ISO/IEC 8825-5:2021

INTERNATIONAL ISO/IEC
STANDARD 8825-5
Fourth edition
2021-06
Information technology — ASN.1
encoding rules —
Part 5:
Mapping W3C XML schema definitions
into ASN.1
Technologies de l'information — Règles de codage ASN.1 —
Partie 5: Mappage en ASN.1 des définitions de schéma XML du W3C
Reference number
ISO/IEC 8825-5:2021(E)
©
ISO/IEC 2021

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ISO/IEC 8825-5:2021(E)

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ISO/IEC 8825-5:2021(E)
Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
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see www.iso.org/iso/foreword.html. In the IEC, see www.iec.ch/understanding-standards.
This document was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 6, Telecommunications and information exchange between systems, in collaboration
with ITU-T. The identical text is published as ITU-T X.694 (02/2021).
This fourth edition cancels and replaces the third edition (ISO/IEC 8825-5:2015), which has been
technically revised.
A list of all parts in the ISO/IEC 8825 series can be found on the ISO and IEC websites.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html and www.iec.ch/national-
committees.

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ISO/IEC 8825-5:2021(E)
CONTENTS
Page
1 Scope . 1
2 Normative references. 1
2.1 Identical Recommendations | International Standards . 1
2.2 Additional references . 2
3 Definitions . 2
3.1 Imported definitions . 2
3.2 Additional definitions . 3
4 Abbreviations . 3
5 Notation . 3
6 Purpose and extent of standardization . 3
7 Mapping XSD Schemas . 4
8 Ignored schema components and properties . 5
9 ASN.1 modules . 6
10 Name conversion . 6
10.1 General . 6
10.2 Generating ASN.1 type definitions that are references to ASN.1 type assignments . 7
10.3 Generating identifiers and type reference names . 7
10.4 Order of the mapping . 9
11 Mapping uses of XSD built-in types . 10
12 Mapping facets. 10
12.1 The length, minLength, and maxLength facets . 11
12.2 The pattern facet . 11
12.3 The whiteSpace facet. 11
12.4 The enumeration facet . 12
12.5 Other facets . 14
13 Mapping simple type definitions . 14
14 Mapping element declarations . 16
15 Mapping attribute declarations . 17
16 Mapping values of simple type definitions . 17
17 Mapping model group definitions . 17
18 Mapping model groups . 17
19 Mapping particles . 18
20 Mapping complex type definitions. 19
21 Mapping wildcards . 20
22 Mapping attribute uses . 22
23 Mapping uses of simple and complex type definitions (general case) . 22
24 Mapping special uses of simple and complex type definitions (substitutable) . 23
25 Mapping special uses of simple and complex type definitions (substitutable, nillable) . 24
26 Mapping special uses of simple type definitions (nillable) . 25
27 Mapping special uses of complex type definitions (nillable) . 26
28 Mapping special uses of element declarations (head of element substitution group) . 27
29 Generating special ASN.1 type assignments for types used in element declarations . 27
30 Generating special ASN.1 type assignments for types belonging to a derivation hierarchy . 29
31 Generating special ASN.1 type assignments for element substitution groups . 29
Annex A – ASN.1 type definitions corresponding to XSD built-in types for the version 1 mapping . 30
Annex B – ASN.1 type definitions corresponding to XSD built-in types for the version 2 mapping . 33
Rec. ITU-T X.694 (02/2021) iii
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ISO/IEC 8825-5:2021(E)
Page
Annex C – Identification of the XSD module. 37
Annex D – Examples of mappings . 38
D.1 A Schema using simple type definitions . 38
D.2 The corresponding ASN.1 definitions . 39
D.3 Further examples . 40
Annex E – Use of the mapping to provide binary encodings for W3C XML Schema . 60
E.1 Encoding XSD Schemas . 60
E.2 Transfer without using the XSD Schema for Schemas . 60
E.3 Transfer using the XSD Schema for Schemas . 60


iv Rec. ITU-T X.694 (02/2021)
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ISO/IEC 8825-5:2021(E)
Introduction
This Recommendation | International Standard specifies version 1 and version 2 of a mapping from a W3C XML Schema
definition (an XSD Schema) into an Abstract Syntax Notation One (ASN.1) schema. The mappings can be applied to any
XSD Schema. Both mappings specify the generation of one or more ASN.1 modules containing type definitions, together
with ASN.1 XER encoding instructions. These are jointly described as an ASN.1 schema for Extensible Markup Language
(XML) documents. This ASN.1 schema (produced by either version of the mapping), when used with the ASN.1 Extended
XML Encoding Rules (EXTENDED-XER), can be used to generate and to validate the same set of W3C XML 1.0
documents as the original XSD Schema. The resulting ASN.1 types and encodings support the same semantic content as
the XSD Schema. Thus ASN.1 tools can be used interchangeably with XSD tools for the generation and processing of
the specified XML documents.
Other standardized ASN.1 encoding rules, such as the Distinguished Encoding Rules (DER) or the Packed Encoding
Rules (PER), can be used in conjunction with this standardized mapping, but produce encodings for version 2 of the
mapping that differ from (and are less verbose than) those produced by version 1 for XSD constructs involving dates and
times or wildcards.
The combination of this Recommendation | International Standard with ASN.1 Encoding Rules provides fully
standardized and vendor-independent compact and canonical binary encodings for data originally defined using an XSD
Schema.
The ASN.1 schema provides a clear separation between the specification of the information content of messages
(their abstract syntax) and the precise form of the XML document (e.g., use of attributes instead of elements). This results
in both a clearer and generally a less verbose schema than the original XSD Schema.
Annex A forms an integral part of this Recommendation | International Standard, and is an ASN.1 module containing a
set of ASN.1 type assignments that correspond to each of the XSD built-in types for version 1 of the mapping. Mappings
of XSD Schemas into ASN.1 schemas either import the type reference names of those type assignments or include the
type definitions in-line.
Annex B also forms an integral part of this Recommendation | International Standard and provides the ASN.1 module for
version 2 of the mapping.
Annex C does not form an integral part of this Recommendation | International Standard, and summarizes the object
identifier, OID internationalized resource identifier and object descriptor values assigned in this Recommendation |
International Standard.
Annex D does not form an integral part of this Recommendation | International Standard, and gives examples of the
mapping of XSD Schemas into ASN.1 schemas.
Annex E does not form an integral part of this Recommendation | International Standard, and describes the use of the
mapping defined in this Recommendation | International Standard, in conjunction with standardized ASN.1 Encoding
Rules, to provide compact and canonical encodings for data defined using an XSD Schema.

 Rec. ITU-T X.694 (02/2021) v
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ISO/IEC 8825-5:2021 (E)
INTERNATIONAL STANDARD
ITU-T RECOMMENDATION
Information technology – ASN.1 encoding rules: Mapping W3C XML
schema definitions into ASN.1
1 Scope
This Recommendation | International Standard specifies two versions of a mapping from any XSD Schema into an
Abstract Syntax Notation One (ASN.1) schema. The ASN.1 schema for both versions support the same semantics and
validate the same set of XML documents.
This Recommendation | International Standard specifies the final XER encoding instructions that are to be applied as part
of the defined mapping to ASN.1 types, but does not specify which syntactic form is to be used for the specification of
those final XER encoding instructions, or the order or manner of their assignment.
NOTE – Implementers of tools generating these mappings may choose any syntactic form or order of assignment that results in the
specified final XER encoding instructions being applied. Examples in this Recommendation | International Standard generally use
the type prefix form, but use of an XER Encoding Control Section may be preferred for the mapping of a complete XSD Schema,
as a matter of style.
There are different ways (syntactically) of assigning XER encoding instructions for use in EXTENDED-XER encodings
(e.g., use of ASN.1 type prefix encoding instructions or use of an XER encoding control section). The choice of these
syntactic forms is a matter of style and lies outside the scope of this Recommendation | International Standard.
2 Normative references
The following Recommendations | International Standards and W3C specifications contain provisions which, through
reference in this text, constitute provisions of this Recommendation | International Standard. At the time of publication,
the editions indicated were valid. All Recommendations, International Standards and W3C specifications are subject to
revision, and parties to agreements based on this Recommendation | International Standard are encouraged to investigate
the possibility of applying the most recent edition of the Recommendations, International Standards and W3C
specifications listed below. Members of IEC and ISO maintain registers of currently valid International Standards. The
Telecommunication Standardization Bureau of the ITU maintains a list of currently valid ITU-T Recommendations. The
W3C maintains a list of currently valid W3C specifications. The reference to a document within this Recommendation |
International Standard does not give it, as a stand-alone document, the status of a Recommendation or International
Standard.
2.1 Identical Recommendations | International Standards
NOTE – The complete set of ASN.1 Recommendations | International Standards is listed in this clause, as these documents can all
be applicable in particular uses of this Recommendation | International Standard. Where these are not directly referenced in the
body of this Recommendation | International Standard, a † symbol is added to the reference.
– Recommendation ITU-T X.680 (2021) |ISO/IEC 8824-1:2021, Information technology – Abstract Syntax
Notation One (ASN.1): Specification of basic notation.
– Recommendation ITU-T X.681 (2021) | ISO/IEC 8824-2:2021, Information technology – Abstract Syntax
Notation One (ASN.1): Information object specification.
– Recommendation ITU-T X.682 (2021) | ISO/IEC 8824-3:2021, Information technology – Abstract Syntax
Notation One (ASN.1): Constraint specification.
– Recommendation ITU-T X.683 (2021) | ISO/IEC 8824-4:2021, Information technology – Abstract Syntax
Notation One (ASN.1): Parameterization of ASN.1 specifications.
– Recommendation ITU-T X.690 (2021) | ISO/IEC 8825-1:2021, Information technology – ASN.1 encoding
Rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished
Encoding Rules (DER).
– Recommendation ITU-T X.691 (2021) | ISO/IEC 8825-2:2021, Information technology – ASN.1 encoding
rules: Specification of Packed Encoding Rules (PER).
– Recommendation ITU-T X.692 (2021) | ISO/IEC 8825-3:2021, Information technology – ASN.1 encoding
rules: Specification of Encoding Control Notation (ECN).
– Recommendation ITU-T X.693 (2021) | ISO/IEC 8825-4:2021, Information technology – ASN.1 encoding
rules: XML Encoding Rules (XER).
 Rec. ITU-T X.694 (02/2021) 1
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ISO/IEC 8825-5:2021 (E)
– Recommendation ITU-T X.891 (2005) | ISO/IEC 24824-1:2007, Information technology – Generic
applications of ASN.1: Fast infoset.
NOTE – The references above shall be interpreted as references to the identified Recommendations | International Standards
together with all their published amendments and technical corrigenda.
2.2 Additional references
– ISO 8601:2019, Date and time – Representation for information interchange – Part 1: Basic rules.
– W3C XML 1.0:2008, Extensible Markup Language (XML) 1.0 (Fifth Edition), W3C Recommendation,
Copyright © [26 November 2008] World Wide Web Consortium (Massachusetts Institute of Technology,
Institut National de Recherche en Informatique et en Automatique, Keio University),
http://www.w3.org/TR/xml/.
– W3C XML Namespaces:1999, Namespaces in XML, W3C Recommendation, Copyright © [14 January
1999] World Wide Web Consortium (Massachusetts Institute of Technology, Institut National de
Recherche en Informatique et en Automatique, Keio University), http://www.w3.org/TR/1999/REC-xml-
names-19990114.
– W3C XML Information Set:2004, XML Information Set (Second Edition), W3C Recommendation,
Copyright © [4 February 2004] World Wide Web Consortium (Massachusetts Institute of Technology,
Institut National de Recherche en Informatique et en Automatique, Keio University),
http://www.w3.org/TR/xml-infoset/.
– W3C XML Schema:2004, XML Schema Part 1: Structures (Second Edition), W3C Recommendation,
Copyright © [28 October 2004] World Wide Web Consortium (Massachusetts Institute of Technology,
Institut National de Recherche en Informatique et en Automatique, Keio University),
http://www.w3.org/TR/xmlschema-1/.
– W3C XML Schema:2004, XML Schema Part 2: Datatypes (Second Edition), W3C Recommendation,
Copyright © [28 October 2004] World Wide Web Consortium (Massachusetts Institute of Technology,
Institut National de Recherche en Informatique et en Automatique, Keio University),
http://www.w3.org/TR/xmlschema-2/.
NOTE – When the reference "W3C XML Schema" is used in this Recommendation | International Standard, it
refers to W3C XML Schema Part 1 and W3C XML Schema Part 2.
– IETF RFC 2396 (1998), Uniform resource identifiers (URI): Generic syntax.
– IETF RFC 3066 (2001), Tags for the Identification of Languages.
3 Definitions
For the purposes of this Recommendation | International Standard, the following definitions apply.
3.1 Imported definitions
3.1.1 This Recommendation | International Standard uses the terms defined in Rec. ITU-T X.680 | ISO/IEC 8824-1
and in Rec. ITU-T X.693 | ISO/IEC 8825-4.
NOTE – In particular, the terms "final encoding instructions", "type prefix" and "XER encoding control section" are defined in the
Recommendations | International Standards mentioned in this clause.
3.1.2 This Recommendation | International Standard also uses the terms defined in W3C XML Schema and W3C
XML Information Set.
NOTE 1 – It is believed that these terms do not conflict with the terms referenced in 3.1.1. If such a conflict occurs, the definition
of the term in 3.1.1 applies.
NOTE 2 – In particular, the terms "schema component" is defined in W3C XML Schema, and the terms "element information
item" and "attribute information item" are defined in W3C XML Information Set.
NOTE 3 – The terms "top-level simple type definition" and "top-level complex type definition" do not include XSD built-in types, when
used in this Recommendation | International Standard.
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ISO/IEC 8825-5:2021 (E)
3.2 Additional definitions
For the purposes of this Recommendation | International Standard, the following additional definitions apply:
3.2.1 XSD namespace: A namespace with a uniform resource identifier of "http://www.w3.org/2001/XMLSchema".
3.2.2 XSI namespace: A namespace with a uniform resource identifier of
"http://www.w3.org/2001/XMLSchema-instance".
3.2.3 XML namespace: A namespace with a uniform resource identifier of "http://www.w3.org/XML/1998/namespace".
4 Abbreviations
For the purposes of this Recommendation | International Standard, the following abbreviations apply:
ASN.1 Abstract Syntax Notation One
BER (ASN.1) Basic Encoding Rules
DER (ASN.1) Distinguished Encoding Rules
OID Object Identifier
PER (ASN.1) Packed Encoding Rules
URI (IETF) Uniform Resource Identifier
XER (ASN.1) XML Encoding Rules
XML (W3C) Extensible Markup Language
XSD (W3C) XML Schema
5 Notation
5.1 This Recommendation | International Standard references the notation defined by Rec. ITU-T X.680 | ISO/IEC
8824-1, Rec. ITU-T X.682 | ISO/IEC 8824-3, W3C XML 1.0 and W3C XML Schema.
5.2 When it is necessary in the body of this Recommendation | International Standard to specify, either formally or
in examples, the assignment of XER encoding instructions, the type prefix notation is generally used (but see 6.3 and 6.4).
In Annex A, an XER encoding control section is used.
5.3 In this Recommendation | International Standard, bold Courier is used for ASN.1 notation and bold Arial is
used for XSD notation and for XSD terms and concepts.
5.4 The XSD Schemas used in the examples in this Recommendation | International Standard use the prefix xsd: to
identify the XSD namespace.
6 Purpose and extent of standardization
6.1 The mapping to ASN.1 that is specified in this Recommendation | International Standard ensures that:
a) any resulting ASN.1 modules generated by tools conforming to this Recommendation | International
Standard (from the same XSD Schema) define the same (structured) abstract values;
b) all BASIC-XER, CXER, EXTENDED-XER, and binary encodings of that resulting ASN.1 specification
will produce the same encodings (subject to encoder's options); and
c) all XML documents that conform to the source XSD Schema are valid EXTENDED-XER encodings of
abstract values of that ASN.1 specification.
6.2 There are many aspects of an ASN.1 definition (such as the use of white-space, or of encoding control sections
or type prefixes) that affect neither the abstract values being defined nor the XER or binary encodings of those values.
Such aspects of the ASN.1 definition are generally not standardized in this Recommendation | International Standard.
6.3 There are many different ways in ASN.1 of assigning an XER encoding instruction to a type, including:
a) use of a type prefix for every encoding instruction to be assigned; or
b) use of an encoding control section, with a separate encoding instruction for each required assignment; or
c) use of an encoding control section, with a single encoding instruction making a global assignment, possibly
supplemented by use of a negating encoding instruction for specific types.
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ISO/IEC 8825-5:2021 (E)
6.4 This Recommendation | International Standard specifies when a final XER encoding instruction shall be present,
and uses the syntax of 6.3 a) in most of its examples. However, the use of the different options in 6.3 is not standardized,
and conforming implementations of the mapping may choose any syntactic form (or a mixture of syntactic forms) for the
assignment of final XER encoding instructions.
NOTE – The choice among these options does not affect the final binary or XML encodings.
6.5 A formal specification of the required mapping is not provided.
6.6 This Recommendation | International Standard is concerned only with the mapping of XSD Schemas that
conform to W3C XML Schema.
NOTE – Such conformance can be either by the provision of one or more W3C XSD schema documents or by other means as
specified in W3C XML Schema.
7 Mapping XSD Schemas
7.1 A mapping is based on a source XSD Schema, which is a set of schema components (see W3C XML Schema
Part 1, 2.2). No particular representation of schema components or sets of schema components is required or assumed for
the mapping, although it is expected that the source XSD Schema will usually be provided as one or more XML schema
documents (see W3C XML Schema Part 1, 3.15.2).
NOTE 1 – The schema components represented in multiple XML schema documents become part of the same XSD Schema through
the use of the xsd:include, xsd:redefine, and xsd:import element information items.
NOTE 2 – Since the mapping is defined in terms of schema components (and not in terms of their XML representation), it is not
affected by details of the XML representation, such as the use of multiple schema documents linked by xsd:include and
xsd:redefine element information items, the placement of element information items in one or another schema documents, the
order of xsd:attribute element information items within a xsd:complexType element information item, and so on.
NOTE 3 – Two sets of schema documents that differ in many aspects but represent the same set of schema components generate
the same set of ASN.1 type assignments, with the same final encoding instructions assigned to them and to their components to
any depth.
7.2 The source XSD Schema shall meet all the constraints imposed by the XSD specification. If the source XSD
Schema is represented (in part or all) as a set of XML schema documents, each schema document shall be valid according
to the XSD Schema for Schemas (see W3C XML Schema Part 1, Appendix A).
7.3 One or more ASN.1 modules shall be generated for a source XSD Schema. The number of ASN.1 modules
generated is an implementation option. Each ASN.1 module shall contain zero or more type assignments corresponding
to top-level schema components (s
...