NETMOD                                                     D. Bogdanovic
Internet-Draft                                      Volta Networks, Inc.
Intended status: Informational                                 B. Claise
Expires: April 2, 2017                                         C. Moberg
                                                     Cisco Systems, Inc.
                                                      September 29, 2016

                       YANG Module Classification


   The YANG [RFC6020] data modeling language is currently being
   considered for a wide variety of applications throughout the
   networking industry at large.  Many standards-defining 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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on April 2, 2017.

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Copyright Notice

   Copyright (c) 2016 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
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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
     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 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.  Adding The Classification Type to YANG Module Catalogs  . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Change log [RFC Editor: Please remove]  . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   The Internet Engineering Steering Group (IESG) has been actively
   encouraging IETF working groups to use the YANG modeling language
   [RFC6020], [RFC7950] and NETCONF protocol [RFC6241] for configuration
   management purposes, especially in new working group charters

   YANG is also gaining wide acceptance as the de-facto standard
   modeling language in the broader industry.  This extends beyond the
   IETF, including many standards development organizations, industry

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   consortia, ad hoc groups, open source projects, vendors, and end-

   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 type of module based on the nature and intent of the content

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

   A number of module types have created substantial discussion during
   the development of this document including those 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 classification
      type 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 by the operators.  Alternatively, it might be a Network
      Element YANG module that contains the L2VPN data definitions
      required to be configured on a single device.

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   o  And thirdly, this taxonomy would help equipment vendors (whether
      physical or virtual), controller vendors, orchestrator vendors to
      explain to their customers the relationship between the different
      YANG modules they propose in their products.  See Figure 1.

1.1.  Terminology

   [RFC7950] specifies:

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

   o  module: A YANG module defines a hierarchy of nodes that can be
      used for NETCONF-based operations.  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.  For the purpose of
   this document we assume that both approaches (bottom-up and top-down)
   will be used as they both provide benefits that appeal to different

   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 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 we will use the term "orchestrator" to describe a
   system implementing such a process.

   As an example, the Network Service YANG Module defined in
   [YANG-Data-Model-for-L3VPN-service-delivery] 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 specific Network Element YANG Modules to configure the
   participating network elements to perform the intent of 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.

   Network Service YANG Modules define service models to be consumed by
   external systems.  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 speed at the
   expense of reusability.

   The component-based approach captures device-centric features (e.g.
   the definition of a 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

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   including a higher degree of reusability at the expense of initial

   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.
   UNI-interface definitions independent of the underlying transport
   network (e.g.  MEF UNI interface or MPLS interface).  The monolithic
   approach would assume a specific set of transport technologies and
   interface definitions.

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 Types

   This document suggests classifying YANG module 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 types of modules.

   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 Types

3.1.  Standard YANG Modules

   Standard YANG Modules are published by standards-defining
   organizations (SDOs).  While there is no formal definition of what
   construes an SDO, a common feature is that they publish
   specifications along specific processes with content that reflects
   some sort of membership consensus.  The specifications are developed
   for wide use among the membership or for audiences beyond that.

   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, the IEEE
   and the MEF.

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

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   as well as open source projects contributing content to SDO

   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 augmented 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 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.  Adding The Classification Type to YANG Module Catalogs

   The suggested classification in this document would be an useful
   information in a catalog of YANG modules.  Such a catalog allows for
   easy lookup and reusability of YANG modules.  Practically, the YANG
   module classification type would be an additional leaf to a YANG
   module specified in [I-D.openconfig-netmod-model-catalog]:

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                leaf module-class{
                  type enum {
                     "Categorization of the YANG module based on

   Note: this leaf should actually be moved to
   [I-D.openconfig-netmod-model-catalog].  Note2: since a YANG module
   can belong to both service and device, the ENUM is not appropriate.
   An extensible list of module type is more appropriate.

   Indeed, without inspecting the YANG module itself, it's difficult to
   determine whether its type is a network service or a network element.
   The YANG module name might provide some useful information but is not
   a definitive answer.

5.  Security Considerations

   This document doesn't have any Security Considerations.

6.  IANA Considerations

   This document has no IANA actions.

7.  Acknowledgements

   Thanks to David Ball and David Hansford for feedback and suggestions.

8.  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.

   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

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9.  References

9.1.  Normative References

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,

   [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,

9.2.  Informative References

              Bogdanovic, D., Koushik, K., Huang, L., and D. Blair,
              "Network Access Control List (ACL) YANG Data Model",
              draft-ietf-netmod-acl-model-08 (work in progress), July

              Yeung, D., Qu, Y., Zhang, Z., Bogdanovic, D., and K.
              Koushik, "Yang Data Model for OSPF Protocol", draft-ietf-
              ospf-yang-05 (work in progress), July 2016.

              D'Souza, K., Shaikh, A., and R. Shakir, "Catalog and
              registry for YANG models", draft-openconfig-netmod-model-
              catalog-01 (work in progress), July 2016.

              "Writable MIB Module IESG Statement",

              "YANG Data Model for L3VPN service delivery",

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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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