Should retransmissions (nbTrans > 1) increment FCnt?

ChirpStack Network Server
1

Network server sends downlinks with macPayload > fOpts > payload > redundancy > nbRep set to 1…3, when ADR algorithm notices good link budget but high packet loss rate.

My lora sensor has a configurable parameter that controls if these retransmissions use the same FCnt or increment it.

I presume that only one of these settings is compatible with default ADR algorithm. If I increment FCnt, then loss rate will be more skewed by by the most recent transmission etc.

What is the recommended setting - should FCnt be incremented on retransmissions or not?

2

A retransmission by definition would be the same packet retransmitted with same fcnt and MIC.

If the FCNT increments how would the network differentiate a retransmission from a new packet.

Payloads of packets with same fcnt are ignored and payload is sent to the application server only once.

Incrementing the fcnt will send the same payload to the application server more than once.

The answer depends on the behavior you wish for recieved retrans.

3

This is true for LoRaWAN 1.0.x, but I believe the MIC might not be the same for LoRaWAN 1.1(.x). This is because the uplink channel (index) is part of the MIC calculation. A re-transmission on a different channel (as the device should channel-hop) will therefore result in a different MIC.

This might actually be a feature, because now the NS is able to distinguish a retransmission from a replay-attack :slight_smile:

4

Mmm, I guess I’ll reconfigure the sensors to not increement fcnt on retransmission.