Source-Specific Multicast

The operation of source-specific multicast is best understood as a contrast with traditional multicast operation.

Traditionally, multicast receivers have expressed interest in receiving traffic to a multicast address. In this model of operation, the multicast network must discover all multicast sources sending to that address and then route data from all sources to all interested receivers. This behavior is particularly well suited to groupware applications where 1) all participants in the group want to aware of all other participants, and 2) the list of participants isn't known in advance. The source discovery burden on the network can become significant for a large number of sources.

Source-specific multicast allows a receiver to express interest in receiving traffic from a specific source sending to multicast address. This relieves the network of discovering where multicast sources are present and reduces the amount of multicast routing information that the network must maintain.

Use of SSM requires support for it in multicast routers, operating systems, and in applications. Interest in multicast traffic from a specific source is conveyed from hosts to routers using IGMPv3 as specified in ===RFC 3376===