Conformance of implementations to protocol specifications is essential to assure interoperability between peers in network communications. Monitoring or passive testing techniques are used when no access to the communication interfaces is available or when the normal operation of the system cannot be interrupted. Most monitoring techniques only consider control portion of exchanged messages, usually ignoring the data part. However, as protocols become more complex and message exchange more data intensive, testing for data relations and constraints between exchanged messages becomes essential. In this paper we propose a novel approach for defining such relations as properties called invariants and show how they can be tested directly on traces using logic programming. Experimental results for SIP protocol traces are provided