Linux Router: TCP slow, UDP fast

[chris at ghostbusters.co.uk (Chris)]

Thanks loads for the quick replies. I'll try and respond individually.
Lee > I recently disabled tcp_window_scaling and it didn't solve the
problem. I don't know enough about it. Should I enable it again ? Settings
differing from defaults are copied in my first post.

Mike > Strangely I'm not seeing any errors on either the ingress or egress
NICs:

          RX packets:3371200609 errors:0 dropped:0 overruns:0 frame:0
          TX packets:3412500706 errors:0 dropped:0 overruns:0 carrier:0

The only errors I see anywhere are similar on both NICs. Both connect to the
same model of switch with the same default config:

     rx_long_byte_count: 1396158525465
     rx_csum_offload_good: 3341342496
     rx_csum_offload_errors: 89459

and it may be worth noting that flow control is on. Are these a reasonable
level of pause frames to be seeing ? They seem to be higher on non-routing
boxes.

Total bytes (TX)2466202288Unicast packets (TX)3436389971Multicast packets
(TX)213310Broadcast packets (TX)4952902Single Collision Frames (TX)0Late
Collisions (TX)0Excessive Collisions (TX)0Transmitted Pause Frames (TX)27806

Florian > They're running without obvious errors. Auto Neg has taken 1Gbps,
Full. Can Auto Neg cause these symptoms do you think ?

Thanks again,

Chris