I think that putting that IPP servers MUST support "ftp:" is a mistake,
since the "ftp:" URL scheme will probably not progress along standards
track without some serious reengineering, and it isn't clear what it
is that you're expecting the printers to be conformant with. MUST they
also support the ";type=" option in FTP URLs, since that's a Proposed
Standard? MUST they also support ftp://user:@host/ forms?
Right now, the choice of binary or ascii transfer methods in FTP is
Frankly, you'd be better off requiring support for URIs of type "https",
and also in specifying other attributes of what a printer might do when
retrieving data via http on behalf of another user, e.g., supplying
credentials, the user-agent string, cookies, etc.