Asymmetric-Path RTT Measurement and Optimization in Mobile Multi-Homed SCTP Multimedia Transport

We investigate the measurement method of round-trip time (RTT) with existing TCP and SCTP, we find that both of them measure RTT under the scenario of symmetric path, which do not adapt to asymmetric path. We also investigate the optimization of asymmetric path choice in mobile multi-homed SCTP multimedia transfer, we find that existing method of asymmetric path choice has some limitation caused by the insufficient of reasonable RTT calculation. Based on this, we make a potential change to standard SCTP by adding RETID (retransmission identifier) chunk and RETID echo chunk and propose a method named acknowledgement-matched asymmetric-path RTT measurement (AAPRM) to match retransmission TSN with its Ack and to obtain correct RTT of asymmetric path. We also propose an algorithm called smallest RTT asymmetric-path first (SRAPF) for transport path selection that takes into account the dynamic asymmetry over the available routes. The time sequence figure demonstrates how asymmetric-path RTT measurement could benefit from AAPRM and an example is provided showing how a multimedia application could benefit from SRAPF.