New YANG Module Update Procedure
draft-clacla-netmod-yang-model-update-00

Abstract

This document specifies a new YANG module update procedure in case of non backward-compatible 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.

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This Internet-Draft will expire on May 3, 2018.

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. The Problems
• 2.1. Slow Standardization
• 2.2. Some YANG Modules are Not Backward Compatible
• 2.3. A Zoo of YANG Modules
• 2.4. YANG Modules Obsolete Relationship
• 2.5. YANG Module Transition Strategy
• 3. The Solution
• 3.1. SEMVER Semantic Versioning
• 3.2. Updating the YANG 1.1 Specifications
• 3.3. The Derived Semantic Version
• 4. Contributors
• 5. Security Considerations
• 6. IANA Considerations
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Authors' Addresses

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 for this document:

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" 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."

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

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 day one (at least the structure), which in turns, might explain why all the IETF YANG modules take so long to standardize. Shooting for perfection (at least the 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 non backward-compatible YANG modules. An example is the YANG data model for L3VPN service delivery [RFC8049], which, based on implementation experience, must be updated in a non backward-compatible way with draft-wu-l3sm-rfc8049bis [I-D.wu-l3sm-rfc8049bis].

While Standardized Development Organization YANG modules are obviously better for the industry, we must recognize that many YANG modules are actually generated YANG modules (for example, from internal database), 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 YANG modules are.

2.3. A Zoo of YANG Modules

Even if we focus on the IETF, we have to observe that many Standard Development Organizations, opensource fora, and vendors develop YANG modules develop YANG modules. This should be considered a success for an IETF developed technoogy. 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.

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/contribe.php>.

The issue is actually the number of YANG modules the operators are offered. At the time of writing these lines, the number of unique YANG modules in the catalog is exactly 2596 (and the 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.

2.4. 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 discovers that a 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";
       

Along the same lines, going through an offline tool such as the yangcatalog.org, in order to discover the obsolete relationship is a possible automatic way, but not ideal.

2.5. YANG Module Transition Strategy

Let's assume for a moment that we change the YANG module, with the specific example of the 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. Firstly, 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 secondly, to ask them to change their import (or service composition) to ietf-routing-2. Hopefully, in the ietf-routing case, all dependent YANG modules are part of the IETF, so the communication is a manageable.

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>

3. The Solution

The solution is composed of two mandator aspects, a semantic versioning YANG extension and an update to RFC7950. An optional additional check, validating the semantic versioning from a syntact point of view, can either assist in determining the correct semantic 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 as already been proposed in [I-D.openconfig-netmod-model-catalog] (cut/paste 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]:

• MAJOR is incremented when the new version of the specification is incompatible with previous versions.
• MINOR is incremented when new functionality is added in a manner that is backward-compatible with previous versions.
• PATCH is incremented when bug fixes are made in a backward-compatible manner.

    // extension statements
     extension openconfig-version {
       argument "semver" {
         yin-element false;
       }
       description
         "The OpenConfig version number for the module. This is
         expressed as a semantic version number of the form:
           x.y.z
         where:
           * x corresponds to the major version,
           * y corresponds to a minor version,
           * z corresponds to a patch version.
         This version corresponds to the model file within which it is
         defined, and does not cover the whole set of OpenConfig models.
         Where several modules are used to build up a single block of
         functionality, the same module version is specified across each
         file that makes up the module.

         A major version number of 0 indicates that this model is still
         in development (whether within OpenConfig or with industry
         partners), 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.

         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.

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

The extension would typically be used this way:

    module yang-module-name {

      yang-version "1";
      namespace "name-space";
      prefix "prefix-name";

      import openconfig-extensions { prefix "oc-ext"; }

      organization
        "to be completed";

      contact
        "to be completed";

      description
        "to be completed";

      oc-ext:openconfig-version "1.1.2";
      
      revision 2017-10-30 {
        description
          "Change the module structure";
        reference "1.1.2";
      }

      revision 2017-07-30 {
        description
          "Fixed unprintable character";
        reference "0.1.2";
      }

      revision 2017-04-03 {
        description
          "Update copyright notice.";
        reference "0.1.1";
      }

      revision 2017-01-26 {
        description
          "Initial module for inet types";
        reference "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.

3.2. Updating the YANG 1.1 Specifications

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, at the condition that the semantic versioning change are clearly indicated based on the SEMVER YANG extension."

3.3. 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] contains this semver semantic:

• the semantic-version leaf contains the value reported as metadata by a specific YANG module.
• the derived-semantic-version leaf is established by examining the the YANG module themselves. As such, only the YANG syntax, as opposed to the implementation changes that lead some some semantic changes.

The derived-semantic-version is based on "pyang --check-update-from", which checks the backward compatibility from a syntax point of view.

Note that the absolute numbers in the semantic-version and derived-semantic-version are actually meaningless: the difference between two YANG module semver fields should be looked at.

4. Contributors

Anees Shaikh, Google

5. Security Considerations

To be completed

6. IANA Considerations

No IANA action is requested.

7. References

7.1. Normative References

7.2. Informative References

Authors' Addresses