An sdfType for Links

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Table of Contents

1. Introduction

The Semantic Definition Format for Data and Interactions of Things (SDF, [I-D.ietf-asdf-sdf]) is a format for domain experts to use in the creation and maintenance of data and interaction models in the Internet of Things.¶

A common data type that occurs in the modeling of IoT devices is the link. [RFC8288] defines the concept of Web Linking, which, apart from the target URI that any link will contain, can provide additional parameters, such as the "link relation type" that explains the relationship expressed by the link, as well as "target attributes" that provide additional information about the target of the link (without a need to "dereference", i.e., follow, the link).¶

This document defines and registers an sdfType "link" for the Semantic Definition Format. This type models an abstract "serialization" [RFC8288] of a link, in a way that is compatible with the way SDF maps information models to its data modeling language.¶

2. The sdfType "link"

The sdfType "link" is intended to be used with the SDF "type" of "object". The members of that object are strings that are named the same as the link parameter (attribute) names. The special parameter name "href" is used to express the link target. (Parameter names specific to the Constrained RESTful Environment (CoRE) are also discussed in [RFC9423].)¶

Note that attribute names and relation type names are case-insensitive in [RFC8288]. Strings that are case-insensitive in [RFC8288] MUST be in their lowercase form when used in this specification.¶

An example for the instance of a link is provided in Section 5 of [RFC6690]:¶

   </sensors/temp>;rt="temperature-c";if="sensor",

To hold a link like this, we could describe an SDF affordance that is specific on the target attributes above, but does not contain instance-specific (run-time) information on the actual URI that points to the link target. An sdfProperty for that could look like:¶

{
 "sdfProperty": {
  "temp-c-link": {
   "type": "object",
   "sdfType": "link",
   "properties": {
     "href": { "type": "string"},
     "rt": { "type": "string", "const": "temperature-c"},
     "if": { "type": "string", "const": "sensor"}
   }
  }
 }
}

3. Discussion

Links play an important role in SDF modeling both during definition time (for adding information to a model, such as in sdfRef) and during run time (for making links to instances into a subject of data and interaction modeling). The present document is an early attempt at addressing the run-time usage of links, in particular links that fit the Web Linking [RFC8288] abstractions. A related draft [I-D.laari-asdf-relations] addresses definition-time links, but does seem to touch modeling run-time use of links as well (e.g., by discussing "writable" link relations).¶

Not all links used in ecosystems are based on URIs. E.g., OMA has "object links", which are basically pairs of numbers (object/instance). These ecosystem links may have some structure that should be modeled in the SDF model (e.g., where the object id part of a link always has to have a specific value). This structure can be mapped into URI strings using some convention, e.g., an OMA object link could be oma-object:3303:0 (where oma-object is placeholder for a URI scheme to be defined). However, burying structural components of the ecosystem-specific link in a string syntax makes it hard to access and control those components from the model.¶

TODO: Examples are needed to show how the OCF collection pattern is addressed by the current specification.¶

4. Security Considerations

The security considerations of [RFC8288] apply in a general way, although modeling a link as a datatype does not incur all of the security considerations that will apply to actually interchanging these links.¶

(TODO)¶

5. IANA Considerations

// RFC Ed.: please replace RFC XXXX with this RFC number and remove this note.¶

IANA is requested to register the sdfType "link" in the "sdfType Values" sub-registry in the "SDF Parameters" registry, with the following completion for the registration template:¶

6. References

Acknowledgments

Discussions in the OneDM liaison organization shaped this proposal.¶

Author's Address