297 research outputs found
Διερεύνηση του ρόλου των τοπογραφικών συνθηκών στη διάδοση σεισμικών κυμάτων μέσω αριθμητικών προσομοιώσεων – εφαρμογή στο λόφο της Ακρόπολης
Divide and Scale: Formalization of Distributed Ledger Sharding Protocols
Sharding distributed ledgers is the most promising on-chain solution for
scaling blockchain technology. In this work, we define and analyze the
properties a sharded distributed ledger should fulfill. More specifically, we
show that a sharded blockchain cannot be scalable under a fully adaptive
adversary, but it can scale up to under an epoch-adaptive
adversary. This is possible only if the distributed ledger creates succinct
proofs of the valid state updates at the end of each epoch. Our model builds
upon and extends the Bitcoin backbone protocol by defining consistency and
scalability. Consistency encompasses the need for atomic execution of
cross-shard transactions to preserve safety, whereas scalability encapsulates
the speedup a sharded system can gain in comparison to a non-sharded system. We
introduce a protocol abstraction and highlight the sufficient components for
secure and efficient sharding in our model. In order to show the power of our
framework, we analyze the most prominent shared blockchains (Elastico,
Monoxide, OmniLedger, RapidChain) and pinpoint where they fail to meet the
desired properties
Cuttlefish: Expressive Fast Path Blockchains with FastUnlock
Cuttlefish addresses several limitations of existing consensus-less and
consensus-minimized decentralized ledgers, including restricted programmability
and the risk of deadlocked assets. The key insight of Cuttlefish is that
consensus in blockchains is necessary due to contention, rather than multiple
owners of an asset as suggested by prior work. Previous proposals proactively
use consensus to prevent contention from blocking assets, taking a pessimistic
approach. In contrast, Cuttlefish introduces collective objects and multi-owner
transactions that can offer most of the functionality of classic blockchains
when objects transacted on are not under contention. Additionally, in case of
contention, Cuttlefish proposes a novel `Unlock' protocol that significantly
reduces the latency of unblocking contented objects. By leveraging these
features, Cuttlefish implements consensus-less protocols for a broader range of
transactions, including asset swaps and multi-signature transactions, which
were previously believed to require consensus
Mandator and Sporades: Robust Wide-Area Consensus with Efficient Request Dissemination
Consensus algorithms are deployed in the wide area to achieve high
availability for geographically replicated applications. Wide-area consensus is
challenging due to two main reasons: (1) low throughput due to the high latency
overhead of client request dissemination and (2) network asynchrony that causes
consensus protocols to lose liveness. In this paper, we propose Mandator and
Sporades, a modular state machine replication algorithm that enables high
performance and resiliency in the wide-area setting.
To address the high client request dissemination overhead challenge, we
propose Mandator, a novel consensus-agnostic asynchronous dissemination layer.
Mandator separates client request dissemination from the critical path of
consensus to obtain high performance. Composing Mandator with Multi-Paxos
(Mandator-Paxos) delivers significantly high throughput under synchronous
networks. However, under asynchronous network conditions, Mandator-Paxos loses
liveness which results in high latency. To achieve low latency and robustness
under asynchrony, we propose Sporades, a novel omission fault-tolerant
consensus algorithm. Sporades consists of two modes of operations --
synchronous and asynchronous -- that always ensure liveness. The combination of
Mandator and Sporades (Mandator-Sporades) provides a robust and high-performing
state machine replication system.
We implement and evaluate Mandator-Sporades in a wide-area deployment running
on Amazon EC2. Our evaluation shows that in the synchronous execution,
Mandator-Sporades achieves 300k tx/sec throughput in less than 900ms latency,
outperforming Multi-Paxos, EPaxos and Rabia by 650\% in throughput, at a modest
expense of latency. Furthermore, we show that Mandator-Sporades outperforms
Mandator-Paxos, Multi-Paxos, and EPaxos in the face of targeted distributed
denial-of-service attacks
Robust and Scalable Consensus for Sharded Distributed Ledgers
ByzCoin, a promising alternative of Bitcoin, is a scalable consensus protocol used as a building block of many research and enterprise-level decentralized systems. In this paper, we show that ByzCoin is unsuitable for deployment in an anopen, adversarial network and instead introduceMOTOR. MOTORis designed as a secure, robust, and scalable consensus suitable for permissionless sharded blockchains. MOTORachieves these properties by making four key design choices: (a) it prioritizes robustness in adversarial environments while maintaining adequate scalability, (b) it employees provably correct cryptography that resists DoS attacks from individual nodes, (c) it deploys unpredictable rotating leaders to defend against mildly-adaptive adversaries and prevents censorship, and (d) it creates an incentive compatible reward mechanism. These choices are materialized as (a) a “rotating subleader” communication pattern that balances the scalability needs with the robustness requirements under failures, (b) deployment of provable secure BLS multi-signatures, (c) use of deterministic thresh-old signatures as a source of randomness and (d) careful design of the reward allocation mechanism. We have implemented MOTORand compare it withByzCoin. We show that MOTORcan scale similar to ByzCoin with an at most2xoverhead whereas it maintains good performance even under high-percentage of faults, unlike ByzCoin
HammerHead: Leader Reputation for Dynamic Scheduling
The need for high throughput and censorship resistance in blockchain
technology has led to research on DAG-based consensus. The Sui blockchain
protocol uses a variant of the Bullshark consensus algorithm due to its lower
latency, but this leader-based protocol causes performance issues when
candidate leaders crash. In this paper, we explore the ideas pioneered by
Carousel on providing Leader-Utilization and present HammerHead. Unlike
Carousel, which is built with a chained and pipelined consensus protocol in
mind, HammerHead does not need to worry about chain quality as it is directly
provided by the DAG, but needs to make sure that even though validators might
commit blocks in different views the safety and liveness is preserved. Our
implementation of HammerHead shows a slight performance increase in a faultless
setting, and a drastic 2x latency reduction and up to 40% throughput increase
when suffering faults (100 validators, 33 faults)
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