1Thin-streams and TCP 2==================== 3A wide range of Internet-based services that use reliable transport 4protocols display what we call thin-stream properties. This means 5that the application sends data with such a low rate that the 6retransmission mechanisms of the transport protocol are not fully 7effective. In time-dependent scenarios (like online games, control 8systems, stock trading etc.) where the user experience depends 9on the data delivery latency, packet loss can be devastating for 10the service quality. Extreme latencies are caused by TCP's 11dependency on the arrival of new data from the application to trigger 12retransmissions effectively through fast retransmit instead of 13waiting for long timeouts. 14 15After analysing a large number of time-dependent interactive 16applications, we have seen that they often produce thin streams 17and also stay with this traffic pattern throughout its entire 18lifespan. The combination of time-dependency and the fact that the 19streams provoke high latencies when using TCP is unfortunate. 20 21In order to reduce application-layer latency when packets are lost, 22a set of mechanisms has been made, which address these latency issues 23for thin streams. In short, if the kernel detects a thin stream, 24the retransmission mechanisms are modified in the following manner: 25 261) If the stream is thin, fast retransmit on the first dupACK. 272) If the stream is thin, do not apply exponential backoff. 28 29These enhancements are applied only if the stream is detected as 30thin. This is accomplished by defining a threshold for the number 31of packets in flight. If there are less than 4 packets in flight, 32fast retransmissions can not be triggered, and the stream is prone 33to experience high retransmission latencies. 34 35Since these mechanisms are targeted at time-dependent applications, 36they must be specifically activated by the application using the 37TCP_THIN_LINEAR_TIMEOUTS and TCP_THIN_DUPACK IOCTLS or the 38tcp_thin_linear_timeouts and tcp_thin_dupack sysctls. Both 39modifications are turned off by default. 40 41References 42========== 43More information on the modifications, as well as a wide range of 44experimental data can be found here: 45"Improving latency for interactive, thin-stream applications over 46reliable transport" 47http://simula.no/research/nd/publications/Simula.nd.477/simula_pdf_file 48