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UDP

Fast, connectionless transport with no delivery guarantees.

Backend Engineer · Mid-levelFrontend Engineer · Mid-levelData Engineer · Mid-level

Also known as: udp, user datagram protocol, datagrams

UDP sends independent datagrams: no handshake, no ordering, no retransmission, no congestion control. Each send is one packet (or a few), fire-and-forget. What you lose in guarantees you gain in control and overhead — no setup round trips, no head-of-line blocking between messages, no backoff imposed by the transport.

TCP: handshake → stream → acks → ordered bytes (or failure)
UDP: datagram → maybe arrives → maybe in order (your logic decides)

That makes UDP the base for traffic that is loss-tolerant, latency-sensitive, or wants its own reliability: DNS queries, live media, gaming, and modern transports like QUIC that rebuild TCP-like guarantees in userspace without TCP’s one-size-fits-all behaviour.

The classic mistakes:

  • Assuming delivery. UDP datagrams vanish silently under congestion. Anything that must arrive needs application-level acks and retries — you’re rebuilding transport features, so budget for it.
  • Oversized datagrams. Past the path MTU they fragment, and one lost fragment kills the whole datagram. Keep well under ~1400 bytes or implement fragmentation yourself.
  • No congestion control. UDP won’t slow down for anyone; an aggressive sender congests the shared network and starves TCP flows. Well-behaved UDP protocols pace themselves.
  • Blocking and throttling. Many networks deprioritise or block UDP outright. Test on real paths; have a TCP fallback where it matters.
  • Using it for “speed” without measuring. For request/response workloads, the handshake savings are one RTT — often dwarfed by everything else. UDP wins where head-of-line blocking and per-message independence matter, not everywhere.
  • Forgetting ordering. Datagrams arrive in any order, duplicated, or not at all. Sequence numbers and dedup are your job.

When to choose it: live media, gaming, DNS-style queries, and as a substrate for custom transports. Default to TCP’s guarantees; graduate to UDP when you specifically need its properties and can implement the reliability you still require.