Past exam of the computer science course of the University of Oxford / 2015 / Computer networks / 2 / d

UDP fits IP multicast because each independent datagram can be delivered to a group without establishing a separate transport connection with every receiver. Ordinary TCP defines a reliable ordered byte stream between two endpoints, with one peer's acknowledgements, receive window and connection state. A group has many receivers with different losses and capacities, so that interface cannot simply be assigned to one group address. A reliable group service can be designed, but ordinary TCP does not provide it. UDP itself adds neither group reliability nor congestion control; applications must provide whatever additional behaviour they require.

A virtual multicast distribution tree lets one packet traverse a shared upstream link and be copied only where branches diverge. Compared with a separate unicast copy per receiver, this saves repeated transmission over the common links. A tree also supplies a unique parent route and avoids forwarding loops when the routing information is consistent. Branches with no interested receivers can be pruned.

For a source-specific multicast distribution tree, a router needs the group identifier and source identifier, the expected incoming interface, the set of outgoing interfaces leading to interested receivers, and information about local group membership. Timers and join/prune information maintain those decisions. This is naturally keyed by $(S,G)$ for source $S$ and group $G$; other multicast routing designs can use a shared group tree. A router needs the relevant local forwarding entries, rather than a complete list of all group members everywhere. The result is