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Networking Fundamentals · Foundational

Three-Way Handshake

The SYN, SYN-ACK, ACK exchange TCP uses to establish a connection and synchronize sequence numbers.

Definition

The client sends a SYN with its initial sequence number, the server replies with SYN-ACK carrying its own initial sequence number and acknowledging the client's, and the client sends an ACK. After this, both sides are in the ESTABLISHED state. Options such as MSS, window scaling and SACK permitted are negotiated in the SYN segments.

Example

A packet capture of a web request shows SYN, SYN-ACK, ACK before the TLS Client Hello.

Exam tip

A flood of SYNs with no final ACK indicates a SYN flood attack or a blocked return path.

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