IGMPv3 carries over the leave group mechanism from IGMPv2,
but with a simplified implementation.
If you recall,
IGMPv2 introduced a new IGMP packet type for the leave group
messages to differentiate them from membership reports.
IGMPv3 recycles the membership message instead.
First,
notice that the source and destination IP addresses are identical between the
leave group message and the join group message above it.
Considering both are technically membership reports, this is expected.
Next, notice the type is still 22 in hex or 34 in decimal.
This is how Wireshark knows that it is still a membership report.
The report contains one group record with some interesting characteristics.
First, notice the mode is Change to Include.
When the membership report was originally sent,
239.1 .1 .1 was marked as an exclude mode group,
effectively saying I want multicast from any source on this group.
Now IGMPv3 is signaling that this group should be changed to include mode.
Notice that the number of sources is 0.
The client is signaling that I want multicast from 0 sources on this group.
This is a crafty way of canceling membership to a
group without a specialized message.
The router, just like in IGMPv2,
needs to ask the remaining clients is anyone still interested in 239.1 .1 .1?
This is accomplished using a group-specific query.
The destination IP address is the specific group being queried,
since such queries cannot be batched similar to membership reports.
The type and max response times are identical to IGMPv2,
which is unsurprising, since the message accomplishes the exact same task.
The embedded data still contains the group being queried against,
which is 239.1 .1 .1.
Again, this is the same as IGMPv2.
The extra details at the end are new to IGMPv3,
but we explored them when discussing the IGMPv3 general query message.
We won't revisit them,
but I do want to draw your attention to the number of sources.
Because this was an ASM group, there are no specific sources being queried.
IGMPv3 is capable of asking more nuanced questions,
like is anyone else interested in this specific group from this specific source? We will see these packets soon.