Parse data

Another questions. My UDP-server gets related packed:
{"rxpk":[{"tmst":506197427,"time":"2015-12-07T08:23:29.136953Z","chan":1,"rfch":1,"freq":868.300000,"stat":1,"modu":"LORA","datr":"SF7BW125","codr":"4/5","lsnr":9.2,"rssi":-32,"size":18,"data":"QGGQkqkABgAUVyPRYbST/naD"}]}

1) I convert "data" in bytes;
2) Then, I use paragraph 4.3.3.1 from LoRa specification 1R0.

1. "data" is FRMPayload from LoRa specification?

2. How i can get TPort number, to choose Nwkskey or Appskey? I understand, that TPort contained MACPayload, but i don't understand, how can I parse this frame?.

3. In this formula:k =ceil(len(pld) / 16), len is "size" in "rxpk" ?