Retransmission back-off correction for re-join

Previously, we had been initializing our time base in
LoRaMac::initialize() and if the device was not power-cycled, that
initial time was always being used in the calculation of the elapsed
time. This introduced a bug mentioned in issue #8921. The bug made the
re-join attempt of a device after a week or so to use wrong back-off
because in calculate_backoff() API we pass the elapsed time. If we do
not set the initial time while trying to connect, the elapsed time will
include previous session time as well and the device will think it has
spend tha much time and will apply harsher duty cycle back off.

Author: Hasnain Virk

features/lorawan/lorastack/mac/LoRaMac.cpp

@@ -1229,6 +1229,7 @@ void LoRaMac::reset_mac_parameters(void)
 
     _params.sys_params.max_duty_cycle = 0;
     _params.sys_params.aggregated_duty_cycle = 1;
+    _params.timers.mac_init_time = get_current_time();
 
     _mac_commands.clear_command_buffer();
     _mac_commands.clear_repeat_buffer();
@@ -1448,6 +1449,8 @@ void LoRaMac::setup_link_check_request()
 
 lorawan_status_t LoRaMac::prepare_join(const lorawan_connect_t *params, bool is_otaa)
 {
+    reset_mac_parameters();
+
     if (params) {
         if (is_otaa) {
             if ((params->connection_u.otaa.dev_eui == NULL)
@@ -1468,8 +1471,6 @@ lorawan_status_t LoRaMac::prepare_join(const lorawan_connect_t *params, bool is_
             // Reset variable JoinRequestTrials
             _params.join_request_trial_counter = 0;
 
-            reset_mac_parameters();
-
             _params.sys_params.channel_data_rate =
                 _lora_phy->get_alternate_DR(_params.join_request_trial_counter + 1);
         } else {
@@ -1503,8 +1504,6 @@ lorawan_status_t LoRaMac::prepare_join(const lorawan_connect_t *params, bool is_
         // Reset variable JoinRequestTrials
         _params.join_request_trial_counter = 0;
 
-        reset_mac_parameters();
-
         _params.sys_params.channel_data_rate =
             _lora_phy->get_alternate_DR(_params.join_request_trial_counter + 1);
 
@@ -1812,8 +1811,6 @@ lorawan_status_t LoRaMac::initialize(EventQueue *queue,
     _lora_time.init(_params.timers.ack_timeout_timer,
                     mbed::callback(this, &LoRaMac::on_ack_timeout_timer_event));
 
-    _params.timers.mac_init_time = _lora_time.get_current_time();
-
     _params.sys_params.adr_on = MBED_CONF_LORA_ADR_ON;
     _params.sys_params.channel_data_rate = _lora_phy->get_default_max_tx_datarate();