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

TCP and UDP Overview

TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are the two primary transport‑layer protocols used on IP networks. Both operate at layer 4, use port numbers to identify applications, and support client‑server communication.

Key Differences

Connection model: TCP is connection‑oriented; UDP is connection‑less.

Reliability: TCP provides reliable, ordered delivery with acknowledgments and retransmissions; UDP offers no reliability guarantees.

Flow and congestion control: TCP implements both; UDP does not.

Handshake: TCP requires a three‑way handshake to establish a connection; UDP sends packets without handshaking.

TCP Mechanisms

Error Detection and Recovery

Each TCP segment contains a 16‑bit checksum. The sender computes the checksum over the segment header and data; the receiver recomputes it and discards the segment if the values differ, triggering a retransmission.

Checksum: validates integrity of every segment.

Acknowledgment (ACK): receiver sends ACK only for correctly received data; missing ACKs cause the sender to retransmit.

Timeout timers: TCP maintains four timers – retransmission, persistence, keep‑alive, and TIME‑WAIT – to decide when a segment is lost and must be resent.

Flow Control

TCP uses a sliding‑window mechanism. The receiver advertises a window size (the amount of buffer space available). The sender must keep the amount of unacknowledged data within this window, preventing buffer overflow at the receiver.

Congestion Control

TCP adjusts its sending rate based on network congestion signals. The most common algorithms are:

Congestion avoidance: gradually increase the congestion window (cwnd) until loss is detected.

Congestion recovery: on packet loss (detected via duplicate ACKs or timeout), reduce cwnd (e.g., multiplicative decrease) and enter slow start.

Connection Establishment – Three‑Way Handshake

Client sends SYN with an initial sequence number (ISN).

Server replies with SYN‑ACK, acknowledging the client’s SYN and providing its own ISN.

Client sends ACK acknowledging the server’s SYN. The connection is now established.

Connection Termination – Four‑Way Handshake

Client sends FIN to indicate it has finished sending data.

Server acknowledges the FIN with ACK.

Server, after sending any remaining data, sends its own FIN.

Client acknowledges the server’s FIN with ACK. The connection is closed.

UDP Characteristics

UDP is a connection‑less, best‑effort protocol. It adds a minimal header (source port, destination port, length, checksum) and delivers datagrams without ordering, reliability, flow control, or congestion control. This low overhead makes UDP suitable for real‑time applications such as audio/video streaming, online gaming, and broadcast services.

Advantages and Disadvantages

TCP

Advantages: reliable delivery, in‑order transmission, error detection/correction, flow control, congestion control.

Disadvantages: higher latency due to handshake and ACK processing, increased CPU and memory overhead.

UDP

Advantages: low latency, minimal header overhead, suitable for time‑sensitive traffic.

Disadvantages: no reliability, possible packet loss or reordering, no built‑in flow or congestion control, which can lead to network congestion if misused.