IPsec

IPsec (IP Security) is a protocol for securing VPN tunnels.

IPsec is described in RFC 3193: Securing L2TP using IPsec.

IPsec Transport Mode and Tunnel Mode

In Transport Mode, IPsec encrypts the payload of each IP packet, but not the packet headers.

In Tunnel Mode, IPsec encrypts the payload and the headers of each IP packet.

Many networks which are not able to support Tunnel Mode are able to successfully support Transport mode.

IPsec and ISAKMP

IPsec relies on ISAKMP (Internet Security Association and Key Management Protocol) for key exchange.

FreeS/WAN IPsec

FreeS/WAN is an implementation of IPsec and IKE for Linux.

The primary objective of the FreeS/WAN project is to help make IPsec widespread by providing source code which is freely available, runs on a range of machines including ubiquitous cheap PCs, and is not subject to US or other nations' export restrictions.