Passive testing techniques are used whenever the system under test cannot be interrupted, or access to its interfaces is unavailable. Under such conditions, communication traces are extracted from points of observation and compared with the expected behavior formally specified as properties. Since most works on the subject come from a formal model context, they are optimized for testing the control part of the communication with a secondary focus on the data parts. In the current work we provide a data-centric approach for black-box testing of network protocols. A syntax and semantics are provided to express and evaluate complex properties in a bottom-up fashion starting from expected data relations in messages. The approach has been implemented into a framework, and some experimental results are briefly discussed