diff -Nur lwip-2.1.3-org/src/core/init.c lwip-2.1.3/src/core/init.c
--- lwip-2.1.3-org/src/core/init.c	2022-12-06 14:40:45.280000000 +0000
+++ lwip-2.1.3/src/core/init.c	2022-12-06 14:41:01.452000000 +0000
@@ -306,7 +306,7 @@
 #if TCP_SNDLOWAT >= TCP_SND_BUF
 #error "lwip_sanity_check: WARNING: TCP_SNDLOWAT must be less than TCP_SND_BUF. If you know what you are doing, define LWIP_DISABLE_TCP_SANITY_CHECKS to 1 to disable this error."
 #endif
-#if TCP_SNDLOWAT >= (0xFFFF - (4 * TCP_MSS))
+#if TCP_SNDLOWAT >= (0xFFFFFFFF - (4 * TCP_MSS))
 #error "lwip_sanity_check: WARNING: TCP_SNDLOWAT must at least be 4*MSS below u16_t overflow!"
 #endif
 #if TCP_SNDQUEUELOWAT >= TCP_SND_QUEUELEN
diff -Nur lwip-2.1.3-org/src/core/pbuf.c lwip-2.1.3/src/core/pbuf.c
--- lwip-2.1.3-org/src/core/pbuf.c	2022-12-06 14:40:45.280000000 +0000
+++ lwip-2.1.3/src/core/pbuf.c	2022-12-06 14:46:04.860000000 +0000
@@ -869,13 +869,13 @@
   /* proceed to last pbuf of chain */
   for (p = h; p->next != NULL; p = p->next) {
     /* add total length of second chain to all totals of first chain */
-    p->tot_len = (u16_t)(p->tot_len + t->tot_len);
+    p->tot_len = p->tot_len + t->tot_len;
   }
   /* { p is last pbuf of first h chain, p->next == NULL } */
   LWIP_ASSERT("p->tot_len == p->len (of last pbuf in chain)", p->tot_len == p->len);
   LWIP_ASSERT("p->next == NULL", p->next == NULL);
   /* add total length of second chain to last pbuf total of first chain */
-  p->tot_len = (u16_t)(p->tot_len + t->tot_len);
+  p->tot_len = p->tot_len + t->tot_len;
   /* chain last pbuf of head (p) with first of tail (t) */
   p->next = t;
   /* p->next now references t, but the caller will drop its reference to t,
@@ -1181,7 +1181,7 @@
     if (r != NULL) {
       /* Update the tot_len field in the first part */
       for (i = p; i != NULL; i = i->next) {
-        i->tot_len = (u16_t)(i->tot_len - r->tot_len);
+        i->tot_len = tot_len_front;
         LWIP_ASSERT("tot_len/len mismatch in last pbuf",
                     (i->next != NULL) || (i->tot_len == i->len));
       }
@@ -1192,6 +1192,9 @@
       /* tot_len field in rest does not need modifications */
       /* reference counters do not need modifications */
       *rest = r;
+      r->tot_len = r->len;
+    }else{
+      p->tot_len = tot_len_front;
     }
   }
 }
diff -Nur lwip-2.1.3-org/src/include/lwip/opt.h lwip-2.1.3/src/include/lwip/opt.h
--- lwip-2.1.3-org/src/include/lwip/opt.h	2022-12-06 14:40:45.292000000 +0000
+++ lwip-2.1.3/src/include/lwip/opt.h	2022-12-06 14:41:01.456000000 +0000
@@ -1482,7 +1482,7 @@
  * send window while having a small receive window only.
  */
 #if !defined LWIP_WND_SCALE || defined __DOXYGEN__
-#define LWIP_WND_SCALE                  0
+#define LWIP_WND_SCALE                  1
 #define TCP_RCV_SCALE                   0
 #endif
 
diff -Nur lwip-2.1.3-org/src/include/lwip/pbuf.h lwip-2.1.3/src/include/lwip/pbuf.h
--- lwip-2.1.3-org/src/include/lwip/pbuf.h	2022-12-06 14:40:45.284000000 +0000
+++ lwip-2.1.3/src/include/lwip/pbuf.h	2022-12-06 14:46:36.720000000 +0000
@@ -197,7 +197,7 @@
    * For non-queue packet chains this is the invariant:
    * p->tot_len == p->len + (p->next? p->next->tot_len: 0)
    */
-  u16_t tot_len;
+  u32_t tot_len;
 
   /** length of this buffer */
   u16_t len;
diff -Nur lwip-2.1.3-org/src/include/lwipopts.h lwip-2.1.3/src/include/lwipopts.h
--- lwip-2.1.3-org/src/include/lwipopts.h	2022-12-06 14:40:45.292000000 +0000
+++ lwip-2.1.3/src/include/lwipopts.h	2022-12-06 14:41:01.456000000 +0000
@@ -97,7 +97,7 @@
 
 #define TCP_WND (40 * TCP_MSS)
 
-#define TCP_SND_BUF (40 * TCP_MSS)
+#define TCP_SND_BUF (2500 * TCP_MSS)
 
 #define TCP_SND_QUEUELEN (8191)