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The format of an IGMP packet is shown below:
IGMP Message Format
Octets
0
8
Type
The IGMP Type codes are shown below:
Type
Meaning
Membership Query (if Group Address is 0.0.0.0)
0x11
Specific Group Membership Query (if Group Address is Present)
0x11
Membership Report (version 2)
0x16
Leave a Group (version 2)
0x17
Membership Report (version 1)
0x12
IGMP packets enable multicast routers to keep track of the membership of multicast groups, on their respective sub networks.
The following outlines what is communicated between a multicast router and a multicast group member using IGMP.
A host sends an IGMP "report" to join a group
A host will never send a report when it wants to leave a group (for version 1).
A host will send a "leave" report when it wants to leave a group (for version 2).
Multicast routers send IGMP queries (to the all-hosts group address: 224.0.0.1) periodically to see whether any group members
exist on their sub networks. If there is no response from a particular group, the router assumes that there are no group members
on the network.
The Time-to-Live (TTL) field of query messages is set to 1 so that the queries will not be forwarded to other sub networks.
IGMP version 2 introduces some enhancements such as a method to elect a multicast queried for each LAN, an explicit leave
message, and query messages that are specific to a given group.
The states a computer will go through to join or to leave a multicast group are shown below:
16
Response Time
Group Address (all zeros if this is a query)
Checksum
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User's Manual of GSW-4804SF
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