mesh.checkConnection via parent not master

- Adjust mesh.checkConnection function to verify connectivity with parent only. Leave old functionality in place with a #define
- Suggestions in place per @2bndy5
closes #249

RF24Mesh.cpp

@@ -154,7 +154,24 @@ void ESBMesh<network_t, radio_t>::setChild(bool allow)
 template<class network_t, class radio_t>
 bool ESBMesh<network_t, radio_t>::checkConnection()
 {
-    // getAddress() doesn't use auto-ack; do a double-check to manually retry 1 more time
+
+    if (!nodeID) return false;
+    if (mesh_address == MESH_DEFAULT_ADDRESS) return false;
+
+// Connection check via parent node
+#if RF24MESH_CONN_CHECK_TYPE == RF24MESH_CONN_CHECK_PARENT
+    RF24NetworkHeader header;
+    header.to_node = network.parent();
+    header.type = NETWORK_PING;
+    for (uint8_t i = 0; i < MESH_CONNECTION_CHECK_ATTEMPTS; i++) {
+        if (network.write(header, 0, 0)) {
+            return true;
+        }
+    }
+    return false;
+
+#else // Connection check via master node
+    // getAddress() doesn't use auto-ack; check connectivity multiple times
     for (uint8_t i = 0; i < MESH_CONNECTION_CHECK_ATTEMPTS; i++) {
 
         int16_t result = getAddress(_nodeID);
@@ -170,6 +187,7 @@ bool ESBMesh<network_t, radio_t>::checkConnection()
         }
     }
     return false;
+#endif
 }
 
 /*****************************************************/

RF24Mesh_config.h

@@ -64,6 +64,23 @@
     #define MESH_CONNECTION_CHECK_ATTEMPTS 3
 #endif
 
+/**
+ * @brief How to verify a connection
+ *
+ * On child nodes, determine how they verify/check their connection periodically, or when writes start to fail via the `mesh.checkConnection();`
+ * function.
+ * Set RF24MESH_CONN_CHECK_TYPE to RF24MESH_CONN_CHECK_PARENT for the new behaviour of verifying connectivity only with their parent node
+ * Set RF24MESH_CONN_CHECK_TYPE to RF24MESH_CONN_CHECK_MASTER for the old behaviour of verifying connectivity with the master node
+ * The old behaviour typically results in more network congestion, more load on the master node, and less reliable networks,
+ * but can work well when radio conditions are good and/or when there are only a small number of nodes on the network and/or in close proximity
+ * to the master node.
+ */
+#define RF24MESH_CONN_CHECK_PARENT 1
+#define RF24MESH_CONN_CHECK_MASTER 0
+#define RF24MESH_CONN_CHECK_TYPE   RF24MESH_CONN_CHECK_PARENT
+// To use old behavior:
+// #define RF24MESH_CONN_CHECK_TYPE RF24MESH_CONN_CHECK_MASTER
+
 /**************************/
 /***       Debug        ***/
 //#define RF24MESH_DEBUG_MINIMAL /** Uncomment for the Master Node to print out address assignments as they are assigned */