RPL dao dynamic timeout

DAO lifetime will dynamic when Imin define 64seconds delay.
Smaller values will use 40 seconds value.
MAX wait delay is cutted to 14.5min based on 540 seconds.

source/RPL/rpl_downward.c

@@ -885,9 +885,24 @@ void rpl_instance_send_dao_update(rpl_instance_t *instance)
     instance->requested_dao_ack = need_ack;
     instance->dao_in_transit = true;
     if (instance->dao_attempt < 16) {
-        uint32_t t = (uint32_t) rpl_policy_initial_dao_ack_wait(instance->domain, mop) << instance->dao_attempt;
-        t = randLIB_randomise_base(t, 0x4000, 0xC000);
-        instance->dao_retry_timer = t <= 0xffff ? t : 0xffff;
+
+        uint32_t delay = rpl_policy_initial_dao_ack_wait(instance->domain, mop);
+        if (delay < dodag->dio_timer_params.Imin) {
+            //Use Imin Based on delay if it is longer than cache retry
+            delay = dodag->dio_timer_params.Imin;
+        }
+        //Multiply Delay by attempt
+        delay = delay << instance->dao_attempt;
+
+        if (delay > 5400) {
+            //MAX base 540 seconds (9min)
+            delay = 5400;
+        }
+
+        // 0.5 - 1.5 *t randomized base delay
+        delay = randLIB_randomise_base(delay, 0x4000, 0xC000);
+
+        instance->dao_retry_timer = delay;
     } else {
         instance->dao_retry_timer = 0xffff;
     }