Internet Core Protocols: The Definitive Guide

Chapter 4. Multicasting and the Internet Group Management Protocol

Summary	IP multicasting allows IP datagrams to be sent to explicit hosts on multiple networks simultaneously. This is different from traditional broadcasting in that not all hosts have to process the data, and hosts on remote networks can also receive the datagrams. The Internet Group Management Protocol provides a mechanism for IP devices to register with multicast routers, indicating the multicast groups they are interested in participating in.
Protocol ID	2
Relevant STDs	2 (http://www.iana.org/); 3 (includes RFCs 1122 and 1123); 5 (includes RFCs 791, 792, 919, 922, 950 and 1112)
Relevant RFCs	1112 (IP Multicasting and IGMPv1); 1122 (Host Network Requirements); 1469 (Token Ring Multicasting); 2236 (IMGP v2)
Related RFCs	1075 (Distance Vector Multicast Routing Protocol); 1256 (Router Discovery); 1584 (Multicast Extensions to OSPF); 2365 (Administratively Scoped IP Multicast); 2588 (IP Multicast and Firewalls)

When a system needs to communicate with other hosts on the local network, it typically does so either by broadcasting a packet to every host on the local network or by sending unicast datagrams to a specific destination.

Another alternative that can be used is “multicasting,” a technique that lets a system send data to a group address. Any hosts that are monitoring that group address will receive the data, while every other device on the network will simply ignore it.

This model is particularly useful for applications ...