Why DatagramSocketImpl joinGroup method takes a Ne

Question

Just curious. Is that redundant? Haven't you already bound to the network you want to use? Perhaps that's for the case when you bound to 0.0.0.0 and now want to listen to multicast packets only from interface X?

Answer 1

1. If you are bound to INADDR_ANY, which is the normal case, the join-group IGMP message goes out via whichever NIC interface the routing tables say gives the shortest route to the multicast address. In multi-homed hosts you may need it to go out via all NICs, so you loop over them joining via each in turn.

2. If you're bound to a specific NIC it doesn't make sense to specify a network interface when joining.

3. If you're bound to the multicast address itself, so that that socket can only receive multicasts, not unicast UDP, you may need to loop as in (1) if you want to join via all interfaces.