Network Working Group                                          B. Claise                                     R. Wilton, Ed.
Internet-Draft                                                 J. Clarke                                            R. Rahman, Ed.
Updates: 7950,8407,8525 (if approved)                          R. Rahman                Cisco Systems, Inc.
Intended status: Standards Track                          R. Wilton,                         B. Lengyel, Ed.
Expires: September 18, 2020 January 11, 2021                                       Ericsson
                                                               J. Clarke
                                                     Cisco Systems, Inc.
                                                              B. Lengyel
                                                                Ericsson
                                                               J. Sterne
                                                                   Nokia
                                                               B. Claise
                                                     Cisco Systems, Inc.
                                                              K. D'Souza
                                                                    AT&T
                                                          March 17,
                                                           July 10, 2020

                 Updated YANG Module Revision Handling
              draft-ietf-netmod-yang-module-versioning-00
              draft-ietf-netmod-yang-module-versioning-01

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 an earliest revision filter to better represent inter-module
   dependencies.  It provides help and 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.  This document updates RFC
   7950, 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 September 18, 2020. January 11, 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Updates to YANG RFCs  . . . . . . . . . . . . . . . . . .   4
   2.  Terminology and Conventions . . . . . . . . . . . . . . . . .   4
   3.  Refinements to YANG revision handling . . . . . . . . . . . .   5
     3.1.  Updating a YANG module with a new revision  . . . . . . .   5
       3.1.1.  Backwards-compatible changes  . . . . . . . . . . . .   5   6
       3.1.2.  Non-backwards-compatible changes  . . . . . . . . . .   6
     3.2.  nbc-changes revision extension statement  . . . . . . . .   6   7
     3.3.  Revision label  . . . . . . . . . . . . . . . . . . . . .   6
     3.4.  YANG status description   7
       3.3.1.  Revision label scheme extension statement . . . . . . .   7
     3.5.   8
     3.4.  Examples for updating the YANG module revision history  .   7   8
   4.  Import by derived revision  . . . . . . . . . . . . . . . . .  10  11
     4.1.  Module import examples  . . . . . . . . . . . . . . . . .  11  12
   5.  Updates to ietf-yang-library  . . . . . . . . . . . . . . . .  13  14
     5.1.  Resolving ambiguous module imports  . . . . . . . . . . .  13  14
     5.2.  YANG library versioning augmentations . . . . . . . . . .  14  15
       5.2.1.  Advertising revision-label  . . . . . . . . . . . . .  14  15
       5.2.2.  Reporting how deprecated and obsolete nodes are
               handled . . . . . . . . . . . . . . . . . . . . . . .  14  15
   6.  Versioning of YANG instance data  . . . . . . . . . . . . . .  15  16
   7.  Guidelines for using the YANG module update rules . . . . . .  15  16
     7.1.  Guidelines for YANG module authors  . . . . . . . . . . .  15  16
       7.1.1.  Making non-backwards-compatible changes to a YANG
               module  . . . . . . . . . . . . . . . . . . . . . . .  16  17
     7.2.  Versioning Considerations for Clients . . . . . . . . . .  17  18
   8.  Module Versioning Extension YANG Modules  . . . . . . . . . .  18
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  25  26
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  26  27
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26  27
     11.1.  YANG Module Registrations  . . . . . . . . . . . . . . .  26
   12. References  27
     11.2.  Instructions . . . . . . . . . . . . . . . . . . . . . .  28

   12. References  . . .  26
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     12.2.  Informative References . . . .  28
     12.1.  Normative References . . . . . . . . . . . . .  27
   Appendix A.  Appendix . . . . .  28
     12.2.  Informative References . . . . . . . . . . . . . . . . .  28
     A.1.  29
   Appendix A.  Examples of guidelines for making NBC changes to a YANG
           module  . . . . . . . . . . . . that are NBC . . . . . . . . . .  29
   Appendix B.  Examples of applying the NBC change guidelines . . .  28
       A.1.1.  30
     B.1.  Removing a data node  . . . . . . . . . . . . . . . .  28
       A.1.2. . .  30
     B.2.  Changing the type of a leaf node  . . . . . . . . . .  29
       A.1.3. . .  31
     B.3.  Reducing the range of a leaf node . . . . . . . . . .  30
       A.1.4. . .  32
     B.4.  Changing the key of a list  . . . . . . . . . . . . .  30
       A.1.5. . .  32
     B.5.  Renaming a node . . . . . . . . . . . . . . . . . . .  31
       A.1.6. . .  33
     B.6.  Changing a default value  . . . . . . . . . . . . . .  32 . .  34
   Appendix C.  Changes between revisions  . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32  34

1.  Introduction

   This document defines a solution to the YANG module lifecycle
   problems described in [I-D.verdt-netmod-yang-versioning-reqs]. [I-D.ietf-netmod-yang-versioning-reqs].
   Complementary documents provide a complete solution to the YANG
   versioning requirements, with the overall relationship of the
   solution drafts described in [I-D.verdt-netmod-yang-solutions]. [I-D.ietf-netmod-yang-solutions].

   Specifically, this document recognises a need (within standards
   organizations, vendors, and the industry) to sometimes allow YANG
   modules to evolve with non-backwards-compatible changes, which could
   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 where these changes occur, and to manage
   their effect on clients and the broader YANG ecosystem.

   The solution 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.

      A YANG extension statement allowing YANG module imports to specify
      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 descriptions, to report how
      "deprecated" and "obsolete" nodes are handled by a server, and to
      clarify how module imports are resolved when multiple versions 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.  Section 3 describes
   modifications to YANG revision handling and update rules, and
   Section 4 describes a YANG extension statement to do import by
   derived revision.

   This document updates [RFC8525] [RFC7950] section 3. 5.6.5.  Section 5 5.1 defines
   how a client of a YANG library datastore schema chooses resolves ambiguous
   imports for modules which revision of
   an import-only module is used to resolve a module import when the
   definition is otherwise ambiguous. are not "import-only".

   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 [RFC7950] section 5.2.  Section 3.3 describes
   the use of a revision label in the name of a file containing a YANG
   module or submodule.

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.

   In addition, this document uses the terminology:

   o  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.

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

   o  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] assumes, but does not explicitly state, that the revision
   history for a YANG module is strictly linear, i.e., it is prohibited
   to have two independent revisions of a YANG module that are both
   directly derived from the same parent revision.

   This document clarifies [RFC7950] to explicitly allow non linear
   development of YANG module revisions, so modules MAY have multiple
   revisions that directly derive from the same parent revision.  As per
   [RFC7950], YANG module revisions continue to be uniquely identified
   by the module's revision date, and hence all revisions of a module
   MUST have unique revision dates.

   A corollary to the above is that the relationship between two module
   revisions cannot be determined by comparing the module revision date
   alone, and the revision history, or revision label, must also be
   taken into consideration.

   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
   MUST 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 [[RFC7895]
   [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 requires that all updates to a YANG module are
   BC to the previous revision of the module.  This document allows for
   more flexible evolution of YANG modules: NBC changes between module
   revisions are allowed and are documented using a new "nbc-changes"
   YANG extension statement in the module revision history.

   Two revisions of a module MAY have identical content except for the
   revision history.  This could occur, for example, if a module 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 to refine the rules for
   permissible changes when a new YANG module revision is created.

   Where pragmatic, updates to YANG modules SHOULD be backwards-
   compatible, following the definition in Section 3.1.1.

   A new module revision MAY contain NBC changes, i.e., the semantics of
   an existing definition MAY be changed in an NBC way without requiring
   a new definition with a new identifier.  A new module revision with
   NBC changes MUST include the "rev:nbc-changes" extension substatement
   to signal the potential for incompatibility to existing module users
   and readers.

3.1.1.  Backwards-compatible changes

   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, updated by the following rules:

   o  A "status" "deprecated" statement MAY be added, or changed from
      "current" to "deprecated", but adding or changing "status" to
      "obsolete" is not a backwards-compatible change.

   o  Obsolete definitions MAY be removed from published modules, and
      are classified as backwards-compatible changes.  In some
      circumstances it may be helpful to retain the obsolete definitions
      to ensure that their identifiers are not reused with a different
      meaning.

   o  In statements that have any data definition statements as
      substatements, those data definition substatements MAY be
      reordered, as long as they do not change the ordering or of any "rpc"
      "input" substatements. or "output" data definition substatements of "rpc" or
      "action" statements.  If new data definition statements are added,
      they can be added anywhere in the sequence of existing
      substatements.

   o  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.  nbc-changes revision extension statement

   The "rev:nbc-changes" 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:nbc-
   changes" extension statement MUST be added as a substatement to the
   "revision" statement.

   Conversely, if a revision does not contain an "rev:nbc-changes"
   extension substatement then all changes, relative to the preceding
   revision in the revision history, MUST be backwards-compatible.

3.3.  Revision label

   This section updates [RFC7950] section 5.2, it explains how a
   revision label can be used in the name of a file containing a YANG
   module or submodule.

   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.

   YANG Semver [I-D.verdt-netmod-yang-semver] [I-D.ietf-netmod-yang-semver] defines a versioning scheme
   based on Semver 2.0.0 [semver] that can be used as a revision label.  All

   Submodules MAY use a revision labels label scheme.  When they use a revision
   label scheme, submodules MAY use a revision label scheme that match the pattern for is
   different from the "version"
   typedef one used in the ietf-yang-semver YANG module MUST be interpreted as
   YANG semantic version numbers. including module.

   The revision date and revision label within a submodule's space of submodules is separate from the revision
   history have no effect on
   label space of the including module's revision.
   Submodules module.  A change in one submodule MUST NOT use
   result in a new revision label schemes of that submodule and the including
   module, but the actual values of the revision labels in the module
   and submodule could be confused
   with 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 including module's revision label scheme. in all included submodules.

   If a revision has an associated revision label, then it may be used
   instead of the revision date in two places:

      In an "rev:revision-or-derived"
   extension statement argument.

      In

   If a revision has an associated revision label, then it may be used
   instead of the revision date in the filename of a YANG module, where
   it takes the form: module-
      or-submodule-name ['@' revision-label]

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

3.4.

       E.g., acme-router-modules@2018-01-25.yang
       E.g., acme-router-modules#2.0.3.yang

   Two file names, one with the revision date and another with the
   revision label, MAY exist for the same module (or submodule)
   revision.

   A specific revision-label identifies a specific revision (variant) of
   the module.  If two YANG status description 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 contents of the two modules MUST
   be identical.

3.3.1.  Revision label scheme extension statement

   The ietf-yang-revision module specifies the YANG "rev:revision-label-scheme" extension statement
   "status-description" that can be is used as to
   indicate which revision-label scheme a substatement of the status
   statement. module or submodule uses.  The
   mandatory argument to this extension statement can contain
   freeform text to help readers statement:

   o  Specifies the revision-label scheme used by the module or
      submodule

   o  Is defined in the document which specifies the revision-label
      scheme

   o  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 understand why or submodule.  If the node
   was deprecated
   revision-label scheme used by a module or made obsolete, when it submodule is anticipated changed to a
   new scheme, then all revision-label statements that do not conform to
   the node
   will no longer be available for use, and potentially reference other
   schema elements that can new scheme MUST be used instead.  An example is shown below.

   leaf imperial-temperature {
     type int64;
     units "degrees Fahrenheit";
     status deprecated {
       rev:status-description
         "Imperial measurements are being phased out in favor
          of their metric equivalents.  Use metric-temperature
          instead.";
     }
     description
       "Temperature in degrees Fahrenheit.";
   }

3.5. replaced or removed.

3.4.  Examples for updating the YANG module revision history

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

   Example YANG module with branched revision history.

          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 version revision
   history might evolve, over time.  For example, the tree might
   represent the following changes, listed in chronological order from
   oldest revision to newest:

   Example module, revision 2019-06-01:

       module example-module {

         namespace "name-space"; "urn:example:module";
         prefix "prefix-name";

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

         description
           "to be completed";

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

         revision 2019-04-01 {
           rev:revision-label 3.0.0;
           rev:nbc-changes;
           description
             "Add new functionality. Remove some deprecated nodes.";
         }

         revision 2019-02-01 {
           rev:revision-label 2.0.0;
           rev:nbc-changes;
           description "Apply bugfix to pattern statement";
         }

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

         //YANG module definition starts here
       }

   Example module, revision 2019-05-01:

       module example-module {

         namespace "name-space"; "urn:example:module";
         prefix "prefix-name";

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

         description
           "to be completed";

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

         revision 2019-03-01 {
           rev:revision-label 2.1.0;
           description "Apply enhancement to older release train.";
         }

         revision 2019-02-01 {
           rev:revision-label 2.0.0;
           rev:nbc-changes;
           description "Apply bugfix to pattern statement";
         }

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

         //YANG module definition starts here
       }

4.  Import by derived revision

   RFC 7950 allows YANG module "import" statements to optionally require
   the imported module to have a particular revision date.  In practice,
   importing a module with an exact revision date is often too
   restrictive because it requires the importing module to be updated
   whenever any change to the imported module occurs.  The alternative
   choice of using an import statement without any revision date
   statement is also not ideal because the importing module may not work
   with all possible revisions of the imported module.

   Instead, it is desirable for a importing module to specify a "minimum
   required revision" of a module that it is compatible with, based on
   the assumption that later revisions derived from that "minimum
   required revision" are also likely to be compatible.  Many possible
   changes to a YANG module do not break importing modules, even if the
   changes themselves are not strictly backwards-compatible.  E.g.,
   fixing an incorrect pattern statement or description for a leaf would
   not break an import, changing the name of a leaf could break an
   import but frequently would not, but removing a container would break
   imports if that container is augmented by another module.

   The ietf-revisions module defines the "revision-or-derived" extension
   statement, a substatement to the YANG "import" statement, to allow
   for a "minium "minimum required revision" to be specified during import:

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

      A particular revision of an imported module satisfies an import's
      "revision-or-derived" extension statement if the imported module's
      revision history contains a revision statement with a matching
      revision date or revision label.

      An "import" statement MUST NOT contain both a "revision-or-
      derived" extension statement and a "revision-date" statement.

      The "revision-or-derived" extension statement MAY be specified
      multiple times, allowing the import to use any module revision
      that satifies satisfies at least one of the "revision-or-derived" extension
      statements.

      The "revision-or-derived" extension statement does not gaurantee guarantee
      that all module revisions that satisfy an import statement are
      necessarily compatible, it only gives an indication that the
      revisions are more likely to be compatible.  Hence, NBC changes to
      an imported module may also require new revisions of any importing
      modules, updated to accommodation those changes, along with
      updated import "revision-or-derived" extension statements to
      depend on the updated imported module revision.

4.1.  Module import examples

   Consider the example module "example-module" from Section 3.5 3.4 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.  Example 1

   This example selects module revisions 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.

   import example-module {
     rev:revision-or-derived 2019-02-01;
   }

   Alternatively, the first example could have used the revision label
   "1.0.0"
   "2.0.0" instead, which selects the same set of revisions/versions. revisions/labels.

   import example-module {
     rev:revision-or-derived 1.0.0; 2.0.0;
   }

4.1.2.  Example 2

   This example selects module revisions 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 version number than
   2019-04-01/2.1.0 it is not a derived revision, and hence it is not a
   valid revision for import.

   import example-module {
     rev:revision-or-derived 2.1.0;
   }

4.1.3.  Example 3

   This example selects revisions 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:revision-or-derived 2019-04-01;
     rev:revision-or-derived 2019-06-01;
   }

5.  Updates to ietf-yang-library

   This document updates YANG library [RFC7895] [RFC8525] [RFC7950] to clarify how ambiguous
   module imports are resolved.  It also defines the YANG module, ietf-yl-revisions ietf-
   yang-library-revisions that augments YANG library [RFC8525] with new
   versioning
   revision-label related meta-data.

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

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

5.2.1.  Advertising revision-label

   The ietf-yl-revisions ietf-yang-library-revisions YANG module augments the "module"
   list in ietf-
   yang-library ietf-yang-library with a "revision-label" leaf to optionally
   declare the revision label associated wth the particular revision of
   each module.

5.2.2.  Reporting how deprecated and obsolete nodes are handled

   The ietf-yl-revisions ietf-yang-library-revisions YANG module augments YANG library
   with two leaves to allow a server to report how it handles status
   "deprecated" and status "obsolete" nodes.  The leaves are:

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

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

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

   If a server does not set the "deprecated-nodes-implemented" leaf, leaf to
   "true", then clients MUST NOT rely solely on the "rev:nbc-changes"
   statements to determine whether two module revisions are backwards-compatible, 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 [I-D.ietf-netmod-yang-instance-file-format] 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.rwilton-netmod-yang-packages]
   [I-D.ietf-netmod-yang-packages] ).

   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.verdt-netmod-yang-semver]. [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 are
   RECOMMENDED to 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:nbc-changes" statement SHOULD MUST be added only 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 depencencies 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 remove, or curtail, the older
   description statements associated with the previous revisions.

   The "rev:revision-or-derived" extension should 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 MUST SHOULD use the "revision-date"
   statement to include specific submodule revisions.  Changing a
   module's include statements to include different  The revision of
   the including module MUST be updated when any included submodule revisions
   requires a has
   changed.  The revision-label substatement used in the new module
   revision MUST indicate the nature of the module. change, i.e. NBC or BC, to
   the module's schema tree.

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 the different ways in which this
   can be done:

   o  NBC changes can be sometimes be done incrementally using the
      "deprecated" status to provide clients time to adapt to NBC
      changes.

   o  NBC changes are done at once, i.e. without using "status"
      statements.  Depending on the change, this may have a big impact
      on clients.

   o  If the server can support multiple versions revisions of the YANG module or
      of YANG packages(as specified in
      [I-D.rwilton-netmod-yang-packages]), [I-D.ietf-netmod-yang-packages]),
      and allows the client to select the version revision (as per
      [I-D.wilton-netmod-yang-ver-selection]),
      [I-D.ietf-netmod-yang-ver-selection]), then NBC changes MAY be
      done without using "status" statements.  Clients would be required
      to select the version revision which they support and the NBC change would
      have no impact on them

   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" and then when support is removed its status
       MUST be changed to "obsolete".  Instead of using the "obsolete"
       status, the data node MAY be removed from the model but this has
       the risk of breaking modules which import the modified module.

   2.  The new "status-description" extension statement SHOULD be used
       for nodes which are "obsolete" or "deprecated".

   3.  For status "deprecated", deprecated data nodes the "status-description" "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 "status-description". "description".  The reason for deprecating the node can also
       be included in the "status-description" "description" if it is deemed to be of
       potential interest to the user.

   4.

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

   5.

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

   See Appendix A.1 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:

   o  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.

   o  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.

   o  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, status description, revision label scheme, and importing by version. revision.

 <CODE BEGINS> file "ietf-yang-revisions@2019-09-18.yang" "ietf-yang-revisions@2020-07-06.yang"
 module ietf-yang-revisions {
   yang-version 1.1;
   namespace "urn:ietf:params:xml:ns:yang:ietf-yang-revisions";
   prefix rev;

   import ietf-yang-types {
     prefix yang;
     reference
       "XXXX [ietf-netmod-rfc6991-bis]: Common YANG Data Types";
   }

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

      Author:   Benoit Claise
                <mailto:bclaise@cisco.com>

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

      Author:   Reshad Rahman
                <mailto:rrahman@cisco.com>

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

      Author:   Kevin D'Souza
                <mailto:kd6913@att.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) 2019 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
      (http://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 (inc above) with actual RFC number and
   // remove this note.

   revision 2019-09-18 2020-07-06 {
     description
       "Initial version.";
     reference
       "XXXX: Updated YANG Module Revision Handling";
   }

   typedef revision-label {
     type string {
       length "1..255";
       pattern '[^\s@]+';
       pattern '\d{4}-\d{2}-\d{2}' {
         modifier invert-match;
       }
     }
     description
       "A label associated with a YANG revision.

        Excludes spaces and '@'.  MUST NOT match revision-date.

          Revision labels that classify as YANG semantic versions,
          as defined by the ietf-yang-semver:version typedef,
          MUST conform to the versioning behaviour defined in
          XXXX [verdt]: YANG Semantic Versioning."; revision-date.";
     reference
       "XXXX: Updated YANG Module Revision Handling;
        Section 3.3, Revision label";
   }

   typedef revision-date-or-label {
     type union {
       type yang:revision-identifier;
       type revision-label;
     }
     description
       "Represents either a YANG revision date or a revision label";
   }

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

          Each 'revision'

        The statement MAY have MUST only be a single substatement of the 'revision'
        statement.
        Zero or one 'nbc-changes'
          substatement. statement per parent statement is
        allowed.
        The statement MUST NOT have any substatements.

        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 RFC-XXX, then
        the 'nbc-changes' statement MUST be added as a substatement to
        the revision statement.

        Conversely, if a revision of a YANG module only contains
        changes, relative to the preceding revision in the revision
        history, that are classified as 'backwards-compatible' then
        the revision statement MUST NOT contain any 'nbc-changes'
        substatement.";

     reference
       "XXXX: Updated YANG Module Revision Handling;
        Section 3.2, nbc-changes revision extension statement";
   }

   extension revision-label {
     argument revision-label;
     description
       "The revision label can be used to provide an additional
        versioning identifier associated with the revision.  E.g., one
        option for a versioning scheme that could be used is [TODO -
        Reference semver draft].

        The format of the revision-label argument MUST conform to the
        pattern defined for the revision-label typedef.

          Each 'revision'

        The statement MAY have MUST only be a single 'revision-label'
          substatement. substatement of the revision
        statement.
        Zero or one revision-label statement per parent statement
        is allowed.
        The statement MUST NOT have any substatements.

        Revision labels MUST be unique amongst all revisions of a
        module.";

     reference
       "XXXX: Updated YANG Module Revision Handling;
        Section 3.3, Revision label";
   }

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

        The mandatory revision-label-scheme-identity 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'.

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

        The statement MUST NOT have any substatements.";

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

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

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

          Each 'import'

        The statement MAY have MUST only be a substatement of the import
        statement.
        Zero, one or more 'revision-or-derived' statement per parent
        statement is allowed.
        The statement MUST NOT have any substatements.

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

        An 'import' statement MUST NOT contain both a
        'revision-or-derived' extension statement and a
        'revision-date' statement.

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

        The 'revision-or-derived' extension statement does not
          gaurantee
        guarantee that all module revisions that satisfy an import
        statement are necessarily compatible, it only gives an
        indication that the revisions are more likely to be
        compatible.";

     reference
       "XXXX: Updated YANG Module Revision Handling;
        Section 4, Import by derived revision";
   }

     extension status-description

   identity revision-label-scheme-base {
       argument description;

       description
         "Freeform text that describes why a given node has been
          deprecated or made obsolete.  E.g., the
     description could be
          used to give the reason for removal, or it could point to an
          alternative schema elements that can be used in lieu of the
          given node.

          Each 'status' statement MAY have a single 'status-description'
          substatement.";
       "Base identity from which all revision label schemes are
        derived.";

       reference
         "XXXX: Updated YANG Module Revision Handling;
          Section 3.4, YANG status description 3.3.1, Revision label scheme extension statement";

   }
 }
 <CODE ENDS>

   YANG module with augmentations to YANG Library to revision labels

<CODE BEGINS> file "ietf-yl-revisions@2019-09-18.yang" "ietf-yang-library-revisions@2020-07-06.yang"
module ietf-yl-revisions ietf-yang-library-revisions {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-yl-revisions"; "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 Libary"; Library";
  }

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

     Author:   Benoit Claise
               <mailto:bclaise@cisco.com>

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

     Author:   Reshad Rahman
               <mailto:rrahman@cisco.com>

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

     Author:   Kevin D'Souza
               <mailto:kd6913@att.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) 2019 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
     (http://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 revision-label version with 1.0.0 and
  // remove this note.
  revision 2019-09-18 2020-07-06 {
    rev:revision-label 0.1.0;
    description
      "Initial revision";
    reference
      "XXXX: Updated YANG Module Revision Handling";
  }

  augment "/yanglib:yang-library/yanglib:module-set/yanglib:module" {
    description
      "Augmentation modules with a revision label";
    leaf revision-label {
      type rev:revision-label;
      description
        "The revision label associated with this module revision.
         The label MUST match the rev: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: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 empty; boolean;
      description
        "If present, set to true, this leaf indicates that all schema nodes with
         a status 'deprecated' child statement are implemented
         equivalently as if they had status 'current', or otherwise
         deviations MUST be used to explicitly remove 'deprecated'
         nodes from the schema.  If this leaf is absent set to false or absent,
         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 empty; boolean;
      description
        "If present, set to true, this leaf indicates that the server does not
         implement any status 'obsolete' nodes.  If this leaf is
            absent
         set to false or absent, 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
<CODE ENDS>

9.  Contributors

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

   o  Balazs Lengyel

   o  Benoit Claise

   o  Ebben Aries

   o  Jason Sterne

   o  Joe Clarke

   o  Juergen Schoenwaelder
   o  Mahesh Jethanandani

   o  Michael (Wangzitao)

   o  Qin Wu

   o  Reshad Rahman

   o  Rob Wilton

   o  Bo Wu

   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.  We would like thank both
   Anees Shaikh and Rob Shakir for their input into this problem space.

   We would also like to thank Martin Bjorklund, Jan Lindblad and Italo
   Busi for their contributions.

10.  Security Considerations

   The document does not define any new protocol or data model.  There
   are no security impacts. considerations beyond those specified in [RFC7950].

11.  IANA Considerations

11.1.  YANG Module Registrations

   The following YANG module is requested to be registred in the "IANA
   Module Names" registry:

   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-yl-revisions ietf-yang-library-revisions module:

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

      Prefix: yl-rev

      Reference: [RFCXXXX]

11.2.  Instructions

   We may need to give new instructions to IANA e.g. how to review
   revision-label statements to make sure they are accurate?  TBD

12.  References

12.1.  Normative References

   [I-D.ietf-netmod-rfc6991-bis]
              Schoenwaelder, J., "Common YANG Data Types", draft-ietf-
              netmod-rfc6991-bis-02
              netmod-rfc6991-bis-04 (work in progress), November 2019.

   [I-D.verdt-netmod-yang-semver] July 2020.

   [I-D.ietf-netmod-yang-semver]
              Claise, B., Clarke, J., Rahman, R., Wilton, R., Lengyel,
              B., Sterne, J., and K. D'Souza, "YANG Semantic
              Versioning", draft-verdt-netmod-yang-semver-01 (work in
              progress), October 2019.

   [I-D.verdt-netmod-yang-versioning-reqs]
              Clarke, J., "YANG Module Versioning Requirements", draft-
              verdt-netmod-yang-versioning-reqs-02 draft-ietf-netmod-yang-semver-00 (work in
              progress),
              November 2018. March 2020.

   [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>.

   [RFC7895]  Bierman, A., Bjorklund, M., and K. Watsen, "YANG Module
              Library", RFC 7895, DOI 10.17487/RFC7895, June 2016,
              <https://www.rfc-editor.org/info/rfc7895>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [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>.

   [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>.

   [RFC8525]  Bierman, A., Bjorklund, M., Schoenwaelder, J., Watsen, K.,
              and R. Wilton, "YANG Library", RFC 8525,
              DOI 10.17487/RFC8525, March 2019,
              <https://www.rfc-editor.org/info/rfc8525>.

12.2.  Informative References

   [I-D.clacla-netmod-yang-model-update]
              Claise, B., Clarke, J., Lengyel, B., and K. D'Souza, "New
              YANG Module Update Procedure", draft-clacla-netmod-yang-
              model-update-06 (work in progress), July 2018.

   [I-D.ietf-netmod-yang-instance-file-format]
              Lengyel, B. and B. Claise, "YANG Instance Data File
              Format", draft-ietf-netmod-yang-instance-file-format-08 draft-ietf-netmod-yang-instance-file-format-12
              (work in progress), March April 2020.

   [I-D.rwilton-netmod-yang-packages]

   [I-D.ietf-netmod-yang-packages]
              Wilton, R., Rahman, R., Clarke, J., Sterne, J., and W. Bo,
              "YANG Packages", draft-rwilton-netmod-yang-packages-03 draft-ietf-netmod-yang-packages-00 (work
              in progress), February March 2020.

   [I-D.verdt-netmod-yang-solutions]

   [I-D.ietf-netmod-yang-solutions]
              Wilton, R., "YANG Versioning Solution Overview", draft-
              verdt-netmod-yang-solutions-03
              ietf-netmod-yang-solutions-00 (work in progress),
              February March
              2020.

   [I-D.wilton-netmod-yang-ver-selection]

   [I-D.ietf-netmod-yang-ver-selection]
              Wilton, R., Rahman, R., Clarke, J., Sterne, J., and W. Bo,
              "YANG Schema Selection", draft-wilton-netmod-yang-ver-
              selection-02 draft-ietf-netmod-yang-ver-
              selection-00 (work in progress), March 2020.

   [I-D.ietf-netmod-yang-versioning-reqs]
              Clarke, J., "YANG Module Versioning Requirements", draft-
              ietf-netmod-yang-versioning-reqs-03 (work in progress), February
              June 2020.

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

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

Appendix A.  Appendix

A.1.  Examples of guidelines for making NBC changes to a YANG module that are NBC

   Examples of NBC changes include:

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

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

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

   o  Any changes that change or reduce 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.

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

   o  Making the statement conditional on if-feature.

Appendix B.  Examples of applying the NBC change guidelines

   The following sections give guidance for how some of these NBC
   changes could be made to a YANG module:

A.1.1. module.  The examples are all for
   "config true" nodes.

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 node's status is changed to "deprecated" and it is supported
       for at least one year.  This is a BC change.

   2.  When the node is not available anymore, its status is changed to
       "obsolete" and the "status-description" "description" updated, this is an NBC change.  The "status-description" is used to explain why the node
       is not available anymore.

   If the server can support NBC versions revisions of the YANG module
   simultaneously using version selection, selection
   [I-D.ietf-netmod-yang-ver-selection], then the changes can be done
   immediately:

   1.  The new revision of the YANG module has the node's status changed
       to "obsolete" and the "status-description" "description" updated, this is an NBC
       change.

   2.  Clients which require the data node select the older module
       revision

A.1.2. YANG package
       containing the schema version they use

B.2.  Changing the type of a leaf node

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

   1.  The status of node "vpn-id" is changed to "deprecated" and the
       node should be available for at least one year.  This is a BC
       change.

   2.  A new 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 where both nodes are available, how the
       server behaves when either node is set is outside the scope of
       this document and will vary on a case by case basis.  Here are
       some options:

       1.  A server may prevent the new node from being set if the old
           node is already set (and vice-versa).  The new node may have
           a when statement to achieve this.  The old node must not have
           a when statement since this would be an NBC change, but the
           server could reject the old node from being set if the new
           node is already set.

       2.  If the new node is set and a client does a get or get-config
           operation on the old node, the server could map the value.
           For example, if the new node "vpn-name" has value "123" then
           the server could return integer value 123 for the old node
           "vpn-id".  However, if the value can not be mapped, we need a
           way mapped then the
           configuration would be incomplete, this is outside the scope
           of returning "unsupported" TBD. this document.

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

   If the server can support NBC versions revisions of the YANG module
   simultaneously using version selection, selection
   [I-D.ietf-netmod-yang-ver-selection], then the changes can be done
   immediately:

   1.  In the new revision of the YANG module, the status of node "vpn-
       id" is changed to "obsolete".  This is an NBC change.

   2.  New node "vpn-name" is added to the same location as described
       above.

   3.  Clients which require the data node select the older module
       revision YANG package
       containing the schema version they use

   4.  A server should not map between the nodes "vpn-id" and "vpn-
       name", i.e. if a client creates a data instance with "vpn-name"
       then that data instance should not be visible to a client using a
       module revision which has "vpn-id" (and vice-versa).

A.1.3.

B.3.  Reducing the range of a leaf node

   Reducing the range of values of a leaf-node. e.g. consider a "vpn-id"
   node of type integer being changed from type uint32 to type uint16:

   1.  If all values which are being removed were never supported, e.g.
       if a vpn-id of 65536 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 65536 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 A.1.2

A.1.4. 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 should be available for at least one year.  This is a BC
       change.

   2.  A new list is created in the same location with the same data but
       with "dest-address" as key.  Finding an appropriate name for the
       new list can be tricky especially if the name of the existing
       list was perfect.  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 where both lists are available, how the
       server behaves when either list is set is outside the scope of
       this document and will vary on a case by case basis.  Here are
       some options:

       1.  A server could prevent the new list from being set if the old
           list already has entries (and vice-versa).

       2.  If the new list is set and a client does a get or get-config
           operation on the old list, the server could map the entries.
           However if the new list has entries which would lead to
           duplicate keys in the old list, the mapping can not be done.

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

   If the server can support NBC versions revisions of the YANG module
   simultaneously using version selection, selection
   [I-D.ietf-netmod-yang-ver-selection], then the changes can be done
   immediately:

   1.  The new revision of the YANG module has the list "sessions"
       modified to have "dest-address" as key, this is an NBC change.

   2.  Clients which require the previous functionality select the older
       module revision

A.1.5.

B.5.  Renaming a node

   A leaf-node or a container 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 the node should be available for at least one
       year.  This is a BC change.

   2.  The new 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 where both nodes are available, how the
       server behaves when either node is set is outside the scope of
       this document and will vary on a case by case basis.  Here are
       some options:

       1.  A server could 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 to achieve this.  The old node must not
           have a when statement since this would be an NBC change, but
           the server could reject the old node from being set if the
           new node is already set.

       2.  If the new node is set and a client does a get or get-config
           operation on the old node, the server could use the value of
           the new node.  For example, if the new node "ip-address" has
           value X then the server may return value X for the old node
           "ip-adress".

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

   If the server can support NBC versions revisions of the YANG module
   simultaneously using version selection, selection
   [I-D.ietf-netmod-yang-ver-selection], then the changes can be done
   immediately:

   1.  The new revision of the YANG module has the node with the new
       name replacing the node with the old name, this is an NBC change.

   2.  Clients which require the previous node name select the older
       module revision

A.1.6.

B.6.  Changing a default value

Appendix C.  Changes between revisions

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

   v00 - v01

   o  Removed status-description

   o  Allowed both revision-date and revision-label in the filename.

   o  New extension revision-label-scheme

   o  To include submodules, inclusion by revision-date changed from
      MUST to SHOULD

   o  Submodules can use revision label scheme and it can be same or
      different as the including module's scheme

   o  Addressed various comments provided at WG adoption on rev-00

Authors' Addresses
   Benoit Claise
   Cisco Systems, Inc.
   De Kleetlaan 6a b1
   1831 Diegem
   Belgium

   Phone: +32 2 704 5622
   Email: bclaise@cisco.com

   Joe Clarke
   Robert Wilton (editor)
   Cisco Systems, Inc.
   7200-12 Kit Creek Rd
   Research Triangle Park, North Carolina
   United States of America

   Phone: +1-919-392-2867

   Email: jclarke@cisco.com rwilton@cisco.com

   Reshad Rahman
   Cisco Systems, Inc.

   Email: rrahman@cisco.com

   Robert Wilton (editor)
   Cisco Systems, Inc.

   Email: rwilton@cisco.com rrahman@cisco.com

   Balazs Lengyel (editor)
   Ericsson
   Magyar Tudosok Korutja
   1117 Budapest
   Hungary

   Phone: +36-70-330-7909

   Email: balazs.lengyel@ericsson.com

   Joe Clarke
   Cisco Systems, Inc.

   Email: jclarke@cisco.com

   Jason Sterne
   Nokia

   Email: jason.sterne@nokia.com

   Benoit Claise
   Cisco Systems, Inc.

   Email: bclaise@cisco.com

   Kevin D'Souza
   AT&T
   200 S. Laurel Ave
   Middletown, NJ
   United States of America

   Email: kd6913@att.com