BitTorrent

By Pralav Bhansali

INTRODUCTION

BitTorrent is a protocol that enables fast downloading of large files using minimum Internet bandwidth. It costs nothing to use and includes no spyware or pop-up advertising.

Programmer Bram Cohen designed the protocol in April 2001 and released the first available version on July 2, 2001. It is now maintained by Cohen's company, BitTorrent, Inc. Currently, numerous BitTorrent clients are available for a variety of computing platforms. As of January 2012, BitTorrent has 150 million active users according to BitTorrent, Inc. Based on this the total number of monthly BitTorrent users can be estimated at more than a quarter billion. At any given instant of time BitTorrent has, on average, more active users than YouTube and Facebook combined. (This refers to the number of active users at any instant and not to the total number of unique users.)

Unlike other download methods, BitTorrent maximizes transfer speed by gathering pieces of the file you want and downloading these pieces simultaneously from people who already have them. This process makes popular and very large files, such as videos and television programs, download much faster than is possible with other protocols.

A BRIEF HISTORY ON FILE SHARING

To understand how BitTorrent works and why it is different from other file-serving methods, let's examine what happens when you download a file from a Web site (Traditional Client Server Downloading).

It works something like this:

• You open a Web page and click a link to download a file to your computer.
• The Web browser software on your computer (the client) tells the server (a central computer that holds the Web page and the file you want to download) to transfer a copy of the file to your computer. 
• The transfer is handled by a protocol (a set of rules), such as FTP (File Transfer Protocol) or HTTP (HyperText Transfer Protocol).

The transfer speed is affected by a number of variables, including the type of protocol, the amount of traffic on the server and the number of other computers that are downloading the file. If the file is both large and popular, the demands on the server are great, and the download will be slow.



On the other hand, Peer-to-peer file sharing is different from traditional file downloading. In peer-to-peer sharing, you use a software program (rather than your Web browser) to locate computers that have the file you want. Because these are ordinary computers like yours, as opposed to servers, they are called peers.

The process works like this:

• You run peer-to-peer file-sharing software on your computer and send out a request for the file you want to download.
• To locate the file, the software queries other computers that are connected to the Internet and running the file-sharing software.
• When the software finds a computer that has the file you want on its hard drive, the download begins.
• Others using the file-sharing software can obtain files they want from your computer's hard drive.

The file-transfer load is distributed between the computers exchanging files, but file searches and transfers from your computer to others can cause bottlenecks. Some people download files and immediately disconnect without allowing others to obtain files from their system, which is called leeching. This limits the number of computers the software can search for the requested file.

WHAT BitTorrent DOES?

Unlike some other peer-to-peer downloading methods, BitTorrent is a protocol that offloads some of the file tracking work to a central server (called a tracker). Another difference is that it uses a principal called tit-for-tat. This means that in order to receive files, you have to give them. This solves the problem of leeching -- one of developer Bram Cohen's primary goals. With BitTorrent, the more files you share with others, the faster your downloads are. Finally, to make better use of available Internet bandwidth (the pipeline for data transmission), BitTorrent downloads different pieces of the file you want simultaneously from multiple computers.

Here's how it works:


• You open a Web page and click on a link for the file you want.
• BitTorrent client software communicates with a tracker to find other computers running BitTorrent that have the complete file (seed computers) and those with a portion of the file (peers that are usually in the process of downloading the file).
• The tracker identifies the swarm, which is the connected computers that have all of or a portion of the file and are in the process of sending or receiving it.
• The tracker helps the client software trade pieces of the file you want with other computers in the swarm. Your computer receives multiple pieces of the file simultaneously.
• If you continue to run the BitTorrent client software after your download is complete, others can receive .torrent files from your computer; your future download rates improve because you are ranked higher in the "tit-for-tat" system.

Downloading pieces of the file at the same time helps solve a common problem with other peer-to-peer download methods: Peers upload at a much slower rate than they download. By downloading multiple pieces at the same time, the overall speed is greatly improved. The more computers involved in the swarm, the faster the file transfer occurs because there are more sources of each piece of the file. For this reason, BitTorrent is especially useful for large, popular files.

IS IT LEGAL?

BitTorrent is perfectly legal to use. However, it is illegal to download copyrighted materials in most countries. So if the file you're downloading is copyrighted, then what you're doing is not legal.

Similar to other peer-to-peer software, BitTorrent can be used to download copyrighted material. Because BitTorrent handles large files remarkably well, it is especially popular for downloading video files. The Motion Picture Association of America has filed countless lawsuits, causing at least many high-traffic .torrent download sites to shut down.

BitTorrent itself is perfectly legal to use. When you select a file to download, however, it is your responsibility to make sure the file not copyrighted. BitTorrent downloads are not anonymous information about your computer's IP address and the files you download can be traced back to you.

Despite its improper use by distributors of copyrighted material, the BitTorrent program itself both legal and innovative. With additions such as tit-for-tat and an open-source philosophy, BitTorrent will likely build a legacy of its own while serving as a bridge to the next generation of file-serving software.

BitTorrent SPEAK

Like most Internet phenomena, BitTorrent has its own jargon. Some of the more common terms related to BitTorrent include:

• Leeches - People who download files but do not share files on their own computer with others
• Seed or seeder - A computer with a complete copy of a BitTorrent file (At least one seed computer is necessary for a BitTorrent download to operate.)
• Swarm - A group of computers simultaneously sending (uploading) or receiving (downloading) the same file
• .torrent - A pointer file that directs your computer to the file you want to download
• Tracker - A server that manages the BitTorrent file-transfer process

EXPENSIVE UPLOADS

Does your ISP charge for uploads? It's rare, but it's possible. Before leaving the BitTorrent client software open overnight, be sure your ISP doesn't charge for uploads -- otherwise, moving up in the tit-for-tat hierarchy could end up costing you an arm and a leg.

SOURCES

• http://computer.howstuffworks.com/bittorrent.htm
• http://en.wikipedia.org/wiki/BitTorrent_(protocol)