net: tcp: Delay ACK if no PSH flag is present

Delay an ACK in case no PSH flag is present in the data packet. as
described in RFC 813. This allows to reduce the number of ACK packets
we send and thus improve the TCP download throughput.

The results achieved on `nucleo_h723zg` board and the zperf sample
are as follows:
Before: 77.14 Mbps
After: 93.14 Mbps

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

Author: rlubos

subsys/net/ip/tcp.c

@@ -2695,7 +2695,7 @@ static void tcp_queue_recv_data(struct tcp *conn, struct net_pkt *pkt,
 }
 
 static enum net_verdict tcp_data_received(struct tcp *conn, struct net_pkt *pkt,
-					  size_t *len)
+					  size_t *len, bool psh)
 {
 	enum net_verdict ret;
 
@@ -2711,7 +2711,7 @@ static enum net_verdict tcp_data_received(struct tcp *conn, struct net_pkt *pkt,
 	/* Delay ACK response in case of small window or missing PSH,
 	 * as described in RFC 813.
 	 */
-	if (tcp_short_window(conn)) {
+	if (tcp_short_window(conn) || !psh) {
 		k_work_schedule_for_queue(&tcp_work_q, &conn->ack_timer,
 					  ACK_DELAY);
 	} else {
@@ -3252,7 +3252,9 @@ static enum net_verdict tcp_in(struct tcp *conn, struct net_pkt *pkt)
 		if (th) {
 			if (th_seq(th) == conn->ack) {
 				if (len > 0) {
-					verdict = tcp_data_received(conn, pkt, &len);
+					bool psh = FL(&fl, &, PSH);
+
+					verdict = tcp_data_received(conn, pkt, &len, psh);
 					if (verdict == NET_OK) {
 						/* net_pkt owned by the recv fifo now */
 						pkt = NULL;