NAT-T - NAT-Traversal

NAT-T (NAT-Traversal) is a process developed for enabling data protected by IPSec to pass through a NAT. Since IPSec either in transport or tunnel mode provides integrity for the entire IP datagram, any changes to the IP addressing (the function of a NAT) will invalidate the data. To overcome this issue NAT-T appends a new IP and a UDP (User Datagram Protocol) header to the incoming datagram thus ensuring that no changes are made to the incoming datagram stream.