What does the ACK flag mean in TCP?

ACK = Acknowledgment - it marks the acknowledgment number field as significant, confirming data was received; set on nearly every segment after the initial SYN.

ACK = Acknowledgment

The ACK flag indicates that the acknowledgment number field is significant.

Purpose:

• Confirms receipt of data
• Used in virtually every TCP segment after the initial SYN

When it appears:

• During connection setup (SYN+ACK, final ACK)
• During data transfer (all segments)
• During connection termination (with FIN)

Tip: ACK is the most common flag - think of it as saying "I got your message!"

Go deeper:

• RFC 9293 §3.1 — Header Format — defines the ACK control bit and when the acknowledgment number field is significant.
• TCP segment structure — Flags — Wikipedia's control-bit table placing ACK in context.