Rethinking the aggressiveness of SCTP switchover in high delay networks

The Stream Control Transmission Protocol (SCTP) is a transport layer protocol which through its multi-homing features can support mobility. SCTP path failover is temporary in nature and is controlled by Retransmission Time Out (RTO) and Path Maximum Retransmission (PMR) parameters. In order to make an SCTP failover permanent (switchover) a Change Primary Threshold (CPT) parameter has been suggested. When SCTP fails over from a primary path, a HEARTBEAT packet is sent to attempt to re-establish primary path communication. CPT defines a limit of HEARTBEAT attempts which, when exceeded, results in configuration of an alternate path as primary. In this paper we illustrate a scenario in which the current understanding of SCTP failover aggressiveness is inappropriate. Studies have suggested that a low PMR=0 results in the greatest throughput even when the alternate path is degraded. We illustrate that when high delay alternate paths with low loss rate are utilised, more conservative parameter configurations PMR=3 give optimal performance. We also suggest that for optimal throughput moderate CPT values should be utilised.