lwipopts: Default TCP_WND too small for large HTTPS downloads

[timg236]

On SDK develop I was deadlocks when downloading large (2MB) files via HTTPS. It seems that the default TCP WND is too small. The following patch seems to avoid the deadlock.

N.B The PBUF_POOL_SIZE might not be essential though

--- a/test/lwipopts.h
+++ b/test/lwipopts.h
@@ -8,7 +8,7 @@
 #define MEM_SIZE                    8000
 #define MEMP_NUM_TCP_SEG            32
 #define MEMP_NUM_ARP_QUEUE          10
-#define PBUF_POOL_SIZE              24
+#define PBUF_POOL_SIZE              28
 #define LWIP_ARP                    1
 #define LWIP_ETHERNET               1
 #define LWIP_ICMP                   1
@@ -80,9 +80,12 @@
 #define SLIP_DEBUG                  LWIP_DBG_OFF
 #define DHCP_DEBUG                  LWIP_DBG_OFF

-/* TCP WND must be at least 16 kb to match TLS record size
-   or you will get a warning "altcp_tls: TCP_WND is smaller than the RX decrypion buffer, connection RX might stall!" */
-#define TCP_WND  16384
+/* TCP_WND must be strictly greater than the maximum raw TLS record size.
+   A 16384-byte plaintext TLS record expands to ~16413 raw bytes (+ header/nonce/auth tag).
+   Setting TCP_WND == 16384 deadlocks: the server fills the window before mbedTLS can
+   complete the record, so altcp_recved is never called and rcv_wnd stays at 0.
+   Use 2x to allow one record in flight while the previous one is being acked. */
+#define TCP_WND  32768

 #define LWIP_ALTCP 1
 #define LWIP_ALTCP_TLS