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IGMP
Internet Group Management Protocol (IGMP) is used to dynamically
register individual hosts in a multicast group on a particular LAN. Hosts
identify group memberships by sending IGMP messages to their local
multicast router. Under IGMP, routers listen to IGMP messages and peri-
odically send out queries to discover which groups are active or inactive
on a particular subnet.
RFC 2236 defines the specification for IGMP Version 2. There are four
types of IGMP messages:
• Membership query
• IGMP Version 1 membership report
• IGMP Version 2 membership report
• Leave group
Hosts send out IGMP membership reports corresponding to a particular
multicast group to indicate that they are interested in joining that group.
The router periodically sends out an IGMP membership query to verify
that at least one host on the subnet is still interested in receiving traffic
directed to that group. When there is no reply to three consecutive IGMP
membership queries, the router times out the group and stops forwarding
traffic directed toward that group.
With leave group message, the hosts can actively communicate to the
local multicast router their intention to leave the group. The router then
sends out a group-specific query and determines whether there are any
remaining hosts interested in receiving the traffic. If there are no replies,
the router times out the group and stops forwarding the traffic.