Ritesh’s Technical Blog

What is Bit Torrent

Posted on: July 28, 2008

Let’s start from some basics

Basic File sharing: Client-Server and P2P

Skip ahead if you know this part.

A few years ago the Internet was pretty easily divided into two groups: servers and clients. (Just like at a restaurant.) There weren’t that many servers, but their primary job was to hold web pages and files to be downloaded. They didn’t do much other than sit there and wait for people like you to request a web page or file. You, as the client, probably didn’t share any files or web pages with anyone. This segregation was primarily due to the fact that it takes a lot of outgoing bandwidth to share stuff. In a time before broadband DSL and cable modems, back in the days of dialup, most people didn’t have the bandwidth, let alone the knowhow to share stuff on their own. This old system was called client-server, appropriately enough, and it is still used today for the vast majority of stuff on the Internet, including web pages.

With the advent of broadband technologies such as DSL and cable modems, the everyday user like you suddenly has a big chunk of bandwidth, not only for download, but also upload. Sharing files directly from your computer (without first sending them to a server) is now a reality. This is where p2p comes in. The acronym p2p stands for peer-to-peer, which basically means client to client. That is, you download files from people like you instead of from big servers, and in turn they download files from you. You share your files, your friends share their files, and everyone talks directly to each other.

Introduction

Bit Torrent is a peer-to-peer file sharing (P2P) communication protocol. A Bit Torrent client is the application which uses that protocol. You can use Bit Torrent to share and download any type of file you want. It uses a principal called tit-for-tat. This means that in order to receive files, you have to give them. Bit Torrent downloads different pieces of the file you want simultaneously from multiple computers.

Unlike other download methods, Bit Torrent 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.

Developer

The protocol is the brainchild of programmer Bram Cohen, who designed it in April 2001 and released a first implementation on 2 July 2001. It is now maintained by Cohen’s company Bit Torrent, Inc.

How Bit Torrent works?

Before understanding how bit torrent works let us know some bit torrent jargon

· Seeder = A computer with a complete copy of a Bit Torrent file

Seeder’s uploads the file; to get a file you need at least 1 seed

· Leecher =People who download files but do not share files on their own computer with others (also known as a peer)

Leecher’s downloads the file uploaded by a seeder (he can also upload the file for others to download)

· Tracker = A program that keeps track of who has what and what pieces or A server that manages the Bit Torrent file-transfer process

· Swarm = the group of computers involved with any one particular torrent. Or A group of computers simultaneously sending (uploading) or receiving (downloading) the same file

The number of computers that the swarm contains is found by adding the number of seeds to the number of peers.

· Ratio = the amount you have downloaded to the amount you have uploaded.

To share a file or group of files, a peer first creates a small file called a “torrent” (e.g.” MyFile.torrent”). This file contains metadata about the files to be shared and about the tracker, the computer that coordinates the file distribution. Peers that want to download the file first obtain a torrent file for it, and connect to the specified tracker, which tells them from which other peers to download the pieces of the file.

Users browse the web to find a torrent of interest, download it, and open it with a Bit Torrent client. The client connects to the tracker(s) specified in the torrent file, from which it receives a list of peers currently transferring pieces of the file(s) specified in the torrent. The client connects to those peers to obtain the various pieces. Such a group of peers connected to each other to share a torrent is called a swarm. If the swarm contains only the initial seeder, the client connects directly to it and begins to request pieces. As peers enter the swarm, they begin to trade pieces with one another, instead of downloading directly from the seeder.

Clients incorporate mechanisms to optimize their download and upload rates; for example they download pieces in a random order to increase the opportunity to exchange data, which is only possible if two peers have different pieces of the file.

Summary

· Bit Torrent client software communicates with a tracker to find other computers running Bit Torrent 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 swarms, 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 Bit Torrent 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, Bit Torrent is especially useful for large, popular files.

The big differences with Torrents compared to other P2P networks are:

a) I don’t have to give the whole file to each person to get it out there – I can give a bit to each person

b) Once I have given a bit out, its availability has just increased, because someone else can then also give it out, and so on and so on. the speed of a torrent will generally get better as more and more people want it – unlike Soul Seek or other P2P apps where you just wait longer to get it!!

c) Torrents technically aren’t illegal. After all, a torrent is just a tiny little file with no copyrighted content. It’s a bit muddy water here though, as they can facilitate the transfer of copyrighted content.

Indexing

The Bit Torrent protocol provides no way to index torrent files. As a result, a comparatively small number of websites have hosted the large majority of torrents linking to (possibly) copyrighted material, rendering those sites especially vulnerable to lawsuits. Several types of websites support the discovery and distribution of data on the Bit Torrent network.

Public tracker sites such as The Pirate Bay allow users to search in and download from their collection of .torrent files; they also run Bit Torrent trackers for those files. Users can typically also upload .torrent files for content they wish to distribute.

Private tracker sites such as Demonoid operate like public ones except that they restrict access to registered users and keep track of the amount of data each user uploads and downloads, in an attempt to reduce leeching.

There are specialized tracker sites such as FlixFlux for films, bitme for educational content, fullcaliber.be for metal music, PureTnA for pornographic content, and TV torrents for television series. Often these will also be private.

Search engines allow the discovery of .torrent files that are hosted and tracked on other sites; examples include Mininova, Monova, BTJunkie, Torrentz and isoHunt. These sites allow the user to ask for content meeting specific criteria (such as containing a given word or phrase) and retrieve a list of links to .torrent files matching those criteria. This list is often sorted with respect to relevance or number of seeders. Bram Cohen launched a Bit Torrent search engine on http://search.bittorrent.com/ that commingles licensed content with search results.

Limitations and security vulnerabilities

Lack of anonymity

Bit Torrent does not offer its users anonymity. It is possible to obtain the IP addresses of all current, and possibly previous, participants in a swarm from the tracker. This may expose users with insecure systems to attacks.

The leech problem

Bit Torrent file sharers, compared to users of client/server technology, often have little incentive to become seeders after they finish downloading. The result of this is that torrent swarms gradually die out, meaning a lower possibility of obtaining older torrents. Some Bit Torrent websites have attempted to address this by recording each user’s download and upload ratio for all or just the user to see, as well as the provision of access to newer torrent files to people with better ratios. Users who have low upload ratios may see slower download speeds until they upload more. This prevents (statistical) leeching, since after a while they become unable to download much faster than 1-10 kB/s on a high-speed connection. Some trackers exempt dial-up users from this policy, because they cannot upload faster than 1-3 kB/s.

The cheater problem

There are “cheating” clients like BitThief which claim to be able to download without uploading, and because of this can sometimes download faster than regular clients. Such exploitation negatively affects the cooperative nature of the Bit Torrent protocol.

Legal issues

There has been much controversy over the use of Bit Torrent trackers. Bit Torrent metafiles themselves do not store copyrighted data, hence Bit Torrent itself is not illegal – it is the use of it to copy copyrighted material that contravenes laws in some locations.

Various jurisdictions have pursued legal action against websites that host Bit Torrent trackers. High-profile examples include the closing of Suprnova.org, torrentspy.org, LokiTorrent, Demonoid (now back online), OiNK.cd and EliteTorrents.org. The Pirate Bay torrent website, formed by a Swedish anti-copyright group, is notorious for the “legal” section of its website in which letters and replies on the subject of alleged copyright infringements are publicly displayed. On May 31, 2006, The Pirate Bay’s servers in Sweden were raided by Swedish police on allegations by the MPAA of copyright infringement; however, the tracker was up and running again three days later.

HBO, in an effort to combat the distribution of its programming on Bit Torrent networks, has sent cease and desist letters to the Internet Service Providers of Bit Torrent users. Many users have reported receiving letters from their ISPs that threatened to cut off their internet service if the alleged infringement continues. HBO, unlike the RIAA, has not been reported to have filed suit against anyone for sharing files as of April 2007. In 2005 HBO began “poisoning” torrents of its show Rome, by providing bad chunks of data to clients.

On November 23, 2005, the movie industry and Bit Torrent Inc. CEO Bram Cohen, signed a deal they hoped would reduce the number of unlicensed copies available through bittorrent.com’s search engine, run by Bit Torrent, Inc. It meant BitTorrent.com had to remove any links to unlicensed copies of films made by seven of Hollywood’s major movie studios.

There are two major differences between Bit Torrent and many other peer-to-peer file-trading systems, which advocates suggest make it less useful to those sharing copyrighted material without authorization. First, Bit Torrent itself does not offer a search facility to find files by name. A user must find the initial torrent file by other means, such as a web search. Second, Bit Torrent makes no attempt to conceal the host ultimately responsible for facilitating the sharing: a person who wishes to make a file available must run a tracker on a specific host or hosts and distribute the tracker address(es) in the .torrent file. Because it is possible to operate a tracker on a server that is located in a jurisdiction where the copyright holder cannot take legal action, the protocol does offer some vulnerability that other protocols lack. It is far easier to request that the server’s ISP shut down the site than it is to find and identify every user sharing a file on a peer-to-peer network. However, with the use of a distributed hash table (DHT), trackers are no longer required, though often used for client software that does not support DHT to connect to the stream.

Some Important Things to Remember

Keep seeding till you achieve at least a 1:1 RATIO!!!

Once you have the file, it’s no good just closing your client and forgetting about it. In order to allow it to be shared for others you MUST continue to seed.

You are not invisible when you are in a swarm. Others, including copyright owners if they so wished, can see your IP when you are downloading or uploading a file.

If you use a torrent site which has a forum attached, take the time to say thanks to the poster of the torrent. It only takes a moment.

The speed at which you download is determined by things such as the Seed to Peer ratio, the seed(ers) upload bandwidth and of course your available download bandwidth. Try to download torrents that have lots of seeders and few peers in order to get faster speeds.

Some good sites

1.Public tracker sites The Pirate Bay.org (my best site)

2.Private tracker sites Demonoid (for registered users)

3.FlixFlux for films

4.bitme for educational content

5.fullcaliber.be for metal music

6.PureTnA for pornographic content

7.ICTorrent.com a forum based torrent site also contains a large no of useful torrents.

Last words

Bit Torrent 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. Also never upload any copyrighted material on net and never seed any such file. If you have read the above article carefully then you will notice that your IP Addresses are not hidden. Help to protect n prevent PIRACY.

References

1. HowStuffWorks.com

2. Wikipedia

3. Basic File sharing: Client-Server and P2P: Don’t remember the source it is somewhat like tweaking Bit torrent and so on…

4. Some Important Things to Remember: Taken from ICTorrent.com Forums but don’t remember the author.

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