Like ICMP, IGMP is a service protocol for IP. IGMP packets are wrapped into IP datagrams. It is used for forwarding multicasts. Currently, version 2 of the IGMP protocol is up to date with RFC 2236 norms.
The following figure shows the structure of an IGMP version 2 packet:
Figure 5.30: IGMP v2 packet
The Type field acquires the values shown in the following table:
|
Value (hexadecimal) |
Significance |
|---|---|
|
11 |
IP Membership query ("Are there any members on the LAN?") |
|
12 |
IGMPv1 Membership Report |
|
16 |
IGMPv2 Membership Report |
|
17 |
IGMPv2 Leave Group |
The MRT (Maximum Response Time) field is only used in router requests and specifies (in tenths of a second) the time that members of the group have to repeat their requests for membership in the group. In all other cases, the MTR field has a value of 0.
The checksum is calculated in the same way as with the ICMP protocol. The IP group address field is zero for a general request, and in all other cases...