Registration Protocols Extensions                            M. Loffredo
Internet-Draft                                             M. Martinelli
Intended status: Standards Track                     IIT-CNR/Registro.it
Expires: August 5, October 13, 2019                                  S. Hollenbeck
                                                           Verisign Labs
                                                        February 1,
                                                          April 11, 2019

  Registration Data Access Protocol (RDAP) Query Parameters for Result
                           Sorting and Paging
              draft-ietf-regext-rdap-sorting-and-paging-00
              draft-ietf-regext-rdap-sorting-and-paging-01

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 response
   values.  This document describes RDAP query extensions that allow
   clients to specify their preferences for sorting and paging result
   sets.

Status of This Memo

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

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   This Internet-Draft will expire on August 5, October 13, 2019.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   4
   2.  RDAP Query Parameter Specification  . . . . . . . . . . . . .   4
     2.1.  Sorting and Paging Metadata . . . . . . . . . . . . . . .   4
     2.2.  "count" Parameter . . . . . . . . . . . . . . . . . . . .   6
     2.3.  "sort" Parameter  . . . . . . . . . . . . . . . . . . . .   7
       2.3.1.  Representing Sorting Links  . . . . . . . . . . . . .  11
     2.4.  "limit" and "offset" Parameters . . . . . . . . . . . . .  12
       2.4.1.  Representing Paging Offset Pagination Links  . . . . . . . .  13
     2.5.  "cursor" Parameter  . . . . . .  13 . . . . . . . . . . . . .  14
       2.5.1.  Representing Cursor Pagination Links  . . . . . . . .  16
   3.  Negative Answers  . . . . . . . . . . . . . . . . . . . . . .  14  17
   4.  RDAP Conformance  . . . . . . . . . . . . . . . . . . . . . .  15  18
   5.  Implementation Considerations . . . . . . . . . . . . . . . .  15
     5.1.  Considerations about Paging Implementation  . . . . . . .  16  18
   6.  Implementation Status . . . . . . . . . . . . . . . . . . . .  19
     6.1.  IIT-CNR/Registro.it . . . . . . . . . . . . . . . . . . .  19  20
     6.2.  Google Registry . . . . . . . . . . . . . . . . . . . . .  19  20
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20  21
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20  21
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20  21
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     10.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  24  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24  25

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:

   o  reducing the server response bandwidth requirements;
   o  improving server response time;
   o  improving query precision and, consequently, obtaining more
      reliable results;
   o  decreasing server query processing load;
   o  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 (i.e.  ODATA protocol
       [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.

   However, there are some drawbacks associated with 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 in the cookies, while the information on the resource
   identification or the search type is included in the query string.
   The second approach is therefore not compliant with the HTTP standard
   [RFC7230].  As a result, this document describes a specification
   based on use of query parameters.

   Currently the RDAP protocol [RFC7482] defines two query types:

   o  lookup: the server returns only one object;
   o  search: the server returns a collection of objects.

   While the lookup query does not raise issues in the management of
   large result sets, the search query can potentially generate a large
   result set that could be truncated according to the limits of the
   server.  In addition, it is not possible to obtain the total number
   of the 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 could traverse the whole result set to
   find the relevant objects or, due to truncation, could not find them
   at all.

   The protocol described in this specification 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 RFC 7480 [RFC7480].

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

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  RDAP Query Parameter Specification

   The new query parameters are OPTIONAL extensions of path segments
   defined in RFC 7482 [RFC7482].  They are as follows:

   o  "count": a boolean value that allows a client to request the total
      number of objects found (that due to truncation can be different
      from the number of returned objects);

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

   o  "limit" and "offset": numeric values that allow a client to
      request a specific portion of the entire result set;

   o  "cursor": a string value representing a pointer to a specific
      fixed size portion of the result set.  It is alternate to "limit"
      and "offset".

   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 the server-provided links to dynamically discover
   available actions and access the resources it needs.  In this way,
   the client is not requested to have prior knowledge of the service
   and, consequently, to hard code the URIs of different resources.
   This would allow the server to make URI changes as the API evolves
   without breaking the clients.  Definitively, a REST service should be
   as self-descriptive as possible.

   Therefore, the implementation of servers implementing the query parameters described in
   this specification recommends servers to SHOULD provide additional information in their
   responses about both the available sorting criteria and the possible
   pagination.  Such information is collected in two new data structures
   named, respectively, "sorting_metadata" and "paging_metadata".

   Obviously, both the new data structures are OPTIONAL because their
   presence in the response not only depends on the implementation of
   sorting and paging query capabilities but also on some situations
   related to the results.  For example, it is quite natural to expect
   that the "paging_metadata" section element will not be present at the last
   result page when the server implements only the forward pagination.

   The "sorting_metadata" structure contains the following fields: properties:

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

   o  "availableSorts": "AvailableSort[]" (OPTIONAL) an array of objects
      each one describing an alternate available sorting criterion: criterion.
      Members are:

      *  "property": "String" (REQUIRED) the name that can be used by
         the client to request the sorting criterion;
      *  "default": "Boolean" (REQUIRED) whether the sorting criterion
         is applied by default;
      *  "jsonPath": "String" (OPTIONAL) the JSON Path of the RDAP field
         corresponding to the property;
      *  "default": whether the sorting criterion is applied by default;
      *  "links": "Link[]" (OPTIONAL) an array of links as described in
         RFC 8288 [RFC8288] containing the query string that applies the
         sorting criterion.

   Both

   At least one between "currentSort" and "availableSorts" are OPTIONAL fields of the
   "sorting_metadata" structure.  In particular, the "currentSort" field
   is provided when the query string contains a valid value for sort
   parameter, while the "availableSorts" field SHOULD MUST be provided when
   the sort parameter is missing in the query string or when it is
   present and the server implements more than a sorting criterion for
   the RDAP object.  At least the "property" field is REQUIRED in each
   item of the "availableSorts" array while the other fields are
   RECOMMENDED.
   present.

   The "paging_metadata" structure contains the following fields:

   o  "totalCount": "Numeric" (OPTIONAL) a numeric value representing
      the total number of objects found; found.  It is provided if the query
      string contains the "count" parameter;

   o  "pageCount": "Numeric" (OPTIONAL) a numeric value representing the
      number of objects returned in the current page;

   o  "offset": a numeric value identifying page.  It is provided
      when the start total number of current objects exceeds the page in size.  This
      property is redundant for clients because the result set;

   o  "nextOffset": a numeric value identifying page size can be
      derived from the start length of the next
      page in the result set or null search results array but it can be
      helpful if the result set has been
      completely scrolled; end user interacts with the server through a web
      browser;

   o  "links": "Link[]" (OPTIONAL) an array of links as described in RFC
      8288 [RFC8288] containing the reference to next page.

   Only the "pageCount" field is REQUIRED in the "paging_metadata"
   structure.  The other fields appear when pagination occurs.  In this
      specification, only the forward pagination is dealt because it is
      considered satisfactory in order to traverse the result set.  If a
   server should also implement backward pagination, an appropriate
   field (e.g. "prevOffset") identifying the start

      Examples of additional references are to: the previous page
   is RECOMMENDED.  Finally, the "totalCount" field is provided if the
   query string contains page, the count parameter.

   FOR DISCUSSION: Should
      first page, the metadata described in this specification last page.

   At least one between "totalCount" and "links" MUST be part of a more general "metadata" section including other contents
   (e.g rate limits, information about the server, information about the
   response, metadata information related to other parameters)? present.

2.2.  "count" Parameter

   Currently the RDAP protocol does not allow a client to determine the
   total number of the results in a query response when the result set
   is truncated.  This is rather inefficient because the user cannot
   evaluate the query precision and, at the same time, cannot receive
   information that could be relevant.

   The count "count" parameter provides additional functionality (Figure 1)
   that allows a client to request information from the server that
   specifies the total number of elements matching a particular search
   pattern.

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

      Figure 1: Example of RDAP query reporting the count "count" parameter

   The ABNF syntax is the following:

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

   A trueValue means that the server MUST provide the total number of
   the objects in the "totalCount" field of the "paging_metadata"
   section
   element (Figure 2).  A falseValue means that the server MUST NOT
   provide this number.

   {
     "rdapConformance": [
           "rdap_level_0",
           "paging_level_0"
     ],
     ...
     "paging_metadata": {
       "totalCount": 73
     },
     "domainSearchResults": [
       ...
     ]
   }

     Figure 2: Example of RDAP response with "paging_metadata" section element
                     containing the "totalCount" field

2.3.  "sort" Parameter

   The RDAP protocol does not provide any capability to specify results
   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 and paging
   features provided by the RDAP server could help avoid truncation of
   relevant results and allow for scrolling the result set using
   subsequent queries.

   The sort "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" EQ sortItem *( "," sortItem )
      sortItem = property-ref [":" ( "a" / "d" ) ]

   "a" means that the ascending sort MUST be applied, "d" means that the
   descending sort MUST be applied.  If the sort direction is absent, an
   ascending sort MUST be applied (Figure 3).

   In the sort "sort" parameter ABNF syntax, property-ref represents a
   reference to a property of an RDAP object.  Such a reference could be
   expressed by using a JSON Path.  The JSON Path in a JSON document
   [RFC8259] is equivalent to the XPath [W3C.CR-xpath-31-20161213] in a
   XML document.  For example, the JSON Path to select the value of the
   ASCII name inside an RDAP domain object is "$.ldhName", where $
   identifies the root of the document (DOM).  Another way to select a
   value inside a JSON document is the JSON Pointer [RFC6901].  While
   JSON Path or JSON Pointer are both standard ways to select any value
   inside JSON data, neither is particularly easy to use (e.g.
   "$.events[?(@.eventAction='registration')].eventDate" is the JSON
   Path expression of the registration date in a an RDAP domain object).

   Therefore, this specification provides a definition of property-ref
   in terms of RDAP properties.  However, not all the RDAP properties
   are suitable to be used in sort criteria, such as:

   o  properties providing service information (e.g. links, notices,
      remarks, etc.);
   o  multivalued properties (e.g. status, roles, variants, etc.);
   o  properties modeling relationships to other objects (e.g.
      entities).

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

   In the following, a list of the proposed properties for sort criteria an RDAP server MAY implement
   is presented.  The properties are divided in two groups: object
   common properties and object specific properties.

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

      *  registrationDate
      *  reregistrationDate
      *  lastChangedDate
      *  expirationDate
      *  deletionDate
      *  reinstantiationDate
      *  transferDate
      *  lockedDate
      *  unlockedDate

   o  Object specific properties.  With regard to the specific
      properties, some of them are already defined among the query
      paths.  In the following the list of the proposed sorting
      properties, grouped by objects, is shown:

      *  Domain: ldhName
      *  Nameserver: ldhName, ipV4, ipV6.
      *  Entity: fn, handle, org, email, voice, country, city.

   In the following, the correspondence between the sorting properties
   and the RDAP fields is shown (Table 1):

   +------------+------------+---------------+------------+------------+
   | Object     | Sorting    | RDAP property | Reference  | Reference  |
   | class      | property   |               | in RFC     | in RFC     |
   |            |            |               | 7483       | 6350       |
   +------------+------------+---------------+------------+------------+
   | Searchable | Common     | eventAction   | 4.5.       |            |
   | objects    | properties | values        |            |            |
   |            |            | suffixed by   |            |            |
   |            |            | "Date"        |            |            |
   |            |            |               |            |            |
   | Domain     | ldhName    | ldhName       | 5.3.       |            |
   |            |            |               |            |            |
   | Nameserver | ldhName    | ldhName       | 5.2.       |            |
   |            | ipV4       | v4 ipAddress  | 5.2.       |            |
   |            | ipV6       | v6 ipAddress  | 5.2.       |            |
   |            |            |               |            |            |
   | Entity     | handle     | handle        | 5.1.       |            |
   |            | fn         | vcard fn      | 5.1.       | 6.2.1      |
   |            | org        | vcard org     | 5.1.       | 6.6.4      |
   |            | voice      | vcard tel     | 5.1.       | 6.4.1      |
   |            |            | with          |            |            |
   |            |            | type="voice"  |            |            |
   |            | email      | vcard email   | 5.1.       | 6.4.2      |
   |            | country    | country name  | 5.1.       | 6.3.1      |
   |            |            | in vcard adr  |            |            |
   |            | city       | locality in   | 5.1.       | 6.3.1      |
   |            |            | vcard adr     |            |            |
   +------------+------------+---------------+------------+------------+

                  Table 1: Sorting properties definition

   With regard to the definitions in Table 1, some further
   considerations must be made to disambiguate cases where the RDAP
   property is multivalued:

   o  Even if a nameserver can have multiple IPv4 and IPv6 addresses,
      the most common configuration includes one address for each IP
      version.  Therefore, the assumption of having a single IPv4 and/or
      IPv6 value for a nameserver cannot be considered too stringent.

   o  With the exception of handle values, all the sorting properties
      defined for entity objects can be multivalued according to the
      definition of vCard as given in RFC6350 [RFC6350].  When more than
      a value is reported, sorting can be applied to the preferred value
      identified by the parameter pref="1".

   Each RDAP provider MAY define other sorting properties than those
   shown in this document.

   The "jsonPath" field in the "sorting_metadata" section is used to
   clarify the RDAP field the sorting property refers to.  In the
   following, the mapping between the sorting properties and the JSON
   Paths of the RDAP fields is shown (Table 2).  The JSON Paths are
   provided according to the Goessner v.0.8.0 specification
   ([GOESSNER-JSON-PATH]):

   +-------+-------------+---------------------------------------------+
   | Objec | Sorting     | JSON Path                                   |
   | t     | property    |                                             |
   | class |             |                                             |
   +-------+-------------+---------------------------------------------+
   | Searc | registratio | "$.domainSearchResults[*].events[?(@.eventA |
   | hable | nDate       | ction=="registration")].eventDate           |
   | objec |             |                                             |
   | ts    |             |                                             |
   |       | reregistrat | "$.domainSearchResults[*].events[?(@.eventA |
   |       | ionDate     | ction=="reregistration")].eventDate         |
   |       | lastChanged | "$.domainSearchResults[*].events[?(@.eventA |
   |       | Date        | ction=="lastChanged")].eventDate            |
   |       | expirationD | "$.domainSearchResults[*].events[?(@.eventA |
   |       | ate         | ction=="expiration")].eventDate             |
   |       | deletionDat | "$.domainSearchResults[*].events[?(@.eventA |
   |       | e           | ction=="deletion")].eventDate               |
   |       | reinstantia | "$.domainSearchResults[*].events[?(@.eventA |
   |       | tionDate    | ction=="reinstantiation")].eventDate        |
   |       | transferDat | "$.domainSearchResults[*].events[?(@.eventA |
   |       | e           | ction=="transfer")].eventDate               |
   |       | lockedDate  | "$.domainSearchResults[*].events[?(@.eventA |
   |       |             | ction=="locked")].eventDate                 |
   |       | unlockedDat | "$.domainSearchResults[*].events[?(@.eventA |
   |       | e           | ction=="unlocked")].eventDate               |
   |       |             |                                             |
   | Domai | ldhName     | $.domainSearchResults[*].ldhName            |
   | n     |             |                                             |
   |       |             |                                             |
   | Names | ldhName     | $.nameserverSearchResults[*].ldhName        |
   | erver |             |                                             |
   |       | ipV4        | $.nameserverSearchResults[*].ipAddresses.v4 |
   |       |             | [0]                                         |
   |       | ipV6        | $.nameserverSearchResults[*].ipAddresses.v6 |
   |       |             | [0]                                         |
   |       |             |                                             |
   | Entit | handle      | $.entitySearchResults[*].handle             |
   | y     |             |                                             |
   |       | 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]                           |
   |       | city        | $.entitySearchResults[*].vcardArray[1][?(@[ |
   |       |             | 0]=="adr")][3][3]                           |
   +-------+-------------+---------------------------------------------+

              Table 2: Sorting properties - JSON Path Mapping

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

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

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

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

      Figure 3: Examples of RDAP query reporting the sort "sort" parameter

2.3.1.  Representing Sorting Links

   An RDAP server MAY use the "links" array of the "sorting_metadata"
   section to provide ready-made references [RFC8288] to the available
   sort criteria (Figure 4).  Each link represents a reference to an
   alternate view of the results.

{
  "rdapConformance": [
    "rdap_level_0",
    "sorting_level_0"
  ],
  ...
  "sorting_metadata": {
     "currentSort": "ldhName",
     "availableSorts": [
     {
     "property": "registrationDate",
     "jsonPath": "$.domainSearchResults[*].events[?(@.eventAction==\"registration\")].eventDate",
     "default": false,
     "links": [
        {
        "value": "https://example.com/rdap/domains?name=*nr.com
                  &sort=ldhName",
        "rel": "alternate",
        "href": "https://example.com/rdap/domains?name=*nr.com
                 &sort=registrationDate",
        "title": "Result Ascending Sort Link",
        "type": "application/rdap+json"
        },
        {
        "value": "https://example.com/rdap/domains?name=*nr.com
                  &sort=ldhName",
        "rel": "alternate",
        "href": "https://example.com/rdap/domains?name=*nr.com
                 &sort=registrationDate:d",
        "title": "Result Descending Sort Link",
        "type": "application/rdap+json"
        }
     ]
  },
  "domainSearchResults": [
    ...
  ]
}

      Figure 4: Example of a "sorting_metadata" instance to implement
                              result sorting

2.4.  "limit" and "offset" Parameters

   An RDAP query could return a response with hundreds of objects,
   especially when partial matching is used.  For that reason, two
   parameters addressing result pagination are defined to make responses
   easier to handle:

   o  "limit": means that the server MUST return the first N objects of
      the result set in the response;
   o  "offset": means that the server MUST skip the first N objects and
      MUST return objects starting from position N+1.

   The ABNF syntax is the following:

      EQ = "="
      limit = "limit" EQ positive-number
      offset = "offset" EQ positive-number
      positive-number = non-zero-digit *digit
      non-zero-digit = "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" /
      "9"
      digit = "0" / non-zero-digit

   When limit and offset are used together, they allow implementation of
   result pagination.  The following examples illustrate requests to
   return, respectively, the first 5 objects, the set of objects
   starting from position 6, and first 5 objects starting from position
   11 of the result set (Figure 5).

   https://example.com/rdap/domains?name=*nr.com&limit=5

   https://example.com/rdap/domains?name=*nr.com&offset=5

   https://example.com/rdap/domains?name=*nr.com&limit=5&offset=10

    Figure 5: Examples of RDAP query reporting the limit "limit" and offset "offset"
                                parameters

2.4.1.  Representing Paging Offset Pagination Links

   An RDAP server MAY SHOULD use the "links" array of the "paging_metadata"
   section
   element to provide a ready-made reference [RFC8288] to the next page
   of the result set (Figure 6).  Examples of additional "rel" values a
   server MAY implements are "first", "last", "prev".

   {
     "rdapConformance": [
       "rdap_level_0",
       "paging_level_0"
     ],
     ...
     "notices": [
       {
         "title": "Search query limits",
         "type": "result set truncated due to excessive load",
         "description": [
         "search results for domains are limited to 10"
         ]
       }
     ],
     "paging_metadata": {
       "totalCount": 73,
       "pageCount": 10,
       "offset": 10,
       "nextOffset": 20,
       "links": [
         {
         "value": "https://example.com/rdap/domains?name=*nr.com",
         "rel": "next",
         "href": "https://example.com/rdap/domains?name=*nr.com&limit=10
                   &offset=10",
         "title": "Result Pagination Link",
         "type": "application/rdap+json"
         }
       ]
     },
     "domainSearchResults": [
       ...
     ]
   }

   Figure 6: Example of a "paging_metadata" instance to implement result
                   pagination based on offset and limit

3.  Negative Answers

2.5.  "cursor" Parameter

   The value constraints for use of limit and offset operators represents the parameters most common way
   to implement results pagination.  However, when offset has a very
   high value, scrolling the result set could take some time.  In
   addition, offset pagination may return inconsistent pages when data
   are defined by their ABNF
   syntax.  Therefore, each request providing an invalid value frequently updated (i.e. real-time data) but this is not the case
   of registration data.  An alternative approach to offset pagination
   is the keyset pagination, a.k.a. seek-method [SEEK] or cursor
   pagination.  This method has been taken as the basis for the
   implementation of a "cursor" parameter SHOULD obtain [CURSOR] by some REST API
   providers (e.g.  [CURSOR-API1],[CURSOR-API2]).  The cursor is an HTTP 400 (Bad Request) response code.  The
   same response SHOULD be returned if
   opaque URL-safe string representing a logical pointer to the client provides an
   unsupported value for first
   result of the sort parameter in both single and multi
   sort.

   The server can provide a different response when it supports next page (Figure 7).

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

    Figure 7: An example of RDAP query reporting the
   limit and/or "cursor" parameter

   However cursor pagination raises some drawbacks with respect to
   offset parameters pagination:

   o  it needs at least one key field;
   o  it does not allow to sort by any field and paginate the client submits values that are
   out of results
      because sorting has to be made on the valid ranges.  The possible cases are: key field;
   o  If the client submits a value for  it does not allow to skip pages because they have to be scrolled
      in sequential order starting from the limit parameter that is
      greater than initial page;
   o  it makes very hard the number navigation of objects the result set in both
      directions because all comparison and sort operations have to be processed, it is
      RECOMMENDED that server returns a response including only
      reversed.

   Furthermore, in the
      processed objects. RDAP context, some additional considerations can
   be made:

   o  If the client submits  an RDAP object is a value conceptual aggregation of information
      collected from more than one data structure (e.g. table) and this
      makes even harder for the offset parameter that is
      greater than developers the number implementation of objects to be processed, it is
      RECOMMENDED the
      seek-method that server returns an HTTP 404 (Not Found) response
      code.

   Optionally, is already quite difficult.  In fact, for
      example, the response MAY include additional entity object can gather information regarding
   the negative answer in from different
      data structures (registrars, registrants, contacts, resellers, and
      so on), each one with its own key field mapping the HTTP entity body.

4. RDAP Conformance

   Servers returning entity
      handle;

   o  depending on the "paging_metadata" section in their responses
   MUST include "paging_level_0" in number of the rdapConformance array page results as well as
   servers returning the "sorting_metadata" section MUST include
   "sorting_level_0".

5.  Implementation Considerations

   The implementation number
      and the complexity of the new parameters 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)
      and, therefore, data selection is technically feasible, as
   operators for counting, a time consuming process.  This
      situation occurs even though the data selection process makes use
      of indexes;

   o  depending on the access levels defined by each RDAP operator, the
      increase of complexity and the decrease of flexibility of cursor
      pagination with respect to the offset pagination could be
      considered impractical.

   Finally, the cursor pagination seems to be a bit inconsistent with
   some capabilities described in this document like sorting and the
   implementation of paging are currently supported by links other than the major RDBMSs.

   In usual "next page" link.
   But, while the following, provisioning of more paging links can be superfluous,
   dropping the match between sorting capability seems quite unreasonable.

   Ultimately, both pagination methods have benefits and drawbacks.
   Therefore, since the new defined parameters "paging_metadata" element keeps the same
   structure and clients merely make use of the SQL operators is shown (Table 3):

   +----------------+--------------------------------------------------+
   | New query      | SQL operator                                     |
   | parameter      |                                                  |
   +----------------+--------------------------------------------------+
   | count          | count(*) query without offset, limit and order   |
   |                | by                                               |
   |                | [MYSQL-COUNT],[POSTGRES-COUNT],[ORACLE-COUNT]    |
   |                |                                                  |
   | sort           | order by                                         |
   |                | [MYSQL-SORT],[POSTGRES-SORT],[ORACLE-SORT]       |
   |                |                                                  |
   | limit          | limit n (in MySql [MYSQL-LIMIT] and Postgres     |
   |                | [POSTGRES-LIMIT])                                |
   |                | FETCH FIRST n ROWS ONLY (in Oracle               |
   |                | [ORACLE-LIMIT])                                  |
   |                |                                                  |
   | offset         | offset m (in Postgres)                           |
   |                | OFFSET m ROWS (in Oracle)                        |
   |                |                                                  |
   | limit + offset | limit n offset m (in MySql and Postgres)         |
   |                | OFFSET m ROWS FETCH NEXT n ROWS ONLY (in Oracle) |
   +----------------+--------------------------------------------------+

              Table 3: New query parameters vs. SQL operators

   With regard to Oracle, Table 3 reports only one of the three methods
   that can be used to implement limit and offset parameters.  The
   others are described in [ORACLE-ROWNUM] and [ORACLE-ROW-NUMBER].

   In addition, similar operators paging links as they are completely or partially supported
   by the most known NoSQL databases (MongoDB, CouchDB, HBase,
   Cassandra, Hadoop) so the implementation of the new parameters seems
   to be practicable by servers working without the use of an RDBMS.

5.1.  Considerations about Paging Implementation

   The use of limit and offset
   provided, this RDAP specification lets operators represents the most common way to implement results pagination.  However, when offset has a high
   value, scrolling
   method according to their needs, the result set could take some time.  In addition, user access levels, the
   submitted queries.

2.5.1.  Representing Cursor Pagination Links

   The same considerations made for offset pagination may return inconsistent pages when data links are
   frequently updated (i.e. real-time data) but this is not the case of
   registration data.  An alternative approach applied
   to offset pagination is
   the keyset pagination, a.k.a. seek-method [SEEK] or cursor based
   pagination.  This method has been taken as the basis for the
   implementation of a cursor parameter [CURSOR] by some REST API
   providers (e.g.  [CURSOR-API1],[CURSOR-API2]).  The cursor parameter
   is an opaque URL-safe string representing a logical pointer to the
   first result of the next page pagination links (Figure 7). 8).

   {
     "rdapConformance": [
       "rdap_level_0",
       "paging_level_0"
     ],
     ...
     "notices": [
       {
         "title": "Search query limits",
         "type": "result set truncated due to excessive load",
         "description": [
         "search results for domains are limited to 10"
         ]
       }
     ],
     "paging_metadata": {
       "totalCount": 73,
       "pageCount": 10,
       "links": [
         {
         "value": "https://example.com/rdap/domains?name=*nr.com",
         "rel": "next",
         "href": "https://example.com/rdap/domains?name=*nr.com&limit=10 "https://example.com/rdap/domains?name=*nr.com
                 &cursor=wJlCDLIl6KTWypN7T6vc6nWEmEYe99Hjf1XY1xmqV-M=",
         "title": "Result Pagination Link",
         "type": "application/rdap+json"
         }
       ]
     },
     "domainSearchResults": [
       ...
     ]
   }

   Figure 7: 8: Example of a "paging_metadata" instance to implement keyset
                                pagination

   But keyset pagination raises some drawbacks with respect to offset
   pagination:

   o  it needs at least one key field;
   o "paging_metadata" instance to implement cursor
                                pagination

3.  Negative Answers

   The value constraints for the parameters are defined by their ABNF
   syntax.  Therefore, each request providing an invalid value for a
   parameter SHOULD obtain an HTTP 400 (Bad Request) response code.  The
   same response SHOULD be returned if the client provides an
   unsupported value for the "sort" parameter in both single and multi
   sort.

   The server can provide a different response when it does not allow to sort by any field supports the
   "limit" and/or "offset" parameters and paginate the results
      because sorting has to be made on client submits values that
   are out of the key field; valid ranges.  The possible cases are:

   o  it does not allow to skip pages because they have to be scrolled
      in sequential order starting from  If the initial page;
   o  it makes very hard client submits a value for the navigation of "limit" parameter that is
      greater than the result set in both
      directions because all comparison and sort operations have number of objects to be
      reversed.

   Furthermore, in processed, it is
      RECOMMENDED that server returns a response including only the RDAP context, some additional considerations can
   be made:
      processed objects.

   o  an RDAP object is  If the client submits a conceptual aggregation of information
      collected from more than one data structure (e.g. table) and this
      makes even harder value for the developers "offset" parameter that is
      greater than the implementation number of objects to be processed as well as an
      invalid value for the
      seek-method that "cursor" parameter, it is already quite difficult.  In fact, for
      example, RECOMMENDED that
      server returns an HTTP 404 (Not Found) response code.

   Optionally, the entity object can gather response MAY include additional information from different
      data structures (registrars, registrants, contacts, resellers, and
      so on), each one with its own key field mapping regarding
   the negative answer in the RDAP HTTP entity
      handle;

   o  depending on body.

4.  RDAP Conformance

   Servers returning the number of "paging_metadata" element in their responses
   MUST include "paging_level_0" in the page results rdapConformance array as well as
   servers returning the number
      and the complexity of the properties "sorting_metadata" element MUST include
   "sorting_level_0".

5.  Implementation Considerations

   The implementation of each RDAP object in the
      response, the time required new parameters is technically feasible, as
   operators for counting, sorting and paging are currently supported by offset pagination to skip the
      previous pages could be much faster than the processing time
      needed to build
   the current page. major RDBMSs.

   In fact, RDAP objects are
      usually formed by information belonging to multiple data
      structures and containing multivalued properties (e.g. arrays)
      and, therefore, data selection is a time consuming process.  This
      situation occurs even though the data selection process makes use
      of indexes;

   o  depending on following, the access levels match between the new defined by each RDAP operator, parameters and
   the
      increase of complexity SQL operators is shown (Table 3):

   +----------------+--------------------------------------------------+
   | New query      | SQL operator                                     |
   | parameter      |                                                  |
   +----------------+--------------------------------------------------+
   | count          | count(*) query without offset, limit and order   |
   |                | by                                               |
   |                | [MYSQL-COUNT],[POSTGRES-COUNT],[ORACLE-COUNT]    |
   |                |                                                  |
   | sort           | order by                                         |
   |                | [MYSQL-SORT],[POSTGRES-SORT],[ORACLE-SORT]       |
   |                |                                                  |
   | limit          | limit n (in MySql [MYSQL-LIMIT] and Postgres     |
   |                | [POSTGRES-LIMIT])                                |
   |                | FETCH FIRST n ROWS ONLY (in Oracle               |
   |                | [ORACLE-LIMIT])                                  |
   |                |                                                  |
   | offset         | offset m (in Postgres)                           |
   |                | OFFSET m ROWS (in Oracle)                        |
   |                |                                                  |
   | limit + offset | limit n offset m (in MySql and the decrease of flexibility of keyset
      pagination with respect Postgres)         |
   |                | OFFSET m ROWS FETCH NEXT n ROWS ONLY (in Oracle) |
   +----------------+--------------------------------------------------+

              Table 3: New query parameters vs. SQL operators

   With regard to the offset pagination could be
      considered impractical.

   Finally, the keyset pagination is not fully compliant with the
   additional RDAP capabilities proposed by this document.  In fact, the
   presence Oracle, Table 3 reports only one of a possible cursor parameter does not seem to be
   consistent with both the sorting capability and the possibility three methods
   that can be used to implement additional ready-made links besides "limit" and "offset" parameters.  The
   others are described in [ORACLE-ROWNUM] and [ORACLE-ROW-NUMBER].

   In addition, similar operators are completely or partially supported
   by the classic "next page"
   link.  But, while most known NoSQL databases (MongoDB, CouchDB, HBase,
   Cassandra, Hadoop) so the provisioning implementation of more paging links can be
   superfluous, dropping the sorting capability seems quite
   unreasonable.

   If pagination is implemented by using a cursor, both "offset" and
   "nextOffset" fields MUST not be included in the "paging_metadata"
   section.

   FOR DISCUSSION: Should RDAP specification reports both offset and
   cursor new parameters and let operators to implement pagination according seems
   to their needs, the user access levels, be practicable by servers working without the submitted queries? use of an RDBMS.

6.  Implementation Status

   NOTE: Please remove this section and the reference to RFC 7942 prior
   to publication as an RFC.

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in RFC 7942
   [RFC7942].  The description of implementations in this section is
   intended to assist the IETF in its decision processes in progressing
   drafts to RFCs.  Please note that the listing of any individual
   implementation here does not imply endorsement by the IETF.
   Furthermore, no effort has been spent to verify the information
   presented here that was supplied by IETF contributors.  This is not
   intended as, and must not be construed to be, a catalog of available
   implementations or their features.  Readers are advised to note that
   other implementations may exist.

   According to RFC 7942, "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

6.1.  IIT-CNR/Registro.it

      Responsible Organization: Institute of Informatics and Telematics
      of National Research Council (IIT-CNR)/Registro.it
      Location: https://rdap.pubtest.nic.it/
      Description: This implementation includes support for RDAP queries
      using data from the public test environment of .it ccTLD.  The
      RDAP server implements both offset and cursor based pagination (the
      latter only when sort "sort" and offset "offset" parameters are not present in
      the query string).
      Level of Maturity: This is a "proof of concept" research
      implementation.
      Coverage: This implementation includes all of the features
      described in this specification.
      Contact Information: Mario Loffredo, mario.loffredo@iit.cnr.it

6.2.  Google Registry

      Responsible Organization: Google Registry
      Location: https://www.registry.google/rdap/
      Description: This implementation includes support for RDAP queries
      for TLDs such as .GOOGLE, .HOW, .SOY, and .xn--q9jyb4c . The RDAP
      server implements cursor based pagination (the number of objects
      per page is fixed so the limit parameter is not available). pagination.  The link used to request the
      next page is included in the notice section of the response.
      Level of Maturity: Production.
      Coverage: This implementation includes the cursor "cursor" parameter
      described in this specification.
      Contact Information: Brian Mountford, mountford@google.com

7.  Security Considerations

   Security services for the operations specified in this document are
   described in RFC 7481 [RFC7481].

   Search query typically requires more server resources (such as
   memory, CPU cycles, and network bandwidth) when compared to lookup
   query.  This increases the risk of server resource exhaustion and
   subsequent denial of service due to abuse.  This risk can be
   mitigated by either restricting search functionality and limiting the
   rate of search requests.  Servers can also reduce their load by
   truncating the results in the response.  However, this last security
   policy can result in a higher inefficiency if the RDAP server does
   not provide any functionality to return the truncated results.

   The new parameters presented in this document provide the RDAP
   operators with a way to implement a secure server without penalizing
   its efficiency.  The "count" parameter gives the user a measure to
   evaluate the query precision and, at the same time, return a
   significant information.  The sort "sort" parameter allows the user to
   obtain the most relevant information at the beginning of the result
   set.  In both cases, the user doesn't need to submit further
   unnecessary search requests.  Finally, the limit "limit" and offset "offset"
   parameters enable the user to scroll the result set by submitting a
   sequence of sustainable queries according to the server limits.

8.  IANA Considerations

   This document has no actions for IANA.

9.  Acknowledgements

   The authors would like to acknowledge Brian Mountford for his
   contribution to the development of this document.

10.  References

10.1.  Normative References

   [ISO.3166.1988]
              International Organization for Standardization, "Codes for
              the representation of names of countries, 3rd edition",
              ISO Standard 3166, August 1988.

   [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>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.

   [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>.

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

   [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>.

   [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>.

10.2.  Informative References

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

   [CURSOR-API1]
              facebook.com, "facebook for developers - Using the Graph
              API", July 2017, <https://developers.facebook.com/docs/
              graph-api/using-graph-api>.

   [CURSOR-API2]
              twitter.com, "Pagination", 2017,
              <https://developer.twitter.com/en/docs/ads/general/guides/
              pagination.html>.

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

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

   [MYSQL-COUNT]
              mysql.com, "MySQL 5.7 Reference Manual, Counting Rows",
              October 2015, <https://dev.mysql.com/doc/refman/5.7/en/
              counting-rows.html>.

   [MYSQL-LIMIT]
              mysql.com, "MySQL 5.7 Reference Manual, SELECT Syntax",
              October 2015,
              <https://dev.mysql.com/doc/refman/5.7/en/select.html>.

   [MYSQL-SORT]
              mysql.com, "MySQL 5.7 Reference Manual, Sorting Rows",
              October 2015, <https://dev.mysql.com/doc/refman/5.7/en/
              sorting-rows.html>.

   [OData-Part1]
              Pizzo, M., Handl, R., and M. Zurmuehl, "OData Version 4.0.
              Part 1: Protocol Plus Errata 03", June 2016,
              <http://docs.oasis-
              open.org/odata/odata/v4.0/errata03/os/complete/part1-
              protocol/
              odata-v4.0-errata03-os-part1-protocol-complete.pdf>.

   [ORACLE-COUNT]
              Oracle Corporation, "Database SQL Language Reference,
              COUNT", March 2016,
              <http://docs.oracle.com/database/122/SQLRF/COUNT.htm>.

   [ORACLE-LIMIT]
              Oracle Corporation, "Database SQL Language Reference,
              SELECT, Row limiting clause", March 2016,
              <http://docs.oracle.com/database/122/SQLRF/SELECT.htm>.

   [ORACLE-ROW-NUMBER]
              Oracle Corporation, "Database SQL Language Reference,
              SELECT, ROW_NUMBER", March 2016,
              <http://docs.oracle.com/database/122/SQLRF/
              ROW_NUMBER.htm#SQLRF06100>.

   [ORACLE-ROWNUM]
              Oracle Corporation, "Database SQL Language Reference,
              SELECT, ROWNUM Pseudocolumn", March 2016,
              <http://docs.oracle.com/database/122/SQLRF/
              ROWNUM-Pseudocolumn.htm#SQLRF00255>.

   [ORACLE-SORT]
              Oracle Corporation, "Database SQL Language Reference,
              SELECT, Order by clause", March 2016,
              <http://docs.oracle.com/database/122/SQLRF/SELECT.htm>.

   [POSTGRES-COUNT]
              postgresql.org, "PostgresSQL, Aggregate Functions",
              September 2016,
              <https://www.postgresql.org/docs/9.6/static/
              functions-aggregate.html>.

   [POSTGRES-LIMIT]
              postgresql.org, "PostgresSQL, LIMIT and OFFSET", September
              2016, <https://www.postgresql.org/docs/9.6/static/
              queries-limit.html>.

   [POSTGRES-SORT]
              postgresql.org, "PostgresSQL, Sorting Rows", September
              2016, <https://www.postgresql.org/docs/9.6/static/
              queries-order.html>.

   [REST]     Fredrich, T., "RESTful Service Best Practices,
              Recommendations for Creating Web Services", April 2012,
              <http://www.restapitutorial.com/media/
              RESTful_Best_Practices-v1_1.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>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

   [SEEK]     EverSQL.com, "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-31-20161213]
              Robie, J., Dyck, M., and J. Spiegel, "XML Path Language
              (XPath) 3.1", World Wide Web Consortium CR CR-xpath-
              31-20161213, December 2016,
              <https://www.w3.org/TR/2016/CR-xpath-31-20161213>.

Appendix A.  Change Log

   00:  Initial working group version ported from draft-loffredo-regext-
      rdap-sorting-and-paging-05
   01:  Removed both "offset" and "nextOffset" to keep "paging_metadata"
      consistent between the pagination methods.  Renamed
      "Considerations about Paging Implementation" section in ""cursor"
      Parameter".  Removed "FOR DISCUSSION" items.  Provided a more
      detailed description of both "sorting_metadata" and
      "paging_metadata" objects.

Authors' Addresses

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

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

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

   Email: maurizio.martinelli@iit.cnr.it
   URI:   http://www.iit.cnr.it
   Scott Hollenbeck
   Verisign Labs
   12061 Bluemont Way
   Reston, VA  20190
   USA

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