Internet DRAFT - draft-ietf-netmod-yang-model-classification
draft-ietf-netmod-yang-model-classification
NETMOD D. Bogdanovic
Internet-Draft Volta Networks, Inc.
Intended status: Informational B. Claise
Expires: December 15, 2017 C. Moberg
Cisco Systems, Inc.
June 13, 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
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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 15, 2017.
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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
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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: 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. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
7. Change log [RFC Editor: Please remove] . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 10
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], [RFC7950] and NETCONF protocol [RFC6241] for
configuration management purposes, especially in new working group
charters [Writable-MIB-Module-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, including many standards development organizations, industry
consortia, ad hoc groups, open source projects, vendors, and end-
users.
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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 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:
o 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.
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, 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.
o And thirdly, this taxonomy would help equipment vendors (whether
physical or virtual), controller vendors, orchestrator vendors to
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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 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:
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
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+--------------------------+
| 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
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 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
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consumption by programmers 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, 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
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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 [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. interface
configuration [RFC7223], OSPF configuration [I-D.ietf-ospf-yang], and
firewall rules definitions [I-D.ietf-netmod-acl-model].
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.
3. Second Dimension: 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.
Figure 2 illustrates the relationship between the three types of
modules.
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+--------------+
| 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). 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.
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 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.
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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 has no 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.
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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. 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. Informative References
[I-D.ietf-netmod-acl-model]
Bogdanovic, D., Koushik, K., Huang, L., and D. Blair,
"Network Access Control List (ACL) YANG Data Model",
draft-ietf-netmod-acl-model-10 (work in progress), March
2017.
[I-D.ietf-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 in progress), March 2017.
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[Writable-MIB-Module-IESG-Statement]
"Writable MIB Module IESG Statement",
<https://www.ietf.org/iesg/statement/writable-mib-
module.html>.
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
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