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