Date: Wed, 15 Nov 2000 08:15:52 -0700 (MST) From: Charles Mott <cmott@scientech.com> To: "Louis A. Mamakos" <louie@TransSys.COM> Cc: Ruslan Ermilov <ru@FreeBSD.ORG>, Archie Cobbs <archie@dellroad.org>, net@FreeBSD.ORG, Ari Suutari <ari@suutari.iki.fi> Subject: Re: libalias: Incremental Update of Internet Checksum Message-ID: <Pine.BSF.4.21.0011150810010.59290-100000@carcassonne.scientech.com> In-Reply-To: <200011151436.eAFEaHG65417@whizzo.transsys.com>
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On Wed, 15 Nov 2000, Louis A. Mamakos wrote: > But the checksum is supposed to be the one's complement of the checksum > of the payload (which is computed using one's complement arithmetic). If > you compute a checksum, and the value is zero, you insert the complemented > value (0xffff) into the packet. > > louie A 1's complement sum of the payload can never be 0x0000 if any components the payload are non-zero. Therefore a checksum of 0xffff is not possible, since at least the word containing the IP protocol byte is non zero. This is explained in RFC 1624. I think that Ruslan has done his homework on this subject, and there is really no point to dispute the matter further. Charles Mott To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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