[Openswan Users] Weird xl2tpd behavior with HSDPA

[Kristian Evensen]

Hello,

I have set up an l2tp tunnel using xl2tpd over an HSDPA-connection and 
have got it working. However, after benchmarking it (by transfering big 
files from a nearby server and measuring the average throughput), I 
noticed something strange - the performance of TCP with the tunnel is 
better than without. Now, I am not complaining, but I am a little bit 
curious as to what causes this. Has anyone experienced similar or have 
any ideas?

What happens, at least according to the packet captures, is that one 
packet is lost which causes several dupAcks to be sent and without the 
tunnels it takes significantly longer before the retransmission is 
received. Also, there seems to be more packet loss without the tunnel, 
but as far as I can tell that is random since l2tp does not provide any 
reliability. Or have I misunderstood something?

For xl2tpd I have disabled flow control and debugging, which did not 
seem to affect the situation. For without tunneling, I have tried 
clamping the MSS to be the same size as in the tunnel (MTU used there is 
1410 bytes) without any effect.

Thanks in advance for any help,
Kristian