RTTPROBS: A RTT Probing Scheme for RTT Aware Multi-path Scheduling

In multi-path scenarios, the default multi-path TCP (MPTCP) scheduler prefers the subflow with the minimum round-trip time (RTT) to send packets. Therefore, the scheduler requires reliable and timely information on the RTTs for the individual subflows to make proper scheduling decisions. We show for various situations that a reliable and timely RTT estimate is not always given. This shortcoming is induced from the original TCP behavior that the RTT is only updated if data is transmitted on a TCP flow. This means for MPTCP, that a subflow, which suffers from a short-term RTT increase, may not be selected again by the scheduler and so never gets an RTT update. Moreover, we demonstrate for various scenarios, that this leads to non-optimal scheduling decisions. We propose an RTT probing scheme to achieve reliable and timely RTT updates. Our probing scheme is designed for a fast reaction as well as little probing traffic. The advantages are demonstrated in emulated as well as real-world vehicular experiments.