HACK: Hierarchical ACKs for Efficient Wireless Medium Utilization

WiFi's physical layer has increased in speed from 802.11b's 11 Mbps to the Gbps rates of emerging 802.11ac. Despite these gains, WiFi's inefficient MAC layer limits achievable end-to-end throughput. The culprit is 802.11's mandatory idle period before each medium acquisition, which has come to dwarf the duration of a packet's transmission. This overhead is especially punishing for TCP traffic, whose every two data packets elicit a short TCP ACK. Even frame aggregation and block link-layer ACKs (introduced in 802.11n) leave significant medium acquisition overhead for TCP ACKs. In this paper, we propose TCP/HACK (Hierarchical ACKnowledgment), a system that applies cross-layer optimization to TCP traffic on WiFi networks by carrying TCP ACKs within WiFi's link-layer acknowledgments. By eliminating all medium acquisitions for TCP ACKs in unidirectional TCP flows, TCP/HACK significantly improves medium utilization, and thus significantly increases achievable capacity for TCP workloads. Our measurements of a real-time, line-speed implementation for 802.11a on the SoRa software-defined radio platform and simulations of 802.11n networks at scale demonstrate that TCP/HACK significantly improves TCP throughput on WiFi networks.