Source-Specific Multicasting (SSM)

Source-Specific Multicasting (SSM)
Within any multicast group, it is possible for two sources to exist. Therefore, as multiple listeners
join the group, they all receive multicast streams from both sources. This can be filtered out, but
possibly not until the last router is reached, in which case considerable unnecessary traffic will
have been transmitted. Source-Specific Multicasting (SSM) is an extension to the PIM protocol
that removes that problem without having to resort to MSDP source discovery. SSM requires the
network be running IGMPv3.
In SSM multicast networks, the router closest to the receiver receives a request from that
receiver to join to a multicast source. The receiver application uses the Include option to specify
the required source. Once the multicast router knows the specific source of the multicast stream,
it no longer needs to communicate via the RP, but can instead forward data to the receiver
directly, using a source-based share tree distribution system.