





Internet Engineering Task Force (IETF)                 M. Bjorklund, Ed.
Request for Comments: 6020                                Tail-f Systems
Category: Standards Track                                   October 2010
ISSN: 2070-1721


                  YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)

Abstract

   YANG is a data modeling language used to model configuration and
   state data manipulated by the Network Configuration Protocol
   (NETCONF), NETCONF remote procedure calls, and NETCONF notifications.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6020.

Copyright Notice

   Copyright (c) 2010 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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RFC 6020                          YANG                      October 2010


   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction ....................................................8
   2. Keywords ........................................................8
   3. Terminology .....................................................8
      3.1. Mandatory Nodes ...........................................10
   4. YANG Overview ..................................................11
      4.1. Functional Overview .......................................11
      4.2. Language Overview .........................................13
           4.2.1. Modules and Submodules .............................13
           4.2.2. Data Modeling Basics ...............................13
           4.2.3. State Data .........................................18
           4.2.4. Built-In Types .....................................18
           4.2.5. Derived Types (typedef) ............................19
           4.2.6. Reusable Node Groups (grouping) ....................20
           4.2.7. Choices ............................................21
           4.2.8. Extending Data Models (augment) ....................22
           4.2.9. RPC Definitions ....................................23
           4.2.10. Notification Definitions ..........................24
   5. Language Concepts ..............................................25
      5.1. Modules and Submodules ....................................25
           5.1.1. Import and Include by Revision .....................26
           5.1.2. Module Hierarchies .................................27
      5.2. File Layout ...............................................28
      5.3. XML Namespaces ............................................29
           5.3.1. YANG XML Namespace .................................29
      5.4. Resolving Grouping, Type, and Identity Names ..............29
      5.5. Nested Typedefs and Groupings .............................29
      5.6. Conformance ...............................................30
           5.6.1. Basic Behavior .....................................31
           5.6.2. Optional Features ..................................31
           5.6.3. Deviations .........................................31
           5.6.4. Announcing Conformance Information in the
                  <hello> Message ....................................32
      5.7. Data Store Modification ...................................34
   6. YANG Syntax ....................................................34



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      6.1. Lexical Tokenization ......................................34
           6.1.1. Comments ...........................................34
           6.1.2. Tokens .............................................34
           6.1.3. Quoting ............................................35
      6.2. Identifiers ...............................................36
           6.2.1. Identifiers and Their Namespaces ...................36
      6.3. Statements ................................................37
           6.3.1. Language Extensions ................................37
      6.4. XPath Evaluations .........................................38
           6.4.1. XPath Context ......................................38
      6.5. Schema Node Identifier ....................................39
   7. YANG Statements ................................................39
      7.1. The module Statement ......................................39
           7.1.1. The module's Substatements .........................41
           7.1.2. The yang-version Statement .........................41
           7.1.3. The namespace Statement ............................42
           7.1.4. The prefix Statement ...............................42
           7.1.5. The import Statement ...............................42
           7.1.6. The include Statement ..............................43
           7.1.7. The organization Statement .........................44
           7.1.8. The contact Statement ..............................44
           7.1.9. The revision Statement .............................44
           7.1.10. Usage Example .....................................45
      7.2. The submodule Statement ...................................46
           7.2.1. The submodule's Substatements ......................48
           7.2.2. The belongs-to Statement ...........................48
           7.2.3. Usage Example ......................................49
      7.3. The typedef Statement .....................................49
           7.3.1. The typedef's Substatements ........................50
           7.3.2. The typedef's type Statement .......................50
           7.3.3. The units Statement ................................50
           7.3.4. The typedef's default Statement ....................50
           7.3.5. Usage Example ......................................51
      7.4. The type Statement ........................................51
           7.4.1. The type's Substatements ...........................51
      7.5. The container Statement ...................................51
           7.5.1. Containers with Presence ...........................52
           7.5.2. The container's Substatements ......................53
           7.5.3. The must Statement .................................53
           7.5.4. The must's Substatements ...........................55
           7.5.5. The presence Statement .............................56
           7.5.6. The container's Child Node Statements ..............56
           7.5.7. XML Mapping Rules ..................................56
           7.5.8. NETCONF <edit-config> Operations ...................56
           7.5.9. Usage Example ......................................57
      7.6. The leaf Statement ........................................58
           7.6.1. The leaf's default value ...........................58
           7.6.2. The leaf's Substatements ...........................59



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           7.6.3. The leaf's type Statement ..........................59
           7.6.4. The leaf's default Statement .......................59
           7.6.5. The leaf's mandatory Statement .....................60
           7.6.6. XML Mapping Rules ..................................60
           7.6.7. NETCONF <edit-config> Operations ...................60
           7.6.8. Usage Example ......................................61
      7.7. The leaf-list Statement ...................................62
           7.7.1. Ordering ...........................................62
           7.7.2. The leaf-list's Substatements ......................63
           7.7.3. The min-elements Statement .........................63
           7.7.4. The max-elements Statement .........................63
           7.7.5. The ordered-by Statement ...........................64
           7.7.6. XML Mapping Rules ..................................64
           7.7.7. NETCONF <edit-config> Operations ...................65
           7.7.8. Usage Example ......................................66
      7.8. The list Statement ........................................67
           7.8.1. The list's Substatements ...........................68
           7.8.2. The list's key Statement ...........................68
           7.8.3. The list's unique Statement ........................69
           7.8.4. The list's Child Node Statements ...................70
           7.8.5. XML Mapping Rules ..................................70
           7.8.6. NETCONF <edit-config> Operations ...................71
           7.8.7. Usage Example ......................................72
      7.9. The choice Statement ......................................75
           7.9.1. The choice's Substatements .........................76
           7.9.2. The choice's case Statement ........................76
           7.9.3. The choice's default Statement .....................77
           7.9.4. The choice's mandatory Statement ...................79
           7.9.5. XML Mapping Rules ..................................79
           7.9.6. NETCONF <edit-config> Operations ...................79
           7.9.7. Usage Example ......................................79
      7.10. The anyxml Statement .....................................80
           7.10.1. The anyxml's Substatements ........................81
           7.10.2. XML Mapping Rules .................................81
           7.10.3. NETCONF <edit-config> Operations ..................81
           7.10.4. Usage Example .....................................82
      7.11. The grouping Statement ...................................82
           7.11.1. The grouping's Substatements ......................83
           7.11.2. Usage Example .....................................84
      7.12. The uses Statement .......................................84
           7.12.1. The uses's Substatements ..........................85
           7.12.2. The refine Statement ..............................85
           7.12.3. XML Mapping Rules .................................86
           7.12.4. Usage Example .....................................86
      7.13. The rpc Statement ........................................87
           7.13.1. The rpc's Substatements ...........................88
           7.13.2. The input Statement ...............................88
           7.13.3. The output Statement ..............................89



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           7.13.4. XML Mapping Rules .................................90
           7.13.5. Usage Example .....................................91
      7.14. The notification Statement ...............................91
           7.14.1. The notification's Substatements ..................92
           7.14.2. XML Mapping Rules .................................92
           7.14.3. Usage Example .....................................93
      7.15. The augment Statement ....................................93
           7.15.1. The augment's Substatements .......................94
           7.15.2. XML Mapping Rules .................................94
           7.15.3. Usage Example .....................................95
      7.16. The identity Statement ...................................97
           7.16.1. The identity's Substatements ......................97
           7.16.2. The base Statement ................................97
           7.16.3. Usage Example .....................................98
      7.17. The extension Statement ..................................98
           7.17.1. The extension's Substatements .....................99
           7.17.2. The argument Statement ............................99
           7.17.3. Usage Example ....................................100
      7.18. Conformance-Related Statements ..........................100
           7.18.1. The feature Statement ............................100
           7.18.2. The if-feature Statement .........................102
           7.18.3. The deviation Statement ..........................102
      7.19. Common Statements .......................................105
           7.19.1. The config Statement .............................105
           7.19.2. The status Statement .............................105
           7.19.3. The description Statement ........................106
           7.19.4. The reference Statement ..........................106
           7.19.5. The when Statement ...............................107
   8. Constraints ...................................................108
      8.1. Constraints on Data ......................................108
      8.2. Hierarchy of Constraints .................................109
      8.3. Constraint Enforcement Model .............................109
           8.3.1. Payload Parsing ...................................109
           8.3.2. NETCONF <edit-config> Processing ..................110
           8.3.3. Validation ........................................111
   9. Built-In Types ................................................111
      9.1. Canonical Representation .................................112
      9.2. The Integer Built-In Types ...............................112
           9.2.1. Lexical Representation ............................113
           9.2.2. Canonical Form ....................................114
           9.2.3. Restrictions ......................................114
           9.2.4. The range Statement ...............................114
           9.2.5. Usage Example .....................................115
      9.3. The decimal64 Built-In Type ..............................115
           9.3.1. Lexical Representation ............................115
           9.3.2. Canonical Form ....................................115
           9.3.3. Restrictions ......................................116
           9.3.4. The fraction-digits Statement .....................116



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           9.3.5. Usage Example .....................................117
      9.4. The string Built-In Type .................................117
           9.4.1. Lexical Representation ............................117
           9.4.2. Canonical Form ....................................117
           9.4.3. Restrictions ......................................117
           9.4.4. The length Statement ..............................117
           9.4.5. Usage Example .....................................118
           9.4.6. The pattern Statement .............................119
           9.4.7. Usage Example .....................................119
      9.5. The boolean Built-In Type ................................120
           9.5.1. Lexical Representation ............................120
           9.5.2. Canonical Form ....................................120
           9.5.3. Restrictions ......................................120
      9.6. The enumeration Built-In Type ............................120
           9.6.1. Lexical Representation ............................120
           9.6.2. Canonical Form ....................................120
           9.6.3. Restrictions ......................................120
           9.6.4. The enum Statement ................................120
           9.6.5. Usage Example .....................................121
      9.7. The bits Built-In Type ...................................122
           9.7.1. Restrictions ......................................122
           9.7.2. Lexical Representation ............................122
           9.7.3. Canonical Form ....................................122
           9.7.4. The bit Statement .................................122
           9.7.5. Usage Example .....................................123
      9.8. The binary Built-In Type .................................123
           9.8.1. Restrictions ......................................124
           9.8.2. Lexical Representation ............................124
           9.8.3. Canonical Form ....................................124
      9.9. The leafref Built-In Type ................................124
           9.9.1. Restrictions ......................................124
           9.9.2. The path Statement ................................124
           9.9.3. Lexical Representation ............................125
           9.9.4. Canonical Form ....................................125
           9.9.5. Usage Example .....................................126
      9.10. The identityref Built-In Type ...........................129
           9.10.1. Restrictions .....................................129
           9.10.2. The identityref's base Statement .................129
           9.10.3. Lexical Representation ...........................130
           9.10.4. Canonical Form ...................................130
           9.10.5. Usage Example ....................................130
      9.11. The empty Built-In Type .................................131
           9.11.1. Restrictions .....................................131
           9.11.2. Lexical Representation ...........................131
           9.11.3. Canonical Form ...................................131
           9.11.4. Usage Example ....................................131
      9.12. The union Built-In Type .................................132
           9.12.1. Restrictions .....................................132



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           9.12.2. Lexical Representation ...........................132
           9.12.3. Canonical Form ...................................133
      9.13. The instance-identifier Built-In Type ...................133
           9.13.1. Restrictions .....................................134
           9.13.2. The require-instance Statement ...................134
           9.13.3. Lexical Representation ...........................134
           9.13.4. Canonical Form ...................................134
           9.13.5. Usage Example ....................................134
   10. Updating a Module ............................................135
   11. YIN ..........................................................137
      11.1. Formal YIN Definition ...................................137
           11.1.1. Usage Example ....................................141
   12. YANG ABNF Grammar ............................................143
   13. Error Responses for YANG Related Errors ......................165
      13.1. Error Message for Data That Violates a unique
            Statement ...............................................165
      13.2. Error Message for Data That Violates a
            max-elements Statement ..................................165
      13.3. Error Message for Data That Violates a
            min-elements Statement ..................................165
      13.4. Error Message for Data That Violates a must Statement ...166
      13.5. Error Message for Data That Violates a
            require-instance Statement ..............................166
      13.6. Error Message for Data That Does Not Match a
            leafref Type ............................................166
      13.7. Error Message for Data That Violates a mandatory
            choice Statement ........................................166
      13.8. Error Message for the "insert" Operation ................167
   14. IANA Considerations ..........................................167
      14.1. Media type application/yang .............................168
      14.2. Media type application/yin+xml ..........................169
   15. Security Considerations ......................................170
   16. Contributors .................................................171
   17. Acknowledgements .............................................171
   18. References ...................................................171
      18.1. Normative References ....................................171
      18.2. Informative References ..................................172














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RFC 6020                          YANG                      October 2010


1.  Introduction

   YANG is a data modeling language used to model configuration and
   state data manipulated by the Network Configuration Protocol
   (NETCONF), NETCONF remote procedure calls, and NETCONF notifications.
   YANG is used to model the operations and content layers of NETCONF
   (see the NETCONF Configuration Protocol [RFC4741], Section 1.1).

   This document describes the syntax and semantics of the YANG
   language, how the data model defined in a YANG module is represented
   in the Extensible Markup Language (XML), and how NETCONF operations
   are used to manipulate the data.

2.  Keywords

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

3.  Terminology

   o  anyxml: A data node that can contain an unknown chunk of XML data.

   o  augment: Adds new schema nodes to a previously defined schema
      node.

   o  base type: The type from which a derived type was derived, which
      may be either a built-in type or another derived type.

   o  built-in type: A YANG data type defined in the YANG language, such
      as uint32 or string.

   o  choice: A schema node where only one of a number of identified
      alternatives is valid.

   o  configuration data: The set of writable data that is required to
      transform a system from its initial default state into its current
      state [RFC4741].

   o  conformance: A measure of how accurately a device follows a data
      model.

   o  container: An interior data node that exists in at most one
      instance in the data tree.  A container has no value, but rather a
      set of child nodes.





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   o  data definition statement: A statement that defines new data
      nodes.  One of container, leaf, leaf-list, list, choice, case,
      augment, uses, and anyxml.

   o  data model: A data model describes how data is represented and
      accessed.

   o  data node: A node in the schema tree that can be instantiated in a
      data tree.  One of container, leaf, leaf-list, list, and anyxml.

   o  data tree: The instantiated tree of configuration and state data
      on a device.

   o  derived type: A type that is derived from a built-in type (such as
      uint32), or another derived type.

   o  device deviation: A failure of the device to implement the module
      faithfully.

   o  extension: An extension attaches non-YANG semantics to statements.
      The extension statement defines new statements to express these
      semantics.

   o  feature: A mechanism for marking a portion of the model as
      optional.  Definitions can be tagged with a feature name and are
      only valid on devices that support that feature.

   o  grouping: A reusable set of schema nodes, which may be used
      locally in the module, in modules that include it, and by other
      modules that import from it.  The grouping statement is not a data
      definition statement and, as such, does not define any nodes in
      the schema tree.

   o  identifier: Used to identify different kinds of YANG items by
      name.

   o  instance identifier: A mechanism for identifying a particular node
      in a data tree.

   o  interior node: Nodes within a hierarchy that are not leaf nodes.

   o  leaf: A data node that exists in at most one instance in the data
      tree.  A leaf has a value but no child nodes.

   o  leaf-list: Like the leaf node but defines a set of uniquely
      identifiable nodes rather than a single node.  Each node has a
      value but no child nodes.




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   o  list: An interior data node that may exist in multiple instances
      in the data tree.  A list has no value, but rather a set of child
      nodes.

   o  module: A YANG module defines a hierarchy of nodes that can be
      used for NETCONF-based operations.  With its definitions and the
      definitions it imports or includes from elsewhere, a module is
      self-contained and "compilable".

   o  RPC: A Remote Procedure Call, as used within the NETCONF protocol.

   o  RPC operation: A specific Remote Procedure Call, as used within
      the NETCONF protocol.  It is also called a protocol operation.

   o  schema node: A node in the schema tree.  One of container, leaf,
      leaf-list, list, choice, case, rpc, input, output, notification,
      and anyxml.

   o  schema node identifier: A mechanism for identifying a particular
      node in the schema tree.

   o  schema tree: The definition hierarchy specified within a module.

   o  state data: The additional data on a system that is not
      configuration data such as read-only status information and
      collected statistics [RFC4741].

   o  submodule: A partial module definition that contributes derived
      types, groupings, data nodes, RPCs, and notifications to a module.
      A YANG module can be constructed from a number of submodules.

   o  top-level data node: A data node where there is no other data node
      between it and a module or submodule statement.

   o  uses: The "uses" statement is used to instantiate the set of
      schema nodes defined in a grouping statement.  The instantiated
      nodes may be refined and augmented to tailor them to any specific
      needs.

3.1.  Mandatory Nodes

   A mandatory node is one of:

   o  A leaf, choice, or anyxml node with a "mandatory" statement with
      the value "true".

   o  A list or leaf-list node with a "min-elements" statement with a
      value greater than zero.



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   o  A container node without a "presence" statement, which has at
      least one mandatory node as a child.

4.  YANG Overview

4.1.  Functional Overview

   YANG is a language used to model data for the NETCONF protocol.  A
   YANG module defines a hierarchy of data that can be used for NETCONF-
   based operations, including configuration, state data, Remote
   Procedure Calls (RPCs), and notifications.  This allows a complete
   description of all data sent between a NETCONF client and server.

   YANG models the hierarchical organization of data as a tree in which
   each node has a name, and either a value or a set of child nodes.
   YANG provides clear and concise descriptions of the nodes, as well as
   the interaction between those nodes.

   YANG structures data models into modules and submodules.  A module
   can import data from other external modules, and include data from
   submodules.  The hierarchy can be augmented, allowing one module to
   add data nodes to the hierarchy defined in another module.  This
   augmentation can be conditional, with new nodes appearing only if
   certain conditions are met.

   YANG models can describe constraints to be enforced on the data,
   restricting the appearance or value of nodes based on the presence or
   value of other nodes in the hierarchy.  These constraints are
   enforceable by either the client or the server, and valid content
   MUST abide by them.

   YANG defines a set of built-in types, and has a type mechanism
   through which additional types may be defined.  Derived types can
   restrict their base type's set of valid values using mechanisms like
   range or pattern restrictions that can be enforced by clients or
   servers.  They can also define usage conventions for use of the
   derived type, such as a string-based type that contains a host name.

   YANG permits the definition of reusable groupings of nodes.  The
   instantiation of these groupings can refine or augment the nodes,
   allowing it to tailor the nodes to its particular needs.  Derived
   types and groupings can be defined in one module or submodule and
   used in either that location or in another module or submodule that
   imports or includes it.







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RFC 6020                          YANG                      October 2010


   YANG data hierarchy constructs include defining lists where list
   entries are identified by keys that distinguish them from each other.
   Such lists may be defined as either sorted by user or automatically
   sorted by the system.  For user-sorted lists, operations are defined
   for manipulating the order of the list entries.

   YANG modules can be translated into an equivalent XML syntax called
   YANG Independent Notation (YIN) (Section 11), allowing applications
   using XML parsers and Extensible Stylesheet Language Transformations
   (XSLT) scripts to operate on the models.  The conversion from YANG to
   YIN is lossless, so content in YIN can be round-tripped back into
   YANG.

   YANG strikes a balance between high-level data modeling and low-level
   bits-on-the-wire encoding.  The reader of a YANG module can see the
   high-level view of the data model while understanding how the data
   will be encoded in NETCONF operations.

   YANG is an extensible language, allowing extension statements to be
   defined by standards bodies, vendors, and individuals.  The statement
   syntax allows these extensions to coexist with standard YANG
   statements in a natural way, while extensions in a YANG module stand
   out sufficiently for the reader to notice them.

   YANG resists the tendency to solve all possible problems, limiting
   the problem space to allow expression of NETCONF data models, not
   arbitrary XML documents or arbitrary data models.  The data models
   described by YANG are designed to be easily operated upon by NETCONF
   operations.

   To the extent possible, YANG maintains compatibility with Simple
   Network Management Protocol's (SNMP's) SMIv2 (Structure of Management
   Information version 2 [RFC2578], [RFC2579]).  SMIv2-based MIB modules
   can be automatically translated into YANG modules for read-only
   access.  However, YANG is not concerned with reverse translation from
   YANG to SMIv2.

   Like NETCONF, YANG targets smooth integration with the device's
   native management infrastructure.  This allows implementations to
   leverage their existing access control mechanisms to protect or
   expose elements of the data model.










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4.2.  Language Overview

   This section introduces some important constructs used in YANG that
   will aid in the understanding of the language specifics in later
   sections.  This progressive approach handles the inter-related nature
   of YANG concepts and statements.  A detailed description of YANG
   statements and syntax begins in Section 7.

4.2.1.  Modules and Submodules

   A module contains three types of statements: module-header
   statements, revision statements, and definition statements.  The
   module header statements describe the module and give information
   about the module itself, the revision statements give information
   about the history of the module, and the definition statements are
   the body of the module where the data model is defined.

   A NETCONF server may implement a number of modules, allowing multiple
   views of the same data, or multiple views of disjoint subsections of
   the device's data.  Alternatively, the server may implement only one
   module that defines all available data.

   A module may be divided into submodules, based on the needs of the
   module owner.  The external view remains that of a single module,
   regardless of the presence or size of its submodules.

   The "include" statement allows a module or submodule to reference
   material in submodules, and the "import" statement allows references
   to material defined in other modules.

4.2.2.  Data Modeling Basics

   YANG defines four types of nodes for data modeling.  In each of the
   following subsections, the example shows the YANG syntax as well as a
   corresponding NETCONF XML representation.

4.2.2.1.  Leaf Nodes

   A leaf node contains simple data like an integer or a string.  It has
   exactly one value of a particular type and no child nodes.

   YANG Example:

       leaf host-name {
           type string;
           description "Hostname for this system";
       }




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   NETCONF XML Example:

       <host-name>my.example.com</host-name>

   The "leaf" statement is covered in Section 7.6.

4.2.2.2.  Leaf-List Nodes

   A leaf-list is a sequence of leaf nodes with exactly one value of a
   particular type per leaf.

   YANG Example:

     leaf-list domain-search {
         type string;
         description "List of domain names to search";
     }

   NETCONF XML Example:

     <domain-search>high.example.com</domain-search>
     <domain-search>low.example.com</domain-search>
     <domain-search>everywhere.example.com</domain-search>

   The "leaf-list" statement is covered in Section 7.7.

4.2.2.3.  Container Nodes

   A container node is used to group related nodes in a subtree.  A
   container has only child nodes and no value.  A container may contain
   any number of child nodes of any type (including leafs, lists,
   containers, and leaf-lists).

   YANG Example:

     container system {
         container login {
             leaf message {
                 type string;
                 description
                     "Message given at start of login session";
             }
         }
     }







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   NETCONF XML Example:

     <system>
       <login>
         <message>Good morning</message>
       </login>
     </system>

   The "container" statement is covered in Section 7.5.

4.2.2.4.  List Nodes

   A list defines a sequence of list entries.  Each entry is like a
   structure or a record instance, and is uniquely identified by the
   values of its key leafs.  A list can define multiple key leafs and
   may contain any number of child nodes of any type (including leafs,
   lists, containers etc.).

   YANG Example:

     list user {
         key "name";
         leaf name {
             type string;
         }
         leaf full-name {
             type string;
         }
         leaf class {
             type string;
         }
     }



















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   NETCONF XML Example:

     <user>
       <name>glocks</name>
       <full-name>Goldie Locks</full-name>
       <class>intruder</class>
     </user>
     <user>
       <name>snowey</name>
       <full-name>Snow White</full-name>
       <class>free-loader</class>
     </user>
     <user>
       <name>rzell</name>
       <full-name>Rapun Zell</full-name>
       <class>tower</class>
     </user>

   The "list" statement is covered in Section 7.8.

4.2.2.5.  Example Module

   These statements are combined to define the module:




























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RFC 6020                          YANG                      October 2010


     // Contents of "acme-system.yang"
     module acme-system {
         namespace "http://acme.example.com/system";
         prefix "acme";

         organization "ACME Inc.";
         contact "joe@acme.example.com";
         description
             "The module for entities implementing the ACME system.";

         revision 2007-06-09 {
             description "Initial revision.";
         }

         container system {
             leaf host-name {
                 type string;
                 description "Hostname for this system";
             }

             leaf-list domain-search {
                 type string;
                 description "List of domain names to search";
             }

             container login {
                 leaf message {
                     type string;
                     description
                         "Message given at start of login session";
                 }

                 list user {
                     key "name";
                     leaf name {
                         type string;
                     }
                     leaf full-name {
                         type string;
                     }
                     leaf class {
                         type string;
                     }
                 }
             }
         }
     }




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4.2.3.  State Data

   YANG can model state data, as well as configuration data, based on
   the "config" statement.  When a node is tagged with "config false",
   its subhierarchy is flagged as state data, to be reported using
   NETCONF's <get> operation, not the <get-config> operation.  Parent
   containers, lists, and key leafs are reported also, giving the
   context for the state data.

   In this example, two leafs are defined for each interface, a
   configured speed and an observed speed.  The observed speed is not
   configuration, so it can be returned with NETCONF <get> operations,
   but not with <get-config> operations.  The observed speed is not
   configuration data, and it cannot be manipulated using <edit-config>.

     list interface {
         key "name";

         leaf name {
             type string;
         }
         leaf speed {
             type enumeration {
                 enum 10m;
                 enum 100m;
                 enum auto;
             }
         }
         leaf observed-speed {
             type uint32;
             config false;
         }
     }

4.2.4.  Built-In Types

   YANG has a set of built-in types, similar to those of many
   programming languages, but with some differences due to special
   requirements from the management domain.  The following table
   summarizes the built-in types discussed in Section 9:











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       +---------------------+-------------------------------------+
       | Name                | Description                         |
       +---------------------+-------------------------------------+
       | binary              | Any binary data                     |
       | bits                | A set of bits or flags              |
       | boolean             | "true" or "false"                   |
       | decimal64           | 64-bit signed decimal number        |
       | empty               | A leaf that does not have any value |
       | enumeration         | Enumerated strings                  |
       | identityref         | A reference to an abstract identity |
       | instance-identifier | References a data tree node         |
       | int8                | 8-bit signed integer                |
       | int16               | 16-bit signed integer               |
       | int32               | 32-bit signed integer               |
       | int64               | 64-bit signed integer               |
       | leafref             | A reference to a leaf instance      |
       | string              | Human-readable string               |
       | uint8               | 8-bit unsigned integer              |
       | uint16              | 16-bit unsigned integer             |
       | uint32              | 32-bit unsigned integer             |
       | uint64              | 64-bit unsigned integer             |
       | union               | Choice of member types              |
       +---------------------+-------------------------------------+

   The "type" statement is covered in Section 7.4.

4.2.5.  Derived Types (typedef)

   YANG can define derived types from base types using the "typedef"
   statement.  A base type can be either a built-in type or a derived
   type, allowing a hierarchy of derived types.

   A derived type can be used as the argument for the "type" statement.

   YANG Example:

     typedef percent {
         type uint8 {
             range "0 .. 100";
         }
         description "Percentage";
     }

     leaf completed {
         type percent;
     }





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   NETCONF XML Example:

     <completed>20</completed>

   The "typedef" statement is covered in Section 7.3.

4.2.6.  Reusable Node Groups (grouping)

   Groups of nodes can be assembled into reusable collections using the
   "grouping" statement.  A grouping defines a set of nodes that are
   instantiated with the "uses" statement:

     grouping target {
         leaf address {
             type inet:ip-address;
             description "Target IP address";
         }
         leaf port {
             type inet:port-number;
             description "Target port number";
         }
     }

     container peer {
         container destination {
             uses target;
         }
     }

   NETCONF XML Example:

     <peer>
       <destination>
         <address>192.0.2.1</address>
         <port>830</port>
       </destination>
     </peer>

   The grouping can be refined as it is used, allowing certain
   statements to be overridden.  In this example, the description is
   refined:










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RFC 6020                          YANG                      October 2010


     container connection {
         container source {
             uses target {
                 refine "address" {
                     description "Source IP address";
                 }
                 refine "port" {
                     description "Source port number";
                 }
             }
         }
         container destination {
             uses target {
                 refine "address" {
                     description "Destination IP address";
                 }
                 refine "port" {
                     description "Destination port number";
                 }
             }
         }
     }

   The "grouping" statement is covered in Section 7.11.

4.2.7.  Choices

   YANG allows the data model to segregate incompatible nodes into
   distinct choices using the "choice" and "case" statements.  The
   "choice" statement contains a set of "case" statements that define
   sets of schema nodes that cannot appear together.  Each "case" may
   contain multiple nodes, but each node may appear in only one "case"
   under a "choice".

   When an element from one case is created, all elements from all other
   cases are implicitly deleted.  The device handles the enforcement of
   the constraint, preventing incompatibilities from existing in the
   configuration.

   The choice and case nodes appear only in the schema tree, not in the
   data tree or NETCONF messages.  The additional levels of hierarchy
   are not needed beyond the conceptual schema.









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   YANG Example:

     container food {
       choice snack {
           case sports-arena {
               leaf pretzel {
                   type empty;
               }
               leaf beer {
                   type empty;
               }
           }
           case late-night {
               leaf chocolate {
                   type enumeration {
                       enum dark;
                       enum milk;
                       enum first-available;
                   }
               }
           }
       }
    }

   NETCONF XML Example:

     <food>
       <pretzel/>
       <beer/>
     </food>

   The "choice" statement is covered in Section 7.9.

4.2.8.  Extending Data Models (augment)

   YANG allows a module to insert additional nodes into data models,
   including both the current module (and its submodules) or an external
   module.  This is useful for example for vendors to add vendor-
   specific parameters to standard data models in an interoperable way.

   The "augment" statement defines the location in the data model
   hierarchy where new nodes are inserted, and the "when" statement
   defines the conditions when the new nodes are valid.








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   YANG Example:

     augment /system/login/user {
         when "class != 'wheel'";
         leaf uid {
             type uint16 {
                 range "1000 .. 30000";
             }
         }
     }

   This example defines a "uid" node that only is valid when the user's
   "class" is not "wheel".

   If a module augments another module, the XML representation of the
   data will reflect the prefix of the augmenting module.  For example,
   if the above augmentation were in a module with prefix "other", the
   XML would look like:

   NETCONF XML Example:

     <user>
       <name>alicew</name>
       <full-name>Alice N. Wonderland</full-name>
       <class>drop-out</class>
       <other:uid>1024</other:uid>
     </user>

   The "augment" statement is covered in Section 7.15.

4.2.9.  RPC Definitions

   YANG allows the definition of NETCONF RPCs.  The operations' names,
   input parameters, and output parameters are modeled using YANG data
   definition statements.
















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   YANG Example:

     rpc activate-software-image {
         input {
             leaf image-name {
                 type string;
             }
         }
         output {
             leaf status {
                 type string;
             }
         }
     }

   NETCONF XML Example:

     <rpc message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       <activate-software-image xmlns="http://acme.example.com/system">
         <image-name>acmefw-2.3</image-name>
      </activate-software-image>
     </rpc>

     <rpc-reply message-id="101"
                xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       <status xmlns="http://acme.example.com/system">
         The image acmefw-2.3 is being installed.
       </status>
     </rpc-reply>

   The "rpc" statement is covered in Section 7.13.

4.2.10.  Notification Definitions

   YANG allows the definition of notifications suitable for NETCONF.
   YANG data definition statements are used to model the content of the
   notification.













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   YANG Example:

     notification link-failure {
         description "A link failure has been detected";
         leaf if-name {
             type leafref {
                 path "/interface/name";
             }
         }
         leaf if-admin-status {
             type admin-status;
         }
         leaf if-oper-status {
             type oper-status;
         }
     }

   NETCONF XML Example:

     <notification
         xmlns="urn:ietf:params:netconf:capability:notification:1.0">
       <eventTime>2007-09-01T10:00:00Z</eventTime>
       <link-failure xmlns="http://acme.example.com/system">
         <if-name>so-1/2/3.0</if-name>
         <if-admin-status>up</if-admin-status>
         <if-oper-status>down</if-oper-status>
       </link-failure>
     </notification>

   The "notification" statement is covered in Section 7.14.

5.  Language Concepts

5.1.  Modules and Submodules

   The module is the base unit of definition in YANG.  A module defines
   a single data model.  A module can define a complete, cohesive model,
   or augment an existing data model with additional nodes.

   Submodules are partial modules that contribute definitions to a
   module.  A module may include any number of submodules, but each
   submodule may belong to only one module.

   The names of all standard modules and submodules MUST be unique.
   Developers of enterprise modules are RECOMMENDED to choose names for
   their modules that will have a low probability of colliding with
   standard or other enterprise modules, e.g., by using the enterprise
   or organization name as a prefix for the module name.



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   A module uses the "include" statement to include its submodules, and
   the "import" statement to reference external modules.  Similarly, a
   submodule uses the "import" statement to reference other modules, and
   uses the "include" statement to reference other submodules within its
   module.  A module or submodule MUST NOT include submodules from other
   modules, and a submodule MUST NOT import its own module.

   The import and include statements are used to make definitions
   available to other modules and submodules:

   o  For a module or submodule to reference definitions in an external
      module, the external module MUST be imported.

   o  For a module to reference definitions in one of its submodules,
      the module MUST include the submodule.

   o  For a submodule to reference definitions in a second submodule of
      the same module, the first submodule MUST include the second
      submodule.

   There MUST NOT be any circular chains of imports or includes.  For
   example, if submodule "a" includes submodule "b", "b" cannot include
   "a".

   When a definition in an external module is referenced, a locally
   defined prefix MUST be used, followed by ":", and then the external
   identifier.  References to definitions in the local module MAY use
   the prefix notation.  Since built-in data types do not belong to any
   module and have no prefix, references to built-in data types (e.g.,
   int32) cannot use the prefix notation.

5.1.1.  Import and Include by Revision

   Published modules evolve independently over time.  In order to allow
   for this evolution, modules need to be imported using specific
   revisions.  When a module is written, it uses the current revisions
   of other modules, based on what is available at the time.  As future
   revisions of the imported modules are published, the importing module
   is unaffected and its contents are unchanged.  When the author of the
   module is prepared to move to the most recently published revision of
   an imported module, the module is republished with an updated
   "import" statement.  By republishing with the new revision, the
   authors explicitly indicate their acceptance of any changes in the
   imported module.







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   For submodules, the issue is related but simpler.  A module or
   submodule that includes submodules needs to specify the revision of
   the included submodules.  If a submodule changes, any module or
   submodule that includes it needs to be updated.

   For example, module "b" imports module "a".

     module a {
         revision 2008-01-01 { ... }
         grouping a {
             leaf eh { .... }
         }
     }

     module b {
         import a {
             prefix p;
             revision-date 2008-01-01;
         }

         container bee {
             uses p:a;
         }
     }

   When the author of "a" publishes a new revision, the changes may not
   be acceptable to the author of "b".  If the new revision is
   acceptable, the author of "b" can republish with an updated revision
   in the "import" statement.

5.1.2.  Module Hierarchies

   YANG allows modeling of data in multiple hierarchies, where data may
   have more than one top-level node.  Models that have multiple top-
   level nodes are sometimes convenient, and are supported by YANG.

   NETCONF is capable of carrying any XML content as the payload in the
   <config> and <data> elements.  The top-level nodes of YANG modules
   are encoded as child elements, in any order, within these elements.
   This encapsulation guarantees that the corresponding NETCONF messages
   are always well-formed XML documents.










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RFC 6020                          YANG                      October 2010


   For example:

     module my-config {
         namespace "http://example.com/schema/config";
         prefix "co";

         container system { ... }
         container routing { ... }
     }

   could be encoded in NETCONF as:

     <rpc message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
          xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
       <edit-config>
         <target>
           <running/>
         </target>
         <config>
           <system xmlns="http://example.com/schema/config">
             <!-- system data here -->
           </system>
           <routing xmlns="http://example.com/schema/config">
             <!-- routing data here -->
           </routing>
         </config>
       </edit-config>
     </rpc>

5.2.  File Layout

   YANG modules and submodules are typically stored in files, one module
   or submodule per file.  The name of the file SHOULD be of the form:

     module-or-submodule-name ['@' revision-date] ( '.yang' / '.yin' )

   YANG compilers can find imported modules and included submodules via
   this convention.  While the YANG language defines modules, tools may
   compile submodules independently for performance and manageability
   reasons.  Errors and warnings that cannot be detected during
   submodule compilation may be delayed until the submodules are linked
   into a cohesive module.








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RFC 6020                          YANG                      October 2010


5.3.  XML Namespaces

   All YANG definitions are specified within a module that is bound to a
   particular XML namespace [XML-NAMES], which is a globally unique URI
   [RFC3986].  A NETCONF client or server uses the namespace during XML
   encoding of data.

   Namespaces for modules published in RFC streams [RFC4844] MUST be
   assigned by IANA, see Section 14.

   Namespaces for private modules are assigned by the organization
   owning the module without a central registry.  Namespace URIs MUST be
   chosen so they cannot collide with standard or other enterprise
   namespaces, for example by using the enterprise or organization name
   in the namespace.

   The "namespace" statement is covered in Section 7.1.3.

5.3.1.  YANG XML Namespace

   YANG defines an XML namespace for NETCONF <edit-config> operations
   and <error-info> content.  The name of this namespace is
   "urn:ietf:params:xml:ns:yang:1".

5.4.  Resolving Grouping, Type, and Identity Names

   Grouping, type, and identity names are resolved in the context in
   which they are defined, rather than the context in which they are
   used.  Users of groupings, typedefs, and identities are not required
   to import modules or include submodules to satisfy all references
   made by the original definition.  This behaves like static scoping in
   a conventional programming language.

   For example, if a module defines a grouping in which a type is
   referenced, when the grouping is used in a second module, the type is
   resolved in the context of the original module, not the second
   module.  There is no worry over conflicts if both modules define the
   type, since there is no ambiguity.

5.5.  Nested Typedefs and Groupings

   Typedefs and groupings may appear nested under many YANG statements,
   allowing these to be lexically scoped by the hierarchy under which
   they appear.  This allows types and groupings to be defined near
   where they are used, rather than placing them at the top level of the
   hierarchy.  The close proximity increases readability.





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RFC 6020                          YANG                      October 2010


   Scoping also allows types to be defined without concern for naming
   conflicts between types in different submodules.  Type names can be
   specified without adding leading strings designed to prevent name
   collisions within large modules.

   Finally, scoping allows the module author to keep types and groupings
   private to their module or submodule, preventing their reuse.  Since
   only top-level types and groupings (i.e., those appearing as
   substatements to a module or submodule statement) can be used outside
   the module or submodule, the developer has more control over what
   pieces of their module are presented to the outside world, supporting
   the need to hide internal information and maintaining a boundary
   between what is shared with the outside world and what is kept
   private.

   Scoped definitions MUST NOT shadow definitions at a higher scope.  A
   type or grouping cannot be defined if a higher level in the schema
   hierarchy has a definition with a matching identifier.

   A reference to an unprefixed type or grouping, or one which uses the
   prefix of the current module, is resolved by locating the closest
   matching "typedef" or "grouping" statement among the immediate
   substatements of each ancestor statement.

5.6.  Conformance

   Conformance is a measure of how accurately a device follows the
   model.  Generally speaking, devices are responsible for implementing
   the model faithfully, allowing applications to treat devices which
   implement the model identically.  Deviations from the model can
   reduce the utility of the model and increase fragility of
   applications that use it.

   YANG modelers have three mechanisms for conformance:

   o  the basic behavior of the model

   o  optional features that are part of the model

   o  deviations from the model

   We will consider each of these in sequence.









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RFC 6020                          YANG                      October 2010


5.6.1.  Basic Behavior

   The model defines a contract between the NETCONF client and server,
   which allows both parties to have faith the other knows the syntax
   and semantics behind the modeled data.  The strength of YANG lies in
   the strength of this contract.

5.6.2.  Optional Features

   In many models, the modeler will allow sections of the model to be
   conditional.  The device controls whether these conditional portions
   of the model are supported or valid for that particular device.

   For example, a syslog data model may choose to include the ability to
   save logs locally, but the modeler will realize that this is only
   possible if the device has local storage.  If there is no local
   storage, an application should not tell the device to save logs.

   YANG supports this conditional mechanism using a construct called
   "feature".  Features give the modeler a mechanism for making portions
   of the module conditional in a manner that is controlled by the
   device.  The model can express constructs that are not universally
   present in all devices.  These features are included in the model
   definition, allowing a consistent view and allowing applications to
   learn which features are supported and tailor their behavior to the
   device.

   A module may declare any number of features, identified by simple
   strings, and may make portions of the module optional based on those
   features.  If the device supports a feature, then the corresponding
   portions of the module are valid for that device.  If the device
   doesn't support the feature, those parts of the module are not valid,
   and applications should behave accordingly.

   Features are defined using the "feature" statement.  Definitions in
   the module that are conditional to the feature are noted by the
   "if-feature" statement with the name of the feature as its argument.

   Further details are available in Section 7.18.1.

5.6.3.  Deviations

   In an ideal world, all devices would be required to implement the
   model exactly as defined, and deviations from the model would not be
   allowed.  But in the real world, devices are often not able or
   designed to implement the model as written.  For YANG-based





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RFC 6020                          YANG                      October 2010


   automation to deal with these device deviations, a mechanism must
   exist for devices to inform applications of the specifics of such
   deviations.

   For example, a BGP module may allow any number of BGP peers, but a
   particular device may only support 16 BGP peers.  Any application
   configuring the 17th peer will receive an error.  While an error may
   suffice to let the application know it cannot add another peer, it
   would be far better if the application had prior knowledge of this
   limitation and could prevent the user from starting down the path
   that could not succeed.

   Device deviations are declared using the "deviation" statement, which
   takes as its argument a string that identifies a node in the schema
   tree.  The contents of the statement details the manner in which the
   device implementation deviates from the contract as defined in the
   module.

   Further details are available in Section 7.18.3.

5.6.4.  Announcing Conformance Information in the <hello> Message

   The namespace URI MUST be advertised as a capability in the NETCONF
   <hello> message to indicate support for the YANG module by a NETCONF
   server.  The capability URI advertised MUST be of the form:

     capability-string   = namespace-uri [ parameter-list ]
     parameter-list      = "?" parameter *( "&" parameter )
     parameter           = revision-parameter /
                           module-parameter /
                           feature-parameter /
                           deviation-parameter
     revision-parameter  = "revision=" revision-date
     module-parameter    = "module=" module-name
     feature-parameter   = "features=" feature *( "," feature )
     deviation-parameter = "deviations=" deviation *( "," deviation )

   Where "revision-date" is the revision of the module (see
   Section 7.1.9) that the NETCONF server implements, "module-name" is
   the name of module as it appears in the "module" statement (see
   Section 7.1), "namespace-uri" is the namespace URI for the module as
   it appears in the "namespace" statement (see Section 7.1.3),
   "feature" is the name of an optional feature implemented by the
   device (see Section 7.18.1), and "deviation" is the name of a module
   defining device deviations (see Section 7.18.3).

   In the parameter list, each named parameter MUST occur at most once.




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5.6.4.1.  Modules

   Servers indicate the names of supported modules via the <hello>
   message.  Module namespaces are encoded as the base URI in the
   capability string, and the module name is encoded as the "module"
   parameter to the base URI.

   A server MUST advertise all revisions of all modules it implements.

   For example, this <hello> message advertises one module "syslog".

   <hello xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <capability>
       http://example.com/syslog?module=syslog&amp;revision=2008-04-01
     </capability>
   </hello>

5.6.4.2.  Features

   Servers indicate the names of supported features via the <hello>
   message.  In <hello> messages, the features are encoded in the
   "features" parameter within the URI.  The value of this parameter is
   a comma-separated list of feature names that the device supports for
   the specific module.

   For example, this <hello> message advertises one module, informing
   the client that it supports the "local-storage" feature of module
   "syslog".
<hello xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
  <capability>
    http://example.com/syslog?module=syslog&amp;features=local-storage
  </capability>
</hello>

5.6.4.3.  Deviations

   Device deviations are announced via the "deviations" parameter.  The
   value of the "deviations" parameter is a comma-separated list of
   modules containing deviations from the capability's module.

   For example, this <hello> message advertises two modules, informing
   the client that it deviates from module "syslog" according to the
   deviations listed in the module "my-devs".








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   <hello xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       <capability>
         http://example.com/syslog?module=syslog&amp;deviations=my-devs
       </capability>
       <capability>
         http://example.com/my-deviations?module=my-devs
       </capability>
     </hello>

5.7.  Data Store Modification

   Data models may allow the server to alter the configuration data
   store in ways not explicitly directed via NETCONF protocol messages.
   For example, a data model may define leafs that are assigned system-
   generated values when the client does not provide one.  A formal
   mechanism for specifying the circumstances where these changes are
   allowed is out of scope for this specification.

6.  YANG Syntax

   The YANG syntax is similar to that of SMIng [RFC3780] and programming
   languages like C and C++.  This C-like syntax was chosen specifically
   for its readability, since YANG values the time and effort of the
   readers of models above those of modules writers and YANG tool-chain
   developers.  This section introduces the YANG syntax.

   YANG modules use the UTF-8 [RFC3629] character encoding.

6.1.  Lexical Tokenization

   YANG modules are parsed as a series of tokens.  This section details
   the rules for recognizing tokens from an input stream.  YANG
   tokenization rules are both simple and powerful.  The simplicity is
   driven by a need to keep the parsers easy to implement, while the
   power is driven by the fact that modelers need to express their
   models in readable formats.

6.1.1.  Comments

   Comments are C++ style.  A single line comment starts with "//" and
   ends at the end of the line.  A block comment is enclosed within "/*"
   and "*/".

6.1.2.  Tokens

   A token in YANG is either a keyword, a string, a semicolon (";"), or
   braces ("{" or "}").  A string can be quoted or unquoted.  A keyword
   is either one of the YANG keywords defined in this document, or a



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   prefix identifier, followed by ":", followed by a language extension
   keyword.  Keywords are case sensitive.  See Section 6.2 for a formal
   definition of identifiers.

6.1.3.  Quoting

   If a string contains any space or tab characters, a semicolon (";"),
   braces ("{" or "}"), or comment sequences ("//", "/*", or "*/"), then
   it MUST be enclosed within double or single quotes.

   If the double-quoted string contains a line break followed by space
   or tab characters that are used to indent the text according to the
   layout in the YANG file, this leading whitespace is stripped from the
   string, up to and including the column of the double quote character,
   or to the first non-whitespace character, whichever occurs first.  In
   this process, a tab character is treated as 8 space characters.

   If the double-quoted string contains space or tab characters before a
   line break, this trailing whitespace is stripped from the string.

   A single-quoted string (enclosed within ' ') preserves each character
   within the quotes.  A single quote character cannot occur in a
   single-quoted string, even when preceded by a backslash.

   Within a double-quoted string (enclosed within " "), a backslash
   character introduces a special character, which depends on the
   character that immediately follows the backslash:

    \n      new line
    \t      a tab character
    \"      a double quote
    \\      a single backslash

   If a quoted string is followed by a plus character ("+"), followed by
   another quoted string, the two strings are concatenated into one
   string, allowing multiple concatenations to build one string.
   Whitespace trimming and substitution of backslash-escaped characters
   in double-quoted strings is done before concatenation.

6.1.3.1.  Quoting Examples

   The following strings are equivalent:

     hello
     "hello"
     'hello'
     "hel" + "lo"
     'hel' + "lo"



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   The following examples show some special strings:

     "\""  - string containing a double quote
     '"'   - string containing a double quote
     "\n"  - string containing a new line character
     '\n'  - string containing a backslash followed
             by the character n

   The following examples show some illegal strings:

     ''''  - a single-quoted string cannot contain single quotes
     """   - a double quote must be escaped in a double-quoted string

   The following strings are equivalent:

         "first line
            second line"

     "first line\n" + "  second line"

6.2.  Identifiers

   Identifiers are used to identify different kinds of YANG items by
   name.  Each identifier starts with an uppercase or lowercase ASCII
   letter or an underscore character, followed by zero or more ASCII
   letters, digits, underscore characters, hyphens, and dots.
   Implementations MUST support identifiers up to 64 characters in
   length.  Identifiers are case sensitive.  The identifier syntax is
   formally defined by the rule "identifier" in Section 12.  Identifiers
   can be specified as quoted or unquoted strings.

6.2.1.  Identifiers and Their Namespaces

   Each identifier is valid in a namespace that depends on the type of
   the YANG item being defined.  All identifiers defined in a namespace
   MUST be unique.

   o  All module and submodule names share the same global module
      identifier namespace.

   o  All extension names defined in a module and its submodules share
      the same extension identifier namespace.

   o  All feature names defined in a module and its submodules share the
      same feature identifier namespace.

   o  All identity names defined in a module and its submodules share
      the same identity identifier namespace.



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   o  All derived type names defined within a parent node or at the top
      level of the module or its submodules share the same type
      identifier namespace.  This namespace is scoped to all descendant
      nodes of the parent node or module.  This means that any
      descendent node may use that typedef, and it MUST NOT define a
      typedef with the same name.

   o  All grouping names defined within a parent node or at the top
      level of the module or its submodules share the same grouping
      identifier namespace.  This namespace is scoped to all descendant
      nodes of the parent node or module.  This means that any
      descendent node may use that grouping, and it MUST NOT define a
      grouping with the same name.

   o  All leafs, leaf-lists, lists, containers, choices, rpcs,
      notifications, and anyxmls defined (directly or through a uses
      statement) within a parent node or at the top level of the module
      or its submodules share the same identifier namespace.  This
      namespace is scoped to the parent node or module, unless the
      parent node is a case node.  In that case, the namespace is scoped
      to the closest ancestor node that is not a case or choice node.

   o  All cases within a choice share the same case identifier
      namespace.  This namespace is scoped to the parent choice node.

   Forward references are allowed in YANG.

6.3.  Statements

   A YANG module contains a sequence of statements.  Each statement
   starts with a keyword, followed by zero or one argument, followed
   either by a semicolon (";") or a block of substatements enclosed
   within braces ("{ }"):

     statement = keyword [argument] (";" / "{" *statement "}")

   The argument is a string, as defined in Section 6.1.2.

6.3.1.  Language Extensions

   A module can introduce YANG extensions by using the "extension"
   keyword (see Section 7.17).  The extensions can be imported by other
   modules with the "import" statement (see Section 7.1.5).  When an
   imported extension is used, the extension's keyword MUST be qualified
   using the prefix with which the extension's module was imported.  If
   an extension is used in the module where it is defined, the
   extension's keyword MUST be qualified with the module's prefix.




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   Since submodules cannot include the parent module, any extensions in
   the module that need to be exposed to submodules MUST be defined in a
   submodule.  Submodules can then include this submodule to find the
   definition of the extension.

   If a YANG compiler does not support a particular extension, which
   appears in a YANG module as an unknown-statement (see Section 12),
   the entire unknown-statement MAY be ignored by the compiler.

6.4.  XPath Evaluations

   YANG relies on XML Path Language (XPath) 1.0 [XPATH] as a notation
   for specifying many inter-node references and dependencies.  NETCONF
   clients and servers are not required to implement an XPath
   interpreter, but MUST ensure that the requirements encoded in the
   data model are enforced.  The manner of enforcement is an
   implementation decision.  The XPath expressions MUST be syntactically
   correct, and all prefixes used MUST be present in the XPath context
   (see Section 6.4.1).  An implementation may choose to implement them
   by hand, rather than using the XPath expression directly.

   The data model used in the XPath expressions is the same as that used
   in XPath 1.0 [XPATH], with the same extension for root node children
   as used by XSLT 1.0 [XSLT] (Section 3.1).  Specifically, it means
   that the root node may have any number of element nodes as its
   children.

6.4.1.  XPath Context

   All YANG XPath expressions share the following XPath context
   definition:

   o  The set of namespace declarations is the set of all "import"
      statements' prefix and namespace pairs in the module where the
      XPath expression is specified, and the "prefix" statement's prefix
      for the "namespace" statement's URI.

   o  Names without a namespace prefix belong to the same namespace as
      the identifier of the current node.  Inside a grouping, that
      namespace is affected by where the grouping is used (see
      Section 7.12).

   o  The function library is the core function library defined in
      [XPATH], and a function "current()" that returns a node set with
      the initial context node.

   o  The set of variable bindings is empty.




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   The mechanism for handling unprefixed names is adopted from XPath 2.0
   [XPATH2.0], and helps simplify XPath expressions in YANG.  No
   ambiguity may ever arise because YANG node identifiers are always
   qualified names with a non-null namespace URI.

   The context node varies with the YANG XPath expression, and is
   specified where the YANG statement with the XPath expression is
   defined.

6.5.  Schema Node Identifier

   A schema node identifier is a string that identifies a node in the
   schema tree.  It has two forms, "absolute" and "descendant", defined
   by the rules "absolute-schema-nodeid" and "descendant-schema-nodeid"
   in Section 12, respectively.  A schema node identifier consists of a
   path of identifiers, separated by slashes ("/").  In an absolute
   schema node identifier, the first identifier after the leading slash
   is any top-level schema node in the local module or in all imported
   modules.

   References to identifiers defined in external modules MUST be
   qualified with appropriate prefixes, and references to identifiers
   defined in the current module and its submodules MAY use a prefix.

   For example, to identify the child node "b" of top-level node "a",
   the string "/a/b" can be used.

7.  YANG Statements

   The following sections describe all of the YANG statements.

   Note that even a statement that does not have any substatements
   defined in YANG can have vendor-specific extensions as substatements.
   For example, the "description" statement does not have any
   substatements defined in YANG, but the following is legal:

     description "some text" {
         acme:documentation-flag 5;
     }

7.1.  The module Statement

   The "module" statement defines the module's name, and groups all
   statements that belong to the module together.  The "module"
   statement's argument is the name of the module, followed by a block
   of substatements that hold detailed module information.  The module
   name follows the rules for identifiers in Section 6.2.




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   Names of modules published in RFC streams [RFC4844] MUST be assigned
   by IANA, see Section 14.

   Private module names are assigned by the organization owning the
   module without a central registry.  It is RECOMMENDED to choose
   module names that will have a low probability of colliding with
   standard or other enterprise modules and submodules, e.g., by using
   the enterprise or organization name as a prefix for the module name.

   A module typically has the following layout:

     module <module-name> {

         // header information
         <yang-version statement>
         <namespace statement>
         <prefix statement>

         // linkage statements
         <import statements>
         <include statements>

         // meta information
         <organization statement>
         <contact statement>
         <description statement>
         <reference statement>

         // revision history
         <revision statements>

         // module definitions
         <other statements>
     }

















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7.1.1.  The module's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | augment      | 7.15    | 0..n        |
                 | choice       | 7.9     | 0..n        |
                 | contact      | 7.1.8   | 0..1        |
                 | container    | 7.5     | 0..n        |
                 | description  | 7.19.3  | 0..1        |
                 | deviation    | 7.18.3  | 0..n        |
                 | extension    | 7.17    | 0..n        |
                 | feature      | 7.18.1  | 0..n        |
                 | grouping     | 7.11    | 0..n        |
                 | identity     | 7.16    | 0..n        |
                 | import       | 7.1.5   | 0..n        |
                 | include      | 7.1.6   | 0..n        |
                 | leaf         | 7.6     | 0..n        |
                 | leaf-list    | 7.7     | 0..n        |
                 | list         | 7.8     | 0..n        |
                 | namespace    | 7.1.3   | 1           |
                 | notification | 7.14    | 0..n        |
                 | organization | 7.1.7   | 0..1        |
                 | prefix       | 7.1.4   | 1           |
                 | reference    | 7.19.4  | 0..1        |
                 | revision     | 7.1.9   | 0..n        |
                 | rpc          | 7.13    | 0..n        |
                 | typedef      | 7.3     | 0..n        |
                 | uses         | 7.12    | 0..n        |
                 | yang-version | 7.1.2   | 0..1        |
                 +--------------+---------+-------------+

7.1.2.  The yang-version Statement

   The optional "yang-version" statement specifies which version of the
   YANG language was used in developing the module.  The statement's
   argument is a string.  If present, it MUST contain the value "1",
   which is the current YANG version and the default value.

   Handling of the "yang-version" statement for versions other than "1"
   (the version defined here) is out of scope for this specification.
   Any document that defines a higher version will need to define the
   backward compatibility of such a higher version.







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7.1.3.  The namespace Statement

   The "namespace" statement defines the XML namespace that all
   identifiers defined by the module are qualified by, with the
   exception of data node identifiers defined inside a grouping (see
   Section 7.12 for details).  The argument to the "namespace" statement
   is the URI of the namespace.

   See also Section 5.3.

7.1.4.  The prefix Statement

   The "prefix" statement is used to define the prefix associated with
   the module and its namespace.  The "prefix" statement's argument is
   the prefix string that is used as a prefix to access a module.  The
   prefix string MAY be used to refer to definitions contained in the
   module, e.g., "if:ifName".  A prefix follows the same rules as an
   identifier (see Section 6.2).

   When used inside the "module" statement, the "prefix" statement
   defines the prefix to be used when this module is imported.  To
   improve readability of the NETCONF XML, a NETCONF client or server
   that generates XML or XPath that use prefixes SHOULD use the prefix
   defined by the module, unless there is a conflict.

   When used inside the "import" statement, the "prefix" statement
   defines the prefix to be used when accessing definitions inside the
   imported module.  When a reference to an identifier from the imported
   module is used, the prefix string for the imported module is used in
   combination with a colon (":") and the identifier, e.g., "if:
   ifIndex".  To improve readability of YANG modules, the prefix defined
   by a module SHOULD be used when the module is imported, unless there
   is a conflict.  If there is a conflict, i.e., two different modules
   that both have defined the same prefix are imported, at least one of
   them MUST be imported with a different prefix.

   All prefixes, including the prefix for the module itself MUST be
   unique within the module or submodule.

7.1.5.  The import Statement

   The "import" statement makes definitions from one module available
   inside another module or submodule.  The argument is the name of the
   module to import, and the statement is followed by a block of
   substatements that holds detailed import information.  When a module
   is imported, the importing module may:





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RFC 6020                          YANG                      October 2010


   o  use any grouping and typedef defined at the top level in the
      imported module or its submodules.

   o  use any extension, feature, and identity defined in the imported
      module or its submodules.

   o  use any node in the imported module's schema tree in "must",
      "path", and "when" statements, or as the target node in "augment"
      and "deviation" statements.

   The mandatory "prefix" substatement assigns a prefix for the imported
   module that is scoped to the importing module or submodule.  Multiple
   "import" statements may be specified to import from different
   modules.

   When the optional "revision-date" substatement is present, any
   typedef, grouping, extension, feature, and identity referenced by
   definitions in the local module are taken from the specified revision
   of the imported module.  It is an error if the specified revision of
   the imported module does not exist.  If no "revision-date"
   substatement is present, it is undefined from which revision of the
   module they are taken.

   Multiple revisions of the same module MUST NOT be imported.

                        The import's Substatements

                 +---------------+---------+-------------+
                 | substatement  | section | cardinality |
                 +---------------+---------+-------------+
                 | prefix        | 7.1.4   | 1           |
                 | revision-date | 7.1.5.1 | 0..1        |
                 +---------------+---------+-------------+

7.1.5.1.  The import's revision-date Statement

   The import's "revision-date" statement is used to specify the exact
   version of the module to import.  The "revision-date" statement MUST
   match the most recent "revision" statement in the imported module.

7.1.6.  The include Statement

   The "include" statement is used to make content from a submodule
   available to that submodule's parent module, or to another submodule
   of that parent module.  The argument is an identifier that is the
   name of the submodule to include.  Modules are only allowed to





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   include submodules that belong to that module, as defined by the
   "belongs-to" statement (see Section 7.2.2).  Submodules are only
   allowed to include other submodules belonging to the same module.

   When a module includes a submodule, it incorporates the contents of
   the submodule into the node hierarchy of the module.  When a
   submodule includes another submodule, the target submodule's
   definitions are made available to the current submodule.

   When the optional "revision-date" substatement is present, the
   specified revision of the submodule is included in the module.  It is
   an error if the specified revision of the submodule does not exist.
   If no "revision-date" substatement is present, it is undefined which
   revision of the submodule is included.

   Multiple revisions of the same submodule MUST NOT be included.

                       The includes's Substatements

                 +---------------+---------+-------------+
                 | substatement  | section | cardinality |
                 +---------------+---------+-------------+
                 | revision-date | 7.1.5.1 | 0..1        |
                 +---------------+---------+-------------+

7.1.7.  The organization Statement

   The "organization" statement defines the party responsible for this
   module.  The argument is a string that is used to specify a textual
   description of the organization(s) under whose auspices this module
   was developed.

7.1.8.  The contact Statement

   The "contact" statement provides contact information for the module.
   The argument is a string that is used to specify contact information
   for the person or persons to whom technical queries concerning this
   module should be sent, such as their name, postal address, telephone
   number, and electronic mail address.

7.1.9.  The revision Statement

   The "revision" statement specifies the editorial revision history of
   the module, including the initial revision.  A series of revision
   statements detail the changes in the module's definition.  The
   argument is a date string in the format "YYYY-MM-DD", followed by a
   block of substatements that holds detailed revision information.  A
   module SHOULD have at least one initial "revision" statement.  For



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   every published editorial change, a new one SHOULD be added in front
   of the revisions sequence, so that all revisions are in reverse
   chronological order.

7.1.9.1.  The revision's Substatement

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | description  | 7.19.3  | 0..1        |
                 | reference    | 7.19.4  | 0..1        |
                 +--------------+---------+-------------+

7.1.10.  Usage Example

     module acme-system {
         namespace "http://acme.example.com/system";
         prefix "acme";

         import ietf-yang-types {
             prefix "yang";
         }

         include acme-types;

         organization "ACME Inc.";
         contact
             "Joe L. User

              ACME, Inc.
              42 Anywhere Drive
              Nowhere, CA 95134
              USA

              Phone: +1 800 555 0100
              EMail: joe@acme.example.com";

         description
             "The module for entities implementing the ACME protocol.";

         revision "2007-06-09" {
             description "Initial revision.";
         }

         // definitions follow...
     }





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7.2.  The submodule Statement

   While the primary unit in YANG is a module, a YANG module can itself
   be constructed out of several submodules.  Submodules allow a module
   designer to split a complex model into several pieces where all the
   submodules contribute to a single namespace, which is defined by the
   module that includes the submodules.

   The "submodule" statement defines the submodule's name, and groups
   all statements that belong to the submodule together.  The
   "submodule" statement's argument is the name of the submodule,
   followed by a block of substatements that hold detailed submodule
   information.  The submodule name follows the rules for identifiers in
   Section 6.2.

   Names of submodules published in RFC streams [RFC4844] MUST be
   assigned by IANA, see Section 14.

   Private submodule names are assigned by the organization owning the
   submodule without a central registry.  It is RECOMMENDED to choose
   submodule names that will have a low probability of colliding with
   standard or other enterprise modules and submodules, e.g., by using
   the enterprise or organization name as a prefix for the submodule
   name.



























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   A submodule typically has the following layout:

     submodule <module-name> {

         <yang-version statement>

         // module identification
         <belongs-to statement>

         // linkage statements
         <import statements>
         <include statements>

         // meta information
         <organization statement>
         <contact statement>
         <description statement>
         <reference statement>

         // revision history
         <revision statements>

         // module definitions
         <other statements>
     }


























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7.2.1.  The submodule's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | augment      | 7.15    | 0..n        |
                 | belongs-to   | 7.2.2   | 1           |
                 | choice       | 7.9     | 0..n        |
                 | contact      | 7.1.8   | 0..1        |
                 | container    | 7.5     | 0..n        |
                 | description  | 7.19.3  | 0..1        |
                 | deviation    | 7.18.3  | 0..n        |
                 | extension    | 7.17    | 0..n        |
                 | feature      | 7.18.1  | 0..n        |
                 | grouping     | 7.11    | 0..n        |
                 | identity     | 7.16    | 0..n        |
                 | import       | 7.1.5   | 0..n        |
                 | include      | 7.1.6   | 0..n        |
                 | leaf         | 7.6     | 0..n        |
                 | leaf-list    | 7.7     | 0..n        |
                 | list         | 7.8     | 0..n        |
                 | notification | 7.14    | 0..n        |
                 | organization | 7.1.7   | 0..1        |
                 | reference    | 7.19.4  | 0..1        |
                 | revision     | 7.1.9   | 0..n        |
                 | rpc          | 7.13    | 0..n        |
                 | typedef      | 7.3     | 0..n        |
                 | uses         | 7.12    | 0..n        |
                 | yang-version | 7.1.2   | 0..1        |
                 +--------------+---------+-------------+

7.2.2.  The belongs-to Statement

   The "belongs-to" statement specifies the module to which the
   submodule belongs.  The argument is an identifier that is the name of
   the module.

   A submodule MUST only be included by the module to which it belongs,
   or by another submodule that belongs to that module.

   The mandatory "prefix" substatement assigns a prefix for the module
   to which the submodule belongs.  All definitions in the local
   submodule and any included submodules can be accessed by using the
   prefix.






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                      The belongs-to's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | prefix       | 7.1.4   | 1           |
                 +--------------+---------+-------------+

7.2.3.  Usage Example

     submodule acme-types {

         belongs-to "acme-system" {
             prefix "acme";
         }

         import ietf-yang-types {
             prefix "yang";
         }

         organization "ACME Inc.";
         contact
             "Joe L. User

              ACME, Inc.
              42 Anywhere Drive
              Nowhere, CA 95134
              USA

              Phone: +1 800 555 0100
              EMail: joe@acme.example.com";

         description
             "This submodule defines common ACME types.";

         revision "2007-06-09" {
             description "Initial revision.";
         }

         // definitions follows...
     }

7.3.  The typedef Statement

   The "typedef" statement defines a new type that may be used locally
   in the module, in modules or submodules which include it, and by
   other modules that import from it, according to the rules in




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   Section 5.5.  The new type is called the "derived type", and the type
   from which it was derived is called the "base type".  All derived
   types can be traced back to a YANG built-in type.

   The "typedef" statement's argument is an identifier that is the name
   of the type to be defined, and MUST be followed by a block of
   substatements that holds detailed typedef information.

   The name of the type MUST NOT be one of the YANG built-in types.  If
   the typedef is defined at the top level of a YANG module or
   submodule, the name of the type to be defined MUST be unique within
   the module.

7.3.1.  The typedef's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | default      | 7.3.4   | 0..1        |
                 | description  | 7.19.3  | 0..1        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | type         | 7.3.2   | 1           |
                 | units        | 7.3.3   | 0..1        |
                 +--------------+---------+-------------+

7.3.2.  The typedef's type Statement

   The "type" statement, which MUST be present, defines the base type
   from which this type is derived.  See Section 7.4 for details.

7.3.3.  The units Statement

   The "units" statement, which is optional, takes as an argument a
   string that contains a textual definition of the units associated
   with the type.

7.3.4.  The typedef's default Statement

   The "default" statement takes as an argument a string that contains a
   default value for the new type.

   The value of the "default" statement MUST be valid according to the
   type specified in the "type" statement.

   If the base type has a default value, and the new derived type does
   not specify a new default value, the base type's default value is
   also the default value of the new derived type.



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   If the type's default value is not valid according to the new
   restrictions specified in a derived type or leaf definition, the
   derived type or leaf definition MUST specify a new default value
   compatible with the restrictions.

7.3.5.  Usage Example

     typedef listen-ipv4-address {
         type inet:ipv4-address;
         default "0.0.0.0";
     }

7.4.  The type Statement

   The "type" statement takes as an argument a string that is the name
   of a YANG built-in type (see Section 9) or a derived type (see
   Section 7.3), followed by an optional block of substatements that are
   used to put further restrictions on the type.

   The restrictions that can be applied depend on the type being
   restricted.  The restriction statements for all built-in types are
   described in the subsections of Section 9.

7.4.1.  The type's Substatements

               +------------------+---------+-------------+
               | substatement     | section | cardinality |
               +------------------+---------+-------------+
               | bit              | 9.7.4   | 0..n        |
               | enum             | 9.6.4   | 0..n        |
               | length           | 9.4.4   | 0..1        |
               | path             | 9.9.2   | 0..1        |
               | pattern          | 9.4.6   | 0..n        |
               | range            | 9.2.4   | 0..1        |
               | require-instance | 9.13.2  | 0..1        |
               | type             | 7.4     | 0..n        |
               +------------------+---------+-------------+

7.5.  The container Statement

   The "container" statement is used to define an interior data node in
   the schema tree.  It takes one argument, which is an identifier,
   followed by a block of substatements that holds detailed container
   information.

   A container node does not have a value, but it has a list of child
   nodes in the data tree.  The child nodes are defined in the
   container's substatements.



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7.5.1.  Containers with Presence

   YANG supports two styles of containers, those that exist only for
   organizing the hierarchy of data nodes, and those whose presence in
   the configuration has an explicit meaning.

   In the first style, the container has no meaning of its own, existing
   only to contain child nodes.  This is the default style.

   For example, the set of scrambling options for Synchronous Optical
   Network (SONET) interfaces may be placed inside a "scrambling"
   container to enhance the organization of the configuration hierarchy,
   and to keep these nodes together.  The "scrambling" node itself has
   no meaning, so removing the node when it becomes empty relieves the
   user from performing this task.

   In the second style, the presence of the container itself is
   configuration data, representing a single bit of configuration data.
   The container acts as both a configuration knob and a means of
   organizing related configuration.  These containers are explicitly
   created and deleted.

   YANG calls this style a "presence container" and it is indicated
   using the "presence" statement, which takes as its argument a text
   string indicating what the presence of the node means.

   For example, an "ssh" container may turn on the ability to log into
   the device using ssh, but can also contain any ssh-related
   configuration knobs, such as connection rates or retry limits.

   The "presence" statement (see Section 7.5.5) is used to give
   semantics to the existence of the container in the data tree.



















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7.5.2.  The container's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | choice       | 7.9     | 0..n        |
                 | config       | 7.19.1  | 0..1        |
                 | container    | 7.5     | 0..n        |
                 | description  | 7.19.3  | 0..1        |
                 | grouping     | 7.11    | 0..n        |
                 | if-feature   | 7.18.2  | 0..n        |
                 | leaf         | 7.6     | 0..n        |
                 | leaf-list    | 7.7     | 0..n        |
                 | list         | 7.8     | 0..n        |
                 | must         | 7.5.3   | 0..n        |
                 | presence     | 7.5.5   | 0..1        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | typedef      | 7.3     | 0..n        |
                 | uses         | 7.12    | 0..n        |
                 | when         | 7.19.5  | 0..1        |
                 +--------------+---------+-------------+

7.5.3.  The must Statement

   The "must" statement, which is optional, takes as an argument a
   string that contains an XPath expression (see Section 6.4).  It is
   used to formally declare a constraint on valid data.  The constraint
   is enforced according to the rules in Section 8.

   When a datastore is validated, all "must" constraints are
   conceptually evaluated once for each data node in the data tree, and
   for all leafs with default values in use (see Section 7.6.1).  If a
   data node does not exist in the data tree, and it does not have a
   default value, its "must" statements are not evaluated.

   All such constraints MUST evaluate to true for the data to be valid.

   The XPath expression is conceptually evaluated in the following
   context, in addition to the definition in Section 6.4.1:

   o  The context node is the node in the data tree for which the "must"
      statement is defined.

   o  The accessible tree is made up of all nodes in the data tree, and
      all leafs with default values in use (see Section 7.6.1).




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   The accessible tree depends on the context node:

   o  If the context node represents configuration, the tree is the data
      in the NETCONF datastore where the context node exists.  The XPath
      root node has all top-level configuration data nodes in all
      modules as children.

   o  If the context node represents state data, the tree is all state
      data on the device, and the <running/> datastore.  The XPath root
      node has all top-level data nodes in all modules as children.

   o  If the context node represents notification content, the tree is
      the notification XML instance document.  The XPath root node has
      the element representing the notification being defined as the
      only child.

   o  If the context node represents RPC input parameters, the tree is
      the RPC XML instance document.  The XPath root node has the
      element representing the RPC operation being defined as the only
      child.

   o  If the context node represents RPC output parameters, the tree is
      the RPC reply instance document.  The XPath root node has the
      elements representing the RPC output parameters as children.

   The result of the XPath expression is converted to a boolean value
   using the standard XPath rules.

   Note that since all leaf values in the data tree are conceptually
   stored in their canonical form (see Sections 7.6 and 7.7), any XPath
   comparisons are done on the canonical value.

   Also note that the XPath expression is conceptually evaluated.  This
   means that an implementation does not have to use an XPath evaluator
   on the device.  How the evaluation is done in practice is an
   implementation decision.















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7.5.4.  The must's Substatements

                 +---------------+---------+-------------+
                 | substatement  | section | cardinality |
                 +---------------+---------+-------------+
                 | description   | 7.19.3  | 0..1        |
                 | error-app-tag | 7.5.4.2 | 0..1        |
                 | error-message | 7.5.4.1 | 0..1        |
                 | reference     | 7.19.4  | 0..1        |
                 +---------------+---------+-------------+

7.5.4.1.  The error-message Statement

   The "error-message" statement, which is optional, takes a string as
   an argument.  If the constraint evaluates to false, the string is
   passed as <error-message> in the <rpc-error>.

7.5.4.2.  The error-app-tag Statement

   The "error-app-tag" statement, which is optional, takes a string as
   an argument.  If the constraint evaluates to false, the string is
   passed as <error-app-tag> in the <rpc-error>.

7.5.4.3.  Usage Example of must and error-message

     container interface {
         leaf ifType {
             type enumeration {
                 enum ethernet;
                 enum atm;
             }
         }
         leaf ifMTU {
             type uint32;
         }
         must "ifType != 'ethernet' or " +
              "(ifType = 'ethernet' and ifMTU = 1500)" {
             error-message "An ethernet MTU must be 1500";
         }
         must "ifType != 'atm' or " +
              "(ifType = 'atm' and ifMTU <= 17966 and ifMTU >= 64)" {
             error-message "An atm MTU must be  64 .. 17966";
         }
     }







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7.5.5.  The presence Statement

   The "presence" statement assigns a meaning to the presence of a
   container in the data tree.  It takes as an argument a string that
   contains a textual description of what the node's presence means.

   If a container has the "presence" statement, the container's
   existence in the data tree carries some meaning.  Otherwise, the
   container is used to give some structure to the data, and it carries
   no meaning by itself.

   See Section 7.5.1 for additional information.

7.5.6.  The container's Child Node Statements

   Within a container, the "container", "leaf", "list", "leaf-list",
   "uses", "choice", and "anyxml" statements can be used to define child
   nodes to the container.

7.5.7.  XML Mapping Rules

   A container node is encoded as an XML element.  The element's local
   name is the container's identifier, and its namespace is the module's
   XML namespace (see Section 7.1.3).

   The container's child nodes are encoded as subelements to the
   container element.  If the container defines RPC input or output
   parameters, these subelements are encoded in the same order as they
   are defined within the "container" statement.  Otherwise, the
   subelements are encoded in any order.

   A NETCONF server that replies to a <get> or <get-config> request MAY
   choose not to send a container element if the container node does not
   have the "presence" statement and no child nodes exist.  Thus, a
   client that receives an <rpc-reply> for a <get> or <get-config>
   request, must be prepared to handle the case that a container node
   without a "presence" statement is not present in the XML.

7.5.8.  NETCONF <edit-config> Operations

   Containers can be created, deleted, replaced, and modified through
   <edit-config>, by using the "operation" attribute (see [RFC4741],
   Section 7.2) in the container's XML element.

   If a container does not have a "presence" statement and the last
   child node is deleted, the NETCONF server MAY delete the container.





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   When a NETCONF server processes an <edit-config> request, the
   elements of procedure for the container node are:

      If the operation is "merge" or "replace", the node is created if
      it does not exist.

      If the operation is "create", the node is created if it does not
      exist.  If the node already exists, a "data-exists" error is
      returned.

      If the operation is "delete", the node is deleted if it exists.
      If the node does not exist, a "data-missing" error is returned.

7.5.9.  Usage Example

   Given the following container definition:

     container system {
         description "Contains various system parameters";
         container services {
             description "Configure externally available services";
             container "ssh" {
                 presence "Enables SSH";
                 description "SSH service specific configuration";
                 // more leafs, containers and stuff here...
             }
         }
     }

   A corresponding XML instance example:

     <system>
       <services>
         <ssh/>
       </services>
     </system>

   Since the <ssh> element is present, ssh is enabled.

   To delete a container with an <edit-config>:











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     <rpc message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
          xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
       <edit-config>
         <target>
           <running/>
         </target>
         <config>
           <system xmlns="http://example.com/schema/config">
             <services>
               <ssh nc:operation="delete"/>
             </services>
           </system>
         </config>
       </edit-config>
     </rpc>

7.6.  The leaf Statement

   The "leaf" statement is used to define a leaf node in the schema
   tree.  It takes one argument, which is an identifier, followed by a
   block of substatements that holds detailed leaf information.

   A leaf node has a value, but no child nodes in the data tree.
   Conceptually, the value in the data tree is always in the canonical
   form (see Section 9.1).

   A leaf node exists in zero or one instances in the data tree.

   The "leaf" statement is used to define a scalar variable of a
   particular built-in or derived type.

7.6.1.  The leaf's default value

   The default value of a leaf is the value that the server uses if the
   leaf does not exist in the data tree.  The usage of the default value
   depends on the leaf's closest ancestor node in the schema tree that
   is not a non-presence container:

   o  If no such ancestor exists in the schema tree, the default value
      MUST be used.

   o  Otherwise, if this ancestor is a case node, the default value MUST
      be used if any node from the case exists in the data tree, or if
      the case node is the choice's default case, and no nodes from any
      other case exist in the data tree.





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   o  Otherwise, the default value MUST be used if the ancestor node
      exists in the data tree.

   In these cases, the default value is said to be in use.

   When the default value is in use, the server MUST operationally
   behave as if the leaf was present in the data tree with the default
   value as its value.

   If a leaf has a "default" statement, the leaf's default value is the
   value of the "default" statement.  Otherwise, if the leaf's type has
   a default value, and the leaf is not mandatory, then the leaf's
   default value is the type's default value.  In all other cases, the
   leaf does not have a default value.

7.6.2.  The leaf's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | config       | 7.19.1  | 0..1        |
                 | default      | 7.6.4   | 0..1        |
                 | description  | 7.19.3  | 0..1        |
                 | if-feature   | 7.18.2  | 0..n        |
                 | mandatory    | 7.6.5   | 0..1        |
                 | must         | 7.5.3   | 0..n        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | type         | 7.6.3   | 1           |
                 | units        | 7.3.3   | 0..1        |
                 | when         | 7.19.5  | 0..1        |
                 +--------------+---------+-------------+

7.6.3.  The leaf's type Statement

   The "type" statement, which MUST be present, takes as an argument the
   name of an existing built-in or derived type.  The optional
   substatements specify restrictions on this type.  See Section 7.4 for
   details.

7.6.4.  The leaf's default Statement

   The "default" statement, which is optional, takes as an argument a
   string that contains a default value for the leaf.

   The value of the "default" statement MUST be valid according to the
   type specified in the leaf's "type" statement.




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   The "default" statement MUST NOT be present on nodes where
   "mandatory" is true.

7.6.5.  The leaf's mandatory Statement

   The "mandatory" statement, which is optional, takes as an argument
   the string "true" or "false", and puts a constraint on valid data.
   If not specified, the default is "false".

   If "mandatory" is "true", the behavior of the constraint depends on
   the type of the leaf's closest ancestor node in the schema tree that
   is not a non-presence container (see Section 7.5.1):

   o  If no such ancestor exists in the schema tree, the leaf MUST
      exist.

   o  Otherwise, if this ancestor is a case node, the leaf MUST exist if
      any node from the case exists in the data tree.

   o  Otherwise, the leaf MUST exist if the ancestor node exists in the
      data tree.

   This constraint is enforced according to the rules in Section 8.

7.6.6.  XML Mapping Rules

   A leaf node is encoded as an XML element.  The element's local name
   is the leaf's identifier, and its namespace is the module's XML
   namespace (see Section 7.1.3).

   The value of the leaf node is encoded to XML according to the type,
   and sent as character data in the element.

   A NETCONF server that replies to a <get> or <get-config> request MAY
   choose not to send the leaf element if its value is the default
   value.  Thus, a client that receives an <rpc-reply> for a <get> or
   <get-config> request, MUST be prepared to handle the case that a leaf
   node with a default value is not present in the XML.  In this case,
   the value used by the server is known to be the default value.

   See Section 7.6.8 for an example.

7.6.7.  NETCONF <edit-config> Operations

   When a NETCONF server processes an <edit-config> request, the
   elements of procedure for the leaf node are:





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      If the operation is "merge" or "replace", the node is created if
      it does not exist, and its value is set to the value found in the
      XML RPC data.

      If the operation is "create", the node is created if it does not
      exist.  If the node already exists, a "data-exists" error is
      returned.

      If the operation is "delete", the node is deleted if it exists.
      If the node does not exist, a "data-missing" error is returned.

7.6.8.  Usage Example

   Given the following "leaf" statement, placed in the previously
   defined "ssh" container (see Section 7.5.9):

     leaf port {
         type inet:port-number;
         default 22;
         description "The port to which the SSH server listens"
     }

   A corresponding XML instance example:

     <port>2022</port>

   To set the value of a leaf with an <edit-config>:

     <rpc message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
          xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
       <edit-config>
         <target>
           <running/>
         </target>
         <config>
           <system xmlns="http://example.com/schema/config">
             <services>
               <ssh>
                 <port>2022</port>
               </ssh>
             </services>
           </system>
         </config>
       </edit-config>
     </rpc>





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7.7.  The leaf-list Statement

   Where the "leaf" statement is used to define a simple scalar variable
   of a particular type, the "leaf-list" statement is used to define an
   array of a particular type.  The "leaf-list" statement takes one
   argument, which is an identifier, followed by a block of
   substatements that holds detailed leaf-list information.

   The values in a leaf-list MUST be unique.

   Conceptually, the values in the data tree are always in the canonical
   form (see Section 9.1).

   If the type referenced by the leaf-list has a default value, it has
   no effect in the leaf-list.

7.7.1.  Ordering

   YANG supports two styles for ordering the entries within lists and
   leaf-lists.  In many lists, the order of list entries does not impact
   the implementation of the list's configuration, and the device is
   free to sort the list entries in any reasonable order.  The
   "description" string for the list may suggest an order to the device
   implementor.  YANG calls this style of list "system ordered" and they
   are indicated with the statement "ordered-by system".

   For example, a list of valid users would typically be sorted
   alphabetically, since the order in which the users appeared in the
   configuration would not impact the creation of those users' accounts.

   In the other style of lists, the order of list entries matters for
   the implementation of the list's configuration and the user is
   responsible for ordering the entries, while the device maintains that
   order.  YANG calls this style of list "user ordered" and they are
   indicated with the statement "ordered-by user".

   For example, the order in which firewall filters entries are applied
   to incoming traffic may affect how that traffic is filtered.  The
   user would need to decide if the filter entry that discards all TCP
   traffic should be applied before or after the filter entry that
   allows all traffic from trusted interfaces.  The choice of order
   would be crucial.

   YANG provides a rich set of facilities within NETCONF's <edit-config>
   operation that allows the order of list entries in user-ordered lists
   to be controlled.  List entries may be inserted or rearranged,
   positioned as the first or last entry in the list, or positioned
   before or after another specific entry.



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   The "ordered-by" statement is covered in Section 7.7.5.

7.7.2.  The leaf-list's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | config       | 7.19.1  | 0..1        |
                 | description  | 7.19.3  | 0..1        |
                 | if-feature   | 7.18.2  | 0..n        |
                 | max-elements | 7.7.4   | 0..1        |
                 | min-elements | 7.7.3   | 0..1        |
                 | must         | 7.5.3   | 0..n        |
                 | ordered-by   | 7.7.5   | 0..1        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | type         | 7.4     | 1           |
                 | units        | 7.3.3   | 0..1        |
                 | when         | 7.19.5  | 0..1        |
                 +--------------+---------+-------------+

7.7.3.  The min-elements Statement

   The "min-elements" statement, which is optional, takes as an argument
   a non-negative integer that puts a constraint on valid list entries.
   A valid leaf-list or list MUST have at least min-elements entries.

   If no "min-elements" statement is present, it defaults to zero.

   The behavior of the constraint depends on the type of the leaf-list's
   or list's closest ancestor node in the schema tree that is not a non-
   presence container (see Section 7.5.1):

   o  If this ancestor is a case node, the constraint is enforced if any
      other node from the case exists.

   o  Otherwise, it is enforced if the ancestor node exists.

   The constraint is further enforced according to the rules in
   Section 8.

7.7.4.  The max-elements Statement

   The "max-elements" statement, which is optional, takes as an argument
   a positive integer or the string "unbounded", which puts a constraint
   on valid list entries.  A valid leaf-list or list always has at most
   max-elements entries.




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   If no "max-elements" statement is present, it defaults to
   "unbounded".

   The "max-elements" constraint is enforced according to the rules in
   Section 8.

7.7.5.  The ordered-by Statement

   The "ordered-by" statement defines whether the order of entries
   within a list are determined by the user or the system.  The argument
   is one of the strings "system" or "user".  If not present, order
   defaults to "system".

   This statement is ignored if the list represents state data, RPC
   output parameters, or notification content.

   See Section 7.7.1 for additional information.

7.7.5.1.  ordered-by system

   The entries in the list are sorted according to an unspecified order.
   Thus, an implementation is free to sort the entries in the most
   appropriate order.  An implementation SHOULD use the same order for
   the same data, regardless of how the data were created.  Using a
   deterministic order will make comparisons possible using simple tools
   like "diff".

   This is the default order.

7.7.5.2.  ordered-by user

   The entries in the list are sorted according to an order defined by
   the user.  This order is controlled by using special XML attributes
   in the <edit-config> request.  See Section 7.7.7 for details.

7.7.6.  XML Mapping Rules

   A leaf-list node is encoded as a series of XML elements.  Each
   element's local name is the leaf-list's identifier, and its namespace
   is the module's XML namespace (see Section 7.1.3).

   The value of each leaf-list entry is encoded to XML according to the
   type, and sent as character data in the element.

   The XML elements representing leaf-list entries MUST appear in the
   order specified by the user if the leaf-list is "ordered-by user";
   otherwise, the order is implementation-dependent.  The XML elements




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   representing leaf-list entries MAY be interleaved with other sibling
   elements, unless the leaf-list defines RPC input or output
   parameters.

   See Section 7.7.8 for an example.

7.7.7.  NETCONF <edit-config> Operations

   Leaf-list entries can be created and deleted, but not modified,
   through <edit-config>, by using the "operation" attribute in the
   leaf-list entry's XML element.

   In an "ordered-by user" leaf-list, the attributes "insert" and
   "value" in the YANG XML namespace (Section 5.3.1) can be used to
   control where in the leaf-list the entry is inserted.  These can be
   used during "create" operations to insert a new leaf-list entry, or
   during "merge" or "replace" operations to insert a new leaf-list
   entry or move an existing one.

   The "insert" attribute can take the values "first", "last", "before",
   and "after".  If the value is "before" or "after", the "value"
   attribute MUST also be used to specify an existing entry in the leaf-
   list.

   If no "insert" attribute is present in the "create" operation, it
   defaults to "last".

   If several entries in an "ordered-by user" leaf-list are modified in
   the same <edit-config> request, the entries are modified one at the
   time, in the order of the XML elements in the request.

   In a <copy-config>, or an <edit-config> with a "replace" operation
   that covers the entire leaf-list, the leaf-list order is the same as
   the order of the XML elements in the request.

   When a NETCONF server processes an <edit-config> request, the
   elements of procedure for a leaf-list node are:

      If the operation is "merge" or "replace", the leaf-list entry is
      created if it does not exist.

      If the operation is "create", the leaf-list entry is created if it
      does not exist.  If the leaf-list entry already exists, a
      "data-exists" error is returned.

      If the operation is "delete", the entry is deleted from the leaf-
      list if it exists.  If the leaf-list entry does not exist, a
      "data-missing" error is returned.



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7.7.8.  Usage Example

     leaf-list allow-user  {
         type string;
         description "A list of user name patterns to allow";
     }

   A corresponding XML instance example:

     <allow-user>alice</allow-user>
     <allow-user>bob</allow-user>

   To create a new element in this list, using the default <edit-config>
   operation "merge":

     <rpc message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
          xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
       <edit-config>
         <target>
           <running/>
         </target>
         <config>
           <system xmlns="http://example.com/schema/config">
             <services>
               <ssh>
                 <allow-user>eric</allow-user>
               </ssh>
             </services>
           </system>
         </config>
       </edit-config>
     </rpc>

   Given the following ordered-by user leaf-list:

     leaf-list cipher  {
         type string;
         ordered-by user;
         description "A list of ciphers";
     }

   The following would be used to insert a new cipher "blowfish-cbc"
   after "3des-cbc":







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     <rpc message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
          xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0"
          xmlns:yang="urn:ietf:params:xml:ns:yang:1">
       <edit-config>
         <target>
           <running/>
         </target>
         <config>
           <system xmlns="http://example.com/schema/config">
             <services>
               <ssh>
                 <cipher nc:operation="create"
                         yang:insert="after"
                         yang:value="3des-cbc">blowfish-cbc</cipher>
               </ssh>
             </services>
           </system>
         </config>
       </edit-config>
     </rpc>

7.8.  The list Statement

   The "list" statement is used to define an interior data node in the
   schema tree.  A list node may exist in multiple instances in the data
   tree.  Each such instance is known as a list entry.  The "list"
   statement takes one argument, which is an identifier, followed by a
   block of substatements that holds detailed list information.

   A list entry is uniquely identified by the values of the list's keys,
   if defined.



















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7.8.1.  The list's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | choice       | 7.9     | 0..n        |
                 | config       | 7.19.1  | 0..1        |
                 | container    | 7.5     | 0..n        |
                 | description  | 7.19.3  | 0..1        |
                 | grouping     | 7.11    | 0..n        |
                 | if-feature   | 7.18.2  | 0..n        |
                 | key          | 7.8.2   | 0..1        |
                 | leaf         | 7.6     | 0..n        |
                 | leaf-list    | 7.7     | 0..n        |
                 | list         | 7.8     | 0..n        |
                 | max-elements | 7.7.4   | 0..1        |
                 | min-elements | 7.7.3   | 0..1        |
                 | must         | 7.5.3   | 0..n        |
                 | ordered-by   | 7.7.5   | 0..1        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | typedef      | 7.3     | 0..n        |
                 | unique       | 7.8.3   | 0..n        |
                 | uses         | 7.12    | 0..n        |
                 | when         | 7.19.5  | 0..1        |
                 +--------------+---------+-------------+

7.8.2.  The list's key Statement

   The "key" statement, which MUST be present if the list represents
   configuration, and MAY be present otherwise, takes as an argument a
   string that specifies a space-separated list of leaf identifiers of
   this list.  A leaf identifier MUST NOT appear more than once in the
   key.  Each such leaf identifier MUST refer to a child leaf of the
   list.  The leafs can be defined directly in substatements to the
   list, or in groupings used in the list.

   The combined values of all the leafs specified in the key are used to
   uniquely identify a list entry.  All key leafs MUST be given values
   when a list entry is created.  Thus, any default values in the key
   leafs or their types are ignored.  It also implies that any mandatory
   statement in the key leafs are ignored.

   A leaf that is part of the key can be of any built-in or derived
   type, except it MUST NOT be the built-in type "empty".





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   All key leafs in a list MUST have the same value for their "config"
   as the list itself.

   The key string syntax is formally defined by the rule "key-arg" in
   Section 12.

7.8.3.  The list's unique Statement

   The "unique" statement is used to put constraints on valid list
   entries.  It takes as an argument a string that contains a space-
   separated list of schema node identifiers, which MUST be given in the
   descendant form (see the rule "descendant-schema-nodeid" in
   Section 12).  Each such schema node identifier MUST refer to a leaf.

   If one of the referenced leafs represents configuration data, then
   all of the referenced leafs MUST represent configuration data.

   The "unique" constraint specifies that the combined values of all the
   leaf instances specified in the argument string, including leafs with
   default values, MUST be unique within all list entry instances in
   which all referenced leafs exist.  The constraint is enforced
   according to the rules in Section 8.

   The unique string syntax is formally defined by the rule "unique-arg"
   in Section 12.

7.8.3.1.  Usage Example

   With the following list:

     list server {
         key "name";
         unique "ip port";
         leaf name {
             type string;
         }
         leaf ip {
             type inet:ip-address;
         }
         leaf port {
             type inet:port-number;
         }
     }








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   The following configuration is not valid:

     <server>
       <name>smtp</name>
       <ip>192.0.2.1</ip>
       <port>25</port>
     </server>

     <server>
       <name>http</name>
       <ip>192.0.2.1</ip>
       <port>25</port>
     </server>

   The following configuration is valid, since the "http" and "ftp" list
   entries do not have a value for all referenced leafs, and are thus
   not taken into account when the "unique" constraint is enforced:

     <server>
       <name>smtp</name>
       <ip>192.0.2.1</ip>
       <port>25</port>
     </server>

     <server>
       <name>http</name>
       <ip>192.0.2.1</ip>
     </server>

     <server>
       <name>ftp</name>
       <ip>192.0.2.1</ip>
     </server>

7.8.4.  The list's Child Node Statements

   Within a list, the "container", "leaf", "list", "leaf-list", "uses",
   "choice", and "anyxml" statements can be used to define child nodes
   to the list.

7.8.5.  XML Mapping Rules

   A list is encoded as a series of XML elements, one for each entry in
   the list.  Each element's local name is the list's identifier, and
   its namespace is the module's XML namespace (see Section 7.1.3).






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   The list's key nodes are encoded as subelements to the list's
   identifier element, in the same order as they are defined within the
   "key" statement.

   The rest of the list's child nodes are encoded as subelements to the
   list element, after the keys.  If the list defines RPC input or
   output parameters, the subelements are encoded in the same order as
   they are defined within the "list" statement.  Otherwise, the
   subelements are encoded in any order.

   The XML elements representing list entries MUST appear in the order
   specified by the user if the list is "ordered-by user", otherwise the
   order is implementation-dependent.  The XML elements representing
   list entries MAY be interleaved with other sibling elements, unless
   the list defines RPC input or output parameters.

7.8.6.  NETCONF <edit-config> Operations

   List entries can be created, deleted, replaced, and modified through
   <edit-config>, by using the "operation" attribute in the list's XML
   element.  In each case, the values of all keys are used to uniquely
   identify a list entry.  If all keys are not specified for a list
   entry, a "missing-element" error is returned.

   In an "ordered-by user" list, the attributes "insert" and "key" in
   the YANG XML namespace (Section 5.3.1) can be used to control where
   in the list the entry is inserted.  These can be used during "create"
   operations to insert a new list entry, or during "merge" or "replace"
   operations to insert a new list entry or move an existing one.

   The "insert" attribute can take the values "first", "last", "before",
   and "after".  If the value is "before" or "after", the "key"
   attribute MUST also be used, to specify an existing element in the
   list.  The value of the "key" attribute is the key predicates of the
   full instance identifier (see Section 9.13) for the list entry.

   If no "insert" attribute is present in the "create" operation, it
   defaults to "last".

   If several entries in an "ordered-by user" list are modified in the
   same <edit-config> request, the entries are modified one at the time,
   in the order of the XML elements in the request.

   In a <copy-config>, or an <edit-config> with a "replace" operation
   that covers the entire list, the list entry order is the same as the
   order of the XML elements in the request.





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   When a NETCONF server processes an <edit-config> request, the
   elements of procedure for a list node are:

      If the operation is "merge" or "replace", the list entry is
      created if it does not exist.  If the list entry already exists
      and the "insert" and "key" attributes are present, the list entry
      is moved according to the values of the "insert" and "key"
      attributes.  If the list entry exists and the "insert" and "key"
      attributes are not present, the list entry is not moved.

      If the operation is "create", the list entry is created if it does
      not exist.  If the list entry already exists, a "data-exists"
      error is returned.

      If the operation is "delete", the entry is deleted from the list
      if it exists.  If the list entry does not exist, a "data-missing"
      error is returned.

7.8.7.  Usage Example

   Given the following list:

     list user {
         key "name";
         config true;
         description "This is a list of users in the system.";

         leaf name {
             type string;
         }
         leaf type {
             type string;
         }
         leaf full-name {
             type string;
         }
     }

   A corresponding XML instance example:

     <user>
       <name>fred</name>
       <type>admin</type>
       <full-name>Fred Flintstone</full-name>
     </user>






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   To create a new user "barney":

     <rpc message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
          xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
       <edit-config>
         <target>
           <running/>
         </target>
         <config>
           <system xmlns="http://example.com/schema/config">
             <user nc:operation="create">
               <name>barney</name>
               <type>admin</type>
               <full-name>Barney Rubble</full-name>
             </user>
           </system>
         </config>
       </edit-config>
     </rpc>

   To change the type of "fred" to "superuser":

     <rpc message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
          xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
       <edit-config>
         <target>
           <running/>
         </target>
         <config>
           <system xmlns="http://example.com/schema/config">
             <user>
               <name>fred</name>
               <type>superuser</type>
             </user>
           </system>
         </config>
       </edit-config>
     </rpc>











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   Given the following ordered-by user list:

     list user {
         description "This is a list of users in the system.";
         ordered-by user;
         config true;

         key "name";

         leaf name {
             type string;
         }
         leaf type {
             type string;
         }
         leaf full-name {
             type string;
         }
     }

   The following would be used to insert a new user "barney" after the
   user "fred":

     <rpc message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
          xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0"
          xmlns:yang="urn:ietf:params:xml:ns:yang:1">
       <edit-config>
         <target>
           <running/>
         </target>
         <config>
           <system xmlns="http://example.com/schema/config"
                xmlns:ex="http://example.com/schema/config">
             <user nc:operation="create"
                   yang:insert="after"
                   yang:key="[ex:name='fred']">
               <name>barney</name>
               <type>admin</type>
               <full-name>Barney Rubble</full-name>
             </user>
           </system>
         </config>
       </edit-config>
     </rpc>






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   The following would be used to move the user "barney" before the user
   "fred":

     <rpc message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
          xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0"
          xmlns:yang="urn:ietf:params:xml:ns:yang:1">
       <edit-config>
         <target>
           <running/>
         </target>
         <config>
           <system xmlns="http://example.com/schema/config"
                xmlns:ex="http://example.com/schema/config">
             <user nc:operation="merge"
                   yang:insert="before"
                   yang:key="[ex:name='fred']">
               <name>barney</name>
             </user>
           </system>
         </config>
       </edit-config>
     </rpc>

7.9.  The choice Statement

   The "choice" statement defines a set of alternatives, only one of
   which may exist at any one time.  The argument is an identifier,
   followed by a block of substatements that holds detailed choice
   information.  The identifier is used to identify the choice node in
   the schema tree.  A choice node does not exist in the data tree.

   A choice consists of a number of branches, defined with the "case"
   substatement.  Each branch contains a number of child nodes.  The
   nodes from at most one of the choice's branches exist at the same
   time.

   See Section 8.3.2 for additional information.













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7.9.1.  The choice's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | case         | 7.9.2   | 0..n        |
                 | config       | 7.19.1  | 0..1        |
                 | container    | 7.5     | 0..n        |
                 | default      | 7.9.3   | 0..1        |
                 | description  | 7.19.3  | 0..1        |
                 | if-feature   | 7.18.2  | 0..n        |
                 | leaf         | 7.6     | 0..n        |
                 | leaf-list    | 7.7     | 0..n        |
                 | list         | 7.8     | 0..n        |
                 | mandatory    | 7.9.4   | 0..1        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | when         | 7.19.5  | 0..1        |
                 +--------------+---------+-------------+

7.9.2.  The choice's case Statement

   The "case" statement is used to define branches of the choice.  It
   takes as an argument an identifier, followed by a block of
   substatements that holds detailed case information.

   The identifier is used to identify the case node in the schema tree.
   A case node does not exist in the data tree.

   Within a "case" statement, the "anyxml", "choice", "container",
   "leaf", "list", "leaf-list", and "uses" statements can be used to
   define child nodes to the case node.  The identifiers of all these
   child nodes MUST be unique within all cases in a choice.  For
   example, the following is illegal:

     choice interface-type {     // This example is illegal YANG
         case a {
             leaf ethernet { ... }
         }
         case b {
             container ethernet { ...}
         }
     }







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   As a shorthand, the "case" statement can be omitted if the branch
   contains a single "anyxml", "container", "leaf", "list", or
   "leaf-list" statement.  In this case, the identifier of the case node
   is the same as the identifier in the branch statement.  The following
   example:

     choice interface-type {
         container ethernet { ... }
     }

   is equivalent to:

     choice interface-type {
         case ethernet {
             container ethernet { ... }
         }
     }

   The case identifier MUST be unique within a choice.

7.9.2.1.  The case's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | choice       | 7.9     | 0..n        |
                 | container    | 7.5     | 0..n        |
                 | description  | 7.19.3  | 0..1        |
                 | if-feature   | 7.18.2  | 0..n        |
                 | leaf         | 7.6     | 0..n        |
                 | leaf-list    | 7.7     | 0..n        |
                 | list         | 7.8     | 0..n        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | uses         | 7.12    | 0..n        |
                 | when         | 7.19.5  | 0..1        |
                 +--------------+---------+-------------+

7.9.3.  The choice's default Statement

   The "default" statement indicates if a case should be considered as
   the default if no child nodes from any of the choice's cases exist.
   The argument is the identifier of the "case" statement.  If the
   "default" statement is missing, there is no default case.

   The "default" statement MUST NOT be present on choices where
   "mandatory" is true.



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   The default case is only important when considering the default
   values of nodes under the cases.  The default values for nodes under
   the default case are used if none of the nodes under any of the cases
   are present.

   There MUST NOT be any mandatory nodes (Section 3.1) directly under
   the default case.

   Default values for child nodes under a case are only used if one of
   the nodes under that case is present, or if that case is the default
   case.  If none of the nodes under a case are present and the case is
   not the default case, the default values of the cases' child nodes
   are ignored.

   In this example, the choice defaults to "interval", and the default
   value will be used if none of "daily", "time-of-day", or "manual" are
   present.  If "daily" is present, the default value for "time-of-day"
   will be used.

     container transfer {
         choice how {
             default interval;
             case interval {
                 leaf interval {
                     type uint16;
                     default 30;
                     units minutes;
                 }
             }
             case daily {
                 leaf daily {
                     type empty;
                 }
                 leaf time-of-day {
                     type string;
                     units 24-hour-clock;
                     default 1am;
                 }
             }
             case manual {
                 leaf manual {
                     type empty;
                 }
             }
         }
     }





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7.9.4.  The choice's mandatory Statement

   The "mandatory" statement, which is optional, takes as an argument
   the string "true" or "false", and puts a constraint on valid data.
   If "mandatory" is "true", at least one node from exactly one of the
   choice's case branches MUST exist.

   If not specified, the default is "false".

   The behavior of the constraint depends on the type of the choice's
   closest ancestor node in the schema tree which is not a non-presence
   container (see Section 7.5.1):

   o  If this ancestor is a case node, the constraint is enforced if any
      other node from the case exists.

   o  Otherwise, it is enforced if the ancestor node exists.

   The constraint is further enforced according to the rules in
   Section 8.

7.9.5.  XML Mapping Rules

   The choice and case nodes are not visible in XML.

   The child nodes of the selected "case" statement MUST be encoded in
   the same order as they are defined in the "case" statement if they
   are part of an RPC input or output parameter definition.  Otherwise,
   the subelements are encoded in any order.

7.9.6.  NETCONF <edit-config> Operations

   Since only one of the choice's cases can be valid at any time, the
   creation of a node from one case implicitly deletes all nodes from
   all other cases.  If an <edit-config> operation creates a node from a
   case, the NETCONF server will delete any existing nodes that are
   defined in other cases inside the choice.

7.9.7.  Usage Example

   Given the following choice:










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     container protocol {
         choice name {
             case a {
                 leaf udp {
                     type empty;
                 }
             }
             case b {
                 leaf tcp {
                    type empty;
                 }
             }
         }
     }

   A corresponding XML instance example:

     <protocol>
       <tcp/>
     </protocol>

   To change the protocol from tcp to udp:

     <rpc message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
          xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
       <edit-config>
         <target>
           <running/>
         </target>
         <config>
           <system xmlns="http://example.com/schema/config">
             <protocol>
               <udp nc:operation="create"/>
             </protocol>
           </system>
         </config>
       </edit-config>
     </rpc>

7.10.  The anyxml Statement

   The "anyxml" statement defines an interior node in the schema tree.
   It takes one argument, which is an identifier, followed by a block of
   substatements that holds detailed anyxml information.






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   The "anyxml" statement is used to represent an unknown chunk of XML.
   No restrictions are placed on the XML.  This can be useful, for
   example, in RPC replies.  An example is the <filter> parameter in the
   <get-config> operation.

   An anyxml node cannot be augmented (see Section 7.15).

   Since the use of anyxml limits the manipulation of the content, it is
   RECOMMENDED that the "anyxml" statement not be used to represent
   configuration data.

   An anyxml node exists in zero or one instances in the data tree.

7.10.1.  The anyxml's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | config       | 7.19.1  | 0..1        |
                 | description  | 7.19.3  | 0..1        |
                 | if-feature   | 7.18.2  | 0..n        |
                 | mandatory    | 7.6.5   | 0..1        |
                 | must         | 7.5.3   | 0..n        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | when         | 7.19.5  | 0..1        |
                 +--------------+---------+-------------+

7.10.2.  XML Mapping Rules

   An anyxml node is encoded as an XML element.  The element's local
   name is the anyxml's identifier, and its namespace is the module's
   XML namespace (see Section 7.1.3).  The value of the anyxml node is
   encoded as XML content of this element.

   Note that any prefixes used in the encoding are local to each
   instance encoding.  This means that the same XML may be encoded
   differently by different implementations.

7.10.3.  NETCONF <edit-config> Operations

   An anyxml node is treated as an opaque chunk of data.  This data can
   be modified in its entirety only.

   Any "operation" attributes present on subelements of an anyxml node
   are ignored by the NETCONF server.





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   When a NETCONF server processes an <edit-config> request, the
   elements of procedure for the anyxml node are:

      If the operation is "merge" or "replace", the node is created if
      it does not exist, and its value is set to the XML content of the
      anyxml node found in the XML RPC data.

      If the operation is "create", the node is created if it does not
      exist, and its value is set to the XML content of the anyxml node
      found in the XML RPC data.  If the node already exists, a
      "data-exists" error is returned.

      If the operation is "delete", the node is deleted if it exists.
      If the node does not exist, a "data-missing" error is returned.

7.10.4.  Usage Example

   Given the following "anyxml" statement:

     anyxml data;

   The following are two valid encodings of the same anyxml value:

     <data xmlns:if="http://example.com/ns/interface">
       <if:interface>
         <if:ifIndex>1</if:ifIndex>
       </if:interface>
     </data>

     <data>
       <interface xmlns="http://example.com/ns/interface">
         <ifIndex>1</ifIndex>
       </interface>
     </data>

7.11.  The grouping Statement

   The "grouping" statement is used to define a reusable block of nodes,
   which may be used locally in the module, in modules that include it,
   and by other modules that import from it, according to the rules in
   Section 5.5.  It takes one argument, which is an identifier, followed
   by a block of substatements that holds detailed grouping information.

   The "grouping" statement is not a data definition statement and, as
   such, does not define any nodes in the schema tree.

   A grouping is like a "structure" or a "record" in conventional
   programming languages.



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   Once a grouping is defined, it can be referenced in a "uses"
   statement (see Section 7.12).  A grouping MUST NOT reference itself,
   neither directly nor indirectly through a chain of other groupings.

   If the grouping is defined at the top level of a YANG module or
   submodule, the grouping's identifier MUST be unique within the
   module.

   A grouping is more than just a mechanism for textual substitution,
   but defines a collection of nodes.  Identifiers appearing inside the
   grouping are resolved relative to the scope in which the grouping is
   defined, not where it is used.  Prefix mappings, type names, grouping
   names, and extension usage are evaluated in the hierarchy where the
   "grouping" statement appears.  For extensions, this means that
   extensions are applied to the grouping node, not the uses node.

7.11.1.  The grouping's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | choice       | 7.9     | 0..n        |
                 | container    | 7.5     | 0..n        |
                 | description  | 7.19.3  | 0..1        |
                 | grouping     | 7.11    | 0..n        |
                 | leaf         | 7.6     | 0..n        |
                 | leaf-list    | 7.7     | 0..n        |
                 | list         | 7.8     | 0..n        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | typedef      | 7.3     | 0..n        |
                 | uses         | 7.12    | 0..n        |
                 +--------------+---------+-------------+

















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7.11.2.  Usage Example

     import ietf-inet-types {
         prefix "inet";
     }

     grouping endpoint {
         description "A reusable endpoint group.";
         leaf ip {
             type inet:ip-address;
         }
         leaf port {
             type inet:port-number;
         }
     }

7.12.  The uses Statement

   The "uses" statement is used to reference a "grouping" definition.
   It takes one argument, which is the name of the grouping.

   The effect of a "uses" reference to a grouping is that the nodes
   defined by the grouping are copied into the current schema tree, and
   then updated according to the "refine" and "augment" statements.

   The identifiers defined in the grouping are not bound to a namespace
   until the contents of the grouping are added to the schema tree via a
   "uses" statement that does not appear inside a "grouping" statement,
   at which point they are bound to the namespace of the current module.






















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7.12.1.  The uses's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | augment      | 7.15    | 0..1        |
                 | description  | 7.19.3  | 0..1        |
                 | if-feature   | 7.18.2  | 0..n        |
                 | refine       | 7.12.2  | 0..1        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | when         | 7.19.5  | 0..1        |
                 +--------------+---------+-------------+

7.12.2.  The refine Statement

   Some of the properties of each node in the grouping can be refined
   with the "refine" statement.  The argument is a string that
   identifies a node in the grouping.  This node is called the refine's
   target node.  If a node in the grouping is not present as a target
   node of a "refine" statement, it is not refined, and thus used
   exactly as it was defined in the grouping.

   The argument string is a descendant schema node identifier (see
   Section 6.5).

   The following refinements can be done:

   o  A leaf or choice node may get a default value, or a new default
      value if it already had one.

   o  Any node may get a specialized "description" string.

   o  Any node may get a specialized "reference" string.

   o  Any node may get a different "config" statement.

   o  A leaf, anyxml, or choice node may get a different "mandatory"
      statement.

   o  A container node may get a "presence" statement.

   o  A leaf, leaf-list, list, container, or anyxml node may get
      additional "must" expressions.

   o  A leaf-list or list node may get a different "min-elements" or
      "max-elements" statement.




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7.12.3.  XML Mapping Rules

   Each node in the grouping is encoded as if it was defined inline,
   even if it is imported from another module with another XML
   namespace.

7.12.4.  Usage Example

   To use the "endpoint" grouping defined in Section 7.11.2 in a
   definition of an HTTP server in some other module, we can do:

     import acme-system {
         prefix "acme";
     }

     container http-server {
         leaf name {
             type string;
         }
         uses acme:endpoint;
     }

   A corresponding XML instance example:

     <http-server>
       <name>extern-web</name>
       <ip>192.0.2.1</ip>
       <port>80</port>
     </http-server>

   If port 80 should be the default for the HTTP server, default can be
   added:

     container http-server {
         leaf name {
             type string;
         }
         uses acme:endpoint {
             refine port {
                 default 80;
             }
         }
     }

   If we want to define a list of servers, and each server has the ip
   and port as keys, we can do:





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     list server {
         key "ip port";
         leaf name {
             type string;
         }
         uses acme:endpoint;
     }

   The following is an error:

     container http-server {
         uses acme:endpoint;
         leaf ip {          // illegal - same identifier "ip" used twice
             type string;
         }
     }

7.13.  The rpc Statement

   The "rpc" statement is used to define a NETCONF RPC operation.  It
   takes one argument, which is an identifier, followed by a block of
   substatements that holds detailed rpc information.  This argument is
   the name of the RPC, and is used as the element name directly under
   the <rpc> element, as designated by the substitution group
   "rpcOperation" in [RFC4741].

   The "rpc" statement defines an rpc node in the schema tree.  Under
   the rpc node, a schema node with the name "input", and a schema node
   with the name "output" are also defined.  The nodes "input" and
   "output" are defined in the module's namespace.





















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7.13.1.  The rpc's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | description  | 7.19.3  | 0..1        |
                 | grouping     | 7.11    | 0..n        |
                 | if-feature   | 7.18.2  | 0..n        |
                 | input        | 7.13.2  | 0..1        |
                 | output       | 7.13.3  | 0..1        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | typedef      | 7.3     | 0..n        |
                 +--------------+---------+-------------+

7.13.2.  The input Statement

   The "input" statement, which is optional, is used to define input
   parameters to the RPC operation.  It does not take an argument.  The
   substatements to "input" define nodes under the RPC's input node.

   If a leaf in the input tree has a "mandatory" statement with the
   value "true", the leaf MUST be present in a NETCONF RPC invocation.
   Otherwise, the server MUST return a "missing-element" error.

   If a leaf in the input tree has a default value, the NETCONF server
   MUST use this value in the same cases as described in Section 7.6.1.
   In these cases, the server MUST operationally behave as if the leaf
   was present in the NETCONF RPC invocation with the default value as
   its value.

   If a "config" statement is present for any node in the input tree,
   the "config" statement is ignored.

   If any node has a "when" statement that would evaluate to false, then
   this node MUST NOT be present in the input tree.















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7.13.2.1.  The input's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | choice       | 7.9     | 0..n        |
                 | container    | 7.5     | 0..n        |
                 | grouping     | 7.11    | 0..n        |
                 | leaf         | 7.6     | 0..n        |
                 | leaf-list    | 7.7     | 0..n        |
                 | list         | 7.8     | 0..n        |
                 | typedef      | 7.3     | 0..n        |
                 | uses         | 7.12    | 0..n        |
                 +--------------+---------+-------------+

7.13.3.  The output Statement

   The "output" statement, which is optional, is used to define output
   parameters to the RPC operation.  It does not take an argument.  The
   substatements to "output" define nodes under the RPC's output node.

   If a leaf in the output tree has a "mandatory" statement with the
   value "true", the leaf MUST be present in a NETCONF RPC reply.

   If a leaf in the output tree has a default value, the NETCONF client
   MUST use this value in the same cases as described in Section 7.6.1.
   In these cases, the client MUST operationally behave as if the leaf
   was present in the NETCONF RPC reply with the default value as its
   value.

   If a "config" statement is present for any node in the output tree,
   the "config" statement is ignored.

   If any node has a "when" statement that would evaluate to false, then
   this node MUST NOT be present in the output tree.















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7.13.3.1.  The output's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | choice       | 7.9     | 0..n        |
                 | container    | 7.5     | 0..n        |
                 | grouping     | 7.11    | 0..n        |
                 | leaf         | 7.6     | 0..n        |
                 | leaf-list    | 7.7     | 0..n        |
                 | list         | 7.8     | 0..n        |
                 | typedef      | 7.3     | 0..n        |
                 | uses         | 7.12    | 0..n        |
                 +--------------+---------+-------------+

7.13.4.  XML Mapping Rules

   An rpc node is encoded as a child XML element to the <rpc> element
   defined in [RFC4741].  The element's local name is the rpc's
   identifier, and its namespace is the module's XML namespace (see
   Section 7.1.3).

   Input parameters are encoded as child XML elements to the rpc node's
   XML element, in the same order as they are defined within the "input"
   statement.

   If the RPC operation invocation succeeded, and no output parameters
   are returned, the <rpc-reply> contains a single <ok/> element defined
   in [RFC4741].  If output parameters are returned, they are encoded as
   child elements to the <rpc-reply> element defined in [RFC4741], in
   the same order as they are defined within the "output" statement.



















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7.13.5.  Usage Example

   The following example defines an RPC operation:

     module rock {
         namespace "http://example.net/rock";
         prefix "rock";

         rpc rock-the-house {
             input {
                 leaf zip-code {
                     type string;
                 }
             }
         }
     }

   A corresponding XML instance example of the complete rpc and rpc-
   reply:

     <rpc message-id="101"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       <rock-the-house xmlns="http://example.net/rock">
         <zip-code>27606-0100</zip-code>
       </rock-the-house>
     </rpc>

     <rpc-reply message-id="101"
                xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       <ok/>
     </rpc-reply>

7.14.  The notification Statement

   The "notification" statement is used to define a NETCONF
   notification.  It takes one argument, which is an identifier,
   followed by a block of substatements that holds detailed notification
   information.  The "notification" statement defines a notification
   node in the schema tree.

   If a leaf in the notification tree has a "mandatory" statement with
   the value "true", the leaf MUST be present in a NETCONF notification.

   If a leaf in the notification tree has a default value, the NETCONF
   client MUST use this value in the same cases as described in
   Section 7.6.1.  In these cases, the client MUST operationally behave
   as if the leaf was present in the NETCONF notification with the
   default value as its value.



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   If a "config" statement is present for any node in the notification
   tree, the "config" statement is ignored.

7.14.1.  The notification's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | choice       | 7.9     | 0..n        |
                 | container    | 7.5     | 0..n        |
                 | description  | 7.19.3  | 0..1        |
                 | grouping     | 7.11    | 0..n        |
                 | if-feature   | 7.18.2  | 0..n        |
                 | leaf         | 7.6     | 0..n        |
                 | leaf-list    | 7.7     | 0..n        |
                 | list         | 7.8     | 0..n        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | typedef      | 7.3     | 0..n        |
                 | uses         | 7.12    | 0..n        |
                 +--------------+---------+-------------+

7.14.2.  XML Mapping Rules

   A notification node is encoded as a child XML element to the
   <notification> element defined in NETCONF Event Notifications
   [RFC5277].  The element's local name is the notification's
   identifier, and its namespace is the module's XML namespace (see
   Section 7.1.3).





















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7.14.3.  Usage Example

   The following example defines a notification:

     module event {

         namespace "http://example.com/event";
         prefix "ev";

         notification event {
             leaf event-class {
                 type string;
             }
             anyxml reporting-entity;
             leaf severity {
                 type string;
             }
         }
     }

   A corresponding XML instance example of the complete notification:

     <notification
       xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
       <eventTime>2008-07-08T00:01:00Z</eventTime>
       <event xmlns="http://example.com/event">
         <event-class>fault</event-class>
         <reporting-entity>
           <card>Ethernet0</card>
         </reporting-entity>
         <severity>major</severity>
       </event>
     </notification>

7.15.  The augment Statement

   The "augment" statement allows a module or submodule to add to the
   schema tree defined in an external module, or the current module and
   its submodules, and to add to the nodes from a grouping in a "uses"
   statement.  The argument is a string that identifies a node in the
   schema tree.  This node is called the augment's target node.  The
   target node MUST be either a container, list, choice, case, input,
   output, or notification node.  It is augmented with the nodes defined
   in the substatements that follow the "augment" statement.

   The argument string is a schema node identifier (see Section 6.5).
   If the "augment" statement is on the top level in a module or
   submodule, the absolute form (defined by the rule



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   "absolute-schema-nodeid" in Section 12) of a schema node identifier
   MUST be used.  If the "augment" statement is a substatement to the
   "uses" statement, the descendant form (defined by the rule
   "descendant-schema-nodeid" in Section 12) MUST be used.

   If the target node is a container, list, case, input, output, or
   notification node, the "container", "leaf", "list", "leaf-list",
   "uses", and "choice" statements can be used within the "augment"
   statement.

   If the target node is a choice node, the "case" statement, or a case
   shorthand statement (see Section 7.9.2) can be used within the
   "augment" statement.

   If the target node is in another module, then nodes added by the
   augmentation MUST NOT be mandatory nodes (see Section 3.1).

   The "augment" statement MUST NOT add multiple nodes with the same
   name from the same module to the target node.

7.15.1.  The augment's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | case         | 7.9.2   | 0..n        |
                 | choice       | 7.9     | 0..n        |
                 | container    | 7.5     | 0..n        |
                 | description  | 7.19.3  | 0..1        |
                 | if-feature   | 7.18.2  | 0..n        |
                 | leaf         | 7.6     | 0..n        |
                 | leaf-list    | 7.7     | 0..n        |
                 | list         | 7.8     | 0..n        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | uses         | 7.12    | 0..n        |
                 | when         | 7.19.5  | 0..1        |
                 +--------------+---------+-------------+

7.15.2.  XML Mapping Rules

   All data nodes defined in the "augment" statement are defined as XML
   elements in the XML namespace of the module where the "augment" is
   specified.

   When a node is augmented, the augmenting child nodes are encoded as
   subelements to the augmented node, in any order.



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7.15.3.  Usage Example

   In namespace http://example.com/schema/interfaces, we have:

     container interfaces {
         list ifEntry {
             key "ifIndex";

             leaf ifIndex {
                 type uint32;
             }
             leaf ifDescr {
                 type string;
             }
             leaf ifType {
                 type iana:IfType;
             }
             leaf ifMtu {
                 type int32;
             }
         }
     }

   Then, in namespace http://example.com/schema/ds0, we have:

     import interface-module {
         prefix "if";
     }
     augment "/if:interfaces/if:ifEntry" {
         when "if:ifType='ds0'";
         leaf ds0ChannelNumber {
             type ChannelNumber;
         }
     }

















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   A corresponding XML instance example:

     <interfaces xmlns="http://example.com/schema/interfaces"
                 xmlns:ds0="http://example.com/schema/ds0">
       <ifEntry>
         <ifIndex>1</ifIndex>
         <ifDescr>Flintstone Inc Ethernet A562</ifDescr>
         <ifType>ethernetCsmacd</ifType>
         <ifMtu>1500</ifMtu>
       </ifEntry>
       <ifEntry>
         <ifIndex>2</ifIndex>
         <ifDescr>Flintstone Inc DS0</ifDescr>
         <ifType>ds0</ifType>
         <ds0:ds0ChannelNumber>1</ds0:ds0ChannelNumber>
       </ifEntry>
     </interfaces>

   As another example, suppose we have the choice defined in
   Section 7.9.7.  The following construct can be used to extend the
   protocol definition:

     augment /ex:system/ex:protocol/ex:name {
         case c {
             leaf smtp {
                 type empty;
             }
         }
     }

   A corresponding XML instance example:

     <ex:system>
       <ex:protocol>
         <ex:tcp/>
       </ex:protocol>
     </ex:system>

   or

     <ex:system>
       <ex:protocol>
         <other:smtp/>
       </ex:protocol>
     </ex:system>






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7.16.  The identity Statement

   The "identity" statement is used to define a new globally unique,
   abstract, and untyped identity.  Its only purpose is to denote its
   name, semantics, and existence.  An identity can either be defined
   from scratch or derived from a base identity.  The identity's
   argument is an identifier that is the name of the identity.  It is
   followed by a block of substatements that holds detailed identity
   information.

   The built-in datatype "identityref" (see Section 9.10) can be used to
   reference identities within a data model.

7.16.1.  The identity's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | base         | 7.16.2  | 0..1        |
                 | description  | 7.19.3  | 0..1        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 +--------------+---------+-------------+

7.16.2.  The base Statement

   The "base" statement, which is optional, takes as an argument a
   string that is the name of an existing identity, from which the new
   identity is derived.  If no "base" statement is present, the identity
   is defined from scratch.

   If a prefix is present on the base name, it refers to an identity
   defined in the module that was imported with that prefix, or the
   local module if the prefix matches the local module's prefix.
   Otherwise, an identity with the matching name MUST be defined in the
   current module or an included submodule.

   Since submodules cannot include the parent module, any identities in
   the module that need to be exposed to submodules MUST be defined in a
   submodule.  Submodules can then include this submodule to find the
   definition of the identity.

   An identity MUST NOT reference itself, neither directly nor
   indirectly through a chain of other identities.







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7.16.3.  Usage Example

     module crypto-base {
         namespace "http://example.com/crypto-base";
         prefix "crypto";

         identity crypto-alg {
             description
                "Base identity from which all crypto algorithms
                 are derived.";
         }
     }

     module des {
         namespace "http://example.com/des";
         prefix "des";

         import "crypto-base" {
             prefix "crypto";
         }

         identity des {
             base "crypto:crypto-alg";
             description "DES crypto algorithm";
         }

         identity des3 {
             base "crypto:crypto-alg";
             description "Triple DES crypto algorithm";
         }
     }

7.17.  The extension Statement

   The "extension" statement allows the definition of new statements
   within the YANG language.  This new statement definition can be
   imported and used by other modules.

   The statement's argument is an identifier that is the new keyword for
   the extension and must be followed by a block of substatements that
   holds detailed extension information.  The purpose of the "extension"
   statement is to define a keyword, so that it can be imported and used
   by other modules.

   The extension can be used like a normal YANG statement, with the
   statement name followed by an argument if one is defined by the
   extension, and an optional block of substatements.  The statement's
   name is created by combining the prefix of the module in which the



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   extension was defined, a colon (":"), and the extension's keyword,
   with no interleaving whitespace.  The substatements of an extension
   are defined by the extension, using some mechanism outside the scope
   of this specification.  Syntactically, the substatements MUST be YANG
   statements, or also defined using "extension" statements.  YANG
   statements in extensions MUST follow the syntactical rules in
   Section 12.

7.17.1.  The extension's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | argument     | 7.17.2  | 0..1        |
                 | description  | 7.19.3  | 0..1        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 +--------------+---------+-------------+

7.17.2.  The argument Statement

   The "argument" statement, which is optional, takes as an argument a
   string that is the name of the argument to the keyword.  If no
   argument statement is present, the keyword expects no argument when
   it is used.

   The argument's name is used in the YIN mapping, where it is used as
   an XML attribute or element name, depending on the argument's "yin-
   element" statement.

7.17.2.1.  The argument's Substatements

                 +--------------+----------+-------------+
                 | substatement | section  | cardinality |
                 +--------------+----------+-------------+
                 | yin-element  | 7.17.2.2 | 0..1        |
                 +--------------+----------+-------------+

7.17.2.2.  The yin-element Statement

   The "yin-element" statement, which is optional, takes as an argument
   the string "true" or "false".  This statement indicates if the
   argument is mapped to an XML element in YIN or to an XML attribute
   (see Section 11).

   If no "yin-element" statement is present, it defaults to "false".





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7.17.3.  Usage Example

   To define an extension:

     module my-extensions {
       ...

       extension c-define {
         description
           "Takes as argument a name string.
           Makes the code generator use the given name in the
           #define.";
         argument "name";
       }
     }

   To use the extension:

     module my-interfaces {
       ...
       import my-extensions {
         prefix "myext";
       }
       ...

       container interfaces {
         ...
         myext:c-define "MY_INTERFACES";
       }
     }

7.18.  Conformance-Related Statements

   This section defines statements related to conformance, as described
   in Section 5.6.

7.18.1.  The feature Statement

   The "feature" statement is used to define a mechanism by which
   portions of the schema are marked as conditional.  A feature name is
   defined that can later be referenced using the "if-feature" statement
   (see Section 7.18.2).  Schema nodes tagged with a feature are ignored
   by the device unless the device supports the given feature.  This
   allows portions of the YANG module to be conditional based on
   conditions on the device.  The model can represent the abilities of
   the device within the model, giving a richer model that allows for
   differing device abilities and roles.




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   The argument to the "feature" statement is the name of the new
   feature, and follows the rules for identifiers in Section 6.2.  This
   name is used by the "if-feature" statement to tie the schema nodes to
   the feature.

   In this example, a feature called "local-storage" represents the
   ability for a device to store syslog messages on local storage of
   some sort.  This feature is used to make the "local-storage-limit"
   leaf conditional on the presence of some sort of local storage.  If
   the device does not report that it supports this feature, the
   "local-storage-limit" node is not supported.

     module syslog {
         ...
         feature local-storage {
             description
                 "This feature means the device supports local
                  storage (memory, flash or disk) that can be used to
                  store syslog messages.";
         }

         container syslog {
             leaf local-storage-limit {
                 if-feature local-storage;
                 type uint64;
                 units "kilobyte";
                 config false;
                 description
                     "The amount of local storage that can be
                      used to hold syslog messages.";
             }
         }
     }

   The "if-feature" statement can be used in many places within the YANG
   syntax.  Definitions tagged with "if-feature" are ignored when the
   device does not support that feature.

   A feature MUST NOT reference itself, neither directly nor indirectly
   through a chain of other features.

   In order for a device to implement a feature that is dependent on any
   other features (i.e., the feature has one or more "if-feature" sub-
   statements), the device MUST also implement all the dependant
   features.






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7.18.1.1.  The feature's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | description  | 7.19.3  | 0..1        |
                 | if-feature   | 7.18.2  | 0..n        |
                 | status       | 7.19.2  | 0..1        |
                 | reference    | 7.19.4  | 0..1        |
                 +--------------+---------+-------------+

7.18.2.  The if-feature Statement

   The "if-feature" statement makes its parent statement conditional.
   The argument is the name of a feature, as defined by a "feature"
   statement.  The parent statement is implemented by servers that
   support this feature.  If a prefix is present on the feature name, it
   refers to a feature defined in the module that was imported with that
   prefix, or the local module if the prefix matches the local module's
   prefix.  Otherwise, a feature with the matching name MUST be defined
   in the current module or an included submodule.

   Since submodules cannot include the parent module, any features in
   the module that need to be exposed to submodules MUST be defined in a
   submodule.  Submodules can then include this submodule to find the
   definition of the feature.

7.18.3.  The deviation Statement

   The "deviation" statement defines a hierarchy of a module that the
   device does not implement faithfully.  The argument is a string that
   identifies the node in the schema tree where a deviation from the
   module occurs.  This node is called the deviation's target node.  The
   contents of the "deviation" statement give details about the
   deviation.

   The argument string is an absolute schema node identifier (see
   Section 6.5).

   Deviations define the way a device or class of devices deviate from a
   standard.  This means that deviations MUST never be part of a
   published standard, since they are the mechanism for learning how
   implementations vary from the standards.








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   Device deviations are strongly discouraged and MUST only be used as a
   last resort.  Telling the application how a device fails to follow a
   standard is no substitute for implementing the standard correctly.  A
   device that deviates from a module is not fully compliant with the
   module.

   However, in some cases, a particular device may not have the hardware
   or software ability to support parts of a standard module.  When this
   occurs, the device makes a choice either to treat attempts to
   configure unsupported parts of the module as an error that is
   reported back to the unsuspecting application or ignore those
   incoming requests.  Neither choice is acceptable.

   Instead, YANG allows devices to document portions of a base module
   that are not supported or supported but with different syntax, by
   using the "deviation" statement.

7.18.3.1.  The deviation's Substatements

                 +--------------+----------+-------------+
                 | substatement | section  | cardinality |
                 +--------------+----------+-------------+
                 | description  | 7.19.3   | 0..1        |
                 | deviate      | 7.18.3.2 | 1..n        |
                 | reference    | 7.19.4   | 0..1        |
                 +--------------+----------+-------------+

7.18.3.2.  The deviate Statement

   The "deviate" statement defines how the device's implementation of
   the target node deviates from its original definition.  The argument
   is one of the strings "not-supported", "add", "replace", or "delete".

   The argument "not-supported" indicates that the target node is not
   implemented by this device.

   The argument "add" adds properties to the target node.  The
   properties to add are identified by substatements to the "deviate"
   statement.  If a property can only appear once, the property MUST NOT
   exist in the target node.

   The argument "replace" replaces properties of the target node.  The
   properties to replace are identified by substatements to the
   "deviate" statement.  The properties to replace MUST exist in the
   target node.






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   The argument "delete" deletes properties from the target node.  The
   properties to delete are identified by substatements to the "delete"
   statement.  The substatement's keyword MUST match a corresponding
   keyword in the target node, and the argument's string MUST be equal
   to the corresponding keyword's argument string in the target node.

                       The deviates's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | config       | 7.19.1  | 0..1        |
                 | default      | 7.6.4   | 0..1        |
                 | mandatory    | 7.6.5   | 0..1        |
                 | max-elements | 7.7.4   | 0..1        |
                 | min-elements | 7.7.3   | 0..1        |
                 | must         | 7.5.3   | 0..n        |
                 | type         | 7.4     | 0..1        |
                 | unique       | 7.8.3   | 0..n        |
                 | units        | 7.3.3   | 0..1        |
                 +--------------+---------+-------------+

7.18.3.3.  Usage Example

   In this example, the device is informing client applications that it
   does not support the "daytime" service in the style of RFC 867.

     deviation /base:system/base:daytime {
         deviate not-supported;
     }

   The following example sets a device-specific default value to a leaf
   that does not have a default value defined:

     deviation /base:system/base:user/base:type {
         deviate add {
             default "admin"; // new users are 'admin' by default
         }
     }

   In this example, the device limits the number of name servers to 3:

     deviation /base:system/base:name-server {
         deviate replace {
             max-elements 3;
         }
     }




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   If the original definition is:

     container system {
         must "daytime or time";
         ...
     }

   a device might remove this must constraint by doing:

     deviation "/base:system" {
         deviate delete {
             must "daytime or time";
         }
     }

7.19.  Common Statements

   This section defines substatements common to several other
   statements.

7.19.1.  The config Statement

   The "config" statement takes as an argument the string "true" or
   "false".  If "config" is "true", the definition represents
   configuration.  Data nodes representing configuration will be part of
   the reply to a <get-config> request, and can be sent in a
   <copy-config> or <edit-config> request.

   If "config" is "false", the definition represents state data.  Data
   nodes representing state data will be part of the reply to a <get>,
   but not to a <get-config> request, and cannot be sent in a
   <copy-config> or <edit-config> request.

   If "config" is not specified, the default is the same as the parent
   schema node's "config" value.  If the parent node is a "case" node,
   the value is the same as the "case" node's parent "choice" node.

   If the top node does not specify a "config" statement, the default is
   "true".

   If a node has "config" set to "false", no node underneath it can have
   "config" set to "true".

7.19.2.  The status Statement

   The "status" statement takes as an argument one of the strings
   "current", "deprecated", or "obsolete".




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   o  "current" means that the definition is current and valid.

   o  "deprecated" indicates an obsolete definition, but it permits new/
      continued implementation in order to foster interoperability with
      older/existing implementations.

   o  "obsolete" means the definition is obsolete and SHOULD NOT be
      implemented and/or can be removed from implementations.

   If no status is specified, the default is "current".

   If a definition is "current", it MUST NOT reference a "deprecated" or
   "obsolete" definition within the same module.

   If a definition is "deprecated", it MUST NOT reference an "obsolete"
   definition within the same module.

   For example, the following is illegal:

     typedef my-type {
       status deprecated;
       type int32;
     }

     leaf my-leaf {
       status current;
       type my-type; // illegal, since my-type is deprecated
     }

7.19.3.  The description Statement

   The "description" statement takes as an argument a string that
   contains a human-readable textual description of this definition.
   The text is provided in a language (or languages) chosen by the
   module developer; for the sake of interoperability, it is RECOMMENDED
   to choose a language that is widely understood among the community of
   network administrators who will use the module.

7.19.4.  The reference Statement

   The "reference" statement takes as an argument a string that is used
   to specify a textual cross-reference to an external document, either
   another module that defines related management information, or a
   document that provides additional information relevant to this
   definition.






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   For example, a typedef for a "uri" data type could look like:

     typedef uri {
       type string;
       reference
         "RFC 3986: Uniform Resource Identifier (URI): Generic Syntax";
       ...
     }

7.19.5.  The when Statement

   The "when" statement makes its parent data definition statement
   conditional.  The node defined by the parent data definition
   statement is only valid when the condition specified by the "when"
   statement is satisfied.  The statement's argument is an XPath
   expression (see Section 6.4), which is used to formally specify this
   condition.  If the XPath expression conceptually evaluates to "true"
   for a particular instance, then the node defined by the parent data
   definition statement is valid; otherwise, it is not.

   See Section 8.3.2 for additional information.

   The XPath expression is conceptually evaluated in the following
   context, in addition to the definition in Section 6.4.1:

   o  If the "when" statement is a child of an "augment" statement, then
      the context node is the augment's target node in the data tree, if
      the target node is a data node.  Otherwise, the context node is
      the closest ancestor node to the target node that is also a data
      node.

   o  If the "when" statement is a child of a "uses", "choice", or
      "case" statement, then the context node is the closest ancestor
      node to the "uses", "choice", or "case" node that is also a data
      node.

   o  If the "when" statement is a child of any other data definition
      statement, the context node is the data definition's node in the
      data tree.

   o  The accessible tree is made up of all nodes in the data tree, and
      all leafs with default values in use (see Section 7.6.1).









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   The accessible tree depends on the context node:

   o  If the context node represents configuration, the tree is the data
      in the NETCONF datastore where the context node exists.  The XPath
      root node has all top-level configuration data nodes in all
      modules as children.

   o  If the context node represents state data, the tree is all state
      data on the device, and the <running/> datastore.  The XPath root
      node has all top-level data nodes in all modules as children.

   o  If the context node represents notification content, the tree is
      the notification XML instance document.  The XPath root node has
      the element representing the notification being defined as the
      only child.

   o  If the context node represents RPC input parameters, the tree is
      the RPC XML instance document.  The XPath root node has the
      element representing the RPC operation being defined as the only
      child.

   o  If the context node represents RPC output parameters, the tree is
      the RPC reply instance document.  The XPath root node has the
      elements representing the RPC output parameters as children.

   The result of the XPath expression is converted to a boolean value
   using the standard XPath rules.

   Note that the XPath expression is conceptually evaluated.  This means
   that an implementation does not have to use an XPath evaluator on the
   device.  The "when" statement can very well be implemented with
   specially written code.

8.  Constraints

8.1.  Constraints on Data

   Several YANG statements define constraints on valid data.  These
   constraints are enforced in different ways, depending on what type of
   data the statement defines.

   o  If the constraint is defined on configuration data, it MUST be
      true in a valid configuration data tree.

   o  If the constraint is defined on state data, it MUST be true in a
      reply to a <get> operation without a filter.





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   o  If the constraint is defined on notification content, it MUST be
      true in any notification instance.

   o  If the constraint is defined on RPC input parameters, it MUST be
      true in an invocation of the RPC operation.

   o  If the constraint is defined on RPC output parameters, it MUST be
      true in the RPC reply.

8.2.  Hierarchy of Constraints

   Conditions on parent nodes affect constraints on child nodes as a
   natural consequence of the hierarchy of nodes. "must", "mandatory",
   "min-elements", and "max-elements" constraints are not enforced if
   the parent node has a "when" or "if-feature" property that is not
   satisfied on the current device.

   In this example, the "mandatory" constraint on the "longitude" leaf
   are not enforced on devices that lack the "has-gps" feature:

       container location {
           if-feature has-gps;
           leaf longitude {
               mandatory true;
               ...
           }
       }

8.3.  Constraint Enforcement Model

   For configuration data, there are three windows when constraints MUST
   be enforced:

   o  during parsing of RPC payloads

   o  during processing of NETCONF operations

   o  during validation

   Each of these scenarios is considered in the following sections.

8.3.1.  Payload Parsing

   When content arrives in RPC payloads, it MUST be well-formed XML,
   following the hierarchy and content rules defined by the set of
   models the device implements.





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   o  If a leaf data value does not match the type constraints for the
      leaf, including those defined in the type's "range", "length", and
      "pattern" properties, the server MUST reply with an
      "invalid-value" error-tag in the rpc-error, and with the error-
      app-tag and error-message associated with the constraint, if any
      exist.

   o  If all keys of a list entry are not present, the server MUST reply
      with a "missing-element" error-tag in the rpc-error.

   o  If data for more than one case branch of a choice is present, the
      server MUST reply with a "bad-element" in the rpc-error.

   o  If data for a node tagged with "if-feature" is present, and the
      feature is not supported by the device, the server MUST reply with
      an "unknown-element" error-tag in the rpc-error.

   o  If data for a node tagged with "when" is present, and the "when"
      condition evaluates to "false", the server MUST reply with an
      "unknown-element" error-tag in the rpc-error.

   o  For insert handling, if the value for the attributes "before" and
      "after" are not valid for the type of the appropriate key leafs,
      the server MUST reply with a "bad-attribute" error-tag in the rpc-
      error.

   o  If the attributes "before" and "after" appears in any element that
      is not a list whose "ordered-by" property is "user", the server
      MUST reply with an "unknown-attribute" error-tag in the rpc-error.

8.3.2.  NETCONF <edit-config> Processing

   After the incoming data is parsed, the NETCONF server performs the
   <edit-config> operation by applying the data to the configuration
   datastore.  During this processing, the following errors MUST be
   detected:

   o  Delete requests for non-existent data.

   o  Create requests for existent data.

   o  Insert requests with "before" or "after" parameters that do not
      exist.








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   During <edit-config> processing:

   o  If the NETCONF operation creates data nodes under a "choice", any
      existing nodes from other "case" branches are deleted by the
      server.

   o  If the NETCONF operation modifies a data node such that any node's
      "when" expression becomes false, then the node with the "when"
      expression is deleted by the server.

8.3.3.  Validation

   When datastore processing is complete, the final contents MUST obey
   all validation constraints.  This validation processing is performed
   at differing times according to the datastore.  If the datastore is
   <running/> or <startup/>, these constraints MUST be enforced at the
   end of the <edit-config> or <copy-config> operation.  If the
   datastore is <candidate/>, the constraint enforcement is delayed
   until a <commit> or <validate> operation.

   o  Any "must" constraints MUST evaluate to "true".

   o  Any referential integrity constraints defined via the "path"
      statement MUST be satisfied.

   o  Any "unique" constraints on lists MUST be satisfied.

   o  The "min-elements" and "max-elements" constraints are enforced for
      lists and leaf-lists.

9.  Built-In Types

   YANG has a set of built-in types, similar to those of many
   programming languages, but with some differences due to special
   requirements from the management information model.

   Additional types may be defined, derived from those built-in types or
   from other derived types.  Derived types may use subtyping to
   formally restrict the set of possible values.

   The different built-in types and their derived types allow different
   kinds of subtyping, namely length and regular expression restrictions
   of strings (Sections 9.4.4 and 9.4.6) and range restrictions of
   numeric types (Section 9.2.4).

   The lexical representation of a value of a certain type is used in
   the NETCONF messages and when specifying default values and numerical
   ranges in YANG modules.



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9.1.  Canonical Representation

   For most types, there is a single canonical representation of the
   type's values.  Some types allow multiple lexical representations of
   the same value, for example, the positive integer "17" can be
   represented as "+17" or "17".  Implementations MUST support all
   lexical representations specified in this document.

   When a NETCONF server sends data, it MUST be in the canonical form.

   Some types have a lexical representation that depends on the XML
   context in which they occur.  These types do not have a canonical
   form.

9.2.  The Integer Built-In Types

   The integer built-in types are int8, int16, int32, int64, uint8,
   uint16, uint32, and uint64.  They represent signed and unsigned
   integers of different sizes:

   int8  represents integer values between -128 and 127, inclusively.

   int16  represents integer values between -32768 and 32767,
      inclusively.

   int32  represents integer values between -2147483648 and 2147483647,
      inclusively.

   int64  represents integer values between -9223372036854775808 and
      9223372036854775807, inclusively.

   uint8  represents integer values between 0 and 255, inclusively.

   uint16  represents integer values between 0 and 65535, inclusively.

   uint32  represents integer values between 0 and 4294967295,
      inclusively.

   uint64  represents integer values between 0 and 18446744073709551615,
      inclusively.











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9.2.1.  Lexical Representation

   An integer value is lexically represented as an optional sign ("+" or
   "-"), followed by a sequence of decimal digits.  If no sign is
   specified, "+" is assumed.

   For convenience, when specifying a default value for an integer in a
   YANG module, an alternative lexical representation can be used, which
   represents the value in a hexadecimal or octal notation.  The
   hexadecimal notation consists of an optional sign ("+" or "-"), the
   characters "0x" followed a number of hexadecimal digits, where
   letters may be uppercase or lowercase.  The octal notation consists
   of an optional sign ("+" or "-"), the character "0" followed a number
   of octal digits.

   Note that if a default value in a YANG module has a leading zero
   ("0"), it is interpreted as an octal number.  In the XML instance
   documents, an integer is always interpreted as a decimal number, and
   leading zeros are allowed.

   Examples:

     // legal values
     +4711                       // legal positive value
     4711                        // legal positive value
     -123                        // legal negative value
     0xf00f                      // legal positive hexadecimal value
     -0xf                        // legal negative hexadecimal value
     052                         // legal positive octal value

     // illegal values
     - 1                         // illegal intermediate space



















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9.2.2.  Canonical Form

   The canonical form of a positive integer does not include the sign
   "+".  Leading zeros are prohibited.  The value zero is represented as
   "0".

9.2.3.  Restrictions

   All integer types can be restricted with the "range" statement
   (Section 9.2.4).

9.2.4.  The range Statement

   The "range" statement, which is an optional substatement to the
   "type" statement, takes as an argument a range expression string.  It
   is used to restrict integer and decimal built-in types, or types
   derived from those.

   A range consists of an explicit value, or a lower-inclusive bound,
   two consecutive dots "..", and an upper-inclusive bound.  Multiple
   values or ranges can be given, separated by "|".  If multiple values
   or ranges are given, they all MUST be disjoint and MUST be in
   ascending order.  If a range restriction is applied to an already
   range-restricted type, the new restriction MUST be equal or more
   limiting, that is raising the lower bounds, reducing the upper
   bounds, removing explicit values or ranges, or splitting ranges into
   multiple ranges with intermediate gaps.  Each explicit value and
   range boundary value given in the range expression MUST match the
   type being restricted, or be one of the special values "min" or
   "max". "min" and "max" mean the minimum and maximum value accepted
   for the type being restricted, respectively.

   The range expression syntax is formally defined by the rule
   "range-arg" in Section 12.

9.2.4.1.  The range's Substatements

                 +---------------+---------+-------------+
                 | substatement  | section | cardinality |
                 +---------------+---------+-------------+
                 | description   | 7.19.3  | 0..1        |
                 | error-app-tag | 7.5.4.2 | 0..1        |
                 | error-message | 7.5.4.1 | 0..1        |
                 | reference     | 7.19.4  | 0..1        |
                 +---------------+---------+-------------+






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9.2.5.  Usage Example

     typedef my-base-int32-type {
         type int32 {
             range "1..4 | 10..20";
         }
     }

     typedef my-type1 {
         type my-base-int32-type {
             // legal range restriction
             range "11..max"; // 11..20
         }
     }

     typedef my-type2 {
         type my-base-int32-type {
             // illegal range restriction
             range "11..100";
         }
     }

9.3.  The decimal64 Built-In Type

   The decimal64 type represents a subset of the real numbers, which can
   be represented by decimal numerals.  The value space of decimal64 is
   the set of numbers that can be obtained by multiplying a 64-bit
   signed integer by a negative power of ten, i.e., expressible as
   "i x 10^-n" where i is an integer64 and n is an integer between 1 and
   18, inclusively.

9.3.1.  Lexical Representation

   A decimal64 value is lexically represented as an optional sign ("+"
   or "-"), followed by a sequence of decimal digits, optionally
   followed by a period ('.') as a decimal indicator and a sequence of
   decimal digits.  If no sign is specified, "+" is assumed.

9.3.2.  Canonical Form

   The canonical form of a positive decimal64 does not include the sign
   "+".  The decimal point is required.  Leading and trailing zeros are
   prohibited, subject to the rule that there MUST be at least one digit
   before and after the decimal point.  The value zero is represented as
   "0.0".






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9.3.3.  Restrictions

   A decimal64 type can be restricted with the "range" statement
   (Section 9.2.4).

9.3.4.  The fraction-digits Statement

   The "fraction-digits" statement, which is a substatement to the
   "type" statement, MUST be present if the type is "decimal64".  It
   takes as an argument an integer between 1 and 18, inclusively.  It
   controls the size of the minimum difference between values of a
   decimal64 type, by restricting the value space to numbers that are
   expressible as "i x 10^-n" where n is the fraction-digits argument.

   The following table lists the minimum and maximum value for each
   fraction-digit value:

     +----------------+-----------------------+----------------------+
     | fraction-digit | min                   | max                  |
     +----------------+-----------------------+----------------------+
     | 1              | -922337203685477580.8 | 922337203685477580.7 |
     | 2              | -92233720368547758.08 | 92233720368547758.07 |
     | 3              | -9223372036854775.808 | 9223372036854775.807 |
     | 4              | -922337203685477.5808 | 922337203685477.5807 |
     | 5              | -92233720368547.75808 | 92233720368547.75807 |
     | 6              | -9223372036854.775808 | 9223372036854.775807 |
     | 7              | -922337203685.4775808 | 922337203685.4775807 |
     | 8              | -92233720368.54775808 | 92233720368.54775807 |
     | 9              | -9223372036.854775808 | 9223372036.854775807 |
     | 10             | -922337203.6854775808 | 922337203.6854775807 |
     | 11             | -92233720.36854775808 | 92233720.36854775807 |
     | 12             | -9223372.036854775808 | 9223372.036854775807 |
     | 13             | -922337.2036854775808 | 922337.2036854775807 |
     | 14             | -92233.72036854775808 | 92233.72036854775807 |
     | 15             | -9223.372036854775808 | 9223.372036854775807 |
     | 16             | -922.3372036854775808 | 922.3372036854775807 |
     | 17             | -92.23372036854775808 | 92.23372036854775807 |
     | 18             | -9.223372036854775808 | 9.223372036854775807 |
     +----------------+-----------------------+----------------------+












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9.3.5.  Usage Example

     typedef my-decimal {
         type decimal64 {
             fraction-digits 2;
             range "1 .. 3.14 | 10 | 20..max";
         }
     }

9.4.  The string Built-In Type

   The string built-in type represents human-readable strings in YANG.
   Legal characters are tab, carriage return, line feed, and the legal
   characters of Unicode and ISO/IEC 10646 [ISO.10646]:

     ;; any Unicode character, excluding the surrogate blocks,
     ;; FFFE, and FFFF.
     string = *char
     char = %x9 / %xA / %xD / %x20-D7FF / %xE000-FFFD /
            %x10000-10FFFF

9.4.1.  Lexical Representation

   A string value is lexically represented as character data in the XML
   instance documents.

9.4.2.  Canonical Form

   The canonical form is the same as the lexical representation.  No
   Unicode normalization is performed of string values.

9.4.3.  Restrictions

   A string can be restricted with the "length" (Section 9.4.4) and
   "pattern" (Section 9.4.6) statements.

9.4.4.  The length Statement

   The "length" statement, which is an optional substatement to the
   "type" statement, takes as an argument a length expression string.
   It is used to restrict the built-in type "string", or types derived
   from "string".

   A "length" statement restricts the number of Unicode characters in
   the string.






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   A length range consists of an explicit value, or a lower bound, two
   consecutive dots "..", and an upper bound.  Multiple values or ranges
   can be given, separated by "|".  Length-restricting values MUST NOT
   be negative.  If multiple values or ranges are given, they all MUST
   be disjoint and MUST be in ascending order.  If a length restriction
   is applied to an already length-restricted type, the new restriction
   MUST be equal or more limiting, that is, raising the lower bounds,
   reducing the upper bounds, removing explicit length values or ranges,
   or splitting ranges into multiple ranges with intermediate gaps.  A
   length value is a non-negative integer, or one of the special values
   "min" or "max". "min" and "max" mean the minimum and maximum length
   accepted for the type being restricted, respectively.  An
   implementation is not required to support a length value larger than
   18446744073709551615.

   The length expression syntax is formally defined by the rule
   "length-arg" in Section 12.

9.4.4.1.  The length's Substatements

                 +---------------+---------+-------------+
                 | substatement  | section | cardinality |
                 +---------------+---------+-------------+
                 | description   | 7.19.3  | 0..1        |
                 | error-app-tag | 7.5.4.2 | 0..1        |
                 | error-message | 7.5.4.1 | 0..1        |
                 | reference     | 7.19.4  | 0..1        |
                 +---------------+---------+-------------+

9.4.5.  Usage Example

     typedef my-base-str-type {
         type string {
             length "1..255";
         }
     }

     type my-base-str-type {
         // legal length refinement
         length "11 | 42..max"; // 11 | 42..255
     }

     type my-base-str-type {
         // illegal length refinement
         length "1..999";
     }





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9.4.6.  The pattern Statement

   The "pattern" statement, which is an optional substatement to the
   "type" statement, takes as an argument a regular expression string,
   as defined in [XSD-TYPES].  It is used to restrict the built-in type
   "string", or types derived from "string", to values that match the
   pattern.

   If the type has multiple "pattern" statements, the expressions are
   ANDed together, i.e., all such expressions have to match.

   If a pattern restriction is applied to an already pattern-restricted
   type, values must match all patterns in the base type, in addition to
   the new patterns.

9.4.6.1.  The pattern's Substatements

                 +---------------+---------+-------------+
                 | substatement  | section | cardinality |
                 +---------------+---------+-------------+
                 | description   | 7.19.3  | 0..1        |
                 | error-app-tag | 7.5.4.2 | 0..1        |
                 | error-message | 7.5.4.1 | 0..1        |
                 | reference     | 7.19.4  | 0..1        |
                 +---------------+---------+-------------+

9.4.7.  Usage Example

   With the following type:

     type string {
         length "0..4";
         pattern "[0-9a-fA-F]*";
     }

   the following strings match:

     AB          // legal
     9A00        // legal

   and the following strings do not match:

     00ABAB      // illegal, too long
     xx00        // illegal, bad characters







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9.5.  The boolean Built-In Type

   The boolean built-in type represents a boolean value.

9.5.1.  Lexical Representation

   The lexical representation of a boolean value is a string with a
   value of "true" or "false".  These values MUST be in lowercase.

9.5.2.  Canonical Form

   The canonical form is the same as the lexical representation.

9.5.3.  Restrictions

   A boolean cannot be restricted.

9.6.  The enumeration Built-In Type

   The enumeration built-in type represents values from a set of
   assigned names.

9.6.1.  Lexical Representation

   The lexical representation of an enumeration value is the assigned
   name string.

9.6.2.  Canonical Form

   The canonical form is the assigned name string.

9.6.3.  Restrictions

   An enumeration cannot be restricted.

9.6.4.  The enum Statement

   The "enum" statement, which is a substatement to the "type"
   statement, MUST be present if the type is "enumeration".  It is
   repeatedly used to specify each assigned name of an enumeration type.
   It takes as an argument a string which is the assigned name.  The
   string MUST NOT be empty and MUST NOT have any leading or trailing
   whitespace characters.  The use of Unicode control codes SHOULD be
   avoided.

   The statement is optionally followed by a block of substatements that
   holds detailed enum information.




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   All assigned names in an enumeration MUST be unique.

9.6.4.1.  The enum's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | description  | 7.19.3  | 0..1        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | value        | 9.6.4.2 | 0..1        |
                 +--------------+---------+-------------+

9.6.4.2.  The value Statement

   The "value" statement, which is optional, is used to associate an
   integer value with the assigned name for the enum.  This integer
   value MUST be in the range -2147483648 to 2147483647, and it MUST be
   unique within the enumeration type.  The value is unused by YANG and
   the XML encoding, but is carried as a convenience to implementors.

   If a value is not specified, then one will be automatically assigned.
   If the "enum" substatement is the first one defined, the assigned
   value is zero (0); otherwise, the assigned value is one greater than
   the current highest enum value.

   If the current highest value is equal to 2147483647, then an enum
   value MUST be specified for "enum" substatements following the one
   with the current highest value.

9.6.5.  Usage Example

     leaf myenum {
         type enumeration {
             enum zero;
             enum one;
             enum seven {
                 value 7;
             }
         }
     }

   The lexical representation of the leaf "myenum" with value "seven"
   is:

     <myenum>seven</myenum>





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9.7.  The bits Built-In Type

   The bits built-in type represents a bit set.  That is, a bits value
   is a set of flags identified by small integer position numbers
   starting at 0.  Each bit number has an assigned name.

9.7.1.  Restrictions

   A bits type cannot be restricted.

9.7.2.  Lexical Representation

   The lexical representation of the bits type is a space-separated list
   of the individual bit values that are set.  An empty string thus
   represents a value where no bits are set.

9.7.3.  Canonical Form

   In the canonical form, the bit values are separated by a single space
   character and they appear ordered by their position (see
   Section 9.7.4.2).

9.7.4.  The bit Statement

   The "bit" statement, which is a substatement to the "type" statement,
   MUST be present if the type is "bits".  It is repeatedly used to
   specify each assigned named bit of a bits type.  It takes as an
   argument a string that is the assigned name of the bit.  It is
   followed by a block of substatements that holds detailed bit
   information.  The assigned name follows the same syntax rules as an
   identifier (see Section 6.2).

   All assigned names in a bits type MUST be unique.

9.7.4.1.  The bit's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | description  | 7.19.3  | 0..1        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | position     | 9.7.4.2 | 0..1        |
                 +--------------+---------+-------------+







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9.7.4.2.  The position Statement

   The "position" statement, which is optional, takes as an argument a
   non-negative integer value that specifies the bit's position within a
   hypothetical bit field.  The position value MUST be in the range 0 to
   4294967295, and it MUST be unique within the bits type.  The value is
   unused by YANG and the NETCONF messages, but is carried as a
   convenience to implementors.

   If a bit position is not specified, then one will be automatically
   assigned.  If the "bit" substatement is the first one defined, the
   assigned value is zero (0); otherwise, the assigned value is one
   greater than the current highest bit position.

   If the current highest bit position value is equal to 4294967295,
   then a position value MUST be specified for "bit" substatements
   following the one with the current highest position value.

9.7.5.  Usage Example

   Given the following leaf:

     leaf mybits {
         type bits {
             bit disable-nagle {
                 position 0;
             }
             bit auto-sense-speed {
                 position 1;
             }
             bit 10-Mb-only {
                 position 2;
             }
         }
         default "auto-sense-speed";
     }

   The lexical representation of this leaf with bit values disable-nagle
   and 10-Mb-only set would be:

     <mybits>disable-nagle 10-Mb-only</mybits>

9.8.  The binary Built-In Type

   The binary built-in type represents any binary data, i.e., a sequence
   of octets.





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9.8.1.  Restrictions

   A binary can be restricted with the "length" (Section 9.4.4)
   statement.  The length of a binary value is the number of octets it
   contains.

9.8.2.  Lexical Representation

   Binary values are encoded with the base64 encoding scheme (see
   [RFC4648], Section 4).

9.8.3.  Canonical Form

   The canonical form of a binary value follows the rules in [RFC4648].

9.9.  The leafref Built-In Type

   The leafref type is used to reference a particular leaf instance in
   the data tree.  The "path" substatement (Section 9.9.2) selects a set
   of leaf instances, and the leafref value space is the set of values
   of these leaf instances.

   If the leaf with the leafref type represents configuration data, the
   leaf it refers to MUST also represent configuration.  Such a leaf
   puts a constraint on valid data.  All leafref nodes MUST reference
   existing leaf instances or leafs with default values in use (see
   Section 7.6.1) for the data to be valid.  This constraint is enforced
   according to the rules in Section 8.

   There MUST NOT be any circular chains of leafrefs.

   If the leaf that the leafref refers to is conditional based on one or
   more features (see Section 7.18.2), then the leaf with the leafref
   type MUST also be conditional based on at least the same set of
   features.

9.9.1.  Restrictions

   A leafref cannot be restricted.

9.9.2.  The path Statement

   The "path" statement, which is a substatement to the "type"
   statement, MUST be present if the type is "leafref".  It takes as an
   argument a string that MUST refer to a leaf or leaf-list node.






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   The syntax for a path argument is a subset of the XPath abbreviated
   syntax.  Predicates are used only for constraining the values for the
   key nodes for list entries.  Each predicate consists of exactly one
   equality test per key, and multiple adjacent predicates MAY be
   present if a list has multiple keys.  The syntax is formally defined
   by the rule "path-arg" in Section 12.

   The predicates are only used when more than one key reference is
   needed to uniquely identify a leaf instance.  This occurs if a list
   has multiple keys, or a reference to a leaf other than the key in a
   list is needed.  In these cases, multiple leafrefs are typically
   specified, and predicates are used to tie them together.

   The "path" expression evaluates to a node set consisting of zero,
   one, or more nodes.  If the leaf with the leafref type represents
   configuration data, this node set MUST be non-empty.

   The "path" XPath expression is conceptually evaluated in the
   following context, in addition to the definition in Section 6.4.1:

   o  The context node is the node in the data tree for which the "path"
      statement is defined.

   The accessible tree depends on the context node:

   o  If the context node represents configuration data, the tree is the
      data in the NETCONF datastore where the context node exists.  The
      XPath root node has all top-level configuration data nodes in all
      modules as children.

   o  Otherwise, the tree is all state data on the device, and the
      <running/> datastore.  The XPath root node has all top-level data
      nodes in all modules as children.

9.9.3.  Lexical Representation

   A leafref value is encoded the same way as the leaf it references.

9.9.4.  Canonical Form

   The canonical form of a leafref is the same as the canonical form of
   the leaf it references.









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9.9.5.  Usage Example

   With the following list:

     list interface {
         key "name";
         leaf name {
             type string;
         }
         leaf admin-status {
             type admin-status;
         }
         list address {
             key "ip";
             leaf ip {
                 type yang:ip-address;
             }
         }
     }

   The following leafref refers to an existing interface:

     leaf mgmt-interface {
         type leafref {
             path "../interface/name";
         }
     }

   An example of a corresponding XML snippet:

     <interface>
       <name>eth0</name>
     </interface>
     <interface>
       <name>lo</name>
     </interface>

     <mgmt-interface>eth0</mgmt-interface>













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   The following leafrefs refer to an existing address of an interface:

     container default-address {
         leaf ifname {
             type leafref {
                 path "../../interface/name";
             }
         }
         leaf address {
             type leafref {
                 path "../../interface[name = current()/../ifname]"
                    + "/address/ip";
             }
         }
     }

   An example of a corresponding XML snippet:

     <interface>
       <name>eth0</name>
       <admin-status>up</admin-status>
       <address>
         <ip>192.0.2.1</ip>
       </address>
       <address>
         <ip>192.0.2.2</ip>
       </address>
     </interface>
     <interface>
       <name>lo</name>
       <admin-status>up</admin-status>
       <address>
         <ip>127.0.0.1</ip>
       </address>
     </interface>

     <default-address>
       <ifname>eth0</ifname>
       <address>192.0.2.2</address>
     </default-address>











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   The following list uses a leafref for one of its keys.  This is
   similar to a foreign key in a relational database.

     list packet-filter {
         key "if-name filter-id";
         leaf if-name {
             type leafref {
                 path "/interface/name";
             }
         }
         leaf filter-id {
             type uint32;
         }
         ...
     }

   An example of a corresponding XML snippet:

     <interface>
       <name>eth0</name>
       <admin-status>up</admin-status>
       <address>
         <ip>192.0.2.1</ip>
       </address>
       <address>
         <ip>192.0.2.2</ip>
       </address>
     </interface>

     <packet-filter>
       <if-name>eth0</if-name>
       <filter-id>1</filter-id>
       ...
     </packet-filter>
     <packet-filter>
       <if-name>eth0</if-name>
       <filter-id>2</filter-id>
       ...
     </packet-filter>












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   The following notification defines two leafrefs to refer to an
   existing admin-status:

     notification link-failure {
         leaf if-name {
             type leafref {
                 path "/interface/name";
             }
         }
         leaf admin-status {
             type leafref {
                 path
                   "/interface[name = current()/../if-name]"
                 + "/admin-status";
             }
         }
     }

   An example of a corresponding XML notification:

     <notification
       xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
       <eventTime>2008-04-01T00:01:00Z</eventTime>
       <link-failure xmlns="http://acme.example.com/system">
         <if-name>eth0</if-name>
         <admin-status>up</admin-status>
       </link-failure>
     </notification>

9.10.  The identityref Built-In Type

   The identityref type is used to reference an existing identity (see
   Section 7.16).

9.10.1.  Restrictions

   An identityref cannot be restricted.

9.10.2.  The identityref's base Statement

   The "base" statement, which is a substatement to the "type"
   statement, MUST be present if the type is "identityref".  The
   argument is the name of an identity, as defined by an "identity"
   statement.  If a prefix is present on the identity name, it refers to
   an identity defined in the module that was imported with that prefix.
   Otherwise, an identity with the matching name MUST be defined in the
   current module or an included submodule.




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   Valid values for an identityref are any identities derived from the
   identityref's base identity.  On a particular server, the valid
   values are further restricted to the set of identities defined in the
   modules supported by the server.

9.10.3.  Lexical Representation

   An identityref is encoded as the referred identity's qualified name
   as defined in [XML-NAMES].  If the prefix is not present, the
   namespace of the identityref is the default namespace in effect on
   the element that contains the identityref value.

   When an identityref is given a default value using the "default"
   statement, the identity name in the default value MAY have a prefix.
   If a prefix is present on the identity name, it refers to an identity
   defined in the module that was imported with that prefix.  Otherwise,
   an identity with the matching name MUST be defined in the current
   module or an included submodule.

9.10.4.  Canonical Form

   Since the lexical form depends on the XML context in which the value
   occurs, this type does not have a canonical form.

9.10.5.  Usage Example

   With the identity definitions in Section 7.16.3 and the following
   module:

     module my-crypto {

         namespace "http://example.com/my-crypto";
         prefix mc;

         import "crypto-base" {
             prefix "crypto";
         }

         identity aes {
             base "crypto:crypto-alg";
         }

         leaf crypto {
             type identityref {
                 base "crypto:crypto-alg";
             }
         }
     }



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   the leaf "crypto" will be encoded as follows, if the value is the
   "des3" identity defined in the "des" module:

     <crypto xmlns:des="http://example.com/des">des:des3</crypto>

   Any prefixes used in the encoding are local to each instance
   encoding.  This means that the same identityref may be encoded
   differently by different implementations.  For example, the following
   example encodes the same leaf as above:

     <crypto xmlns:x="http://example.com/des">x:des3</crypto>

   If the "crypto" leaf's value instead is "aes" defined in the
   "my-crypto" module, it can be encoded as:

     <crypto xmlns:mc="http://example.com/my-crypto">mc:aes</crypto>

   or, using the default namespace:

     <crypto>aes</crypto>

9.11.  The empty Built-In Type

   The empty built-in type represents a leaf that does not have any
   value, it conveys information by its presence or absence.

   An empty type cannot have a default value.

9.11.1.  Restrictions

   An empty type cannot be restricted.

9.11.2.  Lexical Representation

   Not applicable.

9.11.3.  Canonical Form

   Not applicable.

9.11.4.  Usage Example

   The following leaf

     leaf enable-qos {
         type empty;
     }




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   will be encoded as

     <enable-qos/>

   if it exists.

9.12.  The union Built-In Type

   The union built-in type represents a value that corresponds to one of
   its member types.

   When the type is "union", the "type" statement (Section 7.4) MUST be
   present.  It is used to repeatedly specify each member type of the
   union.  It takes as an argument a string that is the name of a member
   type.

   A member type can be of any built-in or derived type, except it MUST
   NOT be one of the built-in types "empty" or "leafref".

   When a string representing a union data type is validated, the string
   is validated against each member type, in the order they are
   specified in the "type" statement, until a match is found.

   Any default value or "units" property defined in the member types is
   not inherited by the union type.

   Example:

     type union {
         type int32;
         type enumeration {
             enum "unbounded";
         }
     }

9.12.1.  Restrictions

   A union cannot be restricted.  However, each member type can be
   restricted, based on the rules defined in Section 9.

9.12.2.  Lexical Representation

   The lexical representation of a union is a value that corresponds to
   the representation of any one of the member types.







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9.12.3.  Canonical Form

   The canonical form of a union value is the same as the canonical form
   of the member type of the value.

9.13.  The instance-identifier Built-In Type

   The instance-identifier built-in type is used to uniquely identify a
   particular instance node in the data tree.

   The syntax for an instance-identifier is a subset of the XPath
   abbreviated syntax, formally defined by the rule
   "instance-identifier" in Section 12.  It is used to uniquely identify
   a node in the data tree.  Predicates are used only for specifying the
   values for the key nodes for list entries, a value of a leaf-list
   entry, or a positional index for a list without keys.  For
   identifying list entries with keys, each predicate consists of one
   equality test per key, and each key MUST have a corresponding
   predicate.

   If the leaf with the instance-identifier type represents
   configuration data, and the "require-instance" property
   (Section 9.13.2) is "true", the node it refers to MUST also represent
   configuration.  Such a leaf puts a constraint on valid data.  All
   such leaf nodes MUST reference existing nodes or leaf nodes with
   their default value in use (see Section 7.6.1) for the data to be
   valid.  This constraint is enforced according to the rules in
   Section 8.

   The "instance-identifier" XPath expression is conceptually evaluated
   in the following context, in addition to the definition in
   Section 6.4.1:

   o  The context node is the root node in the accessible tree.

   The accessible tree depends on the leaf with the instance-identifier
   type:

   o  If this leaf represents configuration data, the tree is the data
      in the NETCONF datastore where the leaf exists.  The XPath root
      node has all top-level configuration data nodes in all modules as
      children.

   o  Otherwise, the tree is all state data on the device, and the
      <running/> datastore.  The XPath root node has all top-level data
      nodes in all modules as children.





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9.13.1.  Restrictions

   An instance-identifier can be restricted with the "require-instance"
   statement (Section 9.13.2).

9.13.2.  The require-instance Statement

   The "require-instance" statement, which is a substatement to the
   "type" statement, MAY be present if the type is
   "instance-identifier".  It takes as an argument the string "true" or
   "false".  If this statement is not present, it defaults to "true".

   If "require-instance" is "true", it means that the instance being
   referred MUST exist for the data to be valid.  This constraint is
   enforced according to the rules in Section 8.

   If "require-instance" is "false", it means that the instance being
   referred MAY exist in valid data.

9.13.3.  Lexical Representation

   An instance-identifier value is lexically represented as a string.
   All node names in an instance-identifier value MUST be qualified with
   explicit namespace prefixes, and these prefixes MUST be declared in
   the XML namespace scope in the instance-identifier's XML element.

   Any prefixes used in the encoding are local to each instance
   encoding.  This means that the same instance-identifier may be
   encoded differently by different implementations.

9.13.4.  Canonical Form

   Since the lexical form depends on the XML context in which the value
   occurs, this type does not have a canonical form.

9.13.5.  Usage Example

   The following are examples of instance identifiers:

     /* instance-identifier for a container */
     /ex:system/ex:services/ex:ssh

     /* instance-identifier for a leaf */
     /ex:system/ex:services/ex:ssh/ex:port

     /* instance-identifier for a list entry */
     /ex:system/ex:user[ex:name='fred']




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     /* instance-identifier for a leaf in a list entry */
     /ex:system/ex:user[ex:name='fred']/ex:type

     /* instance-identifier for a list entry with two keys */
     /ex:system/ex:server[ex:ip='192.0.2.1'][ex:port='80']

     /* instance-identifier for a leaf-list entry */
     /ex:system/ex:services/ex:ssh/ex:cipher[.='blowfish-cbc']

     /* instance-identifier for a list entry without keys */
     /ex:stats/ex:port[3]

10.  Updating a Module

   As experience is gained with a module, it may be desirable to revise
   that module.  However, changes are not allowed if they have any
   potential to cause interoperability problems between a client using
   an original specification and a server using an updated
   specification.

   For any published change, a new "revision" statement (Section 7.1.9)
   MUST be included in front of the existing "revision" statements.  If
   there are no existing "revision" statements, then one MUST be added
   to identify the new revision.  Furthermore, any necessary changes
   MUST be applied to any meta-data statements, including the
   "organization" and "contact" statements (Sections 7.1.7, 7.1.8).

   Note that definitions contained in a module are available to be
   imported by any other module, and are referenced in "import"
   statements via the module name.  Thus, a module name MUST NOT be
   changed.  Furthermore, the "namespace" statement MUST NOT be changed,
   since all XML elements are qualified by the namespace.

   Obsolete definitions MUST NOT be removed from modules since their
   identifiers may still be referenced by other modules.

   A definition may be revised in any of the following ways:

   o  An "enumeration" type may have new enums added, provided the old
      enums's values do not change.

   o  A "bits" type may have new bits added, provided the old bit
      positions do not change.

   o  A "range", "length", or "pattern" statement may expand the allowed
      value space.





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   o  A "default" statement may be added to a leaf that does not have a
      default value (either directly or indirectly through its type).

   o  A "units" statement may be added.

   o  A "reference" statement may be added or updated.

   o  A "must" statement may be removed or its constraint relaxed.

   o  A "mandatory" statement may be removed or changed from "true" to
      "false".

   o  A "min-elements" statement may be removed, or changed to require
      fewer elements.

   o  A "max-elements" statement may be removed, or changed to allow
      more elements.

   o  A "description" statement may be added or clarified without
      changing the semantics of the definition.

   o  New typedefs, groupings, rpcs, notifications, extensions,
      features, and identities may be added.

   o  New data definition statements may be added if they do not add
      mandatory nodes (Section 3.1) to existing nodes or at the top
      level in a module or submodule, or if they are conditionally
      dependent on a new feature (i.e., have an "if-feature" statement
      that refers to a new feature).

   o  A new "case" statement may be added.

   o  A node that represented state data may be changed to represent
      configuration, provided it is not mandatory (Section 3.1).

   o  An "if-feature" statement may be removed, provided its node is not
      mandatory (Section 3.1).

   o  A "status" statement may be added, or changed from "current" to
      "deprecated" or "obsolete", or from "deprecated" to "obsolete".

   o  A "type" statement may be replaced with another "type" statement
      that does not change the syntax or semantics of the type.  For
      example, an inline type definition may be replaced with a typedef,
      but an int8 type cannot be replaced by an int16, since the syntax
      would change.





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   o  Any set of data definition nodes may be replaced with another set
      of syntactically and semantically equivalent nodes.  For example,
      a set of leafs may be replaced by a uses of a grouping with the
      same leafs.

   o  A module may be split into a set of submodules, or a submodule may
      be removed, provided the definitions in the module do not change
      in any other way than allowed here.

   o  The "prefix" statement may be changed, provided all local uses of
      the prefix also are changed.

   Otherwise, if the semantics of any previous definition are changed
   (i.e., if a non-editorial change is made to any definition other than
   those specifically allowed above), then this MUST be achieved by a
   new definition with a new identifier.

   In statements that have any data definition statements as
   substatements, those data definition substatements MUST NOT be
   reordered.

11.  YIN

   A YANG module can be translated into an alternative XML-based syntax
   called YIN.  The translated module is called a YIN module.  This
   section describes symmetric mapping rules between the two formats.

   The YANG and YIN formats contain equivalent information using
   different notations.  The YIN notation enables developers to
   represent YANG data models in XML and therefore use the rich set of
   XML-based tools for data filtering and validation, automated
   generation of code and documentation, and other tasks.  Tools like
   XSLT or XML validators can be utilized.

   The mapping between YANG and YIN does not modify the information
   content of the model.  Comments and whitespace are not preserved.

11.1.  Formal YIN Definition

   There is a one-to-one correspondence between YANG keywords and YIN
   elements.  The local name of a YIN element is identical to the
   corresponding YANG keyword.  This means, in particular, that the
   document element (root) of a YIN document is always <module> or
   <submodule>.

   YIN elements corresponding to the YANG keywords belong to the
   namespace whose associated URI is
   "urn:ietf:params:xml:ns:yang:yin:1".



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   YIN elements corresponding to extension keywords belong to the
   namespace of the YANG module where the extension keyword is declared
   via the "extension" statement.

   The names of all YIN elements MUST be properly qualified with their
   namespaces specified above using the standard mechanisms of
   [XML-NAMES], i.e., "xmlns" and "xmlns:xxx" attributes.

   The argument of a YANG statement is represented in YIN either as an
   XML attribute or a subelement of the keyword element.  Table 1
   defines the mapping for the set of YANG keywords.  For extensions,
   the argument mapping is specified within the "extension" statement
   (see Section 7.17).  The following rules hold for arguments:

   o  If the argument is represented as an attribute, this attribute has
      no namespace.

   o  If the argument is represented as an element, it is qualified by
      the same namespace as its parent keyword element.

   o  If the argument is represented as an element, it MUST be the first
      child of the keyword element.

   Substatements of a YANG statement are represented as (additional)
   children of the keyword element and their relative order MUST be the
   same as the order of substatements in YANG.

   Comments in YANG MAY be mapped to XML comments.























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               Mapping of arguments of the YANG statements.

            +------------------+---------------+-------------+
            | keyword          | argument name | yin-element |
            +------------------+---------------+-------------+
            | anyxml           | name          | false       |
            | argument         | name          | false       |
            | augment          | target-node   | false       |
            | base             | name          | false       |
            | belongs-to       | module        | false       |
            | bit              | name          | false       |
            | case             | name          | false       |
            | choice           | name          | false       |
            | config           | value         | false       |
            | contact          | text          | true        |
            | container        | name          | false       |
            | default          | value         | false       |
            | description      | text          | true        |
            | deviate          | value         | false       |
            | deviation        | target-node   | false       |
            | enum             | name          | false       |
            | error-app-tag    | value         | false       |
            | error-message    | value         | true        |
            | extension        | name          | false       |
            | feature          | name          | false       |
            | fraction-digits  | value         | false       |
            | grouping         | name          | false       |
            | identity         | name          | false       |
            | if-feature       | name          | false       |
            | import           | module        | false       |
            | include          | module        | false       |
            | input            | <no argument> | n/a         |
            | key              | value         | false       |
            | leaf             | name          | false       |
            | leaf-list        | name          | false       |
            | length           | value         | false       |
            | list             | name          | false       |
            | mandatory        | value         | false       |
            | max-elements     | value         | false       |
            | min-elements     | value         | false       |
            | module           | name          | false       |
            | must             | condition     | false       |
            | namespace        | uri           | false       |
            | notification     | name          | false       |
            | ordered-by       | value         | false       |
            | organization     | text          | true        |
            | output           | <no argument> | n/a         |
            | path             | value         | false       |



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            | pattern          | value         | false       |
            | position         | value         | false       |
            | prefix           | value         | false       |
            | presence         | value         | false       |
            | range            | value         | false       |
            | reference        | text          | true        |
            | refine           | target-node   | false       |
            | require-instance | value         | false       |
            | revision         | date          | false       |
            | revision-date    | date          | false       |
            | rpc              | name          | false       |
            | status           | value         | false       |
            | submodule        | name          | false       |
            | type             | name          | false       |
            | typedef          | name          | false       |
            | unique           | tag           | false       |
            | units            | name          | false       |
            | uses             | name          | false       |
            | value            | value         | false       |
            | when             | condition     | false       |
            | yang-version     | value         | false       |
            | yin-element      | value         | false       |
            +------------------+---------------+-------------+

                                  Table 1


























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11.1.1.  Usage Example

   The following YANG module:

     module acme-foo {
         namespace "http://acme.example.com/foo";
         prefix "acfoo";

         import my-extensions {
             prefix "myext";
         }

         list interface {
             key "name";
             leaf name {
                 type string;
             }

             leaf mtu {
                 type uint32;
                 description "The MTU of the interface.";
                 myext:c-define "MY_MTU";
             }
         }
     }

   where the extension "c-define" is defined in Section 7.17.3, is
   translated into the following YIN:























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     <module name="acme-foo"
             xmlns="urn:ietf:params:xml:ns:yang:yin:1"
             xmlns:acfoo="http://acme.example.com/foo"
             xmlns:myext="http://example.com/my-extensions">

       <namespace uri="http://acme.example.com/foo"/>
       <prefix value="acfoo"/>

       <import module="my-extensions">
         <prefix value="myext"/>
       </import>

       <list name="interface">
         <key value="name"/>
         <leaf name="name">
           <type name="string"/>
         </leaf>
         <leaf name="mtu">
           <type name="uint32"/>
           <description>
             <text>The MTU of the interface.</text>
           </description>
           <myext:c-define name="MY_MTU"/>
         </leaf>
       </list>
     </module>

























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12.  YANG ABNF Grammar

   In YANG, almost all statements are unordered.  The ABNF grammar
   [RFC5234] defines the canonical order.  To improve module
   readability, it is RECOMMENDED that clauses be entered in this order.

   Within the ABNF grammar, unordered statements are marked with
   comments.

   This grammar assumes that the scanner replaces YANG comments with a
   single space character.

   <CODE BEGINS> file "yang.abnf"

   module-stmt         = optsep module-keyword sep identifier-arg-str
                         optsep
                         "{" stmtsep
                             module-header-stmts
                             linkage-stmts
                             meta-stmts
                             revision-stmts
                             body-stmts
                         "}" optsep

   submodule-stmt      = optsep submodule-keyword sep identifier-arg-str
                         optsep
                         "{" stmtsep
                             submodule-header-stmts
                             linkage-stmts
                             meta-stmts
                             revision-stmts
                             body-stmts
                         "}" optsep

   module-header-stmts = ;; these stmts can appear in any order
                         [yang-version-stmt stmtsep]
                          namespace-stmt stmtsep
                          prefix-stmt stmtsep

   submodule-header-stmts =
                         ;; these stmts can appear in any order
                         [yang-version-stmt stmtsep]
                          belongs-to-stmt stmtsep








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   meta-stmts          = ;; these stmts can appear in any order
                         [organization-stmt stmtsep]
                         [contact-stmt stmtsep]
                         [description-stmt stmtsep]
                         [reference-stmt stmtsep]

   linkage-stmts       = ;; these stmts can appear in any order
                         *(import-stmt stmtsep)
                         *(include-stmt stmtsep)

   revision-stmts      = *(revision-stmt stmtsep)

   body-stmts          = *((extension-stmt /
                            feature-stmt /
                            identity-stmt /
                            typedef-stmt /
                            grouping-stmt /
                            data-def-stmt /
                            augment-stmt /
                            rpc-stmt /
                            notification-stmt /
                            deviation-stmt) stmtsep)

   data-def-stmt       = container-stmt /
                         leaf-stmt /
                         leaf-list-stmt /
                         list-stmt /
                         choice-stmt /
                         anyxml-stmt /
                         uses-stmt

   yang-version-stmt   = yang-version-keyword sep yang-version-arg-str
                         optsep stmtend

   yang-version-arg-str = < a string that matches the rule
                           yang-version-arg >

   yang-version-arg    = "1"

   import-stmt         = import-keyword sep identifier-arg-str optsep
                         "{" stmtsep
                             prefix-stmt stmtsep
                             [revision-date-stmt stmtsep]
                         "}"







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   include-stmt        = include-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              [revision-date-stmt stmtsep]
                          "}")

   namespace-stmt      = namespace-keyword sep uri-str optsep stmtend

   uri-str             = < a string that matches the rule
                           URI in RFC 3986 >

   prefix-stmt         = prefix-keyword sep prefix-arg-str
                         optsep stmtend

   belongs-to-stmt     = belongs-to-keyword sep identifier-arg-str
                         optsep
                         "{" stmtsep
                             prefix-stmt stmtsep
                         "}"

   organization-stmt   = organization-keyword sep string
                         optsep stmtend

   contact-stmt        = contact-keyword sep string optsep stmtend

   description-stmt    = description-keyword sep string optsep
                         stmtend

   reference-stmt      = reference-keyword sep string optsep stmtend

   units-stmt          = units-keyword sep string optsep stmtend

   revision-stmt       = revision-keyword sep revision-date optsep
                         (";" /
                          "{" stmtsep
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                          "}")

   revision-date       =  date-arg-str

   revision-date-stmt = revision-date-keyword sep revision-date stmtend









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   extension-stmt      = extension-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [argument-stmt stmtsep]
                              [status-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                          "}")

   argument-stmt       = argument-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              [yin-element-stmt stmtsep]
                          "}")

   yin-element-stmt    = yin-element-keyword sep yin-element-arg-str
                         stmtend

   yin-element-arg-str = < a string that matches the rule
                           yin-element-arg >

   yin-element-arg     = true-keyword / false-keyword

   identity-stmt       = identity-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [base-stmt stmtsep]
                              [status-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                          "}")

   base-stmt           = base-keyword sep identifier-ref-arg-str
                         optsep stmtend

   feature-stmt        = feature-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              *(if-feature-stmt stmtsep)
                              [status-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                          "}")





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   if-feature-stmt     = if-feature-keyword sep identifier-ref-arg-str
                         optsep stmtend

   typedef-stmt        = typedef-keyword sep identifier-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             type-stmt stmtsep
                             [units-stmt stmtsep]
                             [default-stmt stmtsep]
                             [status-stmt stmtsep]
                             [description-stmt stmtsep]
                             [reference-stmt stmtsep]
                          "}"

   type-stmt           = type-keyword sep identifier-ref-arg-str optsep
                         (";" /
                          "{" stmtsep
                              type-body-stmts
                          "}")

   type-body-stmts     = numerical-restrictions /
                         decimal64-specification /
                         string-restrictions /
                         enum-specification /
                         leafref-specification /
                         identityref-specification /
                         instance-identifier-specification /
                         bits-specification /
                         union-specification

   numerical-restrictions = range-stmt stmtsep

   range-stmt          = range-keyword sep range-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [error-message-stmt stmtsep]
                              [error-app-tag-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                           "}")

   decimal64-specification = fraction-digits-stmt

   fraction-digits-stmt = fraction-digits-keyword sep
                          fraction-digits-arg-str stmtend





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   fraction-digits-arg-str = < a string that matches the rule
                              fraction-digits-arg >

   fraction-digits-arg = ("1" ["0" / "1" / "2" / "3" / "4" /
                               "5" / "6" / "7" / "8"])
                         / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9"

   string-restrictions = ;; these stmts can appear in any order
                         [length-stmt stmtsep]
                         *(pattern-stmt stmtsep)

   length-stmt         = length-keyword sep length-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [error-message-stmt stmtsep]
                              [error-app-tag-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                           "}")

   pattern-stmt        = pattern-keyword sep string optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [error-message-stmt stmtsep]
                              [error-app-tag-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                           "}")

   default-stmt        = default-keyword sep string stmtend

   enum-specification  = 1*(enum-stmt stmtsep)

   enum-stmt           = enum-keyword sep string optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [value-stmt stmtsep]
                              [status-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                           "}")







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   leafref-specification =
                         ;; these stmts can appear in any order
                         path-stmt stmtsep
                         [require-instance-stmt stmtsep]

   path-stmt           = path-keyword sep path-arg-str stmtend

   require-instance-stmt = require-instance-keyword sep
                            require-instance-arg-str stmtend

   require-instance-arg-str = < a string that matches the rule
                              require-instance-arg >

   require-instance-arg = true-keyword / false-keyword


   instance-identifier-specification =
                         [require-instance-stmt stmtsep]

   identityref-specification =
                         base-stmt stmtsep

   union-specification = 1*(type-stmt stmtsep)

   bits-specification  = 1*(bit-stmt stmtsep)

   bit-stmt            = bit-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [position-stmt stmtsep]
                              [status-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                            "}"
                          "}")

   position-stmt       = position-keyword sep
                         position-value-arg-str stmtend

   position-value-arg-str = < a string that matches the rule
                              position-value-arg >

   position-value-arg  = non-negative-integer-value

   status-stmt         = status-keyword sep status-arg-str stmtend





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   status-arg-str      = < a string that matches the rule
                           status-arg >

   status-arg          = current-keyword /
                         obsolete-keyword /
                         deprecated-keyword

   config-stmt         = config-keyword sep
                         config-arg-str stmtend

   config-arg-str      = < a string that matches the rule
                           config-arg >

   config-arg          = true-keyword / false-keyword

   mandatory-stmt      = mandatory-keyword sep
                         mandatory-arg-str stmtend

   mandatory-arg-str   = < a string that matches the rule
                           mandatory-arg >

   mandatory-arg       = true-keyword / false-keyword

   presence-stmt       = presence-keyword sep string stmtend

   ordered-by-stmt     = ordered-by-keyword sep
                         ordered-by-arg-str stmtend

   ordered-by-arg-str  = < a string that matches the rule
                           ordered-by-arg >

   ordered-by-arg      = user-keyword / system-keyword

   must-stmt           = must-keyword sep string optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [error-message-stmt stmtsep]
                              [error-app-tag-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                           "}")

   error-message-stmt  = error-message-keyword sep string stmtend

   error-app-tag-stmt  = error-app-tag-keyword sep string stmtend





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RFC 6020                          YANG                      October 2010


   min-elements-stmt   = min-elements-keyword sep
                         min-value-arg-str stmtend

   min-value-arg-str   = < a string that matches the rule
                           min-value-arg >

   min-value-arg       = non-negative-integer-value

   max-elements-stmt   = max-elements-keyword sep
                         max-value-arg-str stmtend

   max-value-arg-str   = < a string that matches the rule
                           max-value-arg >

   max-value-arg       = unbounded-keyword /
                         positive-integer-value

   value-stmt          = value-keyword sep integer-value stmtend

   grouping-stmt       = grouping-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [status-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                              *((typedef-stmt /
                                 grouping-stmt) stmtsep)
                              *(data-def-stmt stmtsep)
                          "}")

   container-stmt      = container-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [when-stmt stmtsep]
                              *(if-feature-stmt stmtsep)
                              *(must-stmt stmtsep)
                              [presence-stmt stmtsep]
                              [config-stmt stmtsep]
                              [status-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                              *((typedef-stmt /
                                 grouping-stmt) stmtsep)
                              *(data-def-stmt stmtsep)
                          "}")




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RFC 6020                          YANG                      October 2010


   leaf-stmt           = leaf-keyword sep identifier-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             [when-stmt stmtsep]
                             *(if-feature-stmt stmtsep)
                             type-stmt stmtsep
                             [units-stmt stmtsep]
                             *(must-stmt stmtsep)
                             [default-stmt stmtsep]
                             [config-stmt stmtsep]
                             [mandatory-stmt stmtsep]
                             [status-stmt stmtsep]
                             [description-stmt stmtsep]
                             [reference-stmt stmtsep]
                          "}"

   leaf-list-stmt      = leaf-list-keyword sep identifier-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             [when-stmt stmtsep]
                             *(if-feature-stmt stmtsep)
                             type-stmt stmtsep
                             [units-stmt stmtsep]
                             *(must-stmt stmtsep)
                             [config-stmt stmtsep]
                             [min-elements-stmt stmtsep]
                             [max-elements-stmt stmtsep]
                             [ordered-by-stmt stmtsep]
                             [status-stmt stmtsep]
                             [description-stmt stmtsep]
                             [reference-stmt stmtsep]
                          "}"

   list-stmt           = list-keyword sep identifier-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             [when-stmt stmtsep]
                             *(if-feature-stmt stmtsep)
                             *(must-stmt stmtsep)
                             [key-stmt stmtsep]
                             *(unique-stmt stmtsep)
                             [config-stmt stmtsep]
                             [min-elements-stmt stmtsep]
                             [max-elements-stmt stmtsep]
                             [ordered-by-stmt stmtsep]
                             [status-stmt stmtsep]
                             [description-stmt stmtsep]
                             [reference-stmt stmtsep]



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RFC 6020                          YANG                      October 2010


                             *((typedef-stmt /
                                grouping-stmt) stmtsep)
                             1*(data-def-stmt stmtsep)
                          "}"

   key-stmt            = key-keyword sep key-arg-str stmtend

   key-arg-str         = < a string that matches the rule
                           key-arg >

   key-arg             = node-identifier *(sep node-identifier)

   unique-stmt         = unique-keyword sep unique-arg-str stmtend

   unique-arg-str      = < a string that matches the rule
                           unique-arg >

   unique-arg          = descendant-schema-nodeid
                         *(sep descendant-schema-nodeid)

   choice-stmt         = choice-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [when-stmt stmtsep]
                              *(if-feature-stmt stmtsep)
                              [default-stmt stmtsep]
                              [config-stmt stmtsep]
                              [mandatory-stmt stmtsep]
                              [status-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                              *((short-case-stmt / case-stmt) stmtsep)
                          "}")

   short-case-stmt     = container-stmt /
                         leaf-stmt /
                         leaf-list-stmt /
                         list-stmt /
                         anyxml-stmt

   case-stmt           = case-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [when-stmt stmtsep]
                              *(if-feature-stmt stmtsep)
                              [status-stmt stmtsep]



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RFC 6020                          YANG                      October 2010


                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                              *(data-def-stmt stmtsep)
                          "}")

   anyxml-stmt         = anyxml-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [when-stmt stmtsep]
                              *(if-feature-stmt stmtsep)
                              *(must-stmt stmtsep)
                              [config-stmt stmtsep]
                              [mandatory-stmt stmtsep]
                              [status-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                           "}")

   uses-stmt           = uses-keyword sep identifier-ref-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              [when-stmt stmtsep]
                              *(if-feature-stmt stmtsep)
                              [status-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                              *(refine-stmt stmtsep)
                              *(uses-augment-stmt stmtsep)
                          "}")

   refine-stmt         = refine-keyword sep refine-arg-str optsep
                         (";" /
                          "{" stmtsep
                              (refine-container-stmts /
                               refine-leaf-stmts /
                               refine-leaf-list-stmts /
                               refine-list-stmts /
                               refine-choice-stmts /
                               refine-case-stmts /
                               refine-anyxml-stmts)
                          "}")

   refine-arg-str      = < a string that matches the rule
                           refine-arg >

   refine-arg          = descendant-schema-nodeid



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RFC 6020                          YANG                      October 2010


   refine-container-stmts =
                         ;; these stmts can appear in any order
                         *(must-stmt stmtsep)
                         [presence-stmt stmtsep]
                         [config-stmt stmtsep]
                         [description-stmt stmtsep]
                         [reference-stmt stmtsep]

   refine-leaf-stmts   = ;; these stmts can appear in any order
                         *(must-stmt stmtsep)
                         [default-stmt stmtsep]
                         [config-stmt stmtsep]
                         [mandatory-stmt stmtsep]
                         [description-stmt stmtsep]
                         [reference-stmt stmtsep]

   refine-leaf-list-stmts =
                         ;; these stmts can appear in any order
                         *(must-stmt stmtsep)
                         [config-stmt stmtsep]
                         [min-elements-stmt stmtsep]
                         [max-elements-stmt stmtsep]
                         [description-stmt stmtsep]
                         [reference-stmt stmtsep]

   refine-list-stmts   = ;; these stmts can appear in any order
                         *(must-stmt stmtsep)
                         [config-stmt stmtsep]
                         [min-elements-stmt stmtsep]
                         [max-elements-stmt stmtsep]
                         [description-stmt stmtsep]
                         [reference-stmt stmtsep]

   refine-choice-stmts = ;; these stmts can appear in any order
                         [default-stmt stmtsep]
                         [config-stmt stmtsep]
                         [mandatory-stmt stmtsep]
                         [description-stmt stmtsep]
                         [reference-stmt stmtsep]

   refine-case-stmts   = ;; these stmts can appear in any order
                         [description-stmt stmtsep]
                         [reference-stmt stmtsep]


   refine-anyxml-stmts = ;; these stmts can appear in any order
                         *(must-stmt stmtsep)
                         [config-stmt stmtsep]



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RFC 6020                          YANG                      October 2010


                         [mandatory-stmt stmtsep]
                         [description-stmt stmtsep]
                         [reference-stmt stmtsep]

   uses-augment-stmt   = augment-keyword sep uses-augment-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             [when-stmt stmtsep]
                             *(if-feature-stmt stmtsep)
                             [status-stmt stmtsep]
                             [description-stmt stmtsep]
                             [reference-stmt stmtsep]
                             1*((data-def-stmt stmtsep) /
                                (case-stmt stmtsep))
                          "}"

   uses-augment-arg-str = < a string that matches the rule
                            uses-augment-arg >

   uses-augment-arg    = descendant-schema-nodeid

   augment-stmt        = augment-keyword sep augment-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             [when-stmt stmtsep]
                             *(if-feature-stmt stmtsep)
                             [status-stmt stmtsep]
                             [description-stmt stmtsep]
                             [reference-stmt stmtsep]
                             1*((data-def-stmt stmtsep) /
                                (case-stmt stmtsep))
                          "}"

   augment-arg-str     = < a string that matches the rule
                           augment-arg >

   augment-arg         = absolute-schema-nodeid

   unknown-statement   = prefix ":" identifier [sep string] optsep
                         (";" / "{" *unknown-statement2 "}")

   unknown-statement2   = [prefix ":"] identifier [sep string] optsep
                         (";" / "{" *unknown-statement2 "}")

   when-stmt           = when-keyword sep string optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order



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RFC 6020                          YANG                      October 2010


                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                           "}")

   rpc-stmt            = rpc-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              *(if-feature-stmt stmtsep)
                              [status-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                              *((typedef-stmt /
                                 grouping-stmt) stmtsep)
                              [input-stmt stmtsep]
                              [output-stmt stmtsep]
                          "}")

   input-stmt          = input-keyword optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             *((typedef-stmt /
                                grouping-stmt) stmtsep)
                             1*(data-def-stmt stmtsep)
                         "}"

   output-stmt         = output-keyword optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             *((typedef-stmt /
                                grouping-stmt) stmtsep)
                             1*(data-def-stmt stmtsep)
                         "}"

   notification-stmt   = notification-keyword sep
                         identifier-arg-str optsep
                         (";" /
                          "{" stmtsep
                              ;; these stmts can appear in any order
                              *(if-feature-stmt stmtsep)
                              [status-stmt stmtsep]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                              *((typedef-stmt /
                                 grouping-stmt) stmtsep)
                              *(data-def-stmt stmtsep)
                          "}")




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RFC 6020                          YANG                      October 2010


   deviation-stmt      = deviation-keyword sep
                         deviation-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             [description-stmt stmtsep]
                             [reference-stmt stmtsep]
                             (deviate-not-supported-stmt /
                               1*(deviate-add-stmt /
                                  deviate-replace-stmt /
                                  deviate-delete-stmt))
                         "}"

   deviation-arg-str   = < a string that matches the rule
                           deviation-arg >

   deviation-arg       = absolute-schema-nodeid

   deviate-not-supported-stmt =
                         deviate-keyword sep
                         not-supported-keyword optsep
                         (";" /
                          "{" stmtsep
                          "}")

   deviate-add-stmt    = deviate-keyword sep add-keyword optsep
                         (";" /
                          "{" stmtsep
                              [units-stmt stmtsep]
                              *(must-stmt stmtsep)
                              *(unique-stmt stmtsep)
                              [default-stmt stmtsep]
                              [config-stmt stmtsep]
                              [mandatory-stmt stmtsep]
                              [min-elements-stmt stmtsep]
                              [max-elements-stmt stmtsep]
                          "}")

   deviate-delete-stmt = deviate-keyword sep delete-keyword optsep
                         (";" /
                          "{" stmtsep
                              [units-stmt stmtsep]
                              *(must-stmt stmtsep)
                              *(unique-stmt stmtsep)
                              [default-stmt stmtsep]
                          "}")






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RFC 6020                          YANG                      October 2010


   deviate-replace-stmt = deviate-keyword sep replace-keyword optsep
                         (";" /
                          "{" stmtsep
                              [type-stmt stmtsep]
                              [units-stmt stmtsep]
                              [default-stmt stmtsep]
                              [config-stmt stmtsep]
                              [mandatory-stmt stmtsep]
                              [min-elements-stmt stmtsep]
                              [max-elements-stmt stmtsep]
                          "}")

   ;; Ranges

   range-arg-str       = < a string that matches the rule
                           range-arg >

   range-arg           = range-part *(optsep "|" optsep range-part)

   range-part          = range-boundary
                         [optsep ".." optsep range-boundary]

   range-boundary      = min-keyword / max-keyword /
                         integer-value / decimal-value

   ;; Lengths

   length-arg-str      = < a string that matches the rule
                           length-arg >

   length-arg          = length-part *(optsep "|" optsep length-part)

   length-part         = length-boundary
                         [optsep ".." optsep length-boundary]

   length-boundary     = min-keyword / max-keyword /
                         non-negative-integer-value

   ;; Date

   date-arg-str        = < a string that matches the rule
                           date-arg >

   date-arg            = 4DIGIT "-" 2DIGIT "-" 2DIGIT







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RFC 6020                          YANG                      October 2010


   ;; Schema Node Identifiers

   schema-nodeid       = absolute-schema-nodeid /
                         descendant-schema-nodeid

   absolute-schema-nodeid = 1*("/" node-identifier)

   descendant-schema-nodeid =
                         node-identifier
                         absolute-schema-nodeid

   node-identifier     = [prefix ":"] identifier


   ;; Instance Identifiers

   instance-identifier = 1*("/" (node-identifier *predicate))

   predicate           = "[" *WSP (predicate-expr / pos) *WSP "]"

   predicate-expr      = (node-identifier / ".") *WSP "=" *WSP
                         ((DQUOTE string DQUOTE) /
                          (SQUOTE string SQUOTE))

   pos                 = non-negative-integer-value

   ;; leafref path

   path-arg-str        = < a string that matches the rule
                           path-arg >

   path-arg            = absolute-path / relative-path

   absolute-path       = 1*("/" (node-identifier *path-predicate))

   relative-path       = 1*(".." "/") descendant-path

   descendant-path     = node-identifier
                         [*path-predicate absolute-path]

   path-predicate      = "[" *WSP path-equality-expr *WSP "]"

   path-equality-expr  = node-identifier *WSP "=" *WSP path-key-expr

   path-key-expr       = current-function-invocation *WSP "/" *WSP
                         rel-path-keyexpr





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RFC 6020                          YANG                      October 2010


   rel-path-keyexpr    = 1*(".." *WSP "/" *WSP)
                         *(node-identifier *WSP "/" *WSP)
                         node-identifier

   ;;; Keywords, using abnfgen's syntax for case-sensitive strings

   ;; statement keywords
   anyxml-keyword      = 'anyxml'
   argument-keyword    = 'argument'
   augment-keyword     = 'augment'
   base-keyword        = 'base'
   belongs-to-keyword  = 'belongs-to'
   bit-keyword         = 'bit'
   case-keyword        = 'case'
   choice-keyword      = 'choice'
   config-keyword      = 'config'
   contact-keyword     = 'contact'
   container-keyword   = 'container'
   default-keyword     = 'default'
   description-keyword = 'description'
   enum-keyword        = 'enum'
   error-app-tag-keyword = 'error-app-tag'
   error-message-keyword = 'error-message'
   extension-keyword   = 'extension'
   deviation-keyword   = 'deviation'
   deviate-keyword     = 'deviate'
   feature-keyword     = 'feature'
   fraction-digits-keyword = 'fraction-digits'
   grouping-keyword    = 'grouping'
   identity-keyword    = 'identity'
   if-feature-keyword  = 'if-feature'
   import-keyword      = 'import'
   include-keyword     = 'include'
   input-keyword       = 'input'
   key-keyword         = 'key'
   leaf-keyword        = 'leaf'
   leaf-list-keyword   = 'leaf-list'
   length-keyword      = 'length'
   list-keyword        = 'list'
   mandatory-keyword   = 'mandatory'
   max-elements-keyword = 'max-elements'
   min-elements-keyword = 'min-elements'
   module-keyword      = 'module'
   must-keyword        = 'must'
   namespace-keyword   = 'namespace'
   notification-keyword= 'notification'
   ordered-by-keyword  = 'ordered-by'
   organization-keyword= 'organization'



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RFC 6020                          YANG                      October 2010


   output-keyword      = 'output'
   path-keyword        = 'path'
   pattern-keyword     = 'pattern'
   position-keyword    = 'position'
   prefix-keyword      = 'prefix'
   presence-keyword    = 'presence'
   range-keyword       = 'range'
   reference-keyword   = 'reference'
   refine-keyword      = 'refine'
   require-instance-keyword = 'require-instance'
   revision-keyword    = 'revision'
   revision-date-keyword = 'revision-date'
   rpc-keyword         = 'rpc'
   status-keyword      = 'status'
   submodule-keyword   = 'submodule'
   type-keyword        = 'type'
   typedef-keyword     = 'typedef'
   unique-keyword      = 'unique'
   units-keyword       = 'units'
   uses-keyword        = 'uses'
   value-keyword       = 'value'
   when-keyword        = 'when'
   yang-version-keyword= 'yang-version'
   yin-element-keyword = 'yin-element'

   ;; other keywords

   add-keyword         = 'add'
   current-keyword     = 'current'
   delete-keyword      = 'delete'
   deprecated-keyword  = 'deprecated'
   false-keyword       = 'false'
   max-keyword         = 'max'
   min-keyword         = 'min'
   not-supported-keyword = 'not-supported'
   obsolete-keyword    = 'obsolete'
   replace-keyword     = 'replace'
   system-keyword      = 'system'
   true-keyword        = 'true'
   unbounded-keyword   = 'unbounded'
   user-keyword        = 'user'










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RFC 6020                          YANG                      October 2010


   current-function-invocation = current-keyword *WSP "(" *WSP ")"

   ;; Basic Rules

   prefix-arg-str      = < a string that matches the rule
                           prefix-arg >

   prefix-arg          = prefix

   prefix              = identifier

   identifier-arg-str  = < a string that matches the rule
                           identifier-arg >

   identifier-arg      = identifier

   ;; An identifier MUST NOT start with (('X'|'x') ('M'|'m') ('L'|'l'))
   identifier          = (ALPHA / "_")
                         *(ALPHA / DIGIT / "_" / "-" / ".")

   identifier-ref-arg-str = < a string that matches the rule
                           identifier-ref-arg >

   identifier-ref-arg  = [prefix ":"] identifier

   string              = < an unquoted string as returned by
                           the scanner >

   integer-value       = ("-" non-negative-integer-value)  /
                          non-negative-integer-value

   non-negative-integer-value = "0" / positive-integer-value

   positive-integer-value = (non-zero-digit *DIGIT)

   zero-integer-value  = 1*DIGIT

   stmtend             = ";" / "{" *unknown-statement "}"

   sep                 = 1*(WSP / line-break)
                         ; unconditional separator

   optsep              = *(WSP / line-break)

   stmtsep             = *(WSP / line-break / unknown-statement)

   line-break          = CRLF / LF




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RFC 6020                          YANG                      October 2010


   non-zero-digit      = %x31-39

   decimal-value       = integer-value ("." zero-integer-value)

   SQUOTE              = %x27
                         ; ' (Single Quote)

   ;;
   ;; RFC 5234 core rules.
   ;;

   ALPHA               = %x41-5A / %x61-7A
                         ; A-Z / a-z

   CR                  = %x0D
                         ; carriage return

   CRLF                = CR LF
                         ; Internet standard new line

   DIGIT               = %x30-39
                         ; 0-9

   DQUOTE              = %x22
                         ; " (Double Quote)

   HEXDIG              = DIGIT /
                         %x61 / %x62 / %x63 / %x64 / %x65 / %x66
                         ; only lower-case a..f

   HTAB                = %x09
                         ; horizontal tab

   LF                  = %x0A
                         ; linefeed

   SP                  = %x20
                         ; space

   VCHAR               = %x21-7E
                         ; visible (printing) characters

   WSP                 = SP / HTAB
                         ; whitespace

   <CODE ENDS>





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RFC 6020                          YANG                      October 2010


13.  Error Responses for YANG Related Errors

   A number of NETCONF error responses are defined for error cases
   related to the data-model handling.  If the relevant YANG statement
   has an "error-app-tag" substatement, that overrides the default value
   specified below.

13.1.  Error Message for Data That Violates a unique Statement

   If a NETCONF operation would result in configuration data where a
   unique constraint is invalidated, the following error is returned:

     error-tag:      operation-failed
     error-app-tag:  data-not-unique
     error-info:     <non-unique>: Contains an instance identifier that
                     points to a leaf that invalidates the unique
                     constraint. This element is present once for each
                     non-unique leaf.

                     The <non-unique> element is in the YANG
                     namespace ("urn:ietf:params:xml:ns:yang:1").

13.2.  Error Message for Data That Violates a max-elements Statement

   If a NETCONF operation would result in configuration data where a
   list or a leaf-list would have too many entries the following error
   is returned:

     error-tag:      operation-failed
     error-app-tag:  too-many-elements

   This error is returned once, with the error-path identifying the list
   node, even if there are more than one extra child present.

13.3.  Error Message for Data That Violates a min-elements Statement

   If a NETCONF operation would result in configuration data where a
   list or a leaf-list would have too few entries the following error is
   returned:

     error-tag:      operation-failed
     error-app-tag:  too-few-elements

   This error is returned once, with the error-path identifying the list
   node, even if there are more than one child missing.






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RFC 6020                          YANG                      October 2010


13.4.  Error Message for Data That Violates a must Statement

   If a NETCONF operation would result in configuration data where the
   restrictions imposed by a "must" statement is violated the following
   error is returned, unless a specific "error-app-tag" substatement is
   present for the "must" statement.

     error-tag:      operation-failed
     error-app-tag:  must-violation

13.5.  Error Message for Data That Violates a require-instance Statement

   If a NETCONF operation would result in configuration data where a
   leaf of type "instance-identifier" marked with require-instance
   "true" refers to a non-existing instance, the following error is
   returned:

     error-tag:      data-missing
     error-app-tag:  instance-required
     error-path:     Path to the instance-identifier leaf.

13.6.  Error Message for Data That Does Not Match a leafref Type

   If a NETCONF operation would result in configuration data where a
   leaf of type "leafref" refers to a non-existing instance, the
   following error is returned:

     error-tag:      data-missing
     error-app-tag:  instance-required
     error-path:     Path to the leafref leaf.

13.7.  Error Message for Data That Violates a mandatory choice Statement

   If a NETCONF operation would result in configuration data where no
   nodes exists in a mandatory choice, the following error is returned:

     error-tag:      data-missing
     error-app-tag:  missing-choice
     error-path:     Path to the element with the missing choice.
     error-info:     <missing-choice>: Contains the name of the missing
                     mandatory choice.

                     The <missing-choice> element is in the YANG
                     namespace ("urn:ietf:params:xml:ns:yang:1").







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13.8.  Error Message for the "insert" Operation

   If the "insert" and "key" or "value" attributes are used in an
   <edit-config> for a list or leaf-list node, and the "key" or "value"
   refers to a non-existing instance, the following error is returned:

     error-tag:      bad-attribute
     error-app-tag:  missing-instance

14.  IANA Considerations

   This document defines a registry for YANG module and submodule names.
   The name of the registry is "YANG Module Names".

   The registry shall record for each entry:

   o  the name of the module or submodule

   o  for modules, the assigned XML namespace

   o  for modules, the prefix of the module

   o  for submodules, the name of the module it belongs to

   o  a reference to the (sub)module's documentation (e.g., the RFC
      number)

   There are no initial assignments.

   For allocation, RFC publication is required as per RFC 5226
   [RFC5226].  All registered YANG module names MUST comply with the
   rules for identifiers stated in Section 6.2, and MUST have a module
   name prefix.

   The module name prefix 'ietf-' is reserved for IETF stream documents
   [RFC4844], while the module name prefix 'irtf-' is reserved for IRTF
   stream documents.  Modules published in other RFC streams MUST have a
   similar suitable prefix.

   All module and submodule names in the registry MUST be unique.

   All XML namespaces in the registry MUST be unique.

   This document registers two URIs for the YANG and YIN XML namespaces
   in the IETF XML registry [RFC3688].  Following the format in RFC
   3688, the following have been registered.

     URI: urn:ietf:params:xml:ns:yang:yin:1



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     URI: urn:ietf:params:xml:ns:yang:1

     Registrant Contact: The IESG.

     XML: N/A, the requested URIs are XML namespaces.

   This document registers two new media types as defined in the
   following sections.

14.1.  Media type application/yang

  MIME media type name:  application

  MIME subtype name:  yang

  Mandatory parameters:  none

  Optional parameters:  none

  Encoding considerations:  8-bit

  Security considerations:  See Section 15 in RFC 6020

  Interoperability considerations:  None

  Published specification:  RFC 6020

  Applications that use this media type:

    YANG module validators, web servers used for downloading YANG
    modules, email clients, etc.

  Additional information:

     Magic Number:  None

     File Extension:  .yang

     Macintosh file type code:  'TEXT'

  Personal and email address for further information:
     Martin Bjorklund <mbj@tail-f.com>

  Intended usage:  COMMON

  Author:
     This specification is a work item of the IETF NETMOD working group,
     with mailing list address <netmod@ietf.org>.



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  Change controller:
     The IESG <iesg@ietf.org>

14.2.  Media type application/yin+xml

  MIME media type name:  application

  MIME subtype name:  yin+xml

  Mandatory parameters:  none

  Optional parameters:

     "charset":  This parameter has identical semantics to the charset
     parameter of the "application/xml" media type as specified in
     [RFC3023].

  Encoding considerations:

     Identical to those of "application/xml" as
     described in [RFC3023], Section 3.2.

  Security considerations:  See Section 15 in RFC 6020

  Interoperability considerations:  None

  Published specification:  RFC 6020

  Applications that use this media type:

    YANG module validators, web servers used for downloading YANG
    modules, email clients, etc.

  Additional information:

     Magic Number:  As specified for "application/xml" in [RFC3023],
                    Section 3.2.

     File Extension:  .yin

     Macintosh file type code:  'TEXT'

  Personal and email address for further information:
     Martin Bjorklund <mbj@tail-f.com>

  Intended usage:  COMMON





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RFC 6020                          YANG                      October 2010


  Author:
     This specification is a work item of the IETF NETMOD working group,
     with mailing list address <netmod@ietf.org>.

  Change controller:
     The IESG <iesg@ietf.org>

15.  Security Considerations

   This document defines a language with which to write and read
   descriptions of management information.  The language itself has no
   security impact on the Internet.

   The same considerations are relevant as for the base NETCONF protocol
   (see [RFC4741], Section 9).

   Data modeled in YANG might contain sensitive information.  RPCs or
   notifications defined in YANG might transfer sensitive information.

   Security issues are related to the usage of data modeled in YANG.
   Such issues shall be dealt with in documents describing the data
   models and documents about the interfaces used to manipulate the data
   e.g., the NETCONF documents.

   Data modeled in YANG is dependent upon:

   o  the security of the transmission infrastructure used to send
      sensitive information.

   o  the security of applications that store or release such sensitive
      information.

   o  adequate authentication and access control mechanisms to restrict
      the usage of sensitive data.

   YANG parsers need to be robust with respect to malformed documents.
   Reading malformed documents from unknown or untrusted sources could
   result in an attacker gaining privileges of the user running the YANG
   parser.  In an extreme situation, the entire machine could be
   compromised.











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16.  Contributors

   The following people all contributed significantly to the initial
   YANG document:

    - Andy Bierman (Brocade)
    - Balazs Lengyel (Ericsson)
    - David Partain (Ericsson)
    - Juergen Schoenwaelder (Jacobs University Bremen)
    - Phil Shafer (Juniper Networks)

17.  Acknowledgements

   The editor wishes to thank the following individuals, who all
   provided helpful comments on various versions of this document:
   Mehmet Ersue, Washam Fan, Joel Halpern, Leif Johansson, Ladislav
   Lhotka, Gerhard Muenz, Tom Petch, Randy Presuhn, David Reid, and Bert
   Wijnen.

18.  References

18.1.  Normative References

   [ISO.10646]  International Organization for Standardization,
                "Information Technology - Universal Multiple-Octet Coded
                Character Set (UCS)", ISO Standard 10646:2003, 2003.

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

   [RFC3023]    Murata, M., St. Laurent, S., and D. Kohn, "XML Media
                Types", RFC 3023, January 2001.

   [RFC3629]    Yergeau, F., "UTF-8, a transformation format of ISO
                10646", STD 63, RFC 3629, November 2003.

   [RFC3688]    Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
                January 2004.

   [RFC3986]    Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
                Resource Identifier (URI): Generic Syntax", STD 66,
                RFC 3986, January 2005.

   [RFC4648]    Josefsson, S., "The Base16, Base32, and Base64 Data
                Encodings", RFC 4648, October 2006.

   [RFC4741]    Enns, R., "NETCONF Configuration Protocol", RFC 4741,
                December 2006.



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   [RFC5226]    Narten, T. and H. Alvestrand, "Guidelines for Writing an
                IANA Considerations Section in RFCs", BCP 26, RFC 5226,
                May 2008.

   [RFC5234]    Crocker, D. and P. Overell, "Augmented BNF for Syntax
                Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5277]    Chisholm, S. and H. Trevino, "NETCONF Event
                Notifications", RFC 5277, July 2008.

   [XML-NAMES]  Hollander, D., Tobin, R., Thompson, H., Bray, T., and A.
                Layman, "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>.

   [XPATH]      Clark, 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>.

   [XSD-TYPES]  Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes
                Second Edition", World Wide Web Consortium
                Recommendation REC-xmlschema-2-20041028, October 2004,
                <http://www.w3.org/TR/2004/REC-xmlschema-2-20041028>.

18.2.  Informative References

   [RFC2578]    McCloghrie, K., Ed., Perkins, D., Ed., and J.
                Schoenwaelder, Ed., "Structure of Management Information
                Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.

   [RFC2579]    McCloghrie, K., Ed., Perkins, D., Ed., and J.
                Schoenwaelder, Ed., "Textual Conventions for SMIv2",
                STD 58, RFC 2579, April 1999.

   [RFC3780]    Strauss, F. and J. Schoenwaelder, "SMIng - Next
                Generation Structure of Management Information",
                RFC 3780, May 2004.

   [RFC4844]    Daigle, L. and Internet Architecture Board, "The RFC
                Series and RFC Editor", RFC 4844, July 2007.

   [XPATH2.0]   Berglund, A., Boag, S., Chamberlin, D., Fernandez, M.,
                Kay, M., Robie, J., and J. Simeon, "XML Path Language
                (XPath) 2.0", World Wide Web Consortium
                Recommendation REC-xpath20-20070123, January 2007,
                <http://www.w3.org/TR/2007/REC-xpath20-20070123>.



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   [XSLT]       Clark, J., "XSL Transformations (XSLT) Version 1.0",
                World Wide Web Consortium Recommendation REC-xslt-
                19991116, November 1999,
                <http://www.w3.org/TR/1999/REC-xslt-19991116>.

Author's Address

   Martin Bjorklund (editor)
   Tail-f Systems

   EMail: mbj@tail-f.com








































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