Date: Wed, 15 Nov 2000 09:36:17 -0500 From: "Louis A. Mamakos" <louie@TransSys.COM> 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> Subject: Re: libalias: Incremental Update of Internet Checksum Message-ID: <200011151436.eAFEaHG65417@whizzo.transsys.com> In-Reply-To: Your message of "Wed, 15 Nov 2000 10:04:07 %2B0200." <20001115100407.D36400@sunbay.com> References: <Pine.BSF.4.21.0011130015100.50906-100000@carcassonne.scientech.com> <200011150315.eAF3FIG60231@whizzo.transsys.com> <20001115100407.D36400@sunbay.com>
next in thread | previous in thread | raw e-mail | index | archive | help
> On Tue, Nov 14, 2000 at 10:15:18PM -0500, Louis A. Mamakos wrote: > > > > Arithmetically, a value of 0xffff is identical to 0x0000 since they > > both represent the value of zero when using a one's complement binary > > representation of values. > > > Except you can't actually receive the value of 0xffff in a checksum field. > One's complement sum is guaranteed to be non-zero except if all items are > zeroes. Since IP Protocol field is a non-zero value that participates in > all checksums (IP, TCP, UDP, ICMP), the checksum value (which is a one's > complement of a one's complement sum) is guaranteed to be non-0xffff. For other than UDP, it really doesn't make a difference if the value of the checksum field is 0x0000 (+0) or 0xffff (-0); they both have the (1's complement arithmetic) value of zero. > > It turns that this property is used in some network protocols (e.g., UDP) > > to distinguish between a checksum value that's computed as zero (represented > > as 0xffff in the packet) from a packet which has no computed checksum at all. > > This was done in the dark ages when it was deemed "too expensive" to > > compute a checksum. > > > >From the above it follows that UDP should better be using 0xffff rather than > 0x0 to indicate that a packet has no computed checksum. It is quite possible > that the computed checksum will have a zero value, in which case the receiving > UDP module will consider such a packet as with no computed checksum, which is > wrong. 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 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200011151436.eAFEaHG65417>