In most of modern enterprise systems, redundancy configuration is often considered to provide availability during the part of such systems is being patched. However, the redundancy may increase the attack surface of the system. In this paper, we model and assess the security and capacity oriented availability of multiple server redundancy designs when applying security patches to the servers. We construct (1) a graphical security model to evaluate the security under potential attacks before and after applying patches, (2) a stochastic reward net model to assess the capacity oriented availability of the system with a patch schedule. We present our approach based on case study and model-based evaluation for multiple design choices. The results show redundancy designs increase capacity oriented availability but decrease security when applying security patches. We define functions that compare values of security metrics and capacity oriented availability with the chosen upper lower bounds to find design choices that satisfy both security and availability requirements.
Kim @cite_7 proposed a hierarchical approach to model the availability of the non-virtualized and virtualized systems using the fault tree for the system and the CTMC for the components. Trivedi @cite_3 presented several case studies on assessing the availability of real systems from Motorola, Cisco and Sun Microsystems via stochastic models.
@cite_3 @cite_7
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This paper develops an availability model of a virtualized system. We construct non-virtualized and virtualized two hosts system models using a two-level hierarchical approach in which fault trees are used in the upper level and homogeneous continuous time Markov chains (CTMC) are used to represent sub-models in lower level. In the models, we incorporate not only hardware failures (e.g., CPU, memory, power, etc) but also software failures including Virtual Machine Monitor (VMM), Virtual Machine (VM), and application failures. We also incorporate high availability (HA) service and VM live migration in the virtualized system. Metrics we use are system steady state availability, downtime in minutes per year and capacity oriented availability. Availability assessment is of paramount importance to guarantee uninterrupted operation of a variety of commercial-grade information and networked systems. In this paper, we present practical case studies that show how to use stochastic analytic modeling approaches to quantitatively assess the availability of such systems. We present non-state-space models, statespace models and hierarchical models. We describe the details of these modeling approaches to assess system availability. Copyright © 2012 John Wiley & Sons, Ltd. We develop a new tractable model for K-tier heterogeneous cellular networks (HetNets), where each base station (BS) is powered solely by a self-contained energy harvesting module. The BSs across tiers differ in terms of the energy harvesting rate, energy storage capacity, transmit power and deployment density. Since a BS may not always have enough energy, it may need to be kept OFF and allowed to recharge while nearby users are served by neighboring BSs that are ON. We show that the fraction of time a k^ th tier BS can be kept ON, termed availability ρ_k, is a fundamental metric of interest. Using tools from random walk theory, fixed point analysis and stochastic geometry, we characterize the set of K-tuples (ρ_1,ρ_2,…ρ_K), termed the availability region, that is achievable by general uncoordinated operational strategies, where the decision to toggle the current ON OFF state of a BS is taken independently of the other BSs. If the availability vector corresponding to the optimal system performance, e.g., in terms of rate, lies in this availability region, there is no performance loss due to the presence of unreliable energy sources. As a part of our analysis, we model the temporal dynamics of the energy level at each BS as a birth-death process, derive the energy utilization rate, and use hitting stopping time analysis to prove that there exists a fundamental limit on ρ_k that cannot be surpassed by any uncoordinated strategy. There is a need to quantify system properties methodically. Dependability and security models have evolved nearly independently. Therefore, it is crucial to develop a classification of dependability and security models which can meet the requirement of professionals in both fault-tolerant computing and security community. In this paper, we present a new classification of dependability and security models. First we present the classification of threats and mitigations in systems and networks. And then we present several individual model types such as availability, confidentiality, integrity, performance, reliability, survivability, safety and maintainability. Finally we show that each model type can be combined and represented by one of the model representation techniques: combinatorial (such as reliability block diagrams (RBD), reliability graphs, fault trees, attack trees), state-space (continuous time Markov chains, stochastic Petri nets, fluid stochastic Petri nets, etc) and hierarchical (e.g., fault trees in the upper level and Markov chains in the lower level). We show case studies for each individual model types as well as composite model types.