TCP/IP Fundamentals

Explain the difference between TCP and UDP. When would you use each?

Networkingbeginner

// interview question

Explain the difference between TCP and UDP. When would you use each?

Sample answer

TCP (Transmission Control Protocol) is connection-oriented, ensuring reliable, ordered delivery with error checking and retransmission. UDP (User Datagram Protocol) is connectionless, offering faster but unreliable delivery without guarantees. Use TCP for web traffic (HTTP), file transfers, and email. Use UDP for streaming, gaming, DNS queries, and real-time applications where speed matters more than reliability.

Why this matters

Understanding TCP vs UDP is fundamental for troubleshooting network issues, configuring firewalls, and designing distributed systems. DevOps engineers must know which protocol applications use to properly monitor, secure, and optimize network traffic.

Code examples

Check open TCP/UDP ports

bash

# List all TCP connections
netstat -tulpn | grep tcp
ss -tulpn | grep tcp

# List all UDP connections
netstat -tulpn | grep udp
ss -tulpn | grep udp

# Test TCP connectivity
nc -zv hostname 80

# Test UDP connectivity
nc -zuv hostname 53

Common mistakes to avoid

Assuming all network traffic uses TCP
Blocking UDP on firewalls without understanding impact on DNS and NTP
Not considering TCP overhead for high-throughput applications

Likely follow-ups

What is the TCP three-way handshake?
Why does DNS use UDP by default but can fall back to TCP?
What are the common TCP states and what do they mean?

Answer out loud first, then check yourself against the model answer.

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