[CLUE-Tech] Network DD

[Timothy C. Klein]

* Keith Hellman (kehellman at yahoo.com) wrote:
> 
> I'm wondering if your accurately counting the actual bytes going across
> the interface.  Your file may be X bytes big but your byte calculation
> doesn't account for the tcp wrapper, ip wrapper, or link layer envelope.
> 
> I'm not even sure where to get these numbers, but I would suggest doing a 
>   >/sbin/ifconfig ethX
>   >dd if=/dev/random bs=4 count=4 |netcat target.ip.address target-port
>   >/sbin/ifconfig ethX
> 
> And compare the total transferred bytes between the two ifconfig outputs. 
> I think they should be more than 16 bytes...

Thanks, I had forgot about that.  It adds some inaccuracy, becuase I
don't want to eliminate all data transfer between the two, but it is
still interesting.  The other overhead, though, does not seem to be
really significant.  Just treid 4 runs with netcat, I get pretty much
the same result.  Right about 30% utilization of the 100 Mb/s. That is
when I measure the difference in bytes between ifconfig outputs, rather
than the file size.  I realize that I may never get all the way to 100%,
but less than 1/3 is bothering me.  I must find out why.  A new
project...

> Also keep in mind, IIRC, netcat is doing VERY dumb byte transfers: give it
> a byte, it sends a byte.  The SSH tools, on the other hand, may (and this
> is pure speculation) be more optimized (especially scp) to moving large
> blocks of data, hence they do it with less protocol overhead, hence
> faster.

Ah, but I want dumb transfers.  I just want to measure how long a known
set of data takes to transfer.  I don't want compression, tons of tcp
overhead, etc.

> Someone knowledgeable about this stuff should comment, I'm really just
> thinking out loud...
>  

Gonna have to test some more.  These machines are separted by 25' of
Cat5 wire that may not be up to 100Mb/s spec, so I am going to try it
with a short piece of Cat 5e wire, just to rule out a lame line.

I have always had the feeling that the stock Linux network drivers seem
to behave *very* conservatively 'out of the box', and I am going to find
out for sure!

Tim
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== Timothy Klein || teece at silverklein.net   ==
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== "Hello, World" 17 Errors, 31 Warnings... ==
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