Protocol Independent Multicast

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Protocol Independent Multicast

There are four variants of PIM:

PIM Sparse Mode (PIM-SM) explicitly builds unidirectional shared trees rooted at a Rendezvous Point (RP) per group, and optionally creates shortest-path trees per source. PIM-SM generally scales fairly well for wide-area usage. See the PIM Internet Standard RFC 4601

PIM Dense Mode (PIM-DM) implicitly builds shortest-path trees by flooding multicast traffic domain wide, and then pruning back branches of the tree where no receivers are present. PIM-DM generally has poor scaling properties. See experimental RFC 3973

Bidirectional PIM explicitly builds shared bi-directional trees. It never builds a shortest path tree, so may have longer end-to-end delays than PIM-SM, but scales well because it needs no source-specific state. see draft-ietf-pim-bidir

PIM Source Specific Multicast (PIM-SSM) builds trees that are rooted in just one source, offering a more secure and scalable model for a limited amount of applications (mostly broadcasting of content). In SSM, an IP datagram is transmitted by a source S to an SSM destination address G, and receivers can receive this datagram by subscribing to channel (S,G). See informational RFC 3569

Of the four, PIM-SM has the widest deployment.