SYN is an abbreviation for “synchronize” in the context of the Transmission Control Protocol (TCP), one of the core protocols governing internet traffic. It refers to a flag that’s used in the TCP header to indicate the initial step in the TCP three-way handshake process, which establishes a connection between a client and a server.

Here’s a brief breakdown:

  1. SYN: A client wanting to initiate a TCP connection with a server sends a TCP segment with the SYN flag set. This signals its desire to open a connection. Along with the SYN flag, the client also sends an initial sequence number.
  2. SYN-ACK: Upon receiving the SYN segment, the server replies with a segment that has both the SYN and ACK (acknowledgment) flags set, indicating that it has received the client’s request and is also ready to establish a connection.
  3. ACK: The client then sends an ACK segment back to the server to acknowledge the server’s response. Once this is done, the TCP connection is established, and the two devices can start exchanging data.

The SYN flag, thus, plays a crucial role in the establishment of TCP connections, ensuring that data transfer only begins once both parties have agreed to communicate.