BLEST: Blocking estimation-based MPTCP scheduler for heterogeneous networks

With the widespread availability of multi-homed devices, multipath transport protocols such as MPTCP are becoming increasingly relevant to support better use of multiple connectivity through capacity aggregation and seamless failover. However, capacity aggregation over heterogeneous paths, such as offered by cellular and Wi-Fi networks, is problematic. It causes packet reordering leading to head-of-line (HoL) blocking at the receiver, increased end-to-end delays and lower application goodput. MPTCP tackles this issue by penalising the use of longer paths, and increasing buffer sizes. This, however, results in suboptimal resource usage. In this paper, we first evaluate and compare the performance of default MPTCP and alternative state-of-the-art schedulers, all implemented in the Linux kernel, for a range of traffic patterns and network environments. This allows us to identify shortcomings of various approaches. We then propose a send-window BLocking ESTimation scheduler, BLEST, which aims to minimise HoL-blocking in heterogeneous networks, thereby increasing the potential for capacity aggregation by reducing the number of spurious retransmissions. The resulting scheduler allows an increase by 12% in application goodput with bulk traffic while reducing unnecessary retransmissions by 80% as compared to default MPTCP and other schedulers.