I've posted a suggested new introduction to the Model Document in
ftp://ftp.pwg.org/pub/pwg/ipp/new_MOD/newintro.txt
This is based on the system diagram that's been discussed here for
the past few days. The objective of this intro is to put IPP into context
with the rest of the components required to implement a printing system,
e.g. Security, Directory, Transport, etc. I'd appreciate any comments.