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SESK: Stuff Everyone Should Know · sesk.info/tech-tcp-handshake

After DNS finds the server's IP address, how does the browser open a TCP connection?

A three-way handshake: SYN, SYN-ACK, ACK.

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  1. It sends the whole page request at once
  2. A single PING message
  3. A three-way handshake: SYN, SYN-ACK, ACK ✔

Why: The browser sends SYN, the server answers SYN-ACK, and the browser replies ACK. Only then can data flow. For HTTPS, a TLS handshake follows to agree on encryption and verify the server.

Huh, didn't know that: The first chunk of a page is usually about 14KB because TCP slow start sends a small amount first and ramps up.

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References

Answer: A three-way handshake: SYN, SYN-ACK, ACK. The browser sends SYN, the server answers SYN-ACK, and the browser replies ACK. Only then can data flow. For HTTPS, a TLS handshake follows to agree on encryption and verify the server.

  1. MDN: How browsers work developer.mozilla.org/en-US/docs/Web/Performance/Guides/How_browsers_work
    Once the IP address is known, the browser sets up a connection to the server via a TCP three-way handshake
    Checked 2026-10-11.

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