NETMOD
Internet Engineering Task Force (IETF)                     D. Bogdanovic
Internet-Draft
Request for Comments: 8199                          Volta Networks, Inc.
Intended status:
Category: Informational                                        B. Claise
Expires: December 15, 2017
ISSN: 2070-1721                                                C. Moberg
                                                     Cisco Systems, Inc.
                                                           June 13,
                                                               July 2017

                       YANG Module Classification
             draft-ietf-netmod-yang-model-classification-08

Abstract

   The YANG data modeling language is currently being considered for a
   wide variety of applications throughout the networking industry at
   large.  Many standards development organizations (SDOs), open source open-source
   software projects, vendors 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 document is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list  It represents the consensus of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a maximum candidate for any level of Internet
   Standard; see Section 2 of six months RFC 7841.

   Information about the current status of this document, any errata,
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   This Internet-Draft will expire on December 15, 2017.
   http://www.rfc-editor.org/info/rfc8199.

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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: YANG Module Origin Types  . . . . . . . . . . . .   7
     3.1.  Standard YANG Modules . . . . . . . . . . . . . . . . . .   8
     3.2.  Vendor-specific  Vendor-Specific YANG Modules and Extensions . . . . . . .   8
     3.3.  User-specific  User-Specific YANG Modules and Extensions . . . . . . . .   9
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Acknowledgements  . . . . . . . .  References  . . . . . . . . . . . . . .   9
   7.  Change log [RFC Editor: Please remove] . . . . . . . . . . .   9
   8.
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     6.2.  Informative References  . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . .  10
   Acknowledgements  . . . . . . .  10
     8.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11  10

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 the Network Configuration Protocol (NETCONF)
   [RFC6241] for configuration management purposes, especially in new
   working group charters [Writable-MIB-Module-IESG-Statement]. [IESG-Statement].

   YANG is also gaining wide acceptance as the de-facto de facto standard data
   modeling language in the broader industry.  This extends beyond the
   IETF, including
   IETF to include many standards development organizations, SDOs, industry consortia, ad hoc groups, open open-
   source projects, vendors, and end- end users.

   There are currently no clear guidelines on how to classify the
   layering of YANG modules according to abstraction, abstraction or how to classify
   modules along the continuum spanning formal standards publications,
   vendor-specific modules modules, and modules provided by end-users. 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 module origin type 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.  While the classification
   boundaries are at times blurry, this document should provide a robust
   starting point as the YANG community gains further experience with
   designing and deploying modules.  To be more explicit, it is expected
   that the classification criteria will change over time.

   A number of modules, for example, modules have concerned with topologies,
   created substantial discussion during the development of this document: for examples, modules concerned with
   topologies.
   document.  Topology modules are useful both on the Network Element network element
   level (e.g. (e.g., link-state database content) as well as and on the Network
   Service network service
   level (e.g. (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 discussions among SDOs and
      industry consortia discussions,
      whose consortia; the goals of such discussions are determined in their
      by the respective areas of work.

   o  Second, operators might look at the YANG module abstraction layers
      to understand which Network Service YANG modules Modules and Network
      Element YANG modules 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 a
      Layer 2 Virtual Private Network (L2VPN) YANG module might be a
      Network Service YANG module, Module, ready to be used as a service model
      by a network operator.  Alternatively, it might be a Network
      Element YANG module Module that contains the L2VPN data definitions
      required to be configured on a single device.

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

1.1.  Terminology

   [RFC7950] specifies:

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

   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
   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 for the development of YANG modules.  This document
   considers both bottom-up and top-down approaches as they are both
   used and they each 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, operations operations, and notifications of specific device-centric
      technologies or features features.

   o  Network Service YANG Modules describe the configuration, state
      data, operations operations, and notifications of abstract representations of
      services implemented on one or multiple network elements elements.

                        +--------------------------+
                        |  Operations and Business |
                        |      Support Systems     |
                        |        (OSS/BSS)      (OSSs and BSSs)     |
                        +--------------------------+

        - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
        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 Abstraction Layers

   Figure 1 illustrates the application of YANG modules at different
   layers of abstraction.  Layering of modules allows for reusability of
   existing lower layer lower-layer modules by higher level 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 Modules (e.g., for e.g. routing or switching protocols
   protocols) requires teams that include developers with experience of
   implementing those protocols.

   On the other hand, Network Service YANG modules Modules are best developed by
   network operators experienced in defining network services for
   consumption by programmers programmers, e.g., those 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, 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 process; the details depending depend on how the
   network operator chooses to realize the service.  For the purpose of
   this document, the term "orchestrator" is used to describe 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 Systems, and
   applications exposed to network service consumers, being
   either consumers (either internal
   network operations people or external customers. customers).  These modules are
   commonly designed, developed 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 development, including development speed
   at the expense of reusability.

   The component-based approach captures device-centric features (e.g.
   the definition of a (e.g.,
   VPN Routing and Forwarding (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 development, including a higher degree of
   reusability at the expense of initial development speed.

   As an example, an L2VPN service can be built on many different types
   of transport network technologies, including e.g. including, e.g., MPLS or carrier
   ethernet. Carrier
   Ethernet.  A component-based approach would allow for reuse of e.g.
   User-Network User-
   Network Interface (UNI) definitions independent of definitions, such as the
   underlying transport network (e.g. MEF UNI interface or
   MPLS
   interface). interface, independent of the underlying transport network.  The
   monolithic approach would assume a specific set of transport
   technologies and interface definitions.

   An example of a Network Service YANG module Module is in [RFC8049].  It
   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 configuration data according to 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. e.g.,
   interface configuration [RFC7223], OSPF configuration [I-D.ietf-ospf-yang], [OSPF-YANG],
   and
   firewall rules definitions [I-D.ietf-netmod-acl-model]. access control list (ACL) configuration [ACL-YANG].

   The module Network Element YANG 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: YANG Module Origin Types

   This document suggests classifying YANG module origin types as
   standard
   Standard YANG modules, vendor-specific Modules, Vendor-Specific YANG modules Modules and extensions, Extensions,
   or user-specific User-Specific YANG modules Modules and extensions 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 module origin types.

   Figure 2 illustrates the relationship between the three types of
   modules.

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

                    Figure 2: YANG Module Origin Types

3.1.  Standard YANG Modules

   Standard YANG Modules are published by standards development
   organizations (SDOs). SDOs.  Most SDOs create
   specifications according to a formal process in order to produce a
   standard that is useful for their constituencies.

   The lifecycle lifecycles of these modules is are driven by the editing cycle cycles of
   the
   specification specifications and not tied to a specific implementation.

   Examples of SDOs in the networking industry are the IETF and the
   IEEE.

3.2.  Vendor-specific  Vendor-Specific YANG Modules and Extensions

   Vendor-specific

   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 organization, for example, vendors of virtual or physical
   equipment, industry consortia, and opensource open-source projects.  The intent
   of these modules range ranges from providing openly published YANG modules
   that may eventually be contributed back to, to or adopted by, by an SDO, SDO to
   strictly internal YANG modules not intended for external consumption.

   The lifecycle lifecycles of these modules are generally aligned with the
   release
   cycle cycles of the product or open source open-source software project
   deliverables.

   It is worth noting that there is an increasing amount of interaction
   between open source open-source projects and SDOs in the networking industry.
   This includes open source open-source projects implementing published standards
   as well as open source open-source projects contributing content to SDO
   processes.

   Vendors also develop Vendor-specific Extensions 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 added 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  User-Specific YANG Modules and Extensions

   User-specific

   User-Specific YANG Modules are developed by organizations that
   operate YANG-based infrastructure including devices and
   orchestrators.  For
   orchestrators, for example, network administrators in enterprises, 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 the ability to describe the service-to-network decomposition
   in orchestrators and the module to configuration module-to-configuration decomposition in
   devices.

   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 does not require any IANA actions.

6.  Acknowledgements

   Thanks to David Ball and Jonathen 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
   RFC6020.

   version 06: updates based on comments from Adrian Farrel about YANG
   Data Model for L3VPN Service Delivery.

   version 07: updates based on comments from Pete Resnick

8.  References

8.1.

6.1.  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,
              <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>.

   [RFC8049]  Litkowski, S., Tomotaki, L., and K. Ogaki, "YANG Data
              Model for L3VPN Service Delivery", RFC 8049,
              DOI 10.17487/RFC8049, February 2017,
              <http://www.rfc-editor.org/info/rfc8049>.

8.2.

6.2.  Informative References

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

   [ACL-YANG]
              Bogdanovic, D., Koushik, K., Jethanandani, M., Huang, L., Agarwal, S.,
              and D. Blair, "Network Access Control List (ACL) YANG Data
              Model",
              draft-ietf-netmod-acl-model-10 (work Work in progress), March Progress, draft-ietf-netmod-acl-model-11,
              June 2017.

   [I-D.ietf-ospf-yang]

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

   [OSPF-YANG]
              Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem,
              "Yang Data Model for OSPF Protocol", draft-ietf-ospf-
              yang-07 (work Work in progress), March Progress,
              draft-ietf-ospf-yang-08, July 2017.

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

Acknowledgements

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

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