2.1. Introduction

The flow and routing of data on the Internet is controlled by a set of protocols called the Transmission Control Protocol/Internet Protocol (TCP/IP) suite. These protocols provide many sorts of services. Some protocols provide a connectionless, best-effort, datagram delivery service. Others provide a connection-based reliable data delivery service. Still others provide routing, name resolution, and network control messaging. Together, the TCP/IP protocols form an infrastructure that applications can use to communicate with peer applications on machines across the room or across the world.

Before beginning our examination of the role of tunnels and VPNs in networking, let's review some of the basic facts about TCP/IP and its use in the Internet. This chapter looks at the major protocols that we will build on later in the text. We will recall their normal operating modes, their data formats, and their operation on the wire. The subject is a large one, of course, so our coverage will be a précis rather than an exhaustive account. A detailed treatment of TCP/IP along the lines of our review is given in [Stevens 1994]. Another excellent account of TCP/IP [Comer 2000], takes a slightly different view.