G.8031/Y.1342 Ethernet Protection Switching学习笔记

This Recommendation specifies linear 1+1 protection switching architecture and linear 1:1 protection switching architecture.
以太网线性保护用于保护一条以太网连接，它是一种专用的端到端保护结构，可以用于不同的网络结构，如网状网，环网等。
G.8031定义了1＋1和1︰1两种保护方式，在1＋1方式中每一个保护资源都对应着一个工作资源，在保护域内，1＋1方式采用双发单收的保护机制；1︰1方式采用保护资源与工作资源彼此切换的机制。

In the linear 1+1 protection switching architecture, a protection transport entity is dedicated to each working transport entity. The normal traffic is copied and fed to both working and protection transport entities with a permanent bridge at the source of the protected domain. The traffic on working and protection transport entities is transmitted simultaneously to the sink of the protected domain, where a selection between the working and protection transport entities is made based on some predetermined criteria, such as server defect indication.
在1＋1结构中，保护连接是每条工作连接专用的，工作连接与保护连接在保护域的源端进行桥接。业务在工作和保护连接上同时发向保护域的宿端，在宿端，基于某种预先确定的准则如缺陷指示来选择接收来自工作或保护连接上的业务。为了避免单点失效，工作连接和保护连接应该走分离的路由。

In the linear 1:1 protection switching architecture, the protection transport entity is dedicated to the working transport entity. However, the normal traffic is transported either on the working transport entity or on the protection transport entity using a selector bridge at the source of the protected domain. The selector at the sink of the protected domain selects the entity which carries the normal traffic.
在1：1结构中，保护连接是每条工作连接专用的，被保护的工作业务由工作或保护连接进行传送。工作和保护连接的选择方法由某种机制决定。为了避免单点失效，工作连接和保护连接应该走分离路由。

In the linear protection architecture defined in this version of the Recommendation, protection switching occurs at the two distinct endpoints of a point-to-point VLAN-based ETH SNC(Subnetwork Connection). Between these endpoints, there will be both "working" and "protection" transport entities.
For a given direction of transmission, the "head end" of the protected entity is capable of performing a bridge function, which will place a copy of a normal traffic signal onto a protection transport entity when required.
The "tail end" will perform a selector function, where it is capable of selecting a normal traffic signal either from its usual working transport entity, or from a protection transport entity.
In the case of bidirectional transmission, where both directions of transmission are protected, both ends of the protected entity will normally provide both bridge and selector functions.

In the case of bidirectional transmission, it is possible to choose either unidirectional or bidirectional switching. With unidirectional switching, the selectors at each end are fully independent. With bidirectional switching, an attempt is made to coordinate the two ends so that both have the same bridge and selector settings, even for a unidirectional failure. Bidirectional switching always requires APS information to coordinate the two endpoints. Unidirectional switching can protect two unidirectional failures in opposite directions on different entities.
以太网线性保护的倒换类型包括单向倒换和双向倒换。
单向倒换时只有受影响的连接方向倒换至保护路径，两端的选择器是独立的。单向保护可以防止在两个独立方向上的单通故障。在单向保护倒换操作模式下，保护倒换由保护域的宿端选择器完全基于本地（即保护宿端）信息来完成。工作（被保护）业务在保护域的源端永久桥接到工作和保护连接上。若使用连接性检查包检测工作和保护连接故障，则它们同时在保护域的源端插入到工作和保护连接上，并在保护域宿端进行检测和提取。需注意无论连接是否被选择器所选择，连接性检查包都会在上面发送。

双向倒换即受影响的和未受影响的连接方向均倒换至保护路径。双向倒换需要自动保护倒换协议（APS）用于协调连接的两端。在双向保护倒换模式下，基于本地或近端信息和来自另一端或远端的APS协议信息，保护倒换由保护域源端选择器桥接和宿端选择器共同来完成。若使用连接性检查包检测工作和保护连接故障，则它们同时在保护域的源端插入到工作和保护连接上，并在保护域宿端进行检测和提取。需要注意的是，无论连接是否被选择器选择，连接性检查包都会在上面发送。

The linear 1+1 protection switching architecture operates with either unidirectional or bidirectional switching.
1)单向1+1保护倒换结构

如果工作连接上发生单向故障（从结点A到结点Z的传输方向），此故障将在保护域宿端结点Z 被检测到，然后结点Z 选择器将倒换至保护连接。

2)双向1＋1保护倒换结构

3)双向1:1保护倒换结构

以太网线性保护的应用

APS Protocol：APS PDU format
APS information is carried within the APS PDU which is one of a suite of Ethernet CFM PDUs.

- Version: 0x00
- OpCode: 0x39
- Flags: 0x00
- TLV Offset: 0x04
- END TLV: 0x00