Improving SCTP retransmission delays for time-dependent thin streams

Abstract

A large number of network services rely on IP and reliable transport protocols. For applications that provide abundant data for transmission, loss is usually handled satisfactorily, even if the application is latency-sensitive (Wang et al. 2004). For data streams where small packets are sent intermittently, however, applications can occasionally experience extreme latencies (Griwodz and Halvorsen 2006). As it is not uncommon that such thin-stream applications are time-dependent, any unnecessarily induced delay can have severe consequences for the service provided. Massively Multiplayer Online Games (MMOGs) are a defining example of thin streams. Many MMOGs (like World of Warcraft and Age of Conan) use TCP for the benefits of reliability, in-order delivery and NAT/firewall traversal. It has been shown that TCP has several shortcomings with respect to the latency requirements of thin streams because of the way it handles retransmissions (Griwodz and Halvorsen 2006). As such, an alternative to TCP may be SCTP (Stewart et al. 2000), which was originally developed to meet the requirements of signaling transport. In this paper, we evaluate the Linux-kernel SCTP implementation in the context of thin streams. To address the identified latency challenges, we propose sender-side only enhancements that reduce the application-layer latency in a manner that is compatible with unmodified receivers. These enhancements can be switched on by applications and are used only when the system identifies the stream as thin. To evaluate the latency performance, we have performed several tests over various real networks and over an emulated network, varying parameters like RTT, packet loss and amount of competing cross traffic. When comparing our modifications with SCTP on Linux and FreeBSD and TCP New Reno, our results show great latency improvements and indicate the need for a separate handling of thin and thick streams.

Improving Throughput in SCTP via Dynamic Optimization of Retransmission Bounds

“…Most enhancements in [11] are highly invasive, modifying fast retransmit and timeout behaviors and firing the fast retransmission mechanism after a single SACK (selective acknowledgment) that reports missing chunks. Other work [12] proposes similar mechanisms. In contrast, the dynamic RTOmin algorithm makes a subtle but important change to the (previously static) value of RTOmin, allowing intelligent management of stream context without invasive modifications.…”

“…For each test iteration, several thousand transfers of chunks were attempted, producing statistically viable data. The shaded areas in Figures 11,12, and Figure 16 show 95% confidence intervals for each test. For the purposes of this paper we focus on the transfer of 50-byte chunks, but other chunk sizes provide similar results.…”

Enhancing SCTP Performance through the Selection of Appropriate Retransmission Policies

“…The authors in [7] presented the working of SCTP over thin an thick streams of data. The modified SCTP according to thin and thick streams.…”

SCTP: designed for timely message delivery?

“…Thus, Early Retransmit is more robust to reordering than related proposals that only lower the threshold (cf. [16]). Furthermore, a number of reordering mitigations have been proposed for Early Retransmit, in [1].…”