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Efficient RDAP Referrals

Document Type Active Internet-Draft (individual)
Authors Gavin Brown , Andy Newton
Last updated 2024-05-23
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Registration Protocols Extensions (regext)                      G. Brown
Internet-Draft                                                 A. Newton
Intended status: Standards Track                                   ICANN
Expires: 24 November 2024                                    23 May 2024

                        Efficient RDAP Referrals


   This document describes how RDAP servers can provide HTTP "Link"
   header fields in RDAP responses to allow RDAP clients to efficiently
   determine the URL of related RDAP records for a resource.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 24 November 2024.

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   Copyright (c) 2024 IETF Trust and the persons identified as the
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  RDAP Link Objects . . . . . . . . . . . . . . . . . . . . . .   3
   3.  HTTP "Link" Header Field  . . . . . . . . . . . . . . . . . .   3
     3.1.  Registrar RDAP "Link" Header  . . . . . . . . . . . . . .   4
   4.  RDAP Responses  . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  RDAP HEAD requests  . . . . . . . . . . . . . . . . . . .   4
   5.  RDAP Conformance  . . . . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   Many Registration Data Access Protocol (RDAP, described in [RFC7480],
   [RFC7481], [RFC9082], [RFC9083] and others) resources contain
   referrals to related RDAP resources.

   For example, in the domain space, an RDAP record for a domain name
   received from the registry operator may include a referral to the
   RDAP record for the same object provided by the sponsoring registrar,
   while in the IP address space, an RDAP record for an address
   allocation may include referrals to enclosing or sibling prefixes.

   In both cases, RDAP service users are often equally if not more
   interested in these related RDAP resources than the resource provided
   by the TLD registry or RIR.

   While RDAP supports redirection of RDAP requests using HTTP
   redirections (which use a 3xx HTTP status and the "Location" header
   field, see Section 15.4 of [RFC9110]), it is not possible for RDAP
   servers to know _a priori_ whether a client requesting an RDAP record
   is doing so because it wants to retrieve a related RDAP record, or
   its own, so it can only respond by providing the full RDAP response.
   The client must then parse that response in order to extract the
   relevant URL from the "links" property of the object.

   This results in the wasteful expenditure of time, compute resources
   and bandwidth on the part of both the client and server.

   This document describes how an RDAP server can use "Link" HTTP header
   fields in responses to HEAD and GET requests to provide RDAP clients
   with the URL of related RDAP records, without the need for a
   signalling mechanism for the client to tell the server that it is
   only interested in retrieving those URLs.

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2.  RDAP Link Objects

   RDAP link objects, described in Section 4.2 of [RFC9083], establish
   unidirectional relationships between an RDAP resource and other web
   resources, which may also be RDAP resources.  The "rel" property
   indicates the nature of the relationship, and its possible values are
   described in [RFC8288].

   If a link object has a "type" property which contains the value
   "application/rdap+json", then clients can assume that the linked
   resource is also an RDAP resource.

   In the domain name space, this allows clients to discover the URL of
   the sponsoring registrar's RDAP record for a given domain name, if
   the "rel" property has the value "related", while in the IP address
   space, the "up" and "down" values allow RDAP clients to navigate the
   hierarchy of address space allocations.

3.  HTTP "Link" Header Field

   "Link" header fields, described in Section 3 of [RFC8288], provide a
   means for describing a relationship between two resources, generally
   between the requested resource and some other resource.  The "Link"
   header field is semantically equivalent to the <link> element in
   HTML, and multiple "Link" headers may be present in the header of an
   HTTP response.

   "Link" header fields may contain most of the parameters that are also
   present in Link objects in RDAP responses (See Section 4.2 of
   [RFC9083]).  So for example, an RDAP link object which has the
   following JSON representation:

     "value" : "",
     "rel" : "self",
     "href" : "",
     "hreflang" : [ "en", "ch" ],
     "title" : "title",
     "media" : "screen",
     "type" : "application/json"

   may be represented as follows:

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   Link: <>;

3.1.  Registrar RDAP "Link" Header

   Following on from the above, the following RDAP link object (which
   represents the RDAP URL of the sponsoring registrar of a resource):

     "value": "",
     "title": "URL of Sponsoring Registrar's RDAP Record",
     "rel": "related",
     "href": "",
     "type": "application/rdap+json"

   may be represented as follows:

   Link: <>;
     title="URL of Sponsoring Registrar's RDAP Record";

4.  RDAP Responses

   In response to GET and HEAD RDAP requests, RDAP servers which
   implement this specification MUST include a "Link" header field for
   each link object which refers to an RDAP resource that is present in
   the "links" array of the object in question.  The server MAY also
   include "Link" headers for link objects which refer to other types of
   resource.  In all cases, the link attributes MUST be the same in both

4.1.  RDAP HEAD requests

   The HTTP HEAD method can be used for obtaining metadata about a
   resource without transferring that resource (see Section 4.3.2 of

   An RDAP client which only wishes to obtain the URLs of related RDAP
   resources can issue a HEAD request for an RDAP resource and check the
   response for the presence of an appropriate "Link" header field.  If
   the link is absent, it may then fall back to performing a GET

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   An RDAP client interested in both the server's record and related
   records can use the traditional method of performing a GET request
   and extracting the link objects from the response.  To improve
   performance, RDAP clients MAY inspect the header of a response,
   extract the link headers, and issue requests for the related record
   in parallel while the request to the server is still in flight.  As
   an example, the cURL library provides the CURLOPT_HEADERFUNCTION
   ( configuration
   option to provide a callback that is invoked as soon as it has
   received header data.

5.  RDAP Conformance

   Servers which implement this specification MUST include the string
   "link_headers" in the "rdapConformance" array in all RDAP responses.

6.  IANA Considerations

   IANA is requested to register the following value in the RDAP
   Extensions Registry:

   *Extension identifier:*  registrar_link_header
   *Registry operator:*  any
   *Published specification:*  this document
   *Contact:*  IETF < (>
   *Intended usage:*  this extension indicates that the server provides
      the URL of the registrar's RDAP record in a "Link" header in
      responses to RDAP queries.

7.  Normative References

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

   [RFC7480]  Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the
              Registration Data Access Protocol (RDAP)", STD 95,
              RFC 7480, DOI 10.17487/RFC7480, March 2015,

   [RFC7481]  Hollenbeck, S. and N. Kong, "Security Services for the
              Registration Data Access Protocol (RDAP)", STD 95,
              RFC 7481, DOI 10.17487/RFC7481, March 2015,

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   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,

   [RFC9082]  Hollenbeck, S. and A. Newton, "Registration Data Access
              Protocol (RDAP) Query Format", STD 95, RFC 9082,
              DOI 10.17487/RFC9082, June 2021,

   [RFC9083]  Hollenbeck, S. and A. Newton, "JSON Responses for the
              Registration Data Access Protocol (RDAP)", STD 95,
              RFC 9083, DOI 10.17487/RFC9083, June 2021,

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,

Authors' Addresses

   Gavin Brown
   12025 Waterfront Drive, Suite 300
   Los Angeles, CA 90292
   United States of America

   Andy Newton
   12025 Waterfront Drive, Suite 300
   Los Angeles, CA 90292
   United States of America

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