IGMP Versions 1 and 2
Users that want to receive multicast packets need to be able to join and leave multicast groups. This is accomplished using IGMP.
Figure 5-18. IGMP Message Format
The IGMP Type codes are shown below:
Type | Meaning |
0x11 | Membership Query (if Group Address is 0.0.0.0) |
0x1 | Specific Group Membership Query (if Group Address |
1 | is Present) |
0x1 | Membership Report (version 2) |
6 |
|
0x1 | Leave a Group (version 2) |
7 |
|
0x1 | Membership Report (version 1) |
2 |
|
| Table |
Multicast routers use IGMP to manage multicast group memberships:
•An IGMP “report” is sent by a user’s computer to join a group
•IGMP version 1 does not have an explicit ‘leave’ message. Group members have an expiration timer, and if this timer expires before a query response is returned, the member is dropped from the group.
•IGMP version 2 introduces an explicit “leave” report. When a user wants to leave a group, this report is sent to the multicast router (for IGMP version 2).
•Multicast routers send IGMP queries (to the
The TTL field of query messages is set to 1 so that the queries do not get forwarded to other subnetworks.
IGMP version 2 introduces a few extensions to IGMP version 1 such as, the election of a single multicast querier for each network, explicit ‘leave’ reports, and queries that are specific to a particular multicast group.
The router with the lowest IP address is elected as the querier. The explicit group leave message is added to decrease latency, and routers can ask for membership reports from a particular multicast group ID.
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