The document provides an overview of the BitTorrent protocol, a peer-to-peer file sharing system that enables users to download and share files by connecting to multiple peers instead of a single server. Key components of the BitTorrent architecture include torrents, peers, and trackers, and the protocol incorporates strategies for efficient data sharing such as piece selection and tit-for-tat cooperation. It concludes that BitTorrent effectively addresses scalability and optimization issues in file distribution.

1. BitTorrentPresented By : Seminar Guide:Ashish Kumar Ms. Preetha M KurupRoll no.: 17Cs – ‘ A ’

2. Contents:IntroductionWorking of BitTorrentTerminologyArchitecture of bitTorrentSimple exampleConclusion References

3. IntroductionBitTorrent is a peer to peer communication protocol for file sharing.The data to be shared is divided into many equal sized portions called pieces.Each piece is further sub-divided into equal-sized sub-pieces called blocks.All clients interested in sharing the same data are grouped into a swarm.It does not require a user to download a file completely from a single server.

4. IntroductionInstead a file can be downloaded from many such users who are indeed downloading the same file.A user who has the complete file, called the seed will initiate the download by transferring pieces of file to the users.Once a user has some considerable number of such pieces of a file then even he can start sharing them with other users who are yet to receive those pieces.Pieces are not downloaded in sequential order.

5. IntroductionThis concept enables a client not to depend on a server completely .It also reduces the overall load on the server.

6. TerminologyTorrent : refers to the small metadata file you receive from the web server.Peer : A peer is another computer on the internet that you connect to and transfer data.Leech: peer that’s downloading the fileSeed: A computer that has a complete copy of a certain torrent. Swarm: The group of machines that are collectively connected for a particular file.

7. TerminologyTracker : A server on the Internet that acts to coordinate the action of BitTorrent clients. The clients are in constant touch with this server to know about the peers in the swarm.Share ratio : This is ratio of amount of a file downloaded to that of uploaded. Choked: It is a state of an uploader where he does not want to send anything on his link. In such cases, the connection is said to be choked.

8. Swarm -snapshot

9. Working of BitTorrentBitTorrent protocol shares data through what are known as torrents.For a torrent to be alive or active it must have several key components to function. These components include a tracker server, a .torrent file, a web server where the .torrent file is stored and a complete copy of the file being exchanged. The first step in the BitTorrent exchange occurs when a peer downloads a .torrent file from a server.

10. Working of BitTorrentThe role of .torrent files is to provide the metadata that allows the protocol to function.The .torrent files contains :The URL of the tracker

11. Pieces <hash1, hash 2,…, hash n>

12. Piece length

13. Name of the file

14. Length of the fileWorking of BitTorrentWhen a .torrent file is opened by the peer’s client software, the peer then connects to the tracker server responsible for coordinating activity for that specific torrent.

15. TrackerWeb Server3214Peer2Peer1556Client6Fig: A Typical BitTorrent System

16. BitTorrent: publishing a fileHarry Potter.torrentBobWeb ServerTrackerDownloader:ASeeder:BDownloader:C

17. Architecture of BitTorrentThe BitTorrent protocol can be split into the following five main components:MetainfoFile - a file which contains all details necessary for the protocol to operate.Tracker - A server which helps manage the BitTorrent protocol.Peers - Users exchanging data via the BitTorrent protocol.Data - The files being transferred across the protocol

18. Architecture of BitTorrentClient - The program which sits on a peers computer and implements the protocolPeers use TCP to communicate and send dataA tracker is a HTTP/HTTPS service and typically works on port 6969. BitTorrent clients communicate with the tracker using HTTP GET requests, which is a standard CGI method.BitTorrent uses ports 6881-6889 to send messages and data between peers.

19. TrackerPeer1Peer3Peer4Peer243210I have piecesI have piecesTrackerDoes anyone have piece 1yes, peers 1 and 4

20. Piece SelectionThere are three stages of piece selection, which change depending on which stage of completion a peer is at.Random First PieceWhen downloading first begins, as the peer has nothing to upload, a piece is selected at random to get the download started.Rarest FirstWhen a peer selects which piece to download next, the rarest piece will be chosen from the current swarm.

21. Piece SelectionEnd-game mode:When requests sent for all sub-pieces, (re)send requests to all peers.To speed up completion of downloadCancel request for downloaded sub-pieces

22. Tit-for-tat as incentive to uploadWant to encourage all peers to contributePeer A said to choke peer B if it (A) decides not to upload to BEach peer (say A) unchokes at most 4 interested peers at any timeThe three with the largest upload rates to AWhere the tit-for-tat comes inAnother randomly chosen (Optimistic Unchoke)To periodically look for better choices

23. Simple example{1,2,3,4,5,6,7,8,9,10}Seeder: A{}{1,2,3}{1,2,3,5}{}{1,2,3}{1,2,3,4}{1,2,3,4,5}Downloader CDownloader B