SYN Message (Request) Definition

What is SYN message in TCP?

SYN message is a TCP request for initiating a connection:

step 1: a connection request from a client to a server (the client sends a TCP packet with the SYN flag set);
step 2: server response (the client is sent a packet with SYN and ACK flags set);
step 3: acknowledgment of the client’s readiness to establish the connection (the server is sent a packet with the ACK flag set).

The connection is deemed to be established if all the 3 steps (the so-called TCP three-way handshake) have been successful.