An In-depth Analysis of Subflow Degradation for Multi-path TCP on High Speed Rails

Recent advances in high-speed rails (HSRs), coupled with user demands for communication on the move, are propelling the need for acceptable quality of experience (QoE) in high-speed mobility environments. However, with throughput declining significantly the QoE on existing HSRs is still far from satisfactory. In order to improve QoE on HSRs, this paper seeks to answer the question regarding which is better of two options: the selection of the best cellular carrier applying single-path TCP or the conjunction of multiple carriers applying Multipath TCP (MPTCP). To this end, we carefully design comparison experiments using the two approaches on HSRs with a peak speed of 310 km/h. Measurement study on MPTCP performance shows that generally carrier conjunction gives similar performance as carrier selection. We take an in-depth analysis of the details of the instances, and for the first time expose the phenomenon called subflow degradation. We further confirm that subflow degradation of MPTCP occurs due to its poor adaptability to frequent handoffs. We believe these insights can provide valuable guidance for the design, implementation, and deployment of transmission protocols in high-speed mobility environments.