Asymmetry or others?

[chongz --at-- positioning-research.com]

Hi all,
Sorry I asked this before but I still can't figure it out:

I have two linux (2.4) boxes in two universities connecting 
via an Education WAN link. I got different bandwidth output
if I put client and server on different boxes. One direction
is about 15Mbps, and the other is 30Mbps. I use simple iperf
(1.6.3) options:
  a>$./iperf -s -w 100k
  b>$./iperf -c a -w 100k

Because it's education network, I really don't think it's
asymmtric although I am asking the administrator about this.
(But it has about 8 hops so it's pretty hard to find out all
the configuration along the path...)

I am thinking if there are other possibilities making this
happen? I already setup the same values of the 
sys.core.[r|w]mem_[default|max] and 
sys.net.ipv4.tcp_[r|w|]mem on the two boxes.

Does anyone have other suggestions except the link asymmetric?

Thanks very much for your help,
-Chong