Semantic Definition Format (SDF) Modeling for Digital Twin
draft-ietf-asdf-digital-twin-05
| Document | Type | Active Internet-Draft (asdf WG) | |
|---|---|---|---|
| Authors | Hyunjeong Lee , Jungha Hong | ||
| Last updated | 2026-09-14 | ||
| Replaces | draft-lee-asdf-digital-twin | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Additional resources |
GitHub Organization
Mailing list discussion |
||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-asdf-digital-twin-05
ASDF H. Lee, Ed.
Internet-Draft J. Hong
Intended status: Informational ETRI
Expires: 18 March 2027 14 September 2026
Semantic Definition Format (SDF) Modeling for Digital Twin
draft-ietf-asdf-digital-twin-05
Abstract
This memo specifies SDF modeling for digital twins, i.e., 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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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 18 March 2027.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
Lee & Hong Expires 18 March 2027 [Page 1]
Internet-Draft SDF modeling for digital twin September 2026
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
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
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 considerations . . . . . . . . . . . . . . . . . . . 11
5.5. Security and access considerations . . . . . . . . . . . 11
5.6. Implementation guidelines . . . . . . . . . . . . . . . . 12
6. Digital twin system in various domains . . . . . . . . . . . 12
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 12
6.2. Marine system . . . . . . . . . . . . . . . . . . . . . . 12
6.3. Healthcare system . . . . . . . . . . . . . . . . . . . . 17
6.4. Smart building system . . . . . . . . . . . . . . . . . . 19
6.5. Smart network system . . . . . . . . . . . . . . . . . . 21
7. Requirements for implementing digital twins . . . . . . . . . 24
8. Procedure for digital twin implementation . . . . . . . . . . 24
8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 24
8.2. Procedure . . . . . . . . . . . . . . . . . . . . . . . . 25
9. Security Considerations . . . . . . . . . . . . . . . . . . . 26
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
11.1. Normative References . . . . . . . . . . . . . . . . . . 26
11.2. Informative References . . . . . . . . . . . . . . . . . 28
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 28
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
Lee & Hong Expires 18 March 2027 [Page 2]
Internet-Draft SDF modeling for digital twin September 2026
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 twins 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.
Digital twin technologies are applied in various domains, including
manufacturing, energy, healthcare, agriculture, and transportation.
A common format is therefore needed to represent objects in these
domains as digital twins. SDF [RFC9880] 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 [RFC9880], 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.
Lee & Hong Expires 18 March 2027 [Page 3]
Internet-Draft SDF modeling for digital twin September 2026
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
Lee & Hong Expires 18 March 2027 [Page 4]
Internet-Draft SDF modeling for digital twin September 2026
4. Motivation and design rationale
The document is based on the underlying structure defined in
[RFC9880], which standardizes the semantic definition format (SDF)
for representing IoT affordances. 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, manufacturer)
These fields can appear in both sdfObject and sdfThing contexts,
follow the same structural pattern as sdfData, and are 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.
Lee & Hong Expires 18 March 2027 [Page 5]
Internet-Draft SDF modeling for digital twin September 2026
+================+==============+==================================+
| 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")
Lee & Hong Expires 18 March 2027 [Page 6]
Internet-Draft SDF modeling for digital twin September 2026
* 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 and may refer to a
term defined in an external ontology (e.g., SAREF[saref4bldg]).
* 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-sdf-protocol-mapping]
Lee & Hong Expires 18 March 2027 [Page 7]
Internet-Draft SDF modeling for digital twin September 2026
{
"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": {
"serviceID": "361c9c4f-22d7-4a1e-824b-8b61045a566a",
"characteristicID": "b7adf665-37c7-44be-8e1c-651a10573613"
}
}
}
}
}
}
Figure 2: An example of sdfRelation
5. Protocol considerations for digital twin realization
Lee & Hong Expires 18 March 2027 [Page 8]
Internet-Draft SDF modeling for digital twin September 2026
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-sdf-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. The current SDF Protocol Mapping document
defines mappings for BLE and Zigbee. Table 2 is intended to provide
examples of protocol types that may be used for digital twin
implementations; it does not imply that SDF protocol mappings have
been defined for all of the listed protocols.
Lee & Hong Expires 18 March 2027 [Page 9]
Internet-Draft SDF modeling for digital twin September 2026
+===========+===========================+===========================+
| Protocol | Role in digital twin | Characteristics |
+===========+===========================+===========================+
| MQTT | Sensor data | Lightweight, publish- |
| | publishing, event | subscribe, suitable for |
| | reporting | 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 |
+-----------+---------------------------+---------------------------+
| Zigbee | Sensor data access, | Uses endpoints, clusters, |
| | event reporting, and | attributes, and commands |
| | device control for | for property, event, and |
| | IoT devices | action mappings |
+-----------+---------------------------+---------------------------+
| HTTP/REST | Enterprise | Rich semantics, widely |
| | integration, cloud | supported, heavier |
| | API | overhead |
+-----------+---------------------------+---------------------------+
| WebSocket | Bi-directional low- | State synchronization, |
| | latency updates | real-time commands |
+-----------+---------------------------+---------------------------+
| NIPC | Standardized control | Useful for industrial, |
| | of non-IP devices | air-gapped, or legacy |
| | | systems |
+-----------+---------------------------+---------------------------+
Table 2: Roles and characteristics for each protocol
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 as shown in Figure 3.
Lee & Hong Expires 18 March 2027 [Page 10]
Internet-Draft SDF modeling for digital twin September 2026
{
"sdfProperty": {
"temperature": {
"type": "number",
"unit": "Cel",
"sdfProtocolMap": {
"ble": {
"serviceID": "00001809-0000-1000-8000-00805f9b34fb",
"characteristicID": "00002a1c-0000-1000-8000-00805f9b34fb"
}
}
}
}
}
Figure 3: An example of protocol binding
5.4. QoS considerations
Quality of service (QoS) settings define the reliability and
frequency of data transfer between digital twins and physical
objects. These settings are particularly important for telemetry
(e.g., sdfProperty updates) and command response flows (e.g.,
sdfAction operations). For example, MQTT defines three QoS levels
that can be selected according to the reliability requirements of
digital twin data exchange, as shown in Table 3.
+===========+============================+=======================+
| 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: Example MQTT QoS levels and recommended usage
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.
Lee & Hong Expires 18 March 2027 [Page 11]
Internet-Draft SDF modeling for digital twin September 2026
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 poll-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. Digital twin system in various domains
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 cover several
domains, including marine systems, healthcare systems, smart
buildings, and smart networks. This consistent modeling approach can
support interoperability across 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.
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
Lee & Hong Expires 18 March 2027 [Page 12]
Internet-Draft SDF modeling for digital twin September 2026
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 enables 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 enable interoperability with other domains.
Lee & Hong Expires 18 March 2027 [Page 13]
Internet-Draft SDF modeling for digital twin September 2026
+=======================+=============+============================+
| Attribute | SDF element | 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, |
| | | relType: regulatedBy |
+-----------------------+-------------+----------------------------+
| Battery-to-Controller | sdfRelation | source: batterySensor, |
| | | target: powerController, |
| | | relType: 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.
Lee & Hong Expires 18 March 2027 [Page 14]
Internet-Draft SDF modeling for digital twin September 2026
{
"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"
}
Lee & Hong Expires 18 March 2027 [Page 15]
Internet-Draft SDF modeling for digital twin September 2026
},
"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",
"relType": "regulatedBy",
"directionality": "unidirectional",
"description": "The current temperature of the heater is regulated by the thermostat's setPoint value."
},
"battery-to-controller": {
Lee & Hong Expires 18 March 2027 [Page 16]
Internet-Draft SDF modeling for digital twin September 2026
"source": "#/sdfObject/batterySensor",
"target": "#/sdfObject/powerController",
"relType": "connectedTo",
"directionality": "unidirectional"
}
}
}
}
}
Figure 4: An example of marine system
6.3. Healthcare system
This case 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 | 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 |
| | | (relType: monitoredBy) |
+-----------+------------------+----------------------------+
Table 5: Components and SDF elements of a healthcare system
Lee & Hong Expires 18 March 2027 [Page 17]
Internet-Draft SDF modeling for digital twin September 2026
A digital twin example of a patient monitoring system with ECG and
infusion pump components is illustrated in Figure 5. In this
healthcare scenario, a biosensor measuring the heart rate is
functionally connected to an alert system that emits a high-heart-
rate warning, enabling 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",
"relType": "monitoredBy",
"directionality": "unidirectional"
}
}
}
}
}
Figure 5: An example of healthcare
Lee & Hong Expires 18 March 2027 [Page 18]
Internet-Draft SDF modeling for digital twin September 2026
6.4. Smart building system
This case shows a digital twin representing a smart lighting control
system within a smart building domain. Contextual information such
as room number, zone, and usage scenario is included to support
location-based control and analysis. The MQTT and CoAP mappings in
this example are illustrative and are not currently defined by the
SDF Protocol Mapping specification.
The SDF elements and related components used in this domain are
described in Table 6.
+============+==================+================================+
| Attribute | SDF element | 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 (illustrative |
| | | mappings) for monitoring and |
| | | control |
+------------+------------------+--------------------------------+
| Relations | sdfRelation | sensor-to-lightController |
| | | (relType: triggers) |
+------------+------------------+--------------------------------+
Table 6: Components and SDF elements of a smart building system
Lee & Hong Expires 18 March 2027 [Page 19]
Internet-Draft SDF modeling for digital twin September 2026
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,
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": {
Lee & Hong Expires 18 March 2027 [Page 20]
Internet-Draft SDF modeling for digital twin September 2026
"type": "boolean",
"description": "Whether the room is currently occupied"
}
},
"sdfEvent": {
"motionDetected": {
"description": "Triggered when motion is detected"
}
}
}
},
"sdfRelation": {
"sensorToLight": {
"source": "#/sdfThing/roomControl001/sdfObject/sensorUnit",
"target": "#/sdfThing/roomControl001/sdfObject/lightController",
"relType": "triggers",
"directionality": "unidirectional"
}
}
}
}
}
Figure 6: An example of smart building lighting system
6.5. Smart network system
This case shows a digital twin-based smart network system that
obtains traffic information associated with a network device and
generates digital twins respectively corresponding to the network
device and the traffic information. The current traffic information
of the network device is synchronized with the digital twin
corresponding to the traffic information. Based on the synchronized
digital twins, a prediction model can be trained to predict an
abnormal condition associated with the network device. In an SDF
representation, the network device can be modeled as an sdfObject and
the traffic information can be represented by an associated sdfObject
or sdfData. Current traffic values can be represented using
sdfProperty, and the association and synchronization between the
network-device twin and the traffic-information twin can be expressed
using sdfRelation. Model training and inference are performed by an
analytics function in the application sublayer, while the SDF model
represents the synchronized input state and the predicted abnormal
condition.
The SDF elements and related components used in this domain are
described in Table 7.
Lee & Hong Expires 18 March 2027 [Page 21]
Internet-Draft SDF modeling for digital twin September 2026
+===================+=============+============================+
| Attribute | SDF element | Properties |
+===================+=============+============================+
| Smart network | sdfThing | smartNetwork001 including |
| system | | networkDeviceTwin and |
| | | trafficTwin |
+-------------------+-------------+----------------------------+
| networkDeviceTwin | sdfObject | device ID, status, |
| | | currentTraffic, |
| | | utilization, latency |
+-------------------+-------------+----------------------------+
| trafficTwin | sdfObject / | currentTraffic, |
| | sdfData | trafficHistory, |
| | | trafficPattern |
+-------------------+-------------+----------------------------+
| Current traffic | sdfProperty | throughput, packetRate, |
| | | utilization, delay |
+-------------------+-------------+----------------------------+
| Twin association | sdfRelation | networkDeviceTwin |
| | | associated with |
| | | trafficTwin |
+-------------------+-------------+----------------------------+
| Synchronization | sdfRelation | currentTraffic |
| | / | synchronized with |
| | sdfProperty | trafficTwin state |
+-------------------+-------------+----------------------------+
| Prediction result | sdfProperty | anomalyScore, |
| | / sdfEvent | abnormalConditionPredicted |
+-------------------+-------------+----------------------------+
Table 7: Components and SDF elements of a smart network system
A digital twin representation of the smart network system is shown in
Figure 7. The case illustrates its core model to the elements of
traffic information associated with a network device, digital twins
corresponding to the network device and the traffic information,
synchronization of current traffic information with the traffic-
information twin, and model training for prediction of an abnormal
condition.
Lee & Hong Expires 18 March 2027 [Page 22]
Internet-Draft SDF modeling for digital twin September 2026
{
"sdfThing": {
"smartNetwork001": {
"sdfObject": {
"networkDeviceTwin": {
"sdfProperty": {
"currentTraffic": { "type": "number", "unit": "Mbit/s" },
"utilization": { "type": "number", "unit": "percent" }
}
},
"trafficTwin": {
"sdfData": {
"trafficHistory": { "type": "array" },
"trafficPattern": { "type": "string" }
},
"sdfProperty": {
"synchronizedCurrentTraffic": { "type": "number", "unit": "Mbit/s" }
}
}
},
"sdfRelation": {
"trafficAssociation": {
"source": "#/sdfObject/networkDeviceTwin",
"target": "#/sdfObject/trafficTwin",
"relType": "associatedWith"
},
"trafficSynchronization": {
"source": "#/sdfObject/networkDeviceTwin/sdfProperty/currentTraffic",
"target": "#/sdfObject/trafficTwin/sdfProperty/synchronizedCurrentTraffic",
"relType": "synchronizedWith"
}
},
"sdfProperty": {
"anomalyScore": { "type": "number" }
},
"sdfEvent": {
"abnormalConditionPredicted": {
"description": "Raised when the trained model predicts an abnormal condition"
}
}
}
}
}
Figure 7: An example of smart network system
Lee & Hong Expires 18 March 2027 [Page 23]
Internet-Draft SDF modeling for digital twin September 2026
7. Requirements for implementing digital twins
A digital twin may be represented using an sdfThing or sdfObject
containing elements 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.
* 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. The
general principles for representing an sdfThing as a digital twin
within a specific domain are 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
Lee & Hong Expires 18 March 2027 [Page 24]
Internet-Draft SDF modeling for digital twin September 2026
* configuring the observable object as a digital twin based on the
collected data and intended purposes
* enabling interworking among digital twins that represent different
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.
* 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.
Lee & Hong Expires 18 March 2027 [Page 25]
Internet-Draft SDF modeling for digital twin September 2026
* 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.
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 be protected using appropriate authentication and authorization
mechanisms.
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>.
Lee & Hong Expires 18 March 2027 [Page 26]
Internet-Draft SDF modeling for digital twin September 2026
[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-04, 25 July 2026,
<https://datatracker.ietf.org/doc/html/I-D.draft-ietf-
asdf-sdf-nonaffordance-04>.
[I-D.draft-ietf-asdf-sdf-protocol-mapping]
Mohan, R., Brinckman, B., and L. Corneo, "SDF Protocol
Mapping", Work in Progress, Internet-Draft, I-D.draft-
ietf-asdf-sdf-protocol-mapping-11, 27 August 2026,
<https://datatracker.ietf.org/doc/html/I-D.draft-ietf-
asdf-sdf-protocol-mapping-11>.
[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>.
[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>.
[RFC9880] Koster, M., Bormann, C., and A. Keränen, "Semantic
Definition Format (SDF) for Data and Interactions of
Things", BCP 14, RFC 9880, DOI 10.17487/RFC9880, January
2026, <https://www.rfc-editor.org/rfc/rfc9880>.
Lee & Hong Expires 18 March 2027 [Page 27]
Internet-Draft SDF modeling for digital twin September 2026
[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
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
Lee & Hong Expires 18 March 2027 [Page 28]
Internet-Draft SDF modeling for digital twin September 2026
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
Lee & Hong Expires 18 March 2027 [Page 29]