HACK: hierarchical ACKs for efficient wireless medium utilization
Lynne Salameh, Astrit Zhushi, Mark James Handley, Kyle Jamieson, Brad Karp · 2014
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 punish-ing 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 signifi-cant medium acquisition overhead for TCP ACKs. In this paper, we propose TCP/HACK (Hierarchical AC-Knowledgment), a system that applies cross-layer opti-mization 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 in-creases achievable capacity for TCP workloads. Our mea-surements 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. 1