gnrc_lorawan: downlink data might not be received when using ABP.

[jia200x]

Description

There are some scenarios where the LoRaWAN Network Server will try to negotiate RX configuration for ABP nodes using Fopts.
GNRC LoRaWAN only implements a minimal sets of Fopts, so it will simply ignore some of this Fopts.

Some Network Servers such as TTNv3 or the newer version of Chirpstack will keep sending these Fopts requests until the node replies, delaying the scheduled downlink forever. Therefore, an application would never receive any downlink data under these circumstances.

This issue DOES NOT affect OTAA nodes. (and in fact, OTAA should be always preferred regardless of the issue due to security reasons.

The only solution would be to implement the following Fopts:

• RXParamSetupReq
• DevStatusReq
• NewChannnelReq
• RXTimingSetup

Exptected results

A node running examples/gnrc_lorawan should be able to receive DL frames when joining with ABP (using TTNv3 or the newest Chirpstack)

Actual results

The node will never receive a downlink under the circumstances.

[stale]

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