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
NETMODserviceelementstandardvendorusercontrollerorchestratorThe 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 the Network Configuration Protocol (NETCONF)
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 to
include many SDOs, 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, 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:
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.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.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. 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 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:Network Element YANG Modules describe the configuration, state
data, operations, and notifications of specific device-centric
technologies or features.Network 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 (e.g., for routing or switching protocols)
requires teams that include developers with experience
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, e.g., those developing 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; 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 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 .
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.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 access control list (ACL) configuration
.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.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. illustrates the relationship between the
three types of 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.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.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 does not require any IANA actions.Network Access Control List (ACL) YANG Data ModelYang Data Model for OSPF ProtocolWritable MIB Module IESG StatementThanks to David Ball and Jonathan Hansford for feedback and
suggestions.