IPSec

IPsec (IP security) is a standardized framework for securing Internet Protocol (IP) communications by encrypting and/or authenticating each IP packet in a data stream. There are two modes of IPsec operation: transport mode and tunnel mode. In transport mode only the payload (message) of the IP packet is encrypted. It is fully-routable since the IP header is sent as plain text; however, it can not cross NAT interfaces, as this will invalidate its hash value. In tunnel mode, the entire IP packet is encrypted. It must then be encapsulated into a new IP packet for routing to work.