(分享)1-1 MSDP - Preparation(上)

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■视频名称：

    《1-1 MSDP - Preparation(上)》

■所属专辑：

    《Developing Interdomain Multicast Networks》（跨域组播）

■视频时长：

    26分06秒

■内容简介：

    在组播PIM-SM模式下，离组播源最近的组播路由器，向RP注册，使RP知道组播源的信息。同时，RP还知道所有组播接受者的信息。这样，RP作为中转站，可以将组播流量从组播源转发给组播接受者。

    在一个组播域中，RP无法知道其他组播域的数据源，因此RP无法处理跨越多个组播域的组播流量。因此，人们发明了MSDP，使RP可以获得其他组播域的数据源的信息。同时，RP还可以获得本域的组播接受者的信息，这样，RP就可以转发跨域的组播流量。MSDP的一个重要特点，就是每个组播域都可以维护自己的RP，不同组播域的RP之间通过MSDP交换数据源信息。

    不同组播域的RP之间，通过TCP建议MSDP邻居关系。当RP发现本组播域的一个数据源在发送数据，该RP会将组播数据封装在Source-Active (SA)消息中，发送给所有MSDP邻居。每个MSDP邻居使用特殊的RPF检查，将SA消息转发给更多的MSDP邻居。如果接收SA消息的MSDP是一个RP，并且本域内有相应的接受者，那么这个RP会向数据源方向建立SPT。当然，这个SPT伸展到其他组播域。建立SPT以后，组播流量就不必通过SA消息封装的方式扩散到全网，而是通过SPT和RPT转发。

    MSDP用于运营商之间部署组播服务。运营商一定不希望依赖其他运营商的RP来部署组播，因此每个运营商都维护自己的RP，这些RP之间使用MSDP来协调部署组播服务。

■Abstract：

    In the PIM-SM model, multicast sources must register with their local RP. Actually, the router closest to a source registers with the RP, but the key point to note is that the RP "knows" about all the sources for any particular group. The RP also needs to know if there are interested receivers and where they are located downstream. The designated router for each receiver network is responsible for sending joins toward the RP.

    RPs in one domain have no way of knowing about sources located in other domains. MSDP is a mechanism that allows RPs to share information about active sources. RPs know about the receivers in their local domain. When RPs in remote domains hear about the active sources, they can pass on that information to their local receivers. Multicast data can then be forwarded between the domains. A useful feature of MSDP is that it allows each domain to maintain an independent RP that does not rely on other domains, but it does enable RPs to forward traffic between domains. PIM-SM is used to forward the traffic between the multicast domains.

    The RP in each domain establishes an MSDP peering session using a TCP connection with the RPs in other domains or with border routers leading to the other domains. When the RP learns about a new multicast source within its own domain (through the normal PIM register mechanism), the RP encapsulates the first data packet in a Source-Active (SA) message and sends the SA to all MSDP peers. Each receiving peer uses a modified RPF check to forward the SA until the SA reaches every MSDP router in the interconnected networks—theoretically the entire multicast Internet. If the receiving MSDP peer is an RP, and the RP has a (*, G) entry for the group in the SA (there is an interested receiver), the RP creates (S, G) state for the source and joins to the shortest path tree for the source. The encapsulated data is decapsulated and forwarded down the shared tree of that RP. When the last hop router (the router closest to the receiver) receives the multicast packet, it may join the shortest path tree to the source. The MSDP speaker periodically sends SAs that include all sources within the RP's domain.

    MSDP was developed for peering between Internet service providers (ISPs). ISPs did not want to rely on an RP maintained by a competing ISP to provide service to their customers. MSDP allows each ISP to have its own local RP and still forward and receive multicast traffic to the Internet.