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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" category="std" ipr="trust200902" docName="draft-ietf-netmod-yang-instance-file-format-21"> docName="draft-ietf-netmod-yang-instance-file-format-21" number="9195" obsoletes="" updates="" submissionType="IETF" xml:lang="en" tocInclude="true" tocDepth="4" symRefs="true" sortRefs="true" version="3" consensus="true">

  <front>
    <title abbrev="YANG Instance Data">YANG Instance Data Data">A File Format</title> Format for YANG Instance Data</title>
    <seriesInfo name="RFC" value="9195"/>
    <author initials="B." surname="Lengyel" fullname="Balazs Lengyel">
      <organization abbrev="Ericsson"> Ericsson </organization> abbrev="Ericsson">Ericsson</organization>
      <address>
	<postal>
  <street>Magyar Tudosok korutja 11</street>
          <city>Budapest</city>
          <country>Hungary</country>
          <code>1117</code>
	</postal>
        <email>balazs.lengyel@ericsson.com</email>
      </address>
    </author>
    <author fullname="Benoit Claise" initials="B" initials="B." surname="Claise">
      <organization>Huawei</organization>
      <address>
        <email>benoit.claise@huawei.com</email>
      </address>
    </author>
    <date year="2021"/> year="2022" month="February"/>
    <area>OPS</area>
    <workgroup>Netmod</workgroup>

<abstract>

      <t>
        There is a need to document data defined in YANG models at design time,
        implementation time time, or when a live server is unavailable.
        This document specifies a standard
        file format for YANG instance data, which follows the syntax and semantics
        of existing YANG models, models and annotates it with metadata.
      </t>
    </abstract>
  </front>
  <middle>
    <section anchor="intro" title="Introduction"> numbered="true" toc="default">
      <name>Introduction</name>
      <t>
        There is a need to document data defined in YANG models when a live server is unavailable.
        Data is often needed at design, design time, implementation time time, or even later
        when a live running server is unavailable.
        To facilitate this offline delivery of data, this document specifies a standard
        format for YANG instance data sets and YANG instance data files.
        The format of the instance data set is defined by the "ietf-yang-instance-data"
        YANG module, module; see <xref target="instance_data_yam"/>. target="instance_data_yam" format="default"/>.
        The YANG data model in this document conforms to the Network
        Management Datastore Architecture (NMDA) defined in <xref target="RFC8342"/> target="RFC8342" format="default"/>.
      </t>
      <t>The following is a list of already implemented already-implemented and potential use cases.
        <list style="format UC%d">
          <t>Documentation
      </t>
      <ol spacing="normal" type="UC%d">
	<li anchor="uc1">Documentation of server capabilities</t>
          <t>Preloading capabilities</li>
        <li anchor="uc2">Preloading default configuration data</t>
          <t>Documenting data</li>
        <li anchor="uc3">Documenting factory default settings</t>
          <t>Storing settings</li>
        <li anchor="uc4">Storing the configuration of a device, e.g., for backup, archive archive, or
            audit purposes</t>
          <t>Storing purposes</li>
        <li anchor="uc5">Storing diagnostics data</t>
          <t>Allowing data</li>
        <li anchor="uc6">Allowing YANG instance data to potentially be carried within other IPC inter-process communication (IPC) message formats</t>
          <t>Default formats</li>
        <li anchor="uc7">Default instance data used as part of a templating solution</t>
          <t>Providing solution</li>
        <li anchor="uc8">Providing data examples in RFCs or internet drafts</t>
        </list>
      </t> drafts</li>
      </ol>
      <t><xref target="detailed_use_cases"/> target="detailed_use_cases" format="default"/> describes the first three use
        cases in detail.
      </t><t>
      </t>
      <t>
        There are  many and varied use cases where YANG instance data
        could be used. This document does not limit future uses of instance data
        sets, so specifying how and when to use YANG instance data
        is out of scope for this document. It is anticipated that other
        documents will define specific use cases. Use cases are listed
        only as examples.
      </t>
      <section title="Terminology" anchor="terminology">
        <t>The anchor="terminology" numbered="true" toc="default">
        <name>Terminology</name>
        <t>
    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
          NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
          "MAY", "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
    NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
    "<bcp14>MAY</bcp14>", and "OPTIONAL" "<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as
    described in BCP 14 BCP&nbsp;14 <xref target="RFC2119"/> <xref target="RFC8174"/>
    when, and only when, they appear in all capitals, as shown here.</t>
        <t>Instance Data: A here.
        </t>
<dl>
	<dt>Instance Data:</dt><dd>A collection of instantiated data nodes.</t>
        <t>Instance nodes.</dd>
        <dt>Instance Data Set:  A Set:</dt><dd>A named set of data items annotated with metadata
          that can be used as instance data in a YANG data tree.</t>
        <t>Instance tree.</dd>
        <dt>Instance Data File:  A File:</dt><dd>A file containing an instance data set formatted
          according to the rules described in this document.</t>
        <t>Content-schema: A document.</dd>
        <dt>Content-schema:</dt><dd>A set of YANG modules with their revision, supported
          features, and deviations for which the instance data set contains instance data.</t>
        <t>Content defining data.</dd>
          <dt>Content-defining YANG module: an Module:</dt><dd>An individual YANG module that is part of the content-schema.</t> content-schema.</dd>
</dl>
        <t>The term "server" is used as defined in <xref target="RFC8342"/>.</t> target="RFC8342" format="default"/>.</t>

      </section>
      <section title="Principles"> numbered="true" toc="default">
        <name>Principles</name>
        <t>The following is a list of the basic principles of the instance data format:
          <list style="format P%d">
            <t>Two
        </t>
        <ol spacing="normal" type="P%d"><li>Two standard formats shall be defined based on the XML and
              JSON encodings.</t>
            <t>Instance encodings.</li>
          <li>Instance data shall reuse existing encoding rules for
              YANG defined
              YANG-defined data.
            </t>
            <t>Metadata
            </li>
          <li>Metadata about the instance data set
              (<xref target="instance_data_set_metadata"/>) target="instance_data_set_metadata" format="default"/>) shall be defined.</t>
            <t>A defined.</li>
          <li>A YANG instance data set shall be allowed to contain data
              for multiple YANG modules.</t>
            <t>Instance modules.</li>
          <li>Instance data shall be allowed to contain configuration data,
              state data, or a mix of the two.</t>
            <t>Partial two.</li>
          <li>Partial data sets shall be allowed.</t>
            <t>The allowed.</li>
          <li>The YANG instance data format shall be usable for any data for which
              YANG module(s) are defined and available to the reader, independent
              of whether the module is implemented by a server.</t>
            <t>It server.</li>
          <li>It shall be possible to report the identity of the datastore with which the
            instance data set is associated.</t>
          </list>
        </t> associated.</li>
        </ol>
      </section>
      <section title="Delivery numbered="true" toc="default">
        <name>Delivery of Instance Data"> Data</name>
        <t>Instance data sets that are produced as
            a result of some sort of specification or design effort
            may be available without the need for a live
            server
            server, e.g., via download from the vendor's website, website or in any other
            way that product documentation is distributed.</t>
        <t>Other instance data sets may be read from or produced by the YANG
            server itself itself, e.g., UC5 <xref target="uc5" format="none">UC5</xref> documenting diagnostic data.</t>
      </section>
      <section title="Data numbered="true" toc="default">
        <name>Data Life cycle"> Cycle</name>
        <t>
              A YANG instance data set is created at a specific
              point of time. If the data changes afterwards, the instance data set
              will no longer represent the current data, data unless it is updated.
              The current values may be
              retrieved at run-time runtime via NETCONF/RESTCONF or
              received
              received, e.g., in YANG-Push notifications.
        </t>
        <t>
              Whether the instance data changes and and, if so, when and how, how
              should be described either in the instance data set's description
              statement or in some other implementation specific implementation-specific manner.
        </t>
      </section>
    </section>
    <section anchor="instance_data_file_format" title="Instance numbered="true" toc="default">
      <name>Instance Data File Format"> Format</name>
      <t>
        A YANG instance data file MUST <bcp14>MUST</bcp14> contain a single instance data set and no
        additional data.
      </t>
      <t>The format of the instance data set is defined  by the
        "ietf-yang-instance-data" YANG module.
        It is made up of a header part and content-data.
        The header part carries metadata for the instance data set.
        The content-data, defined as an anydata data node, carries
        the instance data that the user wants to document/provide. document and/or provide.
        The syntax and semantics of content-data is are defined by the content-schema.
      </t>
      <t>Two formats are specified based on the XML and JSON YANG encodings. The
        file formats are achieved by applying the respective XML and JSON
        encoding rules for the YANG structure included in this document.
        Later, as other YANG encodings (e.g., CBOR) are defined, further
        instance data formats may be specified.
      </t>
      <t>The content-data part MUST <bcp14>MUST</bcp14> conform to the content-schema, content-schema while allowing for the
        exceptions listed below. The content-data part SHALL <bcp14>SHALL</bcp14> follow the
        encoding rules defined in  <xref target="RFC7950"/> target="RFC7950" format="default"/> for XML and
        <xref target="RFC7951"/> target="RFC7951" format="default"/> for JSON and MUST <bcp14>MUST</bcp14> use UTF-8 character encoding.
        Content-data MAY <bcp14>MAY</bcp14> include:
        <list style="symbols">
          <t>metadata,
      </t>
      <ul spacing="normal">
        <li>metadata, as defined by <xref target="RFC7952"/>.</t>
          <t>origin target="RFC7952" format="default"/>.</li>
        <li>origin metadata, as specified in
          <xref target="RFC8526"/> target="RFC8526" format="default"/> and <xref target="RFC8527"/></t>
          <t>implementation specific target="RFC8527" format="default"/>.</li>
        <li>implementation-specific metadata relevant to individual
            data nodes. Unknown metadata MUST <bcp14>MUST</bcp14> be ignored by users of
            instance data, allowing it to be used
            later for other purposes.</t>
        </list>
      </t> purposes.</li>
      </ul>
      <t>An instance data set MAY <bcp14>MAY</bcp14> contain data for any number of
        YANG modules; if needed needed, it MAY <bcp14>MAY</bcp14> carry the complete configuration and state
        data for a server.
        Default values should be excluded where they do not provide
        additional useful data.
        </t><t>
      </t>
      <t>
        Configuration ("config true") and operational state data ("config false")
        MAY
        <bcp14>MAY</bcp14> be mixed in the instance data file.
        </t><t>
      </t>
      <t>
        Instance data files MAY <bcp14>MAY</bcp14> contain partial data sets. This means "mandatory",
        "min-elements", "require-instance true", "must" "must", and "when" constraints MAY <bcp14>MAY</bcp14> be violated.
      </t>
      <t>The name of the instance data file SHOULD <bcp14>SHOULD</bcp14> be of the following form (using ABNF notation <xref target="RFC5234"/>):
        <figure align="center" anchor="filename" suppress-title="true">
          <artwork align="left" name="filename-abnf">
<![CDATA[ target="RFC5234" format="default"/>):
      </t>
      <sourcecode name="filename-abnf" type="abnf"><![CDATA[
   instance-data-set-name ["@" ( revision-date / timestamp ) ]
                  ( ".xml" / ".json" )

      E.g.,
]]></sourcecode>

<t>Examples include:</t>
<artwork><![CDATA[
      acme-router-modules.xml
      E.g.,
      acme-router-modules@2018-01-25.xml
      E.g.,
      acme-router-modules@2018-01-25T15_06_34_3+01_00.json
]]>
          </artwork>
        </figure>
]]></artwork>
      <t>
        If the leaf "name" is present in the instance data header,
        its value SHOULD <bcp14>SHOULD</bcp14> be used for the "instance-data-set-name" in the filename.

        If the "revision-date" is present in the filename filename, it MUST <bcp14>MUST</bcp14> conform to
        the format of the revision-date leaf in the YANG model.
        If the "revision-date" is present in both the filename and in the
        instance data header, the revision date in the file name MUST filename <bcp14>MUST</bcp14> be
        set to the latest revision date inside the instance data set.

        If the "timestamp" is present in the filename filename, it MUST <bcp14>MUST</bcp14> conform to
        the format of the timestamp leaf in the YANG model except for
        replacing colons as described below.
        If the "timestamp" is present both in both the filename and in the
        instance data header, the timestamp in the file name SHOULD filename <bcp14>SHOULD</bcp14> be
        set to the timestamp inside the instance data set; any colons,
        if present, shall be replaced by underscores.
      </t>

      <t anchor="instance_data_set_metadata">Metadata, information about the
      data set itself MUST itself, <bcp14>MUST</bcp14> be included. Some metadata items are
      defined in the YANG module "ietf-yang-instance-data", but other items MAY <bcp14>MAY</bcp14>
      be used.</t><t> used.</t>
      <t>
      Metadata MUST <bcp14>MUST</bcp14> include:
        <list><t><list style="symbols">
          <t>Version
      </t>
      <ul empty="true" spacing="normal">
        <li>
          <ul spacing="normal">
            <li>Version of the YANG Instance Data instance data format (if not explicitly present present, the default value is used)</t>
        </list></t></list> used).</li>
          </ul>
        </li>
      </ul>
      <t>
      Metadata SHOULD <bcp14>SHOULD</bcp14> include:
        <list><t><list style="symbols">
          <t>Name
      </t>
      <ul empty="true" spacing="normal">
        <li>
          <ul spacing="normal">
            <li>Name of the data set</t>
          <t>Content-schema set.</li>
            <li>Content-schema specification (i.e., the "content-schema" node)</t>
          <t>Description node).</li>
            <li>Description of the instance data set. The description SHOULD <bcp14>SHOULD</bcp14>
            contain information on whether and how the data can change during
            the lifetime of the server
          </t>
          <t>An server.
          </li>
            <li>An indication of whether default values are included.
            The default handling uses the concepts defined in <xref target="RFC6243"/>,
            however target="RFC6243" format="default"/>;
            however, as only concepts are re-used, users of instance data sets, sets
            do not need to support RFC 6243.
          </t>
        </list></t></list>
        </t> <xref target="RFC6243"/>.
          </li>
          </ul>
        </li>
      </ul>
      <section title="Specifying numbered="true" toc="default">
        <name>Specifying the Content Schema"> Schema</name>
        <t>To properly understand and use an instance data set, the user needs to
          know the content-schema. The content-schema can be either specified either in external documents or within the instance data set.
          In the latter case case, one of the following methods MUST <bcp14>MUST</bcp14> be used:
          <list>
           <t>Inline method: Include
        </t>
        <dl spacing="normal">
          <dt>Inline method:</dt><dd>Include the needed information as part of the
          instance data set.</t>
           <t>Simplified-Inline method: Include set.</dd>
          <dt>Simplified-inline method:</dt><dd>Include the needed information as part of
            the instance data set; short specification, only the module modules' name and revision-date is used.</t>
           <t>URI method: Include are used.</dd>
          <dt>URI method:</dt><dd>Include a URI that references another YANG instance
            data file. This instance data file will use the same content-schema
            as the referenced YANG instance data file. file (if you don't want to repeat the info again and again)</t>
          </list> again).</dd>
        </dl>
        <t>
          Additional methods methods, e.g., a YANG-package based YANG-package-based solution may be added later.
        </t>
        <t>Note,
        <t>Note that the specified content-schema only indicates the set of
          modules that were used to define this YANG instance data set.
          Sometimes instance data may be used for a server supporting a
          different YANG module set (e.g., for the "Preloading default
          configuration data" use-case, UC2 use case, <xref target="uc2" format="none">UC2</xref> in <xref target="intro"/>, target="intro" format="default"/>, the instance
          data set may not be updated every time the YANG modules on the
          server are updated).

          Whether an instance data set originally defined using a specific
          content-schema is usable with a different other another schema
          depends on many factors factors, including the amount number of differences and the
          compatibility between the original and the
          other schema, schema when considering modules, revisions, features,
          deviations, the scope of the instance data, etc.
        </t>
        <section title="Inline Method"> numbered="true" toc="default">
          <name>Inline Method</name>
          <t>The inline-yang-library "inline-yang-library" anydata data node carries instance data (conforming to
            ietf-yang-library@2019-01-04)
            "ietf-yang-library@2019-01-04") <xref target="RFC8525"/> target="RFC8525" format="default"/> that specifies the content
            defining content-defining YANG modules modules, including revision,
            supported features, deviations deviations, and any relevant additional relevant data.
            An example of the "inline" inline method is provided in <xref target="acme-router-modules-example"/>. target="acme-router-modules-example" format="default"/>.
          </t>
        </section>
        <section title="Simplified-Inline Method"> numbered="true" toc="default">
          <name>Simplified-Inline Method</name>
          <t>The instance data set contains a list of content defining content-defining YANG
             modules
             modules, including the revision date for each.
             Usage of this method implies that the modules are
             used without any deviations and with all features
             supported. YANG modules that are only required to satisfy
             import-only dependencies MAY <bcp14>MAY</bcp14> be excluded from the leaf-list.
             If they are excluded excluded, then the consumer of the instance data
             set has to apply the YANG language rules to resolve the imports.
             An example of the "simplified-inline" simplified-inline method is provided in <xref target="read-only-acm-rules-example"/>. target="read-only-acm-rules-example" format="default"/>.
          </t>
        </section>
        <section title="URI Method"> numbered="true" toc="default">
          <name>URI Method</name>
          <t>The "same-schema-as-file" leaf SHALL <bcp14>SHALL</bcp14> contain a URI that references another YANG
            instance data file. The current instance data file will use the same
            content-schema as the referenced file.
          </t><t>
          </t>
          <t>
            The referenced instance data file MAY <bcp14>MAY</bcp14> have no content-data if it is
            used solely for specifying the content-schema.
          </t><t>
          </t>
          <t>
            If a referenced instance data file is unavailable, the content-schema
            is unknown.
          </t><t>
          </t>
          <t>
            The URI method is advantageous when the user wants to avoid the
            overhead of specifying the content-schema in each instance data
            file: E.g., In UC6,
            file -- for example, in <xref target="uc6" format="none">UC6</xref>, when the system creates a diagnostic file every minute to document the state of the server.
          </t>
          <t>An example of the "URI" URI method is provided in <xref target="acme-router-netconf-diagnostics-example"/>.</t> target="acme-router-netconf-diagnostics-example" format="default"/>.</t>
        </section>
      </section>
      <section title="Examples"  anchor="examples"> anchor="examples" numbered="true" toc="default">
        <name>Examples</name>
        <section title="Documentation anchor="acme-router-modules-example" numbered="true" toc="default">
          <name>Documentation of server capabilities" anchor="acme-router-modules-example"> Server Capabilities</name>
          <t>The example file acme-router-modules@2021-09-16.xml acme-router-modules@2022-01-20.xml reflects UC1 <xref target="uc1" format="none">UC1</xref> in <xref target="intro"/>. target="intro" format="default"/>.
            It provides a list of supported YANG modules and NETCONF capabilities for a server.
            It uses the "inline" inline method to specify the content-schema.</t>
          <t>The example uses artwork folding <xref target="RFC8792"/>.</t> target="RFC8792" format="default"/>.</t>
          <figure align="center" anchor="acme-router-modules">
            <artwork align="left" name="acme-router-modules@2021-09-16.xml">
<![CDATA[==========
            <sourcecode name="acme-router-modules@2022-01-20.xml" type="xml"><![CDATA[
========== NOTE: '\' line wrapping per RFC 8792 ===========

<?xml version="1.0" encoding="UTF-8"?>
<instance-data-set xmlns=\
    "urn:ietf:params:xml:ns:yang:ietf-yang-instance-data">
  <name>acme-router-modules</name>
  <content-schema>
    <inline-yang-library>
      <modules-state \
        xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library">
        <module>
          <name>ietf-yang-library</name>
          <revision>2019-01-04</revision>
        </module>
        <module>
          <name>ietf-netconf-monitoring</name>
          <revision>2010-10-04</revision>
        </module>
      </modules-state>
    </inline-yang-library>
  </content-schema>
  <revision>
    <date>2020-10-23</date>
    <description>Initial version</description>
  </revision>
  <description>Defines the minimal set of modules that any \
      acme-router will contain. This minimal set will \
      only change when a new SW software release is \
      introduced.</description>
  <contact>info@acme.example.com</contact>
  <content-data>
    <modules-state \
        xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library">
      <module>
        <name>ietf-yang-library</name>
        <revision>2019-01-04</revision>
        <namespace>\
          urn:ietf:params:xml:ns:yang:ietf-yang-library\
        </namespace>
        <conformance-type>implement</conformance-type>
      </module>
      <module>
        <name>ietf-system</name>
        <revision>2014-08-06</revision>
       <namespace>urn:ietf:params:xml:ns:yang:ietf-system</namespace>
        <feature>sys:authentication</feature>
        <feature>sys:local-users</feature>
        <deviation>
          <name>acme-system-ext</name>
          <revision>2018-08-06</revision>
        </deviation>
        <conformance-type>implement</conformance-type>
      </module>
      <module>
        <name>ietf-netconf-monitoring</name>
        <revision>2010-10-04</revision>
        <namespace>\
          urn:ietf:params:xml:ns:yang:ietf-netconf-monitoring\
        </namespace>
        <conformance-type>implement</conformance-type>
      </module>
      <module>
        <name>ietf-yang-types</name>
        <revision>2013-07-15</revision>
        <namespace>urn:ietf:params:xml:ns:yang:ietf-yang-types\
          </namespace>
        <conformance-type>import</conformance-type>
      </module>
      <module>
        <name>acme-system-ext</name>
        <revision>2018-08-06</revision>
        <namespace>\
          urn:rdns:acme.example.com:oammodel:acme-system-ext\
        </namespace>
        <conformance-type>implement</conformance-type>
      </module>
    </modules-state>
    <netconf-state \
        xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-monitoring">
      <capabilities>
        <capability>\
          urn:ietf:params:netconf:capability:validate:1.1\
        </capability>
      </capabilities>
    </netconf-state>
  </content-data>
</instance-data-set>
]]></artwork>
]]></sourcecode>
          </figure>
        </section>
        <section title="Preloading default configuration data"  anchor="read-only-acm-rules-example"> anchor="read-only-acm-rules-example" numbered="true" toc="default">
          <name>Preloading Default Configuration Data</name>
          <t>The example file read-only-acm-rules@2021-09-16.xml read-only-acm-rules@2022-01-20.xml reflects UC2 <xref target="uc2" format="none">UC2</xref> in <xref target="intro"/>. target="intro" format="default"/>.
            It provides a default rule set for a read-only operator role.
            It uses the "simplified-inline" simplified-inline method for specifying the content-schema.</t>
          <figure align="center" anchor="read-only-acm-rules">
            <artwork align="left" name="read-only-acm-rules@2021-09-16.xml">
<![CDATA[==========
            <sourcecode name="read-only-acm-rules@2022-01-20.xml" type="xml"><![CDATA[
========== NOTE: '\' line wrapping per RFC 8792 ===========

<?xml version="1.0" encoding="UTF-8"?>
<instance-data-set
    xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-instance-data">
  <name>read-only-acm-rules</name>
  <content-schema>
    <module>ietf-netconf-acm@2018-02-14</module>
  </content-schema>
  <revision>
    <date>2018-07-04</date>
    <description>Initial version</description>
  </revision>
  <description>Default access control rules for a read-only \
      role. This set of rules will only change when a new  \
      SW
      software release is introduced.</description>
  <content-data>
    <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm">
      <enable-nacm>true</enable-nacm>
      <read-default>deny</read-default>
      <exec-default>deny</exec-default>
      <rule-list>
        <name>read-only-role</name>
        <group>read-only-group</group>
        <rule>
          <name>read-all</name>
          <module-name>*</module-name>
          <access-operation>read</access-operation>
          <action>permit</action>
        </rule>
      </rule-list>
    </nacm>
  </content-data>
</instance-data-set>
]]></artwork>
]]></sourcecode>
          </figure>
        </section>
        <section title="Storing diagnostics data"  anchor="acme-router-netconf-diagnostics-example"> anchor="acme-router-netconf-diagnostics-example" numbered="true" toc="default">
          <name>Storing Diagnostics Data</name>
          <t>The example file acme-router-netconf-diagnostics@2018-01-25T17_00_38Z.json
            reflects UC5 <xref target="uc5" format="none">UC5</xref> in <xref target="intro"/>. target="intro" format="default"/>.
            An instance data set that contains
            statistics about the NETCONF server is produced by the server every 15 minutes that contains
            statistics about the NETCONF server. minutes. As a new set is produced periodically many times a day day, a revision-date
            would be useless; instead instead, a timestamp is included.</t>
          <figure align="center" anchor="acme-router-netconf-diagnostics">
          <artwork align="left" name="acme-router-netconf-diagnostics@2018-01-25T17_00_38Z.json">
<![CDATA[==========
            <sourcecode name="acme-router-netconf-diagnostics@2018-01-25T17_00_38Z.json" type=""><![CDATA[
========== NOTE: '\' line wrapping per RFC 8792 ===========

{
  "ietf-yang-instance-data:instance-data-set": {
    "name": "acme-router-netconf-diagnostics",
    "content-schema": {
      "same-schema-as-file": "file:///acme-diagnostics-schema.json"
    },
    "timestamp": "2018-01-25T17:00:38Z",
    "description":  ["NETCONF statistics, \
        The data may change at any time."],
    "content-data": {
      "ietf-netconf-monitoring:netconf-state": {
        "statistics": {
          "netconf-start-time ": "2018-12-05T17:45:00Z",
          "in-bad-hellos ": "32",
          "in-sessions ": "397",
          "dropped-sessions ": "87",
          "in-rpcs ": "8711",
          "in-bad-rpcs ": "408",
          "out-rpc-errors ": "408",
          "out-notifications": "39007"
        }
      }
    }
  }
}
]]></artwork>
]]></sourcecode>
          </figure>
        </section>
      </section>
    </section>
    <section anchor="instance_data_yam" title="YANG numbered="true" toc="default">
      <name>YANG Instance Data Model"> Model</name>
      <section title="Tree Diagram"> numbered="true" toc="default">
        <name>Tree Diagram</name>
        <t>
          The following tree diagram <xref target="RFC8340"/> target="RFC8340" format="default"/>
          provides an overview of the data model.
        </t>
        <t>
            <figure>
              <artwork><![CDATA[
        <sourcecode name="" type="yangtree"><![CDATA[
module: ietf-yang-instance-data
  structure instance-data-set:
    +-- name?
    +--name?                string
    +-- format-version?
    +--format-version?      string
    +-- includes-defaults?
    +--includes-defaults?   enumeration
    +-- content-schema
    +--content-schema
    |  +-- (content-schema-spec)?  +--(content-schema-spec)?
    |     +--:(simplified-inline)
    |     |  +-- module*  +--module*                module-with-revision-date
    |     +--:(inline)
    |     |  +-- inline-yang-library  +--inline-yang-library    <anydata>
    |     +--:(uri)
    |        +-- same-schema-as-file?        +--same-schema-as-file?   inet:uri
    +-- description*
    +--description*         string
    +-- contact?
    +--contact?             string
    +-- organization?
    +--organization?        string
    +-- datastore?
    +--datastore?           ds:datastore-ref
    +-- revision*
    +--revision* [date]
    |  +-- date  +--date           string
    |  +-- description?  +--description?   string
    +-- timestamp?
    +--timestamp?           yang:date-and-time
    +-- content-data?
    +--content-data?        <anydata>
]]></artwork>
            </figure>
        </t>
]]></sourcecode>
      </section>
      <section title="YANG Model" anchor="yang-model"> anchor="yang-model" numbered="true" toc="default">
        <name>YANG Model</name>
        <t>
             This YANG module imports typedefs from <xref target="RFC6991"/>, target="RFC6991" format="default"/>,
             <xref target="RFC6243"/>, target="RFC6243" format="default"/>,
             identities from <xref target="RFC8342"/> target="RFC8342" format="default"/>, and
             the "structure" extension from <xref target="RFC8791"/>. target="RFC8791" format="default"/>.
             It also references <xref target="RFC8525"/>.
          </t><t>
              <figure>
                  <artwork align="left" name="ietf-yang-instance-data@2021-09-16.yang"><![CDATA[
<CODE BEGINS> file "ietf-yang-instance-data@2021-09-16.yang"
// RFC Ed.: replace the date above if the module gets changed in
//any way during reviews or RFC editor process and remove this note target="RFC8525" format="default"/>.
        </t>
        <sourcecode name="ietf-yang-instance-data@2022-01-20.yang" type="yang" markers="true"><![CDATA[
module ietf-yang-instance-data {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-yang-instance-data";
  prefix yid;

  import ietf-yang-structure-ext {
    prefix sx;
    reference
      "YANG
      "RFC 8791: YANG Data Structure Extensions:
       draft-ietf-netmod-yang-data-ext-05"; Extensions";
  }
  import ietf-datastores {
    prefix ds;
    reference
      "RFC 8342: Network Management Datastore Architecture (NMDA)";
  }
  import ietf-inet-types {
    prefix inet;
    reference
      "RFC 6991: Common YANG Data Types";
  }
  import ietf-yang-types {
    prefix yang;
    reference
      "RFC 6991: Common YANG Data Types";
  }
  import ietf-netconf-with-defaults {
    prefix ncwd;
    reference
      "RFC 6243: With-defaults Capability for NETCONF";
  }

  organization
    "IETF NETMOD Working Group";
  contact
    "WG Web:   <https://datatracker.ietf.org/wg/netmod/>
     WG List:  <mailto:netmod@ietf.org>

     Author:  Balazs Lengyel
        <mailto:balazs.lengyel@ericsson.com>

     Author:  Benoit Claise
        <mailto:benoit.claise@huawei.com>";
  description
    "The module defines the structure and content of YANG
     instance data sets.

     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 (RFC 2119)
     (RFC 8174) when, and only when, they appear in all
     capitals, as shown here.

     Copyright (c) 2021 2022 IETF Trust and the persons identified as
     authors of the code.  All rights reserved.

     Redistribution and use in source and binary forms, with or
     without modification, is permitted pursuant to, and subject
     to the license terms contained in, the Simplified Revised BSD License
     set forth in Section 4.c of the IETF Trust's
     Legal Provisions Relating to IETF Documents
     (http://trustee.ietf.org/license-info).
     (https://trustee.ietf.org/license-info).

     This version of this YANG module is part of RFC XXXX; 9195
     (https://www.rfc-editor.org/info/rfc9195); see the RFC itself
     for full legal notices.";
  // RFC Ed.: replace XXXX with RFC number and remove this note

  revision 2021-09-16 2022-01-20 {
    // RFC Ed.: replace the date above if the module gets changed in
    // anyway during reviews or RFC editor process and remove
    //this note
    description
      "Initial revision.";
    reference
      "RFC XXXX: 9195: YANG Instance Data File Format";
    // RFC Ed.: replace XXXX with RFC number and remove this note
  }

  typedef module-with-revision-date {
    type string {
      pattern '[a-zA-Z_][a-zA-Z0-9\-_.]*'
            + '(@\d{4}-(1[0-2]|0[1-9])-(0[1-9]|[1|2][0-9]|3[0-1]))?';
      pattern '.|..|[^xX].*|.[^mM].*|..[^lL].*';
    }
    description
      "A type defining a module name and an optional revision
       date, e.g. ietf-yang-library@2019-01-04"; e.g., ietf-yang-library@2019-01-04.";
  }

  sx:structure "instance-data-set" instance-data-set {
    description
      "A data structure to define a format for YANG instance
       data.  The majority of the YANG nodes provide meta-data provides metadata
       about the instance data; the instance data itself is
       contained only in the 'content-data' node.";
    leaf name {
      type string;
      description
        "An arbitrary name for the YANG instance data set.  This
         value is primarily used for descriptive purposes.  However,
         when the instance data set is saved to a file, then the
         filename MUST encode the name's value, value per Section 2
         of RFC XXXX.";
      // RFC Ed.: replace XXXX with RFC number and remove this note 9195.";
    }
    leaf format-version {
      type string {
        pattern '\d{4}-(1[0-2]|0[1-9])-(0[1-9]|[1|2][0-9]|3[0-1])';
      }
      default "2021-09-16";
      // RFC Ed.: replace the date above if the module gets changed
      // in anyway during reviews or RFC editor process and remove
      // this note "2022-01-20";
      description
        "The 'revision' of the 'ietf-yang-instance-data' module
         used to encode this 'instance-data-set'.";
    }
    leaf includes-defaults {
      type ncwd:with-defaults-mode;
      default report-all; "report-all";
      description "
        Indicates
        "Indicates how data nodes with default values are
         represented for all data nodes contained in the
         instance-data-set.

         It uses the same definitions as per section Section 3 of RFC 6243, 6243
         but applied in the context of an instance data file rather
         than a NETCONF request using the <with-defaults>
         parameter.

         For JSON files, the encoding of the 'report-all-tagged'
         option is as defined in section Section 4.8.9 of RFC 8040.";
      reference "With-defaults
        "RFC 6243: With-defaults Capability for NETCONF, RFC 6243."; NETCONF";
    }
    container content-schema {
      description
        "The content schema (i.e., YANG modules) used to create
         the instance data set.
         If not present present, the user needs to obtain the information
         through external documents.";
      choice content-schema-spec {
        description
          "Specification of the content-schema.";
        case simplified-inline {
          leaf-list module {
            type module-with-revision-date;
            min-elements 1;
            description
              "The list of content defining content-defining YANG modules.

               The value SHALL start with the module name.
               If the module contains a revision statement statement, the
               revision date SHALL be included in the leaf-list
               entry.

               E.g., ietf-yang-library@2019-01-04
               entry, e.g., ietf-yang-library@2019-01-04.

               Usage of this leaf-list implies the modules are
               used without any deviations and with all features
               supported. Multiple revisions of the same module
               MUST NOT be specified.";
          }
        }
        case inline {
          anydata inline-yang-library {
            mandatory true;
            description
              "Instance data corresponding to the
               ietf-yang-library@2019-01-04 defining
               the set of content defining content-defining YANG modules for
               this instance-data-set.";
          }
        }
        case uri {
          leaf same-schema-as-file {
            type inet:uri;
            description
              "A reference to another YANG instance data file.
               This instance data file uses the same
               content schema as the referenced file.

               Referenced files using the 'inline' or the
               'simplified-inline' methods MUST be supported.
               Referenced files using the 'URI Method' method' MAY be
               supported.

               The URL schemes 'file://' and 'https://' MUST
               be supported, supported; other schemes MAY also be
               supported.";
          }
        }
      }
    }
    leaf-list description {
      type string;
      description
        "Description of the instance data set.";
    }
    leaf contact {
      type string;
      description
        "Contact information for the person or
         organization to whom queries concerning this
         instance data set should be sent.";
    }
    leaf organization {
      type string;
      description
        "Organization responsible for the instance
         data set.";
    }
    leaf datastore {
      type ds:datastore-ref;
      description
        "The identity of the datastore with which the
         instance data set is associated, e.g., the datastore from
         where the data was read or read, the datastore into which the data
         may be loaded loaded, or the datastore which that is being documented.
         If a single specific datastore cannot be specified, the
         leaf MUST be absent.

         If this leaf is absent, then the datastore to which the
         instance data belongs is unspecified.";
    }
    list revision {
      key "date";
      description
        "Instance data sets that are produced as
         a result of some sort of specification or design effort
         SHOULD have at least one revision entry.  For every
         published editorial change, a new unique revision SHOULD
         be added in front of the revisions sequence so that all
         revisions are in reverse chronological order.

         In case cases of instance data sets that are read from
         or produced by a server or otherwise subject to
         frequent updates or changes, revision
         SHOULD NOT be present"; present.";
      leaf date {
        type string {
          pattern '\d{4}-(1[0-2]|0[1-9])-(0[1-9]|[1|2][0-9]|3[0-1])';
        }
        description
          "Specifies the date the instance data set
           was last modified. Formatted as YYYY-MM-DD"; YYYY-MM-DD.";
      }
      leaf description {
        type string;
        description
          "Description of this revision of the instance data set.";
      }
    }
    leaf timestamp {
      type yang:date-and-time;
      description
        "The date and time when the instance data set
         was last modified.

         In case cases of instance data sets that are read from or
         produced by a server or otherwise subject to frequent
         updates or changes, the timestamp SHOULD be present.

         If both a revision list entry and timestamp are present present,
         the timestamp SHOULD contain the same date as the
         latest revision statement.";
    }
    anydata content-data {
      description
        "Contains the real instance data.
         The data MUST conform to the relevant YANG modules
         specified either in the content-schema or in some other
         implementation specific
         implementation-specific manner.";
    }
  }
}
<CODE ENDS>
]]></artwork>
              </figure>
          </t>
]]></sourcecode>
      </section>
    </section>
    <section anchor="security" title="Security Considerations"> numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>The YANG module defined in this document only defines a wrapper structure
        specifying a format and a metadata header for YANG
        instance data defined by the content-schema. Because of this this,
        the security considerations template for YANG models in
        section 3.7.1 in
        <xref target="RFC8407"/> target="RFC8407" sectionFormat="of" section="3.7.1"/> is not followed.
        The instance data is designed to be accessed as a stored file or
        over any file access method or protocol.
      </t>
      <t>The document does not specify any method to influence the
        behavior of a server.</t>
      <t>
        The header part is usually not security sensitive, however sensitive; however, sensitive
        information may be included, in which case it needs to be handled securely securely,
        as mentioned below. Information to consider includes:
        <list style="symbol">
          <t>If
      </t>
      <ul spacing="normal">
        <li>If the URI method is used for specification of the content-schema and
            the URI includes a userinfo subcomponent</t>
          <t>Any subcomponent</li>
        <li>Any description text</t>
        </list>
      </t> text</li>
      </ul>
      <t>The content part may contain sensitive data.
        The security sensitivity of this data is completely dependent on
        the content-schema.
        Depending on the nature of the instance data, instance data
        files MAY <bcp14>MAY</bcp14> need to be handled securely.
        The same kind of handling should be applied to this file at-rest at rest and
        in-transit
        in transit that would be needed for the result of a read operation
        returning the same data.  These in-transit protection mechanisms will
        also mitigate integrity issues when transporting the file.
      </t>
      <t>Instance data files should be protected against modification or
        unauthorized access using normal file handling file-handling mechanisms.
        Care should be taken, when
        When copying the original files or providing file access for
	additional users, care should be taken not to reveal information
	unintentionally.
      </t>
      <t>If the URI method is used for specification of the content-schema,
        there is a risk that the config schema section in the referenced YANG
        instance data file may be altered maliciously or even as part of its normal
        handling. In this case case, the content-schema might differ from the one
        expected. Protecting the integrity and stability of the referenced
        file should be ensured.
      </t>
    </section>
    <section anchor="iana" title="IANA Considerations"> numbered="true" toc="default">
      <name>IANA Considerations</name>
      <t>This document registers one URI and one YANG module.</t>
      <section title="URI Registration"> numbered="true" toc="default">
        <name>URI Registration</name>
        <t>This document registers one the following URI in the <xref target="RFC3688">
          IETF target="RFC3688" format="default">
          "IETF XML registry </xref>. Following the format in RFC 3688, the
          following registration is requested to be made:</t>
        <t><figure><artwork>
URI: urn:ietf:params:xml:ns:yang:ietf-yang-instance-data
Registrant Contact: The IESG.
XML: N/A, Registry"</xref>:</t>
<dl spacing="compact">
<dt>URI:</dt><dd>urn:ietf:params:xml:ns:yang:ietf-yang-instance-data</dd>
<dt>Registrant Contact:</dt><dd>The IESG.</dd>
<dt>XML:</dt><dd>N/A, the requested URI is an XML namespace.
        </artwork></figure></t> namespace.</dd>
</dl>
      </section>
      <section title="YANG numbered="true" toc="default">
        <name>YANG Module Name Registration"> Registration</name>
        <t>This document registers one the following YANG module in the YANG "YANG Module Names Names"
          registry <xref target="RFC6020"/>. Following the format in [RFC6020],
          the following registrations are requested:
        </t>
        <t><figure><artwork>
name: ietf-yang-instance-data
namespace: urn:ietf:params:xml:ns:yang:ietf-yang-instance-data
prefix: yid
reference: RFC XXXX
// RFC Ed.: replace XXXX with RFC number and remove this note
        </artwork></figure></t>
      </section> target="RFC6020" format="default"/>:</t>
<dl spacing="compact">
<dt>Name:</dt><dd>ietf-yang-instance-data</dd>
<dt>Namespace:</dt><dd>urn:ietf:params:xml:ns:yang:ietf-yang-instance-data</dd>
<dt>Prefix:</dt><dd>yid</dd>
<dt>Reference:</dt><dd>RFC 9195</dd>
</dl>
      </section>
    <section title="Acknowledgments">
        <t>For their valuable comments, discussions, and feedback, we wish to
        acknowledge Andy Bierman, Juergen Schoenwaelder, Rob Wilton, Joe Clarke, Kent Watsen,
        Martin Bjorklund, Ladislav Lhotka, Qin Wu and other members of the Netmod WG.</t>
    </section>
  </middle>
  <?rfc needLines="20"?>
  <back>
    <references title="Normative References">
      <?rfc include='reference.RFC.7950'?>
      <?rfc include='reference.RFC.7951'?>
      <?rfc include='reference.RFC.7952'?>
      <?rfc include='reference.RFC.8526'?>
      <?rfc include='reference.RFC.8527'?>
      <?rfc include='reference.RFC.6243'?>
      <?rfc include="reference.RFC.8342"?>
      <?rfc include='reference.RFC.2119'?>
      <?rfc include='reference.RFC.8174'?>
      <?rfc include='reference.RFC.6991'?>
      <?rfc include='reference.RFC.8525'?>
      <?rfc include='reference.RFC.8791'?>
      <?rfc include='reference.RFC.6020'?>
      <?rfc include='reference.RFC.5234'?>
    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7950.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7951.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7952.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8526.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8527.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6243.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8342.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6991.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8525.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8791.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6020.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5234.xml"/>
      </references>
      <references>
        <name>Informative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8641.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8632.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3688.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8340.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8407.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8792.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8808.xml"/>
      </references>
    <references title="Informative References">
      <?rfc include='reference.RFC.8641'?>
      <?rfc include='reference.RFC.8632'?>
      <?rfc include='reference.RFC.3688'?>
      <?rfc include="reference.RFC.8340"?>
      <?rfc include="reference.RFC.8407"?>
      <?rfc include='reference.RFC.8792'?>
      <?rfc include='reference.RFC.8808'?>
    </references>
    <?rfc needLines="100"?>

    <section title="Changes between revisions">
      <t>RFC Ed.: Remove section "Changes between revisions"</t>
      <t>v20 - v21
        <list style="symbols">
          <t>Minor updates for IESG comments</t>
          <t>Added ABNF as a normative reference for the filename's definition.</t>
          <t>Enhanced security considerations</t>
          <t>Added data change information to the description of the examples.</t>
        </list>
      </t>
      <t>v19 - v20
        <list style="symbols">
          <t>Minor updates for IESG comments</t>
        </list>
      </t>
      <t>v18 - v19
        <list style="symbols">
          <t>Updated leaf includes-defaults </t>
        </list>
      </t>
      <t>v17 - v18
        <list style="symbols">
          <t>Added the report-all-tagged mode to the leaf includes-defaults</t>
        </list>
      </t>
      <t>v16 - v17
        <list style="symbols">
          <t>Removed default statement from includes-default</t>
        </list>
      </t>
      <t>v15 - v16
        <list style="symbols">
          <t>Editorial changes</t>
        </list>
      </t>
      <t>v14 - v15
        <list style="symbols">
          <t>Removed reference to revision-label</t>
          <t>For the inline method made the usage of
            ietf-yang-library@2019-01-04 mandatory.
            Simplified the case "inline" in the YANG module.</t>
          <t>Removed the "inline-module" leaf as it does not carry
            useful information anymore.</t>
          <t>Removed YANG feature</t>
        </list>
      </t>
      <t>v13 - v14
        <list style="symbols">
          <t>Added leaf includes-defaults</t>
          <t>Many small changes based on AD review</t>
        </list>
      </t>
      <t>v09 - v13
        <list style="symbols">
          <t>Editorial updates</t>
        </list>
      </t>
      <t>v08 - v09
        <list style="symbols">
          <t>Removed statement "the format will be similar to the response of a
            NETCONF get operation"</t>
          <t>Introduced artwork folding in the examples</t>
        </list>
      </t>
      <t>v07 - v08
        <list style="symbols">
          <t>Moved compatibility into appendix</t>
          <t>Renamed yid-version to format-version.
            Changed format to date of the YANG module</t>
          <t>Made support of ietf-yang-library mandatory if
          inline-content-schema is supported</t>
          <t>Many small changes based on WGLC</t>
        </list>
      </t>
      <t>v06 - v07
        <list style="symbols">
          <t>Updated terminology, use-cases</t>
          <t>Many small changes based on WGLC</t>
        </list>
      </t>
      <t>v05 - v06
        <list style="symbols">
          <t>Modified module name format, removed .yin or .yang extension</t>
          <t>Removed pattern for module and inline-module. The
            usage of revision-label should also be allowed.
          </t>
        </list>
      </t>
      <t>v04 - v05
        <list style="symbols">
          <t>Updated according to YANG-Doctor review</t>
          <t>Updated security considerations</t>
          <t>Added a wrapping container for the schema, and renamed the data
            nodes in the inline and uri cases.
          </t>
          <t>Allowed .yin for simplified-inline schema naming.
            Made date optional if it is unavailable in the YANG module.</t>
          <t>Added a mandatory yid-version to the header metadata to allow
            later updates of the module.</t>
        </list>
      </t>
      <t>v03 - v04
        <list style="symbols">
          <t>removed entity-tag and last-modified timestamp</t>
          <t>Added simplified-inline method of content-schema specification</t>
        </list>
      </t>
      <t>v02 - v03
        <list style="symbols">
          <t>target renamed to "content-schema" and "content defining YANG
            module(s)"</t>
          <t>Made name of instance data set optional</t>
          <t>Updated according to draft-ietf-netmod-yang-data-ext-03</t>
          <t>Clarified that entity-tag and last-modified timestamp are encoded
          as metadata. While they contain useful data, the HTTP-header based
          encoding from Restconf is not suitable.</t>
        </list>
      </t>
      <t>v01 - v02
        <list style="symbols">
          <t>Removed design time from terminology</t>
          <t>Defined the format of the content-data part by referencing various
          RFCs and drafts instead of the result of the get-data and get operations.</t>
          <t>Changed target-ptr to a choice</t>
          <t>Inline target-ptr may include augmenting modules and alternatives to ietf-yang-library</t>
          <t>Moved list of target modules into a separate &lt;target-modules> element.</t>
          <t>Added backwards compatibility considerations</t>
        </list>
      </t>
      <t>v00 - v01
        <list style="symbols">
          <t>Added the target-ptr metadata with 3 methods</t>
          <t>Added timestamp metadata</t>
          <t>Removed usage of dedicated .yid file extension</t>
          <t>Added list of use cases</t>
          <t>Added list of principles</t>
          <t>Updated examples</t>
          <t>Moved detailed use case descriptions to appendix</t>
        </list>
      </t>
    </section>
    <section title="Backwards Compatibility"> numbered="true" toc="default">
      <name>Backwards Compatibility</name>
      <t>The concept of backwards compatibility "backwards compatibility" and what changes are backwards
      compatible are not defined for "instance instance data sets" sets as it is they are highly
      dependent on the specific use case and the content-schema.
      </t><t>
      </t>
      <t>
      In case of "instance data sets" that are the result of design or specification
      activity, some changes that may be good to avoid are listed below.
      </t><t>
      </t>
      <t>
      YANG uses the concept of managed entities identified by key
      values; if the connection between the represented entity and the key
      value is not preserved during an update, this may lead to the following problems.
        <list style="symbols">
          <t>If
      </t>
      <ul spacing="normal">
        <li>If the key value of a list entry that represents the same
            managed entity as before is changed, the user may mistakenly
            identify the list entry as new.</t>
          <t>If new.</li>
        <li>If the meaning of a list entry is changed, changed but the key values
            are not (e.g., redefining an alarm-type but not changing its
            alarm-type-id)
            alarm-type-id), the change may not be noticed.</t>
          <t>If noticed.</li>
        <li>If the key value of a previously removed list entry is reused
            for a different entity, the change may be misinterpreted as
            reintroducing the previous entity.</t>
        </list>
      </t> entity.</li>
      </ul>
    </section>
    <section title="Detailed anchor="detailed_use_cases" numbered="true" toc="default">
      <name>Detailed Use Cases" anchor="detailed_use_cases"> Cases</name>
      <t>This section is non-normative.</t>
      <section title="Use numbered="true" toc="default">
        <name>Use Case 1: Early Documentation of Server Capabilities"> Capabilities</name>
        <t> A server has a number of server-capabilities server capabilities that are defined
          in YANG modules and can be retrieved from the server
          using protocols like NETCONF or RESTCONF. Server capabilities include:
          <list style="symbols">
            <t>
        </t>
        <ul spacing="normal">
          <li> data defined in "ietf-yang-library": YANG modules, submodules,
              features, deviations, schema-mounts, and datastores
              supported (<xref target="RFC8525"/>)</t>
            <t>alarms target="RFC8525" format="default"/>).</li>
          <li>alarms supported (<xref target="RFC8632"/>)</t>
            <t>data target="RFC8632" format="default"/>).</li>
          <li>data nodes and subtrees that support or do not support on-change
            notifications (<xref target="RFC8641"/>)</t>
            <t>netconf-capabilities target="RFC8641" format="default"/>).</li>
          <li>netconf-capabilities in ietf-netconf-monitoring.</t>
          </list> ietf-netconf-monitoring.</li>
        </ul>
        <t>
          While it is good practice to allow a client to query these capabilities
          from the live server, that is often not possible.
        </t>
        <t>
          Often when a network node is released, an associated NMS
          (network management system) Network Management System (NMS)
          is also released with it. The NMS depends on the capabilities of the server.
          During NMS implementation, information about server capabilities is needed.
          If the information is unavailable early in some offline
          document,
          document but only as instance data from the live network node, the NMS implementation will be
          delayed,
          delayed because it has to wait until the network node is ready. Also Also, assuming
          that all NMS implementors will have a correctly configured network nodes
          from which data can be retrieved, retrieved is a very expensive proposition. (An NMS may handle dozens
          of node types.)
        </t>
        <t>
          Network operators often build their own home-grown homegrown NMS systems that
          need to be integrated with a vendor's network node.  The operator
          needs to know the network node's server capabilities in order to do
          this. Moreover, the network operator's decision to buy a vendor's
          product may even be influenced by the network node's OAM Operations, Administration, and Maintenance (OAM) feature set
          documented as the server's capabilities.
        </t>
        <t>
          Beside NMS implementors, system integrators and many others also need the same
          information early.  Examples could be model driven model-driven testing, generating documentation, etc.
        </t>
        <t>
          Most server-capabilities server capabilities are relatively stable and change only during
          upgrade or due to licensing or the addition or removal of hardware. They are
          usually defined by a vendor at design time, before the product is released.

          It is feasible and advantageous to define/document define and document them early early,
          e.g., in a YANG instance data File. file.
        </t>
        <t>It is anticipated that a separate IETF document will define in
          detail how and which set of server capabilities should be documented.
        </t>
      </section>
      <section title="Use numbered="true" toc="default">
        <name>Use Case 2: Preloading Data"> Data</name>
        <t>
          There are parts of the configuration that must be fully configurable
          by the operator. However, often a simple default configuration often will
          be sufficient.
        </t><t>
        </t>
        <t>
          One example is access control groups/roles and related rules.
          While a sophisticated operator may define dozens of different groups,
          often a basic (read-only operator, read-write system administrator,
          security-administrator) triplet will be enough.
          Vendors will often provide such default configuration data to make
          device configuration easier for an operator.
        </t>
        <t>
          The device vendor may define a set of default groups (/nacm:nacm/groups) and rules
          for these groups to access specific parts of the common models (/nacm:nacm/rule-list/rule).
        </t>
        <t>
          YANG instance data files can be used to document and/or preload the
          default configuration.
        </t>
      </section>
      <section title="Use numbered="true" toc="default">
        <name>Use Case 3: Documenting Factory Default Settings"> Settings</name>
        <t>
          Nearly every server has a factory default configuration. If the
          system is really badly misconfigured or if the current configuration
          is to be abandoned, the system can be reset to the default factory
          configuration.
        </t><t>
        </t>
        <t>
          YANG instance data can be used to document the factory default
          configuration. See <xref target="RFC8808"></xref>. target="RFC8808" format="default"/>.
        </t>
      </section>
    </section>
    <section numbered="false" toc="default">
      <name>Acknowledgments</name>
      <t>For their valuable comments, discussions, and feedback, we wish to
        acknowledge <contact fullname="Andy Bierman"/>, <contact fullname="Juergen Schoenwaelder"/>, <contact fullname="Rob Wilton"/>,  <contact fullname="Joe Clarke"/>, <contact fullname="Kent Watsen"/>,     <contact fullname="Martin Bjorklund"/>, <contact fullname="Ladislav Lhotka"/>, <contact fullname="Qin Wu"/>, and other members of the Netmod Working Group.</t>
    </section>
  </back>

</rfc>
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