Source-specific multicast

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Source-specific multicast (SSM) is a method of delivering multicast packets originating in only a specific source address S to any receiver that requests it. It limits the original multicast model to be originated from just one host, simplifiying the requirements on the network and easing security of multicast delivery.

It requires that the receivers specify the source address, something that is only possible in IGMPv3 and IPv6's MLD.

To simplify the idea of SSM, we define it in terms of the difference between SSM and Any-Source Multicast (ASM).

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

all participants in the group want to be aware of all other participants, and
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.

Use of SSM requires support for it in last-hop routers and receivers' operating system. The support of ssm is not required in the rest of network items (the other routers and even the sender). Interest in multicast traffic from a specific source is conveyed from hosts to routers using IGMPv3 as specified in RFC 3376.

SSM destination addresses must be in the ranges 232.0.0.0/8 for IPv4 and FF3x::/96 for IPv6.