RDAP Extensions

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Table of Contents

1. Background

The Registration Data Access Protocol (RDAP) defines a uniform means to access data from Internet operations registries, specifically Domain Name Registries (DNRs) and Internet Number Registries (INRs). The queries for DNRs and INRs are defined in [RFC9082] and the responses for DNRs and INRs are defined in [RFC9083].¶

RDAP contains a means to define extensions for queries not found in [RFC9082] and responses not found in [RFC9083].¶

2. The RDAP Extension Identifier

Section 6 of [RFC7480] describes the identifier used to signify RDAP extensions and the IANA registry into which RDAP extensions are to be registered.¶

In brief, RDAP extensions identifiers start with an alphabetic character and may contain alphanumeric characters and "_" (underscore) characters. This formulation was explicitly chosen to allow compatibility with variable names in programming languages and transliteration with XML.¶

RDAP extension identifiers have no explicit structure and are opaque in that no inner-meaning can be "seen" in them.¶

When in use in RDAP, extension identifiers are prepended to both URL path segments and JSON attribute names. In both cases, the extension identifier acts as a namespace preventing collisions between extension elements.¶

3. Usage in Queries

Section 5 of [RFC9082] describes the use of extension identifiers in formulating URIs to query RDAP servers. The extension identifiers are to be prepended to the path segments they use. For example, if an extension uses the identifier foobar, then the path segments used in that extension are prepended with foobar_. If the foobar extension defines paths fizz and fazz, the URIs for this extension might take the following form:¶

https://base.example/foobar_fizz
https://base.example/foobar_fazz

Although [RFC9082] describes the use of URI query strings, it does not define their use with extensions. [RFC7480] instructs servers to ignore unknown query parameters. Therefore, the use of query parameters, prefixed or not with an extension identifier, is undefined. Despite this, there are several extensions that do specify query parameters.¶

4. Usage in JSON

Section 2 of [RFC9083] describes the use of extension identifiers in the JSON returned by RDAP servers. Just as in URIs, the extension identifier is prepended to JSON names to create a namespace so that the JSON name from one extension will not collide with the JSON name of another extension. And just as with URIs, clients are to ignore unknown JSON names.¶

The example given in [RFC9083] is as follows:¶

{
  "handle" : "ABC123",
  "lunarNIC_beforeOneSmallStep" : "TRUE THAT!",
  "remarks" :
  [
    {
      "description" :
      [
        "She sells sea shells down by the sea shore.",
        "Originally written by Terry Sullivan."
      ]
    }
  ],
  "lunarNIC_harshMistressNotes" :
  [
    "In space,",
    "nobody can hear you scream."
  ]
}

In this example, the extension identified by lunarNIC is prepended to the names of both a JSON string and a JSON array.¶

The following example shows this use with a JSON object.¶

{
  "handle" : "ABC123",
  "remarks" :
  [
    {
      "description" :
      [
        "She sells sea shells down by the sea shore.",
        "Originally written by Terry Sullivan."
      ]
    }
  ],
  "lunarNIC_author" :
  {
    "firstInitial": "J",
    "lastName": "Heinlein"
  }
}

Here the JSON name "lunarNic_author" will separate the JSON from other extensions that may have an "author" structure. But the JSON contained within "lunarNIC_author" need not be prepended as the extension collision is avoided by "lunarNIC_author".¶

5. Camel Casing

The styling convention used in [RFC9083] for JSON names is often called "camel casing", in reference to the hump of a camel. In this style, the first letter of every word, except the first word, composing a name is capitalized. This convention was adopted to visually separate the namespace from the name, with an underscore between them.¶

Though there is no explicit guidance to use camel case names, extensions would be wise to continue the style.¶

6. Two Classes of Extensions

Though all RDAP extensions are to be registered in the IANA RDAP extensions registry, there is an implicit two-class system of extensions that comes from the inherit ownership of the RDAP specifications by the IETF: extensions created by the IETF and extensions not created by the IETF.¶

In the perspective of how extensions identifiers are used as namespace separators, extensions created by the IETF are not required to be prefixed with an extension identifier as the IETF can coordinate its own activities to avoid name collisions. In practice, extensions owned by the IETF do use extension identifiers.¶

7. Extension Versioning

Because RDAP extensions are opaque, they posses no explicit version despite the fact that some extension identifiers include trailing numbers. That is, RDAP extensions are opaquely versioned.¶

For example, fizzbuzz_1 may be the successor to fizzbuzz_0, but it may also be an extension for a completely separate purpose. Only consultation of the definition of fizzbuzz_1 will determine its relationship with fizzbuzz_0. Additionally, fizzbuzz_99 may be the predecessor of fizzbuzz_0.¶

8. Extension Definitions

Extensions must be documented in an RFC or in some other permanent and readily available reference, in sufficient detail that interoperability between independent implementations is possible.¶

Though RDAP gives each extension its own namespace, the definition of an extension may re-use definitions found in the base RDAP specification or in any other properly registered extension.¶

[RFC9083] notes that the extension identifiers provide a "hint" to the client as to how to interpret the response. This wording does not intentionally restrict the extension to defining only JSON values within the extensions namespace. Therefore, an extension may define the use of its own JSON values together with the use of JSON values from other extensions or RDAP specifications. As with the ICANN profile or RIR profile extensions, the extension may simply signal policy applied to already defined RDAP structures.¶

Authors' Addresses