6    Support of the MAC Service

MAC Bridges interconnect the separate IEEE 802 LANs that compose a Bridged Local Area Network by relaying and filtering frames between the separate MACs of the bridged LANs. The position of the bridging function within the MAC Sublayer is shown in Figure 6-1.

MAC网桥把彼此独立的IEEE 802 LAN连接在一起，构成桥接本地局域网，MAC网桥对于不同LAN之间的数据进行转发和过滤，如Figure 6-1。
 
 

RSTP协议——第6章(6-6.2)_RSTP

This clause discusses the following aspects of service provision:
 
a) Provision of the MAC Service to end stations.
b) Preservation of the MAC Service.
c) Maintenance of QoS.
d) Provision of the Internal Sublayer Service within the MAC Bridge.

e) Support of the Internal Sublayer Service by specific MAC procedures.

f) Filtering services.
 
本章讨论如下方面的规定：
a)   MAC服务到终端工作站的规定。
b)   MAC服务的保存。
c)   维护QoS。
d)   在MAC网桥内的内部子层服务的规定。
e)   使用特定MAC函数支持内部子层服务。
f)   过滤服务。
6.1         Support of the MAC Service

The MAC Service provided to end stations attached to a Bridged Local Area Network is the (unconfirmed) connectionless-mode MAC Service defined in ISO/IEC 15802-1. The MAC Service is defined as an abstraction of the features common to a number of specific MAC Services; it describes the transfer of user data between source and destination end stations, via MA-UNITDATA request primitives and corresponding MA-UNITDATA indication primitives issued at MAC service access points. Each MA-UNITDATA request and indication primitive has four parameters: Destination Address, Source Address, MAC Service data unit (MSDU), and Priority.

提供给终端工作站，连接到桥接LAN的MAC服务是ISO/IEC 15802-1所定义的非正式的无连接模式MAC服务。MAC服务是对多种具体MAC服务共同特性的抽象，它描述了用户数据在源和目的终端工作站之间的传输，使用MA-UNITDATA请求和对应的MA-UNITDATA指示原语来描述。每个MA-UNITDATA请求和指示原语都有4个参数：目的地址、源地址、MAC服务数据单元（MSDU）和优先级。
 

To improve the availability of the MAC Service to end stations and support network management, MAC Bridges can be configured:

为了提升MAC服务到终端工作站和网管的可用性，MAC网桥可以配置为：
 

a) To provide redundant paths between end stations to enable the network to continue to provide the MAC Service in the event of component failure (of Bridge or LAN).

a)  支持终端工作站之间冗余的路径，以便网络一部分发生故障时还能继续提供MAC服务。
 

b) So that the paths supported between end stations are predictable and configurable given the availability of network components.

b)  在给定网络可用性的条件下，在终端工作站之间的路径是可预测和可配置的。
 

A MAC Bridge may restrict the provision of the MAC Service to authenticated and authorized devices. Unauthorized devices can be denied access to a bridged LAN, except as necessary for the purpose of conducting protocol exchanges required by an authentication process.

MAC网桥可以通过对设备进行认证和授权来显示MAC服务。除了认证协议外，非授权设备被禁止访问桥接LAN。
 

NOTE—Authentication and authorization to access a LAN may be achieved by administrative or management mechanisms, or by means of an active authorization mechanism, such as is defined in IEEE Std 802.1X.

注意：认证和对LAN访问的认证参见IEEE Std 802.1X。

6.2         Preservation of the MAC Service

The MAC Service provided by a Bridged Local Area Network is similar to that provided by a single LAN (6.3). In consequence,

桥接LAN提供的MAC服务类似于单个LAN(6.3)所提供的服务。如下：
 

a) A Bridge is not directly addressed by communicating end stations, except as an end station for management purposes—frames transmitted between end stations carry the MAC Address of the peer-end station in their Destination Address field, not a MAC Address of the Bridge.

a)  网桥不能通过与终端工作站直接通讯而进行寻址，除非终端工作站处于管理目的，在终端工作站之间传输的数据包在目的地址字段带着对端工作站的MAC地址，不是网桥的MAC地址。
 
b) All MAC Addresses need to be unique within the network.
b) 所有MAC地址在网络内都是唯一的。
 

c) MAC Addresses of end stations are not restricted by the topology and configuration of the network.

c) 网络拓扑和配置对终端工作站的MAC地址没有限制。