Salameh, L; Zhushi, A; Handley, M; Jamieson, K; Karp, B; (2014) HACK: Hierarchical ACKs for Efficient Wireless Medium Utilization. In: Gibson, G and Zeldovich, N, (eds.) Proceedings of USENIX ATC ’14: 2014 USENIX Annual Technical Conference. (pp. pp. 359-370). USENIX Association

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Abstract

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 signifi- cant 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.

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