ISO/IEC 21000-3:2025

International
Standard
ISO/IEC 21000-3
Second edition
Information technology —
Multimedia framework (MPEG-21) —
2025-08
Part 3:
Digital Item Identification
Technologies de l'information — Cadre multimédia (MPEG-21) —
Partie 3: Identification des éléments numériques
Reference number
© ISO/IEC 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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© ISO/IEC 2025 – All rights reserved
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and abbreviated terms . 1
3.1 Terms and definitions .1
3.2 Abbreviated terms .3
4 Identification of digital items and their parts. 4
4.1 Identifier element .4
4.1.1 Syntax . .4
4.1.2 Semantics .4
4.1.3 Notes .4
4.2 RelatedIdentifier element .5
4.2.1 Syntax . .5
4.2.2 Semantics .5
4.2.3 Example .5
4.3 Relationships element .5
4.3.1 Syntax . .6
4.3.2 Semantics .6
4.3.3 MPEG-21 core ontology for DI relationships .6
4.4 Type element .7
4.4.1 Syntax . .7
4.4.2 Semantics .7
4.4.3 Example .7
4.5 Digital Item identification XML schema definition .7
4.6 Examples .8
4.6.1 Example: Identification of a music release .9
4.6.2 Example: Referring to Digital Item in external DID document .9
4.6.3 Example: Referring to resources stored on external server .11
4.6.4 Example: Identifying a sound recording and the related underlying musical work .11
4.6.5 Example: Identifying a Digital Item type .11
5 Identification of description schemes .12
Annex A (informative) Interoperability considerations on resolution systems .13
Annex B (informative) List of existing identification and description schemes . 17
Annex C (informative) Usage of this document in the MPEG-21 multimedia framework .25
Annex D (informative) Relationships element employment example .28
Bibliography .34

© ISO/IEC 2025 – All rights reserved
iii
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.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of document should be noted. This document was drafted in accordance with the editorial rules of the ISO/
IEC Directives, Part 2 (see www.iso.org/directives or www.iec.ch/members_experts/refdocs).
ISO and IEC draw attention to the possibility that the implementation of this document may involve the
use of (a) patent(s). ISO and IEC take no position concerning the evidence, validity or applicability of any
claimed patent rights in respect thereof. As of the date of publication of this document, ISO and IEC had
received notice of (a) patent(s) which may be required to implement this document. However, implementers
are cautioned that this may not represent the latest information, which may be obtained from the patent
database available at www.iso.org/patents and https://patents.iec.ch. ISO and IEC shall not be held
responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT) 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 29, Coding of audio, picture, multimedia and hypermedia information.
This second edition cancels and replaces the first edition (ISO/IEC 21000-3:2003), which has been technically
revised. It also incorporates ISO/IEC 21000-3:2003/Amd 1:2007 and ISO/IEC 21000-3:2003/Amd 2:2013.
The main changes are as follows:
— removal of Annex A (normative) - Requirements for the Registration Authority for Digital Item
Identification Systems, and related descriptions that requires ISO/IEC 21000-3 to employ Registration
Authority.
— updating information on ISO 15706-2 (ISAN), which has recently been approved as an International
Standard.
— provision of MPEG-21 DII with the ability to explicitly and unambiguously describe existing relationships
between different MPEG-21 Digital Items. It also conveys the base inter-DI relationship taxonomy in the
form of an RDF/OWL ontology.
A list of all parts in the ISO/IEC 21000 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.

© ISO/IEC 2025 – All rights reserved
iv
Introduction
0.1  Executive summary for MPEG-21
Today, many elements exist to build an infrastructure for the delivery and consumption of multimedia
content. The aim for MPEG-21 multimedia framework (ISO/IEC 21000) is to describe how these various
elements fit together.
The result is an open framework for multimedia delivery and consumption, with both the content creator
and content consumer as focal points. This open framework provides content creators and service providers
with equal opportunities in the MPEG-21 enabled open market. This will also be to the benefit of the content
consumer providing them access to a large variety of content in an interoperable manner.
The vision for MPEG-21 is to define a multimedia framework to enable transparent and augmented use of
multimedia resources across a wide range of networks and devices used by different communities.
This document specifies how Digital Items (see ISO/IEC 21000-2) and parts and collections thereof can be
uniquely identified.
0.2  Organisation of the document
This introduction contains an overview of MPEG-21 Digital Items and the relation between ISO/IEC 21000-2
and this document.
Clause 4 specifies how to uniquely identify Digital Items, how to associate related identifiers with Digital
Items and how to identify different types of Digital Items. Clause 5 then specifies how to associate metadata
with Digital Items by using description scheme identifiers.
Annex A contains an example of how to resolve a unique identifier to appropriate metadata. Annex B contains
a list of existing identification schemes that can be used by this document. Annex C provides an approach to
dealing with varying functional granularities for identifying Digital Items. Annex D provides an example of
how to express the relationship between two Digital Items.
0.3  Introduction to Digital Items
Within any system (such as MPEG-21) that proposes to facilitate a wide range of actions involving Digital
Items, there is a need for a very precise description for defining exactly what constitutes such an “item”.
Clearly there are many kinds of content, and probably just as many possible ways of describing it to reflect
its context of use. This presents a strong challenge to lay out a powerful and flexible model for Digital Items
which can accommodate the myriad forms that content can take (and the new forms it will assume in the
future). Such a model is only truly useful if it yields a format that can be used to represent any Digital Items
defined within the model unambiguously and communicate them, and information about them, successfully.
ISO/IEC 21000-2 provides such flexibility for representing Digital Items.
0.4  Example of a Digital Item
This subclause provides a simple example of a Digital Item. More complex examples can be found in
ISO/IEC 21000-2.
This example uses ISO/IEC 21000 to create an "MPEG-21 music album" comprising a series of resources:
— Three audio files (coded in MPEG-2 AAC, as specified in ISO/IEC 13818-3) representing the "tracks" that
form the basis of the album;
— Two text files (in Unicode, as specified in ISO/IEC 10646) representing the lyrics to two of the tracks;
— Two images (in JPEG, as specified in the ISO/IEC 10918 series) representing the cover photograph and
other artwork of the album;
[24]
— A text file (in HTML, as specified in W3C, HTML 4.0 Specification ) representing the introductory text
for the album.
© ISO/IEC 2025 – All rights reserved
v
The relationship between these resources and how they relate to the Digital Item itself is expressed in
ISO/IEC 21000-2 (DID). The DID contains, besides the references to the resources, information about the
item and/or parts thereof. These metadata elements are associated through DID mechanisms to the item/
resources as shown in Figure 1.
Figure 1 — MPEG-21 music album
0.5  Relationship between Digital Item Declaration and Digital Item Identification
Identifiers covered by this document can be associated with Digital Items, containers, components, and/
or fragments thereof by including them in a specific place in the Digital Item Declaration. This place is the
Statement element. Examples of likely Statements include descriptive, control, revision tracking and/or
identifying information.
Figure 2 shows this relationship. The shaded boxes are subject of this document while the bold boxes are
defined in ISO/IEC 21000-2.
Several elements within a Digital Item Declaration can have zero, one or more Descriptors (as specified in
ISO/IEC 21000-2). Each Descriptor may contain one Statement which can contain one identifier relating
to the parent element of the Statement. In Figure 2, the two statements shown are used to identify a
Component (left hand side of the diagram) and an Item (right hand side of the diagram).

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vi
Figure 2 — Relationship between Digital Item Declaration and Digital Item Identification
Figure 3 gives an example of a DID descriptor containing one identifier. The use of the DII schema (identified
by the DII namespace) is defined in subclause 4.4.
Figure 3 — Example: Uniquely identifying a Digital Item
0.6  Linking identifiers with associated information
Users may link identifiers to related entities (e.g. related metadata, related Digital Items and parts thereof,
etc). One mechanism for achieving this is by using an online resolution service such as the Domain Name
System (DNS) Resolution system (as specified in IETF RFC 1738).

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vii
While some of the identification systems that are used to uniquely identify "content" have the capabilities to
resolve an identifier online to appropriate metadata (e.g. cIDf, DOI), others do not have this capability (e.g.
ISBN, ISRC). The latter identification systems still enable users to link the identifier to appropriate metadata
offline. ISO/IEC 21000-2 does not mandate or specify such linking mechanism. Annex A provides an example
of how such linking can be done online.

© ISO/IEC 2025 – All rights reserved
viii
International Standard ISO/IEC 21000-3:2025(en)
Information technology — Multimedia framework (MPEG-21) —
Part 3:
Digital Item Identification
1 Scope
This document specifies:
— How to uniquely identify Digital Items (and parts thereof);
— How to uniquely identify IP related to the Digital Items (and parts thereof), for example abstractions;
— How to express the relationship between the two above identifiers;
— How to deal with varying levels of functional granularity for Digital Item identifiers;
— How to uniquely identify description schemes;
— The relationship between Digital Items (and parts thereof) and existing identification systems. Annex C
contains a list of relevant identification systems. This is not an exhaustive list and is subject to change
over time;
— How to express the relationship between two Digital Items.
This document does not specify:
— New identification systems for the content elements for which identification and description schemes
already exist and are in use (e.g. this document does not attempt to replace the ISRC, as defined in
ISO 3901, for sound recordings);
— Normative description schemes for describing content.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO/IEC 21000-2, Information technology — Multimedia framework (MPEG-21) — Part 2: Digital Item
Declaration
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/

© ISO/IEC 2025 – All rights reserved
3.1.1
component
binding a resource to a set of descriptors
Note 1 to entry: These descriptors are information related to all or part of the specific resource instance. Such
descriptors will typically contain control or structural information about the resource (such as bit rate, character set,
start points or encryption information) but not information describing the “content” within. A component itself is not
an item; components are building blocks of items.
3.1.2
descriptor
associating information with the enclosing element
Note 1 to entry: This information may be a component (such as a thumbnail of an image, or a text component), or a
textual statement.
3.1.3
DI
digital item
structured digital objects, including a standard representation, identification and metadata
Note 1 to entry: This entity is the fundamental unit of distribution and transaction within the MPEG-21 framework as
a whole.
3.1.4
electronic media
representing a digital media resource in a given media format for electronic distribution
3.1.5
entity
anything that can be uniquely identified regardless of its nature, type or granularity (e.g. digital resources,
individuals and organisations, transactions, etc)
3.1.6
functional granularity
uniquely identified Digital Item
Note 1 to entry: The functional granularity principle states that the identification of a Digital Item can be different for
different users whenever it needs to be distinguished from another Digital Item.
3.1.7
identification scheme
associating identifiers with entities (both as defined herein), e.g. ISRC, ISBN, etc
3.1.8
identifier
unique label allocated to an entity within a given namespace
Note 1 to entry: A label associated with a specific entity, e.g. a string "ISRC GB-XYZ-01-00001" whose function is to
distinguish one entity from another.
3.1.9
inter-DI relationship
logical connection or association between two Digital Items, pertaining to the semantics of the role that one
DI plays towards another DI
3.1.10
item
grouping of sub-items and/or components that are bound to relevant descriptors
Note 1 to entry: The term item is a technical term, and is, as such, a narrower term than Digital Item.

© ISO/IEC 2025 – All rights reserved
3.1.11
media resource
content directly capable of digital representation
3.1.12
physical media
representing either an analogue or a digital media resource in a given media format for physical distribution.
3.1.13
relator
describes the relationship between two entities
3.1.14
resolution system
act of submitting an identifier to a network service and receiving in return one or more pieces of some
information (which includes resources, descriptions, another identifier, Digital Item, etc.) related to the
identifier
3.1.15
resource
individually identifiable asset such as a video or audio clip, an image, or a textual asset
Note 1 to entry: A resource may also potentially be a physical object. All resources shall be locatable via an
unambiguous address.
3.1.16
statement
literal textual value that contains information, but not an asset
Note 1 to entry: Examples of likely statements include descriptive, control, revision tracking or identifying information
(such as an identifier as described in this document).
3.2 Abbreviated terms
cIDf Content ID forum
CIS Common information system
CISAC International confederation of societies of authors and composers
(confédération internationale des sociétés d'auteurs et compositeurs)
DCMI Dublin core metadata initiative
DID Digital item declaration
DII Digital item identification
DOI Digital object identifier
EAN European article number
IEC International electrotechnical commission
ISAN International standard audiovisual number
ISBN International standard book number
ISO International organization for standardization
ISRC International standard recording code

© ISO/IEC 2025 – All rights reserved
ISSN International standard serial number
ISTC International standard textual work code
ISWC International standard musical work code
MPEG Moving picture expert group
SMPTE Society of motion picture and television engineers
UCC Uniform code council
UPC Universal product code
URI Uniform resource identifier
URL Uniform resource locator
URN Uniform resource name
V-ISAN Version identifier for ISAN
4 Identification of digital items and their parts
4.1 Identifier element
Digital Items and their parts within the MPEG-21 multimedia framework are identified by encapsulating
Uniform Resource Identifiers (URIs), as specified in IETF RFC 2396, into the Identifier element.
4.1.1 Syntax
<--! #############################
ISO/IEC 21000-3 DI Identification Element
#############################>

4.1.2 Semantics
This element contains an identifier for a Digital Item, container, component, and/or fragment thereof in the
form of a URI.
A Registration Authority may be set up for maintaining a list of identification schemes to be used within this
document. However, the set up and maintenance of such a Registration Authority is out of the scope of this
document.
Identifiers are not required to be registered with such a Registration Authority to be conformant to
subclause 4.1.
4.1.3 Notes
A Uniform Resource Identifier (URI) is a compact string of characters for identifying an abstract or physical
resource, where a resource is defined as “anything that has identity”.
The requirement that a Digital Item identifier be a URI is also consistent with the statement that the MPEG-
21 identifier may be a Uniform Resource Locator (URL), since the specification of URI is designed to meet
the recommendations laid out in IETF RFC 1738 (URL specification). The term URL refers to a specific subset
of URI that is in use today as pointers to information on the Internet; it allows for long-term to short-term
persistence depending on the business case.

© ISO/IEC 2025 – All rights reserved
4.2 RelatedIdentifier element
While the Identifier element is intended to enable the unique identification of Digital Items (or parts
thereof), the RelatedIdentifier element conveys identification information that is related to the Digital
Item (or parts thereof) which may be required by certain users. This relationship is expressed through the
use of relators as defined in ISO/IEC 21000-6.
One example of this is the identification of an abstraction of the work (e.g. composition used when creating
a sound recording).
4.2.1 Syntax









4.2.2 Semantics
The RelatedIdentifier element allows associating identifiers that are related to the Digital Item, container,
component, and/or fragment thereof but do not directly identify the Digital Item (or part thereof). The value
shall be in the form of a URI.
A Registration Authority may be set up for maintaining a list of identification schemes to be used within this
document. However, the set up and maintenance of such a Registration Authority is out of the scope of this
document.
Values of the RelatedIdentifier element are not required to be registered with such a Registration
Authority to be conformant to subclause 4.2.
The RelatedIdentifier element shall not be used for identifying the Digital Item (or part thereof) itself.
This shall be done using the Identifier element.
The relationshipType attribute enables expressing the relationship of the resource identified by a
RelatedIdentifier to the Digital Item (or part thereof) bearing the RelatedIdentifier. The value shall be
in the form of a URI. A relator shall be identified with a URI of the form urn:mpeg:mpeg21:2002:01-RDD-
NS:x, as specified in ISO/IEC 21000-6.
4.2.3 Example
See Figure 8 for a specific example for the RelatedIdentifier element.
4.3 Relationships element
The Relationships element is intended to enable the description of one, or more, relationships between
properly identified Digital Items. Each such relationship logically connects two DIs (referring them through
their DII identifiers), to each other through a specific, directional logical connection of the subject-predicate-
object type. One of the DIs plays the role of the subject, the other DI plays the role of the object and the
relationship itself plays the role of the logical predicate that relates subject to object.
One example of such a relationship is that which exists between a “Book DI” and its corresponding “Errata
DI”. The “Errata DI” is the subject in a “correction” relationship (or predicate) where the “Book DI” is the object.

© ISO/IEC 2025 – All rights reserved
4.3.1 Syntax



xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
ref="rdf:RDF" minOccurs="1" maxOccurs="1"/>



A Relationship element has to appear as a child of a Statement element (defined in ISO/IEC 21000-2).
4.3.2 Semantics
The Relationships element allows the semantically precise definition of any number of inter-DI relationships.
Together, these relationships form the relational context of the Digital Item that this information is
associated with.
The content of the Relationships element is RDF/OWL metadata, which performs the actual description of
inter-DI relationships.
To be in accordance with this clause, this metadata shall conform to a universal (MPEG-21 wide) ontology,
termed, MPEG-21 Core Ontology for DI Relationships (MPEG-21 CODIR), defined in 4.3.3.
4.3.3 MPEG-21 core ontology for DI relationships







]>
xml:base="mpeg21:corerelationalontology"
xmlns:owl2xml="https://www.w3.org/2006/12/owl2-xml#"
xmlns:mpeg21coreRelOnt="mpeg21:corerelationalontology"
xmlns:xsd="https://www.w3.org/2001/XMLSchema#"
xmlns:rdfs="https://www.w3.org/2000/01/rdf-schema#"
xmlns:rdf="https://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:owl="https://www.w3.org/2002/07/owl#">







© ISO/IEC 2025 – All rights reserved




The MPEG-21 Core Ontology for DI Relationships (textually presented above and graphically depicted in
Figure 4) only defines the fundamental taxonomy of possible inter-DI relationships. This may be extended
by other domain-specific ontologies, which will add the taxonomic definition for their domain specific inter-
DI relationships.
Figure 4 — MPEG-21 CODIR’s graphical depiction
4.4 Type element
4.4.1 Syntax
<--! ################################
ISO/IEC 21000-3 Type Element
################################>

The Type element shall only appear as a child element of a Statement that shall appear as a child element of a
Descriptor that shall appear as a child element of an Item.
4.4.2 Semantics
The Type element will allow identifying special types of Digital Items. Such types can be defined, for example,
by different parts of ISO/IEC 21000 or ISO/IEC 23000.
4.4.3 Example
See Figure 9 for a specific example for the Type element.
4.5 Digital Item identification XML schema definition
This clause defines the XML schema for this document.






xmlns="urn:mpeg:mpeg21:2002:01-DII-NS"
targetNamespace="urn:mpeg:mpeg21:2002:01-DII-NS"
version="0.02">
© ISO/IEC 2025 – All rights reserved
















xmlns:rdf="https://www.w3.org/1999/02/22-rdf-syntax-ns#"
ref="rdf:RDF" minOccurs="1" maxOccurs="1"/>






This document defines its own namespace urn:mpeg:mpeg21:2002:01-DII-NS. The “01” represents a serial
number that is expected to change as the schema evolves along with the rest of ISO/IEC 21000. The "2002"
in the namespace identifier is not expected to change when subsequent versions of the ISO/IEC 21000-2
schema are developed.
4.6 Examples
The following examples show how the Identifier, RelatedIdentifier and Type elements are used to
identify Digital Items and parts thereof, how related identifiers are associated with Digital Items and how
special Digital Item types are identified.

© ISO/IEC 2025 – All rights reserved
The following examples also illustrate how such identifiers are used to reference resources.
4.6.1 Example: Identification of a music release
This example shows how the Identifier element is used to uniquely identify a Digital Item containing a
music release. The identifier uses stems from a proprietary identification scheme using the namespace myID.
The myID is not a registered URN namespace.
Figure 5 — Example: Identification of a music release
4.6.2 Example: Referring to Digital Item in external DID document
Digital Item identifiers can be used to refer to Digital Item, container, component, fragment thereof, or DID
documents that are externally defined. This is an example to show how Digital Item identifiers can be used
for these purposes. In this example, it is assumed that a Registration Authority for Digital Item Identification
Systems uses the form urn:mpegRA:mpeg21:dii:sss:nnn, where the string sss denotes the identifier for an
identification system and nnn denotes a unique identifier within that identification system. The namespace
identifier "urn:mpegRA" is only used as an example.

© ISO/IEC 2025 – All rights reserved
Figure 6 — Example: Referring to Digital Item in external DID document

© ISO/IEC 2025 – All rights reserved
4.6.3 Example: Referring to resources stored on external server
Digital Item identifiers can also be used to refer to media resources stored on external servers. In this case,
the association of identifier with media resources are not explicitly declared in DID as in Figure 6 but made
by external identifier providers. Identifiers in this example work just like URLs. The same assumption is
used in Figure 6 for the URN form.
Figure 7 — Example: Referring to resources stored on external server
4.6.4 Example: Identifying a sound recording and the related underlying musical work
This example shows how a resource is uniquely identified (an MPEG Audio Layer III-coded sound recording)
within a Digital Item using an International Standard Recording Code (ISRC).
This example also highlights how a related identifier is associated (i.e., identifying the underlying music
work with an International Standard Work Code - ISWC) with such a resource.
Figure 8 — Example: Identifying a sound recording and the related underlying musical work
4.6.5 Example: Identifying a Digital Item type
This example shows how a special Digital Item type “Foo Digital Item” is identified, which is assumed for
this example to have been specified in one of the parts of ISO/IEC 21000.

© ISO/IEC 2025 – All rights reserved
A terminal receiving this Digital Item will therefore be able to detect that it is a “Foo Digital Item” and act
accordingly.
Figure 9 — Example: Identifying a Digital Item type
5 Identification of description schemes
[22]
The XML schema definition specifies how to use XML-based description schemes within Digital Items by
identifying them through their XML namespace identifier.
ISO/IEC 21000-2 allows to include non-XML-based descriptors into Digital Items by identifying the
description scheme through a unique namespace identifier (i.e. a URI).
Example 3: A digital music album (ISO/IEC 21000-2:2005, subclause 9.3) gives an example of how to identify
description schemes and to include metadata from such identification schemes into a Digital Item.

© ISO/IEC 2025 – All rights reserved
Annex A
(informative)
Interoperability considerations on resolution systems
A.1 General
This document mandates that any identification scheme shall be a URI. The vision for ISO/IEC 21000,
however, is to define a multimedia framework to enable transparent and augmented use of multimedia
resources across a wide range of networks and devices used by different communities, not just to describe
Digital Items superficially. In that sense, it is meaningful to show how existing ID schemes, as actually used
in different communities, can be used as URN-based identifiers in accordance with ISO/IEC 21000-2. In
order to provide a practical view, a real implementation example with online resolution is provided in this
informative annex.
A.2 Necessary functions and resolution system switcher
Figure A.1 shows the relation between DID documents, media resources, and identifiers. Two DIDL elements:
Reference which refers to external elements such as Container, Item, Component, Descriptor, Anchor,
or Annotation, and Resource, which specifies an external media resource of a Component, are carried
within URI identifiers (e.g. URL or URN) in DID documents. When a Resource or Reference element
is selected by a user with the DID browser, the referred element or the specified media resource can be
obtained through the network if an online resolution service is available.
If the identifier is a URN, the URN is first converted into the corresponding locator such as a URL, and
then the referred element or specified media resource is obtained. The problem is, however, that there are
[19]
several different kinds of ID resolution methods such as http, handle system, and Z39.50. Accordingly, the
following two actions are necessary to handle URN-based IDs in DID documents.
— The ID scheme managing the identifier is identified from the URN string (identifier).
— The relation of the identifier to the URL, individually managed in the specific ID scheme, is resolved and
the URL is acquired through the network.

© ISO/IEC 2025 – All rights reserved
Figure A.1 — Relation of DID documents, media resources, and identifiers
An implementation example, called Resolution System Switcher (RSS), for these two functions is considered.
RSS proceeds in the following way as depicted in Figure A.2: Reference or Reference element is selected
in a DID document by user with DID browser.
The URN string, which refers to an external DIDL element in Reference or specifies an external media
resource in Reference, is sent to RSS:
— The ID scheme is identified by URN parsing;
— The corresponding online ID resolution method is selected from a repository;
— URL is determined using the selected online ID resolution method for the specified ID scheme;
— The URL string is returned to the DID browser;
— The referred external DIDL element or the specified external media resource is acquired by using the
URL and displayed.
© ISO/IEC 2025 – All rights reserved
Figure A.2 — An implementation example: Resolution system switcher (RSS)
Two implementation models are possible for RSS. The first one is the Remote RSS model (the client-server
model), where most functions are offered by a common server. This allows different kinds of terminal to
share it (terminal architecture independent), and also makes it easier to adopt new ID schemes because
no updating on the user side is needed. The other implementation model is the Local RSS model (plug-in
model), where all functions are realized on the terminal side. This makes implementation simpler, and in
addition processing overheads are lighter than is true with the Remote RSS model.
A.3 An overall example of a DID browsing system with resolution system switcher
An example of an overall DID browsing system that includes RSS is shown in Figure A.3. The system consists
of four parts:
— DIDBS (DID Browsing Server) is used to convert DID documents stored in the server into HTML
equivalents (called “visualization”) so that ordinary HTML browsers such as Internet Explorer and
Netscape Navigator can display them on the terminal's screen, and to extract URN strings from DID
documents. DIDBS also mounts RSS (Remote RSS model) that sends a resolve request with the URN to
the selected online ID resolution system.
— The online ID resolution system translates the ID into its corresponding URL. It is assumed that each ID
scheme has its own online resolution system based on its own communication protocols. The example
uses two different URN-based ID schemes: “Content ID” provided by cIDf and “DOI” provided by IDF, both
of which are listed in Annex B.
— Content Providers are WWW servers that store DID documents and/or media resources.
— User terminal is an ordinary personal computer for the end-user. It is connected to the Internet and has
a Web browser to access the DIDBS.

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Figure A.3 — Overall configuration of DID browsing system w
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