An Evaluation of Multipath TCP with Highly Asymmetric Subflows

Abstract

Multipath TCP (MPTCP) is an extension of the Transmission Control Protocol (TCP) that allows the simultaneous use of multiple available network interfaces to transmit and receive data. MPTCP can improve the throughput, lower the latency, and provide higher resilience to network failures. MPTCP creates a number of network connections (subflows) between the destination and presents a single endpoint to the application. MPTCP schedulers multiplexe data over subflows based on their end-to-end path metrics. In this study, we found that the presence of asymmetric links within an MPTCP connection can lead to suboptimal performance. We explored the architecture of the Linux implementation of MPTCP and identified the design choices that lead MPTCP to underperform in the presence of highly asymmetric links. To test the behaviour of MPTCP an emulation testbed was built using the Mininet emulator. We conducted comprehensive experiments in this controlled environment to analyze MPTCP behavior under asymmetric subflows in terms of bandwidth and latency. We designed a novel scheduling algorithm tailored to mitigate the impact of asymmetric subflows and implemented it in the Linux kernel. Building a scheduling algorithm for MPTCP in the Linux kernel is not a straightforward task. Several iterations of the algorithm had to be investigated in order to develop a practically deployable algorithm. The proposed algorithms were implemented in the Linux Kernel and were tested in the testbed. These algorithms were tested for their suitability to be used over highly asymmetric links under several test scenarios. Finally, we proposed the “Extended Dynamic Scheduler Algorithm” which observes the MPTCP connection and adjusts its subflows to limit the effect of asymmetric subflows in the MPTCP connection. The algorithm also has its own kickback policy where the throughput of the connection starts to improve when the asymmetry of the subflows decreases.