Crash-Quiescent Failure Detection

Due to the multiplicity of loci of control, a main issue distributed systems have to cope with lies in the uncertainty on the system state created by the adversaries that are asynchrony, failures, dynamicity, mobility, etc. Considering message-passing systems, this paper considers the uncertainty created by the net effect of asynchrony and process crash failures in systems where the processes are anonymous (i.e., processes have no identity and locally execute the same algorithm). Trivially, agreement problems such as consensus, that cannot be solved in non-anonymous asynchronous systems prone to process failures, cannot be solved either if the system is anonymous. The paper investigates failure detectors that allow processes to circumvent this impossibility. It has several contributions. It first presents four failure detectors (denoted AP, $${\overline{AP}}$$ , AΩ, and AΣ) and show that they are the “identity-free” counterparts of perfect failure detectors, eventual leader failure detectors, and quorum failure detectors, respectively. AΣ is new and showing that AΣ and Σ have the same computability power in a non-anonymous system is not trivial. The paper also shows that the notion of failure detector reduction is related to the computation model. Then, the paper presents and proves correct a uniform anonymous consensus algorithm based on the failure detector pair (AΩ, AΣ) (“uniform” means here that not only processes have no identity, but no process is aware of the total number of processes). This new algorithm is not a simple “straightforward extension” of an algorithm designed for non-anonymous systems. To benefit from AΣ, it uses a novel broadcast facility which encapsulates an AΣ-based message exchange pattern that provides the processes with an interesting intersection property on the set of messages they have exchanged. Finally, the paper discusses the notions of failure detector hierarchy, weakest failure detector for anonymous consensus, and the implementation of identity-free failure detectors in anonymous systems.

While lots of consensus algorithms have been proposed for crash-prone asynchronous message-passing systems enriched with a failure detector of the class Ω (the class of eventual leader failure detectors), very few algorithms have been proposed for systems enriched with a failure detector of the class P (the class of perfect failure detectors). Moreover, (to the best of our knowledge) the early decision and stopping notion has not been investigated in such systems. This paper presents an early-deciding/stopping P-based consensus algorithm. A process that does not crash decides (and stops) in at most min(f+2, t+1) rounds, where t is the maximum number of processes that may crash, and f the actual number of crashes (0≤ f≤ t). Differently from what occurs in a synchronous system, a perfect failure detector notifies failures asynchronously. This makes the design of an early deciding (and stopping) algorithm not trivial. Interestingly enough, the proposed algorithm meets the lower bound on the number of rounds for early decision in synchronous systems. In that sense, it is optimal. The paper then presents an original algorithm that implements a perfect failure detector in the Theta model, an interesting model that achieves some form of synchrony without relying on physical clocks. Hence, the stacking of these algorithms provides an algorithm that solves consensus in the Theta model in min(f+2, t+1) communication rounds, i.e., in two rounds when there are no failures, which is clearly optimal.

This paper is about the weakest failure detector to solve the Election problem in asynchronous distributed systems. We first discuss the relationship between the Election problem and the Consensus problem in asynchronous distributed systems with unreliable failure detectors. Chandra and Toueg have stated that Consensus is solvable in asynchronous systems with unreliable failure detectors. But, in contrast to the Consensus problem, the Election problem is impossible to solve with unreliable failure detectors even with a single crash failure. More precisely, the weakest failure detector that is needed to solve this problem is a Perfect Failure Detector, which is strictly stronger than the weakest failure detector that is needed to solve Consensus.

A non-blocking implementation of a concurrent object is an implementation that does not prevent concurrent accesses to the internal representation of the object, while guaranteeing the deadlock-freedom progress condition without using locks. Considering a failure free context, G. Taubenfeld has introduced (DISC 2013) a simple modular approach, captured under a new problem called the it fair synchronization problem, to transform a non-blocking implementation into a starvation-free implementation satisfying a strong fairness requirement. This paper extends this approach in several directions. It first generalizes the fair synchronization problem to read/write asynchronous systems where any number of processes may crash. Then, it introduces a new failure detector and uses it to solve the fair synchronization problem when processes may crash. This failure detector, denoted QP (Quasi Perfect), is very close to, but strictly weaker than, the perfect failure detector. Last but not least, the paper shows that the proposed failure detector QP is optimal in the sense that the information on failures it provides to the processes can be extracted from any algorithm solving the fair synchronization problem in the presence of any number of process crash failures.

This paper defines the Non-Blocking Atomic Commitment problem in a message-passing asynchronous system and determines a failure detector to solve the problem. This failure detector, which we call the modal failure detector star, and which we denote by M*, is strictly weaker than the perfect failure detector P but strictly stronger than the eventually perfect failure detector ◇P. The paper shows that at any environment, the problem is solvable with M*.

This paper is about the relationship between the Election Problem and Failure Detectors in asynchronous distributed systems. It is stated in [7] that a Perfect Failure Detector P is needed to solve the Election problem. But in contrast to the result, there is a failure detector that solves Election weaker than the Perfect Failure Detector. We introduce the Confirmatory failure detector C. We show that to solve Election, C is necessary while P is not, whereas C+$\diamondsuit$S is sufficient when a majority of the processes are correct.

This research work was designed to examine the effect of legislative oversight function on the performance of the Nigerian Communication Commission (NCC). The major objectives of the study include to examine the effect of legislative oversight function on NCC in promoting the implementation of national communications in Nigeria; to determine the effect of legislative oversight function in ensuring NCC promotes reliable and accessible communication services in Nigeria; to understand how legislative oversight function assists NCC in protecting the rights and interest of service providers and consumers within Nigeria; and to investigate the influence of legislative oversight function in creating an effective, impartial and independent regulatory authority. A survey design method was used for this study. A sample of 221 staff were drawn from Legislative Aides, Staff of NCC and staff of telecommunication companies. Relevant data for this research work was collected from both primary and secondary sources of data. The study adopted principal agent theory as a theoretical framework. Based on these broad objectives, the legislative oversight function on NCC has a positive effect in promoting the implementation of the national communications in Nigeria. Furthermore, it was revealed that the oversight functions of the legislature on NCC has a positive effect on the promotion of reliable and accessible communication services in Nigeria, and also, it was revealed that legislative oversight function has influenced the creation of an effective, impartial and independent regulatory authority. The research therefore concluded and recommended among others that the committee on communication should be careful not to erode the autonomy enjoyed by NCC as this could bring about hitches in service delivery. Secondly, NCC must at all times seek to improve their performance to ensure that national communication systems are reliable and accessible to the people. Finally, the committee on communication should ensure continuous protection of the rights and interest of service providers and consumers within Nigeria.

This paper proposes a novel multi-agent deep reinforcement learning (MADRL)-based positioning algorithm for multiple unmanned aerial vehicles (UAVs) collaboration in mobile access applications where the UAVs work as mobile base stations. The primary objective of the proposed algorithm is to establish reliable mobile access networks for vehicle-to-everything (V2X) communications. This paper jointly considers energy-efficient UAV operation and reliable wireless communication services for realizing robust mobile access services. For the energy-efficient UAV operation, the reward function formulation of our proposed MADRL algorithm contains the features for UAV energy consumption models in order to realize efficient operations. Furthermore, for the reliable wireless communication services, the quality of service (QoS) requirements of individual users are considered as a part of reward function. Furthermore, this paper considers 60, GHz millimeter-wave (mmWave) mobile access for utilizing the benefits of i) ultra-wide-bandwidth for multi-Gbps high-speed communications and ii) high-directional communications for spatial reuse that is obviously good for avoiding interference among densely deployed users. Lastly, the comprehensive and data-intensive performance evaluation of the proposed MADRL-based algorithm for multi-UAV positioning is conducted. The results of these evaluations demonstrate that the proposed algorithm outperforms other existing algorithms.

A necessary and sufficient condition for transforming limited accuracy failure detectors

Under the current international security situation, national communication information security is facing a severe test. At the same time, our traffic network is developing rapidly, and it is also an urgent problem to achieve high quality communication services under the high-speed mobile scene. Traditional solutions include building a dedicated mobile communication system such as GSM-R and increasing the coverage area of mobile network base stations, but these solutions are either limited in scope of application or too high in construction cost, which cannot meet the development needs. In order to provide secure and reliable communication services in high-speed mobile scenarios, we design a device that can integrate multiple wireless heterogeneous networks based on the intelligent fusion network theory. This paper will introduce the design and implementation process of the wireless heterogeneous network access mechanism of the device. The device can access multiple wireless heterogeneous networks at the same time and use multiple links to provide high-quality network services for users. The device can also select the network with the best performance to provide services for users. After a lot of tests and long-term practical operation, the mechanism can better realize the network access and network selection of devices, and then realize to provide users with reliable and high-quality network services.KeywordsSmart mobile routerNetwork accessMulti-attribute decision making

It is shown that, in a precise sense, if there is no bound on the number of faulty processes in a system with unreliable but fair communication, Uniform Distributed Coordination (UDC) can be attained if and only if a system has perfect failure detectors. This result is generalized to the case where there is a bound t on the number of faulty processes. It is shown that a certain type of generalized failure detector is necessary and sufficient for achieving UDC in a context with at most t faulty processes. Reasoning about processes' knowledge as to which other processes are faulty plays a key role in the analysis.

The DLMS (Device Language Message Specification) standard is widely adopted in smart metering systems for utility services, as it defines the mechanisms that enable the standardized and interoperable exchange of data between metering devices. Wi-SUN FAN (Wireless Smart Ubiquitous Network Field Area Network) is a wireless communication standard that has gained increasing attention in the field of smart metering networks, due to its capability to operate in mesh topologies and support multi-hop communications in an efficient and scalable manner. To date, no studies have been reported that evaluate the combined performance of the DLMS and Wi-SUN FAN standards. In this context, this work evaluates the performance of the DLMS standard over a Wi-SUN FAN network through detailed simulations conducted using the Contiki-NG/Cooja platform. The study implements the DLMS data transmission process and evaluates key performance metrics such as Packet Delivery Rate (PDR) and latency, considering both linear and mesh network topologies. The results demonstrate that a Wi-SUN FAN network can provide efficient and reliable communication services, achieving PDR values above 86.02% in both topologies, thereby confirming its feasibility for DLMS-based smart metering applications.

The 3GPP Long Term Evolution (LTE) is now being considered as a promising communications technology for military and public safely applications to provide quality and reliable communication services. In this paper, we consider an LTE-in-a-Box (LIB) system, which is the preferred architecture for military and public safety applications and study how deliberate attacks or natural disasters could cause service disruptions. We present some design alternatives that could anticipate and evolve from disruptions to provide a degraded, but acceptable performance when adverse events occur. Our design alternatives conform to the 3GPP specifications, and can be accomplished using simple software upgrades.

This paper presents an overview of the hardware and software components of the Crystal multicomputer project. The goal of the Crystal project is to design and implement a vehicle that serves a variety of research projects involving distributed computation. Crystal can be used simultaneously by multiple research projects by partitioning the available processors according to the requirements of each project. Users can employ the Crystal multicomputer in several ways. Projects such as operating systems and database machines that need direct control of processor resources (clock, memory management, communication devices) can be implemented using a reliable communication service (the "nugget" that resides on each node processor. Projects that prefer a higher-level interface can be implemented using the Charlotte distributed operating system. Finally, users interested in Crystal principally as a cycle server can run UNIX® jobs on node machines using the "remote" unix service. Development, debugging, and execution of projects can take place remotely under the control of any of several UNIX hosts. Acquiring a partition of machines, resetting each machine, and then loading an application onto each machine is performed by invoking a UNIX-resident program (the "nuggetmaster"). Communication with node machines in a partition is facilitated by a virtual terminal and window mechanism. Crystal is fully operational and has been used to support a variety of research projects. To illustrate the flexibility provided by the Crystal environment, four of these projects are described.

Maintaining efficient and reliable communication services in vehicle networks is challenging due to the high mobility of vehicle users. One of the key issues is to deal with the large amount of channel feedback and make better use of limited wireless resources in real time. In this letter, we propose a trajectory data driven method to maximize the total user utilities, which is characterized by intelligent channel state information (CSI) feedback reduction and vehicle-to-vehicle (V2V)/vehicle-to-infrastructure (V2I) mode switching. A deep neural network (DNN) model is trained using optimization approaches and vehicles’ trajectory data. Based on the well trained model, V2V/V2I mode is selected adaptively and bandwidth is allocated optimally with partial channel feedback. Simulation results show that our proposed method outperforms the comparison baseline and can achieve 96.23% of the performance obtained by exhausting searching with full CSI feedback.