Date: Fri, 8 Jun 2001 11:40:52 -0500 (CDT) From: Jonathan Lemon <jlemon@flugsvamp.com> To: Bsdguru@aol.com, hackers@freebsd.org Subject: Re: How to disable software TCP checksumming? Message-ID: <200106081640.f58Geqr63121@prism.flugsvamp.com> In-Reply-To: <local.mail.freebsd-hackers/84.1717a060.28525287@aol.com>
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In article <local.mail.freebsd-hackers/84.1717a060.28525287@aol.com> you write:
>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.
I think there's some confusion here. There are two types of checksums:
1. CRC link-level checksum on the ethernet packet.
2. Upper layer checksums (TCP/IP)
All hardware that I'm aware of automatically performs #1, I'm not sure
if there is anything that will permit disabling this.
Step 2 is usually known as the "end-to-end" checksum, and is traditionally
performed by the host CPU. Depending on your view of what constitutes
an "endpoint", these checksums may be performed by the network adapter
itself. I believe that this is what the original meant by disabling
software checksums, in the sense that his hardware would do this work.
Dispensing with TCP/IP checksum altogether is a fairly bad idea, but
I don't think that is what was being proposed here.
--
Jonathan
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