I’m a little confused. I think the language is there in the spec:

Spec v32 says:

encoding

Enum.

Values are IANA charsets or CCSID[MJB1] ^{^[1]}s.

This property can be computed by way of an expression which returns the appropriate string.

Note that there is, deliberately, no concept of 'native' encoding^{^[2]}.

Conforming DFDL v1.0 processors must accept at least 'UTF-8'', “UTF-16”, “UTF-16BE', ”UTF-16LE', ”ASCII”, and 'ISO-8859-1' [MJB2] as encoding names. Encoding names are case-insensitive, so “utf-8” and “UTF-8” are equivalent. The “UTF-16” encoding requires that dfdl:byteOrder is defined.

Annotation: dfdl:format

In the references it lists:

IANA character set encoding names: (http://www.iana.org/assignments/character-sets)

I agree that neither the minimum list above in the box, nor the reference to the IANA list are sufficient.

I did not find any “x- “ character sets described here.

Also, searching the IANA list I find no mention of BOM. So what list of encodings are you referring to?

I did find mention of UTF-16/UCS-2 requiring a BOM in the ICU. This may be a usage pattern that ICU supports, and if I were coding up something hoping it would be useful this might be what I would have done too; however, I have not seen data with this behavior, so I have to question whether this is in any way in use anywhere. Is it?

While we’re trimming options on encodings, with some web searching I wasn’t able to find the standard for CCSID other than at an IBM web site. So while there is a CCSID for iso-8859-1 encoding, that doesn’t mean CCSIDs are an ISO standard, rather just that they have some conformant sets.

Based on this I suggest dropping CCSID support since it is a vendor standard only (If I’m correct.) If this is, however, a de-facto standard even outside of IBM context then I’ll retract this suggestion.

W.r.t. BOMs, I spent quite a lot of time on BOMs, mostly due to the hassle that Unicode specifically says they are not characters; hence, I was shooting for a semantics where a 10 character string could have either 10 or 11 codepoints in it due to a BOM being present or absent thereby turning many fixed length things into variable length. Length determination gets pretty complex if you do this. You have to look at quite a few properties just to decide whether something is fixed or variable length.

The last proposal before we dropped BOMs altogether was to have a special character set UTF-16-VL (for variable length) which means there may or may not be a BOM. We concluded that this doesn’t belong in DFDL, I do think the right way to solve this BOM problem is with identification of encodings that allow/require/prohibit use of BOMs since a BOM is not a character it must be part of the character set encoding. E.g., UTF-16-BOM-required, UTF-16-BOM-prohibited, UTF-16-BOM-allowed, etc. Somebody other than DFDL should pick the names. The same issue comes up with the UTF-16 with and without the surrogate-pairs crud. I.e., do you want number of codepoints or do you want the surrogate-pairs considered to be one character. We used to have a lengthUnits=”fullUnicodeCharacters” to specify this behavior. This has been dropped as too complex also. Again UTF-16-VL was the last suggested way to fix this, i.e., VL for variable length meaning interpret the BOMs, the surrogate pairs, etc.

One other issue of this kind is the weird variant of utf-8 where surrogate pairs are encoded as 3 bytes each rather than using the 4-byte standard utf-8 way of encoding a 20-bit character code. Again, this should be a new character set encoding name. E.g., utf-8-encoded-surrogate-pairs. There’s java’s funny utf-8 variant also where zero is encoded as 2 bytes also. These are all issues where there is a funny encoding but no standard IANA name for it.

If someone would like to co-author a suggestion for some new IANA charset encoding names to propose to whomever that is, I would happily contribute.

At this point, I’m pretty convinced that we should just say for DFDL v1 a BOM is a codepoint and we treat it like any other codepoint.

I also haven’t seen any real use of BOMs. In memory people use native forms and don’t have these, and externally UTF-8 seems preferred. I’d like to hear of real BOM usage examples.

I also don’t think we “have to” support them in that a BOM can be treated like an optional element that might or might not exist before a string. Using a combination of valueCalc properties and defaults and a calculated value for the byteOrder property one can, I believe, achieve every combination of optional or required BOM and generate them on output or omit in whatever situations. It will be clumsy, but I prefer this to putting a bunch of speculative features into the standard where we don’t really have a strong usage model in mind.

…mike

From: dfdl-wg-bounces@ogf.org [mailto:dfdl-wg-bounces@ogf.org] On Behalf Of Ian W Parkinson
Sent: Tuesday, June 24, 2008 9:26 AM
To: dfdl-wg@ogf.org
Subject: Re: [DFDL-WG] Required encodings and testing

Hi all,

I'd suggest that we only need worry about those character sets described at http://www.iana.org/assignments/character-sets. Are the ones beginning "x-" specific to ICU? I think this would simplify the matter of BOMs somewhat, as we wouldn't need to deal explicitly with character sets that must have a BOM (presumably the -BOM variants) and so make the 'spec-twister' a non-issue.

Unicode BOMs would remain a complex issue, though. If the schema specifies encoding="UTF-16BE" or "UTF16-LE" then our behaviour is clear enough going by the spec at http://www.ietf.org/rfc/rfc2781.txt - we never generate a BOM, and any BOM encountered is treated as a character. If the schema specifies just "UTF-16" (in wihch the BOM is strictly optional) then we'd honour any BOM at the top of the text field, defaulting to the specified dfdl:byteOrder value. On unparse we can choose whether or not to include a BOM - I'd suggest we always include a BOM and use dfdl:byteOrder (*). If a particular schema needs to control this more explicitly then they can use an expression to compute UTF-16BE or UTF-16LE as appropriate.

That would leave the following edge-case: a schema which wants to generate BOMless data so specifies (e.g.) UTF-16LE, but wants to tolerate and honour any BOM present on parse. Do we need to deal with this unusual situation? It perhaps could be handled through an optional hidden field, but would we want to make it easier to achieve?

(*) the alternative would be to leave the byte order up to the implementation, potentially allowing data to be output with the endianness in which it was received. This may be beneficial in some situations but would leave the schema author without a way to specify the byteOrder while still requiring a BOM to be generated.

Cheers,

Ian

Ian Parkinson
WebSphere ESB Development
Mail Point 211, Hursley Park, Hursley, Winchester, SO21 2JN, UK

From:	"RPost" <rp0428@pacbell.net>
To:	<dfdl-wg@ogf.org>
Date:	24/06/2008 01:58
Subject:	[DFDL-WG] Required encodings and testing

Thanks for the response re encodings and issues. Very helpful.

I put my responses in the attachment but here is the first part about encoding.

Your response: We haven't picked a basic set that all conforming implementations must support other than that UTF-8 and USASCII must be supported. We might require more than this though.

That’s a relief!

The current spec mentions UTF-8, ebcdic-cp-us (IBM037), and UTF-16BE.

Since Java 1.6 supports 160 encodings using 686 aliases I've no doubt you see the reason for my question about which encodings need initial support.

ICU supports even more encodings and requiring some of these could implicitly require implementors to support ICU. Not an issue if that is truly needed but that requirement alone could dissuade some from participating in the project.

The encodings I have examined/tested so far are: US-ASCII, ISO-8859-1, UTF-8, UTF-16, UTF-16BE, UTF-16LE, UTF-32, UTF-32BE, UTF-32LE, IBM1047, IBM500, IBM037, x-UTF-16LE-BOM, X-UTF-32BE-BOM, X-UTF-32LE-BOM.

I have not run across any issues with any of the above encodings.

ICU includes 175 UCM files of which 135 are for SBCS encodings. I have not tested or examined all of these but would not expect them to be an issue either.

Also not examined are the 27 UCM files for MBCS encodings. A brief review shows that many of these should not be an issue.

BIG5 or GB18030 could definitely be an issue and there are several others like these that might require a custom effort to support. Ok if you really need it but better delayed initially if you don't.

Glad to know we don't need to visit these for the short term. I'm sure implementers would much rather concentrate on the DFDL aspect of things rather than become encoding experts. I know I would.
--
dfdl-wg mailing list
dfdl-wg@ogf.org
http://www.ogf.org/mailman/listinfo/dfdl-wg

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

^{^[1]} CCSID stands for Coded Character Set ID, a 3 digit representation for a codepage specifier. TBD: cite relevant standard for CCSIDs here.

^{^[2]} The concept of native character encoding is avoided in DFDL since a DFDL schema containing such a property binding does not contain a complete description of data, but rather an incomplete one which is parameterized by characteristics of the operating environment where the DFDL processor executes. In DFDL this same behavior is achieved through use of true parameterization, for example by use of Selectors to choose among annotations specifying different character set encoding property bindings.