An analysis of update ordering in distributed replication systems

This paper analyses update ordering and its impact on the performance of a distributed replication system. We propose a model for update orderings and constraints and develop a number of algorithms for implementing different ordering constraints. A performance study is then carried out to analyse the update-ordering model. We show that our model allows the definition of an ordering constraint on each update operation, and the ordering implementation takes account of detailed inter-operation semantics denoted by commutative operations and causal operations to reduce unnecessary delay and results in a better response time for update requests.

Distributed defense against distributed denial-of-service attacks

Distributed defense is a promising way to neutralize the distributed Denial-of-Service attacks by detecting and responding the attacking sources widespread around the Internet. Components of the distributed defense system will cooperate with each other to combat the attacks. Compared with the centralized defense systems, distributed defense systems can discover the attacks more timely from both source end and victim end, fight the attacks with more resources and take advantage of more flexible strategies. This paper investigates 7 distributed defense systems which make use of various strategies to mitigate the DDoS attacks. Different architectures are designed in these 7 systems to provide distributed DDoS defense solutions. We evaluate these systems in terms of deployment, detection, response, security, robustness and implementation. For each criteria, we give a recommendation on which technologies are best suitable for a successful distributed defense system based on the analysis result. Finally we propose our idea on the design of an effective distributed defense system.

A comparative study at the logical level of centralised and distributed recovery in clusters

Cluster systems are becoming more prevalent in today’s computer society and users are beginning to request that these systems be reliable. Currently, most clusters have been designed to provide high performance at the cost of providing little to no reliability. To combat this, this report looks at how a recovery facility, based on either a centralised or distributed approach could be implemented into a cluster that is supported by a check pointing facility. This recovery facility can then recover failed user processes by using checkpoints of the processes that have been taken during failure free execution.

Phylogenetic relationships of the globally distributed freshwater prawn genus Macrobrachium (Crustacea: Decapoda: Palaemonidae): biogeography, taxonomy, and the convergent evolution of abbreviated larval development

There has hitherto been little research into evolutionary and taxonomic relationships amongst species of the freshwater prawn genus Macrobrachium Bate across its global distribution. Previous work by the authors demonstrated that the endemic Australian species did not evolve from a single ancestral lineage. To examine whether other regional Macrobrachium faunas also reflect this pattern of multiple origins, the phylogeny of 30 Macrobrachium species from Asia, Central/South America and Australia was inferred from mitochondrial 16S rRNA sequences. Phylogenetic relationships demonstrate that, despite some evidence for regional diversification, Australia, Asia and South America clearly contain Macrobrachium species that do not share a common ancestry, suggesting that large-scale dispersal has been a major feature of the evolutionary history of the genus. The evolution of abbreviated larval development (ALD), associated with the transition from an estuarine into a purely freshwater lifecycle, was also mapped onto the phylogeny and was shown to be a relatively homoplasious trait and not taxonomically informative. Other taxonomic issues, as well as the evolutionary origins of Macrobrachium, are also discussed.

Protecting web applications from DDoS attacks by an active distributed defense system

In the last a few years a number of highly publicized incidents of Distributed Denial of Service (DDoS) attacks against high-profile government and commercial websites have made people aware of the importance of providing data and services security to users. A DDoS attack is an availability attack, which is characterized by an explicit attempt from an attacker to prevent legitimate users of a service from using the desired resources. This paper introduces the vulnerability of web applications to DDoS attacks, and presents an active distributed defense system that has a deployment mixture of sub-systems to protect web applications from DDoS attacks. According to the simulation experiments, this system is effective in that it is able to defend web applications against attacks. It can avoid overall network congestion and provide more resources to legitimate web users.

Object oriented finite element analysis: a distributed approach to mesh generation

The object-oriented finite element method (OOFEM) has attracted the attention of many researchers. Compared with the traditional finite element method, OOFEM software has the advantages of maintenance and reuse. Moreover, it is easier to expand the architecture to a distributed one. In this paper, we introduce a distributed architecture of a object-oriented finite element preprocessor. A comparison between the distributed system and the centralised system shows that the former, presented in the paper, greatly improves the performance of mesh generation. Other finite element analysis modules could be expanded according to this architecture.

Synergy : a comprehensive software distributed shared memory system

Considerable research and development has been invested in software Distributed Shared Memory (DSM). The primary focus of this work has traditionally been on high performance and consistency protocols. Unfortunately, clusters present a number of challenges for any DSM systems not solvable through consistency protocols alone. These challenges relate to the ability of DSM systems to adjust to load fluctuations, computers being added/removed from the cluster, to deal with faults, and the ability to use DSM objects larger than the available physical memory. This paper introduces the Synergy DSM System and its integration with the virtual memory, group communication and process migration services of the Genesis Cluster Operating System.

Mark-aided distributed filtering by using neural network for DDoS defense

Currently Distributed Denial of Service (DDoS) attacks have been identified as one of the most serious problems on the Internet. The aim of DDoS attacks is to prevent legitimate users from accessing desired resources, such as network bandwidth. Hence the immediate task of DDoS defense is to provide as much resources as possible to legitimate users when there is an attack. Unfortunately most current defense approaches can not efficiently detect and filter out attack traffic. Our approach is to find the network anomalies by using neural network, deploy the system at distributed routers, identify the attack packets, and then filter them. The marks in the IP header that are generated by a group of IP traceback schemes, Deterministic Packet Marking (DPM)/Flexible Deterministic Packet Marking (FDPM), assist this process of identifying attack packets. The experimental results show that this approach can be used to defend against both intensive and subtle DDoS attacks, and can catch DDoS attacks’ characteristic of starting from multiple sources to a single victim. According to results, we find the marks in IP headers can enhance the sensitivity and accuracy of detection, thus improve the legitimate traffic throughput and reduce attack traffic throughput. Therefore, it can perform well in filtering DDoS attack traffic precisely and effectively.

An efficient replicated data access approach for large-scale distributed systems

In data-intensive distributed systems, replication is the most widely used approach to offer high data availability, low bandwidth consumption, increased fault-tolerance and improved scalability of the overall system. Replication-based systems implement replica control protocols that enforce a specified semantics of accessing the data. Also, the performance depends on a number of factors, the chief of which is the protocol used to maintain consistency among object replica. In this paper, we propose a new low-cost and high data availability protocol called the box-shaped grid structure for maintaining consistency of replicated data on networked distributed computing systems. We show that the proposed protocol provides high data availability, low communication costs, and increased fault-tolerance as compared to the baseline replica control protocols.

Distributed power control in cellular mobile radio systems with time-varying link gains

A simple distributed power control algorithm for communication systems with mobile users and unknown time-varying link gains is proposed. We prove that the proposed algorithm is exponentially converging. Furthermore, we show that the algorithm significantly outperforms the well-known Foschini and Miljanic algorithm in the case of quickly moving mobile users.

Investigating the shared visual context in distributed collaborative work

Distributed collaborators require computermediated communication (CMC) technologies in order to work together. Various systems have been developed for this express purpose with varying degrees of success. A basic method of evaluating the usability of a system is to compare it with face-to-face interaction. To replicate the face-to-face context, it is necessary to investigate how visual information plays a role in supporting collaborators performing tasks.
This research examines the effects of visual information and its role in both face-to-face and video generated visual contexts. The results were generated by asking participants to collaboratively solve visual tasks in either of the two contexts. The results show that both the face-to-face and the video conferencing contexts have similar effects on subjects’ ability to perform tasks. Task outcomes exhibited no significant difference between these two contexts. Awareness and conversational grounding had positive effects on the subject’s task performance and communication. On the other hand, presence had mixed effects on a subject’s task performance and communication behaviors.