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TCP vs UDP: Key Differences Explained

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This article explores the key differences between TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). Both protocols operate at Layer 4 (the Transport Layer) of the OSI model, sitting just above the IP layer.

Unlike TCP, UDP is connectionless. It doesn’t guarantee reliability, doesn’t implement windowing, and doesn’t ensure that data is received in the same order it was transmitted.

However, UDP offers functionalities such as data transfer and multiplexing, with fewer overhead bytes in its data packets. This smaller overhead means UDP requires less time to process each packet and less memory. The absence of an acknowledgement field also speeds things up, as UDP doesn’t need to wait for ACKs or hold data in memory until they are acknowledged.

Let’s break down the differences in more detail:

Feature	TCP	UDP
Definition	Defined in RFC793	Defined in RFC768
Full Form	Transmission Control Protocol	User Datagram Protocol
Connection Type	Connection-oriented service	Connectionless service
Reliability	Reliable, in-order byte-stream protocol	Unreliable, un-ordered packet protocol
Reliability Mechanisms	Considered highly reliable due to SYN and acknowledgement fields	Considered an unreliable service protocol
Protocol Field in IP Header	0x06 (decimal 6)	0x11 (decimal 17)
Header Structure	Consists of source port, destination port, sequence number, acknowledgement number, header length, flags (U, A, P, R, S, F), window, checksum, pointer, and padding bits. See image below.	Consists of source port, destination port, segment length, and checksum. See image below.
Data Transfer	Continuous stream of ordered data, segmented before transmission.	Message-based datagram delivery.
Multiplexing	Receiving hosts determine the correct application for the data based on the port number field (2 bytes).	Receiving hosts determine the correct application for the data based on the port number field (2 bytes).
Flow Control	Uses window size to protect buffer space and routing devices.	Not supported in UDP.