Date: Sat, 9 Jun 2001 17:27:03 EDT From: Bsdguru@aol.com To: tlambert2@mindspring.com Cc: freebsd-hackers@freebsd.org Subject: Re: How to disable software TCP checksumming? Message-ID: <99.16194e84.2853eea7@aol.com>
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In a message dated 06/09/2001 1:21:45 PM Eastern Daylight Time, tlambert2@mindspring.com writes: > Bsdguru@aol.com wrote: > > > > This thread is baffling. The bottom line is that you cant > > trust data coming into your machine, and you have to > > checksum it. The link level check only verifies that what > > was sent by the last forwarding point is the same as what > > you got, but in NO WAY implies that all of the data is > > valid. A link level checksum pass is a necessary, but not > > a sufficient condition for the data being acceptable. There > > are scads of reasons that it could be bad. Disabling > > checksumming is a kludge that you may chose to do, but its > > never the right thing to do. > > Tell me: at what point are you willing to trust the data? > > Is "the card plus the motherboard" my machine, or is it > just "the motherboard"? > > The cards support offloading the checksum to hardware; > you are saying that it is never reasonable to do this. > > Fine. That basically means that we need to checksum the > contents of memory once they have been loaded into the > L2 cache, to make sure they arrived safely from memory. > > Wait... now we need to checksum them in L1 cache, once > they have arrived from L2. > no, if your system is bad, you are sunk in general. This discussion was transforming into a "the link level check is good enough" discussion, and that what I was commenting on. Of course if the hardware does the IP and tcp checksumming its adaquate, although unreported DMA errors are not that uncommon, so it is safer to do the checks AFTER the data has been copied to memory. It may be a mistake to assume that all bus transfers are 100% reliable. Bryan To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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