net: tcp: Add exponential backoff for ZWP probing

Persist timer should implement exponential backoff, as per RFC 1122:

    The transmitting host SHOULD send the first zero-window
    probe when a zero window has existed for the retransmission
    timeout period (see Section 4.2.2.15), and SHOULD increase
    exponentially the interval between successive probes.

Implement this, by following Linux behaviour, and simply double the
timeout or each probe transmission.

Additionally, prevent reseting the persist timer in case an
acknowledgment is received with zero window size, and the timer is
already running.

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

subsys/net/ip/tcp.c

@@ -30,6 +30,7 @@ LOG_MODULE_REGISTER(net_tcp, CONFIG_NET_TCP_LOG_LEVEL);
 #define ACK_TIMEOUT K_MSEC(ACK_TIMEOUT_MS)
 #define FIN_TIMEOUT K_MSEC(tcp_fin_timeout_ms)
 #define ACK_DELAY K_MSEC(100)
+#define ZWP_MAX_DELAY_MS 120000
 
 static int tcp_rto = CONFIG_NET_TCP_INIT_RETRANSMISSION_TIMEOUT;
 static int tcp_retries = CONFIG_NET_TCP_RETRY_COUNT;
@@ -1346,8 +1347,20 @@ static void tcp_send_zwp(struct k_work *work)
 	tcp_derive_rto(conn);
 
 	if (conn->send_win == 0) {
+		uint64_t timeout;
+
+		/* Make sure the retry counter does not overflow. */
+		if (conn->zwp_retries < UINT8_MAX) {
+			conn->zwp_retries++;
+		}
+
+		timeout = TCP_RTO_MS << conn->zwp_retries;
+		if (timeout == 0 || timeout > ZWP_MAX_DELAY_MS) {
+			timeout = ZWP_MAX_DELAY_MS;
+		}
+
 		(void)k_work_reschedule_for_queue(
-			&tcp_work_q, &conn->persist_timer, K_MSEC(TCP_RTO_MS));
+			&tcp_work_q, &conn->persist_timer, K_MSEC(timeout));
 	}
 
 	k_mutex_unlock(&conn->lock);
@@ -2008,8 +2021,12 @@ static enum net_verdict tcp_in(struct tcp *conn, struct net_pkt *pkt)
 		}
 
 		if (conn->send_win == 0) {
-			(void)k_work_reschedule_for_queue(
-				&tcp_work_q, &conn->persist_timer, K_MSEC(TCP_RTO_MS));
+			if (!k_work_delayable_is_pending(&conn->persist_timer)) {
+				conn->zwp_retries = 0;
+				(void)k_work_reschedule_for_queue(
+					&tcp_work_q, &conn->persist_timer,
+					K_MSEC(TCP_RTO_MS));
+			}
 		} else {
 			(void)k_work_cancel_delayable(&conn->persist_timer);
 		}

subsys/net/ip/tcp_private.h

@@ -273,6 +273,7 @@ struct tcp { /* TCP connection */
 	uint16_t rto;
 #endif
 	uint8_t send_data_retries;
+	uint8_t zwp_retries;
 	bool in_retransmission : 1;
 	bool in_connect : 1;
 	bool in_close : 1;