Index: sys/netinet/in_proto.c
===================================================================
--- sys/netinet/in_proto.c
+++ sys/netinet/in_proto.c
@@ -120,7 +120,6 @@
 	.pr_flags =		PR_CONNREQUIRED|PR_IMPLOPCL|PR_WANTRCVD|
 				    PR_CAPATTACH,
 	.pr_ctloutput =		tcp_ctloutput,
-	.pr_slowtimo =		tcp_slowtimo,
 	.pr_drain =		tcp_drain,
 	.pr_usrreqs =		&tcp_usrreqs
 },
Index: sys/netinet/tcp_subr.c
===================================================================
--- sys/netinet/tcp_subr.c
+++ sys/netinet/tcp_subr.c
@@ -1568,6 +1568,9 @@
 #ifdef INET6
 	IP6PROTO_REGISTER(IPPROTO_TCP, tcp6_input, tcp6_ctlinput);
 #endif
+
+	callout_init(&tcpslow_callout, 1);
+	callout_reset(&tcpslow_callout, hz / 2, tcp_slowtimo, NULL);
 }
 SYSINIT(tcp_init, SI_SUB_PROTO_DOMAIN, SI_ORDER_THIRD, tcp_init, NULL);
 
Index: sys/netinet/tcp_timer.c
===================================================================
--- sys/netinet/tcp_timer.c
+++ sys/netinet/tcp_timer.c
@@ -233,15 +233,17 @@
 }
 
 /*
- * Tcp protocol timeout routine called every 500 ms.
- * Updates timestamps used for TCP
- * causes finite state machine actions if timers expire.
+ * Legacy TCP global callout routine called every 500 ms.
+ * Used to cleanup timewait states, which lack their own callouts.
  */
+struct callout tcpslow_callout;
 void
-tcp_slowtimo(void)
+tcp_slowtimo(void *arg __unused)
 {
+	struct epoch_tracker et;
 	VNET_ITERATOR_DECL(vnet_iter);
 
+	NET_EPOCH_ENTER(et);
 	VNET_LIST_RLOCK_NOSLEEP();
 	VNET_FOREACH(vnet_iter) {
 		CURVNET_SET(vnet_iter);
@@ -249,6 +251,9 @@
 		CURVNET_RESTORE();
 	}
 	VNET_LIST_RUNLOCK_NOSLEEP();
+	NET_EPOCH_EXIT(et);
+
+	callout_reset(&tcpslow_callout, hz / 2, tcp_slowtimo, NULL);
 }
 
 int	tcp_backoff[TCP_MAXRXTSHIFT + 1] =
Index: sys/netinet/tcp_var.h
===================================================================
--- sys/netinet/tcp_var.h
+++ sys/netinet/tcp_var.h
@@ -1187,7 +1187,8 @@
 int	 tcp_twcheck(struct inpcb *, struct tcpopt *, struct tcphdr *,
 	    struct mbuf *, int);
 void	 tcp_setpersist(struct tcpcb *);
-void	 tcp_slowtimo(void);
+extern struct callout tcpslow_callout;
+void	 tcp_slowtimo(void *);
 void	 tcp_record_dsack(struct tcpcb *tp, tcp_seq start, tcp_seq end, int tlp);
 struct tcptemp *
 	 tcpip_maketemplate(struct inpcb *);
Index: sys/netinet6/in6_proto.c
===================================================================
--- sys/netinet6/in6_proto.c
+++ sys/netinet6/in6_proto.c
@@ -163,7 +163,6 @@
 				    PR_LISTEN|PR_CAPATTACH,
 	.pr_ctloutput =		tcp_ctloutput,
 #ifndef INET	/* don't call initialization, timeout, and drain routines twice */
-	.pr_slowtimo =		tcp_slowtimo,
 	.pr_drain =		tcp_drain,
 #endif
 	.pr_usrreqs =		&tcp6_usrreqs,