TCP vs UDP, and When to Reach for Each

Loading this page used TCP. Resolving the domain to an IP first used UDP. Two protocols, one request, picked for different jobs.

The usual line is “TCP is reliable, UDP is fast.” True enough to pass an interview, shallow enough to lead you to the wrong choice.

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TCP is a phone call

Both sides shake hands before any data moves:

client  ──  SYN     ──▶  server
client  ◀── SYN-ACK ──   server
client  ──  ACK     ──▶  server   connected

After that, TCP numbers every byte, acknowledges what arrives, resends what gets lost, and slows down when the network is congested. You program against a clean abstraction: an ordered stream of bytes that either arrives intact or fails loudly.

import net from "node:net";

net.createServer((socket) => {
    socket.on("data", (chunk) => socket.write(chunk)); // echo
}).listen(4000);

The catch: TCP has no message boundaries. Send "hello" then "world" and one data event might fire with "helloworld", or split as "hel" then "loworld". You get ordering for free and pay for it by framing your own messages (a delimiter or a length prefix).

UDP is a postcard

No handshake, no ACKs, no retransmission. Send it and walk away. A datagram might be lost, duplicated, or arrive out of order, and nobody tells you. The header is 8 bytes vs TCP’s 20+.

import dgram from "node:dgram";

const server = dgram.createSocket("udp4");
server.on("message", (msg, rinfo) => server.send(msg, rinfo.port, rinfo.address));
server.bind(4000);

The one thing UDP guarantees: each message is exactly one whole datagram. UDP keeps message boundaries; TCP does not.

Head-of-line blocking

Because TCP delivers strictly in order, one lost segment stalls every segment behind it until it is resent. Great for a file (useless out of order), wrong for a live call (the resent packet is already stale, you would rather skip it). This is why HTTP/3 runs on UDP and rebuilds only the reliability it wants. More on that in QUIC, TCP rebuilt on UDP.

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Which one?

	TCP	UDP
Delivery	Guaranteed or errors	Best effort
Ordering	In order	As they arrive
Boundaries	Byte stream, you frame	One datagram per message
Overhead	20+ byte header	8 byte header

Default to TCP: web, APIs, files, anything where a missing byte corrupts the result.

Use UDP when late data is worthless: DNS, live video and voice, games, telemetry. The deciding question is not “do I want reliability” but what should happen to data that arrives late. Worth waiting for, use TCP. Useless once late, UDP lets you move on.