The key distinction between TCP and UDP is reliability. The reliability of TCP communication is obtained through the use of connection-oriented sessions. Before a host using TCP sends data to another host, TCP initiates a process to create a connection with the destination. This stateful connection enables the tracking of a session, or communication stream between the hosts. This process ensures that each host is aware of and prepared for the communication stream. A TCP conversation requires the establishment of a session between the hosts in both directions, as shown in the animation.
After a session has been established, and data transfer begins, the destination sends acknowledgements to the source for the segments that it receives. These acknowledgements form the basis of reliability within the TCP session. When the source receives an acknowledgement, it knows that the data has been successfully delivered and can quit tracking that data. If the source does not receive an acknowledgement within a predetermined amount of time, it retransmits that data to the destination.
Part of the additional overhead of using TCP is the network traffic generated by acknowledgements and retransmissions. The establishment of sessions creates overhead in the form of additional segments being exchanged. There is also additional overhead on the individual hosts created by the necessity to keep track of which segments are awaiting acknowledgement and by the retransmission process.