Delivery Confirmation, Retransmission, and Ordering

TCP and UDP are two ways to send packets. With TCP, the receiver sends back an ACK — an acknowledgment — for each packet it receives, and the sender retransmits any packet whose ACK doesn't arrive. This way, every packet eventually arrives in order, even if some are lost. UDP has no such delivery confirmation or retransmission — a lost packet simply stays missing, and ordering is not guaranteed either.