This article may be too technical for most readers to understand. Please help improve it to make it understandable to non-experts, without removing the technical details. (October 2021) (Learn how and when to remove this message)

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:

Guidelines and supporting documentation

The following requests for comments provide guidelines and supporting documentation:

IETF usage

Standards-track protocol specifications

The following requests for comments define standards-track protocols that are (partially) defined using YANG modules:

Standards-track data models

The following RFCs define standards-track YANG data models:

Experimental specifications

The following requests for comments are experimental specifications that use or extend YANG:

Implementations

Open source implementations (sorted by name):

Closed source implementations (sorted by name):

References

  1. ^ Björklund, Martin (2010). YANG -A Data Modeling Language for the Network Configuration Protocol (NETCONF) (Technical report). IETF. doi:10.17487/RFC6020. RFC6020.
  2. ^ a b Björklund, Martin (2016). The YANG 1.1 Data Modeling Language (Technical report). IETF. doi:10.17487/RFC7950. RFC7950.
  3. ^ RFC 8328 : Policy-Based Management Framework for the Simplified Use of Policy Abstractions (SUPA)
  4. ^ Enns, Rob; Björklund, Martin; Schönwälder, Jürgen; Bierman, Andy (2011). Network Configuration Protocol (NETCONF) (Technical report). IETF. doi:10.17487/RFC6241. RFC6241.
  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.

External links

Categories