Internet Group Management Protocol

Internet Group Management Protocol (IGMP) is the protocol used to support multicasting. To use multicasting, a process on a host must be able to join and leave a group. A process is a user program that is using the network. Group access is identified by the group address and the interface (NIC). A host must keep track of the groups that at least one process belongs to and the number of processes that belong to the group. IGMP is defined in RFC 1112.

IGMP messages are used by multicast routers to track group memberships on each of its networks. It uses these rules:

1. The first time a process on a host joins a multicast group, the host will send an IGMP report. This means that every time the host needs to receive messages from a new group to support its processes, it will send a report.
2. Multicast routers will send IGMP queries regularly to determine whether any hosts are running processes that belong to any groups. The group address of the query is set to 0, the TTL field is set to 1, and the destination IP address is 224.0.0.1 which is the all hosts group address which address all the multicast capable routers and hosts on a network.
3. A host sends one IGMP response for each group that contains one or more processes. The router expects one response from each host for each group that one or more of its processes require access to.
4. A host does not send a report when its last process leaves a group (when the group access is no longer required by a process). The multicast router relies on query responses to update this information.

IGMP is defined in RFC 1112. Hosts and routers use IGMP to support multicasting. Multicast routers must know which hosts belong to what group at any given point of time. The IGMP message is 8 bytes. consisting of:

1. Bits 0 to 3 - IGMP version number
2. Bits 4 to 7 - IGMP type. 1=query sent by a multicast router. 2 is a response sent by a host.
3. Bits 8 to 15 - unused
4. Bits 16 to 31 - Checksum
5. The last 4 bytes - 32 bit group address which is the same as the class D IP address.

IGMP message formats are encapsulated in an IP datagram which contain a time to live (TTL) field. The default is to set the TTL field to 1 which means the datagram will not leave its subnetwork. an application can increase its TTL field in a message to locate a server distance in terms of hops.

Addresses from 224.0.0.0 to 224.0.0.255 are not forwarded by multicast routers since these addresses are intended for applications that do not need to communicate with other networks. Therefore these addresses can be used for group multicasting on private networks with no concern for addresses being used for multicasting on other networks.