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11.2 | |
The Application Layer | |
| 11.2.4 | |
HTTP |
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Hypertext Transfer Protocol (HTTP) works
with the World Wide Web, which is the fastest growing and most used
part of the Internet. One of the main reasons for the extraordinary
growth of the Web is the ease with which it allows access to
information. A Web browser is a client-server application, which means
that it requires both a client and a server component in order to
function. A Web browser presents data in multimedia formats on Web
pages that use text, graphics, sound, and video. The Web pages are
created with a format language called Hypertext Markup Language
(HTML). HTML directs a Web browser on a particular Web page to produce
the appearance of the page in a specific manner. In addition, HTML
specifies locations for the placement of text, files, and objects that
are to be transferred from the Web server to the Web browser.
Hyperlinks make the World Wide Web easy to navigate. A hyperlink is an object, word, phrase, or picture, on a Web page. When that hyperlink is clicked, it directs the browser to a new Web page. The Web page contains, often hidden within its HTML description, an address location known as a Uniform Resource Locator (URL). In the URL http://www.cisco.com/edu/,
the "http://" tells the browser which protocol to use. The second
part, "www", is the hostname or name of a specific machine with a
specific IP address. The last part, /education identifies the
specific folder location on the server that contains the default web
page. A Web browser usually opens to a starting or "home" page. The URL of the home page has already been stored in the configuration area of the Web browser and can be changed at any time. From the starting page, click on one of the Web page hyperlinks, or type a URL in the address bar of the browser. The Web browser examines the protocol to determine if it needs to open another program, and then determines the IP address of the Web server using DNS. Then the transport layer, network layer, data link layer, and physical layer work together to initiate a session with the Web server. The data that is transferred to the HTTP server contains the folder name of the Web page location. The data can also contain a specific file name for an HTML page. If no name is given, then the default name as specified in the configuration on the server is used. The server responds to the request by sending to the Web client all of the text, audio, video, and graphic files specified in the HTML instructions. The client browser reassembles all the files to create a view of the Web page, and then terminates the session. If another page that is located on the same or a different server is clicked, the whole process begins again.
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Web Links