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Versions: 00 01

NETMOD                                                         L. Lhotka
Internet-Draft                                                    CZ.NIC
Intended status: Standards Track                          April 02, 2012
Expires: October 4, 2012


                      Modeling JSON Text with YANG
                       draft-lhotka-yang-json-00

Abstract

   This document defines rules for mapping data models expressed in YANG
   to 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
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   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 October 4, 2012.

Copyright Notice

   Copyright (c) 2012 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
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   (http://trustee.ietf.org/license-info) in effect on the date of
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   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 . . . . . . . . . . . . . . . . . . .  4
   3.  Specification of the Translation Procedure . . . . . . . . . .  5
     3.1.  Names and Namespaces . . . . . . . . . . . . . . . . . . .  5
     3.2.  Mapping XML Elements to JSON Objects . . . . . . . . . . .  6
     3.3.  Mapping YANG Datatypes to JSON Values  . . . . . . . . . .  7
       3.3.1.  Numeric Types  . . . . . . . . . . . . . . . . . . . .  7
       3.3.2.  The "string" Type  . . . . . . . . . . . . . . . . . .  7
       3.3.3.  The "boolean" Type . . . . . . . . . . . . . . . . . .  7
       3.3.4.  The "enumeration" Type . . . . . . . . . . . . . . . .  7
       3.3.5.  The "bits" Type  . . . . . . . . . . . . . . . . . . .  7
       3.3.6.  The "binary" Type  . . . . . . . . . . . . . . . . . .  7
       3.3.7.  The "leafref" Type . . . . . . . . . . . . . . . . . .  7
       3.3.8.  The "identityref" Type . . . . . . . . . . . . . . . .  7
       3.3.9.  The "empty" Type . . . . . . . . . . . . . . . . . . .  8
       3.3.10. The "union" Type . . . . . . . . . . . . . . . . . . .  8
       3.3.11. The "instance-identifier" Type . . . . . . . . . . . .  8
     3.4.  Example  . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . 10
     3.6.  Security Considerations  . . . . . . . . . . . . . . . . . 10
     3.7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . 10
   4.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     4.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
























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1.  Introduction

   The aim of this document is define rules for mapping data models
   expressed in the YANG data modeling language [RFC6020] to JavaScript
   Object Notation (JSON) text [RFC4627].  The result can be potentially
   applied in two different ways:

   1.  JSON can be used in the context of the NETCONF protocol [RFC6241]
       and with existing data models expressed in YANG.

   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.

   The former use case is not currently possible without further work
   because all NETCONF and YANG standards so far assume that XML [XML]
   is used for encoding both protocol messages and configuration data.

   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
   heavily 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 to translate 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.  Translation of YANG namespaces is supported.

   2.  The information about datatypes of leaf nodes is used for
       translating the leaf values to the most appropriate JSON
       datatype.

   3.  Translation of XML attributes, mixed content, comments and
       processing instructions is not supported.

   Properties 1 and 2 mean that the translation is driven by a YANG data
   model which must therefore be known in advance.





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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 model

   o  data node

   o  data tree

   o  datatype

   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 following subsections contain mainly rules for translating XML
   documents to JSON text.  Rules for the inverse translation are stated
   only where needed, otherwise they can be easily inferred.

   XML documents may be translated to JSON text only if they are valid
   instances of a YANG data model, which must therefore be known
   beforehand.  There is one extra restriction beyond the standard YANG
   rules: XML documents with mixed content - which is marginal in YANG
   and allowed only in anyxml nodes - cannot be translated by this
   procedure.

   The semantics of several YANG statements, such as "rpc",
   "notification", "config", "if-feature" or "default", are more or less
   specific to NETCONF and may or may not be meaningful in a non-NETCONF
   context.  If such a statement appears in a data model that is used
   for the translation procedure, then all YANG rules concerning this
   statement MUST be observed.

   For example, an application that uses YANG for validating JSON text
   may decide to use the "default" statement for defining default values
   of JSON's primitive datatypes (string, number or boolean).  If it
   does so, the default values SHALL also be taken into account for the
   validation of semantic constraints such as those defined by "must"
   statements.

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.

   Most of the time, the namespace of JSON names is implicit.  The
   namespace MUST be explicitly specified whenever the namespace of a
   name differs from that of its parent object, or whenever no parent
   object exists.  The namespace MUST NOT be explicitly specified
   elsewhere.



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   In YANG terms, the namespace SHALL be specified only in the names of
   (i) top-level objects and (ii) objects that augment the data tree of
   another module (see Section 7.15 in [RFC6020]).

   Where the namespace has to be specified in JSON text, it SHALL be
   done in the following form:

           <module name>:<local name>

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

   The translation procedure MUST correctly map YANG namespace URIs to
   YANG module names and vice versa.

   When mapping namespaces from JSON text to XML, the resulting XML
   document may use default namespace declarations (via the "xmlns"
   attribute), prefix-based namespace declarations (via attributes
   beginning with "xmlns:"), or any combination thereof following the
   rules stated in [XMLNS].  If prefixed names are used, their prefix
   SHOULD be the one defined by the "prefix" statement in the YANG
   module where each data node is defined.

3.2.  Mapping XML Elements to JSON Objects

   XML elements are translated to JSON objects in a straightforward way:

   o  XML elements that have no siblings of the same name correspond
      either to a name/value pair or to a JSON object:

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

      *  An XML element which is modeled as a container in YANG is
         translated to an object.

   o  A sequence of sibling XML elements with the same name (modeled as
      a list or leaf-list in YANG) corresponds to a JSON array.  If the
      sequence is modeled as a leaf-list in YANG, then the array
      elements are primitive values (strings, numbers or booleans) whose
      type depends on the datatype of the leaf-list (see Section 3.3).
      If the sequence is modeled as a list in YANG, then the array
      elements are JSON objects.







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3.3.  Mapping YANG Datatypes to JSON Values

3.3.1.  Numeric Types

   YANG numeric types ("int8", "int16", "int32", "int64", "uint8",
   "uint16", "uint32", "uint64" and "decimal64") are mapped to JSON
   numbers whose decimal representation is the YANG canonical form of
   the number.  Hexadecimal values MUST be converted to decimal.

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., a space-separated list of
   bit values.

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
   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, subject to the same constraints as the
   XML value.

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



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   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'.

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.

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.  Example

   Consider a simple data model defined by the following YANG module:
















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   module ex-json {
     namespace "http://example.com/ex-json";
     prefix ej;
     import ietf-inet-types {
       prefix inet;
     }
     container top {
       list address {
         key "seqno";
         leaf seqno {
           type uint8;
         }
         leaf ip {
           type inet:ip-address;
           mandatory true;
         }
       }
       container phases {
         typedef angle {
           type decimal64 {
             fraction-digits 2;
           }
           units "radians";
         }
         leaf max-phase {
           default "6.28";
           type angle;
         }
         leaf-list phase {
           type angle;
           must ". <= ../max-phase";
           min-elements 1;
         }
       }
     }
   }

                      Figure 2: Example YANG module.

   By using the translation procedure defined in this document, we can
   conclude that the following JSON text is valid according to the data
   model:









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   {
     "ex-json:top": {
       "address": [
         {
           "seqno": 1,
           "ip": "192.0.2.1"
         },
         {
           "seqno": 2,
           "ip": "2001:db8:0:1::1"
         }
       ],
       "phases": {
         "phase": [0.79, 1.04, 3.14]
       }
     }
   }

                       Figure 3: Example JSON text.

   Note that the semantic constraint specified by the "must" statement
   in Figure 2 is satisfied by all elements of the "phase" array because
   the default value of 6.28 is used for the absent "max-phase" leaf.

3.5.  IANA Considerations

   TBD.

3.6.  Security Considerations

   TBD.

3.7.  Acknowledgments

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















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4.  References

4.1.  Normative References

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

   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, July 2006.

   [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

   [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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Author's Address

   Ladislav Lhotka
   CZ.NIC

   Email: lhotka@nic.cz













































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