Impact of Packet Loss and Timing Errors on Scheduled Periodic Traffic with Time-Aware Shaping (TAS) in Time-Sensitive Networking (TSN)

TL;DR

The paper investigates the fragility of Per-Stream Scheduled Traffic (PSST) within Time-Sensitive Networking (TSN) when Time-Aware Shaper (TAS) schedules and talker timing are imperfect. It introduces a visualization approach for TAS schedules, analyzes how missing, additional, early, or late frames can propagate delays across a network, and uses OMNeT++ simulations to quantify these effects. The findings show that even small timing deviations can trigger cascading delays or packet loss, underscoring a fundamental fragility in TSN scheduled traffic. The authors propose mitigations via time-based PSFP filtering and frame isolation, while noting current hardware limitations and outlining directions for protecting TSN networks against misbehavior and potential attacks.

Abstract

Time-Sensitive Networking (TSN) is a collection of mechanisms to enhance the realtime transmission capability of Ethernet networks. TSN combines priority queuing, traffic scheduling, and the Time-Aware Shaper (TAS) to carry periodic traffic with ultra-low latency and jitter. That is, so-called Talkers send periodic traffic with highest priority according to a schedule. The schedule is designed such that the scheduled traffic is forwarded by the TSN bridges with no or only little queuing delay. To protect that traffic against other frames, the TAS is configured on all interfaces such that lower-priority queues can send only when high-priority traffic is not supposed to be forwarded. In the literature on scheduling algorithms for the TAS there is mostly the explicit or implicit assumption that the TAS also limits transmission slots of high-priority traffic. In this paper we show that this assumption can lead to tremendous problems like very long queuing delay or even packet loss in case of faulty frames. A faulty frame arrives too early or too late according to the schedule, it is missing or additional. We construct minimal examples to illustrate basic effects of faulty frames on a single link and demonstrate how this effect can propagate through the networks and cause remote problems. We further show using simulations that a single slightly delayed frame may lead to frame loss on multiple links. We show that these problems can be alleviated or avoided when TAS-based transmission slots for high-priority traffic are configured longer than needed or if they are not limited at all.

Figures (11)

• Figure 1: Priority queuing and gate mechanism of the IEEE802.1Qbv Enhancements for scheduled traffic.
• Figure 2: Per-Stream Filtering and Policing
• Figure 3: Queue occupancy over time on an egress queue in the presence of TAS and periodic traffic w/o faulty frames. Sudden increases are frame arrivals, linear decreases indicate frame transmissions. Gate state: red means closed gate, green means open gate.
• Figure 4: When a single link is considered, multiple talkers send traffic over a single bridge to a listener. The queuing on the link from the bridge to the listener is considered.
• Figure 5: In the second period an additional orange frame arrives before the magenta frame. As a consequence, the orange frame takes the time slot in that period and the magenta frame is significantly delayed. The time slot taken by the additional frame is missing in the future so that the queue will contain at least one frame and cause delay for all future frames.