TCP vs UDP — cheat sheet

A one-screen cheat sheet. Just headings, lists and a checklist — the simplest kind of note.

TCP

• Connection-oriented: three-way handshake before any data.
• Reliable: retransmits lost packets, delivers in order.
• Flow and congestion control so it doesn't flood the network.
• Used by: HTTP/1.1 and HTTP/2, SSH, email, databases.

UDP

• Connectionless: just send datagrams.
• No delivery or ordering guarantees — the app handles loss if it cares.
• Low overhead and latency.
• Used by: DNS, video calls, online games, QUIC (HTTP/3).

Pick UDP when…

• Late data is worse than lost data (voice, video, games)
• Messages are tiny and fit in one packet (DNS)
• You need reliable, ordered bytes (file transfer)

Rule of thumb: TCP for correctness, UDP for timeliness.