RFC 9195

Internet Engineering Task Force (IETF)                        B. Lengyel
Request for Comments: 9195                                      Ericsson
Category: Standards Track                                      B. Claise
ISSN: 2070-1721                                                   Huawei
                                                           February 2022

                  A File Format for YANG Instance Data


   There is a need to document data defined in YANG models at design
   time, implementation 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
   and annotates it with metadata.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
     1.2.  Principles
     1.3.  Delivery of Instance Data
     1.4.  Data Life Cycle
   2.  Instance Data File Format
     2.1.  Specifying the Content Schema
       2.1.1.  Inline Method
       2.1.2.  Simplified-Inline Method
       2.1.3.  URI Method
     2.2.  Examples
       2.2.1.  Documentation of Server Capabilities
       2.2.2.  Preloading Default Configuration Data
       2.2.3.  Storing Diagnostics Data
   3.  YANG Instance Data Model
     3.1.  Tree Diagram
     3.2.  YANG Model
   4.  Security Considerations
   5.  IANA Considerations
     5.1.  URI Registration
     5.2.  YANG Module Name Registration
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Appendix A.  Backwards Compatibility
   Appendix B.  Detailed Use Cases
     B.1.  Use Case 1: Early Documentation of Server Capabilities
     B.2.  Use Case 2: Preloading Data
     B.3.  Use Case 3: Documenting Factory Default Settings

   Authors' Addresses

1.  Introduction

   There is a need to document data defined in YANG models when a live
   server is unavailable.  Data is often needed at design time,
   implementation 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; see Section 3.
   The YANG data model in this document conforms to the Network
   Management Datastore Architecture (NMDA) defined in [RFC8342].

   The following is a list of already-implemented and potential use

   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 inter-process communication (IPC) message formats

   UC7  Default instance data used as part of a templating solution

   UC8  Providing data examples in RFCs or internet drafts

   Appendix B describes the first three use cases in detail.

   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.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

   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].

1.2.  Principles

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

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

   P2  Instance data shall reuse existing encoding rules for YANG-
       defined data.

   P3  Metadata about the instance data set (Section 2, Paragraph 14)
       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 implemented by a server.

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

1.3.  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.

1.4.  Data Life Cycle

   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 unless it is updated.  The current values
   may be retrieved at runtime 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.

2.  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 the user wants to document and/or provide.  The
   syntax and semantics of content-data are defined by the content-

   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.

   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].

   *  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 for
   a server.  Default values should be excluded where they do not
   provide additional useful data.

   Configuration ("config true") and operational state data ("config
   false") 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" constraints MAY be violated.

   The name of the instance data file SHOULD be of the following form
   (using ABNF notation [RFC5234]):

      instance-data-set-name ["@" ( revision-date / timestamp ) ]
                     ( ".xml" / ".json" )

   Examples include:


   If the leaf "name" is present in the instance data header, its value
   SHOULD be used for the "instance-data-set-name" in the filename.  If
   the "revision-date" is present in the filename, it MUST conform to
   the format of the revision-date leaf in the YANG model.  If the
   "revision-date" is present in both the filename and the instance data
   header, the revision date in the filename MUST be set to the latest
   revision date inside the instance data set.  If the "timestamp" is
   present in the filename, it MUST conform to the format of the
   timestamp leaf in the YANG model except for replacing colons as
   described below.  If the "timestamp" is present in both the filename
   and the instance data header, the timestamp in the filename SHOULD be
   set to the timestamp inside the instance data set; any colons, if
   present, shall be replaced by underscores.

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

   Metadata MUST include:

      -  Version of the YANG instance data format (if not explicitly
         present, the default value is used).

   Metadata SHOULD include:

      -  Name of the data set.

      -  Content-schema specification (i.e., the "content-schema" node).

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

      -  An indication of whether default values are included.  The
         default handling uses the concepts defined in [RFC6243];
         however, as only concepts are re-used, users of instance data
         sets do not need to support [RFC6243].

2.1.  Specifying the Content Schema

   To properly understand and use an instance data set, the user needs
   to know the content-schema.  The content-schema can be specified
   either in external documents or within the instance data set.  In the
   latter case, one of the following methods MUST 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; only the modules' name and revision-date
      are used.

   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).

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

   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 in Section 1, 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 another schema depends on many
   factors, including the number of differences and the compatibility
   between the original and the other schema when considering modules,
   revisions, features, deviations, the scope of the instance data, etc.

2.1.1.  Inline Method

   The "inline-yang-library" anydata data node carries instance data
   (conforming to "ietf-yang-library@2019-01-04") [RFC8525] that
   specifies the content-defining YANG modules, including revision,
   supported features, deviations, and any additional relevant data.  An
   example of the inline method is provided in Section 2.2.1.

2.1.2.  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.  YANG modules that are only required to satisfy
   import-only dependencies MAY be excluded from the leaf-list.  If they
   are 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 method is provided in Section 2.2.2.

2.1.3.  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 unavailable, the 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
   -- for example, in UC6, when the system creates a diagnostic file
   every minute to document the state of the server.

   An example of the URI method is provided in Section 2.2.3.

2.2.  Examples

2.2.1.  Documentation of Server Capabilities

   The example file acme-router-modules@2022-01-20.xml reflects UC1 in
   Section 1.  It provides a list of supported YANG modules and NETCONF
   capabilities for a server.  It uses the inline method to specify the

   The example uses artwork folding [RFC8792].

   ========== NOTE: '\' line wrapping per RFC 8792 ===========

   <?xml version="1.0" encoding="UTF-8"?>
   <instance-data-set xmlns=\
         <modules-state \
       <description>Initial version</description>
     <description>Defines the minimal set of modules that any \
         acme-router will contain. This minimal set will \
         only change when a new software release is \
       <modules-state \
       <netconf-state \

                                  Figure 1

2.2.2.  Preloading Default Configuration Data

   The example file read-only-acm-rules@2022-01-20.xml reflects UC2 in
   Section 1.  It provides a default rule set for a read-only operator
   role.  It uses the simplified-inline method for specifying the

   ========== NOTE: '\' line wrapping per RFC 8792 ===========

   <?xml version="1.0" encoding="UTF-8"?>
       <description>Initial version</description>
     <description>Default access control rules for a read-only \
         role. This set of rules will only change when a new  \
         software release is introduced.</description>
       <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm">

                                  Figure 2

2.2.3.  Storing Diagnostics Data

   The example file acme-router-netconf-
   diagnostics@2018-01-25T17_00_38Z.json reflects UC5 in Section 1.  An
   instance data set that contains statistics about the NETCONF server
   is produced by the server every 15 minutes.  As a new set is produced
   periodically many times a day, a revision-date would be useless;
   instead, a timestamp is included.

   ========== 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"

                                  Figure 3

3.  YANG Instance Data Model

3.1.  Tree Diagram

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

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

3.2.  YANG Model

   This YANG module imports typedefs from [RFC6991], [RFC6243],
   identities from [RFC8342], and the "structure" extension from
   [RFC8791].  It also references [RFC8525].

   <CODE BEGINS> file "ietf-yang-instance-data@2022-02-17.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;
         "RFC 8791: YANG Data Structure Extensions";
     import ietf-datastores {
       prefix ds;
         "RFC 8342: Network Management Datastore Architecture (NMDA)";
     import ietf-inet-types {
       prefix inet;
         "RFC 6991: Common YANG Data Types";
     import ietf-yang-types {
       prefix yang;
         "RFC 6991: Common YANG Data Types";
     import ietf-netconf-with-defaults {
       prefix ncwd;
         "RFC 6243: With-defaults Capability for NETCONF";

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

        Author:  Balazs Lengyel

        Author:  Benoit Claise
       "The module defines the structure and content of YANG
        instance data sets.

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
        '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) 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 Revised BSD License
        set forth in Section 4.c of the IETF Trust's
        Legal Provisions Relating to IETF Documents

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

     revision 2022-02-17 {
         "Initial revision.";
         "RFC 9195: YANG Instance Data File Format";

     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].*';
         "A type defining a module name and an optional revision
          date, e.g., ietf-yang-library@2019-01-04.";

     sx:structure instance-data-set {
         "A data structure to define a format for YANG instance
          data.  The majority of the YANG nodes provides metadata
          about the instance data; the instance data itself is
          contained only in the 'content-data' node.";
       leaf name {
         type string;
           "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 per Section 2
            of RFC 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 "2022-01-20";
           "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";
           "Indicates how data nodes with default values are
            represented for all data nodes contained in the

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

            For JSON files, the encoding of the 'report-all-tagged'
            option is as defined in Section 4.8.9 of RFC 8040.";
           "RFC 6243: With-defaults Capability for NETCONF";
       container content-schema {
           "The content schema (i.e., YANG modules) used to create
            the instance data set.
            If not present, the user needs to obtain the information
            through external documents.";
         choice content-schema-spec {
             "Specification of the content-schema.";
           case simplified-inline {
             leaf-list module {
               type module-with-revision-date;
               min-elements 1;
                 "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, 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;
                 "Instance data corresponding to the
                  ietf-yang-library@2019-01-04 defining
                  the set of content-defining YANG modules for
                  this instance-data-set.";
           case uri {
             leaf same-schema-as-file {
               type inet:uri;
                 "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' MAY be

                  The URL schemes 'file://' and 'https://' MUST
                  be supported; other schemes MAY also be
       leaf-list description {
         type string;
           "Description of the instance data set.";
       leaf contact {
         type string;
           "Contact information for the person or
            organization to whom queries concerning this
            instance data set should be sent.";
       leaf organization {
         type string;
           "Organization responsible for the instance
            data set.";
       leaf datastore {
         type ds:datastore-ref;
           "The identity of the datastore with which the
            instance data set is associated, e.g., the datastore from
            where the data was read, the datastore into which the data
            may be loaded, or the datastore 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";
           "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 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.";
         leaf date {
           type string {
             pattern '\d{4}-(1[0-2]|0[1-9])-(0[1-9]|[1|2][0-9]|3[0-1])';
             "Specifies the date the instance data set
              was last modified. Formatted as YYYY-MM-DD.";
         leaf description {
           type string;
             "Description of this revision of the instance data set.";
       leaf timestamp {
         type yang:date-and-time;
           "The date and time when the instance data set
            was last modified.

            In 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,
            the timestamp SHOULD contain the same date as the
            latest revision statement.";
       anydata content-data {
           "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 manner.";

4.  Security Considerations

   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, the security
   considerations template for YANG models in Section 3.7.1 of [RFC8407]
   is not followed.  The instance data is designed to be accessed as a
   stored file or over any file access method or protocol.

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

   The header part is usually not security sensitive; however, sensitive
   information may be included, in which case it needs to be handled
   securely, as mentioned below.  Information to consider includes:

   *  If the URI method is used for specification of the content-schema
      and the URI includes a userinfo subcomponent

   *  Any description text

   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 need to be handled securely.  The same kind of handling
   should be applied to this file at rest and 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.

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

   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, the content-schema might differ from
   the one expected.  Protecting the integrity and stability of the
   referenced file should be ensured.

5.  IANA Considerations

   This document registers one URI and one YANG module.

5.1.  URI Registration

   This document registers the following URI in the "IETF XML Registry"

   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.

5.2.  YANG Module Name Registration

   This document registers the following 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 9195

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,

   [RFC6243]  Bierman, A. and B. Lengyel, "With-defaults Capability for
              NETCONF", RFC 6243, DOI 10.17487/RFC6243, June 2011,

   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              RFC 6991, DOI 10.17487/RFC6991, July 2013,

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,

   [RFC7951]  Lhotka, L., "JSON Encoding of Data Modeled with YANG",
              RFC 7951, DOI 10.17487/RFC7951, August 2016,

   [RFC7952]  Lhotka, L., "Defining and Using Metadata with YANG",
              RFC 7952, DOI 10.17487/RFC7952, August 2016,

   [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>.

   [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,

   [RFC8525]  Bierman, A., Bjorklund, M., Schoenwaelder, J., Watsen, K.,
              and R. Wilton, "YANG Library", RFC 8525,
              DOI 10.17487/RFC8525, March 2019,

   [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,

   [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,

   [RFC8791]  Bierman, A., Björklund, M., and K. Watsen, "YANG Data
              Structure Extensions", RFC 8791, DOI 10.17487/RFC8791,
              June 2020, <https://www.rfc-editor.org/info/rfc8791>.

6.2.  Informative References

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,

   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,

   [RFC8407]  Bierman, A., "Guidelines for Authors and Reviewers of
              Documents Containing YANG Data Models", BCP 216, RFC 8407,
              DOI 10.17487/RFC8407, October 2018,

   [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>.

   [RFC8792]  Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
              "Handling Long Lines in Content of Internet-Drafts and
              RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,

   [RFC8808]  Wu, Q., Lengyel, B., and Y. Niu, "A YANG Data Model for
              Factory Default Settings", RFC 8808, DOI 10.17487/RFC8808,
              August 2020, <https://www.rfc-editor.org/info/rfc8808>.

Appendix A.  Backwards Compatibility

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

   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.

   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

   *  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.

   *  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.

   *  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 B.  Detailed Use Cases

   This section is non-normative.

B.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:

   *  data defined in "ietf-yang-library": YANG modules, submodules,
      features, deviations, schema-mounts, and datastores supported

   *  alarms supported ([RFC8632]).

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

   *  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 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 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 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 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 Operations,
   Administration, and Maintenance (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 and
   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.

B.2.  Use Case 2: Preloading Data

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

   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).

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

B.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 to the default factory

   YANG instance data can be used to document the factory default
   configuration.  See [RFC8808].


   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 Working Group.

Authors' Addresses

   Balazs Lengyel
   Magyar Tudosok korutja 11
   Email: balazs.lengyel@ericsson.com

   Benoit Claise
   Email: benoit.claise@huawei.com