Network Working Group M. Bjorklund, Ed.
Internet-Draft Tail-f Systems
Intended status: Standards Track February 1, 2008
Expires: August 4, 2008
YANG - A data modeling language for NETCONF
draft-bjorklund-netconf-yang-01
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Abstract
YANG is a data modeling language used to model configuration and
state data manipulated by the NETCONF protocol, NETCONF remote
procedure calls, and NETCONF notifications.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 7
2. Key Words . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 9
4. YANG Overview . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1. Functional Overview . . . . . . . . . . . . . . . . . . . 12
4.2. Language Overview . . . . . . . . . . . . . . . . . . . . 13
4.2.1. Modules and Submodules . . . . . . . . . . . . . . . 13
4.2.2. Data Modeling Basics . . . . . . . . . . . . . . . . 14
4.2.3. Operational 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. Module Hierarchies . . . . . . . . . . . . . . . . . 27
5.2. File Layout . . . . . . . . . . . . . . . . . . . . . . . 27
5.3. Object Based View of YANG . . . . . . . . . . . . . . . . 28
5.4. XML Namespaces . . . . . . . . . . . . . . . . . . . . . 28
5.4.1. YANG Namespace . . . . . . . . . . . . . . . . . . . 29
5.5. Ordering . . . . . . . . . . . . . . . . . . . . . . . . 29
5.6. Containers with Presence . . . . . . . . . . . . . . . . 30
5.7. Scoping . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.8. Nested Typedefs and Groupings . . . . . . . . . . . . . . 31
6. YANG syntax . . . . . . . . . . . . . . . . . . . . . . . . . 32
6.1. Lexicographical Tokenization . . . . . . . . . . . . . . 32
6.1.1. Comments . . . . . . . . . . . . . . . . . . . . . . 32
6.1.2. Tokens . . . . . . . . . . . . . . . . . . . . . . . 32
6.1.3. Quoting . . . . . . . . . . . . . . . . . . . . . . . 32
6.2. Identifiers . . . . . . . . . . . . . . . . . . . . . . . 34
6.2.1. Identifiers and their namespaces . . . . . . . . . . 34
6.3. Statements . . . . . . . . . . . . . . . . . . . . . . . 34
6.3.1. Language Extensions . . . . . . . . . . . . . . . . . 35
6.4. XPath Evaluations . . . . . . . . . . . . . . . . . . . . 35
7. YANG Statements . . . . . . . . . . . . . . . . . . . . . . . 36
7.1. The module Statement . . . . . . . . . . . . . . . . . . 36
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7.1.1. The module's Substatements . . . . . . . . . . . . . 37
7.1.2. The yang-version Statement . . . . . . . . . . . . . 38
7.1.3. The namespace Statement . . . . . . . . . . . . . . . 38
7.1.4. The prefix Statement . . . . . . . . . . . . . . . . 39
7.1.5. The import Statement . . . . . . . . . . . . . . . . 39
7.1.6. The include Statement . . . . . . . . . . . . . . . . 40
7.1.7. The organization Statement . . . . . . . . . . . . . 40
7.1.8. The contact Statement . . . . . . . . . . . . . . . . 40
7.1.9. The revision Statement . . . . . . . . . . . . . . . 40
7.1.10. Usage Example . . . . . . . . . . . . . . . . . . . . 41
7.2. The submodule Statement . . . . . . . . . . . . . . . . . 41
7.2.1. The submodule's Substatements . . . . . . . . . . . . 43
7.2.2. The belongs-to Statement . . . . . . . . . . . . . . 44
7.2.3. Usage Example . . . . . . . . . . . . . . . . . . . . 45
7.3. The typedef Statement . . . . . . . . . . . . . . . . . . 45
7.3.1. The typedef's Substatements . . . . . . . . . . . . . 46
7.3.2. The typedef's type Statement . . . . . . . . . . . . 46
7.3.3. The units Statement . . . . . . . . . . . . . . . . . 46
7.3.4. The typedef's default Statement . . . . . . . . . . . 46
7.3.5. Usage Example . . . . . . . . . . . . . . . . . . . . 47
7.4. The type Statement . . . . . . . . . . . . . . . . . . . 47
7.4.1. The type's Substatements . . . . . . . . . . . . . . 47
7.5. The container Statement . . . . . . . . . . . . . . . . . 47
7.5.1. The container's Substatements . . . . . . . . . . . . 48
7.5.2. The must Statement . . . . . . . . . . . . . . . . . 48
7.5.3. The must's Substatements . . . . . . . . . . . . . . 49
7.5.4. The presence Statement . . . . . . . . . . . . . . . 50
7.5.5. The container's Child Node Statements . . . . . . . . 50
7.5.6. XML Encoding Rules . . . . . . . . . . . . . . . . . 50
7.5.7. NETCONF <edit-config> Operations . . . . . . . . . . 51
7.5.8. Usage Example . . . . . . . . . . . . . . . . . . . . 51
7.6. The leaf Statement . . . . . . . . . . . . . . . . . . . 52
7.6.1. The leaf's Substatements . . . . . . . . . . . . . . 53
7.6.2. The leaf's type Statement . . . . . . . . . . . . . . 53
7.6.3. The leaf's default Statement . . . . . . . . . . . . 53
7.6.4. The leaf's mandatory Statement . . . . . . . . . . . 53
7.6.5. XML Encoding Rules . . . . . . . . . . . . . . . . . 54
7.6.6. NETCONF <edit-config> Operations . . . . . . . . . . 54
7.6.7. Usage Example . . . . . . . . . . . . . . . . . . . . 54
7.7. The leaf-list Statement . . . . . . . . . . . . . . . . . 55
7.7.1. The leaf-list's Substatements . . . . . . . . . . . . 56
7.7.2. The min-elements Statement . . . . . . . . . . . . . 56
7.7.3. The max-elements Statement . . . . . . . . . . . . . 56
7.7.4. The ordered-by Statement . . . . . . . . . . . . . . 56
7.7.5. XML Encoding Rules . . . . . . . . . . . . . . . . . 57
7.7.6. NETCONF <edit-config> operations . . . . . . . . . . 57
7.7.7. Usage Example . . . . . . . . . . . . . . . . . . . . 58
7.8. The list Statement . . . . . . . . . . . . . . . . . . . 59
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7.8.1. The list's Substatements . . . . . . . . . . . . . . 60
7.8.2. The list's key Statement . . . . . . . . . . . . . . 60
7.8.3. The lists's unique Statement . . . . . . . . . . . . 61
7.8.4. The list's Child Node Statements . . . . . . . . . . 62
7.8.5. XML Encoding Rules . . . . . . . . . . . . . . . . . 62
7.8.6. NETCONF <edit-config> operations . . . . . . . . . . 62
7.8.7. Usage Example . . . . . . . . . . . . . . . . . . . . 63
7.9. The choice Statement . . . . . . . . . . . . . . . . . . 66
7.9.1. The choice's Substatements . . . . . . . . . . . . . 66
7.9.2. The choice's case Statement . . . . . . . . . . . . . 66
7.9.3. The choice's default Statement . . . . . . . . . . . 68
7.9.4. The choice's mandatory Statement . . . . . . . . . . 69
7.9.5. XML Encoding Rules . . . . . . . . . . . . . . . . . 69
7.9.6. NETCONF <edit-config> operations . . . . . . . . . . 69
7.9.7. Usage Example . . . . . . . . . . . . . . . . . . . . 70
7.10. The anyxml Statement . . . . . . . . . . . . . . . . . . 71
7.10.1. The anyxml's Substatements . . . . . . . . . . . . . 71
7.10.2. XML Encoding Rules . . . . . . . . . . . . . . . . . 71
7.10.3. NETCONF <edit-config> operations . . . . . . . . . . 71
7.10.4. Usage Example . . . . . . . . . . . . . . . . . . . . 72
7.11. The grouping Statement . . . . . . . . . . . . . . . . . 72
7.11.1. The grouping's Substatements . . . . . . . . . . . . 73
7.11.2. Usage Example . . . . . . . . . . . . . . . . . . . . 74
7.12. The uses Statement . . . . . . . . . . . . . . . . . . . 74
7.12.1. The uses's Substatements . . . . . . . . . . . . . . 75
7.12.2. The uses's Refinement Statements . . . . . . . . . . 75
7.12.3. XML Encoding Rules . . . . . . . . . . . . . . . . . 76
7.12.4. Usage Example . . . . . . . . . . . . . . . . . . . . 76
7.13. The rpc Statement . . . . . . . . . . . . . . . . . . . . 77
7.13.1. The rpc's Substatements . . . . . . . . . . . . . . . 77
7.13.2. The input Statement . . . . . . . . . . . . . . . . . 77
7.13.3. The output Statement . . . . . . . . . . . . . . . . 78
7.14. The notification Statement . . . . . . . . . . . . . . . 79
7.14.1. The notification's Substatements . . . . . . . . . . 80
7.15. The augment Statement . . . . . . . . . . . . . . . . . . 80
7.15.1. The augment's Substatements . . . . . . . . . . . . . 81
7.15.2. The when Statement . . . . . . . . . . . . . . . . . 81
7.15.3. XML Encoding Rules . . . . . . . . . . . . . . . . . 82
7.15.4. Usage Example . . . . . . . . . . . . . . . . . . . . 82
7.16. The extension Statement . . . . . . . . . . . . . . . . . 84
7.16.1. The extension's Substatements . . . . . . . . . . . . 84
7.16.2. The argument Statement . . . . . . . . . . . . . . . 84
7.16.3. Usage Example . . . . . . . . . . . . . . . . . . . . 85
7.17. Common Statements . . . . . . . . . . . . . . . . . . . . 86
7.17.1. The config Statement . . . . . . . . . . . . . . . . 86
7.17.2. The status Statement . . . . . . . . . . . . . . . . 86
7.17.3. The description Statement . . . . . . . . . . . . . . 87
7.17.4. The reference Statement . . . . . . . . . . . . . . . 87
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8. Built-in Types . . . . . . . . . . . . . . . . . . . . . . . 88
8.1. The Integer Built-in Types . . . . . . . . . . . . . . . 88
8.1.1. Lexicographic Representation . . . . . . . . . . . . 89
8.1.2. Restrictions . . . . . . . . . . . . . . . . . . . . 89
8.1.3. The range Statement . . . . . . . . . . . . . . . . . 89
8.1.4. Usage Example . . . . . . . . . . . . . . . . . . . . 90
8.2. The Floating Point Built-in Types . . . . . . . . . . . . 90
8.2.1. Lexicographic Representation . . . . . . . . . . . . 90
8.2.2. Restrictions . . . . . . . . . . . . . . . . . . . . 90
8.2.3. Usage Example . . . . . . . . . . . . . . . . . . . . 91
8.3. The string Built-in Type . . . . . . . . . . . . . . . . 91
8.3.1. Lexicographic Representation . . . . . . . . . . . . 91
8.3.2. Restrictions . . . . . . . . . . . . . . . . . . . . 91
8.3.3. The length Statement . . . . . . . . . . . . . . . . 91
8.3.4. The pattern Statement . . . . . . . . . . . . . . . . 92
8.3.5. Usage Example . . . . . . . . . . . . . . . . . . . . 93
8.4. The boolean Built-in Type . . . . . . . . . . . . . . . . 93
8.4.1. Lexicographic Representation . . . . . . . . . . . . 93
8.4.2. Restrictions . . . . . . . . . . . . . . . . . . . . 93
8.5. The enumeration Built-in Type . . . . . . . . . . . . . . 93
8.5.1. Lexicographic Representation . . . . . . . . . . . . 93
8.5.2. Restrictions . . . . . . . . . . . . . . . . . . . . 93
8.5.3. The enum Statement . . . . . . . . . . . . . . . . . 94
8.5.4. Usage Example . . . . . . . . . . . . . . . . . . . . 95
8.6. The bits Built-in Type . . . . . . . . . . . . . . . . . 95
8.6.1. Restrictions . . . . . . . . . . . . . . . . . . . . 95
8.6.2. Lexicographic Representation . . . . . . . . . . . . 95
8.6.3. The bit Statement . . . . . . . . . . . . . . . . . . 95
8.6.4. Usage Example . . . . . . . . . . . . . . . . . . . . 96
8.7. The binary Built-in Type . . . . . . . . . . . . . . . . 97
8.7.1. Restrictions . . . . . . . . . . . . . . . . . . . . 97
8.7.2. Lexicographic Representation . . . . . . . . . . . . 97
8.8. The keyref Built-in Type . . . . . . . . . . . . . . . . 97
8.8.1. Restrictions . . . . . . . . . . . . . . . . . . . . 97
8.8.2. The path Statement . . . . . . . . . . . . . . . . . 97
8.8.3. Lexicographic Representation . . . . . . . . . . . . 98
8.8.4. Usage Example . . . . . . . . . . . . . . . . . . . . 98
8.9. The empty Built-in Type . . . . . . . . . . . . . . . . . 99
8.9.1. Restrictions . . . . . . . . . . . . . . . . . . . . 99
8.9.2. Lexicographic Representation . . . . . . . . . . . . 100
8.9.3. Usage Example . . . . . . . . . . . . . . . . . . . . 100
8.10. The union Built-in Type . . . . . . . . . . . . . . . . . 100
8.10.1. Restrictions . . . . . . . . . . . . . . . . . . . . 100
8.10.2. Lexicographic Representation . . . . . . . . . . . . 101
8.11. The instance-identifier Built-in Type . . . . . . . . . . 101
8.11.1. Restrictions . . . . . . . . . . . . . . . . . . . . 101
8.11.2. Lexicographic Representation . . . . . . . . . . . . 101
8.11.3. Usage Example . . . . . . . . . . . . . . . . . . . . 101
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9. Updating a Module . . . . . . . . . . . . . . . . . . . . . . 103
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 104
11. Security Considerations . . . . . . . . . . . . . . . . . . . 105
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 106
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 107
13.1. Normative References . . . . . . . . . . . . . . . . . . 107
13.2. Non-Normative References . . . . . . . . . . . . . . . . 108
Appendix A. Derived YANG Types . . . . . . . . . . . . . . . . . 109
A.1. Core YANG Derived Types . . . . . . . . . . . . . . . . . 109
A.2. Internet Specific Derived Types . . . . . . . . . . . . . 114
A.3. IEEE 802 Specific Derived Types . . . . . . . . . . . . . 120
Appendix B. YIN . . . . . . . . . . . . . . . . . . . . . . . . 123
B.1. Formal YIN Definition . . . . . . . . . . . . . . . . . . 123
B.2. Transformation Algorithm YANG-2-YIN . . . . . . . . . . . 123
B.2.1. Usage Example . . . . . . . . . . . . . . . . . . . . 125
B.3. Transformation Algorithm YIN-2-YANG . . . . . . . . . . . 125
B.3.1. Tabulation, Formatting . . . . . . . . . . . . . . . 126
Appendix C. XML Schema Considerations . . . . . . . . . . . . . 127
Appendix D. YANG ABNF Grammar . . . . . . . . . . . . . . . . . 128
Appendix E. Error Responses for YANG Related Errors . . . . . . 146
E.1. Error Message for Data that Violates a YANG unique
Statement: . . . . . . . . . . . . . . . . . . . . . . . 146
E.2. Error Message for Data that Violates a YANG
max-elements Statement: . . . . . . . . . . . . . . . . . 146
E.3. Error Message for Data that Violates a YANG
min-elements Statement: . . . . . . . . . . . . . . . . . 146
E.4. Error Message for Data that Violates a YANG must or
when statement, a length, range or pattern restriction: . 146
E.5. Error Message for the "insert" Operation . . . . . . . . 147
Appendix F. Why We Need a New Modeling Language . . . . . . . . 148
F.1. Why not XSD? . . . . . . . . . . . . . . . . . . . . . . 148
F.2. Why not RelaxNG . . . . . . . . . . . . . . . . . . . . . 149
F.3. Why not SMIng . . . . . . . . . . . . . . . . . . . . . . 149
Appendix G. ChangeLog . . . . . . . . . . . . . . . . . . . . . 151
G.1. Version -01 . . . . . . . . . . . . . . . . . . . . . . . 151
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 153
Intellectual Property and Copyright Statements . . . . . . . . . 154
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1. Introduction
Today, the NETCONF protocol [RFC4741] lacks a standardized way to
create data models. Instead, vendors are forced to use proprietary
solutions. In order for NETCONF to be a interoperable protocol,
models must be defined in a vendor-neutral way. YANG provides the
language and rules for defining such models for use with NETCONF.
YANG is a data modeling language used to model configuration and
state data manipulated by the NETCONF protocol, NETCONF remote
procedure calls, and NETCONF notifications. This document describes
the syntax and semantics of the YANG language, how the data model
defined in a YANG module is represented in XML, and how NETCONF
operations are being used to manipulate the data.
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2. Key Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14, [RFC2119].
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3. Terminology
o augment: Adds new nodes to a previously defined node.
o base type: The type from which a derived type was derived, which
may be either a built-in type or another derived type.
o built-in type: A YANG data type defined in the YANG language, such
as uint32 or string.
o choice: A node where only one of a number of identified
alternative values is valid.
o container: An interior node in the data tree which exist in zero
or 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, augment, uses,
and anyxml.
o data model: Formal representation of the application-specific
components of a conceptual network management programmatic
interface.
o data model module: Container of definitions pertaining to a
specific data model.
o data model object: A definition within a data model module that
represents a conceptual construct which can be accessed via a
network management protocol. Also called an object.
o data node: A node in the schema tree that can be instantiated in a
data tree. One of container, leaf, leaf-list, and list.
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 extension: An extension attaches non-YANG semantics to nodes. The
extension statement defines new statements to express these
semantics.
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.
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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 nodes: Nodes within a hierarchy that are not leaf nodes.
o leaf: A node in the data tree with a value but no child nodes.
o leaf-list: Like the leaf node but defines a set of uniquely
identifiable nodes rather than a single node. Each node has a
value but no child nodes.
o list: Interior nodes in the data tree which may exist in multiple
instances. A list node has no value, but rather a set of child
nodes.
o MIB: A Management Information Base, traditionally referring to a
management information defined using SNMP's SMI.
o module: A YANG module defines a hierarchy of nodes which can be
used for NETCONF-based operations. With its definitions and the
definitions it imports or includes from elsewhere, a module is
self-contained and "compilable".
o node: A logical location in a hierarchy of data elements.
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, and
notification.
o schema node identifier: A mechanism for identifying a particular
node in the schema tree.
o schema tree: The definition hierarchy specified within a module.
o submodule: A partial module definition which contributes derived
types, groupings, data nodes, RPCs, and notifications to a module.
A YANG module can be constructed from a number of submodules.
o uses: The "uses" statement is used to instantiate the set of nodes
defined in a grouping statement. The instantiated nodes may be
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refined and augmented to tailor them to any specific needs.
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4. YANG Overview
4.1. Functional Overview
YANG is a language used to model data for the NETCONF protocol. A
YANG module defines a hierarchy of nodes which can be used for
NETCONF-based operations, including configuration, state data, remote
procedure calls (RPCs), and notifications. This allows a complete
description of all data sent between a NETCONF client and server.
YANG models the hierarchical organization of data as a tree in which
each node has a name, and either a value or a set of child nodes.
YANG provides clear and concise descriptions of the nodes, as well as
the interaction between those nodes.
YANG structures data models into modules and submodules. A module
can import data from other external modules, and include data from
submodules. The hierarchy can be extended, allowing one module to
add data nodes to the hierarchy defined in another module. This
augmentation can be conditional, with new nodes to appearing only if
certain conditions are met.
YANG models can describe constraints to be enforced on the data,
restricting the appearance or value of nodes based the presence or
value of other nodes in the hierarchy. These constraints are
enforceable by either the client or the server, and valid content
must abide by them.
YANG defines a set of built-in types, and has a type mechanism
through which additional types may be defined. Derived types can
restrict their base type's set of valid values using mechanisms like
range or pattern restrictions that can be enforced by clients or
servers. They can also define usage conventions for use of the
derived type, such as a string-based type that contains a host name.
YANG permits the definition of complex types using reusable grouping
of nodes. The instantiation of these groupings can refine or augment
the nodes, allowing it to tailor the nodes to its particular needs.
Derived types and groupings can be defined in one module or submodule
and used in either that location or in another module or submodule
that imports or includes it.
YANG organizational constructs include defining lists of nodes with
the same names and identifying the keys which distinguish list
members from each other. Such lists may be defined as either sorted
by user or automatically sorted by the system. For user-sorted
lists, operations are defined for manipulating the order of the
nodes.
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YANG modules can be translated into an XML format called YIN
(Appendix B), allowing applications using XML parsers and XSLT
scripts to operate on the models.
XML Schema [XSD] files can be generated from YANG modules, giving a
precise description of the XML representation of the data modeled in
YANG modules.
YANG strikes a balance between high-level object-oriented modeling
and low-level bits-on-the-wire encoding. The reader of a YANG module
can easily see the high-level view of the data model while seeing how
the object will be encoded in NETCONF operations.
YANG is an extensible language, allowing extension statements to be
defined by standards bodies, vendors, and individuals. The statement
syntax allows these extensions to coexist with standard YANG
statements in a natural way, while making extensions stand out
sufficiently for the reader to notice them.
YANG resists the tendency to solve all possible problems, limiting
the problem space to allow expression of NETCONF data models, not
arbitrary XML documents or arbitrary data models. The data models
described by YANG are designed to be easily operated upon by NETCONF
operations.
To the extent possible, YANG maintains compatibility with SNMP's
SMIv2 (Structure of Management Information version 2 [RFC2578],
[RFC2579]). SMIv2-based MIB modules can be automatically translated
into YANG modules for read-only access. However YANG is not
concerned with reverse translation from YANG to SMIv2.
Like NETCONF, YANG targets smooth integration with device's native
management infrastructure. This allows implementations to leverage
their existing access control mechanisms to protect or expose
elements of the data model.
4.2. Language Overview
This section introduces some important constructs used in YANG that
will aid in the understanding of the language specifics in later
sections.
4.2.1. Modules and Submodules
YANG defines modules using the "module" statement. This statement
defines the name of the module, which is typically used as the base
name of the file containing the module. The file suffix ".yang" is
typically used for YANG files. A module contains three types of
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statements: module-header statements, revision statements, and
definition statements. The module header statements describe the
module and give information about the module itself, the revision
statements give information about the history of the module, and the
definition statements are the body of the module where the data model
is defined.
Submodule are partial modules that contribute derived types,
groupings, data nodes, RPCs and notifications to a module. A module
may include a number of submodules, but each submodule may belong to
only one module. The "include" statement allows a module or
submodule to reference material in submodules, and the "import"
statement allows references to material defined in other modules.
To reference an item that is defined in an external module it MUST be
imported. Identifiers that are neither defined nor imported MUST NOT
be visible in the local module.
To reference an item that is defined in one of its submodules, the
module MUST include the submodule.
A submodule that needs to reference an item defined in another
submodule of the same module, MUST include this submodule.
There MUST NOT be any circular chains of imports or includes. For
example, if submodule "a" includes submodule "b", "b" cannot include
"a".
When a definition in an external module is referenced, a locally
defined prefix MUST be used, followed by ":", and then the external
identifier. References to definitions in the local module MAY use
the prefix notation. References to built-in data types (e.g., int32)
MUST NOT use the prefix notation.
Forward references are allowed in YANG.
4.2.2. Data Modeling Basics
YANG defines four types of nodes for data modeling. In each of the
following subsections, the example shows the YANG syntax as well as a
corresponding NETCONF XML representation.
4.2.2.1. Leaf Nodes
A leaf node contains simple data like an integer or a string. It has
exactly one value of a particular type, and no child nodes.
YANG Example:
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leaf host-name {
type string;
description "Hostname for this system";
}
NETCONF XML Encoding:
<host-name>my.example.com</host-name>
The "leaf" statement is covered in Section 7.6.
4.2.2.2. Leaf-list Nodes
A leaf-list is a sequence of leaf nodes with exactly one value of a
particular type per leaf.
YANG Example:
leaf-list domain-search {
type string;
description "List of domain names to search";
}
NETCONF XML Encoding:
<domain-search>high.example.com</domain-search>
<domain-search>low.example.com</domain-search>
<domain-search>everywhere.example.com</domain-search>
The "leaf-list" statement is covered in Section 7.7.
4.2.2.3. Container Nodes
A container node is used to group related nodes in a subtree. A
container has only child nodes and no value. A container may contain
any number of child nodes of any type (including leafs, lists,
containers, and leaf-lists).
YANG Example:
container system {
container login {
leaf message {
type string;
description
"Message given at start of login session";
}
}
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}
NETCONF XML Encoding:
<system>
<login>
<message>Good morning, Dave</message>
</login>
</system>
The "container" statement is covered in Section 7.5.
4.2.2.4. List Nodes
A list is a sequence of list entries. An entry is like a structure
or a record. A list entry is uniquely identified by its key(s). A
list entry may contain any number of child nodes of any type
(including leafs, lists, containers etc.).
YANG Example:
list user {
key "name";
leaf name {
type string;
}
leaf full-name {
type string;
}
leaf class {
type string;
}
}
NETCONF XML Encoding:
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<user>
<name>glocks</name>
<full-name>Goldie Locks</full-name>
<class>intruder</class>
</user>
<user>
<name>snowey</name>
<full-name>Snow White</full-name>
<class>free-loader</class>
</user>
<user>
<name>rzull</name>
<full-name>Repun Zell</full-name>
<class>tower</class>
</user>
The "list" statement is covered in Section 7.8.
4.2.2.5. Example Module
These statements are combined to define the module:
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// Contents of "acme-system.yang"
module acme-system {
namespace "http://acme.example.com/system";
prefix "acme";
organization "ACME Inc.";
contact "joe@acme.example.com";
description
"The module for entities implementing the ACME system.";
revision 2007-06-09 {
description "Initial revision.";
}
container system {
leaf host-name {
type string;
description "Hostname for this system";
}
leaf-list domain-search {
type string;
description "List of domain names to search";
}
container login {
leaf message {
type string;
description
"Message given at start of login session";
}
list user {
key "name";
leaf name {
type string;
}
leaf full-name {
type string;
}
leaf class {
type string;
}
}
}
}
}
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4.2.3. Operational Data
YANG can model operational data, as well as configuration data, based
on the "config" statement. When a node is tagged with "config
false", its subhierarchy is flagged as operational data, to be
reported using NETCONF's <get> operation, not the <get-config>
operation. Parent containers, lists, and key leafs are reported
also, giving the context for the operational data.
In this example, two leafs are defined for each interface, a
configured speed and an observed speed. The observed speed is not
configuration, so it can be returned with NETCONF <get> operations,
but not with <get-config> operations. The observed speed is not
configuration data, and cannot be manipulated using <edit-config>.
list interface {
key "name";
config true;
leaf name {
type string;
}
leaf speed {
type enumeration {
enum 10m;
enum 100m;
enum auto;
}
}
leaf observed-speed {
type uint32;
config false;
}
}
4.2.4. Built-in Types
YANG has a set of built-in types, similar to those of many
programming languages, but with some differences due to special
requirements from the management information model. The following
table summarizes the built-in types discussed in Section 8:
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+---------------------+-------------+-------------------------------+
| Name | Type | Description |
+---------------------+-------------+-------------------------------+
| int8 | Number | 8-bit signed integer |
| int16 | Number | 16-bit signed integer |
| int32 | Number | 32-bit signed integer |
| int64 | Number | 64-bit signed integer |
| uint8 | Number | 8-bit unsigned integer |
| uint16 | Number | 16-bit unsigned integer |
| uint32 | Number | 32-bit unsigned integer |
| uint64 | Number | 64-bit unsigned integer |
| float32 | Number | 32-bit IEEE floating point |
| | | real number |
| float64 | Number | 64-bit IEEE floating point |
| | | real number |
| string | Text | Human readable string |
| boolean | Text | "true" or "false" |
| enumeration | Text/Number | Enumerated strings with |
| | | associated numeric values |
| bits | Text/Number | A set of bits or flags |
| binary | Text | Any binary data |
| keyref | Text/Number | A reference to a list's key |
| | | value |
| empty | Empty | A leaf that does not have any |
| | | value |
| union | Text/Number | Choice of member types |
| instance-identifier | Text | References a data tree node |
+---------------------+-------------+-------------------------------+
The "type" statement is covered in Section 8.
4.2.5. Derived Types (typedef)
YANG can define derived types from base types using the "typedef"
statement. A base type can be either a built-in type or a derived
type, allowing a hierarchy of derived types.
A derived type can be used as the argument for the "type" statement.
YANG Example:
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typedef percent {
type uint16 {
range "0 .. 100";
}
description "Percentage";
}
leaf completed {
type percent;
}
NETCONF XML Encoding:
<completed>20</completed>
The "typedef" statement is covered in Section 7.3.
4.2.6. Reusable Node Groups (grouping)
Groups of nodes can be assembled into the equivalent of complex types
using the "grouping" statement. "grouping" defines a set of nodes
that are instantiated with the "uses" statement:
grouping target {
leaf address {
type inet:ip-address;
description "Target IP address";
}
leaf port {
type inet:port-number;
description "Target port number";
}
}
container peer {
container destination {
uses target;
}
}
NETCONF XML Encoding:
<peer>
<destination>
<address>192.0.2.1</address>
<port>830</port>
</destination>
</peer>
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The grouping can be refined as it is used, allowing certain
statements to be overridden. In this example the description is
refined:
container connection {
container source {
uses target {
leaf address {
description "Source IP address";
}
leaf port {
description "Source port number";
}
}
}
container destination {
uses target {
leaf address {
description "Destination IP address";
}
leaf port {
description "Destination port number";
}
}
}
}
The "grouping" statement is covered in Section 7.11.
4.2.7. Choices
YANG allows the data model to segregate incompatible nodes into
distinct choices using the "choice" and "case" statements. The
"choice" statement contains a set of "case" statements which define
sets of schema nodes that cannot appear together. Each "case" may
contain multiple nodes, but each node may appear in only one "case"
under a "choice".
When an element from one case is created, all elements from all other
cases are implicitly deleted. The device handles the enforcement of
the constraint, preventing incompatibilities from existing in the
configuration.
The choice and case nodes appear only in the schema tree, not in the
data tree or XML encoding. The additional levels of hierarchy are
not needed beyond the conceptual schema.
YANG Example:
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choice snack {
mandatory true;
case sports-arena {
leaf pretzel {
type empty;
}
leaf beer {
type empty;
}
}
case late-night {
leaf chocolate {
type enumeration {
enum dark;
enum milk;
enum first-available;
}
}
}
}
NETCONF XML Encoding:
<chocolate>first-available</chocolate>
The "choice" statement is covered in Section 7.9.
4.2.8. Extending Data Models (augment)
YANG allows a module to insert additional nodes into data models,
including both the current module (and its submodules) or an external
module. This is useful e.g. for vendors to add vendor-specific
parameters to standard data models in an interoperable way.
The "augment" statement defines the location in the data model
hierarchy where new nodes are inserted, and the "when" statement
defines the conditions when the new nodes are valid.
YANG Example:
augment system/login/user {
when "class != 'wheel'";
leaf uid {
type uint16 {
range "1000 .. 30000";
}
}
}
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This example defines a "uid" node that only is valid when the user's
"class" is not "wheel".
If a module augments another model, the XML representation of the
data will reflect the prefix of the augmenting model. For example,
if the above augmentation were in a module with prefix "other", the
XML would look like:
NETCONF XML Encoding:
<user>
<name>alicew</name>
<full-name>Alice N. Wonderland</full-name>
<class>drop-out</class>
<other:uid>1024</other:uid>
</user>
The "augment" statement is covered in Section 7.15.
4.2.9. RPC Definitions
YANG allows the definition of NETCONF RPCs. The method names, input
parameters and output parameters are modeled using YANG data
definition statements.
YANG Example:
rpc activate-software-image {
input {
leaf image-name {
type string;
}
}
output {
leaf status {
type string;
}
}
}
NETCONF XML Encoding:
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<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<activate-software-image xmlns="http://acme.example.com/system">
<name>acmefw-2.3</name>
</activate-software-image>
</rpc>
<rpc-reply message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<data>
<status xmlns="http://acme.example.com/system">
The image acmefw-2.3 is being installed.
</status>
</data>
</rpc-reply>
The "rpc" statement is covered in Section 7.13.
4.2.10. Notification Definitions
YANG allows the definition of notifications suitable for NETCONF.
YANG data definition statements are used to model the content of the
notification.
YANG Example:
notification link-failure {
description "A link failure has been detected";
leaf if-index {
type int32 { range "1 .. max"; }
}
leaf if-name {
type keyref {
path "/interfaces/interface/name";
}
}
}
NETCONF XML Encoding:
<notification
xmlns="urn:ietf:params:netconf:capability:notification:1.0">
<eventTime>2007-09-01T10:00:00Z</eventTime>
<link-failure xmlns="http://acme.example.com/system">
<if-name>so-1/2/3.0</if-name>
</link-failure>
</notification>
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The "notification" statement is covered in Section 7.14.
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5. Language Concepts
5.1. Modules and Submodules
The module is the base unit of definition in YANG. A module defines
a single data model. A module can define a complete, cohesive model,
or augment an existing data model with additional nodes.
A NETCONF server may implement a number of modules, allowing multiple
views of the same data, or multiple views of disjoint subsections of
the device's data. Alternatively, the server may implement only one
module that defines all available data. Any modules that are
implemented MUST be available for all defined datastores.
A module may be divided into submodules, based on the needs of the
module owner. The external view remains that of a single module,
regardless of the presence or size of its submodules.
A module uses the "include" statement to include its submodules, and
the "import" statement to reference external modules. Similarly, a
submodule may use the "import" statement to reference other modules,
and may use the "include" statement to reference other submodules
within its module. A module or submodule may not include submodules
from other modules, nor may a submodule import its own module.
The names of all standard modules must be unique, but different
revisions of the same module should have the same name. Developers
of enterprise modules are encouraged to choose names for their
modules that will have a low probability of colliding with standard
or other enterprise modules, e.g., by using the enterprise or
organization name as a prefix.
5.1.1. Module Hierarchies
YANG allows modeling of data in multiple hierarchies, where data may
have more than one root node. While it is recommended to use a model
with a single root node, models that have multiple roots nodes are
sometimes convenient, and are supported by YANG.
Due to the possibility of multiple roots the modeled data does not
necessarily map to a well-formed XML document. Often a conceptual
root node (e.g. <data> or <config> element in NETCONF RPCs) is added
to overcome this problem.
5.2. File Layout
YANG modules and submodules are typically stored in files, one module
or submodule per file, with the name of the file given by the
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concatenation of the module or submodule name and the file suffix
".yang". YANG compilers can find imported modules and included
submodules via this convention. While the YANG language defines
modules, tools may compile submodules independently for performance
and manageability reasons. Many errors and warnings that cannot be
detected during submodule compilation may be delayed until the
submodules are linked into a cohesive module.
5.3. Object Based View of YANG
While YANG models the configuration as a data tree, it can be used in
an object-based manner as well.
The configuration and state data of the device is modeled as a tree
of object instances (objects for short). Each object in the tree has
a type name (or managed object class name), a namespace, a (possibly
empty) set of attributes and a (possibly empty) set of child objects.
A managed object class could be defined as a grouping, containing
just one list. Attributes should be defined as leafs inside the
list. Child objects should be defined with the corresponding uses
statements.
A defined grouping unambiguously defines its properties, it has its
own unique name, so when it is referred to in the "uses" statement it
is always the same well defined set of properties that we are using.
The data tree can be defined as one or more top level containers
containing managed object classes defined as groupings. All further
levels of the data tree are defined by managed object classes
containing further managed objects.
5.4. XML Namespaces
All YANG definitions are specified within a particular XML Namespace.
Each module defines an XML namespace as a globally unique URI
[RFC3986]. A NETCONF client or server uses the namespace during XML
encoding of data.
The namespace URI is advertised as a capability in the NETCONF
<hello> message to indicate support for the YANG module by a NETCONF
server.
Namespaces for standard module names will be assigned by IANA. They
MUST be unique (but different revisions of the same module should
have the same namespace).
Namespaces for private module names will be assigned by the
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organization owning the module without a central registry. It is
recommended to choose namespaces that will have a low probability of
colliding with standard or other enterprise modules, e.g. by using
the enterprise or organization name in the namespace.
The "namespace" statement is covered in Section 7.1.3.
5.4.1. YANG Namespace
YANG defines its own namespace for NETCONF <edit-config> operations.
This namespace is "urn:ietf:params:xml:ns:yang:1" [XXX IANA].
5.5. Ordering
YANG supports two styles for ordering the entries within a list. In
many lists, the order of list entries does not impact the
implementation of the list's configuration, and the device is free to
sort the list entries in any reasonable order. The "description"
string for the list may suggest an order. YANG calls this style of
list "system ordered" and they are indicated with the statement
"ordered-by system".
For example, a list of valid users would typically be sorted
alphabetically, since the order in which the users appeared in the
configuration would not impact the creation of those users' accounts.
In the other style of lists, the order of list entries matters for
the implementation of the list's configuration and the user is
responsible for ordering the entries, while the device maintains that
order. YANG calls this style of list "user ordered" and they are
indicated with the statement "ordered-by user".
For example, the order in which firewall filters entries are applied
to incoming traffic may affect how that traffic is filtered. The
user would need to decide if the filter entry that discards all TCP
traffic should be applied before or after the filter entry that
allows all traffic from trusted interfaces. The choice of order
would be crucial.
YANG provides a rich set of facilities within NETCONF's <edit-config>
operation which allow the order of list entries in user-ordered lists
to be controlled. List entries may be inserted or rearranged,
positioned as the first or last entry in the list, or positioned
before or after another specific entry.
The "ordered-by" statement is covered in Section 7.7.4.
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5.6. Containers with Presence
YANG supports two styles of containers, those which exist only for
organizing the hierarchy of data nodes, and those whose presence in
the configuration has an explicit meaning.
In the first style, the container has no meaning of its own, existing
only to contain child nodes. The container data node is implicitly
created when the first child data node is created. The data node is
implicitly deleted when the last non-key child is deleted, since an
empty container has no meaning.
For example, the set of scrambling options for SONET interfaces may
be placed inside a "scrambling" container to enhance the organization
of the configuration hierarchy, and to keep these nodes together.
The "scrambling" node itself has no meaning, so removing the node
when it becomes empty relieves the user from the task of performing
this task.
In the second style, the presence of the container itself is
configuration data, representing a single bit of configuration data.
The container acts as both a configuration knob and a means of
organizing related configuration. These containers are explicitly
created and deleted.
YANG calls this style a "presence container" and they are indicated
using the "presence" statement, which takes as its argument a text
string indicating what the presence of the node means.
For example, an "ssh" container may turn on the ability to log into
the device using ssh, but can also contain any ssh-related
configuration knobs, such as connection rates or retry limits.
The "presence" statement is covered in Section 7.5.4.
5.7. Scoping
YANG uses static scoping. Grouping definitions are resolved in the
context in which they are defined, rather than the context in which
they are used. Users of groupings are not required to import modules
or include submodules to satisfy all references made by the grouping.
For example, if a module defines a grouping in which a type is
referenced, when the grouping is used in a second module, the type is
resolved in the original module, not the second module. There is no
worry over conflicts if both modules define the type, since there is
no ambiguity.
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5.8. Nested Typedefs and Groupings
Typedefs and groupings may appear nested under many YANG statements,
allowing these to be lexically scoped by the hierarchy under which
they appear. This allows types and groupings to be defined near
where they are used, rather than placing them at the top level of the
hierarchy. The close proximity increases readability.
Scoping also allows types to be defined without concern for naming
conflicts between types in different submodules. Type names can be
specified without adding leading strings designed to prevent name
collisions within large modules.
Finally, scoping allows the module author to keep types and groupings
private to their module or submodule, preventing their reuse. Since
only top-level types and groupings can be used outside the module or
submodule, the developer has more control over what pieces of their
module are presented to the outside world, supporting the need to
hide internal information and maintaining a boundary between what is
shared with the outside world and what is kept private.
Scoped definitions MUST NOT shadow definitions at a higher scope. A
type or group cannot be defined if a higher level in the schema
hierarchy has a definition with a matching identifier.
When a YANG implementation resolves a reference to an unprefixed type
or grouping, it searches up the levels of hierarchy in the schema
tree, starting at the current level, for the definition of the type
or grouping.
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6. YANG syntax
The YANG syntax is similar to that of SMIng [RFC3780] and programming
languages like C and C++. This C-like syntax was chosen specifically
for its readability, since YANG values the time and effort of the
readers of models above those of modules writers and YANG tool-chain
developers. This section introduces the YANG syntax.
YANG modules are written in the UTF-8 [RFC3629] character set.
6.1. Lexicographical Tokenization
YANG modules are parsed as a series of tokens. This section details
the rules for recognizing tokens from an input stream. YANG
tokenization rules are both simple and powerful. The simplicity is
driven by a need to keep the parsers easy to implement, while the
power is driven by the fact that modelers need to express their
models in readable formats.
6.1.1. Comments
Comments are C++ style. A single line comment starts with "//" and
ends at the end of the line. A block comment is enclosed within "/*"
and "*/".
6.1.2. Tokens
A token in YANG is either a keyword, a string, ";", "{", or "}". A
string can be quoted or unquoted. A keyword is either one of the
core YANG keywords defined in this document, or a prefix identifier,
followed by ":", followed by a language extension keyword. Keywords
are case sensitive. See Section 6.2 for a formal definition of
identifiers.
6.1.3. Quoting
If a string contains any whitespace characters, a semicolon (";"),
curly braces ("{ }"), or comment sequences ("//", "/*", or "*/"),
then it MUST be enclosed within double or single quotes.
If the double quoted string contains a line break followed by
whitespace which is used to indent the text according to the layout
in the YANG file, this leading whitespace is stripped from the
string, up to at most the same column of the double quote character.
If the double quoted string contains whitespace before a line break,
this trailing whitespace is stripped from the string.
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A single quoted string (enclosed within ' ') preserves each character
within the quotes. A single quote character can not occur in a
single quoted string, even when preceded by a backslash.
If a quoted string is followed by a plus character ("+"), followed by
another quoted string, the two strings are concatenated into one
quoted string, allowing multiple concatenations to build one quoted
string. Whitespace trimming of double quoted strings is done before
concatenation.
Within a double quoted string (enclosed within " "), a backslash
character introduces a special character, which depends on the
character that immediately follows the backslash:
\n new line
\t a tab character
\" a double quote
\\ a single backslash
6.1.3.1. Quoting Examples
The following strings are equivalent:
hello
"hello"
'hello'
"hel" + "lo"
'hel' + "lo"
The following examples show some special strings:
"\"" - string containing a double quote
'"' - string containing a double quote
"\n" - string containing a newline character
'\n' - string containing a backslash followed
by the character n
The following examples show some illegal strings:
'''' - a single-quoted string cannot contain single quotes
""" - a double quote must be escaped in a double quoted string
The following strings are equivalent:
"first line
second line"
"first line\n" + " second line"
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6.2. Identifiers
Identifiers are used to identify different kinds of YANG items by
name. Each identifier starts with an upper-case or lower-case ASCII
letter or an underscore character, followed by zero or more ASCII
letters, digits, underscore characters, hyphens, and dots.
Implementations MUST support identifiers up to 63 characters in
length. Identifiers are case sensitive. The identifier syntax is
formally defined by the rule "identifier" in Appendix D. Identifiers
can be specified as quoted or unquoted strings.
6.2.1. Identifiers and their namespaces
Each identifier is valid in a namespace which depends on the type of
the YANG item being defined:
o All module and submodule names share the same global module
identifier namespace.
o All extension names defined in a module and its submodules share
the same extension identifier namespace.
o All derived type names defined within a parent node or at the top-
level of the module or its submodules share the same type
identifier namespace. This namespace is scoped to the parent node
or module.
o All groupings defined within a parent node or at the top-level of
the module or its submodules share the same grouping identifier
namespace. This namespace is scoped to the parent node or module.
o All leafs, leaf-lists, lists, containers, choices, rpcs, and
notifications defined within a parent node or at the top-level of
the module or its submodules share the same identifier namespace.
This namespace is scoped to the parent node or module, unless the
parent node is a case node. In that case, the namespace is scoped
to the parent node of the case node's parent choice node.
o All cases within a choice share the same case identifier
namespace. This namespace is scoped to the parent choice node.
All identifiers defined in a namespace MUST be unique.
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
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within curly braces ("{ }"):
statement = keyword [argument] (";" / "{" *statement "}")
The argument is a string, as defined in Section 6.1.2.
6.3.1. Language Extensions
A module can introduce YANG extensions by using the "extension"
keyword (see Section 7.16). The extensions can be imported by other
modules with the "import" statement (see Section 7.1.5). When an
imported extension is used, the keyword must be qualified using the
prefix with which the extension's module was imported.
6.4. XPath Evaluations
YANG relies on XPath as a notation for specifying many inter-node
references and dependencies. NETCONF clients and servers are not
required to implement an XPath interpreter, but MUST ensure that the
requirements encoded in the data model are enforced. The manner of
enforcement is an implementation decision. The XPath expressions
MUST be valid, but any implementation may choose to implement them by
hand, rather than using the XPath expression directly.
XPath expressions are evaluated in the context of the current node,
with the namespace of the current module defined as the null
namespace. References to identifiers in external modules MUST be
qualified with appropriate prefixes, and references to the current
module and its submodules MAY use a prefix.
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7. YANG Statements
The following sections describe all of the YANG core statements.
Note that even a statement which does not have any substatements
defined in core YANG can have vendor-specific extensions as
substatements. For example, the "description" statement does not
have any substatements defined in core YANG, but the following is
legal:
description "some text" {
acme:documentation-flag 5;
}
7.1. The module Statement
The "module" statement defines the module's name, and groups all
statements which belong to the module together. The "module"
statement's argument is the name of the module, followed by a block
of substatements that hold detailed module information. The module
name follows the rules for identifiers in Section 6.2.
Standard module names will be assigned by IANA. The names of all
standard modules MUST be unique (but different revisions of the same
module should have the same name).
Private module names will be assigned by the organization owning the
module without a central registry. It is recommended to choose names
for their modules that will have a low probability of colliding with
standard or other enterprise modules, e.g. by using the enterprise or
organization name as a prefix.
A module SHOULD have the following layout:
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module <module-name> {
// header information
<yang-version statement>
<namespace statement>
<prefix statement>
// linkage statements
<import statements>
<include statements>
// meta information
<organization statement>
<contact statement>
<description statement>
<reference statement>
// revision history
<revision statements>
// module definitions
<extension statements>
<typedef statements>
<grouping statements>
<container statements>
<leaf statements>
<leaf-list statements>
<list statements>
<choice statements>
<uses statements>
<rpc statements>
<notification statements>
<augment statements>
}
7.1.1. The module's Substatements
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+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| anyxml | 7.10 | 0..n |
| augment | 7.15 | 0..n |
| choice | 7.9 | 0..n |
| contact | 7.1.8 | 0..1 |
| container | 7.5 | 0..n |
| description | 7.17.3 | 0..1 |
| extension | 7.16 | 0..n |
| grouping | 7.11 | 0..n |
| import | 7.1.5 | 0..n |
| include | 7.1.6 | 0..n |
| leaf | 7.6 | 0..n |
| leaf-list | 7.7 | 0..n |
| list | 7.8 | 0..n |
| namespace | 7.1.3 | 1 |
| notification | 7.14 | 0..n |
| organization | 7.1.7 | 0..1 |
| prefix | 7.1.4 | 1 |
| reference | 7.17.4 | 0..1 |
| revision | 7.1.9 | 0..n |
| rpc | 7.13 | 0..n |
| typedef | 7.3 | 0..n |
| uses | 7.12 | 0..n |
| yang-version | 7.1.2 | 0..1 |
+--------------+---------+-------------+
7.1.2. The yang-version Statement
The "yang-version" statement specifies which version of the YANG
language was used in developing the module. The statement's argument
contains value "1", which is the current yang version and the default
value.
This statement is intended for future-proofing the syntax of YANG
against possible changes in later versions of YANG. Since the
current version is the default value, the statement need not appear
in YANG modules until a future version is defined. When a new
version is defined, YANG modules can either use version 2 features
and add the "yang-version 2" statement, or remain within the version
1 feature set and continue to use the default setting of "yang-
version 1".
7.1.3. The namespace Statement
The "namespace" statement defines the XML namespace for all XML
elements defined by the module. Its argument is the URI of the
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namespace.
The namespace URI is advertised as a capability in the NETCONF
<hello> message to indicate support for the YANG module by a NETCONF
server. The capability URI advertised SHOULD be on the form:
namespace-uri "?" revision
Where "revision" is the revision of the module (see Section 7.1.9)
that the server implements.
See also Section 5.4.
7.1.4. The prefix Statement
The "prefix" statement is used to define the prefix associated with
the namespace of a module. The "prefix" statement's argument is the
prefix string which is used as a prefix to access a module. The
prefix string may be used to refer to definitions contained in the
module, e.g. "if:ifName". A prefix follows the same rules as an
identifier (see Section 6.2).
When used inside the "module" statement, the "prefix" statement
defines the prefix to be used when this module is imported. To
improve readability of the NETCONF XML, a NETCONF client or server
which generates XML or XPath that use prefixes, the prefix defined by
a module SHOULD be used, unless there is a conflict.
When used inside the "import" statement, the "prefix" statement
defines the prefix to be used when accessing data inside the imported
module. When a reference to an identifier from the imported module
is used, the prefix string for the module from which objects are
being imported is used in combination with a colon (":") and the
identifier, e.g. "if:ifIndex". To improve readability of YANG
modules, the prefix defined by a module SHOULD be used when the
module is imported, unless there is a conflict.
All prefixes, including the prefix for the module itself MUST be
unique within the module or submodule.
7.1.5. The import Statement
The "import" statement makes content from one module available inside
another module or submodule. The argument is the name of the module
to import, and the statement is followed by a block of substatements
that holds detailed import information.
All identifiers contained in an imported module are imported into the
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current module or submodule, so that they can be referenced by
definitions in the current module or submodule. The mandatory
"prefix" substatement assigns a prefix for the imported module which
is scoped to the importing module or submodule. Multiple "import"
statements may be specified to import from different modules.
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| prefix | 7.1.4 | 1 |
+--------------+---------+-------------+
7.1.6. The include Statement
The "include" statement is used to make content from a submodule
available to the module. The argument is an identifier which is the
name of the submodule to include. Modules are only allowed to
include submodules that belong to that module, as defined by the
"belongs-to" statement (see Section 7.2.2).
7.1.7. The organization Statement
The "organization" statement defines the party responsible for this
module. The argument is a string which is used to specify a textual
description of the organization(s) under whose auspices this module
was developed.
7.1.8. The contact Statement
The "contact" statement provides contact information for the module.
The argument is a string which is used to specify the name, postal
address, telephone number, and electronic mail address of the person
to whom technical queries concerning this module should be sent.
7.1.9. The revision Statement
The "revision" statement specifies the editorial revision history of
the module, including the initial revision. A series of revisions
statements detail the changes in the module's definition. The
argument is a date string in the format "YYYY-MM-DD", followed by a
block of substatements that holds detailed revision information. A
module SHOULD have at least one initial "revision" statement. For
every editorial change, a new one SHOULD be added in front of the
revisions sequence, so that all revisions are in reverse
chronological order.
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7.1.9.1. The revision's Substatement
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| description | 7.17.3 | 0..1 |
+--------------+---------+-------------+
7.1.10. Usage Example
module acme-system {
namespace "http://acme.example.com/system";
prefix "acme";
import yang-types {
prefix "yang";
}
include acme-types;
organization "ACME Inc.";
contact
"Joe L. User
ACME, Inc.
42 Anywhere Drive
Nowhere, CA 95134
USA
Phone: +1 800 555 0815
EMail: joe@acme.example.com";
description
"The module for entities implementing the ACME protocol.";
revision "2007-06-09" {
description "Initial revision.";
}
// definitions follows...
}
7.2. The submodule Statement
While the primary unit in YANG is a module, a YANG module can itself
be constructed out of several submodules. Submodules allow to split
a complex module in several pieces where all the submodules
contribute to a single namespace, which is defined by the module
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including the submodules.
The "submodule" statement is used to give the submodule a name, and
to group all statements which belong to the submodule together.
The "submodule" statement, which must be present at most once, takes
as an argument an identifier which is the name of the submodule,
followed by a block of substatements that hold detailed submodule
information.
Standard submodule names will be assigned by IANA. Name of all
standard submodules must be unique and in addition not conflict with
module names (but different revisions of the same submodule should
have the same name).
Private submodule names will be assigned by the organization owning
the submodule without a central registry. It is recommended to
choose names for their submodules that will have a low probability of
colliding with standard or other enterprise modules and submodules,
e.g. by using the enterprise or organization name as a prefix.
A submodule SHOULD have the following layout:
submodule <module-name> {
<yang-version statement>
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// module identification
<belongs-to statement>
// linkage statements
<import statements>
<include statements>
// meta information
<organization statement>
<contact statement>
<description statement>
<reference statement>
// revision history
<revision statements>
// module definitions
<extension statements>
<typedef statements>
<grouping statements>
<container statements>
<leaf statements>
<leaf-list statements>
<list statements>
<choice statements>
<uses statements>
<rpc statements>
<notification statements>
<augment statements>
}
7.2.1. The submodule's Substatements
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+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| anyxml | 7.10 | 0..n |
| augment | 7.15 | 0..n |
| belongs-to | 7.2.2 | 1 |
| choice | 7.9 | 0..n |
| contact | 7.1.8 | 0..1 |
| container | 7.5 | 0..n |
| description | 7.17.3 | 0..1 |
| extension | 7.16 | 0..n |
| grouping | 7.11 | 0..n |
| import | 7.1.5 | 0..n |
| include | 7.1.6 | 0..n |
| leaf | 7.6 | 0..n |
| leaf-list | 7.7 | 0..n |
| list | 7.8 | 0..n |
| notification | 7.14 | 0..n |
| organization | 7.1.7 | 0..1 |
| reference | 7.17.4 | 0..1 |
| revision | 7.1.9 | 0..n |
| rpc | 7.13 | 0..n |
| typedef | 7.3 | 0..n |
| uses | 7.12 | 0..n |
| yang-version | 7.1.2 | 0..1 |
+--------------+---------+-------------+
7.2.2. The belongs-to Statement
The "belongs-to" statement specifies the module to which the
submodule belongs. The argument is an identifier which is the name
of the module. Only the module to which a submodule belongs, or
another submodule that belongs to the same module, are allowed to
include that submodule.
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7.2.3. Usage Example
submodule acme-types {
belongs-to "acme-system";
import yang-types {
prefix "yang";
}
organization "ACME Inc.";
contact
"Joe L. User
ACME, Inc.
42 Anywhere Drive
Nowhere, CA 95134
USA
Phone: +1 800 555 0815
EMail: joe@acme.example.com";
description
"This submodule defines common ACME types.";
revision "2007-06-09" {
description "Initial revision.";
}
// definitions follows...
}
7.3. The typedef Statement
The "typedef" statement defines a new type which may be used locally
in the module, in modules or submodules which include it, and by
other modules which import from it. The new type is called the
"derived type", and the type from which it was derived is called the
"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
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the module. For details about scoping for nested typedef, see
Section 5.8.
7.3.1. The typedef's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| default | 7.3.4 | 0..1 |
| description | 7.17.3 | 0..1 |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 0..1 |
| type | 7.3.2 | 1 |
| units | 7.3.3 | 0..1 |
+--------------+---------+-------------+
7.3.2. The typedef's type Statement
The "type" statement, which must be present, defines the base type
from which this type is derived. See Section 7.4 for details.
7.3.3. The units Statement
The "units" statement, which is optional, takes as an argument a
string which contains a textual definition of the units associated
with the type.
7.3.4. The typedef's default Statement
The "default" statement takes as an argument a string which contains
a default value for the new type.
The value of the "default" statement MUST correspond to the type
specified in the "type" statement.
If the base type has a default value, and the new derived type does
not specify a new default value, the base type's default value is
also the default value of the new derived type. The default value
MUST correspond to any restrictions in the derived type.
If the base type's default value does not correspond to the new
restrictions, the derived type MUST define a new default value.
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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 8) or a derived type (see
Section 7.3), followed by an optional block of substatements that are
used to put further restrictions on the type.
The restrictions that can be applied depends on the type being
restricted. All restriction statements are described in conjunction
with the built-in types in Section 8.
7.4.1. The type's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| bit | 8.6.3 | 0..n |
| enum | 8.5.3 | 0..n |
| length | 8.3.3 | 0..1 |
| path | 8.8.2 | 0..1 |
| pattern | 8.3.4 | 0..1 |
| range | 8.1.3 | 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.
By default, a container does not carry any information, but is used
to organize and give structure to the data being defined. The
"presence" statement (see Section 7.5.4) is used to give semantics to
the existence of the container in the data tree.
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7.5.1. The container's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| anyxml | 7.10 | 0..n |
| augment | 7.15 | 0..n |
| choice | 7.9 | 0..n |
| config | 7.17.1 | 0..1 |
| container | 7.5 | 0..n |
| description | 7.17.3 | 0..1 |
| grouping | 7.11 | 0..n |
| leaf | 7.6 | 0..n |
| leaf-list | 7.7 | 0..n |
| list | 7.8 | 0..n |
| must | 7.5.2 | 0..n |
| presence | 7.5.4 | 0..1 |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 0..1 |
| typedef | 7.3 | 0..n |
| uses | 7.12 | 0..n |
+--------------+---------+-------------+
7.5.2. The must Statement
The "must" statement, which is optional, takes as an argument a
string which contains an XPath expression. It is used to formally
declare a constraint on the configuration data. When a configuration
datastore is validated, all "must" constraints are conceptually
evaluated once for each corresponding instance in the datastore's
data tree, and for all leafs with default values in effect. If an
instance does not exist in the data tree, and it does not have a
default value, its "must" statements are not evaluated. Within a
"must" expression,
All such constraints MUST evaluate to true for the configuration to
be valid.
The "must" statement is ignored if the data does not represent
configuration.
The XPath expression is conceptually evaluated in the following
context:
o The context node is the node in the data tree for which the "must"
statement is defined.
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o The accessible tree is made up of all nodes in the data tree, and
all leafs with default values.
o The set of namespace declarations is the set of all "import"
statements' prefix and namespace pairs, and the "prefix"
statement's prefix for the "namespace" statement's URI.
o The null namespace is defined to be the namespace of the current
module.
o One variable "this", which is the context node, is defined.
The result of the XPath expression is converted to a boolean value
using the standard XPath rules.
Note that the XPath expression is conceptually evaluated. This means
that an implementation does not have to use an XPath evaluator on the
device. How the evaluation is done in practice is an implementation
decision.
7.5.3. The must's Substatements
+---------------+---------+-------------+
| substatement | section | cardinality |
+---------------+---------+-------------+
| description | 7.17.3 | 0..1 |
| error-app-tag | 7.5.3.2 | 0..1 |
| error-message | 7.5.3.1 | 0..1 |
| reference | 7.17.4 | 0..1 |
+---------------+---------+-------------+
7.5.3.1. The error-message Statement
The "error-message" statement, which is optional, takes a string as
an argument. If the constraint evaluates to false, the string is
passed as <error-message> in the <rpc-error>.
7.5.3.2. The error-app-tag Statement
The "error-app-tag" statement, which is optional, takes a string as
an argument. If the constraint evaluates to false, the string is
passed as <error-app-tag> in the <rpc-error>.
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7.5.3.3. Usage Example of must and error-message
container interface {
leaf ifType {
type enumeration {
enum ethernet;
enum atm;
}
}
leaf ifMTU {
type uint32;
}
must "ifType != ethernet or " +
"(ifType == ethernet and ifMTU == 1500)" {
error-message "An ethernet MTU must be 1500";
}
must "ifType != atm or " +
"(ifType == atm and ifMTU <= 17966 and ifMTU >= 64)" {
error-message "An atm MTU must be 64 .. 17966";
}
}
7.5.4. The presence Statement
The "presence" statement assigns a meaning to the presence of a
container in the data tree. It takes as an argument a string which
contains a textual description of what the node's presence means.
If a container has the "presence" statement, the container's
existence in the data tree carries some meaning. Otherwise, the
container is used to give some structure to the data, and it carries
no meaning by itself.
See Section 5.6 for additional information.
7.5.5. The container's Child Node Statements
Within a container, the "container", "leaf", "list", "leaf-list",
"uses", and "choice" statements can be used to define child nodes to
the container.
7.5.6. XML Encoding Rules
A container node is encoded as an XML element. The element's name is
the container's identifier, and its XML namespace is the module's XML
namespace.
The container's child nodes are encoded as subelements to the
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container element, in the same order as they are defined within the
container statement.
A NETCONF server that replies to a <get> or <get-config> request MAY
choose not to send a container element if the container node does not
have the "presence" statement and no child nodes exist. Thus, a
client that receives an <rpc-reply> for a <get> or <get-config>
request, must be prepared to handle the case that a container node
without a presence statement is not present in the XML.
7.5.7. NETCONF <edit-config> Operations
When a NETCONF server processes an <edit-config> request, the
elements of procedure for the container node are:
If the operation is "merge" the node is created if it does not
exist.
If the operation is "replace" and the node exists, all child nodes
not present in the XML are deleted, and child nodes present in the
XML but not present in the datastore are created.
If the operation is "create" the node is created if it does not
exist.
If the operation is "delete" the node is deleted if it exists.
If the container has a "presence" statement, it may be implicitly
created if it does not exist, even if the operation is "none".
If a container has a "presence" statement and the last child node
is deleted, the NETCONF server MAY delete the container.
7.5.8. Usage Example
Given the following container definition:
container system {
description "Contains various system parameters";
container services {
description "Configure externally available services";
container "ssh" {
presence "Enables SSH";
description "SSH service specific configuration";
// more leafs, containers and stuff here...
}
}
}
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A corresponding XML encoding would look like this:
<system>
<services>
<ssh/>
</services>
</system>
Since the <ssh> element is present, ssh is enabled.
To delete a container with an <edit-config>:
<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
<edit-config>
<target>
<running/>
</target>
<config>
<system xmlns="http://example.com/schema/config">
<services>
<ssh nc:operation="delete"/>
</services>
</system>
</config>
</edit-config>
</rpc>
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.
A leaf node exists in zero or one instances in the data tree,
depending on the value of the "mandatory" statement.
The "leaf" statement is used to define a scalar variable of a
particular built-in or derived type.
If a leaf has a "default" statement, the leaf's default value is set
to the value of the "default" statement. Otherwise, if the leaf's
type has a default value, and the leaf is not mandatory, then the
leaf's default value is set to the type's default value. In all
other cases, the leaf does not have a default value.
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If the leaf has a default value, the server MUST use this value
internally if no value is provided by the NETCONF client when the
instance is created.
7.6.1. The leaf's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| config | 7.17.1 | 0..1 |
| default | 7.6.3 | 0..1 |
| description | 7.17.3 | 0..1 |
| mandatory | 7.6.4 | 0..1 |
| must | 7.5.2 | 0..n |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 0..1 |
| type | 7.6.2 | 1 |
| units | 7.3.3 | 0..1 |
+--------------+---------+-------------+
7.6.2. The leaf's type Statement
The "type" statement, which must be present, takes as an argument the
name of an existing built-in or derived type. The optional
substatements specify restrictions on this type. See Section 7.4 for
details.
7.6.3. The leaf's default Statement
The "default" statement, which is optional, takes as an argument a
string which contains a default value for the leaf.
The value of the "default" statement MUST correspond to the type
specified in the leaf's "type" statement.
The "default" statement MUST NOT be present on nodes where
"mandatory" is true.
7.6.4. The leaf's mandatory Statement
The "mandatory" statement, which is optional, takes as an argument
the string "true" or "false". If "mandatory" is "true", the node
must exist in a valid configuration if its parent node exists. Since
containers without a "presence" statement are implicitly created and
deleted when needed, they are ignored when performing mandatory tests
for leafs. A mandatory leaf within such a container is mandatory
even if the container's data node does not exist.
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If not specified, the default is "false".
7.6.5. XML Encoding Rules
A leaf node is encoded as an XML element. The element's name is the
leaf's identifier, and its XML namespace is the module's XML
namespace.
The value of the leaf node is encoded to XML according to the type,
and sent as character data in the element.
A NETCONF server that replies to a <get> or <get-config> request MAY
choose not to send the leaf element if its value is the default
value. Thus, a client that receives an <rpc-reply> for a <get> or
<get-config> request, must be prepared to handle the case that a leaf
node with a default value is not present in the XML. In this case,
the value used by the server is known to be the default value.
See Section 7.6.7 for an example.
7.6.6. NETCONF <edit-config> Operations
When a NETCONF server processes an <edit-config> request, the
elements of procedure for the leaf node are:
If the operation is "merge", the node is created if it does not
exist, and its value is set to the value found in the XML RPC
data.
If the operation is "replace", the node is created if it does not
exist, and its value is set to the value found in the XML RPC
data.
If the operation is "create" the node is created if it does not
exist.
If the operation is "delete" the node is deleted if it exists.
7.6.7. Usage Example
Given the following leaf statement:
leaf port {
type inet:port-number;
default 22;
description "The port which the SSH server listens to"
}
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A corresponding XML encoding:
<port>2022</port>
To create a leaf with an edit-config:
<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
<edit-config>
<target>
<running/>
</target>
<config>
<system xmlns="http://example.com/schema/config">
<services>
<ssh>
<port>2022</port>
</ssh>
</services>
</system>
</config>
</edit-config>
</rpc>
7.7. The leaf-list Statement
Where the "leaf" statement is used to define a simple scalar variable
of a particular type, the "leaf-list" statement is used to define an
array of a particular type. The "leaf-list" statement takes one
argument, which is an identifier, followed by a block of
substatements that holds detailed leaf-list information.
The values in a leaf-list MUST be unique.
If the type referenced by the leaf-list has a default value, it has
no effect in the leaf-list.
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7.7.1. The leaf-list's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| config | 7.17.1 | 0..1 |
| description | 7.17.3 | 0..1 |
| max-elements | 7.7.3 | 0..1 |
| min-elements | 7.7.2 | 0..1 |
| must | 7.5.2 | 0..n |
| ordered-by | 7.7.4 | 0..1 |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 0..1 |
| type | 7.4 | 1 |
| units | 7.3.3 | 0..1 |
+--------------+---------+-------------+
7.7.2. The min-elements Statement
The "min-elements" statement, which is optional, takes as an argument
a non-negative integer which puts a constraint on a valid
configuration. A valid configuration always has at least min-
elements values in the leaf-list or list.
If no "min-elements" statement is present, it defaults to zero.
7.7.3. The max-elements Statement
The "max-elements" statement, which is optional, takes as an argument
a positive integer or the string "unbounded", which puts a constraint
on a valid configuration. A valid configuration always has at most
max-elements values in the leaf-list or list.
If no "max-elements" statement is present, it defaults to
"unbounded".
7.7.4. The ordered-by Statement
The "ordered-by" statement defines whether the order of entries
within a list are determined by the user or the system. The argument
is one of the strings "system" or "user". If not present, order
defaults to "system".
See Section 5.5 for additional information.
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7.7.4.1. ordered-by system
The entries in the list are sorted according to an unspecified order.
Thus an implementation is free to sort the entries in the most
appropriate order. An implementation SHOULD use the same order for
the same data, regardless of how the data were created. Using a
deterministic order will makes comparisons possible using simple
tools like "diff".
This is the default order.
7.7.4.2. ordered-by user
The entries in the list are sorted according to an order defined by
the user. This order is controlled by using special XML attributes
in the <edit-config> request. See Section 7.7.6 for details.
7.7.5. XML Encoding Rules
A leaf-list node is encoded as a series of XML elements. Each
element's name is the leaf-list's identifier, and its XML namespace
is the module's XML namespace.
The value of the leaf-list node is encoded to XML according to the
type, and sent as character data in the element.
See Section 7.7.7 for an example.
7.7.6. NETCONF <edit-config> operations
Leaf-list entries can be created and deleted, but not modified,
through <edit-config>, by using the "operation" attribute in the
leaf-list entry's XML element.
In an "ordered-by user" leaf-list, the attributes "insert" and
"value" in the YANG namespace (Section 5.4.1) can be used to control
where in the leaf-list the entry is inserted. These can be used
during "create" operations to insert a new leaf-list entry, or during
"merge" or "replace" operations to insert a new leaf-list entry or
move an existing one.
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".
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In a <copy-config>, or an <edit-config> with a "replace" operation
which covers the entire leaf-list, the leaf-list order is the same as
the order of the XML elements in the request.
When a NETCONF server processes an <edit-config> request, the
elements of procedure for a leaf-list node are:
If the operation is "merge" or "replace" the leaf-list entry is
created if it does not exist.
If the operation is "create" the leaf-list entry is created if it
does not exist.
If the operation is "delete" the entry is deleted from the leaf-
list if it exists.
7.7.7. Usage Example
leaf-list allow-user {
type string;
description "A list of user name patterns to allow";
}
A corresponding XML encoding:
<allow-user>alice</allow-user>
<allow-user>bob</allow-user>
To create a new element in the list:
<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
<edit-config>
<target>
<running/>
</target>
<config>
<system xmlns="http://example.com/schema/config">
<services>
<ssh>
<allow-user>eric</allow-user>
</ssh>
</services>
</system>
</config>
</edit-config>
</rpc>
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Given the following ordered-by user leaf-list:
leaf-list ciphers {
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":
<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0"
xmlns:yang="urn:ietf:params:xml:ns:yang:1">
<edit-config>
<target>
<running/>
</target>
<config>
<system xmlns="http://example.com/schema/config">
<services>
<ssh>
<cipher nc:operation="create"
yang:insert="after"
yang:value="3des-cbc">blowfish-cbc</cipher>
</ssh>
</services>
</system>
</config>
</edit-config>
</rpc>
7.8. The list Statement
The "list" statement is used to define interior nodes in the schema
tree. A list node may exist in multiple instances in the data tree.
Each such instance is known as a list entry. The "list" statement
takes one argument which is an identifier, followed by a block of
substatements that holds detailed list information.
A list entry is uniquely identified by the values of the list's keys.
A list is similar to a table where each list entry is a row in the
table.
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7.8.1. The list's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| anyxml | 7.10 | 0..n |
| augment | 7.15 | 0..n |
| choice | 7.9 | 0..n |
| config | 7.17.1 | 0..1 |
| container | 7.5 | 0..n |
| description | 7.17.3 | 0..1 |
| grouping | 7.11 | 0..n |
| key | 7.8.2 | 0..1 |
| leaf | 7.6 | 0..n |
| leaf-list | 7.7 | 0..n |
| list | 7.8 | 0..n |
| max-elements | 7.7.3 | 0..1 |
| min-elements | 7.7.2 | 0..1 |
| must | 7.5.2 | 0..n |
| ordered-by | 7.7.4 | 0..1 |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 0..1 |
| typedef | 7.3 | 0..n |
| unique | 7.8.3 | 0..n |
| uses | 7.12 | 0..n |
+--------------+---------+-------------+
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.
The combined values of all the leafs specified in the key are used to
uniquely identify a list entry. All key leafs MUST be given values
when a list entry is created. Thus, any default values in the key
leafs or their types are ignored. It also implies that any mandatory
statement in the key leafs are ignored.
A leaf that is part of the key can be of any built-in or derived
type, except it MUST NOT be the built-in type "empty".
All key leafs in a list MUST have the same value for their "config"
as the list itself.
The key string syntax is formally defined by the rule "key-arg" in
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Appendix D.
7.8.3. The lists's unique Statement
The "unique" statement is used to put constraints on valid
configurations. 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. Each such schema node identifier MUST refer
to a leaf.
In a valid configuration, the combined values of all the leaf
instances specified in the string MUST be unique within all list
entry instances.
The unique string syntax is formally defined by the rule "unique-arg"
in Appendix D.
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:
<server>
<name>smtp</name>
<ip>192.0.2.1</ip>
<port>25</port>
</server>
<server>
<name>http</name>
<ip>192.0.2.1</ip>
<port>25</port>
</server>
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7.8.4. The list's Child Node Statements
Within a list, the "container", "leaf", "list", "leaf-list", "uses",
and "choice" statements can be used to define child nodes to the
list.
7.8.5. XML Encoding Rules
A list is encoded as a series of XML elements, one for each entry in
the list. Each element's name is the list's identifier, and its XML
namespace is the module's XML namespace.
The list's key nodes are encoded as subelements to the list's
identifier element, in the same order as they are defined within the
key statement.
The rest of the list's child nodes are encoded as subelements to the
list element, after the keys, in the same order as they are defined
within the list statement.
7.8.6. NETCONF <edit-config> operations
List entries can be created, deleted, replaced and modified through
<edit-config>, by using the "operation" attribute in the list's XML
element. In each case, the values of all keys are used to uniquely
identify a list entry. If all keys are not specified for a list
entry, a "missing-element" error is returned.
In an "ordered-by user" list, the attributes "insert" and "key" in
the YANG namespace (Section 5.4.1) can be used to control where in
the list the entry is inserted. These can be used during "create"
operations to insert a new list entry, or during "merge" or "replace"
operations to insert a new list entry or move an existing one.
The "insert" attribute can take the values "first", "last", "before",
and "after". If the value is "before" or "after", the "key"
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 8.11) for the list entry.
If no "insert" attribute is present in the "create" operation, it
defaults to "last".
In a <copy-config>, or an <edit-config> with a "replace" operation
which covers the entire list, the list entry order is the same as the
order of the XML elements in the request.
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7.8.7. Usage Example
Given the following list:
list user {
key "name";
config true;
description "This is a list of users in the system.";
leaf name {
type string;
}
leaf type {
type string;
}
leaf full-name {
type string;
}
}
A corresponding XML encoding:
<user>
<name>fred</name>
<type>admin</type>
<full-name>Fred Flintstone</full-name>
</name>
To create a new user "barney":
<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
<edit-config>
<target>
<running/>
</target>
<config>
<system xmlns="http://example.com/schema/config">
<user nc:operation="create">
<name>barney</name>
<type>admin</type>
<full-name>Barney Rubble</full-name>
</user>
</system>
</config>
</edit-config>
</rpc>
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To change the type of "fred" to "superuser":
<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
<edit-config>
<target>
<running/>
</target>
<config>
<system xmlns="http://example.com/schema/config">
<user>
<name>fred</name>
<type>superuser</type>
</user>
</system>
</config>
</edit-config>
</rpc>
Given the following ordered-by user list:
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":
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<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0"
xmlns:yang="urn:ietf:params:xml:ns:yang:1">
<edit-config>
<target>
<running/>
</target>
<config>
<system xmlns="http://example.com/schema/config"
xmlns:ex="http://example.com/schema/config">
<user nc:operation="create"
yang:insert="after"
yang:key="[ex:name='fred']">
<name>barney</name>
<type>admin</type>
<full-name>Barney Rubble</full-name>
</user>
</system>
</config>
</edit-config>
</rpc>
The following would be used to move the user "barney" before the user
"fred":
<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0"
xmlns:yang="urn:ietf:params:xml:ns:yang:1">
<edit-config>
<target>
<running/>
</target>
<config>
<system xmlns="http://example.com/schema/config"
xmlns:ex="http://example.com/schema/config">
<user nc:operation="merge"
yang:insert="before"
yang:key="[ex:name='fred']">
<name>barney</name>
</user>
</system>
</config>
</edit-config>
</rpc>
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7.9. The choice Statement
The "choice" statement defines a set of alternatives, only one of
which may exist at any one time. The argument is an identifier,
followed by a block of substatements that holds detailed choice
information. The identifier is used to identify the choice node in
the schema tree. A choice node does not exist in the data tree.
A choice consists of a number of branches, defined with the case
substatement. Each branch contains a number of child nodes. The
"choice" statement puts a constraint on a valid configuration. In a
valid configuration, the nodes from at most one of the choice's
branches exist at the same time.
If a choice is marked with "mandatory true", at least one node from
one of the cases must be present in a valid configuration.
See Section 4.2.7 for additional information.
7.9.1. The choice's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| anyxml | 7.10 | 0..n |
| case | 7.9.2 | 0..n |
| container | 7.5 | 0..n |
| default | 7.9.3 | 0..1 |
| description | 7.17.3 | 0..1 |
| leaf | 7.6 | 0..n |
| leaf-list | 7.7 | 0..n |
| list | 7.8 | 0..n |
| mandatory | 7.9.4 | 0..1 |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 0..1 |
+--------------+---------+-------------+
7.9.2. The choice's case Statement
The "case" statement is used to define branches of the choice. It
takes as an argument an identifier, followed by a block of
substatements that holds detailed case information.
The identifier is used to identify the case node in the schema tree.
A case node does not exist in the data tree.
Within a "case" statement, the "anyxml", "container", "leaf", "list",
"leaf-list", "uses", and "augment" statements can be used to define
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child nodes to the case node. The identifiers of all these child
nodes must be unique within all cases in a choice. For example, the
following is illegal:
choice interface-type { // This example is illegal YANG
case a {
leaf ethernet { ... }
}
case b {
container ethernet { ...}
}
}
As a shorthand, the "case" statement can be omitted if the branch
contains a single "anyxml", "container", "leaf", "list", or "leaf-
list" statement. In this case, the identifier of the case node is
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.
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7.9.2.1. The case's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| anyxml | 7.10 | 0..n |
| augment | 7.15 | 0..n |
| container | 7.5 | 0..n |
| description | 7.17.3 | 0..1 |
| leaf | 7.6 | 0..n |
| leaf-list | 7.7 | 0..n |
| list | 7.8 | 0..n |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 0..1 |
| uses | 7.12 | 0..n |
+--------------+---------+-------------+
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 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 child nodes 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.
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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". If "mandatory" is "true", nodes from
exactly one of the choice's case branches MUST exist in a valid
configuration.
If not specified, the default is "false".
7.9.5. XML Encoding Rules
The choice and case nodes are not visible in XML.
7.9.6. NETCONF <edit-config> operations
Since only one of the choices cases can be valid at any time, the
creation of a node from one case implicitly deletes all nodes from
all other cases. If an <edit-config> operation creates a node, the
NETCONF server will delete any existing nodes that are defined in
other cases inside the choice.
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7.9.7. Usage Example
Given the following choice:
container protocol {
choice name {
case a {
leaf udp {
type empty;
}
}
case b {
leaf tcp {
type empty;
}
}
}
}
A corresponding XML encoding:
<protocol>
<tcp/>
</protocol>
To change the protocol from tcp to udp:
<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
<edit-config>
<target>
<running/>
</target>
<config>
<system xmlns="http://example.com/schema/config">
<protocol>
<udp nc:operation="create"/>
</protocol>
</system>
</config>
</edit-config>
</rpc>
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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.
This can be useful in e.g. RPC replies. An example is the <filter>
parameter in the <get-config> operation.
An anyxml node cannot be augmented.
It is NOT RECOMMENDED that the anyxml statement is used to represent
configuration data.
7.10.1. The anyxml's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| config | 7.17.1 | 0..1 |
| description | 7.17.3 | 0..1 |
| mandatory | 7.6.4 | 0..1 |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 0..1 |
+--------------+---------+-------------+
7.10.2. XML Encoding Rules
An anyxml node is encoded as an XML element. The element's name is
the anyxml's identifier, and its XML namespace is the module's XML
namespace. The value of the anyxml node is encoded as XML content of
this element.
Note that any prefixes used in the encoding are local to each
instance encoding. This means that the same XML may be encoded
differently by different implementations.
7.10.3. NETCONF <edit-config> operations
An anyxml node is treated as an opaque chunk of data. This data can
be modified in its entirety only.
Any "operation" attributes within the XML value of an anyxml node are
ignored by the NETCONF server.
When a NETCONF server processes an <edit-config> request, the
elements of procedure for the anyxml node are:
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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.
If the operation is "delete" the node is deleted if it exists.
7.10.4. Usage Example
Given the following anyxml statement:
anyxml data;
The following are two valid encodings of the same anyxml value:
<data xmlns:if="http://example.com/ns/interface">
<if:interface>
<if:ifIndex>1</if:ifIndex>
</if:interface>
</data>
<data>
<interface xmlns="http://example.com/ns/interface">
<ifIndex>1</ifIndex>
</interface>
</data>
7.11. The grouping Statement
The "grouping" statement is used to define a reusable block of nodes,
which may be used locally in the module, in modules which include it,
and by other modules which import from it. It takes one argument
which is an identifier, followed by a block of substatements that
holds detailed grouping information.
The grouping statement is not a data definition statement and, as
such, does not define any nodes in the schema tree.
A grouping is like a "structure" or a "record" in conventional
programming languages.
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Once a grouping is defined, it can be referenced in a "uses"
statement (see Section 7.12).
If the grouping is defined at the top level of a YANG module or
submodule, the grouping's identifier MUST be unique within the
module. For details about scoping for nested groupings, see
Section 5.8.
A grouping is more than just a mechanism for textual substitution,
but defines a collection of nodes. References from inside the
grouping are relative to the scope in which the grouping is defined,
not where it is used. Prefix mappings, type names, grouping names,
and extension usage are evaluated in the hierarchy where the grouping
statement appears. For extensions, this means that extensions are
applied to the grouping node, not the use node.
7.11.1. The grouping's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| anyxml | 7.10 | 0..n |
| augment | 7.15 | 0..n |
| choice | 7.9 | 0..n |
| container | 7.5 | 0..n |
| description | 7.17.3 | 0..1 |
| grouping | 7.11 | 0..n |
| leaf | 7.6 | 0..n |
| leaf-list | 7.7 | 0..n |
| list | 7.8 | 0..n |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 0..1 |
| typedef | 7.3 | 0..n |
| uses | 7.12 | 0..n |
+--------------+---------+-------------+
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7.11.2. Usage Example
import inet-types {
prefix "inet";
}
grouping address {
description "A reusable address group.";
leaf ip {
type inet:ip-address;
}
leaf port {
type inet:port-number;
}
}
7.12. The uses Statement
The "uses" statement is used to reference a "grouping" definition.
It takes one argument, which is the name of the grouping.
The effect of a "uses" reference to a grouping is that the nodes
defined by the grouping are copied into the current schema tree, and
then updated according to the refinement statements. Thus, the
identifiers defined in the grouping are copied into the current
module's namespace, even if the grouping is imported from some other
module.
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7.12.1. The uses's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| anyxml | 7.10 | 0..n |
| choice | 7.9 | 0..n |
| container | 7.5 | 0..n |
| description | 7.17.3 | 0..1 |
| leaf | 7.6 | 0..n |
| leaf-list | 7.7 | 0..n |
| list | 7.8 | 0..n |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 0..1 |
| uses | 7.12 | 0..n |
+--------------+---------+-------------+
7.12.2. The uses's Refinement Statements
Some of the properties of each node in the grouping can be refined in
substatements to "uses". If a node is not present in a substatement,
it is not refined, and thus used exactly as it was defined in the
"grouping". The refinement substatements MUST be specified in the
same order as in the grouping, and a node can be refined only once.
The following refinements can be done:
o A leaf or choice node may get a default value, or a new default
value if it already had one.
o Any node may get a specialized "description" string.
o Any node may get a specialized "reference" string.
o Any node may get a different "config" statement.
o A leaf or choice node may get a different "mandatory" statement.
o A container node may get a "presence" statement.
o A leaf, leaf-list, list or container node may get additional
"must" expressions.
o A leaf-list or list node may get a different "min-elements" or
"max-elements" statement.
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7.12.3. XML Encoding Rules
Each node in the grouping is encoded as if it was defined inline,
even if it is imported from another module with another XML
namespace.
7.12.4. Usage Example
To use the "address" grouping defined in Section 7.11.2 in a
definition of an HTTP server in some other module, we can do:
import acme-system {
prefix acme;
}
container http-server {
leaf name {
type string;
}
uses acme:address;
}
A corresponding XML encoding:
<http-server>
<name>extern-web</name>
<ip>192.0.2.1</ip>
<port>80</port>
</http-server>
If port 80 should be the default for the HTTP server, default can be
added:
container http-server {
leaf name {
type string;
}
uses acme:address {
leaf port {
default 80;
}
}
}
If we want to define a list of servers, and each server has the ip
and port as keys, we can do:
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list server {
key "ip port";
leaf name {
type string;
}
uses acme:address;
}
The following is an error:
container http-server {
uses acme:address;
leaf ip { // illegal - same identifier "ip" used twice
type string;
}
}
7.13. The rpc Statement
The "rpc" statement is used to define a NETCONF RPC method. It takes
one argument, which is an identifier, followed by a block of
substatements that holds detailed rpc information. This argument is
the name of the RPC, and is used as the element name directly under
the <rpc> element, as designated by the substitution group
"rpcOperation" in [RFC4741].
The rpc "statement" defines an rpc node in the schema tree.
7.13.1. The rpc's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| description | 7.17.3 | 0..1 |
| grouping | 7.11 | 0..n |
| input | 7.13.2 | 0..1 |
| output | 7.13.3 | 0..1 |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 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
"input" statement defines an input node in the schema tree, with the
name "input".
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If a container in the input tree has a "presence" statement, the
container need not be present in a NETCONF RPC invocation.
If a leaf in the input tree has a "mandatory" statement with the
value "true", the leaf MUST be present in a NETCONF RPC invocation.
If a leaf in the input tree has a default value, the NETCONF server
MUST internally use this default if the leaf is not present in a
NETCONF RPC invocation.
If a "config" or "must" statement is present for any node in the
input tree, it is ignored.
7.13.2.1. The input's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| anyxml | 7.10 | 0..n |
| augment | 7.15 | 0..n |
| choice | 7.9 | 0..n |
| container | 7.5 | 0..n |
| grouping | 7.11 | 0..n |
| leaf | 7.6 | 0..n |
| leaf-list | 7.7 | 0..n |
| list | 7.8 | 0..n |
| typedef | 7.3 | 0..n |
| uses | 7.12 | 0..n |
+--------------+---------+-------------+
7.13.3. The output Statement
The "output" statement, which is optional, is used to define output
parameters to the RPC method. It does not take an argument. The
"output" statement defines an output node in the schema tree, with
the name "output".
If a container in the output tree has a "presence" statement, the
container need not be present in a NETCONF RPC reply
If a leaf in the output tree has a "mandatory" statement with the
value "true", the leaf MUST be present in a NETCONF RPC reply.
If a leaf in the output tree has a default value, the NETCONF client
MUST internally use this default if the leaf is not present in a
NETCONF RPC reply.
If a "config" or "must" statement is present for any node in the
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output tree, it is ignored.
7.13.3.1. The output's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| anyxml | 7.10 | 0..n |
| augment | 7.15 | 0..n |
| choice | 7.9 | 0..n |
| container | 7.5 | 0..n |
| grouping | 7.11 | 0..n |
| leaf | 7.6 | 0..n |
| leaf-list | 7.7 | 0..n |
| list | 7.8 | 0..n |
| typedef | 7.3 | 0..n |
| uses | 7.12 | 0..n |
+--------------+---------+-------------+
7.14. The notification Statement
The "notification" statement is used to define a NETCONF
notification. It takes one argument, which is an identifier,
followed by a block of substatements that holds detailed notification
information. The notification "statement" defines a notification
node in the schema tree.
If a container in the notification tree has a "presence" statement,
the container need not be present in a NETCONF notification.
If a leaf in the notification tree has a "mandatory" statement with
the value "true", the leaf MUST be present in a NETCONF notification.
If a leaf in the notification tree has a default value, the NETCONF
server MUST internally use this default if the leaf is not present in
a NETCONF notification.
If a "config" or "must" statement is present for any node in the
notification tree, it is ignored.
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7.14.1. The notification's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| anyxml | 7.10 | 0..n |
| augment | 7.15 | 0..n |
| choice | 7.9 | 0..n |
| container | 7.5 | 0..n |
| description | 7.17.3 | 0..1 |
| grouping | 7.11 | 0..n |
| leaf | 7.6 | 0..n |
| leaf-list | 7.7 | 0..n |
| list | 7.8 | 0..n |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 0..1 |
| typedef | 7.3 | 0..n |
| uses | 7.12 | 0..n |
+--------------+---------+-------------+
7.15. The augment Statement
The "augment" statement allows a module or submodule to add to the
schema tree defined in another module or submodule. The argument is
a string which identifies a node in the schema tree. This node is
called the augment's target node. The target node MUST be either a
container, list, choice, case, rpc, input, output, or notification
node. It is augmented with the nodes defined in the substatements
that follow the "augment" statement.
The augment string is a schema node identifier. The syntax is
formally defined by the rule "augment-arg" in Appendix D. If the
"augment" statement is on the top-level in a module or submodule, the
absolute form of a schema node identifier MAY be used. Otherwise,
the descendant form MUST be used.
The syntax for a schema node identifier is a subset of the XPath
syntax. It is an absolute or relative XPath location path in
abbreviated syntax, where axes and predicates are not permitted.
If the target node is a container, list, case, input, output, or
notification node, the "container", "leaf", "list", "leaf-list",
"uses", and "choice" statements can be used within the "augment"
statement.
If the target node is a choice node, the "case" statement can be used
within the "augment" statement.
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If the target node is rpc node, the "input" and "output" statements
can be used within the "augment" statement.
7.15.1. The augment's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| anyxml | 7.10 | 0..n |
| augment | 7.15 | 0..n |
| case | 7.9.2 | 0..n |
| choice | 7.9 | 0..n |
| container | 7.5 | 0..n |
| description | 7.17.3 | 0..1 |
| leaf | 7.6 | 0..n |
| leaf-list | 7.7 | 0..n |
| list | 7.8 | 0..n |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 0..1 |
| uses | 7.12 | 0..n |
| when | 7.15.2 | 0..1 |
+--------------+---------+-------------+
7.15.2. The when Statement
The "when" statement allows the augmentation to be conditional, with
the nodes only being valid when a specific criteria is satisfied.
The statement's argument is an XPath expression, which is used to
formally specify constraints on which instances in the data tree will
be augmented by this statement. If the XPath expression conceptually
evaluates to "true" for a particular instance, then it is augmented,
otherwise it is not.
The XPath expression is conceptually evaluated in the following
context:
o 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 The set of namespace declarations is the set of all "import"
statements' prefix and namespace pairs, and the "prefix"
statement's prefix for the "namespace" statement's URI.
o The null namespace is defined to be the namespace of the current
module.
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o One variable "this", which is the context node, is defined.
The result of the XPath expression is converted to a boolean value
using the standard XPath rules.
Note that the XPath expression is conceptually evaluated. This means
that an implementation does not have to use an XPath evaluator on the
device. The augment can very well be implemented with specially
written code.
7.15.3. XML Encoding Rules
All data nodes defined in the "augment" statement are defined as XML
elements in the XML namespace of the module where the "augment" is
specified.
When a node is augmented, the augmented child nodes are encoded after
all normal child nodes. If the node is augmented more than once, the
blocks of augmented child nodes are sorted (in alphanumeric order)
according to their namespace URI and name of the first child node in
each block.
7.15.4. Usage Example
In namespace http://example.com/schema/interfaces, we have:
container interfaces {
list ifEntry {
key "ifIndex";
leaf ifIndex {
type uint32;
}
leaf ifDescr {
type string;
}
leaf ifType {
type iana:IfType;
}
leaf ifMtu {
type int32;
}
}
}
Then in namespace http://example.com/schema/ds0, we have:
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import interface-module {
prefix if;
}
augment "/if:interfaces/if:ifEntry" {
when "if:ifType='ds0'";
leaf ds0ChannelNumber {
type ChannelNumber;
}
}
A corresponding XML encoding:
<interfaces xmlns="http://example.com/schema/interfaces"
xmlns:ds0="http://example.com/schema/ds0"
<ifEntry>
<ifIndex>1</ifIndex>
<ifDescr>Flintstone Inc Ethernet A562</ifDescr>
<ifType>ethernetCsmacd</ifType>
<ifMtu>1500</ifMtu>
</ifEntry>
<ifEntry>
<ifIndex>2</ifIndex>
<ifDescr>Flintstone Inc DS0</ifDescr>
<ifType>ds0</ifType>
<ds0:ds0ChannelNumber>1</ds0:ds0ChannelNumber>
</ifEntry>
</interfaces>
As another example, suppose we have the choice defined in
Section 7.9.7. The following construct can be used to extend the
protocol definition:
augment /ex:system/ex:protocol/ex:name {
case c {
leaf smtp {
type empty;
}
}
}
A corresponding XML encoding:
<ex:system>
<ex:protocol>
<ex:tcp/>
</ex:protocol>
</ex:system>
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or
<ex:system>
<ex:protocol>
<other:smtp/>
</ex:protocol>
</ex:system>
7.16. 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 by 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
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 Appendix D.
7.16.1. The extension's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| argument | 7.16.2 | 0..1 |
| description | 7.17.3 | 0..1 |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 0..1 |
+--------------+---------+-------------+
7.16.2. 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
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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.
7.16.2.1. The argument's Substatements
+--------------+----------+-------------+
| substatement | section | cardinality |
+--------------+----------+-------------+
| yin-element | 7.16.2.2 | 0..1 |
+--------------+----------+-------------+
7.16.2.2. The yin-element Statement
The "yin-element" statement, which is optional, takes as an argument
the string "true" or "false". This statement indicates if the
argument should be mapped to an XML element in YIN or to an XML
attribute. (see Appendix B).
If no "yin-element" statement is present, it defaults to "false".
7.16.3. 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:
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module my-interfaces {
...
import my-extensions {
prefix "myext";
}
...
container interfaces {
...
myext:c-define "MY_INTERFACES";
}
}
7.17. Common Statements
This section defines sub-statements common to several other
statements.
7.17.1. The config Statement
The "config" statement takes as an argument the string "true" or
"false". If "config" is "true", the definition represents
configuration, and will be part of the reply to a <get-config>
request, and may be sent in a <copy-config> or <edit-config> request.
If "config" is "false", it represents state data, and will be part of
the reply to a <get>, but not to a <get-config> request.
If "config" is not specified, the default is the same as the parent
node's (in the data model) "config" value. If the top node does not
specify a "config" statement, the default is "true".
If a node has "config" "false", no node underneath it can have
"config" set to "true".
7.17.2. The status Statement
The "status" statement takes as an argument one of the strings
"current", "deprecated", or "obsolete".
o "current" means that the definition is current and valid.
o "deprecated" indicates an obsolete definition, but it permits new/
continued implementation in order to foster interoperability with
older/existing implementations.
o "obsolete" means the definition is obsolete and should not be
implemented and/or can be removed if previously implemented.
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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.
7.17.3. The description Statement
The "description" statement takes as an argument a string which
contains a high-level textual description of this definition.
7.17.4. 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.
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8. 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. An initial set of
commonly used derived types is defined in the YANG standard modules
"yang-types" (Appendix A.1), "inet-types" (Appendix A.2), and "ieee-
types" (Appendix A.3).
The different built-in types and their derived types allow different
kinds of subtyping, namely length and regular expression restrictions
of strings (Section 8.3.3, Section 8.3.4) and range restrictions of
numeric types (Section 8.1.3).
The lexicographic representation of a value of a certain type is used
in the XML encoding over NETCONF, and when specifying default values
in a YANG module.
8.1. The Integer Built-in Types
The integer built-in types are int8, int16, int32, int64, uint8,
uint16, uint32, and uint64. They represent signed and unsigned
integers of different sizes:
int8 represents integer values between -128 and 127, inclusively.
int16 represents integer values between -32768 and 32767,
inclusively.
int32 represents integer values between -2147483648 and 2147483647,
inclusively.
int64 represents integer values between -9223372036854775808 and
9223372036854775807, inclusively.
uint8 represents integer values between 0 and 255, inclusively.
uint16 represents integer values between 0 and 65535, inclusively.
uint32 represents integer values between 0 and 4294967295,
inclusively.
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uint64 represents integer values between 0 and 18446744073709551615,
inclusively.
8.1.1. Lexicographic Representation
An integer value is lexicographically represented as an optional sign
("+" or "-"), followed by a sequence of decimal digits. If no sign
is specified, "+" is assumed.
For convenience, when specifying a default value for an integer in a
YANG module, an alternative lexicographic representation can be used,
which represents the value in a hexadecimal or octal notation. The
hexadecimal notation consists of an optional sign ("+" or "-"), the
characters "0x" followed a number of hexadecimal digits, where
letters may be upper- or lowercase. The octal notation consists of
an optional sign ("+" or "-"), the character "0" followed a number of
octal digits.
Examples:
// legal values
+4711 // legal positive value
4711 // legal positive value
-123 // legal negative value
0xf00f // legal positive hexadecimal value
-0xf // legal negative hexadecimal value
052 // legal positive octal value
// illegal values
- 1 // illegal intermediate space
8.1.2. Restrictions
All integer types can be restricted with the "range" statement
(Section 8.1.3).
8.1.3. The range Statement
The "range" statement, which is an optional substatement to the
"type" statement, takes as an argument a range expression string. It
is used to restrict integer and floating point built-in types, or
types derived from those.
A range consists of an explicit value, or a lower inclusive bound,
two consecutive dots "..", and an upper inclusive bound. Multiple
values or ranges can be given, separated by "|". If multiple values
or ranges are given they all MUST be disjoint and MUST be in
ascending order. If a value restriction is applied to an already
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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 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-
expr" in Appendix D.
8.1.3.1. The range's Substatements
+---------------+---------+-------------+
| substatement | section | cardinality |
+---------------+---------+-------------+
| description | 7.17.3 | 0..1 |
| error-app-tag | 7.5.3.2 | 0..1 |
| error-message | 7.5.3.1 | 0..1 |
| reference | 7.17.4 | 0..1 |
+---------------+---------+-------------+
8.1.4. Usage Example
type int32 {
range "1..4 | 10 | 20..max";
}
8.2. The Floating Point Built-in Types
The floating point built-in types are float32 and float64. They
represent floating point values of single and double precision as
defined in [IEEE.754]. Special values are positive and negative
infinity, and not-a-number.
8.2.1. Lexicographic Representation
A floating point value is lexicographically represented as consisting
of a decimal mantissa followed, optionally, by the character "E" or
"e", followed by an integer exponent. The special values positive
and negative infinity and not-a-number have lexical representations
INF, -INF and NaN, respectively. The minimal value accepted for a
float is -INF, and the maximal value accepted for a float is INF.
8.2.2. Restrictions
All floating point types can be restricted with the "range" statement
(Section 8.1.3).
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8.2.3. Usage Example
type float32 {
range "1..4.5 | 10 | 20..INF";
}
is equivalent to
type float32 {
range "1..4.5 | 10 | 20..max";
}
8.3. 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
8.3.1. Lexicographic Representation
A string value is lexicographically represented as character data in
the XML encoding.
8.3.2. Restrictions
A string can be restricted with the "length" (Section 8.3.3) and
"pattern" (Section 8.3.4) statements.
8.3.3. 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
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be disjoint and MUST be in ascending order. If a length restriction
is applied to an already length restricted type, the new restriction
MUST be equal or more limiting, that is, raising the lower bounds,
reducing the upper bounds, removing explicit length values or ranges,
or splitting ranges into multiple ranges with intermediate gaps. A
length value is a non-negative integer, or one of the special values
"min" or "max". "min" and "max" means the minimum and maximum length
accepted for the type being restricted, respectively. An
implementation is not required to support a length value larger than
18446744073709551615.
The length expression syntax is formally defined by the rule "length-
expr" in Appendix D.
8.3.3.1. The length's Substatements
+---------------+---------+-------------+
| substatement | section | cardinality |
+---------------+---------+-------------+
| description | 7.17.3 | 0..1 |
| error-app-tag | 7.5.3.2 | 0..1 |
| error-message | 7.5.3.1 | 0..1 |
| reference | 7.17.4 | 0..1 |
+---------------+---------+-------------+
8.3.4. 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.
8.3.4.1. The pattern's Substatements
+---------------+---------+-------------+
| substatement | section | cardinality |
+---------------+---------+-------------+
| description | 7.17.3 | 0..1 |
| error-app-tag | 7.5.3.2 | 0..1 |
| error-message | 7.5.3.1 | 0..1 |
| reference | 7.17.4 | 0..1 |
+---------------+---------+-------------+
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8.3.5. Usage Example
With the following type:
type string {
length "0..4";
pattern "[0-9a-fA-F]*";
}
the following strings match:
AB // legal
9A00 // legal
and the following strings do not match:
00ABAB // illegal
xx00 // illegal
8.4. The boolean Built-in Type
The boolean built-in type represents a boolean value.
8.4.1. Lexicographic Representation
The lexicographical representation of a boolean value is the strings
"true" and "false".
8.4.2. Restrictions
A boolean cannot be restricted.
8.5. The enumeration Built-in Type
The enumeration built-in type represents values from a set of
assigned names.
8.5.1. Lexicographic Representation
The lexicographical representation of an enumeration value is the
assigned name string.
8.5.2. Restrictions
An enumeration cannot be restricted.
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8.5.3. 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. It is
optionally followed by a block of substatements which holds detailed
enum information.
All assigned names in an enumeration MUST be unique.
8.5.3.1. The enum's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| description | 7.17.3 | 0..1 |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 0..1 |
| value | 8.5.3.2 | 0..1 |
+--------------+---------+-------------+
8.5.3.2. The value Statement
The "value" statement, which is optional, is used to associate an
integer value with the assigned name for the enum. This integer
value MUST be in the range -2147483648 to 2147483647, and it MUST be
unique within the enumeration type.
If a value is not specified, then one will be automatically assigned.
If the enum sub-statement is the first one defined, the assigned
value is zero (0), otherwise the assigned value is one greater than
the current highest enum value.
If the current highest value is equal to 2147483647, then an enum
value MUST be specified for enum sub-statements following the one
with the current highest value.
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8.5.4. Usage Example
type enumeration {
enum enabled {
value 1;
}
enum disabled {
value 2;
}
}
type enumeration {
enum zero;
enum one;
enum seven {
value 7;
}
}
8.6. 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.
8.6.1. Restrictions
A bits type cannot be restricted.
8.6.2. 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.
8.6.3. The bit Statement
The "bit" statement, which is a substatement to the "type" statement,
MUST be present if the type is "bits". It is repeatedly used to
specify each assigned named bit of a bits type. It takes as an
argument a string which is the assigned name of the bit. It is
followed by a block of substatements which holds detailed bit
information. A bit name follows the same syntax rules as an
identifier (see Section 6.2).
All bit names in a bits type MUST be unique.
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8.6.3.1. The bit's Substatements
+--------------+---------+-------------+
| substatement | section | cardinality |
+--------------+---------+-------------+
| description | 7.17.3 | 0..1 |
| reference | 7.17.4 | 0..1 |
| status | 7.17.2 | 0..1 |
| position | 8.6.3.2 | 0..1 |
+--------------+---------+-------------+
8.6.3.2. The position Statement
The "position" statement, which is optional, takes as an argument a
non-negative integer value which specifies the bit's position within
a hypothetical bit field. The position value MUST be in the range 0
to 4294967295, and it MUST be unique within the bits type. The value
is unused by YANG and the XML encoding, but is carried as a
convenience to implementors.
If a bit position is not specified, then one will be automatically
assigned. If the bit sub-statement is the first one defined, the
assigned value is zero (0), otherwise the assigned value is one
greater than the current highest bit position.
If the current highest bit position value is equal to 4294967295,
then a position value MUST be specified for bit sub-statements
following the one with the current highest position value.
8.6.4. 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";
}
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The lexicographic representation of this leaf with bit values
disable-nagle and 10-Mb-only set would be:
<mybits>disable-nagle 10-Mb-only</mybits>
8.7. The binary Built-in Type
The binary built-in type represents any binary data, i.e. a sequence
of octets.
8.7.1. Restrictions
A binary can be restricted with the "length" (Section 8.3.3)
statement. The length of a binary value is the number of octets it
contains.
8.7.2. Lexicographic Representation
Binary values are encoded with the base64 encoding scheme [RFC4648].
8.8. The keyref Built-in Type
The keyref type is used to reference a particular list entry in the
data tree. Its value is constrained to be the same as the key of an
existing list entry.
If the leaf with the keyref type represents configuration, the list
entry it refers to MUST also represent configuration. Such a leaf
puts a constraint on a valid configuration. In a valid
configuration, all keyref nodes MUST reference existing list entries.
8.8.1. Restrictions
A keyref cannot be restricted.
8.8.2. The path Statement
The "path" statement, which is a substatement to the "type"
statement, MUST be present if the type is "keyref". It takes as an
argument a string which MUST refer to one key node of a list entry.
The syntax for a path argument is a subset of the XPath syntax. It
is an absolute or relative XPath location path in abbreviated syntax,
where axes are not permitted, and predicates are used only for
constraining the values for the key nodes for list entries. Each
predicate consists of at most one equality test per key.
The predicates are only used when more than one key reference is
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needed to uniquely identify a list entry. This occurs if the list
has multiple keys, or a reference to a list within a list is needed.
In these cases, multiple keyref leafs are typically specified, and
predicates are used to tie them together.
The syntax is formally defined by the rule "path-arg" in Appendix D.
8.8.3. Lexicographic Representation
A keyref value is encoded the same way as the key it references.
8.8.4. Usage Example
With the following list:
list interface {
key "name";
leaf name {
type string;
}
list address {
key "ip";
leaf ip {
type yang:ip-address;
}
}
}
The following keyref refers to an existing interface:
leaf mgmt-interface {
type keyref {
path "../interface/name";
}
}
A corresponding XML snippet is e.g.:
<interface>
<name>eth0</name>
</interface>
<interface>
<name>lo</name>
</interface>
<mgmt-interface>eth0</mgmt-interface>
The following keyrefs refer to an existing address of an interface:
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container default-address {
leaf ifname {
type keyref {
path "../../interface/name";
}
}
leaf address {
type keyref {
path "../../interface[name = $this/../ifname]/address/ip";
}
}
}
A corresponding XML snippet is e.g.:
<interface>
<name>eth0</name>
<address>
<ip>192.0.2.1</ip>
</address>
<address>
<ip>192.0.2.2</ip>
</address>
</interface>
<interface>
<name>lo</name>
<address>
<ip>127.0.0.1</ip>
</address>
</interface>
<default-address>
<ifname>eth0</ifname>
<address>192.0.2.2</address>
</default-address>
8.9. 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.
8.9.1. Restrictions
An empty type cannot be restricted.
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8.9.2. Lexicographic Representation
Not applicable.
8.9.3. Usage Example
The following leaf
leaf enable-qos {
type empty;
}
will be encoded as
<enable-qos/>
if it exists.
8.10. The union Built-in Type
The union built-in type represents a value that corresponds to one of
its member types.
When the type is "union", the "type" statement (Section 7.4) MUST be
present. It is used to repeatedly specify each member type of the
union. It takes as an argument a string which is the name of a
member type.
A member type can be of any built-in or derived type, except it MUST
NOT be one of the built-in types "empty" or "keyref".
Example:
type union {
type int32;
type enumeration {
enum "unbounded";
}
}
8.10.1. Restrictions
A union can not be restricted. However, each member type can be
restricted, based on the rules defined in Section 8 chapter.
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8.10.2. Lexicographic Representation
The lexicographical representation of an union is a value that
corresponds to the representation of any one of the member types.
8.11. The instance-identifier Built-in Type
The instance-identifier built-in type is used to uniquely identify a
particular instance node in the data tree.
The syntax for an instance-identifier is a subset of the XPath
syntax, which is used to uniquely identify a node in the data tree.
It is an absolute XPath location path in abbreviated syntax, where
axes are not permitted, and predicates are used only for specifying
the values for the key nodes for list entries, or a value of a leaf-
list. Each predicate consists of one equality test per key. Each
key MUST have a corresponding predicate.
The syntax is formally defined by the rule "absolute-instid" in
Appendix D.
8.11.1. Restrictions
An instance-identifier cannot be restricted.
8.11.2. Lexicographic Representation
An instance-identifier value is lexicographically represented as a
string in the XML encoding. The namespace prefixes used in the
encoding MUST be declared in the XML namespace scope in the instance-
idenfitier's XML element.
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.
8.11.3. Usage Example
The following are examples of instance identifiers:
/ex:system/ex:services/ex:ssh/ex:port
/ex:system/ex:user[ex:name='fred']
/ex:system/ex:user[ex:name='fred']/ex:type
/ex:system/ex:server[ex:ip='192.0.2.1'][ex:port='80']
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/ex:system/ex:services/ex:ssh/ex:cipher[.='blowfish-cbc']
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9. Updating a Module
[Editor's Note: add versioning rules, i.e. what can be done w/o
changing the module name and the namespace]
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10. IANA Considerations
A registry for standard YANG modules shall be set up. Each entry
shall contain the unique module name, the unique XML namespace from
the YANG URI Scheme and some reference to the module's documentation.
This document registers five URIs for the YANG XML namespace in the
IETF XML registry [RFC3688].
URI: urn:ietf:params:xml:ns:yang:ieee-types
URI: urn:ietf:params:xml:ns:yang:inet-types
URI: urn:ietf:params:xml:ns:yang:yang-types
URI: urn:ietf:params:xml:ns:yang:yin:1
URI: urn:ietf:params:xml:ns:yang:1
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11. Security Considerations
This document defines a language with which to write and read
descriptions of management information. The language itself has no
security impact on the Internet.
Data modeled in YANG might contain sensitive information. RPCs or
notifications defined in YANG might transfer sensitive information.
Security issues are related to the usage of data modeled in YANG.
Such issues shall be dealt with in documents describing the data
models and documents about the interfaces used to manipulate the data
e.g. the NETCONF documents.
YANG is dependent upon:
o the security of the transmission infrastructure used to send
sensitive information
o the security of applications which store or release such sensitive
information.
o adequate authentication access control mechanisms to restrict the
usage of sensitive data.
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12. Contributors
The following people all contributed significantly to the initial
YANG draft:
- Andy Bierman (andybierman.com)
- Balazs Lengyel (Ericsson)
- David Partain (Ericsson)
- Juergen Schoenwaelder (Jacobs University Bremen)
- Phil Shafer (Juniper Networks)
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13. References
13.1. Normative References
[IEEE.754]
Institute of Electrical and Electronics Engineers,
"Standard for Binary Floating-Point Arithmetic",
IEEE Standard 754, August 1985.
[ISO.10646]
International Organization for Standardization,
"Information Technology - Universal Multiple-octet coded
Character Set (UCS) - Part 1: Architecture and Basic
Multilingual Plane", ISO Standard 10646-1, May 1993.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC4741] Enns, R., "NETCONF Configuration Protocol", RFC 4741,
December 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[XSD] Maloney, M., Beech, D., Thompson, H., and N. Mendelsohn,
"XML Schema Part 1: Structures Second Edition", W3C
REC REC-xmlschema-1-20041021, October 2004.
[XSD-TYPES]
Biron, P V. and A. Malhotra, "XML Schema Part 2: Datatypes
Second Edition", W3C REC REC-xmlschema-2-20041028,
October 2004.
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13.2. Non-Normative References
[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
[RFC2579] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Textual Conventions for SMIv2",
STD 58, RFC 2579, April 1999.
[RFC3780] Strauss, F. and J. Schoenwaelder, "SMIng - Next Generation
Structure of Management Information", RFC 3780, May 2004.
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Appendix A. Derived YANG Types
Most YANG modules are built on top of the definitions of some
commonly used derived types. The definitions of these derived types
are contained in the "yang-types", "inet-types", and "ieee-types"
modules which are defined below. Their derived types are generally
applicable for modeling all areas of management information.
A.1. Core YANG Derived Types
module yang-types {
// XXX namespace to be allocated by IANA
namespace "urn:ietf:params:xml:ns:yang:yang-types";
prefix "yang";
organization
"YANG Language Design Team";
contact
"Martin Bjorklund (Editor) <mbj@tail-f.com>";
description
"This module contains standard derived YANG types.";
revision 2007-10-02 {
description "Initial revision.";
}
/*
* collection of counter and gauge types
*/
typedef counter32 {
type uint32;
description
"The counter32 type represents a non-negative integer
which monotonically increases until it reaches a
maximum value of 2^32-1 (4294967295 decimal), when it
wraps around and starts increasing again from zero.
Counters have no defined `initial' value, and thus, a
single value of a counter has (in general) no information
content. Discontinuities in the monotonically increasing
value normally occur at re-initialization of the
management system, and at other times as specified in the
description of an object instance using this type. If
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such other times can occur, for example, the creation of
an object instance of type counter32 at times other than
re-initialization, then a corresponding object should be
defined, with an appropriate type, to indicate the last
discontinuity.
The counter32 type should not be used for configuration
objects. A default statement should not be used for
attributes with a type value of counter32.";
reference
"RFC 2578 (STD 58)";
}
typedef zero-based-counter32 {
type yang:counter32;
default "0";
description
"The zero-based-counter32 type represents a counter32
which has the defined `initial' value zero.";
reference
"RFC 2021";
}
typedef counter64 {
type uint64;
description
"The counter64 type represents a non-negative integer
which monotonically increases until it reaches a
maximum value of 2^64-1 (18446744073709551615), when
it wraps around and starts increasing again from zero.
Counters have no defined `initial' value, and thus, a
single value of a counter has (in general) no information
content. Discontinuities in the monotonically increasing
value normally occur at re-initialization of the
management system, and at other times as specified in the
description of an object instance using this type. If
such other times can occur, for example, the creation of
an object instance of type counter64 at times other than
re-initialization, then a corresponding object should be
defined, with an appropriate type, to indicate the last
discontinuity.
The counter64 type should not be used for configuration
objects. A default statement should not be used for
attributes with a type value of counter64.";
reference
"RFC 2578 (STD 58)";
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}
typedef zero-based-counter64 {
type yang:counter64;
default "0";
description
"The zero-based-counter64 type represents a counter64
which has the defined `initial' value zero.";
reference
"RFC 2856";
}
typedef gauge32 {
type uint32;
description
"The gauge32 type represents a non-negative integer,
which may increase or decrease, but shall never
exceed a maximum value, nor fall below a minimum
value. The maximum value can not be greater than
2^32-1 (4294967295 decimal), and the minimum value
can not be smaller than 0. The value of a gauge32
has its maximum value whenever the information
being modeled is greater than or equal to its
maximum value, and has its minimum value whenever
the information being modeled is smaller than or
equal to its minimum value. If the information
being modeled subsequently decreases below
(increases above) the maximum (minimum) value, the
gauge32 also decreases (increases).";
reference
"RFC 2578 (STD 58)";
}
typedef gauge64 {
type uint64;
description
"The gauge64 type represents a non-negative integer,
which may increase or decrease, but shall never
exceed a maximum value, nor fall below a minimum
value. The maximum value can not be greater than
2^64-1 (18446744073709551615), and the minimum value
can not be smaller than 0. The value of a gauge64
has its maximum value whenever the information
being modeled is greater than or equal to its
maximum value, and has its minimum value whenever
the information being modeled is smaller than or
equal to its minimum value. If the information
being modeled subsequently decreases below
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(increases above) the maximum (minimum) value, the
gauge64 also decreases (increases).";
reference
"RFC 2856";
}
/*
* collection of identifier related types
*/
typedef uri {
type string;
description
"A uri type represents Uniform Resource Identifier (URI)
as defined by STD 66.
Objects using this type MUST be in US-ASCII encoding, and
MUST be normalized as described by RFC 3986 Sections
6.2.1, 6.2.2.1, and 6.2.2.2. All unnecessary
percent-encoding is removed, and all case-insensitive
characters are set to lowercase except for hexadecimal
digits, which are normalized to uppercase as described in
Section 6.2.2.1.
The purpose of this normalization is to help provide unique
URIs. Note that this normalization is not sufficient to
provide uniqueness. Two URIs that are textually distinct
after this normalization may still be equivalent.
Objects using this type MAY restrict the schemes that they
permit. For example, 'data:' and 'urn:' schemes might not
be appropriate.
A zero-length URI is not a valid URI. This can be used to
express 'URI absent' where required, for example when used
as an index field.";
reference
"RFC 3986 (STD 66), RFC 3305, and RFC 5017";
}
typedef object-identifier {
type string {
pattern '[0-2](\.\d+)+';
}
description
"The object-identifier type represents administratively
assigned names in a registration-hierarchical-name tree.
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Values of this type are denoted as a sequence of numerical
non-negative sub-identifier values. Each sub-identifier
value MUST NOT exceed 2^32-1 (4294967295). Sub-identifiers
are separated by single dots and without any intermediate
white space.
Although the number of sub-identifiers is not limited,
module designers should realize that there may be
implementations that stick with the SMIv1/v2 limit of 128
sub-identifiers.";
reference
"ITU-T Recommendation X.660 / ISO/IEC 9834-1";
}
/*
* collection of date and time related types
*/
typedef date-and-time {
type string {
pattern '\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(\.d*)?'
+ '(Z|(\+|-)\d{2}:\d{2})';
}
description
'The date-and-time type is a profile of the ISO 8601
standard for representation of dates and times using the
Gregorian calendar. The format is most easily described
using the following ABFN (see RFC 3339):
date-fullyear = 4DIGIT
date-month = 2DIGIT ; 01-12
date-mday = 2DIGIT ; 01-28, 01-29, 01-30, 01-31
time-hour = 2DIGIT ; 00-23
time-minute = 2DIGIT ; 00-59
time-second = 2DIGIT ; 00-58, 00-59, 00-60
time-secfrac = "." 1*DIGIT
time-numoffset = ("+" / "-") time-hour ":" time-minute
time-offset = "Z" / time-numoffset
partial-time = time-hour ":" time-minute ":" time-second
[time-secfrac]
full-date = date-fullyear "-" date-month "-" date-mday
full-time = partial-time time-offset
date-time = full-date "T" full-time';
reference "RFC 3339";
}
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typedef timeticks {
type uint32;
description
"The timeticks type represents a non-negative integer
which represents the time, modulo 2^32 (4294967296
decimal), in hundredths of a second between two epochs.
When objects are defined which use this type, the
description of the object identifies both of the reference
epochs.";
reference
"RFC 2578 (STD 58)";
}
typedef timestamp {
type yang:timeticks;
description
"The timestamp type represents the value of an associated
timeticks object at which a specific occurrence
happened. The specific occurrence must be defined in the
description of any object defined using this type. When
the specific occurrence occurred prior to the last time
the associated timeticks attribute was zero, then the
timestamp value is zero. Note that this requires all
timestamp values to be reset to zero when the value of
the associated timeticks attribute reaches 497+ days and
wraps around to zero.
The associated timeticks object must be specified
in the description of any object using this type.";
reference
"RFC 2579 (STD 58)";
}
/*
* collection of generic address types
*/
typedef phys-address {
type string;
description
"Represents media- or physical-level addresses.";
reference
"RFC 2579 (STD 58)";
}
}
A.2. Internet Specific Derived Types
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module inet-types {
// XXX namespace to be allocated by IANA
namespace "urn:ietf:params:xml:ns:yang:inet-types";
prefix "inet";
organization
"YANG Language Design Team";
contact
"Martin Bjorklund (Editor) <mbj@tail-f.com>";
description
"This module contains standard derived YANG types
for Internet addresses and related things.";
revision 2007-10-02 {
description "Initial revision.";
}
/*
* collection of protocol field related types
*/
typedef ip-version {
type enumeration {
enum unknown {
value 0;
description
"An unknown or unspecified version of the
Internet protocol.";
}
enum ipv4 {
value 1;
description
"The IPv4 protocol as defined in RFC 791.";
}
enum ipv6 {
value 2;
description
"The IPv6 protocol as defined in RFC 2460.";
}
}
description
"This value represents the version of the IP protocol.";
reference
"RFC 791 (STD 5), RFC 2460";
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}
typedef dscp {
type uint8 {
range "0..63";
}
description
"The dscp type represents a Differentiated Services
Code-Point that may be used for marking a traffic
stream.";
reference
"RFC 3289, RFC 2474, RFC 2780";
}
typedef flow-label {
type uint32 {
range "0..1048575";
}
description
"The flow-label type represents flow identifier or
Flow Label in an IPv6 packet header that may be
used to discriminate traffic flows.";
reference
"RFC 2460";
}
typedef port-number {
type uint16 {
range "1..65535";
}
description
"The port-number type represents a 16-bit port
number of an Internet transport layer protocol
such as UDP, TCP, DCCP or SCTP. Port numbers are
assigned by IANA. A current list of all
assignments is available from
<http://www.iana.org/>.
Note that the value zero is not a valid port
number. A union type might be used in situations
where the value zero is meaningful.";
reference
"RFC 4001";
}
/*
* collection of autonomous system related types
*/
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typedef asn {
type uint32;
description
"The asn type represents autonomous system numbers which
identify an Autonomous System (AS). An AS is a set of
routers under a single technical administration, using an
interior gateway protocol and common metrics to route
packets within the AS, and using an exterior gateway
protocol to route packets to other ASs'. IANA maintains
the AS number space and has delegated large parts to the
regional registries.
Autonomous system numbers are currently limited to 16 bits
(0..65535). There is however work in progress to enlarge
the autonomous system number space to 32 bits. This
textual convention therefore uses an uint32 base type
without a range restriction in order to support a larger
autonomous system number space.";
reference
"RFC 1771, RFC 1930, RFC 4001";
}
/*
* collection of IP address and hostname related types
*/
typedef ip-address {
type union {
type inet:ipv4-address;
type inet:ipv6-address;
}
description
"The ip-address type represents an IP address and
is IP version neutral. The format of the textual
representations implies the IP version.";
}
typedef ipv4-address {
type string {
pattern
'(([0-1]?[0-9]?[0-9]|2[0-4][0-9]|25[0-5])\.){3}'
+ '([0-1]?[0-9]?[0-9]|2[0-4][0-9]|25[0-5])'
+ '(%[\p{N}\p{L}]+)?';
}
description
"The ipv4-address type represents an IPv4 address in
dotted-quad notation. The IPv4 address may include
a zone index, separated by a % sign.";
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}
typedef ipv6-address {
type string {
pattern
/* full */
'((([0-9a-fA-F]{1,4}:){7})([0-9a-fA-F]{1,4})'
+ '(%[\p{N}\p{L}]+)?)'
/* mixed */
+ '|((([0-9a-fA-F]{1,4}:){6})(([0-9]{1,3}\.'
+ '[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}))'
+ '(%[\p{N}\p{L}]+)?)'
/* shortened */
+ '|((([0-9a-fA-F]{1,4}:)*([0-9a-fA-F]{1,4}))*(::)'
+ '(([0-9a-fA-F]{1,4}:)*([0-9a-fA-F]{1,4}))*'
+ '(%[\p{N}\p{L}]+)?)'
/* shortened mixed */
+ '((([0-9a-fA-F]{1,4}:)*([0-9a-fA-F]{1,4}))*(::)'
+ '(([0-9a-fA-F]{1,4}:)*([0-9a-fA-F]{1,4}))*'
+ '(([0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}))'
+ '(%[\p{N}\p{L}]+)?)';
}
description
"The ipv6-address type represents an IPv6 address in
full, mixed, shortened and shortened mixed notation.
The IPv6 address may include a zone index, separated
by a % sign.";
}
typedef ip-prefix {
type union {
type inet:ipv4-prefix;
type inet:ipv6-prefix;
}
description
"The ip-prefix type represents an IP prefix and
is IP version neutral. The format of the textual
representations implies the IP version.";
}
typedef ipv4-prefix {
type string {
pattern
'(([0-1]?[0-9]?[0-9]|2[0-4][0-9]|25[0-5])\.){3}'
+ '([0-1]?[0-9]?[0-9]|2[0-4][0-9]|25[0-5])'
+ '/\p{N}+';
}
description
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"The ipv4-prefix type represents an IPv4 address prefix.
The prefix length is given by the number following the
slash character and must be less than or equal 32.
Values larger than 32 should be treated as 32.
A prefix length value of n corresponds to an IP address
mask which has n contiguous 1-bits from the most
significant bit (MSB) and all other bits set to 0.
The IPv4 address represented in dotted quad notation
should have all bits that do not belong to the prefix
set to zero.";
}
typedef ipv6-prefix {
type string {
pattern
/* full */
'((([0-9a-fA-F]{1,4}:){7})([0-9a-fA-F]{1,4})'
+ '/\p{N}+)'
/* mixed */
+ '|((([0-9a-fA-F]{1,4}:){6})(([0-9]{1,3}\.'
+ '[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}))'
+ '/\p{N}+)'
/* shortened */
+ '|((([0-9a-fA-F]{1,4}:)*([0-9a-fA-F]{1,4}))*(::)'
+ '(([0-9a-fA-F]{1,4}:)*([0-9a-fA-F]{1,4}))*'
+ '/\p{N}+)'
/* shortened mixed */
+ '((([0-9a-fA-F]{1,4}:)*([0-9a-fA-F]{1,4}))*(::)'
+ '(([0-9a-fA-F]{1,4}:)*([0-9a-fA-F]{1,4}))*'
+ '(([0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}))'
+ '/\p{N}+)';
}
description
"The ipv6-prefix type represents an IPv6 address prefix.
The prefix length is given by the number following the
slash character and must be less than or equal 128.
Values larger than 128 should be treated as 128.
A prefix length value of n corresponds to an IP address
mask which has n contiguous 1-bits from the most
significant bit (MSB) and all other bits set to 0.
The IPv6 address represented in dotted quad notation
should have all bits that do not belong to the prefix
set to zero.";
}
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typedef domain-name {
type string {
pattern '([\p{L}\p{N}]+\.)*[\p{L}\p{N}]';
}
description
"The domain-name type represents a DNS domain
name. The name SHOULD be fully qualified
whenever possible.
The description clause of objects using the
domain-name type MUST describe how (and when)
these names are resolved to IP addresses.
Note that the resolution of a domain-name value
may require to query multiple DNS records (e.g.,
A for IPv4 and AAAA for IPv6). The order of the
resolution process and which DNS record takes
precedence depends on the configuration of the
resolver.";
reference
"RFC 1034";
}
typedef host {
type union {
type inet:ip-address;
type inet:domain-name;
}
description
"The host type represents either an IP address
or a DNS domain name.";
}
}
A.3. IEEE 802 Specific Derived Types
module ieee-types {
// XXX namespace to be allocated by IANA
namespace "urn:ietf:params:xml:ns:yang:ieee-types";
prefix "ieee";
import yang-types {
prefix yang;
}
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organization
"YANG Language Design Team";
contact
"Martin Bjorklund (Editor) <mbj@tail-f.com>";
description
"This module contains standard derived YANG types
for IEEE 802 addresses and related things.";
revision 2007-10-02 {
description "Initial revision.";
}
/*
* collection of IEEE address type definitions
*/
typedef mac-address {
type yang:phys-address {
pattern '([0-9a-fA-F]{2}:){5}[0-9a-fA-F]{2}';
}
description
"The mac-address type represents an 802 MAC address
represented in the `canonical' order defined by
IEEE 802.1a, i.e., as if it were transmitted least
significant bit first, even though 802.5 (in contrast
to other 802.x protocols) requires MAC addresses to
be transmitted most significant bit first.";
reference
"RFC 2579 STD 58";
}
/*
* collection of IEEE 802 related identifier types
*/
typedef bridgeid {
type string {
pattern '[0-9a-fA-F]{4}:'
+ '([0-9a-fA-F]{2}:){5}[0-9a-fA-F]{2}';
}
description
"The bridgeid type represents identifers that uniquely
identify a bridge. Its first four hexadecimal digits
contain a priority value followed by a colon. The
remaining characters contain the MAC address used to
refer to a bridge in a unique fashion (typically, the
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numerically smallest MAC address of all ports on the
bridge).";
reference
"RFC 4188";
}
typedef vlanid {
type uint16 {
range "1..4094";
}
description
"The vlanid type uniquely identifies a VLAN. This is
the 12-bit VLAN-ID used in the VLAN Tag header. The
range is defined by the referenced specification.";
reference
"IEEE Std 802.1Q 2003 Edition, Virtual Bridged Local
Area Networks.";
}
}
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Appendix B. YIN
A YANG module can be specified in an alternative XML-based syntax
called YIN. This appendix describes symmetric mapping rules between
the two formats.
The YANG and YIN formats contain equivalent information using
different notations. The purpose of the YIN notation is to allow the
user to translate YANG into YIN, use the rich set of XML based tools
on the YIN format to transform, or filter the model information.
Tools like XSLT or XML validators can be utilized. After this the
model can be transformed back to the YANG format if needed, which
provides a more concise and readable format.
The YANG-2-YIN and the YIN-2-YANG transformations will not modify the
information content of the model.
B.1. Formal YIN Definition
YIN is described by an algorithm that transforms YANG to YIN.
B.2. Transformation Algorithm YANG-2-YIN
Every keyword results in a new XML element. The name of the element
is the keyword. All core YANG elements are defined in the namespace
"urn:ietf:params:xml:ns:yang:yin:1". [XXX IANA]
The top-level element is always <module> or <submodule>.
Elements which represent keywords that are imported extensions from
other modules MUST be properly namespace qualified, where the
namespace is the namespace of the imported module. Translators
SHOULD use the same prefix as used in the YANG module.
If the keyword has an argument, its encoding depends on the value of
the argument's "yin-element". If "yin-element" is false, the
argument is encoded as an XML attribute to the keyword's element. If
"yin-element" is true, the argument is encoded as a subelement to the
keyword's element. The name of the attribute or element is the name
of the argument.
The core YANG keywords have arguments according to the table below.
Extension keywords have arguments according to Section 7.16.2.
YANG to YIN keyword map
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+---------------+---------------+-------------+
| keyword | argument name | yin-element |
+---------------+---------------+-------------+
| anyxml | name | false |
| argument | name | false |
| augment | target-node | false |
| belongs-to | module | false |
| bit | name | false |
| case | name | false |
| choice | name | false |
| config | value | false |
| contact | info | true |
| container | name | false |
| default | value | false |
| description | text | true |
| enum | name | false |
| error-app-tag | value | false |
| error-message | value | true |
| extension | name | false |
| grouping | name | false |
| import | module | false |
| include | module | false |
| input | <no argument> | n/a |
| key | value | false |
| leaf | name | false |
| leaf-list | name | false |
| length | value | false |
| list | name | false |
| mandatory | value | false |
| max-elements | value | false |
| min-elements | value | false |
| module | name | false |
| must | condition | false |
| namespace | uri | false |
| notification | name | false |
| ordered-by | value | false |
| organization | info | true |
| output | <no argument> | n/a |
| path | value | false |
| pattern | value | false |
| position | value | false |
| prefix | value | false |
| range | value | false |
| reference | info | false |
| revision | date | false |
| rpc | name | false |
| status | value | false |
| submodule | name | false |
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| 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 30
If a statement is followed by substatements, those substatements are
subelements in the YIN mapping.
Comments in YANG MAY be transformed into XML comments.
B.2.1. Usage Example
The following YANG snippet:
leaf mtu {
type uint32;
description "The MTU of the interface.";
}
is translated into the following YIN snippet:
<leaf name="mtu">
<type name="uint32"/>
<description>
<text>The MTU of the interface."</text>
</description>
</leaf>
B.3. Transformation Algorithm YIN-2-YANG
The transformation is based on a recursive algorithm that is started
on the <module> or <submodule> element.
The element is transformed into a YANG keyword. If the keyword in
Table 30 is marked as yin-element true, the subelement with the
keyword's argument name in Table 30 contains the YANG keyword's
argument as text content. If the keyword in Table 30 is marked as
yin-element false, the element's attribute with keyword's argument
name in Table 30 contains the YANG keyword's argument.
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If there are no other subelements to the element, the YANG statement
is closed with a ";". Otherwise, each such subelement is
transformed, according to the same algorithm, as substatements to the
current YANG statement, enclosed within "{" and "}".
XML comments in YIN MAY be transformed into YANG comments.
B.3.1. Tabulation, Formatting
To get a readable YANG module the YANG output will have to be
indented with appropriate whitespace characters.
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Appendix C. XML Schema Considerations
It is possible to generate an XSD document representing the data
model defined in YANG. The XSD will specify the structure of the
model, the used types, cardinality, etc. but a good part of the
information in YANG can not be represented in standard XSD constructs
e.g. config, status, default, keyref.
The above information will be added to XSD inside annotation
statements (which can not be handled by standard XML tools). The
exact form of annotations is for further study.
Data models defined in YANG might have multiple top-level elements.
To make it possible to validate the XML encoded data, both for the
generated XSD describing the data model and for the config and state
data received in NETCONF RPCs a single top level element <data> shall
be added.
From a YANG model, many different XSD schema can be generated
defining the same data model. The exact rules to generate XSD from
YANG are outside the scope of this document, however some general
considerations are needed.
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Appendix D. YANG ABNF Grammar
In YANG, almost all statements are unordered. The ABNF grammar
[RFC5234] defines the canonical order. To improve module
readability, it is RECOMMENDED that clauses be entered in this order.
Within the ABNF grammar, unordered statements are marked with
comments.
This grammar assumes that the scanner replaces YANG comments with a
single space character.
module = module-keyword sep identifier-str optsep
"{" stmtsep
module-header-stmts
linkage-stmts
meta-stmts
revision-stmts
body-stmts
"}" optsep
submodule = submodule-keyword sep identifier-str optsep
"{" stmtsep
submodule-header-stmts
linkage-stmts
submodule-meta-stmts
revision-stmts
body-stmts
"}" optsep
module-header-stmts = ;; these stmts can appear in any order
[yang-version-stmt stmtsep]
namespace-stmt stmtsep
prefix-stmt stmtsep
submodule-header-stmts = ;; these stmts can appear in any order
[yang-version-stmt stmtsep]
belongs-to-stmt stmtsep
meta-stmts = ;; these stmts can appear in any order
[organization-stmt stmtsep]
[contact-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
submodule-meta-stmts = ;; these stmts can appear in any order
[organization-stmt stmtsep]
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[contact-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
linkage-stmts = ;; these stmts can appear in any order
*(import-stmt stmtsep)
*(include-stmt stmtsep)
revision-stmts = *(revision-stmt stmtsep)
body-stmts = *((extension-stmt /
typedef-stmt /
grouping-stmt /
data-def-stmt /
rpc-stmt /
notification-stmt) stmtsep)
data-def-stmt = container-stmt /
leaf-stmt /
leaf-list-stmt /
list-stmt /
choice-stmt /
anyxml-stmt /
uses-stmt /
augment-stmt
case-data-def-stmt = container-stmt /
leaf-stmt /
leaf-list-stmt /
list-stmt /
anyxml-stmt /
uses-stmt /
augment-stmt
yang-version-stmt = yang-version-keyword sep "1" optsep stmtend
import-stmt = import-keyword sep identifier-str optsep
"{" stmtsep
prefix-stmt stmtsep
"}"
include-stmt = include-keyword sep identifier-str optsep
stmtend
namespace-stmt = namespace-keyword sep uri-str optsep stmtend
uri-str = < a string which matches the rule
URI in RFC 3986 >
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prefix-stmt = prefix-keyword sep prefix-str optsep stmtend
belongs-to-stmt = belongs-to-keyword sep identifier-str
optsep stmtend
organization-stmt = organization-keyword sep string
optsep stmtend
contact-stmt = contact-keyword sep string optsep stmtend
description-stmt = description-keyword sep string optsep
stmtend
reference-stmt = reference-keyword sep string optsep stmtend
units-stmt = units-keyword sep string optsep stmtend
revision-stmt = revision-keyword sep date-expr-str optsep
(";" /
"{" stmtsep
[description-stmt stmtsep]
"}")
extension-stmt = extension-keyword sep identifier-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-str optsep
(";" /
"{" stmtsep
[yin-element-stmt stmtsep]
"}")
yin-element-stmt = yin-element-keyword sep yin-element-arg-str
stmtend
yin-element-arg-str = < a string which matches the rule
yin-element-arg >
yin-element-arg = true-keyword / false-keyword
typedef-stmt = typedef-keyword sep identifier-str optsep
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"{" stmtsep
;; these stmts can appear in any order
type-stmt stmtsep
[units-stmt stmtsep]
[default-stmt stmtsep]
[status-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
"}"
type-stmt = type-keyword sep identifier-ref-str optsep
(";" /
"{" stmtsep
( numerical-restrictions /
string-restrictions /
enum-specification /
keyref-specification /
bits-specification /
union-specification )
stmtsep
"}")
numerical-restrictions = range-stmt stmtsep
range-stmt = range-keyword sep range-expr-str optsep
(";" /
"{" stmtsep
;; these stmts can appear in any order
[error-message-stmt stmtsep]
[error-app-tag-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
"}")
string-restrictions = ;; these stmts can appear in any order
[length-stmt stmtsep]
[pattern-stmt stmtsep]
length-stmt = length-keyword sep length-expr-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]
"}")
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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 identifier-str optsep
(";" /
"{" stmtsep
;; these stmts can appear in any order
[value-stmt stmtsep]
[status-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
"}")
keyref-specification = path-stmt stmtsep
path-stmt = path-keyword sep path-arg-str stmtend
union-specification = 1*(type-stmt stmtsep)
bits-specification = 1*(bit-stmt stmtsep)
bit-stmt = bit-keyword sep identifier-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-str stmtend
position-value-str = < a string which matches the rule
position-value >
position-value = non-negative-decimal-value
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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 >
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
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error-app-tag-stmt = error-app-tag-keyword sep string stmtend
min-elements-stmt = min-elements-keyword sep
min-value-str stmtend;
min-value-str = < a string which matches the rule
min-value >
min-value = non-negative-decimal-value
max-elements-stmt = max-elements-keyword sep
max-value-str stmtend;
max-value-str = < a string which matches the rule
max-value >
max-value = unbounded-keyword / positive-decimal-value
value-stmt = value-keyword sep decimal-value stmtend
grouping-stmt = grouping-keyword sep identifier-str optsep
(";" /
"{" stmtsep
;; these stmts can appear in any order
[status-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
*((typedef-stmt /
grouping-stmt) stmtsep)
*(data-def-stmt stmtsep)
"}")
container-stmt = container-keyword sep identifier-str optsep
(";" /
"{" stmtsep
;; these stmts can appear in any order
*(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-str optsep
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"{" stmtsep
;; these stmts can appear in any order
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-str optsep
"{" stmtsep
;; these stmts can appear in any order
type-stmt stmtsep
[units-stmt stmtsep]
*(must-stmt stmtsep)
[config-stmt stmtsep]
[min-elements-stmt stmtsep]
[max-elements-stmt stmtsep]
[ordered-by-stmt stmtsep]
[status-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
"}"
list-stmt = list-keyword sep identifier-str optsep
"{" stmtsep
;; these stmts can appear in any order
*(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
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key-arg-str = < a string which matches the rule
key-arg >
key-arg = 1*(identifier sep)
unique-stmt = unique-keyword sep unique-arg-str stmtend
unique-arg-str = < a string which matches the rule
unique-arg >
unique-arg = 1*(descendant-schema-nodeid 1*sp)
choice-stmt = choice-keyword sep identifier-str optsep
"{" stmtsep
;; these stmts can appear in any order
[default-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-str optsep
(";" /
"{" stmtsep
;; these stmts can appear in any order
[status-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
*(case-data-def-stmt stmtsep)
"}")
anyxml-stmt = anyxml-keyword sep identifier-str optsep
(";" /
"{" stmtsep
;; these stmts can appear in any order
[config-stmt stmtsep]
[mandatory-stmt stmtsep]
[status-stmt stmtsep]
[description-stmt stmtsep]
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[reference-stmt stmtsep]
"}")
uses-stmt = uses-keyword sep identifier-ref-str optsep
"{" stmtsep
;; these stmts can appear in any order
[status-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
*(refinement-stmt stmtsep)
"}"
refinement-stmt = refine-container-stmt /
refine-leaf-stmt /
refine-leaf-list-stmt /
refine-list-stmt /
refine-choice-stmt /
refine-anyxml-stmt
refine-leaf-stmt = leaf-keyword sep identifier-str optsep
"{" stmtsep
;; 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-stmt = leaf-list-keyword sep identifier-str optsep
"{" stmtsep
;; these stmts can appear in any order
*(must-stmt stmtsep)
[config-stmt stmtsep]
[min-elements-stmt stmtsep]
[max-elements-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
"}"
refine-list-stmt = list-keyword sep identifier-str optsep
"{" stmtsep
;; these stmts can appear in any order
*(must-stmt stmtsep)
[config-stmt stmtsep]
[min-elements-stmt stmtsep]
[max-elements-stmt stmtsep]
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[description-stmt stmtsep]
[reference-stmt stmtsep]
*(refinement-stmt stmtsep)
"}"
refine-choice-stmt = choice-keyword sep identifier-str optsep
"{" stmtsep
;; these stmts can appear in any order
[default-stmt stmtsep]
[mandatory-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
*(refine-case-stmt stmtsep)
"}"
refine-case-stmt = case-keyword sep identifier-str optsep
"{" stmtsep
;; these stmts can appear in any order
[description-stmt stmtsep]
[reference-stmt stmtsep]
*(refinement-stmt stmtsep)
"}"
refine-container-stmt = container-keyword sep identifier-str optsep
"{" stmtsep
;; these stmts can appear in any order
*(must-stmt stmtsep)
[presence-stmt stmtsep]
[config-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
*(refinement-stmt stmtsep)
"}"
refine-anyxml-stmt = anyxml-keyword sep identifier-str optsep
"{" stmtsep
;; these stmts can appear in any order
[config-stmt stmtsep]
[mandatory-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
"}"
unknown-statement = prefix ":" identifier [sep string] optsep
(";" / "{" *unknown-statement "}")
augment-stmt = augment-keyword sep augment-arg-str optsep
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"{" stmtsep
;; these stmts can appear in any order
[when-stmt stmtsep]
[status-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
(([input-stmt stmtsep]
[output-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 /
descendant-schema-nodeid
when-stmt = when-keyword sep string stmtend
rpc-stmt = rpc-keyword sep identifier-str optsep
(";" /
"{" stmtsep
;; these stmts can appear in any order
[status-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
*((typedef-stmt /
grouping-stmt) stmtsep)
[input-stmt stmtsep]
[output-stmt stmtsep]
"}")
input-stmt = input-keyword optsep
"{" stmtsep
;; these stmts can appear in any order
*((typedef-stmt /
grouping-stmt) stmtsep)
1*(data-def-stmt stmtsep)
"}"
output-stmt = output-keyword optsep
"{" stmtsep
;; these stmts can appear in any order
*((typedef-stmt /
grouping-stmt) stmtsep)
1*(data-def-stmt stmtsep)
"}"
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notification-stmt = notification-keyword sep identifier-str optsep
(";" /
"{" stmtsep
;; these stmts can appear in any order
[status-stmt stmtsep]
[description-stmt stmtsep]
[reference-stmt stmtsep]
*((typedef-stmt /
grouping-stmt) stmtsep)
*(data-def-stmt stmtsep)
"}")
;; Ranges
range-expr-str = < a string which matches the rule
range-expr >
range-expr = optsep range-part
*(optsep "|" optsep range-part)
optsep
range-part = range-boundary
[optsep ".." optsep range-boundary]
range-boundary = neginf-keyword / posinf-keyword /
min-keyword / max-keyword /
decimal-value / float-value
;; Lengths
length-expr-str = < a string which matches the rule
length-expr >
length-expr = optsep length-part *(optsep "|"
optsep length-part) optsep
length-part = length-boundary
[optsep ".." optsep length-boundary]
length-boundary = min-keyword / max-keyword /
non-negative-decimal-value
;; Date
date-expr-str = < a string which matches the rule
date-expr >
date-expr = 4DIGIT "-" 2DIGIT "-" 2DIGIT
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;; Schema Node Identifiers
schema-nodeid = absolute-schema-nodeid /
relative-schema-nodeid
absolute-schema-nodeid
= 1*("/" node-identifier)
relative-schema-nodeid
= descendant-schema-nodeid /
(("." / "..") "/"
*relative-schema-nodeid)
descendant-schema-nodeid
= node-identifier
absolute-schema-nodeid
node-identifier = [prefix ":"] identifier
;; Instance Identifiers
instance-identifier-str
= < a string which matches the rule
instance-identifier >
instance-identifier = absolute-instid /
relative-instid
absolute-instid = 1*("/" (node-identifier *predicate))
relative-instid = descendant-instid /
(("." / "..") "/"
*relative-instid)
descendant-instid = node-identifier *predicate
absolute-instid
predicate = "[" *WSP predicate-expr *WSP "]"
predicate-expr = (node-identifier / ".") *WSP "=" *WSP
((DQUOTE string DQUOTE) /
(SQUOTE string SQUOTE))
;; keyref path
path-arg-str = < a string which matches the rule
path-arg >
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path-arg = absolute-path-arg /
relative-path-arg
absolute-path-arg = 1*("/" (node-identifier *path-predicate))
relative-path-arg = descendant-path-arg /
(".." "/"
*relative-path-arg)
descendant-path-arg = node-identifier *path-predicate
absolute-path-arg
path-predicate = "[" *WSP path-equality-expr *WSP "]"
path-equality-expr = node-identifier *WSP "=" *WSP path-key-expr
path-key-expr = this-variable-keyword "/" rel-path-keyexpr
rel-path-keyexpr = 1*(".." "/") *(node-identifier "/")
node-identifier
;;; Keywords, using abnfgen's syntax for case-sensitive strings
;; statment keywords
anyxml-keyword = 'anyxml'
argument-keyword = 'argument'
augment-keyword = 'augment'
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'
grouping-keyword = 'grouping'
import-keyword = 'import'
include-keyword = 'include'
input-keyword = 'input'
key-keyword = 'key'
leaf-keyword = 'leaf'
leaf-list-keyword = 'leaf-list'
length-keyword = 'length'
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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'
revision-keyword = 'revision'
rpc-keyword = 'rpc'
status-keyword = 'status'
submodule-keyword = 'submodule'
type-keyword = 'type'
typedef-keyword = 'typedef'
unique-keyword = 'unique'
units-keyword = 'units'
uses-keyword = 'uses'
value-keyword = 'value'
when-keyword = 'when'
yang-version-keyword = 'yang-version'
yin-element-keyword = 'yin-element'
;; other keywords
current-keyword = 'current'
deprecated-keyword = 'deprecated'
false-keyword = 'false'
max-keyword = 'max'
min-keyword = 'min'
nan-keyword = 'NaN'
neginf-keyword = '-INF'
obsolete-keyword = 'obsolete'
posinf-keyword = 'INF'
system-keyword = 'system'
this-variable-keyword = '$this'
true-keyword = 'true'
unbounded-keyword = 'unbounded'
user-keyword = 'user'
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;; Basic Rules
keyword = [prefix ":"] identifier
prefix-str = < a string which matches the rule
prefix >
prefix = identifier
identifier-str = < a string which matches the rule
identifier >
identifier = (ALPHA / "_")
*(ALPHA / DIGIT / "_" / "-" / ".")
identifier-ref-str = < a string which matches the rule
identifier-ref
identifier-ref = [prefix ":"] identifier
string = < an unquoted string as returned by
the scanner >
decimal-value = ("-" non-negative-decimal-value) /
non-negative-decimal-value
non-negative-decimal-value = "0" / positive-decimal-value
positive-decimal-value = (non-zero-digit *DIGIT)
zero-decimal-value = 1*DIGIT
stmtend = ";" / "{" *unknown-statement "}"
sep = 1*(WSP / line-break)
; unconditional separator
optsep = *(WSP / line-break)
stmtsep = *(WSP / line-break / unknown-statement)
line-break = CRLF / LF
non-zero-digit = %x31-39
float-value = neginf-keyword /
posinf-keyword /
nan-keyword /
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decimal-value "." zero-decimal-value
*1("E" ("+"/"-") zero-decimal-value)
SQUOTE = %x27
; ' (Single Quote)
;;
;; RFC 4234 core rules.
;;
ALPHA = %x41-5A / %x61-7A
; A-Z / a-z
CR = %x0D
; carriage return
CRLF = CR LF
; Internet standard newline
DIGIT = %x30-39
; 0-9
DQUOTE = %x22
; " (Double Quote)
HEXDIG = DIGIT /
%x61 / %x62 / %x63 / %x64 / %x65 / %x66
; only lower-case a..f
HTAB = %x09
; horizontal tab
LF = %x0A
; linefeed
SP = %x20
; space
VCHAR = %x21-7E
; visible (printing) characters
WSP = SP / HTAB
; white space
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Appendix E. 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.
E.1. Error Message for Data that Violates a YANG unique Statement:
If a NETCONF operation would result in configuration data where a
unique constraint is invalidated, the following error is returned:
Tag: operation-failed
Error-app-tag: data-not-unique
Error-info: <non-unique>: Contains an instance identifier which
points to a leaf which invalidates the unique
constraint. This element is present once for each
leaf invalidating the unique constraint.
The <non-unique> element is in the YANG
namespace ("urn:ietf:params:xml:ns:yang:1"
[XXX IANA]).
E.2. Error Message for Data that Violates a YANG max-elements
Statement:
If a NETCONF operation would result in configuration data where a
list or a leaf-list would have too many entries the following error
is returned:
Tag: operation-failed
Error-app-tag: too-many-elements
E.3. Error Message for Data that Violates a YANG min-elements
Statement:
If a NETCONF operation would result in configuration data where a
list or a leaf-list would have too few entries the following error is
returned:
Tag: operation-failed
Error-app-tag: too-few-elements
E.4. Error Message for Data that Violates a YANG must or when
statement, a length, range or pattern restriction:
If a NETCONF operation would result in configuration data where the
restrictions imposed by a "must", "when", "length", "range" or
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"pattern" statement is violated the following error is returned:
Tag: operation-failed
Error-app-tag: data-restriction-violation
E.5. Error Message for the "insert" Operation
If the "insert" and "key" or "value" attributes are used in an <edit-
config> for a list or leaf-list node, and the "key" or "value" refers
to a non-existing instance, the following error is returned:
Tag: bad-attribute
Error-app-tag: missing-instance
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Appendix F. Why We Need a New Modeling Language
There have been many discussions about whether the IETF should design
its own language or use an existing one. The YANG designers believe
strongly that existing languages are not the right answer.
YANG is based on languages used actively for development of NETCONF-
based management systems. As such, the design of YANG is based
heavily on requirements placed upon those languages by their users
and the experience of writing NETCONF data models in vendor specific
languages.
During previous implementations that were input to YANG, developers
realized that they didn't want to read or write data models written
with XSD (or RelaxNG or RelaxNG compact), so the languages were based
on something "home grown" designed for the developers who would be
writing models and implementing them on devices.
F.1. Why not XSD?
There are several reasons for not using XSD, such as:
o XSD is too expressive, and gives too much freedom in allowable XML
content. Without restrictions on its use and very clear
guidelines, we think it will be very difficult to make
interoperable XSD models. For example, there are constructs such
as xs:redefine that would have to be disallowed.
o XSD is not expressive enough for NETCONF data modeling.
Additional semantics, such as state vs. config, integrity
constraints, error-messages, list order semantics, remote
procedure call and remote procedure call parameter definitions,
and notification definitions would have to be put in appinfo
elements, essentially creating a new language contained in appinfo
elements.
o Operators and developers want models that are simple to read and
text-based. XSD is very difficult to read and debug. In the
NETCONF work, obvious bugs in the relatively simple protocol XSD
went undetected for years. The phrase many seem to use is that
XSD is "write-only".
o Defenders of XSD often counter claims of complexity with
availability of advanced tools, but the IETF cannot require such
tools to read/write models. Rather, the IETF needs a simpler
language that is text-based, patch-friendly, and grep-able.
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o From a process perspective, the IETF lacks any IETF change control
and real input into the update process for XSD.
Does all of this mean that XSD has no place in the NETCONF management
picture? Clearly, the answer is no. XSD is an excellent formal
description mechanism that applications can consume. XSD can be
generated from the models defined in YANG.
F.2. Why not RelaxNG
RelaxNG is considered by many people a much simpler to understand and
to use schema notation for XML documents compared to XSD. However,
the reasons for not using RelaxNG are similar as for XSD:
o RelaxNG is like XSD too expressive and gives too much freedom in
allowable XML content (see above).
o RelaxNG is like XSD not expressive enough for NETCONF data
modeling (see above).
o While RelaxNG may be simpler to read (especially the compact
notation), when fully annotated with all needed features, it will
likely become as difficult to read as XSD.
o The IETF has no change control over RelaxNG.
Like in the XSD case, it will be possible to generate RelaxNG from
the models defined in YANG for the purpose to feed RelaxNG tools.
But this will only cover the information needed to validate instance
documents and some NETCONF specific information will be lost or
buried in extensions generic tools will not understand.
F.3. Why not SMIng
SMIng has been designed to be protocol independent so that SMIng data
models can be used with several management protocols. This is
achieved by avoiding protocol details in the data model and by
providing protocol binding information in so called protocol
mappings. Unfortunately, protocol independence does not came for
free and introduces complexity:
o Protocol independence requires to work with an abstract naming
system for managed objects which complicates the construction of
data models and their mappings.
o Error and exception handling can only be specified in abstract
term in protocol independent data models.
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o Management protocols have different features to express
persistency of (changes to) management objects.
o All the items listed above make it non-trivial to write and review
truly protocol independent data models.
A NETCONF specific data modeling language like YANG makes it much
easier to describe data models in a way that maps to NETCONF in a
very straight-forward manner and has therefore been chosen as the
best approach. Note that the design of YANG actually borrows heavily
from the SMIng work.
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Appendix G. ChangeLog
G.1. Version -01
o Moved text about imports and includes from Identifiers Section 6.2
to Modules and submodules Section 4.2.1.
o Clarified how presence containers behave with edit-config
operation "none".
o Clarified how mandatory leafs behave within non-presence
containers.
o Added a shorthand syntax for singleton cases within a "choice"
statement.
o Removed the type anyxml, and added a new data definition statement
"anyxml".
o Clarified that "grouping" is more than simple textual
substitution.
o Clarified how core YANG statements can be used within extensions.
o Clarified that ranges are inclusive.
o Added some missing refinements in the "uses" statement.
o Added reference to [RFC3688] for URIs.
o Added "error-message" and "error-app-tag" to the "range"
statement.
o Made the value statement optional for enums, and clarified how the
value is derived if no explicit value is given.
o Made the "position" statement optional for bits, and clarified how
the position is derived if no explicit position is given.
o Changed the XML lexicographical representation of bit value into a
space separated list. This makes it consistent with how all other
types are encoded.
o Clarified that a keyref must not point from configuration to non-
configuration.
o Added the built-in type instance-identifier.
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o Made all meta-statements optional.
o Fixed typos and made several minor edits.
o Several minor grammar fixes.
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Author's Address
Martin Bjorklund (editor)
Tail-f Systems
Email: mbj@tail-f.com
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Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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Bjorklund Expires August 4, 2008 [Page 154]