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From: david@dcl-cs.UUCP
Newsgroups: comp.dcom.lans
Subject: Internet Protocol header checksum implementation problem
Message-ID: <328@dcl-csvax.comp.lancs.ac.uk>
Date: Tue, 7-Apr-87 09:16:50 EST
Article-I.D.: dcl-csva.328
Posted: Tue Apr  7 09:16:50 1987
Date-Received: Sat, 11-Apr-87 06:01:14 EST
Reply-To: david@comp.lancs.ac.uk (David Coffield)
Distribution: world
Organization: Department of Computing at Lancaster University, UK.
Lines: 31


I'm posting this on behalf of a project student so if anyone
has any thoughts on it, could they channel them through me. Thanks.
--

INTERNET HEADER CHECKSUM

I am having a no doubt trivial problem calculating the header checksum
within an internet packet and would be grateful if someone could enlighten
me as to my mistake.

To calculate the checksum I take each 16-bits within the header  
( starting with the version/IHL octet and ending with the octet immediately
preceding the data portion of the Internet packet).  To each of these
I take the 16-bit one's complement and add them all up giving an 
unsigned 16-bit value.  This is then one's complemented to give me 
the header checksum which is placed in the internet header checksum field
(as per the IP spec).

When calculating the checksum the checksum field is taken as containing zero.

I have applied this procedure to packets taken directly from the network,
the final checksum is nothing like what it should be, however, the value
before taking the final one's complement is sometimes 7 sometimes 8 lower
than what is in the checksum field of the packet read.

Any clues? Thanks.
-- 
janet:  david@uk.ac.lancs.comp           Department of Computing
arpa:   david@comp.lancs.ac.uk           University of Lancaster
uucp:   ...!mcvax!ukc!dcl-cs!david       Bailrigg, Lancaster, LA1 4YR, UK