Path: utzoo!attcan!uunet!ginosko!gem.mps.ohio-state.edu!usc!apple!agate!ucbvax!cs.UMD.EDU!chris
From: chris@cs.UMD.EDU (Chris Torek)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Checksum Byte Order...What is it?
Message-ID: <8910012110.AA26188@gyre.UMD.EDU>
Date: 1 Oct 89 21:10:58 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 14

Barry Margolin notes that
>There's a rigorous proof (which I don't know) that this always produces
>the right result.  I think the end-around carry is the part of the
>checksum algorithm that does it ...

It is.  In fact, the whole thing becomes obvious (hence not needing
anything other than rigorous handwaving :-) ) if you view the bits
as being wrapped around a 16-segment cylinder.  When carries out of
slot 15 are added back to slot 0, it becomes clear that all the bits
are treated identically, and the whole thing is isomorphic under
arbitrary rotations (as long as the input and output have the same
rotation, that is).

Chris