Disabling IP multicast

The UDP layer of the JChannel protocol stack by default uses IP multicast to broadcast messages to all group members. However, in some environments, using IP multicast may not be possible:

• IP multicasting is disabled between subnets in a local network. IP multicasting is enabled only within the same subnets, bridges drop IP multicast packets.

• Members of a group span multiple geographically separate networks, some routers between the networks drop IP multicast packets.

• Some members are behind a firewall (see section 3.8).

• As of the JDK 1.1.X, IP multicasting is not allowed in applets. Therefore applets using JavaGroups would have to disable IP multicast (or use tunneling, see 3.8).

In these cases, instead of sending an IP multicast message to n group members, n unicast messages can be sent. Although not as efficient (and using more bandwidth), it is sometimes the only possibility to reach group members. To disable the use of IP multicast, UDP has to be instructed to use unicast instead of multicast messages to send messages to the group (see 3.7.1). To find out whether IP multicasting is enabled at all, the demo program JavaGroups/tests/Mcast.java can be used: it has to be started on each host, and messages sent by one Mcast process should be received by all others.