Semantic Definition Format (SDF) modeling for Digital Twin
draft-ietf-asdf-digital-twin-00

ASDF                                                         H. Lee, Ed.
Internet-Draft                                                   J. Hong
Intended status: Informational                                      ETRI
Expires: 19 February 2026                                 18 August 2025

       Semantic Definition Format (SDF) modeling for Digital Twin
                    draft-ietf-asdf-digital-twin-00

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

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Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Boat modeling . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Relationship modeling . . . . . . . . . . . . . . . . . .   7
   6.  Requirements for digital twin . . . . . . . . . . . . . . . .   9
   7.  Procedure for digital twin implementation . . . . . . . . . .  10
     7.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.2.  Identifying and scoping physical assets . . . . . . . . .  11
     7.3.  Defining a digital twin . . . . . . . . . . . . . . . . .  11
     7.4.  Metadata and contextualization  . . . . . . . . . . . . .  11
     7.5.  Binding Interfaces and Communications . . . . . . . . . .  11
     7.6.  Verification and compliance . . . . . . . . . . . . . . .  11
     7.7.  Deployment and registration . . . . . . . . . . . . . . .  11
     7.8.  Runtime monitoring and updating . . . . . . . . . . . . .  12
     7.9.  Lifecycle and governance management . . . . . . . . . . .  12
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     10.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

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

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   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.

   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   | sdfRelation      | Inter-object/inter-twin      |
   |                |                  | relationships                |
   +----------------+------------------+------------------------------+

              Table 1: Digital twin modeling within sdfThing

5.  Examples

5.1.  Boat 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.

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                 {
                   "sdfThing": {
                     "boat007": {
                       "label": "Digital Twin of Boat #007",
                       "sdfRequired": {
                         "heater1": "#/sdfObject/heater"
                       },
                       "sdfNonAffordance": {
                         "identifier": { "type": "string", "const": "urn:boat:007:heater:1" },
                         "location": { "type": "string", "default": "Deck A, Port Side" },
                         "owner": { "type": "string", "default": "OceanTech Ltd." }
                       }
                     }
                   },
                   "sdfObject": {
                     "heater": {
                       "label": "Cabin Heater",
                       "sdfProperty": {
                         "status": { "type": "string", "enum": ["on", "off", "error"], "default": "off" },
                         "characteristic": { "type": "string", "default": "12V electric heater, 800W" }
                       },
                       "sdfEvent": {
                         "maintenanceSchedule": { "type": "string", "format": "date-time" }
                       },
                       "nipcProtocol": "zigbee"
                     }
                   },
                   "contextSnapshot": {
                     "thingId": "boat-007",
                     "timestamp": "2025-06-20T09:00:00Z",
                     "context": {
                       "location": { "lat": "35.2988", "lon": "129.2547", "alt": "0.0" },
                       "installationInfo": { "floor": 1, "mountType": "wall" }
                     }
                   }
                 }

            Figure 2: An example of SDF mapping for digital twin

5.2.  Relationship modeling

   To enable advanced modeling of inter-object and inter-twin
   relationships, this document adopts the sdfRelation extension as
   defined in [I-D.draft-laari-asdf-relations].  The sdfRelation keyword
   allows describing complex relationships beyond just the parent-child
   hierarchy.  These relationships can include:

   *  Physical relations (e.g., "inside", "next to")

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   *  Functional relations (e.g., "controls", "is controlled by")

   *  Semantic relations (e.g., "similar to", "same as")

   The sdfRelation definition can include the following fields as
   defined in [I-D.draft-laari-asdf-relations]:

   *  relType: Specifies the type of relationship that can an external
      ontologies (e.g., SAREF[saref4bldg]) can refer to.

   *  target: Points to the SDF object or an external ontology term that
      is the target of the relationship.

   *  description: Provides a detailed textual explanation of the
      relationship.

   *  label: A short human-readable label for the relationship.

   *  property: Additional properties describing the relationship
      context.

   *  $comment: Optional properties including implementers notes.

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                     {
                       "namespace": {
                         "saref": "https://saref.etsi.org/saref4bldg/v1.1.2/"
                       },
                       "sdfObject": {
                         "lightbulb": {
                           "description": "A smart lightbulb",
                           "sdfProperty": {
                             "adjacent-node": {
                               "type": "object",
                               "sdfType": "link"
                             }
                           },
                           "sdfRelation": {
                             "sameRoomAsThermostat": {
                               "relType": "saref:isLocatedIn",
                               "target": "#/sdfObject/thermostat",
                               "description": "This lightbulb is located in the same room as the thermostat.",
                               "label": "Located together"
                             }
                           }
                         },
                         "thermostat": {
                           "description": "A thermostat in the same room",
                           "sdfProperty": {
                             "adjacent-node": {
                               "type": "object",
                               "sdfType": "link"
                             }
                           }
                         }
                       }
                     }

                    Figure 3: An example of sdfRelation

6.  Requirements for digital twin

   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.

   *  Identification: sdfThings and sdfObjects should contain data that
      uniquely identify them as digital twins.

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   *  Data acquisition: data related to sdfThing and sdfObject, such as
      sdfProperty, sdfEvent, and sdfAction, should be collected from IP
      and non-IP devices.

   *  Data analysis: collected data needs to be analyzed to understand
      the state of sdfThing and sdfObject.

   *  Accuracy: The sdfThings and sdfObjects should be represented as
      digital twins with appropriate levels of detail and accuracy,
      depending on the application.

   *  Synchronization: sdfThings and sdfObjects should be synchronized
      with the digital twin at intervals appropriate to the requirements
      of each application.  Newly added or deleted sdfThings and
      sdfObjects should be recognized and reflected in the digital twin.

7.  Procedure for digital twin implementation

7.1.  Overview

   It is essential to define a standardized implementation procedure to
   ensure interoperability, scalability, and effective lifecycle
   management across digital twin systems.  This section outlines a
   step-by-step approach aligned with the Semantic Definition Format
   (SDF) model and its architecture, enabling consistent modeling,
   integration, and operation of digital twins in IoT environments.  A
   recommended procedure for representing an sdfThing as a digital twin
   within a specific domain is outlined as follows:

   *  defining a purpose for expressing the observable object, as known
      as a physical asset or an object of interest, as a digital twin in
      the domain

   *  organizing data based on the roles of the observable object in the
      domain

   *  configuring the observable object into the digital twin based on
      the data for the purposes

   *  interworking with a digital twin of each of other domains in which
      the observable object performs a different role

   *  synchronizing the observable object and the digital twin

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7.2.  Identifying and scoping physical assets

   The first step is to clearly identify the physical assets that will
   be represented as digital twins.  This step includes assigning a
   globally unique identifier, such as a URN or UUID, and determining
   the extent of modeling.  It also involves deciding whether the unique
   identifier will cover the entire system or focus on a specific
   subsystem or component.  Although all assets in space can be
   represented by digital twins, it is cost-effective to select assets
   for implementation purposes and configure them as digital twins.

7.3.  Defining a digital twin

   A detailed digital twin should be defined using SDF structures,
   including sdfThing and sdfObject.  This step requires specifying
   affordances such as sdfProperty, sdfAction, and sdfEvent, as well as
   non-affordance metadata like location, owner, and other descriptive
   elements through sdfNonAffordance.

7.4.  Metadata and contextualization

   This step adds metadata that enriches the context of the digital
   twin, such as geographic location, ownership details, manufacturing
   information, and feature summaries.  It can also support advanced
   analytics and management, including contextual attributes such as
   production schedules or maintenance periods.

7.5.  Binding Interfaces and Communications

   Digital twins are bound to real-world communication interfaces and
   protocols such as MQTT, CoAP, and HTTP.  This allows affordance of
   SDF models to interact with real-world data sources, APIs, and
   physical assets in a smooth and reliable manner.

7.6.  Verification and compliance

   Once an asset is defined and bound as a digital twin, it should be
   validated against syntax and semantic rules using tools such as JSON
   schema validators or CDDL definitions.  Compliance with specific SDF
   profiles or domain-specific standards must also be verified to ensure
   interoperability.

7.7.  Deployment and registration

   After verification, the digital twins are deployed in a digital twin
   registry, edge system, or cloud infrastructure.  This step involves
   registering the model with the discovery service for integration and
   use by other systems or stakeholders.

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7.8.  Runtime monitoring and updating

   During operations, digital twins need to continuously monitor real
   data and update their status accordingly.  Properties updates, event
   processing, and partial updates using contextPatch messages should be
   supported for efficient and lightweight synchronization.

7.9.  Lifecycle and governance management

   The life cycle of the digital twin is managed through version
   tracking, audit logs, and compliance documents.  This step ensures
   safe and transparent governance and enables proper disposal and
   deregistration when assets are no longer available.

8.  Security Considerations

   Only authorized users should have the authority to manage digital
   twins, sdfThings and sdfObjects.  Also, Secure communication and
   metadata integrity are essential when implementing digital twins.
   All context messages, including contextPatch and identityManifest,
   must have mechanisms such as authentication and authorization
   applied.

9.  IANA Considerations

   This document has no IANA actions.

10.  References

10.1.  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-02, 8 April 2025,
              <https://datatracker.ietf.org/doc/html/I-D.draft-hong-
              asdf-sdf-nonaffordance-02>.

   [I-D.draft-laari-asdf-relations]
              Laari, P., "Extended relation information for Semantic
              Definition Format (SDF)", Work in Progress, Internet-
              Draft, I-D.draft-laari-asdf-relations-04, 28 January 2025,
              <https://datatracker.ietf.org/doc/html/I-D.draft-laari-
              asdf-relations-04>.

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   [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>.

   [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.

10.2.  Informative References

   [saref4bldg]
              Poveda-Villaln, M. and R. Garcia-Castro, "SAREF extension
              for building", 5 June 2020,
              <https://saref.etsi.org/saref4bldg>.

Acknowledgements

   This specification is based on work by the One Data Model group.

Contributors

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   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

Authors' Addresses

   Hyunjeong Lee (editor)
   Electronics and Telecommunications Research Institute
   218 Gajeong-ro, Yuseong-gu
   Daejeon
   34129
   South Korea
   Phone: +82 42 860 1213
   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

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