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Applications and Real-Time Area (ART) Registration Protocols Extensions (regext) This document defines a media type for RDAP that can be used to describe RDAP content with RDAP extensions. Additionally, this document describes the usage of this media type with RDAP for the purposes of signalling RDAP extensions during content negotiation.

Background defines the 'application/rdap+json' media type to be used with RDAP. This document defines a new media type to be used in conjunction with the current media type when an RDAP extension needs to be described during HTTP content negotiation. Though there maybe other purposes, the usage of this media type enables an RDAP client to signal to an RDAP server the list of RDAP extensions supported by that client. For example, an RDAP client that supports the "foo" extension may use this mechanism as a signal to an RDAP server, thus allowing the server to serve data using the "foo" extension only when it can be assured the client can understand it. By using this method, there is no need for every RDAP extension to define their own unique signaling mechanism. Additionally, this method is designed to be backwards compatible with the deployed RDAP ecosystem (see for further information).

Document Terms 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 when, and only when, they appear in all capitals, as shown here.

RDAP-X: The RDAP With Extensions Media Type The media type defined by this document is 'application/rdap-x+json'. This media type has a parameter of "extensions" which is a whitespace-separated list of RDAP extensions as defined in the IANA RDAP Extensions registry. Here is an example: For readability, this document will refer to this media type, RDAP With Extensions, as RDAP-X.

Using The RDAP-X Media Type specifies the usage of 'application/json', 'application/rdap+json' or both with HTTP accept header. When using the media type defined by this document, the 'application/rdap+json' media type MUST also be used in the accept header. An example: When a server is programmed to understand the RDAP-X media type, it SHOULD respond with this media type in the content-type header. By doing so, clients will be able to detect if the server recognizes the media type. Otherwise, the server will use the 'application/rdap+json' media type signalling to the client that the RDAP-X media type is not recognized by the server. This updates the usage of the content-type header with RDAP defined in RFC 7480, but this usage is backward compatible. If both a client and server support the RDAP-X media type, and the client requests an extension that is unimplemented by the server, the server SHOULD respond with the RDAP-X media type using only extensions implemented by the server. This behavior is backward compatible as RDAP clients must ignore unknown extensions as specified by . Responding with an HTTP 406 Not Acceptable status code is NOT RECOMMENDED. When the RDAP-X media type is used in the content-type header, the values in the media type's extension parameter SHOULD match the values in the rdapConformance array in the return JSON. When there is a mismatch between extension parameters and the rdapConformance array, clients SHOULD give preference to the rdapConformance array. Just as servers should not put extensions into the rdapConformance array for which they do not support, servers SHOULD NOT list extensions in the RDAP-X media type for which they do not support. Nothing in this specification sidesteps or obviates the HTTP content negotiation defined in for RDAP. Specifically, if a client gives RDAP-X a lower qvalue than any other media type, that is a signal not to use RDAP-X. Likewise, nothing in this specification sidesteps or obviates the HTTP caching mechanisms defined in . Further advice on the vary header may be found in . Some RDAP extensions, such as , have other protocol elements passed from the client to the server, and the presence of these protocol elements may be used by servers to determine a client's capability to handle the RDAP extension. This specification does not require the usage of those extensions identifiers in the extensions parameter, though clients SHOULD list the extension identifier in the extensions parameter when using other protocol elements of those extensions. Servers SHOULD NOT require the usage of extension identifiers in the extensions paramater when other extension protocol elements are used.

Usage in RDAP Links defines a link structure used in RDAP. The type attribute signals to a client the expected media type of the resource referenced in the href attribute, and some clients use this information to determine if the URI in the href attribute should be dereferenced. Servers MAY use the RDAP-X media type in the type attribute if a client has negotiated content with the server using the RDAP-X media type, the resource referenced by the URI matches the RDAP-X media type, and the resource referenced by the URI is served by a server compliant with this specification. Otherwise, use of the application/rdap+json media type is RECOMMENDED when the URI references RDAP resources.

RDAP-X Extension This document defines an RDAP "profile" extension using the identifier "rdapx" (hyphen characters are not allowed in RDAP extension identifiers). This RDAP extension defines no additional RDAP queries or response structures. The purpose of this RDAP extension is to allow servers to signal support for RDAP-X in rdapConformance arrays of responses to /help (aka "service discovery").

Security Considerations As stated in , this specification does not override the protocol elements of RDAP security extensions, such as , nor does it override the protocol elements of other security features of HTTP. This specification does contrast with solutions using query parameters in that those solutions require servers to blindly copy query parameters into redirect URLs in situations where such copying could cause harm, such as copying an API key intended for one server into the redirect URL of another server.

IANA Considerations Type name: application Subtype name: rdap-x+json Required parameters: This media type has a parameter of "extensions" which is a whitespace-separated list of RDAP extensions as defined in the IANA RDAP Extensions registry. Optional parameters: N/A Encoding considerations: See Section 3.1 of . Security considerations: The media represented by this identifier does not have security considerations beyond that found in Section 12 of . Interoperability considerations: There are no known interoperability problems regarding this media format. Published specification: This document. Applications that use this media type: Implementations of the Registration Data Access Protocol (RDAP) with Extensions. Additional information: This media type is a product of the IETF REGEXT Working Group. The REGEXT charter, information on the REGEXT mailing list, and other documents produced by the REGEXT Working Group can be found at https://datatracker.ietf.org/wg/regext/. Person & email address to contact for further information: IESG <iesg&ietf.org> Intended usage: COMMON Restrictions on usage: none Author: Andy Newton Change controller: IETF Provisional Registration: No

Acknowledgements Pawel Kowalik and James Mitchell have provided ideas and feedbacks that have contributed to the content of this document.

References Normative References Informative References

Using the Vary Header Server implementers may want to consider using the vary header depending on the caching behavior desired of shared caches (i.e. middleboxes, not client caches). Consider the following scenario where user Bob and user Alice send queries to the same RDAP server that is routed through a middlebox network element implementing a shared HTTP cache. User Bob sends a query for the domain example.com (http://regy.example/domain/example.com) without RDAP-X. The accept header sent for Bob's query would be accept: application/rdap+json or accept: application/json.
User Alice later sends a query for the same domain, however her client uses RDAP-X. The accept header returned by Alice might be accept: application/rdap-x+json, application/rdap+json. If no vary header is set in the response for these queries, the shared cache will compare only the URL of the query when processing cache items and therefore user Bob and user Alice would receive the same answer. In other words, since both queried http://regy.example/domain/example.com the shared cache would return the answer of the first query to the second query and all other subsequent queries until the item expired out of the cache. If server implementers do not desire this behaviour and would signal that caches consider each query separately, servers should also return a vary: accept header to inform the cache that the accept header should also be considered when processing cache items. Server implementers should also consult regarding caching and other uses of the vary header.

Design Considerations

Not Reusing the Existing Media Type strongly discourages the creation of new parameters on existing media types to enable new features. As RDAP has always had extensions, it could be argued that adding an "extensions" parameter to the existing application/rdap+json media type is not adding a new feature to RDAP. However, the opposite could be argued that adding the capability for clients to signal desired RDAP extensions is a new feature. More practically, there is concern that adding a new parameter to the existing media type would not be backward compatible with some server software. That is, servers examining media types as exact string matches may incorrectly conclude that the existing media type with an unknown, new parameter may not be the same as the existing media type without parameters. A similar, though less likely, concern exists for clients. As servers are required to handle multiple media types according to and , it therefore seems reasonable to conclude that defining a new media type for use with the existing media type is best to preserve backward compatibility.

Query Parameters Considered Harmful Another design approach to communicating RDAP extensions from the client to the server would be the use of URI query parameters:

Copy and Paste Consider two RDAP users, Alice and Bob. Alice has an RDAP client that supports the extensions "fizzbuzz", and Bob has an RDAP client that does not support this extension. Now consider the scenario where Alice copies and pastes the RDAP URL from above into an email and sends it to Bob. When Bob uses that URL with his RDAP client, it will be communicating to the server that the extension "fizzbuzz" is understood by Bob's client when it is not. In this scenario, Bob's client will be unable to render the RDAP extension regardless of the usage or not of the query parameter. However, if the server is using the query parameter for secondary purposes, such as gathering metrics and statistics, then the capabilities of Bob's client will have been incorrectly signalled to the server.

Redirects The RDAP ecosystem uses redirects in many situations. discusses "aggregators", which are RDAP servers used to help clients find authoritative RDAP servers using the RDAP bootstrap registries. Redirects are also heavily used by the RIRs when IP addresses or autonomous system numbers are transferred from one RIR to another. Within HTTP, URI query parameters are not explicitly preserved during a redirect (probably due to architecture considerations, see the section below). Specific to RDAP, instructs RDAP servers to ignore unknown query parameters and instructs clients not to transform a URL of a redirect. Therefore, query parameters denoting RDAP extensions will not survive redirects. This can be readily observed in currently deployed RDAP servers: To further demonstrate that query parameters do not automatically survive redirects but that media types do, consider the code found here. This code consists of a simple client and a simple server. The client sets both a new media type and query parameters. The servers listen on two ports, redirecting the client from a URL on the first port to a URL on the second port. Here is the output of the client. It shows that the query parameters are not automatically preserved but that the media type is automatically preserved. Here is the output of the server. It shows that the client, upon redirect, automatically sends the media type but does not automatically preserve the query parameters. Preservation of query parameters is not a guaranteed feature of HTTP client and server libraries, whereas preservation of media types is.

Referral Compatibility It is common in the RDAP ecosystem to link from one RDAP resource to another. These are typically conveyed in the link structure defined in and use the "application/rdap+json" media type. One common usage is to link to a domain registration in a domain registrar from a domain registration in a domain registry. Usage of the RDAP-X media type does not require clients to conduct further processing of these referrals, whereas a query parameter approach would require clients to process and deconflict any other query parameters if present.

Architectural Violations As noted in , URI query parameters are meant to be part of the identity of the resource being identified by a URI and pointed to by the location of a URL. RDAP extensions change the portions of JSON returned by the server but are not intended to change the resource being identified. That is, a domain registration is the same domain registration regardless of whether the postal address in that domain registration is communicated via JCard or a new RDAP extension for JSContact. Changing how the content of a resource is conveyed is called content negotiation and is discussed in detail in using media types. Readers should note that protocol design is not a "priestly affair" in which architectural violations are strictly forbidden. Every design decision is a trade-off. However, following the architecture of an ecosystem generally makes re-use of software and systems easier, and often eases the adoption of newer features in the future. When given the choice between two designs, the design that does not violate architecture should be preferred when all other considerations are equal.

RDAP Extension Versioning It is beyond the scope of this document to define the versioning of RDAP extensions. However, there is design intent to allow the use of explicitly versioned RDAP extension identifiers where they are also compatible with the identifiers used in the rdapConformance array of RDAP. Consider the scenario in which the IETF decides that RDAP extension identifiers suffixed with the character string __V denotes RDAP extensions versioned using a semantic versioning scheme. In this scenario, the RDAP extension identifier fizzbuzz__V is registered with IANA. The __V suffix indicates that when the identifier is used in the rdapConformance array, it must appear appended with a character string denoting the semantic version of the extension. For example, fizzbuzz__V_2_1 denotes version 2.1 of the fizzbuzz extension. In RDAP JSON, the conformance would appear as: The usage with the rdap-x media type would be: Readers should note that this scenario is provided to show design intent and is not a full-fledged extension versioning design. Additionally, the new media type defined in this document has utility with existing, opaquely versioned RDAP extensions and does not depend on the definition of a new versioning scheme for RDAP extensions.