Mike

Some further thoughts from IBM on your recommendations, after more internal discussion here.

Preferable to have dfdl:bitOrder as a separate property rather to handle it via new dfdl:byteOrder enums. Although new properties pose validation issues for existing schemas, this should not compromise the language design. DFDL can choose what bitOrder/byteOrder combinations are supported.
OK with with new dfdl:byteOrder enum for littleEndianAtomic16Bit though can we improve the name?
dfdl:encoding has an architected system for extra encodings so US-ASCII-7-Bit-Packed should be x-US-ASCII-7-Bit-Packed, and the spec updated to remove specific mention of US-ASCII-7-Bit-Packed.

We discussed proposed new dfdl:lengthKind 'fixedLengthOrTerminated'. A new enum implies that it can be used in any scenario, so the following need to be specified.

dfdl:terminator must be set and can not be empty string or contain ES on its own
If xs:string or xs:hexBinary, can maxLength facet be used instead of dfdl:length? (Suggest no - this is variable length data so min/maxLength are for validation only).
Can dfdl:length be an expression? (Suggest no unless specific use case identified)
Any special rules for emptyValueDelimiterPolicy and nilValueDelimiterPolicy ?
Use on complex element. Presumably dfdl:length is first used to extract a 'box' but within that box does parser immediately scan for the dfdl:terminator or does it descend into the complex type and parse the children, expecting to either consume all the box or to find the terminator at the end? (Suggest the latter).
Use on complex element. Last child can not be dfdl:lengthKind 'endOfParent'.
Scanning rules: Use of this new dfdl:lengthKind switches off any in-scope stack of terminating markup in force at that point. Put another way, when we are scanning for the dfdl:terminator, we are not looking for any markup from an outer scope.

So there's plenty to think about with this new dfdl:lengthKind. A good rule for deciding whether a new dfdl:length or dfdl:occursCountKind should be added is whether it bends some other part of the spec out of shape. The new dfdl:lengthKind looks ok so far.

However we *think* we have come up with an alternative model which is simpler than you one you state in the document. Example for field 'varstr' with max length 100:

<xs:sequence dfdl:terminator="{if (fn:str-len(varstr) eq 100) then '%ES;' else '%DEL'}" ...>
<xs:element name="varstr" type="xs:string" dfdl:lengthKind="pattern" dfdl:pattern="([^\x7F].\x7F)|(.{100})" ... />
</xs:sequence>

Can't put dfdl:terminator with a self-referencing expression on the element. Might need fn:exists in the dfdl:terminator expression to handle optionality. Does that work?

Regards

Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848
----- Forwarded by Steve Hanson/UK/IBM on 11/07/2014 13:09 -----

From: Steve Hanson/UK/IBM
To: Mike Beckerle <mbeckerle.dfdl@gmail.com>,
Cc: "dfdl-wg@ogf.org" <dfdl-wg@ogf.org>
Date: 08/07/2014 13:31
Subject: Re: [DFDL-WG] Action 233 (deferred) - "byte order not sufficient..." - draft document on experience with binary format MIL-STD-2045

Mike

Please find attached IBM's initial comments to your experience document, as Word comments. We only got as far as the 3 x required extensions, not looked at the optional usability stuff in detail yet.

We think we have our collective heads around the least significant bit ordering concept, but we think the explanation could be clearer and show the bits on-the-wire. Some debate as to whether this could be considered some variation of byteOrder but you've obviously thought this through and concluded a separate property is best. Also should bit order apply to text reps, given that byteOrder is binary rep only and any byte ordering variations in encodings are handled as separate encodings (eg, UTF-16LE and UTF-16BE).

Regarding the US-ASCII-7-Bit-Packed encoding enum, this was added via erratum previously using the idea of DFDL-specific named encoding. But we are thinking that this could have been handled as an x- encoding, rather than specifically adding it to the spec. And thinking further on that same thread, should byteOrder be made to work like encoding and allow x- enums, then the new byteOrder would become a x- enum. The Wikipedia article you cite on Endianness mentions other byte orders (eg, Middle-Endian, PDP-Endian).

Regards

Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848

From: Mike Beckerle <mbeckerle.dfdl@gmail.com>
To: "dfdl-wg@ogf.org" <dfdl-wg@ogf.org>,
Date: 24/06/2014 20:27
Subject: [DFDL-WG] Action 233 (deferred) - "byte order not sufficient..." - draft document on experience with binary format MIL-STD-2045
Sent by: dfdl-wg-bounces@ogf.org

I have created an experience document about the "bit order" issue, which was a deferred action 233, and the subject of a public comment.

The document is here: http://redmine.ogf.org/dmsf_files/13268. The public comment item is http://redmine.ogf.org/boards/15/topics/43.

It recommends a new dfdl:bitOrder property, and a new dfdl:byteOrder enum value, without which it is impossible to model these data formats. It also recommends several other improvements to DFDL to facilitate handling these data formats.

The formats in question are a variety of MIL-STD formats which are all densely packed binary data. These formats are in broad use. MIL-STD-2045 is one part of this family and this particular format specification is generally available without any restrictions from a US DoD web site (http://assistdocs.com) so I made this specific format the subject of the document as it illustrates all the problematic issues.

We have implemented the dfdl:bitOrder property in Daffodil, and it works with some useful tests now passing.

We have also enhanced our TDML implementation to enable creation of tests for this feature (and in the process actually found two bugs in the MIL-STD-2045 spec!).

Both the property and this TDML enhancement are described in the document.

The sponsors of the Daffodil project are extremely keen to get this needed binary support into the DFDL v1.0 standard so as to have multiple DFDL implementations support it.

...mikeb

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | www.tresys.com
Please note: Contributions to the DFDL Workgroup's email discussions are subject to the OGF Intellectual Property Policy
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Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU