One document matched: draft-ietf-netmod-yang-model-classification-03.xml
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<rfc ipr="trust200902" docName="draft-ietf-netmod-yang-model-classification-03" category="info" >
<front>
<title abbrev="YANG Module Classification">YANG Module Classification</title>
<author fullname="Dean Bogdanovic" initials="D." surname="Bogdanovic">
<organization abbrev="Volta Networks, Inc.">
Volta Networks, Inc.
</organization>
<address>
<email>dean@voltanet.io</email>
</address>
</author>
<author initials="B." surname="Claise" fullname="Benoit Claise">
<organization abbrev="Cisco Systems, Inc.">
Cisco Systems, Inc.
</organization>
<address>
<postal>
<street>De Kleetlaan 6a b1</street>
<city>1831 Diegem</city>
<country>Belgium</country>
</postal>
<phone>+32 2 704 5622</phone>
<email>bclaise@cisco.com</email>
</address>
</author>
<author fullname="Carl Moberg" initials="C." surname="Moberg">
<organization>Cisco Systems, Inc.</organization>
<address>
<email>camoberg@cisco.com</email>
</address>
</author>
<!-- Let xml2rfc figure out the date -->
<date/>
<area>Operations and Management</area>
<workgroup>NETMOD</workgroup>
<abstract>
<t>The YANG <xref target="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 for a wide variety of applications. At
the same time, there is currently no well-known terminology to categorize
various types of YANG modules.</t>
<t>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.</t>
<t>This document describes a set of concepts and associated terms to
support consistent classification of YANG modules.</t>
</abstract>
</front>
<middle>
<section anchor="introduction" title="Introduction">
<t>The Internet Engineering Steering Group (IESG) has been actively
encouraging IETF working groups to use the YANG modeling language <xref
target="RFC6020"/>, <xref target="RFC7950"/> and
NETCONF protocol <xref target="RFC6241"/> for configuration
management purposes, especially in new working group charters
<xref target="Writable-MIB-Module-IESG-Statement"/>.</t>
<t>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 consortia,
ad hoc groups, open source projects, vendors, and end-users.</t>
<t>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.</t>
<t>This document presents a set of concepts and terms to form a useful
taxonomy for consistent classification of YANG modules in two dimensions:
<list style="symbols">
<t>The layering of modules based on their abstraction levels</t>
<t>The type of module based on the nature and intent of the content</t>
</list>
</t>
<t>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
gains further experience with designing and deploying modules. To be more
explicit, the authors believe that the classification criteria will
change over time.</t>
<t>A number of module types have created substantial discussion during
the development of this document including those 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.</t>
<t>This document should provide benefits to multiple audiences:
<list style="symbols">
<t>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.</t>
<t>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.</t>
<t>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
<xref target="modulelayers"/>.</t>
</list>
</t>
<section anchor="terminology" title="Terminology">
<t><xref target='RFC7950'/> specifies:
<list style="symbols">
<t>data model: A data model describes how data is represented and
accessed.</t>
<t>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".
</t>
</list>
</t>
</section>
</section>
<section anchor="firstdimension" title="First Dimension: YANG Module Abstraction Layers">
<t>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.</t>
<t>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.
</t>
<t>For layering purposes, this document suggests the classification of
YANG modules into two distinct abstraction layers:</t>
<t>
<list style="symbols">
<t>Network Element YANG Modules describe the configuration, state
data, operations and notifications of specific device-centric
technologies or features</t>
<t>Network Service YANG Modules describe the configuration, state
data, operations and notifications of abstract representations of services
implemented on one or multiple network elements</t>
</list>
</t>
<figure title="YANG Module Layers" anchor="modulelayers">
<artwork align="center">
+--------------------------+
| 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
</artwork>
</figure>
<t><xref target="modulelayers"/> 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.</t>
<t>For module developers, per-layer modeling allows for separation of
concern across editing teams focusing on specific areas.</t>
<t>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.</t>
<t>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.</t>
<section anchor="networkservice" title="Network Service YANG Modules">
<t>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 we
will use the term "orchestrator" to describe a system implementing such
a process.</t>
<t>As an example, the Network Service YANG Module defined in
<xref target='YANG-Data-Model-for-L3VPN-service-delivery'/> 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 specific Network Element YANG 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.</t>
<t>Network Service YANG Modules define service models to be consumed
by external systems. These modules are commonly designed, developed and
deployed by network infrastructure teams.</t>
<t>YANG allows for different design patterns to describe network services,
ranging from monolithic to component-based approaches.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
</section>
<section anchor="networkelement" title="Network Element YANG Modules">
<t>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
<xref target='RFC7223'/>, OSPF configuration
<xref target='I-D.ietf-ospf-yang'/>, and firewall rules definitions
<xref target='I-D.ietf-netmod-acl-model'/>.</t>
<t>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.</t>
</section>
</section>
<section title="Second Dimension: Module Types">
<t>This document suggests classifying YANG module types as standard
YANG modules, vendor-specific YANG modules and extensions, or
user-specific YANG modules and extensions</t>
<t>The suggested classification applies to both Network Element YANG
Modules and Network Service YANG Modules.</t>
<t>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 types of modules.</t>
<t><xref target="moduletypes"/> illustrates the relationship between the
three types of modules.</t>
<figure title="YANG Module Types" anchor="moduletypes">
<artwork align="center" type="ascii-art">
+--------------+
| User |
| Extensions |
+------+-------+
Augments
+------+-------+ +--------------+ +--------------+
| Vendor | | User | | User |
| Extensions | | Extensions | | Extensions |
+------+-------+ +------+-------+ +------+-------+
Augments Augments Augments
+------+-----------------+-------+ +------+-------+ +--------------+
| Standard | | Vendor | | User |
| Modules | | Modules | | Modules |
+--------------------------------+ +--------------+ +--------------+
</artwork>
</figure>
<section title="Standard YANG Modules">
<t>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.</t>
<t>The lifecycle of these modules is driven by the editing cycle of the
specification and not tied to a specific implementation.</t>
<t>Examples of SDOs in the networking industry are the IETF, the IEEE
and the MEF.</t>
</section>
<section title="Vendor-specific YANG Modules and Extensions">
<t>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.</t>
<t>The lifecycle of these modules are generally aligned with the release
cycle of the product or open source software project deliverables.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="User-specific YANG Modules and Extensions">
<t>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.
</t>
<t>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.
</t>
<t>The lifecycles of these modules are generally aligned with the change
cadence of the infrastructure.</t>
</section>
</section>
<section anchor="catalog" title="Adding The Classification Type to YANG Module Catalogs">
<t>The suggested classification in this document would be an useful
information in a catalog of YANG modules. Such a catalog allows for
easy lookup and reusability of YANG modules. Practically, the YANG
module classification type would be an additional leaf to a YANG
module specified in <xref target='I-D.openconfig-netmod-model-catalog'/>:</t>
<figure>
<artwork align="center"><![CDATA[
leaf module-class{
type enum {
service
device
notApplicable
}
description
"Categorization of the YANG module based on
draft-ietf-netmod-yang-model-classification.";
}
]]></artwork>
</figure>
<t>Note: this leaf should actually be moved to <xref target='I-D.openconfig-netmod-model-catalog'/>.
Note2: since a YANG module can belong to both service and device, the ENUM is not appropriate.
An extensible list of module type is more appropriate.</t>
<t>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 definitive answer.</t>
</section>
<section anchor="security" title="Security Considerations">
<t>This document doesn't have any Security Considerations.</t>
</section>
<section anchor="iana" title="IANA Considerations">
<t>This document has no IANA actions.</t>
</section>
<section anchor="ack" title="Acknowledgements">
<t>Thanks to David Ball and David Hansford for feedback and suggestions.</t>
</section>
<section anchor ="changes" title="Change log [RFC Editor: Please remove]">
<t>version 00: Renamed and small fixes based on WG feedback.</t>
<t>version 01: Language fixes, collapsing of vendor data models and extensions,
and the introduction of user data models and extensions.</t>
<t>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.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include='reference.RFC.6020'?>
<?rfc include='reference.RFC.6241'?>
<?rfc include='reference.RFC.7950'?>
<?rfc include='reference.RFC.7223'?>
</references>
<references title="Informative References">
<?rfc include='reference.I-D.ietf-netmod-acl-model'?>
<?rfc include='reference.I-D.ietf-ospf-yang'?>
<?rfc include='reference.I-D.openconfig-netmod-model-catalog'?>
<reference anchor="Writable-MIB-Module-IESG-Statement"
target="https://www.ietf.org/iesg/statement/writable-mib-module.html">
<front>
<title>Writable MIB Module IESG Statement</title>
<author/>
<date/>
</front>
</reference>
<reference anchor="YANG-Data-Model-for-L3VPN-service-delivery"
target="https://tools.ietf.org/id/draft-l3vpn-service-yang">
<front>
<title>YANG Data Model for L3VPN service delivery</title>
<author/>
<date/>
</front>
</reference>
</references>
</back>
</rfc>
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