Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 256
Receiving Multicast Datagrams

Multicast IP datagrams are received using the same Receive IP operation as normal, unicast datagrams. However, before any datagrams destined to a particular group can be received, an upper-layer protocol (an application) must ask the IP module to join that group. Thus, the IP service interface must be extended to provide two new operations: JoinHostGroup (group-address, interface) and LeaveHostGroup (group-address, interface).

The JoinHostGroup operation requests that this host become a member of the host group identified by “group-address” on the given network interface. The LeaveGroup operation requests that this host give up its membership in the host group identified by “group-address” on the given network interface. The interface specifies a unique interface for those IP hosts having more than one interface. If you have more than one interface, you can join the same group on each of the interfaces; however, this will allow you to receive duplicate multicast datagrams. More than one application may request to join the same group; the port numbers will differentiate the applications.

The possibility exists for each operation to not work. Since an application can join any group, multiple applications or one application may join many groups, which can lead to resource problems. JoinHostGroup may fail due to lack of local resources. A multicast membership may persist even after an application has requested the LeaveHostGroup, due to the fact that other applications may be using that host group. Remember, a group is a range of hosts that can receive and transmit IP datagrams using a specific IP class D address. A group of hosts all use the same (unique) class D address to indicate the group.