YANG Instance Data File Format
draft-ietf-netmod-yang-instance-file-format-12

Netmod                                                        B. Lengyel
Internet-Draft                                                  Ericsson
Intended status: Standards Track                               B. Claise
Expires: September 10, 2020                          Cisco Systems, Inc.
                                                           March 9, 2020


                     YANG Instance Data File Format
             draft-ietf-netmod-yang-instance-file-format-08

Abstract

   There is a need to document data defined in YANG models when a live
   server is not available.  Data is often needed already at design or
   implementation time or needed by groups that do not have a live
   running server available.  This document specifies a standard file
   format for YANG instance data, which follows the syntax and semantics
   of existing YANG models, and annotates it with metadata.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 10, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Principles  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Delivery of Instance Data . . . . . . . . . . . . . . . .   4
     2.3.  Data Life cycle . . . . . . . . . . . . . . . . . . . . .   5
   3.  Instance Data File Format . . . . . . . . . . . . . . . . . .   5
     3.1.  Specifying the Content Schema . . . . . . . . . . . . . .   7
       3.1.1.  Inline Method . . . . . . . . . . . . . . . . . . . .   7
       3.1.2.  Simplified-Inline Method  . . . . . . . . . . . . . .   8
       3.1.3.  URI Method  . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  YANG Instance Data Model  . . . . . . . . . . . . . . . . . .  12
     4.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . . . .  12
     4.2.  YANG Model  . . . . . . . . . . . . . . . . . . . . . . .  13
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  URI Registration  . . . . . . . . . . . . . . . . . . . .  19
     6.2.  YANG Module Name Registration . . . . . . . . . . . . . .  19
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  20
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Appendix A.  Open Issues  . . . . . . . . . . . . . . . . . . . .  22
   Appendix B.  Changes between revisions  . . . . . . . . . . . . .  22
   Appendix C.  Backwards Compatibility  . . . . . . . . . . . . . .  24
   Appendix D.  Detailed Use Cases - Non-Normative . . . . . . . . .  24
     D.1.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .  24
       D.1.1.  Use Case 1: Early Documentation of Server
               Capabilities  . . . . . . . . . . . . . . . . . . . .  24
       D.1.2.  Use Case 2: Preloading Data . . . . . . . . . . . . .  26
       D.1.3.  Use Case 3: Documenting Factory Default Settings  . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Instance Data: A collection of instantiated data nodes.




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   Instance Data Set: A named set of data items annotated with metadata
   that can be used as instance data in a YANG data tree.

   Instance Data File: A file containing an instance data set formatted
   according to the rules described in this document.

   Content-schema: A set of YANG modules with their revision, supported
   features, and deviations for which the instance data set contains
   instance data.

   Content defining YANG module: an individual YANG module that is part
   of the content-schema.

   The term Server is used as defined in [RFC8342].

2.  Introduction

   There is a need to document data defined in YANG models when a live
   server is not available.  Data is often needed already at design or
   implementation time or needed by groups that do not have a live
   running server available.  To facilitate this offline delivery of
   data, this document specifies a standard format for YANG instance
   data sets and YANG instance data files.

   The following is a list of already implemented and potential use
   cases.

   UC1  Documentation of server capabilities

   UC2  Preloading default configuration data

   UC3  Documenting Factory Default Settings

   UC4  Storing the configuration of a device, e.g., for backup, archive
        or audit purposes

   UC5  Storing diagnostics data

   UC6  Allowing YANG instance data to potentially be carried within
        other IPC message formats

   UC7  Default instance data used as part of a templating solution

   UC8  Providing data examples in RFCs or internet drafts

   In Appendix D we describe the first three use cases in detail.





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   There are many and varied use cases where YANG instance data could be
   used.  We do not want to 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 here only to indicate the
   need for this work.

2.1.  Principles

   The following is a list of the basic principles of the instance data
   format:

   P1  Two standard formats shall be defined based on the XML and JSON
       encodings.

   P2  Instance data shall reuse existing encoding rules for YANG
       defined data.  Its format will be similar to the response of a
       NETCONF <get> operation or the RESTCONF response to a GET method
       invocation on the (unified) datastore resource.

   P3  Metadata about the instance data set (Section 3, Paragraph 9)
       shall be defined.

   P4  A YANG instance data set shall be allowed to contain data for
       multiple YANG modules.

   P5  Instance data shall be allowed to contain configuration data,
       state data, or a mix of the two.

   P6  Partial data sets shall be allowed.

   P7  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 actually implemented by a
       server.

   P8  It shall be possible to report the identity of the datastore with
       which the instance data set is associated.

2.2.  Delivery of Instance Data

   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 e.g., via download from the vendor's website, or in any
   other way that product documentation is distributed.

   Other instance data sets may be read from or produced by the YANG
   server itself e.g., UC5 documenting diagnostic data.



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2.3.  Data Life cycle

   A YANG instance data set is created at a specific point of time.  If
   the data changes afterwards, this is not represented in the instance
   data set anymore.  The current values may be retrieved at run-time
   via NETCONF/RESTCONF or received e.g., in YANG-Push notifications.

   Whether the instance data changes and if so, when and how, should be
   described either in the instance data set's description statement or
   in some other implementation specific manner.

3.  Instance Data File Format

   A YANG instance data file MUST contain a single instance data set and
   no additional data.

   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 we want to document/provide.  The syntax and
   semantics of content-data is defined by the content-schema.

   Two formats are specified based on the XML and JSON YANG encodings.
   Later as other YANG encodings (e.g., CBOR) are defined, further
   instance data formats may be specified.

   The content-data part MUST conform to the content-schema, while
   allowing for the exceptions listed below.  The content-data part
   SHALL follow the encoding rules defined in [RFC7950] for XML and
   [RFC7951] for JSON and MUST use UTF-8 character encoding.  Content-
   data MAY include:

      metadata as defined by [RFC7952].

      a default attribute as defined in [RFC6243] section 6. and in
      [RFC8040] section 4.8.9.

      origin metadata as specified in [RFC8526] and [RFC8527]

      implementation specific metadata relevant to individual data
      nodes.  Unknown metadata MUST be ignored by users of instance
      data, allowing it to be used later for other purposes.

   An instance data set MAY contain data for any number of YANG modules;
   if needed it MAY carry the complete configuration and state data set
   for a server.  Default values SHOULD NOT be included.




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   Config=true and config=false data MAY be mixed in the instance data
   file.

   Instance data files MAY contain partial data sets.  This means
   mandatory, min-elements, require-instance=true, must and when
   constrains MAY be violated.

   The name of the instance data file SHOULD take one of the following
   two forms:

      If revision information inside the data set is present

      *  instance-data-set-name ['@' revision-date] '.filetype'

      *  E.g., acme-router-modules@2018-01-25.xml

      If the leaf "name" is present in the instance data header, this
      MUST be used.  If the "revision-date" is present in both the
      filename and in the instance data header, the revision date in the
      file name MUST be set to the latest revision date inside the
      instance data set.

      If timestamp information inside the data set is present

      *  instance-data-set-name ['@' timestamp] '.filetype'

      *  E.g., acme-router-modules@2018-01-25T15_06_34_3+01_00.json

      If the leaf "name" is present in the instance data header, this
      MUST be used.  If the "timestamp" is present both in the filename
      and in the instance data header, the timestamp in the file name
      MUST be set to the timestamp inside the instance data set; the
      semicolons and the decimal point, if present, shall be replaced by
      underscores.

   The revision date or timestamp is optional. ".filetype" SHALL be
   ".json" or ".xml" according to the format used.

   Metadata, information about the data set itself SHOULD be included in
   the instance data set.  Some metadata items are defined in the YANG
   module ietf-yang-instance-data, but other items MAY also be used.
   Metadata MUST include:

      Version of the YANG Instance Data format

   Metadata SHOULD include:

   o  Name of the data set



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   o  Content schema specification

   o  Description of the instance data set.  The description SHOULD
      contain information whether and how the data can change during the
      lifetime of the server.

3.1.  Specifying the Content Schema

   To properly understand and use an instance data set, the user needs
   to know the content-schema.  One of the following methods SHOULD be
   used:

      Inline method: Include the needed information as part of the
      instance data set.

      Simplified-Inline method: Include the needed information as part
      of the instance data set; short specification.

      URI method: 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.  (if you don't
      want to repeat the info again and again)

      External Method: Do not include the content-schema, the user needs
      to obtain the information through external documents.

   Additional methods e.g., a YANG-package based solution may be added
   later.

   Note, 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 "UC2 Preloading Data" 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 schema
   depends on many factors including the amount of differences and the
   compatibility between the original and the other schema, considering
   modules, revisions, features, deviations, the scope of the instance
   data, etc.

3.1.1.  Inline Method

   One or more inline-module elements define YANG module(s) used to
   specify the content defining YANG modules.

      E.g., ietf-yang-library@2016-06-21




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   The anydata inline-schema carries instance data (conforming to the
   inline-modules) that actually specifies the content defining YANG
   modules including revision, supported features, deviations and any
   relevant additional data (e.g., revision labels
   [I-D.verdt-netmod-yang-module-versioning]).  See Section 3.2.

3.1.2.  Simplified-Inline Method

   The instance data set contains a list of content defining YANG
   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.

3.1.3.  URI Method

   The same-schema-as-file leaf SHALL 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.

   The referenced instance data file MAY have no content-data if it is
   used solely for specifying the content-schema.

   If a referenced instance data file is not available, content-schema
   is unknown.

   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 Use Case 6, when the system creates a diagnostic file every
   minute to document the state of the server.

3.2.  Examples

   The following example is based on "UC1, Documenting Server
   Capabilities".  It provides (a shortened) list of supported YANG
   modules and NETCONF capabilities for a server.  It uses the inline
   method to specify the content-schema.

  <?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>
    <format-version>2020-03-06</format-version>
    <content-schema>
      <inline-module>
        ietf-yang-library@2016-06-21
      </inline-module>
      <inline-schema>
        <modules-state



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          xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library">
          <module>
            <name>ietf-yang-library</name>
            <revision>2016-06-21</revision>
          </module>
          <module>
            <name>ietf-netconf-monitoring</name>
            <revision>2010-10-04</revision>
          </module>
        </modules-state>
      </inline-schema>
    <content-schema>
    <revision>
      <date>1956-10-23</date>
      <description>Initial version</description>
    </revision>
    <description>Defines the minimal set of modules that any acme-router
        will contain.</description>
    <contact>info@acme.com</contact>
    <content-data>
      <!-- The example lists only 4 modules, but it could list the
        full set of supported modules for a server, potentially many
        dozens of modules -->
      <modules-state
          xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library">
        <module>
          <name>ietf-yang-library</name>
          <revision>2016-06-21</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-yang-types</name>
          <revision>2013-07-15</revision>



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          <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.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>

     Figure 1: XML Instance Data Set - Use case 1, Documenting server
                               capabilities

   The following example is based on "UC2, Preloading Default
   Configuration".  It provides a (shortened) default rule set for a
   read-only operator role.  It uses the inline method for specifying
   the content-schema.





















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 <?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>
   <format-version>2020-03-06</format-version>
   <content-schema>
     <inline-module>ietf-yang-library@2019-01-04</inline-module>
     <inline-schema>
       <yang-library
           xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library">
         <module-set>
           <name>all</name>
           <module>
             <name>ietf-netconf-acm</name>
             <revision>2012-02-22</revision>
           </module>
         </module-set>
       </yang-library>
     </inline-schema>
   </content-schema>
   <revision>
     <date>1776-07-04</date>
     <description>Initial version</description>
   </revision>
   <description>Access control rules for a read-only role.</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>

      Figure 2: XML Instance Data Set - Use case 2, Preloading access
                               control data





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   The following example is based on UC5 Storing diagnostics data.  An
   instance data set is produced by the server every 15 minutes that
   contains statistics about NETCONF.  As a new set is produced
   periodically many times a day a revision-date would be useless;
   instead a timestamp is included.

   {
     "ietf-yang-instance-data:instance-data-set": {
       "name": "acme-router-netconf-diagnostics",
       "format-version": "2020-03-06",
       "content-schema": {
         "same-schema-as-file": "file:///acme-diagnostics-schema.json",
       },
       "timestamp": "2018-01-25T17:00:38Z",
       "description":
         "NETCONF statistics",
       "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"
           }
         }
       }
     }
   }

    Figure 3: JSON Instance Data File example - UC5 Storing diagnostics
                                   data

4.  YANG Instance Data Model

4.1.  Tree Diagram

   The following tree diagram [RFC8340] provides an overview of the data
   model.









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   module: ietf-yang-instance-data
     structure instance-data-set:
        +--rw name?                         string
        +--rw format-version                string
        +--rw content-schema
        |  +--rw (content-schema-spec)?
        |     +--:(simplified-inline)
        |     |  +--rw module*                 string
        |     +--:(inline)         {inline-content-schema}?
        |     |  +--rw inline-module*           string
        |     |  +--rw inline-schema            <anydata>
        |     +--:(uri)
        |        +--rw same-schema-as-file?     inet:uri
        +--rw description*          string
        +--rw contact?              string
        +--rw organization?         string
        +--rw datastore?            ds:datastore-ref
        +--rw revision* [date]
        |  +--rw date               string
        |  +--rw description?       string
        +--rw timestamp?            yang:date-and-time
        +--rw content-data?         <anydata>

4.2.  YANG Model

   This YANG module imports typedefs from [RFC6991], identities from
   [RFC8342] and the "structure" extension from
   [I-D.ietf-netmod-yang-data-ext].

   The YANG data model in this document conforms to the Network
   Management Datastore Architecture (NMDA) defined in [RFC8342]

   <CODE BEGINS> file "ietf-yang-instance-data@2020-03-06.yang"
   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;
     }
     import ietf-datastores {
       prefix ds;
     }
     import ietf-inet-types {
       prefix inet;
     }
     import ietf-yang-types {



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       prefix yang;
     }

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

        Author:  Balazs Lengyel
           <mailto:balazs.lengyel@ericsson.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) 2020 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 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).

        This version of this YANG module is part of RFC XXXX; see
        the RFC itself for full legal notices.";

     revision 2020-03-06 {
       description
         "Initial revision.";
       reference
         "RFC XXXX: YANG Instance Data Format";
     }

     feature inline-content-schema {
       description
         "This feature indicates that inline content-schema
             option is supported.";
     }




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     sx:structure "instance-data-set" {
       description
         "A data structure to define a format for
          YANG instance data sets. Consists of meta-data about
          the instance data set and the real content-data.";
       leaf name {
         type string;
         description
           "Name of the YANG instance data set.";
       }
       leaf format-version {
         type string;
         default "2020-03-06";
         description
           "Version of the 'YANG Instance Data format'.

            It SHALL contain the revision date of the
            ietf-yang-instance-data module used when creating the
            instance data set in a YYYY-MM-DD format";
       }
       container content-schema {
         description
           "The content schema used to create the instance data set";
         choice content-schema-spec {
           description
             "Specification of the content-schema";
           case simplified-inline {
             leaf-list module {
               type string;
               description
                 "The list of content defining YANG modules.

                  The value SHALL start with the module name.
                  If the module contains a revision statement the
                  revision date SHALL be included in the leaf-list
                  entry. If other methods (e.g., revision-label) are
                  defined to identify individual module revisions
                  those MAY be used instead of using a revision date.

                  E.g., ietf-yang-library@2016-06-21

                  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 {



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             if-feature "inline-content-schema";
             leaf-list inline-module {
               type string;
               min-elements 1;
               ordered-by user;
               description
                 "Indicates that content defining YANG modules
                  are specified inline.

                  The value SHALL start with the module name.
                  If the module contains a revision statement the
                  revision date SHALL be included in the leaf-list
                  entry. If other methods (e.g., revision-label) are
                  defined to identify individual module revisions
                  those MAY be used instead of using a revision date.

                  E.g., ietf-yang-library@2016-06-21

                  The first item is either ietf-yang-library or some
                  other YANG module that contains a list of YANG modules
                  with their name, revision-date, supported-features,
                  and deviations. The usage of
                  ietf-yang-library@2019-01-04 MUST be supported.
                  Using other modules, module versions MAY also be
                  supported.

                  As some versions of ietf-yang-library MAY contain
                  different module-sets for different datastores, if
                  multiple module-sets are included, the instance data
                  set's meta-data MUST contain the datastore information
                  and instance data for the ietf-yang-library MUST also
                  contain information specifying the module-set for the
                  relevant datastore.

                  Subsequent items MAY specify YANG modules augmenting
                  the first module with useful data
                  (e.g., revision label).";
             }
             anydata inline-schema {
               mandatory true;
               description
                 "Instance data corresponding to the YANG modules
                  specified in the inline-module nodes defining the set
                  of content defining YANG modules for this
                  instance-data-set.";
             }
           }
           case uri {



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             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.";
             }
           }
         }
       }
       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 the datastore into which the data
            may be loaded or the datastore which 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 undefined.";
       }
       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 one SHOULD be added



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            in front of the revisions sequence so that all
            revisions are in reverse chronological order.

            For 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";
         leaf date {
           type string {
             pattern '\d{4}-\d{2}-\d{2}';
           }
           description
             "Specifies the date the instance data set
              was last modified. Formatted as 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.

            For instance data sets that are read from or produced
            by a server or otherwise subject to frequent
            updates or changes, timestamp SHOULD be present";
       }
       anydata content-data {
         description
           "Contains the real instance data.
            The data MUST conform to the relevant YANG Modules specified
            either in the content-schema-spec or in some other
            implementation specific manner.";
       }
     }
   }
   <CODE ENDS>

5.  Security Considerations

   The YANG module defined in this document is designed as a wrapper
   specifying a format and a metadata header for YANG instance data
   defined by the content-schema.  The data is designed to be accessed
   as a stored file or over any file access method or protocol.



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   The document does not specify any method to influence the behavior of
   a server.

   Instance data files may contain sensitive data.

   The header part is not security sensitive.

   The security sensitivity of the instance data in the content part is
   completely dependent on the content schema.  Depending on the nature
   of the instance data, instance data files MAY need to be handled in a
   secure way.  The same kind of handling should be applied, that would
   be needed for the result of a read operation returning the same data.

   Instance data files should be protected against modification or
   unauthorized access using normal file handling mechanisms.  Care
   should be taken, when copying the original files or providing file
   access for additional users, not to reveal information
   unintentionally.

6.  IANA Considerations

   This document registers one URI and one YANG module.

6.1.  URI Registration

   This document registers one URI in the IETF XML registry [RFC3688].
   Following the format in RFC 3688, the following registration is
   requested to be made:

   URI: urn:ietf:params:xml:ns:yang:ietf-yang-instance-data

   Registrant Contact: The IESG.

   XML: N/A, the requested URI is an XML namespace.

6.2.  YANG Module Name Registration

   This document registers one YANG module in the YANG Module Names
   registry [RFC6020].

   name: ietf-yang-instance-data
   namespace: urn:ietf:params:xml:ns:yang:ietf-yang-instance-data
   prefix: yid
   reference: RFC XXXX







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7.  Acknowledgments

   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.

8.  References

8.1.  Normative References

   [I-D.ietf-netmod-yang-data-ext]
              Bierman, A., Bjorklund, M., and K. Watsen, "YANG Data
              Structure Extensions", draft-ietf-netmod-yang-data-ext-05
              (work in progress), December 2019.

   [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>.

   [RFC6243]  Bierman, A. and B. Lengyel, "With-defaults Capability for
              NETCONF", RFC 6243, DOI 10.17487/RFC6243, June 2011,
              <https://www.rfc-editor.org/info/rfc6243>.

   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              RFC 6991, DOI 10.17487/RFC6991, July 2013,
              <https://www.rfc-editor.org/info/rfc6991>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [RFC7951]  Lhotka, L., "JSON Encoding of Data Modeled with YANG",
              RFC 7951, DOI 10.17487/RFC7951, August 2016,
              <https://www.rfc-editor.org/info/rfc7951>.

   [RFC7952]  Lhotka, L., "Defining and Using Metadata with YANG",
              RFC 7952, DOI 10.17487/RFC7952, August 2016,
              <https://www.rfc-editor.org/info/rfc7952>.

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

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



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   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
              <https://www.rfc-editor.org/info/rfc8340>.

   [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Network Management Datastore Architecture
              (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
              <https://www.rfc-editor.org/info/rfc8342>.

   [RFC8525]  Bierman, A., Bjorklund, M., Schoenwaelder, J., Watsen, K.,
              and R. Wilton, "YANG Library", RFC 8525,
              DOI 10.17487/RFC8525, March 2019,
              <https://www.rfc-editor.org/info/rfc8525>.

   [RFC8526]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "NETCONF Extensions to Support the Network
              Management Datastore Architecture", RFC 8526,
              DOI 10.17487/RFC8526, March 2019,
              <https://www.rfc-editor.org/info/rfc8526>.

   [RFC8527]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "RESTCONF Extensions to Support the Network
              Management Datastore Architecture", RFC 8527,
              DOI 10.17487/RFC8527, March 2019,
              <https://www.rfc-editor.org/info/rfc8527>.

8.2.  Informative References

   [I-D.ietf-netmod-factory-default]
              WU, Q., Lengyel, B., and Y. Niu, "A YANG Data Model for
              Factory Default Settings", draft-ietf-netmod-factory-
              default-14 (work in progress), February 2020.

   [I-D.verdt-netmod-yang-module-versioning]
              Claise, B., Clarke, J., Rahman, R., Wilton, R., Lengyel,
              B., Sterne, J., and K. D'Souza, "Updated YANG Module
              Revision Handling", draft-verdt-netmod-yang-module-
              versioning-01 (work in progress), October 2019.

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

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.




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   [RFC8632]  Vallin, S. and M. Bjorklund, "A YANG Data Model for Alarm
              Management", RFC 8632, DOI 10.17487/RFC8632, September
              2019, <https://www.rfc-editor.org/info/rfc8632>.

   [RFC8641]  Clemm, A. and E. Voit, "Subscription to YANG Notifications
              for Datastore Updates", RFC 8641, DOI 10.17487/RFC8641,
              September 2019, <https://www.rfc-editor.org/info/rfc8641>.

Appendix A.  Open Issues

   o  -

Appendix B.  Changes between revisions

   v07 - v08

   o  Moved compatibility into appendix

   o  Renamed yid-version to format-version as "yid" can be regarded as
      a racial slur.  Changed format to date of the YANG module

   o  Made support of ietf-yang-library mandatory if inline-content-
      schema is supported

   o  Many small changes based on WGLC

   v06 - v07

   o  Updated terminology, use-cases

   o  Many small changes based on WGLC

   v05 - v06

   o  Modified module name format, removed .yin or .yang extension

   o  Removed pattern for module and inline-module.  We want to allow
      the usage of revision-label later

   v04 - v05

   o  Updated according to YANG-Doctor review

   o  Updated security considerations

   o  Added a wrapping container for the schema, and renamed the data
      nodes in the inline and uri cases.




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   o  Allowed .yin for simplified-inline schema naming.  Made date
      optional if it is not available in the YANG module.

   o  Added a mandatory yid-version to the header metadata to allow
      later updates of the module.

   v03 - v04

   o  removed entity-tag and last-modified timestamp

   o  Added simplified-inline method of content-schema specification

   v02 - v03

   o  target renamed to "content-schema" and "content defining YANG
      module(s)"

   o  Made name of instance data set optional

   o  Updated according to draft-ietf-netmod-yang-data-ext-03

   o  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.

   v01 - v02

   o  Removed design time from terminology

   o  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.

   o  Changed target-ptr to a choice

   o  Inline target-ptr may include augmenting modules and alternatives
      to ietf-yang-library

   o  Moved list of target modules into a separate <target-modules>
      element.

   o  Added backwards compatibility considerations

   v00 - v01

   o  Added the target-ptr metadata with 3 methods

   o  Added timestamp metadata



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   o  Removed usage of dedicated .yid file extension

   o  Added list of use cases

   o  Added list of principles

   o  Updated examples

   o  Moved detailed use case descriptions to appendix

Appendix C.  Backwards Compatibility

   The concept of backwards compatibility and what changes are backwards
   compatible are not defined for instance data sets as it is highly
   dependent on the specific use case and the content-schema.

   For instance data that is the result of a design or specification
   activity, some changes that may be good to avoid are listed.  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 problems.

   o  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.

   o  If the meaning of a list entry is changed, but the key values are
      not (e.g., redefining an alarm-type but not changing its alarm-
      type-id) the change may not be noticed.

   o  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.

Appendix D.  Detailed Use Cases - Non-Normative

D.1.  Use Cases

   We present a number of use cases were YANG instance data is needed.

D.1.1.  Use Case 1: Early Documentation of Server Capabilities

   A server has a number of server-capabilities that are defined in YANG
   modules and can be retrieved from the server using protocols like
   NETCONF or RESTCONF.  Server capabilities include:






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   o  data defined in ietf-yang-library: YANG modules, submodules,
      features, deviations, schema-mounts, and datastores supported
      ([RFC8525])

   o  alarms supported ([RFC8632])

   o  data nodes and subtrees that support or do not support on-change
      notifications ([RFC8641])

   o  netconf-capabilities in ietf-netconf-monitoring

   While it is good practice to allow a client to query these
   capabilities from the live server, that is often not possible.

   Often when a network node is released, an associated NMS (network
   management system) 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 not
   available early in some offline document, but only as instance data
   from the live network node, the NMS implementation will be delayed,
   because it has to wait until the network node is ready.  Also
   assuming that all NMS implementors will have a correctly configured
   network nodes from which data can be retrieved, is a very expensive
   proposition.  (An NMS may handle dozens of node types.)

   Network operators often build their own home-grown 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 feature set
   documented as the server's capabilities.

   Beside NMS implementors, system integrators and many others also need
   the same information early.  Examples could be model driven testing,
   generating documentation, etc.

   Most 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 them early e.g., in a YANG instance data File.

   It is anticipated that a separate IETF document will define in detail
   how and which set of server capabilities should be documented.







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D.1.2.  Use Case 2: Preloading Data

   There are parts of the configuration that must be fully configurable
   by the operator.  However, often a simple default configuration will
   be sufficient.

   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.

   Defining access control data is a complex task.  To help, the device
   vendor predefines a set of default groups (/nacm:nacm/groups) and
   rules for these groups to access specific parts of common models
   (/nacm:nacm/rule-list/rule).

   YANG instance data files are used to document and/or preload the
   default configuration.

D.1.3.  Use Case 3: Documenting Factory Default Settings

   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 the default factory
   configuration.

   In NETCONF, the <delete-config> operation can already be used to
   reset the startup datastore.  There are ongoing efforts to introduce
   a new, more generic factory-reset operation for the same purpose
   [I-D.ietf-netmod-factory-default]

   The operator currently has no way to know what the default
   configuration actually contains.  YANG instance data can also be used
   to document the factory default configuration.

Authors' Addresses

   Balazs Lengyel
   Ericsson
   Magyar Tudosok korutja 11
   1117 Budapest
   Hungary

   Phone: +36-70-330-7909
   Email: balazs.lengyel@ericsson.com




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   Benoit Claise
   Cisco Systems, Inc.
   De Kleetlaan 6a b1
   1831 Diegem
   Belgium

   Phone: +32 2 704 5622
   Email: bclaise@cisco.com











































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