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YANG  is a data modeling language for the NETCONF network configuration protocol. The YANG data modeling language was developed by the NETMOD  working group in the Internet Engineering Task Force (IETF) 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.
YANG is a modular language representing data structures in an XML tree format. The data modeling language comes with a number of builtin data types. Additional application specific data types can be derived from the builtin 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.
Many network management protocols have associated data modeling languages. The first widely deployed Internet standard for network management was the Simple Network Management Protocol (SNMP). The data modeling language associated to SNMP was called the Structure of Management Information (SMI). The SMI language itself was based on the 1988 version of the Abstract Syntax Notation One (ASN.1). 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 (RFC 3780, RFC 3781).
Soon after the development of the NETCONF protocol in the IETF, it became clear that a data modeling language is 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. Currently, the NETMOD working group is working on YANG 1.1, which is expected to be completed in 2015.
The YANG data modeling language has been used by the following Request for Comments (RFCs):
- RFC 6022: YANG Module for NETCONF Monitoring
- RFC 6991: Common YANG Data Types
- RFC 6087: Guidelines for Authors and Reviewers of YANG Data Model Documents
- RFC 6095: Extending YANG with Language Abstractions
- RFC 6110: Mapping YANG to Document Schema Definition Languages and Validating NETCONF Content
- RFC 6241: Network Configuration Protocol (NETCONF)
- RFC 6243: With-defaults Capability for NETCONF
- RFC 6470: Network Configuration Protocol (NETCONF) Base Notifications
- RFC 6536: Network Configuration Protocol (NETCONF) Access Control Model
- RFC 6643: Translation of Structure of Management Information Version 2 (SMIv2) MIB Modules to YANG Modules
- RFC 7223: A YANG Data Model for Interface Management
- RFC 7224: IANA Interface Type YANG Module
- RFC 7277: A YANG Data Model for IP Management
- pyang - an extensible YANG validator and converter written in Python.
- MG-SOFT Visual YANG Designer - a user friendly YANG definition file creator/editor/modeler/builder/designer, based on MG-SOFT's own YANG compiler, implemented in Java.
- ChampNMS MasterYANG - a YANG data model designer, visualizer and editor.
- yangbuilder - a groovy builder for YANG.
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