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Whenever your click on a link or type an address into your web browser you're making a "request" for that particular page. That request is handled by what is known as the Hyper Text Transfer Protocol (HTTP). The request gets routed over the internet to the appropriate server and if everything goes according to plan, that server will send the page requested back to your browser. HTTP is an integral part of the Internet Protocol (IP) suite. It's used by browser software to connect to the server that holds the website you want to see. The server watched for these requests by monitoring what is called a TCP port (port 80 in most cases). Transmission Control Protocl (TCP) is the protocol that creates connections between two computer over the internet, allowing them to pass data back and forth. TCP is made to allow the transmitted data to be reassembled into the proper form when it reached its destination. There are a number of TCP ports that monitor specific protocols. For example, TCP port 21 is normally used for FTP (File Transfer Protocol) and port 80 is normally used for HTTP (web browsing). If a web server gets a request on port 80 in the form of GET / HTTP/1.1 it sends a response code that tells the requesting computer whether the page is available or not. A typical request will look like this: GET /faq.html HTTP/1.1
Host: http://www.mywebsite.com This request is for a web page at http://www.mywebsite.com/faq.html. The "host" is specified to identify a particular website on that server (in the case of shared hosting, there can be many sites on one server). If the faq.html page is available, the response will be: HTTP/1.1 200 OK
Date: Mon, 15 November 2005 13:22:54 GMT
Server: Apache/1.3.27 (Unix) (Red-Hat/Linux)
Last-Modified: Wed, 23 Aug 2005 08:23:33 GMT ...after which the contents of the actual web page will follow. Let's break that response down... HTTP/1.1 200 OK means that particular web page is available. Other codes may be returned. For example, the code 404 means the requested page cannot be found. You've probably seen 404 errors when you've clicked a link to a non-existent page or misspelled a web site when you entered in in your browser. The date is self explanatory and the server is information about the software running on the server that is hosting the web page. When the web page is being sent to your browser, it gets sent in "packets" of information. The header also contains information that specifies its order in the stream. These packets can be sent through various routes to get to your computer, so they all get reassembled by your browser when they arrive. This normally all takes place in seconds, so you see the web page as it was meant to be seen even though all the various packets may have taken completely different paths from the server to your computer. John Lenaghan writes about Unix & Windows web hosting and related topics on the Hosting Report website. Find out more at http://www.hostingreport.org
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