How do TTN handlers know which port and data format to use for a Downlink message?

Hello all,

I am interesting on sending a message back to my motes, so the downlink; I was wondering how the TTN handlers know what port to use in order to perform such as action, or if it is not the responsible for such an action who is and how does that.

TTN-Backend-Components.png1840x432

I am interesting on knowing it since if I have my own application (A) connected to the Handler, I would be interested to see the format in which I should push that data, wiki says that it is like below

{ "payload": "SGVsbG8gd29ybGQK",
  "port": 1,
  "ttl": "1h" }

But should I wrap it with something more? Like for instance the device DevEUI . Is it possible to do the same with the old TTN back-end?

Thanks in advance!

Regards!