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Network Working Group                                  M. Bjorklund, Ed.
Internet-Draft                                            Tail-f Systems
Intended status: Standards Track                        November 3, 2008
Expires: May 7, 2009


              YANG - A data modeling language for NETCONF
                       draft-ietf-netmod-yang-02

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Copyright Notice

   Copyright (C) The IETF Trust (2008).














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Abstract

   YANG is a data modeling language used to model configuration and
   state data manipulated by the NETCONF protocol, NETCONF remote
   procedure calls, and NETCONF notifications.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   8
   2.  Key Words . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.1.  Mandatory nodes . . . . . . . . . . . . . . . . . . . . .  12
   4.  YANG Overview . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  Functional Overview . . . . . . . . . . . . . . . . . . .  13
     4.2.  Language Overview . . . . . . . . . . . . . . . . . . . .  14
       4.2.1.  Modules and Submodules  . . . . . . . . . . . . . . .  14
       4.2.2.  Data Modeling Basics  . . . . . . . . . . . . . . . .  15
       4.2.3.  Operational Data  . . . . . . . . . . . . . . . . . .  20
       4.2.4.  Built-in Types  . . . . . . . . . . . . . . . . . . .  20
       4.2.5.  Derived Types (typedef) . . . . . . . . . . . . . . .  21
       4.2.6.  Reusable Node Groups (grouping) . . . . . . . . . . .  22
       4.2.7.  Choices . . . . . . . . . . . . . . . . . . . . . . .  23
       4.2.8.  Extending Data Models (augment) . . . . . . . . . . .  24
       4.2.9.  RPC Definitions . . . . . . . . . . . . . . . . . . .  25
       4.2.10. Notification Definitions  . . . . . . . . . . . . . .  26
   5.  Language Concepts . . . . . . . . . . . . . . . . . . . . . .  28
     5.1.  Modules and Submodules  . . . . . . . . . . . . . . . . .  28
       5.1.1.  Module Hierarchies  . . . . . . . . . . . . . . . . .  28
     5.2.  File Layout . . . . . . . . . . . . . . . . . . . . . . .  28
     5.3.  Object Based View of YANG . . . . . . . . . . . . . . . .  29
     5.4.  XML Namespaces  . . . . . . . . . . . . . . . . . . . . .  29
       5.4.1.  YANG Namespace  . . . . . . . . . . . . . . . . . . .  30
     5.5.  Ordering  . . . . . . . . . . . . . . . . . . . . . . . .  30
     5.6.  Containers with Presence  . . . . . . . . . . . . . . . .  31
     5.7.  Scoping . . . . . . . . . . . . . . . . . . . . . . . . .  32
     5.8.  Nested Typedefs and Groupings . . . . . . . . . . . . . .  32
     5.9.  Conformance . . . . . . . . . . . . . . . . . . . . . . .  32
       5.9.1.  Basic Behavior  . . . . . . . . . . . . . . . . . . .  33
       5.9.2.  Optional Features . . . . . . . . . . . . . . . . . .  33
       5.9.3.  Deviations  . . . . . . . . . . . . . . . . . . . . .  34
       5.9.4.  Announcing Conformance Information in the <hello>
               Message . . . . . . . . . . . . . . . . . . . . . . .  34
   6.  YANG syntax . . . . . . . . . . . . . . . . . . . . . . . . .  36
     6.1.  Lexicographical Tokenization  . . . . . . . . . . . . . .  36
       6.1.1.  Comments  . . . . . . . . . . . . . . . . . . . . . .  36
       6.1.2.  Tokens  . . . . . . . . . . . . . . . . . . . . . . .  36
       6.1.3.  Quoting . . . . . . . . . . . . . . . . . . . . . . .  36



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     6.2.  Identifiers . . . . . . . . . . . . . . . . . . . . . . .  38
       6.2.1.  Identifiers and their namespaces  . . . . . . . . . .  38
     6.3.  Statements  . . . . . . . . . . . . . . . . . . . . . . .  39
       6.3.1.  Language Extensions . . . . . . . . . . . . . . . . .  39
     6.4.  XPath Evaluations . . . . . . . . . . . . . . . . . . . .  39
   7.  YANG Statements . . . . . . . . . . . . . . . . . . . . . . .  40
     7.1.  The module Statement  . . . . . . . . . . . . . . . . . .  40
       7.1.1.  The module's Substatements  . . . . . . . . . . . . .  41
       7.1.2.  The yang-version Statement  . . . . . . . . . . . . .  42
       7.1.3.  The namespace Statement . . . . . . . . . . . . . . .  43
       7.1.4.  The prefix Statement  . . . . . . . . . . . . . . . .  43
       7.1.5.  The import Statement  . . . . . . . . . . . . . . . .  43
       7.1.6.  The include Statement . . . . . . . . . . . . . . . .  44
       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 . . . . . . . . . . . . . . . . .  45
       7.2.1.  The submodule's Substatements . . . . . . . . . . . .  47
       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.  The container's Substatements . . . . . . . . . . . .  52
       7.5.2.  The must Statement  . . . . . . . . . . . . . . . . .  52
       7.5.3.  The must's Substatements  . . . . . . . . . . . . . .  53
       7.5.4.  The presence Statement  . . . . . . . . . . . . . . .  54
       7.5.5.  The container's Child Node Statements . . . . . . . .  55
       7.5.6.  XML Encoding Rules  . . . . . . . . . . . . . . . . .  55
       7.5.7.  NETCONF <edit-config> Operations  . . . . . . . . . .  55
       7.5.8.  Usage Example . . . . . . . . . . . . . . . . . . . .  56
     7.6.  The leaf Statement  . . . . . . . . . . . . . . . . . . .  57
       7.6.1.  The leaf's Substatements  . . . . . . . . . . . . . .  58
       7.6.2.  The leaf's type Statement . . . . . . . . . . . . . .  58
       7.6.3.  The leaf's default Statement  . . . . . . . . . . . .  58
       7.6.4.  The leaf's mandatory Statement  . . . . . . . . . . .  58
       7.6.5.  XML Encoding Rules  . . . . . . . . . . . . . . . . .  59
       7.6.6.  NETCONF <edit-config> Operations  . . . . . . . . . .  59
       7.6.7.  Usage Example . . . . . . . . . . . . . . . . . . . .  59
     7.7.  The leaf-list Statement . . . . . . . . . . . . . . . . .  60
       7.7.1.  The leaf-list's Substatements . . . . . . . . . . . .  61



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       7.7.2.  The min-elements Statement  . . . . . . . . . . . . .  61
       7.7.3.  The max-elements Statement  . . . . . . . . . . . . .  61
       7.7.4.  The ordered-by Statement  . . . . . . . . . . . . . .  61
       7.7.5.  XML Encoding Rules  . . . . . . . . . . . . . . . . .  62
       7.7.6.  NETCONF <edit-config> operations  . . . . . . . . . .  62
       7.7.7.  Usage Example . . . . . . . . . . . . . . . . . . . .  63
     7.8.  The list Statement  . . . . . . . . . . . . . . . . . . .  65
       7.8.1.  The list's Substatements  . . . . . . . . . . . . . .  65
       7.8.2.  The list's key Statement  . . . . . . . . . . . . . .  66
       7.8.3.  The lists's unique Statement  . . . . . . . . . . . .  67
       7.8.4.  The list's Child Node Statements  . . . . . . . . . .  68
       7.8.5.  XML Encoding Rules  . . . . . . . . . . . . . . . . .  68
       7.8.6.  NETCONF <edit-config> operations  . . . . . . . . . .  68
       7.8.7.  Usage Example . . . . . . . . . . . . . . . . . . . .  69
     7.9.  The choice Statement  . . . . . . . . . . . . . . . . . .  72
       7.9.1.  The choice's Substatements  . . . . . . . . . . . . .  73
       7.9.2.  The choice's case Statement . . . . . . . . . . . . .  73
       7.9.3.  The choice's default Statement  . . . . . . . . . . .  74
       7.9.4.  The choice's mandatory Statement  . . . . . . . . . .  76
       7.9.5.  XML Encoding Rules  . . . . . . . . . . . . . . . . .  76
       7.9.6.  NETCONF <edit-config> operations  . . . . . . . . . .  76
       7.9.7.  Usage Example . . . . . . . . . . . . . . . . . . . .  76
     7.10. The anyxml Statement  . . . . . . . . . . . . . . . . . .  77
       7.10.1. The anyxml's Substatements  . . . . . . . . . . . . .  78
       7.10.2. XML Encoding Rules  . . . . . . . . . . . . . . . . .  78
       7.10.3. NETCONF <edit-config> operations  . . . . . . . . . .  78
       7.10.4. Usage Example . . . . . . . . . . . . . . . . . . . .  79
     7.11. The grouping Statement  . . . . . . . . . . . . . . . . .  79
       7.11.1. The grouping's Substatements  . . . . . . . . . . . .  80
       7.11.2. Usage Example . . . . . . . . . . . . . . . . . . . .  80
     7.12. The uses Statement  . . . . . . . . . . . . . . . . . . .  80
       7.12.1. The uses's Substatements  . . . . . . . . . . . . . .  81
       7.12.2. The refine Statement  . . . . . . . . . . . . . . . .  81
       7.12.3. XML Encoding Rules  . . . . . . . . . . . . . . . . .  82
       7.12.4. Usage Example . . . . . . . . . . . . . . . . . . . .  82
     7.13. The rpc Statement . . . . . . . . . . . . . . . . . . . .  83
       7.13.1. The rpc's Substatements . . . . . . . . . . . . . . .  84
       7.13.2. The input Statement . . . . . . . . . . . . . . . . .  84
       7.13.3. The output Statement  . . . . . . . . . . . . . . . .  85
       7.13.4. XML Encoding Rules  . . . . . . . . . . . . . . . . .  86
       7.13.5. Usage Example . . . . . . . . . . . . . . . . . . . .  86
     7.14. The notification Statement  . . . . . . . . . . . . . . .  87
       7.14.1. The notification's Substatements  . . . . . . . . . .  88
       7.14.2. XML Encoding Rules  . . . . . . . . . . . . . . . . .  88
       7.14.3. Usage Example . . . . . . . . . . . . . . . . . . . .  88
     7.15. The augment Statement . . . . . . . . . . . . . . . . . .  89
       7.15.1. The augment's Substatements . . . . . . . . . . . . .  90
       7.15.2. XML Encoding Rules  . . . . . . . . . . . . . . . . .  90



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       7.15.3. Usage Example . . . . . . . . . . . . . . . . . . . .  91
     7.16. The identity Statement  . . . . . . . . . . . . . . . . .  93
       7.16.1. The identity's Substatements  . . . . . . . . . . . .  93
       7.16.2. The base Statement  . . . . . . . . . . . . . . . . .  93
       7.16.3. Usage Example . . . . . . . . . . . . . . . . . . . .  94
     7.17. The extension Statement . . . . . . . . . . . . . . . . .  94
       7.17.1. The extension's Substatements . . . . . . . . . . . .  95
       7.17.2. The argument Statement  . . . . . . . . . . . . . . .  95
       7.17.3. Usage Example . . . . . . . . . . . . . . . . . . . .  96
     7.18. Conformance-related Statements  . . . . . . . . . . . . .  96
       7.18.1. The feature Statement . . . . . . . . . . . . . . . .  96
       7.18.2. The if-feature Statement  . . . . . . . . . . . . . .  98
       7.18.3. The deviation Statement . . . . . . . . . . . . . . .  98
     7.19. Common Statements . . . . . . . . . . . . . . . . . . . . 101
       7.19.1. The config Statement  . . . . . . . . . . . . . . . . 101
       7.19.2. The status Statement  . . . . . . . . . . . . . . . . 101
       7.19.3. The description Statement . . . . . . . . . . . . . . 102
       7.19.4. The reference Statement . . . . . . . . . . . . . . . 102
       7.19.5. The when Statement  . . . . . . . . . . . . . . . . . 102
   8.  Constraints . . . . . . . . . . . . . . . . . . . . . . . . . 104
   9.  Built-in Types  . . . . . . . . . . . . . . . . . . . . . . . 105
     9.1.  Canonical representation  . . . . . . . . . . . . . . . . 105
     9.2.  The Integer Built-in Types  . . . . . . . . . . . . . . . 105
       9.2.1.  Lexicographic Representation  . . . . . . . . . . . . 106
       9.2.2.  Canonical Form  . . . . . . . . . . . . . . . . . . . 106
       9.2.3.  Restrictions  . . . . . . . . . . . . . . . . . . . . 106
       9.2.4.  The range Statement . . . . . . . . . . . . . . . . . 107
       9.2.5.  Usage Example . . . . . . . . . . . . . . . . . . . . 108
     9.3.  The Floating Point Built-in Types . . . . . . . . . . . . 108
       9.3.1.  Lexicographic Representation  . . . . . . . . . . . . 108
       9.3.2.  Canonical Form  . . . . . . . . . . . . . . . . . . . 108
       9.3.3.  Restrictions  . . . . . . . . . . . . . . . . . . . . 108
       9.3.4.  Usage Example . . . . . . . . . . . . . . . . . . . . 109
     9.4.  The string Built-in Type  . . . . . . . . . . . . . . . . 109
       9.4.1.  Lexicographic Representation  . . . . . . . . . . . . 109
       9.4.2.  Canonical Form  . . . . . . . . . . . . . . . . . . . 109
       9.4.3.  Restrictions  . . . . . . . . . . . . . . . . . . . . 109
       9.4.4.  The length Statement  . . . . . . . . . . . . . . . . 109
       9.4.5.  Usage Example . . . . . . . . . . . . . . . . . . . . 110
       9.4.6.  The pattern Statement . . . . . . . . . . . . . . . . 111
       9.4.7.  Usage Example . . . . . . . . . . . . . . . . . . . . 111
     9.5.  The boolean Built-in Type . . . . . . . . . . . . . . . . 111
       9.5.1.  Lexicographic Representation  . . . . . . . . . . . . 112
       9.5.2.  Restrictions  . . . . . . . . . . . . . . . . . . . . 112
     9.6.  The enumeration Built-in Type . . . . . . . . . . . . . . 112
       9.6.1.  Lexicographic Representation  . . . . . . . . . . . . 112
       9.6.2.  Canonical Form  . . . . . . . . . . . . . . . . . . . 112
       9.6.3.  Restrictions  . . . . . . . . . . . . . . . . . . . . 112



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       9.6.4.  The enum Statement  . . . . . . . . . . . . . . . . . 112
       9.6.5.  Usage Example . . . . . . . . . . . . . . . . . . . . 113
     9.7.  The bits Built-in Type  . . . . . . . . . . . . . . . . . 114
       9.7.1.  Restrictions  . . . . . . . . . . . . . . . . . . . . 114
       9.7.2.  Lexicographic Representation  . . . . . . . . . . . . 114
       9.7.3.  Canonical Form  . . . . . . . . . . . . . . . . . . . 114
       9.7.4.  The bit Statement . . . . . . . . . . . . . . . . . . 114
       9.7.5.  Usage Example . . . . . . . . . . . . . . . . . . . . 115
     9.8.  The binary Built-in Type  . . . . . . . . . . . . . . . . 115
       9.8.1.  Restrictions  . . . . . . . . . . . . . . . . . . . . 116
       9.8.2.  Lexicographic Representation  . . . . . . . . . . . . 116
       9.8.3.  Canonical Form  . . . . . . . . . . . . . . . . . . . 116
     9.9.  The keyref Built-in Type  . . . . . . . . . . . . . . . . 116
       9.9.1.  Restrictions  . . . . . . . . . . . . . . . . . . . . 116
       9.9.2.  The path Statement  . . . . . . . . . . . . . . . . . 116
       9.9.3.  Lexicographic Representation  . . . . . . . . . . . . 117
       9.9.4.  Canonical Form  . . . . . . . . . . . . . . . . . . . 117
       9.9.5.  Usage Example . . . . . . . . . . . . . . . . . . . . 117
     9.10. The identityref Built-in Type . . . . . . . . . . . . . . 118
       9.10.1. Restrictions  . . . . . . . . . . . . . . . . . . . . 118
       9.10.2. The identityref's base Statement  . . . . . . . . . . 119
       9.10.3. Lexicographic Representation  . . . . . . . . . . . . 119
       9.10.4. Usage Example . . . . . . . . . . . . . . . . . . . . 119
     9.11. The empty Built-in Type . . . . . . . . . . . . . . . . . 120
       9.11.1. Restrictions  . . . . . . . . . . . . . . . . . . . . 120
       9.11.2. Lexicographic Representation  . . . . . . . . . . . . 120
       9.11.3. Canonical Representation  . . . . . . . . . . . . . . 120
       9.11.4. Usage Example . . . . . . . . . . . . . . . . . . . . 120
     9.12. The union Built-in Type . . . . . . . . . . . . . . . . . 120
       9.12.1. Restrictions  . . . . . . . . . . . . . . . . . . . . 121
       9.12.2. Lexicographic Representation  . . . . . . . . . . . . 121
       9.12.3. Canonical Form  . . . . . . . . . . . . . . . . . . . 121
     9.13. The instance-identifier Built-in Type . . . . . . . . . . 121
       9.13.1. Restrictions  . . . . . . . . . . . . . . . . . . . . 121
       9.13.2. Lexicographic Representation  . . . . . . . . . . . . 121
       9.13.3. Canonical Form  . . . . . . . . . . . . . . . . . . . 122
       9.13.4. Usage Example . . . . . . . . . . . . . . . . . . . . 122
   10. Updating a Module . . . . . . . . . . . . . . . . . . . . . . 123
   11. YIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
     11.1. Formal YIN Definition . . . . . . . . . . . . . . . . . . 126
     11.2. Transformation Algorithm YANG-2-YIN . . . . . . . . . . . 126
       11.2.1. Usage Example . . . . . . . . . . . . . . . . . . . . 128
     11.3. Transformation Algorithm YIN-2-YANG . . . . . . . . . . . 129
       11.3.1. Tabulation, Formatting  . . . . . . . . . . . . . . . 129
   12. YANG ABNF Grammar . . . . . . . . . . . . . . . . . . . . . . 130
   13. Error Responses for YANG Related Errors . . . . . . . . . . . 151
     13.1. Error Message for Data that Violates a YANG unique
           Statement:  . . . . . . . . . . . . . . . . . . . . . . . 151



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     13.2. Error Message for Data that Violates a YANG
           max-elements Statement: . . . . . . . . . . . . . . . . . 151
     13.3. Error Message for Data that Violates a YANG
           min-elements Statement: . . . . . . . . . . . . . . . . . 151
     13.4. Error Message for Data that Violates a YANG must
           statement:  . . . . . . . . . . . . . . . . . . . . . . . 152
     13.5. Error Message for the "insert" Operation  . . . . . . . . 152
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 153
   15. Security Considerations . . . . . . . . . . . . . . . . . . . 154
   16. Contributors  . . . . . . . . . . . . . . . . . . . . . . . . 155
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . . 156
     17.1. Normative References  . . . . . . . . . . . . . . . . . . 156
     17.2. Non-Normative References  . . . . . . . . . . . . . . . . 157
   Appendix A.  ChangeLog  . . . . . . . . . . . . . . . . . . . . . 158
     A.1.  Version -02 . . . . . . . . . . . . . . . . . . . . . . . 158
     A.2.  Version -01 . . . . . . . . . . . . . . . . . . . . . . . 158
     A.3.  Version -00 . . . . . . . . . . . . . . . . . . . . . . . 159
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 160
   Intellectual Property and Copyright Statements  . . . . . . . . . 161
































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1.  Introduction

   Today, the NETCONF protocol [RFC4741] lacks a standardized way to
   create data models.  Instead, vendors are forced to use proprietary
   solutions.  In order for NETCONF to be a interoperable protocol,
   models must be defined in a vendor-neutral way.  YANG provides the
   language and rules for defining such models for use with NETCONF.

   YANG is a data modeling language used to model configuration and
   state data manipulated by the NETCONF protocol, NETCONF remote
   procedure calls, and NETCONF notifications.  This document describes
   the syntax and semantics of the YANG language, how the data model
   defined in a YANG module is represented in XML, and how NETCONF
   operations are used to manipulate the data.





































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2.  Key Words

   The key words "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].













































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3.  Terminology

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

   o  augment: Adds new nodes to a previously defined 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 node where only one of a number of identified
      alternative values 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 the
      model.

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

   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 model object: A definition within a module that represents a
      construct which can be accessed via a network management protocol.
      Also called an object.

   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 which 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.



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   o  extension: An extension attaches non-YANG semantics to nodes.  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 which support that feature.

   o  grouping: A reusable set of nodes, which may be used locally in
      the module, in modules which include it, and by other modules
      which import from it.

   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 node in the data tree with 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.

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

   o  MIB: A Management Information Base, traditionally referring to a
      management information defined using SNMP's SMI.

   o  module: A YANG module defines a hierarchy of nodes which 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 method: A specific Remote Procedure Call, as used within the
      NETCONF protocol.  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.





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   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 which contributes derived
      types, groupings, data nodes, RPCs, and notifications to a module.
      A YANG module can be constructed from a number of submodules.

   o  uses: The "uses" statement is used to instantiate the set of 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 or choice 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.

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






















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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 nodes which 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 extended, allowing one module to
   add data nodes to the hierarchy defined in another module.  This
   augmentation can be conditional, with new nodes to 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 complex types using reusable grouping
   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.

   YANG organizational constructs include defining lists of nodes with
   the same names and identifying the keys which distinguish list
   members 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
   nodes.



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   YANG modules can be translated into an XML format called YIN
   (Section 11), allowing applications using XML parsers and XSLT
   scripts to operate on the models.

   YANG strikes a balance between high-level object-oriented modeling
   and low-level bits-on-the-wire encoding.  The reader of a YANG module
   can easily see the high-level view of the data model while seeing how
   the object 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 making extensions 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 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 device's native
   management infrastructure.  This allows implementations to leverage
   their existing access control mechanisms to protect or expose
   elements of the data model.

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.

4.2.1.  Modules and Submodules

   YANG defines modules using the "module" statement.  This statement
   defines the name of the module, which is typically used as the base
   name of the file containing the module.  The file suffix ".yang" is
   typically used for YANG files.  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



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   definition statements are the body of the module where the data model
   is defined.

   Submodule are partial modules that contribute derived types,
   groupings, data nodes, RPCs and notifications to a module.  A module
   may include a number of submodules, but each submodule may belong to
   only one module.  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.

   To reference an item that is defined in an external module it MUST be
   imported.  Identifiers that are neither defined, imported nor
   included MUST NOT be visible in the local module.

   To reference an item that is defined in one of its submodules, the
   module MUST include the submodule.

   A submodule that needs to reference an item defined in another
   submodule of the same module, MUST include this 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.  References to built-in data types (e.g., int32)
   MUST NOT use the prefix notation.

   Forward references are allowed in YANG.

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 Encoding:

       <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 Encoding:

     <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";
             }
         }
     }

   NETCONF XML Encoding:



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

   The "container" statement is covered in Section 7.5.

4.2.2.4.  List Nodes

   A list is a sequence of list entries.  An entry is like a structure
   or a record.  A list entry is uniquely identified by the values of
   its key leafs.  A list entry can have multiple keys.  A list entry
   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;
         }
     }

   NETCONF XML Encoding:

     <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>rzull</name>
       <full-name>Repun Zell</full-name>
       <class>tower</class>
     </user>



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   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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     // 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.  Operational Data

   YANG can model operational data, as well as configuration data, based
   on the "config" statement.  When a node is tagged with "config
   false", its subhierarchy is flagged as operational 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 operational 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 cannot be manipulated using <edit-config>.

     list interface {
         key "name";
         config true;

         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                | Type        | Description                   |
   +---------------------+-------------+-------------------------------+
   | binary              | Text        | Any binary data               |
   | bits                | Text/Number | A set of bits or flags        |
   | boolean             | Text        | "true" or "false"             |
   | empty               | Empty       | A leaf that does not have any |
   |                     |             | value                         |
   | enumeration         | Text/Number | Enumerated strings with       |
   |                     |             | associated numeric values     |
   | float32             | Number      | 32-bit IEEE floating point    |
   |                     |             | real number                   |
   | float64             | Number      | 64-bit IEEE floating point    |
   |                     |             | real number                   |
   | identityref         | Text        | A reference to an abstract    |
   |                     |             | identity                      |
   | instance-identifier | Text        | References a data tree node   |
   | int8                | Number      | 8-bit signed integer          |
   | int16               | Number      | 16-bit signed integer         |
   | int32               | Number      | 32-bit signed integer         |
   | int64               | Number      | 64-bit signed integer         |
   | keyref              | Text/Number | A reference to a list's key   |
   |                     |             | value                         |
   | string              | Text        | Human readable string         |
   | uint8               | Number      | 8-bit unsigned integer        |
   | uint16              | Number      | 16-bit unsigned integer       |
   | uint32              | Number      | 32-bit unsigned integer       |
   | uint64              | Number      | 64-bit unsigned integer       |
   | union               | Text/Number | Choice of member types        |
   +---------------------+-------------+-------------------------------+

   The "type" statement is covered in Section 9.

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:









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     typedef percent {
         type uint16 {
             range "0 .. 100";
         }
         description "Percentage";
     }

     leaf completed {
         type percent;
     }

   NETCONF XML Encoding:

     <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 the equivalent of complex types
   using the "grouping" statement. "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 Encoding:

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



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

     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 which 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 XML encoding.  The additional levels of hierarchy are
   not needed beyond the conceptual schema.

   YANG Example:



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     container food {
       choice snack {
           mandatory true;
           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 Encoding:

     <food>
       <chocolate>first-available</chocolate>
     </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 e.g. 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.

   YANG Example:








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

   NETCONF XML Encoding:

     <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 method names, input
   parameters and output parameters are modeled using YANG data
   definition statements.

   YANG Example:

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



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   NETCONF XML Encoding:

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

   YANG Example:

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

   NETCONF XML Encoding:

     <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>
       </link-failure>
     </notification>



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   The "notification" statement is covered in Section 7.14.


















































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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.

   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.  Any modules that are
   implemented MUST be available for all defined datastores.

   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.

   A module uses the "include" statement to include its submodules, and
   the "import" statement to reference external modules.  Similarly, a
   submodule may use the "import" statement to reference other modules,
   and may use the "include" statement to reference other submodules
   within its module.  A module or submodule may not include submodules
   from other modules, nor may a submodule import its own module.

   The names of all standard modules must be unique, but different
   revisions of the same module should have the same name.  Developers
   of enterprise modules are encouraged 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.

5.1.1.  Module Hierarchies

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

5.2.  File Layout

   YANG modules and submodules are typically stored in files, one module
   or submodule per file, with the name of the file given by the
   concatenation of the module or submodule name and the file suffix
   ".yang".  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.  Many errors and warnings that cannot be
   detected during submodule compilation may be delayed until the



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   submodules are linked into a cohesive module.

5.3.  Object Based View of YANG

   While YANG models the configuration as a data tree, it can be used in
   an object-based manner as well.

   The configuration and state data of the device is modeled as a tree
   of object instances (objects for short).  Each object in the tree has
   a type name (or managed object class name), a namespace, a (possibly
   empty) set of attributes and a (possibly empty) set of child objects.

   A managed object class could be defined as a grouping, containing
   just one list.  Attributes should be defined as leafs inside the
   list.  Child objects should be defined with the corresponding uses
   statements.

   A defined grouping unambiguously defines its properties, it has its
   own unique name, so when it is referred to in the "uses" statement it
   is always the same well defined set of properties that we are using.

   The data tree can be defined as one or more top level containers
   containing managed object classes defined as groupings.  All further
   levels of the data tree are defined by managed object classes
   containing further managed objects.

5.4.  XML Namespaces

   All YANG definitions are specified within a particular XML Namespace.
   Each module defines an XML namespace as a globally unique URI
   [RFC3986].  A NETCONF client or server uses the namespace during XML
   encoding of data.

   The namespace URI is 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 on 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-number
     module-parameter    = "module=" module-name
     feature-parameter   = "features=" feature *( "," feature )
     deviation-parameter = "deviations=" deviation *( "," deviation )




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   Where "revision-number" is the revision of the module (see
   Section 7.1.9) that the 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 for the module as it appears in the
   "namespace" statement, "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).

   Namespaces for standard module names will be assigned by IANA.  They
   MUST be unique (but different revisions of the same module should
   have the same namespace).

   Namespaces for private module names will be assigned by the
   organization owning the module without a central registry.  It is
   recommended to choose namespaces that will have a low probability of
   colliding with standard or other enterprise modules, e.g. by using
   the enterprise or organization name in the namespace.

   The "namespace" statement is covered in Section 7.1.3.

5.4.1.  YANG Namespace

   YANG defines its own namespace for NETCONF <edit-config> operations.
   This namespace is "urn:ietf:params:xml:ns:yang:1" [XXX IANA].

5.5.  Ordering

   YANG supports two styles for ordering the entries within a list.  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.  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



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   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 which allow 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.

   The "ordered-by" statement is covered in Section 7.7.4.

5.6.  Containers with Presence

   YANG supports two styles of containers, those which 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.  The container data node is implicitly
   created when the first child data node is created.  The data node is
   implicitly deleted when the last non-key child is deleted, since an
   empty container has no meaning.

   For example, the set of scrambling options for 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 the task of 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 they are 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 is covered in Section 7.5.4.





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5.7.  Scoping

   YANG uses static scoping.  Grouping definitions are resolved in the
   context in which they are defined, rather than the context in which
   they are used.  Users of groupings are not required to import modules
   or include submodules to satisfy all references made by the grouping.

   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 original module, not the second module.  There is no
   worry over conflicts if both modules define the type, since there is
   no ambiguity.

5.8.  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.

   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 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 group cannot be defined if a higher level in the schema
   hierarchy has a definition with a matching identifier.

   When a YANG implementation resolves a reference to an unprefixed type
   or grouping, or one which uses the prefix of the local module, it
   searches up the levels of hierarchy in the schema tree, starting at
   the current level, for the definition of the type or grouping.

5.9.  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



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   implement the model identically.  Deviations from the model can
   reduce the utility of the model and increase fragility into
   applications that use it.

   YANG modelers have three levels of 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.

5.9.1.  Basic Behavior

   The model defines a contract between the 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 and the mindless devotion with which
   implementers follow it.

5.9.2.  Optional Features

   In many models, the modeler will allow sections of the model to be
   conditional, based on the device.  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
   "features".  A module may declare any number of features, identified
   by simple strings, and may make portions of the module optional based
   on those feature.  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 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 which 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.



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   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.

5.9.3.  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
   willing to implement the model as written.  For YANG-based 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 which 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.

5.9.4.  Announcing Conformance Information in the <hello> Message

   Devices indicate the names of supported features and device
   deviations 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 which the
   device supports for the specific module.

     <hello>
       <capability>
         http://example.com/mod/mcp?features=feat1&module=mcp
       </capability>
       <capability>
         http://example.com/mod/some?module=some&features=one,two,three
       </capability>
     </hello>

   Device deviations are announced via the "deviations" parameter.  The
   value of the deviations parameter is a comma-separated list of



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   modules containing deviations from the capability's module.

     <hello>
       <capability>
         http://example.com/abc?deviations=my-devs
       </capability>
     </hello>












































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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 are written in the UTF-8 [RFC3629] character set.

6.1.  Lexicographical 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, ";", "{", or "}".  A
   string can be quoted or unquoted.  A keyword is either one of the
   core YANG keywords defined in this document, or a 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 whitespace characters, a semicolon (";"),
   curly 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
   whitespace which is used to indent the text according to the layout
   in the YANG file, this leading whitespace is stripped from the
   string, up to at most the same column of the double quote character.

   If the double quoted string contains whitespace before a line break,
   this trailing whitespace is stripped from the string.




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

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

   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

6.1.3.1.  Quoting Examples

   The following strings are equivalent:

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

   The following examples show some special strings:

     "\""  - string containing a double quote
     '"'   - string containing a double quote
     "\n"  - string containing a newline 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"



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6.2.  Identifiers

   Identifiers are used to identify different kinds of YANG items by
   name.  Each identifier starts with an upper-case or lower-case 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 63 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 which depends on the type of
   the YANG item being defined:

   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.

   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 the parent node
      or module.

   o  All groupings 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 the parent node or module.

   o  All leafs, leaf-lists, lists, containers, choices, rpcs, and
      notifications defined 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 parent node of the case node's parent choice node.

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

   All identifiers defined in a namespace MUST be unique.



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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 curly 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.

   Since submodules cannot include the parent module, any extensions in
   the module which 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.

6.4.  XPath Evaluations

   YANG relies on 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 valid, but any implementation may choose to
   implement them by hand, rather than using the XPath expression
   directly.

   XPath expressions are evaluated in the context of the current node,
   with the namespace of the current module defined as the null
   namespace.  References to identifiers in external modules MUST be
   qualified with appropriate prefixes, and references to the current
   module and its submodules MAY use a prefix.









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7.  YANG Statements

   The following sections describe all of the YANG core statements.

   Note that even a statement which does not have any substatements
   defined in core YANG can have vendor-specific extensions as
   substatements.  For example, the "description" statement does not
   have any substatements defined in core 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 which 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.

   Standard module names will be assigned by IANA.  The names of all
   standard modules MUST be unique (but different revisions of the same
   module should have the same name).

   Private module names will be assigned by the organization owning the
   module without a central registry.  It is 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.

   A module SHOULD have the following layout:


















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     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
         <extension statements>
         <feature statements>
         <typedef statements>
         <grouping statements>
         <container statements>
         <leaf statements>
         <leaf-list statements>
         <list statements>
         <choice statements>
         <uses statements>
         <rpc statements>
         <notification statements>
         <augment statements>
         <deviation statements>
     }

7.1.1.  The module's Substatements













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                 +--------------+---------+-------------+
                 | 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        |
                 | 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 "yang-version" statement specifies which version of the YANG
   language was used in developing the module.  The statement's argument
   contains value "1", which is the current yang version and the default
   value.

   This statement is intended for future-proofing the syntax of YANG
   against possible changes in later versions of YANG.  Since the
   current version is the default value, the statement need not appear
   in YANG modules until a future version is defined.  When a new
   version is defined, YANG modules can either use version 2 statements
   and add the "yang-version 2" statement, or remain within the version
   1 feature set and continue to use the default setting of "yang-
   version 1".






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7.1.3.  The namespace Statement

   The "namespace" statement defines the XML namespace for all XML
   elements defined by the module.  Its argument is the URI of the
   namespace.

   See also Section 5.4.

7.1.4.  The prefix Statement

   The "prefix" statement is used to define the prefix associated with
   the namespace of a module.  The "prefix" statement's argument is the
   prefix string which 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
   which generates XML or XPath that use prefixes, the prefix defined by
   a module SHOULD be used, 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 module from which objects
   are being imported 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.

   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.

   All identifiers contained in an imported module are imported into the
   current module or submodule, so that they can be referenced by
   definitions in the current module or submodule.  The mandatory
   "prefix" substatement assigns a prefix for the imported module which
   is scoped to the importing module or submodule.  Multiple "import"
   statements may be specified to import from different modules.



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                        The import's Substatements

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

7.1.6.  The include Statement

   The "include" statement is used to make content from a submodule
   available to the module.  The argument is an identifier which is the
   name of the submodule to include.  Modules are only allowed to
   include submodules that belong to that module, as defined by the
   "belongs-to" statement (see Section 7.2.2).

   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.

7.1.7.  The organization Statement

   The "organization" statement defines the party responsible for this
   module.  The argument is a string which 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 which is used to specify the name, postal
   address, telephone number, and electronic mail address of the person
   to whom technical queries concerning this module should be sent.

7.1.9.  The revision Statement

   The "revision" statement specifies the editorial revision history of
   the module, including the initial revision.  A series of revisions
   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
   every editorial change, a new one SHOULD be added in front of the
   revisions sequence, so that all revisions are in reverse
   chronological order.





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7.1.9.1.  The revision's Substatement

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

7.1.10.  Usage Example

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

         import 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 0815
              EMail: joe@acme.example.com";

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

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

         // definitions follows...
     }

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 to split
   a complex module in several pieces where all the submodules
   contribute to a single namespace, which is defined by the module



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   including the submodules.

   The "submodule" statement is used to give the submodule a name, and
   to group all statements which belong to the submodule together.

   The "submodule" statement, which must be present at most once, takes
   as an argument an identifier which is the name of the submodule,
   followed by a block of substatements that hold detailed submodule
   information.

   Standard submodule names will be assigned by IANA.  Name of all
   standard submodules must be unique and in addition not conflict with
   module names (but different revisions of the same submodule should
   have the same name).

   Private submodule names will be assigned by the organization owning
   the submodule without a central registry.  It is recommended to
   choose names for their submodules 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.

   A submodule SHOULD have the following layout:

     submodule <module-name> {


         <yang-version statement>
























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         // 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
         <extension statements>
         <feature statements>
         <typedef statements>
         <grouping statements>
         <container statements>
         <leaf statements>
         <leaf-list statements>
         <list statements>
         <choice statements>
         <uses statements>
         <rpc statements>
         <notification statements>
         <augment statements>
         <deviation statements>
     }

7.2.1.  The submodule's Substatements

















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                 +--------------+---------+-------------+
                 | 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        |
                 | 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 which is the name
   of the module.  Only the module to which a submodule belongs, or
   another submodule that belongs to the same module, are allowed to
   include that submodule.

   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 yang-types {
             prefix "yang";
         }

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

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

              Phone: +1 800 555 0815
              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 which may be used locally
   in the module, in modules or submodules which include it, and by
   other modules which import from it.  The new type is called the
   "derived type", and the type from which it was derived is called the



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   "base type".  All derived types can be traced back to a YANG built-in
   type.

   The "typedef" statement's argument is an identifier which 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.  For details about scoping for nested typedef, see
   Section 5.8.

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 which 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 which contains
   a default value for the new type.

   The value of the "default" statement MUST correspond 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.  The default value



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   MUST correspond to any restrictions in the derived type.

   If the base type's default value does not correspond to the new
   restrictions, the derived type MUST define a new default value.

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 which 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 depends on the type being
   restricted.  All restriction statements are described in conjunction
   with the built-in types in 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        |
                 | type         | 7.4     | 0..n        |
                 +--------------+---------+-------------+

7.5.  The container Statement

   The "container" statement is used to define an interior 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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   By default, a container does not carry any information, but is used
   to organize and give structure to the data being defined.  The
   "presence" statement (see Section 7.5.4) is used to give semantics to
   the existence of the container in the data tree.

7.5.1.  The container's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | augment      | 7.15    | 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         | 7.5.2   | 0..n        |
                 | presence     | 7.5.4   | 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        |
                 +--------------+---------+-------------+

7.5.2.  The must Statement

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

   When a data is validated, all "must" constraints are conceptually
   evaluated once for each corresponding instance in the data tree, and
   for all leafs with default values in effect.  If an instance 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 "must" statement is ignored if the data does not represent
   configuration.




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   The XPath expression is conceptually evaluated in the following
   context:

   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.

   o  The set of namespace declarations is the set of all "import"
      statements' prefix and namespace pairs, and the "prefix"
      statement's prefix for the "namespace" statement's URI.

   o  Elements without a namespace refer to nodes in the current module.

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

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

   If the node with the must statement represents configuration data,
   any node referenced in the XPath expression MUST also represent
   configuration.

   Note that since all leaf values in the data tree are conceptually
   stored in their canonical form (see Section 7.6 and Section 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.

7.5.3.  The must's Substatements

                 +---------------+---------+-------------+
                 | substatement  | section | cardinality |
                 +---------------+---------+-------------+
                 | description   | 7.19.3  | 0..1        |
                 | error-app-tag | 7.5.3.2 | 0..1        |
                 | error-message | 7.5.3.1 | 0..1        |
                 | reference     | 7.19.4  | 0..1        |
                 +---------------+---------+-------------+






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7.5.3.1.  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>.

7.5.3.2.  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>.

7.5.3.3.  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";
         }
     }

7.5.4.  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 which
   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 5.6 for additional information.





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7.5.5.  The container's Child Node Statements

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

7.5.6.  XML Encoding Rules

   A container node is encoded as an XML element.  The element's name is
   the container's identifier, and its XML namespace is the module's XML
   namespace.

   The container's child nodes are encoded as subelements to the
   container element, in the same order as they are defined within the
   container statement.

   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.

7.5.7.  NETCONF <edit-config> Operations

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

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

      If the operation is "replace" and the node exists, all child nodes
      not present in the XML are deleted, and child nodes present in the
      XML but not present in the datastore are created.

      If the operation is "create" the node is created if it does not
      exist.

      If the operation is "delete" the node is deleted if it exists.

      If the container has a "presence" statement, it may be implicitly
      created if it does not exist, even if the operation is "none".

      If a container has a "presence" statement and the last child node
      is deleted, the NETCONF server MAY delete the container.






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7.5.8.  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 encoding would look like this:

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

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

   To delete a container 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 nc:operation="delete"/>
             </services>
           </system>
         </config>
       </edit-config>
     </rpc>







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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,
   depending on the value of the "mandatory" statement.

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

   If a leaf has a "default" statement, the leaf's default value is set
   to 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 set to the type's default value.  In all
   other cases, the leaf does not have a default value.

   If the leaf has a default value, the server MUST use this value
   internally if no value is provided by the NETCONF client whenthe
   instance is created.


























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7.6.1.  The leaf's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | config       | 7.19.1  | 0..1        |
                 | default      | 7.6.3   | 0..1        |
                 | description  | 7.19.3  | 0..1        |
                 | if-feature   | 7.18.2  | 0..n        |
                 | mandatory    | 7.6.4   | 0..1        |
                 | must         | 7.5.2   | 0..n        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | type         | 7.6.2   | 1           |
                 | units        | 7.3.3   | 0..1        |
                 | when         | 7.19.5  | 0..1        |
                 +--------------+---------+-------------+

7.6.2.  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.

7.6.3.  The leaf's default Statement

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

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

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

7.6.4.  The leaf's mandatory Statement

   The "mandatory" statement, which is optional, takes as an argument
   the string "true" or "false".  If not specified, the default is
   "false".

   If "mandatory" is "true", the node MUST exist if its parent node
   exists.  This constraint is enforced according to the rules in
   Section 8.

   Since containers without a "presence" statement are implicitly
   created and deleted when needed, they are ignored when performing



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   mandatory tests for leafs.  A mandatory leaf within such a container
   is mandatory even if the container's data node does not exist.

7.6.5.  XML Encoding Rules

   A leaf node is encoded as an XML element.  The element's name is the
   leaf's identifier, and its XML namespace is the module's XML
   namespace.

   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 7.6.7 for an example.

7.6.6.  NETCONF <edit-config> Operations

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

      If the operation is "merge", 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 "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 operation is "delete" the node is deleted if it exists.

7.6.7.  Usage Example

   Given the following leaf statement:

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



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   A corresponding XML encoding:

     <port>2022</port>

   To create 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>

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.











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7.7.1.  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 | 7.7.3   | 0..1        |
                 | min-elements | 7.7.2   | 0..1        |
                 | must         | 7.5.2   | 0..n        |
                 | ordered-by   | 7.7.4   | 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        |
                 +--------------+---------+-------------+

7.7.2.  The min-elements Statement

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

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

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

7.7.3.  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.

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

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

7.7.4.  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



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   defaults to "system".

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

   See Section 5.5 for additional information.

7.7.4.1.  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 makes comparisons possible using simple
   tools like "diff".

   This is the default order.

7.7.4.2.  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 7.7.6 for details.

7.7.5.  XML Encoding Rules

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

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

   See Section 7.7.7 for an example.

7.7.6.  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 namespace (Section 5.4.1) 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.




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   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".

   In a <copy-config>, or an <edit-config> with a "replace" operation
   which 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 operation is "delete" the entry is deleted from the leaf-
      list if it exists.

7.7.7.  Usage Example

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

   A corresponding XML encoding:

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

   To create a new element in the list:














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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>
                 <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 interior nodes 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.

   A list is similar to a table where each list entry is a row in the
   table.

7.8.1.  The list's Substatements















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                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | augment      | 7.15    | 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 | 7.7.3   | 0..1        |
                 | min-elements | 7.7.2   | 0..1        |
                 | must         | 7.5.2   | 0..n        |
                 | ordered-by   | 7.7.4   | 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 which 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 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".

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




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

7.8.3.  The lists's unique Statement

   The "unique" statement is used to put constraints on valid list
   entries.  It takes as an argument a string which 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.  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;
         }
     }

   The following configuration is not valid:










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     <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>

7.8.4.  The list's Child Node Statements

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

7.8.5.  XML Encoding Rules

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

   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, in the same order as they are defined
   within the list statement.

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 namespace (Section 5.4.1) 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"



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   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".

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

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 encoding:

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

   To create a new user "barney":











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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">
             <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>

   Given the following ordered-by user list:











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     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>

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





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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"
                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
   "choice" statement puts a constraint on a valid configuration.  In a
   valid configuration, the nodes from at most one of the choice's
   branches exist at the same time.

   See Section 4.2.7 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", "container", "leaf", "list",
   "leaf-list", "uses", and "augment" 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 { ...}
         }
     }

   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



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   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        |
                 | augment      | 7.15    | 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 exists.
   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.

   The default case is only important when considering the default
   values of nodes under the cases.  The default values for nodes under



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   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".  If "mandatory" is "true", at least one
   node from exactly one of the choice's case branches MUST exist.  This
   constraint is enforced according to the rules in Section 8.

   If not specified, the default is "false".

7.9.5.  XML Encoding Rules

   The choice and case nodes are not visible in XML.

7.9.6.  NETCONF <edit-config> operations

   Since only one of the choices 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, 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:

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

   A corresponding XML encoding:

     <protocol>
       <tcp/>
     </protocol>

   To change the protocol from tcp to udp:




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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">
             <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.

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

   An anyxml node cannot be augmented.

   It is NOT RECOMMENDED that the anyxml statement is used to represent
   configuration data.



















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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    | 7.6.4   | 0..1        |
                 | reference    | 7.19.4  | 0..1        |
                 | status       | 7.19.2  | 0..1        |
                 | when         | 7.19.5  | 0..1        |
                 +--------------+---------+-------------+

7.10.2.  XML Encoding Rules

   An anyxml node is encoded as an XML element.  The element's name is
   the anyxml's identifier, and its XML namespace is the module's XML
   namespace.  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 within the XML value of an anyxml node are
   ignored by the NETCONF server.

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

      If the operation is "merge", 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 "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.




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      If the operation is "delete" the node is deleted if it exists.

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 which include it,
   and by other modules which import from it.  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.

   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.  For details about scoping for nested groupings, see
   Section 5.8.

   A grouping is more than just a mechanism for textual substitution,
   but defines a collection of nodes.  References from inside the
   grouping are relative to the scope in which the grouping is defined,



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   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 use node.

7.11.1.  The grouping's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | augment      | 7.15    | 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        |
                 +--------------+---------+-------------+

7.11.2.  Usage Example

     import inet-types {
         prefix "inet";
     }

     grouping address {
         description "A reusable address 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



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   then updated according to the refinement statements.  Thus, the
   identifiers defined in the grouping are copied into the current
   module's namespace, even if the grouping is imported from some other
   module.

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 a string which
   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 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 schema node identifier.  The syntax is
   formally defined by the rule "descendant-schema-nodeid" in
   Section 12.

   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 or choice node may get a different "mandatory" statement.

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





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

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

7.12.3.  XML Encoding 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 "address" 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:address;
     }

   A corresponding XML encoding:

     <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:address {
             refine port {
                 default 80;
             }
         }



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     }

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

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

   The following is an error:

     container http-server {
         uses acme:address;
         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 method.  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, an input node with the name "input", and an output 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 method.  It does not take an argument.  The
   substatements to "input" defines nodes under the RPC's input node.

   If a container in the input tree has a "presence" statement, the
   container need not be present in a NETCONF RPC invocation.

   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.

   If a leaf in the input tree has a default value, the NETCONF server
   MUST internally use this default if the leaf is not present in a
   NETCONF RPC invocation.

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


















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7.13.2.1.  The input's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | augment      | 7.15    | 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 method.  It does not take an argument.  The
   substatements to "output" defines nodes under the RPC's output node.

   If a container in the output tree has a "presence" statement, the
   container need not be present in a NETCONF RPC reply

   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 internally use this default if the leaf is not present in a
   NETCONF RPC reply.

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
















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7.13.3.1.  The output's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | augment      | 7.15    | 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 Encoding Rules

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

   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 method 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.

7.13.5.  Usage Example

   The following example defines an RPC method:

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

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



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     }

   A corresponding XML encoding 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 container in the notification tree has a "presence" statement,
   the container need not be present in a NETCONF notification.

   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
   server MUST internally use this default if the leaf is not present in
   a NETCONF notification.

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















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7.14.1.  The notification's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | augment      | 7.15    | 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 Encoding Rules

   A notification node is encoded as a child XML element to the
   <notification> element defined in [RFC5277].  The element's name is
   the notification's identifier, and its XML namespace is the module's
   XML namespace.

   The notifications's child nodes are encoded as subelements to the
   notification node's XML element, in the same order as they are
   defined within the notification statement.

7.14.3.  Usage Example

   The following example defines a notification:
















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     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 encoding 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 another module or submodule.  The argument is
   a string which 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.  The syntax is
   formally defined by the rule "augment-arg" in Section 12.  If the
   "augment" statement is on the top-level in a module or submodule, the
   absolute form (defined by the rule "absolute-schema-nodeid" in
   Section 12) of a schema node identifier MUST be used.  If the
   "augment" statement is in a "uses" statement, the descendant form
   (defined by the rule "descendant-schema-nodeid" in Section 12) MUST
   be used.



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   The syntax for a schema node identifier is a subset of the XPath
   syntax.  It is an absolute or relative XPath location path in
   abbreviated syntax, where axes and predicates are not permitted.

   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 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).

7.15.1.  The augment's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | anyxml       | 7.10    | 0..n        |
                 | augment      | 7.15    | 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 Encoding 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 augmented child nodes are encoded after
   all normal child nodes.  If the node is augmented more than once, the
   blocks of augmented child nodes are sorted (in alphanumeric order)
   according to their namespace URI and name of the first child node in
   each block.




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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;
         }
     }

   A corresponding XML encoding:















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     <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 encoding:

     <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 be defined either
   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
   which 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 which 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 which 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.










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7.16.3.  Usage Example

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

         identitiy 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



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   name is created by combining the the prefix of the module in which
   the 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 core YANG statements, or also defined using "extension"
   statements.  Core 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 which 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 text
   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 should be 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 5.9.

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
   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 my-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;
                 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.

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        |
                 +--------------+---------+-------------+







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7.18.2.  The if-feature Statement

   The "if-feature" statement is used to mark portions of the model as
   conditional.  The argument is the name of a feature, as defined by a
   "feature" statement.  If a prefix is present on the feature name, it
   refers to a feature defined the module which 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 which 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 the module which 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's syntax is formally defined by the rule "deviation-arg"
   in Section 12.

   Deviations define the way a device or class of devices deviate from
   the 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.

   Device deviations are strongly discouraged and should only be used as
   a last resort.  Telling the application how a device fails to follow
   the standard is no substitute for implementing the standard directly.

   However in some cases the cost of following the standard is heavy and
   the payoff may be small.  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 to treat attempts to
   configure unsupported parts of the module as either an error that is
   reported back to the unsuspecting application, or ignore that
   incoming requests.  Neither choice is acceptable.

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





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7.18.3.1.  The deviation's Substatements

                 +--------------+----------+-------------+
                 | substatement | section  | cardinality |
                 +--------------+----------+-------------+
                 | description  | 7.19.3   | 0..1        |
                 | deviate      | 7.18.3.2 | 0..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 as a substatement to the "deviate"
   statement.  If the 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 property to replace MUST exist in the
   target node.

   The argument "delete" deletes properties from the target node.  The
   properties to delete are identified by substatement to "delete".  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.

















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                       The deviates's Substatements

                 +--------------+---------+-------------+
                 | substatement | section | cardinality |
                 +--------------+---------+-------------+
                 | config       | 7.19.1  | 0..1        |
                 | default      | 7.6.3   | 0..1        |
                 | mandatory    | 7.6.4   | 0..1        |
                 | max-elements | 7.7.3   | 0..1        |
                 | min-elements | 7.7.2   | 0..1        |
                 | must         | 7.5.2   | 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 old RFC867-style "daytime" service.

     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;
         }
     }

   If the original definition is:

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



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   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 sub-statements 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, and will be part of the reply to a <get-config>
   request, and may be sent in a <copy-config> or <edit-config> request.
   If "config" is "false", it represents state data, and will be part of
   the reply to a <get>, but not to a <get-config> request.

   If "config" is not specified, the default is the same as the parent
   node's (in the data model) "config" value.  If the top node does not
   specify a "config" statement, the default is "true".

   If a node has "config" "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".

   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 if previously implemented.

   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.




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

7.19.3.  The description Statement

   The "description" statement takes as an argument a string which
   contains a high-level textual description of this definition.

7.19.4.  The reference Statement

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

7.19.5.  The when Statement

   The "when" statement allows a data definition statement to be
   conditional, with the node(s) defined by the data defined statement
   only being valid when a specific criteria is satisfied.  The
   statement's argument is an XPath expression, which is used to
   formally specify this criteria.  If the XPath expression conceptually
   evaluates to "true" for a particular instance, then the nodes defined
   by the data definition statement are valid, otherwise they are not.

   The XPath expression is conceptually evaluated in the following
   context:

   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 which is also a data
      node.

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

   o  If the "when" statement is a child of any other data definition
      statement, teh 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.

   o  The set of namespace declarations is the set of all "import"
      statements' prefix and namespace pairs, and the "prefix"



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      statement's prefix for the "namespace" statement's URI.

   o  Elements without a namespace refer to nodes in the current module.

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

   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 augment can very well be implemented with specially
   written code.




































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8.  Constraints

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

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

   If the constraint is defined on state data, it MUST be true in a
   reply to the <get> command.

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

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

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































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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 (Section 9.4.4, Section 9.4.6) and range restrictions of
   numeric types (Section 9.2.4).

   The lexicographic representation of a value of a certain type is used
   in the XML encoding over NETCONF, and when specifying default values
   in a YANG module.

9.1.  Canonical representation

   For each type, there is a single canonical representation of the
   type's values.  Some types allow multiple lexicographic
   representations of the same value, for example the positive integer
   "17" can be represented as "+17" or "17".

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.







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   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.

9.2.1.  Lexicographic Representation

   An integer value is lexicographically 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 lexicographic 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 upper- or lowercase.  The octal notation consists of
   an optional sign ("+" or "-"), the character "0" followed a number of
   octal digits.

   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

9.2.2.  Canonical Form

   The canonical form of a positive integer does not include the sign
   "+".

9.2.3.  Restrictions

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






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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 floating point 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 value restriction is applied to an already
   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" means
   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 | 7.5.3.2 | 0..1        |
                 | error-message | 7.5.3.1 | 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";
         }
     }

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

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

9.3.  The Floating Point Built-in Types

   The floating point built-in types are float32 and float64.  They
   represent floating point values of single and double precision as
   defined in [IEEE.754].  Special values are positive and negative
   infinity, and not-a-number.

9.3.1.  Lexicographic Representation

   A floating point value is lexicographically represented as consisting
   of a decimal mantissa followed, optionally, by the character "E" or
   "e", followed by an integer exponent.  The special values positive
   and negative infinity and not-a-number have lexical representations
   INF, -INF and NaN, respectively.  The minimal value accepted for a
   float is -INF, and the maximal value accepted for a float is INF.

9.3.2.  Canonical Form

   [Editor's Note: TBD]

9.3.3.  Restrictions

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









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9.3.4.  Usage Example

     type float32 {
         range "1..4.5 | 10 | 20..INF";
     }

    is equivalent to

     type float32 {
         range "1..4.5 | 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-DFFF / %xE000-FFFD /
            %x10000-10FFFF

9.4.1.  Lexicographic Representation

   A string value is lexicographically represented as character data in
   the XML encoding.

9.4.2.  Canonical Form

   The canonical form is the same as the lexicographical 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 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" means 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 | 7.5.3.2 | 0..1        |
                 | error-message | 7.5.3.1 | 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 completely
   matches the pattern.

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

9.4.6.1.  The pattern's Substatements

                 +---------------+---------+-------------+
                 | substatement  | section | cardinality |
                 +---------------+---------+-------------+
                 | description   | 7.19.3  | 0..1        |
                 | error-app-tag | 7.5.3.2 | 0..1        |
                 | error-message | 7.5.3.1 | 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
     xx00        // illegal

9.5.  The boolean Built-in Type

   The boolean built-in type represents a boolean value.







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9.5.1.  Lexicographic Representation

   The lexicographical representation of a boolean value is the strings
   "true" and "false".

9.5.2.  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.  Lexicographic Representation

   The lexicographical 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 control codes SHOULD be avoided.

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

   All assigned names in an enumeration MUST be unique.











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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.

   If a value is not specified, then one will be automatically assigned.
   If the enum sub-statement 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 sub-statements following the one
   with the current highest value.

9.6.5.  Usage Example

     type enumeration {
         enum enabled {
             value 1;
         }
         enum disabled {
             value 2;
         }
     }


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





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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.  Lexicographic Representation

   The lexicographical 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 in the space separated list
   appear in the same order as they are specified in the "bits"
   statement.

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 which is the assigned name of the bit.  It is
   followed by a block of substatements which holds detailed bit
   information.  A bit name follows the same syntax rules as an
   identifier (see Section 6.2).

   All bit 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 which 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 XML encoding, but is carried as a
   convenience to implementors.

   If a bit position is not specified, then one will be automatically
   assigned.  If the bit sub-statement 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 sub-statements
   following the one with the current highest position value.

9.7.5.  Usage Example

   Given the following type:

     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 lexicographic 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.  Lexicographic Representation

   Binary values are encoded with the base64 encoding scheme [RFC4648].

9.8.3.  Canonical Form

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

9.9.  The keyref Built-in Type

   The keyref type is used to reference a particular list entry in the
   data tree.  Its value is constrained to be the same as the key of an
   existing list entry.

   If the leaf with the keyref type represents configuration data, the
   list entry it refers to MUST also represent configuration.  Such a
   leaf puts a constraint on valid data.  All keyref nodes MUST
   reference existing list entries for the data to be valid.  This
   constraint is enforced according to the rules in Section 8.

9.9.1.  Restrictions

   A keyref 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 "keyref".  It takes as an
   argument a string which MUST refer to one key node of a list entry.

   The syntax for a path argument is a subset of the XPath syntax.  It
   is an absolute or relative XPath location path in abbreviated syntax,
   where axes are not permitted, and predicates are used only for
   constraining the values for the key nodes for list entries.  Each
   predicate consists of at most one equality test per key.

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




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

9.9.3.  Lexicographic Representation

   A keyref value is encoded the same way as the key it references.

9.9.4.  Canonical Form

   The canonical form of a keyref is the same as the canonical form of
   the key it references.

9.9.5.  Usage Example

   With the following list:

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

   The following keyref refers to an existing interface:

     leaf mgmt-interface {
         type keyref {
             path "../interface/name";
         }
     }

   A corresponding XML snippet is e.g.:

     <interface>
       <name>eth0</name>
     </interface>
     <interface>
       <name>lo</name>
     </interface>

     <mgmt-interface>eth0</mgmt-interface>

   The following keyrefs refer to an existing address of an interface:



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     container default-address {
         leaf ifname {
             type keyref {
                 path "../../interface/name";
             }
         }
         leaf address {
             type keyref {
                 path "../../interface[name = current()/../ifname]"
                    + "/address/ip";
             }
         }
     }

   A corresponding XML snippet is e.g.:

     <interface>
       <name>eth0</name>
       <address>
         <ip>192.0.2.1</ip>
       </address>
       <address>
         <ip>192.0.2.2</ip>
       </address>
     </interface>
     <interface>
       <name>lo</name>
       <address>
         <ip>127.0.0.1</ip>
       </address>
     </interface>

     <default-address>
       <ifname>eth0</ifname>
       <address>192.0.2.2</address>
     </default-address>

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.






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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 the module which was imported with that prefix.
   Otherwise an identity with the matching name must be defined in the
   current module or an included submodule.

   Valid values for an identityref are any identities derived from the
   identityref's base identity.

9.10.3.  Lexicographic Representation

   An identityref is encoded as the referred identity's qualified name
   [XML-NAMES].

   [Editor's Note: TBD.  How to handle prefixes?]

9.10.4.  Usage Example

   With the identity definitions in Section 7.16.3, the leaf:

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

     leaf crypto {
         type identity-ref {
             base "crypto:crypto-alg";
         }
     }

   will be encoded as:

     <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>







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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.  Lexicographic Representation

   Not applicable.

9.11.3.  Canonical Representation

   Not applicable.

9.11.4.  Usage Example

   The following leaf

     leaf enable-qos {
         type empty;
     }

   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 which 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 "keyref".

   Example:




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     type union {
         type int32;
         type enumeration {
             enum "unbounded";
         }
     }

9.12.1.  Restrictions

   A union can not be restricted.  However, each member type can be
   restricted, based on the rules defined in Section 9 chapter.

9.12.2.  Lexicographic Representation

   The lexicographical representation of an union is a value that
   corresponds to the representation of any one of the member types.

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
   syntax, which is used to uniquely identify a node in the data tree.
   It is an absolute XPath location path in abbreviated syntax, where
   axes are not permitted, and predicates are used only for specifying
   the values for the key nodes for list entries, or a value of a leaf-
   list.  Each predicate consists of one equality test per key.  Each
   key MUST have a corresponding predicate.

   The syntax is formally defined by the rule "absolute-instid" in
   Section 12.

9.13.1.  Restrictions

   An instance-identifier cannot be restricted.

9.13.2.  Lexicographic Representation

   An instance-identifier value is lexicographically represented as a
   string in the XML encoding.  The namespace prefixes used in the
   encoding MUST be declared in the XML namespace scope in the instance-
   idenfitier's XML element.



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   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.

9.13.3.  Canonical Form

   [Editor's Note: TBD.  How to handle prefixes?]

9.13.4.  Usage Example

   The following are examples of instance identifiers:

     /ex:system/ex:services/ex:ssh/ex:port

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

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

     /ex:system/ex:server[ex:ip='192.0.2.1'][ex:port='80']

     /ex:system/ex:services/ex:ssh/ex:cipher[.='blowfish-cbc']






























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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)
   SHOULD be included in front of the existing revision statements.
   Furthermore, any necessary changes MUST be applied to any meta
   statements, including the "organization" and "contact" statements
   (Section 7.1.7, Section 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 encoded in 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 bits's
      positions do not change.

   o  A "range", "length", or "pattern" statement may expand the allowed
      value space.

   o  A "default" statement may be added.

   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
      less elements.




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   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, rpc, 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 a "if-feature" statement
      which 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
      which does not change the syntax or semantics of the type.  For
      example, an inline type definition may be replaced with a typedef,
      but a int8 type cannot be replaced by a int16, since the syntax
      would change.

   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 submodule may
      be removed, provided the definitions in the module do not change
      in any other way than allowed here.

   o  The "prefix" statment 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.



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   In statements which have any data definition statements as
   substatements, those data definition substatements MUST NOT be
   reordered.

   [Editor's Note: These rules work as long as we have import/include by
   revision]













































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11.  YIN

   A YANG module can be specified in an alternative XML-based syntax
   called YIN.  This section describes symmetric mapping rules between
   the two formats.

   The YANG and YIN formats contain equivalent information using
   different notations.  The purpose of the YIN notation is to allow the
   user to translate YANG into YIN, use the rich set of XML based tools
   on the YIN format to transform, or filter the model information.
   Tools like XSLT or XML validators can be utilized.  After this the
   model can be transformed back to the YANG format if needed, which
   provides a more concise and readable format.

   The YANG-2-YIN and the YIN-2-YANG transformations will not modify the
   information content of the model.

11.1.  Formal YIN Definition

   YIN is described by an algorithm that transforms YANG to YIN.

11.2.  Transformation Algorithm YANG-2-YIN

   Every keyword results in a new XML element.  The name of the element
   is the keyword.  All core YANG elements are defined in the namespace
   "urn:ietf:params:xml:ns:yang:yin:1".  [XXX IANA]

   The top-level element is always <module> or <submodule>.

   Elements which represent keywords that are imported extensions from
   other modules MUST be properly namespace qualified, where the
   namespace is the namespace of the imported module.  The XML prefix
   for such extensions MUST be the same as the prefix defined in the
   module's "import" statement.

   Elements which represent keywords that are included extensions from
   other submodules MUST be properly namespace qualified, where the
   namespace is the namespace of the module that the submodule belongs
   to.  The XML prefix for such extensions MUST be the same as the local
   prefix, i.e. for a module it is as defined in the "prefix" statement,
   and for a submodule, as defined in the submodule's "belongs-to"
   statement.

   If the keyword has an argument, its encoding depends on the value of
   the argument's "yin-element".  If "yin-element" is false, the
   argument is encoded as an XML attribute to the keyword's element.  If
   "yin-element" is true, the argument is encoded as a subelement to the
   keyword's element.  The name of the attribute or element is the name



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   of the argument.

   The core YANG keywords have arguments according to the table below.
   Extension keywords have arguments according to Section 7.17.2.

                          YANG to YIN keyword map

              +---------------+---------------+-------------+
              | 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       | info          | true        |
              | container     | name          | false       |
              | default       | value         | false       |
              | description   | text          | true        |
              | enum          | name          | false       |
              | error-app-tag | value         | false       |
              | error-message | value         | true        |
              | extension     | name          | false       |
              | deviate       | target-node   | false       |
              | deviation     | value         | false       |
              | feature       | name          | 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       |



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              | ordered-by    | value         | false       |
              | organization  | info          | true        |
              | output        | <no argument> | n/a         |
              | path          | value         | false       |
              | pattern       | value         | false       |
              | position      | value         | false       |
              | prefix        | value         | false       |
              | presence      | value         | false       |
              | range         | value         | false       |
              | reference     | info          | false       |
              | refine        | target-node   | false       |
              | revision      | 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 35

   If a statement is followed by substatements, those substatements are
   subelements in the YIN mapping.

   Comments in YANG MAY be transformed into XML comments.

11.2.1.  Usage Example

   The following YANG snippet:

     leaf mtu {
         type uint32;
         description "The MTU of the interface.";
     }

   is translated into the following YIN snippet:








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     <leaf name="mtu">
       <type name="uint32"/>
       <description>
         <text>The MTU of the interface."</text>
       </description>
     </leaf>

11.3.  Transformation Algorithm YIN-2-YANG

   The transformation is based on a recursive algorithm that is started
   on the <module> or <submodule> element.

   The element is transformed into a YANG keyword.  If the keyword in
   Table 35 is marked as yin-element true, the subelement with the
   keyword's argument name in Table 35 contains the YANG keyword's
   argument as text content.  If the keyword in Table 35 is marked as
   yin-element false, the element's attribute with keyword's argument
   name in Table 35 contains the YANG keyword's argument.

   If there are no other subelements to the element, the YANG statement
   is closed with a ";".  Otherwise, each such subelement is
   transformed, according to the same algorithm, as substatements to the
   current YANG statement, enclosed within "{" and "}".

   XML comments in YIN MAY be transformed into YANG comments.

11.3.1.  Tabulation, Formatting

   To get a readable YANG module the YANG output will have to be
   indented with appropriate whitespace characters.





















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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.

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

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



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                         *(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 /

case-data-def-stmt     = container-stmt /
                         leaf-stmt /
                         leaf-list-stmt /
                         list-stmt /
                         anyxml-stmt /
                         uses-stmt

yang-version-stmt      = yang-version-keyword sep yang-version-arg-str
                         optsep stmtend

yang-version-arg-str   = < a string which matches the rule
                           yang-version-arg >

yang-version-arg       = "1"

import-stmt            = import-keyword sep identifier-arg-str optsep
                         "{" stmtsep
                             prefix-stmt stmtsep
                         "}"

include-stmt           = include-keyword sep identifier-arg-str optsep
                         stmtend

namespace-stmt         = namespace-keyword sep uri-str optsep stmtend



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uri-str                = < a string which 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 date-arg-str optsep
                         (";" /
                          "{" stmtsep
                              [description-stmt stmtsep]
                          "}")

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



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yin-element-arg-str    = < a string which 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]
                          "}")

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
                          "}")




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type-body-stmts        = numerical-restrictions /
                         string-restrictions /
                         enum-specification /
                         keyref-specification /
                         identityref-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]
                           "}")

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)




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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]
                           "}")

keyref-specification   = path-stmt stmtsep

path-stmt              = path-keyword sep path-arg-str stmtend

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 which matches the rule
                           position-value-arg >

position-value-arg     = non-negative-decimal-value

status-stmt            = status-keyword sep status-arg-str stmtend

status-arg-str         = < a string which matches the rule
                           status-arg >

status-arg             = current-keyword /
                         obsolete-keyword /
                         deprecated-keyword




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config-stmt            = config-keyword sep
                         config-arg-str stmtend

config-arg-str         = < a string which 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 which 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 which 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

min-elements-stmt      = min-elements-keyword sep
                         min-value-arg-str stmtend;

min-value-arg-str      = < a string which matches the rule
                           min-value-arg >

min-value-arg          = non-negative-decimal-value




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max-elements-stmt      = max-elements-keyword sep
                         max-value-arg-str stmtend;

max-value-arg-str      = < a string which matches the rule
                           max-value-arg >

max-value-arg          = unbounded-keyword /
                         positive-decimal-value

value-stmt             = value-keyword sep decimal-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)
                          "}")

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)



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                             [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]
                             *((typedef-stmt /
                                grouping-stmt) stmtsep)
                             1*(data-def-stmt stmtsep)
                          "}"

key-stmt               = key-keyword sep key-arg-str stmtend

key-arg-str            = < a string which matches the rule



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                           key-arg >

key-arg                = identifier *(sep identifier)

unique-stmt            = unique-keyword sep unique-arg-str stmtend

unique-arg-str         = < a string which 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]
                              [description-stmt stmtsep]
                              [reference-stmt stmtsep]
                              *(case-data-def-stmt stmtsep)
                          "}")

anyxml-stmt            = anyxml-keyword sep identifier-arg-str optsep
                         (";" /
                          "{" stmtsep



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                              ;; these stmts can appear in any order
                              [when-stmt stmtsep]
                              *(if-feature-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]
                              *(refinement-stmt stmtsep)
                              *(uses-augment-stmt stmtsep)
                          "}")

refinement-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 which matches the rule
                           refine-arg >

refine-arg             = descendant-schema-nodeid


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



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                         *(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]
                         [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
                         [config-stmt stmtsep]
                         [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]



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                             1*((data-def-stmt stmtsep) /
                                (case-stmt stmtsep))
                          "}"

uses-augment-arg-str   = < a string which 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 which matches the rule
                           augment-arg >

augment-arg            = absolute-schema-nodeid

unknown-statement      = prefix ":" identifier [sep string] optsep
                         (";" / "{" *unknown-statement "}")

when-stmt              = when-keyword sep string stmtend

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



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                             *((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)
                          "}")

deviation-stmt         = deviation-keyword sep
                         deviation-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             [description-stmt stmtsep]
                             [reference-stmt stmtsep]
                             *(deviate-stmt stmtsep)
                         "}"

deviation-arg-str      = < a string which matches the rule
                           deviation-arg >

deviation-arg          = absolute-schema-nodeid

deviate-stmt           = deviate-keyword sep deviate-arg-str optsep
                         "{" stmtsep
                             ;; these stmts can appear in any order
                             [type-stmt stmtsep]
                             [units-stmt stmtsep]
                             *(must-stmt stmtsep)
                             *(unique-stmt stmtsep)



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                             [default-stmt stmtsep]
                             [config-stmt stmtsep]
                             [mandatory-stmt stmtsep]
                             [min-elements-stmt stmtsep]
                             [max-elements-stmt stmtsep]
                         "}"

deviate-arg-str        = < a string which matches the rule
                           deviate-arg >

deviate-arg            = add-keyword /
                         delete-keyword /
                         replace-keyword /
                         not-supported-keyword

;; Ranges

range-arg-str          = < a string which matches the rule
                           range-arg >

range-arg              = range-part *(optsep "|" optsep range-part)

range-part             = range-boundary
                         [optsep ".." optsep range-boundary]

range-boundary         = neginf-keyword / posinf-keyword /
                         min-keyword / max-keyword /
                         decimal-value / float-value

;; Lengths

length-arg-str         = < a string which 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-decimal-value

;; Date

date-arg-str           = < a string which matches the rule
                           date-arg >

date-arg               = 4DIGIT "-" 2DIGIT "-" 2DIGIT



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;; Schema Node Identifiers

schema-nodeid          = absolute-schema-nodeid /
                         relative-schema-nodeid

absolute-schema-nodeid
                       = 1*("/" node-identifier)

relative-schema-nodeid
                       = descendant-schema-nodeid /
                         (("." / "..") "/"
                         *relative-schema-nodeid)

descendant-schema-nodeid
                       = node-identifier
                         absolute-schema-nodeid

node-identifier        = [prefix ":"] identifier


;; Instance Identifiers

instance-identifier-str
                       = < a string which matches the rule
                           instance-identifier >

instance-identifier    = absolute-instid / relative-instid

absolute-instid        = 1*("/" (node-identifier *predicate))

relative-instid        = descendant-instid /
                         (("." / "..") "/"
                          *relative-instid)

descendant-instid      = node-identifier *predicate
                         absolute-instid

predicate              = "[" *WSP predicate-expr *WSP "]"

predicate-expr         = (node-identifier / ".") *WSP "=" *WSP
                         ((DQUOTE string DQUOTE) /
                          (SQUOTE string SQUOTE))

;; keyref path

path-arg-str           = < a string which matches the rule
                           path-arg >




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path-arg               = absolute-path / relative-path

absolute-path          = 1*("/" (node-identifier *path-predicate))

relative-path          = descendant-path /
                         (".." "/"
                         *relative-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 "/"
                         rel-path-keyexpr

rel-path-keyexpr       = 1*(".." "/") *(node-identifier "/")
                         node-identifier

;;; Keywords, using abnfgen's syntax for case-sensitive strings

;; statment 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'
grouping-keyword       = 'grouping'
identity-keyword       = 'identity'
if-feature-keyword     = 'if-feature'
import-keyword         = 'import'



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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'
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'
revision-keyword       = 'revision'
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'



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nan-keyword            = 'NaN'
neginf-keyword         = '-INF'
not-supported-keyword  = 'not-supported'
obsolete-keyword       = 'obsolete'
posinf-keyword         = 'INF'
replace-keyword        = 'replace'
system-keyword         = 'system'
true-keyword           = 'true'
unbounded-keyword      = 'unbounded'
user-keyword           = 'user'

current-function-invocation = 'current()'

;; Basic Rules

keyword                = [prefix ":"] identifier

prefix-arg-str         = < a string which matches the rule
                           prefix-arg >

prefix-arg             = prefix

prefix                 = identifier

identifier-arg-str     = < a string which matches the rule
                           identifier-arg >

identifier-arg         = identifier

identifier             = (ALPHA / "_")
                         *(ALPHA / DIGIT / "_" / "-" / ".")

identifier-ref-arg-str = < a string which matches the rule
                           identifier-ref-arg >

identifier-ref-arg     = [prefix ":"] identifier

string                 = < an unquoted string as returned by
                           the scanner >

decimal-value          = ("-" non-negative-decimal-value)  /
                          non-negative-decimal-value

non-negative-decimal-value = "0" / positive-decimal-value

positive-decimal-value = (non-zero-digit *DIGIT)

zero-decimal-value     = 1*DIGIT



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stmtend                = ";" / "{" *unknown-statement "}"

sep                    = 1*(WSP / line-break)
                         ; unconditional separator

optsep                 = *(WSP / line-break)

stmtsep                = *(WSP / line-break / unknown-statement)

line-break             = CRLF / LF

non-zero-digit         = %x31-39

float-value            = neginf-keyword /
                         posinf-keyword /
                         nan-keyword /
                         decimal-value "." zero-decimal-value
                            *1("E" ("+"/"-") zero-decimal-value)

SQUOTE                 = %x27
                         ; ' (Single Quote)

;;
;; RFC 4234 core rules.
;;

ALPHA                  = %x41-5A / %x61-7A
                         ; A-Z / a-z

CR                     = %x0D
                         ; carriage return

CRLF                   = CR LF
                         ; Internet standard newline

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




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LF                     = %x0A
                         ; linefeed

SP                     = %x20
                         ; space

VCHAR                  = %x21-7E
                         ; visible (printing) characters

WSP                    = SP / HTAB
                         ; white space








































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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 YANG unique Statement:

   If a NETCONF operation would result in configuration data where a
   unique constraint is invalidated, the following error is returned:

     Tag:            operation-failed
     Error-app-tag:  data-not-unique
     Error-info:     <non-unique>: Contains an instance identifier which
                     points to a leaf which invalidates the unique
                     constraint. This element is present once for each
                     leaf invalidating the unique constraint.

                     The <non-unique> element is in the YANG
                     namespace ("urn:ietf:params:xml:ns:yang:1"
                     [XXX IANA]).

13.2.  Error Message for Data that Violates a YANG 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:

     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 YANG 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:

     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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13.4.  Error Message for Data that Violates a YANG 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.

     Tag:            operation-failed
     Error-app-tag:  must-violation

13.5.  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:

     Tag:            bad-attribute
     Error-app-tag:  missing-instance

































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14.  IANA Considerations

   This document registers two URIs for the YANG XML namespace in the
   IETF XML registry [RFC3688].

     URI: urn:ietf:params:xml:ns:yang:yin:1

     URI: urn:ietf:params:xml:ns:yang:1











































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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.

   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.

   YANG is dependent upon:

   o  the security of the transmission infrastructure used to send
      sensitive information

   o  the security of applications which store or release such sensitive
      information.

   o  adequate authentication and access control mechanisms to restrict
      the usage of sensitive data.



























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16.  Contributors

   The following people all contributed significantly to the initial
   YANG draft:

    - Andy Bierman (andybierman.com)
    - Balazs Lengyel (Ericsson)
    - David Partain (Ericsson)
    - Juergen Schoenwaelder (Jacobs University Bremen)
    - Phil Shafer (Juniper Networks)









































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17.  References

17.1.  Normative References

   [IEEE.754]
              Institute of Electrical and Electronics Engineers,
              "Standard for Binary Floating-Point Arithmetic",
              IEEE Standard 754, August 1985.

   [ISO.10646]
              International Organization for Standardization,
              "Information Technology - Universal Multiple-octet coded
              Character Set (UCS) - Part 1: Architecture and Basic
              Multilingual Plane", ISO Standard 10646-1, May 1993.

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

   [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.

   [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]
              Tobin, R., Bray, T., Hollander, D., and A. Layman,
              "Namespaces in XML 1.0 (Second Edition)", World Wide Web
              Consortium Recommendation REC-xml-names-20060816,
              August 2006,
              <http://www.w3.org/TR/2006/REC-xml-names-20060816>.

   [XPATH]    Clark, J. and S. DeRose, "XML Path Language (XPath)



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              Version 1.0", World Wide Web Consortium
              Recommendation REC-xpath-19991116, November 1999,
              <http://www.w3.org/TR/1999/REC-xpath-19991116>.

   [XSD-TYPES]
              Biron, P V. and A. Malhotra, "XML Schema Part 2: Datatypes
              Second Edition", W3C REC REC-xmlschema-2-20041028,
              October 2004.

17.2.  Non-Normative 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.






























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Appendix A.  ChangeLog

A.1.  Version -02

   o  Added module update rules. (yang-00000)

   o  Added "refine" statement as a substatement to "uses". (yang-00088)

   o  Allow "augment" on top-level and in "uses" only. (yang-00088)

   o  Allow "when" on all data defintion statements. (yang-00088)

   o  Added section "Constraints" and clarified when constraints are
      enforced. (yang-00172)

   o  Added "feature" and "if-feature" statements. (yang-00750)

   o  Added "prefix" as a substatement to "belongs-to". (yang-00755)

   o  Added section on Conformance. (yang-01281)

   o  Added "deviation" statement. (yang-01281)

   o  Added "identity" statement and "identityref" type. (yang-01339)

   o  Aligned grammar for "enum" with text.

A.2.  Version -01

   o  Removed "Appendix A.  Derived YANG Types".

   o  Removed "Appendix C.  XML Schema Considerations".

   o  Removed "Appendix F.  Why We Need a New Modeling Language".

   o  Moved "Appendix B.  YIN" to its own section.

   o  Moved "Appendix D.  YANG ABNF Grammar" to its own section.

   o  Moved "Appendix E.  Error Responses for YANG Related Errors" into
      its own section.

   o  The "input" and "output" nodes are now implicitly created by the
      "rpc" statement, in order for augmentation of these nodes to work
      correctly.

   o  Allow "config" in "choice".




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   o  Added reference to XPath 1.0.

   o  Using an XPath function "current()" instead of the variable
      "$this".

   o  Clarified that a "must" expression in a configuration node must
      not reference non-configuration nodes.

   o  Added XML encoding rules and usage examples for rpc and
      notification.

   o  Removed requirement that refinements are specified in the same
      order as in the original grouping's definition.

   o  Fixed whitespace issues in the ABNF grammar.

   o  Added the term "mandatory node", and refer to it in the
      description of augment (see Section 7.15), and choice (see
      Section 7.9.3).

   o  Added support for multiple "pattern" statements in "type".

   o  Several clarifications and fixed typos.

A.3.  Version -00

   Changes from draft-bjorklund-netconf-yang-02.txt

   o  Fixed bug in grammar for bit-stmt

   o  Fixed bugs in example XPath expressions

   o  Added keyword 'presence' to the YIN mapping table


















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Author's Address

   Martin Bjorklund (editor)
   Tail-f Systems

   Email: mbj@tail-f.com













































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Full Copyright Statement

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