Question

Computer networks have been around since the 1950s. If you put cables between two or more computers and allow them to send data back and forth through these cables, you can do all kinds of wonderful things.

If connecting two machines in the same building allows us to do wonderful things, connecting machines all over the planet should be even better. The technology to start implementing this vision was developed in the 1980s, and the resulting network is called the Internet. It has lived up to its promise.

A computer can use this network to spew bits at another computer. For any effective communication to arise out of this bit-spewing, the computers at both ends must know what the bits are supposed to represent. The meaning of any given sequence of bits depends entirely on the kind of thing that it is trying to express and on the encoding mechanism used.

A network protocol describes a style of communication over a network. There are protocols for sending email, for fetching email, for sharing files, or even for controlling computers that happen to be infected by malicious software.

For example, a simple chat protocol might consist of one computer sending the bits that represent the text “CHAT?” to another machine and the other responding with “OK!” to confirm that it understands the protocol. They can then proceed to send each other strings of text, read the text sent by the other from the network, and display whatever they receive on their screens.

Most protocols are built on top of other protocols. Our example chat protocol treats the network as a streamlike device into which you can put bits and have them arrive at the correct destination in the correct order. Ensuring those things is already a rather difficult technical problem.

The Transmission Control Protocol (TCP) is a protocol that solves this problem. All Internet-connected devices “speak” it, and most communication on the Internet is built on top of it.

A TCP connection works as follows: one computer must be waiting, or listening, for other computers to start talking to it. To be able to listen for different kinds of communication at the same time on a single machine, each listener has a number (called a port) associated with it. Most protocols specify which port should be used by default. For example, when we want to send an email using the SMTP protocol, the machine through which we send it is expected to be listening on port 25.

Another computer can then establish a connection by connecting to the target machine using the correct port number. If the target machine can be reached and is listening on that port, the connection is successfully created. The listening computer is called the server, and the connecting computer is called the client.

Such a connection acts as a two-way pipe through which bits can flow—the machines on both ends can put data into it. Once the bits are successfully transmitted, they can be read out again by the machine on the other side. This is a convenient model. You could say that TCP provides an abstraction of the network.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.