The modern Web browser, such as Netscape or the Internet Explorer, represents the pinnacle, to date, of connectivity software. It manages to manipulate half a dozen Internet protocols and twice as many file formats, all through a connection that can range in speeds from 1 kilobyte per second all the way to 10 megabits per second, and seems to do all this with deceptive ease. Even a novice computer or Internet user can use one of these programs to retrieve Internet-based data with almost no previous instruction梚t's almost entirely a point-and-click affair.
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Yet this simple interface belies an exceptionally complex latticework of programming that is the ultimate communications program梩hat's what a Web browser is really all about. All a Web browser does is communicate to various Internet servers, first requesting servers to communicate back to establish a basic link, then asking the server to transmit information that, in and of itself, requires little computing power to display to the Web browser's user. Of course, this is a brief summary of the process of displaying a Web page, but by breaking it down, you'll see that a Web browser is little more than a communications-based, graphically oriented, Internet walkie-talkie.
Here are all the steps a Web browser takes to display a simple Web page once you give it a URL. Don't worry梩his will be in plain English. Trust me, it's interesting:
The browser looks at the URL and breaks it down into its component parts. It determines that if you gave it a URL of http://www.cris.com/~wernst/index.html, you want to use the HyperText Transfer Protocol (http) to try to get a Web page named index.html from an Internet host (or server) named www.cris.com in a subdirectory called ~wernst. You see, there's very little computing involved in breaking down the URL梩he browser merely performs the equivalent of diagramming a sentence, as you used to do in grammer school.
The browser looks at your network configuration settings to find the Internet address of the nearest predefined Domain Name Server (DNS), a computer that has a big database or table of all the Internet servers out there. It communicates to the DNS the name of the server you want (in this case, www.cris.com), and the DNS tells the browser what the numeric Internet address (called an IP address) is (which in this case is 199.3.12.172) through the TCP protocol. At this point, only two lookup tables have been accessed, and there's been some communication between the DNS and the browser梟ot much real computing.
Next, the browser takes the addressing information from the DNS and communicates to www.cris.com through its IP address of 199.3.12.172 with HTTP because it was specified in the URL. This initial communique basically breaks down into "Hello www.cris.com. I am a Web browser named (Netscape, Internet Explorer, or something similar) and would like some Web pages (or other information) from your directories. Is this OK? Are you alive?" Usually, the server replies with the equivalent of "Sure. What do you want?" At this point, you've now had two more lines of communication from start and finish and very little computation.http://wowgold.tomore.com/
The browser, now knowing the server is serving, requests the file: "I would like the file index.html from your subdirectory '~wernst,' please." The server looks through its directories for the file, finds it, checks to make sure it's a valid file type with its own set of lookup tables, sends the message "I found it. Here it comes," and sends the file. If it can't find the requested file, it sends the message "Huh? I don't have the file you've asked for," (or other error message, depending on the type of problem) and the communication stops. At this point, you just have more communication and table lookups梟ot much computation.
Assuming the file exists, the browser receives the Web page in the form of an HTML file and starts to decode it. HTML files are human-readable text files, essentially, requiring almost no real computation to decode and display. When the browser finds an embedded image, for example, it sends another communiqué to the server: "Just a minute. I also need the file 'image1.gif' for this page. Can you send it now?" The server, upon receiving this new message, checks its directories and file type tables to make sure the requested file can be sent, says "OK," and sends the inline image file. This process repeats for every inline image in the HTML file and ends the communiqué once the browser says it has all the inline images. As the files are sent (with lots more communication), the browser builds the page onscreen with the codes in the HTML file as a guide and the inline images as they're received, using a fraction of your CPU's computing power to generate onscreen fonts, colors, and images.
Figure 1.1 graphically represents all these steps.