The BitTorrent protocol

1. The BitTorrent protocol A peer-to-peer file sharing protocol.

2. Problem Addressed • How do we transfer a piece of data quickly to a group of people interested in it. • Web server • Distributors • Redhat patches • Soln: BitTorrent based file swarming

3. Philosophy • Author: Bram Cohen • Based on Tit-for-tat • Incentive - Uploading while downloading • Pieces of files

4. Tracker Web Server Web page with link to .torrent .torrent C A Peer [Seed] Peer [Leech] Downloader “US” B Peer [Leech] Overall Architecture

5. Tracker Web Server Web page with link to .torrent Get-announce C A Peer [Seed] Peer [Leech] Downloader “US” B Peer [Leech] Overall Architecture

6. Tracker Web Server Web page with link to .torrent Response-peer list C A Peer [Seed] Peer [Leech] Downloader “US” B Peer [Leech] Overall Architecture

7. Tracker Web Server Web page with link to .torrent Shake-hand C A Peer [Seed] Peer [Leech] Downloader “US” Shake-hand B Peer [Leech] Overall Architecture

8. Tracker Web Server Web page with link to .torrent pieces C A pieces Peer [Seed] Peer [Leech] Downloader “US” B Peer [Leech] Overall Architecture

9. Tracker Web Server Web page with link to .torrent pieces C A pieces Peer [Seed] Peer [Leech] Downloader “US” pieces B Peer [Leech] Overall Architecture

10. Tracker Web Server Web page with link to .torrent Get-announce Response-peer list pieces C A pieces Peer [Seed] Peer [Leech] Downloader “US” pieces B Peer [Leech] Overall Architecture

11. Messages • Peer – Peer messages • TCP Sockets • Peer – Tracker messages • HTTP Request/Response • B-encoding

12. .torrent • url of the tracker • Pieces <hash1,hash2,….hashn> • Piece length • Name • Length • Files • Path • length

13. Tracker • Peer cache • IP, port, peer id • State information • Completed • Downloading • Returns random list

15. Peer Operation • Choking algorithm • Choke/Unchoke • Preferred peers • Optimistic unchoke • Snubbing behavior • Prevented by Anti-snubbing. • Upload to interested peers who are not choking.

16. Peer Operation • Verify on receiving complete piece • Endgame Behavior • Cancel

17. Experimental Client

19. Strengths • Better bandwidth utilization • Never before speeds. • Up to 7 MB/s from the Internet. • Limit free riding – tit-for-tat • Limit leech attack – coupling upload & download • Spurious files not propagated • Ability to resume a download

20. Drawbacks • Small files – latency, overhead • Random list of peers - naive • Scalability • Millions of peers – Tracker behavior (uses 1/1000 of bandwidth) • Single point of failure • Robustness • System progress dependent on altruistic nature of seeds (and peers) • Malicious attacks and leeches.

21. MNL’s Interest • Robustness • Devise possible attacks and find loopholes • Secure against attacks • Scalability • Sharing across files • Millions of users • Possible extensions • Peer optimization • Upload just enough to maximize your download. • Intelligent tracker • Peer selection policy • Dead sites • Share html content

22. Optimism - hack • Connect to as many peers as possible • Benefit • Optimistic unchoking • Probability of free download increases

23. download upload Peer upload optimization • Upload enough to stay within the preferred list of as many peers as possible • Get the best download deal with minimum required upload

24. Smarter tracker • Classify peers • Pick at random from each of these subsets • No compromise on random choice • Will lead to a more progressive peer network.

25. Interesting links • Official site: http://bitconjurer.org/BitTorrent • BitTorrent FAQ: http://btfaq.com • Torrent sites • http://f.scarywater.net • http://www.suprnova.org • http://tvtorrents.com Remember • leave your download windows open • Big brother is watching!