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 +268,15 @@
     &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:
+ * - Omniswitch 63xx and 64xx models (negotiating TS, but not sending them).
+ * - HP LaserJet 500 color M551dn (negotiating TS, but not sending them).
+ * - IPFW TCP keep-alive injection (not sending TS when injecting packets).
+ * - Mac OS 11.4 (negotiating TS, but not sending TS on keep-alive packets).
+ * 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");