NETMOD                                                     D. Bogdanovic
Intended status: Informational                                 B. Claise
Expires: June 9, October 6, 2016                                       C. Moberg
                                                     Cisco Systems, Inc.
                                                        December 7, 2015
                                                           April 4, 2016

                       YANG Model 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, standards-defining organizations
   (SDOs), open source software projects, and vendors and users are using
   YANG to develop and publish models of configuration, state data and
   operations for a wide variety of applications.  At the same time,
   there is currently no well-known terminology to categorize various
   types of YANG models.

   A consistent terminology would help with the categorization of
   models, assist in the analysis 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 models.

Status of This Memo

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   This Internet-Draft will expire on June 9, October 6, 2016.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  First Dimension: YANG Model Abstraction Layers  . . . . . . .   3
     2.1.  Network Service YANG Data Models  . . . . . . . . . . . .   4
     2.2.  Network Element YANG Data models  . . . . . . . . . . . .   5
   3.  Second Dimension: Model Types . . . . . . . . . . . . . . . .   6
     3.1.  Standard YANG Models  . . . . . . . . . . . . . . . . . .   6
     3.2.  Vendor-specific YANG Models and Extensions  . . . . . . .   6
     3.3.  User-specific YANG Models and Extensions  . . . . . . . .   7
     3.4.  Adding Models to Catalogs .   6
     3.3.  Vendor-specific Extensions . . . . . . . . . . . . . . .   7
     3.5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.   8
     3.6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.   8
     3.7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.   8
     3.8.  Change log [RFC Editor: Please remove]  . . . . . . . . . . .   7
   8.   8
   4.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     4.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8   9

1.  Introduction

   The Internet Engineering Steering Group (IESG) has been actively
   encouraging IETF working groups to use the NETCONF [RFC6241] and YANG
   standards 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
   modeling language in the broader industry.  This extends beyond the
   IETF, including many standards development organizations, industry
   consortia, ad hoc groups, open source projects projects, vendors, and vendors. end-

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

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

   o  The layering of models based on their abstraction levels

   o  The type of model based on the nature and intent of the content

   The two categories are covered in the next two sections.

2.  First Dimension: YANG Model Abstraction Layers

   Model developers have taken two approaches to developing YANG model: models:
   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 models.  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 appeals appeal to different

   For layering purposes, this documents document suggests the classification of
   data models into two distinct abstraction layers:

   o  Network Element YANG Models describe the configuration, state data
      and operations of a specific device-centric technology technologies or feature. features

   o  Network Service YANG Models describes describe the configuration, state data
      and operations of an abstract representation of a service
      implemented on one or multiple network elements

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

   For model 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 models for e.g e.g. routing or switching protocols
   requires teams that include developers with experience from of
   implementing those protocols.

   On the other hand, network service models are best developed by
   people experienced in defining network services for consumption by
   programmers developing e.g. flow-through provisioning systems or
   self-service portals.

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

       - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
       Network Service YANG data models

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

       - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
       Network Element YANG data models

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

       Fig. 1 YANG Model Layers

2.1.  Network Service YANG Data Models

   Network Service YANG Data Models describe the characteristics of a
   service, as agreed upon with consumers of that service.  That is, a
   service model 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 data models 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 model included in
   [YANG-Data-Model-for-L3VPN-service-delivery] provides an abstracted
   model for Layer 3 IP VPN service configuration.  An orchestrator
   receives operations on service instances according to the service
   model and decomposes the data into specific Network Element models to
   configure the participating network elements to perform the intent of
   the service.

   Network Service YANG models defines define services models to be consumed by
   external systems.  These models 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 model
   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 services.  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
   approach would assume a specific set of transport technologies and
   interface definitions.

2.2.  Network Element YANG Data models

   Network Element YANG Data Models describe the configuration, state
   data and operations of a network device as defined by the vendor of
   that device.  The models 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 model provides a coherent data
   model representation of what is commonly a very mixed 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
   management framework that implements the YANG model.

3.  Second Dimension: Model Types

   This document suggests classifying YANG model types as either
   standard YANG models, vendor-specific YANG models, or vendor-specific
   extensions. models and extensions, and
   user-specific YANG models and extensions

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

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

3.1.  Standard YANG Models

   Standard YANG models are published by standards defining 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 models are 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 Models and Extensions

   Vendor-specific YANG models are developed by organizations with the
   intent to support a specific set of implementations under control of
   that organization.  The intent of these models range from providing
   openly published YANG models that may eventually be contributed back
   to, or adopted by an SDO, to strictly internal YANG models not
   intended for external consumption.

   The lifecycle of these models 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

3.3.  Vendor-specific Extensions

   Vendors also develop Vendor-specific Extensions to standard models
   using YANG constructs for extending data definitions of previously
   published models.  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 data models to cover the full
   scope of features in implementations, which commonly is broader than
   what is covered by the standard model.


3.3.  User-specific YANG Models and Extensions

   User-specific YANG models are developed by organizations that operate
   YANG-based infrastructure including devices and orchestrators.  The
   intent of these models is to express the specific needs for a certain
   implementation, above and beyond what is provided by vendors.

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

   The lifecycle of these models are generally aligned with the change
   cadence of the infrastructure.

3.4.  Adding Models to Catalogs

   The suggested classification in this document supports the creation
   of catalogs, such as proposed in
   [I-D.openconfig-netmod-model-catalog].  Such catalogs allows for easy
   lookup and reusability of YANG models.  SDO-classified models also
   provide an educational resource providing architectural guidelines
   for model development, based on a membership reviewn and consensus.

3.5.  Security Considerations

   At this stage, authors of the draft didn't look into security


3.6.  IANA Considerations

   This document requests no action by IANA.


3.7.  Acknowledgements

   Thanks to David Ball and David Hansford for his enlightenments on Metro Ethernet Forum
   service aspects.

7. feedback and suggestions.

3.8.  Change log [RFC Editor: Please remove]

   version 1: restructure the document, add the two dimensions, add the
   interaction with the different SDOs 00: Renamed and opensource projects, add the
   definitions. small fixes based on WG feedback.

   version 2: added definitions for config and service 01: Language fixes, collapsing of vendor models clarified
   second dimension and
   extensions, and the introduction of model classification.  fixed typos user models and extensions.

   version 3: restructure 02: Added two sections, Model Catalog and partial rewrite Benefits of section 2.

   version 4: Language fixes

8. model

4.  References


4.1.  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,


4.2.  Informative References

              Bogdanovic, D., Sreenivasa, Koushik, K., Huang, L., and D. Blair,
              "Network Access Control List (ACL) YANG Data Model",
              draft-ietf-netmod-acl-model-07 (work in progress), October
              2015. March

              Yeung, D., Qu, Y., Zhang, J., Bogdanovic, D., and K.
              Koushik, "Yang Data Model for OSPF Protocol", draft-ietf-
              ospf-yang-04 (work in progress), March 2016.

              D'Souza, K., Shaikh, A., and R. Shakir, "Catalog and
              registry for YANG models", draft-openconfig-netmod-model-
              catalog-00 (work in progress), October 2015.

   [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,

              "Writable MIB Module IESG Statement",

              "YANG Data Model for L3VPN service delivery",

Authors' Addresses

   Dean Bogdanovic


   Benoit Claise
   Cisco Systems, Inc.


   Carl Moberg
   Cisco Systems, Inc.