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TCP Three-Way Handshake: Purpose of the SYN / SYN+ACK / ACK Exchange

TCP Three-Way Handshake: Purpose of the SYN / SYN+ACK / ACK Exchange

Verified Sources
Nov 4, 2026

The exchange you listed—Client sends SYN, Server replies SYN+ACK, and Client responds with ACK—is the TCP three-way handshake. Its purpose is to establish a TCP connection by synchronizing initial sequence numbers, confirming reachability, and preparing both endpoints to send and receive data reliably over the newly formed connection.2

Key terms to recognize:

  • TCP handshake
  • SYN
  • ACK
  • Initial sequence number

Footnotes

  1. The TCP/IP Guide — TCP Connection Establishment (Three-Way Handshake): https://www.tcpipguide.com/free/t_TCPConnectionEstablishmentTheThreeWayHandsh.htm - Explains SYN → SYN/ACK → ACK as TCP three-way handshake for connection establishment and sequence number synchronization. ↩

  2. RFC 793 - Transmission Control Protocol (TCP): https://www.rfc-editor.org/rfc/rfc793 - Defines TCP behavior including control flags (SYN/ACK) and establishes connection state transitions. ↩

TCP Three-Way Handshake Explained

Why this exchange exists (what it accomplishes)

At a protocol level, the handshake ensures both sides agree on:

  1. That the peer is reachable (the other endpoint can receive packets).
  2. The starting sequence numbers for ordered delivery and acknowledgment tracking.
  3. That each side is prepared to proceed with the TCP connection state machine (before sending data).2

This is fundamentally connection establishment, not fragmentation and not termination.

Footnotes

  1. The TCP/IP Guide — TCP Connection Establishment (Three-Way Handshake): https://www.tcpipguide.com/free/t_TCPConnectionEstablishmentTheThreeWayHandsh.htm - Explains SYN → SYN/ACK → ACK as TCP three-way handshake for connection establishment and sequence number synchronization. ↩ ↩2

  2. RFC 793 - Transmission Control Protocol (TCP): https://www.rfc-editor.org/rfc/rfc793 - Defines TCP behavior including control flags (SYN/ACK) and establishes connection state transitions. ↩

TCP Connection Establishment (Three-Way Handshake)

  1. 1
    Step 1

    The client initiates connection setup and includes an initial sequence number (ISN).

  2. 2
    Step 2

    The server acknowledges the client’s SYN and sends its own ISN, indicating readiness to establish the connection.

  3. 3
    Step 3

    The client acknowledges the server’s SYN; after this, both sides have confirmed each other’s sequence parameters.

  4. 4
    Step 4

    Once the handshake completes, the TCP connection is established and the endpoints transition to data transfer behavior.

Eliminating the answer choices

Your options map to distinct TCP/UDP behaviors:

ChoiceMeaningCorrect?
(i) IP fragmentationA network-layer process for splitting datagrams that exceed MTUNo
(ii) TCP connection terminationDone with FIN/ACK (and related handshake variants) to close a TCP sessionNo
(iii) UDP connection establishmentUDP is connectionless; there is no built-in handshake like TCP’sNo
(iv) TCP connection establishmentThree-way handshake establishes sequence synchronization and readinessYes

The SYN / SYN+ACK / ACK sequence is explicitly the TCP connection establishment mechanism (three-way handshake).2

Footnotes

  1. The TCP/IP Guide — TCP Connection Establishment (Three-Way Handshake): https://www.tcpipguide.com/free/t_TCPConnectionEstablishmentTheThreeWayHandsh.htm - Explains SYN → SYN/ACK → ACK as TCP three-way handshake for connection establishment and sequence number synchronization. ↩

  2. RFC 793 - Transmission Control Protocol (TCP): https://www.rfc-editor.org/rfc/rfc793 - Defines TCP behavior including control flags (SYN/ACK) and establishes connection state transitions. ↩

Pro Tip: Recognize TCP by its flags

If you see SYN → SYN+ACK → ACK, think “TCP connection setup.” If you see FIN/ACK, think “TCP teardown/termination.”

Common confusion: UDP doesn’t “connect”

UDP may use an application-level concept of “session,” but TCP’s SYN/SYN+ACK/ACK is not a UDP mechanism. UDP has no standardized handshake to establish a reliable connection.

Footnotes

  1. RFC 768 - User Datagram Protocol (UDP): https://www.rfc-editor.org/rfc/rfc768 - Describes UDP as connectionless, contrasting with TCP’s connection-oriented behavior. ↩

TCP Session Lifecycle (High-Level)

Connection establishment

Start

Three-way handshake: SYN → SYN+ACK → ACK.2"

Footnotes

  1. The TCP/IP Guide — TCP Connection Establishment (Three-Way Handshake): https://www.tcpipguide.com/free/t_TCPConnectionEstablishmentTheThreeWayHandsh.htm - Explains SYN → SYN/ACK → ACK as TCP three-way handshake for connection establishment and sequence number synchronization. ↩

  2. RFC 793 - Transmission Control Protocol (TCP): https://www.rfc-editor.org/rfc/rfc793 - Defines TCP behavior including control flags (SYN/ACK) and establishes connection state transitions. ↩

Reliable data transfer

Middle

Ordered delivery with acknowledgments and retransmission as needed."

Connection termination

End

Typically uses FIN (plus ACK), closing the connection statefully."

FAQ

Knowledge Check

Question 1 of 4
Q1Single choice

In the TCP exchange SYN → SYN+ACK → ACK, what is the purpose?

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