YANG Module Classification
draft-ietf-netmod-yang-model-classification-03

+--------------------------+
                        |  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
   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.
   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
   modules.

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  +--------------+
  |     User     |
  |   Extensions |
  +------+-------+
      Augments
  +------+-------+  +--------------+  +--------------+
  |   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
   processes.

   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
   devices.

   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 {
                       service
                       device
                       notApplicable
                  }
                  description
                     "Categorization of the YANG module based on
                      draft-ietf-netmod-yang-model-classification.";
                }

   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
   RFC6020.

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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,
              <http://www.rfc-editor.org/info/rfc6020>.

   [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,
              <http://www.rfc-editor.org/info/rfc6241>.

   [RFC7223]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,
              <http://www.rfc-editor.org/info/rfc7223>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <http://www.rfc-editor.org/info/rfc7950>.

9.2.  Informative References

   [I-D.ietf-netmod-acl-model]
              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
              2016.

   [I-D.ietf-ospf-yang]
              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.

   [I-D.openconfig-netmod-model-catalog]
              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]
              "Writable MIB Module IESG Statement",
              <https://www.ietf.org/iesg/statement/writable-mib-
              module.html>.

   [YANG-Data-Model-for-L3VPN-service-delivery]
              "YANG Data Model for L3VPN service delivery",
              <https://tools.ietf.org/id/draft-l3vpn-service-yang>.

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Authors' Addresses

   Dean Bogdanovic
   Volta Networks, Inc.

   Email: dean@voltanet.io

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

   Phone: +32 2 704 5622
   Email: bclaise@cisco.com

   Carl Moberg
   Cisco Systems, Inc.

   Email: camoberg@cisco.com

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