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