Internet Group Management Protocol (IGMP)

IGMP is the extension to the ICMP and it occupies the same place as the ICMP in the IP protocol stack.

The IP data field will contain the 8-byte IGMP message in the format .

Systems participating in IGMP fall into two types: hosts and multicast routers.

As described in Multicasting, in order to receive multicast datagrams, a host must join a host group. When a host is multi-homed, it may join any group on one or more of its interfaces.

The multicast messages that the host receives from the same group on two different subnets may be different.

To join a group, the host sends a report on an interface. The report is addressed to the multicast group of interest.

Multicast routers on the same subnet receive the report and set a flag to indicate that at least one host on that subnet is a member of that group.

Multicast routers regularly, but infrequently , send out a query to the all hosts multicast address.

Each host which still wishes to be a member of one or more groups replies once for each group of interest.

Each reply is sent after a random delay to ensure that IGMP does not cause bursts of traffic on the subnet.