Index: sys/netinet/cc/cc_newreno.c
===================================================================
--- sys/netinet/cc/cc_newreno.c
+++ sys/netinet/cc/cc_newreno.c
@@ -134,7 +134,7 @@
 	if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
 	    (ccv->flags & CCF_CWND_LIMITED)) {
 		u_int cw = CCV(ccv, snd_cwnd);
-		u_int incr = CCV(ccv, t_maxseg);
+		u_int incr = 0;
 
 		/*
 		 * Regular in-order ACK, open the congestion window.
@@ -169,7 +169,7 @@
 					ccv->flags &= ~CCF_ABC_SENTAWND;
 				else
 					incr = 0;
-			} else
+			} else if (ccv->bytes_this_ack > 0)
 				incr = max((incr * incr / cw), 1);
 		} else if (V_tcp_do_rfc3465) {
 			/*
@@ -188,7 +188,8 @@
 				    CCV(ccv, t_maxseg));
 			else
 				incr = min(ccv->bytes_this_ack, CCV(ccv, t_maxseg));
-		}
+		} else if (ccv->bytes_this_ack > 0)
+			incr = CCV(ccv, t_maxseg);
 		/* ABC is on by default, so incr equals 0 frequently. */
 		if (incr > 0)
 			CCV(ccv, snd_cwnd) = min(cw + incr,
Index: sys/netinet/tcp_input.c
===================================================================
--- sys/netinet/tcp_input.c
+++ sys/netinet/tcp_input.c
@@ -2031,6 +2031,9 @@
 				TCP_PROBE5(connect__established, NULL, tp,
 				    m, tp, th);
 				cc_conn_init(tp);
+				if ((tlen == 0) &&
+				    (tp->snd_una+1 == th->th_ack))
+					tp->snd_una++;
 				tcp_timer_activate(tp, TT_KEEP,
 				    TP_KEEPIDLE(tp));
 			}
@@ -2416,8 +2419,12 @@
 			 * snd_cwnd reduction that occurs when a TFO SYN|ACK
 			 * is retransmitted.
 			 */
-			if (!IS_FASTOPEN(tp->t_flags))
+			if (!IS_FASTOPEN(tp->t_flags)) {
 				cc_conn_init(tp);
+				if ((tlen == 0) &&
+				    (tp->snd_una+1 == th->th_ack))
+					tp->snd_una++;
+			}
 			tcp_timer_activate(tp, TT_KEEP, TP_KEEPIDLE(tp));
 		}
 		/*
Index: sys/netinet/tcp_stacks/rack.c
===================================================================
--- sys/netinet/tcp_stacks/rack.c
+++ sys/netinet/tcp_stacks/rack.c
@@ -5266,6 +5266,9 @@
 			TCP_PROBE5(connect__established, NULL, tp,
 			    mtod(m, const char *), tp, th);
 			cc_conn_init(tp);
+			if ((tlen == 0) &&
+			    (tp->snd_una+1 == th->th_ack))
+				tp->snd_una++;
 		}
 	} else {
 		/*
@@ -5481,8 +5484,12 @@
 		 * is not harmless as it would undo the snd_cwnd reduction
 		 * that occurs when a TFO SYN|ACK is retransmitted.
 		 */
-		if (!IS_FASTOPEN(tp->t_flags))
+		if (!IS_FASTOPEN(tp->t_flags)) {
 			cc_conn_init(tp);
+			if ((tlen == 0) &&
+			    (tp->snd_una+1 == th->th_ack))
+				tp->snd_una++;
+		}
 	}
 	/*
 	 * If segment contains data or ACK, will call tcp_reass() later; if