1 / 44

The Bittorrent Protocol

The Bittorrent Protocol. A Technical Look. Presented by Cody Sand. What is Bittorrent ?. Peer-to-Peer (P2P) File Transfer Protocol Centralized Discovery, Decentralized Distribution Currently uses TCP transfer.

akasma
Download Presentation

The Bittorrent Protocol

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. The Bittorrent Protocol A Technical Look Presented by Cody Sand

  2. What is Bittorrent? • Peer-to-Peer (P2P) File Transfer Protocol • Centralized Discovery, Decentralized Distribution • Currently uses TCP transfer. • Designed to “facilitate file transfers among multiple peers across unreliable networks. “ • Tit-for-Tat protocol • Any downloaders MUST upload, discourages “leeching”. • Uses • Content Distribution (Downloading) • Music, Movies, Software • Valve/Blizzard Downloads • Streaming video (beta)

  3. History • Designed by Bram Cohen in 2001. • Assisted in Development of Valve’s Steam platform, before focusing on Bittorrent fulltime. • First implementation released in July 2001. • Accounted for roughly 33% of all internet traffic today. • Nearly 100,000,000 users worldwide.

  4. How it works • Peer distributing the file breaks it into small pieces, typically 64k – 1MB in size. • Pieces over 512k tend to decrease protocol efficiency • Peer creates a checksum for each piece and records it in a file (a .torrent file) • When another peer receives a piece, it is checked against this file for integrity purposes. • This model allows uploading/downloading of one or more files by many peers.

  5. Why Bittorrent? • For large, popular files, more customers require more bandwidth. • Bandwidth costs go up. • Bittorrent’s solution: • Unload some distribution to users • Allows for savings in bandwidth and costs.

  6. Structure • Ordinary Web Server • A static metainfo file (.torrent). • A bittorrent tracker • An original “downloader” or the “seed” • Original content provider • Web Browser • Consumer (Downloader) with client software.

  7. Web Server / Bittorrent Tracker • Web Server • Hosts the tracker, which is the most essential piece. • Trackers implemented to run with all major servers. • Tracker • Assists communication between peers. • Responds to HTTP Get Requests. • Used to track peers, download statistics.

  8. Tracker GET Requests • Standard HTTP Get Request • Parameters • info_hash – 20byte SHA-1 hash code to ID torrent file • peer_id – 20 character string, identifies user/client. • ip – IP address of peer. • port – the port the client is listening on. • uploaded – total amount uploaded so far. • downloaded – total amount downloaded so far • left – number of bytes left to download • event – state of torrent (ie. started, completed, stopped) • numwant – Optional key stating the number of desired peers. default is 50.

  9. Example GET Request http://some.tracker.com:999/announce ?info_hash=12345678901234567890 &peer_id=UTABCXYZ &ip=174.250.2.22 &port=6888 &downloaded=1234 &left=98765 &event=stopped

  10. Tracker Response • Error Message (Failure) • 3-digit error code • human-readable error message • List of peers (Success) • Dictionary format • interval – time client should wait before rerequesting • peers – list of dictionaries coresponding to peers • id – the peer_id of remote peer • ip – IP address of remote peer • port – port number

  11. Torrent file (.torrent) • Static metainfo in BEncoded format. • Provide client with information • Tracker Location (URL) • File(s) Information • Storage information

  12. BEncoding • Format for loosely structured data • 4 types • Byte strings • Integers • Lists • Dictionaries

  13. BEncoded Byte String • A sequence of bytes, not necessarily characters • Format • <length>:<contents> • Example • 5:hello • 7:seminar

  14. BEncoded Integer • Format • i<number>e • Example • i42e

  15. BEncoded List • Format • l<contents>e • Example • l5:helloi42e7:seminare • List: • 5:hell0 • I42e • 7:seminar

  16. BEncoded Dictionary • Associative array • Format • d<contents>e • <contents> alternating key/value • Example • d5:hello5:class4:falli2009ee • hello => class • fall =>2009

  17. .torrent Files • Structure • BEncoded Dictionary • announce – string value containing tracker URL. • info – points to dictionary contain info about files • Optional fields • announce-list – a list of alternate trackers • comment – any comment the author wants to include • createdby – author of the .torrent file • creationdate – the date the .torrent was created. • Others • Can put ANY field into .torrent. Ignored if not supported.

  18. Single File Torrents • info dictionary fields • length – size of the file, in bytes • name – a string containing the filename • piece length – number of bytes in each piece • pieces – string value containing concatenated 20-byte SHA-1 hashes for each piece of file.

  19. Multiple File Torrents • info dictionary fields • files – dictionary containing • length – size of the file, in bytes • path – path to the file, including filename • name – name of top-most directory • piece length – number of bytes in each piece • pieces – string value containing concatenated 20-byte SHA-1 hashes for each piece of file. For verification.

  20. Peer Messaging • Handshaking • Peer connects to remote peer, citing specific file • Peer states • (Un)Choked – Is the remote peer allowed to send data? • (Un)Interested – Is the peer interested in receiving data? • Data Transfer takes place when one side is interested, and the other is not choking. • Peer-Wire Messages • Used to communicate between peers

  21. Peer Wire Messaging • Choke • Inform the other peer they are being choked • Unchoke • Inform the other peer they are no longer being choked • Interested • Inform remote peer that data is desired • Uninterested • Inform remote peer that data transfer is not desired

  22. Peer Wire Messaging • Have • Announce that peer has a piece of the torrent • Remote peer may then request that piece • Bitfield • Send a bitfield representing pieces peer needs/has. • Request • Official request for a specific piece(s). • Piece • Transfer of a piece(s) • Cancel • Sent if peer requested data, but is no longer interested.

  23. Process Tracker goes online

  24. Process Content Provider creates .torrent file using bittorrent client software

  25. Process Client uploads the .torrent file to the server.

  26. Process Index 029317AHBS: first.torrent Tracker indexes the torrent Client re-downloads the .torrent file from the server and opens it using a bittorrent client.

  27. Process Index 029317AHBS: first.torrent Client software sends HTTP Get request, sending information to server; PeerID, and lets tracker know we have full file

  28. Process Index 029317AHBS: first.torrent Peers 123.234.12.34 Adds seed to peer list Client software sends HTTP Get request, sending information to server; PeerID, and lets tracker know we have full file

  29. Process Index 029317AHBS: first.torrent Peers 123.234.12.34 More peers download the .torrent file

  30. Process Index 029317AHBS: first.torrent Peers 123.234.12.34 Peers open the .torrent file, and each send an HTTP Get request identifying that the want to start downloading the file.

  31. Process Index 029317AHBS: first.torrent Peers 123.234.12.34 111.222.33.44 222.124.232.1 125.55.66.42 122.45.77.99 Tracker adds peers to list

  32. Process Index 029317AHBS: first.torrent Peers 123.234.12.34 111.222.33.44 222.124.232.1 125.55.66.42 122.45.77.99 Tracker returns the list of peers to the new peers.

  33. Process Index 029317AHBS: first.torrent Peers 123.234.12.34 111.222.33.44 222.124.232.1 125.55.66.42 122.45.77.99 Peers begin forming direct connections with other peers, forming a “swarm”.

  34. Process Index 029317AHBS: first.torrent Peers 123.234.12.34 111.222.33.44 222.124.232.1 125.55.66.42 122.45.77.99 Peers begin exchanging messages ((choked)) ((unchoked)) ((Uninterested)) ((Interested))

  35. Process Index 029317AHBS: first.torrent Peers 123.234.12.34 111.222.33.44 222.124.232.1 125.55.66.42 122.45.77.99 Peers begin exchanging messages ((have)) ((bitfield)) ((Request)) ((Piece))

  36. Peer to Peer Process State: Choked/Uninterested Handshake State: Choked/Interested ((interested)) State: Unchoked/Uninterested ((Unchoked)) Optional ((Bitfield)) / ((Have)) ((Request)) Data Transfer Begins ((Piece))

  37. Process Index 029317AHBS: first.torrent Peers 123.234.12.34 111.222.33.44 222.124.232.1 125.55.66.42 122.45.77.99 55.121.33.65 75.45.65.95 111.85.243.11 As peers continue to connect to tracker, more peers are added to the peer list.

  38. Process Index 029317AHBS: first.torrent Peers 123.234.12.34 111.222.33.44 222.124.232.1 125.55.66.42 122.45.77.99 55.121.33.65 75.45.65.95 111.85.243.11 New peers receive up-to-date peer lists. Existing peers periodically report to tracker, also receiving new peer list.

  39. Process Index 029317AHBS: first.torrent Peers 123.234.12.34 111.222.33.44 222.124.232.1 125.55.66.42 122.45.77.99 55.121.33.65 75.45.65.95 111.85.243.11 Peers continue exchanging messages and data until all peers receive the entire file. As pieces are received, the program calculates the SHA-1 hash for each piece and checks it against .torrent file. Any piece with incorrect checksum must be discarded and redownloaded.

  40. Process Index 029317AHBS: first.torrent Peers 123.234.12.34 111.222.33.44 222.124.232.1 125.55.66.42 122.45.77.99 55.121.33.65 75.45.65.95 111.85.243.11 As peers disconnect, they send another HTTP request, stating that they’re no longer sending.

  41. Process Index 029317AHBS: first.torrent Peers 123.234.12.34 111.222.33.44 222.124.232.1 125.55.66.42 122.45.77.99 55.121.33.65 75.45.65.95 Tracker removes peer As peers disconnect, they send another HTTP request, stating that they’re no longer sending.

  42. Process Index 029317AHBS: first.torrent Peers 123.234.12.34 111.222.33.44 222.124.232.1 125.55.66.42 As peers disconnect, they send another HTTP request, stating that they’re no longer sending.

  43. Future of Bittorrent • Bittorrent continues to increase its presence online. • Bram Cohen’s company, Bittorrent, Inc. is now in control of the protocol. • UDP implementation • Increased speeds without TCP’s congestion control • Streaming Video • Piece selection algorithms to increase speed and ensure sequential delivery.

  44. The End.Thank You.

More Related