In this chapter, we introduce the notion of tunnels and tunneling. Tunneling is a way of forming a virtual network on top of a physical network. That’s a bit murky for a definition, but let’s agree to accept it provisionally until we look at an example.
Suppose that we have two Novell Netware local area networks (LANs) in separate locations that we would like to connect through the Internet. Novell networks use IPX packets for their network-layer encapsulation and thus cannot route packets on an IP network directly.
IPX provides a connectionless unreliable datagram service similar to that provided by IP in TCP/IP.
There are several ways of linking the two LANs; one way, suggested in RFC 1234 [Provan 1991], is ...