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Chapter 30Multicast RoutingThis section describes the usage and the internals of multicast routing implementation in ns. We first describe the userinterface to enable multicast routing (Section 30.1), specify the multicast routing protocol to be used and the various methodsand configuration parameters specific to the protocols currently supported in ns. We then describe in detail the internals andthe architecture of the multicast routing implementation in ns (Section 30.2).The procedures and functions described in this chapter can be found in various files in the directories ~ns/tcl/mcast, ~ns/tcl/ctrmcast;additional support routines are in ~ns/mcast_ctrl.{cc,h}, ~ns/tcl/lib/ns-lib.tcl, and ~ns/tcl/lib/ns-node.tcl.30.1 Multicast APIMulticast forwarding requires enhancements to the nodes and links in the topology. Therefore, the user must specify multicastrequirements to the Simulator class before creating the topology. This is done in one of two ways:orset ns [new Simulator -multicast on]set ns [new Simulator]$ns multicastWhen multicast extensions are thus enabled, nodes will be created with additional classifiers and replicators for multicastforwarding, and links will contain elements to assign incoming interface labels to all packets entering a node.A multicast routing strategy is the mechanism by which the multicast distribution tree is computed in the simulation. nssupports three multiast route computation strategies: centralised, dense mode(DM) or shared tree mode(ST).The method mrtproto{} in the Class Simulator specifies either the route computation strategy, for centralised multicastrouting, or the specific detailed multicast routing protocol that should be used.The following are examples of valid invocations of multicast routing in ns:set cmc [$ns mrtproto CtrMcast]$ns mrtproto DM;# specify centralized multicast for all nodes;# cmc is the handle for multicast protocol object;# specify dense mode multicast for all nodes261