Why UDP Is the Wild West of Networking and How TCP Tames It

Preface

In the internet world, UDP behaves like a reckless swordsman, cutting through data streams without a connection, sacrificing reliability for speed; TCP is like a gentleman, establishing a three‑handshake connection, using retransmission and flow control to ensure reliable transmission. Both complement each other: UDP for real‑time video, TCP for web page loading.

The Wild Man: UDP Protocol

Imagine sending a parcel: with UDP you only need the recipient’s address and you ship immediately, without any prior handshake.

Connection‑less

Connection‑less (data is sent as soon as the destination IP is known)

Just a packet carrier

UDP does not generate its own data; it simply forwards the already‑processed payload without adding extra header bytes (only an 8‑byte header).

Only carries data packets (header overhead is minimal, just 8 bytes)

No ordering or reliability

Because UDP sends packets immediately, it cannot guarantee that packets arrive in order or even arrive at all; packet loss may occur.

Does not guarantee packet ordering Does not guarantee reliability (packet loss)

No congestion control

UDP transmits at a constant rate regardless of network conditions, which can lead to packet loss when the network is congested.

Efficiency

The lack of congestion control makes UDP efficient for streaming video or live broadcasts where speed matters more than perfect delivery.

UDP Summary

Connection‑less (send data as soon as the destination IP is known) Only carries data packets (no extra header, only 8 bytes) No packet ordering No reliability (possible packet loss) No congestion control Efficient (the previous five points contribute to efficiency)

Typical applications: live streaming, video, online gaming, and other latency‑sensitive services.

The Gentleman: TCP Protocol

TCP establishes a connection through a three‑handshake process and ends it with a four‑handshake termination, ensuring reliable communication.

Header information

TCP adds several fields to each segment:

Sequence Number

Acknowledgment Number

Window Size (how many bytes the current network can handle)

Flags

Common flag meanings:

URG – urgent packet

ACK – acknowledgment, indicates the segment is valid

PSH – push, tells the receiver to pass data to the application immediately

SYN – synchronize, used to establish a connection

RST – reset, indicates the need to reconnect

FIN – finish, used to close a connection

TCP Three‑Way Handshake

When a client wants to connect to a server, the process is:

TCP three‑way handshake: Client sends SYN, enters SYN‑SEND state. Server replies with SYN+ACK, enters SYN‑RECEIVED state. Client sends ACK, both sides enter ESTABLISHED state.

Why three handshakes?

If the first SYN is lost, the client retransmits; without the final ACK, the server could remain in a half‑open state, wasting resources.

TCP Four‑Way Termination

The connection is closed in four steps:

TCP four‑way termination: Client sends FIN, enters FIN‑WAIT‑1. Server replies ACK, enters CLOSE‑WAIT. Server sends its own FIN after sending remaining data, enters LAST‑ACK. Client replies ACK, enters TIME‑WAIT, then both sides reach CLOSED.

TCP Summary

Reliable and connection‑oriented Slow start and congestion control (adjusts sending rate based on network conditions) Ordered delivery (sequence numbers ensure correct order)