Recommendations for Creating IANA-Maintained YANG Modules

IANA maintains a set of registries that are key for interoperability. The content of these registries are usually available using various formats (e.g., plain text, XML). However, there were some confusion in the past about whether the content of some registries is dependent on a specific representation format. For example, Section 5 of was published to clarify that MIB and YANG modules are merely additional formats in which the "Interface Types (ifType)" and "Tunnel Types (tunnelType)" registries are available. The MIB and YANG modules are not separate registries, and the same values are always present in all formats of the same registry. Also, some YANG modules include parameters and values directly in a module that is not maintained by IANA while these are populated in an IANA registry. Such a design is suboptimal as it creates another source of information that may deviate from the IANA registry as new values are assigned or some values are deprecated. For the sake of consistency, better flexibility to support new values, and maintaining IANA registries as the unique authoritative source of information, when such an information is maintained in a registry, this document encourages the use of IANA-maintained modules. updates the guidelines in .

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. This document makes use of the terms defined in Section 2 of .

When designing a YANG module for a functionality governed by a protocol for which IANA maintains a registry, it is RECOMMENDED to specify an IANA-maintained module that echoes the content of that registry. This is superior to including that content in an IETF-maintained module. When one or multiple sub-registries are available under the same registry, it is RECOMMENDED to define an IANA-maintained module for each sub-registry. However, module designers MAY consider defining one single IANA-maintained module that covers all sub-registries if maintaining that single module is manageable (e.g., very few values are present or expected to be present for each sub-registry). An example of such a module is documented in Section 5.2 of . An IANA-maintained module may use identities (e.g., ) or enumerations (e.g., ). The decision about which type to use is left to the module designers and should be made based upon specifics related to the intended use of the IANA-maintained module. For example, identities are useful if the registry entries are organized hierarchically, possibly including multiple inheritances. It is RECOMMENDED that the reasoning for the design choice is documented in the companion specification that registers an IANA-maintained module. For example, defines an IANA-maintained module that uses enumerations for the following reason:

Designers of IANA-maintained modules MAY supply the full initial version of the module in a specification document that registers the module or only a script to be used (including by IANA) for generating the module (e.g., an XSLT stylesheet as in Appendix A of ). When a script is used, the Internet-Draft that defines an IANA-maintained module SHOULD include an appendix with the initial full version of the module. Including such an appendix in pre-RFC versions is meant to assess the correctness of the outcome of the supplied script. The authors MUST include a note to the RFC Editor requesting that the appendix be removed before publication as RFC. Initial versions of IANA-maintained modules that are published in RFCs may be misused despite the appropriate language to refer to the IANA registry to retrieve the up-to-date module. This is problematic for interoperability, e.g., when values are deprecated or are associated with a new meaning. Note: provides XSLT 1.0 stylesheets and other tools for translating IANA registries to YANG modules. The tools can be used to generate up-to-date revisions of an IANA-maintained module based upon the XML representation of an IANA registry.

This document does not require any IANA action.

This document does not introduce new concerns other than those already discussed in Section 15 of .

This document is triggered by a discussion the author had with Dhruv Dhody and Jensen Zhang. Thanks to Jürgen Schönwälder, Ladislav Lhotka, and Qin Wu for the discussion and valuable comments. Special thanks to Ladislav Lhotka for sharing more context that led to the design documented in . Thanks for Andy Bierman for the comments.