"An IPP object MUST be able to accept any of the attribute syntaxes
defined in Section 4.1, including their full range and any
value tags that are undefined in this document, in any operation
in which a client may supply attributes or the system ..."
The places where one may encounter a value tag in an IPP packet are the same places where one may encounter attribute group tags. The IPP documentation states that IPP applications should accept and ignore new/unknown attribute group tags. This is a reasonable requirement because whenever my application runs into an "unknown" tag it assumes it's a new attribute group. But if we allow the flexibility that I think you're proposing, parsing an IPP packet becomes much more complex and dangerous. When I run into an "unknown tag", am I looking at ..
a) The start of a new attribute group?
b) An attribute of the current attribute group with an unknown syntax?
c) A malformed packet?
d) If two unknown tags are next to each other are they two empty new attribute groups or a new syntax and its attribute name length?
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