LoRaWAN: Downlinks Fail After Disconnecting and Connecting Again

[mattbrown015]

Description

Here's what I've just tried:

1. Connect to the TTN using OTAA.
2. Watch some successful uplinks and downlinks.
3. Call LoRaWANInterface::disconnect().
4. Wait a minute.
5. Call LoRaWANInterface::connect(), receiving the join-accept message.
6. Watch some successful uplinks.
7. Do something that requires a downlink, for example add a link check request.
8. Downlinks fail with [ERR ][LMAC]: MIC failed.

My feeling is that there is a problem with downlink_counter; while processing the first downlink it has the value of 65537.

I also tried disconnecting and then connecting again to a different TTN app, the behaviour was the same.

The documentation has a great deal to say in **NOTES ON RECONNECTION:** but I'm not sure what any of it means for me. Just calling 'disconnect' and 'connect` seems like the obvious thing to do.

SHA 2a824a1

Issue request type

[ ] Question
[ ] Enhancement
[X] Bug

[mattbrown015]

I added a call to reset_mac_parameters() to LoRaMac::prepare_join and this meant the link check response was received.

The important detail here is that I'm specifying the connection parameters rather than relying on the ones defined in mbed_app.json. In other words, I'm running the top part of LoRaMac::prepare_join.

diff --git a/features/lorawan/lorastack/mac/LoRaMac.cpp b/features/lorawan/lorastack/mac/LoRaMac.cpp
index 833f7ccc7..a17a175d3 100644
--- a/features/lorawan/lorastack/mac/LoRaMac.cpp
+++ b/features/lorawan/lorastack/mac/LoRaMac.cpp
@@ -1419,6 +1419,8 @@ 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();
         } else {
             if ((params->connection_u.abp.dev_addr == 0)
                     || (params->connection_u.abp.nwk_id == 0)

[ciarmcom]

ARM Internal Ref: MBOTRIAGE-1510

[kivaisan]

@mattbrown015 Good finding. I'll prepare a fix for this.