While doing a technical assessment to determine network vulnerabilities, you used the TCP XMAS scan. What would be the response of all open ports?

ACK packets are used within established TCP sessions for acknowledgment and are not sent by open ports in response to unsolicited XMAS scan probes with no existing session context.

A SYN packet initiates a new TCP connection and is never sent by a target host in response to an inbound probe - it would violate the TCP state machine entirely.

RFC 793 specifies that a TCP stack must not respond to incoming packets that do not carry SYN, RST, or ACK flags when the target port is in an open state - such packets are silently discarded. The XMAS scan deliberately sets FIN, PSH, and URG flags to exploit this behavior; the lack of any response indicates the port is open. This technique works reliably on RFC 793-compliant systems but not on Windows hosts, which send RST for all unexpected packets regardless of port state.

RST/ACK is the response sent by CLOSED ports when they receive unexpected XMAS packets, not by open ports - this RST response is actually how XMAS scans identify closed ports.