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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 , but this usage is backwards-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 backwards-compatible as RDAP clients must ignore unknown extensions as specified by . Responding with an HTTP 406 Not Acceptable status code is NOT RECOMMENDED. Likewise, if a server is required to use an extension in a response that was not requested by the client, the server SHOULD respond as if the client had requested the extension. This behavior is backwards-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 "extensions" parameter SHOULD match the values in the "rdapConformance" array in the returned JSON. When there is a mismatch between the "extensions" parameter 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. The contents of the "extensions" parameter mirrors the content of the "rdapConformance" array in server responses. This includes the identifier "rdap_level_0", which is not an extension identifier by an identifier for the base RDAP specifications. Servers MUST follow the same rules for placing "rdap_level_0" in the content of the "extensions" parameter and the "rdapConformance" array. Clients SHOULD interpret an "extensions" parameter without "rdap_level_0" or one of its successor identifiers (e.g. "rdap_level_1") in the same manner as the interpretation of the "rdapConformance" array without "rdap_level_0" or one of its successors. Nothing in this specification sidesteps or obviates the HTTP content negotiation defined in for RDAP. Specifically, if a client gives RDAP-X a lower q value 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 can 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 extension 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" parameter when other extension protocol elements are used.

Examples The following examples use the HTTP/1.1 message exchange syntax as seen in .

Classic Negotiation This example demonstrates the negotiation of the "applicaton/rdap+json" media type as defined in using an RDAP "/help" query. This example also demonstrates the negotiation in which a client does not support RDAP-X but a server does support RDAP-X. Client Request: Server Response:

RDAP-X Negotiation In this example, both the client and server support RDAP-X and a fictional extension of "foo". Client Request: Server Response:

No Server Support Negotiation In this example, only the client supports RDAP-X, along with a fictional extension of "foo". Client Request: Server Response:

Differing Extension Negotiation In this example, both the client and server support RDAP-X. The client supports the extensions "foo" and "bar" while the server only support "foo". Client Request: Server Response:

Extension Versioning and Meta-data For scenarios where the "versioning" extension, as defined by , is used, the extension identifiers in the client request may not be exact or case-insensitive matches for the extension identifiers in the server response (unlike scenarios where the "versioning" extension is not used). That is, the extension identifiers used by the client have prepended versioning information, but the extension identifiers returned by the server do not have prepended versioning information (such information is in the "versioning" JSON). Client Request: Server Response: Servers might also use the "versioning" extension to describe meta-data about supported extensions even if the servers do not explicitly support extension versioning.

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 de-referenced. 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.

Example With RDAP-X Negotiation In this example, both the client and server support RDAP-X and the server has opted to return links with the RDAP-X media type in a domain query. Client Request: Server Response:

Example Without RDAP-X Negotiation In this example, the client does not support RDAP-X. Though the server may support RDAP-X, it will not signal RDAP-X because the client does not request it. Client Request: Server Response:

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 IANA is requested to register the following value in the Media Types Registry at https://www.iana.org/assignments/media-types/: 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 (https://www.iana.org/assignments/rdap-extensions/rdap-extensions.xhtml). 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 sent for Alice's query 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 behavior 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 backwards-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.

Inappropriate Use of Query Parameters Another design approach to communicating RDAP extensions from the client to the server would be the use of URI query parameters: However, there are a few problems with using query parameters for this scenario. Some of these problems are specific to RDAP and are also documented in . The following sections also describe the problems.

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 the 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 de-conflict 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.