Overview of Computer Networks

5.2.1 Basic Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5.2.2 Advanced Use of Sockets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

5.3 Nonblocking I/O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.4 Debugging Client/Server Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

6 Packet/Frame Formats 24

6.1 TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

6.2 IP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

6.3 Ethernet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

7 Putting It All Together 25

8 Application-Layer Protocols 27

9 Routing Issues 28

2

1 Significance of Networks

1.1 History

The key to the computer revolution of the 1980s was the invention of the microprocessor in the 1970s. This

gave rise to the cheap computers which have become ubiquitous in homes, schools and offices, and to the

embedded computers which serve as controllers inside cameras, cars, washing machines and so on.

The computer revolution of the late 1990s, now continuing int the 21st century, involves computer networks,

whose existence is enriching our society in countless different ways.

1.2 What Are Networks Used For?

A simplified but worthwhile description of the uses of computer networks might be as follows:

• Sharing of hardware: For example, several PCs might be networked together in a wired or wireless

local area network (LAN) to share a printer.

• Sharing of information: Distributed databases, e-mail, the World Wide Web and so on are examples

of this. Here the sharing involves both LANs and wide area networks (WANs), especially the latter.

1.3 Which Aspects of Networks Are Important to Know?

Networks touch upon virtually every aspect of computing today. A good working knowledge (i.e. not of the

"partial credit on an extra problem" type) is of enormous value.

It should be noted carefully that it is important to know both the software and non-software aspects of

networks-the latter meaning the hardware, the protocols,1 the quantitative performance issues and so on.

If you interview for a job involving networks, you can expect that at least half of the questions will be

on the non-software aspects, even if the job itself deals mainly with software. Moreover, many jobs you

might interview for might be as system administrators; in such positions, knowing the physical structure of

networks is just as important as knowing how the network software works.

2 An Introductory Example

Consider the following structure:

1A protocol is simply an organized set of rules for performing some task, usually with two entities cooperating with each other.