UDP is a connectionless transport protocol that sends datagrams without delivery guarantees, used by DNS, video streaming, and QUIC for low-latency communication.

What is UDP? Definition, examples and how it works

UDP (User Datagram Protocol) is a transport-layer protocol defined in RFC 768 (1980). It is connectionless and message-oriented, meaning data is transmitted as discrete packets called datagrams. Unlike TCP, UDP does not establish a session or provide reliable delivery, ordering, or duplicate protection. Its header is minimal, consisting only of source port, destination port, length, and an optional checksum (required in IPv6).

Applications using UDP must handle reliability themselves. The checksum is optional in IPv4 but mandatory in IPv6. Common services include DNS queries (port 53), DHCP (ports 67/68), SNMP, VoIP, and video streaming. Notably, QUIC, the transport for HTTP/3, is built on UDP. UDP also supports broadcast and multicast transmissions, which are not possible with TCP.

UDP sits between the network layer (IP) and the application layer in the TCP/IP stack. Its low overhead and connectionless nature make it ideal for real-time applications where speed is critical and occasional packet loss is acceptable. It offers no congestion control at the transport level, so applications must implement their own mechanisms if needed.

Key facts

Defined in RFC 768 (1980) by David P. Reed.
Header is only 8 bytes (no options).
No connection setup or teardown (no handshake).
Used by DNS, DHCP, SNMP, NTP, RTSP, and QUIC.
Supports broadcast (to all hosts) and multicast (to a group).

How it works in practice

A DNS client sends a query in a single UDP datagram to port 53. The server replies with another datagram containing the resolved IP. If the response is lost, the client retransmits after a timeout. No connection state is kept, making the exchange fast and lightweight.

Related terms

TCP IP QUIC DNS Datagram SCTP IPv6

References

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