Multicast Routing Protocols

Multicast routing protocols enable a collection of multicast routers to build (join) distribution trees when a host on a directly attached subnet, typically a LAN, wants to receive traffic from a certain multicast group.

There are five multicast routing protocols:

• DVMRP—The first of the multicast routing protocols and hampered by a number of limitations that make this method unattractive for large-scale Internet use. DVMRP is a dense-mode-only protocol, and uses the flood-and-prune or implicit join method to deliver traffic everywhere and then determine where the uninterested receivers are. DVMRP uses source-based distribution trees in the form (S,G).
• MOSPF—Extends OSPF for multicast use, but only for dense mode. However, MOSPF has an explicit join message, so routers do not have to flood their entire domain with multicast traffic from every source. MOSPF uses source-based distribution trees in the form (S,G).
• PIM dense mode—This is PIM operating in dense mode (PIM DM), but the differences from PIM sparse mode are profound enough to consider the two modes separately. PIM also supports sparse-dense mode, with mixed sparse and dense groups, but there is no special notation for that operational mode. In contrast to DVMRP and MOSPF, PIM dense mode allows a router to use any unicast routing protocol and performs RPF checks using the unicast routing table. PIM dense mode has an implicit join message, so routers use the flood-and-prune method to deliver traffic everywhere and then determine where the uninterested receivers are. PIM dense mode uses source-based distribution trees in the form (S,G), as do all dense-mode protocols.
• PIM sparse mode—Allows a router to use any unicast routing protocol and performs RPF checks using the unicast routing table. However, PIM sparse mode has an explicit join message, so routers determine where the interested receivers are and send join messages upstream to their neighbors, building trees from receivers to RP. PIM sparse mode uses an RP router as the initial source of multicast group traffic and therefore builds distribution trees in the form (*,G), as do all sparse-mode protocols. However, PIM sparse mode migrates to an (S,G) source-based tree if that path is shorter than through the RP for a particular multicast group's traffic.
• Core-based tree (CBT)—Shares all of the characteristics of PIM sparse mode (sparse mode, explicit join, and shared (*,G) trees), but is said to be more efficient at finding sources than PIM sparse mode. CBT is rarely encountered outside academic discussions. There are no large-scale deployments of CBT, commercial or otherwise.

The differences among the five multicast routing protocols are summarized in Table 2.

Table 2: Multicast Routing Protocols Compared

Multicast Routing Protocol	Dense Mode	Sparse Mode	Implicit Join	Explicit Join	(S,G) SBT	(*,G) Shared Tree
DVMRP	Yes	No	Yes	No	Yes	No
MOSPF	Yes	No	No	Yes	Yes	No
PIM dense mode	Yes	No	Yes	No	Yes	No
PIM sparse mode	No	Yes	No	Yes	Yes, maybe	Yes, initially
CBT	No	Yes	No	Yes	No	Yes

It is important to realize that retransmissions due to a high bit-error rate on a link or overloaded router can make multicast as inefficient as repeated unicast. Therefore, there is a trade-off in many multicast applications regarding the session support when using TCP (but TCP always resends missing segments) or the simple drop-and-continue strategy of the UDP datagram service (but reordering can become an issue). Modern multicast uses UDP almost exclusively.