Index: head/sys/netinet/tcp_input.c
===================================================================
--- head/sys/netinet/tcp_input.c
+++ head/sys/netinet/tcp_input.c
@@ -1481,7 +1481,7 @@
     struct tcpcb *tp, int drop_hdrlen, int tlen, uint8_t iptos,
     int ti_locked)
 {
-	int thflags, acked, ourfinisacked, needoutput = 0;
+	int thflags, acked, ourfinisacked, needoutput = 0, sack_changed;
 	int rstreason, todrop, win;
 	u_long tiwin;
 	char *s;
@@ -1501,6 +1501,7 @@
 	thflags = th->th_flags;
 	inc = &tp->t_inpcb->inp_inc;
 	tp->sackhint.last_sack_ack = 0;
+	sack_changed = 0;
 
 	/*
 	 * If this is either a state-changing packet or current state isn't
@@ -2424,7 +2425,7 @@
 		if ((tp->t_flags & TF_SACK_PERMIT) &&
 		    ((to.to_flags & TOF_SACK) ||
 		     !TAILQ_EMPTY(&tp->snd_holes)))
-			tcp_sack_doack(tp, &to, th->th_ack);
+			sack_changed = tcp_sack_doack(tp, &to, th->th_ack);
 		else
 			/*
 			 * Reset the value so that previous (valid) value
@@ -2436,7 +2437,9 @@
 		hhook_run_tcp_est_in(tp, th, &to);
 
 		if (SEQ_LEQ(th->th_ack, tp->snd_una)) {
-			if (tlen == 0 && tiwin == tp->snd_wnd) {
+			if (tlen == 0 &&
+			    (tiwin == tp->snd_wnd ||
+			    (tp->t_flags & TF_SACK_PERMIT))) {
 				/*
 				 * If this is the first time we've seen a
 				 * FIN from the remote, this is not a
@@ -2478,8 +2481,20 @@
 				 * When using TCP ECN, notify the peer that
 				 * we reduced the cwnd.
 				 */
-				if (!tcp_timer_active(tp, TT_REXMT) ||
-				    th->th_ack != tp->snd_una)
+				/*
+				 * Following 2 kinds of acks should not affect
+				 * dupack counting:
+				 * 1) Old acks
+				 * 2) Acks with SACK but without any new SACK
+				 * information in them. These could result from
+				 * any anomaly in the network like a switch
+				 * duplicating packets or a possible DoS attack.
+				 */
+				if (th->th_ack != tp->snd_una ||
+				    ((tp->t_flags & TF_SACK_PERMIT) &&
+				    !sack_changed))
+					break;
+				else if (!tcp_timer_active(tp, TT_REXMT))
 					tp->t_dupacks = 0;
 				else if (++tp->t_dupacks > tcprexmtthresh ||
 				     IN_FASTRECOVERY(tp->t_flags)) {
@@ -2608,9 +2623,20 @@
 					tp->snd_cwnd = oldcwnd;
 					goto drop;
 				}
-			} else
-				tp->t_dupacks = 0;
+			}
 			break;
+		} else {
+			/*
+			 * This ack is advancing the left edge, reset the
+			 * counter.
+			 */
+			tp->t_dupacks = 0;
+			/*
+			 * If this ack also has new SACK info, increment the
+			 * counter as per rfc6675.
+			 */
+			if ((tp->t_flags & TF_SACK_PERMIT) && sack_changed)
+				tp->t_dupacks++;
 		}
 
 		KASSERT(SEQ_GT(th->th_ack, tp->snd_una),
@@ -2629,7 +2655,6 @@
 			} else
 				cc_post_recovery(tp, th);
 		}
-		tp->t_dupacks = 0;
 		/*
 		 * If we reach this point, ACK is not a duplicate,
 		 *     i.e., it ACKs something we sent.
Index: head/sys/netinet/tcp_sack.c
===================================================================
--- head/sys/netinet/tcp_sack.c
+++ head/sys/netinet/tcp_sack.c
@@ -345,17 +345,22 @@
  * Process cumulative ACK and the TCP SACK option to update the scoreboard.
  * tp->snd_holes is an ordered list of holes (oldest to newest, in terms of
  * the sequence space).
+ * Returns 1 if incoming ACK has previously unknown SACK information,
+ * 0 otherwise. Note: We treat (snd_una, th_ack) as a sack block so any changes
+ * to that (i.e. left edge moving) would also be considered a change in SACK
+ * information which is slightly different than rfc6675.
  */
-void
+int
 tcp_sack_doack(struct tcpcb *tp, struct tcpopt *to, tcp_seq th_ack)
 {
 	struct sackhole *cur, *temp;
 	struct sackblk sack, sack_blocks[TCP_MAX_SACK + 1], *sblkp;
-	int i, j, num_sack_blks;
+	int i, j, num_sack_blks, sack_changed;
 
 	INP_WLOCK_ASSERT(tp->t_inpcb);
 
 	num_sack_blks = 0;
+	sack_changed = 0;
 	/*
 	 * If SND.UNA will be advanced by SEG.ACK, and if SACK holes exist,
 	 * treat [SND.UNA, SEG.ACK) as if it is a SACK block.
@@ -392,7 +397,7 @@
 	 * received.
 	 */
 	if (num_sack_blks == 0)
-		return;
+		return (sack_changed);
 
 	/*
 	 * Sort the SACK blocks so we can update the scoreboard with just one
@@ -443,6 +448,7 @@
 			tp->snd_fack = sblkp->end;
 			/* Go to the previous sack block. */
 			sblkp--;
+			sack_changed = 1;
 		} else {
 			/* 
 			 * We failed to add a new hole based on the current 
@@ -459,9 +465,11 @@
 			    SEQ_LT(tp->snd_fack, sblkp->end))
 				tp->snd_fack = sblkp->end;
 		}
-	} else if (SEQ_LT(tp->snd_fack, sblkp->end))
+	} else if (SEQ_LT(tp->snd_fack, sblkp->end)) {
 		/* fack is advanced. */
 		tp->snd_fack = sblkp->end;
+		sack_changed = 1;
+	}
 	/* We must have at least one SACK hole in scoreboard. */
 	KASSERT(!TAILQ_EMPTY(&tp->snd_holes),
 	    ("SACK scoreboard must not be empty"));
@@ -490,6 +498,7 @@
 		tp->sackhint.sack_bytes_rexmit -= (cur->rxmit - cur->start);
 		KASSERT(tp->sackhint.sack_bytes_rexmit >= 0,
 		    ("sackhint bytes rtx >= 0"));
+		sack_changed = 1;
 		if (SEQ_LEQ(sblkp->start, cur->start)) {
 			/* Data acks at least the beginning of hole. */
 			if (SEQ_GEQ(sblkp->end, cur->end)) {
@@ -545,6 +554,7 @@
 		else
 			sblkp--;
 	}
+	return (sack_changed);
 }
 
 /*
Index: head/sys/netinet/tcp_var.h
===================================================================
--- head/sys/netinet/tcp_var.h
+++ head/sys/netinet/tcp_var.h
@@ -741,7 +741,7 @@
 extern	struct pr_usrreqs tcp_usrreqs;
 tcp_seq tcp_new_isn(struct tcpcb *);
 
-void	 tcp_sack_doack(struct tcpcb *, struct tcpopt *, tcp_seq);
+int	 tcp_sack_doack(struct tcpcb *, struct tcpopt *, tcp_seq);
 void	 tcp_update_sack_list(struct tcpcb *tp, tcp_seq rcv_laststart, tcp_seq rcv_lastend);
 void	 tcp_clean_sackreport(struct tcpcb *tp);
 void	 tcp_sack_adjust(struct tcpcb *tp);