Yet Another Next Generation (YANG, /jæŋ/) is a data modeling language for the definition of data sent over network management protocols such as the NETCONF and RESTCONF. Developed and maintained by the NETMOD working group in the Internet Engineering Task Force (IETF), YANG was initially published as RFC 6020 in October 2010, with a significant update to version 1.1 in August 2016 (RFC 7950).
YANG enables comprehensive network automation by providing a standardized way to model the configuration and state data of network elements. The language can be used to define the format of event notifications emitted by network devices and allows data modelers to define the signature of RPCs that can be invoked on network elements via the NETCONF protocol. Being protocol-independent, YANG models can be converted into various encoding formats, including XML, JSON, and CBOR, depending on the network configuration protocol's support.
YANG is a modular language and represents data structures in a hierarchical tree format. It includes numerous built-in data types, with the capability for users to derive additional application-specific types. More complex reusable data structures can be represented as "groupings," which promote model reusability and consistency. YANG data models can use XPath expressions to define constraints on the elements of a YANG data model, enabling validation of configuration data before it is committed to devices.
YANG has become the de facto standard for modeling network device configurations across the telecommunications industry and is widely supported by major network equipment manufacturers. It plays a crucial role in software-defined networking (SDN) and network function virtualization (NFV) environments by providing a consistent interface for programmatic network management.
Contents
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 (SNMP). The data modeling language associated with 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, 2579 and 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, 3781).
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 the 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.
JSON encoding
The code block below shows the JSON representation of an instantiation of the example-sports data model.
XML encoding
The code block below shows the XML representation of an instantiation of the example-sports data model.
Documentation
Language specifications and architectural documents
The following Request for Comments (RFCs) define the YANG language and some basic extensions:
RFC 6020: YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)
RFC 6110: Mapping YANG to Document Schema Definition Languages and Validating NETCONF Content
RFC 7950: The YANG 1.1 Data Modeling Language [does not replace RFC 6020]
RFC 7951: JSON Encoding of Data Modeled with YANG
RFC 7952: Defining and Using Metadata with YANG
RFC 8342: Network Management Datastore Architecture (NMDA)
RFC 8525: YANG Library [obsoletes RFC 7895]
RFC 8528: YANG Schema Mount
RFC 8791: YANG Data Structure Extensions
RFC 9254: Encoding of Data Modeled with YANG in the Concise Binary Object Representation (CBOR)
RFC 9595: YANG Schema Item iDentifier (YANG SID)
Guidelines and supporting documentation
The following requests for comments provide guidelines and supporting documentation:
RFC 9907: Guidelines for Authors and Reviewers of Documents Containing YANG Data Models [obsoletes RFC 8407]
RFC 8199: YANG Module Classification
RFC 8340: YANG Tree Diagrams
RFC 8969: A Framework for Automating Service and Network Management with YANG
RFC 9195: A File Format for YANG Instance Data
RFC 9890: An Update to YANG Module Names Registration
IETF usage
Standards-track protocol specifications
The following requests for comments define standards-track protocols that are (partially) defined using YANG modules:
RFC 6241: Network Configuration Protocol (NETCONF)
RFC 6243: With-defaults Capability for NETCONF
RFC 6470: Network Configuration Protocol (NETCONF) Base Notifications
RFC 8040: RESTCONF Protocol
RFC 8071: NETCONF Call Home and RESTCONF Call Home
RFC 8072: YANG Patch Media Type
RFC 8341: Network Configuration Access Control Model [obsoletes RFC 6536]
RFC 8526: NETCONF Extensions to Support the Network Management Datastore Architecture
RFC 8527: RESTCONF Extensions to Support the Network Management Datastore Architecture
RFC 8572: Secure Zero Touch Provisioning (SZTP)
RFC 8639: Subscription to YANG Notifications
RFC 8640: Dynamic Subscription to YANG Events and Datastores over NETCONF
RFC 8641: Subscription to YANG Notifications for Datastore Updates
RFC 8650: Dynamic Subscription to YANG Events and Datastores over RESTCONF
RFC 10016: System-Defined Configuration
Standards-track data models
The following RFCs define standards-track YANG data models:
RFC 6022: YANG Module for NETCONF Monitoring
RFC 6643: Translation of Structure of Management Information Version 2 (SMIv2) MIB Modules to YANG Modules
RFC 6728: Configuration Data Model for the IP Flow Information Export (IPFIX) and Packet Sampling (PSAMP) Protocols
RFC 7224: IANA Interface Type YANG Module
RFC 7317: A YANG Data Model for System Management
RFC 7407: A YANG Data Model for SNMP Configuration
RFC 8177: YANG Data Model for Key Chains
RFC 8194: A YANG Data Model for LMAP Measurement Agents
RFC 8294: Common YANG Data Types for the Routing Area
RFC 8299: YANG Data Model for L3VPN Service Delivery
RFC 8343: A YANG Data Model for Interface Management [obsoletes RFC 7223]
RFC 8344: A YANG Data Model for IP Management [obsoletes RFC 7277]
RFC 8345: A YANG Data Model for Network Topologies
RFC 8346: A YANG Data Model for Layer 3 Topologies
RFC 8347: A YANG Data Model for the Virtual Router Redundancy Protocol (VRRP)
Experimental specifications
The following requests for comments are experimental specifications that use or extend YANG:
RFC 6095: Extending YANG with Language Abstractions
RFC 7758: Time Capability in NETCONF