The purpose of interoperability testing is to ensure that interconnected protocol implementations communicate correctly while providing the expected services. To perform interoperability testing, conventional approaches rely on the active testing method, which stimulates the system under test to accomplish a diagnose. However, the arbitrary stimuli injected by using active testing method disturb inevitably the normal operation of the system under test. On the contrary, passive testing is a technique for the veri cation of behavioral properties of systems by only observing their external behavior. However, passive testing method often encounters observation problems due to the asynchronous test environment. In fact, the delay in such environment may in uence the observation of the real order of the outputs produced by the implementations under test. Consequently, the verdict emitted by the test system maybe permissive or bias. As passive testing depends only on observation, issues here are then to keep a high level of observation in the testing activity, ensuring that no biased or permissive verdict is introduced. To solve the observation problems in passive interoperability testing, this report proposes a method based on logical clocks. Each output produced by the implementations under test is associated with a time stamp, which is carried on the y during its transmission. By analyzing the time stamps, the tester is able to recreate the true sequence of the observed events so as to give a correct verdict.