Section 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 network 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 network 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, ARPA, which is part of the U.S. Department of Defense, became Defense Advanced Research Projects Agency (DARPA). Whether it is known as ARPA or DARPA, the agency and its mission of funding advanced research have 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. Still, 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 in 1995 the NSFNet ceased its role as a primary Internet backbone network.

Today the Internet is larger than ever and encompasses hundreds of thousands of networks worldwide. It is no longer dependent on a core (or backbone) network or on governmental support. Today's Internet is built by commercial providers. National network providers, called tier-one providers, and regional network providers create the infrastructure. Internet Service Providers (ISPs) provide local access and user services. This network of networks is linked together in the United States at several major interconnection points called Network Access Points (NAPs).

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 1983doubling 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 IP. 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 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 spurred interest in TCP/IP. As more organizations became familiar with TCP/IP, they saw that its power can be applied in other network applications as well. 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 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 components, 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 DSL connection, a dial-up line, an optical network, 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 countryonly the natives are truly at home in it. TCP/IP creates 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 an open standards development process and publicly available standards documents. Internet standards are developed by the Internet Engineering Task Force (IETF) in open, public meetings. The protocols developed in this process are published as Requests for Comments (RFCs).[3] As the title "Request for Comments" implies, the style and content of these documents are much less rigid than in 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. There are three basic types of RFCs: standards (STD), best current practices (BCP), and informational (FYI).

[3] Interested in finding out how Internet standards are created? Read RFC 2026, The Internet Standards Process.

RFCs that define official protocol standards are STDs and are given an STD number in addition to an RFC number. Creating an official Internet standard is a rigorous process. Standards track RFCs pass through three maturity levels before becoming standards:

Proposed Standard

This is a protocol specification that is important enough and has received enough Internet community support to be considered for a standard. The specification is stable and well understood, but it is not yet a standard and may be withdrawn from consideration to be a standard.

Draft Standard

This is a protocol specification for which at least two independent, interoperable implementations exist. A draft standard is a final specification undergoing widespread testing. It will change only if the testing forces a change.

Internet Standard

A specification is declared a standard only after extensive testing and only if the protocol defined in the specification is considered to be of significant benefit to the Internet community.

There are two categories of standards. A Technical Specification (TS) defines a protocol. An Applicability Statement (AS) defines when the protocol is to be used. There are three requirement levels that define the applicability of a standard:


This standard protocol is a required part of every TCP/IP implementation. It must be included for the TCP/IP stack to be compliant.


This standard protocol should be included in every TCP/IP implementation, although it is not required for minimal compliance.


This standard is optional. It is up to the software vendor to implement it or not.

Two other requirements levels (limited use and not recommended) apply to RFCs that are not part of the standards track. A "limited use" protocol is used only in special circumstances, such as during an experiment. A protocol is "not recommended " when it has limited functionality or is outdated. There are three types of non-standards track RFCs:


An experimental RFC is limited to use in research and development.


A historic RFC is outdated and no longer recommended for use.


An informational RFC provides information of general interest to the Internet community; it does not define an Internet standard protocol.

A subset of the informational RFCs is called the FYI (For Your Information) notes. An FYI document is given an FYI number in addition to an RFC number. FYI documents provide introductory and background material about the Internet and TCP/IP networks. FYI documents are not mentioned in RFC 2026 and are not included in the Internet standards process. But there are several interesting FYI documents available.[4]

[4] To find out more about FYI documents, read RFC 1150, FYI on FYI: An Introduction to the FYI Notes.

Another group of RFCs that go beyond documenting protocols are the Best Current Practices (BCP) RFCs. BCPs formally document techniques and procedures. Some of these document the way that the IETF conducts itself; RFC 2026 is an example of this type of BCP. Others provide guidelines for the operation of a network or service; RFC 1918, Address Allocation for Private Internets, is an example of this type of BCP. BCPs that provide operational guidelines are often of great interest to network administrators.

There are now more than 3,000 RFCs. As a network system administrator, you will no doubt read several. It is as important to know which ones to read as it is to understand them when you do read them. Use the RFC categories and the requirements levels to help you determine which RFCs are applicable to your situation. (A good starting point is to focus on those RFCs that also have an STD number.) To understand what you read, you need to understand the language of data communications. RFCs contain protocol implementation specifications defined in terminology that is unique to data communications.