Semantic Definition Format (SDF) Extension for Non-Affordance Information

3.1. Motivation

In the current SDF framework, the primary focus is on defining affordances - interactive elements such as Properties, Actions, and Events. While this approach effectively captures the dynamic capabilities of a Thing, it overlooks essential non-interactive attributes that are vital for a comprehensive device representation. These non-affordance attributes encompass contextual information and descriptive metadata, including dimensions, weight, location, manufacturer details, and operational constraints. The absence of a standardized representation for such static information can lead to fragmented device models, hindering interoperability and seamless integration across diverse IoT ecosystems. Although it is technically possible to model such information using 'sdfProperty', this approach introduces several forms of semantic confusion:¶

1. Users may misinterpret the field as observable or interactive: When interactive properties (e.g. sensor readings or actuators) and fixed attributes (e.g. a device’s physical dimensions or serial number) are all represented as properties, it becomes unclear which elements are meant to change or be acted upon and which are immutable context. This ambiguity forces developers and tools to infer intent manually, making models harder to interpret and maintain. Over time, such models require extra documentation and care to ensure that static fields are not mistakenly treated as dynamic, adding to the maintenance burden.¶

2. Developers may implement unnecessary runtime I/O interfaces: Many IoT frameworks and tools automatically create API handlers (e.g., REST endpoints or CoAP resources) for each defined Property. If static metadata like a device's model name or install location is modeled as a property, a tool might erroneously generate read/write accessors for it. This is problematic because such metadata is meant to be read-only context, not an interactive affordance. The result is superfluous or misleading interface endpoints that do not reflect the device's real capabilities, potentially causing confusion or security issues. In short, using sdfProperty for static fields violates the expectation that those fields remain non-interactive, since the default affordance treatment would imply they can be polled or even written to.¶

3. Tools and UIs may treat static metadata as operational data: SDF is meant to clearly convey a Thing's interactive capabilities versus its contextual attributes. When both are blended under the same construct, developers and automated tools may misinterpret the purpose of a given field. For example, a field representing location or manufacturer might be misconstrued as an operational parameter rather than informative metadata. This blurring of semantics makes it harder to build consistent tooling and to map SDF models to platform implementations, since one cannot reliably distinguish which elements require interactive handling. In essence, the lack of separation between affordances and non-affordances dilutes the semantic clarity of SDF models , undermining the SDF goal of an unambiguous, self-descriptive device model.¶

To address these concerns, this document introduces 'sdfNonAffordance' as a dedicated top-level keyword to define static, descriptive, and non-interactive metadata. This construct enables a clear semantic distinction from 'sdfProperty', making the data model more expressive, machine-readable, and robust to implementation assumptions.¶

While the primary focus of this document is the introduction of a static model extension via 'sdfNonAffordance', practical use of such metadata in real-world deployments often requires runtime mechanisms for metadata exchange. Use cases such as device onboarding, dynamic environment configuration, and regulatory audits benefit from the ability to transmit static context data as part of operational protocols. To that end, this draft also introduces optional runtime messages -'contextSnapshot', 'identityManifest', and 'contextPatch'- that can convey non-affordance attributes at appropriate times.¶

These messages are not the core of the SDF extension but are essential for practical interoperability, especially in systems where device metadata needs to be programmatically discovered, validated, or synchronized. Their inclusion supports real-world use cases that rely on the seamless integration of descriptive metadata into operational contexts. Thus, the proposal reflects both a modeling advancement for SDF and a runtime integration pattern to enable widespread adoption. This design accommodates ecosystem-specific metadata such as regulatory certifications, deployment regions, or vendor-specific constraints by allowing flexible attributes to be included and mapped according to the needs of each ecosystem.¶

3.2. Use Cases

3.2.1. Asset Management and Tracking¶

• Scenario: In logistics and warehouse systems, physical containers and pallets are often equipped with IoT sensors that monitor temperature, vibration, or location. However, these containers also possess static physical attributes-such as size, weight, and capacity-which influence how and where they can be deployed. These characteristics are not part of the interactive sensing interface but are essential for automated decision-making.¶

• Requirements: The system must be able to describe and transmit non-affordance information such as dimensions, maximum load, and designated location zones in a way that is readable by other systems but not intended for modification. This data must also be available for use during onboarding or audit processes.¶

• Solution: Incorporating non-affordance information into SDF allows for the uniform representation of these attributes, facilitating efficient asset tracking, optimizing load planning, and ensuring compliance with transportation regulations. These properties are modeled using the 'sdfNonAffordance' keyword within an 'sdfObject'. Section 4.1.1 provides an example showing how to represent this information, while Section 4.2.3 demonstrates how location data might be reported at runtime using a 'contextPatch' message.¶

3.2.2. Environmental Context Awareness¶

• Scenario: Environmental sensors in smart buildings, such as temperature, CO2, or motion detectors, are typically installed in diverse physical contexts that affect how sensor values should be interpreted. A sensor on the third floor mounted to a window may behave differently than one on a concrete wall.¶

• Requirements: The system needs to express and exchange metadata such as installation floor, mounting surface, indoor/outdoor classification, and nearby materials. These fields are static yet contextual, impacting calibration, analytics, or filtering algorithms.¶

• Solution: By extending SDF to include environmental context as non-affordance information, the system can dynamically adjust operations based on device placement and environmental factors, enhancing occupant comfort and energy efficiency. These characteristics are defined under 'sdfNonAffordance' and transmitted as part of the 'contextSnapshot' message during onboarding. Section 4.1.2 provides an example model.¶

3.2.3. Regulatory Compliance and Certification¶

• Scenario: In regulated environments such as healthcare, aviation, or industrial automation, each device must include traceable identity and compliance information. These include manufacturer names, model numbers, firmware versions, and certification IDs.¶

• Requirements: These attributes must be immutable and readable, stored in a way that supports device registration, update tracking, and policy enforcement. They are not typically modifiable at runtime, but may need to be transmitted securely during verification or integration steps.¶

• Solution: Embedding this non-affordance information within SDF ensures that all relevant metadata is consistently available, simplifying compliance reporting and facilitating timely maintenance and recalls when necessary. The 'sdfNonAffordance' keyword can be used at the 'sdfThing' level to encapsulate such identity metadata. An example is shown in Section 4.1.3, and the 'identityManifest' format in Section 4.2.2 supports secure exchange of this data.¶

By integrating non-affordance information into SDF, these use cases demonstrate how a more holistic device model enhances interoperability, operational efficiency, and compliance across various IoT applications.¶

4.1. Static Model Definition (sdfNonAffordance)

​sdfNonAffordance is introduced as a peer to sdfProperty, sdfAction, and sdfEvent. It exists solely at design-time and captures metadata that must remain read-only in any generated interface. The construct may appear either at the sdfThing level (global context) or inside an individual sdfObject (component-specific context).¶

This subsection presents SDF examples that statically describe non-affordance information associated with different types of devices and use cases. Each example corresponds to a use case described in Section 3.2.¶

4.1.1. Example: Asset Management and Tracking¶

The following example corresponds to the use case in Section 3.2.1. It models static metadata for a shipping container, including physical dimensions and capacity.¶

{
  "sdfObject": {
    "assetContainer": {
      "description": "Shipping container with embedded sensors",
      "sdfNonAffordance": {
        "physicalSpecs": {
          "description": "Physical dimensions and capacity",
          "type": "object",
          "properties": {
            "length": { "type": "number", "unit": "m" },
            "width": { "type": "number", "unit": "m" },
            "height": { "type": "number", "unit": "m" },
            "maxLoadKg": { "type": "number", "unit": "kg" }
          },
          "required": ["length", "width", "height", "maxLoadKg"]
        },
        "location": {
          "type": "object",
          "properties": {
            "lat": { "type": "number" },
            "lon": { "type": "number" }
          }
        }
      }
    }
  }
}

4.1.2. Example: Environmental Context Awareness¶

This example corresponds to Section 3.2.2. It describes installation-related metadata for a building-mounted environmental sensor.¶

           {
             "sdfObject": {
               "envSensor": {
                 "description": "Environmental sensor unit",
                 "sdfNonAffordance": {
                   "installationInfo": {
                     "type": "object",
                     "properties": {
                       "floor": { "type": "integer" },
                       "mountType": {
                         "type": "string",
                         "enum": ["wall", "ceiling", "window"]
                       },
                       "indoorOutdoor": {
                         "type": "string",
                         "enum": ["indoor", "outdoor"]
                       }
                     },
                     "required": ["floor", "mountType"]
                   }
                 }
               }
             }
           }

4.1.3. Example: Regulatory Compliance and Certification¶

Aligned with Section 3.2.3, the following SDF model defines immutable identity and certification data of a regulated device.¶

{
  "sdfThing": {
    "deviceMetadata": {
      "sdfNonAffordance": {
        "identity": {
          "type": "object",
          "properties": {
            "manufacturer": { "type": "string" },
            "model": { "type": "string" },
            "firmwareVersion": { "type": "string" }
          }
        },
        "certifications": {
          "type": "array",
          "items": {
            "type": "object",
            "properties": {
              "scheme": { "type": "string" },
              "certId": { "type": "string" },
              "region": { "type": "string" }
            }
          }
        }
      }
    }
  }
}

4.2. Run-Time Context Messages

During operation, some contextual values change (e.g., a device is moved to a new room) or must be declared for audit purposes. To communicate those facts without re-classifying them as affordances, three transport-agnostic JSON envelopes for run-time context exchange are defined:¶

• contextSnapshot: conveys the full, current set of non-affordance fields-such as installation information or geographic coordinates-and is typically sent at boot, on request, or during periodic audits.¶

• identityManifest: declares immutable identity data (model, manufacturer, capability tags, certifications) and is normally issued once at commissioning or whenever a permanent attribute is added, for example after a firmware upgrade that introduces a new capability.¶

• contextPatch: transmits only the keys that have changed since the last snapshot, minimising bandwidth when a device is moved, re-mounted, or otherwise updated in context.¶

All envelopes carry a thingId and an timestamp.¶

4.2.1. contextSnapshot Message¶

The 'contextSnapshot' message provides a complete view of a device's static non-affordance metadata. This message is typically sent upon onboarding or registration to inform the system of all contextual properties.¶

{
  "thingId": "envSensor:abc123",
  "timestamp": "2025-07-01T12:00:00Z",
  "contextSnapshot": {
    "installationInfo": {
      "floor": 3,
      "mountType": "ceiling",
      "indoorOutdoor": "indoor"
    }
  }

4.2.2. identityManifest Message¶

The 'identityManifest' message describes immutable identity attributes of a device or asset. It can be used for device authentication or registry lookup.¶

{
  "thingId": "medDevice:unit42",
  "timestamp": "2025-07-01T08:15:00Z",
  "identityManifest": {
    "manufacturer": "HealthTech Inc.",
    "model": "HT-2025-M",
    "firmwareVersion": "1.4.3",
    "certifications": [
      { "scheme": "FDA", "certId": "FDA123456", "region": "US" },
      { "scheme": "CE", "certId": "CE987654", "region": "EU" }
    ]
  }
}

4.2.3. contextPatch Message¶

The 'contextPatch' message reports changes to specific non-affordance attributes. This allows for efficient partial updates without resending the entire snapshot.¶

{
   "thingId": "assetContainer:box001",
   "timestamp": "2025-07-01T14:23:00Z",
   "contextPatch": {
     "location": {
       "lat": 37.5665,
       "lon": 126.9780
     }
   }
 }