An Active-Passive Measurement Study of TCP Performance over LTE on High-speed Rails

We conduct a large-scale active-passive measurement study of TCP performance over LTE on HSR. Our measurement targets the HSR routes in China operating at above 300 km/h. We performed extensive data collection through both controlled setting and passive monitoring, obtaining 1732.9 GB data collected over 135719 km of trips. Leveraging such a unique dataset, we measure important performance metrics such as TCP goodput, latency, loss rate, as well as key characteristics of TCP flows, application breakdown, and users' behaviors. We further quantitatively study the impact of frequent cellular handover on HSR networking performance, and conduct in-depth examination of the performance of two widely deployed transport-layer protocols: TCP CUBIC and TCP BBR. Our findings reveal the performance of today's commercial HSR networks "in the wild", as well as identify several performance inefficiencies, which motivate us to design a simple yet effective congestion control algorithm based on BBR to further boost the throughput by up to 36.5%. They together highlight the need to develop dedicated protocol mechanisms that are friendly to extreme mobility.

Our high-level experimental methodology is to perform bulk data download over TCP. We compare the performance and incurred cross-layer interactions between CUBIC and BBR. We measure two types of TCP downlink flows, including long flows (i.e., fixed duration of 150 seconds) and fixed size of 64 KB (corresponding to typical web page). We run tshark on both client and server to collect packet-level TCP traces. We also instrument the client phones using MobileInsight to collect lower layer information including PHY rate and handover events. See more information in our paper [PDF]

Testbed

We deploy two powerful co-located servers running different TCP variants and tether two Android phones to one laptop via USB 3.0. We programmatically run two experiments simultaneously on the two phones, which appear as network interfaces on the laptop and function as link-layer devices.

We carried out experiments on the Beijing-Shanghai (300/350 km/h) HSR route as it represents the state-of-art HSR networking environments in terms of train speed and track-side cellular infrastructure. We collected 357.9 GB data by traveling 51367 km on the trains. Note that there exists tens of ms delay between the on-chip time of LTE protocol message (reported by MobileInsight) and the system time of TCP pcap trace. Download TCP and LTE traces from Mega.

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