Incentives Build Robustness in BitTorrent?

1
Incentives Build Robustness in BitTorrent?

• Analysis on Bit Torrent Client performance
• By Jimmy Wong

2
Agenda

• Introduction
• BitTorrent Basics
• Bit Tyrant A strategic client
• Bit Thief A free-riding client
• Other Tricks of BT Client
• Real World Experiments
• Avoiding of Tricks
• Conclusion

3
Introduction

• BitTorrent core incentive
• Upload More, Download More
• Problem Definition
• I am a selfish user
• Can I gain without pay / pay less?

4
BitTorrent Basics

• Step 0 Publishing Content
• Create Torrent file
• Tracker URL
• SHA-1 Hash Checksum
• Assign a Tracker
• Distribute Torrent to users through Web Site,
  Newsgroup or Forum

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BitTorrent Basics

• Step 1 Joining
• Peers get the Torrent
• Connect to Tracker
• Look for available peers
• DHT (Distributed Hash Table)
• Create connection with other peers
• Start with Others Optimistic Unchoking
• Look for better peers
• Let other peers join the swarm

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BitTorrent Basics

• Step 2 to N-1 Pipelining
• Exchange bitmap about which chunks are available
• Pay more, get more
• To build incentive for peers to upload
• Reciprocal
• Rarest first
• To preserve File completeness among the swarm
• To build up peers reputation

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BitTorrent Basics

• Choking
• Send data to only few good peers
• Contribute data to you
• Have pieces that you wants
• A resourceful peers
• It will be nice if I can pay minimal cost to be
  un-choke!!!!

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BitTorrent Basics

• Optimistic Un-choking
• Allow new peers to get some pieces for exchange
• Allow discovery of better exchanging partners
• Most peers are so friendly
• I can gain without contribute!!!!

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BitTorrent Basics

• Ending Phase
• Toward the end of download
• Request for missing pieces
• Search for available peers
• 20-30 pieces left

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Bit Tyrant A strategic client

• Incentive
• 100KB/s ? 15KB/s
• 1 drop
• 15KB/s ? 10KB/s
• 40 drop
• About 1.9Mbps World Wide Bandwidth
• http//arstechnica.com/news.ars/post/20070529-surv
  ey-average-broadband-speed-in-us-is-1-9mbps.html
• Why do I still need to give u so much?

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Bit Tyrant A strategic client

• Resource Management
• Bandwidth fixed
• Conn x Speed Constant
• Current Official BT Protocol (lt5.0)
• 4 un-choking peers
• 1 optimistic un-choking
• A Optimization Problem!!

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Bit Tyrant A strategic client

• Bit Tyrant Un-chocking algorithm
• Mark Dp be download rate of conn p
• Mark Up be upload rate of conn p
• For each round
• Calculate performance metric Dp / Up
• Un-choking the conn with the highest metric
• At the end of each round
• If peer doesnt un-choking us ? Up ?(1delta)Up
• If peer un-choking us ? Dp downlod rate
• If peer has un-choked us for r round ? Up ?
  (1-lamda)Up

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Bit Tyrant A strategic client

• Very simple algorithm
• Net Effect
• Select friendly peers
• Filter out unfriendly peers
• Gain from peers most of the time

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Bit Tyrant A strategic client

• Dynamic sizing of active set
• Serving the best peers first
• The 2nd best next and so on
• Until upload rate is exhausted
• Local Optimal trading
• Not global, because only concern your local peers
  group

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BitThief A free-riding client

• Download only
• No copy right issue (Only Contributor is sued)
• Conserve Resource
• Use to spoil the community
• Basic Idea
• Gain from optimistic un-choking
• Gain from free sharing of Seeder

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BitThief A free-riding client

• Gain From Optimistic Un-choking
• Request for as many clients as possible
• May be block by the Tracker
• SIMD like query to Tracker
• Connect to all client available
• Higher Chance of being un-choking
• Always pretend to be a new comer
• A bitmap with no available piece
• Try to download whatever available
• Most Clients are so nice

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BitThief A free-riding client

• Gain From Free Sharing of Seeder
• Seeder select peers in two ways
• The Highest Bandwidth (from tracker)
• Round Robin
• Bit Thief report high upload rate
• of Seeder gt of Leecher ? Even better
• Gain from both methodology

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Other Tricks of BT Client

• Client Discrimination
• Higher priority for Similar Client
• Delibrately choke other client
• Pretend to be Local Neighbor
• Fake tracker with wrong upload rate
• Retrieve from Seed

• Rapid Disconnect and Reconnect
• Hope to gain from optimistic unchoking
• Remove from black-list
• Multiple Connection to the same client
• For high speed BB
• Gain the most from the same client

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Experiment with BT Client

• Testing Environment
• Athlon 64 2800
• 1GB Ram
• 120GB 7200 rpm HDD
• Windows XP SP2 w/ TCP Conn Crack
• BT Software
• Utorrent / Mainline
• Bit Comet
• Bit Tryant
• iCable internet Connection

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Experiment with BT Client

• Gentoo AMD 64 Minimal ISO
• 54.4MB
• 34 Seeders, 0 Leechers
• Ubuntu 7.10 i386 Desktop ISO
• 695MB
• 928 Seeders / 78 Leechers
• NBA Match
• 921MB
• 120 Seeders, 714 Leechers

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Experiment with BT Client
Gentoo Upload Time Avg Speed KB/s Conn Peer Seed/Leech
Bit Comet 0 1840 50.65 10/0
uTorrent 0 1701 55.62 12/0
Bit Tyrant 0 1504 62.75 16/0
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Experiment with BT Client
Ubuntu Upload Time Avg Speed KB/s Conn Peer Seed/Leech
Bit Comet 32MB 1520 773.57 75/70
uTorrent 112KB 6913 171.37 36/1
Bit Tyrant 100KB 2628 448.16 40/1
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Experiment with BT Client
NBA Upload Time Avg Speed KB/s Conn Peer Seed/Leech
Bit Comet 1070MB 5232 299 11/53
uTorrent 1035MB 14250 152 7/29
Bit Tyrant 540MB 12010 196 10/22
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Experiment with BT Client

• Bit Thief
• Achieve a high download rate (150KB/s) for
  Ubuntu and NBA
• Suck at 25

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Avoiding Tricks

• Target
• Stop Fake Information
• Identify Free-riding
• Difficulties
• Bit Torrent is so wide spread
• Too many implementations
• Incentive driven works again!!!
• Co-operative identify bad client
• Block bad peers, Share with good peers

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Avoiding Tricks

• Sharing with Peers
• Search for local optimal best peers
• Search for potential bad peers
• Broadcast of bad peers (In a majority basics)
• It will fail if more than half of clients are
  in-honest

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Conclusion

• BT Robustness
• Most peers are friendly
• Ways to Crack
• Gain from cracking the protocol
• Bit Tyrant Download gt Upload
• Bit Thief Free Riding
• Proposed Solution
• Decentralized Client Solution

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Reference

• Incentives Build Robustness in BitTorrent
  http//www.bittorrent.org/bittorrentecon.pdf
• Do incentives build robustness in BitTorrent?
  http//www.cs.washington.edu/homes/piatek/papers/B
  itTyrant.pdf
• Free Riding in BitTorrent is Cheap
  http//dcg.ethz.ch/publications/hotnets06.pdf