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

Abstract

  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
  https://www.rfc-editor.org/info/rfc8982.

Copyright Notice

  Copyright (c) 2021 IETF Trust and the persons identified as the
  document authors.  All rights reserved.

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  described in the Simplified BSD License.

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
  Acknowledgements
  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",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
  "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"
  parameter:

  https://example.com/rdap/domains?name=example*.com&fieldSet=afieldset

  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
     members:

     "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": [
      "rdap_level_0",
      "subsetting"
    ],
    ...
    "subsetting_metadata": {
      "currentFieldSet": "afieldset",
      "availableFieldSets": [
        {
        "name": "anotherfieldset",
        "description": "Contains some fields",
        "default": false,
        "links": [
          {
          "value": "https://example.com/rdap/domains?name=example*.com
                    &fieldSet=afieldset",
          "rel": "alternate",
          "href": "https://example.com/rdap/domains?name=example*.com
                   &fieldSet=anotherfieldset",
          "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": [
      "rdap_level_0",
      "subsetting"
    ],
    ...
    "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"
  registry:

  Extension identifier:  subsetting
  Registry operator:  Any
  Published specification:  RFC 8982
  Contact:  IETF <[email protected]>
  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,
             <https://www.rfc-editor.org/info/rfc2119>.

  [RFC5890]  Klensin, J., "Internationalized Domain Names for
             Applications (IDNA): Definitions and Document Framework",
             RFC 5890, DOI 10.17487/RFC5890, August 2010,
             <https://www.rfc-editor.org/info/rfc5890>.

  [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,
             <https://www.rfc-editor.org/info/rfc7230>.

  [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,
             <https://www.rfc-editor.org/info/rfc7480>.

  [RFC7481]  Hollenbeck, S. and N. Kong, "Security Services for the
             Registration Data Access Protocol (RDAP)", RFC 7481,
             DOI 10.17487/RFC7481, March 2015,
             <https://www.rfc-editor.org/info/rfc7481>.

  [RFC7482]  Newton, A. and S. Hollenbeck, "Registration Data Access
             Protocol (RDAP) Query Format", RFC 7482,
             DOI 10.17487/RFC7482, March 2015,
             <https://www.rfc-editor.org/info/rfc7482>.

  [RFC7483]  Newton, A. and S. Hollenbeck, "JSON Responses for the
             Registration Data Access Protocol (RDAP)", RFC 7483,
             DOI 10.17487/RFC7483, March 2015,
             <https://www.rfc-editor.org/info/rfc7483>.

  [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,
             <https://www.rfc-editor.org/info/rfc8288>.

8.2.  Informative References

  [CQL]      Whitaker, G., "Catnap Query Language Reference", commit
             d4f402c, September 2017,
             <https://github.com/gregwhitaker/catnap/wiki/Catnap-Query-
             Language-Reference>.

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

  [REST]     Fielding, R., "Architectural Styles and the Design of
             Network-based Software Architectures", Ph.D. Dissertation,
             University of California, Irvine, 2000,
             <https://www.ics.uci.edu/~fielding/pubs/dissertation/
             fielding_dissertation.pdf>.

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
  drawbacks:

  *  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
  issues:

  *  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.

  https://example.com/rdap/domains?name=example*.com
          &fields=domainSearchResults(objectClassName,ldhName)

  https://example.com/rdap/domains?name=example*.com
          &fields=domainSearchResults(objectClassName,ldhName,
                  unicodeName,
                  status,
                  events(eventAction,eventDate),
                  entities(objectClassName,handle,roles),
                  nameservers(objectClassName,ldhName))

     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.

Acknowledgements

  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
  IIT-CNR/Registro.it
  Via Moruzzi,1
  56124 Pisa
  Italy

  Email: [email protected]
  URI:   https://www.iit.cnr.it


  Maurizio Martinelli
  IIT-CNR/Registro.it
  Via Moruzzi,1
  56124 Pisa
  Italy

  Email: [email protected]
  URI:   https://www.iit.cnr.it

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