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Commonly confused · Networking Fundamentals

TCP vs UDP: What's the Difference?

TCP is connection-oriented and guarantees ordered, reliable delivery using a three-way handshake, acknowledgments, and retransmission. UDP is connectionless and simply sends datagrams with no delivery guarantee, trading reliability for lower overhead and latency.

Transmission Control Protocol (TCP)

Full definition
vs

User Datagram Protocol (UDP)

Full definition
Transmission Control ProtocolUser Datagram Protocol
ConnectionConnection-oriented (SYN, SYN-ACK, ACK)Connectionless, no handshake
ReliabilityAcknowledgments and retransmission of lost segmentsBest effort; lost datagrams are not resent
OrderingSequence numbers reassemble data in orderNo ordering guarantee
Flow controlSliding window flow and congestion controlNone
Header size20 bytes minimum8 bytes
Typical usesHTTP/HTTPS, SSH, SMTP, FTPDNS queries, DHCP, VoIP, video streaming, SNMP, TFTP
Remember it

TCP is a phone call with confirmation; UDP is shouting into the room and hoping it lands.

Exam tip

Expect questions that give an application (VoIP, DNS, file transfer) and ask which transport it uses. Real-time traffic that cannot wait for retransmission points to UDP; anything that must arrive intact points to TCP.

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