NETMOD                                                     D. Bogdanovic
Internet-Draft                                      Volta Networks, Inc.
Intended status: Informational                                 B. Claise
Expires: October 29, 2017                                      C. Moberg
                                                     Cisco Systems, Inc.
                                                          April 27, 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-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

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   This Internet-Draft will expire on October 29, 2017.

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Copyright Notice

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   Provisions Relating to IETF Documents
   ( in effect on the date of
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   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.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Change log [RFC Editor: Please remove]  . . . . . . . . . . .  10
   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 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 as a service model by 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.  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 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.  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.

   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 peole or extarnal 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 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
   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

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

   Another example for a network service model is [RFC8049].  Although
   it provides information that can be used to achieve customer service
   service level agreement, which is more then network service module
   classification describes in this document, it provides an abstract
   model for Layer 3 IP VPN service configuration which is a good
   network service model.  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 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].

   Although the [RFC7950], [RFC7950] doesn't explain the relationship of
   the terms '(YANG) data model' and '(YANG) module', the authors
   understand there is a 1:1 relationship between a data model and a
   YANG module, but a data model may also be expressed using a
   collection of YANG modules (and submodules).  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.

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   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

  |     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.

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   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

   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.  Security Considerations

   This document doesn't have any Security Considerations.

5.  IANA Considerations

   This document has no IANA actions.

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6.  Acknowledgements

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

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.

   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.

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

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              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.

              "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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