So this is an abstract view of a UDT instance. The timer is used to trigger various timeout events. In order to send user data to another UDT instance, a single UDP channel is set up. (Note that this channel only exists logically, as UDP is not connection-oriented.)
Sender A sends user data in UDP packets to Receiver B. Each UDP packet is assigned a unique sequence number and a timestamp. Receiver B will periodically feedback acknowledgments and loss reports as well.