Document

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

Convex Consensus (CC) allows a set of parties to agree on a value v inside the convex hull of their inputs with respect to a predefined abstract convexity notion, even in the presence of byzantine parties. In this work, we focus on achieving CC in the best-of-both-worlds paradigm, i.e., simultaneously tolerating at most t_s corruptions if communication is synchronous, and at most t_a ≤ t_s corruptions if it is asynchronous. Our protocol is randomized, which is a requirement under asynchrony, and we prove that it achieves optimal resilience. In the process, we introduce communication primitives tailored to the network-agnostic model. These are a deterministic primitive allowing parties to obtain intersecting views (Gather), and a randomized primitive leading to identical views (Agreement on a Core-Set). Our primitives provide stronger guarantees than previous counterparts, making them of independent interest.

Andrei Constantinescu, Diana Ghinea, Roger Wattenhofer, and Floris Westermann. Convex Consensus with Asynchronous Fallback. In 38th International Symposium on Distributed Computing (DISC 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 319, pp. 15:1-15:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{constantinescu_et_al:LIPIcs.DISC.2024.15, author = {Constantinescu, Andrei and Ghinea, Diana and Wattenhofer, Roger and Westermann, Floris}, title = {{Convex Consensus with Asynchronous Fallback}}, booktitle = {38th International Symposium on Distributed Computing (DISC 2024)}, pages = {15:1--15:23}, 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.2024.15}, URN = {urn:nbn:de:0030-drops-212411}, doi = {10.4230/LIPIcs.DISC.2024.15}, annote = {Keywords: convex consensus, network-agnostic protocols, agreement on a core-set} }

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Brief Announcement

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

The distributed computing literature considers multiple options for modeling communication. Most simply, communication is categorized as either synchronous or asynchronous. Synchronous communication assumes that messages get delivered within a publicly known timeframe and that parties' clocks are synchronized. Asynchronous communication, on the other hand, only assumes that messages get delivered eventually. A more nuanced approach, or a middle ground between the two extremes, is given by the partially synchronous model, which is arguably the most realistic option. This model comes in two commonly considered flavors:
ii) The Global Stabilization Time (GST) model: after an (unknown) amount of time, the network becomes synchronous. This captures scenarios where network issues are transient.
iii) The Unknown Latency (UL) model: the network is, in fact, synchronous, but the message delay bound is unknown. This work formally establishes that any time-agnostic property that can be achieved by a protocol in the UL model can also be achieved by a (possibly different) protocol in the GST model. By time-agnostic, we mean properties that can depend on the order in which events happen but not on time as measured by the parties. Most properties considered in distributed computing are time-agnostic. The converse was already known, even without the time-agnostic requirement, so our result shows that the two network conditions are, under one sensible assumption, equally demanding.

Andrei Constantinescu, Diana Ghinea, Jakub Sliwinski, and Roger Wattenhofer. Brief Announcement: Unifying Partial Synchrony. In 38th International Symposium on Distributed Computing (DISC 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 319, pp. 43:1-43:7, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{constantinescu_et_al:LIPIcs.DISC.2024.43, author = {Constantinescu, Andrei and Ghinea, Diana and Sliwinski, Jakub and Wattenhofer, Roger}, title = {{Brief Announcement: Unifying Partial Synchrony}}, booktitle = {38th International Symposium on Distributed Computing (DISC 2024)}, pages = {43:1--43:7}, 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.2024.43}, URN = {urn:nbn:de:0030-drops-212717}, doi = {10.4230/LIPIcs.DISC.2024.43}, annote = {Keywords: partial synchrony, unknown latency, global stabilization time} }

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**Published in:** LIPIcs, Volume 316, 6th Conference on Advances in Financial Technologies (AFT 2024)

Decentralized Autonomous Organizations (DAOs) are blockchain-based organizations that facilitate decentralized governance. Today, DAOs not only hold billions of dollars in their treasury but also govern many of the most popular Decentralized Finance (DeFi) protocols. This paper systematically analyses security threats to DAOs, focusing on the types of attacks they face. We study attacks on DAOs that took place in the past, attacks that have been theorized to be possible, and potential attacks that were uncovered and prevented in audits. For each of these (potential) attacks, we describe and categorize the attack vectors utilized into four categories. This reveals that while many attacks on DAOs take advantage of the less tangible and more complex human nature involved in governance, audits tend to focus on code and protocol vulnerabilities. Thus, additionally, the paper examines empirical data on DAO vulnerabilities, outlines risk factors contributing to these attacks, and suggests mitigation strategies to safeguard against such vulnerabilities.

Rainer Feichtinger, Robin Fritsch, Lioba Heimbach, Yann Vonlanthen, and Roger Wattenhofer. SoK: Attacks on DAOs. In 6th Conference on Advances in Financial Technologies (AFT 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 316, pp. 28:1-28:27, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{feichtinger_et_al:LIPIcs.AFT.2024.28, author = {Feichtinger, Rainer and Fritsch, Robin and Heimbach, Lioba and Vonlanthen, Yann and Wattenhofer, Roger}, title = {{SoK: Attacks on DAOs}}, booktitle = {6th Conference on Advances in Financial Technologies (AFT 2024)}, pages = {28:1--28:27}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-345-4}, ISSN = {1868-8969}, year = {2024}, volume = {316}, editor = {B\"{o}hme, Rainer and Kiffer, Lucianna}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.AFT.2024.28}, URN = {urn:nbn:de:0030-drops-209640}, doi = {10.4230/LIPIcs.AFT.2024.28}, annote = {Keywords: blockchain, DAO, governance, security, measurements, voting systems} }

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**Published in:** LIPIcs, Volume 292, 3rd Symposium on Algorithmic Foundations of Dynamic Networks (SAND 2024)

Many fundamental distributed computing problems require coordinated access to a shared resource. A distributed directory is an overlay data structure on an asynchronous graph G that helps to access a shared token t. The directory supports three basic operations: publish, to initialize the directory, lookup, to read the contents of the token, and move, to get exclusive update access to the token. There are known directory schemes that achieve message complexity within polylog factors of the optimal cost with respect to the number of nodes n and the diameter D of G. Motivated by fault-tolerant distributed computing implementations, we consider the impact of edge failures on distributed directories. We give a distributed directory overlay data structure that can tolerate edge failures without disrupting the directory operations. The directory can be repaired concurrently while it processes directory operations. We analyze the impact of the faults on the amortized cost of the three directory operations compared to the optimal cost. We show that f edges failures increase the amortized competitive ratio of the operations by at most factor f. We also analyze the message complexity to repair the overlay structure, in terms of the number of messages that are sent and the maximum distance a message traverses. For an edge failure, the repair mechanism uses messages of size 𝒪(log n) that traverse distance at most D', the graph diameter after the fault. To our knowledge, this is the first asymptotic analysis of a fault-tolerant distributed directory.

Judith Beestermöller, Costas Busch, and Roger Wattenhofer. Fault-Tolerant Distributed Directories. In 3rd Symposium on Algorithmic Foundations of Dynamic Networks (SAND 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 292, pp. 5:1-5:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{beestermoller_et_al:LIPIcs.SAND.2024.5, author = {Beesterm\"{o}ller, Judith and Busch, Costas and Wattenhofer, Roger}, title = {{Fault-Tolerant Distributed Directories}}, booktitle = {3rd Symposium on Algorithmic Foundations of Dynamic Networks (SAND 2024)}, pages = {5:1--5:20}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-315-7}, ISSN = {1868-8969}, year = {2024}, volume = {292}, editor = {Casteigts, Arnaud and Kuhn, Fabian}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SAND.2024.5}, URN = {urn:nbn:de:0030-drops-198833}, doi = {10.4230/LIPIcs.SAND.2024.5}, annote = {Keywords: distributed directory, sparse partition, fault tolerance, message complexity, path dilation} }

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Invited Talk

**Published in:** LIPIcs, Volume 286, 27th International Conference on Principles of Distributed Systems (OPODIS 2023)

With the book Thinking Fast and Slow, Daniel Kahneman popularized the idea that the human brain can think in two different modes. The fast mode is instinctive and automatic, while the slow mode is deliberative and logical. As of 2023, one can argue that machine learning understands how to think fast. Deep neural networks are remarkably successful in rapidly classifying and regressing data. Thinking slow on the other hand is still a mystery. Large language models may provide an illusion of being able to think slow. However, prompts that need multiple deductive steps are generally beyond the capabilities of large language models. Distributed algorithms have the potential to help understanding deductive reasoning. Distributed algorithms usually consist of several little steps, iteratively applied, each step being easily learnable. As such distributed computing may provide an interesting bridge towards understanding deduction, extrapolation, reasoning, and everything else needed to think slow. In the talk, we will discuss some exciting case studies from graph generation to origami folding.

Roger Wattenhofer. Distributed Algorithms as a Gateway To Deductive Learning (Invited Talk). In 27th International Conference on Principles of Distributed Systems (OPODIS 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 286, p. 3:1, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{wattenhofer:LIPIcs.OPODIS.2023.3, author = {Wattenhofer, Roger}, title = {{Distributed Algorithms as a Gateway To Deductive Learning}}, booktitle = {27th International Conference on Principles of Distributed Systems (OPODIS 2023)}, pages = {3:1--3:1}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-308-9}, ISSN = {1868-8969}, year = {2024}, volume = {286}, editor = {Bessani, Alysson and D\'{e}fago, Xavier and Nakamura, Junya and Wada, Koichi and Yamauchi, Yukiko}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2023.3}, URN = {urn:nbn:de:0030-drops-194936}, doi = {10.4230/LIPIcs.OPODIS.2023.3}, annote = {Keywords: abstract visual reasoning, agent-based reasoning, classic algorithm benchmarks, differentiable status registers, explainable graphs, graph generation algorithms, integer sequences, neural combinatorial circuits, recurrent network algorithms, origami folding, Tatham’s puzzles} }

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**Published in:** LIPIcs, Volume 286, 27th International Conference on Principles of Distributed Systems (OPODIS 2023)

Traditional blockchain design gives miners or validators full control over transaction ordering, i.e., they can freely choose which transactions to include or exclude, as well as in which order. While not an issue initially, the emergence of decentralized finance has introduced new transaction order dependencies allowing parties in control of the ordering to make a profit by front-running others' transactions. In this work, we present the Decentralized Clock Network, a new approach for achieving fair transaction ordering. Users submit their transactions to the network’s clocks, which run an agreement protocol that provides each transaction with a timestamp of receipt which is then used to define the transactions' order. By separating agreement from ordering, our protocol is efficient and has a simpler design compared to other available solutions. Moreover, our protocol brings to the blockchain world the paradigm of asynchronous fallback, where the algorithm operates with stronger fairness guarantees during periods of synchronous use, switching to an asynchronous mode only during times of increased network delay.

Andrei Constantinescu, Diana Ghinea, Lioba Heimbach, Zilin Wang, and Roger Wattenhofer. A Fair and Resilient Decentralized Clock Network for Transaction Ordering. In 27th International Conference on Principles of Distributed Systems (OPODIS 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 286, pp. 8:1-8:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{constantinescu_et_al:LIPIcs.OPODIS.2023.8, author = {Constantinescu, Andrei and Ghinea, Diana and Heimbach, Lioba and Wang, Zilin and Wattenhofer, Roger}, title = {{A Fair and Resilient Decentralized Clock Network for Transaction Ordering}}, booktitle = {27th International Conference on Principles of Distributed Systems (OPODIS 2023)}, pages = {8:1--8:20}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-308-9}, ISSN = {1868-8969}, year = {2024}, volume = {286}, editor = {Bessani, Alysson and D\'{e}fago, Xavier and Nakamura, Junya and Wada, Koichi and Yamauchi, Yukiko}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2023.8}, URN = {urn:nbn:de:0030-drops-194989}, doi = {10.4230/LIPIcs.OPODIS.2023.8}, annote = {Keywords: Median Validity, Blockchain, Fair Ordering, Front-running Prevention, Miner Extractable Value} }

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**Published in:** LIPIcs, Volume 282, 5th Conference on Advances in Financial Technologies (AFT 2023)

Lending protocols in decentralized finance enable the permissionless exchange of capital from lenders to borrowers without relying on a trusted third party for clearing or market-making. Interest rates are set by the supply and demand of capital according to a pre-defined function. In the lead-up to The Merge: Ethereum blockchain’s transition from proof-of-work (PoW) to proof-of-stake (PoS), a fraction of the Ethereum ecosystem announced plans of continuing with a PoW-chain. Owners of ETH - whether their ETH was borrowed or not - would hold the native tokens on each chain. This development alarmed lending protocols. They feared spiking ETH borrowing rates would lead to mass liquidations which could undermine their viability. Thus, the decentralized autonomous organization running the protocols saw no alternative to intervention - restricting users' ability to borrow.
We investigate the effects of the merge and the aforementioned intervention on the two biggest lending protocols on Ethereum: AAVE and Compound. Our analysis finds that borrowing rates were extremely volatile, jumping by two orders of magnitude, and borrowing at times reached 100% of the available funds. Despite this, no spike in mass liquidations or irretrievable loans materialized. Further, we are the first to quantify and analyze hard-fork-arbitrage, profiting from holding debt in the native blockchain token during a hard fork. We find that arbitrageurs transferred tokens to centralized exchanges which at the time were worth more than 13 Mio US$, money that was effectively extracted from the platforms' lenders.

Lioba Heimbach, Eric Schertenleib, and Roger Wattenhofer. DeFi Lending During The Merge. In 5th Conference on Advances in Financial Technologies (AFT 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 282, pp. 9:1-9:25, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{heimbach_et_al:LIPIcs.AFT.2023.9, author = {Heimbach, Lioba and Schertenleib, Eric and Wattenhofer, Roger}, title = {{DeFi Lending During The Merge}}, booktitle = {5th Conference on Advances in Financial Technologies (AFT 2023)}, pages = {9:1--9:25}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-303-4}, ISSN = {1868-8969}, year = {2023}, volume = {282}, editor = {Bonneau, Joseph and Weinberg, S. Matthew}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.AFT.2023.9}, URN = {urn:nbn:de:0030-drops-191985}, doi = {10.4230/LIPIcs.AFT.2023.9}, annote = {Keywords: blockchain, Ethereum, lending protocol, hard fork} }

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**Published in:** Dagstuhl Reports, Volume 12, Issue 11 (2023)

This report documents the program and outcomes of Dagstuhl Seminar 22452 "Computational Social Dynamics". The seminar addressed social and dynamic problems in the field of algorithmic game theory, and their implications in numerous applications, such as fair division, financial networks, or behavioral game theory. We summarize organizational aspects of the seminar, the talk abstracts, and the problems that were discussed in the open problem sessions.

Martin Hoefer, Sigal Oren, Roger Wattenhofer, and Giovanna Varricchio. Computational Social Dynamics (Dagstuhl Seminar 22452). In Dagstuhl Reports, Volume 12, Issue 11, pp. 28-44, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@Article{hoefer_et_al:DagRep.12.11.28, author = {Hoefer, Martin and Oren, Sigal and Wattenhofer, Roger and Varricchio, Giovanna}, title = {{Computational Social Dynamics (Dagstuhl Seminar 22452)}}, pages = {28--44}, journal = {Dagstuhl Reports}, ISSN = {2192-5283}, year = {2023}, volume = {12}, number = {11}, editor = {Hoefer, Martin and Oren, Sigal and Wattenhofer, Roger and Varricchio, Giovanna}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/DagRep.12.11.28}, URN = {urn:nbn:de:0030-drops-178346}, doi = {10.4230/DagRep.12.11.28}, annote = {Keywords: algorithmic game theory, behavioral economics, fair division, financial networks, social networks} }

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Invited Talk

**Published in:** LIPIcs, Volume 221, 1st Symposium on Algorithmic Foundations of Dynamic Networks (SAND 2022)

Networks are notoriously difficult to understand, and adding dynamics does not help. Can the current wonder weapon of computation (yes, machine learning) come to the rescue? Unfortunately, learning with networks is generally not well understood. "Neural network networks" (better and less confusingly known as graph neural networks) can learn simple graph patterns, but they are a far cry from their impressive machine learning cousins in the image- or the game-domain. In my opinion, the most astonishing graph neural networks are in fact dealing with dynamic networks: They simulate sand (the granular material, not the symposium) quite naturally. In my talk, I will discuss and compare different computational objects and paradigms: networks, dynamics, algorithms, and learning. What are the differences? And what can they learn from each other? In the technical part of the talk, I will present DropGNN, our new algorithm-inspired approach for handling graph neural networks. But mostly I will vent about misunderstandings and mistakes, and I will propose open questions, and new research directions. DropGNN is joint work with Pál András Papp, Karolis Martinkus, and Lukas Faber, published at NeurIPS, December 2021.

Roger Wattenhofer. Networks, Dynamics, Algorithms, and Learning (Invited Talk). In 1st Symposium on Algorithmic Foundations of Dynamic Networks (SAND 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 221, p. 3:1, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{wattenhofer:LIPIcs.SAND.2022.3, author = {Wattenhofer, Roger}, title = {{Networks, Dynamics, Algorithms, and Learning}}, booktitle = {1st Symposium on Algorithmic Foundations of Dynamic Networks (SAND 2022)}, pages = {3:1--3:1}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-224-2}, ISSN = {1868-8969}, year = {2022}, volume = {221}, editor = {Aspnes, James and Michail, Othon}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SAND.2022.3}, URN = {urn:nbn:de:0030-drops-159451}, doi = {10.4230/LIPIcs.SAND.2022.3}, annote = {Keywords: graph neural networks} }

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**Published in:** LIPIcs, Volume 202, 46th International Symposium on Mathematical Foundations of Computer Science (MFCS 2021)

We study the stabilization time of two common types of influence propagation. In majority processes, nodes in a graph want to switch to the most frequent state in their neighborhood, while in minority processes, nodes want to switch to the least frequent state in their neighborhood. We consider the sequential model of these processes, and assume that every node starts out from a uniform random state.
We first show that if nodes change their state for any small improvement in the process, then stabilization can last for up to Θ(n²) steps in both cases. Furthermore, we also study the proportional switching case, when nodes only decide to change their state if they are in conflict with a (1+λ)/2 fraction of their neighbors, for some parameter λ ∈ (0,1). In this case, we show that if λ < 1/3, then there is a construction where stabilization can indeed last for Ω(n^{1+c}) steps for some constant c > 0. On the other hand, if λ > 1/2, we prove that the stabilization time of the processes is upper-bounded by O(n ⋅ log n).

Pál András Papp and Roger Wattenhofer. Stabilization Bounds for Influence Propagation from a Random Initial State. In 46th International Symposium on Mathematical Foundations of Computer Science (MFCS 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 202, pp. 83:1-83:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{papp_et_al:LIPIcs.MFCS.2021.83, author = {Papp, P\'{a}l Andr\'{a}s and Wattenhofer, Roger}, title = {{Stabilization Bounds for Influence Propagation from a Random Initial State}}, booktitle = {46th International Symposium on Mathematical Foundations of Computer Science (MFCS 2021)}, pages = {83:1--83:15}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-201-3}, ISSN = {1868-8969}, year = {2021}, volume = {202}, editor = {Bonchi, Filippo and Puglisi, Simon J.}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.MFCS.2021.83}, URN = {urn:nbn:de:0030-drops-145239}, doi = {10.4230/LIPIcs.MFCS.2021.83}, annote = {Keywords: Majority process, Minority process, Stabilization time, Random initialization, Asynchronous model} }

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**Published in:** LIPIcs, Volume 185, 12th Innovations in Theoretical Computer Science Conference (ITCS 2021)

We consider financial networks, where banks are connected by contracts such as debts or credit default swaps. We study the clearing problem in these systems: we want to know which banks end up in a default, and what portion of their liabilities can these defaulting banks fulfill. We analyze these networks in a sequential model where banks announce their default one at a time, and the system evolves in a step-by-step manner.
We first consider the reversible model of these systems, where banks may return from a default. We show that the stabilization time in this model can heavily depend on the ordering of announcements. However, we also show that there are systems where for any choice of ordering, the process lasts for an exponential number of steps before an eventual stabilization. We also show that finding the ordering with the smallest (or largest) number of banks ending up in default is an NP-hard problem. Furthermore, we prove that defaulting early can be an advantageous strategy for banks in some cases, and in general, finding the best time for a default announcement is NP-hard. Finally, we discuss how changing some properties of this setting affects the stabilization time of the process, and then use these techniques to devise a monotone model of the systems, which ensures that every network stabilizes eventually.

Pál András Papp and Roger Wattenhofer. Sequential Defaulting in Financial Networks. In 12th Innovations in Theoretical Computer Science Conference (ITCS 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 185, pp. 52:1-52:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{papp_et_al:LIPIcs.ITCS.2021.52, author = {Papp, P\'{a}l Andr\'{a}s and Wattenhofer, Roger}, title = {{Sequential Defaulting in Financial Networks}}, booktitle = {12th Innovations in Theoretical Computer Science Conference (ITCS 2021)}, pages = {52:1--52:20}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-177-1}, ISSN = {1868-8969}, year = {2021}, volume = {185}, editor = {Lee, James R.}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITCS.2021.52}, URN = {urn:nbn:de:0030-drops-135919}, doi = {10.4230/LIPIcs.ITCS.2021.52}, annote = {Keywords: Financial Network, Sequential Defaulting, Credit Default Swap, Clearing Problem, Stabilization Time} }

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**Published in:** LIPIcs, Volume 181, 31st International Symposium on Algorithms and Computation (ISAAC 2020)

In this paper, we present tight bounds for the k-server problem with delays in the uniform metric space. The problem is defined on n+k nodes in the uniform metric space which can issue requests over time. These requests can be served directly or with some delay using k servers, by moving a server to the corresponding node with an open request. The task is to find an online algorithm that can serve the requests while minimizing the total moving and delay costs. We first provide a lower bound by showing that the competitive ratio of any deterministic online algorithm cannot be better than (2k+1) in the clairvoyant setting. We will then show that conservative algorithms (without delay) can be equipped with an accumulative delay function such that all such algorithms become (2k+1)-competitive in the non-clairvoyant setting. Together, the two bounds establish a tight result for both, the clairvoyant and the non-clairvoyant settings.

Predrag Krnetić, Darya Melnyk, Yuyi Wang, and Roger Wattenhofer. The k-Server Problem with Delays on the Uniform Metric Space. In 31st International Symposium on Algorithms and Computation (ISAAC 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 181, pp. 61:1-61:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{krnetic_et_al:LIPIcs.ISAAC.2020.61, author = {Krneti\'{c}, Predrag and Melnyk, Darya and Wang, Yuyi and Wattenhofer, Roger}, title = {{The k-Server Problem with Delays on the Uniform Metric Space}}, booktitle = {31st International Symposium on Algorithms and Computation (ISAAC 2020)}, pages = {61:1--61:13}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-173-3}, ISSN = {1868-8969}, year = {2020}, volume = {181}, editor = {Cao, Yixin and Cheng, Siu-Wing and Li, Minming}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2020.61}, URN = {urn:nbn:de:0030-drops-134056}, doi = {10.4230/LIPIcs.ISAAC.2020.61}, annote = {Keywords: Online k-Server, Paging, Delayed Service, Conservative Algorithms} }

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Track A: Algorithms, Complexity and Games

**Published in:** LIPIcs, Volume 168, 47th International Colloquium on Automata, Languages, and Programming (ICALP 2020)

We study the stabilization time of a wide class of processes on graphs, in which each node can only switch its state if it is motivated to do so by at least a (1+λ)/2 fraction of its neighbors, for some 0 < λ < 1. Two examples of such processes are well-studied dynamically changing colorings in graphs: in majority processes, nodes switch to the most frequent color in their neighborhood, while in minority processes, nodes switch to the least frequent color in their neighborhood. We describe a non-elementary function f(λ), and we show that in the sequential model, the worst-case stabilization time of these processes can completely be characterized by f(λ). More precisely, we prove that for any ε > 0, O(n^(1+f(λ)+ε)) is an upper bound on the stabilization time of any proportional majority/minority process, and we also show that there are graph constructions where stabilization indeed takes Ω(n^(1+f(λ)-ε)) steps.

Pál András Papp and Roger Wattenhofer. A General Stabilization Bound for Influence Propagation in Graphs. In 47th International Colloquium on Automata, Languages, and Programming (ICALP 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 168, pp. 90:1-90:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{papp_et_al:LIPIcs.ICALP.2020.90, author = {Papp, P\'{a}l Andr\'{a}s and Wattenhofer, Roger}, title = {{A General Stabilization Bound for Influence Propagation in Graphs}}, booktitle = {47th International Colloquium on Automata, Languages, and Programming (ICALP 2020)}, pages = {90:1--90:15}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-138-2}, ISSN = {1868-8969}, year = {2020}, volume = {168}, editor = {Czumaj, Artur and Dawar, Anuj and Merelli, Emanuela}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2020.90}, URN = {urn:nbn:de:0030-drops-124978}, doi = {10.4230/LIPIcs.ICALP.2020.90}, annote = {Keywords: Minority process, Majority process} }

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Track A: Algorithms, Complexity and Games

**Published in:** LIPIcs, Volume 168, 47th International Colloquium on Automata, Languages, and Programming (ICALP 2020)

We study the incentives of banks in a financial network, where the network consists of debt contracts and credit default swaps (CDSs) between banks. One of the most important questions in such a system is the problem of deciding which of the banks are in default, and how much of their liabilities these banks can pay. We study the payoff and preferences of the banks in the different solutions to this problem. We also introduce a more refined model which allows assigning priorities to payment obligations; this provides a more expressive and realistic model of real-life financial systems, while it always ensures the existence of a solution.
The main focus of the paper is an analysis of the actions that a single bank can execute in a financial system in order to influence the outcome to its advantage. We show that removing an incoming debt, or donating funds to another bank can result in a single new solution that is strictly more favorable to the acting bank. We also show that increasing the bank’s external funds or modifying the priorities of outgoing payments cannot introduce a more favorable new solution into the system, but may allow the bank to remove some unfavorable solutions, or to increase its recovery rate. Finally, we show how the actions of two banks in a simple financial system can result in classical game theoretic situations like the prisoner’s dilemma or the dollar auction, demonstrating the wide expressive capability of the financial system model.

Pál András Papp and Roger Wattenhofer. Network-Aware Strategies in Financial Systems. In 47th International Colloquium on Automata, Languages, and Programming (ICALP 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 168, pp. 91:1-91:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{papp_et_al:LIPIcs.ICALP.2020.91, author = {Papp, P\'{a}l Andr\'{a}s and Wattenhofer, Roger}, title = {{Network-Aware Strategies in Financial Systems}}, booktitle = {47th International Colloquium on Automata, Languages, and Programming (ICALP 2020)}, pages = {91:1--91:17}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-138-2}, ISSN = {1868-8969}, year = {2020}, volume = {168}, editor = {Czumaj, Artur and Dawar, Anuj and Merelli, Emanuela}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2020.91}, URN = {urn:nbn:de:0030-drops-124988}, doi = {10.4230/LIPIcs.ICALP.2020.91}, annote = {Keywords: Financial network, credit default swap, creditor priority, clearing problem, prisoner’s dilemma, dollar auction} }

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**Published in:** LIPIcs, Volume 149, 30th International Symposium on Algorithms and Computation (ISAAC 2019)

We analyze the stabilization time of minority processes in graphs. A minority process is a dynamically changing coloring, where each node repeatedly changes its color to the color which is least frequent in its neighborhood. First, we present a simple Omega(n^2) stabilization time lower bound in the sequential adversarial model. Our main contribution is a graph construction which proves a Omega(n^(2-epsilon)) stabilization time lower bound for any epsilon>0. This lower bound holds even if the order of nodes is chosen benevolently, not only in the sequential model, but also in any reasonable concurrent model of the process.

Pál András Papp and Roger Wattenhofer. Stabilization Time in Minority Processes. In 30th International Symposium on Algorithms and Computation (ISAAC 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 149, pp. 43:1-43:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

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@InProceedings{papp_et_al:LIPIcs.ISAAC.2019.43, author = {Papp, P\'{a}l Andr\'{a}s and Wattenhofer, Roger}, title = {{Stabilization Time in Minority Processes}}, booktitle = {30th International Symposium on Algorithms and Computation (ISAAC 2019)}, pages = {43:1--43:19}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-130-6}, ISSN = {1868-8969}, year = {2019}, volume = {149}, editor = {Lu, Pinyan and Zhang, Guochuan}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2019.43}, URN = {urn:nbn:de:0030-drops-115393}, doi = {10.4230/LIPIcs.ISAAC.2019.43}, annote = {Keywords: Minority process, Benevolent model} }

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**Published in:** LIPIcs, Volume 126, 36th International Symposium on Theoretical Aspects of Computer Science (STACS 2019)

A minority process in a weighted graph is a dynamically changing coloring. Each node repeatedly changes its color in order to minimize the sum of weighted conflicts with its neighbors. We study the number of steps until such a process stabilizes. Our main contribution is an exponential lower bound on stabilization time. We first present a construction showing this bound in the adversarial sequential model, and then we show how to extend the construction to establish the same bound in the benevolent sequential model, as well as in any reasonable concurrent model. Furthermore, we show that the stabilization time of our construction remains exponential even for very strict switching conditions, namely, if a node only changes color when almost all (i.e., any specific fraction) of its neighbors have the same color. Our lower bound works in a wide range of settings, both for node-weighted and edge-weighted graphs, or if we restrict minority processes to the class of sparse graphs.

Pál András Papp and Roger Wattenhofer. Stabilization Time in Weighted Minority Processes. In 36th International Symposium on Theoretical Aspects of Computer Science (STACS 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 126, pp. 54:1-54:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

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@InProceedings{papp_et_al:LIPIcs.STACS.2019.54, author = {Papp, P\'{a}l Andr\'{a}s and Wattenhofer, Roger}, title = {{Stabilization Time in Weighted Minority Processes}}, booktitle = {36th International Symposium on Theoretical Aspects of Computer Science (STACS 2019)}, pages = {54:1--54:15}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-100-9}, ISSN = {1868-8969}, year = {2019}, volume = {126}, editor = {Niedermeier, Rolf and Paul, Christophe}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2019.54}, URN = {urn:nbn:de:0030-drops-102938}, doi = {10.4230/LIPIcs.STACS.2019.54}, annote = {Keywords: Minority process, Benevolent model} }

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**Published in:** LIPIcs, Volume 123, 29th International Symposium on Algorithms and Computation (ISAAC 2018)

Payment networks, also known as channels, are a most promising solution to the throughput problem of cryptocurrencies. In this paper we study the design of capital-efficient payment networks, offline as well as online variants. We want to know how to compute an efficient payment network topology, how capital should be assigned to the individual edges, and how to decide which transactions to accept. Towards this end, we present a flurry of interesting results, basic but generally applicable insights on the one hand, and hardness results and approximation algorithms on the other hand.

Georgia Avarikioti, Yuyi Wang, and Roger Wattenhofer. Algorithmic Channel Design. In 29th International Symposium on Algorithms and Computation (ISAAC 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 123, pp. 16:1-16:12, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{avarikioti_et_al:LIPIcs.ISAAC.2018.16, author = {Avarikioti, Georgia and Wang, Yuyi and Wattenhofer, Roger}, title = {{Algorithmic Channel Design}}, booktitle = {29th International Symposium on Algorithms and Computation (ISAAC 2018)}, pages = {16:1--16:12}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-094-1}, ISSN = {1868-8969}, year = {2018}, volume = {123}, editor = {Hsu, Wen-Lian and Lee, Der-Tsai and Liao, Chung-Shou}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2018.16}, URN = {urn:nbn:de:0030-drops-99648}, doi = {10.4230/LIPIcs.ISAAC.2018.16}, annote = {Keywords: blockchain, payment channels, layer 2 solution, network design, payment hubs, routing} }

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**Published in:** LIPIcs, Volume 123, 29th International Symposium on Algorithms and Computation (ISAAC 2018)

We investigate the problem of Min-cost Perfect Matching with Delays (MPMD) in which requests are pairwise matched in an online fashion with the objective to minimize the sum of space cost and time cost. Though linear-MPMD (i.e., time cost is linear in delay) has been thoroughly studied in the literature, it does not well model impatient requests that are common in practice. Thus, we propose convex-MPMD where time cost functions are convex, capturing the situation where time cost increases faster and faster. Since the existing algorithms for linear-MPMD are not competitive any more, we devise a new deterministic algorithm for convex-MPMD problems. For a large class of convex time cost functions, our algorithm achieves a competitive ratio of O(k) on any k-point uniform metric space. Moreover, our deterministic algorithm is asymptotically optimal, which uncover a substantial difference between convex-MPMD and linear-MPMD which allows a deterministic algorithm with constant competitive ratio on any uniform metric space.

Xingwu Liu, Zhida Pan, Yuyi Wang, and Roger Wattenhofer. Impatient Online Matching. In 29th International Symposium on Algorithms and Computation (ISAAC 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 123, pp. 62:1-62:12, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{liu_et_al:LIPIcs.ISAAC.2018.62, author = {Liu, Xingwu and Pan, Zhida and Wang, Yuyi and Wattenhofer, Roger}, title = {{Impatient Online Matching}}, booktitle = {29th International Symposium on Algorithms and Computation (ISAAC 2018)}, pages = {62:1--62:12}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-094-1}, ISSN = {1868-8969}, year = {2018}, volume = {123}, editor = {Hsu, Wen-Lian and Lee, Der-Tsai and Liao, Chung-Shou}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2018.62}, URN = {urn:nbn:de:0030-drops-100107}, doi = {10.4230/LIPIcs.ISAAC.2018.62}, annote = {Keywords: online algorithm, online matching, convex function, competitive analysis, lower bound} }

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**Published in:** LIPIcs, Volume 121, 32nd International Symposium on Distributed Computing (DISC 2018)

Recently, there has been a growing interest in grid exploration by agents with limited capabilities. We show that the grid cannot be explored by three semi-synchronous finite automata, answering an open question by Emek et al. [TCS'15] in the negative.
In the setting we consider, time is divided into discrete steps, where in each step, an adversarially selected subset of the agents executes one look-compute-move cycle. The agents operate according to a shared finite automaton, where every agent is allowed to have a distinct initial state. The only means of communication is to sense the states of the agents sharing the same grid cell. The agents are equipped with a global compass and whenever an agent moves, the destination cell of the movement is chosen by the agent's automaton from the set of neighboring grid cells. In contrast to the four agent protocol by Emek et al., we show that three agents do not suffice for grid exploration.

Sebastian Brandt, Jara Uitto, and Roger Wattenhofer. A Tight Lower Bound for Semi-Synchronous Collaborative Grid Exploration. In 32nd International Symposium on Distributed Computing (DISC 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 121, pp. 13:1-13:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{brandt_et_al:LIPIcs.DISC.2018.13, author = {Brandt, Sebastian and Uitto, Jara and Wattenhofer, Roger}, title = {{A Tight Lower Bound for Semi-Synchronous Collaborative Grid Exploration}}, booktitle = {32nd International Symposium on Distributed Computing (DISC 2018)}, pages = {13:1--13:17}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-092-7}, ISSN = {1868-8969}, year = {2018}, volume = {121}, editor = {Schmid, Ulrich and Widder, Josef}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2018.13}, URN = {urn:nbn:de:0030-drops-98029}, doi = {10.4230/LIPIcs.DISC.2018.13}, annote = {Keywords: Finite automata, Graph exploration, Mobile robots} }

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**Published in:** LIPIcs, Volume 95, 21st International Conference on Principles of Distributed Systems (OPODIS 2017)

We present a new fault-tolerant distributed state machine to inherit the best features of its “parents in spirit”: Paxos, providing strong consistency, and a blockchain, providing simplicity and availability. Our proposal is simple as it does not include any heavy weight distributed failure handling protocols such as leader election. In addition, our proposal has a few other valuable features, e.g., it is responsive, it scales well, and it does not send any overhead messages.

Conrad Burchert and Roger Wattenhofer. piChain: When a Blockchain meets Paxos. In 21st International Conference on Principles of Distributed Systems (OPODIS 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 95, pp. 2:1-2:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{burchert_et_al:LIPIcs.OPODIS.2017.2, author = {Burchert, Conrad and Wattenhofer, Roger}, title = {{piChain: When a Blockchain meets Paxos}}, booktitle = {21st International Conference on Principles of Distributed Systems (OPODIS 2017)}, pages = {2:1--2:13}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-061-3}, ISSN = {1868-8969}, year = {2018}, volume = {95}, editor = {Aspnes, James and Bessani, Alysson and Felber, Pascal and Leit\~{a}o, Jo\~{a}o}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2017.2}, URN = {urn:nbn:de:0030-drops-86543}, doi = {10.4230/LIPIcs.OPODIS.2017.2}, annote = {Keywords: Consensus, Crash Failures, Availability, Network Partition, Consistency} }

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Brief Announcement

**Published in:** LIPIcs, Volume 91, 31st International Symposium on Distributed Computing (DISC 2017)

Contrary to common belief, a recent work by Ellen, Gelashvili, Shavit, and Zhu has shown that computability does not require multicore architectures to support "strong" synchronization instructions like compare-and-swap, as opposed to combinations of "weaker" instructions like decrement and multiply. However, this is the status quo, and in turn, most efficient concurrent data-structures heavily rely on compare-and-swap (e.g. for swinging pointers).
We show that this need not be the case, by designing and implementing a concurrent linearizable Log data-structure (also known as a History object), supporting two operations: append(item), which appends the item to the log, and get-log(), which returns the appended items so far, in order. Readers are wait-free and writers are lock-free, hence this data-structure can be used in a lock-free universal construction to implement any concurrent object with a given sequential specification. Our implementation uses atomic read, xor, decrement, and fetch-and-increment instructions supported on X86 architectures, and provides similar performance to a compare-and-swap-based solution on today's hardware. This raises a fundamental question about minimal set of synchronization instructions that the architectures have to support.

Rati Gelashvili, Idit Keidar, Alexander Spiegelman, and Roger Wattenhofer. Brief Announcement: Towards Reduced Instruction Sets for Synchronization. In 31st International Symposium on Distributed Computing (DISC 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 91, pp. 53:1-53:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{gelashvili_et_al:LIPIcs.DISC.2017.53, author = {Gelashvili, Rati and Keidar, Idit and Spiegelman, Alexander and Wattenhofer, Roger}, title = {{Brief Announcement: Towards Reduced Instruction Sets for Synchronization}}, booktitle = {31st International Symposium on Distributed Computing (DISC 2017)}, pages = {53:1--53:4}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-053-8}, ISSN = {1868-8969}, year = {2017}, volume = {91}, editor = {Richa, Andr\'{e}a}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2017.53}, URN = {urn:nbn:de:0030-drops-80201}, doi = {10.4230/LIPIcs.DISC.2017.53}, annote = {Keywords: Consensus hierarchy, universal construction, synchronization instruction.} }

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**Published in:** LIPIcs, Volume 81, Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques (APPROX/RANDOM 2017)

In the min-cost bipartite perfect matching with delays (MBPMD) problem, requests arrive online at points of a finite metric space. Each request is either positive or negative and has to be matched to a request of opposite polarity. As opposed to traditional online matching problems, the algorithm does not have to serve requests as they arrive, and may choose to match them later at a cost. Our objective is to minimize the sum of the distances between matched pairs of requests (the connection cost) and the sum of the waiting times of the requests (the delay cost). This objective exhibits a natural tradeoff between minimizing the distances and the cost of waiting for better matches. This tradeoff appears in many real-life scenarios, notably, ride-sharing platforms. MBPMD is related to its non-bipartite variant, min-cost perfect matching with delays (MPMD), in which each request can be matched to any other request. MPMD was introduced by Emek et al. (STOC'16), who showed an O(log^2(n)+log(Delta))-competitive randomized algorithm on n-point metric spaces with aspect ratio Delta.
Our contribution is threefold. First, we present a new lower bound construction for MPMD and MBPMD. We get a lower bound of Omega(sqrt(log(n)/log(log(n)))) on the competitive ratio of any randomized algorithm for MBPMD. For MPMD, we improve the lower bound from Omega(sqrt(log(n))) (shown by Azar et al., SODA'17) to Omega(log(n)/log(log(n))), thus, almost matching their upper bound of O(log(n)). Second, we adapt the algorithm of Emek et al. to the bipartite case, and provide a simplified analysis that improves the competitive ratio to O(log(n)). The key ingredient of the algorithm is an O(h)-competitive randomized algorithm for MBPMD on weighted trees of height h. Third, we provide an O(h)-competitive deterministic algorithm for MBPMD on weighted trees of height h. This algorithm is obtained by adapting the algorithm for MPMD by Azar et al. to the apparently more complicated bipartite setting.

Itai Ashlagi, Yossi Azar, Moses Charikar, Ashish Chiplunkar, Ofir Geri, Haim Kaplan, Rahul Makhijani, Yuyi Wang, and Roger Wattenhofer. Min-Cost Bipartite Perfect Matching with Delays. In Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques (APPROX/RANDOM 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 81, pp. 1:1-1:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{ashlagi_et_al:LIPIcs.APPROX-RANDOM.2017.1, author = {Ashlagi, Itai and Azar, Yossi and Charikar, Moses and Chiplunkar, Ashish and Geri, Ofir and Kaplan, Haim and Makhijani, Rahul and Wang, Yuyi and Wattenhofer, Roger}, title = {{Min-Cost Bipartite Perfect Matching with Delays}}, booktitle = {Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques (APPROX/RANDOM 2017)}, pages = {1:1--1:20}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-044-6}, ISSN = {1868-8969}, year = {2017}, volume = {81}, editor = {Jansen, Klaus and Rolim, Jos\'{e} D. P. and Williamson, David P. and Vempala, Santosh S.}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.APPROX-RANDOM.2017.1}, URN = {urn:nbn:de:0030-drops-75509}, doi = {10.4230/LIPIcs.APPROX-RANDOM.2017.1}, annote = {Keywords: online algorithms with delayed service, bipartite matching, competitive analysis} }

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**Published in:** LIPIcs, Volume 80, 44th International Colloquium on Automata, Languages, and Programming (ICALP 2017)

For the game of Cops and Robbers, it is known that in 1-cop-win graphs, the cop can capture the robber in O(n) time, and that there exist graphs in which this capture time is tight. When k >= 2, a simple counting argument shows that in k-cop-win graphs, the capture time is at most O(n^{k + 1}), however, no non-trivial lower bounds were previously known; indeed, in their 2011 book, Bonato and Nowakowski ask whether this upper bound can be improved. In this paper, the question of Bonato and Nowakowski is answered on the negative, proving that the O(n^{k + 1}) bound is asymptotically tight for any constant k >= 2. This yields a surprising gap in the capture time complexities between the 1-cop and the 2-cop cases.

Sebastian Brandt, Yuval Emek, Jara Uitto, and Roger Wattenhofer. A Tight Lower Bound for the Capture Time of the Cops and Robbers Game. In 44th International Colloquium on Automata, Languages, and Programming (ICALP 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 80, pp. 82:1-82:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{brandt_et_al:LIPIcs.ICALP.2017.82, author = {Brandt, Sebastian and Emek, Yuval and Uitto, Jara and Wattenhofer, Roger}, title = {{A Tight Lower Bound for the Capture Time of the Cops and Robbers Game}}, booktitle = {44th International Colloquium on Automata, Languages, and Programming (ICALP 2017)}, pages = {82:1--82:13}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-041-5}, ISSN = {1868-8969}, year = {2017}, volume = {80}, editor = {Chatzigiannakis, Ioannis and Indyk, Piotr and Kuhn, Fabian and Muscholl, Anca}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2017.82}, URN = {urn:nbn:de:0030-drops-74134}, doi = {10.4230/LIPIcs.ICALP.2017.82}, annote = {Keywords: cops and robbers, capture time, lower bound} }

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**Published in:** LIPIcs, Volume 70, 20th International Conference on Principles of Distributed Systems (OPODIS 2016)

Consider a complete bipartite graph of 2n nodes with n nodes on each side. In a round, each node can either send at most one message to a neighbor or receive at most one message from a neighbor. Each node has a preference list that ranks all its neighbors in a strict order from 1 to n. We introduce a non-negative similarity parameter D < n for the preference lists of nodes on one side only. For D = 0, these preference lists are same and for D = n-1, they can be completely arbitrary. There is no restriction on the preference lists of the other side. We show that each node can compute its partner in a stable matching by receiving O(n(D + 1)) messages of size O(log n) each. We also show that this is optimal (up to a logarithmic factor) if D is constant.

Pankaj Khanchandani and Roger Wattenhofer. Distributed Stable Matching with Similar Preference Lists. In 20th International Conference on Principles of Distributed Systems (OPODIS 2016). Leibniz International Proceedings in Informatics (LIPIcs), Volume 70, pp. 12:1-12:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{khanchandani_et_al:LIPIcs.OPODIS.2016.12, author = {Khanchandani, Pankaj and Wattenhofer, Roger}, title = {{Distributed Stable Matching with Similar Preference Lists}}, booktitle = {20th International Conference on Principles of Distributed Systems (OPODIS 2016)}, pages = {12:1--12:16}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-031-6}, ISSN = {1868-8969}, year = {2017}, volume = {70}, editor = {Fatourou, Panagiota and Jim\'{e}nez, Ernesto and Pedone, Fernando}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2016.12}, URN = {urn:nbn:de:0030-drops-70811}, doi = {10.4230/LIPIcs.OPODIS.2016.12}, annote = {Keywords: distributed stable matching, similar preference lists, stable matching, stable marriage} }

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**Published in:** LIPIcs, Volume 46, 19th International Conference on Principles of Distributed Systems (OPODIS 2015)

Consider a group of mobile finite automata, referred to as agents, located in the origin of an infinite grid. The grid is occupied by obstacles, i.e., sets of cells that can not be entered by the agents. In every step, an agent can sense the states of the co-located agents and is allowed to move to any neighboring cell of the grid not blocked by an obstacle. We assume that the circumference of each obstacle is finite but allow the number of obstacles to be unbounded. The task of the agents is to cooperatively find a treasure, hidden in the grid by an adversary.
In this work, we show how the agents can utilize their simple means of communication and their constant memory to systematically explore the grid and to locate the treasure in finite time. As integral part of the agents' behavior, we present a method that allows a group of six agents to follow a straight line, even if the line is partially obstructed by obstacles, and to discover all free cells along this line. In total, our search protocol requires nine agents.

Tobias Langner, Barbara Keller, Jara Uitto, and Roger Wattenhofer. Overcoming Obstacles with Ants. In 19th International Conference on Principles of Distributed Systems (OPODIS 2015). Leibniz International Proceedings in Informatics (LIPIcs), Volume 46, pp. 9:1-9:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2016)

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@InProceedings{langner_et_al:LIPIcs.OPODIS.2015.9, author = {Langner, Tobias and Keller, Barbara and Uitto, Jara and Wattenhofer, Roger}, title = {{Overcoming Obstacles with Ants}}, booktitle = {19th International Conference on Principles of Distributed Systems (OPODIS 2015)}, pages = {9:1--9:17}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-98-9}, ISSN = {1868-8969}, year = {2016}, volume = {46}, editor = {Anceaume, Emmanuelle and Cachin, Christian and Potop-Butucaru, Maria}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2015.9}, URN = {urn:nbn:de:0030-drops-66005}, doi = {10.4230/LIPIcs.OPODIS.2015.9}, annote = {Keywords: Mobile agents, algorithms, treasure search} }

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**Published in:** LIPIcs, Volume 46, 19th International Conference on Principles of Distributed Systems (OPODIS 2015)

We introduce a stronger validity property for the byzantine agreement problem with orderable initial values: The median validity property. In particular, the decision value is required to be close to the median of the initial values of the non-byzantine nodes. The proximity to the median scales with the desired level of fault-tolerance: If no fault-tolerance is required, algorithms have to decide for the true median. If the number of failures is maximal, algorithms must still decide on a value within the range of the input values of the non-byzantine nodes. We present a deterministic algorithm satisfying this property for n >= 3t+1 within t+1 phases, where t is the maximum number of byzantine nodes and n is the total number of nodes.

David Stolz and Roger Wattenhofer. Byzantine Agreement with Median Validity. In 19th International Conference on Principles of Distributed Systems (OPODIS 2015). Leibniz International Proceedings in Informatics (LIPIcs), Volume 46, pp. 22:1-22:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2016)

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@InProceedings{stolz_et_al:LIPIcs.OPODIS.2015.22, author = {Stolz, David and Wattenhofer, Roger}, title = {{Byzantine Agreement with Median Validity}}, booktitle = {19th International Conference on Principles of Distributed Systems (OPODIS 2015)}, pages = {22:1--22:14}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-98-9}, ISSN = {1868-8969}, year = {2016}, volume = {46}, editor = {Anceaume, Emmanuelle and Cachin, Christian and Potop-Butucaru, Maria}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2015.22}, URN = {urn:nbn:de:0030-drops-65911}, doi = {10.4230/LIPIcs.OPODIS.2015.22}, annote = {Keywords: Reliability, fault-tolerance, median, consensus} }

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**Published in:** Dagstuhl Seminar Proceedings, Volume 10211, Flexible Network Design (2010)

From Monday 24.05.2010---Friday 28.05.2010, the Dagstuhl Seminar 10211 ``Flexible Network Design '' was held in Schloss Dagstuhl~--~Leibniz Center for Informatics.
During the seminar, several participants presented their current
research, and ongoing work and open problems were discussed. Abstracts of
the presentations given during the seminar as well as abstracts of
seminar results and ideas are put together in this paper. The first section
describes the seminar topics and goals in general.
Links to extended abstracts or full papers are provided, if available.

Anupam Gupta, Stefano Leonardi, Berthold Vöcking, and Roger Wattenhofer. 10211 Abstracts Collection – Flexible Network Design. In Flexible Network Design. Dagstuhl Seminar Proceedings, Volume 10211, pp. 1-15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2010)

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@InProceedings{gupta_et_al:DagSemProc.10211.1, author = {Gupta, Anupam and Leonardi, Stefano and V\"{o}cking, Berthold and Wattenhofer, Roger}, title = {{10211 Abstracts Collection – Flexible Network Design}}, booktitle = {Flexible Network Design}, pages = {1--15}, series = {Dagstuhl Seminar Proceedings (DagSemProc)}, ISSN = {1862-4405}, year = {2010}, volume = {10211}, editor = {Anupam Gupta and Stefano Leonardi and Berthold V\"{o}cking and Roger Wattenhofer}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/DagSemProc.10211.1}, URN = {urn:nbn:de:0030-drops-27279}, doi = {10.4230/DagSemProc.10211.1}, annote = {Keywords: Network Design, Approximation Algorithms, Game Theory and Mechanism Design, Wireless Networks} }

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**Published in:** Dagstuhl Seminar Proceedings, Volume 9371, Algorithmic Methods for Distributed Cooperative Systems (2010)

An introduction to distributed algorithms, in particular local algorithms. Essentially a practice talk of my SSS 2009 invited talk.

Christoph Lenzen, Jukka Suomela, and Roger Wattenhofer. Local Algorithms: Self-Stabilization on Speed. In Algorithmic Methods for Distributed Cooperative Systems. Dagstuhl Seminar Proceedings, Volume 9371, pp. 1-18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2010)

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@InProceedings{lenzen_et_al:DagSemProc.09371.3, author = {Lenzen, Christoph and Suomela, Jukka and Wattenhofer, Roger}, title = {{Local Algorithms: Self-Stabilization on Speed}}, booktitle = {Algorithmic Methods for Distributed Cooperative Systems}, pages = {1--18}, series = {Dagstuhl Seminar Proceedings (DagSemProc)}, ISSN = {1862-4405}, year = {2010}, volume = {9371}, editor = {S\'{a}ndor Fekete and Stefan Fischer and Martin Riedmiller and Suri Subhash}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/DagSemProc.09371.3}, URN = {urn:nbn:de:0030-drops-24257}, doi = {10.4230/DagSemProc.09371.3}, annote = {Keywords: Local Algorithms, Self-Stabilization, Lower Bounds, Upper Bounds, MIS} }

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**Published in:** Dagstuhl Seminar Proceedings, Volume 7151, Geometry in Sensor Networks (2007)

From 9.4.2007 to 13.4.07, the Dagstuhl Seminar 07151 ``Geometry in Sensor
Networks'' was held in the International Conference and Research Center
(IBFI), Schloss Dagstuhl.
During the seminar, several participants presented their current
research, and ongoing work and open problems were discussed. Abstracts of
the presentations given during the seminar as well as abstracts of
seminar results and ideas are put together in this paper. The first
section describes the seminar topics and goals in general.
Links to extended abstracts or full papers are provided, if available.

Subhash Suri, Roger Wattenhofer, and Peter Widmayer. 07151 Abstracts Collection – Geometry in Sensor Networks. In Geometry in Sensor Networks. Dagstuhl Seminar Proceedings, Volume 7151, pp. 1-11, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2007)

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@InProceedings{suri_et_al:DagSemProc.07151.1, author = {Suri, Subhash and Wattenhofer, Roger and Widmayer, Peter}, title = {{07151 Abstracts Collection – Geometry in Sensor Networks}}, booktitle = {Geometry in Sensor Networks}, pages = {1--11}, series = {Dagstuhl Seminar Proceedings (DagSemProc)}, ISSN = {1862-4405}, year = {2007}, volume = {7151}, editor = {Subhash Suri and Roger Wattenhofer and Peter Widmayer}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/DagSemProc.07151.1}, URN = {urn:nbn:de:0030-drops-11206}, doi = {10.4230/DagSemProc.07151.1}, annote = {Keywords: Sensor networks, computational geometry, distributed algorithms, mobile computing} }

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**Published in:** Dagstuhl Seminar Proceedings, Volume 6131, Peer-to-Peer-Systems and -Applications (2006)

Many P2P systems are only proven efficient for static environments.
However, in practice, P2P systems are often very dynamic in the sense that
peers can join and leave a system at any time and concurrently. In the first
part of my talk, I will present a DHT we have developed recently in our
group which maintains desirable properties under worst-case churn. In the
second part of my talk, we will briefly look at another challenge of prime
importance in P2P computing, namely selfishness. Concretely, some results
are presented concerning the impact of selfish behavior on the performance
of P2P topologies.

Stefan Schmid, Thomas Moscibroda, and Roger Wattenhofer. On the Topologies Formed by Selfish Peers. In Peer-to-Peer-Systems and -Applications. Dagstuhl Seminar Proceedings, Volume 6131, pp. 1-5, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2006)

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@InProceedings{schmid_et_al:DagSemProc.06131.4, author = {Schmid, Stefan and Moscibroda, Thomas and Wattenhofer, Roger}, title = {{On the Topologies Formed by Selfish Peers}}, booktitle = {Peer-to-Peer-Systems and -Applications}, pages = {1--5}, series = {Dagstuhl Seminar Proceedings (DagSemProc)}, ISSN = {1862-4405}, year = {2006}, volume = {6131}, editor = {Anthony D. Joseph and Ralf Steinmetz and Klaus Wehrle}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/DagSemProc.06131.4}, URN = {urn:nbn:de:0030-drops-6431}, doi = {10.4230/DagSemProc.06131.4}, annote = {Keywords: Churn, Selfishness, P2P Topologies} }

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**Published in:** Dagstuhl Seminar Proceedings, Volume 6131, Peer-to-Peer-Systems and -Applications (2006)

Peer-to-peer systems are often faced with the problem of frequent membership changes. However, many systems are only proven efficient or correct in static environments. In my talk, I will present techniques to maintain desirable properties of a distributed hash table (low peer degree, low network diameter) in spite of ongoing and concurrent dynamics. I will then go on and study the effect of peers not acting according to our protocols. Concretely, I assume that peers are selfish and choose the behavior which maximizes their utility. I will report on our results concerning the impact of selfishness on the peer-to-peer topology.

Stefan Schmid, Fabian Kuhn, Thomas Moscibroda, and Roger Wattenhofer. Taming Dynamic and Selfish Peers. In Peer-to-Peer-Systems and -Applications. Dagstuhl Seminar Proceedings, Volume 6131, pp. 1-14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2006)

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@InProceedings{schmid_et_al:DagSemProc.06131.5, author = {Schmid, Stefan and Kuhn, Fabian and Moscibroda, Thomas and Wattenhofer, Roger}, title = {{Taming Dynamic and Selfish Peers}}, booktitle = {Peer-to-Peer-Systems and -Applications}, pages = {1--14}, series = {Dagstuhl Seminar Proceedings (DagSemProc)}, ISSN = {1862-4405}, year = {2006}, volume = {6131}, editor = {Anthony D. Joseph and Ralf Steinmetz and Klaus Wehrle}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/DagSemProc.06131.5}, URN = {urn:nbn:de:0030-drops-6477}, doi = {10.4230/DagSemProc.06131.5}, annote = {Keywords: Churn, Selfishness, P2P Topologies} }

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