Date: Fri, 20 Aug 2004 20:36:08 +0100 From: David Malone <dwmalone@maths.tcd.ie> To: "David W. Hankins" <David_Hankins@isc.org> Cc: current@freebsd.org Subject: Re: on amd64 tcp4 cksums are bad (FYI) Message-ID: <20040820193608.GA94415@walton.maths.tcd.ie> In-Reply-To: <20040820190525.GA21626@isc.org> References: <20040820190525.GA21626@isc.org>
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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.
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