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rfc:rfc8199

Internet Engineering Task Force (IETF) D. Bogdanovic Request for Comments: 8199 Volta Networks, Inc. Category: Informational B. Claise ISSN: 2070-1721 C. Moberg

                                                   Cisco Systems, Inc.
                                                             July 2017
                     YANG Module Classification

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
 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

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of 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 candidate for any level of Internet
 Standard; see Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc8199.

Bogdanovic, et al. Informational [Page 1] RFC 8199 YANG Module Classification July 2017

Copyright Notice

 Copyright (c) 2017 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 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 YANG Modules and Extensions . . . . . . .   8
   3.3.  User-Specific YANG Modules and Extensions . . . . . . . .   9
 4.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
 5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
 6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
   6.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
   6.2.  Informative References  . . . . . . . . . . . . . . . . .  10
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
 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] and the Network Configuration Protocol (NETCONF)
 [RFC6241] for configuration management purposes, especially in new
 working group charters [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 to include many SDOs, industry consortia, ad hoc groups, open-
 source projects, vendors, and end users.

Bogdanovic, et al. Informational [Page 2] RFC 8199 YANG Module Classification July 2017

 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 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 concerned with topologies,
 created substantial discussion during the development of this
 document.  Topology modules are useful both on the network element
 level (e.g., link-state database content) and 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 discussions among SDOs and
    industry consortia; the goals of such discussions are determined
    by the respective areas of work.
 o  Second, operators might look at the YANG module abstraction layers
    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, a
    Layer 2 Virtual Private Network (L2VPN) YANG module might be a
    Network Service YANG Module, ready to be used as a service model
    by a 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.

Bogdanovic, et al. Informational [Page 3] RFC 8199 YANG Module Classification July 2017

 o  Third, this taxonomy 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 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 or well-defined best
 practices 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, 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.

Bogdanovic, et al. Informational [Page 4] RFC 8199 YANG Module Classification July 2017

                      +--------------------------+
                      |  Operations and Business |
                      |      Support Systems     |
                      |      (OSSs and BSSs)     |
                      +--------------------------+
  1. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Network Service YANG Modules

           +------------+      +-------------+      +-------------+
           |            |      |             |      |             |
           |  - L2VPN   |      |   - L2VPN   |      |    L3VPN    |
           |  - VPWS    |      |   - VPLS    |      |             |
           |            |      |             |      |             |
           +------------+      +-------------+      +-------------+
  1. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

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 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 (e.g., for routing or switching
 protocols) requires teams that include developers with experience
 implementing those protocols.

Bogdanovic, et al. Informational [Page 5] RFC 8199 YANG Module Classification July 2017

 On the other hand, Network Service YANG Modules are best developed by
 network operators experienced in defining network services for
 consumption by programmers, e.g., those developing 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; the details depend on how the
 network operator chooses to realize the service.  For the purpose of
 this document, the term "orchestrator" is used to describe a system
 implementing such a process.
 External systems can be provisioning systems, service orchestrators,
 Operations Support Systems, Business Support Systems, and
 applications exposed to network service consumers (either internal
 network operations people or external 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 development speed
 at the expense of reusability.
 The component-based approach captures device-centric features (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, 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., MPLS or Carrier
 Ethernet.  A component-based approach would allow for reuse of User-
 Network Interface (UNI) definitions, such as the MEF UNI interface or

Bogdanovic, et al. Informational [Page 6] RFC 8199 YANG Module Classification July 2017

 MPLS 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 is in [RFC8049].  It
 provides an abstract model for Layer 3 IP VPN service configuration.
 This module includes 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.,
 interface configuration [RFC7223], OSPF configuration [OSPF-YANG],
 and access control list (ACL) configuration [ACL-YANG].
 The 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 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 module origin types.

Bogdanovic, et al. Informational [Page 7] RFC 8199 YANG Module Classification July 2017

 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 SDOs.  Most SDOs create
 specifications according to a formal process in order to produce a
 standard that is useful for their constituencies.
 The lifecycles of these modules are driven by the editing cycles of
 the 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 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 open-source projects.  The intent
 of these modules ranges 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 lifecycles of these modules are generally aligned with the
 release cycles of the product or open-source software project
 deliverables.

Bogdanovic, et al. Informational [Page 8] RFC 8199 YANG Module Classification July 2017

 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
 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 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 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 the 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. Security Considerations

 This document doesn't have any Security Considerations.

5. IANA Considerations

 This document does not require any IANA actions.

Bogdanovic, et al. Informational [Page 9] RFC 8199 YANG Module Classification July 2017

6. References

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>.

6.2. Informative References

 [ACL-YANG]
            Bogdanovic, D., Jethanandani, M., Huang, L., Agarwal, S.,
            and D. Blair, "Network Access Control List (ACL) YANG Data
            Model", Work in Progress, draft-ietf-netmod-acl-model-11,
            June 2017.
 [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", Work in Progress,
            draft-ietf-ospf-yang-08, July 2017.

Bogdanovic, et al. Informational [Page 10] RFC 8199 YANG Module Classification July 2017

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

Bogdanovic, et al. Informational [Page 11]

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