tcp: provide sysctl for the maximum retransmission timeout

Reviewed by:		tuexen
Differential Revision:	https://reviews.freebsd.org/D50891

share/man/man4/tcp.4

@@ -31,7 +31,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd January 10, 2025
+.Dd June 16, 2025
 .Dt TCP 4
 .Os
 .Sh NAME
@@ -871,10 +871,13 @@ segment is lost (default and maximum is 12).
 .It Va rexmit_drop_options
 Drop TCP options from third and later retransmitted SYN segments
 of a connection.
-.It Va rexmit_initial , rexmit_min , rexmit_slop
+.It Va rexmit_initial , rexmit_min , rexmit_slop , rexmit_max
 Adjust the retransmit timer calculation for
 .Tn TCP .
-The slop is
+A new connection starts with timer set to
+.Va rexmit_initial .
+The
+.Va rexmit_slop
 typically added to the raw calculation to take into account
 occasional variances that the
 .Tn SRTT
@@ -894,6 +897,11 @@ For this reason, we use 200ms of slop and a near-0
 minimum, which gives us an effective minimum of 200ms (similar to
 .Tn Linux ) .
 The initial value is used before an RTT measurement has been performed.
+The
+.Va rexmit_min
+and
+.Va rexmit_max
+set minimum and maximum timer values that a connection may have.
 .It Va rfc1323
 Implement the window scaling and timestamp options of RFC 1323/RFC 7323
 (default is 1).

sys/netinet/tcp_input.c

@@ -383,7 +383,7 @@ cc_conn_init(struct tcpcb *tp)
 		}
 		TCPT_RANGESET(tp->t_rxtcur,
 		    ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1,
-		    tp->t_rttmin, TCPTV_REXMTMAX);
+		    tp->t_rttmin, tcp_rexmit_max);
 	}
 	if (metrics.hc_ssthresh) {
 		/*
@@ -3732,7 +3732,7 @@ tcp_xmit_timer(struct tcpcb *tp, int rtt)
 	 * the minimum feasible timer (which is 2 ticks).
 	 */
 	TCPT_RANGESET(tp->t_rxtcur, TCP_REXMTVAL(tp),
-		      max(tp->t_rttmin, rtt + 2), TCPTV_REXMTMAX);
+	    max(tp->t_rttmin, rtt + 2), tcp_rexmit_max);
 
 	/*
 	 * We received an ack for a packet that wasn't retransmitted;

sys/netinet/tcp_stacks/bbr.c

@@ -10150,7 +10150,7 @@ bbr_init(struct tcpcb *tp, void **ptr)
 	tcp_change_time_units(tp, TCP_TMR_GRANULARITY_TICKS);
 	TCPT_RANGESET(tp->t_rxtcur,
 	    ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1,
-	    tp->t_rttmin, TCPTV_REXMTMAX);
+	    tp->t_rttmin, tcp_rexmit_max);
 	bbr_start_hpts_timer(bbr, tp, cts, 5, 0, 0);
 	return (0);
 }

sys/netinet/tcp_stacks/rack.c

@@ -6347,7 +6347,7 @@ rack_timer_start(struct tcpcb *tp, struct tcp_rack *rack, uint32_t cts, int sup_
 		if (to < rack_tlp_min) {
 			to = rack_tlp_min;
 		}
-		if (to > TICKS_2_USEC(TCPTV_REXMTMAX)) {
+		if (to > TICKS_2_USEC(tcp_rexmit_max)) {
 			/*
 			 * If the TLP time works out to larger than the max
 			 * RTO lets not do TLP.. just RTO.

sys/netinet/tcp_subr.c

@@ -1052,7 +1052,8 @@ tcp_default_fb_init(struct tcpcb *tp, void **ptr)
 	if (tp->t_rxtshift == 0)
 		tp->t_rxtcur = rexmt;
 	else
-		TCPT_RANGESET(tp->t_rxtcur, rexmt, tp->t_rttmin, TCPTV_REXMTMAX);
+		TCPT_RANGESET(tp->t_rxtcur, rexmt, tp->t_rttmin,
+		    tcp_rexmit_max);
 
 	/*
 	 * Nothing to do for ESTABLISHED or LISTEN states. And, we don't
@@ -1478,6 +1479,7 @@ tcp_init(void *arg __unused)
 	tcp_rexmit_min = TCPTV_MIN;
 	if (tcp_rexmit_min < 1)
 		tcp_rexmit_min = 1;
+	tcp_rexmit_max = TCPTV_REXMTMAX;
 	tcp_persmin = TCPTV_PERSMIN;
 	tcp_persmax = TCPTV_PERSMAX;
 	tcp_rexmit_slop = TCPTV_CPU_VAR;

sys/netinet/tcp_syncache.c

@@ -443,7 +443,7 @@ syncache_timeout(struct syncache *sc, struct syncache_head *sch, int docallout)
 	else
 		TCPT_RANGESET(rexmt,
 		    tcp_rexmit_initial * tcp_backoff[sc->sc_rxmits],
-		    tcp_rexmit_min, TCPTV_REXMTMAX);
+		    tcp_rexmit_min, tcp_rexmit_max);
 	sc->sc_rxttime = ticks + rexmt;
 	sc->sc_rxmits++;
 	if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc)) {

sys/netinet/tcp_timer.c

@@ -119,6 +119,11 @@ SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_min, CTLTYPE_INT | CTLFLAG_RW,
     &tcp_rexmit_min, 0, sysctl_msec_to_ticks, "I",
     "Minimum Retransmission Timeout");
 
+int	tcp_rexmit_max;
+SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_max, CTLTYPE_INT | CTLFLAG_RW,
+    &tcp_rexmit_max, 0, sysctl_msec_to_ticks, "I",
+    "Maximum Retransmission Timeout");
+
 int	tcp_rexmit_slop;
 SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_slop, CTLTYPE_INT | CTLFLAG_RW,
     &tcp_rexmit_slop, 0, sysctl_msec_to_ticks, "I",
@@ -618,8 +623,7 @@ tcp_timer_rexmt(struct tcpcb *tp)
 		rexmt = tcp_rexmit_initial * tcp_backoff[tp->t_rxtshift];
 	else
 		rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
-	TCPT_RANGESET(tp->t_rxtcur, rexmt,
-		      tp->t_rttmin, TCPTV_REXMTMAX);
+	TCPT_RANGESET(tp->t_rxtcur, rexmt, tp->t_rttmin, tcp_rexmit_max);
 
 	/*
 	 * We enter the path for PLMTUD if connection is established or, if

sys/netinet/tcp_timer.h

@@ -163,6 +163,7 @@ extern int tcp_maxunacktime;		/* max time without making progress */
 extern int tcp_maxpersistidle;
 extern int tcp_rexmit_initial;
 extern int tcp_rexmit_min;
+extern int tcp_rexmit_max;
 extern int tcp_rexmit_slop;
 extern int tcp_ttl;			/* time to live for TCP segs */
 extern int tcp_backoff[];