



ASDF                                                         H. Lee, Ed.
Internet-Draft                                                   J. Hong
Intended status: Informational                                      ETRI
Expires: 23 October 2026                                   21 April 2026


       Semantic Definition Format (SDF) Modeling for Digital Twin
                    draft-ietf-asdf-digital-twin-04

Abstract

   This memo specifies SDF modeling for digital twins, i.e. a digital
   twin systems, and their 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 digital twins 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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 23 October 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.











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   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.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  SDF structure for digital twin  . . . . . . . . . . . . . . .   3
   4.  Motivation and design rationale . . . . . . . . . . . . . . .   5
     4.1.  Introduction to sdfContext  . . . . . . . . . . . . . . .   5
     4.2.  Digital twin modeling using SDF elements  . . . . . . . .   5
     4.3.  Relationship modeling . . . . . . . . . . . . . . . . . .   6
   5.  Protocol considerations for digital twin realization  . . . .   8
     5.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Supported protocol types  . . . . . . . . . . . . . . . .   9
     5.3.  Protocol binding in SDF . . . . . . . . . . . . . . . . .  10
     5.4.  QoS and Synchronization Semantics . . . . . . . . . . . .  10
     5.5.  Security and access considerations  . . . . . . . . . . .  11
     5.6.  Implementation guidelines . . . . . . . . . . . . . . . .  11
   6.  Examples of digital twin system . . . . . . . . . . . . . . .  11
     6.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.2.  Marine system . . . . . . . . . . . . . . . . . . . . . .  11
     6.3.  Healthcare system . . . . . . . . . . . . . . . . . . . .  16
     6.4.  Smart building system . . . . . . . . . . . . . . . . . .  18
   7.  Requirements for implenmenting digital twin . . . . . . . . .  20
   8.  Procedure for digital twin implementation . . . . . . . . . .  21
     8.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  21
     8.2.  Procedure . . . . . . . . . . . . . . . . . . . . . . . .  21
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     11.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  24
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25









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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.

   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 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 sublayer is structured into three sublayers: the
   device communication sublayer, the digital twin sublayer, and the
   application sublayer.




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   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 to facilitate informed decision making.

        +---------------------------------------------+ - - - - - - - - - - -
        |            Application sublayer             |
        | +----------+ +------+ +--------+ +--------+ |
        | |  Human   | | HMI  | |  Apps  | |  Peers | |
        | +----------+ +------+ +--------+ +--------+ |
        +---------------------------------------------+
        |           Digital twin sublayer             |
        | +----------+ +-------------+ +------------+ |
        | | Operation| | Application | |  Resource  | |
        | |    and   | |     and     | | access and | |
        | |management| |   service   | |interchange | |
        | +----------+ +-------------+ +------------+ |
        | +-----------------------------------------+ |  Digital twin layer
        | |           Digital representation        | |
        | |   +-------------+   +----------------+  | |
        | |   |  Affordance |   | Non-affordance |  | |
        | |   |   objects   |   |    objects     |  | |
        | |   +-------------+   +----------------+  | |
        | +-----------------------------------------+ |
        +---------------------------------------------+
        |        Device communication sublayer        |
        |     +-------------+   +----------------+    |
        |     |    Data     |   |     Object     |    |
        |     | collection  |   |     control    |    |
        |     +-------------+   +----------------+    |
        +---------------------------------------------+ - - - - - - - - - - -
        |     +-------------+   +----------------+    |
        |     |  Affordance |   |  sdfContext    |    |
        |     |   objects   |   |    objects     |    |     Physical layer
        |     +-------------+   +----------------+    |
        +---------------------------------------------+ - - - - - - - - - - -

             Figure 1: Basic Architecture of digital twin






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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 objects, including
   metadata, identity, contextual relationships, historical data, as
   well as device interfaces.

4.1.  Introduction to sdfContext

   A new SDF keyword sdfContext described in
   [I-D.draft-ietf-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 modeling using SDF elements

   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 example mapping of digital twin attributes to SDF elements is
   shown in Table 1.










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   +================+==============+==================================+
   | Attribute      | Recommended  | Description                      |
   |                | Mapping      |                                  |
   +================+==============+==================================+
   | Identifier     | sdfContext   | Globally unique digital twin ID  |
   |                |              | (e.g., URN)                      |
   +----------------+--------------+----------------------------------+
   | Characteristic | sdfProperty  | General description or domain    |
   |                | or sdfData   | properties                       |
   +----------------+--------------+----------------------------------+
   | Schedule       | sdfEvent or  | Time-based actions,              |
   |                | sdfData      | availability, or maintenance     |
   +----------------+--------------+----------------------------------+
   | Status         | sdfAction or | Actual or calculated operating   |
   |                | sdfProperty  | conditions                       |
   +----------------+--------------+----------------------------------+
   | Location       | sdfContext   | Physical or logical location     |
   |                |              | information                      |
   +----------------+--------------+----------------------------------+
   | Report         | sdfData      | Measurement summaries,           |
   |                |              | analytics, or logs               |
   +----------------+--------------+----------------------------------+
   | owner          | sdfContext   | Organization or entity           |
   |                |              | responsible for the digital twin |
   +----------------+--------------+----------------------------------+
   | Relationship   | sdfRelation  | Inter-object/inter-twin          |
   |                |              | relationships                    |
   +----------------+--------------+----------------------------------+

        Table 1: Digital twin modeling using elements of SDF model

4.3.  Relationship modeling

   The sdfRelation, defined in [I-D.draft-laari-asdf-relations], is a
   structure for specifying logical or physical relationships between
   objects within an SDF model.  If conventional sdfThing, sdfObject,
   and sdfProperty focus on defining the properties of individual
   digital twins, sdfRelation is a means of expressing interactions and
   structural links between them.  Since these relationships go beyond a
   single digital twin definition, they must be managed in a separate
   structure, where sdfRelation is used.  The sdfRelation keyword allows
   describing complex relationships beyond just the parent-child
   hierarchy.  These relationships can include:

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

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




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   *  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.

   An example of sdfRelation is shown in Figure 2.  The sdfProtocolMap
   in this example is described in [I-D.draft-ietf-asdf-nipc] and
   [I-D.draft-ietf-asdf-protocol-mapping]



























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   {
      "sdfThing": {
         "Room001": {
             "description": "Contains lightbult and thermostat"
              "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 is in the same room as the lightbulb",
                  "sdfProperty": {
                     "adjacent-node": {"type": "object","sdfType": "link"}
                   }
                 },
                 "sdfProtocolMap": {
                   "description": "Protocol between the lightbulb and thermostat",
                   "ble": {
                     "services":
                         [{"serviceID": "361c9c4f-22d7-4a1e-824b-8b61045a566a"}],
                     "cached": false,
                     "cacheIdlePurge": 3600,
                     "unit": "Second",
                     "autoUpdate": true,
                     "bonding": "default"
                 }
               }
            }
        }
     }
   }

                    Figure 2: An example of sdfRelation

5.  Protocol considerations for digital twin realization






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5.1.  Motivation

   Digital twins require continuous and reliable communication with
   physical objects.  To support synchronization, monitoring, control,
   and event notification, appropriate network protocols should be
   selected and semantically bound to modeled elements in SDF
   structures.  This clause outlines the main protocol types, roles in
   digital twin operations, and guidelines for representing these
   bindings using [I-D.draft-ietf-asdf-protocol-mapping].

5.2.  Supported protocol types

   Digital twin applications can use different types of protocols
   depending on device performance, data volume, latency sensitivity,
   and network topology:

   +===========+=========================+=============================+
   | Protocol  | Role in digital twin    | Characteristics             |
   +===========+=========================+=============================+
   | MQTT      | Sensor data publishing, | Lightweight, publish-       |
   |           | event reporting         | subscribe, suitable for     |
   |           |                         | IoT                         |
   +-----------+-------------------------+-----------------------------+
   | CoAP      | REST-like access to     | UDP-based, compact,         |
   |           | constrained devices     | supports observe/notify     |
   +-----------+-------------------------+-----------------------------+
   | BLE       | Local data exchange,    | Low energy, short range,    |
   |           | control for wearables   | uses characteristics/       |
   |           | or embedded devices     | services                    |
   +-----------+-------------------------+-----------------------------+
   | HTTP/REST | Enterprise integration, | Rich semantics, widely      |
   |           | cloud API               | supported, heavier          |
   |           |                         | overhead                    |
   +-----------+-------------------------+-----------------------------+
   | WebSocket | Bi-directional low-     | State synchronization,      |
   |           | latency updates         | real-time commands          |
   +-----------+-------------------------+-----------------------------+
   | NIPC      | Standardized control of | Useful for industrial,      |
   |           | non-IP devices          | air-gapped, or legacy       |
   |           |                         | systems                     |
   +-----------+-------------------------+-----------------------------+

            Table 2: Roles and characteristics for each protocol








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5.3.  Protocol binding in SDF

   To model the way digital twins communicate with their physical
   objects, sdfProtocolMap can be defined within the sdfProperty,
   sdfAction or sdfEvent levels.  Each protocol entry can specify a
   communication topic, path, security mechanism, QoS settings, and
   timing parameters.

                {
                "sdfProtocolMap": {
                   "mqtt": {
                     "topic": "boat007/heater1/status",
                     "qos": 1,
                     "updateInterval": 10,
                     "unit": "seconds"
                   },
                   "ble": {
                     "characteristicUUID": "00002a6e-0000-1000-8000-00805f9b34fb",
                     "serviceUUID": "00001809-0000-1000-8000-00805f9b34fb",
                     "read": true,
                     "write": true,
                     "notify": true
                   }
                }

                  Figure 3: An example of protocol binding

5.4.  QoS and Synchronization Semantics

   Quality of service (QoS) settings help define how reliable and
   frequently data is transferred between digital twins and physical
   objects, which are particularly important for telemetry (e.g.,
   sdfProperty updates) and command response flows (sdfAction).

    +===========+============================+=======================+
    | QoS level | Meaning                    | Recommended Usage     |
    +===========+============================+=======================+
    | 0         | At most once (best effort) | Periodic sensor data  |
    +-----------+----------------------------+-----------------------+
    | 1         | At least once              | State updates, events |
    +-----------+----------------------------+-----------------------+
    | 2         | Exactly once               | Control command, AI   |
    |           |                            | action                |
    +-----------+----------------------------+-----------------------+

                 Table 3: Meaning and usage of QoS levels





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5.5.  Security and access considerations

   When binding protocols, sdfSecurityMap can be used to include
   security parameters (e.g., authentication tokens, OSCORE for CoAP,
   BLE pairing status).  Role-based access control for specific protocol
   endpoints is also recommended.

5.6.  Implementation guidelines

   The protocol is essential to realizing the digital twin in operation,
   and it is recommended that the following considerations are taken
   into account:

   *  Use protocol mapping appropriate for device classes and network
      environments

   *  Provide both push-based (e.g., MQTT) and pool-based (e.g., HTTP)
      bindings, if applicable

   *  Reuse standardized subject structures, UUIDs, or endpoint URIs to
      ensure interoperability

   *  Avoid duplicate or conflicting bindings, and define protocol
      preferences via metadata if necessary.

6.  Examples of digital twin system

6.1.  Overview

   Various examples are included to show how SDF-based digital twin
   models can be applied to real-world scenarios.  These examples show
   how to represent physical objects using SDF elements.  In addition,
   sdfContext and sdfRelation are used to describe additional
   information such as the context of components, and the relationship
   between components (sdfRelation).  The examples include several
   domains such as marine systems, healthcare, smart buildings, and
   energy environments.  This consistent modeling approach can support
   interoperability among applications.

6.2.  Marine system

   Table 4 describes an example of how a maritime vessel, referred to as
   Boat007, can be described as a digital twin using the SDF model.  In
   this example, individual physical parts such as heaters and batteries
   are treated as separate sdfObjects, while the entire vessel is
   represented as a single sdfThing that groups these components
   together.




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   In a vessel modeled with SDF, each component is described using
   elements such as attributes, actions, and events.  These elements are
   used to represent how the component behaves and what state it is in
   at a given time.  The connections between components are also
   included in the model.  For example, a battery may be linked to a
   controller, which helps show how different parts are related and
   interact with each other.  This kind of representation makes it
   easier to follow the condition of devices over time.  It also allows
   operational data to be used more effectively for monitoring, while
   keeping the model compatible with other systems built in a similar
   way.

   This structure enable developers and systems integrators to:

   *  Seamlessly capture and communicate the features and state of the
      device

   *  Consolidate operational data for real-time monitoring and analysis

   *  Standardized semantics enables interoperability with other
      domains.






























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   +=======================+=============+============================+
   | Attribute             | SDF element | Example properties         |
   +=======================+=============+============================+
   | Boat007               | sdfThing    | id, name, model, includes  |
   |                       |             | heater1 and battery1       |
   +-----------------------+-------------+----------------------------+
   | Heater1               | sdfObject   | status (sdfProperty),      |
   |                       |             | temperature (sdfProperty), |
   |                       |             | turnOn (sdfAction)         |
   +-----------------------+-------------+----------------------------+
   | Thermostat1           | sdfObject   | setPoint, mode             |
   |                       |             | (sdfProperty)              |
   +-----------------------+-------------+----------------------------+
   | Battery1              | sdfObject   | voltage (sdfProperty),     |
   |                       |             | chargeLevel (sdfProperty), |
   |                       |             | battery-to-controller      |
   |                       |             | (sdfRelation)              |
   +-----------------------+-------------+----------------------------+
   | Controller            | sdfObject   | status (sdfProperty),      |
   |                       |             | controlMode (sdfProperty)  |
   +-----------------------+-------------+----------------------------+
   | Temp-to-Thermostat    | sdfRelation | source:                    |
   |                       |             | heater1.temperature,       |
   |                       |             | target:                    |
   |                       |             | thermostat1.setPoint,      |
   |                       |             | relationType: regulatedBy  |
   +-----------------------+-------------+----------------------------+
   | Battery-to-Controller | sdfRelation | source: batterySensor,     |
   |                       |             | target: powerController,   |
   |                       |             | relationType: connectedTo  |
   +-----------------------+-------------+----------------------------+
   | Location              | sdfContext  | latitude, longitude,       |
   |                       |             | dockedAt (e.g., port007)   |
   +-----------------------+-------------+----------------------------+

         Table 4: Components and SDF elements of a marine system

   In the context of Boat007, shown in Figure 4, such a Digital twin can
   support various applications, including predictive maintenance,
   energy optimization, and fleet-level coordination, demonstrating the
   practicality and scalability of SDF-based Digital twin modeling for
   mobility and transportation systems.









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             {
               "sdfThing": {
                   "boat007": {
                   "label": "Boat #007 with a heater",
                   "description": "Contains heaters, fans, battery, etc."
                   "sdfProperty": {
                      "status": {
                         "type": "boolean",
                         "description": "Indicates if the boat is powered"
                       }
                   },
                   "sdfObject": {
                      "heater1": {
                       "description": "A heater ",
                       "identityManifest": {
                          "manufacturer": "HeaterTech Inc.",
                           "model": "HEATER-2025-V1",
                           "firmwareVersion": "1.4.3",
                           "dateOfManufacture": "2025-04-20T09:00:00Z",
                           "certifications": [
                             { "scheme": "KS", "certId": "KS123", "region": "KR" } ]
                         },
                         "contextSnapshot": {
                           "thingId": "heater:unit5689",
                           "timestamp": "2025-05-23T10:20:00Z",
                           "installationInfo": {
                              "room": "kitchen",
                               "floor": 1,
                               "mountType": "freestanding",
                               "installationDate": "2025-06-01"
                           },
                           "usageProfile": {
                               "type": "residential",
                               "powerCircuit": "230V@60Hz",
                               "energyRating": "A++"
                           },
                           "location": {"lat": 35.1796, "lon": 129.0756 }
                         },
                         "sdfProperty": {
                             "status": {
                               "type": "boolean"
                               "description":"Whether the heater is powered"
                             },
                             "temperature": {
                                "type": "number",
                                "unit": "degreeCelsius",
                                "description": "Temperature of the heater"
                               }



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                         },
                         "sdfAction": {
                             "turnOn": { "description": "Activate the heater" },
                             "turnOff": { "description": "Deactivate the heater" }
                         },
                         "contextPatch": {
                             "thingId": "heater:unit5689",
                             "timestamp": "2025-06-20T09:00:00Z",
                             "location": {"lat": "35.2988", "lon": "129.2547" },
                             "installationInfo": {"floor": 1, "mountType": "wall" }
                         }
                       },
                       "thermostat": {
                            "maintenanceSchedule": {
                               "timestamp": "2025-05-20T10:00:00Z"
                               "description": "Last maintained date"
                           }
                       },
                       "batterySensor1": {
                         "sdfProperty": {
                             "chargeLevel": {
                               "type": "number",
                               "unit": "percent",
                               "description": "Battery charge level"
                             },
                             "voltage": {
                               "type": "number",
                               "unit": "volt",
                               "description": "Battery voltage"
                           }
                         }
                       },
                       "powerController1": {
                          "sdfAction": {
                             "connect": {"description": "Connect power from the battery" },
                             "disconnect": {"description": "Disconn power from the battery"}
                             }
                         }
                     },
                     "sdfRelation": {
                       "temperature-control": {
                          "source": "#/sdfObject/heater1/sdfProperty/temperature",
                          "target": "#/sdfObject/thermostat1/sdfProperty/setPoint",
                          "relationType": "regulatedBy",
                          "directionality": "unidirectional",
                          "description": "The current temperature of the heater is regulated by the thermostat's setPoint value."
                       },
                       "battery-to-controller": {



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                          "source": "#/sdfObject/batterySensor",
                          "target": "#/sdfObject/powerController",
                          "relationType": "connectedTo",
                          "directionality": "unidirectional"
                      }
                   }
                 }
               }
             }

                   Figure 4: An example of marine system

6.3.  Healthcare system

   This example represents a digital twin for a patient health monitor
   system (patientMonitor001) assigned to a patient.  The system reports
   real-time health properties while referencing contextual patient
   information with the components and elements shown in Table 5.

       +===========+==================+============================+
       | Attribute | SDF element      | Example properties         |
       +===========+==================+============================+
       | Patient   | sdfThing         | patientMonitor001 as a     |
       | monitor   |                  | digital twin               |
       +-----------+------------------+----------------------------+
       | ECG       | sdfObject        | heartRate, rhythmType,     |
       | Module    |                  | signalStrength             |
       +-----------+------------------+----------------------------+
       | Infusion  | sdfObject        | flowRate, volumeRemaining, |
       | Pump      |                  | alarmStatus                |
       +-----------+------------------+----------------------------+
       | Property  | sdfProperty      | e.g., temperature,         |
       |           |                  | bloodPressureSystolic,     |
       |           |                  | oxygenSaturation           |
       +-----------+------------------+----------------------------+
       | Context   | sdfContext       | bedNumber, wardLocation,   |
       | info      |                  | patientID, usageScenario   |
       +-----------+------------------+----------------------------+
       | Identity  | identityManifest | systemType,                |
       | info      |                  | firmwareVersion,           |
       |           |                  | hospitalAssetTag           |
       +-----------+------------------+----------------------------+
       | Relations | sdfRelation      | ECG → AlarmSystem          |
       |           |                  | (relationType:             |
       |           |                  | monitoredBy)               |
       +-----------+------------------+----------------------------+

        Table 5: Components and SDF elements of a healthcare system



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   A digital twin example of a patient monitoring system with ECG and
   infusion pump components is illustated in Figure 5.  in the
   healthcare domain, where a biosensor measuring the heart rate is
   functionally connected to an alert system that emits a high heart
   rate warning.  This enables real-time patient monitoring in medical
   environments.

             {
               "sdfThing": {
                 "patientMonitor001": {
                 "sdfObject": {
                     "ecg": {
                         "sdfProperty": {
                             "heartRate": { "type": "number", "unit": "bpm" },
                             "rhythmType": { "type": "string" }
                           }
                       },
                       "infusionPump": {
                           "sdfProperty": {
                             "flowRate": { "type": "number", "unit": "ml/h" },
                             "volumeRemaining": { "type": "number", "unit": "ml" }
                           }
                       }
                     },
                     "sdfContext": {
                         "wardLocation": { "const": "ICU-5A" },
                         "patientID": { "const": "PT123456" }
                     },
                     "identityManifest": {
                       "manufacturer": "MediTech",
                       "model": "IM-500",
                       "serialNumber": "MT-IM500-00789"
                     },
                     "sdfRelation": {
                       "heartRate-to-alertSystem": {
                           "description": "The heart rate data from the biosensor is monitored by the alert system, which triggers a warning event when a high heart rate is detected.",
                           "source": "#/sdfObject/biosensor/sdfProperty/heartRate",
                           "target": "#/sdfObject/alertSystem/sdfEvent/highHeartRateAlert",
                           "relationType": "monitoredBy",
                           "directionality": "unidirectional"
                       }
                   }
                 }
               }
             }

                     Figure 5: An example of healthcare




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6.4.  Smart building system

   This example shows a digital twin representing a smart lighting
   control system within a smart building domain.  The system uses both
   the MQTT protocol and the CoAP protocol to integrate lighting devices
   and occupancy-based controls.  Contextual information such as room
   number, zone, and usage scenario is included to support location-
   based control and analysis.

   The SDF elements and related components used in this domain are
   described in Table 6.

    +============+==================+================================+
    | Attribute  | SDF element      | Example properties             |
    +============+==================+================================+
    | Smart room | sdfThing         | roomControl001 as a digital    |
    |            |                  | twin, including                |
    |            |                  | lightController and sensorUnit |
    +------------+------------------+--------------------------------+
    | Light      | sdfObject        | brightness (sdfProperty),      |
    | controller |                  | toggle (sdfAction)             |
    +------------+------------------+--------------------------------+
    | Sensor     | sdfObject        | occupancy (sdfProperty),       |
    | unit       |                  | motionDetected (sdfEvent)      |
    +------------+------------------+--------------------------------+
    | Property   | sdfProperty      | brightness:percent,            |
    |            |                  | occupancy:boolean              |
    +------------+------------------+--------------------------------+
    | Action     | sdfAction        | toggle (on/off), dimTo (level) |
    +------------+------------------+--------------------------------+
    | Context    | sdfContext       | roomNumber: “101”, zone:       |
    | info       |                  | “eastWing”, usage: “office”    |
    +------------+------------------+--------------------------------+
    | Identity   | identityManifest | vendor: “SmartBuild Inc.”,     |
    | info       |                  | firmware: “v2.1.0”             |
    +------------+------------------+--------------------------------+
    | Protocol   | sdfProtocolMap   | MQTT + CoAP for monitoring and |
    |            |                  | control                        |
    +------------+------------------+--------------------------------+
    | Relations  | sdfRelation      | sensor-to-lightController      |
    |            |                  | (relationType: triggers)       |
    +------------+------------------+--------------------------------+

     Table 6: Components and SDF elements of a smart building system

   A digital twin representation of the smart building example is shown
   in Figure 6.  In this configuration, occupancy sensors trigger
   lighting control action through functional relationships,



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   demonstrating real-time and context-aware behavior.  Such modeling
   can be applicable to energy optimization, comfort control, and
   responsive automation in smart buildings.

               {
                 "sdfThing": {
                   "roomControl001": {
                     "sdfContext": {
                       "roomNumber": "101",
                       "zone": "eastWing",
                       "usage": "office"
                     },
                     "sdfObject": {
                       "lightController": {
                         "sdfProperty": {
                           "brightness": {
                             "type": "integer",
                             "unit": "percent",
                             "description": "Current brightness level of the light"
                           }
                         },
                         "sdfAction": {
                           "toggle": {
                             "description": "Turns the light on or off"
                           },
                           "dimTo": {
                             "description": "Dims the light to the specified brightness"
                           }
                         },
                         "sdfProtocolMap": {
                           "mqtt": {
                             "topic": "building/room101/light",
                             "qos": 1,
                             "updateInterval": 5,
                             "unit": "seconds"
                           },
                           "coap": {
                             "method": "POST",
                             "href": "/room101/light/toggle"
                           }
                         }
                       },
                       "sensorUnit": {
                         "sdfProperty": {
                           "occupancy": {
                             "type": "boolean",
                             "description": "Whether the room is currently occupied"
                           }



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                         },
                         "sdfEvent": {
                           "motionDetected": {
                             "description": "Triggered when motion is detected"
                           }
                         }
                       }
                     },
                     "sdfRelation": {
                       "sensorToLight": {
                         "source": "#/sdfThing/roomControl001/sdfObject/sensorUnit",
                         "target": "#/sdfThing/roomControl001/sdfObject/lightController",
                         "relationType": "triggers",
                         "directionality": "unidirectional"
                       }
                     }
                   }
                 }
               }

           Figure 6: An example of smart building lighting system

7.  Requirements for implenmenting 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.

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

   *  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.







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   *  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.

8.  Procedure for digital twin implementation

8.1.  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
   general principles 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 a
      digital twin in the domain

   *  collecting and mapping 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 among digital twins reflecting various roles of the
      observable object

   *  synchronizing the observable object and the digital twin

8.2.  Procedure

   The procedure of digitally twinning the space and the objects
   contained in it is described.

   *  Identifying and scoping physical objects: The first step is to
      clearly identify the physical objects 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 objects in space can be represented by
      digital twins, it is cost-effective to select objects for
      implementation purposes and configure them as digital twins.





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   *  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 sdfContext.

   *  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.

   *  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 objects in a
      smooth and reliable manner.

   *  Verification and compliance: Once an object 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.

   *  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.

   *  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.

   *  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
      objects are no longer available.








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9.  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.

10.  IANA Considerations

   This document has no IANA actions.

11.  References

11.1.  Normative References

   [I-D.draft-ietf-asdf-nipc]
              Brinckman, B., Mohan, R., and B. Sanford, "An Application
              Layer Interface for Non-IP device control (NIPC)", Work in
              Progress, Internet-Draft, I-D.draft-ietf-asdf-nipc-13, 20
              September 2025, <https://datatracker.ietf.org/doc/html/I-
              D.draft-ietf-asdf-nipc-13>.

   [I-D.draft-ietf-asdf-protocol-mapping]
              Mohan, R., Brinckman, B., and L. Corneo, "SDF Protocol
              Mapping", Work in Progress, Internet-Draft, I-D.draft-
              ietf-asdf-protocol-mapping-06, 2 March 2026,
              <https://datatracker.ietf.org/doc/html/I-D.draft-ietf-
              asdf-protocol-mapping-06>.

   [I-D.draft-ietf-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-ietf-asdf-sdf-
              nonaffordance-01, 21 September 2025,
              <https://datatracker.ietf.org/doc/html/I-D.draft-ietf-
              asdf-sdf-nonaffordance-01>.

   [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-25, 13 October 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-asdf-
              sdf-25>.

   [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.

11.2.  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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