hi i am having a error while configuring l2tp vpn

i am installing ubuntu 9.01 on one virtual machines and windows xp on another..

prob1
i am unable to ping from windows xp to ubuntu virtual machine

prob second

while verifying ipsec through command

ipsec verify

i am getting this error

oppurtunistic encryption support[disabled]

i want to know what is the role of oppurtunistic support