UDP is not a reliable protocol. TCP is reliable protocol. - If you need to keep track of packets TCP is better because the recipient automatically acknowledges the sender when message is received. - TCP uses the 3 way handshake: SYN, SYN-ACK, ACK - IP packets can be lost or fail during retransmissions TCP detects these problems, requests retransmission of lost data, rearranges out-of-order data, and even helps minimize network congestion to reduce the occurrence of the other problems.