a sybil-proof and time-sensi4ve incen4ve tree mechanism ...ruozhouy/docs/gc-15-slides-xiang.pdf ·...
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ASybil-proofandTime-sensi4veIncen4veTreeMechanismforCrowdsourcing
Xiang Zhang, Guoliang Xue, and Ruozhou YuArizona State University
Dejun YangColorado School of Mines
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Outline
PerformanceEvalua3on
Conclusion
CrowdsourcingModel
Incen3veTreeMechanism
Introduc3on&Mo3va3on
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Crowdsourcing
Crowdsourcing is the act of taking a job
traditionally performed by a designated agent
(usually an employee) and outsourcing it to an
undefined, generally large group of people in the
form of an open call. [1]”
“
[1]JeffHowe,“Crowdsourcing:WhythePoweroftheCrowdIsDrivingthefutureofBusiness,”CrownPublishingGroup,2008.
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Crowdsourcing Applications
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The power of a single provider is limited • A crowdsourcing task requires a large
number of service providers
Graph source:h Q p s : / /g igaom.com/2010/01/15/crowdsourcing-offers-the-best-c e l l u l a r -c o v e r a g e -maps-yet/
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Incentive Tree Mechanism
…… ……
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DARPA Network Challenge
• In 2009, DARPA launched the DARPA Network Challenge [2], which offered $40,000 for the team to find 10 red balloons randomly deployed in the U.S. in the shortest time.
• An MIT team wan the challenge by applying an incentive tree mechanism [3] to recruit balloon finders and found all balloons in the shortest time.
• [2]hQp://archive.darpa.mil/networkchallenge• [3]G.Pickard,W.Pan,I.Rahwan,M.Cebrian,R.Crane,A.Madan,andA.Pentland,
“Time-cri3calSocialMobiliza3on,”Science,vol.334,pp.509-512,2011.
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MIT Strategy • MIT Strategy rewards each balloon finder $2000,
the inviter of the finder $1000, the inviter of the inviter $500, and so on…
2000
1000 2000
2000+500+1000=3500
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Sybil Attacks
• A Sybil a=ack is that a dishonest individual who creates multiple fake identities for extra rewards with no extra contribution devoted.
• Many incentive tree mechanisms are vulnerable to sybil a]acks, and it blocks the purpose of solicitation, and encourages providers to generate fake identities instead of make contribution.
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• After the sybil a]ack, the a]acker has a reward
of $3000, which is more than what he deserves ($2000)
Vulnerability of MIT Strategy
2000
1000
2000
2000+500+500=3000
1000
Total new reward is 1000+2000=3000
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Submodular & Time-sensitive Contribution
• Most of the research uses a linear summation contribution model when calculating the contribution from a set of providers
• However, this is not always true.• A submodular contribution and time sensitive
model is applied• A function is submodular if and only if, for
any element and any sets ,
• Time-sensitivity: with the same efforts, the contribution of a provider changes with time
TS ⊆v)(}){()(}){( TfvTfSfvSf −∪≥−∪
)(⋅f
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Contribution
• The main contributions of this paper:– To the best of our knowledge, we are the first
to consider the sybil-proof incentive tree under the submodular and time-sensitive contribution model
– We propose a reward function for the incentive tree, and prove that it satisfies the desired economic properties
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Outline
PerformanceEvalua3on
Conclusion
CrowdsourcingModel
Incen3veTreeMechanism
Introduc3on&Mo3va3on
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System Model • A task owner requires one task to be finished• potential provider set• joins at time • is the submodular contribution function
over a subset of providers• is the set of providers joining before • is the time-sensitive marginal contribution
of provider joining at time :
• is the incentive tree; is the set of providers who are descendants of
},...,,{ 21 npppP =ip it)(SC
PS ⊆itS it
),( ii tpcitip
)(}){(),(ii titii SCpSCtpc −∪=
T 'iTip
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Desired Properties of Incentive Tree • Continuing Contribution Incentive (CCI)
• Continuing Solicitation Incentive (CSI)
• θ-Reward Proportional to Contribution (θ-RPC)
• Early Contribution Incentive (ECI)
• Sybil-proofness
Aprovideralwayshastheincen3vetomakemorecontribu3on
Aprovideralwayshastheincen3vetomakemoresolicita3on
Aprovider’srewardisatleastθ times its contribution
Aprovideralwayshastheincen3vetojoinearlier
Aprovideralwaysnoincen3vetolaunchanysybilaQack
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Outline
PerformanceEvalua3on
Conclusion
CrowdsourcingModel
Incen3veTreeMechanism
Introduc3on&Mo3va3on
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Reward Function
• We propose the reward function as• The mechanism works as follows:
– The platform announces the task;– Each providers makes contribution and solicitation– The platform decides the reward for each provider
according to this reward function
),( ii tpR
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Theoretic Result
Theorem:Therewardfunc3onguaranteesCCI,CSI,θ-RPC,ECI,andsybil-proofness.
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Outline
PerformanceEvalua3on
Conclusion
CrowdsourcingModel
Incen3veTreeMechanism
Introduc3on&Mo3va3on
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Simulation Setup
• Task size 1000, each provider covers (0,20]– The marginal contribution of each provider is
the marginal coverage size, i.e., the size that is first covered by the provider
• The number of children for each non-leaf provider is uniformly and randomly distributed over (0,5]
• Averaged over 10,000 instances
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Simulation Result
• Total rewards
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Simulation Result
• Sybil-proofness
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Simulation Result
• Time-sensitivity
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Outline
PerformanceEvalua3on
Conclusion
CrowdsourcingModel
Incen3veTreeMechanism
Introduc3on&Mo3va3on
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Conclusion
• We propose a sybil-proof, submodular, and time sensitive incentive tree mechanism for crowdsourcing;
• We proved that the mechanism satisfies CCI, CSI, θ-RPC, ECI, and sybil-proofness;
• Extensive evaluation results further confirm our analysis.
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Thank you!
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