Sentinel: Proactive Elastic Controller Scaling with MARL-Guided Load Redistribution for Resilient SD-WANs

Controller placement in software-defined wide area networks (SD-WANs) is commonly optimized assuming continuously available controllers, leaving the control plane vulnerable to overload and abrupt reassignment after controller unavailability. This paper presents Sentinel, an auditable supervisory mechanism that extends GMM-MARL placement into proactive elastic controller management through utilization monitoring, hysteresis-based scaling, learned aiding-controller placement, and bounded incremental migration. Deterministic supervision decides when scaling is authorized, whereas the learned policy selects placement and assignment subject to capacity and latency constraints. Evaluation uses a 95-node composite topology derived from OS3E, Darkstrand, and CRL. An operational track compares proactive Sentinel scale-out with reactive CCA-PSO, and an equal-resource ablation separates resource restoration from placement quality. Sentinel reduces operational average case latency (ACL) from 3988 to 3008 µs and limits maximum utilization to 87.5%, versus 98.4% under CCA-PSO. Under equal resources, Sentinel achieves 3453 ± 627 µs ACL, within 2.9% of static GMM-MARL re-optimization. An auxiliary matched RL application benchmark on OS3E against a reconstruction (RL-HCP*) shows lower ACL and worst-case latency for RL-HCP*, while Sentinel retains competitive load and coordination behavior. Overall, proactive controller headroom restoration, learned placement, and bounded migration provide resilient control-plane management without disruptive global reassignment.