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Updated YANG Module Revision Handling
draft-ietf-netmod-yang-module-versioning-08

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Robert Wilton , Reshad Rahman , Balázs Lengyel , Joe Clarke , Jason Sterne
Last updated 2023-01-12
Replaces draft-verdt-netmod-yang-module-versioning
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draft-ietf-netmod-yang-module-versioning-08
Network Working Group                                     R. Wilton, Ed.
Internet-Draft                                       Cisco Systems, Inc.
Updates: 6020, 7950, 8407, 8525 (if approved)             R. Rahman, Ed.
Intended status: Standards Track                               Graphiant
Expires: 16 July 2023                                    B. Lengyel, Ed.
                                                                Ericsson
                                                               J. Clarke
                                                     Cisco Systems, Inc.
                                                               J. Sterne
                                                                   Nokia
                                                         12 January 2023

                 Updated YANG Module Revision Handling
              draft-ietf-netmod-yang-module-versioning-08

Abstract

   This document specifies a new YANG module update procedure that can
   document when non-backwards-compatible changes have occurred during
   the evolution of a YANG module.  It extends the YANG import statement
   with a minimum revision suggestion to help document inter-module
   dependencies.  It provides guidelines for managing the lifecycle of
   YANG modules and individual schema nodes.  It provides a mechanism,
   via the revision label YANG extension, to specify a revision
   identifier for YANG modules and submodules.  This document updates
   RFC 7950, RFC 6020, RFC 8407 and RFC 8525.

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 16 July 2023.

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Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Updates to YANG RFCs  . . . . . . . . . . . . . . . . . .   4
   2.  Terminology and Conventions . . . . . . . . . . . . . . . . .   5
   3.  Refinements to YANG revision handling . . . . . . . . . . . .   5
     3.1.  Updating a YANG module with a new revision  . . . . . . .   6
       3.1.1.  Backwards-compatible rules  . . . . . . . . . . . . .   7
       3.1.2.  Non-backwards-compatible changes  . . . . . . . . . .   8
     3.2.  non-backwards-compatible extension statement  . . . . . .   8
     3.3.  Removing revisions from the revision history  . . . . . .   8
     3.4.  Revision label  . . . . . . . . . . . . . . . . . . . . .  10
       3.4.1.  File names  . . . . . . . . . . . . . . . . . . . . .  10
       3.4.2.  Revision label scheme extension statement . . . . . .  11
     3.5.  Examples for updating the YANG module revision history  .  11
   4.  Guidance for revision selection on imports  . . . . . . . . .  14
     4.1.  Recommending a minimum revision for module imports  . . .  15
       4.1.1.  Module import examples  . . . . . . . . . . . . . . .  16
   5.  Updates to ietf-yang-library  . . . . . . . . . . . . . . . .  17
     5.1.  Resolving ambiguous module imports  . . . . . . . . . . .  18
     5.2.  YANG library versioning augmentations . . . . . . . . . .  18
       5.2.1.  Advertising revision-label  . . . . . . . . . . . . .  19
       5.2.2.  Reporting how deprecated and obsolete nodes are
               handled . . . . . . . . . . . . . . . . . . . . . . .  19
   6.  Versioning of YANG instance data  . . . . . . . . . . . . . .  19
   7.  Guidelines for using the YANG module update rules . . . . . .  20
     7.1.  Guidelines for YANG module authors  . . . . . . . . . . .  20
       7.1.1.  Making non-backwards-compatible changes to a YANG
               module  . . . . . . . . . . . . . . . . . . . . . . .  21
     7.2.  Versioning Considerations for Clients . . . . . . . . . .  22
   8.  Module Versioning Extension YANG Modules  . . . . . . . . . .  22
   9.  Security considerations . . . . . . . . . . . . . . . . . . .  31
     9.1.  Security considerations for module revisions  . . . . . .  31

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     9.2.  Security considerations for the modules defined in this
           document  . . . . . . . . . . . . . . . . . . . . . . . .  32
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  33
     10.1.  YANG Module Registrations  . . . . . . . . . . . . . . .  33
     10.2.  Guidance for versioning in IANA maintained YANG
            modules  . . . . . . . . . . . . . . . . . . . . . . . .  34
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  35
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  35
     11.2.  Informative References . . . . . . . . . . . . . . . . .  36
   Appendix A.  Examples of changes that are NBC . . . . . . . . . .  38
   Appendix B.  Examples of applying the NBC change guidelines . . .  38
     B.1.  Removing a data node  . . . . . . . . . . . . . . . . . .  39
     B.2.  Changing the type of a leaf node  . . . . . . . . . . . .  39
     B.3.  Reducing the range of a leaf node . . . . . . . . . . . .  40
     B.4.  Changing the key of a list  . . . . . . . . . . . . . . .  40
     B.5.  Renaming a node . . . . . . . . . . . . . . . . . . . . .  41
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  42

1.  Introduction

   The current YANG [RFC7950] module update rules require that updates
   of YANG modules preserve strict backwards compatibility.  This has
   caused problems as described in
   [I-D.ietf-netmod-yang-versioning-reqs].  This document recognizes the
   need to sometimes allow YANG modules to evolve with non-backwards-
   compatible changes, which can cause breakage to clients and importing
   YANG modules.  Accepting that non-backwards-compatible changes do
   sometimes occur, it is important to have mechanisms to report when
   these changes occur, and to manage their effect on clients and the
   broader YANG ecosystem.

   This document defines a flexible versioning solution.  Several other
   documents build on this solution with additional capabilities.
   [I-D.ietf-netmod-yang-schema-comparison] specifies an algorithm that
   can be used to compare two revisions of a YANG schema and provide
   granular information to allow module users to determine if they are
   impacted by changes between the revisions.  The
   [I-D.ietf-netmod-yang-semver] document extends the module versioning
   work by introducing a revision label scheme based on semantic
   versioning.  YANG packages [I-D.ietf-netmod-yang-packages] provides a
   mechanism to group sets of related YANG modules together in order to
   manage schema and conformance of YANG modules as a cohesive set
   instead of individually.  Finally,
   [I-D.ietf-netmod-yang-ver-selection] provides a schema selection
   mechanism that allows a client to choose which schemas to use when
   interacting with a server from the available schema that are
   supported and advertised by the server.  These other documents are

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   mentioned here as informative references.  Support of the other
   documents is not required in an implementation in order to take
   advantage of the mechanisms and functionality offered by this module
   versioning document.

   The document comprises five parts:

   *  Refinements to the YANG 1.1 module revision update procedure,
      supported by new extension statements to indicate when a revision
      contains non-backwards-compatible changes, and an optional
      revision label.

   *  Updated guidance for revision selection on imports and a YANG
      extension statement allowing YANG module imports to document an
      earliest module revision that may satisfy the import dependency.

   *  Updates and augmentations to ietf-yang-library to include the
      revision label in the module and submodule descriptions, to report
      how "deprecated" and "obsolete" nodes are handled by a server, and
      to clarify how module imports are resolved when multiple revisions
      could otherwise be chosen.

   *  Considerations of how versioning applies to YANG instance data.

   *  Guidelines for how the YANG module update rules defined in this
      document should be used, along with examples.

   Note to RFC Editor (To be removed by RFC Editor)

   Open issues are tracked at https://github.com/netmod-wg/yang-ver-dt/
   issues.

1.1.  Updates to YANG RFCs

   This document updates [RFC7950] section 11 and [RFC6020] section 10.
   Section 3 describes modifications to YANG revision handling and
   update rules, and Section 4.1 describes a YANG extension statement to
   describe potential YANG import revision dependencies.

   This document updates [RFC7950] section 5.2, [RFC6020] section 5.2
   and [RFC8407] section 3.2.  Section 3.4.1 describes the use of a
   revision label in the name of a file containing a YANG module or
   submodule.

   This document updates [RFC7950] section 5.6.5 and [RFC8525].
   Section 5.1 defines how a client of a YANG library datastore schema
   resolves ambiguous imports for modules which are not "import-only".

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   This document updates [RFC8407] section 4.7.  Section 7 provides
   guidelines on managing the lifecycle of YANG modules that may contain
   non-backwards-compatible changes and a branched revision history.

   This document updates [RFC8525] with augmentations to include
   revision labels in the YANG library data and two boolean leafs to
   indicate whether status deprecated and status obsolete schema nodes
   are implemented by the server.

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.

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

   *  schema node

   In addition, this document uses the following terminology:

   *  YANG module revision: An instance of a YANG module, uniquely
      identified with a revision date, with no implied ordering or
      backwards compatibility between different revisions of the same
      module.

   *  Backwards-compatible (BC) change: A backwards-compatible change
      between two YANG module revisions, as defined in Section 3.1.1

   *  Non-backwards-compatible (NBC) change: A non-backwards-compatible
      change between two YANG module revisions, as defined in
      Section 3.1.2

3.  Refinements to YANG revision handling

   [RFC7950] and [RFC6020] assume, but do not explicitly state, that the
   revision history for a YANG module or submodule is strictly linear,
   i.e., it is prohibited to have two independent revisions of a YANG
   module or submodule that are both directly derived from the same
   parent revision.

   This document clarifies [RFC7950] and [RFC6020] to explicitly allow
   non-linear development of YANG module and submodule revisions, so
   that they MAY have multiple revisions that directly derive from the
   same parent revision.  As per [RFC7950] and [RFC6020], YANG module

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   and submodule revisions continue to be uniquely identified by their
   revision date, and hence all revisions of a given module or submodule
   MUST have unique revision dates.

   For a given YANG module revision, revision B is defined as being
   derived from revision A, if revision A is listed in the revision
   history of revision B.  Although this document allows for a branched
   revision history, a given YANG module revision history does not
   contain all revisions in all possible branches, it only lists those
   from which is was derived, i.e., the module revision's history
   describes a single path of derived revisions back to the root of the
   module's revision history.

   A corollary to the text above is that the ancestry (derived
   relationship) between two module or submodule revisions cannot be
   determined by comparing the module or submodule revision date or
   label alone - the revision history must be consulted.

   A module's name and revision date identifies a specific immutable
   definition of that module within its revision history.  Hence, if a
   module includes submodules then to ensure that the module's content
   is uniquely defined, the module's "include" statements SHOULD use
   "revision-date" substatements to specify the exact revision date of
   each included submodule.  When a module does not include its
   submodules by revision-date, the revision of submodules used cannot
   be derived from the including module.  Mechanisms such as YANG
   packages [I-D.ietf-netmod-yang-packages], and YANG library [RFC8525],
   MAY be used to specify the exact submodule revisions used when the
   submodule revision date is not constrained by the "include"
   statement.

   [RFC7950] section 11 and [RFC6020] section 10 require that all
   updates to a YANG module are BC to the previous revision of the
   module.  This document introduces a method to indicate that an NBC
   change has occurred between module revisions: this is done by using a
   new "non-backwards-compatible" YANG extension statement in the module
   revision history.

   Two revisions of a module or submodule MAY have identical content
   except for the revision history.  This could occur, for example, if a
   module or submodule has a branched history and identical changes are
   applied in multiple branches.

3.1.  Updating a YANG module with a new revision

   This section updates [RFC7950] section 11 and [RFC6020] section 10 to
   refine the rules for permissible changes when a new YANG module
   revision is created.

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   A new module revision MAY contain NBC changes, e.g., the semantics of
   an existing data-node definition MAY be changed in an NBC manner
   without requiring a new data-node definition with a new identifier.
   A YANG extension, defined in Section 3.2, is used to signal the
   potential for incompatibility to existing module users and readers.

   Note that NBC changes often create problems for clients, thus it is
   recommended to avoid making them.

   As per [RFC7950] and [RFC6020], all published revisions of a module
   are given a new unique revision date.  This applies even for module
   revisions containing (in the module or included submodules) only
   changes to any whitespace, formatting, comments or line endings
   (e.g., DOS vs UNIX).

3.1.1.  Backwards-compatible rules

   A change between two module revisions is defined as being "backwards-
   compatible" if the change conforms to the module update rules
   specified in [RFC7950] section 11 and [RFC6020] section 10, updated
   by the following rules:

   *  A "status" "deprecated" statement MAY be added, or changed from
      "current" to "deprecated", but adding or changing "status" to
      "obsolete" is a non-backwards-compatible change.

   *  YANG schema nodes with a "status" "obsolete" substatement MAY be
      removed from published modules, and the removal is classified as a
      backwards-compatible change.  In some circumstances it may be
      helpful to retain the obsolete definitions since their identifiers
      may still be referenced by other modules and to ensure that their
      identifiers are not reused with a different meaning.

   *  A statement that is defined using the YANG "extension" statement
      MAY be added, removed, or changed, if it does not change the
      semantics of the module.  Extension statement definitions SHOULD
      specify whether adding, removing, or changing statements defined
      by that extension are backwards-compatible or non-backwards-
      compatible.

   *  Any change made to the "revision-date" or "recommended-min"
      substatements of an "import" statement, including adding new
      "revision-date" or "recommended-min" substatements, changing the
      argument of any "revision-date" or "recommended-min"
      substatetements, or removing any "revision-date" or "recommended-
      min" substatements, is classified as backwards-compatible.

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   *  Any changes (including whitespace or formatting changes) that do
      not change the semantic meaning of the module are backwards-
      compatible.

3.1.2.  Non-backwards-compatible changes

   Any changes to YANG modules that are not defined by Section 3.1.1 as
   being backwards-compatible are classified as "non-backwards-
   compatible" changes.

3.2.  non-backwards-compatible extension statement

   The "rev:non-backwards-compatible" extension statement is used to
   indicate YANG module revisions that contain NBC changes.

   If a revision of a YANG module contains changes, relative to the
   preceding revision in the revision history, that do not conform to
   the module update rules defined in Section 3.1.1, then a "rev:non-
   backwards-compatible" extension statement MUST be added as a
   substatement to the "revision" statement.

   Adding, modifying or removing a "rev:non-backwards-compatible"
   extension statement is considered to be a BC change.

3.3.  Removing revisions from the revision history

   Authors may wish to remove revision statements from a module or
   submodule.  Removal of revision information may be desirable for a
   number of reasons including reducing the size of a large revision
   history, or removing a revision that should no longer be used or
   imported.  Removing revision statements is allowed, but can cause
   issues and SHOULD NOT be done without careful analysis of the
   potential impact to users of the module or submodule.  Doing so can
   lead to import breakages when import by recommended-min is used.
   Moreover, truncating history may cause loss of visibility of when
   non-backwards-compatible changes were introduced.

   An author MAY remove a contiguous sequence of entries from the end
   (i.e., oldest entries) of the revision history.  This is acceptable
   even if the first remaining (oldest) revision entry in the revision
   history contains a rev:non-backwards-compatible substatement.

   An author MAY remove a contiguous sequence of entries in the revision
   history as long as the presence or absence of any existing rev:non-
   backwards-compatible substatements on all remaining entries still
   accurately reflect the compatibility relationship to their preceding
   entries remaining in the revision history.

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   The author MUST NOT remove the first (i.e., newest) revision entry in
   the revision history.

   Example revision history:

   revision 2020-11-11 {
     rev:label 4.0.0;
     rev:non-backwards-compatible;
   }

   revision 2020-08-09 {
     rev:label 3.0.0;
     rev:non-backwards-compatible;
   }

   revision 2020-06-07 {
     rev:label 2.1.0;
   }

   revision 2020-02-10 {
     rev:label 2.0.0;
     rev:non-backwards-compatible;
   }

   revision 2019-10-21 {
     rev:label 1.1.3;
   }

   revision 2019-03-04 {
     rev:label 1.1.2;
   }

   revision 2019-01-02 {
     rev:label 1.1.1;
   }

   In the revision history example above, removing the revision history
   entry for 2020-02-10 would also remove the rev:non-backwards-
   compatible annotation and hence the resulting revision history would
   incorrectly indicate that revision 2020-06-07 is backwards-compatible
   with revisions 2019-01-02 through 2019-10-21 when it is not, and so
   this change cannot be made.  Conversely, removing one or more
   revisions out of 2019-03-04, 2019-10-21 and 2020-08-09 from the
   revision history would still retain a consistent revision history,
   and is acceptable, subject to an awareness of the concerns raised in
   the first paragraph of this section.

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3.4.  Revision label

   Each revision entry in a module or submodule MAY have a revision
   label associated with it, providing an alternative alias to identify
   a particular revision of a module or submodule.  The revision label
   could be used to provide an additional versioning identifier
   associated with the revision.

   A revision label scheme is a set of rules describing how a particular
   type of revision label operates for versioning YANG modules and
   submodules.  For example, YANG Semver [I-D.ietf-netmod-yang-semver]
   defines a revision label scheme based on Semver 2.0.0 [semver].
   Other documents may define other YANG revision label schemes.

   Submodules MAY use a revision label scheme.  When they use a revision
   label scheme, submodules MAY use a revision label scheme that is
   different from the one used in the including module.

   The revision label space of submodules is separate from the revision
   label space of the including module.  A change in one submodule MUST
   result in a new revision label of that submodule and the including
   module, but the actual values of the revision labels in the module
   and submodule could be completely different.  A change in one
   submodule does not result in a new revision label in another
   submodule.  A change in a module revision label does not necessarily
   mean a change to the revision label in all included submodules.

   If a revision has an associated revision label, then it may be used
   instead of the revision date in a "rev:recommended-min" extension
   statement argument.

   A specific revision label identifies a specific revision of the
   module.  If two YANG modules contain the same module name and the
   same revision label (and hence also the same revision-date) in their
   latest revision statement, then the file contents of the two modules,
   including the revision history, MUST be identical.

3.4.1.  File names

   This section updates [RFC7950] section 5.2, [RFC6020] section 5.2 and
   [RFC8407] section 3.2

   If a revision has an associated revision label, then it is
   RECOMMENDED that the name of the file for that revision be of the
   form:

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     module-or-submodule-name ['#' revision-label] ( '.yang' / '.yin' )

       E.g., acme-router-module#2.0.3.yang

   YANG module (or submodule) files may be identified using either the
   revision-date (as per [RFC8407] section 3.2) or the revision label.

3.4.2.  Revision label scheme extension statement

   The optional "rev:revision-label-scheme" extension statement is used
   to indicate which revision label scheme a module or submodule uses.
   There MUST NOT be more than one revision label scheme in a module or
   submodule.  The mandatory argument to this extension statement:

   *  specifies the revision label scheme used by the module or
      submodule

   *  is defined in the document which specifies the revision label
      scheme

   *  MUST be an identity derived from "revision-label-scheme-base".

   The revision label scheme used by a module or submodule SHOULD NOT
   change during the lifetime of the module or submodule.  If the
   revision label scheme used by a module or submodule is changed to a
   new scheme, then all revision label statements that do not conform to
   the new scheme MUST be replaced or removed.

3.5.  Examples for updating the YANG module revision history

   The following diagram, explanation, and module history illustrates
   how the branched revision history, "non-backwards-compatible"
   extension statement, and revision "label" extension statement could
   be used:

   Example YANG module with branched revision history.

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          Module revision date        Revision label
            2019-01-01                 <- 1.0.0
                |
            2019-02-01                 <- 2.0.0
                |      \
            2019-03-01  \              <- 3.0.0
                |        \
                |       2019-04-01     <- 2.1.0
                |           |
                |       2019-05-01     <- 2.2.0
                |
            2019-06-01                 <- 3.1.0

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

   Example module, revision 2019-06-01:

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   module example-module {

     namespace "urn:example:module";
     prefix "prefix-name";
     rev:revision-label-scheme "yangver:yang-semver";

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

     description
       "to be completed";

     revision 2019-06-01 {
       rev:label 3.1.0;
       description "Add new functionality.";
     }

     revision 2019-03-01 {
       rev:label 3.0.0;
       rev:non-backwards-compatible;
       description
         "Add new functionality. Remove some deprecated nodes.";
     }

     revision 2019-02-01 {
       rev:label 2.0.0;
       rev:non-backwards-compatible;
       description "Apply bugfix to pattern statement";
     }

     revision 2019-01-01 {
       rev:label 1.0.0;
       description "Initial revision";
     }

     //YANG module definition starts here
   }

   Example module, revision 2019-05-01:

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   module example-module {

     namespace "urn:example:module";
     prefix "prefix-name";
     rev:revision-label-scheme "yangver:yang-semver";

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

     description
       "to be completed";

     revision 2019-05-01 {
       rev:label 2.2.0;
       description "Backwards-compatible bugfix to enhancement.";
     }

     revision 2019-04-01 {
       rev:label 2.1.0;
       description "Apply enhancement to older release train.";
     }

     revision 2019-02-01 {
       rev:label 2.0.0;
       rev:non-backwards-compatible;
       description "Apply bugfix to pattern statement";
     }

     revision 2019-01-01 {
       rev:label 1.0.0;
       description "Initial revision";
     }

     //YANG module definition starts here
   }

4.  Guidance for revision selection on imports

   [RFC7950] and [RFC6020] allow YANG module "import" statements to
   optionally require the imported module to have a specific revision
   date.  In practice, importing a module with an exact revision date
   can be too restrictive because it requires the importing module to be
   updated whenever any change to the imported module occurs, and hence
   section Section 7.1 suggests that authors do not restrict YANG module
   imports to exact revision dates.

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   Instead, for conformance purposes (section 5.6 of [RFC7950]), the
   recommended approach for defining the relationship between specific
   YANG module revisions is to specify the relationships outside of the
   YANG modules, e.g., via YANG library [RFC8525], YANG packages
   [I-D.ietf-netmod-yang-packages], a filesystem directory containing a
   set of consistent YANG module revisions, or a revision control system
   commit label.

4.1.  Recommending a minimum revision for module imports

   Although the previous section indicates that the actual relationship
   constraints between different revisions of YANG modules should be
   specified outside of the modules, in some scenarios YANG modules are
   designed to be loosely coupled, and implementors may wish to select
   sets of YANG module revisions that are expected to work together.
   For these cases it can be helpful for a module author to provide
   guidance on a recommended minimum revision that is expected to
   satisfy an YANG import.  E.g., the module author may know of a
   dependency on a type or grouping that has been introduced in a
   particular imported YANG module revision.  Although there can be no
   guarantee that all derived future revisions from the particular
   imported module will necessarily also be compatible, older revisions
   of the particular imported module are very unlikely to ever be
   compatible.

   This document introduces a new YANG extension statement to provide
   guidance to module implementors on a recommended minimum module
   revision of an imported module that is anticipated to be compatible.
   This statement has been designed to be machine-readable so that tools
   can parse the minimum revision extension statement and generate
   warnings if appropriate, but this extension statement does not alter
   YANG module conformance of valid YANG module versions in any way, and
   specifically it does not alter the behavior of the YANG module import
   statement from that specified in [RFC7950].

   The ietf-revisions module defines the "recommended-min" extension
   statement, a substatement to the YANG "import" statement, to allow
   for a "minimum recommended revision" to be documented:

      The argument to the "recommended-min" extension statement is a
      revision date or a revision label.

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      A particular revision of an imported module adheres to an import's
      "recommended-min" extension statement if the imported module's
      revision history contains a revision statement with a matching
      revision date or revision label.  Removing entries from a module's
      revision history may cause a particular revision to no longer
      satisfy an import's "recommended-min" statement if the revision-
      date or label is no longer present in the module's revision
      history; further described in Section 3.3 and Section 7.1.

      The "recommended-min" extension statement MAY be specified
      multiple times, allowing a set of recommended minimum revisions to
      be documented.  Module implementors are recommended to pick a
      module revision that adheres to any of the "recommended-min"
      statements.

      Adding, modifying or removing a "recommended-min" extension
      statement is a BC change.

4.1.1.  Module import examples

   Consider the example module "example-module" from Section 3.5 that is
   hypothetically available in the following revision/label pairings:
   2019-01-01/1.0.0, 2019-02-01/2.0.0, 2019-03-01/3.0.0,
   2019-04-01/2.1.0, 2019-05-01/2.2.0 and 2019-06-01/3.1.0.  The
   relationship between the revisions is as before:

          Module revision date        Revision label
            2019-01-01                 <- 1.0.0
                |
            2019-02-01                 <- 2.0.0
                |      \
            2019-03-01  \              <- 3.0.0
                |        \
                |       2019-04-01     <- 2.1.0
                |           |
                |       2019-05-01     <- 2.2.0
                |
            2019-06-01                 <- 3.1.0

4.1.1.1.  Example 1

   This example recommends module revisions for import that match, or
   are derived from the revision 2019-02-01.  E.g., this dependency
   might be used if there was a new container added in revision
   2019-02-01 that is augmented by the importing module.  It includes
   revisions/labels: 2019-02-01/2.0.0, 2019-03-01/3.0.0,
   2019-04-01/2.1.0, 2019-05-01/2.2.0 and 2019-06-01/3.1.0.

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   import example-module {
     rev:recommended-min 2019-02-01;
   }

   Alternatively, the first example could have used the revision label
   "2.0.0" instead, which selects the same set of revisions/labels.

   import example-module {
     rev:recommended-min 2.0.0;
   }

4.1.1.2.  Example 2

   This example recommends module revisions for import that are derived
   from 2019-04-01 by using the revision label 2.1.0.  It includes
   revisions/labels: 2019-04-01/2.1.0 and 2019-05-01/2.2.0.  Even though
   2019-06-01/3.1.0 has a higher revision label number than
   2019-04-01/2.1.0 it is not a derived revision, and hence it is not a
   recommended revision for import.

   import example-module {
     rev:recommended-min 2.1.0;
   }

4.1.1.3.  Example 3

   This example recommends module revisions for import that are derived
   from either 2019-04-01 or 2019-06-01.  It includes revisions/labels:
   2019-04-01/2.1.0, 2019-05-01/2.2.0, and 2019-06-01/3.1.0.

   import example-module {
     rev:recommended-min 2019-04-01;
     rev:recommended-min 2019-06-01;
   }

5.  Updates to ietf-yang-library

   This document updates YANG 1.1 [RFC7950] and YANG library [RFC8525]
   to clarify how ambiguous module imports are resolved.  It also
   defines the YANG module, ietf-yang-library-revisions, that augments
   YANG library [RFC8525] with revision labels and two leafs to indicate
   how a server implements deprecated and obsolete schema nodes.

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5.1.  Resolving ambiguous module imports

   A YANG datastore schema, defined in [RFC8525], can specify multiple
   revisions of a YANG module in the schema using the "import-only"
   list, with the requirement from [RFC7950] section 5.6.5 that only a
   single revision of a YANG module may be implemented.

   If a YANG module import statement does not specify a specific
   revision within the datastore schema then it could be ambiguous as to
   which module revision the import statement should resolve to.  Hence,
   a datastore schema constructed by a client using the information
   contained in YANG library may not exactly match the datastore schema
   actually used by the server.

   The following two rules remove the ambiguity:

   If a module import statement could resolve to more than one module
   revision defined in the datastore schema, and one of those revisions
   is implemented (i.e., not an "import-only" module), then the import
   statement MUST resolve to the revision of the module that is defined
   as being implemented by the datastore schema.

   If a module import statement could resolve to more than one module
   revision defined in the datastore schema, and none of those revisions
   are implemented, then the import MUST resolve to the module revision
   with the latest revision date.

5.2.  YANG library versioning augmentations

   The "ietf-yang-library-revisions" YANG module has the following
   structure (using the notation defined in [RFC8340]):

   module: ietf-yang-library-revisions
     augment /yanglib:yang-library/yanglib:module-set/yanglib:module:
       +--ro revision-label?   rev:revision-label
     augment /yanglib:yang-library/yanglib:module-set/yanglib:module
               /yanglib:submodule:
       +--ro revision-label?   rev:revision-label
     augment /yanglib:yang-library/yanglib:module-set
               /yanglib:import-only-module/yanglib:submodule:
       +--ro revision-label?   rev:revision-label
     augment /yanglib:yang-library/yanglib:schema:
       +--ro deprecated-nodes-implemented?   boolean
       +--ro obsolete-nodes-absent?          boolean

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5.2.1.  Advertising revision-label

   The ietf-yang-library-revisions YANG module augments the "module" and
   "submodule" lists in ietf-yang-library with "revision-label" leafs to
   optionally declare the revision label associated with each module and
   submodule.

5.2.2.  Reporting how deprecated and obsolete nodes are handled

   The ietf-yang-library-revisions YANG module augments YANG library
   with two boolean leafs to allow a server to report how it implements
   status "deprecated" and status "obsolete" schema nodes.  The leafs
   are:

   deprecated-nodes-implemented:  If set to "true", this leaf indicates
      that all schema nodes with a status "deprecated" are implemented
      equivalently as if they had status "current"; otherwise deviations
      MUST be used to explicitly remove "deprecated" nodes from the
      schema.  If this leaf is set to "false" or absent, then the
      behavior is unspecified.

   obsolete-nodes-absent:  If set to "true", this leaf indicates that
      the server does not implement any status "obsolete" schema nodes.
      If this leaf is set to "false" or absent, then the behaviour is
      unspecified.

   Servers SHOULD set both the "deprecated-nodes-implemented" and
   "obsolete-nodes-absent" leafs to "true".

   If a server does not set the "deprecated-nodes-implemented" leaf to
   "true", then clients MUST NOT rely solely on the "rev:non-backwards-
   compatible" statements to determine whether two module revisions are
   backwards-compatible, and MUST also consider whether the status of
   any nodes has changed to "deprecated" and whether those nodes are
   implemented by the server.

6.  Versioning of YANG instance data

   Instance data sets [RFC9195] do not directly make use of the updated
   revision handling rules described in this document, as compatibility
   for instance data is undefined.

   However, instance data specifies the content-schema of the data-set.
   This schema SHOULD make use of versioning using revision dates and/or
   revision labels for the individual YANG modules that comprise the
   schema or potentially for the entire schema itself (e.g.,
   [I-D.ietf-netmod-yang-packages]).

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   In this way, the versioning of a content-schema associated with an
   instance data set may help a client to determine whether the instance
   data could also be used in conjunction with other revisions of the
   YANG schema, or other revisions of the modules that define the
   schema.

7.  Guidelines for using the YANG module update rules

   The following text updates section 4.7 of [RFC8407] to revise the
   guidelines for updating YANG modules.

7.1.  Guidelines for YANG module authors

   All IETF YANG modules MUST include revision label statements for all
   newly published YANG modules, and all newly published revisions of
   existing YANG modules.  The revision label MUST take the form of a
   YANG semantic version number [I-D.ietf-netmod-yang-semver].

   NBC changes to YANG modules may cause problems to clients, who are
   consumers of YANG models, and hence YANG module authors SHOULD
   minimize NBC changes and keep changes BC whenever possible.

   When NBC changes are introduced, consideration should be given to the
   impact on clients and YANG module authors SHOULD try to mitigate that
   impact.

   A "rev:non-backwards-compatible" statement MUST be added if there are
   NBC changes relative to the previous revision.

   Removing old revision statements from a module's revision history
   could break import by revision, and hence it is RECOMMENDED to retain
   them.  If all dependencies have been updated to not import specific
   revisions of a module, then the corresponding revision statements can
   be removed from that module.  An alternative solution, if the
   revision section is too long, would be to remove, or curtail, the
   older description statements associated with the previous revisions.

   The "rev:recommended-min" extension MAY be used in YANG module
   imports to indicate revision dependencies between modules in
   preference to the "revision-date" statement, which causes overly
   strict import dependencies and SHOULD NOT be used.

   A module that includes submodules SHOULD use the "revision-date"
   statement to include specific submodule revisions.  The revision of
   the including module MUST be updated when any included submodule has
   changed.

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   In some cases a module or submodule revision that is not strictly NBC
   by the definition in Section 3.1.2 of this specification may include
   the "non-backwards-compatible" statement.  Here is an example when
   adding the statement may be desirable:

   *  A "config false" leaf had its value space expanded (for example, a
      range was increased, or additional enum values were added) and the
      author or server implementor feels there is a significant
      compatibility impact for clients and users of the module or
      submodule

7.1.1.  Making non-backwards-compatible changes to a YANG module

   There are various valid situations where a YANG module has to be
   modified in an NBC way.  Here are some guidelines on how non-
   backwards-compatible changes can be made incrementally, with the
   assumption that deprecated nodes are implemented by the server, and
   obsolete nodes are not:

   1.  The changes should be made gradually, e.g., a data node's status
       SHOULD NOT be changed directly from "current" to "obsolete" (see
       Section 4.7 of [RFC8407]), instead the status SHOULD first be
       marked "deprecated".  At some point in the future, when support
       is removed for the data node, there are two options.  The first,
       and preferred, option is to keep the data node definition in the
       model and change the status to "obsolete".  The second option is
       to simply remove the data node from the model, but this has the
       risk of breaking modules which import the modified module, and
       the removed identifier may be accidently reused in a future
       revision.

   2.  For deprecated data nodes the "description" statement SHOULD also
       indicate until when support for the node is guaranteed (if
       known).  If there is a replacement data node, rpc, action or
       notification for the deprecated node, this SHOULD be stated in
       the "description".  The reason for deprecating the node can also
       be included in the "description" if it is deemed to be of
       potential interest to the user.

   3.  For obsolete data nodes, it is RECOMMENDED to keep the above
       information, from when the node had status "deprecated", which is
       still relevant.

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   4.  When obsoleting or deprecating data nodes, the "deprecated" or
       "obsolete" status SHOULD be applied at the highest possible level
       in the data tree.  For clarity, the "status" statement SHOULD
       also be applied to all descendent data nodes, but the additional
       status related information does not need to be repeated if it
       does not introduce any additional information.

   5.  NBC changes which can break imports SHOULD be avoided because of
       the impact on the importing module.  The importing modules could
       get broken, e.g., if an augmented node in the importing module
       has been removed from the imported module.  Alternatively, the
       schema of the importing modules could undergo an NBC change due
       to the NBC change in the imported module, e.g., if a node in a
       grouping has been removed.  As described in Appendix B.1, instead
       of removing a node, that node SHOULD first be deprecated and then
       obsoleted.

   See Appendix B for examples on how NBC changes can be made.

7.2.  Versioning Considerations for Clients

   Guidelines for clients of modules using the new module revision
   update procedure:

   *  Clients SHOULD be liberal when processing data received from a
      server.  For example, the server may have increased the range of
      an operational node causing the client to receive a value which is
      outside the range of the YANG model revision it was coded against.

   *  Clients SHOULD monitor changes to published YANG modules through
      their revision history, and use appropriate tooling to understand
      the specific changes between module revision.  In particular,
      clients SHOULD NOT migrate to NBC revisions of a module without
      understanding any potential impact of the specific NBC changes.

   *  Clients SHOULD plan to make changes to match published status
      changes.  When a node's status changes from "current" to
      "deprecated", clients SHOULD plan to stop using that node in a
      timely fashion.  When a node's status changes to "obsolete",
      clients MUST stop using that node.

8.  Module Versioning Extension YANG Modules

   YANG module with extension statements for annotating NBC changes,
   revision label, revision label scheme, and importing by revision.

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   <CODE BEGINS> file "ietf-yang-revisions@2022-11-29.yang"
   module ietf-yang-revisions {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-yang-revisions";
     prefix rev;

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

        Author:   Joe Clarke
                 <mailto:jclarke@cisco.com>

        Author:   Reshad Rahman
                 <mailto:reshad@yahoo.com>

        Author:   Robert Wilton
                 <mailto:rwilton@cisco.com>

        Author:   Balazs Lengyel
                 <mailto:balazs.lengyel@ericsson.com>

        Author:   Jason Sterne
                 <mailto:jason.sterne@nokia.com>";
     description
       "This YANG 1.1 module contains definitions and extensions to
        support updated YANG revision handling.

        Copyright (c) 2002 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; 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.";

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     // 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 2022-11-29 {
       rev:label "1.0.0-draft-ietf-netmod-yang-module-versioning-08";
       description
         "Initial version.";
       reference
         "XXXX: Updated YANG Module Revision Handling";
     }

     typedef revision-date {
       type string {
         pattern '[0-9]{4}-(1[0-2]|0[1-9])-(0[1-9]|[1-2][0-9]|3[0-1])';
       }
       description
         "A date associated with a YANG revision.

          Matches dates formatted as YYYY-MM-DD.";
       reference
         "RFC 7950: The YANG 1.1 Data Modeling Language";
     }

     typedef revision-label {
       type string {
         length "1..255";
         pattern '[a-zA-Z0-9,\-_.+]+';
         pattern '[0-9]{4}-[0-9]{2}-[0-9]{2}' {
           modifier "invert-match";
           error-message
             "The revision-label must not match a revision-date.";
         }
       }
       description
         "A label associated with a YANG revision.

          Alphanumeric characters, comma, hyphen, underscore, period
          and plus are the only accepted characters. MUST NOT match
          revision-date or pattern similar to a date.";
       reference
         "XXXX: Updated YANG Module Revision Handling;
          Section 3.3, Revision label";
     }

     typedef revision-date-or-label {
       type union {

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         type revision-date;
         type revision-label;
       }
       description
         "Represents either a YANG revision date or a revision label";
     }

     extension non-backwards-compatible {
       description
         "This statement is used to indicate YANG module revisions that
          contain non-backwards-compatible changes.

          The statement MUST only be a substatement of the 'revision'
          statement.  Zero or one 'non-backwards-compatible' statements
          per parent statement is allowed.  No substatements for this
          extension have been standardized.

          If a revision of a YANG module contains changes, relative to
          the preceding revision in the revision history, that do not
          conform to the backwards-compatible module update rules
          defined in RFC-XXX, then the 'non-backwards-compatible'
          statement MUST be added as a substatement to the revision
          statement.

          Conversely, if a revision does not contain a
          'non-backwards-compatible' statement then all changes,
          relative to the preceding revision in the revision history,
          MUST be backwards-compatible.

          A new module revision that only contains changes that are
          backwards-compatible SHOULD NOT include the
          'non-backwards-compatible' statement.  An example of when an
          author might add the 'non-backwards-compatible' statement is
          if they believe a change could negatively impact clients even
          though the backwards compatibility rules defined in RFC-XXXX
          classify it as a backwards-compatible change.

          Add, removing, or changing a 'non-backwards-compatible'
          statement is a backwards-compatible version change.";
       reference
         "XXXX: Updated YANG Module Revision Handling;
          Section 3.2,
          non-backwards-compatible revision extension statement";
     }

     extension label {
       argument revision-label;
       description

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         "The revision label can be used to provide an additional
          versioning identifier associated with a module or submodule
          revision.  One such scheme that could be used is [XXXX:
          ietf-netmod-yang-semver].

          The format of the revision label argument MUST conform to the
          pattern defined for the revision label typedef in this module.

          The statement MUST only be a substatement of the revision
          statement.  Zero or one revision label statements per parent
          statement are allowed.  No substatements for this extension
          have been standardized.

          Revision labels MUST be unique amongst all revisions of a
          module or submodule.

          Adding a revision label is a backwards-compatible version
          change.  Changing or removing an existing revision label in
          the revision history is a non-backwards-compatible version
          change, because it could impact any references to that
          revision label.";
       reference
         "XXXX: Updated YANG Module Revision Handling;
          Section 3.3, Revision label";
     }

     extension revision-label-scheme {
       argument revision-label-scheme-base;
       description
         "The revision label scheme specifies which revision label
          scheme the module or submodule uses.

          The mandatory revision-label-scheme-base argument MUST be an
          identity derived from revision-label-scheme-base.

          This extension is only valid as a top-level statement, i.e.,
          given as as a substatement to 'module' or 'submodule'.  No
          substatements for this extension have been standardized.

          This extension MUST be used if there is a revision label
          statement in the module or submodule.

          Adding a revision label scheme is a backwards-compatible
          version change.  Changing a revision label scheme is a
          non-backwards-compatible version change, unless the new
          revision label scheme is backwards-compatible with the
          replaced revision label scheme.  Removing a revision label
          scheme is a non-backwards-compatible version change.";

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       reference
         "XXXX: Updated YANG Module Revision Handling;
          Section 3.3.1, Revision label scheme extension statement";
     }

     extension recommended-min {
       argument revision-date-or-label;
       description
         "Recommends the revision of the module that may be imported to
          one that matches or is derived from the specified
          revision-date or revision label.

          The argument value MUST conform to the
          'revision-date-or-label' defined type.

          The statement MUST only be a substatement of the import
          statement.  Zero, one or more 'recommended-min' statements per
          parent statement are allowed.  No substatements for this
          extension have been standardized.

          If specified multiple times, then any module revision that
          satisfies at least one of the 'recommended-min' statements is
          an acceptable recommended revision for import.

          A particular revision of an imported module adheres to an
          import's 'recommended-min' extension statement if the imported
          module's revision history contains a revision statement with a
          matching revision date or revision label.

          Adding, removing or updating a 'recommended-min' statement to
          an import is a backwards-compatible change.";
       reference
         "XXXX: Updated YANG Module Revision Handling; Section 4,
          Recommending a minimum revision for module imports";
     }

     identity revision-label-scheme-base {
       description
         "Base identity from which all revision label schemes are
          derived.";
       reference
         "XXXX: Updated YANG Module Revision Handling;
          Section 3.3.1, Revision label scheme extension statement";
     }
   }
   <CODE ENDS>

   YANG module with augmentations to YANG Library to revision labels

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   <CODE BEGINS> file "ietf-yang-library-revisions@2021-11-04.yang"
   module ietf-yang-library-revisions {
     yang-version 1.1;
     namespace
       "urn:ietf:params:xml:ns:yang:ietf-yang-library-revisions";
     prefix yl-rev;

     import ietf-yang-revisions {
       prefix rev;
       reference
         "XXXX: Updated YANG Module Revision Handling";
     }
     import ietf-yang-library {
       prefix yanglib;
       reference
         "RFC 8525: YANG Library";
     }

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

        Author:   Joe Clarke
                  <mailto:jclarke@cisco.com>

        Author:   Reshad Rahman
                  <mailto:reshad@yahoo.com>

        Author:   Robert Wilton
                  <mailto:rwilton@cisco.com>

        Author:   Balazs Lengyel
                  <mailto:balazs.lengyel@ericsson.com>

        Author:   Jason Sterne
                  <mailto:jason.sterne@nokia.com>";
     description
       "This module contains augmentations to YANG Library to add module
        level revision label and to provide an indication of how
        deprecated and obsolete nodes are handled by the server.

        Copyright (c) 2002 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

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        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; 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 (including in the imports above) with
     // actual RFC number and remove this note.
     // RFC Ed.: please replace label version with 1.0.0 and
     // remove this note.

     revision 2021-11-04 {
       rev:label "1.0.0-draft-ietf-netmod-yang-module-versioning-05";
       description
         "Initial revision";
       reference
         "XXXX: Updated YANG Module Revision Handling";
     }

     // library 1.0 modules-state is not augmented with revision-label

     augment "/yanglib:yang-library/yanglib:module-set/yanglib:module" {
       description
         "Add a revision label to module information";
       leaf revision-label {
         type rev:revision-label;
         description
           "The revision label associated with this module revision.
            The label MUST match the revision label value in the
            specific revision of the module loaded in this module-set.";
         reference
           "XXXX: Updated YANG Module Revision Handling;
            Section 5.2.1, Advertising revision-label";
       }
     }

     augment
       "/yanglib:yang-library/yanglib:module-set/yanglib:module/"

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     + "yanglib:submodule" {
       description
         "Add a revision label to submodule information";
       leaf revision-label {
         type rev:revision-label;
         description
           "The revision label associated with this submodule revision.
            The label MUST match the revision label value in the
            specific revision of the submodule included by the module
            loaded in this module-set.";
         reference
           "XXXX: Updated YANG Module Revision Handling;
            Section 5.2.1, Advertising revision-label";
       }
     }

     augment "/yanglib:yang-library/yanglib:module-set/"
           + "yanglib:import-only-module" {
       description
         "Add a revision label to module information";
       leaf revision-label {
         type rev:revision-label;
         description
           "The revision label associated with this module revision.
            The label MUST match the revision label value in the
            specific revision of the module included in this
            module-set.";
         reference
           "XXXX: Updated YANG Module Revision Handling;
            Section 5.2.1, Advertising revision-label";
       }
     }

     augment "/yanglib:yang-library/yanglib:module-set/"
           + "yanglib:import-only-module/yanglib:submodule" {
       description
         "Add a revision label to submodule information";
       leaf revision-label {
         type rev:revision-label;
         description
           "The revision label associated with this submodule revision.
            The label MUST match the rev:label value in the specific
            revision of the submodule included by the import-only-module
            loaded in this module-set.";
         reference
           "XXXX: Updated YANG Module Revision Handling;
            Section 5.2.1, Advertising revision-label";
       }

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     }

     augment "/yanglib:yang-library/yanglib:schema" {
       description
         "Augmentations to the ietf-yang-library module to indicate how
          deprecated and obsoleted nodes are handled for each datastore
          schema supported by the server.";
       leaf deprecated-nodes-implemented {
         type boolean;
         description
           "If set to true, this leaf indicates that all schema nodes
            with a status 'deprecated' are implemented equivalently as
            if they had status 'current'; otherwise deviations MUST be
            used to explicitly remove deprecated nodes from the schema.
            If this leaf is absent or set to false, then the behavior is
            unspecified.";
         reference
           "XXXX: Updated YANG Module Revision Handling;
            Section 5.2.2, Reporting how deprecated and obsolete nodes
            are handled";
       }
       leaf obsolete-nodes-absent {
         type boolean;
         description
           "If set to true, this leaf indicates that the server does not
            implement any status 'obsolete' schema nodes.  If this leaf
            is absent or set to false, then the behaviour is
            unspecified.";
         reference
           "XXXX: Updated YANG Module Revision Handling; Section 5.2.2,
            Reporting how deprecated and obsolete nodes are handled";
       }
     }
   }
   <CODE ENDS>

9.  Security considerations

9.1.  Security considerations for module revisions

   As discussed in the introduction of this document, YANG modules
   occasionally undergo changes that are not backwards compatible.  This
   occurs in both standards and vendor YANG modules despite the
   prohibitions in RFC 7950.  RFC 7950 also allows nodes to change to
   status 'obsolete' which can change behavior and compatibility for a
   client.

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   The fact that YANG modules change in a non-backwards-compatible
   manner may have security implications.  Such changes should be
   carefully considered, including the scenarios described below.  The
   rev:non-backwards-compatible extension statement introduced in this
   document provides an alert that the module or submodule may contain
   changes that impact users and need to be examined more closely for
   both compatibility and potential security implications.  Flagging the
   change reduces the risk of introducing silent exploitable
   vulnerabilities.

   When a module undergoes a non-backwards-compatible change, a server
   may implement different semantics for a given leaf than a client
   using an older version of the module is expecting.  If the particular
   leaf controls any security functions of the device, or is related to
   parts of the configuration or state that are sensitive from a
   security point of view, then the difference in behavior between the
   old and new revisions needs to be considered carefully.  In
   particular, changes to the default of the leaf should be examined.

   Implementors and users should also consider impact to data node
   access control rules (e.g.  The Network Configuration Access Control
   Model (NACM) [RFC8341]) in the face of non-backwards-compatible
   changes.  Access rules may need to be adjusted when a new module
   revision is introduced that contains a non-backwards-compatible
   change.

   If the changes to a module or submodule have security implications,
   it is recommended to highlight those implications in the description
   of the revision statement.

9.2.  Security considerations for the modules defined in this document

   The YANG module specified in this document defines a schema for data
   that is designed to 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 NETCONF access control model [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.

   This document does not define any new protocol or data nodes that are
   writable.

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   This document updates YANG Library [RFC8525] with augmentations to
   include revision labels in the YANG library data and two boolean
   leafs to indicate whether status deprecated and status obsolete
   schema nodes are implemented by the server.  These read-only
   augmentations do not add any new security considerations beyond those
   already present in [RFC8525].

10.  IANA Considerations

10.1.  YANG Module Registrations

   This document requests IANA to registers a URI in the "IETF XML
   Registry" [RFC3688].  Following the format in RFC 3688, the following
   registrations are requested.

      URI: urn:ietf:params:xml:ns:yang:ietf-yang-revisions
      Registrant Contact: The IESG.
      XML: N/A, the requested URI is an XML namespace.

      URI: urn:ietf:params:xml:ns:yang:ietf-yang-library-revisions
      Registrant Contact: The IESG.
      XML: N/A, the requested URI is an XML namespace.

   The following YANG module is requested to be registred in the "IANA
   Module Names" [RFC6020].  Following the format in RFC 6020, the
   following registrations are requested:

   The ietf-yang-revisions module:

      Name: ietf-yang-revisions

      XML Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-revisions

      Prefix: rev

      Reference: [RFCXXXX]

   The ietf-yang-library-revisions module:

      Name: ietf-yang-library-revisions

      XML Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-library-
      revisions

      Prefix: yl-rev

      Reference: [RFCXXXX]

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10.2.  Guidance for versioning in IANA maintained YANG modules

   Note for IANA (to be removed by the RFC editor): Please check that
   the registries and IANA YANG modules are referenced in the
   appropriate way.

   IANA is responsible for maintaining and versioning YANG modules that
   are derived from other IANA registries.  For example,
   "iana-if-type.yang" [IfTypeYang] is derived from the "Interface Types
   (ifType) IANA registry" [IfTypesReg], and "iana-routing-types.yang"
   [RoutingTypesYang] is derived from the "Address Family Numbers"
   [AddrFamilyReg] and "Subsequent Address Family Identifiers (SAFI)
   Parameters" [SAFIReg] IANA registries.

   Normally, updates to the registries cause any derived YANG modules to
   be updated in a backwards-compatible way, but there are some cases
   where the registry updates can cause non-backward-compatible updates
   to the derived YANG module.  An example of such an update is the
   2020-12-31 revision of iana-routing-types.yang
   [RoutingTypesDecRevision], where the enum name for two SAFI values
   was changed.

   In all cases, IANA MUST follow the versioning guidance specified in
   Section 3.1, and MUST include a "rev:non-backwards-compatible"
   substatement to the latest revision statement whenever an IANA
   maintained module is updated in a non-backwards-compatible way, as
   described in Section 3.2.

   Note: For published IANA maintained YANG modules that contain non-
   backwards-compatible changes between revisions, a new revision should
   be published with the "rev:non-backwards-compatible" substatement
   retrospectively added to any revisions containing non-backwards-
   compatible changes.

   Non-normative examples of updates to enumeration types in IANA
   maintained modules that would be classified as non-backwards-
   compatible changes are: Changing the status of an enumeration typedef
   to obsolete, changing the status of an enum entry to obsolete,
   removing an enum entry, changing the identifier of an enum entry, or
   changing the described meaning of an enum entry.

   Non-normative examples of updates to enumeration types in IANA
   maintained modules that would be classified as backwards-compatible
   changes are: Adding a new enum entry to the end of the enumeration,
   changing the status or an enum entry to deprecated, or improving the
   description of an enumeration that does not change its defined
   meaning.

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   Non-normative examples of updates to identity types in IANA
   maintained modules that would be classified as non-backwards-
   compatible changes are: Changing the status of an identity to
   obsolete, removing an identity, renaming an identity, or changing the
   described meaning of an identity.

   Non-normative examples of updates to identity types in IANA
   maintained modules that would be classified as backwards-compatible
   changes are: Adding a new identity, changing the status or an
   identity to deprecated, or improving the description of an identity
   that does not change its defined meaning.

11.  References

11.1.  Normative References

   [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-
              08, 24 October 2022, <https://www.ietf.org/archive/id/
              draft-ietf-netmod-yang-semver-08.txt>.

   [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. and RFC Publisher, "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.,
              Bierman, A., Ed., and RFC Publisher, "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>.

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   [RFC7950]  Bjorklund, M., Ed. and RFC Publisher, "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>.

   [RFC8341]  Bierman, A., Bjorklund, M., and RFC Publisher, "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. and RFC Publisher, "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>.

11.2.  Informative References

   [AddrFamilyReg]
              "Address Family Numbers IANA Registry",
              <https://www.iana.org/assignments/address-family-numbers/
              address-family-numbers.xhtml>.

   [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://www.ietf.org/archive/id/draft-clacla-
              netmod-yang-model-update-06.txt>.

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   [I-D.ietf-netmod-yang-packages]
              Wilton, R., Rahman, R., Clarke, J., Sterne, J., and B. Wu,
              "YANG Packages", Work in Progress, Internet-Draft, draft-
              ietf-netmod-yang-packages-03, 4 March 2022,
              <https://www.ietf.org/archive/id/draft-ietf-netmod-yang-
              packages-03.txt>.

   [I-D.ietf-netmod-yang-schema-comparison]
              Wilton, R., "YANG Schema Comparison", Work in Progress,
              Internet-Draft, draft-ietf-netmod-yang-schema-comparison-
              01, 2 November 2020, <https://www.ietf.org/archive/id/
              draft-ietf-netmod-yang-schema-comparison-01.txt>.

   [I-D.ietf-netmod-yang-ver-selection]
              Wilton, R., Rahman, R., Clarke, J., Sterne, J., and B. Wu,
              "YANG Schema Selection", Work in Progress, Internet-Draft,
              draft-ietf-netmod-yang-ver-selection-00, 17 March 2020,
              <https://www.ietf.org/archive/id/draft-ietf-netmod-yang-
              ver-selection-00.txt>.

   [I-D.ietf-netmod-yang-versioning-reqs]
              Clarke, J., "YANG Module Versioning Requirements", Work in
              Progress, Internet-Draft, draft-ietf-netmod-yang-
              versioning-reqs-07, 10 July 2022,
              <https://www.ietf.org/archive/id/draft-ietf-netmod-yang-
              versioning-reqs-07.txt>.

   [IfTypesReg]
              "Interface Types (ifType) IANA Registry",
              <https://www.iana.org/assignments/smi-numbers/smi-
              numbers.xhtml#smi-numbers-5>.

   [IfTypeYang]
              "iana-if-type YANG Module",
              <https://www.iana.org/assignments/iana-if-type/iana-if-
              type.xhtml>.

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

   [RFC9195]  Lengyel, B. and B. Claise, "A File Format for YANG
              Instance Data", RFC 9195, DOI 10.17487/RFC9195, February
              2022, <https://www.rfc-editor.org/info/rfc9195>.

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   [RoutingTypesDecRevision]
              "2020-12-31 revision of iana-routing-types.yang",
              <https://www.iana.org/assignments/yang-parameters/iana-
              routing-types@2020-12-31.yang>.

   [RoutingTypesYang]
              "iana-routing-types YANG Module",
              <https://www.iana.org/assignments/iana-routing-types/iana-
              routing-types.xhtml>.

   [SAFIReg]  "Subsequent Address Family Identifiers (SAFI) Parameters
              IANA Registry", <https://www.iana.org/assignments/safi-
              namespace/safi-namespace.xhtml>.

   [semver]   "Semantic Versioning 2.0.0", <https://www.semver.org>.

Appendix A.  Examples of changes that are NBC

   Examples of NBC changes include:

   *  Deleting a data node, or changing it to status obsolete.

   *  Changing the name, type, or units of a data node.

   *  Modifying the description in a way that changes the semantic
      meaning of the data node.

   *  Any changes that remove any previously allowed values from the
      allowed value set of the data node, either through changes in the
      type definition, or the addition or changes to "must" statements,
      or changes in the description.

   *  Adding or modifying "when" statements that reduce when the data
      node is available in the schema.

   *  Making the statement conditional on if-feature.

Appendix B.  Examples of applying the NBC change guidelines

   The following sections give steps that could be taken for making NBC
   changes to a YANG module or submodule using the incremental approach
   described in section Section 7.1.1.

   The examples are all for "config true" nodes.

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B.1.  Removing a data node

   Removing a leaf or container from the data tree, e.g., because
   support for the corresponding feature is being removed:

   1.  The schema node's status is changed to "deprecated" and the node
       is supported for some period of time (e.g. one year).  This is a
       BC change.

   2.  When the schema node is not supported anymore, its status is
       changed to "obsolete" and the "description" updated.  This is an
       NBC change.

B.2.  Changing the type of a leaf node

   Changing the type of a leaf node. e.g., a "vpn-id" node of type
   integer being changed to a string:

   1.  The status of schema node "vpn-id" is changed to "deprecated" and
       the node is supported for some period of time (e.g. one year).
       This is a BC change.  The description is updated to indicate that
       "vpn-name" is replacing this node.

   2.  A new schema node, e.g., "vpn-name", of type string is added to
       the same location as the existing node "vpn-id".  This new node
       has status "current" and its description explains that it is
       replacing node "vpn-id".

   3.  During the period of time when both schema nodes are supported,
       the interactions between the two nodes is outside the scope of
       this document and will vary on a case by case basis.  One
       possible option is to have the server prevent the new node from
       being set if the old node is already set (and vice-versa).  The
       new node could have a "when" statement added to it to achieve
       this.  The old node, however, must not have a "when" statement
       added, or an existing "when" modified to be more restrictive,
       since this would be an NBC change.  In any case, the server could
       reject the old node from being set if the new node is already
       set.

   4.  When the schema node "vpn-id" is not supported anymore, its
       status is changed to "obsolete" and the "description" is updated.
       This is an NBC change.

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B.3.  Reducing the range of a leaf node

   Reducing the range of values of a leaf-node, e.g., consider a "vpn-
   id" schema node of type uint32 being changed from range 1..5000 to
   range 1..2000:

   1.  If all values which are being removed were never supported, e.g.,
       if a vpn-id of 2001 or higher was never accepted, this is a BC
       change for the functionality (no functionality change).  Even if
       it is an NBC change for the YANG model, there should be no impact
       for clients using that YANG model.

   2.  If one or more values being removed was previously supported,
       e.g., if a vpn-id of 3333 was accepted previously, this is an NBC
       change for the YANG model.  Clients using the old YANG model will
       be impacted, so a change of this nature should be done carefully,
       e.g., by using the steps described in Appendix B.2

B.4.  Changing the key of a list

   Changing the key of a list has a big impact to the client.  For
   example, consider a "sessions" list which has a key "interface" and
   there is a need to change the key to "dest-address".  Such a change
   can be done in steps:

   1.  The status of list "sessions" is changed to "deprecated" and the
       list is supported for some period of time (e.g. one year).  This
       is a BC change.  The description is updated to indicate the new
       list that is replacing this list.

   2.  A new list is created in the same location with the same
       descendant schema nodes but with "dest-address" as key.  Finding
       an appropriate name for the new list can be difficult.  In this
       case the new list is called "sessions-address", has status
       "current" and its description should explain that it is replacing
       list "session".

   3.  During the period of time when both lists are supported, the
       interactions between the two lists is outside the scope of this
       document and will vary on a case by case basis.  One possible
       option is to have the server prevent entries in the new list from
       being created if the old list already has entries (and vice-
       versa).

   4.  When list "sessions" is not available anymore, its status is
       changed to "obsolete" and the "description" is updated.  This is
       an NBC change.

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B.5.  Renaming a node

   A leaf or container schema node may be renamed, either due to a
   spelling error in the previous name or because of a better name.  For
   example a node "ip-adress" could be renamed to "ip-address":

   1.  The status of the existing node "ip-adress" is changed to
       "deprecated" and is supported for some period of time (e.g. one
       year).  This is a BC change.  The description is updated to
       indicate the node that is replacing this node.

   2.  The new schema node "ip-address" is added to the same location as
       the existing node "ip-adress".  This new node has status
       "current" and its description should explain that it is replacing
       node "ip-adress".

   3.  During the period of time when both nodes are available, the
       interactions between the two nodes is outside the scope of this
       document and will vary on a case by case basis.  One possible
       option is to have the server prevent the new node from being set
       if the old node is already set (and vice-versa).  The new node
       could have a "when" statement added to it to achieve this.  The
       old node, however, must not have a "when" statement added, or an
       existing "when" modified to be more restrictive, since this would
       be an NBC change.  In any case, the server could reject the old
       node from being set if the new node is already set.

   4.  When node "ip-adress" is not available anymore, its status is
       changed to "obsolete" and the "description" is updated.  This is
       an NBC change.

Contributors

   This document grew out of the YANG module versioning design team that
   started after IETF 101.  The authors and the following individuals
   are (or have been) members of the design team and have worked on the
   YANG versioning project:

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     Benoit Claise
     benoit.claise@huawei.com

     Bo Wu
     lana.wubo@huawei.com

     Ebben Aries
     exa@juniper.net

     Jan Lindblad
     lindbla@cisco.com

     Juergen Schoenwaelder
     j.shoenwaelder@jacobs-university.de

     Mahesh Jethanandani
     mjethanandani@gmail.com

     Michael (Wangzitao)
     wangzitao@huawei.com

     Per Andersson
     perander@cisco.com

     Qin Wu
     bill.wu@huawei.com

   The initial revision of this document was refactored and built upon
   [I-D.clacla-netmod-yang-model-update].  We would like to thank Kevin
   D'Souza and Benoit Claise for their initial work in this problem
   space.

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

   We would also like to thank Lou Berger, Andy Bierman, Martin
   Bjorklund, Italo Busi, Tom Hill, Scott Mansfield, and Kent Watsen for
   their contributions and review comments.

Authors' Addresses

   Robert Wilton (editor)
   Cisco Systems, Inc.
   Email: rwilton@cisco.com

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   Reshad Rahman (editor)
   Graphiant
   Email: reshad@yahoo.com

   Balazs Lengyel (editor)
   Ericsson
   Email: balazs.lengyel@ericsson.com

   Joe Clarke
   Cisco Systems, Inc.
   Email: jclarke@cisco.com

   Jason Sterne
   Nokia
   Email: jason.sterne@nokia.com

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