4 SSM - SSM-Mapping

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■视频名称：

    《4 SSM - SSM-Mapping》

■所属专辑：

    《Developing Interdomain Multicast Networks》（跨域组播）

■视频时长：

    7分10秒

■内容简介：

    部署SSM时，如果组播终端不支持IGMPv3或URD，或组播终端不允许部署SSM，那么需要最后一跳路由器将ISM映射为SSM，即SSM mapping。最典型的场合，是IPTV系统。IPTV核心网采用SSM，但是终端不支持SSM且不便修改，所以采用SSM Mapping是最可行的解决方案。

    SSM Mapping适合更为苛刻的场景，部署SSM困难重重：（1）组播终端的操作系统不支持IGMPv3；（2）组播终端软件升级可能性不大，例如机顶盒数量庞大；（3）组播终端软件不能通过web浏览器启动，因此无法部署URD。

    SSM Mapping使最后一跳路由器可以主动发现数据源。当配置了SSM Mapping以后，如果路由器接收到IGMPv1或IGMPv2成员通告，那么路由器会自动对应到一个或多个（S，G）。

■Abstract：

    SSM mapping supports SSM transition in cases where neither URD nor IGMP v3lite is available, or when supporting SSM on the end system is impossible or unwanted due to administrative or technical reasons. Using SSM to deliver live streaming video to legacy STBs that do not support IGMPv3 is a typical application of SSM mapping.

    Prior to the introduction of SSM mapping, the following conditions would have prevented SSM transition in the case of legacy STB deployments with STB receivers that only support IGMPv1 or IGMPv2: (1)The operating system on the receivers do not support IGMPv3; thus, IGMPv3 cannot be used to support SSM. (2)Moreover, the application running on the receivers cannot be upgraded to support SSM; thus, IGMPv3 lite cannot be used to support SSM transition. (3)Furthermore, the application itself cannot be started through a web browser; thus, URD cannot be used to support SSM transition.

    SSM mapping introduces a means for the last hop router to discover sources sending to groups. When SSM mapping is configured, if a router receives an IGMPv1 or IGMPv2 membership report for a particular group G, the router translates this report into one or more (S, G) channel memberships for the well-known sources associated with this group.