Quick Tip: URLs - Uniform Resource Locator

URLs, or Uniform Resource Locators, are the method by which documents or data are addressed in the World Wide Web. The URL contains the following information:

- the protocol.
- the DNS name of the machine on which the page is located.
- the local name uniquely indicating the specific page.
- the location of the resource in the directory structure of the server.

To make a piece of text clickable, the page writer must provide two items of information : the clickable text to be displayed and the URL of the page to go to if the text is selected. Once the text is selected, the browser looks up the host name using DNS. Now armed with the host's IP address, the browser establish TCP connection to host. Over that connection, it sends the file name using the specified protocol.
The URL scheme is open to have protocols other than HTTP also. In short, URL's have been designed to not only allow users to navigate the Web, but to deal with FTP, news, Gopher, email, and telnet as well, making all the specialized user interface programs for those other services unnecessary, and thus integrating nearly all Internet access into a single program, the Web Browser.
The growing use of the Web has turned up an weakness in URL scheme. A URL points to one specific host. For pages that are heavily referenced, it is desirable to have multiple copies far apart, to reduce network traffic. The advent of systems such as Akami are meant to meet that need, distributing content over multiple servers on a global level.

WWW - The Server Side

For all the incoming connectionts from different clients, every website is associated with a server process listening to TCP port 80. The client sends a request after the connection is made and the server sends the reply and then the connection is released. The protocol that is responsible for requests and replies is called HTTP.

The steps that occur between the user clicking and a page being displayed are:
- The browser determines the URL.
- The browser asks DNS for IP address.
- DNS replies.
- Browser makes a TCP connection to the port.
- It then sendsthe GET command.
- The server sends the file.
- The TCP connection is released.
- The browser displays the text of the file.
- The browser fetchesand displays all images of the file.

Not all servers speak HTTP, Old servers use FTP, Gopher or other protocols. Given the number of different protocols, it was thought impractical to make browser understand different protocols. However, since there is a need to make information available (where the server talks in protocols other than HTTP), a solution was required. This solution is something called a proxy server. A proxy server takes a HTTP request from the browser and translates these requests into the FTP/Gopher/other protocols. The proxy server is a separate logical server.
A proxy server also serves to provide an important function called caching. Through caching, a proxy server keeps a local copy of the pages that pass through it. If a user requests for a page, if the page is present on the cache of the proxy server, it serves the page to the user. this way it serves to reduce load on final server.

Introduction: WWW - The Client Side

The Internet / World Wide Web consists of pages and each page contains links or pointers to other pages. Users follow the link by clicking on them. This process can be repeated indefinitely, possibly traversing hundreds of linked pages. Pages that point to other pages are said to use "hypertext".

Pages are viewed with a program called a browser. When a page is requested, the browser fetches, interprets the text and formats the commands that it contains and displays the page. Strings of text that are links to other pages are called hyperlinks.

Most browsers have numerous buttons and features to navigate the Web. In addition to having ordinary text and hypertext, web pages also contain icons, line drawings, maps and photographs. Some pages also consist of audio tracks, video clips, or both. When hypertext pages are combined with other media, the result is called hypermedia. Many Web pages consists of large images which take a long time to load.

Some browsers deal with slow loading of images by first fetching and displaying the text, then getting the images. Some Web pages contain forms that request the user to enter information. Some browsers use the local disk to cache pages that they have fetched. A check is made before a page is fetched to see if it is in the local cache. If so, check if it is up to date. If so,there is no need to load the page again.

To host a web browser, a machine must be directly connected to Internet or have a SLIP or PPP connection to a router or other machine that is directly on Internet.

Quick Introduction to the World Wide Web (WWW) / Internet

The World Wide Web is an architectural framework for accessing linked documents spread out over thousands of machines all over the Internet. The World Wide Web began as a networked information project at CERN, where Tim Berners-Lee, now Director of the World Wide Web Consortium [W3C], developed a vision of the project.

The Web has a body of software, and a set of protocols and conventions. Through the use of hypertext and multimedia techniques, the web is easy for anyone to roam, browse, and contribute to.

World Wide Web can also be defined as system of Internet servers that support specially formatted documents. The documents are formatted in a markup language called HTML (HyperText Markup Language) that supports links to other documents, as well as graphics, audio, and video files. You can jump from one document to another simply by clicking on hot spots. Not all Internet servers are part of the World Wide Web. Web is basically a client-server system. Web pages are written in HTML and java.