Internet Engineering Task Force (IETF) L. Martini
Request for Comments: 6073 C. Metz
Category: Standards Track Cisco Systems, Inc.
ISSN: 2070-1721 T. Nadeau
LucidVision
M. Bocci
M. Aissaoui
Alcatel-Lucent
January 2011
Internet Engineering Task Force (IETF) L. Martini
Request for Comments: 6073 C. Metz
Category: Standards Track Cisco Systems, Inc.
ISSN: 2070-1721 T. Nadeau
LucidVision
M. Bocci
M. Aissaoui
Alcatel-Lucent
January 2011
Segmented Pseudowire
分段假丝
Abstract
摘要
This document describes how to connect pseudowires (PWs) between different Packet Switched Network (PSN) domains or between two or more distinct PW control plane domains, where a control plane domain uses a common control plane protocol or instance of that protocol for a given PW. The different PW control plane domains may belong to independent autonomous systems, or the PSN technology is heterogeneous, or a PW might need to be aggregated at a specific PSN point. The PW packet data units are simply switched from one PW to another without changing the PW payload.
本文档描述了如何在不同的分组交换网络（PSN）域之间或两个或多个不同的PW控制平面域之间连接伪线（PW），其中控制平面域对给定PW使用公共控制平面协议或该协议的实例。不同的PW控制平面域可能属于独立的自治系统，或者PSN技术是异构的，或者PW可能需要在特定的PSN点聚合。PW分组数据单元仅从一个PW切换到另一个PW，而不改变PW有效负载。
Status of This Memo
关于下段备忘
This is an Internet Standards Track document.
这是一份互联网标准跟踪文件。
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
本文件是互联网工程任务组（IETF）的产品。它代表了IETF社区的共识。它已经接受了公众审查，并已被互联网工程指导小组（IESG）批准出版。有关互联网标准的更多信息，请参见RFC 5741第2节。
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6073.
有关本文件当前状态、任何勘误表以及如何提供反馈的信息，请访问http://www.rfc-editor.org/info/rfc6073.
Copyright Notice
版权公告
Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.
版权所有（c）2011 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 (http://trustee.ietf.org/license-info) 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. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
本文件受BCP 78和IETF信托有关IETF文件的法律规定的约束(http://trustee.ietf.org/license-info)自本文件出版之日起生效。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。从本文件中提取的代码组件必须包括信托法律条款第4.e节中所述的简化BSD许可证文本，并提供简化BSD许可证中所述的无担保。
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.
本文件可能包含2008年11月10日之前发布或公开的IETF文件或IETF贡献中的材料。控制某些材料版权的人员可能未授予IETF信托允许在IETF标准流程之外修改此类材料的权利。在未从控制此类材料版权的人员处获得充分许可的情况下，不得在IETF标准流程之外修改本文件，也不得在IETF标准流程之外创建其衍生作品，除了将其格式化以RFC形式发布或将其翻译成英语以外的其他语言。
Table of Contents
目录
1. Introduction ....................................................4
2. Specification of Requirements ...................................5
3. Terminology .....................................................5
4. General Description .............................................6
5. PW Switching and Attachment Circuit Type ........................9
6. Applicability ...................................................9
7. MPLS-PW to MPLS-PW Switching ...................................10
7.1. Static Control Plane Switching ............................10
7.2. Two LDP Control Planes Using the Same FEC Type ............11
7.2.1. FEC 129 Active/Passive T-PE Election Procedure .....11
7.3. LDP Using FEC 128 to LDP Using the Generalized FEC 129 ....12
7.4. LDP SP-PE TLV .............................................12
7.4.1. PW Switching Point PE Sub-TLVs .....................14
7.4.2. Adaptation of Interface Parameters .................15
7.5. Group ID ..................................................16
7.6. PW Loop Detection .........................................16
8. MPLS-PW to L2TPv3-PW Control Plane Switching ...................16
8.1. Static MPLS and L2TPv3 PWs ................................17
8.2. Static MPLS PW and Dynamic L2TPv3 PW ......................17
1. Introduction ....................................................4
2. Specification of Requirements ...................................5
3. Terminology .....................................................5
4. General Description .............................................6
5. PW Switching and Attachment Circuit Type ........................9
6. Applicability ...................................................9
7. MPLS-PW to MPLS-PW Switching ...................................10
7.1. Static Control Plane Switching ............................10
7.2. Two LDP Control Planes Using the Same FEC Type ............11
7.2.1. FEC 129 Active/Passive T-PE Election Procedure .....11
7.3. LDP Using FEC 128 to LDP Using the Generalized FEC 129 ....12
7.4. LDP SP-PE TLV .............................................12
7.4.1. PW Switching Point PE Sub-TLVs .....................14
7.4.2. Adaptation of Interface Parameters .................15
7.5. Group ID ..................................................16
7.6. PW Loop Detection .........................................16
8. MPLS-PW to L2TPv3-PW Control Plane Switching ...................16
8.1. Static MPLS and L2TPv3 PWs ................................17
8.2. Static MPLS PW and Dynamic L2TPv3 PW ......................17
8.3. Static L2TPv3 PW and Dynamic LDP/MPLS PW ..................17
8.4. Dynamic LDP/MPLS and L2TPv3 PWs ...........................17
8.4.1. Session Establishment ..............................18
8.4.2. Adaptation of PW Status message ....................18
8.4.3. Session Tear Down ..................................18
8.5. Adaptation of L2TPv3 AVPs to Interface Parameters .........19
8.6. Switching Point TLV in L2TPv3 .............................20
8.7. L2TPv3 and MPLS PW Data Plane .............................20
8.7.1. Mapping the MPLS Control Word to L2TP ..............21
9. Operations, Administration, and Maintenance (OAM) ..............22
9.1. Extensions to VCCV to Support MS-PWs ......................22
9.2. OAM from MPLS PW to L2TPv3 PW .............................22
9.3. OAM Data Plane Indication from MPLS PW to MPLS PW .........22
9.4. Signaling OAM Capabilities for Switched Pseudowires .......23
9.5. OAM Capability for MS-PWs Demultiplexed Using MPLS ........23
9.5.1. MS-PW and VCCV CC Type 1 ...........................24
9.5.2. MS-PW and VCCV CC Type 2 ...........................24
9.5.3. MS-PW and VCCV CC Type 3 ...........................24
9.6. MS-PW VCCV Operations .....................................24
9.6.1. VCCV Echo Message Processing .......................25
9.6.2. Detailed VCCV Procedures ...........................27
10. Mapping Switched Pseudowire Status ............................31
10.1. PW Status Messages Initiated by the S-PE .................32
10.1.1. Local PW2 Transmit Direction Fault ................33
10.1.2. Local PW1 Transmit Direction Fault ................34
10.1.3. Local PW2 Receive Direction Fault .................34
10.1.4. Local PW1 Receive Direction Fault .................34
10.1.5. Clearing Faults ...................................34
10.2. PW Status Messages and SP-PE TLV Processing ..............35
10.3. T-PE Processing of PW Status Messages ....................35
10.4. Pseudowire Status Negotiation Procedures .................35
10.5. Status Dampening .........................................35
11. Peering between Autonomous Systems ............................35
12. Congestion Considerations .....................................36
13. Security Considerations .......................................36
13.1. Data Plane Security ......................................36
13.1.1. VCCV Security Considerations ......................36
13.2. Control Protocol Security ................................37
14. IANA Considerations ...........................................38
14.1. L2TPv3 AVP ...............................................38
14.2. LDP TLV TYPE .............................................38
14.3. LDP Status Codes .........................................38
14.4. L2TPv3 Result Codes ......................................38
14.5. New IANA Registries ......................................39
15. Normative References ..........................................39
16. Informative References ........................................40
17. Acknowledgments ...............................................42
18. Contributors ..................................................42
8.3. Static L2TPv3 PW and Dynamic LDP/MPLS PW ..................17
8.4. Dynamic LDP/MPLS and L2TPv3 PWs ...........................17
8.4.1. Session Establishment ..............................18
8.4.2. Adaptation of PW Status message ....................18
8.4.3. Session Tear Down ..................................18
8.5. Adaptation of L2TPv3 AVPs to Interface Parameters .........19
8.6. Switching Point TLV in L2TPv3 .............................20
8.7. L2TPv3 and MPLS PW Data Plane .............................20
8.7.1. Mapping the MPLS Control Word to L2TP ..............21
9. Operations, Administration, and Maintenance (OAM) ..............22
9.1. Extensions to VCCV to Support MS-PWs ......................22
9.2. OAM from MPLS PW to L2TPv3 PW .............................22
9.3. OAM Data Plane Indication from MPLS PW to MPLS PW .........22
9.4. Signaling OAM Capabilities for Switched Pseudowires .......23
9.5. OAM Capability for MS-PWs Demultiplexed Using MPLS ........23
9.5.1. MS-PW and VCCV CC Type 1 ...........................24
9.5.2. MS-PW and VCCV CC Type 2 ...........................24
9.5.3. MS-PW and VCCV CC Type 3 ...........................24
9.6. MS-PW VCCV Operations .....................................24
9.6.1. VCCV Echo Message Processing .......................25
9.6.2. Detailed VCCV Procedures ...........................27
10. Mapping Switched Pseudowire Status ............................31
10.1. PW Status Messages Initiated by the S-PE .................32
10.1.1. Local PW2 Transmit Direction Fault ................33
10.1.2. Local PW1 Transmit Direction Fault ................34
10.1.3. Local PW2 Receive Direction Fault .................34
10.1.4. Local PW1 Receive Direction Fault .................34
10.1.5. Clearing Faults ...................................34
10.2. PW Status Messages and SP-PE TLV Processing ..............35
10.3. T-PE Processing of PW Status Messages ....................35
10.4. Pseudowire Status Negotiation Procedures .................35
10.5. Status Dampening .........................................35
11. Peering between Autonomous Systems ............................35
12. Congestion Considerations .....................................36
13. Security Considerations .......................................36
13.1. Data Plane Security ......................................36
13.1.1. VCCV Security Considerations ......................36
13.2. Control Protocol Security ................................37
14. IANA Considerations ...........................................38
14.1. L2TPv3 AVP ...............................................38
14.2. LDP TLV TYPE .............................................38
14.3. LDP Status Codes .........................................38
14.4. L2TPv3 Result Codes ......................................38
14.5. New IANA Registries ......................................39
15. Normative References ..........................................39
16. Informative References ........................................40
17. Acknowledgments ...............................................42
18. Contributors ..................................................42
The PWE3 Architecture [RFC3985] defines the signaling and encapsulation techniques for establishing Single-Segment Pseudowires (SS-PWs) between a pair of terminating PEs. Multi-Segment Pseudowires (MS-PWs) are most useful in two general cases:
PWE3体系结构[RFC3985]定义了用于在一对终端PE之间建立单段伪线（SS PW）的信令和封装技术。多段伪导线（MS PWs）在两种一般情况下最有用：
-i. In some cases it is not possible, desirable, or feasible to establish a PW control channel between the terminating source and destination PEs. At a minimum, PW control channel establishment requires knowledge of and reachability to the remote (terminating) PE IP address. The local (terminating) PE may not have access to this information because of topology, operational, or security constraints.
-一,。在某些情况下，在终端源和目标PEs之间建立PW控制信道是不可能、不可取或不可行的。至少，PW控制信道的建立需要了解远程（终端）PE IP地址，并且能够访问该地址。由于拓扑、操作或安全限制，本地（终止）PE可能无法访问此信息。
An example is the inter-AS L2VPN scenario where the terminating PEs reside in different provider networks (ASes) and it is the practice to cryptographically sign all control traffic exchanged between two networks. Technically, an SS-PW could be used but this would require cryptographic signatures on ALL terminating source and destination PE nodes. An MS-PW allows the providers to confine key administration to just the PW switching points connecting the two domains.
一个例子是inter-AS L2VPN场景，其中终端PE驻留在不同的提供商网络（ASE）中，实践是对两个网络之间交换的所有控制流量进行加密签名。从技术上讲，可以使用SS-PW，但这需要在所有终止的源和目标PE节点上进行加密签名。MS-PW允许提供者将密钥管理仅限于连接两个域的PW交换点。
A second example might involve a single AS where the PW setup path between the terminating PEs is computed by an external entity. Assume that a full mesh of PWE3 control channels is established between PE-A, PE-B, and PE-C. A client-layer L2 connection tunneled through a PW is required between terminating PE-A and PE-C. The external entity computes a PW setup path that passes through PE-B. This results in two discrete PW segments being built: one between PE-A and PE-B and one between PE-B and PE-C. The successful client-layer L2 connection between terminating PE-A and terminating PE-C requires that PE-B performs the PWE3 switching process.
第二个示例可能涉及单个AS，其中端接PE之间的PW设置路径由外部实体计算。假设在PE-a、PE-B之间建立了PWE3控制通道的完整网格，和PE-C。端接PE-A和PE-C之间需要通过PW隧道的客户端层L2连接。外部实体计算通过PE-B的PW设置路径。这将导致构建两个离散PW段：一个在PE-A和PE-B之间，另一个在PE-B和PE-C之间。成功的客户端层L2连接端接PE-A和端接PE-C要求PE-B执行PWE3切换过程。
A third example involves the use of PWs in hierarchical IP/MPLS networks. Access networks connected to a backbone use PWs to transport customer payloads between customer sites serviced by the same access network and up to the edge of the backbone where they can be terminated or switched onto a succeeding PW segment crossing the backbone. The use of PWE3 switching between the access and backbone networks can potentially reduce the PWE3 control channels and routing information processed by the access network T-PEs.
第三个例子涉及在分层IP/MPLS网络中使用PWs。连接到主干网的接入网使用PWs在由同一接入网提供服务的客户站点之间传输客户有效负载，直至主干网的边缘，在那里，客户有效负载可以被终止或切换到穿过主干网的后续PW段。在接入网和骨干网之间使用PWE3交换可以潜在地减少由接入网T-PEs处理的PWE3控制信道和路由信息。
It should be noted that PWE3 switching does not help in any way to reduce the amount of PW state supported by each access network T-PE.
应当注意，PWE3交换在任何方面都无助于减少每个接入网络T-PE支持的PW状态量。
-ii. In some applications, the signaling protocol and encapsulation on each segment of the PW are different. The terminating PEs are connected to networks employing different PW signaling and encapsulation protocols. In this case, it is not possible to use an SS-PW. An MS-PW with the appropriate signaling protocol interworking performed at the PW switching points can enable PW connectivity between the terminating PEs in this scenario.
-二,。在某些应用中，PW的每个段上的信令协议和封装是不同的。终端PE连接到采用不同PW信令和封装协议的网络。在这种情况下，不可能使用SS-PW。在这种情况下，具有在PW交换点执行的适当信令协议互通的MS-PW可以实现终端PE之间的PW连接。
A more detailed discussion of the requirements pertaining to MS-PWs can be found in [RFC5254].
有关MS PWs要求的更详细讨论，请参见[RFC5254]。
There are four different mechanisms to establish PWs:
建立PWs有四种不同的机制：
-i. Static configuration of the PW (MPLS or Layer 2 Tunneling Protocol version 3 (L2TPv3)) -ii. LDP using FEC 128 (PWid FEC Element) -iii. LDP using FEC 129 (Generalized PWid FEC Element) -iv. L2TPv3
-一,。PW的静态配置（MPLS或第2层隧道协议版本3（L2TPv3））-ii。使用FEC 128的LDP（PWid FEC元件）-iii.使用FEC 129的LDP（通用PWid FEC元件）-iv.L2TPv3
While MS-PWs are composed of PW segments, each PW segment cannot function independently, as the PW service is always instantiated across the complete MS-PW. Hence, no PW segments can be signaled or be operational without the complete MS-PW being signaled at once.
虽然MS PW由PW段组成，但每个PW段不能独立运行，因为PW服务始终在整个MS-PW中实例化。因此，如果不立即向整个MS-PW发送信号，任何PW段都不能发送信号或运行。
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 [RFC2119].
本文件中的关键词“必须”、“不得”、“必需”、“应”、“不应”、“应”、“不应”、“建议”、“可”和“可选”应按照[RFC2119]中所述进行解释。
- PW Terminating Provider Edge (T-PE). A PE where the customer-facing attachment circuits (ACs) are bound to a PW forwarder. A Terminating PE is present in the first and last segments of a MS-PW. This incorporates the functionality of a PE as defined in [RFC3985].
- PW端接提供程序边缘（T-PE）。一种PE，其中面向客户的连接电路（ACs）绑定到PW转发器。终端PE出现在MS-PW的第一段和最后一段中。这包括[RFC3985]中定义的PE功能。
- Single-Segment Pseudowire (SS-PW). A PW set up directly between two T-PE devices. The PW label is unchanged between the originating and terminating T-PEs.
- 单段伪导线（SS-PW）。直接在两个T-PE设备之间设置的PW。起始和终止T-PE之间的PW标签保持不变。
- Multi-Segment Pseudowire (MS-PW). A static or dynamically configured set of two or more contiguous PW segments that behave and function as a single point-to-point PW. Each end of an MS-PW by definition MUST terminate on a T-PE.
- 多段伪导线（MS-PW）。由两个或多个连续PW段组成的静态或动态配置集，其行为和功能类似于单个点对点PW。根据定义，MS-PW的每一端必须在T-PE上终止。
- PW Segment. A part of a single-segment or multi-segment PW, which traverses one PSN tunnel in each direction between two PE devices, T-PEs and/or S-PEs (switching PE).
- PW段。单段或多段PW的一部分，在两个PE设备T-PE和/或S-PE（交换PE）之间的每个方向上穿过一个PSN隧道。
- PW Switching Provider Edge (S-PE). A PE capable of switching the control and data planes of the preceding and succeeding PW segments in an MS-PW. The S-PE terminates the PSN tunnels of the preceding and succeeding segments of the MS-PW. It therefore includes a PW switching point for an MS-PW. A PW switching point is never the S-PE and the T-PE for the same MS-PW. A PW switching point runs necessary protocols to set up and manage PW segments with other PW switching points and terminating PEs. An S-PE can exist anywhere a PW must be processed or policy applied. It is therefore not limited to the edge of a provider network.
- PW交换提供程序边缘（S-PE）。一种能够在MS-PW中切换前一个PW段和后一个PW段的控制平面和数据平面的PE。S-PE终止MS-PW之前和后续段的PSN隧道。因此，它包括MS-PW的PW开关点。PW切换点绝不是同一MS-PW的S-PE和T-PE。PW交换点运行必要的协议，以设置和管理与其他PW交换点和终端PE的PW段。S-PE可以存在于必须处理PW或应用策略的任何位置。因此，它不限于提供商网络的边缘。
- MS-PW path. The set of S-PEs that will be traversed in sequence to form the MS-PW.
- MS-PW路径。将按顺序遍历以形成MS-PW的一组S-PE。
A pseudowire (PW) is a mechanism that carries the essential elements of an emulated service from one PE to one or more other PEs over a PSN as described in Figure 1 and in [RFC3985]. Many providers have deployed PWs as a means of migrating existing (or building new) L2VPN services (e.g., Frame Relay, ATM, or Ethernet) onto a PSN.
伪线（PW）是一种机制，它通过PSN将模拟服务的基本元素从一个PE传送到一个或多个其他PE，如图1和[RFC3985]中所述。许多提供商已将PWs部署为将现有（或构建新的）L2VPN服务（例如，帧中继、ATM或以太网）迁移到PSN上的一种手段。
PWs may span multiple domains of the same or different provider networks. In these scenarios, PW control channels (i.e., targeted LDP, L2TPv3) and PWs will cross AS boundaries.
PWs可以跨越相同或不同提供商网络的多个域。在这些场景中，PW控制信道（即目标LDP、L2TPv3）和PW将作为边界跨越。
Inter-AS L2VPN functionality is currently supported, and several techniques employing MPLS encapsulation and LDP signaling have been documented [RFC4364]. It is also straightforward to support the same inter-AS L2VPN functionality employing L2TPv3. In this document, we define a methodology to switch a PW between different Packet Switched Network (PSN) domains or between two or more distinct PW control plane domains.
目前支持AS间L2VPN功能，并记录了几种采用MPLS封装和LDP信令的技术[RFC4364]。使用L2TPv3支持与L2VPN相同的inter-AS功能也很简单。在本文中，我们定义了一种在不同的分组交换网络（PSN）域之间或在两个或多个不同的PW控制平面域之间切换PW的方法。
|<-------------- Emulated Service ---------------->|
| |
| |<-------- Pseudowire ------>| |
| | | |
| | |<-- PSN Tunnel -->| | |
| V V V V |
V AC +----+ +----+ AC V
+-----+ | | PE1|==================| PE2| | +-----+
| |----------|............PW1.............|----------| |
| CE1 | | | | | | | | CE2 |
| |----------|............PW2.............|----------| |
+-----+ ^ | | |==================| | | ^ +-----+
^ | +----+ +----+ | | ^
| | Provider Edge 1 Provider Edge 2 | |
| | | |
Customer | | Customer
Edge 1 | | Edge 2
| |
native service native service
|<-------------- Emulated Service ---------------->|
| |
| |<-------- Pseudowire ------>| |
| | | |
| | |<-- PSN Tunnel -->| | |
| V V V V |
V AC +----+ +----+ AC V
+-----+ | | PE1|==================| PE2| | +-----+
| |----------|............PW1.............|----------| |
| CE1 | | | | | | | | CE2 |
| |----------|............PW2.............|----------| |
+-----+ ^ | | |==================| | | ^ +-----+
^ | +----+ +----+ | | ^
| | Provider Edge 1 Provider Edge 2 | |
| | | |
Customer | | Customer
Edge 1 | | Edge 2
| |
native service native service
Figure 1: PWE3 Reference Model
图1:PWE3参考模型
There are two methods for switching a PW between two PW domains. In the first method (Figure 2), the two separate control plane domains terminate on different PEs.
在两个PW域之间切换PW有两种方法。在第一种方法中（图2），两个独立的控制平面域终止于不同的PE。
|<-------Multi-Segment Pseudowire------->|
| PSN PSN |
AC | |<-1->| |<-2->| | AC
| V V V V V V |
| +----+ +-----+ +----+ +----+ |
+----+ | | |=====| | | |=====| | | +----+
| |-------|......PW1.......|--AC1--|......PW2......|-------| |
| CE1| | | | | | | | | | | |CE2 |
| |-------|......PW3.......|--AC2--|......PW4......|-------| |
+----+ | | |=====| | | |=====| | | +----+
^ +----+ +-----+ +----+ +----+ ^
| PE1 PE2 PE3 PE4 |
| ^ ^ |
| | | |
| PW switching points |
| |
| |
|<-------------------- Emulated Service ---------------->|
|<-------Multi-Segment Pseudowire------->|
| PSN PSN |
AC | |<-1->| |<-2->| | AC
| V V V V V V |
| +----+ +-----+ +----+ +----+ |
+----+ | | |=====| | | |=====| | | +----+
| |-------|......PW1.......|--AC1--|......PW2......|-------| |
| CE1| | | | | | | | | | | |CE2 |
| |-------|......PW3.......|--AC2--|......PW4......|-------| |
+----+ | | |=====| | | |=====| | | +----+
^ +----+ +-----+ +----+ +----+ ^
| PE1 PE2 PE3 PE4 |
| ^ ^ |
| | | |
| PW switching points |
| |
| |
|<-------------------- Emulated Service ---------------->|
Figure 2: PW Switching Using AC Reference Model
图2：使用交流参考模型的PW切换
In Figure 2, pseudowires in two separate PSNs are stitched together using native service attachment circuits. PE2 and PE3 only run the control plane for the PSN to which they are directly attached. At PE2 and PE3, PW1 and PW2 are connected using attachment circuit AC1, while PW3 and PW4 are connected using attachment circuit AC2.
在图2中，使用本机服务连接电路将两个单独PSN中的伪线缝合在一起。PE2和PE3仅为其直接连接的PSN运行控制平面。在PE2和PE3处，PW1和PW2使用附件电路AC1连接，而PW3和PW4使用附件电路AC2连接。
Native |<-----Multi-Segment Pseudowire------>| Native
Service | PSN PSN | Service
(AC) | |<-Tunnel->| |<-Tunnel->| | (AC)
| V V 1 V V 2 V V |
| +----+ +-----+ +----+ |
+----+ | |TPE1|==========|SPE1 |==========|TPE2| | +----+
| |------|.....PW.Seg't1....X....PW.Seg't3.....|-------| |
| CE1| | | | | | | | | |CE2 |
| |------|.....PW.Seg't2....X....PW.Seg't4.....|-------| |
+----+ | | |==========| |==========| | | +----+
^ +----+ +-----+ +----+ ^
| Provider Edge 1 ^ Provider Edge 2 |
| | |
| | |
| PW switching point |
| |
|<----------------- Emulated Service --------------->|
Native |<-----Multi-Segment Pseudowire------>| Native
Service | PSN PSN | Service
(AC) | |<-Tunnel->| |<-Tunnel->| | (AC)
| V V 1 V V 2 V V |
| +----+ +-----+ +----+ |
+----+ | |TPE1|==========|SPE1 |==========|TPE2| | +----+
| |------|.....PW.Seg't1....X....PW.Seg't3.....|-------| |
| CE1| | | | | | | | | |CE2 |
| |------|.....PW.Seg't2....X....PW.Seg't4.....|-------| |
+----+ | | |==========| |==========| | | +----+
^ +----+ +-----+ +----+ ^
| Provider Edge 1 ^ Provider Edge 2 |
| | |
| | |
| PW switching point |
| |
|<----------------- Emulated Service --------------->|
Figure 3: MS-PW Reference Model
图3:MS-PW参考模型
In Figure 3, SPE1 runs two separate control planes: one toward TPE1, and one toward TPE2. The PW switching point (S-PE) is configured to connect PW Segment 1 and PW Segment 3 together to complete the multi-segment PW between TPE1 and TPE2. PW Segment 1 and PW Segment 3 MUST be of the same PW type, but PSN Tunnel 1 and PSN Tunnel 2 need not be the same technology. In the latter case, if the PW is switched to a different technology, the PEs must adapt the PDU encapsulation between the different PSN technologies. In the case where PSN Tunnel 1 and PSN Tunnel 2 are the same technology, the PW PDU does not need to be modified, and PDUs are then switched between the pseudowires at the PW label level.
在图3中，SPE1运行两个单独的控制平面：一个朝向TPE1，另一个朝向TPE2。PW开关点（S-PE）配置为将PW段1和PW段3连接在一起，以完成TPE1和TPE2之间的多段PW。PW段1和PW段3必须为相同的PW类型，但PSN隧道1和PSN隧道2不需要采用相同的技术。在后一种情况下，如果PW切换到不同的技术，PEs必须在不同的PSN技术之间调整PDU封装。在PSN隧道1和PSN隧道2是相同技术的情况下，不需要修改PW PDU，然后在PW标签级别的伪线之间切换PDU。
It should be noted that it is possible to adapt one PSN technology to a different one, for example, MPLS over an IP encapsulation or Generic Routing Encapsulation (GRE) [RFC4023], but this is outside the scope of this document. Further, one could perform an interworking function on the PWs themselves at the S-PE, allowing conversion from one PW type to another, but this is also outside the scope of this document.
应该注意的是，可以将一种PSN技术调整为不同的技术，例如，IP封装上的MPLS或通用路由封装（GRE）[RFC4023]，但这超出了本文档的范围。此外，可以在S-PE上对PWs本身执行互通功能，允许从一种PW类型转换为另一种PW类型，但这也不在本文件的范围内。
This document describes procedures for building multi-segment pseudowires using manual configuration of the switching point PE1.
本文件描述了使用手动配置开关点PE1构建多段伪导线的程序。
Other documents may build on this base specification to automate the configuration and selection of S-PE1. All elements of the establishment of end-to-end MS-PWs including routing and signaling are out of scope of this document, and any discussion in this document serves purely as examples. It should also be noted that a PW can traverse multiple PW switching points along it's path, and the edge PEs will not require any specific knowledge of how many S-PEs the PW has traversed (though this may be reported for troubleshooting purposes).
其他文档可基于此基本规范来自动配置和选择S-PE1。建立端到端MS PWs的所有要素（包括路由和信令）均不在本文件范围内，本文件中的任何讨论仅作为示例。还应注意的是，一个PW可以沿着其路径穿过多个PW开关点，边缘PE不需要任何关于PW穿过多少s-PE的具体知识（尽管出于故障排除目的可能会报告）。
The general approach taken for MS-PWs is to connect the individual control planes by passing along any signaling information immediately upon reception. First, the S-PE is configured to switch a PW segment from a specific peer to another PW segment destined for a different peer. No control messages are exchanged yet, as the S-PE does not have enough information to actually initiate the PW setup messages. However, if a session does not already exist, a control protocol (LDP/L2TP) session MAY be setup. In this model, the MS-PW setup is starting from the T-PE devices. Once the T-PE is configured, it sends the PW control setup messages. These messages are received by the S-PE, and immediately used to form the PW setup messages for the next SS-PW of the MS-PW.
MS PWs采用的一般方法是在接收到信号后立即通过传递任何信令信息来连接各个控制平面。首先，S-PE被配置为将PW段从特定对等机切换到目的地为不同对等机的另一PW段。由于S-PE没有足够的信息来实际启动PW设置消息，因此尚未交换任何控制消息。然而，如果会话不存在，则可以设置控制协议（LDP/L2TP）会话。在此模型中，MS-PW设置从T-PE设备开始。一旦配置了T-PE，它将发送PW控制设置消息。这些消息由S-PE接收，并立即用于形成MS-PW下一个SS-PW的PW设置消息。
The PWs in each PSN are established independently, with each PSN being treated as a separate PW domain. For example, in Figure 2 for the case of MPLS PSNs, PW1 is setup between PE1 and PE2 using the LDP targeted session as described in [RFC4447], and at the same time a separate pseudowire, PW2, is setup between PE3 and PE4. The ACs for PW1 and PW2 at PE2 and PE3 MUST be configured such that they are the same PW type, e.g., ATM Virtual Channel Connection (VCC), Ethernet VLAN, etc.
每个PSN中的PW独立建立，每个PSN被视为一个单独的PW域。例如，在图2中，对于MPLS PSN的情况，使用[RFC4447]中所述的LDP目标会话在PE1和PE2之间设置PW1，同时在PE3和PE4之间设置单独的伪线PW2。PE2和PE3处PW1和PW2的ACs必须配置为相同的PW类型，例如ATM虚拟通道连接（VCC）、以太网VLAN等。
The general applicability of MS-PWs and their relationship to L2VPNs are described in [RFC5659]. The applicability of a PW type, as specified in the relevant RFC for that encapsulation (e.g., [RFC4717] for ATM), applies to each segment. This section describes further applicability considerations.
[RFC5659]中描述了MS PWs的一般适用性及其与L2VPN的关系。PW类型的适用性，如相关RFC中针对该封装的规定（例如，[RFC4717]适用于ATM），适用于每个段。本节描述了进一步的适用性注意事项。
As with SS-PWs, the performance of any segment will be limited by the performance of the underlying PSN. The performance may be further degraded by the emulation process, and performance degradation may be further increased by traversing multiple PW segments. Furthermore, the overall performance of an MS-PW is no better than the worst-performing segment of that MS-PW.
与SS PWs一样，任何细分市场的业绩都将受到基础PSN业绩的限制。性能可通过仿真过程进一步降低，并且性能降低可通过遍历多个PW段而进一步增加。此外，MS-PW的整体性能并不比该MS-PW中性能最差的部分好。
Since different PSN types may be able to achieve different maximum performance objectives, it is necessary to carefully consider which PSN types are used along the path of an MS-PW.
由于不同的PSN类型可能能够实现不同的最大性能目标，因此需要仔细考虑哪些PSN类型沿MS-PW的路径使用。
Referencing Figure 3, T-PE1 set up PW Segment 1 using the LDP targeted session as described in [RFC4447], at the same time a separate pseudowire, PW Segment 3, is setup to T-PE2. Each PW is configured independently on the PEs, but on S-PE1, PW Segment 1 is connected to PW Segment 3. PDUs are then switched between the pseudowires at the PW label level. Hence, the data plane does not need any special knowledge of the specific pseudowire type. A simple standard MPLS label swap operation is sufficient to connect the two PWs, and in this case the PW adaptation function cannot be used. However, when pushing a new PSN label, the Time to Live (TTL) SHOULD be set to 255, or some other locally configured fixed value.
参考图3，T-PE1使用[RFC4447]中所述的LDP目标会话设置PW段1，同时将单独的伪线PW段3设置到T-PE2。每个PW在PEs上独立配置，但在S-PE1上，PW段1连接到PW段3。然后在PW标签级别的伪线之间切换PDU。因此，数据平面不需要特定伪线类型的任何特殊知识。简单的标准MPLS标签交换操作足以连接两个PW，在这种情况下，不能使用PW自适应功能。但是，在推送新的PSN标签时，生存时间（TTL）应设置为255，或其他一些本地配置的固定值。
This process can be repeated as many times as necessary; the only limitation to the number of S-PEs traversed is imposed by the TTL field of the PW MPLS label. The setting of the TTL of the PW MPLS label is a matter of local policy on the originating PE, but SHOULD be set to 255. However, if the PW PDU contains an Operations, Administration, and Maintenance (OAM) packet, then the TTL can be set to the required value as explained later in this document.
这个过程可以根据需要重复多次；对经过的S-PE数量的唯一限制是PW MPLS标签的TTL字段。PW MPLS标签的TTL设置是原始PE上的本地策略问题，但应设置为255。但是，如果PW PDU包含操作、管理和维护（OAM）数据包，则可以将TTL设置为所需的值，如本文档后面所述。
There are three different mechanisms for MPLS-to-MPLS PW setup:
MPLS到MPLS PW设置有三种不同的机制：
-i. Static configuration of the PW -ii. LDP using FEC 128 -iii. LDP using the generalized FEC 129
-一,。PW-ii的静态配置。使用FEC128-iii的LDP。使用广义FEC129的LDP
This results in four distinct PW switching situations that are significantly different and must be considered in detail:
这导致了四种不同的PW切换情况，这些情况明显不同，必须详细考虑：
-i. Switching between two static control planes -ii. Switching between a static and a dynamic LDP control plane -iii. Switching between two LDP control planes using the same FEC type -iv. Switching between LDP using FEC 128 and LDP using the generalized FEC 129
-一,。在两个静态控制平面之间切换-ii。在静态和动态LDP控制平面之间切换-iii.使用相同FEC类型在两个LDP控制平面之间切换-iv.使用FEC 128在LDP和使用通用FEC 129在LDP之间切换
In the case of two static control planes, the S-PE MUST be configured to direct the MPLS packets from one PW into the other. There is no control protocol involved in this case. When one of the control planes is a simple static PW configuration and the other control
在两个静态控制平面的情况下，必须将S-PE配置为将MPLS数据包从一个PW定向到另一个PW。本案不涉及控制协议。当其中一个控制平面为简单的静态PW配置，而另一个控制平面为
plane is a dynamic LDP FEC 128 or generalized PW FEC, then the static control plane should be considered similar to an attachment circuit (AC) in the reference model of Figure 1. The switching point PE SHOULD signal the appropriate PW status if it detects a failure in sending or receiving packets over the static PW segment. In the absence of a PW status communication mechanism when the PW is statically configured, the status communicated to the dynamic LDP PW will be limited to local interface failures. In this case, the S-PE PE behaves in a very similar manner to a T-PE, assuming an active signaling role. This means that the S-PE will immediately send the LDP Label Mapping message if the static PW is deemed to be UP.
平面为动态LDP FEC 128或广义PW FEC，则静态控制平面应视为类似于图1参考模型中的连接电路（AC）。如果切换点PE在通过静态PW段发送或接收数据包时检测到故障，则应向相应的PW状态发送信号。在静态配置PW时没有PW状态通信机制的情况下，传送到动态LDP PW的状态将限于本地接口故障。在这种情况下，S-PE以与T-PE非常相似的方式表现，承担主动信令角色。这意味着，如果静态PW被认为已启动，则S-PE将立即发送LDP标签映射消息。
The S-PE SHOULD assume an initial passive role. This means that when independent PWs are configured on the switching point, the Label Switching Router (LSR) does not advertise the LDP PW FEC mapping until it has received at least one of the two PW LDP FECs from a remote PE. This is necessary because the switching point LSR does not know a priori what the interface parameter field in the initial FEC advertisement will contain.
S-PE应承担最初的被动角色。这意味着，当在交换点上配置独立的PW时，标签交换路由器（LSR）在从远程PE接收到两个PW-LDP-FEC中的至少一个之前不播发LDP-PW-FEC映射。这是必要的，因为切换点LSR事先不知道初始FEC广告中的接口参数字段将包含什么。
If one of the S-PEs doesn't accept an LDP Label Mapping message, then a Label Release message may be sent back to the originator T-PE depending on the cause of the error. LDP liberal label retention mode still applies; hence, if a PE is simply not configured yet, the label mapping is stored for future use. An MS-PW is declared UP only when all the constituent SS-PWs are UP.
如果其中一个S-PE不接受LDP标签映射消息，则可根据错误原因将标签释放消息发送回发起人t-PE。LDP自由标签保留模式仍然适用；因此，如果还没有配置PE，则存储标签映射以供将来使用。MS-PW仅在所有组成SS PW都启动时才宣布启动。
The Pseudowire Identifier (PWid), as defined in [RFC4447], is a unique number between each pair of PEs. Hence, each SS-PW that forms an MS-PW may have a different PWid. In the case of the generalized PW FEC, the Attachment Group Identifier (AGI) / Source Attachment Identifier (SAI) / Target Attachment Identifier (TAI) may have to also be different for some, or sometimes all, SS-PWs.
[RFC4447]中定义的伪线标识符（PWid）是每对PE之间的唯一编号。因此，形成MS-PW的每个SS-PW可以具有不同的PWid。在通用PW FEC的情况下，对于某些或有时全部SS PW，附件组标识符（AGI）/源附件标识符（SAI）/目标附件标识符（TAI）可能也必须不同。
When an MS-PW is signaled using FEC 129, each T-PE might independently start signaling the MS-PW. If the MS-PW path is not statically configured, in certain cases the signaling procedure could result in an attempt to set up each direction of the MS-PW through different S-PEs. If an operator wishes to avoid this situation, one of the T-PEs MUST start the PW signaling (active role), while the other waits to receive the LDP label mapping before sending the respective PW LDP Label Mapping message (passive role). When the MS-PW path is not statically configured, the active T-PE (the Source
当使用FEC 129向MS-PW发送信号时，每个T-PE可以独立地开始向MS-PW发送信号。如果MS-PW路径不是静态配置的，在某些情况下，信令程序可能导致尝试通过不同的S-PE设置MS-PW的每个方向。如果运营商希望避免这种情况，其中一个T-PE必须启动PW信令（主动角色），而另一个在发送相应的PW LDP标签映射消息（被动角色）之前等待接收LDP标签映射。当MS-PW路径未静态配置时，活动T-PE（源
T-PE) and the passive T-PE (the Target T-PE) MUST be identified before signaling is initiated for a given MS-PW.
在为给定MS-PW启动信令之前，必须识别T-PE）和无源T-PE（目标T-PE）。
The determination of which T-PE assumes the active role SHOULD be done as follows:
确定哪个T-PE承担积极作用应如下所示：
The SAII and TAII are compared as unsigned integers; if the SAII is larger, then the T-PE assumes the active role.
SAII和TAII作为无符号整数进行比较；如果SAII较大，则T-PE承担主动作用。
The selection process to determine which T-PE assumes the active role MAY be superseded by manual provisioning. In this case, one of the T-PEs MUST be set to the active role, and the other one MUST be set to the passive role.
确定哪个T-PE担任主动角色的选择过程可能会被手动资源调配所取代。在这种情况下，一个T-PE必须设置为主动角色，另一个必须设置为被动角色。
When a PE is using the generalized FEC 129, there are two distinct roles that a PE can assume: active and passive. A PE that assumes the active role will send the LDP PW setup message, while a passive role PE will simply reply to an incoming LDP PW setup message. The S-PE will always remain passive until a PWid FEC 128 LDP message is received, which will cause the corresponding generalized PW FEC LDP message to be formed and sent. If a generalized FEC PW LDP message is received while the switching point PE is in a passive role, the corresponding PW FEC 128 LDP message will be formed and sent.
当PE使用通用FEC 129时，PE可以承担两个不同的角色：主动和被动。承担主动角色的PE将发送LDP PW设置消息，而被动角色PE将只回复传入的LDP PW设置消息。在接收到PWid FEC 128 LDP消息之前，S-PE将始终保持被动，这将导致形成并发送相应的通用PW FEC LDP消息。如果在交换点PE处于被动角色时接收到广义FEC PW LDP消息，则将形成并发送相应的PW FEC 128 LDP消息。
PWids need to be mapped to the corresponding AGI/TAI/SAI and vice versa. This can be accomplished by local S-PE configuration, or by some other means, such as some form of auto discovery. Such other means are outside the scope of this document.
PWID需要映射到相应的AGI/TAI/SAI，反之亦然。这可以通过本地S-PE配置或其他一些方式（如某种形式的自动发现）来实现。此类其他方式不在本文件范围内。
The edge-to-edge PW might traverse several switching points, in separate administrative domains. For management and troubleshooting reasons, it is useful to record information about the switching points at the S-PEs that the PW traverses. This is accomplished by using a PW Switching Point PE TLV (SP-PE TLV).
边到边PW可能在单独的管理域中穿越多个切换点。出于管理和故障排除的原因，记录PW经过的S-PEs处的开关点信息非常有用。这是通过使用PW开关点PE TLV（SP-PE TLV）实现的。
Sending the SP-PE TLV is OPTIONAL; however, the PE or S-PE MUST process the TLV upon reception. The "U" bit MUST be set for backward compatibility with T-PEs that do not support the MS-PW extensions described in the document. The SP-PE TLV MAY appear only once for each switching point traversed, and it cannot be of length zero. The SP-PE TLV is appended to the PW FEC at each S-PE, and the order of the SP-PE TLVs in the LDP message MUST be preserved. The SP-PE TLV
发送SP-PE TLV是可选的；但是，PE或S-PE必须在接收时处理TLV。必须将“U”位设置为与不支持本文件所述MS-PW扩展的T-PEs向后兼容。SP-PE TLV对于经过的每个开关点只能出现一次，且长度不能为零。SP-PE TLV附加到每个S-PE的PW FEC，并且必须保留LDP消息中SP-PE TLV的顺序。SP-PE TLV
is necessary to support some of the Virtual Circuit Connectivity Verification (VCCV) functions for MS-PWs. See Section 9.5 for more details. The SP-PE TLV is encoded as follows:
对于支持MS PWs的某些虚拟电路连接验证（VCCV）功能是必需的。详见第9.5节。SP-PE TLV编码如下：
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| SP-PE TLV (0x096D) | SP-PE TLV Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLV Type | Length | Variable Length Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Variable Length Value |
| " " " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| SP-PE TLV (0x096D) | SP-PE TLV Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLV Type | Length | Variable Length Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Variable Length Value |
| " " " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- SP-PE TLV Length
- SP-PE TLV长度
Specifies the total length of all the following SP-PE TLV fields in octets.
指定以下所有SP-PE TLV字段的总长度（以八位字节为单位）。
- Sub-TLV Type
- 亚TLV型
Encodes how the Value field is to be interpreted.
编码如何解释值字段。
- Length
- 长
Specifies the length of the Value field in octets.
以八位字节为单位指定值字段的长度。
- Value
- 价值
Octet string of Length octets that encodes information to be interpreted as specified by the Type field.
八位字节长度八位字节的字符串，该字符串对要按照类型字段指定的方式解释的信息进行编码。
PW Switching Point PE sub-TLV Types are assigned by IANA according to the process defined in Section 14 (IANA Considerations) below.
PW开关点PE子TLV类型由IANA根据下文第14节（IANA注意事项）中定义的流程分配。
For local policy reasons, a particular S-PE can filter out all SP-PE TLVs in a Label Mapping message that traverses it and not include its own SP-PE TLV. In this case, from any upstream PE, it will appear as if this particular S-PE is the T-PE. This might be necessary, depending on local policy, if the S-PE is at the service provider administrative boundary. It should also be noted that because there are no SP-PE TLVs describing the path beyond the S-PE that removed them, VCCV will only work as far as that S-PE.
出于本地策略原因，特定的S-PE可以过滤掉标签映射消息中的所有SP-PE TLV，该消息将遍历该S-PE，而不包括其自己的SP-PE TLV。在这种情况下，从任何上游PE来看，似乎该特定S-PE就是T-PE。如果S-PE位于服务提供商管理边界，则这可能是必要的，具体取决于本地策略。还应注意的是，由于没有SP-PE TLV描述移除它们的S-PE之外的路径，VCCV仅在S-PE范围内工作。
The SP-PE TLV contains sub-TLVs that describe various characteristics of the S-PE traversed. The SP-PE TLV MUST contain the appropriate mandatory sub-TLVs specified below. The definitions of the PW Switching Point PE sub-TLVs are as follows:
SP-PE TLV包含子TLV，描述所穿越S-PE的各种特征。SP-PE TLV必须包含以下规定的相应强制性子TLV。PW开关点PE子TLV的定义如下：
- PWid of last PW segment traversed.
- 最后经过的PW段的PWid。
This is only applicable if the last PW segment traversed used LDP FEC 128 to signal the PW. This sub-TLV type contains a PWid in the format of the PWid described in [RFC4447]. This is just a 32-bit unsigned integer number.
这仅适用于最后穿过的PW段使用LDP FEC 128向PW发送信号的情况。此子TLV类型包含[RFC4447]中描述的PWid格式的PWid。这只是一个32位无符号整数。
- PW Switching Point description string.
- PW开关点描述字符串。
An OPTIONAL description string of text up to 80 characters long. Human-readable text MUST be provided in the UTF-8 character set using the Default Language [RFC2277].
一个可选的描述字符串，包含最多80个字符的文本。必须使用默认语言[RFC2277]在UTF-8字符集中提供人类可读文本。
- Local IP address of PW Switching Point.
- PW交换点的本地IP地址。
The local IPv4 or IPv6 address of the PW Switching Point. This is an OPTIONAL Sub-TLV. In most cases, this will be the local LDP session IP address of the S-PE.
PW交换点的本地IPv4或IPv6地址。这是一个可选的子TLV。在大多数情况下，这将是S-PE的本地LDP会话IP地址。
- Remote IP address of the last PW Switching Point traversed or of the T-PE.
- 最后经过的PW交换点或T-PE的远程IP地址。
The IPv4 or IPv6 address of the last PW Switching Point traversed or of the T-PE. This is an OPTIONAL Sub-TLV. In most cases, this will be the remote IP address of the LDP session. This Sub-TLV SHOULD only be included if there are no other SP-PE TLVs present from other S-PEs, or if the remote IP address of the LDP session does not correspond to the "Local IP address of PW Switching Point" TLV value contained in the last SP-PE TLV.
最后经过的PW交换点或T-PE的IPv4或IPv6地址。这是一个可选的子TLV。在大多数情况下，这将是LDP会话的远程IP地址。仅当其他S-PE不存在其他SP-PE TLV，或者LDP会话的远程IP地址与最后一个SP-PE TLV中包含的“PW交换点的本地IP地址”TLV值不一致时，才应包括该子TLV。
- The FEC element of last PW segment traversed.
- 最后经过的PW段的FEC元素。
This is only applicable if the last PW segment traversed used LDP FEC 129 to signal the PW.
这仅适用于通过的最后一个PW段使用LDP FEC 129向PW发送信号的情况。
The FEC element of the last PW segment traversed. This is encoded in the following format:
最后经过的PW段的FEC元素。这是按以下格式编码的：
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AGI Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ AGI Value (contd.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SAII Value (contd.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ TAII Value (contd.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AGI Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ AGI Value (contd.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SAII Value (contd.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ TAII Value (contd.) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- L2 PW address of the PW Switching Point (recommended format).
- PW开关点的L2 PW地址（推荐格式）。
This sub-TLV type contains an L2 PW address of PW Switching Point in the format described in Section 3.2 of [RFC5003]. This includes the AII type field and length, as well as the L2 PW address with the AC ID field set to zero.
该子TLV类型包含PW开关点的L2 PW地址，格式如[RFC5003]第3.2节所述。这包括AII类型字段和长度，以及AC ID字段设置为零的L2 PW地址。
[RFC4447] defines several interface parameters, which are used by the Network Service Processing (NSP) to adapt the PW to the attachment circuit (AC). The interface parameters are only used at the endpoints, and MUST be passed unchanged across the S-PE. However, the following interface parameters MAY be modified as follows:
[RFC4447]定义了几个接口参数，网络服务处理（NSP）使用这些参数使PW适应连接电路（AC）。接口参数仅在端点处使用，并且必须在S-PE中以不变的方式传递。但是，以下接口参数可以修改如下：
- 0x03 Optional Interface Description string This Interface parameter MAY be modified or altogether removed from the FEC element depending on local configuration policies.
- 0x03可选接口描述字符串根据本地配置策略，此接口参数可以从FEC元素中修改或完全删除。
- 0x09 Fragmentation indicator This parameter MAY be inserted in the FEC by the switching point if it is capable of re-assembly of fragmented PW frames according to [RFC4623].
- 0x09碎片指示符如果该参数能够根据[RFC4623]重新组装碎片PW帧，则该参数可由切换点插入FEC中。
- 0x0C VCCV parameter This Parameter contains the Control Channel (CC) type and Connectivity Verification (CV) type bit fields. The CV type bit field MUST be reset to reflect the CV type supported by the S-PE. The CC type bit field MUST have bit 1 "Type 2: MPLS Router Alert Label" set to 0. The other bit fields MUST be reset to reflect the CC type supported by the S-PE.
- 0x0C VCCV参数此参数包含控制通道（CC）类型和连接验证（CV）类型位字段。CV类型位字段必须重置以反映S-PE支持的CV类型。CC类型位字段的位1“类型2:MPLS路由器警报标签”必须设置为0。其他位字段必须重置以反映S-PE支持的CC类型。
The Group ID (GR ID) is used to reduce the number of status messages that need to be sent by the PE advertising the PW FEC. The GR ID has local significance only, and therefore MUST be mapped to a unique GR ID allocated by the S-PE.
组ID（GR ID）用于减少PE向PW FEC发送广告时需要发送的状态消息的数量。GR ID仅具有本地意义，因此必须映射到S-PE分配的唯一GR ID。
A switching point PE SHOULD inspect the PW Switching Point PE TLV, to verify that its own IP address does not appear in it. If the PE's IP address appears in a received PW Switching Point PE TLV, the PE SHOULD break the loop and send a label release message with the following error code:
切换点PE应检查PW切换点PE TLV，以验证其自身的IP地址未出现在其中。如果PE的IP地址出现在收到的PW交换点PE TLV中，则PE应断开循环并发送带有以下错误代码的标签释放消息：
Value E Description 0x0000003A 0 PW Loop Detected
检测到值E说明0x0000003A 0 PW循环
If an S-PE along the MS-PW removed all SP-PE TLVs, as mentioned above, this loop detection method will fail.
如上文所述，如果MS-PW沿线的S-PE移除了所有SP-PE TLV，则该环路检测方法将失败。
Both MPLS and L2TPv3 PWs may be static or dynamic. This results in four possibilities when switching between L2TPv3 and MPLS.
MPLS和L2TPv3 PWs可以是静态的，也可以是动态的。这导致在L2TPv3和MPLS之间切换时出现四种可能性。
-i. Switching between static MPLS and L2TPv3 PWs -ii. Switching between a static MPLS PW and a dynamic L2TPv3 PW -iii. Switching between a static L2TPv3 PW and a dynamic LDP/MPLS PW -iv. Switching between a dynamic LDP/MPLS PW and a dynamic L2TPv3 PW
-一,。在静态MPLS和L2TPv3 PWs-ii之间切换。在静态MPLS PW和动态L2TPv3 PW之间切换-iii.在静态L2TPv3 PW和动态LDP/MPLS PW之间切换-iv.在动态LDP/MPLS PW和动态L2TPv3 PW之间切换
In the case of two static control planes, the S-PE MUST be configured to direct packets from one PW into the other. There is no control protocol involved in this case. The configuration MUST include which MPLS PW Label maps to which L2TPv3 Session ID (and associated Cookie, if present) as well as which MPLS Tunnel Label maps to which PE destination IP address.
在两个静态控制平面的情况下，必须将S-PE配置为将数据包从一个PW定向到另一个PW。本案不涉及控制协议。配置必须包括哪个MPLS PW标签映射到哪个L2TPv3会话ID（以及相关的Cookie，如果存在），以及哪个MPLS隧道标签映射到哪个PE目标IP地址。
When a statically configured MPLS PW is switched to a dynamic L2TPv3 PW, the static control plane should be considered identical to an attachment circuit (AC) in the reference model of Figure 1. The switching point PE SHOULD signal the appropriate PW status if it detects a failure in sending or receiving packets over the static PW. Because the PW is statically configured, the status communicated to the dynamic L2TPv3 PW will be limited to local interface failures. In this case, the S-PE behaves in a very similar manner to a T-PE, assuming an active role.
当静态配置的MPLS PW切换到动态L2TPv3 PW时，静态控制平面应视为与图1参考模型中的连接电路（AC）相同。如果切换点PE在通过静态PW发送或接收数据包时检测到故障，则应向相应的PW状态发送信号。由于PW是静态配置的，因此与动态L2TPv3 PW通信的状态将限于本地接口故障。在这种情况下，S-PE的行为方式与T-PE非常相似，扮演着积极的角色。
When a statically configured L2TPv3 PW is switched to a dynamic LDP/MPLS PW, then the static control plane should be considered identical to an attachment circuit (AC) in the reference model of Figure 1. The switching point PE SHOULD signal the appropriate PW status (via an L2TPv3 Set-Link-Info (SLI) message) if it detects a failure in sending or receiving packets over the static PW. Because the PW is statically configured, the status communicated to the dynamic LDP/MPLS PW will be limited to local interface failures. In this case, the S-PE behaves in a very similar manner to a T-PE, assuming an active role.
当静态配置的L2TPv3 PW切换为动态LDP/MPLS PW时，静态控制平面应视为与图1参考模型中的连接电路（AC）相同。如果切换点PE在通过静态PW发送或接收数据包时检测到故障，则应（通过L2TPv3 Set Link Info（SLI）消息）向适当的PW状态发送信号。由于PW是静态配置的，因此与动态LDP/MPLS PW通信的状态将限于本地接口故障。在这种情况下，S-PE的行为方式与T-PE非常相似，扮演着积极的角色。
When switching between dynamic PWs, the switching point always assumes an initial passive role. Thus, it does not initiate an LDP/MPLS or L2TPv3 PW until it has received a connection request (Label Mapping or Incoming-Call-Request (ICRQ)) from one side of the node. Note that while MPLS PWs are made up of two unidirectional Label Switched Paths (LSPs) bonded together by FEC identifiers, L2TPv3 PWs are bidirectional in nature, setup via a three-message exchange (ICRQ, Incoming-Call-Reply (ICRP), and Incoming-Call-Connected (ICCN)). Details of Session Establishment, Tear Down, and PW Status signaling are detailed below.
在动态PW之间切换时，切换点始终承担初始被动角色。因此，在从节点的一侧接收到连接请求（标签映射或传入呼叫请求（ICRQ））之前，它不会启动LDP/MPLS或L2TPv3 PW。注意，虽然MPLS PW由两个由FEC标识符连接在一起的单向标签交换路径（LSP）组成，但L2TPv3 PW本质上是双向的，通过三个消息交换（ICRQ、传入呼叫应答（ICRP）和传入呼叫连接（ICCN））进行设置。会话建立、中断和PW状态信令的详细信息如下。
When the S-PE receives an L2TPv3 ICRQ message, the identifying AVPs included in the message are mapped to FEC identifiers and sent in an LDP Label Mapping message. Conversely, if an LDP Label Mapping message is received, it is either mapped to an ICRP message or causes an L2TPv3 session to be initiated by sending an ICRQ.
当S-PE接收到L2TPv3 ICRQ消息时，包括在该消息中的识别avp被映射到FEC标识符并在LDP标签映射消息中发送。相反，如果接收到LDP标签映射消息，则它要么映射到ICRP消息，要么通过发送ICRQ来启动L2TPv3会话。
Following are two example exchanges of messages between LDP and L2TPv3. The first is a case where an L2TPv3 T-PE initiates an MS-PW; the second is a case where an MPLS T-PE initiates an MS-PW.
以下是LDP和L2TPv3之间的两个消息交换示例。第一个是L2TPv3 T-PE发起MS-PW的情况；第二个是MPLS T-PE发起MS-PW的情况。
PE 1 (L2TPv3) PW Switching Node PE3 (MPLS/LDP)
PE 1（L2TPv3）PW交换节点PE3（MPLS/LDP）
AC "Up"
L2TPv3 ICRQ --->
LDP Label Mapping --->
AC "Up"
<--- LDP Label Mapping
<--- L2TPv3 ICRP
L2TPv3 ICCN --->
<-------------------- MS-PW Established ------------------>
PE 1 (MPLS/LDP) PW Switching Node PE3 (L2TPv3)
AC "Up"
L2TPv3 ICRQ --->
LDP Label Mapping --->
AC "Up"
<--- LDP Label Mapping
<--- L2TPv3 ICRP
L2TPv3 ICCN --->
<-------------------- MS-PW Established ------------------>
PE 1 (MPLS/LDP) PW Switching Node PE3 (L2TPv3)
AC "Up"
LDP Label Mapping --->
L2TPv3 ICRQ --->
<--- L2TPv3 ICRP
<--- LDP Label Mapping
L2TPv3 ICCN --->
AC "Up"
<-------------------- MS-PW Established ------------------>
AC "Up"
LDP Label Mapping --->
L2TPv3 ICRQ --->
<--- L2TPv3 ICRP
<--- LDP Label Mapping
L2TPv3 ICCN --->
AC "Up"
<-------------------- MS-PW Established ------------------>
L2TPv3 uses the SLI message to indicate an interface status change (such as the interface transitioning from "Up" or "Down"). MPLS/LDP PWs either signal this via an LDP Label Withdraw or the PW Status Notification message defined in Section 4.4 of [RFC4447]. The LDP status TLV bit SHOULD be mapped to the L2TPv3 equivalent Extended Circuit Status Values TLV specified in [RFC5641].
L2TPv3使用SLI消息指示接口状态更改（例如接口从“向上”或“向下”转换）。MPLS/LDP PWs可通过LDP标签撤回或[RFC4447]第4.4节中定义的PW状态通知消息发出此信号。LDP状态TLV位应映射到[RFC5641]中指定的L2TPv3等效扩展电路状态值TLV。
L2TPv3 uses a single message, Call-Disconnect-Notify (CDN), to tear down a pseudowire. The CDN message translates to a Label Withdraw message in LDP. Following are two example exchanges of messages
L2TPv3使用一条消息，即Call Disconnect Notify（CDN），来断开一条伪线。CDN消息在LDP中转换为标签撤回消息。下面是两个消息交换示例
between LDP and L2TPv3. The first is a case where an L2TPv3 T-PE initiates the termination of an MS-PW; the second is a case where an MPLS T-PE initiates the termination of an MS-PW.
在LDP和L2TPv3之间。第一个是L2TPv3 T-PE发起MS-PW终止的情况；第二个是MPLS T-PE发起MS-PW的终止的情况。
PE 1 (L2TPv3) PW Switching Node PE3 (MPLS/LDP)
PE 1（L2TPv3）PW交换节点PE3（MPLS/LDP）
AC "Down"
L2TPv3 CDN --->
LDP Label Withdraw --->
AC "Down"
<-- LDP Label Release
AC "Down"
L2TPv3 CDN --->
LDP Label Withdraw --->
AC "Down"
<-- LDP Label Release
<--------------- MS-PW Data Path Down ------------------>
PE 1 (MPLS LDP) PW Switching Node PE3 (L2TPv3)
<--------------- MS-PW Data Path Down ------------------>
PE 1 (MPLS LDP) PW Switching Node PE3 (L2TPv3)
AC "Down"
LDP Label Withdraw --->
L2TPv3 CDN -->
<-- LDP Label Release
AC "Down"
AC "Down"
LDP Label Withdraw --->
L2TPv3 CDN -->
<-- LDP Label Release
AC "Down"
<---------------- MS-PW Data Path Down ------------------>
<---------------- MS-PW Data Path Down ------------------>
[RFC4447] defines several interface parameters that MUST be mapped to the equivalent AVPs in L2TPv3 setup messages.
[RFC4447]定义了几个接口参数，这些参数必须映射到L2TPv3设置消息中的等效AVP。
* Interface MTU
* 接口MTU
The Interface MTU parameter is mapped directly to the L2TP "Interface Maximum Transmission Unit" AVP defined in [RFC4667].
接口MTU参数直接映射到[RFC4667]中定义的L2TP“接口最大传输单元”AVP。
* Max Number of Concatenated ATM cells
* 级联ATM信元的最大数目
This interface parameter is mapped directly to the L2TP "ATM Maximum Concatenated Cells AVP" described in Section 6 of [RFC4454].
该接口参数直接映射到[RFC4454]第6节中描述的L2TP“ATM最大级联信元AVP”。
* PW Type
* PW型
The PW Type defined in [RFC4446] is mapped to the L2TPv3 "Pseudowire Type" AVP defined in [RFC3931].
[RFC4446]中定义的PW类型映射到[RFC3931]中定义的L2TPv3“伪线类型”AVP。
* PWid (FEC 128)
* PWid（FEC 128）
For FEC 128, the PWid is mapped directly to the L2TPv3 "Remote End ID" AVP defined in [RFC3931].
对于FEC 128，PWid直接映射到[RFC3931]中定义的L2TPv3“远程端ID”AVP。
* Generalized FEC 129 SAI/TAI
* 通用FEC 129 SAI/TAI
Section 4.3 of [RFC4667] defines how to encode the SAI and TAI parameters. These can be mapped directly.
[RFC4667]第4.3节定义了如何对SAI和TAI参数进行编码。这些可以直接映射。
Other interface parameter mappings are unsupported when switching between LDP/MPLS and L2TPv3 PWs.
在LDP/MPLS和L2TPv3 PWs之间切换时，不支持其他接口参数映射。
When translating between LDP and L2TPv3 control messages, the PW Switching Point PE TLV described earlier in this document is carried in a single variable-length L2TP AVP present in the ICRQ and ICRP messages, and optionally in the ICCN message.
当在LDP和L2TPv3控制消息之间转换时，本文档前面描述的PW切换点PE TLV在ICRQ和ICRP消息中存在的单个可变长度L2TP AVP中携带，并且可选地在ICCN消息中携带。
The L2TP "PW Switching Point AVP" is Attribute Type 101. The AVP MAY be hidden (the L2TP AVP H-bit may be 0 or 1), the length of the AVP is 6 plus the length of the series of Switching Point PE sub-TLVs included in the AVP, and the AVP MUST NOT be marked Mandatory (the L2TP AVP M-bit MUST be 0).
L2TP“PW开关点AVP”是属性类型101。AVP可以是隐藏的（L2TP AVP H位可以是0或1），AVP的长度是6加上AVP中包含的一系列开关点PE子TLV的长度，并且AVP不得被标记为强制（L2TP AVP M位必须是0）。
When switching between an MPLS and L2TP PW, packets are sent in their entirety from one PW to the other, replacing the MPLS label stack with the L2TPv3 and IP header or vice versa.
在MPLS和L2TP PW之间切换时，数据包从一个PW整体发送到另一个PW，用L2TPv3和IP报头替换MPLS标签堆栈，反之亦然。
Section 5.4 of [RFC3985] discusses the purpose of the various shim headers necessary for enabling a pseudowire over an IP or MPLS PSN. For L2TPv3, the Payload Convergence and Sequencing function is carried out via the Default L2-Specific Sublayer defined in [RFC3931]. For MPLS, these two functions (together with PSN Convergence) are carried out via the MPLS Control Word. Since these functions are different between MPLS and L2TPv3, interworking between the two may be necessary.
[RFC3985]第5.4节讨论了在IP或MPLS PSN上启用伪线所需的各种垫片头的用途。对于L2TPv3，有效负载聚合和排序功能通过[RFC3931]中定义的默认L2特定子层执行。对于MPLS，这两个功能（以及PSN聚合）通过MPLS控制字执行。由于MPLS和L2TPv3之间的功能不同，因此可能需要两者之间的互通。
The L2TP L2-Specific Sublayer and MPLS Control Word are shim headers, which in some cases are not necessary to be present at all. For example, an Ethernet PW with sequencing disabled will generally not require an MPLS Control Word or L2TP Default L2-Specific Sublayer to be present at all. In this case, Ethernet frames are simply sent from one PW to the other without any modification beyond the MPLS and L2TP/IP encapsulation and decapsulation.
L2TP L2特定子层和MPLS控制字是垫片头，在某些情况下根本不需要存在垫片头。例如，禁用排序的以太网PW通常不需要存在MPLS控制字或L2TP默认L2特定子层。在这种情况下，以太网帧仅从一个PW发送到另一个PW，而无需进行MPLS和L2TP/IP封装和去封装以外的任何修改。
The following section offers guidelines for how to interwork between L2TP and MPLS for those cases where the Payload Convergence, Sequencing, or PSN Convergence functions are necessary on one or both sides of the switching node.
以下部分提供了在交换节点的一侧或两侧需要有效负载聚合、排序或PSN聚合功能的情况下，L2TP和MPLS之间如何互通的指南。
The MPLS Control Word consists of (from left to right):
MPLS控制字包括（从左到右）：
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Reserved | Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Reserved | Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-i. These bits are always zero in an MPLS PW PDU. It is not necessary to map them to L2TP.
-一,。在MPLS PW PDU中，这些位始终为零。不需要将它们映射到L2TP。
-ii. These six bits may be used for Payload Convergence depending on the PW type. For ATM, the first four of these bits are defined in [RFC4717]. These map directly to the bits defined in [RFC4454]. For Frame Relay, these bits indicate how to set the bits in the Frame Relay header that must be regenerated for L2TP as it carries the Frame Relay header intact.
-二,。根据PW类型，这六位可用于有效负载聚合。对于ATM，这些位中的前四位在[RFC4717]中定义。这些直接映射到[RFC4454]中定义的位。对于帧中继，这些位指示如何在帧中继报头中设置必须为L2TP重新生成的位，因为它携带完整的帧中继报头。
-iii. L2TP determines its payload length from IP. Thus, this Length field need not be carried directly to L2TP. This Length field will have to be calculated and inserted for MPLS when necessary.
-iii.L2TP根据IP确定其有效负载长度。因此，该长度字段不需要直接携带到L2TP。必要时，必须为MPLS计算并插入此长度字段。
-iv. The Default L2-Specific Sublayer has a sequence number with different semantics than that of the MPLS Control Word. This difference eliminates the possibility of supporting sequencing across the MS-PW by simply carrying the sequence number through the switching point transparently. As such, sequence numbers MAY be supported by checking the sequence numbers of packets arriving at the switching point and regenerating a new sequence number in the appropriate format for the PW on egress. If this type of sequence interworking at the switching node is not supported, and a T-PE requests sequencing of all packets via the L2TP control channel during session setup, the switching node SHOULD NOT allow the session to be established by sending a CDN message with Result Code set to 31 "Sequencing not supported".
-iv.默认的L2特定子层具有与MPLS控制字不同语义的序列号。这种差异通过简单地透明地携带序列号通过切换点，消除了支持跨MS-PW排序的可能性。因此，可以通过检查到达交换点的分组的序列号并以适当格式为出口上的PW重新生成新序列号来支持序列号。如果不支持交换节点处的这种序列互通，并且T-PE在会话设置期间请求通过L2TP控制信道对所有分组进行排序，则交换节点不应允许通过发送结果代码设置为31“sequencing not supported”（排序不支持）的CDN消息来建立会话。
Single-segment pseudowires are signaled using the Virtual Circuit Connectivity Verification (VCCV) parameter included in the interface parameter field of the PWid FEC TLV or the interface parameter sub-TLV of the Generalized PWid FEC TLV as described in [RFC5085]. When a switching point exists between PE nodes, it is required to be able to continue operating VCCV end-to-end across a switching point and to provide the ability to trace the path of the MS-PW over any number of segments.
如[RFC5085]所述，使用PWid FEC TLV的接口参数字段中包含的虚拟电路连接验证（VCCV）参数或通用PWid FEC TLV的接口参数sub TLV向单段伪线发送信号。当PE节点之间存在切换点时，需要能够在切换点上继续端到端操作VCCV，并提供在任意数量段上跟踪MS-PW路径的能力。
This document provides a method for achieving these two objectives. This method is based on reusing the existing VCCV Control Word (CW) and decrementing the TTL of the PW label at each S-PE in the path of the MS-PW.
本文件提供了实现这两个目标的方法。该方法基于重用现有VCCV控制字（CW）并减少MS-PW路径中每个S-PE处PW标签的TTL。
When an MS-PW includes SS-PWs that use the L2TPv3, the MPLS PW OAM MUST be terminated at the S-PE connecting the L2TPv3 and MPLS segments. Status information received in a particular PW segment can then be used to generate the appropriate status messages on the following PW segment. In the case of L2TPV3, the status bits in the circuit status AVP defined in Section 5.4.5 of [RFC3931] and Extended Circuit Status Values defined in [RFC5641] can be mapped directly to the PW status bits defined in Section 5.4.3 of [RFC4447].
当MS-PW包括使用L2TPv3的SS PW时，MPLS PW OAM必须在连接L2TPv3和MPLS段的S-PE处终止。在特定PW段中接收的状态信息可用于生成下一PW段上的适当状态消息。在L2TPV3的情况下，[RFC3931]第5.4.5节中定义的电路状态AVP中的状态位和[RFC5641]中定义的扩展电路状态值可以直接映射到[RFC4447]第5.4.3节中定义的PW状态位。
VCCV messages are specific to the MPLS data plane and cannot be used for an L2TPv3 PW segment. Therefore, the S-PE MUST NOT send the VCCV parameter included in the interface parameter field of the PWid FEC TLV or the sub-TLV interface parameter of the Generalized PWid FEC TLV. It might be possible to translate VCCV messages from L2TPv3 PW segments to MPLS PW segments and vice versa; however, this topic is left for further study.
VCCV消息特定于MPLS数据平面，不能用于L2TPv3 PW段。因此，S-PE不得发送PWid FEC TLV接口参数字段中包含的VCCV参数或通用PWid FEC TLV的sub TLV接口参数。可以将VCCV消息从L2TPv3 PW段转换为MPLS PW段，反之亦然；然而，这一主题还有待进一步研究。
As stated above, the S-PE MUST perform a standard MPLS label swap operation on the MPLS PW label. By the rules defined in [RFC3032], the PW label TTL MUST be decreased at every S-PE. Once the PW label TTL reaches the value of 0, the packet is sent to the control plane to be processed. Hence, by controlling the PW TTL value of the PW label, it is possible to select exactly which S-PE will respond to the VCCV packet.
如上所述，S-PE必须在MPLS PW标签上执行标准MPLS标签交换操作。根据[RFC3032]中定义的规则，必须在每个S-PE降低PW标签TTL。一旦PW标签TTL达到值0，数据包被发送到控制平面进行处理。因此，通过控制PW标签的PW TTL值，可以准确地选择哪个S-PE将响应VCCV分组。
Similarly to SS-PW, MS-PW VCCV capabilities are signaled using the VCCV parameter included in the interface parameter field of the PWid FEC TLV or the sub-TLV interface parameter of the Generalized PWid FEC TLV as described in [RFC5085].
与SS-PW类似，MS-PW VCCV能力使用PWid FEC TLV的接口参数字段中包含的VCCV参数或通用PWid FEC TLV的子TLV接口参数发出信号，如[RFC5085]所述。
In Figure 3, T-PE1 uses the VCCV parameter included in the interface parameter field of the PWid FEC TLV or the sub-TLV interface parameter of the Generalized PWid FEC TLV to indicate to the far-end T-PE2 what VCCV capabilities T-PE1 supports. This is the same VCCV parameter as would be used if T-PE1 and T-PE2 were connected directly. S-PE2, which is a PW switching point, as part of the adaptation function for interface parameters, processes locally the VCCV parameter then passes it to T-PE2. If there were multiple S-PEs on the path between T-PE1 and T-PE2, each would carry out the same processing, passing along the VCCV parameter. The local processing of the VCCV parameter removes CC Types specified by the originating T-PE that are not supported on the S-PE. For example, if T-PE1 indicates that it supports CC Types 1, 2, and 3, then the S-PE removes the Router Alert CC Type 2, leaving the rest of the TLV unchanged, and passes the modified VCCV parameter to the next S-PE along the path.
在图3中，T-PE1使用PWid FEC TLV接口参数字段中包含的VCCV参数或通用PWid FEC TLV的sub TLV接口参数向远端T-PE2指示T-PE1支持的VCCV功能。这与直接连接T-PE1和T-PE2时使用的VCCV参数相同。S-PE2是PW开关点，作为接口参数自适应功能的一部分，本地处理VCCV参数，然后将其传递给T-PE2。如果T-PE1和T-PE2之间的路径上有多个S-PE，则每个S-PE将执行相同的处理，并传递VCCV参数。VCCV参数的本地处理将删除原始T-PE指定的、S-PE不支持的CC类型。例如，如果T-PE1表示它支持CC类型1、2和3，则S-PE将删除路由器警报CC类型2，保持TLV的其余部分不变，并将修改后的VCCV参数沿路径传递给下一个S-PE。
The far end T-PE (T-PE2) receives the VCCV parameter indicating only the CC Types that are supported by the initial T-PE (T-PE1) and all S-PEs along the PW path.
远端T-PE（T-PE2）接收VCCV参数，该参数仅指示初始T-PE（T-PE1）和沿PW路径的所有S-PE支持的CC类型。
The VCCV parameter ID is defined as follows in [RFC4446]:
VCCV参数ID在[RFC4446]中定义如下：
Parameter ID Length Description 0x0c 4 VCCV
参数ID长度说明0x0c 4 VCCV
The format of the VCCV parameter field is as follows:
VCCV参数字段的格式如下所示：
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0c | 0x04 | CC Types | CV Types |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0c | 0x04 | CC Types | CV Types |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Bit 0 (0x01) - Type 1: PWE3 Control Word with 0001b as first nibble as defined in [RFC4385] Bit 1 (0x02) - Type 2: MPLS Router Alert Label Bit 2 (0x04) - Type 3: MPLS Demultiplexor PW Label with TTL == 1 (Type 3).
位0（0x01）-类型1:PWE3控制字，0001b作为[RFC4385]位1（0x02）中定义的第一个半字节-类型2:MPLS路由器警报标签位2（0x04）-类型3:TTL==1的MPLS解复用器PW标签（类型3）。
VCCV CC Type 1 can be used for MS-PWs. However, if the CW is enabled on user packets, VCCV CC Type 1 MUST be used according to the rules in [RFC5085]. When using CC Type 1 for MS-PWs, the PE transmitting the VCCV packet MUST set the TTL to the appropriate value to reach the destination S-PE. However, if the packet is destined for the T-PE, the TTL can be set to any value that is sufficient for the packet to reach the T-PE.
VCCV CC类型1可用于MS PWs。但是，如果在用户数据包上启用CW，则必须根据[RFC5085]中的规则使用VCCV CC类型1。当MS PWs使用CC Type 1时，发送VCCV数据包的PE必须将TTL设置为适当的值，以到达目的地S-PE。然而，如果分组目的地是T-PE，则TTL可以被设置为足以使分组到达T-PE的任何值。
VCCV CC Type 2 is not supported for MS-PWs and MUST be removed from a VCCV parameter field by the S-PE.
MS PWs不支持VCCV CC类型2，必须由S-PE从VCCV参数字段中删除。
VCCV CC Type 3 can be used for MS-PWs; however, if the CW is enabled, VCCV Type 1 is preferred according to the rules in [RFC5085]. Note that for using the VCCV Type 3, TTL method, the PE will set the PW label TTL to the appropriate value necessary to reach the target PE; otherwise, the VCCV packet might be forwarded over the AC to the Customer Premise Equipment (CPE).
VCCV CC类型3可用于MS PWs；但是，如果启用CW，根据[RFC5085]中的规则，首选VCCV类型1。注意，对于使用VCCV类型3，TTL方法，PE将PW标签TTL设置为达到目标PE所需的适当值；否则，VCCV分组可能会通过AC转发到用户端设备（CPE）。
This document specifies four VCCV operations:
本文件规定了四种VCCV操作：
-i. End-to-end MS-PW connectivity verification. This operation enables the connectivity of the MS-PW to be tested from source T-PE to destination T-PE. In order to do this, the sending T-PE must include the FEC used in the last segment of the MS-PW to the destination T-PE in the VCCV-Ping echo request. This information is either configured at the sending T-PE or is obtained by processing the corresponding sub-TLVs of the optional SP-PE TLV, as described below.
-一,。端到端MS-PW连接验证。此操作使MS-PW的连接性能够从源T-PE测试到目标T-PE。为此，发送T-PE必须将MS-PW最后一段中使用的FEC包括在VCCV Ping echo请求中发送到目的地T-PE。该信息要么在发送T-PE处配置，要么通过处理可选SP-PE TLV的相应子TLV获得，如下所述。
-ii. Partial MS-PW connectivity verification. This operation enables the connectivity of any contiguous subset of the segments of an MS-PW to be tested from the source T-PE or a source S-PE to a destination S-PE or T-PE. Again, the FEC used on the last segment to be tested must be included in the VCCV-Ping echo request message. This information is determined by the sending T-PE or S-PE as in (i) above.
-二,。部分MS-PW连接验证。此操作允许从源T-PE或源S-PE到目标S-PE或T-PE测试MS-PW段的任何连续子集的连接性。同样，要测试的最后一段上使用的FEC必须包含在VCCV Ping echo请求消息中。该信息由发送的T-PE或S-PE确定，如上文（i）所述。
-iii. MS-PW path verification. This operation verifies the path of the MS-PW, as returned by the SP-PE TLV, against the actual data path of the MS-PW. The sending T-PE or S-PE
-iii.MS-PW路径验证。此操作根据MS-PW的实际数据路径验证SP-PE TLV返回的MS-PW路径。发送的T-PE或S-PE
iteratively sends a VCCV echo request to each S-PE along the MS-PW path, using the FEC for the corresponding MS-PW segment in the SP-PE TLV. If the SP-PE TLV information is correct, then a VCCV echo reply showing that this is a valid router for the FEC will be received. However, if the SP-PE TLV information is incorrect, then this operation enables the first incorrect switching point to be determined, but not the actual path of the MS-PW beyond that. This operation cannot be used when the MS-PW is statically configured or when the SP-PE TLV is not supported. The processing of the PW Switching Point PE TLV used for this operation is described below. This operation is OPTIONAL.
使用SP-PE TLV中对应MS-PW段的FEC，沿MS-PW路径向每个S-PE迭代发送VCCV回波请求。如果SP-PE TLV信息正确，则会收到VCCV回音回复，表明这是FEC的有效路由器。但是，如果SP-PE TLV信息不正确，则此操作可确定第一个不正确的开关点，但不能确定MS-PW的实际路径。当MS-PW静态配置或SP-PE TLV不受支持时，不能使用此操作。用于此操作的PW开关点PE TLV的处理如下所述。此操作是可选的。
-iv. MS-PW path trace. This operation traces the data path of the MS-PW using FECs included in the Target FEC stack TLV [RFC4379] returned by S-PEs or T-PEs in an echo reply message. The sending T-PE or S-PE uses this information to recursively test each S-PE along the path of the MS-PW in a single operation in a similar manner to LSP trace. This operation is able to determine the actual data path of the MS-PW, and can be used for both statically configured and signaled MS-PWs. Support for this operation is OPTIONAL.
-iv.MS-PW路径跟踪。此操作使用由S-PEs或T-PEs在回显回复消息中返回的目标FEC堆栈TLV[RFC4379]中包含的FEC跟踪MS-PW的数据路径。发送T-PE或S-PE使用该信息，以与LSP跟踪类似的方式，在单个操作中沿MS-PW路径递归测试每个S-PE。此操作能够确定MS-PW的实际数据路径，并可用于静态配置和信号MS-PW。对该操作的支持是可选的。
Note that the above operations rely on intermediate S-PEs and/or the destination T-PE to include the PW Switching Point PE TLV as a part of the MS-PW setup process, or to include the Target FEC stack TLV in the VCCV echo reply message. For various reasons, e.g., privacy or security of the S-PE/T-PE, this information may not be available to the source T-PE. In these cases, manual configuration of the FEC MAY still be used.
注意，上述操作依赖于中间S-PE和/或目的地T-PE，以将PW切换点PE TLV包括在MS-PW设置过程中，或将目标FEC堆栈TLV包括在VCCV回波回复消息中。出于各种原因，例如S-PE/T-PE的隐私或安全性，源T-PE可能无法获得该信息。在这些情况下，仍然可以使用FEC的手动配置。
The challenge for the control plane is to be able to build the VCCV echo request packet with the necessary information to reach the desired S-PE or T-PE, for example, the target FEC 128 PW sub-TLV of the downstream PW segment that the packet is destined for. This could be even more difficult in situations in which the MS-PW spans different providers and Autonomous Systems.
控制平面面临的挑战是能够使用必要的信息构建VCCV回波请求分组，以到达期望的S-PE或T-PE，例如，分组目的地下游PW段的目标FEC 128 PW子TLV。在MS-PW跨越不同提供商和自治系统的情况下，这可能更加困难。
For example, in Figure 3, T-PE1 has the FEC 128 of the segment (PW segment 1), but it does not readily have the information required to compose the FEC 128 of the following segment (PW segment 3), if a VCCV echo request is to be sent to T-PE2. This can be achieved by the methods described in the following subsections.
例如，在图3中，T-PE1具有该段（PW段1）的FEC 128，但如果要向T-PE2发送VCCV回波请求，则T-PE1不具备构成下一段（PW段3）的FEC 128所需的信息。这可以通过以下小节中描述的方法实现。
When performing a partial or end-to-end connectivity or path verification, the sender of the echo request message requires the FEC of the last segment to the target S-PE/T-PE node. This information can either be configured manually or be obtained by inspecting the corresponding sub-TLVs of the PW Switching Point PE TLV.
当执行部分或端到端连接或路径验证时，echo请求消息的发送方需要到目标S-PE/T-PE节点的最后一段的FEC。该信息可以手动配置，也可以通过检查PW开关点PE TLV的相应子TLV获得。
The necessary SP-PE sub-TLVs are:
必要的SP-PE子TLV包括：
Type Description 0x01 PWid of last PW segment traversed 0x03 Local IP address of PW Switching Point 0x04 Remote IP address of last PW Switching Point traversed or of the T-PE
类型描述0x01最后经过的PW段的PWid 0x03最后经过的PW交换点的本地IP地址0x04最后经过的PW交换点或T-PE的远程IP地址
When performing an OPTIONAL MS-PW path trace operation, the T-PE will automatically learn the target FEC by probing, one by one, the S-PEs of the MS-PW path, using the FEC returned in the Target FEC stack of the previous VCCV echo reply.
当执行可选的MS-PW路径跟踪操作时，T-PE将使用上一个VCCV回波应答的目标FEC堆栈中返回的FEC，通过逐个探测MS-PW路径的S-PE，自动学习目标FEC。
Upon receiving a VCCV echo request, the control plane on S-PEs (or the target node of each segment of the MS-PW) validates the request and responds to the request with an echo reply consisting of a return code of 8 (label switched at stack depth) indicating that it is an S-PE and not the egress router for the MS-PW.
收到VCCV回送请求后，S-PEs上的控制平面（或MS-PW各段的目标节点）验证该请求，并用回送回复响应该请求，该回送回复包含返回码8（标签在堆栈深度处交换），指示它是S-PE，而不是MS-PW的出口路由器。
S-PEs that wish to reveal their downstream next-hop in a trace operation should include the FEC of the downstream PW segment in the Target FEC stack (as per Sections 3.2 and 4.5 of [RFC4379]) of the echo reply message. FEC 128 PWs MUST use the format shown in Section 3.2.9 of [RFC4379] for the sub-TLV in the Target FEC stack, while FEC 129 PWs MUST use the format shown in Section 3.2.10 of [RFC4379] for the sub-TLV in the Target FEC stack. Note that an S-PE MUST NOT include this FEC information in the reply if it has been configured not to do so for administrative reasons or for reasons explained previously.
希望在跟踪操作中显示其下游下一跳的S-PE应包括回波回复消息的目标FEC堆栈中下游PW段的FEC（根据[RFC4379]第3.2节和第4.5节）。对于目标FEC堆栈中的子TLV，FEC 128 PW必须使用[RFC4379]第3.2.9节中所示的格式，而对于目标FEC堆栈中的子TLV，FEC 129 PW必须使用[RFC4379]第3.2.10节中所示的格式。请注意，如果S-PE由于管理原因或前面解释的原因被配置为不在回复中包含此FEC信息，则S-PE不得在回复中包含此FEC信息。
If the node is the T-PE or the egress node of the MS-PW, it responds to the echo request with an echo reply with a return code of 3 (Egress Router).
如果该节点是T-PE或MS-PW的出口节点，则它使用返回码为3（出口路由器）的回送回复来响应回送请求。
The operation to be taken by the node receiving the echo reply in response to an echo request depends on the VCCV mode of operation described above. See Section 9.5.2 for detailed procedures.
接收回显回复的节点响应回显请求所采取的操作取决于上述VCCV操作模式。详细程序见第9.5.2节。
There are two similar methods of verifying the MS-PW path: Path Trace and Path Verification. Path Trace does not use the LDP control plane to obtain information on the path to verify, so this method is well suited if portions of the MS-PW are statically configured SS-PWs. The Path Verification method relies on information obtained from the LDP control plane, and hence offers better verification of the current forwarding behavior compared to the LDP signaled forwarding information of the MS-PW path. However, in the case where there are statically signaled SS-PWs in the MS-PW path, the path information is unavailable and must be programmed manually.
验证MS-PW路径有两种类似的方法：路径跟踪和路径验证。路径跟踪不使用LDP控制平面来获取要验证的路径信息，因此如果MS-PW的部分是静态配置的SS PW，则此方法非常适合。路径验证方法依赖于从LDP控制平面获得的信息，因此与MS-PW路径的LDP信号转发信息相比，能够更好地验证当前转发行为。但是，在MS-PW路径中存在静态信号SS PW的情况下，路径信息不可用，必须手动编程。
In Figure 3, if T-PE1, S-PE, and T-PE2 support Control Word, the PW control plane will automatically negotiate the use of the CW. VCCV CC Type 3 will function correctly whether or not the CW is enabled on the PW. However, VCCV Type 1 (which can be use for end-to-end verification only) is only supported if the CW is enabled.
在图3中，如果T-PE1、S-PE和T-PE2支持控制字，PW控制平面将自动协商CW的使用。无论PW上是否启用CW，VCCV CC类型3都将正常工作。但是，只有启用CW时，才支持VCCV类型1（只能用于端到端验证）。
At the S-PE, the data path operations include an outer label pop, inner label swap, and new outer label push. Note that there is no requirement for the S-PE to inspect the CW. Thus, the end-to-end connectivity of the multi-segment pseudowire can be verified by performing all of the following steps:
在S-PE上，数据路径操作包括外部标签弹出、内部标签交换和新的外部标签推送。注意，S-PE无需检查CW。因此，可以通过执行以下所有步骤来验证多段伪线的端到端连接：
-i. The T-PE forms a VCCV-Ping echo request message with the FEC matching that of the last PW segment to the destination T-PE.
-一,。T-PE形成VCCV Ping echo请求消息，FEC将最后一个PW段的FEC与目标T-PE匹配。
-ii. The T-PE sets the inner PW label TTL to the exact value to allow the packet to reach the far-end T-PE. (The value is determined by counting the number of S-PEs from the control plane information.) Alternatively, if CC Type 1 is supported, the packet can be encapsulated according to CC Type 1 in [RFC5085].
-二,。T-PE将内部PW标签TTL设置为精确值，以允许数据包到达远端T-PE。（该值通过从控制平面信息中计算S-PE的数量来确定。）或者，如果支持CC类型1，则可以根据[RFC5085]中的CC类型1封装数据包。
-iii. The T-PE sends a VCCV packet that will follow the exact same data path at each S-PE as that taken by data packets.
-iii.T-PE发送一个VCCV数据包，该数据包将在每个S-PE处遵循与数据包所采用的数据路径完全相同的数据路径。
-iv. The S-PE may perform an outer label pop, if Penultimate Hop Popping (PHP) is disabled, and will perform an inner label swap with TTL decrement and a new outer label push.
-iv.如果倒数第二跳弹出（PHP）被禁用，S-PE可以执行外部标签弹出，并将执行带有TTL减量的内部标签交换和新的外部标签推送。
-v. There is no requirement for the S-PE to inspect the CW.
-五,。S-PE无需检查CW。
-vi. The VCCV packet is diverted to VCCV control processing at the destination T-PE.
-vi.VCCV数据包被转移到目的地T-PE的VCCV控制处理。
-vii. The destination T-PE replies using the specified reply mode, i.e., reverse PW path or IP path.
-七,。目标T-PE使用指定的应答模式进行应答，即反向PW路径或IP路径。
In order to trace part of the multi-segment pseudowire, the TTL of the PW label may be used to force the VCCV message to 'pop out' at an intermediate node. When the TTL expires, the S-PE can determine that the packet is a VCCV packet either by checking the CW or (if the CW is not in use) by checking for a valid IP header with UDP destination port 3503. The packet should then be diverted to VCCV processing.
为了跟踪多段伪线的一部分，可使用PW标签的TTL强制VCCV消息在中间节点“弹出”。当TTL到期时，S-PE可以通过检查CW或（如果CW未被使用）通过检查具有UDP目的地端口3503的有效IP报头来确定该分组是VCCV分组。然后，数据包应转移到VCCV处理。
In Figure 3, if T-PE1 sends a VCCV message with the TTL of the PW label equal to 1, the TTL will expire at the S-PE. T-PE1 can thus verify the first segment of the pseudowire.
在图3中，如果T-PE1发送的VCCV消息中PW标签的TTL等于1，则TTL将在S-PE处过期。因此，T-PE1可以验证伪导线的第一段。
The VCCV packet is built according to [RFC4379], Section 3.2.9 for FEC 128, or Section 3.2.10 for FEC 129. All the information necessary to build the VCCV LSP ping packet is collected by inspecting the S-PE TLVs.
VCCV数据包根据[RFC4379]、第3.2.9节（FEC 128）或第3.2.10节（FEC 129）构建。通过检查S-PE TLV收集构建VCCV LSP ping数据包所需的所有信息。
Note that this use of the TTL is subject to the caution expressed in [RFC5085]. If a penultimate LSR between S-PEs or between an S-PE and a T-PE manipulates the PW label TTL, the VCCV message may not emerge from the MS-PW at the correct S-PE.
请注意，TTL的使用应遵守[RFC5085]中所述的注意事项。如果S-PE之间或S-PE和T-PE之间的倒数第二个LSR操纵PW标签TTL，则VCCV消息可能不会在正确的S-PE处从MS-PW中出现。
Assuming that all nodes along an MS-PW support the Control Word CC Type 3, VCCV between S-PEs may be accomplished using the PW label TTL as described above. In Figure 3, the S-PE may verify the path between it and T-PE2 by sending a VCCV message with the PW label TTL set to 1. Given a more complex network with multiple S-PEs, an S-PE may verify the connectivity between it and an S-PE two segments away by sending a VCCV message with the PW label TTL set to 2. Thus, an S-PE can diagnose connectivity problems by successively increasing the TTL. All the information needed to build the proper VCCV echo
假设沿着MS-PW的所有节点都支持控制字CC Type 3，则S-PE之间的VCCV可以使用如上所述的PW标签TTL来实现。在图3中，S-PE可通过发送PW标签TTL设置为1的VCCV消息来验证其与T-PE2之间的路径。给定具有多个S-PE的更复杂网络，S-PE可通过发送PW标签TTL设置为2的VCCV消息来验证其与两段之外的S-PE之间的连接。因此，S-PE可以通过连续增加TTL来诊断连接问题。建立正确VCCV回波所需的所有信息
request packet (as described in [RFC4379], Sections 3.2.9 or 3.2.10) is obtained automatically from the LDP label mapping that contains S-PE TLVs.
请求数据包（如[RFC4379]第3.2.9或3.2.10节所述）自动从包含S-PE TLV的LDP标签映射中获得。
As an example, in Figure 3, VCCV trace can be performed on the MS-PW originating from T-PE1 by a single operational command. The following process ensues:
例如，在图3中，可以通过单个操作命令对源自T-PE1的MS-PW执行VCCV跟踪。随后进行以下过程：
-i. T-PE1 sends a VCCV echo request with TTL set to 1 and a FEC containing the pseudowire information of the first segment (PW1 between T-PE1 and S-PE) to S-PE for validation. If FEC Stack Validation is enabled, the request may also include an additional sub-TLV such as LDP Prefix and/or RSVP LSP, dependent on the type of transport tunnel the segmented PW is riding on.
-一,。T-PE1向S-PE发送TTL设置为1的VCCV回波请求和包含第一段伪线信息（T-PE1和S-PE之间的PW1）的FEC，以进行验证。如果启用了FEC堆栈验证，则请求还可以包括附加的子TLV，例如LDP前缀和/或RSVP LSP，这取决于分段PW所乘坐的传输隧道的类型。
-ii. S-PE validates the echo request with the FEC. Since it is a switching point between the first and second segment, it builds an echo reply with a return code of 8 and sends the echo reply back to T-PE1.
-二,。S-PE使用FEC验证回显请求。由于它是第一段和第二段之间的切换点，因此它构建一个返回码为8的回音应答，并将回音应答发送回T-PE1。
-iii. T-PE1 builds a second VCCV echo request based on the information obtained from the control plane (SP-PE TLV). It then increments the TTL and sends it out to T-PE2. Note that the VCCV echo request packet is switched at the S-PE data path and forwarded to the next downstream segment without any involvement from the control plane.
-iii.T-PE1基于从控制平面（SP-PE TLV）获得的信息，构建第二个VCCV回波请求。然后，它增加TTL并将其发送给T-PE2。请注意，VCCV回波请求数据包在S-PE数据路径上交换，并转发到下一个下游段，而不涉及控制平面。
-iv. T-PE2 receives and validates the echo request with the FEC. Since T-PE2 is the destination node or the egress node of the MS-PW, it replies to T-PE1 with an echo reply with a return code of 3 (Egress Router).
-iv.T-PE2通过FEC接收并验证回声请求。因为T-PE2是MS-PW的目的地节点或出口节点，所以它用返回码为3（出口路由器）的回音回复T-PE1。
-v. T-PE1 receives the echo reply from T-PE2. T-PE1 is made aware that T-PE2 is the destination of the MS-PW because the echo reply has a return code of 3. The trace process is completed.
-五,。T-PE1接收来自T-PE2的回音应答。T-PE1知道T-PE2是MS-PW的目的地，因为回声应答的返回码为3。跟踪过程已完成。
If no echo reply is received, or an error code is received from a particular PE, the trace process MUST stop immediately, and packets MUST NOT be sent further along the MS-PW.
如果未收到回音回复，或从特定PE收到错误代码，则跟踪过程必须立即停止，并且数据包不得沿MS-PW进一步发送。
For more detail on the format of the VCCV echo packet, refer to [RFC5085] and [RFC4379]. The TTL here refers to that of the inner (PW) label TTL.
有关VCCV回波数据包格式的更多详细信息，请参阅[RFC5085]和[RFC4379]。这里的TTL是指内部（PW）标签TTL的TTL。
As an example, in Figure 3, VCCV trace can be performed on the MS-PW originating from T-PE1 by a single operational command. The following OPTIONAL process ensues:
例如，在图3中，可以通过单个操作命令对源自T-PE1的MS-PW执行VCCV跟踪。随后进行以下可选过程：
-i. T-PE1 sends a VCCV echo request with TTL set to 1 and a FEC containing the pseudowire information of the first segment (PW1 between T-PE1 and S-PE) to S-PE for validation. If FEC Stack Validation is enabled, the request may also include an additional sub-TLV such as LDP Prefix and/or RSVP LSP, dependent on the type of transport tunnel the segmented PW is riding on.
-一,。T-PE1向S-PE发送TTL设置为1的VCCV回波请求和包含第一段伪线信息（T-PE1和S-PE之间的PW1）的FEC，以进行验证。如果启用了FEC堆栈验证，则请求还可以包括附加的子TLV，例如LDP前缀和/或RSVP LSP，这取决于分段PW所乘坐的传输隧道的类型。
-ii. The S-PE validates the echo request with the FEC.
-二,。S-PE使用FEC验证回显请求。
-iii. The S-PE builds an echo reply with a return code of 8 and sends the echo reply back to T-PE1, appending the FEC 128 information for the next segment along the MS-PW to the VCCV echo reply packet using the Target FEC stack TLV (as per Sections 3.2 and 4.5 of [RFC4379]).
-iii.S-PE构建一个返回码为8的回音应答，并将回音应答发送回T-PE1，使用目标FEC堆栈TLV（根据[RFC4379]第3.2节和第4.5节）将MS-PW上下一段的FEC 128信息附加到VCCV回音应答包。
-iv. T-PE1 builds a second VCCV echo request based on the information obtained from the FEC stack TLV received in the previous VCCV echo reply. It then increments the TTL and sends it out to T-PE2. Note that the VCCV echo request packet is switched at the S-PE data path and forwarded to the next downstream segment without any involvement from the control plane.
-iv.T-PE1基于从前一个VCCV回波回复中接收的FEC堆栈TLV获得的信息，构建第二个VCCV回波请求。然后，它增加TTL并将其发送给T-PE2。请注意，VCCV回波请求数据包在S-PE数据路径上交换，并转发到下一个下游段，而不涉及控制平面。
-v. T-PE2 receives and validates the echo request with the FEC. Since T-PE2 is the destination node or the egress node of the MS-PW, it replies to T-PE1 with an echo reply with a return code of 3 (Egress Router).
-五,。T-PE2通过FEC接收并验证回声请求。因为T-PE2是MS-PW的目的地节点或出口节点，所以它用返回码为3（出口路由器）的回音回复T-PE1。
-vi. T-PE1 receives the echo reply from T-PE2. T-PE1 is made aware that T-PE2 is the destination of the MS-PW because the echo reply has a return code of 3. The trace process is completed.
-vi.T-PE1接收来自T-PE2的回音回复。T-PE1知道T-PE2是MS-PW的目的地，因为回声应答的返回码为3。跟踪过程已完成。
If no echo reply is received, or an error code is received from a particular PE, the trace process MUST stop immediately, and packets MUST NOT be sent further along the MS-PW.
如果未收到回音回复，或从特定PE收到错误代码，则跟踪过程必须立即停止，并且数据包不得沿MS-PW进一步发送。
For more detail on the format of the VCCV echo packet, refer to [RFC5085] and [RFC4379]. The TTL here refers to that of the inner (PW) label TTL.
有关VCCV回波数据包格式的更多详细信息，请参阅[RFC5085]和[RFC4379]。这里的TTL是指内部（PW）标签TTL的TTL。
In the PW switching with attachment circuits case (Figure 2), PW status messages indicating PW or attachment circuit faults MUST be mapped to fault indications or OAM messages on the connecting AC as defined in [PW-MSG-MAP].
在带有附件电路的PW切换情况下（图2），指示PW或附件电路故障的PW状态消息必须映射到[PW-MSG-MAP]中定义的连接AC上的故障指示或OAM消息。
In the PW control plane switching case (Figure 3), there is no attachment circuit at the S-PE, but the two PWs are connected together. Similarly, the status of the PWs is forwarded unchanged from one PW to the other by the control plane switching function. However, it may sometimes be necessary to communicate fault status of one of the locally attached PW segments at an S-PE. For LDP, this can be accomplished by sending an LDP notification message containing the PW status TLV, as well as an OPTIONAL PW Switching Point PE TLV as follows:
在PW控制平面切换情况下（图3），S-PE处没有连接电路，但两个PW连接在一起。类似地，通过控制平面切换功能，PWs的状态从一个PW不变地转发到另一个PW。然而，有时可能需要在S-PE上传达一个本地连接PW段的故障状态。对于LDP，这可以通过发送包含PW状态TLV的LDP通知消息以及可选PW切换点PE TLV来实现，如下所示：
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Notification (0x0001) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1| Status (0x0300) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1| Status Code=0x00000028 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type=0 | PW Status TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Status TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Status TLV | PWid FEC or Generalized ID FEC|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| PWid FEC or Generalized ID FEC (contd.) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| SP-PE TLV (0x096D) | SP-PE TLV Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Variable Length Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Notification (0x0001) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1| Status (0x0300) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1| Status Code=0x00000028 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type=0 | PW Status TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Status TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Status TLV | PWid FEC or Generalized ID FEC|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| PWid FEC or Generalized ID FEC (contd.) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| SP-PE TLV (0x096D) | SP-PE TLV Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Variable Length Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Only one SP-PE TLV can be present in this message. This message is then relayed by each S-PE unchanged. The T-PE decodes the status message and the included SP-PE TLV to detect exactly where the fault occurred. At the T-PE, if there is no SP-PE TLV included in the LDP status notification, then the status message can be assumed to have originated at the remote T-PE.
此消息中只能存在一个SP-PE TLV。然后，该消息由每个S-PE不加更改地中继。T-PE对状态信息和包含的SP-PE TLV进行解码，以准确检测故障发生的位置。在T-PE，如果LDP状态通知中不包括SP-PE TLV，则可以假定状态消息起源于远程T-PE。
The merging of the received LDP status and the local status for the PW segments at an S-PE can be summarized as follows:
在S-PE处接收到的LDP状态和PW段的本地状态的合并可以总结如下：
-i. When the local status for both PW segments is UP, the S-PE passes any received AC or PW status bits unchanged, i.e., the status notification TLV is unchanged, but the PWid in the case of a FEC 128 TLV is set to the value of the PW segment of the next hop.
-一,。当两个PW段的本地状态为UP时，S-PE传递任何接收到的AC或PW状态位不变，即状态通知TLV不变，但在FEC 128 TLV的情况下，PWid设置为下一跳的PW段的值。
-ii. When the local status for any of the PW segments is at fault, the S-PE always sends the local status bits regardless of whether the received status bits from the remote node indicated that an upstream fault has cleared. AC status bits are passed along unchanged.
-二,。当任何PW段的本地状态为故障时，S-PE始终发送本地状态位，而不管从远程节点接收到的状态位是否指示上游故障已清除。交流状态位的传递方式不变。
The PW fault directions are defined as follows:
PW故障方向定义如下：
+-------+
---PW1 Receive---->| |-----PW2 Transmit---->
S-PE1 | S-PE2 | S-PE3
<--PW1 Transmit----| |<----PW2 Receive------
+-------+
+-------+
---PW1 Receive---->| |-----PW2 Transmit---->
S-PE1 | S-PE2 | S-PE3
<--PW1 Transmit----| |<----PW2 Receive------
+-------+
Figure 4: S-PE and PW Transmission/Reception Directions
图4:S-PE和PW传输/接收方向
When a local fault is detected by the S-PE, a PW status message is sent in both directions along the PW. Since there are no attachment circuits on an S-PE, only the following status messages are relevant:
当S-PE检测到局部故障时，沿PW向两个方向发送PW状态消息。由于S-PE上没有连接电路，因此只有以下状态消息相关：
0x00000008 - Local PSN-facing PW (ingress) Receive Fault 0x00000010 - Local PSN-facing PW (egress) Transmit Fault
0x00000008-面向本地PSN的PW（入口）接收故障0x00000010-面向本地PSN的PW（出口）传输故障
Each S-PE needs to store only two 32-bit PW status words for each PW segment: one for local failures and one for remote failures (normally received from another PE). The first failure will set the appropriate bit in the 32-bit status word, and each subsequent failure will be ORed to the appropriate PW status word. In the case
每个S-PE只需为每个PW段存储两个32位PW状态字：一个用于本地故障，一个用于远程故障（通常从另一个PE接收）。第一个故障将在32位状态字中设置适当的位，随后的每个故障都将被OR到适当的PW状态字。在这种情况下
that the PW status word stores remote failures, this rule has the effect of a logical OR operation with the first failure received on the particular PW segment.
由于PW状态字存储远程故障，此规则具有逻辑OR操作的效果，在特定PW段上接收到第一个故障。
It should be noted that remote failures received on an S-PE are just passed along the MS-PW unchanged, while local failures detected an S-PE are signaled on both PW segments.
应注意的是，在S-PE上接收到的远程故障只会在MS-PW上传递，而在两个PW段上都会发出S-PE检测到的本地故障的信号。
A T-PE can receive multiple failures from S-PEs along the MS-PW; however, only the failure from the remote closest S-PE will be stored (last PW status message received). The PW status word received is just ORed to any existing remote PW status already stored on the T-PE.
T-PE可接收MS-PW沿线S-PE的多个故障；但是，仅存储来自远程最近S-PE的故障（收到最后一条PW状态消息）。接收到的PW状态字仅与已存储在T-PE上的任何现有远程PW状态进行OR运算。
Given that there are two PW segments at a particular S-PE for a particular MS-PW (referring to Figure 4), there are four possible failure cases as follows:
鉴于特定MS-PW的特定S-PE处有两个PW段（参考图4），有以下四种可能的故障情况：
-i. PW2 Transmit direction fault -ii. PW1 Transmit direction fault -iii. PW2 Receive direction fault -iv. PW1 Receive direction fault
-一,。PW2传输方向故障-ii。PW1发送方向故障-iii.PW2接收方向故障-iv.PW1接收方向故障
Once a PW status notification message is initiated at an S-PE for a particular PW status bit, any further status message for the same status bit (and received from an upstream neighbor) is processed locally and not forwarded until the S-PE original status error state is cleared.
一旦在S-PE上针对特定PW状态位启动PW状态通知消息，则针对相同状态位（并从上游邻居处接收）的任何进一步状态消息将在本地处理，并且在清除S-PE原始状态错误状态之前不会转发。
Each S-PE along the MS-PW MUST store any PW status messages transiting it. If more than one status message with the same PW status bit set is received by a T-PE or S-PE, only the last PW status message is stored.
MS-PW沿线的每个S-PE必须存储传输它的任何PW状态消息。如果T-PE或S-PE接收到具有相同PW状态位设置的多条状态消息，则仅存储最后一条PW状态消息。
When this failure occurs, the S-PE will take the following actions:
发生此故障时，S-PE将采取以下措施：
* Send a PW status message to S-PE3 containing "0x00000010 - Local PSN-facing PW (egress) Transmit Fault".
* 向S-PE3发送包含“0x00000010-面向PW（出口）传输故障的本地PSN”的PW状态消息。
* Send a PW status message to S-PE1 containing "0x00000008 - Local PSN-facing PW (ingress) Receive Fault".
* 向S-PE1发送包含“0x00000008-面向PW（入口）的本地PSN接收故障”的PW状态消息。
* Store 0x00000010 in the local PW status word for the PW segment toward S-PE3.
* 在朝向S-PE3的PW段的本地PW状态字中存储0x00000010。
When this failure occurs, the S-PE will take the following actions:
发生此故障时，S-PE将采取以下措施：
* Send a PW status message to S-PE1 containing "0x00000010 - Local PSN-facing PW (egress) Transmit Fault".
* 向S-PE1发送包含“0x00000010-面向PW（出口）传输故障的本地PSN”的PW状态消息。
* Send a PW status message to S-PE3 containing "0x00000008 - Local PSN-facing PW (ingress) Receive Fault".
* 向S-PE3发送包含“0x00000008-面向PW（入口）的本地PSN接收故障”的PW状态消息。
* Store 0x00000010 in the local PW status word for the PW segment toward S-PE1.
* 在朝向S-PE1的PW段的本地PW状态字中存储0x00000010。
When this failure occurs, the S-PE will take the following actions:
发生此故障时，S-PE将采取以下措施：
* Send a PW status message to S-PE3 containing "0x00000008 - Local PSN-facing PW (ingress) Receive Fault".
* 向S-PE3发送包含“0x00000008-面向PW（入口）的本地PSN接收故障”的PW状态消息。
* Send a PW status message to S-PE1 containing "0x00000010 - Local PSN-facing PW (egress) Transmit Fault".
* 向S-PE1发送包含“0x00000010-面向PW（出口）传输故障的本地PSN”的PW状态消息。
* Store 0x00000008 in the local PW status word for the PW segment toward S-PE3.
* 在朝向S-PE3的PW段的本地PW状态字中存储0x00000008。
When this failure occurs, the S-PE will take the following actions:
发生此故障时，S-PE将采取以下措施：
* Send a PW status message to S-PE1 containing "0x00000008 - Local PSN-facing PW (ingress) Receive Fault".
* 向S-PE1发送包含“0x00000008-面向PW（入口）的本地PSN接收故障”的PW状态消息。
* Send a PW status message to S-PE3 containing "0x00000010 - Local PSN-facing PW (egress) Transmit Fault".
* 向S-PE3发送包含“0x00000010-面向PW（出口）传输故障的本地PSN”的PW状态消息。
* Store 0x00000008 in the local PW status word for the PW segment toward S-PE1.
* 在朝向S-PE1的PW段的本地PW状态字中存储0x00000008。
Remote PW status fault clearing messages received by an S-PE will only be forwarded if there are no corresponding local faults on the S-PE. (Local faults always supersede remote faults.)
只有在S-PE上没有相应的本地故障时，才会转发S-PE接收到的远程PW状态故障清除消息。（本地故障始终取代远程故障。）
Once the local fault has cleared, and there is no corresponding (same PW status bit set) remote fault, a PW status message is sent out to the adjacent PEs, clearing the fault.
一旦本地故障被清除，并且没有相应的（相同的PW状态位设置）远程故障，PW状态信息被发送到相邻的PEs，从而清除故障。
When a PW status fault clearing message is forwarded, the S-PE will always send the SP-PE TLV associated with the PE that cleared the fault.
转发PW状态故障清除消息时，S-PE将始终发送与清除故障的PE相关的SP-PE TLV。
When a PW status message is received that includes an SP-PE TLV, the SP-PE TLV information MAY be stored, along with the contents of the PW status Word according to the procedures described above. The SP-PE TLV stored is always the SP-PE TLV that is associated with the PE that set that particular last fault. If subsequent PW status messages for the same PW status bit are received, the SP-PE TLV will overwrite the previously stored SP-PE TLV.
当接收到包括SP-PE TLV的PW状态消息时，可根据上述过程存储SP-PE TLV信息以及PW状态字的内容。存储的SP-PE TLV始终是与设置该特定上次故障的PE关联的SP-PE TLV。如果收到相同PW状态位的后续PW状态消息，SP-PE TLV将覆盖先前存储的SP-PE TLV。
The PW switching architecture is based on the concept that the T-PE should process the PW LDP messages in the same manner as if it were participating in the setup of a PW segment. However, a T-PE participating in an MS-PW SHOULD be able to process the SP-PE TLV. Otherwise, the processing of PW status messages and other PW setup messages is exactly as described in [RFC4447].
PW交换体系结构基于这样的概念，即T-PE应以与参与PW段设置相同的方式处理PW LDP消息。但是，参与MS-PW的T-PE应能够处理SP-PE TLV。否则，PW状态消息和其他PW设置消息的处理完全如[RFC4447]中所述。
Pseudowire status signaling methodology, defined in [RFC4447], SHOULD be transparent to the switching point.
[RFC4447]中定义的伪线状态信令方法应对开关点透明。
When the PW control plane switching methodology is used to cross an administrative boundary, it might be necessary to prevent excessive status signaling changes from being propagated across the administrative boundary. This can be achieved by using a similar method as commonly employed for the BGP route advertisement dampening. The details of this OPTIONAL algorithm are a matter of implementation and are outside the scope of this document.
当PW控制平面切换方法用于跨越管理边界时，可能需要防止过多的状态信令更改跨管理边界传播。这可以通过使用通常用于BGP路由广告抑制的类似方法来实现。此可选算法的详细信息属于实现问题，不在本文档范围内。
The procedures outlined in this document can be employed to provision and manage MS-PWs crossing AS boundaries. The use of more advanced mechanisms involving auto-discovery and ordered PWE3 MS-PW signaling will be covered in a separate document.
本文件中概述的程序可用于提供和管理MS PWs作为边界。涉及自动发现和有序PWE3 MS-PW信令的更高级机制的使用将在单独的文档中介绍。
Each PSN carrying the PW may be subject to congestion. The congestion considerations in [RFC3985] apply to PW segments as well. Each PW segment will handle any congestion experienced by the PW traffic independently of the other MS-PW segments. It is possible that passing knowledge of congestion between segments and to the T-PEs can result in more efficient edge-to-edge congestion mitigation systems. However, any specific methods of congestion mitigation are outside the scope of this document and left for further study.
每个承载PW的PSN可能会出现拥塞。[RFC3985]中的拥塞注意事项也适用于PW段。每个PW段将独立于其他MS-PW段处理PW流量遇到的任何拥塞。将路段之间和T-PEs之间的拥塞信息传递给T-PEs可能会产生更有效的边到边拥塞缓解系统。然而，任何缓解拥塞的具体方法都不在本文件的范围内，有待进一步研究。
This document specifies the LDP, L2TPv3, and VCCV extensions that are needed for setting up and maintaining pseudowires. The purpose of setting up pseudowires is to enable Layer 2 frames to be encapsulated and transmitted from one end of a pseudowire to the other. Therefore, we discuss the security considerations for both the data plane and the control plane in the following sections. The guidelines and security considerations specified in [RFC5920] also apply to MS-PW when the PSN is MPLS.
本文档指定了设置和维护伪线所需的LDP、L2TPv3和VCCV扩展。设置伪线的目的是使第2层帧能够被封装并从伪线的一端传输到另一端。因此，我们将在以下部分讨论数据平面和控制平面的安全注意事项。当PSN为MPLS时，[RFC5920]中规定的指南和安全注意事项也适用于MS-PW。
Data plane security considerations as discussed in [RFC4447], [RFC3931], and [RFC3985] apply to this extension without any changes.
[RFC4447]、[RFC3931]和[RFC3985]中讨论的数据平面安全注意事项适用于此扩展，无任何更改。
The VCCV technology for MS-PW offers a method for the service provider to verify the data path of a specific PW. This involves sending a packet to a specific PE and receiving an answer that either confirms the information contained in the packet or indicates that it is incorrect. This is a very similar process to the commonly used IP ICMP ping and TTL expired methods for IP networks. It should be noted that when using VCCV Type 3 for PW when the CW is not enabled, if a packet is crafted with a TTL greater than the number of hops along the MS-PW path, or an S-PE along the path mis-processes the TTL, the packet could mistakenly be forwarded out of the attachment circuit as a native PW packet. This packet would most likely be treated as an error packet by the CE. However, if this possibility is not acceptable, the CW should be enabled to guarantee that a VCCV packet will never be mistakenly forwarded to the AC.
MS-PW的VCCV技术为服务提供商提供了验证特定PW的数据路径的方法。这涉及向特定PE发送数据包，并接收确认数据包中包含的信息或指示其不正确的回答。这与IP网络常用的IP ICMP ping和TTL过期方法非常相似。应该注意的是，当CW未启用时，在PW中使用VCCV类型3时，如果一个数据包的TTL大于MS-PW路径上的跳数，或者沿路径的S-PE错误地处理了TTL，则该数据包可能会被错误地作为本机PW数据包转发出连接电路。此数据包很可能被CE视为错误数据包。但是，如果这种可能性不可接受，则应启用CW以确保VCCV数据包不会被错误地转发到AC。
General security considerations with regard to the use of LDP are specified in Section 5 of RFC 5036. Security considerations with regard to the L2TPv3 control plane are specified in [RFC3931]. These considerations apply as well to the case where LDP or L2TPv3 is used to set up PWs.
RFC 5036第5节规定了使用LDP的一般安全注意事项。[RFC3931]中规定了L2TPv3控制平面的安全注意事项。这些注意事项也适用于使用LDP或L2TPv3设置PWs的情况。
A pseudowire connects two attachment circuits. It is important to make sure that LDP connections are not arbitrarily accepted from anywhere, or else a local attachment circuit might get connected to an arbitrary remote attachment circuit. Therefore, an incoming session request MUST NOT be accepted unless its IP source address is known to be the source of an "eligible" peer. The set of eligible peers could be pre-configured (either as a list of IP addresses or as a list of address/mask combinations), or it could be discovered dynamically via an auto-discovery protocol that is itself trusted. (Note that if the auto-discovery protocol were not trusted, the set of "eligible peers" it produces could not be trusted.)
伪导线连接两个连接电路。务必确保LDP连接不会从任何地方被任意接受，否则本地连接电路可能会连接到任意远程连接电路。因此，除非已知传入会话请求的IP源地址是“合格”对等方的源，否则不得接受该请求。可以预先配置符合条件的对等点集（作为IP地址列表或地址/掩码组合列表），也可以通过自身受信任的自动发现协议动态发现该对等点集。（请注意，如果自动发现协议不受信任，则无法信任它生成的“合格对等方”集。）
Even if a connection request appears to come from an eligible peer, its source address may have been spoofed. So some means of preventing source address spoofing must be in place. For example, if all the eligible peers are in the same network, source address filtering at the border routers of that network could eliminate the possibility of source address spoofing.
即使连接请求似乎来自合格的对等方，其源地址也可能被欺骗。因此，必须采取一些措施防止源地址欺骗。例如，如果所有符合条件的对等方都在同一网络中，则在该网络的边界路由器上进行源地址过滤可以消除源地址欺骗的可能性。
For a greater degree of security, the LDP authentication option, as described in Section 2.9 of [RFC5036], or the Control Message Authentication option of [RFC3931], MAY be used. This provides integrity and authentication for the control messages, and eliminates the possibility of source address spoofing. Use of the message authentication option does not provide privacy, but privacy of control messages is not usually considered to be highly important. Both the LDP and L2TPv3 message authentication options rely on the configuration of pre-shared keys, making it difficult to deploy when the set of eligible neighbors is determined by an auto-configuration protocol.
为了更大程度的安全性，可使用[RFC5036]第2.9节中所述的LDP认证选项或[RFC3931]的控制消息认证选项。这为控制消息提供了完整性和身份验证，并消除了源地址欺骗的可能性。消息身份验证选项的使用不提供隐私，但控制消息的隐私通常不被认为是非常重要的。LDP和L2TPv3消息身份验证选项都依赖于预共享密钥的配置，这使得在自动配置协议确定合格邻居集时难以部署。
The protocol described in this document relies on the LDP MD5 authentication key option, as described in Section 2.9 of [RFC5036], to provide integrity and authentication for the LDP messages and protect against source address spoofing. This mechanism relies on the configuration of pre-shared keys, which typically introduces some fragility. In the specific case of MS-PW, the number of links that leave an organization will be limited in practice, so the reliance on pre-shared keys should be manageable.
本文件中描述的协议依赖于[RFC5036]第2.9节中描述的LDP MD5认证密钥选项，以提供LDP消息的完整性和认证，并防止源地址欺骗。这种机制依赖于预共享密钥的配置，这通常会引入一些脆弱性。在MS-PW的特定情况下，离开组织的链接数量实际上是有限的，因此对预共享密钥的依赖应该是可控的。
When the Generalized PWid FEC Element is used, it is possible that a particular peer may be one of the eligible peers, but may not be the right one to connect to the particular attachment circuit identified by the particular instance of the Generalized ID FEC element. However, given that the peer is known to be one of the eligible peers (as discussed above), this would be the result of a configuration error, rather than a security problem. Nevertheless, it may be advisable for a PE to associate each of its local attachment circuits with a set of eligible peers, rather than have just a single set of eligible peers associated with the PE as a whole.
当使用广义PWid FEC元件时，特定对等方可能是合格对等方之一，但可能不是连接到由广义ID FEC元件的特定实例标识的特定连接电路的正确对等方。然而，鉴于已知该对等方是合格对等方之一（如上所述），这将是配置错误的结果，而不是安全问题。然而，PE最好将其每个本地连接电路与一组合格对等点相关联，而不是仅将一组合格对等点作为一个整体与PE相关联。
This document uses a new L2TP parameter; IANA already maintains the registry "Control Message Attribute Value Pairs" defined by [RFC3438]. The following new value has been assigned:
本文档使用了一个新的L2TP参数；IANA已经维护了[RFC3438]定义的注册表“控制消息属性值对”。已指定以下新值：
101 PW Switching Point AVP
101 PW开关点AVP
This document uses a new LDP TLV type; IANA already maintains the registry "TLV TYPE NAME SPACE" defined by RFC 5036. The following value has been assigned:
本文件使用新的LDP TLV类型；IANA已经维护了RFC 5036定义的注册表“TLV类型名称空间”。已指定以下值：
TLV type Description 0x096D Pseudowire Switching Point PE TLV
TLV类型说明0x096D伪线开关点PE TLV
This document uses a new LDP status code; IANA already maintains the registry "STATUS CODE NAME SPACE" defined by RFC 5036. The following value has been assigned:
本文件使用新的LDP状态码；IANA已经维护RFC 5036定义的注册表“状态代码名称空间”。已指定以下值：
Assignment E Description 0x0000003A 0 PW Loop Detected
检测到分配E说明0x0000003A 0 PW循环
This document uses a new L2TPv3 Result Code for the CDN message, as assigned by IANA in the "Result Code AVP (Attribute Type 1) Values" registry.
本文档使用IANA在“结果代码AVP（属性类型1）值”注册表中分配的CDN消息的新L2TPv3结果代码。
Registry Name: Result Code AVP (Attribute Type 1) Values Defined Result Code values for the CDN message are:
注册表名称：结果代码AVP（属性类型1）值为CDN消息定义的结果代码值为：
Assignment Description 31 Sequencing not supported
不支持分配说明31排序
IANA has set up a registry named "Pseudowire Switching Point PE sub-TLV Type". These are 8-bit values. Type values 1 through 6 are defined in this document. Type values 7 through 64 are to be assigned by IANA using the "Expert Review" policy defined in [RFC5226]. Type values 65 through 127, as well as 0 and 255, are to be allocated using the IETF consensus policy defined in RFC 5226. Type values 128 through 254 are reserved for vendor proprietary extensions and are to be assigned by IANA, using the "First Come First Served" policy defined in RFC 5226.
IANA已经建立了一个名为“伪线交换点PE sub TLV类型”的注册表。这些是8位值。本文档中定义了类型值1到6。IANA将使用[RFC5226]中定义的“专家评审”政策分配类型值7至64。使用RFC 5226中定义的IETF共识策略分配类型值65到127以及0和255。类型值128到254保留给供应商专有扩展，由IANA使用RFC 5226中定义的“先到先得”策略分配。
The Type Values are assigned as follows:
类型值的分配如下所示：
Type Length Description
类型长度描述
0x01 4 PWid of last PW segment traversed 0x02 variable PW Switching Point description string 0x03 4/16 Local IP address of PW Switching Point 0x04 4/16 Remote IP address of last PW Switching Point traversed or of the T-PE 0x05 variable FEC Element of last PW segment traversed 0x06 12 L2 PW address of PW Switching Point
0x01 4经过的最后一个PW段的PWid 0x02变量PW开关点描述字符串0x03 4/16 PW开关点的本地IP地址0x04 4/16经过的最后一个PW开关点的远程IP地址或经过0x06 12 PW开关点的最后一个PW段的T-PE 0x05变量FEC元素的远程IP地址
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2119]Bradner，S.，“RFC中用于表示需求水平的关键词”，BCP 14，RFC 2119，1997年3月。
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, January 1998.
[RFC2277]Alvestrand，H.，“IETF字符集和语言政策”，BCP 18，RFC 2277，1998年1月。
[RFC3931] Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed., "Layer Two Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
[RFC3931]Lau，J.，Ed.，Townsley，M.，Ed.，和I.Goyret，Ed.，“第二层隧道协议-版本3（L2TPv3）”，RFC 39312005年3月。
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private Networks (VPNs)", RFC 4364, February 2006.
[RFC4364]Rosen，E.和Y.Rekhter，“BGP/MPLS IP虚拟专用网络（VPN）”，RFC 4364，2006年2月。
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures", RFC 4379, February 2006.
[RFC4379]Kompella，K.和G.Swallow，“检测多协议标签交换（MPLS）数据平面故障”，RFC 4379，2006年2月。
[RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson, "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN", RFC 4385, February 2006.
[RFC4385]Bryant，S.，Swallow，G.，Martini，L.，和D.McPherson，“用于MPLS PSN的伪线仿真边到边（PWE3）控制字”，RFC 43852006年2月。
[RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
[RFC4446]Martini，L.，“伪线边到边仿真（PWE3）的IANA分配”，BCP 116，RFC 4446，2006年4月。
[RFC4447] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and G. Heron, "Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)", RFC 4447, April 2006.
[RFC4447]Martini，L.，Ed.，Rosen，E.，El Aawar，N.，Smith，T.，和G.Heron，“使用标签分发协议（LDP）的伪线设置和维护”，RFC 4447，2006年4月。
[RFC5003] Metz, C., Martini, L., Balus, F., and J. Sugimoto, "Attachment Individual Identifier (AII) Types for Aggregation", RFC 5003, September 2007.
[RFC5003]Metz，C.，Martini，L.，Balus，F.，和J.Sugimoto，“聚合的附件个人标识符（AII）类型”，RFC 5003，2007年9月。
[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed., "LDP Specification", RFC 5036, October 2007.
[RFC5036]Andersson，L.，Ed.，Minei，I.，Ed.，和B.Thomas，Ed.，“LDP规范”，RFC 5036，2007年10月。
[RFC5085] Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire Virtual Circuit Connectivity Verification (VCCV): A Control Channel for Pseudowires", RFC 5085, December 2007.
[RFC5085]Nadeau，T.，Ed.，和C.Pignataro，Ed.，“伪线虚拟电路连接验证（VCCV）：伪线的控制通道”，RFC 5085，2007年12月。
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
[RFC5226]Narten，T.和H.Alvestrand，“在RFCs中编写IANA注意事项部分的指南”，BCP 26，RFC 5226，2008年5月。
[RFC5641] McGill, N. and C. Pignataro, "Layer 2 Tunneling Protocol Version 3 (L2TPv3) Extended Circuit Status Values", RFC 5641, August 2009.
[RFC5641]McGill，N.和C.Pignataro，“第2层隧道协议版本3（L2TPv3）扩展电路状态值”，RFC 56412009年8月。
[PW-MSG-MAP] Aissaoui, M., Busschbach, P., Morrow, M., Martini, L., Stein, Y(J)., Allan, D., and T. Nadeau, "Pseudowire (PW) OAM Message Mapping", Work in Progress, October 2010.
[PW-MSG-MAP]Aissaoui，M.，Busschbach，P.，Morrow，M.，Martini，L.，Stein，Y（J.），Allan，D.，和T.Nadeau，“伪线（PW）OAM消息映射”，正在进行的工作，2010年10月。
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack Encoding", RFC 3032, January 2001.
[RFC3032]Rosen，E.，Tappan，D.，Fedorkow，G.，Rekhter，Y.，Farinaci，D.，Li，T.，和A.Conta，“MPLS标签堆栈编码”，RFC 3032，2001年1月。
[RFC3438] Townsley, W., "Layer Two Tunneling Protocol (L2TP) Internet Assigned Numbers Authority (IANA) Considerations Update", BCP 68, RFC 3438, December 2002.
[RFC3438]汤斯利，W.“第二层隧道协议（L2TP）互联网分配号码管理局（IANA）注意事项更新”，BCP 68，RFC 3438，2002年12月。
[RFC3985] Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC3985]Bryant，S.，Ed.，和P.Pate，Ed.，“伪线仿真边到边（PWE3）架构”，RFC 39852005年3月。
[RFC4023] Worster, T., Rekhter, Y., and E. Rosen, Ed., "Encapsulating MPLS in IP or Generic Routing Encapsulation (GRE)", RFC 4023, March 2005.
[RFC4023]Worster，T.，Rekhter，Y.，和E.Rosen，编辑，“在IP或通用路由封装（GRE）中封装MPLS”，RFC4023，2005年3月。
[RFC4454] Singh, S., Townsley, M., and C. Pignataro, "Asynchronous Transfer Mode (ATM) over Layer 2 Tunneling Protocol Version 3 (L2TPv3)", RFC 4454, May 2006.
[RFC4454]Singh，S.，Townsley，M.，和C.Pignataro，“第2层隧道协议第3版（L2TPv3）上的异步传输模式（ATM）”，RFC 4454，2006年5月。
[RFC4623] Malis, A. and M. Townsley, "Pseudowire Emulation Edge-to-Edge (PWE3) Fragmentation and Reassembly", RFC 4623, August 2006.
[RFC4623]Malis，A.和M.Townsley，“伪线仿真边到边（PWE3）碎片化和重组”，RFC 46232006年8月。
[RFC4667] Luo, W., "Layer 2 Virtual Private Network (L2VPN) Extensions for Layer 2 Tunneling Protocol (L2TP)", RFC 4667, September 2006.
[RFC4667]Luo，W.“第二层隧道协议（L2TP）的第二层虚拟专用网络（L2VPN）扩展”，RFC 4667，2006年9月。
[RFC4717] Martini, L., Jayakumar, J., Bocci, M., El-Aawar, N., Brayley, J., and G. Koleyni, "Encapsulation Methods for Transport of Asynchronous Transfer Mode (ATM) over MPLS Networks", RFC 4717, December 2006.
[RFC4717]Martini，L.，Jayakumar，J.，Bocci，M.，El-Aawar，N.，Brayley，J.，和G.Koleyni，“MPLS网络上异步传输模式（ATM）传输的封装方法”，RFC 47172006年12月。
[RFC5254] Bitar, N., Ed., Bocci, M., Ed., and L. Martini, Ed., "Requirements for Multi-Segment Pseudowire Emulation Edge-to-Edge (PWE3)", RFC 5254, October 2008.
[RFC5254]Bitar，N.，Ed.，Bocci，M.，Ed.，和L.Martini，Ed.，“多段伪线仿真边到边（PWE3）的要求”，RFC 5254，2008年10月。
[RFC5659] Bocci, M. and S. Bryant, "An Architecture for Multi-Segment Pseudowire Emulation Edge-to-Edge", RFC 5659, October 2009.
[RFC5659]Bocci，M.和S.Bryant，“多段伪线边到边仿真的体系结构”，RFC 5659，2009年10月。
[RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS Networks", RFC 5920, July 2010.
[RFC5920]方，L.，编辑，“MPLS和GMPLS网络的安全框架”，RFC 5920，2010年7月。
The authors wish to acknowledge the contributions of Satoru Matsushima, Wei Luo, Neil Mcgill, Skip Booth, Neil Hart, Michael Hua, and Tiberiu Grigoriu.
作者希望感谢松岛佐藤、罗伟、尼尔·麦吉尔、斯基普·布斯、尼尔·哈特、迈克尔·华和提比略·格里戈里奥的贡献。
The following people also contributed text to this document:
以下人员也为本文件提供了文本：
Florin Balus Alcatel-Lucent 701 East Middlefield Rd. Mountain View, CA 94043 US EMail: florin.balus@alcatel-lucent.com
Florin Balus Alcatel-Lucent加利福尼亚州山景城东米德菲尔德路701号，邮编94043美国电子邮件：Florin。balus@alcatel-朗讯网
Mike Duckett Bellsouth Lindbergh Center, D481 575 Morosgo Dr Atlanta, GA 30324 US EMail: mduckett@bellsouth.net
Mike Duckett Bellbergh South Lindbergh Center，D481 575，佐治亚州亚特兰大莫洛斯哥博士，邮编30324美国电子邮件：mduckett@bellsouth.net
Authors' Addresses
作者地址
Luca Martini Cisco Systems, Inc. 9155 East Nichols Avenue, Suite 400 Englewood, CO 80112 US EMail: lmartini@cisco.com
Luca Martini Cisco Systems，Inc.地址：美国科罗拉多州恩格尔伍德东尼科尔斯大道9155号400室，邮编：80112电子邮件：lmartini@cisco.com
Chris Metz Cisco Systems, Inc. EMail: chmetz@cisco.com
Chris Metz Cisco Systems，Inc.电子邮件：chmetz@cisco.com
Thomas D. Nadeau EMail: tnadeau@lucidvision.com
Thomas D.Nadeau电子邮件：tnadeau@lucidvision.com
Matthew Bocci Alcatel-Lucent Grove House, Waltham Road Rd White Waltham, Berks SL6 3TN UK EMail: matthew.bocci@alcatel-lucent.co.uk
Matthew Bocci Alcatel-Lucent Grove House，沃尔瑟姆路，伯克斯怀特沃尔瑟姆SL6 3TN英国电子邮件：Matthew。bocci@alcatel-朗讯公司
Mustapha Aissaoui Alcatel-Lucent 600, March Road, Kanata, ON Canada EMail: mustapha.aissaoui@alcatel-lucent.com
Mustapha Aissaoui Alcatel-Lucent 600，卡纳塔三月路，加拿大电子邮件：Mustapha。aissaoui@alcatel-朗讯网