RFC 4911
Encoding Instructions for the Robust XML Encoding Rules (RXER)
Pages: 91
Experimental
Experimental
Part 1 of 4 – Pages 1 to 20
Network Working Group S. Legg Request for Comments: 4911 eB2Bcom Category: Experimental July 2007 Encoding Instructions for the Robust XML Encoding Rules (RXER) Status of This Memo This memo defines an Experimental Protocol for the Internet community. It does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The IETF Trust (2007).Abstract
This document defines encoding instructions that may be used in an Abstract Syntax Notation One (ASN.1) specification to alter how ASN.1 values are encoded by the Robust XML Encoding Rules (RXER) and Canonical Robust XML Encoding Rules (CRXER), for example, to encode a component of an ASN.1 value as an Extensible Markup Language (XML) attribute rather than as a child element. Some of these encoding instructions also affect how an ASN.1 specification is translated into an Abstract Syntax Notation X (ASN.X) specification. Encoding instructions that allow an ASN.1 specification to reference definitions in other XML schema languages are also defined.
Table of Contents
1. Introduction ....................................................3 2. Conventions .....................................................3 3. Definitions .....................................................4 4. Notation for RXER Encoding Instructions .........................4 5. Component Encoding Instructions .................................6 6. Reference Encoding Instructions .................................8 7. Expanded Names of Components ...................................10 8. The ATTRIBUTE Encoding Instruction .............................11 9. The ATTRIBUTE-REF Encoding Instruction .........................12 10. The COMPONENT-REF Encoding Instruction ........................13 11. The ELEMENT-REF Encoding Instruction ..........................16 12. The LIST Encoding Instruction .................................17 13. The NAME Encoding Instruction .................................19 14. The REF-AS-ELEMENT Encoding Instruction .......................19 15. The REF-AS-TYPE Encoding Instruction ..........................20 16. The SCHEMA-IDENTITY Encoding Instruction ......................22 17. The SIMPLE-CONTENT Encoding Instruction .......................22 18. The TARGET-NAMESPACE Encoding Instruction .....................23 19. The TYPE-AS-VERSION Encoding Instruction ......................24 20. The TYPE-REF Encoding Instruction .............................25 21. The UNION Encoding Instruction ................................26 22. The VALUES Encoding Instruction ...............................27 23. Insertion Encoding Instructions ...............................29 24. The VERSION-INDICATOR Encoding Instruction ....................32 25. The GROUP Encoding Instruction ................................34 25.1. Unambiguous Encodings ....................................36 25.1.1. Grammar Construction ..............................37 25.1.2. Unique Component Attribution ......................47 25.1.3. Deterministic Grammars ............................52 25.1.4. Attributes in Unknown Extensions ..................54 26. Security Considerations .......................................56 27. References ....................................................56 27.1. Normative References .....................................56 27.2. Informative References ...................................57 Appendix A. GROUP Encoding Instruction Examples ...................58 Appendix B. Insertion Encoding Instruction Examples ...............74 Appendix C. Extension and Versioning Examples .....................87
1. Introduction
This document defines encoding instructions [X.680-1] that may be used in an Abstract Syntax Notation One (ASN.1) [X.680] specification to alter how ASN.1 values are encoded by the Robust XML Encoding Rules (RXER) [RXER] and Canonical Robust XML Encoding Rules (CRXER) [RXER], for example, to encode a component of an ASN.1 value as an Extensible Markup Language (XML) [XML10] attribute rather than as a child element. Some of these encoding instructions also affect how an ASN.1 specification is translated into an Abstract Syntax Notation X (ASN.X) specification [ASN.X]. This document also defines encoding instructions that allow an ASN.1 specification to incorporate the definitions of types, elements, and attributes in specifications written in other XML schema languages. References to XML Schema [XSD1] types, elements, and attributes, RELAX NG [RNG] named patterns and elements, and XML document type definition (DTD) [XML10] element types are supported. In most cases, the effect of an encoding instruction is only briefly mentioned in this document. The precise effects of these encoding instructions are described fully in the specifications for RXER [RXER] and ASN.X [ASN.X], at the points where they apply.2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED" and "MAY" in this document are to be interpreted as described in BCP 14, RFC 2119 [BCP14]. The key word "OPTIONAL" is exclusively used with its ASN.1 meaning. 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, unless qualified otherwise. A reference to an ASN.1 production [X.680] (e.g., Type, NamedType) is a reference to text in an ASN.1 specification corresponding to that production. Throughout this document, "component" is synonymous with NamedType. This document uses the namespace prefix "xsi:" to stand for the namespace name [XMLNS10] "http://www.w3.org/2001/XMLSchema-instance". Example ASN.1 definitions in this document are assumed to be defined in an ASN.1 module with a TagDefault of "AUTOMATIC TAGS" and an EncodingReferenceDefault [X.680-1] of "RXER INSTRUCTIONS".
3. Definitions
The following definition of base type is used in specifying a number of encoding instructions. Definition (base type): If a type, T, is a constrained type, then the base type of T is the base type of the type that is constrained; else if T is a prefixed type, then the base type of T is the base type of the type that is prefixed; else if T is a type notation that references or denotes another type (i.e., DefinedType, ObjectClassFieldType, SelectionType, TypeFromObject, or ValueSetFromObjects), then the base type of T is the base type of the type that is referenced or denoted; otherwise, the base type of T is T itself. Aside: A tagged type is a special case of a prefixed type.4. Notation for RXER Encoding Instructions
The grammar of ASN.1 permits the application of encoding instructions [X.680-1], through type prefixes and encoding control sections, that modify how abstract values are encoded by nominated encoding rules. The generic notation for type prefixes and encoding control sections is defined by the ASN.1 basic notation [X.680] [X.680-1], and includes an encoding reference to identify the specific encoding rules that are affected by the encoding instruction. The encoding reference that identifies the Robust XML Encoding rules is literally RXER. An RXER encoding instruction applies equally to both RXER and CRXER encodings. The specific notation for an encoding instruction for a specific set of encoding rules is left to the specification of those encoding rules. Consequently, this companion document to the RXER specification [RXER] defines the notation for RXER encoding instructions. Specifically, it elaborates the EncodingInstruction and EncodingInstructionAssignmentList placeholder productions of the ASN.1 basic notation. In the context of the RXER encoding reference, the EncodingInstruction production is defined as follows, using the conventions of the ASN.1 basic notation:
EncodingInstruction ::=
AttributeInstruction |
AttributeRefInstruction |
ComponentRefInstruction |
ElementRefInstruction |
GroupInstruction |
InsertionsInstruction |
ListInstruction |
NameInstruction |
RefAsElementInstruction |
RefAsTypeInstruction |
SimpleContentInstruction |
TypeAsVersionInstruction |
TypeRefInstruction |
UnionInstruction |
ValuesInstruction |
VersionIndicatorInstruction
In the context of the RXER encoding reference, the
EncodingInstructionAssignmentList production (which only appears in
an encoding control section) is defined as follows:
EncodingInstructionAssignmentList ::=
SchemaIdentityInstruction ?
TargetNamespaceInstruction ?
TopLevelComponents ?
TopLevelComponents ::= TopLevelComponent TopLevelComponents ?
TopLevelComponent ::= "COMPONENT" NamedType
Definition (top-level NamedType): A NamedType is a top-level
NamedType (equivalently, a top-level component) if and only if it is
the NamedType in a TopLevelComponent. A NamedType nested within the
Type of the NamedType of a TopLevelComponent is not itself a
top-level NamedType.
Aside: Specification writers should note that non-trivial types
defined within a top-level NamedType will not be visible to ASN.1
tools that do not understand RXER.
Although a top-level NamedType only appears in an RXER encoding
control section, the default encoding reference for the module
[X.680-1] still applies when parsing a top-level NamedType.
Each top-level NamedType within a module SHALL have a distinct
identifier.
The NamedType production is defined by the ASN.1 basic notation. The
other productions are described in subsequent sections and make use
of the following productions:
NCNameValue ::= Value
AnyURIValue ::= Value
QNameValue ::= Value
NameValue ::= Value
The Value production is defined by the ASN.1 basic notation.
The governing type for the Value in an NCNameValue is the NCName type
from the AdditionalBasicDefinitions module [RXER].
The governing type for the Value in an AnyURIValue is the AnyURI type
from the AdditionalBasicDefinitions module.
The governing type for the Value in a QNameValue is the QName type
from the AdditionalBasicDefinitions module.
The governing type for the Value in a NameValue is the Name type from
the AdditionalBasicDefinitions module.
The Value in an NCNameValue, AnyURIValue, QNameValue, or NameValue
SHALL NOT be a DummyReference [X.683] and SHALL NOT textually contain
a nested DummyReference.
Aside: Thus, encoding instructions are not permitted to be
parameterized in any way. This restriction will become important
if a future specification for ASN.X explicitly represents
parameterized definitions and parameterized references instead of
expanding out parameterized references as in the current
specification. A parameterized definition could not be directly
translated into ASN.X if it contained encoding instructions that
were not fully specified.
5. Component Encoding Instructions
Certain of the RXER encoding instructions are categorized as
component encoding instructions. The component encoding instructions
are the ATTRIBUTE, ATTRIBUTE-REF, COMPONENT-REF, GROUP, ELEMENT-REF,
NAME, REF-AS-ELEMENT, SIMPLE-CONTENT, TYPE-AS-VERSION, and
VERSION-INDICATOR encoding instructions (whose notations are
described respectively by AttributeInstruction,
AttributeRefInstruction, ComponentRefInstruction, GroupInstruction,
ElementRefInstruction, NameInstruction, RefAsElementInstruction,
SimpleContentInstruction, TypeAsVersionInstruction, and
VersionIndicatorInstruction).
The Type in the EncodingPrefixedType for a component encoding
instruction SHALL be either:
(1) the Type in a NamedType, or
(2) the Type in an EncodingPrefixedType in a PrefixedType in a
BuiltinType in a Type that is one of (1) to (4), or
(3) the Type in an TaggedType in a PrefixedType in a BuiltinType in a
Type that is one of (1) to (4), or
(4) the Type in a ConstrainedType (excluding a TypeWithConstraint) in
a Type that is one of (1) to (4).
Aside: The effect of this condition is to force the component
encoding instructions to be textually within the NamedType to
which they apply. Only case (2) can be true on the first
iteration as the Type belongs to an EncodingPrefixedType; however,
any of (1) to (4) can be true on subsequent iterations.
Case (4) is not permitted when the encoding instruction is the
ATTRIBUTE-REF, COMPONENT-REF, ELEMENT-REF, or REF-AS-ELEMENT encoding
instruction.
The NamedType in case (1) is said to be "subject to" the component
encoding instruction.
A top-level NamedType SHALL NOT be subject to an ATTRIBUTE-REF,
COMPONENT-REF, GROUP, ELEMENT-REF, REF-AS-ELEMENT, or SIMPLE-CONTENT
encoding instruction.
Aside: This condition does not preclude these encoding
instructions being used on a nested NamedType.
A NamedType SHALL NOT be subject to two or more component encoding
instructions of the same kind, e.g., a NamedType is not permitted to
be subject to two NAME encoding instructions.
The ATTRIBUTE, ATTRIBUTE-REF, COMPONENT-REF, GROUP, ELEMENT-REF,
REF-AS-ELEMENT, SIMPLE-CONTENT, and TYPE-AS-VERSION encoding
instructions are mutually exclusive. The NAME, ATTRIBUTE-REF,
COMPONENT-REF, ELEMENT-REF, and REF-AS-ELEMENT encoding instructions
are mutually exclusive. A NamedType SHALL NOT be subject to two or
more encoding instructions that are mutually exclusive.
A SelectionType [X.680] SHALL NOT be used to select the Type from a NamedType that is subject to an ATTRIBUTE-REF, COMPONENT-REF, ELEMENT-REF or REF-AS-ELEMENT encoding instruction. The other component encoding instructions are not inherited by the type denoted by a SelectionType. Definition (attribute component): An attribute component is a NamedType that is subject to an ATTRIBUTE or ATTRIBUTE-REF encoding instruction, or subject to a COMPONENT-REF encoding instruction that references a top-level NamedType that is subject to an ATTRIBUTE encoding instruction. Definition (element component): An element component is a NamedType that is not subject to an ATTRIBUTE, ATTRIBUTE-REF, GROUP, or SIMPLE-CONTENT encoding instruction, and not subject to a COMPONENT-REF encoding instruction that references a top-level NamedType that is subject to an ATTRIBUTE encoding instruction. Aside: A NamedType subject to a GROUP or SIMPLE-CONTENT encoding instruction is neither an attribute component nor an element component.6. Reference Encoding Instructions
Certain of the RXER encoding instructions are categorized as reference encoding instructions. The reference encoding instructions are the ATTRIBUTE-REF, COMPONENT-REF, ELEMENT-REF, REF-AS-ELEMENT, REF-AS-TYPE, and TYPE-REF encoding instructions (whose notations are described respectively by AttributeRefInstruction, ComponentRefInstruction, ElementRefInstruction, RefAsElementInstruction, RefAsTypeInstruction, and TypeRefInstruction). These encoding instructions (except COMPONENT-REF) allow an ASN.1 specification to incorporate the definitions of types, elements, and attributes in specifications written in other XML schema languages, through implied constraints on the markup that may appear in values of the Markup ASN.1 type from the AdditionalBasicDefinitions module [RXER] (for ELEMENT-REF, REF-AS-ELEMENT, REF-AS-TYPE, and TYPE-REF) or the UTF8String type (for ATTRIBUTE-REF). References to XML Schema [XSD1] types, elements, and attributes, RELAX NG [RNG] named patterns and elements, and XML document type definition (DTD) [XML10] element types are supported. References to ASN.1 types and top-level components are also permitted. The COMPONENT-REF encoding instruction provides a more direct method of referencing a top-level component. The Type in the EncodingPrefixedType for an ELEMENT-REF, REF-AS-ELEMENT, REF-AS-TYPE, or TYPE-REF encoding instruction SHALL be either:
(1) a ReferencedType that is a DefinedType that is a typereference
(not a DummyReference) or ExternalTypeReference that references
the Markup ASN.1 type from the AdditionalBasicDefinitions module
[RXER], or
(2) a BuiltinType that is a PrefixedType that is a TaggedType where
the Type in the TaggedType is one of (1) to (3), or
(3) a BuiltinType that is a PrefixedType that is an
EncodingPrefixedType where the Type in the EncodingPrefixedType
is one of (1) to (3) and the EncodingPrefix in the
EncodingPrefixedType does not contain a reference encoding
instruction.
Aside: Case (3) and similar cases for the ATTRIBUTE-REF and
COMPONENT-REF encoding instructions have the effect of making the
reference encoding instructions mutually exclusive as well as
singly occurring.
With respect to the REF-AS-TYPE and TYPE-REF encoding instructions,
the DefinedType in case (1) is said to be "subject to" the encoding
instruction.
The restrictions on the Type in the EncodingPrefixedType for an
ATTRIBUTE-REF encoding instruction are specified in Section 9. The
restrictions on the Type in the EncodingPrefixedType for a
COMPONENT-REF encoding instruction are specified in Section 10.
The reference encoding instructions make use of a common production
defined as follows:
RefParameters ::= ContextParameter ?
ContextParameter ::= "CONTEXT" AnyURIValue
A RefParameters instance provides extra information about a reference
to a definition. A ContextParameter is used when a reference is
ambiguous, i.e., refers to definitions in more than one schema
document or external DTD subset. This situation would occur, for
example, when importing types with the same name from independently
developed XML Schemas defined without a target namespace [XSD1].
When used in conjunction with a reference to an element type in an
external DTD subset, the AnyURIValue in the ContextParameter is the
system identifier (a Uniform Resource Identifier or URI [URI]) of the
external DTD subset; otherwise, the AnyURIValue is a URI that
indicates the intended schema document, either an XML Schema
specification, a RELAX NG specification, or an ASN.1 or ASN.X
specification.
7. Expanded Names of Components
Each NamedType has an associated expanded name [XMLNS10], determined as follows: (1) if the NamedType is subject to a NAME encoding instruction, then the local name of the expanded name is the character string specified by the NCNameValue of the NAME encoding instruction, (2) else if the NamedType is subject to a COMPONENT-REF encoding instruction, then the expanded name is the same as the expanded name of the referenced top-level NamedType, (3) else if the NamedType is subject to an ATTRIBUTE-REF or ELEMENT-REF encoding instruction, then the namespace name of the expanded name is equal to the namespace-name component of the QNameValue of the encoding instruction, and the local name is equal to the local-name component of the QNameValue, (4) else if the NamedType is subject to a REF-AS-ELEMENT encoding instruction, then the local name of the expanded name is the LocalPart [XMLNS10] of the qualified name specified by the NameValue of the encoding instruction, (5) otherwise, the local name of the expanded name is the identifier of the NamedType. In cases (1) and (5), if the NamedType is a top-level NamedType and the module containing the NamedType has a TARGET-NAMESPACE encoding instruction, then the namespace name of the expanded name is the character string specified by the AnyURIValue of the TARGET-NAMESPACE encoding instruction; otherwise, the namespace name has no value. Aside: Thus, the TARGET-NAMESPACE encoding instruction applies to a top-level NamedType but not to any other NamedType. In case (4), if the encoding instruction contains a Namespace, then the namespace name of the expanded name is the character string specified by the AnyURIValue of the Namespace; otherwise, the namespace name has no value. The expanded names for the attribute components of a CHOICE, SEQUENCE, or SET type MUST be distinct. The expanded names for the components of a CHOICE, SEQUENCE, or SET type that are not attribute components MUST be distinct. These tests are applied after the COMPONENTS OF transformation specified in X.680, Clause 24.4 [X.680].
Aside: Two components of the same CHOICE, SEQUENCE, or SET type
may have the same expanded name if one of them is an attribute
component and the other is not. Note that the "not" case includes
components that are subject to a GROUP or SIMPLE-CONTENT encoding
instruction.
The expanded name of a top-level NamedType subject to an ATTRIBUTE
encoding instruction MUST be distinct from the expanded name of every
other top-level NamedType subject to an ATTRIBUTE encoding
instruction in the same module.
The expanded name of a top-level NamedType not subject to an
ATTRIBUTE encoding instruction MUST be distinct from the expanded
name of every other top-level NamedType not subject to an ATTRIBUTE
encoding instruction in the same module.
Aside: Two top-level components may have the same expanded name if
one of them is an attribute component and the other is not.
8. The ATTRIBUTE Encoding Instruction
The ATTRIBUTE encoding instruction causes an RXER encoder to encode a
value of the component to which it is applied as an XML attribute
instead of as a child element.
The notation for an ATTRIBUTE encoding instruction is defined as
follows:
AttributeInstruction ::= "ATTRIBUTE"
The base type of the type of a NamedType that is subject to an
ATTRIBUTE encoding instruction SHALL NOT be:
(1) a CHOICE, SET, or SET OF type, or
(2) a SEQUENCE type other than the one defining the QName type from
the AdditionalBasicDefinitions module [RXER] (i.e., QName is
allowed), or
(3) a SEQUENCE OF type where the SequenceOfType is not subject to a
LIST encoding instruction, or
(4) an open type.
Example
PersonalDetails ::= SEQUENCE {
firstName [ATTRIBUTE] UTF8String,
middleName [ATTRIBUTE] UTF8String,
surname [ATTRIBUTE] UTF8String
}
9. The ATTRIBUTE-REF Encoding Instruction
The ATTRIBUTE-REF encoding instruction causes an RXER encoder to
encode a value of the component to which it is applied as an XML
attribute instead of as a child element, where the attribute's name
is a qualified name of the attribute declaration referenced by the
encoding instruction. In addition, the ATTRIBUTE-REF encoding
instruction causes values of the UTF8String type to be restricted to
conform to the type of the attribute declaration.
The notation for an ATTRIBUTE-REF encoding instruction is defined as
follows:
AttributeRefInstruction ::=
"ATTRIBUTE-REF" QNameValue RefParameters
Taken together, the QNameValue and the ContextParameter in the
RefParameters (if present) MUST reference an XML Schema attribute
declaration or a top-level NamedType that is subject to an ATTRIBUTE
encoding instruction.
The type of a referenced XML Schema attribute declaration SHALL NOT
be, either directly or by derivation, the XML Schema type QName,
NOTATION, ENTITY, ENTITIES, or anySimpleType.
Aside: Values of these types require information from the context
of the attribute for interpretation. Because an ATTRIBUTE-REF
encoding instruction is restricted to prefixing the ASN.1
UTF8String type, there is no mechanism to capture such context.
The type of a referenced top-level NamedType SHALL NOT be, either
directly or by subtyping, the QName type from the
AdditionalBasicDefinitions module [RXER].
The Type in the EncodingPrefixedType for an ATTRIBUTE-REF encoding
instruction SHALL be either:
(1) the UTF8String type, or
(2) a BuiltinType that is a PrefixedType that is a TaggedType where
the Type in the TaggedType is one of (1) to (3), or
(3) a BuiltinType that is a PrefixedType that is an
EncodingPrefixedType where the Type in the EncodingPrefixedType
is one of (1) to (3) and the EncodingPrefix in the
EncodingPrefixedType does not contain a reference encoding
instruction.
The identifier of a NamedType subject to an ATTRIBUTE-REF encoding
instruction does not contribute to the name of attributes in an RXER
encoding. For the sake of consistency, the identifier SHOULD, where
possible, be the same as the local name of the referenced attribute
declaration.
10. The COMPONENT-REF Encoding Instruction
The ASN.1 basic notation does not have a concept of a top-level
NamedType and therefore does not have a mechanism to reference a
top-level NamedType. The COMPONENT-REF encoding instruction provides
a way to specify that a NamedType within a combining type definition
is equivalent to a referenced top-level NamedType.
The notation for a COMPONENT-REF encoding instruction is defined as
follows:
ComponentRefInstruction ::= "COMPONENT-REF" ComponentReference
ComponentReference ::=
InternalComponentReference |
ExternalComponentReference
InternalComponentReference ::= identifier FromModule ?
FromModule ::= "FROM" GlobalModuleReference
ExternalComponentReference ::= modulereference "." identifier
The GlobalModuleReference production is defined by the ASN.1 basic
notation [X.680]. If the GlobalModuleReference is absent from an
InternalComponentReference, then the identifier MUST be the
identifier of a top-level NamedType in the same module. If the
GlobalModuleReference is present in an InternalComponentReference,
then the identifier MUST be the identifier of a top-level NamedType
in the referenced module.
The modulereference in an ExternalComponentReference is used in the
same way as a modulereference in an ExternalTypeReference. The
identifier in an ExternalComponentReference MUST be the identifier of
a top-level NamedType in the referenced module.
The Type in the EncodingPrefixedType for a COMPONENT-REF encoding
instruction SHALL be either:
(1) a ReferencedType that is a DefinedType that is a typereference
(not a DummyReference) or an ExternalTypeReference, or
(2) a BuiltinType or ReferencedType that is one of the productions in
Table 1 in Section 5 of the specification for RXER [RXER], or
(3) a BuiltinType that is a PrefixedType that is a TaggedType where
the Type in the TaggedType is one of (1) to (4), or
(4) a BuiltinType that is a PrefixedType that is an
EncodingPrefixedType where the Type in the EncodingPrefixedType
is one of (1) to (4) and the EncodingPrefix in the
EncodingPrefixedType does not contain a reference encoding
instruction.
The restrictions on the use of RXER encoding instructions are such
that no other RXER encoding instruction is permitted within a
NamedType if the NamedType is subject to a COMPONENT-REF encoding
instruction.
The Type in the top-level NamedType referenced by the COMPONENT-REF
encoding instruction MUST be either:
(a) if the preceding case (1) is used, a ReferencedType that is a
DefinedType that is a typereference or ExternalTypeReference that
references the same type as the DefinedType in case (1), or
(b) if the preceding case (2) is used, a BuiltinType or
ReferencedType that is the same as the BuiltinType or
ReferencedType in case (2), or
(c) a BuiltinType that is a PrefixedType that is an
EncodingPrefixedType where the Type in the EncodingPrefixedType
is one of (a) to (c), and the EncodingPrefix in the
EncodingPrefixedType contains an RXER encoding instruction.
In principle, the COMPONENT-REF encoding instruction creates a
notional NamedType where the expanded name is that of the referenced
top-level NamedType and the Type in case (1) or (2) is substituted by
the Type of the referenced top-level NamedType.
In practice, it is sufficient for non-RXER encoders and decoders to
use the original NamedType rather than the notional NamedType because
the Type in case (1) or (2) can only differ from the Type of the
referenced top-level NamedType by having fewer RXER encoding
instructions, and RXER encoding instructions are ignored by non-RXER
encoders and decoders.
Although any prefixes for the Type in case (1) or (2) would be
bypassed, it is sufficient for RXER encoders and decoders to use the
referenced top-level NamedType instead of the notional NamedType
because these prefixes cannot be RXER encoding instructions (except,
of course, for the COMPONENT-REF encoding instruction) and can have
no effect on an RXER encoding.
Example
Modules ::= SEQUENCE OF
module [COMPONENT-REF module
FROM AbstractSyntaxNotation-X
{ 1 3 6 1 4 1 21472 1 0 1 }]
ModuleDefinition
Note that the "module" top-level NamedType in the
AbstractSyntaxNotation-X module is defined like so:
COMPONENT module ModuleDefinition
The ASN.X translation of the SEQUENCE OF type definition provides
a more natural representation:
<namedType xmlns:asnx="urn:ietf:params:xml:ns:asnx"
name="Modules">
<sequenceOf>
<element ref="asnx:module"/>
</sequenceOf>
</namedType>
Aside: The <namedType> element in ASN.X corresponds to a
TypeAssignment, not a NamedType.
The identifier of a NamedType subject to a COMPONENT-REF encoding
instruction does not contribute to an RXER encoding. For the sake of
consistency with other encoding rules, the identifier SHOULD be the
same as the identifier in the ComponentRefInstruction.
11. The ELEMENT-REF Encoding Instruction
The ELEMENT-REF encoding instruction causes an RXER encoder to encode a value of the component to which it is applied as an element where the element's name is a qualified name of the element declaration referenced by the encoding instruction. In addition, the ELEMENT-REF encoding instruction causes values of the Markup ASN.1 type to be restricted to conform to the type of the element declaration. The notation for an ELEMENT-REF encoding instruction is defined as follows: ElementRefInstruction ::= "ELEMENT-REF" QNameValue RefParameters Taken together, the QNameValue and the ContextParameter in the RefParameters (if present) MUST reference an XML Schema element declaration, a RELAX NG element definition, or a top-level NamedType that is not subject to an ATTRIBUTE encoding instruction. A referenced XML Schema element declaration MUST NOT have a type that requires the presence of values for the XML Schema ENTITY or ENTITIES types. Aside: Entity declarations are not supported by CRXER. Example AnySchema ::= CHOICE { module [ELEMENT-REF { namespace-name "urn:ietf:params:xml:ns:asnx", local-name "module" }] Markup, schema [ELEMENT-REF { namespace-name "http://www.w3.org/2001/XMLSchema", local-name "schema" }] Markup, grammar [ELEMENT-REF { namespace-name "http://relaxng.org/ns/structure/1.0", local-name "grammar" }] Markup } The ASN.X translation of the choice type definition provides a more natural representation:
<namedType xmlns:asnx="urn:ietf:params:xml:ns:asnx"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:rng="http://relaxng.org/ns/structure/1.0"
name="AnySchema">
<choice>
<element ref="asnx:module" embedded="true"/>
<element ref="xs:schema" embedded="true"/>
<element ref="rng:grammar" embedded="true"/>
</choice>
</namedType>
The identifier of a NamedType subject to an ELEMENT-REF encoding
instruction does not contribute to the name of an element in an RXER
encoding. For the sake of consistency, the identifier SHOULD, where
possible, be the same as the local name of the referenced element
declaration.
12. The LIST Encoding Instruction
The LIST encoding instruction causes an RXER encoder to encode a
value of a SEQUENCE OF type as a white-space-separated list of the
component values.
The notation for a LIST encoding instruction is defined as follows:
ListInstruction ::= "LIST"
The Type in an EncodingPrefixedType for a LIST encoding instruction
SHALL be either:
(1) a BuiltinType that is a SequenceOfType of the
"SEQUENCE OF NamedType" form, or
(2) a ConstrainedType that is a TypeWithConstraint of the
"SEQUENCE Constraint OF NamedType" form or
"SEQUENCE SizeConstraint OF NamedType" form, or
(3) a ConstrainedType that is not a TypeWithConstraint where the Type
in the ConstrainedType is one of (1) to (5), or
(4) a BuiltinType that is a PrefixedType that is a TaggedType where
the Type in the TaggedType is one of (1) to (5), or
(5) a BuiltinType that is a PrefixedType that is an
EncodingPrefixedType where the Type in the EncodingPrefixedType
is one of (1) to (5).
The effect of this condition is to force the LIST encoding
instruction to be textually co-located with the SequenceOfType or
TypeWithConstraint to which it applies.
Aside: This makes it clear to a reader that the encoding
instruction applies to every use of the type no matter how it
might be referenced.
The SequenceOfType in case (1) and the TypeWithConstraint in case (2)
are said to be "subject to" the LIST encoding instruction.
A SequenceOfType or TypeWithConstraint SHALL NOT be subject to more
than one LIST encoding instruction.
The base type of the component type of a SequenceOfType or
TypeWithConstraint that is subject to a LIST encoding instruction
MUST be one of the following:
(1) the BOOLEAN, INTEGER, ENUMERATED, REAL, OBJECT IDENTIFIER,
RELATIVE-OID, GeneralizedTime, or UTCTime type, or
(2) the NCName, AnyURI, Name, or QName type from the
AdditionalBasicDefinitions module [RXER].
Aside: While it would be feasible to allow the component type to
also be any character string type that is constrained such that
all its abstract values have a length greater than zero and none
of its abstract values contain any white space characters, testing
whether this condition is satisfied can be quite involved. For
the sake of simplicity, only certain immediately useful
constrained UTF8String types, which are known to be suitable, are
permitted (i.e., NCName, AnyURI, and Name).
The NamedType in a SequenceOfType or TypeWithConstraint that is
subject to a LIST encoding instruction MUST NOT be subject to an
ATTRIBUTE, ATTRIBUTE-REF, COMPONENT-REF, GROUP, ELEMENT-REF,
REF-AS-ELEMENT, SIMPLE-CONTENT, or TYPE-AS-VERSION encoding
instruction.
Example
UpdateTimes ::= [LIST] SEQUENCE OF updateTime GeneralizedTime
13. The NAME Encoding Instruction
The NAME encoding instruction causes an RXER encoder to use a nominated character string instead of a component's identifier wherever that identifier would otherwise appear in the encoding (e.g., as an element or attribute name). The notation for a NAME encoding instruction is defined as follows: NameInstruction ::= "NAME" "AS"? NCNameValue Example CHOICE { foo-att [ATTRIBUTE] [NAME AS "Foo"] INTEGER, foo-elem [NAME "Foo"] INTEGER }14. The REF-AS-ELEMENT Encoding Instruction
The REF-AS-ELEMENT encoding instruction causes an RXER encoder to encode a value of the component to which it is applied as an element where the element's name is the name of the external DTD subset element type declaration referenced by the encoding instruction. In addition, the REF-AS-ELEMENT encoding instruction causes values of the Markup ASN.1 type to be restricted to conform to the content and attributes permitted by that element type declaration and its associated attribute-list declarations. The notation for a REF-AS-ELEMENT encoding instruction is defined as follows: RefAsElementInstruction ::= "REF-AS-ELEMENT" NameValue Namespace ? RefParameters Namespace ::= "NAMESPACE" AnyURIValue Taken together, the NameValue and the ContextParameter in the RefParameters (if present) MUST reference an element type declaration in an external DTD subset that is conformant with Namespaces in XML 1.0 [XMLNS10]. The Namespace is present if and only if the Name of the referenced element type declaration conforms to a PrefixedName (a QName) [XMLNS10], in which case the Namespace specifies the namespace name to be associated with the Prefix of the PrefixedName.
The referenced element type declaration MUST NOT require the presence
of attributes of type ENTITY or ENTITIES.
Aside: Entity declarations are not supported by CRXER.
Example
Suppose that the following external DTD subset has been defined
with a system identifier of "http://www.example.com/inventory":
<?xml version='1.0'?>
<!ELEMENT product EMPTY>
<!ATTLIST product
name CDATA #IMPLIED
partNumber CDATA #REQUIRED
quantity CDATA #REQUIRED >
The product element type declaration can be referenced as an
element in an ASN.1 type definition:
CHOICE {
product [REF-AS-ELEMENT "product"
CONTEXT "http://www.example.com/inventory"]
Markup
}
Here is the ASN.X translation of this ASN.1 type definition:
<type>
<choice>
<element elementType="product"
context="http://www.example.com/inventory"/>
</choice>
</type>
The identifier of a NamedType subject to a REF-AS-ELEMENT encoding
instruction does not contribute to the name of an element in an RXER
encoding. For the sake of consistency, the identifier SHOULD, where
possible, be the same as the Name of the referenced element type
declaration (or the LocalPart if the Name conforms to a
PrefixedName).
(page 20 continued on part 2)