I had been trying to find that again! It was instrumental in some RE/VR I did last year on tmp and the differences between the UDP socket (available without auth) and the TCP socket. Thanks for making that.

I can't remember the details of the scheme, but it also allows you to authenticate using your TPLink cloud credential. If my memory is correct, the username is md5(tplink_account_email) and the password is the cloud account password. If you care, I can find my notes on that to confirm.