[strongSwan] how does pluto and charon share port 500

[Simon Chan]

Greetings,

I am running tests with User-Mode Linux. I face the problem that if I start
both pluto and charon then charon can never establish connection. Tcpdump at 
both
ends indicate the response does come back but charon just can't receive it. 
I can also
see in syslog that pluto is receiving message from the far-end.
If I put "plutostart=no" in ipsec.conf than things work normally.
"lsof" shows both pluto and charon are listening on port 500.

I am currently trying Debian 6.0.3 root filesystem but I have previously 
used the Gentoo
filesystem downloaded from StrongSwan UML site with same results.
Can someone explain what's going on?

Here is the syslog showing pluto received the response message but charon 
got nothing:

Jan 31 22:18:07 station01 charon: 05[IKE] retransmit 5 of request with 
message ID 0
Jan 31 22:18:07 station01 charon: 05[NET] sending packet: from 
192.168.18.1[500] to 192.168.3.195[500]
Jan 31 22:18:07 station01 charon: 14[NET] sending packet: from 
192.168.18.1[500] to 192.168.3.195[500]
Jan 31 22:18:07 station01 pluto[1578]: |
Jan 31 22:18:07 station01 pluto[1578]: | *received 376 bytes from 
192.168.3.195:500 on eth0
Jan 31 22:18:07 station01 pluto[1578]: | next event EVENT_REINIT_SECRET in 
3086 seconds
Jan 31 22:19:23 station01 charon: 02[IKE] giving up after 5 retransmits
Jan 31 22:19:23 station01 charon: 02[IKE] peer not responding, trying again 
(3/0)

 Thanks for help.
Simon