Network Working Group                                    P. Eardley, Ed.
Request for Comments: 5559                                            BT
Category: Informational                                        June 2009
Network Working Group                                    P. Eardley, Ed.
Request for Comments: 5559                                            BT
Category: Informational                                        June 2009

Pre-Congestion Notification (PCN) Architecture


Status of This Memo


This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.


Copyright Notice


Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved.

版权所有(c)2009 IETF信托基金和确定为文件作者的人员。版权所有。

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document ( Please review these documents carefully, as they describe your rights and restrictions with respect to this document.

本文件受BCP 78和IETF信托在本文件出版之日生效的与IETF文件有关的法律规定的约束( 请仔细阅读这些文件,因为它们描述了您对本文件的权利和限制。

This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.




This document describes a general architecture for flow admission and termination based on pre-congestion information in order to protect the quality of service of established, inelastic flows within a single Diffserv domain.


Table of Contents


   1. Introduction ....................................................3
      1.1. Overview of PCN ............................................3
      1.2. Example Use Case for PCN ...................................4
      1.3. Applicability of PCN .......................................7
      1.4. Documents about PCN ........................................8
   2. Terminology .....................................................9
   3. High-Level Functional Architecture .............................11
      3.1. Flow Admission ............................................13
      3.2. Flow Termination ..........................................14
      3.3. Flow Admission and/or Flow Termination When There Are Only
           Two PCN Encoding States ...................................15
      3.4. Information Transport .....................................16
      3.5. PCN-Traffic ...............................................16
      3.6. Backwards Compatibility ...................................17
   4. Detailed Functional Architecture ...............................18
      4.1. PCN-Interior-Node Functions ...............................19
      4.2. PCN-Ingress-Node Functions ................................19
      4.3. PCN-Egress-Node Functions .................................20
      4.4. Admission Control Functions ...............................21
      4.5. Flow Termination Functions ................................22
      4.6. Addressing ................................................22
      4.7. Tunnelling ................................................23
      4.8. Fault Handling ............................................25
   5. Operations and Management ......................................25
      5.1. Fault Operations and Management ...........................25
      5.2. Configuration Operations and Management ...................26
           5.2.1. System Options .....................................27
           5.2.2. Parameters .........................................28
      5.3. Accounting Operations and Management ......................30
      5.4. Performance and Provisioning Operations and Management ....30
      5.5. Security Operations and Management ........................31
   6. Applicability of PCN ...........................................32
      6.1. Benefits ..................................................32
      6.2. Deployment Scenarios ......................................33
      6.3. Assumptions and Constraints on Scope ......................35
           6.3.1. Assumption 1: Trust and Support of PCN -
                  Controlled Environment .............................36
           6.3.2. Assumption 2: Real-Time Applications ...............36
           6.3.3. Assumption 3: Many Flows and Additional Load .......37
           6.3.4. Assumption 4: Emergency Use Out of Scope ...........37
      6.4. Challenges ................................................37
   7. Security Considerations ........................................40
   8. Conclusions ....................................................41
   9. Acknowledgements ...............................................41
   1. Introduction ....................................................3
      1.1. Overview of PCN ............................................3
      1.2. Example Use Case for PCN ...................................4
      1.3. Applicability of PCN .......................................7
      1.4. Documents about PCN ........................................8
   2. Terminology .....................................................9
   3. High-Level Functional Architecture .............................11
      3.1. Flow Admission ............................................13
      3.2. Flow Termination ..........................................14
      3.3. Flow Admission and/or Flow Termination When There Are Only
           Two PCN Encoding States ...................................15
      3.4. Information Transport .....................................16
      3.5. PCN-Traffic ...............................................16
      3.6. Backwards Compatibility ...................................17
   4. Detailed Functional Architecture ...............................18
      4.1. PCN-Interior-Node Functions ...............................19
      4.2. PCN-Ingress-Node Functions ................................19
      4.3. PCN-Egress-Node Functions .................................20
      4.4. Admission Control Functions ...............................21
      4.5. Flow Termination Functions ................................22
      4.6. Addressing ................................................22
      4.7. Tunnelling ................................................23
      4.8. Fault Handling ............................................25
   5. Operations and Management ......................................25
      5.1. Fault Operations and Management ...........................25
      5.2. Configuration Operations and Management ...................26
           5.2.1. System Options .....................................27
           5.2.2. Parameters .........................................28
      5.3. Accounting Operations and Management ......................30
      5.4. Performance and Provisioning Operations and Management ....30
      5.5. Security Operations and Management ........................31
   6. Applicability of PCN ...........................................32
      6.1. Benefits ..................................................32
      6.2. Deployment Scenarios ......................................33
      6.3. Assumptions and Constraints on Scope ......................35
           6.3.1. Assumption 1: Trust and Support of PCN -
                  Controlled Environment .............................36
           6.3.2. Assumption 2: Real-Time Applications ...............36
           6.3.3. Assumption 3: Many Flows and Additional Load .......37
           6.3.4. Assumption 4: Emergency Use Out of Scope ...........37
      6.4. Challenges ................................................37
   7. Security Considerations ........................................40
   8. Conclusions ....................................................41
   9. Acknowledgements ...............................................41
   10. References ....................................................42
      10.1. Normative References .....................................42
      10.2. Informative References ...................................42
   Appendix A.  Possible Future Work Items ...........................48
       A.1.  Probing .................................................50
             A.1.1.  Introduction ....................................50
             A.1.2.  Probing Functions ...............................50
             A.1.3.  Discussion of Rationale for Probing, Its
                     Downsides and Open Issues .......................51
   10. References ....................................................42
      10.1. Normative References .....................................42
      10.2. Informative References ...................................42
   Appendix A.  Possible Future Work Items ...........................48
       A.1.  Probing .................................................50
             A.1.1.  Introduction ....................................50
             A.1.2.  Probing Functions ...............................50
             A.1.3.  Discussion of Rationale for Probing, Its
                     Downsides and Open Issues .......................51
1. Introduction
1. 介绍
1.1. Overview of PCN
1.1. PCN概述

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 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 a PCN-domain, PCN-traffic is forwarded in a prioritised Diffserv traffic class. Every link in the PCN-domain is configured with two rates (PCN-threshold-rate and PCN-excess-rate). If the overall rate of PCN-traffic on a link exceeds a configured rate, then a PCN-interior-node marks PCN-packets appropriately. The PCN-egress-nodes use this information to make admission control and flow termination decisions. Flow admission control determines whether a new flow can be admitted without any impact, in normal circumstances, on the QoS of existing PCN-flows. However, in abnormal circumstances (for instance, a disaster affecting multiple nodes and causing traffic re-routes), the QoS on existing PCN-flows may degrade even though care was exercised when admitting those flows. The flow termination mechanism removes sufficient traffic in order to protect the QoS of the remaining PCN-flows. All PCN-boundary-nodes and PCN-interior-nodes are PCN-enabled and are trusted for correct PCN operation. PCN-ingress-nodes police arriving packets to check that they are part of an admitted PCN-flow that keeps within its agreed flowspec, and hence they maintain per-flow state. PCN-interior-nodes meter all PCN-traffic, and hence do not need to maintain any per-flow


state. Decisions about flow admission and termination are made for a particular pair of PCN-boundary-nodes, and hence PCN-egress-nodes must be able to identify which PCN-ingress-node sent each PCN-packet.


1.2. Example Use Case for PCN
1.2. PCN的示例用例

This section outlines an end-to-end QoS scenario that uses the PCN mechanisms within one domain. The parts outside the PCN-domain are out of scope for PCN, but are included to help clarify how PCN could be used. Note that this section is only an example -- in particular, there are other possibilities (see Section 3) for how the PCN-boundary-nodes perform admission control and flow termination.


As a fundamental building block, each link of the PCN-domain operates the following. Please refer to [Eardley09] and Figure 1.


o A threshold meter and marker, which marks all PCN-packets if the rate of PCN-traffic is greater than a first configured rate, the PCN-threshold-rate. The admission control mechanism limits the PCN-traffic on each link to *roughly* its PCN-threshold-rate.

o 阈值计和标记器,如果PCN流量的速率大于第一个配置的速率(PCN阈值速率),则标记所有PCN数据包。接纳控制机制将每个链路上的PCN流量限制为*大约*其PCN阈值速率。

o An excess-traffic meter and marker, which marks a proportion of PCN-packets such that the amount marked equals the traffic rate in excess of a second configured rate, the PCN-excess-rate. The flow termination mechanism limits the PCN-traffic on each link to *roughly* its PCN-excess-rate.

o 一种超额流量表和标记器,用于标记PCN数据包的一部分,以使标记的量等于超过第二个配置速率(PCN超额速率)的流量速率。流终止机制将每条链路上的PCN流量限制为*大约*其PCN超额率。

Overall, the aim is to give an "early warning" of potential congestion before there is any significant build-up of PCN-packets in the queue on the link; we term this "Pre-Congestion Notification" by analogy with ECN (Explicit Congestion Notification, [RFC3168]). Note that the link only meters the bulk PCN-traffic (and not per flow).


                          ==   Metering &    ==
                          ==Marking behaviour==       ==PCN mechanisms==
           Rate of     ^
      PCN-traffic on   |
     bottleneck link   |
                       |       Some pkts                  Terminate some
                       |  excess-traffic-marked           admitted flows
                       |           &                            &
                       |     Rest of pkts                Block new flows
                       |   threshold-marked
     PCN-excess-rate  -|------------------------------------------------
                       |       All pkts                  Block new flows
                       |   threshold-marked
   PCN-threshold-rate -|------------------------------------------------
                       |        No pkts                  Admit new flows
                       |      PCN-marked
                          ==   Metering &    ==
                          ==Marking behaviour==       ==PCN mechanisms==
           Rate of     ^
      PCN-traffic on   |
     bottleneck link   |
                       |       Some pkts                  Terminate some
                       |  excess-traffic-marked           admitted flows
                       |           &                            &
                       |     Rest of pkts                Block new flows
                       |   threshold-marked
     PCN-excess-rate  -|------------------------------------------------
                       |       All pkts                  Block new flows
                       |   threshold-marked
   PCN-threshold-rate -|------------------------------------------------
                       |        No pkts                  Admit new flows
                       |      PCN-marked

Figure 1: Example of how the PCN admission control and flow termination mechanisms operate as the rate of PCN-traffic increases.


The two forms of PCN-marking are indicated by setting the ECN and DSCP (Differentiated Services Codepoint [RFC2474]) fields to known values, which are configured for the domain. Thus, the PCN-egress-nodes can monitor the PCN-markings in order to measure the severity of pre-congestion. In addition, the PCN-ingress-nodes need to set the ECN and DSCP fields to that configured for an unmarked PCN-packet, and the PCN-egress-nodes need to revert to values appropriate outside the PCN-domain.


For admission control, we assume end-to-end RSVP (Resource Reservation Protocol) [RFC2205]) signalling in this example. The PCN-domain is a single RSVP hop. The PCN-domain operates Diffserv, and we assume that PCN-traffic is scheduled with the expedited forwarding (EF) per-hop behaviour [RFC3246]. Hence, the overall solution is in line with the "IntServ over Diffserv" framework defined in [RFC2998], as shown in Figure 2.

对于接纳控制,在本例中,我们假设端到端RSVP(资源预留协议)[RFC2205])信令。PCN域是单个RSVP跳。PCN域操作Diffserv,并且我们假设PCN流量是通过每跳加速转发(EF)行为进行调度的[RFC3246]。因此,总体解决方案符合[RFC2998]中定义的“IntServ over Diffserv”框架,如图2所示。

   ___    ___    _______________________________________    ____    ___
  |   |  |   |  | PCN-             PCN-            PCN- |  |    |  |   |
  |   |  |   |  |ingress         interior         egress|  |    |  |   |
  |   |  |   |  | -node           -nodes          -node |  |    |  |   |
  |   |  |   |  |-------+  +-------+  +-------+  +------|  |    |  |   |
  |   |  |   |  |       |  | PCN   |  | PCN   |  |      |  |    |  |   |
  |   |..|   |..|Ingress|..|meter &|..|meter &|..|Egress|..|    |..|   |
  |   |..|   |..|Policer|..|marker |..|marker |..|Meter |..|    |..|   |
  |   |  |   |  |-------+  +-------+  +-------+  +------|  |    |  |   |
  |   |  |   |  |  \                                 /  |  |    |  |   |
  |   |  |   |  |   \                               /   |  |    |  |   |
  |   |  |   |  |    \  PCN-feedback-information   /    |  |    |  |   |
  |   |  |   |  |     \  (for admission control)  /     |  |    |  |   |
  |   |  |   |  |      --<-----<----<----<-----<--      |  |    |  |   |
  |   |  |   |  |       PCN-feedback-information        |  |    |  |   |
  |   |  |   |  |        (for flow termination)         |  |    |  |   |
  |___|  |___|  |_______________________________________|  |____|  |___|
   ___    ___    _______________________________________    ____    ___
  |   |  |   |  | PCN-             PCN-            PCN- |  |    |  |   |
  |   |  |   |  |ingress         interior         egress|  |    |  |   |
  |   |  |   |  | -node           -nodes          -node |  |    |  |   |
  |   |  |   |  |-------+  +-------+  +-------+  +------|  |    |  |   |
  |   |  |   |  |       |  | PCN   |  | PCN   |  |      |  |    |  |   |
  |   |..|   |..|Ingress|..|meter &|..|meter &|..|Egress|..|    |..|   |
  |   |..|   |..|Policer|..|marker |..|marker |..|Meter |..|    |..|   |
  |   |  |   |  |-------+  +-------+  +-------+  +------|  |    |  |   |
  |   |  |   |  |  \                                 /  |  |    |  |   |
  |   |  |   |  |   \                               /   |  |    |  |   |
  |   |  |   |  |    \  PCN-feedback-information   /    |  |    |  |   |
  |   |  |   |  |     \  (for admission control)  /     |  |    |  |   |
  |   |  |   |  |      --<-----<----<----<-----<--      |  |    |  |   |
  |   |  |   |  |       PCN-feedback-information        |  |    |  |   |
  |   |  |   |  |        (for flow termination)         |  |    |  |   |
  |___|  |___|  |_______________________________________|  |____|  |___|
  Sx     Access               PCN-domain                   Access    Rx
  End    Network                                          Network   End
  Host                                                              Host
                  <---- signalling across PCN-domain--->
                (for admission control & flow termination)
  Sx     Access               PCN-domain                   Access    Rx
  End    Network                                          Network   End
  Host                                                              Host
                  <---- signalling across PCN-domain--->
                (for admission control & flow termination)
  <-------------------end-to-end QoS signalling protocol--------------->
  <-------------------end-to-end QoS signalling protocol--------------->

Figure 2: Example of possible overall QoS architecture.


A source wanting to start a new QoS flow sends an RSVP PATH message. Normal hop-by-hop IntServ [RFC1633] is used outside the PCN-domain (we assume successfully). The PATH message travels across the PCN-domain; the PCN-egress-node reads the PHOP (previous RSVP hop) object to discover the specific PCN-ingress-node for this flow. The RESV message travels back from the receiver, and triggers the PCN-egress-node to check what fraction of the PCN-traffic from the relevant PCN-ingress-node is currently being threshold-marked. It adds an object with this information onto the RESV message, and hence the PCN-ingress-node learns about the level of pre-congestion on the path. If this level is below some threshold, then the PCN-ingress-node admits the new flow into the PCN-domain. The RSVP message triggers the PCN-ingress-node to install two normal IntServ items: five-tuple information, so that it can subsequently identify data packets that are part of a previously admitted PCN-flow, and a traffic profile, so that it can police the flow to within its reservation. Similarly, the RSVP message triggers the PCN-egress-node to install five-tuple and PHOP information so that it can identify packets as part of a flow from a specific PCN-ingress-node.

想要启动新QoS流的源发送RSVP PATH消息。正常的逐跳IntServ[RFC1633]在PCN域之外使用(我们假设成功)。路径消息穿过PCN域;PCN出口节点读取PHOP(先前RSVP跃点)对象,以发现此流的特定PCN入口节点。RESV消息从接收器传回,并触发PCN出口节点,以检查来自相关PCN入口节点的PCN流量的哪个部分当前被标记为阈值。它将带有此信息的对象添加到RESV消息中,因此PCN入口节点了解路径上的预拥塞级别。如果此级别低于某个阈值,则PCN入口节点允许新的流进入PCN域。RSVP消息触发PCN入口节点安装两个正常的IntServ项:五元组信息,以便它随后能够识别作为先前被接纳的PCN流的一部分的数据包,以及流量配置文件,以便它能够在其保留范围内监控流。类似地,RSVP消息触发PCN出口节点安装五元组和PHOP信息,以便它能够将分组识别为来自特定PCN入口节点的流的一部分。

The flow termination mechanism may happen when some abnormal circumstance causes a link to become so pre-congested that it excess-traffic-marks (and perhaps also drops) PCN-packets. In this example, when a PCN-egress-node observes such a packet, it then, with some probability, terminates this PCN-flow; the probability is configured low enough to avoid over termination and high enough to ensure rapid termination of enough flows. It also informs the relevant PCN-ingress-node so that it can block any further traffic on the terminated flow.


1.3. Applicability of PCN
1.3. PCN的适用性

Compared with alternative QoS mechanisms, PCN has certain advantages and disadvantages that will make it appropriate in particular scenarios. For example, compared with hop-by-hop IntServ [RFC1633], PCN only requires per-flow state at the PCN-ingress-nodes. Compared with the Diffserv architecture [RFC2475], an operator needs to be less accurate and/or conservative in its prediction of the traffic matrix. The Diffserv architecture's traffic-conditioning agreements are static and coarse; they are defined at subscription time and are used (for instance) to limit the total traffic at each ingress of the domain, regardless of the egress for the traffic. On the other hand, PCN firstly uses admission control based on measurements of the current conditions between the specific pair of PCN-boundary-nodes, and secondly, in case of a disaster, PCN protects the QoS of most flows by terminating a few selected ones.


PCN's admission control is a measurement-based mechanism. Hence, it assumes that the present is a reasonable prediction of the future: the network conditions are measured at the time of a new flow request, but the actual network performance must be acceptable during the call some time later. Hence, PCN is unsuitable in several circumstances:


o If the source adapts its bit rate dependent on the level of pre-congestion, because then the aggregate traffic might become unstable. The assumption in this document is that PCN-packets come from real-time applications generating inelastic traffic, such as the Controlled Load Service [RFC2211].

o 如果源根据拥塞前的级别调整其比特率,因为这样总流量可能变得不稳定。本文中的假设是,PCN数据包来自产生非弹性流量的实时应用程序,如受控负载服务[RFC2211]。

o If a potential bottleneck link has capacity for only a few flows, because then a new flow can move a link directly from no pre-congestion to being so overloaded that it has to drop packets. The assumption in this document is that this isn't a problem.

o 如果一个潜在的瓶颈链路只有几个流的容量,因为一个新的流可以直接将一个链路从没有预拥塞移动到过载,以至于它不得不丢弃数据包。本文档中的假设是,这不是一个问题。

o If there is the danger of a "flash crowd", in which many admission requests arrive within the reaction time of PCN's admission mechanism, because then they all might get admitted and so

o 如果存在“快闪人群”的危险,其中许多录取请求在PCN录取机制的反应时间内到达,因为这样他们都可能被录取

overload the network. The assumption in this document is that, if it is necessary, then flash crowds are limited in some fashion beyond the scope of this document, for instance by rate-limiting QoS requests.


The applicability of PCN is discussed further in Section 6.


1.4. Documents about PCN
1.4. 关于PCN的文件

The purpose of this document is to describe a general architecture for flow admission and termination based on (pre-)congestion information in order to protect the quality of service of flows within a Diffserv domain. This document describes the PCN architecture at a high level (Section 3) and in more detail (Section 4). It also defines some terminology, and provides considerations about operations, management, and security. Section 6 considers the applicability of PCN in more detail, covering its benefits, deployment scenarios, assumptions, and potential challenges. The Appendix covers some potential future work items.


Aspects of PCN are also documented elsewhere:


o Metering and marking: [Eardley09] standardises threshold metering and marking and excess-traffic metering and marking. A PCN-packet may be marked, depending on the metering results.

o 计量和标记:[Eardley09]标准化了阈值计量和标记以及超额流量计量和标记。根据计量结果,可标记PCN数据包。

o Encoding: the "baseline" encoding is described in [Moncaster09-1], which standardises two PCN encoding states (PCN-marked and not PCN-marked), whilst (experimental) extensions to the baseline encoding can provide three encoding states (threshold-marked, excess-traffic-marked, or not PCN-marked), for instance, see [Moncaster09-2]. (There may be further encoding states as suggested in [Westberg08].) Section 3.6 considers the backwards compatibility of PCN encoding with ECN.

o 编码:在[Moncaster 09-1]中描述了“基线”编码,它标准化了两种PCN编码状态(有PCN标记和无PCN标记),而基线编码的(实验性)扩展可以提供三种编码状态(阈值标记、过量流量标记或无PCN标记),例如,请参见[Moncaster 09-2]。(可能存在[Westberg08]中建议的其他编码状态)第3.6节考虑了PCN编码与ECN的向后兼容性。

o PCN-boundary-node behaviour: how the PCN-boundary-nodes convert the PCN-markings into decisions about flow admission and flow termination, as described in Informational documents such as [Taylor09] and [Charny07-2]. The concept is that the standardised metering and marking by PCN-nodes allows several possible PCN-boundary-node behaviours. A number of possibilities are outlined in this document; detailed descriptions and comparisons are in [Charny07-1] and [Menth09-2].

o PCN边界节点行为:如[Taylor09]和[Charny07-2]等信息性文件所述,PCN边界节点如何将PCN标记转换为关于流量进入和流量终止的决策。概念是,PCN节点的标准化计量和标记允许几种可能的PCN边界节点行为。本文件概述了一些可能性;详细说明和比较见[Charny07-1]和[Menth09-2]。

o Signalling between PCN-boundary-nodes: signalling is needed to transport PCN-feedback-information between the PCN-boundary-nodes (in the example above, this is the fraction of traffic, between the pair of PCN-boundary-nodes, that is PCN-marked). The exact

o PCN边界节点之间的信令:需要信令在PCN边界节点之间传输PCN反馈信息(在上面的示例中,这是PCN边界节点对之间的通信量的分数,即PCN标记)。确切的

details vary for different PCN-boundary-node behaviours, and so should be described in those documents. It may require an extension to the signalling protocol -- standardisation is out of scope of the PCN WG.


o The interface by which the PCN-boundary-nodes learn identification information about the admitted flows: the exact requirements vary for different PCN-boundary-node behaviours and for different signalling protocols, and so should be described in those documents. They will be similar to those described in the example above -- a PCN-ingress-node needs to be able to identify that a packet is part of a previously admitted flow (typically from its five-tuple) and each PCN-boundary-node needs to be able to identify the other PCN-boundary-node for the flow.

o PCN边界节点通过其了解关于允许流的识别信息的接口:对于不同的PCN边界节点行为和不同的信令协议,确切的要求各不相同,因此应在这些文件中进行描述。它们将类似于上述示例中描述的那些——PCN入口节点需要能够识别一个数据包是先前允许的流的一部分(通常来自其五个元组),并且每个PCN边界节点需要能够识别该流的另一个PCN边界节点。

2. Terminology
2. 术语

o PCN-domain: a PCN-capable domain; a contiguous set of PCN-enabled nodes that perform Diffserv scheduling [RFC2474]; the complete set of PCN-nodes that in principle can, through PCN-marking packets, influence decisions about flow admission and termination for the PCN-domain; includes the PCN-egress-nodes, which measure these PCN-marks, and the PCN-ingress-nodes.

o PCN域:具有PCN功能的域;一组连续的启用PCN的节点,执行区分服务调度[RFC2474];原则上可以通过PCN标记数据包影响PCN域的流量接纳和终止决策的完整PCN节点集;包括测量这些PCN标记的PCN出口节点和PCN入口节点。

o PCN-boundary-node: a PCN-node that connects one PCN-domain to a node either in another PCN-domain or in a non-PCN-domain.

o PCN边界节点:将一个PCN域连接到另一个PCN域或非PCN域中的节点的PCN节点。

o PCN-interior-node: a node in a PCN-domain that is not a PCN-boundary-node.

o PCN内部节点:PCN域中不是PCN边界节点的节点。

o PCN-node: a PCN-boundary-node or a PCN-interior-node.

o PCN节点:PCN边界节点或PCN内部节点。

o PCN-egress-node: a PCN-boundary-node in its role in handling traffic as it leaves a PCN-domain.

o PCN出口节点:一个PCN边界节点,在其离开PCN域时处理流量。

o PCN-ingress-node: a PCN-boundary-node in its role in handling traffic as it enters a PCN-domain.

o PCN入口节点:一个PCN边界节点,在其进入PCN域时处理流量。

o PCN-traffic, PCN-packets, PCN-BA: a PCN-domain carries traffic of different Diffserv behaviour aggregates (BAs) [RFC2474]. The PCN-BA uses the PCN mechanisms to carry PCN-traffic, and the corresponding packets are PCN-packets. The same network will carry traffic of other Diffserv BAs. The PCN-BA is distinguished by a combination of the Diffserv codepoint (DSCP) and ECN fields.

o PCN流量,PCN数据包,PCN-BA:PCN域承载不同区分服务行为聚合(BAs)的流量[RFC2474]。PCN-BA使用PCN机制承载PCN流量,相应的数据包为PCN数据包。同一网络将承载其他Diffserv BAs的流量。PCN-BA通过区分服务代码点(DSCP)和ECN字段的组合来区分。

o PCN-flow: the unit of PCN-traffic that the PCN-boundary-node admits (or terminates); the unit could be a single microflow (as defined in [RFC2474]) or some identifiable collection of microflows.

o PCN流量:PCN边界节点允许(或终止)的PCN流量单位;该单元可以是单个微流(定义见[RFC2474])或一些可识别的微流集合。

o Pre-congestion: a condition of a link within a PCN-domain such that the PCN-node performs PCN-marking, in order to provide an "early warning" of potential congestion before there is any significant build-up of PCN-packets in the real queue. (Hence, by analogy with ECN, we call our mechanism Pre-Congestion Notification.)

o 预拥塞:PCN域内链路的一种状态,使得PCN节点执行PCN标记,以便在实际队列中出现任何明显的PCN数据包累积之前提供潜在拥塞的“早期警告”。(因此,与ECN类似,我们称我们的机制为拥塞前通知。)

o PCN-marking: the process of setting the header in a PCN-packet based on defined rules, in reaction to pre-congestion; either threshold-marking or excess-traffic-marking. Such a packet is then called PCN-marked.

o PCN标记:根据定义的规则在PCN数据包中设置报头的过程,以应对预拥塞;阈值标记或过量交通标记。这样的数据包称为PCN标记。

o Threshold-metering: a metering behaviour that, if the PCN-traffic exceeds the PCN-threshold-rate, indicates that all PCN-traffic is to be threshold-marked.

o 阈值计量:一种计量行为,如果PCN流量超过PCN阈值速率,则表明所有PCN流量都将被标记为阈值。

o PCN-threshold-rate: the reference rate of a threshold-meter, which is configured for each link in the PCN-domain and which is lower than the PCN-excess-rate.

o PCN阈值速率:阈值计的参考速率,为PCN域中的每个链路配置,低于PCN超额速率。

o Threshold-marking: the setting of the header in a PCN-packet to a specific encoding, based on indications from the threshold-meter. Such a packet is then called threshold-marked.

o 阈值标记:根据阈值计的指示,将PCN数据包中的报头设置为特定编码。这样的数据包被称为阈值标记。

o Excess-traffic-metering: a metering behaviour that, if the PCN-traffic exceeds the PCN-excess-rate, indicates that the amount of PCN-traffic to be excess-traffic-marked is equal to the amount in excess of the PCN-excess-rate.

o 超额流量计量:一种计量行为,如果PCN流量超过PCN超额率,则表明要标记的超额流量的PCN流量等于超出PCN超额率的流量。

o PCN-excess-rate: the reference rate of an excess-traffic-meter, which is a configured for each link in the PCN-domain and which is higher than the PCN-threshold-rate.

o PCN超额率:超额流量表的参考率,为PCN域中的每个链路配置,高于PCN阈值率。

o Excess-traffic-marking: the setting of the header in a PCN-packet to a specific encoding, based on indications from the excess-traffic-meter. Such a packet is then called excess-traffic-marked.

o 超额流量标记:根据超额流量表的指示,将PCN数据包中的报头设置为特定编码。这样的数据包被称为超额流量标记。

o PCN-colouring: the process of setting the header in a PCN-packet by a PCN-boundary-node; performed by a PCN-ingress-node so that PCN-nodes can easily identify PCN-packets; performed by a PCN-egress-node so that the header is appropriate for nodes beyond the PCN-domain.

o PCN着色:通过PCN边界节点在PCN数据包中设置报头的过程;由PCN入口节点执行,使得PCN节点能够容易地识别PCN分组;由PCN出口节点执行,以便报头适合PCN域以外的节点。

o Ingress-egress-aggregate: The collection of PCN-packets from all PCN-flows that travel in one direction between a specific pair of PCN-boundary-nodes.

o 入口-出口聚合:从在一对特定PCN边界节点之间沿一个方向移动的所有PCN流收集PCN数据包。

o PCN-feedback-information: information signalled by a PCN-egress-node to a PCN-ingress-node (or a central control node), which is needed for the flow admission and flow termination mechanisms.

o PCN反馈信息:由PCN出口节点向PCN入口节点(或中央控制节点)发送信号的信息,这是流量接纳和流量终止机制所需的信息。

o PCN-admissible-rate: the rate of PCN-traffic on a link up to which PCN admission control should accept new PCN-flows.

o PCN接纳率:PCN接纳控制应接受新PCN流的链路上PCN流量的速率。

o PCN-supportable-rate: the rate of PCN-traffic on a link down to which PCN flow termination should, if necessary, terminate already admitted PCN-flows.

o PCN可支持速率:链路上的PCN流量速率,如有必要,PCN流终止应终止已接纳的PCN流。

3. High-Level Functional Architecture
3. 高级功能体系结构

The high-level approach is to split functionality between:


o PCN-interior-nodes "inside" the PCN-domain, which monitor their own state of pre-congestion and mark PCN-packets as appropriate. They are not flow-aware, nor are they aware of ingress-egress-aggregates. The functionality is also done by PCN-ingress-nodes for their outgoing interfaces (ie, those "inside" the PCN-domain).

o PCN内部节点“位于”PCN域内,监控其自身的预拥塞状态,并根据需要标记PCN数据包。他们不知道流量,也不知道进出集料。该功能也由PCN入口节点为其输出接口(即“在”PCN域内的接口)完成。

o PCN-boundary-nodes at the edge of the PCN-domain, which control admission of new PCN-flows and termination of existing PCN-flows, based on information from PCN-interior-nodes. This information is in the form of the PCN-marked data packets (which are intercepted by the PCN-egress-nodes) and is not in signalling messages. Generally, PCN-ingress-nodes are flow-aware.

o PCN域边缘的PCN边界节点,根据来自PCN内部节点的信息控制新PCN流的接纳和现有PCN流的终止。该信息以PCN标记的数据包(由PCN出口节点截获)的形式存在,不在信令消息中。通常,PCN入口节点是流感知的。

The aim of this split is to keep the bulk of the network simple, scalable, and robust, whilst confining policy, application-level, and security interactions to the edge of the PCN-domain. For example, the lack of flow awareness means that the PCN-interior-nodes don't care about the flow information associated with PCN-packets, nor do the PCN-boundary-nodes care about which PCN-interior-nodes its ingress-egress-aggregates traverse.


In order to generate information about the current state of the PCN-domain, each PCN-node PCN-marks packets if it is "pre-congested". Exactly when a PCN-node decides if it is "pre-congested" (the algorithm) and exactly how packets are "PCN-marked" (the encoding) will be defined in separate Standards Track documents, but at a high level it is as follows:


o the algorithms: a PCN-node meters the amount of PCN-traffic on each one of its outgoing (or incoming) links. The measurement is made as an aggregate of all PCN-packets, not per flow. There are two algorithms: one for threshold-metering and one for excess-traffic-metering. The meters trigger PCN-marking as necessary.

o 算法:PCN节点测量其每个传出(或传入)链路上的PCN流量。测量是作为所有PCN数据包的集合进行的,而不是每个流。有两种算法:一种用于阈值计量,另一种用于超额流量计量。必要时,仪表触发PCN标记。

o the encoding(s): a PCN-node PCN-marks a PCN-packet by modifying a combination of the DSCP and ECN fields. In the "baseline" encoding [Moncaster09-1], the ECN field is set to 11 and the DSCP is not altered. Extension encodings may be defined that, at most, use a second DSCP (eg, as in [Moncaster09-2]) and/or set the ECN field to values other than 11 (eg, as in [Menth08-2]).

o 编码:PCN节点PCN通过修改DSCP和ECN字段的组合来标记PCN数据包。在“基线”编码[Moncaster09-1]中,ECN字段设置为11,DSCP不改变。扩展编码可以定义为最多使用第二个DSCP(例如,如[Moncaster 09-2])和/或将ECN字段设置为11以外的值(例如,如[Menth08-2])。

In a PCN-domain, the operator may have two or three encoding states available. The baseline encoding provides two encoding states (not PCN-marked and PCN-marked), whilst extended encodings can provide three encoding states (not PCN-marked, threshold-marked, and excess-traffic-marked).


An operator may choose to deploy either admission control or flow termination or both. Although designed to work together, they are independent mechanisms, and the use of one does not require or prevent the use of the other. Three encoding states naturally allows both flow admission and flow termination. If there are only two encoding states, then there are several options -- see Section 3.3.


The PCN-boundary-nodes monitor the PCN-marked packets in order to extract information about the current state of the PCN-domain. Based on this monitoring, a distributed decision is made about whether to admit a prospective new flow or terminate existing flow(s). Sections 4.4 and 4.5 mention various possibilities for how the functionality could be distributed.


PCN-metering and PCN-marking need to be configured on all (potentially pre-congested) links in the PCN-domain to ensure that the PCN mechanisms protect all links. The actual functionality can be configured on the outgoing or incoming interfaces of PCN-nodes -- or one algorithm could be configured on the outgoing interface and the other on the incoming interface. The important point is that a consistent choice is made across the PCN-domain to ensure that the PCN mechanisms protect all links. See [Eardley09] for further discussion.


The objective of threshold-marking, as triggered by the threshold-metering algorithm, is to threshold-mark all PCN-packets whenever the bit rate of PCN-packets is greater than some configured rate, the PCN-threshold-rate. The objective of excess-traffic-metering, as triggered by the excess-traffic-marking algorithm, is to excess-


traffic-mark PCN-packets at a rate equal to the difference between the bit rate of PCN-packets and some configured rate, the PCN-excess-rate. Note that this description reflects the overall intent of the algorithms rather than their instantaneous behaviour, since the rate measured at a particular moment depends on the detailed algorithm, its implementation, and the traffic's variance as well as its rate (eg, marking may well continue after a recent overload, even after the instantaneous rate has dropped). The algorithms are specified in [Eardley09].


Admission and termination approaches are detailed and compared in [Charny07-1] and [Menth09-2]. The discussion below is just a brief summary. Sections 3.1 and 3.2 assume there are three encoding states available, whilst Section 3.3 assumes there are two encoding states available.


From the perspective of the outside world, a PCN-domain essentially looks like a Diffserv domain, but without the Diffserv architecture's traffic-conditioning agreements. PCN-traffic is either transported across it transparently or policed at the PCN-ingress-node (ie, dropped or carried at a lower QoS). One difference is that PCN-traffic has better QoS guarantees than normal Diffserv traffic because the PCN mechanisms better protect the QoS of admitted flows. Another difference may occur in the rare circumstance when there is a failure: on the one hand, some PCN-flows may get terminated but, on the other hand, other flows will get their QoS restored. Non-PCN-traffic is treated transparently, ie, the PCN-domain is a normal Diffserv domain.


3.1. Flow Admission
3.1. 流量入口

The objective of PCN's flow admission control mechanism is to limit the PCN-traffic on each link in the PCN-domain to *roughly* its PCN-admissible-rate by admitting or blocking prospective new flows, in order to protect the QoS of existing PCN-flows. With three encoding states available, the PCN-threshold-rate is configured by the operator as equal to the PCN-admissible-rate on each link. It is set lower than the traffic rate at which the link becomes congested and the node drops packets.


Exactly how the admission control decision is made will be defined separately in Informational documents. This document describes two approaches (others might be possible):


o The PCN-egress-node measures (possibly as a moving average) the fraction of the PCN-traffic that is threshold-marked. The fraction is measured for a specific ingress-egress-aggregate. If the fraction is below a threshold value, then the new flow is

o PCN出口节点测量(可能作为移动平均值)标记为阈值的PCN流量的分数。针对特定的入口-出口骨料测量该分数。如果分数低于阈值,则新的流将被忽略

admitted; if the fraction is above the threshold value, then it is blocked. The fraction could be measured as an EWMA (exponentially weighted moving average), which has sometimes been called the "congestion level estimate".


o The PCN-egress-node monitors PCN-traffic and if it receives one (or several) threshold-marked packets, then the new flow is blocked; otherwise, it is admitted. One possibility may be to react to the marking state of an initial flow-setup packet (eg, RSVP PATH). Another is that after one (or several) threshold-marks, all flows are blocked until after a specific period of no congestion.

o PCN出口节点监控PCN流量,如果它接收到一个(或多个)带有阈值标记的数据包,则新流量被阻塞;否则,它被承认。一种可能是对初始流设置包(例如,RSVP路径)的标记状态作出反应。另一种情况是,在一个(或多个)阈值标记之后,所有流都会被阻塞,直到一段特定的无阻塞期之后。

Note that the admission control decision is made for a particular pair of PCN-boundary-nodes. So it is quite possible for a new flow to be admitted between one pair of PCN-boundary-nodes, whilst at the same time another admission request is blocked between a different pair of PCN-boundary-nodes.


3.2. Flow Termination
3.2. 流量终止

The objective of PCN's flow termination mechanism is to limit the PCN-traffic on each link to *roughly* its PCN-supportable-rate, by terminating some existing PCN-flows, in order to protect the QoS of the remaining PCN-flows. With three encoding states available, the PCN-excess-rate is configured by the operator as equal to the PCN-supportable-rate on each link. It may be set lower than the traffic rate at which the link becomes congested and at which the node drops packets.


Exactly how the flow termination decision is made will be defined separately in Informational documents. This document describes several approaches (others might be possible):


o In one approach, the PCN-egress-node measures the rate of PCN-traffic that is not excess-traffic-marked, which is the amount of PCN-traffic that can actually be supported, and communicates this to the PCN-ingress-node. Also, the PCN-ingress-node measures the rate of PCN-traffic that is destined for this specific PCN-egress-node. The difference represents the excess amount that should be terminated.

o 在一种方法中,PCN出口节点测量未标记过多通信量的PCN通信量的速率,该速率是实际可支持的PCN通信量,并将其传送给PCN入口节点。此外,PCN入口节点测量目的地为该特定PCN出口节点的PCN通信量的速率。差额代表应终止的超额金额。

o Another approach instead measures the rate of excess-traffic-marked traffic and terminates this amount of traffic. This terminates less traffic than the previous approach, if some nodes are dropping PCN-traffic.

o 另一种方法是测量标记为traffic的多余流量的速率,并终止此流量。如果某些节点正在丢弃PCN流量,则此方法终止的流量比前一种方法少。

o Another approach monitors PCN-packets and terminates some of the PCN-flows that have an excess-traffic-marked packet. (If all such flows were terminated, far too much traffic would be terminated, so a random selection needs to be made from those with an excess-traffic-marked packet [Menth08-1].)

o 另一种方法监视PCN数据包,并终止具有标记为数据包的过量流量的一些PCN流。(如果所有这些流都被终止,那么会有太多的流量被终止,因此需要从标记有过量流量的数据包中进行随机选择[Menth08-1]。)

Since flow termination is designed for "abnormal" circumstances, it is quite likely that some PCN-nodes are congested and, hence, that packets are being dropped and/or significantly queued. The flow termination mechanism must accommodate this.


Note also that the termination control decision is made for a particular pair of PCN-boundary-nodes. So it is quite possible for PCN-flows to be terminated between one pair of PCN-boundary-nodes, whilst at the same time none are terminated between a different pair of PCN-boundary-nodes.


3.3. Flow Admission and/or Flow Termination When There Are Only Two PCN Encoding States

3.3. 只有两种PCN编码状态时的流允许和/或流终止

If a PCN-domain has only two encoding states available (PCN-marked and not PCN-marked), ie, it is using the baseline encoding [Moncaster09-1], then an operator has three options (others might be possible):

如果一个PCN域只有两种可用的编码状态(PCN标记和未标记),即它使用的是基线编码[Moncaster 09-1],那么操作员有三个选项(其他选项也可以):

o admission control only: PCN-marking means threshold-marking, ie, only the threshold-metering algorithm triggers PCN-marking. Only PCN admission control is available.

o 仅限准入控制:PCN标记表示阈值标记,即只有阈值计量算法触发PCN标记。只有PCN准入控制可用。

o flow termination only: PCN-marking means excess-traffic-marking, ie, only the excess-traffic-metering algorithm triggers PCN-marking. Only PCN termination control is available.

o 仅限流量终止:PCN标记表示超额流量标记,即只有超额流量计量算法触发PCN标记。只有PCN终止控制可用。

o both admission control and flow termination: only the excess-traffic-metering algorithm triggers PCN-marking; however, the configured rate (PCN-excess-rate) is set equal to the PCN-admissible-rate, as shown in Figure 3. [Charny07-2] describes how both admission control and flow termination can be triggered in this case and also gives some pros and cons of this approach. The main downside is that admission control is less accurate.

o 准入控制和流量终止:只有超额流量计量算法触发PCN标记;但是,配置的速率(PCN超额速率)设置为等于PCN允许速率,如图3所示。[Charny07-2]描述了在这种情况下如何触发准入控制和流量终止,并给出了这种方法的一些优缺点。主要的缺点是准入控制不太准确。

                          ==   Metering &    ==
                          ==Marking behaviour==       ==PCN mechanisms==
           Rate of     ^
      PCN-traffic on   |
     bottleneck link   |                                  Terminate some
                       |                                  admitted flows
                       |                                         &
                       |                                 Block new flows
                       |       Some pkts
   U*PCN-excess-rate  -|  excess-traffic-marked        -----------------
                       |                                 Block new flows
     PCN-excess-rate  -|------------------------------------------------
                       |         No pkts                 Admit new flows
                       |       PCN-marked
                          ==   Metering &    ==
                          ==Marking behaviour==       ==PCN mechanisms==
           Rate of     ^
      PCN-traffic on   |
     bottleneck link   |                                  Terminate some
                       |                                  admitted flows
                       |                                         &
                       |                                 Block new flows
                       |       Some pkts
   U*PCN-excess-rate  -|  excess-traffic-marked        -----------------
                       |                                 Block new flows
     PCN-excess-rate  -|------------------------------------------------
                       |         No pkts                 Admit new flows
                       |       PCN-marked

Figure 3: Schematic of how the PCN admission control and flow termination mechanisms operate as the rate of PCN-traffic increases, for a PCN-domain with two encoding states and using the approach of [Charny07-2]. Note: U is a global parameter for all links in the PCN-domain.


3.4. Information Transport
3.4. 信息传输

The transport of pre-congestion information from a PCN-node to a PCN-egress-node is through PCN-markings in data packet headers, ie, "in-band"; no signalling protocol messaging is needed. Signalling is needed to transport PCN-feedback-information -- for example, to convey the fraction of PCN-marked traffic from a PCN-egress-node to the relevant PCN-ingress-node. Exactly what information needs to be transported will be described in future documents about possible boundary mechanisms. The signalling could be done by an extension of RSVP or NSIS (Next Steps in Signalling), for instance; [Lefaucheur06] describes the extensions needed for RSVP.


3.5. PCN-Traffic
3.5. PCN流量

The following are some high-level points about how PCN works:


o There needs to be a way for a PCN-node to distinguish PCN-traffic from other traffic. This is through a combination of the DSCP field and/or ECN field.

o PCN节点需要有一种方法来区分PCN流量和其他流量。这是通过DSCP字段和/或ECN字段的组合实现的。

o It is not advised to have competing-non-PCN-traffic but, if there is such traffic, there needs to be a mechanism to limit it. "Competing-non-PCN-traffic" means traffic that shares a link with PCN-traffic and competes for its forwarding bandwidth. Hence, more competing-non-PCN-traffic results in poorer QoS for PCN. Further, 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.

o 不建议存在竞争性非PCN流量,但如果存在此类流量,则需要有一种机制来限制。“竞争性非PCN流量”是指与PCN流量共享链路并竞争其转发带宽的流量。因此,更多竞争性非PCN业务导致PCN的QoS较差。此外,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。

o Two examples of such competing-non-PCN-traffic are:

o 此类竞争性非PCN流量的两个示例是:

1. traffic that is priority scheduled over PCN (perhaps a particular application or an operator's control messages);

1. 优先于PCN调度的流量(可能是特定应用程序或操作员的控制消息);

2. traffic that is scheduled at the same priority as PCN (for example, if the Voice-Admit codepoint is used for PCN-traffic [Moncaster09-1] and there is non-PCN, voice-admit traffic in the PCN-domain).

2. 以与PCN相同的优先级调度的流量(例如,如果语音接纳码点用于PCN流量[Moncaster 09-1],并且PCN域中存在非PCN、语音接纳流量)。

o If there is such competing-non-PCN-traffic, then PCN's mechanisms should take account of it, in order to improve the accuracy of the decision about whether to admit (or terminate) a PCN-flow. For example, one mechanism is that such competing-non-PCN-traffic contributes to the PCN-meters (ie, is metered by the threshold-marking and excess-traffic-marking algorithms).

o 如果存在此类竞争性的非PCN流量,则PCN的机制应考虑到这一点,以提高关于是否接纳(或终止)PCN流量的决策的准确性。例如,一种机制是,这种竞争的非PCN流量有助于PCN计量器(即,由阈值标记和过量流量标记算法计量)。

o There will be other non-PCN-traffic that doesn't compete for the same forwarding bandwidth as PCN-traffic, because it is forwarded at lower priority. Hence, it shouldn't contribute to the PCN-meters. Examples are best-effort and assured-forwarding traffic. However, a PCN-node should dedicate some capacity to lower-priority traffic so that it isn't starved.

o 将有其他非PCN流量与PCN流量不竞争相同的转发带宽,因为它以较低的优先级转发。因此,它不应该有助于PCN仪表。例如,尽最大努力和有保证的转发流量。然而,PCN节点应该为低优先级的流量分配一些容量,这样它就不会饿死。

o This document assumes that the PCN mechanisms are applied to a single behaviour aggregate in the PCN-domain. However, it would also be possible to apply them independently to more than one behaviour aggregate, which are distinguished by DSCP.

o 本文档假设PCN机制应用于PCN域中的单个行为聚合。但是,也可以将它们单独应用于多个行为集合,这些行为集合由DSCP区分。

3.6. Backwards Compatibility
3.6. 向后兼容性

PCN specifies semantics for the ECN field that differ from the default semantics of [RFC3168]. A particular PCN encoding scheme needs to describe how it meets the guidelines of BCP 124 [RFC4774] for specifying alternative semantics for the ECN field. In summary, the approach is to:

PCN为ECN字段指定不同于[RFC3168]默认语义的语义。一个特定的PCN编码方案需要描述它如何满足BCP 124[RFC4774]关于为ECN字段指定替代语义的指南。总之,方法是:

o use a DSCP to allow PCN-nodes to distinguish PCN-traffic that uses the alternative ECN semantics;

o 使用DSCP允许PCN节点区分使用替代ECN语义的PCN流量;

o define these semantics for use within a controlled region, the PCN-domain;

o 定义这些语义,以便在受控区域(PCN域)内使用;

o take appropriate action if ECN-capable, non-PCN-traffic arrives at a PCN-ingress-node with the DSCP used by PCN.

o 如果支持ECN的非PCN流量到达PCN入口节点,且PCN使用DSCP,则采取适当的措施。

For the baseline encoding [Moncaster09-1], the "appropriate action" is to block ECN-capable traffic that uses the same DSCP as PCN from entering the PCN-domain directly. "Blocking" means it is dropped or downgraded to a lower-priority behaviour aggregate, or alternatively such traffic may be tunnelled through the PCN-domain. The reason that "appropriate action" is needed is that the PCN-egress-node clears the ECN field to 00.

对于基线编码[Moncaster 09-1],适当的措施是阻止使用与PCN相同DSCP的支持ECN的流量直接进入PCN域。“阻塞”意味着它被丢弃或降级为较低优先级的行为聚合,或者这种流量可以通过PCN域进行隧道传输。需要“适当操作”的原因是PCN出口节点将ECN字段清除为00。

Extended encoding schemes may need to take different "appropriate action".


4. Detailed Functional Architecture
4. 详细的功能架构

This section is intended to provide a systematic summary of the new functional architecture in the PCN-domain. First, it describes functions needed at the three specific types of PCN-node; these are data plane functions and are in addition to the normal router functions for PCN-nodes. Then, it describes the further functionality needed for both flow admission control and flow termination; these are signalling and decision-making functions, and there are various possibilities for where the functions are physically located. The section is split into:


1. functions needed at PCN-interior-nodes

1. PCN内部节点需要的功能

2. functions needed at PCN-ingress-nodes

2. PCN入口节点所需的功能

3. functions needed at PCN-egress-nodes

3. PCN出口节点所需的功能

4. other functions needed for flow admission control

4. 流量允许控制所需的其他功能

5. other functions needed for flow termination control

5. 流量终止控制所需的其他功能

Note: Probing is covered in the Appendix.


The section then discusses some other detailed topics:


1. addressing

1. 寻址

2. tunnelling

2. 隧道掘进

3. fault handling

3. 故障处理

4.1. PCN-Interior-Node Functions
4.1. 内部节点函数

Each link of the PCN-domain is configured with the following functionality:


o Behaviour aggregate classification - determine whether or not an incoming packet is a PCN-packet.

o 行为聚合分类-确定传入数据包是否为PCN数据包。

o PCN-meter - measure the "amount of PCN-traffic". The measurement is made on the overall PCN-traffic, not per flow. Algorithms determine whether to indicate to the PCN-marking functionality that packets should be PCN-marked.

o PCN表-测量“PCN流量”。该测量是在整个PCN流量上进行的,而不是在每个流量上。算法确定是否向PCN标记功能指示数据包应进行PCN标记。

o PCN-mark - as triggered by indications from the PCN-meter functionality; if necessary, PCN-mark packets with the appropriate encoding.

o PCN标记-由PCN仪表功能指示触发;如有必要,PCN用适当的编码标记数据包。

o Drop - if the queue overflows, then naturally packets are dropped. In addition, the link may be configured with a maximum rate for PCN-traffic (below the physical link rate), above which PCN-packets are dropped.

o Drop-如果队列溢出,则自然丢弃数据包。此外,链路可配置有PCN业务的最大速率(低于物理链路速率),高于该速率PCN分组被丢弃。

The functions are defined in [Eardley09] and the baseline encoding in [Moncaster09-1] (extended encodings are to be defined in other documents).


                                       +---------+   Result
                                    |  |  Meter  |       |
                                    |  +---------+       V
         +----------+   +- - - - -+  |                +------+
         |   BA     |   |         |  |                |      |    Marked
Packet =>|Classifier|==>| Dropper |==?===============>|Marker|==> Packet
Stream   |          |   |         |  |                |      |    Stream
         +----------+   +- - - - -+  |                +------+
                                    |  +---------+       ^
                                    |  | Excess  |       |
                                    +->| Traffic |-------+
                                       |  Meter  |   Result
                                       +---------+   Result
                                    |  |  Meter  |       |
                                    |  +---------+       V
         +----------+   +- - - - -+  |                +------+
         |   BA     |   |         |  |                |      |    Marked
Packet =>|Classifier|==>| Dropper |==?===============>|Marker|==> Packet
Stream   |          |   |         |  |                |      |    Stream
         +----------+   +- - - - -+  |                +------+
                                    |  +---------+       ^
                                    |  | Excess  |       |
                                    +->| Traffic |-------+
                                       |  Meter  |   Result

Figure 4: Schematic of PCN-interior-node functionality.


4.2. PCN-Ingress-Node Functions
4.2. PCN入口节点功能

Each ingress link of the PCN-domain is configured with the following functionality:


o Packet classification - determine whether an incoming packet is part of a previously admitted flow by using a filter spec (eg, DSCP, source and destination addresses, port numbers, and protocol).

o 数据包分类-通过使用筛选器规范(例如,DSCP、源和目标地址、端口号和协议)确定传入数据包是否是先前允许的流的一部分。

o Police - police, by dropping any packets received with a DSCP indicating PCN transport that do not belong to an admitted flow. (A prospective PCN-flow that is rejected could be blocked or admitted into a lower-priority behaviour aggregate.) Similarly, police packets that are part of a previously admitted flow, to check that the flow keeps to the agreed rate or flowspec (eg, see [RFC1633] for a microflow and its NSIS equivalent).

o 警察-警察,通过丢弃使用DSCP接收的任何数据包,该数据包指示不属于已接纳流的PCN传输。(被拒绝的预期PCN流可能被阻止或允许进入较低优先级的行为集合。)同样,对先前允许的流的一部分的数据包进行监控,以检查该流是否保持商定的速率或流量规格(例如,有关微流及其NSIS等效物,请参见[RFC1633])。

o PCN-colour - set the DSCP and ECN fields appropriately for the PCN-domain, for example, as in [Moncaster09-1].

o PCN颜色-为PCN域适当设置DSCP和ECN字段,例如,如[Moncaster09-1]中所示。

o Meter - some approaches to flow termination require the PCN-ingress-node to measure the (aggregate) rate of PCN-traffic towards a particular PCN-egress-node.

o 仪表-某些流量终止方法要求PCN入口节点测量特定PCN出口节点的PCN流量(聚合)速率。

The first two are policing functions, needed to make sure that PCN-packets admitted into the PCN-domain belong to a flow that has been admitted and to ensure that the flow keeps to the flowspec agreed (eg, doesn't exceed an agreed maximum rate and is inelastic traffic). Installing the filter spec will typically be done by the signalling protocol, as will re-installing the filter, for example, after a re-route that changes the PCN-ingress-node (see [Briscoe06] for an example using RSVP). PCN-colouring allows the rest of the PCN-domain to recognise PCN-packets.


4.3. PCN-Egress-Node Functions
4.3. 出口节点函数

Each egress link of the PCN-domain is configured with the following functionality:


o Packet classify - determine which PCN-ingress-node a PCN-packet has come from.

o 数据包分类-确定PCN数据包来自哪个PCN入口节点。

o Meter - "measure PCN-traffic" or "monitor PCN-marks".

o 仪表-“测量PCN流量”或“监控PCN标记”。

o PCN-colour - for PCN-packets, set the DSCP and ECN fields to the appropriate values for use outside the PCN-domain.

o PCN颜色-对于PCN数据包,将DSCP和ECN字段设置为适当的值,以便在PCN域之外使用。

The metering functionality, of course, depends on whether it is targeted at admission control or flow termination. Alternatives involve the PCN-egress-node "measuring", as an aggregate (ie, not per flow), all PCN-packets from a particular PCN-ingress-node, or "monitoring" the PCN-traffic and reacting to one (or several) PCN-


marked packets. For PCN-colouring, [Moncaster09-1] specifies that the PCN-egress-node resets the ECN field to 00; other encodings may define different behaviour.


4.4. Admission Control Functions
4.4. 接纳控制功能

As well as the functions covered above, other specific admission control functions need to be performed (others might be possible):


o Make decision about admission - based on the output of the PCN-egress-node's meter function. In the case where it "measures PCN-traffic", the measured traffic on the ingress-egress-aggregate is compared with some reference level. In the case where it "monitors PCN-marks", the decision is based on whether or not one (or several) packets are PCN-marked (eg, the RSVP PATH message). In either case, the admission decision also takes account of policy and application-layer requirements [RFC2753].

o 根据PCN出口节点的仪表功能的输出,做出准入决策。在其“测量PCN流量”的情况下,将入口-出口集合上的测量流量与一些参考水平进行比较。在“监控PCN标记”的情况下,决定基于是否有一个(或多个)数据包被PCN标记(例如,RSVP路径消息)。在这两种情况下,接纳决定还考虑了策略和应用层要求[RFC2753]。

o Communicate decision about admission - signal the decision to the node making the admission control request (which may be outside the PCN-domain) and to the policer (PCN-ingress-node function) for enforcement of the decision.

o 通信关于接纳的决策-将决策发信号给发出接纳控制请求的节点(可能在PCN域之外)和警察(PCN入口节点功能),以强制执行决策。

There are various possibilities for how the functionality could be distributed (we assume the operator will configure which is used):


o The decision is made at the PCN-egress-node and the decision (admit or block) is signalled to the PCN-ingress-node.

o 该决定在PCN出口节点作出,该决定(允许或阻止)通过信号发送给PCN入口节点。

o The decision is recommended by the PCN-egress-node (admit or block), but the decision is definitively made by the PCN-ingress-node. The rationale is that the PCN-egress-node naturally has the necessary information about the amount of PCN-marks on the ingress-egress-aggregate, whereas the PCN-ingress-node is the policy enforcement point [RFC2753] that polices incoming traffic to ensure it is part of an admitted PCN-flow.

o 该决定由PCN出口节点(允许或阻止)推荐,但该决定最终由PCN入口节点做出。基本原理是,PCN出口节点自然具有关于入口-出口聚合上的PCN标记量的必要信息,而PCN入口节点是策略实施点[RFC2753],它对进入的流量进行策略管理,以确保它是允许的PCN流的一部分。

o The decision is made at the PCN-ingress-node, which requires that the PCN-egress-node signals PCN-feedback-information to the PCN-ingress-node. For example, it could signal the current fraction of PCN-traffic that is PCN-marked.

o 该决定在PCN入口节点作出,这要求PCN出口节点向PCN入口节点发送PCN反馈信息信号。例如,它可以向PCN标记的PCN流量的当前部分发送信号。

o The decision is made at a centralised node (see Appendix).

o 决策是在一个集中节点上做出的(见附录)。

Note: Admission control functionality is not performed by normal PCN-interior-nodes.


4.5. Flow Termination Functions
4.5. 流终止函数

As well as the functions covered above, other specific termination control functions need to be performed (others might be possible):


o PCN-meter at PCN-egress-node - similarly to flow admission, there are two types of possibilities: to "measure PCN-traffic" on the ingress-egress-aggregate, or to "monitor PCN-marks" and react to one (or several) PCN-marks.

o PCN出口节点处的PCN仪表-与流量入口类似,有两种可能性:在进出口集合上“测量PCN流量”,或“监控PCN标记”并对一个(或多个)PCN标记作出反应。

o (if required) PCN-meter at PCN-ingress-node - make "measurements of PCN-traffic" being sent towards a particular PCN-egress-node; again, this is done for the ingress-egress-aggregate and not per flow.

o (如果需要)PCN入口节点处的PCN仪表-对发送至特定PCN出口节点的“PCN流量进行测量”;同样,这是针对入口-出口骨料而不是每个流进行的。

o (if required) Communicate PCN-feedback-information to the node that makes the flow termination decision - for example, as in [Briscoe06], communicate the PCN-egress-node's measurements to the PCN-ingress-node.

o (如果需要)将PCN反馈信息传达给做出流量终止决策的节点——例如,如[Briscoe06]中所述,将PCN出口节点的测量结果传达给PCN入口节点。

o Make decision about flow termination - use the information from the PCN-meter(s) to decide which PCN-flow or PCN-flows to terminate. The decision takes account of policy and application-layer requirements [RFC2753].

o 决定流量终止-使用PCN流量计中的信息来决定终止哪个PCN流量或PCN流量。该决策考虑了策略和应用层需求[RFC2753]。

o Communicate decision about flow termination - signal the decision to the node that is able to terminate the flow (which may be outside the PCN-domain) and to the policer (PCN-ingress-node function) for enforcement of the decision.

o 关于流终止的通信决策-将决策发送给能够终止流的节点(可能在PCN域之外)和策略(PCN入口节点功能)以执行决策。

There are various possibilities for how the functionality could be distributed, similar to those discussed above in Section 4.4.


Note: Flow termination functionality is not performed by normal PCN-interior-nodes.


4.6. Addressing
4.6. 寻址

PCN-nodes may need to know the address of other PCN-nodes. Note that PCN-interior-nodes don't need to know the address of other PCN-nodes (except their next-hop neighbours for routing purposes).


At a minimum, the PCN-egress-node needs to know the address of the PCN-ingress-node associated with a flow so that the PCN-ingress-node can be informed of the admission decision (and any flow termination decision) and enforce it through policing. There are various


possibilities for how the PCN-egress-node can do this, ie, associate the received packet to the correct ingress-egress-aggregate. It is not the intention of this document to mandate a particular mechanism.


o The addressing information can be gathered from signalling -- for example, through the regular processing of an RSVP PATH message, as the PCN-ingress-node is the previous RSVP hop (PHOP) ([Lefaucheur06]). Another option is that the PCN-ingress-node could signal its address to the PCN-egress-node.

o 寻址信息可以通过信令收集——例如,通过RSVP路径消息的常规处理,因为PCN入口节点是前一个RSVP跳(PHOP)([Lefaucheur06])。另一种选择是,PCN入口节点可以向PCN出口节点发送其地址信号。

o Always tunnel PCN-traffic across the PCN-domain. Then the PCN-ingress-node's address is simply the source address of the outer packet header. The PCN-ingress-node needs to learn the address of the PCN-egress-node, either by manual configuration or by one of the automated tunnel endpoint discovery mechanisms (such as signalling or probing over the data route, interrogating routing, or using a centralised broker).

o 始终在PCN域中隧道PCN流量。然后,PCN入口节点的地址就是外部数据包报头的源地址。PCN入口节点需要通过手动配置或一种自动隧道端点发现机制(例如通过数据路由发送信号或探测、询问路由或使用集中代理)了解PCN出口节点的地址。

4.7. Tunnelling
4.7. 隧道掘进

Tunnels may originate and/or terminate within a PCN-domain (eg, IP over IP, IP over MPLS). It is important that the PCN-marking of any packet can potentially influence PCN's flow admission control and termination -- it shouldn't matter whether the packet happens to be tunnelled at the PCN-node that PCN-marks the packet, or indeed whether it's decapsulated or encapsulated by a subsequent PCN-node. This suggests that the "uniform conceptual model" described in [RFC2983] should be re-applied in the PCN context. In line with both this and the approach of [RFC4303] and [Briscoe09], the following rule is applied if encapsulation is done within the PCN-domain:

隧道可以在PCN域内发起和/或终止(例如,IP over IP、IP over MPLS)。重要的是,任何数据包的PCN标记都可能影响PCN的流量允许控制和终止——不管该数据包是否恰好在PCN标记该数据包的PCN节点处进行隧道传输,也不管它是否被后续PCN节点解封或封装。这表明[RFC2983]中描述的“统一概念模型”应重新应用于PCN环境中。根据这一点以及[RFC4303]和[Briscoe09]的方法,如果在PCN域内进行封装,则应用以下规则:

o Any PCN-marking is copied into the outer header.

o 任何PCN标记都会复制到外部标题中。

Note: A tunnel will not provide this behaviour if it complies with [RFC3168] tunnelling in either mode, but it will if it complies with [RFC4301] IPsec tunnelling.


Similarly, in line with the "uniform conceptual model" of [RFC2983], with the "full-functionality option" of [RFC3168], and with [RFC4301], the following rule is applied if decapsulation is done within the PCN-domain:


o If the outer header's marking state is more severe, then it is copied onto the inner header.

o 如果外部收割台的标记状态更严重,则会将其复制到内部收割台上。

Note that the order of increasing severity is: not PCN-marked, threshold-marked, and excess-traffic-marked.


An operator may wish to tunnel PCN-traffic from PCN-ingress-nodes to PCN-egress-nodes. The PCN-marks shouldn't be visible outside the PCN-domain, which can be achieved by the PCN-egress-node doing the PCN-colouring function (Section 4.3) after all the other (PCN and tunnelling) functions. The potential reasons for doing such tunnelling are: the PCN-egress-node then automatically knows the address of the relevant PCN-ingress-node for a flow, and, even if ECMP (Equal Cost Multi-Path) is running, all PCN-packets on a particular ingress-egress-aggregate follow the same path (for more on ECMP, see Section 6.4). But such tunnelling also has drawbacks, for example, the additional overhead in terms of bandwidth and processing as well as the cost of setting up a mesh of tunnels between PCN-boundary-nodes (there is an N^2 scaling issue).


Potential issues arise for a "partially PCN-capable tunnel", ie, where only one tunnel endpoint is in the PCN-domain:


1. The tunnel originates outside a PCN-domain and ends inside it. If the packet arrives at the tunnel ingress with the same encoding as used within the PCN-domain to indicate PCN-marking, then this could lead the PCN-egress-node to falsely measure pre-congestion.

1. 隧道起始于PCN域之外,结束于PCN域内部。如果数据包到达隧道入口时使用的编码与PCN域中用于指示PCN标记的编码相同,则这可能导致PCN出口节点错误地测量预拥塞。

2. The tunnel originates inside a PCN-domain and ends outside it. If the packet arrives at the tunnel ingress already PCN-marked, then it will still have the same encoding when it's decapsulated, which could potentially confuse nodes beyond the tunnel egress.

2. 隧道起始于PCN域内部,结束于PCN域外部。如果数据包到达已被PCN标记的隧道入口,那么在解除封装时它仍将具有相同的编码,这可能会混淆隧道出口以外的节点。

In line with the solution for partially capable Diffserv tunnels in [RFC2983], the following rules are applied:


o For case (1), the tunnel egress node clears any PCN-marking on the inner header. This rule is applied before the "copy on decapsulation" rule above.

o 对于情况(1),隧道出口节点清除内部集管上的任何PCN标记。此规则在上述“拆封时复制”规则之前应用。

o For case (2), the tunnel ingress node clears any PCN-marking on the inner header. This rule is applied after the "copy on encapsulation" rule above.

o 对于情况(2),隧道入口节点清除内部收割台上的任何PCN标记。此规则在上面的“封装时复制”规则之后应用。

Note that the above implies that one has to know, or determine, the characteristics of the other end of the tunnel as part of establishing it.


Tunnelling constraints were a major factor in the choice of the baseline encoding. As explained in [Moncaster09-1], with current tunnelling endpoints, only the 11 codepoint of the ECN field survives decapsulation, and hence the baseline encoding only uses the 11 codepoint to indicate PCN-marking. Extended encoding schemes need to

隧道限制是选择基线编码的主要因素。如[Moncaster 09-1]中所述,对于当前隧道端点,只有ECN字段的11个代码点能够在去封装后存活,因此基线编码仅使用11个代码点来指示PCN标记。扩展编码方案需要

explain their interactions with (or assumptions about) tunnelling. A lengthy discussion of all the issues associated with layered encapsulation of congestion notification (for ECN as well as PCN) is in [Briscoe09].


4.8. Fault Handling
4.8. 故障处理

If a PCN-interior-node (or one of its links) fails, then lower-layer protection mechanisms or the regular IP routing protocol will eventually re-route around it. If the new route can carry all the admitted traffic, flows will gracefully continue. If instead this causes early warning of pre-congestion on the new route, then admission control based on Pre-Congestion Notification will ensure that new flows will not be admitted until enough existing flows have departed. Re-routing may result in heavy (pre-)congestion, which will cause the flow termination mechanism to kick in.


If a PCN-boundary-node fails, then we would like the regular QoS signalling protocol to be responsible for taking appropriate action. As an example, [Briscoe09] considers what happens if RSVP is the QoS signalling protocol.


5. Operations and Management
5. 业务和管理

This section considers operations and management issues, under the FCAPS headings: Faults, Configuration, Accounting, Performance, and Security. Provisioning is discussed with performance.


5.1. Fault Operations and Management
5.1. 故障操作和管理

Fault Operations and Management is about preventing faults, telling the management system (or manual operator) that the system has recovered (or not) from a failure, and about maintaining information to aid fault diagnosis.


Admission blocking and, particularly, flow termination mechanisms should rarely be needed in practice. It would be unfortunate if they didn't work after an option had been accidentally disabled. Therefore, it will be necessary to regularly test that the live system works as intended (devising a meaningful test is left as an exercise for the operator).


Section 4 describes how the PCN architecture has been designed to ensure admitted flows continue gracefully after recovering automatically from link or node failures. The need to record and monitor re-routing events affecting signalling is unchanged by the


addition of PCN to a Diffserv domain. Similarly, re-routing events within the PCN-domain will be recorded and monitored just as they would be without PCN.


PCN-marking does make it possible to record "near-misses". For instance, at the PCN-egress-node a "reporting threshold" could be set to monitor how often -- and for how long -- the system comes close to triggering flow blocking without actually doing so. Similarly, bursts of flow termination marking could be recorded even if they are not sufficiently sustained to trigger flow termination. Such statistics could be correlated with per-queue counts of marking volume (Section 5.2) to upgrade resources in danger of causing service degradation or to trigger manual tracing of intermittent incipient errors that would otherwise have gone unnoticed.


Finally, of course, many faults are caused by failings in the management process ("human error"): a wrongly configured address in a node, a wrong address given in a signalling protocol, a wrongly configured parameter in a queueing algorithm, a node set into a different mode from other nodes, and so on. Generally, a clean design with few configurable options ensures this class of faults can be traced more easily and prevented more often. Sound management practice at run-time also helps. For instance, a management system should be used that constrains configuration changes within system rules (eg, preventing an option setting inconsistent with other nodes), configuration options should be recorded in an offline database, and regular automatic consistency checks between live systems and the database should be performed. PCN adds nothing specific to this class of problems.


5.2. Configuration Operations and Management
5.2. 配置操作和管理

Threshold-metering and -marking and excess-traffic-metering and -marking are standardised in [Eardley09]. However, more diversity in PCN-boundary-node behaviours is expected, in order to interface with diverse industry architectures. It may be possible to have different PCN-boundary-node behaviours for different ingress-egress-aggregates within the same PCN-domain.


PCN-metering behaviour is enabled on either the egress or the ingress interfaces of PCN-nodes. A consistent choice must be made across the PCN-domain to ensure that the PCN mechanisms protect all links.


PCN configuration control variables fall into the following categories:


o system options (enabling or disabling behaviours)

o 系统选项(启用或禁用行为)

o parameters (setting levels, addresses, etc.)

o 参数(设置级别、地址等)

One possibility is that all configurable variables sit within an SNMP (Simple Network Management Protocol) management framework [RFC3411], being structured within a defined management information base (MIB) on each node, and being remotely readable and settable via a suitably secure management protocol (such as SNMPv3).


Some configuration options and parameters have to be set once to "globally" control the whole PCN-domain. Where possible, these are identified below. This may affect operational complexity and the chances of interoperability problems between equipment from different vendors.


It may be possible for an operator to configure some PCN-interior-nodes so that they don't run the PCN mechanisms, if it knows that these links will never become (pre-)congested.


5.2.1. System Options
5.2.1. 系统选项

On PCN-interior-nodes there will be very few system options:


o Whether two PCN-markings (threshold-marked and excess-traffic-marked) are enabled or only one. Typically, all nodes throughout a PCN-domain will be configured the same in this respect. However, exceptions could be made. For example, if most PCN-nodes used both markings but some legacy hardware was incapable of running two algorithms, an operator might be willing to configure these legacy nodes solely for excess-traffic-marking to enable flow termination as a back-stop. It would be sensible to place such nodes where they could be provisioned with a greater leeway over expected traffic levels.

o 是否启用两个PCN标记(阈值标记和超额流量标记)或仅启用一个PCN标记。通常,整个PCN域中的所有节点在这方面的配置都相同。但是,也可以有例外。例如,如果大多数PCN节点使用两种标记,但某些传统硬件无法运行两种算法,则运营商可能愿意将这些传统节点单独配置为多余的流量标记,以使流量终止成为一个后挡。明智的做法是将此类节点放置在可以提供比预期流量水平更大的回旋余地的位置。

o In the case where only one PCN-marking is enabled, all nodes must be configured to generate PCN-marks from the same meter (ie, either the threshold meter or the excess-traffic meter).

o 在仅启用一个PCN标记的情况下,必须将所有节点配置为从同一仪表(即阈值仪表或过量流量仪表)生成PCN标记。

PCN-boundary-nodes (ingress and egress) will have more system options:


o Which of admission and flow termination are enabled. If any PCN-interior-node is configured to generate a marking, all PCN-boundary-nodes must be able to interpret that marking (which

o 启用了哪种入口和流量终止。如果任何PCN内部节点配置为生成标记,则所有PCN边界节点必须能够解释该标记(该标记

includes understanding, in a PCN-domain that uses only one type of PCN-marking, whether they are generated by PCN-interior-nodes' threshold meters or their excess-traffic meters). Therefore, all PCN-boundary-nodes must be configured the same in this respect.


o Where flow admission and termination decisions are made: at PCN-ingress-nodes or at PCN-egress-nodes (or at a centralised node, see Appendix). Theoretically, this configuration choice could be negotiated for each pair of PCN-boundary-nodes, but we cannot imagine why such complexity would be required, except perhaps in future inter-domain scenarios.

o 在作出流量接纳和终止决定的地方:在PCN入口节点或PCN出口节点(或在集中节点,见附录)。理论上,可以为每对PCN边界节点协商此配置选择,但我们无法想象为什么需要如此复杂,除非可能在未来的域间场景中。

o How PCN-markings are translated into admission control and flow termination decisions (see Sections 3.1 and 3.2).

o PCN标记如何转化为准入控制和流量终止决策(见第3.1节和第3.2节)。

PCN-egress-nodes will have further system options:


o How the mapping should be established between each packet and its aggregate (eg, by MPLS label and by IP packet filter spec) and how to take account of ECMP.

o 应如何在每个数据包及其聚合(例如,通过MPLS标签和IP数据包过滤器规范)之间建立映射,以及如何考虑ECMP。

o If an equipment vendor provides a choice, there may be options for selecting which smoothing algorithm to use for measurements.

o 如果设备供应商提供选择,则可能存在选择用于测量的平滑算法的选项。

5.2.2. Parameters
5.2.2. 参数

Like any Diffserv domain, every node within a PCN-domain will need to be configured with the DSCP(s) used to identify PCN-packets. On each interior link, the main configuration parameters are the PCN-threshold-rate and PCN-excess-rate. A larger PCN-threshold-rate enables more PCN-traffic to be admitted on a link, hence improving capacity utilisation. A PCN-excess-rate set further above the PCN-threshold-rate allows greater increases in traffic (whether due to natural fluctuations or some unexpected event) before any flows are terminated, ie, minimises the chances of unnecessarily triggering the termination mechanism. For instance, an operator may want to design their network so that it can cope with a failure of any single PCN-node without terminating any flows.


Setting these rates on the first deployment of PCN will be very similar to the traditional process for sizing an admission-controlled network, depending on: the operator's requirements for minimising flow blocking (grade of service), the expected PCN-traffic load on each link and its statistical characteristics (the traffic matrix), contingency for re-routing the PCN-traffic matrix in the event of single or multiple failures, and the expected load from other classes relative to link capacities [Menth09-1]. But, once a domain is in operation, a PCN design goal is to be able to determine growth in


these configured rates much more simply, by monitoring PCN-marking rates from actual rather than expected traffic (see Section 5.4 on Performance and Provisioning).


Operators may also wish to configure a rate greater than the PCN-excess-rate that is the absolute maximum rate that a link allows for PCN-traffic. This may simply be the physical link rate, but some operators may wish to configure a logical limit to prevent starvation of other traffic classes during any brief period after PCN-traffic exceeds the PCN-excess-rate but before flow termination brings it back below this rate.


Threshold-metering requires a threshold token bucket depth to be configured, excess-traffic-metering requires a value for the MTU (maximum size of a PCN-packet on the link), and both require setting a maximum size of their token buckets. It is preferable to have rules that set defaults for these parameters but to then allow operators to change them -- for instance, if average traffic characteristics change over time.


The PCN-egress-node may allow configuration of:


o how it smooths metering of PCN-markings (eg, EWMA parameters)

o 如何平滑PCN标记的计量(例如EWMA参数)

Whichever node makes admission and flow termination decisions will contain algorithms for converting PCN-marking levels into admission or flow termination decisions. These will also require configurable parameters, for instance:


o An admission control algorithm that is based on the fraction of marked packets will at least require a marking threshold setting above which it denies admission to new flows.

o 基于标记分组的分数的接纳控制算法将至少需要标记阈值设置,超过该设置它将拒绝接纳新流。

o Flow termination algorithms will probably require a parameter to delay termination of any flows until it is more certain that an anomalous event is not transient.

o 流终止算法可能需要一个参数来延迟任何流的终止,直到更确定异常事件不是瞬态的。

o A parameter to control the trade-off between how quickly excess flows are terminated and over-termination.

o 一个参数,用于控制超额流量终止和超额终止之间的权衡。

One particular approach [Charny07-2] would require a global parameter to be defined on all PCN-nodes, but would only need one PCN-marking rate to be configured on each link. The global parameter is a scaling factor between admission and termination (the rate of PCN-traffic on a link up to which flows are admitted vs. the rate above which flows are terminated). [Charny07-2] discusses in full the impact of this particular approach on the operation of PCN.


5.3. Accounting Operations and Management
5.3. 会计业务和管理

Accounting is only done at trust boundaries so it is out of scope of this document, which is confined to intra-domain issues. Use of PCN internal to a domain makes no difference to the flow signalling events crossing trust boundaries outside the PCN-domain, which are typically used for accounting.


5.4. Performance and Provisioning Operations and Management
5.4. 性能和资源调配操作与管理

Monitoring of performance factors measurable from *outside* the PCN-domain will be no different with PCN than with any other packet-based, flow admission control system, both at the flow level (blocking probability, etc.) and the packet level (jitter [RFC3393], [Y.1541], loss rate [RFC4656], mean opinion score [P.800], etc.). The difference is that PCN is intentionally designed to indicate *internally* which exact resource(s) are the cause of performance problems and by how much.


Even better, PCN indicates which resources will probably cause problems if they are not upgraded soon. This can be achieved by the management system monitoring the total amount (in bytes) of PCN-marking generated by each queue over a period. Given possible long provisioning lead times, pre-congestion volume is the best metric to reveal whether sufficient persistent demand has occurred to warrant an upgrade because, even before utilisation becomes problematic, the statistical variability of traffic will cause occasional bursts of pre-congestion. This "early warning system" decouples the process of adding customers from the provisioning process. This should cut the time to add a customer when compared against admission control that is provided over native Diffserv [RFC2998] because it saves having to verify the capacity-planning process before adding each customer.


Alternatively, before triggering an upgrade, the long-term pre-congestion volume on each link can be used to balance traffic load across the PCN-domain by adjusting the link weights of the routing system. When an upgrade to a link's configured PCN-rates is required, it may also be necessary to upgrade the physical capacity available to other classes. However, there will usually be sufficient physical capacity for the upgrade to go ahead as a simple configuration change. Alternatively, [Songhurst06] describes an adaptive rather than preconfigured system, where the configured PCN-threshold-rate is replaced with a high and low water mark and the marking algorithm automatically optimises how physical capacity is shared, using the relative loads from PCN and other traffic classes.


All the above processes require just three extra counters associated with each PCN queue: threshold-markings, excess-traffic-markings, and drops. Every time a PCN-packet is marked or dropped, its size in bytes should be added to the appropriate counter. Then the management system can read the counters at any time and subtract a previous reading to establish the incremental volume of each type of (pre-)congestion. Readings should be taken frequently so that anomalous events (eg, re-routes) can be distinguished from regular fluctuating demand, if required.


5.5. Security Operations and Management
5.5. 安保业务和管理

Security Operations and Management is about using secure operational practices as well as being able to track security breaches or near-misses at run-time. PCN adds few specifics to the general good practice required in this field [RFC4778]. The correct functions of the system should be monitored (Section 5.4) in multiple independent ways and correlated to detect possible security breaches. Persistent (pre-)congestion marking should raise an alarm (both on the node doing the marking and on the PCN-egress-node metering it). Similarly, persistently poor external QoS metrics (such as jitter or mean opinion score) should raise an alarm. The following are examples of symptoms that may be the result of innocent faults, rather than attacks; however, until diagnosed, they should be logged and should trigger a security alarm:


o Anomalous patterns of non-conforming incoming signals and packets rejected at the PCN-ingress-nodes (eg, packets already marked PCN-capable or traffic persistently starving token bucket policers).

o PCN入口节点拒绝的不一致传入信号和数据包的异常模式(例如,已标记为支持PCN的数据包或持续缺乏令牌桶策略的流量)。

o PCN-capable packets arriving at a PCN-egress-node with no associated state for mapping them to a valid ingress-egress-aggregate.

o 到达PCN出口节点的支持PCN的数据包,没有用于将其映射到有效入口出口聚合的关联状态。

o A PCN-ingress-node receiving feedback signals that are about the pre-congestion level on a non-existent aggregate or that are inconsistent with other signals (eg, unexpected sequence numbers, inconsistent addressing, conflicting reports of the pre-congestion level, etc.).

o PCN入口节点接收关于不存在的聚合上的预拥塞级别或与其他信号不一致的反馈信号(例如,意外的序列号、不一致的寻址、预拥塞级别的冲突报告等)。

o Pre-congestion marking arriving at a PCN-egress-node with (pre-)congestion markings focused on particular flows, rather than randomly distributed throughout the aggregate.

o 到达PCN出口节点的预拥塞标记(预)拥塞标记集中于特定流,而不是随机分布在整个聚合中。

6. Applicability of PCN
6. PCN的适用性
6.1. Benefits
6.1. 利益

The key benefits of the PCN mechanisms are that they are simple, scalable, and robust, because:


o Per-flow state is only required at the PCN-ingress-nodes ("stateless core"). This is required for policing purposes (to prevent non-admitted PCN-traffic from entering the PCN-domain) and so on. It is not generally required that other network entities are aware of individual flows (although they may be in particular deployment scenarios).

o 每个流状态仅在PCN入口节点(“无状态核心”)处需要。这是出于监管目的(防止未被接纳的PCN流量进入PCN域)等所必需的。通常不要求其他网络实体知道单个流(尽管它们可能在特定部署场景中)。

o Admission control is resilient: with PCN, QoS is decoupled from the routing system. Hence, in general, admitted flows can survive capacity, routing, or topology changes without additional signalling. The PCN-admissible-rate on each link can be chosen to be small enough that admitted traffic can still be carried after a re-routing in most failure cases [Menth09-1]. This is an important feature, as QoS violations in core networks due to link failures are more likely than QoS violations due to increased traffic volume [Iyer03].

o 接纳控制具有弹性:通过PCN,QoS与路由系统解耦。因此,一般来说,允许的流可以在没有额外信令的情况下经受住容量、路由或拓扑变化。每个链路上的PCN容许率可选择为足够小,以便在大多数故障情况下,重新路由后仍能承载允许的流量[Menth09-1]。这是一个重要特性,因为在核心网络中,由于链路故障而导致的QoS违规比由于通信量增加而导致的QoS违规更为可能[Iyer03]。

o The PCN-metering behaviours only operate on the overall PCN-traffic on the link, not per flow.

o PCN计量行为仅对链路上的总体PCN流量起作用,而不是对每个流量起作用。

o The information of these measurements is signalled to the PCN-egress-nodes by the PCN-marks in the packet headers, ie, "in-band". No additional signalling protocol is required for transporting the PCN-marks. Therefore, no secure binding is required between data packets and separate congestion messages.

o 这些测量的信息通过分组报头(即“带内”)中的PCN标记用信号发送给PCN出口节点。传输PCN标记不需要额外的信令协议。因此,数据包和单独的拥塞消息之间不需要安全绑定。

o The PCN-egress-nodes make separate measurements, operating on the aggregate PCN-traffic from each PCN-ingress-node, ie, not per flow. Similarly, signalling by the PCN-egress-node of PCN-feedback-information (which is used for flow admission and termination decisions) is at the granularity of the ingress-egress-aggregate. An alternative approach is that the PCN-egress-nodes monitor the PCN-traffic and signal PCN-feedback-information (which is used for flow admission and termination decisions) at the granularity of one (or a few) PCN-marks.

o PCN出口节点进行单独的测量,对来自每个PCN入口节点的聚合PCN流量(即,不是每个流量)进行操作。类似地,由PCN出口节点发送的PCN反馈信息(用于流接纳和终止决策)的信令处于入口-出口聚合的粒度。另一种方法是,PCN出口节点以一个(或几个)PCN标记的粒度监视PCN流量和信号PCN反馈信息(用于流接纳和终止决策)。

o The admitted PCN-load is controlled dynamically. Therefore, it adapts as the traffic matrix changes. It also adapts if the network topology changes (eg, after a link failure). Hence, an operator can be less conservative when deploying network capacity and less accurate in their prediction of the PCN-traffic matrix.

o 允许的PCN负载是动态控制的。因此,它会随着流量矩阵的变化而调整。如果网络拓扑发生变化(例如,链路故障后),它也会进行调整。因此,运营商在部署网络容量时可能不太保守,对PCN流量矩阵的预测也不太准确。

o The termination mechanism complements admission control. It allows the network to recover from sudden unexpected surges of PCN-traffic on some links, thus restoring QoS to the remaining flows. Such scenarios are expected to be rare but not impossible. They can be caused by large network failures that redirect lots of admitted PCN-traffic to other links or by the malfunction of measurement-based admission control in the presence of admitted flows that send for a while with an atypically low rate and then increase their rates in a correlated way.

o 终止机制补充了准入控制。它允许网络从某些链路上突发的意外PCN流量激增中恢复,从而恢复剩余流量的QoS。这种情况预计很少见,但并非不可能。它们可能是由于大型网络故障导致的,这些故障将大量已接纳的PCN流量重定向到其他链路,或者是由于在已接纳的流量存在的情况下,基于测量的接纳控制发生故障,这些流量以不典型的低速率发送一段时间,然后以相关方式增加其速率。

o Flow termination can also enable an operator to be less conservative when deploying network capacity. It is an alternative to running links at low utilisation in order to protect against link or node failures. This is especially the case with SRLGs (shared risk link groups), which are links that share a resource, such as a fibre, whose failure affects all links in that group [RFC4216]). Fully protecting traffic against a single SRLG failure requires low utilisation (~10%) of the link bandwidth on some links before failure [Charny08].

o 流量终止还可以使运营商在部署网络容量时不那么保守。它是在低利用率下运行链路的替代方案,以防止链路或节点故障。SRLGs(共享风险链路组)尤其如此,SRLGs是共享资源(如光纤)的链路,其故障会影响该组中的所有链路[RFC4216])。针对单个SRLG故障完全保护流量需要在故障发生之前在某些链路上利用率较低(~10%)的链路带宽[Charny08]。

o The PCN-supportable-rate may be set below the maximum rate that PCN-traffic can be transmitted on a link in order to trigger the termination of some PCN-flows before loss (or excessive delay) of PCN-packets occurs, or to keep the maximum PCN-load on a link below a level configured by the operator.

o 可将PCN支持速率设置为低于链路上可传输PCN流量的最大速率,以便在PCN数据包丢失(或过度延迟)之前触发某些PCN流的终止,或将链路上的最大PCN负载保持在操作员配置的水平以下。

o Provisioning of the network is decoupled from the process of adding new customers. By contrast, with the Diffserv architecture [RFC2475], operators rely on subscription-time Service Level Agreements, which statically define the parameters of the traffic that will be accepted from a customer. This way, the operator has to verify that provision is sufficient each time a new customer is added to check that the Service Level Agreement can be fulfilled. A PCN-domain doesn't need such traffic conditioning.

o 网络的供应与添加新客户的过程是分离的。相比之下,在Diffserv体系结构[RFC2475]中,运营商依赖于订阅时间服务级别协议,该协议静态地定义将从客户处接受的流量参数。这样,运营商必须在每次添加新客户时验证供应是否足够,以检查服务级别协议是否能够得到满足。PCN域不需要这样的流量调节。

6.2. Deployment Scenarios
6.2. 部署场景

Operators of networks will want to use the PCN mechanisms in various arrangements depending, for instance, on how they are performing admission control outside the PCN-domain (users after all are concerned about QoS end-to-end), what their particular goals and assumptions are, how many PCN encoding states are available, and so on.


A PCN-domain may have three encoding states (or pedantically, an operator may choose to use up three encoding states for PCN): not PCN-marked, threshold-marked, and excess-traffic-marked. This way, both PCN admission control and flow termination can be supported. As


illustrated in Figure 1, admission control accepts new flows until the PCN-traffic rate on the bottleneck link rises above the PCN-threshold-rate, whilst, if necessary, the flow termination mechanism terminates flows down to the PCN-excess-rate on the bottleneck link.


On the other hand, a PCN-domain may have two encoding states (as in [Moncaster09-1]) (or pedantically, an operator may choose to use up two encoding states for PCN): not PCN-marked and PCN-marked. This way, there are three possibilities, as discussed in the following paragraphs (see also Section 3.3).

另一方面,PCN域可能有两种编码状态(如[Moncaster 09-1]中所述)(或者,操作员可能会选择为PCN使用两种编码状态):未标记PCN和标记PCN。通过这种方式,有三种可能性,如下段所述(另见第3.3节)。

First, an operator could just use PCN's admission control, solving heavy congestion (caused by re-routing) by "just waiting" -- as sessions end, PCN-traffic naturally reduces; meanwhile, the admission control mechanism will prevent admission of new flows that use the affected links. So, the PCN-domain will naturally return to normal operation, but with reduced capacity. The drawback of this approach would be that, until sufficient sessions have ended to relieve the congestion, all PCN-flows as well as lower-priority services will be adversely affected.


Second, an operator could just rely on statically provisioned capacity per PCN-ingress-node (regardless of the PCN-egress-node of a flow) for admission control, as is typical in the hose model of the Diffserv architecture [Kumar01]. Such traffic-conditioning agreements can lead to focused overload: many flows happen to focus on a particular link and then all flows through the congested link fail catastrophically. PCN's flow termination mechanism could then be used to counteract such a problem.


Third, both admission control and flow termination can be triggered from the single type of PCN-marking; the main downside here is that admission control is less accurate [Charny07-2]. This possibility is illustrated in Figure 3.


Within the PCN-domain, there is some flexibility about how the decision-making functionality is distributed. These possibilities are outlined in Section 4.4 and are also discussed elsewhere, such as in [Menth09-2].


The flow admission and termination decisions need to be enforced through per-flow policing by the PCN-ingress-nodes. If there are several PCN-domains on the end-to-end path, then each needs to police at its PCN-ingress-nodes. One exception is if the operator runs both the access network (not a PCN-domain) and the core network (a PCN-domain); per-flow policing could be devolved to the access network


and not be done at the PCN-ingress-node. Note that, to aid readability, the rest of this document assumes that policing is done by the PCN-ingress-nodes.


PCN admission control has to fit with the overall approach to admission control. For instance, [Briscoe06] describes the case where RSVP signalling runs end-to-end. The PCN-domain is a single RSVP hop, ie, only the PCN-boundary-nodes process RSVP messages, with RSVP messages processed on each hop outside the PCN-domain, as in IntServ over Diffserv [RFC2998]. It would also be possible for the RSVP signalling to be originated and/or terminated by proxies, with application-layer signalling between the end user and the proxy (eg, SIP signalling with a home hub). A similar example would use NSIS (Next Steps in Signalling) [RFC3726] instead of RSVP.


It is possible that a user wants its inelastic traffic to use the PCN mechanisms but also react to ECN markings outside the PCN-domain [Sarker08]. Two possible ways to do this are to tunnel all PCN-packets across the PCN-domain, so that the ECN marks are carried transparently across the PCN-domain, or to use an encoding like [Moncaster09-2]. Tunnelling is discussed further in Section 4.7.

用户可能希望其非弹性流量使用PCN机制,但也对PCN域外的ECN标记作出反应[Sarker08]。实现这一点的两种可能方法是通过隧道将所有PCN数据包传输到PCN域,以便ECN标记在PCN域中透明地传输,或者使用类似于[Moncaster 09-2]的编码。第4.7节将进一步讨论隧道开挖。

Some further possible deployment models are outlined in the Appendix.


6.3. Assumptions and Constraints on Scope
6.3. 关于范围的假设和限制

The scope of this document is restricted by the following assumptions:


1. These components are deployed in a single Diffserv domain, within which all PCN-nodes are PCN-enabled and are trusted for truthful PCN-marking and transport.

1. 这些组件部署在单个Diffserv域中,其中所有PCN节点都启用了PCN,并且受信任进行真实的PCN标记和传输。

2. All flows handled by these mechanisms are inelastic and constrained to a known peak rate through policing or shaping.

2. 由这些机制处理的所有流都是非弹性的,并且通过控制或整形限制在已知的峰值速率。

3. The number of PCN-flows across any potential bottleneck link is sufficiently large that stateless, statistical mechanisms can be effective. To put it another way, the aggregate bit rate of PCN-traffic across any potential bottleneck link needs to be sufficiently large, relative to the maximum additional bit rate added by one flow. This is the basic assumption of measurement-based admission control.

3. 通过任何潜在瓶颈链路的PCN流的数量足够大,因此无状态的统计机制是有效的。换句话说,相对于一个流添加的最大附加比特率,任何潜在瓶颈链路上的PCN流量的聚合比特率都需要足够大。这是基于测量的接纳控制的基本假设。

4. PCN-flows may have different precedence, but the applicability of the PCN mechanisms for emergency use (911, GETS (Government Telecommunications Service), WPS (Wireless Priority Service), MLPP (Multilevel Precedence and Premption), etc.) is out of scope.

4. PCN流可能具有不同的优先级,但应急使用(911、GETS(政府电信服务)、WPS(无线优先级服务)、MLPP(多级优先级和预选项)等PCN机制的适用性超出范围。

6.3.1. Assumption 1: Trust and Support of PCN - Controlled Environment
6.3.1. 假设1:PCN控制环境的信任和支持

It is assumed that the PCN-domain is a controlled environment, ie, all the nodes in a PCN-domain run PCN and are trusted. There are several reasons for this assumption:


o The PCN-domain has to be encircled by a ring of PCN-boundary-nodes; otherwise, traffic could enter a PCN-BA without being subject to admission control, which would potentially degrade the QoS of existing PCN-flows.

o PCN域必须被一圈PCN边界节点包围;否则,流量可以进入PCN-BA而不受准入控制,这可能会降低现有PCN流的QoS。

o Similarly, a PCN-boundary-node has to trust that all the PCN-nodes mark PCN-traffic consistently. A node not performing PCN-marking wouldn't be able to send an alert when it suffered pre-congestion, which potentially would lead to too many PCN-flows being admitted (or too few being terminated). Worse, a rogue node could perform various attacks, as discussed in Section 7.

o 类似地,PCN边界节点必须信任所有PCN节点一致地标记PCN流量。未执行PCN标记的节点在遇到预拥塞时将无法发送警报,这可能会导致接纳太多PCN流(或终止太少PCN流)。更糟糕的是,流氓节点可能会执行各种攻击,如第7节所述。

One way of assuring the above two points are in effect is to have the entire PCN-domain run by a single operator. Another way is to have several operators that trust each other in their handling of PCN-traffic.


Note: All PCN-nodes need to be trustworthy. However, if it is known that an interface cannot become pre-congested, then it is not strictly necessary for it to be capable of PCN-marking, but this must be known even in unusual circumstances, eg, after the failure of some links.


6.3.2. Assumption 2: Real-Time Applications
6.3.2. 假设2:实时应用程序

It is assumed that any variation of source bit rate is independent of the level of pre-congestion. We assume that PCN-packets come from real-time applications generating inelastic traffic, ie, sending packets at the rate the codec produces them, regardless of the availability of capacity [RFC4594]. Examples of such real-time applications include voice and video requiring low delay, jitter, and packet loss, the Controlled Load Service [RFC2211], and the Telephony service class [RFC4594]. This assumption is to help focus the effort where it looks like PCN would be most useful, ie, the sorts of


applications where per-flow QoS is a known requirement. In other words, we focus on PCN providing a benefit to inelastic traffic (PCN may or may not provide a benefit to other types of traffic).


As a consequence, it is assumed that PCN-metering and PCN-marking is being applied to traffic scheduled with an expedited forwarding per-hop behaviour [RFC3246] or with a per-hop behaviour with similar characteristics.


6.3.3. Assumption 3: Many Flows and Additional Load
6.3.3. 假设3:大量流量和额外负载

It is assumed that there are many PCN-flows on any bottleneck link in the PCN-domain (or, to put it another way, the aggregate bit rate of PCN-traffic across any potential bottleneck link is sufficiently large, relative to the maximum additional bit rate added by one PCN-flow). Measurement-based admission control assumes that the present is a reasonable prediction of the future: the network conditions are measured at the time of a new flow request, but the actual network performance must be acceptable during the call some time later. One issue is that if there are only a few variable rate flows, then the aggregate traffic level may vary a lot, perhaps enough to cause some packets to get dropped. If there are many flows, then the aggregate traffic level should be statistically smoothed. How many flows is enough depends on a number of factors, such as the variation in each flow's rate, the total rate of PCN-traffic, and the size of the "safety margin" between the traffic level at which we start admission-marking and at which packets are dropped or significantly delayed.


No explicit assumptions are made about how many PCN-flows are in each ingress-egress-aggregate. Performance-evaluation work may clarify whether it is necessary to make any additional assumptions on aggregation at the ingress-egress-aggregate level.


6.3.4. Assumption 4: Emergency Use Out of Scope
6.3.4. 假设4:超出范围的紧急使用

PCN-flows may have different precedence, but the applicability of the PCN mechanisms for emergency use (911, GETS, WPS, MLPP, etc.) is out of scope for this document.


6.4. Challenges
6.4. 挑战

Prior work on PCN and similar mechanisms has led to a number of considerations about PCN's design goals (things PCN should be good at) and some issues that have been hard to solve in a fully satisfactory manner. Taken as a whole, PCN represents a list of


trade-offs (it is unlikely that they can all be 100% achieved) and perhaps a list of evaluation criteria to help an operator (or the IETF) decide between options.


The following are open issues. They are mainly taken from [Briscoe06], which also describes some possible solutions. Note that some may be considered unimportant in general or in specific deployment scenarios, or by some operators.


Note: Potential solutions are out of scope for this document.


o ECMP (Equal Cost Multi-Path) Routing: The level of pre-congestion is measured on a specific ingress-egress-aggregate. However, if the PCN-domain runs ECMP, then traffic on this ingress-egress-aggregate may follow several different paths -- some of the paths could be pre-congested whilst others are not. There are three potential problems:

o ECMP(等成本多路径)路由:在特定的入口-出口聚合上测量预拥塞级别。但是,如果PCN域运行ECMP,则此入口-出口聚合上的流量可能会遵循多条不同的路径——其中一些路径可能会预拥塞,而其他路径则不会。有三个潜在问题:

1. over-admission: a new flow is admitted (because the pre-congestion level measured by the PCN-egress-node is sufficiently diluted by unmarked packets from non-congested paths that a new flow is admitted), but its packets travel through a pre-congested PCN-node.

1. 过度接纳:接纳新的流(因为PCN出口节点测量的预拥塞水平被来自允许新流的非拥塞路径的未标记数据包充分稀释),但其数据包通过预拥塞PCN节点。

2. under-admission: a new flow is blocked (because the pre-congestion level measured by the PCN-egress-node is sufficiently increased by PCN-marked packets from pre-congested paths that a new flow is blocked), but its packets travel along an uncongested path.

2. 在接纳下:新流被阻塞(因为来自新流被阻塞的预拥塞路径的PCN标记的数据包充分增加了PCN出口节点测量的预拥塞水平),但其数据包沿着未阻塞路径移动。

3. ineffective termination: a flow is terminated but its path doesn't travel through the (pre-)congested router(s). Since flow termination is a "last resort", which protects the network should over-admission occur, this problem is probably more important to solve than the other two.

3. 无效终止:流被终止,但其路径不通过(预)拥塞的路由器。由于流终止是一种“最后手段”,它可以在出现过度接入时保护网络,因此解决这个问题可能比解决另外两个问题更重要。

o ECMP and Signalling: It is possible that, in a PCN-domain running ECMP, the signalling packets (eg, RSVP, NSIS) follow a different path than the data packets, which could matter if the signalling packets are used as probes. Whether this is an issue depends on which fields the ECMP algorithm uses; if the ECMP algorithm is restricted to the source and destination IP addresses, then it will not be an issue. ECMP and signalling interactions are a specific instance of a general issue for non-traditional routing combined with resource management along a path [Hancock02].

o ECMP和信令:在运行ECMP的PCN域中,信令包(例如,RSVP、NSIS)可能遵循与数据包不同的路径,这可能与信令包是否用作探测器有关。这是否是一个问题取决于ECMP算法使用的字段;如果ECMP算法仅限于源IP地址和目标IP地址,那么这将不是问题。ECMP和信令交互是非传统路由与沿路径的资源管理相结合的一般问题的具体实例[Hancock02]。

o Tunnelling: There are scenarios where tunnelling makes it difficult to determine the path in the PCN-domain. The problem, its impact, and the potential solutions are similar to those for ECMP.

o 隧道:在某些情况下,隧道使确定PCN域中的路径变得困难。问题、影响和潜在解决方案与ECMP类似。

o Scenarios with only one tunnel endpoint in the PCN-domain: Such scenarios may make it harder for the PCN-egress-node to gather from the signalling messages (eg, RSVP, NSIS) the identity of the PCN-ingress-node.

o PCN域中只有一个隧道端点的场景:此类场景可能使PCN出口节点更难从信令消息(例如,RSVP、NSIS)收集PCN入口节点的标识。

o Bi-Directional Sessions: Many applications have bi-directional sessions -- hence, there are two microflows that should be admitted (or terminated) as a pair -- for instance, a bi-directional voice call only makes sense if microflows in both directions are admitted. However, the PCN mechanisms concern admission and termination of a single flow, and coordination of the decision for both flows is a matter for the signalling protocol and out of scope for PCN. One possible example would use SIP pre-conditions. However, there are others.

o 双向会话:许多应用程序都有双向会话——因此,有两个微流应该成对接纳(或终止)——例如,双向语音呼叫只有在接纳两个方向的微流时才有意义。然而,PCN机制涉及单个流的接纳和终止,两个流的决策协调是信令协议的问题,超出了PCN的范围。一个可能的例子是使用SIP前置条件。然而,还有其他的。

o Global Coordination: PCN makes its admission decision based on PCN-markings on a particular ingress-egress-aggregate. Decisions about flows through a different ingress-egress-aggregate are made independently. However, one can imagine network topologies and traffic matrices where, from a global perspective, it would be better to make a coordinated decision across all the ingress-egress-aggregates for the whole PCN-domain. For example, to block (or even terminate) flows on one ingress-egress-aggregate so that more important flows through a different ingress-egress-aggregate could be admitted. The problem may well be relatively insignificant.

o 全球协调:PCN根据特定进出口集料上的PCN标记做出准入决定。关于通过不同进出集料的流量的决定是独立作出的。然而,可以想象网络拓扑和流量矩阵,从全局的角度来看,最好在整个PCN域的所有入口-出口聚合中做出协调决策。例如,阻止(甚至终止)一个入口-出口聚合上的流,以便可以允许通过不同入口-出口聚合的更重要的流。这个问题很可能相对无关紧要。

o Aggregate Traffic Characteristics: Even when the number of flows is stable, the traffic level through the PCN-domain will vary because the sources vary their traffic rates. PCN works best when there is not too much variability in the total traffic level at a PCN-node's interface (ie, in the aggregate traffic from all sources). Too much variation means that a node may (at one moment) not be doing any PCN-marking and then (at another moment) drop packets because it is overloaded. This makes it hard to tune the admission control scheme to stop admitting new flows at the right time. Therefore, the problem is more likely with fewer, burstier flows.

o 聚合流量特征:即使流量数量稳定,通过PCN域的流量水平也会有所不同,因为源的流量率不同。当PCN节点接口的总流量水平(即来自所有来源的总流量)没有太多变化时,PCN工作得最好。太多的变化意味着节点可能(在某一时刻)没有进行任何PCN标记,然后(在另一时刻)由于过载而丢弃数据包。这使得很难调整接纳控制方案以在正确的时间停止接纳新的流。因此,流量越少、波动越大,问题就越有可能出现。

o Flash crowds and Speed of Reaction: PCN is a measurement-based mechanism and so there is an inherent delay between packet marking by PCN-interior-nodes and any admission control reaction at PCN-boundary-nodes. For example, if a big burst of admission requests

o 闪光人群和反应速度:PCN是一种基于测量的机制,因此在PCN内部节点的数据包标记和PCN边界节点的任何接纳控制反应之间存在固有的延迟。例如,如果大量的入学申请

potentially occurs in a very short space of time (eg, prompted by a televote), they could all get admitted before enough PCN-marks are seen to block new flows. In other words, any additional load offered within the reaction time of the mechanism must not move the PCN-domain directly from a no congestion state to overload. This "vulnerability period" may have an impact at the signalling level, for instance, QoS requests should be rate-limited to bound the number of requests able to arrive within the vulnerability period.


o Silent at Start: After a successful admission request, the source may wait some time before sending data (eg, waiting for the called party to answer). Then the risk is that, in some circumstances, PCN's measurements underestimate what the pre-congestion level will be when the source does start sending data.

o 开始时静音:在成功的接纳请求之后,源可能会等待一段时间再发送数据(例如,等待被叫方应答)。然后,风险在于,在某些情况下,PCN的测量低估了当数据源确实开始发送数据时的拥塞前水平。

7. Security Considerations
7. 安全考虑

Security considerations essentially come from the Trust Assumption Section 6.3.1, ie, that all PCN-nodes are PCN-enabled and are trusted for truthful PCN-metering and PCN-marking. PCN splits functionality between PCN-interior-nodes and PCN-boundary-nodes, and the security considerations are somewhat different for each, mainly because PCN-boundary-nodes are flow-aware and PCN-interior-nodes are not.


o Because PCN-boundary-nodes are flow-aware, they are trusted to use that awareness correctly. The degree of trust required depends on the kinds of decisions they have to make and the kinds of information they need to make them. There is nothing specific to PCN.

o 因为PCN边界节点是流感知的,所以可以信任它们正确使用该感知。所需的信任程度取决于他们必须做出的决策的种类以及做出决策所需的信息的种类。没有任何特定于PCN的内容。

o The PCN-ingress-nodes police packets to ensure a PCN-flow sticks within its agreed limit, and to ensure that only PCN-flows that have been admitted contribute PCN-traffic into the PCN-domain. The policer must drop (or perhaps downgrade to a different DSCP) any PCN-packets received that are outside this remit. This is similar to the existing IntServ behaviour. Between them, the PCN-boundary-nodes must encircle the PCN-domain; otherwise, PCN-packets could enter the PCN-domain without being subject to admission control, which would potentially destroy the QoS of existing flows.

o PCN入口节点对数据包进行监控,以确保PCN流保持在其约定的限制范围内,并确保只有已被接纳的PCN流将PCN流量贡献到PCN域。警察必须删除(或者可能降级到不同的DSCP)收到的任何超出此权限的PCN数据包。这与现有的IntServ行为类似。在它们之间,PCN边界节点必须包围PCN域;否则,PCN数据包可以进入PCN域而不受准入控制,这可能会破坏现有流的QoS。

o 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 DoS or eavesdropping.

o 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DoS或窃听。

o The PCN-boundary-nodes rely on correct PCN-marking by the PCN-interior-nodes. For instance, a rogue 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. More subtly, the rogue PCN-interior-node could perform these attacks selectively on particular flows, or it could PCN-mark the correct fraction overall but carefully choose which flows it marked.

o PCN边界节点依赖于PCN内部节点正确的PCN标记。例如,流氓PCN内部节点可以对所有数据包进行PCN标记,从而不允许任何流。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。更微妙的是,流氓PCN内部节点可以选择性地对特定流执行这些攻击,或者PCN可以标记正确的分数,但要仔细选择标记的流。

o The PCN-boundary-nodes should be able to deal with DoS attacks and state exhaustion attacks based on fast changes in per-flow signalling.

o PCN边界节点应能够基于每流信令的快速变化处理DoS攻击和状态耗尽攻击。

o The signalling between the PCN-boundary-nodes must be protected from attacks. For example, the recipient needs to validate that the message is indeed from the node that claims to have sent it. Possible measures include digest authentication and protection against replay and man-in-the-middle attacks. For the RSVP protocol specifically, hop-by-hop authentication is in [RFC2747], and [Behringer09] may also be useful.

o 必须保护PCN边界节点之间的信令免受攻击。例如,收件人需要验证消息是否确实来自声称已发送消息的节点。可能的措施包括摘要身份验证和防止重播和中间人攻击。具体而言,对于RSVP协议,[RFC2747]中提供了逐跳认证,[Behringer09]也可能有用。

Operational security advice is given in Section 5.5.


8. Conclusions
8. 结论

This document describes a general architecture for flow admission and termination based on pre-congestion information, in order to protect the quality of service of established, inelastic flows within a single Diffserv domain. The main topic is the functional architecture. This document also mentions other topics like the assumptions and open issues associated with the PCN architecture.


9. Acknowledgements
9. 致谢

This document is a revised version of an earlier individual working draft authored by: P. Eardley, J. Babiarz, K. Chan, A. Charny, R. Geib, G. Karagiannis, M. Menth, and T. Tsou. They are therefore contributors to this document.


Thanks to those who have made comments on this document: Lachlan Andrew, Joe Babiarz, Fred Baker, David Black, Steven Blake, Ron Bonica, Scott Bradner, Bob Briscoe, Ross Callon, Jason Canon, Ken Carlberg, Anna Charny, Joachim Charzinski, Andras Csaszar, Francis Dupont, Lars Eggert, Pasi Eronen, Adrian Farrel, Ruediger Geib, Wei Gengyu, Robert Hancock, Fortune Huang, Christian Hublet, Cullen Jennings, Ingemar Johansson, Georgios Karagiannis, Hein Mekkes, Michael Menth, Toby Moncaster, Dimitri Papadimitriou, Dan Romascanu, Daisuke Satoh, Ben Strulo, Tom Taylor, Hannes Tschofenig, Tina Tsou,


David Ward, Lars Westberg, Magnus Westerlund, and Delei Yu. Thanks to Bob Briscoe who extensively revised the Operations and Management section.

David Ward、Lars Westberg、Magnus Westerlund和Delei Yu。感谢鲍勃·布里斯科(Bob Briscoe),他广泛修订了运营和管理部分。

This document is the result of discussions in the PCN WG and forerunner activity in the TSVWG. A number of previous drafts were presented to TSVWG; their authors were: B. Briscoe, P. Eardley, D. Songhurst, F. Le Faucheur, A. Charny, J. Babiarz, K. Chan, S. Dudley, G. Karagiannis, A. Bader, L. Westberg, J. Zhang, V. Liatsos, X-G. Liu, and A. Bhargava.


The admission control mechanism evolved from the work led by Martin Karsten on the Guaranteed Stream Provider developed in the M3I project [Karsten02] [M3I], which in turn was based on the theoretical work of Gibbens and Kelly [Gibbens99].

准入控制机制是由Martin Karsten在M3I项目[Karsten02][M3I]中开发的担保流提供商上领导的工作演变而来的,该项目反过来又基于Gibbens和Kelly[Gibbens99]的理论工作。

10. References
10. 工具书类
10.1. Normative References
10.1. 规范性引用文件

[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。

[RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec, J., Courtney, W., Davari, S., Firoiu, V., and D. Stiliadis, "An Expedited Forwarding PHB (Per-Hop Behavior)", RFC 3246, March 2002.

[RFC3246]Davie,B.,Charny,A.,Bennet,J.,Benson,K.,Le Boudec,J.,Courtney,W.,Davari,S.,Firoiu,V.,和D.Stiliadis,“快速转发PHB(每跳行为)”,RFC 32462002年3月。

10.2. Informative References
10.2. 资料性引用

[RFC1633] Braden, B., Clark, D., and S. Shenker, "Integrated Services in the Internet Architecture: an Overview", RFC 1633, June 1994.


[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification", RFC 2205, September 1997.

[RFC2205]Braden,B.,Zhang,L.,Berson,S.,Herzog,S.,和S.Jamin,“资源预留协议(RSVP)——第1版功能规范”,RFC 22052997年9月。

[RFC2211] Wroclawski, J., "Specification of the Controlled-Load Network Element Service", RFC 2211, September 1997.


[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。

[RFC2747] Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic Authentication", RFC 2747, January 2000.

[RFC2747]Baker,F.,Lindell,B.和M.Talwar,“RSVP加密认证”,RFC 2747,2000年1月。

[RFC2753] Yavatkar, R., Pendarakis, D., and R. Guerin, "A Framework for Policy-based Admission Control", RFC 2753, January 2000.

[RFC2753]Yavatkar,R.,Pendarakis,D.,和R.Guerin,“基于政策的准入控制框架”,RFC 2753,2000年1月。

[RFC2983] Black, D., "Differentiated Services and Tunnels", RFC 2983, October 2000.

[RFC2983]Black,D.,“差异化服务和隧道”,RFC 29832000年10月。

[RFC2998] Bernet, Y., Ford, P., Yavatkar, R., Baker, F., Zhang, L., Speer, M., Braden, R., Davie, B., Wroclawski, J., and E. Felstaine, "A Framework for Integrated Services Operation over Diffserv Networks", RFC 2998, November 2000.

[RFC2998]Bernet,Y.,Ford,P.,Yavatkar,R.,Baker,F.,Zhang,L.,Speer,M.,Braden,R.,Davie,B.,Wroclawski,J.,和E.Felstaine,“区分服务网络上的综合服务运营框架”,RFC 2998,2000年11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。

[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen, P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-Protocol Label Switching (MPLS) Support of Differentiated Services", RFC 3270, May 2002.

[RFC3270]Le Faucheur,F.,Wu,L.,Davie,B.,Davari,S.,Vaananen,P.,Krishnan,R.,Cheval,P.,和J.Heinanen,“区分服务的多协议标签交换(MPLS)支持”,RFC 32702002年5月。

[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation Metric for IP Performance Metrics (IPPM)", RFC 3393, November 2002.

[RFC3393]Demichelis,C.和P.Chimento,“IP性能度量的IP数据包延迟变化度量(IPPM)”,RFC 3393,2002年11月。

[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks", STD 62, RFC 3411, December 2002.

[RFC3411]Harrington,D.,Presohn,R.,和B.Wijnen,“描述简单网络管理协议(SNMP)管理框架的体系结构”,STD 62,RFC 3411,2002年12月。

[RFC3726] Brunner, M., "Requirements for Signaling Protocols", RFC 3726, April 2004.

[RFC3726]Brunner,M.,“信令协议的要求”,RFC 3726,2004年4月。

[RFC4216] Zhang, R. and J. Vasseur, "MPLS Inter-Autonomous System (AS) Traffic Engineering (TE) Requirements", RFC 4216, November 2005.

[RFC4216]Zhang,R.和J.Vasseur,“MPLS自治系统间(AS)流量工程(TE)要求”,RFC 42162005年11月。

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005.

[RFC4301]Kent,S.和K.Seo,“互联网协议的安全架构”,RFC 43012005年12月。

[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005.

[RFC4303]Kent,S.,“IP封装安全有效载荷(ESP)”,RFC 4303,2005年12月。

[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration Guidelines for DiffServ Service Classes", RFC 4594, August 2006.

[RFC4594]Babiarz,J.,Chan,K.,和F.Baker,“区分服务服务类的配置指南”,RFC 45942006年8月。

[RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M. Zekauskas, "A One-way Active Measurement Protocol (OWAMP)", RFC 4656, September 2006.

[RFC4656]Shalunov,S.,Teitelbaum,B.,Karp,A.,Boote,J.,和M.Zekauskas,“单向主动测量协议(OWAMP)”,RFC 46562006年9月。

[RFC4774] Floyd, S., "Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field", BCP 124, RFC 4774, November 2006.

[RFC4774]Floyd,S.,“为显式拥塞通知(ECN)字段指定替代语义”,BCP 124,RFC 4774,2006年11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。

[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion Marking in MPLS", RFC 5129, January 2008.

[RFC5129]Davie,B.,Briscoe,B.,和J.Tay,“MPLS中的显式拥塞标记”,RFC 5129,2008年1月。

[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic Class" Field", RFC 5462, February 2009.

[RFC5462]Andersson,L.和R.Asati,“多协议标签交换(MPLS)标签堆栈条目:“EXP”字段重命名为“流量类”字段”,RFC 5462,2009年2月。

[P.800] "Methods for subjective determination of transmission quality", ITU-T Recommendation P.800, August 1996.


[Y.1541] "Network Performance Objectives for IP-based Services", ITU-T Recommendation Y.1541, February 2006.


[Babiarz06] Babiarz, J., Chan, K., Karagiannis, G., and P. Eardley, "SIP Controlled Admission and Preemption", Work in Progress, October 2006.


[Behringer09] Behringer, M. and F. Le Faucheur, "Applicability of Keying Methods for RSVP Security", Work in Progress, March 2009.

[Behringer09]Behringer,M.和F.Le Faucheur,“RSVP安全的键控方法的适用性”,正在进行的工作,2009年3月。

[Briscoe06] 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]Briscoe,B.,Eardley,P.,Songhurst,D.,Le Faucheur,F.,Charny,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。

[Briscoe08] Briscoe, B., "Emulating Border Flow Policing using Re-PCN on Bulk Data", Work in Progress, September 2008.

[Briscoe08]Briscoe,B.,“在批量数据上使用Re PCN模拟边境流量监管”,正在进行的工作,2008年9月。

[Briscoe09] Briscoe, B., "Tunnelling of Explicit Congestion Notification", Work in Progress, March 2009.


[Bryant08] Bryant, S., Davie, B., Martini, L., and E. Rosen, "Pseudowire Congestion Control Framework", Work in Progress, May 2008.


[Charny07-1] Charny, A., Babiarz, J., Menth, M., and X. Zhang, "Comparison of Proposed PCN Approaches", Work in Progress, November 2007.


[Charny07-2] Charny, A., Zhang, X., Le Faucheur, F., and V. Liatsos, "Pre-Congestion Notification Using Single Marking for Admission and Termination", Work in Progress, November 2007.

[Charny07-2]Charny,A.,Zhang,X.,Le Faucheur,F.,和V.Liatsos,“使用单标记进入和终止的拥堵前通知”,在建工程,2007年11月。

[Charny07-3] Charny, A., "Email to PCN WG mailing list", November 2007, < web/pcn/current/msg00871.html>.

[Charny07-3]Charny,A.,“发送至PCN工作组邮件列表的电子邮件”,2007年11月< web/pcn/current/msg00871.html>。

[Charny08] Charny, A., "Email to PCN WG mailing list", March 2008, < pcn/current/msg01359.html>.

[Charny08]Charny,A.,“发送至PCN工作组邮件列表的电子邮件”,2008年3月< pcn/current/msg01359.html>。

[Eardley07] Eardley, P., "Email to PCN WG mailing list", October 2007, < web/pcn/current/msg00831.html>.

[Eardley 07]Eardley,P.,“发送至PCN工作组邮件列表的电子邮件”,2007年10月< web/pcn/current/msg00831.html>。

[Eardley09] Eardley, P., "Metering and marking behaviour of PCN-nodes", Work in Progress, May 2009.

[Eardley 09]Eardley,P.,“PCN节点的计量和标记行为”,正在进行的工作,2009年5月。

[Gibbens99] Gibbens, R. and F. Kelly, "Distributed connection acceptance control for a connectionless network", Proceedings International Teletraffic Congress (ITC16), Edinburgh, pp. 941-952, 1999.


[Hancock02] Hancock, R. and E. Hepworth, "Slide 14 of 'NSIS: An Outline Framework for QoS Signalling'", May 2002, <h ttp:// nsis-framework-outline.ppt>.

[Hancock02]Hancock,R.和E.Hepworth,“NSIS:QoS信令框架概要”幻灯片14,2002年5月,< nsis框架大纲.ppt>。

[Iyer03] Iyer, S., Bhattacharyya, S., Taft, N., and C. Diot, "An approach to alleviate link overload as observed on an IP backbone", IEEE INFOCOM, 2003, <>.

[Iyer03]Iyer,S.,Bhattacharyya,S.,Taft,N.,和C.Diot,“一种减轻IP主干上的链路过载的方法”,IEEE INFOCOM,2003<>.

[Karsten02] Karsten, M. and J. Schmitt, "Admission Control Based on Packet Marking and Feedback Signalling -- Mechanisms, Implementation and Experiments", TU-Darmstadt Technical Report TR-KOM-2002-03, May 2002, < publications/abstracts/KS02-5.html>.

[Karsten02]Karsten,M.和J.Schmitt,“基于数据包标记和反馈信令的接纳控制——机制、实施和实验”,TU Darmstadt技术报告TR-KOM-2002-03,2002年5月< 出版物/摘要/KS02-5.html>。

[Kumar01] Kumar, A., Rastogi, R., Silberschatz, A., and B. Yener, "Algorithms for Provisioning Virtual Private Networks in the Hose Model", Proceedings ACM SIGCOMM (ITC16), , 2001.

[Kumar01]Kumar,A.,Rastogi,R.,Silberschatz,A.,和B.Yener,“软管模型中配置虚拟专用网络的算法”,ACM SIGCOMM论文集(ITC16),2001年。

[Lefaucheur06] Le Faucheur, F., Charny, A., Briscoe, B., Eardley, P., Babiarz, J., and K. Chan, "RSVP Extensions for Admission Control over Diffserv using Pre-congestion Notification (PCN)", Work in Progress, June 2006.

[Lefaucheur06]Le Faucheur,F.,Charny,A.,Briscoe,B.,Eardley,P.,Babiarz,J.,和K.Chan,“使用拥塞前通知(PCN)对区分服务进行准入控制的RSVP扩展”,正在进行的工作,2006年6月。

[M3I] "M3I - Market Managed Multiservice Internet", <>.


[Menth08-1] Menth, M., Lehrieder, F., Eardley, P., Charny, A., and J. Babiarz, "Edge-Assisted Marked Flow Termination", Work in Progress, February 2008.


[Menth08-2] Menth, M., Babiarz, J., Moncaster, T., and B. Briscoe, "PCN Encoding for Packet-Specific Dual Marking (PSDM)", Work in Progress, July 2008.


[Menth09-1] Menth, M. and M. Hartmann, "Threshold Configuration and Routing Optimization for PCN-Based Resilient Admission Control", Computer Networks, 2009, <>.


[Menth09-2] 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, <http:// Publications/papers/Menth08-PCN-Overview.pdf>>.

[Menth09-2]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,< PCN Overview.pdf>>。

[Moncaster09-1] Moncaster, T., Briscoe, B., and M. Menth, "Baseline Encoding and Transport of Pre-Congestion Information", Work in Progress, May 2009.

[Moncaster 09-1]Moncaster,T.,Briscoe,B.,和M.Minth,“拥堵前信息的基线编码和传输”,正在进行的工作,2009年5月。

[Moncaster09-2] Moncaster, T., Briscoe, B., and M. Menth, "A PCN encoding using 2 DSCPs to provide 3 or more states", Work in Progress, April 2009.

[Moncaster 09-2]Moncaster,T.,Briscoe,B.,和M.Minth,“使用2个DSCP提供3个或更多状态的PCN编码”,正在进行的工作,2009年4月。

[Sarker08] Sarker, Z. and I. Johansson, "Usecases and Benefits of end to end ECN support in PCN Domains", Work in Progress, November 2008.


[Songhurst06] Songhurst, DJ., Eardley, P., Briscoe, B., Di Cairano Gilfedder, C., and J. Tay, "Guaranteed QoS Synthesis for Admission Control with Shared Capacity", BT Technical Report TR-CXR9-2006-001, Feburary 2006, < B.Briscoe/projects/ipe2eqos/gqs/papers/ GQS_shared_tr.pdf>.

[Songhurst 06]Songhurst,DJ.,Eardley,P.,Briscoe,B.,Di Cairano Gilfedder,C.,和J.Tay,“共享容量接入控制的保证QoS综合”,BT技术报告TR-CXR9-2006-001,2006年2月< B.Briscoe/projects/ipe2eqos/gqs/papers/gqs\u shared\u tr.pdf>。

[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.


[Tsou08] Tsou, T., Huang, F., and T. Taylor, "Applicability Statement for the Use of Pre-Congestion Notification in a Resource-Controlled Network", Work in Progress, November 2008.


[Westberg08] Westberg, L., Bhargava, A., Bader, A., Karagiannis, G., and H. Mekkes, "LC-PCN: The Load Control PCN Solution", Work in Progress, November 2008.


Appendix A. Possible Future Work Items

This section mentions some topics that are outside the PCN WG's current charter but that have been mentioned as areas of interest. They might be work items for the PCN WG after a future re-chartering, some other IETF WG, another standards body, or an operator-specific usage that is not standardised.


Note: It should be crystal clear that this section discusses possibilities only.


The first set of possibilities relate to the restrictions described in Section 6.3:


o A single PCN-domain encompasses several autonomous systems that do not trust each other. A possible solution is a mechanism like re-PCN [Briscoe08].

o 单个PCN域包含多个相互不信任的自治系统。一种可能的解决方案是一种类似re PCN的机制[Briscoe08]。

o Not all the nodes run PCN. For example, the PCN-domain is a multi-site enterprise network. The sites are connected by a VPN tunnel; although PCN doesn't operate inside the tunnel, the PCN mechanisms still work properly because of the good QoS on the virtual link (the tunnel). Another example is that PCN is deployed on the general Internet (ie, widely but not universally deployed).

o 并非所有节点都运行PCN。例如,PCN域是一个多站点企业网络。这些站点通过VPN隧道连接;尽管PCN不在隧道内运行,但由于虚拟链路(隧道)具有良好的QoS,PCN机制仍能正常工作。另一个例子是,PCN部署在通用Internet上(即,广泛但不普遍部署)。

o Applying the PCN mechanisms to other types of traffic, ie, beyond inelastic traffic -- for instance, applying the PCN mechanisms to traffic scheduled with the Assured Forwarding per-hop behaviour. One example could be flow-rate adaptation by elastic applications that adapt according to the pre-congestion information.

o 将PCN机制应用于其他类型的流量,即非弹性流量之外的流量——例如,将PCN机制应用于具有保证每跳转发行为的调度流量。一个例子是通过弹性应用程序的流量自适应,该应用程序根据拥塞前信息进行自适应。

o The aggregation assumption doesn't hold, because the link capacity is too low. Measurement-based admission control is less accurate, with a greater risk of over-admission for instance.

o 聚合假设不成立,因为链路容量太低。基于测量的准入控制不太准确,例如,过度准入的风险更大。

o The applicability of PCN mechanisms for emergency use (911, GETS, WPS, MLPP, etc.).

o 应急使用PCN机制(911、GETS、WPS、MLPP等)的适用性。

Other possibilities include:


o Probing. This is discussed in Appendix A.1 below.

o 探测。下文附录A.1对此进行了讨论。

o The PCN-domain extends to the end users. This scenario is described in [Babiarz06]. The end users need to be trusted to do their own policing. If there is sufficient traffic, then the aggregation assumption may hold. A variant is that the PCN-domain extends out as far as the LAN edge switch.

o PCN域扩展到最终用户。[Babiarz06]中描述了这种情况。需要信任最终用户来执行他们自己的策略。如果存在足够的通信量,则聚合假设可能成立。一种变体是PCN域延伸到LAN边缘交换机。

o Indicating pre-congestion through signalling messages rather than in-band (in the form of PCN-marked packets).

o 通过信令消息而不是带内(以PCN标记的数据包的形式)指示预拥塞。

o The decision-making functionality is at a centralised node rather than at the PCN-boundary-nodes. This requires that the PCN-egress-node signals PCN-feedback-information to the centralised node, and that the centralised node signals to the PCN-ingress-node the decision about admission (or termination). Such possibility may need the centralised node and the PCN-boundary-nodes to be configured with each other's addresses. The centralised case is described further in [Tsou08].

o 决策功能位于集中节点,而不是PCN边界节点。这要求PCN出口节点向集中式节点发送PCN反馈信息的信号,集中式节点向PCN入口节点发送关于接纳(或终止)的决定的信号。这种可能性可能需要使用彼此的地址配置集中节点和PCN边界节点。[Tsou08]中进一步描述了集中案例。

o Signalling extensions for specific protocols (eg, RSVP and NSIS) -- for example, the details of how the signalling protocol installs the flowspec at the PCN-ingress-node for an admitted PCN-flow, and how the signalling protocol carries the PCN-feedback-information. Perhaps also for other functions such as for coping with failure of a PCN-boundary-node ([Briscoe06] considers what happens if RSVP is the QoS signalling protocol) and for establishing a tunnel across the PCN-domain if it is necessary to carry ECN marks transparently.

o 特定协议(例如,RSVP和NSIS)的信令扩展——例如,信令协议如何在PCN入口节点为已接纳的PCN流安装flowspec的详细信息,以及信令协议如何携带PCN反馈信息。可能也适用于其他功能,如处理PCN边界节点的故障([Briscoe06]考虑如果RSVP是QoS信令协议会发生什么),以及在需要透明地携带ECN标记时在PCN域上建立隧道。

o Policing by the PCN-ingress-node may not be needed if the PCN-domain can trust that the upstream network has already policed the traffic on its behalf.

o 如果PCN域可以相信上游网络已经代表其对流量进行了监控,则可能不需要PCN入口节点进行监控。

o PCN for Pseudowire. PCN may be used as a congestion avoidance mechanism for edge-to-edge pseudowire emulations [Bryant08].

o 假丝的PCN。PCN可用作边到边伪线仿真的拥塞避免机制[Bryant08]。

o PCN for MPLS. [RFC3270] defines how to support the Diffserv architecture in MPLS (Multiprotocol Label Switching) networks. [RFC5129] describes how to add PCN for admission control of microflows into a set of MPLS aggregates. PCN-marking is done in MPLS's EXP field (which [RFC5462] re-names the Class of Service (CoS) field).

o MPLS的PCN。[RFC3270]定义了如何在MPLS(多协议标签交换)网络中支持区分服务体系结构。[RFC5129]描述了如何将用于微流准入控制的PCN添加到一组MPLS聚合中。PCN标记在MPLS的EXP字段中完成(该字段[RFC5462]重新命名服务类别(CoS)字段)。

o PCN for Ethernet. Similarly, it may be possible to extend PCN into Ethernet networks, where PCN-marking is done in the Ethernet header. Note: Specific consideration of this extension is outside of the IETF's remit.

o 以太网的PCN。类似地,也可以将PCN扩展到以太网网络,其中PCN标记在以太网报头中完成。注:此扩展的具体考虑不在IETF的职权范围内。

A.1. Probing
A.1. 探查
A.1.1. Introduction
A.1.1. 介绍

Probing is a potential mechanism to assist admission control.


PCN's admission control, as described so far, is essentially a reactive mechanism where the PCN-egress-node monitors the pre-congestion level for traffic from each PCN-ingress-node; if the level rises, then it blocks new flows on that ingress-egress-aggregate. However, it's possible that an ingress-egress-aggregate carries no traffic, and so the PCN-egress-node can't make an admission decision using the usual method described earlier.


One approach is to be "optimistic" and simply admit the new flow. However, it's possible to envisage a scenario where the traffic levels on other ingress-egress-aggregates are already so high that they're blocking new PCN-flows, and admitting a new flow onto this "empty" ingress-egress-aggregate adds extra traffic onto a link that is already pre-congested. This may 'tip the balance' so that PCN's flow termination mechanism is activated or some packets are dropped. This risk could be lessened by configuring, on each link, a sufficient 'safety margin' above the PCN-threshold-rate.


An alternative approach is to make PCN a more proactive mechanism. The PCN-ingress-node explicitly determines, before admitting the prospective new flow, whether the ingress-egress-aggregate can support it. This can be seen as a "pessimistic" approach, in contrast to the "optimism" of the approach above. It involves probing: a PCN-ingress-node generates and sends probe packets in order to test the pre-congestion level that the flow would experience.


One possibility is that a probe packet is just a dummy data packet, generated by the PCN-ingress-node and addressed to the PCN-egress-node.


A.1.2. Probing Functions
A.1.2. 探测功能

The probing functions are:


o Make the decision that probing is needed. As described above, this is when the ingress-egress-aggregate (or the ECMP path -- see Section 6.4) carries no PCN-traffic. An alternative is to always probe, ie, probe before admitting any PCN-flow.

o 做出需要探测的决定。如上所述,这是当入口-出口聚合(或ECMP路径——参见第6.4节)不承载PCN流量时。另一种方法是始终进行探测,即在允许任何PCN流之前进行探测。

o (if required) Communicate the request that probing is needed; the PCN-egress-node signals to the PCN-ingress-node that probing is needed.

o (如果需要)传达需要探测的请求;PCN出口节点向PCN入口节点发出需要探测的信号。

o (if required) Generate probe traffic; the PCN-ingress-node generates the probe traffic. The appropriate number (or rate) of probe packets will depend on the PCN-metering algorithm; for example, an excess-traffic-metering algorithm triggers fewer PCN-marks than a threshold-metering algorithm, and so will need more probe packets.

o (如果需要)生成探测流量;PCN入口节点生成探测流量。探测包的适当数量(或速率)将取决于PCN计量算法;例如,过量流量计量算法触发的PCN标记少于阈值计量算法,因此需要更多的探测数据包。

o Forward probe packets; as far as PCN-interior-nodes are concerned, probe packets are handled the same as (ordinary data) PCN-packets in terms of routing, scheduling, and PCN-marking.

o 转发探测包;就PCN内部节点而言,探测数据包在路由、调度和PCN标记方面的处理与(普通数据)PCN数据包相同。

o Consume probe packets; the PCN-egress-node consumes probe packets to ensure that they don't travel beyond the PCN-domain.

o 使用探测包;PCN出口节点使用探测数据包以确保它们不会超出PCN域。

A.1.3. Discussion of Rationale for Probing, Its Downsides and Open Issues

A.1.3. 讨论调查的基本原理、缺点和未决问题

It is an unresolved question whether probing is really needed, but two viewpoints have been put forward as to why it is useful. The first is perhaps the most obvious: there is no PCN-traffic on the ingress-egress-aggregate. The second assumes that multipath routing (eg, ECMP) is running in the PCN-domain. We now consider each in turn.


The first viewpoint assumes the following:


o There is no PCN-traffic on the ingress-egress-aggregate (so a normal admission decision cannot be made).

o 入口-出口聚合上没有PCN流量(因此无法做出正常的准入决定)。

o Simply admitting the new flow has a significant risk of leading to overload: packets dropped or flows terminated.

o 简单地承认新的流有导致过载的重大风险:数据包丢失或流终止。

On the former bullet, [Eardley07] suggests that, during the future busy hour of a national network with about 100 PCN-boundary-nodes, there are likely to be significant numbers of aggregates with very few flows under nearly all circumstances.


The latter bullet could occur if new flows start on many of the empty ingress-egress-aggregates, which together overload a link in the PCN-domain. To be a problem, this would probably have to happen in a short time period (flash crowd) because, after the reaction time of the system, other (non-empty) ingress-egress-aggregates that pass through the link will measure pre-congestion and so block new flows. Also, flows naturally end anyway.


The downsides of probing for this viewpoint are:


o Probing adds delay to the admission control process.

o 探测增加了准入控制过程的延迟。

o Sufficient probing traffic has to be generated to test the pre-congestion level of the ingress-egress-aggregate. But the probing traffic itself may cause pre-congestion, causing other PCN-flows to be blocked or even terminated -- and, in the flash crowd scenario, there will be probing on many ingress-egress-aggregates.

o 必须生成足够的探测流量,以测试入口-出口聚合的预拥塞水平。但是探测流量本身可能会导致预拥塞,导致其他PCN流被阻塞甚至终止——并且,在flash群组场景中,将对许多入口-出口聚合进行探测。

The second viewpoint applies in the case where there is multipath routing (eg, ECMP) in the PCN-domain. Note that ECMP is often used on core networks. There are two possibilities:


(1) If admission control is based on measurements of the ingress-egress-aggregate, then the viewpoint that probing is useful assumes:

(1) 如果准入控制基于入口-出口聚合的测量,那么探测有用的观点假设:

* There's a significant chance that the traffic is unevenly balanced across the ECMP paths and, hence, there's a significant risk of admitting a flow that should be blocked (because it follows an ECMP path that is pre-congested) or of blocking a flow that should be admitted.

* ECMP路径上的流量很有可能不均衡,因此,存在接纳应阻止的流量(因为该流量遵循预拥挤的ECMP路径)或阻塞应接纳的流量的重大风险。

Note: [Charny07-3] suggests unbalanced traffic is quite possible, even with quite a large number of flows on a PCN-link (eg, 1000), when Assumption 3 (aggregation) is likely to be satisfied.


(2) If admission control is based on measurements of pre-congestion on specific ECMP paths, then the viewpoint that probing is useful assumes:

(2) 如果准入控制基于特定ECMP路径上的预拥塞测量,那么探测有用的观点假设:

* There is no PCN-traffic on the ECMP path on which to base an admission decision.

* 在ECMP路径上没有PCN通信量作为准入决定的基础。

* Simply admitting the new flow has a significant risk of leading to overload.

* 简单地承认新流量有导致过载的重大风险。

* The PCN-egress-node can match a packet to an ECMP path.

* PCN出口节点可以将数据包与ECMP路径匹配。

Note: This is similar to the first viewpoint and so, similarly, could occur in a flash crowd if a new flow starts more or less simultaneously on many of the empty ECMP paths. Because there are several ECMP paths between each pair of PCN-boundary-nodes, it's presumably more likely that an ECMP path is "empty" than an ingress-egress-aggregate is. To constrain the number of ECMP paths, a few tunnels could be set up between each pair of PCN-


boundary-nodes. Tunnelling also solves the issue in the point immediately above (which is otherwise hard to solve because an ECMP routing decision is made independently on each node).


The downsides of probing for this viewpoint are:


o Probing adds delay to the admission control process.

o 探测增加了准入控制过程的延迟。

o Sufficient probing traffic has to be generated to test the pre-congestion level of the ECMP path. But there's the risk that the probing traffic itself may cause pre-congestion, causing other PCN-flows to be blocked or even terminated.

o 必须生成足够的探测流量,以测试ECMP路径的预拥塞水平。但是,探测流量本身也有可能导致预拥塞,导致其他PCN流被阻塞甚至终止。

o The PCN-egress-node needs to consume the probe packets to ensure they don't travel beyond the PCN-domain, since they might confuse the destination end node. This is non-trivial, since probe packets are addressed to the destination end node in order to test the relevant ECMP path (ie, they are not addressed to the PCN-egress-node, unlike the first viewpoint above).

o PCN出口节点需要使用探测数据包,以确保它们不会超出PCN域,因为它们可能会混淆目标端节点。这是非常重要的,因为探测包被寻址到目的地端节点以测试相关的ECMP路径(即,与上面的第一个观点不同,它们不被寻址到PCN出口节点)。

The open issues associated with these viewpoints include:


o What rate and pattern of probe packets does the PCN-ingress-node need to generate so that there's enough traffic to make the admission decision?

o PCN入口节点需要生成多少速率和模式的探测数据包,以便有足够的流量来做出接纳决策?

o What difficulty does the delay (whilst probing is done), and possible packet drops, cause applications?

o 延迟(进行探测时)和可能的数据包丢失会给应用程序带来什么困难?

o Can the delay be alleviated by automatically and periodically probing on the ingress-egress-aggregate? Or does this add too much overhead?

o 是否可以通过自动和周期性地探测进出口聚合来缓解延迟?还是这会增加太多的开销?

o Are there other ways of dealing with the flash crowd scenario? For instance, by limiting the rate at which new flows are admitted, or perhaps by a PCN-egress-node blocking new flows on its empty ingress-egress-aggregates when its non-empty ones are pre-congested.

o 有没有其他方法来处理闪电人群场景?例如,通过限制新流被接纳的速率,或者可能通过PCN出口节点在其空入口-出口聚合上阻塞新流,当其非空入口-出口聚合预先拥塞时。

o (Second viewpoint only) How does the PCN-egress-node disambiguate probe packets from data packets (so it can consume the former)? The PCN-egress-node must match the characteristic setting of particular bits in the probe packet's header or body, but these bits must not be used by any PCN-interior-node's ECMP algorithm. In the general case, this isn't possible, but it should be possible for a typical ECMP algorithm (which examines the source and destination IP addresses and port numbers, the protocol ID, and the DSCP).

o (仅第二种观点)PCN出口节点如何消除探测数据包与数据包之间的歧义(以便使用前者)?PCN出口节点必须匹配探测包头或正文中特定位的特征设置,但这些位不得由任何PCN内部节点的ECMP算法使用。在一般情况下,这是不可能的,但对于典型的ECMP算法(它检查源和目标IP地址和端口号、协议ID和DSCP)应该是可能的。

Author's Address


Philip Eardley (editor) BT B54/77, Sirius House Adastral Park Martlesham Heath Ipswich, Suffolk IP5 3RE United Kingdom

Philip Eardley(编辑)英国电信B54/77,英国萨福克郡马特勒沙姆·希思·伊普斯维奇市天狼星之家Adastral公园IP5