Communication on the Internet is based on the TCP/IP suite. TCP/IP is covered in detail in Chapter 25, "Overview of the TCP/IP Protocol Suite"; other services, applications, and tools designed to work with TCP/IP are discussed in Chapters 26, "Basic TCP/IP Services and Applications," 28, "Troubleshooting Tools for TCP/IP Networks," and 29, "BOOTP and Dynamic Host Configuration Protocol (DHCP)." Although TCP/IP is a great LAN/WAN networking solution, it does not provide for dial-up connections. A dial-up connection is a point-to-point link using a phone line. Because of this, a router or server on the remote network will be your connection point to that network using a modem. The remote access server at the Internet service provider (ISP, or maybe your corporate dial-in modem bank) creates point-to-point connections with dial-in clients . This connection needs a method for sending IP or other protocols across this point-to-point connection (where addressing doesn't matter because the conversation has only two parties) transparently to the actual 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. However, far fewer use SLIP as compared to the Point-to-Point Protocol (PPP). PPP is more robust and has generally replaced SLIP in all but the most unique cases during the past few years . PPP makes up for many of the shortcomings of SLIP. For example:
Both of these protocols are nonroutable, due to their point-to-point connection. There are only two parties to the connection, so there is no need for routing. Of course, this applies only to PPP or SLIP. Both can encapsulate other protocols that are then passed to routers or other devices. 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.
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