Sarah Brocklehurst

Recalibrating software reliability models

There is no universally applicable software reliability growth model which can be trusted to give accurate predictions of reliability in all circumstances. A technique of analyzing predictive accuracy called the u-plot allows a user to estimate the relationship between the predicted reliability and the true reliability. It is shown how this can be used to improve reliability predictions in a very general way by a process of recalibration. Simulation results show that the technique gives improved reliability predictions in a large proportion of cases. However, a user does not need to trust the efficacy of recalibration, since the new reliability estimates produced by the technique are truly predictive and their accuracy in a particular application can be judged using the earlier methods. The generality of this approach suggests its use whenever a software reliability model is used. Indeed, although this work arose from the need to address the poor performance of software reliability models, it is likely to have applicability in other areas such as reliability growth modeling for hardware. >

Towards Operational Measures of Computer Security: Concepts

Ideally, a measure of the security of a system should capture quantitatively the intuitive notion of `the ability of the system to resist attack’. That is, it should be operational,reflecting the degree to which the system can be expected to remain free of security breaches under particular conditions of operation (including attack). Instead, current security levels at best merely reflect the extensiveness of safeguards introduced during the design and development of a system. Whilst we might expect a system developed to a higher level than another to exhibit ‘more secure behaviour’ in operation, this cannot be guaranteed; more particularly, we cannot infer what the actual security behaviour will be from knowledge of such a level. In the paper we discuss similarities between reliability and security with the intention of working towards measures of ‘operational security’ similar to those that we have for reliability of systems. Very informally, these measures could involve expressions such as the rate of occurrence of security breaches (cf rate of occurrence of failures in reliability), or the probability that a specified ‘mission’ can be accomplished without a security breach (cf reliability function). This new approach is based on the analogy between system failure and security breach. A number of other analogies to support this view are introduced. We examine this duality critically, and have identified a number of important open questions that need to be answered before this quantitative approach can be taken further. The work described here is therefore somewhat tentative, and one of our major intentions is to invite discussion about the plausibility and feasibility of this new approach.

Combination of Predictions Obtained from Different Software Reliability Growth Models

In the development of techniques for software reliability measurement and prediction, many software reliability growth models have been proposed. Application of these models to real data sources has shown that there is commonly great disagreement in predictions, while none of them has been shown to be more trustworthy than others in terms of predictive quality in all applications. Recent work has largely overcome this problem through the development of specialized techniques which analyse the accuracy of predictions from reliability models. Such techniques allow the user to choose, for future predictions for a particular data source, those models which gave the best predictions in the past, for this data.