UDP
also: User Datagram Protocol
A lightweight, connectionless transport protocol that sends packets fast with no delivery guarantees.
UDP (User Datagram Protocol) is the minimal transport protocol: it fires individual datagrams at a destination with no handshake, no ordering, no retransmission, and no congestion control. It trades reliability for speed and low overhead, leaving any guarantees the application needs to the application itself.
Worked example: live video calls, online games, and DNS lookups use UDP because a slightly dropped frame or a quick retry is better than waiting for TCP to redeliver stale data — a late packet is worse than a lost one. Gotcha: "no guarantees" does not mean "no ordering ever" — protocols built on UDP like QUIC (HTTP/3) reimplement reliability and ordering on top, keeping UDP's low overhead while adding back only the guarantees they actually want.