ISO/IEC 8825-4:2015

Information technology - ASN.1 encoding rules: XML Encoding Rules (XER) - Part 4:

ISO/IEC 8825-4:2015 specifies a set of basic XML Encoding Rules (BASIC-XER) that may be used to derive a transfer syntax for values of types defined in Rec. ITU-T X.680 | ISO/IEC 8824-1 and Rec. ITU-T X.681 | ISO/IEC 8824-2. This Recommendation | International Standard also specifies a set of Canonical XML Encoding Rules (CXER) which provide constraints on the basic XML Encoding Rules and produce a unique encoding for any given ASN.1 value. This Recommendation | International Standard further specifies a set of extended XML Encoding Rules (EXTENDED-XER) which adds further encoders options, and also allows the ASN.1 specifier to vary the encoding that would be produced by BASIC-XER. It is implicit in the specification of these encoding rules that they are also used for decoding. The encoding rules specified in this Recommendation | International Standard: - are used at the time of communication; - are intended for use in circumstances where displaying of values and/or processing them using commonly available XML tools (such as browsers) is the major concern in the choice of encoding rules; - allow the extension of an abstract syntax by addition of extra values for all forms of extensibility described in Rec. ITU-T X.680 | ISO/IEC 8824‑1. ISO/IEC 8825-4:2015 also specifies the syntax and semantics of XER encoding instructions, and the rules for their assignment and combination. XER encoding instructions can be used to control the EXTENDED-XER encoding for specific ASN.1 types.

Technologies de l'information — Règles de codage ASN.1: Règles de codage XML (XER) — Partie 4:

General Information

• Status: Withdrawn
• Publication Date: 11-Nov-2015
• Withdrawal Date: 11-Nov-2015
• ICS: 35.100.60 - Presentation layer
• Technical Committee: ISO/IEC JTC 1/SC 6 - Telecommunications and information exchange between systems
• Drafting Committee: ISO/IEC JTC 1/SC 6/WG 7 - Network, transport and future network
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 30-Jun-2021
• Completion Date: 30-Oct-2025

Relations

• Revised: ISO/IEC 8825-4:2021 - Information technology - ASN.1 encoding rules - Part 4: XML Encoding Rules (XER)
• Effective Date: 15-Aug-2020
• Revises: ISO/IEC 8825-4:2008 - Information technology - ASN.1 encoding rules: XML Encoding Rules (XER) - Part 4:
• Effective Date: 05-Nov-2015

Standards Content (Sample)

INTERNATIONAL ISO/IEC
STANDARD 8825-4
Third edition
2015-11-15
Information technology — ASN.1
encoding rules: XML Encoding Rules
(XER)
Technologies de l'information — Règles de codage ASN.1: Règles de
codage XML (XER)
Reference number
©
ISO/IEC 2015
©  ISO/IEC 2015
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form or by any
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ii © ISO/IEC 2015 – All rights reserved

Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that are members of
ISO or IEC participate in the development of International Standards through technical committees
established by the respective organization to deal with particular fields of technical activity. ISO and IEC
technical committees collaborate in fields of mutual interest. Other international organizations, governmental
and non-governmental, in liaison with ISO and IEC, also take part in the work. In the field of information
technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of the joint technical committee is to prepare International Standards. Draft International
Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as
an International Standard requires approval by at least 75 % of the national bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.
This third edition cancels and replaces the second edition of ISO/IEC 8825-4:2008 which has been technically
revised. It also incorporates ISO/IEC 8825-4:2008/Cor.1:2012 and ISO/IEC 8825-4:2008/Cor.2:2014.
ISO/IEC 8825-4 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.693 (08/2015).
© ISO/IEC 2015 – All rights reserved iii

Int e rnat ional Telecommunicat ion U n ion

ITU-T X.693
(08/2015)
TELECOMMUNICATION
STANDARDIZATION SECTOR
OF ITU
SERIES X: DATA NETWORKS, OPEN SYSTEM
COMMUNICATIONS AND SECURITY
OSI networking and system aspects – Abstract Syntax
Notation One (ASN.1)
Information technology – ASN.1 encoding rules:
XML Encoding Rules (XER)
Recommendation ITU-T X.693
ITU-T X-SERIES RECOMMENDATIONS
DATA NETWORKS, OPEN SYSTEM COMMUNICATIONS AND SECURITY

PUBLIC DATA NETWORKS
Services and facilities X.1–X.19
Interfaces X.20–X.49
Transmission, signalling and switching X.50–X.89
Network aspects X.90–X.149
Maintenance X.150–X.179
Administrative arrangements X.180–X.199
OPEN SYSTEMS INTERCONNECTION
Model and notation X.200–X.209
Service definitions X.210–X.219
Connection-mode protocol specifications X.220–X.229
Connectionless-mode protocol specifications X.230–X.239
PICS proformas X.240–X.259
Protocol Identification X.260–X.269
Security Protocols X.270–X.279
Layer Managed Objects X.280–X.289
Conformance testing X.290–X.299
INTERWORKING BETWEEN NETWORKS
General X.300–X.349
Satellite data transmission systems X.350–X.369
IP-based networks X.370–X.379
MESSAGE HANDLING SYSTEMS X.400–X.499
DIRECTORY X.500–X.599
OSI NETWORKING AND SYSTEM ASPECTS
Networking X.600–X.629
Efficiency X.630–X.639
Quality of service X.640–X.649
Naming, Addressing and Registration X.650–X.679
Abstract Syntax Notation One (ASN.1) X.680–X.699
OSI MANAGEMENT
Systems management framework and architecture X.700–X.709
Management communication service and protocol X.710–X.719
Structure of management information X.720–X.729
Management functions and ODMA functions X.730–X.799
SECURITY X.800–X.849
OSI APPLICATIONS
Commitment, concurrency and recovery X.850–X.859
Transaction processing X.860–X.879
Remote operations X.880–X.889
Generic applications of ASN.1 X.890–X.899
OPEN DISTRIBUTED PROCESSING X.900–X.999
INFORMATION AND NETWORK SECURITY X.1000–X.1099
SECURE APPLICATIONS AND SERVICES X.1100–X.1199
CYBERSPACE SECURITY X.1200–X.1299
SECURE APPLICATIONS AND SERVICES X.1300–X.1399
CYBERSECURITY INFORMATION EXCHANGE X.1500–X.1599
CLOUD COMPUTING SECURITY X.1600–X.1699

For further details, please refer to the list of ITU-T Recommendations.

INTERNATIONAL STANDARD ISO/IEC 8825-4
RECOMMENDATION ITU-T X.693
Information technology – ASN.1 encoding rules:
XML Encoding Rules (XER)
Summary
Recommendation ITU-T X.693 | ISO/IEC 8825-4 specifies rules for encoding values of ASN.1 types using the Extensible
Markup Language (XML).
History
*
Edition Recommendation Approval Study Group Unique ID
1.0 ITU-T X.693 2001-12-22 7 11.1002/1000/5565
1.1 ITU-T X.693 (2001) Amd. 1 2003-10-29 17 11.1002/1000/7023
1.2 ITU-T X.693 (2001) Amd. 2 2006-06-13 17 11.1002/1000/8841
1.3 ITU-T X.693 (2001) Amd. 3 2007-05-29 17 11.1002/1000/9110
2.0 ITU-T X.693 2008-11-13 17 11.1002/1000/9611
2.1 ITU-T X.693 (2008) Cor. 1 2011-10-14 17 11.1002/1000/11381
2.2 ITU-T X.693 (2008) Cor. 2 2014-03-01 17 11.1002/1000/12148
3.0 ITU-T X.693 2015-08-13 17 11.1002/1000/12489

BBBBBBBBBBBBBBBB
*
To access the Recommendation, type the URL http://handle.itu.int/ in the address field of your web browser, followed by the
Recommendation's unique ID. For example, http://handle.itu.int/11.1002/1000/11830-en.
Rec. ITU-T X.693 (08/2015) i
FOREWORD
The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of
telecommunications, information and communication technologies (ICTs). The ITU Telecommunication
Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical,
operating and tariff questions and issuing Recommendations on them with a view to standardizing
telecommunications on a worldwide basis.
The World Telecommunication Standardization Assembly (WTSA), which meets every four years,
establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on
these topics.
The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1.
In some areas of information technology which fall within ITU-T's purview, the necessary standards are
prepared on a collaborative basis with ISO and IEC.

NOTE
In this Recommendation, the expression "Administration" is used for conciseness to indicate both a
telecommunication administration and a recognized operating agency.
Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain
mandatory provisions (to ensure, e.g., interoperability or applicability) and compliance with the
Recommendation is achieved when all of these mandatory provisions are met. The words "shall" or some
other obligatory language such as "must" and the negative equivalents are used to express requirements. The
use of such words does not suggest that compliance with the Recommendation is required of any party.

INTELLECTUAL PROPERTY RIGHTS
ITU draws attention to the possibility that the practice or implementation of this Recommendation may
involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence,
validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others
outside of the Recommendation development process.
As of the date of approval of this Recommendation, ITU had not received notice of intellectual property,
protected by patents, which may be required to implement this Recommendation. However, implementers
are cautioned that this may not represent the latest information and are therefore strongly urged to consult
the TSB patent database at http://www.itu.int/ITU-T/ipr/.

¤ ITU 2015
All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the
prior written permission of ITU.
ii Rec. ITU-T X.693 (08/2015)
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 ASN.1 BASIC ENCODING RULES (BER) . 2
3.2 ADDITIONAL DEFINITIONS . 2
4 ABBREVIATIONS . 4
5 ENCODINGS SPECIFIED BY THIS RECOMMENDATION | INTERNATIONAL STANDARD. 4
6 ENCODING INSTRUCTIONS SPECIFIED BY THIS RECOMMENDATION | INTERNATIONAL
STANDARD . 5
7 CONFORMANCE . 5
8 BASIC XML ENCODING RULES . 5
8.1 PRODUCTION OF A COMPLETE BASIC-XER ENCODING . 5
8.2 THE XML PROLOG . 6
8.3 THE XML DOCUMENT ELEMENT . 6
8.4 ENCODING OF THE EXTERNAL TYPE . 7
8.5 ENCODING OF THE OPEN TYPE . 7
8.6 DECODING OF TYPES WITH EXTENSION MARKERS . 7
9 CANONICAL XML ENCODING RULES . 7
9.1 GENERAL RULES FOR CANONICAL XER . 7
9.2 REAL VALUES . 8
9.3 BITSTRING VALUE . 8
9.4 OCTETSTRING VALUE . 8
9.5 SEQUENCE VALUE . 8
9.6 SET VALUE . 8
9.7 SET-OF VALUE . 9
9.8 OBJECT IDENTIFIER VALUE . 9
9.9 RELATIVE OBJECT IDENTIFIER VALUE . 9
9.10 GENERALIZEDTIME . 9
9.11 UTCTIME . 9
9.12 OPEN TYPE VALUE . 10
9.13 THE TIME TYPE AND THE USEFUL TIME TYPES . 10
10 EXTENDED XML ENCODING RULES . 10
10.1 GENERAL . 10
10.2 EXTENDED-XER CONFORMANCE . 11
10.3 STRUCTURE OF AN EXTENDED-XER ENCODING . 13
11 NOTATION, CHARACTER SET AND LEXICAL ITEMS USED IN XER ENCODING
INSTRUCTIONS . 14
Rec. ITU-T X.693 (08/2015) iii

12 KEYWORDS . 14
13 ASSIGNING AN XER ENCODING INSTRUCTION TO AN ASN.1 TYPE USING A TYPE
PREFIX . 15
14 ASSIGNING AN XER ENCODING INSTRUCTION USING AN XER ENCODING CONTROL
SECTION . 18
14.1 THE ENCODING INSTRUCTION ASSIGNMENT LIST . 18
14.2 IDENTIFICATION OF THE TARGETS FOR AN XER ENCODING INSTRUCTION USING
A TARGET LIST . 18
14.2.1 GENERAL RULES . 18
14.2.2 TARGET IDENTIFICATION USING AN ASN.1 TYPE REFERENCE AND
IDENTIFIERS . 21
14.2.3 TARGET IDENTIFICATION USING A BUILT-IN TYPE NAME . 22
14.2.4 USE OF IDENTIFIERS IN CONTEXT. 23
14.2.5 USE OF IMPORTED TYPES IDENTIFICATION . 24
15 MULTIPLE ASSIGNMENT OF XER ENCODING INSTRUCTIONS . 24
15.1 ORDER IN WHICH MULTIPLE ASSIGNMENTS ARE CONSIDERED . 24
15.2 EFFECT OF ASSIGNING A NEGATING ENCODING INSTRUCTION . 25
15.3 MULTIPLE ASSIGNMENT OF ENCODING INSTRUCTIONS WITH MULTIPLE
CATEGORIES . 25
15.4 MULTIPLE ASSIGNMENT OF XER ENCODING INSTRUCTIONS OF THE SAME
CATEGORY . 25
15.5 PERMITTED COMBINATIONS OF FINAL ENCODING INSTRUCTIONS . 26
16 XER ENCODING INSTRUCTION SUPPORT FOR XML NAMESPACES AND QUALIFIED
NAMES . 27
17 SPECIFICATION OF EXTENDED-XER ENCODINGS . 28
17.1 THE XML DOCUMENT ELEMENT . 29
17.2 THE "TYPENAMEORMODIFIEDTYPENAME" PRODUCTION . 29
17.3 THE "ATTRIBUTELIST" PRODUCTION . 29
17.4 THE "EXTENDEDXMLVALUE" PRODUCTION. 29
17.5 THE "EXTENDEDXMLCHOICEVALUE" PRODUCTION . 31
17.6 THE "EXTENDEDXMLSEQUENCEVALUE" AND "EXTENDEDXMLSETVALUE"
PRODUCTIONS . 31
17.7 THE "EXTENDEDXMLSEQUENCEOFVALUE" AND "EXTENDEDXMLSETOFVALUE"
PRODUCTIONS . 32
17.8 THE "MODIFIEDXMLINTEGERVALUE" PRODUCTION . 33
17.9 THE "MODIFIEDXMLREALVALUE" PRODUCTION . 34
18 THE ANY-ATTRIBUTES ENCODING INSTRUCTION . 34
18.1 GENERAL . 34
18.2 RESTRICTIONS . 35
18.3 EFFECT ON ENCODINGS . 36
19 THE ANY-ELEMENT ENCODING INSTRUCTION . 36
19.1 GENERAL . 36
19.2 RESTRICTIONS . 36
19.3 EFFECT ON ENCODINGS . 37
20 THE ATTRIBUTE ENCODING INSTRUCTION . 38
20.1 GENERAL . 38
iv Rec. ITU-T X.693 (08/2015)
20.2 RESTRICTIONS . 38
20.3 EFFECT ON ENCODINGS . 38
21 THE BASE64 ENCODING INSTRUCTION . 40
21.1 GENERAL . 40
21.2 RESTRICTIONS . 40
21.3 EFFECT ON ENCODINGS . 40
22 THE DECIMAL ENCODING INSTRUCTION . 41
22.1 GENERAL . 41
22.2 RESTRICTIONS . 41
22.3 EFFECT ON ENCODINGS . 42
23 THE DEFAULT-FOR-EMPTY ENCODING INSTRUCTION . 42
23.1 GENERAL . 42
23.2 RESTRICTIONS . 43
23.3 EFFECT ON ENCODINGS . 44
24 THE ELEMENT ENCODING INSTRUCTION . 44
24.1 GENERAL . 44
24.2 RESTRICTIONS . 44
24.3 EFFECT ON ENCODINGS . 44
25 THE EMBED-VALUES ENCODING INSTRUCTION . 44
25.1 GENERAL . 44
25.2 RESTRICTIONS . 45
25.3 EFFECT ON ENCODINGS . 45
26 THE GLOBAL-DEFAULTS ENCODING INSTRUCTION . 46
26.1 GENERAL . 46
26.2 RESTRICTIONS . 46
26.3 EFFECT ON ENCODINGS . 46
27 THE LIST ENCODING INSTRUCTION . 47
27.1 GENERAL . 47
27.2 RESTRICTIONS . 47
27.3 EFFECT ON ENCODINGS . 47
28 THE NAME ENCODING INSTRUCTION . 48
28.1 GENERAL . 48
28.2 RESTRICTIONS . 49
28.3 EFFECT ON ENCODINGS . 49
29 THE NAMESPACE ENCODING INSTRUCTION . 50
29.1 GENERAL . 50
29.2 RESTRICTIONS . 50
29.3 EFFECT ON ENCODINGS . 51
30 THE PI-OR-COMMENT ENCODING INSTRUCTION . 51
30.1 GENERAL . 51
30.2 RESTRICTIONS . 52
30.3 EFFECT ON THE ENCODINGS . 52
31 THE TEXT ENCODING INSTRUCTION . 52
Rec. ITU-T X.693 (08/2015) v
31.1 GENERAL . 52
31.2 RESTRICTIONS . 53
31.3 EFFECT ON ENCODINGS . 53
32 THE UNTAGGED ENCODING INSTRUCTION . 54
32.1 GENERAL . 54
32.2 RESTRICTIONS . 55
32.3 EFFECT ON ENCODINGS . 55
33 THE USE-NIL ENCODING INSTRUCTION . 56
33.1 GENERAL . 56
33.2 RESTRICTIONS . 56
33.3 EFFECT ON ENCODINGS . 57
34 THE USE-NUMBER ENCODING INSTRUCTION . 57
34.1 GENERAL . 57
34.2 RESTRICTIONS . 57
34.3 EFFECT ON ENCODINGS . 57
35 THE USE-ORDER ENCODING INSTRUCTION . 58
35.1 GENERAL . 58
35.2 RESTRICTIONS . 58
35.3 EFFECT ON ENCODINGS . 59
36 THE USE-QNAME ENCODING INSTRUCTION . 59
36.1 GENERAL . 59
36.2 RESTRICTIONS . 59
36.3 EFFECT ON ENCODINGS . 60
37 THE USE-TYPE ENCODING INSTRUCTION . 60
37.1 GENERAL . 60
37.2 RESTRICTIONS . 60
37.3 EFFECT ON ENCODINGS . 61
38 THE USE-UNION ENCODING INSTRUCTION . 61
38.1 GENERAL . 61
38.2 RESTRICTIONS . 61
38.3 EFFECT ON ENCODINGS . 62
39 THE WHITESPACE ENCODING INSTRUCTION . 63
39.1 GENERAL . 63
39.2 RESTRICTIONS . 63
39.3 EFFECT ON ENCODINGS . 64
40 IDENTIFICATION OF THE ENCODING RULES . 64
ANNEX A – EXAMPLES OF BASIC-XER AND CXER ENCODINGS . 65
A.1 ASN.1 DESCRIPTION OF THE RECORD STRUCTURE . 65
A.2 ASN.1 DESCRIPTION OF A RECORD VALUE . 65
A.3 BASIC XML REPRESENTATION OF THIS RECORD VALUE . 65
A.4 CANONICAL XML REPRESENTATION OF THIS RECORD VALUE . 66
ANNEX B – PARTIAL XML CONTENT AND DETERMINISTIC ENCODINGS . 67
B.1 PARTIAL XML CONTENT . 67
vi Rec. ITU-T X.693 (08/2015)
B.2 RECOMMENDED RESTRICTIONS ON ENCODINGS PRODUCING PARTIAL XML
ELEMENT CONTENT . 67
ANNEX C – EXAMPLES OF EXTENDED-XER ENCODINGS USING XER
ENCODING INSTRUCTIONS . 70
C.1 INTRODUCTION . 70
C.2 SIMPLE EXAMPLES. 70
C.2.1 A BASE-BALL CARD . 70
C.2.2 AN EMPLOYEE . 71
C.3 MORE COMPLEX EXAMPLES . 71
C.3.1 USING A UNION OF TWO SIMPLE TYPES . 71
C.3.2 USING A TYPE IDENTIFICATION ATTRIBUTE . 72
C.3.3 USING ENUMERATION VALUES . 72
C.3.4 USING AN EMPTY ENCODING FOR A DEFAULT VALUE . 72
C.3.5 USING EMBEDDED-VALUES FOR NOTIFICATION OF A PAYMENT DUE . 72

Rec. ITU-T X.693 (08/2015) vii

1 Introduction
Rec. ITU-T X.680 | ISO/IEC 8824-1, Rec. ITU-T X.681 | ISO/IEC 8824-2, Rec. ITU-T X.682 | ISO/IEC 8824-3, Rec.
ITU-T X.683 | ISO/IEC 8824-4 together describe Abstract Syntax Notation One (ASN.1), a notation for the definition
of messages to be exchanged between peer applications.
This Recommendation | International Standard defines encoding rules that may be applied to values of ASN.1 types
defined using the notation specified in Rec. ITU-T X.680 | ISO/IEC 8824-1 and Rec. ITU-T X.681 | ISO/IEC 8824-2.
Application of these encoding rules produces a transfer syntax for such values. It is implicit in the specification of these
encoding rules that they are also to be used for decoding.
There is more than one set of encoding rules that can be applied to values of ASN.1 types. This Recommendation |
International Standard defines three sets of encoding rules that use the Extensible Markup Language (XML). These
encoding rules all produce an XML document compliant to W3C XML 1.0. The first set is called the Basic XML
Encoding Rules (BASIC-XER). The second set is called the Canonical XML Encoding Rules (CANONICAL-XER, or
CXER) because there is only one way of encoding an ASN.1 value using these encoding rules. (Canonical encoding
rules are generally used for applications using security-related features such as digital signatures.) The third set is called
the extended XML Encoding Rules (EXTENDED-XER). The extended XML Encoding Rules allow additional encoders
options, and take account of encoding instructions that specify variations of the BASIC-XER encodings in order to
support specific styles of XML documents (see below). The extended XML Encoding Rules are not canonical, and there
is no canonical form for these rules defined in this Recommendation | International Standard.
There are many aspects of an XML representation of data (such as the use of XML attributes instead of child elements,
or the use of white-space delimited lists) whose use is a matter of style and XML designer choice. If a type defined in an
ASN.1 specification is encoded by BASIC-XER or by CXER, then there is a single fixed style used for the XML
representation, with no user control of stylistic features. This Recommendation | International Standard specifies the
syntax and semantics of XER encoding instructions which specify the stylistic features of the XML in an EXTENDED-
XER encoding. XER encoding instructions can also be used to determine the possible insertion of XML processing
instructions in an EXTENDED-XER encoding. XER encoding instructions are ignored by BASIC-XER and by CXER,
but are used by EXTENDED-XER.
NOTE – "Stylistic features", such as use of attributes or white-space delimited lists, can also affect the size of an encoding and
the ease with which it can be processed, so use of such features is not just a matter of style. Where such issues are important,
EXTENDED-XER with encoding instructions may be preferred over BASIC-XER or CXER.
Clause 8 specifies the BASIC-XER encoding of ASN.1 types.
Clause 9 specifies the CXER encoding of ASN.1 types.
Clause 10 specifies the EXTENDED-XER encoding of ASN.1 types, referencing later clauses which define the XER
encoding instructions.
Clauses 11 to 14 list and categorize the XER encoding instructions and specify the syntax for their assignment to an
ASN.1 type or component using either an XER type prefix (see Rec. ITU-T X.680 | ISO/IEC 8824-1, 31.3) or an XER
encoding control section (see Rec. ITU-T X.680 | ISO/IEC 8824-1, clause 54).
Clause 15 defines the order of precedence if XER encoding instructions are present in both an XER type prefix and in
an XER encoding control section.
Clause 16 specifies the XER encoding instruction support for XML namespaces when using EXTENDED-XER.
Clause 17 specifies EXTENDED-XER encodings.
Clauses 18 to 39 specify:
a) the syntax of each XER encoding instruction used in a type prefix or an XER encoding control section;
b) restrictions on the XER encoding instructions that can be associated with a particular ASN.1 type
(resulting from inheritance and multiple assignments);
c) modifications to the XER encoding rules that are required in an EXTENDED-XER encoding when an
XER encoding instruction is applied.
Annex A is informative and contains examples of BASIC-XER and CXER encodings.
viii Rec. ITU-T X.693 (08/2015)

Annex B is informative and contains a description of the partial XML content that is produced when constructions such
as sequence and sequence-of have their surrounding tags removed, together with restrictions on EXTENDED-XER
specifications that enable easy determination of the ASN.1 component that an XML element is associated with.
Annex C is informative and contains examples of XER encoding instructions and of the corresponding
EXTENDED-XER encodings.
Rec. ITU-T X.693 (08/2015) ix
INTERNATIONAL STANDARD
ITU-T RECOMMENDATION
Information technology –
ASN.1 encoding rules:
XML Encoding Rules (XER)
1 Scope
This Recommendation | International Standard specifies a set of basic XML Encoding Rules (BASIC-XER) that may be
used to derive a transfer syntax for values of types defined in Rec. ITU-T X.680 | ISO/IEC 8824-1 and Rec. ITU-T X.681
| ISO/IEC 8824-2. This Recommendation | International Standard also specifies a set of Canonical XML Encoding Rules
(CXER) which provide constraints on the basic XML Encoding Rules and produce a unique encoding for any given
ASN.1 value. This Recommendation | International Standard further specifies a set of extended XML Encoding Rules
(EXTENDED-XER) which adds further encoders options, and also allows the ASN.1 specifier to vary the encoding that
would be produced by BASIC-XER. It is implicit in the specification of these encoding rules that they are also used for
decoding.
The encoding rules specified in this Recommendation | International Standard:
– are used at the time of communication;
– are intended for use in circumstances where displaying of values and/or processing them using commonly
available XML tools (such as browsers) is the major concern in the choice of encoding rules;
– allow the extension of an abstract syntax by addition of extra values for all forms of extensibility described
in Rec. ITU-T X.680 | ISO/IEC 8824-1.
This Recommendation | International Standard also specifies the syntax and semantics of XER encoding instructions, and
the rules for their assignment and combination. XER encoding instructions can be used to control the EXTENDED-XER
encoding for specific ASN.1 types.
2 Normative references
The following Recommendations and International Standards 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 and Standards 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 and Standards 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.
NOTE – This Recommendation | International Standard is based on ISO/IEC 10646:2003 and the Unicode standard version
3.2.0:2002. It cannot be applied using later versions of these two standards.
2.1 Identical Recommendations | International Standards
– Recommendation ITU-T X.680 (2015) | ISO/IEC 8824-1:2015, Information technology – Abstract Syntax
Notation One (ASN.1): Specification of basic notation.
– Recommendation ITU-T X.681 (2015) | ISO/IEC 8824-2:2015, Information technology – Abstract Syntax
Notation One (ASN.1): Information object specification.
– Recommendation ITU-T X.682 (2015) | ISO/IEC 8824-3:2015, Information technology – Abstract Syntax
Notation One (ASN.1): Constraint specification.
– Recommendation ITU-T X.683 (2015) | ISO/IEC 8824-4:2015, Information technology – Abstract Syntax
Notation One (ASN.1): Parameterization of ASN.1 specifications.
– Recommendation ITU-T X.690 (2015) | ISO/IEC 8825-1:2015, Information technology – ASN.1 encoding
rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished
Encoding Rules (DER).
Rec. ITU-T X.693 (08/2015) 1
– Recommendation ITU-T X.691 (2015) | ISO/IEC 8825-2:2015, Information technology – ASN.1 encoding
rules: Specification of Packed Encoding Rules (PER).
– Recommendation ITU-T X.692 (2015) | ISO/IEC 8825-3:2015, Information technology – ASN.1 encoding
rules: Specification of Encoding Control Notation (ECN).
2.2 Additional references
– IETF RFC 2045 (1996), Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet
Message Bodies.
– IETF RFC 2141 (1997), URN Syntax.
– IETF RFC 2396 (1998), Uniform Resource Identifiers (URI): Generic Syntax.
– IETF RFC 3061 (2001), A URN Namespace of Object Identifiers.
– ISO/IEC 10646:2003, Information technology – Universal Multiple-Octet Coded Character Set (UCS).
– The Unicode Standard, Version 3.2.0, The Unicode Consortium. (Reading, MA, Addison-Wesley)
NOTE 1 – The graphics characters (and their encodings) defined by the above reference are identical to those
defined by ISO/IEC 10646, but the above reference is included because it also specifies the names of control
characters.
– W3C XML 1.0:2000, Extensible Markup Language (XML) 1.0 (Second Edition), W3C Recommendation,
Copyright © [6 October 2000] World Wide Web Consortium, (Massachusetts Institute of Technology,
Institut National de Recherche en Informatique et en Automatique, Keio University),
http://www.w3.org/TR/2000/REC-xml-20001006.
– 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.
NOTE 2 – 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.
3 Definitions
For the purposes of this Recommendation | International Standard, the definitions of Rec. ITU-T X.680 | ISO/IEC 8824-1
and the following definitions apply.
3.1 ASN.1 Basic Encoding Rules (BER)
This Recommendation | International Standard makes use of the following terms defined in Rec. ITU-T X.690 |
ISO/IEC 8825-1:
a) data value;
b) dynamic conformance;
c) encoding (of a data value);
d) receiver;
e) sender;
f) static conformance.
3.2 Additional definitions
For the purposes of this Recommendation | International Standard, the following definitions apply.
3.2.1 ASN.1 schema:  The definition of the content and structure of data using an ASN.1 type definition.
NOTE – This enables encoding rules to produce binary encodings of the values of an ASN.1 type, or encodings using XML.
3.2.2 associated empt
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