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

Chapter 260
Protocols

Review RFC 1112. Naturally, there are a few protocols involved in order to make multicasting work. We will start out with the most prevalent protocols and then work our way into others. No matter which router-router protocol is used, one protocol is used with all of them: the Internet Group Management Protocol (IGMP). To support IGMP, a host must join the “all-hosts” group (address 224.0.0.1) on each network interface at initialization time and must remain a member for as long as the host is active. Therefore, there is at least one multicast address that every multicast host should be a member of.

IGMP is the protocol that runs between multicast hosts and their adjacent multicast routers. (Router manufacturers can choose whether to implement multicast or not. Routers that participate in multicast must run a multicast protocol (beyond RIP2, OSPF, etc.) Most major router manufacturers have or are in the process of implementing these protocols.) IGMP is used to keep neighboring multicast routers informed of the host group memberships present on a particular local network. The IGMP header is used for all multicast communication, whether it is between hosts or routers.

In order for an interface to receive a multicast datagram, it must have previously been set up to receive and process multicast datagrams. Since IGMP does not use a transport layer such as TCP or UDP, the IP Protocol field is set to 2 (as reserved by IANA RFC 1700) in order to identify the process (IGMP) using the IP service. Therefore, before any multicast packets are received, the upper-layer software must ensure that IP and the MAC layer interfaces are set up to received multicast datagrams.

A host may be a member of more than one group; in fact, there is no upper limit on the number of groups allowed (except for the upper limit of the IP multicast address). NICs have a very limited capability for receiving multicast packets. In other words, when the user installs the version of IP for multicast, it must also be able to set up the NIC to receive multicast packets as well. Each host group will have a different multicast address and therefore it will be mapped to a multicast MAC address as well (discussed previously). But the NIC card may only be able to hold a finite number of multicast addresses. In this case, check with the manufacturer. Some have implemented the ability to receive all multicast packets. In this way, it will be up to the IP layer software to filter out unwanted packets.