What type of inbound connection send an ACK back by default?

Web services normally use an HTTP status code; for example, an ACK would be 200 OK for REST/HTTP and would be available through the %Net.HttpResponse Object in the StatusCode/StatusLine properties. SOAP usually provides some sort of payload along with the status code, and that would be found in the Data property. The type of response would likely be identified in the source/target system's WSDL for the SOAP interface.