Asymmetric ethernet throughput?

[Mike Meyer]

I was doing some network testing, and noticed something odd: The
network throughput was asymmetric. I was sending between 2 and 5 times
as much data as I was receiving. Swapping the ends of the test didn't
change that. Changing the non-FBSD box for a FBSD box made the
throughput symmetric.

Details:

One end is always a FreeBSD 5.5 box with a RealTek 8138 nic. Testing
is done with netstrain, built from ports.

I first saw this with an OSX 10.4.6 box, a G4 powerbook. The FreeBSD
box was sending ~10MB/sec on a 100BaseT network, which I regard as
excellent. But I was only receiving 3M/sec. I tried swapping the
server and client ends of netstrain, and got the same results: I was
sending 10MB/sec to OSX, and receiving about 3MB/sec from it.

I then tried making the other end a FBSD 6.1-RELEASE box, and the
throughput was symmetric, with both ends sending and receiving
10MB/sec.

I rebooted the 6.1 box into Ubuntu lunix running a 2.6.12
kernel. Again, I got asymmetric results - but this time it was the
other way: I was receving ~10MB/sec, but only sending about 3MB/sec.

Trying this between the Linux box and the OSX box (sorry, I've only
got one of each) gave me symmetric ~10MB/sec throughput.

I'm just curious if there's an explanation for this.

	Thanks,
	<mike
-- 
Mike Meyer <mwm at mired.org>		http://www.mired.org/consulting.html
Independent Network/Unix/Perforce consultant, email for more information.