Substatements of "md:annotation".

substatement	RFC 6020 section	cardinality
description	7.19.3	0..1
reference	7.19.4	0..1
status	7.19.2	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.

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

module example-inactive {
  namespace "http://example.org/example-inactive";
  prefix "ein";
  import ietf-yang-metadata {
    prefix "md";
  }
  md:annotation inactive {
    type boolean;
    description
      "If this annotation is attached to a configuration data node,
       and its value is 'true', then the server MUST behave
       as if the data subtree rooted at this node was not
       present.";
  }
}

Metadata annotations defined with the "md:annotation" statement may be attached to any valid instance of a data node, i.e., container, leaf, list, leaf-list or anyxml, throughout the data model. Metadata annotations are always optional.

4. The Encoding of Annotations

XML attributes are a natural choice for encoding metadata in XML instance documents. For JSON [6], 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 [7].

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.

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

    <foo xmlns:ein="http://example.org/example-inactive"
         ein:inactive="true">
      ...
    </foo>
	  

4.2. JSON Encoding

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

1. The encoding of YANG data node instances as defined in [7] does not change.
2. Namespaces of metadata annotations are encoded in the same way as namespaces of YANG data node instances, see [7].

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.

    MODULE_NAME:LOCAL_NAME
	    

The name of a metadata annotation (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.

4.2.2. Adding Annotations to Container, Anyxml and List Instances

For an instance that is translated to a JSON object (i.e., a container, anyxml 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",
        ...
      }
    ]
	      

Examples:

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

4.2.3. Adding Annotations to Leaf Instances

For a leaf instance, the metadata object is added as a sibling name/value pair whose the name is the symbol "@" concatenated with the identifier of the leaf.

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

For example, if "flag" is a leaf node:

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 the leaf-list instance, where the name is the symbol "@" concatenated with the identifier of the leaf-list, 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 the array, i.e., those following the last non-null metadata object, MAY be omitted.

    "folio": [6, 3, 7, 8],
    "@folio": [
      null,
      {"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

RFC 6110 [4] defines a standard mapping of YANG data models to Document Schema Definition Languages (DSDL) [9]. 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">
      ...
    </rng:attribute>
	  

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 [4]), 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 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 [4].

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">
        <rng:choice>
          <rng:value>true</rng:value>
          <rng:value>false</rng:value>
        </rng:choice>
      </rng:attribute>
    </rng:define>
	

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__"/>
      ...
    </rng:element>
	

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 Ed.: 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 "yang-metadata@2014-09-11.yang"

module ietf-yang-metadata {

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

  prefix "md";

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

  contact
    "Editor:   Ladislav Lhotka
               <mailto:lhotka@nic.cz>";

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

  revision 2014-09-11 {
    description
      "Initial revision.";
    reference
      "RFC XXXX: Defining and Using Metadata with YANG";
  }

  extension annotation {
    argument name;
    description
      "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 may be
       specified using the following standard YANG substatements (all
       are optional and may appear only once): 'type', 'description',
       'reference', 'status' and 'units'. If the 'type' statement is
       not present, the built-in 'string' type is used by default.

       A server announces 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).
       Depending on the prescribed usage patterns, the annotation
       then may be attached by the server and/or client to any valid
       instance of a data node defined by the server's data model.

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

<CODE ENDS>

7. IANA Considerations

RFC Ed.: In this section, replace all occurrences of 'XXXX' with the actual RFC number (and remove this note).

This document registers the following namespace URI in the IETF XML registry [2]:

----------------------------------------------------------
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 [3]:

-------------------------------------------------------------------
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 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.

9. References

9.1. Normative References

[1]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[2]	Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, January 2004.
[3]	Bjorklund, M., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, October 2010.
[4]	Lhotka, L., "Mapping YANG to Document Schema Definition Languages and Validating NETCONF Content", RFC 6110, February 2011.
[5]	Enns, R., Bjorklund, M., Schoenwaelder, J. and A. Bierman, "Network Configuration Protocol (NETCONF)", RFC 6241, June 2011.
[6]	Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, March 2014.
[7]	Lhotka, L., "JSON Encoding of Data Modeled with YANG", Internet-Draft draft-ietf-netmod-yang-json-00, April 2014.
[8]	Cowan, J. and R. Tobin, "XML Information Set (Second Edition)", World Wide Web Consortium Recommendation REC-xml-infoset-20040204, February 2004.

9.2. Informative References

[9]	International Organization for Standardization, "Document Schema Definition Languages (DSDL) - Part 1: Overview", ISO/IEC 19757-1, November 2004.
[10]	Bierman, A., Bjorklund, M., Watsen, K. and R. Fernando, "RESTCONF Protocol", Internet-Draft draft-ietf-netconf-restconf-01, July 2014.

Author's Address