Time For Some Alphabet Soup

Whenever you type an address into your web browser, or click on a link in a web page, you are making a request for a certain document. Handled by the Hyper Text Transfer Protocol (HTTP), your request is sent over the Internet to the server that holds the document you want. Assuming all goes well, the server will respond by sending the document, usually a web page consisting of text and graphics.

Exactly what is HTTP? It is part of the Internet Protocol (IP) suite, and is used by a "client," such as a web browser, to establish a connection with the server that hosts a particular website. The server monitors TCP port 80 as it waits for incoming requests.

Connections on the Internet that allow 2 computers to exchange data are created by the Transmission Control Protocol (TCP). TCP is equipped to identify the requesting computer, and to properly transmit data to its destination.

Server To Web Browser -- Behind The Scenes

Several TCP ports are available with standardized uses. For example, TCP Port 21 is usually reserved for FTP (File Transfer Protocol) for uploading and downloading files. Port 80 is usually used for HTTP.

If the server receives a request string on TCP port 80 in the form of GET / HTTP/1.1 it will send a response code, depending on whether the requested web page is available or not. A typical request will look like this:

GET /faq.html HTTP/1.1

Host: http://www.mywebsite.com

This is a request for the page "faq" on the host site "mywebsite". The "host" must be specified to distinguish between websites that are hosted on shared servers. If faq.html is available, the server will respond something like:

HTTP/1.1 200 OK

Date: Mon, 12 October 2005 22:38:34 GMT

Server: Apache/1.3.27 (Unix) (Red-Hat/Linux)

Last-Modified: Wed, 08 Jan 2003 23:11:55 GMT ... followed by the actual web page.

How Data Gets Where It's Going

The first line above, HTTP/1.1 200 OK, means that the requested web page is available. Other codes may also be returned. For example, the code 404 means the server cannot find the requested page.

When found, the web page is sent via TCP as a series of data packets, each with a header that specifies its destination and its order in the data stream. The various packets can take different paths to reach their destination.

Each is sent through a router, which polls other routers close by. If a connection with the first router is unavailable, the data will be sent through another one. This allows the data to reach its destination as quickly as possible.

What Happens When It Gets There

When the web browser receives the data, it sends back an acknowledgement. This insures that all the packets have been received within a certain time. If not, they will be re-transmitted by the server. TCP also checks to be sure the data is undamaged.

The data is then reassembled in the correct order, thanks to the sequence number of each data packet.

And Presto! The web page appears on your computer screen, usually in a few seconds.

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Copyright 2005 Ron King. This article may be reprinted if the resource box is left intact and the links live.