What does the URG flag mean in TCP?

URG = Urgent - it marks the urgent pointer field as significant so flagged data is processed out of band (rarely used in practice).

URG = Urgent

The URG flag indicates that the urgent pointer field is significant and data should be processed out of band.

Purpose:

• Marks data as urgent/high priority
• Receiver processes urgent data before normal data
• Uses the urgent pointer to indicate urgent data location

Rarely used in practice:

• Mostly used in legacy protocols
• Telnet uses it for interrupt commands (Ctrl+C)

Tip: URG is the "interrupt" flag - "Stop what you're doing, this is urgent!"

Go deeper:

• RFC 9293 §3.1 — Header Format — defines the URG control bit and the separate Urgent Pointer field it activates.
• TCP segment structure — Flags — Wikipedia's control-bit table placing URG in context.