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Networking Fundamentals · Foundational

User Datagram Protocol (UDP)

A connectionless Layer 4 protocol that sends datagrams with minimal overhead and no delivery, ordering or retransmission guarantees.

Definition

UDP (RFC 768) adds only source port, destination port, length and checksum in an 8-byte header. It has no handshake, acknowledgments, flow control or congestion control, so it is fast and suitable for real-time and query-response traffic. Protocols using UDP include DNS queries, DHCP, TFTP, SNMP, NTP, Syslog, RTP and QUIC.

Example

DHCP clients and servers communicate over UDP ports 67 and 68.

Exam tip

DNS uses UDP 53 for most queries but TCP 53 for zone transfers and large responses, so "both" is often the right answer.

Don't confuse it with

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