Change tcp.inflight_min from 1024 to a production default of 6144. Create

a sysctl for the stabilization value for the bandwidth delay product (inflight)
algorithm and document it.

MFC after:	3 days

2 files changed, 28 insertions, 0 deletions

diff --git a/share/man/man4/tcp.4 b/share/man/man4/tcp.4
index bd3b755..1024271 100644
--- a/share/man/man4/tcp.4
+++ b/share/man/man4/tcp.4
@@ -343,6 +343,19 @@ This value should not generally be modified but may be used to set a
 global per-connection limit on queued data, potentially allowing you to
 intentionally set a less then optimum limit to smooth data flow over a
 network while still being able to specify huge internal TCP buffers.
+.It tcp.inflight_stab
+The bandwidth delay product algorithm requires a slightly larger window
+then it otherwise calculates for stability.  This parameter determines the
+extra window in maximal packets / 10.  The default value of 20 represents 
+2 maximal packets.  Reducing this value is not recommended but you may
+come across a situation with very slow links where the ping time
+reduction of the default inflight code is not sufficient.  If this case
+occurs you should first try reducing tcp.inflight_min and, if that does not
+work, reduce both tcp.inflight_min and tcp.inflight_stab, trying values of
+15, 10, or 5 for the latter.  Never use a value less then 5.  Reducing
+tcp.inflight_stab can lead to upwards of a 20% underutilization of the link
+as well as reducing the algorithm's ability to adapt to changing
+situations and should only be done as a last resort.
 .El
 .Sh ERRORS
 A socket operation may fail with one of the following errors returned:
diff --git a/share/man/man7/tuning.7 b/share/man/man7/tuning.7
index 092658b..eead2a8 100644
--- a/share/man/man7/tuning.7
+++ b/share/man/man7/tuning.7
@@ -587,6 +587,21 @@ only effects data transmission (uploading / server-side).
 It does not
 effect data reception (downloading).
 .Pp
+Adjusting
+.Va net.inet.tcp.inflight_stab
+is not recommended.
+This parameter defaults to 20, representing 2 maximal packets added
+to the bandwidth delay product window calculation.  The additional
+window is required to stabilize the algorithm and improve responsiveness
+to changing conditions, but it can also result in higher ping times
+over slow links (though still much lower then you would get without
+the inflight algorithm).  In such cases you may
+wish to try reducing this parameter to 15, 10, or 5, and you may also
+have to reduce
+.Va net.inet.tcp.inflight_min
+(for example, to 3500) to get the desired effect.  Reducing these parameters
+should be done as a last resort only.
+.Pp
 The
 .Va net.inet.ip.portrange.*
 sysctls control the port number ranges automatically bound to TCP and UDP