Internet-Draft RDAP Sorting and Paging October 2020
Loffredo, et al. Expires 5 April 2021 [Page]
Registration Protocols Extensions
Intended Status:
Standards Track
M. Loffredo
M. Martinelli
S. Hollenbeck
Verisign Labs

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


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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 5 April 2021.

Table of Contents

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:

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. 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 [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 to have a prior knowledge of the server implementation which is widely considered a REST design anti-pattern. As a result, this document describes a specification based on the use of query parameters.

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

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:

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 not required to have prior knowledge of the service and, 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 "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) 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, 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 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:*.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 the objects in the "totalCount" field of the "paging_metadata" element (Figure 1). A falseValue means that the server MUST NOT provide this number.

  "rdapConformance": [
  "paging_metadata": {
    "totalCount": 43
  "domainSearchResults": [
Figure 1: Example of RDAP response with "paging_metadata" element containing the "totalCount" field

2.3. "sort" Parameter

The RDAP protocol 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 including the "sort" parameter:*.com&sort=name*.com&sort=registrationDate:d*.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: 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. Builtin 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-31-20161213], 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 or JSON Pointer are both commonly adopted notations to select any value inside JSON data, neither is particularly coincise and easy to use (e.g. "$.domainSearchResults[*].events[?(@.eventAction='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, such as:

  • properties providing service information (e.g. links, notices, remarks);

  • multivalued properties (e.g. status, roles, 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, being jCard the JSON format for vCard [RFC6350], the corresponding definitions are included in vCard specification.

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

Table 1: Sorting properties definition
Object class Sorting property RDAP property RFC 7483 RFC 6350 RFC 8605
Searchable objects Common properties eventAction values suffixed by "Date" 4.5
Domain name unicodeName/ ldhName 5.3
Nameserver name unicodeName/ ldhName 5.2
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 type="voice" 5.1 6.4.1
email jCard email 5.1 6.4.2
country country name in jCard adr 5.1 6.3.1
cc country code in jCard adr 5.1 3.1
city locality in jCard adr 5.1 6.3.1

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 reported, 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 reported, 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






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













  • Domain



  • Nameserver







  • Entity








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









Additional notes on the reported 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]

2.4. "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 "" whereby the key value identifies the last row of the current page.

Note that this specification uses a Base64 encoding for cursor encryption just for example. RDAP servers are NOT RECOMMENDED to encrypt a 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 reported 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:*.com &cursor=wJlCDLIl6KTWypN7T6vc6nWEmEYe99Hjf1XY1xmqV-M=

3. Negative Answers

The constraints for the parameters values 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:

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

    "errorCode": 400,
    "title": "Domain sorting property 'unknownproperty' 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). Additional implementation notes are included in Appendix C.

5. IANA Considerations

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

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 [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/

  • Responsible Organization: Institute of Informatics and Telematics of the National Research Council (IIT-CNR)/
  • Location:
  • Description: This implementation includes support for RDAP queries using data from .it public test environment.
  • Level of Maturity: This is an "alpha" test implementation.
  • Coverage: This implementation includes all of the features described in this specification.
  • Contact Information: Mario Loffredo,

6.2. APNIC

  • Responsible Organization: Asia-Pacific Network Information Centre
  • Location:
  • Description: A proof-of-concept for RDAP mirroring.
  • Level of Maturity: This is a proof-of-concept implementation.
  • Coverage: This implementation includes all of the features described in the specification except for nameserver sorting and unicodeName sorting.
  • Contact Information: Tom Harrison,

7. 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.

8. References

8.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, , <>.
Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, DOI 10.17487/RFC5890, , <>.
Perreault, S., "vCard Format Specification", RFC 6350, DOI 10.17487/RFC6350, , <>.
Kewisch, P., "jCard: The JSON Format for vCard", RFC 7095, DOI 10.17487/RFC7095, , <>.
Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, , <>.
Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, , <>.
Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the Registration Data Access Protocol (RDAP)", RFC 7480, DOI 10.17487/RFC7480, , <>.
Hollenbeck, S. and N. Kong, "Security Services for the Registration Data Access Protocol (RDAP)", RFC 7481, DOI 10.17487/RFC7481, , <>.
Newton, A. and S. Hollenbeck, "Registration Data Access Protocol (RDAP) Query Format", RFC 7482, DOI 10.17487/RFC7482, , <>.
Newton, A. and S. Hollenbeck, "JSON Responses for the Registration Data Access Protocol (RDAP)", RFC 7483, DOI 10.17487/RFC7483, , <>.
Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", BCP 205, RFC 7942, DOI 10.17487/RFC7942, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, , <>.
Nottingham, M., "Web Linking", RFC 8288, DOI 10.17487/RFC8288, , <>.
Hollenbeck, S. and R. Carney, "vCard Format Extensions: ICANN Extensions for the Registration Data Access Protocol (RDAP)", RFC 8605, DOI 10.17487/RFC8605, , <>.

8.2. Informative References

Nimesh, R., "Paginating Real-Time Data with Keyset Pagination", , <>.
[CURSOR-API1], "facebook for developers - Using the Graph API", , <>.
[CURSOR-API2], "Pagination", , <>.
Goessner, S., "JSONPath - XPath for JSON", , <>.
Jedrzejewski, B., "HATEOAS - a simple explanation", , <>.
Pizzo, M., Handl, R., and M. Zurmuehl, "OData Version 4.0. Part 1: Protocol Plus Errata 03", , <>.
Fielding, R., "Architectural Styles and the Design of Network-based Software Architectures", , <>.
Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed., "JavaScript Object Notation (JSON) Pointer", RFC 6901, DOI 10.17487/RFC6901, , <>.
[SEEK], "Faster Pagination in Mysql - Why Order By With Limit and Offset is Slow?", , <>.
Robie, J., Dyck, M., and J. Spiegel, "XML Path Language (XPath) 3.1", World Wide Web Consortium CR CR-xpath-31-20161213, , <>.

Appendix A. JSONPath operators

The jsonpaths used in this document are provided according to the Goessner v.0.8.0 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, and their subelements, respectively, object members and array items. 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 Table 2 and Table 3 respectively.

Table 2: JSONPath Top-Level Operators
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 3: JSONPath Filter 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

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:

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:

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 an opaque to client URL-safe string representing a logical pointer to the first result of the next page.

Nevertheless, even keyset pagination can be troublesome:

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) and, therefore, data selection might therefore 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. Additional Implementation Notes

This section contains an overview of the main choices made during the implementation of the capabilities defined above in the RDAP public test server of at the Institute of Informatics and Telematics of the National Research Counci (IIT-CNR). The content of this section can represent a guidance for those implementers who plan to provide RDAP users with those capabilities. The RDAP public test server can be accessed at Further documentation about the server features is available at

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, "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, the 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, fetching always 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, through offset pagination otherwise. As most of the 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.


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

Change Log

Initial working group version ported from draft-loffredo-regext-rdap-sorting-and-paging-05
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.
Removed both "offset" and "limit" parameters. Added ABNF syntax of the cursor parameter. Rearranged the layout of some sections. Removed some items from "Informative References" section. Changed "IANA Considerations" section.
Added "cc" to the list of sorting properties in "Sorting Properties Declaration" section. Added RFC8605 to the list of "Informative References".
Replaced "ldhName" with "name" in the "Sorting Properties Declaration" section. Clarified the sorting logic for the JSON value types and the sorting policy for multivalued fields.
Clarified the logic of sorting on IP addresses. Clarified the mapping between the sorting properties and the RDAP response fields. Updated "Acknowledgements" section.
Renamed "pageCount" to "pageSize" and added "pageNumber" in the "paging_metadata" object.
Added "Paging Responses to POST Requests" section.
Added "Approaches to Result Pagination" section to appendix. Added the case of requesting a sort on a property not included in the response to the errors listed in the "Negative Answers" section.

Updated the "Implementation Status" section to include APNIC implementation. Moved the "RDAP Conformance" section up in the document. Removed the "Paging Responses to POST Requests" section. Updated the "Acknowledgements" section. Removed unused references. In the "Sorting Properties Declaration" section:

  • clarified the logic of sorting on events;
  • corrected the jsonpath of the "lastChanged" sorting property;
  • provided a JSONPath example taking into account the vCard "pref" parameter.
Corrected the jsonpaths of both "fn" and "org" sorting properties in Table 2. Corrected JSON content in Figure 2. Moved [W3C.CR-xpath-31-20161213] and [RFC7942] to the "Normative References". Changed the rdapConformance tags "sorting_level_0" and "paging_level_0" to "sorting" and "paging" respectively.
Added the "JSONPath operators" section to appendix.
Changed the content of "JSONPath operators" section.
Minor pre-AD review edits.
Additionl minor pre-AD review edits.
In section ""sort" Parameter" added a paragraph providing conversions of IP addresses into their numerical representations. In section "Sorting Properties Declaration" rearranged Table 2 in a list to make the content more readable. Other minor edits due to AD review.
In section "Introduction" replaced "... large result set that could be truncated ..." with "... large result set that is often truncated ..." as suggested by Gen-ART reviewer. Added Appendix C.

Edits made:

  • in the "Sorting and Paging Metadata" section:

    • replaced "Members are:" with "The AvailableSort object includes the following members:";
    • clarified that an RDAP server MUST define only one default sorting property for each object class;
  • in the "Negative Answers" section:

    • replaced the phrase "the response MAY include additional information regarding the negative answer" with the phrase "the response MAY include additional information regarding either the supported sorting properties or the correct cursor value";
    • added a new example;
  • clarified the required members of a Link object in the "Representing Sorting Links" section;
  • corrected the [REST] reference in the "Informative References" section;
  • replaced the phrase "and subsequent denial of service due to abuse" with the phrase "and subsequent denial of service" in "Security Considerations" section.

Edits made:

  • in the "Introduction" section:

    • revised the reasons for using query parameters instead of HTTP headers;
  • in the "Sorting and Paging Metadata" section:

    • replaced the phrase "number of objects returned in the current page" with the phrase "number of objects that should have been returned in the current page" in the definition of the "pageSize" field;
  • in the ""sort" Parameter" section:

    • clarified the sorting logic for values denoting dates and times;
    • replaced the IPv6 address "2001:0db8:85a3:0000:0000:8a2e:0370:7334" with "2001:0db8:85a3:0:0:8a2e:0370:7334";
  • in the "Sorting Properties Declaration" section:

    • replaced the sorting properties "ipV4" and "ipV6" with "ipv4" and "ipv6";
    • replaced the sentence "Therefore, the assumption of having a single IPv4 and/or IPv6 value for a nameserver cannot be considered too stringent." with the sentence "Therefore, this specification makes the assumption that nameservers have a single IPv4 and/or IPv6 value."
    • clarified that the sorting properties MUST NOT be used with a with a meaning other than the one described this document;
    • specified that JSONPath operators used in this section are those defined in "Appendix A";
  • in the ""cursor" Parameter" section:

    • corrected the Base64 encoding of "offset=100,limit=50";
    • clarified that RDAP servers are NOT RECOMMENDED to encrypt a cursor value through a mere Base64 encoding;
  • changed last sentence of second paragraph of the "Security Considerations" section;
  • updated the "Acknowledgements" section;
  • in "Appendix A":

    • changed introductory paragraph;
    • replaced "opaque URL-safe string" with "opaque to client URL-safe string";
  • added JSONPath union operator in Table 2 of "Appendix B"
  • changed the explanation of offset and limit operators in "Appendix B";
  • converted the figures containing only RDAP queries into texts;
  • changed the wildcard prefixed patterns into wildcard suffixed in all the RDAP queries;
  • cleaned the text.

Authors' Addresses

Mario Loffredo
Via Moruzzi,1
56124 Pisa
Maurizio Martinelli
Via Moruzzi,1
56124 Pisa
Scott Hollenbeck
Verisign Labs
12061 Bluemont Way
Reston, VA 20190
United States of America