Dense and Sparse Modes for Multicast Networks

The actions of receivers suggest two basic strategies for protocols to handle joining and pruning branches among a collection of multicast routers:

• Dense-mode multicast—The assumption could be made that almost all possible subnets have at least one receiver wanting to receive the multicast traffic from a source, so the network is flooded with traffic on all possible branches, then pruned back when branches do not express an interest in receiving the packets, explicitly (by message) or implicitly (time-out silence). This is the dense mode of multicast operation. LANs are appropriate networks for dense-mode operation.
• Sparse-mode multicast—Alternatively, the assumption could be made that very few of the possible receivers want packets from this source, so the network establishes and sends packets only on branches that have at least one leaf indicating (by message) a desire for the traffic. This is the sparse mode of multicast operation. WANs are appropriate networks for sparse-mode operation, and indeed a common multicast guideline is not to run dense mode on a WAN under any circumstances.

Some multicast routing protocols, especially older ones, support only dense-mode operation, which makes them inappropriate for use on the public Internet. Others allow sparse mode as well. If sparse-dense mode is supported, the multicast routing protocol allows some multicast groups to be sparse and other groups to be dense.

There is also a difference between the multicast protocols used between host and router and between the multicast routers themselves. Hosts on a given subnetwork need to inform their router only whether or not they are interested in receiving packets from a certain multicast group. The source host needs to inform its routers only that it is the source of traffic for a particular multicast group. In other words, no detailed knowledge of the distribution tree is needed by any hosts, only a group membership protocol to inform routers of their participation in a multicast group. Between adjacent routers, on the other hand, the multicast routing protocols must avoid loops as they build a detailed sense of the network topology and distribution tree from source to leaf. So, different multicast protocols are used for the host-router portion and the router-router portion of the multicast network.