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Multicast Routing

The goal of multicast routing is to find a tree between the routers attached to the senders and receivers of a multicast group. In practice two approaches have been adopted for determining the multicast routing tree. They differ according to whether a single group-shared tree is used for all senders in the group or whether a source-specific tree is constructed for each individual sender.

Group-Shared Tree
In order to build a single, shared routing tree from all senders, this approach first selects a core router (rendezvous point). The multicast tree is constructed when edge routers with attached hosts send join messages addressed to the core router. Data packets must be sent first to the core router, which forwards them along the multicast tree to the receivers.

Source-based Tree
In practice, a so-called reverse path forwarding algorithm is used to build a tree from a specific source to all receivers. When a router receives a multicast packet over the link, which is on its own shortest path to the sender, it transmits it on all outgoing links (except the one on which it was received). Otherwise it simply discards the incoming packet. By this approach, multicast packets are flooded over the network to all receivers, but furthermore to routers with no attached receivers for that group. The solution to the problem of receiving unwanted multicast packets is known as pruning. Each router that has no attached hosts for this group sends a prune message to its upstream router.

In this application you are able to select senders and receivers of a multicast group and see for the selected scenario how group-based and source-based multicast trees are constructed.