Index: head/sys/netinet/tcp_lro.c
===================================================================
--- head/sys/netinet/tcp_lro.c
+++ head/sys/netinet/tcp_lro.c
@@ -57,6 +57,7 @@
 #include <netinet/ip.h>
 #include <netinet/ip_var.h>
 #include <netinet/tcp.h>
+#include <netinet/tcp_seq.h>
 #include <netinet/tcp_lro.h>
 #include <netinet/tcp_var.h>
 
@@ -794,7 +795,9 @@
 
 		/* Try to append the new segment. */
 		if (__predict_false(seq != le->next_seq ||
-		    (tcp_data_len == 0 && le->ack_seq == th->th_ack))) {
+		    (tcp_data_len == 0 &&
+		    le->ack_seq == th->th_ack &&
+		    le->window == th->th_win))) {
 			/* Out of order packet or duplicate ACK. */
 			tcp_lro_active_remove(le);
 			tcp_lro_flush(lc, le);
@@ -811,12 +814,20 @@
 			le->tsval = tsval;
 			le->tsecr = *(ts_ptr + 2);
 		}
-
-		le->next_seq += tcp_data_len;
-		le->ack_seq = th->th_ack;
-		le->window = th->th_win;
-		le->append_cnt++;
-
+		if (tcp_data_len || SEQ_GT(ntohl(th->th_ack), ntohl(le->ack_seq))) {
+			le->next_seq += tcp_data_len;
+			le->ack_seq = th->th_ack;
+			le->window = th->th_win;
+			le->append_cnt++;
+		} else if (th->th_ack == le->ack_seq) {
+			le->window = WIN_MAX(le->window, th->th_win);
+			le->append_cnt++;
+		} else {
+			/* no data and old ack */
+			le->append_cnt++;
+			m_freem(m);
+			return (0);
+		}
 #ifdef TCP_LRO_UPDATE_CSUM
 		le->ulp_csum += tcp_lro_rx_csum_fixup(le, l3hdr, th,
 		    tcp_data_len, ~csum);
Index: head/sys/netinet/tcp_seq.h
===================================================================
--- head/sys/netinet/tcp_seq.h
+++ head/sys/netinet/tcp_seq.h
@@ -47,6 +47,14 @@
 #define	SEQ_MIN(a, b)	((SEQ_LT(a, b)) ? (a) : (b))
 #define	SEQ_MAX(a, b)	((SEQ_GT(a, b)) ? (a) : (b))
 
+#define	WIN_LT(a,b)	((short)(ntohs(a)-ntohs(b)) < 0)
+#define	WIN_LEQ(a,b)	((short)(ntohs(a)-ntohs(b)) <= 0)
+#define	WIN_GT(a,b)	((short)(ntohs(a)-ntohs(b)) > 0)
+#define	WIN_GEQ(a,b)	((short)(ntohs(a)-ntohs(b)) >= 0)
+
+#define	WIN_MIN(a, b)	((WIN_LT(a, b)) ? (a) : (b))
+#define	WIN_MAX(a, b)	((WIN_GT(a, b)) ? (a) : (b))
+
 /* for modulo comparisons of timestamps */
 #define TSTMP_LT(a,b)	((int)((a)-(b)) < 0)
 #define TSTMP_GT(a,b)	((int)((a)-(b)) > 0)