Network Working Group P. Eardley, Ed.
Request for Comments: 5670 BT
Category: Standards Track November 2009
Network Working Group P. Eardley, Ed.
Request for Comments: 5670 BT
Category: Standards Track November 2009
Metering and Marking Behaviour of PCN-Nodes
PCN节点的计量和标记行为
Abstract
摘要
The objective of Pre-Congestion Notification (PCN) is to protect the quality of service (QoS) of inelastic flows within a Diffserv domain in a simple, scalable, and robust fashion. This document defines the two metering and marking behaviours of PCN-nodes. Threshold-metering and -marking marks all PCN-packets if the rate of PCN-traffic is greater than a configured rate ("PCN-threshold-rate"). Excess-traffic-metering and -marking marks a proportion of PCN-packets, such that the amount marked equals the rate of PCN-traffic in excess of a configured rate ("PCN-excess-rate"). The level of marking allows PCN-boundary-nodes to make decisions about whether to admit or terminate PCN-flows.
预拥塞通知（PCN）的目标是以一种简单、可扩展和健壮的方式保护Diffserv域内非弹性流的服务质量（QoS）。本文件定义了PCN节点的两种计量和标记行为。阈值计量和标记-如果PCN流量的速率大于配置的速率（“PCN阈值速率”），则标记所有PCN数据包。超额流量计量和标记标记PCN数据包的一部分，以便标记的数量等于超过配置速率的PCN流量速率（“PCN超额速率”）。标记级别允许PCN边界节点决定是否接纳或终止PCN流。
Status of This Memo
关于下段备忘
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) 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。
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Table of Contents
目录
1. Introduction ....................................................2
1.1. Terminology ................................................4
1.1.1. Requirements Language ...............................5
2. Specified PCN-Metering and -Marking Behaviours ..................5
2.1. Behaviour Aggregate Classification Function ................5
2.2. Dropping Function ..........................................5
2.3. Threshold-Meter Function ...................................6
2.4. Excess-Traffic-Meter Function ..............................6
2.5. Marking Function ...........................................7
3. Security Considerations .........................................7
4. Acknowledgements ................................................8
5. References ......................................................8
5.1. Normative Reference ........................................8
5.2. Informative References .....................................8
Appendix A. Example Algorithms ...................................11
A.1. Threshold-Metering and -Marking ...........................11
A.2. Excess-Traffic-Metering and -Marking ......................12
Appendix B. Implementation Notes .................................13
B.1. Competing-Non-PCN-Traffic .................................13
B.2. Scope .....................................................14
B.3. Behaviour Aggregate Classification ........................15
B.4. Dropping ..................................................15
B.5. Threshold-Metering ........................................17
B.6. Excess-Traffic-Metering ...................................18
B.7. Marking ...................................................19
1. Introduction ....................................................2
1.1. Terminology ................................................4
1.1.1. Requirements Language ...............................5
2. Specified PCN-Metering and -Marking Behaviours ..................5
2.1. Behaviour Aggregate Classification Function ................5
2.2. Dropping Function ..........................................5
2.3. Threshold-Meter Function ...................................6
2.4. Excess-Traffic-Meter Function ..............................6
2.5. Marking Function ...........................................7
3. Security Considerations .........................................7
4. Acknowledgements ................................................8
5. References ......................................................8
5.1. Normative Reference ........................................8
5.2. Informative References .....................................8
Appendix A. Example Algorithms ...................................11
A.1. Threshold-Metering and -Marking ...........................11
A.2. Excess-Traffic-Metering and -Marking ......................12
Appendix B. Implementation Notes .................................13
B.1. Competing-Non-PCN-Traffic .................................13
B.2. Scope .....................................................14
B.3. Behaviour Aggregate Classification ........................15
B.4. Dropping ..................................................15
B.5. Threshold-Metering ........................................17
B.6. Excess-Traffic-Metering ...................................18
B.7. Marking ...................................................19
The objective of Pre-Congestion Notification (PCN) is to protect the quality of service (QoS) of inelastic flows within a Diffserv domain in a simple, scalable, and robust fashion. Two mechanisms are used: admission control to decide whether to admit or block a new flow request, and (in abnormal circumstances) flow termination to decide whether to terminate some of the existing flows. To achieve this, the overall rate of PCN-traffic is metered on every link in the domain, and PCN-packets are appropriately marked when certain configured rates are exceeded. These configured rates are below the rate of the link, thus providing notification to boundary nodes about
预拥塞通知（PCN）的目标是以一种简单、可扩展和健壮的方式保护Diffserv域内非弹性流的服务质量（QoS）。使用两种机制：准入控制来决定是否接纳或阻止新的流请求，以及（在异常情况下）流终止来决定是否终止某些现有流。为了实现这一点，在域中的每个链路上测量PCN流量的总速率，并且当超过某些配置速率时，适当地标记PCN数据包。这些配置的速率低于链路的速率，因此向边界节点提供关于链路的通知
overloads before any congestion occurs (hence "Pre-Congestion Notification"). The level of marking allows boundary nodes to make decisions about whether to admit or terminate. Within the domain, PCN-traffic is forwarded in a prioritised Diffserv traffic class [RFC2475].
在发生任何拥塞之前过载（因此称为“拥塞前通知”）。标记级别允许边界节点决定是允许还是终止。在域内，PCN流量以优先区分服务流量类别[RFC2475]转发。
This document defines the two metering and marking behaviours of PCN-nodes. Their aim is to enable PCN-nodes to give an "early warning" of potential congestion before there is any significant build-up of PCN-packets in their queues. In summary, their objectives are:
本文件定义了PCN节点的两种计量和标记行为。他们的目标是使PCN节点能够在其队列中出现任何明显的PCN数据包累积之前，对潜在的拥塞发出“早期警告”。总之，他们的目标是：
o Threshold-metering and -marking: to mark all PCN-packets (with a "threshold-mark") when the bit rate of PCN-traffic is greater than its configured reference rate ("PCN-threshold-rate").
o 阈值计量和标记：当PCN流量的比特率大于其配置的参考速率（“PCN阈值速率”）时，标记所有PCN数据包（带有“阈值标记”）。
o Excess-traffic-metering and -marking: when the bit rate of PCN-packets is greater than its configured reference rate ("PCN-excess-rate"), to mark PCN-packets (with an "excess-traffic-mark") at a rate equal to the difference between the rate of PCN-traffic and the PCN-excess-rate.
o 超额流量计量和标记：当PCN数据包的比特率大于其配置的参考速率（“PCN超额速率”）时，以等于PCN流量速率和PCN超额速率之差的速率标记PCN数据包（带有“超额流量标记”）。
Note that although [RFC3168] defines a broadly RED-like (Random Early Detection) default congestion marking behaviour, it allows alternatives to be defined; this document defines such an alternative.
注意，尽管[RFC3168]定义了广泛的红色（随机早期检测）默认拥塞标记行为，但它允许定义备选方案；本文件定义了此类替代方案。
Section 2 below describes the functions involved, which in outline (see Figure 1) are:
下面第2节描述了所涉及的功能，其概要（见图1）为：
o Behaviour aggregate (BA) classification: decide whether or not an incoming packet is a PCN-packet.
o 行为聚合（BA）分类：确定传入数据包是否为PCN数据包。
o Dropping (optional): drop packets if the link is overloaded.
o 丢弃（可选）：如果链路过载，则丢弃数据包。
o Threshold-meter: determine whether the bit rate of PCN-traffic exceeds its configured reference rate (PCN-threshold-rate). The meter operates on all PCN-packets on the link, and not on individual flows.
o 阈值表：确定PCN流量的比特率是否超过其配置的参考速率（PCN阈值速率）。仪表在链路上的所有PCN数据包上运行，而不是在单个流上运行。
o Excess-traffic-meter: measure by how much the bit rate of PCN-traffic exceeds its configured reference rate (PCN-excess-rate). The meter operates on all PCN-packets on the link, and not on individual flows.
o 超额流量表：通过PCN流量的比特率超过其配置的参考速率（PCN超额速率）的多少来测量。仪表在链路上的所有PCN数据包上运行，而不是在单个流上运行。
o PCN-mark: actually mark the PCN-packets, if the meter functions indicate to do so.
o PCN标记：如果仪表功能指示，则实际标记PCN数据包。
+---------+ Result
+->|Threshold|-------+
| | Meter | |
| +---------+ V
+----------+ +- - - - -+ | +------+
| BA | | | | | | Marked
Packet =>|Classifier|==>| Dropper |==?===============>|Marker|==> Packet
Stream | | | | | | | Stream
+----------+ +- - - - -+ | +------+
| +---------+ ^
| | Excess | |
+->| Traffic |-------+
| Meter | Result
+---------+
+---------+ Result
+->|Threshold|-------+
| | Meter | |
| +---------+ V
+----------+ +- - - - -+ | +------+
| BA | | | | | | Marked
Packet =>|Classifier|==>| Dropper |==?===============>|Marker|==> Packet
Stream | | | | | | | Stream
+----------+ +- - - - -+ | +------+
| +---------+ ^
| | Excess | |
+->| Traffic |-------+
| Meter | Result
+---------+
Figure 1: Schematic of PCN-interior-node functionality
图1:PCN内部节点功能示意图
Appendix A gives an example of algorithms that fulfil the specification of Section 2, and Appendix B provides some explanations of and comments on Section 2. Both the Appendices are informative.
附录A给出了满足第2节规范的算法示例，附录B提供了第2节的一些解释和评论。两个附录都提供了信息。
The general architecture for PCN is described in [RFC5559], whilst [Menth10] is an overview of PCN.
[RFC5559]中描述了PCN的一般架构，而[Menth10]是PCN的概述。
In addition to the terminology defined in [RFC5559] and [RFC2474], the following terms are defined:
除[RFC5559]和[RFC2474]中定义的术语外，还定义了以下术语：
o Competing-non-PCN-packet: a non-PCN-packet that shares a link with PCN-packets and competes with them for its forwarding bandwidth. Competing-non-PCN-packets MUST NOT be PCN-marked (only PCN-packets can be PCN-marked).
o 竞争性非PCN数据包：与PCN数据包共享链路并与之竞争转发带宽的非PCN数据包。竞争性非PCN数据包不得带有PCN标记（只有PCN数据包可以带有PCN标记）。
Note: In general, it is not advised to have any competing-non-PCN-traffic.
注：一般来说，不建议有任何竞争性的非PCN流量。
Note: There is likely to be traffic (such as best effort) that is forwarded at lower priority than PCN-traffic; although it shares the link with PCN-traffic, it doesn't compete for forwarding bandwidth, and hence it is not competing-non-PCN-traffic. See Appendix B.1 for further discussion about competing-non-PCN-traffic.
注意：可能存在以低于PCN流量的优先级转发的流量（如尽力而为）；虽然它与PCN流量共享链路，但它不竞争转发带宽，因此它不竞争非PCN流量。有关竞争性非PCN流量的进一步讨论，请参见附录B.1。
o Metered-packet: a packet that is metered by the metering functions specified in Sections 2.3 and 2.4. A PCN-packet MUST be treated as a metered-packet (with the minor exception noted below in Section 2.4). A competing-non-PCN-packet MAY be treated as a metered-packet.
o 计量数据包：由第2.3节和第2.4节规定的计量功能计量的数据包。PCN数据包必须视为计量数据包（以下第2.4节所述的次要例外情况除外）。竞争的非PCN分组可被视为经计量的分组。
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。
This section defines the two PCN-metering and -marking behaviours. The descriptions are functional and are not intended to restrict the implementation. The informative Appendices supplement this section.
本节定义了两种PCN计量和标记行为。这些描述是功能性的，并不是为了限制实现。资料性附录补充了本节内容。
A PCN-node MUST classify a packet as a PCN-packet if the value of its Differentiated Services Code Point (DSCP) and Explicit Congestion Notification (ECN) fields correspond to a PCN-enabled codepoint, as defined in the encoding scheme applicable to the PCN-domain (for example, [RFC5696] defines the baseline encoding). Otherwise, the packet MUST NOT be classified as a PCN-packet.
如果PCN节点的区分服务代码点（DSCP）和显式拥塞通知（ECN）字段的值对应于PCN启用的代码点，则PCN节点必须将数据包分类为PCN数据包，如适用于PCN域的编码方案中所定义（例如，[RFC5696]定义基线编码）。否则，数据包不得被分类为PCN数据包。
A PCN-node MUST classify a packet as a competing-non-PCN-packet if it is not a PCN-packet and it competes with PCN-packets for its forwarding bandwidth on a link.
如果数据包不是PCN数据包，并且它在链路上与PCN数据包竞争转发带宽，则PCN节点必须将数据包分类为竞争性非PCN数据包。
Note: If the PCN-node's queue overflows, then naturally packets are dropped. This section describes additional action.
注意：如果PCN节点的队列溢出，则自然丢弃数据包。本节介绍其他操作。
On all links in the PCN-domain, dropping MAY be done by first metering all metered-packets to determine if the rate of metered-traffic on the link is greater than the rate allowed for such traffic; if the rate of metered-traffic is too high, then drop metered-packets.
在PCN域中的所有链路上，可以通过首先计量所有计量分组来完成丢弃，以确定链路上的计量流量的速率是否大于此类流量允许的速率；如果按流量计费的速率过高，则丢弃按流量计费的数据包。
If the PCN-node drops PCN-packets, then:
如果PCN节点丢弃PCN数据包，则：
o PCN-packets that arrive at the PCN-node already excess-traffic-marked SHOULD be preferentially dropped.
o 到达PCN节点的PCN数据包已标记过多流量，应优先丢弃。
o the PCN-node's excess-traffic-meter SHOULD NOT meter the PCN-packets that it drops.
o PCN节点的过量流量表不应测量其丢弃的PCN数据包。
A PCN-node MUST implement a threshold-meter that has behaviour functionally equivalent to the following.
PCN节点必须实现一个阈值测量仪，其行为在功能上等同于以下各项。
The meter acts like a token bucket, which is sized in bits and has a configured reference rate (bits per second). The amount of tokens in the token bucket is termed F_tm. Tokens are added at the reference rate (PCN-threshold-rate), to a maximum value BS_tm. Tokens are removed equal to the size in bits of the metered-packet, to a minimum F_tm = 0. (Explanation of abbreviations: F is short for Fill of the token bucket, BS for bucket size, and tm for threshold-meter.)
仪表的作用类似于令牌桶，其大小以位为单位，并具有配置的参考速率（位/秒）。令牌桶中的令牌量称为F_tm。令牌以参考速率（PCN阈值速率）添加到最大值BS_tm。令牌被移除，其大小等于计量数据包的比特数，最小F_tm=0。（缩写说明：F是令牌桶填充的缩写，BS是桶大小的缩写，tm是阈值表的缩写。）
The token bucket has a configured intermediate depth, termed threshold. If F_tm < threshold, then the meter indicates to the marking function that the packet is to be threshold-marked; otherwise, it does not.
令牌桶具有配置的中间深度，称为阈值。如果F_tm<阈值，则仪表向标记功能指示数据包要进行阈值标记；否则，它不会。
A packet SHOULD NOT be metered (by this excess-traffic-meter function) in the following two cases:
在以下两种情况下，不应（通过此过量流量表功能）对数据包进行计量：
o if the PCN-packet is already excess-traffic-marked on arrival at the PCN-node.
o 如果PCN数据包在到达PCN节点时已经标记了过量流量。
o if this PCN-node drops the packet.
o 如果此PCN节点丢弃数据包。
Otherwise, the PCN-packet MUST be treated as a metered-packet -- that is, it is metered by the excess-traffic-meter.
否则，PCN数据包必须被视为已计量的数据包——也就是说，它由超额流量表计量。
A PCN-node MUST implement an excess-traffic-meter. The excess-traffic-meter SHOULD indicate packets to be excess-traffic-marked, independent of their size ("packet size independent marking"); if "packet size independent marking" is not implemented, then the excess-traffic-meter MUST use the "classic" metering behaviour.
PCN节点必须实现超额流量表。超额流量表应指示标记超额流量的数据包，与数据包大小无关（“数据包大小独立标记”）；如果未实施“数据包大小独立标记”，则超额流量表必须使用“经典”计量行为。
For the "classic" metering behaviour, the excess-traffic-meter has behaviour functionally equivalent to the following.
对于“经典”计量行为，超额交通量计的行为在功能上等同于以下行为。
The meter acts like a token bucket, which is sized in bits and has a configured reference rate (bits per second). The amount of tokens in the token bucket is termed F_etm. Tokens are added at the reference rate (PCN-excess-rate), to a maximum value BS_etm. Tokens are removed equal to the size in bits of the metered-packet, to a minimum
仪表的作用类似于令牌桶，其大小以位为单位，并具有配置的参考速率（位/秒）。代币桶中代币的数量称为F_etm。代币按参考汇率（PCN超额汇率）添加至最大值BS_etm。令牌被移除的最小值等于计量数据包的比特大小
F_etm = 0. If the token bucket is empty (F_etm = 0), then the meter indicates to the marking function that the packet is to be excess-traffic-marked. (Explanation of abbreviations: F is short for Fill of the token bucket, BS for bucket size, and etm for excess-traffic-meter.)
F_etm=0。如果令牌桶为空（F_etm=0），则仪表向标记功能指示该数据包将被标记为多余流量。（缩写说明：F是令牌桶填充的缩写，BS是桶大小的缩写，etm是超额流量表的缩写。）
For "packet size independent marking", the excess-traffic-meter has behaviour functionally equivalent to the following.
对于“与数据包大小无关的标记”，超额流量表的功能等同于以下各项。
The meter acts like a token bucket, which is sized in bits and has a configured reference rate (bits per second). The amount of tokens in the token bucket is termed F_etm. Tokens are added at the reference rate (PCN-excess-rate), to a maximum value BS_etm. If the token bucket is not negative, then tokens are removed equal to the size in bits of the metered-packet (and the meter does not indicate to the marking function that the packet is to be excess-traffic-marked). If the token bucket is negative (F_etm < 0), then the meter indicates to the marking function that the packet is to be excess-traffic-marked (and no tokens are removed). (Explanation of abbreviations: F is short for Fill of the token bucket, BS for bucket size, and etm for excess-traffic-meter.)
仪表的作用类似于令牌桶，其大小以位为单位，并具有配置的参考速率（位/秒）。代币桶中代币的数量称为F_etm。代币按参考汇率（PCN超额汇率）添加至最大值BS_etm。如果令牌桶不是负的，则令牌被移除，其大小等于计量数据包的位大小（并且计量器不会向标记功能指示该数据包将被标记为过量流量）。如果令牌桶为负（F_etm<0），则仪表向标记功能指示该数据包将被标记为多余的通信量（并且不移除令牌）。（缩写说明：F是令牌桶填充的缩写，BS是桶大小的缩写，etm是超额流量表的缩写。）
Otherwise, the meter MUST NOT indicate marking.
否则，仪表不得显示标记。
A PCN-packet MUST be marked to reflect the metering results by setting its encoding state appropriately, as specified by the specific encoding scheme that applies in the PCN-domain. A consistent choice of encoding scheme MUST be made throughout a PCN-domain.
必须根据PCN域中应用的特定编码方案的规定，通过适当设置其编码状态来标记PCN数据包，以反映计量结果。必须在整个PCN域中选择一致的编码方案。
A PCN-node MUST NOT:
PCN节点不得：
o PCN-mark a packet that is not a PCN-packet;
o PCN标记不是PCN数据包的数据包；
o change a non-PCN-packet into a PCN-packet;
o 将非PCN分组更改为PCN分组；
o change a PCN-packet into a non-PCN-packet.
o 将PCN数据包更改为非PCN数据包。
Note: Although competing-non-PCN-packets MAY be metered, they MUST NOT be PCN-marked.
注意：尽管竞争的非PCN数据包可能会被计量，但它们不得带有PCN标记。
It is assumed that all PCN-nodes are PCN-enabled and are trusted for truthful PCN-metering and PCN-marking. If this isn't the case, then there are numerous potential attacks. For instance, a rogue PCN-
假设所有PCN节点都启用了PCN，并且可信，可以进行真实的PCN计量和PCN标记。如果不是这样的话，那么就有很多潜在的攻击。例如，流氓PCN-
interior-node could PCN-mark all packets so that no flows were admitted. Another possibility is that it doesn't PCN-mark any packets, even when it is pre-congested.
内部节点可以对所有数据包进行PCN标记，以便不允许任何流。另一种可能性是它不会标记任何数据包，即使是在预拥塞的情况下。
Note that PCN-interior-nodes are not flow-aware. This prevents some security attacks where an attacker targets specific flows in the data plane -- for instance, for Denial-of-Service (DoS) or eavesdropping.
请注意，PCN内部节点不支持流。这可以防止攻击者以数据平面中的特定流为目标的某些安全攻击，例如拒绝服务（DoS）或窃听。
As regards Security Operations and Management, PCN adds few specifics to the general good practice required in this field [RFC4778]. For example, it may be sensible for a PCN-node to raise an alarm if it is persistently PCN-marking.
关于安全运营和管理，PCN在该领域所需的一般良好实践中增加了一些细节[RFC4778]。例如，如果PCN节点持续进行PCN标记，则可能会发出警报。
Security considerations are further discussed in [RFC5559].
[RFC5559]中进一步讨论了安全注意事项。
This document is the result of extensive collaboration within the PCN WG. Amongst the most active other contributors to the development of the ideas specified in this document have been Jozef Babiarz, Bob Briscoe, Kwok-Ho Chan, Anna Charny, Georgios Karagiannis, Michael Menth, Toby Moncaster, Daisuke Satoh, and Joy Zhang. Appendix A is based on text from Michael Menth.
本文件是PCN工作组广泛合作的结果。本文件中规定的创意发展最活跃的其他贡献者包括Jozef Babiarz、Bob Briscoe、郭浩灿、Anna Charny、Georgios Karagiannis、Michael Meth、Toby Moncaster、Daisuke Satoh和Joy Zhang。附录A基于Michael Menth的文本。
This document is a development of [Briscoe06-2]. Its authors are therefore also contributors to this document: Jozef Babiarz, Attila Bader, Bob Briscoe, Kwok-Ho Chan, Anna Charny, Stephen Dudley, Philip Eardley, Georgios Karagiannis, Francois Le Faucheur, Vassilis Liatsos, Dave Songhurst, and Lars Westberg.
本文件是[Briscoe06-2]的发展。因此，其作者也是本文件的贡献者：Jozef Babiarz、Attila Bader、Bob Briscoe、郭浩灿、Anna Chary、Stephen Dudley、Philip Eardley、Georgios Karagiannis、Francois Le Faucheur、Vassilis Liatsos、Dave Songhurst和Lars Westberg。
Thanks to those who've made comments on the document: Joe Babiarz, Fred Baker, David Black, Bob Briscoe, Ken Carlberg, Anna Charny, Ralph Droms, Mehmet Ersue, Adrian Farrel, Ruediger Geib, Wei Gengyu, Fortune Huang, Christian Hublet, Ingemar Johansson, Georgios Karagiannis, Alexey Melnikov, Michael Menth, Toby Moncaster, Dimitri Papadimitriou, Tim Polk, Daisuke Satoh, and Magnus Westerlund.
感谢那些对这份文件发表评论的人：乔·巴比亚斯、弗雷德·贝克、大卫·布莱克、鲍勃·布里斯科、肯·卡尔伯格、安娜·查尼、拉尔夫·德罗姆斯、迈赫迈特·厄苏、阿德里安·法雷尔、鲁迪格·盖布、魏根玉、黄福琼、克里斯蒂安·哈勃、英格玛·约翰逊、乔治亚·卡拉吉安尼斯、亚历克西·梅尔尼科夫、迈克尔·门特、托比·蒙卡斯特、迪米特里·帕季米特里欧，蒂姆·波尔克、大辅·佐藤和马格纳斯·韦斯特隆德。
[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。
[Baker08] Baker, F., Polk, J., and M. Dolly, "DSCP for Capacity-Admitted Traffic", Work in Progress, November 2008.
[Baker08]Baker，F.，Polk，J.，和M.Dolly，“允许通行能力的DSCP”，正在进行的工作，2008年11月。
[Briscoe06-1] Briscoe, B., Eardley, P., Songhurst, D., Le Faucheur, F., Charny, A., Babiarz, J., Chan, K., Dudley, S., Karagiannis, G., Bader, A., and L. Westberg, "An edge-to-edge Deployment Model for Pre-Congestion Notification: Admission Control over a DiffServ Region", Work in Progress, October 2006.
[Briscoe06-1]Briscoe，B.，Eardley，P.，Songhurst，D.，Le Faucheur，F.，Chary，A.，Babiarz，J.，Chan，K.，Dudley，S.，Karagiannis，G.，Bader，A.，和L.Westberg，“拥塞前通知的边到边部署模型：区分服务区域的准入控制”，正在进行的工作，2006年10月。
[Briscoe06-2] Briscoe, B., Eardley, P., Songhurst, D., Le Faucheur, F., Charny, A., Liatsos, V., Babiarz, J., Chan, K., Dudley, S., Karagiannis, G., Bader, A., and L. Westberg, "Pre-Congestion Notification marking", Work in Progress, October 2006.
[Briscoe06-2]Briscoe，B.，Eardley，P.，Songhurst，D.，Le Faucheur，F.，Chary，A.，Liatsos，V.，Babiarz，J.，Chan，K.，Dudley，S.，Karagiannis，G.，Bader，A.，L.Westberg，“拥堵前通知标记”，正在进行中，2006年10月。
[Briscoe08] Briscoe, B., "Byte and Packet Congestion Notification", Work in Progress, August 2008.
[Briscoe08]Briscoe，B.，“字节和数据包拥塞通知”，正在进行的工作，2008年8月。
[Charny07] Charny, A., Babiarz, J., Menth, M., and X. Zhang, "Comparison of Proposed PCN Approaches", Work in Progress, November 2007.
[Charny07]Charny，A.，Babiarz，J.，Minth，M.，和X.Zhang，“拟议PCN方法的比较”，正在进行的工作，2007年11月。
[Menth10] Menth, M., Lehrieder, F., Briscoe, B., Eardley, P., Moncaster, T., Babiarz, J., Chan, K., Charny, A., Karagiannis, G., Zhang, X., Taylor, T., Satoh, D., and R. Geib, "A Survey of PCN-Based Admission Control and Flow Termination", IEEE Communications Surveys and Tutorials, 2010 (third issue), <http:// www3.informatik.uni-wuerzburg.de/staff/menth/ Publications/papers/Menth08-PCN-Overview.pdf>.
[Menth10]Minth，M.，Lehrieder，F.，Briscoe，B.，Eardley，P.，Moncaster，T.，Babiarz，J.，Chan，K.，Charny，A.，Karagiannis，G.，Zhang，X.，Taylor，T.，Satoh，D.，和R.Geib，“基于PCN的接纳控制和流终止的调查”，IEEE通信调查和教程，2010年（第三期），<http://ww3.informatik.uni-wuerzburg.de/staff/menth/Publications/papers/Menth08 PCN Overview.pdf>。
[RFC2474] 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.
[RFC2474]Nichols，K.，Blake，S.，Baker，F.，和D.Black，“IPv4和IPv6头中区分服务字段（DS字段）的定义”，RFC 2474，1998年12月。
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., and W. Weiss, "An Architecture for Differentiated Services", RFC 2475, December 1998.
[RFC2475]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。
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001.
[RFC3168]Ramakrishnan，K.，Floyd，S.，和D.Black，“向IP添加显式拥塞通知（ECN）”，RFC 3168，2001年9月。
[RFC4778] Kaeo, M., "Operational Security Current Practices in Internet Service Provider Environments", RFC 4778, January 2007.
[RFC4778]Kaeo，M.，“互联网服务提供商环境中的运营安全当前实践”，RFC 4778，2007年1月。
[RFC5127] Chan, K., Babiarz, J., and F. Baker, "Aggregation of DiffServ Service Classes", RFC 5127, February 2008.
[RFC5127]Chan，K.，Babiarz，J.，和F.Baker，“区分服务类的聚合”，RFC 5127，2008年2月。
[RFC5559] Eardley, P., "Pre-Congestion Notification (PCN) Architecture", RFC 5559, June 2009.
[RFC5559]Eardley，P.，“拥塞前通知（PCN）体系结构”，RFC555592009年6月。
[RFC5696] Moncaster, T., Briscoe, B., and M. Menth, "Baseline Encoding and Transport of Pre-Congestion Information", RFC 5696, November 2009.
[RFC5696]Moncaster，T.，Briscoe，B.，和M.Minth，“拥堵前信息的基线编码和传输”，RFC 56962009年11月。
[Taylor09] Charny, A., Huang, F., Menth, M., and T. Taylor, "PCN Boundary Node Behaviour for the Controlled Load (CL) Mode of Operation", Work in Progress, March 2009.
[Taylor09]Charny，A.，Huang，F.，Minth，M.，和T.Taylor，“受控负荷（CL）运行模式下的PCN边界节点行为”，在建工程，2009年3月。
Note: This Appendix is informative, not normative. It is an example of algorithms that implement Section 2 and is based on [Charny07] and [Menth10].
注：本附录为资料性附录，非规范性附录。这是实现第2节的算法示例，基于[Charny07]和[Menth10]。
There is no attempt to optimise the algorithms. The metering and marking functions are implemented together. It is assumed that three encoding states are available (one for threshold-marked, one for excess-traffic-marked, and one for not-marked). It is assumed that all metered-packets are PCN-packets and that the link is never overloaded. For excess-traffic-marking, "packet size independent marking" applies.
没有人试图优化算法。计量和标记功能一起实现。假设有三种编码状态可用（一种用于标记阈值，一种用于标记过量流量，一种用于未标记）。假设所有计量数据包都是PCN数据包，并且链路从未过载。对于超额流量标记，“数据包大小独立标记”适用。
A token bucket with the following parameters:
具有以下参数的令牌桶：
* PCN-threshold-rate: token rate of token bucket (bits/second)
* PCN门限速率：令牌桶的令牌速率（位/秒）
* BS_tm: depth of token bucket (bits)
* BS_tm：令牌桶深度（位）
* threshold: marking threshold of token bucket (bits)
* 阈值：标记令牌桶的阈值（位）
* lastUpdate: time the token bucket was last updated (seconds)
* lastUpdate：上次更新令牌存储桶的时间（秒）
* F_tm: amount of tokens in token bucket (bits)
* F_tm：令牌桶中的令牌数量（位）
A PCN-packet has the following parameters:
PCN数据包具有以下参数：
* packet_size: the size of the PCN-packet (bits)
* 数据包大小：PCN数据包的大小（位）
* packet_mark: the PCN encoding state of the packet
* 数据包标记：数据包的PCN编码状态
In addition there is the parameter:
此外，还有一个参数：
now: the current time (seconds)
现在：当前时间（秒）
The following steps are performed when a PCN-packet arrives on a link:
当PCN数据包到达链路时，执行以下步骤：
* F_tm = min(BS_tm, F_tm + (now - lastUpdate) * PCN-threshold-rate); // add tokens to token bucket
* F_tm=min（BS_tm，F_tm+（现在-最新更新）*PCN阈值率）；//将令牌添加到令牌桶
* F_tm = max(0, F_tm - packet_size); // remove tokens from token bucket
* F_tm=最大值（0，F_tm-数据包大小）；//从令牌桶中移除令牌
* if ((F_tm < threshold) AND (packet_mark != excess-traffic-marked)) then packet_mark = threshold-marked; // do threshold-marking, but don't re-mark packets that are already excess-traffic-marked
* 如果（（F_tm<threshold）和（packet_mark！=标记的过量流量）），则packet_mark=标记的threshold；//进行阈值标记，但不要重新标记已标记过多流量的数据包
* lastUpdate = now // Note: 'now' has the same value as in step 1
* lastUpdate=now//注意：“now”的值与步骤1中的值相同
A token bucket with the following parameters:
具有以下参数的令牌桶：
* PCN-excess-rate: token rate of token bucket (bits/second)
* PCN超额率：令牌桶的令牌率（位/秒）
* BS_etm: depth of TB in token bucket (bits)
* BS_etm：令牌桶中TB的深度（位）
* lastUpdate: time the token bucket was last updated (seconds)
* lastUpdate：上次更新令牌存储桶的时间（秒）
* F_etm: amount of tokens in token bucket (bits)
* F_etm：令牌桶中的令牌数量（位）
A PCN-packet has the following parameters:
PCN数据包具有以下参数：
* packet_size: the size of the PCN-packet (bits)
* 数据包大小：PCN数据包的大小（位）
* packet_mark: the PCN encoding state of the packet
* 数据包标记：数据包的PCN编码状态
In addition there is the parameter:
此外，还有一个参数：
* now: the current time (seconds)
* 现在：当前时间（秒）
The following steps are performed when a PCN-packet arrives on a link:
当PCN数据包到达链路时，执行以下步骤：
* F_etm = min(BS_etm, F_etm + (now - lastUpdate) * PCN-excess-rate); // add tokens to token bucket
* F_etm=min（BS_etm，F_etm+（现在-最新更新）*PCN超额率）；//将令牌添加到令牌桶
* if (packet_mark != excess-traffic-marked) then // do not meter packets that are already excess-traffic-marked
* 如果（packet_mark！=标记了过量流量），则//不要测量已经标记了过量流量的数据包
+ if (F_etm < 0) then packet_mark = excess-traffic-marked; // do excess-traffic-marking. The algorithm ensures this is independent of packet size
+ 如果（F_etm<0），则数据包标记=标记的过量流量；//不要做过多的交通标志。该算法确保这与数据包大小无关
+ else F_etm = F_etm - packet_size; // remove tokens from token bucket if don't mark packet
+ else F_etm=F_etm-数据包大小；//如果不标记数据包，则从令牌桶中移除令牌
* lastUpdate = now // Note: 'now' has the same value as in step 1
* lastUpdate=now//注意：“now”的值与步骤1中的值相同
Note: This Appendix is informative, not normative. It comments on Section 2, including reasoning about whether MUSTs or SHOULDs are required. For guidance on Operations and Management considerations, please see [RFC5559].
注：本附录为资料性附录，非规范性附录。它对第2节进行了评论，包括关于是否需要“必须”或“应该”的推理。有关操作和管理注意事项的指导，请参阅[RFC5559]。
In general, it is not advised to have any competing-non-PCN-traffic, essentially because the unpredictable amount of competing-non-PCN-traffic makes the PCN mechanisms less accurate and so reduces PCN's ability to protect the QoS of admitted PCN-flows [RFC5559]. But if there is competing-non-PCN-traffic, then:
一般来说，不建议存在任何竞争性非PCN流量，这主要是因为竞争性非PCN流量的不可预测量使得PCN机制不太准确，从而降低了PCN保护已接纳PCN流的QoS的能力[RFC5559]。但如果存在竞争性非PCN流量，则：
1. There should be a mechanism to limit it, for example:
1. 应该有一个机制来限制它，例如：
* limit the rate at which competing-non-PCN-traffic can be forwarded on each link in the PCN-domain. One method for achieving this is to queue competing-non-PCN-packets separately from PCN-packets and to limit the scheduling rate of the former. Another method is to drop competing-non-PCN-packets in excess of some rate.
* 限制竞争性非PCN流量在PCN域中每个链路上的转发速率。实现这一点的一种方法是将竞争的非PCN分组与PCN分组分开排队，并限制前者的调度速率。另一种方法是丢弃超过一定速率的竞争性非PCN数据包。
* police competing-non-PCN-traffic at the PCN-ingress-nodes, as in the Diffserv architecture, for example. However, Diffserv's static traffic conditioning agreements risk a focused overload of traffic from several PCN-ingress-nodes onto one link.
* 例如，在区分服务体系结构中，对PCN入口节点处的非PCN流量进行监控。然而，Diffserv的静态流量调节协议存在着将多个PCN入口节点的流量集中过载到一条链路上的风险。
* by design, it is known that the level of competing-non-PCN-traffic is always very small -- perhaps it consists of operator control messages only.
* 通过设计，已知竞争的非PCN通信量的水平总是非常小的——可能它仅由操作员控制消息组成。
2. In general, PCN's mechanisms should take account of competing-non-PCN-traffic, in order to improve the accuracy of the decision about whether to admit (or terminate) a PCN-flow. For example:
2. 一般而言，PCN的机制应考虑竞争性非PCN流量，以提高关于是否接纳（或终止）PCN流的决策的准确性。例如：
* competing-non-PCN-traffic contributes to the PCN-meters; competing-non-PCN-packets are treated as metered-packets.
* 竞争性非PCN流量有助于PCN计量器；竞争的非PCN数据包被视为计量数据包。
* each PCN-node, on its links: (1) reduces the reference rates (PCN-threshold-rate and PCN-excess-rate), in order to allow 'headroom' for the competing-non-PCN-traffic; (2) limits the maximum forwarding rate of competing-non-PCN-traffic to be less than the 'headroom'. In this case, competing-non-PCN-packets are not treated as metered-packets.
* 每个PCN节点在其链路上：（1）降低参考速率（PCN阈值速率和PCN超额速率），以便为竞争的非PCN流量留出“净空”；（2） 将竞争性非PCN流量的最大转发速率限制为小于“净空”。在这种情况下，竞争的非PCN分组不被视为计量分组。
3. The operator should decide on appropriate action. Dropping is discussed further in Appendix B.4.
3. 操作员应决定采取适当的措施。附录B.4中进一步讨论了跌落。
One specific example of competing-non-PCN-traffic occurs if the PCN-compatible Diffserv codepoint is one of those that [Baker08] defines as suitable for use with admission control and there is such non-PCN-traffic in the PCN-domain. A similar example could occur for Diffserv codepoints of the Real-Time Treatment Aggregate [RFC5127]. In such cases, PCN-traffic and competing-non-PCN-traffic are distinguished by different values of the ECN field [RFC5696].
如果与PCN兼容的Diffserv代码点是[Baker08]定义为适合用于准入控制的代码点之一，并且PCN域中存在此类非PCN流量，则会出现竞争性非PCN流量的一个具体示例。实时治疗聚合[RFC5127]的Diffserv代码点也可能出现类似的例子。在这种情况下，通过ECN字段[RFC5696]的不同值来区分PCN流量和竞争性非PCN流量。
Another example would occur if there is more than one PCN-compatible Diffserv codepoint in a PCN-domain. For instance, suppose there are two PCN-BAs treated at different priorities. Then as far as the lower priority PCN-BA is concerned, the higher priority PCN-traffic needs to be treated as competing-non-PCN-traffic.
如果一个PCN域中有多个与PCN兼容的Diffserv码点，则会出现另一个示例。例如，假设有两个PCN BAs以不同的优先级处理。然后，就低优先级PCN-BA而言，高优先级PCN流量需要被视为竞争性非PCN流量。
It may be known, for instance by the design of the network topology, that some links can never be pre-congested (even in unusual circumstances, such as after the failure of some links). There is then no need to deploy the PCN-metering and -marking behaviour on those links.
例如，通过网络拓扑的设计，可以知道某些链路永远不会预拥塞（即使在异常情况下，例如在某些链路发生故障后）。因此，无需在这些链路上部署PCN计量和标记行为。
The meters can be implemented on the ingoing or outgoing interface of a PCN-node. It may be that existing hardware can support only one meter per ingoing interface and one per outgoing interface. Then, for instance, threshold-metering could be run on all the ingoing interfaces and excess-traffic-metering on all the outgoing interfaces; note that the same choice must be made for all the links in a PCN-domain to ensure that the two metering behaviours are applied exactly once for all the links.
仪表可以在PCN节点的输入或输出接口上实现。可能现有硬件只能支持每个输入接口一个仪表，每个输出接口一个仪表。然后，例如，可以在所有输入接口上运行阈值计量，在所有输出接口上运行多余流量计量；请注意，必须对PCN域中的所有链路进行相同的选择，以确保两种计量行为对所有链路应用一次。
The baseline encoding [RFC5696] specifies only two encoding states (PCN-marked and not-marked). In this case, "excess-traffic-marked" means a packet that is PCN-marked as a result of the excess-traffic-meter function, and "threshold-marked" means a packet that is PCN-marked as a result of the threshold-meter function. As far as terminology is concerned, this interpretation is consistent with that defined in [RFC5559]. Note that a deployment needs to make a consistent choice throughout the PCN-domain whether PCN-marked is interpreted as excess-traffic-marked or threshold-marked.
The baseline encoding [RFC5696] specifies only two encoding states (PCN-marked and not-marked). In this case, "excess-traffic-marked" means a packet that is PCN-marked as a result of the excess-traffic-meter function, and "threshold-marked" means a packet that is PCN-marked as a result of the threshold-meter function. As far as terminology is concerned, this interpretation is consistent with that defined in [RFC5559]. Note that a deployment needs to make a consistent choice throughout the PCN-domain whether PCN-marked is interpreted as excess-traffic-marked or threshold-marked.translate error, please retry
Note that even if there are only two encoding states, it is still required that both the meters are implemented, in order to ease compatibility between equipment and to remove a configuration option and associated complexity. Hardware with limited availability of
请注意，即使只有两种编码状态，仍然需要实现两种仪表，以简化设备之间的兼容性，并消除配置选项和相关复杂性。可用性有限的硬件
token buckets could be configured to run only one of the meters, but it must be possible to enable either meter. Although, in the scenario with two encoding states, indications from one of the meters are ignored by the marking function, they may be logged or acted upon in some other way, for example, by the management system or an explicit signalling protocol; such considerations are out of the scope of this document.
令牌桶可以配置为仅运行一个仪表，但必须能够启用任一仪表。尽管在具有两种编码状态的情况下，标记功能会忽略其中一个仪表的指示，但可以通过其他方式记录或处理指示，例如，通过管理系统或显式信令协议；此类考虑不在本文件范围内。
Configuration of PCN-nodes will define what values of the DSCP and ECN fields indicate a PCN-packet in a particular PCN-domain. For instance, [RFC5696] defines the baseline encoding.
PCN节点的配置将定义DSCP和ECN字段的哪些值表示特定PCN域中的PCN数据包。例如，[RFC5696]定义了基线编码。
Configuration will also define what values of the DSCP and ECN fields indicate a competing-non-PCN-packet in a particular PCN-domain.
配置还将定义DSCP和ECN字段的哪些值表示特定PCN域中存在竞争的非PCN数据包。
The objective of the dropping function is to minimise the queueing delay suffered by metered-traffic at a PCN-node, since PCN-traffic (and perhaps competing-non-PCN-traffic) is expected to be inelastic traffic generated by real-time applications. In practice, it would be defined as exceeding a specific traffic profile, typically based on a token bucket.
丢弃功能的目标是最小化PCN节点处计量流量所遭受的排队延迟，因为PCN流量（以及可能竞争的非PCN流量）预计为实时应用程序生成的非弹性流量。在实践中，它将被定义为超过特定的流量配置文件，通常基于令牌桶。
If there is no competing-non-PCN-traffic, then it is not expected that the dropping function is needed, since PCN's flow admission and termination mechanisms limit the amount of PCN-traffic. Even so, it still might be implemented as a back stop against misconfiguration of the PCN-domain, for instance.
如果没有竞争的非PCN流量，则预计不需要丢弃功能，因为PCN的流量接纳和终止机制限制了PCN流量。即使如此，它仍然可能被实现为防止PCN域配置错误的一个后盾。
If there is competing-non-PCN-traffic, then the details of the dropping function will depend on how the router's implementation handles the two sorts of traffic:
如果存在竞争性的非PCN流量，则丢弃功能的细节将取决于路由器的实现如何处理这两种流量：
1. a common queue for PCN-traffic and competing-non-PCN-traffic, with a traffic conditioner for the competing-non-PCN-traffic; or
1. PCN流量和竞争性非PCN流量的公共队列，竞争性非PCN流量的流量调节器；或
2. separate queues, in which case the amount of competing-non-PCN-traffic can be limited by limiting the rate at which the scheduler (for the competing-non-PCN-traffic) forwards packets.
2. 独立队列，在这种情况下，可以通过限制调度程序（用于竞争性非PCN流量）转发数据包的速率来限制竞争性非PCN流量的数量。
(The discussion here is based on that in [Baker08].)
（此处的讨论基于[08]中的讨论。）
Note that only dropping of packets is allowed. Downgrading of packets to a lower priority BA is not allowed (see Appendix B.7), since it would lead to packet mis-ordering. Shaping ("the process of delaying packets" [RFC2475]) is not suitable if the traffic comes from real-time applications.
请注意，只允许丢弃数据包。不允许将数据包降级为较低优先级的BA（见附录B.7），因为这将导致数据包错误排序。如果流量来自实时应用程序，则整形（“延迟数据包的过程”[RFC2475]）不合适。
Preferential dropping of competing-non-PCN-traffic: In general, it is reasonable for competing-non-PCN-traffic to get harsher treatment than PCN-traffic (that is, competing-non-PCN-packets are preferentially dropped) because PCN's flow admission and termination mechanisms are stronger than the mechanisms that are likely to be applied to the competing-non-PCN-traffic. The PCN mechanisms also mean that a dropper should not be needed for the PCN-traffic.
竞争性非PCN流量优先丢弃：一般来说，竞争性非PCN流量比PCN流量受到更严厉的待遇是合理的（即竞争性非PCN数据包优先丢弃）因为PCN的流量接纳和终止机制比可能应用于竞争性非PCN流量的机制更强。PCN机制还意味着PCN流量不需要滴管。
Preferential dropping of excess-traffic-marked packets: Section 2.2 specifies, "If the PCN-node drops PCN-packets, then ... PCN-packets that arrive at the PCN-node already excess-traffic-marked SHOULD be preferentially dropped". In brief, the reason is that, with the "controlled load" edge behaviour [Taylor09], this avoids over-termination in the event of multiple bottlenecks in the PCN-domain [Charny07]. A fuller explanation is as follows. The optimal dropping behaviour depends on the particular edge behaviour [Menth10]. A single dropping behaviour is defined, as it is simpler to standardise, implement, and operate. The standardised dropping behaviour is at least adequate for all edge behaviours (and good for some), whereas others are not (for example, with tail dropping, far too much traffic may be terminated with the "controlled load" edge behaviour, in the event of multiple bottlenecks in the PCN-domain [Charny07]). The dropping behaviour is defined as a 'SHOULD', rather than a 'MUST', in recognition that other dropping behaviour may be preferred in particular circumstances, for example: (1) with the "marked flow" termination edge behaviour, preferential dropping of unmarked packets may be better [Menth10]; (2) tail dropping may make PCN-marking behaviour easier to implement on current routers.
优先丢弃多余的标记流量的数据包：第2.2节规定，“如果PCN节点丢弃PCN数据包，那么……到达已经标记多余流量的PCN节点的PCN数据包应优先丢弃”。简而言之，原因是，通过“受控负载”边缘行为[Taylor09]，这避免了PCN域中出现多个瓶颈时的过度终止[Charny07]。更全面的解释如下。最佳跌落行为取决于特定的边缘行为[Menth10]。由于标准化、实施和操作更为简单，因此定义了单次跌落行为。标准化下降行为至少适用于所有边缘行为（并且对某些边缘行为有利），而其他边缘行为则不适用（例如，在出现PCN域中的多个瓶颈的情况下，尾部下降时，过多的流量可能会因“受控负载”边缘行为而终止[Charny07]）。丢弃行为被定义为“应该”，而不是“必须”，因为在特定情况下，其他丢弃行为可能是首选的，例如：（1）对于“标记流”终止边缘行为，未标记数据包的优先丢弃可能更好[Menth10]；（2） 尾部脱落可能使PCN标记行为更容易在当前路由器上实现。
Exactly what "preferentially dropped" means is left to the implementation. It is also left to the implementation what to do if there are no excess-traffic-marked PCN-packets available at a particular instant.
“优先删除”的确切含义留给实现。如果在特定时刻没有多余的带有流量标记的PCN数据包可用，那么该怎么办也留给实现来做。
Section 2.2 also specifies, "the PCN-node's excess-traffic-meter SHOULD NOT meter the PCN-packets that it drops." This avoids over-termination [Menth10]. Effectively, it means that the dropping function (if present) should be done before the meter functions -- which is natural.
第2.2节还规定，“PCN节点的超额流量表不应测量其丢弃的PCN数据包。”这避免了过度终止[Menth10]。实际上，这意味着下降功能（如果存在）应该在仪表工作之前完成——这是很自然的。
The description is in terms of a 'token bucket with threshold' (which [Briscoe06-1] views as a virtual queue). However, the description is not intended to standardise implementation.
描述是以“具有阈值的令牌桶”（其[Briscoe06-1]视为虚拟队列）的形式进行的。然而，该描述并非旨在标准化实施。
The reference rate of the threshold-meter (PCN-threshold-rate) is configured at less than the rate allocated to the PCN-traffic class. Also, the PCN-threshold-rate is less than, or possibly equal to, the PCN-excess-rate.
阈值表的参考速率（PCN阈值速率）配置为小于分配给PCN流量等级的速率。此外，PCN阈值率小于或可能等于PCN超额率。
Section 2.3 specifies, "If F_tm < threshold, then the meter indicates to the marking function that the packet is to be threshold-marked; otherwise, it does not." Note that a PCN-packet is marked without explicit additional bias for the packet's size.
第2.3节规定，“如果F_tm<阈值，则仪表向标记功能指示数据包将被标记为阈值；否则，它不会。”注意，PCN数据包的标记没有明确的数据包大小附加偏差。
The behaviour must be functionally equivalent to the description in Section 2.3. "Functionally equivalent" means the observable 'black box' behaviour is the same or very similar, for example, if either precisely the same set of packets is marked or if the set is shifted by one packet. It is intended to allow implementation freedom over matters such as:
该行为必须在功能上等同于第2.3节中的描述。“功能等效”是指可观察到的“黑盒”行为相同或非常相似，例如，如果标记的是完全相同的数据包集，或者该数据包集被移动了一个数据包。其目的是允许在以下事项上自由实施：
o whether tokens are added to the token bucket at regular time intervals or only when a packet is processed.
o 令牌是以固定的时间间隔添加到令牌桶，还是仅在处理数据包时添加。
o whether the new token bucket depth is calculated before or after it is decided whether to PCN-mark the packet. The effect of this is simply to shift the sequence of marks by one packet.
o 新令牌桶深度是在决定是否对数据包进行PCN标记之前还是之后计算的。这样做的效果只是将标记序列移动一个数据包。
o when the token bucket is very nearly empty and a packet arrives larger than F_tm, then the precise change in F_tm is up to the implementation. For instance:
o 当令牌桶几乎为空且到达的数据包大于F_tm时，F_tm的精确变化取决于实现。例如：
* set F_tm = 0 and indicate threshold-mark to the marking function.
* 设置F_tm=0，并向标记功能指示阈值标记。
* check whether F_tm < threshold and if it is, then indicate threshold-mark to the marking function; then set F_tm = 0.
* 检查F_tm是否<阈值，如果是，则向标记功能指示阈值标记；然后设置F_tm=0。
* leave F_tm unaltered and indicate threshold-mark to the marking function.
* 保持F_tm不变，并向标记功能指示阈值标记。
o similarly, when the token bucket is very nearly full and a packet arrives larger than (BS_tm - F_tm), then the precise change in F_tm is up to the implementation.
o 类似地，当令牌桶非常接近满且到达的数据包大于（BS_tm-F_tm）时，F_tm的精确变化取决于实现。
Note that all PCN-packets, even if already marked, are metered by the threshold-meter function (unlike the excess-traffic-meter function), because all packets should contribute to the decision whether there is room for a new flow.
请注意，所有PCN数据包，即使已经标记，也由阈值计量器功能计量（与过量流量计量器功能不同），因为所有数据包都应该有助于决定是否有空间容纳新流量。
The description is in terms of a token bucket, however the implementation is not standardised.
该描述是以令牌桶的形式进行的，但其实现并不是标准化的。
The reference rate of the excess-traffic-meter (PCN-excess-rate) is configured at less than (or possibly equal to) the rate allocated to the PCN-traffic class. Also, the PCN-excess-rate is greater than, or possibly equal to, the PCN-threshold-rate.
超额流量表的参考速率（PCN超额速率）配置为小于（或可能等于）分配给PCN流量等级的速率。此外，PCN超标率大于或可能等于PCN阈值率。
As in Section B.5, "functionally equivalent" allows some implementation flexibility, for example, the exact algorithm when the token bucket is very nearly empty or very nearly full.
如第B.5节所述，“功能等效”允许一些实现灵活性，例如，当令牌桶非常接近空或非常接近满时的精确算法。
Section 2.4 specifies, "A packet SHOULD NOT be metered (by this excess-traffic-meter function) ... if the packet is already excess-traffic-marked on arrival at the PCN-node". This avoids over-termination (with some edge behaviours) in the event that the PCN-traffic passes through multiple bottlenecks in the PCN-domain [Charny07]. Note that an implementation could determine whether the packet is already excess-traffic-marked as an integral part of its BA classification function. The behaviour is defined as a 'SHOULD NOT', rather than a 'MUST NOT', because it may be slightly harder to implement than a metering function that is blind to previous packet markings.
第2.4节规定，“如果数据包在到达PCN节点时已标记为过量流量，则不应（通过该过量流量表功能）对数据包进行计量”。这避免了PCN流量通过PCN域中多个瓶颈时的过度终止（具有某些边缘行为）[Charny07]。注意，实现可以确定包是否已经被标记为其BA分类功能的组成部分的过量流量。该行为被定义为“不应该”，而不是“不得”，因为它可能比不知道以前数据包标记的计量功能更难实现。
Section 2.4 specifies, "A packet SHOULD NOT be metered (by this excess-traffic-meter function) ... if this PCN-node drops the packet." This avoids over-termination [Menth10]. (A similar statement could also be made for the threshold-meter function but is irrelevant, as a link that is overloaded will already be substantially pre-congested and hence threshold-marking all packets.) It seems natural to perform the dropping function before the metering functions, although for some equipment it may be harder to implement; hence, the behaviour is defined as a 'SHOULD NOT', rather than a 'MUST NOT'.
第2.4节规定，“如果此PCN节点丢弃数据包，则不应（通过此过量流量表功能）对数据包进行计量。”这避免了过度终止[Menth10]。（也可以对阈值计量器功能进行类似的说明，但与此无关，因为过载的链路已经基本上处于预拥塞状态，因此会对所有数据包进行阈值标记。）在计量功能之前执行丢弃功能似乎是自然的，尽管对于某些设备来说可能更难实现；因此，该行为被定义为“不应该”，而不是“不得”。
"Packet size independent marking" -- excess-traffic-marking that is independent of packet size -- is specified as a 'SHOULD' rather than a 'MUST' in Section 2.4 because it may be slightly harder for some equipment to implement, and the impact of not doing so is undesirable but moderate (sufficient traffic is terminated, but flows with large packets are more likely to be terminated). With the "classic"
“与数据包大小无关的标记”——与数据包大小无关的过量流量标记——在第2.4节中被指定为“应该”而不是“必须”，因为某些设备可能会稍微难以实施，不这样做的影响是不希望的，但是中等的（足够的流量被终止，但大数据包的流量更有可能被终止）
excess-traffic-meter behaviour, large packets are more likely to be excess-traffic-marked than small packets (because packets are marked if the number of tokens in the token bucket is smaller than the packet size). This means that, with some edge behaviours, flows with large packets are more likely to be terminated than flows with small packets ([Briscoe08], [Menth10]). "Packet size independent marking" can be achieved by a small modification of the "classic" excess-traffic-meter. The number of tokens in the bucket can become negative; if this number is negative at a packet's arrival, the packet is marked; otherwise, the amount of tokens equal to the packet size is removed from the bucket. Note that with "packet size independent marking", either the packet is marked or tokens are removed -- never both. Hence, the token bucket cannot become more negative than the maximum packet size on the link. The algorithm described in Appendix A implements this behaviour.
过量流量表行为，大数据包比小数据包更有可能标记过量流量（因为如果令牌桶中的令牌数量小于数据包大小，则标记数据包）。这意味着，对于某些边缘行为，具有大数据包的流比具有小数据包的流更有可能被终止（[Briscoe08]，[Menth10]）。“包大小独立标记”可以通过对“经典”超额流量表的一个小修改来实现。桶中的令牌数可能变为负数；如果数据包到达时该数字为负数，则标记该数据包；否则，等于数据包大小的令牌量将从存储桶中移除。请注意，使用“与数据包大小无关的标记”时，要么标记数据包，要么删除令牌——决不能同时标记两者。因此，令牌桶不能变得比链路上的最大数据包大小更负。附录A中描述的算法实现了这种行为。
Note that BS_etm is independent of BS_tm, F_etm is independent of F_tm (except in that a packet can change both), and the two configured rates (PCN-excess-rate and PCN-threshold-rate) are independent (except that PCN-excess-rate >= PCN-threshold-rate).
请注意，BS_etm独立于BS_tm，F_etm独立于F_tm（除非一个数据包可以同时改变两者），并且两个配置的速率（PCN超额速率和PCN阈值速率）是独立的（除非PCN超额速率>=PCN阈值速率）。
Section 2.5 defines, "A PCN-node MUST NOT ...change a PCN-packet into a non-PCN-packet". This means that a PCN-node is not allowed to downgrade a PCN-packet into a lower priority Diffserv BA (hence, downgrading is not allowed as an alternative to dropping).
第2.5节规定，“PCN节点不得……将PCN数据包更改为非PCN数据包”。这意味着不允许PCN节点将PCN数据包降级为较低优先级的Diffserv BA（因此，不允许降级作为丢弃的替代方案）。
Section 2.5 defines, "A PCN-node MUST NOT ...PCN-mark a packet that is not a PCN-packet". This means that in the scenario where competing-non-PCN-packets are treated as metered-packets, a meter may indicate a packet is to be PCN-marked, but the marking function knows it cannot be marked. It is left open to the implementation exactly what to do in this case; one simple possibility is to mark the next PCN-packet. Note that unless the PCN-packets are a large fraction of all the metered-packets, the PCN mechanisms may not work well.
第2.5节定义，“PCN节点不得…PCN标记非PCN数据包的数据包”。这意味着，在竞争性非PCN数据包被视为计量数据包的情况下，计量器可能指示数据包将被PCN标记，但标记功能知道它不能被标记。在这种情况下，具体做什么由实现决定；一种简单的可能性是标记下一个PCN数据包。请注意，除非PCN数据包占所有计量数据包的很大一部分，否则PCN机制可能无法正常工作。
Although the metering functions are described separately from the marking function, they can be implemented in an integrated fashion.
尽管计量功能与标记功能分开描述，但它们可以以集成方式实现。
Author's Address
作者地址
Philip Eardley (editor) BT Adastral Park, Martlesham Heath Ipswich IP5 3RE UK
Philip Eardley（编辑）英国电信阿达斯特拉尔公园，马特勒沙姆希思伊普斯维奇IP5 3RE英国
EMail: philip.eardley@bt.com
EMail: philip.eardley@bt.com