Internet-Draft YANG Schema Comparison October 2025
Andersson, et al. Expires 23 April 2026 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-ietf-netmod-yang-schema-comparison-05
Updates:
7950 (if approved)
Published:
Intended Status:
Standards Track
Expires:
Authors:
P. Andersson, Ed.
Ionio Systems
R. Wilton
Cisco Systems, Inc.
M. Vaško
CESNET

YANG Schema Comparison

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 23 April 2026.

▲

Table of Contents

1. 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.

1.1. Override/per-node tags

1.2. Separate rules for config vs state

2. 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 an algorithm for comparing either only the compiled schema or both the parsed and 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 gRPC.

But, the generated data can also be used by YANG module authors to check all the changes made between 2 revisions. Additionally, it can assist with deciding what semantic version should the new revision of the module use based on the [I-D.ietf-netmod-yang-semver] document.

3. 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.

This document makes use of the following terminology introduced in the YANG 1.1 Data Modeling Language [RFC7950]:

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]:

In addition, this document defines the terminology:

4. 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:

After the compilation the result is still a valid YANG module without the following statements:

5. YANG schema tree comparison algorithm

This algorithm is defined for separate YANG modules. When comparing the compiled schema, all the non data-definition statements are first considered and then all the data-definition statements, recursively. These schema-only statements are skipped:

When comparing the parsed schema, the algorithm operates in the same fashion but compares all the YANG statements. However, only in the selected YANG module ignoring imported and included modules.

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.

5.1. Module identification

When generating the comparison output of two compiled schemas, unique identification of the modules in question in included. 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.

5.2. 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 unchanged 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.

5.3. 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.

6. Comparison tooling

The `yanglint` tool from the libyang library (in separate Git branch schema_comparison) is able to generate the `ietf-yang-schema-comparison` YANG data of two revisions of a YANG module.

7. Schema Comparison YANG Module

YANG module with nodes describing the differences between two revisions of YANG schemas. The "ietf-yang-schema-comparison" YANG module imports definitions from the "ietf-yang-types" module defined in [RFC6991] and "ietf-yang-library" module defined in [RFC8525], and "ietf-yang-structure-ext" module defined in [RFC8791].

<CODE BEGINS> file "ietf-yang-schema-comparison@2025-10-20.yang"

module ietf-yang-schema-comparison {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-yang-schema-comparison";
  prefix schema-cmp;

  import ietf-yang-types {
    prefix yang;
    reference
      "RFC 6991: Common YANG Data types";
  }
  import ietf-yang-library {
    prefix yanglib;
    reference
      "RFC 8525: YANG Library";
  }
  import ietf-yang-structure-ext {
    prefix sx;
    reference
      "RFC 8791: YANG Data Structure Extensions";
  }

  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 schemas. The
     output of the comparison algorithm is described in this YANG module.
     The base schema nodes describe changes in a compiled YANG module
     with all of its included submodules and imported modules. The rest
     of the schema nodes are optional and describe changes in all the
     statements of a single 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)
     (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 2025-10-20 {
    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";
  }

  feature parsed-schema {
    description
      "Compare also the statements removed from the compiled schema to get all
       the backwards-compatible and non-backwards-compatible changes possible
       for a YANG module.";
    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 schema-path {
    type string;
    description
      "Node schema path (node-id). A generic type is used to not enforce their
       evaluation or validation in this context.";
  }

  typedef stmt-type {
    type enumeration {
      enum "base" {
        description
          "YANG statement 'base'.";
      }
      enum "bit" {
        description
          "YANG statement 'bit'.";
      }
      enum "config" {
        description
          "YANG statement 'config'.";
      }
      enum "contact" {
        description
          "YANG statement 'contact'.";
      }
      enum "default" {
        description
          "YANG statement 'default'.";
      }
      enum "description" {
        description
          "YANG statement 'description'.";
      }
      enum "enum" {
        description
          "YANG statement 'enum'.";
      }
      enum "error-app-tag" {
        description
          "YANG statement 'error-app-tag'.";
      }
      enum "error-message" {
        description
          "YANG statement 'error-message'.";
      }
      enum "extension-instance" {
        description
          "Any extension instance YANG statement.";
      }
      enum "fraction-digits" {
        description
          "YANG statement 'fraction-digits'.";
      }
      enum "identity" {
        description
          "YANG statement 'identity'.";
      }
      enum "length" {
        description
          "YANG statement 'length'.";
      }
      enum "mandatory" {
        description
          "YANG statement 'mandatory'.";
      }
      enum "max-elements" {
        description
          "YANG statement 'max-elements'.";
      }
      enum "min-elements" {
        description
          "YANG statement 'min-elements'.";
      }
      enum "must" {
        description
          "YANG statement 'must'.";
      }
      enum "node" {
        description
          "YANG statement 'container', 'leaf', 'leaf-list', 'list', 'anydata',
           'anyxml', 'rpc', 'action', or 'notification'. If the parsed schema
           is also compared, it can additionally be 'choice', 'case', 'uses' or
           'grouping'.";
      }
      enum "ordered-by" {
        description
          "YANG statement 'ordered-by'.";
      }
      enum "organization" {
        description
          "YANG statement 'organization'.";
      }
      enum "path" {
        description
          "YANG statement 'path'.";
      }
      enum "pattern" {
        description
          "YANG statement 'pattern'.";
      }
      enum "presence" {
        description
          "YANG statement 'presence'.";
      }
      enum "range" {
        description
          "YANG statement 'range'.";
      }
      enum "reference" {
        description
          "YANG statement 'reference'.";
      }
      enum "require-instance" {
        description
          "YANG statement 'require-instance'.";
      }
      enum "status" {
        description
          "YANG statement 'status'.";
      }
      enum "type" {
        description
          "YANG statement 'type'.";
      }
      enum "units" {
        description
          "YANG statement 'units'.";
      }
      enum "unique" {
        description
          "YANG statement 'unique'.";
      }
      enum "when" {
        description
          "YANG statement 'when'.";
      }
    }
    description
      "Type of the statement that a change affects.";
  }

  typedef revision-or-empty {
    type union {
      type empty;
      type yanglib:revision-identifier;
    }
    description
      "Module or submodule revision. If it has none, an empty value is
       used.";
  }

  grouping if-feature-stmts {
    description
      "Describes changes of all the if-features on a statement.";
    leaf-list if-feature {
      type string;
      description
        "Value of an if-feature statement.";
    }
  }

  grouping typedef-stmts {
    description
      "Describes changes of all the typedefs on a specific level and parent.";
    list typedef {
      key "name";
      description
        "List of typedefs.";
      leaf name {
        type yang:yang-identifier;
        description
          "Name of the typedef.";
      }
      leaf default {
        type string;
        description
          "List of default substatement values.";
      }
      leaf description {
        type string;
        description
          "Description substatement value.";
      }
      leaf reference {
        type string;
        description
          "Reference substatement value.";
      }
      uses status-stmt;
      container type {
        description
          "Type substatement.";
        uses type-substmts;
        list union-type {
          description
            "List of a union type substatements.";
          uses type-substmts;
        }
      }
      leaf units {
        type string;
        description
          "Units substatement value.";
      }
    }
  }

  grouping augment-stmts {
    description
      "Describes changes of all the augment statements on a specific level and
       parent.";
    list augment {
      if-feature parsed-schema;
      description
        "List of augment substatements.";
      leaf target {
        type string;
        mandatory true;
        description
          "Augment target argument.";
      }
      uses node-info;
    }
  }

  grouping ext-instance-stmts {
    description
      "Describes changes of all the extension instances.";
    list ext-instance {
      description
        "List of extension-instance substatements.";
      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-stmt {
    description
      "Describes a status statement change.";
    leaf status {
      type enumeration {
        enum "current" {
          description
            "Current effective status of a statement.";
        }
        enum "deprecated" {
          description
            "Deprecated effective status of a statement.";
        }
        enum "obsolete" {
          description
            "Obsolete effective status of a statement.";
        }
      }
      mandatory true;
      description
        "Status substatement value.";
    }
  }

  grouping must-stmts {
    description
      "Describes changes of all the must statements of a specific parent
       statement.";
    list must {
      description
        "List of must substatements.";
      leaf condition {
        type string;
        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.";
      }
      uses ext-instance-stmts;
    }
  }

  grouping when-stmts {
    description
      "Describes changes of all the must statements of a specific parent
       statement.";
    list when {
      description
        "List of when substatements.";
      leaf condition {
        type string;
        description
          "Condition substatement value.";
      }
      leaf description {
        type string;
        description
          "Description substatement value.";
      }
      leaf reference {
        type string;
        description
          "Reference substatement value.";
      }
      uses status-stmt;
      uses ext-instance-stmts;
    }
  }

  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.";
    }
    uses ext-instance-stmts;
  }

  grouping type-substmts {
    description
      "Describes a type statement change.";
    leaf base-type {
      type enumeration {
        enum "int8" {
          description
            "YANG built-in type 'int8'.";
        }
        enum "int16" {
          description
            "YANG built-in type 'int16'.";
        }
        enum "int32" {
          description
            "YANG built-in type 'int32'.";
        }
        enum "int64" {
          description
            "YANG built-in type 'int64'.";
        }
        enum "uint8" {
          description
            "YANG built-in type 'uint8'.";
        }
        enum "uint16" {
          description
            "YANG built-in type 'uint16'.";
        }
        enum "uint32" {
          description
            "YANG built-in type 'uint32'.";
        }
        enum "uint64" {
          description
            "YANG built-in type 'uint64'.";
        }
        enum "decimal64" {
          description
            "YANG built-in type 'decimal64'.";
        }
        enum "string" {
          description
            "YANG built-in type 'string'.";
        }
        enum "boolean" {
          description
            "YANG built-in type 'boolean'.";
        }
        enum "enumeration" {
          description
            "YANG built-in type 'enumeration'.";
        }
        enum "bits" {
          description
            "YANG built-in type 'bits'.";
        }
        enum "binary" {
          description
            "YANG built-in type 'binary'.";
        }
        enum "leafref" {
          description
            "YANG built-in type 'leafref'.";
        }
        enum "identityref" {
          description
            "YANG built-in type 'identityref'.";
        }
        enum "empty" {
          description
            "YANG built-in type 'empty'.";
        }
        enum "union" {
          description
            "YANG built-in type 'union'.";
        }
        enum "instance-identifier" {
          description
            "YANG built-in type '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;
          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 {
      description
        "List of pattern statements.";
      leaf expression {
        type string;
        description
          "Pattern substatement value.";
      }
      leaf inverted {
        type boolean;
        description
          "Inverted substatement value.";
      }
      uses restriction-substmts;
    }
    list enum {
      key "name";
      ordered-by user;
      description
        "List of enum statements. Keeps the same order of the statements as in
         the module.";
      leaf name {
        type yang:yang-identifier;
        description
          "Enum statement name.";
      }
      uses if-feature-stmts {
        if-feature parsed-schema;
      }
      leaf description {
        type string;
        description
          "Description substatement value.";
      }
      leaf reference {
        type string;
        description
          "Reference substatement value.";
      }
      leaf value {
        type int32;
        description
          "Value substatement value.";
      }
      uses status-stmt;
      uses ext-instance-stmts;
    }
    list bit {
      key "name";
      ordered-by user;
      description
        "List of bit statements. Keeps the same order of the statements as in
         the module.";
      leaf name {
        type yang:yang-identifier;
        description
          "Bit statement name.";
      }
      uses if-feature-stmts {
        if-feature parsed-schema;
      }
      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-stmt;
      uses ext-instance-stmts;
    }
    leaf path {
      type string;
      description
        "Path substatement value.";
    }
    leaf require-instance {
      type boolean;
      description
        "Require-instance substatement value.";
    }
    leaf-list base {
      type yang:yang-identifier;
      description
        "List of base substatement values.";
    }
    uses ext-instance-stmts;
  }

  grouping refine-deviate-node-substmts {
    description
      "All substatements of a data-definition node that can be changed by both
       a refine and deviate statement.";
    uses must-stmts;
    leaf-list default {
      type string;
      description
        "List of default substatement values.";
    }
    leaf config {
      type boolean;
      description
        "Config substatement value.";
    }
    leaf mandatory {
      type boolean;
      description
        "Mandatory substatement value.";
    }
    leaf min-elements {
      type uint32;
      description
        "Min-elements substatement value.";
    }
    leaf max-elements {
      type uint32;
      description
        "Max-elements substatement value.";
    }
  }

  grouping refine-node-substmts {
    description
      "All substatements of a data-definition node that can be changed by a
       refine statement.";
    uses if-feature-stmts {
      if-feature parsed-schema;
    }
    leaf presence {
      type boolean;
      description
        "Presence substatement existence.";
    }
    leaf description {
      type string;
      description
        "Description substatement value.";
    }
    leaf reference {
      type string;
      description
        "Reference substatement value.";
    }
    uses refine-deviate-node-substmts;
    uses ext-instance-stmts;
  }

  grouping deviate-only-node-substmts {
    description
      "All substatements of a data-definition node that can be changed only by
       a deviate statement.";
    container type {
      description
        "Type substatement.";
      uses type-substmts;
      list union-type {
        description
          "List of a union type substatements.";
        uses type-substmts;
      }
    }
    leaf units {
      type string;
      description
        "Units 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.";
      }
    }
  }

  grouping node-substmts {
    description
      "All substatements of a data-definition node.";
    uses when-stmts;
    leaf-list key {
      type yang:yang-identifier;
      description
        "Leaf-list of nodes referenced by a key statement.";
    }
    leaf ordered-by {
      type enumeration {
        enum "system" {
          description
            "System-ordered list or leaf-list of items.";
        }
        enum "user" {
          description
            "User-ordered list or leaf-list of items.";
        }
      }
      description
        "Ordered-by substatement value.";
    }
    uses status-stmt;
    uses refine-node-substmts;
    uses deviate-only-node-substmts;
    uses typedef-stmts {
      if-feature parsed-schema;
    }
    anydata groupings {
      if-feature parsed-schema;
      description
        "Describes changes of all the augment statement on a specific level and
         parent. Follows the exact same YANG schema node structure as the
         grouping 'node-info'. There is no 'uses' statement to break a cyclic
         uses-grouping.";
    }
    anydata augments {
      if-feature parsed-schema;
      description
        "Describes changes of all the augment statement on a specific level and
         parent. Follows the exact same YANG schema node structure as the
         grouping 'augment-stmts'. There is no 'uses' statement to break a
         cyclic uses-grouping.";
    }
    list refine {
      if-feature parsed-schema;
      description
        "List of refine substatements.";
      leaf target {
        type string;
        mandatory true;
        description
          "Refine target path.";
      }
      uses refine-node-substmts;
    }
  }

  grouping module-substmts {
    description
      "All compiled non-data-definition substatements of a module.";
    leaf prefix {
      if-feature parsed-schema;
      type string;
      description
        "Prefix substatement value.";
    }
    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 extension {
      if-feature parsed-schema;
      key "name";
      description
        "List of extension substatements.";
      leaf name {
        type yang:yang-identifier;
        description
          "Extension name.";
      }
      leaf argument {
        type yang:yang-identifier;
        description
          "Argument substatement value.";
      }
      uses status-stmt;
      leaf description {
        type string;
        description
          "Description substatement value.";
      }
      leaf reference {
        type string;
        description
          "Reference substatement value.";
      }
    }
    list feature {
      if-feature parsed-schema;
      key "name";
      description
        "List of feature substatements.";
      leaf name {
        type yang:yang-identifier;
        description
          "Feature name.";
      }
      uses if-feature-stmts;
      uses status-stmt;
      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.";
      }
      uses if-feature-stmts {
        if-feature parsed-schema;
      }
      leaf-list base {
        if-feature parsed-schema;
        type yang:yang-identifier;
        description
          "List of bases of an identity.";
      }
      uses ext-instance-stmts;
    }
    uses augment-stmts;
    list deviation {
      if-feature parsed-schema;
      description
        "List of deviation substatements.";
      leaf target {
        type string;
        mandatory true;
        description
          "Deviation target argument.";
      }
      list deviate {
        min-elements 1;
        description
          "List of deviate substatements.";
        leaf argument {
          type enumeration {
            enum not-supported {
              description
                "Deviate 'not-supported' argument value.";
            }
            enum add {
              description
                "Deviate 'add' argument value.";
            }
            enum replace {
              description
                "Deviate 'replace' argument value.";
            }
            enum delete {
              description
                "Deviate 'delete' argument value.";
            }
          }
          mandatory true;
          description
            "Deviate argument value.";
        }
        uses refine-deviate-node-substmts;
        uses deviate-only-node-substmts;
      }
      leaf description {
        type string;
        description
          "Description substatement value.";
      }
      leaf reference {
        type string;
        description
          "Reference substatement value.";
      }
    }
    uses typedef-stmts {
      if-feature parsed-schema;
    }
    uses ext-instance-stmts;
  }

  grouping conformance-type {
    description
      "Conformance type of a change.";
    leaf conformance {
      type enumeration {
        enum "backwards-compatible" {
          description
            "Backwards-compatible or editorial change.";
        }
        enum "non-backwards-compatible" {
          description
            "Non-backwards-compatible change.";
        }
      }
      mandatory true;
      description
        "Conformance information of a change in the compared YANG modules.";
    }
  }

  grouping change-info {
    description
      "Details of a single change.";
    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.";
          }
        }
        mandatory true;
        description
          "Type of change of the statement.";
      }
      uses conformance-type;
    }
  }

  grouping module-params {
    description
      "Identification details of a processed YANG module.";
    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.";
    }
  }

  grouping node-info {
    description
      "Information about all the changed nodes.";
    list node-comparison {
      key "node";
      ordered-by user;
      description
        "Information about data-definition (node) statement substatement
         changes. Keeps the traversed order of the nodes by the algorithm.
         It SHOULD follow the depth-first search order.";
      leaf node {
        type schema-path;
        description
          "Path to the changed data-definition statement.";
      }
      leaf node-type {
        type enumeration {
          enum "container" {
            description
              "YANG statement 'container' node.";
          }
          enum "leaf" {
            description
              "YANG statement 'leaf' node.";
          }
          enum "leaf-list" {
            description
              "YANG statement 'leaf-list' node.";
          }
          enum "list" {
            description
              "YANG statement 'list' node.";
          }
          enum "anydata" {
            description
              "YANG statement 'anydata' node.";
          }
          enum "anyxml" {
            description
              "YANG statement 'anyxml' node.";
          }
          enum "rpc" {
            description
              "YANG statement 'rpc' node.";
          }
          enum "action" {
            description
              "YANG statement 'action' node.";
          }
          enum "notification" {
            description
              "YANG statement 'notification' node.";
          }
          enum "choice" {
            if-feature parsed-schema;
            description
              "YANG statement 'choice' node.";
          }
          enum "case" {
            if-feature parsed-schema;
            description
              "YANG statement 'case' node.";
          }
          enum "uses" {
            if-feature parsed-schema;
            description
              "YANG statement 'uses' node.";
          }
          enum "grouping" {
            if-feature parsed-schema;
            description
              "YANG statement 'grouping' node.";
          }
        }
        mandatory true;
        description
          "Type of the changed data-definition statement.";
      }
      uses change-info;
      container old {
        description
          "All the node substatements in the older revision of the YANG
           module.";
        uses node-substmts;
      }
      container new {
        description
          "All the node substatements in the newer revision of the YANG
           module.";
        uses node-substmts;
      }
    }
  }

  sx:structure schema-comparison {
    description
      "Schema comparison details.";
    list schema {
      description
        "Instance of a schema comparison of 2 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-comparison {
        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-substmts;
        }
        container new {
          description
            "Changed statements in the newer revision of the YANG module.";
          uses module-substmts;
        }
      }
      uses node-info;
    }
  }
}

<CODE ENDS>

8. 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:

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.

9. Security Considerations

This section follows the template defined in Section 3.7.1 of [RFC8407].

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 specified 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 structure "schema-comparison" contains all the groupings reflecting the changes between YANG modules. If the data of this structure are published in online tooling, care needs to be taken so that knowledge of YANG modules is not leaked.

Since the module does not define any RPCs, actions, or notifications, the security considerations for such statements are not provided here.

10. IANA Considerations

10.1. 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-yang-schema-comparison
Registrant Contact: The IESG.
XML: N/A, the requested URI is an XML namespace.

10.2. 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-yang-schema-comparison
XML Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-schema-comparison
Prefix: schema-cmp
Reference: RFC XXXX

11. References

11.1. 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-15, , <https://datatracker.ietf.org/doc/html/draft-ietf-netmod-yang-module-versioning-15>.
[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, , <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, , <https://datatracker.ietf.org/doc/html/draft-ietf-netmod-yang-semver-24>.
[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, , <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, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC3688]
Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, , <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, , <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, , <https://www.rfc-editor.org/info/rfc6241>.
[RFC6242]
Wasserman, M., "Using the NETCONF Protocol over Secure Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, , <https://www.rfc-editor.org/info/rfc6242>.
[RFC6991]
Schoenwaelder, J., Ed., "Common YANG Data Types", RFC 6991, DOI 10.17487/RFC6991, , <https://www.rfc-editor.org/info/rfc6991>.
[RFC7950]
Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, , <https://www.rfc-editor.org/info/rfc7950>.
[RFC8040]
Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, , <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, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC8341]
Bierman, A. and M. Bjorklund, "Network Configuration Access Control Model", STD 91, RFC 8341, DOI 10.17487/RFC8341, , <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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc8525>.
[RFC8791]
Bierman, A., Björklund, M., and K. Watsen, "YANG Data Structure Extensions", RFC 8791, DOI 10.17487/RFC8791, , <https://www.rfc-editor.org/info/rfc8791>.

11.2. 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, , <https://datatracker.ietf.org/doc/html/draft-clacla-netmod-yang-model-update-06>.

Appendix A. Comparison Examples

This section provides various examples of the output of the comparison algorithm defined in this document to illustrate it on actual YANG modules.

A.1. Compiled Comparison Examples

This section provides examples of comparing only compiled schema trees of YANG modules.

A.1.1. Simple Example Module

An example simple module comparison with a typedef change and a new leaf addition.

<CODE BEGINS> file "mod@2025-01-01.yang"

module mod {
    namespace "urn:mod";
    prefix m;
    yang-version 1.1;

    revision 2025-01-01;

    typedef my-string {
        type string {
            length "1..10";
        }
    }

    container cont {
        leaf l {
            type my-string;
        }
    }
}

<CODE ENDS>
<CODE BEGINS> file "mod@2025-06-01.yang"

module mod {
    namespace "urn:mod";
    prefix m;
    yang-version 1.1;

    revision 2025-06-01;
    revision 2025-01-01;

    typedef my-string {
        type string {
            length "1..20";
        }
    }

    container cont {
        leaf l {
            type my-string;
        }

        leaf l2 {
            type int32;
        }
    }
}

<CODE ENDS>
<CODE BEGINS> file "mod_comparison.json"

{
  "ietf-yang-schema-comparison:schema-comparison": {
    "schema": [
      {
        "source": {
          "module": "mod",
          "revision": "2025-01-01"
        },
        "target": {
          "module": "mod",
          "revision": "2025-06-01"
        },
        "conformance": "backwards-compatible",
        "node-comparison": [
          {
            "node": "/mod:cont/l",
            "node-type": "leaf",
            "changed": [
              {
                "stmt": "length",
                "change": "modified",
                "conformance": "backwards-compatible"
              }
            ],
            "old": {
              "status": "current",
              "config": true,
              "mandatory": false,
              "type": {
                "base-type": "string",
                "length": {
                  "interval": [
                    {
                      "min": "1",
                      "max": "10"
                    }
                  ]
                }
              }
            },
            "new": {
              "status": "current",
              "config": true,
              "mandatory": false,
              "type": {
                "base-type": "string",
                "length": {
                  "interval": [
                    {
                      "min": "1",
                      "max": "20"
                    }
                  ]
                }
              }
            }
          },
          {
            "node": "/mod:cont/l",
            "node-type": "leaf",
            "changed": [
              {
                "stmt": "node",
                "change": "added",
                "conformance": "backwards-compatible"
              }
            ],
            "new": {
              "status": "current",
              "config": true,
              "mandatory": false,
              "type": {
                "base-type": "string",
                "length": {
                  "interval": [
                    {
                      "min": "1",
                      "max": "20"
                    }
                  ]
                }
              }
            }
          }
        ]
      }
    ]
  }
}

<CODE ENDS>

Authors' Addresses

Per Andersson (editor)
Ionio Systems
Robert Wilton
Cisco Systems, Inc.
Michal Vaško
CESNET