Our dependence on the internet for simple and time critical applications make availability of services a top security concern. There are many attacks to threaten the availability of service, but Distributed Denial of Service (DDoS) attack constitute one of the major and the most growing threat to s everely degrade the system performance. DDoS defense has been matured over time and many solutions have been proposed to solve this problem. However, most of the research is focused on detecting and blocking this attack, which results in significant disruption before regaining system control and are not suitable for critical infrastructures. So, researchers come up with an interesting alternative, overlay based defensive approaches, which try to survive the attack by providing sustainable performance to the legitimate clients in attack situations. This article provides a review of the overlay based defensive techniques by categorizing them into two classes based on their underlying strategy: Proactive and Reactive approaches. The techniques within each category are discussed along with their advantages and disadvantages, and are studied comparatively with respect to relevant performance criteria. This paper also discusses the vulnerabilities of these architectures to guide the development of more robust procedures to combat DDoS attacks.

In this paper, we have considered an optimal design and provisioning of WDM networks for the grooming of sub-wavelengths traffic requests. As higher layer electronic ports, such as transceivers and optical splitters are dominant cost factors of an optical network, it is essential to reduce their numbers of use while grooming the multicast traffic requests into high bandwidth light-trees. This paper provides an optimal cost design of WDM networks with multicast traffic grooming under static traffic demand. We develop a unified framework for the optimal provisioning of different practical scenarios of multicast traffic grooming in a static traffic scenario. In this study, we design an Integer linear Programming (ILP) formulation for multicast traffic grooming to minimize the cost associated with the higher layer electronic ports such as transceivers, splitters and wavelengths, and simultaneously maximize the bandwidth utilization of the network. We propose a heuristic algorithm called Efficient Light-Tree based Multicast Traffic Grooming (ELT-MTG) algorithm to achieve scalability in large size optical networks. Simulations are conducted on several standard well-known WDM mesh networks to study the design cost (based on number of transceivers, optical splitters and wavelengths) used in the networks. The result, thus obtained by comparison, helped us to conclude that the proposed approach ELT-MTG gives better performance than well-known existing logical-first sequential routing with single-hop grooming (LFSEQSH) and logical-first sequential routing with multi-hop grooming (LFSEQMH) algorithms in a static traffic environment.

Throughput improvement of the wireless LANs has been a constant area of research. Most of the work in this area focuses on designing throughput optimal schemes for fully connected networks (no hidden nodes). The schemes are optimal in a sense that they maximize the sum throughput subject to giving equal channel access opportunity to each user. But, we demonstrate that the optimal schemes proposed in the literature, though perform optimally in fully connected network, achieve significantly lesser throughput even than that of standard IEEE 802.11 in a network with hidden nodes. This motivates designing schemes that provide near optimal performance even when hidden nodes are present. This problem is particularly challenging as mathematical models for networks with hidden nodes do not exist. Here, we consider the weighted fair throughput allocation policy that maximizes system throughput while guaranteeing that every user achieves a throughput that is proportional to their weights. We prove the optimality of the proposed scheme in a fully connected network. However, the specialty of the algorithm, as observed in the simulation study, is that it can provide the optimal throughput even in networks with hidden terminals. Unlike existing techniques, which use the mathematical model to derive invariant features, we attempt to maximize the throughput directly using stochastic approximation techniques, and hence does not require the estimation of any system parameters which can result in inaccurate predictions. Simulations show that our proposed policies perform better than the standard IEEE 802.11 protocol without or with hidden nodes present. Moreover, our proposed schemes outperform existing schemes when the network is not fully connected.

In multi-channel WLANs, the MAC protocol allocates a channel to each transmission pair while avoiding interference. In the channel allocation phase, if the MAC protocol allocates one channel iteratively to multiple narrowly spaced transmission pairs, both channel utilization and network throughput can be improved. However, such an approach leads to traffic overload; thus, if a transmission takes place over a transmission channel, the number of terminals that can receive the transmission decreases. B-MAC, a multi-channel MAC protocol designed for broadcast, starts a new broadcast transmission after waiting for a certain period of time; however, this waiting time increases the transmission overhead and degrades channel utilization. In this paper, we propose CA-BMAC, a new multi-channel MAC protocol that reduces the waiting time associated with B-MAC, and thus, the transmission overhead. Performance evaluations of the proposed protocol clarify its advantages over B-MAC.

Fault tolerance is actually an essential issue in cloud computing to face failures and minimize their damages. The checkpointing is a powerful fault tolerance technique that consists of saving the transient state of a computation system on a persistent storage from which the execution state can be restarted in case of failure. The coordinated checkpointing is an efficient checkpointing strategy because it is domino effect-free and it needs only the last stored checkpoint to ensure a consistent state. In this paper we propose a lightweight coordinated checkpointing for cloud computing that minimizes the overload of classical coordinated checkpointing by minimizing the number of the participating virtual machines (VMs) in each checkpointing interval. The experimental results prove that our proposal reduces the overload and improves the system performances.

Wireless Sensor Networks have proven their capability to deal with problems where wide and hardly accessible areas need to be monitored. Among the other systems there are also sensor networks in wh...

In IEEE 802.16 WiMAX, it is a challenging task to achieve quality of service (QoS) by using scheduling technique as it requires understanding the traffic features and service requirements. Also most of existing works have considered only one of the service flows among rtPS, ertPS, nrtPS, UGS and BE. In addition, the existing priority estimation methods are not so accurate. To overcome these problems, in this paper, we propose a fuzzy based scheduling technique for multiple service flows in IEEE 802.16 WiMAX networks. This technique involves two entities namely evaluator and slot allocator. When the service flow enters into the network, the evaluator distinguishes the service flow as real time or non real time and estimates its priority using fuzzy logic technique. Then the evaluator arranges the real time and non real time service flows in two sub queues and then sorts them by their priority levels. The slot allocator obtains the priority information of service flows from the evaluator to allocate slots. By simulation results, we show that the proposed approach is better in terms of the measured channel utilization and received bandwidth for real-time and non real time flows.

Telecommunication networks are evolving due to rapid growth of internet traffic and the necessity to satisfy the new requirements of emerging packet services. Upgrading of network devices so as they can allow scalable and full line rate traffic aggregation and eliminate any internal performance bottlenecks is crucial. In packet switching devices, since data plane functionalities need to be executed for each incoming packet and power per given bandwidth is strongly related to the amount of processing on this data, packet processing at ultra-high rate is becoming the major challenge. The main trend to address such power consumption and scalability issues is to bypass packet switches by switching traffic at lower layers. This results in the packet optical transport network approach, where packet switching provides flexible end-to-end connectivity based on tunnel encapsulation while wavelength switching, exploiting optical bypass, allows reducing electrical switch size at transit nodes. There are also opportunities consisting in simplifying packet switch functionalities or designing completely different packet switch architectures. In this paper a new Ethernet aggregation and switching solution with potentialities to simplify and scale Ethernet switch forwarding functionality is proposed. This solution, based on a burst-basis transmission compliant with the Ethernet Standard, is able to maintain flexibility and any to any connectivity deriving from the connectionless nature of Ethernet. At the same time, it provides Ethernet technology with efficient aggregation capabilities allowing to reduce processing of transit traffic. This is allowed thanks to an Ethernet burst structure conceived as a variable number of consecutive frames of the same connection preceded by a proprietary burst control frame carrying information necessary for burst data frames classification. As a result, burst control frames experience the conventional Ethernet switch packet processing while data frames are mapped on the corresponding queue/output port according to the result of control frames classification. The proposed solution also provides the possibility to recognize data frames of the same burst through a proprietary inter-frame gap inserted among them; that allows to dynamically adapt burst size to the available bandwidth at transit nodes in order to limit frame delay and jitter and to support intermediate grooming. These features make it a very competitive approach in the context of packet optical transport being able to support dynamic multi-granular switching. The proposed solution has been validated by estimating its efficiency in terms of energy consumption with respect to a commercial packet switch. The impact of burst transmission on packet delay and jitter across ring and mesh networks has been also evaluated through different sets of simulations.

Over the past decade, the ever-increasing energy demands of IT infrastructures have posed significant challenges for the research community in terms of reducing their total power consumption and minimizing their environmental impact. Optical communication networks are envisioned to be promising candidates to help preventing this problem affecting the Internet backbone, as well as for distributed applications such as computational grids. In this paper, we propose an adaptive and distributed scheme for the establishment of energy-efficient lightpaths in computational grids. The grid is deployed over an optical circuit-switched backbone network, defining an optical grid network. Each node of the backbone network maintains two different dynamic thresholds values and estimates the changes in network performance by evaluating the moving average of the total wavelength channel occupancy on all its input/output links. The nodes have the ability of reducing the energy consumption by entering into an Energy Saving Mode (ESM) on the basis of a comparison between their channel occupancy and the thresholds. Furthermore, we extend our framework by allowing the thresholds to be dynamically adapted depending on the network performance in terms of blocking probability. We show that the proposed method achieves considerable energy savings when compared to a normal energy-unaware operational mode and still allows to maintain an acceptable level of network performance in terms of blocking probability and end-to-end delay. Numerical results are obtained with a Java event-driven simulator of two different optical network topologies.

In performing video multicast over WiMAX networks, AMC enables the BSs to be able to select an appropriate modulation method in accordance with the transmission objective, namely to improve the video quality or to maximize the bandwidth utilization efficiency. However, by choosing the modulation method which maximizes the video quality at all of the SSs, the bandwidth utilization efficiency is reduced and vice versa. In addition, because wireless channels are noisy, FEC is always employed for video multicast. However, by choosing high FEC ratio, the video quality is improved but the bandwidth utilization efficiency is degraded and vice versa. In other words, a tradeoff exists between the video quality and the bandwidth utilization efficiency for video multicast over WiMAX networks. Accordingly, the present study proposes a scheme designated as Performance-Driven Robust Video Multicast (PDRVM), in which both the modulation method and the FEC ratio are determined adaptively in accordance with the network status by mathematical analyses. Simulation results show that PDRVM not only ensures that most of the SSs acquire a good video quality, but also makes the best possible use of the available transmission resources.