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Source-Specific Multicast

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Source-specific multicast (SSM) is a service model for multicast operation. It is best understood as a contrast with the traditional or any-source multicast (ASM) service model.

In the ASM service model, receiver express interest in traffic to a multicast address. 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 be 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.

The SSM service model allows a receiver to express interest in receiving traffic from a specific source sending to a 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.

See RFC 3569 for an overview of SSM.

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.

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