Upgrading and Repairing Networks

The Point-to-Point Tunneling Protocol (PPTP)

The Point-to-Point Protocol (PPP) is an error-checking protocol used for dial-up connections to the Internet. PPP allows for the encapsulation of data packets from multiple protocols for simple transmission across a dedicated link, such as a phone line when you dial in to the Internet. PPP performs no routing functions, but merely encapsulates the protocol packets it receives by attaching its own header and sends them to the other endpoint of the connection.

	For more information about PPP, see Chapter 14, "Dial-Up Connections."

PPTP extends the capabilities of PPP so that a tunnel can be created through a packet-switched network, such as the Internet, instead of across a serial link. The concepts are similar. PPTP encapsulates another protocol packet and the PPTP packet is then routed through the network. The endpoints that use the PPTP connection don't have to be aware that they are at opposite ends of a large packet-switched network. Instead, it is as if both computers are on the same network.

Another difference between PPP and PPTP is that PPTP allows for the encryption of the payload portion of the packet so that IP (or other protocol) datagrams can be protected from prying eyes as they travel.

For example, a home user who wants to connect to a server on his company's network first makes a dial-up connection to an Internet service provider (ISP). After the connection is set up, another set of protocol negotiations begins to set up the PPTP tunnel, over the PPP link. The endpoints for the PPP link are the home user and the ISP. The endpoints for the PPTP link are the home user's computer and the company's remote access service (RAS) that uses the Internet. At the RAS endpoint, the PPTP packets are unpackaged and the contents decrypted to reveal the original IP (or other protocol) packet, which can then be sent onto the company network. Thus, the home user can operate as if he were directly connected to the company network.

This protocol was used in Windows NT 4.0, and although it's still supported in Windows 2000 and Windows XP, the newer L2TP is the preferred method for newer implementations for Windows clients.