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The advent of multicast and the growth and complexity of the Internet has complicated network protocol design and evaluation. Evaluation of Internet protocols usually uses random scenarios or scenarios based on designers' intuition. Such an approach may be useful for average case analysis but does not cover boundary-point (worst or best case) scenarios. To synthesize boundary-point scenarios, a more systematic approach is needed. In this paper, we present a method for automatic synthesis of worst and best case scenarios for protocol boundary-point evaluation. Our method uses a fault-oriented test generation (FOTG) algorithm for searching the protocol and system state space to synthesize these scenarios. The algorithm is based on a global finite state machine (FSM) model. We extend the algorithm with timing semantics to handle end-to-end delays and address performance criteria. We introduce the notion of a virtual LAN to represent delays of the underlying multicast distribution tree. Our algorithms utilize implicit backward search using branch and bound techniques and start from given target events. As a case study, we use our method to evaluate variants of the timer suppression mechanism, used in various multicast protocols, with respect to two performance criteria: overhead of response messages and response time. Simulation results for reliable multicast protocols show that our method provides a scalable way for synthesizing worst case scenarios automatically. Results obtained using stress scenarios differ dramatically from those obtained through average case analyses. We hope for our method to serve as a model for applying systematic evaluation to other multicast protocols.
Date of Publication: Feb. 2004