The only protocol to build multicast trees is PIM-SMv2. It is specified in draft-ietf-pim-sm-v2-new-11.txt [FHHK05], and is at this time of writing almost published as an RFC (this new RFC will obsolete RFC 2362). PIM-SMv2 supports IPv4 and IPv6, and supports both the ASM and SSM model. PIM-SMv2 is usually called PIM-SSM when working in the SSM mode. The mode of PIM-SMv2 (ASM or SSM) is subject to the addresses used inside the backbone. Also, to work in the ASM mode, PIM-SMv2 requires enabling of one or more Rendezvous Points (RPs).
As PIM-SMv2 is defined for IPv4 and IPv6, there are no differences with tree building for IPv4 and IPv6. The reader can refer to other documents to have information about PIM-SMv2. The small changes reside in the definition of the RP. CISCO RP discovery, which is a CISCO protocol to automatically advertise an RP, is not defined for IPv6. On the other hand, Embedded-RP proposes a new group-to-RP mapping solution.
A PIM-SSM router only needs to implement the upstream and downstream (S,G) state machine from the specification. It needs to implement the (S,G) assert and state machine for multiple PIM routers sharing a common LAN. Also Hello messages, neighbour discovery DR discovery and packet forwarding rules must be implemented.
With this subset, the implementation is greatly simplified compared to the (*,G), (S,G,RPT), (*,*,RP)
downstream and upstream state machine, the register state machine the (*,G), assert state machine, the bootstrap RP election, the Keep alive Timer and the SptBit from a PIM-SM router. Also, the
forwarding rules are more complex in PIM-SM.
By putting source discovery and address allocation outside of the network, SSM multicast routing does exactly what its name indicates: multi-destination routing. Because the multicast network function is greatly simplified, debugging, securing and deploying an SSM multicast network is greatly simplified too.
Network operators may choose to deploy PIM-SSM only or PIM-SM, but if PIM-SM is deployed, PIM-SSM is implicitly deployed too. Note that PIM-SM and PIM-SSM can be used simultaneously since they use different address ranges.