Extensions Parameter for the RDAP Media Type
draft-ietf-regext-rdap-x-media-type-04

Registration Protocols Extensions (regext)                     A. Newton
Internet-Draft                                                     ICANN
Intended status: Standards Track                                J. Singh
Expires: 3 January 2026                                             ARIN
                                                             2 July 2025

              Extensions Parameter for the RDAP Media Type
                 draft-ietf-regext-rdap-x-media-type-04

Abstract

   This document defines a new parameter for the RDAP media type that
   can be used to describe RDAP content with RDAP extensions.
   Additionally, this document describes the usage of this parameter
   with RDAP for the purposes of signalling RDAP extensions during
   content negotiation.

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 3 January 2026.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Document Terms  . . . . . . . . . . . . . . . . . . . . .   3
   2.  The RDAP Media Type With Extensions Parameter . . . . . . . .   3
   3.  Using The Extensions Parameter  . . . . . . . . . . . . . . .   3
     3.1.  Extension Identifier  . . . . . . . . . . . . . . . . . .   5
     3.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   5
       3.2.1.  Classic Negotiation . . . . . . . . . . . . . . . . .   5
       3.2.2.  Negotiation of an RDAP Extension  . . . . . . . . . .   6
       3.2.3.  Negotiation of an RDAP Extension Without
               "Content-Type"  . . . . . . . . . . . . . . . . . . .   6
       3.2.4.  No Server Support for exts_list Parameter . . . . . .   7
       3.2.5.  Differing Extension Negotiation . . . . . . . . . . .   7
       3.2.6.  Extension Versioning and Meta-data  . . . . . . . . .   7
   4.  Usage in RDAP Links . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  RDAP Extension Registry . . . . . . . . . . . . . . . . .   9
     6.2.  Addition of Parameter to RDAP Media Type  . . . . . . . .   9
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Using the Vary Header  . . . . . . . . . . . . . . .  11
   Appendix B.  Design Considerations  . . . . . . . . . . . . . . .  12
     B.1.  Reusing the Existing Media Type . . . . . . . . . . . . .  12
     B.2.  Inappropriate Use of Query Parameters . . . . . . . . . .  13
       B.2.1.  Copy and Paste  . . . . . . . . . . . . . . . . . . .  13
       B.2.2.  Redirects . . . . . . . . . . . . . . . . . . . . . .  13
       B.2.3.  Referral Compatibility  . . . . . . . . . . . . . . .  14
       B.2.4.  Architectural Violations  . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Background

   [RFC7480] defines the "application/rdap+json" media type to be used
   with RDAP.  This document defines a new parameter for this media type
   when an RDAP extension needs to be described during HTTP content
   negotiation.

   This parameter enables an RDAP client to signal to an RDAP server the
   list of RDAP extensions supported by that client.  For example, an
   RDAP client that supports the "foo" extension may use this mechanism
   as a signal to an RDAP server, thus allowing the server to respond
   with data using the "foo" extension inside an RDAP response only when
   it can be assured the client can understand the extension.

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   By using this method, there is no need for every RDAP extension to
   define their own unique signaling mechanism.  Additionally, this
   method is designed to be backwards-compatible with the deployed RDAP
   ecosystem (see Appendix B for further information).

1.1.  Document Terms

   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.  The RDAP Media Type With Extensions Parameter

   The RDAP media type, "application/rdap+json", may have an optional
   parameter named "exts_list".  This parameter is a whitespace-
   separated list of RDAP extension identifiers (as would be found in
   the "rdapConformance" array).

   Here is an example:

   application/rdap+json;exts_list="rdap_level_0 exts fred"

3.  Using The Extensions Parameter

   [RFC7480] specifies the usage of "application/json", "application/
   rdap+json" or both with HTTP "accept" header.  The "exts_list"
   parameter may only be used with the "application/rdap+json" media
   type.

   This is an example of the "accept" header using the RDAP media type
   with an "extensions" parameter:

   accept: application/json;q=0.9,
       application/rdap+json;exts_list="rdap_level_0 exts fred";q=1

   If both a client and a server support the "exts_list" parameter, and
   the client requests an extension that is unimplemented by the server,
   the server MUST respond with only extensions included in the response
   by the server.  This behavior is backwards-compatible as RDAP clients
   must ignore unknown RDAP extensions as specified by [RFC9083].
   Responding with an HTTP 406 Not Acceptable status code is NOT
   RECOMMENDED because an RDAP client could interpret this status code
   to mean that the server does not understand RDAP in its entirety.

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   Likewise, if a server is required to use an extension in a response
   that was not requested by the client, the server MUST respond as if
   the client had requested the extension.  This behavior is backwards-
   compatible as RDAP clients must ignore unknown extensions as
   specified by [RFC9083].  Responding with an HTTP 406 Not Acceptable
   status code is NOT RECOMMENDED for the reason stated above.

   When the "exts_list" parameter is used in the RDAP media type in the
   "content-type" header, the values in the media type's "exts_list"
   parameter MUST match the values in the "rdapConformance" array in the
   returned JSON.  However, implementation experience has shown that
   some HTTP server libraries do not support modification of the
   "content-type" header per query type.  Therefore use of the
   "exts_list" parameter with the media type of the "content-type"
   header is NOT REQUIRED.  That is, when used in the "content-type"
   header, the values of the "exts_list" parameter must match that of
   the "rdapConformance" array but server may opt to omit the
   "exts_list" parameter from the media type in the "content-type"
   header.

   The contents of the "exts_list" parameter mirrors the content of the
   "rdapConformance" array in server responses.  This includes the
   identifier "rdap_level_0", which is not an extension identifier but
   an identifier for the base RDAP specifications.  Servers MUST follow
   the same rules for placing "rdap_level_0" in the content of the
   "exts_list" parameter and the "rdapConformance" array.  Clients MUST
   interpret an "exts_list" parameter without "rdap_level_0" or one of
   its successor identifiers (e.g. "rdap_level_1") in the same manner as
   the interpretation of the "rdapConformance" array without
   "rdap_level_0" or one of its successors.

   Nothing in this specification sidesteps or obviates the HTTP content
   negotiation defined in [RFC9110] for RDAP.

   Likewise, nothing in this specification sidesteps or obviates the
   HTTP caching mechanisms defined in [RFC9110].  Further advice on the
   "vary" header can be found in Appendix A.

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   Some RDAP extensions, such as [RFC9560], have other protocol elements
   (e.g. extension-specific query parameters) passed from the client to
   the server, and the presence of these protocol elements may be used
   by servers to determine a client's capability to handle the related
   RDAP extension(s).  This specification does not require the usage of
   those extension identifiers in the "exts_list" parameter, though
   clients SHOULD list the extension identifier in the "exts_list"
   parameter when using other protocol elements of those extensions for
   better compatibility with servers recognizing the "exts_list"
   parameter.  Servers SHOULD NOT require the usage of extension
   identifiers in the "exts_list" parameter when other extension
   protocol elements are used for backwards-compatibility purposes.

3.1.  Extension Identifier

   This document defines an RDAP extension using the identifier "exts".
   This RDAP extension defines no additional RDAP queries or response
   structures.

   The purpose of this RDAP extension is to allow servers to signal
   support for the "exts_list" parameter in "rdapConformance" arrays of
   responses to "/help" (aka "service discovery").

3.2.  Examples

   The following examples use the HTTP/1.1 message exchange syntax as
   seen in [RFC9110].

3.2.1.  Classic Negotiation

   This example demonstrates the negotiation of the "application/
   rdap+json" media type as defined in [RFC7480] using an RDAP "/help"
   query.  This example also demonstrates the negotiation in which a
   client does not support the "exts_list" parameter but a server does
   support the "exts_list" parameter.

   Client Request:

   GET /help HTTP/1.1
   accept: application/rdap+json

   Server Response:

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   HTTP/1.1 200 OK
   content-type: application/rdap+json;exts_list="rdap_level_0 exts"

   { "rdapConformance" : [ "rdap_level_0", "exts" ],
     "notices" : [
       { "description" : [ "my content includes a trailing CRLF" ] } ] }

3.2.2.  Negotiation of an RDAP Extension

   In this example, both the client and server support the "exts_list"
   parameter and a fictional extension of "foo".

   Client Request:

   GET /help HTTP/1.1
   accept: application/rdap+json;exts_list="rdap_level_0 exts foo"

   Server Response:

   HTTP/1.1 200 OK
   content-type: application/rdap+json;exts_list="rdap_level_0 exts foo"

   { "rdapConformance" : [ "rdap_level_0", "exts", "foo" ],
     "notices" : [
       { "description" : [ "my content includes a trailing CRLF" ] } ] }

3.2.3.  Negotiation of an RDAP Extension Without "Content-Type"

   In this example, both the client and server support the "exts_list"
   parameter and a fictional extension of "foo".  However, the server
   does not support the "exts_list" parameter in the "content-type"
   header.

   Client Request:

   GET /help HTTP/1.1
   accept: application/rdap+json;exts_list="rdap_level_0 exts foo"

   Server Response:

   HTTP/1.1 200 OK
   content-type: application/rdap+json

   { "rdapConformance" : [ "rdap_level_0", "exts", "foo" ],
     "notices" : [
       { "description" : [ "my content includes a trailing CRLF" ] } ] }

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3.2.4.  No Server Support for exts_list Parameter

   In this example, only the client supports the "exts_list" parameter,
   along with a fictional extension of "foo" by both.

   Client Request:

   GET /help HTTP/1.1
   accept: application/rdap+json;exts_list="rdap_level_0 exts foo"

   Server Response:

   HTTP/1.1 200 OK
   content-type: application/rdap+json

   { "rdapConformance" : [ "rdap_level_0", "foo" ],
     "notices" : [
       { "description" : [ "my content includes a trailing CRLF" ] } ] }

3.2.5.  Differing Extension Negotiation

   In this example, both the client and server support the "exts_list"
   parameter.  The client supports the extensions "foo" and "bar" while
   the server only support "foo".

   Client Request:

   GET /help HTTP/1.1
   accept: application/rdap+json;exts_list="rdap_level_0 exts foo bar"

   Server Response:

   HTTP/1.1 200 OK
   content-type: application/rdap+json;exts_list="rdap_level_0 exts foo"

   { "rdapConformance" : [ "rdap_level_0", "exts", "foo" ],
     "notices" : [
       { "description" : [ "my content includes a trailing CRLF" ] } ] }

3.2.6.  Extension Versioning and Meta-data

   For scenarios where the "versioning" extension, as defined by
   [I-D.ietf-regext-rdap-versioning], is used, the extension identifiers
   in the client request may not be exact or case-insensitive matches
   for the extension identifiers in the server response (unlike
   scenarios where the "versioning" extension is not used).  That is,
   the extension identifiers used by the client have appended versioning
   information, but the extension identifiers returned by the server do

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   not have appended versioning information (such information is in the
   "versioning" JSON).

   Client Request:

GET /domain/example.com HTTP/1.1
accept: application/rdap+json;exts_list="rdap_level_0 exts versioning_0_2"

   Server Response:

HTTP/1.1 200 OK
content-type: application/rdap+json;exts_list="rdap_level_0 exts versioning"

{ "rdapConformance" : [ "rdap_level_0", "exts", "versioning" ],
  "objectClassName": "domain",
  "ldhName": "example.com",
  "versioning": [ {
    "extension": "versioning",
    "type": "semantic",
    "version": "versioning_0_2" } ]
}

   Servers might also use the "versioning" extension to describe meta-
   data about supported extensions even if the servers do not explicitly
   support extension versioning.

4.  Usage in RDAP Links

   Section 4.2 of [RFC9083] defines a link structure used in RDAP.

   {
     "value": "https://example.com/context_uri",
     "rel": "self",
     "href": "https://example.com/target_uri",
     "hreflang": [ "en", "ch" ],
     "title": "title",
     "media": "screen",
     "type": "application/json"
   }

   The type attribute signals to a client the expected media type of the
   resource referenced in the href attribute, and some clients use this
   information to determine if the URI in the href attribute should be
   de-referenced.

   Usage of the "exts_list" parameter in the media type of the "type"
   attribute is allowed but the "type" attribute as a whole is only a
   hint, as noted by [RFC8288]:

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   |  The "type" attribute, when present, is a hint indicating what the
   |  media type of the result of dereferencing the link should be.
   |  Note that this is only a hint; for example, it does not override
   |  the Content-Type header field of a HTTP response obtained by
   |  actually following the link.

5.  Security Considerations

   As stated in Section 3, this specification does not override the
   protocol elements of RDAP security extensions, such as [RFC9560], nor
   does it override the protocol elements of other security features of
   HTTP.

   This specification does contrast with solutions using query
   parameters in that those solutions require servers to blindly copy
   query parameters into redirect URLs in situations where such copying
   could cause harm, such as copying an API key intended for one server
   into the redirect URL of another server.

6.  IANA Considerations

6.1.  RDAP Extension Registry

   The IETF requests the IANA to register the following extension in the
   RDAP Extensions Registry at [RDAP-EXTENSIONS]:

   Extension identifier: exts

   Registry operator: ALL

   Published specification: [RFC Reference Once Published]

   Person & email address to contact for further information:
   The Internet Engineering Steering Group <iesg@ietf.org>

   Intended usage: COMMON

6.2.  Addition of Parameter to RDAP Media Type

   This document defines the optional parameter "exts_list" for the
   media type "application/rdap+json" as described in Section 2.

   The IETF requests the IANA to add this document as an additional
   reference to the IANA Media Type registry at [MEDIA-TYPES] for the
   media type "application/rdap+json".

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7.  Acknowledgements

   Pawel Kowalik provided extensive review of this document and
   conducted a study that forms the basis of re-using the existing RDAP
   media type.  Mario Loffredo and James Mitchell have provided ideas
   and feedbacks that have contributed to the content of this document
   based on their implementation experience.  Murray Kucherawy and
   Alexey Melnikov provided guidance on the use of media types and media
   type parameters.  Jame Gould provided feedback that contributed to
   the content of this document.

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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7480]  Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the
              Registration Data Access Protocol (RDAP)", STD 95,
              RFC 7480, DOI 10.17487/RFC7480, March 2015,
              <https://www.rfc-editor.org/info/rfc7480>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

8.2.  Informative References

   [I-D.ietf-regext-rdap-extensions]
              Newton, A., Singh, J., and T. Harrison, "RDAP Extensions",
              Work in Progress, Internet-Draft, draft-ietf-regext-rdap-
              extensions-06, 29 April 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-regext-
              rdap-extensions-06>.

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   [I-D.ietf-regext-rdap-versioning]
              Gould, J., Keathley, D., and M. Loffredo, "Versioning in
              the Registration Data Access Protocol (RDAP)", Work in
              Progress, Internet-Draft, draft-ietf-regext-rdap-
              versioning-03, 23 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-regext-
              rdap-versioning-03>.

   [MEDIA-TYPES]
              IANA, "Media Types",
              <https://www.iana.org/assignments/media-types/>.

   [RDAP-EXTENSIONS]
              IANA, "RDAP Extensions",
              <https://www.iana.org/assignments/rdap-extensions/>.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC9083]  Hollenbeck, S. and A. Newton, "JSON Responses for the
              Registration Data Access Protocol (RDAP)", STD 95,
              RFC 9083, DOI 10.17487/RFC9083, June 2021,
              <https://www.rfc-editor.org/info/rfc9083>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

   [RFC9560]  Hollenbeck, S., "Federated Authentication for the
              Registration Data Access Protocol (RDAP) Using OpenID
              Connect", RFC 9560, DOI 10.17487/RFC9560, April 2024,
              <https://www.rfc-editor.org/info/rfc9560>.

Appendix A.  Using the Vary Header

   Server implementers may want to consider using the "vary" header
   depending on the caching behavior desired of shared caches (i.e.
   middleboxes, not client caches).

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   Consider the following scenario where user Bob and user Alice send
   queries to the same RDAP server that is routed through a middlebox
   network element implementing a shared HTTP cache.

   User Bob sends a query for the domain "example.com"
   (http://regy.example/domain/example.com (http://regy.example/domain/
   example.com)) without the "exts_list" parameter.  The "accept" header
   sent for Bob's query would be accept: application/rdap+json or
   accept: application/json.

   User Alice later sends a query for the same domain, however her
   client uses the "exts_list" parameter.  The "accept" header sent for
   Alice's query might be accept: application/
   rdap+json;exts_list="rdap_level_0 exts foo".

   If no "vary" header is set in the response for these queries, the
   shared cache will compare only the URL of the query when processing
   cache items and therefore user Bob and user Alice would receive the
   same answer.  In other words, since both queried
   "http://regy.example/domain/example.com" (http://regy.example/domain/
   example.com") the shared cache would return the answer of the first
   query to the second query and all other subsequent queries until the
   item expired out of the cache.

   If server implementers do not desire this behavior and would signal
   that caches consider each query separately, servers should also
   return a "vary: accept" header to inform the cache that the "accept"
   header should also be considered when processing cache items.  Server
   implementers should also consult [RFC9110] regarding caching and
   other uses of the "vary" header.

Appendix B.  Design Considerations

B.1.  Reusing the Existing Media Type

   Earliest versions of this document specified a new media type because
   the authors believed the addition of new parameter on the existing
   RDAP media type may be backwards-incompatible with many RDAP servers.
   However, a study conducted by Pawel Kowalik concluded that 99.65% of
   RDAP servers are compatible with a new parameter on the existing RDAP
   media type.

   Additionally, [RFC2045] requires that the server ignore unknown
   parameters.

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B.2.  Inappropriate Use of Query Parameters

   Another design approach to communicating RDAP extensions from the
   client to the server would be the use of URI query parameters:

   https://rdap.example/domain/foo.example?extensions=fizzbuzz

   However, there are a few problems with using query parameters for
   this scenario.  Some of these problems are specific to RDAP and are
   also documented in [I-D.ietf-regext-rdap-extensions].  The following
   sections also describe the problems.

B.2.1.  Copy and Paste

   Consider two RDAP users, Alice and Bob. Alice has an RDAP client that
   supports the extension "fizzbuzz", and Bob has an RDAP client that
   does not support this extension.

   Now consider the scenario where Alice copies and pastes the RDAP URL
   from above into an email and sends it to Bob. When Bob uses that URL
   with his RDAP client, it will be communicating to the server that the
   extension "fizzbuzz" is understood by Bob's client when it is not.

   In this scenario, Bob's client will be unable to render the RDAP
   extension regardless of the usage or not of the query parameter.
   However, if the server is using the query parameter for secondary
   purposes, such as gathering metrics and statistics, then the
   capabilities of Bob's client will have been incorrectly signalled to
   the server.

B.2.2.  Redirects

   The RDAP ecosystem uses redirects in many situations.  [RFC7480]
   discusses "aggregators", which are RDAP servers used to help clients
   find authoritative RDAP servers using the RDAP bootstrap registries.
   Redirects are also heavily used by the RIRs when IP addresses or
   autonomous system numbers are transferred from one RIR to another.

   Within HTTP, URI query parameters are not explicitly preserved during
   a redirect (probably due to architecture considerations, see the
   section below).  Specific to RDAP, [RFC7480] instructs RDAP servers
   to ignore unknown query parameters and instructs clients not to
   transform the URL of a redirect.

   Therefore, query parameters denoting RDAP extensions should not
   survive redirects in RDAP.  This can be readily observed in currently
   deployed RDAP servers:

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curl -v https://rdap-bootstrap.arin.net/bootstrap/autnum/2830?extension=fizzbuzz

   To further demonstrate that query parameters do not automatically
   survive redirects but that media types do, consider the code found
   here (https://github.com/anewton1998/draft-regext-ext-json-media-
   type).  This code consists of a simple client and a simple server.
   The client sets both a new media type and query parameters.  The
   servers listen on two ports, redirecting the client from a URL on the
   first port to a URL on the second port.

   Preservation of query parameters is not a guaranteed feature of HTTP
   client and server libraries, whereas preservation of media types is
   much more likely to occur.

B.2.3.  Referral Compatibility

   It is common in the RDAP ecosystem to link from one RDAP resource to
   another.  These are typically conveyed in the link structure defined
   in Section 4.2 of [RFC9083] and use the "application/rdap+json" media
   type.  One common usage is to link to a domain registration in a
   domain registrar from a domain registration in a domain registry.

   {
     "value" : "https://regy.example/domain/foo.example",
     "rel" : "related",
     "href" : "https://regr.example/domain/foo.example",
     "type" : "application/rdap+json"
   }

   Usage of the "exts_list" parameter does not require clients to
   conduct further processing of these referrals, whereas a query
   parameter approach would require clients to process and de-conflict
   any other query parameters if present.

B.2.4.  Architectural Violations

   Section 3.4 of [RFC3986] states the following:

   |  The query component contains non-hierarchical data that, along
   |  with data in the path component (Section 3.3), serves to identify
   |  a resource within the scope of the URI's scheme and naming
   |  authority (if any).

   Therefore, URI query parameters are meant to be part of the identity
   of the resource being identified by a URI and pointed to by the
   location of a URL.  RDAP extensions change the portions of JSON
   returned by the server but are not intended to change the resource
   being identified.  That is, a domain registration is the same domain

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   registration regardless of whether the postal address in that domain
   registration is communicated via JCard or a new RDAP extension for
   JSContact.

   Changing how the content of a resource is conveyed is called content
   negotiation and is discussed in detail in [RFC9110] using media
   types.

   Readers should note that protocol design is not a "priestly affair"
   in which architectural violations are strictly forbidden.  Every
   design decision is a trade-off.  However, following the architecture
   of an ecosystem generally makes re-use of software and systems
   easier, and often eases the adoption of newer features in the future.
   When given the choice between two designs, the design that does not
   violate architecture should be preferred when all other
   considerations are equal.

Authors' Addresses

   Andy Newton
   ICANN
   Email: andy@hxr.us

   Jasdip Singh
   ARIN
   Email: jasdips@arin.net

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