Zarko Milosevic

S-Paxos: Offloading the Leader for High Throughput State Machine Replication

Implementations of state machine replication are prevalently using variants of Paxos or other leader-based protocols. Typically these protocols are also leader-centric, in the sense that the leader performs more work than the non-leader replicas. Such protocols scale poorly, because as the number of replicas or the load on the system increases, the leader replica quickly reaches the limits of one of its resources. In this paper we show that much of the work performed by the leader in a leader-centric protocol can in fact be evenly distributed among all the replicas, thereby leaving the leader only with minimal additional workload. This is done (i) by distributing the work of handling client communication among all replicas, (ii) by disseminating client requests among replicas in a distributed fashion, and (iii) by executing the ordering protocol on ids. We derive a variant of Paxos incorporating these ideas. Compared to leader-centric protocols, our protocol not only achieves significantly higher throughput for any given number of replicas, but also increases its throughput with the number of replicas.

On the Reduction of Atomic Broadcast to Consensus with Byzantine Faults

We investigate the reduction of atomic broadcast to consensus in systems with Byzantine faults. Among the several definitions of Byzantine consensus that differ only by their validity property, we identify those equivalent to atomic broadcast. Finally, we give the first deterministic atomic broadcast reduction with a constant time complexity with respect to consensus.

Unifying Byzantine Consensus Algorithms with Weak Interactive Consistency

The paper considers the consensus problem in a partially synchronous system with Byzantine processes. In this context, the literature distinguishes authenticated Byzantine faults, where messages can be signed by the sending process (with the assumption that the signature cannot be forged by any other process), and Byzantine faults, where there is no mechanism for signatures (but the receiver of a message knows the identity of the sender). The paper proposes an abstraction called weak interactive consistency (WIC ) that unifies consensus algorithms with and without signed messages. WIC can be implemented with and without signatures. The power of WIC is illustrated on two seminal Byzantine consensus algorithms: the Castro-Liskov PBFT algorithm (no signatures) and the Martin-Alvisi FaB Paxos algorithms (signatures). WIC allows a very concise expression of these two algorithms.

Distal: A framework for implementing fault-tolerant distributed algorithms

We introduce Distal, a new framework that simplifies turning pseudocode of fault tolerant distributed algorithms into efficient executable code. Without proper tool support, even small amounts of pseudocode normally ends up in several thousands of non-trivial lines of Java or C++. Distal is implemented as a library in Scala and consists of two main parts: a domain specific language (DSL) in which algorithms are expressed and an efficient messaging layer that deals with low level issues such as connection management, threading and (de)serialization. The DSL is designed such that implementations of distributed algorithms highly resemble the pseudocode found in research papers. By writing code that is close to the protocol description, one can be more convinced that the implemented system really reflects the protocol specification on paper. Distal does not only make it simple and intuitive to implement distributed algorithms but it also leads to efficient implementations.

Generic construction of consensus algorithms for benign and Byzantine faults

The paper proposes a generic consensus algorithm that highlights the basic and common features of known consensus algorithms. The parameters of the generic algorithm encapsulate the core differences between various consensus algorithms, including leader-based and leader-free algorithms, addressing benign faults, authenticated Byzantine faults and Byzantine faults. This leads to the identification of three classes of consensus algorithms. With the proposed classification, Paxos and PBFT indeed belong to the same class, while FaB Paxos belongs to a different class. Interestingly, the classification allowed us to identify a new Byzantine consensus algorithm that requires n > 4b, where b is the maximum number of Byzantine processes.

Bounded Delay in Byzantine-Tolerant State Machine Replication

The paper proposes a new state machine replication protocol for the partially synchronous system model with Byzantine faults. The algorithm, called BFT-Mencius, guarantees that the latency of updates initiated by correct processes is eventually upper-bounded, even in the presence of Byzantine processes. BFTMencius is based on a new communication primitive, Abortable Timely Announced Broadcast (ATAB), and does not use signatures. We evaluate the performance of BFT-Mencius in cluster settings, and show that it provides bounded latency and good throughput, being comparable to the state-of-the-art algorithms such as PBFT and Spinning in fault-free configurations and outperforming them under performance attacks by Byzantine processes.

Securing every bit: authenticated broadcast in radio networks

This paper studies non-cryptographic authenticated broadcast in radio networks subject to malicious failures. We introduce two protocols that address this problem. The first, NeighborWatchRB, makes use of a novel strategy in which honest devices monitor their neighbors for malicious behavior. Second, we present a more robust variant, MultiPathRB, that tolerates the maximum possible density of malicious devices per region, using an elaborate voting strategy. We also introduce a new proof technique to show that both protocols ensure asymptotically optimal running time. We demonstrate the fault tolerance of our protocols through extensive simulation. Simulations show the practical superiority of the NeighborWatchRB protocol (an advantage hidden in the constants of the asymptotic complexity). The NeighborWatchRB protocol even performs relatively well when compared to the simple, fast epidemic protocols commonly used in the radio setting, protocols that tolerate no malicious faults. We therefore believe that the overhead for ensuring authenticated broadcast is reasonable, especially in applications that use authenticated broadcast only when necessary, such as distributing an authenticated digest

Tolerating permanent and transient value faults

Transmission faults allow us to reason about permanent and transient value faults in a uniform way. However, all existing solutions to consensus in this model are either in the synchronous system, or require strong conditions for termination, that exclude the case where all messages of a process can be corrupted. In this paper we introduce eventual consistency in order to overcome this limitation. Eventual consistency denotes the existence of rounds in which processes receive the same set of messages. We show how eventually consistent rounds can be simulated from eventually synchronous rounds, and how eventually consistent rounds can be used to solve consensus. Depending on the nature and number of permanent and transient transmission faults, we obtain different conditions on

Student mini-kernel project based on an FPGA board

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