Improve NewReno after_idle ssthresh per RFC2861 Sec.2

[rscheff]

Summary

This fix was originally included in the set of patches for
Cubic (https://reviews.freebsd.org/D18982?vs=on&id=54610#change-sMaGTxkKtcaO),
but when splitting them up into D18954, D18982, D19118 this was
unfortuately missed.

The rationale for adjusting ssthresh to 3/4 of the value
of cwnd before an idle period is given in RFC2861 - provided
that cwnd did not grow beyond bounds due to the flow being
application limited (see D21798, discovered later)

As cc_newreno is the default "fall-through" reaction, most tcp
variants will see a benefit from this.

Using cwnd-cwnd/4 arithmetic to stay within the integer boundaries of
cwnd, instead of (3*cwnd)/4

Test Plan

This change has been part of the internal Cubic patch set, and
yields significant performance improvements for request-response
IO workloads with frequent tcp idle periods.

[tuexen]

sys/netinet/cc/cc_newreno.c

221 ↗

(On Diff #64530)

Can you add a comment describing where this modification comes from? Right now, the above comment does not describe what you added.

[rscheff]

• adding comment referencing relevant RFC

[rscheff]

expanded the comment to reference RFC2861 as for the rationale to adjust the ssthresh here also. Since this is dependent on D21798 (restrict unbounded cwnd growth), should this be mentioned also?

[rscheff]

• Merge branch 'master'