TCP vs UDP

TCP and UDP are the two fundamental transport protocols of the internet. TCP provides reliable, ordered delivery at the cost of latency. UDP provides fast, best-effort delivery without reliability guarantees.

TCP: Reliability Above All

TCP establishes a connection with a three-way handshake, numbers every byte of data, requires acknowledgment of received data, and retransmits lost packets. The receiver always gets data in the correct order with nothing missing.

This reliability is essential for web browsing (every HTML byte matters), databases (every query result must be complete), and file transfers (a missing byte corrupts the file).

UDP: Speed Above All

UDP sends datagrams without establishing a connection, without acknowledgments, and without retransmission. If a packet is lost, the sender never knows. If packets arrive out of order, the receiver must deal with it.

This is perfect for real-time applications where speed matters more than completeness. In a video call, retransmitting a lost frame from 200ms ago is useless — the moment has passed. In a game, the latest position update matters; the one from 100ms ago does not.

Building on UDP

Modern protocols build reliability features on top of UDP selectively. QUIC (used by HTTP/3) provides reliable, ordered streams over UDP without TCP's head-of-line blocking. WebRTC uses UDP for media with selective retransmission for key frames only.

For more on networking fundamentals, see our concepts library and system design interview guide. Explore pricing for practice.

The Bottom Line

Use TCP when reliability and ordering are non-negotiable. Use UDP when low latency matters more than guaranteed delivery. Modern protocols like QUIC show that the future is building smart reliability on top of UDP.