Registration Protocols Extensions M. Loffredo
Internet-Draft M. Martinelli
Intended status: Standards Track IIT-CNR/
Expires: September 24, 2018 S. Hollenbeck
Verisign Labs
March 23, 2018

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 (subsetting) 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

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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 (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:

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 mechanism 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 parameters is technically feasible, as operators for counting, sorting and paging rows are currently supported by the major RDBMSs. Impact on the current state of RDAP implementation is estimated to be quite low.

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

These new query parameters are OPTIONAL extensions of path segments defined in RFC 7482 [RFC7482]. The query parameters 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. "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 new parameter "count" provides additional functionality (Figure 1) that allows a client to request information from the server that specifies the number of elements found to match a particular 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 "paging_count" property of the response (Figure 2). A falseValue means that the server MUST NOT provide this number.

  "rdapConformance": [
  "domainSearchResults": [

Figure 2: Example of RDAP response with "paging_count" property

2.2. "sortby" Parameter

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

The "sortby" 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 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 sortby 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 ldhName property 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 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:

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 is presented. The properties are divided in two groups: object common properties and object specific properties.

In the following, the correspondence between the values of sortby parameter and the RDAP object properties is shown (Table 1):

Sorting properties definition
Object class sortby value Object property Reference in RFC 7483 Reference in RFC 6350
Searchable objects Common properties eventAction values suffixed by "Date" 4.5.
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
tel vcard tel with type="voice" 5.1. 6.4.1
email vcard email 5.1. 6.4.2
country country code (as given in ISO.3166.1988 [ISO.3166.1988]) of the country name in vcard adr 5.1. 6.3.1
countryName country name in vcard adr 5.1. 6.3.1
locality locality in vcard adr 5.1. 6.3.1

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

Each RDAP provider MAY define other sorting properties than those shown in this document. (FOR DISCUSSION: how should those properties be defined? Is an IANA registry appropriate?)

Figure 3: Examples of RDAP query reporting the sortby parameter

2.3. "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:

The ABNF syntax is the following:

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 4).

Figure 4: Examples of RDAP query reporting the limit and offset parameters

2.3.1. Use of "paging_links" Property

An RDAP server MAY use the new "paging_links" property to provide a ready-made reference [RFC8288] to the next page of the result set (Figure 5). Examples of additional "rel" values are "first", "last", "prev".

  "rdapConformance": [
  "notices": [
      "title":"Search query limits",
      "type":"result set truncated due to excessive load",
      "description": [
      "search results for domains are limited to 10"
  "paging_links": [
      "title": "Result Pagination Link",
  "domainSearchResults": [

Figure 5: Example of "paging_links" property to implement result 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 sortby parameter in both single and multi sort.

The server can provide a different response when it supports the limit and/or offset parameters and the client submits values that are out of the valid ranges. The possible cases are:

Optionally, the response MAY include additional information regarding the negative answer in the HTTP entity body.

4. RDAP Conformance

Servers returning "paging_links" and "paging_count" properties MUST include "paging_level_0" in the rdapConformance array of their responses.

5. Implementation Considerations

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

In the following, the match between the new defined parameters and the SQL operators is shown (Table 2):

New query parameters vs. SQL operators
New query parameter SQL operator
count count(*) query without offset, limit and order by
sortby order by
limit limit n (in MySql [MYSQL-LIMIT] and Postgres [POSTGRES-LIMIT])
offset offset m (in Postgres)
OFFSET m ROWS (in Oracle)
limit + offset limit n offset m (in MySql and Postgres)

With regard to Oracle, Table 2 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 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 operators represents the most common way to implement results pagination. However, when offset has a high value, scrolling the result set could take some time. In addition, offset pagination may return inconsistent pages when data are 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 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 (Figure 6).

  "rdapConformance": [
  "notices": [
      "title":"Search query limits",
      "type":"result set truncated due to excessive load",
      "description": [
      "search results for domains are limited to 10"
  "paging_links": [
      "title": "Result Pagination Link",
  "domainSearchResults": [

Figure 6: Example of "paging_links" property to implement keyset pagination

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

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

Finally, the keyset pagination is not fully compliant with the additional RDAP capabilities proposed by this document. In fact, the presence of a possible cursor parameter does not seem to be consistent with both the sorting capability and the possibility to implement additional ready-made links besides the classic "next page" link. But, while the provisioning of more paging links can be superfluous, dropping the sorting capability seems quite unreasonable.

FOR DISCUSSION: Should RDAP specification reports both offset and cursor parameters and let operators to implement pagination according to their needs, the user access levels, the submitted queries?

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/

6.2. Google Registry

7. IANA Considerations

FOR DISCUSSION: An IANA-maintained registry could be used to document all of the properties that could be used as sort criteria. Such a registry could also be used to identify the properties for RDAP query filtering capabilities.

7.1. Registry of References to RDAP Properties

Entries in this registry could contain the following:

In the following, an example of the entry about the reference "ipV4" is reported:

8. 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 "sortby" 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" and "offset" parameters enable the user to scroll the result set by submitting a sequence of sustainable queries according to the server limits.

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.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[RFC5266] Devarapalli, V. and P. Eronen, "Secure Connectivity and Mobility Using Mobile IPv4 and IKEv2 Mobility and Multihoming (MOBIKE)", BCP 136, RFC 5266, DOI 10.17487/RFC5266, June 2008.
[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.
[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.

10.2. 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], "Working with timelines", 2017.
[MYSQL-COUNT], "MySQL 5.7 Reference Manual, Counting Rows", October 2015.
[MYSQL-LIMIT], "MySQL 5.7 Reference Manual, SELECT Syntax", October 2015.
[MYSQL-SORT], "MySQL 5.7 Reference Manual, Sorting Rows", October 2015.
[OData-Part1] Pizzo, M., Handl, R. and M. Zurmuehl, "OData Version 4.0. Part 1: Protocol Plus Errata 03", June 2016.
[ORACLE-COUNT] Oracle Corporation, "Database SQL Language Reference, COUNT", March 2016.
[ORACLE-LIMIT] Oracle Corporation, "Database SQL Language Reference, SELECT, Row limiting clause", March 2016.
[ORACLE-ROW-NUMBER] Oracle Corporation, "Database SQL Language Reference, SELECT, ROW_NUMBER", March 2016.
[ORACLE-ROWNUM] Oracle Corporation, "Database SQL Language Reference, SELECT, ROWNUM Pseudocolumn", March 2016.
[ORACLE-SORT] Oracle Corporation, "Database SQL Language Reference, SELECT, Order by clause", March 2016.
[POSTGRES-COUNT], "PostgresSQL, Aggregate Functions", September 2016.
[POSTGRES-LIMIT], "PostgresSQL, LIMIT and OFFSET", September 2016.
[POSTGRES-SORT], "PostgresSQL, Sorting Rows", September 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.
[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.
[SEEK], "Faster Pagination in Mysql - Why Order By With Limit and Offset is Slow?", July 2017.
[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.

Appendix A. Change Log

Initial version.
Added the paragraph "Considerations about Paging Implementation" to "Implementation Considerations" section. Added "Implementation Status" section. Added acknowledgements. Renamed the property reporting the paging links.
Corrected the value of "title" field in "paging_links" property. Updated references to RFC5988 (obsoleted by RFC 8288) and RFC7159 (obsoleted by RFC 8259). Revised some sentences.
Added the paragraph "Google Registry" to "Implementation Status" section.

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: