net/tcp: add kconfig to support retransmission at a fixed time

the maximum retransmission interval allowed for car projects
cannot exceed 6 seconds, and allows for fixed retransmission intervals.
according to the previous algorithm, the retransmission interval
may be 0.5 * 2 ^ 4 = 8 seconds, which does not meet the requirements.

Signed-off-by: zhanghongyu <zhanghongyu@xiaomi.com>

net/tcp/Kconfig

@@ -105,7 +105,21 @@ config NET_TCP_RTO
 	int "RTO of TCP/IP connections"
 	default 3
 	---help---
-		RTO of TCP/IP connections (all tasks)
+		Default retransmission timeout (RTO) of TCP/IP connections (all tasks).
+		In units of half seconds. When the same data packet is retransmitted
+		multiple times due to timeout, the RTO will be doubled. but the maximum
+		RTO is NET_TCP_RTO * 16. When this packet is ACKed, the RTO will be
+		reset to this value. This algorithm helps to reduce network congestion
+		to some extent.
+
+config NET_TCP_FIXED_RTO
+	bool "Use fixed RTO"
+	default n
+	---help---
+		Use fixed RTO for TCP/IP connections (all tasks), i.e. do not use
+		the RTO exponentially increasing algorithm. This is useful for projects
+		that require a fixed RTO value or have restrictions on the maximum
+		retransmission time.
 
 config NET_TCP_MAXRTX
 	int "Maximum retransmitted number of TCP/IP data packet"

net/tcp/tcp_input.c

@@ -1082,6 +1082,7 @@ found:
         }
 #endif
 
+#ifndef CONFIG_NET_TCP_FIXED_RTO
       /* Do RTT estimation, unless we have done retransmissions. */
 
       if (conn->nrtx == 0)
@@ -1102,6 +1103,7 @@ found:
           conn->sv += m;
           conn->rto = (conn->sa >> 3) + conn->sv;
         }
+#endif
 
       /* Set the acknowledged flag. */
 

net/tcp/tcp_timer.c

@@ -579,7 +579,9 @@ void tcp_timer(FAR struct net_driver_s *dev, FAR struct tcp_conn_s *conn)
 
               /* Exponential backoff. */
 
+#ifndef CONFIG_NET_TCP_FIXED_RTO
               conn->rto = TCP_RTO << (conn->nrtx > 4 ? 4: conn->nrtx);
+#endif
               tcp_update_retrantimer(conn, conn->rto);
               conn->nrtx++;