Asymetric network speed question

[David G. Miller]

Over the past couple of weeks I have been upgrading my home network to 
CAT6/gigabit (I now understand why no one likes to pull cable).  Almost 
everything is working as expected with one exception.  iperf reports 
significantly different speeds between my server (mutilate) and a 
workstation (bend) on the network depending on which one is the iperf 
server and which is the client. 

The server is running CentOS 4.3 and the workstation is running FC4.  
Both systems have all current updates applied.  The network cards are 
3com 3c2000-t's and they connect through a 3com OfficeConnect gigabit 
switch.  The switch says the connections are gigabit and ethtool says 
the same on the workstation (ethtool doesn't like the sk98lin module on 
the CentOS box).

If I make the workstation, bend, the iperf server I see pretty much what 
I expect:

[dave at mutilate ~]# iperf -c bend
------------------------------------------------------------
Client connecting to bend, TCP port 5001
TCP window size: 27.5 KByte (default)
------------------------------------------------------------
[  3] local 192.168.255.254 port 33315 connected with 192.168.0.187 port 
5001
[  3]  0.0-10.0 sec    814 MBytes    683 Mbits/sec

However, if I reverse the roles and make bend the client and mutilate 
the server I see:

[dave at bend ~]# iperf -c mutilate
------------------------------------------------------------
Client connecting to mutilate, TCP port 5001
TCP window size: 27.4 KByte (default)
------------------------------------------------------------
[  3] local 192.168.0.187 port 39728 connected with 192.168.255.254 port 
5001
[  3]  0.0-10.0 sec    232 MBytes    194 Mbits/sec

If I go to an old PIII/733 box I put on the network for this testing I see:

[dave at spindle ~]# iperf -c mutilate
------------------------------------------------------------
Client connecting to mutilate, TCP port 5001
TCP window size: 27.5 KByte (default)
------------------------------------------------------------
[  3] local 192.168.0.188 port 33183 connected with 192.168.255.254 port 
5001
[  3]  0.0-10.0 sec    490 MBytes    411 Mbits/sec

somewhat slower but not bad given the old hardware.

Anyone have a reason why iperf says the connection slows down when bend 
is the server but not when bend is the client?  Could a bad cable cause 
this or is it the skge module on bend (spindle and mutilate both run 
CentOS 4.3 with the sk98lin module and iperf says the speed is about the 
same in both directions between them)?  A bad cable seems odd given the 
speed is fine in one direction.

Thanks,
Dave

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