tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
net: TCP thin-stream detection
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>
authored by
Andreas Petlund
and committed by
David S. Miller
5aa4b32f
16cad981
+55
+47
Documentation/networking/tcp-thin.txt
+47
Documentation/networking/tcp-thin.txt
···
1
+
Thin-streams and TCP
2
+
====================
3
+
A wide range of Internet-based services that use reliable transport
4
+
protocols display what we call thin-stream properties. This means
5
+
that the application sends data with such a low rate that the
6
+
retransmission mechanisms of the transport protocol are not fully
7
+
effective. In time-dependent scenarios (like online games, control
8
+
systems, stock trading etc.) where the user experience depends
9
+
on the data delivery latency, packet loss can be devastating for
10
+
the service quality. Extreme latencies are caused by TCP's
11
+
dependency on the arrival of new data from the application to trigger
12
+
retransmissions effectively through fast retransmit instead of
13
+
waiting for long timeouts.
14
+
15
+
After analysing a large number of time-dependent interactive
16
+
applications, we have seen that they often produce thin streams
17
+
and also stay with this traffic pattern throughout its entire
18
+
lifespan. The combination of time-dependency and the fact that the
19
+
streams provoke high latencies when using TCP is unfortunate.
20
+
21
+
In order to reduce application-layer latency when packets are lost,
22
+
a set of mechanisms has been made, which address these latency issues
23
+
for thin streams. In short, if the kernel detects a thin stream,
24
+
the retransmission mechanisms are modified in the following manner:
25
+
26
+
1) If the stream is thin, fast retransmit on the first dupACK.
27
+
2) If the stream is thin, do not apply exponential backoff.
28
+
29
+
These enhancements are applied only if the stream is detected as
30
+
thin. This is accomplished by defining a threshold for the number
31
+
of packets in flight. If there are less than 4 packets in flight,
32
+
fast retransmissions can not be triggered, and the stream is prone
33
+
to experience high retransmission latencies.
34
+
35
+
Since these mechanisms are targeted at time-dependent applications,
36
+
they must be specifically activated by the application using the
37
+
TCP_THIN_LINEAR_TIMEOUTS and TCP_THIN_DUPACK IOCTLS or the
38
+
tcp_thin_linear_timeouts and tcp_thin_dupack sysctls. Both
39
+
modifications are turned off by default.
40
+
41
+
References
42
+
==========
43
+
More information on the modifications, as well as a wide range of
44
+
experimental data can be found here:
45
+
"Improving latency for interactive, thin-stream applications over
46
+
reliable transport"
47
+
http://simula.no/research/nd/publications/Simula.nd.477/simula_pdf_file
+8
include/net/tcp.h
+8
include/net/tcp.h
···
1386
1386
tcp_sk(sk)->highest_sack = new;
1387
1387
}
1388
1388
1389
+
/* Determines whether this is a thin stream (which may suffer from
1390
+
* increased latency). Used to trigger latency-reducing mechanisms.
1391
+
*/
1392
+
static inline unsigned int tcp_stream_is_thin(struct tcp_sock *tp)
1393
+
{
1394
+
return tp->packets_out < 4 && !tcp_in_initial_slowstart(tp);
1395
+
}
1396
+
1389
1397
/* /proc */
1390
1398
enum tcp_seq_states {
1391
1399
TCP_SEQ_STATE_LISTENING,