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.
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Full name
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Transmission Control Protocol (TCP) · User Datagram Protocol (UDP)
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Connection
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TCP: Connection-oriented (handshake first) · UDP: Connectionless (fire and forget)
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Reliability
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TCP: Guaranteed delivery, ordered, error-checked · UDP: Best effort — no guarantees
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Speed
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TCP: Slower (overhead) · UDP: Faster (minimal overhead)
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Header size
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TCP: 20 bytes · UDP: 8 bytes
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Use when
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TCP: Accuracy matters · UDP: Speed matters
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Step 1: SYN
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Client → Server
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Step 2: SYN/ACK
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Server → Client
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Step 3: ACK
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Client → Server
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Teardown
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FIN → FIN-ACK → ACK
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Web browsing
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HTTP/HTTPS
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Email
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SMTP, IMAP, POP3
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File transfer
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FTP, SFTP
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Remote admin
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SSH, RDP
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DNS lookups
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Port 53
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DHCP
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Ports 67/68
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Streaming / VoIP / gaming
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Speed matters more than perfection
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NTP / SNMP / TFTP
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Ports 123 / 161 / 69
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"Which is connectionless?"
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UDP
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SYN flood attack
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Abuses the TCP handshake (half-open connections)
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Sequencing + ACKs
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TCP uses them; UDP has neither
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DNS transport
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UDP for queries, TCP for zone transfers
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