A method of communicatively connecting first and second endpoints across a NAT and/or PAT router to form an IPSec encrypted tunnel is disclosed. A message is received by the first endpoint from the second endpoint. The message includes an encrypted portion including a source port, a destination port, a source IP address, and a destination IP address. It is determined whether a table entry exists for the message. If Yes, it is determined by the first endpoint whether a NAT router and/or a PAT router is between the first endpoint and the second endpoint based, at least in part, on the table entry and the encrypted portion of the message. If Yes, an IPSec encrypted tunnel is created using IPSec transport mode for further communications between the first and second endpoints. An apparatus and a computer program product are also disclosed.