In passive testing approach faults are detected while the System Under Test (SUT) is observed during its normal operation. Usually traces produced by SUT are recorded and compared with those of the specification. The traditional approach to passive testing has a low performance if non-deterministic specifications are considered. Therefore, in this paper it is proposed an approach to passive testing in order to express invariants for network protocols, such as session maintenance protocols. In the proposed technique, critical properties are represented as a set of invariants that an Implementation Under Test (IUT) should fulfill. Furthermore, it is proposed a mechanism to get around the problem to determine from which state it is started the observation of execution traces. In order to validate the effectiveness of the proposed approach, it is made use of the Managed Session Protocol as a real-life case study.