In Software Defined Networks (SDNs), the control plane of a network is decoupled from its data plane. For scalability and robustness, the logically centralized control plane is implemented by physically placing different controllers throughout the network. The determination of the number and placement of controllers is known as the Controller Placement Problem (CPP). In the regular (i.e., failure-free) state, the control plane must guarantee a given maximum delay between every switch and its primary controller and a given maximum delay between every pair of controllers. In general, these delay bounds allow multiple solutions and, so, other goals can be used to determine the best CPP solution. In this paper, we assess the connectivity-based resilience to malicious attacks against multiple network nodes of the CPP solutions obtained with three different aims: the regular state delay optimization without any concern about attacks, the regular state delay optimization taking into consideration the worst-case attacks and the resilience optimization to attacks against multiple nodes. We assess the CPP solutions considering attacks of targeted nature (when the attacker has complete knowledge of the data plane) and attacks of non-targeted nature (i.e., random and epidemic attacks). We present computational results providing an analysis of the CPP solutions to the different types of attacks. The main conclusion is that the connectivity-based resilience between the different CPP solutions strongly depends on the network topology, the regular state delay bounds and the type of attacks. Finally, we provide insights on how SDN operators can consider the conducted assessment when deciding the controller placements in their networks.
This paper addresses two related problems in the context of transparent optical networks. In the network design problem, the aim is to identify a set of fiber links to connect a given set of nodes. In the network upgrade problem, the aim is to identify a set of new fiber links to add to a given network topology. For a given fiber length budget, the aim in both problems is to maximize the network resilience to the simultaneous failure of its critical nodes. The resilience is evaluated by the average 2‐terminal reliability (A2TR) against a set of critical node failures and the critical nodes are the ones that minimize the A2TR of the network. So, the design/upgrade problem is a bi‐level max‐min optimization problem. Recently, a multi‐start greedy randomized heuristic was proposed for both problems. Here, we propose an alternative method based on a greedy deterministic algorithm and we provide computational results showing that the new method obtains better solutions. The results show that the resiliency difference between existing network topologies and the best network design solutions is very high but this difference can be significantly reduced by network upgrades with small fiber length budgets.
This chapter is dedicated to the description of structural methods aiming to improve the robustness of anycast communications to large-scale failures, either due to natural disasters or malicious human activities. The chapter considers both software-defined networks (SDNs) where the anycast nodes are the nodes hosting SDN controllers, and content delivery networks (CDNs) where the anycast nodes are the nodes hosting content replicas. Most of the structural methods described in this chapter aim to optimally select the anycast nodes in a given network. The chapter first addresses the robustness of anycast communications to natural disasters based on geodiversity routing. Then, different methods are described to select the SDN controller locations aiming to maximize the SDN control plane robustness to malicious node attacks. Finally, the chapter addresses the robustness of CDNs to malicious link cuts by describing methods for the network upgrade (based either on the addition of new links or new replica locations) and for the optimal selection of content replica locations.
Disaster-based resilience issues can severely interrupt communication in networks, making their functions unavailable. Such interruptions may include hardware-/software- related failures or malicious attacks. Especially, the latter is becoming more and more visible with higher intensity and more massive scale. In order to prevent it, new technology concepts and new mitigation strategies are needed. In this chapter, we present the most common cyber-attacks that affect networks based on the concepts of the software defined network (SDN), the content delivery network (CDN) and the information-centric network (ICN). We then indicate solutions to these problems. Finally, we discuss the future opportunities of how the communication networks can be updated to decrease the topological vulnerability to attacks.
Disasters can cause, intentionally or unintentionally, the failure of several network components at the same time. A vast body of literature focuses on understanding the impact of disasters on the network infrastructure to enable the design of more robust networks. However, these multiple failures also affect the applications running over the network infrastructure. Even when the impact of a disaster on the structural performance indicators is insignificant, the functional implications can be substantial. More importantly, a small degradation in network performance can result in severe disruptions of overlay applications, or even completely prevent their proper functioning. Therefore, it is essential to analyze the impact of a disaster on the functional aspects of the network, i.e. the Quality of Service (QoS) offered to the applications and the Quality of Experience (QoE) perceived by the users. In this chapter, we review the functional metrics for evaluating the impact of disasters on applications and users. We specify relevant packet- and network-based functional metrics as well as perceived subjective metrics, and demonstrate the impact of disasters on QoS and QoE metrics in a case study.
On-Demand cloud resources are highly available and reliable since most common cloud service providers organize their clouds as a network of several regions (data centres) and multiple availability zones in each region. This redundant and highly distributed resource pool guarantees users high availability and reliability, even in case of disasters. In order to increase revenues, cloud service providers offer their unused computing resources for much cheaper prices than On-Demand resources, in the form of volatile cloud resources. The trade-off for the high discount is their volatile ability, i.e. lower availability and lower reliability. This means that a user can lose part or all volatile resources at any time, similar to a large-scale technology-related massive failure (disaster). This chapter introduces volatile cloud resources, their life cycle, pros and cons. It also presents several resilient techniques against volatile cloud resources’ disruptions and multiple failures.
The focus of this chapter is on communication (and partially, computing) solutions which allow satisfying demands from the immediate aftermath of a disaster until full restoration of pre-disaster communication infrastructure and services. As traffic demand might differ substantially from the one in the pre-disaster scenario, due to the specific needs of post-disaster scenarios, it appears evident that a simple restoration of existing infrastructure and services might not be sufficient to satisfy it, and that specific solutions are required. This chapter reviews the most relevant post-disaster scenarios, outlining a set of reference use cases and their communication requirements. Then, it presents an overview of the state of the art for emergency and post-disaster communications. Finally, it focuses on a set of specific solutions of special relevance for disaster scenarios, outlining the main research challenges which are open to date.
Due to the increasing dependence on network services of our society, research has recently been concentrating on enhancing traditional protection strategies to withstand largescale failures, as in case of disaster events. The recently-formed EU-funded RECODIS project aims at coordinating and fostering research collaboration in Europe on disaster resiliency in communication networks. In particular, the Working Group (WG) 2 of the RECODIS project focuses on developing new network resiliency strategies to survive weather-based disruptions. As a first step, WG2 members have conducted a comprehensive literature survey on existing studies on this topic. This paper classifies and summarizes the most relevant studies collected by WG2 members in this first phase of the project. While the majority of studies regarding weather-based disruptions deals with wireless network (as wireless channel is directly affected by weather conditions), in this survey we cover also disaster resiliency approaches designed for wired network if they leverage network reconfiguration based on disaster “alerts”, considering that many weather-based disruptions grant an “alert” thanks to weather forecast.
Computer networks and data sharing applications are vital for our current society and fundamental for any available ICT solution, so that networking is considered as one of the key critical infrastructures and its correct behavior should be always enforced, even in case of disasters or severe execution conditions. Resilience is a strongly demanding nonfunctional requirement for current computer networks, and one of the key factors to provide it is represented by loss tolerance. From wired to wireless networks, exchanged packets can be lost due to routing anomalies or temporary malfunctioning at the networking hardware or software. The experienced loss pattern can be exacerbated by severe weather conditions in wireless environments and/or due to effects of a disaster. Proper recovery schemes must be put in place in order to provide loss tolerance and move a step forward realizing disaster-resilient networks. This work contributes to such an objective by proposing a loss-tolerant scheme based on forward error correction and properly implementing it in a distributed manner, by having multiple nodes generating coded information to be used for the reconstruction of the lost data without retransmissions. The problem of placing and tuning such coding nodes within a communication infrastructure has been tackled according to a repeated game theory formulation, and a preliminary evaluation demonstrates the achievable improvements in the delivery success rate with limited costs.
Communication networks are subject to many challenges leading to single or multiple failures of its elements. Example failure scenarios include unintentional failures due to scheduled maintenance activities or massive failures caused by disaster‐induced events. Therefore, it is crucial to enhance the networks with redundancy and resilience mechanisms able to maintain the availability of network services after a failure. As these problems are often inherently hard to solve to optimality, the role of time‐efficient approximation schemes is essential. In this paper, we highlight the selected problems of communication networks resilience addressed in this issue of Networks.
Content Delivery Networks (CDNs) are a major enabler of large-scale content distribution for Internet applications. Many of these applications require high bandwidth and low latency for a satisfactory user experience, e.g, cloud gaming, augmented reality, tactile Internet and vehicular communications. Replication is one of the most prominent solutions to meet the requirements of latency-sensitive applications. However, infrastructure disruptions can greatly degrade the performance of such applications, or even cease their proper execution. The extent of degradation can be exacerbated by malicious attackers that target the critical elements of the CDN physical infrastructure to disconnect or severely degrade services. In this work, we assess the effects of physical-layer attacks performed by cutting optical fiber links on content accessibility and latency in CDNs. We perform preliminary experiments on the Germany50 network with 50 nodes and 88 links, considering the scenario where attackers cut the links with highest importance, i.e., betweenness centrality (denoted by the number of shortest paths that traverse a link), in order to increase the effectiveness of the attack. By cutting links that are traversed by the majority of shortest paths, the attack forces a larger part of services to use longer paths, incurring higher latency. We consider the cases where each content has between 1 and 4 replicas placed at the network nodes with highest closeness centrality, i.e., nodes closest to all other nodes.
High throughput, resilience, and low latency requirements drive the development of 5G‐enabled content delivery networks (CDNs) which combine core data centers (cDCs) with edge data centers (eDCs) that cache the most popular content closer to the end users for traffic load and latency reduction. Deployed over the existing optical network infrastructure, CDNs are vulnerable to link cut attacks aimed at disrupting the overlay services. Planning a CDN to balance the stringent service requirements and increase resilience to attacks in a cost‐efficient way entails solving the content placement problem (CPP) across the cDCs and eDCs. This article proposes a framework for finding Pareto‐optimal solutions with minimal user‐to‐content distance and maximal robustness to targeted link cuts, under a defined budget. We formulate two optimization problems as integer linear programming (ILP) models. The first, denoted as K‐best CPP with minimal distance (K‐CPP‐minD), identifies the eDC/cDC placement solutions with minimal user‐to‐content distance. The second performs critical link set detection to evaluate the resilience of the K‐CPP‐minD solutions to targeted fiber cuts. Extensive simulations verify that the eDC/cDC selection obtained by our models improves network resilience to link cut attacks without adversely affecting the user‐to‐content distances or the core network traffic mitigation benefits.
Content Delivery Networks (CDNs) are a key enabler for the distribution of large amounts of data with high capacity and low latency. For instance, content streaming companies extensively use geographical distribution and replication to meet the ever-growing demand for media. Optical networks are the only future-proof technology available that meets the reach and capacity requirements of CDNs. However, the robustness of optical networks becomes a point of concern, as they can be a target of deliberate link cuts that can severely degrade network connectivity and cause large-scale service disruption. To mitigate the vulnerabilities, actions can be taken in the optical and/or cloud infrastructures. The replication of content across geographically diverse data centers results in an intrinsic increase of content accessibility. At the network infrastructure level, robustness to attacks can be enhanced by increasing the topology connectivity through link addition. This work focuses on the latter approach and its effectiveness in increasing content accessibility in the presence of deliberate link cuts. The paper proposes a framework for evaluation and enhancement of content accessibility in CDNs by sparse link addition. First, a content accessibility measure called μ-ACA is introduced to gauge the content accessibility of a given network topology under a set of link cut attack scenarios. Based on this measure, a new link addition strategy is defined aimed at maximizing the content accessibility for a given number of extra links. Simulation results on real-world reference topologies show that the proposed strategy can significantly improve content accessibility by adding a very limited number of optical fiber links.
The problem of how to provide, in a cost-efficient manner, high levels of availability and service differentiation in communication networks was investigated in Tipper (Telecommun Syst 56(1): 5–16 2014), Gomes et al. (2014), and Alashaikh et al. (Comput Netw 82:4–19 2015). The strategy adopted was to embed in the physical layer topology a high availability set of links and nodes (termed the “spine”). The spine enables through protection, routing, and cross-layer mapping, the provisioning of differentiated classes of resilience with varying levels of end-to-end availability. Here, we present an optimization model formulation of the spine design problem, considering link availability and the cost of upgrading link availability. The design problem seeks to minimize the cost while attaining a desired target flow availability. Extensive numerical results illustrate the benefits of modifying the availability of a subset of links of the network to implement quality of resilience classes.
Disaster-based failures can seriously disrupt any communication network, making its services unavailable. Such disruptions may be caused by natural disasters, technology-related failures, or malicious attacks, and they are observably increasing in number, intensity and scale. When network services that are a part of critical infrastructure become unavailable, commercial and/or societal problems are inevitable. The issue of limiting the impact of disaster-based failures needs to be urgently addressed due to the lack of suitable mechanisms deployed in the current networks. The COST CA15127 (RECODIS) Action will fill this gap by developing appropriate solutions to provide cost-efficient resilient communications in the presence of disaster-based disruptions considering both existing and emerging communication network architectures. It will be driven by researchers from academia and industry in strong cooperation with governmental bodies. In this paper, we highlight the objectives of RECODIS, its structure, as well as planned outcomes.
Optical Wireless communication systems are a good competitor to other wireless communication technologies in relation of its capacity to deliver high-speed broadband traffic. The way optical wireless transceivers operate is more or less the same as fiber optics ones; however, since laser signals are transferred through the atmosphere, the path loss between the transmitter and the receiver is getting raised due to various external factors (conditions) that appear on weather. The characteristics of optical wireless systems and its changes in the face of different weather conditions strongly affect the parameters of Quality of Service. Also, this influence provides the possibility to quantify the significance of the service disruption impact to the metrics of Quality of Experience. Due to this, this paper gives a new approach to the relation of the characteristics of optical wireless communication system, known as Free Space Optics, affected during the weather-based disruptions with the parameters of Quality of Service. Furthermore, this relation is used in estimation of Quality of Experience metrics.
The successful career of graduates, which focus on development and maintenance of adaptive systems, apart to the main background in electrical engineering, requires strong abilities to work with users of developed device, entrepreneurship skills, knowledge of marketing and ability to develop market oriented product. This paper demonstrates the approach, which encourages creative skills to build the prototypes of new products, motivates students to create new ideas for start-ups, and allows to develop entrepreneurship skills during the education process. The case study of the Riga Technical University RTU master level program "Adaptronics" concerning the development of entrepreneurship skills is described in this work. The case study of student' survey at the University of Split is also discussed in the paper. The survey results demonstrate the students' level of entrepreneurial orientation in correlation with their working experience and their perception about opportunity to find a job after graduating the university.