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Documents authored by Halpern, Joseph Y.

Document
DOI: 10.4230/LIPIcs.DISC.2024.2
A Knowledge-Based Analysis of Intersection Protocols

Authors: Kaya Alpturer, Joseph Y. Halpern, and Ron van der Meyden

Published in: LIPIcs, Volume 319, 38th International Symposium on Distributed Computing (DISC 2024)

Abstract
The increasing wireless communication capabilities of vehicles creates opportunities for more efficient intersection management strategies. One promising approach is the replacement of traffic lights with a system wherein vehicles run protocols among themselves to determine right of way. In this paper, we define the intersection problem to model this scenario abstractly, without any assumptions on the specific structure of the intersection or a bound on the number of vehicles. Protocols solving the intersection problem must guarantee safety (no collisions) and liveness (every vehicle eventually goes through). In addition, we would like these protocols to satisfy various optimality criteria, some of which turn out to be achievable only in a subset of the contexts. In particular, we show a partial equivalence between eliminating unnecessary waiting, a criterion of interest in the distributed mutual-exclusion literature, and a notion of optimality that we define called lexicographical optimality. We then introduce a framework to design protocols for the intersection problem by converting an intersection policy, which is based on a global view of the intersection, to a protocol that can be run by the vehicles through the use of knowledge-based programs. Our protocols are shown to guarantee safety and liveness while also being optimal under sufficient conditions on the context. Finally, we investigate protocols in the presence of faulty vehicles that experience communication failures and older vehicles with limited communication capabilities. We show that intersection protocols can be made safe, live and optimal even in the presence of faulty behavior.
Cite as

Kaya Alpturer, Joseph Y. Halpern, and Ron van der Meyden. A Knowledge-Based Analysis of Intersection Protocols. In 38th International Symposium on Distributed Computing (DISC 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 319, pp. 2:1-2:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{alpturer_et_al:LIPIcs.DISC.2024.2,
  author =	{Alpturer, Kaya and Halpern, Joseph Y. and van der Meyden, Ron},
  title =	{{A Knowledge-Based Analysis of Intersection Protocols}},
  booktitle =	{38th International Symposium on Distributed Computing (DISC 2024)},
  pages =	{2:1--2:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-352-2},
  ISSN =	{1868-8969},
  year =	{2024},
  volume =	{319},
  editor =	{Alistarh, Dan},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2024.2},
  URN =		{urn:nbn:de:0030-drops-212291},
  doi =		{10.4230/LIPIcs.DISC.2024.2},
  annote =	{Keywords: Intersection management, Autonomous vehicles, Distributed algorithms, Epistemic logic, Fault tolerance}
}
Document
DOI: 10.4230/LIPIcs.DISC.2023.1
Colordag: An Incentive-Compatible Blockchain

Authors: Ittai Abraham, Danny Dolev, Ittay Eyal, and Joseph Y. Halpern

Published in: LIPIcs, Volume 281, 37th International Symposium on Distributed Computing (DISC 2023)

Abstract
We present Colordag, a blockchain protocol where following the prescribed strategy is, with high probability, a best response as long as all miners have less than 1/2 of the mining power. We prove the correctness of Colordag even if there is an extremely powerful adversary who knows future actions of the scheduler: specifically, when agents will generate blocks and when messages will arrive. The state-of-the-art protocol, Fruitchain, is an ε-Nash equilibrium as long as all miners have less than 1/2 of the mining power. However, there is a simple deviation that guarantees that deviators are never worse off than they would be by following Fruitchain, and can sometimes do better. Thus, agents are motivated to deviate. Colordag implements a solution concept that we call ε-sure Nash equilibrium and does not suffer from this problem. Because it is an ε-sure Nash equilibrium, Colordag is an ε-Nash equilibrium and with probability 1-ε is a best response.
Cite as

Ittai Abraham, Danny Dolev, Ittay Eyal, and Joseph Y. Halpern. Colordag: An Incentive-Compatible Blockchain. In 37th International Symposium on Distributed Computing (DISC 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 281, pp. 1:1-1:22, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{abraham_et_al:LIPIcs.DISC.2023.1,
  author =	{Abraham, Ittai and Dolev, Danny and Eyal, Ittay and Halpern, Joseph Y.},
  title =	{{Colordag: An Incentive-Compatible Blockchain}},
  booktitle =	{37th International Symposium on Distributed Computing (DISC 2023)},
  pages =	{1:1--1:22},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-301-0},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{281},
  editor =	{Oshman, Rotem},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2023.1},
  URN =		{urn:nbn:de:0030-drops-191272},
  doi =		{10.4230/LIPIcs.DISC.2023.1},
  annote =	{Keywords: Game theory, incentives, blockchain}
}
Document
Invited Talk
DOI: 10.4230/OASIcs.Tokenomics.2021.1
Distributed Computing Meets Game Theory: Fault Tolerance and Implementation with Cheap Talk (Invited Talk)

Authors: Joseph Y. Halpern

Published in: OASIcs, Volume 97, 3rd International Conference on Blockchain Economics, Security and Protocols (Tokenomics 2021)

Abstract
Traditionally, work in distributed computing has divided the agents into "good guys" and "bad guys". The good guys follow the protocol; the bad guys do everything in their power to make sure it does not work. By way of contrast, game theory has focused on "rational" agents, who try to maximize their utilities. Here I try to combine these viewpoints. Specifically, following the work of Abraham et al. [I. Abraham et al., 2006], I consider (k,t)-robust protocols/strategies, which tolerate coalitions of rational players of size up to k and up to t malicious players. I focus in particular on the problem that economists have called implementing a mediator. That is, can the players in the system, just talking among themselves (using what economists call "cheap talk") simulate the effects of the mediator (see, e.g., [I. Barany, 1992; E. Ben-Porath, 2003; Forges, 1990; D. Gerardi, 2004; Y. Heller, 2005; A. Urbano and J. E. Vila, 2002; A. Urbano and J. E. Vila, 2004]). In computer science, this essentially amounts to multiparty computation [O. Goldreich et al., 1987; A. Shamir et al., 1981; A. Yao, 1982]. Ideas from cryptography and distributed computing allow us to prove results on how many agents are required to implement a (k,t)-robust mediator just using cheap talk. These results subsume (and, in some cases, correct) results from the game theory literature. The results of Abraham et al. [I. Abraham et al., 2006] were proved for what are called synchronous systems in the distributed computing community; this is also the case for all the results in the economics literature cited above. In synchronous systems, communication proceeds in atomic rounds, and all messages sent during round r are received by round r + 1. But many systems in the real world are asynchronous. In an asynchronous setting, there are no rounds; messages sent by the players may take arbitrarily long to get to their recipients. Markets and the internet are best viewed as asynchronous. Blockchain implementations assume partial synchrony, where there is an upper bound on how long messages take to arrive. The partial synchronous setting already shows some of the difficulty of moving away from synchrony: An agent i can wait to take its action until it receives a message from j (on which its action can depend). This cannot happen in a synchronous setting. Abraham, Dolev, Geffner, abnd Halpern [I. Abraham et al., 2019] extend the results on implementing mediators to the asynchronous setting.
Cite as

Joseph Y. Halpern. Distributed Computing Meets Game Theory: Fault Tolerance and Implementation with Cheap Talk (Invited Talk). In 3rd International Conference on Blockchain Economics, Security and Protocols (Tokenomics 2021). Open Access Series in Informatics (OASIcs), Volume 97, pp. 1:1-1:2, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{halpern:OASIcs.Tokenomics.2021.1,
  author =	{Halpern, Joseph Y.},
  title =	{{Distributed Computing Meets Game Theory: Fault Tolerance and Implementation with Cheap Talk}},
  booktitle =	{3rd International Conference on Blockchain Economics, Security and Protocols (Tokenomics 2021)},
  pages =	{1:1--1:2},
  series =	{Open Access Series in Informatics (OASIcs)},
  ISBN =	{978-3-95977-220-4},
  ISSN =	{2190-6807},
  year =	{2022},
  volume =	{97},
  editor =	{Gramoli, Vincent and Halaburda, Hanna and Pass, Rafael},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/OASIcs.Tokenomics.2021.1},
  URN =		{urn:nbn:de:0030-drops-158981},
  doi =		{10.4230/OASIcs.Tokenomics.2021.1},
  annote =	{Keywords: robust equilibrium, implementing mediators, asynchronous systems}
}
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