YANG Module Classification
Volta Networks, Inc.
dean@voltanet.io
Cisco Systems, Inc.
De Kleetlaan 6a b11831 DiegemBelgium+32 2 704 5622bclaise@cisco.comCisco Systems, Inc.camoberg@cisco.com
Operations and Management
NETMODThe 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.The Internet Engineering Steering Group (IESG) has been actively
encouraging IETF working groups to use the YANG data modeling language , and
NETCONF protocol for configuration
management purposes, especially in new working group charters
.YANG is also gaining wide acceptance as the de-facto standard data modeling
language in the broader industry. This extends beyond the IETF,
including many standards development organizations, industry 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:
The layering of modules based on their abstraction levelsThe module origin type based on the nature and intent of the contentThe 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 have created substantial discussion during
the development of this document: for examples, modules concerned with
topologies. Topology modules are useful both on the Network Element
level (e.g. link-state database content) as well as 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:
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.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, an
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.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 support in their products. specifies:
data model: A data model describes how data is represented and
accessed.
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".
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 on, or well-defined best
practices around 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:Network Element YANG Modules describe the configuration, state
data, operations and notifications of specific device-centric
technologies or featuresNetwork Service YANG Modules describe the configuration, state
data, operations and notifications of abstract representations of services
implemented on one or multiple network elements 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 consumption by
programmers developing e.g. flow-through provisioning systems or self-service
portals.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, 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
and applications exposed to network service consumers, being 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.
the definition of a 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 e.g. User-Network Interface
(UNI) 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.An example of a Network Service YANG module is in .
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.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
, OSPF configuration
, and firewall rules definitions
.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.This document suggests classifying YANG module origin types as standard
YANG modules, vendor-specific YANG modules and extensions, or
user-specific YANG modules and extensionsThe 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. illustrates the relationship between the
three types of modules.Standard YANG Modules are published by standards development
organizations (SDOs). Most SDOs create specifications according to a
formal process in order to produce a standard that is useful for their
constituencies.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 and the IEEE.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 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.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.This document doesn't have any Security Considerations.This document has no IANA actions.Thanks to David Ball and Jonathen Hansford for feedback and
suggestions.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 ResnickWritable MIB Module IESG Statement