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The
(IGMP) runs between hosts and their immediately-neighboring multicast routers.
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The
mechanisms of the protocol allow a host to inform its local router that it
wishes to receive transmissions addressed to a specific multicast group. Also,
routers periodically query the LAN to determine if any group members are still
active.
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If
there is more than one IP multicast router on the LAN, one of the routers is
elected "querier" and assumes the responsibility of querying the LAN
for the presence of any group members.
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Based
on the group membership information learned from the IGMP, a router is able to determine
which (if any) multicast traffic needs to be forwarded to each of its
"leaf" subnetworks.
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"Leaf"
subnetworks are those that have no further downstream routers; they either contain
receivers for some set of groups, or they do not.
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Multicast
routers use the information derived from IGMP, along with a multicast routing protocol,
to support IP multicasting across the MBone.
Figure: IGMP
message format
Version: This field
identifies the version number
Type: This field
identifies the message type. There are two message types, type1 indicates a query
message sent by router and type 2 indicates a report sent by host
Unused: This field must
be set to zero.
Checksum: This field
contains checksum for all eight bytes of IGMP message
Group Address: This address is
the class D IPV4 address. This field is set to zero in a query message and is
set to a valid group address in the response.
Multicast
routers periodically transmit Host Membership Query messages to determine which
host groups have members on their directly-attached networks.
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