Introduce two new sysctl's:

net.inet.tcp.rexmit_min (default 3 ticks equiv)

    This sysctl is the retransmit timer RTO minimum,
    specified in milliseconds.  This value is
    designed for algorithmic stability only.

net.inet.tcp.rexmit_slop (default 200ms)

    This sysctl is the retransmit timer RTO slop
    which is added to every retransmit timeout and
    is designed to handle protocol stack overheads
    and delayed ack issues.

Note that the *original* code applied a 1-second
RTO minimum but never applied real slop to the RTO
calculation, so any RTO calculation over one second
would have no slop and thus not account for
protocol stack overheads (TCP timestamps are not
a measure of protocol turnaround!).  Essentially,
the original code made the RTO calculation almost
completely irrelevant.

Please note that the 200ms slop is debateable.
This commit is not meant to be a line in the sand,
and if the community winds up deciding that increasing
it is the correct solution then it's easy to do.
Note that larger values will destroy performance
on lossy networks while smaller values may result in
a greater number of unnecessary retransmits.

1 files changed, 3 insertions, 1 deletions

diff --git a/sys/netinet/tcp_subr.c b/sys/netinet/tcp_subr.c
index 9dab996..c2799d5 100644
--- a/sys/netinet/tcp_subr.c
+++ b/sys/netinet/tcp_subr.c
@@ -196,6 +196,8 @@ tcp_init()
 	tcp_keepintvl = TCPTV_KEEPINTVL;
 	tcp_maxpersistidle = TCPTV_KEEP_IDLE;
 	tcp_msl = TCPTV_MSL;
+	tcp_rexmit_min = TCPTV_MIN;
+	tcp_rexmit_slop = TCPTV_CPU_VAR;
 
 	INP_INFO_LOCK_INIT(&tcbinfo, "tcp");
 	LIST_INIT(&tcb);
@@ -542,7 +544,7 @@ tcp_newtcpcb(inp)
 	 */
 	tp->t_srtt = TCPTV_SRTTBASE;
 	tp->t_rttvar = ((TCPTV_RTOBASE - TCPTV_SRTTBASE) << TCP_RTTVAR_SHIFT) / 4;
-	tp->t_rttmin = TCPTV_MIN;
+	tp->t_rttmin = tcp_rexmit_min;
 	tp->t_rxtcur = TCPTV_RTOBASE;
 	tp->snd_cwnd = TCP_MAXWIN << TCP_MAX_WINSHIFT;
 	tp->snd_ssthresh = TCP_MAXWIN << TCP_MAX_WINSHIFT;