I'm dissatisfied with "application/ipp" as the name for the
Internet media type registration, because it's somewhat
redundant (i = internet in an internet protocol) and misdirected
(the content is not a 'protocol', it's a description of a
print job used in one protocol but useful possibly in others).
So I suggest renaming it: application/print-job. That is,
an 'application/print-job' is a description of a job that the
sender wants the recipient to print.
It might be possible to send one of these in an internet fax,
or in just regular email. You're designing it to work
in a particular kind of messaging (one in which the result
of the message is something that lets you query status and
modify the job later), but it's just a message.
As far as a 'transaction identifier' that Randy Turner was
proposing, I thought that the transaction was a resource, and
that you would identify it using a uniform resource identifier
(aka URI aka URL).
A 'raw IPP' shouldn't be so raw as to not allow interaction
with printer resources using ipp://host URLs instead of http://host
URLs, but for the most part, which protocol you were using
would just be determined by the name of the scheme.