Here are attributes and their values, as described in various RFCs related to IPP.
mime media type as described in RFC 2046: Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types. (Updated by RFC2646, RFC3798).
not limited to list, as An administrator MAY define additional values using the 'name' or 'keyword' attribute syntax, depending on implementation.
The better way to determine which media sizes are supported is to parse the media-supported attribute after getPrinterAttributes() (eg. $ipp->printer_attributes->media_supported).
The following standard values are defined for input-trays (from ISO DPA and the Printer MIB):
The following standard values are defined for media sizes (from ISO DPA):
This attribute specifies the number of print-stream pages to impose upon a single side of an instance of a selected medium.
See file readable-attributes.html
See file cups-attributes.html