What Is UDP? A Plain-English Explanation
UDP is what voice chat, online games and video calls run on. It guarantees nothing: no delivery confirmation, no reordering, no retransmission of anything lost. Here is what UDP is in plain English, and why "unreliable" is a feature here rather than a flaw.
What UDP does
Almost nothing — and that is the entire point. UDP (User Datagram Protocol) takes your data, stamps an address and a port on it, sends it and forgets about it.
It does none of the work TCP busies itself with:
- no connection setup — you can start sending immediately;
- no waiting for an acknowledgement;
- no resending what got lost;
- no reordering of packets;
- no slowing down when the network is congested.
The packet leaves, and after that it is luck. Arrives, does not arrive, shows up second instead of first — UDP does not care.
Why that is useful
Because waiting is often worse than losing.
Picture a voice call. One packet holding 20 milliseconds of audio goes missing. TCP has exactly one move: stall the stream and wait for the retransmission. Meanwhile the other person hears silence, and when the packet finally lands it is useless — the conversation moved on. The result is stuttering and latency that keeps climbing.
UDP just keeps going. Those 20 milliseconds either pass unnoticed or register as a tiny click. The audio stays live.
Games work the same way. A player position that arrives half a second late is worthless — three newer ones have come in since. Better to drop the stale packet than delay the current one.
What runs on UDP
| Service | Port |
|---|---|
| CS 1.6, Day of Defeat, Condition Zero, Sven Co-op | 27015 |
| Valheim | 2456–2458 |
| Rust | 28015 |
| Minecraft Bedrock Edition | 19132 |
| TeamSpeak 3 (voice) | 9987 |
| Mumble | 64738 |
| DNS (domain name lookups) | 53 |
| WireGuard | 51820 |
Note what is missing: Minecraft Java Edition is not on this list, because it speaks TCP. Bedrock, on the other hand, is UDP. Same game, different editions, different protocols.
Who handles reliability, if not UDP
The application does — when it needs to at all. A game engine decides for itself what a missing packet means: usually it predicts movement and fills the gap from the next update. Modern protocols built on UDP (QUIC, which carries HTTP/3) implement their own acknowledgements — but on their terms, not under TCP's rigid rules.
The point is that UDP does not impose a decision; it leaves it to the application.
What a UDP port is
The same as a TCP port: a number from 1 to 65535 that tells the operating system which program owns the incoming data. The numbering is independent between the two protocols — UDP 27015 and TCP 27015 are different ports and may belong to different programs. GoldSrc servers do exactly that: the game runs on UDP 27015 while RCON uses TCP 27015.
FAQ
Is UDP worse than TCP? No, it is different. For a file, TCP wins; for voice, UDP does.
Why is UDP called unreliable? That is terminology, not a verdict: the protocol makes no delivery guarantees. It says nothing about connection quality.
Is UDP faster? Not in throughput, but in latency: it spends no time on handshakes, acknowledgements or waiting for retransmissions.
Why is forwarding UDP ports harder? UDP has no connection, so a router has nothing solid to track when deciding where to send a reply. NAT works around it with timers, and that is noticeably more temperamental than with TCP.
Want to open a UDP port to the internet?
A tunnel removes port forwarding and the need for a public IP entirely — see How to open a UDP port with a tunnel.
Related: what is TCP · TCP vs UDP