Thanks Mike,
I agree with almost all of that. A couple
of things remain to be cleared up, though.
- The term 'Unicode Signature' is understood
by a very small percentage of developers and is ( in practice ) interchangeable
with 'Byte Order Mark', Although it sounds like a misuse of the term, various
documents issued by the Unicode consortium talk about the 'UTF8 BOM'. That
being the case, I think DFDL should use the better-known term 'Byte Order
Mark' both in the description and the property name.
- There is no way for a client application
to find out what the actual byte order was, which will make it difficult
to serialize a parsed infoset while preserving the byte order of the original
data stream.
regards,
Tim Kimber, Common Transformation Team,
Hursley, UK
Internet: kimbert@uk.ibm.com
Tel. 01962-816742
Internal tel. 246742
From:
"Mike Beckerle"
<mbeckerle.dfdl@gmail.com>
To:
<dfdl-wg@ogf.org>
Date:
25/08/2011 01:01
Subject:
[DFDL-WG] Action
151 - BOM disposal
Sent by:
dfdl-wg-bounces@ogf.org
My stab at BOMs:
On our last call, when discussing
the BOMs aka Unicode signature, we concluded that the grammar for document
changes to
Document = UnicodeSignature
Element
Below is my proposal (based
on an earlier proposal by Steve, but modified to be “Unicode signature”
oriented instead of “BOM” oriented)
The UnicodeSignature field
is optional. It is only of non-zero size when the encoding is exactly one
of UTF-8, UTF-16, or UTF-32, and the following Element begins with textual
data (meaning it is of type string, or any other type with representation=”text”,
or when complex, its first child is always textual data inductively). In
all other cases the Unicode Signature is assumed to be absent (zero length)
when parsing, and it is not generated on unparsing.
The UnicodeSignature must
contain one of the allowed byte sequences for a UnicodeSignature. If it
does, then it is removed and does not contribute to characters parsed or
otherwise placed into the infoset. These are the valid byte sequences:
Bytes
| Encoding
Form
|
00 00 FE FF
| UTF-32, big-endian
|
FF FE 00 00
| UTF-32, little-endian
|
FE FF
| UTF-16, big-endian
|
FF FE
| UTF-16, little-endian
|
EF BB BF
| UTF-8 |
If the byte pattern indicates
little endian, then encoding=”UTF-xx” is interpreted throughout the document
as if it said “UTF-xxLE”, and similarly UTF-xxBE if the signature indicates
big endian. (shorthand xx = 16 or xx = 32)
If the byte pattern is the
signature for utf-8, and the encoding is utf-8, then the signature is simply
removed, and is not placed into the infoset, but as there is no byte order
for utf-8, there is nothing else to do.
If the bytes are not a Unicode
signature, and the dfdl:encoding is utf-16 or utf-32, then the dfdl:byteOrder
property is used to determine the byte order. If dfdl:byteOrder is not
defined, then bigEndian is assumed throughout the document.
We need a property to determine
whether we generate a Unicode signature or not. Suggest documentOutputUnicodeSignature=yes/no
as the property, and this property is only interpreted when the encoding
is utf-8, utf-16, or utf-32, and the document element begins with textual
data (defined as above). In the output case, for utf-16 and utf-32, something
has to decide what endianness to use, so the dfdl:byteOrder property is
used to determine the output encoding to be used. If dfdl:byteOrder is
not defined, then bigEndian is used.
…mikeb
From: Steve Hanson [mailto:smh@uk.ibm.com]
Sent: Tuesday, August 23, 2011 5:04 AM
To: Mike Beckerle
Cc: 'Stephanie Fetzer'; Tim Kimber
Subject: RE: Fw: BOM disposal
We have documentFinalTerminatorCanBeMissing so that modellers don't have
the headache of explicitly modeling an 'optional' <CR><LF>
at the end of a document. I don't see why we shouldn't assist Unicode modellers
in a similar way. But only at document level.
UTF-16/32. I think when U+FEFF is encountered at any place other
than the start of a DFDL described document, then it should be interpreted
as ZWNBS. This
is in keeping with the intent of the Unicode standard (as quoted by you
in the other e-mail you forwarded).
UTF-8. I can go either way on this. Although not strictly a byte order
control, it is something that may or may not appear at the start of a UTF-8
document and I can see Tim's argument for handling it seamlessly.
Regards
Steve Hanson
Architect, Data Format Description Language (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:
| Tim Kimber/UK/IBM@IBMGB, Steve Hanson/UK/IBM@IBMGB
|
Cc:
| "'Stephanie Fetzer'" <sfetzer@us.ibm.com>
|
Date:
| 23/08/2011 03:29
|
Subject:
| RE: Fw: BOM disposal |
Ok, so that’s interesting.
So we’re down to the issue of whether in a fixed-length string context,
does a BOM count as one of the fixed length of characters, or not.
IMHO, I think BOM/ZWNBS should just be treated as another codepoint to
us, and we shouldn’t be removing them, or treating them as “non-characters”.
As to whether to generate them, I think we should not. That is to
say, regardless of whether we interpret them to determine the encoding,
they should still be codepoints that appear in the infoset, both when parsing,
and when unparsing. This means that they interact badly with things like
initiators and padding. Hence, they’re very likely to be modeled as separate
string elements containing only the BOM.
When encoding is UTF-16 or UTF-32, there is the question of whether one
must have a BOM for every single string, or whether one must compute the
byteOrder property from data, or if there is some “magic sticky behavior”
where some prior string can have a BOM, and have this respected by subsequent
string elements.
I suggest the following definition of “has a BOM to specify the byte order”
(1) The element, of type string, begins with the BOM codepoint.
This changes the DFDL grammar. The Byte-order-mark field in the grammar
would appear before the initiator of the element, and before any pad characters.
(2) An enclosing sequence has a nearest (greatest index) prior
sibling which recursively “has a BOM to specify the byte order”
Inductively, this means the first element of a sequence can have a BOM,
and all subsequent elements in that sequence as direct children or within
subsequences/choices, and sub-elements generally, will all pick up their
byteOrder from that same BOM.
It also lets you concatenate two representations, one of which is BOM big-endian,
the other BOM little-endian.
This does add some overhead. In every case if encoding is utf-16, then
for every string, you must check for a BOM.
From: Tim Kimber [mailto:KIMBERT@uk.ibm.com]
Sent: Monday, August 22, 2011 5:32 AM
To: Steve Hanson
Cc: mbeckerle.dfdl@gmail.com;
Stephanie Fetzer
Subject: Re: Fw: BOM disposal
I disagree.
The term 'Byte Order Mark' is potentially misleading. It does not only
indicate byte order - it also indicates the encoding of the stream, A BOM
can legally be used at the start of a UTF-8 document, when it is more properly
called a 'Unicode Signature'. Some text editors mark all their UTF-8 documents
in this way ( including Eclipse on Linux, apparently ).
The Unicode standard 6.0 (http://www.unicode.org/versions/Unicode6.0.0/UnicodeStandard-6.0.pdf)
says:
Unicode Signature. An initial BOM may also serve as an implicit
marker to identify a file as
containing Unicode text. For UTF-16, the sequence FE16
FF16
(or its byte-reversed counterpart,
FF16 FE16)
is exceedingly rare at the outset of text files that use other character
encodings. The corresponding UTF-8 BOM sequence, EF16
BB16
BF16,
is also exceedingly
rare. In either case, it is therefore unlikely to be confused with real
text data. The same is
true for both single-byte and multibyte encodings.
Data streams (or files) that begin with the U+FEFF byte order mark are
likely to contain
Unicode characters. It is recommended that applications sending or receiving
untyped data
streams of coded characters use this signature. If other signaling methods
are used, signatures
should not be employed.
Conformance to the Unicode Standard does not require the use of the BOM
as such a signature.
See Section 16.8, Specials, for more information on the byte order
mark and its use
as an encoding signature.
This paragraph could be taken to imply that UTF-8 with a BOM is rare, but
that does not appear to be the case in the real world:
While there is obviously no need for a byte order signature when using
UTF-8,
there are occasions when processes convert UTF-16 or UTF-32 data containing
a byte order mark into UTF-8. When represented in UTF-8, the byte order
mark turns into the byte sequence <EF BB BF>. Its usage at the beginning
of a
UTF-8 data stream is neither required nor recommended by the Unicode Standard,
but its presence does not affect conformance to the UTF-8 encoding
scheme. Identification of the <EF BB BF> byte sequence at the beginning
of a
data stream can, however, be taken as a near-certain indication that the
data
stream is using the UTF-8 encoding scheme.
regards,
Tim Kimber, Common Transformation Team,
Hursley, UK
Internet: kimbert@uk.ibm.com
Tel. 01962-816742
Internal tel. 246742
From: Steve
Hanson/UK/IBM
To: mbeckerle.dfdl@gmail.com
Cc: Tim Kimber/UK/IBM@IBMGB,
Stephanie Fetzer/Charlotte/IBM@IBMUS
Date: 18/08/2011
09:03
Subject: Fw:
BOM disposal
Hi Mike
I've re-read the BOM and UTF-8 material and I agree with you. Explicit
modelling of a ZWNBS character suffices for UTF-8.
Regards
Steve Hanson
Architect, Data Format Description Language (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 18/08/2011 08:58 -----
From:
| Mike Beckerle <mbeckerle.dfdl@gmail.com>
|
To:
| Steve Hanson/UK/IBM@IBMGB
|
Date:
| 17/08/2011 23:00
|
Subject:
| RE: BOM disposal |
I think it is OK to add BOM control but I think the reference to utf8 and
BOMs is wrong. We should never encode a BOM into utf8 and if a zwnbs is
encoded in utf8 even as the first codepoint it should not ever be considered
to be a BOM and should always go into the infoset.
----- Forwarded by Steve Hanson/UK/IBM on 18/08/2011 08:58 -----
From:
| Steve Hanson/UK/IBM
|
To:
| "Mike Beckerle" <mbeckerle.dfdl@gmail.com>
|
Cc:
| "'Stephanie Fetzer'" <sfetzer@us.ibm.com>,
Tim Kimber/UK/IBM@IBMGB
|
Date:
| 17/08/2011 17:57
|
Subject:
| RE: BOM disposal |
Hi Mike
I've read below and also the historical e-mail that you forwarded.
I am happy that when U+FEFF is encountered at any place other than the
start of a DFDL described document, then it is interpreted as ZWNBS.
But I am concerned that we are making life harder than it need be for modellers
who have to handle Unicode documents that start with a BOM.
Take the simple example of wanting to read in a file in one encoding, look
at the DFDL infoset in order to make some routing decision, and then send
it on in a different encoding. As the spec stands, for all encodings except
those with a BOM the modeller can create a single DFDL model that uses
external variable $encoding to control the output. But once you make
one of the document's encoding Unicode with the possibility of a BOM then
the model has to change to accomodate this in a non-trivial way. That's
not very usable, and further I don't think it is in the spirit of another
paragraph in RFC 2781...
All applications that process text with the "UTF-16" charset
label
MUST be able to read at least the first two octets of the text and be
able to process those octets in order to determine the serialization
order of the text. Applications that process text with the "UTF-16"
charset label MUST NOT assume the serialization without first
checking the first two octets to see if they are a big-endian BOM, a
little-endian BOM, or not a BOM. All applications that process text
with the "UTF-16" charset label MUST be able to interpret both
big-
endian and little-endian text.
Proposal:
On parsing: If encoding is set when starting
to process the model, and is UTF-8, UTF-16, UTF-32 (including BE/LE variants)
then the DFDL parser looks for a BOM.
If a BOM is found at the very start of the document then it is not
added to the infoset, and:
- UTF-16, UTF-32: The DFDL byteOrder property is ignored for text data
of those encodings throughout the rest of the document and the BOM implies
the byte order
- UTF-8: The BOM is ignored as byte order is not used anyway.
- LE/BE variants. Processing error as this contravenes the Unicode standard..
If there is no BOM then byteOrder property behaves as currently stated
for UTF-16 and UTF-32.
On unparsing: If encoding is set when starting to process the model, and
is UTF-8, UTF-16 or UTF-32 (excluding BE/LE variants), then the DFDL unparser
optionally outputs a BOM, under the control .of a new document-level property
**, documentOutputBOM = yes/no. The BOM that is output depends on the setting
of byteOrder.
There is one issue with this. I deliberately used the phrase 'if encoding
is set when starting to process the model'. We have to define what this
means. DFDL encoding applies to all text elements and all objects that
have text delimiters. One option is to say that BOM processing only takes
place if encoding is actually to be used by the first element in the model.
So if I started my data with binary data that did not have an initiator
then no BOM processing would take place. Another option is to say that
BOM processing only takes place if there is a default dfdl:format in the
xsd with encoding set (then you can imagine the BOM as an implicit hidden
optional element that gets encoding from scope).
** (We already document level properties - documentFinalTerminatorCanBeMissing).
Regards
Steve Hanson
Architect, Data Format Description Language (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:
| Tim Kimber/UK/IBM@IBMGB
|
Cc:
| Steve Hanson/UK/IBM@IBMGB, "'Stephanie
Fetzer'" <sfetzer@us.ibm.com>
|
Date:
| 15/08/2011 21:54
|
Subject:
| RE: BOM disposal |
I stand corrected on the BOM character. This ZWNBS stuff means it *is*
a character regardless of the Unicode folks having deprecated it (see http://en.wikipedia.org/wiki/Zero-width_non-breaking_space
) , or their goal of BOMs somehow being non-characters.
Though my guess is that it mostly would come up because UTF-16 with BOM
was converted to UTF-8, with the BOM at the front converted to the UTF-8
encoding of a BOM. Concatenate some of these, and you’ll have ZWNBS characters
embedded in the string.
I think there is a flock more cases beyond the ones Tim enumerated having
to do with whether you remove the BOM or it takes up space in the string.
E.g., if I have fixed length data with properties that say there is an
optional BOM, is that data now variable length? I’d rather not go there.
If I ask the length in characters of a string, do I count BOMs or not?
Either way, the point is that there is good reason to just treat these
BOM/ZWNBS as characters, and to just fix the language in the spec about
UTF-8 BOMs, which is just fixing a turn of phrase.
Stripping these characters out, that’s a calculation an application can
easily do. (I could be talked into an XPath function in DFDL to do exactly
this.)
The 2nd paragraph about BOMs in the spec mentions they can be
modeled. I believe the BOM-based behaviors described in Tim’s mail can
all be modeled relatively easily as separate elements. They can then compute
the value of the byteOrder property with an expression that references
the elements. (I am assuming we allow byteOrder to be computed…. ). To
be concrete about it:
E.g.,
<sequence>
<element name=”bom1” type=”byte” dfdl:representation=’binary’
Dfdl:outputValueCalc=”{0xFE}”/>
<element name=”bom2” type=”byte” dfdl:representation=’binary’
Dfdl:outputValueCalc=”{0xFF}”/>
<element name=”data” type=”string” dfdl:encoding=”utf-16”
Dfdl:byteOrder=”{ if (../bom1 = 0xFE and ../bom2 = 0xFF) then
‘bigEndian’
Else if (../bom1 = 0xFF
and ../bom2 = 0xFE then ‘littleEndian’
Else error(‘no BOM found’)
}”
/>
</sequence>
One could even create a situation where BOM’s are accepted and tolerated:
<choice>
<…. The above sequence is one arm of the choice …>
<element name=”data” type=”string” dfdl:encoding=”utf-16be”/>
</choice>
This would cause a BOM to be accepted and used if present, and default
to bigEndian otherwise. Output would always be bigEndian.
With some clever use of variables and type definitions, I suspect this
can even be made reasonably compact.
These things are clumsy, but the alternative is more properties, and of
all the cases Tim enumerated, we’re not even sure we have them all, or
if anyone will use them.
Some much earlier DFDL draft had a unicodeByteOrderMarkPolicy property,….
I believe it was dropped for lack of clarity on exactly what the use cases
needed to be. It was like ‘prohibited’ ‘tolerated’ ‘required’ ‘ignored’
‘generated’ or some enumeration like that.
…mikeb
From: Tim Kimber [mailto:KIMBERT@uk.ibm.com]
Sent: Wednesday, July 27, 2011 4:32 PM
To: mbeckerle.dfdl@gmail.com
Cc: Steve Hanson; Stephanie Fetzer
Subject: BOM disposal
Key points about BOMs are:
- For all Unicode encodings, the "Zero Width Non-breaking Space"
character corresponds to the byte sequence of a BOM, but...
- a BOM is not considered to be a part of the data
My own assumptions about BOMs are:
- some input documents will have a BOM by accident, just because the application
that wrote it did not explicitly tell the encoder to omit the BOM.
- some users will expect a BOM at the start of an input document to be
honoured
- most users will be surprised if they get a ZWNBSP in the info set. Some
may even get a little annoyed if they find that they cannot prevent it,
because the Unicode specification is pretty clear that BOMs are not data.
I think we need to modify the DFDL rules about handling of BOMs. I don't
have all the answers, but I do think the following scenarios are likely
to crop up:
Parsing:
a) there is a BOM at the start of the input document.{1} The user
wants the DFDL parser to act as though the dfdl:encoding external variable
had been set to the encoding implied by the BOM.
b) there is sometimes a BOM at the start of the input document. The character
encoding is defined by the schema so the BOM is redundant. The user doesn't
care whether it is there or not, and would like DFDL to completely ignore
it.
c) at some point within the document ( not at the start ) there is a BOM
at the beginning of an element. The user wants the BOM to be ignored.
d) at some point within the document ( not at the start ) there is a BOM
at the beginning of an element. The user wants the encoding of the element
to be defined by the BOM
e) the user wants a BOM to be treated exactly like an ordinary character
( probably with the aim of ensuring that the document round-trips without
losing BOMs ).
Serializing
f) the user always wants the output document to start with a BOM when the
encoding is one of the Unicode encodings
g) the user wants an element within the document to start with a BOM that
signals its encoding
Feel free to come with other scenarios if you think I've missed any.
{1} I think I've done quite well to avoid any Monty Python 'Life of
Brian' references so far...
regards,
Tim Kimber, Common Transformation Team,
Hursley, UK
Internet: kimbert@uk.ibm.com
Tel. 01962-816742
Internal tel. 246742
----- Forwarded by Tim Kimber/UK/IBM on 27/07/2011 20:59 -----
From: Steve
Hanson/UK/IBM
To:
mbeckerle.dfdl@gmail.com
Cc: Tim Kimber/UK/IBM@IBMGB
Date: 27/07/2011
19:15
Subject: OGF
DFDL WG Call Agenda 2011-08-09
Hi Mike
I've posted a draft agenda on GridForge below for 9th Aug call.
The last of the spec issues you raised concerned section 12.3.7.1.3 about
BOMs. I know that Tim is not happy with this either, and has done some
thinking in this area. However he is on vacation 9th Aug. It might be worth
you two getting together before then and discussing?
-----------------------------------------------------------------------------------------------------------
Please find agenda for the above call on GridForge at:
http://forge.gridforum.org/sf/docman/do/downloadDocument/projects.dfdl-wg/docman.root.current_0.calls/doc16305/1
As per action 144 an errata to the spec has been created here: http://forge.gridforum.org/sf/go/doc16280?nav=1
Regards
Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF
DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848
Unless stated otherwise above:
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741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6
3AU
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
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
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
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