diff --git a/sys/netinet/tcp_lro.h b/sys/netinet/tcp_lro.h
--- a/sys/netinet/tcp_lro.h
+++ b/sys/netinet/tcp_lro.h
@@ -146,7 +146,8 @@
 	uint16_t		compressed;
 	uint16_t		uncompressed;
 	uint16_t		window;
-	uint16_t		timestamp;	/* flag, not a TCP hdr field. */
+	uint16_t		timestamp : 1;
+	uint16_t		needs_merge : 1;
 	struct bintime		alloc_time;	/* time when entry was allocated */
 };
 
diff --git a/sys/netinet/tcp_lro.c b/sys/netinet/tcp_lro.c
--- a/sys/netinet/tcp_lro.c
+++ b/sys/netinet/tcp_lro.c
@@ -820,7 +820,7 @@
 tcp_flush_out_entry(struct lro_ctrl *lc, struct lro_entry *le)
 {
 	/* Check if we need to recompute any checksums. */
-	if (le->m_head->m_pkthdr.lro_nsegs > 1) {
+	if (le->needs_merge) {
 		uint16_t csum;
 
 		switch (le->inner.data.lro_type) {
@@ -921,6 +921,7 @@
 	le->next_seq = ntohl(th->th_seq) + tcp_data_len;
 	le->ack_seq = th->th_ack;
 	le->window = th->th_win;
+	le->needs_merge = 0;
 
 	/* Setup new data pointers. */
 	le->m_head = m;
@@ -962,10 +963,12 @@
 }
 
 static void
-tcp_lro_mbuf_append_pkthdr(struct mbuf *m, const struct mbuf *p)
+tcp_lro_mbuf_append_pkthdr(struct lro_entry *le, const struct mbuf *p)
 {
+	struct mbuf *m;
 	uint32_t csum;
 
+	m = le->m_head;
 	if (m->m_pkthdr.lro_nsegs == 1) {
 		/* Compute relative checksum. */
 		csum = p->m_pkthdr.lro_tcp_d_csum;
@@ -982,6 +985,7 @@
 	m->m_pkthdr.lro_tcp_d_csum = csum;
 	m->m_pkthdr.lro_tcp_d_len += p->m_pkthdr.lro_tcp_d_len;
 	m->m_pkthdr.lro_nsegs += p->m_pkthdr.lro_nsegs;
+	le->needs_merge = 1;
 }
 
 static void
@@ -1100,8 +1104,12 @@
 			le->next_seq += tcp_data_len;
 			le->ack_seq = th->th_ack;
 			le->window = th->th_win;
+			le->needs_merge = 1;
 		} else if (th->th_ack == le->ack_seq) {
-			le->window = WIN_MAX(le->window, th->th_win);
+			if (WIN_GT(th->th_win, le->window)) {
+				le->window = th->th_win;
+				le->needs_merge = 1;
+			}
 		}
 
 		if (tcp_data_len == 0) {
@@ -1110,7 +1118,7 @@
 		}
 
 		/* Merge TCP data checksum and length to head mbuf. */
-		tcp_lro_mbuf_append_pkthdr(le->m_head, m);
+		tcp_lro_mbuf_append_pkthdr(le, m);
 
 		/*
 		 * Adjust the mbuf so that m_data points to the first byte of