Ethereal-users: Re: [Ethereal-users] confusion about TCP checksum calculation

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From: "Ronnie Sahlberg" <sahlberg@xxxxxxxxxxxxxxxx>
Date: Wed, 2 Jan 2002 09:03:59 +1100
Hi,

>I can't figure out why changing the link layer headers causes the
>TCP checksum to change? Or is there a bug in the calculation
>somewhere?

TCP cheacksums are based on both the TCP layer as well as a IP pseudo
header.
The IP pseudo header contains some fileds as IP address and such.
Please see RFC793

If you change some fields in the IP layer, then the TCP checksum will also
be modified.