Ethereal-dev: Re: [Ethereal-dev] new ICE dissector (second try)

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From: Guy Harris <gharris@xxxxxxxxx>
Date: Wed, 06 Oct 2004 10:45:00 -0700
francesco fondelli wrote:


seems fine but I had a try, forcing desegmentation, and I got a
"[Unreassembled Packet (incorrect TCP checksum): ICEP]"
error in the column info. I have no idea where the problem is.
I attach a dump with error.
The problem is probably that you're running Ethereal on an OS that marks loopback interfaces as interfaces that offload TCP checksumming, but that don't actually do any checksumming (because you shouldn't have to worry about network errors for packets that aren't actually transmitted on a network) so as not to waste CPU time doing checksumming on those packeets, and capturing traffic that machine is sending to itself (and therefore going over the loopback interface), so the TCP checksum value is bogus.
This is also a problem with non-loopback interfaces that do checksum offloading, so there's an option for the TCP dissector to turn off the check of the checksum: 

	http://www.ethereal.com/faq.html#q5.14