netdev.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
* [net-next PATCH v4 1/3] net: TCP thin-stream detection
@ 2010-02-16 14:40 Andreas Petlund
  2010-02-18  0:32 ` David Miller
  2010-02-21 10:21 ` Pavel Machek
  0 siblings, 2 replies; 11+ messages in thread
From: Andreas Petlund @ 2010-02-16 14:40 UTC (permalink / raw)
  To: netdev@vger.kernel.org
  Cc: Ilpo Järvinen, Eric Dumazet, Arnd Hannemann, LKML,
	shemminger, David Miller, william.allen.simpson, damian,
	Eric W. Biederman

Major changes: Added thin-stream info in new file:
Documentation/networking/tcp-thin.txt

Signed-off-by: Andreas Petlund <apetlund@simula.no>
---
 Documentation/networking/tcp-thin.txt |   47 +++++++++++++++++++++++++++++++++
 include/net/tcp.h                     |    7 +++++
 2 files changed, 54 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/networking/tcp-thin.txt

diff --git a/Documentation/networking/tcp-thin.txt b/Documentation/networking/tcp-thin.txt
new file mode 100644
index 0000000..151e229
--- /dev/null
+++ b/Documentation/networking/tcp-thin.txt
@@ -0,0 +1,47 @@
+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
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 87d164b..e5e2056 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1386,6 +1386,13 @@ static inline void tcp_highest_sack_combine(struct sock *sk,
 		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 */
 enum tcp_seq_states {
 	TCP_SEQ_STATE_LISTENING,
-- 
1.6.3.3


^ permalink raw reply related	[flat|nested] 11+ messages in thread
end of thread, other threads:[~2010-02-22 22:12 UTC | newest]

Thread overview: 11+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2010-02-16 14:40 [net-next PATCH v4 1/3] net: TCP thin-stream detection Andreas Petlund
2010-02-18  0:32 ` David Miller
2010-02-21 10:21 ` Pavel Machek
2010-02-21 11:23   ` Alexander Zimmermann
     [not found]     ` <2EC47E8B-BC4C-4DAB-86ED-F702D67F8AD0@nokia.com>
     [not found]       ` <20100221.150311.193707249.davem@davemloft.net>
2010-02-22  6:41         ` Lars Eggert
2010-02-22 15:17           ` Andi Kleen
2010-02-22 15:35             ` Alexander Zimmermann
2010-02-22 15:40             ` Andreas Petlund
2010-02-22 15:51             ` Andreas Petlund
2010-02-22 16:06             ` Hagen Paul Pfeifer
2010-02-22 22:13             ` David Miller

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).