IGMP

IGMP, the Internet Group Management Protocol, has IP protocol number 2.
The message format for v1 (rfc1112) was

• 4 bits version number (0, rfc988 (obsolete); 1, rfc1112)
• 4 bits type (1 = Host Membership Query; 2 = Host Membership Report)
• 8 bits unused, sent zeroed
• 16 bits checksum
• 32 bits Group Address (zeroed for a Host Membership Query)

For v2 (rfc2236) this changed to:

• 8 bits type (0x11 Membership Query , 0x16 v2 Membership Report, 0x17 Leave Group, an additional type, 0x12 v1 Membership Report is for backward compatibility only)
• 8 bits Max Resp Time, only used in Membership Query messages, zeroed otherwise
• 16 bits payload checksum (paload may be over 8 octets but is ignored if type is v2 valid)
• 32 bits Group Address (zeroed for a General Membership Query, set for a specific one)

A Host Membership Query is sent to the all hosts IP group address (224.0.0.1) with TTL 1 by a multicast router on startup and at intervals (not usually less than 60s) thereafter. All hosts in a multicast group respond to a Host Membership Query with a Host Membership Report (or send one when they want to join a multicast group for the first time). For ICMP v2 the IP TTL must be 1 and the IP Router Alert Option (rfc2113) must be set in the IP header. In v2 each multicast router on a single network will assume the role of either "Querier" or "Non-Querier"; the multicast router with the lowqest IP address should be the Querier. When a host leaves a group it must send a Leave Group message to the all-routers group (224.0.0.2) if it was the last host to reply to a Query (it may send one anyway).

www.ansdell.org.uk -> networks -> network management -> igmp.html