Coordinating access to shared resources is a challenging task, in particular if real-time and security aspects have to be integrated into the same system. However, rather than exacerbating the problem, we found that considering real-time guarantees actually simplifies the security problem of preventing information leakage over shared-resource covert channels. We introduce a transformation for standard real-time resource locking protocols and show that protocols transformed in this way preserve the confidentiality guarantees of the schedulers on which they are based. Through this transformation, we were able to prove that four out of the seven investigated protocols are information-flow secure.

A practically feasible modification to fixed-priority schedulers allows to avoid timing channels despite threads having access to precise clocks. This modification is rather simple: we compute at admission time a static predicate that states whether a thread may possibly leak information; if such a thread blocks we switch to the idle thread instead. We describe the modified scheduler, provide a mechanical PVS-based proof of noninterference and show how common admission algorithms can be reused to give real-time guarantees for this modified scheduler. While providing similar isolation guarantees, our approach outperforms timepartitioning schedulers in terms of achieved real-time guarantees.

Existing real-time research focuses on how to formulate. model and enforce timeliness guarantees for task sets whose correctness has a temporal aspect. However; the resulting systems often exhibit poor resource utilization due to the resource scheduler reserving more resources than required in order to ensure that admitted schedules can be satisfied under worst case conditions. Weakening the guarantees leads to the known concepts of firm and soft real-time tasks, butt we think the paradigm needs to be shifted further,: reifying efficient utilization. With Quality-Assuring Scheduling (QAS) we presented such an algorithm. However: its practical applicability is restricted to uniform and harmonic periods, due to its complexity for arbitrary periods. To overcome this limitation, we introduce Quality-Rate-Monotonic Scheduling (QRMS), which, although slightly more pessimistic, is less complex compared to QAS. Thee admission control is again based on a probabilistic model to ensure that a requested fraction of jobs is successfully executed. Thus the amount of missed deadlines can be externally controlled, even in sustained overload situations.

We present a unified model for admission and scheduling, applicable for various active resources such as CPU or disk to assure a requested quality in situations of temporary overload. The model allows us to predict and control the behavior of applications based on given quality requirements. It uses the variations in the execution time, i.e., the time any active resource is needed We split resource requirements into a mandatory part which must be available and an optional part which should be available as often as possible but at least with a certain percentage. In combination with a given distribution for the execution time we can move away from worst-case reservations and drastically reduce the amount of reserved resources for applications which can tolerate occasional deadline misses. This increases the number of admittable applications. For example, with negligible loss of quality our system can admit more than two times the disk bandwidth than a system based on the worst-case. Finally, we validated the predictions of our model by measurements using a prototype real-time system and observed a high accuracy between predicted and measured values.

An increasing number of application systems can be characterized by their requirement to process sequences of events in real-time. These sequences are principally of constant rate, but may vary within given limits. Hence, several parameter sets that seem to differ only slightly have been proposed to describe such sequences. This includes the parameter sets used in the Tenet Protocol Suite, in the traffic description of an ATM connection, and in the model of linear bounded arrival processes (LBAP) for transferring continuous media. The existence of several parameter sets raises the question whether or not the parameter sets differ in principle or in notation only. To answer this question, the paper proposes a generalized model for jitter constrained periodic streams. That model subsumes the parameter sets mentioned above, allows to prove their equivalence and to transform the different sets of parameters each to another.