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SESK: Stuff Everyone Should Know · sesk.info/aplus-tcp-three-way

How many messages make up the TCP connection setup, known as the handshake?

Three: SYN, SYN-ACK, then ACK.

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  1. Two
  2. Three ✔
  3. Four

Why: TCP sets up a connection with a three-way handshake. The middle steps are combined in one message, so the whole exchange is three messages. After that, data flows reliably.

Huh, didn't know that: RFC 9293 describes TCP as a reliable, in-order, byte-stream service.

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References

Answer: Three. TCP sets up a connection with a three-way handshake. The middle steps are combined in one message, so the whole exchange is three messages. After that, data flows reliably.

  1. RFC 9293 www.rfc-editor.org/rfc/rfc9293
    Because steps 2 and 3 can be combined in a single message this is called the three-way (or three message) handshake (3WHS).
    Checked 2026-10-10.
  2. RFC 9293 www.rfc-editor.org/rfc/rfc9293
    TCP provides a reliable, in-order, byte-stream service to applications.
    Checked 2026-10-10.

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