36 Search Results for "Wattenhofer, Roger"

Document
DOI: 10.4230/LIPIcs.DISC.2024.15
Convex Consensus with Asynchronous Fallback

Authors: Andrei Constantinescu, Diana Ghinea, Roger Wattenhofer, and Floris Westermann

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

Abstract
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.
Cite as

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}
}
Document
Brief Announcement
DOI: 10.4230/LIPIcs.DISC.2024.43
Brief Announcement: Unifying Partial Synchrony

Authors: Andrei Constantinescu, Diana Ghinea, Jakub Sliwinski, and Roger Wattenhofer

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

Abstract
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.
Cite as

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}
}
Document
DOI: 10.4230/LIPIcs.AFT.2024.28
SoK: Attacks on DAOs

Authors: Rainer Feichtinger, Robin Fritsch, Lioba Heimbach, Yann Vonlanthen, and Roger Wattenhofer

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

Abstract
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.
Cite as

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}
}
Document
Track A: Algorithms, Complexity and Games
DOI: 10.4230/LIPIcs.ICALP.2024.48
Solving Woeginger’s Hiking Problem: Wonderful Partitions in Anonymous Hedonic Games

Authors: Andrei Constantinescu, Pascal Lenzner, Rebecca Reiffenhäuser, Daniel Schmand, and Giovanna Varricchio

Published in: LIPIcs, Volume 297, 51st International Colloquium on Automata, Languages, and Programming (ICALP 2024)

Abstract
A decade ago, Gerhard Woeginger posed an open problem that became well-known as "Woeginger’s Hiking Problem": Consider a group of n people that want to go hiking; everyone expresses preferences over the size of their hiking group in the form of an interval between 1 and n. Is it possible to efficiently assign the n people to a set of hiking subgroups so that every person approves the size of their assigned subgroup? The problem is also known as efficiently deciding if an instance of an anonymous Hedonic Game with interval approval preferences admits a wonderful partition. We resolve the open problem in the affirmative by presenting an O(n⁵) time algorithm for Woeginger’s Hiking Problem. Our solution is based on employing a dynamic programming approach for a specific rectangle stabbing problem from computational geometry. Moreover, we propose natural, more demanding extensions of the problem, e.g., maximizing the number of satisfied participants and variants with single-peaked preferences, and show that they are also efficiently solvable. Last but not least, we employ our solution to efficiently compute a partition that maximizes the egalitarian welfare for anonymous single-peaked Hedonic Games.
Cite as

Andrei Constantinescu, Pascal Lenzner, Rebecca Reiffenhäuser, Daniel Schmand, and Giovanna Varricchio. Solving Woeginger’s Hiking Problem: Wonderful Partitions in Anonymous Hedonic Games. In 51st International Colloquium on Automata, Languages, and Programming (ICALP 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 297, pp. 48:1-48:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{constantinescu_et_al:LIPIcs.ICALP.2024.48,
  author =	{Constantinescu, Andrei and Lenzner, Pascal and Reiffenh\"{a}user, Rebecca and Schmand, Daniel and Varricchio, Giovanna},
  title =	{{Solving Woeginger’s Hiking Problem: Wonderful Partitions in Anonymous Hedonic Games}},
  booktitle =	{51st International Colloquium on Automata, Languages, and Programming (ICALP 2024)},
  pages =	{48:1--48:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-322-5},
  ISSN =	{1868-8969},
  year =	{2024},
  volume =	{297},
  editor =	{Bringmann, Karl and Grohe, Martin and Puppis, Gabriele and Svensson, Ola},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2024.48},
  URN =		{urn:nbn:de:0030-drops-201910},
  doi =		{10.4230/LIPIcs.ICALP.2024.48},
  annote =	{Keywords: Algorithmic Game Theory, Dynamic Programming, Anonymous Hedonic Games, Single-Peaked Preferences, Social Optimum, Wonderful Partitions}
}
Document
DOI: 10.4230/LIPIcs.SAND.2024.5
Fault-Tolerant Distributed Directories

Authors: Judith Beestermöller, Costas Busch, and Roger Wattenhofer

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

Abstract
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.
Cite as

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}
}
Document
Invited Talk
DOI: 10.4230/LIPIcs.OPODIS.2023.3
Distributed Algorithms as a Gateway To Deductive Learning (Invited Talk)

Authors: Roger Wattenhofer

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

Abstract
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.
Cite as

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}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2023.8
A Fair and Resilient Decentralized Clock Network for Transaction Ordering

Authors: Andrei Constantinescu, Diana Ghinea, Lioba Heimbach, Zilin Wang, and Roger Wattenhofer

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

Abstract
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.
Cite as

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}
}
Document
DOI: 10.4230/LIPIcs.AFT.2023.9
DeFi Lending During The Merge

Authors: Lioba Heimbach, Eric Schertenleib, and Roger Wattenhofer

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

Abstract
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.
Cite as

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}
}
Document
DOI: 10.4230/DagRep.12.11.28
Computational Social Dynamics (Dagstuhl Seminar 22452)

Authors: Martin Hoefer, Sigal Oren, Roger Wattenhofer, and Giovanna Varricchio

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

Abstract
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.
Cite as

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}
}
Document
Invited Talk
DOI: 10.4230/LIPIcs.SAND.2022.3
Networks, Dynamics, Algorithms, and Learning (Invited Talk)

Authors: Roger Wattenhofer

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

Abstract
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.
Cite as

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}
}
Document
DOI: 10.4230/LIPIcs.DISC.2021.33
Algorithms for the Minimum Dominating Set Problem in Bounded Arboricity Graphs: Simpler, Faster, and Combinatorial

Authors: Adir Morgan, Shay Solomon, and Nicole Wein

Published in: LIPIcs, Volume 209, 35th International Symposium on Distributed Computing (DISC 2021)

Abstract
We revisit the minimum dominating set problem on graphs with arboricity bounded by α. In the (standard) centralized setting, Bansal and Umboh [Bansal and Umboh, 2017] gave an O(α)-approximation LP rounding algorithm, which also translates into a near-linear time algorithm using general-purpose approximation results for explicit mixed packing and covering or pure covering LPs [Koufogiannakis and Young, 2014; Young, 2014; Allen-Zhu and Orecchia, 2019; Quanrud, 2020]. Moreover, [Bansal and Umboh, 2017] showed that it is NP-hard to achieve an asymptotic improvement for the approximation factor. On the other hand, the previous two non-LP-based algorithms, by Lenzen and Wattenhofer [Christoph Lenzen and Roger Wattenhofer, 2010], and Jones et al. [Jones et al., 2013], achieve an approximation factor of O(α²) in linear time. There is a similar situation in the distributed setting: While there is an O(log² n)-round LP-based O(α)-approximation algorithm implied in [Kuhn et al., 2006], the best non-LP-based algorithm by Lenzen and Wattenhofer [Christoph Lenzen and Roger Wattenhofer, 2010] is an implementation of their centralized algorithm, providing an O(α²)-approximation within O(log n) rounds. We address the questions of whether one can achieve an O(α)-approximation algorithm that is elementary, i.e., not based on any LP-based methods, either in the centralized setting or in the distributed setting. We resolve both questions in the affirmative, and en route achieve algorithms that are faster than the state-of-the-art LP-based algorithms. Our contribution is two-fold: 1) In the centralized setting, we provide a surprisingly simple combinatorial algorithm that is asymptotically optimal in terms of both approximation factor and running time: an O(α)-approximation in linear time. The previous state-of-the-art O(α)-approximation algorithms are (1) LP-based, (2) more complicated, and (3) have super-linear running time. 2) Based on our centralized algorithm, we design a distributed combinatorial O(α)-approximation algorithm in the CONGEST model that runs in O(αlog n) rounds with high probability. Not only does this result provide the first nontrivial non-LP-based distributed o(α²)-approximation algorithm for this problem, it also outperforms the best LP-based distributed algorithm for a wide range of parameters.
Cite as

Adir Morgan, Shay Solomon, and Nicole Wein. Algorithms for the Minimum Dominating Set Problem in Bounded Arboricity Graphs: Simpler, Faster, and Combinatorial. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 33:1-33:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{morgan_et_al:LIPIcs.DISC.2021.33,
  author =	{Morgan, Adir and Solomon, Shay and Wein, Nicole},
  title =	{{Algorithms for the Minimum Dominating Set Problem in Bounded Arboricity Graphs: Simpler, Faster, and Combinatorial}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{33:1--33:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-210-5},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{209},
  editor =	{Gilbert, Seth},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.33},
  URN =		{urn:nbn:de:0030-drops-148353},
  doi =		{10.4230/LIPIcs.DISC.2021.33},
  annote =	{Keywords: Graph Algorithms, Dominating Set, Bounded Arboricity, Linear time algorithms}
}
Document
DOI: 10.4230/LIPIcs.MFCS.2021.83
Stabilization Bounds for Influence Propagation from a Random Initial State

Authors: Pál András Papp and Roger Wattenhofer

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

Abstract
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).
Cite as

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}
}
Document
DOI: 10.4230/LIPIcs.ITCS.2021.52
Sequential Defaulting in Financial Networks

Authors: Pál András Papp and Roger Wattenhofer

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

Abstract
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.
Cite as

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}
}
Document
DOI: 10.4230/LIPIcs.ISAAC.2020.61
The k-Server Problem with Delays on the Uniform Metric Space

Authors: Predrag Krnetić, Darya Melnyk, Yuyi Wang, and Roger Wattenhofer

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

Abstract
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.
Cite as

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}
}
Document
Track A: Algorithms, Complexity and Games
DOI: 10.4230/LIPIcs.ICALP.2020.90
A General Stabilization Bound for Influence Propagation in Graphs

Authors: Pál András Papp and Roger Wattenhofer

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

Abstract
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.
Cite as

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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