Browsing by Author "GIBBONS, Phillip B."

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    DCast: Sustaining Collaboration despite Rational Collusion
    (2011-02-07) YU, Haifeng; GIBBONS, Phillip B.; SHI, Chenwei
    A key and well-known problem in large-scale collaborative distributed systems is how to incentivize the rational/selfish users so that they will properly collaborate. Within such context, this paper focuses on designing incentive mechanisms for overlay multicast systems. A key limitation shared by existing proposals on the problem is that they are no longer able to provide proper incentives and thus will collapse when rational users collude or launch sybil attacks. This work explicitly aims to properly sustain collaboration despite collusion and sybil attacks by rational users. To this end, we propose a new decentralized DCast multicast protocol that uses a novel mechanism with debt-links and circulating debts. We formally prove that the protocol offers a novel concept of safety-net guarantee: A user running the protocol will always obtain a reasonably good utility despite the deviation of any number of rational users that potentially collude or launch sybil attacks. Our prototyping as well as simulation demonstrates the feasibility and safety-net guarantee of our design in practice.
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    SybilLimit: A Near-Optimal Social Network Defense against Sybil Attacks
    (2008-02-05) YU, Haifeng; GIBBONS, Phillip B.; XIAO, Feng
    Decentralized distributed systems such as peer-to-peer systems are particularly vulnerable to {\em sybil attacks}, where a malicious user pretends to have multiple identities (called {\em sybil nodes}). Without a trusted central authority, defending against sybil attacks is quite challenging. Among the small number of decentralized approaches, our recent SybilGuard protocol leverages a key insight on social networks to bound the number of sybil nodes accepted. Although its direction is promising, SybilGuard can allow a large number of sybil nodes to be accepted. Furthermore, SybilGuard assumes that social networks are fast mixing, which has never been confirmed in the real world. This paper presents the novel SybilLimit protocol that leverages the same insight as SybilGuard but offers dramatically improved and near-optimal guarantees. The number of sybil nodes accepted is reduced by a factor of $\Theta(\sqrt{n})$, or around 200 times in our experiments for a million-node system. We further prove that SybilLimit's guarantee is at most a $\log n$ factor away from optimal, when considering approaches based on fast-mixing social networks. Finally, based on three large-scale real-world social networks, we provide the first evidence that real-world social networks are indeed fast mixing. This validates the fundamental assumption behind SybilLimit's and SybilGuard's approach.