Index: share/man/man4/tcp.4
===================================================================
--- share/man/man4/tcp.4
+++ share/man/man4/tcp.4
@@ -34,7 +34,7 @@
 .\"     From: @(#)tcp.4	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd December 19, 2020
+.Dd January 14, 2021
 .Dt TCP 4
 .Os
 .Sh NAME
@@ -369,8 +369,17 @@
 .Bl -tag -width ".Va TCPCTL_DO_RFC1323"
 .It Dv TCPCTL_DO_RFC1323
 .Pq Va rfc1323
-Implement the window scaling and timestamp options of RFC 1323
+Implement the window scaling and timestamp options of RFC 1323/RFC 7323
 (default is true).
+.It Va tolerate_missing_ts
+Tolerate the missing of timestamps (RFC 1323/RFC 7323) for
+.Tn TCP
+segments belonging to
+.Tn TCP
+connections for which support of
+.Tn TCP
+timestamps has been negotiated.
+(default is 0, i.e., the missing of timestamps is not tolerated).
 .It Dv TCPCTL_MSSDFLT
 .Pq Va mssdflt
 The default value used for the maximum segment size
@@ -728,12 +737,20 @@
 .Xr tcp_functions 9
 .Rs
 .%A "V. Jacobson"
-.%A "R. Braden"
+.%A "B. Braden"
 .%A "D. Borman"
 .%T "TCP Extensions for High Performance"
 .%O "RFC 1323"
 .Re
 .Rs
+.%A "D. Borman"
+.%A "B. Braden"
+.%A "V. Jacobson"
+.%A "R. Scheffenegger"
+.%T "TCP Extensions for High Performance"
+.%O "RFC 7323"
+.Re
+.Rs
 .%A "A. Heffernan"
 .%T "Protection of BGP Sessions via the TCP MD5 Signature Option"
 .%O "RFC 2385"
Index: sys/netinet/tcp_input.c
===================================================================
--- sys/netinet/tcp_input.c
+++ sys/netinet/tcp_input.c
@@ -1692,11 +1692,12 @@
 	/*
 	 * If timestamps were negotiated during SYN/ACK and a
 	 * segment without a timestamp is received, silently drop
-	 * the segment, unless it is a RST segment.
+	 * the segment, unless it is a RST segment or missing timestamps are
+	 * tolerated.
 	 * See section 3.2 of RFC 7323.
 	 */
 	if ((tp->t_flags & TF_RCVD_TSTMP) && !(to.to_flags & TOF_TS)) {
-		if ((thflags & TH_RST) != 0) {
+		if (((thflags & TH_RST) != 0) || V_tcp_tolerate_missing_ts) {
 			if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 				log(LOG_DEBUG, "%s; %s: Timestamp missing, "
 				    "segment processed normally\n",
Index: sys/netinet/tcp_stacks/bbr.c
===================================================================
--- sys/netinet/tcp_stacks/bbr.c
+++ sys/netinet/tcp_stacks/bbr.c
@@ -11463,11 +11463,12 @@
 	/*
 	 * If timestamps were negotiated during SYN/ACK and a
 	 * segment without a timestamp is received, silently drop
-	 * the segment, unless it is a RST segment.
+	 * the segment, unless it is a RST segment or missing timestamps are
+	 * tolerated.
 	 * See section 3.2 of RFC 7323.
 	 */
 	if ((tp->t_flags & TF_RCVD_TSTMP) && !(to.to_flags & TOF_TS) &&
-	    ((thflags & TH_RST) == 0)) {
+	    ((thflags & TH_RST) == 0) && (V_tcp_tolerate_missing_ts == 0)) {
 		retval = 0;
 		goto done_with_input;
 	}
Index: sys/netinet/tcp_stacks/rack.c
===================================================================
--- sys/netinet/tcp_stacks/rack.c
+++ sys/netinet/tcp_stacks/rack.c
@@ -10879,11 +10879,12 @@
 	/*
 	 * If timestamps were negotiated during SYN/ACK and a
 	 * segment without a timestamp is received, silently drop
-	 * the segment, unless it is a RST segment.
+	 * the segment, unless it is a RST segment or missing timestamps are
+	 * tolerated.
 	 * See section 3.2 of RFC 7323.
 	 */
 	if ((tp->t_flags & TF_RCVD_TSTMP) && !(to.to_flags & TOF_TS) &&
-	    ((thflags & TH_RST) == 0)) {
+	    ((thflags & TH_RST) == 0) && (V_tcp_tolerate_missing_ts == 0)) {
 		way_out = 5;
 		retval = 0;
 		goto done_with_input;
Index: sys/netinet/tcp_subr.c
===================================================================
--- sys/netinet/tcp_subr.c
+++ sys/netinet/tcp_subr.c
@@ -256,6 +256,11 @@
     &VNET_NAME(tcp_do_rfc1323), 0,
     "Enable rfc1323 (high performance TCP) extensions");
 
+VNET_DEFINE(int, tcp_tolerate_missing_ts) = 0;
+SYSCTL_INT(_net_inet_tcp, OID_AUTO, tolerate_missing_ts, CTLFLAG_VNET | CTLFLAG_RW,
+    &VNET_NAME(tcp_tolerate_missing_ts), 0,
+    "Tolerate missing TCP timestamps");
+
 VNET_DEFINE(int, tcp_ts_offset_per_conn) = 1;
 SYSCTL_INT(_net_inet_tcp, OID_AUTO, ts_offset_per_conn, CTLFLAG_VNET | CTLFLAG_RW,
     &VNET_NAME(tcp_ts_offset_per_conn), 0,
Index: sys/netinet/tcp_syncache.c
===================================================================
--- sys/netinet/tcp_syncache.c
+++ sys/netinet/tcp_syncache.c
@@ -1231,18 +1231,30 @@
 		/*
 		 * If timestamps were negotiated during SYN/ACK and a
 		 * segment without a timestamp is received, silently drop
-		 * the segment.
+		 * the segment, unless the missing timestamps are tolerated.
 		 * See section 3.2 of RFC 7323.
 		 */
 		if ((sc->sc_flags & SCF_TIMESTAMP) &&
 		    !(to->to_flags & TOF_TS)) {
-			SCH_UNLOCK(sch);
-			if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
-				log(LOG_DEBUG, "%s; %s: Timestamp missing, "
-				    "segment silently dropped\n", s, __func__);
-				free(s, M_TCPLOG);
+			if (V_tcp_tolerate_missing_ts) {
+				if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
+					log(LOG_DEBUG,
+					    "%s; %s: Timestamp missing, "
+					    "segment processed normally\n",
+					    s, __func__);
+					free(s, M_TCPLOG);
+				}
+			} else {
+				SCH_UNLOCK(sch);
+				if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
+					log(LOG_DEBUG,
+					    "%s; %s: Timestamp missing, "
+					    "segment silently dropped\n",
+					    s, __func__);
+					free(s, M_TCPLOG);
+				}
+				return (-1);  /* Do not send RST */
 			}
-			return (-1);  /* Do not send RST */
 		}
 
 		/*
Index: sys/netinet/tcp_timewait.c
===================================================================
--- sys/netinet/tcp_timewait.c
+++ sys/netinet/tcp_timewait.c
@@ -451,10 +451,11 @@
 	/*
 	 * If timestamps were negotiated during SYN/ACK and a
 	 * segment without a timestamp is received, silently drop
-	 * the segment.
+	 * the segment, unless the missing timestamps are tolerated.
 	 * See section 3.2 of RFC 7323.
 	 */
-	if (((to->to_flags & TOF_TS) == 0) && (tw->t_recent != 0)) {
+	if (((to->to_flags & TOF_TS) == 0) && (tw->t_recent != 0) &&
+	    (V_tcp_tolerate_missing_ts == 0)) {
 		goto drop;
 	}
 
Index: sys/netinet/tcp_var.h
===================================================================
--- sys/netinet/tcp_var.h
+++ sys/netinet/tcp_var.h
@@ -840,6 +840,7 @@
 VNET_DECLARE(int, tcp_do_ecn);
 VNET_DECLARE(int, tcp_do_newcwv);
 VNET_DECLARE(int, tcp_do_rfc1323);
+VNET_DECLARE(int, tcp_tolerate_missing_ts);
 VNET_DECLARE(int, tcp_do_rfc3042);
 VNET_DECLARE(int, tcp_do_rfc3390);
 VNET_DECLARE(int, tcp_do_rfc3465);
@@ -883,6 +884,7 @@
 #define	V_tcp_do_autosndbuf		VNET(tcp_do_autosndbuf)
 #define	V_tcp_do_ecn			VNET(tcp_do_ecn)
 #define	V_tcp_do_rfc1323		VNET(tcp_do_rfc1323)
+#define	V_tcp_tolerate_missing_ts	VNET(tcp_tolerate_missing_ts)
 #define V_tcp_ts_offset_per_conn	VNET(tcp_ts_offset_per_conn)
 #define	V_tcp_do_rfc3042		VNET(tcp_do_rfc3042)
 #define	V_tcp_do_rfc3390		VNET(tcp_do_rfc3390)