Impact of frame size and deadlines on WRR scheduling in a switched Ethernet network with critical and non-critical flows

In a real-time network shared between flows with different criticality, the service discipline in network elements highly impacts the overall performance of the system. Weighted Round Robin (WRR) is a well known simple scheduling policy. It has much lower complexity and cost than Deficit Round Robin (DRR) and exhibits GPS like fairness in a fixed packet size environment.In this paper, we consider WRR in an avionics context where data flows of critical avionic functions must co-exist with noncritical flows in a switched Ethernet network, called AFDX. These flows can have heterogeneous delay constraints and features. Upper bounding the worst-case delays of avionic flows is a mandatory part of the certification process. Meanwhile, sharing AFDX with non-critical flows is envisioned to improve bandwidth utilisation. We show that configuring WRR in such environment is challenging, since frame sizes and deadlines can be very different. We propose a heuristic to distribute critical flows in classes and an exhaustive approach to allocate weights to these classes. Our goal is to achieve maximum bandwidth for noncritical flows while respecting the critical flow deadlines.