YANG Module Classification
draft-ietf-netmod-yang-model-classification-02

NETMOD                                                     D. Bogdanovic
Internet-Draft                                      Volta Networks, Inc.
Intended status: Informational                                 B. Claise
Expires: December 24, 2016                                     C. Moberg
                                                     Cisco Systems, Inc.
                                                           June 22, 2016

                       YANG Module Classification
             draft-ietf-netmod-yang-model-classification-02

Abstract

   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 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 modules.

   A consistent terminology would help with the categorization of YANG
   modules, 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 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 http://datatracker.ietf.org/drafts/current/.

   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 24, 2016.

Bogdanovic, et al.      Expires December 24, 2016               [Page 1]
Internet-Draft         YANG Module Classification              June 2016

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
   (http://trustee.ietf.org/license-info) 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 Data Model 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 [RFC6020]
   [I-D.ietf-netmod-rfc6020bis] and 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

Bogdanovic, et al.      Expires December 24, 2016               [Page 2]
Internet-Draft         YANG Module Classification              June 2016

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

   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
   dimensions:

   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 gain further experience with designing and deploying
   modules.  To be more explicit, the authors believe that the
   classification criteria will change over time.

   An example of a type of module that have created substantial
   discussion during the development of this document is topologies.
   Topology models are useful both on the Network Element level (e.g.
   link-state database content) as well as in 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.

Bogdanovic, et al.      Expires December 24, 2016               [Page 3]
Internet-Draft         YANG Module Classification              June 2016

   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

   RFC6020bis [I-D.ietf-netmod-rfc6020bis] specifies:

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

   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 Data Model Abstraction Layers

   Model 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
   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 and operations of specific device-centric technologies or
      features

   o  Network Service YANG Modules 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 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.

Bogdanovic, et al.      Expires December 24, 2016               [Page 4]
Internet-Draft         YANG Module Classification              June 2016

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

                       +--------------------------+
                       |  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

Bogdanovic, et al.      Expires December 24, 2016               [Page 5]
Internet-Draft         YANG Module Classification              June 2016

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 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 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 included in
   [YANG-Data-Model-for-L3VPN-service-delivery] provides an abstract
   model for Layer 3 IP VPN service configuration.  This model 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 model and decomposes the
   data into specific Network Element 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 services 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
   including a higher degree of reusability at the expense of initial
   speed.

Bogdanovic, et al.      Expires December 24, 2016               [Page 6]
Internet-Draft         YANG Module Classification              June 2016

   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 configuration, state data
   and operations 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 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 module.

3.  Second Dimension: Module Types

   This document suggests classifying YANG module types as either
   standard YANG modules, vendor-specific YANG modules and extensions,
   and 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 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.

Bogdanovic, et al.      Expires December 24, 2016               [Page 7]
Internet-Draft         YANG Module Classification              June 2016

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

                        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
   (Institute of Electrical and Electronics Engineers) 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

Bogdanovic, et al.      Expires December 24, 2016               [Page 8]
Internet-Draft         YANG Module Classification              June 2016

   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 what
   is 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 operators 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 lifecycle 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 catalog allows for
   easy lookup and reusability of YANG modules.  Practically, the YANG
   module classification type would be an additional leaf to YANG module
   specified in [I-D.openconfig-netmod-model-catalog]:

Bogdanovic, et al.      Expires December 24, 2016               [Page 9]
Internet-Draft         YANG Module Classification              June 2016

                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.
   A 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 definite 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), epxlain 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.

Bogdanovic, et al.      Expires December 24, 2016              [Page 10]
Internet-Draft         YANG Module Classification              June 2016

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>.

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-07 (work in progress), March
              2016.

   [I-D.ietf-netmod-rfc6020bis]
              Bjorklund, M., "The YANG 1.1 Data Modeling Language",
              draft-ietf-netmod-rfc6020bis-14 (work in progress), June
              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-04 (work in progress), March 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-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,
              <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>.

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

Bogdanovic, et al.      Expires December 24, 2016              [Page 11]
Internet-Draft         YANG Module Classification              June 2016

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

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

Bogdanovic, et al.      Expires December 24, 2016              [Page 12]