[strongSwan] why i cannot download files through l2tp server after being connected for a couple of minutes?

[mnz hz]

i run xl2tpd -D on server side, after i am connected to xl2tpd server, i
can download using vpn for about 2 minutes, but after that , i cannot
download anything using vpn anymore, but the vpn is still connected, why?
and the following is what output by xl2tpd daemon? what it tells? which
staff could such error message? and how to solve this problem?


xl2tpd[27406]: Call established with 111.111.xx.xx, Local: 47476, Remote:
1, Serial: 0
xl2tpd[27406]: Maximum retries exceeded for tunnel 50936.  Closing.
xl2tpd[27406]: Terminating pppd: sending TERM signal to pid 27415
xl2tpd[27406]: Connection 1 closed to 111.111.xx.xx, port 1701 (Timeout)
xl2tpd[27406]: get_call: can't find call 47476 in tunnel 50936
 (ref=0/0)xl2tpd[27406]: get_call: can't find call 47476 in tunnel 50936
 (ref=0/0)xl2tpd[27406]: get_call: can't find call 47476 in tunnel 50936
 (ref=0/0)xl2tpd[27406]: get_call: can't find call 47476 in tunnel 50936
 (ref=0/0)xl2tpd[27406]: get_call: can't find call 47476 in tunnel 50936

...

 (ref=0/0)xl2tpd[27406]: Unable to deliver closing message for tunnel
50936. Destroying anyway.
xl2tpd[27406]: Can not find tunnel 50936 (refhim=0)
xl2tpd[27406]: network_thread: unable to find call or tunnel to handle
packet.  call = 47476, tunnel = 50936 Dumping.
xl2tpd[27406]: Can not find tunnel 50936 (refhim=0)
xl2tpd[27406]: network_thread: unable to find call or tunnel to handle
packet.  call = 47476, tunnel = 50936 Dumping....

...
detailed message here:
http://pastie.org/10198998
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