Within the scope of networking, multicast is a simultaneous delivery of data to a group of computers from a single transmission. Among the most common application that utilizes this includes internet-based streaming media and internet television. In IP multicast, the process is handled at the routing level, where routers prepare optimal distribution paths for multicast data. One problem in multicasting is the issue of flooding, which arises when each node sends a broadcast copy to all its neighbors upon receiving a multicast packet.
To prevent the redundancy resulting from this, a solution is the creation of a spanning tree. The procedure requires the construction of a spanning tree, after which the packet is transmitted to nodes along the tree. The tree is created by having each node forward a join message to the center node. Of the nodes that have forwarded the message, the tree is completed when the center node receives a message from a node already belonging to the group. In the case where the source acts as a central node, this is a source-based tree as opposed to a group shared tree. A source-based tree relies on the shortest path algorithm and is best suited for a small group of computers on a network. In cases where “AS” are included in the recipients’ list, this algorithm is poorly suited. In the case where “AS” is involved, the best option is a shared tree where a router acts as the central node. The nodes send their join messages to this node to create the tree. The path taken by the join message becomes a new branch for this tree. This mode is best suited for multicasting across networks.