If rfu 3 bit of MAC header is set to 110

ChirpStack (v4)
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hello. I am currently using chirpstack v4, device lora wan 1.0.3 version. Set 3 bits to 110 in the rfu field in the MAC header. Send unconfirmed date up message from device in ABP mode. However, I get a mic error. I don’t know why the mic error occurs.

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Device registration was correct, and normal operation was confirmed when 3 bits were set to 000 in the rfu field.

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This is probably because one side does include the RFU bits in the MIC calculation, and the other side does not, resulting in two different MICs.

See also the specification:

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