Network Working Group                                           E. Rosen
Request for Comments: 3031                           Cisco Systems, Inc.
Category: Standards Track                                 A. Viswanathan
                                                  Force10 Networks, Inc.
                                                               R. Callon
                                                  Juniper Networks, Inc.
                                                            January 2001
        
Network Working Group                                           E. Rosen
Request for Comments: 3031                           Cisco Systems, Inc.
Category: Standards Track                                 A. Viswanathan
                                                  Force10 Networks, Inc.
                                                               R. Callon
                                                  Juniper Networks, Inc.
                                                            January 2001
        

Multiprotocol Label Switching Architecture

多协议标签交换体系结构

Status of this Memo

本备忘录的状况

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

本文件规定了互联网社区的互联网标准跟踪协议,并要求进行讨论和提出改进建议。有关本协议的标准化状态和状态,请参考当前版本的“互联网官方协议标准”(STD 1)。本备忘录的分发不受限制。

Copyright Notice

版权公告

Copyright (C) The Internet Society (2001). All Rights Reserved.

版权所有(C)互联网协会(2001年)。版权所有。

Abstract

摘要

This document specifies the architecture for Multiprotocol Label Switching (MPLS).

本文件规定了多协议标签交换(MPLS)的体系结构。

Table of Contents

目录

   1          Specification  ......................................   3
   2          Introduction to MPLS  ...............................   3
   2.1        Overview  ...........................................   4
   2.2        Terminology  ........................................   6
   2.3        Acronyms and Abbreviations  .........................   9
   2.4        Acknowledgments  ....................................   9
   3          MPLS Basics  ........................................   9
   3.1        Labels  .............................................   9
   3.2        Upstream and Downstream LSRs  .......................  10
   3.3        Labeled Packet  .....................................  11
   3.4        Label Assignment and Distribution  ..................  11
   3.5        Attributes of a Label Binding  ......................  11
   3.6        Label Distribution Protocols  .......................  11
   3.7        Unsolicited Downstream vs. Downstream-on-Demand  ....  12
   3.8        Label Retention Mode  ...............................  12
   3.9        The Label Stack  ....................................  13
   3.10       The Next Hop Label Forwarding Entry (NHLFE)  ........  13
   3.11       Incoming Label Map (ILM)  ...........................  14
        
   1          Specification  ......................................   3
   2          Introduction to MPLS  ...............................   3
   2.1        Overview  ...........................................   4
   2.2        Terminology  ........................................   6
   2.3        Acronyms and Abbreviations  .........................   9
   2.4        Acknowledgments  ....................................   9
   3          MPLS Basics  ........................................   9
   3.1        Labels  .............................................   9
   3.2        Upstream and Downstream LSRs  .......................  10
   3.3        Labeled Packet  .....................................  11
   3.4        Label Assignment and Distribution  ..................  11
   3.5        Attributes of a Label Binding  ......................  11
   3.6        Label Distribution Protocols  .......................  11
   3.7        Unsolicited Downstream vs. Downstream-on-Demand  ....  12
   3.8        Label Retention Mode  ...............................  12
   3.9        The Label Stack  ....................................  13
   3.10       The Next Hop Label Forwarding Entry (NHLFE)  ........  13
   3.11       Incoming Label Map (ILM)  ...........................  14
        
   3.12       FEC-to-NHLFE Map (FTN)  .............................  14
   3.13       Label Swapping  .....................................  15
   3.14       Scope and Uniqueness of Labels  .....................  15
   3.15       Label Switched Path (LSP), LSP Ingress, LSP Egress  .  16
   3.16       Penultimate Hop Popping  ............................  18
   3.17       LSP Next Hop  .......................................  20
   3.18       Invalid Incoming Labels  ............................  20
   3.19       LSP Control: Ordered versus Independent  ............  20
   3.20       Aggregation  ........................................  21
   3.21       Route Selection  ....................................  23
   3.22       Lack of Outgoing Label  .............................  24
   3.23       Time-to-Live (TTL)  .................................  24
   3.24       Loop Control  .......................................  25
   3.25       Label Encodings  ....................................  26
   3.25.1     MPLS-specific Hardware and/or Software  .............  26
   3.25.2     ATM Switches as LSRs  ...............................  26
   3.25.3     Interoperability among Encoding Techniques  .........  28
   3.26       Label Merging  ......................................  28
   3.26.1     Non-merging LSRs  ...................................  29
   3.26.2     Labels for Merging and Non-Merging LSRs  ............  30
   3.26.3     Merge over ATM  .....................................  31
   3.26.3.1   Methods of Eliminating Cell Interleave  .............  31
   3.26.3.2   Interoperation: VC Merge, VP Merge, and Non-Merge  ..  31
   3.27       Tunnels and Hierarchy  ..............................  32
   3.27.1     Hop-by-Hop Routed Tunnel  ...........................  32
   3.27.2     Explicitly Routed Tunnel  ...........................  33
   3.27.3     LSP Tunnels  ........................................  33
   3.27.4     Hierarchy: LSP Tunnels within LSPs  .................  33
   3.27.5     Label Distribution Peering and Hierarchy  ...........  34
   3.28       Label Distribution Protocol Transport  ..............  35
   3.29       Why More than one Label Distribution Protocol?  .....  36
   3.29.1     BGP and LDP  ........................................  36
   3.29.2     Labels for RSVP Flowspecs  ..........................  36
   3.29.3     Labels for Explicitly Routed LSPs  ..................  36
   3.30       Multicast  ..........................................  37
   4          Some Applications of MPLS  ..........................  37
   4.1        MPLS and Hop by Hop Routed Traffic  .................  37
   4.1.1      Labels for Address Prefixes  ........................  37
   4.1.2      Distributing Labels for Address Prefixes  ...........  37
   4.1.2.1    Label Distribution Peers for an Address Prefix  .....  37
   4.1.2.2    Distributing Labels  ................................  38
   4.1.3      Using the Hop by Hop path as the LSP  ...............  39
   4.1.4      LSP Egress and LSP Proxy Egress  ....................  39
   4.1.5      The Implicit NULL Label  ............................  40
   4.1.6      Option: Egress-Targeted Label Assignment  ...........  40
   4.2        MPLS and Explicitly Routed LSPs  ....................  42
   4.2.1      Explicitly Routed LSP Tunnels  ......................  42
   4.3        Label Stacks and Implicit Peering  ..................  43
        
   3.12       FEC-to-NHLFE Map (FTN)  .............................  14
   3.13       Label Swapping  .....................................  15
   3.14       Scope and Uniqueness of Labels  .....................  15
   3.15       Label Switched Path (LSP), LSP Ingress, LSP Egress  .  16
   3.16       Penultimate Hop Popping  ............................  18
   3.17       LSP Next Hop  .......................................  20
   3.18       Invalid Incoming Labels  ............................  20
   3.19       LSP Control: Ordered versus Independent  ............  20
   3.20       Aggregation  ........................................  21
   3.21       Route Selection  ....................................  23
   3.22       Lack of Outgoing Label  .............................  24
   3.23       Time-to-Live (TTL)  .................................  24
   3.24       Loop Control  .......................................  25
   3.25       Label Encodings  ....................................  26
   3.25.1     MPLS-specific Hardware and/or Software  .............  26
   3.25.2     ATM Switches as LSRs  ...............................  26
   3.25.3     Interoperability among Encoding Techniques  .........  28
   3.26       Label Merging  ......................................  28
   3.26.1     Non-merging LSRs  ...................................  29
   3.26.2     Labels for Merging and Non-Merging LSRs  ............  30
   3.26.3     Merge over ATM  .....................................  31
   3.26.3.1   Methods of Eliminating Cell Interleave  .............  31
   3.26.3.2   Interoperation: VC Merge, VP Merge, and Non-Merge  ..  31
   3.27       Tunnels and Hierarchy  ..............................  32
   3.27.1     Hop-by-Hop Routed Tunnel  ...........................  32
   3.27.2     Explicitly Routed Tunnel  ...........................  33
   3.27.3     LSP Tunnels  ........................................  33
   3.27.4     Hierarchy: LSP Tunnels within LSPs  .................  33
   3.27.5     Label Distribution Peering and Hierarchy  ...........  34
   3.28       Label Distribution Protocol Transport  ..............  35
   3.29       Why More than one Label Distribution Protocol?  .....  36
   3.29.1     BGP and LDP  ........................................  36
   3.29.2     Labels for RSVP Flowspecs  ..........................  36
   3.29.3     Labels for Explicitly Routed LSPs  ..................  36
   3.30       Multicast  ..........................................  37
   4          Some Applications of MPLS  ..........................  37
   4.1        MPLS and Hop by Hop Routed Traffic  .................  37
   4.1.1      Labels for Address Prefixes  ........................  37
   4.1.2      Distributing Labels for Address Prefixes  ...........  37
   4.1.2.1    Label Distribution Peers for an Address Prefix  .....  37
   4.1.2.2    Distributing Labels  ................................  38
   4.1.3      Using the Hop by Hop path as the LSP  ...............  39
   4.1.4      LSP Egress and LSP Proxy Egress  ....................  39
   4.1.5      The Implicit NULL Label  ............................  40
   4.1.6      Option: Egress-Targeted Label Assignment  ...........  40
   4.2        MPLS and Explicitly Routed LSPs  ....................  42
   4.2.1      Explicitly Routed LSP Tunnels  ......................  42
   4.3        Label Stacks and Implicit Peering  ..................  43
        
   4.4        MPLS and Multi-Path Routing  ........................  44
   4.5        LSP Trees as Multipoint-to-Point Entities  ..........  44
   4.6        LSP Tunneling between BGP Border Routers  ...........  45
   4.7        Other Uses of Hop-by-Hop Routed LSP Tunnels  ........  47
   4.8        MPLS and Multicast  .................................  47
   5          Label Distribution Procedures (Hop-by-Hop)  .........  47
   5.1        The Procedures for Advertising and Using labels  ....  48
   5.1.1      Downstream LSR: Distribution Procedure  .............  48
   5.1.1.1    PushUnconditional  ..................................  49
   5.1.1.2    PushConditional  ....................................  49
   5.1.1.3    PulledUnconditional  ................................  49
   5.1.1.4    PulledConditional  ..................................  50
   5.1.2      Upstream LSR: Request Procedure  ....................  51
   5.1.2.1    RequestNever  .......................................  51
   5.1.2.2    RequestWhenNeeded  ..................................  51
   5.1.2.3    RequestOnRequest  ...................................  51
   5.1.3      Upstream LSR: NotAvailable Procedure  ...............  52
   5.1.3.1    RequestRetry  .......................................  52
   5.1.3.2    RequestNoRetry  .....................................  52
   5.1.4      Upstream LSR: Release Procedure  ....................  52
   5.1.4.1    ReleaseOnChange  ....................................  52
   5.1.4.2    NoReleaseOnChange  ..................................  53
   5.1.5      Upstream LSR: labelUse Procedure  ...................  53
   5.1.5.1    UseImmediate  .......................................  53
   5.1.5.2    UseIfLoopNotDetected  ...............................  53
   5.1.6      Downstream LSR: Withdraw Procedure  .................  53
   5.2        MPLS Schemes: Supported Combinations of Procedures  .  54
   5.2.1      Schemes for LSRs that Support Label Merging  ........  55
   5.2.2      Schemes for LSRs that do not Support Label Merging  .  56
   5.2.3      Interoperability Considerations  ....................  57
   6          Security Considerations  ............................  58
   7          Intellectual Property  ..............................  58
   8          Authors' Addresses  .................................  59
   9          References  .........................................  59
   10         Full Copyright Statement  ...........................  61
        
   4.4        MPLS and Multi-Path Routing  ........................  44
   4.5        LSP Trees as Multipoint-to-Point Entities  ..........  44
   4.6        LSP Tunneling between BGP Border Routers  ...........  45
   4.7        Other Uses of Hop-by-Hop Routed LSP Tunnels  ........  47
   4.8        MPLS and Multicast  .................................  47
   5          Label Distribution Procedures (Hop-by-Hop)  .........  47
   5.1        The Procedures for Advertising and Using labels  ....  48
   5.1.1      Downstream LSR: Distribution Procedure  .............  48
   5.1.1.1    PushUnconditional  ..................................  49
   5.1.1.2    PushConditional  ....................................  49
   5.1.1.3    PulledUnconditional  ................................  49
   5.1.1.4    PulledConditional  ..................................  50
   5.1.2      Upstream LSR: Request Procedure  ....................  51
   5.1.2.1    RequestNever  .......................................  51
   5.1.2.2    RequestWhenNeeded  ..................................  51
   5.1.2.3    RequestOnRequest  ...................................  51
   5.1.3      Upstream LSR: NotAvailable Procedure  ...............  52
   5.1.3.1    RequestRetry  .......................................  52
   5.1.3.2    RequestNoRetry  .....................................  52
   5.1.4      Upstream LSR: Release Procedure  ....................  52
   5.1.4.1    ReleaseOnChange  ....................................  52
   5.1.4.2    NoReleaseOnChange  ..................................  53
   5.1.5      Upstream LSR: labelUse Procedure  ...................  53
   5.1.5.1    UseImmediate  .......................................  53
   5.1.5.2    UseIfLoopNotDetected  ...............................  53
   5.1.6      Downstream LSR: Withdraw Procedure  .................  53
   5.2        MPLS Schemes: Supported Combinations of Procedures  .  54
   5.2.1      Schemes for LSRs that Support Label Merging  ........  55
   5.2.2      Schemes for LSRs that do not Support Label Merging  .  56
   5.2.3      Interoperability Considerations  ....................  57
   6          Security Considerations  ............................  58
   7          Intellectual Property  ..............................  58
   8          Authors' Addresses  .................................  59
   9          References  .........................................  59
   10         Full Copyright Statement  ...........................  61
        
1. Specification
1. 规格

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

本文件中的关键词“必须”、“不得”、“要求”、“应”、“不得”、“应”、“不应”、“建议”、“可”和“可选”应按照RFC 2119中的说明进行解释。

2. Introduction to MPLS
2. MPLS简介

This document specifies the architecture for Multiprotocol Label Switching (MPLS).

本文件规定了多协议标签交换(MPLS)的体系结构。

Note that the use of MPLS for multicast is left for further study.

请注意,对于多播使用MPLS还有待进一步研究。

2.1. Overview
2.1. 概述

As a packet of a connectionless network layer protocol travels from one router to the next, each router makes an independent forwarding decision for that packet. That is, each router analyzes the packet's header, and each router runs a network layer routing algorithm. Each router independently chooses a next hop for the packet, based on its analysis of the packet's header and the results of running the routing algorithm.

当无连接网络层协议的数据包从一个路由器传输到下一个路由器时,每个路由器对该数据包做出独立的转发决策。也就是说,每个路由器分析数据包的报头,每个路由器运行一个网络层路由算法。每个路由器根据对数据包报头的分析和运行路由算法的结果,独立地为数据包选择下一跳。

Packet headers contain considerably more information than is needed simply to choose the next hop. Choosing the next hop can therefore be thought of as the composition of two functions. The first function partitions the entire set of possible packets into a set of "Forwarding Equivalence Classes (FECs)". The second maps each FEC to a next hop. Insofar as the forwarding decision is concerned, different packets which get mapped into the same FEC are indistinguishable. All packets which belong to a particular FEC and which travel from a particular node will follow the same path (or if certain kinds of multi-path routing are in use, they will all follow one of a set of paths associated with the FEC).

数据包头包含的信息远远多于选择下一跳所需的信息。因此,选择下一跳可以被认为是两个函数的组合。第一个函数将整个可能的数据包集划分为一组“转发等价类(FEC)”。第二个将每个FEC映射到下一个跃点。就转发决策而言,映射到同一FEC的不同分组是不可区分的。属于特定FEC且从特定节点传输的所有分组将遵循相同的路径(或者,如果正在使用某些类型的多路径路由,则它们将全部遵循与FEC相关联的一组路径之一)。

In conventional IP forwarding, a particular router will typically consider two packets to be in the same FEC if there is some address prefix X in that router's routing tables such that X is the "longest match" for each packet's destination address. As the packet traverses the network, each hop in turn reexamines the packet and assigns it to a FEC.

在传统的IP转发中,如果路由器的路由表中有一些地址前缀X,那么X将是每个包的目标地址的“最长匹配”,特定路由器通常会考虑两个包处于相同的FEC中。当数据包穿越网络时,每个跃点依次重新检查数据包并将其分配给FEC。

In MPLS, the assignment of a particular packet to a particular FEC is done just once, as the packet enters the network. The FEC to which the packet is assigned is encoded as a short fixed length value known as a "label". When a packet is forwarded to its next hop, the label is sent along with it; that is, the packets are "labeled" before they are forwarded.

在MPLS中,当数据包进入网络时,将特定数据包分配给特定FEC只需一次。分组被分配到的FEC被编码为称为“标签”的短固定长度值。当一个数据包被转发到它的下一个跃点时,标签也随之发送;也就是说,数据包在转发之前会被“标记”。

At subsequent hops, there is no further analysis of the packet's network layer header. Rather, the label is used as an index into a table which specifies the next hop, and a new label. The old label is replaced with the new label, and the packet is forwarded to its next hop.

在随后的跳数中,对数据包的网络层报头没有进一步的分析。相反,标签用作表的索引,表指定下一个跃点和新标签。旧标签被新标签替换,数据包被转发到下一跳。

In the MPLS forwarding paradigm, once a packet is assigned to a FEC, no further header analysis is done by subsequent routers; all forwarding is driven by the labels. This has a number of advantages over conventional network layer forwarding.

在MPLS转发范例中,一旦分组被分配给FEC,后续路由器就不再进行进一步的报头分析;所有的转发都是由标签驱动的。与传统的网络层转发相比,这有许多优点。

- MPLS forwarding can be done by switches which are capable of doing label lookup and replacement, but are either not capable of analyzing the network layer headers, or are not capable of analyzing the network layer headers at adequate speed.

- MPLS转发可以由能够进行标签查找和替换的交换机完成,但不能分析网络层报头,或者不能以足够的速度分析网络层报头。

- Since a packet is assigned to a FEC when it enters the network, the ingress router may use, in determining the assignment, any information it has about the packet, even if that information cannot be gleaned from the network layer header. For example, packets arriving on different ports may be assigned to different FECs. Conventional forwarding, on the other hand, can only consider information which travels with the packet in the packet header.

- 由于分组在进入网络时被分配给FEC,因此在确定分配时,入口路由器可以使用其关于分组的任何信息,即使该信息不能从网络层报头收集。例如,到达不同端口的分组可以被分配给不同的fec。另一方面,传统的转发只能考虑在分组报头中与分组一起传播的信息。

- A packet that enters the network at a particular router can be labeled differently than the same packet entering the network at a different router, and as a result forwarding decisions that depend on the ingress router can be easily made. This cannot be done with conventional forwarding, since the identity of a packet's ingress router does not travel with the packet.

- 在特定路由器进入网络的分组可以被标记为不同于在不同路由器进入网络的相同分组,并且因此可以容易地做出依赖于入口路由器的转发决策。这不能通过传统的转发来实现,因为数据包的入口路由器的身份不随数据包一起传输。

- The considerations that determine how a packet is assigned to a FEC can become ever more and more complicated, without any impact at all on the routers that merely forward labeled packets.

- 决定如何将数据包分配给FEC的考虑因素可能变得越来越复杂,对仅转发标记数据包的路由器没有任何影响。

- Sometimes it is desirable to force a packet to follow a particular route which is explicitly chosen at or before the time the packet enters the network, rather than being chosen by the normal dynamic routing algorithm as the packet travels through the network. This may be done as a matter of policy, or to support traffic engineering. In conventional forwarding, this requires the packet to carry an encoding of its route along with it ("source routing"). In MPLS, a label can be used to represent the route, so that the identity of the explicit route need not be carried with the packet.

- 有时,希望强制分组遵循在分组进入网络时或之前明确选择的特定路由,而不是在分组通过网络时由正常动态路由算法选择。这可以作为一项政策,或支持交通工程。在传统转发中,这要求数据包携带其路由的编码(“源路由”)。在MPLS中,可以使用标签来表示路由,因此显式路由的标识不需要与数据包一起携带。

Some routers analyze a packet's network layer header not merely to choose the packet's next hop, but also to determine a packet's "precedence" or "class of service". They may then apply different discard thresholds or scheduling disciplines to different packets. MPLS allows (but does not require) the precedence or class of service to be fully or partially inferred from the label. In this case, one may say that the label represents the combination of a FEC and a precedence or class of service.

一些路由器分析数据包的网络层报头,不仅是为了选择数据包的下一跳,还为了确定数据包的“优先级”或“服务级别”。然后,他们可以对不同的数据包应用不同的丢弃阈值或调度规程。MPLS允许(但不要求)从标签中完全或部分推断服务的优先级或类别。在这种情况下,可以说标签表示FEC和优先级或服务类别的组合。

MPLS stands for "Multiprotocol" Label Switching, multiprotocol because its techniques are applicable to ANY network layer protocol. In this document, however, we focus on the use of IP as the network layer protocol.

MPLS代表“多协议”标签交换,多协议,因为其技术适用于任何网络层协议。然而,在本文档中,我们将重点介绍IP作为网络层协议的使用。

A router which supports MPLS is known as a "Label Switching Router", or LSR.

支持MPLS的路由器称为“标签交换路由器”,或LSR。

2.2. Terminology
2.2. 术语

This section gives a general conceptual overview of the terms used in this document. Some of these terms are more precisely defined in later sections of the document.

本节给出了本文件中所用术语的一般概念概述。其中一些术语在本文件后面的章节中有更精确的定义。

DLCI a label used in Frame Relay networks to identify frame relay circuits

帧中继网络中用于识别帧中继电路的标签

forwarding equivalence class a group of IP packets which are forwarded in the same manner (e.g., over the same path, with the same forwarding treatment)

转发等价类以相同方式转发的一组IP数据包(例如,通过相同路径,采用相同的转发处理)

frame merge label merging, when it is applied to operation over frame based media, so that the potential problem of cell interleave is not an issue.

帧合并标签合并,当它应用于基于帧的媒体上的操作时,因此单元交织的潜在问题不是问题。

label a short fixed length physically contiguous identifier which is used to identify a FEC, usually of local significance.

标签一个短的固定长度的物理连续标识符,用于识别FEC,通常具有局部意义。

label merging the replacement of multiple incoming labels for a particular FEC with a single outgoing label

标签合并将特定FEC的多个传入标签替换为单个传出标签

label swap the basic forwarding operation consisting of looking up an incoming label to determine the outgoing label, encapsulation, port, and other data handling information.

标签交换基本转发操作,包括查找传入标签以确定传出标签、封装、端口和其他数据处理信息。

label swapping a forwarding paradigm allowing streamlined forwarding of data by using labels to identify classes of data packets which are treated indistinguishably when forwarding.

标签交换一种转发模式,通过使用标签识别转发时不区分处理的数据包类别,简化数据转发。

label switched hop the hop between two MPLS nodes, on which forwarding is done using labels.

标签交换跃点两个MPLS节点之间的跃点,在其上使用标签进行转发。

label switched path The path through one or more LSRs at one level of the hierarchy followed by a packets in a particular FEC.

标签交换路径在层次结构的一个级别上通过一个或多个LSR的路径,后跟特定FEC中的数据包。

label switching router an MPLS node which is capable of forwarding native L3 packets

标签交换路由器能够转发本机L3数据包的MPLS节点

layer 2 the protocol layer under layer 3 (which therefore offers the services used by layer 3). Forwarding, when done by the swapping of short fixed length labels, occurs at layer 2 regardless of whether the label being examined is an ATM VPI/VCI, a frame relay DLCI, or an MPLS label.

第2层是第3层下的协议层(因此提供第3层使用的服务)。通过交换固定长度的短标签进行转发时,无论正在检查的标签是ATM VPI/VCI、帧中继DLCI还是MPLS标签,都会在第2层进行转发。

layer 3 the protocol layer at which IP and its associated routing protocols operate link layer synonymous with layer 2

第3层IP及其相关路由协议运行的协议层链路层与第2层同义

loop detection a method of dealing with loops in which loops are allowed to be set up, and data may be transmitted over the loop, but the loop is later detected

环路检测一种处理环路的方法,允许设置环路,数据可以在环路上传输,但随后会检测到环路

loop prevention a method of dealing with loops in which data is never transmitted over a loop

环路预防处理数据从未通过环路传输的环路的方法

label stack an ordered set of labels

标签堆栈一组有序的标签

merge point a node at which label merging is done

合并点完成标签合并的节点

MPLS domain a contiguous set of nodes which operate MPLS routing and forwarding and which are also in one Routing or Administrative Domain

MPLS域操作MPLS路由和转发的一组连续节点,它们也位于一个路由或管理域中

MPLS edge node an MPLS node that connects an MPLS domain with a node which is outside of the domain, either because it does not run MPLS, and/or because it is in a different domain. Note that if an LSR has a neighboring host which is not running MPLS, that that LSR is an MPLS edge node.

MPLS边缘节点连接MPLS域与域外节点的MPLS节点,原因可能是它不运行MPLS,和/或它位于不同的域中。请注意,如果LSR有一个不运行MPLS的相邻主机,则该LSR是一个MPLS边缘节点。

MPLS egress node an MPLS edge node in its role in handling traffic as it leaves an MPLS domain

MPLS出口节点在离开MPLS域时处理流量的MPLS边缘节点

MPLS ingress node an MPLS edge node in its role in handling traffic as it enters an MPLS domain

MPLS入口节点在进入MPLS域时处理流量的MPLS边缘节点

MPLS label a label which is carried in a packet header, and which represents the packet's FEC

MPLS标签数据包头中携带的标签,表示数据包的FEC

MPLS node a node which is running MPLS. An MPLS node will be aware of MPLS control protocols, will operate one or more L3 routing protocols, and will be capable of forwarding packets based on labels. An MPLS node may optionally be also capable of forwarding native L3 packets.

MPLS节点运行MPLS的节点。MPLS节点将知道MPLS控制协议,将操作一个或多个L3路由协议,并且将能够基于标签转发数据包。MPLS节点还可以可选地能够转发本机L3分组。

MultiProtocol Label Switching an IETF working group and the effort associated with the working group

IETF工作组的多协议标签交换及其相关工作

network layer synonymous with layer 3

网络层与第3层同义

stack synonymous with label stack

堆栈与标签堆栈同义

switched path synonymous with label switched path

交换路径与标签交换路径同义

virtual circuit a circuit used by a connection-oriented layer 2 technology such as ATM or Frame Relay, requiring the maintenance of state information in layer 2 switches.

虚拟电路面向连接的第2层技术(如ATM或帧中继)使用的电路,需要维护第2层交换机中的状态信息。

VC merge label merging where the MPLS label is carried in the ATM VCI field (or combined VPI/VCI field), so as to allow multiple VCs to merge into one single VC

VC合并标签合并,其中MPLS标签在ATM VCI字段(或组合VPI/VCI字段)中携带,以便允许多个VC合并为一个VC

VP merge label merging where the MPLS label is carried din the ATM VPI field, so as to allow multiple VPs to be merged into one single VP. In this case two cells would have the same VCI value only if they originated from the same node. This allows cells from different sources to be distinguished via the VCI.

VP合并标签合并,其中在ATM VPI字段中携带MPLS标签,以便允许将多个VP合并到一个VP中。在这种情况下,只有当两个单元来自同一节点时,它们才会具有相同的VCI值。这允许通过VCI区分来自不同来源的细胞。

VPI/VCI a label used in ATM networks to identify circuits

VPI/VCI在ATM网络中用于识别电路的标签

2.3. Acronyms and Abbreviations
2.3. 缩略语

ATM Asynchronous Transfer Mode BGP Border Gateway Protocol DLCI Data Link Circuit Identifier FEC Forwarding Equivalence Class FTN FEC to NHLFE Map IGP Interior Gateway Protocol ILM Incoming Label Map IP Internet Protocol LDP Label Distribution Protocol L2 Layer 2 L3 Layer 3 LSP Label Switched Path LSR Label Switching Router MPLS MultiProtocol Label Switching NHLFE Next Hop Label Forwarding Entry SVC Switched Virtual Circuit SVP Switched Virtual Path TTL Time-To-Live VC Virtual Circuit VCI Virtual Circuit Identifier VP Virtual Path VPI Virtual Path Identifier

ATM异步传输模式BGP边界网关协议DLCI数据链路电路标识符FEC转发等价类FTN FEC到NHLFE映射IGP内部网关协议ILM传入标签映射IP互联网协议LDP标签分发协议L2层2 L3层3 LSP标签交换路径LSR标签交换路由器MPLS多协议标签交换NHLFE下一跳标签转发条目SVC交换虚拟电路SVP交换虚拟路径TTL生存时间VC虚拟电路VCI虚拟电路标识符VP虚拟路径VPI虚拟路径标识符

2.4. Acknowledgments
2.4. 致谢

The ideas and text in this document have been collected from a number of sources and comments received. We would like to thank Rick Boivie, Paul Doolan, Nancy Feldman, Yakov Rekhter, Vijay Srinivasan, and George Swallow for their inputs and ideas.

本文件中的想法和文本是从许多来源和收到的评论中收集的。我们要感谢Rick Boivie、Paul Doolan、Nancy Feldman、Yakov Rekhter、Vijay Srinivasan和George Swallow的投入和想法。

3. MPLS Basics
3. MPLS基础

In this section, we introduce some of the basic concepts of MPLS and describe the general approach to be used.

在本节中,我们将介绍MPLS的一些基本概念,并描述将要使用的一般方法。

3.1. Labels
3.1. 标签

A label is a short, fixed length, locally significant identifier which is used to identify a FEC. The label which is put on a particular packet represents the Forwarding Equivalence Class to which that packet is assigned.

标签是一个短的、固定长度的、局部有效的标识符,用于标识FEC。放置在特定数据包上的标签表示该数据包被分配到的转发等价类。

Most commonly, a packet is assigned to a FEC based (completely or partially) on its network layer destination address. However, the label is never an encoding of that address.

最常见的情况是,数据包根据其网络层目标地址(完全或部分)分配给FEC。但是,标签永远不是该地址的编码。

If Ru and Rd are LSRs, they may agree that when Ru transmits a packet to Rd, Ru will label with packet with label value L if and only if the packet is a member of a particular FEC F. That is, they can agree to a "binding" between label L and FEC F for packets moving from Ru to Rd. As a result of such an agreement, L becomes Ru's "outgoing label" representing FEC F, and L becomes Rd's "incoming label" representing FEC F.

如果Ru和Rd是LSR,他们可能同意,当Ru向Rd发送数据包时,Ru将使用标签值为L的数据包进行标记,当且仅当该数据包是特定FEC F的成员时。也就是说,他们可以同意标签L和FEC F之间对于从Ru移动到Rd的数据包的“绑定”。由于这种协议,L成为Ru的“传出标签”表示FEC F,L成为Rd表示FEC F的“传入标签”。

Note that L does not necessarily represent FEC F for any packets other than those which are being sent from Ru to Rd. L is an arbitrary value whose binding to F is local to Ru and Rd.

注意,对于从Ru发送到Rd的数据包以外的任何数据包,L不一定表示FEC F。L是任意值,其与F的绑定是Ru和Rd的本地绑定。

When we speak above of packets "being sent" from Ru to Rd, we do not imply either that the packet originated at Ru or that its destination is Rd. Rather, we mean to include packets which are "transit packets" at one or both of the LSRs.

当我们在上面提到从Ru“发送”到Rd的包时,我们并不意味着该包起源于Ru或其目的地是Rd。相反,我们的意思是包括在一个或两个lsr处的“传输包”的包。

Sometimes it may be difficult or even impossible for Rd to tell, of an arriving packet carrying label L, that the label L was placed in the packet by Ru, rather than by some other LSR. (This will typically be the case when Ru and Rd are not direct neighbors.) In such cases, Rd must make sure that the binding from label to FEC is one-to-one. That is, Rd MUST NOT agree with Ru1 to bind L to FEC F1, while also agreeing with some other LSR Ru2 to bind L to a different FEC F2, UNLESS Rd can always tell, when it receives a packet with incoming label L, whether the label was put on the packet by Ru1 or whether it was put on by Ru2.

有时,对于携带标签L的到达分组,Rd可能很难或甚至不可能判断标签L是由Ru而不是某个其他LSR放置在分组中的。(当Ru和Rd不是直接邻居时,通常会出现这种情况。)在这种情况下,Rd必须确保从标签到FEC的绑定是一对一的。也就是说,Rd不得同意Ru1将L绑定到FEC F1,同时也不得同意某些其他LSR Ru2将L绑定到不同的FEC F2,除非Rd在接收到带有传入标签L的数据包时,总是能够判断标签是由Ru1还是由Ru2放在数据包上。

It is the responsibility of each LSR to ensure that it can uniquely interpret its incoming labels.

每个LSR都有责任确保能够唯一地解释其传入标签。

3.2. Upstream and Downstream LSRs
3.2. 上游和下游LSR

Suppose Ru and Rd have agreed to bind label L to FEC F, for packets sent from Ru to Rd. Then with respect to this binding, Ru is the "upstream LSR", and Rd is the "downstream LSR".

假设Ru和Rd已经同意将标签L绑定到FEC F,用于从Ru发送到Rd的数据包。那么关于该绑定,Ru是“上游LSR”,Rd是“下游LSR”。

To say that one node is upstream and one is downstream with respect to a given binding means only that a particular label represents a particular FEC in packets travelling from the upstream node to the downstream node. This is NOT meant to imply that packets in that FEC would actually be routed from the upstream node to the downstream node.

对于给定绑定来说,一个节点是上游的,一个是下游的,这仅意味着特定标签表示从上游节点到下游节点的分组中的特定FEC。这并不意味着该FEC中的分组实际上将从上游节点路由到下游节点。

3.3. Labeled Packet
3.3. 标签包

A "labeled packet" is