UDP
termnetworking
Stands for: User Datagram Protocol
The minimal transport that wraps a packet with just ports and a checksum and sends it with no handshake, no ordering, and no retransmission: fire and forget.
Jon Postel defined UDP in RFC 768 on August 28, 1980 as the thinnest possible layer over IP, an eight-byte header carrying source and destination ports, a length, and an optional checksum. It guarantees nothing about delivery or duplicates, which is exactly why latency-sensitive and query-response traffic prefers it: its first users were the internet name service and trivial file transfer, and its descendants carry DNS, DHCP, VoIP, gaming, and the QUIC transport under modern HTTP. Where TCP is a phone call, UDP is a postcard.
UDP is the protocol that does almost nothing, and that is precisely its value. It adds ports and a checksum to IP and stops there: no handshake, no ordering, no retransmission, no congestion control. Anything an application needs beyond delivery-if-possible, it must build itself.
That minimalism is why the latency-sensitive world lives on it. Voice and video prefer a lost packet over a late one, since a retransmitted audio sample arrives after the moment it belonged to. DNS uses it because a query and answer fit in one exchange and a connection handshake would double the cost. QUIC, and therefore HTTP/3, runs over UDP for a subtler reason: building reliability in userspace escapes the ossification of middleboxes that have opinions about TCP.
The dangerous property is that UDP is trivially spoofable and often amplifying. A small forged query to an open resolver or NTP server can produce a large response aimed at a victim, which is the mechanism behind the biggest reflection attacks on record. That is why source address validation at network edges matters, and why any UDP service exposed to the internet needs response rate limiting as a matter of course.
Also known as: User Datagram Protocol