RE: IGMP join/leave time variability

From: Torrey Hoffman (
Date: Thu Jul 26 2001 - 12:47:05 EST

Alan Cox wrote:
> Read the IGMP RFC documents they discuss in detail the cases
> where time delays and randomness are needed and important.

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].

>From this, I infer that there should be _no_ initial delay on sending
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.


Torrey Hoffman
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This archive was generated by hypermail 2b29 : Tue Jul 31 2001 - 21:00:28 EST