YANG

Yet Another Next Generation (YANG, /jæŋ/, which rhymes with "hang")^[1]^[2]^[3] is a data modeling language for the definition of data sent over network management protocols such as the NETCONF^[4] and RESTCONF.^[5] The YANG data modeling language is maintained by the NETMOD ^[6] working group in the Internet Engineering Task Force (IETF) and initially was published as RFC 6020 in October 2010, with an update in August 2016 (RFC 7950). 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 (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.^[7] 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.^[2]

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 {

  namespace "http://example.com/example-sports";
  prefix sports;

  import ietf-yang-types { prefix yang; }

  typedef season {
    type string;
    description
      "The name of a sports season, including the type and the year, e.g,
       'Champions League 2014/2015'.";
  }

  container sports {
    config true;

    list person {
      key "name";
      leaf name { type string; }
      leaf birthday { type yang:date-and-time; mandatory true; }
    }

    list team {
      key "name";
      leaf name { type string; }
      list player {
        key "name season";
        unique number;
        leaf name { type leafref { path "/sports/person/name"; }  }
        leaf season { type season; }
        leaf number { type uint16; mandatory true; }
        leaf scores { type uint16; default 0; }
      }
    }
  }
}

JSON encoding

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

{
  "example-sports:sports": {
    "person": [
      {
        "name": "Lionel Andrés Messi",
        "birthday": "1987-06-24T00:00:00-00:00"
      },
      {
        "name": "Cristiano Ronaldo",
        "birthday": "1985-02-05T00:00:00-00:00"
      }
    ],
    "team": [
      {
        "name": "FC Barcelona",
        "player": [
          {
            "name": "Lionel Andrés Messi",
            "season": "Champions League 2014/2015",
            "number": 10,
            "scores": 43
          }
        ]
      },
      {
        "name": "Real Madrid",
        "player": [
          {
            "name": "Cristiano Ronaldo",
            "season": "Champions League 2014/2015",
            "number": 7,
            "scores": 48
          }
        ]
      }
    ]
  }
}

XML encoding

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

<data xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">

  <sports xmlns="http://example.com/example-sports">
    <person>
      <name>Lionel Andrés Messi</name>
      <birthday>1987-06-24T00:00:00-00:00</birthday>
    </person>
    <person>
      <name>Cristiano Ronaldo</name>
      <birthday>1985-02-05T00:00:00-00:00</birthday>
    </person>
    <team>
      <name>FC Barcelona</name>
      <player>
        <name>Lionel Andrés Messi</name>
        <season>Champions League 2014/2015</season>
        <number>10</number>
        <scores>43</scores>
      </player>
    </team>
    <team>
      <name>Real Madrid</name>
      <player>
        <name>Cristiano Ronaldo</name>
        <season>Champions League 2014/2015</season>
        <number>7</number>
        <scores>48</scores>
      </player>
    </team>
  </sports>

</data>

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 8407: Guidelines for Authors and Reviewers of Documents Containing YANG Data Models [obsoletes RFC 6087]
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

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

Standards-track data models

The following RFCs define standards-track YANG data models:

RFC 6022: YANG Module for NETCONF Monitoring
RFC 6991: Common YANG Data Types [obsoletes RFC 6021]
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)
RFC 8348: A YANG Data Model for Hardware Management
RFC 8349: A YANG Data Model for Routing Management (NMDA Version) [obsoletes RFC 8022]
RFC 8366: A Voucher Artifact for Bootstrapping Protocols
RFC 8431: A YANG Data Model for the Routing Information Base (RIB)
RFC 8466: A YANG Data Model for Layer 2 Virtual Private Network (L2VPN) Service Delivery
RFC 8512: A YANG Module for Network Address Translation (NAT) and Network Prefix Translation (NPT)
RFC 8513: A YANG Data Model for Dual-Stack Lite (DS-Lite)
RFC 8519: YANG Data Model for Network Access Control Lists (ACLs)
RFC 8520: Manufacturer Usage Description Specification
RFC 8529: YANG Data Model for Network Instances
RFC 8530: YANG Model for Logical Network Elements
RFC 8531: Generic YANG Data Model for Connection-Oriented Operations, Administration, and Maintenance (OAM) Protocols
RFC 8532: Generic YANG Data Model for the Management of Operations, Administration, and Maintenance (OAM) Protocols That Use Connectionless Communications
RFC 8533: A YANG Data Model for Retrieval Methods for the Management of Operations, Administration, and Maintenance (OAM) Protocols That Use Connectionless Communication
RFC 8542: A YANG Data Model for Fabric Topology in Data-Center Networks
RFC 8561: A YANG Data Model for Microwave Radio Link
RFC 8575: YANG Data Model for the Precision Time Protocol (PTP)
RFC 8632: A YANG Data Model for Alarm Management
RFC 8652: A YANG Data Model for the Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD)
RFC 8675: A YANG Data Model for Tunnel Interface Types
RFC 8676: YANG Modules for IPv4-in-IPv6 Address plus Port (A+P) Softwires
RFC 8695: A YANG Data Model for the Routing Information Protocol (RIP)
RFC 8776: Common YANG Data Types for Traffic Engineering
RFC 8795: YANG Data Model for Traffic Engineering (TE) Topologies
RFC 8808: A YANG Data Model for Factory Default Settings
RFC 8916: A YANG Data Model for the Multicast Source Discovery Protocol (MSDP)
RFC 8944: A YANG Data Model for Layer 2 Network Topologies
RFC 8960: A YANG Data Model for MPLS Base
RFC 8819: YANG Module Tags
RFC 9020: YANG Data Model for Segment Routing
RFC 9061: A YANG Data Model for IPsec Flow Protection Based on Software-Defined Networking (SDN)
RFC 9067: A YANG Data Model for Routing Policy
RFC 9070: YANG Data Model for MPLS LDP
RFC 9093: A YANG Data Model for Layer 0 Types
RFC 9094: A YANG Data Model for Wavelength Switched Optical Networks (WSONs)
RFC 9105: A YANG Data Model for Terminal Access Controller Access-Control System Plus (TACACS+)
RFC 9108: YANG Types for DNS Classes and Resource Record Types
RFC 9166: A YANG Data Model for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping
RFC 9179: A YANG Grouping for Geographic Locations
RFC 9181: A Common YANG Data Model for Layer 2 and Layer 3 VPNs
RFC 9196: YANG Modules Describing Capabilities for Systems and Datastore Update Notifications
RFC 9243: A YANG Data Model for DHCPv6 Configuration
RFC 9249: A YANG Data Model for NTP
RFC 9291: A YANG Network Data Model for Layer 2 VPNs
RFC 9314: YANG Data Model for Bidirectional Forwarding Detection (BFD)
RFC 9128: YANG Data Model for Protocol Independent Multicast (PIM)
RFC 9129: YANG Data Model for the OSPF Protocol
RFC 9130: YANG Data Model for the IS-IS Protocol
RFC 9194: A YANG Module for IS-IS Reverse Metric
RFC 9348: A YANG Data Model for IP Traffic Flow Security
RFC 9363: A YANG Data Model for Static Context Header Compression (SCHC)
RFC 9375: A YANG Data Model for Network and VPN Service Performance Monitoring
RFC 9398: A YANG Data Model for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Proxy Devices
RFC 9403: A YANG Data Model for RIB Extensions
RFC 9408: A YANG Network Data Model for Service Attachment Points (SAPs)
RFC 9418: A YANG Data Model for Service Assurance
RFC 9472: A YANG Data Model for Reporting Software Bills of Materials (SBOMs) and Vulnerability Information
RFC 9587: YANG Data Model for OSPFv3 Extended Link State Advertisements (LSAs)
RFC 9617: A YANG Data Model for In Situ Operations, Administration, and Maintenance
RFC 9633: Deterministic Networking (DetNet) YANG Data Model
RFC 9640: YANG Data Types and Groupings for Cryptography
RFC 9641: A YANG Data Model for a Truststore
RFC 9642: A YANG Data Model for a Keystore
RFC 9643: YANG Groupings for TCP Clients and TCP Servers
RFC 9644: YANG Groupings for SSH Clients and SSH Servers
RFC 9645: YANG Groupings for TLS Clients and TLS Servers
RFC 9646: Conveying a Certificate Signing Request (CSR) in a Secure Zero-Touch Provisioning (SZTP)
RFC 9647: A YANG Data Model for Babel
RFC 9648: YANG Data Model for TCP
RFC 9656: A YANG Data Model for Microwave Topology
RFC 9684: A YANG Data Model for Challenge-Response-Based Remote Attestation (CHARRA) Procedures Using Trusted Platform Modules (TPMs)

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

References

^ Björklund, Martin (2010). YANG -A Data Modeling Language for the Network Configuration Protocol (NETCONF) (Technical report). IETF. doi:10.17487/RFC6020. RFC6020.
^ ^a ^b Björklund, Martin (2016). The YANG 1.1 Data Modeling Language (Technical report). IETF. doi:10.17487/RFC7950. RFC7950.
^ RFC 8328 : Policy-Based Management Framework for the Simplified Use of Policy Abstractions (SUPA)
^ Enns, Rob; Björklund, Martin; Schönwälder, Jürgen; Bierman, Andy (2011). Network Configuration Protocol (NETCONF) (Technical report). IETF. doi:10.17487/RFC6241. RFC6241.
^ Bierman, Andy; Björklund, Martin; Watsen, Kent (2017). RESTCONF Protocol (Technical report). IETF. doi:10.17487/RFC8040. RFC8040.
^ "Network Modeling Working Group". IETF.
^ Schönwälder, Jürgen; Björklund, Martin; Shafer, Phil (2010). "Network Configuration Management using NETCONF and YANG". IEEE Communications Magazine. 48 (9): 166–173. doi:10.1109/MCOM.2010.5560601. S2CID 736431.

External links

Carl Moberg (2015-11-05). YANG by Example (YouTube). Yokohama: IETF EDU Team. Archived from the original on 2021-12-21.
Stefan Wallin (2014-10-18). YANG Tutorial (YouTube). Stockholm: tail-f. Archived from the original on 2021-12-21.
YANG Model Catalog - Online catalog with a REST API
YANG central - YANG information and tutorials
YANG Discussion Forum - ConfD User Community Forum for discussing YANG related questions.
For a list of YANG-based clients and servers see the NETCONF page.
ISBN 978-0135180396 - "Network Programmability with YANG: The Structure of Network Automation with YANG, NETCONF, RESTCONF, and gNMI"

Implementations

Open source implementations (sorted by name):

clixon is a YANG-based toolchain including interactive CLI, NETCONF and RESTCONF interfaces written in C.
goyang is a YANG parser and compiler written in Go to produce Go language objects
jnc is a pyang-based YANG-to-Java-API compiler
libyang is a YANG parser and toolkit written in C and providing API in C
pyang is an extensible YANG validator and converter written in Python
pyangbind is a pyang-based Python binding generator
Sysrepo is a YANG-based configuration and operational datastore for Unix/Linux applications.
yangbuilder is a builder for YANG, implemented in Apache Groovy (generate yang data models with Apache Groovy, maintain similar models with one source code base)
yang-ide is an Eclipse plugin for editing and visualizing YANG models
yang-js is a YANG parser and evaluator written in CoffeeScript/JavaScript for Node.js and the web browser
- yang-express is a yang-js based Express.js web framework generator
- yang-swagger is a yang-js based Swagger/OpenAPI specification generator
yangson is a Python 3 library for working with JSON encoded configuration and state data modeled using the YANG data modeling language.
YANG Tools is an OpenDaylight Project toolset written in Java
Yang-Explorer - is a pyang-based Yang Browser and RPC Builder Application
ydk-gen is a YANG-to-API compiler generating APIs in multiple languages (e.g. Python, C++)
yuma123 is netconf/YANG toolchain written in C providing: libyuma - API for development of applications supporting runtime compilation of YANG modules, netconfd - modular server, yangcli - interactive command line tool

Closed source implementations (sorted by name):

confd is a commercial management agent toolkit including a YANG compiler
MasterYANG is a YANG model designer, visualizer, and editor for Windows
Visual YANG Designer is a YANG definition file creator/editor/modeler/builder/designer and YANG compiler implemented in Java
yumapro is a commercial management agent toolkit including a YANG compiler

[yang-1-1] Björklund, Martin (2010). YANG -A Data Modeling Language for the Network Configuration Protocol (NETCONF) (Technical report). IETF. doi:10.17487/RFC6020. RFC6020.

[yang-1.1-2] Björklund, Martin (2016). The YANG 1.1 Data Modeling Language (Technical report). IETF. doi:10.17487/RFC7950. RFC7950.

[:0-3] RFC 8328 : Policy-Based Management Framework for the Simplified Use of Policy Abstractions (SUPA)

[NETCONF-4] Enns, Rob; Björklund, Martin; Schönwälder, Jürgen; Bierman, Andy (2011). Network Configuration Protocol (NETCONF) (Technical report). IETF. doi:10.17487/RFC6241. RFC6241.

[RESTCONF-5] Bierman, Andy; Björklund, Martin; Watsen, Kent (2017). RESTCONF Protocol (Technical report). IETF. doi:10.17487/RFC8040. RFC8040.

[6] "Network Modeling Working Group". IETF.

[7] Schönwälder, Jürgen; Björklund, Martin; Shafer, Phil (2010). "Network Configuration Management using NETCONF and YANG". IEEE Communications Magazine. 48 (9): 166–173. doi:10.1109/MCOM.2010.5560601. S2CID 736431.

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