Network Working Group J. Heinanen
Request for Comments: 2597 Telia Finland
Category: Standards Track F. Baker
Cisco Systems
W. Weiss
Lucent Technologies
J. Wroclawski
MIT LCS
June 1999
Network Working Group J. Heinanen
Request for Comments: 2597 Telia Finland
Category: Standards Track F. Baker
Cisco Systems
W. Weiss
Lucent Technologies
J. Wroclawski
MIT LCS
June 1999
Assured Forwarding PHB Group
保证转发PHB组
Status of this Memo
本备忘录的状况
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
本文件规定了互联网社区的互联网标准跟踪协议，并要求进行讨论和提出改进建议。有关本协议的标准化状态和状态，请参考当前版本的“互联网官方协议标准”（STD 1）。本备忘录的分发不受限制。
Copyright Notice
版权公告
Copyright (C) The Internet Society (1999). All Rights Reserved.
版权所有（C）互联网协会（1999年）。版权所有。
Abstract
摘要
This document defines a general use Differentiated Services (DS) [Blake] Per-Hop-Behavior (PHB) Group called Assured Forwarding (AF). The AF PHB group provides delivery of IP packets in four independently forwarded AF classes. Within each AF class, an IP packet can be assigned one of three different levels of drop precedence. A DS node does not reorder IP packets of the same microflow if they belong to the same AF class.
本文档定义了一个称为保证转发（AF）的通用区分服务（DS）[Blake]每跳行为（PHB）组。AF PHB组在四个独立转发的AF类中提供IP分组的传送。在每个AF类中，可以为IP数据包分配三个不同的丢弃优先级级别中的一个。如果属于同一AF类，DS节点不会对同一微流的IP数据包重新排序。
There is a demand to provide assured forwarding of IP packets over the Internet. In a typical application, a company uses the Internet to interconnect its geographically distributed sites and wants an assurance that IP packets within this intranet are forwarded with high probability as long as the aggregate traffic from each site does not exceed the subscribed information rate (profile). It is desirable that a site may exceed the subscribed profile with the understanding that the excess traffic is not delivered with as high probability as the traffic that is within the profile. It is also
有一种需求是在互联网上提供有保证的IP数据包转发。在一个典型的应用中，一家公司使用互联网互连其地理分布的站点，并希望保证只要每个站点的总流量不超过订阅的信息速率（profile），该intranet内的IP数据包就有高概率转发。一个站点可能会超过订阅的配置文件，这是可取的，因为需要理解的是，多余的流量不会以与配置文件内的流量一样高的概率交付。也是
important that the network does not reorder packets that belong to the same microflow, as defined in [Nichols], no matter if they are in or out of the profile.
重要的是，网络不会对属于[Nichols]中定义的同一微流的数据包重新排序，无论它们在配置文件中还是在配置文件之外。
Assured Forwarding (AF) PHB group is a means for a provider DS domain to offer different levels of forwarding assurances for IP packets received from a customer DS domain. Four AF classes are defined, where each AF class is in each DS node allocated a certain amount of forwarding resources (buffer space and bandwidth). IP packets that wish to use the services provided by the AF PHB group are assigned by the customer or the provider DS domain into one or more of these AF classes according to the services that the customer has subscribed to. Further background about this capability and some ways to use it may be found in [Clark].
保证转发（AF）PHB组是提供商DS域为从客户DS域接收的IP分组提供不同级别的转发保证的一种手段。定义了四个AF类，其中每个AF类在每个DS节点中分配了一定数量的转发资源（缓冲区空间和带宽）。希望使用AF PHB组提供的服务的IP分组由客户或提供商DS域根据客户已订阅的服务分配到这些AF类中的一个或多个中。有关此功能的更多背景信息和使用方法，请参见[Clark]。
Within each AF class IP packets are marked (again by the customer or the provider DS domain) with one of three possible drop precedence values. In case of congestion, the drop precedence of a packet determines the relative importance of the packet within the AF class. A congested DS node tries to protect packets with a lower drop precedence value from being lost by preferably discarding packets with a higher drop precedence value.
在每个AF类中，IP数据包（同样由客户或提供商DS域）用三个可能的丢弃优先级值之一进行标记。在拥塞的情况下，数据包的丢弃优先级决定了数据包在AF类中的相对重要性。拥塞的DS节点试图通过优选地丢弃具有较高丢弃优先级值的数据包来保护具有较低丢弃优先级值的数据包不丢失。
In a DS node, the level of forwarding assurance of an IP packet thus depends on (1) how much forwarding resources has been allocated to the AF class that the packet belongs to, (2) what is the current load of the AF class, and, in case of congestion within the class, (3) what is the drop precedence of the packet.
在DS节点中，IP分组的转发保证的级别因此取决于（1）该分组所属的AF类分配了多少转发资源，（2）AF类的当前负载是什么，并且，在该类内发生拥塞的情况下，（3）分组的丢弃优先级是什么。
For example, if traffic conditioning actions at the ingress of the provider DS domain make sure that an AF class in the DS nodes is only moderately loaded by packets with the lowest drop precedence value and is not overloaded by packets with the two lowest drop precedence values, then the AF class can offer a high level of forwarding assurance for packets that are within the subscribed profile (i.e., marked with the lowest drop precedence value) and offer up to two lower levels of forwarding assurance for the excess traffic.
例如，如果在提供者DS域的入口处的流量调节动作确保DS节点中的AF类仅适度地由具有最低丢弃优先级值的分组加载，并且不由具有两个最低丢弃优先级值的分组过载，然后，AF类可以为订阅的简档内的分组提供高水平的转发保证（即，用最低的丢弃优先级值标记），并为多余的业务提供高达两个较低水平的转发保证。
This document describes the AF PHB group. An otherwise DS-compliant node is not required to implement this PHB group in order to be considered DS-compliant, but when a DS-compliant node is said to implement an AF PHB group, it must conform to the specification in this document.
本文档描述了AF PHB组。若要被视为DS兼容，则不需要DS兼容节点来实现该PHB组，但当DS兼容节点被称为实现AF PHB组时，它必须符合本文档中的规范。
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 [Bradner].
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[Bradner]中所述进行解释。
Assured Forwarding (AF) PHB group provides forwarding of IP packets in N independent AF classes. Within each AF class, an IP packet is assigned one of M different levels of drop precedence. An IP packet that belongs to an AF class i and has drop precedence j is marked with the AF codepoint AFij, where 1 <= i <= N and 1 <= j <= M. Currently, four classes (N=4) with three levels of drop precedence in each class (M=3) are defined for general use. More AF classes or levels of drop precedence MAY be defined for local use.
保证转发（AF）PHB组在N个独立的AF类中提供IP数据包的转发。在每个AF类中，IP分组被分配M个不同的丢弃优先级中的一个。属于AF类i且具有丢弃优先级j的IP分组用AF码点AFij标记，其中1<=i<=N和1<=j<=M。目前，定义了四个类（N=4），每个类（M=3）具有三个丢弃优先级。可以为本地使用定义更多的AF类或丢弃优先级级别。
A DS node SHOULD implement all four general use AF classes. Packets in one AF class MUST be forwarded independently from packets in another AF class, i.e., a DS node MUST NOT aggregate two or more AF classes together.
DS节点应该实现所有四个通用AF类。一个AF类中的数据包必须独立于另一个AF类中的数据包进行转发，即DS节点不得将两个或多个AF类聚合在一起。
A DS node MUST allocate a configurable, minimum amount of forwarding resources (buffer space and bandwidth) to each implemented AF class. Each class SHOULD be serviced in a manner to achieve the configured service rate (bandwidth) over both small and large time scales.
DS节点必须为每个实现的AF类分配可配置的最小转发资源量（缓冲区空间和带宽）。每个类的服务方式应能在小时间尺度和大时间尺度上实现配置的服务速率（带宽）。
An AF class MAY also be configurable to receive more forwarding resources than the minimum when excess resources are available either from other AF classes or from other PHB groups. This memo does not specify how the excess resources should be allocated, but implementations MUST specify what algorithms are actually supported and how they can be parameterized.
当从其他AF类或从其他PHB组获得多余资源时，AF类还可配置为接收比最小值更多的转发资源。此备忘录没有指定应如何分配多余的资源，但实现必须指定实际支持哪些算法以及如何对其进行参数化。
Within an AF class, a DS node MUST NOT forward an IP packet with smaller probability if it contains a drop precedence value p than if it contains a drop precedence value q when p < q. Note that this requirement can be fulfilled without needing to dequeue and discard already-queued packets.
在AF类中，DS节点如果包含丢弃优先级值p，则转发IP分组的概率不得小于p<q时包含丢弃优先级值q的概率。请注意，可以满足此要求，而无需退出队列并丢弃已排队的数据包。
Within each AF class, a DS node MUST accept all three drop precedence codepoints and they MUST yield at least two different levels of loss probability. In some networks, particularly in enterprise networks, where transient congestion is a rare and brief occurrence, it may be reasonable for a DS node to implement only two different levels of loss probability per AF class. While this may suffice for some networks, three different levels of loss probability SHOULD be supported in DS domains where congestion is a common occurrence.
在每个AF类中，DS节点必须接受所有三个丢弃优先码点，并且它们必须产生至少两个不同级别的丢失概率。在一些网络中，特别是在企业网络中，短暂的暂时拥塞很少发生，DS节点在每个AF类中仅实现两个不同级别的丢失概率可能是合理的。虽然这对于某些网络来说可能已经足够了，但在经常发生拥塞的DS域中，应该支持三种不同级别的丢失概率。
If a DS node only implements two different levels of loss probability for an AF class x, the codepoint AFx1 MUST yield the lower loss probability and the codepoints AFx2 and AFx3 MUST yield the higher loss probability.
如果DS节点对于AF类x仅实现两个不同级别的丢失概率，则码点AFx1必须产生较低的丢失概率，而码点AFx2和AFx3必须产生较高的丢失概率。
A DS node MUST NOT reorder AF packets of the same microflow when they belong to the same AF class regardless of their drop precedence. There are no quantifiable timing requirements (delay or delay variation) associated with the forwarding of AF packets.
当属于同一AF类时，DS节点不得对同一微流的AF数据包重新排序，无论其丢弃优先级如何。不存在与AF分组的转发相关联的可量化的定时要求（延迟或延迟变化）。
The relationship between AF classes and other PHBs is described in Section 7 of this memo.
本备忘录第7节描述了AF类和其他PHB之间的关系。
The AF PHB group MAY be used to implement both end-to-end and domain edge-to-domain edge services.
AF PHB组可用于实现端到端和域边缘到域边缘服务。
A DS domain MAY at the edge of a domain control the amount of AF traffic that enters or exits the domain at various levels of drop precedence. Such traffic conditioning actions MAY include traffic shaping, discarding of packets, increasing or decreasing the drop precedence of packets, and reassigning of packets to other AF classes. However, the traffic conditioning actions MUST NOT cause reordering of packets of the same microflow.
DS域可以在域的边缘以不同的丢弃优先级控制进入或退出域的AF通信量。此类流量调节动作可包括流量整形、丢弃分组、增加或减少分组的丢弃优先级以及将分组重新分配给其他AF类。但是，流量调节操作不得导致相同微流的数据包重新排序。
This section defines the queueing and discard behavior of the AF PHB group. Other aspects of the PHB group's behavior are defined in Section 2.
本节定义AF PHB组的排队和丢弃行为。PHB集团行为的其他方面见第2节。
An AF implementation MUST attempt to minimize long-term congestion within each class, while allowing short-term congestion resulting from bursts. This requires an active queue management algorithm. An example of such an algorithm is Random Early Drop (RED) [Floyd]. This memo does not specify the use of a particular algorithm, but does require that several properties hold.
AF实现必须尽量减少每个类中的长期拥塞，同时允许突发导致的短期拥塞。这需要一个主动队列管理算法。这种算法的一个例子是随机早期丢弃（RED）[Floyd]。此备忘录未指定特定算法的使用，但要求保留多个属性。
An AF implementation MUST detect and respond to long-term congestion within each class by dropping packets, while handling short-term congestion (packet bursts) by queueing packets. This implies the presence of a smoothing or filtering function that monitors the instantaneous congestion level and computes a smoothed congestion level. The dropping algorithm uses this smoothed congestion level to determine when packets should be discarded.
AF实现必须通过丢弃数据包来检测和响应每个类内的长期拥塞，同时通过对数据包排队来处理短期拥塞（数据包突发）。这意味着存在一个平滑或过滤功能，用于监控瞬时拥塞级别并计算平滑的拥塞级别。丢弃算法使用平滑的拥塞级别来确定何时丢弃数据包。
The dropping algorithm MUST be insensitive to the short-term traffic characteristics of the microflows using an AF class. That is, flows with different short-term burst shapes but identical longer-term packet rates should have packets discarded with essentially equal probability. One way to achieve this is to use randomness within the dropping function.
丢弃算法必须对使用AF类的微流的短期流量特性不敏感。也就是说，具有不同短期突发形状但相同长期分组速率的流应该以基本相同的概率丢弃分组。实现这一点的一种方法是在Droping函数中使用随机性。
The dropping algorithm MUST treat all packets within a single class and precedence level identically. This implies that for any given smoothed congestion level, the discard rate of a particular microflow's packets within a single precedence level will be proportional to that flow's percentage of the total amount of traffic passing through that precedence level.
丢弃算法必须相同地处理单个类和优先级中的所有数据包。这意味着，对于任何给定的平滑拥塞级别，单个优先级别内特定微流数据包的丢弃率将与该流占通过该优先级别的总流量的百分比成正比。
The congestion indication feedback to the end nodes, and thus the level of packet discard at each drop precedence in relation to congestion, MUST be gradual rather than abrupt, to allow the overall system to reach a stable operating point. One way to do this (RED) uses two (configurable) smoothed congestion level thresholds. When the smoothed congestion level is below the first threshold, no packets of the relevant precedence are discarded. When the smoothed congestion level is between the first and the second threshold, packets are discarded with linearly increasing probability, ranging from zero to a configurable value reached just prior to the second threshold. When the smoothed congestion level is above the second threshold, packets of the relevant precedence are discarded with 100% probability.
到终端节点的拥塞指示反馈，以及与拥塞相关的每个丢弃优先级的分组丢弃水平，必须是渐进的，而不是突然的，以允许整个系统达到稳定的工作点。一种方法是（红色）使用两个（可配置的）平滑拥塞级别阈值。当平滑的拥塞级别低于第一个阈值时，不会丢弃具有相关优先级的数据包。当平滑拥塞水平在第一阈值和第二阈值之间时，分组以线性增加的概率丢弃，其范围从零到刚好在第二阈值之前达到的可配置值。当平滑拥塞级别高于第二阈值时，相关优先级的数据包以100%的概率被丢弃。
To allow the AF PHB to be used in many different operating environments, the dropping algorithm control parameters MUST be independently configurable for each packet drop precedence and for each AF class.
为了允许AF PHB在许多不同的操作环境中使用，必须为每个分组丢弃优先级和每个AF类独立配置丢弃算法控制参数。
Within the limits above, this specification allows for a range of packet discard behaviors. Inconsistent discard behaviors lead to inconsistent end-to-end service semantics and limit the range of possible uses of the AF PHB in a multi-vendor environment. As experience is gained, future versions of this document may more tightly define specific aspects of the desirable behavior.
在上述限制范围内，此规范允许一系列数据包丢弃行为。不一致的丢弃行为导致端到端服务语义不一致，并限制了AF PHB在多供应商环境中的可能使用范围。随着经验的积累，本文档的未来版本可能会更严格地定义理想行为的特定方面。
When AF packets are tunneled, the PHB of the tunneling packet MUST NOT reduce the forwarding assurance of the tunneled AF packet nor cause reordering of AF packets belonging to the same microflow.
当AF分组被隧道化时，隧道化分组的PHB不得减少隧道化AF分组的转发保证，也不得导致属于相同微流的AF分组的重新排序。
Recommended codepoints for the four general use AF classes are given below. These codepoints do not overlap with any other general use PHB groups.
下面给出了四种通用AF类的推荐代码点。这些代码点不与任何其他通用PHB组重叠。
The RECOMMENDED values of the AF codepoints are as follows: AF11 = ' 001010', AF12 = '001100', AF13 = '001110', AF21 = '010010', AF22 = ' 010100', AF23 = '010110', AF31 = '011010', AF32 = '011100', AF33 = ' 011110', AF41 = '100010', AF42 = '100100', and AF43 = '100110'. The table below summarizes the recommended AF codepoint values.
AF代码点的建议值如下：AF11='001010'，AF12='001100'，AF13='001110'，AF21='010010'，AF22='010100'，AF23='010110'，AF31='011010'，AF32='011100'，AF33='011110'，AF41='100010'，AF42='100100'，AF43='100110'。下表总结了推荐的AF代码点值。
Class 1 Class 2 Class 3 Class 4
+----------+----------+----------+----------+
Low Drop Prec | 001010 | 010010 | 011010 | 100010 |
Medium Drop Prec | 001100 | 010100 | 011100 | 100100 |
High Drop Prec | 001110 | 010110 | 011110 | 100110 |
+----------+----------+----------+----------+
Class 1 Class 2 Class 3 Class 4
+----------+----------+----------+----------+
Low Drop Prec | 001010 | 010010 | 011010 | 100010 |
Medium Drop Prec | 001100 | 010100 | 011100 | 100100 |
High Drop Prec | 001110 | 010110 | 011110 | 100110 |
+----------+----------+----------+----------+
The AF codepoint mappings recommended above do not interfere with the local use spaces nor the Class Selector codepoints recommended in [Nichols]. The PHBs selected by those Class Selector codepoints may thus coexist with the AF PHB group and retain the forwarding behavior and relationships that was defined for them. In particular, the Default PHB codepoint of '000000' may remain to be used for conventional best effort traffic. Similarly, the codepoints '11x000' may remain to be used for network control traffic.
上面推荐的AF码点映射不会干扰本地使用空间，也不会干扰[Nichols]中推荐的类选择器码点。因此，由这些类选择器代码点选择的PHB可以与AF PHB组共存，并保留为它们定义的转发行为和关系。特别是，默认PHB代码点“000000”可能仍然用于传统的尽力而为流量。类似地，代码点“11x000”可继续用于网络控制流量。
The AF PHB group, in conjunction with edge traffic conditioning actions that limit the amount of traffic in each AF class to a (generally different) percentage of the class's allocated resources, can be used to obtain the overall behavior implied by the Class Selector PHBs. In this case it may be appropriate within a DS domain to use some or all of the Class Selector codepoints as aliases of AF codepoints.
AF PHB组与边缘流量调节动作结合使用，这些动作将每个AF类中的流量限制为该类已分配资源的（通常不同）百分比，可用于获得类选择器PHB暗示的总体行为。在这种情况下，在DS域中使用一些或所有类选择器码点作为AF码点的别名可能是合适的。
In addition to the Class Selector PHBs, any other PHB groups may co-exist with the AF PHB group within the same DS domain. However, any AF PHB group implementation should document the following:
除了类选择器PHB之外，任何其他PHB组都可以与同一DS域中的AF PHB组共存。但是，任何AF PHB集团实施应记录以下内容：
(a) Which, if any, other PHB groups may preempt the forwarding resources specifically allocated to each AF PHB class. This preemption MUST NOT happen in normal network operation, but may be appropriate in certain unusual situations - for example, the '11x000' codepoint may preempt AF forwarding resources, to give precedence to unexpectedly high levels of network control traffic when required.
(a) 如果有的话，其他PHB组可以抢占专门分配给每个AF PHB类的转发资源。此抢占不得发生在正常网络操作中，但可能适用于某些异常情况-例如，“11x000”代码点可能抢占AF转发资源，以便在需要时优先考虑意外高水平的网络控制流量。
(b) How "excess" resources are allocated between the AF PHB group and other implemented PHB groups. For example, once the minimum allocations are given to each AF class, any remaining resources could be allocated evenly between the AF classes and the Default PHB. In an alternative example, any remaining resources could be allocated to forwarding excess AF traffic, with resources devoted to the Default PHB only when all AF demand is met.
(b) 如何在AF PHB组和其他实施的PHB组之间分配“多余”资源。例如，一旦给每个AF类分配了最小分配，任何剩余资源都可以在AF类和默认PHB之间平均分配。在另一个示例中，任何剩余资源都可以分配给转发多余的AF流量，只有在满足所有AF需求时，才将资源专用于默认PHB。
This memo does not specify that any particular relationship hold between AF PHB groups and other implemented PHB groups; it requires only that whatever relationship is chosen be documented. Implementations MAY allow either or both of these relationships to be configurable. It is expected that this level of configuration flexibility will prove valuable to many network administrators.
本备忘录未规定AF PHB组与其他已实施PHB组之间存在任何特定关系；它只要求记录所选择的任何关系。实现可能允许配置其中一个或两个关系。预计这一级别的配置灵活性将对许多网络管理员具有价值。
In order to protect itself against denial of service attacks, a provider DS domain SHOULD limit the traffic entering the domain to the subscribed profiles. Also, in order to protect a link to a customer DS domain from denial of service attacks, the provider DS domain SHOULD allow the customer DS domain to specify how the resources of the link are allocated to AF packets. If a service offering requires that traffic marked with an AF codepoint be limited by such attributes as source or destination address, it is the responsibility of the ingress node in a network to verify validity of such attributes.
为了保护自身免受拒绝服务攻击，提供商DS域应将进入域的流量限制为订阅的配置文件。此外，为了保护到客户DS域的链路免受拒绝服务攻击，提供商DS域应允许客户DS域指定如何将链路的资源分配给AF数据包。如果服务提供要求用AF码点标记的通信量受到诸如源地址或目的地地址之类的属性的限制，则网络中的入口节点负责验证此类属性的有效性。
Other security considerations are covered in [Blake] and [Nichols].
[Blake]和[Nichols]中介绍了其他安全注意事项。
The IETF has been notified of intellectual property rights claimed in regard to some or all of the specification contained in this document. For more information, consult the online list of claimed rights.
IETF已收到关于本文件所含部分或全部规范的知识产权声明。有关更多信息，请查阅在线权利主张列表。
This document allocates twelve codepoints, listed in section 6, in Pool 1 of the code space defined by [Nichols].
本文档在[Nichols]定义的代码空间的池1中分配了第6节中列出的12个代码点。
Appendix: Example Services
附录：示例服务
The AF PHB group could be used to implement, for example, the so-called Olympic service, which consists of three service classes: bronze, silver, and gold. Packets are assigned to these three classes so that packets in the gold class experience lighter load (and thus have greater probability for timely forwarding) than packets assigned to the silver class. Same kind of relationship exists between the silver class and the bronze class. If desired, packets within each class may be further separated by giving them either low, medium, or high drop precedence.
例如，AF PHB集团可用于实施所谓的奥运会服务，该服务由三个服务类别组成：铜牌、银牌和金牌。数据包被分配给这三个类别，因此黄金类别的数据包比白银类别的数据包负载更轻（因此具有更大的及时转发概率）。银级和青铜级之间存在着同样的关系。如果需要，每个类中的数据包可以通过提供低、中或高丢弃优先级来进一步分离。
The bronze, silver, and gold service classes could in the network be mapped to the AF classes 1, 2, and 3. Similarly, low, medium, and high drop precedence may be mapped to AF drop precedence levels 1, 2, or 3.
网络中的青铜、白银和黄金服务等级可以映射到AF等级1、2和3。类似地，低、中、高丢弃优先级可以映射到AF丢弃优先级级别1、2或3。
The drop precedence level of a packet could be assigned, for example, by using a leaky bucket traffic policer, which has as its parameters a rate and a size, which is the sum of two burst values: a committed burst size and an excess burst size. A packet is assigned low drop precedence if the number of tokens in the bucket is greater than the excess burst size, medium drop precedence if the number of tokens in the bucket is greater than zero, but at most the excess burst size, and high drop precedence if the bucket is empty. It may also be necessary to set an upper limit to the amount of high drop precedence traffic from a customer DS domain in order to avoid the situation where an avalanche of undeliverable high drop precedence packets from one customer DS domain can deny service to possibly deliverable high drop precedence packets from other domains.
例如，可以通过使用泄漏桶流量策略来分配分组的丢弃优先级，泄漏桶流量策略的参数是速率和大小，大小是两个突发值的总和：提交的突发大小和多余的突发大小。如果bucket中的令牌数大于多余的突发大小，则为数据包分配低丢弃优先级；如果bucket中的令牌数大于零，则为中等丢弃优先级，但最多为多余的突发大小；如果bucket为空，则为数据包分配高丢弃优先级。还可能需要设置来自客户DS域的高丢弃优先级通信量的上限，以避免来自一个客户DS域的不可递送的高丢弃优先级分组的雪崩可以拒绝来自其他域的可能可递送的高丢弃优先级分组的服务的情况。
Another way to assign the drop precedence level of a packet could be to limit the user traffic of an Olympic service class to a given peak rate and distribute it evenly across each level of drop precedence. This would yield a proportional bandwidth service, which equally apportions available capacity during times of congestion under the assumption that customers with high bandwidth microflows have subscribed to higher peak rates than customers with low bandwidth microflows.
分配数据包丢弃优先级的另一种方法是将奥林匹克服务类的用户流量限制在给定的峰值速率，并将其均匀分布在丢弃优先级的每个级别上。这将产生一种成比例的带宽服务，在假设具有高带宽微流量的客户订阅的峰值速率高于具有低带宽微流量的客户的情况下，该服务在拥塞期间平均分配可用容量。
The AF PHB group could also be used to implement a loss and low latency service using an over provisioned AF class, if the maximum arrival rate to that class is known a priori in each DS node. Specification of the required admission control services, however, is beyond the scope of this document. If low loss is not an objective, a low latency service could be implemented without over provisioning by setting a low maximum limit to the buffer space available for an AF class.
如果在每个DS节点中先验地知道到该类的最大到达速率，则AF PHB组还可用于使用过度配置的AF类来实现丢失和低延迟服务。然而，所需准入控制服务的规范超出了本文件的范围。如果低损耗不是目标，则可以通过为AF类的可用缓冲区空间设置较低的最大限制来实现低延迟服务，而无需过度配置。
References
工具书类
[Blake] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z. and W. Weiss, "An Architecture for Differentiated Services", RFC 2475, December 1998.
[Blake]Blake，S.，Black，D.，Carlson，M.，Davies，E.，Wang，Z.和W.Weiss，“差异化服务架构”，RFC 24751998年12月。
[Bradner] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[Bradner]Bradner，S.，“RFC中用于表示需求水平的关键词”，BCP 14，RFC 2119，1997年3月。
[Clark] Clark, D. and Fang, W., Explicit Allocation of Best Effort Packet Delivery Service. IEEE/ACM Transactions on Networking, Volume 6, Number 4, August 1998, pp. 362-373.
[Clark]Clark，D.和Fang，W.，尽力而为数据包交付服务的显式分配。IEEE/ACM网络交易，第6卷，第4期，1998年8月，第362-373页。
[Floyd] Floyd, S., and Jacobson, V., Random Early Detection gateways for Congestion Avoidance. IEEE/ACM Transactions on Networking, Volume 1, Number 4, August 1993, pp. 397-413.
[Floyd]Floyd，S.，和Jacobson，V.，用于避免拥塞的随机早期检测网关。IEEE/ACM网络交易，第1卷，第4期，1993年8月，第397-413页。
[Nichols] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998.
[Nichols]Nichols，K.，Blake，S.，Baker，F.和D.Black，“IPv4和IPv6报头中区分服务字段（DS字段）的定义”，RFC 2474，1998年12月。
Authors' Addresses
作者地址
Juha Heinanen Telia Finland Myyrmaentie 2 01600 Vantaa, Finland
朱哈·海纳南·泰利亚芬兰迈尔马恩蒂2 01600芬兰万塔
EMail: jh@telia.fi
EMail: jh@telia.fi
Fred Baker Cisco Systems 519 Lado Drive Santa Barbara, California 93111
Fred Baker Cisco Systems 519 Lado Drive Santa Barbara，加利福尼亚93111
EMail: fred@cisco.com
EMail: fred@cisco.com
Walter Weiss Lucent Technologies 300 Baker Avenue, Suite 100, Concord, MA 01742-2168
Walter Weiss-Lucent Technologies马萨诸塞州康科德贝克大道300号100室01742-2168
EMail: wweiss@lucent.com
EMail: wweiss@lucent.com
John Wroclawski MIT Laboratory for Computer Science 545 Technology Sq. Cambridge, MA 02139
约翰·沃克罗夫斯基麻省理工学院计算机科学实验室，马萨诸塞州剑桥545技术广场，邮编：02139
EMail: jtw@lcs.mit.edu
EMail: jtw@lcs.mit.edu
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Acknowledgement
确认
Funding for the RFC Editor function is currently provided by the Internet Society.
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