.: Advanced Linux Networking :.

The final class of network devices is the dial-up device. Most typically, this is a conventional telephone modem used in conjunction with the Point-to-Point Protocol (PPP) to establish a connection to the Internet via an ISP. Such connections are established via command-line or GUI tools, as described in Chapter 2. In addition to these tools, though, the Linux kernel requires support for the dial-up connection.

To activate this support, you must select the PPP (Point-to-Point Protocol) Support option in the Network Device Support menu. When you select this option, several suboptions will become available, such as PPP Support for Async Serial Ports and PPP Deflate Compression. These options aren't usually strictly necessary, but sometimes they can improve a connection, such as by automatically compressing highly compressible data like text for higher net throughput. The experimental PPP over Ethernet option is required if you intend to use the kernel's PPPoE features for some DSL connections, but this option is not required with some add-on PPPoE packages, like Roaring Penguin.

PPP is sometimes used on connections that don't involve modems. For instance, you can use it to network two computers via their serial ports. Such configurations are seldom worthwhile with desktop systems, because Ethernet cards are inexpensive and provide much faster connections. You might want to use this type of link when connecting a desktop system to a palmtop computer, though, or for a temporary connection if you don't want to bother installing network cards.

PPP isn't the only type of dial-up connection that Linux supports. The kernel includes support for the older Serial Line Internet Protocol (SLIP), which serves much the same function as PPP. SLIP has been largely abandoned by ISPs, so it's unlikely you'll need to use it over a modem. A few Linux tools use it locally, though; for instance, some types of dial-on-demand utilities (which dial a PPP connection whenever network activity is detected) use SLIP to detect outgoing connection attempts.

Another protocol that's akin to PPP and SLIP is the Parallel Line Internet Protocol (PLIP). As you might guess by the name, this protocol lets you connect two Linux computers via their parallel (printer) ports. Because these ports are much faster than are RS-232 serial ports, PLIP offers a speed advantage over PPP or SLIP for two-computer local networks. Ethernet is still faster, though. To use PLIP, you must select the PLIP (Parallel Port) Support option in the Network Device Support menu. To do this, you must first activate the Parallel Port Support option in the menu of the same name, including the PC-Style Hardware option (if you're using an x86 computer). If you need to use PLIP networking, you should consult the PLIP Mini-HOWTO (http://www.linuxdoc.org/HOWTO/mini/PLIP.html) for further details, including wiring for the necessary cable, if you can't find a Turbo Laplink cable.