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More application layer BitTorrent (Quick Overview). Tahir Azim. BitTorrent. Torrent file (.torrent) describes file to download Created by the originating “seeder” of the file Specifies the URL of the tracker The tracker describes peers who are participating

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More application layer bittorrent quick overview

More application layerBitTorrent (Quick Overview)

Tahir Azim

Courtesy: Philip Levis/David Mazieres, Stanford


Bittorrent
BitTorrent

  • Torrent file (.torrent) describes file to download

    • Created by the originating “seeder” of the file

    • Specifies the URL of the tracker

      • The tracker describes peers who are participating

    • Specifies file length, piece length, SHA1 hashes of pieces

    • Additional metadata (who created torrent, etc)

  • Client contacts tracker, starts communicating with peers

    d8:announce45:http://open.tracker.thepiratebay.org/announce13:announce-listll45:http://open.tracker.thepiratebay.org/announceel45:http://www.torrent-downloads.to:2710/announceel42:http://denis.stalker.h3q.com:6969/announceel35:http://www.sumotracker.com/announceel26:http://www.prq.to/announceel41:http://inferno.demonoid.com:3408/announceee7:comment9:XXXXXX XX10:created by13:uTorrent/170013:creation datei1183825001e8:encoding5:UTF-84:infod5:filesld6:lengthi724176896e4:pathl13:XXXXXX XX.avieed6:lengthi47e4:pathl42:Torrent downloaded from Demonoid%2ecom.txteee4:name9:XXXXXX XX12:piece lengthi1048576e6:pieces13820:Xtˆ„#T …

Courtesy: Philip Levis/David Mazieres, Stanford


Pieces and sub pieces
Pieces and sub-pieces

  • BitTorrent breaks a file into N pieces

    • For throughput, pieces are large: 256KB-1MB

      • Pieces are the unit of replication

    • For latency, broken into subpieces

      • Subpieces are the unit of transmission

  • Hashes of pieces in torrent provide end-to-end integrity

    • Hashes compute a short summary of a piece of data

    • Cryptographically strong hashes: hard to find collisions or data that produces a hash

Courtesy: Philip Levis/David Mazieres, Stanford


Whom to talk to
Whom to talk to?

  • Uses a tit-for-tat (TFT) policy: upload only to those who give you data

    • Implemented slightly differently by different clients

  • Most peers are “choked” and get no data

    • Determine download rate from all peers

    • Order unchoked peers by download rate, choke all but P best (by default, 4)

  • Occasionally unchoke a random peer (might find its way into P best)

Courtesy: Philip Levis/David Mazieres, Stanford


What to say
What to Say?

  • Peers exchange metadata on what pieces they have

  • Download “rarest” pieces first

    • Pieces held by the fewest peers

    • Improves reliability

  • When down to the last few pieces, ask for them from multiple peers

    • Why?

Courtesy: Philip Levis/David Mazieres, Stanford


Bittorrent communication
BitTorrent Communication

Courtesy: Philip Levis/David Mazieres, Stanford


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