Semantic Definition Format (SDF) modeling for Digital Twin
draft-lee-asdf-digital-twin-08
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| Document | Type | Active Internet-Draft (asdf WG) | |
|---|---|---|---|
| Authors | Hyunjeong Lee , Jungha Hong , Joo-Sang Youn , Yong-Geun Hong | ||
| Last updated | 2025-06-14 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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draft-lee-asdf-digital-twin-08
ASDF H. Lee, Ed.
Internet-Draft J. Hong
Intended status: Standards Track ETRI
Expires: 16 December 2025 J-S. Youn
Dong-eui Univ
Y-G. Hong
Daejeon University
14 June 2025
Semantic Definition Format (SDF) modeling for Digital Twin
draft-lee-asdf-digital-twin-08
Abstract
This memo specifies SDF modeling for a digital twin, i.e. a digital
twin system, and its Things. An SDF is a format that is used to
create and maintain data and interaction, and to represent the
various kinds of data that is exchanged for these interactions. The
SDF format can be used to model the characteristics, behavior and
interactions of Things, i.e. physical objects, in a digital twin that
contain Things as components.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 16 December 2025.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. SDF structure for digital twin . . . . . . . . . . . . . . . 3
4. Motivation and design rationale . . . . . . . . . . . . . . . 4
4.1. Introduction of sdfNonAffordance . . . . . . . . . . . . 5
4.2. Digital Twin-Centric Modeling within sdfThing . . . . . . 5
4.3. Example modeling . . . . . . . . . . . . . . . . . . . . 6
5. Requirements for digital twin . . . . . . . . . . . . . . . . 8
5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Data acquisition . . . . . . . . . . . . . . . . . . . . 8
5.3. Data analysis . . . . . . . . . . . . . . . . . . . . . . 8
5.4. Identification . . . . . . . . . . . . . . . . . . . . . 9
5.5. Accuracy . . . . . . . . . . . . . . . . . . . . . . . . 9
5.6. Synchronization . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
A digital twin is defined as a digital representation of an object of
interest and may require different capabilities, for example,
synchronization and real-time support, according to the specific
domain of application. [Y.4600]. Digital twin help organizations
improve important functional objectives, including real-time control,
off-line analytics, and predictive maintenance, by modeling and
simulating objects in the real world. Therefore, it is important for
a digital twin to represent as much real-world information about the
object as possible when digitally representing the object.
Nowadays, digital twin technologies are applied in various domains
including manufacturing, energy, medical, farm, transportation, etc.
And a common format is needed to represent the objects in the domains
as digital twins. SDF [I-D.ietf-asdf-sdf] can be used for modeling
objects as digital twins.
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This document specifies the modeling and guidance on how to use SDF
to represent objects as digital twins.
2. Terminology
This specification uses the terminology specified in
[I-D.ietf-asdf-sdf] in particular "Class Name Keyword", "Object", and
"Affordance".
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. SDF structure for digital twin
This section describes SDF structure with the new Class Name Keyword,
sdfNonAffordance, to represent a thing or an object as a digital
twin. The architecture of a digital twin based on the SDF model is
illustrated in Figure 1, , following the guidelines of [ISO23247-3].
The Physical Layer comprises affordance and non-affordance objects.
From the real-world objects, only those deemed relevant are selected
for representation as digital twins. The Digital Twin Layer is
structured into three sublayers: the Device Communication Sublayer,
the Digital Twin Sublayer, and the Application Sublayer. The Device
Communication Sublayer is responsible for monitoring and collecting
data from both affordance and non-affordance objects. This sublayer
provides the necessary data to synchronize the physical objects with
their digital twin counterparts. The Digital Twin Sublayer ensures
synchronization between the affordance and non-affordance objects and
their respective digital twins using the data provided by the Device
Communication Sublayer. The Application Sublayer presents the
synchronized values of the digital twins to users, facilitating
informed decision-making.
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+---------------------------------------------+ - - - - - - - - - - -
| Application Sublayer |
| +----------+ +------+ +--------+ +--------+ |
| | Human | | HMI | | Apps | | Peers | |
| +----------+ +------+ +--------+ +--------+ |
+---------------------------------------------+
| Digital Twin Sublayer |
| +----------+ +-------------+ +------------+ |
| | Operation| | Application | | Resource | |
| | and | | and | | access and | |
| |management| | service | |interchange | |
| +----------+ +-------------+ +------------+ |
| +-----------------------------------------+ | Digital twin Layer
| | Digital representation of objects | |
| | +-------------+ +----------------+ | |
| | | Affordance | | NonAffordance | | |
| | | objects | | objects | | |
| | +-------------+ +----------------+ | |
| +-----------------------------------------+ |
+---------------------------------------------+
| Device Communication Sublayer |
| +-------------+ +----------------+ |
| | Data | | Object | |
| | collection | | control | |
| +-------------+ +----------------+ |
+---------------------------------------------+ - - - - - - - - - - -
| +-------------+ +----------------+ |
| | Affordance | | sdfNonAffordance | |
| | objects | | objects | | Physical Layer
| +-------------+ +----------------+ |
+---------------------------------------------+ - - - - - - - - - - -
Figure 1: Basic Architecture of digital twin
4. Motivation and design rationale
The document is based on the underlying structure defined in
[I-D.ietf-asdf-sdf], which which standardizes the semantic definition
format (SDF) for representing IoT affordance. This specification
provides a strong basis for representing individual devices and their
features (sdfProperty, sdfAction, sdfEvent, etc.), but additional
mechanisms are needed to address the unique requirements of digital
twin modeling.
Digital twin systems defined in [ISO23247-3] often have to describe
virtual representations of various physical assets, including
metadata, identity, contextual relationships, historical data, as
well as device interfaces.
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4.1. Introduction of sdfNonAffordance
A new SDF keyword sdfNonAffordance described in
[I-D.draft-hong-asdf-sdf-nonaffordance] is introduced to represent
non-functional or metadata elements that describe a device or
component without implying direct interaction:
* Identifier (e.g., UUID, URN)
* Location (e.g. site, zone, GPS tag)
* Owner (e.g., representative, ,anufacturer)
These field can appear in both sdfObject and sdfThing contexts and
follow the same structural pattern as sdfData and is designed for
scalability.
4.2. Digital Twin-Centric Modeling within sdfThing
To support hierarchical representations (e.g., a boat composed of
heater, GPS, and battery subsystems), this document encourages use of
sdfThing to aggregate related sdfObject components, along with
metadata.
The mapping of digital twin attributes to sdf qualities are shown in
Table 1.
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+================+==================+==============================+
| Attribute | Recommended | Description |
| | Mapping | |
+================+==================+==============================+
| Identifier | sdfNonAffordance | Globally unique digital twin |
| | | ID (e.g., URN) |
+----------------+------------------+------------------------------+
| Characteristic | sdfProperty or | General description or |
| | sdfData | domain properties |
+----------------+------------------+------------------------------+
| Schedule | sdfEvent or | Time-based actions, |
| | sdfData | availability, or maintenance |
+----------------+------------------+------------------------------+
| Status | sdfAction or | Actual or calculated |
| | sdfProperty | operating conditions |
+----------------+------------------+------------------------------+
| Location | sdfNonAffordance | Physical or logical location |
| | | information |
+----------------+------------------+------------------------------+
| Report | sdfData | Measurement summaries, |
| | | analytics, or logs |
+----------------+------------------+------------------------------+
| owner | sdfNonAffordance | Organization or entity |
| | | responsible for the digital |
| | | twin |
+----------------+------------------+------------------------------+
| Relationship | [TBD] | Inter-object/inter-twin |
| | | relationships |
+----------------+------------------+------------------------------+
Table 1: Digital twin modeling within sdfThing
4.3. Example modeling
The example of boat007 Figure 2illustrates how a Digital Twin
representation can be constructed for a heater component (heater1)
installed on a specific vessel (boat007). The Digital Twin is
modeled using the sdfThing structure, which references the heater
object defined in the sdfObject section.
{
"info": {
"title": "An example of the heater #1 in the boat #007",
"version": "2025-01-27",
"copyright": "Copyright 2025. All rights reserved.",
},
"namespace": {
"heater1": "https://example.com/boat007/heater1"
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},
"defaultNamespace": "heater1",
"sdfThing": {
"boat007": {
"label": "Digital Twin of Boat #007",
"description" : "A ship equipped with heating and navigation systems",
"sdfRequired": {
"heater1": "#/sdfObject/heater"
},
"sdfNonAffordance": {
"identifier": {
"type": "string",
"const": "urn:boat:007:heater:1"
},
location:{
"wgs84": {
"latitude": "35.2988233791372",
"longitude": "129.25478376484913",
"altitude": "0.0"
},
"postal": {
"city": "Ulsan",
"post-code": "44110",
"country": "South Korea"
},
"w3w": {
"what3words":"toggle.mopped.garages"
}
},
"owner": {
"type": "string",
"default": "ExamTech Ltd."
}
},
"sdfObject": {
"heater1": {
"label": "Cabin Heater",
"description": "Temperature control system for cabin heating",
"sdfProperty": {
"characteristic": {
"description": "Technical summary of the heater",
"type": "string",
"default": "12V electric heater, 800W, automatic cutoff"
},
"status": {
"description": "Current operational status",
"type": "string",
"enum": ["on", "off", "error"],
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"default": "off"
}
},
"sdfEvent": {
"maintenanceSchedule": {
"description": "Next scheduled maintenance date",
"type": "string",
"format": "date-time"
}
}
"sdfData" : {
report: {
"On February 24, 2025, the boat #007's heater #1 was on from 9 a.m. to 6 p.m."
}
}
}
}
}
}
}
Figure 2: An example of SDF mapping for digital twin
5. Requirements for digital twin
5.1. Overview
A digital twin is a partial representation of sdfThing or sdfObject
that contains attributes such as sdfProperty, sdfAction and
sdfEvent[ISO23247-1]. By representing sdfThing as a digital twin,
crucial events that require appropriate action can be quickly
detected and controlled. The requirements defined in [ISO23247-1]
are applied to represent sdfThings and sdfObjects as digital twins.
5.2. Data acquisition
Data related to sdfThing and sdfObject, such as sdfProperty,
sdfEvent, and sdfAction, should be collected from IP and non-IP
devices.
5.3. Data analysis
The collected data needs to be analyzed to understand the state of
sdfThing and sdfObject.
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5.4. Identification
The sdfThings and sdfObjects should contain data that uniquely
identifies them as digital twins.
5.5. Accuracy
The sdfThings and sdfObjects should be represented as digital twins
with appropriate levels of detail and accuracy, depending on the
application.
5.6. Synchronization
The sdfThings and sdfObjects should be synchronized with their
digital twins in real-time as appropriate for the application. Newly
added or removed sdfThings and sdfObjects should be recognized and
reflected in the digital twin.
6. Security Considerations
Only authorized users should have the authority to manage sdfThings
and sdfObjects.
7. IANA Considerations
This document has no IANA actions.
8. References
8.1. Normative References
[I-D.draft-hong-asdf-sdf-nonaffordance]
Hong, J. and H. Lee, "Semantic Definition Format (SDF)
Extension for Non-Affordance Information", Work in
Progress, Internet-Draft, I-D.draft-hong-asdf-sdf-
nonaffordance-00, 8 April 2025,
<https://datatracker.ietf.org/doc/html/I-D.draft-hong-
asdf-sdf-nonaffordance-00>.
[I-D.ietf-asdf-sdf]
Koster, M., Bormann, C., and A. Keränen, "Semantic
Definition Format (SDF) for Data and Interactions of
Things", Work in Progress, Internet-Draft, draft-ietf-
asdf-sdf-23, 17 March 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-asdf-
sdf-23>.
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[I-D.ietf-laari-asdf-relations]
Laari, P., "Extended relation information for Semantic
Definition Format (SDF)", Work in Progress, Internet-
Draft, I-D.ietf-laari-asdf-relations-04, 28 January 2025,
<https://datatracker.ietf.org/doc/html/I-D.ietf-laari-
asdf-relations-04>.
[ISO23247-1]
"Automation systems and integration Digital twin framework
for manufacturing - Part 1: Overview and general
principles, ISO 23247-1.", October 2021,
<https://www.iso.org/standard/75066.html>.
[ISO23247-3]
"Automation systems and integration Digital twin framework
for manufacturing - Part 3: Digital representation of
manufacturing elements, ISO 23247-3.", October 2021,
<https://www.iso.org/standard/78744.html>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[Y.4600] Union, I. T., ""Recommendation ITU-T Y.4600 (2022),
Requirements and capabilities of a digital twin system for
smart cities.", August 2022.
8.2. Informative References
[saref4bldg]
Poveda-Villaln, M. and R. Garcia-Castro, "SAREF extension
for building", 5 June 2020,
<https://saref.etsi.org/saref4bldg>.
Authors' Addresses
Hyunjeong Lee (editor)
Electronics and Telecommunications Research Institute
218 Gajeong-ro, Yuseong-gu
Daejeon
34129
South Korea
Phone: +82 42 860 1213
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Email: hjlee294@etri.re.kr
Jungha Hong
Electronics and Telecommunications Research Institute
218 Gajeong-ro, Yuseong-gu
Daejeon
34129
South Korea
Phone: +82 42 860 0926
Email: jhong@etri.re.kr
Joo-Sang Youn
DONG-EUI University
176 Eomgwangno Busan_jin_gu
Busan
47340
South Korea
Phone: +82 51 890 1993
Email: joosang.youn@gmail.com
Yong-Geun Hong
Daejeon University
62 Daehak-ro, Dong-gu
Daejeon
34520
South Korea
Phone: +82 42 280 4841
Email: yonggeun.hong@gmail.com
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