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


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

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This Internet-Draft will expire on December 5, 2015.

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Table of Contents

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 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:

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:

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.

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 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 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 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.

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 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 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.

            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

            Routing-instance VRF1 {
              Protocols isis {

            Routing-instance VRF2 {
              Protocols isis {


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

        |                      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.

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

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", Internet-Draft draft-ietf-netmod-acl-model-02, March 2015.
[I-D.ietf-netmod-routing-cfg] Lhotka, L. and A. Lindem, "A YANG Data Model for Routing Management", Internet-Draft draft-ietf-netmod-routing-cfg-19, May 2015.
[I-D.ietf-ospf-yang] Yeung, D., Qu, Y., Zhang, J., Bogdanovic, D. and K. Sreenivasa, Yang Data Model for OSPF Protocol", Internet-Draft draft-ietf-ospf-yang-00, 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"

Authors' Addresses

Dean Bogdanovic Juniper Networks EMail:
Benoit Claise Cisco Systems, Inc. EMail:
Carl Moberg Cisco Systems, Inc. EMail: