[Chapter 1] Overview of TCP/IP
Author: Craig Hunt
Date: 1999-02-04T01:30:49Z

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| Chapter 1 |

# 1. Overview of TCP/IP

Contents:
TCP/IP and the Internet
A Data Communications Model
TCP/IP Protocol Architecture
Network Access Layer
Internet Layer
Transport Layer
Application Layer
Summary

All of us who use a UNIX desktop system - engineers, educators, scientists, and business people - have second careers as UNIX system administrators. Networking these computers gives us new tasks as network administrators.

Network administration and system administration are two different jobs. System administration tasks such as adding users and doing backups are isolated to one independent computer system. Not so with network administration. Once you place your computer on a network, it interacts with many other systems. The way you do network administration tasks has effects, good and bad, not only on your system but on other systems on the network. A sound understanding of basic network administration benefits everyone.

Networking computers dramatically enhances their ability to communicate - and most computers are used more for communication than computation. Many mainframes and supercomputers are busy crunching the numbers for business and science, but the number of such systems pales in comparison to the millions of systems busy moving mail to a remote colleague or retrieving information from a remote repository. Further, when you think of the hundreds of millions of desktop systems that are used primarily for preparing documents to communicate ideas from one person to another, it is easy to see why most computers can be viewed as communications devices.

The positive impact of computer communications increases with the number and type of computers that participate in the network. One of the great benefits of TCP/IP is that it provides interoperable communications between all types of hardware and all kinds of operating systems.

This book is a practical, step-by-step guide to configuring and managing TCP/IP networking software on UNIX computer systems. TCP/IP is the software package that dominates UNIX data communications. It is the leading communications software for UNIX local area networks and enterprise intranets, and for the foundation of the worldwide Internet.

The name "TCP/IP" refers to an entire suite of data communications protocols. The suite gets its name from two of the protocols that belong to it: the Transmission Control Protocol and the Internet Protocol. Although there are many other protocols in the suite, TCP and IP are certainly two of the most important.

The first part of this book discusses the basics of TCP/IP and how it moves data across a network. The second part explains how to configure and run TCP/IP on a UNIX system. Let's start with a little history.

## 1.1 TCP/IP and the Internet

In 1969 the Advanced Research Projects Agency (ARPA) funded a research and development project to create an experimental packet-switching network. This network, called the ARPANET, was built to study techniques for providing robust, reliable, vendor-independent data communications. Many techniques of modern data communications were developed in the ARPANET.

The experimental ARPANET was so successful that many of the organizations attached to it began to use it for daily data communications. In 1975 the ARPANET was converted from an experimental network to an operational network, and the responsibility for administering the network was given to the Defense Communications Agency (DCA). [1] However, development of the ARPANET did not stop just because it was being used as an operational network; the basic TCP/IP protocols were developed after the ARPANET was operational.

> [1] DCA has since changed its name to Defense Information Systems Agency (DISA).

The TCP/IP protocols were adopted as Military Standards (MIL STD) in 1983, and all hosts connected to the network were required to convert to the new protocols. To ease this conversion, DARPA [2] funded Bolt, Beranek, and Newman (BBN) to implement TCP/IP in Berkeley (BSD) UNIX. Thus began the marriage of UNIX and TCP/IP.

> [2] During the 1980s and early 1990s, ARPA, which is part of the U.S. Department of Defense, was named Defense Advanced Research Projects Agency (DARPA). Currently known as ARPA, the agency is again preparing to change its name to DARPA. Whether it is known as ARPA or DARPA, the agency and its mission of funding advanced research has remained the same.

About the time that TCP/IP was adopted as a standard, the term Internet came into common usage. In 1983, the old ARPANET was divided into MILNET, the unclassified part of the Defense Data Network (DDN), and a new, smaller ARPANET. "Internet" was used to refer to the entire network: MILNET plus ARPANET.

In 1985 the National Science Foundation (NSF) created NSFNet and connected it to the then-existing Internet. The original NSFNet linked together the five NSF supercomputer centers. It was smaller than the ARPANET and no faster - 56Kbps. Nonetheless, the creation of the NSFNet was a significant event in the history of the Internet because NSF brought with it a new vision of the use of the Internet. NSF wanted to extend the network to every scientist and engineer in the United States. To accomplish this, in 1987 NSF created a new, faster backbone and a three-tiered network topology that included the backbone, regional networks, and local networks.

In 1990, the ARPANET formally passed out of existence, and the NSFNet ceased its role as a primary Internet backbone network in 1995. Still, today the Internet is larger than ever and encompasses more than 95,000 networks worldwide. This network of networks is linked together in the United States at several major interconnection points:

- The three Network Access Points (NAPs) created by the NSF to ensure continued broad-based access to the Internet.

- The Federal Information Exchanges (FIXs) interconnect U.S. government networks.

- The Commercial Information Exchange (CIX) was the first interconnect specifically for commercial Internet Service Providers (ISPs).

- The Metropolitan Area Exchanges (MAEs) were also created to interconnect commercial ISPs.

The Internet has grown far beyond its original scope. The original networks and agencies that built the Internet no longer play an essential role for the current network. The Internet has evolved from a simple backbone network, through a three-tiered hierarchical structure, to a huge network of interconnected, distributed network hubs. It has grown exponentially since 1983 - doubling in size every year. Through all of this incredible change one thing has remained constant: the Internet is built on the TCP/IP protocol suite.

A sign of the network's success is the confusion that surrounds the term internet. Originally it was used only as the name of the network built upon the Internet Protocol. Now internet is a generic term used to refer to an entire class of networks. An internet (lowercase "i") is any collection of separate physical networks, interconnected by a common protocol, to form a single logical network. The Internet (uppercase "I") is the worldwide collection of interconnected networks, which grew out of the original ARPANET, that uses Internet Protocol (IP) to link the various physical networks into a single logical network. In this book, both "internet" and "Internet" refer to networks that are interconnected by TCP/IP.

Because TCP/IP is required for Internet connection, the growth of the Internet has spurred interest in TCP/IP. As more organizations become familiar with TCP/IP, they see that its power can be applied in other network applications. The Internet protocols are often used for local area networking, even when the local network is not connected to the Internet. TCP/IP is also widely used to build enterprise networks. TCP/IP-based enterprise networks that use Internet techniques and World Wide Web tools to disseminate internal corporate information are called intranets. TCP/IP is the foundation of all of these varied networks.

### 1.1.1 TCP/IP Features

The popularity of the TCP/IP protocols did not grow rapidly just because the protocols were there, or because connecting to the Internet mandated their use. They met an important need (worldwide data communication) at the right time, and they had several important features that allowed them to meet this need. These features are:

- Open protocol standards, freely available and developed independently from any specific computer hardware or operating system. Because it is so widely supported, TCP/IP is ideal for uniting different hardware and software, even if you don't communicate over the Internet.

- Independence from specific physical network hardware. This allows TCP/IP to integrate many different kinds of networks. TCP/IP can be run over an Ethernet, a token ring, a dial-up line, an FDDI net, and virtually any other kind of physical transmission medium.

- A common addressing scheme that allows any TCP/IP device to uniquely address any other device in the entire network, even if the network is as large as the worldwide Internet.

- Standardized high-level protocols for consistent, widely available user services.

### 1.1.2 Protocol Standards

Protocols are formal rules of behavior. In international relations, protocols minimize the problems caused by cultural differences when various nations work together. By agreeing to a common set of rules that are widely known and independent of any nation's customs, diplomatic protocols minimize misunderstandings; everyone knows how to act and how to interpret the actions of others. Similarly, when computers communicate, it is necessary to define a set of rules to govern their communications.

In data communications these sets of rules are also called protocols. In homogeneous networks, a single computer vendor specifies a set of communications rules designed to use the strengths of the vendor's operating system and hardware architecture. But homogeneous networks are like the culture of a single country - only the natives are truly at home in it. TCP/IP attempts to create a heterogeneous network with open protocols that are independent of operating system and architectural differences. TCP/IP protocols are available to everyone, and are developed and changed by consensus - not by the fiat of one manufacturer. Everyone is free to develop products to meet these open protocol specifications.

The open nature of TCP/IP protocols requires publicly available standards documents. All protocols in the TCP/IP protocol suite are defined in one of three Internet standards publications. A number of the protocols have been adopted as Military Standards (MIL STD). Others were published as Internet Engineering Notes (IEN) - though the IEN form of publication has now been abandoned. But most information about TCP/IP protocols is published as Requests for Comments (RFCs). RFCs contain the latest versions of the specifications of all standard TCP/IP protocols. [3] As the title "Request for Comments" implies, the style and content of these documents is much less rigid than most standards documents. RFCs contain a wide range of interesting and useful information, and are not limited to the formal specification of data communications protocols.

> [3] Interested in finding out how Internet standards are created? Read The Internet Standards Process, RFC 1310.

As a network system administrator, you will no doubt read many of the RFCs yourself. Some contain practical advice and guidance that is simple to understand. Other RFCs contain protocol implementation specifications defined in terminology that is unique to data communications.

Acknowledgments |  | 1.2 A Data Communications Model