The establishment of a TCP connection involves a negotiation called three-way handshake. What type of message does the client send to the server in order to begin this negotiation?

ACK (acknowledge) is the third and final message in the handshake, sent by the client after receiving the server's SYN-ACK, not the message that initiates the connection.

In the TCP three-way handshake, the client begins connection establishment by sending a SYN (synchronize) segment containing a randomly chosen initial sequence number. The server then replies with a SYN-ACK acknowledging the client's sequence number and providing its own, and finally the client sends an ACK to complete the connection - making SYN the correct initiating message.

RST (reset) is a control flag used to abruptly terminate an existing or invalid connection, not to initiate a new one.

SYN-ACK is the server's combined response to the client's initial SYN, representing the second step of the handshake rather than the client's opening message.