Internet DRAFT - draft-ietf-netmod-yang-module-versioning
draft-ietf-netmod-yang-module-versioning
Network Working Group R. Wilton, Ed.
Internet-Draft Cisco Systems, Inc.
Updates: 6020, 7950, 8407, 8525 (if approved) R. Rahman, Ed.
Intended status: Standards Track Equinix
Expires: 2 September 2024 B. Lengyel, Ed.
Ericsson
J. Clarke
Cisco Systems, Inc.
J. Sterne
Nokia
1 March 2024
Updated YANG Module Revision Handling
draft-ietf-netmod-yang-module-versioning-11
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 minimum
revision suggestion to help 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 8407 and RFC 8525.
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
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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 2 September 2024.
Copyright Notice
Copyright (c) 2024 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
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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 . . . . . . . . . . . . . . . . . 4
3. Refinements to YANG revision handling . . . . . . . . . . . . 5
3.1. Updating a YANG module with a new revision . . . . . . . 6
3.1.1. Backwards-compatible rules . . . . . . . . . . . . . 7
3.1.2. Non-backwards-compatible changes . . . . . . . . . . 8
3.2. non-backwards-compatible extension statement . . . . . . 8
3.3. Removing revisions from the revision history . . . . . . 8
3.4. Examples for updating the YANG module revision history . 9
4. Guidance for revision selection on imports . . . . . . . . . 12
4.1. Recommending a minimum revision for module imports . . . 13
4.1.1. Module import examples . . . . . . . . . . . . . . . 14
5. New ietf-yang-status-conformance YANG module . . . . . . . . 15
5.1. Reporting how deprecated and obsolete nodes are
handled . . . . . . . . . . . . . . . . . . . . . . . . . 15
6. Guidelines for using the YANG module update rules . . . . . . 16
6.1. Guidelines for YANG module authors . . . . . . . . . . . 16
6.1.1. Making non-backwards-compatible changes to a YANG
module . . . . . . . . . . . . . . . . . . . . . . . 17
6.2. Versioning Considerations for Clients . . . . . . . . . . 18
7. Module Versioning Extension YANG Modules . . . . . . . . . . 18
8. Security considerations . . . . . . . . . . . . . . . . . . . 24
8.1. Security considerations for module revisions . . . . . . 24
8.2. Security considerations for the modules defined in this
document . . . . . . . . . . . . . . . . . . . . . . . . 25
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
9.1. YANG Module Registrations . . . . . . . . . . . . . . . . 25
9.2. Guidance for versioning in IANA maintained YANG
modules . . . . . . . . . . . . . . . . . . . . . . . . . 26
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
10.1. Normative References . . . . . . . . . . . . . . . . . . 27
10.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. Examples of changes that are NBC . . . . . . . . . . 30
Appendix B. Examples of applying the NBC change guidelines . . . 31
B.1. Removing a data node . . . . . . . . . . . . . . . . . . 31
B.2. Changing the type of a leaf node . . . . . . . . . . . . 31
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B.3. Reducing the range of a leaf node . . . . . . . . . . . . 32
B.4. Changing the key of a list . . . . . . . . . . . . . . . 32
B.5. Renaming a node . . . . . . . . . . . . . . . . . . . . . 33
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction
The current YANG [RFC7950] module update rules 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.
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] provides 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 1.1 module revision update procedure,
supported by new extension statements to indicate when a revision
contains non-backwards-compatible changes.
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* Updated guidance for revision selection on imports and a YANG
extension statement allowing YANG module imports to document an
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.
This document updates [RFC8407] section 4.7. 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.
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
In addition, this document uses the following terminology:
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* 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
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, revision B is defined as being
derived from revision A, if revision A is listed in the revision
history of revision B. 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 is was derived, i.e., the module revision's history
describes a single path of derived revisions back to the root of the
module's revision history.
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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 identifies 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
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 an 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.
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* 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.
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.
* 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, if it does not change the
semantics of the module. Extension statement definitions SHOULD
specify whether adding, removing, or changing statements defined
by that extension are 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"
substatetements, 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.
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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 the
preceding revision in the revision history, 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
Authors may wish to remove revision statements from a 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.
An author MAY remove a contiguous sequence of entries from the end
(i.e., 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.
An author 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.
The author MUST NOT remove the first (i.e., newest) revision entry in
the revision history.
Example revision history:
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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]:
Example YANG module with branched revision history using version
identifiers defined in [I-D.ietf-netmod-yang-semver].
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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 "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 "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 require 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 occurs, and hence
section Section 6.1 suggests that authors do not restrict YANG module
imports to exact revision dates.
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
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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 an 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 are very unlikely to ever be
compatible.
This module introduces, 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.
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.
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Adding, modifying or removing a "recommended-min-date" extension
statement is a BC change.
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. E.g., 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;
}
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.
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import example-module {
rev:recommended-min-date 2019-04-01;
}
5. New ietf-yang-status-conformance YANG module
This document defines the YANG module, ietf-yang-status-conformance,
that augments YANG library [RFC8525] with two leafs to indicate how a
server implements deprecated and obsolete schema nodes.
The "ietf-yang-status-conformance" YANG module has the following
structure (using the notation defined in [RFC8340]):
module: ietf-yang-status-conformance
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-status-conformance 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"; otherwise deviations
MUST be used by the server to explicitly remove "deprecated" nodes
from the schema. If this leaf is set to "false" or absent, then
the behavior is unspecified.
obsolete-nodes-absent: If set to "true", this leaf indicates that
the server does not implement any status "obsolete" schema nodes.
If this leaf is set to "false" or absent, then the behaviour is
unspecified.
Servers SHOULD set both the "deprecated-nodes-implemented" and
"obsolete-nodes-absent" leafs to "true", which allows clients to
determine the exact schema used by the server.
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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 has changed to "deprecated" and whether those nodes are
implemented by the server.
6. Guidelines for using the YANG module update rules
The following text updates section 4.7 of [RFC8407] 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 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.
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.
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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 an 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 data node's status
SHOULD NOT be changed directly from "current" to "obsolete" (see
Section 4.7 of [RFC8407]), instead the status SHOULD first be
marked "deprecated". At some point in the future, when support
is removed for the data node, there are two options. The first,
and preferred, option is to keep the data node definition in the
model and change the status to “obsolete”. The second option is
to simply remove the data node from the model, but this has the
risk of breaking modules which import the modified module, and
the removed identifier may be accidentally reused in a future
revision.
2. For deprecated data nodes the "description" statement SHOULD also
indicate until when support for the node is guaranteed (if
known). If there is a replacement data node, rpc, action or
notification for the deprecated node, this SHOULD be stated in
the "description". The reason for deprecating the node can also
be included in the "description" if it is deemed to be of
potential interest to the user.
3. For obsolete data nodes, it is RECOMMENDED to keep the above
information, from when the node had status "deprecated", which is
still relevant.
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4. When obsoleting or deprecating data 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 data 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 an 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 on how NBC changes can be made.
6.2. Versioning Considerations for Clients
Guidelines for clients of modules using the new module revision
update procedure:
* Clients SHOULD be liberal when processing data received from a
server. For example, the server may have increased the range of
an operational node causing the client to receive a value which is
outside the range of the YANG model revision it was coded against.
* 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
YANG module with extension statements for annotating NBC changes and
importing by revision.
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<CODE BEGINS> file "ietf-yang-revisions@2024-02-19.yang"
module ietf-yang-revisions {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-yang-revisions";
prefix rev;
organization
"IETF NETMOD (Network Modeling) Working Group";
contact
"WG Web: <https://datatracker.ietf.org/wg/netmod/>
WG List: <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>
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) 2024 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).
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.";
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// 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 2024-02-19 {
description
"Initial version.";
reference
"XXXX: Updated YANG Module Revision Handling";
}
typedef revision-date {
type string {
pattern '[0-9]{4}-(1[0-2]|0[1-9])-(0[1-9]|[1-2][0-9]|3[0-1])';
}
description
"A date associated with a YANG revision.
Matches dates formatted as YYYY-MM-DD.";
reference
"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-XXX, 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
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'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 revision 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.
The argument value MUST conform to the 'revision-date' defined
type.
The statement MUST only be a substatement of the import
statement. Zero, one or more 'recommended-min-date'
statements per parent statement are allowed. No substatements
for this extension have been standardized.
Zero or one 'recommended-min-date' extension statement is
allowed for each parent 'import' statement.
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,
Recommending a minimum revision for module imports";
}
}
<CODE ENDS>
YANG module for status conformance
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<CODE BEGINS> file "ietf-yang-status-conformance@2024-02-14.yang"
module ietf-yang-status-conformance {
yang-version 1.1;
namespace
"urn:ietf:params:xml:ns:yang:ietf-yang-status-conformance";
prefix ys-conf;
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: <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>
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) 2024 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).
This version of this YANG module is part of RFC XXXX; see
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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 2024-02-14 {
description
"Initial revision";
reference
"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;
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'; otherwise deviations MUST be
used to explicitly remove deprecated nodes from the schema.
If this leaf is absent or set to 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;
description
"If set to true, this leaf indicates that the server does not
implement any status 'obsolete' schema nodes. If this leaf
is absent or set to false, then the behaviour is
unspecified.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 5.1, Reporting how deprecated and obsolete nodes
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are handled";
}
}
}
<CODE ENDS>
8. Security considerations
8.1. 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
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.
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8.2. Security considerations for the modules defined in this document
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.
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].
9. IANA Considerations
9.1. YANG Module Registrations
This document requests IANA to registers a URI in the "IETF XML
Registry" [RFC3688]. Following the format in RFC 3688, the following
registrations are requested.
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-status-conformance
Registrant Contact: The IESG.
XML: N/A, the requested URI is an XML namespace.
The following YANG module is requested to be registred in the "IANA
Module Names" [RFC6020]. Following the format in RFC 6020, the
following registrations are requested:
The ietf-yang-revisions module:
Name: ietf-yang-revisions
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XML Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-revisions
Prefix: rev
Reference: [RFCXXXX]
The ietf-yang-status-conformance module:
Name: ietf-yang-status-conformance
XML Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-status-
conformance
Prefix: ys-conf
Reference: [RFCXXXX]
9.2. 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.
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 MUST follow the versioning guidance specified in
Section 3.1, and MUST include 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.
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Note: For published IANA maintained YANG modules that contain non-
backwards-compatible changes between revisions, a new revision should
be published with the "rev:non-backwards-compatible" substatement
retrospectively added to any revisions containing non-backwards-
compatible changes.
Non-normative examples of updates to enumeration types in IANA
maintained modules that would be classified as non-backwards-
compatible changes are: Changing the status of an enumeration typedef
to obsolete, changing the status of an enum entry to obsolete,
removing an enum entry, changing the identifier of an enum entry, or
changing the described meaning of an enum entry.
Non-normative examples of updates to enumeration types in IANA
maintained modules that would be classified as backwards-compatible
changes are: Adding a new enum entry to the end of the enumeration,
changing the status or an enum entry to deprecated, or improving the
description of an enumeration that does not change its defined
meaning.
Non-normative examples of updates to identity types in IANA
maintained modules that would be classified as non-backwards-
compatible changes are: Changing the status of an identity to
obsolete, removing an identity, renaming an identity, or changing the
described meaning of an identity.
Non-normative examples of updates to identity types in IANA
maintained modules that would be classified as backwards-compatible
changes are: Adding a new identity, changing the status or an
identity to deprecated, or improving the description of an identity
that does not change its defined meaning.
10. References
10.1. 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>.
[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>.
[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>.
[RFC8407] Bierman, A., "Guidelines for Authors and Reviewers of
Documents Containing YANG Data Models", BCP 216, RFC 8407,
DOI 10.17487/RFC8407, October 2018,
<https://www.rfc-editor.org/info/rfc8407>.
[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>.
[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>.
10.2. Informative References
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[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>.
[I-D.ietf-netmod-yang-packages]
Wilton, R., Rahman, R., Clarke, J., Sterne, J., and B. Wu,
"YANG Packages", Work in Progress, Internet-Draft, draft-
ietf-netmod-yang-packages-03, 4 March 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
yang-packages-03>.
[I-D.ietf-netmod-yang-schema-comparison]
Andersson, P. and R. Wilton, "YANG Schema Comparison",
Work in Progress, Internet-Draft, draft-ietf-netmod-yang-
schema-comparison-02, 14 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
yang-schema-comparison-02>.
[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-
12, 2 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
yang-semver-12>.
[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-09, 14 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
yang-versioning-reqs-09>.
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[IfTypesReg]
"Interface Types (ifType) IANA Registry",
<https://www.iana.org/assignments/smi-numbers/smi-
numbers.xhtml#smi-numbers-5>.
[IfTypeYang]
"iana-if-type YANG Module",
<https://www.iana.org/assignments/iana-if-type/iana-if-
type.xhtml>.
[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>.
[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>.
Appendix A. Examples of changes that are NBC
Examples of NBC changes include:
* Deleting a data node, or changing it to status obsolete.
* Changing the name, type, or units of a data node.
* 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.
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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 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 an
NBC change.
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 an NBC change. In any case, the server could
reject the old node from being set if the new node is already
set.
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4. When the schema node "vpn-id" is not supported anymore, its
status is changed to "obsolete" and the "description" is updated.
This is an 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 an 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 an 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
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).
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4. When list "sessions" is not available anymore, its status is
changed to "obsolete" and the "description" is updated. This is
an 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.
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 an NBC change. In any case, the server could reject the old
node from being set if the new node is already set.
4. When node "ip-adress" is not available anymore, its status is
changed to "obsolete" and the "description" is updated. This is
an NBC change.
Contributors
The following people made substantial contributions to this document:
Bo Wu
lana.wubo@huawei.com
Jan Lindblad
jlindbla@cisco.com
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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:
Benoit Claise
benoit.claise@huawei.com
Ebben Aries
exa@juniper.net
Juergen Schoenwaelder
j.shoenwaelder@jacobs-university.de
Mahesh Jethanandani
mjethanandani@gmail.com
Michael (Wangzitao)
wangzitao@huawei.com
Per Andersson
perander@cisco.com
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.
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.
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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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