Registration Protocols Extensions M. Loffredo
Internet-Draft M. Martinelli
Intended status: Standards Track IIT-CNR/
Expires: August 5, 2019 February 1, 2019

Registration Data Access Protocol (RDAP) Partial Response


The Registration Data Access Protocol (RDAP) does not include capabilities to request partial responses. In fact, according to the user authorization, the server can only return full responses. Partial responses capability, especially in the case of search queries, could bring benefits to both clients and servers. This document describes a RDAP query extension that allows clients to specify their preference for obtaining a partial response.

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Table of Contents

1. Introduction

The use of partial response in RESTful API ([REST]) design is very common. The rationale is quite simple: instead of returning objects in API responses with all data fields, only a subset is returned. The benefit is obvious: less data transferred over the network mean less bandwidth usage, faster server response, less CPU time spent both on the server and the client, as well as less memory usage on the client.

Several leading APIs providers (e.g. LinkedIn [LINKEDIN], Facebook [FACEBOOK], Google [GOOGLE]) implement the partial response feature by providing an optional query parameter by which users require the fields they wish to receive. Partial response is also considered a leading principle by many best practices guidelines in REST APIs implementation ([REST-API1], [REST-API2]) in order to improve performance, save on bandwidth and possibly accelerate the overall interaction. In other contexts, for example in digital libraries and bibliographic catalogues, servers can provide responses according to different element sets (i.e. "brief" to get back a short response and "full" to get back the complete response)

Currently, RDAP does not provide a client with any way to request a partial response: the server can only provide the client with the full response ([RFC7483]). Furthermore, servers cannot define the limits of the results according to partial responses and this causes strong inefficiencies.

The protocol described in this specification extends RDAP search capabilities to enable partial responses, by adding a new query parameter and using a RESTful web service. The service is implemented using the Hypertext Transfer Protocol (HTTP) ([RFC7230]) and the conventions described in RFC 7480 ([RFC7480]).

Impact on the current state of RDAP implementation is 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 RFC 2119 ([RFC2119]).

2. Approaches to Partial Response Implementation

Looking at the implementation experiences described above, two approaches to the implementation of partial response can be detected:

The former is more flexible than the latter, because clients can specify all the data fields they need. Anyway, it has some drawbacks:

In addition to those listed above, RDAP responses raise some specific issues:

As an example compliant to the first approach, the Catnap Query Language ([CQL]) is a comprehensive expression language that can be used to customize the JSON response of a RESTful web service. The practical application of CQL to RDAP responses points out that declaring explicitly the output fields would still be acceptable when a few fields are requested but it would become very complicated if the fields should be more. In the following, two CQL expressions for a search domain query are shown (Figure 1): in the first, only objectClassName and ldhName are requested, in the second, the fields of a possible WHOIS-like response are listed.*.com

Figure 1: Examples of CQL expressions for a search domain query

The latter approach seems to facilitate RDAP interoperability. In fact, servers can define some basic field sets which, if known to the clients, can increase the probability to get a valid response. The usage of field sets lets the query string be less complex. In addition, the definition of pre-defined sets of fields makes easier to establish the results limits.

Finally, considering that there is not a real need for RDAP users to have the maximum flexibility in defining all the possible sets of logically connected fields (for example, users interested in domains usually need to know the status, the creation date, the expire date of each domain), the latter approach is preferred.

3. RDAP Path Segment Specification

The new query parameter is an OPTIONAL extension of search path segments defined in RFC 7482 ([RFC7482]). The query parameter is "fieldSet" whose value is a string identifying a server pre-defined set of fields (Figure 2). Values REQUIRED to be implemented are:

Fields belonging to brief and full field sets should be provided according to users access levels. RDAP providers MAY add any property providing service information. Servers MAY implement additional field sets not included in the list above. Servers SHOULD also define a "default" field set.*.com&fieldSet=id

Figure 2: Example of RDAP search query reporting the fieldSet parameter

  "rdapConformance": [
  "domainSearchResults": [
      "objectClassName": "domain",
      "ldhName": ""
      "objectClassName": "domain",
      "ldhName": ""

Figure 3: Example of RDAP response according to the "id" field set

3.1. Brief Field Set

In order to ensure the highest degree of interoperability, the brief field set should contain the most commonly used data elements. Based on the assumption that an RDAP server will return almost the same data as those replied by the corresponding Whois service, the elements included in the brief field set could be those identified in RFC 7485 ([RFC7485]) as mostly supported (i.e. by more than one third of contacted services).

Therefore, RDAP servers are RECOMMENDED to return the following elements in the brief field set (Table 1):

Elements included in brief field set
Object class Whois property RDAP property
Domain Domain Name ldhName
Domain Status status
Creation Date event whose eventAction type is "registration"
Expiration Date event whose eventAction type is "expiration"
Update Date event whose eventAction type is "last update"
Nameserver Name Server ldhName
Entity Entity ID handle
Entity Name vcard fn
Entity Organization vcard org
Entity Email vcard email
Entity Phone vcard tel with type="voice"
Entity Fax vcard tel with type="fax"
Entity Country country name in vcard adr
Entity City locality in vcard adr
Entity Postal Code postal code in vcard adr

The term "Entity" refers to any kind of contact.

3.2. Full Field Set

With regards to the full field set, some additional considerations can be made about how second level objects could be represented. In fact, since the topmost objects could be returned according to different field sets, the same thing could go for their related objects. As a consequence, the full response could range from the one containing no relationship up to a response where each related object is in turn in full format.

FOR DISCUSSION: Should this specification furtherly detail the full field set according to the different representations of the related objects?

3.3. Subsetting 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 self-descriptive as much as possible.

Therefore, the implementation of the query parameter described in this specification recommends servers to provide additional information in their responses about the available field sets. Such information is collected in a new data structures named "subsetting_metadata" containing the following fields:

Both "currentFieldSet" and "availableFieldSets" are OPTIONAL fields of the "subsetting_metadata" structure. In particular, the "currentFieldSet" field is provided when the query string contains a valid value for fieldSet parameter, while the "availableFieldSets" field SHOULD be provided when the fieldSet parameter is missing in the query string or when it is present and the server implements more than a field set for the RDAP object. At least the "name" field is REQUIRED in each item of the "availableFieldSets" array while the other fields are RECOMMENDED.

3.3.1. Representing Subsetting Links

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

  "rdapConformance": [
  "subsetting_metadata": {
     "currentFieldSet": "brief",
     "availableFieldSets": [
     "name": "id",
     "description": "Contains "objectClassName" and the key field",
     "default": false,
     "links": [
        "value": "*
        "rel": "alternate",
        "href": "*
        "title": "Result Subset Link",
        "type": "application/rdap+json"
  "domainSearchResults": [

Figure 4: Example of a "subsetting_metadata" instance

4. RDAP Conformance

Servers returning the "subsetting_metadata" section in their responses MUST include "subsetting_level_0" in the rdapConformance array.

5. 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".

5.1. IIT-CNR/

6. Security Considerations

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. Partial response can contribute together with other strategies (e.g. restricting search functionality, limiting the rate of search requests, truncating and paging results) to mitigate this risk.

Furthermore, partial response can help RDAP operators to regulate access control based on client identification, implemented by HTTP basic or digest authentication as described in RFC 7481 ([RFC7481]) or by a federated authentication system ([I-D.hollenbeck-regext-rdap-openid]). In fact, RDAP operators can follow different, not alternative, approaches to the building of responses according to the user access levels:

Servers can also define different results limits according to the available field sets, so a more flexible truncation strategy can be realized and users can take advantage of a more efficient results paging implementation ([I-D.loffredo-regext-rdap-sorting-and-paging]).

Therefore, the new parameter presented in this document provides the RDAP operators with a way to implement a secure server without penalizing its efficiency.

7. IANA Considerations

This document has no actions for IANA.

8. Acknowledgements

The authors would like to acknowledge Scott Hollenbeck for his contribution to this document.

9. References

9.1. 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.
[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.
[RFC7485] Zhou, L., Kong, N., Shen, S., Sheng, S. and A. Servin, "Inventory and Analysis of WHOIS Registration Objects", RFC 7485, DOI 10.17487/RFC7485, March 2015.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288, DOI 10.17487/RFC8288, October 2017.

9.2. Informative References

[CQL] Whitaker, G., "Catnap Query Language Reference", September 2017.
[FACEBOOK], "facebook for developers - Using the Graph API", July 2017.
[GOOGLE], "Making APIs Faster: Introducing Partial Response and Partial Update", March 2010.
[HATEOAS] Jedrzejewski, B., "HATEOAS - a simple explanation", 2018.
[I-D.hollenbeck-regext-rdap-openid] Hollenbeck, S., "Federated Authentication for the Registration Data Access Protocol (RDAP) using OpenID Connect", Internet-Draft draft-hollenbeck-regext-rdap-openid-10, August 2018.
[I-D.loffredo-regext-rdap-sorting-and-paging] Loffredo, M., Martinelli, M. and S. Hollenbeck, "Registration Data Access Protocol (RDAP) Query Parameters for Result Sorting and Paging", Internet-Draft draft-loffredo-regext-rdap-sorting-and-paging-05, September 2018.
[LINKEDIN], "Java One 2009: Building Consistent RESTful APIs in a High Performance Environment", July 2009.
[REST] Fielding, R., "Architectural Styles and the Design of Network-based Software Architectures", 2000.
[REST-API1] Jobinesh, P., "RESTful Java Web Services - Second Edition", September 2015.
[REST-API2] Masse, M., "REST API Design Rulebook", October 2011.
[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.

Appendix A. Change Log

Initial version.
Added Catnap Query Language as an example of language that can be used to declare explicitly the output fields of RDAP responses. Revised some sentences and references.
Added "Subsetting Metadata" and "RDAP Conformance" sections.
Added "Brief Field Set" and "Full Field Set" sections.

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: