Tunneling and encryption: L2TP
Layer 2 Tunneling Protocol (L2TP) is a combination of PPTP and Cisco L2F technology. L2TP, as the name suggests, uses tunneling to deliver data. It authenticates the client in a two-phase process: It authenticates the computer and then the user. By authenticating the computer, it prevents the data from being intercepted, changed, and returned to the user in what is known as a man-in-the-middle attack. L2TP ensures both parties that the data they receive is exactly the data sent by the originator.
L2TP offers two-phase authentication—once for the computer and once for the user. This helps prevent man-in-the-middle attacks.
Layer 2 Tunneling Protocol (L2TP) is a VPN protocol that lacks security features, such as encryption. However, L2TP can still be used for a secure VPN connection if it is combined with another protocol that does provide encryption.
Unlike IPSec, which operates at the network layer of the OSI model, L2TP operates at the data link layer, making it protocol-independent. This means that an L2TP connection can even support protocols other than TCP/IP, such as AppleTalk and Novell's legacy IPX.