diff -r 3774a7ad837d sys/netinet/tcp_input.c
--- a/sys/netinet/tcp_input.c	Fri Mar 18 17:27:05 2011 +1100
+++ b/sys/netinet/tcp_input.c	Mon Mar 21 08:58:28 2011 +1100
@@ -1564,7 +1564,9 @@
 			}
 		} else if (th->th_ack == tp->snd_una &&
 		    tlen <= sbspace(&so->so_rcv)) {
-			int newsize = 0;	/* automatic sockbuf scaling */
+			int newsize, nrxbytes, nrxmaxsegs;
+
+			newsize = 0; /* Automatic sockbuf scaling. */
 
 			/*
 			 * This is a pure, in-sequence data packet with
@@ -1584,17 +1586,12 @@
 			if ((tp->t_flags & TF_SACK_PERMIT) && tp->rcv_numsacks)
 				tcp_clean_sackreport(tp);
 			TCPSTAT_INC(tcps_preddat);
-			tp->rcv_nxt += tlen;
+
 			/*
 			 * Pull snd_wl1 up to prevent seq wrap relative to
 			 * th_seq.
 			 */
 			tp->snd_wl1 = th->th_seq;
-			/*
-			 * Pull rcv_up up to prevent seq wrap relative to
-			 * rcv_nxt.
-			 */
-			tp->rcv_up = tp->rcv_nxt;
 			TCPSTAT_INC(tcps_rcvpack);
 			TCPSTAT_ADD(tcps_rcvbyte, tlen);
 			ND6_HINT(tp);	/* Some progress has been made */
@@ -1675,12 +1672,37 @@
 			}
 			/* NB: sorwakeup_locked() does an implicit unlock. */
 			sorwakeup_locked(so);
-			if (DELAY_ACK(tp)) {
-				tp->t_flags |= TF_DELACK;
-			} else {
-				tp->t_flags |= TF_ACKNOW;
-				tcp_output(tp);
-			}
+
+			nrxbytes = tlen;
+			nrxmaxsegs = tlen / tp->t_maxseg;
+
+			/* For LRO, we need to send multiple ACKs. */
+			do {
+				if (nrxmaxsegs < 2) {
+					/* Regular segment. */
+					tp->rcv_nxt += nrxbytes;
+				} else {
+					/* LRO segment. */
+					tp->rcv_nxt += V_tcp_delack_enabled ?
+					    2 * tp->t_maxseg : tp->t_maxseg;
+					nrxbytes -= V_tcp_delack_enabled ?
+					    2 * tp->t_maxseg : tp->t_maxseg;
+				}
+
+				if (nrxmaxsegs < 2 && DELAY_ACK(tp)) {
+					tp->t_flags |= TF_DELACK;
+				} else {
+					tp->t_flags |= TF_ACKNOW;
+					tcp_output(tp);
+				}
+
+				nrxmaxsegs -= V_tcp_delack_enabled ? 2 : 1;
+
+			} while (nrxmaxsegs > 0);
+
+			/* Prevent seq wrap relative to rcv_nxt. */
+			tp->rcv_up = tp->rcv_nxt;
+
 			goto check_delack;
 		}
 	}