What does the ACK character do?

ACK (ASCII 6) is a handshake signal meaning 'message received successfully.' In serial protocols like XMODEM, the receiver sends ACK after each valid data block so the sender knows it's safe to transmit the next one.

Is ACK still used in modern networking?

The byte itself isn't, but the concept is everywhere. TCP's three-way handshake uses ACK flags, HTTP/2 has SETTINGS acknowledgments, and WebSocket pings expect pongs — all descendants of the same idea.

What happens if a device never sends ACK?

The sender typically retries a set number of times, then gives up and reports a timeout error. Without ACK, there's no way to confirm delivery, so protocols fall back to resending or dropping the connection.

How is ACK different from NAK?

ACK (ASCII 6) means 'got it, all good.' NAK (ASCII 21) means 'got it, but something's wrong — send it again.' Together they form the simplest error-recovery loop in serial communications.

Acknowledge

ACK (Acknowledge) is ASCII code point 6 (0x06), Unicode U+0006. It is a C0 control character — the affirmative reply in serial handshaking. Originally, a teleprinter sent ACK after receiving a valid data block in response to an ENQ (0x05) poll, confirming the transfer could continue. In IBM BSC framing, alternating ACK0/ACK1 responses provide sequence-level confirmation alongside CRC-16 or LRC checks. UTF-8 encodes this code point as the single byte 0x06, identical to ASCII; Unicode assigns it General Category Cc (Control) with the alias ACKNOWLEDGE.

Control Character

Technical Details

Code Example

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unsigned char ack = 0x06;            // ACK control byte
unsigned char frame[3] = {0x02, 'A', 0x03}; // STX ... ETX
if (frame[0] == 0x02 && frame[2] == 0x03) out[i++] = ack; // acknowledge valid frame

Frequently Asked Questions

ACK (ASCII 6) is a handshake signal meaning 'message received successfully.' In serial protocols like XMODEM, the receiver sends ACK after each valid data block so the sender knows it's safe to transmit the next one.
The byte itself isn't, but the concept is everywhere. TCP's three-way handshake uses ACK flags, HTTP/2 has SETTINGS acknowledgments, and WebSocket pings expect pongs — all descendants of the same idea.
The sender typically retries a set number of times, then gives up and reports a timeout error. Without ACK, there's no way to confirm delivery, so protocols fall back to resending or dropping the connection.
ACK (ASCII 6) means 'got it, all good.' NAK (ASCII 21) means 'got it, but something's wrong — send it again.' Together they form the simplest error-recovery loop in serial communications.

Acknowledge

Control Character