Virtual Private Networks, Second Edition

Chapter 4. Implementing Layer 2 Connections

This chapter will discuss some of the major tunneling protocols used by VPN vendors. These protocols are the Point-to-Point Tunneling Protocol (PPTP), Layer 2 Forwarding (L2F), and the Layer 2 Tunneling Protocol (L2TP).

Tunneling protocols essentially make square pegs fit into round holes. Imagine you have a round pipe and you want to send a cube through it. The cube is just going to get stuck, or isn’t going to fit at all. The way to get around this is to encapsulate the cube within a sphere, then send it through the pipe. In other words, you take something that your transport medium can’t work with and package it within something it can. All computer networking works this way, in one fashion or another.

All of these tunneling protocols operate by tunneling Layer 2 of the OSI Reference Model for communications protocols, also known as the Data Link Layer, over IP. It is at this layer that protocols such as PPP operate. As you may know, PPP is commonly used to transport IP and other protocols over serial and digital connections. Typically PPP connections are made between a client and a remote host, such as a remote access server. Likewise, PPTP, L2F, and L2TP are all used to tunnel PPP connections over the Internet so that they may be terminated on a remote host. In this case, the tunnel essentially acts in place of the line. Because they use existing PPP infrastructure, these protocols gain the advantages of the PPP protocol, including ...