One document matched: draft-ietf-netmod-yang-usage-00.xml
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<front>
<title abbrev="YANG Usage Guidelines">
Guidelines for Authors and Reviewers of YANG Data Model Documents
</title>
<author fullname="Andy Bierman" initials="A.B."
surname="Bierman">
<organization>Netconf Central</organization>
<address>
<postal>
<street></street>
<city>Simi Valley</city>
<region>CA</region>
<code></code>
<country>USA</country>
</postal>
<email>andy@netconfcentral.com</email>
</address>
</author>
<date month="May" year="2009" />
<area>Management</area>
<workgroup>Internet Engineering Task Force</workgroup>
<keyword>NETMOD</keyword>
<keyword>NETCONF</keyword>
<keyword>XML</keyword>
<keyword>YANG</keyword>
<abstract>
<t>
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 NETCONF implementations which utilize
YANG data model modules.
</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>
The standardization of network configuration interfaces for use
with the <xref target="RFC4741">NETCONF</xref> protocol
requires a modular set of data models, which can be reused
and extended over time.
</t>
<t>
This document defines a set of usage guidelines for
standards track documents containing
<xref target="I-D.ietf-netmod-yang">
YANG</xref> data models. It is similar to
the MIB usage guidelines specification
<xref target="RFC4181"/> in intent and structure.
</t>
<t>
Many YANG constructs are defined as optional to use, such as
the description clause. However, in order to
maximize interoperability of NETCONF implementations
utilizing YANG data models, it is desirable to
define a set of usage guidelines which may require
a higher level of compliance than the minimum level
defined in the YANG specification.
</t>
<t>
<figure anchor="NETCONF_stack">
<artwork>
<![CDATA[
The NETCONF stack can be conceptually partitioned into four layers.
Layer Example
+-------------+ +--------------------+ +-------------------+
(4) | Content | | Configuration data | | Notification data |
+-------------+ +--------------------+ +-------------------+
| | |
+-------------+ +-----------------+ +---------------+
(3) | Operations | | <edit-config> | | <eventType> |
+-------------+ +-----------------+ +---------------+
| | |
+-------------+ +--------------------+ +----------------+
(2) | RPC | | <rpc>, <rpc-reply> | | <notification> |
+-------------+ +--------------------+ +----------------+
| | |
+-------------+ +-----------------------------+
(1) | Transport | | BEEP, SSH, SSL, console |
| Protocol | | |
+-------------+ +-----------------------------+
]]>
</artwork>
</figure>
</t>
<t>
This document defines usage guidelines related to
the NETCONF operations layer (3), and NETCONF
content layer (4).
</t>
</section>
<section title="Terminology">
<section title="Requirements Notation">
<t>
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 <xref target="RFC2119"/>.
</t>
</section>
<section title="NETCONF Terms">
<t>
The following terms are defined in <xref target="RFC4741"/>
and are not redefined here:
<list style="symbols">
<t>agent</t>
<t>application</t>
<t>capabilities</t>
<t>manager</t>
<t>operation</t>
<t>RPC</t>
</list>
</t>
</section>
<section title="YANG Terms">
<t>
The following terms are defined in <xref target="I-D.ietf-netmod-yang"/>
and are not redefined here:
<list style="symbols">
<t>data node</t>
<t>module</t>
<t>submodule</t>
<t>namespace</t>
<t>version</t>
</list>
</t>
</section>
<section title="Terms">
<t>
The following terms are used throughout this document:
<list style="symbols">
<t>
module:
Generic term for a YANG data model module or submodule.
When describing properties which are specific to submodules,
the term 'YANG submodule', or simply 'submodule' is used instead.
</t>
<t>
Published Document:
A stable release of a module, usually contained in an RFC.
</t>
<t>
Unpublished Document:
An unstable release of a module, usually contained in
an Internet Draft.
</t>
</list>
</t>
</section>
</section>
<section title="General Documentation Guidelines" anchor="GenGuidelines">
<t>
YANG data model modules under review are likely to be
contained in Internet Drafts. All guidelines for
Internet Draft authors MUST be followed. These
guidelines are available online at:
</t>
<t>
http://www.isi.edu/in-notes/rfc-editor/instructions2authors.txt
</t>
<t>
The following sections MUST be present in an Internet Draft
containing a module:
<list style="symbols">
<t>YANG data model boilerplate section</t>
<t>Narrative sections</t>
<t>Definitions section</t>
<t>Security Considerations section</t>
<t>IANA Considerations section</t>
<t>References section</t>
</list>
</t>
<section title="YANG Data Model Boilerplate Section">
<t>
This section MUST contain a verbatim copy of the latest approved
Internet-Standard Management Framework boilerplate, which is
available on-line at [ed: URL TBD].
</t>
</section>
<section title="Narrative Sections">
<t>
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 module modules. The narrative part SHOULD include one or more
sections to briefly describe the structure of the modules defined
in the specification.
</t>
<t>
If the module(s) defined by the specification import definitions
from other modules (except for those defined in the
<xref target="I-D.ietf-netmod-yang">YANG</xref> or
<xref target="I-D.ietf-netmod-yang-types">YANG Types</xref>
documents) or are always implemented in
conjunction with other modules, then those facts MUST be noted in
the overview section, as MUST any special interpretations of objects
in other modules.
</t>
</section>
<section title="Definitions Section">
<t>
This section contains the module(s) defined by the specification.
These modules MUST be written in YANG
<xref target="I-D.ietf-netmod-yang"/>.
</t>
<t>
See <xref target="YangGuidelines"/> for guidelines on YANG usage.
</t>
</section>
<section title="Security Considerations Section">
<t>
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 [ed: URL TBD]).
</t>
<t>
In particular, writable module objects that could be especially
disruptive if abused MUST be explicitly listed by name and the
associated security risks MUST be spelled out; similarly, readable
module objects 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.
</t>
</section>
<section title="IANA Considerations Section">
<t>
In order to comply with IESG policy as set forth in
http://www.ietf.org/ID-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 what actions are required of the IANA.
</t>
<section title="Documents that Create a New Name Space">
<t>
If an Internet-Draft defines a new name space that is to be
administered by the IANA, then the document MUST include an IANA
Considerations section, specifies how the name space is to be
administered.
</t>
<t>
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 [ed: procedure TBD].
</t>
</section>
<section title="Documents that Extend an Existing Name Space">
<t>
If an Internet-Draft defines any extensions to a YANG
Namespace already administered by the IANA,
then the document MUST include an IANA
Considerations section, specifies how the name space extension
is to be administered.
</t>
<t>
Specifically, if any YANG submodule belongs-to value contained
in the document is associated with a module that contains
a namespace statement value equal to a YANG Namespace
already administered by the IANA, then a new YANG Module registry
entry and YANG Namespace Update Procedure must be requested from the
IANA [ed: procedure TBD].
</t>
</section>
</section>
<section title="Reference Sections">
<t>
[ed: 2223bis text TBD]
</t>
<t>
For every import or include statement which 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.
</t>
<t>
For every reference statement which 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.
</t>
</section>
<section title="Copyright Notices">
<t>
The proper copyright notices MUST be present in the module
description statement. [ed.: See RFC 4181, 3.7. Exact
text for insertion is TBD.]
</t>
</section>
<section title="Intellectual Property Section">
<t>
The proper IPR statements MUST be present in the document,
according to the most current Internet Draft boilerplate.
[ed.: actual IETF IPR text reference TBD]
</t>
</section>
</section>
<section title="YANG Usage Guidelines" anchor="YangGuidelines">
<t>
In general, modules in IETF standards-track specifications MUST
comply with all syntactic and semantic requirements of YANG.
<xref target="I-D.ietf-netmod-yang"/>.
The guidelines in this section are intended
to supplement the YANG specification, which is
intended to define a minimum set of conformance
requirements.
</t>
<t>
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.
</t>
<t>
Only guidelines which clarify or restrict the
minimum conformance requirements are included here.
</t>
<section title="Module Naming Conventions">
<t>
Modules contained in standards track documents
SHOULD be named with the prefix 'ietf-'.
Other types of modules MUST NOT use the 'ietf-'
prefix string.
</t>
<t>
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.
</t>
<t>
All published module names MUST be unique.
</t>
<t>
Once a module name is published, it MUST not be reused,
even if the RFC containing the module is reclassified
to 'Historic' status.
</t>
</section>
<section title="Identifiers">
<t>
Identifiers for modules, submodules, typedefs,
groupings, data objects, rpcs, and notifications
MUST be between 1 and 64 characters in length.
</t>
</section>
<section title="Defaults">
<t>
In general, it is suggested that sub-statements
containing default values SHOULD NOT be present.
For example, 'status current;',
'config true;', 'mandatory false;',
and 'max-elements unbounded;'
are common defaults which would make the module difficult
to read if used everywhere they are allowed.
</t>
<t>
Instead, it is suggested that common
statements SHOULD only be used when being set to a
value other than the default value.
</t>
</section>
<section title="Conditional Statements">
<t>
A module may be conceptually partitioned in several
ways, using the 'if-feature' and/or 'when' statements.
In addition, NETCONF capabilities are designed to
identify optional functionality.
</t>
<t>
Data model designers need to carefully consider all
modularity aspects, including the use of YANG conditional
statements.
</t>
<t>
Objects SHOULD NOT directly reference NETCONF capabilities,
in order to specify optional behavior. Instead, a 'feature'
statement
SHOULD be defined to represent the NETCONF capability,
and the 'if-feature' statement SHOULD be used within
the object definition.
</t>
<t>
If the condition associated with the desired semantics
is not dependent on any particular instance value
within the database, then an 'if-feature' statement
SHOULD be used instead of a 'when' statement.
</t>
<t>
All 'must' and 'when' statements MUST contain valid XPath.
If any name tests are present, they MUST contain
valid module prefixes and/or data node names.
</t>
<t>
The 'attribute' and 'namespace' axis SHOULD NOT be used
because the associated XML node types are not supported in YANG,
and may not be supported consistently across NETCONF
agent implementations.
</t>
<t>
The 'position' and 'last' functions SHOULD NOT be used.
Also, the 'preceding',
'preceding-sibling', 'following',
and 'following-sibling' axis SHOULD NOT be used.
These constructs rely on XML document order within a NETCONF agent
configuration database, which may not be supported
consistently or produce reliable results across implementations.
Predicate expressions based on static node
properties (e.g., name, value, ancestors,
descendants) SHOULD be used instead.
</t>
<t>
Implicit 'position' function calls within predicates
SHOULD NOT be used. (e.g., //chapter[42]).
</t>
<t>
Data nodes which use the 'int64' and 'uint64' built-in
type SHOULD NOT be used within relational expressions.
There are boundary conditions in which the translation
from the YANG 64-bit type to an XPath number can cause
incorrect results.
</t>
<t>
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.
</t>
<t>
Explicit XPath data type conversions MAY be used
(e.g., 'string', 'boolean', or 'number' functions),
instead of implicit XPath data type conversions.
</t>
</section>
<section title="Module Life-cycle Management">
<t>
The status statement SHOULD NOT be present if its value
is 'current'. It MUST be present if its value
is 'deprecated' or 'obsolete'.
</t>
<t>
The module or submodule name MUST NOT be changed, once
the document containing the module or submodule is published.
</t>
<t>
The module namespace URI value SHOULD NOT be changed,
once the document containing the module is published.
</t>
<t>
The revision-date sub-statement (within the imports
statement) SHOULD be present.
It MUST be present (in all published modules) if any
groupings are used from the external module.
</t>
<t>
The revision-date sub-statement (within the include
statement) MAY be present.
It SHOULD be present (in all published modules) if any
groupings are used from the external sub-module.
</t>
</section>
<section title="Header Contents">
<t>
<list style="symbols">
<t>
The namespace MUST be a globally unique
URI, as defined in <xref target="RFC3986"/>.
This value is usually assigned by the IANA.
</t>
<t>
Until a URI is assigned by the IANA,
a temporary namespace URI
MAY be selected which is not likely to
collide with other YANG namespaces, such as
the filename of the Internet Draft containing
the module. This value MUST be a
valid URI (e.g., 'file:///draft-ietf-foo-bar-00').
</t>
<t>
The organization statement MUST be present.
</t>
<t>
The contact statement MUST be present.
</t>
<t>
The description statement MUST be present.
</t>
<t>
If the module represents a model defined in one or more
external documents, then a reference statement MUST
be present.
</t>
<t>
A revision statement MUST be present for each published
version of the module.
</t>
<t>
Each new revision MUST include a revision date which
is higher than any other revision date in the module.
</t>
<t>
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-published.
</t>
</list>
</t>
</section>
<section title="Data Types">
<t>
<list style="symbols">
<t>
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.
</t>
<t>
Data model designers SHOULD use the most appropriate
built-in data type for the particular application.
</t>
<t>
If extensibility of enumerated values is required,
then the identityref data type SHOULD be used
instead of an enumeration or other built-in type.
</t>
<t>
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.
</t>
<t>
For string data types, if the length of the string
is not required to be unbounded in all implementations,
then a length statement SHOULD be present.
[ed: should the 'resource-denied' error be mentioned here?]
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
</list>
</t>
</section>
<section title="Reusable Type Definitions">
<t>
<list style="symbols">
<t>
If an appropriate derived type exists in any
standard module, such as <xref target="I-D.ietf-netmod-yang-types"/>,
then it SHOULD be used instead of defining a new derived type.
</t>
<t>
If an appropriate units identifier can be associated
with the desired semantics, then a units statement
SHOULD be present.
</t>
<t>
If an appropriate default value can be associated
with the desired semantics, then a default statement
SHOULD be present.
</t>
<t>
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.
</t>
<t>
The description statement MUST be present.
</t>
<t>
If the type definition semantics are defined
in an external document, then the reference
statement SHOULD be present.
</t>
</list>
</t>
</section>
<section title="Object Definitions">
<t>
<list style="symbols">
<t>
The description statement MUST be present in the following
body statements:
<list style="symbols">
<t>extension</t>
<t>feature</t>
<t>identity</t>
<t>typedef</t>
<t>grouping</t>
<t>augment</t>
<t>rpc</t>
<t>notification</t>
</list>
</t>
<t>
The description statement MUST be present in the following
data definition constructs:
<list style="symbols">
<t>container</t>
<t>leaf</t>
<t>leaf-list</t>
<t>list</t>
<t>choice</t>
<t>anyxml</t>
</list>
</t>
<t>
If the object semantics are defined in an external document,
then a reference statement SHOULD be present.
</t>
<t>
The 'anyxml' construct MUST NOT be used within
configuration data.
</t>
<t>
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.
</t>
<t>
For list and leaf-list objects, if the number of possible instances
is not required to be unbounded for all implementations,
then the max-elements statement SHOULD be present.
</t>
<t>
If any must or when statements are used within the
object definition, then the object description statement
SHOULD describe the purpose of each one.
</t>
</list>
</t>
</section>
<section title="RPC Definitions">
<t>
<list style="symbols">
<t>
The description statement MUST be present.
</t>
<t>
If the RPC method semantics are defined in an external document,
then a reference statement SHOULD be present.
</t>
<t>
If the RPC method impacts system behavior in some way,
it SHOULD be mentioned in the description statement.
</t>
<t>
If the RPC method is potentially harmful to system
behavior in some way,
it MUST be mentioned in the Security Considerations
section of the document.
</t>
</list>
</t>
</section>
<section title="Notification Definitions">
<t>
<list style="symbols">
<t>
The description statement MUST be present.
</t>
<t>
If the notification semantics are defined in an external document,
then a reference statement SHOULD be present.
</t>
</list>
</t>
</section>
</section>
<!-- Possibly a 'Contributors' section ... -->
<section anchor="IANA" title="IANA Considerations">
<t>
There are no actions requested of IANA at this time.
</t>
</section>
<section anchor="Security" title="Security Considerations">
<t>
This document defines documentation guidelines for
NETCONF content defined with the YANG data modeling
language. It does not introduce
any new or increased security risks into
the management system.
[ed: RFC 4181 style security section TBD]
</t>
</section>
<section title="Acknowledgments">
<t>
The structure and contents of this document are adapted from
<xref target="RFC4181">
Guidelines for MIB Documents
</xref>, by C. M. Heard.
</t>
</section>
</middle>
<!-- ***** BACK MATTER ***** -->
<back>
<references title="Normative References">
&rfc2119;
&rfc3986;
&rfc4741;
&yangspec;
&yangtypes;
</references>
<references title="Informative References">
&rfc4181;
</references>
<section title="Module Review Checklist">
<t>
This section is adapted from RFC 4181.
</t>
<t>
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
a draft document:
</t>
<t>
<list style="numbers">
<t>
I-D Boilerplate --
verify that the draft contains the required
Internet-Draft boilerplate
(see http://www.ietf.org/ietf/1id-guidelines.txt),
including the appropriate statement to permit
publication as an RFC, and that I-D boilerplate does
not contain references or section numbers.
</t>
<t>
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/ietf/1id-guidelines.txt.
</t>
<t>
YANG Module Boilerplate --
verify that the draft contains the latest
approved SNMP Network Management Framework boilerplate from the OPS
area web site (http://www.ops.ietf.org/mib-boilerplate.html).
[ed: real URL TBD]
</t>
<t>
Security Considerations Section
-- verify that the draft uses the
latest approved template from the OPS area web site
(http://www.ops.ietf.org/mib-security.html) and that the guidelines
therein have been followed.
</t>
<t>
IANA Considerations Section
-- this section must always be
present. If the draft requires no action from the IANA, ensure that
this is explicitly noted. If the draft requires URI values to be
assigned, ensure that the IANA Considerations section contains the
information specified in [TBD] of these guidelines. If the
draft contains the initial version of an IANA-maintained module,
verify that the [TBD] invocation contains maintenance
instructions that comply with the requirements in RFC 2434. In the
latter case, the IANA Considerations section that will appear in the
RFC MUST contain a pointer to the actual IANA-maintained module.
</t>
<t>
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).
</t>
<t>
Copyright Notices
-- verify that the draft contains an
abbreviated copyright notice in the description statement of each
YANG module or sub-module, and that it contains the full copyright
notice and disclaimer specified in Sections 5.4 and 5.5 of RFC 3978
at the end of the document. Make sure that the correct year is used
in all copyright dates.
</t>
<t>
IPR Notice
-- if the draft does not contains a verbatim copy of
the IPR notice specified in Section 5 of RFC 3979, recommend that the
IPR notice be included.
</t>
<t>
Other Issues
-- check for any issues mentioned in
http://www.ietf.org/ID-Checklist.html that are not covered elsewhere.
</t>
<t>
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; see
[YANG tool URL TBD] for more information.
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.
</t>
</list>
</t>
</section>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-24 01:06:52 |