Updated YANG Module Revision Handling
draft-ietf-netmod-yang-module-versioning-17
| Document | Type | Active Internet-Draft (netmod WG) | |
|---|---|---|---|
| Authors | Robert Wilton , Reshad Rahman , Balázs Lengyel , Joe Clarke , Jason Sterne | ||
| Last updated | 2026-06-29 | ||
| Replaces | draft-verdt-netmod-yang-module-versioning | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Yang Validation | 0 errors, 0 warnings | ||
| Reviews |
SECDIR IETF Last Call Review due 2025-11-12
Incomplete
|
||
| Additional resources |
Yang catalog entry for ietf-yang-library-revisions@2020-07-06.yang
Yang catalog entry for ietf-yang-revisions@2020-07-06.yang Yang impact analysis for draft-ietf-netmod-yang-module-versioning Mailing list discussion |
||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Lou Berger | ||
| Shepherd write-up | Show Last changed 2026-02-16 | ||
| IESG | IESG state | IESG Evaluation::AD Followup | |
| Action Holder | |||
| Consensus boilerplate | Yes | ||
| Telechat date |
(None)
Has a DISCUSS. Has enough positions to pass once DISCUSS positions are resolved. |
||
| Responsible AD | Mohamed Boucadair | ||
| Send notices to | lberger@labn.net | ||
| IANA | IANA review state | Version Changed - Review Needed | |
| IANA expert review state | Expert Reviews OK |
draft-ietf-netmod-yang-module-versioning-17
Network Working Group R. Wilton, Ed.
Internet-Draft Cisco Systems, Inc.
Updates: 6020, 7950, 8525, 9907 (if approved) R. Rahman, Ed.
Intended status: Standards Track Equinix
Expires: 31 December 2026 B. Lengyel, Ed.
Ericsson
J. Clarke
Cisco Systems, Inc.
J. Sterne
Nokia
29 June 2026
Updated YANG Module Revision Handling
draft-ietf-netmod-yang-module-versioning-17
Abstract
This document refines the RFC 7950 module update rules. It specifies
a new YANG module update procedure that can document when non-
backwards-compatible changes have occurred during the evolution of a
YANG module. It extends the YANG import statement with a suggestion
for a minimum revision. This helps document inter-module
dependencies. It provides guidelines for managing the lifecycle of
YANG modules and individual schema nodes. This document updates RFC
7950, RFC 6020, RFC 8525, and RFC 9907.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 31 December 2026.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Updates to YANG RFCs . . . . . . . . . . . . . . . . . . 4
2. Terminology and Conventions . . . . . . . . . . . . . . . . . 5
3. Refinements to YANG revision handling . . . . . . . . . . . . 6
3.1. Updating a YANG module with a new revision . . . . . . . 7
3.1.1. Backwards-compatible rules . . . . . . . . . . . . . 8
3.1.2. Non-backwards-compatible changes . . . . . . . . . . 9
3.2. Non-backwards-compatible extension statement . . . . . . 9
3.3. Removing revisions from the revision history of published
modules and submodules . . . . . . . . . . . . . . . . . 9
3.4. Examples for updating the YANG module revision history . 10
4. Guidance for revision selection on imports . . . . . . . . . 13
4.1. Recommending a minimum revision for module imports . . . 14
4.1.1. Module import examples . . . . . . . . . . . . . . . 15
5. The ietf-yang-library-status YANG module . . . . . . . . . . 16
5.1. Reporting how deprecated and obsolete nodes are
handled . . . . . . . . . . . . . . . . . . . . . . . . . 16
6. Guidelines for using the YANG module update rules . . . . . . 17
6.1. Guidelines for YANG module authors . . . . . . . . . . . 17
6.1.1. Making non-backwards-compatible changes to a YANG
module . . . . . . . . . . . . . . . . . . . . . . . 18
6.2. Versioning Considerations for Clients . . . . . . . . . . 19
7. Module Versioning Extension YANG Modules . . . . . . . . . . 20
7.1. Module ietf-yang-revisions . . . . . . . . . . . . . . . 20
7.2. Module ietf-yang-library-status . . . . . . . . . . . . . 23
8. Operational considerations . . . . . . . . . . . . . . . . . 25
9. Security considerations . . . . . . . . . . . . . . . . . . . 26
9.1. Security considerations for module revisions . . . . . . 26
9.2. Security considerations for the modules defined in this
document . . . . . . . . . . . . . . . . . . . . . . . . 27
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
10.1. YANG Module Registrations . . . . . . . . . . . . . . . 28
10.2. Guidance for versioning in IANA-maintained YANG
modules . . . . . . . . . . . . . . . . . . . . . . . . 28
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
11.1. Normative References . . . . . . . . . . . . . . . . . . 29
11.2. Informative References . . . . . . . . . . . . . . . . . 30
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Appendix A. Examples of changes that are NBC . . . . . . . . . . 33
Appendix B. Examples of applying the NBC change guidelines . . . 33
B.1. Removing a data node . . . . . . . . . . . . . . . . . . 33
B.2. Changing the type of a leaf node . . . . . . . . . . . . 34
B.3. Reducing the range of a leaf node . . . . . . . . . . . . 34
B.4. Changing the key of a list . . . . . . . . . . . . . . . 35
B.5. Renaming a node . . . . . . . . . . . . . . . . . . . . . 35
Appendix C. Examples when breaking NBC changes have to be
made . . . . . . . . . . . . . . . . . . . . . . . . . . 36
C.1. Removing a data node . . . . . . . . . . . . . . . . . . 36
C.2. Changing the type of a leaf node . . . . . . . . . . . . 36
Appendix D. Example instance-data for ietf-yang-library-status
module . . . . . . . . . . . . . . . . . . . . . . . . . 37
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
The YANG module update rules per [RFC7950] require that updates of
YANG modules preserve strict backwards compatibility. This causes
problems as described in [I-D.ietf-netmod-yang-versioning-reqs].
This document recognizes the need to sometimes allow YANG modules to
evolve with non-backwards-compatible changes, which can cause
breakage to clients and when importing YANG modules. Accepting that
non-backwards-compatible changes do sometimes occur -- e.g., for
bugfixes -- it is important to have mechanisms to report when these
changes occur, and to manage their effect on clients and the broader
YANG ecosystem.
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Several other documents build on this document with additional
capabilities. [I-D.ietf-netmod-yang-schema-comparison] specifies an
algorithm that can be used to compare two revisions of a YANG schema
and provide granular information to allow module users to determine
if they are impacted by changes between the revisions. The
[I-D.ietf-netmod-yang-semver] document defines a YANG extension that
tags a YANG artifact with a version identifier based on semantic
versioning. YANG packages [I-D.ietf-netmod-yang-packages] provide a
mechanism to group sets of related YANG modules together in order to
manage schema and conformance of YANG modules as a cohesive set
instead of individually. Finally,
[I-D.ietf-netmod-yang-ver-selection] provides a schema selection
mechanism that allows a client to choose which schemas to use when
interacting with a server from the available schema that are
supported and advertised by the server. These other documents are
mentioned here as informative references. Support of the other
documents is not required in an implementation in order to take
advantage of the mechanisms and functionality offered by this module
versioning document.
The document comprises four parts:
* Refinements to the YANG 1 and YANG 1.1 module revision update
procedure, supported by a new extension statement to indicate when
a revision contains non-backwards-compatible changes.
* Updated guidance for revision selection on imports and a YANG
extension statement allowing YANG module imports to document a
recommended earliest module revision that may satisfy the import
dependency.
* Updates and augmentations to ietf-yang-library to report how
"deprecated" and "obsolete" nodes are handled by a server.
* Guidelines for how the YANG module update rules defined in this
document should be used, along with examples.
Note to RFC Editor (To be removed by RFC Editor)
Open issues are tracked at https://github.com/netmod-wg/yang-ver-dt/
issues.
1.1. Updates to YANG RFCs
This document updates [RFC7950] section 11 and [RFC6020] section 10.
Section 3 describes modifications to YANG revision handling and
update rules, and Section 4.1 describes a YANG extension statement to
describe potential YANG import revision dependencies.
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This document updates Sections 4.7 and 4.8 of [RFC9907]. Section 6
provides guidelines on managing the lifecycle of YANG modules that
may contain non-backwards-compatible changes and a branched revision
history.
This document updates [RFC8525] with augmentations to include two
boolean leafs to indicate whether status deprecated and status
obsolete schema nodes are implemented by the server. Note, an RFC
"updates" tag is used to highlight to readers of [RFC8525] about the
relationship between these documents because [RFC8525] is a core
manageability specification and this document sets the future
direction of YANG versioning.
2. Terminology and Conventions
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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document makes use of the following terminology introduced in
the YANG 1.1 Data Modeling Language [RFC7950]:
* schema node
Also, this document makes use of the following terminology defined in
[RFC9907]:
* IANA-maintained YANG module
* published
In addition, this document uses the following terminology:
* YANG module revision: An instance of a YANG module, uniquely
identified with a revision date, with no implied ordering or
backwards compatibility between different revisions of the same
module.
* Backwards-compatible (BC) change: A backwards-compatible change
between two YANG module revisions, as defined in Section 3.1.1
* Non-backwards-compatible (NBC) change: A non-backwards-compatible
change between two YANG module revisions, as defined in
Section 3.1.2
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3. Refinements to YANG revision handling
[RFC7950] and [RFC6020] assume, but do not explicitly state, that the
revision history for a YANG module or submodule is strictly linear,
i.e., it is prohibited to have two independent revisions of a YANG
module or submodule that are both directly derived from the same
parent revision.
This document clarifies [RFC7950] and [RFC6020] to explicitly allow
non-linear development of YANG module and submodule revisions, so
that they MAY have multiple revisions that directly derive from the
same parent revision. As per [RFC7950] and [RFC6020], YANG module
and submodule revisions continue to be uniquely identified by their
revision date, and hence all revisions of a given module or submodule
MUST have unique revision dates.
However, using revision dates alone to identify revisions of a YANG
module versioned with a branched revision history is likely to be
confusing because the relationship between module revisions is no
longer guaranteed to be chronologically ordered. Instead, for
modules that may use a branched revision history, it is RECOMMENDED
to use a version identifier, such as the one described in
[I-D.ietf-netmod-yang-semver], that better describes the semantic
relationship between the revisions.
For a given YANG module, revision B is derived from revision A if
revision A is listed in revision B's revision history, or would have
been listed there had it not been removed (see Section 3.3).
Although this document allows for a branched revision history, a
given YANG module revision history does not contain all revisions in
all possible branches, it only lists those from which it was derived,
i.e., the module revision's history describes a single path of linear
derived revisions (or a subset of them if one or more have been
removed) back to the root of the module's revision history.
A corollary to the text above is that the ancestry (derived
relationship) between two module or submodule revisions cannot be
determined by comparing the module or submodule revision date or
version identifier alone - the revision history must be consulted.
A module's name and revision date identify a specific immutable
definition of that module within its revision history. Hence, if a
module includes submodules then to ensure that the module's content
is uniquely defined, the module's "include" statements SHOULD use
"revision-date" substatements to specify the exact revision date of
each included submodule. When a module does not include its
submodules by revision-date, the revision of submodules used cannot
be derived from the including module. Mechanisms such as YANG
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packages [I-D.ietf-netmod-yang-packages] and YANG library [RFC8525]
could be used to specify the exact submodule revisions used when the
submodule revision date is not constrained by the "include"
statement.
[RFC7950] section 11 and [RFC6020] section 10 require that all
updates to a YANG module are backwards-compatible (BC) to the
previous revision of the module. This document introduces a method
to indicate that a non-backwards-compatible (NBC) change has occurred
between module revisions: this is done by using a new "non-backwards-
compatible" YANG extension statement in the module revision history.
Two revisions of a module or submodule MAY have identical content
except for the revision history. This could occur, for example, if a
module or submodule has a branched history and identical changes are
applied in multiple branches.
3.1. Updating a YANG module with a new revision
This section updates [RFC7950] section 11 and [RFC6020] section 10 to
refine the rules for permissible changes when a new YANG module
revision is created.
New module revisions SHOULD NOT contain NBC changes because they
often create problems for clients, however they can be helpful in
some scenarios, and hence are discouraged, but allowed. For example:
* Bugfixes, particularly where the likely client impact is low or
the module is changed to reflect current server behavior.
* To mark nodes as obsolete (or remove them), after a suitable
deprecation period.
* To refine new and unstable modules (or new and unstable nodes
within existing, stable modules).
* Restructuring a module to add new functionality where the cost of
adding the functionality in a BC manner is disproportionate to the
expected benefits of greater client backwards compatibility.
A YANG extension, defined in Section 3.2, is used to signal the
potential for incompatibility to existing module users and readers.
As per [RFC7950] and [RFC6020], all published revisions of a module
are given a new unique revision date.
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3.1.1. Backwards-compatible rules
A change between two module revisions is defined as being "backwards-
compatible" if the change conforms to the module update rules
specified in [RFC7950] section 11 and [RFC6020] section 10, updated
by the following rules:
* A "status" "deprecated" statement MAY be added, or changed from
"current" to "deprecated", but adding or changing "status" to
"obsolete" is a non-backwards-compatible change. See [RFC9907]
and Section 6.1.1 for guidance.
* YANG schema nodes with a "status" "obsolete" substatement MAY be
removed from published modules, and the removal is classified as a
backwards-compatible change. In some circumstances it MAY be
helpful to retain the obsolete definitions since their identifiers
may still be referenced by other modules and to ensure that their
identifiers are not reused with a different meaning.
* A statement that is defined using the YANG "extension" statement
may be added, removed, or changed. While extensions may be
ignored by some clients, other clients may support a given
extension and thus be affected if a module adds, removes or
changes how it uses the extension statement. Therefore, extension
statement definitions SHOULD specify whether adding, removing, or
changing statements defined by that extension are considered
backwards-compatible or non-backwards-compatible.
* Any change made to the "revision-date" or "recommended-min-date"
substatements of an "import" statement, including adding new
"revision-date" or "recommended-min-date" substatements, changing
the argument of any "revision-date" or "recommended-min-date"
substatements, or removing any "revision-date" or "recommended-
min-date" substatements, is classified as backwards-compatible.
* Any changes (including whitespace or formatting changes) that do
not change the semantic meaning of the module are backwards-
compatible.
Only changes in the module itself are considered for determining
backwards compatibility. Changes in other imported modules do not
have a bearing on the backwards compatibility status of the importing
module.
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3.1.2. Non-backwards-compatible changes
Any changes to YANG modules that are not defined by Section 3.1.1 as
being backwards-compatible are classified as "non-backwards-
compatible" changes.
3.2. Non-backwards-compatible extension statement
The "rev:non-backwards-compatible" extension statement is used to
indicate YANG module revisions that contain NBC changes.
If a revision of a YANG module contains changes, relative to its
parent revision, that do not conform to the module update rules
defined in Section 3.1.1, then a "rev:non-backwards-compatible"
extension statement MUST be added as a substatement to the "revision"
statement.
Adding, modifying or removing a "rev:non-backwards-compatible"
extension statement is considered to be a BC change.
3.3. Removing revisions from the revision history of published modules
and submodules
The author considerations in this section update Section 4.8 of
[RFC9907].
Authors may wish to remove revision statements from a published
module or submodule. Removal of revision information may be
desirable for a number of reasons including reducing the size of a
large revision history, or removing a revision that should no longer
be used or imported. Removing revision statements is allowed, but
can cause issues and SHOULD NOT be done without careful analysis of
the potential impact to users of the module or submodule since it may
cause loss of visibility of when non-backwards-compatible changes
were introduced.
Authors MAY remove a contiguous sequence of entries from the end
(i.e., the oldest entries) of the revision history. This is
acceptable even if the first remaining (oldest) revision entry in the
revision history contains a rev:non-backwards-compatible
substatement.
Authors MAY remove a contiguous sequence of entries in the revision
history as long as the presence or absence of any existing rev:non-
backwards-compatible substatements on all remaining entries still
accurately reflect the compatibility relationship to their preceding
entries remaining in the revision history.
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Authors MUST NOT remove the first (i.e., newest) revision entry in
the revision history.
Example revision history:
revision 2020-11-11 {
rev:non-backwards-compatible;
}
revision 2020-08-09 {
rev:non-backwards-compatible;
}
revision 2020-06-07 {
}
revision 2020-02-10 {
rev:non-backwards-compatible;
}
revision 2019-10-21 {
}
revision 2019-03-04 {
}
revision 2019-01-02 {
}
In the revision history example above (with revision descriptions
omitted for clarity), removing the revision history entry for
2020-02-10 would also remove the rev:non-backwards-compatible
annotation and hence the resulting revision history would incorrectly
indicate that revision 2020-06-07 is backwards-compatible with
revisions 2019-01-02 through 2019-10-21 when it is not, and so this
change cannot be made. Conversely, removing one or more revisions
out of 2019-03-04, 2019-10-21 and 2020-08-09 from the revision
history would still retain a consistent revision history, and is
acceptable, subject to an awareness of the concerns raised in the
first paragraph of this section.
3.4. Examples for updating the YANG module revision history
The following diagram, explanation, and module history illustrates
how a branched revision history for a YANG module could be
represented chronologically. To aid clarity, it makes use of both
the "non-backwards-compatible" extension statement, and the "version"
extension statement defined in [I-D.ietf-netmod-yang-semver]:
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Example YANG module with branched revision history using version
identifiers defined in [I-D.ietf-netmod-yang-semver].
Module revision date Example version identifier
2019-01-01 <- 1.0.0
|
2019-02-01 <- 2.0.0
| \
2019-03-01 \ <- 3.0.0
| \
| 2019-04-01 <- 2.1.0
| |
2019-05-01 | <- 3.1.0
|
2019-06-01 <- 2.2.0
The tree diagram above illustrates how an example module's revision
history might evolve over time. For example, the tree might
represent the following changes, listed in chronological order from
the oldest revision to the newest revision:
Example module, revision 2019-05-01:
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module example-module {
namespace "urn:example:module";
prefix "ex-prefix-name";
import ietf-yang-revisions { prefix "rev"; }
import ietf-yang-semver { prefix "ys"; }
description
"to be completed";
revision 2019-05-01 {
ys:version 3.1.0;
description "Add new functionality.";
}
revision 2019-03-01 {
ys:version 3.0.0;
rev:non-backwards-compatible;
description
"Add new functionality. Remove some deprecated nodes.";
}
revision 2019-02-01 {
ys:version 2.0.0;
rev:non-backwards-compatible;
description "Apply bugfix to pattern statement";
}
revision 2019-01-01 {
ys:version 1.0.0;
description "Initial revision";
}
//YANG module definition starts here
}
Example module, revision 2019-06-01:
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module example-module {
namespace "urn:example:module";
prefix "ex-prefix-name";
import ietf-yang-revisions { prefix "rev"; }
import ietf-yang-semver { prefix "ys"; }
description
"to be completed";
revision 2019-06-01 {
ys:version 2.2.0;
description "Backwards-compatible bugfix to enhancement.";
}
revision 2019-04-01 {
ys:version 2.1.0;
description "Apply enhancement to older release train.";
}
revision 2019-02-01 {
ys:version 2.0.0;
rev:non-backwards-compatible;
description "Apply bugfix to pattern statement";
}
revision 2019-01-01 {
ys:version 1.0.0;
description "Initial revision";
}
//YANG module definition starts here
}
4. Guidance for revision selection on imports
[RFC7950] and [RFC6020] allow YANG module "import" statements to
optionally have a revision-statement which requires the imported
module to have a specific revision date. In practice, importing a
module with an exact revision date can be too restrictive because it
requires the importing module to be updated whenever any change to
the imported module is needed by the importing module, and hence
Section 6.1 suggests that authors do not restrict YANG module imports
to exact revision dates.
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Instead, for conformance purposes (section 5.6 of [RFC7950]), the
recommended approach for defining the relationship between specific
YANG module revisions is to specify the relationships outside of the
YANG modules, e.g., via YANG library [RFC8525], YANG packages
[I-D.ietf-netmod-yang-packages], a filesystem directory containing a
set of consistent YANG module revisions, or a revision control system
commit label.
4.1. Recommending a minimum revision for module imports
Although the previous section indicates that the actual relationship
constraints between different revisions of YANG modules should be
specified outside of the modules, in some scenarios YANG modules are
designed to be loosely coupled, and implementors may wish to select
sets of YANG module revisions that are expected to work together.
For these cases it can be helpful for a module author to provide
guidance on a recommended minimum revision that is expected to
satisfy a YANG import. E.g., the module author may know of a
dependency on a type or grouping that has been introduced in a
particular imported YANG module revision. Although there can be no
guarantee that all derived future revisions from the particular
imported module will necessarily also be compatible, older revisions
of the particular imported module may not be compatible.
This document introduces, primarily for modules with a linear
revision history that are versioned using revision dates, a new YANG
extension statement to provide guidance to module implementors on a
recommended minimum module revision of an imported module that is
anticipated to be compatible. This statement has been designed to be
machine-readable so that tools can parse the minimum revision
extension statement and generate warnings if appropriate, but this
extension statement does not alter YANG module conformance of valid
YANG module versions in any way, and specifically it does not alter
the behavior of the YANG module import statement from that specified
in [RFC7950].
The ietf-revisions module defines the "recommended-min-date"
extension statement, a substatement to the YANG "import" statement,
to allow for a "minimum recommended date" to be documented:
The argument to the "recommended-min-date" extension statement is
a revision date which uses the "date-no-zone" type defined in
[RFC9911].
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A particular revision of an imported module adheres to an import's
"recommended-min-date" extension statement if the imported
module's revision date is equal to or later than the revision date
argument of the "recommended-min-date" extension statement in the
importing module.
Zero or one "recommended-min-date" extension statement is allowed
for each parent "import" statement.
Adding, modifying or removing a "recommended-min-date" extension
statement is a BC change.
If a "recommended-min-date" extension statement is ignored by a YANG
parser, the import statement is processed according to the rules
documented in Section 7.1.5 of [RFC7950].
4.1.1. Module import examples
Consider the example module "example-module" from Section 3.4 that is
hypothetically available in the following revisions: 2019-01-01,
2019-02-01, 2019-03-01, 2019-04-01, 2019-05-01 and 2019-06-01. The
relationship between the revisions is as before:
Module revision date
2019-01-01
|
2019-02-01
| \
2019-03-01 \
| \
| 2019-04-01
| |
2019-05-01 |
|
2019-06-01
4.1.1.1. Example 1
This example recommends module revisions for import whose revision
date is or comes after 2019-02-01. For example, this dependency
might be used if there was a new container added in revision
2019-02-01 that is augmented by the importing module. It includes
the following revisions: 2019-02-01, 2019-03-01, 2019-04-01,
2019-05-01 and 2019-06-01.
import example-module {
rev:recommended-min-date 2019-02-01;
}
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4.1.1.2. Example 2
This example recommends module revisions for import whose revision
date is or comes after 2019-04-01. It includes the following
revisions: 2019-04-01, 2019-05-01 and 2019-06-01, even though
revision 2019-05-01 may not contain what is desired from 2019-04-01.
This shows that "recommended-min-date" is not well suited for a
branched revision history, and is most helpful when a module is
restricted to a linear chronological development history.
import example-module {
rev:recommended-min-date 2019-04-01;
}
5. The ietf-yang-library-status YANG module
This document defines the YANG module, ietf-yang-library-status, that
augments YANG library [RFC8525] with two leafs to indicate how a
server implements deprecated and obsolete schema nodes.
The "ietf-yang-library-status" YANG module has the following
structure (using the notation defined in [RFC8340]):
module: ietf-yang-library-status
augment /yanglib:yang-library/yanglib:schema:
+--ro deprecated-nodes-implemented? boolean
+--ro obsolete-nodes-absent? boolean
5.1. Reporting how deprecated and obsolete nodes are handled
The ietf-yang-library-status YANG module augments YANG library with
two boolean leafs to allow a server to report how it implements
status "deprecated" and status "obsolete" schema nodes. The leafs
are:
deprecated-nodes-implemented: If set to "true", this leaf indicates
that all schema nodes with a status "deprecated" are implemented
equivalently as if they had status "current" unless this is
overridden by a deviation statement. Concretely, when this leaf
is set to "true", deviations MUST be used by the server to
explicitly remove any "deprecated" nodes from the schema that are
not implemented. If this leaf value is "false", the leaf's
default value, then the behavior is unspecified.
obsolete-nodes-absent: If set to "true", this leaf indicates that
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the server does not implement any status "obsolete" schema nodes.
If this leaf value is "false", the leaf's default value, then the
behavior is unspecified.
In order to allow clients to determine the exact schema used by
servers, it is RECOMMENDED that servers set both the "deprecated-
nodes-implemented" and "obsolete-nodes-absent" leafs to "true". Not
adhering with this recommendation leads to an ambiguity about the
exact data nodes supported by a server.
If a server does not set the "deprecated-nodes-implemented" leaf to
"true", then clients MUST NOT rely solely on the "rev:non-backwards-
compatible" statements to determine whether two module revisions are
backwards-compatible, and MUST also consider whether the status of
any nodes have changed to "deprecated" and whether those nodes are
implemented by the server. An implementation may choose to add
deviations to more clearly indicate to the clients that these
deprecated nodes are not supported.
6. Guidelines for using the YANG module update rules
The following text updates section 4.7 of [RFC9907] to revise the
guidelines for updating YANG modules.
6.1. Guidelines for YANG module authors
All IETF YANG modules MUST conform to this specification. In
particular, sections: Section 3, Section 4, and the guidelines
documented in this section.
NBC changes to YANG modules may cause problems to clients, who are
consumers of YANG data models, and hence YANG module authors SHOULD
minimize NBC changes and keep changes BC whenever possible.
When NBC changes are introduced, consideration should be given to the
impact on clients and YANG module authors SHOULD try to mitigate that
impact.
A "rev:non-backwards-compatible" statement MUST be added if there are
NBC changes relative to the previous revision.
Removing old revision statements from a module's revision history can
cause a loss of visibility of when non-backwards-compatible changes
were made, and hence it is RECOMMENDED to retain them. An
alternative solution, if the revision section is too long, would be
to remove, or curtail, the older description statements associated
with the previous revisions.
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In cases where a revision dependency is helpful for a module import,
the "rev:recommended-min-date" extension SHOULD be used in preference
to the "revision-date" statement, which causes overly strict import
dependencies and SHOULD NOT be used.
A module that includes submodules SHOULD use the "revision-date"
statement to include specific submodule revisions. The revision of
the including module MUST be updated when any included submodule has
changed.
In some cases a module or submodule revision that is not strictly NBC
by the definition in Section 3.1.2 of this specification may include
the "non-backwards-compatible" statement. Here is an example when
adding the statement may be desirable:
* A "config false" leaf had its value space expanded (for example, a
range was increased, or additional enum values were added) and the
author or server implementor feels there is a significant
compatibility impact for clients and users of the module or
submodule
6.1.1. Making non-backwards-compatible changes to a YANG module
There are various valid situations where a YANG module has to be
modified in a NBC way. Here are some guidelines on how non-
backwards-compatible changes can be made incrementally, with the
assumption that deprecated nodes are implemented by the server, and
obsolete nodes are not:
1. The changes should be made gradually, e.g., a schema node's
status SHOULD NOT be changed directly from "current" to
"obsolete" (see Section 4.7 of [RFC9907]). Instead, the status
SHOULD first be marked "deprecated". At some point in the
future, when support is removed for the schema node, there are
two options. The first, and preferred, option is to keep the
schema node definition in the module and change the status to
“obsolete”. The second option is to simply remove the schema node
from the module, but this has the risk of breaking modules that
import the modified module, and the removed identifier may be
accidentally reused in a future revision.
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2. For deprecated schema nodes the "description" statement SHOULD
also indicate until when support for the schema node is
guaranteed (if known). If there is a replacement schema node for
the deprecated schema node, this MUST either be stated in an
updated "description" or preferably via a standardised machine-
readable alternative YANG statement, if that becomes available.
The reason for deprecating the schema node can also be included
in the "description" if it is deemed to be of potential interest
to the user.
3. For obsolete schema nodes, it is RECOMMENDED to keep the above
information, from when the node had status "deprecated", which is
still relevant.
4. When obsoleting or deprecating schema nodes, the "deprecated" or
"obsolete" status SHOULD be applied at the highest possible level
in the data tree. For clarity, the "status" statement SHOULD
also be applied to all descendent schema nodes, but the
additional status related information does not need to be
repeated if it does not introduce any additional information.
5. NBC changes which can break imports SHOULD be avoided because of
the impact on the importing module. The importing modules could
get broken, e.g., if an augmented node in the importing module
has been removed from the imported module. Alternatively, the
schema of the importing modules could undergo a NBC change due to
the NBC change in the imported module, e.g., if a node in a
grouping has been removed. As described in Appendix B.1, instead
of removing a node, that node SHOULD first be deprecated and then
obsoleted.
See Appendix B for examples of how NBC changes can be made.
6.2. Versioning Considerations for Clients
Guidelines for the developers of clients of modules using the new
module revision update procedure:
* When interacting with a server that implements a non-backwards-
compatible revision of a YANG module, a client may receive data
that is valid per the server's schema but outside the value
constraints of the module revision the client was coded against.
For example, the server may have increased the range of an
operational node. Clients SHOULD handle such values defensively,
taking care to guard against issues such as buffer overruns or
other unexpected behavior arising from out-of-range values.
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* Clients SHOULD monitor changes to published YANG modules through
their revision history, and use appropriate tooling to understand
the specific changes between module revision. In particular,
clients SHOULD NOT migrate to NBC revisions of a module without
understanding any potential impact of the specific NBC changes.
* Clients SHOULD plan to make changes to match published status
changes. When a node's status changes from "current" to
"deprecated", clients SHOULD plan to stop using that node in a
timely fashion. When a node's status changes to "obsolete",
clients MUST stop using that node.
7. Module Versioning Extension YANG Modules
7.1. Module ietf-yang-revisions
YANG module with extension statements for annotating NBC changes and
importing by revision.
<CODE BEGINS> file "ietf-yang-revisions@2026-06-26.yang"
module ietf-yang-revisions {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-yang-revisions";
prefix rev;
import ietf-yang-types {
prefix yang;
reference
"RFC 9911: Common YANG Data Types";
}
organization
"IETF NETMOD (Network Modeling) Working Group";
contact
"WG Web: <https://datatracker.ietf.org/wg/netmod/>
WG List: NETMOD <mailto:netmod@ietf.org>
Author: Joe Clarke
<mailto:jclarke@cisco.com>
Author: Reshad Rahman
<mailto:reshad@yahoo.com>
Author: Robert Wilton
<mailto:rwilton@cisco.com>
Author: Balazs Lengyel
<mailto:balazs.lengyel@ericsson.com>
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Author: Jason Sterne
<mailto:jason.sterne@nokia.com>";
description
"This YANG 1.1 module contains definitions and extensions to
support updated YANG revision handling.
Copyright (c) 2026 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 Revised BSD License set
forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(https://trustee.ietf.org/license-info).
All revisions of IETF and IANA published modules can be found
at the YANG Parameters registry group
(https://www.iana.org/assignments/yang-parameters).
This version of this YANG module is part of RFC XXXX; see
the RFC itself for full legal notices.
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 (RFC 2119) (RFC 8174) when, and only when,
they appear in all capitals, as shown here.";
// RFC Ed.: update the date below with the date of RFC publication
// and remove this note.
// RFC Ed.: replace XXXX (inc above) with actual RFC number and
// remove this note.
revision 2026-06-26 {
description
"Initial version.";
reference
"RFC XXXX: Updated YANG Module Revision Handling";
}
typedef revision-date {
type yang:date-no-zone;
description
"A date associated with a YANG revision.
Matches dates formatted as YYYY-MM-DD.";
reference
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"RFC 7950: The YANG 1.1 Data Modeling Language";
}
extension non-backwards-compatible {
description
"This statement is used to indicate YANG module revisions that
contain non-backwards-compatible changes.
The statement MUST only be a substatement of the 'revision'
statement. Zero or one 'non-backwards-compatible' statements
per parent statement is allowed. No substatements for this
extension have been standardized.
If a revision of a YANG module contains changes, relative to
the preceding revision in the revision history, that do not
conform to the backwards-compatible module update rules
defined in RFC-XXXX, then the 'non-backwards-compatible'
statement MUST be added as a substatement to the revision
statement.
Conversely, if a revision does not contain a
'non-backwards-compatible' statement then all changes,
relative to the preceding revision in the revision history,
MUST be backwards-compatible.
A new module revision that only contains changes that are
backwards-compatible SHOULD NOT include the
'non-backwards-compatible' statement. An example of when an
author might add the 'non-backwards-compatible' statement is
if they believe a change could negatively impact clients even
though the backwards compatibility rules defined in RFC-XXXX
classify it as a backwards-compatible change.
Add, removing, or changing a 'non-backwards-compatible'
statement is a backwards-compatible version change.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 3.2,
non-backwards-compatible extension statement";
}
extension recommended-min-date {
argument revision-date;
description
"Recommends the revision of the module that may be imported to
one whose revision date matches or is after the specified
revision-date.
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The argument value MUST conform to the 'revision-date' defined
type.
The statement MUST only be a substatement of the import
statement. Zero or one 'recommended-min-date' extension
statement is allowed for each parent 'import' statement.
No substatements for this extension have been standardized.
A particular revision of an imported module adheres to an
import's 'recommended-min-date' extension statement if the
imported module's revision date is equal to or later than
the revision date argument of the 'recommended-min-date'
extension statement in the importing module.
Adding, removing or updating a 'recommended-min-date'
statement to an import is a backwards-compatible change.";
reference
"XXXX: Updated YANG Module Revision Handling; Section 4,
Guidance for revision selection on imports";
}
}
<CODE ENDS>
7.2. Module ietf-yang-library-status
YANG module for status conformance.
<CODE BEGINS> file "ietf-yang-library-status@2026-06-26.yang"
module ietf-yang-library-status {
yang-version 1.1;
namespace
"urn:ietf:params:xml:ns:yang:ietf-yang-library-status";
prefix yls;
import ietf-yang-library {
prefix "yanglib";
reference
"RFC 8525: YANG Library";
}
organization
"IETF NETMOD (Network Modeling) Working Group";
contact
"WG Web: <https://datatracker.ietf.org/wg/netmod/>
WG List: NETMOD <mailto:netmod@ietf.org>
Author: Joe Clarke
<mailto:jclarke@cisco.com>
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Author: Reshad Rahman
<mailto:reshad@yahoo.com>
Author: Robert Wilton
<mailto:rwilton@cisco.com>
Author: Balazs Lengyel
<mailto:balazs.lengyel@ericsson.com>
Author: Jason Sterne
<mailto:jason.sterne@nokia.com>";
description
"This module contains augmentations to YANG Library to provide an
indication of how deprecated and obsolete nodes are handled by
the server.
Copyright (c) 2026 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 Revised BSD License set
forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(https://trustee.ietf.org/license-info).
All revisions of IETF and IANA published modules can be found
at the YANG Parameters registry group
(https://www.iana.org/assignments/yang-parameters).
This version of this YANG module is part of RFC XXXX; see
the RFC itself for full legal notices.
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 (RFC 2119) (RFC 8174) when, and only when,
they appear in all capitals, as shown here.";
// RFC Ed.: update the date below with the date of RFC publication
// and remove this note.
// RFC Ed.: replace XXXX (including in the imports above) with
// actual RFC number and remove this note.
revision 2026-06-26 {
description
"Initial revision";
reference
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"XXXX: Updated YANG Module Revision Handling";
}
augment "/yanglib:yang-library/yanglib:schema" {
description
"Augmentations to the ietf-yang-library module to indicate how
deprecated and obsoleted nodes are handled by the server.";
leaf deprecated-nodes-implemented {
type boolean;
default false;
description
"If set to true, this leaf indicates that all schema nodes
with a status 'deprecated' are implemented equivalently as
if they had status 'current' unless this is overridden by a
deviation statement. Concretely, when this leaf is set to
true, deviations MUST be used to explicitly remove any
deprecated nodes from the schema that are not implemented.
If this leaf is false, then the behavior is unspecified.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 5.1, Reporting how deprecated and obsolete nodes
are handled";
}
leaf obsolete-nodes-absent {
type boolean;
default false;
description
"If set to true, this leaf indicates that the server does not
implement any status 'obsolete' schema nodes. If this leaf
is false, then the behavior is unspecified.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 5.1, Reporting how deprecated and obsolete nodes
are handled";
}
}
}
<CODE ENDS>
8. Operational considerations
This document specifies mechanisms to identify, describe, and select
YANG module revisions, including revisions that contain non-
backwards-compatible changes. These mechanisms are intended to
improve the ability of operators and management applications to
assess the impact of using, upgrading to, or importing a particular
module revision.
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When a YANG module or submodule is revised, it is useful for authors
to describe any relevant operational impacts of the changes in the
revision description. In particular, such descriptions can highlight
changes that may affect existing deployments, configuration,
monitoring, automation, or interoperability between clients and
servers. This information can assist operators and tooling in
evaluating whether a change can be safely deployed, whether migration
actions may be required, and whether clients need to be updated.
In environments where multiple module revisions may coexist across
devices or network domains, differences between revisions can
introduce version skew between clients and servers. Awareness of
such differences can help avoid configuration errors, unexpected
behavior, or reduced interoperability.
Tooling and automation systems may use the mechanisms defined in this
document to detect revision compatibility and to select appropriate
revisions when interacting with devices.
This document does not introduce new operational procedures or
manageability requirements beyond enabling improved visibility into
revision differences.
9. Security considerations
9.1. Security considerations for module revisions
As discussed in the introduction of this document, YANG modules
occasionally undergo changes that are not backwards compatible. This
occurs in both standards and vendor YANG modules despite the
prohibitions in RFC 7950. RFC 7950 also allows nodes to change to
status 'obsolete' which can change behavior and compatibility for a
client.
The fact that YANG modules change in a non-backwards-compatible
manner may have security implications. Such changes should be
carefully considered, including the scenarios described below. The
rev:non-backwards-compatible extension statement introduced in this
document provides an alert that the module or submodule may contain
changes that impact users and need to be examined more closely for
both compatibility and potential security implications. Flagging the
change reduces the risk of introducing silent exploitable
vulnerabilities.
When a module undergoes a non-backwards-compatible change, a server
may implement different semantics for a given leaf than a client
using an older version of the module is expecting. If the particular
leaf controls any security functions of the device, or is related to
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parts of the configuration or state that are sensitive from a
security point of view, then the difference in behavior between the
old and new revisions needs to be considered carefully. In
particular, changes to the default of the leaf should be examined.
Implementors and users should also consider impact to data node
access control rules (e.g. The Network Configuration Access Control
Model (NACM) [RFC8341]) in the face of non-backwards-compatible
changes. Access rules may need to be adjusted when a new module
revision is introduced that contains a non-backwards-compatible
change.
If the changes to a module or submodule have security implications,
it is recommended to highlight those implications in the description
of the revision statement.
9.2. Security considerations for the modules defined in this document
This section uses the template described in Section 3.7.1 of
[RFC9907].
The "ietf-yang-library-status" YANG module specified in this document
defines a schema for data that is designed to be accessed via YANG-
based management protocols, such as NETCONF [RFC6241] or RESTCONF
[RFC8040]. These YANG-based management protocols (1) have to use a
secure transport layer (e.g., Secure Shell (SSH) [RFC4252], TLS
[RFC8446], and QUIC [RFC9000]) and (2) have to use mutual
authentication.
The Network Configuration Access Control Model (NACM) [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.
This document does not define any new protocol or data nodes that are
writable.
This document updates YANG Library [RFC8525] with augmentations to
include two boolean leafs that indicate whether status deprecated and
status obsolete schema nodes are implemented by the server. These
read-only augmentations do not add any new security considerations
beyond those already present in [RFC8525].
10. IANA Considerations
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10.1. YANG Module Registrations
IANA is requested to register the following URIs in the "ns" registry
within the "IETF XML Registry" group [RFC3688]:
URI: urn:ietf:params:xml:ns:yang:ietf-yang-revisions
Registrant Contact: The IESG.
XML: N/A, the requested URI is an XML namespace.
URI: urn:ietf:params:xml:ns:yang:ietf-yang-library-status
Registrant Contact: The IESG.
XML: N/A, the requested URI is an XML namespace.
IANA is requested to register the following YANG modules in the "YANG
Module Names" registry [RFC6020] within the "YANG Parameters"
registry group:
The ietf-yang-revisions module:
Name: ietf-yang-revisions
Maintained by IANA? N
XML Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-revisions
Prefix: rev
Reference: [RFCXXXX]
The ietf-yang-library-status module:
Name: ietf-yang-library-status
Maintained by IANA? N
XML Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-library-
status
Prefix: yls
Reference: [RFCXXXX]
10.2. Guidance for versioning in IANA-maintained YANG modules
Note for IANA (to be removed by the RFC editor): Please check that
the registries and IANA YANG modules are referenced in the
appropriate way.
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IANA is responsible for maintaining and versioning YANG modules that
are derived from other IANA registries. For example,
"iana-if-type.yang" [IfTypeYang] is derived from the "Interface Types
(ifType) IANA registry" [IfTypesReg], and "iana-routing-types.yang"
[RoutingTypesYang] is derived from the "Address Family Numbers"
[AddrFamilyReg] and "Subsequent Address Family Identifiers (SAFI)
Parameters" [SAFIReg] IANA registries.
Normally, updates to the registries cause any derived YANG modules to
be updated in a backwards-compatible way, but there are some cases
where the registry updates can cause non-backward-compatible updates
to the derived YANG module. An example of such an update is the
2020-12-31 revision of iana-routing-types.yang
[RoutingTypesDecRevision], where the enum name for two SAFI values
was changed.
In all cases, IANA follows the versioning guidance specified in
Section 3.1, and includes a "rev:non-backwards-compatible"
substatement to the latest revision statement whenever an IANA-
maintained module is updated in a non-backwards-compatible way, as
described in Section 3.2.
For published IANA-maintained YANG modules that contain non-
backwards-compatible changes between revisions, when the next
revision is created, the "rev:non-backwards-compatible" substatement
should be retroactively added to any prior revisions containing non-
backwards-compatible changes.
This document defines the normative requirements for versioning of
IANA-maintained YANG modules. [I-D.ietf-netmod-iana-yang-guidance]
provides operational guidance on how to achieve these normative
requirements for IANA-maintained YANG modules.
11. References
11.1. Normative References
[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>.
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[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>.
[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>.
[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>.
[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>.
[RFC8525] Bierman, A., Bjorklund, M., Schoenwaelder, J., Watsen, K.,
and R. Wilton, "YANG Library", RFC 8525,
DOI 10.17487/RFC8525, March 2019,
<https://www.rfc-editor.org/info/rfc8525>.
[RFC9907] Bierman, A., Boucadair, M., Ed., and Q. Wu, "Guidelines
for Authors and Reviewers of Documents Containing YANG
Data Models", BCP 216, RFC 9907, DOI 10.17487/RFC9907,
March 2026, <https://www.rfc-editor.org/info/rfc9907>.
[RFC9911] Schönwälder, J., Ed., "Common YANG Data Types", RFC 9911,
DOI 10.17487/RFC9911, December 2025,
<https://www.rfc-editor.org/info/rfc9911>.
11.2. Informative References
[AddrFamilyReg]
"Address Family Numbers IANA Registry",
<https://www.iana.org/assignments/address-family-numbers/
address-family-numbers.xhtml>.
[I-D.clacla-netmod-yang-model-update]
Claise, B., Clarke, J., Lengyel, B., and K. D'Souza, "New
YANG Module Update Procedure", Work in Progress, Internet-
Draft, draft-clacla-netmod-yang-model-update-06, 2 July
2018, <https://datatracker.ietf.org/doc/html/draft-clacla-
netmod-yang-model-update-06>.
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[I-D.ietf-netmod-iana-yang-guidance]
Wilton, R., "Guidance for Managing YANG Modules in RFCs
and IANA Registries", Work in Progress, Internet-Draft,
draft-ietf-netmod-iana-yang-guidance-02, 14 April 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
iana-yang-guidance-02>.
[I-D.ietf-netmod-yang-packages]
Wilton, R., Rahman, R., Clarke, J., and J. Sterne, "YANG
Packages", Work in Progress, Internet-Draft, draft-ietf-
netmod-yang-packages-07, 2 March 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
yang-packages-07>.
[I-D.ietf-netmod-yang-schema-comparison]
Andersson, P., Wilton, R., and M. Vaško, "YANG Schema
Comparison", Work in Progress, Internet-Draft, draft-ietf-
netmod-yang-schema-comparison-08, 27 May 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
yang-schema-comparison-08>.
[I-D.ietf-netmod-yang-semver]
Clarke, J., Wilton, R., Rahman, R., Lengyel, B., Sterne,
J., and B. Claise, "YANG Semantic Versioning", Work in
Progress, Internet-Draft, draft-ietf-netmod-yang-semver-
26, 7 May 2026, <https://datatracker.ietf.org/doc/html/
draft-ietf-netmod-yang-semver-26>.
[I-D.ietf-netmod-yang-ver-selection]
Wilton, R., Rahman, R., Clarke, J., Sterne, J., and B. Wu,
"YANG Schema Selection", Work in Progress, Internet-Draft,
draft-ietf-netmod-yang-ver-selection-00, 17 March 2020,
<https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
yang-ver-selection-00>.
[I-D.ietf-netmod-yang-versioning-reqs]
Clarke, J., "YANG Module Versioning Requirements", Work in
Progress, Internet-Draft, draft-ietf-netmod-yang-
versioning-reqs-13, 18 January 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
yang-versioning-reqs-13>.
[IfTypesReg]
"Interface Types (ifType) IANA Registry",
<https://www.iana.org/assignments/smi-numbers/smi-
numbers.xhtml#smi-numbers-5>.
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[IfTypeYang]
"iana-if-type YANG Module",
<https://www.iana.org/assignments/iana-if-type/iana-if-
type.xhtml>.
[RFC4252] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252,
January 2006, <https://www.rfc-editor.org/info/rfc4252>.
[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>.
[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>.
[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>.
[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>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
[RoutingTypesDecRevision]
"2020-12-31 revision of iana-routing-types.yang",
<https://www.iana.org/assignments/yang-parameters/iana-
routing-types@2020-12-31.yang>.
[RoutingTypesYang]
"iana-routing-types YANG Module",
<https://www.iana.org/assignments/iana-routing-types/iana-
routing-types.xhtml>.
[SAFIReg] "Subsequent Address Family Identifiers (SAFI) Parameters
IANA Registry", <https://www.iana.org/assignments/safi-
namespace/safi-namespace.xhtml>.
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Appendix A. Examples of changes that are NBC
Examples of NBC changes include (this list is for illustrative
purposes and not intended to be complete):
* Deleting a data node, or changing it to status obsolete.
* Changing the name or units of a data node.
* Changing the type of a data node if that leads to a change in the
syntax or semantics of the type. See Section 11 of [RFC7950].
* Modifying the description in a way that changes the semantic
meaning of the data node.
* Any changes that remove any previously allowed values from the
allowed value set of the data node, either through changes in the
type definition, or the addition or changes to "must" statements,
or changes in the description.
* Adding or modifying "when" statements that reduce when the data
node is available in the schema.
* Making the statement conditional on if-feature.
Appendix B. Examples of applying the NBC change guidelines
The following sections give steps that could be taken for making NBC
changes to a YANG module or submodule using the incremental approach
described in Section 6.1.1.
The examples are all for "config true" nodes.
B.1. Removing a data node
Removing a leaf or container from the data tree, e.g., because
support for the corresponding feature is being removed:
1. The schema node's status is changed to "deprecated" and the node
is supported for some period of time (e.g. one year). This is a
BC change.
2. When the schema node is not supported anymore, its status is
changed to "obsolete" and the "description" updated. This is a
NBC change.
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B.2. Changing the type of a leaf node
Changing the type of a leaf node. e.g., a "vpn-id" node of type
integer being changed to a string:
1. The status of schema node "vpn-id" is changed to "deprecated" and
the node is supported for some period of time (e.g. one year).
This is a BC change. The description is updated to indicate that
“vpn-name” is replacing this node.
2. A new schema node, e.g., "vpn-name", of type string is added to
the same location as the existing node "vpn-id". This new node
has status "current" and its description explains that it is
replacing node "vpn-id".
3. During the period of time when both schema nodes are supported,
the interactions between the two nodes is outside the scope of
this document and will vary on a case by case basis. One
possible option is to have the server prevent the new node from
being set if the old node is already set (and vice versa). The
new node could have a "when" statement added to it to achieve
this. The old node, however, must not have a "when" statement
added, or an existing "when" modified to be more restrictive,
since this would be a NBC change. In any case, the server could
reject the old node from being set if the new node is already
set.
4. When the schema node "vpn-id" is not supported anymore, its
status is changed to "obsolete" and the "description" is updated.
This is a NBC change.
B.3. Reducing the range of a leaf node
Reducing the range of values of a leaf-node, e.g., consider a "vpn-
id" schema node of type uint32 being changed from range 1..5000 to
range 1..2000:
1. If all values which are being removed were never supported, e.g.,
if a vpn-id of 2001 or higher was never accepted, this is a BC
change for the functionality (no functionality change). Even if
it is a NBC change for the YANG model, there should be no impact
for clients using that YANG model.
2. If one or more values being removed was previously supported,
e.g., if a vpn-id of 3333 was accepted previously, this is a NBC
change for the YANG model. Clients using the old YANG model will
be impacted, so a change of this nature should be done carefully,
e.g., by using the steps described in Appendix B.2
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In both cases above, the "rev:non-backwards-compatible" extension
statement is used to indicate that the YANG module contains a NBC
change.
B.4. Changing the key of a list
Changing the key of a list has a big impact to the client. For
example, consider a "sessions" list which has a key "interface" and
there is a need to change the key to "dest-address". Such a change
can be done in steps:
1. The status of list "sessions" is changed to "deprecated" and the
list is supported for some period of time (e.g. one year). This
is a BC change. The description is updated to indicate the new
list that is replacing this list.
2. A new list is created in the same location with the same
descendant schema nodes but with "dest-address" as key. Finding
an appropriate name for the new list can be difficult. In this
case the new list is called "sessions-address", has status
"current" and its description should explain that it is replacing
list "session".
3. During the period of time when both lists are supported, the
interactions between the two lists is outside the scope of this
document and will vary on a case by case basis. One possible
option is to have the server prevent entries in the new list from
being created if the old list already has entries (and vice
versa).
4. When the list "sessions" is not available anymore, its status is
changed to "obsolete" and its "description" is updated. This is
a NBC change.
B.5. Renaming a node
A leaf or container schema node may be renamed, either due to a
spelling error in the previous name or because of a better name. For
example a node "ip-adress" could be renamed to "ip-address":
1. The status of the existing node "ip-adress" is changed to
"deprecated" and is supported for some period of time (e.g. one
year). This is a BC change. The description is updated to
indicate the node that is replacing this node.
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2. The new schema node "ip-address" is added to the same location as
the existing node "ip-adress". This new node has status
"current" and its description should explain that it is replacing
node "ip-adress".
3. During the period of time when both nodes are available, the
interactions between the two nodes is outside the scope of this
document and will vary on a case by case basis. One possible
option is to have the server prevent the new node from being set
if the old node is already set (and vice versa). The new node
could have a "when" statement added to it to achieve this. The
old node, however, must not have a "when" statement added, or an
existing "when" modified to be more restrictive, since this would
be a NBC change. In any case, the server could reject the old
node from being set if the new node is already set.
4. When the node "ip-adress" is not available anymore, its status is
changed to "obsolete" and its "description" is updated. This is
a NBC change.
Appendix C. Examples when breaking NBC changes have to be made
The following sections give examples when breaking NBC changes have
to be made, i.e. when the incremental approach described in
Section 6.1.1 can not be followed.
The examples are all for "config true" nodes.
C.1. Removing a data node
Removing a leaf or container immediately from the data tree, e.g.,
because the corresponding functionality is no longer supported:
1. The schema node's status is changed to "obsolete" and the
"description" updated. This is a NBC change.
C.2. Changing the type of a leaf node
Changing the type of a leaf node. e.g., a "vpn-id" node of type
integer being changed to a string. The change can not be made
incrementally because the system can not support both types
simultaneously. The following change is made:
1. The type of leaf "vpn-id" is changed from integer to string.
This is a NBC change.
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Appendix D. Example instance-data for ietf-yang-library-status module
This section provides an illustrative example of instance-data for
the ietf-yang-library-status module. It is based on the example in
Appendix B of [RFC8525], with the "module-set" instances removed for
brevity.
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<?xml version="1.0" encoding="UTF-8"?>
<yang-library
xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library"
xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores"
xmlns:yls="urn:ietf:params:xml:ns:yang:ietf-yang-library-status">
<schema>
<name>config-schema</name>
<module-set>config-modules</module-set>
<yls:deprecated-nodes-implemented>
true
</yls:deprecated-nodes-implemented>
<yls:obsolete-nodes-absent>true</yls:obsolete-nodes-absent>
</schema>
<schema>
<name>state-schema</name>
<module-set>config-modules</module-set>
<module-set>state-modules</module-set>
<yls:deprecated-nodes-implemented>
true
</yls:deprecated-nodes-implemented>
<yls:obsolete-nodes-absent>
true
</yls:obsolete-nodes-absent>
</schema>
<datastore>
<name>ds:startup</name>
<schema>config-schema</schema>
</datastore>
<datastore>
<name>ds:running</name>
<schema>config-schema</schema>
</datastore>
<datastore>
<name>ds:operational</name>
<schema>state-schema</schema>
</datastore>
<content-id>
75a43df9bd56b92aacc156a2958fbe12312fb285
</content-id>
</yang-library>
Contributors
The following people made substantial contributions to this document:
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Bo Wu
lana.wubo@huawei.com
Jan Lindblad
jan.lindblad+ietf@for.eco
Acknowledgments
This document grew out of the YANG module versioning design team that
started after IETF 101. The authors, contributors and the following
individuals are (or have been) members of the design team and have
worked on the YANG versioning project:
An LLM was used to generate, under guidance and subsequently reviewed
by the authors, the operational considerations section of this
document.
Benoit Claise
benoit@everything-ops.net
Ebben Aries
exa@juniper.net
Juergen Schoenwaelder
j.shoenwaelder@jacobs-university.de
Mahesh Jethanandani
mjethanandani@gmail.com
Michael (Wangzitao)
wangzitao@huawei.com
Per Andersson
per.ietf@ionio.se
Qin Wu
bill.wu@huawei.com
The initial revision of this document was refactored and built upon
[I-D.clacla-netmod-yang-model-update]. We would like to thank Kevin
D'Souza and Benoit Claise for their initial work in this problem
space.
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Discussions on the use of Semver for YANG versioning has been held
with authors of the OpenConfig YANG models. We would like to thank
both Anees Shaikh and Rob Shakir for their input into this problem
space.
We would also like to thank Lou Berger, Andy Bierman, Martin
Bjorklund, Italo Busi, Tom Hill, Scott Mansfield, and Kent Watsen for
their contributions and review comments.
Authors' Addresses
Robert Wilton (editor)
Cisco Systems, Inc.
Email: rwilton@cisco.com
Reshad Rahman (editor)
Equinix
Email: reshad@yahoo.com
Balazs Lengyel (editor)
Ericsson
Email: balazs.lengyel@ericsson.com
Joe Clarke
Cisco Systems, Inc.
Email: jclarke@cisco.com
Jason Sterne
Nokia
Email: jason.sterne@nokia.com
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