Virtual Private Networks, Second Edition

How PPTP Works

As a tunneling protocol, PPTP encapsulates network protocol datagrams within an IP envelope. After the packet is encapsulated, any router or machine that encounters it from that point on will treat it as an IP packet. The benefit of IP encapsulation is that it allows many different protocols to be routed across an IP-only medium, such as the Internet.

The first thing to understand about PPTP is that it revolves around Microsoft RAS for Windows NT. RAS allows a network administrator to set up a Windows NT server with a modem bank as a dial-in point for remote users. Authentication for the RAS users takes place on the NT server, and a network session is set up using the PPP protocol. Through the PPP connection, all of the protocols allowed by RAS can be transported: TCP/IP, NetBEUI, and IPX/SPX. To the RAS users it appears as though they’re directly connected to the corporate LAN; they notice no difference between RAS through direct dial-in and RAS over the Internet.

PPTP was designed to allow users to connect to a RAS server from any point on the Internet and still have the same authentication, encryption, and corporate LAN access they’d have from dialing directly into it. Instead of dialing into a modem connected to the RAS server, the end users dial into their ISPs and use PPTP to set up a “call” to the server over the Internet. PPTP and RAS use authentication and encryption methods to create a virtual private network.

There are two common scenarios for this type of ...