Lora network server sending downlink message payload greater than 11 bytes

[Charles Oram]

I am running the Lora network server 1.0.41-r1.0 on a Conduit AEP gateway with firmware version 1.4.3 and the Conduit is configured for the AS923 Frequency plan.
When I enable confirmed messages for my Lora nodes and configure the Conduit with “Dwell time up”=1(400ms), “Dwell time down”=1(400ms) and “Additional channels”=922.6 then the Conduit sends a message to the node with 2 NewChannelReq MAC commands and one TxParamSetupReq MAC command in it (after the node has joined). This gives a payload length of 14 bytes, which is larger than the maximum that should be sent when the Lora node is not yet configured (11 bytes, which is 8 bytes less than the maximum payload size given in table 53 and 54 in section 2.7.6 “AS923 Maximum payload size” in the “LoRaWAN Regional Parameters v1.02.rB). This causes the node to reject the message.
I’m not sure why the node checks this, because I don’t think it is the node’s responsibility to ensure that the network server sticks to the Lora specs, but this probably needs to be fixed in the Lora network server since most Lora nodes will be based on the Semtech stack which does this check.
I think that, in theory, the message being sent by the Conduit may be OK, because the message has FOptsLen=0 so can really include up to 19 bytes, and maybe this problem should be fixed in the node firmware by getting it to consider FOptsLen when checking the payload length. Or, just take the check out.
This causes a problem with both an xDot (using mbed5) and with an ST B-L072Z-LRWAN1.

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