[wg11] Minutes of Part 21 E3 DIS testing telecon on June 5, 2914

[Martin Hardwick]

*P21 e3 DIS Testing*

Minutes of June 5, 2104 Telecon


    Attendees

Martin Hardwick <hardwick at steptools.com> <mailto:hardwick at steptools.com>
Robert Lipman <robert.lipman at nist.gov> <mailto:robert.lipman at nist.gov>
Michael Benda <mjbenda at rockwellcollins.com> 
<mailto:mjbenda at rockwellcollins.com>
Hedlind Mikael <mikael.hedlind at scania.com> 
<mailto:mikael.hedlind at scania.com>
Dave Loffredo <loffredo at steptools.com> <mailto:loffredo at steptools.com>
Ed Paff <ejp at transcendata.com> <mailto:ejp at transcendata.com>
Tom Thurman <thomas.r.thurman at imonmail.com> 
<mailto:thomas.r.thurman at imonmail.com>


    Apologies for Absence

Jochen Fritz <jfritz at steptools.com> <mailto:jfritz at steptools.com>

Nicolas Figay <nicolas.figay at eads.net> <mailto:nicolas.figay at eads.net>


    PMI Splitting

ITI has split the sp3_boxy file produced by NIST into a geometry file 
and a pmi file. STEP Tools has viewed the geometry file and is working 
on a viewer for the separated pmi file. The two files and the original 
NIST data can be found at the following URL:

ftp://www.steptools.com/private/P21e3_DIS_testing/PMI/

STEP Tools is sponsoring the development of open source programs to 
split and merge P21 files using the Edition 3 specification. The 
programs are available at the following URL: http://tinyurl.com/thundercode.


    ZIP Assemblies

No visible progress has been made on this test case since the last 
telecom, but the PMI merge and split code is being updated to handle 
this case as well.

ftp://www.steptools.com/private/P21e3_DIS_testing/ZIP_Assembly/

In the last telecom:

·We decided the sub-tree directory names should include the NUAO 
identifier(s) for their corresponding node in the assembly tree.

·We learned that the LZMA algorithm has been shown to make STEP files 
three times more compressed than the more commonly used deflate 
algorithm, but at the cost of an increase in the compression time.

·We learned that each compressed file is required to document the 
algorithm used for its compression in its header and that different 
components in the same ZIP can used different compression algorithms.

·We recommended that the choice of the best compression algorithm should 
be left to the end user and that the standard should be silent on the 
matter.

**


    Unit Definitions

The unit definitions have been merged into a single file called units.stp.

ftp://www.steptools.com/private/P21e3_DIS_testing/Units/

We continued our discussion on whether unit references should be encoded 
as a URL or a URN. The following two lines of code show equivalent URL's 
and URN's.

<http://standards.iso.org/iso/10303/tech/reference_data/41/si_base_units.stp#METRE>;

<urn:iso:std:iso:10303:-41:tech:unit:metre>;

Explanation of URN:

urnIndicates this URI is a URN, instead of the more common URL (http)

iso URN namespace  (other examples are oid, usbn)

stdISO standard

isooriginating organization (other examples are iec, iso-ies, iso-cie)

10303STEP standard

-41part of multipart standard (hyphens required)

Techassociated or embedded resource defined by committee that created 
the standard

<the rest>unspecified -- controlled by committee.

In the above example, the URN has small advantage over the URL but 
perhaps the following URN will be acceptable because a file does not 
have to exist to define a URN.

<urn:iso:metre>;

It is also conceivable that the following URN might be acceptable even 
though there are currently small technical issues stopping the 
deployment of unit definitions for a Fahrenheit measure in a STEP file.

<urn:iso:Fahrenheit>;

We further discussed the requirements if all the unit definitions are to 
be defined in STEP files. The definition of literal constants such as 
PI, and EXPRESS constants such as negri_pi, have both led to extensive 
additions to the edition 3 format for which other strong use cases are 
currently lacking.

On the other hand the problem might be that we have not been 
sufficiently ambitious. It was pointed out that if constants can be 
defined then we are close to allowing expressions to be defined. The use 
of the PLIB schema for expressions has never been very satisfactory 
because the expressions are too hard to read when encoded into the STEP 
files. However, if Part 21 has an expression evaluation capability 
(probably in the anchors) then potential STEP applications such as 
parametrics and construction history might become more tractable. One 
interesting possibility might be to use Modelica as the expression language.


    Digital Signatures

STEP Tools gave a demonstration of how to make a digital signature using 
a private key and a signing certificate and how to verify that a file 
has not been edited since it was signed using a public key.

ftp://www.steptools.com/private/P21e3_DIS_testing/Digital_signatures/

We discussed white space and the issues that might arise if a file is 
read into Notepad/Wordpad and converted from line-feeds to carriage 
returns or vice versa. These characters are explicitly excluded from the 
Part 21 character set so they will not be included when computing and 
verifying the hash value.

We discussed supporting multiple signatures. The easiest procedure is 
for each signature to be applied to all of the characters that precede 
that signature. Thus each new signee is also verifying the signature of 
the previous signees.

For this to be consistent it would be best if the signatures came after 
the END-ISO-10303-21 keyword as shown in the example on the ftp site.

·The next conference call will be held on Friday June 27 at 4PM Paris, 
3PM London, 10AM New York and 7AM Seattle.


    Action Items

1.Complete the first ZIP assembly example.**

2.Consider the best approach for unit definitions: URN's or URLs'?**

3.Consider if Edition 3 should offer any support to the definition of 
parametrics and if so how?**

4.Extend the digital signatures example to include the creation of 
signing certificates.

5.Demonstrate signing at one site (ITI) and verification at another site 
(STEP Tools).

As recorded by Martin Hardwick <hardwick at steptools.com> 
<mailto:hardwick at steptools.com>

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