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Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: comp.dcom.lans
Subject: Re: Internet Protocol header checksum implementation problem
Message-ID: <7899@utzoo.UUCP>
Date: Sat, 11-Apr-87 22:07:13 EST
Article-I.D.: utzoo.7899
Posted: Sat Apr 11 22:07:13 1987
Date-Received: Sat, 11-Apr-87 22:07:13 EST
References: <328@dcl-csvax.comp.lancs.ac.uk>
Organization: U of Toronto Zoology
Lines: 21

> To calculate the checksum I take each 16-bits within the header  ...
> 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 ... (as per the IP spec).

The problem is that you have misunderstooding the meaning of the term
"one's-complement addition".  This does not mean adding the complements
of the numbers.  One's-complement addition is addition done by treating
the 16-bit lumps as numbers in the obsolete one's-complement number
representation, rather than the two's-complement representation that
most modern computers use.  See any good book on digital systems design
for the details of what one's-complement representation is all about.
The simple way to fake one's-complement addition on a two's-complement
machine is to do a regular add and then add the carry to the result; on a
32-bit machine it's often better to do a 32-bit sum of all the 16-bit
values and then add the top 16 bits to the bottom 16 (repeating this
until the top 16 are zero).
-- 
"We must choose: the stars or	Henry Spencer @ U of Toronto Zoology
the dust.  Which shall it be?"	{allegra,ihnp4,decvax,pyramid}!utzoo!henry