The Fast and Furious Protocol
UDP (User Datagram Protocol) is the lightweight alternative to TCP. It sends data without establishing a connection, without acknowledgments, without ordering, and without guarantees. It's the "fire and forget" protocol.
If TCP is certified mail with tracking and signature confirmation, UDP is tossing a letter over a fence — it might arrive, it might not, and you won't know either way.
How UDP Works
UDP is beautifully simple. The application writes data, UDP wraps it in a header with source and destination ports and a checksum, and sends it. That's it. No handshake, no ACKs, no retransmission.
┌────────────┬────────────┬────────────┬────────────┐
│ Src Port │ Dst Port │ Length │ Checksum │
│ (16 bits) │ (16 bits) │ (16 bits) │ (16 bits) │
└────────────┴────────────┴────────────┴────────────┘
│
▼
┌──────────┐
│ Data │
└──────────┘
The UDP header is just 8 bytes — compared to TCP's 20-60 bytes. Less overhead means faster transmission.
When to Use UDP
UDP is perfect when speed matters more than reliability:
- Video streaming — If a frame is lost, it's better to skip it and show the next one than to wait for a retransmission. The viewer might see a brief glitch, but the video keeps playing.
- Online gaming — Game updates need to arrive fast. A 50ms delay from TCP retransmission could mean the difference between hitting and missing a target. If a packet is lost, the next one will have updated data anyway.
- DNS queries — A DNS lookup is a single request and response. There's no stream to order — you either get the answer or you don't. UDP is perfect for this.
- Voice over IP (VoIP) — Real-time voice can't wait for retransmissions. Dropped packets result in brief audio gaps, which is acceptable compared to the delays TCP would introduce.
- Broadcast/Multicast — UDP supports broadcast and multicast; TCP doesn't.
What UDP Doesn't Do
- No connection setup — No handshake. Data is sent immediately.
- No reliability — Lost packets are gone forever. The application must handle this if it matters.
- No ordering — Packets might arrive out of order. The application must sort them if needed.
- No flow control — The sender can blast data as fast as it wants, potentially overwhelming the receiver.
- No congestion control — UDP doesn't care if the network is congested. It sends at full speed regardless.
This isn't a flaw — it's a design choice. UDP gives applications control over what reliability mechanisms they need, rather than imposing a one-size-fits-all solution.
Application-Level Reliability
Many applications that use UDP add their own reliability mechanisms. For example, modern video streaming protocols (like QUIC, which underlies HTTP/3) build reliability on top of UDP while maintaining its speed advantages.
The philosophy is: let the application decide what reliability means for its specific use case, rather than forcing TCP's heavy guarantees on everything.