Have you heard about Hack Frontend Community?Join us on Telegram!

By continuing to use the platform, you accept the terms of the Privacy Policy and the use of cookies.

TCP Three-Way Handshake: SYN, SYN-ACK, ACK Explained

Three-way Handshake is connection establishment process between client and server via TCP (Transmission Control Protocol).
This mechanism ensures reliable and ordered connection between two parties before data exchange begins.

How Does Three-way Handshake Work?

TCP connection establishment happens in three steps:

1. SYN (from client)

Client initiates connection and sends SYN packet (synchronize) to server with initial sequence number.

2. SYN-ACK (from server)

Server receives SYN, confirms it with ACK (acknowledgment), and simultaneously sends its SYN — i.e., server says: "I received your request and want to establish connection".

3. ACK (from client)

Client receives SYN-ACK, confirms it by sending ACK packet. After this connection is considered established, and data exchange begins.

Three-way Handshake Diagram

Why is Three-way Handshake Needed?

Establishes reliable connection between client and server
Agrees initial sequence numbers to avoid data loss
Checks that both parties are ready for data transmission

Application

When opening web page — your browser first establishes TCP connection with server using this process
Used in HTTPS, FTP, SMTP and other protocols over TCP

Important:

Three-way handshake applies only to TCP. UDP protocol doesn't require connection establishment.

Conclusion

Stage	What it Does
`SYN`	Client requests connection
`SYN-ACK`	Server confirms and offers connection
`ACK`	Client confirms connection

After this data transmission can begin.

Back

Forward

Content

How Does Three-way Handshake Work?Three-way Handshake Diagram Why is Three-way Handshake Needed?Application Conclusion