Comparison of methods for accelerating the response time of TCP sessions over long delay paths

Many modern Internet applications require fast client-server interaction to maximise user interactivity. This need has caused new work to be proposed to working groups of the Internet Engineering Task Force (IETF), which seek to remove bottlenecks when starting sessions. RFC 6298 improves Transmission Control Protocol (TCP) responsiveness by retransmitting lost SYNs, or first data segments sent after the three-way handshake (3WHS), with a reduced initial retransmission timeout (RTO) of 1 second from the previous value of 3 seconds. This benefits a large number of Internet connections. However, there is growing use of wireless and other long-haul radio technologies that yield round trip times (RTT) greater than 1 second. This means a significant number of connections will suffer spurious retransmissions with associated performance penalties. This paper examines whether RFC 6298 can cooperate with a set of proposed methods to extend performance benefits to long delay paths that may experience startup loss. It was found that a slight modification of TCP congestion-state initialization after the 3WHS significantly improves performance, especially if combined with SYN duplication and an increased Initial Window (IW). We further explain how a deprecated proposal to seed the RTO based on the SYN round trip time (RTT) can be updated to become suited for general deployment. Proposed modifications are evaluated, by analysis and using ns2 simulations, showing improved overall responsiveness for short delay paths as well as networks paths with shared bandwidth and appreciable delay (e.g. wireless/satellite networks).