Optimizing PR-SCTP performance using NR-SACKs

A partially reliable extension of SCTP, PR-SCTP, has been considered as a candidate for prioritizing content sensitive traffic at the transport layer. PR-SCTP offers a flexible QoS trade-off between timeliness and reliability. Several applications such as streaming multimedia, IPTV transmission, and SIP signaling have been shown to benefit from this. Our previous work, however, suggests that the performance gain can be very much reduced in a network with competing traffic. One of the most important factors in this case is the inefficiency in the forward_tsn mechanism in PR-SCTP. In this paper, we thoroughly examine the forward_tsn inefficiency and propose a solution to overcome it that takes advantage of the NR-SACKs mechanism available in FreeBSD. Moreover, we implement and evaluate the proposed solution. Our initial set of results show a significant performance gain for PR-SCTP with NR-SACKs. In some scenarios the average message transfer delay is reduced by more than 75%.