TCP vs UDP: what's the difference?

TCP and UDP are the two main transport protocols of the internet. TCP (Transmission Control Protocol) guarantees that data arrives complete and in order — at the cost of speed. UDP (User Datagram Protocol) sends data fast without guarantees — packets can be lost, reordered, or duplicated. Web pages, email, and file transfers use TCP. Video calls, online games, and DNS lookups use UDP.

What they have in common

Both protocols run on top of IP (Internet Protocol). IP delivers packets to the right address, but it does not handle packet ordering or error checking — that is exactly the gap TCP and UDP fill differently. Both also use port numbers to tell a computer which application a packet belongs to, so a server can run many services at once.

How TCP works: the reliable option

TCP lets two hosts connect and exchange a stream of data. Before anything is sent, the two computers establish a connection with a three-way handshake:

1. The source sends a SYN packet to start the dialogue.
2. The target replies with SYN-ACK to agree.
3. The source sends ACK to confirm — after which the data can flow.

TCP then guarantees the delivery of packets in the same order they were sent. If a packet goes missing, TCP asks for it to be re-sent. It also performs congestion control: transfers start small and grow until they reach what the network can support. This reliability is why email, web browsing, and file downloads use TCP — a corrupted download is useless.

How UDP works: the fast option

UDP is a connectionless protocol. There is no handshake, no connection setup: one computer simply starts sending data. UDP packets are called datagrams, and each one is addressed and sent independently. UDP provides checksums for data integrity and port numbers for addressing, but it does not guarantee delivery, ordering, or duplicate protection — that burden moves to the application.

The trade-off is speed. Without handshakes, retransmissions, and ordering bookkeeping, UDP is much faster and adds far less overhead. That makes it ideal for time-sensitive traffic, where dropping a packet is better than waiting for a delayed retransmission.

Comparison table

When to use TCP

Use TCP whenever the data must arrive complete and in the right order:

• Web pages (HTTP/HTTPS)
• Email (SMTP, IMAP)
• File transfers
• Anything where a partial result is useless

When to use UDP

Use UDP whenever speed matters more than perfection, and the application can tolerate some loss:

• VoIP phone calls — a slightly static-y call beats a crystal-clear but delayed one
• Video streaming and conferencing
• Online gaming
• DNS lookups — they need to be fast and efficient

A real-world example

Loading a web page uses TCP: the page's bytes must arrive intact, in order, or the page is broken. A video call uses UDP: frames travel as fast as possible, and the occasional dropped frame just produces a tiny flicker — nobody wants the call to pause while a missing frame is re-sent.

FAQ

Which is faster, TCP or UDP?

UDP is faster. It has no handshake, no retransmission, and no ordering bookkeeping, so it adds far less overhead than TCP. That raw speed is why UDP is chosen for time-sensitive traffic where a dropped packet is better than waiting for a delayed retransmission.

Can TCP and UDP use the same port number?

Yes. Both protocols use port numbers to tell a computer which application a packet belongs to, and a server can run many services at once. Because TCP and UDP are separate protocols, a TCP connection and a UDP socket can use the same port number simultaneously without conflicting.

Why is UDP used for video calls and online games?

Because those applications are time-sensitive. Video calls, conferencing, and online games care more about low latency than about perfect delivery — an occasional lost frame or packet just causes a small flicker, whereas waiting for a delayed retransmission would make the call or game feel unresponsive.

Which is more reliable, TCP or UDP?

TCP. It establishes a connection with a three-way handshake, guarantees packets arrive complete and in order, and re-sends anything that goes missing. UDP provides no such guarantees — packets can be lost, reordered, or duplicated — which is why web pages, email, and file transfers use TCP.

Summary

TCP and UDP solve the same problem — moving data over IP — with opposite priorities. TCP guarantees delivery and order, and pays for it with handshakes, retransmissions, and overhead. UDP trades all guarantees for raw speed. Pick TCP when completeness matters; pick UDP when latency matters more.