From owner-freebsd-net  Wed Nov 15  7:48:22 2000
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To: Ruslan Ermilov <ru@FreeBSD.ORG>
Cc: Charles Mott <cmott@scientech.com>,
	Archie Cobbs <archie@dellroad.org>, net@FreeBSD.ORG,
	Ari Suutari <ari@suutari.iki.fi>
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From: "Louis A. Mamakos" <louie@TransSys.COM>
Subject: Re: libalias: Incremental Update of Internet Checksum 
References: <Pine.BSF.4.21.0011130015100.50906-100000@carcassonne.scientech.com> <200011150315.eAF3FIG60231@whizzo.transsys.com> <20001115100407.D36400@sunbay.com> <200011151436.eAFEaHG65417@whizzo.transsys.com> <20001115172435.A9724@sunbay.com> 
In-reply-to: Your message of "Wed, 15 Nov 2000 17:24:35 +0200."
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> If I compute the sum on IP PACKET, then sum can't be zero, so checksum can
> never be a 0xffff.  This is all explained in RFC 1624, BTW :-)

I can see you you would think this, given the errornous statement
from the RFC:

   In one's complement, there are two representations of zero: the all
   zero and the all one bit values, often referred to as +0 and -0.
   One's complement addition of non-zero inputs can produce -0 as a
   result, but never +0.  Since there is guaranteed to be at least one
   non-zero field in the IP header, and the checksum field in the
   protocol header is the complement of the sum, the checksum field can
   never contain ~(+0), which is -0 (0xFFFF).  It can, however, contain
   ~(-0), which is +0 (0x0000).

However, having actually used 1's complement-based ALU's, I can assure
you that one's complement addition of non-zero inputs certainlly can
produce +0 as the result.  I have the core plane from that machine in
my office as proof :-)

louie


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