net: TCP thin-stream detection

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

		tcp_sk(sk)->highest_sack = new;

		tcp_sk(sk)->highest_sack = new;

}

}

/* Determines whether this is a thin stream (which may suffer from

 * increased latency). Used to trigger latency-reducing mechanisms.

 */

static inline unsigned int tcp_stream_is_thin(struct tcp_sock *tp)

{

	return tp->packets_out < 4 && !tcp_in_initial_slowstart(tp);

}

/* /proc */

/* /proc */

enum tcp_seq_states {

enum tcp_seq_states {

	TCP_SEQ_STATE_LISTENING,

	TCP_SEQ_STATE_LISTENING,