Downstream data packet

@ndarkness
I have made a NodeJS server using dgram to receive UDP packets from my gateway. These are then decrypted with some code from node-red https://github.com/willem4ever/node-red-ttn/blob/master/90-TTN.js. (and working fine so far)

The main difficulty I have now is figuring out how to exactly create the macframe
(MHDR | FHDR | PORT | Payload | MIC)

My txpk currently looks like this:

{
	"txpk": {
		"tmst": 1632335508,
		"freq": 869.525,
		"rfch": 0,
		"powe": 27,
		"modu": "LORA",
		"datr": "SF9BW125",
		"codr": "4/5",
		"ipol": true,
		"ncrc": true,
		"size": 39,
		"data": "YAAbevggAAADKrTC7sh6+lUVj4ONHthNPEPVfQ="
	}
}

While the one received from semtech looks like this (same payload string):

{
	"txpk": {
		"tmst": 1632335508,
		"freq": 869.525000,
		"rfch": 0,
		"powe": 27,
		"modu": "LORA",
		"datr": "SF9BW125",
		"codr": "4/5",
		"ipol": true,
		"ncrc": true,
		"size": 28,
		"data": "oPh6GwAgEQADuwrLrt0DrKc3rYizpLhWx2Blng"
	}
}

When using semtech, I am getting an mac_tx_ok and/or an mac_rx 3 . It is just really hard to find out what is going wrong with creating that data packet since there is a lot of encryption/base64 and hex going on