TCP is the default for anything that must arrive intact and in order; the kernel handles retransmission, ordering, flow control, and congestion control for you. UDP is the right base when you would rather handle loss yourself than pay for head-of-line blocking — real-time media, DNS, and modern protocols like QUIC all build on it.
These are not competitors so much as two different contracts with the network. TCP promises a reliable ordered stream and hides the messy parts. UDP promises almost nothing and hands you the raw datagram. The choice is really a question of who should own reliability: the kernel, or your application.
Connection-oriented. A three-way handshake (SYN, SYN-ACK, ACK) establishes state on both ends before any data flows, and a four-way teardown closes it.
Connectionless. Each datagram is independent; there is no handshake and no shared state to set up or tear down.
Guaranteed delivery via sequence numbers, acknowledgements, and retransmission of lost segments.
None. Lost, duplicated, or reordered datagrams are the application’s problem to detect and handle.
Bytes are delivered to the application in the exact order sent.
No ordering guarantee; datagrams can arrive in any order.
A single lost segment stalls everything behind it until it is retransmitted — painful when multiplexing many streams over one connection (HTTP/2).
No HoL blocking; a lost datagram never holds up the next one.
Built in: a receive window for flow control plus congestion control (CUBIC, BBR) that backs off under loss.
None by default. Applications that flood UDP without their own pacing can cause congestion collapse.
20-byte minimum header, plus connection state and handshake round-trips before data.
8-byte header, no handshake — the lightest standard transport.
HTTP/1.1 and HTTP/2, TLS, SSH, SMTP, database wire protocols, file transfer.
DNS, DHCP, NTP, VoIP/RTP, video streaming, gaming, QUIC (and thus HTTP/3).
A web API or anything over HTTP
HTTP/1.1 and HTTP/2 ride TCP; reliability and ordering are exactly what request/response needs.
Live video conferencing or multiplayer games
A dropped frame should be skipped, not resent. UDP plus app-level recovery is the standard shape.
A new high-performance protocol
Build on UDP like QUIC did: you get reliability without TCP’s per-connection head-of-line blocking.
DNS resolution
One datagram each way; fall back to TCP only for responses too large for a single packet.