NETMOD Working Group                                           L. Lhotka
Internet-Draft                                                    CZ.NIC
Intended status: Standards Track                          April 21, 2014
Expires:                        October 23, 13, 2014
Expires: April 16, 2015

                JSON Encoding of Data Modeled with YANG
                     draft-ietf-netmod-yang-json-00
                     draft-ietf-netmod-yang-json-01

Abstract

   This document defines encoding rules for representing configuration and configuration,
   state
   data data, RPC input and output parameters, and notifications
   defined using YANG as JSON JavaScript Object Notation (JSON) text.  It does so by specifying a
   procedure for translating the subset of YANG-compatible XML documents
   to JSON text, and vice versa.  A JSON encoding of XML attributes is
   also defined so as to allow for including metadata in JSON documents.

Status of This Memo

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   This Internet-Draft will expire on October 23, 2014. April 16, 2015.

Copyright Notice

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

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology and Notation  . . . . . . . . . . . . . . . . . .   4   3
   3.  Specification  Validation of the Translation Procedure JSON-encoded
       Instance Data . . . . . . . . . . . . . . . . . . .   5
     3.1. . . . . .   3
   4.  Names and Namespaces  . . . . . . . . . . . . . . . . . .   6
     3.2.  Mapping XML Elements to JSON Objects . .   4
   5.  Encoding of YANG Data Node Instances  . . . . . . . . . .   8
       3.2.1. . .   6
     5.1.  The "leaf" Data Node  . . . . . . . . . . . . . . . .   8
       3.2.2. . .   6
     5.2.  The "container" Data Node . . . . . . . . . . . . . .   8
       3.2.3. . .   7
     5.3.  The "leaf-list" Data Node . . . . . . . . . . . . . .   9
       3.2.4. . .   7
     5.4.  The "list" Data Node  . . . . . . . . . . . . . . . .   9
       3.2.5. . .   7
     5.5.  The "anyxml" Data Node  . . . . . . . . . . . . . . .  10
     3.3. . .   8
   6.  The Mapping of YANG Datatypes to JSON Values  . . . . . . . . . .  11
       3.3.1.   8
     6.1.  Numeric Datatypes . . . . . . . . . . . . . . . . . .  11
       3.3.2. . .   9
     6.2.  The "string" Type . . . . . . . . . . . . . . . . . .  11
       3.3.3. . .   9
     6.3.  The "boolean" Type  . . . . . . . . . . . . . . . . .  11
       3.3.4. . .   9
     6.4.  The "enumeration" Type  . . . . . . . . . . . . . . .  11
       3.3.5. . .   9
     6.5.  The "bits" Type . . . . . . . . . . . . . . . . . . .  12
       3.3.6. . .   9
     6.6.  The "binary" Type . . . . . . . . . . . . . . . . . .  12
       3.3.7. . .   9
     6.7.  The "leafref" Type  . . . . . . . . . . . . . . . . .  12
       3.3.8. . .  10
     6.8.  The "identityref" Type  . . . . . . . . . . . . . . .  12
       3.3.9. . .  10
     6.9.  The "empty" Type  . . . . . . . . . . . . . . . . . .  12
       3.3.10. . .  10
     6.10. The "union" Type  . . . . . . . . . . . . . . . . . .  13
       3.3.11. . .  11
     6.11. The "instance-identifier" Type  . . . . . . . . . . .  13
   4.  Encoding Metadata in JSON . .  11
   7.  I-JSON Compliance . . . . . . . . . . . . . . . . . .  14
   5.  IANA . . . .  12
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   9.  Acknowledgments . .  16
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   7.  Acknowledgments . .  13
   10. References  . . . . . . . . . . . . . . . . . . . . .  17
   8. . . . .  13
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     10.2.  Informative References . . . . . . . .  17
     8.1.  Normative References . . . . . . . . .  14
   Appendix A.  A Complete Example . . . . . . . . . .  17
     8.2.  Informative References . . . . . . .  14
   Appendix B.  Change Log . . . . . . . . . .  17
   Appendix A.  A Complete Example . . . . . . . . . . .  16
     B.1.  Changes Between Revisions -00 and -01 . . . . . . . .  18 . .  16
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20  17

1.  Introduction

   The aim of this document is define rules NETCONF protocol [RFC6241] uses XML [W3C.REC-xml-20081126] for representing
   configuration and state
   encoding data defined in its Content Layer.  Other management protocols might
   want to use other encodings while still benefiting from using the YANG data modeling
   language
   [RFC6020] as JavaScript Object Notation (JSON)
   text [RFC7159].  The result can be potentially applied in two
   different ways:

   1.  JSON may be used instead of the standard XML [XML] encoding in
       the context of the NETCONF protocol [RFC6241] and/or with
       existing data models expressed in YANG.  An example application
       is modeling language.

   For example, the RESTCONF Protocol [RESTCONF].

   2.  Other documents that choose JSON to represent structured data can
       use YANG for defining the data model, i.e., both syntactic and
       semantic constraints that the data have to satisfy.

   JSON mapping rules could be specified in a similar way as the XML
   mapping rules in [RFC6020].  This would however require solving
   several problems.  To begin with, YANG uses XPath [XPath] quite
   extensively, but XPath is not defined for JSON and such a definition
   would be far from straightforward.

   In order to avoid these technical difficulties, this document employs
   an alternative approach: it defines a relatively simple procedure
   which allows for translating the subset of protocol [I-D.ietf-netconf-restconf]
   supports two encodings: XML that can be modeled
   using YANG to JSON, (media type "application/yang.data+xml")
   and vice versa.  Consequently, validation of a
   JSON text against a data model can done by translating the JSON text
   to XML, which is then validated according to the rules stated in
   [RFC6020].

   The translation procedure is adapted to YANG specifics and
   requirements, namely:

   1. (media type "application/yang.data+json).

   The translation is driven by a concrete specification of the YANG data model and uses
       information about modelling language [RFC6020]
   defines only XML encoding for data types to achieve better results than
       generic XML-JSON translation procedures.

   2.  Various document types are supported, namely configuration data, instances, i.e. contents of
   configuration + datastores, state data, RPC RFC input and output
   parameters, and event notifications.

   3.  XML namespaces specified in the data model are mapped to
       namespaces of JSON objects.  However, explicit namespace
       identifiers are rarely needed in JSON text.

   4.  Section 4 defines JSON encoding  The aim of XML attributes.  Although XML
       attributes cannot be modeled with YANG, they are often used for
       attaching metadata this document is to elements, and a standard JSON
   define rules for encoding is
       therefore needed.

   5.  Translation of XML mixed content, comments and processing
       instructions is outside the scope of this document.

   Item 1 above also means that, depending on the data model, the same
   XML element can be translated data as JavaScript Object Notation
   (JSON) text [RFC7159].

   In order to different JSON objects.  For
   example,

       <foo>123</foo>

   is translated achieve maximum interoperability while allowing
   implementations to
       "foo": 123

   if the "foo" node is defined as use a leaf with the "uint8" datatype, or
   to

       "foo": ["123"]

   if variety of available JSON parsers, the "foo" node is defined JSON
   encoding rules follow, as a leaf-list with the "string"
   datatype, and much as possible, the <foo> element has no siblings constraints of the same name.
   I-JSON restricted profile [I-D.ietf-json-i-json].  Section Section 7
   discusses I-JSON conformance in more detail.

2.  Terminology and Notation

   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 [RFC6020]:

   o  anyxml

   o  augment

   o  container

   o  data node

   o  data tree

   o  datatype

   o  feature

   o  identity

   o  instance identifier

   o  leaf

   o  leaf-list

   o  list

   o  module

   o  submodule

   The following terms are defined in [XMLNS]:

   o  local name

   o  prefixed name

   o  qualified name

3.  Specification of the Translation Procedure

   The translation procedure defines a 1-1 correspondence between the
   subset of YANG-compatible XML documents and JSON text.  This means
   that the translation can be applied in both directions and it is
   always invertible.

   The translation procedure is applicable only to data hierarchies that
   are modelled by a YANG data model.  An input XML document MAY contain
   enclosing elements representing NETCONF "Operations" and "Messages"
   layers.  However, these enclosing elements do not appear in the
   resulting JSON document.

   Any YANG-compatible XML document can be translated, except documents
   with mixed content.  This is only a minor limitation since mixed
   content is marginal in YANG - it is allowed only in anyxml data
   nodes.

   The following sections specify rules mainly for translating XML
   documents to JSON text.  Rules for the inverse translation are stated
   only where necessary, otherwise they can in this
   document are to be easily inferred.

   REQUIRED parameters of the translation procedure are: interpreted as described in [RFC2119].

   The following terms are defined in [RFC6020]:

   o  anyxml

   o  augment

   o  container

   o  YANG  data model consisting of a set of YANG modules, node

   o  type of the input document,  identity

   o  optional features (defined via the "feature" statement) that are
      considered active.

   The permissible types  instance identifier

   o  leaf

   o  leaf-list

   o  list

   o  module

   o  submodule

3.  Validation of input documents JSON-encoded Instance Data

   Instance data validation as defined in [RFC6020] is only applicable
   to XML-encoded data.  For one, semantic constraints in "must"
   statements are listed expressed using XPath 1.0 [W3C.REC-xpath-19991116],
   which can be properly interpreted only in Table 1
   together the XML context.

   This document along with the corresponding part "XML Mapping Rules"
   sections from [RFC6020] also define an implicit schema-driven mapping
   of the data model that is used
   for the translation.

    +------------------------------+---------------------------------+
    | Document Type                | Data Model Section              |
    +------------------------------+---------------------------------+
    | configuration and state data | main data tree                  |
    |                              |                                 |
    | configuration                | main data tree ("config true")  |
    |                              |                                 |
    | RPC input parameters         | "input" data nodes under "rpc"  |
    |                              |                                 |
    | RPC output parameters        | "output" data nodes under "rpc" |
    |                              |                                 |
    | notification                 | "notification" data nodes       |
    +------------------------------+---------------------------------+

                       Table 1: YANG Document Types

   When translating XML JSON-encoded instances to JSON, the type of the input XML-encoded instances (and vice versa).
   This mapping is mostly straightforward.  In cases where doubts could
   arise, this document can
   often be determined form the encapsulating elements belonging gives explicit instructions for mapping JSON-
   encoded instances to the
   "Operations" or "Messages" layer as defined by the NETCONF protocol
   (see Sec. 1.2 in [RFC6241]).

   A particular application MAY decide XML.

   In order to support only validate a subset of
   document types from Table 1.

   XML documents can JSON instance document, it MUST first be translated
   mapped, at least conceptually, to JSON text only if they are valid
   instances of the YANG data model and selected document type, also
   taking into account corresponding XML instance
   document.  By definition, the active features, if there are any.

   The resulting JSON document is always a single object ([RFC7159],
   Sec. 4) whose members are translated from then valid if and only
   if the original XML document
   using is valid according to the rules specified stated in the following sections.

3.1.
   [RFC6020].

4.  Names and Namespaces

   The local part

   Instances of YANG data nodes (leafs, containers, leaf-lists, lists
   and anyxml nodes) are always encoded as members of a JSON object,
   i.e., as name/value pairs.  This section defines how the name part is
   formed, and the following sections deal with the value part.

   Except in the cases specified below, the member name is always identical to
   the local name identifier of the corresponding XML element.

   Each JSON YANG data node.  Every such name lives in
   belongs to a namespace which is uniquely identified by
   the name of associated with the YANG module where
   the corresponding data node is defined.  If the data node is defined
   in a submodule, then the namespace identifier is the name of determined by the main module
   to which the submodule belongs.  The translation procedure MUST correctly map YANG

   If the namespace URIs of a member name has to YANG be explicitly specified, the
   module names and vice versa.

   The namespace name SHALL be expressed in JSON text by prefixing used as a prefix to the local (local) member name.
   Both parts of the member name in SHALL be separated with a colon
   character (":").  In other words, the namespace-qualified name will
   have the following way: form:

           <module name>:<local name>

       Figure 1: Encoding a namespace identifier with a local name.

   The

   Names with namespace identifier identifiers in the form shown in Figure 1 MUST
   be used for local names that are
   ambiguous, i.e., whenever the data model permits a sibling all top-level YANG data nodes, and also for all nodes
   whose parent node
   with the same local name. belongs to a different namespace.  Otherwise, the names
   with namespace identifier is
   OPTIONAL. identifiers MUST NOT be used.

   For example, consider the following YANG module:

   module foomod {

     namespace "http://example.com/foomod";

     prefix "fm"; "foo";

     container foo top {
       leaf bar foo {
         type boolean; uint8;
       }
     }
   }

   If the data model consists only of this module, then the following is
   a valid JSON document: JSON-encoded configuration:

   {
         "foo":
     "foomod:top": {
           "bar": true
       "foo": 54
     }
   }

   Note that the top-level container instance contains the namespace
   identifier (module name) but the "foo" leaf doesn't because it is
   defined in the same module as its parent container.

   Now, assume the container "foo" "top" is augmented from another module: module,
   "barmod":

   module barmod {

     namespace "http://example.com/barmod";

     prefix "bm"; "bar";

     import foomod {
       prefix fm; "foo";
     }

     augment "/fm:foo" "/foo:top" {
       leaf bar {
         type uint8; boolean;
       }
     }
   }

   In the data model combining "foomod" and "barmod", we have two
   sibling data nodes with the same local name, namely "bar".  In this
   case, a

   A valid JSON document has to specify an explicit namespace
   identifier (module name) for JSON-encoded configuration containing both leaves: leafs may then
   look like this:

   {
         "foo":
     "foomod:top": {
           "foomod:bar": true,
       "foo": 54,
       "barmod:bar": 123 true
     }
   }

3.2.  Mapping XML Elements

   The name of the "bar" leaf must be prefixed with the namespace
   identifier because its parent is defined in a different module, hence
   it belongs to JSON Objects

   An XML element that another namespace.

   Explicit namespace identifiers are sometimes needed when encoding
   values of the "identityref" and "instances-identifier" types.  The
   same form as shown in Figure 1 is modelled then used as well.  See Sections
   6.8 and 6.11 for details.

5.  Encoding of YANG Data Node Instances

   Every complete JSON instance document, such as a configuration
   datastore content, is an object.  Its members are instances of all
   top-level data nodes defined by the YANG data model.

   Character encoding MUST be UTF-8.

   Any data node instance is translated to encoded as a name/value pair where the name
   is formed from the name of the XML
   element data node identifier using the rules in of Section 3.1. 4.
   The value depends on the
   type category of the data node as specified explained in
   the following sections.

3.2.1. subsections.

5.1.  The "leaf" Data Node

   An XML element that is modeled as YANG

   A leaf instance is translated to encoded as a name/
   value name/value pair and the type of where the value is derived from can
   be a string, number, literal 'true' or 'false' or the YANG
   datatype special array
   '[null]', depending on the type of the leaf (see Section 3.3 6 for the datatype mapping
   type encoding rules).

   Example: For the leaf node definition

   leaf foo {
     type uint8;
   }

   the XML element

       <foo>123</foo>

   corresponds to the JSON name/value pair following is a valid JSON-encoded instance:

   "foo": 123

3.2.2.

5.2.  The "container" Data Node

   An XML element that is modeled as YANG container instance is translated to encoded as a name/object pair.  The
   container's child data nodes are encoded as members of the object.

   Example: For the container definition

   container bar {
     leaf foo {
       type uint8;
     }
   }

   the XML element

       <bar>
         <foo>123</foo>
       </bar>

   corresponds to the JSON name/value pair following is a valid instance:

   "bar": {
     "foo": 123
   }

3.2.3.

5.3.  The "leaf-list" Data Node

   A sequence of one or more sibling XML elements with the same
   qualified name that is modeled as YANG leaf-list is translated to encoded as a name/array pair, and the array elements
   are primitive values whose type depends on the datatype of the leaf-list (see
   Section 3.3). 6).

   Example: For the leaf-list definition

   leaf-list foo {
     type uint8;
   }

   the XML elements

       <foo>123</foo>
       <foo>0</foo

   correspond to the JSON name/value pair following is a valid instance:

   "foo": [123, 0]

3.2.4.

5.4.  The "list" Data Node

   A sequence of one or more sibling XML elements with the same
   qualified name that is modeled as YANG list instance is translated to encoded as a name/
   array name/array pair, and the array
   elements are JSON objects.

   Unlike the XML encoding, where the list keys are required to come
   before precede any
   other siblings, and to appear in the order specified by the data
   model, the order of members within a JSON JSON-encoded list entry is arbitrary,
   arbitrary because JSON objects are fundamentally unordered
   collections of members.

   Example: For the list definition
   list bar {
     key foo;
     leaf foo {
       type uint8;
     }
     leaf baz {
       type string;
     }
   }

   the XML elements

       <bar>
         <foo>123</foo>
         <baz>zig</baz>
       </bar>
       <bar>
         <foo>0</foo>
         <baz>zag</baz>
       </bar>

   correspond to the JSON name/value pair following is a valid instance:

   "bar": [
     {
       "foo": 123,
       "baz": "zig"
     },
     {
           "foo": 0,
       "baz": "zag" "zag",
       "foo": 0
     }
   ]

3.2.5.

5.5.  The "anyxml" Data Node

   An XML element that is modeled as a YANG anyxml data node instance is translated to a name/object name/value pair.  The content of such an element is
   not modelled by YANG, and there may not value can
   be a straightforward mapping
   to of any valid JSON text (e.g., if it is a mixed XML content).  Therefore,
   translation type, i.e. an object, array, number, string or
   any of anyxml contents is necessarily application-specific the literals 'true', 'false' and outside 'null'.

   This document defines no mapping between the scope contents of this document. JSON- and
   XML-encoded anyxml instances.  It is not necessary because anyxml
   contents are not subject to YANG-based validation (see sec. 7.10 in
   [RFC6020]).

   Example: For the anyxml definition

   anyxml bar;

   the XML element

       <bar>
         <p xmlns="http://www.w3.org/1999/xhtml">
           This is <em>very</em> cool.
         </p>
       </bar>

   may be translated to the following JSON name/value pair:

       {
         "bar": {
           "p": "This is *very* cool."
         }
       }

3.3. a valid instance:

   "bar": [true, null, true]

6.  The Mapping of YANG Datatypes to JSON Values

3.3.1.

   The type of the JSON value in an instance of the leaf or leaf-list
   data node depends on the datatype of that data node as specified in
   the following subsections.

6.1.  Numeric Datatypes

   A value of one of the YANG numeric datatypes ("int8", "int8", "int16", "int32", "int64", "uint8", "uint16", "uint32", "uint16" is
   represented as a JSON number.

   A value of the "int64", "uint64" and
   "decimal64") or "decimal64" type is mapped to encoded as a
   JSON number using string whose contents is the same lexical
   representation.

3.3.2. representation of that
   numeric value as specified in sections 9.2.1 and 9.3.1 of [RFC6020].

   For example, if the type of the leaf "foo" in Section 5.1 was
   "unit64" instead of "uint8", the instance would have to be encoded as

   "foo": "123"

   The special handling of 64-bit numbers follows from I-JSON
   recommendation to encode numbers exceeding the IEEE 754-2000 double
   precision range as strings, see sec. 2.2 in [I-D.ietf-json-i-json].

6.2.  The "string" Type

   A "string" value is mapped to an identical encoded as a JSON string, subject to JSON encoding
   rules.

3.3.3.

6.3.  The "boolean" Type

   A "boolean" value is mapped to the corresponding JSON value literal name
   'true' or 'false'.

3.3.4.

6.4.  The "enumeration" Type

   An "enumeration" value is mapped in the same way as a string except
   that the permitted values are defined by "enum" statements in YANG.

3.3.5.
   See sec. 9.6 in [RFC6020].

6.5.  The "bits" Type

   A "bits" value is mapped to a JSON string identical to the lexical
   representation of this value in XML, i.e., space-separated names
   representing the individual bit values that are set.

3.3.6.  See sec. 9.7 in
   [RFC6020].

6.6.  The "binary" Type

   A "binary" value is mapped to a JSON string identical to the lexical
   representation of this value in XML, i.e., base64-encoded binary
   data.

3.3.7.  See sec. 9.8 in [RFC6020].

6.7.  The "leafref" Type

   A "leafref" value is mapped according to the same rules as the type
   of the leaf being referred to.

3.3.8.

6.8.  The "identityref" Type

   An "identityref" value is mapped to a string representing the
   qualified name of the
   an identity.  Its namespace MAY MUST be expressed as shown in Figure 1.  If the namespace part 1 if
   it is not present, different from the namespace of the name leaf node containing the
   identityref value, and MAY be expressed otherwise.

   For example, consider the following schematic module:

   module exmod {
     ...
     import ietf-interfaces {
       prefix if;
     }
     import iana-if-type {
       prefix ianaift;
     }
     ...
     leaf type {
       type identityref {
         base "if:interface-type";
       }
     }
   }

   A valid instance of the JSON object containing "type" leaf is then encoded as follows:

   "type": "iana-if-type:ethernetCsmacd"

   The namespace identifier "iana-if-type" must be present in this case
   because the "ethernetCsmacd" identity is not defined in the same
   module as the value is
   assumed.

3.3.9. "type" leaf.

6.9.  The "empty" Type

   An "empty" value is mapped to '[null]', i.e., an array with the
   'null' value literal being its only element.

   This encoding was chosen instead of using simply 'null' in order to
   facilitate the use of empty leafs in common programming languages.
   When used in a boolean context, the '[null]' value, unlike 'null',
   evaluates to 'true'.

   Example: For the leaf definition

       leaf foo {
           type empty;
       }

   the XML element

       <foo/>

   corresponds to the JSON name/value pair

       "foo": [null]

3.3.10.  The "union" Type

   YANG "union" type represents a choice among multiple alternative
   types.  The actual type of the XML value MUST be determined using the
   procedure specified in Sec. 9.12 of [RFC6020] and the mapping rules
   for that type are used.

   For example, consider the following YANG definition:

       leaf-list bar {
           type union {
               type uint16;
               type string;
           }
       }

   The sequence of three XML elements

       <bar>6378</bar>
       <bar>14.5</bar>
       <bar>infinity</bar>

   will then be translated to this name/array pair:

       "bar": [6378, "14.5", "infinity"]

3.3.11.  The "instance-identifier" Type

   An "instance-identifier" value is a string representing a simplified
   XPath specification.  It is mapped to an analogical JSON string in
   which all occurrences of XML namespace prefixes are either removed or
   replaced with the corresponding module name according to the rules of
   Section 3.1.

   When translating such a value from JSON to XML, all components of the
   instance-identifier MUST be given appropriate XML namespace prefixes.
   It is RECOMMENDED that these prefixes be those defined via the
   "prefix" statement in the corresponding YANG modules.

   For example, assume "ex" is the prefix defined for the "example"
   module.  Then the XML-encoded instance identifier

       /ex:system/ex:user[ex:name='fred']

   corresponds to the following JSON-encoded instance identifier:

       /example:system/example:user[example:name='fred']

   or simply

       /system/user[name='fred']

   if the local names of the data nodes "system", "user" and "name" are
   unambiguous.

4.  Encoding Metadata in JSON

   By design, YANG does not allow for modeling XML attributes.  However,
   attributes are often used in XML instance documents for attaching
   various types of metadata information to elements.  It is therefore
   desirable to have a standard way for representing attributes in JSON
   documents as well.

   The metadata encoding defined in the rest of this section satisfies boolean context, the following two important requirements:

   1.  There has '[null]' value, unlike 'null',
   evaluates to be true.

   Example: For the leaf definition

   leaf foo {
     type empty;
   }

   a way for adding metadata to instances of all
       types of YANG data nodes, i.e., leafs, containers, list and leaf-
       list entries, and anyxml nodes.

   2. valid instance is

   "foo": [null]

6.10.  The encoding "union" Type

   A value of YANG data node instances as defined in the
       previous sections must not change.

   Existing proposals for metadata encoding in JSON, such "union" type is encoded as
   [JSON-META], are oriented on rather specific uses the value of any of metadata, and
   fall short with respect to the first requirement.

   All attributes assigned to an XML element are mapped in
   member types.

   Unlike XML, JSON to
   members (name/value pairs) conveys part of a single object, henceforth denoted as the metadata object.  The placement type information already in the
   encoding.  When validating a value of the "union" type, this object depends on
   information MUST also be taken into account.

   For example, consider the following YANG definition:

   leaf bar {
     type of union {
       type uint16;
       type string;
     }
   }

   In RESTCONF [I-D.ietf-netconf-restconf], it is fully acceptable to
   set the element from YANG viewpoint, as specified value of "bar" in the following paragraphs.

   For way when using the
   "application/yang.data+xml" media type:

   <bar>13.5</bar>

   because the value may be interpreted as a string, i.e., the second
   member type of the union.  When using the "application/
   yang.data+json" media type, however, this is an XML element that error:

   "bar": 13.5

   In this case, the JSON encoding indicates the value is translated supposed to be
   a JSON object (i.e., a
   container, anyxml node and list entry), the metadata object number rather than string.

6.11.  The "instance-identifier" Type

   An "instance-identifier" value is added encoded as a new member of string that object with the name "@".

   Examples:

   o  If "cask" is a container or anyxml node,
   analogical to the lexical representation in XML instance with
      attributes

       <cask foo="a" bar="b">
         ...
       </cask> encoding, see
   sec. 9.13.3 in [RFC6020].  The only difference is mapped that XML namespace
   prefixes used for qualifying node names in the instance-identifier
   value are replaced by the corresponding module names according to the following JSON object:

       "cask": {
         "@": {
           "foo": "a",
           "bar": "b"
         }
         ...
       }

   o  If "seq" is
   rules of Section 4.

   Conversely, when translating such a list, then value from JSON to XML, the pair
   namespace identifier (YANG module name) in each component of XML elements

       <seq foo="a">
         <name>one</name>
       </seq>
       <seq bar="b">
         <name>two</name>
       </seq>

      is mapped to the following JSON array:

       "seq": [
         {
           "@": {
             "foo": "a"
           },
           "name": "one"
         },
         {
           "@": {
             "bar": "b"
           },
           "name": "two"
         }
       ]

   In order to assign attributes to a leaf instance, a sibling name/
   value pair is added, where
   instance-identifier MUST be replaced by the name XML namespace prefix that
   is the symbol "@" concatenated associated with the identifier namespace URI reference of the leaf. module.

   For example, assume "ex" is the prefix associated with the element

       <flag foo="a" bar="b">true</foo> namespace
   URI that is mapped defined in the "example" module.  Then the XML-encoded
   instance-identifier

   /ex:system/ex:user[ex:name='fred']

   corresponds to the following two name/value pairs:

       "flag": true,
       "@flag": {
         "foo": "a",
         "bar": "b"
       }

   Finally, for a leaf-list instance, which JSON-encoded instance-identifier:

   /example:system/example:user[example:name='fred']

7.  I-JSON Compliance

   I-JSON [I-D.ietf-json-i-json] is represented as a restricted profile of JSON
   array with primitive values, attributes may be assigned to one or
   more entries by adding a sibling name/value pair, where that
   guarantees maximum interoperability for protocols that use JSON in
   their messages, no matter what JSON encoders/decoders are used in
   protocol implementations.  The encoding defined in this document
   therefore observes the name is I-JSON requirements and recommendations as
   closely as possible.

   In particular, the symbol "@" concatenated with following properties are guaranteed:

   o  Character encoding is UTF-8.

   o  Member names within the identifier same JSON object are always unique.

   o  The order of the leaf-list, and
   the value JSON object members is never relied upon.

   o  Numbers of any type supported by YANG can be exchanged reliably.
      See Section 6.1 for details.

   The only two cases where a JSON array whose i-th element is the metadata object
   with attributes assigned instance document encoded according
   to this document may deviate from I-JSON were dictated by the i-th entry of the leaf-list, or nil
   if need to
   be able to encode the i-th entry has no attributes.

   Trailing nil values same instance data in both JSON and XML.  These
   two exceptions are:

   o  Leaf values encoded as strings may contain code points identifying
      Noncharacters that belong to the array, i.e., those following XML character set (see sec. 2.2
      in [W3C.REC-xml-20081126]).

   o  Values of the last non-
   nil metadata object, MAY be omitted.

   For example, a leaf-list instance "binary" type are encoded with four entries

       <folio>6</folio>
       <folio foo="a">3</folio>
       <folio bar="b">7</folio>
       <folio>8</folio>

   is mapped to the following two name/value pairs:

       "folio": [6, 3, 7, 8],
       "@folio": [nil, {"foo": "a"}, {"bar": "b"}]

   The base64 encoding
      scheme (see sec. 9.8.2 in [RFC6020]) whereas I-JSON recommends
      base64url instead.  However, the use of attributes as specified above has base64 should not cause
      any interoperability problems because these values never appear in
      an URL.

8.  Security Considerations

   This document defines an alternative encoding for data modeled in the following two
   limitations:

   o  Mapping
   YANG data modeling language.  As such, it doesn't contribute any new
   security issues beyond those discussed in sec. 15 of namespaces [RFC6020].

   JSON is rather different from XML, and JSON parsers may thus suffer
   from other types of vulnerabilities than their XML attributes counterparts.  To
   minimize these security risks, it is undefined.

   o  Attribute values can only important that client and server
   software supporting JSON encoding behaves as required in sec. 3 of
   [I-D.ietf-json-i-json].  That is, any received JSON data that violate
   any of I-JSON strict constraints MUST NOT be strings, other trusted nor acted upon.
   Violations due to the presence of Unicode Noncharacters in the data types are not
      supported.

5.  IANA Considerations

   TBD - register application/yang.data+json media type?

6.  Security Considerations

   TBD.

7.
   exceptions (see Section 7) SHOULD be carefully examined.

9.  Acknowledgments

   The author wishes to thank Andy Bierman, Martin Bjorklund Bjorklund, Juergen
   Schoenwaelder and Phil Shafer for their helpful comments and
   suggestions.

8.

10.  References

8.1.

10.1.  Normative References

   [I-D.ietf-json-i-json]
              Bray, T., "The I-JSON Message Format", draft-ietf-json-
              i-json-03 (work in progress), August 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for the
              Network Configuration Protocol (NETCONF)", RFC 6020,
              September
              October 2010.

   [RFC6241]  Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
              Bierman, "NETCONF "Network Configuration Protocol", Protocol (NETCONF)", RFC
              6241, June 2011.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, March 2014.

   [XMLNS]    Bray, T., Hollander, D., Layman, A., Tobin, R., and H.
              Thompson, "Namespaces in XML 1.0 (Third Edition)", World
              Wide Web Consortium Recommendation REC-xml-names-20091208,
              December 2009,
              <http://www.w3.org/TR/2009/REC-xml-names-20091208>.

   [XML]

   [W3C.REC-xml-20081126]
              Bray, T., Paoli, J., Sperberg-McQueen, C., M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20081126, November 2008,
              <http://www.w3.org/TR/2006/REC-xml-20060816>.

8.2.
              <http://www.w3.org/TR/2008/REC-xml-20081126>.

10.2.  Informative References

   [IF-CFG]   Bjorklund, M., "A YANG Data Model for Interface
              Management", draft-ietf-netmod-interfaces-cfg-16 (work in
              progress), January 2014.

   [JSON-META]
              Sakimura, N., "JSON Metadata", draft-sakimura-json-
              metadata-01 (work in progress), November 2013.

   [RESTCONF]

   [I-D.ietf-netconf-restconf]
              Bierman, A., Bjorklund, M., Watsen, K., and R. Fernando, K. Watsen, "RESTCONF
              Protocol", draft-ietf-netconf-restconf-00 draft-ietf-netconf-restconf-02 (work in
              progress), March October 2014.

   [RFC7223]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 7223, May 2014.

   [XPath]

   [W3C.REC-xpath-19991116]
              Clark, J., J. and S. DeRose, "XML Path Language (XPath)
              Version 1.0", World Wide Web Consortium Recommendation
              REC-xpath-19991116, November 1999,
              <http://www.w3.org/TR/1999/REC-xpath-19991116>.

Appendix A.  A Complete Example

   The JSON document shown below was translated from a represents the same data as the reply
   to the NETCONF <get> request that can be found appearing in Appendix D of [IF-CFG]. [RFC7223].
   The data model is a combination of two YANG modules: "ietf-
   interfaces" and "ex-vlan" (the latter is an example module from
   Appendix C of [IF-CFG]). [RFC7223]).  The "if-mib" feature defined in the "ietf-
   interfaces" module is considered to be active.

   {
     "interfaces":
     "ietf-interfaces:interfaces": {
       "interface": [
         {
           "name": "eth0",
           "type": "iana-if-type:ethernetCsmacd",
           "enabled": false
         },
         {
           "name": "eth1",
           "type": "iana-if-type:ethernetCsmacd",
           "enabled": true,
           "vlan-tagging":
           "ex-vlan:vlan-tagging": true
         },
         {
           "name": "eth1.10",
           "type": "iana-if-type:l2vlan",
           "enabled": true,
           "base-interface":
           "ex-vlan:base-interface": "eth1",
           "vlan-id":
           "ex-vlan:vlan-id": 10
         },
         {
           "name": "lo1",
           "type": "iana-if-type:softwareLoopback",
           "enabled": true
         }
       ]
     },
     "interfaces-state":
     "ietf-interfaces:interfaces-state": {
       "interface": [
         {
           "name": "eth0",
           "type": "iana-if-type:ethernetCsmacd",
           "admin-status": "down",
           "oper-status": "down",
           "if-index": 2,
           "phys-address": "00:01:02:03:04:05",
           "statistics": {
             "discontinuity-time": "2013-04-01T03:00:00+00:00"
           }
         },
         {
           "name": "eth1",
           "type": "iana-if-type:ethernetCsmacd",
           "admin-status": "up",
           "oper-status": "up",
           "if-index": 7,
           "phys-address": "00:01:02:03:04:06",
           "higher-layer-if": [
             "eth1.10"
           ],
           "statistics": {
             "discontinuity-time": "2013-04-01T03:00:00+00:00"
           }
         },
         {
           "name": "eth1.10",
           "type": "iana-if-type:l2vlan",
           "admin-status": "up",
           "oper-status": "up",
           "if-index": 9,
           "lower-layer-if": [
             "eth1"
           ],
           "statistics": {
             "discontinuity-time": "2013-04-01T03:00:00+00:00"
           }
         },
         {
           "name": "eth2",
           "type": "iana-if-type:ethernetCsmacd",
           "admin-status": "down",
           "oper-status": "down",
           "if-index": 8,
           "phys-address": "00:01:02:03:04:07",
           "statistics": {
             "discontinuity-time": "2013-04-01T03:00:00+00:00"
           }
         },
         {
           "name": "lo1",
           "type": "iana-if-type:softwareLoopback",
           "admin-status": "up",
           "oper-status": "up",
           "if-index": 1,
           "statistics": {
             "discontinuity-time": "2013-04-01T03:00:00+00:00"
           }
         }
       ]
     }
   }

Appendix B.  Change Log

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

B.1.  Changes Between Revisions -00 and -01

   o  Metadata encoding was moved to a separate I-D, draft-lhotka-
      netmod-yang-metadata.

   o  JSON encoding is now defined directly rather than via XML-JSON
      mapping.

   o  The rules for namespace encoding has changed.  This affect both
      node instance names and instance-identifiers.

   o  I-JSON-related changes.  The most significant is the string
      encoding of 64-bit numbers.

   o  When validating union type, the partial type info present in JSON
      encoding is taken into account.

   o  Added section about I-JSON compliance.

   o  Updated the example in appendix.

   o  Wrote Security Considerations.

   o  Removed IANA Considerations as there are none.

Author's Address

   Ladislav Lhotka
   CZ.NIC

   Email: lhotka@nic.cz