Registration Data Access Protocol (RDAP) Query Parameters for Result Sorting and Paging
RFC 8977



Internet Engineering Task Force (IETF)                       M. Loffredo
Request for Comments: 8977                                 M. Martinelli
Category: Standards Track                            IIT-CNR/Registro.it
ISSN: 2070-1721                                            S. Hollenbeck
                                                           Verisign Labs
                                                            January 2021

  Registration Data Access Protocol (RDAP) Query Parameters for Result
                           Sorting and Paging

Abstract

   The Registration Data Access Protocol (RDAP) does not include core
   functionality for clients to provide sorting and paging parameters
   for control of large result sets.  This omission can lead to
   unpredictable server processing of queries and client processing of
   responses.  This unpredictability can be greatly reduced if clients
   can provide servers with their preferences for managing large
   responses.  This document describes RDAP query extensions that allow
   clients to specify their preferences for sorting and paging result
   sets.

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/rfc8977.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Conventions Used in This Document
   2.  RDAP Query Parameter Specification
     2.1.  Sorting and Paging Metadata
       2.1.1.  RDAP Conformance
     2.2.  "count" Parameter
     2.3.  "sort" Parameter
       2.3.1.  Sorting Properties Declaration
       2.3.2.  Representing Sorting Links
     2.4.  "cursor" Parameter
       2.4.1.  Representing Paging Links
   3.  Negative Answers
   4.  Implementation Considerations
   5.  IANA Considerations
   6.  Security Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Appendix A.  JSONPath Operators
   Appendix B.  Approaches to Result Pagination
     B.1.  Specific Issues Raised by RDAP
   Appendix C.  Implementation Notes
     C.1.  Sorting
     C.2.  Counting
     C.3.  Paging
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The availability of functionality for result sorting and paging
   provides benefits to both clients and servers in the implementation
   of RESTful services [REST].  These benefits include:

   *  reducing the server response bandwidth requirements

   *  improving server response time

   *  improving query precision and, consequently, obtaining more
      relevant results

   *  decreasing server query processing load

   *  reducing client response processing time

   Approaches to implementing features for result sorting and paging can
   be grouped into two main categories:

   1.  Sorting and paging are implemented through the introduction of
       additional parameters in the query string (e.g., the Open Data
       Protocol (ODATA) [ODATA-PART1]).

   2.  Information related to the number of results and the specific
       portion of the result set to be returned, in addition to a set of
       ready-made links for the result set scrolling, are inserted in
       the HTTP header of the request/response [RFC7231].

   However, there are some drawbacks associated with the use of the HTTP
   header.  First, the header properties cannot be set directly from a
   web browser.  Moreover, in an HTTP session, the information on the
   status (i.e., the session identifier) is usually inserted in the
   header or a cookie, while the information on the resource
   identification or the search type is included in the query string.
   Finally, providing custom information through HTTP headers assumes
   the client has prior knowledge of the server implementation, which is
   widely considered a Representational State Transfer (REST) design
   anti-pattern.  As a result, this document describes a specification
   based on the use of query parameters.

   Currently, RDAP [RFC7482] defines two query types:

   lookup:  the server returns only one object

   search:  the server returns a collection of objects

   While the lookup query does not raise issues regarding response size
   management, the search query can potentially generate a large result
   set that is often truncated according to server limits.  Besides, it
   is not possible to obtain the total number of objects found that
   might be returned in a search query response [RFC7483].  Lastly,
   there is no way to specify sort criteria to return the most relevant
   objects at the beginning of the result set.  Therefore, the client
   might traverse the whole result set to find the relevant objects or,
   due to truncation, might not find them at all.

   The specification described in this document extends RDAP query
   capabilities to enable result sorting and paging by adding new query
   parameters that can be applied to RDAP search path segments.  The
   service is implemented using the Hypertext Transfer Protocol (HTTP)
   [RFC7230] and the conventions described in [RFC7480].

   The implementation of the new parameters is technically feasible, as
   operators for counting, sorting, and paging rows are currently
   supported by the major relational database management systems.

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 Query Parameter Specification

   The new query parameters are OPTIONAL extensions of path segments
   defined in [RFC7482].  They are as follows:

   "count":  a boolean value that allows a client to request the return
      of the total number of objects found

   "sort":  a string value that allows a client to request a specific
      sort order for the result set

   "cursor":  a string value representing a pointer to a specific fixed-
      size portion of the result set

   Augmented Backus-Naur Form (ABNF) [RFC5234] is used in the following
   sections to describe the formal syntax of these new parameters.

2.1.  Sorting and Paging Metadata

   According to most advanced principles in REST design, collectively
   known as HATEOAS (Hypermedia as the Engine of Application State)
   [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 neither 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 parameters described in
   this specification SHOULD provide additional information in their
   responses about both the available sorting criteria and possible
   pagination.  Such information is collected in two OPTIONAL response
   elements named "sorting_metadata" and "paging_metadata".

   The "sorting_metadata" element contains the following properties:

   "currentSort": "String" (OPTIONAL)
      Either the value of the "sort" parameter as specified in the query
      string or the sort applied by default, if any.

   "availableSorts": "AvailableSort[]" (OPTIONAL)
      An array of objects, with each element describing an available
      sort criterion.  The AvailableSort object includes the following
      members:

      "property": "String" (REQUIRED)
         The name that can be used by the client to request the sort
         criterion.

      "default": "Boolean" (REQUIRED)
         Indicator of whether the sort criterion is applied by default.
         An RDAP server MUST define only one default sorting property
         for each object class.

      "jsonPath": "String" (OPTIONAL)
         The JSONPath expression of the RDAP field corresponding to the
         property.

      "links": "Link[]" (OPTIONAL)
         An array of links as described in [RFC8288] containing the
         query string that applies the sort criterion.

   At least one of the "currentSort" and "availableSorts" properties
   MUST be present.

   The "paging_metadata" element contains the following fields:

   "totalCount": "Numeric" (OPTIONAL)
      A numeric value representing the total number of objects found.
      It MUST be provided if and only if the query string contains the
      "count" parameter.

   "pageSize": "Numeric" (OPTIONAL)
      A numeric value representing the number of objects that should
      have been returned in the current page.  It MUST be provided if
      and only if the total number of objects exceeds the page size.
      This property is redundant for RDAP clients because the page size
      can be derived from the length of the search results array, but it
      can be helpful if the end user interacts with the server through a
      web browser.

   "pageNumber": "Numeric" (OPTIONAL)
      A numeric value representing the number of the current page in the
      result set.  It MUST be provided if and only if the total number
      of objects found exceeds the page size.

   "links": "Link[]" (OPTIONAL)
      An array of links as described in [RFC8288] containing the
      reference to the next page.  In this specification, only forward
      pagination is described because it is all that is necessary to
      traverse the result set.

2.1.1.  RDAP Conformance

   Servers returning the "paging_metadata" element in their response
   MUST include the string literal "paging" in the rdapConformance
   array.  Servers returning the "sorting_metadata" element MUST include
   the string literal "sorting".

2.2.  "count" Parameter

   Currently, RDAP does not allow a client to determine the total number
   of results in a query response when the result set is truncated.
   This is inefficient because the user cannot determine if the result
   set is complete.

   The "count" parameter provides additional functionality that allows a
   client to request information from the server that specifies the
   total number of objects matching the search pattern.

   The following is an example of an RDAP query including the "count"
   parameter:

   https://example.com/rdap/domains?name=example*.com&count=true

   The ABNF syntax is the following:

      count = "count=" ( trueValue / falseValue )
      trueValue = ("true" / "yes" / "1")
      falseValue = ("false" / "no" / "0")

   A trueValue means that the server MUST provide the total number of
   objects in the "totalCount" field of the "paging_metadata" element
   (Figure 1).  A falseValue means that the server MUST NOT provide this
   number.

   {
     "rdapConformance": [
           "rdap_level_0",
           "paging"
     ],
     ...
     "paging_metadata": {
       "totalCount": 43
     },
     "domainSearchResults": [
       ...
     ]
   }

     Figure 1: Example of RDAP Response with "paging_metadata" Element
                     Containing the "totalCount" Field

2.3.  "sort" Parameter

   RDAP does not provide any capability to specify the result set sort
   criteria.  A server could implement a default sorting scheme
   according to the object class, but this feature is not mandatory and
   might not meet user requirements.  Sorting can be addressed by the
   client, but this solution is rather inefficient.  Sorting features
   provided by the RDAP server could help avoid truncation of relevant
   results.

   The "sort" parameter allows the client to ask the server to sort the
   results according to the values of one or more properties and
   according to the sort direction of each property.  The ABNF syntax is
   the following:

      sort = "sort=" sortItem *( "," sortItem )
      sortItem = property-ref [":" ( "a" / "d" ) ]
      property-ref = ALPHA *( ALPHA / DIGIT / "_" )

   "a" means that an ascending sort MUST be applied; "d" means that a
   descending sort MUST be applied.  If the sort direction is absent, an
   ascending sort MUST be applied.

   The following are examples of RDAP queries that include the "sort"
   parameter:

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

   https://example.com/rdap/
   domains?name=example*.com&sort=registrationDate:d

   https://example.com/rdap/
   domains?name=example*.com&sort=lockedDate,name

   Except for sorting IP addresses and values denoting dates and times,
   servers MUST implement sorting according to the JSON value type of
   the RDAP field the sorting property refers to.  That is, JSON strings
   MUST be sorted lexicographically, and JSON numbers MUST be sorted
   numerically.  Values denoting dates and times MUST be sorted in
   chronological order.  If IP addresses are represented as JSON
   strings, they MUST be sorted based on their numeric conversion.

   The conversion of an IPv4 address to a number is possible since each
   dotted format IPv4 address is a representation of a number written in
   a 256-based manner; for example, 192.168.0.1 means 1*256^0 + 0*256^1
   + 168*256^2 + 192*256^3 = 3232235521.  Similarly, an IPv6 address can
   be converted into a number by applying the base 65536.  Therefore,
   the numerical representation of the IPv6 address
   2001:0db8:85a3:0:0:8a2e:0370:7334 is
   42540766452641154071740215577757643572.  Built-in functions and
   libraries for converting IP addresses into numbers are available in
   most known programming languages and relational database management
   systems.

   If the "sort" parameter presents an allowed sorting property, it MUST
   be provided in the "currentSort" field of the "sorting_metadata"
   element.

2.3.1.  Sorting Properties Declaration

   In the "sort" parameter ABNF syntax, the element named "property-ref"
   represents a reference to a property of an RDAP object.  Such a
   reference could be expressed by using a JSONPath expression (named
   "jsonpath" in the following).

   JSONPath is a syntax, originally based on the XML XPath notation
   [W3C.CR-xpath-31-20170321], which represents a path to select an
   element (or a set of elements) in a JSON document [RFC8259].  For
   example, the jsonpath to select the value of the ASCII name inside an
   RDAP domain lookup response is "$.ldhName", where $ identifies the
   root of the document object model (DOM).  Another way to select a
   value inside a JSON document is the JSON Pointer [RFC6901].

   While JSONPath and JSON Pointer are both commonly adopted notations
   to select any value inside JSON data, neither is particularly concise
   and easy to use (e.g., "$.domainSearchResults[*].events[?(@.eventActi
   on='registration')].eventDate" is the jsonpath of the registration
   date in an RDAP domain search response).

   Therefore, this specification defines the "property-ref" element in
   terms of names identifying RDAP properties.  However, not all the
   RDAP properties are suitable to be used in sort criteria.  These
   properties include:

   *  properties providing service information (e.g., links, notices,
      and remarks)

   *  multivalued properties (e.g., status, roles, and variants)

   *  properties representing relationships to other objects (e.g.,
      entities)

   On the contrary, properties expressed as values of other properties
   (e.g., registration date) could be used in such a context.

   A list of properties an RDAP server MAY implement is defined.  The
   properties are divided into two groups: object-common properties and
   object-specific properties.

   *  Object-common properties.  Object-common properties are derived
      from merging the "eventAction" and the "eventDate" properties.
      The following values of the "sort" parameter are defined:

      -  registrationDate

      -  reregistrationDate

      -  lastChangedDate

      -  expirationDate

      -  deletionDate

      -  reinstantiationDate

      -  transferDate

      -  lockedDate

      -  unlockedDate

   *  Object-specific properties.  Note that some of these properties
      are also defined as query path segments.  These properties
      include:

      -  Domain: name

      -  Nameserver: name, ipv4, ipv6

      -  Entity: fn, handle, org, email, voice, country, cc, city

   The correspondence between these sorting properties and the RDAP
   object classes is shown in Table 1.  Some of the sorting properties
   defined for the RDAP entity class are related to jCard elements
   [RFC7095], but because jCard is the JSON format for vCard, the
   corresponding definitions are included in the vCard specification
   [RFC6350].

   An RDAP server MUST NOT use the defined sorting properties with a
   meaning other than that described in Table 1.

    +============+============+=================+======+=======+======+
    | Object     | Sorting    | RDAP property   | RFC  | RFC   | RFC  |
    | class      | property   |                 | 7483 | 6350  | 8605 |
    +============+============+=================+======+=======+======+
    | Searchable | Common     | eventAction     | 4.5  |       |      |
    | objects    | properties | values suffixed |      |       |      |
    |            |            | by "Date"       |      |       |      |
    +------------+------------+-----------------+------+-------+------+
    | Domain     | name       | unicodeName/    | 5.3  |       |      |
    |            |            | ldhName         |      |       |      |
    +------------+------------+-----------------+------+-------+------+
    | Nameserver | name       | unicodeName/    | 5.2  |       |      |
    |            |            | ldhName         |      |       |      |
    +------------+------------+-----------------+------+-------+------+
    |            | ipv4       | v4 ipAddress    | 5.2  |       |      |
    +------------+------------+-----------------+------+-------+------+
    |            | ipv6       | v6 ipAddress    | 5.2  |       |      |
    +------------+------------+-----------------+------+-------+------+
    | Entity     | handle     | handle          | 5.1  |       |      |
    +------------+------------+-----------------+------+-------+------+
    |            | fn         | jCard fn        | 5.1  | 6.2.1 |      |
    +------------+------------+-----------------+------+-------+------+
    |            | org        | jCard org       | 5.1  | 6.6.4 |      |
    +------------+------------+-----------------+------+-------+------+
    |            | voice      | jCard tel with  | 5.1  | 6.4.1 |      |
    |            |            | type="voice"    |      |       |      |
    +------------+------------+-----------------+------+-------+------+
    |            | email      | jCard email     | 5.1  | 6.4.2 |      |
    +------------+------------+-----------------+------+-------+------+
    |            | country    | country name in | 5.1  | 6.3.1 |      |
    |            |            | jCard adr       |      |       |      |
    +------------+------------+-----------------+------+-------+------+
    |            | cc         | country code in | 5.1  |       | 3.1  |
    |            |            | jCard adr       |      |       |      |
    +------------+------------+-----------------+------+-------+------+
    |            | city       | locality in     | 5.1  | 6.3.1 |      |
    |            |            | jCard adr       |      |       |      |
    +------------+------------+-----------------+------+-------+------+

                 Table 1: Definitions of Sorting Properties

   Regarding the definitions in Table 1, some further considerations are
   needed to disambiguate some cases:

   *  Since the response to a search on either domains or nameservers
      might include both A-labels and U-labels [RFC5890] in general, a
      consistent sorting policy MUST treat the unicodeName and ldhName
      as two representations of the same value.  The unicodeName value
      MUST be used while sorting if it is present; when the unicodeName
      is unavailable, the value of the ldhName MUST be used instead.

   *  The jCard "sort-as" parameter MUST be ignored for the sorting
      capability described in this document.

   *  Even if a nameserver can have multiple IPv4 and IPv6 addresses,
      the most common configuration includes one address for each IP
      version.  Therefore, this specification makes the assumption that
      nameservers have a single IPv4 and/or IPv6 value.  When more than
      one address per IP version is presented, sorting MUST be applied
      to the first value.

   *  Multiple events with a given action on an object might be
      returned.  If this occurs, sorting MUST be applied to the most
      recent event.

   *  Except for handle values, all the sorting properties defined for
      entity objects can be multivalued according to the definition of
      vCard as given in [RFC6350].  When more than one value is
      presented, sorting MUST be applied to the preferred value
      identified by the parameter pref="1".  If the "pref" parameter is
      missing, sorting MUST be applied to the first value.

   The "jsonPath" field in the "sorting_metadata" element is used to
   clarify the RDAP response field the sorting property refers to.  The
   mapping between the sorting properties and the jsonpaths of the RDAP
   response fields is shown below.  The JSONPath operators used herein
   are described in Appendix A.

   *  Searchable objects

      registrationDate
         $.domainSearchResults[*].events[?(@.eventAction=="registration"
         )].eventDate

      reregistrationDate
         $.domainSearchResults[*].events[?(@.eventAction=="reregistratio
         n")].eventDate

      lastChangedDate
         $.domainSearchResults[*].events[?(@.eventAction=="last
         changed")].eventDate

      expirationDate
         $.domainSearchResults[*].events[?(@.eventAction=="expiration")]
         .eventDate

      deletionDate
         $.domainSearchResults[*].events[?(@.eventAction=="deletion")].e
         ventDate

      reinstantiationDate
         $.domainSearchResults[*].events[?(@.eventAction=="reinstantiati
         on")].eventDate

      transferDate
         $.domainSearchResults[*].events[?(@.eventAction=="transfer")].e
         ventDate

      lockedDate
         $.domainSearchResults[*].events[?(@.eventAction=="locked")].eve
         ntDate

      unlockedDate
         $.domainSearchResults[*].events[?(@.eventAction=="unlocked")].e
         ventDate

   *  Domain

      name
         $.domainSearchResults[*].[unicodeName,ldhName]

   *  Nameserver

      name
         $.nameserverSearchResults[*].[unicodeName,ldhName]

      ipv4
         $.nameserverSearchResults[*].ipAddresses.v4[0]

      ipv6
         $.nameserverSearchResults[*].ipAddresses.v6[0]

   *  Entity

      handle
         $.entitySearchResults[*].handle

      fn
         $.entitySearchResults[*].vcardArray[1][?(@[0]=="fn")][3]

      org
         $.entitySearchResults[*].vcardArray[1][?(@[0]=="org")][3]

      voice
         $.entitySearchResults[*].vcardArray[1][?(@[0]=="tel" &&
         @[1].type=="voice")][3]

      email
         $.entitySearchResults[*].vcardArray[1][?(@[0]=="email")][3]

      country
         $.entitySearchResults[*].vcardArray[1][?(@[0]=="adr")][3][6]

      cc
         $.entitySearchResults[*].vcardArray[1][?(@[0]=="adr")][1].cc

      city
         $.entitySearchResults[*].vcardArray[1][?(@[0]=="adr")][3][3]

   Additional notes on the provided jsonpaths:

   *  Those related to the event dates are defined only for the "domain"
      object.  To obtain the equivalent jsonpaths for "entity" and
      "nameserver", the path segment "domainSearchResults" must be
      replaced with "entitySearchResults" and "nameserverSearchResults",
      respectively.

   *  Those related to jCard elements are specified without taking into
      account the "pref" parameter.  Servers that sort those values
      identified by the "pref" parameter SHOULD update a jsonpath by
      adding an appropriate filter.  For example, if the email values
      identified by pref="1" are considered for sorting, the jsonpath of
      the "email" sorting property should be
      $.entitySearchResults[*].vcardArray[1][?(@[0]=="email" &&
      @[1].pref=="1")][3].

2.3.2.  Representing Sorting Links

   An RDAP server MAY use the "links" array of the "sorting_metadata"
   element to provide ready-made references [RFC8288] to the available
   sort criteria (Figure 2).  Each link represents a reference to an
   alternate view of the results.

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

   {
     "rdapConformance": [
       "rdap_level_0",
       "sorting"
     ],
     ...
     "sorting_metadata": {
        "currentSort": "name",
        "availableSorts": [
          {
          "property": "registrationDate",
          "jsonPath": "$.domainSearchResults[*]
             .events[?(@.eventAction==\"registration\")].eventDate",
          "default": false,
          "links": [
            {
            "value": "https://example.com/rdap/domains?name=example*.com
                      &sort=name",
            "rel": "alternate",
            "href": "https://example.com/rdap/domains?name=example*.com
                     &sort=registrationDate",
            "title": "Result Ascending Sort Link",
            "type": "application/rdap+json"
            },
            {
            "value": "https://example.com/rdap/domains?name=example*.com
                      &sort=name",
            "rel": "alternate",
            "href": "https://example.com/rdap/domains?name=example*.com
                     &sort=registrationDate:d",
            "title": "Result Descending Sort Link",
            "type": "application/rdap+json"
            }
          ]
          },
          ...
        ]
     },
     "domainSearchResults": [
       ...
     ]
   }

      Figure 2: Example of a "sorting_metadata" Instance to Implement
                               Result Sorting

2.4.  "cursor" Parameter

   The "cursor" parameter defined in this specification can be used to
   encode information about any pagination method.  For example, in the
   case of a simple implementation of the "cursor" parameter to
   represent offset pagination information, the "cursor" value
   "b2Zmc2V0PTEwMCxsaW1pdD01MA==" is the base64 encoding of
   "offset=100,limit=50".  Likewise, in a simple implementation to
   represent keyset pagination information, the "cursor" value
   "ZXhhbXBsZS1OLmNvbQ==" represents the base64 encoding of
   "key=example-N.com" whereby the key value identifies the last row of
   the current page.

   Note that this specification uses a base64 encoding for cursor
   obfuscation just for example.  RDAP servers are NOT RECOMMENDED to
   obfuscate the "cursor" value through a mere base64 encoding.

   This solution lets RDAP providers implement a pagination method
   according to their needs, a user's access level, and the submitted
   query.  Besides, servers can change the method over time without
   announcing anything to clients.  The considerations that have led to
   this solution are described in more detail in Appendix B.

   The ABNF syntax of the "cursor" parameter is the following:

      cursor = "cursor=" 1*( ALPHA / DIGIT / "/" / "=" / "-" / "_" )

   The following is an example of an RDAP query including the "cursor"
   parameter:

   https://example.com/rdap/domains?name=example*.com
   &cursor=wJlCDLIl6KTWypN7T6vc6nWEmEYe99Hjf1XY1xmqV-M=

2.4.1.  Representing Paging Links

   An RDAP server SHOULD use the "links" array of the "paging_metadata"
   element to provide a ready-made reference [RFC8288] to the next page
   of the result set (Figure 3).  Examples of additional "rel" values a
   server MAY implement are "first", "last", and "prev".

   {
     "rdapConformance": [
       "rdap_level_0",
       "paging"
     ],
     ...
     "notices": [
       {
         "title": "Search query limits",
         "type": "result set truncated due to excessive load",
         "description": [
         "search results for domains are limited to 50"
         ]
       }
     ],
     "paging_metadata": {
       "totalCount": 73,
       "pageSize": 50,
       "pageNumber": 1,
       "links": [
         {
         "value": "https://example.com/rdap/domains?name=example*.com",
         "rel": "next",
         "href": "https://example.com/rdap/domains?name=example*.com
                 &cursor=wJlCDLIl6KTWypN7T6vc6nWEmEYe99Hjf1XY1xmqV-M=",
         "title": "Result Pagination Link",
         "type": "application/rdap+json"
         }
       ]
     },
     "domainSearchResults": [
       ...
     ]
   }

   Figure 3: Example of a "paging_metadata" Instance to Implement Cursor
                                 Pagination

3.  Negative Answers

   The constraints for the values of parameters are defined by their
   ABNF syntax.  Therefore, each request that includes an invalid value
   for a parameter SHOULD produce an HTTP 400 (Bad Request) response
   code.  The same response SHOULD be returned in the following cases:

   *  if sorting by either single or multiple properties, the client
      provides an unsupported value for the "sort" parameter, as well as
      a value related to an object property not included in the response

   *  if the client submits an invalid value for the "cursor" parameter

   Optionally, the response MAY include additional information regarding
   either the supported sorting properties or the correct "cursor" value
   in the HTTP entity body (Figure 4).

   {
       "errorCode": 400,
       "title": "Domain sorting property 'unknown' is not valid",
       "description": [
           "Supported domain sorting properties are:"
           "'aproperty', 'anotherproperty'"
       ]

   }

     Figure 4: Example of RDAP Error Response Due to an Invalid Domain
                  Sorting Property Included in the Request

4.  Implementation Considerations

   Implementation of the new parameters is technically feasible, as
   operators for counting, sorting, and paging are currently supported
   by the major relational database management systems.  Similar
   operators are completely or partially supported by the most well-
   known NoSQL databases (e.g., MongoDB, CouchDB, HBase, Cassandra,
   Hadoop, etc.).  Additional implementation notes are included in
   Appendix C.

5.  IANA Considerations

   IANA has registered the following values in the "RDAP Extensions"
   registry:

   Extension identifier:  paging
   Registry operator:  Any
   Published specification:  RFC 8977
   Contact:  IETF <iesg@ietf.org>
   Intended usage:  This extension describes a best practice for result
      set paging.

   Extension identifier:  sorting
   Registry operator:  Any
   Published specification:  RFC 8977
   Contact:  IETF <iesg@ietf.org>
   Intended usage:  This extension describes a best practice for result
      set sorting.

6.  Security Considerations

   Security services for the operations specified in this document are
   described in [RFC7481].

   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 either
   restricting search functionality or limiting the rate of search
   requests.  Servers can also reduce their load by truncating the
   results in a response.  However, this last security policy can result
   in a higher inefficiency or risk due to acting on incomplete
   information if the RDAP server does not provide any functionality to
   return the truncated results.

   The new parameters presented in this document provide RDAP operators
   with a way to implement a server that reduces inefficiency risks.
   The "count" parameter gives the client the ability to evaluate the
   completeness of a response.  The "sort" parameter allows the client
   to obtain the most relevant information at the beginning of the
   result set.  This can reduce the number of unnecessary search
   requests.  Finally, the "cursor" parameter enables the user to scroll
   the result set by submitting a sequence of sustainable queries within
   server-acceptable limits.

7.  References

7.1.  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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [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>.

   [RFC6350]  Perreault, S., "vCard Format Specification", RFC 6350,
              DOI 10.17487/RFC6350, August 2011,
              <https://www.rfc-editor.org/info/rfc6350>.

   [RFC7095]  Kewisch, P., "jCard: The JSON Format for vCard", RFC 7095,
              DOI 10.17487/RFC7095, January 2014,
              <https://www.rfc-editor.org/info/rfc7095>.

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc7231>.

   [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>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

   [RFC8605]  Hollenbeck, S. and R. Carney, "vCard Format Extensions:
              ICANN Extensions for the Registration Data Access Protocol
              (RDAP)", RFC 8605, DOI 10.17487/RFC8605, May 2019,
              <https://www.rfc-editor.org/info/rfc8605>.

7.2.  Informative References

   [CURSOR]   Nimesh, R., "Paginating Real-Time Data with Cursor Based
              Pagination", July 2014, <https://www.sitepoint.com/
              paginating-real-time-data-cursor-based-pagination/>.

   [CURSOR-API1]
              Facebook, "Facebook for Developers -- Using the Graph
              API", <https://developers.facebook.com/docs/graph-api/
              using-graph-api>.

   [CURSOR-API2]
              Twitter, "Twitter Ads API",
              <https://developer.twitter.com/en/docs/twitter-ads-api>.

   [GOESSNER-JSON-PATH]
              Goessner, S., "JSONPath - XPath for JSON", February 2007,
              <https://goessner.net/articles/JsonPath/>.

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

   [JSONPATH-COMPARISON]
              "JSONPath Comparison",
              <https://cburgmer.github.io/json-path-comparison/>.

   [JSONPATH-WG]
              IETF, "JSON Path (jsonpath)",
              <https://datatracker.ietf.org/wg/jsonpath/about/>.

   [ODATA-PART1]
              Pizzo, M., Handl, R., and M. Zurmuehl, "OData Version 4.0.
              Part 1: Protocol Plus Errata 03", June 2016,
              <https://docs.oasis-
              open.org/odata/odata/v4.0/errata03/os/complete/part1-
              protocol/odata-v4.0-errata03-os-part1-protocol-
              complete.pdf>.

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

   [RFC6901]  Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
              "JavaScript Object Notation (JSON) Pointer", RFC 6901,
              DOI 10.17487/RFC6901, April 2013,
              <https://www.rfc-editor.org/info/rfc6901>.

   [SEEK]     EverSQL, "Faster Pagination in Mysql - Why Order By With
              Limit and Offset is Slow?", July 2017,
              <https://www.eversql.com/faster-pagination-in-mysql-why-
              order-by-with-limit-and-offset-is-slow/>.

   [W3C.CR-xpath-31-20170321]
              Robie, J., Dyck, M., and J. Spiegel, "XML Path Language
              (XPath) 3.1", World Wide Web Consortium Recommendation
              REC-xpath-31-20170321, March 2017,
              <https://www.w3.org/TR/2017/REC-xpath-31-20170321/>.

Appendix A.  JSONPath Operators

   The jsonpaths used in this document are provided according to the
   Goessner proposal [GOESSNER-JSON-PATH].

   Such specification requires that implementations support a set of
   "basic operators".  These operators are used to access the elements
   of a JSON structure like objects and arrays, as well as their
   subelements (object members and array items, respectively).  No
   operations are defined for retrieving parent or sibling elements of a
   given element.  The root element is always referred to as $
   regardless of it being an object or array.

   Additionally, the specification permits implementations to support
   arbitrary script expressions.  These can be used to index into an
   object or array, or to filter elements from an array.  While script
   expression behavior is implementation-defined, most implementations
   support the basic relational and logical operators as well as both
   object member and array item access, sufficiently similar for the
   purpose of this document.  Commonly supported operators/functions
   divided into "top-level operators" and "filter operators" are
   documented in Tables 2 and 3, respectively.

   For more information on implementation interoperability issues, see
   [JSONPATH-COMPARISON].  At the time of writing, work is beginning on
   a standardization effort too (see [JSONPATH-WG]).

      +===================+=========================================+
      | Operator          | Description                             |
      +===================+=========================================+
      | $                 | Root element                            |
      +-------------------+-----------------------------------------+
      | .<name>           | Object member access (dot-notation)     |
      +-------------------+-----------------------------------------+
      | ['<name>']        | Object member access (bracket-notation) |
      +-------------------+-----------------------------------------+
      | [<number>]        | Array item access                       |
      +-------------------+-----------------------------------------+
      | *                 | All elements within the specified scope |
      +-------------------+-----------------------------------------+
      | [?(<expression>)] | Filter expression                       |
      +-------------------+-----------------------------------------+

                   Table 2: JSONPath Top-Level Operators

      +====================+========================================+
      | Operator           | Description                            |
      +====================+========================================+
      | @                  | Current element being processed        |
      +--------------------+----------------------------------------+
      | .<name>            | Object member access                   |
      +--------------------+----------------------------------------+
      | .[<name1>,<name2>] | Union of object members                |
      +--------------------+----------------------------------------+
      | [<number>]         | Array item access                      |
      +--------------------+----------------------------------------+
      | ==                 | Left is equal to right                 |
      +--------------------+----------------------------------------+
      | !=                 | Left is not equal to right             |
      +--------------------+----------------------------------------+
      | <                  | Left is less than right                |
      +--------------------+----------------------------------------+
      | <=                 | Left is less than or equal to right    |
      +--------------------+----------------------------------------+
      | >                  | Left is greater than right             |
      +--------------------+----------------------------------------+
      | >=                 | Left is greater than or equal to right |
      +--------------------+----------------------------------------+
      | &&                 | Logical conjunction                    |
      +--------------------+----------------------------------------+
      | ||                 | Logical disjunction                    |
      +--------------------+----------------------------------------+

                     Table 3: JSONPath Filter Operators

Appendix B.  Approaches to Result Pagination

   An RDAP query could return a response with hundreds, even thousands,
   of objects, especially when partial matching is used.  For this
   reason, the "cursor" parameter addressing result pagination is
   defined to make responses easier to handle.

   Presently, the most popular methods to implement pagination in a REST
   API include offset pagination and keyset pagination.  Neither
   pagination method requires the server to handle the result set in a
   storage area across multiple requests since a new result set is
   generated each time a request is submitted.  Therefore, they are
   preferred to any other method requiring the management of a REST
   session.

   Using limit and offset operators represents the traditionally used
   method to implement result pagination.  Both of them can be used
   individually:

   "limit=N":  means that the server returns the first N objects of the
      result set

   "offset=N":  means that the server skips the first N objects and
      returns objects starting from position N+1

   When limit and offset are used together, they provide the ability to
   identify a specific portion of the result set.  For example, the pair
   "offset=100,limit=50" returns the first 50 objects starting from
   position 101 of the result set.

   Though easy to implement, offset pagination also includes drawbacks:

   *  When offset has a very high value, scrolling the result set could
      take some time.

   *  It always requires fetching all rows before dropping as many rows
      as specified by offset.

   *  It may return inconsistent pages when data are frequently updated
      (i.e., real-time data).

   Keyset pagination [SEEK] adds a query condition that enables the
   selection of the only data not yet returned.  This method has been
   taken as the basis for the implementation of a "cursor" parameter
   [CURSOR] by some REST API providers [CURSOR-API1] [CURSOR-API2].  The
   cursor is a URL-safe string opaque to the client and representing a
   logical pointer to the first result of the next page.

   Nevertheless, even keyset pagination can be troublesome:

   *  It needs at least one key field.

   *  It does not allow sorting simply by any field because the sorting
      criterion must contain a key.

   *  It works best with full composite values supported by database
      management systems (i.e., [x,y]>[a,b]); emulation is possible but
      inelegant and less efficient.

   *  It does not allow direct navigation to arbitrary pages because the
      result set must be scrolled in sequential order starting from the
      initial page.

   *  Implementing bidirectional navigation is tedious because all
      comparison and sort operations have to be reversed.

B.1.  Specific Issues Raised by RDAP

   Some additional considerations can be made in the RDAP context:

   *  An RDAP object is a conceptual aggregation of information
      generally collected from more than one data structure (e.g.,
      table), and this makes it even harder to implement keyset
      pagination, a task that is already quite difficult.  For example,
      the entity object can include information from different data
      structures (registrars, registrants, contacts, resellers), each
      one with its key field mapping the RDAP entity handle.

   *  Depending on the number of page results as well as the number and
      the complexity of the properties of each RDAP object in the
      response, the time required by offset pagination to skip the
      previous pages could be much faster than the processing time
      needed to build the current page.  In fact, RDAP objects are
      usually formed by information belonging to multiple data
      structures and containing multivalued properties (i.e., arrays);
      therefore, data selection might be a time-consuming process.  This
      situation occurs even though the selection is supported by
      indexes.

   *  Depending on the access levels defined by each RDAP operator, the
      increase in complexity and the decrease in flexibility of keyset
      pagination in comparison to offset pagination could be considered
      impractical.

   Ultimately, both pagination methods have benefits and drawbacks.

Appendix C.  Implementation Notes

   This section contains an overview of the main choices made during the
   implementation of the capabilities defined in this document in the
   RDAP public test server of Registro.it at the Institute of
   Informatics and Telematics of the National Research Council (IIT-
   CNR).  The content of this section can represent guidance for
   implementers who plan to provide RDAP users with those capabilities.
   The RDAP public test server can be accessed at
   <https://rdap.pubtest.nic.it/>.  Further documentation about the
   server features is available at <https://rdap.pubtest.nic.it/doc/
   README.html>.

C.1.  Sorting

   If no sort criterion is specified in the query string, the results
   are sorted by a default property: "name" for domains and nameservers,
   and "handle" for entities.  The server supports multiple property
   sorting but the "sorting_metadata" object includes only the links to
   alternative result set views sorted by a single property just to show
   the list of sorting properties allowed for each searchable object.
   The server supports all the object-specific sorting properties
   described in the specification except for nameserver sorting based on
   unicodeName, that is, the "name" sorting property is mapped onto the
   "ldhName" response field.  Regarding the object-common properties,
   sorting by registrationDate, expirationDate, lastChangedDate, and
   transferDate is supported.

C.2.  Counting

   The counting operation is implemented through a separate query.  Some
   relational database management systems support custom operators to
   get the total count together with the rows, but the resulting query
   can be considerably more expensive than that performed without the
   total count.  Therefore, as "totalCount" is an optional response
   information, always fetching the total number of rows has been
   considered an inefficient solution.  Furthermore, to avoid the
   processing of unnecessary queries, when the "count" parameter is
   included in the submitted query, it is not also repeated in the query
   strings of the "links" array provided in both "paging_metadata" and
   "sorting_metadata" objects.

C.3.  Paging

   The server implements the cursor pagination through the keyset
   pagination when sorting by a unique property is requested or the
   default sort is applied.  Otherwise, it implements the cursor
   pagination through the offset pagination.  As most relational
   database management systems don't support the comparison of full
   composite values natively, the implementation of full keyset
   pagination seem to be troublesome so, at least initially, a selective
   applicability of keyset pagination is advisable.  Moreover, the
   "cursor" value encodes not only information about pagination but also
   about the search pattern and the other query parameters in order to
   check the consistency of the entire query string.  If the "cursor"
   value is inconsistent with the rest of the query string, the server
   returns an error response.

Acknowledgements

   The authors would like to acknowledge Brian Mountford, Tom Harrison,
   Karl Heinz Wolf, Jasdip Singh, Erik Kline, Éric Vyncke, Benjamin
   Kaduk, and Roman Danyliw for their contributions to the development
   of this document.

Authors' Addresses

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

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

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

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

   Scott Hollenbeck
   Verisign Labs
   12061 Bluemont Way
   Reston, VA 20190
   United States of America

   Email: shollenbeck@verisign.com
   URI:   https://www.verisignlabs.com/