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From: don.provan@A.CS.CMU.EDU
Newsgroups: mod.protocols.tcp-ip
Subject: Re: IP, ICMP, TCP, and UDP checksumming.
Message-ID: <23Oct85.223317.DP0N@A.CS.CMU.EDU>
Date: Wed, 23-Oct-85 22:33:00 EDT
Article-I.D.: A.23Oct85.223317.DP0N
Posted: Wed Oct 23 22:33:00 1985
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if you checksum the entire message (or header in IP), including the
checksum field, you'll always get -0 regardless of what the sending
entity put in the checksum field (assuming the packet isn't entirely
zero, which isn't legal in IP and TCP and, i think, ICMP and UDP).
if everyone used this approach (which, i believe, works just as well
if the machine does your ones complement addition, although you'd
check for +0 on a machine that normalized), we shouldn't have to
worry about which one is "right".  of course, when they change the
checksum we'll all be up a creek...