RFC 8407
Guidelines for Authors and Reviewers of Documents Containing YANG Data Models
Guidelines for Authors and Reviewers of Documents Containing YANG Data Models 🎀
part 1 of 11 🧚
Okay bestie, let's start with the title block 💅. This doc is RFC 8407, made by the Internet Engineering Task Force (IETF). A. Bierman from YumaWorks wrote it, and it was published in October 2018 🍓. It's a Best Current Practice (that's the category), and it's labelled BCP 216 and ISSN 2070-1721 🔥. It also obsoletes RFC 6087, which means this one officially replaces it 😭. The header doesn't list any Updates, so it doesn't touch anything else.
Abstract ✨
So the abstract is basically the TL;DR 🦋. This memo gives guidelines for authors and reviewers of specs that contain YANG modules. It defines recommendations and procedures meant to make Network Configuration Protocol (NETCONF) and RESTCONF implementations that use YANG modules more interoperable and easier to use 💜. And yep, it says again that it obsoletes RFC 6087 💀.
Status of This Memo 🌈
Okay so this part is the official stuff, but it's giving "this is a real standard" energy 👑. This memo documents an Internet Best Current Practice.
This document is a product of the IETF. It represents the consensus of the IETF community, it got public review, and the Internet Engineering Steering Group (IESG) approved it for publication ⭐. If you want more about BCPs, check out Section 2 of RFC 7841 🧚.
For the current status, any errata, and how to send feedback, go to https://www.rfc-editor.org/info/rfc8407 🫶.
Copyright Notice 💖
Copyright (c) 2018 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, which you can read at https://trustee.ietf.org/license-info as they were on the publication date 📜. Please actually read them, because they explain what you're allowed to do and what you're not 🙈. Code Components pulled out of this document must include the Simplified BSD License text, as described in Section 4.e of the Trust Legal Provisions, and they come with no warranty, as the Simplified BSD License says 💅.
Table of Contents 🦋
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Changes since RFC 6087 . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. NETCONF Terms . . . . . . . . . . . . . . . . . . . . . . 7
2.2. YANG Terms . . . . . . . . . . . . . . . . . . . . . . . 7
2.3. NMDA Terms . . . . . . . . . . . . . . . . . . . . . . . 7
2.4. Requirements Notation . . . . . . . . . . . . . . . . . . 8
3. General Documentation Guidelines . . . . . . . . . . . . . . 8
3.1. Module Copyright . . . . . . . . . . . . . . . . . . . . 9
3.2. Code Components . . . . . . . . . . . . . . . . . . . . . 9
3.2.1. Example Modules . . . . . . . . . . . . . . . . . . . 9
3.3. Terminology Section . . . . . . . . . . . . . . . . . . . 10
3.4. Tree Diagrams . . . . . . . . . . . . . . . . . . . . . . 10
3.5. Narrative Sections . . . . . . . . . . . . . . . . . . . 10
3.6. Definitions Section . . . . . . . . . . . . . . . . . . . 11
3.7. Security Considerations Section . . . . . . . . . . . . . 11
3.7.1. Security Considerations Section Template . . . . . . 12
3.8. IANA Considerations Section . . . . . . . . . . . . . . . 13
3.8.1. Documents That Create a New Namespace . . . . . . . . 14
3.8.2. Documents That Extend an Existing Namespace . . . . . 14
3.9. References Sections . . . . . . . . . . . . . . . . . . . 14
3.10. Validation Tools . . . . . . . . . . . . . . . . . . . . 14
3.11. Module Extraction Tools . . . . . . . . . . . . . . . . . 15
3.12. Module Usage Examples . . . . . . . . . . . . . . . . . . 15
4. YANG Usage Guidelines . . . . . . . . . . . . . . . . . . . . 15
4.1. Module Naming Conventions . . . . . . . . . . . . . . . . 16
4.2. Prefixes . . . . . . . . . . . . . . . . . . . . . . . . 17
4.3. Identifiers . . . . . . . . . . . . . . . . . . . . . . . 18
4.3.1. Identifier Naming Conventions . . . . . . . . . . . . 18
4.4. Defaults . . . . . . . . . . . . . . . . . . . . . . . . 19
4.5. Conditional Statements . . . . . . . . . . . . . . . . . 19
4.6. XPath Usage . . . . . . . . . . . . . . . . . . . . . . . 20
4.6.1. XPath Evaluation Contexts . . . . . . . . . . . . . . 20
4.6.2. Function Library . . . . . . . . . . . . . . . . . . 21
4.6.3. Axes . . . . . . . . . . . . . . . . . . . . . . . . 22
4.6.4. Types . . . . . . . . . . . . . . . . . . . . . . . . 23
4.6.5. Wildcards . . . . . . . . . . . . . . . . . . . . . . 24
4.6.6. Boolean Expressions . . . . . . . . . . . . . . . . . 24
4.7. YANG Definition Lifecycle Management . . . . . . . . . . 25
4.8. Module Header, Meta, and Revision Statements . . . . . . 26
4.9. Namespace Assignments . . . . . . . . . . . . . . . . . . 28
4.10. Top-Level Data Definitions . . . . . . . . . . . . . . . 29
4.11. Data Types . . . . . . . . . . . . . . . . . . . . . . . 30
4.11.1. Fixed-Value Extensibility . . . . . . . . . . . . . 30
4.11.2. Patterns and Ranges . . . . . . . . . . . . . . . . 31
4.11.3. Enumerations and Bits . . . . . . . . . . . . . . . 32
4.11.4. Union Types . . . . . . . . . . . . . . . . . . . . 33
4.11.5. Empty and Boolean . . . . . . . . . . . . . . . . . 34
4.12. Reusable Type Definitions . . . . . . . . . . . . . . . . 35
4.13. Reusable Groupings . . . . . . . . . . . . . . . . . . . 35
4.14. Data Definitions . . . . . . . . . . . . . . . . . . . . 36
4.14.1. Non-Presence Containers . . . . . . . . . . . . . . 38
4.14.2. Top-Level Data Nodes . . . . . . . . . . . . . . . . 38
4.15. Operation Definitions . . . . . . . . . . . . . . . . . . 39
4.16. Notification Definitions . . . . . . . . . . . . . . . . 39
4.17. Feature Definitions . . . . . . . . . . . . . . . . . . . 40
4.18. YANG Data Node Constraints . . . . . . . . . . . . . . . 41
4.18.1. Controlling Quantity . . . . . . . . . . . . . . . . 41
4.18.2. "must" versus "when" . . . . . . . . . . . . . . . . 41
4.19. "augment" Statements . . . . . . . . . . . . . . . . . . 41
4.19.1. Conditional Augment Statements . . . . . . . . . . . 41
4.19.2. Conditionally Mandatory Data Definition Statements . 42
4.20. Deviation Statements . . . . . . . . . . . . . . . . . . 43
4.21. Extension Statements . . . . . . . . . . . . . . . . . . 44
4.22. Data Correlation . . . . . . . . . . . . . . . . . . . . 45
4.22.1. Use of "leafref" for Key Correlation . . . . . . . . 46
4.23. Operational State . . . . . . . . . . . . . . . . . . . . 47
4.23.1. Combining Operational State and Configuration Data . 47
4.23.2. Representing Operational Values of Configuration
Data . . . . . . . . . . . . . . . . . . . . . . . . 47
4.23.3. NMDA Transition Guidelines . . . . . . . . . . . . . 48
4.24. Performance Considerations . . . . . . . . . . . . . . . 52
4.25. Open Systems Considerations . . . . . . . . . . . . . . . 52
4.26. Guidelines for Constructs Specific to YANG 1.1 . . . . . 53
4.26.1. Importing Multiple Revisions . . . . . . . . . . . . 53
4.26.2. Using Feature Logic . . . . . . . . . . . . . . . . 53
4.26.3. "anyxml" versus "anydata" . . . . . . . . . . . . . 53
4.26.4. "action" versus "rpc" . . . . . . . . . . . . . . . 53
4.27. Updating YANG Modules (Published versus Unpublished) . . 54
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 55
6. Security Considerations . . . . . . . . . . . . . . . . . . . 55
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 56
7.1. Normative References . . . . . . . . . . . . . . . . . . 56
7.2. Informative References . . . . . . . . . . . . . . . . . 57
Appendix A. Module Review Checklist . . . . . . . . . . . . . . 59
Appendix B. YANG Module Template . . . . . . . . . . . . . . . . 61
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 62
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 631. Introduction ✨
Let's get into it 🌸. Standardizing network configuration interfaces for management protocols like the Network Configuration Protocol [RFC6241] and the RESTCONF protocol [RFC8040] needs a modular set of data models 🧚. Those models have to be reusable and extendable over time, because nobody wants to start from scratch every year 💅.
This document sets out a bunch of usage guidelines for documents that contain YANG 1.1 [RFC7950] and YANG 1.0 [RFC6020] data models 💖. YANG is what defines the data structures, protocol operations, and notification content used inside a NETCONF and/or RESTCONF server 🦋. A server that supports a particular YANG module will support the client's NETCONF and/or RESTCONF operation requests, as shown by what the module says 🎀.
Lots of YANG constructs are optional to use, like the "description" statement 🙈. But if you want YANG modules to be actually useful, it's worth defining guidelines that ask for a higher level of compliance than the bare minimum in the YANG specification [RFC7950] 🔥.
YANG also allows stuff like infinitely long identifiers and string values, or top-level mandatory nodes, and a compliant server doesn't have to support those 💀. So only constructs that every server is required to support can be used in IETF YANG modules 🫶.
This document covers usage guidelines for the NETCONF operations layer and NETCONF content layer, as defined in [RFC6241], and for the RESTCONF methods and RESTCONF resources, as defined in [RFC8040] 🌈.
These guidelines are for authors and reviewers, to make published YANG data models easier to read and more interoperable 💜.
Heads up: this doc is not a YANG tutorial! The reader is expected to already know the YANG data modeling language before putting this guidance into practice 🧚.
1.1. Changes since RFC 6087 🎀
okay so the bestie list of everything that got glowed up since [RFC6087] 💅✨
- Updated NETCONF reference from RFC 4741 to RFC 6241
- Updated NETCONF over the Secure Shell (SSH) citation from RFC 4742 to RFC 6242
- Updated YANG Types reference from RFC 6021 to RFC 6991
- Updated obsolete URLs for IETF resources
- Changed top-level data node guideline
- Clarified XML Path Language (XPath) usage for a literal value representing a YANG identity
- Clarified XPath usage for a when-stmt
- Clarified XPath usage for "preceding-sibling" and "following-sibling" axes
- Added terminology guidelines
- Added mention of RFC 8174, which updates RFC 2119 by clarifying the use of capitalized key words
- Added YANG tree diagram guidelines
- Updated XPath guidelines for type conversions and function library usage
- Updated "Data Types" section
- Updated "Notification Definitions" section
- Clarified conditional key leaf nodes
- Clarified usage of "uint64" and "int64" data types
- Added text on YANG feature usage
- Added "Identifier Naming Conventions" section
- Clarified use of mandatory nodes with conditional augmentations
- Clarified namespace and domain conventions for example modules
- Clarified conventions for identifying code components
- Added YANG 1.1 guidelines
- Added "YANG Data Node Constraints" section
- Added mention of the RESTCONF protocol
- Added guidelines for datastores revised by the Network Management Datastore Architecture (NMDA)
2. Terminology 📖
the terms we use all over this doc, bestie 🦋
- published: A stable release of a module or submodule. For example, the "Request for Comments" described in Section 2.1 of [RFC2026] is considered a stable publication.
- unpublished: An unstable release of a module or submodule. For example the "Internet-Draft" described in Section 2.2 of [RFC2026] is considered an unstable publication that is a work in progress, subject to change at any time.
- YANG fragment: A set of YANG statements that are not intended to represent a complete YANG module or submodule. These statements are not intended for actual use, except to provide an example of YANG statement usage. The invalid syntax
...is sometimes used to indicate that additional YANG statements would be present in a real YANG module. - YANG tree diagram: A diagram representing the contents of a YANG module, as defined in [RFC8340]. It is also called a "tree diagram".
2.1. NETCONF Terms 🌸
these ones are defined in [RFC6241], so we're not redefining them here 🫶
- capabilities
- client
- operation
- server
2.2. YANG Terms 💜
these ones come from [RFC7950], so same deal, we're not redoing them
- data node
- module
- namespace
- submodule
- version
- YANG
- YIN
note that the term 'module' can be used as a generic term for a YANG module or submodule. when we're describing stuff that's specific to submodules, we use the term 'submodule' instead 💅
2.3. NMDA Terms 🧚
these are defined in [RFC8342] and we're not redefining them, periodt
- configuration
- conventional configuration datastore
- datastore
- operational state
- operational state datastore
2.4. Requirements Notation 🔥
okay this part is important, so listen up 👑 the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document get interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they show up in all capitals, just like here. no cap 🧢
3. General Documentation Guidelines 📝
YANG modules under review are likely to live in Internet-Drafts (I-Ds). All guidelines for I-D authors [ID-Guidelines] MUST be followed. The guidelines for RFCs should be followed too, and they're defined in [RFC7322] (and any future RFCs that obsolete it), [RFC-STYLE], and [RFC7841] 📚
The following sections MUST be present in an I-D containing a module:
- Narrative sections
- Definition sections
- Security Considerations section
- IANA Considerations section
- References section
there are three usage scenarios for YANG that can show up in an I-D or RFC:
- normative module or submodule
- example module or submodule
- example YANG fragment not part of any module or submodule
the guidelines in this document mainly refer to a normative module or submodule, but they may be applicable to example modules and YANG fragments as well 🌈
3.1. Module Copyright ©️
the module "description" statement MUST contain a reference to the latest approved IETF Trust Copyright statement, which is available online at:
https://trustee.ietf.org/license-info/
3.2. Code Components 💖
each normative YANG module or submodule in an I-D or RFC counts as 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 [RFC7950]. The name string form that includes the revision date SHOULD be used. The revision date MUST match the date used in the most recent revision of the module.
here's an example for the "2016-03-20" revision of the "ietf-foo" module ✨
<CODE BEGINS> file "ietf-foo@2016-03-20.yang"
module ietf-foo {
namespace "urn:ietf:params:xml:ns:yang:ietf-foo";
prefix "foo";
organization "...";
contact "...";
description "...";
revision 2016-03-20 {
description "Latest revision";
reference "RFC XXXX: Foo Protocol";
}
// ... more statements
}
<CODE ENDS>3.2.1. Example Modules 🦋
example modules are not code components, so the <CODE BEGINS> convention MUST NOT be used for example modules 🙈
an example module SHOULD be named using the term "example", followed by a hyphen, followed by a descriptive name, e.g., "example-toaster".
see Section 4.9 for the namespace guidelines for example modules 💅
3.3. 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. no skipping it, bestie 🫶
3.4. Tree Diagrams 🌳
YANG tree diagrams give a concise representation of a YANG module and SHOULD be included to help readers understand the structure of a module 🧠 guidelines on tree diagrams can be found in Section 3 of [RFC8340].
if YANG tree diagrams are used, then an informative reference to the YANG tree diagrams specification MUST be included in the document. Refer to Section 2.2 of [RFC8349] for an example of such a reference.
3.5. 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 that briefly describe the structure of the modules defined in the specification.
if the module or modules defined by the specification import definitions from other modules (except for those defined in [RFC7950] or [RFC6991]) or are always implemented in conjunction with other modules, then those facts MUST be noted in the overview section. any special interpretations of definitions in other modules MUST be noted as well. Refer to Section 2.3 of [RFC8349] for an example of this overview section.
if the document contains a YANG module(s) that is compliant with NMDA [RFC8342], then the Introduction section should mention this fact. here's an example of how that could look 👑
The YANG data model in this document conforms to the Network
Management Datastore Architecture defined in
RFC 8342.consistent indentation SHOULD be used for all examples, including YANG fragments and protocol message instance data. if line wrapping is done for formatting purposes, then this SHOULD be noted, as shown in the following example 🌸
[note: '\' line wrapping for formatting only]
<myleaf xmlns="tag:example.com,2017:example-two">\
this is a long value so the line needs to wrap to stay\
within 72 characters\
</myleaf>3.6. Definitions Section 💜
this section holds the module(s) defined by the specification. these modules SHOULD be written using the YANG 1.1 [RFC7950] syntax. YANG 1.0 [RFC6020] syntax MAY be used if no YANG 1.1 constructs or semantics are needed in the module. If any of the imported YANG modules are written using YANG 1.1, then the module MUST be written using YANG 1.1 🔥
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 Structure of Management Information Version 2 (SMIv2), which are not affected by these guidelines 🌈
note that if the module itself is considered normative and not an example module or example YANG fragment, then all YANG statements within a YANG module are considered normative. The use of keywords defined in [RFC2119] and [RFC8174] applies to YANG "description" statements in normative modules exactly as it would in any other normative section.
example YANG modules and example YANG fragments MUST NOT contain any normative text, including any all-uppercase reserved words from [RFC2119] and [RFC8174] 🙈
consistent indentation and formatting SHOULD be used in all YANG statements within a module.
see Section 4 for guidelines on YANG usage 🧚
3.7. Security Considerations Section 🚨
this one is serious, bestie, so no skimming 😭 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, which is available at https://trac.ietf.org/trac/ops/wiki/yang-security-guidelines. Section 3.7.1 contains the security considerations template dated 2013-05-08 and last updated on 2018-07-02. Authors MUST check the web page at the URL listed above in case there is a more recent version available 👀
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 explained.
- 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.
- 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.
3.7.1. Security Considerations Section Template 🔐
Okay bestie, this is the template for the Security Considerations section, so you basically fill in the blanks. The YANG module is for data you reach through NETCONF or RESTCONF, and the template has comments in it telling you what to describe for writable data, readable data, and RPC operations. Here's the whole thing, copy-paste ready 💅
X. Security Considerations
The YANG module specified in this document defines a schema for data
that is designed to be accessed via network management protocols such
as NETCONF [RFC6241] or RESTCONF [RFC8040]. The lowest NETCONF layer
is the secure transport layer, and the mandatory-to-implement secure
transport is Secure Shell (SSH) [RFC6242]. The lowest RESTCONF layer
is HTTPS, and the mandatory-to-implement secure transport is TLS
[RFC8446].
The NETCONF access control model [RFC8341] provides the means to
restrict access for particular NETCONF or RESTCONF users to a
preconfigured subset of all available NETCONF or RESTCONF protocol
operations and content.
-- 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.
There are a number of data nodes defined in this YANG module that 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>3.8. IANA Considerations Section 🦋
Every I-D that gets submitted to the IESG for publication MUST contain an IANA Considerations section, no exceptions 🫶 You can see the IESG policy at <https://www.ietf.org/id-info/checklist.html>. What the section needs depends on what IANA is being asked to do. If there's nothing for IANA to do, the section just says "This document has no IANA actions". For more details, check the guidelines in [RFC8126].
Each normative YANG module MUST be registered in both the "IETF XML Registry" [RFC3688] [IANA-XML] and the "YANG Module Names" registry [RFC6020] [IANA-MOD-NAMES]. This applies to brand-new modules and updated ones too. There's an example of an update registration for the "ietf-template" module in Section 5, so go look at that 🌈
3.8.1. Documents That Create a New Namespace 🌸
If an I-D defines a new namespace that IANA is going to administer, then the document MUST include an IANA Considerations section that explains how that namespace gets administered 💖
So basically, if any YANG module namespace statement value in the document isn't already registered with IANA, then a new entry in the "ns" subregistry within the "IETF XML Registry" MUST be requested from IANA ✨
3.8.2. Documents That Extend an Existing Namespace 🧚
You can extend an existing namespace with a YANG submodule that belongs to a module IANA already administers. When you do that, the document with the main module MUST be updated to use the latest revision of the submodule. Keep it current, bestie 🍓
3.9. References Sections 📚
For every import or include statement in a module that's part of the spec and points at a module in another document, there MUST be a matching normative reference to that document in the Normative References section. That reference MUST match the exact module version the spec actually uses 🔥
For every normative reference statement in a module that points at a separate document, there SHOULD be a matching normative reference in the Normative References section. That one SHOULD match the exact document version the spec actually uses. If the reference statement points at an informative reference in a separate document, a matching informative reference MAY show up in the Informative References section 🫧
3.10. Validation Tools 💅
All modules need to be validated before they go into an I-D, so no skipping this step 😭 The 'pyang' YANG compiler is freely available on GitHub at <https://github.com/mbj4668/pyang>.
If you use 'pyang' to validate a normative module, the "--ietf" command-line option MUST be used so it catches any IETF guideline issues. If you use it on an example module, the "--ietf" option MAY be used to catch those same issues.
The "yanglint" program is also freely available on GitHub at <https://github.com/CESNET/libyang>. It's the one you want for validating XPath statements inside YANG modules 🦋
3.11. Module Extraction Tools 🔍
There's a version of 'rfcstrip' that pulls YANG modules out of an I-D or RFC, and it's freely available at <https://github.com/mbj4668/rfcstrip>. It's handy for checking that the "<CODE BEGINS>" and "<CODE ENDS>" tags are used properly and that the normative YANG modules extract cleanly 🧚
The "xym" tool is also freely available on GitHub, and it can extract YANG modules from a document too: <https://github.com/xym-tool/xym> 🌈
3.12. Module Usage Examples 🎀
Each specification that defines one or more modules SHOULD include usage examples, either spread throughout the document or in an appendix. That includes example instance document snippets in an appropriate encoding (like XML and/or JSON) that show how the YANG module(s) are meant to be used 💖 Example modules MUST be validated, and Section 3.10 has the tools for that. If you use IP addresses, then a mix of IPv4 and IPv6 addresses, or IPv6 addresses only, SHOULD be used in the examples 🌸
4. YANG Usage Guidelines 👑
Modules in IETF Standards Track specifications MUST comply with all syntactic and semantic requirements of YANG 1.1 [RFC7950]. Check Section 3.6 for the exception for YANG 1.0 🫶 The guidelines in this section are meant to supplement the YANG specification [RFC7950], which only sets out a minimum set of conformance requirements.
To promote interoperability and build a set of practices from past experience, the following sections set up usage guidelines for specific YANG constructs ✨
Only guidelines that clarify or restrict the minimum conformance requirements are included here, so don't expect a whole rulebook 💅
4.1. Module Naming Conventions 🏷️
Normative modules in Standards Track documents MUST be named according to the guidelines in the IANA Considerations section of [RFC7950] 🔥
A distinctive word or abbreviation, like a protocol name or working group abbreviation, SHOULD be used in the module name. If you're defining new stuff that extends one or more existing modules, then the same word or abbreviation should be reused, instead of making a new one 🧚
All published module names MUST be unique. For a YANG module published in an RFC, IANA guarantees that uniqueness. 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's suggested that you pick a stable prefix that represents the whole organization. All normative YANG modules published by the IETF MUST begin with the prefix "ietf-". Another standards organization, like the IEEE, might use the prefix "ieee-" for all of its YANG modules 🌈
Once a module name is published, it MUST NOT be reused, even if the RFC containing the module gets reclassified to "Historic" status. You can't change a module name in YANG, and trying would be treated as a brand-new module, not a rename 💀
4.2. Prefixes 🎀
All YANG definitions are scoped by the module that contains the definition being referenced. That means definitions from multiple modules can be used together, even when the names aren't unique. In the example below, the identifier "foo" is used in all three modules 🦄
module example-foo {
namespace "tag:example.com,2017:example-foo";
prefix f;
container foo;
}
module example-bar {
namespace "tag:example.com,2017:example-bar";
prefix b;
typedef foo { type uint32; }
}
module example-one {
namespace "tag:example.com,2017:example-one";
prefix one;
import example-foo { prefix f; }
import example-bar { prefix b; }
augment "/f:foo" {
leaf foo { type b:foo; }
}
}YANG sets out these rules for prefix usage 🧚
- Prefixes are never used for built-in data types and YANG keywords.
- A prefix MUST be used for any external statement, meaning a statement defined with the YANG "extension" statement.
- The proper module prefix MUST be used for all identifiers imported from other modules.
- The proper module prefix MUST be used for all identifiers included from a submodule.
These guidelines apply to prefix usage of the current (local) module 💖
- The local module prefix SHOULD be used instead of no prefix in all path expressions.
- The local module prefix MUST be used instead of no prefix in all "default" statements for an "identityref" or "instance-identifier" data type.
- The local module prefix MAY be used for references to typedefs, groupings, extensions, features, and identities defined in the module.
Prefix values SHOULD be short but are also likely to be unique. Prefix values SHOULD NOT conflict with known modules that have already been published 🫶
4.3. Identifiers 🦄
Identifiers for all YANG identifiers in published modules MUST be between 1 and 64 characters in length. That includes any construct specified as an "identifier-arg-str" token in the ABNF in Section 14 of [RFC7950] ✨
4.3.1. Identifier Naming Conventions 🧚
Okay bestie, identifiers SHOULD follow one consistent naming pattern all through the module 💅 Only lowercase letters, numbers, and dashes SHOULD be used in names ✨ Uppercase characters, the period character, and the underscore character MAY be used, but only if the identifier represents a well-known value that uses them 🦋 YANG doesn't allow any other characters in identifiers, no cap 🙈
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, because that's giving repetitive 😭 YANG identifiers are hierarchical and only need to be unique among sibling nodes defined in the same module namespace 🧩
Using common identifiers like "name" or "id" in data definition statements is totally fine 💖 Especially if those data nodes share a common data type ✨
Identifiers SHOULD NOT carry any special semantics that identify data modeling properties 🙅♀️ Only YANG statements and YANG extension statements are designed to convey machine-readable data modeling properties 🫶 For example, naming something "config" or "state" does not change whether it's 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 👑
4.4. Defaults 🍓
In general, it's 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 they were used everywhere they're allowed 😭 Here's the tea:
+--------------+---------------+
| Statement | Default Value |
+--------------+---------------+
| config | true |
| mandatory | false |
| max-elements | unbounded |
| min-elements | 0 |
| ordered-by | system |
| status | current |
| yin-element | false |
+--------------+---------------+
Statement Defaults4.5. Conditional Statements 🌈
A module may be conceptually partitioned in several ways, using the "if-feature" and/or "when" statements ✨
Data model designers need to think carefully about all the modularity aspects, including the use of YANG conditional statements 🧠
If a data definition is optional, depending on server support for a NETCONF or RESTCONF protocol capability, then a YANG "feature" statement SHOULD be defined. The defined "feature" statement SHOULD then be used in the conditional "if-feature" statement referencing the optional data definition 💅
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 leafs that do not also apply to the parent list node 🚫
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 🧹
4.6. XPath Usage 🎀
This section describes guidelines for using the XML Path Language (XPath) [W3C.REC-xpath] within YANG modules 💜
4.6.1. XPath Evaluation Contexts 🦋
YANG defines five separate contexts for evaluating XPath statements, and they're all different vibes 🌈
- 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".
- 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.
- Notification: an event notification document. The document root is the notification element.
- RPC Input: The document root is the conceptual "input" node, which is the parent of all RPC input parameter definitions.
- 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 okay because when-stmt context is this notification
when "/if:interfaces/if:interface[name='eth0']";
type leafref {
// Okay because path-stmt has a different context
path "/if:interfaces/if:interface/if:mtu";
}
}
}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 🏷️
4.6.2. 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 that defines the must or when expression containing the "local-name" function 🙅♀️ Example of a "local-name" function that should not be used:
/*[local-name()='foo']The "derived-from-or-self" function SHOULD be used instead of an equality expression for identityref values 👑 This allows the identities to be conceptually augmented 🦋
Example:
// do not use
when "md-name-format = 'name-format-null'";
// this is preferred
when "derived-from-or-self(md-name-format, 'name-format-null')";4.6.3. Axes 🌸
The "attribute" and "namespace" axes are not supported in YANG and MAY be empty in a NETCONF or RESTCONF server implementation 🧃
The "preceding" and "following" axes SHOULD NOT be used 🙈 These constructs rely on XML document order within a NETCONF or RESTCONF 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, and "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 be 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" 🌷
4.6.4. 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 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
}Okay bestie, let's keep going 💅✨
4.6.5. Wildcards 🌸
Okay so here's a plot twist 😭 you can write an XPath expression that works totally fine when you look at just one module, but then it acts totally different once a server combines a bunch of modules together 🫠 That's because other modules can augment nodes, which adds new stuff into the mix.
The thing is, a server expands the wildcard against all the nodes from all the namespaces, not just the one module. So a "must" or "when" expression using the * operator can literally always come out false if you only check it inside a single YANG module 💀 When that happens, the "description" statement SHOULD clarify that augmenting objects are expected to match the wildcard expansion. Like this 🎀:
when /foo/services/*/active {
description
"No services directly defined in this module.
Matches objects that have augmented the services container.";
}4.6.6. Boolean Expressions 🦋
Okay this one is giving main character energy 👑 The YANG "must" and "when" statements use an XPath boolean expression to set the test condition. It's super important to write these so they don't accidentally change their result when the objects they reference get updated in a future revision of the module 😩
For example, leaf "foo2" must exist if leaf "foo1" is equal 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;
}Now in the next revision of the module, leaf "foo1" gets extended with a new enum named "four" 👀
leaf foo1 {
type enumeration {
enum one;
enum two;
enum three;
enum four;
}
}And the first XPath expression will now allow "four" to be accepted too, on top of "one" and "three" 🙈 That's exactly why the "!=" version is the INCORRECT one, bestie. 💀
4.7. YANG Definition Lifecycle Management 🌈
Okay so the YANG status statement MUST be present within a definition if its value is "deprecated" or "obsolete" 📝 The status SHOULD NOT be changed straight from "current" to "obsolete" 🙅♀️ An object SHOULD be available for at least one year with a "deprecated" status before it gets changed to "obsolete" ⏳
The module or submodule name MUST NOT be changed once the document containing it is published 🔒
The module namespace URI value MUST NOT be changed once the document containing the module is published 🔒
The revision date substatement within the import 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 an import statement is for a module from a stable source (e.g., an RFC for an IETF module), then a reference-stmt SHOULD be present within that import statement 🫶
import ietf-yang-types {
prefix yang;
reference "RFC 6991: Common YANG Data Types";
}If submodules are used, then the document containing the main module MUST be updated so that the main module revision date is equal to or more recent than the revision date of any submodule that is (directly or indirectly) included by the main module 💜
Definitions for future use SHOULD NOT be specified in a module. Please don't add placeholder objects like this "reserved" example 🙅♀️💀
leaf reserved {
type string;
description
"This object has no purpose at this time, but a future
revision of this module might define a purpose
for this object.";
}
}4.8. 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 (WG) 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 WG web and mailing information SHOULD 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. There's no need to include the contact info for WG Chairs 📬
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 3.1 ⚖️
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 those 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, and it MUST identify the published document that contains the module. Modules get extracted from their original documents all the time, so it's super useful for devs and operators to know how to find the original source document in a consistent way 🔎 The "revision" statement MAY have a "description" substatement 💖
Here's an example revision statement for a published YANG module:
revision "2012-02-22" {
description
"Initial version";
reference
"RFC 8341: Network Configuration
Access Control Model";
}For an unpublished module, you don't need a complete history of every unpublished revision. So within a sequence of draft versions, only the most recent revision needs to be recorded in the module. Do not remove or reuse a revision statement for a published module 🚫 A new revision date is not required unless the module contents have changed. If they have changed, then the revision date of that new version MUST be updated to a date later than the previous version's date 📅
Here are the two revision statements for an unpublished update to a published YANG module:
revision "2017-12-11" {
description
"Added support for YANG 1.1 actions and notifications tied to
data nodes. Clarify how NACM extensions can be used by other
data models.";
reference
"RFC 8407: Network Configuration Protocol (NETCONF)
Access Control Model";
}
revision "2012-02-22" {
description
"Initial version";
reference
"RFC 8341: Network Configuration
Access Control Model";
}4.9. Namespace Assignments 🎀
It is RECOMMENDED that only valid YANG modules be included in documents, whether or not the modules are published yet. This allows:
- the module to compile correctly instead of generating disruptive fatal errors.
- early implementors to use the modules without picking a random value for the XML namespace.
- 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 Names" 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:These 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-netconfNote that a different URN prefix string SHOULD be used for modules that are not Standards Track. The string SHOULD be selected according to the guidelines in [RFC7950] 📖
The following URIs show what modules that are not Standards Track might use. The domain "example.com" SHOULD be used by example modules in IETF I-Ds. These URIs are not meant to be dereferenced; they're only for module namespace identification 🧚
Example URIs using URLs per [RFC3986]:
https://example.com/ns/example-interfaces
https://example.com/ns/example-systemExample URIs using tags per [RFC4151]:
tag:example.com,2017:example-interfaces
tag:example.com,2017:example-system4.10. Top-Level Data Definitions 🌷
The top-level data organization SHOULD be considered carefully, in advance. Data model designers need to think about 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's sometimes useful to define separate top-level containers for configuration and non-configuration data. For some existing top-level data nodes, configuration data wasn't in scope, so only one container representing operational state was created. Refer to NMDA [RFC8342] for details 🔍
The number of top-level data nodes within a module SHOULD be minimized. It's often useful to retrieve related information within a single subtree, and if data is too distributed it gets hard to retrieve everything 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 it may change over time 🙅♀️
A mandatory database data definition is a node that a client must provide for the database to be valid. The server isn't 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 happen when the server boots or when a module is loaded dynamically at runtime ⚠️
4.11. Data Types 💜
Picking an appropriate data type (built-in, existing derived, or new derived) is very subjective, so few requirements can be set on that subject 🤷♀️
Data model designers SHOULD use the most appropriate built-in data type for the particular application ✨
The signed numeric data types ("int8", "int16", "int32", and "int64") SHOULD NOT be used unless negative values are allowed for the desired semantics 🔢🚫
4.11.1. Fixed-Value Extensibility 💖
Okay so if your set of values is fixed AND one single naming authority controls the data type contents, then an enumeration data type SHOULD be used. ✨
leaf foo {
type enumeration {
enum one;
enum two;
}
}If you need extensibility of enumerated values, 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. 🎀
4.11.2. 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. Bestie, if the patterns in a type definition have known limitations like false negative or false positive matches, then these limitations SHOULD be documented within the typedef or data definition. 🫶
The following typedef from [RFC6991] shows how to use the "pattern" statement properly:
typedef ipv4-address-no-zone {
type inet:ipv4-address {
pattern '[0-9\.]*';
}
...
}For string data types, if the length of the string has to be bounded in all implementations, then a length statement MUST be present. 💅
The following typedef from [RFC6991] shows how to use the "length" statement properly:
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] shows how to use the "range" statement properly:
typedef dscp {
type uint8 {
range "0..63";
}
...
}4.11.3. 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. This part is so important, no cap. 👑
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... ";
}4.11.4. Union Types ⭐
The YANG "union" type is evaluated by testing a value against each member type in the union. The first type definition that accepts a value as valid is the member type used. In general, member types SHOULD be ordered from most restrictive to least restrictive types. It's giving order of operations, iykyk. 💜
In the following example, the "enumeration" type will never be matched, because the "string" type before it matches everything. Girl, that's a whole plot hole 💀
Incorrect:
type union {
type string;
type enumeration {
enum up;
enum down;
}
}Correct:
type union {
type enumeration {
enum up;
enum down;
}
type string;
}It is possible for different member types to match, depending on the input encoding format. In XML, all values are passed as string nodes, but in JSON there are different value types for numbers, booleans, and strings. 🌈
In the following example, a JSON numeric value will always be matched by the "int32" type, but in XML the string value representing a number will be matched by the "string" type. The second version will match the "int32" member type no matter how the input is encoded. Main character energy for the second version 🦋
Incorrect:
type union {
type string;
type int32;
}Correct:
type union {
type int32;
type string;
}4.11.5. Empty and Boolean 🙈
YANG provides an "empty" data type, which has one value (i.e., present). The default is "not present", which is not actually a value. When used within a list key, only one value can (and must) exist for this key leaf. The type "empty" SHOULD NOT be used for a key leaf, since it is pointless. 🫠
There is really no difference between a leaf of type "empty" and a leaf-list of type "empty". Both are limited to one instance. The type "empty" SHOULD NOT be used for a leaf-list. 🙅♀️
The advantage of using type "empty" instead of type "boolean" is that the default (not present) does not take up any bytes in a representation. The disadvantage is that the client may not be sure if an empty leaf is missing because it was filtered somehow or not implemented. The client may not have a complete and accurate schema for the data returned by the server, and may not be aware of the missing leaf. Ngl that's a little bit chaotic 😭
The YANG "boolean" data type provides two values ("true" and "false"). When used within a list key, two entries can exist for this key leaf. Default values are ignored for key leafs, but a default statement is often used for plain boolean leafs. The advantage of the "boolean" type is that the leaf or leaf-list has a clear representation for both values. The default value is usually not returned unless the client explicitly requests it, so no bytes are used in a typical representation. ✨
In general, the "boolean" data type SHOULD be used instead of the "empty" data type, as shown in the example below:
Incorrect:
leaf flag1 {
type empty;
}Correct:
leaf flag2 {
type boolean;
default false;
}4.12. 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. Why reinvent the wheel bestie 🙄
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. That way reuse is easier without unnecessary coupling 🌸
The "description" statement MUST be present. Non-negotiable, periodt 💅
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. 📚
4.13. 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 in which it is defined. It just 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, so they stay as robust as possible:
- Clearly identify the purpose of the grouping in the "description" statement.
- There are five 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.
- Do not reference data outside the grouping in any "path", "must", or "when" statements.
- Do not include a "default" substatement on a leaf or choice unless the value applies on all possible contexts.
- Do not include a "config" substatement on a data node unless the value applies on all possible contexts.
- Clearly identify any external dependencies in the grouping "description" statement, such as nodes referenced by an absolute path from a "path", "must", or "when" statement.
4.14. Data Definitions 🦋
The "description" statement MUST be present in the following YANG statements:
- anyxml
- augment
- choice
- container
- extension
- feature
- grouping
- identity
- leaf
- leaf-list
- list
- notification
- rpc
- 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 paper trail is everything 📚
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. Use the cute tool for the job 🎀
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 that configuration anyxml nodes will not be supported. Literally the most chaotic statement in this whole section 💀
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. Explain yourself, babe ✨
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" 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. Nested choices are giving drama, handle with care 🙈
If a list defines any key leafs, then these leafs SHOULD be defined in order, as the first child nodes within the list. The key leafs MAY be in a different order in some cases, for example when they are defined in a grouping and not inline in the list statement. Flexibility is a whole vibe 🧚
4.14.1. Non-Presence Containers ✨
okay so a non-presence container is basically a folder 📁 that exists only to group data into specific subtrees 🌸. It doesn't have any semantics in the data model beyond that job, although YANG still lets you put stuff in it (like a "must" statement inside it) 💅. that's just YANG being YANG.
Example using container wrappers:
container top {
container foos {
list foo { ... }
}
container bars {
list bar { ... }
}
}Example without container wrappers:
container top {
list foo { ... }
list bar { ... }
}Using non-presence containers to organize data is a total vibe-check thing, kind of like making subdirectories in a file system 🗂️. Even though these containers have no semantics, they can still affect protocol operations for the descendant data nodes inside them, so using them SHOULD be considered carefully 🧐.
The NETCONF and RESTCONF protocols can't currently delete all list (or leaf-list) entries at once 😭. People sometimes work around this with a parent container, since deleting the container also removes all of its child entries 🫶.
4.14.2. Top-Level Data Nodes
Top-level objects need to be handled with care 🦋:
- top-level siblings are not ordered
- top-level siblings are not static and depend on the modules that are loaded
- for subtree filtering, retrieval of a top-level leaf-list will be treated as a content-match node for all top-level-siblings
- a top-level list with many instances may impact performance
4.15. 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 🚨. this one is serious, bestie, so don't skip it 💀.
4.16. 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 💜.
4.17. 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 description-stmt within a feature-stmt MUST specify any interactions with other features 🫧.
The set of YANG features defined in a module should be considered carefully 🧠. Very fine granular features increase interoperability complexity and should be avoided 🙅♀️. A likely misuse of the feature mechanism is tagging individual leafs (e.g., counters) with separate features, which is a big no-no 🙈.
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 📦. The set of features within a module is easy for the reader to discover, 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 thing to think about when deciding between a new module and a YANG feature is how stable the module is 🪨. It may be desirable to have a stable base module that doesn't change often. 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";
}4.18. YANG Data Node Constraints 🎀
4.18.1. Controlling Quantity
The "min-elements" and "max-elements" statements can be used to control how many list or leaf-list instances are required for a particular data node 🔢. YANG constraint statements SHOULD be used to identify conditions that apply to all implementations of the data model 🌟. If platform-specific limitations (e.g., the "max-elements" supported for a particular list) are relevant to operations, then a data model definition statement (e.g., "max-ports" leaf) SHOULD be used to identify the limit 🧾.
4.18.2. "must" versus "when"
"must" and "when" YANG statements are used to provide cross-object referential tests 🔀. They behave very differently, so pay attention 👑. The "when" statement causes data node instances to be silently deleted as soon as the condition becomes false 🫥. A false "when" expression is not considered to be an error, so no panic 🫶.
The "when" statement SHOULD be used together with "augment" or "uses" statements to achieve conditional model composition 🧩. The condition SHOULD be based on static properties of the augmented entry (e.g., list key leafs) 🗝️.
The "must" statement causes a datastore validation error if the condition is false ❌. This statement SHOULD be used for enforcing parameter value restrictions that involve more than one data node (e.g., end-time parameter must be after the start-time parameter) ⏰.
4.19. "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 🪞.
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 ✨.
4.19.1. 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 {
...
}
}
}4.19.2. 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 keep 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 so that the old client wouldn't know about the new condition and wouldn't specify it 🫣. The conditional "augment" statement can contain mandatory objects only if the condition is false, unless the client explicitly requests otherwise 🔒.
Only a conditional "augment" statement that uses the "when" statement form of a 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 of the target data node, because the client has no control over that external data 🚧.
In the following dummy example, it is okay to augment the "interface" entry with "mandatory-leaf" because the augmentation depends on support for "some-new-iftype" 🌈. The old client doesn't know about this type, so it would never select it, and therefore it would not add a mandatory data node 🧚.
module example-module {
yang-version 1.1;
namespace "tag:example.com,2017:example-module";
prefix mymod;
import iana-if-type { prefix iana; }
import ietf-interfaces { prefix if; }
identity some-new-iftype {
base iana:iana-interface-type;
}
augment "/if:interfaces/if:interface" {
when "if:type = 'mymod:some-new-iftype'";
leaf mandatory-leaf {
type string;
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 😭💅.
4.20. Deviation Statements 🎀
okay so per RFC 7950, Section 7.20.3, the YANG deviation statement is not allowed to show up in IETF YANG modules 🙅♀️ but it's lowkey super useful for documenting what a server can and can't do ✨ Deviation statements aren't reusable and usually aren't shared across all platforms, so they're very much a one-platform kinda vibe 💅
There are a bunch of reasons you might need deviations in an implementation 🤔 like, an object just can't be supported on every platform, or feature delivery happens in multiple development phases. Deviation statements can also add annotations to a module, and that does not affect the conformance requirements for the module 📝
It is suggested that deviation statements be defined in separate modules from regular YANG definitions 🧁 That way, the deviations can be platform specific and/or temporary 🦋
The order deviation statements get evaluated in can change the result 🌀 So multiple deviation statements in the same module, for the same target object, SHOULD NOT be used 🙅♀️
The max-elements statement is meant to describe an architectural limit on the number of list entries. It is not meant to describe platform limitations 🙈 If a platform has a hard resource limit, it's better to use a deviation statement for it 💜
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;
}
}4.21. Extension Statements 🧩
The YANG extension statement is how you specify external definitions ✨ In YANG syntax it shows up as an unknown-statement. Using extension statements in a published module needs some serious careful thought 🤔
okay, here are the guidelines for using YANG extensions:
- The semantics of the extension MUST NOT contradict any YANG statements. Extensions can add semantics that aren't covered by the normal YANG statements 🌈
- 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 not, identify what conditions apply that would require implementation of the extension 📋
- The extension MUST clearly identify where it can be used within other YANG statements 📍
- The extension MUST clearly identify if YANG statements or other extensions are allowed or required within the extension as substatements 💅
4.22. Data Correlation 🦋
Data can be correlated in a bunch of 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 how tightly coupled the old and new functionality is:
- 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 ✨
- 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 🌸
- mirror: create new objects in a new module or the original module, except use a new naming scheme and data location. The naming can be coupled in different ways. Tight coupling is achieved with a
leafrefdata type, with therequire-instancesubstatement set totrue. 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 substatement 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 substatement to false 🍓
The relationship between configuration and operational state has been clarified in NMDA [RFC8342] 💖
4.22.1. Use of "leafref" for Key Correlation
Sometimes it is not practical to augment a data structure 🙃 For example, the correlated data could have different keys or contain mandatory nodes.
The following example shows the use of the "leafref" data type for data correlation purposes 🧚
Not preferred:
list foo {
key name;
leaf name {
type string;
}
...
}
list foo-addon {
key name;
config false;
leaf name {
type string;
}
...
}Preferred:
list foo {
key name;
leaf name {
type string;
}
...
}
list foo-addon {
key name;
config false;
leaf name {
type leafref {
path "/foo/name";
require-instance false;
}
}
leaf addon {
type string;
mandatory true;
}
}4.23. Operational State 🌸
The modeling of operational state with YANG has glowed up over time 💫 At first, only data with a config statement value of false was considered to be operational state. This data wasn't considered part of any datastore, which made the YANG XPath definition way more complicated 😭
Operational state is now modeled using YANG according to the new NMDA [RFC8342] and is conceptually contained in the operational state datastore, which also includes the operational values of configuration data 🎀 There's no longer any need to duplicate data structures to provide separate configuration and operational state sections. Iconic, honestly ✨
This section describes some data modeling issues related to operational state, plus guidelines for transitioning YANG data model design to be NMDA compatible 🧠
4.23.1. Combining Operational State and Configuration Data
If possible, operational state SHOULD be combined with its associated configuration data 💅 This prevents duplication of key leafs and ancestor nodes. It also prevents race conditions for retrieval of dynamic entries, and it allows configuration and operational state to be retrieved together with minimal message overhead 🚀
container foo {
...
// contains "config true" and "config false" nodes that have
// no corresponding "config true" object (e.g., counters)
}4.23.2. Representing Operational Values of Configuration Data
If possible, the same data type SHOULD be used to represent the configured value and the operational value, for a given leaf or leaf-list object 🦄
Sometimes the configured value set is different than the operational value set for that object, for example, the admin-status and oper-status leafs in [RFC8343]. In this case, a separate object MAY be used to represent the configured and operational values 🌈
Sometimes the list keys are not identical for configuration data and the corresponding operational state. In this case, separate lists MAY be used to represent the configured and operational values 🍭
If it is not possible to combine configuration and operational state, then the keys used to represent list entries SHOULD be the same type. The leafref data type SHOULD be used in operational state for key leafs that have corresponding configuration instances. The require-instance statement MAY be set to false (in YANG 1.1 modules only) to indicate instances are allowed in the operational state that do not exist in the associated configuration data 🙌
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 and justify any NMDA exceptions in detail, such as the use of separate subtrees and/or separate leafs 🫶 The description statements for both the configuration and the operational state SHOULD be used for this purpose 📝
4.23.3. NMDA Transition Guidelines
YANG modules SHOULD be designed with the assumption that they will be used on servers supporting the operational state datastore 💜 With this in mind, YANG modules SHOULD define config false nodes wherever they make sense to the data model. Config false nodes SHOULD NOT be defined to provide the operational value for configuration nodes, except when the value space of a configured and operational value may differ. In that case, a distinct config false node SHOULD be defined to hold the operational value for the configured node 🧁
The following guidelines are meant to help modelers develop YANG modules that maximize the utility of the model with both current and new implementations 🌷
New modules, and modules that are not concerned with the operational state of configuration information, SHOULD immediately be structured to be NMDA compatible, as described in Section 4.23.1. This transition MAY be deferred if the module does not contain any configuration datastore objects ⭐
The remaining items are options that MAY be followed during the time that NMDA mechanisms are being defined 🕰️
- (a) Modules that require immediate support for the NMDA features SHOULD be structured for NMDA. A temporary non-NMDA version of this type of module MAY exist, either as an existing model or as a model created by hand or with suitable tools that mirror the current modeling strategies. Both the NMDA and the non-NMDA modules SHOULD be published in the same document, with NMDA modules in the document main body and the non-NMDA modules in a non-normative appendix. The use of the non-NMDA module will allow temporary bridging of the time period until NMDA implementations are available 🌉
- (b) For published models, the model should be republished with an NMDA-compatible structure, deprecating non-NMDA constructs. For example, the
ietf-interfacesmodel in [RFC7223] has been restructured as an NMDA-compatible model in [RFC8343]. The/interfaces-statehierarchy has been markedstatus deprecated. Models that mark their/foo-statehierarchy withstatus deprecatedwill allow NMDA-capable implementations to avoid the cost of duplicating the state nodes, while enabling non-NMDA-capable implementations to use them for access to the operational values 🎀 - (c) For models that augment models that have not been structured with the NMDA, the modeler will have to consider the structure of the base model and the guidelines listed above. Where possible, such models should move to new revisions of the base model that are NMDA compatible. When that is not possible, augmenting
statecontainers SHOULD be avoided, with the expectation that the base model will be re-released with the state containers marked as deprecated. It is RECOMMENDED to augment only the/foohierarchy of the base model. Where this recommendation cannot be followed, any newstateelements SHOULD be included in their own module 🧚♀️
4.23.3.1. Temporary Non-NMDA Modules ✨
okay so a temporary non-NMDA module lets a non-NMDA-aware client peek at operational state on an NMDA-compliant server 👀. It holds the top-level "config false" data nodes that would have lived in a legacy YANG module (the pre-NMDA kind) 💅.
A server that has to support both NMDA and non-NMDA clients can advertise the new NMDA module and the temporary non-NMDA module 🫶. A non-NMDA client can use separate "foo" and "foo-state" subtrees, except the "foo-state" subtree lives in a different (temporary) module 🧚. The NMDA module can still be used by a non-NMDA client to reach the conventional configuration datastores and the deprecated <get> operation, which accesses nested "config false" data nodes 💜.
To make the temporary non-NMDA model from an NMDA model, you can take these steps:
- Change the module name by appending
"-state"to the original module name - Change the namespace by appending
"-state"to the original namespace value - Change the prefix by appending
"-s"to the original prefix value - Add an import to the original module (e.g., for typedef definitions)
- Retain or create only the top-level nodes that have a
configstatement value offalse. These subtrees represent"config false"data nodes that were combined into the configuration subtree, so they are not available to non-NMDA aware clients 🙈. Set thestatusstatement to"deprecated"for each new node. - The module description SHOULD clearly identify the module as a temporary non-NMDA module 🍓
4.23.3.2. Example: Create a New NMDA Module 🦋
Create an NMDA-compliant module, using combined configuration and state subtrees, whenever possible 🌸
module example-foo {
namespace "urn:example.com:params:xml:ns:yang:example-foo";
prefix "foo";
container foo {
// configuration data child nodes
// operational value in operational state datastore only
// may contain "config false" nodes as needed
}
}4.23.3.3. Example: Convert an Old Non-NMDA Module 🧹
Do not remove non-compliant objects from existing modules. Instead, change the status to "deprecated". At some point, usually after 1 year, the status MAY be changed to "obsolete" 💀
Old Module:
module example-foo {
namespace "urn:example.com:params:xml:ns:yang:example-foo";
prefix "foo";
container foo {
// configuration data child nodes
}
container foo-state {
config false;
// operational state child nodes
}
}Converted NMDA Module:
module example-foo {
namespace "urn:example.com:params:xml:ns:yang:example-foo";
prefix "foo";
container foo {
// configuration data child nodes
// operational value in operational state datastore only
// may contain "config false" nodes as needed
// will contain any data nodes from old foo-state
}
// keep original foo-state but change status to deprecated
container foo-state {
config false;
status deprecated;
// operational state child nodes
}
}4.23.3.4. Example: Create a Temporary NMDA Module 🎀
Create a new module that contains the top-level operational state data nodes that would have been available before they were combined with configuration data nodes (to be NMDA compliant) 🧚
module example-foo-state {
namespace "urn:example.com:params:xml:ns:yang:example-foo-state";
prefix "foo-s";
// import new or converted module; not used in this example
import example-foo { prefix foo; }
container foo-state {
config false;
status deprecated;
// operational state child nodes
}
}4.24. Performance Considerations ⏱️
It is generally likely that certain YANG statements require more runtime resources than other statements. There are no performance requirements for YANG validation, but the following information MAY be considered when designing YANG data models 💅
- Lists are generally more expensive than containers
"when"statement evaluation is generally more expensive than"if-feature"or"choice"statements"must"statements are generally more expensive than"min-entries","max-entries","mandatory", or"unique"statements"identityref"leafs are generally more expensive than"enumeration"leafs"leafref"and"instance-identifier"types with"require-instance"set to true are generally more expensive than if"require-instance"is set to false
4.25. Open Systems Considerations 🌈
Only the modules imported by a particular module can be assumed to be present in an implementation. An open system MAY include any combination of YANG modules 🫧
4.26. Guidelines for Constructs Specific to YANG 1.1 📚
The set of guidelines for YANG 1.1 will grow as operational experience is gained with the new language features. This section contains an initial set of guidelines for new YANG 1.1 language features 🌸
4.26.1. Importing Multiple Revisions 🔁
Standard modules SHOULD NOT import multiple revisions of the same module into a module. This MAY be done if independent definitions (e.g., enumeration typedefs) from specific revisions are needed in the importing module 🙈
4.26.2. 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 🫶
"feature"s SHOULD be used instead of the "when" statement, if possible. Features are advertised by the server, and objects conditional by the "if-feature" statement are conceptually grouped together. There is no such commonality supported for "when" statements 💜
Features generally require less server implementation complexity and runtime resources than objects that use "when" statements. Features are generally static (i.e., set when a module is loaded and not changed at runtime). However, every client edit might cause a "when" statement result to change 🥹
4.26.3. "anyxml" versus "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 🔥
4.26.4. "action" versus "rpc" 🧚
The use of "action" statements or "rpc" statements is a subjective design decision. RPC operations are not associated with any particular data node. Actions are associated with a specific data node definition. An "action" statement SHOULD be used if the protocol operation is specific to a subset of all data nodes instead of all possible data nodes 👑
The same action name MAY be used in different definitions within different data node. For example, a "reset" action defined with a data node definition for an interface might have different parameters than for a power supply or a VLAN. The same action name SHOULD be used to represent similar semantics 🦋
The NETCONF Access Control Model (NACM) [RFC8341] does not support parameter-based access control for RPC operations. The user is given permission (or not) to invoke the RPC operation with any parameters. For example, if each client is only allowed to reset their own interface, then NACM cannot be used 🔐
For example, NACM cannot enforce access control based on the value of the "interface" parameter, only the "reset" operation itself 😭
rpc reset {
input {
leaf interface {
type if:interface-ref;
mandatory true;
description "The interface to reset.";
}
}
}However, NACM can enforce access control for individual interface instances, using a "reset" action. If the user does not have read access to the specific "interface" instance, then it cannot invoke the "reset" action for that interface instance ✨
container interfaces {
list interface {
...
action reset { }
}
}4.27. Updating YANG Modules (Published versus Unpublished) 🗂️
YANG modules can change over time. Typically, new data model definitions are needed to support new features. YANG update rules defined in Section 11 of [RFC7950] MUST be followed for published modules. They MAY be followed for unpublished modules 💖
The YANG update rules only apply to published module revisions. Each organization will have their own way to identify published work that is considered to be stable and unpublished work that is considered to be unstable. For example, in the IETF, the RFC document is used for published work, and the I-D is used for unpublished work 🌷
5. IANA Considerations 🎀
The following registration in the "ns" subregistry of the "IETF XML Registry" [RFC3688] was detailed in [RFC6087] and has been updated by IANA to reference this document 💅
URI: urn:ietf:params:xml:ns:yang:ietf-template
Registrant Contact: The IESG.
XML: N/A, the requested URI is an XML namespace.The following assignment was detailed in [RFC6087] and has been updated by IANA in the "YANG Module Names" registry. This document has also been added as a reference for the "YANG Module Names" registry itself, as it contains the template necessary for registration in Appendix B 🧾
+-----------+-------------------------------------------+
| Field | Value |
+-----------+-------------------------------------------+
| Name | ietf-template |
| Namespace | urn:ietf:params:xml:ns:yang:ietf-template |
| Prefix | temp |
| Reference | RFC 8407 |
+-----------+-------------------------------------------+
YANG Registry Assignment6. Security Considerations 🔒
This document defines documentation guidelines for NETCONF or RESTCONF content defined with the YANG data modeling language. Therefore, it does not introduce any new or increased security risks into the management system 🛡️💜
7. References 📖
7.1. Normative References
- [ID-Guidelines] Housley, R., "Guidelines to Authors of Internet-Drafts", December 2010,
https://www.ietf.org/standards/ids/guidelines/. 📝✨ - [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
https://www.rfc-editor.org/info/rfc2119. 🔑💖 - [RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, January 2004,
https://www.rfc-editor.org/info/rfc3688. 🗂️ - [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005,
https://www.rfc-editor.org/info/rfc3986. 🌐 - [RFC5378] Bradner, S., Ed. and J. Contreras, Ed., "Rights Contributors Provide to the IETF Trust", BCP 78, RFC 5378, DOI 10.17487/RFC5378, November 2008,
https://www.rfc-editor.org/info/rfc5378. 🎀 - [RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, DOI 10.17487/RFC6020, October 2010,
https://www.rfc-editor.org/info/rfc6020. 🧚 - [RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed., and A. Bierman, Ed., "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
https://www.rfc-editor.org/info/rfc6241. 🦋 - [RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
https://www.rfc-editor.org/info/rfc6242. 🔐 - [RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, August 2016,
https://www.rfc-editor.org/info/rfc7950. 💜 - [RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
https://www.rfc-editor.org/info/rfc8040. 🌸 - [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017,
https://www.rfc-editor.org/info/rfc8174. 🔠 - [RFC8342] Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K., and R. Wilton, "Network Management Datastore Architecture (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
https://www.rfc-editor.org/info/rfc8342. 🗄️ - [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
https://www.rfc-editor.org/info/rfc8446. 🔒 - [W3C.REC-xpath] Clark, J. and S. DeRose, "XML Path Language (XPath) Version 1.0", W3C Recommendation REC-xpath-19991116, November 1999,
http://www.w3.org/TR/1999/REC-xpath-19991116. 🛤️
7.2. Informative References
- [IANA-MOD-NAMES] IANA, "YANG Module Names",
https://www.iana.org/assignments/yang-parameters/. 📋 - [IANA-XML] IANA, "IETF XML Registry",
https://www.iana.org/assignments/xml-registry/. 📇 - [RFC-STYLE] RFC Editor, "Style Guide",
http://www.rfc-editor.org/styleguide/. ✏️ - [RFC2026] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996,
https://www.rfc-editor.org/info/rfc2026. 🏛️ - [RFC4151] Kindberg, T. and S. Hawke, "The 'tag' URI Scheme", RFC 4151, DOI 10.17487/RFC4151, October 2005,
https://www.rfc-editor.org/info/rfc4151. 🏷️ - [RFC4181] Heard, C., Ed., "Guidelines for Authors and Reviewers of MIB Documents", BCP 111, RFC 4181, DOI 10.17487/RFC4181, September 2005,
https://www.rfc-editor.org/info/rfc4181. 👀 - [RFC6087] Bierman, A., "Guidelines for Authors and Reviewers of YANG Data Model Documents", RFC 6087, DOI 10.17487/RFC6087, January 2011,
https://www.rfc-editor.org/info/rfc6087. 🌟 - [RFC6991] Schoenwaelder, J., Ed., "Common YANG Data Types", RFC 6991, DOI 10.17487/RFC6991, July 2013,
https://www.rfc-editor.org/info/rfc6991. 🧁 - [RFC7223] Bjorklund, M., "A YANG Data Model for Interface Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,
https://www.rfc-editor.org/info/rfc7223. 🔌 - [RFC7322] Flanagan, H. and S. Ginoza, "RFC Style Guide", RFC 7322, DOI 10.17487/RFC7322, September 2014,
https://www.rfc-editor.org/info/rfc7322. 💅 - [RFC7841] Halpern, J., Ed., Daigle, L., Ed., and O. Kolkman, Ed., "RFC Streams, Headers, and Boilerplates", RFC 7841, DOI 10.17487/RFC7841, May 2016,
https://www.rfc-editor.org/info/rfc7841. 📦 - [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017,
https://www.rfc-editor.org/info/rfc8126. 🧾 - [RFC8340] Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams", BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
https://www.rfc-editor.org/info/rfc8340. 🌳 - [RFC8341] Bierman, A. and M. Bjorklund, "Network Configuration Access Control Model", STD 91, RFC 8341, DOI 10.17487/RFC8341, March 2018,
https://www.rfc-editor.org/info/rfc8341. 🛡️ - [RFC8343] Bjorklund, M., "A YANG Data Model for Interface Management", RFC 8343, DOI 10.17487/RFC8343, March 2018,
https://www.rfc-editor.org/info/rfc8343. 🔗 - [RFC8349] Lhotka, L., Lindem, A., and Y. Qu, "A YANG Data Model for Routing Management (NMDA Version)", RFC 8349, DOI 10.17487/RFC8349, March 2018,
https://www.rfc-editor.org/info/rfc8349. 🛣️
Appendix A. Module Review Checklist ✨
This section is adapted from RFC 4181. 🦋
The whole point of a YANG module review is to check the module for both technical correctness and adherence to IETF documentation requirements. 💅 The checklist below may be helpful when reviewing an I-D:
- I-D Boilerplate 📝 -- verify that the document contains the required I-D boilerplate (see
https://www.ietf.org/id-info/guidelines.html), including the appropriate statement to permit publication as an RFC, and that the I-D boilerplate does not contain references or section numbers. - 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
https://www.ietf.org/id-info/guidelines.html. - Copyright Notice ©️ -- verify that the document has the appropriate text regarding the rights that document contributors 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:
- Security Considerations section 🔐 -- verify that the document uses the latest approved template from the Operations and Management (OPS) area website (see
https://trac.ietf.org/area/ops/trac/wiki/yang-security-guidelines) and that the guidelines therein have been followed. - 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 [RFC6020].
- References 📚 -- verify that the references are properly divided between normative and informative references, that RFCs 2119 and 8174 are included as normative references 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 okay 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). If a YANG module contains reference or "description" statements that refer to an I-D, then the I-D is included as an informative reference.
- License 🎀 -- verify that the document contains the Simplified BSD License in each YANG module or submodule. Some guidelines related to this requirement are described in Section 3.1. Make sure that the correct year is used in all copyright dates. Use the approved text from the latest TLP document, which can be found at:
- Other Issues 🔍 -- check for any issues mentioned in
https://www.ietf.org/id-info/checklist.htmlthat are not covered elsewhere. - 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, including formatting advice, can be found at:
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. Read it like you're the implementor, bestie! 🫶
Appendix B. YANG Module Template ✨
okay bestie, this is the cheat sheet you copy when you start your very own YANG module 🎀 It's a skeleton with the must-have bits already written and little notes telling you what to swap out. Here it is, verbatim, because the details matter 💖
<CODE BEGINS> file "ietf-template@2016-03-20.yang"
module ietf-template {
yang-version 1.1;
// 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://datatracker.ietf.org/wg/your-wg-name/>
WG List: <mailto:your-wg-name@ietf.org>
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
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
// replace '2016-03-20' with the module publication date
// the format is (year-month-day)
revision 2016-03-20 {
description
"what changed in this revision";
reference "RFC XXXX: <Replace With Document Title>";
}
// 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>Acknowledgments 🫶
this whole template is adapted from "Guidelines for Authors and Reviewers of MIB Documents" [RFC4181], by C. M. Heard. Shoutout to that work, it's the blueprint behind all of this 🌸
the working group also gave a huge thank-you 💜 to Martin Bjorklund, Juergen Schoenwaelder, Ladislav Lhotka, Jernej Tuljak, Lou Berger, Robert Wilton, Kent Watsen, and William Lupton for their extensive reviews and contributions to this document. literally the reviewers who made it so good 👑
Author's Address 💌
Andy Bierman
YumaWorks
Email: andy@yumaworks.com
📦 where this page comes from
- ietf: rfc8407 2018-10
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms