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

Handling congestion in blockchain systems is a fundamental problem given that the security and decentralization objectives of such systems lead to designs that compromise on (horizontal) scalability (what sometimes is referred to as the "blockchain trilemma"). Motivated by this, we focus on the question whether it is possible to design a transaction inclusion policy for block producers that facilitates fee and delay predictability while being incentive compatible at the same time.
Reconciling these three properties is seemingly paradoxical given that the dominant approach to transaction processing is based on first-price auctions (e.g., as in Bitcoin) or dynamic adjustment of the minimum admissible fee (e.g. as in Ethereum EIP-1559) something that breaks fee predictability. At the same time, in fixed fee mechanisms (e.g., as in Cardano), fees are trivially predictable but are subject to relatively inexpensive bribing or denial of service attacks where transactions may be delayed indefinitely by a well funded attacker, hence breaking delay predictability.
In this work, we set out to address this problem by putting forward blockchain space tokenization (BST), namely a new capability of a blockchain system to tokenize its capacity for transactions and allocate it to interested users who are willing to pay ahead of time for the ability to post transactions regularly for a period of time. We analyze our system in the face of worst-case transaction-processing attacks by introducing a security game played between the mempool mechanism and an adversary. Leveraging this framework, we prove that BST offers predictable and asymptotically optimal delays, predictable fees, and is incentive compatible, thus answering the question posed in the affirmative.

Aggelos Kiayias, Elias Koutsoupias, Philip Lazos, and Giorgos Panagiotakos. Blockchain Space Tokenization. In 6th Conference on Advances in Financial Technologies (AFT 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 316, pp. 9:1-9:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{kiayias_et_al:LIPIcs.AFT.2024.9, author = {Kiayias, Aggelos and Koutsoupias, Elias and Lazos, Philip and Panagiotakos, Giorgos}, title = {{Blockchain Space Tokenization}}, booktitle = {6th Conference on Advances in Financial Technologies (AFT 2024)}, pages = {9:1--9:20}, 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.2024.9}, URN = {urn:nbn:de:0030-drops-209453}, doi = {10.4230/LIPIcs.AFT.2024.9}, annote = {Keywords: Blockchain protocols, Predictable Service, Transaction Fees} }

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

We study a variant of the k-server problem, the infinite server problem, in which infinitely many servers reside initially at a particular point of the metric space and serve a sequence of requests. In the framework of competitive analysis, we show a surprisingly tight connection between this problem and the (h,k)-server problem, in which an online algorithm with k servers competes against an offline algorithm with h servers. Specifically, we show that the infinite server problem has bounded competitive ratio if and only if the (h,k)-server problem has bounded competitive ratio for some k=O(h). We give a lower bound of 3.146 for the competitive ratio of the infinite server problem, which implies the same lower bound for the (h,k)-server problem even when k>>h and holds also for the line metric; the previous known bounds were 2.4 for general metric spaces and 2 for the line. For weighted trees and layered graphs we obtain upper bounds, although they depend on the depth. Of particular interest is the infinite server problem on the line, which we show to be equivalent to the seemingly easier case in which all requests are in a fixed bounded interval away from the original position of the servers. This is a special case of a more general reduction from arbitrary metric spaces to bounded subspaces. Unfortunately, classical approaches (double coverage and generalizations, work function algorithm, balancing algorithms) fail even for this special case.

Christian Coester, Elias Koutsoupias, and Philip Lazos. The Infinite Server Problem. In 44th International Colloquium on Automata, Languages, and Programming (ICALP 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 80, pp. 14:1-14:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{coester_et_al:LIPIcs.ICALP.2017.14, author = {Coester, Christian and Koutsoupias, Elias and Lazos, Philip}, title = {{The Infinite Server Problem}}, booktitle = {44th International Colloquium on Automata, Languages, and Programming (ICALP 2017)}, pages = {14:1--14:14}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-041-5}, ISSN = {1868-8969}, year = {2017}, volume = {80}, editor = {Chatzigiannakis, Ioannis and Indyk, Piotr and Kuhn, Fabian and Muscholl, Anca}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2017.14}, URN = {urn:nbn:de:0030-drops-74563}, doi = {10.4230/LIPIcs.ICALP.2017.14}, annote = {Keywords: Online Algorithms, k-Server, Resource Augmentation} }

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

We study a dynamic market setting where an intermediary interacts with an unknown large sequence of agents that can be either sellers or buyers: their identities, as well as the sequence length n, are decided in an adversarial, online way. Each agent is interested in trading a single item, and all items in the market are identical. The intermediary has some prior, incomplete knowledge of the agents' values for the items: all seller values are independently drawn from the same distribution F_S, and all buyer values from F_B. The two distributions may differ, and we make common regularity assumptions, namely that F_B is MHR and F_S is log-concave.
We focus on online, posted-price mechanisms, and analyse two objectives: that of maximizing the intermediary's profit and that of maximizing the social welfare, under a competitive analysis benchmark. First, on the negative side, for general agent sequences we prove tight competitive ratios of Theta(\sqrt(n)) and Theta(\ln n), respectively for the two objectives. On the other hand, under the extra assumption that the intermediary knows some bound \alpha on the ratio between the number of sellers and buyers, we design asymptotically optimal online mechanisms with competitive ratios of 1+o(1) and 4, respectively. Additionally, we study the model where the number of items that can be stored in stock throughout the execution is bounded, in which case the competitive ratio for the profit is improved to O(ln n).

Yiannis Giannakopoulos, Elias Koutsoupias, and Philip Lazos. Online Market Intermediation. In 44th International Colloquium on Automata, Languages, and Programming (ICALP 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 80, pp. 47:1-47:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{giannakopoulos_et_al:LIPIcs.ICALP.2017.47, author = {Giannakopoulos, Yiannis and Koutsoupias, Elias and Lazos, Philip}, title = {{Online Market Intermediation}}, booktitle = {44th International Colloquium on Automata, Languages, and Programming (ICALP 2017)}, pages = {47:1--47:14}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-041-5}, ISSN = {1868-8969}, year = {2017}, volume = {80}, editor = {Chatzigiannakis, Ioannis and Indyk, Piotr and Kuhn, Fabian and Muscholl, Anca}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2017.47}, URN = {urn:nbn:de:0030-drops-74815}, doi = {10.4230/LIPIcs.ICALP.2017.47}, annote = {Keywords: optimal auctions, bilateral trade, sequential auctions, online algorithms, competitive analysis} }

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