Using 802.11 MAC Retransmissions for Path Selection in Multi-Homed Transport Layer Protocols

The stream control transmission protocol (SCTP) is a transport layer protocol which can support mobility through its multi-homing feature. One of the key parameters used by SCTP to manage path selection is retransmission time out (RTO). We illustrate that SCTP's evaluation of underlying paths using end to end metrics, results in the calculation of excessive RTO for degraded wireless paths. This excessive RTO causes SCTP to behave in a counter intuitive manner which delays path switchover. Our investigation shows that the increased round trip time (RTT) is as a result of retransmissions at the 802.11 MAC. We illustrate that when a connectionless 802.2 LLC service is utilized, 802.11 independently implements positive frame acknowledgement resulting in significantly increased RTT. We propose a cross layer path selection algorithm, which recognizes increased 802.11 MAC retransmissions as an indicator of imminent path failure. Results presented indicate that our cross layer approach has excellent performance in comparison to standard SCTP strategies.