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Re: node types etc.



On Tue, 16 Jan 2001, Philipp Rumpf wrote:
> > bjorn.wesen@xxxxxxx.com said:
> > >  How can something be cheaper than a single addition ? :) 
> > 
> > TCP checksums aren't just a simple addition. You have to fold back into 16 
> 
> Yes.  They're a simple addition of one's-complement 16 bit units (at least
> in the actual loop).

Which, it turns out, is fortunately implementable using the normal 2's
complement add with carry. 

The folding into 16-bits is just an extra addition done at the end of the
checksum, nothing expensive compared to the number of other additions
done.

> > bits. I'm told that CRC can be cheaper on 32-bit machines. A straight 
> > 32-bit checksum, on the other hand, can quite possibly be cheaper than 
> > either.

The 32-bit TCP checksum IS a "straight 32-bit checksum", with an
additional add at the end of the block. It's only difficult to implement
if the machine in question lack the ability to do carry additions.

-BW


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