Network Working Group B. Claise
Internet-Draft J. Clarke, Ed.
Updates: 8407 (if approved) R. Rahman
Intended status: Standards Track R. Wilton, Ed.
Expires: January 14, 2021 Cisco Systems, Inc.
B. Lengyel
J. Sterne
K. D'Souza
July 13, 2020

YANG Semantic Versioning


This document specifies a scheme and guidelines for applying a modified set of semantic versioning rules to revisions of YANG modules. Additionally, this document defines a revision-label for this modified semver scheme.

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

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 January 14, 2021.

Copyright Notice

Copyright (c) 2020 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 ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

1. Introduction

[I-D.ietf-netmod-yang-module-versioning] puts forth a number of concepts relating to modified rules for updating modules, a means to signal when a new revision of a module has non-backwards-compatible (NBC) changes compared to its previous revision, and a versioning scheme that uses the revision history as a lineage for determining from where a specific revision of a YANG module is derived. Additionally, section 3.3 of [I-D.ietf-netmod-yang-module-versioning] defines a revision label which can be used as an overlay or alias to provide additional context or an additional way to refer to a specific revision.

This document defines a revision-label scheme that uses modified [semver] rules for YANG artifacts (i.e., YANG modules and YANG packages [I-D.ietf-netmod-yang-packages]) as well as the revision label definition for using this scheme. The goal of this is to add a human readable version label that provides compatibility information for the YANG artifact without one needing to compare or parse its body. The label and rules defined herein represent the RECOMMENDED revision label scheme for IETF YANG artifacts.

2. 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.

Additionally, this document uses the following terminology:

3. YANG Semantic Versioning

This section defines YANG Semantic Versioning, explains how it is used with YANG artifacts, and the rules associated with changing an artifact's semantic version number when its contents are updated.

3.1. YANG Semantic Versioning Pattern

YANG artifacts that employ semantic versioning as defined in this document MUST use a version string (e.g., in revision-label or as a package version) that corresponds to the following pattern: X.Y.Z_COMPAT. Where:

Additionally, [semver] defines two specific types of metadata that may be appended to a semantic version string. Pre-release metadata MAY be appended to a semver string after a trailing '-' character. Build metadata MAY be appended after a trailing '+' character. If both pre-release and build metadata are present, then build metadata MUST follow pre-release metadata. While build metadata MUST be ignored by YANG semver parsers, pre-release metadata MUST be used during module development and MUST be considered base on Section 5. Both pre-release and build metadata are allowed in order to support all of the [semver] rules. Thus, a version lineage that follows strict [semver] rules is allowed for a YANG artifact.

To signal the use of this versioning scheme, modules MUST set the revision-label-scheme extension as defined in [I-D.ietf-netmod-yang-module-versioning] to the identity "yang-semver". That identity value is defined in the ietf-yang-semver module below.

Additionally, this ietf-yang-semver module defines a typedef that formally specifies the syntax of the YANG semver version string.

3.2. Semantic Versioning Scheme for YANG Artifacts

This document defines the YANG semantic versioning scheme that is used for YANG artifacts that employ the YANG semver label. The versioning scheme has the following properties:

Every YANG module versioned using the YANG semantic versioning scheme specifies the module's semantic version number as the argument to the 'rev:revision-label' statement.

Because the rules put forth in [I-D.ietf-netmod-yang-module-versioning] are designed to work well with existing versions of YANG and allow for artifact authors to migrate to this scheme, it is not expected that all revisions of a given YANG artifact will have a semantic version label. For example, the first revision of a module may have been produced before this scheme was available.

YANG packages that make use of this semantic versioning scheme will have their semantic version as the value of the "revision_label" property.

As stated above, the YANG semver version number is expressed as a string of the form: 'X.Y.Z_COMPAT'; where X, Y, and Z each represent non-negative integers smaller than 2147483647 without leading zeroes, and _COMPAT represents an optional suffix of either "_compatible" or "_non_compatible".

The YANG artifact name and YANG semantic version number uniquely identify a revision of said artifact. There MUST NOT be multiple instances of a YANG artifact definition with the same name and YANG semantic version number but different content (and in the case of modules, different revision dates).

There MUST NOT be multiple versions of a YANG artifact that have the same MAJOR, MINOR and PATCH version numbers, but different patch modifier strings. E.g., artifact version "1.2.3_non_compatible" MUST NOT be defined if artifact version "1.2.3" has already been defined.

3.2.1. Examples for YANG semantic version numbers

The following diagram and explanation illustrates how YANG semantic version numbers work.

Example YANG semantic version numbers for an example artifact:

          |  \
          |   1.1.0 -> 1.1.1_compatible -> 1.1.2_non_compatible
          |    |
          |   1.2.0 -> 1.2.1_non_compatible -> 1.2.2_non_compatible
          |    |
          |   1.3.0 -> 1.3.1

Assume the tree diagram above illustrates how an example YANG module's version history might evolve. For example, the tree might represent the following changes, listed in chronological order from oldest revision to newest:

The partial ordering relationships based on the semantic versioning numbers can be defined as follows:

There is no ordering relationship between 1.1.1_non_compatible and either 1.2.0 or 1.2.1_non_compatible, except that they share the common ancestor of 1.1.0.

Looking at the version number alone, the module definition in 2.0.0 does not necessarily contain the contents of 1.3.0. However, the module revision history in 2.0.0 may well indicate that it was edited from module version 1.3.0.

3.3. YANG Semantic Version Update Rules

When a new revision of an artifact is produced, then the following rules define how the YANG semantic version number for the new artifact revision is calculated, based on the changes between the two artifact revisions, and the YANG semantic version number of the base artifact revision from which the changes are derived:

  1. If an artifact is being updated in a non-backwards-compatible way, then the artifact version "X.Y.Z[_compatible|_non_compatible]" MUST be updated to "X+1.0.0" unless that artifact version has already been defined with different content, in which case the artifact version "X.Y.Z+1_non_compatible" MUST be used instead.
  2. Under some circumstances (e.g., to avoid adding a "_compatible" modifier) an artifact author MAY also update the MAJOR version when the only changes are backwards-compatible. This is where tooling is important to highlight all changes. Because, while avoiding the "_compatible" and "_non_compatible" modifiers have a clear advantage, bumping a MAJOR version when changes are entirely backwards-compatible may confuse end users.
  3. If an artifact is being updated in a backwards-compatible way, then the next version number depends on the format of the current version number:
    "X.Y.Z" - the artifact version MUST be updated to "X.Y+1.0", unless that artifact version has already been defined with different content, when the artifact version MUST be updated to "X.Y.Z+1_compatible"" instead.
    "X.Y.Z_compatible" - the artifact version MUST be updated to "X.Y.Z+1_compatible".
    "X.Y.Z_non_compatible" - the artifact version MUST be updated to "X.Y.Z+1_non_compatible".

  4. If an artifact is being updated in an editorial way, then the next version number depends on the format of the current version number:
    "X.Y.Z" - the artifact version MUST be updated to "X.Y.Z+1"
    "X.Y.Z_compatible" - the artifact version MUST be updated to "X.Y.Z+1_compatible".
    "X.Y.Z_non_compatible" - the artifact version MUST be updated to "X.Y.Z+1_non_compatible".

  5. YANG artifact semantic version numbers beginning with 0, i.e "0.X.Y" are regarded as beta definitions and need not follow the rules above. Either the MINOR or PATCH version numbers may be updated, regardless of whether the changes are non-backwards-compatible, backwards-compatible, or editorial. See Section 5 for more details on using this notation during module development.

3.4. Examples of the YANG Semver Label

3.4.1. Example Module Using YANG Semver

Below is a sample YANG module that uses the YANG semver revision label based on the rules defined in this document.

    module example-versioned-module {
      yang-version 1.1;
      namespace "urn:example:versioned:module";
      prefix "exvermod";
      rev:revision-label-scheme "yangver:yang-semver";

      import ietf-yang-revisions { prefix "rev"; }
      import ietf-yang-semver { prefix "yangver"; }

        "to be completed";

      revision 2018-02-28 {
        description "Added leaf 'wobble'";
        rev:revision-label "3.1.0";

      revision 2017-12-31 {
        description "Rename 'baz' to 'bar', added leaf 'wibble'";
        rev:revision-label "3.0.0";

      revision 2017-10-30 {
        description "Change the module structure";
        rev:revision-label "2.0.0";

      revision 2017-08-30 {
        description "Clarified description of 'foo-64' leaf";
        rev:revision-label "1.3.1";

      revision 2017-07-30 {
        description "Added leaf foo-64";
        rev:revision-label "1.3.0";

      revision 2017-04-20 {
        description "Add new functionality, leaf 'baz'";
        rev:revision-label "1.2.0";

      revision 2017-04-03 {
        description "Add new functionality, leaf 'foo'";
        rev:revision-label "1.1.0";

      revision 2017-04-03 {
        description "First release version.";
        rev:revision-label "1.0.0";

      // Note: semver rules do not apply to 0.X.Y labels.

      revision 2017-01-30 {
        description "NBC changes to initial revision";
        semver:module-version "0.2.0";

      revision 2017-01-26 {
        description "Initial module version";
        semver:module-version "0.1.0";

      //YANG module definition starts here

3.4.2. Example of Package Using YANG Semver

Below is an example YANG package that uses the semver revision label based on the rules defined in this document.

     "ietf-yang-instance-data:instance-data-set": {
       "name": "example-yang-pkg",
       "target-ptr": "TBD",
       "timestamp": "2018-09-06T17:00:00Z",
       "description": "Example IETF package definition",
       "content-data": {
         "ietf-yang-package:yang-package": {
           "name": "example-yang-pkg",
           "version": "1.3.1",

4. Import Module by Semantic Version

[I-D.ietf-netmod-yang-module-versioning] allows for imports to be done based on a module or a derived revision of a module. The rev:revision-or-derived statement can specify either a revision date or a revision label. When importing by semver, the YANG semver revision label value MAY be used as an argument to rev:revision-or-derived. In so, any module which has that semver label as its latest revision label or has that label in its revision history can be used to satisfy the import requirement. For example:

        import example-module {
          rev:revision-or-derived "3.0.0";

Note: the import lookup does not stop when a non-backward-compatible change is encountered. That is, if module B imports a module A at or derived from version 2.0.0, resolving that import will pass through a revision of module A with version 2.1.0_non_compatible in order to determine if the present instance of module A derives from 2.0.0.

5. Guidelines for Using Semver During Module Development

This section and the IETF-specific sub-section below provides YANG semver-specific guidelines to consider when developing new YANG modules. As such this section updates [RFC8407].

Development of a brand new YANG module outside of the IETF that uses YANG semver as its revision-label scheme SHOULD begin with a 0 for the MAJOR version component. This allows the module to disregard strict semver rules with respect to non-backwards-compatible changes during its initial development. However, module developers MAY choose to use the semver pre-release syntax instead with a 1 for the MAJOR version component. For example, an initial module revision-label might be either 0.0.1 or 1.0.0-alpha.1. If the authors choose to use the 0 MAJOR version component scheme, they MAY switch to the pre-release scheme with a MAJOR version component of 1 when the module is nearing initial release (e.g., a module's revision label may transition from 0.3.0 to 1.0.0-beta.1 to indicate it is more mature and ready for testing).

When using pre-release notation, the format MUST include at least one alphabetic component and MUST end with a '.' and then one or more digits. These alphanumeric components will be used when deciding pre-release precedence. The following are examples of valid pre-release versions

When developing a new revision of an existing module using the YANG semver revision-label scheme, the intended target semver version MUST be used along with pre-release notation. For example, if a released module which has a current revision-label of 1.0.0 is being modified with the intent to make non-backwards-compatible changes, the first development MAJOR version component must be 2 with some pre-release notation such as -alpha.1, making the version 2.0.0-alpha.1. That said, every publicly available release of a module MUST have a unique YANG semver revision-label (where a publicly available release is one that could be implemented by a vendor or consumed by an end user). Therefore, it may be prudent to include the year or year and month development began (e.g., 2.0.0-201907-alpha.1). As a module undergoes development, it is possible that the original intent changes. For example, a 1.0.0 version of a module that was destined to become 2.0.0 after a development cycle may have had a scope change such that the final version has no non-backwards-compatible changes and becomes 1.1.0 instead. This change is acceptable to make during the development phase so long as pre-release notation is present in both versions (e.g., 2.0.0-alpha.3 becomes 1.1.0-alpha.4). However, on the next development cycle (after 1.1.0 is released), if again the new target release is 2.0.0, new pre-release components must be used such that every revision-label for a given module MUST be unique throughout its entire lifecycle (e.g., the first pre-release version might be 2.0.0-202005-alpha.1 if keeping the same year and month notation mentioned above).

5.1. Pre-release Version Precedence

[TODO: Describe precedence considering there could be changes during development and parallel development tracks.]

5.2. YANG Semver in IETF Modules

Net new module development within the IETF SHOULD begin with the 0 MAJOR number scheme as described above. When revising an existing IETF module, the revision-label MUST use the target (i.e., intended) MAJOR and MINOR version components with a 0 patch version component. If the intended ratified release will be non-backward-compatible with the current ratified release, the MINOR version component MUST be 0.

All IETF modules in development MUST use the whole document name as a pre-release version string, including the current document revision. For example, if a module which is currently released at version 1.0.0 is being revised to include non-backwards-compatible changes in draft-user-netmod-foo, its development revision-labels MUST include 2.0.0-draft-user-netmod-foo followed by the document's revision (e.g., 2.0.0-draft-user-netmod-foo-02). This will ensure each pre-release version is unique across the lifecycle of the module. Even when using the 0 MAJOR version for initial module development (where MINOR and PATCH can change), appending the draft name as a pre-release component helps to ensure uniqueness when there are perhaps multiple, parallel efforts creating the same module.

If a module is being revised and the original module never had a revision-label (i.e., you wish to start using YANG semver in future module revisions), choose a semver value that makes the most sense based on the module's history. For example, if a module started out in the pre-NMDA ([RFC8342]) world, and then had NMDA support added without removing any legacy "state" branches -- and you are looking to add additional new features -- a sensible choice for the target YANG semver would be 1.2.0 (since 1.0.0 would have been the initial, pre-NMDA release, and 1.1.0 would have been the NMDA revision).

See Appendix A for a detailed example of IETF pre-release versions.

6. YANG Module

This YANG module contains the typedef for the YANG semantic version.

<CODE BEGINS> file "ietf-yang-semver@2019-09-06.yang"
  module ietf-yang-semver {
    yang-version 1.1;
    namespace "urn:ietf:params:xml:ns:yang:ietf-yang-semver";
    prefix yangver;
    rev:revision-label-scheme "yang-semver";

    import ietf-yang-revisions {
      prefix rev;

      "IETF NETMOD (Network Modeling) Working Group";
      "WG Web:   <>
       WG List:  <>

       Author:   Joe Clarke
      "This module provides type and grouping definitions for YANG

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

       Redistribution and use in source and binary forms, with or
       without modification, is permitted pursuant to, and subject
       to the license terms contained in, the Simplified BSD License
       set forth in Section 4.c of the IETF Trust's Legal Provisions
       Relating to IETF Documents

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

    // RFC Ed.: update the date below with the date of RFC publication
    // and remove this note.
    // RFC Ed.: replace XXXX with actual RFC number and remove this
    // note.

    revision 2020-06-30 {
      rev:revision-label "1.0.0-draft-ietf-netmod-yang-semver-01";
        "Initial revision";
        "RFC XXXX: YANG Semantic Versioning.";

     * Identities

    identity yang-semver {
      base rev:revision-label-scheme-base-identity;
        "The revision-label scheme corresponds to the YANG semver scheme
         which is defined by the pattern in the 'version' typedef below.
         The rules governing this revision-label scheme are defined in the
         reference for this identity.";
        "RFC XXXX: YANG Semantic Versioning.";

     * Typedefs

    typedef version {
      type string {
        pattern '\d+[.]\d+[.]\d+(_(non_)?compatible)?(-[\w\d.]+)?([+][\w\d\.]+)?';
        "Represents a YANG semantic version number.  The rules governing the
         use of this revision label scheme are defined in the reference for
         this typedef.";
        "RFC XXXX: YANG Semantic Versioning.";

7. Contributors

This document grew out of the YANG module versioning design team that started after IETF 101. The design team consists of the following members whom have worked on the YANG versioning project:

The initial revision of this document was refactored and built upon [I-D.clacla-netmod-yang-model-update].

Discussons on the use of Semver for YANG versioning has been held with authors of the OpenConfig YANG models based on their own [openconfigsemver]. We would like thank both Anees Shaikh and Rob Shakir for their input into this problem space.

8. Security Considerations

The document does not define any new protocol or data model. There are no security impacts.

9. IANA Considerations


10. References

10.1. Normative References

[I-D.ietf-netmod-yang-module-versioning] Wilton, R., Rahman, R., Lengyel, B., Clarke, J., Sterne, J., Claise, B. and K. D'Souza, "Updated YANG Module Revision Handling", Internet-Draft draft-ietf-netmod-yang-module-versioning-01, July 2020.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[RFC8407] Bierman, A., "Guidelines for Authors and Reviewers of Documents Containing YANG Data Models", BCP 216, RFC 8407, DOI 10.17487/RFC8407, October 2018.

10.2. Informative References

[I-D.clacla-netmod-yang-model-update] Claise, B., Clarke, J., Lengyel, B. and K. D'Souza, "New YANG Module Update Procedure", Internet-Draft draft-clacla-netmod-yang-model-update-06, July 2018.
[I-D.ietf-netmod-yang-packages] Wilton, R., Rahman, R., Clarke, J., Sterne, J. and W. Bo, "YANG Packages", Internet-Draft draft-ietf-netmod-yang-packages-00, March 2020.
[openconfigsemver] "Semantic Versioning for Openconfig Models"
[RFC8342] Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K. and R. Wilton, "Network Management Datastore Architecture (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018.
[semver] "Semantic Versioning 2.0.0"

Appendix A. Example IETF Module Development

Assume a new YANG module is being developed in the netmod working group in the IETF. Initially, this module is being developed in an individual internet draft, draft-jdoe-netmod-example-module. The following represents the initial version tree (i.e., value of revision-label) of the module as it's being initially developed.

Version lineage for initial module development:


At this point, development stabilizes, and the workgroup adopts the draft. Thus now the draft becomes draft-ietf-netmod-example-module. The initial pre-release lineage continues as follows.

Continued version lineage after adoption:


At this point, the draft is ratified and becomes RFC12345 and the YANG module version number becomes 1.0.0.

A time later, the module needs to be revised to add additional capabilities. Development will be done in a backwards-compatible way. Two new individual drafts are proposed to go about adding the capabilities in different ways: draft-jdoe-netmod-exmod-enhancements and draft-jadoe-netmod-exmod-changes. These are initially developed in parallel with the following versions.

Parallel development for next module revision:

      1.1.0-draft-jdoe-netmod-exmod-enhancements-00 || 1.1.0-draft-jadoe-netmod-exmod-changes-00
        |                                                |
      1.1.0-draft-jdoe-netmod-exmod-enhancements-01 || 1.1.0-draft-jadoe-netmod-exmod-changes-01

At this point, the WG decides to merge some aspects of both and adopt the work in jadoe's draft as draft-ietf-netmod-exmod-changes. A single version lineage continues.


The draft is ratified, and the new module version becomes 1.1.0.

Authors' Addresses

Benoit Claise Cisco Systems, Inc. De Kleetlaan 6a b1 1831 Diegem, Belgium Phone: +32 2 704 5622 EMail:
Joe Clarke (editor) Cisco Systems, Inc. 7200-12 Kit Creek Rd Research Triangle Park, North Carolina United States of America Phone: +1-919-392-2867 EMail:
Reshad Rahman Cisco Systems, Inc. EMail:
Robert Wilton (editor) Cisco Systems, Inc. EMail:
Balazs Lengyel Ericsson Magyar Tudosok Korutja 1117 Budapest, Hungary Phone: +36-70-330-7909 EMail:
Jason Sterne Nokia EMail:
Kevin D'Souza AT&T 200 S. Laurel Ave Middletown, NJ United States of America EMail: