Understanding TCP vs UDP: Key Differences, Handshakes, and When to Use Each

1. Introduction to TCP/UDP

TCP and UDP are two common Internet transport protocols that operate on the IP network layer.

TCP (Transmission Control Protocol) is a connection-oriented reliable protocol that provides ordered, complete data transmission, flow control, and congestion control. It establishes connections with a three‑way handshake and terminates with a four‑way handshake. Data is sent and received in order, ensuring reliable delivery, and is suitable for applications such as web browsing, file transfer, email, and remote login.

UDP (User Datagram Protocol) is a connectionless, unreliable protocol that offers a simple, best‑effort delivery without guaranteeing order, completeness, flow control, or congestion control. Data is sent as datagrams without establishing a connection, making UDP ideal for real‑time applications like audio/video streaming, online gaming, and broadcasting.

2. Characteristics of TCP and UDP

Common points: Both run on the IP network as transport‑layer protocols, use port numbers to identify applications, and support client‑server communication.

Differences:

TCP is connection‑oriented; UDP is connectionless.

TCP provides reliable data transfer; UDP does not guarantee reliability.

TCP includes flow control and congestion control; UDP lacks these mechanisms.

TCP requires a three‑way handshake to establish a connection; UDP does not.

TCP Error Detection and Recovery

TCP uses a checksum to detect data corruption. The sender calculates the checksum and includes it in the TCP header; the receiver recomputes it and compares it to the received value. If they differ, the receiver requests retransmission.

Key fields for error handling:

Checksum – verifies each segment; corrupted segments are discarded.

Acknowledgment – the receiver sends ACK only for correctly received data.

Timeout – the sender retransmits if an ACK is not received within a timeout, using timers such as retransmission, persist, keep‑alive, and time‑wait.

TCP Flow Control and Congestion Control

Flow control uses a sliding‑window mechanism where the receiver advertises the amount of data it can accept, and the sender adjusts its transmission rate accordingly.

Congestion control employs algorithms (e.g., congestion avoidance and congestion control) to detect network congestion and adjust the sending window to prevent overload.

TCP Connection Management

Three‑way handshake:

Client sends SYN with an initial sequence number.

Server replies with SYN‑ACK.

Client sends ACK, completing the connection.

Four‑way termination:

Client sends FIN to indicate it has finished sending data.

Server acknowledges with ACK.

Server sends its own FIN.

Client acknowledges with ACK, and the connection closes.

3. Summary

Advantages of TCP: reliability, error detection and correction, flow control, and congestion control.

Disadvantages of TCP: higher latency due to handshake and acknowledgment overhead, and increased processing load.

Advantages of UDP: lower latency, faster transmission, and lightweight header.

Disadvantages of UDP: lack of reliability, possible packet loss or reordering, and no flow or congestion control.