Guidelines for YANG Documents

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.

Status of this Memo

This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). 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 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 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

1. Introduction
2. Terminology
    2.1. Requirements Notation
    2.2. NETCONF Terms
    2.3. YANG Terms
    2.4. Terms
3. General Documentation Guidelines
    3.1. Module Copyright
    3.2. Narrative Sections
    3.3. Definitions Section
    3.4. Security Considerations Section
    3.5. IANA Considerations Section
        3.5.1. Documents that Create a New Name Space
        3.5.2. Documents that Extend an Existing Name Space
    3.6. Reference Sections
    3.7. Intellectual Property Section
4. YANG Usage Guidelines
    4.1. Module Naming Conventions
    4.2. Identifiers
    4.3. Defaults
    4.4. Conditional Statements
    4.5. XPath Usage
    4.6. Lifecycle Management
    4.7. Module Header, Meta, and Revision Statements
    4.8. Namespace Assignments
    4.9. Top Level Data Definitions
    4.10. Data Types
    4.11. Reusable Type Definitions
    4.12. Data Definitions
    4.13. Operation Definitions
    4.14. Notification Definitions
5. IANA Considerations
6. Security Considerations
7. Acknowledgments
8. References
    8.1. Normative References
    8.2. Informative References
Appendix A. Module Review Checklist
Appendix B. YANG Module Template
Appendix C. Change Log
    C.1. Changes from 05 to 06
    C.2. Changes from 04 to 05
    C.3. Changes from 03 to 04
    C.4. Changes from 02 to 03
    C.5. Changes from 01 to 02
    C.6. Changes from 00 to 01
§ Author's Address

1. Introduction

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 which 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 which 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 [RFC4741] (Enns, R., “NETCONF Configuration Protocol,” December 2006.). These guidelines are intended to be used by authors and reviewers to improve the readability and interoperability of published YANG data models.

2. Terminology

2.1. Requirements Notation

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

2.3. YANG Terms

Note that the term 'module' may be used as a generic term for a YANG module or submodule. When describing properties which are specific to submodules, the term 'submodule' is used instead.

2.4. Terms

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.rfc-editor.org/rfc-editor/instructions2authors.txt

The following sections MUST be present in an Internet-Draft containing a module:

3.1. Module Copyright

The module description statement MUST contain a reference to the latest approved IETF Trust Copyright statement, which is available on-line 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.

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 YANG modules. The narrative part SHOULD include one or more sections to briefly describe the structure of the modules defined in the specification.

3.3. Definitions Section

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 http://www.ops.ietf.org/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.

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 which has action items for IANA MUST contain an IANA Considerations section. The requirements for this section vary depending what actions are required of the IANA. If there are no IANA considerations applicable to the document, then the IANA Considerations section is not required. Refer to the guidelines in [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.) for more details.

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, that 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. The YANG specification includes the procedure for this purpose in its IANA Considerations section.

3.5.2. Documents that Extend an Existing Name Space

It is possible to extend an existing namespace using a YANG submodule which 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.

3.6. Reference Sections

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 normative 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. 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.

3.7. Intellectual Property Section

The proper IPR statements MUST be present in the document, according to the most current Internet-Draft boilerplate. Refer to the IETF Trust Legal Provision for the exact legal text that needs to be included.

4. YANG Usage Guidelines

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

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.

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

4.3. Defaults

In general, it is suggested that sub-statements containing very common default values SHOULD NOT be present. The following sub-statements are commonly used with the default value, which would make the module difficult to read if used everywhere they are allowed.

4.4. 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 the NETCONF capability is supported within the data model.

4.5. XPath Usage

The 'attribute' and 'namespace' axes are not supported in YANG, and MAY be empty in a NETCONF server implementation.

The 'position' and 'last' functions MAY be used with caution. A server is not required to maintain any particular XML document order for system-ordered data nodes. 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', 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., name, value, ancestors, descendants) SHOULD be used instead.

The 'preceding-sibling' and 'following-sibling' axes MAY be used, with caution. A server is not required to maintain a persistent or deterministic XML document order, which will affect use of these axes.

Implicit 'position' function calls within predicates MAY be used with caution. (e.g., //chapter[42]). Note that a NETCONF server is only required to maintain relative document order for related instances of a user-ordered list or leaf-list data definition (i.e., 'ordered-by' statement set to 'user').

Data nodes which use the 'int64' and 'uint64' built-in type MAY be used with caution, 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. 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.

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.

4.6. Lifecycle Management

The status statement 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 MUST NOT be changed, once the document containing the module is published.

The revision-date sub-statement within the imports statement SHOULD be present if any groupings are used from the external module.

The revision-date sub-statement within the include statement SHOULD be present if any groupings are used from the external sub-module.

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 which is (directly or indirectly) included by the main module.

4.7. Module Header, Meta, and Revision Statements

The organization statement MUST be present. If the module is contained in a documented intended for standards-track status, then the organization SHOULD be the IETF working group chartered to write the document.

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.

Statement	Default Value
config	true
mandatory	false
max-elements	unbounded
min-elements	0
ordered-by	system
status	current
yin-element	false

The description statement MUST be present. The appropriate IETF Trust Copyright text MUST be present, as described in Section 3.1 (Module Copyright).

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 which 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.

Each new revision MUST include a revision date which is higher than any other revision date in the module.

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.

4.8. Namespace Assignments

It is RECOMMENDED that only valid YANG modules are included in documents, whether they are published yet or not. This allows:

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 which 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 temporary namespace statement values for standards-track modules:

The following examples of non-standards track modules are only suggestions. There are no guidelines for this type of URN in this document:

4.9. Top Level Data Definitions

There SHOULD only be one top-level data node defined in each YANG module, if any data nodes are defined at all.

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 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.

4.10. 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.

If extensibility of enumerated values is required, then the identityref data type SHOULD be used instead of an enumeration or other built-in type.

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.

For string data types, if the length of the string is required to bounded in all implementations, then a length statement MUST be present.

For string data types, data definition semantics SHOULD NOT rely on preservation of leading and trailing whitespace characters.

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 signed numeric data types (i.e., 'int8', 'int16', 'int32', and 'int64') SHOULD NOT be used unless negative values are allowed for the desired semantics.

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.

4.11. Reusable Type Definitions

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.

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.

4.12. Data Definitions

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 used with caution within configuration data. This may be useful to represent an HTML banner containing markup elements, such as <b> and </b>. However, this construct SHOULD NOT be used if other YANG data node types can be used instead to represent the desired syntax and semantics.

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.

4.13. 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.

4.14. Notification Definitions

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.

5. IANA Considerations

This document registers one URI in the IETF XML registry [RFC3688] (Mealling, M., “The IETF XML Registry,” January 2004.). The following registration is requested:

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.

This document requests the following assignment in the YANG Module Names Registry for the YANG module template in Appendix B (YANG Module Template).

6. Security Considerations

Field	Value
name	ietf-template
namespace	urn:ietf:params:xml:ns:yang:ietf-template
prefix	temp
reference	RFCXXXX

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://www.ops.ietf.org/yang-security-considerations.txt

This document does not introduce any new or increased security risks into the management system.

7. Acknowledgments

The working group thanks Martin Bjorklund and Juergen Schoenwaelder for their extensive reviews and contributions to this document.

8. References

8.1. Normative References

8.2. Informative References

Appendix A. Module Review Checklist

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:

Appendix B. 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) 2010 IETF Trust and the persons identified as
      the document authors.  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
      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 draft-ietf-netmod-yang-usage-04.txt


    // 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>

[RFC2119]	Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).
[RFC3688]	Mealling, M., “The IETF XML Registry,” BCP 81, RFC 3688, January 2004 (TXT).
[RFC3986]	Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” STD 66, RFC 3986, January 2005 (TXT, HTML, XML).
[RFC4741]	Enns, R., “NETCONF Configuration Protocol,” RFC 4741, December 2006 (TXT).
[RFC5378]	Bradner, S. and J. Contreras, “Rights Contributors Provide to the IETF Trust,” BCP 78, RFC 5378, November 2008 (TXT).
[W3C.REC-xpath-19991116]	DeRose, S. and J. Clark, “XML Path Language (XPath) Version 1.0,” World Wide Web Consortium Recommendation REC-xpath-19991116, November 1999 (HTML).
[I-D.ietf-netmod-yang]	Bjorklund, M., “YANG - A data modeling language for the Network Configuration Protocol (NETCONF),” draft-ietf-netmod-yang-13 (work in progress), June 2010 (TXT).
[I-D.ietf-netmod-yang-types]	Schoenwaelder, J., “Common YANG Data Types,” draft-ietf-netmod-yang-types-09 (work in progress), April 2010 (TXT).

[RFC4181]	Heard, C., “Guidelines for Authors and Reviewers of MIB Documents,” BCP 111, RFC 4181, September 2005 (TXT).
[RFC5226]	Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” BCP 26, RFC 5226, May 2008 (TXT).

	Andy Bierman
	InterWorking Labs
Email:	andyb@iwl.com

Guidelines for YANG Documentsdraft-ietf-netmod-yang-usage-06

Abstract