Date: Wed, 06 Jun 2001 10:19:15 -0400 From: "Louis A. Mamakos" <louie@TransSys.COM> To: "Daniel C. Sobral" <dcs@newsguy.com> Cc: Giorgos Keramidas <keramidi@otenet.gr>, Bob Willcox <bob@immure.com>, Jesper Skriver <jesper@skriver.dk>, hackers list <freebsd-hackers@FreeBSD.ORG> Subject: Re: How to disable software TCP checksumming? Message-ID: <200106061419.f56EJFn82513@whizzo.transsys.com> In-Reply-To: Your message of "Wed, 06 Jun 2001 06:58:07 -0300." <3B1DFEAF.927E1DC3@newsguy.com> References: <20010529144114.I19771@luke.immure.com> <20010529221107.C49875@skriver.dk> <20010529155212.M19771@luke.immure.com> <20010530045200.A1031@hades.hell.gr> <3B1CACDA.96599BD5@newsguy.com> <200106051422.f55EM4n78505@whizzo.transsys.com> <3B1DFEAF.927E1DC3@newsguy.com>
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> "Louis A. Mamakos" wrote: > > > > > > > > It seems to me to be kind of moot to check the same value twice, unless > > > you suspect hardware problems. Aren't you talking about two different > > > checks over the same data instead of checksum off-loading? > > > > Suspect hardware problem? Of course you should! That's why memory > > systems have parity or ECC, and I/O buses are similarlly protected. At > > least on real computers. > > > > The link-level CRC only protects the data as it goes over the link > ^^^^^^^^^^ > > After reading the rest of his messages, I'm not so sure, but I would > think he was talking about _transport_ level check sum, and verifying > that with hardward (NIC) instead of software (IP stack). If I'm not mistaken the message that started this thread inquired about adding an option to prevent TCP from doing a checksum, since the fancy gigabit hardware performed reliable link-level error detection itself. I argue that since TCP is an end-to-end transport protocol that individual error detection on a per-hop basis is not sufficient either theoretically or practically. My last message illustrated a number of cases where the transport of the packet over a link was reliably done, but the contents of the packet were corrupted by malfunctioning software or hardware which the end-to-end TCP checksum detected. I have less of an issue with the endpoints of the TCP connection offloading checksum computation to the NIC card, though you're still exposed to a certain class of error, like the PR I referenced. The problem is what happend to your data in intermediate network elements (routers, etc.) between the endpoints of the TCP connection. Louis Mamakos To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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