The Point-to-Point Protocol and the Serial Line Internet Protocol

Communication on the Internet is based on the TCP/IP suite. TCP/IP is covered in detail in Chapter 24, "Overview of the TCP/IP Protocol Suite"; other services, applications, and tools designed to work with TCP/IP are discussed in Chapters 25, "Basic TCP/IP Services and Applications," 27, "Troubleshooting Tools for TCP/IP Networks," and 28, "BOOTP and Dynamic Host Configuration Protocol (DHCP)."

A dial-up connection is a point-to-point link using a phone line. A router or server on the remote network is your connection point to that network using a modem or a bank of modems grouped together. The remote access server at the ISP (or maybe your corporate dial-in modem bank) creates point-to-point connections with dial-in clients. After a point-to-point connection is established, you need a method for sending data across it. Addressing at this point doesn't necessarily matter because the conversation has only two parties, and they are transparent to the network transport protocol.

The Serial Line Internet Protocol (SLIP) was the first widely adopted protocol, and was initially found mainly on Unix systems. Many operating systems (including Unix, Linux, and Windows) still support SLIP today, although it's not commonly in use. Although you will find SLIP in use on older Unix-based systems, some systems are either too outdated or are end of life (EOL). Consequently, support on those systems is no longer offered. Because of these facts, you find SLIP in use when working heavily with antiquated Unix-based systems. On the other hand, if you are working with modern network and system infrastructure, it's a good bet that you will be working with PPP. PPP is more robust and has replaced SLIP in all but the most unique cases.

PPP makes up for many of the shortcomings of SLIP. For example:

• PPP offers synchronous as well as asynchronous communications, whereas SLIP offers only synchronous.

• PPP includes error correction. SLIP relies on either the error-correction capabilities of the hardware used to make the connection or the error-correction capabilities of TCP/IP.

• PPP provides for automatic, dynamic address assignment. SLIP must be manually configured prior to dialing or during the initial session setup.

• PPP provides for compression. SLIPfor the most partdoes not. There are variations of SLIP, such as Compressed SLIP, or CSLIP, that enable compression, but these are uncommon and often proprietary implementations.

• Multiple protocol stacks can use the same PPP link (such as IP and IPX). Higher layer protocols, such as the network layer's IP and IPX, are able to traverse the link whereas SLIP is limited to IP only. Things come full circle, though. As more and more standardization is done with TCP/IP and its suite of protocols, you will find that IP is the only protocol you will see and use regardless. Even factoring that in, however, PPP still brings much more to the table than does SLIP.

SLIP and PPP are nonroutable, due to their point-to-point connection. Both SLIP and PPP can encapsulate protocols such as IP, and then pass them on to routers or other devices so the data can reach its intended destination. At the receiving end of a connection, the PPP or SLIP information is stripped off and the protocol (such as IP) that was sent across the serial link (such as a modem) is then transmitted through a network as if it were coming from a computer or another device attached to the network.

Note

Encapsulate is a networking term that means a particular protocol is carrying, in its payload section, another packet generated by a different protocol. SLIP is one of the simplest of the protocols developed for this purpose, using only two characters to successfully transmit other protocols across a point-to-point connection. PPP is more complicated, adding its own protocol header information to the packet (and stripping it off at the other endpoint), to make communications a little more flexible and reliable.