Network Working Group A. Charny
Request for Comments: 3247 Cisco Systems, Inc.
Category: Informational J.C.R. Bennett
Motorola
K. Benson
Tellabs
J.Y. Le Boudec
EPFL
A. Chiu
Celion Networks
W. Courtney
TRW
S. Davari
PMC-Sierra
V. Firoiu
Nortel Networks
C. Kalmanek
AT&T Research
K.K. Ramakrishnan
TeraOptic Networks
March 2002
Network Working Group A. Charny
Request for Comments: 3247 Cisco Systems, Inc.
Category: Informational J.C.R. Bennett
Motorola
K. Benson
Tellabs
J.Y. Le Boudec
EPFL
A. Chiu
Celion Networks
W. Courtney
TRW
S. Davari
PMC-Sierra
V. Firoiu
Nortel Networks
C. Kalmanek
AT&T Research
K.K. Ramakrishnan
TeraOptic Networks
March 2002
Supplemental Information for the New Definition of the EF PHB (Expedited Forwarding Per-Hop Behavior)
EF PHB（每跳加速转发行为）新定义的补充信息
Status of this Memo
本备忘录的状况
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) The Internet Society (2001). All Rights Reserved.
版权所有（C）互联网协会（2001年）。版权所有。
Abstract
摘要
This document was written during the process of clarification of RFC2598 "An Expedited Forwarding PHB" that led to the publication of revised specification of EF "An Expedited Forwarding PHB". Its primary motivation is providing additional explanation to the revised EF definition and its properties. The document also provides additional implementation examples and gives some guidance for computation of the numerical parameters of the new definition for several well known schedulers and router architectures.
本文件是在澄清RFC2598“快速转运PHB”的过程中编写的，该澄清导致EF“快速转运PHB”修订规范的发布。其主要目的是对修订后的环境足迹定义及其性质进行补充解释。该文档还提供了其他实现示例，并为计算几个著名调度器和路由器架构的新定义的数值参数提供了一些指导。
Table of Contents
目录
1 Introduction ........................................... 2
2 Definition of EF PHB ................................... 3
2.1 The formal definition .................................. 3
2.2 Relation to Packet Scale Rate Guarantee ................ 6
2.3 The need for dual characterization of EF PHB ........... 7
3 Per Packet delay ....................................... 9
3.1 Single hop delay bound ................................. 9
3.2 Multi-hop worst case delay ............................. 10
4 Packet loss ............................................ 10
5 Implementation considerations .......................... 11
5.1 The output buffered model with EF FIFO at the output. .. 12
5.1.1 Strict Non-preemptive Priority Queue ................... 12
5.1.2 WF2Q ................................................... 13
5.1.3 Deficit Round Robin (DRR) .............................. 13
5.1.4 Start-Time Fair Queuing and Self-Clocked Fair Queuing .. 13
5.2 Router with Internal Delay and EF FIFO at the output ... 13
6 Security Considerations ................................ 14
7 References ............................................. 14
Appendix A. Difficulties with the RFC 2598 EF PHB Definition .. 16
Appendix B. Alternative Characterization of Packet Scale Rate
Guarantee ......................................... 20
Acknowledgements .............................................. 22
Authors' Addresses ............................................ 22
Full Copyright Statement ...................................... 24
1 Introduction ........................................... 2
2 Definition of EF PHB ................................... 3
2.1 The formal definition .................................. 3
2.2 Relation to Packet Scale Rate Guarantee ................ 6
2.3 The need for dual characterization of EF PHB ........... 7
3 Per Packet delay ....................................... 9
3.1 Single hop delay bound ................................. 9
3.2 Multi-hop worst case delay ............................. 10
4 Packet loss ............................................ 10
5 Implementation considerations .......................... 11
5.1 The output buffered model with EF FIFO at the output. .. 12
5.1.1 Strict Non-preemptive Priority Queue ................... 12
5.1.2 WF2Q ................................................... 13
5.1.3 Deficit Round Robin (DRR) .............................. 13
5.1.4 Start-Time Fair Queuing and Self-Clocked Fair Queuing .. 13
5.2 Router with Internal Delay and EF FIFO at the output ... 13
6 Security Considerations ................................ 14
7 References ............................................. 14
Appendix A. Difficulties with the RFC 2598 EF PHB Definition .. 16
Appendix B. Alternative Characterization of Packet Scale Rate
Guarantee ......................................... 20
Acknowledgements .............................................. 22
Authors' Addresses ............................................ 22
Full Copyright Statement ...................................... 24
The Expedited Forwarding (EF) Per-Hop Behavior (PHB) was designed to be used to build a low-loss, low-latency, low-jitter, assured bandwidth service. The potential benefits of this service, and therefore the EF PHB, are enormous. Because of the great value of this PHB, it is critical that the forwarding behavior required of and delivered by an EF-compliant node be specific, quantifiable, and unambiguous.
加速转发（EF）每跳行为（PHB）设计用于构建低损耗、低延迟、低抖动、有保证的带宽服务。这项服务以及EF PHB的潜在好处是巨大的。由于此PHB的巨大价值，因此EF兼容节点所需和提供的转发行为必须是特定的、可量化的和明确的。
Unfortunately, the definition of EF PHB in the original RFC2598 [10] was not sufficiently precise (see Appendix A and [4]). A more precise definition is given in [6]. This document is intended to aid in the understanding of the properties of the new definition and provide supplemental information not included in the text of [6] for sake of brevity.
不幸的是，原始RFC2598[10]中EF PHB的定义不够精确（见附录A和[4]）。[6]给出了更精确的定义。为了简洁起见，本文件旨在帮助理解新定义的性质，并提供[6]文本中未包含的补充信息。
This document is outlined as follows. In section 2, we briefly restate the definition for EF PHB of [6]. We then provide some additional discussion of this definition and describe some of its properties. We discuss the issues associated with per-packet delay
本文件概述如下。在第2节中，我们简要重申了[6]中EF PHB的定义。然后，我们对这个定义进行了一些额外的讨论，并描述了它的一些性质。我们讨论与每包延迟相关的问题
and loss in sections 3 and 4. In section 5 we discuss the impact of known scheduling architectures on the critical parameters of the new definition. We also discuss the impact of deviation of real devices from the ideal output-buffered model on the magnitude of the critical parameters in the definition.
以及第3和第4节中的损失。在第5节中，我们讨论了已知调度架构对新定义的关键参数的影响。我们还讨论了实际设备与理想输出缓冲模型的偏差对定义中关键参数大小的影响。
An intuitive explanation of the new EF definition is described in [6]. Here we restate the formal definition from [6] verbatim.
[6]中描述了新EF定义的直观解释。这里我们逐字重述[6]中的正式定义。
A node that supports EF on an interface I at some configured rate R MUST satisfy the following equations:
在接口I上以某种配置速率R支持EF的节点必须满足以下等式：
d_j <= f_j + E_a for all j>0 (eq_1)
d_j <= f_j + E_a for all j>0 (eq_1)
where f_j is defined iteratively by
其中f_j由以下公式迭代定义：
f_0 = 0, d_0 = 0
f_0=0，d_0=0
f_j = max(a_j, min(d_j-1, f_j-1)) + l_j/R, for all j > 0 (eq_2)
f_j = max(a_j, min(d_j-1, f_j-1)) + l_j/R, for all j > 0 (eq_2)
In this definition:
在这一定义中：
- d_j is the time that the last bit of the j-th EF packet to depart actually leaves the node from the interface I.
- d_j是第j个EF包的最后一位实际离开接口I的时间。
- f_j is the target departure time for the j-th EF packet to depart from I, the "ideal" time at or before which the last bit of that packet should leave the node.
- f_j是第j个EF数据包离开I的目标离开时间，即该数据包的最后一位离开节点时或之前的“理想”时间。
- a_j is the time that the last bit of the j-th EF packet destined to the output I actually arrives at the node.
- a_j是发送到输出I的第j个EF包的最后一位实际到达节点的时间。
- l_j is the size (bits) of the j-th EF packet to depart from I. l_j is measured on the IP datagram (IP header plus payload) and does not include any lower layer (e.g. MAC layer) overhead.
- l_j是要离开I的第j个EF数据包的大小（位）。l_j在IP数据报（IP报头加有效载荷）上测量，不包括任何较低层（例如MAC层）开销。
- R is the EF configured rate at output I (in bits/second).
- R是输出I处的EF配置速率（以位/秒为单位）。
- E_a is the error term for the treatment of the EF aggregate. Note that E_a represents the worst case deviation between actual departure time of an EF packet and ideal departure time of the same packet, i.e. E_a provides an upper bound on (d_j - f_j) for all j.
- E_a是EF骨料处理的误差项。注意，E_a表示EF分组的实际出发时间和同一分组的理想出发时间之间的最坏情况偏差，即E_a为所有j提供了（d_j-f_j）的上界。
- d_0 and f_0 do not refer to a real packet departure but are used purely for the purposes of the recursion. The time origin should be chosen such that no EF packets are in the system at time 0.
- d_0和f_0并不表示实际的数据包离开，而是纯粹用于递归。时间原点的选择应确保在时间0时系统中没有EF数据包。
- for the definitions of a_j and d_j, the "last bit" of the packet includes the layer 2 trailer if present, because a packet cannot generally be considered available for forwarding until such a trailer has been received.
- 对于a_j和d_j的定义，分组的“最后一位”包括第2层尾部（如果存在），因为在接收到这样的尾部之前，通常不能认为分组可用于转发。
An EF-compliant node MUST be able to be characterized by the range of possible R values that it can support on each of its interfaces while conforming to these equations, and the value of E_a that can be met on each interface. R may be line rate or less. E_a MAY be specified as a worst-case value for all possible R values or MAY be expressed as a function of R.
EF兼容节点必须能够通过其在符合这些等式的同时在其每个接口上支持的可能R值的范围以及在每个接口上可以满足的E_a值来表征。R可以是行速率或更低。E_a可指定为所有可能R值的最坏情况值，或可表示为R的函数。
Note also that, since a node may have multiple inputs and complex internal scheduling, the j-th EF packet to arrive at the node destined for a certain interface may not be the j-th EF packet to depart from that interface. It is in this sense that eq_1 and eq_2 are unaware of packet identity.
还注意，由于节点可以具有多个输入和复杂的内部调度，因此到达目的地为某个接口的节点的j-th EF分组可能不是离开该接口的j-th EF分组。正是在这种意义上，eq_1和eq_2不知道分组标识。
In addition, a node that supports EF on an interface I at some configured rate R MUST satisfy the following equations:
此外，在接口I上以某种配置速率R支持EF的节点必须满足以下等式：
D_j <= F_j + E_p for all j>0 (eq_3)
D_j <= F_j + E_p for all j>0 (eq_3)
where F_j is defined iteratively by
其中F_j由以下公式迭代定义：
F_0 = 0, D_0 = 0
F_0=0，D_0=0
F_j = max(A_j, min(D_j-1, F_j-1)) + L_j/R, for all j > 0 (eq_4)
F_j = max(A_j, min(D_j-1, F_j-1)) + L_j/R, for all j > 0 (eq_4)
In this definition:
在这一定义中：
- D_j is the actual departure time of the individual EF packet that arrived at the node destined for interface I at time A_j, i.e., given a packet which was the j-th EF packet destined for I to arrive at the node via any input, D_j is the time at which the last bit of that individual packet actually leaves the node from the interface I.
- D_j是在时间A_j到达目的地为接口I的节点的单个EF分组的实际离开时间，即，给定作为目的地为I的第j个EF分组的分组经由任何输入到达节点，D_j是该单个分组的最后一位实际离开接口I的时间。
- F_j is the target departure time for the individual EF packet that arrived at the node destined for interface I at time A_j.
- F_j是在时间A_j到达目的地为接口I的节点的单个EF分组的目标离开时间。
- A_j is the time that the last bit of the j-th EF packet destined to the output I to arrive actually arrives at the node.
- A_j是发送到输出I的第j个EF分组的最后一位实际到达节点的时间。
- L_j is the size (bits) of the j-th EF packet to arrive at the node that is destined to output I. L_j is measured on the IP datagram (IP header plus payload) and does not include any lower layer (e.g. MAC layer) overhead.
- L_j是到达目的地为输出I的节点的第j个EF分组的大小（位）。L_j在IP数据报（IP报头加有效载荷）上测量，不包括任何较低层（例如MAC层）开销。
- R is the EF configured rate at output I (in bits/second).
- R是输出I处的EF配置速率（以位/秒为单位）。
- E_p is the error term for the treatment of individual EF packets. Note that E_p represents the worst case deviation between the actual departure time of an EF packet and the ideal departure time of the same packet, i.e. E_p provides an upper bound on (D_j - F_j) for all j.
- E_p是用于处理单个EF数据包的错误项。注意，E_p表示EF数据包的实际出发时间和同一数据包的理想出发时间之间的最坏情况偏差，即E_p为所有j提供了（D_j-F_j）的上界。
- D_0 and F_0 do not refer to a real packet departure but are used purely for the purposes of the recursion. The time origin should be chosen such that no EF packets are in the system at time 0.
- D_0和F_0并不表示实际的数据包离开，而是纯粹用于递归。时间原点的选择应确保在时间0时系统中没有EF数据包。
- for the definitions of A_j and D_j, the "last bit" of the packet includes the layer 2 trailer if present, because a packet cannot generally be considered available for forwarding until such a trailer has been received.
- 对于A_j和D_j的定义，分组的“最后一位”包括第2层尾部（如果存在），因为在接收到这样的尾部之前，通常不能认为分组可用于转发。
It is the fact that D_j and F_j refer to departure times for the j-th packet to arrive that makes eq_3 and eq_4 aware of packet identity. This is the critical distinction between the last two equations and the first two.
事实上，D_j和F_j指的是第j个分组到达的离开时间，这使得eq_3和eq_4知道分组标识。这是后两个方程和前两个方程之间的关键区别。
An EF-compliant node SHOULD be able to be characterized by the range of possible R values that it can support on each of its interfaces while conforming to these equations, and the value of E_p that can be met on each interface. E_p MAY be specified as a worst-case value for all possible R values or MAY be expressed as a function of R. An E_p value of "undefined" MAY be specified.
EF兼容节点应能够通过其在符合这些等式的同时在其每个接口上支持的可能R值的范围以及在每个接口上可以满足的E_p值来表征。E_p可指定为所有可能R值的最坏情况值，或可表示为R的函数。可指定“未定义”的E_p值。
Finally, there is an additional recommendation in [6] that an EF compliant node SHOULD NOT reorder packets within a micorflow.
最后，在[6]中还有一个额外的建议，即EF兼容节点不应在micorflow中重新排序数据包。
The definitions described in this section are referred to as aggregate and packet-identity-aware packet scale rate guarantee [4],[2]. An alternative mathematical characterization of packet scale rate guarantee is given in Appendix B.
本节中描述的定义称为聚合和分组标识感知分组规模速率保证[4]，[2]。附录B中给出了分组规模速率保证的另一种数学特性。
Consider the case of an ideal output-buffered device with an EF FIFO at the output. For such a device, the i-th packet to arrive to the device is also the i-th packet to depart from the device. Therefore, in this ideal model the aggregate behavior and packet-identity-aware characteristics are identical, and E_a = E_p. In this section we therefore omit the subscript and refer to the latency term simply as E.
考虑在输出端具有EF FIFO的理想输出缓冲装置的情况。对于这样的设备，到达该设备的第i个分组也是离开该设备的第i个分组。因此，在这个理想模型中，聚合行为和包标识感知特征是相同的，E_a=E_p。因此，在本节中，我们省略了下标，并将延迟项简单地称为E。
It could be shown that for such an ideal device the definition of section 2.1 is stronger than the well-known rate-latency curve [2] in the sense that if a scheduler satisfies the EF definition it also satisfies the rate-latency curve. As a result, all the properties known for the rate-latency curve also apply to the modified EF definition. However, we argue below that the definition of section 2.1 is more suitable to reflect the intent of EF PHB than the rate-latency curve.
可以证明，对于这样一个理想设备，第2.1节的定义比众所周知的速率延迟曲线[2]更强，因为如果调度器满足EF定义，它也满足速率延迟曲线。因此，速率-延迟曲线的所有已知属性也适用于修改后的EF定义。然而，我们在下文中认为，第2.1节的定义比速率-延迟曲线更适合反映EF PHB的意图。
It is shown in [2] that the rate-latency curve is equivalent to the following definition:
如[2]所示，速率-延迟曲线相当于以下定义：
Definition of Rate Latency Curve (RLC):
速率-延迟曲线（RLC）的定义：
D(j) <= F'(j) + E (eq_5)
D(j) <= F'(j) + E (eq_5)
where
哪里
F'(0)=0, F'(j)=max(a(j), F'(j-1))+ L(j)/R for all j>0 (eq_6)
F'(0)=0, F'(j)=max(a(j), F'(j-1))+ L(j)/R for all j>0 (eq_6)
It can be easily verified that the EF definition of section 2.1 is stronger than RLC by noticing that for all j, F'(j) >= F(j).
通过注意到对于所有j，F'（j）>=F（j），可以很容易地验证第2.1节中的EF定义比RLC更强。
It is easy to see that F'(j) in the definition of RLC corresponds to the time the j-th departure should have occurred should the EF aggregate be constantly served exactly at its configured rate R. Following the common convention, we refer to F'(j) as the "fluid finish time" of the j-th packet to depart.
很容易看出，RLC定义中的F’（j）对应于如果EF聚合始终以其配置的速率R提供服务，则第j个离开应该发生的时间。按照常见惯例，我们将F’（j）称为第j个离开数据包的“流体完成时间”。
The intuitive meaning of the rate-latency curve of RLC is that any packet is served at most time E later than this packet would finish service in the fluid model.
RLC速率-延迟曲线的直观含义是，在流体模型中，任何数据包的服务时间都比该数据包完成服务的时间晚。
For RLC (and hence for the stronger EF definition) it holds that in any interval (0,t) the EF aggregate gets close to the desired service rate R (as long as there is enough traffic to sustain this rate). The discrepancy between the ideal and the actual service in this interval depends on the latency term E, which in turn depends on the
对于RLC（因此对于更强的EF定义），它认为在任何间隔（0，t）内，EF聚合接近期望的服务速率R（只要有足够的流量来维持该速率）。在这个时间间隔内，理想服务和实际服务之间的差异取决于延迟项E，而延迟项E又取决于
scheduling implementation. The smaller E, the smaller the difference between the configured rate and the actual rate achieved by the scheduler.
调度执行。E越小，调度程序实现的配置速率和实际速率之间的差异就越小。
While RLC guarantees the desired rate to the EF aggregate in all intervals (0,t) to within a specified error, it may nevertheless result in large gaps in service. For example, suppose that (a large number) N of identical EF packets of length L arrived from different interfaces to the EF queue in the absence of any non-EF traffic. Then any work-conserving scheduler will serve all N packets at link speed. When the last packet is sent at time NL/C, where C is the capacity of output link, F'(N) will be equal to NL/R. That is, the scheduler is running ahead of ideal, since NL/C < NL/R for R < C. Suppose now that at time NL/C a large number of non-EF packets arrive, followed by a single EF packet. Then the scheduler can legitimately delay starting to send the EF packet until time F'(N+1)=(N+1)L/R + E - L/C. This means that the EF aggregate will have no service at all in the interval (NL/C, (N+1)L/R + E - L/C). This interval can be quite large if R is substantially smaller than C. In essence, the EF aggregate can be "punished" by a gap in service for receiving faster service than its configured rate at the beginning.
尽管RLC保证EF聚合在所有间隔（0，t）内的期望速率在指定的误差范围内，但它仍可能导致服务中的较大差距。例如，假设在没有任何非EF通信量的情况下，长度为L的相同EF数据包（大量）N从不同接口到达EF队列。然后，任何节省工作的调度器将以链路速度为所有N个数据包提供服务。当最后一个数据包在时间NL/C发送时，其中C是输出链路的容量，F’（N）将等于NL/R。也就是说，由于NL/C<NL/R for R<C，调度程序在理想情况下提前运行。现在假设在时间NL/C到达大量非EF数据包，然后是单个EF数据包。然后，调度器可以合法地延迟开始发送EF数据包，直到时间F’（N+1）=（N+1）L/R+E-L/C。这意味着EF聚合在时间间隔（NL/C，（N+1）L/R+E-L/C）内将没有任何服务。如果R明显小于C，则该间隔可能相当大。本质上，EF聚合可能会受到服务差距的“惩罚”，因为它在开始时接收的服务比其配置的速率更快。
The new EF definition alleviates this problem by introducing the term min(D(j-1), F(j-1)) in the recursion. Essentially, this means that the fluid finishing time is "reset" if that packet is sent before its "ideal" departure time. As a consequence of that, for the case where the EF aggregate is served in the FIFO order, suppose a packet arrives at time t to a server satisfying the EF definition. The packet will be transmitted no later than time t + Q(t)/R + E, where Q(t) is the EF queue size at time t (including the packet under discussion)[4].
新的EF定义通过在递归中引入术语min（D（j-1），F（j-1））来缓解这个问题。本质上，这意味着，如果该数据包在其“理想”离开时间之前发送，则流体完成时间为“重置”。因此，对于以FIFO顺序服务EF聚合的情况，假设数据包在时间t到达满足EF定义的服务器。数据包将在不晚于时间t+Q（t）/R+E的情况下传输，其中Q（t）是时间t（包括讨论中的数据包）的EF队列大小[4]。
In a more general case, where either the output scheduler does not serve the EF packets in a FIFO order, or the variable internal delay in the device reorders packets while delivering them to the output (or both), the i-th packet destined to a given output interface to arrive to the device may no longer be the i-th packet to depart from that interface. In that case the packet-identity-aware and the aggregate definitions are no longer identical.
在更一般的情况下，如果输出调度器不以FIFO顺序为EF数据包提供服务，或者设备中的可变内部延迟在将数据包交付到输出（或两者）时对数据包进行重新排序，目的地为给定输出接口以到达设备的第i个分组可能不再是离开该接口的第i个分组。在这种情况下，包标识感知和聚合定义不再相同。
The aggregate behavior definition can be viewed as a truly aggregate characteristic of the service provided to EF packets. For an analogy, consider a dark reservoir to which all arriving packets are placed. A scheduler is allowed to pick a packet from the reservoir in a random order, without any knowledge of the order of packet
聚合行为定义可以被视为提供给EF数据包的服务的真正聚合特性。作为一个类比，考虑一个黑暗的水库，所有到达的数据包被放置。允许调度器以随机顺序从库中拾取数据包，而不知道数据包的顺序
arrivals. The aggregate part of the definition measures the accuracy of the output rate provided to the EF aggregate as a whole. The smaller E_a, the more accurate is the assurance that the reservoir is drained at least at the configured rate.
到达。定义的聚合部分衡量提供给整个EF聚合的输出率的准确性。E_a越小，确保储液罐至少以配置速率排水就越准确。
Note that in this reservoir analogy packets of EF aggregate may be arbitrarily reordered. However, the definition of EF PHB given in [6] explicitly requires that no packet reordering occur within a microflow. This requirement restricts the scheduling implementations, or, in the reservoir analogy, the order of pulling packets out of the reservoir to make sure that packets within a microflow are not reordered, but it still allows reordering at the aggregate level.
注意，在此情况下，EF骨料的数据包可以任意重新排序。然而，在[6]中给出的EF PHB的定义明确要求微流中不发生数据包重新排序。这一要求限制了调度实现，或者在水库类比中，限制了从水库中拉出数据包的顺序，以确保微流中的数据包不会重新排序，但它仍然允许在聚合级别重新排序。
Note that reordering within the aggregate, as long as there is no flow-level reordering, does not necessarily reflect a "bad" service. Consider for example a scheduler that arbitrates among 10 different EF "flows" with diverse rates. A scheduler that is aware of the rate requirements may choose to send a packet of the faster flow before a packet of the slower flow to maintain lower jitter at the flow level. In particular, an ideal "flow"-aware WFQ scheduler will cause reordering within the aggregate, while maintaining packet ordering and small jitter at the flow level.
请注意，只要没有流级重新排序，聚合内的重新排序就不一定反映“坏”服务。例如，考虑一个调度器，它在10个不同的EF“流”中以不同的速率进行仲裁。知道速率要求的调度器可以选择在较慢流的分组之前发送较快流的分组，以在流级别保持较低的抖动。特别是，一个理想的“流”感知WFQ调度器将导致聚合内的重新排序，同时在流级别保持数据包排序和小抖动。
It is intuitively clear that for such a scheduler, as well as for a simpler FIFO scheduler, the "accuracy" of the service rate is crucial for minimizing "flow"-level jitter. The packet-identity-aware definition quantifies this accuracy of the service rate.
直观地看，对于这样的调度器，以及对于更简单的FIFO调度器，服务速率的“准确性”对于最小化“流”级抖动至关重要。包标识感知定义量化了服务速率的这种准确性。
However, the small value of E_a does not give any assurances about the absolute value of per-packet delay. In fact, if the input rate exceeds the configured rate, the aggregate behavior definition may result in arbitrarily large delay of a subset of packets. This is the primary motivation for the packet-identity-aware definition.
然而，E_a的小值不能保证每个分组延迟的绝对值。事实上，如果输入速率超过配置的速率，聚合行为定义可能导致数据包子集的任意大延迟。这是包标识感知定义的主要动机。
The primary goal of the packet-aware characterization of the EF implementation is that, unlike the aggregate behavior characterization, it provides a way to find a per-packet delay bound as a function of input traffic parameters.
EF实现的包感知特征化的主要目标是，与聚合行为特征化不同，它提供了一种方法来找到作为输入流量参数函数的每包延迟界限。
While the aggregate behavior definition characterizes the accuracy of the service rate of the entire EF aggregate, the packet-identity-aware part of the definition characterizes the deviation of the device from an ideal server that serves the EF aggregate in FIFO order at least at the configured rate.
虽然聚合行为定义表征了整个EF聚合的服务速率的准确性，但定义的包标识感知部分表征了设备与理想服务器的偏差，该理想服务器至少以配置的速率以FIFO顺序为EF聚合提供服务。
The value of E_p in the packet-identity-aware definition is therefore affected by two factors: the accuracy of the aggregate rate service
因此，包标识感知定义中的E_p值受两个因素的影响：聚合速率服务的准确性
and the degree of packet reordering within the EF aggregate (under the constraint that packets within the same microflow are not reordered). Therefore, a sub-aggregate aware device that provides an ideal service rate to the aggregate, and also provides an ideal rate service for each of the sub-aggregates, may nevertheless have a very large value of E_p (in this case E_p must be at least equal to the ratio of the maximum packet size divided by the smallest rate of any sub aggregate). As a result, a large value of E_p does not necessarily mean that the service provided to EF aggregate is bad - rather it may be an indication that the service is good, but non-FIFO. On the other hand, a large value of E_p may also mean that the aggregate service is very inaccurate (bursty), and hence in this case the large value of E_p reflects a poor quality of implementation.
以及EF聚合内的数据包重新排序的程度（在相同微流内的数据包不重新排序的约束下）。因此，尽管如此，向集合提供理想服务速率并且还为每个子集合提供理想速率服务的子集合感知设备可以具有非常大的E_p值（在这种情况下，E_p必须至少等于最大分组大小除以任何子集合的最小速率的比率）。因此，E_p的大值并不一定意味着提供给EF aggregate的服务是坏的——相反，它可能表明服务是好的，但不是FIFO。另一方面，较大的E_p值也可能意味着聚合服务非常不准确（突发），因此在这种情况下，较大的E_p值反映了较差的实现质量。
As a result, a large number of E_p does not necessarily provide any guidance on the quality of the EF implementation. However, a small value of E_p does indicate a high quality FIFO implementation.
因此，大量的E_p不一定能对环境足迹实施的质量提供任何指导。然而，E_p的一个小值确实表示高质量FIFO实现。
Since E_p and E_a relate to different aspects of the EF implementation, they should be considered together to determine the quality of the implementation.
由于E_p和E_a涉及环境足迹实施的不同方面，因此应将它们结合起来考虑，以确定实施的质量。
The primary motivation for the packet-identity-aware definition is that it allows quantification of the per-packet delay bound. This section discusses the issues with computing per-packet delay.
包标识感知定义的主要动机是它允许量化每个包的延迟界限。本节讨论计算每个数据包延迟的问题。
If the total traffic arriving to an output port I from all inputs is constrained by a leaky bucket with parameters (R, B), where R is the configured rate at I, and B is the bucket depth (burst), then the delay of any packet departing from I is bounded by D_p, given by
如果从所有输入到达输出端口I的总通信量受到带有参数（R，B）的泄漏桶的约束，其中R是I处的配置速率，B是桶深度（突发），则离开I的任何数据包的延迟由D_p限定，由
D_p = B/R + E_p (eq_7)
D_p = B/R + E_p (eq_7)
(see appendix B).
（见附录B）。
Because the delay bound depends on the configured rate R and the input burstiness B, it is desirable for both of these parameters to be visible to a user of the device. A PDB desiring a particular delay bound may need to limit the range of configured rates and allowed burstiness that it can support in order to deliver such bound. Equation (eq_7) provides a means for determining an acceptable operating region for the device with a given E_p. It may also be useful to limit the total offered load to a given output to some rate R_1 < R (e.g. to obtain end-to-end delay bounds [5]). It
由于延迟界限取决于配置的速率R和输入突发性B，因此期望这两个参数对设备的用户可见。想要特定延迟边界的PDB可能需要限制其能够支持的配置速率和允许的突发性的范围，以便提供这种边界。等式（等式7）提供了一种方法，用于确定具有给定E_p的设备的可接受工作区域。将给定输出的总提供负载限制在某个速率R_1<R（例如，获得端到端延迟边界[5]）也是有用的。信息技术
is important to realize that, while R_1 may also be a configurable parameter of the device, the delay bound in (eq_7) does not depend on it. It may be possible to get better bounds explicitly using the bound on the input rate, but the bound (eq_7) does not take advantage of this information.
重要的是要认识到，尽管R_1也可以是设备的可配置参数，但（eq_7）中的延迟边界并不依赖于它。使用输入速率上的界限可以显式地获得更好的界限，但是界限（eq_7）没有利用这一信息。
Although the PHB defines inherently local behavior, in this section we briefly discuss the issue of per-packet delay as the packet traverses several hops implementing EF PHB. Given a delay bound (eq_7) at a single hop, it is tempting to conclude that per-packet bound across h hops is simply h times the bound (eq_7). However, this is not necessarily the case, unless B represents the worst case input burstiness across all nodes in the network.
虽然PHB定义了固有的本地行为，但在本节中，我们简要讨论了当数据包通过几个实现EF PHB的跃点时，每个数据包的延迟问题。给定一个单跳的延迟界限（eq_7），很容易得出这样的结论：跨越h个跳的每个数据包界限就是界限（eq_7）的h倍。然而，情况未必如此，除非B表示网络中所有节点的最坏输入突发性。
Unfortunately, obtaining such a worst case value of B is not trivial. If EF PHB is implemented using aggregate class-based scheduling where all EF packets share a single FIFO, the effect of jitter accumulation may result in an increase in burstiness from hop to hop. In particular, it can be shown that unless severe restrictions on EF utilization are imposed, even if all EF flows are ideally shaped at the ingress, then for any value of delay D it is possible to construct a network where EF utilization on any link is bounded not to exceed a given factor, no flow traverses more than a specified number of hops, but there exists a packet that experiences a delay more than D [5]. This result implies that the ability to limit the worst case burstiness and the resulting end-to-end delay across several hops may require not only limiting EF utilization on all links, but also constraining the global network topology. Such topology constraints would need to be specified in the definition of any PDB built on top of EF PHB, if such PDB requires a strict worst case delay bound.
不幸的是，获得这样一个最坏情况下的B值并非易事。如果EF PHB使用基于聚合类的调度来实现，其中所有EF数据包共享一个FIFO，则抖动累积的影响可能会导致从一个跳到另一个跳的突发性增加。特别地，可以表明，除非对EF利用率施加严格限制，即使所有EF流在入口处都是理想形状，那么对于延迟D的任何值，都可以构建一个网络，其中任何链路上的EF利用率都有界，不超过给定的因子，没有流通过超过指定跳数，但是存在一个延迟超过D的数据包[5]。这一结果意味着，要限制最坏情况下的突发性以及跨多个跃点产生的端到端延迟，可能不仅需要限制所有链路上的EF利用率，还需要限制全局网络拓扑。如果此类PDB需要严格的最坏情况延迟界限，则需要在EF PHB之上构建的任何PDB的定义中指定此类拓扑约束。
Any device with finite buffering may need to drop packets if the input burstiness becomes sufficiently high. To meet the low loss objective of EF, a node may be characterized by the operating region in which loss of EF due to congestion will not occur. This may be specified as a token bucket of rate r <= R and burst size B that can be offered from all inputs to a given output interface without loss.
如果输入突发性足够高，任何具有有限缓冲的设备都可能需要丢弃数据包。为了满足EF的低损耗目标，节点的特征可以是不会发生由于拥塞导致的EF损耗的操作区域。这可以指定为速率r<=r和突发大小B的令牌桶，该令牌桶可以从所有输入提供给给定的输出接口而不会丢失。
However, as discussed in the previous section, the phenomenon of jitter accumulation makes it generally difficult to guarantee that the input burstiness never exceeds the specified operating region for nodes internal to the DiffServ domain. A no-loss guarantee across multiple hops may require specification of constraints on network
然而，如前一节所讨论的，抖动累积现象使得通常难以保证输入突发性不会超过DiffServ域内部节点的指定操作区域。跨多个跃点的无丢失保证可能需要指定网络上的约束
topology which are outside the scope of inherently local definition of a PHB. Thus, it must be possible to establish whether a device conforms to the EF definition even when some packets are lost.
超出PHB固有局部定义范围的拓扑。因此，即使某些数据包丢失，也必须能够确定设备是否符合EF定义。
This can be done by performing an "off-line" test of conformance to equations (eq_1)- (eq_4). After observing a sequence of packets entering and leaving the node, the packets which did not leave are assumed lost and are notionally removed from the input stream. The remaining packets now constitute the arrival stream and the packets which left the node constitute the departure stream. Conformance to the equations can thus be verified by considering only those packets that successfully passed through the node.
这可以通过执行符合方程式（等式1）-（等式4）的“离线”测试来实现。在观察到进入和离开节点的数据包序列之后，假设没有离开的数据包丢失，并从输入流中名义上移除。剩下的分组现在构成到达流，离开节点的分组构成离开流。因此，可以通过仅考虑成功通过节点的那些分组来验证与方程的一致性。
Note that specification of which packets are lost in the case when loss does occur is beyond the scope of the definition of EF PHB. However, those packets that were not lost must conform to the equations definition of EF PHB in section 2.1.
请注意，在确实发生丢失的情况下丢失哪些数据包的规范超出了EF PHB定义的范围。但是，未丢失的数据包必须符合第2.1节中EF PHB的方程式定义。
A packet passing through a router will experience delay for a number of reasons. Two familiar components of this delay are the time the packet spends in a buffer at an outgoing link waiting for the scheduler to select it and the time it takes to actually transmit the packet on the outgoing line.
由于多种原因，通过路由器的数据包将经历延迟。这种延迟的两个常见组成部分是数据包在传出链路的缓冲区中等待调度器选择它的时间，以及在传出线路上实际传输数据包所需的时间。
There may be other components of a packet's delay through a router, however. A router might have to do some amount of header processing before the packet can be given to the correct output scheduler, for example. In another case a router may have a FIFO buffer (called a transmission queue in [7]) where the packet sits after being selected by the output scheduler but before it is transmitted. In cases such as these, the extra delay a packet may experience can be accounted for by absorbing it into the latency terms E_a and E_p.
然而，通过路由器的数据包延迟可能还有其他成分。例如，在将数据包发送给正确的输出调度器之前，路由器可能必须进行一定量的报头处理。在另一种情况下，路由器可能有一个FIFO缓冲区（在[7]中称为传输队列），其中数据包在被输出调度器选择之后但在被传输之前位于该缓冲区。在这样的情况下，分组可能经历的额外延迟可以通过将其吸收到延迟项E_a和E_p中来解释。
Implementing EF on a router with a multi-stage switch fabric requires special attention. A packet may experience additional delays due to the fact that it must compete with other traffic for forwarding resources at multiple contention points in the switch core. The delay an EF packet may experience before it even reaches the output-link scheduler should be included in the latency term. Input-buffered and input/output-buffered routers based on crossbar design may also require modification of their latency terms. The factors such as the speedup factor and the choice of crossbar arbitration algorithms may affect the latency terms substantially.
在具有多级交换结构的路由器上实现EF需要特别注意。数据包可能会经历额外的延迟，因为它必须在交换机核心的多个争用点上与其他流量竞争转发资源。EF数据包在到达输出链路调度器之前可能经历的延迟应包括在延迟项中。基于交叉设计的输入缓冲路由器和输入/输出缓冲路由器也可能需要修改其延迟条款。诸如加速因子和crossbar仲裁算法的选择等因素可能会实质性地影响延迟项。
Delay in the switch core comes from two sources, both of which must be considered. The first part of this delay is the fixed delay a packet experiences regardless of the other traffic. This component of the delay includes the time it takes for things such as packet segmentation and reassembly in cell based cores, enqueueing and dequeuing at each stage, and transmission between stages. The second part of the switch core delay is variable and depends on the type and amount of other traffic traversing the core. This delay comes about if the stages in the core mix traffic flowing between different input/output port pairs. Thus, EF packets must compete against other traffic for forwarding resources in the core. Some of this competing traffic may even be traffic from other, non-EF aggregates. This introduces extra delay, that can also be absorbed by the latency term in the definition.
交换机内核中的延迟来自两个来源，必须考虑这两个来源。该延迟的第一部分是一个数据包经历的固定延迟，与其他流量无关。延迟的这一部分包括在基于单元的核心中进行数据包分割和重组、在每个阶段排队和退队以及阶段之间的传输等所需的时间。交换机核心延迟的第二部分是可变的，取决于穿过核心的其他通信量的类型和数量。如果核心中的各个阶段混合了不同输入/输出端口对之间的流量，则会出现此延迟。因此，EF数据包必须与其他流量竞争，以在核心中转发资源。其中一些竞争流量甚至可能是来自其他非EF聚合的流量。这引入了额外的延迟，也可以被定义中的延迟项吸收。
To capture these considerations, in this section we will consider two simplified implementation examples. The first is an ideal output buffered node where packets entering the device from an input interface are immediately delivered to the output scheduler. In this model the properties of the output scheduler fully define the values of the parameters E_a and E_p. We will consider the case where the output scheduler implements aggregate class-based queuing, so that all EF packets share a single queue. We will discuss the values of E_a and E_p for a variety of class-based schedulers widely considered.
为了捕捉这些考虑，在本节中，我们将考虑两个简化的实现示例。第一个是理想的输出缓冲节点，其中从输入接口进入设备的数据包将立即传递到输出调度器。在这个模型中，输出调度器的属性完全定义了参数E_a和E_p的值。我们将考虑输出调度器实现基于聚合类的队列的情况，以便所有EF分组共享一个队列。我们将讨论广泛考虑的各种基于类的调度器的E_a和E_p的值。
The second example will consider a router modeled as a black box with a known bound on the variable delay a packet can experience from the time it arrives to an input to the time it is delivered to its destination output. The output scheduler in isolation is assumed to be an aggregate scheduler where all EF packets share a single FIFO queue, with a known value of E_a(S)=E_p(S)=E(S). This model provides a reasonable abstraction to a large class of router implementations.
第二个例子将考虑一个路由器，该路由器被建模为一个黑盒，它具有一个已知的变量，该变量可以从一个包到达它到达目的地输出的时间到它的输入时经历的可变延迟。隔离的输出调度器被假定为一个聚合调度器，其中所有EF数据包共享一个FIFO队列，其已知值为E_a（S）=E_p（S）=E（S）。该模型为一大类路由器实现提供了合理的抽象。
5.1. The output buffered model with EF FIFO at the output.
5.1. 输出端带有EF FIFO的输出缓冲模型。
As has been mentioned earlier, in this model E_a = E_p, so we shall omit the subscript and refer to both terms as latency E. The remainder of this subsection discusses E for a number of scheduling implementations.
如前所述，在该模型中，E_a=E_p，因此我们将省略下标，并将这两个术语称为延迟E。本小节的其余部分讨论了许多调度实现的延迟E。
A Strict Priority scheduler in which all EF packets share a single FIFO queue which is served at strict non-preemptive priority over other queues satisfies the EF definition with the latency term E = MTU/C where MTU is the maximum packet size and C is the speed of the output link.
严格优先级调度器，其中所有EF数据包共享一个FIFO队列，该队列以严格的非抢占优先级服务于其他队列，满足EF定义，延迟项E=MTU/C，其中MTU是最大数据包大小，C是输出链路的速度。
Another scheduler that satisfies the EF definition with a small latency term is WF2Q described in [1]. A class-based WF2Q scheduler, in which all EF traffic shares a single queue with the weight corresponding to the configured rate of the EF aggregate satisfies the EF definition with the latency term E = MTU/C+MTU/R.
[1]中描述的另一个满足EF定义且具有小延迟项的调度器是WF2Q。一种基于类的WF2Q调度器，其中所有EF流量共享一个队列，其权重对应于EF聚合的配置速率，满足EF定义，延迟项为E=MTU/C+MTU/R。
For DRR [12], E can be shown to grow linearly with N*(r_max/r_min)*MTU, where r_min and r_max denote the smallest and the largest rate among the rate assignments of all queues in the scheduler, and N is the number of queues in the scheduler.
对于DRR[12]，E可以显示为与N*（r_max/r_min）*MTU线性增长，其中r_min和r_max表示调度器中所有队列的速率分配中的最小速率和最大速率，N是调度器中的队列数。
For Start-Time Fair Queuing (SFQ) [9] and Self-Clocked Fair Queuing (SCFQ) [8] E can be shown to grow linearly with the number of queues in the scheduler.
对于开始时间公平队列（SFQ）[9]和自时钟公平队列（SCFQ）[8]，E可以显示为随着调度程序中队列的数量线性增长。
In this section we consider a router which is modeled as follows. A packet entering the router may experience a variable delay D_v with a known upper bound D. That is, 0<=D_v<=D. At the output all EF packets share a single class queue. Class queues are scheduled by a scheduler with a known value E_p(S)=E(S) (where E(S) corresponds to the model where this scheduler is implemented in an ideal output buffered device).
在这一节中，我们考虑如下建模的路由器。进入路由器的数据包可能会经历一个具有已知上界D的可变延迟D_v。即，0<=D_v<=D。在输出端，所有EF数据包共享一个类队列。类队列由具有已知值E_p（S）=E（S）（其中E（S）对应于该调度器在理想输出缓冲设备中实现的模型）的调度器进行调度。
The computation of E_p is more complicated in this case. For such device, it can be shown that E_p = E(S)+2D+2B/R (see [13]).
在这种情况下，E_p的计算更加复杂。对于这样的装置，可以显示E_p=E（S）+2D+2B/R（见[13]）。
Recall from the discussion of section 3 that bounding input burstiness B may not be easy in a general topology. In the absence of the knowledge of a bound on B one can bound E_p as E_p = E(S) + D*C_inp/R (see [13]).
回顾第3节的讨论，在一般拓扑中，边界输入突发性B可能并不容易。在不知道B上的一个界的情况下，可以将E_p作为E_p=E（S）+D*C_inp/R（见[13]）来定义。
Note also that the bounds in this section are derived using only the bound on the variable portion of the interval delay and the error bound of the output scheduler. If more details about the architecture of a device are available, it may be possible to compute better bounds.
还请注意，本节中的边界仅使用间隔延迟可变部分的边界和输出调度器的错误边界导出。如果可以获得关于设备架构的更多细节，就有可能计算出更好的边界。
This informational document provides additional information to aid in understanding of the EF PHB described in [6]. It adds no new functions to it. As a result, it adds no security issues to those described in that specification.
本信息性文件提供了其他信息，有助于理解[6]中所述的EF PHB。它没有添加任何新功能。因此，它不会给该规范中描述的安全问题增加任何安全问题。
[1] J.C.R. Bennett and H. Zhang, "WF2Q: Worst-case Fair Weighted Fair Queuing", INFOCOM'96, March 1996.
[1] J.C.R.Bennett和H.Zhang，“WF2Q：最坏情况公平加权公平排队”，INFOCOM'96，1996年3月。
[2] J.-Y. Le Boudec, P. Thiran, "Network Calculus", Springer Verlag Lecture Notes in Computer Science volume 2050, June 2001 (available online at http://lcawww.epfl.ch).
[2] J.-Y.Le Boudec，P.Thiran，“网络微积分”，计算机科学2050卷中的Springer Verlag课堂讲稿，2001年6月（可在线访问http://lcawww.epfl.ch).
[3] Bradner, S., "Key Words for Use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[3] Bradner，S.，“RFC中用于表示需求水平的关键词”，BCP 14，RFC 2119，1997年3月。
[4] J.C.R. Bennett, K. Benson, A. Charny, W. Courtney, J.Y. Le Boudec, "Delay Jitter Bounds and Packet Scale Rate Guarantee for Expedited Forwarding", Proc. Infocom 2001, April 2001.
[4] J.C.R.Bennett，K.Benson，A.Charny，W.Courtney，J.Y.Le Boudec，“加速转发的延迟抖动界限和数据包规模速率保证”，Proc。InfoCom2001，2001年4月。
[5] A. Charny, J.-Y. Le Boudec "Delay Bounds in a Network with Aggregate Scheduling". Proc. of QoFIS'2000, September 25-26, 2000, Berlin, Germany.
[5] A.Charny，J.-Y.Le Boudec“具有聚合调度的网络中的延迟边界”。过程。2000年9月25日至26日，德国柏林。
[6] Davie, B., Charny, A., Baker, F., Bennett, J.C.R., Benson, K., Boudec, J., Chiu, A., Courtney, W., Davari, S., Firoiu, V., Kalmanek, C., Ramakrishnan, K.K. and D. Stiliadis, "An Expedited Forwarding PHB (Per-Hop Behavior)", RFC 3246, March 2002.
[6] Davie，B.，Charny，A.，Baker，F.，Bennett，J.C.R.，Benson，K.，Boudec，J.，Chiu，A.，Courtney，W.，Davari，S.，Firoiu，V.，Kalmanek，C.，Ramakrishnan，K.K.和D.Stiliadis，“快速转发PHB（每跳行为）”，RFC 32462002年3月。
[7] T. Ferrari and P. F. Chimento, "A Measurement-Based Analysis of Expedited Forwarding PHB Mechanisms," Eighth International Workshop on Quality of Service, Pittsburgh, PA, June 2000.
[7] T.Ferrari和P.F.Chimento，“基于测量的快速转发PHB机制分析”，第八届服务质量国际研讨会，宾夕法尼亚州匹兹堡，2000年6月。
[8] S.J. Golestani. "A Self-clocked Fair Queuing Scheme for Broad-band Applications". In Proceedings of IEEE INFOCOM'94, pages 636-646, Toronto, CA, April 1994.
[8] S.J.戈莱斯塔尼。“宽带应用的自时钟公平排队方案”。《IEEE INFOCOM'94会议录》，第636-646页，加利福尼亚州多伦多，1994年4月。
[9] P. Goyal, H.M. Vin, and H. Chen. "Start-time Fair Queuing: A Scheduling Algorithm for Integrated Services". In Proceedings of the ACM-SIGCOMM 96, pages 157-168, Palo Alto, CA, August 1996.
[9] P.戈亚尔、H.M.文和H.陈。“开始时间公平排队：综合业务的调度算法”。1996年8月，加利福尼亚州帕洛阿尔托，ACM-SIGCOM96会议记录，第157-168页。
[10] Jacobson, V., Nichols, K. and K. Poduri, "An Expedited Forwarding PHB", RFC 2598, June 1999.
[10] Jacobson，V.，Nichols，K.和K.Poduri，“快速转发PHB”，RFC 25981999年6月。
[11] Jacobson, V., Nichols, K. and K. Poduri, "The 'Virtual Wire' Behavior Aggregate", Work in Progress.
[11] Jacobson，V.，Nichols，K.和K.Poduri，“虚拟线路”行为聚合”，正在进行中。
[12] M. Shreedhar and G. Varghese. "Efficient Fair Queuing Using Deficit Round Robin". In Proceedings of SIGCOMM'95, pages 231-243, Boston, MA, September 1995.
[12] M.Shreedhar和G.Varghese。“使用赤字循环的高效公平排队”。1995年9月，马萨诸塞州波士顿，SIGCOMM'95会议记录，第231-243页。
[13] Le Boudec, J.-Y., Charny, A. "Packet Scale Rate Guarantee for non-FIFO Nodes", Infocom 2002, New York, June 2002.
[13] Le Boudec，J.-Y.，Charny，A.“非FIFO节点的数据包规模速率保证”，Infocom 2002，纽约，2002年6月。
The definition of the EF PHB as given in [10] states:
[10]中给出的EF PHB定义说明：
"The EF PHB is defined as a forwarding treatment for a particular diffserv aggregate where the departure rate of the aggregate's packets from any diffserv node must equal or exceed a configurable rate. The EF traffic SHOULD receive this rate independent of the intensity of any other traffic attempting to transit the node. It [the EF PHB departure rate] SHOULD average at least the configured rate when measured over any time interval equal to or longer than the time it takes to send an output link MTU sized packet at the configured rate."
“EF PHB被定义为特定diffserv聚合的转发处理，其中来自任何diffserv节点的聚合数据包的离开率必须等于或超过可配置的速率。EF流量应接收该速率，而与试图传输该节点的任何其他流量的强度无关。它[EF PHB离开率]在任何时间间隔内测量时，至少应平均配置的速率，该时间间隔等于或大于以配置的速率发送输出链路MTU大小的数据包所需的时间。”
A literal interpretation of the definition would consider the behaviors given in the next two subsections as non-compliant. The definition also unnecessarily constrains the maximum configurable rate of an EF aggregate.
定义的字面解释将考虑在以下两个部分中给出的行为不符合。该定义还不必要地限制了EF聚合的最大可配置速率。
Consider the following stream forwarded from a router with EF-configured rate R=C/2, where C is the output line rate. In the illustration, E is an MTU-sized EF packet while x is a non-EF packet or unused capacity, also of size MTU.
考虑从EF配置速率R＝C／2的路由器转发的以下流，其中C是输出线速率。在图示中，E是MTU大小的EF分组，而x是非EF分组或未使用的容量，也具有MTU大小。
E x E x E x E x E x E x...
|-----|
E x E x E x E x E x E x...
|-----|
The interval between the vertical bars is 3*MTU/C, which is greater than MTU/(C/2), and so is subject to the EF PHB definition. During this interval, 3*MTU/2 bits of the EF aggregate should be forwarded, but only MTU bits are forwarded. Therefore, while this forwarding pattern should be considered compliant under any reasonable interpretation of the EF PHB, it actually does not formally comply with the definition of RFC 2598.
垂直条之间的间隔为3*MTU/C，大于MTU/（C/2），因此受EF PHB定义的约束。在此间隔期间，应转发EF聚合的3*MTU/2位，但仅转发MTU位。因此，尽管根据EF PHB的任何合理解释，该转发模式应被视为符合要求，但它实际上并不符合RFC 2598的定义。
Note that this forwarding pattern can occur in any work-conserving scheduler in an ideal output-buffered architecture where EF packets arrive in a perfectly clocked manner according to the above pattern and are forwarded according to exactly the same pattern in the absence of any non-EF traffic.
请注意，此转发模式可以发生在理想输出缓冲体系结构中的任何节省工作的调度器中，其中EF分组根据上述模式以完全计时的方式到达，并且在没有任何非EF通信的情况下根据完全相同的模式转发。
Trivial as this example may be, it reveals the lack of mathematical precision in the formal definition. The fact that no work-conserving scheduler can formally comply with the definition is unfortunate, and appears to warrant some changes to the definition that would correct this problem.
尽管这个例子可能微不足道，但它揭示了形式定义中缺乏数学精度。不幸的是，没有一个节省工作的调度器能够正式遵守定义，并且似乎需要对定义进行一些更改，以纠正此问题。
The underlying reason for the problem described here is quite simple - one can only expect that the EF aggregate is served at configured rate in some interval where there is enough backlog of EF packets to sustain that rate. In the example above the packets come in exactly at the rate at which they are served, and so there is no persistent backlog. Certainly, if the input rate is even smaller than the configured rate of the EF aggregate, there will be no backlog as well, and a similar formal difficulty will occur.
这里描述的问题的根本原因很简单——人们只能期望EF聚合在某个间隔内以配置的速率提供服务，在该间隔内有足够的EF数据包积压来维持该速率。在上面的示例中，数据包以其被服务的速率进入，因此没有持久的积压。当然，如果输入速率甚至小于EF聚合的配置速率，那么也不会有积压工作，并且会出现类似的正式困难。
A seemingly simple solution to this difficulty might be to require that the EF aggregate is served at its configured rate only when the queue is backlogged. However, as we show in the remainder of this section, this solution does not suffice.
解决这一难题的一个看似简单的解决方案可能是，仅当队列积压时，才要求以其配置的速率提供EF聚合。但是，正如我们在本节剩余部分中所示，这种解决方案还不够。
We now argue that the example considered in the previous section is not as trivial as it may seem at first glance.
我们现在认为，上一节中考虑的示例并不像乍看起来那么琐碎。
Consider a router with EF configured rate R = C/2 as in the previous example, but with an internal delay of 3T (where T = MTU/C) between the time that a packet arrives at the router and the time that it is first eligible for forwarding at the output link. Such things as header processing, route look-up, and delay in switching through a multi-layer fabric could cause this delay. Now suppose that EF traffic arrives regularly at a rate of (2/3)R = C/3. The router will perform as shown below.
考虑一个具有EF配置速率R＝C／2的路由器，如在前面的例子中一样，但是在包到达路由器的时间和它在输出链路上首次有资格转发的时间之间具有3T（其中T＝MTU/C）的内部延迟。诸如报头处理、路由查找以及通过多层结构进行切换的延迟都可能导致这种延迟。现在假设EF流量以（2/3）R=C/3的速率定期到达。路由器将执行如下所示的操作。
EF Packet Number 1 2 3 4 5 6 ...
EF包编号1 2 3 4 5 6。。。
Arrival (at router) 0 3T 6T 9T 12T 15T ...
到达（路由器）0 3T 6T 9T 12T 15T。。。
Arrival (at scheduler) 3T 6T 9T 12T 15T 18T ...
到达（在调度台）3T 6T 9T 12T 15T 18T。。。
Departure 4T 7T 10T 13T 16T 19T ...
出发时间4T 7T 10T 13T 16T 19T。。。
Again, the output does not satisfy the RFC 2598 definition of EF PHB. As in the previous example, the underlying reason for this problem is that the scheduler cannot forward EF traffic faster than it arrives. However, it can be easily seen that the existence of internal delay causes one packet to be inside the router at all times. An external observer will rightfully conclude that the number of EF packets that arrived to the router is always at least one greater than the number of EF packets that left the router, and therefore the EF aggregate is constantly backlogged. However, while the EF aggregate is continuously backlogged, the observed output rate is nevertheless strictly less that the configured rate.
同样，输出不满足EF PHB的RFC 2598定义。与前一个示例一样，此问题的根本原因是调度器转发EF流量的速度不能超过它到达的速度。然而，可以很容易地看出，内部延迟的存在导致一个数据包始终在路由器内部。外部观察者将正确地得出结论，到达路由器的EF数据包的数量始终至少比离开路由器的EF数据包的数量大一个，因此EF聚合经常被积压。然而，尽管EF聚合持续积压，但观察到的输出速率仍严格低于配置速率。
This example indicates that the simple addition of the condition that EF aggregate must receive its configured rate only when the EF aggregate is backlogged does not suffice in this case.
此示例表明，仅在EF聚合积压时，简单添加EF聚合必须接收其配置速率的条件在这种情况下是不够的。
Yet, the problem described here is of fundamental importance in practice. Most routers have a certain amount of internal delay. A vendor declaring EF compliance is not expected to simultaneously declare the details of the internals of the router. Therefore, the existence of internal delay may cause a perfectly reasonable EF implementation to display seemingly non-conformant behavior, which is clearly undesirable.
然而，这里描述的问题在实践中具有根本重要性。大多数路由器都有一定的内部延迟。声明EF合规性的供应商不应同时声明路由器内部的详细信息。因此，内部延迟的存在可能会导致完全合理的EF实现显示出看似不一致的行为，这显然是不可取的。
It is well understood that with any non-preemptive scheduler, the RFC-2598-compliant configurable rate for an EF aggregate cannot exceed C/2 [11]. This is because an MTU-sized EF packet may arrive to an empty queue at time t just as an MTU-sized non-EF packet begins service. The maximum number of EF bits that could be forwarded during the interval [t, t + 2*MTU/C] is MTU. But if configured rate R > C/2, then this interval would be of length greater than MTU/R, and more than MTU EF bits would have to be served during this interval for the router to be compliant. Thus, R must be no greater than C/2.
众所周知，对于任何非抢占式调度程序，EF聚合的RFC-2598兼容可配置速率不能超过C/2[11]。这是因为MTU大小的EF数据包可能在时间t到达空队列，就像MTU大小的非EF数据包开始服务一样。在间隔[t，t+2*MTU/C]期间可转发的EF比特的最大数量为MTU。但是，如果配置的速率R>C/2，那么该间隔的长度将大于MTU/R，并且在该间隔期间必须提供超过MTU EF位的服务，路由器才能兼容。因此，R不得大于C/2。
It can be shown that for schedulers other than PQ, such as various implementations of WFQ, the maximum compliant configured rate may be much smaller than 50%. For example, for SCFQ [8] the maximum configured rate cannot exceed C/N, where N is the number of queues in the scheduler. For WRR, mentioned as compliant in section 2.2 of RFC 2598, this limitation is even more severe. This is because in these schedulers a packet arriving to an empty EF queue may be forced to wait until one packet from each other queue (in the case of SCFQ) or until several packets from each other queue (in the case of WRR) are served before it will finally be forwarded.
可以看出，对于PQ以外的调度器，例如WFQ的各种实现，最大兼容配置率可能远小于50%。例如，对于SCFQ[8]，最大配置速率不能超过C/N，其中N是计划程序中的队列数。对于RFC 2598第2.2节中提到的WRR，该限制更为严格。这是因为在这些调度器中，到达空EF队列的数据包可能会被迫等待，直到来自每个队列的一个数据包（对于SCFQ）或来自每个队列的多个数据包（对于WRR）被服务，然后才最终被转发。
While it is frequently assumed that the configured rate of EF traffic will be substantially smaller than the link bandwidth, the requirement that this rate should never exceed 50% of the link bandwidth appears unnecessarily limiting. For example, in a fully connected mesh network, where any flow traverses a single link on its way from source to its destination there seems no compelling reason to limit the amount of EF traffic to 50% (or an even smaller percentage for some schedulers) of the link bandwidth.
虽然经常假设EF流量的配置速率将大大小于链路带宽，但该速率不得超过链路带宽的50%的要求似乎是不必要的限制。例如，在完全连接的网状网络中，当任何流在从源到目的地的途中穿过单个链路时，似乎没有令人信服的理由将EF通信量限制在链路带宽的50%（或某些调度器的更小百分比）。
Another, perhaps even more striking example is the fact that even a TDM circuit with dedicated slots cannot be configured to forward EF packets at more than 50% of the link speed without violating RFC 2598
另一个可能更显著的例子是，即使具有专用时隙的TDM电路也不能配置为在不违反RFC 2598的情况下以超过链路速度50%的速率转发EF分组
(unless the entire link is configured for EF). If the configured rate of EF traffic is greater than 50% (but less than the link speed), there will always exist an interval longer than MTU/R in which less than the configured rate is achieved. For example, suppose the configured rate of the EF aggregate is 2C/3. Then the forwarding pattern of the TDM circuit might be
（除非为EF配置了整个链路）。如果EF流量的配置速率大于50%（但小于链路速度），则始终存在一个比MTU/R长的间隔，在该间隔内，达到的速率小于配置速率。例如，假设EF聚合的配置速率为2C/3。然后，TDM电路的转发模式可能是
E E x E E x E E x ...
|---|
E E x E E x E E x ...
|---|
where only one packet is served in the marked interval of length 2T = 2MTU/C. But at least 4/3 MTU would have to be served during this interval by a router in compliance with the definition in RFC 2598. The fact that even a TDM line cannot be booked over 50% by EF traffic indicates that the restriction is artificial and unnecessary.
其中，在标记的长度为2T=2MTU/C的间隔内，仅服务一个数据包。但根据RFC 2598中的定义，路由器必须在此间隔内服务至少4/3 MTU。即使是TDM线路也不能被EF流量预订超过50%，这表明该限制是人为的，不必要的。
One possibility to correct the problems discussed in the previous sections might be to attempt to clarify the definition of the intervals to which the definition applied or by averaging over multiple intervals. However, an attempt to do so meets with considerable analytical and implementation difficulties. For example, attempting to align interval start times with some epochs of the forwarded stream appears to require a certain degree of global clock synchronization and is fraught with the risk of misinterpretation and mistake in practice.
纠正前几节中讨论的问题的一种可能性是尝试澄清定义适用的区间的定义，或通过对多个区间进行平均。然而，这样做的尝试在分析和执行方面遇到了相当大的困难。例如，试图将间隔开始时间与转发流的某些时段对齐似乎需要一定程度的全局时钟同步，并且在实践中充满了误解和错误的风险。
Another approach might be to allow averaging of the rates over some larger time scale. However, it is unclear exactly what finite time scale would suffice in all reasonable cases. Furthermore, this approach would compromise the notion of very short-term time scale guarantees that are the essence of EF PHB.
另一种方法可能是允许在更大的时间范围内平均费率。然而，目前尚不清楚在所有合理的情况下，有限的时间尺度究竟能满足什么要求。此外，这种方法将损害作为EF PHB本质的非常短期的时间尺度保证的概念。
We also explored a combination of two simple fixes. The first is the addition of the condition that the only intervals subject to the definition are those that fall inside a period during which the EF aggregate is continuously backlogged in the router (i.e., when an EF packet is in the router). The second is the addition of an error (latency) term that could serve as a figure-of-merit in the advertising of EF services.
我们还探索了两个简单修复的组合。第一个是增加了一个条件，即受定义约束的唯一间隔是在EF聚合在路由器中连续积压的时间段内的间隔（即，当EF分组在路由器中时）。第二个是增加了一个错误（延迟）术语，该术语可以作为EF服务广告中的价值指标。
With the addition of these two changes the candidate definition becomes as follows:
添加这两项更改后，候选定义如下所示：
In any interval of time (t1, t2) in which EF traffic is continuously backlogged, at least R(t2 - t1 - E) bits of EF traffic must be served, where R is the configured rate for the EF aggregate and E is an implementation-specific latency term.
在EF流量连续积压的任何时间间隔（t1、t2）中，必须至少服务EF流量的R（t2-t1-E）位，其中R是EF聚合的配置速率，E是特定于实现的延迟项。
The "continuously backlogged" condition eliminates the insufficient-packets-to-forward difficulty while the addition of the latency term of size MTU/C resolves the perfectly-clocked forwarding example (section A.1), and also removes the limitation on EF configured rate.
“连续积压”条件消除了转发数据包不足的困难，同时添加大小为MTU/C的延迟项解决了完全时钟转发示例（第A.1节），并消除了对EF配置速率的限制。
However, neither fix (nor the two of them together) resolves the example of section A.2. To see this, recall that in the example of section A.2 the EF aggregate is continuously backlogged, but the service rate of the EF aggregate is consistently smaller than the configured rate, and therefore no finite latency term will suffice to bring the example into conformance.
但是，无论是fix（或两者一起）都无法解决第A.2节的示例。要了解这一点，请回想一下，在第A.2节的示例中，EF聚合连续积压，但EF聚合的服务速率始终小于配置的速率，因此没有有限的延迟期限足以使示例符合要求。
The proofs of several bounds in this document can be found in [13]. These proofs use an algebraic characterization of the aggregate definition given by (eq_1), (eq_2), and packet identity aware definition given by (eq_3), (eq_4). Since this characterization is of interest on its own, we present it in this section.
本文中几个界限的证明见[13]。这些证明使用了（eq_1）、（eq_2）给出的聚合定义的代数特征，以及（eq_3）、（eq_4）给出的包标识感知定义。由于这种特性本身就很有趣，我们将在本节中介绍它。
Theorem B1. Characterization of the aggregate definition without f_n.
定理B1。无f_n的骨料定义的表征。
Consider a system where packets are numbered 1, 2, ... in order of arrival. As in the aggregate definition, call a_n the n-th arrival time, d_n - the n-th departure time, and l_n the size of the n-th packet to depart. Define by convention d_0=0. The aggregate definition with rate R and latency E_a is equivalent to saying that for all n and all 0<=j<= n-1:
考虑一个系统，其中的数据包编号为1, 2，…按到达顺序。在聚合定义中，调用a_n表示第n个到达时间，d_n表示第n个离开时间，l_n表示要离开的第n个数据包的大小。按约定定义d_0=0。速率R和延迟E_a的聚合定义等同于表示对于所有n和所有0<=j<=n-1：
d_n <= E_a + d_j + (l_(j+1) + ... + l_n)/R (eq_b1)
d_n <= E_a + d_j + (l_(j+1) + ... + l_n)/R (eq_b1)
or
或
there exists some j+1 <= k <= n such that
there exists some j+1 <= k <= n such that
d_n <= E_a + a_k + (l_k + ... + l_n)/R (eq_b2)
d_n <= E_a + a_k + (l_k + ... + l_n)/R (eq_b2)
Theorem B2. Characterization of packet-identity-aware definition without F_n.
定理B2。无F_n的分组身份感知定义的特征化。
Consider a system where packets are numbered 1, 2, ... in order of arrival. As in the packet-identity-aware definition, call A_n, D_n the arrival and departure times for the n-th packet, and L_n the size of this packet. Define by convention D_0=0. The packet identity aware definition with rate R and latency E_p is equivalent to saying that for all n and all 0<=j<= n-1:
考虑一个系统，其中的数据包编号为1, 2，…按到达顺序。在包标识感知定义中，调用A_n，D_n表示第n个包的到达和离开时间，L_n表示该包的大小。按约定定义D_0=0。具有速率R和延迟E_p的分组标识感知定义等同于表示对于所有n和所有0<=j<=n-1：
D_n <= E_p + D_j + (L_{j+1} + ... + L_n)/R (eq_b3)
D_n <= E_p + D_j + (L_{j+1} + ... + L_n)/R (eq_b3)
or
或
there exists some j+1 <= k <= n such that
there exists some j+1 <= k <= n such that
D_n <= E_p + A_k + (L_k + ... + L_n)/R (eq_b4)
D_n <= E_p + A_k + (L_k + ... + L_n)/R (eq_b4)
For the proofs of both Theorems, see [13].
关于这两个定理的证明，请参见[13]。
Acknowledgements
致谢
This document could not have been written without Fred Baker, Bruce Davie and Dimitrios Stiliadis. Their time, support and insightful comments were invaluable.
没有弗雷德·贝克、布鲁斯·戴维斯和迪米特里奥斯·斯蒂里亚迪斯，这份文件就不可能写成。他们的时间、支持和富有洞察力的评论是无价的。
Authors' Addresses
作者地址
Anna Charny Cisco Systems 300 Apollo Drive Chelmsford, MA 01824
马萨诸塞州切姆斯福德阿波罗大道300号安娜查尼思科系统公司01824
EMail: acharny@cisco.com
EMail: acharny@cisco.com
Jon Bennett Motorola 3 Highwood Drive East Tewksbury, MA 01876
乔恩·贝内特：马萨诸塞州特克斯伯里市东海伍德大道3号摩托罗拉01876
EMail: jcrb@motorola.com
EMail: jcrb@motorola.com
Kent Benson Tellabs Research Center 3740 Edison Lake Parkway #101 Mishawaka, IN 46545
肯特·本森·特拉布研究中心3740爱迪生湖公园路#101米沙瓦卡，46545
EMail: Kent.Benson@tellabs.com
EMail: Kent.Benson@tellabs.com
Jean-Yves Le Boudec ICA-EPFL, INN Ecublens, CH-1015 Lausanne-EPFL, Switzerland
Jean-Yves Le Boudec ICA-EPFL，瑞士洛桑EPFL CH-1015埃克布伦斯酒店
EMail: jean-yves.leboudec@epfl.ch
EMail: jean-yves.leboudec@epfl.ch
Angela Chiu Celion Networks 1 Sheila Drive, Suite 2 Tinton Falls, NJ 07724
Angela Chiu Celion Networks 1 Sheila Drive，2号套房，新泽西州丁顿瀑布，邮编07724
EMail: angela.chiu@celion.com
EMail: angela.chiu@celion.com
Bill Courtney TRW Bldg. 201/3702 One Space Park Redondo Beach, CA 90278
加利福尼亚州雷东多海滩太空公园一号比尔·考特尼TRW大厦201/3702号，邮编90278
EMail: bill.courtney@trw.com
EMail: bill.courtney@trw.com
Shahram Davari PMC-Sierra Inc 411 Legget Drive Ottawa, ON K2K 3C9, Canada
加拿大K2K 3C9渥太华Legget大道411号Shahram Davari PMC Sierra Inc
EMail: shahram_davari@pmc-sierra.com
EMail: shahram_davari@pmc-sierra.com
Victor Firoiu Nortel Networks 600 Tech Park Billerica, MA 01821
Victor Firoiu Nortel Networks 600科技园马萨诸塞州比尔里卡01821
EMail: vfiroiu@nortelnetworks.com
EMail: vfiroiu@nortelnetworks.com
Charles Kalmanek AT&T Labs-Research 180 Park Avenue, Room A113, Florham Park NJ
Charles Kalmanek AT&T实验室研究室新泽西州弗洛勒姆公园大道180号A113室
EMail: crk@research.att.com
EMail: crk@research.att.com
K.K. Ramakrishnan TeraOptic Networks, Inc. 686 W. Maude Ave Sunnyvale, CA 94086
K.K.罗摩克里希南Terapologic Networks，Inc.加利福尼亚州桑尼维尔莫德大道西686号，邮编94086
EMail: kk@teraoptic.com
EMail: kk@teraoptic.com
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Acknowledgement
确认
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