If an IPP printer responds to any protocol when an attempt to use the
protocol is made, then the responses to that protocol should be
conforming responses.
By this it is meant that
(a) a printer does not need to respond to any attempt to use any
protocol on any port that the printer is not supporting.
(b) if the printer is capable of doing the protocol (say HTTP), but an
administrator has configured the printer to not authorized use of that
protocol, then the printer may either refuse to participate in the
protocol or give an appropriate error message for that protocol. In the
case of HTTP, the printer might respond with an error message 401 - Not
Authorized.