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→External links: updated link information about pimd to indicate that it is no longer a separate module, but now merged as an official module of the Quagga suite |
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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.<ref>{{cite web |url=http://technet.microsoft.com/en-us/library/bb742462.aspx |title=PIM-SM Multicast Routing Protocol |publisher=[[Microsoft]] |accessdate=2014-03-26}}</ref>
* '''PIM Dense Mode''' (PIM-DM) uses [[dense multicast]] routing. It 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 is straightforward to implement but generally has poor scaling properties. The first multicast routing protocol, [[DVMRP]] used dense-mode multicast routing.<ref>{{cite web |url=http://www.multicasttech.com/faq/ |title=Frequently Asked Questions (FAQ) File for Multicasting |publisher=Multicast Tech |archiveurl=
* '''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 Bidirectional PIM Internet Standard RFC 5015.
* '''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.
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