Online testing approaches are becoming crucial in today's complex systems. By that way, testing a protocol at run-time has to be performed during a normal use of the system without disturbing the process. The traces are observed and analyzed on-the-fly to provide test verdicts and no trace sets should be studied as a posteriori to the testing process. In this process, it is a challenging work to keep the same preciseness in conformance testing and the same efficiency in performance testing. In this paper, aiming to find a solution, we present a novel online passive testing approach based on Horn-Logic. In order to evaluate and assess our approach, we also developed a prototype and experimented it with a set of Session Initiation Protocol properties in a real IP Multimedia Subsystem environment. Finally, the preliminary results and discussions are provided.