how IPSEC header is created in xfrm in kernel 2.6.9???

Hello,
       Can anybody help me How IpSec calls occurred in
Linux kernel 2.6.9. I want to know actually how packet
forms AFTER APPLYING ESP/UDP to Forming Normal UDP
Packet in Linux Kernel.
      Please anybody provide me packet path in XFRM to
form IPSEC header on packet.
        Thanks in advance.
regards,
linux_lover

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