Testing a protocol at runtime in an online way is a complex and challenging work. It requires the ability to handle numerous messages in a short time, and also requires the same offline testing preciseness. Meanwhile, since online testing is a long term continuously process, the tester has to undergo severe conditions when dealing with large amount of nonstop traces. As a matter, of course, it raises an interesting issue of finding out an online testing approach which can efficiently and accurately test the protocols. In this paper, we present a novel logic-based online passive testing approach to test, at runtime, the protocol conformance and performance through formally specified properties with new definitions of verdicts. In order to evaluate and assess our methodology, we experimented our approach with several Session Initiation Protocol properties in a real IP Multimedia Subsystem environment. Relevant verdicts and discussions are provided