UDP (User Datagram Protocol)

UDP (User Datagram Protocol) is a transport protocol that sends self-contained datagrams with only a small header, no connection setup, no retransmission and no ordering. Reliability, if needed, is left to the application.

Why it matters

UDP has low overhead and no handshake delay, so it suits traffic where freshness matters more than perfection: DNS queries, voice and video calls, online games and QUIC, which builds its own reliability on top. The downside is that nothing tells the sender a datagram was lost, and UDP can be abused for amplification DDoS attacks.

Example

$ dig @1.1.1.1 example.com A

This sends one UDP datagram to port 53 and expects one back. If no reply arrives, the client simply retries, possibly over TCP for large answers.

Common mistakes

• Testing a UDP service with a TCP port check. A closed UDP port is hard to distinguish from a filtered one.
• Sending datagrams larger than the path MTU and relying on fragmentation.
• Exposing open UDP services such as NTP or memcached that can be used for reflection attacks.

Related

• TCP
• Port
• DDoS
• Jitter

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