Spinning the World-Wide Web

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World Wide Web Protocols

Technically the World-Wide Web hinges on three enabling protocols, the HyperText Markup Language (HTML) that specifies a simple markup language for describing hypertext pages, the Hypertext Transfer Protocol (HTTP) which is used by web browsers to communicate with web clients, and Uniform Resource Locators (URL's) which are used to specify the links between documents.

HyperText Markup Language

The hypertext pages on the web are all written using the Hypertext Markup Language (HTML), a simple language consisting of a small number of tags to delineate logical constructs within the text. Unlike a procedural language such as postscript (move 1 inch to the right, 2 inches down, and create a green WWW in 15 pointer bold helvetica font), HTML deals with higher level constructs such as "headings," "lists," "images," etc. This leaves individual browsers free to format text in the most appropriate way for their particular environment; for example, the same document can be viewed on a MAC, on a PC, or on a linemode terminal, and while the content of the document remains the same, the precise way in which it is displayed will vary between the different environments.

The earliest version of HTML (subsequently labeled HTML1), was deliberately kept very simple to make the task of browser developers easier. Subsequent versions of HTML will allow more advanced features. HTML2 (approximately what most browsers support today) includes the ability to embed images in documents, layout fill-in forms, and nest lists to arbitrary depths. HTML3 (currently being defined) will allow still more advanced features such as mathematical equations, tables, and figures with captions and flow-around text.

Hypertext Transfer Protocol

Although most Web browsers are able to communicate using a variety of protocols, such as FTP, Gopher and WAIS, the most common protocol in use on the Web is that designed specifically for the WWW project, the HyperText Transfer Protocol. In order to give the fast response time needed for hypertext applications, a very simple protocol which uses a single round trip between the client and the server is used.

In the first phase of a HTTP transfer the browser sends a request for a document to the server. Included in this request is the description of the document being requested, as well as a list of document types that the browser is capable of handling. The Multipurpose Internet Mail Extensions (MIME) standard is used to specify the document types that the browser can handle, typically a variety of video, audio, and image formats in addition to plain text and HTML. The browser is able to specify weights for each document type, in order to inform the server about the relative desirability of different document types.

In response to a query the server returns the document to the browser using one of the formats acceptable to the browser. If necessary the server can translate the document from the format it is stored in into a format acceptable to the browser. For example the server might have an image stored in the highly compressed JPEG image format, and if a browser capable of displaying JPEG images requests the image it would be returned in this format. However if a browser capable of displaying images only if they are in GIF format requested the same document the server would be able to translate the image and return the (larger) GIF image. This provides a way of introducing more sophisticated document formats in future but still enabling older or less advanced browser to access the same information.

In addition to the basic "GET" transaction described above the HTTP is also able to support a number of other transaction types, such as "POST" for sending the data for fill-out forms back to the server and "PUT" which might be used in the future to allow authors to save modified versions of documents back to the server.

Uniform Resource Locators

The final key to the World-Wide Web is the URL's which allow the hypertext documents to point to other documents located anywhere on the web. A URL consists of 3 major components:

The first component specifies the protocol to be used to access the document, for example, HTTP, FTP, or Gopher, etc. The second component specifies the node on the network from which the document is to be obtained, and the third component specifies the location of the document on the remote machine. The third component of the URL is passed without modification by the browser to the server, and the interpretation of this component is performed by the server, so while a documents location is often specified as a Unix-like file specification, there is no requirement that this is how it is actually interpreted by the server.

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