On simplifying congestion window handling for CMT-SCTP

Abstract

Currently Concurrent Multipath Transfer for Stream Control Transmission Protocol draft RFC (CMT-SCTP) [2] specifies that the sender should implement a new algorithm, named Cwnd Update for CMT (CUC), section 3.2, in order to cope with the increased amount of gap blocks present in SCTP S/ACK(s), naturally caused by multi-path usage. The algorithm was proposed in order to cope with the traditional view that gap blocks indicate packet loss and to mitigate the effects that natural reordering on CMT had on it. SCTP now has an errata [13] (section 3.22) that better deals with such S/ACK(s) in congestion avoidance phase, as it allows increasing congestion window even if the cumulative ACK did not advance. It did not change, though, the slow start phase, and that is what we examine in this paper. Our results indicate the performance is similar for low round trip times (RTTs) and improved for higher ones, while reducing the total amount of retransmissions. Therefore, our proposal is to make the Fast Recovery flag to be per transport, to allow increasing the congestion window without new cumulative ACK(s) also in slow start phase and to remove CUC indication from CMT-SCTP specification.