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

Chapter 255
Extensions to the IP Service Interface

The normal IP transmission logic is as simple as:

If the IP destination is on the same local network, send the datagram locally directly to the destination. If not, send the datagram locally to a router.

Since multicasting is nothing more than an extension of the IP protocol, the logic is simply expanded:

If the IP destination is on the same local network or if it is a host group, send the datagram directly to the destination. If neither, send the datagram locally to a router.

Notice that the multicast host does not specifically look for a router, even though members of the host group may be multiple hops away. Multicast datagrams are not addressed to a router, but multicast datagrams can be reached through an internet—they do not have to remain local. Multicast datagrams that span subnets require routers and these routers must be running a special multicasting protocol (a few of which will be explained next. When a host transmits a multicast packet, it simply transmits the packet out its interface using the normal IP datagram transmission (shown). In this way, all the hosts that belong to the same group on the local network receive and process this datagram. If the TTL field (known as the scope ) is greater than 1, the multicast routers receive and forward this packet out their interfaces towards all other networks that belong to that group. (How the router determines which interfaces belong to that group is discussed in the section, “DVMRP.”) Therefore, the router is also a member of the host group. The receiving router decrements the TTL and forwards the packet as a local multicast on its networks that are participating in that group.

In multicasting, a router is considered part of the group as well as individual hosts.