Interval-based clock synchronization with optimal precision

AbstractWe present description and analysis of a novel optimal precision clock synchronization algorithm (OP), which takes care of both precision and accuracy with respect to external time. It relies upon the generic interval-based algorithm of Schmid and Schossmaier [Real-Time Syst. 12 (2) (1997) 173] and utilizes a convergence function based on the orthogonal accuracy algorithm of Schmid [Chicago J. Theor. Comput. Sci. 3 (2000) 3]. As far as precision is concerned, we show that OP achieves optimal worst case precision, optimal maximum clock adjustment, and optimal rate, as does the algorithm of Fetzer and Cristian [Proceedings 10th Annual IEEE Conference on Computer Assurance, Gaithersburg, MD, 1995]. However, relying upon a perception-based hybrid fault model and a fairly realistic system model, our results are valid for a wide variety of node and link faults and apply to very high-precision applications as well: Impairments due to clock granularity and discrete rate adjustment cannot be ignored here anymore. Our accuracy analysis focuses on the nodes’ local accuracy interval, which provides the atop running application with an on-line bound on the current deviation from external time. We show that this bound could get larger than twice the necessary lower bound (“traditional accuracy”), hence OP is considerably suboptimal in this respect

The customizable fault/error model for dependable distributed systems

Dependability is a qualitative term referring to a system's ability to meet its service requirements in the presence of faults. The types and number of faults covered by a system play a primary role in determining the level of dependability which that system can potentially provide. Given the variety and multiplicity of fault types, to simplify the design process, the system algorithm design often focuses on specific fault types, resulting in either over-optimistic (all fault permanent) or over-pessimistic (all faults malicious) dependable system designs. A more practical and realistic approach is to recognize that faults of varied severity levels and of differing occurrence probabilities may appear as combinations rather than the assumed single fault type occurrences. The ability to allow the user to select/customize a particular combination of fault types of varied severity characterizes the proposed customizable fault/error model (CFEM). The CFEM organizes diverse fault categories into a cohesive framework by classifying faults according to the effect they have on the required system services rather than by targeting the source of the fault condition. In this paper, we develop (a) the complete framework for the CFEM fault classification, (b) the voting functions applicable under the CFEM, and (c) the fundamental distributed services of consensus and convergence under the CFEM on which dependable distributed functionality can be supported. © 2002 Elsevier Science B.V. All rights reserved