What does the SYN flag mean in TCP?

SYN = Synchronize - it synchronizes sequence numbers to open a connection (first step of the three-way handshake).

SYN = Synchronize

The SYN flag is used during connection establishment (the three-way handshake).

Purpose:

• Requests to establish a connection
• Synchronizes sequence numbers between devices
• First step in the TCP three-way handshake

When it appears:

• Step 1: Client → Server sends SYN
• Step 2: Server → Client sends SYN+ACK

Tip: Think "SYN = Start Your Numbers" - it's about synchronizing sequence numbers to start the conversation.

Go deeper:

• RFC 9293 §3.5 — Establishing a Connection — how the SYN flag opens a connection and synchronizes initial sequence numbers.
• TCP segment structure — Flags — Wikipedia's control-bit table placing SYN in context.