Out-of-sync Schedule Robustness for Time-sensitive Networks

Time-Sensitive Networks (TSN) enable real-time communication guarantees over Ethernet by defining a timed-gate mechanism (802.1Qbv), binding frame transmissions to a global schedule, and a synchronization protocol (802.1AS), which aligns the time of each node to a reference clock. When the reference clock is lost, the individual clocks drift apart until a new master clock is elected (re-synchronization interval). This may lead to a complete loss of determinism due to an inconsistent alignment of the transmission patterns between nodes.In this paper, we address the problem of creating time-triggered schedules for 802.1Qbv with an enhanced robustness against the temporary loss of synchronization. We analyze the protocol behavior upon a loss of synchronization and derive new constraints for the computation of schedules able to tolerate the worst-case drift of individual clocks during the resynchronization interval. Moreover, we formulate the problem as a design space exploration to synthesize schedules with the maximum tolerance towards loss-of-sync issues. We have developed a scheduler tool based on the Z3 SMT/OMT solver and conducted a series of experiments with a set of synthetic use-cases. Our evaluation exposes a series of trade-offs between the robustness of the schedule and the feasibility and computation run-time of the scheduler as well as the end-to-end communication latency.