CodeHS The Internet Practice Test

During the TCP three-way handshake, which packet is sent first?

ACK SYN-ACK FIN SYN

Correct Answer:

SYN

Explanation:

Connecting over TCP starts with a SYN packet sent by the client to the server to initiate the handshake and synchronize sequence numbers. This first SYN signals the intent to open a connection and establishes the initial sequence number space for both sides. The server then replies with a SYN-ACK to acknowledge and provide its own sequence number, and finally the client sends an ACK to finish establishing the connection. Fin is used to close connections, and a plain ACK isn’t enough to start a connection, so the initial packet must be the SYN.

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