Index: sys/netinet/in_kdtrace.h
===================================================================
--- sys/netinet/in_kdtrace.h
+++ sys/netinet/in_kdtrace.h
@@ -65,6 +65,8 @@
 SDT_PROBE_DECLARE(tcp, , , debug__output);
 SDT_PROBE_DECLARE(tcp, , , debug__user);
 SDT_PROBE_DECLARE(tcp, , , debug__drop);
+SDT_PROBE_DECLARE(tcp, , , receive__autoresize);
+SDT_PROBE_DECLARE(tcp, , , xmit__timer);
 
 SDT_PROBE_DECLARE(udp, , , receive);
 SDT_PROBE_DECLARE(udp, , , send);
Index: sys/netinet/in_kdtrace.c
===================================================================
--- sys/netinet/in_kdtrace.c
+++ sys/netinet/in_kdtrace.c
@@ -132,6 +132,22 @@
     "void *", "void *",
     "int", "tcplsinfo_t *");
 
+SDT_PROBE_DEFINE6_XLATE(tcp, , , receive__autoresize,
+    "u_int32_t", "u_int32_t",
+    "struct tcpcb *", "csinfo_t *",
+    "u_int", "u_int",
+    "struct tcpcb *", "tcpsinfo_t *" ,
+    "struct tcphdr *", "tcpinfoh_t *",
+    "int", "int");
+
+SDT_PROBE_DEFINE6_XLATE(tcp, , , xmit__timer,
+    "void *", "pktinfo_t *",
+    "struct inpcb *", "csinfo_t *",
+    "uint8_t *", "ipinfo_t *",
+    "struct inpcb *", "udpsinfo_t *",
+    "struct udphdr *", "udpinfo_t *",
+    "int", "int");
+
 SDT_PROBE_DEFINE5_XLATE(udp, , , receive,
     "void *", "pktinfo_t *",
     "struct inpcb *", "csinfo_t *",
Index: sys/netinet/tcp_input.c
===================================================================
--- sys/netinet/tcp_input.c
+++ sys/netinet/tcp_input.c
@@ -1885,24 +1885,51 @@
 		 * the buffer to better manage the socket buffer resources.
 		 */
 			if (V_tcp_do_autorcvbuf &&
-			    (to.to_flags & TOF_TS) &&
-			    to.to_tsecr &&
 			    (so->so_rcv.sb_flags & SB_AUTOSIZE)) {
-				if (TSTMP_GT(to.to_tsecr, tp->rfbuf_ts) &&
-				    to.to_tsecr - tp->rfbuf_ts < hz) {
+				int rtt_check = 0;
+				uint32_t t = 0;
+
+				if ((to.to_flags & TOF_TS) != 0 &&
+				    to.to_tsecr) {
+					/* Timestamp based RTT. */
+					if (TSTMP_GT(to.to_tsecr,
+					    tp->rfbuf_ts) &&
+					    to.to_tsecr - tp->rfbuf_ts < hz)
+						rtt_check = 3;
+					else
+						rtt_check = 1;
+				} else {
+					/* Estimated RTT. */
+					t = tcp_ts_getticks();
+					if (tp->t_srtt != 0 &&
+					    tp->rfbuf_ts != 0 &&
+					    t - tp->rfbuf_ts >
+					    (tp->t_srtt >> TCP_RTT_SHIFT))
+						rtt_check = 4;
+					else
+						rtt_check = 2;
+				}
+
+				TCP_PROBE6(receive__autoresize, NULL,
+				    tp, t, tp, th, rtt_check);
+
+				if (rtt_check > 2) {
 					if (tp->rfbuf_cnt >
 					    (so->so_rcv.sb_hiwat / 8 * 7) &&
 					    so->so_rcv.sb_hiwat <
 					    V_tcp_autorcvbuf_max) {
-						newsize =
-						    min(so->so_rcv.sb_hiwat +
+						newsize = min(
+						    so->so_rcv.sb_hiwat +
 						    V_tcp_autorcvbuf_inc,
 						    V_tcp_autorcvbuf_max);
+						TCP_PROBE6(receive__autoresize,
+						    newsize, tp, t, tp, th,
+						    rtt_check);
 					}
 					/* Start over with next RTT. */
 					tp->rfbuf_ts = 0;
 					tp->rfbuf_cnt = 0;
-				} else
+				} else if (rtt_check != 0)
 					tp->rfbuf_cnt += tlen;	/* add up */
 			}
 
@@ -3492,6 +3519,8 @@
 
 	INP_WLOCK_ASSERT(tp->t_inpcb);
 
+	TCP_PROBE6(xmit__timer, NULL, tp, NULL, tp, NULL, rtt);
+
 	TCPSTAT_INC(tcps_rttupdated);
 	tp->t_rttupdated++;
 	if ((tp->t_srtt != 0) && (tp->t_rxtshift <= TCP_RTT_INVALIDATE)) {
Index: sys/netinet/tcp_output.c
===================================================================
--- sys/netinet/tcp_output.c
+++ sys/netinet/tcp_output.c
@@ -829,11 +829,13 @@
 			to.to_tsval = tcp_ts_getticks() + tp->ts_offset;
 			to.to_tsecr = tp->ts_recent;
 			to.to_flags |= TOF_TS;
-			/* Set receive buffer autosizing timestamp. */
-			if (tp->rfbuf_ts == 0 &&
-			    (so->so_rcv.sb_flags & SB_AUTOSIZE))
-				tp->rfbuf_ts = tcp_ts_getticks();
 		}
+
+		/* Set receive buffer autosizing timestamp. */
+		if (tp->rfbuf_ts == 0 &&
+		    (so->so_rcv.sb_flags & SB_AUTOSIZE))
+			tp->rfbuf_ts = tcp_ts_getticks();
+
 		/* Selective ACK's. */
 		if (tp->t_flags & TF_SACK_PERMIT) {
 			if (flags & TH_SYN)