Network Working Group A. Bierman
Internet-Draft YumaWorks
Intended status: Standards Track October 19, 2015
Expires: April 21, 2016
Guidelines for Authors and Reviewers of YANG Data Model Documents
draft-ietf-netmod-rfc6087bis-05
Abstract
This memo provides guidelines for authors and reviewers of Standards
Track specifications containing YANG data model modules. Applicable
portions may be used as a basis for reviews of other YANG data model
documents. Recommendations and procedures are defined, which are
intended to increase interoperability and usability of Network
Configuration Protocol (NETCONF) implementations that utilize YANG
data model modules.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 21, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Requirements Notation . . . . . . . . . . . . . . . . . . 5
2.2. NETCONF Terms . . . . . . . . . . . . . . . . . . . . . . 5
2.3. YANG Terms . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. YANG Tree Diagrams . . . . . . . . . . . . . . . . . . . . . . 7
4. General Documentation Guidelines . . . . . . . . . . . . . . . 8
4.1. Module Copyright . . . . . . . . . . . . . . . . . . . . . 8
4.2. Terminology Section . . . . . . . . . . . . . . . . . . . 9
4.3. Tree Diagrams . . . . . . . . . . . . . . . . . . . . . . 9
4.4. Narrative Sections . . . . . . . . . . . . . . . . . . . . 10
4.5. Definitions Section . . . . . . . . . . . . . . . . . . . 10
4.6. Security Considerations Section . . . . . . . . . . . . . 10
4.7. IANA Considerations Section . . . . . . . . . . . . . . . 11
4.7.1. Documents that Create a New Namespace . . . . . . . . 11
4.7.2. Documents that Extend an Existing Namespace . . . . . 12
4.8. Reference Sections . . . . . . . . . . . . . . . . . . . . 12
4.9. Validation Tools . . . . . . . . . . . . . . . . . . . . . 12
4.10. Module Extraction Tools . . . . . . . . . . . . . . . . . 12
5. YANG Usage Guidelines . . . . . . . . . . . . . . . . . . . . 13
5.1. Module Naming Conventions . . . . . . . . . . . . . . . . 13
5.2. Prefixes . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.3. Identifiers . . . . . . . . . . . . . . . . . . . . . . . 15
5.3.1. Identifier Naming Conventions . . . . . . . . . . . . 15
5.4. Defaults . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.5. Conditional Statements . . . . . . . . . . . . . . . . . . 16
5.6. XPath Usage . . . . . . . . . . . . . . . . . . . . . . . 17
5.6.1. XPath Evaluation Contexts . . . . . . . . . . . . . . 17
5.6.2. Function Library . . . . . . . . . . . . . . . . . . . 18
5.6.3. Axes . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.6.4. Types . . . . . . . . . . . . . . . . . . . . . . . . 19
5.6.5. Wildcards . . . . . . . . . . . . . . . . . . . . . . 20
5.6.6. Boolean Expressions . . . . . . . . . . . . . . . . . 20
5.7. Lifecycle Management . . . . . . . . . . . . . . . . . . . 21
5.8. Module Header, Meta, and Revision Statements . . . . . . . 22
5.9. Namespace Assignments . . . . . . . . . . . . . . . . . . 23
5.10. Top-Level Data Definitions . . . . . . . . . . . . . . . . 24
5.11. Data Types . . . . . . . . . . . . . . . . . . . . . . . . 24
5.11.1. Fixed Value Extensibility . . . . . . . . . . . . . . 25
5.11.2. Patterns and Ranges . . . . . . . . . . . . . . . . . 25
5.11.3. Enumerations and Bits . . . . . . . . . . . . . . . . 26
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5.12. Reusable Type Definitions . . . . . . . . . . . . . . . . 27
5.13. Reusable Groupings . . . . . . . . . . . . . . . . . . . . 28
5.14. Data Definitions . . . . . . . . . . . . . . . . . . . . . 28
5.15. Operation Definitions . . . . . . . . . . . . . . . . . . 30
5.16. Notification Definitions . . . . . . . . . . . . . . . . . 30
5.17. Feature Definitions . . . . . . . . . . . . . . . . . . . 31
5.18. Augment Statements . . . . . . . . . . . . . . . . . . . . 31
5.18.1. Conditional Augment Statements . . . . . . . . . . . . 32
5.18.2. Conditionally Mandatory Data Definition Statements . . 32
5.19. Deviation Statements . . . . . . . . . . . . . . . . . . . 33
5.20. Extension Statements . . . . . . . . . . . . . . . . . . . 34
5.21. Data Correlation . . . . . . . . . . . . . . . . . . . . . 35
5.22. Operational State . . . . . . . . . . . . . . . . . . . . 36
5.23. Performance Considerations . . . . . . . . . . . . . . . . 38
5.24. YANG 1.1 Guidelines . . . . . . . . . . . . . . . . . . . 38
5.24.1. Importing Multiple Revisions . . . . . . . . . . . . . 39
5.24.2. Using Feature Logic . . . . . . . . . . . . . . . . . 39
5.24.3. anyxml vs. anydata . . . . . . . . . . . . . . . . . . 39
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40
7. Security Considerations . . . . . . . . . . . . . . . . . . . 41
7.1. Security Considerations Section Template . . . . . . . . . 41
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 43
9. Changes Since RFC 6087 . . . . . . . . . . . . . . . . . . . . 44
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45
10.1. Normative References . . . . . . . . . . . . . . . . . . . 45
10.2. Informative References . . . . . . . . . . . . . . . . . . 46
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 47
A.1. 04 to 05 . . . . . . . . . . . . . . . . . . . . . . . . . 47
A.2. 03 ot 04 . . . . . . . . . . . . . . . . . . . . . . . . . 47
A.3. 02 to 03 . . . . . . . . . . . . . . . . . . . . . . . . . 47
A.4. 01 to 02 . . . . . . . . . . . . . . . . . . . . . . . . . 47
A.5. 00 to 01 . . . . . . . . . . . . . . . . . . . . . . . . . 48
Appendix B. Module Review Checklist . . . . . . . . . . . . . . . 49
Appendix C. YANG Module Template . . . . . . . . . . . . . . . . 51
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 53
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1. Introduction
The standardization of network configuration interfaces for use with
the Network Configuration Protocol [RFC6241] requires a modular set
of data models, which can be reused and extended over time.
This document defines a set of usage guidelines for Standards Track
documents containing [I-D.ietf-netmod-rfc6020bis] data models. YANG
is used to define the data structures, protocol operations, and
notification content used within a NETCONF server. A server that
supports a particular YANG module will support client NETCONF
operation requests, as indicated by the specific content defined in
the YANG module.
This document is similar to the Structure of Management Information
version 2 (SMIv2) usage guidelines specification [RFC4181] in intent
and structure. However, since that document was written a decade
after SMIv2 modules had been in use, it was published as a 'Best
Current Practice' (BCP). This document is not a BCP, but rather an
informational reference, intended to promote consistency in documents
containing YANG modules.
Many YANG constructs are defined as optional to use, such as the
description statement. However, in order to maximize
interoperability of NETCONF implementations utilizing YANG data
models, it is desirable to define a set of usage guidelines that may
require a higher level of compliance than the minimum level defined
in the YANG specification.
In addition, YANG allows constructs such as infinite length
identifiers and string values, or top-level mandatory nodes, that a
compliant server is not required to support. Only constructs that
all servers are required to support can be used in IETF YANG modules.
This document defines usage guidelines related to the NETCONF
operations layer and NETCONF content layer, as defined in [RFC6241].
These guidelines are intended to be used by authors and reviewers to
improve the readability and interoperability of published YANG data
models.
Note that this document is not a YANG tutorial and the reader is
expected to know the YANG data modeling language before using this
document.
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2. Terminology
2.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
RFC 2119 language is used here to express the views of the NETMOD
working group regarding content for YANG modules. YANG modules
complying with this document will treat the RFC 2119 terminology as
if it were describing best current practices.
2.2. NETCONF Terms
The following terms are defined in [RFC6241] and are not redefined
here:
o capabilities
o client
o operation
o server
2.3. YANG Terms
The following terms are defined in [I-D.ietf-netmod-rfc6020bis] and
are not redefined here:
o data node
o module
o namespace
o submodule
o version
o YANG
o YIN
Note that the term 'module' may be used as a generic term for a YANG
module or submodule. When describing properties that are specific to
submodules, the term 'submodule' is used instead.
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2.4. Terms
The following terms are used throughout this document:
o published: A stable release of a module or submodule, usually
contained in an RFC.
o unpublished: An unstable release of a module or submodule, usually
contained in an Internet-Draft.
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3. YANG Tree Diagrams
YANG tree diagrams provide a concise representation of a YANG module
to help readers understand the module structure.
The meaning of the symbols in YANG tree diagrams is as follows:
o Brackets "[" and "]" enclose list keys.
o Abbreviations before data node names: "rw" means configuration
(read-write) and "ro" state data (read-only).
o Symbols after data node names: "?" means an optional node, "!"
means a presence container, and "*" denotes a list and leaf-list.
o Parentheses enclose choice and case nodes, and case nodes are also
marked with a colon (":").
o Ellipsis ("...") stands for contents of subtrees that are not
shown.
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4. General Documentation Guidelines
YANG data model modules under review are likely to be contained in
Internet-Drafts. All guidelines for Internet-Draft authors MUST be
followed. The RFC Editor provides guidelines for authors of RFCs,
which are first published as Internet-Drafts. These guidelines
should be followed and are defined in [RFC2223] and updated in
[RFC5741] and "RFC Document Style" [RFC-STYLE].
The following sections MUST be present in an Internet-Draft
containing a module:
o Narrative sections
o Definitions section
o Security Considerations section
o IANA Considerations section
o References section
4.1. Module Copyright
The module description statement MUST contain a reference to the
latest approved IETF Trust Copyright statement, which is available
online at:
http://trustee.ietf.org/license-info/
Each YANG module or submodule contained within an Internet-Draft or
RFC is considered to be a code component. The strings "<CODE
BEGINS>" and "<CODE ENDS>" MUST be used to identify each code
component.
The "<CODE BEGINS>" tag SHOULD be followed by a string identifying
the file name specified in Section 5.2 of
[I-D.ietf-netmod-rfc6020bis]. The following example is for the
'2010-01-18' revision of the 'ietf-foo' module:
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<CODE BEGINS> file "ietf-foo@2010-01-18.yang"
module ietf-foo {
// ...
revision 2010-01-18 {
description "Latest revision";
reference "RFC XXXX";
}
// ...
}
<CODE ENDS>
Note that this convention MUST NOT be used for example modules or
module fragments.
4.2. Terminology Section
A terminology section MUST be present if any terms are defined in the
document or if any terms are imported from other documents.
If YANG tree diagrams are used, then a sub-section explaining the
YANG tree diagram syntax MUST be present, containing the following
text:
A simplified graphical representation of the data model is used in
this document. The meaning of the symbols in these diagrams is
defined in [RFCXXXX].
-- RFC Editor: Replace XXXX with RFC number and remove note
4.3. Tree Diagrams
YANG tree diagrams provide a concise representation of a YANG module,
and SHOULD be included to help readers understand YANG module
structure. Tree diagrams MAY be split into sections to correspond to
document structure.
The following example shows a simple YANG tree diagram:
+--rw top-level-config-container
| +--rw config-list* [key-name]
| +--rw key-name string
| +--rw optional-parm? string
| +--rw mandatory-parm identityref
| +--ro read-only-leaf string
+--ro top-level-nonconfig-container
+--ro nonconfig-list* [name]
+--ro name string
+--ro type string
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4.4. Narrative Sections
The narrative part MUST include an overview section that describes
the scope and field of application of the module(s) defined by the
specification and that specifies the relationship (if any) of these
modules to other standards, particularly to standards containing
other YANG modules. The narrative part SHOULD include one or more
sections to briefly describe the structure of the modules defined in
the specification.
If the module(s) defined by the specification imports definitions
from other modules (except for those defined in the
[I-D.ietf-netmod-rfc6020bis] or [RFC6991] documents), or are always
implemented in conjunction with other modules, then those facts MUST
be noted in the overview section, as MUST be noted any special
interpretations of definitions in other modules.
4.5. Definitions Section
This section contains the module(s) defined by the specification.
These modules SHOULD be written using the YANG syntax defined in
[I-D.ietf-netmod-rfc6020bis]. YANG 1.0 [RFC6020] MAY be used if no
YANG 1.1 constructs or semantics are needed in the module.
A YIN syntax version of the module MAY also be present in the
document. There MAY also be other types of modules present in the
document, such as SMIv2, which are not affected by these guidelines.
Note that all YANG statements within a YANG module are considered
normative, if the module itself is considered normative, and not an
example module. The use of keywords defined in [RFC2119] apply to
YANG description statements in normative modules exactly as they
would in any other normative section.
Example YANG modules MUST NOT contain any normative text, including
any reserved words from [RFC2119].
See Section 5 for guidelines on YANG usage.
4.6. Security Considerations Section
Each specification that defines one or more modules MUST contain a
section that discusses security considerations relevant to those
modules.
This section MUST be patterned after the latest approved template
(available at http://trac.tools.ietf.org/area/ops/trac/wiki/
yang-security-guidelines). Section 7.1 contains the security
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considerations template dated 2013-05-08. Authors MUST check the
webpage at the URL listed above in case there is a more recent
version available.
In particular:
o Writable data nodes that could be especially disruptive if abused
MUST be explicitly listed by name and the associated security
risks MUST be explained.
o Readable data nodes that contain especially sensitive information
or that raise significant privacy concerns MUST be explicitly
listed by name and the reasons for the sensitivity/privacy
concerns MUST be explained.
o Operations (i.e., YANG 'rpc' statements) that are potentially
harmful to system behavior or that raise significant privacy
concerns MUST be explicitly listed by name and the reasons for the
sensitivity/privacy concerns MUST be explained.
4.7. IANA Considerations Section
In order to comply with IESG policy as set forth in
http://www.ietf.org/id-info/checklist.html, every Internet-Draft that
is submitted to the IESG for publication MUST contain an IANA
Considerations section. The requirements for this section vary
depending on what actions are required of the IANA. If there are no
IANA considerations applicable to the document, then the IANA
Considerations section stating that there are no actions is removed
by the RFC Editor before publication. Refer to the guidelines in
[RFC5226] for more details.
4.7.1. Documents that Create a New Namespace
If an Internet-Draft defines a new namespace that is to be
administered by the IANA, then the document MUST include an IANA
Considerations section that specifies how the namespace is to be
administered.
Specifically, if any YANG module namespace statement value contained
in the document is not already registered with IANA, then a new YANG
Namespace registry entry MUST be requested from the IANA. The
[I-D.ietf-netmod-rfc6020bis] specification includes the procedure for
this purpose in its IANA Considerations section.
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4.7.2. Documents that Extend an Existing Namespace
It is possible to extend an existing namespace using a YANG submodule
that belongs to an existing module already administered by IANA. In
this case, the document containing the main module MUST be updated to
use the latest revision of the submodule.
4.8. Reference Sections
For every import or include statement that appears in a module
contained in the specification, which identifies a module in a
separate document, a corresponding normative reference to that
document MUST appear in the Normative References section. The
reference MUST correspond to the specific module version actually
used within the specification.
For every normative reference statement that appears in a module
contained in the specification, which identifies a separate document,
a corresponding normative reference to that document SHOULD appear in
the Normative References section. The reference SHOULD correspond to
the specific document version actually used within the specification.
If the reference statement identifies an informative reference, which
identifies a separate document, a corresponding informative reference
to that document MAY appear in the Informative References section.
4.9. Validation Tools
All modules need to be validated before submission in an Internet
Draft. The 'pyang' YANG compiler is freely available from github:
https://github.com/mbj4668/pyang
If the 'pyang' compiler is used, then the "--ietf" command line
option SHOULD be used to identify any IETF guideline issues.
4.10. Module Extraction Tools
A version of 'rfcstrip' is available which will extract YANG modules
from an Internet Draft or RFC. The 'rfcstrip' tool which supports
YANG module extraction is freely available:
http://www.yang-central.org/twiki/pub/Main/YangTools/rfcstrip
This tool can be used to verify that the "<CODE BEGINS>" and "<CODE
ENDS>" tags are used correctly and that the normative YANG modules
can be extracted correctly.
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5. YANG Usage Guidelines
In general, modules in IETF Standards Track specifications MUST
comply with all syntactic and semantic requirements of YANG
[I-D.ietf-netmod-rfc6020bis]. The guidelines in this section are
intended to supplement the YANG specification, which is intended to
define a minimum set of conformance requirements.
In order to promote interoperability and establish a set of practices
based on previous experience, the following sections establish usage
guidelines for specific YANG constructs.
Only guidelines that clarify or restrict the minimum conformance
requirements are included here.
5.1. Module Naming Conventions
Normative modules contained in Standards Track documents MUST be
named according to the guidelines in the IANA Considerations section
of [I-D.ietf-netmod-rfc6020bis].
A distinctive word or acronym (e.g., protocol name or working group
acronym) SHOULD be used in the module name. If new definitions are
being defined to extend one or more existing modules, then the same
word or acronym should be reused, instead of creating a new one.
All published module names MUST be unique. For a YANG module
published in an RFC, this uniqueness is guaranteed by IANA. For
unpublished modules, the authors need to check that no other work in
progress is using the same module name.
Example modules are non-normative, and SHOULD be named with the
prefix "example-".
It is suggested that a stable prefix be selected representing the
entire organization. All normative YANG modules published by the
IETF MUST begin with the prefix "ietf-". Another standards
organization, such as the IEEE, might use the prefix "ieee-" for all
YANG modules.
Once a module name is published, it MUST NOT be reused, even if the
RFC containing the module is reclassified to 'Historic' status. A
module name cannot be changed in YANG, and this would be treated as a
a new module, not a name change.
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5.2. Prefixes
All YANG definitions are scoped by the module containing the
definition being referenced. This allows definitions from multiple
modules to be used, even if the names are not unique. In the example
below, the identifier "foo" is used in all 3 modules:
module example-foo {
namespace "http://example.com/ns/foo";
prefix f;
container foo;
}
module example-bar {
namespace "http://example.com/ns/bar";
prefix b;
typedef foo { type uint32; }
}
module example-one {
namespace "http://example.com/ns/one";
prefix one;
import example-foo { prefix f; }
import example-bar { prefix b; }
augment "/f:foo" {
leaf foo { type b:foo; }
}
}
YANG defines the following rules for prefix usage:
o Prefixes are never allowed for built in data types and YANG
keywords.
o A prefix MUST be used for any external statement (i.e., a
statement defined with the YANG "extension" statement)
o The proper module prefix MUST be used for all identifiers imported
from other modules
o The proper module prefix MUST be used for all identifiers included
from a submodule.
The following guidelines apply to prefix usage of the current (local)
module:
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o The local module prefix SHOULD be used instead of no prefix in all
path expressions.
o The local module prefix MUST be used instead of no prefix in all
"default" statements for an "identityref" or "instance-identifier"
data type
o The lcaol module prefix MAY be used for references to typedefs,
groupings, extensions, features, and identities defined in the
module.
Prefix values SHOULD be short, but also likely to be unique. Prefix
values SHOULD NOT conflict with known modules that have been
previously published.
5.3. Identifiers
Identifiers for all YANG identifiers in published modules MUST be
between 1 and 64 characters in length. These include any construct
specified as an 'identifier-arg-str' token in the ABNF in Section 13
of [I-D.ietf-netmod-rfc6020bis].
5.3.1. Identifier Naming Conventions
Identifiers SHOULD follow a consistent naming pattern throughout the
module. Only lower-case letters, numbers, and dashes SHOULD be used
in identifier names. Upper-case characters and the underscore
character MAY be used if the identifier represents a well-known value
that uses these characters.
Identifiers SHOULD include complete words and/or well-known acronyms
or abbreviations. Child nodes within a container or list SHOULD NOT
replicate the parent identifier. YANG identifiers are hierarchical
and are only meant to be unique within the the set of sibling nodes
defined in the same module namespace.
It is permissible to use common identifiers such as "name" or "id" in
data definition statements, especially if these data nodes share a
common data type.
Identifiers SHOULD NOT carry any special semantics that identify data
modelling properties. Only YANG statements and YANG extension
statements are designed to convey machine readable data modelling
properties. For example, naming an object "config" or "state" does
not change whether it is configuration data or state data. Only
defined YANG statements or YANG extension statements can be used to
assign semantics in a machine readable format in YANG.
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5.4. Defaults
In general, it is suggested that substatements containing very common
default values SHOULD NOT be present. The following substatements
are commonly used with the default value, which would make the module
difficult to read if used everywhere they are allowed.
+--------------+---------------+
| Statement | Default Value |
+--------------+---------------+
| config | true |
| mandatory | false |
| max-elements | unbounded |
| min-elements | 0 |
| ordered-by | system |
| status | current |
| yin-element | false |
+--------------+---------------+
Statement Defaults
5.5. Conditional Statements
A module may be conceptually partitioned in several ways, using the
'if-feature' and/or 'when' statements.
Data model designers need to carefully consider all modularity
aspects, including the use of YANG conditional statements.
If a data definition is optional, depending on server support for a
NETCONF protocol capability, then a YANG 'feature' statement SHOULD
be defined to indicate that the NETCONF capability is supported
within the data model.
If any notification data, or any data definition, for a non-
configuration data node is not mandatory, then the server may or may
not be required to return an instance of this data node. If any
conditional requirements exist for returning the data node in a
notification payload or retrieval request, they MUST be documented
somewhere. For example, a 'when' or 'if-feature' statement could
apply to the data node, or the conditional requirements could be
explained in a 'description' statement within the data node or one of
its ancestors (if any).
If any 'if-feature' statements apply to a list node, then the same
'if-feature' statements MUST apply to any key leaf nodes for the
list. There MUST NOT be any 'if-feature' statements applied to any
key leaf that do not also apply to the parent list node.
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There SHOULD NOT be any 'when' statements applied to a key leaf node.
It is possible that a 'when' statement for an ancestor node of a key
leaf will have the exact node-set result as the key leaf. In such a
case, the 'when' statement for the key leaf is redundant and SHOULD
be avoided.
5.6. XPath Usage
This section describes guidelines for using the XML Path Language
[W3C.REC-xpath-19991116] (XPath) within YANG modules.
5.6.1. XPath Evaluation Contexts
YANG defines 5 separate contexts for evaluation of XPath statements:
1) The "running" datastore: collection of all YANG configuration data
nodes. The document root is the conceptual container, (e.g.,
"config" in the "edit-config" operation), which is the parent of all
top-level data definition statements with a "config" statement value
of "true".
2) State data + the "running" datastore: collection of all YANG data
nodes. The document root is the conceptual container, parent of all
top-level data definition statements.
3) Notification: an event notification document. The document root
is the notification element.
4) RPC Input: The document root is the conceptual "input" node, which
is the parent of all RPC input parameter definitions.
5) RPC Output: The document root is the conceptual "output" node,
which is the parent of all RPC output parameter definitions.
Note that these XPath contexts cannot be mixed. For example, a
"when" statement in a notification context cannot reference
configuration data.
notification foo {
leaf mtu {
// NOT OK because when-stmt context is this notification
when "/if:interfaces/if:interface[name='eth0']";
type leafref {
// OK because path-stmt has a different context
path "/if:interfaces/if:interface/if:mtu";
}
}
}
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It is especially important to consider the XPath evaluation context
for XPath expressions defined in groupings. An XPath expression
defined in a grouping may not be portable, meaning it cannot be used
in multiple contexts and produce proper results.
If the XPath expressions defined in a grouping are intended for a
particular context, then this context SHOULD be identified in the
"description" statement for the grouping.
5.6.2. Function Library
The 'position' and 'last' functions SHOULD NOT be used. This applies
to implicit use of the 'position' function as well (e.g.,
'//chapter[42]'). A server is only required to maintain the relative
XML document order of all instances of a particular user-ordered list
or leaf-list. The 'position' and 'last' functions MAY be used if
they are evaluated in a context where the context node is a user-
ordered 'list' or 'leaf-list'.
The 'id' function SHOULD NOT be used. The 'ID' attribute is not
present in YANG documents so this function has no meaning. The YANG
compiler SHOULD return an empty string for this function.
The 'namespace-uri' and 'name' functions SHOULD NOT be used.
Expanded names in XPath are different than YANG. A specific
canonical representation of a YANG expanded name does not exist.
The 'lang' function SHOULD NOT be used. This function does not apply
to YANG because there is no 'lang' attribute set with the document.
The YANG compiler SHOULD return 'false' for this function.
The 'local-name', 'namespace-uri', 'name', 'string', and 'number'
functions SHOULD NOT be used if the argument is a node-set. If so,
the function result will be determined by the document order of the
node-set. Since this order can be different on each server, the
function results can also be different. Any function call that
implicitly converts a node-set to a string will also have this issue.
The 'local-name' function SHOULD NOT be used to reference local names
outside of the YANG module defining the must or when expression
containing the 'local-name' function. Example of a local-name
function that should not be used:
/*[local-name()='foo']
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5.6.3. Axes
The 'attribute' and 'namespace' axes are not supported in YANG, and
MAY be empty in a NETCONF server implementation.
The 'preceding', and 'following' axes SHOULD NOT be used. These
constructs rely on XML document order within a NETCONF server
configuration database, which may not be supported consistently or
produce reliable results across implementations. Predicate
expressions based on static node properties (e.g., element name or
value, 'ancestor' or 'descendant' axes) SHOULD be used instead. The
'preceding' and 'following' axes MAY be used if document order is not
relevant to the outcome of the expression (e.g., check for global
uniqueness of a parameter value).
The 'preceding-sibling' and 'following-sibling' axes SHOULD NOT used,
however they MAY be used if document order is not relevant to the
outcome of the expression.
A server is only required to maintain the relative XML document order
of all instances of a particular user-ordered list or leaf-list. The
'preceding-sibling' and 'following-sibling' axes MAY be used if they
are evaluated in a context where the context node is a user-ordered
'list' or 'leaf-list'.
5.6.4. Types
Data nodes that use the 'int64' and 'uint64' built-in type SHOULD NOT
be used within numeric or boolean expressions. There are boundary
conditions in which the translation from the YANG 64-bit type to an
XPath number can cause incorrect results. Specifically, an XPath
'double' precision floating point number cannot represent very large
positive or negative 64-bit numbers because it only provides a total
precision of 53 bits. The 'int64' and 'uint64' data types MAY be
used in numeric expressions if the value can be represented with no
more than 53 bits of precision.
Data modelers need to be careful not to confuse the YANG value space
and the XPath value space. The data types are not the same in both,
and conversion between YANG and XPath data types SHOULD be considered
carefully.
Explicit XPath data type conversions MAY be used (e.g., 'string',
'boolean', or 'number' functions), instead of implicit XPath data
type conversions.
XPath expressions that contain a literal value representing a YANG
identity SHOULD always include the declared prefix of the module
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where the identity is defined.
XPath expressions for 'when' statements SHOULD NOT reference the
context node or any descendant nodes of the context node. They MAY
reference descendant nodes if the 'when' statement is contained
within an 'augment' statement, and the referenced nodes are not
defined within the 'augment' statement.
Example:
augment "/rt:active-route/rt:input/rt:destination-address" {
when "rt:address-family='v4ur:ipv4-unicast'" {
description
"This augment is valid only for IPv4 unicast.";
}
// nodes defined here within the augment-stmt
// cannot be referenced in the when-stmt
}
5.6.5. Wildcards
It is possible to construct XPath expressions that will evaluate
differently when combined with several modules within a server
implementation, then when evaluated within the single module. This
is due to augmenting nodes from other modules.
Wildcard expansion is done within a server against all the nodes from
all namespaces, so it is possible for a 'must' or 'when' expression
that uses the '*' operator will always evaluate to false if processed
within a single YANG module. In such cases, the 'description'
statement SHOULD clarify that augmenting objects are expected to
match the wildcard expansion.
when /foo/services/*/active {
description
"No services directly defined in this module.
Matches objects that have augmented the services container.";
}
5.6.6. Boolean Expressions
The YANG "must" and "when" statements use an XPath boolean expression
to define the test condition for the statement. It is important to
specify these expressions in a way that will not cause inadvertent
changes in the result if the objects referenced in the expression are
updated in future revisions of the module.
For example, the leaf "foo2" must exist if the leaf "foo1" is equal
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to "one" or "three":
leaf foo1 {
type enumeration {
enum one;
enum two;
enum three;
}
}
leaf foo2 {
// INCORRECT
must "/f:foo1 != 'two'";
type string;
}
leaf foo2 {
// CORRECT
must "/f:foo1 = 'one' or /f:foo1 = 'three'";
type string;
}
In the next revision of the module, leaf "foo1" is extended with a
nem enum named "four":
leaf foo1 {
type enumeration {
enum one;
enum two;
enum three;
enum four;
}
}
Now the first XPath expression will allow the enum "four" to be
accepted in addition to the "one" and "three" enum values.
5.7. Lifecycle Management
The status statement MUST be present if its value is 'deprecated' or
'obsolete'. The status SHOULD NOT be changed from 'current' directly
to 'obsolete'. An object SHOULD be available for at least one year
with 'deprecated' status before it is changed to 'obsolete'.
The module or submodule name MUST NOT be changed, once the document
containing the module or submodule is published.
The module namespace URI value MUST NOT be changed, once the document
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containing the module is published.
The revision-date substatement within the imports statement SHOULD be
present if any groupings are used from the external module.
The revision-date substatement within the include statement SHOULD be
present if any groupings are used from the external submodule.
If submodules are used, then the document containing the main module
MUST be updated so that the main module revision date is equal or
more recent than the revision date of any submodule that is (directly
or indirectly) included by the main module.
5.8. Module Header, Meta, and Revision Statements
For published modules, the namespace MUST be a globally unique URI,
as defined in [RFC3986]. This value is usually assigned by the IANA.
The organization statement MUST be present. If the module is
contained in a document intended for IETF Standards Track status,
then the organization SHOULD be the IETF working group chartered to
write the document. For other standards organizations, a similar
approach is also suggested.
The contact statement MUST be present. If the module is contained in
a document intended for Standards Track status, then the working
group web and mailing information MUST be present, and the main
document author or editor contact information SHOULD be present. If
additional authors or editors exist, their contact information MAY be
present. In addition, the Area Director and other contact
information MAY be present.
The description statement MUST be present. For modules published
within IETF documents, the appropriate IETF Trust Copyright text MUST
be present, as described in Section 4.1.
If the module relies on information contained in other documents,
which are not the same documents implied by the import statements
present in the module, then these documents MUST be identified in the
reference statement.
A revision statement MUST be present for each published version of
the module. The revision statement MUST have a reference
substatement. It MUST identify the published document that contains
the module. Modules are often extracted from their original
documents, and it is useful for developers and operators to know how
to find the original source document in a consistent manner. The
revision statement MAY have a description substatement.
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Each new revision MUST include a revision date that is higher than
any other revision date in the module. The revision date does not
need to be updated if the module contents do not change in the new
document revision.
It is acceptable to reuse the same revision statement within
unpublished versions (i.e., Internet-Drafts), but the revision date
MUST be updated to a higher value each time the Internet-Draft is re-
posted.
5.9. Namespace Assignments
It is RECOMMENDED that only valid YANG modules be included in
documents, whether or not they are published yet. This allows:
o the module to compile correctly instead of generating disruptive
fatal errors.
o early implementors to use the modules without picking a random
value for the XML namespace.
o early interoperability testing since independent implementations
will use the same XML namespace value.
Until a URI is assigned by the IANA, a proposed namespace URI MUST be
provided for the namespace statement in a YANG module. A value
SHOULD be selected that is not likely to collide with other YANG
namespaces. Standard module names, prefixes, and URI strings already
listed in the YANG Module Registry MUST NOT be used.
A standard namespace statement value SHOULD have the following form:
<URN prefix string>:<module-name>
The following URN prefix string SHOULD be used for published and
unpublished YANG modules:
urn:ietf:params:xml:ns:yang:
The following example URNs would be valid namespace statement values
for Standards Track modules:
urn:ietf:params:xml:ns:yang:ietf-netconf-partial-lock
urn:ietf:params:xml:ns:yang:ietf-netconf-state
urn:ietf:params:xml:ns:yang:ietf-netconf
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Note that a different URN prefix string SHOULD be used for non-
Standards-Track modules. The string SHOULD be selected according to
the guidelines in [I-D.ietf-netmod-rfc6020bis].
The following examples are for non-Standards-Track modules. The
domain "example.com" SHOULD be used in all namespace URIs for example
modules.
http://example.com/ns/example-interfaces
http://example.com/ns/example-system
5.10. Top-Level Data Definitions
The top-level data organization SHOULD be considered carefully, in
advance. Data model designers need to consider how the functionality
for a given protocol or protocol family will grow over time.
The separation of configuration data and operational state SHOULD be
considered carefully. It is often useful to define separate top-
level containers for configuration and non-configuration data.
The number of top-level data nodes within a module SHOULD be
minimized. It is often useful to retrieve related information within
a single subtree. If data is too distributed, is becomes difficult
to retrieve all at once.
The names and data organization SHOULD reflect persistent
information, such as the name of a protocol. The name of the working
group SHOULD NOT be used because this may change over time.
A mandatory database data definition is defined as a node that a
client must provide for the database to be valid. The server is not
required to provide a value.
Top-level database data definitions MUST NOT be mandatory. If a
mandatory node appears at the top level, it will immediately cause
the database to be invalid. This can occur when the server boots or
when a module is loaded dynamically at runtime.
5.11. Data Types
Selection of an appropriate data type (i.e., built-in type, existing
derived type, or new derived type) is very subjective, and therefore
few requirements can be specified on that subject.
Data model designers SHOULD use the most appropriate built-in data
type for the particular application.
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The signed numeric data types (i.e., 'int8', 'int16', 'int32', and
'int64') SHOULD NOT be used unless negative values are allowed for
the desired semantics.
5.11.1. Fixed Value Extensibility
If the set of values is fixed and the data type contents are
controlled by a single naming authority, then an enumeration data
type SHOULD be used.
leaf foo {
type enumeration {
enum one;
enum two;
}
}
If extensibility of enumerated values is required, then the
'identityref' data type SHOULD be used instead of an enumeration or
other built-in type.
identity foo-type {
description "Base for the extensible type";
}
identity one {
base f:foo-type;
}
identity two {
base f:foo-type;
}
leaf foo {
type identityref {
base f:foo-type;
}
}
Note that any module can declare an identity with base "foo-type"
that is valid for the "foo" leaf. Identityref values are considered
to be qualified names.
5.11.2. Patterns and Ranges
For string data types, if a machine-readable pattern can be defined
for the desired semantics, then one or more pattern statements SHOULD
be present. A single quoted string SHOULD be used to specify the
pattern, since a double-quoted string can modify the content.
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The following typedef from [RFC6991] demonstrates the proper use of
the "pattern" statement:
typedef ipv4-address-no-zone {
type inet:ipv4-address {
pattern '[0-9\.]*';
}
...
}
For string data types, if the length of the string is required to be
bounded in all implementations, then a length statement MUST be
present.
The following typedef from [RFC6991] demonstrates the proper use of
the "length" statement:
typedef yang-identifier {
type string {
length "1..max";
pattern '[a-zA-Z_][a-zA-Z0-9\-_.]*';
pattern '.|..|[^xX].*|.[^mM].*|..[^lL].*';
}
...
}
For numeric data types, if the values allowed by the intended
semantics are different than those allowed by the unbounded intrinsic
data type (e.g., 'int32'), then a range statement SHOULD be present.
The following typedef from [RFC6991] demonstrates the proper use of
the "range" statement:
typedef dscp {
type uint8 {
range "0..63";
}
...
}
5.11.3. Enumerations and Bits
For 'enumeration' or 'bits' data types, the semantics for each 'enum'
or 'bit' SHOULD be documented. A separate description statement
(within each 'enum' or 'bit' statement) SHOULD be present.
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leaf foo {
// INCORRECT
type enumeration {
enum one;
enum two;
}
description
"The foo enum...
one: The first enum
two: The second enum";
}
leaf foo {
// CORRECT
type enumeration {
enum one {
description "The first enum";
}
enum two {
description "The second enum";
}
}
description
"The foo enum... ";
}
5.12. Reusable Type Definitions
If an appropriate derived type exists in any standard module, such as
[RFC6991], then it SHOULD be used instead of defining a new derived
type.
If an appropriate units identifier can be associated with the desired
semantics, then a units statement SHOULD be present.
If an appropriate default value can be associated with the desired
semantics, then a default statement SHOULD be present.
If a significant number of derived types are defined, and it is
anticipated that these data types will be reused by multiple modules,
then these derived types SHOULD be contained in a separate module or
submodule, to allow easier reuse without unnecessary coupling.
The description statement MUST be present.
If the type definition semantics are defined in an external document
(other than another YANG module indicated by an import statement),
then the reference statement MUST be present.
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5.13. Reusable Groupings
A reusable grouping is a YANG grouping that can be imported by
another module, and is intended for use by other modules. This is
not the same as a grouping that is used within the module it is
defined, but happens to be exportable to another module because it is
defined at the top-level of the YANG module.
The following guidelines apply to reusable groupings, in order to
make them as robust as possible:
o Clearly identify the purpose of the grouping in the "description"
statement.
o There are 5 different XPath contexts in YANG (rpc/input, rpc/
output, notification, config=true data nodes, and all data nodes).
Clearly identify which XPath contexts are applicable or excluded
for the grouping.
o Do not reference data outside the grouping in any "path", "must",
or "when" statements.
o Do not include a "default" sub-statement on a leaf or choice
unless the value applies on all possible contexts.
o Do not include a "config" sub-statement on a data node unless the
value applies on all possible contexts.
o Clearly identify any external dependencies in the grouping
"description" statement, such as nodes referenced by absolute path
from a "path", "must", or "when" statement.
5.14. Data Definitions
The description statement MUST be present in the following YANG
statements:
o anyxml
o augment
o choice
o container
o extension
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o feature
o grouping
o identity
o leaf
o leaf-list
o list
o notification
o rpc
o typedef
If the data definition semantics are defined in an external document,
(other than another YANG module indicated by an import statement),
then a reference statement MUST be present.
The 'anyxml' construct may be useful to represent an HTML banner
containing markup elements, such as '<b>' and '</b>', and
MAY be used in such cases. However, this construct SHOULD NOT be
used if other YANG data node types can be used instead to represent
the desired syntax and semantics.
It has been found that the 'anyxml' statement is not implemented
consistently across all servers. It is possible that mixed mode XML
will not be supported, or configuration anyxml nodes will not
supported.
If there are referential integrity constraints associated with the
desired semantics that can be represented with XPath, then one or
more 'must' statements SHOULD be present.
For list and leaf-list data definitions, if the number of possible
instances is required to be bounded for all implementations, then the
max-elements statements SHOULD be present.
If any 'must' or 'when' statements are used within the data
definition, then the data definition description statement SHOULD
describe the purpose of each one.
The "choice" statement is allowed to be directly present within a
"case" statement in YANG 1.1. This needs to be considered carefully.
Consider simply including the nested "choice" as additional "case"
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statements within the parent "choice" statement. Note that the
"mandatory" and "default" statements within a nested "choice"
statement only apply if the "case" containing the nested "choice"
statement is first selected.
5.15. Operation Definitions
If the operation semantics are defined in an external document (other
than another YANG module indicated by an import statement), then a
reference statement MUST be present.
If the operation impacts system behavior in some way, it SHOULD be
mentioned in the description statement.
If the operation is potentially harmful to system behavior in some
way, it MUST be mentioned in the Security Considerations section of
the document.
5.16. Notification Definitions
The description statement MUST be present.
If the notification semantics are defined in an external document
(other than another YANG module indicated by an import statement),
then a reference statement MUST be present.
If the notification refers to a specific resource instance, then this
instance SHOULD be identified in the notification data. This is
usually done by including 'leafref' leaf nodes with the key leaf
values for the resource instance. For example:
notification interface-up {
description "Sent when an interface is activated.";
leaf name {
type leafref {
path "/if:interfaces/if:interface/if:name";
}
}
}
Note that there are no formal YANG statements to identify any data
node resources associated with a notification. The description
statement for the notification SHOULD specify if and how the
notification identifies any data node resources associated with the
specific event.
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5.17. Feature Definitions
The YANG "feature" statement is used to define a label for a set of
optional functionality within a module. The "if-feature" statement
is used in the YANG statements associated with a feature.
The set of YANG features available in a module should be considered
carefully. The description-stmt within a feature-stmt MUST specify
any interactions with other features.
If there is a large set of objects associated with a YANG feature,
then consider moving those objects to a separate module, instead of
using a YANG feature. Note that the set of features within a module
is easily discovered by the reader, but the set of related modules
within the entire YANG library is not as easy to identity. Module
names with a common prefix can help readers identity the set of
related modules, but this assumes the reader will have discovered and
installed all the relevant modules.
Another consideration for deciding whether to create a new module or
add a YANG feature is the stability of the module in question. It
may be desirable to have a stable base module that is not changed
frequently. If new functionality is placed in a separate module,
then the base module does not need to be republished. If it is
designed as a YANG feature then the module will need to be
republished.
If one feature requires implementation of another feature, then an
"if-feature" statement SHOULD be used in the dependent "feature"
statement.
For example, feature2 requires implementation of feature1:
feature feature1 {
description "Some protocol feature";
}
feature feature2 {
if-feature "feature1";
description "Another protocol feature";
}
5.18. Augment Statements
The YANG "augment" statement is used to define a set of data
definition statements that will be added as child nodes of a target
data node. The module namespace for these data nodes will be the
augmenting module, not the augmented module.
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A top-level "augment" statement SHOULD NOT be used if the target data
node is in the same module or submodule as the evaluated "augment"
statement. The data definition statements SHOULD be added inline
instead.
5.18.1. Conditional Augment Statements
The "augment" statement is often used together with the "when"
statement and/or "if-feature" statement to make the augmentation
conditional on some portion of the data model.
The following example from [RFC7223] shows how a conditional
container called "ethernet" is added to the "interface" list only for
entries of the type "ethernetCsmacd".
augment "/if:interfaces/if:interface" {
when "if:type = 'ianaift:ethernetCsmacd'";
container ethernet {
leaf duplex {
...
}
}
}
5.18.2. Conditionally Mandatory Data Definition Statements
YANG has very specific rules about how configuration data can be
updated in new releases of a module. These rules allow an "old
client" to continue interoperating with a "new server".
If data nodes are added to an existing entry, the old client MUST NOT
be required to provide any mandatory parameters that were not in the
original module definition.
It is possible to add conditional augment statements such that the
old client would not know about the new condition, and would not
specify the new condition. The conditional augment statement can
contain mandatory objects only if the condition is false unless
explicitly requested by the client.
Only a conditional augment statement that uses the "when" statement
form of condition can be used in this manner. The YANG features
enabled on the server cannot be controlled by the client in any way,
so it is not safe to add mandatory augmenting data nodes based on the
"if-feature" statement.
The XPath "when" statement condition MUST NOT reference data outside
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of target data node because the client does not have any control over
this external data.
In the following dummy example, it is OK to augment the "interface"
entry with "mandatory-leaf" because the augmentation depends on
support for "some-new-iftype". The old client does not know about
this type so it would never select this type, and therefore not be
adding a mandatory data node.
module my-module {
...
identity some-new-iftype {
base iana:iana-interface-type;
}
augment "/if:interfaces/if:interface" {
when "if:type = 'mymod:some-new-iftype'";
leaf mandatory-leaf {
mandatory true;
...
}
}
}
Note that this practice is safe only for creating data resources. It
is not safe for replacing or modifying resources if the client does
not know about the new condition. The YANG data model MUST be
packaged in a way that requires the client to be aware of the
mandatory data nodes if it is aware of the condition for this data.
In the example above, the "some-new-iftype" identity is defined in
the same module as the "mandatory-leaf" data definition statement.
This practice is not safe for identities defined in a common module
such as "iana-if-type" because the client is not required to know
about "my-module" just because it knows about the "iana-if-type"
module.
5.19. Deviation Statements
The YANG "deviation" statement cannot appear in IETF YANG modules,
but it can be useful for documenting server capabilities. Deviation
statements are not reusable and typically not shared across all
platforms.
There are several reasons that deviations might be needed in an
implementation, e.g., an object cannot be supported on all platforms,
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or feature delivery is done in multiple development phases.
It is suggested that deviation statements be defined in separate
modules from regular YANG definitions. This allows the deviations to
be platform-specific and/or temporary.
The "max-elements" statement is intended to describe an architectural
limit to the number of list entries. It is not intended to describe
platform limitations. It is better to use a "deviation" statement
for the platforms that have a hard resource limit.
Example documenting platform resource limits:
Wrong: (max-elements in the list itself)
container backups {
list backup {
...
max-elements 10;
...
}
}
Correct: (max-elements in a deviation)
deviation /bk:backups/bk:backup {
deviate add {
max-elements 10;
}
}
5.20. Extension Statements
The YANG "extension" statement is used to specify external
definitions. This appears in the YANG syntax as an
"unknown-statement". Usage of extension statements in a published
module needs to be considered carefully.
The following guidelines apply to the usage of YANG extensions:
o The semantics of the extension MUST NOT contradict any YANG
statements. Extensions can add semantics not covered by the
normal YANG statements.
o The module containing the extension statement MUST clearly
identify the conformance requirements for the extension. It
should be clear whether all implementations of the YANG module
containing the extension need to also implement the extension. If
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not, identify what conditions apply that would require
implementation of the extension.
o The extension MUST clearly identify where it can be used within
other YANG statements.
o The extension MUST clearly identify if YANG statements or other
extensions are allowed or required within the extension as sub-
statements.
5.21. Data Correlation
Data can be correlated in various ways, using common data types,
common data naming, and common data organization. There are several
ways to extend the functionality of a module, based on the degree of
coupling between the old and new functionality:
o inline: update the module with new protocol-accessible objects.
The naming and data organization of the original objects is used.
The new objects are in the original module namespace.
o augment: create a new module with new protocol-accessible objects
that augment the original data structure. The naming and data
organization of the original objects is used. The new objects are
in the new module namespace.
o mirror: create new objects in a new module or the original module,
except use new a naming scheme and data location. The naming can
be coupled in different ways. Tight coupling is achieved with a
"leafref" data type, with the "require-instance" sub-statement set
to "true". This method SHOULD be used.
If the new data instances are not limited to the values in use in the
original data structure, then the "require-instance" sub-statement
MUST be set to "false". Loose coupling is achieved by using key
leafs with the same data type as the original data structure. This
has the same semantics as setting the "require-instance" sub-
statement to "false".
It is sometimes useful to separate configuration and operational
state, so that they do not not even share the exact same naming
characteristics. The correlation between configuration the
operational state data that is affected by changes in configuration
is a complex problem. There may not be a simple 1:1 relationship
between a configuration data node and an operational data node.
Further work is needed in YANG to clarify this relationship.
Protocol work may also be needed to allow a client to retrieve this
type of information from a server. At this time the best practice is
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to clearly document any relationship to other data structures in the
"description" statement.
5.22. Operational State
In YANG, any data that has a "config" statement value of "false"
could be considered operational state. The relationship between
configuration (i.e., "config" statement has a value of "true") and
operational state can be complex.
One challenge for client developers is determining if the configured
value is being used, which requires the developer to know which
operational state parameters are associated with the particular
configuration object (or group of objects).
The simplest interaction between configuration and operational state
is "none". For example, the arbitrary administrative name or
sequence number assigned to an access control rule. The configured
value is always the value that is being used by the system.
However, some configuration parameters interact with routing and
other signalling protocols, such that the operational value in use by
the system may not be the same as the configured value. Other
parameters specify the desired state, but environmental and other
factors can cause the actual state to be different.
For example a "temperature" configuration setting only represents the
desired temperature. An operational state parameter is needed that
reports the actual temperature in order to determine if the cooling
system is operating correctly. YANG has no mechanism other than the
"description" statement to associate the desired temperature and the
actual temperature.
Careful consideration needs to be given to the location of
operational state data. It can either be located within the
configuration subtree for which it applies, or it can be located
outside the particular configuration subtree. Placing operation
state within the configuration subtree is appropriate if the
operational values can only exist if the configuration exists.
The "interfaces" and "interfaces-state" subtrees defined in [RFC7223]
are an example of a complex relationship between configuration and
operational state. The operational values can include interface
entries that have been discovered or initialized by the system. An
interface may be in use that has not been configured at all.
Therefore, the operational state for an interface cannot be located
within the configuration for that same interface.
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Sometimes the configured value represents some sort of procedure to
be followed, in which the system will select an actual value, based
on protocol negotiation.
leaf duplex-admin-mode {
type enumeration {
enum auto;
enum half;
enum full;
}
}
leaf duplex-oper-mode {
config false;
type enumeration {
enum half;
enum full;
}
}
For example a "duplex" mode configuration may be "auto" to auto-
negotiate the actual value to be used. The operational parameter
will never contain the value "auto". It will always contain the
result of the auto-negotiation, such as "half" or "full". This is
just one way in which the configuration data model is not exactly the
same as the operational data model. Another is if the detailed
properties of the data are different for configured vs. learned
entries.
If all the data model properties are aligned between configuration
and operational data, then it can be useful to define the
configuration parameters within a grouping, and then replicate that
grouping within the operational state portion of the data model.
grouping parms {
// do not use config-stmt in any of the nodes
// placed in this grouping
}
container foo {
uses parms; // these are all config=true by default
state {
config false; // only exists if foo config exists
uses parms;
}
}
Note that this mechanism can also be used if the configuration and
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operational state data are in separate sub-trees:
container bar { // bar config can exist without bar-state
config true;
uses parms;
}
container bar-state { // bar-state can exist without bar
config false;
uses parms;
}
The need to replicate objects or define different operational state
objects depends on the data model. It is not possible to define one
approach that will be optimal for all data models. Designers SHOULD
describe the relationship in detail between configuration objects and
any associated operational state objects. The "description"
statements for both the configuration and the operational state
SHOULD be used for this purpose.
5.23. Performance Considerations
It is generally likely that certain YANG statements require more
runtime resources than other statements. Although there are no
performance requirements for YANG validation, the following
information MAY be considered when designing YANG data models:
o Lists are generally more expensive than containers
o "when-stmt" evaluation is generally more expensive than
"if-feature" or "choice" statements
o "must" statement is generally more expensive than "min-entries",
"max-entries", "mandatory", or "unique" statements
o "identityref" leafs are generally more expensive than
"enumeration" leafs
o "leafref" and "instance-identifier" types with "requite-instance"
set to true are generally more expensive than if
"require-instance" is set to false
5.24. YANG 1.1 Guidelines
TODO: need more input on YANG 1.1 guidelines
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5.24.1. Importing Multiple Revisions
Standard modules SHOULD NOT import multiple revisions of the same
module into a module. This MAY be done if the authors can
demonstrate that the "avoided" definitions from most recent of the
multiple revisions are somehow broken or harmful to interoperability.
5.24.2. Using Feature Logic
The YANG 1.1 feature logic is much more expressive than YANG 1.0. A
"description" statement SHOULD describe the "if-feature" logic in
text, to help readers understand the module.
YANG features SHOULD be used instead of the "when" statement, if
possible. This reduces server implementation complexity and might
reduce runtime resource requirements as well.
5.24.3. anyxml vs. anydata
The "anyxml" statement MUST NOT be used to represent a conceptual
subtree of YANG data nodes. The "anydata" statement MUST be used for
this purpose.
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6. IANA Considerations
This document registers one URI in the IETF XML registry [RFC3688].
The following registration has been made:
URI: urn:ietf:params:xml:ns:yang:ietf-template
Registrant Contact: The NETMOD WG of the IETF.
XML: N/A, the requested URI is an XML namespace.
Per this document, the following assignment has been made in the YANG
Module Names Registry for the YANG module template in Appendix C.
+-----------+-------------------------------------------+
| Field | Value |
+-----------+-------------------------------------------+
| Name | ietf-template |
| Namespace | urn:ietf:params:xml:ns:yang:ietf-template |
| Prefix | temp |
| Reference | RFC XXXX |
+-----------+-------------------------------------------+
YANG Registry Assignment
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7. Security Considerations
This document defines documentation guidelines for NETCONF content
defined with the YANG data modeling language. The guidelines for how
to write a Security Considerations section for a YANG module are
defined in the online document
http://trac.tools.ietf.org/area/ops/trac/wiki/
yang-security-guidelines
This document does not introduce any new or increased security risks
into the management system.
The following section contains the security considerations template
dated 2010-06-16. Be sure to check the webpage at the URL listed
above in case there is a more recent version available.
Each specification that defines one or more YANG modules MUST contain
a section that discusses security considerations relevant to those
modules. This section MUST be patterned after the latest approved
template (available at
http://www.ops.ietf.org/netconf/yang-security-considerations.txt).
In particular, writable data nodes that could be especially
disruptive if abused MUST be explicitly listed by name and the
associated security risks MUST be spelled out.
Similarly, readable data nodes that contain especially sensitive
information or that raise significant privacy concerns MUST be
explicitly listed by name and the reasons for the sensitivity/privacy
concerns MUST be explained.
Further, if new RPC operations have been defined, then the security
considerations of each new RPC operation MUST be explained.
7.1. Security Considerations Section Template
X. Security Considerations
The YANG module defined in this memo is designed to be accessed via
the NETCONF protocol [RFC6241]. The lowest NETCONF layer is the
secure transport layer and the mandatory-to-implement secure
transport is SSH [RFC6242].
-- if you have any writable data nodes (those are all the
-- "config true" nodes, and remember, that is the default)
-- describe their specific sensitivity or vulnerability.
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There are a number of data nodes defined in this YANG module which
are writable/creatable/deletable (i.e., config true, which is the
default). These data nodes may be considered sensitive or vulnerable
in some network environments. Write operations (e.g., edit-config)
to these data nodes without proper protection can have a negative
effect on network operations. These are the subtrees and data nodes
and their sensitivity/vulnerability:
<list subtrees and data nodes and state why they are sensitive>
-- for all YANG modules you must evaluate whether any readable data
-- nodes (those are all the "config false" nodes, but also all other
-- nodes, because they can also be read via operations like get or
-- get-config) are sensitive or vulnerable (for instance, if they
-- might reveal customer information or violate personal privacy
-- laws such as those of the European Union if exposed to
-- unauthorized parties)
Some of the readable data nodes in this YANG module may be considered
sensitive or vulnerable in some network environments. It is thus
important to control read access (e.g., via get, get-config, or
notification) to these data nodes. These are the subtrees and data
nodes and their sensitivity/vulnerability:
<list subtrees and data nodes and state why they are sensitive>
-- if your YANG module has defined any rpc operations
-- describe their specific sensitivity or vulnerability.
Some of the RPC operations in this YANG module may be considered
sensitive or vulnerable in some network environments. It is thus
important to control access to these operations. These are the
operations and their sensitivity/vulnerability:
<list RPC operations and state why they are sensitive>
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8. Acknowledgments
The structure and contents of this document are adapted from
[RFC4181], guidelines for MIB Documents, by C. M. Heard.
The working group thanks Martin Bjorklund, Juergen Schoenwaelder,
Ladislav Lhotka, and Jernej Tuljak for their extensive reviews and
contributions to this document.
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9. Changes Since RFC 6087
The following changes have been made to the guidelines published in
[RFC6087]:
o Updated NETCONF reference from RFC 4741 to RFC 6241
o Updated NETCONF over SSH citation from RFC 4742 to RFC 6242
o Updated YANG Types reference from RFC 6021 to RFC 6991
o Updated obsolete URLs for IETF resources
o Changed top-level data node guideline
o Clarified XPath usage for a literal value representing a YANG
identity
o Clarified XPath usage for a when-stmt
o Clarified XPath usage for 'proceeding-sibling' and
'following-sibling' axes
o Added terminology guidelines
o Added YANG tree diagram definition and guideline
o Updated XPath guidelines for type conversions and function library
usage.
o Updated data types section
o Updated notifications section
o Clarified conditional key leaf nodes
o Clarify usage of 'uint64' and 'int64' data types
o Added text on YANG feature usage
o Added Identifier Naming Conventions
o Clarified use of mandatory nodes with conditional augmentations
o Clarified namespace and domain conventions for example modules
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10. References
10.1. Normative References
[I-D.ietf-netmod-rfc6020bis]
Bjorklund, M., "YANG - A Data Modeling Language for the
Network Configuration Protocol (NETCONF)",
draft-ietf-netmod-rfc6020bis-07 (work in progress),
September 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2223] Postel, J. and J. Reynolds, "Instructions to RFC Authors",
RFC 2223, October 1997.
[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.
[RFC5378] Bradner, S. and J. Contreras, "Rights Contributors Provide
to the IETF Trust", BCP 78, RFC 5378, November 2008.
[RFC5741] Daigle, L., Kolkman, O., and IAB, "RFC Streams, Headers,
and Boilerplates", RFC 5741, December 2009.
[RFC6020] Bjorklund, M., "YANG - A Data Modeling Language for the
Network Configuration Protocol (NETCONF)", RFC 6020,
October 2010.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, June 2011.
[RFC6991] Schoenwaelder, J., "Common YANG Data Types", RFC 6991,
July 2013.
[W3C.REC-xpath-19991116]
Clark, J. and S. DeRose, "XML Path Language (XPath)
Version 1.0", World Wide Web Consortium
Recommendation REC-xpath-19991116, November 1999,
<http://www.w3.org/TR/1999/REC-xpath-19991116>.
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10.2. Informative References
[RFC-STYLE]
Braden, R., Ginoza, S., and A. Hagens, "RFC Document
Style", September 2009,
<http://www.rfc-editor.org/rfc-style-guide/rfc-style>.
[RFC4181] Heard, C., "Guidelines for Authors and Reviewers of MIB
Documents", BCP 111, RFC 4181, September 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC6087] Bierman, A., "Guidelines for Authors and Reviewers of YANG
Data Model Documents", RFC 6087, January 2011.
[RFC7223] Bjorklund, M., "A YANG Data Model for Interface
Management", RFC 7223, May 2014.
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Appendix A. Change Log
-- RFC Ed.: remove this section before publication.
A.1. 04 to 05
o Clarified that YANG 1.1 SHOULD be used but YANG 1.0 MAY be used if
no YANG 1.1 features needed
o Changed SHOULD follow YANG naming conventions to MUST follow (for
standards track documents only)
o Clarified module naming conventions for normative modules, example
modules, and modules from other SDOs.
o Added prefix value selection guidelines
o Added new section on guidelines for reusable groupings
o Made header guidelines less IETF-specific
o Added new section on guidelines for extension statements
o Added guidelines for nested "choice" statement within a "case"
statement
A.2. 03 ot 04
o Added sections for deviation statements and performance
considerations
o Added YANG 1.1 section
o Updated YANG reference from 1.0 to 1.1
A.3. 02 to 03
o Updated draft based on github data tracker issues added by Benoit
Clause (Issues 12 - 18)
A.4. 01 to 02
o Updated draft based on mailing list comments.
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A.5. 00 to 01
All issues from the issue tracker have been addressed.
https://github.com/netmod-wg/rfc6087bis/issues
o Issue 1: Tree Diagrams: Added Section 3 so RFCs with YANG modules
can use an Informative reference to this RFC for tree diagrams.
Updated guidelines to reference this RFC when tree diagrams are
used
o Issue 2: XPath function restrictions: Added paragraphs in XPath
usage section for 'id', 'namespace-uri', 'name', and 'lang'
functions
o Issue 3: XPath function document order issues: Added paragraph in
XPath usage section about node-set ordering for 'local-name',
'namespace-uri', 'name', 'string' and 'number' functions. Also
any function that implicitly converts a node-set to a string.
o Issue 4: XPath preceding-sibling and following-sibling: Checked
and text in XPath usage section already has proposed text from
Lada.
o Issue 5: XPath 'when-stmt' reference to descendant nodes: Added
exception and example in XPath Usage section for augmented nodes.
o Issue 6: XPath numeric conversions: Changed 'numeric expressions'
to 'numeric and boolean expressions'
o Issue 7: XPath module containment: Added sub-section on XPath
wildcards
o Issue 8: status-stmt usage: Added text to Lifecycle Management
section about transitioning from active to deprecated and then to
obsolete.
o Issue 9: resource identification in notifications: Add text to
Notifications section about identifying resources and using the
leafref data type.
o Issue 10: single quoted strings: Added text to Data Types section
about using a single-quoted string for patterns.
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Appendix B. Module Review Checklist
This section is adapted from RFC 4181.
The purpose of a YANG module review is to review the YANG module both
for technical correctness and for adherence to IETF documentation
requirements. The following checklist may be helpful when reviewing
an Internet-Draft:
o I-D Boilerplate -- verify that the draft contains the required
Internet-Draft boilerplate (see
http://www.ietf.org/id-info/guidelines.html), including the
appropriate statement to permit publication as an RFC, and that
I-D boilerplate does not contain references or section numbers.
o Abstract -- verify that the abstract does not contain references,
that it does not have a section number, and that its content
follows the guidelines in
http://www.ietf.org/id-info/guidelines.html.
o Copyright Notice -- verify that the draft has the appropriate text
regarding the rights that document contributers provide to the
IETF Trust [RFC5378]. Verify that it contains the full IETF Trust
copyright notice at the beginning of the document. The IETF Trust
Legal Provisions (TLP) can be found at:
http://trustee.ietf.org/license-info/
o Security Considerations section -- verify that the draft uses the
latest approved template from the OPS area website (http://
trac.tools.ietf.org/area/ops/trac/wiki/yang-security-guidelines)
and that the guidelines therein have been followed.
o IANA Considerations section -- this section must always be
present. For each module within the document, ensure that the
IANA Considerations section contains entries for the following
IANA registries:
XML Namespace Registry: Register the YANG module namespace.
YANG Module Registry: Register the YANG module name, prefix,
namespace, and RFC number, according to the rules specified
in [I-D.ietf-netmod-rfc6020bis].
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o References -- verify that the references are properly divided
between normative and informative references, that RFC 2119 is
included as a normative reference if the terminology defined
therein is used in the document, that all references required by
the boilerplate are present, that all YANG modules containing
imported items are cited as normative references, and that all
citations point to the most current RFCs unless there is a valid
reason to do otherwise (for example, it is OK to include an
informative reference to a previous version of a specification to
help explain a feature included for backward compatibility). Be
sure citations for all imported modules are present somewhere in
the document text (outside the YANG module).
o License -- verify that the draft contains the Simplified BSD
License in each YANG module or submodule. Some guidelines related
to this requirement are described in Section 4.1. Make sure that
the correct year is used in all copyright dates. Use the approved
text from the latest Trust Legal Provisions (TLP) document, which
can be found at:
http://trustee.ietf.org/license-info/
o Other Issues -- check for any issues mentioned in
http://www.ietf.org/id-info/checklist.html that are not covered
elsewhere.
o Technical Content -- review the actual technical content for
compliance with the guidelines in this document. The use of a
YANG module compiler is recommended when checking for syntax
errors. A list of freely available tools and other information
can be found at:
http://trac.tools.ietf.org/wg/netconf/trac/wiki
Checking for correct syntax, however, is only part of the job.
It is just as important to actually read the YANG module document
from the point of view of a potential implementor. It is
particularly important to check that description statements are
sufficiently clear and unambiguous to allow interoperable
implementations to be created.
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Appendix C. YANG Module Template
<CODE BEGINS> file "ietf-template@2010-05-18.yang"
module ietf-template {
// replace this string with a unique namespace URN value
namespace
"urn:ietf:params:xml:ns:yang:ietf-template";
// replace this string, and try to pick a unique prefix
prefix "temp";
// import statements here: e.g.,
// import ietf-yang-types { prefix yang; }
// import ietf-inet-types { prefix inet; }
// identify the IETF working group if applicable
organization
"IETF NETMOD (NETCONF Data Modeling Language) Working Group";
// update this contact statement with your info
contact
"WG Web: <http://tools.ietf.org/wg/your-wg-name/>
WG List: <mailto:your-wg-name@ietf.org>
WG Chair: your-WG-chair
<mailto:your-WG-chair@example.com>
Editor: your-name
<mailto:your-email@example.com>";
// replace the first sentence in this description statement.
// replace the copyright notice with the most recent
// version, if it has been updated since the publication
// of this document
description
"This module defines a template for other YANG modules.
Copyright (c) <insert year> IETF Trust and the persons
identified as authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or
without modification, is permitted pursuant to, and subject
to the license terms contained in, the Simplified BSD License
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set forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(http://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC XXXX; see
the RFC itself for full legal notices.";
// RFC Ed.: replace XXXX with actual RFC number and remove
// this note
reference "RFC XXXX";
// RFC Ed.: remove this note
// Note: extracted from RFC XXXX
// replace '2010-05-18' with the module publication date
// The format is (year-month-day)
revision "2010-05-18" {
description
"Initial version";
}
// extension statements
// feature statements
// identity statements
// typedef statements
// grouping statements
// data definition statements
// augment statements
// rpc statements
// notification statements
// DO NOT put deviation statements in a published module
}
<CODE ENDS>
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Author's Address
Andy Bierman
YumaWorks
Email: andy@yumaworks.com
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