Internet Engineering Task Force A. Bierman
Internet-Draft Netconf Central
Intended status: Informational May 18, 2009
Expires: November 19, 2009
Guidelines for Authors and Reviewers of YANG Data Model Documents
draft-ietf-netmod-yang-usage-00
Status of this Memo
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Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 NETCONF
implementations which utilize YANG data model modules.
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 . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. General Documentation Guidelines . . . . . . . . . . . . . . . 7
3.1. YANG Data Model Boilerplate Section . . . . . . . . . . . 7
3.2. Narrative Sections . . . . . . . . . . . . . . . . . . . . 7
3.3. Definitions Section . . . . . . . . . . . . . . . . . . . 8
3.4. Security Considerations Section . . . . . . . . . . . . . 8
3.5. IANA Considerations Section . . . . . . . . . . . . . . . 8
3.5.1. Documents that Create a New Name Space . . . . . . . . 8
3.5.2. Documents that Extend an Existing Name Space . . . . . 8
3.6. Reference Sections . . . . . . . . . . . . . . . . . . . . 9
3.7. Copyright Notices . . . . . . . . . . . . . . . . . . . . 9
3.8. Intellectual Property Section . . . . . . . . . . . . . . 9
4. YANG Usage Guidelines . . . . . . . . . . . . . . . . . . . . 10
4.1. Module Naming Conventions . . . . . . . . . . . . . . . . 10
4.2. Identifiers . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Defaults . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4. Conditional Statements . . . . . . . . . . . . . . . . . . 11
4.5. Module Life-cycle Management . . . . . . . . . . . . . . . 12
4.6. Header Contents . . . . . . . . . . . . . . . . . . . . . 12
4.7. Data Types . . . . . . . . . . . . . . . . . . . . . . . . 13
4.8. Reusable Type Definitions . . . . . . . . . . . . . . . . 13
4.9. Object Definitions . . . . . . . . . . . . . . . . . . . . 14
4.10. RPC Definitions . . . . . . . . . . . . . . . . . . . . . 15
4.11. Notification Definitions . . . . . . . . . . . . . . . . . 15
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. Normative References . . . . . . . . . . . . . . . . . . . 19
8.2. Informative References . . . . . . . . . . . . . . . . . . 19
Appendix A. Module Review Checklist . . . . . . . . . . . . . . . 20
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Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
The standardization of network configuration interfaces for use with
the NETCONF [RFC4741] protocol 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 YANG [I-D.ietf-netmod-yang] data models. It is
similar to the MIB usage guidelines specification [RFC4181] in intent
and structure.
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.
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 | | |
+-------------+ +-----------------------------+
Figure 1
This document defines usage guidelines related to the NETCONF
operations layer (3), and NETCONF content layer (4).
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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].
2.2. NETCONF Terms
The following terms are defined in [RFC4741] and are not redefined
here:
o agent
o application
o capabilities
o manager
o operation
o RPC
2.3. YANG Terms
The following terms are defined in [I-D.ietf-netmod-yang] and are not
redefined here:
o data node
o module
o submodule
o namespace
o version
2.4. Terms
The following terms are used throughout this document:
o 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.
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o Published Document: A stable release of a module, usually
contained in an RFC.
o Unpublished Document: An unstable release of a module, usually
contained in an Internet Draft.
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3. 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. These guidelines are available online at:
http://www.isi.edu/in-notes/rfc-editor/instructions2authors.txt
The following sections MUST be present in an Internet Draft
containing a module:
o YANG data model boilerplate section
o Narrative sections
o Definitions section
o Security Considerations section
o IANA Considerations section
o References section
3.1. YANG Data Model Boilerplate Section
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].
3.2. 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 module 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 import definitions from
other modules (except for those defined in the YANG
[I-D.ietf-netmod-yang] or YANG Types [I-D.ietf-netmod-yang-types]
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.
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3.3. Definitions Section
This section contains the module(s) defined by the specification.
These modules MUST be written in YANG [I-D.ietf-netmod-yang].
See Section 4 for guidelines on YANG usage.
3.4. 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 [ed: URL TBD]).
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.
3.5. IANA Considerations Section
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.
3.5.1. Documents that Create a New Name Space
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.
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].
3.5.2. Documents that Extend an Existing Name Space
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.
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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].
3.6. Reference Sections
[ed: 2223bis text TBD]
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.
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.
3.7. Copyright Notices
The proper copyright notices MUST be present in the module
description statement. [ed.: See RFC 4181, 3.7. Exact text for
insertion is TBD.]
3.8. Intellectual Property Section
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]
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4. 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-yang]. 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 which clarify or restrict the minimum conformance
requirements are included here.
4.1. Module Naming Conventions
Modules contained in standards track documents SHOULD be named with
the prefix 'ietf-'. Other types of modules MUST NOT use the 'ietf-'
prefix string.
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.
Once a module name is published, it MUST not be reused, even if the
RFC containing the module is reclassified to 'Historic' status.
4.2. Identifiers
Identifiers for modules, submodules, typedefs, groupings, data
objects, rpcs, and notifications MUST be between 1 and 64 characters
in length.
4.3. Defaults
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.
Instead, it is suggested that common statements SHOULD only be used
when being set to a value other than the default value.
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4.4. Conditional Statements
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.
Data model designers need to carefully consider all modularity
aspects, including the use of YANG conditional statements.
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.
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.
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.
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.
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.
Implicit 'position' function calls within predicates SHOULD NOT be
used. (e.g., //chapter[42]).
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.
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.
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Explicit XPath data type conversions MAY be used (e.g., 'string',
'boolean', or 'number' functions), instead of implicit XPath data
type conversions.
4.5. Module Life-cycle Management
The status statement SHOULD NOT be present if its value is 'current'.
It MUST be present if its value is 'deprecated' or '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 SHOULD NOT be changed, once the
document containing the module is published.
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.
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.
4.6. Header Contents
o The namespace MUST be a globally unique URI, as defined in
[RFC3986]. This value is usually assigned by the IANA.
o 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').
o The organization statement MUST be present.
o The contact statement MUST be present.
o The description statement MUST be present.
o If the module represents a model defined in one or more external
documents, then a reference statement MUST be present.
o A revision statement MUST be present for each published version of
the module.
o Each new revision MUST include a revision date which is higher
than any other revision date in the module.
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o 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.
4.7. Data Types
o 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.
o Data model designers SHOULD use the most appropriate built-in data
type for the particular application.
o If extensibility of enumerated values is required, then the
identityref data type SHOULD be used instead of an enumeration or
other built-in type.
o 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.
o 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?]
o 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.
o 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.
o 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.
4.8. Reusable Type Definitions
o If an appropriate derived type exists in any standard module, such
as [I-D.ietf-netmod-yang-types], then it SHOULD be used instead of
defining a new derived type.
o If an appropriate units identifier can be associated with the
desired semantics, then a units statement SHOULD be present.
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o If an appropriate default value can be associated with the desired
semantics, then a default statement SHOULD be present.
o 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.
o The description statement MUST be present.
o If the type definition semantics are defined in an external
document, then the reference statement SHOULD be present.
4.9. Object Definitions
o The description statement MUST be present in the following body
statements:
* extension
* feature
* identity
* typedef
* grouping
* augment
* rpc
* notification
o The description statement MUST be present in the following data
definition constructs:
* container
* leaf
* leaf-list
* list
* choice
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* anyxml
o If the object semantics are defined in an external document, then
a reference statement SHOULD be present.
o The 'anyxml' construct MUST NOT be used within configuration data.
o 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.
o 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.
o If any must or when statements are used within the object
definition, then the object description statement SHOULD describe
the purpose of each one.
4.10. RPC Definitions
o The description statement MUST be present.
o If the RPC method semantics are defined in an external document,
then a reference statement SHOULD be present.
o If the RPC method impacts system behavior in some way, it SHOULD
be mentioned in the description statement.
o 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.
4.11. Notification Definitions
o The description statement MUST be present.
o If the notification semantics are defined in an external document,
then a reference statement SHOULD be present.
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5. IANA Considerations
There are no actions requested of IANA at this time.
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6. Security Considerations
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]
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7. Acknowledgments
The structure and contents of this document are adapted from
Guidelines for MIB Documents [RFC4181], by C. M. Heard.
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4741] Enns, R., "NETCONF Configuration Protocol", RFC 4741,
December 2006.
[I-D.ietf-netmod-yang]
Bjorklund, M., "YANG - A data modeling language for
NETCONF", draft-ietf-netmod-yang-05 (work in progress),
April 2009.
[I-D.ietf-netmod-yang-types]
Schoenwaelder, J., "Common YANG Data Types",
draft-ietf-netmod-yang-types-03 (work in progress),
May 2009.
8.2. Informative References
[RFC4181] Heard, C., "Guidelines for Authors and Reviewers of MIB
Documents", BCP 111, RFC 4181, September 2005.
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Appendix A. 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
a draft document:
1. 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.
2. 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.
3. 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]
4. 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.
5. 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.
6. 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
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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).
7. 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.
8. 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.
9. Other Issues -- check for any issues mentioned in
http://www.ietf.org/ID-Checklist.html that are not covered
elsewhere.
10. 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.
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Author's Address
Andy Bierman
Netconf Central
Simi Valley, CA
USA
Email: andy@netconfcentral.com
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