Registration Protocols Extensions M. Loffredo
Internet-Draft M. Martinelli
Intended status: Standards Track IIT-CNR/
Expires: December 19, 2020 S. Hollenbeck
Verisign Labs
June 17, 2020

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.

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This Internet-Draft will expire on December 19, 2020.

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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.

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. The second approach is therefore not compliant with the HTTP standard [RFC7230]. 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 could be 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 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 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, respectively, "sorting_metadata" and "paging_metadata".

The "sorting_metadata" element contains the following properties:

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

The "paging_metadata" element contains the following fields:

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 (Figure 1) that allows a client to request information from the server that specifies the total number of objects matching the search pattern.

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

The ABNF syntax is the following:

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

  "rdapConformance": [
  "paging_metadata": {
    "totalCount": 43
  "domainSearchResults": [

Figure 2: 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:

"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 (Figure 3).

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

Except for sorting IP addresses, 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. If IP addresses are represented as JSON strings, they MUST be sorted based on their numeric conversion.

If the "sort" parameter reports 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, property-ref represents a reference to a property of an RDAP object. Such a reference could be expressed by using a JSONPath. The JSONPath in a JSON document [RFC8259] is equivalent to the XPath [W3C.CR-xpath-31-20161213] in a XML document. For example, the JSONPath 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 JSONPath 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 JSONPath expression of the registration date in an RDAP domain object).

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

On the contrary, properties expressed as values of other properties (e.g. registration date) could be used in such a context. The list of properties an RDAP server MAY implement are divided into two groups: object common properties and object specific properties.

The correspondence between these sorting properties and the RDAP object classes is shown in 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 vcard fn 5.1. 6.2.1
org vcard org 5.1. 6.6.4
voice vcard tel with type="voice" 5.1. 6.4.1
email vcard email 5.1. 6.4.2
country country name in vcard adr 5.1. 6.3.1
cc country code in vcard adr 5.1. 3.1
city locality in vcard adr 5.1. 6.3.1

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

The "jsonPath" field in the "sorting_metadata" element is used to clarify the RDAP field the sorting property refers to. The mapping between the sorting properties and the JSONPaths of the RDAP fields is shown in Table 2. The JSONPaths are provided according to the Goessner v.0.8.0 specification [GOESSNER-JSON-PATH]. Further documentation about JSONPath operators used in Table 2 is included in Appendix A.

Sorting properties - JSONPath Mapping
Object class Sorting property JSONPath
Searchable objects registrationDate $.domainSearchResults[*].events[?(@.eventAction=="registration")].eventDate
reregistrationDate $.domainSearchResults[*].events[?(@.eventAction=="reregistration")].eventDate
lastChangedDate $.domainSearchResults[*].events[?(@.eventAction=="last changed")].eventDate
expirationDate $.domainSearchResults[*].events[?(@.eventAction=="expiration")].eventDate
deletionDate $.domainSearchResults[*].events[?(@.eventAction=="deletion")].eventDate
reinstantiationDate $.domainSearchResults[*].events[?(@.eventAction=="reinstantiation")].eventDate
transferDate $.domainSearchResults[*].events[?(@.eventAction=="transfer")].eventDate
lockedDate $.domainSearchResults[*].events[?(@.eventAction=="locked")].eventDate
unlockedDate $.domainSearchResults[*].events[?(@.eventAction=="unlocked")].eventDate
Domain name $.domainSearchResults[*].unicodeName
Nameserver name $.nameserverSearchResults[*].unicodeName
ipV4 $.nameserverSearchResults[*].ipAddresses.v4[0]
ipV6 $.nameserverSearchResults[*].ipAddresses.v6[0]
Entity handle $.entitySearchResults[*].handle
fn $.entitySearchResults[*].vcardArray[1][?(@[0]=="fn")][3]
org $.entitySearchResults[*].vcardArray[1][?(@[0]=="org")][3]
voice $.entitySearchResults[*].vcardArray[1][?(@[0]=="tel" && @[1].type=="voice")][3]
email $.entitySearchResults[*].vcardArray[1][?(@[0]=="email")][3]
country $.entitySearchResults[*].vcardArray[1][?(@[0]=="adr")][3][6]
cc $.entitySearchResults[*].vcardArray[1][?(@[0]=="adr")][1].cc
city $.entitySearchResults[*].vcardArray[1][?(@[0]=="adr")][3][3]

Table 2 JSONPath notes:

2.3.2. Representing Sorting Links

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

  "rdapConformance": [
  "sorting_metadata": {
     "currentSort": "name",
     "availableSorts": [
       "property": "registrationDate",
       "jsonPath": "$.domainSearchResults[*]
       "default": false,
       "links": [
         "value": "*
         "rel": "alternate",
         "href": "*
         "title": "Result Ascending Sort Link",
         "type": "application/rdap+json"
         "value": "*
         "rel": "alternate",
         "href": "*
         "title": "Result Descending Sort Link",
         "type": "application/rdap+json"
  "domainSearchResults": [

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

2.4. "cursor" Parameter

The cursor parameter defined in this specification can be used to encode information about any pagination method. For example, in the case of a simple implementation of the cursor parameter to represent offset pagination information, the cursor value "b2Zmc2V0PTEwMCxsaW1pdD01MAo=" is the Base64 encoding of "offset=100,limit=50". Likewise, in a simple implementation to represent keyset pagination information, the cursor value "a2V5PXRoZWxhc3Rkb21haW5vZnRoZXBhZ2UuY29t=" represents the Base64 encoding of "" whereby the key value identifies the last row of the current page.

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:

Figure 5: An example of an RDAP query reporting the "cursor" parameter

2.4.1. Representing Paging Links

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

  "rdapConformance": [
  "notices": [
      "title": "Search query limits",
      "type": "result set truncated due to excessive load",
      "description": [
      "search results for domains are limited to 50"
  "paging_metadata": {
    "totalCount": 73,
    "pageSize": 50,
    "pageNumber": 1, 
    "links": [
      "value": "*",
      "rel": "next",
      "href": "*
      "title": "Result Pagination Link",
      "type": "application/rdap+json"
  "domainSearchResults": [

Figure 6: Example of a "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 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 the negative answer in the HTTP entity body.

4. Implementation Considerations

Implementation of the new parameters is technically feasible, as operators for counting, sorting and paging are currently supported by the major RDBMSs. Similar operators are completely or partially supported by the most well-known NoSQL databases (e.g. MongoDB, CouchDB, HBase, Cassandra, Hadoop).

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/

6.2. APNIC

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 due to abuse. 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 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

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, DOI 10.17487/RFC5890, August 2010.
[RFC6350] Perreault, S., "vCard Format Specification", RFC 6350, DOI 10.17487/RFC6350, August 2011.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014.
[RFC7480] Newton, A., Ellacott, B. and N. Kong, "HTTP Usage in the Registration Data Access Protocol (RDAP)", RFC 7480, DOI 10.17487/RFC7480, March 2015.
[RFC7481] Hollenbeck, S. and N. Kong, "Security Services for the Registration Data Access Protocol (RDAP)", RFC 7481, DOI 10.17487/RFC7481, March 2015.
[RFC7482] Newton, A. and S. Hollenbeck, "Registration Data Access Protocol (RDAP) Query Format", RFC 7482, DOI 10.17487/RFC7482, March 2015.
[RFC7483] Newton, A. and S. Hollenbeck, "JSON Responses for the Registration Data Access Protocol (RDAP)", RFC 7483, DOI 10.17487/RFC7483, March 2015.
[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.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[RFC8259] Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, December 2017.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288, DOI 10.17487/RFC8288, October 2017.
[RFC8605] Hollenbeck, S. and R. Carney, "vCard Format Extensions: ICANN Extensions for the Registration Data Access Protocol (RDAP)", RFC 8605, DOI 10.17487/RFC8605, May 2019.
[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.

8.2. Informative References

[CURSOR] Nimesh, R., "Paginating Real-Time Data with Keyset Pagination", July 2014.
[CURSOR-API1], "facebook for developers - Using the Graph API", July 2017.
[CURSOR-API2], "Pagination", 2017.
[GOESSNER-JSON-PATH] Goessner, S., "JSONPath - XPath for JSON", 2007.
[HATEOAS] Jedrzejewski, B., "HATEOAS - a simple explanation", 2018.
[OData-Part1] Pizzo, M., Handl, R. and M. Zurmuehl, "OData Version 4.0. Part 1: Protocol Plus Errata 03", June 2016.
[REST] Fredrich, T., "RESTful Service Best Practices, Recommendations for Creating Web Services", April 2012.
[RFC6901] Bryan, P., Zyp, K. and M. Nottingham, "JavaScript Object Notation (JSON) Pointer", RFC 6901, DOI 10.17487/RFC6901, April 2013.
[SEEK], "Faster Pagination in Mysql - Why Order By With Limit and Offset is Slow?", July 2017.

Appendix A. JSONPath operators

A JSONPath expression represents a path to find an element (or a set of elements) in a JSON content.

The base JSONPath 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 3 and Table 4 respectively.

JSONPath Top-Level Operators
Operator Descritpion
$ 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
JSONPath Filter Operators
Operator Descritpion
@ Current element being processed
.<name> Object member access
[<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 URL-safe string representing a logical pointer to the first result of the next page (Figure 5).

Nevertheless, even keyset pagination can be troublesome:

B.1. Specific Issues Raised by RDAP

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

Ultimately, both pagination methods have benefits and drawbacks.


The authors would like to acknowledge Brian Mountford, Tom Harrison, Karl Heinz Wolf and Jasdip Singh 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 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:

Corrected the JSONPaths of both "fn" and "org" sorting properties in Table 2. Corrected JSON content in Figure 4. 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.

Authors' Addresses

Mario Loffredo IIT-CNR/ Via Moruzzi,1 Pisa, 56124 IT EMail: URI:
Maurizio Martinelli IIT-CNR/ Via Moruzzi,1 Pisa, 56124 IT EMail: URI:
Scott Hollenbeck Verisign Labs 12061 Bluemont Way Reston, VA 20190 USA EMail: URI: