COMPUTER NETWROKS EDITION 3

1.1 Applications
Most people know the internet through its applications: the www, email, streaming audio and video, chat room, and music (file) sharing. The Web, for example, present an intuitively simple interface. Users view pages full of textual and graphical objects, click on objects that want to learn more about, and a corresponding new page appears. Most people are also aware that just under the covers, each seletable objects on a page is bound to an identifier for the next page to be viewed. This identifier, called a uniform resource locator (URL), uniquely names every possible page that can be viewed from your Web browser.

Is the URL for a page representing this book at Morgan Kaufmann: The string http indicates that the Hyper Text Transfer  Protocol (HTTP) should be used to  download the page, www.mkp.com is the name of the machine that server the page,  pd3e uniquely identifies the page at the  publisher’s site.

          What most Web users are not aware of, however, is that by clicking on just one such URL, as many as 17 messages may be exchanged over the internet, and this the assumes the page it self is small enough to fit in a single message. This number includes up to six messages to translate the server name ( www.mkp.com ) into its internet address (213.38.165.180), there messages to sep up a Transission Control Protocol (TCP) connection betwen your browser and this server, four messages for your browser to send the HTTP “get” request and the server to respond with the requested page (and for each side to acknowledge receipt of that message), and four messages to tear down the TCP connection. Of course, this does not include the millions of messages exchanged by internet nodes throughout the day, just to let each other know that they exist and are ready to server Web pages, translate name to addresses, and forward messages toward their ultimated destination.