Network Working Group                                     R. Wilton, Ed.
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Informational                         February 19, 2020
Expires: August 22, 2020

                   YANG Versioning Solution Overview


   This document gives an overview of the different documents that
   comprise a full solution to the YANG versioning requirements
   document.  The purpose of this document is to help readers understand
   how the discrete parts of the YANG versioning solution fit together
   during working group development of the solution documents.

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   This Internet-Draft will expire on August 22, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Solution Documents  . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Updated YANG Module Revision Handling . . . . . . . . . .   3
     2.2.  YANG Semantic Versioning  . . . . . . . . . . . . . . . .   4
     2.3.  Versioned YANG packages . . . . . . . . . . . . . . . . .   4
     2.4.  Dynamic YANG schema selection . . . . . . . . . . . . . .   5
     2.5.  YANG Schema Comparison  . . . . . . . . . . . . . . . . .   6
   3.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   [I-D.ietf-netmod-yang-versioning-reqs] documents the requirements for
   any solution to the YANG [RFC7950] versioning problem.  In
   particular, chapter 5 lists the formal requirements that a solution

   The complete solution to all of the YANG versioning requirements is
   comprised of five documents, each addressing different aspects of the
   solution.  These documents are:

   1.  [I-D.verdt-netmod-yang-module-versioning]

   2.  [I-D.verdt-netmod-yang-semver]

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

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

   5.  [I-D.verdt-netmod-yang-schema-comparison]

   The aim of this document is to help readers understand how these
   different solution documents fit together, and also which documents
   contribute solutions that address particular individual requirements.

   Open issues, across all of the solution documents are tracked at

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2.  Solution Documents

2.1.  Updated YANG Module Revision Handling

   In summary, [I-D.verdt-netmod-yang-module-versioning] specifies
   minimal extensions and updates to the YANG language, YANG Library,
   and YANG author guidelines to provide more flexible YANG module
   revision handling.  The intent is that these changes and extensions
   could be folded into future revisions of the updated specifications.
   The document provides a base solution for all requirements except Req
   2.2, Req 3.1 and Req 3.2.

   The extensions and changes in the document can be summarized thus:

   o  It defines a YANG extension statement to indicate where non-
      backwards-compatible changes have occurred in a module's revision

   o  It relaxes the rules for the module revision history to allow for
      a non-linear module revision history.  I.e., any given module
      revision may have multiple revisions directly derived from it.

   o  It defines a new import extension statement that restricts the
      allowed module revisions that satisfy the import to only those
      derived from a specified module revision.

   o  It defines a revision label extension statement to allow an
      informative name to be associated with a particular revision date,
      and to be used in import statements, YANG module filenames, and is
      available in YANG library.  One example of how the revision label
      could be used is to associate a semantic versioning scheme to YANG
      module revisions.

   o  It updates the YANG rules for changes between module revisions
      that are allowed to be classified as backwards-compatible.  In
      particular, marking a node as obsolete is no longer classified as
      a backwards compatible change.

   o  It provides updated guidance on how servers handle deprecated and
      obsolete YANG nodes and augments YANG library with additional
      leaves to report the server's behavior to clients.

   o  It provides an extension statement to allow a description
      statement to be associated with a YANG status statement, providing
      more information about why the status has changed.

   o  It defines how versioning relates to YANG instance data.

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   o  It refines the guidelines for updating modules, taking into
      consideration that non-backwards-compatible changes are sometimes
      necessary for various pragmatic reasons.

2.2.  YANG Semantic Versioning

   [I-D.verdt-netmod-yang-semver] defines a semantic versioning scheme,
   derived from the 2.0.0 specification, that can be used in
   conjunction with the revision label extension statement defined in
   Section 2.1 to allow semantic version numbers to be used to manage
   the revision lifecycle of YANG modules and other related YANG assets,
   e.g., YANG packages.  This document provides an enhanced solution for
   Req 2.1, but organizations authoring modules are not obliged to use
   this specific versioning scheme, and could choose a different
   overlaid versioning scheme, or none at all and rely solely on
   revision dates.

   The aims of the YANG semantic versioning scheme are:

   o  to generally allow clients to determine whether NBC changes have
      occurred between two revisions from the version number alone,
      without having to check the full revision history;

   o  to give a more informative identifier for a branched revision
      history over revision dates alone;

   o  to allow revision branches that contain fixes for published non-
      latest releases.

2.3.  Versioned YANG packages

   The two previous solution documents primarily address version and
   revision management of individual modules.
   [I-D.rwilton-netmod-yang-packages] provides a mechanism to group sets
   of related YANG modules revisions together, into constructs called
   YANG packages, and to apply a versioning scheme to the groups.

   The core part of this document are YANG module definitions that
   define a YANG package.  The definitions are used as an augmentation
   to YANG library and also in YANG instance data documents for offline

   The principle aims of YANG packages are:

      To define an efficient hierarchical structure that can precisely
      specify a YANG schema.

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      To provide an simple alternative mechanism to manage conformance
      of modules.  Rather than checking conformance against a set of
      individual YANG module revisions and enabled features, it should
      be easier to check for conformance against a much smaller set of
      YANG package versions.

      To provide a more efficient mechanism for servers to share
      conformance information with clients.  Rather that downloading and
      comparing all individual module revisions and features via YANG
      library, the client can just check whether the package version is
      compatible instead.  The package definition could be retrieved and
      cached from multiple sources.

      To define constructs that can be used for YANG schema selection.

   Although the YANG packages document does not satisfy any versioning
   requirements directly, it provides foundational building blocks for
   the schema selection solution, described in Section 2.4, that does
   address two of the requirements.

2.4.  Dynamic YANG schema selection

   [I-D.wilton-netmod-yang-ver-selection] specifies a solution for
   requirements 3.1 and 3.2 via the use of
   [I-D.rwilton-netmod-yang-packages] and a model and protocol based
   schema selection scheme that can be used by clients to choose which
   schemas to use when interacting with the device from the available
   schema that are supported and advertised by the server.

   The dynamic YANG schema selection solution:

      allows servers to define named 'schema-sets' which specify the
      schema for each supported datastore via references to YANG

      can support clients choosing a single default schema-set (from
      those advertised by the server) that is used for all NETCONF/
      RESTCONF protocol sessions;

      can support clients enabling multiple compatible secondary schema-
      sets that can be used on separate NETCONF/RESTCONF protocol

      can support clients configuring named custom schema-sets that can
      be selected as default or secondary schema-sets;

      can support different module versions via placing them in
      different schema-sets;

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      can support different schema families (e.g., IETF YANG modules ,
      native vendor, or OpenConfig);

      allows considerable freedom in the schema selection capabilities
      that servers choose to support.

2.5.  YANG Schema Comparison

   The final piece of the solution jigsaw is a document that describes
   how to algorithmically compare YANG schema, addressing Req 2.2.

   [I-D.verdt-netmod-yang-schema-comparison] specifies an algorithm that
   can be used to compare two revisions of a YANG schema to determine
   the overall scope of the changes, and a list of the specific changes,
   between the two revisions.

   The YANG Schema Comparison solution:

      defines a algorithm for comparing two YANG schema, identifying the
      differences and classifying them as backwards-compatible, non-
      backwards-compatible or editorial;

      can be used to compare individual YANG module revisions;

      can be used to compare YANG schema defined using YANG packages;

      can filter the comparison output to the subset of the schema nodes
      that are of interest, providing a more precise answer for clients
      to determine whether they would likely be affected when upgrading
      between two schema versions;

      defines YANG extensions to improve the accuracy of the comparison
      algorithm by explicitly annotating the type of change to
      statements within a YANG module, for use where the type of change
      would otherwise be ambiguous to a simple programmatic comparison

3.  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 that design team and have contributed to defining the
   problem, specifying the requirements, and working on a solution:

   o  Balazs Lengyel

   o  Benoit Claise

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   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  Susan Hares

   o  Wu Bo

4.  Security Considerations

   The document does not define any new protocol or data model.  There
   is no security impact.

5.  IANA Considerations


6.  References

6.1.  Normative References

              Clarke, J., "YANG Module Versioning Requirements", draft-
              ietf-netmod-yang-versioning-reqs-02 (work in progress),
              December 2019.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,

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6.2.  Informative References

              Wilton, R., "YANG Packages", draft-rwilton-netmod-yang-
              packages-02 (work in progress), October 2019.

              Claise, B., Clarke, J., Rahman, R., Wilton, R., Lengyel,
              B., Sterne, J., and K. D'Souza, "Updated YANG Module
              Revision Handling", draft-verdt-netmod-yang-module-
              versioning-01 (work in progress), October 2019.

              Wilton, R., "YANG Schema Comparison", draft-verdt-netmod-
              yang-schema-comparison-00 (work in progress), November

              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.

              Wilton, R., Rahman, R., and J. Clarke, "YANG Schema
              Version Selection", draft-wilton-netmod-yang-ver-
              selection-01 (work in progress), November 2019.

Author's Address

   Robert Wilton (editor)
   Cisco Systems, Inc.


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