Index: head/sys/netinet/tcp_input.c
===================================================================
--- head/sys/netinet/tcp_input.c
+++ head/sys/netinet/tcp_input.c
@@ -2264,7 +2264,8 @@
 		 * DSACK - add SACK block for dropped range
 		 */
 		if (tp->t_flags & TF_SACK_PERMIT) {
-			tcp_update_sack_list(tp, th->th_seq, th->th_seq+tlen);
+			tcp_update_sack_list(tp, th->th_seq,
+			    th->th_seq + todrop);
 			/*
 			 * ACK now, as the next in-sequence segment
 			 * will clear the DSACK block again
@@ -3051,21 +3052,29 @@
 				 * DSACK actually handled in the fastpath
 				 * above.
 				 */
-				tcp_update_sack_list(tp, save_start, save_start + save_tlen);
-			} else
-			if ((tlen > 0) && SEQ_GT(tp->rcv_nxt, save_rnxt)) {
+				tcp_update_sack_list(tp, save_start,
+				    save_start + save_tlen);
+			} else if ((tlen > 0) && SEQ_GT(tp->rcv_nxt, save_rnxt)) {
 				/*
 				 * Cleaning sackblks by using zero length
 				 * update.
 				 */
-				tcp_update_sack_list(tp, save_start, save_start);
-			} else
-			if ((tlen > 0) && (tlen >= save_tlen)) {
+				if ((tp->rcv_numsacks >= 1) &&
+				    (tp->sackblks[0].end == save_start)) {
+					/* partial overlap, recorded at todrop above */
+					tcp_update_sack_list(tp, tp->sackblks[0].start,
+					    tp->sackblks[0].end);
+				} else {
+					tcp_update_dsack_list(tp, save_start,
+					    save_start + save_tlen);
+				}
+			} else if ((tlen > 0) && (tlen >= save_tlen)) {
 				/* Update of sackblks. */
-				tcp_update_sack_list(tp, save_start, save_start + save_tlen);
-			} else
-			if (tlen > 0) {
-				tcp_update_sack_list(tp, save_start, save_start+tlen);
+				tcp_update_dsack_list(tp, save_start,
+				    save_start + save_tlen);
+			} else if (tlen > 0) {
+				tcp_update_dsack_list(tp, save_start,
+				    save_start + tlen);
 			}
 		}
 #if 0