Net-P4ct: Enhanced WAN Bandwidth Fair Sharing Using P4 Programmable Switches

At growing internet companies like ByteDance, Wide Area Network (WAN) bandwidth sharing across diverse services with varying SLO requirements is a fundamental challenge. Conventional host-based enforcement systems, where agents identify and throttle traffic at the server end, face practical challenges such as "blind spot" traffic, kernel-dependent operational complexity, and significant server resource overhead. To address these issues, we present Net-P4ct, an in-network bandwidth enforcement system using P4 programmable switches. Net-P4ct improves both bandwidth guarantees and fair sharing by shifting dynamic QoS control into the switch data plane. Specifically, it achieves broader traffic coverage by combining host-side traffic tagging with a P4-switch pipeline, where service classification and QoS class assignment are performed. Based on observed traffic metrics, a centralized control plane determines real-time policy updates according to the max-min fair bandwidth allocation. We demonstrate the system's benefits including improved bandwidth utilization, reduced operational complexity, and lower per-byte processing cost. Net-P4ct has been deployed in ByteDance's production WAN for nearly a year, and we hope to share our experience with the community.