Internet Group Management Protocol. IGMP is one of the protocols used to make IP Multicast work. It is used between a multicast configured router and a device (host) that wants to receive a multicast stream. IGMP has a TTL (Time To Live) of 1, so it will only get as far as the nearest router.
With IGMP, the router will advertise IGMP queries (really announcements), which list the multicast sessions available. The device will advertise which sessions it wants to join. From these join reports the router decides to which outputs to send the multicast traffic.
IGMPv2 is defined in RFC 2236
IGMPv3 which support Source Specific Multicasting is defied in RFC 3376
See IGMP Snoop, Multicast, PIM
