Bittorrent

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Bittorrent

• Zachary Neyland

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Definitions

• Client An application
• Tracker A server that assists in communication
  between peers
• Peer Someone that is currently downloading
  while sharing
• Seed Someone who is only sharing 100 complete
• Leecher Someone who ONLY downloads and does not
  share ?
• Swarm The group of computers uploading/downloadi
  ng

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What is BT?

• BT is a Peer-2-Peer protocol
• Created by Bram Cohen in April 2001, released
  July 2001
• Peers/Seeds VS a centralized file server

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General Idea

• File is broken in to many different pieces
• Each piece is downloaded in a rarest first
  order
• As peers get more pieces they seed those until
  100 - then they become a seed

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Trackers Role

• Assists in communication between peers
• It manages connections
• Informs the swarm of a new client
• Informs the new client of the swarm
• Does NOT contain any information about the file
  being downloaded
• Allows for large number of clients to connect
  without using much power

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Clients Role

• Determines what order to download pieces in
• Creates and manages a list of peers/seeds
• Reassembles file once all of the pieces have
  arrived

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Client -gt Tracker communication(Announcement)

• Info_hash The file that the client is looking
  to download
• peer_id 20 characters that typically represent
  the client used
• Usually AAXXXX-
• port/ip The port and IP address that the client
  is running on
• Up/downloaded Amount in bytes that the client
  has done
• Events
• Started
• Stopped
• Completed
• Left amount of bytes left to download

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Tracker Response

• Interval How often the client should announce
  itself
• Complete Number of seeders
• Incomplete Number of peers
• Peers A list of all clients connected (seeds
  and peers)
• Two ways of sending the information
• Binary and Dictionary

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Binary VS Dictionary

• Binary
• A string consisting of multiples of 6 bytes
• 4 Bytes are the IP
• 2 Bytes are the port
• Dictionary
• Plain text consisting of
• Peer_id
• IP
• Port

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Example

• 4 peers and 1 seed
• 4 pieces
• Seed only sends needs to send a total of one copy

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BT Indexers

• Lists .torrent files
• Typically a website
• Usually provides a means for finding specific
  torrents
• Searching
• Forums
• Often double as a tracker

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Legal Issues

• Trackers do not actually host any files
• Peers may not actually send 100 of a file to
  another peer

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Applications

• Piracy
• Games
• Movies
• Music
• TV
• Legit
• Patches (WOW)
• OS Distribution (Linux)
• Music
• Games
• Movies

14
Questions

• Who created the BT protocol?
• Is ThePirateBay an indexer or a tracker?