How the BitTorrent protocol works, along with PHP code on how to write a torrent tracker.

1. Introduction to the BitTorrent Protocol Tommy Montgomery // March 2, 2010

2. What is BitTorrent?

3. What is BitTorrent? NOT illegal

4. What is BitTorrent? NOT illegal Peer-to-peer (P2P) file transfer

5. What is BitTorrent? NOT illegal Peer-to-peer (P2P) file transfer Awesome

6. What is BitTorrent? NOT illegal Peer-to-peer (P2P) file transfer Awesome

7. Poppycock! BitTorrent historically has a bad reputation, due to its decentralized method of file transfer This makes it difficult to “remove” a download, such as one that is illegal BitTorrent is not inherently evil People are inherently evil I like bullet points

8. How does it work?

9. How does it work: Consumer Download torrent: lots-o-cool-stuff-n-things.lulz.torrent Open it in your favorite client:

10. How does it work Files are downloaded piecemeal, from many different places instead of just one Trackers are the central hub of control for each torrent Each tracker manages your “peers”, which are other people who are trying to download the same file

11. Glossary

12. Glossary Seeder

13. Glossary Seeder Someone who has finished downloading and is only uploading

14. Glossary Seeder Someone who has finished downloading and is only uploading Leecher

15. Glossary Seeder Someone who has finished downloading and is only uploading Leecher Someone who is downloading

16. Glossary Seeder Someone who has finished downloading and is only uploading Leecher Someone who is downloading Peer

17. Glossary Seeder Someone who has finished downloading and is only uploading Leecher Someone who is downloading Peer The collective term for seeders and leechers

18. What is a torrent file? A Bencoded metadata file containing information about one or more files Name/size/date of each file # of “pieces” of each file SHA1 hash of each piece Trackers for the torrent

19. What is a “piece”? Torrents are made up of many small pieces You never download all the pieces from one place (unless there’s only one seed) Pieces are usually less than a couple megabytes each

20. Bencoding Dictionaries Prefixed with “d”, end with “e” Lists Prefixed with “l”, end with “e” Integers Prefixed with “i”, end with “e” Strings Prefixed with string length + “:”

21. Bencoding d3:foo3:bar4:lulzi8el5:hello5:worldee

24. String

26. String

27. Integer

29. Trackers HTTP service that responds to GET requests Response is text/plain Bencoded dictionary Usually listens on port 6969

30. Tracker Announce Request Usually has a billion GET params: info_hash peer_id port uploaded downloaded left compact no_peer_id event ip numwant key trackerid

31. Tracker Announce Request Important GET params: info_hash the SHA1 hash of the piece to download peer_id unique identifier of the person doing the downloading ip/port the IP address and port of the peer uploaded/downloaded/left used for calculating “share ratio”, i.e. how much the tracker likes them

32. Tracker Announce Response A Bencoded dictionary with these keys: complete incomplete tracker id (yes, that is a space) peers warning message failure reason (only if an error occurred) interval min interval

33. Tracker Announce Response The relevant parts: peers Dictionary containing each peer’s IP address and port Or, if the client wants it compact, a binary string of each peer’s IP address and port interval/min interval the interval in seconds which the client is allowed to make a request complete/incomplete number of seeders/leechers

34. Tracker Implementation PHP/MySQL/Apache Why not?

35. First, of course… We need a Bencoding library. Let’s roll our own.

37. yay.

38. It’s tracker time We need a way to keep track of peers, and who’s downloaded what and stuff Let’s use MySQL

40. It’s tracker time Request is pretty boring, as it’s just a glorified array with error handling

41. It’s tracker time Response is a Bencoded dictionary, and is also a glorified array

43. It’s tracker time Now we can set up an exception handler that will send a proper response back in the event of a catastrophe

44. It’s tracker time Now we can make the tracker announce the peers

47. Set up the torrent Now, stuff your tracker on a server somewhere, listen on port 6969 Then add your tracker’s announce URL to a torrent e.g. http://likeaboss.biz:6969/announce

48. Other stuff Trackers are also facilitators and the supreme overlords over the torrents they track The list of peers that gets sent back to the client determines who downloads from whom They will use download/upload statistics to determine who isn’t uploading/seeding, and punish them by giving them crappy peers

49. Open source trackers OpenTracker (used by what’s left of The Pirate Bay) XBT Tracker – implemented using UDP instead of HTTP for ultimate efficiency MonoTorrent – cross-platform C# implementation (client as well) PeerTracker – PHP 5 implementation

50. In short… Unless you want to implement some weird rules for who gets to download stuff, don’t write your own. I only did it for fun. Because it’s fun.

51. Questions?