Running TCP/IP Over a Serial Port:

Of much greater utility than the capability to connect a terminal to your server's serial port is the capability to run TCP/IP over a serial port. Doing so allows you to run TCP/IP over a telephone line with a modem. This is, of course, the way that most people connect to the Internet through a local ISP. But it is also a way for you to provide connectivity into the Internet or your enterprise network for a remote field office or for users working at home.

Point-to-Point Protocol (PPP) provides the framing mechanism for sending IP datagrams over a telephone line. PPP uses a three-layered architecture to accomplish this:

• Data Link layer: PPP uses a High-Level Data Link Control (HDLC) protocol to provide reliable data delivery over any type of serial line.
• Link Control Layer: A Link Control Protocol (LCP) was specifically developed for PPP. It opens and closes connections, monitors link quality, and negotiates the link configuration parameters.
• Network Control Layer: PPP is designed to carry a wide variety of network protocols. Protocols in this layer provide the control information that is necessary to customize the PPP link for the type of network traffic it is carrying. The network control protocol for TCP/IP is the Internet Protocol Control Protocol (IPCP).

Properly configuring a PPP service requires that all of these layers are correctly installed as well, and that the serial port and modem over which the traffic passes are properly configured. The remainder of this chapter looks at how these things are done on a Linux system.