The format of an IGMP packet is shown below: |
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IGMP Message Format |
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Octets |
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0 | 8 | 16 | 31 |
Type |
| Response Time | Checksum |
Group Address (all zeros if this is a query)
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| Figure 6- 15. IGMP Message Format |
The IGMP Type codes are shown below: |
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| 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) |
Table 6- 1. IGMP Type Codes
IGMP packets enable multicast routers to keep track of the membership of multicast groups, on their respective subnetworks.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
The
IGMP version 2 introduces some enhancements such as a method to elect a multicast querier 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:
IGMP State
Transitions
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| Member | ||||
Leave Group |
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| Join Group | ||
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Stop Timer |
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| Start Timer | ||
Count |
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Becomes | Delaying |
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Zero | |||||
Member | |||||
Leave | |||||
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Group |
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Query Received |
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| Timer Expires | ||
Start Timer |
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| Send Response | ||
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Member
Figure 6- 16. IGMP State Transitions
Another Host
Responds
Stop Timer
Allied Telesyn | 39 |
