The User Datagram Protocol is one of the two core transport protocols of the Internet, the counterpart to TCP. It was defined by Jon Postel of USC’s Information Sciences Institute in RFC 768, dated 28 August 1980, one of the shortest foundational standards in the entire RFC series at roughly three pages. The specification opens by describing UDP as a protocol that “provides a procedure for application programs to send messages to other programs with a minimum of protocol mechanism.”
UDP is deliberately spare. It adds only four fields to a message handed to IP: source port, destination port, length, and checksum. There is no connection setup, no acknowledgment of receipt, no retransmission of lost packets, no ordering of packets, and no flow or congestion control. A UDP datagram is sent and either arrives or does not. The protocol’s own text warns that “this protocol provides a procedure … where delivery and duplicate protection are not guaranteed.” The application, not the transport layer, is responsible for any reliability it needs.
That minimalism is precisely the point. The ports let many programs on one host share a single IP address, and the optional checksum guards against corruption, but the protocol otherwise stays out of the way. For applications where speed and low overhead matter more than guaranteed delivery, the round-trip cost of TCP’s three-way handshake and its retransmission machinery would be a liability rather than a benefit. UDP lets a single small request and response cross the network with no setup cost.
The Domain Name System is the archetypal example: a DNS query and its reply usually fit in one datagram each, and the resolver simply retries if no answer comes back. Real-time media, voice and video conferencing, and online games also favor UDP, because a late packet is useless and resending it would only add delay; dropping it and moving on is the right behavior. Network management protocols such as SNMP and bootstrap protocols like DHCP also ride on UDP.
UDP’s continued relevance is striking given its age. Decades after RFC 768, Google’s QUIC protocol, which became the basis for HTTP/3, was built on top of UDP precisely because UDP imposes no opinions of its own. By implementing connection management, reliability, and congestion control in user space above a bare UDP datagram, QUIC could evolve faster than protocols frozen into operating-system kernels. Postel’s three-page specification thus remains a live foundation for the newest layers of the Internet.