From owner-freebsd-hackers  Wed Jun  6 22:49:29 2001
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From: Bill Fenner <fenner@research.att.com>
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To: louie@transsys.com
Subject: Re: How to disable software TCP checksumming?
Cc: freebsd-hackers@freebsd.org
References:  <20010529144114.I19771@luke.immure.com> <20010529221107.C49875@skriver.dk> <20010529155212.M19771@luke.immure.com> <20010530045200.A1031@hades.hell.gr> <20010529235215.A60177@luke.immure.com> <20010530085155.B24096@Odin.AC.HMC.Edu> <200105310204.f4V248n15260@whizzo.transsys.com>
Date: Wed, 6 Jun 2001 22:49:18 -0700
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>The TCP checksum protects more than just the contents of the packet
>on the wire; it's also a (somewhat) weak check on the contents
>of your packet sitting in memory, and as it's going over the bus
>in your computer between memory and peripherals and for other end-to-end
>sorts of issues. 

In fact, Jonathan Stone at Stanford did some measurements of checksum
errors for his thesis, and found that on a given link in the Stanford
residences there were a surprising (small, but measurable) number of
packets with IP header checksum errors with what appeared to be DMA errors
-- e.g. 4 bytes missing from the middle of the header.  When the end to
end checksum is really memory to memory, it can catch errors like this...

  Bill

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