1. RFC 8982
Internet Engineering Task Force (IETF)                       M. Loffredo
Request for Comments: 8982                                 M. Martinelli
Category: Standards Track                            IIT-CNR/Registro.it
ISSN: 2070-1721                                            February 2021

       Registration Data Access Protocol (RDAP) Partial Response


   The Registration Data Access Protocol (RDAP) does not include
   capabilities to request partial responses.  Servers will only return
   full responses that include all of the information that a client is
   authorized to receive.  A partial response capability that limits the
   amount of information returned, especially in the case of search
   queries, could bring benefits to both clients and servers.  This
   document describes an RDAP query extension that allows clients to
   specify their preference for obtaining a partial response.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Table of Contents

   1.  Introduction
     1.1.  Conventions Used in This Document
   2.  RDAP Path Segment Specification
     2.1.  Subsetting Metadata
       2.1.1.  RDAP Conformance
       2.1.2.  Representing Subsetting Links
   3.  Dealing with Relationships
   4.  Basic Field Sets
   5.  Negative Answers
   6.  IANA Considerations
   7.  Security Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Appendix A.  Approaches to Partial Response Implementation
     A.1.  Specific Issues Raised by RDAP
   Authors' Addresses

1.  Introduction

   The use of partial responses in RESTful API [REST] design is very
   common.  The rationale is quite simple: instead of returning objects
   in API responses with all data fields, only a subset of the fields in
   each result object is returned.  The benefit is obvious: less data
   transferred over the network means less bandwidth usage, faster
   server responses, less CPU time spent both on the server and the
   client, and less memory usage on the client.

   Currently, RDAP does not provide a client with any way to request a
   partial response.  Servers can only provide the client with a full
   response [RFC7483].  Servers cannot limit the amount of information
   returned in a response based on a client's preferences, and this
   creates inefficiencies.

   The protocol described in this specification extends RDAP search
   capabilities to enable partial responses through the provisioning of
   predefined sets of fields that clients can submit to an RDAP service
   by adding a new query parameter.  The service is implemented using
   the Hypertext Transfer Protocol (HTTP) [RFC7230] and the conventions
   described in [RFC7480].

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  RDAP Path Segment Specification

   The path segment defined in this section is an OPTIONAL extension of
   search path segments defined in [RFC7482].  This document defines an
   RDAP query parameter, "fieldSet", whose value is a non-empty string
   identifying a server-defined set of fields returned in place of the
   full response.  The field sets supported by a server are usually
   described in out-of-band documents (e.g., RDAP profile) together with
   other features.  Moreover, this document defines in Section 2.1 an
   in-band mechanism by means of which servers can provide clients with
   basic information about the supported field sets.

   The following is an example of an RDAP query including the "fieldSet"


   This solution can be implemented by RDAP providers with less effort
   than field selection and is easily requested by clients.  The
   considerations that have led to this solution are described in more
   detail in Appendix A.

2.1.  Subsetting Metadata

   According to most advanced principles in REST design, collectively
   known as "Hypermedia as the Engine of Application State" (HATEOAS)
   [HATEOAS], a client entering a REST application through an initial
   URI should use server-provided links to dynamically discover
   available actions and access the resources it needs.  In this way,
   the client is not required to have prior knowledge of the service
   nor, consequently, to hard-code the URIs of different resources.
   This allows the server to make URI changes as the API evolves without
   breaking clients.  Definitively, a REST service should be as self-
   descriptive as possible.

   Therefore, servers implementing the query parameter described in this
   specification SHOULD provide additional information in their
   responses about the available field sets.  Such information is
   collected in a new JSON data structure named "subsetting_metadata"
   containing the following properties:

   "currentFieldSet": "String" (REQUIRED)
      either the value of the "fieldSet" parameter as specified in the
      query string, or the field set applied by default.

   "availableFieldSets": "AvailableFieldSet[]" (OPTIONAL)
      an array of objects, with each element describing an available
      field set.  The AvailableFieldSet object includes the following

      "name": "String" (REQUIRED)
         the field set name.

      "default": "Boolean" (REQUIRED)
         indicator of whether the field set is applied by default.  An
         RDAP server MUST define only one default field set.

      "description": "String" (OPTIONAL)
         a human-readable description of the field set.

      "links": "Link[]" (OPTIONAL)
         an array of links as described in [RFC8288] containing the
         query string that applies the field set (see Section 2.1.2).

2.1.1.  RDAP Conformance

   Servers returning the "subsetting_metadata" section in their
   responses MUST include "subsetting" in the rdapConformance array.

2.1.2.  Representing Subsetting Links

   An RDAP server MAY use the "links" array of the "subsetting_metadata"
   element to provide ready-made references [RFC8288] to the available
   field sets (Figure 1).  The target URI in each link is the reference
   to an alternative to the current view of results identified by the
   context URI.

   The "value", "rel", and "href" JSON values MUST be specified.  All
   other JSON values are OPTIONAL.

     "rdapConformance": [
     "subsetting_metadata": {
       "currentFieldSet": "afieldset",
       "availableFieldSets": [
         "name": "anotherfieldset",
         "description": "Contains some fields",
         "default": false,
         "links": [
           "value": "https://example.com/rdap/domains?name=example*.com
           "rel": "alternate",
           "href": "https://example.com/rdap/domains?name=example*.com
           "title": "Result Subset Link",
           "type": "application/rdap+json"
     "domainSearchResults": [

           Figure 1: Example of a "subsetting_metadata" Instance

3.  Dealing with Relationships

   Representation of second-level objects within a field set produces
   additional considerations.  Since the representation of the topmost
   returned objects will vary according to the field set in use, the
   response may contain no relationships (e.g., for an abbreviated field
   set) or may contain associated objects as in a normal RDAP query
   response.  Each field set can indicate the format of the additional
   objects to be returned, in the same manner that the format of the
   topmost objects is controlled by the field set.

4.  Basic Field Sets

   This section defines three basic field sets that servers MAY
   implement to facilitate their interaction with clients:

   "id":  The server provides only the key field; "handle" for entities,
      and "ldhName" for domains and nameservers.  If a returned domain
      or nameserver is an Internationalized Domain Name (IDN) [RFC5890],
      then the "unicodeName" field MUST additionally be included in the
      response.  This field set could be used when the client wants to
      obtain a collection of object identifiers (Figure 2).

   "brief":  The field set contains the fields that can be included in a
      "short" response.  This field set could be used when the client is
      asking for a subset of the full response that provides only basic
      knowledge of each object.

   "full":  The field set contains all of the information the server can
      provide for a particular object.

   The "objectClassName" field is implicitly included in each of the
   above field sets.  RDAP providers SHOULD include a "links" field
   indicating the "self" link relationship.  RDAP providers MAY also add
   any property providing service information.

   Fields included in the "brief" and "full" field set responses MUST
   take into account the user's access and authorization levels.

     "rdapConformance": [
     "domainSearchResults": [
         "objectClassName": "domain",
         "ldhName": "example1.com",
         "links": [
           "value": "https://example.com/rdap/domain/example1.com",
           "rel": "self",
           "href": "https://example.com/rdap/domain/example1.com",
           "type": "application/rdap+json"
         "objectClassName": "domain",
         "ldhName": "example2.com",
         "links": [
           "value": "https://example.com/rdap/domain/example2.com",
           "rel": "self",
           "href": "https://example.com/rdap/domain/example2.com",
           "type": "application/rdap+json"

     Figure 2: Example of RDAP Response According to the "id" Field Set

5.  Negative Answers

   Each request including an empty or unsupported "fieldSet" value MUST
   produce an HTTP 400 (Bad Request) response code.  Optionally, the
   response MAY include additional information regarding the supported
   field sets in the HTTP entity body (Figure 3).

       "errorCode": 400,
       "title": "Field set 'unknownfieldset' is not valid",
       "description": [
           "Supported field sets are: 'afieldset', 'anotherfieldset'."


      Figure 3: Example of RDAP Error Response Due to an Invalid Field
                        Set Included in the Request

6.  IANA Considerations

   IANA has registered the following value in the "RDAP Extensions"

   Extension identifier:  subsetting
   Registry operator:  Any
   Published specification:  RFC 8982
   Contact:  IETF <iesg@ietf.org>
   Intended usage:  This extension describes a best practice for partial
      response provisioning.

7.  Security Considerations

   A search query typically requires more server resources (such as
   memory, CPU cycles, and network bandwidth) when compared to a lookup
   query.  This increases the risk of server resource exhaustion and
   subsequent denial of service.  This risk can be mitigated by
   supporting the return of partial responses combined with other
   strategies (e.g., restricting search functionality, limiting the rate
   of search requests, and truncating and paging results).

   Support for partial responses gives RDAP operators the ability to
   implement data access control policies based on the HTTP
   authentication mechanisms described in [RFC7481].  RDAP operators can
   vary the information returned in RDAP responses based on a client's
   access and authorization levels.  For example:

   *  the list of fields for each set can differ based on the client's
      access and authorization levels;

   *  the set of available field sets could be restricted based on the
      client's access and authorization levels.

   Servers can also define different result limits according to the
   available field sets, so a more flexible truncation strategy can be
   implemented.  The new query parameter presented in this document
   provides RDAP operators with a way to implement a server that reduces
   inefficiency risks.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

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

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

   [RFC7482]  Newton, A. and S. Hollenbeck, "Registration Data Access
              Protocol (RDAP) Query Format", RFC 7482,
              DOI 10.17487/RFC7482, March 2015,

   [RFC7483]  Newton, A. and S. Hollenbeck, "JSON Responses for the
              Registration Data Access Protocol (RDAP)", RFC 7483,
              DOI 10.17487/RFC7483, March 2015,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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

8.2.  Informative References

   [CQL]      Whitaker, G., "Catnap Query Language Reference", commit
              d4f402c, September 2017,

   [HATEOAS]  Jedrzejewski, B., "HATEOAS - a simple explanation",
              February 2018, <https://www.e4developer.com/2018/02/16/

   [REST]     Fielding, R., "Architectural Styles and the Design of
              Network-based Software Architectures", Ph.D. Dissertation,
              University of California, Irvine, 2000,

Appendix A.  Approaches to Partial Response Implementation

   Looking at the implementation experiences of partial responses
   offered by data providers on the web, two approaches are observed:

   *  the client explicitly describes the data fields to be returned;

   *  the client describes a name identifying a server-defined set of
      data fields.

   The former is more flexible than the latter because clients can
   specify all the data fields they need.  However, it has some

   *  Fields have to be declared according to a given syntax.  This is a
      simple task when the data structure of the object is flat, but it
      is much more difficult when the object has a tree structure like
      that of a JSON object.  The presence of arrays and deep nested
      objects complicate both the syntax definition of the query and,
      consequently, the processing required on the server side.

   *  Clients need to recognize the returned data structure to avoid
      cases when the requested fields are invalid.

   *  The request of some fields might not match the client's access and
      authorization levels.  Clients might request unauthorized fields,
      and servers have to define a strategy for responding such as
      always returning an error response or returning a response that
      ignores the unauthorized fields.

A.1.  Specific Issues Raised by RDAP

   In addition to those listed above, RDAP responses raise some specific

   *  Relevant entity object information is included in a jCard, but
      such information cannot be easily selected because it is split
      into the items of a jagged array.

   *  RDAP responses contain some properties providing service
      information (e.g., rdapConformance, links, notices, remarks,
      etc.), which are not normally selected but are just as important.
      They could be returned anyway but, in this case, the server would
      provide unrequested data.

   It is possible to address these issues.  For example, the Catnap
   Query Language [CQL] is a comprehensive expression language that can
   be used to customize the JSON response of a RESTful web service.
   Application of CQL to RDAP responses would explicitly identify the
   output fields that would be acceptable when a few fields are
   requested but it would become very complicated when processing a
   larger number of fields.  In the following, two CQL expressions for a
   domain search query are shown (Figure 4).  In the first, only
   objectClassName and ldhName are requested.  In the second, the fields
   of a possible WHOIS-like response are listed.



      Figure 4: Examples of CQL Expressions for a Domain Search Query

   The field set approach seems to facilitate RDAP interoperability.
   Servers can define basic field sets that, if known to clients, can
   increase the probability of obtaining a valid response.  The usage of
   field sets makes the query string less complex.  Moreover, the
   definition of predefined sets of fields makes it easier to establish
   result limits.

   Finally, considering that there is no real need for RDAP users to
   have the maximum flexibility in defining all the possible sets of
   logically connected fields (e.g., users interested in domains usually
   need to know the status, the creation date, and the expiry date of
   each domain), the field set approach is preferred.


   The authors would like to acknowledge Scott Hollenbeck, Tom Harrison,
   Karl Heinz Wolf, Jasdip Singh, Patrick Mevzek, Benjamin Kaduk, Roman
   Danyliw, Murray Kucherawy, Erik Kline, and Robert Wilton for their
   contribution to this document.

Authors' Addresses

   Mario Loffredo
   Via Moruzzi,1
   56124 Pisa

   Email: mario.loffredo@iit.cnr.it
   URI:   https://www.iit.cnr.it

   Maurizio Martinelli
   Via Moruzzi,1
   56124 Pisa

   Email: maurizio.martinelli@iit.cnr.it
   URI:   https://www.iit.cnr.it
  1. RFC 8982