YANG Schema Comparison
draft-ietf-netmod-yang-schema-comparison-03

Network Working Group                                  P. Andersson, Ed.
Internet-Draft                                             Ionio Systems
Updates: 7950 (if approved)                                    R. Wilton
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: 12 April 2026                                          M. Vaško
                                                                  CESNET
                                                          9 October 2025
                         YANG Schema Comparison
              draft-ietf-netmod-yang-schema-comparison-03
Abstract
   This document specifies an algorithm for comparing two revisions of a
   YANG schema to determine the scope of changes, and a list of changes,
   between the revisions.  The output of the algorithm can be used to
   help select an appropriate revision-label or YANG semantic version
   number for a new revision.  Included is also a YANG module describing
   the output of this algorithm.
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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.
   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."
   This Internet-Draft will expire on 12 April 2026.
Copyright Notice
   Copyright (c) 2025 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
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   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.  Key Issues  . . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  On-wire vs Schema analysis  . . . . . . . . . . . . . . .   3
     1.2.  Comparison on module or full schema (YANG artifact,
           arbitrary blob.  Questions  . . . . . . . . . . . . . . .   4
   2.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Override/per-node tags  . . . . . . . . . . . . . . . . .   4
     2.2.  Separate rules for config vs state  . . . . . . . . . . .   4
     2.3.  Tool/report verbosity . . . . . . . . . . . . . . . . . .   4
     2.4.  sub-modules . . . . . . . . . . . . . . . . . . . . . . .   4
     2.5.  Write algorithm in pseudo code or just describe the rules/
           goals in text?  . . . . . . . . . . . . . . . . . . . . .   4
     2.6.  Categories in the report: bc, nbc, potentially-nbc,
           editorial.  Allow filtering in the draft without defining
           it? . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.7.  Only for YANG 1.1?  . . . . . . . . . . . . . . . . . . .   5
   3.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Terminology and Conventions . . . . . . . . . . . . . . . . .   5
   5.  Compiled YANG schema tree . . . . . . . . . . . . . . . . . .   6
   6.  Compiled YANG schema tree comparison algorithm  . . . . . . .   7
     6.1.  Module identification . . . . . . . . . . . . . . . . . .   8
     6.2.  Change content  . . . . . . . . . . . . . . . . . . . . .   8
     6.3.  Change conformance  . . . . . . . . . . . . . . . . . . .   9
   7.  Comparison tooling  . . . . . . . . . . . . . . . . . . . . .   9
   8.  Schema Comparison YANG Module . . . . . . . . . . . . . . . .   9
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  26
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  26
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     11.1.  The "IETF XML" Registry  . . . . . . . . . . . . . . . .  27
     11.2.  The "YANG Module Names" Registry . . . . . . . . . . . .  28
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  28
     12.2.  Informative References . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30
1.  Key Issues
   { This section is only to present the current ongoing work, not part
   of the final draft. }
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   The contributors have identified several key issues that need
   attention.  This section presents selected key issues which have been
   discussed together with suggestions for proposed solution or
   requirements.
1.1.  On-wire vs Schema analysis
   Should one algorithm be used or two?  The consesus reached was to
   define two separate algorithms, one for on-wire format and one for
   schema.
   Schema: any changes that affect the YANG schema in an NBC manner
   according to the full rules of
   [I-D.ietf-netmod-yang-module-versioning].  This may be important for
   clients that, for example, automatically generate code using the YANG
   and where the change of a typedef name or a choice name could be
   significant.  Also important for other modules that may augment or
   deviate the schema being compared.
   Changes to the module that aren't semantic should raise that there
   has been editorial changes
   Ordering in the schema, RFC 7950 doesn't allow reordering; thus an
   NBC change.
   Open Questions:
   Groupings / uses
   typedefs, namespaces, choice names, prefixes, module metadata.
   *  typedef renaming (on-wire, same base type etc)
   *  Should all editorial (text) diffs be reported?
   *  What about editorial changes that might change semantics, e.g.  a
      description of a leaf?
   *  Metadata arguments which relies on the formatted input text.  E.g
      description, contact (etc), extension (how does the user want to
      tune verbosity level for editorial changes: whitespace, spelling,
      editorial, potentially-nbc?
   *  XPath, must, when: don't normalize XPath expressions
   *  presence statements
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1.2.  Comparison on module or full schema (YANG artifact, arbitrary
      blob.  Questions
   *  features
   *  packages vs directories vs libraries vs artifact
   *  package specific comparison, package metadata or only looking at
      the modules
   *  import only or implemented module
   Filter out comparison for a specific subrtree, path etc.  Use case
   for on-wire e.g. yang subscriptions, did the model change fro what is
   subscribed on?
2.  Open Issues
   { This section is only to present the current ongoing work, not part
   of the final draft. }
   The following issues have not ben discussed in any wider extent yet.
2.1.  Override/per-node tags
2.2.  Separate rules for config vs state
2.3.  Tool/report verbosity
   *  where to report changes (module, grouping, typedef, uses)
   *  output level (conceptual level or exact strings)
   *  granularity: error/warning/info level per reported change category
2.4.  sub-modules
2.5.  Write algorithm in pseudo code or just describe the rules/goals in
      text?
2.6.  Categories in the report: bc, nbc, potentially-nbc, editorial.
      Allow filtering in the draft without defining it?
   One option can be to have a tool option that presents the reason
   behind the decision, e.g. --details could be used to explain to the
   user why a certain change was marked as nbc.
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   Another option is to present reasoning and analysis in deeper levels
   of verbosity; e.g. one extra level of verbosity, -v, could present
   the reason for categorizing a change nbc, and an additional extra
   level of verbosity, e.g. -vv, could also present the detailed
   analysis the tool made to categorize the change.
2.7.  Only for YANG 1.1?
3.  Introduction
   This document defines the algorithm for comparing two revisions of a
   YANG schema.  There are two kinds of YANG schemas: on-the-wire or
   compiled schema and text or parsed schema.
   The compiled schema is what tools use when working with any kinds of
   YANG data.  Generally, in all the YANG modules, all the schema nodes
   need to be fully resolved and have their final form.  For example,
   every "uses" statement is replaced with the nodes from the
   corresponding "grouping", every type referencing a "typedef" is
   replaced with the type of the "typedef" and so on.  On the other
   hand, the parsed schema is comprised of all the YANG modules together
   as they are.  That means no statements are resolved and they remain
   in exactly the same text form as present in the YANG modules.
   For determining whether a new revision of a YANG 1.1 module is or is
   not backwards-compatible based on all the rules in [RFC7950] and
   [I-D.ietf-netmod-yang-module-versioning] , both of these schemas
   would be required.  Similarly for YANG 1.0, the rules in [RFC6020] .
   This document defines only the algorithm for comparing the compiled
   schema.  The output of this algorithm can be utilized by tools
   processing YANG instance data to determine what exact changes should
   they expect and even how to transform instance data valid for an old
   revision of a YANG module to be valid for a newer revision of the
   same YANG module.  Such tools are typically clients or servers of
   YANG-driven protocols, e.g.  NETCONF, RESTCONF, or gNMI.
   In addition, the comparison algorithm can assist YANG module authors
   with updating their modules and be used to verify the semantic
   version of the new revision of the YANG module based on the
   [I-D.ietf-netmod-yang-semver] document.
4.  Terminology and Conventions
   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.
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   This document makes use of the following terminology introduced in
   the YANG 1.1 Data Modeling Language [RFC7950]:
   *  schema node
   This document uses terminology introduced in the YANG versioning
   requirements document [I-D.ietf-netmod-yang-versioning-reqs].
   This document makes of the following terminology introduced in the
   YANG Packages [I-D.ietf-netmod-yang-packages]:
   *  YANG schema
   In addition, this document defines the terminology:
   *  Change scope: Whether a change between two revisions is classified
      as non-backwards-compatible or backwards-compatible.
   *  Node compatibility statement: An extension statements (e.g. nbc-
      change-at) that can be used to indicate the backwards
      compatibility of individual schema nodes and specific YANG
      statements.
5.  Compiled YANG schema tree
   The compiled schema tree resolves all the statements used for
   syntactic abstraction and simplification to obtain one schema node
   tree with only data definition statements.  Textual fields are kept
   as well because they may affect backwards compatibility of the whole
   YANG module.
   The steps taken when compiling a schema:
   *  'Import' statements are resolved by finding the referenced YANG
      modules and making their contents available.
   *  All submodules of a single module are merged to create one large
      YANG module.
   *  'Uses' statements are replaced by the referenced 'groupings' and
      the YANG nodes they define.
   *  'Types' referencing any 'typedef' statements are replaced with the
      actual type details.
   *  'Augments' are resolved by placing the included YANG nodes in
      their targets.
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   *  'Deviation' and 'refine' statements are applied by changing the
      appropriate schema nodes.
   *  All the 'if-feature' statements are evaluated based on the enabled
      'features' of implemented YANG modules and the disabled schema
      nodes are removed.
   After the compilation the result is still a valid YANG module without
   the following statements:
   *  import
   *  include
   *  revision
   *  typedef
   *  grouping
   *  augment
   *  deviation
   *  feature
   *  if-feature
   *  refine
   *  extension
6.  Compiled YANG schema tree comparison algorithm
   This algorithm is defined for separate compiled YANG modules.  It
   first compares all the non data-definition statements in the module
   and then all the data-definition statements recursively.  These
   schema-only statements are skipped:
   *  choice
   *  case
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   The non data-definition statements are considered modified when their
   content is changed or any of their substatements are changed.  If a
   statement is present in the old revision of a module and missing in
   the new revision, it is considered removed.  Vice-versa, if missing
   in the old revision but present in the new revision, it is considered
   added.
   Data-definition statements are compared similarly except that a
   change in a descendant data-definition statement is not considered a
   change of the parent data-definition statement but a separate change
   instead.
   Only statements with any changes are reported and included in the
   algorithm output.
6.1.  Module identification
   When generating the comparison output of two compiled YANG modules,
   it includes unique identification of the modules in question.  For
   both the old and the new revision of the module, its name, revision,
   list of submodules, and a leaf-list of enabled features is included.
   In addition, the same information is reported for all the imported
   YANG modules, recursively.  This ensures that if any other output is
   produced with a different set of changes for two YANG modules, their
   identification information must also differ.
6.2.  Change content
   Non-data-definition statements are uniquely identified with their
   YANG identifier and data-definition statements with their absolute
   schema data node-ids.
   For every change, the changed statement and the parent of the changed
   statement are included.  While redundant in many cases, it allows to
   uniquely report changes in nested statements, e.g. an extension
   instance change in the type of a leaf statement.  Also, all the
   substatements with their value or content are included for every
   changed statement, for both older and newer revision of the YANG
   module, if applicable.  This ensures that the output includes all the
   possibly required information without relying on learning it from the
   YANG modules themselves.
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6.3.  Change conformance
   Every change is classified as either backwards-compatible or non-
   backwards-compatible based on sections 3.1.1 and 3.1.2 in
   [I-D.ietf-netmod-yang-module-versioning].  But, there are cases when
   it is impossible for tooling to determine whether a change is
   backwards-compatible or not.  The affected statements are:
   *  pattern
   *  when
   *  must
   *  description
   *  extension instance
   Normally, any change in these statement is reported as non-backwards-
   compatible.  However, if the author of the new revision of a YANG
   module determines that the change of an aforementioned statement is
   backwards-compatible based on the compatibility rules for the
   specific statement, they can decide to use the 'schema-cmp:backwards-
   compatible' extension to signify this fact.  Then, this change must
   be reported as backwards-compatible.
7.  Comparison tooling
   The `yanglint` tool from the libyang library is able to generate the
   `ietf-schema-comparison` YANG data of two revisions of a YANG module.
8.  Schema Comparison YANG Module
   YANG module with nodes describing the differences between two
   revisions of compiled YANG modules.  The "ietf-schema-comparison"
   YANG module imports definitions from the "ietf-yang-types" module
   defined in [RFC6991] and "ietf-yang-library" module defined in
   [RFC8525].
   <CODE BEGINS> file "ietf-schema-comparison@2025-10-09.yang"
   module ietf-schema-comparison {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-schema-comparison";
     prefix schema-cmp;
     import ietf-yang-types {
       prefix yang;
       reference
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         "RFC 6991: Common YANG Data types";
     }
     import ietf-yang-library {
       prefix yanglib;
       reference
         "RFC 8525: YANG Library";
     }
     organization
       "IETF NETMOD (Network Modeling) Working Group";
     contact
       "WG List: NETCONF WG list <mailto:netmod@ietf.org>
        WG Web:  https://datatracker.ietf.org/wg/netmod
        Author:  Michal Vaško
                 <mailto:mvasko@cesnet.cz>
        Author:  Rob Wilton
                 <mailto:rwilton@cisco.com>
        Author:  Per Andersson
                 <mailto:per.ietf@ionio.se>";
     description
       "This YANG 1.1 module contains definitions and extensions to
        support comparison between different versions of YANG modules. The
        output of the comparison algorithm is described in this YANG module.
        Copyright (c) 2025 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
        (https://trustee.ietf.org/license-info).
        This version of this YANG module is part of RFC XXXX
        (https://www.rfc-editor.org/info/rfcXXXX); see the RFC
        itself for full legal notices.
        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)
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        (RFC 8174) when, and only when, they appear in all
        capitals, as shown here.";
     // RFC Ed.: update the date below with the date of RFC publication
     // and remove this note.
     // RFC Ed.: replace XXXX (inc above) with actual RFC number and
     // remove this note.
     revision 2025-10-09 {
       description
         "Initial revision.";
       reference
         "RFC XXXX: YANG Schema Comparison";
     }
     extension backwards-compatible {
       description
         "Marks statements in a new revision of a module
          that were changed but in a backwards-compatible
          manner according to YANG 1.1 and 1.0 rules for
          updating a module.
          When used as a substatement of a 'pattern'
          statement, it means its allowed value space has
          been expanded.
          When used as a substatement of a 'when' or
          'must' statement, it means their constraint has
          been relaxed.
          When used as a substatement of a 'description'
          statement, it means it has been changed in a
          backwards-compatible way.
          When used as a substatement of an extension
          instance, it means adding, modifying, or removing
          it is a backwards-compatible change.
          Not allowed to be instiantiated for any other
          statements.";
       reference
         "RFC 7950: The YANG 1.1 Data Modeling Language, section 11;
          RFC 6020: YANG - A Data Modeling Language for the Network
          Configuration Protocol (NETCONF), section 10";
     }
     typedef data-path {
       type string;
       description
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         "Used for any data paths and XPath expressions in the compared YANG
          modules. A generic type is used to not enforce their evaluation or
          validation in this context.";
     }
     typedef stmt-type {
       type enumeration {
         enum "base";
         enum "bit";
         enum "config";
         enum "contact";
         enum "default";
         enum "description";
         enum "enum";
         enum "error-app-tag";
         enum "error-message";
         enum "extension-instance";
         enum "fraction-digits";
         enum "identity";
         enum "length";
         enum "mandatory";
         enum "max-elements";
         enum "min-elements";
         enum "must";
         enum "node";
         enum "ordered-by";
         enum "organization";
         enum "path";
         enum "pattern";
         enum "presence";
         enum "range";
         enum "reference";
         enum "require-instance";
         enum "status";
         enum "type";
         enum "units";
         enum "unique";
         enum "when";
       }
       description
         "Type of the statement that a change affects.";
     }
     typedef revision-or-empty {
       type union {
         type empty;
         type yanglib:revision-identifier;
       }
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       description
         "Module or submodule revision. If it has none, an empty value is
          used.";
     }
     grouping ext-instance-changes {
       description
         "Describes an extension-instance statement change.";
       leaf module {
         type yang:yang-identifier;
         mandatory true;
         description
           "Module with the extension definition of an extension instance.";
       }
       leaf name {
         type yang:yang-identifier;
         mandatory true;
         description
           "Name of the extension defition of an extension instance.";
       }
       leaf argument {
         type string;
         description
           "Argument used of an extension definition.";
       }
       anydata substatements {
         description
           "Any substatements of the extension instance.";
       }
     }
     grouping status-changes {
       description
         "Describes a status statement change.";
       leaf status {
         type enumeration {
           enum "current";
           enum "deprecated";
           enum "obsolete";
         }
         mandatory true;
         description
           "Status substatement value.";
       }
     }
     grouping must-changes {
       description
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         "Describes a must statement change.";
       leaf condition {
         type data-path;
         description
           "Condition substatement value.";
       }
       leaf description {
         type string;
         description
           "Description substatement value.";
       }
       leaf reference {
         type string;
         description
           "Reference substatement value.";
       }
       leaf error-message {
         type string;
         description
           "Error-message substatement value.";
       }
       leaf error-app-tag {
         type string;
         description
           "Error-app-tag substatement value.";
       }
       list ext-instance {
         description
           "List of extension-instance substatements.";
         uses ext-instance-changes;
       }
     }
     grouping when-changes {
       description
         "Describes a when statement change.";
       leaf condition {
         type data-path;
         description
           "Condition substatement value.";
       }
       leaf description {
         type string;
         description
           "Description substatement value.";
       }
       leaf reference {
         type string;
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         description
           "Reference substatement value.";
       }
       uses status-changes;
       list ext-instance {
         description
           "List of extension-instance substatements.";
         uses ext-instance-changes;
       }
     }
     grouping restriction-substmts {
       description
         "Common substatements shared by all restriction statements.";
       leaf description {
         type string;
         description
           "Description substatement value.";
       }
       leaf reference {
         type string;
         description
           "Reference substatement value.";
       }
       leaf error-message {
         type string;
         description
           "Error-message substatement value.";
       }
       leaf error-app-tag {
         type string;
         description
           "Error-app-tag substatement value.";
       }
       list ext-instance {
         description
           "List of extension-instance substatements.";
         uses ext-instance-changes;
       }
     }
     grouping type-changes {
       description
         "Describes a type statement change.";
       leaf base-type {
         type enumeration {
           enum "int8";
           enum "int16";
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           enum "int32";
           enum "int64";
           enum "uint8";
           enum "uint16";
           enum "uint32";
           enum "uint64";
           enum "decimal64";
           enum "string";
           enum "boolean";
           enum "enumeration";
           enum "bits";
           enum "binary";
           enum "leafref";
           enum "identityref";
           enum "empty";
           enum "union";
           enum "instance-identifier";
         }
         description
           "Type substatement value.";
       }
       container range {
         description
           "Range statement substatements.";
         list interval {
           description
             "Intervals of a range statement.";
           leaf min {
             type int64;
             description
               "Lower boundary of an interval of a range statement.";
           }
           leaf max {
             type int64;
             description
               "Upper boundary of an interval of a range statement.";
           }
         }
         uses restriction-substmts;
       }
       container length {
         description
           "Length statement substatements.";
         list interval {
           description
             "Intervals of a length statement.";
           leaf min {
             type uint64;
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             description
               "Lower boundary of an interval of a length statement.";
           }
           leaf max {
             type uint64;
             description
               "Upper boundary of an interval of a length statement.";
           }
         }
         uses restriction-substmts;
       }
       leaf fraction-digits {
         type uint8;
         description
           "Fraction-digits substatement value.";
       }
       list pattern {
         leaf expression {
           type string;
           description
             "Pattern substatement value.";
         }
         leaf inverted {
           type boolean;
           description
             "Inverted substatement value.";
         }
         uses restriction-substmts;
       }
       list enum {
         key "name";
         description
           "List of enum statements.";
         leaf name {
           type yang:yang-identifier;
           description
             "Enum statement name.";
         }
         leaf description {
           type string;
           description
             "Description substatement value.";
         }
         leaf reference {
           type string;
           description
             "Reference substatement value.";
         }
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         leaf value {
           type int32;
           description
             "Value substatement value.";
         }
         uses status-changes;
         list ext-instance {
           description
             "List of extension-instance substatements.";
           uses ext-instance-changes;
         }
       }
       list bit {
         key "name";
         leaf name {
           type yang:yang-identifier;
         }
         leaf description {
           type string;
           description
             "Description substatement value.";
         }
         leaf reference {
           type string;
           description
             "Reference substatement value.";
         }
         leaf position {
           type uint32;
           description
             "Position substatement value.";
         }
         uses status-changes;
         list ext-instance {
           description
             "List of extension-sintance substatements.";
           uses ext-instance-changes;
         }
       }
       leaf path {
         type data-path;
         description
           "Path substatement value.";
       }
       leaf require-instance {
         type boolean;
         description
           "Require-instance substatement value.";
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       }
       leaf-list base {
         type yang:yang-identifier;
         description
           "List of base substatement values.";
       }
       list ext-instance {
         description
           "List of extension-instance substatements.";
         uses ext-instance-changes;
       }
     }
     grouping node-changes {
       leaf config {
         type boolean;
         description
           "Config substatement value.";
       }
       leaf description {
         type string;
         description
           "Description substatement value.";
       }
       leaf mandatory {
         type boolean;
         description
           "Mandatory substatement value.";
       }
       list must {
         description
           "List of must substatements.";
         uses must-changes;
       }
       leaf presence {
         type boolean;
         description
           "Presence substatement existence.";
       }
       leaf reference {
         type string;
         description
           "Reference substatement value.";
       }
       uses status-changes;
       list when {
         description
           "List of when substatements.";
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         uses when-changes;
       }
       container type {
         description
           "Type substatement.";
         uses type-changes;
         list union-type {
           description
             "List of a union type substatements.";
           uses type-changes;
         }
       }
       leaf units {
         type string;
         description
           "Units substatement value.";
       }
       leaf ordered-by {
         type enumeration {
           enum "system";
           enum "user";
         }
         description
           "Ordered-by substatement value.";
       }
       leaf-list default {
         type string;
         description
           "List of default substatement values.";
       }
       leaf min-elements {
         type uint32;
         description
           "Min-elements substatement value.";
       }
       leaf max-elements {
         type uint32;
         description
           "Max-elements substatement value.";
       }
       list unique {
         description
           "List of unique substatements.";
         leaf-list node {
           type yang:yang-identifier;
           description
             "List of nodes referenced by a unique substatement.";
         }
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       }
       list ext-instance {
         description
           "List of extension-instance substatements.";
         uses ext-instance-changes;
       }
     }
     grouping module-changes {
       leaf organization {
         type string;
         description
           "Organization substatement value.";
       }
       leaf contact {
         type string;
         description
           "Contact substatement value.";
       }
       leaf description {
         type string;
         description
           "Description substatement value.";
       }
       leaf reference {
         type string;
         description
           "Reference substatement value.";
       }
       list identity {
         key "name";
         description
           "List of identity substatements.";
         leaf name {
           type yang:yang-identifier;
           description
             "Identity statement name.";
         }
         list ext-instance {
           description
             "List of extension-instance substatements.";
           uses ext-instance-changes;
         }
       }
       list ext-instance {
         description
           "List of extension-instance substatements.";
         uses ext-instance-changes;
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       }
     }
     grouping conformance-type {
       leaf conformance {
         type enumeration {
           enum "backwards-compatible";
           enum "non-backwards-compatible";
         }
         mandatory true;
         description
           "Conformance information of a change in the compared YANG modules.";
       }
     }
     grouping change-info {
       list changed {
         key "stmt";
         min-elements 1;
         description
           "List of changes.";
         leaf stmt {
           type stmt-type;
           description
             "Changed statement.";
         }
         leaf parent-stmt {
           type stmt-type;
           description
             "Parent statement of the changed statement.";
         }
         leaf change {
           type enumeration {
             enum "modified" {
               description
                 "Statement existed and was modified.";
             }
             enum "added" {
               description
                 "Statement was added.";
             }
             enum "removed" {
               description
                 "Statement was removed.";
             }
             enum "moved" {
               description
                 "Statement was moved and the position affects the meaning.";
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             }
           }
           mandatory true;
           description
             "Type of change of the statement.";
         }
         uses conformance-type;
       }
     }
     grouping module-params {
       leaf module {
         type yang:yang-identifier;
         mandatory true;
         description
           "Compared module name.";
       }
       leaf revision {
         type revision-or-empty;
         mandatory true;
         description
           "Compared module revision.";
       }
       list submodule {
         key "name revision";
         leaf name {
           type yang:yang-identifier;
           mandatory true;
           description
             "Submodule name.";
         }
         leaf revision {
           type revision-or-empty;
           mandatory true;
           description
             "Submodule revision.";
         }
         description
           "Submodule of the main module.";
       }
       leaf-list enabled-feature {
         type yang:yang-identifier;
         description
           "All the enabled features of the module.";
       }
     }
     container schema-comparison {
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       config false;
       description
         "Schema comparison details.";
       list compiled-diff {
         description
           "Instance of a schema comparison of two YANG modules in different
            revisions with all their imports.";
         container source {
           description
             "Source module information.";
           uses module-params;
         }
         list source-import {
           key "module revision";
           description
             "List of source module imported modules information.";
           uses module-params;
         }
         container target {
           description
             "Target module information.";
           uses module-params;
         }
         list target-import {
           key "module revision";
           description
             "List of target module imported modules information.";
           uses module-params;
         }
         uses conformance-type;
         container module-diff {
           description
             "Information about direct module statement substatement changes.";
           uses change-info;
           container old {
             description
               "Changed statements in the older revision of the YANG module.";
             uses module-changes;
           }
           container new {
             description
               "Changed statements in the newer revision of the YANG module.";
             uses module-changes;
           }
         }
         list node-diff {
           key "node";
           description
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             "Information about data-definition (node) statement substatement
              changes.";
           leaf node {
             type data-path;
             description
               "Path to the changed data-definition statement.";
           }
           leaf node-type {
             type enumeration {
               enum "container";
               enum "leaf";
               enum "leaf-list";
               enum "list";
               enum "anydata";
               enum "anyxml";
               enum "rpc";
               enum "action";
               enum "notification";
             }
             mandatory true;
             description
               "Type of the changed data-definition statement.";
           }
           leaf in-rpc-action {
             type enumeration {
               enum "input";
               enum "output";
             }
             description
               "Present if the changed data-definition statement is a
                descendant of an action or rpc statement.";
           }
           uses change-info;
           container old {
             description
               "All the node substatements in the older revision of the YANG
                module.";
             uses node-changes;
           }
           container new {
             description
               "All the node substatements in the newer revision of the YANG
                module.";
             uses node-changes;
           }
         }
       }
     }
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   }
   <CODE ENDS>
9.  Contributors
   This document grew out of the YANG module versioning design team that
   started after IETF 101.  The following individuals are (or have been)
   members of the design team and have worked on the YANG versioning
   project:
   *  Balazs Lengyel
   *  Benoit Claise
   *  Bo Wu
   *  Ebben Aries
   *  Jason Sterne
   *  Joe Clarke
   *  Juergen Schoenwaelder
   *  Mahesh Jethanandani
   *  Michael Wang
   *  Qin Wu
   *  Reshad Rahman
   *  Rob Wilton
   *  Jan Lindblad
   *  Per Andersson
   The ideas for a tooling based comparison of YANG module revisions was
   first described in [I-D.clacla-netmod-yang-model-update].  This
   document extends upon those initial ideas.
10.  Security Considerations
   This section follows the template defined in Section 3.7.1 of
   [RFC8407].
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   The YANG module specified in this document defines a schema for data
   that is designed to be used by offline tooling to generate output for
   differences in supplied YANG modules.
   The YANG module specifiod in this document MAY be accessed via
   network management protocols such as NETCONF [RFC6241] or RESTCONF
   [RFC8040].  The lowest NETCONF layer is the secure transport layer,
   and the mandatory-to-implement secure transport is Secure Shell (SSH)
   [RFC6242].  The lowest RESTCONF layer is HTTPS, and the mandatory-to-
   implement secure transport is TLS [RFC8446].
   The Network Configuration Access Control Model (NACM) [RFC8341]
   provides the means to restrict access for particular NETCONF or
   RESTCONF users to a preconfigured subset of all available NETCONF or
   RESTCONF protocol operations and content.
   When the module is used for offline tooling there are no security
   considerations, since the user has full access to all YANG modules
   used.
   The container "schema-comparison" contains all the groupings
   reflecting the changes between YANG modules.  If this data node is
   published in online tooling, care needs to be taken so that knowledge
   of YANG modules are not leaked.
   Since the modules does not define any RPCs or actions or
   notifications, and thus the security considerations for such are not
   provided here.
11.  IANA Considerations
11.1.  The "IETF XML" Registry
   This document register one URI in the "ns" subregistry of the IETF
   XML Registry [RFC3688] maintained at
   https://www.iana.org/assignments/xml-registry/xml-registry.xhtml#ns.
   Following the format in [RFC3688], the following registration is
   requested:
          URI: urn:ietf:params:xml:ns:yang:ietf-schema-comparison
          Registrant Contact: The IESG.
          XML: N/A, the requested URI is an XML namespace.
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11.2.  The "YANG Module Names" Registry
   This document registers one YANG module in the YANG Module Names
   registry [RFC6020] maintained at https://www.iana.org/assignments/
   yang-parameters/yang-paramaters.xhtml.  Following the format defined
   in [RFC6020], the below registration is requested:
     Name: ietf-schema-comparison
     XML Namespace: urn:ietf:params:xml:ns:yang:ietf-schema-comparison
     Prefix: schema-cmp
     Reference: RFC XXXX
12.  References
12.1.  Normative References
   [I-D.ietf-netmod-yang-module-versioning]
              Wilton, R., Rahman, R., Lengyel, B., Clarke, J., and J.
              Sterne, "Updated YANG Module Revision Handling", Work in
              Progress, Internet-Draft, draft-ietf-netmod-yang-module-
              versioning-13, 6 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
              yang-module-versioning-13>.
   [I-D.ietf-netmod-yang-packages]
              Wilton, R., Rahman, R., Clarke, J., and J. Sterne, "YANG
              Packages", Work in Progress, Internet-Draft, draft-ietf-
              netmod-yang-packages-06, 7 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
              yang-packages-06>.
   [I-D.ietf-netmod-yang-semver]
              Clarke, J., Wilton, R., Rahman, R., Lengyel, B., Sterne,
              J., and B. Claise, "YANG Semantic Versioning", Work in
              Progress, Internet-Draft, draft-ietf-netmod-yang-semver-
              24, 29 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
              yang-semver-24>.
   [I-D.ietf-netmod-yang-solutions]
              Wilton, R., "YANG Versioning Solution Overview", Work in
              Progress, Internet-Draft, draft-ietf-netmod-yang-
              solutions-01, 2 November 2020,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
              yang-solutions-01>.
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   [I-D.ietf-netmod-yang-versioning-reqs]
              Clarke, J., "YANG Module Versioning Requirements", Work in
              Progress, Internet-Draft, draft-ietf-netmod-yang-
              versioning-reqs-12, 21 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
              yang-versioning-reqs-12>.
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.
   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.
   [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>.
   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.
   [RFC6242]  Wasserman, M., "Using the NETCONF Protocol over Secure
              Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
              <https://www.rfc-editor.org/info/rfc6242>.
   [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>.
   [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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   [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
              Access Control Model", STD 91, RFC 8341,
              DOI 10.17487/RFC8341, March 2018,
              <https://www.rfc-editor.org/info/rfc8341>.
   [RFC8407]  Bierman, A., "Guidelines for Authors and Reviewers of
              Documents Containing YANG Data Models", BCP 216, RFC 8407,
              DOI 10.17487/RFC8407, October 2018,
              <https://www.rfc-editor.org/info/rfc8407>.
   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.
   [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>.
12.2.  Informative References
   [I-D.clacla-netmod-yang-model-update]
              Claise, B., Clarke, J., Lengyel, B., and K. D'Souza, "New
              YANG Module Update Procedure", Work in Progress, Internet-
              Draft, draft-clacla-netmod-yang-model-update-06, 2 July
              2018, <https://datatracker.ietf.org/doc/html/draft-clacla-
              netmod-yang-model-update-06>.
Authors' Addresses
   Per Andersson (editor)
   Ionio Systems
   Email: per.ietf@ionio.se
   Robert Wilton
   Cisco Systems, Inc.
   Email: rwilton@cisco.com
   Michal Vaško
   CESNET
   Email: mvasko@cesnet.cz
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