TCP IP Networks

Modern networking applications require a sophisticated approach to carrying data from one machine to another. If you are managing a Linux machine that has many users, each of whom may wish to simultaneously connect to remote hosts on a network, you need a way of allowing them to share your network connection without interfering with each other. The approach that a large number of modern networking protocols uses is called packet-switching. A packet is a small chunk of data that is transferred from one machine to another across the network. The switching occurs as the datagram is carried across each link in the network. A packet-switched network shares a single network link among many users by alternately sending packets from one user to another across that link.
The solution that Unix systems, and subsequently many non-Unix systems, have adopted is known as TCP IP. When talking about TCP IP networks you will hearthe term datagram, which technically has a special meaning but is often used interchangeably with packet. In this section, we will have a look at underlying concepts of the TCP IP protocols.

TCP IP traces its origins to a research project funded by the United States Defense Advanced Research Projects Agency (DARPA) in 1969. The ARPANET was an experimental network that was converted into an operational one in 1975 after it had proven to be a success.
In 1983, the new protocol suite TCP IP was adopted as a standard, and all hosts on the network were required to use it. When ARPANET finally grew into the Internet (with ARPANET itself passing out of existence in 1990), the use of TCP IP had spread to networks beyond the Internet itself. Many companies have now built corporate TCP IP networks, and the Internet has grown to a point at which it could almost be considered a mainstream consumer technology. It is difficult to read a newspaper or magazine now without seeing reference to the Internet; almost everyone can now use it.
For something concrete to look at as we discuss TCP IP throughout the following sections, we will consider Groucho Marx University (GMU), situated somewhere in Fredland, as an example. Most departments run their own Local Area Networks, while some share one and others run several of them. They are all interconnected and hooked to the Internet through a single high-speed link.
Suppose your Linux box is connected to a LAN of Unix hosts at the Mathematics department, and its name is erdos. To access a host at the Physics department, say quark, you enter the following command:
$ rlogin quark. physics
Welcome to the Physics Department at GMU
(ttyq2) login:
At the prompt, you enter your login name, say andres, and your password. You are then given a shell[2] on quark, to which you can type as if you were sitting at the system's console. After you exit the shell, you are returned to your own machine's prompt. You have just used one of the instantaneous, interactive applications that TCP/IP provides: remote login. The shell is a command-line interface to the Unix operating system. It's similar to the DOS prompt in a Microsoft Windows environment, albeit much more powerful.
While being logged into quark, you might also want to run a graphical user interface application, like a word processing program, a graphics drawing program, or even a World Wide Web browser. The X windows system is a fully network-aware graphical user environment, and it is available for many different computing systems. To tell this application that you want to have its windows displayed on your host's screen, you have to set the DISPLAY environment variable:

$ DISPLAY=erdos.maths:0.0
$ export DISPLAY
If you now start your application, it will contact your X server instead of quark's, and display all its windows on your screen. Of course, this requires that you have X11 runnning on erdos. The point here is that TCP IP allows quark and erdos to send X11 packets back and forth to give you the illusion that you're on a single system. The network is almost transparent here. Another very important application in TCP IP networks is NFS, which stands for Network File System. It is another form of making the network transparent, because it basically allows you to treat directory hierarchies from other hosts as if they were local file systems and look like any other directories on your host. For example, all users' home directories can be kept on a central server machine from
which all other hosts on the LAN mount them. The effect is that users can log in to any machine and find themselves in the same home directory. Similarly, it is possible to share large amounts of data (such as a database, documentation or application programs) among many hosts by maintaining one copy of the data on a server and allowing other hosts to access it.The Network File System.
Of course, these are only examples of what you can do with TCP IP networks. The possibilities are almost limitless, and we'll introduce you to more as you read on through the book. We will now have a closer look at the way TCP IP works. This information will help you understand how and why you have to configure your machine. We will start by xamining the hardware, and slowly work our way up.