine | "Pseudo-range" | No reliable relationship, |
| De-Jitter | reveals this | but some heuristics |
| Buffer Size | property by | |
| Required | anchoring the | |
| | distribution at the | |
| | minimum delay | |
|---------------|----------------------|----------------------------|
| Estimate of | Distribution has | No reliable relationship |
| Queuing Time | one-to-one | |
| and Variation | relationship on a | |
| | stable path, | |
| | especially when | |
| | sample min = true | |
| | min | |
|---------------|----------------------|----------------------------|
| Specification | One constraint | Distribution is two-sided, |
| Simplicity: | needed for | usually has zero mean, and |
| Single Number | single-sided | no universal summary |
| SLA | distribution, and | statistic that relates to |
| | easily related to | a physical quantity |
| | quantities above | |
+---------------+----------------------+----------------------------+
+---------------+----------------------+----------------------------+
| Comparison | PDV = D(i)-D(min) | IPDV = D(i)-D(i-1) |
| Area | | |
+---------------+----------------------+----------------------------+
| Challenging | Less sensitive to | Preferred when path |
| Circumstances | packet loss, and | changes are frequent or |
| | simplifies analysis | when measurement clocks |
| | when load balancing | exhibit some skew |
| | or multiple paths | |
| | are present | |
|---------------|----------------------|----------------------------|
| Spatial | All validated | Has sensitivity to |
| Composition | methods use this | sequence and spacing |
| of DV metric | form | changes, which tends to |
| | | break the requirement for |
| | | independent distributions |
| | | between path segments |
|---------------|----------------------|----------------------------|
| Determine | "Pseudo-range" | No reliable relationship, |
| De-Jitter | reveals this | but some heuristics |
| Buffer Size | property by | |
| Required | anchoring the | |
| | distribution at the | |
| | minimum delay | |
|---------------|----------------------|----------------------------|
| Estimate of | Distribution has | No reliable relationship |
| Queuing Time | one-to-one | |
| and Variation | relationship on a | |
| | stable path, | |
| | especially when | |
| | sample min = true | |
| | min | |
|---------------|----------------------|----------------------------|
| Specification | One constraint | Distribution is two-sided, |
| Simplicity: | needed for | usually has zero mean, and |
| Single Number | single-sided | no universal summary |
| SLA | distribution, and | statistic that relates to |
| | easily related to | a physical quantity |
| | quantities above | |
+---------------+----------------------+----------------------------+
Summary of Comparisons
比较摘要
This section discusses the practical aspects of delay variation measurement, with special attention to the two formulations compared in this memo.
本节讨论延迟变化测量的实际方面，特别注意本备忘录中比较的两种公式。
As stated in Section 1.2, there is a strong dependency between the active measurement stream characteristics and the results. The IPPM literature includes two primary methods for collecting samples: Poisson sampling described in [RFC2330], and Periodic sampling in [RFC3432]. The Poisson method was intended to collect an unbiased sample of performance, while the Periodic method addresses a "known bias of interest". Periodic streams are required to have random start times and limited stream duration, in order to avoid unwanted synchronization with some other periodic process, or cause congestion-aware senders to synchronize with the stream and produce atypical results. The random start time should be different for each new stream.
如第1.2节所述，主动测量流特性与结果之间存在强烈的相关性。IPPM文献包括两种主要的样本采集方法：[RFC2330]中描述的泊松采样和[RFC3432]中描述的周期采样。泊松法旨在收集无偏的绩效样本，而周期法则处理“已知的利益偏差”。周期性流要求具有随机的开始时间和有限的流持续时间，以避免与某些其他周期性进程进行不必要的同步，或导致拥塞感知发送方与流同步并产生非典型结果。每个新流的随机开始时间应该不同。
It is worth noting that [RFC3393] was developed in parallel with [RFC3432]. As a result, all the stream metrics defined in [RFC3393] specify the Poisson sampling method.
值得注意的是，[RFC3393]是与[RFC3432]并行开发的。因此，[RFC3393]中定义的所有流度量都指定了泊松采样方法。
Periodic sampling is frequently used in measurements of delay variation. Several factors foster this choice:
周期采样常用于延迟变化的测量。有几个因素促成了这一选择：
1. Many application streams that are sensitive to delay variation also exhibit periodicity, and so exemplify the bias of interest. If the application has a constant packet spacing, this constant spacing can be the inter-packet gap for the test stream. VoIP streams often use 20 ms spacing, so this is an obvious choice for an Active stream. This applies to both IPDV and PDV forms.
1. 许多对延迟变化敏感的应用程序流也表现出周期性，因此举例说明了兴趣的偏差。如果应用程序具有恒定的分组间隔，则该恒定间隔可以是测试流的分组间间隙。VoIP流通常使用20毫秒的间隔，因此这对于活动流来说是一个明显的选择。这适用于IPDV和PDV表格。
2. The spacing between packets in the stream will influence whether the stream experiences short-range dependency, or only long-range dependency, as investigated in [Li.Mills]. The packet spacing also influences the IPDV distribution and the stream's sensitivity to reordering. For example, with a 20 ms spacing the IPDV distribution cannot go below -20 ms without packet reordering.
2. 流中数据包之间的间隔将影响流是经历短期依赖，还是仅经历长期依赖，如[Li.Mills]中所研究的。包间距还影响IPDV分布和流对重新排序的敏感性。例如，如果间隔为20毫秒，IPDV分发不能低于-20毫秒，而不进行数据包重新排序。
3. The measurement process may make several simplifying assumptions when the send spacing and send rate are constant. For example, the inter-arrival times at the destination can be compared with an ideal sending schedule, and allowing a one-point measurement
3. 当发送间隔和发送速率恒定时，测量过程可能会做出一些简化假设。例如，目的地的到达时间可以与理想的发送时间表进行比较，并允许单点测量
of delay variation (described in [Y.1540]) that approximates the IPDV form. Simplified methods that approximate PDV are possible as well (some are discussed in Appendix II of [Y.1541]).
近似IPDV形式的延迟变化（如[Y.1540]所述）。近似PDV的简化方法也是可能的（一些在[Y.1541]的附录II中讨论）。
4. Analysis of truncated, or non-symmetrical IPDV distributions is simplified. Delay variations in excess of the periodic sending interval can cause multiple singleton values at the negative limit of the packet spacing (see Section 5.2 and [Cia03]). Only packet reordering can cause the negative spacing limit to be exceeded.
4. 简化了截断或非对称IPDV分布的分析。超过周期性发送间隔的延迟变化可导致多个单态值处于数据包间隔的负极限（见第5.2节和[Cia03]）。只有数据包重新排序才能导致超出负间隔限制。
Despite the emphasis on inter-packet delay differences with IPDV, both Poisson [Demichelis] and Periodic [Li.Mills] streams have been used, and these references illustrate the different analyses that are possible.
尽管IPDV强调包间延迟差异，但使用了泊松[Demichelis]流和周期[Li.Mills]流，这些参考文献说明了可能的不同分析。
The advantages of using a Poisson distribution are discussed in [RFC2330]. The main properties are to avoid predicting the sample times, avoid synchronization with periodic events that are present in networks, and avoid inducing synchronization with congestion-aware senders. When a Poisson stream is used with IPDV, the distribution will reflect inter-packet delay variation on many different time scales (or packet spacings). The unbiased Poisson sampling brings a new layer of complexity in the analysis of IPDV distributions.
[RFC2330]中讨论了使用泊松分布的优点。其主要特性是避免预测采样时间，避免与网络中存在的周期性事件同步，并避免诱导与拥塞感知发送方同步。当泊松流与IPDV一起使用时，分布将反映许多不同时间尺度（或分组间隔）上的分组间延迟变化。无偏泊松抽样为IPDV分布的分析带来了新的复杂性。
One key aspect of measurement devices is their ability to store singletons (or individual measurements). This feature usually is closely related to local calculation capabilities. For example, an embedded measurement device with limited storage will like provide only a few statistics on the delay variation distribution, while dedicated measurement systems store all the singletons and allow detailed analysis (later calculation of either form of delay variation is possible with the original singletons).
测量设备的一个关键方面是其存储单个（或单个）测量值的能力。此功能通常与本地计算能力密切相关。例如，具有有限存储的嵌入式测量设备将仅提供关于延迟变化分布的一些统计信息，而专用测量系统存储所有单态并允许详细分析（稍后可以使用原始单态计算任何形式的延迟变化）。
Therefore, systems with limited storage must choose their metrics and summary statistics in advance. If both IPDV and PDV statistics are desired, the supporting information must be collected as packets arrive. For example, the PDV range and high percentiles can be determined later if the minimum and several of the largest delays are stored while the measurement is in-progress.
因此，存储有限的系统必须提前选择其度量和汇总统计数据。如果需要IPDV和PDV统计数据，则必须在数据包到达时收集支持信息。例如，如果在测量过程中存储了最小延迟和几个最大延迟，则可以稍后确定PDV范围和高百分位数。
Both IPDV and PDV can be summarized as a range in milliseconds.
IPDV和PDV都可以总结为以毫秒为单位的范围。
With IPDV, it is interesting to report on a positive percentile, and an inter-quantile range is appropriate to reflect both positive and negative tails (e.g., 5% to 95%). If the IPDV distribution is symmetric around a mean of zero, then it is sufficient to report on the positive side of the distribution.
有了IPDV，报告正百分位数是很有趣的，分位数间范围适合反映正尾和负尾（例如，5%到95%）。如果IPDV分布围绕零的平均值对称，则报告分布的正侧就足够了。
With PDV, it is sufficient to specify the upper percentile (e.g., 99.9%).
对于PDV，指定上百分位（例如99.9%）就足够了。
At several points in this memo, we have recommended use of test intervals on the order of minutes. In their paper examining the stability of Internet path properties [Zhang.Duff], Zhang et al. concluded that consistency was present on the order of minutes for the performance metrics considered (loss, delay, and throughput) for the paths they measured.
在本备忘录的几点中，我们建议使用分钟的测试间隔。在他们研究互联网路径属性稳定性的论文[Zhang.Duff]中，Zhang等人得出结论，他们所测量的路径的性能指标（损失、延迟和吞吐量）的一致性是以分钟为单位的。
The topic of temporal aggregation of performance measured in small intervals to estimate some larger interval is described in the Metric Composition Framework [IPPM-Framework].
度量组合框架[IPPM框架]中描述了在小间隔内测量的性能的时间聚集以估计较大间隔的主题。
The primary recommendation here is to test using durations that are similar in length to the session time of interest. This applies to both IPDV and PDV, but is possibly more relevant for PDV since the duration determines how often the D_min will be determined, and the size of the associated sample.
这里的主要建议是使用与感兴趣的会话时间长度相似的持续时间进行测试。这适用于IPDV和PDV，但可能与PDV更相关，因为持续时间决定了确定D_min的频率以及相关样本的大小。
As with one-way-delay measurements, local clock synchronization is an important matter for delay variation measurements.
与单向延迟测量一样，本地时钟同步是延迟变化测量的一个重要问题。
There are several options available:
有几个选项可供选择：
1. Global Positioning System receivers
1. 全球定位系统接收机
2. In some parts of the world, Cellular Code Division Multiple Access (CDMA) systems distribute timing signals that are derived from GPS and traceable to UTC.
2. 在世界上的一些地区，蜂窝码分多址（CDMA）系统分发来自GPS并可追溯到UTC的定时信号。
3. Network Time Protocol [RFC1305] is a convenient choice in many cases, but usually offers lower accuracy than the options above.
3. 网络时间协议[RFC1305]在许多情况下是一个方便的选择，但通常提供的精确度低于上述选项。
When clock synchronization is inconvenient or subject to appreciable errors, then round-trip measurements may give a cumulative indication of the delay variation present on both directions of the path. However, delay distributions are rarely symmetrical, so it is difficult to infer much about the one-way-delay variation from round-trip measurements. Also, measurements on asymmetrical paths add complications for the one-way-delay metric.
当时钟同步不方便或存在明显误差时，往返测量可给出路径两个方向上存在的延迟变化的累积指示。然而，延迟分布很少是对称的，因此很难从往返测量中推断出单向延迟变化。此外，不对称路径上的测量增加了单向延迟度量的复杂性。
Lost and delayed packets are separated by a waiting time threshold. Packets that arrive at the measurement destination within their waiting time have finite delay and are not lost. Otherwise, packets are designated lost and their delay is undefined. Guidance on setting the waiting time threshold may be found in [RFC2680] and [IPPM-Reporting].
丢失和延迟的数据包由等待时间阈值分隔。在等待时间内到达测量目的地的数据包具有有限延迟且不会丢失。否则，数据包被指定为丢失，其延迟未定义。有关设置等待时间阈值的指导，请参见[RFC2680]和[IPPM报告]。
In essence, [IPPM-Reporting] suggests to use a long waiting time to serve network characterization and revise results for specific application delay thresholds as needed.
本质上，[IPPM报告]建议使用较长的等待时间来服务于网络特性描述，并根据需要修改特定应用程序延迟阈值的结果。
Packet reordering, defined in [RFC4737], is essentially an extreme form of delay variation where the packet stream arrival order differs from the sending order.
[RFC4737]中定义的包重排序本质上是延迟变化的一种极端形式，其中包流到达顺序不同于发送顺序。
PDV results are not sensitive to packet arrival order, and are not affected by reordering other than to reflect the more extreme variation.
PDV结果对数据包到达顺序不敏感，并且不受重新排序的影响，只是为了反映更极端的变化。
IPDV results will change if reordering is present because they are sensitive to the sequence of delays of arriving packets. The main example of this sensitivity is in the truncation of the negative tail of the distribution.
如果存在重新排序，IPDV结果将发生变化，因为它们对到达数据包的延迟序列很敏感。这种敏感性的主要例子是分布负尾的截断。
o When there is no reordering, the negative tail is limited by the sending time spacing between packets.
o 当没有重新排序时，负尾受到数据包之间发送时间间隔的限制。
o If reordering occurs (and the reordered packets are not discarded), the negative tail can take on any value (in principal).
o 如果发生重新排序（并且重新排序的数据包没有被丢弃），负尾可以接受任何值（原则上）。
In general, measurement systems should have the capability to detect when sequence has changed. If IPDV measurements are made without regard to packet arrival order, the IPDV will be under-reported when reordering occurs.
一般来说，测量系统应具有检测序列何时发生变化的能力。如果IPDV测量是在不考虑数据包到达顺序的情况下进行的，则当发生重新排序时，IPDV将被低估。
All of the references that discuss or define delay variation suggest ways to represent or report the results, and interested readers should review the various possibilities.
所有讨论或定义延迟变化的参考文献都提出了表示或报告结果的方法，感兴趣的读者应该回顾各种可能性。
For example, [IPPM-Reporting] suggests reporting a pseudo-range of delay variation based on calculating the difference between a high percentile of delay and the minimum delay. The 99.9th percentile minus the minimum will give a value that can be compared with objectives in [Y.1541].
例如，[IPPM报告]建议根据计算高百分位延迟和最小延迟之间的差异报告延迟变化的伪范围。99.9%减去最小值将给出一个值，该值可与[Y.1541]中的目标进行比较。
The security considerations that apply to any active measurement of live networks are relevant here as well. See the "Security Considerations" sections in [RFC2330], [RFC2679], [RFC3393], [RFC3432], and [RFC4656].
适用于实时网络的任何主动测量的安全注意事项也与此相关。请参阅[RFC2330]、[RFC2679]、[RFC3393]、[RFC3432]和[RFC4656]中的“安全注意事项”部分。
Security considerations do not contribute to the selection of PDV or IPDV forms of delay variation, because measurements using these metrics involve exactly the same security issues.
安全方面的考虑不利于选择PDV或IPDV形式的延迟变化，因为使用这些度量的测量涉及完全相同的安全问题。
The authors would like to thank Phil Chimento for his suggestion to employ the convention of conditional distributions of delay to deal with packet loss, and his encouragement to "write the memo" after hearing "the talk" on this topic at IETF 65. We also acknowledge constructive comments from Alan Clark, Loki Jorgenson, Carsten Schmoll, and Robert Holley.
作者要感谢Phil Chimento的建议，他建议采用延迟条件分布的约定来处理数据包丢失，并鼓励他在IETF 65上听到关于这个主题的“演讲”后“写备忘录”。我们还感谢Alan Clark、Loki Jorgenson、Carsten Schmoll和Robert Holley提出的建设性意见。
Practitioners have raised several questions that this section intends to answer:
从业人员提出了本节打算回答的几个问题：
- How is this D_min calculated? Is it DV(99%) as mentioned in [Krzanowski]?
- 这个D_min是如何计算的？是[Krzanowski]中提到的DV（99%）吗？
- Do we need to keep all the values from the interval, then take the minimum? Or do we keep the minimum from previous intervals?
- 我们是否需要保留区间中的所有值，然后取最小值？还是我们保持以前间隔的最小值？
The value of D_min used as the reference delay for PDV calculations is simply the minimum delay of all packets in the current sample. The usual single value summary of the PDV distribution is D_(99.9th percentile) minus D_min.
用作PDV计算参考延迟的D_min值只是当前样本中所有数据包的最小延迟。PDV分布的通常单值汇总为D_（99.9%位）减去D_min。
It may be appropriate to segregate sub-sets and revise the minimum value during a sample. For example, if it can be determined with certainty that the path has changed by monitoring the Time to Live or Hop Count of arriving packets, this may be sufficient justification to reset the minimum for packets on the new path. There is also a simpler approach to solving this problem: use samples collected over short evaluation intervals (on the order of minutes). Intervals with path changes may be more interesting from the loss or one-way-delay perspective (possibly failing to meet one or more SLAs), and it may not be necessary to conduct delay variation analysis. Short evaluation intervals are preferred for measurements that serve as a basis for troubleshooting, since the results are available to report soon after collection.
在取样过程中，分离子集并修改最小值可能是合适的。例如，如果可以通过监测到达的分组的生存时间或跳数来确定路径已经改变，则这可能是重置新路径上分组的最小值的充分理由。还有一种更简单的方法来解决这个问题：使用在较短的评估间隔内收集的样本（以分钟为单位）。从丢失或单向延迟的角度来看（可能无法满足一个或多个SLA），具有路径变化的间隔可能更有趣，并且可能没有必要进行延迟变化分析。对于作为故障排除基础的测量，最好使用较短的评估间隔，因为在收集结果后可以立即报告结果。
It is not necessary to store all delay values in a sample when storage is a major concern. D_min can be found by comparing each new singleton value with the current value and replacing it when required. In a sample with 5000 packets, evaluation of the 99.9th percentile can also be achieved with limited storage. One method calls for storing the top 50 delay singletons and revising the top value list each time 50 more packets arrive.
当存储是一个主要问题时，不必在样本中存储所有延迟值。D_min可以通过将每个新的单例值与当前值进行比较并在需要时进行替换来找到。在5000个数据包的样本中，99.9%的评估也可以通过有限的存储实现。一种方法要求存储前50个延迟单例，并在每次有50多个数据包到达时修改顶值列表。
[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。
[RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis, "Framework for IP Performance Metrics", RFC 2330, May 1998.
[RFC2330]Paxson，V.，Almes，G.，Mahdavi，J.，和M.Mathis，“IP性能度量框架”，RFC 2330，1998年5月。
[RFC2679] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way Delay Metric for IPPM", RFC 2679, September 1999.
[RFC2679]Almes，G.，Kalidini，S.，和M.Zekauskas，“IPPM的单向延迟度量”，RFC 2679，1999年9月。
[RFC2680] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way Packet Loss Metric for IPPM", RFC 2680, September 1999.
[RFC2680]Almes，G.，Kalidini，S.，和M.Zekauskas，“IPPM的单向数据包丢失度量”，RFC 2680，1999年9月。
[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation Metric for IP Performance Metrics (IPPM)", RFC 3393, November 2002.
[RFC3393]Demichelis，C.和P.Chimento，“IP性能度量的IP数据包延迟变化度量（IPPM）”，RFC 3393，2002年11月。
[RFC3432] Raisanen, V., Grotefeld, G., and A. Morton, "Network performance measurement with periodic streams", RFC 3432, November 2002.
[RFC3432]Raisanen，V.，Grotefeld，G.，和A.Morton，“周期流的网络性能测量”，RFC 3432，2002年11月。
[RFC4090] Pan, P., Swallow, G., and A. Atlas, "Fast Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090, May 2005.
[RFC4090]Pan，P.，Swallow，G.，和A.Atlas，“LSP隧道RSVP-TE快速重路由扩展”，RFC 40902005年5月。
[RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M. Zekauskas, "A One-way Active Measurement Protocol (OWAMP)", RFC 4656, September 2006.
[RFC4656]Shalunov，S.，Teitelbaum，B.，Karp，A.，Boote，J.，和M.Zekauskas，“单向主动测量协议（OWAMP）”，RFC 46562006年9月。
[RFC4737] Morton, A., Ciavattone, L., Ramachandran, G., Shalunov, S., and J. Perser, "Packet Reordering Metrics", RFC 4737, November 2006.
[RFC4737]Morton，A.，Ciavattone，L.，Ramachandran，G.，Shalunov，S.，和J.Perser，“数据包重新排序度量”，RFC 4737，2006年11月。
[COM12.D98] Clark, A., "Analysis, measurement and modelling of Jitter", ITU-T Delayed Contribution COM 12 - D98, January 2003.
[COM12.D98]Clark，A.，“抖动的分析、测量和建模”，ITU-T延迟贡献COM 12-D98，2003年1月。
[Casner] Casner, S., Alaettinoglu, C., and C. Kuan, "A Fine-Grained View of High Performance Networking", NANOG 22, May 20-22, 2001, <http://www.nanog.org/mtg-0105/agenda.html>.
[Casner]Casner，S.，Alaettinoglu，C.和C.Kuan，“高性能网络的细粒度视图”，NANOG 22，2001年5月20-22日<http://www.nanog.org/mtg-0105/agenda.html>.
[Cia03] Ciavattone, L., Morton, A., and G. Ramachandran, "Standardized Active Measurements on a Tier 1 IP Backbone", IEEE Communications Magazine, p. 90-97, June 2003.
[Cia03]Ciavattone，L.，Morton，A.，和G.Ramachandran，“第1层IP主干上的标准化主动测量”，IEEE通信杂志，第页。2003年6月90日至97日。
[Demichelis] Demichelis, C., "Packet Delay Variation Comparison between ITU-T and IETF Draft Definitions", November 2000, <http://www.advanced.org/ippm/ archive.3/att-0075/01-pap02.doc>.
[Demichelis]Demichelis，C.，“ITU-T和IETF草案定义之间的数据包延迟变化比较”，2000年11月<http://www.advanced.org/ippm/ 归档文件3/att-0075/01-pap02.doc>。
[G.1020] ITU-T, "Performance parameter definitions for the quality of speech and other voiceband applications utilizing IP networks", ITU-T Recommendation G.1020, 2006.
[G.1020]ITU-T，“利用IP网络的语音质量和其他语音带应用的性能参数定义”，ITU-T建议G.1020，2006年。
[G.1050] ITU-T, "Network model for evaluating multimedia transmission performance over Internet Protocol", ITU-T Recommendation G.1050, November 2005.
[G.1050]ITU-T，“评估互联网协议多媒体传输性能的网络模型”，ITU-T建议G.1050，2005年11月。
[I.356] ITU-T, "B-ISDN ATM Layer Cell Transfer Performance", ITU-T Recommendation I.356, March 2000.
[I.356]ITU-T，“B-ISDN ATM层信元传输性能”，ITU-T建议I.356，2000年3月。
[IPPM-Framework] Morton, A., "Framework for Metric Composition", Work in Progress, October 2008.
[IPPM框架]Morton，A.，“度量组合框架”，正在进行的工作，2008年10月。
[IPPM-Reporting] Morton, A., Ramachandran, G., and G. Maguluri, "Reporting Metrics: Different Points of View", Work in Progress, January 2009.
[IPPM报告]Morton，A.，Ramachandran，G.，和G.Maguluri，“报告指标：不同观点”，正在进行的工作，2009年1月。
[IPPM-Spatial] Morton, A. and E. Stephan, "Spatial Composition of Metrics", Work in Progress, July 2008.
[IPPM Spatial]Morton，A.和E.Stephan，“度量的空间构成”，正在进行的工作，2008年7月。
[Krzanowski] Presentation at IPPM, IETF-64, "Jitter Definitions: What is What?", November 2005.
[Krzanowski]在IPPM上的演讲，IETF-64，“抖动定义：什么是什么？”，2005年11月。
[Li.Mills] Li, Q. and D. Mills, "The Implications of Short-Range Dependency on Delay Variation Measurement", Second IEEE Symposium on Network Computing and Applications, 2003.
[Li.Mills]Li，Q.和D.Mills，“延迟变化测量的短程依赖性的影响”，第二届IEEE网络计算和应用研讨会，2003年。
[Morton06] Morton, A., "A Brief Jitter Metrics Comparison, and not the last word, by any means...", slide presentation at IETF 65, IPPM Session, March 2006.
[Morton06]Morton，A.，“一个简单的抖动度量比较，而不是最后一句话，无论如何……”，IETF 65，IPPM会议上的幻灯片演示，2006年3月。
[RFC1305] Mills, D., "Network Time Protocol (Version 3) Specification, Implementation", RFC 1305, March 1992.
[RFC1305]Mills，D.，“网络时间协议（版本3）规范，实施”，RFC1305，1992年3月。
[RFC3357] Koodli, R. and R. Ravikanth, "One-way Loss Pattern Sample Metrics", RFC 3357, August 2002.
[RFC3357]Koodli，R.和R.Ravikanth，“单向损失模式样本度量”，RFC 3357，2002年8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。
[Y.1540] ITU-T, "Internet protocol data communication service - IP packet transfer and availability performance parameters", ITU-T Recommendation Y.1540, November 2007.
[Y.1540]ITU-T，“互联网协议数据通信服务——IP数据包传输和可用性性能参数”，ITU-T建议Y.1540，2007年11月。
[Y.1541] ITU-T, "Network Performance Objectives for IP-Based Services", ITU-T Recommendation Y.1541, February 2006.
[Y.1541]ITU-T，“基于IP的服务的网络性能目标”，ITU-T建议Y.1541，2006年2月。
[Zhang.Duff] Zhang, Y., Duffield, N., Paxson, V., and S. Shenker, "On the Constancy of Internet Path Properties", Proceedings of ACM SIGCOMM Internet Measurement Workshop, November 2001.
[Zhang.Duff]Zhang，Y.，Duffield，N.，Paxson，V.，和S.Shenker，“关于互联网路径属性的恒定性”，ACM SIGCOMM互联网测量研讨会论文集，2001年11月。
Authors' Addresses
作者地址
Al Morton AT&T Labs 200 Laurel Avenue South Middletown, NJ 07748 USA
美国新泽西州劳雷尔大道南米德尔顿200号阿尔莫顿AT&T实验室，邮编：07748
Phone: +1 732 420 1571
Fax: +1 732 368 1192
EMail: acmorton@att.com
URI: http://home.comcast.net/~acmacm/
Phone: +1 732 420 1571
Fax: +1 732 368 1192
EMail: acmorton@att.com
URI: http://home.comcast.net/~acmacm/
Benoit Claise Cisco Systems, Inc. De Kleetlaan 6a b1 Diegem, 1831 Belgium
Benoit Claise Cisco Systems，Inc.De Kleetlaan 6a b1 Diegem，1831比利时
Phone: +32 2 704 5622
EMail: bclaise@cisco.com
Phone: +32 2 704 5622
EMail: bclaise@cisco.com