NETMOD D. Bogdanovic
Internet-Draft Juniper Networks
Intended status: Informational B. Claise
Expires: December 5, 2015 C. Moberg
Cisco Systems, Inc.
June 3, 2015
YANG model classification
draft-bogdanovic-netmod-yang-model-classification-03
Abstract
The YANG [RFC6020] data modeling language is currently being
considered for a wide variety of applications throughout the
networking industry at large. Many standards organizations and open
source projects are using YANG to develop and publish models of
configuration, state data and operations for a wide variety of
networking applications. At the same time, there is currently no
well-known terminology to categorize the various types of YANG models
that are being worked on.
A consistent terminology would help with the categorization of
models, assist in the analysis the YANG data modeling efforts in the
IETF and in other places, 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 models.
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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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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 5, 2015.
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Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. First Dimension: YANG Data Model Layering . . . . . . . . . . 3
2.1. Network Service YANG Data Models . . . . . . . . . . . . 4
2.2. Network Element YANG Data models . . . . . . . . . . . . 5
3. Second Dimension: Model Type . . . . . . . . . . . . . . . . 6
3.1. Standard YANG model . . . . . . . . . . . . . . . . . . . 7
3.2. Standard Extension YANG Model . . . . . . . . . . . . . . 7
3.3. Proprietary Extension to Standard YANG Model . . . . . . 7
3.4. Vendor configuration model . . . . . . . . . . . . . . . 7
3.5. Proprietary YANG Model . . . . . . . . . . . . . . . . . 9
4. Typical Architecture . . . . . . . . . . . . . . . . . . . . 9
5. IETF, Other SDOs, and open source . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
9. Change log [RFC Editor: Please remove] . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
The Internet Engineering Steering Group (IESG) has been actively
encouraging IETF working groups to use the NETCONF [RFC6241] and YANG
standards for configuration management purposes, especially in new
charters [Writable-MIB-Module-IESG-Statement].
YANG is also gaining wide acceptance as the de-facto standard
modeling language in the broader industry. This extends beyond the
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IETF, including many standard development organizations, industry
consortia, ad hoc groups, and open source projects.
There are no clear guidelines on how to classify the layering of YANG
models according to abstraction, or how to classify models along the
continuum spanning industry standard and vendor-specific models.
This document presents a set of concepts and terms to form a useful
taxonomy for consistent classification of YANG models in two
dimensions:
o Based on the level of abstraction in the model
o Based on the applicability of the model
The two categories are covered in the next two sections.
2. First Dimension: YANG Data Model Layering
Model developers have taken two approaches to development: 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 develoment of models. 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 appeals to different
groups.
For layering purposes, this documents suggests the classification of
data models into two distinct abstraction layers:
o Network Element YANG Models describe the configuration, state data
and operations of a specific device-centric technology or feature.
o Network Service YANG Models describes the configuration, state
data and operations of an abstract representation of a service
implemented on one or multiple network elements
Figure 1 illustrates the application of YANG models at different
layers of abstraction. Layering of models allow for reusability of
existing lower layer models in higher level models while limiting
duplication of features across layers.
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For model 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 models for e.g routing or switching protocols
requires teams that include developers with experience from
implementing those protocols.
On the other hand, network service models are best developed by
people experienced in defining network services for consumption by
programmers developing e.g. flow-through provisioning systems or
self-service portals.
+-----------------------+
| |
| OSS/BSS |
| |
+-----------------------+
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Network Service YANG data models
+------------+ +-------------+ +-------------+
| | | | | |
| - VPWS | | - VPLS | | L3VPN |
| - L2VPN | | - L2VPN | | |
| | | | | |
+------------+ +-------------+ +-------------+
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Network Element YANG data models
+------------+ +-----------+ +-------------+ +-----------+
| | | | | | | |
| MPLS | | BGP | | Interface | | Routing |
| | | | | | | |
+------------+ +-----------+ +-------------+ +-----------+
Fig. 1 YANG Model Layers
2.1. Network Service YANG Data Models
Network Service YANG Data Models are created to describe the
characteristics of a service, as agreed upon with consumers of that
service. That is, a service model does not expose the detailed
configuration parameters of all participating network elements, but
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describes an abstract model that allows instances of the service to
be decomposed into instance data according to the Network Element
data models 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.
As an example, the Network Service model included in
http://datatracker.ietf.org/doc/draft-l3vpn-service-yang/ provides an
abstracted view of a Layer 3 IP VPN service configuration components.
An orchestrator receives operations on instances according to the
service model as input and decomposes the data into specific Network
Element models to configure the participating network elements to
perform the service.
Network Service YANG models defines complete service to be consumed
by external systems. These models 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 model
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.
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 services. 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.
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.
2.2. Network Element YANG Data models
Network Element YANG Data Models describe the configuration, state
data and operations of a network device as defined by the vendor of
that device. The models are commonly structured around features of
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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 model provides a coherent
representation of what is commonly a very mixed software environment
that consists 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 management
framework that implements the YANG model.
3. Second Dimension: Model Type
At very high level, models can be divided into proprietary and
standard. Each vendor, consortium, open source project can publish
their models and those are considered proprietary models. When an
SDO, such as IETF or IEEE, publishes an accepted model document, then
this is a standard model. There are use cases where a consortium has
published work which de facto became standard, such as Linux kernel,
but for the clarity in this document, authors are making a separation
between models based on the above description.
Standard YANG Model: YANG model defined by an Standard Development
organization (SDO), e.g. IETF, IEEE.
Standard Extension YANG Model: YANG Model that describes a
standard extension, example route filter, to standard filter YANG
model.
Proprietary Extension to Standard YANG Model: As the Standard YANG
Models contains a subset of all the Vendor Configuration Models,
proprietary extensions must complement the Standard YANG Models to
represent a Vendor Configuration Model.
Proprietary YANG Model: A non Standard YANG Model.
Vendor Configuration Model: It describes all configurable
capabilities of the device and what device vendor exposes for
configuration. The vendor configuration model can be CLI or YANG-
based.
As mentioned earlier in this document, there are two ways of
designing models, top down and bottom up with one restriction.
Everything is dependent on the vendor data model. That model
describes all the possibilities and if model developers prefers, they
can use vendor model only to design service components, network
service and business service. Using vendor model provides all
capabilities today, but it comes with restrictions of portability
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between vendors and to certain extent devices. On the other hand,
only standard models and standard extensions can be used, but this
might result in less feature rich or less efficient services.
Service model developer has a choice to reuse service components or
write a model completely based on vendor data model.
3.1. Standard YANG model
With YANG we have a common language, that enables different
communities to express data models that are widely understandable
without lot of additional explanation. This enables different
groups, such as IETF, to standardize data models, defined as an IETF
RFC, and vendors to support them, which will make it easier to for
network operators to manage their network configuration
programmatically. The Standard YANG Models can distinguished between
the core YANG models, such as the YANG Data Model for Interface
Management [RFC7223], and the technology specific YANG models, such
as the Configuration Data Model for the IP Flow Information Export
(IPFIX) and Packet Sampling (PSAMP) Protocols [RFC6728].
3.2. Standard Extension YANG Model
Standard Extension is the conditional portion of a Standard YANG
Model, expressed with the feature, if-feature, augment YANG
statements [RFC6020]. An example of such standard extension is
policy based routing (PBR). PBR is found in many vendor
implementations and have many common features, but not all vendors
support PBR on all of their devices.
3.3. Proprietary Extension to Standard YANG Model
Proprietary extension is a conditional portion of a Standard YANG
Model, expressed with feature, if-feature, augment YANG statements
[RFC6020]. Proprietary extensions are required as the Standard YANG
model will not cover all the possible configuration parameters of the
different vendors. Proprietary extension can be a feature depending
on hardware platform capabilities and it is not available by other
vendors. Such an example could be match condition for packet
classification used for PBR.
3.4. Vendor configuration model
Base model for all other models is the vendor configuration model.
It describes all configurable capabilities of the device and what
device vendor exposes for configuration.
The standard configuration model is a subset of vendor configuration
model. The standard configuration model can be broken into base
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model and standard extension models, where the base is common data
model and standard extensions are standard features that are not
implemented by all vendors. Example of standard base model is Access
Control List and routing filter is a standard extension on ACL. Or
another example: encryption algorithm is standard feature, but the
different types, like md5, hmac-md5, hmac-sha1, etc are standard
extensions, as it is not that all vendors have all encryption
algorithm types implemented.
Although all vendors provide very similar functionality using
standards, implementations are different. One of basic examples are
dynamic routing protocols. We can see today two main types of
routing protocol configuration.
protocol centric - all the protocol related config is contained
with the protocol itself. Especially in case of multiple
instances of the routing protocol running in different routing-
instances (routing-instance as described in core routing model
[I-D.ietf-netmod-routing-cfg]), all the routing-instance protocol
config is contained in the default routing instance.
Router ospf 10
Default-metric 100
Address-family ipv4 vrf VRF1
Network x.x.x.x area 0
Address-family ipv4 vrf VRF2
Network x.x.x.x area 0
Address-family ipv4
Network x.x.x.x area 1
In term of YANG model, the routing protocol configuration will be
defined within the default routing-instance and the routing-
protocol config will contain multiple instances referring to other
routing-instances.
VRF centric - All the protocol related config for a routing-
instance is contained within this routing-instance.
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Routing-instance VRF1 {
Protocols isis {
}
}
Routing-instance VRF2 {
Protocols isis {
}
}
In terms of YANG model, the routing protocol configuration for a
routing-instance will be defined within the associated routing-
instance.
The bottom line message is that, even if YANG models are
standardized, they will provide different CLI outcomes, simply
because the CLI among vendors is not standardized.
3.5. Proprietary YANG Model
While waiting for the Standard YANG Models to be published, the
different vendors might offer Proprietary YANG Models.
4. Typical Architecture
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+--------------------------------------------------------+
| OSS/BSS |
+--------------------------------------------------------+
+--------------------------------------------------------+
| Orchestrator |
| +------------------------------------------------+ |
| | Network Service Model | |
| | | |
| +------------------------------------------------+ |
+--------------------------------------------------------+
+--------------------------------------------------------+
| Network Element |
| | |
| +-----------------------+ | +-------------------+ |
| | Standard YANG model | | | Proprietary | |
| | | | | YANG Model | |
| +-----------------------+ | | | |
| | | | |
| +-----------------------+ | | | |
| | Proprietary Extension | | | | |
| | To YANG Standard | | | | |
| | Model | | | | |
| +-----------------------+ | +-------------------+ |
| | |
| +-------------------------------------------------+ |
| | Vendor Configuration Model | |
| +-------------------------------------------------+ |
+--------------------------------------------------------+
Fig. 2 Typical Architecture
The OSS/BSS may contains business related models. Those models,
which may or may not be written in YANG, are outside the scope of the
IETF work
5. IETF, Other SDOs, and open source
IETF, as a standard defining organization (SDO), is well positioned
to standardize Network Element YANG models. With a wide range of
expertise found within its working groups focused on those technology
definitions. As IETF participants implement those protocols, they
have deep expertise about the implementation and finding a common
base standard configuration model between vendors should be a very
viable goal.
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In some situation where the protocols are standardized by different
SDOs, those SDOs should be responsible for its YANG data modeling
effort. For example, the IETF has transferred the responsibility for
some IEEE technology-related MIB modules to the IEEE 802.1 and 802.3
Working Group [RFC4663], [RFC7448]. Similarly, the IEEE should be
responsible for similar YANG data modeling efforts.
Developing Network Service YANG Models requires network operations
expertise. When those operators participate in IETF work, the right
working group can be formed, and those Service YANG Models can be
developed within IETF. However, some other groups, like Metro
Ethernet Forum or CableLabs, could be better positioned for service
modeling related to their area of expertise.
Today there are many open source projects and some of them are
becoming de facto standards, like the Linux kernel. Many such open
source projects, like Open Daylight, OpenStack, etc, are doing very
good work and their work is being accepted and deployed in production
environments. They bring a lot of very valuable experience to other
groups. From IETF perspective, if there is such a work present, it
can be used as a very good starting point for modeling within IETF.
6. Security Considerations
At this stage, authors of the draft didn't look into security
considerations.
7. IANA Considerations
This document requests no action by IANA.
8. Acknowledgements
Thanks to David Ball for his enlightenments on Metro Ethernet Forum
service aspects.
9. Change log [RFC Editor: Please remove]
version 1: restructure the document, add the two dimensions, add the
interaction with the different SDOs and opensource projects, add the
definitions.
version 2: added definitions for config and service models clarified
second dimension of model classification. fixed typos
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10. References
10.1. Normative References
[RFC6020] Bjorklund, M., "YANG - A Data Modeling Language for the
Network Configuration Protocol (NETCONF)", RFC 6020,
October 2010.
10.2. Informative References
[I-D.ietf-netmod-acl-model]
Bogdanovic, D., Sreenivasa, K., Huang, L., and D. Blair,
"Network Access Control List (ACL) YANG Data Model",
draft-ietf-netmod-acl-model-02 (work in progress), March
2015.
[I-D.ietf-netmod-routing-cfg]
Lhotka, L. and A. Lindem, "A YANG Data Model for Routing
Management", draft-ietf-netmod-routing-cfg-19 (work in
progress), May 2015.
[I-D.ietf-ospf-yang]
Yeung, D., Qu, Y., Zhang, J., Bogdanovic, D., and K.
Sreenivasa, "Yang Data Model for OSPF Protocol", draft-
ietf-ospf-yang-00 (work in progress), March 2015.
[RFC4663] Harrington, D., "Transferring MIB Work from IETF Bridge
MIB WG to IEEE 802.1 WG", RFC 4663, September 2006.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
Bierman, "Network Configuration Protocol (NETCONF)", RFC
6241, June 2011.
[RFC6728] Muenz, G., Claise, B., and P. Aitken, "Configuration Data
Model for the IP Flow Information Export (IPFIX) and
Packet Sampling (PSAMP) Protocols", RFC 6728, October
2012.
[RFC7223] Bjorklund, M., "A YANG Data Model for Interface
Management", RFC 7223, May 2014.
[RFC7448] Taylor, T. and D. Romascanu, "MIB Transfer from the IETF
to the IEEE 802.3 WG", RFC 7448, February 2015.
[Writable-MIB-Module-IESG-Statement]
"Writable MIB Module IESG Statement",
<https://www.ietf.org/iesg/statement/writable-mib-
module.html>.
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Authors' Addresses
Dean Bogdanovic
Juniper Networks
Email: deanb@juniper.net
Benoit Claise
Cisco Systems, Inc.
Email: bclaise@cisco.com
Carl Moberg
Cisco Systems, Inc.
Email: camoberg@cisco.com
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