udp

UDP, or User Datagram Protocol, is a transport layer protocol in the Internet protocol suite. It provides a simple, connectionless, best‑effort service for delivering datagrams between hosts. UDP emphasizes low latency and minimal overhead, at the expense of reliability and ordering. It is commonly used for time‑sensitive or lossy data transmissions.

UDP operates above IP and does not establish a connection before sending data. It preserves message boundaries; each send yields a separate datagram that is delivered intact or not at all. The UDP header is 8 bytes and contains four 16‑bit fields: source port, destination port, length, and checksum. The length field covers the header and data; the checksum protects header and payload using a pseudo‑header derived from IP addresses and protocol. In IPv4, the checksum is optional; in IPv6 it is mandatory.

The maximum UDP datagram size is 65,535 bytes, including the 8‑byte header, yielding a maximum payload of 65,527 bytes. In practice, the IP MTU limits datagram size, and large datagrams may be fragmented at the IP layer; loss of any fragment causes the whole datagram to be discarded.

Common applications include DNS, DHCP, SNMP, TFTP, and media transport with RTP or streaming protocols. UDP also supports multicast and broadcast. Many newer protocols, such as QUIC, run over UDP to combine low overhead with reliability implemented at the application layer.

Compared with TCP, UDP provides no guaranteed delivery, ordering, or congestion control. Its simplicity and speed make it suitable for real-time applications and services that tolerate some loss, where lower latency is prioritized over perfect accuracy.