3.1 WWW Overview
Pick up your pen, mouse or favorite pointing device and press it on a reference in this document - perhaps to the author's name, organization, or some related work. Suppose you are directly presented with the background material - other papers, the author's coordinates, the organization's address and its entire telephone directory. Suppose each of these documents has the same property of being linked to other original documents all over the world. You would have at your fingertips all you need to know about electronic publishing, high-energy physics or for that matter Asian culture. As you are reading this book on paper, you can only dream, but read on. -- Introduction of "World-Wide Web: The Information Universe"
Tim Berners-Lee et al., 1992 [Ber92]
When Tim Berners-Lee wrote the above sentences in 1991, the scenario he described was still mostly visionary. In the last five years, the Internet has experienced a phenomenal growth, mostly due to the enormous popularity of the web, such that the dream described by Tim Berners-Lee has now, just five years later, come true.
WWW is the most successful experiment so far that realizes Vannevar Bush's vision of creating the information universe by linking together all knowledge known to mankind. It implements a bottom-up approach by defining an architecture and light-weight protocol that runs on a variety of platforms. There have been other visionaries before, most prominently Theodore Nelson, who has been working on his Xanadu system for the last 25 years. Contrary to Xanadu [Nel90], which is incompatible with all existing systems, WWW offers gateways to link existing systems like Gopher [McC92], NetNews, WAIS [Kah89], etc., into the World Wide Web (fig. I.10).
Figure I.10 WWW architectural overview
The WWW architecture copes with a wide range of hardware platforms, operating systems, network protocols, information systems, and graphical user interfaces using the client-server model. The client's task is to resolve a document address into the actual document using its repertoire of network protocols and then to display the retrieved document. WWW clients currently understand, besides the native HTTP (hypertext transfer protocol), FTP and NNTP (network news transfer protocol). The server provides the basic data in a format that has to be negotiated with the client. WWW thus covers:
- http://www.w3.org/pub/WWW/Addressing/Addressing.html
- A common naming scheme for documents (URL--Uniform Resource Locators)
- http://www.w3.org/pub/WWW/Protocols/
- Common network access protocols (HTTP--HyperText Transfer Protocol)
- http://www.w3.org/pub/WWW/MarkUp/
- Common data formats for hypertext (HTML--HyperText Markup Language)
Almost any existing information system can be represented using the WWW data model. WWW offers an architecture that stretches seamlessly from small, personal notes to large information networks on other continents. Documents on the web do not have to exist as actual documents, but they can be created on the fly by the server as an answer to a query and can, e.g., represent views of a database, or snapshots of changing data.
Figure I.11. WWW hypertext encoding and display
Figure I.11 illustrates different representations of an original WWW document which is specified in HTML (Hypertext Mark-up Language), a variant of SGML. The document contains, in addition to the text body, embedded links that are mapped into the different user interfaces to the web, the so-called web browsers, depending on the capabilities of the actual system. These links may point to any other document integrated into the web, anywhere on the world.