From owner-freebsd-net Wed Nov 15 7:17:38 2000 Delivered-To: freebsd-net@freebsd.org Received: from if.scientech.com (mail.rv.scientech.com [198.60.89.9]) by hub.freebsd.org (Postfix) with ESMTP id 2D9C737B657; Wed, 15 Nov 2000 07:17:35 -0800 (PST) Received: from carcassonne (carcassonne.scientech.com [10.10.25.250]) by if.scientech.com (8.9.3/8.9.3) with ESMTP id IAA19007; Wed, 15 Nov 2000 08:15:52 -0700 Date: Wed, 15 Nov 2000 08:15:52 -0700 (MST) From: Charles Mott To: "Louis A. Mamakos" Cc: Ruslan Ermilov , Archie Cobbs , net@FreeBSD.ORG, Ari Suutari Subject: Re: libalias: Incremental Update of Internet Checksum In-Reply-To: <200011151436.eAFEaHG65417@whizzo.transsys.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-freebsd-net@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.org 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