I'm one of Nat's co-workers, also looking at this problem.
RFC 2236, the IGMPv2 spec, states:
When a host joins a multicast group, it should immediately transmit
an unsolicited Version 2 Membership Report for that group, in case it
is the first member of that group on the network. To cover the
possibility of the initial Membership Report being lost or damaged,
it is recommended that it be repeated once or twice after short
delays [Unsolicited Report Interval].
the IGMP join. In fact, a quick peek at the source confirms this:
#define IGMP_Initial_Report_Delay (1*HZ)
/* IGMP_Initial_Report_Delay is not from IGMP specs!
* IGMP specs require to report membership immediately after
* joining a group, but we delay the first report by a
* small interval. It seems more natural and still does not
* contradict to specs provided this delay is small enough.
But this "small interval" is actually very noticeable in our application.
I think we'll take it out of our version, and I believe it should be
removed from the standard kernel.
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