The critical requirement on reliability, fault-tolerance and security of network devices highlights the necessity of protocol robustness testing. Mutation testing of protocol messages is an important part of robustness testing, but related theory and practices are not well developed. This paper builds a NFSM model for mutation testing of protocol messages and proposes two types of Normal-Verification Sequence to enhance verdict mechanism. For single-field mutation testing of protocol messages, we propose the concept of compound anomalous test case to further simplify test sequences. As a standard test specification language, TTCN-3 reveals strong excellence in conformance testing, so we apply TTCN-3 to mutation testing and extend it according to test requirements. Using our method we test OSPFv2 sufficiently with a test system based on extended TTCN-3. The results indicate that our method has good capability of error-finding.