diff --git a/share/man/man4/tcp.4 b/share/man/man4/tcp.4
--- a/share/man/man4/tcp.4
+++ b/share/man/man4/tcp.4
@@ -34,7 +34,7 @@
 .\"     From: @(#)tcp.4	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd April 24, 2021
+.Dd June 19, 2021
 .Dt TCP 4
 .Os
 .Sh NAME
@@ -382,7 +382,9 @@
 connections for which support of
 .Tn TCP
 timestamps has been negotiated.
-(default is 0, i.e., the missing of timestamps is not tolerated).
+As of June 2021, several TCP stacks are known to violate RFC 7323, including
+modern widely deployed ones.
+Therefore the default is 1, i.e., the missing of timestamps is tolerated.
 .It Dv TCPCTL_MSSDFLT
 .Pq Va mssdflt
 The default value used for the maximum segment size
diff --git a/sys/netinet/tcp_subr.c b/sys/netinet/tcp_subr.c
--- a/sys/netinet/tcp_subr.c
+++ b/sys/netinet/tcp_subr.c
@@ -268,7 +268,14 @@
     &VNET_NAME(tcp_do_rfc1323), 0,
     "Enable rfc1323 (high performance TCP) extensions");
 
-VNET_DEFINE(int, tcp_tolerate_missing_ts) = 0;
+/*
+ * As of June 2021, several TCP stacks violate RFC 7323 from September 2014.
+ * Some stacks negotiate TS, but never send them after connection setup. Some
+ * stacks negotiate TS, but don't send them when sending keep-alive segments.
+ * These include modern widely deployed TCP stacks.
+ * Therefore tolerating violations for now...
+ */
+VNET_DEFINE(int, tcp_tolerate_missing_ts) = 1;
 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");