T(SN)-Ray: Gauging TAS and PSFP Delays of TSN Switches for Predictable Deterministic Networking

Time-Sensitive Networking (TSN) allows for scheduling periodic traffic in Ethernet networks, i.e., sending the traffic such that the traffic encounters minimal delay in intermediate nodes. TSN has two novel features Time-Aware Shaper (TAS) and Per-Stream Filtering and Policing (PSFP) that support time-triggered forwarding at a timescale of nanoseconds. The TAS safeguards scheduled traffic from other priority traffic by opening and closing of transmission gates in TSN switches. PSFP accepts scheduled traffic only when it arrives in the scheduled intervals. As switch-internal forwarding delays are also at this timescale, their knowledge is needed for the computation of schedules that define (1) the transmission of Ethernet frames at Talkers, (2) the opening and closing of the transmission gates in TSN switches, and (3) the configuration of the time-based filter in PSFP also in TSN switches. While it is known that such delays exist and matter, they are not indicated on data sheets, and a measurement methodology to derive them is still lacking. This work fills this gap with T-Ray, a black-box measurement approach. A delay model for TSN switches is proposed together with a measurement methodology to quantify the parameters of the delay model for a given TSN switch. The suggested gauging is illustrated with switches from Kontron and Relyum, and the results reveal operation delays of ∼ 1 μs.