Testing a protocol at runtime in an online way is a complex and challenging work. It requires the same preciseness in conformance testing and efficiency in performance testing, where conformance testing is a functional test which verifies whether the behaviors of the protocol satisfy defined requirements, and performance testing is a qualitative and quantitative test which checks whether the performance requirements of the protocol have been satisfied under certain conditions. As a matter of course, it raises an interesting issue of converging these two kinds of testing by using the same formal approach, and applying the approach online. In this paper, we present a novel logic-based online testing approach to test the protocol conformance and performance through formally specified properties. In order to evaluate and assess our methodology, we developed a prototype and experimented it with a set of Session Initiation Protocol properties in a real IP Multimedia Subsystem environment. Finally, the relevant verdicts and discussions are provided