Cellular: Fix to prefer IPv6 single stack with fallback to IPv4

Author: Ari Parkkila

features/cellular/framework/AT/AT_CellularNetwork.cpp

@@ -459,7 +459,7 @@ bool AT_CellularNetwork::set_new_context(int cid)
             strncpy(pdp_type, "IPV6", sizeof(pdp_type));
             break;
         case IPV4V6_STACK:
-            strncpy(pdp_type, "IPV4V6", sizeof(pdp_type));
+            strncpy(pdp_type, "IPV6", sizeof(pdp_type)); // try first IPV6 and then fall-back to IPv4
             break;
         default:
             break;

features/cellular/framework/AT/AT_CellularStack.cpp

@@ -81,6 +81,11 @@ const char * AT_CellularStack::get_ip_address()
     return _ip;
 }
 
+nsapi_error_t AT_CellularStack::socket_stack_init()
+{
+    return NSAPI_ERROR_OK;
+}
+
 nsapi_error_t AT_CellularStack::socket_open(nsapi_socket_t *handle, nsapi_protocol_t proto)
 {
     if (!is_protocol_supported(proto) || !handle) {
@@ -90,6 +95,10 @@ nsapi_error_t AT_CellularStack::socket_open(nsapi_socket_t *handle, nsapi_protoc
     int max_socket_count = get_max_socket_count();
 
     if (!_socket) {
+        if (socket_stack_init() != NSAPI_ERROR_OK) {
+            return NSAPI_ERROR_NO_SOCKET;
+        }
+
         _socket = new CellularSocket*[max_socket_count];
         if (!_socket) {
             tr_error("No memory to open socket!");

features/cellular/framework/AT/AT_CellularStack.h

@@ -46,6 +46,13 @@ class AT_CellularStack : public NetworkStack, public AT_CellularBase
     virtual const char *get_ip_address();
 protected: // NetworkStack
 
+    /**
+     * Modem specific socket stack initialization
+     *
+     *  @return 0 on success
+     */
+    virtual nsapi_error_t socket_stack_init();
+
     virtual nsapi_error_t socket_open(nsapi_socket_t *handle, nsapi_protocol_t proto);
 
     virtual nsapi_error_t socket_close(nsapi_socket_t handle);
@@ -87,8 +94,10 @@ class AT_CellularStack : public NetworkStack, public AT_CellularBase
         SocketAddress localAddress;
         void (*_cb)(void *);
         void *_data;
-        bool created;
-        bool rx_avail; // used to synchronize reading from modem
+        bool created; // socket has been created on modem stack
+        bool started; // socket has been opened on modem stack
+        bool tx_ready; // socket is ready for sending on modem stack
+        bool rx_avail; // socket has data for reading on modem stack
     };
 
     /**