The same as tcp, accept tcp connection, then wait udp connection. After get udp address from udp register message, the ut connection was complete here.
Ut Connection receive message is different from normal tcp/udp. It's receive tcp and udp message both. So there is very very important on delay time. We use delay time to differentiate between tcp and udp message.
If you just use, don't care this step. If you want to know more details about ut, you have to read close function source code