Understanding TCP vs UDP: Handshakes, Data Transfer, and Key Differences

Purpose

Both TCP and UDP operate at the transport layer and are used for data exchange between programs, handling files, video streams, images, and other data types.

Key Differences

TCP is connection‑oriented and provides reliable, ordered delivery, making it suitable for file transfers, email, and web browsing. UDP is connectionless and faster but may lose packets, fitting real‑time applications such as DNS queries, voice calls, video streaming, and tunnel networks like VXLAN.

An analogy: sending a UDP packet is like mailing a letter without knowing if it arrives, while a TCP connection resembles a phone call with immediate feedback and confirmation.

TCP Communication Process

TCP communication consists of three main steps: a three‑way handshake, data transmission with acknowledgments, and a four‑way termination.

Three‑Way Handshake

The client initiates a connection with a SYN packet, the server replies with a SYN‑ACK, and the client completes the handshake with an ACK, establishing a reliable connection.

The three‑handshake design prevents old or duplicate requests from causing state inconsistencies in unreliable networks.

Data Transmission

TCP splits large data into segments, assigns sequence numbers, and uses a send buffer. The receiver acknowledges each segment with an ACK containing the next expected sequence number, ensuring ordered and reliable delivery.

Four‑Way Termination

Either side can initiate connection closure. The process involves four steps: FIN from the initiator, ACK from the peer, FIN from the peer, and final ACK from the initiator, with a timeout to ensure the final ACK is received.

UDP Overview

UDP sends packets without establishing a connection, resulting in minimal overhead and low latency but no guarantee of delivery or ordering. It is ideal for applications where speed is critical and occasional loss is acceptable.

In summary, TCP offers reliable, ordered transmission for quality‑critical tasks, while UDP provides fast, lightweight delivery for real‑time or low‑latency scenarios.