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In this paper we present a finite state machine (FSM) based implementation of SIP protocol stack for SIP user agent. Design decisions made during the development process, and the resulting characteristics of software product are discussed. Some characteristics of FSM-based framework that has been used in development are presented. We compare quantitatively our implementation with a well known open source SIP implementation, resiprocate. Comparison is focused on the following parameters: lines of code (LOC) and cyclomatic complexity. Results show viability of FSM-based approach, most importantly its support for modularization.