BitTorrent Protocol
- 1. Peer to Peer file transfer protocol Transfer of large files Files are shared by many users Active participation of all users Huge success in the file sharing domain 35% of internet traffic Introduction
- 2. Striking Characteristics Not dependent on a single source A central server is not needed No single point of failure More the number of users, greater will be the transfer rate Data available in pieces, not as a single large file Tit for tat strategy
- 3. Terminology Torrent Peer Seed Leecher Swarm Reseed Tracker Share Ratio
- 5. HTTP Peer – Tracker TCP Peer – Peer Components of BitTorrent System Metainfo File Data Tracker Peers Client
- 6. Metainfo File Contains all information about a torrent File with a .torrent extension It has the following keys info announce creation date comment created by Keys are encoded before they are sent
- 7. Encoding scheme used by BitTorrent is called as Bencoding Hash of all the pieces are present in info field of metainfo file File Distribution Files are uploaded in public sites by seeds Users download this file via HTTP and can participate in the torrent transfer
- 8. Data Data can be a single file or multiple files contained in a directory Data is split into many pieces of equal size Each piece is further divided into many blocks A piece will have a hash, needed for data integrity check Common piece sizes are 256kb, 512kb and 1mb
- 9. Tracker Manages users participating in a torrent Helps peers find each other It is a HTTP service that works on port 6969 Peers request tracker for other peers having the required pieces Tracker responds with a list of peers that have the requested piece A tracker can manage many torrents
- 11. HTTP GET message from a peer to a tracker has the following parameters info_hash peer_id uploaded downloaded left port Response message from tracker has the following keys interval tracker_id peers
- 12. Scraping Querying the state of a given torrent Tracker responds with a scrape page Announce URL : http://example.com/announce Scrape URL : http://example.com/scrape Scrape page has the following keys complete incomplete downloaded
- 13. Peers Ports 6881 – 6889 are used by peers Peers speak TCP Following are the key strategies used by peers while sharing files Random First Piece Rarest First Endgame Mode Peer Distribution Choking Optimistic Unchoking
- 14. Peers perform a handshake to start the communication Handshaking involves these messages Character 19 followed by ‘BitTorrent Protocol’ 20 bytes hash of the info field from metainfo file 20 bytes peer id Peers start talking to each other They pass a number of messages between them to send data and control the interactions
- 15. Client Executable program running on user’s machine Co ordinates with OS to perform read write operations A .torrent file must be opened by the client Peers with same client perform better
- 16. Sub Protocols Tracker HTTP Protocol (THP) Client sends GET request to tracker URL Tracker responds with a dictionary having all the information Client sends requests at regular intervals Peer Wire Protocol (PWP) Peers keep track of each others state A state has two values, interested and choking
- 17. Working of BitTorrent 1. Seeder generates a torrent file and uploads torrent to a web server. 2. The seeder notifies the tracker that it is sharing the file described in the torrent file.
- 18. 3. A leecher downloads the torrent file from the web server. 4. The leecher connects to the tracker specified in the torrent file.
- 19. 5. The leecher connects to its peers to retrieve pieces of the files.
- 20. Attacks on BitTorrent Pollution Attack The peers receive the peer list from the tracker. One peer contacts the attacker for a chunk of the file. The attacker sends back a false chunk. Attacker requests all chunks from swarm and wastes their upload bandwidth.
- 22. DDOS Attack The attacker downloads a large number of torrent files from a web server. Attacker spoofs IP address and port with that of victim and notifies the tracker Tracker directs peers towards victim Victim will be flooded with requests from other peers
- 24. Bandwidth Shaping This is done by user’s ISP Unencrypted BitTorrent packets are easily identified and filtered. Sophisticated filtering software can detect BitTorrent like behavior. Comcast has recently admitted to filtering BitTorrent traffic.
- 25. Current Solutions Pollution Attack Blacklisting Achieved using software such as Peer Guardian or moBlock. Blocks connections from blacklisted IPs which are downloaded from an online database.
- 26. DDOS Attack Spoofing needs to be avoided in the first place This can be done by using filters Make the tracker validate a peer whether it has the torrent or not
- 27. Bandwidth Shaping Encryption Most BitTorrent clients can encrypt the packets they send. Simple filters which simply look at the contents of the packet can easily be traversed Tunneling Using VPN software to connect to an unfiltered network. Such tunnels which are free from filters provide easy path to BitTorrent packets
- 28. Other File Transfer Methods Typical HTTP file transfer Download Accelerator Plus (DAP) Other Peer to Peer systems Kazaa, eDonkey, Direct Connect ….
- 29. Conclusion BitTorrent is a popular P2P technology deployed across the Internet. The protocol has found a niche as a preferred method for the decentralized distribution of large files. It provides data integrity which ensures that the data will always be genuine and free from flaws. BitTorrent uses tit for tat exchange to increase cooperation among peers and has developed a reputation for being efficient, scalable, and robust.