GE -DSH-73 User Manual page 116

Chapter 5: Web-Based Management
IGMP Versions 1 and 2
Multicast groups allow members to join or leave at any time. IGMP provides the
method for members and multicast routers to communicate when joining or leaving
a multicast group.
IGMP version 1 is defined in RFC 1112. It has a fixed packet size and no optional data.
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
0x11 Membership Query (if Group Address is 0.0.0.0).
0x11 Specific Group Membership Query (if Group Address is Present).
0x16 Membership Report (version 2).
0x17 Leave a Group (version 2).
0x12 Membership Report (version 1).
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.
112
16
Response Time
Group Address (all zeros if this is a query).
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