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1.. SPDX-License-Identifier: GPL-2.02 3====================4Thin-streams and TCP5====================6 7A wide range of Internet-based services that use reliable transport8protocols display what we call thin-stream properties. This means9that the application sends data with such a low rate that the10retransmission mechanisms of the transport protocol are not fully11effective. In time-dependent scenarios (like online games, control12systems, stock trading etc.) where the user experience depends13on the data delivery latency, packet loss can be devastating for14the service quality. Extreme latencies are caused by TCP's15dependency on the arrival of new data from the application to trigger16retransmissions effectively through fast retransmit instead of17waiting for long timeouts.18 19After analysing a large number of time-dependent interactive20applications, we have seen that they often produce thin streams21and also stay with this traffic pattern throughout its entire22lifespan. The combination of time-dependency and the fact that the23streams 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 issues27for 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 as34thin. This is accomplished by defining a threshold for the number35of packets in flight. If there are less than 4 packets in flight,36fast retransmissions can not be triggered, and the stream is prone37to experience high retransmission latencies.38 39Since these mechanisms are targeted at time-dependent applications,40they must be specifically activated by the application using the41TCP_THIN_LINEAR_TIMEOUTS and TCP_THIN_DUPACK IOCTLS or the42tcp_thin_linear_timeouts and tcp_thin_dupack sysctls. Both43modifications are turned off by default.44 45References46==========47More information on the modifications, as well as a wide range of48experimental data can be found here:49 50"Improving latency for interactive, thin-stream applications over51reliable transport"52http://simula.no/research/nd/publications/Simula.nd.477/simula_pdf_file53