NETMOD Working Group L. Lhotka
Internet-Draft CZ.NIC
Intended status: Standards Track April 16, 2015
Expires: October 18, 2015

Defining and Using Metadata with YANG


This document defines a YANG extension statement that allows for defining syntax of metadata annotions in YANG modules. The document also specifies XML and JSON encoding of annotations and other rules for annotating instances of YANG data nodes.

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Table of Contents

1. Introduction

There is a need to be able to annotate instances of YANG [I-D.ietf-netmod-rfc6020bis] data nodes with metadata. Typical use cases are:

However, metadata annotations could potentially lead to interoperability problems if they are used in an ad hoc fashion by different organizations and/or without proper documentation. A sound metadata framework for YANG should therefore satisfy these requirements:

  1. The set of annotations must be extensible in a distributed manner so as to allow for defining new annotations without running into the risk of collisions with annotations defined and used by others.
  2. Syntax and semantics of annotations must be documented and the documentation must be easily accessible.
  3. Clients of network management protocols such as NETCONF [RFC6241] or RESTCONF [I-D.ietf-netconf-restconf] must be able to learn all annotations supported by a given server and identify each of them correctly.

This document proposes a systematic way for defining the syntax of metadata annotations. For this purpose, YANG extension statement "annotation" is defined in the module "ietf-yang-metadata" (Section 6). Other YANG modules importing this module can use the "annotation" statement for defining the syntax of one or more annotations.

The benefits of defining the syntax of metadata annotations in a YANG module are the following:

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

The following terms are defined in [RFC6241]:

The following terms are defined in [I-D.ietf-netmod-rfc6020bis]:

The following terms are defined in [W3C.REC-xml-infoset-20040204]:

The following terms are defined in [RFC7159]:

In the following text, XML element names and YANG extension statements are always written with explicit namespace prefixes that are assumed to be bound to URI references as shown in Table 1.

Used namespace prefixes and corresponding URI references
Prefix URI Reference
md urn:ietf:params:xml:ns:yang:ietf-yang-metadata

3. Defining Annotations in YANG

Metadata annotations are defined by YANG extension statement "md:annotation". This YANG language extension is defined in the module "ietf-yang-metadata" (Section 6).

Substatements of "md:annotation" are shown in Table 2. They are all core YANG statements, and the numbers in the second column refer to the corresponding sections in [I-D.ietf-netmod-rfc6020bis] where each statement is described.

Substatements of "md:annotation".
substatement RFC 6020bis section cardinality
description 7.20.3 0..1
if-feature 7.19.2 0..n
reference 7.20.4 0..1
status 7.20.2 0..1
type 7.6.3 0..1
units 7.3.3 0..1

Using the "type" statement, a type may be specified for the annotation value according to the same rules as for YANG leaf or leaf-list types. However, the "type" statement is optional as a substatement of "md:annotation" statement. If it is not present, the built-in "string" type is the default.

The "if-feature" statement, if present, makes the annotation conditional: it is supported only by servers that advertise the corresponding feature.

For example, the following module defines the "inactive" annotation:

module example-inactive {
  namespace "";
  prefix "ein";
  import ietf-yang-metadata {
    prefix "md";
  md:annotation inactive {
    type boolean;
      "If this annotation is attached to a configuration data node,
       then the data subtree rooted at this node is deactivated.";

By advertising a YANG module in which metadata annotation A is defined using the "md:annotation" statement, a server specifies support for the syntax of annotation A. This means that configuration or state data, RPC messages and notifications will be considered syntactically valid if annotation A is attached to any data node instance, provided that all rules stated in this document are observed.

However, the semantics of an annotation, usage rules, and expected behavior of clients and servers MUST be specified separately by other means that are outside the scope of this document.

4. The Encoding of Annotations

XML attributes are a natural choice for encoding metadata in XML instance documents. For JSON [RFC7159], there is no generally established method for encoding metadata. This document thus introduces a special encoding method that is consistent with the JSON encoding of YANG data node instances as defined in [I-D.ietf-netmod-yang-json].

4.1. XML Encoding

Metadata annotations are added to XML-encoded instances of YANG data nodes as XML attributes according to these rules:

  • The local name of the attribute SHALL be the same as the name of the annotation specified in the argument of the corresponding "md:annotation" statement.
  • The namespace of the attribute SHALL be identified by the URI that appears as the argument of the "namespace" statement in the YANG module where the annotation is defined. It is RECOMMENDED that the prefix specified by the "prefix" statement in the same module is used in the qualified name of the attribute.
  • The attribute value SHALL be encoded in the same way as the value of a YANG leaf instance having the same type, see [I-D.ietf-netmod-rfc6020bis], sec. 9.

For example, the "inactive" annotation defined in Section 3 may be encoded as follows:

<foo xmlns:ein=""

4.2. JSON Encoding

The JSON metadata encoding defined in this section has the following properties:

  1. The encoding of YANG data node instances as defined in [I-D.ietf-netmod-yang-json] does not change.
  2. Namespaces of metadata annotations are encoded in the same way as namespaces of YANG data node instances, see [I-D.ietf-netmod-yang-json].

4.2.1. Metadata Object and Annotations

All metadata annotations assigned to a YANG data node instance are encoded as members (name/value pairs) of a single JSON object, henceforth denoted as the metadata object. The placement and name of this object depends on the type of the data node as specified in the following subsections.


The name of a metadata annotation (as a member of the metadata object) SHALL be of the following form:

Note that unlike YANG data node instances, for annotations the explicit namespace identifier (MODULE_NAME) must always be used.

The value of a metadata annotation SHALL be encoded in exactly the same way as the value of a YANG leaf node having the same type as the annotation, see [I-D.ietf-netmod-yang-json], sec. 6.

4.2.2. Adding Annotations to Container and List Instances

For an instance that is encoded as a JSON object (i.e., a container or list entry), the metadata object is added as a new member of that object with the name "@".

"cask": {
  "@": {
    "example-inactive:inactive": true
"seq": [
    "@": {
      "example-inactive:inactive": true
    "name": "one",
    "name": "two",


  • "cask" is a container node:
  • "seq" is a list whose key is "name", annotation "inactive" is added only to the first entry:

4.2.3. Adding Annotations to Anyxml and Leaf Instances

For an anyxml or leaf instance, the metadata object is added as a sibling name/value pair whose the name is the symbol "@" concatenated with the name of the leaf or anyxml member that is being annotated.

"flag": true,
"@flag": {
  "example-inactive:inactive": true

For example, if "flag" is a leaf node of the "boolean" type:

4.2.4. Adding Annotations to Leaf-list Instances

For a leaf-list instance, which is represented as a JSON array with values of a primitive type, annotations may be assigned to one or more entries by adding a name/array pair as a sibling of the leaf-list instance, where the name is symbol "@" concatenated with the name of the leaf-list that is being annotated, and the value is a JSON array whose i-th element is the metadata object with annotations assigned to the i-th entry of the leaf-list instance, or null if the i-th entry has no annotations.

Trailing null values in that array, i.e., those following the last non-null metadata object, MAY be omitted.

"bibliomod:folio": [6, 3, 7, 8],
"@bibliomod:folio": [
  {"example-inactive:inactive": true},
  {"example-inactive:inactive": true}

For example, in the following leaf-list instance with four entries, the "inactive" annotation is added to the second and third entry in the following way:

5. Representing Annotations in DSDL Schemas

[RFC6110] defines the standard mapping of YANG data models to Document Schema Definition Languages (DSDL) [ISO.19757-1]. This section specifies the mapping for the extension statement "md:annotation" (Section 6), which enables validation of XML instance documents containing metadata annotations.

    md:annotation ARGUMENT;
    md:annotation ARGUMENT {
    <rng:attribute name="PREFIX:ARGUMENT">

The first step of the DSDL mapping procedure, i.e., the transformation of the YANG data model to the hybrid schema (see sec. 6 in [RFC6110]), is modified as follows:

  1. If the data model contains at least one "md:annotation" statement, then a RELAX NG named pattern definition MUST be added as a child of the root <rng:grammar> element in the hybrid schema. It is RECOMMENDED to use the name "__yang_metadata__" for this named pattern.
  2. A reference to the named pattern described in item 1 MUST be included as a child of every <rng:element> pattern that corresponds to a container, leaf, list or leaf-list data node.
  3. Every metadata annotation definition in the form 1.
  4. Substatements of "md:annotation", if there are any, SHALL be mapped to children of the "rng:attribute" pattern exactly as described in sec. 10 of [RFC6110].

For example, the named pattern definition (item 1), when constructed only for the "inactive" annotation, will have the following form:

<rng:define name="__yang_metadata__">
    <rng:attribute name="ein:inactive">

Every "rng:element" pattern that corresponds to a container, leaf, list or leaf-list data node will then contain a reference to the above named pattern, for example

<rng:element name="foo:bar">
  <rng:ref name="__yang_metadata__"/>

Note that it is not necessary to use such a reference for "rng:element" patterns corresponding to anyxml data nodes because they already permit any XML attributes to be attached to their instances.

The second step of the DSDL mapping procedure, i.e., the transformation of the hybrid schema to RELAX NG, Schematron and DSRL schemas, is unaffected by the inclusion of "md:annotation".

6. Metadata YANG Module

RFC Editor: In this section, replace all occurrences of 'XXXX' with the actual RFC number and all occurrences of the revision date below with the date of RFC publication (and remove this note).

<CODE BEGINS> file "ietf-yang-metadata@2015-04-16.yang"

module ietf-yang-metadata {

  namespace "urn:ietf:params:xml:ns:yang:ietf-yang-metadata";

  prefix "md";

    "IETF NETMOD (NETCONF Data Modeling Language) Working Group";

    "WG Web:   <>
     WG List:  <>

     WG Chair: Thomas Nadeau

     WG Chair: Juergen Schoenwaelder

     Editor:   Ladislav Lhotka

    "This YANG module defines an extension statement that allows for
     defining metadata annotations.

     Copyright (c) 2014 IETF Trust and the persons identified as
     authors of the code. All rights reserved.

     Redistribution and use in source and binary forms, with or
     without modification, is permitted pursuant to, and subject to
     the license terms contained in, the Simplified BSD License set
     forth in Section 4.c of the IETF Trust's Legal Provisions
     Relating to IETF Documents

     The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
     'OPTIONAL' in the module text are to be interpreted as described
     in RFC 2119 (

     This version of this YANG module is part of RFC XXXX
     (; see the RFC itself for
     full legal notices.";

  revision 2015-04-16 {
      "Initial revision.";
      "RFC XXXX: Defining and Using Metadata with YANG";

  extension annotation {
    argument name;
      "This extension allows for defining metadata annotations in
       YANG modules. The 'md:annotation' statement can appear only at
       the top level of a YANG module.

       An annotation defined with this extension statement inherits
       the namespace and other context from the YANG module in which
       it is defined.

       Other properties of the annotation and documentation are
       specified using the following standard YANG substatements (all
       are optional): 'description', 'if-feature', 'reference',
       'status', 'type' and 'units'. If the 'type' statement is not
       present, the built-in 'string' type is used by default.

       A server announces syntactic support for a particular
       annotation by including the module in which the annotation is
       defined among the advertised YANG modules (e.g., in NETCONF
       hello message).

       Semantics and usage rules for an annotation MUST be specified
       separately. The 'description' and/or 'reference' statements
       SHOULD provide corresponding links.

       XML and JSON encoding of annotations is defined in
       RFC XXXX.";


7. IANA Considerations

RFC Editor: In this section, replace all occurrences of 'XXXX' with the actual RFC number and all occurrences of the revision date below with the date of RFC publication (and remove this note).

This document registers the following namespace URI in the IETF XML registry [RFC3688]:

URI: urn:ietf:params:xml:ns:yang:ietf-yang-metadata

Registrant Contact: The IESG.

XML: N/A, the requested URI is an XML namespace.

This document registers the following YANG module in the YANG Module Names registry [I-D.ietf-netmod-rfc6020bis]:

name:         ietf-yang-metadata
namespace:    urn:ietf:params:xml:ns:yang:ietf-yang-metadata
prefix:       md
reference:    RFC XXXX

8. Security Considerations

This document introduces a mechanism for defining the syntax of metadata annotations in YANG modules and using them with instances of YANG data nodes. By itself, this mechanism represents no security threat. Security implications of a particular annotation defined using this mechanism have to be duly considered and documented in the specification of the annotation's semantics.

9. Acknowledgments

The author wishes to thank Andy Bierman, Martin Bjorklund and Kent Watsen for their helpful comments and suggestions.

10. References

10.1. Normative References

[I-D.ietf-netmod-rfc6020bis] Bjorklund, M., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", Internet-Draft draft-ietf-netmod-rfc6020bis-04, March 2015.
[I-D.ietf-netmod-yang-json] Lhotka, L., "JSON Encoding of Data Modeled with YANG", Internet-Draft draft-ietf-netmod-yang-json-03, February 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, January 2004.
[RFC6110] Lhotka, L., "Mapping YANG to Document Schema Definition Languages and Validating NETCONF Content", RFC 6110, February 2011.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J. and A. Bierman, "Network Configuration Protocol (NETCONF)", RFC 6241, June 2011.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, March 2014.
[W3C.REC-xml-infoset-20040204] Cowan, J. and R. Tobin, "XML Information Set (Second Edition)", World Wide Web Consortium Recommendation REC-xml-infoset-20040204, February 2004.

10.2. Informative References

[I-D.ietf-netconf-restconf] Bierman, A., Bjorklund, M. and K. Watsen, "RESTCONF Protocol", Internet-Draft draft-ietf-netconf-restconf-04, January 2015.
[ISO.19757-1] International Organization for Standardization, "Document Schema Definition Languages (DSDL) - Part 1: Overview", ISO/IEC 19757-1, November 2004.

Appendix A. Change Log

RFC Editor: Remove this section upon publication as an RFC.

A.1. Changes Between draft-lhotka-netmod-yang-metadata-01 and draft-ietf-netmod-yang-metadata-00

  • References to RFC 6020 were changed to the 6021bis I-D.
  • Text about RFC 2119 key words was added to "ietf-yang-metadata" module description.

A.2. Changes Between draft-lhotka-netmod-yang-metadata-00 and -01

  • Encoding of annotations for anyxml nodes was changed to be the same as for leafs. This was necessary because anyxml value now needn't be an object.
  • It is stated that "md:annotation" statement defines only the syntax of an annotation.
  • Allowed "if-feature" as a substatement of "md:annotation".

Author's Address

Ladislav Lhotka CZ.NIC EMail: