From mboxrd@z Thu Jan  1 00:00:00 1970
From: rigoberto perez <rigo666beast@yahoo.com>
Subject: Why the tcp ckecsum involves the skuff checksum?
Date: Thu, 11 Dec 2008 08:19:04 -0800 (PST)
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To: netfilter <netfilter@vger.kernel.org>

Friends:

In Linux implementation of the TCP checksum the functions using the checksum of the skbuff. Theoretically the TCP pseudo header only contains the both IP directions, the protocol (6), 1 byte reserved full of zeros and the segment length. So, in what form is used the skbuff checksum in this computation?. 

I suppose that the TCP level is not concern about netfilter modules. It's transparent. I mean, if I change the packet content in out direction and restore it in direction this change should be invisible to the uppers levels - including TCP level.

Thanks