5aa4b32fc8
Inline function to dynamically detect thin streams based on the number of packets in flight. Used to dynamically trigger thin-stream mechanisms if enabled by ioctl or sysctl. Signed-off-by: Andreas Petlund <apetlund@simula.no> Signed-off-by: David S. Miller <davem@davemloft.net>
47 lines
2.3 KiB
Text
47 lines
2.3 KiB
Text
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:
|
|
|
|
1) If the stream is thin, fast retransmit on the first dupACK.
|
|
2) 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
|