--- tcp_output.c.~1.205.~	2018-04-18 06:10:31.000000000 +0300
+++ tcp_output.c	2018-04-18 15:16:20.213923329 +0300
@@ -988,16 +988,27 @@
 		 * taking into account that we are limited by
 		 * TCP_MAXWIN << tp->rcv_scale.
 		 */
-		long adv = min(win, (long)TCP_MAXWIN << tp->rcv_scale) -
-			(tp->rcv_adv - tp->rcv_nxt);
+		long recwin = min(win, (long)TCP_MAXWIN << tp->rcv_scale);
+		long oldwin, adv;
 
 		/*
-		 * If the new window size ends up being the same as the old
-		 * size when it is scaled, then don't force a window update.
+		 * rcv_nxt may overtake rcv_adv when we accept a
+		 * zero-window probe.
 		 */
-		if ((tp->rcv_adv - tp->rcv_nxt) >> tp->rcv_scale ==
-		    (adv + tp->rcv_adv - tp->rcv_nxt) >> tp->rcv_scale)
+		if (SEQ_GT(tp->rcv_adv, tp->rcv_nxt))
+			oldwin = tp->rcv_adv - tp->rcv_nxt;
+		else
+			oldwin = 0;
+
+		/*
+		 * If the new window size ends up being the same as or
+		 * less than the old size when it is scaled, then
+		 * don't force a window update.
+		 */
+		if (recwin >> tp->rcv_scale <= oldwin >> tp->rcv_scale)
 			goto dontupdate;
+
+		adv = recwin - oldwin;
 		if (adv >= (long) (2 * rxsegsize))
 			goto send;
 		if (2 * adv >= (long) so->so_rcv.sb_hiwat)