BitTorrent is a peer-to-peer protocol for distributing large files across the Internet. It allows users to download files from multiple sources at once, reducing bandwidth strain on any single source. The protocol works by breaking files into pieces that can be downloaded in any order from other users who already have pieces of the file. This "swarming" approach scales well as more users join a download. BitTorrent has proven highly effective for distributing popular content to millions of users and accounts for a significant portion of Internet traffic today.

1. The BitTorrent Protocol

2. Common ScenarioMillions want to download the same popular huge files (for free)SoftwaresMedia (the real example!)Client-server model failsSingle server failsCan’t afford to deploy enough servers

3. Client-ServerOverloaded!SourceRouter“Interested” End-host

4. Peer-to-PeerA model of communication where every node in the network acts alike.

5. As opposed to the Client-Server model, where one node provides services and other nodes use the services. Advantages of P2P ComputingNo central point of failureE.g., the Internet and the Web do not have a central point of failure.Most internet and web services use the client-server model (e.g. HTTP), so a specific service does have a central point of failure.ScalabilitySince every peer is alike, it is possible to add more peers to the system and scale to larger networks.Disadvantage of P2P ComputingDecentralized coordination.

6. All nodes are not created equal.BitTorrentWritten by Bram Cohen in 2001

7. Designed to transfer large files

8. 160 million clients, 100 million active users

9. Used by many different people and organisations

10. The more popular a large video, audio or software file, the faster and cheaper it can be transferred with BitTorrent“Pull-based” “swarming” approachEach file split into smaller piecesNodes request desired pieces from neighborsAs opposed to parents pushing data that they receivePieces not downloaded in sequential orderEncourages contribution by all nodesPeer-to-peer in natureEven if clients join simultaneously (“flash crowd”)BitTorrent protocol is implemented in applications called BitTorrent Clients such as uTorrent, Bit Comet.

11. BitTorrent TerminologyPeers – A node or computer that does not have the complete fileSeed or seeder - A computer with a complete copy of a BitTorrent fileSwarm- 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

12. BitTorrent SwarmSwarm

13. Set of peers all downloading the same file

14. Organized as a random mesh

15. Each node knows list of pieces downloaded by neighbors

16. Node requests pieces it does not own from neighbors

18. A *.torrent guides users to owners of a fileUser obtains *.torrent file. File contains meta info about a target file. 1Armed with a list of peers holding pieces of the file, user downloads from many peers4User loads *.torrent file into BitTorrent client, which then looks up the named client23Tracker coordinates peers.

19. All peers act as a sourceA machine with a complete copy (the seed) can distribute incomplete pieces to multiple peersSeedPeers exchange different pieces of the file with one another until they assemble a wholeAs soon as the user has a piece of the file on his machine, he can become a source of that piece to other peers, helping speed download

20. The key ingredients of the *.torrent file are the tracker’s address and the unique SHA1 hash All data in a metainfo file is encoded. info: a dictionary that describes the file(s) of the torrent. announce: contains the URL of the “tracker”creation dateComments from the author(optional)created by: (optional)piece length: number of bytes in each piece (integer)pieces: string consisting of the concatenation of all 20-byte SHA1 hash values, one per piece

21. Bit Torrent Download Download and install the BitTorrent client software

22. Check and configure firewall and/or router for BitTorrent (if applicable)

23. Find files to download

24. Download and open the .torrent pointer file

25. Let BitTorrent give and receive pieces of the file

26. Stay connected after the download completes to share your .torrent files with othersUpload and Publish File Download and install the BitTorrent client software

27. Create a New .torrent filePublish the .torrent file on torrent search Index sites such as PirateBay

28. Peer-peer transactions:Choosing pieces to requestRarest-first: Look at all pieces at all peers, and request piece that’s owned by fewest peersIncreases diversity in the pieces downloadedavoids case where a node and each of its peers have exactly the same pieces; increases throughputIncreases likelihood all pieces still available even if original seed leaves before any one node has downloaded entire file