Extensible Provisioning Protocol (EPP) mapping for DNS Time-To-Live (TTL) values
draft-ietf-regext-epp-ttl-04

Registration Protocols Extensions (regext)                      G. Brown
Internet-Draft                                                     ICANN
Intended status: Standards Track                        14 December 2023
Expires: 16 June 2024

  Extensible Provisioning Protocol (EPP) mapping for DNS Time-To-Live
                              (TTL) values
                      draft-ietf-regext-epp-ttl-04

Abstract

   This document describes an extension to the Extensible Provisioning
   Protocol (EPP) that allows EPP clients to manage the Time-To-Live
   (TTL) value for domain name delegation records.

About this draft

   This note is to be removed before publishing as an RFC.

   The source for this draft, and an issue tracker, may can be found at
   https://github.com/gbxyz/epp-ttl-extension.

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 16 June 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.

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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions used in this document . . . . . . . . . . . .   3
     1.2.  Extension elements  . . . . . . . . . . . . . . . . . . .   4
       1.2.1.  Element content . . . . . . . . . . . . . . . . . . .   5
       1.2.2.  Supported DNS record types  . . . . . . . . . . . . .   5
         1.2.2.1.  Glue records  . . . . . . . . . . . . . . . . . .   6
       1.2.3.  Examples  . . . . . . . . . . . . . . . . . . . . . .   6
         1.2.3.1.  Explicit TTL value  . . . . . . . . . . . . . . .   6
         1.2.3.2.  Empty value indicating default TTL  . . . . . . .   6
         1.2.3.3.  Custom record type  . . . . . . . . . . . . . . .   6
   2.  EPP command mapping . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  EPP query commands  . . . . . . . . . . . . . . . . . . .   6
       2.1.1.  EPP <info> command  . . . . . . . . . . . . . . . . .   6
     2.2.  EPP transform commands  . . . . . . . . . . . . . . . . .   8
       2.2.1.  EPP <create> command  . . . . . . . . . . . . . . . .   9
       2.2.2.  EPP <update> command  . . . . . . . . . . . . . . . .  10
   3.  Server processing of TTL values . . . . . . . . . . . . . . .  12
     3.1.  Permitted record types  . . . . . . . . . . . . . . . . .  12
     3.2.  Use of TTL values in delegation records . . . . . . . . .  12
   4.  Out-of-band changes to TTL values . . . . . . . . . . . . . .  12
   5.  Operational considerations  . . . . . . . . . . . . . . . . .  12
     5.1.  Operational impact of TTL values  . . . . . . . . . . . .  12
     5.2.  When TTL values should be changed . . . . . . . . . . . .  13
   6.  Security considerations . . . . . . . . . . . . . . . . . . .  13
   7.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  XML namespace . . . . . . . . . . . . . . . . . . . . . .  13
     7.2.  EPP extension registry  . . . . . . . . . . . . . . . . .  14
   8.  Formal syntax . . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  Implementation status . . . . . . . . . . . . . . . . . . . .  17
   10. Change log  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     10.1.  Change from 03 to 04 . . . . . . . . . . . . . . . . . .  18
     10.2.  Change from 02 to 03 . . . . . . . . . . . . . . . . . .  18
     10.3.  Change from 01 to 02 . . . . . . . . . . . . . . . . . .  18
     10.4.  Change from 00 to 01 . . . . . . . . . . . . . . . . . .  18
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.1.  Normative references . . . . . . . . . . . . . . . . . .  19
     12.2.  Informative references . . . . . . . . . . . . . . . . .  20
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20

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1.  Introduction

   The principal output of any domain name provisioning system is a DNS
   zone file, which contains the delegation record(s) for names
   registered within a zone (such as a top-level domain).  These records
   typically include one or more NS records, but may also include DS
   records for domains secured with DNSSEC ([RFC9364]), and DNAME
   records for IDN variants ([RFC6927]).  Where glue (see Section 7 of
   [RFC8499]) is required, A and/or AAAA records may also be published
   for nameservers.

   Typically, the Time-To-Live value (TTL, see Section 5 of [RFC8499])
   of these records is determined by the registry operator.  However, in
   some circumstances it may be desirable to allow the sponsoring client
   of a domain name to change the TTL values used for that domain's
   delegation: for example, to reduce the amount of time required to
   complete a change of DNS servers, DNSSEC deployment or key rollover,
   or to allow for fast rollback of such changes.

   This document describes an EPP extension to the domain name and host
   object mappings (described in [RFC5731] and [RFC5732], respectively)
   which allows the sponsor of a domain name or host object to change
   the TTL values of the resource record(s) associated with that object.
   It also describes how EPP servers should handle TTLs specified by EPP
   clients, and how both parties co-ordinate to manage TTL values in
   response to changes in operational or security requirements.

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].

   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 are provided only to illustrate element
   relationships and are not required features of this protocol.

   A protocol client that is authorized to manage an existing object is
   described as a "sponsoring" client throughout this document.

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

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   EPP uses XML namespaces to provide an extensible object management
   framework and to identify schemas required for XML instance parsing
   and validation.  These namespaces and schema definitions are used to
   identify both the base protocol schema and the schemas for managed
   objects.

   The XML namespace prefixes used in examples (such as the string ttl
   in ttl:create) are solely for illustrative purposes.  A conforming
   implementation MUST NOT require the use of these or any other
   specific namespace prefixes.

1.2.  Extension elements

   This extension adds additional elements to the EPP domain and host
   mappings.

   The <ttl:ttl> element is used to define TTL values for the DNS
   resource records associated with domain and host objects.

   The <ttl:ttl> element may have the following attributes, depending on
   whether it appears in a command or response frame:

   1.  "for", which is REQUIRED in both commands and responses, and
       which specifies the DNS record type to which the TTL value
       pertains.  This attribute MUST have one of the following values:
       "NS", "DS", "DNAME", "A", "AAAA" or "custom";

   2.  If the value of the "for" attribute is "custom", then the
       <ttl:ttl> element MUST also have a "custom" attribute containing
       a DNS record type conforming with the regular expression in
       Section 1.3 of [RFC6895].  Additionally, the record type MUST be
       registered with IANA.

   3.  "min", which MUST NOT be present in commands frames but MUST be
       present in response frames, and which is used by the server to
       indicate the lowest value that may be set;

   4.  "default", which MUST NOT be present in commands frames but MUST
       be present in response frames, and which is used by the server to
       indicate the default value;

   5.  "max", which MUST NOT be present in commands frames but MUST be
       present in response frames, and which is used by the server to
       indicate the highest value that may be set;

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1.2.1.  Element content

   The XML schema found in Section 8 of this document restricts the
   content of <ttl:ttl> elements to be either:

   1.  a non-negative integer, indicating the value of the TTL in
       seconds, or

   2.  empty, in which case the server's default TTL for the given
       record type is to be applied.

1.2.2.  Supported DNS record types

   To facilitate forward compatibility with future changes to the DNS
   protocol, this document does not enumerate or restrict the DNS record
   types that can be included in the "for" and "custom" attributes of
   <ttl:ttl> elements.

   The regular expression which is used to validate the values of the
   "custom" attribute is based on the expression found in Section 1.3 of
   [RFC6895], and is intended to match any existing and future RRTYPE
   mnemonic registered with the IANA: this eliminates the need to update
   this document in the event that a new DNS record that exists above a
   zone cut is specified.

   Nevertheless, EPP servers which implement this extension MUST
   restrict the DNS record types that are accepted in <create> and
   <update> commands, and included in <info> responses, allowing only
   those types that are actually published in the DNS for domain and
   host objects.

   A server which receives a <create> or <update> command which includes
   an invalid DNS record type MUST respond with a 2004 "Parameter value
   range" error.

   As an illustrative example, a server MAY allow clients to specify TTL
   values for the following record types for domain objects:

   1.  NS;

   2.  DS (if the server also implements [RFC5910]);

   3.  DNAME (if the registry implements IDN variants using DNAME
       records).

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1.2.2.1.  Glue records

   Servers which implements host objects ([RFC5732]) MAY allow clients
   to specify TTL values for A and AAAA records for host objects.  These
   servers MUST reject commands which attempt to set TTL values for
   these record types for domain objects.

   EPP servers which uses the "host attribute" model (described in
   Section 1.1 of [RFC5731]) MAY allow clients to specify TTL values for
   A and AAAA records for domain objects using a 2004 "Parameter value
   range" error.

1.2.3.  Examples

1.2.3.1.  Explicit TTL value

   <ttl:ttl for="NS">3600<ttl:ttl>

1.2.3.2.  Empty value indicating default TTL

   <ttl:ttl for="NS"/>

1.2.3.3.  Custom record type

   <ttl:ttl
     for="custom"
     custom="DELEG">3600<ttl:ttl>

2.  EPP command mapping

2.1.  EPP query commands

2.1.1.  EPP <info> command

   This extension defines an additional element for EPP <info> responses
   for domain and host objects.

   When a server receives a domain or host <info> command from a client
   which included the extension namespace in the <svcExtension> element
   of the <login> command, the response MUST contain an <extension>
   element, which MUST contain a <ttl:infData> element.  This element
   MUST contain one or more <ttl:ttl> records as described in
   Section 1.2.

   Example domain <info> response:

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   S:<?xml version="1.0" encoding="utf-8" standalone="no"?>
   S:<epp xmlns="urn:ietf:params:xml:ns:epp-1.0">
   S:  <response>
   S:    <result code="1000">
   S:      <msg>Command completed successfully</msg>
   S:    </result>
   S:    <resData>
   S:      <domain:infData
   S:        xmlns:domain="urn:ietf:params:xml:ns:domain-1.0">
   S:        <domain:name>example.com</domain:name>
   S:        <domain:roid>EXAMPLE1-REP</domain:roid>
   S:        <domain:status s="ok"/>
   S:        <domain:ns>
   S:          <domain:hostObj>ns1.example.com</domain:hostObj>
   S:          <domain:hostObj>ns1.example.net</domain:hostObj>
   S:        </domain:ns>
   S:        <domain:clID>ClientX</domain:clID>
   S:        <domain:crID>ClientX</domain:crID>
   S:        <domain:crDate>2023-11-08T10:14:55.0Z</domain:crDate>
   S:        <domain:exDate>2024-11-08T10:14:55.0Z</domain:exDate>
   S:      </domain:infData>
   S:    </resData>
   S:    <extension>
   S:      <ttl:infData
   S:        xmlns:ttl="urn:ietf:params:xml:ns:epp:ttl-1.0">
   S:        <ttl:ttl for="NS"
   S:          min="3600"
   S:          default="86400"
   S:          max="172800">172800</ttl:ttl>
   S:        <ttl:ttl for="DS"
   S:          min="60"
   S:          default="86400"
   S:          max="172800">300</ttl:ttl>
   S:      </ttl:infData>
   S:      <secDNS:infData
   S:        xmlns:secDNS="urn:ietf:params:xml:ns:secDNS-1.1">
   S:        <secDNS:dsData>
   S:          <secDNS:keyTag>12345</secDNS:keyTag>
   S:          <secDNS:alg>13</secDNS:alg>
   S:          <secDNS:digestType>2</secDNS:digestType>
   S:          <secDNS:digest>49FD46E6C4B45C55D4AC</secDNS:digest>
   S:        </secDNS:dsData>
   S:      </secDNS:infData>
   S:    </extension>
   S:    <trID>
   S:      <clTRID>ABC-12345</clTRID>
   S:      <svTRID>54322-XYZ</svTRID>
   S:    </trID>

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   S:  </response>
   S:</epp>

   Example host <info> response:

   S:<?xml version="1.0" encoding="UTF-8" standalone="no"?>
   S:<epp xmlns="urn:ietf:params:xml:ns:epp-1.0">
   S:  <response>
   S:    <result code="1000">
   S:      <msg>Command completed successfully</msg>
   S:    </result>
   S:    <resData>
   S:      <host:infData
   S:        xmlns:host="urn:ietf:params:xml:ns:host-1.0">
   S:        <host:name>ns1.example.com</host:name>
   S:        <host:roid>NS1_EXAMPLE1-REP</host:roid>
   S:        <host:status s="ok"/>
   S:        <host:addr ip="v4">192.0.2.2</host:addr>
   S:        <host:addr ip="v6">1080::8:800:200C:417A</host:addr>
   S:        <host:clID>ClientX</host:clID>
   S:        <host:crID>ClientX</host:crID>
   S:        <host:crDate>2023-11-08T10:14:55.0Z</host:crDate>
   S:      </host:infData>
   S:    </resData>
   S:    <extension>
   S:      <ttl:infData
   S:        xmlns:ttl="urn:ietf:params:xml:ns:epp:ttl-1.0">
   S:        <ttl:ttl for="A"
   S:          min="3600"
   S:          default="86400"
   S:          max="172800"/>
   S:        <ttl:ttl for="AAAA"
   S:          min="3600"
   S:          default="86400"
   S:          max="172800">86400</ttl:ttl>
   S:      </ttl:infData>
   S:    </extension>
   S:    <trID>
   S:      <clTRID>ABC-12345</clTRID>
   S:      <svTRID>54322-XYZ</svTRID>
   S:    </trID>
   S:  </response>
   S:</epp>

2.2.  EPP transform commands

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2.2.1.  EPP <create> command

   This extension defines an additional element for EPP <create>
   commands for domain and host objects.

   The <command> element of the <create> command frame MAY contain an
   <extension> element which MAY contain a <ttl:create> element.  This
   element MUST contain one or more <ttl:ttl> records as described in
   Section 1.2.

   Example domain <create> command:

   C:<?xml version="1.0" encoding="UTF-8" standalone="no"?>
   C:<epp xmlns="urn:ietf:params:xml:ns:epp-1.0">
   C:  <command>
   C:    <create>
   C:      <domain:create
   C:        xmlns:domain="urn:ietf:params:xml:ns:domain-1.0">
   C:        <domain:name>example.com</domain:name>
   C:        <domain:period unit="y">1</domain:period>
   C:        <domain:ns>
   C:          <domain:hostObj>ns1.example.com</domain:hostObj>
   C:          <domain:hostObj>ns1.example.net</domain:hostObj>
   C:        </domain:ns>
   C:        <domain:authInfo>
   C:          <domain:pw/>
   C:        </domain:authInfo>
   C:      </domain:create>
   C:    </create>
   C:    <extension>
   C:      <ttl:create
   C:        xmlns:ttl="urn:ietf:params:xml:ns:epp:ttl-1.0">
   C:        <ttl:ttl for="NS">172800</ttl:ttl>
   C:        <ttl:ttl for="DS">300</ttl:ttl>
   C:      </ttl:create>
   C:      <secDNS:create
   C:        xmlns:secDNS="urn:ietf:params:xml:ns:secDNS-1.1">
   C:        <secDNS:dsData>
   C:          <secDNS:keyTag>12345</secDNS:keyTag>
   C:          <secDNS:alg>13</secDNS:alg>
   C:          <secDNS:digestType>2</secDNS:digestType>
   C:          <secDNS:digest>49FD46E6C4B45C55D4AC</secDNS:digest>
   C:        </secDNS:dsData>
   C:      </secDNS:create>
   C:    </extension>
   C:    <clTRID>ABC-12345</clTRID>
   C:  </command>
   C:</epp>

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   Example host <create> command:

   C:<?xml version="1.0" encoding="UTF-8" standalone="no"?>
   C:<epp xmlns="urn:ietf:params:xml:ns:epp-1.0">
   C:  <command>
   C:    <create>
   C:      <host:create
   C:        xmlns:host="urn:ietf:params:xml:ns:host-1.0">
   C:        <host:name>ns1.example.com</host:name>
   C:        <host:addr ip="v4">192.0.2.2</host:addr>
   C:        <host:addr ip="v6">1080::8:800:200C:417A</host:addr>
   C:      </host:create>
   C:    </create>
   C:    <extension>
   C:      <ttl:create
   C:        xmlns:ttl="urn:ietf:params:xml:ns:epp:ttl-1.0">
   C:        <ttl:ttl for="A"/>
   C:        <ttl:ttl for="AAAA">86400</ttl:ttl>
   C:      </ttl:create>
   C:    </extension>
   C:    <clTRID>ABC-12345</clTRID>
   C:  </command>
   C:</epp>

   If an EPP server receives a <create> command containing a TTL value
   that is outside the server's permitted range, it MUST reject the
   command with a 2306 "Parameter value policy error" response.

2.2.2.  EPP <update> command

   This extension defines an additional element for EPP <update>
   commands for domain and host objects.

   The <command> element of the <update> command frame MAY contain an
   <extension> element which MAY contain a <ttl:update> element.  This
   element MUST contain one or more <ttl:ttl> records as described in
   Section 1.2.

   Example domain <update> command:

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   C:<?xml version="1.0" encoding="UTF-8" standalone="no"?>
   C:<epp xmlns="urn:ietf:params:xml:ns:epp-1.0">
   C:  <command>
   C:    <update>
   C:      <domain:update
   C:        xmlns:domain="urn:ietf:params:xml:ns:domain-1.0">
   C:        <domain:name>example.com</domain:name>
   C:      </domain:update>
   C:    </update>
   C:    <extension>
   C:      <ttl:update
   C:        xmlns:ttl="urn:ietf:params:xml:ns:epp:ttl-1.0">
   C:        <ttl:ttl for="NS"/>
   C:      </ttl:update>
   C:    </extension>
   C:    <clTRID>ABC-12345</clTRID>
   C:  </command>
   C:</epp>

   Example host <update> command:

   C:<?xml version="1.0" encoding="UTF-8" standalone="no"?>
   C:<epp xmlns="urn:ietf:params:xml:ns:epp-1.0">
   C:  <command>
   C:    <update>
   C:      <host:update
   C:        xmlns:host="urn:ietf:params:xml:ns:host-1.0">
   C:        <host:name>ns1.example.com</host:name>
   C:      </host:update>
   C:    </update>
   C:    <extension>
   C:      <ttl:update
   C:        xmlns:ttl="urn:ietf:params:xml:ns:epp:ttl-1.0">
   C:        <ttl:ttl for="A">86400</ttl:ttl>
   C:        <ttl:ttl for="AAAA">3600</ttl:ttl>
   C:      </ttl:update>
   C:    </extension>
   C:    <clTRID>ABC-12345</clTRID>
   C:  </command>
   C:</epp>

   If an EPP server receives an <update> command containing a TTL value
   that is outside the server's permitted range, it MUST reject the
   command with a 2306 "Parameter value policy error" response.

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3.  Server processing of TTL values

3.1.  Permitted record types

   Servers MAY restrict the supported DNS record types in accordance
   with their own operational needs.  For example, a server MAY allow
   clients to specify TTL values for DS records only.

   A server which receives a <create> or <update> command which includes
   a restricted record type MUST respond with a 2306 "Parameter value
   policy" error.

3.2.  Use of TTL values in delegation records

   EPP servers which implement this extension SHOULD use the values
   provided by EPP clients for the TTL values records published in the
   DNS for domain and and objects.

   EPP servers that use the "host attribute" model SHOULD use any A and/
   or AAAA TTL values specified for the domain object when publishing
   NS, A and AAAA records derived from host attributes.

4.  Out-of-band changes to TTL values

   EPP server operators MAY, in order to address operational or security
   issues, make changes to TTL values out-of-band (that is, not in
   response to an <update> command received from the sponsoring client).

   Server operators MAY also implement automatic reset of TTL values, so
   that they revert to the default value a certain amount of time after
   an update has been made.

   If a TTL value is changed out-of-band, EPP server operators MAY
   notify the sponsoring client using the EPP Change Poll extension
   ([RFC8590]), which provides a generalised method for EPP servers to
   notify clients of changes to objects under their sponsorship.

5.  Operational considerations

5.1.  Operational impact of TTL values

   Domain registry operators must strike a balance between, on the one
   hand, the desire of registrants for changes to their domains to be
   visible in the DNS quickly, and on the other, the increased DNS query
   traffic that short TTLs can bring.  Historically, registry operators
   have used a global TTL value which was applied to all delegations
   within their zones, which could then be tuned to an optimum value.

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   Domain registry operators SHOULD implement limits on the maximum and
   minimum accepted TTL values that are narrower than the values
   permitted in the XML schema in the Formal syntax (which were chosen
   to allow any TTL permitted in DNS records), in order to prevent
   scenarios where an excessively high or low TTL causes operational
   issues on either side of the zone cut.

   Section 4 describes how server operators MAY unilaterally change TTL
   values in order to address operational or security issues, or only
   permit changes for limited time periods (after which TTLs revert to
   the default).

5.2.  When TTL values should be changed

   A common operational mistake is changing of DNS record TTLs during or
   after the planned change to the records themselves.  This arises due
   to a misunderstanding about how TTLs work.

   Implementations of this specification SHOULD ensure that the user
   understands that changes to a TTL are only effective in shortening
   transition periods if implemented a period of time — at least equal
   to the current TTL — _before_ the planned change.

6.  Security considerations

   Some malicious actors use a technique called "fast flux DNS"
   ([SAC-025]) to rapidly change the DNS configuration for a zone in
   order to evade takedown and law enforcement activity.

   Registry operators SHOULD take this into consideration when setting
   the lower limit on TTL values, since a short TTL on delegations may
   enhance the effectiveness of fast flux techniques on evasion.

7.  IANA considerations

7.1.  XML namespace

   This document uses URNs to describe XML namespaces and XML schemas
   conforming to a registry mechanism described in [RFC3688].  The
   following URI assignment has been made by IANA:

   Registration for the TTL namespace:

      *URI:* urn:ietf:params:xml:ns:epp:ttl-1.0

      *Registrant Contact:* See the author of this document

      *XML:* None.  Namespace URIs do not represent an XML specification

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   Registration for the TTL XML schema:

      *URI:* urn:ietf:params:xml:schema:epp:ttl-1.0

      *Registrant Contact:* See the author of this document

      *XML:* See the "Formal syntax" section of this document

7.2.  EPP extension registry

   The EPP extension described in this document has been registered by
   the IANA in the Extensions for the "Extensible Provisioning Protocol
   (EPP)" registry described in [RFC7451].  The details of the
   registration are as follows:

      *Name of Extension:* Extensible Provisioning Protocol (EPP)
      Mapping for DNS Time-To-Live (TTL) values

      *Document Status:* Standards Track

      *Reference:* URL of this document

      *Registrant Name and Email Address:* See the author of this
      document

      *TLDs:* Any

      *IPR Disclosure:* None

      *Status:* Active

      *Notes:* None

8.  Formal syntax

   The formal syntax presented here is a complete schema representation
   of the extension suitable for automated validation of EPP XML
   instances.

   <?xml version="1.0" encoding="UTF-8"?>
   <schema
     xmlns="http://www.w3.org/2001/XMLSchema"
     targetNamespace="urn:ietf:params:xml:ns:epp:ttl-1.0"
     xmlns:ttl="urn:ietf:params:xml:ns:epp:ttl-1.0"
     elementFormDefault="qualified">
     <annotation>
       <documentation>
         Extensible Provisioning Protocol v1.0 extension

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         schema for Time-To-Live (TTL) values for domain
         and host objects.
       </documentation>
     </annotation>

     <!--
       extension elements can appear in <create> and
       <update> commands, and <info> responses
     -->

     <element name="create" type="ttl:commandContainer">
       <unique name="uniqueRRTypeForCreate">
         <selector xpath="ttl:ttl"/>
         <field xpath="@for"/>
       </unique>
     </element>

     <element name="update" type="ttl:commandContainer">
       <unique name="uniqueRRTypeForUpdate">
         <selector xpath="ttl:ttl"/>
         <field xpath="@for"/>
       </unique>
     </element>

     <element name="infData" type="ttl:responseContainer">
       <unique name="uniqueRRTypeForInfo">
         <selector xpath="ttl:ttl"/>
         <field xpath="@for"/>
       </unique>
     </element>

     <complexType name="commandContainer">
       <sequence>
         <element
           name="ttl"
           type="ttl:commandTTLType"
           minOccurs="1"
           maxOccurs="unbounded"/>
       </sequence>
     </complexType>

     <complexType name="responseContainer">
       <sequence>
         <element
           name="ttl"
           type="ttl:responseTTLType"
           minOccurs="1"
           maxOccurs="unbounded"/>

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       </sequence>
     </complexType>

     <complexType name="commandTTLType">
       <simpleContent>
         <extension base="ttl:ttlOrNull">
           <attribute
             name="for"
             type="ttl:rrType"
             use="required"/>
         </extension>
       </simpleContent>
     </complexType>

     <complexType name="responseTTLType">
       <simpleContent>
         <extension base="ttl:ttlOrNull">
           <attribute
             name="for"
             type="ttl:rrType"
             use="required"/>

           <attribute
             name="custom"
             type="ttl:customRRType"/>

           <attribute
             name="min"
             type="ttl:ttlValue"
             use="required"/>

           <attribute
             name="default"
             type="ttl:ttlValue"
             use="required"/>

           <attribute
             name="max"
             type="ttl:ttlValue"
             use="required"/>
         </extension>
       </simpleContent>
     </complexType>

     <!--
       union type allowing the element to either contain
       nothing or a TTL value
     -->

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     <simpleType name="ttlOrNull">
       <union
         memberTypes="ttl:emptyValue ttl:ttlValue"/>
     </simpleType>

     <!-- empty value type -->
     <simpleType name="emptyValue">
       <restriction base="token">
         <length value="0"/>
       </restriction>
     </simpleType>

     <!-- TTL value type -->
     <simpleType name="ttlValue">
       <restriction base="nonNegativeInteger">
         <minInclusive value="0"/>
         <maxInclusive value="2147483647"/>
       </restriction>
     </simpleType>

     <!-- resource record mnemonic type -->
     <simpleType name="rrType">
       <restriction base="token">
         <enumeration value="NS" />
         <enumeration value="DS" />
         <enumeration value="DNAME" />
         <enumeration value="A" />
         <enumeration value="AAAA" />
         <enumeration value="custom" />
       </restriction>
     </simpleType>

     <!-- custom resource record type -->
     <simpleType name="customRRType">
       <restriction base="token">
         <pattern value="A|[A-Z][A-Z0-9\-]*[A-Z0-9]"/>
       </restriction>
     </simpleType>
   </schema>

9.  Implementation status

   This section is to be removed before publishing as an RFC.

   None currently reported - if you have implemented this extension,
   please let me know!

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10.  Change log

   This section is to be removed before publishing as an RFC.

10.1.  Change from 03 to 04

   1.  Changed the for attribute to be an enumeration and added the
       custom attribute.

   2.  Added the min, default and max attributes.

   3.  Apply feedback from Jim Gould.

10.2.  Change from 02 to 03

   1.  Rolled back the "straw man" syntax from 02. ttl:ttl now has a for
       attribute which can be any DNS record type.  Section 1.2.2
       describes how the set of supported record types may be limited.

   2.  Removed the global/explicit models and just use the explicit
       model.

   3.  Removed the cascading effect where a TTL set on a domain affects
       subordinate hosts.

10.3.  Change from 01 to 02

   1.  Renamed the ttl:seconds XSD type to ttl:container, and the
       ttl:nonNegativeInteger type to ttl:ttlType, to permit multiple
       TTL values.

   2.  Converted XML instances from artwork to source code.

10.4.  Change from 00 to 01

   1.  Incorporate feedback from Jim Gould.

   2.  Add wording to describe how TTL values are jointly managed by
       both clients and servers.

   3.  Fix minimum/maximum TTL value and schema namespace (thanks
       Patrick Mevzek).

   4.  Moved text on how the server should handle impermissible TTL
       values from the top of Section 4 to Sections 3.2.1 and 3.2.2
       (thanks Rick Wilhelm).

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   5.  Namespace changed from urn:ietf:params:xml:ns:ttl-1.0 to
       urn:ietf:params:xml:ns:epp:ttl-1.0.

   6.  Added discussion on EPP servers which use the host attribute
       model in Section 3.2 (thanks Hugo Salgado).

   7.  Added a Change Log (Section 10).

11.  Acknowledgements

   The author wishes to thank the following people for their advice and
   feedback during the development of this document:

   1.  Hugo Salgado;

   2.  Patrick Mevzek;

   3.  Rick Wilhelm;

   4.  James Gould;

   5.  Marc Groeneweg;

   6.  Ties de Kock.

12.  References

12.1.  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>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.

   [RFC5731]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Domain Name Mapping", STD 69, RFC 5731,
              DOI 10.17487/RFC5731, August 2009,
              <https://www.rfc-editor.org/info/rfc5731>.

   [RFC5732]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Host Mapping", STD 69, RFC 5732, DOI 10.17487/RFC5732,
              August 2009, <https://www.rfc-editor.org/info/rfc5732>.

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   [RFC5910]  Gould, J. and S. Hollenbeck, "Domain Name System (DNS)
              Security Extensions Mapping for the Extensible
              Provisioning Protocol (EPP)", RFC 5910,
              DOI 10.17487/RFC5910, May 2010,
              <https://www.rfc-editor.org/info/rfc5910>.

   [RFC6895]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <https://www.rfc-editor.org/info/rfc6895>.

   [RFC7451]  Hollenbeck, S., "Extension Registry for the Extensible
              Provisioning Protocol", RFC 7451, DOI 10.17487/RFC7451,
              February 2015, <https://www.rfc-editor.org/info/rfc7451>.

12.2.  Informative references

   [RFC6927]  Levine, J. and P. Hoffman, "Variants in Second-Level Names
              Registered in Top-Level Domains", RFC 6927,
              DOI 10.17487/RFC6927, May 2013,
              <https://www.rfc-editor.org/info/rfc6927>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

   [RFC8590]  Gould, J. and K. Feher, "Change Poll Extension for the
              Extensible Provisioning Protocol (EPP)", RFC 8590,
              DOI 10.17487/RFC8590, May 2019,
              <https://www.rfc-editor.org/info/rfc8590>.

   [RFC9364]  Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
              RFC 9364, DOI 10.17487/RFC9364, February 2023,
              <https://www.rfc-editor.org/info/rfc9364>.

   [SAC-025]  ICANN Security and Stability Advisory Committee (SSAC),
              "SSAC Advisory on Fast Flux Hosting and DNS", SAC 25,
              January 2008,
              <https://www.icann.org/en/system/files/files/sac-
              025-en.pdf>.

Author's Address

   Gavin Brown
   ICANN
   12025 Waterfront Drive, Suite 300
   Los Angeles, CA 90292
   United States of America
   Email: gavin.brown@icann.org

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   URI:   https://www.icann.org/

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