1

Incentives Build Robustness in BitTorrent?

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

5

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 peer’s 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-survey-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 (<=5.0)– 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 doesn’t un-choking us Up (1+delta)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 > # of Leecher Even bette

r• Gain from both methodology

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Other Tricks of BT Client

• Client Discrimination– Higher priority for Si

milar Client– Delibrately choke ot

her client

• Pretend to be Local Neighbor– Fake tracker with wr

ong upload rate– Retrieve from Seed

• Rapid Disconnect and Reconnect– Hope to gain from o

ptimistic unchoking– Remove from black-l

ist

• Multiple Connection to the same client– For high speed BB– Gain the most from t

he 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 TimeAvg Speed

KB/s

Conn Peer

Seed/Leech

Bit Comet 0 18:40 50.65 10/0

uTorrent 0 17:01 55.62 12/0

Bit Tyrant 0 15:04 62.75 16/0

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Experiment with BT Client

Ubuntu Upload TimeAvg Speed

KB/s

Conn Peer

Seed/Leech

Bit Comet 32MB 15:20 773.57 75/70

uTorrent 112KB 69:13 171.37 36/1

Bit Tyrant 100KB 26:28 448.16 40/1

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Experiment with BT Client

NBA Upload TimeAvg Speed

KB/s

Conn Peer

Seed/Leech

Bit Comet 1070MB 52:32 299 11/53

uTorrent 1035MB 1:42:50 152 7/29

Bit Tyrant 540MB 1:20:10 196 10/22

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Experiment with BT Client

• Bit Thief – Achieve a high download rate (~150KB/s) for Ub

untu 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 > Upload– Bit Thief: Free Riding

• Proposed Solution– Decentralized Client Solution

28

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/BitTyrant.pdf

• Free Riding in BitTorrent is Cheap http://dcg.ethz.ch/publications/hotnets06.pdf