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YANG Module Classification

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8199.
Authors Dean Bogdanović , Benoît Claise , Carl Moberg
Last updated 2019-12-03 (Latest revision 2017-06-13)
Replaces draft-bogdanovic-netmod-yang-model-classification
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Informational
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Lou Berger
Shepherd write-up Show Last changed 2017-04-24
IESG IESG state Became RFC 8199 (Informational)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Warren "Ace" Kumari
Send notices to "Lou Berger" <>
IANA IANA review state Version Changed - Review Needed
IANA action state No IANA Actions
NETMOD                                                     D. Bogdanovic
Internet-Draft                                      Volta Networks, Inc.
Intended status: Informational                                 B. Claise
Expires: December 15, 2017                                     C. Moberg
                                                     Cisco Systems, Inc.
                                                           June 13, 2017

                       YANG Module Classification


   The YANG data modeling language is currently being considered for a
   wide variety of applications throughout the networking industry at
   large.  Many standards development organizations (SDOs), open source
   software projects, vendors and users are using YANG to develop and
   publish YANG modules for a wide variety of applications.  At the same
   time, there is currently no well-known terminology to categorize
   various types of YANG modules.

   A consistent terminology would help with the categorization of YANG
   modules, assist in the analysis of the YANG data modeling efforts in
   the IETF and other organizations, and bring clarity to the YANG-
   related discussions between the different groups.

   This document describes a set of concepts and associated terms to
   support consistent classification of YANG modules.

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

   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 December 15, 2017.

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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
   ( 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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  First Dimension: YANG Module Abstraction Layers . . . . . . .   4
     2.1.  Network Service YANG Modules  . . . . . . . . . . . . . .   6
     2.2.  Network Element YANG Modules  . . . . . . . . . . . . . .   7
   3.  Second Dimension: Module Origin Types . . . . . . . . . . . .   7
     3.1.  Standard YANG Modules . . . . . . . . . . . . . . . . . .   8
     3.2.  Vendor-specific YANG Modules and Extensions . . . . . . .   8
     3.3.  User-specific YANG Modules and Extensions . . . . . . . .   9
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Change log [RFC Editor: Please remove]  . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The Internet Engineering Steering Group (IESG) has been actively
   encouraging IETF working groups to use the YANG data modeling
   language [RFC7950], [RFC7950] and NETCONF protocol [RFC6241] for
   configuration management purposes, especially in new working group
   charters [Writable-MIB-Module-IESG-Statement].

   YANG is also gaining wide acceptance as the de-facto standard data
   modeling language in the broader industry.  This extends beyond the
   IETF, including many standards development organizations, industry
   consortia, ad hoc groups, open source projects, vendors, and end-

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   There are currently no clear guidelines on how to classify the
   layering of YANG modules according to abstraction, or how to classify
   modules along the continuum spanning formal standards publications,
   vendor-specific modules and modules provided by end-users.

   This document presents a set of concepts and terms to form a useful
   taxonomy for consistent classification of YANG modules in two

   o  The layering of modules based on their abstraction levels

   o  The module origin type based on the nature and intent of the

   The intent of this document is to provide a taxonomy to simplify
   human communication around YANG modules.  While the classification
   boundaries are at times blurry, this document should provide a robust
   starting point as the YANG community gains further experience with
   designing and deploying modules.  To be more explicit, it is expected
   that the classification criteria will change over time.

   A number of modules have created substantial discussion during the
   development of this document: for examples, modules concerned with
   topologies.  Topology modules are useful both on the Network Element
   level (e.g. link-state database content) as well as on the Network
   Service level (e.g. network-wide, configured topologies).  In the
   end, it is the module developer that classifies the module according
   to the initial intent of the module content.

   This document should provide benefits to multiple audiences:

   o  First, a common taxonomy helps with the different standards
      development organizations and industry consortia discussions,
      whose goals are determined in their respective areas of work.

   o  Second, operators might look at the YANG module abstraction layers
      to understand which Network Service YANG modules and Network
      Element YANG modules are available for their service composition.
      It is difficult to determine the module type without inspecting
      the YANG module itself.  The YANG module name might provide some
      useful information but is not a definite answer.  For example, an
      L2VPN YANG module might be a Network Service YANG module, ready to
      be used as a service model by a network operator.  Alternatively,
      it might be a Network Element YANG module that contains the L2VPN
      data definitions required to be configured on a single device.

   o  And thirdly, this taxonomy would help equipment vendors (whether
      physical or virtual), controller vendors, orchestrator vendors to

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      explain to their customers the relationship between the different
      YANG modules they support in their products.

1.1.  Terminology

   [RFC7950] specifies:

   o  data model: A data model describes how data is represented and

   o  module: A YANG module defines hierarchies of schema nodes.  With
      its definitions and the definitions it imports or includes from
      elsewhere, a module is self-contained and "compilable".

2.  First Dimension: YANG Module Abstraction Layers

   Module developers have taken two approaches to developing YANG
   modules: top-down and bottom-up.  The top-down approach starts with
   high level abstractions modeling business or customer requirements
   and maps them to specific networking technologies.  The bottom-up
   approach starts with fundamental networking technologies and maps
   them into more abstract constructs.

   There are currently no specific requirements on, or well-defined best
   practices around the development of YANG modules.  This document
   considers both bottom-up and top-down approaches as they are both
   used and they each provide benefits that appeal to different groups.

   For layering purposes, this document suggests the classification of
   YANG modules into two distinct abstraction layers:

   o  Network Element YANG Modules describe the configuration, state
      data, operations and notifications of specific device-centric
      technologies or features

   o  Network Service YANG Modules describe the configuration, state
      data, operations and notifications of abstract representations of
      services implemented on one or multiple network elements

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                        |  Operations and Business |
                        |      Support Systems     |
                        |        (OSS/BSS)         |

        - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
        Network Service YANG Modules

             +------------+      +-------------+      +-------------+
             |            |      |             |      |             |
             |  - L2VPN   |      |   - L2VPN   |      |    L3VPN    |
             |  - VPWS    |      |   - VPLS    |      |             |
             |            |      |             |      |             |
             +------------+      +-------------+      +-------------+

        - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
        Network Element YANG Modules

        +------------+  +------------+  +-------------+  +------------+
        |            |  |            |  |             |  |            |
        |    MPLS    |  |    BGP     |  | IPv4 / IPv6 |  |  Ethernet  |
        |            |  |            |  |             |  |            |
        +------------+  +------------+  +-------------+  +------------+

          L2VPN: Layer 2 Virtual Private Network
          L3VPN: Layer 3 Virtual Private Network
          VPWS: Virtual Private Wire Service
          VPLS: Virtual Private LAN Service

                 Figure 1: YANG Module Abstraction Layers

   Figure 1 illustrates the application of YANG modules at different
   layers of abstraction.  Layering of modules allows for reusability of
   existing lower layer modules by higher level modules while limiting
   duplication of features across layers.

   For module developers, per-layer modeling allows for separation of
   concern across editing teams focusing on specific areas.

   As an example, experience from the IETF shows that creating useful
   Network Element YANG modules for e.g. routing or switching protocols
   requires teams that include developers with experience of
   implementing those protocols.

   On the other hand, Network Service YANG modules are best developed by
   network operators experienced in defining network services for

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   consumption by programmers developing e.g. flow-through provisioning
   systems or self-service portals.

2.1.  Network Service YANG Modules

   Network Service YANG Modules describe the characteristics of a
   service, as agreed upon with consumers of that service.  That is, a
   service module does not expose the detailed configuration parameters
   of all participating network elements and features, but describes an
   abstract model that allows instances of the service to be decomposed
   into instance data according to the Network Element YANG Modules of
   the participating network elements.  The service-to-element
   decomposition is a separate process with details depending on how the
   network operator chooses to realize the service.  For the purpose of
   this document, the term "orchestrator" is used to describe to
   describe a system implementing such a process.

   Network Service YANG Modules define service models to be consumed by
   external systems.  External systems can be provisioning systems,
   service orchestrators, Operations Support Systems, Business Support
   Systems and applications exposed to network service consumers, being
   either internal network operations people or external customers.
   These modules are commonly designed, developed and deployed by
   network infrastructure teams.

   YANG allows for different design patterns to describe network
   services, ranging from monolithic to component-based approaches.

   The monolithic approach captures the entire service in a single
   module and does not put focus on reusability of internal data
   definitions and groupings.  The monolithic approach has the
   advantages of single-purpose development including development speed
   at the expense of reusability.

   The component-based approach captures device-centric features (e.g.
   the definition of a VPN Routing and Forwarding (VRF), routing
   protocols, or packet filtering) in a vendor-independent manner.  The
   components are designed for reuse across many service modules.  The
   set of components required for a specific service is then composed
   into the higher-level service.  The component-based approach has the
   advantages of modular development including a higher degree of
   reusability at the expense of initial development speed.

   As an example, an L2VPN service can be built on many different types
   of transport network technologies, including e.g.  MPLS or carrier
   ethernet.  A component-based approach would allow for reuse of e.g.
   User-Network Interface (UNI) definitions independent of the
   underlying transport network (e.g.  MEF UNI interface or MPLS

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   interface).  The monolithic approach would assume a specific set of
   transport technologies and interface definitions.

   An example of a Network Service YANG module is in [RFC8049].  It
   provides an abstract model for Layer 3 IP VPN service configuration.
   This module includes e.g. the concept of a 'site-network-access' to
   represent bearer and connection parameters.  An orchestrator receives
   operations on service instances according to the service module and
   decomposes the data into configuration data according to specific
   Network Element YANG Modules to configure the participating network
   elements to the service.  In the case of the L3VPN module, this would
   include translating the 'site-network-access' parameters to the
   appropriate parameters in the Network Element YANG Module implemented
   on the constituent elements.

2.2.  Network Element YANG Modules

   Network Element YANG Modules describe the characteristics of a
   network device as defined by the vendor of that device.  The modules
   are commonly structured around features of the device, e.g. interface
   configuration [RFC7223], OSPF configuration [I-D.ietf-ospf-yang], and
   firewall rules definitions [I-D.ietf-netmod-acl-model].

   The module provides a coherent data model representation of the
   software environment consisting of the operating system and
   applications running on the device.  The decomposition, ordering, and
   execution of changes to the operating system and application
   configuration is the task of the agent that implements the module.

3.  Second Dimension: Module Origin Types

   This document suggests classifying YANG module origin types as
   standard YANG modules, vendor-specific YANG modules and extensions,
   or user-specific YANG modules and extensions

   The suggested classification applies to both Network Element YANG
   Modules and Network Service YANG Modules.

   It is to be expected that real-world implementations of both Network
   Service YANG Modules and Network Element YANG Modules will include a
   mix of all three module origin types.

   Figure 2 illustrates the relationship between the three types of

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  |     User     |
  |   Extensions |
  +------+-------+  +--------------+  +--------------+
  |   Vendor     |  |     User     |  |     User     |
  |  Extensions  |  |  Extensions  |  |  Extensions  |
  +------+-------+  +------+-------+  +------+-------+
      Augments          Augments          Augments
  +------+-----------------+-------+  +------+-------+  +--------------+
  |            Standard            |  |    Vendor    |  |    User      |
  |            Modules             |  |    Modules   |  |   Modules    |
  +--------------------------------+  +--------------+  +--------------+

                    Figure 2: YANG Module Origin Types

3.1.  Standard YANG Modules

   Standard YANG Modules are published by standards development
   organizations (SDOs).  Most SDOs create specifications according to a
   formal process in order to produce a standard that is useful for
   their constituencies.

   The lifecycle of these modules is driven by the editing cycle of the
   specification and not tied to a specific implementation.

   Examples of SDOs in the networking industry are the IETF and the

3.2.  Vendor-specific YANG Modules and Extensions

   Vendor-specific YANG Modules are developed by organizations with the
   intent to support a specific set of implementations under control of
   that organization.  For example vendors of virtual or physical
   equipment, industry consortia, and opensource projects.  The intent
   of these modules range from providing openly published YANG modules
   that may eventually be contributed back to, or adopted by, an SDO, to
   strictly internal YANG modules not intended for external consumption.

   The lifecycle of these modules are generally aligned with the release
   cycle of the product or open source software project deliverables.

   It is worth noting that there is an increasing amount of interaction
   between open source projects and SDOs in the networking industry.
   This includes open source projects implementing published standards
   as well as open source projects contributing content to SDO

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   Vendors also develop Vendor-specific Extensions to standard modules
   using YANG constructs for extending data definitions of previously
   published modules.  This is done using the 'augment' statement that
   allows locally defined data trees to be added into locations in
   externally defined data trees.

   Vendors use this to extend standard modules to cover the full scope
   of features in implementations, which commonly is broader than that
   covered by the standard module.

3.3.  User-specific YANG Modules and Extensions

   User-specific YANG Modules are developed by organizations that
   operate YANG-based infrastructure including devices and
   orchestrators.  For example, network administrators in enterprises,
   or at service providers.  The intent of these modules is to express
   the specific needs for a certain implementation, above and beyond
   what is provided by vendors.

   This module type obviously requires the infrastructure to support the
   introduction of user-provided modules and extensions.  This would
   include the ability to describe the service-to-network decomposition
   in orchestrators and the module to configuration decomposition in

   The lifecycles of these modules are generally aligned with the change
   cadence of the infrastructure.

4.  Security Considerations

   This document doesn't have any Security Considerations.

5.  IANA Considerations

   This document has no IANA actions.

6.  Acknowledgements

   Thanks to David Ball and Jonathen Hansford for feedback and

7.  Change log [RFC Editor: Please remove]

   version 00: Renamed and small fixes based on WG feedback.

   version 01: Language fixes, collapsing of vendor data models and
   extensions, and the introduction of user data models and extensions.

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   version 02: Updated the YANG Module Catalog section, terminology
   alignment (YANG data model versus YANG module), explain better the
   distinction between the Network Element and Service YANG data models
   even if sometimes there are grey areas, editorial pass.  Changed the
   use of the term 'model' to 'module' to be better aligned with

   version 06: updates based on comments from Adrian Farrel about YANG
   Data Model for L3VPN Service Delivery.

   version 07: updates based on comments from Pete Resnick

8.  References

8.1.  Normative References

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,

   [RFC7223]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,

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

   [RFC8049]  Litkowski, S., Tomotaki, L., and K. Ogaki, "YANG Data
              Model for L3VPN Service Delivery", RFC 8049,
              DOI 10.17487/RFC8049, February 2017,

8.2.  Informative References

              Bogdanovic, D., Koushik, K., Huang, L., and D. Blair,
              "Network Access Control List (ACL) YANG Data Model",
              draft-ietf-netmod-acl-model-10 (work in progress), March

              Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem,
              "Yang Data Model for OSPF Protocol", draft-ietf-ospf-
              yang-07 (work in progress), March 2017.

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              "Writable MIB Module IESG Statement",

Authors' Addresses

   Dean Bogdanovic
   Volta Networks, Inc.


   Benoit Claise
   Cisco Systems, Inc.
   De Kleetlaan 6a b1
   1831 Diegem

   Phone: +32 2 704 5622

   Carl Moberg
   Cisco Systems, Inc.


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