Holdover Improvements to the linuxptp Servo for Network Adapters with High-Precision Oscillators

The Linux PTP Project (linuxptp) provides the de facto standard application framework for Precision Time Protocol (IEEE 1588 and its profiles) implementation on Linux. The ptp4l user-space application of linuxptp realizes PTP-compliant clocks (Ordinary, Boundary, or Transparent). The ptp4l application disciplines a timeReceiver clock with a servo algorithm to a time Transmitter clock. It is possible in regular operation that ptp4l goes from timeReceiver role to time Transmitter role, in which case the system is in holdover mode, i.e., it keeps time based only on its oscillator, and that kept time is transmitted to time Receivers over the PTP protocol. The holdover performance of ptp4l is not investigated in detail up to now; however, new network adapters and add-on cards with High-Performance Oscillators (HPO) are developed for applications in which it is a common mode of operation. The paper presents and investigates modified servo algorithms for ptp4l to improve holdover performance and compares them to the built-in servos. The new algorithms are tested with an Intel E810-XXVDA4T Network Adapter with HPO. The results apply to other network adapters with HPO in our view, and they can also improve the performance of regular Network Adapters in all application fields. The new servo implementations are planned to be submitted for inclusion into the linuxptp code base.