Path: utzoo!attcan!uunet!ncrlnk!ncrcae!hubcap!gatech!rutgers!tut.cis.ohio-state.edu!ucbvax!UM.CC.UMICH.EDU!Dave_Katz
From: Dave_Katz@UM.CC.UMICH.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: TCP-IP zero checksum (with segue question)
Message-ID: <4109706@um.cc.umich.edu>
Date: 10 Mar 89 14:09:18 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 14

> The reason for jamming in 0xFFFF into the checksum field when the computed sum
> is zero is to allow the receiving machine to be able to distinguish between
> an incoming TCP packet that has a checksum to verify against a packet that had
> not computed the checksum before putting it out on the net.
 
The spec calls for the "ones complement of the ones complement sum."  Ones
complement arithmetic never results in -0 (0xFFFF) as a result, so the
ones complement of that never results in 0.  If you can really do ones
complement arithmetic (don't throw away those PDP 11s!), there's no
"jamming" involved;  it just comes out that way.  This is handy for
detecting a zero checksum (if the semantics of that are meaningful),
but it's also handy because you can verify the checksum of a received
packet by running the algorithm with the received checksum in place;
a result of zero means that the checksum was valid.