19.1 The World Wide Web

Electronic hypertext contains links to expanded or related information embedded at relevant points in a document. The links are analogous to footnotes in a traditional paper document, but the electronic nature of these documents allows easier physical access to the links. As early as 1945, Vannevar Bush proposed linked systems for documents on microfiche [18], but electronic hypertext systems did not take hold until the 1960s and 1970s.

In 1980, Tim Berners-Lee wrote a notebook program for CERN called ENQUIRE that had bidirectional links between nodes representing information. In 1989, he proposed a system for browsing the CERN Computer Center's documentation and help service. Tim Berners-Lee and Robert Cailliau developed a prototype GUI browser-editor for the system in 1990 and coined the name "World Wide Web." The initial system was released in 1991. At the beginning of 1993 there were 50 known web servers, a number that grew to 500 by the end of 1993 and to 650,000 by 1997. Today, web browsers have become an integral interface to information, and the Internet has millions of web servers.

The World Wide Web is a collection of clients and servers that have agreed to interact and exchange information in a certain format. The client (an application such as a browser) first establishes a connection with a server (an application that accepts connections and responds). Once it has established a connection, the client sends an initial request asking for service. The server responds with the requested information or an error.

As described so far, the World Wide Web is a simple client-server architecture, no different from many others. Its attractiveness lies in the simplicity of the rules for locating resources (URIs), communicating (HTTP) and presenting information (HTML). The next section describes URLs, the most common format for resource location on the Web. Section 19.3 gives an overview of HTTP, the web communication protocol. HTML, the actual format for web pages, is not within the scope of this book. Section 19.4 discusses tunnels, gateways and caching. The chapter project explores various aspects of tunnels, proxies and gateways. Sections 19.5 and 19.6 guide you through the implementation of a tunnel that might be used in a firewall. Section 19.7 describes a driver for testing the programs. Section 19.8 discusses the HTTP parsing needed for the proxy servers. Sections 19.9 and 19.10 describe a proxy server that monitors the traffic generated by the browsers that use it. Sections 19.12 and 19.13 explore the use of gateways for firewalls and load balancing, respectively.