One embodiment of the present invention sets forth a method for autonomously validating the time and frequency data obtained from multiple sources, and generating a suitable estimate of the frequency difference between the client clock and the source. The method includes the steps of protocol data unit validation, offset measurement, minimum offset filtering, and frequency filtering. With these steps, the negative effects of packet delay variation may be mitigated and a frequency estimate is determined for the source in question, together with an associated validity status. Consequently, quality control of the local clock is achieved in packet networks at significantly reduced cost and decreased level of complexity relative to prior art approaches.