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Versions: 00 01 02 draft-ietf-netmod-yang-json

NETMOD                                                         L. Lhotka
Internet-Draft                                                    CZ.NIC
Intended status: Standards Track                      September 23, 2013
Expires: March 27, 2014


                      Modeling JSON Text with YANG
                    draft-lhotka-netmod-yang-json-02

Abstract

   This document defines rules for presenting configuration and
   operational state data defined using YANG as JSON text.  It does so
   by specifying a procedure for translating the subset of YANG-
   compatible XML documents to JSON text, and vice versa.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 27, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology and Notation . . . . . . . . . . . . . . . . . . .  5
   3.  Specification of the Translation Procedure . . . . . . . . . .  6
     3.1.  Names and Namespaces . . . . . . . . . . . . . . . . . . .  7
     3.2.  Mapping XML Elements to JSON Objects . . . . . . . . . . .  9
       3.2.1.  The "leaf" Data Node . . . . . . . . . . . . . . . . .  9
       3.2.2.  The "container" Data Node  . . . . . . . . . . . . . .  9
       3.2.3.  The "leaf-list" Data Node  . . . . . . . . . . . . . . 10
       3.2.4.  The "list" Data Node . . . . . . . . . . . . . . . . . 10
       3.2.5.  The "anyxml" Data Node . . . . . . . . . . . . . . . . 11
     3.3.  Mapping YANG Datatypes to JSON Values  . . . . . . . . . . 12
       3.3.1.  Numeric Datatypes  . . . . . . . . . . . . . . . . . . 12
       3.3.2.  The "string" Type  . . . . . . . . . . . . . . . . . . 12
       3.3.3.  The "boolean" Type . . . . . . . . . . . . . . . . . . 12
       3.3.4.  The "enumeration" Type . . . . . . . . . . . . . . . . 12
       3.3.5.  The "bits" Type  . . . . . . . . . . . . . . . . . . . 12
       3.3.6.  The "binary" Type  . . . . . . . . . . . . . . . . . . 12
       3.3.7.  The "leafref" Type . . . . . . . . . . . . . . . . . . 13
       3.3.8.  The "identityref" Type . . . . . . . . . . . . . . . . 13
       3.3.9.  The "empty" Type . . . . . . . . . . . . . . . . . . . 13
       3.3.10. The "union" Type . . . . . . . . . . . . . . . . . . . 13
       3.3.11. The "instance-identifier" Type . . . . . . . . . . . . 14
     3.4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . 14
     3.5.  Security Considerations  . . . . . . . . . . . . . . . . . 14
     3.6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . 14
   4.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     4.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     4.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Appendix A.  A Complete Example  . . . . . . . . . . . . . . . . . 16
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19



















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1.  Introduction

   The aim of this document is define rules for presenting configuration
   and operational state data defined in the YANG data modeling
   language [RFC6020] as JavaScript Object Notation (JSON) text [JSON].
   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 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 XML that can be modeled
   using YANG to JSON, 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.  The translation is driven by a concrete YANG data model and uses
       information about data types to achieve better results than
       generic XML-JSON translation procedures.

   2.  Various document types are supported, namely configuration data,
       configuration + state data, RPC input and output parameters, and
       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.  Translation of XML attributes, mixed content, comments and
       processing instructions is outside the scope of this document.




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   Item 1 above also means that, depending on the data model, the same
   XML element can be translated to different JSON objects.  For
   example,

       <foo>123</foo>

   is translated to

       "foo": 123

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

       "foo": ["123"]

   if the "foo" node is defined as a leaf-list with the "string"
   datatype, and the <foo> element has no siblings of the same name.


































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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








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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 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 are ignored by the
   translation procedure and 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" 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 be easily inferred.

   REQUIRED parameters of the translation procedure are:

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

   o  type of the input document,

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

   The permissible types of input documents are listed in Table 1
   together with the corresponding part of the data model that is used
   for the translation.

















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

                       Table 1: YANG Document Types

   A particular application MAY decide to support only a subset of
   document types from Table 1.  For instance, RESTCONF Protocol
   [RESTCONF] does not use notifications.

   XML documents can be translated to JSON text only if they are valid
   instances of the YANG data model and selected document type, also
   taking into account the active features, if there are any.

   The resulting JSON document is always a single object ([JSON],
   Sec. 4) whose members are translated from the original XML document
   using the rules specified in the following sections.

3.1.  Names and Namespaces

   The local part of a JSON name is always identical to the local name
   of the corresponding XML element.

   Each JSON name lives in a namespace which is uniquely identified by
   the name of 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 the main module to which the
   submodule belongs.  The translation procedure MUST correctly map YANG
   namespace URIs to YANG module names and vice versa.

   The namespace SHALL be expressed in JSON text by prefixing the local
   name in the following way:

           <module name>:<local name>

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

   The namespace identifier MUST be used for local names that are



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   ambiguous, i.e., whenever the data model permits a sibling node with
   the same local name.  Otherwise, the namespace identifier is
   OPTIONAL.

   For example, consider the following YANG module:

       module foomod {
           namespace "http://example.com/foomod";
           prefix "fm";
           container foo {
               leaf bar {
                   type boolean;
               }
           }
       }

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

       {
         "foo": {
           "bar": true
         }
       }

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

       module barmod {
           namespace "http://example.com/barmod";
           prefix "bm";
           import foomod {
               prefix fm;
           }
           augment "/fm:foo" {
               leaf bar {
                   type uint8;
               }
           }
       }

   In the data model combining "foomod" and "barmod", we have two
   sibling nodes with the same local name, namely "bar".  In this case,
   a valid JSON document has to specify an explicit namespace identifier
   (module name) for both leaves:







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

3.2.  Mapping XML Elements to JSON Objects

   XML elements that are modelled as YANG data nodes are translated to a
   name/value pair where the name is formed from the name of the XML
   element using the rules in Section 3.1.  The value depends on the
   type of the data node as specified in the following sections.

3.2.1.  The "leaf" Data Node

   An XML element that is modeled as YANG leaf is translated to a name/
   value pair and the type of the value is derived from the YANG
   datatype of the leaf (see Section 3.3 for the datatype mapping
   rules).

   Example: For the leaf node definition

       leaf foo {
           type uint8;
       }

   the XML element

       <foo>123</foo>

   corresponds to the JSON name/value pair

       "foo": 123

3.2.2.  The "container" Data Node

   An XML element that is modeled as YANG container is translated to a
   name/object pair.

   Example: For the container node definition

       container bar {
           leaf foo {
               type uint8;
           }
       }




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   the XML element

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

   corresponds to the JSON name/value pair

       "bar": {
         "foo": 123
       }

3.2.3.  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 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).

   Example: For the leaf-list node definition

       leaf-list foo {
           type uint8;
       }

   the XML elements

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

   corresponds to the JSON name/value pair

       "foo": [123, 0]

3.2.4.  The "list" Data Node

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

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

   Example: For the list node definition




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       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>

   corresponds to the JSON name/value pair

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

3.2.5.  The "anyxml" Data Node

   An XML element that is modeled as a YANG anyxml node is translated to
   a name/object pair.  The content of such an element is not modelled
   by YANG, and there may not be a straightforward mapping to JSON text
   (e.g., if it is a mixed XML content).  Therefore, translation of
   anyxml contents is necessarily application-specific and outside the
   scope of this document.

   Example: For the anyxml node definition

       anyxml bar;

   the XML element



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       <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.  Mapping YANG Datatypes to JSON Values

3.3.1.  Numeric Datatypes

   A value of one of the YANG numeric datatypes ("int8", "int16",
   "int32", "int64", "uint8", "uint16", "uint32", "uint64" and
   "decimal64") is mapped to a JSON number using the same lexical
   representation.

3.3.2.  The "string" Type

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

3.3.3.  The "boolean" Type

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

3.3.4.  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.  The "bits" Type

   A "bits" value is mapped to a 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.  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



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   data.

3.3.7.  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.  The "identityref" Type

   An "identityref" value is mapped to a string representing the
   qualified name of the identity.  Its namespace MAY be expressed as
   shown in Figure 1.  If the namespace part is not present, the
   namespace of the name of the JSON object containing the value is
   assumed.

3.3.9.  The "empty" Type

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

   This representation 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 node 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:




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       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.

3.4.  IANA Considerations

   TBD.

3.5.  Security Considerations

   TBD.

3.6.  Acknowledgments

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









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4.  References

4.1.  Normative References

   [JSON]     Bray, T., Ed., "The JSON Data Interchange Format",
              draft-ietf-json-rfc4627bis-03 (work in progress),
              September 2013.

   [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
              Network Configuration Protocol (NETCONF)", RFC 6020,
              September 2010.

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

   [XML]      Bray, T., Paoli, J., Sperberg-McQueen, C., 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>.

   [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>.

4.2.  Informative References

   [IF-CFG]   Bjorklund, M., "A YANG Data Model for Interface
              Management", draft-ietf-netmod-interfaces-cfg-12 (work in
              progress), July 2013.

   [RESTCONF]
              Bierman, A., Bjorklund, M., Watsen, K., and R. Fernando,
              "RESTCONF Protocol", draft-bierman-netconf-restconf-01
              (work in progress), September 2013.

   [XPath]    Clark, J., "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>.





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Appendix A.  A Complete Example

   The JSON document shown below was translated from a reply to the
   NETCONF <get> request that can be found in Appendix D of [IF-CFG].
   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]).  The "if-mib" feature defined in the "ietf-
   interfaces" module is considered to be active.

   {
     "interfaces": {
       "interface": [
         {
           "name": "eth0",
           "type": "ethernetCsmacd",
           "enabled": false
         },
         {
           "name": "eth1",
           "type": "ethernetCsmacd",
           "enabled": true,
           "vlan-tagging": true
         },
         {
           "name": "eth1.10",
           "type": "l2vlan",
           "enabled": true,
           "base-interface": "eth1",
           "vlan-id": 10
         },
         {
           "name": "lo1",
           "type": "softwareLoopback",
           "enabled": true
         }
       ]
     },
     "interfaces-state": {
       "interface": [
         {
           "name": "eth0",
           "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"



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           }
         },
         {
           "name": "eth1",
           "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": "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": "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": "softwareLoopback",
           "admin-status": "up",
           "oper-status": "up",
           "if-index": 1,
           "statistics": {
             "discontinuity-time": "2013-04-01T03:00:00+00:00"



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           }
         }
       ]
     }
   }














































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Author's Address

   Ladislav Lhotka
   CZ.NIC

   Email: lhotka@nic.cz













































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