A P2P file distribution system

1
A P2P file distribution system BitTorrent

• Pegasus Team
• CMPE 208

2
Outline

• 1 What is it
• 2 Why do we need BitTorrent
• 3 Architecture And Operation
• 4 Why does it work so well
• 5 Strengths Shortcomings
• 6 Conclusion

3
Overview

• Peer 2 Peer File transfer Protocol
• Works over public Internet.
• Critical components
• Web Server (MetaInfo Server)
• MetaData file (.torrent)
• Central Tracker
• End User Peers

4
Critical Elements

• 1 A web server
• To provide the metainfo file by HTTP
• For example
• http//my.torrentInfo.net
• http//smashhitsongs.com/

Web Server
Snakes on the Plane torrent
US open final torrent
5
Critical Elements

• 2 The .torrent file
• Static metainfo file to contain necessary
  information
• Name
• Size
• Checksum
• IP address of the Tracker

6
Critical Elements

• 3 A BitTorrent tracker
• Non-content-sharing node
• Track peers
• For example
• http//my.moviestracker.com8080/announce
• http//games.records.org6969/announce

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Critical Elements

• 4 An end user (peer)
• Guys who want to use BitTorrent must install
  corresponding software or plug-in for web
  browsers.
• Downloader (leecher) Peer has only a part ( or
  none ) of the file.
• Seeder Peer has the complete file, and chooses
  to stay in the system to allow other peers to
  download

8
Outline

• 1 What is it
• 2 Problem And BitTorrent Solution
• 3 Architecture And Operation
• 4 Why does it work so well
• 5 Strengths Shortcomings
• 6 Conclusion

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Problems

• Traditional Client/Server Sharing
• Performance deteriorates rapidly as the number of
  clients increases
• Free-riding in P2P network
• Free riders only download without contributing
  to the network.

10
BitTorrent

• Scalability of BitTorrent is very good. Even
  more peers means better performance.
• Strong incentives to prevent free-riding.
• Priority downloading
• High availability of downloads

11
Outline

• 1 What is it
• 2 Why do we need BitTorrent
• 3 Architecture And Operation
• 4 Why does it work so well
• 5 Strengths Shortcomings
• 6 Conclusion

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Connectivity
Web Server
John Doe
Tracker
Downloader Joe
Seeder Mike
Downloader Michelle
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Piece

• A file is cut into pieces of fixed size,
  typically 256Kb
• Each downloader reports to all of its peers what
  pieces it has.
• To verify data, Hash codes are used for all the
  pieces, included in .torrent files.

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A trivial example
1,2,3,4,5,6,7,8,9,10
Seeder John

1,2,3
1,2,3,5

1,2,3
1,2,3,4
1,2,3,4,5
Downloader Joe
Downloader Mike
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Messages

• Peer Peer messages
• TCP Sockets
• Peer Tracker messages
• HTTP Request/Response
• B-encoding
• The "B" encoding is identical to the "BASE64"
  encoding defined by RFC 1521.

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Tracker Function

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

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Peer Functions

• File space allocated
• Connect to peers
• Bitfield
• have(1) and not have(0)
• Have ltpiecegt
• Advertise pieces
• Interest /Not_interested
• Expressing interest in the pieces published by
  the peer
• Requesting for a chunk
• Request ltindex, offset, lengthgt

18
Outline

• 1 What is it
• 2 Why do we need BitTorrent
• 3 Architecture And Operation
• 4 Why does it work so well
• 5 Strengths Shortcomings
• 6 Conclusion

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Peer Selection

• Built-in incentive mechanism (where all the magic
  happens)
• Choking Algorithm
• Optimistic Unchoking
• Anti-snubbing

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Choking Algorithm

• Choking is a temporal refusal to upload
• Choking evaluation is performed every 10 seconds.
  
• Each peer unchokes a fixed number of peers
  (default 4)
• The decision on which peers to un/choke is based
  solely on download rate, which is evaluated on a
  rolling, 20-second average

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Reasons for Choking

• TCP congestion control.
• To ensure the peers to get a consistent download
  rate.

Joe
Choked
Choked
mike
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Optimistic Unchoking

• a BitTorrent peer has a single optimistic
  unchoke which is uploaded regardless of the
  current download rate from it. This peer rotates
  avery 30s
• Reason
• To discover currently unused connections are
  better than the ones being used

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Anti-snubbing

• When a peer received no data in 60s, we assume it
  is choked by all other peers, and refuse to
  upload to it except for the optimistic unchoking.
• Reason
• It may cause several concurrent optimistic
  unchokes.

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Example
mike
40kb/s
70kb/s
70kb/s
10kb/s
110kb/s
Downloader Joe
10kb/s
30kb/s
5kb/s
Downloader Rob
Downloader Ron
Downloader Kelly
Downloader Albert
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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

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Shortcomings

• 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.

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Conclusion

• BitTorrent is a well thought-out protocol that
  embraces aspects of cooperation and
  self-optimizing mechanisms.
• BitTorrent propose solutions for current
  optimization and scalability problems

28
References

• 1 Bram Cohen, Incentives build robustness in
  Bittorrent, May 2003
• 2 Stefan Saroiu, P. Krishna Gummadi, Steven D.
  Gribble, A Measurement Study of Peer-to-Peer File
  sharing Systems, Jan 2002
• 3 Bram Cohen, Bitconjurer.org, BitTorrent
  Protocol Specification
• 4 J R. Douceur, The Sybil Attack
• 5 A W Tucker, Prisoners Dilemma, Serendip.com
• 6 V Vishnumurthy, S Chandrakumar, EG Sirer,
  KARMA A Secure Echonomic Framework for
  Peer-to-Peer Resource Sharing.
• 7 E. Adar and B.A. Huberman, Free Riding on
  Gnutella.
• 8 www.kazaa.com