Ethereal-users: Re: [Ethereal-users] TCP Checksum Incorrect

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From: Guy Harris <guy@xxxxxxxxxx>
Date: Tue, 25 Feb 2003 14:32:41 -0800
On Wed, Feb 26, 2003 at 09:19:30AM +1100, Brett Wheeler wrote:
> Running Ethereal 0.9.3 on Win 2k with WinPcap 2.3 over a Netware 6.0
> Network.
> 
> I have been noticing lately there have been quite a few (appx 30%) TCP
> packets showing an incorrect checksum.

Are they all packets being sent by the machine running Ethereal?

Does your network interface support TCP checksum offloading?  (Windows
2000 can support checksum offloading on network interfaces.)

If both are the case, this is probably normal - for outgoing packets
sent on an interface that does checksum offloading, the TCP code in the
OS will *not* compute a checksum for the packet, and will *not* set the
checksum in the packet, before it's sent to the network interface, which
means it will also not set it before it's handed to the WinPcap driver.