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Date: Fri, 20 Aug 2004 20:36:08 +0100
From: David Malone <dwmalone@maths.tcd.ie>
To: "David W. Hankins" <David_Hankins@isc.org>
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cc: current@freebsd.org
Subject: Re: on amd64 tcp4 cksums are bad (FYI)
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On Fri, Aug 20, 2004 at 12:05:25PM -0700, David W. Hankins wrote:
> This is as observed via tcpdump on [client], which is what is producing
> the bad checksums.  Obviously it doesn't cause a problem since no one
> listens to TCP checksums, but it's interesting.

Actually, most things pay attention to TCP checksums. What you are
actually seeing is that the checksum hasn't been calculated at the
time that tcpdump gets to see the packet. This is probably because
your ethernet card supports TCP checksum offloading, and so the
checksum isn't actually calculated until the packet hits the wire
(which is slightly after bpf feeds the packet to tcpdump....)

If you run tcpdump on the server, you should see that the checksum
is OK (though the packets transmitted by the server may seem to
have bad checksums for similar reasons).

	David.