Let's review the functionalities first.
The reliability control is provided by sequencing and acknowledgment. Each UDT packet is assigned a unique increasing sequence number. The receiver will send back acknowledgments and loss reports according to packet arrival. Lost packets will be retransmitted.
The streaming service is implemented in buffer management.
UDT also has a series of mechanisms to set up, maintain, and tear down a UDT connection.
Finally, each UDT instance has both a sender and receiver, in order to provide duplex data transfer service.