High UDP Loss issues

[Company 2210]

Hi, 
    I have two FreeBSD 5.0-RELEASE boxes connected over an IPSEC (With racoon) wireless link ( both are gateways). Now, TCP traffic flows perfectly, and at the expected speeds ~4.2Mbps. However UDP traffic flows at 10.6Mbps with enormous loss. An output from netperf is shown below:

$ ./netperf -f K -H 10.0.0.2 -t UDP_STREAM
UDP UNIDIRECTIONAL SEND TEST to 10.0.0.2
Socket  Message  Elapsed      Messages                
Size    Size     Time         Okay Errors   Throughput
bytes   bytes    secs            #      #   KBytes/sec

  8192    8192   10.00       13084 416919    10462.48
 42080           10.00           9              7.20


I've tried playing with the MTU on both boxes (network cards and gif tunnel interfaces are set to 1500), but it has made no difference. Now, this is where the plot thickens, I have a second wireless link which connects to a Windows XP box (Layout below):

FREEBSD-BOXA <----------IPSEC--------> FREEBSD-BOXB <------Unencrypted Wireless Link------> Windows XP - BOX

When I run netperf from the Windows XP box to FreeBSD BOXB I get no UDP errrors:

C:\PROGRA~1\NETPERF>netperf -f K -H 81.19.79.1 -t UDP_STREAM
UDP UNIDIRECTIONAL SEND TEST to 81.19.79.1
Socket  Message  Elapsed      Messages
Size    Size     Time         Okay Errors   Throughput
bytes   bytes    secs            #      #   KBytes/sec

  8192    8192   10.00        1465      0    1172.00
 42080           10.00           9              7.20


C:\PROGRA~1\NETPERF>


I am totally at a loss as to why all UDP traffic between the freebsd boxes has an ultra high error rate, but all other traffic doesn't. I'm guessing it may be a flow control issue? but I'm not sure how this could be rectified. Any help greatly appericated.


Colin.