Chapter 1. Introduction to Network Protocols

Just as diplomats use diplomatic protocols in their meetings, computers use network protocols to communicate in computer networks. There are many network protocols in existence; TCP/IP is a family of network protocols that are used for the Internet.

A network protocol is a standard written down on a piece of paper (or, more precisely, with a text editor in a computer). The standards that are used for the Internet are called Requests For Comment (RFC) . RFCs are numbered from 1 onwards. There are more than 4,500 RFCs today. Many of them have become out of date, so only a handful of the first thousand RFCs are still used today.

The International Standardization Office (ISO) has standardized a system of network protocols called as ISO OSI. Another organization that issues communication standards is the International Telecommunication Union (ITU) located in Geneva. The ITU was formerly known as the CCITT and, being founded in 1865, is one of the oldest worldwide organizations (for comparison, the Red Cross was founded in 1863). Some standards are also issued by the Institute of Electrical and Electronics Engineers (IEEE) . RFC, standards released by RIPE (Réseaux IP Européens), and PKCS (Public Key Cryptography Standard) are freely available on the Internet and are easy to get hold of. Other organizations (ISO, ITU, and so on) do not provide their standards free of charge—you have to pay for them. If that presents a problem, then you have to spend some time doing some library research.

First of all, let’s have a look at why network communication is divided into several protocols. The answer is simple although this is a very complex problem that reaches across many different professions. Most books concerning network protocols explain the problem using a metaphor of two foreigners (or philosophers, doctors, and so on) trying to communicate with each other. Each of the two can only communicate in his or her respective language. In order for them to be able to communicate with each other, they need a translator as shown in the following figure:

Figure 1.1: Three-layer communication architecture

The two foreigners exchange ideas, i.e., they communicate. But they only do so virtually. In reality, they are both handing over information to their interpreters, who then transmit this information by sending vibrations through the surrounding air with their vocal cords. Or if the parties are far away from each other, the interpreters communicate over the phone; thus the information is physically transmitted over phone lines. We can therefore talk about virtual communication in the horizontal direction (philosophical communication, the shared language between interpreters, and electronic signals transmitted via phone lines) and real communication in the vertical direction (foreigner-to-interpreter and interpreter-to-phone). We can thus distinguish three levels of communication:

Communication between the two foreigners and between the two interpreters is only virtual. In fact, the only real communication happens between the foreigner and his or her interpreter.

Even more layers are used in computer networks. The number of layers depends on which system of network protocols you choose to use. The system of network protocols is sometimes referred to as the network model. You most commonly work with a system that uses the Internet, which is also referred to as the TCP/IP family. In addition to TCP/IP, we will also come across the ISO OSI model that was standardized by the ISO.

Figure 1.2: Comparison of TCP/IP and ISO OSI network models

The TCP/IP family uses four layers while ISO OSI uses seven layers as shown in the figure above. The TCP/IP and ISO OSI systems differ from each other significantly, although they are very similar on the network and transport layers.

Except for some exceptions like SLIP or PPP, the TCP/IP family does not deal with the link and physical layers. Therefore, even on the Internet, we use the link and physical protocols of the ISO OSI model.