This paper presents a new approach to the task of passive protocol tracing. The method called FollowSM for the first time meets all requirements of practical in-field use, including the checking of time constraints, the independence of the current state when starting the analysis, the admittance of nondeterminism, and on-line real time analysis capability. This is achieved by a suitable modeling of the implementation under test and the generalization of the tracing algorithm to operate on state information with any degree of uncertainty. FollowSM has been implemented as a prototype system and proved capable of minimizing the time required for troubleshooting. Keywords Trace analysis, time constraints, EFSM model, real time, passive monitoring