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Versions: 00 01 02 03 04 05 06 draft-verdt-netmod-yang-semver

Network Working Group                                          B. Claise
Internet-Draft                                                 J. Clarke
Updates: 7950 (if approved)                          Cisco Systems, Inc.
Intended status: Standards Track                              B. Lengyel
Expires: June 18, 2018                                          Ericsson
                                                              K. D'Souza
                                                                    AT&T
                                                       December 15, 2017


                    New YANG Module Update Procedure
                draft-clacla-netmod-yang-model-update-03

Abstract

   This document specifies a new YANG module update procedure in case of
   backward-incompatible changes, as an alternative proposal to the YANG
   1.1 specifications.  This document updates RFC 7950.

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 http://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 June 18, 2018.

Copyright Notice

   Copyright (c) 2017 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
   (http://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 Simplified BSD License text as described in Section 4.e of



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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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The Problems  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Slow Standardization  . . . . . . . . . . . . . . . . . .   3
     2.2.  Some YANG Modules are Not Backward Compatible . . . . . .   3
     2.3.  Non-Backward Compatible Errors  . . . . . . . . . . . . .   4
     2.4.  A Zoo of YANG Modules . . . . . . . . . . . . . . . . . .   4
     2.5.  YANG Modules Obsolete Relationship  . . . . . . . . . . .   5
     2.6.  YANG Module Transition Strategy . . . . . . . . . . . . .   6
     2.7.  Need to Allow Non-Backward Compatible changes . . . . . .   6
     2.8.  Problematic Handling of Status Statement  . . . . . . . .   7
     2.9.  No way to easily decide whether a change is Backward
           Compatible  . . . . . . . . . . . . . . . . . . . . . . .   7
     2.10. Early Warning about Removal . . . . . . . . . . . . . . .   8
   3.  The Solution  . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  SEMVER Semantic Versioning  . . . . . . . . . . . . . . .   9
     3.2.  Updates to YANG 1.1 status statement  . . . . . . . . . .  11
     3.3.  Updating the YANG 1.1 Module Update rules . . . . . . . .  11
     3.4.  The Derived Semantic Version  . . . . . . . . . . . . . .  11
     3.5.  Import by Semantic Version  . . . . . . . . . . . . . . .  12
   4.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .  13
   5.  Semantic Version Extension YANG Module  . . . . . . . . . . .  14
   6.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  16
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The YANG data modeling language [RFC7950] specifies strict rules for
   updating YANG modules (see section 11 "Updating a Module").  Citing a
   few of the relevant rules:

   1.  "As experience is gained with a module, it may be desirable to
       revise that module.  However, changes to published modules are
       not allowed if they have any potential to cause interoperability
       problems between a client using an original specification and a
       server using an updated specification."

   2.  "Note that definitions contained in a module are available to be
       imported by any other module and are referenced in "import"



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       statements via the module name.  Thus, a module name MUST NOT be
       changed.  Furthermore, the "namespace" statement MUST NOT be
       changed, since all XML elements are qualified by the namespace."

   3.  "Otherwise, if the semantics of any previous definition are
       changed (i.e., if a non-editorial change is made to any
       definition other than those specifically allowed above), then
       this MUST be achieved by a new definition with a new identifier."

   4.  "deprecated indicates an obsolete definition, but it permits new/
       continued implementation in order to foster interoperability with
       older/existing implementations."

   What are the consequences?

   1.  Ideally, the YANG module names should not be changed due the
       importance of not changing the automation code in case of import
       statements or service composition at the orchestration layer.

   2.  When the same YANG module name is kept, its new revision must be
       updated in a backward-compatible way.

   3.  While most of the non-backward compatible changes are prohibited,
       a client still does not know if a changed module is backward
       compatible, as a server may remove parts of a module after
       marking it deprecated or obsolete.

2.  The Problems

   This section lists a series of problems, hopefully listed in a
   logical order, which leads to the solution in the next section.

2.1.  Slow Standardization

   The conclusions drawn in the introduction lead to the logical
   conclusion that the standardized YANG modules have to be perfect on
   day one (at least the structure), which in turn might explain why all
   the IETF YANG modules take so long to standardize.  Shooting for
   perfection (at least in structure) is obviously a noble goal, but if
   the perfect standard comes too late, it doesn't help the industry.

2.2.  Some YANG Modules are Not Backward Compatible

   As we learn from our mistakes, we're going to face more and more
   backward-incompatible YANG modules.  An example is the YANG data
   model for L3VPN service delivery [RFC8049], which, based on
   implementation experience, must be updated in a backward-incompatible
   way with draft-wu-l3sm-rfc8049bis [I-D.wu-l3sm-rfc8049bis].



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   While Standards Development Organization (SDO) YANG modules are
   obviously better for the industry, we must recognize that many YANG
   modules are actually generated YANG modules (for example, from
   internal databases), also known as native YANG modules, or vendor
   modules [RFC8199].  From time to time, the new YANG modules are not
   backward-compatible.

   In such cases, it would be better to indicate how backward-compatible
   a given YANG module actually is.

2.3.  Non-Backward Compatible Errors

   Sometimes small errors force us to make non-backward compatible
   updates.  As an example imagine that we have a string with a complex
   pattern (e.g., an IP address).  Let's assume the initial pattern
   incorrectly allows IP addresses to start with 355.  In the next
   version this is corrected to disallow addresses starting with 355.
   Formally this is an non-backward compatible change as the value space
   of the string is decreased.  In reality an IP address and the
   implementation behind it was never capable of handling an address
   starting with 355.  So practically this is a backward compatible
   change, just like a correction of the description statement.  Still
   current YANG rules would force a module name change.

2.4.  A Zoo of YANG Modules

   Even if we focus on the IETF, we have to observe that many SDOs,
   opensource fora, and vendors develop YANG modules.  This should be
   considered a success for an IETF developed technology.  However, the
   operators are faced with this problem: how to select the YANG modules
   to take into account for their service developments.

   The site <https://www.yangcatalog.org> (and the YANG catalog that it
   provides: YANG module for yangcatalog.org,
   [I-D.clacla-netmod-model-catalog]) is an attempt to become a
   reference for all YANG modules available in the industry, for both
   YANG developers to search on what exists already) and for operators
   (to discover the more mature YANG models to automate services).  This
   YANG catalog should not only contain pointers to the YANG modules
   themselves, but also contain metadata related to those YANG modules:
   What is the module type (service model or not?); what is the maturity
   level? (e.g., for the IETF: is this an RFC, a working group document
   or an individual draft?); is this module implemented?; who is the
   contact?; is there open-source code available?  And we expect many
   more in the future.  The industry has begun to understand that the
   metadata related to YANG models become equally important as the YANG
   models themselves.




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   The yangcatalog.org instantiation of the catalog provides a means for
   module authors and vendors implementing modules to upload their
   metadata, which is then searchable via an API, as well as using a
   variety of web-based tools.  The instructions for contributing and
   searching for metadata can be found at <https://www.yangcatalog.org/
   contribute.php>.

   The issue is actually the number of YANG modules the operators are
   offered.  At the time of writing this document, the number of unique
   YANG modules in the catalog is exactly 2596 (and that number keeps
   growing), while the IETF has standardized or is busy standardizing a
   small subset of those.  Therefore, it's important to distinguish the
   relevant YANG modules with the pack and to understand the
   relationship between the YANG modules.

2.5.  YANG Modules Obsolete Relationship

   So the operators use the yangcatalog.org to discover which YANG
   modules they can use NOW.  They base their selection not only on the
   YANG module content, but also on the related metadata.  When faced
   with the zoo of the YANG modules, it's difficult to understand the
   relationship between YANG modules.  As an example: how could an
   operator discover that YANG-MODULE-B obsoletes YANG-MODULE-A?
   Indeed, both have different YANG module names.  The only available
   information is an "obsolete" tag in the published RFC containing
   YANG-MODULE-B: this tag would point to YANG-MODULE-A.  In the world
   of automation, going through a published RFC as a level of
   indirection to understand the YANG module obsolete relationship is a
   non-starter.  Food for thought: the IETF should stop thinking that
   the metric for success is an RFC number, as opposed to the contained
   YANG module(s).

   We need an automatic way to discover that a YANG-MODULE-B obsoletes
   YANG-MODULE-A, so that YANG-MODULE-A should not be given any
   attention.

   The following example is not an automatic way.

       description
           "This YANG module defines a generic service configuration
           model for Layer 3 VPNs. This model is common across all
           vendor implementations. This obsoletes the RFC8049 YANG
           module, ietf-l3vpn-svc@2017-01-2";
       revision 2017-09-14 {
           description
           "First revision of RFC8049.";
           reference
           "RFC xxxx: YANG Data Model for L3VPN Service Delivery";



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   Along the same lines, while going through an out-of-band tool such as
   the yangcatalog.org in order to discover the obsolete relationship is
   a possible automatic way, it is not ideal.

2.6.  YANG Module Transition Strategy

   Let's assume for a moment that we change the YANG module, with the
   specific example of ietf-routing, which some propose to update to
   ietf-routing-2.

   Here are all the ietf-routing dependent YANG modules (those modules
   that depend on ietf-routing) <https://www.yangcatalog.org/yang-
   search/impact_analysis.php?modules[]=ietf-
   routing&recurse=0&rfcs=1&show_subm=1&show_dir=dependents>.  So many
   YANG modules.

   Let's look at the difference for ietf-routing-2:
   <https://www.yangcatalog.org/yang-search/
   impact_analysis.php?modules[]=ietf-routing-
   2&recurse=0&rfcs=1&show_subm=1&show_dir=dependents>.

   Changing the module name from ietf-routing to ietf-routing-2 implies
   that the we have to warn all draft authors of ietf-routing YANG
   dependent modules.  First, to make sure they are aware of ietf-
   routing-2 (publishing a RFC8022bis mentioning in the module
   description that this module is not compatible with the NMDA
   architecture, and providing a pointer to ietf-routing-2 ... is not an
   automatic way... so barely useful).  And second, to ask them to
   change their import (or service composition) to ietf-routing-2.
   Hopefully, in the ietf-routing case, most dependent YANG modules are
   part of the IETF, so the communication is a manageable.  For the
   already existing dependent vendor modules the problem is worse.

   Changing the ietf-interfaces YANG module name would be a different
   challenge, as it's used throughout the industry:
   <https://www.yangcatalog.org/yang-search/
   impact_analysis.php?modules[]=ietf-
   interfaces&recurse=0&rfcs=1&show_subm=1&show_dir=dependents>

2.7.  Need to Allow Non-Backward Compatible changes

   As described in the previous sections, there is a need to allow non-
   backward compatible changes without changing a module's name.  This
   would avoid many of the above problems.  In most cases even after
   non-backward compatible updates a module should keep its name.
   However, for really major changes renaming the module is still the
   proper way to go:




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      when splitting a module into two separate modules

      when removing 80% of a module's schema

      when a standard module is moved from one organization to another
      (e.g., from ietf to ieee)

      when a company's name is changed and new versions of the module
      are renamed to reflect that

   Allowing non-backward compatible changes to happen without a module
   name change will decrease the number of separate modules to handle
   and will make it a trivial task to track these non-backward
   compatible changes.

2.8.  Problematic Handling of Status Statement

   The current definition of deprecated and obsolete in [RFC7950] (as
   quoted below) is problematic and should be corrected.

   o  "deprecated" indicates an obsolete definition, but it permits new/
      continued implementation in order to foster interoperability with
      older/existing implementations.

   o  "obsolete" means that the definition is obsolete and SHOULD NOT be
      implemented and/or can be removed from implementations.

   YANG is considered an interface contract between the server and the
   client.  The current definitions of deprecated and obsolete mean that
   a schema node that is either deprecated or obsolete may or may not be
   implemented.  The client has no way to find out which is the case
   except for by trying to write or read data at the leaf in question.
   This probing would need to be done for each separate data-node, which
   not a trivial thing to do.  This "may or may not" is unacceptable in
   a contract.  In effect, this works as if there would be an if-feature
   statement on each deprecated schema node where the server does not
   advertise whether the feature is supported or not.  Why is it not
   advertised?

2.9.  No way to easily decide whether a change is Backward Compatible

   A management system, SDN controller or any other user of a module
   should be capable of easily determining the compatibility between two
   module versions.  Higher level logic for a network function,
   something that can not be implemented in a purely model driven way,
   is always dependent on a specific version of the module.  If the
   client finds that the module has been updated on the network node, it
   has to decide if it tries to handle it as it handled the previous



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   version of the model or if it just stops, to avoid problems.  To make
   this decision the client needs to know if the module was updated in a
   backward compatible way or not.

   This is not possible to decide today because of the following:

   o  It is possible to change the semantic behavior of a data node,
      action or rpc while the YANG definition does not change (with the
      possible exception of the description statement).  In such a case
      it is impossible to determine whether the change is backward
      compatible just by looking at the YANG statements.  Its only the
      human model designer that can decide.

   o  Problems with the status statement, Section 2.8

   o  Modelers might decide to violate YANG 1.1 update rules for some of
      the reasons above

   Finding status changes or violations of update rules need a line by
   line comparision of the old and new modules, no easy task.

2.10.  Early Warning about Removal

   If a schema part is considered old/bad we need to be able to give
   advance warning that it will be removed.  As this is an advance
   warning the part shall still be present and usable in the current
   revision; however, it will be removed in one of the next revisions.
   We need the advance warning to allow users of the module time enough
   to plan/execute migration away from the deprecated functionality.
   Often deprecation will be accompanied by information whether the
   functionality will just disappear or that there is an alternative,
   possibly more advanced solution that should be used.

   Vendors use such warnings often, but the NMDA related redesign of
   IETF modules is also an example where it would be useful.  (As
   another example see the usage of deprecated in the Java programing
   language.)  The current definition of the deprecated status does not
   serve this purpose as described in Section 2.8.  The definition of
   "deprecated" in the status statement shall be changed to address this
   issue.

3.  The Solution

   The solution is composed of four parts, a semantic versioning YANG
   extension, updates to the YANG 1.1. status statement and module
   update rules and the import by version statement.  An optional
   additional check, validating the semantic versioning from a syntactic
   point of view, can either assist in determining the correct semantic



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   versioning values, or can help in determining the values for YANG
   modules that don't support this extension.

3.1.  SEMVER Semantic Versioning

   The semantic versioning solution proposed here has already been
   proposed in [I-D.openconfig-netmod-model-catalog] (included here with
   the authors permission)which itself is based on [openconfigsemver].
   The goal is to indicate the YANG module backwards (in)compatibility,
   following semver.org semantic versioning [semver]:

   "The SEMVER version number for the module is introduced.  This is
   expressed as a semantic version number of the form: x.y.z

   o  x is the MAJOR version.  It is incremented when the new version of
      the specification is incompatible with previous versions.

   o  y is the MINOR version.  It is incremented when new functionality
      is added in a manner that is backward-compatible with previous
      versions.

   o  z is the PATCH level.  It is incremented when bug fixes are made
      in a backward-compatible manner.

   Along these lines, we propose the following YANG 1.1 extension for a
   more generic semantic version.  The formal definition is found at the
   end of this document.

           extension module-version {
               argument "semver" {
                   yin-element false;
               }
           }

   The extension would typically be used this way:
















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       module yang-module-name {

         namespace "name-space";
         prefix "prefix-name";

         import ietf-semver { prefix "semver"; }

         description
           "to be completed";


         revision 2017-10-30 {
           description
             "Change the module structure";
           semver:module-version "2.0.0";
         }

         revision 2017-07-30 {
           description
             "Added new feature XXX";
           semver:module-version "1.2.0";";
         }

         revision 2017-04-03 {
           description
             "Update copyright notice.";
           semver:module-version "1.0.1";;
         }

         revision 2017-04-03 {
           description
             "First release version.";
           semver:module-version "1.0.0";;
         }

         revision 2017-01-26 {
           description
             "Initial module for inet types";
           semver:module-version "0.1.0";;
         }


         //YANG module definition starts here

   See also "Semantic Versioning and Structure for IETF Specifications"
   [I-D.claise-semver] for a mechanism to combine the semantic
   versioning, the github tools, and a potential change to the IETF
   process.



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3.2.  Updates to YANG 1.1 status statement

   RFC 7950 section 11, must be updated to change the definition of
   deprecated and obsolete.  In both cases the client must be able to
   determine whether the relevant parts are implemented or not without
   probing.  The following definition is proposed:

   o  Deprecated schema nodes MUST still work as defined.  The
      deprecated status serves only as a a warning that the schema node
      will be removed or obsoleted in the future.

   o  Obsolete schema nodes MUST be removed from the implementation.
      Requests concerning these schema nodes MUST be rejected with:

      *  error-tag: operation-failed

      *  error-app-tag: obsolete element

   If there is a need to allow the server to decide whether a
   deprecated/obsolete schema part is implemented YANG already has a
   facility for that: the feature statement.  Use it!

3.3.  Updating the YANG 1.1 Module Update rules

   RFC 7950 section 11, must be updated to express:

   "As experience is gained with a module, it may be desirable to revise
   that module.  Changes to published modules are allowed, even if they
   have some potential to cause interoperability problems, if the
   module-version YANG extension is used in the revision statement to
   clearly indicate the nature of the change."

3.4.  The Derived Semantic Version

   The YANG catalog contains not only the most up-to-date YANG module
   revision of a given module, but keeps all previous revisions as well.
   With APIs in mind, it's important to understand whether different
   YANG module revisions are backward compatible (this is specifically
   imported for native YANG modules, i.e. the ones where generated-from
   = native), even for the YANG modules that don't support the YANG
   extension specified in this document.

   Two distinct leaves in the YANG module
   [I-D.clacla-netmod-model-catalog] contain this semver notation:

   o  the semantic-version leaf contains the value embedded within a
      YANG module (if it is available).




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   o  the derived-semantic-version leaf is established by examining the
      the YANG module themselves.  As such derived-semantic-version only
      takes syntax into account as opposed to the meaning of various
      elements when it computes the semantic version.

   o  The algorithm used to produce the derived-semantic-version is as
      follows:

      1.  Order all modules of the same name by revision from oldest to
          newest.  Include module revisions that are not available, but
          which are defined in the revision statements in one of the
          available module versions.

      2.  If module A, revision N+1 has failed compilation, bump its
          derived semantic MAJOR version.  For unavailable module
          versions assume non-backward compatible changes were done.,
          thus bump its derived semantic MAJOR version.

      3.  Else, run "pyang --check-update-from" on module A, revision N
          and revision N+1 to see if backward-incompatible changes
          exist.

      4.  If backward-incompatible changes exist, bump module A,
          revision N+1's derived MAJOR semantic version.

      5.  If no backward-incompatible changes exist, compare the pyang
          trees of module A, revision N and revision N+1.

      6.  If there are structural differences (e.g., new nodes), bump
          module A, revision N+1's derived MINOR semantic version.

      7.  If no structural differences exist, bump module A, revision
          N+1's derived PATCH semantic version.

   Note that the absolute numbers in the semantic-version and derived-
   semantic-version are actually meaningless by themselves.  That is,
   one must compare two different semver values for a given module to
   understand the compatibility between them.

3.5.  Import by Semantic Version

   If a module is imported by another one, it is usually not specified
   which version of the imported module should be used.  However not all
   versions may be acceptable.  Today YANG 1.1 allows us to specify the
   revision date of the imported module, but that is too specific, as
   even a small spelling correction of the imported module results in a
   change to its revision date, thus making the module revision
   ineligible for import.



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   Using semantic versioning to indicate the acceptable imported module
   versions is much more flexible.

   o  We might indicate that any compatible module-version after e.g.
      3.1.0 is acceptable

   o  We might indicate that any compatible module-version of the 3.1.0,
      4.0.0 or 5.0.0 major versions is acceptable.  Later depending on
      updates in the 6.0.0 series we might allow those revisions also to
      be imported.  As an non-backward compatible change in the 6.0.0
      line might just change a small part of the imported module, the
      non-backward compatible changes may or may not affect the
      importer.

   The module-version statement SHOULD be a substatement of the import
   statement.  An import statement MUST NOT contain both a module-
   version and a revision substatement.  The use of the revision
   substatement of import should be discouraged/deprecated.

4.  Open Issues

   There are a number of open issues to be disccused.  These include the
   following:

   o  Do we need include-by-Semver?

   o  Should IETF/IANA officially generate derived semantic versions for
      their own modules?  As they are the owner of the modules it should
      be their responsibility, but how to document it?

   o  There are cases where the module's name should be changed but we
      still want to formally document the connection between the old and
      the new module names.  For these cases shall we introduce a new
      YANG extension statement

   o  "replaces-module ietf-vlan;" ?

   o  We could consider a new naming convention for module files.
      Today, module files are named using a module@revision.yang
      notation.  We could consider a module%semver.yang variant.  Re-
      using the '@' for version is not ideal, so another separator
      character should be used.  In this manner, both version and
      revision could be used.

   o  Taking another page from Openconfig, the notion of a module bundle
      could be considered.  That is, there may need to be a way to
      enumerate modules that are part of a bundle and are known to




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      interoperate.  This may not be as critical if a rich import-by-
      version is defined.
      While the issue is interesting, it will be not be handled in this
      document.

   o  Similarly, the concept of a feature bundle should be considered.
      Typically, operators combine and test YANG modules to build value-
      add services.  These bundles form releases for specific features
      or services, and it is critical to ensure as the modules evolve,
      the bundles can coherently evolve with them.
      While the issue is interesting, it will be not be handled in this
      document.

   o  When we'll start using this new procedure for a new YANG module
      revision, will we have to update all the dependent YANG modules to
      start using this new procedure, along with the new import
      statement?  Is this a moot point, as a new YANG module name would
      suffer from the same symptoms?
      We see no need for updating other dependent modules.  It is a good
      idea to update them, as they will benefit from using SEMVER,
      however there is no specific need to update them.

5.  Semantic Version Extension YANG Module

   The extension described in this module is defined in the YANG module
   below.

<CODE BEGINS> file "ietf-semver@2017-12-15.yang"
  module ietf-semver {
    yang-version 1.1;
    namespace "urn:ietf:params:xml:ns:yang:ietf-semver";
    prefix semver;

    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:   Kevin D'Souza
                 <mailto:kd6913@att.com>




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       Author:   Balazs Lengyel
                 <mailto:balazs.lengyel@ericsson.com>";
    description
      "This module contains a definition for a YANG 1.1 extension to
       express the semantic version of YANG modules.";

    revision 2017-12-15 {
      description
        "Initial revision.";
      reference "draft-clacla-netmod-yang-model-update:
               New YANG Module Update Procedure";
      semver:module-semver 0.1.1;
    }

    extension module-version {
      argument semver;
      description
        "The version number for the module revision it is used in.
        This is expressed as a semantic version string in the form:
          x.y.z
         where:
          * x corresponds to the major version,
          * y corresponds to a minor version,
          * z corresponds to a patch version.

         A major version number of 0 indicates that this model is still
         in development, and is potentially subject to change.

         Following a release of major version 1, all modules will
         increment major revision number where backwards incompatible
         changes to the model are made.

         The minor version is changed when features are added to the
         model that do not impact current clients use of the model.
         When major version is stepped, the minor version is reset to 0.

         The patch-level version is incremented when non-feature changes
         (such as bugfixes or clarifications to human-readable
         descriptions that do not impact model functionality) are made
         that maintain backwards compatibility.
         When major or minor version is stepped, the patch-level is
         reset to 0.

         The version number is stored in the module meta-data.

         By comparing the module-version between two revisions of a
         given module, one can know if revision N+1 is backwards
         compatible or not relative to revision N, as well as



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         whether or not new features have been added to revision N+1.

         If a module contains this extension it indicates that for this
         module the updated status and update rules as this described in
         RFC XXXX are used.

         The statement MUST only be a substatement of the revision,
         import or include statements.
         Zero or One module-version statement is allowed per parent
         statement. NO substatements are allowed.
         ";

      reference "http://semver.org/ : Semantic Versioning 2.0.0";
    }

    augment /yanglib:modules-state/yanglib:module {
        leaf module-version {
            type string {
                pattern "[0-9]+.[0-9]+.[0-9]+";
            }
        }
    }

    augment /yanglib:modules-state/yanglib:module/yanglib:submodule {
        leaf submodule-version {
            type string {
                pattern "[0-9]+.[0-9]+.[0-9]+";
            }
        }
    }

  }
<CODE ENDS>

6.  Contributors

   o  Anees Shaikh, Google

   o  Rob Shakir, Google

7.  Security Considerations

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







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8.  IANA Considerations

   No IANA action is requested.

9.  References

9.1.  Normative References

   [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>.

9.2.  Informative References

   [I-D.clacla-netmod-model-catalog]
              Clarke, J. and B. Claise, "YANG module for
              yangcatalog.org", draft-clacla-netmod-model-catalog-02
              (work in progress), October 2017.

   [I-D.claise-semver]
              Claise, B., Barnes, R., and J. Clarke, "Semantic
              Versioning and Structure for IETF Specifications", draft-
              claise-semver-01 (work in progress), July 2017.

   [I-D.openconfig-netmod-model-catalog]
              Shaikh, A., Shakir, R., and K. D'Souza, "Catalog and
              registry for YANG models", draft-openconfig-netmod-model-
              catalog-02 (work in progress), March 2017.

   [I-D.wu-l3sm-rfc8049bis]
              Wu, Q., Litkowski, S., Tomotaki, L., and K. Ogaki, "YANG
              Data Model for L3VPN Service Delivery", draft-wu-l3sm-
              rfc8049bis-11 (work in progress), December 2017.

   [openconfigsemver]
              "Semantic Versioning for Openconfig Models",
              <http://www.openconfig.net/docs/semver/>.

   [RFC8049]  Litkowski, S., Tomotaki, L., and K. Ogaki, "YANG Data
              Model for L3VPN Service Delivery", RFC 8049,
              DOI 10.17487/RFC8049, February 2017, <https://www.rfc-
              editor.org/info/rfc8049>.

   [RFC8199]  Bogdanovic, D., Claise, B., and C. Moberg, "YANG Module
              Classification", RFC 8199, DOI 10.17487/RFC8199, July
              2017, <https://www.rfc-editor.org/info/rfc8199>.

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



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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
   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


   Balazs Lengyel
   Ericsson
   Magyar Tudosok Korutja
   1117 Budapest
   Hungary

   Phone: +36-70-330-7909
   Email: balazs.lengyel@ericsson.com


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

   Email: kd6913@att.com












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