TCP's Connection Establishment

In TCP, one side initiates the connection (the client), and one side listens on a given port, and accepts connections (the server).

The connection is established in a 3-way handshake. The client sends a 'SYN' message to the server. The server sends an 'ACK' back to the client, and the client then sends a 'SYN-ACK' back to the server. Only then the connection is open, and traffic may flow in both directions.

If a client tries to send a packet to a server before a connection is established, the server will send a 'RST' (Reset) message back to the client, to abort the transmission.