What are the key differences between TCP and UDP, and when would you use each protocol?

Question

Accepted Answer

TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are the two primary transport layer protocols in the TCP/IP suite. They serve fundamentally different purposes, and choosing between them depends on whether your application prioritizes reliability or speed.

TCP is a connection-oriented protocol, meaning it establishes a connection between sender and receiver before transmitting data. This is done through a three-way handshake: the client sends a SYN packet, the server responds with SYN-ACK, and the client confirms with ACK. TCP guarantees that data arrives in order, without duplication, and retransmits any lost packets. It also implements flow control using a sliding window mechanism to prevent the sender from overwhelming the receiver, and congestion control to avoid flooding the network.

UDP is a connectionless protocol that sends data without establishing a connection first. There is no handshake, no acknowledgment of received packets, no retransmission of lost data, and no guaranteed ordering. This makes UDP much faster and lighter than TCP, but it means the application must handle any reliability requirements itself.

You would use TCP when data integrity and order matter more than speed. Common TCP applications include web browsing (HTTP/HTTPS on port 80/443), email (SMTP on port 25, IMAP on port 143), file transfers (FTP on port 21), and SSH (port 22). Any time you cannot afford to lose data, TCP is the right choice.

You would use UDP when speed and low latency are more important than guaranteed delivery. Common UDP applications include DNS queries (port 53), video streaming, VoIP and video conferencing, online gaming, and SNMP (port 161). In these cases, a small amount of packet loss is acceptable, and retransmitting old data would actually be worse than skipping it.

DNS is an interesting case because it uses UDP for standard queries to keep them fast, but switches to TCP for zone transfers and responses larger than 512 bytes. This shows that many real-world applications use both protocols depending on the situation.

The header size also differs: TCP headers are 20-60 bytes due to all the control information, while UDP headers are only 8 bytes, contributing to UDP's lower overhead.