(分享)1-2 SSM - Preparation(2)

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■视频名称：

    《1-2 SSM - Preparation(2)》

■所属专辑：

    《Developing Interdomain Multicast Networks》（跨域组播）

■视频时长：

    32分26秒

■内容简介：

    SSM（Source Specific Multicast）扩展了传统的PIM协议，更适合单对多通信。SSM允许接受者通过控制层获得数据源的地址以后，直接向数据源方向建立SPT，而不经过RP。

    SSM不需要使用MSDP得知其他组播域的数据源，甚至不需要RP。

    在传统的组播网中，组播数据是根据组播组地址转发的，因此接收者必须加入一个组播组。如果两个数据源向同一个组播组发数据，那么接收者会接收改组所有的数据。但是，接受者往往只需要其中一个数据源的数据，所以接受者需要过滤不需要的数据。该功能可以通过配置接收者实现，但是被过滤的组播流量会造成网络资源的浪费。

    在SSM网络中，接收者不仅能够选择需要加入的组播组，而且可以选择接收哪个数据源的数据。接收者通过IGMPv3的INCLUDE模式表示希望接收哪个数据源发送的哪个组播组数据，最后一条路由器根据IGMPv3协议获得接收者的请求，并向该指定的数据源建立SPT。

    开启了SSM功能的组播路由器，在建立组播转发树的时候，不是象传统PIM-SM那样向RP建立RPT，而是直接向数据源方向建立SPT。通过这种方式，指定数据源发送的组播数据，可以通过SPT直接发送给接收者，而其他数据源向该组播组发送数据，则由于不存在SPT的缘故而无法转发给接收者。

    对于组播地址冲突，SSM具有重要意义。如果两个应用占用了相同的组播地址，那么可以采用SSM将组播数据分离开。对于需要大量组播地址的应用，不方便分配很多组播地址，也可以使用SSM部署应用。

■Abstract：

    SSM is an extension of the PIM protocol that allows for an efficient data delivery mechanism in one-to-many communications. SSM enables a receiving client, once it has learned about a particular multicast source through a directory service, to receive content directly from the source, rather than receiving it using a shared RP.

    SSM removes the requirement of MSDP to discover the active sources in other PIM domains. An out-of-band service at the application level, such as a Web server, can perform source discovery. SSM also removes the requirement to have an RP.

    In traditional multicast implementations, applications must join to an IP multicast group address because traffic is distributed to an entire IP multicast group. If two applications with different sources and receivers use the same IP multicast group address, receivers of both applications will receive traffic from the senders of both the applications. Even though the receivers, if programmed appropriately, can filter out the unwanted traffic, this situation would likely generate noticeable levels of unwanted network traffic.

    In an SSM-enhanced multicast network, the router closest to the receiver will see a request from the receiving application to join to a particular multicast source. The receiver application can signal its intention to join a particular source by using the INCLUDE mode in IGMPv3.

    SSM also solves IP multicast address collision issues associated with one-to-many applications. An address collision can occur when two organization use the same multicast destination address (group address) for different applications. Routers running in SSM mode route data streams based on the full (S, G) address. Assuming that a source has a unique IP address to send on the Internet, any (S, G) from this source would be unique.