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Bit torrent is a peer to peer (P2P) file sharing protocol used for distributing large files over internet.

It is not a software.The software that one use to download files through Bit torrent protocol is called Bit Torrent Client.

Some common Bit Torrent Clients are Utorrent,limewire,frostwire,flashget,etc.Among all Utorrent is the most commonly used Bit Torrent Client.

Files being distributed  are divided into segments called pieces.When a peer receives a new piece of file then that peer become a source of that new piece of file to the other peers.Each piece is protected by a cryptographic hash contained in the torrent descriptor file.

How does Peer to Peer File Sharing Works?

A Peer is a pc running any one of the bittorrent clients.

In peer to peer file sharing every bittorrent client is potential to create,request,and circulate any computer file over network via bittorrent protocol.

In this file sharing a peer first creates a very short file called torrent in order to share a file or group of file.This short file,i.e. torrent,contains details about the file or files to be shared,so-called metadata, and about the trackers.

A Tracker is a server that helps the peers to communicate with each other via bittorrent protocol.

A peer that wants to download a file must first download the torrent file of that particular file,and then connect to the specified tracker.

Is this Bittorrent Protocol download is different from https or FTP download ?

Yup! Bittorrent Protocol download is different from the https/FTP download.

Bittorrent protocol download make small data request over various TCP connections to different machines whereas https/FTP request data over single TCP connections to single machine.

Bittorrent protocol download  is done in a random manner whereas https/FTP download is done in a sequential manner i.e. one after another.

Pros of Bit Torrent Protocol
1)
Transferring files via torrent is, in most cases, much faster than downloading files from a single host. This is because the demand for data, or bandwidth, is distributed evenly amongst many different users.
Imagine a group of workers and a single worker are trying to drag an immense block. While a single worker may be able to shift a block inches at a time under great strain, hundreds of workers working together will be able to move a large block with relative ease.
2)
The tracking system of the disparate pieces of file also allow for easily resumed transfers.If the transfers is interrupted by some reason we can still resume the transfer from where we left off.In this way one can download very large files over several sessions without letting their computer on for huge amount of time.
3)
The deconcentration of files makes the retrieval of files more reliable. In cases where a file is only hosted on one machine, an outage of that particular server makes the retrieval of that file an impossibility. Meanwhile, with a torrent, if one machine goes down, users can easily retrieve the needed piece from another machine.

A Fact
Bittorrent protocol download takes time to reach the maximum speed as it takes time for enough peer connections to be established.

In bittorrent protocol download speed can varies during the downloading process.

https/FTP download reach the maximum speed very quickly.

In https/FTP download speed remain almost constant throughout the downloading process.

Only a few sites or servers support p2p download as it requires peers to be available all the time but in https one server is online all the time.

Hope, that this might helped you in getting the fundamentals of Bittorrent Protocol.Thanks!!!