Asymmetrical bandwidth

In preparation for a different project, I was experimenting with network
bandwidth tests on a few computers, and I'd like to know why I get such
asymmetrical numbers in iperf's dual mode.

Here's scoop:

Testing between a Windows XP box and linux box (ubuntu 6.06), with 100
Mbit cards and switch.

iperf -c 192.168.0.155 -r (tests sending first, then receiving) gives me
consistent results, ranging from 86-92 Mbits/sec each way. Same results
regardless of whether linux is iperf server or client.

But when I run iperf -c 192.168.0.155 -d which tests dual mode
(simultaneously both directions), I get about 90 Mbits/sec leaving the
linux box and 20 Mbits/s coming in, and I get exactly the same results
if I reverse the tests (first time XP was iperf server and linux
connects to it, second time linux is iperf server and XP connects to it).

So regardless of which machine I test from, I get about 90 Mb/s in
either direction.
But in dual mode, the data leaving the linux box is 90 Mb/s, and coming
in is 20 Mb/s.

Who can help me make sense of this?

Thanks,

      Les...
Received on Sun Jun 1 16:34:12 2008