- Clone is repository on your local unix machine and step in the main directory
$ git clone https://github.com/kishanpatel22/bittorrent.git
$ cd bittorrent
- If you don't have pipenv package then install it using command, else ignore below command
$ pip3 install pipenv
- Enter the virtual enviornment and install all dependencies
$ pipenv shell
$ pipenv install --dev
- Change directory to source in bittorrent folder and inorder to display help options
$ cd src
$ python3 main.py --help
- Example for downloading torrent file given destination path for downloading file
$ python3 main.py input_file.torrent -d destination_path/
- Example for seeding torrent file given given destination path of existing file
$ python3 main.py input_file.torrent -s existing_file_path/
- Downloading movie, music perhaps game or very large size software if pretty fun activity using Bittorrent communication protocol which helps in distributing large chunks of data over Internet. Fact is that one third of internet traffic contains Bittorrent data packets, which makes it one of most interesting and trending topics.
-
Traditional client-server model is obviously not scalable when many clients are requesting single server, this leads us to P2P architecture in which instead of centrailzed server, peers or clients or end hosts itself participate in distributing data among themself.
Peer-to-Peer Architecture 
-
Bittorrent is protocol defined for sharing/exchange of data in peer to peer architecture designed by designed by Bram Cohen. The purpose of Bittorrent Protocol or BTP/1.0 was to achieve advantage over plain HTTP when multiple downloads are possible. I would recommend reading this page which gives details about BTP/1.0
-
Amazon Web Services(AWS) Simple Storage Service(S3) is scalable internet based storage service with interface equipped with built-in Bittorrent.
-
Facebook uses BitTorrent to distribute updates to Facebook servers.
-
Open-source free software support their file sharing using Bittorrent inorder to reduce load on servers. Example - ubantu-iso files.
-
Government of UK used Bittorrent to distribute the details of the tax money spend by the cittizens.
Fun Fact - In 2011, BitTorrent had 100 million users and a greater share of network bandwidth than Netflix and Hulu combined.
-
Bittorrent protocol is application layer protocol, since it replies on the services provided by the lower 4 levels of TCP/IP Internet model for sharing the data. It is the application layer where the Bittorrent Client is written such that following a set of rules or protocols it will exchange the data among the peers(ends hosts).
-
In the project, I have made bittorrent client application program in python3, which follows the Bittorrent protocols for downloading and uploading files among serveral peers.
| Entity Names | Significance |
|---|---|
| .torrent file | contains details of trackers, file name, size, info hash, etc regarding file being distributed |
| Tracker | It keeps state of the peers which are active in swarm and what part of files each peer has |
| peers | End systems which may or may not have compelete file but are participating the file distribution |
| PWP | protocol that end system need to follow inorder to distribute the file among other peers |
-
Peers participating in file sharing lead into dense graph like structure called swarm. The peers are classified into two types leechers(one who download only) and seeders(one who upload only).
-
The large file being distributed is divided into number of pieces which inturn is divided into number of different chunks/blocks. The data chunks/blocks are actually shared among the peers by which the whole file gets downloaded.
Distribution of large file in pieces 
-
Parising .torrent : Get .torrent file after that read the .torrent file and decode the bencoded information. From the extracted information get Tracker URLs, file size, name, piece length, info hash, files(if any), etc.
-
Tracker : Communicate with the trackers inorder to know which peers are participating in the download. The tracker response contains the peer information which are currently in the swarm.
-
PWP : Peer wire Protocol (PWP) is used to communicate with all the peers and using peer wire messages(PWM) and download file pieces from the peers.
-
Files have extention .torrent and contain data about trackers URL, file name, file size, piece length, info hash and some additional information about file being distributed.
-
Torrent files are bencoded thus one requires to parse the torrent file and extract the information about the orginal file being download. Thus bittorrent client needs write parser for decoding torrent files.
-
Torrent files can be generated for single/multiple files being distributed. The given below image is output of the torrent parser, however note the torrent file may contain much more data value than as displayed in image.
Torrent file bencoded data Torrent file decoded data 
-
Bittorrent client needs to know which all peers are participating in swarm for distributing the file. The Tracker is server which keeps the track of all the peers joining, leaving or currently active in the swarm.
-
Tracker also knows how many seeders and leetchers are prenent in the swarm for distributing a pariticular torrnet file. However note that the tracker doesn't actual have the torrent file.
Centrailized Tracker 
-
- Trackers which speak the HTTP protocol for responding the swarm(all the peers) information.
- Client needs to do HTTP get request to the tracker and recieved HTTP response containing information about the swarm.
- Such type of tracker introduces significant overhead when dealing with multiple clients at time.
-
- Trackers which speak UDP protocol, such type of trackers are optimized and provide less load on tracker severs.
- Since UDP is stateless and provides unreliable service to clients we need to make multiple request to tracker.
- Client needs to communicate twice to the UDP tracker, once for connection ID and subsequently using that connectionID recieved for annouce request for getting swarm information.
-
-
Trackers urls in torrent files can be absoulte, so client needs to communicate with all trackers unless it recieves swarm data.
-
A typical tracker response contains random 50 or less peers in the swarm along with some additional parametes as given below.
Tracker Response 
-
According to BTP/1.0 PWP facilitates the exchange of pieces of the file. The Bittorrent Client maintains the state information for each connection that it has with a remote peer.
States Signifance am choking client is choking the peer peer choking peer is choking the client am interested client interested in peer peer interested peer interested in client -
Choking here means peer or client being choked will not recieve messages for their request until unchoke message is recieved.
peer choking peer unchoking 
-
Initial state of client for downloading will be peer choking = 1 and client interested = 0, similarly for uploading the state will be client choking = 1 and peer interested = 0. Now we start communicating with peer with these states using Peer Wire Messages (PWM)
-
All the peer wire message are send over TCP connection which provides inorder and reliable bidirectional data communication.
-
Question : The peer IP and port addresses recieved from tracker will be behind the NATED gateway how are you going to establish TCP connection ?
Message Signifance handshake Initial message to be shared after TCP connection keep alive message indicating that the peer connection is still active choke peer is choking client unchoke peer is unchoking client interested peer is interested in client uninterested peer is uninterested in client bitfield pieces that peer has have piece that peer has request request for piece to peer piece piece data response from peer -
Along with these there are few more PWM which are not discussed since they are not implemented in the code. The ideal sequence or exchange of message after establishing successful TCP connection are given below
| Message Sequnece | Client side | to/from | Peer side |
|---|---|---|---|
| 1 | Handshake request | ---> | |
| 2 | <--- | Handshake response | |
| 3 | <--- | Bitfield | |
| 4 | <--- | Have(optional) | |
| 5 | Interested | ---> | |
| 6 | <--- | Unchoke/Choke | |
| 7 | Request1 | ---> | |
| 8 | <--- | Piece1 | |
| 9 | Request2 | ---> | |
| 10 | <--- | Piece2 | |
| k | ... | ---> | |
| k + 1 | <--- | ... |
-
However this is ideal case where all the validation of peer wire message is successful and peer TCP connection is not closed. Example peer could send choke message in that case subsequent message will get not reply, and sometimes even handshake message are invalid !
-
Inorder to solve such issues Downloading Finite State Machine is designed taken into consideration the order in which the messages are exchanged and client state. The Finite state machine with each state having timeout is implemented as given below
Finite State Machine for downloading 
-
Typical sequence of event for the downloading woudl look like as given below.
Sequence of messages in Downloading 
-
Question : How to write into file after receiving the piece message ? The solution is initialize the file with the size with null values and as and when you recieve any of the pieces from the peers write at the proper location in the file. What to generate is file with many holes.
-
Firstly torrenting is Legal ! It is the protocol used in peer to peer architecture, but the problem is in the content that we share on torrent that needs to monitored and must hold permission to share by owner.
-
Copyrighted contents like any new NetFlix web series, any new movie, songs, some gamming softwares, etc generate many legal issues since first of all downloading such copyrighted content is illegal (owner can sue you). And it is actually very hard to monitor over internet the copyrighted bittorrent traffic, there is even no way to know which people have copy righted content.
-
Make sure whatever you download by Bittorrent doesn't have such issues, and lastly enjoy torrenting !.