Guidelines for Extending the Extensible Provisioning Protocol (EPP)

Network Working Group                                      S. Hollenbeck
Request for Comments: 3735                                VeriSign, Inc.
Category: Informational                                       March 2004


  Guidelines for Extending the Extensible Provisioning Protocol (EPP)

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   The Extensible Provisioning Protocol (EPP) is an application layer
   client-server protocol for the provisioning and management of objects
   stored in a shared central repository.  Specified in XML, the
   protocol defines generic object management operations and an
   extensible framework that maps protocol operations to objects.  This
   document presents guidelines for use of EPP's extension mechanisms to
   define new features and object management capabilities.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . .  2
       1.1.  Conventions Used In This Document. . . . . . . . . . .  2
   2.  Principles of Protocol Extension . . . . . . . . . . . . . .  3
       2.1.  Documenting Extensions . . . . . . . . . . . . . . . .  3
       2.2.  Identifying Extensions . . . . . . . . . . . . . . . .  4
             2.2.1.  Standards Track Extensions . . . . . . . . . .  4
             2.2.2.  Other Extensions . . . . . . . . . . . . . . .  5
       2.3.  Extension Announcement and Selection . . . . . . . . .  5
       2.4.  Protocol-level Extension . . . . . . . . . . . . . . .  7
       2.5.  Object-level Extension . . . . . . . . . . . . . . . .  7
       2.6.  Command-Response Extension . . . . . . . . . . . . . .  7
       2.7.  Authentication Information Extension . . . . . . . . .  7
   3.  Selecting an Extension Mechanism . . . . . . . . . . . . . .  8
       3.1.   Mapping and Extension Archives  . . . . . . . . . . .  9
   4.  Internationalization Considerations  . . . . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . 10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . 10
       7.1.  Normative References . . . . . . . . . . . . . . . . . 10
       7.2.  Informative References . . . . . . . . . . . . . . . . 11
   8.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  Author's Address . . . . . . . . . . . . . . . . . . . . . . 12
   10. Full Copyright Statement . . . . . . . . . . . . . . . . . . 13

1. Introduction

   The Extensible Provisioning Protocol (EPP, [2]) was originally
   designed to provide a standard Internet domain name registration
   protocol for use between Internet domain name registrars and domain
   name registries.  However, specific design decisions were made to
   ensure that the protocol could also be used in other provisioning
   environments.  Specifically:

   o  Extensibility has been a design goal from the very beginning.  EPP
      is represented in the Extensible Markup Language (XML, [3]), and
      is specified in XML Schema ([4] and [5]) with XML namespaces [6]
      used to identify protocol grammars.

   o  The EPP core protocol specification describes general protocol
      functions, not objects to be managed by the protocol.  Managed
      object definitions, such as the mapping for Internet domain names
      [10] (itself a protocol extension), are loosely coupled to the
      core specification.

   o  A concentrated effort was made to separate required minimum
      protocol functionality from object management operating logic.

   o  Several extension mechanisms were included to allow designers to
      add new features or to customize existing features for different
      operating environments.

   This document describes EPP's extensibility features in detail and
   provides guidelines for their use.  Though written specifically for
   protocol designers considering EPP as the solution to a provisioning
   problem, anyone interested in using XML to represent IETF protocols
   might find these guidelines useful.

   XML is case sensitive.  Unless stated otherwise, XML instances and
   examples provided in this document MUST be interpreted in the
   character case presented to develop a conforming implementation.

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 [1].

   In examples, "C:" represents lines sent by a protocol client and "S:"
   represents lines returned by a protocol server.  Indentation and
   white space in examples is provided only to illustrate element
   relationships and is not a REQUIRED feature of this specification.

2.  Principles of Protocol Extension

   The EPP extension model is based on the XML representation for a
   wildcard schema component using an <any> element information item (as
   described in section 3.10.2 of [4]) and XML namespaces [6].  This
   section provides guidelines for the development of protocol
   extensions and describes the extension model in detail.

   Extending a protocol implies the addition of features without
   changing the protocol itself.  In EPP that means that an extension
   MUST NOT alter an existing protocol schema as changes may result in
   new versions of an existing schema, not extensions of an existing
   schema.  For example, a designer MUST NOT add new elements to an
   existing schema and call the result an "extension" of the protocol.
   The result is a new, non-backwards-compatible version of an existing
   schema.  Extensions MUST adhere to the principles described in this
   section to be considered valid protocol extensions.

   EPP extensions MUST be specified in XML.  This ensures that parsers
   capable of processing EPP structures will also be capable of
   processing EPP extensions.  Guidelines for the use of XML in IETF
   protocols (thus good information for extension designers) can be
   found in RFC 3470 [11].

   A designer MUST remember that extensions themselves MAY also be
   extensible.  A good chain of extensions is one in which the protocol
   schemas evolve from general functionality to more specific (perhaps
   even more limited) functionality.

2.1.  Documenting Extensions

   The EPP core specification [2] has an appendix that contains a
   suggested outline to document protocol extensions.  Designers are
   free to use this template or any other format as they see fit, but
   the extension document SHOULD at a minimum address all of the topics
   listed in the template.

   Extension designers need to consider the intended audience and
   consumers of their extensions.  Extensions MAY be documented as
   Internet-Draft and RFC documents if the designer is facing
   requirements for coordination, interoperability, and broad
   distribution, though the intended maturity level (informational,
   proposed standard, etc.) largely depends on what is being extended
   and the amount of general interest in the extension.  An extension to
   a standards-track specification with broad interest might well be a
   candidate for standards track publication, whereas an extension to a
   standards track specification with limited interest might be better
   suited for informational publication.

   Extensions need not be published as Internet-Draft or RFC documents
   if they are intended for operation in a closed environment or are
   otherwise intended for a limited audience.  In such cases extensions
   MAY be documented in a file and structural format that is appropriate
   for the intended audience.

2.2.  Identifying Extensions

   An EPP extension is uniquely identified by a Uniform Resource
   Identifier (URI, defined in RFC 2396 [7]).  The URI used to identify
   the extension MUST also be used to identify the XML namespace
   associated with the extension.  Any valid URI MAY be used to identify
   an EPP extension, though the selection of a URI form (Uniform
   Resource Locator (URL) vs. Uniform Resource Name (URN), hierarchical
   vs. relative, etc.) SHOULD depend on factors such as organizational
   policies on change control and a balance between locating resources
   and requirements for persistence.  An extension namespace MAY
   describe multiple extension mechanisms, such as definition of new
   protocol features, objects, or elements, within the schema used to
   define the namespace.

   The following are sample extension-identifying URIs:

      urn:ietf:params:xml:ns:foo-ext1

      http://custom/obj1ext-1.0

   Extension designers MAY include version information in the URI used
   to identify an extension.  If version information is included in the
   URI, the URI itself will need to change as the extension is revised
   or updated.

2.2.1.  Standards Track Extensions

   URIs for extensions intended for IETF standards track use MUST
   conform to the URN syntax specifications and registration procedures
   described in [8].

2.2.2.  Other Extensions

   URIs for extensions that are not intended for IETF standards track
   use MUST conform to the URI syntax specifications described in RFC
   2396.

2.3.  Extension Announcement and Selection

   An EPP server MUST announce extensions that are available for client
   use as part of a <greeting> element that is sent to a client before
   the client establishes an interactive session with the server.  The
   <greeting> element contains zero or more <svcExtension> elements
   that, if present, contain a URI identifying an available extension:

   S:<?xml version="1.0" encoding="UTF-8" standalone="no"?>
   S:<epp xmlns="urn:ietf:params:xml:ns:epp-1.0"
   S:     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   S:     xsi:schemaLocation="urn:ietf:params:xml:ns:epp-1.0
   S:     epp-1.0.xsd">
   S:  <greeting>
   S:    <svID>Example EPP server epp.example.com</svID>
   S:    <svDate>2000-06-08T22:00:00.0Z</svDate>
   S:    <svcMenu>
   S:      <version>1.0</version>
   S:      <lang>en</lang>
   S:      <lang>fr</lang>
   S:      <objURI>urn:ietf:params:xml:ns:obj1</objURI>
   S:      <objURI>urn:ietf:params:xml:ns:obj2</objURI>
   S:      <objURI>urn:ietf:params:xml:ns:obj3</objURI>
   S:      <svcExtension>
   S:        <extURI>urn:ietf:params:xml:ns:foo-ext1</extURI>
   S:        <extURI>http://custom/obj1ext-1.0</extURI>
   S:      </svcExtension>
   S:    </svcMenu>
   S:    <dcp>
   S:      <access><all/></access>
   S:      <statement>
   S:        <purpose><admin/><prov/></purpose>
   S:        <recipient><ours/><public/></recipient>
   S:        <retention><stated/></retention>
   S:      </statement>
   S:    </dcp>
   S:  </greeting>
   S:</epp>

   In the example above, the server is announcing the availability of
   two extensions:

      urn:ietf:params:xml:ns:foo-ext1, and

      http://custom/obj1ext-1.0

   An EPP client MUST establish a session with an EPP server using the
   EPP <login> command before attempting to use any standard commands or
   extensions.  The <login> command contains zero or more <svcExtension>
   elements that, if present, contain a URI identifying an available
   extension that the client wishes to use during the course of the
   session:

   C:<?xml version="1.0" encoding="UTF-8" standalone="no"?>
   C:<epp xmlns="urn:ietf:params:xml:ns:epp-1.0"
   C:     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   C:     xsi:schemaLocation="urn:ietf:params:xml:ns:epp-1.0
   C:     epp-1.0.xsd">
   C:  <command>
   C:    <login>
   C:      <clID>ClientX</clID>
   C:      <pw>foo-BAR2</pw>
   C:      <newPW>bar-FOO2</newPW>
   C:      <options>
   C:        <version>1.0</version>
   C:        <lang>en</lang>
   C:      </options>
   C:      <svcs>
   C:        <objURI>urn:ietf:params:xml:ns:obj1</objURI>
   C:        <objURI>urn:ietf:params:xml:ns:obj2</objURI>
   C:        <objURI>urn:ietf:params:xml:ns:obj3</objURI>
   C:        <svcExtension>
   C:          <extURI>http://custom/obj1ext-1.0</extURI>
   C:        </svcExtension>
   C:      </svcs>
   C:    </login>
   C:    <clTRID>ABC-12345</clTRID>
   C:  </command>
   C:</epp>

   In the example above, the client indicates that it wishes to use an
   extension identified by the http://custom/obj1ext-1.0 URI during the
   session established upon successful completion of the <login>
   command.

   An EPP server MUST announce all extensions that are publicly
   available for client use.  An EPP client MUST NOT request an
   extension that has not been announced by the server.  An EPP server
   MAY restrict a client's ability to select an extension based on a
   client's identity and authorizations granted by the server operator.

2.4.  Protocol-level Extension

   EPP defines a set of structures for client-server command-response
   interaction, but additional structures MAY be added to the protocol.
   New structure definition is a matter of defining a schema for the
   structures that defines needed functionality and assigning a URI to
   uniquely identify the object namespace and schema.  Specific
   protocol-level extension mechanisms are described in section 2.7.1 of
   the EPP core protocol specification [2].

2.5.  Object-level Extension

   EPP commands and responses do not contain attributes that are
   specific to any managed object.  Every command and response MUST
   contain elements bound to an object namespace.  Object definition is
   a matter of defining a schema for the object that defines
   functionality for each needed command and associated response, and
   assigning a URI to uniquely identify the object namespace and schema.
   Specific object-level extension mechanisms are described in section
   2.7.2 of the EPP core protocol specification [2].

2.6.  Command-Response Extension

   EPP command and response structures defined in existing object
   mappings MAY also be extended.  For example, an object mapping that
   describes general functionality for the provisioning of Internet
   domain names can be extended to included additional command and
   response elements needed for the provisioning of domain names that
   represent E.164 telephone numbers [12].  Specific command-response
   extension mechanisms are described in section 2.7.3 of the EPP core
   protocol specification [2].

2.7.  Authentication Information Extension

   Some EPP object mappings, such as the Internet domain name mapping
   [10], include elements to associate authentication information (such
   as a password) with an object.  The schema for any object mapping
   that supports authentication information SHOULD be flexible enough to
   specify multiple forms of authentication information.  With XML
   Schema ([4] and [5]), this can be accomplished by offering an element
   choice that includes an <any> element information item:

      <any namespace="##other"/>

3.  Selecting an Extension Mechanism

   Extensibility is a powerful feature of XML, but it also provides
   multiple opportunities to make poor design decisions.  There are
   typically several different ways to accomplish a single task, and
   while all may "work" (for some definition of "work") one extension
   form will usually be more appropriate than others to complete a given
   task.  The following sequence of steps can be followed to select an
   appropriate extension form to solve an extension problem:

   o  Command-Response Extension: Adding elements to an existing object
      mapping is the simplest form of extension available, and is thus
      the form that should be explored before any other form is
      considered.  The first question to ask when considering an
      extension form is thus:

         Can the task be accomplished by adding to an existing object
         mapping or changing an existing object mapping slightly?

      If the answer to this question is "yes", you should consider
      extending an existing object mapping to complete your task.
      Knowing where to find object mappings is critical to being able to
      answer this question; see section Section 3.1 for information
      describing mapping archives.  If the answer to this question is
      "no", consider an object-level extension next.

   o  Object-level Extension: If there is no existing object mapping
      that can be extended to meet your requirements, consider
      developing a new object mapping.  The second question to ask when
      considering an extension form is thus:

         Can the task be accomplished using the existing EPP command and
         response structures applied to a new object?

      If the answer to this question is "yes", you should consider
      developing a new object mapping to complete your task.  A new
      object mapping should differ significantly from existing object
      mappings; if you find that a new mapping is replicating a
      significant number of structures found in an existing mapping you
      probably answered the command-response question incorrectly.  If
      the answer to this question is "no", consider a protocol-level
      extension next.

   o  Protocol-level Extension: If there is no existing object mapping
      that can be extended to meet your requirements and the existing
      EPP command and response structures are insufficient, consider
      developing new protocol commands, responses, or other structures.
      The third and final question to ask when considering an extension
      form is thus:

         Can the task be accomplished by adding new EPP commands,
         responses, or other structures applied to new or existing
         objects?

      If the answer to this question is "no", EPP can not be used
      directly to complete your task.  If the answer to this question is
      "yes", extend the protocol by defining new operational structures.

   The extension forms and decision points listed here are presented in
   order of complexity.  Selecting an extension form without careful
   consideration of the available extension options can add complexity
   without any gain in functionality.

3.1.  Mapping and Extension Archives

   Existing object mappings are a critical resource when trying to
   select an appropriate extension form.  Existing mappings or
   extensions can provide a solid basis for further extension, but
   designers have to know where to find them to consider them for use.

   Several organizations maintain archives of XML structures that can be
   useful extension platforms.  These include:

   o  The IETF: Object mappings and other extensions have been
      documented in RFCs and Internet-Drafts.

   o  IANA: Guidelines and registration procedures for an IANA XML
      registry used by the IETF are described in "The IETF XML Registry"
      [8].

   o  OASIS [16]: OASIS maintains an XML archive containing schema
      definitions for use in the business applications of XML.

   o  XML.org [17]: XML.org maintains an XML archive containing schema
      definitions for use in multiple industries.

   o  Other archives are likely in the future.  Consult your favorite
      Internet search engine for additional resources.

4.  Internationalization Considerations

   EPP is represented in XML [3], which requires conforming parsers to
   recognize both UTF-8 [13] and UTF-16 [14]; support for other
   character encodings is also possible.  EPP extensions MUST observe
   both the Internationalization Considerations described in the EPP
   core protocol specification [2] and IETF policy on the use of
   character sets and languages described in RFC 2277 [9].

5.  IANA Considerations

   This memo has no direct impact on the IANA.  Guidelines for
   extensions that require IANA action are described in Section 2.2.1.

6. Security Considerations

   EPP extensions inherit the security services of the protocol
   structure that's being extended.  For example, an extension of an
   object mapping inherits all of the security services of the object
   mapping.  Extensions MAY specify additional security services, such
   as services for peer entity authentication, confidentiality, data
   integrity, authorization, access control, or non-repudiation.
   Extensions MUST NOT mandate removal of security services available in
   the protocol structure being extended.

   Protocol designers developing EPP extensions need to be aware of the
   security threats to be faced in their intended operating environment
   so that appropriate security services can be provided.  Guidelines
   for designers to consider and suggestions for writing an appropriate
   Security Considerations section can be found in RFC 3552 [15].

7.  References

7.1.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)", RFC
        3730, March 2004.

   [3]  Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler,
        "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C REC-xml,
        October 2000, <http://www.w3.org/TR/REC-xml>.

   [4]  Thompson, H., Beech, D., Maloney, M. and N. Mendelsohn, "XML
        Schema Part 1: Structures", W3C REC-xmlschema-1, May 2001,
        <http://www.w3.org/TR/xmlschema-1/>.

   [5]  Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes", W3C
        REC-xmlschema-2, May 2001, <http://www.w3.org/TR/xmlschema-2/>.

   [6]  Bray, T., Hollander, D. and A. Layman, "Namespaces in XML", W3C
        REC-xml-names, January 1999, <http://www.w3.org/TR/REC-xml-
        names>.

   [7]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
        Identifiers (URI): Generic Syntax", RFC 2396, August 1998.

   [8]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, January
        2004.

   [9]  Alvestrand, H., "IETF Policy on Character Sets and Languages",
        BCP 18, RFC 2277, January 1998.

7.2.  Informative References

   [10] Hollenbeck, S., "Extensible Provisioning Protocol (EPP) Domain
        Name Mapping", RFC 3731, March 2004.

   [11] Hollenbeck, S., Rose, M. and L. Masinter, "Guidelines for the
        Use of Extensible Markup Language (XML) within IETF Protocols",
        BCP 70, RFC 3470, January 2003.

   [12] Hollenbeck, S., "Extensible Provisioning Protocol E.164 Number
        Mapping", Work in Progress, February 2003.

   [13] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
        2279, January 1998.

   [14] Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO 10646",
        RFC 2781, February 2000.

   [15] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on
        Security Considerations", BCP 72, RFC 3552, July 2003.

8.  URIs

   [16]  <http://oasis-open.org/>

   [17]  <http://xml.org/>

9.  Author's Address

   Scott Hollenbeck
   VeriSign, Inc.
   21345 Ridgetop Circle
   Dulles, VA  20166-6503
   USA

   EMail: shollenbeck@verisign.com

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