Concord: Airtime-Aware Contention Control for Taming Tail Latency from Wi-Fi Frame Bursting

In congested Wi-Fi, sending less does not guarantee lower latency. Based on measurements on commodity Wi-Fi routers in the wild, sparse microflows can suffer bulk-like tail latency even at negligible load. This latency is driven by MAC-level contention dynamics rather than a flow's sending rate, rendering rate-based congestion control ineffective. We present Concord, a Wi-Fi MAC mechanism that makes burst airtime an explicit control signal and penalizes excessive medium holding. Concord operates entirely within the Wi-Fi driver, requires no flow classification, no client or protocol changes, and incurs only O(1) work per burst. Concord shows that controlling medium-holding time, rather than transmission rate, is key to tail latency in WLANs. With four saturated downlink contenders, Concord reduces the 99.9th-percentile enqueue-to-ACK latency of 100 B microflows from 298/461 ms (IEEE baseline / vendor bursting) to 42 ms without sacrificing bulk throughput. For interactive workloads (cloud gaming), it cuts 99.9th-percentile latency from 231/441 ms to 92 ms and reduces starvation by up to 10× versus the default IEEE stack.