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TCP vs UDP Cheat Sheet by

TCP vs UDP, side by side: reliability, speed, headers, the three-way handshake, and real-world use cases — when to use each and why the exam loves asking. Answer the classic comparison questions with confidence. More free study resources at bytebarhq.com.

Side by Side

Full name
Transm­ission Control Protocol (TCP) · User Datagram Protocol (UDP)
Connection
TCP: Connec­tio­n-o­riented (handshake first) · UDP: Connec­tio­nless (fire and forget)
Reliab­ility
TCP: Guaranteed delivery, ordered, error-­checked · UDP: Best effort — no guarantees
Speed
TCP: Slower (overhead) · UDP: Faster (minimal overhead)
Header size
TCP: 20 bytes · UDP: 8 bytes
Use when
TCP: Accuracy matters · UDP: Speed matters
The head-t­o-head every Network+, Security+, and CCNA exam asks about.

The Three-Way Handshake

Step 1: SYN
Client → Server
Step 2: SYN/ACK
Server → Client
Step 3: ACK
Client → Server
Teardown
FIN → FIN-ACK → ACK
SYN flood attacks abuse this by leaving connec­tions half-open.

Use TCP When…

Web browsing
HTTP/HTTPS
Email
SMTP, IMAP, POP3
File transfer
FTP, SFTP
Remote admin
SSH, RDP
Use TCP when accuracy matters.

Use UDP When…

DNS lookups
Port 53
DHCP
Ports 67/68
Streaming / VoIP / gaming
Speed matters more than perfection
NTP / SNMP / TFTP
Ports 123 / 161 / 69
Use UDP when speed matters.

Exam Traps

"­Which is connec­tio­nle­ss?­"
UDP
SYN flood attack
Abuses the TCP handshake (half-open connec­tions)
Sequencing + ACKs
TCP uses them; UDP has neither
DNS transport
UDP for queries, TCP for zone transfers
Read the question twice — these are designed to trick you.
 

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  • Published: 14th September, 2026

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