feat(lwip): Add kconfig to congigure initial RTO time and set default value to 1000

components/lwip/Kconfig

@@ -479,6 +479,13 @@ config LWIP_TCP_TIMESTAMPS
         really used locally. Therefore, it is only enabled when a TS option is
         received in the initial SYN packet from a remote host.
 
+config LWIP_TCP_RTO_TIME
+    int "Default TCP RTO TIME"
+    default 1000
+    help
+        Set default TCP RTO time for a reasonable initial RTO.
+        Advise to set initial RTO to 1 second
+
 endmenu # TCP
 
 menu "UDP"

components/lwip/lwip/src/core/tcp.c

@@ -1630,8 +1630,9 @@ tcp_alloc(u8_t prio)
     /* As initial send MSS, we use TCP_MSS but limit it to 536.
        The send MSS is updated when an MSS option is received. */
     pcb->mss = INITIAL_MSS;
-    pcb->rto = 3000 / TCP_SLOW_INTERVAL;
+    /* make TCP's retransmission time to be configurable */
-    pcb->sv = 3000 / TCP_SLOW_INTERVAL;
+    pcb->rto = LWIP_TCP_RTO_TIME / TCP_SLOW_INTERVAL;
+    pcb->sv = LWIP_TCP_RTO_TIME / TCP_SLOW_INTERVAL;
     pcb->rtime = -1;
     pcb->cwnd = 1;
     pcb->tmr = tcp_ticks;

components/lwip/lwip/src/include/lwip/opt.h

@@ -1350,6 +1350,11 @@
 #define LWIP_WND_SCALE                  0
 #define TCP_RCV_SCALE                   0
 #endif
+
+#if !defined LWIP_TCP_RTO_TIME || defined __DOXYGEN__
+#define LWIP_TCP_RTO_TIME             3000
+#endif
+
 /**
  * @}
  */

components/lwip/port/esp8266/include/lwipopts.h

@@ -1097,6 +1097,12 @@ size_t memp_malloc_get_size(size_t type);
  */
 #define LWIP_WND_SCALE                  0
 #define TCP_RCV_SCALE                   0
+
+/**
+ * LWIP_TCP_RTO_TIME: make TCP RTO time configurable.
+ * Default is 1 second.
+ */
+#define LWIP_TCP_RTO_TIME             CONFIG_LWIP_TCP_RTO_TIME
 /**
  * @}
  */