Network Working Group M. Bjorklund, Ed. Internet-Draft Tail-f Systems Intended status: Standards Track October 22, 2009 Expires: April 25, 2010 YANG - A data modeling language for NETCONF draft-ietf-netmod-yang-08 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 25, 2010. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Bjorklund Expires April 25, 2010 [Page 1] Internet-Draft YANG October 2009 Abstract YANG is a data modeling language used to model configuration and state data manipulated by the Network Configuration Protocol (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. State Data . . . . . . . . . . . . . . . . . . . . . 19 4.2.4. Built-in Types . . . . . . . . . . . . . . . . . . . 19 4.2.5. Derived Types (typedef) . . . . . . . . . . . . . . . 20 4.2.6. Reusable Node Groups (grouping) . . . . . . . . . . . 21 4.2.7. Choices . . . . . . . . . . . . . . . . . . . . . . . 22 4.2.8. Extending Data Models (augment) . . . . . . . . . . . 23 4.2.9. RPC Definitions . . . . . . . . . . . . . . . . . . . 24 4.2.10. Notification Definitions . . . . . . . . . . . . . . 25 5. Language Concepts . . . . . . . . . . . . . . . . . . . . . . 27 5.1. Modules and Submodules . . . . . . . . . . . . . . . . . 27 5.1.1. Import and Include by Revision . . . . . . . . . . . 28 5.1.2. Module Hierarchies . . . . . . . . . . . . . . . . . 28 5.2. File Layout . . . . . . . . . . . . . . . . . . . . . . . 29 5.3. XML Namespaces . . . . . . . . . . . . . . . . . . . . . 30 5.3.1. YANG XML Namespace . . . . . . . . . . . . . . . . . 30 5.4. Resolving Grouping, Type, and Identity Names . . . . . . 30 5.5. Nested Typedefs and Groupings . . . . . . . . . . . . . . 30 5.6. Conformance . . . . . . . . . . . . . . . . . . . . . . . 31 5.6.1. Basic Behavior . . . . . . . . . . . . . . . . . . . 32 5.6.2. Optional Features . . . . . . . . . . . . . . . . . . 32 5.6.3. Deviations . . . . . . . . . . . . . . . . . . . . . 32 5.6.4. Announcing Conformance Information in the Message . . . . . . . . . . . . . . . . . . . . . . . 33 6. YANG syntax . . . . . . . . . . . . . . . . . . . . . . . . . 36 6.1. Lexicographical Tokenization . . . . . . . . . . . . . . 36 6.1.1. Comments . . . . . . . . . . . . . . . . . . . . . . 36 6.1.2. Tokens . . . . . . . . . . . . . . . . . . . . . . . 36 6.1.3. Quoting . . . . . . . . . . . . . . . . . . . . . . . 36 6.2. Identifiers . . . . . . . . . . . . . . . . . . . . . . . 38 Bjorklund Expires April 25, 2010 [Page 2] Internet-Draft YANG October 2009 6.2.1. Identifiers and their namespaces . . . . . . . . . . 38 6.3. Statements . . . . . . . . . . . . . . . . . . . . . . . 39 6.3.1. Language Extensions . . . . . . . . . . . . . . . . . 39 6.4. XPath Evaluations . . . . . . . . . . . . . . . . . . . . 39 6.4.1. XPath Context . . . . . . . . . . . . . . . . . . . . 40 6.5. Schema Node Identifier . . . . . . . . . . . . . . . . . 40 7. YANG Statements . . . . . . . . . . . . . . . . . . . . . . . 42 7.1. The module Statement . . . . . . . . . . . . . . . . . . 42 7.1.1. The module's Substatements . . . . . . . . . . . . . 43 7.1.2. The yang-version Statement . . . . . . . . . . . . . 44 7.1.3. The namespace Statement . . . . . . . . . . . . . . . 44 7.1.4. The prefix Statement . . . . . . . . . . . . . . . . 45 7.1.5. The import Statement . . . . . . . . . . . . . . . . 45 7.1.6. The include Statement . . . . . . . . . . . . . . . . 46 7.1.7. The organization Statement . . . . . . . . . . . . . 47 7.1.8. The contact Statement . . . . . . . . . . . . . . . . 47 7.1.9. The revision Statement . . . . . . . . . . . . . . . 47 7.1.10. Usage Example . . . . . . . . . . . . . . . . . . . . 48 7.2. The submodule Statement . . . . . . . . . . . . . . . . . 48 7.2.1. The submodule's Substatements . . . . . . . . . . . . 49 7.2.2. The belongs-to Statement . . . . . . . . . . . . . . 50 7.2.3. Usage Example . . . . . . . . . . . . . . . . . . . . 51 7.3. The typedef Statement . . . . . . . . . . . . . . . . . . 51 7.3.1. The typedef's Substatements . . . . . . . . . . . . . 52 7.3.2. The typedef's type Statement . . . . . . . . . . . . 52 7.3.3. The units Statement . . . . . . . . . . . . . . . . . 52 7.3.4. The typedef's default Statement . . . . . . . . . . . 52 7.3.5. Usage Example . . . . . . . . . . . . . . . . . . . . 53 7.4. The type Statement . . . . . . . . . . . . . . . . . . . 53 7.4.1. The type's Substatements . . . . . . . . . . . . . . 53 7.5. The container Statement . . . . . . . . . . . . . . . . . 53 7.5.1. Containers with Presence . . . . . . . . . . . . . . 54 7.5.2. The container's Substatements . . . . . . . . . . . . 55 7.5.3. The must Statement . . . . . . . . . . . . . . . . . 55 7.5.4. The must's Substatements . . . . . . . . . . . . . . 57 7.5.5. The presence Statement . . . . . . . . . . . . . . . 58 7.5.6. The container's Child Node Statements . . . . . . . . 58 7.5.7. XML Mapping Rules . . . . . . . . . . . . . . . . . . 58 7.5.8. NETCONF Operations . . . . . . . . . . 58 7.5.9. Usage Example . . . . . . . . . . . . . . . . . . . . 59 7.6. The leaf Statement . . . . . . . . . . . . . . . . . . . 60 7.6.1. The leaf's default value . . . . . . . . . . . . . . 60 7.6.2. The leaf's Substatements . . . . . . . . . . . . . . 61 7.6.3. The leaf's type Statement . . . . . . . . . . . . . . 61 7.6.4. The leaf's default Statement . . . . . . . . . . . . 61 7.6.5. The leaf's mandatory Statement . . . . . . . . . . . 61 7.6.6. XML Mapping Rules . . . . . . . . . . . . . . . . . . 62 7.6.7. NETCONF Operations . . . . . . . . . . 62 Bjorklund Expires April 25, 2010 [Page 3] Internet-Draft YANG October 2009 7.6.8. Usage Example . . . . . . . . . . . . . . . . . . . . 63 7.7. The leaf-list Statement . . . . . . . . . . . . . . . . . 63 7.7.1. Ordering . . . . . . . . . . . . . . . . . . . . . . 64 7.7.2. The leaf-list's Substatements . . . . . . . . . . . . 65 7.7.3. The min-elements Statement . . . . . . . . . . . . . 65 7.7.4. The max-elements Statement . . . . . . . . . . . . . 65 7.7.5. The ordered-by Statement . . . . . . . . . . . . . . 66 7.7.6. XML Mapping Rules . . . . . . . . . . . . . . . . . . 66 7.7.7. NETCONF operations . . . . . . . . . . 66 7.7.8. Usage Example . . . . . . . . . . . . . . . . . . . . 67 7.8. The list Statement . . . . . . . . . . . . . . . . . . . 69 7.8.1. The list's Substatements . . . . . . . . . . . . . . 70 7.8.2. The list's key Statement . . . . . . . . . . . . . . 70 7.8.3. The list's unique Statement . . . . . . . . . . . . . 71 7.8.4. The list's Child Node Statements . . . . . . . . . . 72 7.8.5. XML Mapping Rules . . . . . . . . . . . . . . . . . . 72 7.8.6. NETCONF operations . . . . . . . . . . 73 7.8.7. Usage Example . . . . . . . . . . . . . . . . . . . . 73 7.9. The choice Statement . . . . . . . . . . . . . . . . . . 77 7.9.1. The choice's Substatements . . . . . . . . . . . . . 77 7.9.2. The choice's case Statement . . . . . . . . . . . . . 77 7.9.3. The choice's default Statement . . . . . . . . . . . 79 7.9.4. The choice's mandatory Statement . . . . . . . . . . 80 7.9.5. XML Mapping Rules . . . . . . . . . . . . . . . . . . 81 7.9.6. NETCONF operations . . . . . . . . . . 81 7.9.7. Usage Example . . . . . . . . . . . . . . . . . . . . 81 7.10. The anyxml Statement . . . . . . . . . . . . . . . . . . 82 7.10.1. The anyxml's Substatements . . . . . . . . . . . . . 83 7.10.2. XML Mapping Rules . . . . . . . . . . . . . . . . . . 83 7.10.3. NETCONF operations . . . . . . . . . . 83 7.10.4. Usage Example . . . . . . . . . . . . . . . . . . . . 84 7.11. The grouping Statement . . . . . . . . . . . . . . . . . 84 7.11.1. The grouping's Substatements . . . . . . . . . . . . 85 7.11.2. Usage Example . . . . . . . . . . . . . . . . . . . . 85 7.12. The uses Statement . . . . . . . . . . . . . . . . . . . 85 7.12.1. The uses's Substatements . . . . . . . . . . . . . . 86 7.12.2. The refine Statement . . . . . . . . . . . . . . . . 86 7.12.3. XML Mapping Rules . . . . . . . . . . . . . . . . . . 87 7.12.4. Usage Example . . . . . . . . . . . . . . . . . . . . 87 7.13. The rpc Statement . . . . . . . . . . . . . . . . . . . . 88 7.13.1. The rpc's Substatements . . . . . . . . . . . . . . . 89 7.13.2. The input Statement . . . . . . . . . . . . . . . . . 89 7.13.3. The output Statement . . . . . . . . . . . . . . . . 90 7.13.4. XML Mapping Rules . . . . . . . . . . . . . . . . . . 91 7.13.5. Usage Example . . . . . . . . . . . . . . . . . . . . 91 7.14. The notification Statement . . . . . . . . . . . . . . . 92 7.14.1. The notification's Substatements . . . . . . . . . . 93 7.14.2. XML Mapping Rules . . . . . . . . . . . . . . . . . . 93 Bjorklund Expires April 25, 2010 [Page 4] Internet-Draft YANG October 2009 7.14.3. Usage Example . . . . . . . . . . . . . . . . . . . . 93 7.15. The augment Statement . . . . . . . . . . . . . . . . . . 94 7.15.1. The augment's Substatements . . . . . . . . . . . . . 95 7.15.2. XML Mapping Rules . . . . . . . . . . . . . . . . . . 95 7.15.3. Usage Example . . . . . . . . . . . . . . . . . . . . 95 7.16. The identity Statement . . . . . . . . . . . . . . . . . 97 7.16.1. The identity's Substatements . . . . . . . . . . . . 98 7.16.2. The base Statement . . . . . . . . . . . . . . . . . 98 7.16.3. Usage Example . . . . . . . . . . . . . . . . . . . . 99 7.17. The extension Statement . . . . . . . . . . . . . . . . . 99 7.17.1. The extension's Substatements . . . . . . . . . . . . 100 7.17.2. The argument Statement . . . . . . . . . . . . . . . 100 7.17.3. Usage Example . . . . . . . . . . . . . . . . . . . . 101 7.18. Conformance-related Statements . . . . . . . . . . . . . 101 7.18.1. The feature Statement . . . . . . . . . . . . . . . . 101 7.18.2. The if-feature Statement . . . . . . . . . . . . . . 103 7.18.3. The deviation Statement . . . . . . . . . . . . . . . 103 7.19. Common Statements . . . . . . . . . . . . . . . . . . . . 106 7.19.1. The config Statement . . . . . . . . . . . . . . . . 106 7.19.2. The status Statement . . . . . . . . . . . . . . . . 106 7.19.3. The description Statement . . . . . . . . . . . . . . 107 7.19.4. The reference Statement . . . . . . . . . . . . . . . 107 7.19.5. The when Statement . . . . . . . . . . . . . . . . . 107 8. Constraints . . . . . . . . . . . . . . . . . . . . . . . . . 109 8.1. Constraints on Data . . . . . . . . . . . . . . . . . . . 109 8.2. Hierarchy of Constraints . . . . . . . . . . . . . . . . 109 8.3. Constraint Enforcement Model . . . . . . . . . . . . . . 109 8.3.1. Payload Parsing . . . . . . . . . . . . . . . . . . . 110 8.3.2. NETCONF Processing . . . . . . . . . . 110 8.3.3. Validation . . . . . . . . . . . . . . . . . . . . . 111 9. Built-in Types . . . . . . . . . . . . . . . . . . . . . . . 112 9.1. Canonical representation . . . . . . . . . . . . . . . . 112 9.2. The Integer Built-in Types . . . . . . . . . . . . . . . 112 9.2.1. Lexicographic Representation . . . . . . . . . . . . 113 9.2.2. Canonical Form . . . . . . . . . . . . . . . . . . . 114 9.2.3. Restrictions . . . . . . . . . . . . . . . . . . . . 114 9.2.4. The range Statement . . . . . . . . . . . . . . . . . 114 9.2.5. Usage Example . . . . . . . . . . . . . . . . . . . . 115 9.3. The decimal64 Built-in Type . . . . . . . . . . . . . . . 115 9.3.1. Lexicographic Representation . . . . . . . . . . . . 115 9.3.2. Canonical Form . . . . . . . . . . . . . . . . . . . 115 9.3.3. Restrictions . . . . . . . . . . . . . . . . . . . . 116 9.3.4. The fraction-digits Statement . . . . . . . . . . . . 116 9.3.5. Usage Example . . . . . . . . . . . . . . . . . . . . 116 9.4. The string Built-in Type . . . . . . . . . . . . . . . . 117 9.4.1. Lexicographic Representation . . . . . . . . . . . . 117 9.4.2. Canonical Form . . . . . . . . . . . . . . . . . . . 117 9.4.3. Restrictions . . . . . . . . . . . . . . . . . . . . 117 Bjorklund Expires April 25, 2010 [Page 5] Internet-Draft YANG October 2009 9.4.4. The length Statement . . . . . . . . . . . . . . . . 117 9.4.5. Usage Example . . . . . . . . . . . . . . . . . . . . 118 9.4.6. The pattern Statement . . . . . . . . . . . . . . . . 118 9.4.7. Usage Example . . . . . . . . . . . . . . . . . . . . 119 9.5. The boolean Built-in Type . . . . . . . . . . . . . . . . 119 9.5.1. Lexicographic Representation . . . . . . . . . . . . 119 9.5.2. Canonical Form . . . . . . . . . . . . . . . . . . . 119 9.5.3. Restrictions . . . . . . . . . . . . . . . . . . . . 120 9.6. The enumeration Built-in Type . . . . . . . . . . . . . . 120 9.6.1. Lexicographic Representation . . . . . . . . . . . . 120 9.6.2. Canonical Form . . . . . . . . . . . . . . . . . . . 120 9.6.3. Restrictions . . . . . . . . . . . . . . . . . . . . 120 9.6.4. The enum Statement . . . . . . . . . . . . . . . . . 120 9.6.5. Usage Example . . . . . . . . . . . . . . . . . . . . 121 9.7. The bits Built-in Type . . . . . . . . . . . . . . . . . 122 9.7.1. Restrictions . . . . . . . . . . . . . . . . . . . . 122 9.7.2. Lexicographic Representation . . . . . . . . . . . . 122 9.7.3. Canonical Form . . . . . . . . . . . . . . . . . . . 122 9.7.4. The bit Statement . . . . . . . . . . . . . . . . . . 122 9.7.5. Usage Example . . . . . . . . . . . . . . . . . . . . 123 9.8. The binary Built-in Type . . . . . . . . . . . . . . . . 124 9.8.1. Restrictions . . . . . . . . . . . . . . . . . . . . 124 9.8.2. Lexicographic Representation . . . . . . . . . . . . 124 9.8.3. Canonical Form . . . . . . . . . . . . . . . . . . . 124 9.9. The leafref Built-in Type . . . . . . . . . . . . . . . . 124 9.9.1. Restrictions . . . . . . . . . . . . . . . . . . . . 125 9.9.2. The path Statement . . . . . . . . . . . . . . . . . 125 9.9.3. Lexicographic Representation . . . . . . . . . . . . 126 9.9.4. Canonical Form . . . . . . . . . . . . . . . . . . . 126 9.9.5. Usage Example . . . . . . . . . . . . . . . . . . . . 126 9.10. The identityref Built-in Type . . . . . . . . . . . . . . 129 9.10.1. Restrictions . . . . . . . . . . . . . . . . . . . . 129 9.10.2. The identityref's base Statement . . . . . . . . . . 129 9.10.3. Lexicographic Representation . . . . . . . . . . . . 130 9.10.4. Canonical Form . . . . . . . . . . . . . . . . . . . 130 9.10.5. Usage Example . . . . . . . . . . . . . . . . . . . . 130 9.11. The empty Built-in Type . . . . . . . . . . . . . . . . . 131 9.11.1. Restrictions . . . . . . . . . . . . . . . . . . . . 131 9.11.2. Lexicographic Representation . . . . . . . . . . . . 131 9.11.3. Canonical Form . . . . . . . . . . . . . . . . . . . 131 9.11.4. Usage Example . . . . . . . . . . . . . . . . . . . . 131 9.12. The union Built-in Type . . . . . . . . . . . . . . . . . 132 9.12.1. Restrictions . . . . . . . . . . . . . . . . . . . . 132 9.12.2. Lexicographic Representation . . . . . . . . . . . . 132 9.12.3. Canonical Form . . . . . . . . . . . . . . . . . . . 132 9.13. The instance-identifier Built-in Type . . . . . . . . . . 133 9.13.1. Restrictions . . . . . . . . . . . . . . . . . . . . 133 9.13.2. The require-instance Statement . . . . . . . . . . . 134 Bjorklund Expires April 25, 2010 [Page 6] Internet-Draft YANG October 2009 9.13.3. Lexicographic Representation . . . . . . . . . . . . 134 9.13.4. Canonical Form . . . . . . . . . . . . . . . . . . . 134 9.13.5. Usage Example . . . . . . . . . . . . . . . . . . . . 134 10. Updating a Module . . . . . . . . . . . . . . . . . . . . . . 136 11. YIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 11.1. Formal YIN Definition . . . . . . . . . . . . . . . . . . 139 11.1.1. Usage Example . . . . . . . . . . . . . . . . . . . . 141 12. YANG ABNF Grammar . . . . . . . . . . . . . . . . . . . . . . 143 13. Error Responses for YANG Related Errors . . . . . . . . . . . 165 13.1. Error Message for Data that Violates a unique Statement . 165 13.2. Error Message for Data that Violates a max-elements Statement . . . . . . . . . . . . . . . . . . . . . . . . 165 13.3. Error Message for Data that Violates a min-elements Statement . . . . . . . . . . . . . . . . . . . . . . . . 165 13.4. Error Message for Data that Violates a must Statement . . 166 13.5. Error Message for Data that Violates a require-instance Statement . . . . . . . . . . . . . . . 166 13.6. Error Message for Data that does not Match a leafref Type . . . . . . . . . . . . . . . . . . . . . . . . . . 166 13.7. Error Message for Data that Violates a mandatory choice Statement . . . . . . . . . . . . . . . . . . . . 166 13.8. Error Message for the "insert" Operation . . . . . . . . 167 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 168 15. Security Considerations . . . . . . . . . . . . . . . . . . . 169 16. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 170 17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 171 18. References . . . . . . . . . . . . . . . . . . . . . . . . . 172 18.1. Normative References . . . . . . . . . . . . . . . . . . 172 18.2. Non-Normative References . . . . . . . . . . . . . . . . 173 Appendix A. ChangeLog . . . . . . . . . . . . . . . . . . . . . 174 A.1. Version -08 . . . . . . . . . . . . . . . . . . . . . . . 174 A.2. Version -07 . . . . . . . . . . . . . . . . . . . . . . . 174 A.3. Version -06 . . . . . . . . . . . . . . . . . . . . . . . 174 A.4. Version -05 . . . . . . . . . . . . . . . . . . . . . . . 174 A.5. Version -04 . . . . . . . . . . . . . . . . . . . . . . . 175 A.6. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 175 A.7. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 176 A.8. Version -01 . . . . . . . . . . . . . . . . . . . . . . . 176 A.9. Version -00 . . . . . . . . . . . . . . . . . . . . . . . 177 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 178 Bjorklund Expires April 25, 2010 [Page 7] Internet-Draft YANG October 2009 1. Introduction YANG is a data modeling language used to model configuration and state data manipulated by the Network Configuration Protocol (NETCONF) protocol, NETCONF remote procedure calls, and NETCONF notifications. YANG is used to model the operations and content layers of NETCONF (see the NETCONF Configuration Protocol [RFC4741], section 1.1). This document describes the syntax and semantics of the YANG language, how the data model defined in a YANG module is represented in XML, and how NETCONF operations are used to manipulate the data. Bjorklund Expires April 25, 2010 [Page 8] Internet-Draft YANG October 2009 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]. Bjorklund Expires April 25, 2010 [Page 9] Internet-Draft YANG October 2009 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 nodes 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 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. o extension: An extension attaches non-YANG semantics to nodes. The extension statement defines new statements to express these semantics. Bjorklund Expires April 25, 2010 [Page 10] Internet-Draft YANG October 2009 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. The grouping statement is not a data definition statement and, as such, does not define any nodes in the schema tree. o identifier: Used to identify different kinds of YANG items by name. o instance identifier: A mechanism for identifying a particular node in a data tree. o interior node: Nodes within a hierarchy that are not leaf nodes. o leaf: A 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 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. 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. Bjorklund Expires April 25, 2010 [Page 11] Internet-Draft YANG October 2009 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 top-level data node: A data node where there is no other data node between it and a module or submodule statement. o uses: The "uses" statement is used to instantiate the set of nodes defined in a grouping statement. The instantiated nodes may be refined and augmented to tailor them to any specific needs. 3.1. Mandatory nodes A mandatory node is one of: o A leaf, choice, or anyxml node with a "mandatory" statement with the value "true". o A list or leaf-list node with a "min-elements" statement with a value greater than zero. o A container node without a "presence" statement, which has at least one mandatory node as a child. Bjorklund Expires April 25, 2010 [Page 12] Internet-Draft YANG October 2009 4. YANG Overview 4.1. Functional Overview YANG is a language used to model data for the NETCONF protocol. A YANG module defines a hierarchy of data 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 augmented, allowing one module to add data nodes to the hierarchy defined in another module. This augmentation can be conditional, with new nodes appearing only if certain conditions are met. YANG models can describe constraints to be enforced on the data, restricting the appearance or value of nodes based on the presence or value of other nodes in the hierarchy. These constraints are enforceable by either the client or the server, and valid content must abide by them. YANG defines a set of built-in types, and has a type mechanism through which additional types may be defined. Derived types can restrict their base type's set of valid values using mechanisms like range or pattern restrictions that can be enforced by clients or servers. They can also define usage conventions for use of the derived type, such as a string-based type that contains a host name. YANG permits the definition of 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 where list entries are identified by keys which distinguish them from each other. Such lists may be defined as either sorted by user or automatically sorted by the system. For user-sorted lists, operations are defined for manipulating the order of the list entries. Bjorklund Expires April 25, 2010 [Page 13] Internet-Draft YANG October 2009 YANG modules can be translated into an XML format called YANG Independent Notation (YIN) (Section 11), allowing applications using XML parsers and XSLT scripts to operate on the models. The conversion from YANG to YIN is loss-less, so content in YIN can be round-tripped back into YANG. YANG strikes a balance between high-level data modeling and low-level bits-on-the-wire encoding. The reader of a YANG module can see the high-level view of the data model while understanding how the data will be encoded in NETCONF operations. YANG is an extensible language, allowing extension statements to be defined by standards bodies, vendors, and individuals. The statement syntax allows these extensions to coexist with standard YANG statements in a natural way, while 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. This progressive approach handles the inter-related nature of YANG concepts and statements. A detailed description of YANG statements and syntax begins in Section 7. 4.2.1. Modules and Submodules A module contains three types of statements: module-header statements, revision statements, and definition statements. The module header statements describe the module and give information about the module itself, the revision statements give information Bjorklund Expires April 25, 2010 [Page 14] Internet-Draft YANG October 2009 about the history of the module, and the definition statements are the body of the module where the data model is defined. A NETCONF server may implement a number of modules, allowing multiple views of the same data, or multiple views of disjoint subsections of the device's data. Alternatively, the server may implement only one module that defines all available data. A module may be divided into submodules, based on the needs of the module owner. The external view remains that of a single module, regardless of the presence or size of its submodules. The "include" statement allows a module or submodule to reference material in submodules, and the "import" statement allows references to material defined in other modules. 4.2.2. Data Modeling Basics YANG defines four types of nodes for data modeling. In each of the following subsections, the example shows the YANG syntax as well as a corresponding NETCONF XML representation. 4.2.2.1. Leaf Nodes A leaf node contains simple data like an integer or a string. It has exactly one value of a particular type, and no child nodes. YANG Example: leaf host-name { type string; description "Hostname for this system"; } NETCONF XML Example: my.example.com 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: Bjorklund Expires April 25, 2010 [Page 15] Internet-Draft YANG October 2009 leaf-list domain-search { type string; description "List of domain names to search"; } NETCONF XML Example: high.example.com low.example.com everywhere.example.com 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 Example: Good morning, Dave The "container" statement is covered in Section 7.5. 4.2.2.4. List Nodes A list defines a sequence of list entries. Each entry is like a structure or a record instance, and is uniquely identified by the values of its key leafs. A list can define multiple keys and may contain any number of child nodes of any type (including leafs, Bjorklund Expires April 25, 2010 [Page 16] Internet-Draft YANG October 2009 lists, containers etc.). YANG Example: list user { key "name"; leaf name { type string; } leaf full-name { type string; } leaf class { type string; } } NETCONF XML Example: glocks Goldie Locks intruder snowey Snow White free-loader rzull Repun Zell tower The "list" statement is covered in Section 7.8. 4.2.2.5. Example Module These statements are combined to define the module: Bjorklund Expires April 25, 2010 [Page 17] Internet-Draft YANG October 2009 // 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; } } } } } Bjorklund Expires April 25, 2010 [Page 18] Internet-Draft YANG October 2009 4.2.3. State Data YANG can model state data, as well as configuration data, based on the "config" statement. When a node is tagged with "config false", its subhierarchy is flagged as state data, to be reported using NETCONF's operation, not the operation. Parent containers, lists, and key leafs are reported also, giving the context for the state data. In this example, two leafs are defined for each interface, a configured speed and an observed speed. The observed speed is not configuration, so it can be returned with NETCONF operations, but not with operations. The observed speed is not configuration data, and cannot be manipulated using . list interface { key "name"; leaf name { type string; } leaf speed { type enumeration { enum 10m; enum 100m; enum auto; } } leaf observed-speed { type uint32; config false; } } 4.2.4. Built-in Types YANG has a set of built-in types, similar to those of many programming languages, but with some differences due to special requirements from the management domain. The following table summarizes the built-in types discussed in Section 9: Bjorklund Expires April 25, 2010 [Page 19] Internet-Draft YANG October 2009 +---------------------+-------------+-------------------------------+ | Name | Type | Description | +---------------------+-------------+-------------------------------+ | binary | Text | Any binary data | | bits | Text/Number | A set of bits or flags | | boolean | Text | "true" or "false" | | decimal64 | Number | 64-bit signed decimal number | | empty | Empty | A leaf that does not have any | | | | value | | enumeration | Text/Number | Enumerated strings with | | | | associated numeric values | | 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 | | leafref | Text/Number | A reference to a leaf | | | | instance | | 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 7.4. 4.2.5. Derived Types (typedef) YANG can define derived types from base types using the "typedef" statement. A base type can be either a built-in type or a derived type, allowing a hierarchy of derived types. A derived type can be used as the argument for the "type" statement. YANG Example: Bjorklund Expires April 25, 2010 [Page 20] Internet-Draft YANG October 2009 typedef percent { type uint8 { range "0 .. 100"; } description "Percentage"; } leaf completed { type percent; } NETCONF XML Example: 20 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 Example:
192.0.2.1
830 Bjorklund Expires April 25, 2010 [Page 21] Internet-Draft YANG October 2009 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 NETCONF PDUs. The additional levels of hierarchy are not needed beyond the conceptual schema. YANG Example: Bjorklund Expires April 25, 2010 [Page 22] Internet-Draft YANG October 2009 container food { choice snack { case sports-arena { leaf pretzel { type empty; } leaf beer { type empty; } } case late-night { leaf chocolate { type enumeration { enum dark; enum milk; enum first-available; } } } } } NETCONF XML Example: The "choice" statement is covered in Section 7.9. 4.2.8. Extending Data Models (augment) YANG allows a module to insert additional nodes into data models, including both the current module (and its submodules) or an external module. This is useful for example for vendors to add vendor- specific parameters to standard data models in an interoperable way. The "augment" statement defines the location in the data model hierarchy where new nodes are inserted, and the "when" statement defines the conditions when the new nodes are valid. YANG Example: Bjorklund Expires April 25, 2010 [Page 23] Internet-Draft YANG October 2009 augment /system/login/user { when "class != 'wheel'"; leaf uid { type uint16 { range "1000 .. 30000"; } } } This example defines a "uid" node that only is valid when the user's "class" is not "wheel". If a module augments another module, the XML representation of the data will reflect the prefix of the augmenting module. For example, if the above augmentation were in a module with prefix "other", the XML would look like: NETCONF XML Example: alicew Alice N. Wonderland drop-out 1024 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; } } } Bjorklund Expires April 25, 2010 [Page 24] Internet-Draft YANG October 2009 NETCONF XML Example: acmefw-2.3 The image acmefw-2.3 is being installed. 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 leafref { path "/interfaces/interface/name"; } } leaf if-admin-status { type admin-status; } leaf if-oper-status { type oper-status; } } NETCONF XML Example: Bjorklund Expires April 25, 2010 [Page 25] Internet-Draft YANG October 2009 2007-09-01T10:00:00Z so-1/2/3.0 up down The "notification" statement is covered in Section 7.14. Bjorklund Expires April 25, 2010 [Page 26] Internet-Draft YANG October 2009 5. Language Concepts 5.1. Modules and Submodules The module is the base unit of definition in YANG. A module defines a single data model. A module can define a complete, cohesive model, or augment an existing data model with additional nodes. Submodules are partial modules that contribute definitions to a module. A module may include any number of submodules, but each submodule may belong to only one module. The names of all standard modules and submodules MUST be unique. Developers of enterprise modules are RECOMMENDED to choose names for their modules that will have a low probability of colliding with standard or other enterprise modules, e.g., by using the enterprise or organization name as a prefix for the module name. A module uses the "include" statement to include its submodules, and the "import" statement to reference external modules. Similarly, a submodule uses the "import" statement to reference other modules, and uses the "include" statement to reference other submodules within its module. A module or submodule MUST NOT include submodules from other modules, and a submodule MUST NOT import its own module. The import and include statements are used to make definitions available to other modules and submodules: o For a module or submodule to reference definitions in an external module, the external module MUST be imported. o For a module to reference definitions in one of its submodules, the module MUST include the submodule. o For a submodule to reference definitions in a second submodule of the same module, the first submodule MUST include the second submodule. There MUST NOT be any circular chains of imports or includes. For example, if submodule "a" includes submodule "b", "b" cannot include "a". When a definition in an external module is referenced, a locally defined prefix MUST be used, followed by ":", and then the external identifier. References to definitions in the local module MAY use the prefix notation. Since built-in data types do not belong to any module and have no prefix, references to built-in data types (e.g., int32) cannot use the prefix notation. Bjorklund Expires April 25, 2010 [Page 27] Internet-Draft YANG October 2009 5.1.1. Import and Include by Revision Published modules evolve independently over time. In order to allow for this evolution, modules need to be imported using specific revisions. When a module is written, it uses the current revisions of other modules, based on what is available at the time. As future revisions of the imported modules are published, the importing module is unaffected and its contents are unchanged. When the author of the module is prepared to move to the most recently published revision of an imported module, the module is republished with an updated import statement. By republishing with the new revision, the authors explicitly indicate their acceptance of any changes in the imported module. For submodules, the issue is related but simpler. A module or submodule that includes submodules need to specify the revision of the included submodules. If a submodule changes, any module or submodule that includes it needs to be updated. For example, module "b" imports module "a". module a { revision 2008-01-01 { ... } grouping a { leaf eh { .... } } } module b { import a { prefix p; revision-date 2008-01-01; } container bee { uses p:a; } } When the author of "a" publishes a new revision, the changes may not be acceptable to the author of "b". If the new revision is acceptable, the author of "b" can republish with an updated revision in the import statement. 5.1.2. Module Hierarchies YANG allows modeling of data in multiple hierarchies, where data may have more than one top-level node. Models that have multiple top- Bjorklund Expires April 25, 2010 [Page 28] Internet-Draft YANG October 2009 level nodes are sometimes convenient, and are supported by YANG. NETCONF is capable of carrying any XML content as the payload in the and elements. The top-level nodes of YANG modules are encoded as child elements within these elements. This encapsulation guarantees that the corresponding NETCONF PDUs are always well-formed XML documents. For example: module my-config { namespace "http://example.com/schema/config"; prefix "co"; container system { ... } container routing { ... } } could be encoded in NETCONF as: 5.2. File Layout YANG modules and submodules are typically stored in files, one module or submodule per file. The name of the file SHOULD be of the form: module-or-submodule-name ['.' revision-date] ( '.yang' / '.yin' ) YANG compilers can find imported modules and included submodules via this convention. While the YANG language defines modules, tools may compile submodules independently for performance and manageability Bjorklund Expires April 25, 2010 [Page 29] Internet-Draft YANG October 2009 reasons. Many errors and warnings that cannot be detected during submodule compilation may be delayed until the submodules are linked into a cohesive module. 5.3. XML Namespaces All YANG definitions are specified within a module that is bound to a particular XML Namespace [XML-NAMES], which is a globally unique URI [RFC3986]. A NETCONF client or server uses the namespace during XML encoding of data. Namespaces for modules published in RFC streams MUST be assigned by IANA, see Section 14. Namespaces for private modules are assigned by the organization owning the module without a central registry. Namespace URIs MUST be choosen so they cannot collide with standard or other enterprise namespaces, for example by using the enterprise or organization name in the namespace. The "namespace" statement is covered in Section 7.1.3. 5.3.1. YANG XML Namespace YANG defines an XML namespace for NETCONF operations and content. This namespace is "urn:ietf:params:xml:ns:yang:1". 5.4. Resolving Grouping, Type, and Identity Names Grouping, type, and identity names are resolved in the context in which they are defined, rather than the context in which they are used. Users of groupings, typedefs, and identities are not required to import modules or include submodules to satisfy all references made by the original definition. This behaves like static scoping in a conventional programming language. For example, if a module defines a grouping in which a type is referenced, when the grouping is used in a second module, the type is resolved in the context of the original module, not the second module. There is no worry over conflicts if both modules define the type, since there is no ambiguity. 5.5. Nested Typedefs and Groupings Typedefs and groupings may appear nested under many YANG statements, allowing these to be lexically scoped by the hierarchy under which they appear. This allows types and groupings to be defined near Bjorklund Expires April 25, 2010 [Page 30] Internet-Draft YANG October 2009 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 (i.e., those appearing as substatements to a module or submodule statement) can be used outside the module or submodule, the developer has more control over what pieces of their module are presented to the outside world, supporting the need to hide internal information and maintaining a boundary between what is shared with the outside world and what is kept private. Scoped definitions MUST NOT shadow definitions at a higher scope. A type or grouping cannot be defined if a higher level in the schema hierarchy has a definition with a matching identifier. A reference to an unprefixed type or grouping, or one which uses the prefix of the current module, is resolved by locating the closest matching "typedef" or "grouping" statement among the immediate substatements of each ancestor statement. 5.6. Conformance Conformance is a measure of how accurately a device follows the model. Generally speaking, devices are responsible for implementing the model faithfully, allowing applications to treat devices which implement the model identically. Deviations from the model can reduce the utility of the model and increase fragility of applications that use it. YANG modelers have three mechanisms for conformance: o the basic behavior of the model o optional features that are part of the model o deviations from the model We will consider each of these in sequence. Bjorklund Expires April 25, 2010 [Page 31] Internet-Draft YANG October 2009 5.6.1. Basic Behavior The model defines a contract between the NETCONF client and server, which allows both parties to have faith the other knows the syntax and semantics behind the modeled data. The strength of YANG lies in the strength of this contract and the mindless devotion with which implementers follow it. 5.6.2. 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 "feature". Features give the modeler a mechanism for making portions of the module conditional in a manner that is controlled by the device. The model can express constructs 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. A module may declare any number of features, identified by simple strings, and may make portions of the module optional based on those features. If the device supports a feature, then the corresponding portions of the module are valid for that device. If the device doesn't support the feature, those parts of the module are not valid, and applications should behave accordingly. Features are defined using the "feature" statement. Definitions in the module that are conditional to the feature are noted by the "if-feature" statement with the name of the feature as its argument. Further details are available in Section 7.18.1. 5.6.3. Deviations In an ideal world, all devices would be required to implement the model exactly as defined, and deviations from the model would not be allowed. But in the real world, devices are often not able or willing to implement the model as written. For YANG-based automation Bjorklund Expires April 25, 2010 [Page 32] Internet-Draft YANG October 2009 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. 5.6.4. Announcing Conformance Information in the Message The namespace URI MUST be advertised as a capability in the NETCONF 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-date module-parameter = "module=" module-name feature-parameter = "features=" feature *( "," feature ) deviation-parameter = "deviations=" deviation *( "," deviation ) Where "revision-date" is the revision of the module (see Section 7.1.9) that the NETCONF server implements, "module-name" is the name of module as it appears in the "module" statement (see Section 7.1), "namespace-uri" is the namespace 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). In the parameter list, each named parameter MUST occur at most once. Bjorklund Expires April 25, 2010 [Page 33] Internet-Draft YANG October 2009 5.6.4.1. Modules Servers indicate the names of supported modules via the message. Module namespaces are encoded as the base URI in the capability string, and the module name is encoded as the "module" parameter to the base URI. A server MUST advertise all revisions of all modules it implements. For example, this message advertises one module "syslog". http://example.com/syslog?module=syslog&revision=2008-04-01 5.6.4.2. Features Servers indicate the names of supported features via the 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. For example, this message advertises one module, informing the client that it supports the "local-storage" feature of module "syslog". http://example.com/syslog?module=syslog&features=local-storage 5.6.4.3. Deviations Device deviations are announced via the "deviations" parameter. The value of the deviations parameter is a comma-separated list of modules containing deviations from the capability's module. For example, this message advertises two modules, informing the client that it deviates from module "syslog" according to the deviations listed in the module "my-devs". Bjorklund Expires April 25, 2010 [Page 34] Internet-Draft YANG October 2009 http://example.com/syslog?module=syslog&deviations=my-devs http://example.com/my-deviations?module=my-devs Bjorklund Expires April 25, 2010 [Page 35] Internet-Draft YANG October 2009 6. YANG syntax The YANG syntax is similar to that of SMIng [RFC3780] and programming languages like C and C++. This C-like syntax was chosen specifically for its readability, since YANG values the time and effort of the readers of models above those of modules writers and YANG tool-chain developers. This section introduces the YANG syntax. YANG modules use the UTF-8 [RFC3629] character encoding. 6.1. 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 space or tab characters, a semicolon (";"), braces ("{" or "}"), or comment sequences ("//", "/*", or "*/"), then it MUST be enclosed within double or single quotes. If the double quoted string contains a line break followed by space or tab characters which are used to indent the text according to the layout in the YANG file, this leading whitespace is stripped from the string, up to the column of the double quote character, or to the first non-whitespace character, whichever occurs first. If the double quoted string contains space or tab characters before a line break, this trailing whitespace is stripped from the string. Bjorklund Expires April 25, 2010 [Page 36] Internet-Draft YANG October 2009 A single quoted string (enclosed within ' ') preserves each character within the quotes. A single quote character cannot occur in a single quoted string, even when preceded by a backslash. Within a double quoted string (enclosed within " "), a backslash character introduces a special character, which depends on the character that immediately follows the backslash: \n new line \t a tab character \" a double quote \\ a single backslash If a quoted string is followed by a plus character ("+"), followed by another quoted string, the two strings are concatenated into one string, allowing multiple concatenations to build one string. Whitespace trimming and substition of backslash-escaped characters in double quoted strings is done before concatenation. 6.1.3.1. Quoting Examples The following strings are equivalent: hello "hello" 'hello' "hel" + "lo" 'hel' + "lo" 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" Bjorklund Expires April 25, 2010 [Page 37] Internet-Draft YANG October 2009 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 64 characters in length. Identifiers are case sensitive. The identifier syntax is formally defined by the rule "identifier" in Section 12. Identifiers can be specified as quoted or unquoted strings. 6.2.1. Identifiers and their namespaces Each identifier is valid in a namespace which depends on the type of the YANG item being defined. All identifiers defined in a namespace MUST be unique. o All module and submodule names share the same global module identifier namespace. o All extension names defined in a module and its submodules share the same extension identifier namespace. o All feature names defined in a module and its submodules share the same feature identifier namespace. o All identity names defined in a module and its submodules share the same identity identifier namespace. o All derived type names defined within a parent node or at the top- level of the module or its submodules share the same type identifier namespace. This namespace is scoped to all descendant nodes of the parent node or module. This means that any descendent node may use that typedef, and it MUST NOT define a typedef with the same name. o All grouping names defined within a parent node or at the top- level of the module or its submodules share the same grouping identifier namespace. This namespace is scoped to all descendant nodes of the parent node or module. This means that any descendent node may use that grouping, and it MUST NOT define a grouping with the same name. o All leafs, leaf-lists, lists, containers, choices, rpcs, notifications, and anyxmls defined (directly or through a uses statement) within a parent node or at the top-level of the module or its submodules share the same identifier namespace. This namespace is scoped to the parent node or module, unless the Bjorklund Expires April 25, 2010 [Page 38] Internet-Draft YANG October 2009 parent node is a case node. In that case, the namespace is scoped to the closest ancestor node which is not a case or choice node. o All cases within a choice share the same case identifier namespace. This namespace is scoped to the parent choice node. Forward references are allowed in YANG. 6.3. Statements A YANG module contains a sequence of statements. Each statement starts with a keyword, followed by zero or one argument, followed either by a semicolon (";") or a block of substatements enclosed within braces ("{ }"): statement = keyword [argument] (";" / "{" *statement "}") The argument is a string, as defined in Section 6.1.2. 6.3.1. Language Extensions A module can introduce YANG extensions by using the "extension" keyword (see Section 7.17). The extensions can be imported by other modules with the "import" statement (see Section 7.1.5). When an imported extension is used, the extension's keyword MUST be qualified using the prefix with which the extension's module was imported. If an extension is used in the module where it is defined, the extension's keyword MUST be qualified with the module's prefix. 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. If a YANG compiler does not support a particular extension, which appears in a YANG module as an unknown-statement (see Section 12), the entire unknown statement MAY be ignored by the compiler. 6.4. XPath Evaluations YANG relies on XPath 1.0 [XPATH] as a notation for specifying many inter-node references and dependencies. NETCONF clients and servers are not required to implement an XPath interpreter, but MUST ensure that the requirements encoded in the data model are enforced. The manner of enforcement is an implementation decision. The XPath expressions MUST be syntactically correct, and all prefixes used MUST be present in the XPath context (see Section 6.4.1). An implementation may choose to implement them by hand, rather than Bjorklund Expires April 25, 2010 [Page 39] Internet-Draft YANG October 2009 using the XPath expression directly. The data model used in the XPath expressions is the same as that used in XPath 1.0 [XPATH], with the same extension for root node children as used by XSLT 1.0 [XSLT] (section 3.1). Specifically, it means that the root node may have any number of element nodes as its children. 6.4.1. XPath Context All YANG XPath expressions share the following XPath context definition: o The set of namespace declarations is the set of all "import" statements' prefix and namespace pairs in the module where the XPath expression is specified, and the "prefix" statement's prefix for the "namespace" statement's URI. o Names without a namespace prefix belong to the same namespace as the identifier of the current node. Inside a grouping, that namespace is affected by where the grouping is used (see Section 7.12). o The function library is the core function library defined in [XPATH], and a function "current()" which returns a node set with the initial context node. o The set of variable bindings is empty. The mechanism for handling unprefixed names is adopted from XPath 2.0 [XPATH2.0], and helps simplify XPath expressions in YANG. No ambiguity may ever arise because YANG node identifiers are always qualified names with a non-null namespace URI. The context node varies with the YANG XPath expression, and is specified where the YANG statement with the XPath expression is defined. 6.5. Schema Node Identifier A schema node identifier is a string that identifies a node in the schema tree. It has two forms, "absolute" and "descendant", defined by the rules "absolute-schema-nodeid" and "descendant-schema-nodeid" in Section 12, respectively. A schema node identifier consists of a path of identifiers, separated by slashes ("/"). In an absolute schema node identifier, the first identifier after the leading slash is any top-level schema node in local module or all imported modules. Bjorklund Expires April 25, 2010 [Page 40] Internet-Draft YANG October 2009 References to identifiers defined in external modules MUST be qualified with appropriate prefixes, and references to identifiers defined in the current module and its submodules MAY use a prefix. For example, to identify the child node "b" of top-level node "a", the string "/a/b" can be used. Bjorklund Expires April 25, 2010 [Page 41] Internet-Draft YANG October 2009 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. Names of modules published in RFC streams MUST be assigned by IANA, see Section 14. Private module names are assigned by the organization owning the module without a central registry. It is RECOMMENDED to choose module names that will have a low probability of colliding with standard or other enterprise modules and submodules, e.g., by using the enterprise or organization name as a prefix. A module SHOULD have the following layout: Bjorklund Expires April 25, 2010 [Page 42] Internet-Draft YANG October 2009 module { // header information // linkage statements // meta information // revision history // module definitions } 7.1.1. The module's Substatements Bjorklund Expires April 25, 2010 [Page 43] Internet-Draft YANG October 2009 +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | anyxml | 7.10 | 0..n | | augment | 7.15 | 0..n | | choice | 7.9 | 0..n | | contact | 7.1.8 | 0..1 | | container | 7.5 | 0..n | | description | 7.19.3 | 0..1 | | deviation | 7.18.3 | 0..n | | extension | 7.17 | 0..n | | feature | 7.18.1 | 0..n | | grouping | 7.11 | 0..n | | identity | 7.16 | 0..n | | import | 7.1.5 | 0..n | | include | 7.1.6 | 0..n | | leaf | 7.6 | 0..n | | leaf-list | 7.7 | 0..n | | list | 7.8 | 0..n | | namespace | 7.1.3 | 1 | | notification | 7.14 | 0..n | | organization | 7.1.7 | 0..1 | | prefix | 7.1.4 | 1 | | reference | 7.19.4 | 0..1 | | revision | 7.1.9 | 0..n | | rpc | 7.13 | 0..n | | typedef | 7.3 | 0..n | | uses | 7.12 | 0..n | | yang-version | 7.1.2 | 0..1 | +--------------+---------+-------------+ 7.1.2. The yang-version Statement The optional "yang-version" statement specifies which version of the YANG language was used in developing the module. The statement's argument is a string. If present, it MUST contain the value "1", which is the current yang version and the default value. 7.1.3. The namespace Statement The "namespace" statement defines the XML namespace to which all identifiers defined by the module belong, with the exception of data node identifiers defined inside a grouping (see Section 7.12 for details). The argument to the "namespace" statement is the URI of the namespace. See also Section 5.3. Bjorklund Expires April 25, 2010 [Page 44] Internet-Draft YANG October 2009 7.1.4. The prefix Statement The "prefix" statement is used to define the prefix associated with the module and its namespace. The "prefix" statement's argument is the prefix string 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 imported module is used in combination with a colon (":") and the identifier, e.g., "if: ifIndex". To improve readability of YANG modules, the prefix defined by a module SHOULD be used when the module is imported, unless there is a conflict. All prefixes, including the prefix for the module itself MUST be unique within the module or submodule. 7.1.5. The import Statement The "import" statement makes definitions from one module available inside another module or submodule. The argument is the name of the module to import, and the statement is followed by a block of substatements that holds detailed import information. When a module is imported, the importing module may: o use any grouping and typedef defined at the top-level in the imported module or its submodules o use any extension, feature, and identity defined in the imported module or its submodules o use any node in the imported module's schema tree in augment, must, path, and when statements 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. Bjorklund Expires April 25, 2010 [Page 45] Internet-Draft YANG October 2009 When the optional "revision-date" substatement is present, any typedef, grouping, extension, feature, and identity referenced by definitions in the local module are taken from the specified revision of the imported module. Otherwise, it is undefined from which revision of the module they are taken. Multiple revisions of the same module MUST NOT be imported. The import's Substatements +---------------+---------+-------------+ | substatement | section | cardinality | +---------------+---------+-------------+ | prefix | 7.1.4 | 1 | | revision-date | 7.1.5.1 | 0..1 | +---------------+---------+-------------+ 7.1.5.1. The import's revision-date Statement The import's "revision-date" statement is used to specify the exact version of the module to import. The "revision-date" statement MUST match the most recent "revision" statement in the imported module. 7.1.6. The include Statement The "include" statement is used to make content from a submodule available to that submodule's parent module, or to another submodule of that parent module. The argument is an identifier 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). Submodules are only allowed to include other submodules belonging to the same module. When a module includes a submodule, it incorporates the contents of the submodule into the node hierarchy of the module. When a submodule includes another submodule, the target submodule's definitions are made available to the current submodule. When the optional "revision-date" is present, the specified revision of the submodule is included in the module. Otherwise, it is undefined which revision of the submodule is included. Multiple revisions of the same submodule MUST NOT be included. Bjorklund Expires April 25, 2010 [Page 46] Internet-Draft YANG October 2009 The includes's Substatements +---------------+---------+-------------+ | substatement | section | cardinality | +---------------+---------+-------------+ | revision-date | 7.1.5.1 | 0..1 | +---------------+---------+-------------+ 7.1.7. The organization Statement The "organization" statement defines the party responsible for this module. The argument is a string 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 published editorial change, a new one SHOULD be added in front of the revisions sequence, so that all revisions are in reverse chronological order. 7.1.9.1. The revision's Substatement +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | description | 7.19.3 | 0..1 | | reference | 7.19.4 | 0..1 | +--------------+---------+-------------+ Bjorklund Expires April 25, 2010 [Page 47] Internet-Draft YANG October 2009 7.1.10. Usage Example module acme-system { namespace "http://acme.example.com/system"; prefix "acme"; import ietf-yang-types { prefix "yang"; } include acme-types; organization "ACME Inc."; contact "Joe L. User ACME, Inc. 42 Anywhere Drive Nowhere, CA 95134 USA Phone: +1 800 555 0100 EMail: joe@acme.example.com"; description "The module for entities implementing the ACME protocol."; revision "2007-06-09" { description "Initial revision."; } // definitions 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 a module designer to split a complex model into several pieces where all the submodules contribute to a single namespace, which is defined by the module that includes the submodules. The "submodule" statement defines the submodule's name, and groups all statements which belong to the submodule together. The "submodule" statement's argument is the name of the submodule, followed by a block of substatements that hold detailed submodule information. The submodule name follows the rules for identifiers in Section 6.2. Bjorklund Expires April 25, 2010 [Page 48] Internet-Draft YANG October 2009 Names of submodules published in RFC streams MUST be assigned by IANA, see Section 14. Private submodule names are assigned by the organization owning the submodule without a central registry. It is RECOMMENDED to choose submodule names that will have a low probability of colliding with standard or other enterprise modules and submodules, e.g., by using the enterprise or organization name as a prefix. A submodule SHOULD have the following layout: submodule { // module identification // linkage statements // meta information // revision history // module definitions } 7.2.1. The submodule's Substatements Bjorklund Expires April 25, 2010 [Page 49] Internet-Draft YANG October 2009 +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | anyxml | 7.10 | 0..n | | augment | 7.15 | 0..n | | belongs-to | 7.2.2 | 1 | | choice | 7.9 | 0..n | | contact | 7.1.8 | 0..1 | | container | 7.5 | 0..n | | description | 7.19.3 | 0..1 | | deviation | 7.18.3 | 0..n | | extension | 7.17 | 0..n | | feature | 7.18.1 | 0..n | | grouping | 7.11 | 0..n | | identity | 7.16 | 0..n | | import | 7.1.5 | 0..n | | include | 7.1.6 | 0..n | | leaf | 7.6 | 0..n | | leaf-list | 7.7 | 0..n | | list | 7.8 | 0..n | | notification | 7.14 | 0..n | | organization | 7.1.7 | 0..1 | | reference | 7.19.4 | 0..1 | | revision | 7.1.9 | 0..n | | rpc | 7.13 | 0..n | | typedef | 7.3 | 0..n | | uses | 7.12 | 0..n | | yang-version | 7.1.2 | 0..1 | +--------------+---------+-------------+ 7.2.2. The belongs-to Statement The "belongs-to" statement specifies the module to which the submodule belongs. The argument is an identifier which is the name of the module. A submodule MUST only be included by the module to which it belongs, or by another submodule which belongs to that module. The mandatory "prefix" substatement assigns a prefix for the module to which the submodule belongs. All definitions in the local submodule and any included submodules can be accessed by using the prefix. Bjorklund Expires April 25, 2010 [Page 50] Internet-Draft YANG October 2009 The belongs-to's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | prefix | 7.1.4 | 1 | +--------------+---------+-------------+ 7.2.3. Usage Example submodule acme-types { belongs-to "acme-system" { prefix "acme"; } import ietf-yang-types { prefix "yang"; } organization "ACME Inc."; contact "Joe L. User ACME, Inc. 42 Anywhere Drive Nowhere, CA 95134 USA Phone: +1 800 555 0100 EMail: joe@acme.example.com"; description "This submodule defines common ACME types."; revision "2007-06-09" { description "Initial revision."; } // definitions follows... } 7.3. The typedef Statement The "typedef" statement defines a new type which may be used locally in the module, in modules or submodules which include it, and by other modules which import from it, according to the rules in Section 5.5. The new type is called the "derived type", and the type Bjorklund Expires April 25, 2010 [Page 51] Internet-Draft YANG October 2009 from which it was derived is called the "base type". All derived types can be traced back to a YANG built-in type. The "typedef" statement's argument is an identifier 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. 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 be valid according to the type specified in the "type" statement. If the base type has a default value, and the new derived type does not specify a new default value, the base type's default value is also the default value of the new derived type. Bjorklund Expires April 25, 2010 [Page 52] Internet-Draft YANG October 2009 If the type's default value is not valid according to the new restrictions specified in a derived type or leaf definition, the derived type or leaf definition MUST specify a new default value compatible with the restrictions. 7.3.5. Usage Example typedef listen-ipv4-address { type inet:ipv4-address; default "0.0.0.0"; } 7.4. The type Statement The "type" statement takes as an argument a string 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 depend on the type being restricted. The restriction statements for all built-in types are described in the subsections of Section 9. 7.4.1. The type's Substatements +------------------+---------+-------------+ | substatement | section | cardinality | +------------------+---------+-------------+ | bit | 9.7.4 | 0..n | | enum | 9.6.4 | 0..n | | length | 9.4.4 | 0..1 | | path | 9.9.2 | 0..1 | | pattern | 9.4.6 | 0..n | | range | 9.2.4 | 0..1 | | require-instance | 9.13.2 | 0..1 | | type | 7.4 | 0..n | +------------------+---------+-------------+ 7.5. The container Statement The "container" statement is used to define an interior 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. Bjorklund Expires April 25, 2010 [Page 53] Internet-Draft YANG October 2009 7.5.1. 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. This is the default style. 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 (see Section 7.5.5) is used to give semantics to the existence of the container in the data tree. Bjorklund Expires April 25, 2010 [Page 54] Internet-Draft YANG October 2009 7.5.2. The container's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | anyxml | 7.10 | 0..n | | choice | 7.9 | 0..n | | config | 7.19.1 | 0..1 | | container | 7.5 | 0..n | | description | 7.19.3 | 0..1 | | grouping | 7.11 | 0..n | | if-feature | 7.18.2 | 0..n | | leaf | 7.6 | 0..n | | leaf-list | 7.7 | 0..n | | list | 7.8 | 0..n | | must | 7.5.3 | 0..n | | presence | 7.5.5 | 0..1 | | reference | 7.19.4 | 0..1 | | status | 7.19.2 | 0..1 | | typedef | 7.3 | 0..n | | uses | 7.12 | 0..n | | when | 7.19.5 | 0..1 | +--------------+---------+-------------+ 7.5.3. The must Statement The "must" statement, which is optional, takes as an argument a string 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 store 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 use (see Section 7.6.1). 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 XPath expression is conceptually evaluated in the following context, in addition to the definition in Section 6.4.1: o The context node is the node in the data tree for which the "must" statement is defined. o The accessible tree is made up of all nodes in the data tree, and all leafs with default values in use (see Section 7.6.1). Bjorklund Expires April 25, 2010 [Page 55] Internet-Draft YANG October 2009 The accessible tree depends on the context node: o If the context node represents configuration, the tree is the data in the NETCONF datastore where the context node exists. The XPath root node has all top-level configuration data nodes in all modules as children. o If the context node represents state data, the tree is all state data on the device, and the datastore. The XPath root node has all top-level data nodes in all modules as children. o If the context node represents notification content, the tree is the notification XML instance document. The XPath root node has the element representing the notification being defined as the only child. o If the context node represents RPC input parameters, the tree is the RPC XML instance document. The XPath root node has the element representing the RPC method being defined as the only child. o If the context node represents RPC output parameters, the tree is the RPC reply instance document. The XPath root node has the elements representing the RPC output parameters as children. The result of the XPath expression is converted to a boolean value using the standard XPath rules. Note that since all leaf values in the data tree are conceptually stored in their canonical form (see 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. Bjorklund Expires April 25, 2010 [Page 56] Internet-Draft YANG October 2009 7.5.4. The must's Substatements +---------------+---------+-------------+ | substatement | section | cardinality | +---------------+---------+-------------+ | description | 7.19.3 | 0..1 | | error-app-tag | 7.5.4.2 | 0..1 | | error-message | 7.5.4.1 | 0..1 | | reference | 7.19.4 | 0..1 | +---------------+---------+-------------+ 7.5.4.1. The error-message Statement The "error-message" statement, which is optional, takes a string as an argument. If the constraint evaluates to false, the string is passed as in the . 7.5.4.2. The error-app-tag Statement The "error-app-tag" statement, which is optional, takes a string as an argument. If the constraint evaluates to false, the string is passed as in the . 7.5.4.3. Usage Example of must and error-message container interface { leaf ifType { type enumeration { enum ethernet; enum atm; } } leaf ifMTU { type uint32; } must "ifType != 'ethernet' or " + "(ifType = 'ethernet' and ifMTU = 1500)" { error-message "An ethernet MTU must be 1500"; } must "ifType != 'atm' or " + "(ifType = 'atm' and ifMTU <= 17966 and ifMTU >= 64)" { error-message "An atm MTU must be 64 .. 17966"; } } Bjorklund Expires April 25, 2010 [Page 57] Internet-Draft YANG October 2009 7.5.5. The presence Statement The "presence" statement assigns a meaning to the presence of a container in the data tree. It takes as an argument a string 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 7.5.1 for additional information. 7.5.6. The container's Child Node Statements Within a container, the "container", "leaf", "list", "leaf-list", "uses", "choice", and "anyxml" statements can be used to define child nodes to the container. 7.5.7. XML Mapping Rules A container node is encoded as an XML element. The element's 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. If the container defines RPC input or output parameters, the subelements are encoded in the same order as they are defined within the container statement. Otherwise, the subelements are encoded in any order. A NETCONF server that replies to a or 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 for a or request, must be prepared to handle the case that a container node without a presence statement is not present in the XML. 7.5.8. NETCONF Operations Containers can be created, deleted, replaced and modified through , by using the "operation" attribute (See [RFC4741], section 7.2) in the container's XML element. If a container does not have a "presence" statement and the last child node is deleted, the NETCONF server MAY delete the container. Bjorklund Expires April 25, 2010 [Page 58] Internet-Draft YANG October 2009 7.5.9. Usage Example Given the following container definition: container system { description "Contains various system parameters"; container services { description "Configure externally available services"; container "ssh" { presence "Enables SSH"; description "SSH service specific configuration"; // more leafs, containers and stuff here... } } } A corresponding XML instance example: Since the element is present, ssh is enabled. To delete a container with an : Bjorklund Expires April 25, 2010 [Page 59] Internet-Draft YANG October 2009 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. 7.6.1. The leaf's default value The default value of a leaf is the value that the server uses if the leaf does not exist in the data tree. The usage of the default value depends on the leaf's closest ancestor node in the schema tree which is not a non-presence container: o If no such ancestor exists in the schema tree, the default value MUST be used. o Otherwise, if this ancestor is a case node, the default value MUST be used if any node from the case exists in the data tree, or if the case node is the choice's default case, and no nodes from any other case exist in the data tree. o Otherwise, the default value MUST be used if the ancestor node exists in the data tree. In these cases, the default value is said to be in use. When the default value is in use, the server MUST operationally behave as is if the leaf was present in the data tree with the default value as its value. If a leaf has a "default" statement, the leaf's default value is the value of the "default" statement. Otherwise, if the leaf's type has a default value, and the leaf is not mandatory, then the leaf's default value is the type's default value. In all other cases, the leaf does not have a default value. Bjorklund Expires April 25, 2010 [Page 60] Internet-Draft YANG October 2009 7.6.2. The leaf's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | config | 7.19.1 | 0..1 | | default | 7.6.4 | 0..1 | | description | 7.19.3 | 0..1 | | if-feature | 7.18.2 | 0..n | | mandatory | 7.6.5 | 0..1 | | must | 7.5.3 | 0..n | | reference | 7.19.4 | 0..1 | | status | 7.19.2 | 0..1 | | type | 7.6.3 | 1 | | units | 7.3.3 | 0..1 | | when | 7.19.5 | 0..1 | +--------------+---------+-------------+ 7.6.3. The leaf's type Statement The "type" statement, which MUST be present, takes as an argument the name of an existing built-in or derived type. The optional substatements specify restrictions on this type. See Section 7.4 for details. 7.6.4. The leaf's default Statement The "default" statement, which is optional, takes as an argument a string which contains a default value for the leaf. The value of the "default" statement MUST be valid according to the type specified in the leaf's "type" statement. The "default" statement MUST NOT be present on nodes where "mandatory" is true. 7.6.5. The leaf's mandatory Statement The "mandatory" statement, which is optional, takes as an argument the string "true" or "false", and puts a constraint on valid data. If not specified, the default is "false". If "mandatory" is "true", the behavior of the constraint depends on the type of the leaf's closest ancestor node in the schema tree which is not a non-presence container (see Section 7.5.1): Bjorklund Expires April 25, 2010 [Page 61] Internet-Draft YANG October 2009 o If no such ancestor exists in the schema tree, the leaf MUST exist. o Otherwise, if this ancestor is a case node, the leaf MUST exist if any node from the case exists in the data tree. o Otherwise, the leaf MUST exist if the ancestor node exists in the data tree. This constraint is enforced according to the rules in Section 8. 7.6.6. XML Mapping Rules A leaf node is encoded as an XML element. The element's 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 or request MAY choose not to send the leaf element if its value is the default value. Thus, a client that receives an for a or request, MUST be prepared to handle the case that a leaf node with a default value is not present in the XML. In this case, the value used by the server is known to be the default value. See Section 7.6.8 for an example. 7.6.7. NETCONF Operations When a NETCONF server processes an 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. Bjorklund Expires April 25, 2010 [Page 62] Internet-Draft YANG October 2009 7.6.8. Usage Example Given the following leaf statement, placed in the previously defined "ssh" container (See Section 7.5.9): leaf port { type inet:port-number; default 22; description "The port which the SSH server listens to" } A corresponding XML instance example: 2022 To create a leaf with an : 2022 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). Bjorklund Expires April 25, 2010 [Page 63] Internet-Draft YANG October 2009 If the type referenced by the leaf-list has a default value, it has no effect in the leaf-list. 7.7.1. Ordering YANG supports two styles for ordering the entries within lists and leaf-lists. In many lists, the order of list entries does not impact the implementation of the list's configuration, and the device is free to sort the list entries in any reasonable order. The "description" string for the list may suggest an order. YANG calls this style of list "system ordered" and they are indicated with the statement "ordered-by system". For example, a list of valid users would typically be sorted alphabetically, since the order in which the users appeared in the configuration would not impact the creation of those users' accounts. In the other style of lists, the order of list entries matters for the implementation of the list's configuration and the user is responsible for ordering the entries, while the device maintains that order. YANG calls this style of list "user ordered" and they are indicated with the statement "ordered-by user". For example, the order in which firewall filters entries are applied to incoming traffic may affect how that traffic is filtered. The user would need to decide if the filter entry that discards all TCP traffic should be applied before or after the filter entry that allows all traffic from trusted interfaces. The choice of order would be crucial. YANG provides a rich set of facilities within NETCONF's 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 7.7.5. Bjorklund Expires April 25, 2010 [Page 64] Internet-Draft YANG October 2009 7.7.2. The leaf-list's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | config | 7.19.1 | 0..1 | | description | 7.19.3 | 0..1 | | if-feature | 7.18.2 | 0..n | | max-elements | 7.7.4 | 0..1 | | min-elements | 7.7.3 | 0..1 | | must | 7.5.3 | 0..n | | ordered-by | 7.7.5 | 0..1 | | reference | 7.19.4 | 0..1 | | status | 7.19.2 | 0..1 | | type | 7.4 | 1 | | units | 7.3.3 | 0..1 | | when | 7.19.5 | 0..1 | +--------------+---------+-------------+ 7.7.3. The min-elements Statement The "min-elements" statement, which is optional, takes as an argument a non-negative integer which puts a constraint on valid list entries. A valid leaf-list or list MUST have at least min-elements entries. If no "min-elements" statement is present, it defaults to zero. The behavior of the constraint depends on the type of the leaf-list's or list's closest ancestor node in the schema tree which is not a non-presence container (see Section 7.5.1): o If this ancestor is a case node, the constraint is enforced if any other node from the case exists. o Otherwise, it is enforced if the ancestor node exists. The constraint is further enforced according to the rules in Section 8. 7.7.4. The max-elements Statement The "max-elements" statement, which is optional, takes as an argument a positive integer or the string "unbounded", which puts a constraint on valid list entries. A valid leaf-list or list always has at most max-elements entries. If no "max-elements" statement is present, it defaults to "unbounded". Bjorklund Expires April 25, 2010 [Page 65] Internet-Draft YANG October 2009 The "max-elements" constraint is enforced according to the rules in Section 8. 7.7.5. The ordered-by Statement The "ordered-by" statement defines whether the order of entries within a list are determined by the user or the system. The argument is one of the strings "system" or "user". If not present, order defaults to "system". This statement is ignored if the list represents state data, RPC output parameters, or notification content. See Section 7.7.1 for additional information. 7.7.5.1. ordered-by system The entries in the list are sorted according to an unspecified order. Thus an implementation is free to sort the entries in the most appropriate order. An implementation SHOULD use the same order for the same data, regardless of how the data were created. Using a deterministic order will make comparisons possible using simple tools like "diff". This is the default order. 7.7.5.2. ordered-by user The entries in the list are sorted according to an order defined by the user. This order is controlled by using special XML attributes in the request. See Section 7.7.7 for details. 7.7.6. XML Mapping Rules A leaf-list node is encoded as a series of XML elements. Each element's 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 7.7.8 for an example. 7.7.7. NETCONF operations Leaf-list entries can be created and deleted, but not modified, through , by using the "operation" attribute in the leaf-list entry's XML element. Bjorklund Expires April 25, 2010 [Page 66] Internet-Draft YANG October 2009 In an "ordered-by user" leaf-list, the attributes "insert" and "value" in the YANG XML namespace (Section 5.3.1) can be used to control where in the leaf-list the entry is inserted. These can be used during "create" operations to insert a new leaf-list entry, or during "merge" or "replace" operations to insert a new leaf-list entry or move an existing one. The "insert" attribute can take the values "first", "last", "before", and "after". If the value is "before" or "after", the "value" attribute MUST also be used to specify an existing entry in the leaf- list. If no "insert" attribute is present in the "create" operation, it defaults to "last". If several entries in an "ordered-by user" leaf-list are modified in the same request, the entries are modified one at the time, in the order of the XML elements in the request. In a , or an 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 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. 7.7.8. Usage Example leaf-list allow-user { type string; description "A list of user name patterns to allow"; } A corresponding XML instance example: alice bob To create a new element in the list: Bjorklund Expires April 25, 2010 [Page 67] Internet-Draft YANG October 2009 eric 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": Bjorklund Expires April 25, 2010 [Page 68] Internet-Draft YANG October 2009 blowfish-cbc 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, if defined. A list is similar to a table where each list entry is a row in the table. Bjorklund Expires April 25, 2010 [Page 69] Internet-Draft YANG October 2009 7.8.1. The list's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | anyxml | 7.10 | 0..n | | choice | 7.9 | 0..n | | config | 7.19.1 | 0..1 | | container | 7.5 | 0..n | | description | 7.19.3 | 0..1 | | grouping | 7.11 | 0..n | | if-feature | 7.18.2 | 0..n | | key | 7.8.2 | 0..1 | | leaf | 7.6 | 0..n | | leaf-list | 7.7 | 0..n | | list | 7.8 | 0..n | | max-elements | 7.7.4 | 0..1 | | min-elements | 7.7.3 | 0..1 | | must | 7.5.3 | 0..n | | ordered-by | 7.7.5 | 0..1 | | reference | 7.19.4 | 0..1 | | status | 7.19.2 | 0..1 | | typedef | 7.3 | 0..n | | unique | 7.8.3 | 0..n | | uses | 7.12 | 0..n | | when | 7.19.5 | 0..1 | +--------------+---------+-------------+ 7.8.2. The list's key Statement The "key" statement, which MUST be present if the list represents configuration, and MAY be present otherwise, takes as an argument a string 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 leafs can be defined directly in substatements to the list, or in groupings used in the list. The combined values of all the leafs specified in the key are used to uniquely identify a list entry. All key leafs MUST be given values when a list entry is created. Thus, any default values in the key leafs or their types are ignored. It also implies that any mandatory statement in the key leafs are ignored. A leaf that is part of the key can be of any built-in or derived type, except it MUST NOT be the built-in type "empty". All key leafs in a list MUST have the same value for their "config" Bjorklund Expires April 25, 2010 [Page 70] Internet-Draft YANG October 2009 as the list itself. The key string syntax is formally defined by the rule "key-arg" in Section 12. 7.8.3. The list's unique Statement The "unique" statement is used to put constraints on valid list entries. It takes as an argument a string 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 in which all referenced leafs exist. The constraint is enforced according to the rules in Section 8. The unique string syntax is formally defined by the rule "unique-arg" in Section 12. 7.8.3.1. Usage Example With the following list: list server { key "name"; unique "ip port"; leaf name { type string; } leaf ip { type inet:ip-address; } leaf port { type inet:port-number; } } The following configuration is not valid: Bjorklund Expires April 25, 2010 [Page 71] Internet-Draft YANG October 2009 smtp 192.0.2.1 25 http 192.0.2.1 25 The following configuration is valid, since the "http" and "ftp" list entries do not have a value for all referenced leafs, and are thus not taken into account when the "unique" constraint is enforced: smtp 192.0.2.1 25 http 192.0.2.1 ftp 192.0.2.1 7.8.4. The list's Child Node Statements Within a list, the "container", "leaf", "list", "leaf-list", "uses", "choice", and "anyxml" statements can be used to define child nodes to the list. 7.8.5. XML Mapping Rules A list is encoded as a series of XML elements, one for each entry in the list. Each element's 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. Bjorklund Expires April 25, 2010 [Page 72] Internet-Draft YANG October 2009 The rest of the list's child nodes are encoded as subelements to the list element, after the keys. If the list defines RPC input or output parameters, the subelements are encoded in the same order as they are defined within the list statement. Otherwise, the subelements are encoded in any order. 7.8.6. NETCONF operations List entries can be created, deleted, replaced and modified through , by using the "operation" attribute in the list's XML element. In each case, the values of all keys are used to uniquely identify a list entry. If all keys are not specified for a list entry, a "missing-element" error is returned. In an "ordered-by user" list, the attributes "insert" and "key" in the YANG XML namespace (Section 5.3.1) can be used to control where in the list the entry is inserted. These can be used during "create" operations to insert a new list entry, or during "merge" or "replace" operations to insert a new list entry or move an existing one. The "insert" attribute can take the values "first", "last", "before", and "after". If the value is "before" or "after", the "key" attribute MUST also be used, to specify an existing element in the list. The value of the "key" attribute is the key predicates of the full instance identifier (see Section 9.13) for the list entry. If no "insert" attribute is present in the "create" operation, it defaults to "last". If several entries in an "ordered-by user" list are modified in the same request, the entries are modified one at the time, in the order of the XML elements in the request. In a , or an 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: Bjorklund Expires April 25, 2010 [Page 73] Internet-Draft YANG October 2009 list user { key "name"; config true; description "This is a list of users in the system."; leaf name { type string; } leaf type { type string; } leaf full-name { type string; } } A corresponding XML instance example: fred admin Fred Flintstone To create a new user "barney": barney admin Barney Rubble To change the type of "fred" to "superuser": Bjorklund Expires April 25, 2010 [Page 74] Internet-Draft YANG October 2009 fred superuser Given the following ordered-by user list: list user { description "This is a list of users in the system."; ordered-by user; config true; key "name"; leaf name { type string; } leaf type { type string; } leaf full-name { type string; } } The following would be used to insert a new user "barney" after the user "fred": Bjorklund Expires April 25, 2010 [Page 75] Internet-Draft YANG October 2009 barney admin Barney Rubble The following would be used to move the user "barney" before the user "fred": barney Bjorklund Expires April 25, 2010 [Page 76] Internet-Draft YANG October 2009 7.9. The choice Statement The "choice" statement defines a set of alternatives, only one of which may exist at any one time. The argument is an identifier, followed by a block of substatements that holds detailed choice information. The identifier is used to identify the choice node in the schema tree. A choice node does not exist in the data tree. A choice consists of a number of branches, defined with the "case" substatement. Each branch contains a number of child nodes. The nodes from at most one of the choice's branches exist at the same time. See Section 8.3.2 for additional information. 7.9.1. The choice's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | anyxml | 7.10 | 0..n | | case | 7.9.2 | 0..n | | config | 7.19.1 | 0..1 | | container | 7.5 | 0..n | | default | 7.9.3 | 0..1 | | description | 7.19.3 | 0..1 | | if-feature | 7.18.2 | 0..n | | leaf | 7.6 | 0..n | | leaf-list | 7.7 | 0..n | | list | 7.8 | 0..n | | mandatory | 7.9.4 | 0..1 | | reference | 7.19.4 | 0..1 | | status | 7.19.2 | 0..1 | | when | 7.19.5 | 0..1 | +--------------+---------+-------------+ 7.9.2. The choice's case Statement The "case" statement is used to define branches of the choice. It takes as an argument an identifier, followed by a block of substatements that holds detailed case information. The identifier is used to identify the case node in the schema tree. A case node does not exist in the data tree. Within a "case" statement, the "anyxml", "choice", "container", "leaf", "list", "leaf-list", and "uses" statements can be used to define child nodes to the case node. The identifiers of all these Bjorklund Expires April 25, 2010 [Page 77] Internet-Draft YANG October 2009 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 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. Bjorklund Expires April 25, 2010 [Page 78] Internet-Draft YANG October 2009 7.9.2.1. The case's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | anyxml | 7.10 | 0..n | | choice | 7.9 | 0..n | | container | 7.5 | 0..n | | description | 7.19.3 | 0..1 | | if-feature | 7.18.2 | 0..n | | leaf | 7.6 | 0..n | | leaf-list | 7.7 | 0..n | | list | 7.8 | 0..n | | reference | 7.19.4 | 0..1 | | status | 7.19.2 | 0..1 | | uses | 7.12 | 0..n | | when | 7.19.5 | 0..1 | +--------------+---------+-------------+ 7.9.3. The choice's default Statement The "default" statement indicates if a case should be considered as the default if no child nodes from any of the choice's cases 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 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. Bjorklund Expires April 25, 2010 [Page 79] Internet-Draft YANG October 2009 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; } } } } 7.9.4. The choice's mandatory Statement The "mandatory" statement, which is optional, takes as an argument the string "true" or "false", and puts a constraint on valid data. If "mandatory" is "true", at least one node from exactly one of the choice's case branches MUST exist. If not specified, the default is "false". The behavior of the constraint depends on the type of the choice's closest ancestor node in the schema tree which is not a non-presence container (see Section 7.5.1): o If this ancestor is a case node, the constraint is enforced if any other node from the case exists. o Otherwise, it is enforced if the ancestor node exists. The constraint is further enforced according to the rules in Section 8. Bjorklund Expires April 25, 2010 [Page 80] Internet-Draft YANG October 2009 7.9.5. XML Mapping Rules The choice and case nodes are not visible in XML. 7.9.6. NETCONF operations Since only one of the choice's cases can be valid at any time, the creation of a node from one case implicitly deletes all nodes from all other cases. If an 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 instance example: To change the protocol from tcp to udp: Bjorklund Expires April 25, 2010 [Page 81] Internet-Draft YANG October 2009 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 for example RPC replies. An example is the parameter in the operation. An anyxml node cannot be augmented. Since the use of anyxml limits the manipulation of the content, it is RECOMMENDED that the anyxml statement not be used to represent configuration data. An anyxml node exists in zero or one instances in the data tree. Bjorklund Expires April 25, 2010 [Page 82] Internet-Draft YANG October 2009 7.10.1. The anyxml's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | config | 7.19.1 | 0..1 | | description | 7.19.3 | 0..1 | | if-feature | 7.18.2 | 0..n | | mandatory | 7.6.5 | 0..1 | | must | 7.5.3 | 0..n | | reference | 7.19.4 | 0..1 | | status | 7.19.2 | 0..1 | | when | 7.19.5 | 0..1 | +--------------+---------+-------------+ 7.10.2. XML Mapping Rules An anyxml node is encoded as an XML element. The element's 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 operations An anyxml node is treated as an opaque chunk of data. This data can be modified in its entirety only. Any "operation" attributes present on subelements of an anyxml node are ignored by the NETCONF server. When a NETCONF server processes an 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. Bjorklund Expires April 25, 2010 [Page 83] Internet-Draft YANG October 2009 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: 1 1 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, according to the rules in Section 5.5. It takes one argument which is an identifier, followed by a block of substatements that holds detailed grouping information. The grouping statement is not a data definition statement and, as such, does not define any nodes in the schema tree. A grouping is like a "structure" or a "record" in conventional programming languages. Once a grouping is defined, it can be referenced in a "uses" statement (see Section 7.12). A grouping MUST NOT reference itself, neither directly nor indirectly through a chain of other groupings. If the grouping is defined at the top level of a YANG module or submodule, the grouping's identifier MUST be unique within the module. A grouping is more than just a mechanism for textual substitution, but defines a collection of nodes. Identifiers appearing inside the grouping are resolved relative to the scope in which the grouping is defined, not where it is used. Prefix mappings, type names, grouping Bjorklund Expires April 25, 2010 [Page 84] Internet-Draft YANG October 2009 names, and extension usage are evaluated in the hierarchy where the grouping statement appears. For extensions, this means that extensions are applied to the grouping node, not the uses node. 7.11.1. The grouping's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | anyxml | 7.10 | 0..n | | choice | 7.9 | 0..n | | container | 7.5 | 0..n | | description | 7.19.3 | 0..1 | | grouping | 7.11 | 0..n | | leaf | 7.6 | 0..n | | leaf-list | 7.7 | 0..n | | list | 7.8 | 0..n | | reference | 7.19.4 | 0..1 | | status | 7.19.2 | 0..1 | | typedef | 7.3 | 0..n | | uses | 7.12 | 0..n | +--------------+---------+-------------+ 7.11.2. Usage Example import ietf-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 then updated according to the refinement and augment statements. Bjorklund Expires April 25, 2010 [Page 85] Internet-Draft YANG October 2009 The identifiers defined in the grouping are not bound to a namespace until the contents of the grouping are added to the schema tree via a "uses" statement that does not appear inside a "grouping" statement, at which point they are bound to the namespace of the current module. 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 descendant schema node identifier (see Section 6.5). The following refinements can be done: o A leaf or choice node may get a default value, or a new default value if it already had one. o Any node may get a specialized "description" string. o Any node may get a specialized "reference" string. o Any node may get a different "config" statement. o A leaf, anyxml, or choice node may get a different "mandatory" statement. o A container node may get a "presence" statement. Bjorklund Expires April 25, 2010 [Page 86] Internet-Draft YANG October 2009 o A leaf, leaf-list, list, container, or anyxml node may get additional "must" expressions. o A leaf-list or list node may get a different "min-elements" or "max-elements" statement. 7.12.3. XML Mapping Rules Each node in the grouping is encoded as if it was defined inline, even if it is imported from another module with another XML namespace. 7.12.4. Usage Example To use the "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 instance example: extern-web 192.0.2.1 80 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; } } Bjorklund Expires April 25, 2010 [Page 87] Internet-Draft YANG October 2009 } 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 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. Bjorklund Expires April 25, 2010 [Page 88] Internet-Draft YANG October 2009 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" define nodes under the RPC's input node. If a leaf in the input tree has a "mandatory" statement with the value "true", the leaf MUST be present in a NETCONF RPC invocation. Otherwise, the server MUST return a "missing-element" error. If a leaf in the input tree has a default value, the NETCONF server MUST use this value in the same cases as described in Section 7.6.1. In these cases, the server MUST operationally behave as if the leaf was present in the NETCONF RPC invocation with the default value as its value. If a "config" statement is present for any node in the input tree, it is ignored. If any node has a "when" statement which would evaluate to false, then this node MUST NOT be present in the input tree. Bjorklund Expires April 25, 2010 [Page 89] Internet-Draft YANG October 2009 7.13.2.1. The input's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | anyxml | 7.10 | 0..n | | choice | 7.9 | 0..n | | container | 7.5 | 0..n | | grouping | 7.11 | 0..n | | leaf | 7.6 | 0..n | | leaf-list | 7.7 | 0..n | | list | 7.8 | 0..n | | typedef | 7.3 | 0..n | | uses | 7.12 | 0..n | +--------------+---------+-------------+ 7.13.3. The output Statement The "output" statement, which is optional, is used to define output parameters to the RPC method. It does not take an argument. The substatements to "output" define nodes under the RPC's output node. If a leaf in the output tree has a "mandatory" statement with the value "true", the leaf MUST be present in a NETCONF RPC reply. If a leaf in the output tree has a default value, the NETCONF client MUST use this value in the same cases as described in Section 7.6.1. In these cases, the client MUST operationally behave as if the leaf was present in the NETCONF RPC reply with the default value as its value. If a "config" statement is present for any node in the output tree, it is ignored. If any node has a "when" statement which would evaluate to false, then this node MUST NOT be present in the output tree. Bjorklund Expires April 25, 2010 [Page 90] Internet-Draft YANG October 2009 7.13.3.1. The output's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | anyxml | 7.10 | 0..n | | choice | 7.9 | 0..n | | container | 7.5 | 0..n | | grouping | 7.11 | 0..n | | leaf | 7.6 | 0..n | | leaf-list | 7.7 | 0..n | | list | 7.8 | 0..n | | typedef | 7.3 | 0..n | | uses | 7.12 | 0..n | +--------------+---------+-------------+ 7.13.4. XML Mapping Rules An rpc node is encoded as a child XML element to the 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 contains a single element defined in [RFC4741]. If output parameters are returned, they are encoded as child elements to the 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; } } } } Bjorklund Expires April 25, 2010 [Page 91] Internet-Draft YANG October 2009 A corresponding XML instance example of the complete rpc and rpc- reply: 27606-0100 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 client MUST use this value in the same cases as described in Section 7.6.1. In these cases, the client MUST operationally behave as if the leaf was present in the NETCONF notification with the default value as its value. If a "config" statement is present for any node in the notification tree, it is ignored. Bjorklund Expires April 25, 2010 [Page 92] Internet-Draft YANG October 2009 7.14.1. The notification's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | anyxml | 7.10 | 0..n | | choice | 7.9 | 0..n | | container | 7.5 | 0..n | | description | 7.19.3 | 0..1 | | grouping | 7.11 | 0..n | | if-feature | 7.18.2 | 0..n | | leaf | 7.6 | 0..n | | leaf-list | 7.7 | 0..n | | list | 7.8 | 0..n | | reference | 7.19.4 | 0..1 | | status | 7.19.2 | 0..1 | | typedef | 7.3 | 0..n | | uses | 7.12 | 0..n | +--------------+---------+-------------+ 7.14.2. XML Mapping Rules A notification node is encoded as a child XML element to the element defined in NETCONF Event Notifications [RFC5277]. The element's name is the notification's identifier, and its XML namespace is the module's XML namespace. 7.14.3. Usage Example The following example defines a notification: module event { namespace "http://example.com/event"; prefix "ev"; notification event { leaf event-class { type string; } anyxml reporting-entity; leaf severity { type string; } } } A corresponding XML instance example of the complete notification: Bjorklund Expires April 25, 2010 [Page 93] Internet-Draft YANG October 2009 2008-07-08T00:01:00Z fault Ethernet0 major 7.15. The augment Statement The "augment" statement allows a module or submodule to add to the schema tree defined in an external module, or the current module and its submodules, and to add to the nodes from a grouping in a "uses" statement. The argument is a string 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 (see Section 6.5). If the "augment" statement is on the top-level in a module or submodule, the absolute form (defined by the rule "absolute-schema-nodeid" in Section 12) of a schema node identifier MUST be used. If the "augment" statement is a substatement to the "uses" statement, the descendant form (defined by the rule "descendant-schema-nodeid" in Section 12) MUST be used. If the target node is a container, list, case, input, output, or notification node, the "container", "leaf", "list", "leaf-list", "uses", and "choice" statements can be used within the "augment" statement. If the target node is a choice node, the "case" statement, or a case shorthand statement (see Section 7.9.2) can be used within the "augment" statement. If the target node is in another module, then nodes added by the augmentation MUST NOT be mandatory nodes (see Section 3.1). The augment statement MUST NOT add multiple nodes with the same name from the same module to the target node. Bjorklund Expires April 25, 2010 [Page 94] Internet-Draft YANG October 2009 7.15.1. The augment's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | anyxml | 7.10 | 0..n | | case | 7.9.2 | 0..n | | choice | 7.9 | 0..n | | container | 7.5 | 0..n | | description | 7.19.3 | 0..1 | | if-feature | 7.18.2 | 0..n | | leaf | 7.6 | 0..n | | leaf-list | 7.7 | 0..n | | list | 7.8 | 0..n | | reference | 7.19.4 | 0..1 | | status | 7.19.2 | 0..1 | | uses | 7.12 | 0..n | | when | 7.19.5 | 0..1 | +--------------+---------+-------------+ 7.15.2. XML Mapping Rules All data nodes defined in the "augment" statement are defined as XML elements in the XML namespace of the module where the "augment" is specified. When a node is augmented, the augmenting child nodes are encoded as subelements to the augmented node, in any order. 7.15.3. Usage Example In namespace http://example.com/schema/interfaces, we have: Bjorklund Expires April 25, 2010 [Page 95] Internet-Draft YANG October 2009 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 instance example: 1 Flintstone Inc Ethernet A562 ethernetCsmacd 1500 2 Flintstone Inc DS0 ds0 1 Bjorklund Expires April 25, 2010 [Page 96] Internet-Draft YANG October 2009 As another example, suppose we have the choice defined in Section 7.9.7. The following construct can be used to extend the protocol definition: augment /ex:system/ex:protocol/ex:name { case c { leaf smtp { type empty; } } } A corresponding XML instance example: or 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. Bjorklund Expires April 25, 2010 [Page 97] Internet-Draft YANG October 2009 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. An identity MUST NOT reference itself, neither directly nor indirectly through a chain of other identities. Bjorklund Expires April 25, 2010 [Page 98] Internet-Draft YANG October 2009 7.16.3. Usage Example module crypto-base { namespace "http://example.com/crypto-base"; prefix "crypto"; identity crypto-alg { description "Base identity from which all crypto algorithms are derived."; } } module des { namespace "http://example.com/des"; prefix "des"; import "crypto-base" { prefix "crypto"; } identity des { base "crypto:crypto-alg"; description "DES crypto algorithm"; } identity des3 { base "crypto:crypto-alg"; description "Triple DES crypto algorithm"; } } 7.17. The extension Statement The "extension" statement allows the definition of new statements within the YANG language. This new statement definition can be imported and used by other modules. The statement's argument is an identifier that is the new keyword for the extension and must be followed by a block of substatements that holds detailed extension information. The purpose of the extension statement is to define a keyword, so that it can be imported and used by other modules. The extension can be used like a normal YANG statement, with the statement name followed by an argument if one is defined by the extension, and an optional block of substatements. The statement's name is created by combining the the prefix of the module in which Bjorklund Expires April 25, 2010 [Page 99] Internet-Draft YANG October 2009 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 is mapped to an XML element in YIN or to an XML attribute. (see Section 11). If no "yin-element" statement is present, it defaults to "false". Bjorklund Expires April 25, 2010 [Page 100] Internet-Draft YANG October 2009 7.17.3. Usage Example To define an extension: module my-extensions { ... extension c-define { description "Takes as argument a name string. Makes the code generator use the given name in the #define."; argument "name"; } } To use the extension: module my-interfaces { ... import my-extensions { prefix "myext"; } ... container interfaces { ... myext:c-define "MY_INTERFACES"; } } 7.18. Conformance-related Statements This section defines statements related to conformance, as described in Section 5.6. 7.18.1. The feature Statement The "feature" statement is used to define a mechanism by which portions of the schema are marked as conditional. A feature name is defined that can later be referenced using the "if-feature" statement (see Section 7.18.2). Schema nodes tagged with a feature are ignored 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. Bjorklund Expires April 25, 2010 [Page 101] Internet-Draft YANG October 2009 The argument to the "feature" statement is the name of the new feature, and follows the rules for identifiers in Section 6.2. This name is used by the "if-feature" statement to tie the schema nodes to the feature. In this example, a feature called "local-storage" represents the ability for a device to store syslog messages on local storage of some sort. This feature is used to make the "local-storage-limit" leaf conditional on the presence of some sort of local storage. If the device does not report that it supports this feature, the "local-storage-limit" node is not supported. module syslog { ... feature local-storage { description "This feature means the device supports local storage (memory, flash or disk) that can be used to store syslog messages."; } container syslog { leaf local-storage-limit { if-feature local-storage; type uint64; units "kilobyte"; config false; description "The amount of local storage that can be used to hold syslog messages."; } } } The "if-feature" statement can be used in many places within the YANG syntax. Definitions tagged with "if-feature" are ignored when the device does not support that feature. A feature MUST NOT reference itself, neither directly nor indirectly through a chain of other features. If a feature is dependent on any other features (i.e., the feature has one or more "if-feature" sub-statements), then all of features it depends on MUST also be implemented by the device. Bjorklund Expires April 25, 2010 [Page 102] Internet-Draft YANG October 2009 7.18.1.1. The feature's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | description | 7.19.3 | 0..1 | | if-feature | 7.18.2 | 0..n | | status | 7.19.2 | 0..1 | | reference | 7.19.4 | 0..1 | +--------------+---------+-------------+ 7.18.2. The if-feature Statement The "if-feature" statement makes its parent statement conditional. The argument is the name of a feature, as defined by a "feature" statement. The parent statement is implemented by servers that support this feature. If a prefix is present on the feature name, it refers to a feature defined in the module 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 string is an absolute schema node identifier (see Section 6.5). 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 correctly. Bjorklund Expires April 25, 2010 [Page 103] Internet-Draft YANG October 2009 However in some cases a particular device may not have the hardware or software ability to support parts of a standard module. When this occurs, the device makes a choice 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. 7.18.3.1. The deviation's Substatements +--------------+----------+-------------+ | substatement | section | cardinality | +--------------+----------+-------------+ | description | 7.19.3 | 0..1 | | deviate | 7.18.3.2 | 1..n | | reference | 7.19.4 | 0..1 | +--------------+----------+-------------+ 7.18.3.2. The deviate Statement The "deviate" statement defines how the device's implementation of the target node deviates from its original definition. The argument is one of the strings "not-supported", "add", "replace", or "delete". The argument "not-supported" indicates that the target node is not implemented by this device. The argument "add" adds properties to the target node. The properties to add are identified 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. Bjorklund Expires April 25, 2010 [Page 104] Internet-Draft YANG October 2009 The deviates's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | config | 7.19.1 | 0..1 | | default | 7.6.4 | 0..1 | | mandatory | 7.6.5 | 0..1 | | max-elements | 7.7.4 | 0..1 | | min-elements | 7.7.3 | 0..1 | | must | 7.5.3 | 0..n | | type | 7.4 | 0..1 | | unique | 7.8.3 | 0..n | | units | 7.3.3 | 0..1 | +--------------+---------+-------------+ 7.18.3.3. Usage Example In this example, the device is informing client applications that it does not support the 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"; ... } Bjorklund Expires April 25, 2010 [Page 105] Internet-Draft YANG October 2009 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. Data nodes representing configuration will be part of the reply to a request, and can be sent in a or request. If "config" is "false", the definition represents state data. Data nodes representing state data will be part of the reply to a , but not to a request. If "config" is not specified, the default is the same as the parent schema node's "config" value. If the parent node is a "case" node, the value is the same as the "case" node's parent "choice" node. If the top node does not specify a "config" statement, the default is "true". If a node has "config" "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 from implementations. Bjorklund Expires April 25, 2010 [Page 106] Internet-Draft YANG October 2009 If no status is specified, the default is "current". If a definition is "current", it MUST NOT reference a "deprecated" or "obsolete" definition within the same module. If a definition is "deprecated", it MUST NOT reference an "obsolete" definition within the same module. 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 makes its parent data definition statement conditional. The node defined by the parent data definition statement is only valid when the condition specified by the "when" statement is satisfied. The statement's argument is an XPath expression, which is used to formally specify this condition. If the XPath expression conceptually evaluates to "true" for a particular instance, then the node defined by the parent data definition statement is valid, otherwise it is not. See Section 8.3.2 for additional information. The XPath expression is conceptually evaluated in the following context, in addition to the definition in Section 6.4.1: o If the "when" statement is a child of an "augment" statement, then the context node is the augment's target node in the data tree, if the target node is a data node. Otherwise, the context node is the closest ancestor node to the target node 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. Bjorklund Expires April 25, 2010 [Page 107] Internet-Draft YANG October 2009 o If the "when" statement is a child of any other data definition statement, the context node is the data definition's node in the data tree. o The accessible tree is made up of all nodes in the data tree, and all leafs with default values in use (see Section 7.6.1). The accessible tree depends on the context node: o If the context node represents configuration, the tree is the data in the NETCONF datastore where the context node exists. The XPath root node has all top-level configuration data nodes in all modules as children. o If the context node represents state data, the tree is all state data on the device, and the datastore. The XPath root node has all top-level data nodes in all modules as children. o If the context node represents notification content, the tree is the notification XML instance document. The XPath root node has the element representing the notification being defined as the only child. o If the context node represents RPC input parameters, the tree is the RPC XML instance document. The XPath root node has the element representing the RPC method being defined as the only child. o If the context node represents RPC output parameters, the tree is the RPC reply instance document. The XPath root node has the elements representing the RPC output parameters as children. The result of the XPath expression is converted to a boolean value using the standard XPath rules. The XPath expression MUST NOT include any references to the context node or any descendants of the context node. 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. Bjorklund Expires April 25, 2010 [Page 108] Internet-Draft YANG October 2009 8. Constraints 8.1. Constraints on Data Several YANG statements define constraints on valid data. These constraints are enforced in different ways, depending on what type of data the statement defines. o If the constraint is defined on configuration data, it MUST be true in a valid configuration data tree. o If the constraint is defined on state data, it MUST be true in a reply to the command. o If the constraint is defined on notification content, it MUST be true in any notification instance. o If the constraint is defined on RPC input parameters, it MUST be true in an invocation of the RPC method. o If the constraint is defined on RPC output parameters, it MUST be true in the RPC reply. 8.2. Hierarchy of Constraints Conditions on parent nodes affect constraints on child nodes as a natural consequence of the hierarchy of nodes. "must", "mandatory", "min-elements", and "max-elements" constraints are not enforced if the parent node has a "when" or "if-feature" property that is not satisfied on the current device. In this example, the mandatory constraints on the "longitude" leaf is not enforced on devices that lack the "has-gps" feature: container location { if-feature has-gps; leaf longitude { mandatory true; ... } } 8.3. Constraint Enforcement Model For configuration data, there are three windows when constraints can be enforced: Bjorklund Expires April 25, 2010 [Page 109] Internet-Draft YANG October 2009 o during parsing of RPC payloads o during processing of NETCONF operations o during validation Each of these scenarios are considered in the following sections. 8.3.1. Payload Parsing When content arrives in RPC payloads, it MUST be well-formed XML, following the hierarchy and content rules defined by the set of models the device implements. o If a leaf data value does not match the type constraints for the leaf, including those defined in the type's range, length, and pattern properties, the server MUST reply with an "invalid-value" error-tag in the rpc-error, and with the error-app-tag and error- message associated with the constraint, if any exist. o If all keys of a list entry are not present, the server MUST reply with a "missing-element" error-tag in the rpc-error. o If data for more than one case branch of a choice is present, the server MUST reply with a "bad-element" in the rpc-error. o If data for a node tagged with "if-feature" is present, and the feature is not supported by the device, the server MUST reply with an "unknown-element" error-tag in the rpc-error. o If data for a node tagged with "when" is present, and the "when" condition evaluates to "false", the server MUST reply with an "unknown-element" error-tag in the rpc-error. o For insert handling, if the value for the attributes "before" and "after" are not valid for the type of the appropriate key leafs, the server MUST reply with a "bad-attribute" error-tag in the rpc- error. o If the attributes "before" and "after" appears in any element that is not a list whose "ordered-by" property is "user", the server MUST reply with an "unknown-attribute" error-tag in the rpc-error. 8.3.2. NETCONF Processing After the incoming data is parsed, the NETCONF server performs the operation by applying the data to the configuration datastore. During this processing the following errors MUST be Bjorklund Expires April 25, 2010 [Page 110] Internet-Draft YANG October 2009 detected: o Delete requests for non-existent data. o Create requests for existent data. o Insert requests with "before" or "after" parameters which do not exist. During processing: o If the NETCONF operation creates data nodes under a "choice", any existing nodes from other "case" branches are deleted by the server. o If the NETCONF operation modifies a data node such that any node's "when" expression becomes false, then that node is deleted by the server. 8.3.3. Validation When datastore processing is complete, the final contents MUST obey all validation constraints. This validation processing is performed at differing time according to the datastore. If the datastore is or , these constraints MUST be enforced at the end of the or operation. If the datastore is , the constraint enforcement is delayed until a or operation. o Any "must" constraints MUST evaluate to "true". o Any referential integrity constraints defined via the "path" statement MUST be satisfied. o Any "unique" constraints on lists MUST be satisfied. o The "min-elements" and "max-elements" constraints are enforced for lists and leaf-lists. Bjorklund Expires April 25, 2010 [Page 111] Internet-Draft YANG October 2009 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 NETCONF PDUs, and when specifying default values and numerical ranges in YANG modules. 9.1. Canonical representation For most types, 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". When a NETCONF server sends data, it MUST be in the canonical form. Some types have a lexical representation that depends on the XML context in which they occur. These types do not have a canonical form. 9.2. The Integer Built-in Types The integer built-in types are int8, int16, int32, int64, uint8, uint16, uint32, and uint64. They represent signed and unsigned integers of different sizes: int8 represents integer values between -128 and 127, inclusively. int16 represents integer values between -32768 and 32767, inclusively. int32 represents integer values between -2147483648 and 2147483647, inclusively. Bjorklund Expires April 25, 2010 [Page 112] Internet-Draft YANG October 2009 int64 represents integer values between -9223372036854775808 and 9223372036854775807, inclusively. uint8 represents integer values between 0 and 255, inclusively. uint16 represents integer values between 0 and 65535, inclusively. uint32 represents integer values between 0 and 4294967295, inclusively. uint64 represents integer values between 0 and 18446744073709551615, inclusively. 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. Note that if a default value in a YANG module has a leading zero ("0"), it is interpreted as an octal number. In the XML instance documents, an integer is always interpreted as a decimal number, and leading zeros are allowed. Examples: // legal values +4711 // legal positive value 4711 // legal positive value -123 // legal negative value 0xf00f // legal positive hexadecimal value -0xf // legal negative hexadecimal value 052 // legal positive octal value // illegal values - 1 // illegal intermediate space Bjorklund Expires April 25, 2010 [Page 113] Internet-Draft YANG October 2009 9.2.2. Canonical Form The canonical form of a positive integer does not include the sign "+". Leading zeros are prohibited. The value zero is represented as "0". 9.2.3. Restrictions All integer types can be restricted with the "range" statement (Section 9.2.4). 9.2.4. The range Statement The "range" statement, which is an optional substatement to the "type" statement, takes as an argument a range expression string. It is used to restrict integer and decimal built-in types, or types derived from those. A range consists of an explicit value, or a lower inclusive bound, two consecutive dots "..", and an upper inclusive bound. Multiple values or ranges can be given, separated by "|". If multiple values or ranges are given they all MUST be disjoint and MUST be in ascending order. If a range restriction is applied to an already range restricted type, the new restriction MUST be equal or more limiting, that is raising the lower bounds, reducing the upper bounds, removing explicit values or ranges, or splitting ranges into multiple ranges with intermediate gaps. Each explicit value and range boundary value given in the range expression MUST match the type being restricted, or be one of the special values "min" or "max". "min" and "max" 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 | 7.5.4.2 | 0..1 | | error-message | 7.5.4.1 | 0..1 | | reference | 7.19.4 | 0..1 | +---------------+---------+-------------+ Bjorklund Expires April 25, 2010 [Page 114] Internet-Draft YANG October 2009 9.2.5. Usage Example typedef my-base-int32-type { type int32 { range "1..4 | 10..20"; } } typedef my-type1 { type my-base-int32-type { // legal range restriction range "11..max"; // 11..20 } } typedef my-type2 { type my-base-int32-type { // illegal range restriction range "11..100"; } } 9.3. The decimal64 Built-in Type The decimal64 type represents a subset of the real numbers, which can be represented by decimal numerals. The value space of decimal64 is the set of numbers that can be obtained by multiplying a 64 bit signed integer by a negative power of ten, i.e., expressible as "i x 10^-n" where i is an integer64 and n is an integer between 1 and 18, inclusively. 9.3.1. Lexicographic Representation A decimal64 value is lexicographically represented as an optional sign ("+" or "-"), followed by a sequence of decimal digits, optionally followed by a period ('.') as a decimal indicator and a sequence of decimal digits. If no sign is specified, "+" is assumed. 9.3.2. Canonical Form The canonical form of a positive decimal64 does not include the sign "+". The decimal point is required. Leading and trailing zeros are prohibited, subject to the rule that there MUST be at least one digit before and after the decimal point. The value zero is represented as "0.0". Bjorklund Expires April 25, 2010 [Page 115] Internet-Draft YANG October 2009 9.3.3. Restrictions A decimal64 type can be restricted with the "range" statement (Section 9.2.4). 9.3.4. The fraction-digits Statement The "fraction-digits" statement, which is a substatement to the "type" statement, MUST be present if the type is "decimal64". It takes as an argument an integer between 1 and 18, inclusively. It controls the size of the minimum difference between values of a decimal64 type, by restricting the value space to numbers that are expressible as "i x 10^-n" where n is the fraction-digits argument. The following table lists the minimum and maximum value for each fraction-digit value: +----------------+-----------------------+----------------------+ | fraction-digit | min | max | +----------------+-----------------------+----------------------+ | 1 | -922337203685477580.8 | 922337203685477580.7 | | 2 | -92233720368547758.08 | 92233720368547758.07 | | 3 | -9223372036854775.808 | 9223372036854775.807 | | 4 | -922337203685477.5808 | 922337203685477.5807 | | 5 | -92233720368547.75808 | 92233720368547.75807 | | 6 | -9223372036854.775808 | 9223372036854.775807 | | 7 | -922337203685.4775808 | 922337203685.4775807 | | 8 | -92233720368.54775808 | 92233720368.54775807 | | 9 | -9223372036.854775808 | 9223372036.854775807 | | 10 | -922337203.6854775808 | 922337203.6854775807 | | 11 | -92233720.36854775808 | 92233720.36854775807 | | 12 | -9223372.036854775808 | 9223372.036854775807 | | 13 | -922337.2036854775808 | 922337.2036854775807 | | 14 | -92233.72036854775808 | 92233.72036854775807 | | 15 | -9223.372036854775808 | 9223.372036854775807 | | 16 | -922.3372036854775808 | 922.3372036854775807 | | 17 | -92.23372036854775808 | 92.23372036854775807 | | 18 | -9.223372036854775808 | 9.223372036854775807 | +----------------+-----------------------+----------------------+ 9.3.5. Usage Example type decimal64 { fraction-digits 2; range "1 .. 3.14 | 10 | 20..max"; } Bjorklund Expires April 25, 2010 [Page 116] Internet-Draft YANG October 2009 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 instance documents. 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. 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 Bjorklund Expires April 25, 2010 [Page 117] Internet-Draft YANG October 2009 accepted for the type being restricted, respectively. An implementation is not required to support a length value larger than 18446744073709551615. The length expression syntax is formally defined by the rule "length-arg" in Section 12. 9.4.4.1. The length's Substatements +---------------+---------+-------------+ | substatement | section | cardinality | +---------------+---------+-------------+ | description | 7.19.3 | 0..1 | | error-app-tag | 7.5.4.2 | 0..1 | | error-message | 7.5.4.1 | 0..1 | | reference | 7.19.4 | 0..1 | +---------------+---------+-------------+ 9.4.5. Usage Example typedef my-base-str-type { type string { length "1..255"; } } type my-base-str-type { // legal length refinement length "11 | 42..max"; // 11 | 42..255 } type my-base-str-type { // illegal length refinement length "1..999"; } 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 ANDed together, i.e., all such expressions have to match. If a pattern restriction is applied to an already pattern restricted Bjorklund Expires April 25, 2010 [Page 118] Internet-Draft YANG October 2009 type, values must match all patterns in the base type, in addition to the new patterns. 9.4.6.1. The pattern's Substatements +---------------+---------+-------------+ | substatement | section | cardinality | +---------------+---------+-------------+ | description | 7.19.3 | 0..1 | | error-app-tag | 7.5.4.2 | 0..1 | | error-message | 7.5.4.1 | 0..1 | | reference | 7.19.4 | 0..1 | +---------------+---------+-------------+ 9.4.7. Usage Example With the following type: type string { length "0..4"; pattern "[0-9a-fA-F]*"; } the following strings match: AB // legal 9A00 // legal and the following strings do not match: 00ABAB // illegal, too long xx00 // illegal, bad characters 9.5. The boolean Built-in Type The boolean built-in type represents a boolean value. 9.5.1. Lexicographic Representation The lexicographical representation of a boolean value is the strings "true" and "false". 9.5.2. Canonical Form The canonical form is the same as the lexicographical representation. Bjorklund Expires April 25, 2010 [Page 119] Internet-Draft YANG October 2009 9.5.3. Restrictions A boolean cannot be restricted. 9.6. The enumeration Built-in Type The enumeration built-in type represents values from a set of assigned names. 9.6.1. 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. Bjorklund Expires April 25, 2010 [Page 120] Internet-Draft YANG October 2009 9.6.4.1. The enum's Substatements +--------------+---------+-------------+ | substatement | section | cardinality | +--------------+---------+-------------+ | description | 7.19.3 | 0..1 | | reference | 7.19.4 | 0..1 | | status | 7.19.2 | 0..1 | | value | 9.6.4.2 | 0..1 | +--------------+---------+-------------+ 9.6.4.2. The value Statement The "value" statement, which is optional, is used to associate an integer value with the assigned name for the enum. This integer value MUST be in the range -2147483648 to 2147483647, and it MUST be unique within the enumeration type. The value is unused by YANG and the XML encoding, but is carried as a convenience to implementors. If a value is not specified, then one will be automatically assigned. If the enum 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; } } leaf myenum { type enumeration { enum zero; enum one; enum seven { value 7; } } } Bjorklund Expires April 25, 2010 [Page 121] Internet-Draft YANG October 2009 The lexicographic representation of the leaf "myenum" with value "seven" is: seven 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 are separated by a single space character and they appear ordered by their position (see Section 9.7.4.2). 9.7.4. The bit Statement The "bit" statement, which is a substatement to the "type" statement, MUST be present if the type is "bits". It is repeatedly used to specify each assigned named bit of a bits type. It takes as an argument a string which is the assigned name of the bit. It is followed by a block of substatements which holds detailed bit information. The assigned name follows the same syntax rules as an identifier (see Section 6.2). All assigned names in a bits type MUST be unique. Bjorklund Expires April 25, 2010 [Page 122] Internet-Draft YANG October 2009 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 | +--------------+---------+-------------+ 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 NETCONF PDUs, 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"; } Bjorklund Expires April 25, 2010 [Page 123] Internet-Draft YANG October 2009 The lexicographic representation of this leaf with bit values disable-nagle and 10-Mb-only set would be: disable-nagle 10-Mb-only 9.8. The binary Built-in Type The binary built-in type represents any binary data, i.e., a sequence of octets. 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 leafref Built-in Type The leafref type is used to reference a particular leaf instance in the data tree. The "path" substatement (Section 9.9.2) selects a set of leaf instances, and the leafref value space is the set of values of these leaf instances. If the leaf with the leafref type represents configuration data, the leaf it refers to MUST also represent configuration. Such a leaf puts a constraint on valid data. All leafref nodes MUST reference existing leaf instances or leafs with default values in use (see Section 7.6.1) for the data to be valid. This constraint is enforced according to the rules in Section 8. There MUST NOT be any circular chains of leafrefs. If the leaf that the leafref refers to is conditional based on one or more feature (see Section 7.18.2), then the leaf with the leafref type MUST also be conditional based on at least the same set of features. Bjorklund Expires April 25, 2010 [Page 124] Internet-Draft YANG October 2009 9.9.1. Restrictions A leafref cannot be restricted. 9.9.2. The path Statement The "path" statement, which is a substatement to the "type" statement, MUST be present if the type is "leafref". It takes as an argument a string which MUST refer to a leaf or leaf-list node. The syntax for a path argument is a subset of the XPath abbreviated syntax. Predicates are used only for constraining the values for the key nodes for list entries. Each predicate consists of exactly one equality test per key, and multiple adjacent predicates MAY be present if a list has multiple keys. The syntax is formally defined by the rule "path-arg" in Section 12. The predicates are only used when more than one key reference is needed to uniquely identify a leaf instance. This occurs if a list has multiple keys, or a reference to a leaf other than the key in a list is needed. In these cases, multiple leafrefs are typically specified, and predicates are used to tie them together. The "path" expression evaluates to a node set consisting of zero, one or more nodes. If the leaf with the leafref type represents configuration data, this node set MUST be non-empty. The "path" XPath expression is conceptually evaluated in the following context, in addition to the definition in Section 6.4.1: o The context node is the node in the data tree for which the "path" statement is defined. The accessible tree depends on the context node: o If the context node represents configuration data, the tree is the data in the NETCONF datastore where the context node exists. The XPath root node has all top-level configuration data nodes in all modules as children. o Otherwise the tree is all state data on the device, and the datastore. The XPath root node has all top-level data nodes in all modules as children. Bjorklund Expires April 25, 2010 [Page 125] Internet-Draft YANG October 2009 9.9.3. Lexicographic Representation A leafref value is encoded the same way as the leaf it references. 9.9.4. Canonical Form The canonical form of a leafref is the same as the canonical form of the leaf it references. 9.9.5. Usage Example With the following list: list interface { key "name"; leaf name { type string; } leaf admin-status { type admin-status; } list address { key "ip"; leaf ip { type yang:ip-address; } } } The following leafref refers to an existing interface: leaf mgmt-interface { type leafref { path "../interface/name"; } } An example of a corresponding XML snippet: eth0 lo eth0 Bjorklund Expires April 25, 2010 [Page 126] Internet-Draft YANG October 2009 The following leafrefs refer to an existing address of an interface: container default-address { leaf ifname { type leafref { path "../../interface/name"; } } leaf address { type leafref { path "../../interface[name = current()/../ifname]" + "/address/ip"; } } } An example of a corresponding XML snippet: eth0 up
192.0.2.1
192.0.2.2
lo up
127.0.0.1
eth0
192.0.2.2
 The following list uses a leafref for one of its keys. This is similar to a foreign key in a relational database. Bjorklund Expires April 25, 2010 [Page 127] Internet-Draft YANG October 2009 list packet-filter { key "if-name filter-id"; leaf if-name { type leafref { path "/interfaces/interface/name"; } } leaf filter-id { type uint32; } ... } An example of a corresponding XML anippet: eth0 up
192.0.2.1
192.0.2.2
eth0 1 ... eth0 2 ... The following notification defines two leafrefs to refer to an existing admin-status: Bjorklund Expires April 25, 2010 [Page 128] Internet-Draft YANG October 2009 notification link-failure { leaf if-name { type leafref { path "/interfaces/interface/name"; } } leaf admin-status { type leafref { path "/interfaces/interface[name = current()/../if-name]" + "/admin-status"; } } } An example of a corresponding XML notification: 2008-04-01T00:01:00Z eth0 up 9.10. The identityref Built-in Type The identityref type is used to reference an existing identity (see Section 7.16). 9.10.1. Restrictions An identityref cannot be restricted. 9.10.2. The identityref's base Statement The "base" statement, which is a substatement to the "type" statement, MUST be present if the type is "identityref". The argument is the name of an identity, as defined by an "identity" statement. If a prefix is present on the identity name, it refers to an identity defined in the module 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. On a particular server, the valid values are further restricted to the set of identities defined in the Bjorklund Expires April 25, 2010 [Page 129] Internet-Draft YANG October 2009 modules supported by the server. 9.10.3. Lexicographic Representation An identityref is encoded as the referred identity's Qualified Name as defined in [XML-NAMES]. If the Prefix is not present, the namespace of the identityref is the default namespace in effect on the element which contains the identityref value. When an identityref is given a default value using the "default" statement, the identity name in the default value MAY have a prefix. If a prefix is present on the identity name, it refers to an identity defined in the module which was imported with that prefix. Otherwise an identity with the matching name MUST be defined in the current module or an included submodule. 9.10.4. Canonical Form Since the lexicographic form depends on the XML context in which the value occurs, this type does not have a canonical form. 9.10.5. Usage Example With the identity definitions in Section 7.16.3, and the following module: module my-crypto { namespace "http://example.com/my-crypto"; prefix mc; import "crypto-base" { prefix "crypto"; } identity aes { base "crypto:crypto-alg"; } leaf crypto { type identityref { base "crypto:crypto-alg"; } } } The leaf "crypto" will be encoded as follows, if the value is the "des3" identity defined in the "des" module: Bjorklund Expires April 25, 2010 [Page 130] Internet-Draft YANG October 2009 des:des3 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: x:des3 If the "crypto" leaf's value instead is "aes" defined in the "my-crypto" module it can be encoded as: mc:aes or, using the default namespace: aes 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 Form Not applicable. 9.11.4. Usage Example The following leaf leaf enable-qos { type empty; } will be encoded as Bjorklund Expires April 25, 2010 [Page 131] Internet-Draft YANG October 2009 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 "leafref". When a string representing a union data type is validated, the string is validated against each member type, in the order they are specified in the type statement, until a match is found. Any default values or units properties defined in the member types are not inherited by the union type. Example: type union { type int32; type enumeration { enum "unbounded"; } } 9.12.1. Restrictions A union cannot be restricted. However, each member type can be restricted, based on the rules defined in Section 9 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. Bjorklund Expires April 25, 2010 [Page 132] Internet-Draft YANG October 2009 9.13. The instance-identifier Built-in Type The instance-identifier built-in type is used to uniquely identify a particular instance node in the data tree. The syntax for an instance-identifier is a subset of the XPath abbreviated syntax, formally defined by the rule "instance-identifier" in Section 12. It is used to uniquely identify a node in the data tree. Predicates are used only for specifying the values for the key nodes for list entries, a value of a leaf-list entry, or a positional index for list without keys. For identifying list entries with keys, each predicate consists of one equality test per key, and each key MUST have a corresponding predicate. If the leaf with the instance-identifier type represents configuration data, and the "require-instance" property (Section 9.13.2) is "true", the node it refers to MUST also represent configuration. Such a leaf puts a constraint on valid data. All such leaf nodes MUST reference existing nodes or leaf nodes with their default value in use (see Section 7.6.1) for the data to be valid. This constraint is enforced according to the rules in Section 8. The "instance-identifier" XPath expression is conceptually evaluated in the following context, in addition to the definition in Section 6.4.1: o The context node is the root node in the accessible tree. The accessible tree depends on the leaf with the instance-identifier type: o If this leaf represents configuration data, the tree is the data in the NETCONF datastore where the leaf exists. The XPath root node has all top-level configuration data nodes in all modules as children. o Otherwise the tree is all state data on the device, and the datastore. The XPath root node has all top-level data nodes in all modules as children. 9.13.1. Restrictions An instance-identifier can be restricted with the "require-instance" statement (Section 9.13.2). Bjorklund Expires April 25, 2010 [Page 133] Internet-Draft YANG October 2009 9.13.2. The require-instance Statement The "require-instance" statement, which is a substatement to the "type" statement, MAY be present if the type is "instance-identifier". It takes as an argument the string "true" or "false". If this statement is not present, it defaults to "true". If "require-instance" is "true", it means that the instance being referred MUST exist for the data to be valid. This constraint is enforced according to the rules in Section 8. If "require-instance" is "false", it means that the instance being referred MAY exist in valid data. 9.13.3. Lexicographic Representation An instance-identifier value is lexicographically represented as a string. All node names in an instance-identifier value MUST be qualified with explicit namespace prefixes and these prefixes MUST be declared in the XML namespace scope in the instance-identifier's XML element. Any prefixes used in the encoding are local to each instance encoding. This means that the same instance-identifier may be encoded differently by different implementations. 9.13.4. Canonical Form Since the lexicographic form depends on the XML context in which the value occurs, this type does not have a canonical form. 9.13.5. Usage Example The following are examples of instance identifiers: Bjorklund Expires April 25, 2010 [Page 134] Internet-Draft YANG October 2009 /* instance-identifier for a container */ /ex:system/ex:services/ex:ssh /* instance-identifier for a leaf */ /ex:system/ex:services/ex:ssh/ex:port /* instance-identifier for a list entry */ /ex:system/ex:user[ex:name='fred'] /* instance-identifier for a leaf in a list entry */ /ex:system/ex:user[ex:name='fred']/ex:type /* instance-identifier for a list entry with two keys */ /ex:system/ex:server[ex:ip='192.0.2.1'][ex:port='80'] /* instance-identifier for a leaf-list entry */ /ex:system/ex:services/ex:ssh/ex:cipher[.='blowfish-cbc'] /* instance-identifier for a list entry without keys */ /ex:stats/ex:port[3] Bjorklund Expires April 25, 2010 [Page 135] Internet-Draft YANG October 2009 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 data 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 bit positions do not change. o A "range", "length", or "pattern" statement may expand the allowed value space. 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. o A "max-elements" statement may be removed, or changed to allow more elements. Bjorklund Expires April 25, 2010 [Page 136] Internet-Draft YANG October 2009 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" statement may be changed, provided all local uses of the prefix also are changed. Otherwise, if the semantics of any previous definition are changed (i.e., if a non-editorial change is made to any definition other than those specifically allowed above), then this MUST be achieved by a new definition with a new identifier. In statements which have any data definition statements as substatements, those data definition substatements MUST NOT be Bjorklund Expires April 25, 2010 [Page 137] Internet-Draft YANG October 2009 reordered. Bjorklund Expires April 25, 2010 [Page 138] Internet-Draft YANG October 2009 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 mapping between YANG and YIN does not modify the information content of the model. Comments and whitespace are not preserved. 11.1. Formal YIN Definition There is a one-to-one correspondence between YANG keywords and YIN elements. The local name of a YIN element is identical to the corresponding YANG keyword. This means in particular that the document element (root) of a YIN document is always or . YIN elements corresponding to the core YANG keywords belong to the namespace whose associated URI is "urn:ietf:params:xml:ns:yang:yin:1". YIN elements corresponding to extension keywords belong to the namespace of the YANG module where the extension keyword is declared via the "extension" statement. The names of all YIN elements MUST be properly qualified with their namespaces specified above using the standard mechanisms of [XML-NAMES], i.e., "xmlns" and "xmlns:xxx" attributes. The argument of a YANG statement is encoded in YIN either as an XML attribute or a subelement of the keyword element. Table 1 defines the encoding for the set of core YANG keywords. For extensions, the argument encoding is specified within the "extension" statement (see Section 7.17). The following rules hold for arguments of both core and extension statements: o If the argument is encoded as an attribute, this attribute has no namespace. Bjorklund Expires April 25, 2010 [Page 139] Internet-Draft YANG October 2009 o If the argument is encoded as an element, it is placed to the same namespace as its parent keyword element. o If the argument is encoded as an element, it MUST be the first child of the keyword element. Substatements of a YANG statement are encoded as (additional) children of the keyword element and their relative order MUST be the same as the order of substatements in YANG. Comments in YANG MAY be mapped to XML comments. Mapping of arguments of the core YANG statements. +------------------+---------------+-------------+ | keyword | argument name | yin-element | +------------------+---------------+-------------+ | anyxml | name | false | | argument | name | false | | augment | target-node | false | | base | name | false | | belongs-to | module | false | | bit | name | false | | case | name | false | | choice | name | false | | config | value | false | | contact | info | true | | container | name | false | | default | value | false | | description | text | true | | deviate | value | false | | deviation | target-node | false | | enum | name | false | | error-app-tag | value | false | | error-message | value | true | | extension | name | false | | feature | name | false | | fraction-digits | value | false | | grouping | name | false | | identity | name | false | | if-feature | name | false | | import | module | false | | include | module | false | | input | | n/a | | key | value | false | | leaf | name | false | | leaf-list | name | false | | length | value | false | Bjorklund Expires April 25, 2010 [Page 140] Internet-Draft YANG October 2009 | list | name | false | | mandatory | value | false | | max-elements | value | false | | min-elements | value | false | | module | name | false | | must | condition | false | | namespace | uri | false | | notification | name | false | | ordered-by | value | false | | organization | info | true | | output | | 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 | | require-instance | value | false | | revision | date | false | | revision-date | date | false | | rpc | name | false | | status | value | false | | submodule | name | false | | type | name | false | | typedef | name | false | | unique | tag | false | | units | name | false | | uses | name | false | | value | value | false | | when | condition | false | | yang-version | value | false | | yin-element | value | false | +------------------+---------------+-------------+ Table 1 11.1.1. Usage Example The following YANG snippet: leaf mtu { type uint32; description "The MTU of the interface."; myext:c-define "MY_MTU"; } Bjorklund Expires April 25, 2010 [Page 141] Internet-Draft YANG October 2009 where the extension "c-define" is defined in Section 7.17.3, is translated into the following YIN snippet: The MTU of the interface. Bjorklund Expires April 25, 2010 [Page 142] Internet-Draft YANG October 2009 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. == begin "yang.abnf" module-stmt = optsep module-keyword sep identifier-arg-str optsep "{" stmtsep module-header-stmts linkage-stmts meta-stmts revision-stmts body-stmts "}" optsep submodule-stmt = optsep submodule-keyword sep identifier-arg-str optsep "{" stmtsep submodule-header-stmts linkage-stmts meta-stmts revision-stmts body-stmts "}" optsep module-header-stmts = ;; these stmts can appear in any order [yang-version-stmt stmtsep] namespace-stmt stmtsep prefix-stmt stmtsep submodule-header-stmts = ;; these stmts can appear in any order [yang-version-stmt stmtsep] belongs-to-stmt stmtsep meta-stmts = ;; these stmts can appear in any order [organization-stmt stmtsep] [contact-stmt stmtsep] [description-stmt stmtsep] Bjorklund Expires April 25, 2010 [Page 143] Internet-Draft YANG October 2009 [reference-stmt stmtsep] linkage-stmts = ;; these stmts can appear in any order *(import-stmt stmtsep) *(include-stmt stmtsep) revision-stmts = *(revision-stmt stmtsep) body-stmts = *((extension-stmt / feature-stmt / identity-stmt / typedef-stmt / grouping-stmt / data-def-stmt / augment-stmt / rpc-stmt / notification-stmt / deviation-stmt) stmtsep) data-def-stmt = container-stmt / leaf-stmt / leaf-list-stmt / list-stmt / choice-stmt / anyxml-stmt / uses-stmt yang-version-stmt = yang-version-keyword sep yang-version-arg-str optsep stmtend yang-version-arg-str = < a string which matches the rule yang-version-arg > yang-version-arg = "1" import-stmt = import-keyword sep identifier-arg-str optsep "{" stmtsep prefix-stmt stmtsep [revision-date-stmt stmtsep] "}" include-stmt = include-keyword sep identifier-arg-str optsep (";" / "{" stmtsep [revision-date-stmt stmtsep] "}") namespace-stmt = namespace-keyword sep uri-str optsep stmtend Bjorklund Expires April 25, 2010 [Page 144] Internet-Draft YANG October 2009 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 revision-date optsep (";" / "{" stmtsep [description-stmt stmtsep] [reference-stmt stmtsep] "}") revision-date = date-arg-str revision-date-stmt = revision-date-keyword sep revision-date stmtend 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 Bjorklund Expires April 25, 2010 [Page 145] Internet-Draft YANG October 2009 [yin-element-stmt stmtsep] "}") yin-element-stmt = yin-element-keyword sep yin-element-arg-str stmtend 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] "}" Bjorklund Expires April 25, 2010 [Page 146] Internet-Draft YANG October 2009 type-stmt = type-keyword sep identifier-ref-arg-str optsep (";" / "{" stmtsep type-body-stmts "}") type-body-stmts = numerical-restrictions / decimal64-specification / string-restrictions / enum-specification / leafref-specification / identityref-specification / instance-identifier-specification / bits-specification / union-specification numerical-restrictions = range-stmt stmtsep range-stmt = range-keyword sep range-arg-str optsep (";" / "{" stmtsep ;; these stmts can appear in any order [error-message-stmt stmtsep] [error-app-tag-stmt stmtsep] [description-stmt stmtsep] [reference-stmt stmtsep] "}") decimal64-specification = fraction-digits-stmt fraction-digits-stmt = fraction-digits-keyword sep fraction-digits-arg-str stmtend fraction-digits-arg-str = < a string which matches the rule fraction-digits-arg > fraction-digits-arg = ("1" ["0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8"]) / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9" string-restrictions = ;; these stmts can appear in any order [length-stmt stmtsep] *(pattern-stmt stmtsep) length-stmt = length-keyword sep length-arg-str optsep (";" / "{" stmtsep ;; these stmts can appear in any order Bjorklund Expires April 25, 2010 [Page 147] Internet-Draft YANG October 2009 [error-message-stmt stmtsep] [error-app-tag-stmt stmtsep] [description-stmt stmtsep] [reference-stmt stmtsep] "}") pattern-stmt = pattern-keyword sep string optsep (";" / "{" stmtsep ;; these stmts can appear in any order [error-message-stmt stmtsep] [error-app-tag-stmt stmtsep] [description-stmt stmtsep] [reference-stmt stmtsep] "}") default-stmt = default-keyword sep string stmtend enum-specification = 1*(enum-stmt stmtsep) enum-stmt = enum-keyword sep string optsep (";" / "{" stmtsep ;; these stmts can appear in any order [value-stmt stmtsep] [status-stmt stmtsep] [description-stmt stmtsep] [reference-stmt stmtsep] "}") leafref-specification = ;; these stmts can appear in any order path-stmt stmtsep [require-instance-stmt stmtsep] path-stmt = path-keyword sep path-arg-str stmtend require-instance-stmt = require-instance-keyword sep require-instance-arg-str stmtend require-instance-arg-str = < a string which matches the rule require-instance-arg > require-instance-arg = true-keyword / false-keyword instance-identifier-specification = [require-instance-stmt stmtsep] Bjorklund Expires April 25, 2010 [Page 148] Internet-Draft YANG October 2009 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-integer-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 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 > Bjorklund Expires April 25, 2010 [Page 149] Internet-Draft YANG October 2009 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-integer-value 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-integer-value value-stmt = value-keyword sep integer-value stmtend grouping-stmt = grouping-keyword sep identifier-arg-str optsep (";" / "{" stmtsep Bjorklund Expires April 25, 2010 [Page 150] Internet-Draft YANG October 2009 ;; 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) [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 Bjorklund Expires April 25, 2010 [Page 151] Internet-Draft YANG October 2009 [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 key-arg > key-arg = node-identifier *(sep node-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 (";" / Bjorklund Expires April 25, 2010 [Page 152] Internet-Draft YANG October 2009 "{" 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] *(data-def-stmt stmtsep) "}") anyxml-stmt = anyxml-keyword sep identifier-arg-str optsep (";" / "{" stmtsep ;; these stmts can appear in any order [when-stmt stmtsep] *(if-feature-stmt stmtsep) *(must-stmt stmtsep) [config-stmt stmtsep] [mandatory-stmt stmtsep] [status-stmt stmtsep] [description-stmt stmtsep] [reference-stmt stmtsep] "}") uses-stmt = uses-keyword sep identifier-ref-arg-str optsep (";" / "{" stmtsep Bjorklund Expires April 25, 2010 [Page 153] Internet-Draft YANG October 2009 ;; these stmts can appear in any order [when-stmt stmtsep] *(if-feature-stmt stmtsep) [status-stmt stmtsep] [description-stmt stmtsep] [reference-stmt stmtsep] *(refine-stmt stmtsep) *(uses-augment-stmt stmtsep) "}") refine-stmt = refine-keyword sep refine-arg-str optsep (";" / "{" stmtsep (refine-container-stmts / refine-leaf-stmts / refine-leaf-list-stmts / refine-list-stmts / refine-choice-stmts / refine-case-stmts / refine-anyxml-stmts) "}") refine-arg-str = < a string 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 *(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] Bjorklund Expires April 25, 2010 [Page 154] Internet-Draft YANG October 2009 [max-elements-stmt stmtsep] [description-stmt stmtsep] [reference-stmt stmtsep] refine-list-stmts = ;; these stmts can appear in any order *(must-stmt stmtsep) [config-stmt stmtsep] [min-elements-stmt stmtsep] [max-elements-stmt stmtsep] [description-stmt stmtsep] [reference-stmt stmtsep] refine-choice-stmts = ;; these stmts can appear in any order [default-stmt stmtsep] [config-stmt stmtsep] [mandatory-stmt stmtsep] [description-stmt stmtsep] [reference-stmt stmtsep] refine-case-stmts = ;; these stmts can appear in any order [description-stmt stmtsep] [reference-stmt stmtsep] refine-anyxml-stmts = ;; these stmts can appear in any order *(must-stmt stmtsep) [config-stmt stmtsep] [mandatory-stmt stmtsep] [description-stmt stmtsep] [reference-stmt stmtsep] uses-augment-stmt = augment-keyword sep uses-augment-arg-str optsep "{" stmtsep ;; these stmts can appear in any order [when-stmt stmtsep] *(if-feature-stmt stmtsep) [status-stmt stmtsep] [description-stmt stmtsep] [reference-stmt stmtsep] 1*((data-def-stmt stmtsep) / (case-stmt stmtsep)) "}" uses-augment-arg-str = < a string which matches the rule uses-augment-arg > uses-augment-arg = descendant-schema-nodeid Bjorklund Expires April 25, 2010 [Page 155] Internet-Draft YANG October 2009 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-statement2 "}") unknown-statement2 = [prefix ":"] identifier [sep string] optsep (";" / "{" *unknown-statement2 "}") 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 *((typedef-stmt / grouping-stmt) stmtsep) 1*(data-def-stmt stmtsep) "}" output-stmt = output-keyword optsep Bjorklund Expires April 25, 2010 [Page 156] Internet-Draft YANG October 2009 "{" 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-not-supported-stmt / 1*(deviate-add-stmt / deviate-replace-stmt / deviate-delete-stmt)) "}" deviation-arg-str = < a string which matches the rule deviation-arg > deviation-arg = absolute-schema-nodeid deviate-not-supported-stmt = deviate-keyword sep not-supported-keyword optsep (";" / "{" stmtsep "}") deviate-add-stmt = deviate-keyword sep add-keyword optsep (";" / "{" stmtsep Bjorklund Expires April 25, 2010 [Page 157] Internet-Draft YANG October 2009 [units-stmt stmtsep] *(must-stmt stmtsep) *(unique-stmt stmtsep) [default-stmt stmtsep] [config-stmt stmtsep] [mandatory-stmt stmtsep] [min-elements-stmt stmtsep] [max-elements-stmt stmtsep] "}") deviate-delete-stmt = deviate-keyword sep delete-keyword optsep (";" / "{" stmtsep [units-stmt stmtsep] *(must-stmt stmtsep) *(unique-stmt stmtsep) [default-stmt stmtsep] "}") deviate-replace-stmt = deviate-keyword sep replace-keyword optsep (";" / "{" stmtsep [type-stmt stmtsep] [units-stmt stmtsep] [default-stmt stmtsep] [config-stmt stmtsep] [mandatory-stmt stmtsep] [min-elements-stmt stmtsep] [max-elements-stmt stmtsep] "}") ;; Ranges range-arg-str = < a string which matches the rule range-arg > range-arg = range-part *(optsep "|" optsep range-part) range-part = range-boundary [optsep ".." optsep range-boundary] range-boundary = min-keyword / max-keyword / integer-value / decimal-value ;; Lengths length-arg-str = < a string which matches the rule length-arg > Bjorklund Expires April 25, 2010 [Page 158] Internet-Draft YANG October 2009 length-arg = length-part *(optsep "|" optsep length-part) length-part = length-boundary [optsep ".." optsep length-boundary] length-boundary = min-keyword / max-keyword / non-negative-integer-value ;; Date date-arg-str = < a string which matches the rule date-arg > date-arg = 4DIGIT "-" 2DIGIT "-" 2DIGIT ;; Schema Node Identifiers schema-nodeid = absolute-schema-nodeid / descendant-schema-nodeid absolute-schema-nodeid = 1*("/" node-identifier) descendant-schema-nodeid = node-identifier absolute-schema-nodeid node-identifier = [prefix ":"] identifier ;; Instance Identifiers instance-identifier = 1*("/" (node-identifier *predicate)) predicate = "[" *WSP (predicate-expr / pos) *WSP "]" predicate-expr = (node-identifier / ".") *WSP "=" *WSP ((DQUOTE string DQUOTE) / (SQUOTE string SQUOTE)) pos = non-negative-integer-value ;; leafref path path-arg-str = < a string which matches the rule path-arg > path-arg = absolute-path / relative-path Bjorklund Expires April 25, 2010 [Page 159] Internet-Draft YANG October 2009 absolute-path = 1*("/" (node-identifier *path-predicate)) relative-path = 1*(".." "/") descendant-path descendant-path = node-identifier [*path-predicate absolute-path] path-predicate = "[" *WSP path-equality-expr *WSP "]" path-equality-expr = node-identifier *WSP "=" *WSP path-key-expr path-key-expr = current-function-invocation *WSP "/" *WSP rel-path-keyexpr rel-path-keyexpr = 1*(".." *WSP "/" *WSP) *(node-identifier *WSP "/" *WSP) 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' fraction-digits-keyword = 'fraction-digits' grouping-keyword = 'grouping' identity-keyword = 'identity' if-feature-keyword = 'if-feature' import-keyword = 'import' include-keyword = 'include' input-keyword = 'input' Bjorklund Expires April 25, 2010 [Page 160] Internet-Draft YANG October 2009 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' require-instance-keyword = 'require-instance' revision-keyword = 'revision' revision-date-keyword = 'revision-date' rpc-keyword = 'rpc' status-keyword = 'status' submodule-keyword = 'submodule' type-keyword = 'type' typedef-keyword = 'typedef' unique-keyword = 'unique' units-keyword = 'units' uses-keyword = 'uses' value-keyword = 'value' when-keyword = 'when' yang-version-keyword= 'yang-version' yin-element-keyword = 'yin-element' ;; other keywords add-keyword = 'add' current-keyword = 'current' delete-keyword = 'delete' deprecated-keyword = 'deprecated' false-keyword = 'false' max-keyword = 'max' min-keyword = 'min' Bjorklund Expires April 25, 2010 [Page 161] Internet-Draft YANG October 2009 not-supported-keyword = 'not-supported' obsolete-keyword = 'obsolete' replace-keyword = 'replace' system-keyword = 'system' true-keyword = 'true' unbounded-keyword = 'unbounded' user-keyword = 'user' current-function-invocation = current-keyword *WSP "(" *WSP ")" ;; Basic Rules 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 > integer-value = ("-" non-negative-integer-value) / non-negative-integer-value non-negative-integer-value = "0" / positive-integer-value positive-integer-value = (non-zero-digit *DIGIT) zero-integer-value = 1*DIGIT stmtend = ";" / "{" *unknown-statement "}" sep = 1*(WSP / line-break) ; unconditional separator Bjorklund Expires April 25, 2010 [Page 162] Internet-Draft YANG October 2009 optsep = *(WSP / line-break) stmtsep = *(WSP / line-break / unknown-statement) line-break = CRLF / LF non-zero-digit = %x31-39 decimal-value = integer-value ("." zero-integer-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 LF = %x0A ; linefeed SP = %x20 ; space VCHAR = %x21-7E ; visible (printing) characters Bjorklund Expires April 25, 2010 [Page 163] Internet-Draft YANG October 2009 WSP = SP / HTAB ; whitespace == end "yang.abnf" Bjorklund Expires April 25, 2010 [Page 164] Internet-Draft YANG October 2009 13. Error Responses for YANG Related Errors A number of NETCONF error responses are defined for error cases related to the data-model handling. If the relevant YANG statement has an "error-app-tag" substatement, that overrides the default value specified below. 13.1. Error Message for Data that Violates a unique Statement If a NETCONF operation would result in configuration data where a unique constraint is invalidated, the following error is returned: Tag: operation-failed Error-app-tag: data-not-unique Error-info: : 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 element is in the YANG namespace ("urn:ietf:params:xml:ns:yang:1"). 13.2. Error Message for Data that Violates a max-elements Statement If a NETCONF operation would result in configuration data where a list or a leaf-list would have too many entries the following error is returned: Tag: operation-failed Error-app-tag: too-many-elements This error is returned once, with the error-path identifying the list node, even if there are more than one extra child present. 13.3. Error Message for Data that Violates a min-elements Statement If a NETCONF operation would result in configuration data where a list or a leaf-list would have too few entries the following error is returned: 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. Bjorklund Expires April 25, 2010 [Page 165] Internet-Draft YANG October 2009 13.4. Error Message for Data that Violates a must Statement If a NETCONF operation would result in configuration data where the restrictions imposed by a "must" statement is violated the following error is returned, unless a specific "error-app-tag" substatement is present for the "must" statement. Tag: operation-failed Error-app-tag: must-violation 13.5. Error Message for Data that Violates a require-instance Statement If a NETCONF operation would result in configuration data where a leaf of type "instance-identifier" marked with require-instance "true" refers to a non-existing instance, the following error is returned: Tag: data-missing Error-app-tag: instance-required Error-path: Path to the instance-identifier leaf. 13.6. Error Message for Data that does not Match a leafref Type If a NETCONF operation would result in configuration data where a leaf of type "leafref" refers to a non-existing instance, the following error is returned: Tag: data-missing Error-app-tag: instance-required Error-path: Path to the leafref leaf. 13.7. Error Message for Data that Violates a mandatory choice Statement If a NETCONF operation would result in configuration data where no nodes exists in a mandatory choice, the following error is returned: Tag: data-missing Error-app-tag: missing-choice Error-path: Path to the element with the missing choice. Error-info: : Contains the name of the missing mandatory choice. The element is in the YANG namespace ("urn:ietf:params:xml:ns:yang:1"). Bjorklund Expires April 25, 2010 [Page 166] Internet-Draft YANG October 2009 13.8. Error Message for the "insert" Operation If the "insert" and "key" or "value" attributes are used in an 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 Bjorklund Expires April 25, 2010 [Page 167] Internet-Draft YANG October 2009 14. IANA Considerations This document defines a registry for YANG module and submodule names. The name of the registry is "YANG Module Names". The registry shall record for each entry: o the name of the module or submodule o for modules, the assigned XML namespace o for modules, the prefix of the module o for submodules, the name of the module it belongs to o a reference to the (sub)module's documentation (the RFC number) For allocation, RFC publication is required as per RFC 5226 [RFC5226]. All registered YANG module names must comply with the rules for identifiers stated in Section 6.2. There are no initial assignments. The module name prefix 'ietf-' is reserved for IETF stream documents while the module name prefix 'irtf-' is reserved for IRTF stream documents. Modules published in other RFC streams must have a similar suitable prefix. The prefix 'iana-' is reserved for modules maintained by IANA. All module and submodule names in the registry MUST be unique. All XML namespaces in the registry MUST be unique. This document registers two URIs for the YANG and YIN XML namespaces in the IETF XML registry [RFC3688]. Following the format in RFC 3688, the following registration is requested. URI: urn:ietf:params:xml:ns:yang:yin:1 URI: urn:ietf:params:xml:ns:yang:1 Registrant Contact: The IESG. XML: N/A, the requested URIs are XML namespaces. Bjorklund Expires April 25, 2010 [Page 168] Internet-Draft YANG October 2009 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. Data modeled in 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. Bjorklund Expires April 25, 2010 [Page 169] Internet-Draft YANG October 2009 16. Contributors The following people all contributed significantly to the initial YANG draft: - Andy Bierman (Netconf Central) - Balazs Lengyel (Ericsson) - David Partain (Ericsson) - Juergen Schoenwaelder (Jacobs University Bremen) - Phil Shafer (Juniper Networks) Bjorklund Expires April 25, 2010 [Page 170] Internet-Draft YANG October 2009 17. Acknowledgements The editor wishes to thank the following individuals, who all provided helpful comments on various versions of this document: Mehmet Ersue, Washam Fan, Joel Halpern, Leif Johansson, Ladislav Lhotka, Gerhard Muenz, Tom Petch, Randy Presuhn, David Reid, and Bert Wijnen. Bjorklund Expires April 25, 2010 [Page 171] Internet-Draft YANG October 2009 18. References 18.1. Normative References [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. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC5277] Chisholm, S. and H. Trevino, "NETCONF Event Notifications", RFC 5277, July 2008. [XML-NAMES] 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, . [XPATH] Clark, J. and S. DeRose, "XML Path Language (XPath) Version 1.0", World Wide Web Consortium Bjorklund Expires April 25, 2010 [Page 172] Internet-Draft YANG October 2009 Recommendation REC-xpath-19991116, November 1999, . [XSD-TYPES] Biron, P V. and A. Malhotra, "XML Schema Part 2: Datatypes Second Edition", W3C REC REC-xmlschema-2-20041028, October 2004. 18.2. 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. [XPATH2.0] Chamberlin, D., Boag, S., Berglund, A., Robie, J., Simeon, J., Fernandez, M., and M. Kay, "XML Path Language (XPath) 2.0", World Wide Web Consortium Recommendation REC- xpath20-20070123, January 2007, . [XSLT] Clark, J., "XSL Transformations (XSLT) Version 1.0", World Wide Web Consortium Recommendation REC-xslt-19991116, November 1999, . Bjorklund Expires April 25, 2010 [Page 173] Internet-Draft YANG October 2009 Appendix A. ChangeLog RFC Editor: remove this section upon publication as an RFC. A.1. Version -08 o Clarified text on leaf deafults. o Added the term "top-level data node" to the terminology section. o Clarified how prefixes are mapped to namespaces in XPath expressions. o Added "reference" as a substatement to "revision". o Added "choice" as a substatement to "case". o Clarified that a leafreaf must be conditional if the leaf it refers to is conditional. o Clarified how identityref default values are represented. o Various bug fixes, clarifications and editorial fixes. A.2. Version -07 o The argument string of "augment", "refine", and "deviation" is described not as an XPath but as a "schema node identifier". o Clarified that there must be no circular references in a leafref. A.3. Version -06 o Various bug fixes, clarifications and editorial fixes after WGLC. o Removed "require-instance" for leafref. o Allow leafrefs to refer to leaf-lists. o Clarified the XPath context in Section 6.4.1, and for "must", "when", "augment", "path" and "instance-identifier". A.4. Version -05 o Replaced the data types "float32" and "float64" with "decimal64". o Specified that the elements in the XML encoding of configuration data can occur in any order. Bjorklund Expires April 25, 2010 [Page 174] Internet-Draft YANG October 2009 o Clarified that "augment" cannot add multiple nodes with the same name. o Clarified what "when" means in RPC input / output. o Wrote the IANA Considerations section. o Changed requirement for minimum identifier length to 64 characters. A.5. Version -04 o Using "revision-date" substatement to "import" and "include". o Corrected grammar and text for instance-identifier. o Fixed bugs in some examples. o Rewrote the YIN description to use a declarative style. o Added two error codes in Section 13. o Clarified that YANG uses the same XPath data model as XSLT. o Specified which errors are generated in Section 8.3.1. o Corrected grammar for key-arg; now allowing optional prefix on each leaf identifier. o Clarified that "unique" constraints only check instances with values for all referenced leafs. o Clarified how mandatory and min-elements constraints are enforced. o Editorial fixes. A.6. Version -03 o Added import by revision (yang-00413) o Changed type "keyref" to "leafref", and added the statement "require-instance" (yang-01253) o Clarified that data sent from the server must be in the canonical form. o Clarified when and how constraints in the models are enforced. Bjorklund Expires April 25, 2010 [Page 175] Internet-Draft YANG October 2009 o Many editorial fixes o Added more strict grammar for the "deviate" statement. A.7. 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.8. 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. Bjorklund Expires April 25, 2010 [Page 176] Internet-Draft YANG October 2009 o Allow "config" in "choice". 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.9. 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 Bjorklund Expires April 25, 2010 [Page 177] Internet-Draft YANG October 2009 Author's Address Martin Bjorklund (editor) Tail-f Systems Email: mbj@tail-f.com Bjorklund Expires April 25, 2010 [Page 178]