Performance and Improvements of TCP CUBIC in Low-Delay Cellular Networks

Abstract

Internet usage is still growing every day and there is an even stronger increase in use of mobile devices on mobile networks to access the web. Ensuring good performance in cellular networks for web traffic, which today often consists of many short-lived TCP flows, is therefore particularly important. For TCP, and likely in the future also for QUIC, CUBIC is the most widely deployed congestion control algorithm and in Linux by default used with HyStart. In this paper we investigate performance impairments that especially happen when the combination of CUBIC and HyStart is used in low-delay cellular networks. To explain the problem in detail, we show measurements of characteristic per-packet round-trip times present in mobile networks due to cellular uplink scheduling. Inherent round-trip time variations often trigger an early exit of Slow Start, causing CUBIC to underutilize the available capacity. Further, we have implemented two straightforward HyStart fixes that enhance the performance of CUBIC significantly. In our LTE test bed, the mean throughput of a 5MB data transfer increases by 10.5 %, and the page loading times decrease respectively. Given the nature of web traffic today, these fixes can improve the web experience of users noticeably, which also shows the importance of measurements, testing, and subsequent optimizations when designing and evaluating transport and congestion control mechanisms, especially for mobile networks.