ISO/IEC 8825-2:2015

INTERNATIONAL ISO/IEC
STANDARD 8825-2
Fifth edition
2015-11-15

Information technology — ASN.1
encoding rules: Specification of Packed
Encoding Rules (PER)
Technologies de l'information — Règles de codage ASN.1:
Spécification des règles de codage compact (PER)





Reference number
ISO/IEC 8825-2:2015(E)
©
ISO/IEC 2015

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ISO/IEC 8825-2:2015(E)
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ii © ISO/IEC 2015 – All rights reserved

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ISO/IEC 8825-2:2015(E)

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 fifth edition cancels and replaces the fourth edition of ISO/IEC 8825-2:2008 which has been technically
revised. It also incorporates ISO/IEC 8825-2:2008/Cor.1:2012, ISO/IEC 8825-5:2008.Cor.2:2012, ISO/IEC
8825-2:2008/Cor.3:2015 and ISO/IEC 8825-2:2008/Cor.4:2015.
ISO/IEC 8825-2 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.691 (08/2015).
© ISO/IEC 2015 – All rights reserved iii

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I n t e r n a t i o n a l T e l e c o m m u n i c a t i o n U n i o n


ITU-T X.691
(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:
Specification of Packed Encoding Rules (PER)

Recommendation ITU-T X.691

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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.

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INTERNATIONAL STANDARD ISO/IEC 8825-2
RECOMMENDATION ITU-T X.691
Information technology – ASN.1 encoding rules:
Specification of Packed Encoding Rules (PER)



Summary
Recommendation ITU-T X.691 | ISO/IEC 8825-2 describes a set of encoding rules that can be applied to values of all
ASN.1 types to achieve a much more compact representation than that achieved by the Basic Encoding Rules and its
derivatives (described in Rec. ITU-T X.690 | ISO/IEC 8825-1).


History
*
Edition Recommendation Approval Study Group Unique ID
1.0 ITU-T X.691 1995-04-10 7 11.1002/1000/3047
1.1 ITU-T X.691 (1995) Technical Cor. 1 1997-12-12 7 11.1002/1000/4184
1.2 ITU-T X.691 (1995) Technical Cor. 2 1997-12-12 7 11.1002/1000/4185
2.0 ITU-T X.691 1997-12-12 7 11.1002/1000/4448
2.1 ITU-T X.691 (1997) Technical Cor. 1 1999-06-18 7 11.1002/1000/4707
2.2 ITU-T X.691 (1997) Amd. 1 1999-06-18 7 11.1002/1000/4706
2.3 ITU-T X.691 (1997) Technical Cor. 2 2001-02-02 7 11.1002/1000/5337
2.4 ITU-T X.691 (1997) Technical Cor. 3 2001-03-15 7 11.1002/1000/5338
3.0 ITU-T X.691 2002-07-14 17 11.1002/1000/6090
3.1 ITU-T X.691 (2002) Amd. 1 2003-10-29 17 11.1002/1000/7022
3.2 ITU-T X.691 (2002) Technical Cor. 1 2005-05-14 17 11.1002/1000/8513
3.3 ITU-T X.691 (2002) Technical Cor. 2 2005-11-29 17 11.1002/1000/8638
3.4 ITU-T X.691 (2002) Amd. 2 2006-06-13 17 11.1002/1000/8839
3.5 ITU-T X.691 (2002) Amd. 3 2007-05-29 17 11.1002/1000/9109
4.0 ITU-T X.691 2008-11-13 17 11.1002/1000/9609
4.1 ITU-T X.691 (2008) Cor. 1 2011-10-14 17 11.1002/1000/11379
4.2 ITU-T X.691 (2008) Cor. 2 2012-04-13 17 11.1002/1000/11585
4.3 ITU-T X.691 (2008) Cor. 3 2014-11-13 17 11.1002/1000/12337
4.4 ITU-T X.691 (2008) Cor. 4 2014-11-13 17 11.1002/1000/12339
5.0 ITU-T X.691 2015-08-13 17 11.1002/1000/12484




*
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.691 (08/2015) i

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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.691 (08/2015)

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CONTENTS
Page
Introduction . vi
1 Scope . 1
2 Normative references . 1
2.1 Identical Recommendations | International Standards . 1
2.2 Additional references . 1
3 Definitions . 2
3.1 Specification of Basic Notation . 2
3.2 Information Object Specification . 2
3.3 Constraint Specification . 2
3.4 Parameterization of ASN.1 Specification . 2
3.5 Basic Encoding Rules . 2
3.6 PER Encoding Instructions . 2
3.7 Additional definitions . 2
4 Abbreviations . 5
5 Notation . 5
6 Convention . 5
7 Encoding rules defined in this Recommendation | International Standard . 5
8 Conformance . 6
9 PER encoding instructions . 6
10 The approach to encoding used for PER . 7
10.1 Use of the type notation. 7
10.2 Use of tags to provide a canonical order . 7
10.3 PER-visible constraints . 7
10.4 Type and value model used for encoding . 9
10.5 Structure of an encoding. 9
10.6 Types to be encoded . 10
11 Encoding procedures . 10
11.1 Production of the complete encoding . 10
11.2 Open type fields . 11
11.3 Encoding as a non-negative-binary-integer . 11
11.4 Encoding as a 2's-complement-binary-integer . 12
11.5 Encoding of a constrained whole number . 12
11.6 Encoding of a normally small non-negative whole number . 13
11.7 Encoding of a semi-constrained whole number . 13
11.8 Encoding of an unconstrained whole number . 14
11.9 General rules for encoding a length determinant . 14
12 Encoding the boolean type . 16
13 Encoding the integer type . 16
14 Encoding the enumerated type . 17
15 Encoding the real type . 18
16 Encoding the bitstring type . 18
17 Encoding the octetstring type . 19
18 Encoding the null type . 19
19 Encoding the sequence type . 19
20 Encoding the sequence-of type . 20
  Rec. ITU-T X.691 (08/2015) iii

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21 Encoding the set type . 21
22 Encoding the set-of type . 21
23 Encoding the choice type . 22
24 Encoding the object identifier type . 22
25 Encoding the relative object identifier type . 23
26 Encoding the internationalized resource reference type . 23
27 Encoding the relative internationalized resource reference type . 23
28 Encoding the embedded-pdv type. 23
29 Encoding of a value of the external type . 23
30 Encoding the restricted character string types . 24
31 Encoding the unrestricted character string type . 26
32 Encoding the time type, the useful time types, the defined time types and the additional time types . 26
32.1 General . 26
32.2 Encoding subtypes with the "Basic=Date" property setting . 31
32.3 Encoding subtypes with the "Basic=Time" property setting . 33
32.4 Encoding subtypes with the "Basic=Date-Time" property setting . 36
32.5 Encoding subtypes with the "Basic=Interval Interval-type=SE" property setting . 36
32.6 Encoding subtypes with the "Basic=Interval Interval-type=D" property setting . 36
32.7 Encoding subtypes with the "Basic=Interval Interval-type=SD" or
"Basic=Interval Interval-type=DE" property setting . 37
32.8 Encoding subtypes with the "Basic=Rec-Interval Interval-type=SE" property setting . 38
32.9 Encoding subtypes with the "Basic=Rec-Interval Interval-type=D" property setting . 39
32.10 Encoding subtypes with the "Basic=Rec-Interval Interval-type=SD" or
"Basic=Rec-Interval Interval-type=DE" property setting . 39
32.11 Encoding subtypes with mixed settings of the Basic property . 40
33 Object identifiers for transfer syntaxes . 43
Annex A Example of encodings . 44
A.1 Record that does not use subtype constraints . 44
A.1.1 ASN.1 description of the record structure . 44
A.1.2 ASN.1 description of a record value . 44
A.1.3 ALIGNED PER representation of this record value . 44
A.1.4 UNALIGNED PER representation of this record value . 45
A.2 Record that uses subtype constraints . 47
A.2.1 ASN.1 description of the record structure . 47
A.2.2 ASN.1 description of a record value . 47
A.2.3 ALIGNED PER representation of this record value . 47
A.2.4 UNALIGNED PER representation of this record value . 48
A.3 Record that uses extension markers . 49
A.3.1 ASN.1 description of the record structure . 49
A.3.2 ASN.1 description of a record value . 50
A.3.3 ALIGNED PER representation of this record value . 50
A.3.4 UNALIGNED PER representation of this record value . 51
A.4 Record that uses extension addition groups . 53
A.4.1 ASN.1 description of the record structure . 53
A.4.2 ASN.1 description of a record value . 53
A.4.3 ALIGNED PER representation of this record value . 53
A.4.4 UNALIGNED PER representation of this record value . 54
Annex B Combining PER-visible and non-PER-visible constraints . 55
B.1 General . 55
B.2 Extensibility and visibility of constraints in PER . 55
B.2.1 General . 55
B.2.2 PER-visibility of constraints . 56
B.2.3 Effective constraints . 57
B.3 Examples . 58
iv Rec. ITU-T X.691 (08/2015)

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Annex C Support for the PER algorithms . 60
Annex D Support for the ASN.1 rules of extensibility . 61
Annex E Tutorial annex on concatenation of PER encodings . 62
Annex F Identification of Encoding Rules . 63

  Rec. ITU-T X.691 (08/2015) v

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Introduction
Specifications 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 types defined
using the notation specified in Rec. ITU-T X.680 | ISO/IEC 8824-1. 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 are more than one set of encoding rules that can be applied to values of ASN.1 types. This Recommendation |
International Standard defines a set of Packed Encoding Rules (PER), so called because they achieve a much more
compact representation than that achieved by the Basic Encoding Rules (BER) and its derivatives described in Rec.
ITU-T X.690 | ISO/IEC 8825-1 which is referenced for some parts of the specification of these Packed Encoding Rules.

vi Rec. ITU-T X.691 (08/2015)

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ISO/IEC 8825-2: 2015 (E)
INTERNATIONAL STANDARD
ITU-T RECOMMENDATION
Information technology –
ASN.1 encoding rules:
Specification of Packed Encoding Rules (PER)
1 Scope
This Recommendation | International Standard specifies a set of Packed Encoding Rules that may be used to derive a
transfer syntax for values of types defined in Rec. ITU-T X.680 | ISO/IEC 8824-1. These Packed Encoding Rules are also
to be applied for decoding such a transfer syntax in order to identify the data values being transferred.
The encoding rules specified in this Recommendation | International Standard:
– are used at the time of communication;
– are intended for use in circumstances where minimizing the size of the representation of values is the
major concern in the choice of encoding rules;
– allow the extension of an abstract syntax by addition of extra values, preserving the encodings
of the existing values, for all forms of extension described in Rec. ITU-T X.680 | ISO/IEC 8824-1;
– can be modified in accordance with the provisions of Rec. ITU-T X.695 | ISO/IEC 8825-6.
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. It cannot be applied using later versions
of this standard.
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).
– Recommendation ITU-T X.695 (2015) | ISO/IEC 8825-6:2015, Information technology – ASN.1 encoding
rules: Registration and application of PER encoding instructions.
2.2 Additional references
– ISO/IEC 646:1991, Information technology – ISO 7-bit coded character set for information interchange.
– ISO/IEC 2022:1994, Information technology – Character code structure and extension techniques.
  Rec. ITU-T X.691 (08/2015) 1

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