YANG


YANG is a data modeling language for the definition of data sent over network management protocols such as the NETCONF and RESTCONF. The YANG data modeling language is maintained by the NETMOD working group in the Internet Engineering Task Force and was published as RFC 6020 in October 2010. The data modeling language can be used to model both configuration data as well as state data of network elements. Furthermore, YANG can be used to define the format of event notifications emitted by network elements and it allows data modelers to define the signature of remote procedure calls that can be invoked on network elements via the NETCONF protocol. The language, being protocol independent, can then be converted into any encoding format, e.g. XML or JSON, that the network configuration protocol supports.
YANG is a modular language representing data structures in an XML tree format. The data modeling language comes with a number of built-in data types. Additional application specific data types can be derived from the built-in data types. More complex reusable data structures can be represented as groupings. YANG data models can use XPATH expressions to define constraints on the elements of a YANG data model.

History

Many network management protocols have associated data modeling languages. The first widely deployed Internet standard for network management was the Simple Network Management Protocol. The data modeling language associated with SNMP was called the Structure of Management Information. The SMI language itself was based on the 1988 version of the Abstract Syntax Notation One. The current version of the SMI language, SMIv2 defined in RFC 2578, RFC 2579, RFC 2580, has developed into an extended subset of ASN.1.
In the late 1990s, a project was started to create a replacement for SMIv2, which was called SMIng. One motivation was to decouple SMIng from the management protocol SNMP and to give SMIng a syntactic structure that is both easy to parse for computer programs and easy to learn for people familiar with programming languages that use a C-like notation. While the SMIng project did not succeed in the IETF, the SMIng specifications were published as experimental documents in May 2004.
Soon after the development of the NETCONF protocol in the IETF, it became clear that a data modeling language was needed to define data models manipulated by the NETCONF protocol. A design team created a proposal that became the basis of the YANG language. The syntactic structure and the base type system was essentially borrowed from SMIng. However, based on the lessons learned from the SMIng project, no attempts were made to make YANG protocol neutral. Instead, YANG ties into concepts of the NETCONF protocol, such as the assumption that data model instances can be serialized into XML. Standardization of YANG started with the formation of the NETMOD working group in April 2008. The YANG 1.0 specification was published as RFC 6020 in October 2010. Recently, the NETMOD working group has been working on YANG 1.1, which has been published in August 2016 in RFC 7950.

Example

The following YANG module example-sports shows a data model for team sports. The module declares a namespace and a prefix and imports the type library module ietf-yang-types before defining the type season. It then defines a container sports that includes a list of persons and a list of teams. A team has a list of players that reference persons via the leafref type and its path restriction.

module example-sports

XML encoding

The code block below shows the XML representation of an instantiation of the example-sports data model.




Lionel Andrés Messi
1987-06-24T00:00:00-00:00


Cristiano Ronaldo
1985-02-05T00:00:00-00:00


FC Barcelona

Lionel Andrés Messi
Champions League 2014/2015
10
43



Real Madrid

Cristiano Ronaldo
Champions League 2014/2015
7
48




JSON encoding

The code block below shows the JSON representation of an instantiation of the example-sports data model.

Documentation

Language specifications and architectural documents

The following requests for comments define the YANG language and some basic extensions:
The following requests for comments provide guidelines and supporting documentation:

Standards-track protocol specifications

The following requests for comments define standards-track protocols that are defined using YANG modules:
The following request for comments define standards-track YANG data models:
The following requests for comments are experimental specifications that use or extend YANG:
Open source implementations :
Closed source implementations :