Path: utzoo!attcan!uunet!mcvax!unido!nixpbe!freiss
From: freiss@nixpbe.uucp
Newsgroups: comp.protocols.tcp-ip
Subject: Re: TCP-IP zero checksum
Message-ID: <23800002@nixpbe>
Date: 10 Mar 89 09:32:00 GMT
References: <107@nixvia.UUCP>
Lines: 24
Nf-ID: #R:nixvia.UUCP:-10700:nixpbe:23800002:000:775
Nf-From: nixpbe.uucp!freiss    Mar 10 10:32:00 1989



Mink writes:
>Can anyone tell me why the checksum in the TCP header isn't allowed to
>be zero ? I can't find it documented in any RFC. Also I can't find this
>conversion back in the tcp_input routine

Hmmm... RFC 793 says (page 16):

...While computing the checksum, the checksum field itself is replaced
with zeros.

Maybe a checksum of zero therefore is regarded as 'something broke while
the checksum was computed' and replaced by the one's complement of zero.
This doesn't sound like a very intelligent way of errorchecking though.
  Anybody have a better idea?

-Martin

--
 Martin Freiss
 Nixdorf Computer AG, Dept. ET2
 Pontanusstr. 55                    USA: ..!uunet!linus!nixbur!freiss.pad
 D-4792 Paderborn, FRG             !USA: ..!mcvax!unido!nixpbe!freiss.pad