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Documents authored by Schmid, Stefan

Document
DOI: 10.4230/LIPIcs.STACS.2024.14
A Subquadratic Bound for Online Bisection

Authors: Marcin Bienkowski and Stefan Schmid

Published in: LIPIcs, Volume 289, 41st International Symposium on Theoretical Aspects of Computer Science (STACS 2024)

Abstract
The online bisection problem is a natural dynamic variant of the classic optimization problem, where one has to dynamically maintain a partition of n elements into two clusters of cardinality n/2. During runtime, an online algorithm is given a sequence of requests, each being a pair of elements: an inter-cluster request costs one unit while an intra-cluster one is free. The algorithm may change the partition, paying a unit cost for each element that changes its cluster. This natural problem admits a simple deterministic O(n²)-competitive algorithm [Avin et al., DISC 2016]. While several significant improvements over this result have been obtained since the original work, all of them either limit the generality of the input or assume some form of resource augmentation (e.g., larger clusters). Moreover, the algorithm of Avin et al. achieves the best known competitive ratio even if randomization is allowed. In this paper, we present the first randomized online algorithm that breaks this natural quadratic barrier and achieves a competitive ratio of Õ(n^{23/12}) without resource augmentation and for an arbitrary sequence of requests.
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Marcin Bienkowski and Stefan Schmid. A Subquadratic Bound for Online Bisection. In 41st International Symposium on Theoretical Aspects of Computer Science (STACS 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 289, pp. 14:1-14:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{bienkowski_et_al:LIPIcs.STACS.2024.14,
  author =	{Bienkowski, Marcin and Schmid, Stefan},
  title =	{{A Subquadratic Bound for Online Bisection}},
  booktitle =	{41st International Symposium on Theoretical Aspects of Computer Science (STACS 2024)},
  pages =	{14:1--14:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-311-9},
  ISSN =	{1868-8969},
  year =	{2024},
  volume =	{289},
  editor =	{Beyersdorff, Olaf and Kant\'{e}, Mamadou Moustapha and Kupferman, Orna and Lokshtanov, Daniel},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2024.14},
  URN =		{urn:nbn:de:0030-drops-197247},
  doi =		{10.4230/LIPIcs.STACS.2024.14},
  annote =	{Keywords: Bisection, Graph Partitioning, online balanced Repartitioning, online Algorithms, competitive Analysis}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2023.11
On the Convergence Time in Graphical Games: A Locality-Sensitive Approach

Authors: Juho Hirvonen, Laura Schmid, Krishnendu Chatterjee, and Stefan Schmid

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

Abstract
Graphical games are a useful framework for modeling the interactions of (selfish) agents who are connected via an underlying topology and whose behaviors influence each other. They have wide applications ranging from computer science to economics and biology. Yet, even though an agent’s payoff only depends on the actions of their direct neighbors in graphical games, computing the Nash equilibria and making statements about the convergence time of "natural" local dynamics in particular can be highly challenging. In this work, we present a novel approach for classifying complexity of Nash equilibria in graphical games by establishing a connection to local graph algorithms, a subfield of distributed computing. In particular, we make the observation that the equilibria of graphical games are equivalent to locally verifiable labelings (LVL) in graphs; vertex labelings which are verifiable with constant-round local algorithms. This connection allows us to derive novel lower bounds on the convergence time to equilibrium of best-response dynamics in graphical games. Since we establish that distributed convergence can sometimes be provably slow, we also introduce and give bounds on an intuitive notion of "time-constrained" inefficiency of best responses. We exemplify how our results can be used in the implementation of mechanisms that ensure convergence of best responses to a Nash equilibrium. Our results thus also give insight into the convergence of strategy-proof algorithms for graphical games, which is still not well understood.
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Juho Hirvonen, Laura Schmid, Krishnendu Chatterjee, and Stefan Schmid. On the Convergence Time in Graphical Games: A Locality-Sensitive Approach. In 27th International Conference on Principles of Distributed Systems (OPODIS 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 286, pp. 11:1-11:24, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{hirvonen_et_al:LIPIcs.OPODIS.2023.11,
  author =	{Hirvonen, Juho and Schmid, Laura and Chatterjee, Krishnendu and Schmid, Stefan},
  title =	{{On the Convergence Time in Graphical Games: A Locality-Sensitive Approach}},
  booktitle =	{27th International Conference on Principles of Distributed Systems (OPODIS 2023)},
  pages =	{11:1--11:24},
  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-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2023.11},
  URN =		{urn:nbn:de:0030-drops-195015},
  doi =		{10.4230/LIPIcs.OPODIS.2023.11},
  annote =	{Keywords: distributed computing, Nash equilibria, mechanism design, best-response dynamics}
}
Document
DOI: 10.4230/LIPIcs.ESA.2023.44
Online Algorithms with Randomly Infused Advice

Authors: Yuval Emek, Yuval Gil, Maciej Pacut, and Stefan Schmid

Published in: LIPIcs, Volume 274, 31st Annual European Symposium on Algorithms (ESA 2023)

Abstract
We introduce a novel method for the rigorous quantitative evaluation of online algorithms that relaxes the "radical worst-case" perspective of classic competitive analysis. In contrast to prior work, our method, referred to as randomly infused advice (RIA), does not make any assumptions about the input sequence and does not rely on the development of designated online algorithms. Rather, it can be applied to existing online randomized algorithms, introducing a means to evaluate their performance in scenarios that lie outside the radical worst-case regime. More concretely, an online algorithm ALG with RIA benefits from pieces of advice generated by an omniscient but not entirely reliable oracle. The crux of the new method is that the advice is provided to ALG by writing it into the buffer ℬ from which ALG normally reads its random bits, hence allowing us to augment it through a very simple and non-intrusive interface. The (un)reliability of the oracle is captured via a parameter 0 ≤ α ≤ 1 that determines the probability (per round) that the advice is successfully infused by the oracle; if the advice is not infused, which occurs with probability 1 - α, then the buffer ℬ contains fresh random bits (as in the classic online setting). The applicability of the new RIA method is demonstrated by applying it to three extensively studied online problems: paging, uniform metrical task systems, and online set cover. For these problems, we establish new upper bounds on the competitive ratio of classic online algorithms that improve as the infusion parameter α increases. These are complemented with (often tight) lower bounds on the competitive ratio of online algorithms with RIA for the three problems.
Cite as

Yuval Emek, Yuval Gil, Maciej Pacut, and Stefan Schmid. Online Algorithms with Randomly Infused Advice. In 31st Annual European Symposium on Algorithms (ESA 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 274, pp. 44:1-44:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{emek_et_al:LIPIcs.ESA.2023.44,
  author =	{Emek, Yuval and Gil, Yuval and Pacut, Maciej and Schmid, Stefan},
  title =	{{Online Algorithms with Randomly Infused Advice}},
  booktitle =	{31st Annual European Symposium on Algorithms (ESA 2023)},
  pages =	{44:1--44:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-295-2},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{274},
  editor =	{G{\o}rtz, Inge Li and Farach-Colton, Martin and Puglisi, Simon J. and Herman, Grzegorz},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.ESA.2023.44},
  URN =		{urn:nbn:de:0030-drops-186970},
  doi =		{10.4230/LIPIcs.ESA.2023.44},
  annote =	{Keywords: Online algorithms, competitive analysis, advice}
}
Document
DOI: 10.4230/DagRep.12.11.96
Towards More Flexible and Automated Communication Networks (Dagstuhl Seminar 22471)

Authors: Artur Hecker, Stefan Schmid, Henning Schulzrinne, Lily Hügerich, Sándor Laki, and Iosif Salem

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

Abstract
This report documents the program and the outcomes of Dagstuhl Seminar 22471 "Towards More Flexible and Automated Communication Networks". Communication network are becoming more and more automated, allowing to overcome human configuration errors (a frequent reason for outages) and enabling a more fine-grained control, potentially improving also efficiency. For example, the percentage of employees of Telecom companies "really touching the network" is decreasing. The goal of this seminar was to bring together experts in the field to identify and discuss the key challenges in making communication networks more autonomous. To this end, the seminar was structured around a small number of enlightning keynote talks, leaving significant time for breakout sessions and discussions, as well as socializing.
Cite as

Artur Hecker, Stefan Schmid, Henning Schulzrinne, Lily Hügerich, Sándor Laki, and Iosif Salem. Towards More Flexible and Automated Communication Networks (Dagstuhl Seminar 22471). In Dagstuhl Reports, Volume 12, Issue 11, pp. 96-108, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@Article{hecker_et_al:DagRep.12.11.96,
  author =	{Hecker, Artur and Schmid, Stefan and Schulzrinne, Henning and H\"{u}gerich, Lily and Laki, S\'{a}ndor and Salem, Iosif},
  title =	{{Towards More Flexible and Automated Communication Networks (Dagstuhl Seminar 22471)}},
  pages =	{96--108},
  journal =	{Dagstuhl Reports},
  ISSN =	{2192-5283},
  year =	{2023},
  volume =	{12},
  number =	{11},
  editor =	{Hecker, Artur and Schmid, Stefan and Schulzrinne, Henning and H\"{u}gerich, Lily and Laki, S\'{a}ndor and Salem, Iosif},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/DagRep.12.11.96},
  URN =		{urn:nbn:de:0030-drops-178379},
  doi =		{10.4230/DagRep.12.11.96},
  annote =	{Keywords: networking, communication technologies, automation, programmability, flexibility}
}
Document
DOI: 10.4230/LIPIcs.STACS.2023.36
Dynamic Maintenance of Monotone Dynamic Programs and Applications

Authors: Monika Henzinger, Stefan Neumann, Harald Räcke, and Stefan Schmid

Published in: LIPIcs, Volume 254, 40th International Symposium on Theoretical Aspects of Computer Science (STACS 2023)

Abstract
Dynamic programming (DP) is one of the fundamental paradigms in algorithm design. However, many DP algorithms have to fill in large DP tables, represented by two-dimensional arrays, which causes at least quadratic running times and space usages. This has led to the development of improved algorithms for special cases when the DPs satisfy additional properties like, e.g., the Monge property or total monotonicity. In this paper, we consider a new condition which assumes (among some other technical assumptions) that the rows of the DP table are monotone. Under this assumption, we introduce a novel data structure for computing (1+ε)-approximate DP solutions in near-linear time and space in the static setting, and with polylogarithmic update times when the DP entries change dynamically. To the best of our knowledge, our new condition is incomparable to previous conditions and is the first which allows to derive dynamic algorithms based on existing DPs. Instead of using two-dimensional arrays to store the DP tables, we store the rows of the DP tables using monotone piecewise constant functions. This allows us to store length-n DP table rows with entries in [0,W] using only polylog(n,W) bits, and to perform operations, such as (min,+)-convolution or rounding, on these functions in polylogarithmic time. We further present several applications of our data structure. For bicriteria versions of k-balanced graph partitioning and simultaneous source location, we obtain the first dynamic algorithms with subpolynomial update times, as well as the first static algorithms using only near-linear time and space. Additionally, we obtain the currently fastest algorithm for fully dynamic knapsack.
Cite as

Monika Henzinger, Stefan Neumann, Harald Räcke, and Stefan Schmid. Dynamic Maintenance of Monotone Dynamic Programs and Applications. In 40th International Symposium on Theoretical Aspects of Computer Science (STACS 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 254, pp. 36:1-36:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{henzinger_et_al:LIPIcs.STACS.2023.36,
  author =	{Henzinger, Monika and Neumann, Stefan and R\"{a}cke, Harald and Schmid, Stefan},
  title =	{{Dynamic Maintenance of Monotone Dynamic Programs and Applications}},
  booktitle =	{40th International Symposium on Theoretical Aspects of Computer Science (STACS 2023)},
  pages =	{36:1--36:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-266-2},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{254},
  editor =	{Berenbrink, Petra and Bouyer, Patricia and Dawar, Anuj and Kant\'{e}, Mamadou Moustapha},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2023.36},
  URN =		{urn:nbn:de:0030-drops-176889},
  doi =		{10.4230/LIPIcs.STACS.2023.36},
  annote =	{Keywords: Dynamic programming, dynamic algorithms, data structures}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2022.25
Chopin: Combining Distributed and Centralized Schedulers for Self-Adjusting Datacenter Networks

Authors: Neta Rozen-Schiff, Klaus-Tycho Foerster, Stefan Schmid, and David Hay

Published in: LIPIcs, Volume 253, 26th International Conference on Principles of Distributed Systems (OPODIS 2022)

Abstract
The performance of distributed and data-centric applications often critically depends on the interconnecting network. Emerging reconfigurable datacenter networks (RDCNs) are a particularly innovative approach to improve datacenter throughput. Relying on a dynamic optical topology which can be adjusted towards the workload in a demand-aware manner, RDCNs allow to exploit temporal and spatial locality in the communication pattern, and to provide topological shortcuts for frequently communicating racks. The key challenge, however, concerns how to realize demand-awareness in RDCNs in a scalable fashion. This paper presents and evaluates Chopin, a hybrid scheduler for self-adjusting networks that provides demand-awareness at low overhead, by combining centralized and distributed approaches. Chopin allocates optical circuits to elephant flows, through its slower centralized scheduler, utilizing global information. Chopin’s distributed scheduler is orders of magnitude faster and can swiftly react to changes in the traffic and adjust the optical circuits accordingly, by using only local information and running at each rack separately.
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Neta Rozen-Schiff, Klaus-Tycho Foerster, Stefan Schmid, and David Hay. Chopin: Combining Distributed and Centralized Schedulers for Self-Adjusting Datacenter Networks. In 26th International Conference on Principles of Distributed Systems (OPODIS 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 253, pp. 25:1-25:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{rozenschiff_et_al:LIPIcs.OPODIS.2022.25,
  author =	{Rozen-Schiff, Neta and Foerster, Klaus-Tycho and Schmid, Stefan and Hay, David},
  title =	{{Chopin: Combining Distributed and Centralized Schedulers for Self-Adjusting Datacenter Networks}},
  booktitle =	{26th International Conference on Principles of Distributed Systems (OPODIS 2022)},
  pages =	{25:1--25:23},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-265-5},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{253},
  editor =	{Hillel, Eshcar and Palmieri, Roberto and Rivi\`{e}re, Etienne},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2022.25},
  URN =		{urn:nbn:de:0030-drops-176457},
  doi =		{10.4230/LIPIcs.OPODIS.2022.25},
  annote =	{Keywords: reconfigurable optical networks, centralized scheduler, distributed scheduler}
}
Document
DOI: 10.4230/LIPIcs.ITCS.2023.47
Asymptotically Tight Bounds on the Time Complexity of Broadcast and Its Variants in Dynamic Networks

Authors: Antoine El-Hayek, Monika Henzinger, and Stefan Schmid

Published in: LIPIcs, Volume 251, 14th Innovations in Theoretical Computer Science Conference (ITCS 2023)

Abstract
Data dissemination is a fundamental task in distributed computing. This paper studies broadcast problems in various innovative models where the communication network connecting n processes is dynamic (e.g., due to mobility or failures) and controlled by an adversary. In the first model, the processes transitively communicate their ids in synchronous rounds along a rooted tree given in each round by the adversary whose goal is to maximize the number of rounds until at least one id is known by all processes. Previous research has shown a ⌈(3n-1)/2⌉-2 lower bound and an O(nlog log n) upper bound. We show the first linear upper bound for this problem, namely ⌈(1+√2) n-1⌉ ≈ 2.4n. We extend these results to the setting where the adversary gives in each round k-disjoint forests and their goal is to maximize the number of rounds until there is a set of k ids such that each process knows of at least one of them. We give a ⌈3(n-k)/2⌉-1 lower bound and a (π²+6)/6 n+1 ≈ 2.6n upper bound for this problem. Finally, we study the setting where the adversary gives in each round a directed graph with k roots and their goal is to maximize the number of rounds until there exist k ids that are known by all processes. We give a ⌈3(n-3k)/2⌉+2 lower bound and a ⌈(1+√2)n⌉+k-1 ≈ 2.4n+k upper bound for this problem. For the two latter problems no upper or lower bounds were previously known.
Cite as

Antoine El-Hayek, Monika Henzinger, and Stefan Schmid. Asymptotically Tight Bounds on the Time Complexity of Broadcast and Its Variants in Dynamic Networks. In 14th Innovations in Theoretical Computer Science Conference (ITCS 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 251, pp. 47:1-47:21, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{elhayek_et_al:LIPIcs.ITCS.2023.47,
  author =	{El-Hayek, Antoine and Henzinger, Monika and Schmid, Stefan},
  title =	{{Asymptotically Tight Bounds on the Time Complexity of Broadcast and Its Variants in Dynamic Networks}},
  booktitle =	{14th Innovations in Theoretical Computer Science Conference (ITCS 2023)},
  pages =	{47:1--47:21},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-263-1},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{251},
  editor =	{Tauman Kalai, Yael},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.ITCS.2023.47},
  URN =		{urn:nbn:de:0030-drops-175502},
  doi =		{10.4230/LIPIcs.ITCS.2023.47},
  annote =	{Keywords: broadcast, cover, k-broadcast, dynamic radius, dynamic graphs, oblivious message adversary, time complexity}
}
Document
DOI: 10.4230/LIPIcs.ITCS.2023.100
The Time Complexity of Consensus Under Oblivious Message Adversaries

Authors: Kyrill Winkler, Ami Paz, Hugo Rincon Galeana, Stefan Schmid, and Ulrich Schmid

Published in: LIPIcs, Volume 251, 14th Innovations in Theoretical Computer Science Conference (ITCS 2023)

Abstract
We study the problem of solving consensus in synchronous directed dynamic networks, in which communication is controlled by an oblivious message adversary that picks the communication graph to be used in a round from a fixed set of graphs 𝐃 arbitrarily. In this fundamental model, determining consensus solvability and designing efficient consensus algorithms is surprisingly difficult. Enabled by a decision procedure that is derived from a well-established previous consensus solvability characterization for a given set 𝐃, we study, for the first time, the time complexity of solving consensus in this model: We provide both upper and lower bounds for this time complexity, and also relate it to the number of iterations required by the decision procedure. Among other results, we find that reaching consensus under an oblivious message adversary can take exponentially longer than both deciding consensus solvability and broadcasting the input value of some unknown process to all other processes.
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Kyrill Winkler, Ami Paz, Hugo Rincon Galeana, Stefan Schmid, and Ulrich Schmid. The Time Complexity of Consensus Under Oblivious Message Adversaries. In 14th Innovations in Theoretical Computer Science Conference (ITCS 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 251, pp. 100:1-100:28, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{winkler_et_al:LIPIcs.ITCS.2023.100,
  author =	{Winkler, Kyrill and Paz, Ami and Rincon Galeana, Hugo and Schmid, Stefan and Schmid, Ulrich},
  title =	{{The Time Complexity of Consensus Under Oblivious Message Adversaries}},
  booktitle =	{14th Innovations in Theoretical Computer Science Conference (ITCS 2023)},
  pages =	{100:1--100:28},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-263-1},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{251},
  editor =	{Tauman Kalai, Yael},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.ITCS.2023.100},
  URN =		{urn:nbn:de:0030-drops-176030},
  doi =		{10.4230/LIPIcs.ITCS.2023.100},
  annote =	{Keywords: dynamic networks, oblivious message adversaries, consensus, time complexity}
}
Document
Brief Announcement
DOI: 10.4230/LIPIcs.DISC.2022.42
Brief Announcement: Minimizing Congestion in Hybrid Demand-Aware Network Topologies

Authors: Wenkai Dai, Michael Dinitz, Klaus-Tycho Foerster, and Stefan Schmid

Published in: LIPIcs, Volume 246, 36th International Symposium on Distributed Computing (DISC 2022)

Abstract
Emerging reconfigurable optical communication technologies enable demand-aware networks: networks whose static topology can be enhanced with demand-aware links optimized towards the traffic pattern the network serves. This paper studies the algorithmic problem of how to jointly optimize the topology and the routing in such demand-aware networks, to minimize congestion. We investigate this problem along two dimensions: (1) whether flows are splittable or unsplittable, and (2) whether routing on the hybrid topology is segregated or not, i.e., whether or not flows either have to use exclusively either the static network or the demand-aware connections. For splittable and segregated routing, we show that the problem is 2-approximable in general, but APX-hard even for uniform demands induced by a bipartite demand graph. For unsplittable and segregated routing, we show an upper bound of O(log m/ log log m) and a lower bound of Ω(log m/ log log m) for polynomial-time approximation algorithms, where m is the number of static links. Under splittable (resp., unsplittable) and non-segregated routing, even for demands of a single source (resp., destination), the problem cannot be approximated better than Ω(c_{max}/c_{min}) unless P=NP, where c_{max} (resp., c_{min}) denotes the maximum (resp., minimum) capacity. It is still NP-hard for uniform capacities, but can be solved efficiently for a single commodity and uniform capacities.
Cite as

Wenkai Dai, Michael Dinitz, Klaus-Tycho Foerster, and Stefan Schmid. Brief Announcement: Minimizing Congestion in Hybrid Demand-Aware Network Topologies. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 42:1-42:3, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{dai_et_al:LIPIcs.DISC.2022.42,
  author =	{Dai, Wenkai and Dinitz, Michael and Foerster, Klaus-Tycho and Schmid, Stefan},
  title =	{{Brief Announcement: Minimizing Congestion in Hybrid Demand-Aware Network Topologies}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{42:1--42:3},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.42},
  URN =		{urn:nbn:de:0030-drops-172330},
  doi =		{10.4230/LIPIcs.DISC.2022.42},
  annote =	{Keywords: Congestion, Reconfigurable Networks, Algorithms, Complexity}
}
Document
Brief Announcement
DOI: 10.4230/LIPIcs.DISC.2022.52
Brief Announcement: Temporal Locality in Online Algorithms

Authors: Maciej Pacut, Mahmoud Parham, Joel Rybicki, Stefan Schmid, Jukka Suomela, and Aleksandr Tereshchenko

Published in: LIPIcs, Volume 246, 36th International Symposium on Distributed Computing (DISC 2022)

Abstract
Online algorithms make decisions based on past inputs, with the goal of being competitive against an algorithm that sees also future inputs. In this work, we introduce time-local online algorithms; these are online algorithms in which the output at any given time is a function of only T latest inputs. Our main observation is that time-local online algorithms are closely connected to local distributed graph algorithms: distributed algorithms make decisions based on the local information in the spatial dimension, while time-local online algorithms make decisions based on the local information in the temporal dimension. We formalize this connection, and show how we can directly use the tools developed to study distributed approximability of graph optimization problems to prove upper and lower bounds on the competitive ratio achieved with time-local online algorithms. Moreover, we show how to use computational techniques to synthesize optimal time-local algorithms.
Cite as

Maciej Pacut, Mahmoud Parham, Joel Rybicki, Stefan Schmid, Jukka Suomela, and Aleksandr Tereshchenko. Brief Announcement: Temporal Locality in Online Algorithms. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 52:1-52:3, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{pacut_et_al:LIPIcs.DISC.2022.52,
  author =	{Pacut, Maciej and Parham, Mahmoud and Rybicki, Joel and Schmid, Stefan and Suomela, Jukka and Tereshchenko, Aleksandr},
  title =	{{Brief Announcement: Temporal Locality in Online Algorithms}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{52:1--52:3},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.52},
  URN =		{urn:nbn:de:0030-drops-172431},
  doi =		{10.4230/LIPIcs.DISC.2022.52},
  annote =	{Keywords: Online algorithms, distributed algorithms}
}
Document
DOI: 10.4230/LIPIcs.DISC.2021.9
Randomized Local Fast Rerouting for Datacenter Networks with Almost Optimal Congestion

Authors: Gregor Bankhamer, Robert Elsässer, and Stefan Schmid

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

Abstract
To ensure high availability, datacenter networks must rely on local fast rerouting mechanisms that allow routers to quickly react to link failures, in a fully decentralized manner. However, configuring these mechanisms to provide a high resilience against multiple failures while avoiding congestion along failover routes is algorithmically challenging, as the rerouting rules can only depend on local failure information and must be defined ahead of time. This paper presents a randomized local fast rerouting algorithm for Clos networks, the predominant datacenter topologies. Given a graph G = (V,E) describing a Clos topology, our algorithm defines local routing rules for each node v ∈ V, which only depend on the packet’s destination and are conditioned on the incident link failures. We prove that as long as number of failures at each node does not exceed a certain bound, our algorithm achieves an asymptotically minimal congestion up to polyloglog factors along failover paths. Our lower bounds are developed under some natural routing assumptions.
Cite as

Gregor Bankhamer, Robert Elsässer, and Stefan Schmid. Randomized Local Fast Rerouting for Datacenter Networks with Almost Optimal Congestion. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 9:1-9:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{bankhamer_et_al:LIPIcs.DISC.2021.9,
  author =	{Bankhamer, Gregor and Els\"{a}sser, Robert and Schmid, Stefan},
  title =	{{Randomized Local Fast Rerouting for Datacenter Networks with Almost Optimal Congestion}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{9:1--9:19},
  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-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.9},
  URN =		{urn:nbn:de:0030-drops-148117},
  doi =		{10.4230/LIPIcs.DISC.2021.9},
  annote =	{Keywords: local failover routing, congestion, randomized algorithms, datacenter networks}
}
Document
Brief Announcement
DOI: 10.4230/LIPIcs.DISC.2021.58
Brief Announcement: Sinkless Orientation Is Hard Also in the Supported LOCAL Model

Authors: Janne H. Korhonen, Ami Paz, Joel Rybicki, Stefan Schmid, and Jukka Suomela

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

Abstract
We show that any algorithm that solves the sinkless orientation problem in the supported LOCAL model requires Ω(log n) rounds, and this is tight. The supported LOCAL is at least as strong as the usual LOCAL model, and as a corollary this also gives a new, short and elementary proof that shows that the round complexity of the sinkless orientation problem in the deterministic LOCAL model is Ω(log n).
Cite as

Janne H. Korhonen, Ami Paz, Joel Rybicki, Stefan Schmid, and Jukka Suomela. Brief Announcement: Sinkless Orientation Is Hard Also in the Supported LOCAL Model. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 58:1-58:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{korhonen_et_al:LIPIcs.DISC.2021.58,
  author =	{Korhonen, Janne H. and Paz, Ami and Rybicki, Joel and Schmid, Stefan and Suomela, Jukka},
  title =	{{Brief Announcement: Sinkless Orientation Is Hard Also in the Supported LOCAL Model}},
  booktitle =	{35th International Symposium on Distributed Computing (DISC 2021)},
  pages =	{58:1--58:4},
  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-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2021.58},
  URN =		{urn:nbn:de:0030-drops-148609},
  doi =		{10.4230/LIPIcs.DISC.2021.58},
  annote =	{Keywords: Supported LOCAL model, sinkless orientation, round elimination}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2020.26
Maximally Resilient Replacement Paths for a Family of Product Graphs

Authors: Mahmoud Parham, Klaus-Tycho Foerster, Petar Kosic, and Stefan Schmid

Published in: LIPIcs, Volume 184, 24th International Conference on Principles of Distributed Systems (OPODIS 2020)

Abstract
Modern communication networks support fast path restoration mechanisms which allow to reroute traffic in case of (possibly multiple) link failures, in a completely decentralized manner and without requiring global route reconvergence. However, devising resilient path restoration algorithms is challenging as these algorithms need to be inherently local. Furthermore, the resulting failover paths often have to fulfill additional requirements related to the policy and function implemented by the network, such as the traversal of certain waypoints (e.g., a firewall). This paper presents local algorithms which ensure a maximally resilient path restoration for a large family of product graphs, including the widely used tori and generalized hypercube topologies. Our algorithms provably ensure that even under multiple link failures, traffic is rerouted to the other endpoint of every failed link whenever possible (i.e. detouring failed links), enforcing waypoints and hence accounting for the network policy. The algorithms are particularly well-suited for emerging segment routing networks based on label stacks.
Cite as

Mahmoud Parham, Klaus-Tycho Foerster, Petar Kosic, and Stefan Schmid. Maximally Resilient Replacement Paths for a Family of Product Graphs. In 24th International Conference on Principles of Distributed Systems (OPODIS 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 184, pp. 26:1-26:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{parham_et_al:LIPIcs.OPODIS.2020.26,
  author =	{Parham, Mahmoud and Foerster, Klaus-Tycho and Kosic, Petar and Schmid, Stefan},
  title =	{{Maximally Resilient Replacement Paths for a Family of Product Graphs}},
  booktitle =	{24th International Conference on Principles of Distributed Systems (OPODIS 2020)},
  pages =	{26:1--26:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-176-4},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{184},
  editor =	{Bramas, Quentin and Oshman, Rotem and Romano, Paolo},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2020.26},
  URN =		{urn:nbn:de:0030-drops-135111},
  doi =		{10.4230/LIPIcs.OPODIS.2020.26},
  annote =	{Keywords: Product Graphs, Resilience, Failures, Routing}
}
Document
Brief Announcement
DOI: 10.4230/LIPIcs.DISC.2020.46
Brief Announcement: What Can(Not) Be Perfectly Rerouted Locally

Authors: Klaus-Tycho Foerster, Juho Hirvonen, Yvonne-Anne Pignolet, Stefan Schmid, and Gilles Tredan

Published in: LIPIcs, Volume 179, 34th International Symposium on Distributed Computing (DISC 2020)

Abstract
In order to provide a high resilience and to react quickly to link failures, modern computer networks support fully decentralized flow rerouting, also known as local fast failover. In a nutshell, the task of a local fast failover algorithm is to pre-define fast failover rules for each node using locally available information only. Ideally, such a local fast failover algorithm provides a perfect resilience deterministically: a packet emitted from any source can reach any target, as long as the underlying network remains connected. Feigenbaum et al. showed [Feigenbaum and others, 2012] that it is not always possible to provide perfect resilience; on the positive side, the authors also presented an efficient algorithm which achieves at least 1-resilience, tolerating a single failure in any network. Interestingly, not much more is known currently about the feasibility of perfect resilience. This brief announcement revisits perfect resilience with local fast failover, both in a model where the source can and cannot be used for forwarding decisions. By establishing a connection between graph minors and resilience, we prove that it is impossible to achieve perfect resilience on any non-planar graph; On the positive side, we can derive perfect resilience for outerplanar and some planar graphs.
Cite as

Klaus-Tycho Foerster, Juho Hirvonen, Yvonne-Anne Pignolet, Stefan Schmid, and Gilles Tredan. Brief Announcement: What Can(Not) Be Perfectly Rerouted Locally. In 34th International Symposium on Distributed Computing (DISC 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 179, pp. 46:1-46:3, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{foerster_et_al:LIPIcs.DISC.2020.46,
  author =	{Foerster, Klaus-Tycho and Hirvonen, Juho and Pignolet, Yvonne-Anne and Schmid, Stefan and Tredan, Gilles},
  title =	{{Brief Announcement: What Can(Not) Be Perfectly Rerouted Locally}},
  booktitle =	{34th International Symposium on Distributed Computing (DISC 2020)},
  pages =	{46:1--46:3},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-168-9},
  ISSN =	{1868-8969},
  year =	{2020},
  volume =	{179},
  editor =	{Attiya, Hagit},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2020.46},
  URN =		{urn:nbn:de:0030-drops-131244},
  doi =		{10.4230/LIPIcs.DISC.2020.46},
  annote =	{Keywords: Resilience, Local Failover}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2019.21
The Evolutionary Price of Anarchy: Locally Bounded Agents in a Dynamic Virus Game

Authors: Laura Schmid, Krishnendu Chatterjee, and Stefan Schmid

Published in: LIPIcs, Volume 153, 23rd International Conference on Principles of Distributed Systems (OPODIS 2019)

Abstract
The Price of Anarchy (PoA) is a well-established game-theoretic concept to shed light on coordination issues arising in open distributed systems. Leaving agents to selfishly optimize comes with the risk of ending up in sub-optimal states (in terms of performance and/or costs), compared to a centralized system design. However, the PoA relies on strong assumptions about agents' rationality (e.g., resources and information) and interactions, whereas in many distributed systems agents interact locally with bounded resources. They do so repeatedly over time (in contrast to "one-shot games"), and their strategies may evolve. Using a more realistic evolutionary game model, this paper introduces a realized evolutionary Price of Anarchy (ePoA). The ePoA allows an exploration of equilibrium selection in dynamic distributed systems with multiple equilibria, based on local interactions of simple memoryless agents. Considering a fundamental game related to virus propagation on networks, we present analytical bounds on the ePoA in basic network topologies and for different strategy update dynamics. In particular, deriving stationary distributions of the stochastic evolutionary process, we find that the Nash equilibria are not always the most abundant states, and that different processes can feature significant off-equilibrium behavior, leading to a significantly higher ePoA compared to the PoA studied traditionally in the literature.
Cite as

Laura Schmid, Krishnendu Chatterjee, and Stefan Schmid. The Evolutionary Price of Anarchy: Locally Bounded Agents in a Dynamic Virus Game. In 23rd International Conference on Principles of Distributed Systems (OPODIS 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 153, pp. 21:1-21:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{schmid_et_al:LIPIcs.OPODIS.2019.21,
  author =	{Schmid, Laura and Chatterjee, Krishnendu and Schmid, Stefan},
  title =	{{The Evolutionary Price of Anarchy: Locally Bounded Agents in a Dynamic Virus Game}},
  booktitle =	{23rd International Conference on Principles of Distributed Systems (OPODIS 2019)},
  pages =	{21:1--21:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-133-7},
  ISSN =	{1868-8969},
  year =	{2020},
  volume =	{153},
  editor =	{Felber, Pascal and Friedman, Roy and Gilbert, Seth and Miller, Avery},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2019.21},
  URN =		{urn:nbn:de:0030-drops-118071},
  doi =		{10.4230/LIPIcs.OPODIS.2019.21},
  annote =	{Keywords: Evolutionary Games, Virus Propagation, Price of Anarchy, Analysis}
}
Document
Brief Announcement
DOI: 10.4230/LIPIcs.DISC.2019.35
Brief Announcement: On Self-Adjusting Skip List Networks

Authors: Chen Avin, Iosif Salem, and Stefan Schmid

Published in: LIPIcs, Volume 146, 33rd International Symposium on Distributed Computing (DISC 2019)

Abstract
This paper explores the design of dynamic network topologies which adjust to the workload they serve, in an online manner. Such self-adjusting networks (SANs) are enabled by emerging optical technologies, and can be found, e.g., in datacenters. SANs can be used to reduce routing costs by moving frequently communicating nodes topologically closer. This paper presents SANs which provide, for the first time, provable working set guarantees: the routing cost between node pairs is proportional to how recently these nodes communicated last time. Our SANs rely on skip lists (which serve as the topology) and provide additional interesting properties such as local routing.
Cite as

Chen Avin, Iosif Salem, and Stefan Schmid. Brief Announcement: On Self-Adjusting Skip List Networks. In 33rd International Symposium on Distributed Computing (DISC 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 146, pp. 35:1-35:3, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

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@InProceedings{avin_et_al:LIPIcs.DISC.2019.35,
  author =	{Avin, Chen and Salem, Iosif and Schmid, Stefan},
  title =	{{Brief Announcement: On Self-Adjusting Skip List Networks}},
  booktitle =	{33rd International Symposium on Distributed Computing (DISC 2019)},
  pages =	{35:1--35:3},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-126-9},
  ISSN =	{1868-8969},
  year =	{2019},
  volume =	{146},
  editor =	{Suomela, Jukka},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2019.35},
  URN =		{urn:nbn:de:0030-drops-113423},
  doi =		{10.4230/LIPIcs.DISC.2019.35},
  annote =	{Keywords: self-adjusting networks, skip lists, working set, online algorithms}
}
Document
DOI: 10.4230/LIPIcs.ESA.2019.75
Compact Oblivious Routing

Authors: Harald Räcke and Stefan Schmid

Published in: LIPIcs, Volume 144, 27th Annual European Symposium on Algorithms (ESA 2019)

Abstract
Oblivious routing is an attractive paradigm for large distributed systems in which centralized control and frequent reconfigurations are infeasible or undesired (e.g., costly). Over the last almost 20 years, much progress has been made in devising oblivious routing schemes that guarantee close to optimal load and also algorithms for constructing such schemes efficiently have been designed. However, a common drawback of existing oblivious routing schemes is that they are not compact: they require large routing tables (of polynomial size), which does not scale. This paper presents the first oblivious routing scheme which guarantees close to optimal load and is compact at the same time - requiring routing tables of polylogarithmic size. Our algorithm maintains the polylogarithmic competitive ratio of existing algorithms, and is hence particularly well-suited for emerging large-scale networks.
Cite as

Harald Räcke and Stefan Schmid. Compact Oblivious Routing. In 27th Annual European Symposium on Algorithms (ESA 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 144, pp. 75:1-75:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

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@InProceedings{racke_et_al:LIPIcs.ESA.2019.75,
  author =	{R\"{a}cke, Harald and Schmid, Stefan},
  title =	{{Compact Oblivious Routing}},
  booktitle =	{27th Annual European Symposium on Algorithms (ESA 2019)},
  pages =	{75:1--75:14},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-124-5},
  ISSN =	{1868-8969},
  year =	{2019},
  volume =	{144},
  editor =	{Bender, Michael A. and Svensson, Ola and Herman, Grzegorz},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.ESA.2019.75},
  URN =		{urn:nbn:de:0030-drops-111968},
  doi =		{10.4230/LIPIcs.ESA.2019.75},
  annote =	{Keywords: Oblivious Routing, Compact Routing, Competitive Analysis}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2018.13
Local Fast Segment Rerouting on Hypercubes

Authors: Klaus-Tycho Foerster, Mahmoud Parham, Stefan Schmid, and Tao Wen

Published in: LIPIcs, Volume 125, 22nd International Conference on Principles of Distributed Systems (OPODIS 2018)

Abstract
Fast rerouting is an essential mechanism in any dependable communication network, allowing to quickly, i.e., locally, recover from network failures, without invoking the control plane. However, while locality ensures a fast reaction, the absence of global information also renders the design of highly resilient fast rerouting algorithms more challenging. In this paper, we study algorithms for fast rerouting in emerging Segment Routing (SR) networks, where intermediate destinations can be added to packets by nodes along the path. Our main contribution is a maximally resilient polynomial-time fast rerouting algorithm for SR networks based on a hypercube topology. Our algorithm is attractive as it preserves the original paths (and hence waypoints traversed along the way), and does not require packets to carry failure information. We complement our results with an integer linear program formulation for general graphs and exploratory simulation results.
Cite as

Klaus-Tycho Foerster, Mahmoud Parham, Stefan Schmid, and Tao Wen. Local Fast Segment Rerouting on Hypercubes. In 22nd International Conference on Principles of Distributed Systems (OPODIS 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 125, pp. 13:1-13:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

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@InProceedings{foerster_et_al:LIPIcs.OPODIS.2018.13,
  author =	{Foerster, Klaus-Tycho and Parham, Mahmoud and Schmid, Stefan and Wen, Tao},
  title =	{{Local Fast Segment Rerouting on Hypercubes}},
  booktitle =	{22nd International Conference on Principles of Distributed Systems (OPODIS 2018)},
  pages =	{13:1--13:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-098-9},
  ISSN =	{1868-8969},
  year =	{2019},
  volume =	{125},
  editor =	{Cao, Jiannong and Ellen, Faith and Rodrigues, Luis and Ferreira, Bernardo},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2018.13},
  URN =		{urn:nbn:de:0030-drops-100739},
  doi =		{10.4230/LIPIcs.OPODIS.2018.13},
  annote =	{Keywords: segment routing, local fast failover, link failures}
}
Document
DOI: 10.4230/LIPIcs.ICALP.2018.143
Congestion-Free Rerouting of Flows on DAGs

Authors: Saeed Akhoondian Amiri, Szymon Dudycz, Stefan Schmid, and Sebastian Wiederrecht

Published in: LIPIcs, Volume 107, 45th International Colloquium on Automata, Languages, and Programming (ICALP 2018)

Abstract
Changing a given configuration in a graph into another one is known as a reconfiguration problem. Such problems have recently received much interest in the context of algorithmic graph theory. We initiate the theoretical study of the following reconfiguration problem: How to reroute k unsplittable flows of a certain demand in a capacitated network from their current paths to their respective new paths, in a congestion-free manner? This problem finds immediate applications, e.g., in traffic engineering in computer networks. We show that the problem is generally NP-hard already for k=2 flows, which motivates us to study rerouting on a most basic class of flow graphs, namely DAGs. Interestingly, we find that for general k, deciding whether an unsplittable multi-commodity flow rerouting schedule exists, is NP-hard even on DAGs. Our main contribution is a polynomial-time (fixed parameter tractable) algorithm to solve the route update problem for a bounded number of flows on DAGs. At the heart of our algorithm lies a novel decomposition of the flow network that allows us to express and resolve reconfiguration dependencies among flows.
Cite as

Saeed Akhoondian Amiri, Szymon Dudycz, Stefan Schmid, and Sebastian Wiederrecht. Congestion-Free Rerouting of Flows on DAGs. In 45th International Colloquium on Automata, Languages, and Programming (ICALP 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 107, pp. 143:1-143:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{akhoondianamiri_et_al:LIPIcs.ICALP.2018.143,
  author =	{Akhoondian Amiri, Saeed and Dudycz, Szymon and Schmid, Stefan and Wiederrecht, Sebastian},
  title =	{{Congestion-Free Rerouting of Flows on DAGs}},
  booktitle =	{45th International Colloquium on Automata, Languages, and Programming (ICALP 2018)},
  pages =	{143:1--143:13},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-076-7},
  ISSN =	{1868-8969},
  year =	{2018},
  volume =	{107},
  editor =	{Chatzigiannakis, Ioannis and Kaklamanis, Christos and Marx, D\'{a}niel and Sannella, Donald},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2018.143},
  URN =		{urn:nbn:de:0030-drops-91471},
  doi =		{10.4230/LIPIcs.ICALP.2018.143},
  annote =	{Keywords: Unsplittable Flows, Reconfiguration, DAGs, FPT, NP-Hardness}
}
Document
DOI: 10.4230/LIPIcs.DISC.2017.5
Demand-Aware Network Designs of Bounded Degree

Authors: Chen Avin, Kaushik Mondal, and Stefan Schmid

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

Abstract
Traditionally, networks such as datacenter interconnects are designed to optimize worst-case performance under arbitrary traffic patterns. Such network designs can however be far from optimal when considering the actual workloads and traffic patterns which they serve. This insight led to the development of demand-aware datacenter interconnects which can be reconfigured depending on the workload. Motivated by these trends, this paper initiates the algorithmic study of demand-aware networks (DANs), and in particular the design of bounded-degree networks. The inputs to the network design problem are a discrete communication request distribution, D, defined over communicating pairs from the node set V, and a bound, d, on the maximum degree. In turn, our objective is to design an (undirected) demand-aware network N = (V,E) of bounded-degree d, which provides short routing paths between frequently communicating nodes distributed across N. In particular, the designed network should minimize the expected path length on N (with respect to D), which is a basic measure of the efficiency of the network. We show that this fundamental network design problem exhibits interesting connections to several classic combinatorial problems and to information theory. We derive a general lower bound based on the entropy of the communication pattern D, and present asymptotically optimal network-aware design algorithms for important distribution families, such as sparse distributions and distributions of locally bounded doubling dimensions.
Cite as

Chen Avin, Kaushik Mondal, and Stefan Schmid. Demand-Aware Network Designs of Bounded Degree. In 31st International Symposium on Distributed Computing (DISC 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 91, pp. 5:1-5:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{avin_et_al:LIPIcs.DISC.2017.5,
  author =	{Avin, Chen and Mondal, Kaushik and Schmid, Stefan},
  title =	{{Demand-Aware Network Designs of Bounded Degree}},
  booktitle =	{31st International Symposium on Distributed Computing (DISC 2017)},
  pages =	{5:1--5:16},
  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-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2017.5},
  URN =		{urn:nbn:de:0030-drops-80153},
  doi =		{10.4230/LIPIcs.DISC.2017.5},
  annote =	{Keywords: Network design, reconfigurable networks, datacenter topology, peer-topeer computing, entropy, sparse spanners}
}
Document
Brief Announcement
DOI: 10.4230/LIPIcs.DISC.2017.58
Brief Announcement: Distributed SplayNets

Authors: Bruna S. Peres, Olga Goussevskaia, Stefan Schmid, and Chen Avin

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

Abstract
SplayNets are reconfigurable networks which adjust to the communication pattern over time. We present DiSplayNets, a distributed (concurrent and decentralized) implementation of SplayNets.
Cite as

Bruna S. Peres, Olga Goussevskaia, Stefan Schmid, and Chen Avin. Brief Announcement: Distributed SplayNets. In 31st International Symposium on Distributed Computing (DISC 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 91, pp. 58:1-58:3, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{peres_et_al:LIPIcs.DISC.2017.58,
  author =	{Peres, Bruna S. and Goussevskaia, Olga and Schmid, Stefan and Avin, Chen},
  title =	{{Brief Announcement: Distributed SplayNets}},
  booktitle =	{31st International Symposium on Distributed Computing (DISC 2017)},
  pages =	{58:1--58:3},
  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-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2017.58},
  URN =		{urn:nbn:de:0030-drops-79661},
  doi =		{10.4230/LIPIcs.DISC.2017.58},
  annote =	{Keywords: Decentralization, Concurrency, Reconfigurable Networks}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2016.18
WNetKAT: A Weighted SDN Programming and Verification Language

Authors: Kim G. Larsen, Stefan Schmid, and Bingtian Xue

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

Abstract
Programmability and verifiability lie at the heart of the software-defined networking paradigm. While OpenFlow and its match-action concept provide primitive operations to manipulate hardware configurations, over the last years, several more expressive network programming languages have been developed. This paper presents WNetKAT, the first network programming language accounting for the fact that networks are inherently weighted, and communications subject to capacity constraints (e.g., in terms of bandwidth) and costs (e.g., latency or monetary costs). WNetKAT is based on a syntactic and semantic extension of the NetKAT algebra. We demonstrate several relevant applications for WNetKAT, including cost and capacity-aware reachability, as well as quality-of-service and fairness aspects. These applications do not only apply to classic, splittable and unsplittable (s,t)-flows, but also generalize to more complex (and stateful) network functions and service chains. For example, WNetKAT allows to model flows which need to traverse certain waypoint functions, which can change the traffic rate. This paper also shows the relationship between the equivalence problem of WNetKAT and the equivalence problem of the weighted finite automata, which implies undecidability of the former. However, this paper also shows the decidability of whether an expression equals to 0, which is sufficient in many practical scenarios, and we initiate the discussion of decidable subsets of the whole language.
Cite as

Kim G. Larsen, Stefan Schmid, and Bingtian Xue. WNetKAT: A Weighted SDN Programming and Verification Language. In 20th International Conference on Principles of Distributed Systems (OPODIS 2016). Leibniz International Proceedings in Informatics (LIPIcs), Volume 70, pp. 18:1-18:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{larsen_et_al:LIPIcs.OPODIS.2016.18,
  author =	{Larsen, Kim G. and Schmid, Stefan and Xue, Bingtian},
  title =	{{WNetKAT: A Weighted SDN Programming and Verification Language}},
  booktitle =	{20th International Conference on Principles of Distributed Systems (OPODIS 2016)},
  pages =	{18:1--18:18},
  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-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2016.18},
  URN =		{urn:nbn:de:0030-drops-70870},
  doi =		{10.4230/LIPIcs.OPODIS.2016.18},
  annote =	{Keywords: Software-Defined Networking, Verification, Reachability, Stateful Processing, Service Chains, Weighted Automata, Decidability, NetKAT}
}
Document
DOI: 10.4230/LIPIcs.CONCUR.2016.19
Topological Self-Stabilization with Name-Passing Process Calculi

Authors: Christina Rickmann, Christoph Wagner, Uwe Nestmann, and Stefan Schmid

Published in: LIPIcs, Volume 59, 27th International Conference on Concurrency Theory (CONCUR 2016)

Abstract
Topological self-stabilization is the ability of a distributed system to have its nodes themselves establish a meaningful overlay network. Independent from the initial network topology, it converges to the desired topology via forwarding, inserting, and deleting links to neighboring nodes. We adapt a linearization algorithm, originally designed for a shared memory model, to asynchronous message-passing. We use an extended localized pi-calculus to model the algorithm and to formally prove its essential self-stabilization properties: closure and weak convergence for every arbitrary initial configuration, and strong convergence for restricted cases.
Cite as

Christina Rickmann, Christoph Wagner, Uwe Nestmann, and Stefan Schmid. Topological Self-Stabilization with Name-Passing Process Calculi. In 27th International Conference on Concurrency Theory (CONCUR 2016). Leibniz International Proceedings in Informatics (LIPIcs), Volume 59, pp. 19:1-19:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2016)

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@InProceedings{rickmann_et_al:LIPIcs.CONCUR.2016.19,
  author =	{Rickmann, Christina and Wagner, Christoph and Nestmann, Uwe and Schmid, Stefan},
  title =	{{Topological Self-Stabilization with Name-Passing Process Calculi}},
  booktitle =	{27th International Conference on Concurrency Theory (CONCUR 2016)},
  pages =	{19:1--19:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-017-0},
  ISSN =	{1868-8969},
  year =	{2016},
  volume =	{59},
  editor =	{Desharnais, Jos\'{e}e and Jagadeesan, Radha},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2016.19},
  URN =		{urn:nbn:de:0030-drops-61761},
  doi =		{10.4230/LIPIcs.CONCUR.2016.19},
  annote =	{Keywords: Distributed Algorithms, Fault Tolerance, Topological Self-Stabilization, Linearization, Process Calculi}
}
Document
DOI: 10.4230/DagRep.5.2.64
Distributed Cloud Computing (Dagstuhl Seminar 15072)

Authors: Yvonne Coady, James Kempf, Rick McGeer, and Stefan Schmid

Published in: Dagstuhl Reports, Volume 5, Issue 2 (2015)

Abstract
This report documents the program and the outcomes of Dagstuhl Seminar 15072 "Distributed Cloud Computing". A distributed cloud connecting multiple, geographically distributed and smaller datacenters, can be an attractive alternative to today's massive, centralized datacenters. A distributed cloud can reduce communication overheads, costs, and latency's by offering nearby computation and storage resources. Better data locality can also improve privacy. In this seminar, we revisit the vision of distributed cloud computing, and identify different use cases as well as research challenges.
Cite as

Yvonne Coady, James Kempf, Rick McGeer, and Stefan Schmid. Distributed Cloud Computing (Dagstuhl Seminar 15072). In Dagstuhl Reports, Volume 5, Issue 2, pp. 64-79, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2015)

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@Article{coady_et_al:DagRep.5.2.64,
  author =	{Coady, Yvonne and Kempf, James and McGeer, Rick and Schmid, Stefan},
  title =	{{Distributed Cloud Computing (Dagstuhl Seminar 15072)}},
  pages =	{64--79},
  journal =	{Dagstuhl Reports},
  ISSN =	{2192-5283},
  year =	{2015},
  volume =	{5},
  number =	{2},
  editor =	{Coady, Yvonne and Kempf, James and McGeer, Rick and Schmid, Stefan},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/DagRep.5.2.64},
  URN =		{urn:nbn:de:0030-drops-50454},
  doi =		{10.4230/DagRep.5.2.64},
  annote =	{Keywords: Distributed Cloud Computing, Distributed Systems}
}
Document
DOI: 10.4230/DagSemProc.06131.4
On the Topologies Formed by Selfish Peers

Authors: Stefan Schmid, Thomas Moscibroda, and Roger Wattenhofer

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

Abstract
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.
Cite as

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-dev.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}
}
Document
DOI: 10.4230/DagSemProc.06131.5
Taming Dynamic and Selfish Peers

Authors: Stefan Schmid, Fabian Kuhn, Thomas Moscibroda, and Roger Wattenhofer

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

Abstract
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.
Cite as

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-dev.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}
}
Document
DOI: 10.4230/DagSemProc.04411.19
Service Specific Overlay Networks Setup and Maintenance using Pattern-based Management

Authors: Eskindir Asmare, Stefan Schmid, and Marcus Brunner

Published in: Dagstuhl Seminar Proceedings, Volume 4411, Service Management and Self-Organization in IP-based Networks (2005)

Abstract
Service specific overlays are overlay networks build and setup for a single service. Since different services need different types of overlays the setup process is pretty different. Current approaches normal allow to send in a overlay specification into the overlay provisioning system (mainly centralized). In our approach, we use pattern-based management in order to run the process completely distributed but still having a basic communication pattern to be reused for other overlays with different requirement. In this talk, we present the concept and a first implementation on the Simpson pattern simulator of KTH.
Cite as

Eskindir Asmare, Stefan Schmid, and Marcus Brunner. Service Specific Overlay Networks Setup and Maintenance using Pattern-based Management. In Service Management and Self-Organization in IP-based Networks. Dagstuhl Seminar Proceedings, Volume 4411, pp. 1-2, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2005)

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@InProceedings{asmare_et_al:DagSemProc.04411.19,
  author =	{Asmare, Eskindir and Schmid, Stefan and Brunner, Marcus},
  title =	{{Service Specific Overlay Networks Setup and Maintenance using Pattern-based Management}},
  booktitle =	{Service Management and Self-Organization in IP-based Networks},
  pages =	{1--2},
  series =	{Dagstuhl Seminar Proceedings (DagSemProc)},
  ISSN =	{1862-4405},
  year =	{2005},
  volume =	{4411},
  editor =	{Matthias Bossardt and Georg Carle and D. Hutchison and Hermann de Meer and Bernhard Plattner},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/DagSemProc.04411.19},
  URN =		{urn:nbn:de:0030-drops-838},
  doi =		{10.4230/DagSemProc.04411.19},
  annote =	{Keywords: overlay networks , management/provisioning of overlays}
}
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