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