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The Internet Group Management Protocol (IGMP)
grew out of the Host Membership Protocol, and the first version,
IGMPv1, was defined in RFC 1112. The most recent version, IGMPv2,
was ratified in November 1997 as a standard by the Internet
Engineering Task Force (IETF) and is covered in RFC 2236.
IGMP messages are used primarily by multicast
hosts to signal their local multicast router when they wish to join
a specific multicast group and begin receiving group traffic, as
shown in the Figure. Hosts may also (with IGMPv2) signal to the
local multicast router that they wish to leave an IP multicast group
and are no longer interested in receiving multicast group traffic.
Using information obtained via IGMP, routers
maintain a list of multicast group memberships on a per-interface
basis. A multicast group membership is active on an interface if at
least one host on that interface has signaled its desire, via IGMP,
to receive the multicast group traffic.
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