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Classical protocols for reliable broadcast and consensus provide security guarantees as long as the number of corrupted parties f is bounded by a single given threshold t. If f > t, these protocols are completely deemed insecure. We consider the relaxed notion of multi-threshold reliable broadcast and consensus where validity, consistency and termination are guaranteed as long as f ≤ t_v, f ≤ t_c and f ≤ t_t respectively. For consensus, we consider both variants of (1-ε)-consensus and almost-surely terminating consensus, where termination is guaranteed with probability (1-ε) and 1, respectively. We give a very complete characterization for these primitives in the asynchronous setting and with no signatures:
- Multi-threshold reliable broadcast is possible if and only if max{t_c,t_v} + 2t_t < n.
- Multi-threshold almost-surely consensus is possible if max{t_c, t_v} + 2t_t < n, 2t_v + t_t < n and t_t < n/3. Assuming a global coin, it is possible if and only if max{t_c, t_v} + 2t_t < n and 2t_v + t_t < n.
- Multi-threshold (1-ε)-consensus is possible if and only if max{t_c, t_v} + 2t_t < n and 2t_v + t_t < n.
@InProceedings{hirt_et_al:LIPIcs.OPODIS.2020.6,
author = {Hirt, Martin and Kastrati, Ard and Liu-Zhang, Chen-Da},
title = {{Multi-Threshold Asynchronous Reliable Broadcast and Consensus}},
booktitle = {24th International Conference on Principles of Distributed Systems (OPODIS 2020)},
pages = {6:1--6:16},
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.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2020.6},
URN = {urn:nbn:de:0030-drops-134917},
doi = {10.4230/LIPIcs.OPODIS.2020.6},
annote = {Keywords: broadcast, byzantine agreement, multi-threshold}
}