All,
I've received a few queries about how the "unknown" and "no-value" attribute value tags are used in IPP recently. What I've been telling people is basically the following, but I'd like to document this somewhere, perhaps as an errata for one of the IPP RFCs (probably RFC 2911 and 3380) along with some updates to the IPP registry for the affected attributes.
Comments?
The "no-value" Attribute Value Tag
This value may be set or returned for any settable Job, Subscription, or Printer attribute. The typical usage is to specify an xxx-default Job Template attribute that has no default value, such as "orientation-requested-default" - in that case the Printer may apply a default based on the document such as automatically applying "orientation-requested" with a value of 4 (landscape) for a landscape document.
The "no-value" value may also be returned for read-only description attributes that are a mandatory part of an object. For example, the Job object's "time-at-completion" attribute can be returned as "no-value" when the Job has not yet entered a terminating state.
The "unknown" Attribute Value Tag
This value may only be set or returned for specific Printer attributes. Currently only the proposed "printer-geo-location" attribute has been defined to potentially have an "unknown" value.
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Michael Sweet, Senior Printing System Engineer, PWG Chair
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