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

Chapter 254
Introduction to IP Multicast

According to RFC 1112, “Host Extensions for IP Multicasting,” the following is the description of IP multicasting:

IP multicasting is the transmission of an IP datagram to a “host group,” a set of zero or more hosts identified by a single IP destination address. A multicast datagram is delivered to all members of its destination host group with the same “best-effort” reliability as regular unicast IP datagrams; i.e., the datagram is not guaranteed to arrive intact at all members of the destination group or in the same order relative to other datagrams.

The membership of a host group is dynamic; that is, hosts may join and leave groups at any time. There is no restriction on the location or number of members in a host group. A host may be a member of more than one group at a time. A host need not be a member of a group to send datagrams to it.

As you learn more about IP multicasting, you will realize, that this protocol is simply an extension of the IP protocol itself. It does not replace the IP protocol and, in fact, it adds a few functions to the IP protocol to allow a host to send and receive multicast datagrams. This is similar to the way ICMP works with IP.

There are three levels associated with multicast:

Level 0: No support for multicast.

Level 1: The ability to send multicast but not receive.

Level 2: The ability to both send and receive multicast packets.