One document matched: draft-legg-xed-dxer-00.txt
INTERNET-DRAFT S. Legg
draft-legg-xed-dxer-00.txt Adacel Technologies
Intended Category: Standard Track D. Prager
Deakin University
August 7, 2003
Directory XML Encoding Rules for ASN.1 Types
Copyright (C) The Internet Society (2003). All Rights Reserved.
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document defines a set of Abstract Syntax Notation One (ASN.1)
encoding rules, called the Directory XML Encoding Rules or DXER, that
produce an Extensible Markup Language (XML) representation for values
of any given ASN.1 data type.
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Table of Contents
1. Introduction .................................................. 2
2. Conventions ................................................... 3
3. Definitions ................................................... 3
4. General Considerations ........................................ 4
5. Standalone DXER Encodings ..................................... 5
6. Encoding Rules ................................................ 6
6.1 Identifiers ............................................... 6
6.2 Type Referencing Notations ................................ 6
6.3 Restricted Character String Types ......................... 7
6.4 BIT STRING ................................................ 7
6.5 BOOLEAN ................................................... 8
6.6 CHARACTER STRING .......................................... 8
6.7 CHOICE .................................................... 8
6.8 EMBEDDED PDV .............................................. 9
6.9 ENUMERATED ................................................ 9
6.10 EXTERNAL ................................................. 9
6.11 GeneralizedTime .......................................... 9
6.12 INSTANCE OF .............................................. 10
6.13 INTEGER .................................................. 10
6.14 NULL ..................................................... 11
6.15 ObjectDescriptor ......................................... 11
6.16 OBJECT IDENTIFIER and RELATIVE-OID ....................... 11
6.17 OCTET STRING ............................................. 11
6.18 REAL ..................................................... 12
6.19 SEQUENCE and SET ......................................... 12
6.20 SEQUENCE OF and SET OF ................................... 13
6.21 UTCTime .................................................. 13
6.22 Open Type ................................................ 14
6.23 AnyType .................................................. 15
7. DXER Transfer Syntax .......................................... 16
8. Relationship to XER ........................................... 16
9. Security Considerations ....................................... 17
10. Acknowledgements ............................................. 17
11. Normative References ......................................... 17
12. Informative References ....................................... 18
13. Intellectual Property Notice ................................. 19
14. Copyright Notice ............................................. 19
15. Authors' Addresses ........................................... 20
1. Introduction
This document defines a set of Abstract Syntax Notation One (ASN.1)
[X680] encoding rules, called the Directory XML Encoding Rules or
DXER, that produce an Extensible Markup Language (XML) [XML]
representation of ASN.1 values of any given arbitrary ASN.1 type.
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An ASN.1 value is regarded as analogous to the content of an element.
The DXER encoding of an ASN.1 value is the well-formed and valid
content of an element in an XML document [XML] conforming to XML
namespaces [XMLNS]. Simple ASN.1 data types such as PrintableString,
INTEGER, BOOLEAN, define character data content while the ASN.1
combining types (i.e. SET, SEQUENCE, SET OF, SEQUENCE OF, and CHOICE)
define element content. The element names are provided by the
identifiers of the components in combining type definitions.
Note that "ASN.1 value" does not mean a Basic Encoding Rules (BER)
[X690] encoded value. The ASN.1 value is an abstract concept that is
independent of any particular encoding. BER is just one possible
encoding of an ASN.1 value. This document defines another possible
encoding.
Rules for canonical DXER encodings will be introduced in a revision
of this document.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [BCP14].
Throughout this document "type" shall be taken to mean an ASN.1 type,
and "value" shall be taken to mean an ASN.1 abstract value.
A reference to a ASN.1 production [X680] (e.g. Type, NamedType) is a
reference to the text in an ASN.1 specification corresponding to that
production.
The specification of DXER makes use of definitions from the XML
Information Set (Infoset) [ISET]. In particular, information item
property names are presented per the Infoset, e.g. [local name]. In
the sections that follow, the term "element" shall be taken to mean
an Infoset element information item.
Literal character strings to be used in the DXER encoding appear
within double quotes, however the double quotes are not part of the
literal value and do not appear in the encoding.
This document uses the namespace prefix "xsi:" to stand for the
namespace name "http://www.w3.org/2001/XMLSchema-instance", though in
practice any valid namespace prefix is permitted in DXER encodings.
3. Definitions
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The root element of an XML document is the [document element] of the
document information item corresponding to the XML document.
The normalized content of an element information item is the list of
information items formed by taking, in order, each character and
element information item in the [children] of the element information
item (thus eliminating any comments or PIs from consideration when
determining the correctness of a DXER encoding).
If the normalized content contains only character information items
then its string value is the sequence of [character codes] of those
character information items in order, otherwise its string value is
empty.
Note that the normalized content definition is for descriptive
purposes only. There is no requirement for DXER encodings to
actually be normalized.
4. General Considerations
A DXER encoding is permitted to contain XML comments, processing
instructions (PIs), CDATA sections, character references and parsed
entity references in any position allowed for a well-formed and valid
XML document [XML].
If entity references (to other than the predefined entities) are used
then the XML document containing the DXER encoding must necessarily
contain a document type declaration and the internal or external
subset of the document type definition (DTD) must contain a
declaration for the entity.
Although comments and PIs are permitted in DXER encodings, there is
no provision for representing comments and PIs in ASN.1 abstract
values, therefore applications using DXER MAY discard any comments or
PIs in received encodings.
Similarly, there is no provision for representing entity references
in ASN.1 abstract values, therefore applications using DXER MAY
replace entity references with their replacement text at any time.
The [attributes] of any element in a DXER encoding are permitted to
contain an attribute information item with the [local name] "type"
and the [namespace name] "http://www.w3.org/2001/XMLSchema-instance"
(i.e. xsi:type [XSD1]) provided the Type of the corresponding
NamedType has an associated qualified name [XMLNS] (see Section 6.22)
that can be used to identify the type.
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Any element in a DXER encoding is permitted to have namespace
declaration attributes [XMLNS]. However note that, with the possible
exception of the root element, the [namespace name] of an element in
a DXER encoding is required to have no value. Thus the applicability
of namespace declarations will be limited to defining the [prefix]
for the root element, and the Prefix for qualified names as values of
the xsi:type attribute.
5. Standalone DXER Encodings
The DXER encoding of some value generates only the content of an
element. When the value being encoded is only part of some larger
XML document (which is, for example, the way ASN.1 Schema [ASD] uses
DXER) then it is the responsibility of the specification invoking
DXER to determine the context of the enclosing element (i.e. its
[local name] and [namespace name]).
DXER can also be used to generate an entire XML document from the
encoding of a value. This is termed a Standalone DXER Encoding of
the value.
ASN.1 does not have a concept analogous to the root element of an XML
document. That is, ASN.1 does not allow a NamedType to appear on its
own, outside of an enclosing combining type. This means that the
rules for encoding the root element in a Standalone DXER Encoding
differ from those that apply to any other element in a DXER encoding.
In a Standalone DXER Encoding the [local name] and [namespace name]
of the root element are determined from the ASN.1 type of the value
being encoded. If the ASN.1 type is not identified with a
typereference (excluding parameterized reference names) [ASN.1] then
the the [local name] of the root element SHALL be "anonymous" and the
[namespace name] of the root element SHALL have no value, otherwise
if the ASN.1 module in which the type is defined does not have a
namespace name [XEDNS] then the [local name] of the root element
SHALL be the typereference and the [namespace name] of the root
element SHALL have no value, otherwise the [local name] of the root
element SHALL be the typereference and the [namespace name] of the
root element SHALL be the namespace name of the module. In the final
case, a namespace declaration will also be required to associate the
namespace name with the chosen [prefix] of the root element.
The [attributes] and [children] of the root element (i.e. its
content) are generated by the normal application of the encoding
rules in Section 6 to the value being encoded.
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6. Encoding Rules
The following sections describe the DXER encoding for values of each
of the ASN.1 type notations. ASN.1 values are uniformly regarded as
analogous to the content of an element, not complete elements in
their own right. In every case described in the following sections,
if the encoding of an ASN.1 value produces no content then the
enclosing element MAY be encoded as an empty element (i.e. using an
empty-element tag).
6.1 Identifiers
An identifier, as defined in ASN.1 notation (Clause 11.3 of X.680
[X680]), is a character string that begins with a latin lowercase
letter (U+0061-U+007A) and is followed by zero, one or more latin
letters (U+0041-U+005A, U+0061-U+007A), decimal digits
(U+0030-U+0039), and hyphens (U+002D). A hyphen is not permitted to
be the last character and a hyphen is not permitted to be followed by
another hyphen. The case of letters in an identifier is always
significant.
ASN.1 identifiers are used for the [local name] of child elements and
may also appear in the character data content of elements.
6.2 Type Referencing Notations
A value of a type with a defined type name is encoded according to
the type definition on the right hand side of the type assignment for
the type name.
A value of a type denoted by the use of a parameterized type with
actual parameters is encoded according to the parameterized type with
the DummyReferences [X683] substituted with the actual parameters.
A value of a tagged or constrained type is encoded as a value of the
type without the tag or constraint, respectively. Tags do not appear
in the XML encodings defined by this document. See X.680 [X680] and
X.682 [X682] for the details of ASN.1 constraint notation.
A value of a fixed type denoted by an ObjectClassFieldType is encoded
according to that fixed type (see Section 6.22 for the case of an
ObjectClassFieldType denoting an open type).
A value of a selection type is encoded according to the type
referenced by the selection type.
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A value of a type described by TypeFromObject notation [X681] is
encoded according to the denoted type.
A value of a type described by ValueSetFromObjects notation [X681] is
encoded according to the governing type.
6.3 Restricted Character String Types
A value of a restricted character string type is encoded such that
the normalized content is the sequence of character information items
representing the characters in the string. Depending on the ASN.1
string type, and an application's internal representation of that
string type, a character may need to be translated to or from the
equivalent ISO 10646 character code [UCS]. The NumericString,
PrintableString, IA5String, VisibleString (ISO646String), BMPString
and UniversalString character encodings use the same character codes
as ISO 10646. For the remaining string types (GeneralString,
GraphicString, TeletexString, T61String and VideotexString) see X.680
[X680].
Note that a consequence of defining the DXER encoding in terms of the
XML Infoset is the implied requirement for ampersand ('&', U+0026)
and left angle bracket ('<', U+003C) characters in string values to
be escaped appropriately [XML].
Certain characters (e.g. control characters) are not legal characters
for XML. These characters are encoded as the replacement character
(U+FFFD). When decoding, the replacement character is retained if it
is a permitted character for the string type, otherwise it is
converted to U+0000 if that character is permitted by the string
type, otherwise it is discarded.
6.4 BIT STRING
A value of the BIT STRING type without a NamedBitList is encoded such
that the string value of the normalized content is either a binary
digit string or a hexadecimal digit string. A hexadecimal digit
string MAY be used only if the number of bits in a BIT STRING value
is a multiple of eight, otherwise a binary digit string is used.
A binary digit string is a sequence of zero, one or more of the
binary digit characters "0" and "1" (i.e. U+0030 and U+0031), Each
bit in the BIT STRING value is encoded as a binary digit in order
from the first bit to the last bit.
A hexadecimal digit string is a sequence of zero, one or more pairs
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of the hexadecimal digit characters "0"-"9", "A"-"F" and "a"-"f"
(i.e. U+0030-U+0039, U+0041-U+0046 and U+0061-U+0066). Each group of
eight bits in the BIT STRING value is encoded as a pair of
hexadecimal digits where the first bit is the most significant. An
odd number of hexadecimal digits is not permitted. If a hexadecimal
digit string is used then the enclosing element's [attributes] SHALL
contain an attribute information item with the [local name] "type"
and the [namespace name] "http://www.w3.org/2001/XMLSchema-instance".
The [normalized value] of this attribute SHALL be the qualified name
with namespace name "http://www.w3.org/2001/XMLSchema" and local part
"hexBinary" (e.g. xsi:type="xsd:hexBinary").
A value of the BIT STRING type with a NamedBitList is encoded such
that the string value of the normalized content is either, as above
for the BIT STRING type without a NamedBitList or, a space separated
list of identifiers. In the latter case, each "1" bit in the
BIT STRING value is represented by its corresponding identifier from
the NamedBitList, in any order.
6.5 BOOLEAN
The BOOLEAN value TRUE is encoded such that the string value of the
normalized content is the literal "true" or "1", at the encoder's
option. The BOOLEAN value FALSE is encoded such that the string
value of the normalized content is the literal "false" or "0", at the
encoder's option.
The DXER encoding of BOOLEAN values is intended to conform to the
lexical representation of the XML Schema boolean datatype.
6.6 CHARACTER STRING
A value of the unrestricted CHARACTER STRING type is encoded
according to the corresponding SEQUENCE type defined in Clause 40.5
of X.680 [X680].
6.7 CHOICE
A value of a CHOICE type other that a ChoiceOfStrings type [GSER] is
encoded such that the normalized content is a single child element
information item - corresponding to the actual chosen alternative -
optionally preceded by and/or followed by white space character
information items.
The chosen alternative corresponds to some NamedType in the CHOICE
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type definition. The [local name] of the child element corresponding
to the chosen alternative SHALL be the identifier of the
corresponding NamedType, the [namespace name] of the child element
SHALL have no value, and the content of the child element SHALL be
the encoding of the value of the chosen alternative according to the
Type of this NamedType.
A value of a ChoiceOfStrings type is encoded such that the string
value of the normalized content is the encoding of the value of the
chosen alternative. The enclosing element's [attributes] MAY contain
an attribute information item with the [local name] "type" and the
[namespace name] "http://www.w3.org/2001/XMLSchema-instance" to
identify the chosen alternative. The [normalized value] of this
attribute SHALL be the qualified name with namespace name
"http://xmled.info/ns/ASN.1/0702/00" and local part either
"BMPString", "GeneralString", "GraphicString", "IA5String",
"ISO646String", "NumericString", "PrintableString", "TeletexString",
"T61String", "UniversalString", "UTF8String", "VideotexString", or
"VisibleString", as appropriate.
If the ChoiceOfStrings value has no character data then the enclosing
element MAY be encoded as an empty element (i.e. using an
empty-element tag).
6.8 EMBEDDED PDV
A value of the EMBEDDED PDV type is encoded according to the
corresponding SEQUENCE type defined in Clause 33.5 of X.680 [X680].
6.9 ENUMERATED
A value of an ENUMERATED type is encoded such that the string value
of the normalized content is the identifier corresponding to the
actual value.
6.10 EXTERNAL
A value of the EXTERNAL type is encoded according to the
corresponding SEQUENCE type defined in Clause 8.18.1 of X.690 [X690].
6.11 GeneralizedTime
A value of the GeneralizedTime type is encoded such that the string
value of the normalized content is a date followed by the letter "T",
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a time of day, optional fractional seconds, and an optional time
zone.
The date is two decimal digits representing the century, followed by
two decimal digits representing the year, "-" (U+002D), two decimal
digits representing the month, "-" (U+002D), and two decimal digits
representing the day.
The time of day is two decimal digits representing the hour, followed
by ":" (U+003A), two decimal digits representing the minutes, ":"
(U+003A), and two decimal digits representing the seconds.
The fractional seconds is a period "." (U+002E) followed by zero, one
or more decimal digits (U+0030-U+0039).
A GeneralizedTime value with fractional hours or minutes is first
converted to the equivalent time with whole minutes and seconds and,
if necessary, fractional seconds.
The minutes are encoded as "00" if the GeneralizedTime value omits
minutes. The seconds are encoded as "00" if the GeneralizedTime
value omits seconds.
The time zone, if present, is either the letter "Z" (U+005A) to
indicate Coordinated Universal Time, a "+" (U+002B) followed by a
time zone differential, or a "-" (U+002D) followed by a time zone
differential.
A time zone differential is two decimal digits representing hours,
":" (U+003A), and two decimal digits representing minutes. The
seconds are encoded as "00" if the GeneralizedTime value omits
seconds from the time zone differential.
The DXER encoding of GeneralizedTime values is intended to conform to
the lexical representation of the XML Schema dateTime datatype.
6.12 INSTANCE OF
A value of the INSTANCE OF type is encoded according to the
corresponding SEQUENCE type defined in Annex C of X.681 [X681].
6.13 INTEGER
A value of the INTEGER type without a NamedNumberList is encoded such
that the string value of the normalized content is a number string
representing the integer value. A number string is a sequence of one
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or more of the decimal digit characters "0" to "9" (U+0030-U+0039),
with an optional leading sign, either "+" (U+002B) or "-" (U+002D).
Multiple leading zero digits are permitted.
A value of an INTEGER type with a NamedNumberList is encoded such
that the string value of the normalized content is either a number
string or the identifier corresponding to the actual INTEGER value.
The DXER encoding of INTEGER values is intended to conform to the
lexical representation of the XML Schema integer datatype.
6.14 NULL
A value of the NULL type is encoded such that the normalized content
is empty.
6.15 ObjectDescriptor
A value of the ObjectDescriptor type is encoded according to the
GraphicString type.
6.16 OBJECT IDENTIFIER and RELATIVE-OID
A value of the OBJECT IDENTIFIER type or RELATIVE-OID type is encoded
such that the string value of the normalized content is a "."
(U+002E) separated list of the object identifier components of the
value. Each object identifier component is encoded as a non-negative
number string. A non-negative number string is either the digit
character "0" (U+0030), or a non-zero decimal digit character
(U+0031-U+0039) followed by zero, one or more of the decimal digit
characters "0" to "9" (U+0030-U+0039).
6.17 OCTET STRING
A value of the OCTET STRING type is encoded such that the string
value of the normalized content is the hexadecimal digit string
representation of the octets. The octets are encoded in order from
the first octet to the last octet. Each octet is encoded as a pair
of the hexadecimal digit characters "0"-"9", "A"-"F" and "a"-"f"
(i.e. U+0030-U+0039, U+0041-U+0046 and U+0061-U+0066). where the
first digit in the pair corresponds to the four most significant bits
of the octet. An odd number of hexadecimal digits is not permitted.
The DXER encoding of OCTET STRING values is intended to conform to
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the lexical representation of the XML Schema hexBinary datatype.
6.18 REAL
A value of the REAL type is encoded such that the string value of the
normalized content is the character string "0" if the value is
positive zero, the character string "-0" if the value is negative
zero, the character string "INF" if the value is positive infinity,
the character string "-INF" if the value is negative infinity, the
character string "NaN" if the value is not a number, or a real number
otherwise.
A real number is the mantissa followed by either "E" (U+0045) or "e"
(U+0065) and the exponent. If the exponent is zero then the "E" or
"e" and exponent MAY be omitted.
The mantissa is a sequence of one or more of the decimal digit
characters "0" to "9" (U+0030-U+0039), with an optional leading sign,
either "+" (U+002B) or "-" (U+002D). Multiple leading zero digits
are permitted.
The exponent is encoded as a number string (see Section 6.13).
The DXER encoding of REAL values is intended to be compatible with
the lexical representation of the XML Schema double datatype (but
allows real values outside the range permitted by double).
6.19 SEQUENCE and SET
A value of a SEQUENCE type (other than the AnyType type [GLUE]) or a
SET type is encoded such that the normalized content is a series of
zero, one or more child element information items - one for each
component value actually present in the SEQUENCE or SET value -
optionally preceded by, followed by, and/or separated by white space
character information items. Each component value corresponds to
some NamedType in the SEQUENCE or SET type definition. The
[local name] of the child element corresponding to the component
value SHALL be the identifier of the corresponding NamedType, the
[namespace name] of the child element SHALL have no value, and the
content of the child element SHALL be the encoding of the component
value according to the Type of the NamedType.
The component values are encoded in the order of their definition in
the type, for both SEQUENCE and SET types. This is a deliberate
departure from BER where the components of a SET can be encoded in
any order.
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If the SEQUENCE or SET type is extensible [X680] then the DXER
decoder must be capable of skipping over any child element with a
name that is not recognised, on the assumption that the sender is
using a more recent definition of the SEQUENCE or SET type.
6.20 SEQUENCE OF and SET OF
A value of a SEQUENCE OF or SET OF ASN.1 type is encoded such that
the normalized content is a series of zero, one or more child
elements - one for each component value - optionally preceded by,
followed by, and/or separated by white space character information
items. The [namespace name] of each child element SHALL have no
value, and the content of each child element SHALL be the encoding of
the corresponding component value according to the Type.
For a value of a SEQUENCE OF NamedType or SET OF NamedType, the
[local name] of each child element SHALL be the identifier of the
NamedType.
For a value of a SEQUENCE OF Type or SET OF Type, the [local name] of
each child element SHALL be the literal "item".
If the SEQUENCE OF or SET OF value has no component values then the
enclosing element MAY be encoded as an empty element (i.e. using an
empty-element tag).
6.21 UTCTime
A value of the UTCTime type is encoded such that the string value of
the normalized content is a date followed by the letter "T", a time
of day, and an optional time zone.
The date is two decimal digits representing the century, followed by
two decimal digits representing the year, "-" (U+002D), two decimal
digits representing the month, "-" (U+002D), and two decimal digits
representing the day.
A UTCTime value does not indicate the century, therefore the century
in the DXER encoding is generated from the year value as follows. If
the year is in the range 50-99 then the century is "19", otherwise
the century is "20". Note that DXER encoded UTCTime values with a
four digit year outside the range 1950 to 2049 are illegal.
The time of day is two decimal digits representing the hour, followed
by ":" (U+003A), two decimal digits representing the minutes, ":"
(U+003A), and two decimal digits representing the seconds.
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The seconds are encoded as "00" if the UTCTime value omits seconds.
The time zone, if present, is either the letter "Z" (U+005A) to
indicate Coordinated Universal Time, a "+" (U+002B) followed by a
time zone differential, or a "-" (U+002D) followed by a time zone
differential.
A time zone differential is two decimal digits representing hours,
":" (U+003A), and two decimal digits representing minutes.
The DXER encoding of UTCTime values is intended to conform to the
lexical representation of the XML Schema dateTime datatype.
6.22 Open Type
A value of an open type denoted by an ObjectClassFieldType [X.681] is
encoded according to the specific Type of the value.
If the encoding of the value does not generate an attribute
information item with the [local name] "type" and the [namespace
name] "http://www.w3.org/2001/XMLSchema-instance" (i.e. xsi:type, see
Sections 6.4 & 6.7) then:
a) if the Type is a typereference (not a DummyReference) or an
ExternalTypeReference in a DefinedType in a ReferencedType and
then and the ASN.1 module in which the referenced type is defined
has a namespace name [XEDNS] then the [attributes] of the
enclosing element SHOULD contain an attribute information item
with the [local name] "type" and the [namespace name]
"http://www.w3.org/2001/XMLSchema-instance" (i.e. xsi:type), where
the [normalized value] of this attribute SHALL be a qualified name
with the namespace name of the module as the namespace name and
the typereference as the local part,
b) if the Type comprises one of the productions in Table 1 of the
specification for ASN.1 Schema [ASD] then the [attributes] of the
enclosing element SHOULD contain an attribute information item
with the [local name] "type" and the [namespace name]
"http://www.w3.org/2001/XMLSchema-instance", where the
[normalized value] of this attribute SHALL be a qualified name
with the namespace name "http://xmled.info/ns/ASN.1/0702/00" and
the local part as indicated in Table 1.
Note that the values of the xsi:type attribute are the same
qualified names that would be used to reference the corresponding
types in the ASN.1 Schema representation [ASD] of the ASN.1
specification, or the XML Schema derivation [CXSD] of the ASN.1
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specification.
Where the xsi:type attribute is present, appropriate namespace
declaration attributes for the namespace names in the attribute's
name and value MUST be added to the enclosing element's
[attributes] if not already in the [in-scope namespaces] for the
element. The namespace prefixes are the encoder's choice.
6.23 AnyType
The AnyType type [GLUE] is used to embed arbitrary XML within
ASN.1 specifications. The DXER encoding of a value of the AnyType
type is intended to be Infoset equivalent to the original XML used
to populate the AnyType value.
The character string in the attributes or context component of an
AnyType value is an XML textual representation of a sequence of
attribute information items. The character string in the content
component of an AnyType value is an XML textual representation of
a sequence of character, comment, processing instruction and child
element information items.
A value of the AnyType type is encoded such that:
a) the [children] of the enclosing element is the same as the
sequence of information items represented by the content
component,
b) the [attributes] of the enclosing element includes the attribute
information items represented by the attributes component, plus
the namespace declarations in the context component that are not
already defined in the [in-scope namespaces] of the enclosing
element.
The character string in the prolog component of an AnyType value is
text conforming to the prolog production of XML [XML]. It is used to
interpret entity references in the context, attributes or content
components of the value.
Any entity references in the context, attributes or content
components of the AnyType value MUST either be replaced in the DXER
encoding by their replacement text, or the corresponding entity
declarations in the prolog component must be added to the DTD of the
XML document containing the DXER encoding. Note that the latter may
not be possible because of a conflict with an existing entity
declaration of the same name. Applications will generally find it
easier to expand out entity references at the earliest opportunity.
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If the content component of the value is absent then the enclosing
element MAY be encoded as an empty element (i.e. using an
empty-element tag).
7. DXER Transfer Syntax
The following OBJECT IDENTIFIER has been assigned by Adacel
Technologies, under an arc assigned to Adacel by Standards Australia,
to identify the Directory XML Encoding Rules:
{ 1 2 36 79672281 0 2 }
This OBJECT IDENTIFIER would be used, for example, to describe the
transfer syntax for a DXER encoded data-value in an EMBEDDED PDV
value.
8. Relationship to XER
DXER and XER [X693] are separate, distinctly different and
incompatible ASN.1 encoding rules for producing XML markup from ASN.1
abstract values. DXER is therefore unrelated to the XML ASN.1 Value
Notation of X.680 [X680].
There is usually a requirement on applications specified in ASN.1 to
maintain backward compatibility with the encodings generated by
previous versions. The encodings in question are typically BER.
Even with the backward compatibility constraint there is still
considerable leeway for specification writers to rewrite the earlier
specification. For example, renaming types, factoring out an in-line
type definition as a named type (or the reverse), or replacing a type
definition with an equivalent parameterized reference. These changes
produce no change to BER, DER, CER, PER [X691], or GSER [GSER]
encodings (so specification writers have felt free to make such
changes to improve their specification), but can change the
[local name] of elements in the XER encoding. Except for the special
case of the root element, the DXER encoding is immune to this
problem, thus DXER encodings are more stable than XER encodings over
successive revisions of an ASN.1 specification. That has an obvious
benefit for interoperability.
DXER allows entity references, comments and PIs in encodings. XER
does not.
DXER is conformant with XML namespaces [XMLNS], while XER does not
allow qualified names at all.
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DXER has also been designed so that is it possible to generate, from
any arbitrary ASN.1 specification, a compatible XML Schema that will
validate correct DXER encodings [CXSD]. The same is not generally
true of XER, except by making changes to the original ASN.1
specification.
9. Security Considerations
DXER does not necessarily enable the exact octet encoding of values
of the TeletexString, VideotexString, GraphicString or GeneralString
types to be reconstructed, so a transformation from DER to DXER and
back to DER may not reproduce the original DER encoding. Therefore
DXER MUST NOT be used to re-encode, whether for storage or
transmission, ASN.1 abstract values whose original binary encoding
must be recoverable. Such recovery is needed for the verification of
digital signatures. In such cases, protocols ought to use DER or a
DER-reversible encoding.
When interpreting security-sensitive fields, and in particular fields
used to grant or deny access, implementations MUST ensure that any
comparisons are done on the underlying abstract value, regardless of
the particular encoding used.
10. Acknowledgements
This document and the technology it describes are a product of a
joint research project between Adacel Technologies Limited and Deakin
University on leveraging existing directory technology to produce an
XML-based directory service.
11. Normative References
[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[XEDNS] Legg, S. and D. Prager, "XED: IANA Considerations",
draft-legg-xed-iana-xx.txt, a work in progress, August 2003.
[GLUE] Legg, S. and D. Prager, "XED: Schema Language Integration",
draft-legg-xed-glue-xx.txt, a work in progress, August 2003.
[X680] ITU-T Recommendation X.680 (07/02) | ISO/IEC 8824-1,
Information technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation.
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[X681] ITU-T Recommendation X.681 (07/02) | ISO/IEC 8824-2,
Information technology - Abstract Syntax Notation One
(ASN.1): Information object specification.
[X682] ITU-T Recommendation X.682 (07/02) | ISO/IEC 8824-3,
Information technology - Abstract Syntax Notation One
(ASN.1): Constraint specification.
[X683] ITU-T Recommendation X.683 (07/02) | ISO/IEC 8824-4,
Information technology - Abstract Syntax Notation One
(ASN.1): Parameterization of ASN.1 specifications.
[X690] ITU-T Recommendation X.690 (07/02) | ISO/IEC 8825-1,
Information technology - ASN.1 encoding rules: Specification
of Basic Encoding Rules (BER), Canonical Encoding Rules
(CER) and Distinguished Encoding Rules (DER).
[UCS] ISO/IEC 10646-1:2000, Information technology - Universal
Multiple-Octet Coded Character Set (UCS) - Part 1:
Architecture and Basic Multilingual Plane.
[XML] Bray, T., Paoli, J., Sperberg-McQueen, M. and E. Maler,
"Extensible Markup Language (XML) 1.0 (Second Edition)", W3C
Recommendation, http://www.w3.org/TR/2000/REC-xml-20001006,
October 2000.
[XMLNS] Bray, T., Hollander, D. and A. Layman, "Namespaces in XML",
http://www.w3.org/TR/1999/REC-xml-names-19990114, January
1999.
[ISET] Cowan, J. and R. Tobin, "XML Information Set", W3C
Recommendation, http://www.w3.org/TR/2001/REC-xml-
infoset-20011024, October 2001.
[XSD1] Thompson, H., Beech, D., Maloney, M. and N. Mendelsohn, "XML
Schema Part 1: Structures", W3C Recommendation,
http://www.w3.org/TR/2001/REC-xmlschema-1-20010502, May
2001.
12. Informative References
[BCP11] Hovey, R. and S. Bradner, "The Organizations Involved in the
IETF Standards Process", BCP 11, RFC 2028, October 1996.
[GSER] Legg, S., "Generic String Encoding Rules for ASN.1 Types",
draft-legg-ldap-gser-xx.txt, a work in progress, June 2003.
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[ASD] Legg, S. and D. Prager, "ASN.1 Schema: An XML Representation
for ASN.1 Specifications", draft-legg-xed-asd-xx.txt, a work
in progress, August 2003.
[CXSD] Legg, S. and D. Prager, "Translation of ASN.1 Specifications
into XML Schema", draft-legg-xed-xsd-xx.txt, a work in
progress, August 2003.
[X691] ITU-T Recommendation X.691 (07/02) | ISO/IEC 8825-4:2002,
Information technology - ASN.1 encoding rules: Specification
of Packed Encoding Rules (PER)
[X693] ITU-T Recommendation X.693 (12/01) | ISO/IEC 8825-4:2002,
Information technology - ASN.1 encoding rules: XML encoding
rules (XER)
13. Intellectual Property Notice
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. [BCP11]
Copies of claims of rights made available for publication and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementors or users of this
specification can be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
14. Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
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kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
15. Authors' Addresses
Dr. Steven Legg
Adacel Technologies Ltd.
250 Bay Street
Brighton, Victoria 3186
AUSTRALIA
Phone: +61 3 8530 7710
Fax: +61 3 8530 7888
EMail: steven.legg@adacel.com.au
Dr. Daniel Prager
C/o Professor Lynn Batten
Department of Computing and Mathematics
Deakin University
Geelong, Victoria 3217
AUSTRALIA
EMail: dan@layabout.net
EMail: lmbatten@deakin.edu.au
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