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The paper addresses a novel method for performance analysis of distributed realtime systems with complex, and especially cyclic data flow graphs. The presented method is based on Network Calculus principles, where flow and service constraint functions are used to bound data flows and processing resources. In acyclic systems flow constraints may be propagated through a system of service models along data flow paths, whereas service constraints are propagated along paths of priority order. For cyclic data flows however constraint propagation becomes equivalently cyclic leading to flow and service constraints implicitely given by a fix point equation in a space of constraint functions. In this paper a method denoted CyNC for obtaining a well defined solution to that problem is presented along with a theoretical justification of the method as well as comparative results for CyNC and alternative methods on a relevant example. The method is implemented in a prototype tool also denoted CyNC providing a graphical user interface for model specification based on the MATLAB/SimuLink framework. [ABSTRACT FROM AUTHOR]