Node Churn Research Articles

Expansion and flooding in dynamic random networks with node churn

AbstractWe study expansion and flooding in evolving graphs, when nodes and edges are continuously created and removed. We consider a model with Poisson node inter‐arrival and exponential node survival times. Upon joining the network, a node connects to random nodes, while an edge disappears whenever one of its endpoints leaves the network. For this model, we show that, although the graph has isolated nodes with large, constant probability, flooding still informs a fraction of the nodes in time . Moreover, at any given time, the graph exhibits a “large‐set expansion” property. We further consider a model in which each edge leaving the network is replaced by a fresh, random one. In this second case, we prove that flooding informs all nodes in time , with high probability. Moreover, the graph is a vertex expander with high probability.

Attestation of Improved SimBlock Node Churn Simulation

Node churn, or the constant joining and leaving of nodes in a network, can impact the performance of a blockchain network. The difficulties of performing research on the actual blockchain network, particularly on a live decentralized global network like Bitcoin, pose challenges that good simulators can overcome. While various tools, such as NS-3 and OMNet++, are useful for simulating network behavior, SimBlock is specifically designed to simulate the complex Bitcoin blockchain network. However, the current implementation of SimBlock has limitations when replicating actual node churn activity. In this study, the SimBlock simulator was improved to simulate node churn more accurately by removing churning nodes and dropping their connections, and increasing additional instrumentation for validation. The methodology used in the study involved modeling the Bitcoin node churn behavior based on previous studies and using the enhanced SimBlock simulator to simulate node churn. Empirical studies were then conducted to determine the suitability and limitations of the node churn simulation. This study found that the improved SimBlock could produce results similar to observed indicators in a 100-node network. However, it still had limitations in replicating node churn behavior accurately. It was discovered that SimBlock limits all nodes to operate as mining nodes and that mining is simulated in a way that does not depict churn accurately at any time but only at specific intervals or under certain conditions. Despite these limitations, the study’s improvements to SimBlock and the identification of its limitations can be useful for future research on node churn in blockchain networks and the development of more effective simulation tools.

Empirical Comparison of Block Relay Protocols

Block relay protocols play a key role in the performance and security of public blockchains. As a result, several such protocols have been deployed in the context of Bitcoin and its variants (e.g. legacy, compact block relay and Graphene) in an attempt to reduce bandwidth utilization. However, the relative performance of these protocols in realistic networking conditions (e.g. with nodes churning -joining and leaving the network) is still not known. This paper aims to fill this key knowledge gap using an experimental testbed of twelve full nodes connected to the Bitcoin Cash blockchain. With the aid of novel logging tools, we contrast the performance of these three protocols, in realistic scenarios, with respect to communication, delay, and block decoding success. Our main findings are that Graphene generally performs the best when nodes remain connected, boasting an average propagation delay of 190 ms (i.e. 29% lower than compact block and 80% lower than the legacy protocol). However, when nodes churn at a high rate, compact blocks may perform better. Through a careful temporal analysis, we identify some root causes of the protocol inefficiencies, together with potential mitigation. We have made our measurement framework and experimental logs publicly available to the broader research community.

Distributed Broadcasting in Dynamic Networks

In this paper, we investigate distributed broadcasting in dynamic networks, where the topology changes continually over time. We propose a network model that captures the dynamicity caused by both churn and mobility of nodes. In contrast to existing work on dynamic networks, our model defines the dynamicity in terms of localized topological changes in the vicinity of each node, rather than a global view of the whole network. Obviously, a local dynamic model suits distributed algorithms better than a global one. The proposed dynamic model uses the more realistic SINR model to depict wireless interference, instead of oversimplified graph-based models adopted in most existing work. We consider the fundamental communication primitive of global broadcast, which is to disseminate a message from a source node to the whole network. Specifically, we present a randomized distributed algorithm that can accomplish dynamic broadcasting in an asymptotically optimal running time of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$O(D_{T})$ </tex-math></inline-formula> with a high probability guarantee, under the assumption of reasonably constant <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dynamicity rate</i> , where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D_{T}$ </tex-math></inline-formula> is the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dynamic diameter</i> , a parameter proposed to depict the complexity of dynamic broadcasting. We believe our local dynamic model can greatly facilitate distributed algorithm studies in mobile and dynamic wireless networks.

Admission Control for Ad-hoc Edge Cloud

The penetration of connected devices in today’s society has reached overwhelming figures particularly in the last decade. At present connected devices are not only becoming available everywhere, but they are rapidly gaining complexity in terms of their ability to hold significant compute and storage capacities. Thus, computing ceases to be confined to certain stationary compute devices in order to allow compute to be embedded and pervasive to everything. This feature combined with the fact that AI processes are multiplying the need for timely processing at the Edge, reveals the urgency to exploit all compute capacity available at the Edge of the network. This represents a significant breakthrough to initial Edge computing developments concentrated in providing low latency compute environments for which IoT devices are solely considered as data sources. Ad-hoc Edge Cloud is a distributed and decentralized Edge computing system dynamically formed out of IoT Edge computing resources, which aims to exploit increasingly available compute capacity at the Edge. The marked characteristics of the IoT devices, which constitute this infrastructure, pose specific challenges to resource management in this context especially due to heterogeneity, dynamicity, and volatility of resources, resulting in the probability of node churn. In this paper , we address particular admission control and service placement issues for Ad-hoc Edge Cloud by presenting a specific Admission Control mechanism for which we define and validate an associated resource availability prediction model.

Impact of Node Churn in the Bitcoin Network

The aim of this work is to evaluate the impact of node churn –nodes leaving and rejoining the network– on the Bitcoin network. We provide a comprehensive analytical model for the churning process. We use a Continuous Time Markov Chain (CTMC) to describe the behavior of a node, and then apply the results to model the changes in connectivity and the impact on network performance. We analyze the time needed to resynchronize a node upon rejoining the network and find that sleep times of the order of hours require synchronization times limited by a minute. We estimate the impact of sleep and synchronization time on overall network connectivity and block/transaction distribution time. Our results show that networks with less than 4000 nodes are sensitive to churn. This occurs due to opposing impact of decrease in network size (and diameter) due to sleep time and increase of communication load per node. However, the impact of churn on network with more than 4000 nodes is noticeable but small enough to make a large Bitcoin network fairly resilient to churn.

An analytical model for churn process in Bitcoin network with ordinary and relay nodes

Dynamic participation of nodes, also known as churn, is quite common in P2P networks, and Bitcoin network is no exception. The impact of churn becomes more pronounced when nodes with different roles and functionality exist. In this paper we analyze node churn in Bitcoin network where ordinary nodes generate transaction traffic and distribute and verify blocks, while relay nodes that just distribute blocks, but have a larger number of connections. We use the tools of probabilistic analysis and Markov chains, and show that churn of relay nodes has a higher impact on traffic performance than that of ordinary nodes. We introduce two different Continuous Time Markov chains (CTMC) for ordinary and relay nodes to model each type of node’s behavior separately. Our results indicate that node sleep impacts the performance more in networks where the proportion of relay nodes is lower, and also that block and transaction delivery times are higher in those cases. Also, we find that the time needed for a node to synchronize upon rejoining the network after sleep is dominated by the time required for block and transaction verification.

CAAVI-RICS model for observing the security of distributed IoT and edge computing systems

The pervasive integration of ‘things‘ in the Internet of Things together with state-of-the-art computer systems provide a stimulating environment for creativity and business opportunities, but also a large range of security challenges. Engineering the security of such systems must acknowledge the peculiar conditions under which such systems operate: low computational capacity, distributed decision-making, significant node churn, etc. These conditions must, therefore, be supported by the techniques and methodologies for building secure and robust IoT systems. With CAAVI-RICS methodology we explore credibility, authentication, authorization, verification, and integrity of IoT and edge computing systems, through explaining the rationale, influence, concerns, and security solutions that accompany them. Our contribution is a complete and detailed systematic categorization and streamlining of security problems, covering the security environment of IoT and edge computing systems. Besides, we contribute to the debate on key aspects of edge computing security and state-of-the-art solutions.

Bioinspired Multiagent Embryonic Architecture for Resilient Edge Networks

With the pervasive introduction of Internet of Everything (IoE) technologies, use cases are frequently being found operating within harsh environmental conditions. This decreases the need for solutions that permit service delivery to operate in a highly resilient manner. This paper presents an architecture for a novel cloud platform designed for resilient service delivery. It supports networks where poor communication links or high node failure will cause services to be delivered in an nonresilient manner. This could be the result of factors such as high node mobility, poor environmental conditions, and unreliable infrastructure from environment disaster or cyber-attack. This biologically inspired architecture uses a purely distributed multiagent approach to provide self-healing and self-organizing properties, modeled on the characteristics of embryonic development and biological cell communication. To permit high levels of a node churn, this multiagent approach uses local-only communication. Probabilistic cellular automata are used to simulate this architecture and evaluate the efficacy of this approach.

Robust and efficient membership management in large-scale dynamic networks

Epidemic protocols are a bio-inspired communication and computation paradigm for large-scale networked systems based on randomised communication. These protocols rely on a membership service to build decentralised and random overlay topologies. In large-scale, dynamic network environments, node churn and failures may have a detrimental effect on the structure of the overlay topologies with negative impact on the efficiency and the accuracy of applications. Most importantly, there exists the risk of a permanent loss of global connectivity that would prevent the correct convergence of applications. This work investigates to what extent a dynamic network environment may negatively affect the performance of Epidemic membership protocols. A novel Enhanced Expander Membership Protocol (EMP+) based on the expansion properties of graphs is presented. The proposed protocol is evaluated against other membership protocols and the comparative analysis shows that EMP+ can support faster application convergence and is the first membership protocol to provide robustness against global network connectivity problems.

Performance analysis of structured peer-to-peer overlays for mobile networks

Distributed Hash Table (DHT) based Peer-to-Peer (P2P) overlays have been widely researched and deployed in many applications such as file sharing, IP telephony, content distribution and media streaming applications. However, their deployment has largely been restricted to fixed, wired networks. This is due to the fact that supporting P2P overlays on wireless networks such as the public mobile data network is more challenging due to constraints in terms of data transmissions on cellular networks, limited battery power of the handsets and increased levels of node churn. However, the proliferation of smartphones makes the use of P2P applications on mobile handsets very desirable. In this article, we have analysed and evaluated the performance and efficiency of five popular DHT based structured P2P overlays (Chord, Pastry, Kademlia, Broose and EpiChord) under conditions as commonly experienced in public mobile data networks. Our results show that the conditions in mobile networks, including a high churn rate and the relatively low bandwidth availability is best matched by Kademlia and EpiChord. These overlays exhibit a high lookup success ratio and low hop count while consuming a moderate amount of bandwidth. These characteristics make these two overlays suitable candidates for use in mobile networks.

Blogracy

The current approach to build social networking systems is to create huge centralized systems owned by a single company. However, such strategy has many drawbacks, e.g., lack of privacy, lack of anonymity, risks of censorship and operating costs. In this paper the authors propose a novel P2P system that leverages existing, widespread and stable technologies such as DHTs and BitTorrent. In particular, they introduce a key-based identity system and a model of social relations for distributing content efficiently among interested readers. The system they propose, Blogracy, is a micro-blogging social networking system focused on: (i) anonymity and resilience to censorship; (ii) authenticatable content; (iii) semantic interoperability using activity streams. The authors have implemented the system and conducted various experiments to study its behaviour. The results are presented here, regarding (i) communication delays for some simulations of node churn, (ii) delays measured in test operations over PlanetLab, in direct communication, and (iii) through the I2P anonymizing network.

Decentralized Time-Synchronized Channel Swapping for Ad Hoc Wireless Networks

Time-synchronized channel hopping (TSCH) is currently the most efficient solution for collision-free interference-avoiding communications in ad hoc wireless networks, such as wireless sensor networks, vehicular networks, and networks of robots or drones. However, all variants of TSCH require some form of centralized coordination to maintain the time–frequency slotting mechanism. This leads to slow convergence to steady state and moderate time–frequency slot utilization, particularly under node churn or mobility. We propose decentralized time-synchronized channel swapping (DT-SCS), which is a novel protocol for medium access control (MAC) in ad hoc wireless networks. Under the proposed protocol, nodes first converge to synchronous beacon packet transmissions across all available channels at the physical layer, with a balanced number of nodes in each channel. This is done by the novel coupling of distributed synchronization and desynchronization mechanisms —which are based on the concept of pulse-coupled oscillators—at the MAC layer. Decentralized channel swapping can then take place via peer-to-peer swap requests/acknowledgments made between concurrent transmitters in neighboring channels. We benchmark the convergence and network throughput of DT-SCS, TSCH, and the efficient multichannel MAC protocol (seen as the state of the art in decentralized, interference-avoiding, and multichannel MAC protocols) under simulated packet losses at the MAC layer. Moreover, performance results via a Contiki-based deployment on TelosB motes reveal that DT-SCS comprises an excellent candidate for decentralized multichannel MAC-layer coordination by providing for quick convergence to steady state, high bandwidth utilization under interference and hidden nodes, as well as high connectivity.

A Framework for Analyzing Anonymous Network Topology

Nowadays, Internet privacy becomes more and more important and sensitive, and has been one of the latest buzz words to hit the Internet world. In response to protect the privacy of Internet users, a variety of privacy enhancing technologies (PETs) have emerged. As one of privacy enhancing technologies, anonymous communication has been extensively studied from various aspects by researchers. Node churn is frequent in anonymous network, which make it difficult to maintain a stable network topology, therefore it is necessary to get more insight into the overall anonymous network topology. In this work, in order to better grasp the present situation of research, we investigate existing organizations and universities which study anonymous communication from the perspective of network topology, and related projects. Then we survey related papers to anonymous communication published in recent years, which focus on the analysis of node selection (especially in the Tor). In addition, we present a framework of anonymous communication network topology, and a visual illustration of this analysis that shows the progression of the research of network topology and node discovery in anonymous communication. Finally, some related problems and follow-up study are presented to be studied deeply in future.

Distributed agreement in dynamic peer-to-peer networks

Motivated by the need for robust and fast distributed computation in highly dynamic Peer-to-Peer (P2P) networks, we present first-known, fully-distributed algorithms for the fundamental distributed agreement problem in dynamic networks that experience heavy node churn (i.e., nodes join and leave the network continuously over time). Our algorithms guarantee stable almost-everywhere agreement with high probability even under high adversarial churn and run in time that is polylogarithmic in n (which is the stable network size). Our first algorithm can tolerate a churn of up to εn per time step, sends only polylogarithmic number of bits per node per time step, and works under an adversary that is oblivious to the algorithm's random choices. Our second algorithm, designed for the more challenging adaptive adversary, can tolerate a churn of up to εn. Being easy to implement, our algorithms could serve as building blocks for other non-trivial distributed computation in dynamic networks.

Overlay live video streaming with heterogeneous bitrate requirements

We study a streaming cloud formed by distributed proxies providing live video service to diverse users (e.g., smart TVs, PCs, tablets, mobile phones, etc.). The proxies form a push-based overlay network, with each proxy serving a certain video bitrate for users to join. To form a proxy overlay serving heterogeneous bitrates, we consider that the video is encoded into multiple MDC (Multiple-Description Coding) streams with the serving bitrate of proxy i being ki description streams. In order to effectively mitigate stream disruption due to node churns, proxy i also joins an additional ri redundant MDC streams (ri⩾0) in such a way that all the (ki+ri) streams are supplied by distinct parents. For live streaming, the critical issue is how to construct the parent-disjoint trees minimizing the assembly delay of the proxies.We present a realistic delay model capturing important system parameters and delay components, formulate the optimization problem and show that it is NP-hard. We propose a centralized algorithm which is useful for a centrally-managed network and serves as a benchmark for comparison (PADTrees-Centralized). For large network, we propose a simple and distributed algorithm which continuously reduces delay through overlay adaptation (PADTrees-Distributed). Through extensive simulation on real Internet topologies, we show that high stream continuity can be achieved with push-based trees in the presence of node churns. Our algorithms are simple and effective, achieving low loss and low delay.

ChordMR: A P2P-based Job Management Scheme in Cloud

MapReduce is a programming model and an associated implementation for processing parallel data, which is widely used in Cloud computing environments. However, the traditional MapReduce system is based on a centralized master-slave structure. While, along with the increase of the number of MapReduce jobs submitted and system scale, the master node will become the bottleneck of the system. To improve this problem, we have proposed a new MapReduce system named ChordMR, which is designed to use a peer-to-peer Chord network to manage master node churn and failure in a decentralized way. More importantly, we propose a job management scheme which contains the aspects of job assignment, job backup, job recovery, etc. In addition, in ChordMR, the job backup scheme makes a number of identical copies for each job and classifies the jobs into K 2 classes to achieve the goals of fault tolerance and load balance. The theoretical and simulation results show that our ChordMR is effective

Leveraging Social Networks for P2P Content-Based File Sharing in Disconnected MANETs

Current peer-to-peer (P2P) file sharing methods in mobile ad hoc networks (MANETs) can be classified into three groups: flooding-based, advertisement-based, and social contact-based. The first two groups of methods can easily have high overhead and low scalability. They are mainly developed for connected MANETs, in which end-to-end connectivity among nodes is ensured. The third group of methods adapts to the opportunistic nature of disconnected MANETs but fails to consider the social interests (i.e., contents) of mobile nodes, which can be exploited to improve the file searching efficiency. In this paper, we propose a P2P content-based file sharing system, namely SPOON, for disconnected MANETs. The system uses an interest extraction algorithm to derive a node's interests from its files for content-based file searching. For efficient file searching, SPOON groups common-interest nodes that frequently meet with each other as communities. It takes advantage of node mobility by designating stable nodes, which have the most frequent contact with community members, as community coordinators for intracommunity searching, and highly mobile nodes that visit other communities frequently as community ambassadors for intercommunity searching. An interest-oriented file searching scheme is proposed for high file searching efficiency. Additional strategies for file prefetching, querying-completion, and loop-prevention, and node churn consideration are discussed to further enhance the file searching efficiency. We first tested our system on the GENI Orbit testbed with a real trace and then conducted event-driven experiment with two real traces and NS2 simulation with simulated disconnected and connected MANET scenarios. The test results show that our system significantly lowers transmission cost and improves file searching success rate compared to current methods.

ReDS: A Framework for Reputation-Enhanced DHTs

Distributed Hash Tables (DHTs) such as Chord and Kademlia offer an efficient solution for locating resources in peer-to-peer networks. Unfortunately, malicious nodes along a lookup path can easily subvert such queries. Several systems, including Halo (based on Chord) and Kad (based on Kademlia), mitigate such attacks by using a combination of redundancy and diversity in the paths taken by redundant lookup queries. Much greater assurance can be provided, however. We describe Reputation for Directory Services (ReDS), a framework for enhancing lookups in redundant DHTs by tracking how well other nodes service lookup requests. We describe how the ReDS technique can be applied to virtually any redundant DHT including Halo and Kad. We also study the collaborative identification and removal of bad lookup paths in a way that does not rely on the sharing of reputation scores --- we show that such sharing is vulnerable to attacks that make it unsuitable for most applications of ReDS. Through extensive simulations we demonstrate that ReDS improves lookup success rates for Halo and Kad by 80% or more over a wide range of conditions, even against strategic attackers attempting to game their reputation scores and in the presence of node churn.

Scale-Free Networks to Search in Unstructured Peer-To-Peer Networks

Unstructured peer-to-peer file sharing networks are very popular in the market. They introduce network traffic. The resultant networks are not perform search efficiently because existing overlay topology formation algorithms are creates unstructured P2P networks without performance guarantees. In this paper, We compares structured and unstructured overlays, Showing through analytical and simulation results how an unstructured solution relying on a scale-free topology is an effective option to deploy for offering services based on equivalent servants. Based on this result, The EQUivalent servAnt locaTOR (EQUATOR) architecture, which overcomes the issues related to the deployment of a scale-free topology for service location in a real network, mainly due to the static nature of the ideal scale-free construction algorithm and the lack of a global knowledge of the participating peers. Simulation results confirmed the effectiveness of EQUATOR, showing how it offers good lookup performance in combination with low message overhead and high resiliency to node churn and failures. Some possible future works are introduced and are related to some complementary issues ranging from the proximity-aware selection of servants to the introduction of proper incentives to encourage nodes to join the EQUATOR overlay and offer their resources.