tcp: Add hystart++ to our cubic implementation.

As promised to the transport call on 11/4/22 here is an implementation
of hystart++ for cubic. It also cleans up the tcp_congestion function
to have a better name. Common variables are moved into the general
cc.h structure so that both cubic and newreno can use them for
hystart++

Reviewed by: Michael Tuexen, Richard Scheffenegger
Sponsored by: Netflix Inc.
Differential Revision: https://reviews.freebsd.org/D33035

2 files changed, 25 insertions, 39 deletions

diff --git a/share/man/man4/cc_newreno.4 b/share/man/man4/cc_newreno.4
index 1c4f4a39cb36..71d3a239ecd1 100644
--- a/share/man/man4/cc_newreno.4
+++ b/share/man/man4/cc_newreno.4
@@ -75,21 +75,10 @@ the congestion window in response to an ECN congestion signal when
 .Va net.inet.tcp.cc.abe=1
 per: cwnd = (cwnd * CC_NEWRENO_BETA_ECN) / 100.
 Default is 80.
-.It Va CC_NEWRENO_ENABLE_HYSTART
-will enable or disable the application of Hystart++.
-The current implementation allows the values 0, 1, 2 and 3.
-A value of 0 (the default) disables the use of Hystart++.
-Setting the value to 1 enables Hystart++.
-Setting the value to 2 enables Hystart++ but also will cause, on exit from Hystart++'s CSS, to
-set the cwnd to the value of where the increase in RTT first began as
-well as setting ssthresh to the flight at send when we exit CSS.
-Setting a value of 3 will keep the setting of the cwnd the same as 2, but will cause ssthresh
-to be set to the average value between the lowest fas rtt (the value cwnd is
-set to) and the fas value at exit of CSS.
 .PP
 Note that currently the only way to enable
 hystart++ is to enable it via socket option.
-When enabling it a value of 1 will enable precise internet-draft behavior
+When enabling it a value of 1 will enable precise internet-draft (version 4) behavior
 (subject to any MIB variable settings), other setting (2 and 3) are experimental.
 .El
 .PP
@@ -120,32 +109,6 @@ the congestion window in response to an ECN congestion signal when
 .Va net.inet.tcp.cc.abe=1
 per: cwnd = (cwnd * beta_ecn) / 100.
 Default is 80.
-.It Va hystartplusplus.bblogs
-This boolean controls if black box logging will be done for hystart++ events. If set
-to zero (the default) no logging is performed.
-If set to one then black box logs will be generated on all hystart++ events.
-.It Va hystartplusplus.css_rounds
-This value controls the number of rounds that CSS runs for.
-The default value matches the current internet-draft of 5.
-.It Va hystartplusplus.css_growth_div
-This value controls the divisor applied to slowstart during CSS.
-The default value matches the current internet-draft of 4.
-.It Va hystartplusplus.n_rttsamples
-This value controls how many rtt samples must be collected in each round for
-hystart++ to be active.
-The default value matches the current internet-draft of 8.
-.It Va hystartplusplus.maxrtt_thresh
-This value controls the maximum rtt variance clamp when considering if CSS is needed.
-The default value matches the current internet-draft of 16000 (in microseconds).
-For further explanation please see the internet-draft.
-.It Va hystartplusplus.minrtt_thresh
-This value controls the minimum rtt variance clamp when considering if CSS is needed.
-The default value matches the current internet-draft of 4000 (in microseconds).
-For further explanation please see the internet-draft.
-.It Va hystartplusplus.lowcwnd
-This value controls what is the lowest congestion window that the tcp
-stack must be at before hystart++ engages.
-The default value matches the current internet-draft of 16.
 .El
 .Sh SEE ALSO
 .Xr cc_cdg 4 ,
diff --git a/share/man/man4/mod_cc.4 b/share/man/man4/mod_cc.4
index 84edf0b2a8ed..ddc7440a4ca7 100644
--- a/share/man/man4/mod_cc.4
+++ b/share/man/man4/mod_cc.4
@@ -83,7 +83,7 @@ The framework exposes the following variables in the
 branch of the
 .Xr sysctl 3
 MIB:
-.Bl -tag -width ".Va abe_frlossreduce"
+.Bl -tag -width ".Va hystartplusplus.css_growth_div"
 .It Va available
 Read-only list of currently available congestion control algorithms by name.
 .It Va algorithm
@@ -102,10 +102,33 @@ support for ABE and for configuration details.
 .It Va abe_frlossreduce
 If non-zero, apply standard beta instead of ABE-beta during ECN-signalled
 congestion recovery episodes if loss also needs to be repaired.
+.It Va hystartplusplus.bblogs
+This boolean controls if black box logging will be done for hystart++ events.
+If set to zero (the default) no logging is performed.
+If set to one then black box logs will be generated on all hystart++ events.
+.It Va hystartplusplus.css_rounds
+This value controls the number of rounds that CSS runs for.
+The default value matches the current internet-draft of 5.
+.It Va hystartplusplus.css_growth_div
+This value controls the divisor applied to slowstart during CSS.
+The default value matches the current internet-draft of 4.
+.It Va hystartplusplus.n_rttsamples
+This value controls how many rtt samples must be collected in each round for
+hystart++ to be active.
+The default value matches the current internet-draft of 8.
+.It Va hystartplusplus.maxrtt_thresh
+This value controls the maximum rtt variance clamp when considering if CSS is needed.
+The default value matches the current internet-draft of 16000 (in microseconds).
+For further explanation please see the internet-draft.
+.It Va hystartplusplus.minrtt_thresh
+This value controls the minimum rtt variance clamp when considering if CSS is needed.
+The default value matches the current internet-draft of 4000 (in microseconds).
+For further explanation please see the internet-draft.
 .El
 .Pp
 Each congestion control module may also expose other MIB variables
 to control their behaviour.
+Note that both newreno and cubic now support hystart++ based on the version 3 of the internet-draft.
 .Sh Kernel Configuration
 .Pp
 All of the available congestion control modules may also be loaded