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.
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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. 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.