1.. SPDX-License-Identifier: GPL-2.0 2 3==================== 4Thin-streams and TCP 5==================== 6 7A wide range of Internet-based services that use reliable transport 8protocols display what we call thin-stream properties. This means 9that the application sends data with such a low rate that the 10retransmission mechanisms of the transport protocol are not fully 11effective. In time-dependent scenarios (like online games, control 12systems, stock trading etc.) where the user experience depends 13on the data delivery latency, packet loss can be devastating for 14the service quality. Extreme latencies are caused by TCP's 15dependency on the arrival of new data from the application to trigger 16retransmissions effectively through fast retransmit instead of 17waiting for long timeouts. 18 19After analysing a large number of time-dependent interactive 20applications, we have seen that they often produce thin streams 21and also stay with this traffic pattern throughout its entire 22lifespan. The combination of time-dependency and the fact that the 23streams provoke high latencies when using TCP is unfortunate. 24 25In order to reduce application-layer latency when packets are lost, 26a set of mechanisms has been made, which address these latency issues 27for thin streams. In short, if the kernel detects a thin stream, 28the retransmission mechanisms are modified in the following manner: 29 301) If the stream is thin, fast retransmit on the first dupACK. 312) If the stream is thin, do not apply exponential backoff. 32 33These enhancements are applied only if the stream is detected as 34thin. This is accomplished by defining a threshold for the number 35of packets in flight. If there are less than 4 packets in flight, 36fast retransmissions can not be triggered, and the stream is prone 37to experience high retransmission latencies. 38 39Since these mechanisms are targeted at time-dependent applications, 40they must be specifically activated by the application using the 41TCP_THIN_LINEAR_TIMEOUTS and TCP_THIN_DUPACK IOCTLS or the 42tcp_thin_linear_timeouts and tcp_thin_dupack sysctls. Both 43modifications are turned off by default. 44 45References 46========== 47More information on the modifications, as well as a wide range of 48experimental data can be found here: 49 50"Improving latency for interactive, thin-stream applications over 51reliable transport" 52http://simula.no/research/nd/publications/Simula.nd.477/simula_pdf_file 53