Rapid Consensus Structure: Continuous Common Knowledge in Asynchronous Distributed Systems

A distributed system guarantees the acceptance of Byzantine fault tolerance (BFT) for information transmission. Practical BFT (pBFT) and asynchronous BFT (aBFT) are among the various available forms of BFT. Distributed systems generally share information with all participating nodes. After information is shared, the systems reshare it. Thus, ensuring BFT consumes a considerable amount of time. Herein, we propose Decision search protocols that apply the gossip protocol, denoted by DecisionBFT, for distributed networks with guaranteed BFT. Each protocol in DecisionBFT is completely asynchronous and leaderless; it has an eventual consensus but no round-robin or proof-of-work. The core concept of the proposed technology is the consensus structure, which is based on the directed acyclic graph (DAG) and gossip protocol. In the most general form, each node in the algorithm has a set of k neighbors of most preference. When receiving transactions, a node creates and connects an event block with all its neighbors. Each event block is signed by the hashes of the creating node and its k peers. The consensus structure of the event blocks utilizes a DAG, which guarantees aBFT. The proposed framework uses Lamport timestamps and concurrent common knowledge. Further, an example of a Decision search algorithm, based on the gossip protocol DecisionBFT, is presented. The proposed DecisionBFT protocol can reach a consensus when 2/3 of all participants agree to an event block without any additional communication overhead. The DecisionBFT protocol relies on a cost function to identify the k peers and generate the DAG-based consensus structure. By creating a dynamic flag table that stores connection information between blocks, the gossip protocol achieves a consensus in fewer steps than that in the case of the existing aBFT protocol.

Tithonus: A Bitcoin Based Censorship Resilient System

Providing reliable and surreptitious communications is difficult in the presence of adaptive and resourceful state level censors. In this paper we introduce Tithonus, a framework that builds on the Bitcoin blockchain and network to provide censorship-resistant communication mechanisms. In contrast to previous approaches, we do not rely solely on the slow and expensive blockchain consensus mechanism but instead fully exploit Bitcoin’s peer-to-peer gossip protocol. We develop adaptive, fast and cost effective data communication solutions that camouflage client requests into inconspicuous Bitcoin transactions. We propose solutions to securely request and transfer content, with unobservability and censorship resistance, and free, pay-per-access and subscription based payment options. When compared to state-of-the-art Bitcoin writing solutions, Tithonus reduces the cost of transferring data to censored clients by 2 orders of magnitude and increases the goodput by 3 to 5 orders of magnitude. We show that Tithonus client initiated transactions are hard to detect, while server initiated transactions cannot be censored without creating split world problems to the Bit-coin blockchain.

When Are Two Gossips the Same?

We provide an in-depth study of the knowledge-theoretic aspects of communication in so-called gossip protocols. Pairs of agents communicate by means of calls in order to spread information—so-called secrets—within the group. Depending on the nature of such calls knowledge spreads in different ways within the group. Systematizing existing literature, we identify 18 different types of communication, and model their epistemic effects through corresponding indistinguishability relations. We then provide a classification of these relations and show its usefulness for an epistemic analysis in presence of different communication types. Finally, we explain how to formalise the assumption that the agents have common knowledge of a distributed epistemic gossip protocol.

On the Computational Complexity of Gossip Protocols

Gossip protocols deal with a group of communicating agents, each holding a private information, and aim at arriving at a situation in which all the agents know each other secrets. Distributed epistemic gossip protocols are particularly simple distributed programs that use formulas from an epistemic logic. Recently, the implementability of these distributed protocols was established (which means that the evaluation of these formulas is decidable), and the problems of their partial correctness and termination were shown to be decidable, but their exact computational complexity was left open. We show that for any monotonic type of calls the implementability of a distributed epistemic gossip protocol is a P^{NP}_{||}-complete problem, while the problems of its partial correctness and termination are in coNP^{NP}.

DYNAMIC RESOURCE MANAGEMENT IN LARGE CLOUD ENVIRONMENTS USING DISTRIBUTED GOSSIP PROTOCOL

Resource management poses particular challenges in large-scale systems, such as server clusters that simultaneously process requests from a large number of clients. We mainly focus on the dynamic resource management in large scale cloud environment. Our core contribution centers around outlining a distributed middleware architecture and presenting one of its key elements, a gossip protocol P* that meets our 3 main design goals: (1) fairness of resource allocation with respect to hosted sites (2) efficient adaptation to load changes and (3) scalability in terms of both the number of machines and sites. We first present a protocol that maximizes the cloud utility under CPU and memory constraints and also minimizes the cost for adapting an allocation. Then, we extend that protocol to have a management control parameter, which can be done with the help of profiling technique. A particular challenge is to develop a gossip protocol that is robust against node failures. In this paper, we present P*, a gossip protocol for continuous monitoring of aggregates, which is robust against discontiguous failures (i.e., under the constraint that neighboring nodes do not fail within a short period of each other)