The Internet Group Management Protocol (IGMP) is used to facilitate the simultaneous transmission of a message to a group of recipients. IGMP helps multicast routers to maintain a list of multicast addresses of groups. ‘Multicasting’ means sending of the same message to more than one receiver simultaneously.
When the router receives a message with a destination address that matches one on the list, it forwards the message, converting the IP multicast address to a physical multicast address. To participate in IP on a local network, the host must inform local multicast routers.
The local routers contact other multicast routers, passing on the membership information and establishing route. IGMP has only two types of messages: report and query. The report message is sent from the host to the router. The query message is sent from the router to the host.
A router sends in an IGMP query to determine if a host wishes to continue membership in a group. The query message is multicast using the multicast address 244.0.0.1. The report message is multicast using a destination address equal to the multicast address being reported.
IP addresses that start with 1110(2) are multicast addresses. Multicast addresses are class D addresses. The IGMP message is encapsulated in an IP datagram with the protocol value of two. When the message is encapsulated in the IP datagram, the value of TTL must be one.
This is required because the domain of IGMP is the LAN. The multicast backbone (MBONE) is a set of routers on the Internet that supports multicasting. MBONE is based on the multicasting capability of IP. Today MBONE uses the services of UDP at the transport layer.