Network Working Group R. Wilton, Ed.
Internet-Draft Cisco Systems, Inc.
Updates: 7950,8407,8525 (if approved) R. Rahman, Ed.
Intended status: Standards Track
Expires: January 13, 2022 B. Lengyel, Ed.
Ericsson
J. Clarke
Cisco Systems, Inc.
J. Sterne
Nokia
July 12, 2021
Updated YANG Module Revision Handling
draft-ietf-netmod-yang-module-versioning-03
Abstract
This document 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 an earliest revision filter to better represent inter-module
dependencies. It provides help and guidelines for managing the
lifecycle of YANG modules and individual schema nodes. It provides a
mechanism, via the revision-label YANG extension, to specify a
revision identifier for YANG modules and submodules. This document
updates RFC 7950, 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
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 January 13, 2022.
Wilton, et al. Expires January 13, 2022 [Page 1]
Internet-Draft Updated YANG Module Revision Handling July 2021
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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 . . . . . . . . . . . . . 6
3.1.2. Non-backwards-compatible changes . . . . . . . . . . 7
3.2. non-backwards-compatible revision extension statement . . 7
3.3. Removing revisions from the revision history . . . . . . 7
3.4. Revision label . . . . . . . . . . . . . . . . . . . . . 8
3.4.1. File names . . . . . . . . . . . . . . . . . . . . . 8
3.4.2. Revision label scheme extension statement . . . . . . 9
3.5. Examples for updating the YANG module revision history . 9
4. Import by derived revision . . . . . . . . . . . . . . . . . 12
4.1. Module import examples . . . . . . . . . . . . . . . . . 14
5. Updates to ietf-yang-library . . . . . . . . . . . . . . . . 15
5.1. Resolving ambiguous module imports . . . . . . . . . . . 15
5.2. YANG library versioning augmentations . . . . . . . . . . 16
5.2.1. Advertising revision-label . . . . . . . . . . . . . 16
5.2.2. Reporting how deprecated and obsolete nodes are
handled . . . . . . . . . . . . . . . . . . . . . . . 16
6. Versioning of YANG instance data . . . . . . . . . . . . . . 17
7. Guidelines for using the YANG module update rules . . . . . . 17
7.1. Guidelines for YANG module authors . . . . . . . . . . . 17
7.1.1. Making non-backwards-compatible changes to a YANG
module . . . . . . . . . . . . . . . . . . . . . . . 18
7.2. Versioning Considerations for Clients . . . . . . . . . . 20
8. Module Versioning Extension YANG Modules . . . . . . . . . . 20
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 29
10. Security Considerations . . . . . . . . . . . . . . . . . . . 30
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
Wilton, et al. Expires January 13, 2022 [Page 2]
Internet-Draft Updated YANG Module Revision Handling July 2021
11.1. YANG Module Registrations . . . . . . . . . . . . . . . 30
11.2. Guidance for versioning in IANA maintained YANG modules 31
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
12.1. Normative References . . . . . . . . . . . . . . . . . . 32
12.2. Informative References . . . . . . . . . . . . . . . . . 33
Appendix A. Examples of changes that are NBC . . . . . . . . . . 34
Appendix B. Examples of applying the NBC change guidelines . . . 35
B.1. Removing a data node . . . . . . . . . . . . . . . . . . 35
B.2. Changing the type of a leaf node . . . . . . . . . . . . 36
B.3. Reducing the range of a leaf node . . . . . . . . . . . . 37
B.4. Changing the key of a list . . . . . . . . . . . . . . . 37
B.5. Renaming a node . . . . . . . . . . . . . . . . . . . . . 38
B.6. Changing a default value . . . . . . . . . . . . . . . . 39
Appendix C. Changes between revisions . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
1. Introduction
This document defines a solution to the YANG module lifecycle
problems described in [I-D.ietf-netmod-yang-versioning-reqs].
Complementary documents provide a complete solution to the YANG
versioning requirements, with the overall relationship of the
solution drafts described in [I-D.ietf-netmod-yang-solutions].
Specifically, this document recognises a need (within standards
organizations, vendors, and the industry) to sometimes allow YANG
modules to evolve with non-backwards-compatible changes, which could
cause breakage to clients and importing YANG modules. Accepting that
non-backwards-compatible changes do sometimes occur, it is important
to have mechanisms to report where these changes occur, and to manage
their effect on clients and the broader YANG ecosystem.
The document comprises five 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, and an optional
revision label.
A YANG extension statement allowing YANG module imports to specify
an earliest module revision that may satisfy the import
dependency.
Updates and augmentations to ietf-yang-library to include the
revision label in the module and submodule descriptions, to report
how "deprecated" and "obsolete" nodes are handled by a server, and
to clarify how module imports are resolved when multiple revisions
could otherwise be chosen.
Wilton, et al. Expires January 13, 2022 [Page 3]
Internet-Draft Updated YANG Module Revision Handling July 2021
Considerations of how versioning applies to YANG instance data.
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. Section 3 describes
modifications to YANG revision handling and update rules, and
Section 4 describes a YANG extension statement to do import by
derived revision.
This document updates [RFC7950] section 5.2. Section 3.4.1 describes
the use of a revision label in the name of a file containing a YANG
module or submodule.
This document updates [RFC7950] section 5.6.5. Section 5.1 defines
how a client of a YANG library datastore schema resolves ambiguous
imports for modules which are not "import-only".
This document updates [RFC8407] section 4.7. Section 7 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
revision labels in the YANG library data and two boolean leaves 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.
In addition, this document uses the terminology:
o 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.
Wilton, et al. Expires January 13, 2022 [Page 4]
Internet-Draft Updated YANG Module Revision Handling July 2021
o Backwards-compatible (BC) change: A backwards-compatible change
between two YANG module revisions, as defined in Section 3.1.1
o 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] assumes, but does 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] 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], 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.
A corollary to the above is that the relationship between two module
or submodule revisions cannot be determined by comparing the module
or submodule revision date alone, and the revision history, or
revision label, must also be taken into consideration.
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 [RFC7895]
[RFC8525], MAY be used to specify the exact submodule revisions used
when the submodule revision date is not constrained by the "include"
statement.
[RFC7950] section 11 requires that all updates to a YANG module are
BC to the previous revision of the module. This document introduces
a method to indicate that an 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.
Wilton, et al. Expires January 13, 2022 [Page 5]
Internet-Draft Updated YANG Module Revision Handling July 2021
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 to refine the rules for
permissible changes when a new YANG module revision is created.
Where pragmatic, updates to YANG modules SHOULD be backwards-
compatible, following the definition in Section 3.1.1.
A new module revision MAY contain NBC changes, e.g., the semantics of
an existing data-node definition MAY be changed in an NBC manner
without requiring a new data-node definition with a new identifier.
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], all published revisions of a module are given a new
unique revision date. This applies even for module revisions
containing (in the module or included submodules) only changes to any
whitespace, formatting, comments or line endings (e.g., DOS vs UNIX).
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, updated by the following rules:
o A "status" "deprecated" statement MAY be added, or changed from
"current" to "deprecated", but adding or changing "status" to
"obsolete" is not a backwards-compatible change.
o YANG schema nodes with a "status" "obsolete" substatement MAY be
removed from published modules, and are classified as backwards-
compatible changes. In some circumstances it may be helpful to
retain the obsolete definitions to ensure that their identifiers
are not reused with a different meaning.
o In statements that have any data definition statements as
substatements, those data definition substatements MAY be
reordered, as long as they do not change the ordering of any
"input" or "output" data definition substatements of "rpc" or
"action" statements. If new data definition statements are added,
they can be added anywhere in the sequence of existing
substatements.
Wilton, et al. Expires January 13, 2022 [Page 6]
Internet-Draft Updated YANG Module Revision Handling July 2021
o Any changes (including whitespace or formatting changes) that do
not change the semantic meaning of the module are backwards
compatible.
o 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.
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 revision 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.
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 desired 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. Doing so can
lead to import breakages when import by revision-or-derived is used.
Moreover, truncating history may cause loss of visibility of when
non-backwards-compatible changes were introduced.
If a revision containing a rev:non-backwards-compatible substatement
is removed from the revision history then a rev:non-backwards-
compatible substatement MUST be added to the nearest newer revision
entry in the revision history that is not being removed.
Wilton, et al. Expires January 13, 2022 [Page 7]
Internet-Draft Updated YANG Module Revision Handling July 2021
3.4. Revision label
Each revision entry in a module or submodule MAY have a revision
label associated with it, providing an alternative alias to identify
a particular revision of a module or submodule. The revision label
could be used to provide an additional versioning identifier
associated with the revision.
YANG Semver [I-D.ietf-netmod-yang-semver] defines a versioning scheme
based on Semver 2.0.0 [semver] that can be used as a revision label.
Submodules MAY use a revision label scheme. When they use a revision
label scheme, submodules MAY use a revision label scheme that is
different from the one used in the including module.
The revision label space of submodules is separate from the revision
label space of the including module. A change in one submodule MUST
result in a new revision label of that submodule and the including
module, but the actual values of the revision labels in the module
and submodule could be completely different. A change in one
submodule does not result in a new revision label in another
submodule. A change in a module revision label does not necessarily
mean a change to the revision label in all included submodules.
If a revision has an associated revision label, then it may be used
instead of the revision date in a "rev:revision-or-derived" extension
statement argument.
A specific revision-label identifies a specific revision (variant) of
the module. If two YANG modules contain the same module name and the
same revision-label (and hence also the same revision-date) in their
latest revision statement, then the file contents of the two modules,
including the revision history, MUST be identical.
3.4.1. File names
This section updates [RFC7950] section 5.2.
If a revision has an associated revision label, then the revision-
label may be used instead of the revision date in the filename of a
YANG file, where it takes the form:
Wilton, et al. Expires January 13, 2022 [Page 8]
Internet-Draft Updated YANG Module Revision Handling July 2021
module-or-submodule-name [['@' revision-date]|['#' revision-label]]
( '.yang' / '.yin' )
E.g., acme-router-module@2018-01-25.yang
E.g., acme-router-module#2.0.3.yang
YANG module (or submodule) files MAY be identified using either
revision-date or revision-label. Typically, only one file name
SHOULD exist for the same module (or submodule) revision. Two file
names, one with the revision date and another with the revision
label, MAY exist for the same module (or submodule) revision, e.g.,
when migrating from one scheme to the other.
3.4.2. Revision label scheme extension statement
The optional "rev:revision-label-scheme" extension statement is used
to indicate which revision-label scheme a module or submodule uses.
There MUST NOT be more than one revision label scheme in a module or
submodule. The mandatory argument to this extension statement:
o specifies the revision-label scheme used by the module or
submodule
o is defined in the document which specifies the revision-label
scheme
o MUST be an identity derived from "revision-label-scheme-base".
The revision-label scheme used by a module or submodule SHOULD NOT
change during the lifetime of the module or submodule. If the
revision-label scheme used by a module or submodule is changed to a
new scheme, then all revision-label statements that do not conform to
the new scheme MUST be replaced or removed.
3.5. Examples for updating the YANG module revision history
The following diagram, explanation, and module history illustrates
how the branched revision history, "non-backwards-compatible"
extension statement, and "revision-label" extension statement could
be used:
Wilton, et al. Expires January 13, 2022 [Page 9]
Internet-Draft Updated YANG Module Revision Handling July 2021
Example YANG module with branched revision history.
Module revision date Revision label
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 <- 2.2.0
|
2019-06-01 <- 3.1.0
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:
Wilton, et al. Expires January 13, 2022 [Page 10]
Internet-Draft Updated YANG Module Revision Handling July 2021
Example module, revision 2019-06-01:
module example-module {
namespace "urn:example:module";
prefix "prefix-name";
rev:revision-label-scheme "yangver:yang-semver";
import ietf-yang-revisions { prefix "rev"; }
import ietf-yang-semver { prefix "yangver"; }
description
"to be completed";
revision 2019-06-01 {
rev:revision-label 3.1.0;
description "Add new functionality.";
}
revision 2019-03-01 {
rev:revision-label 3.0.0;
rev:non-backwards-compatible;
description
"Add new functionality. Remove some deprecated nodes.";
}
revision 2019-02-01 {
rev:revision-label 2.0.0;
rev:non-backwards-compatible;
description "Apply bugfix to pattern statement";
}
revision 2019-01-01 {
rev:revision-label 1.0.0;
description "Initial revision";
}
//YANG module definition starts here
}
Wilton, et al. Expires January 13, 2022 [Page 11]
Internet-Draft Updated YANG Module Revision Handling July 2021
Example module, revision 2019-05-01:
module example-module {
namespace "urn:example:module";
prefix "prefix-name";
rev:revision-label-scheme "yangver:yang-semver";
import ietf-yang-revisions { prefix "rev"; }
import ietf-yang-semver { prefix "yangver"; }
description
"to be completed";
revision 2019-05-01 {
rev:revision-label 2.2.0;
description "Backwards-compatible bugfix to enhancement.";
}
revision 2019-04-01 {
rev:revision-label 2.1.0;
description "Apply enhancement to older release train.";
}
revision 2019-02-01 {
rev:revision-label 2.0.0;
rev:non-backwards-compatible;
description "Apply bugfix to pattern statement";
}
revision 2019-01-01 {
rev:revision-label 1.0.0;
description "Initial revision";
}
//YANG module definition starts here
}
4. Import by derived revision
RFC 7950 allows YANG module "import" statements to optionally require
the imported module to have a particular revision date. In practice,
importing a module with an exact revision date is often too
restrictive because it requires the importing module to be updated
whenever any change to the imported module occurs. The alternative
choice of using an import statement without any revision date
statement is also not ideal because the importing module may not work
with all possible revisions of the imported module.
Wilton, et al. Expires January 13, 2022 [Page 12]
Internet-Draft Updated YANG Module Revision Handling July 2021
Instead, it is desirable for an importing module to specify a
"minimum required revision" of a module that it is compatible with,
based on the assumption that later revisions derived from that
"minimum required revision" are also likely to be compatible. Many
possible changes to a YANG module do not break importing modules,
even if the changes themselves are not strictly backwards-compatible.
E.g., fixing an incorrect pattern statement or description for a leaf
would not break an import, changing the name of a leaf could break an
import but frequently would not, but removing a container would break
imports if that container is augmented by another module.
The ietf-revisions module defines the "revision-or-derived" extension
statement, a substatement to the YANG "import" statement, to allow
for a "minimum required revision" to be specified during import:
The argument to the "revision-or-derived" extension statement is a
revision date or a revision label.
A particular revision of an imported module satisfies an import's
"revision-or-derived" extension statement if the imported module's
revision history contains a revision statement with a matching
revision date or revision label.
An "import" statement MUST NOT contain both a "revision-or-
derived" extension statement and a "revision-date" statement.
The "revision-or-derived" extension statement MAY be specified
multiple times, allowing the import to use any module revision
that satisfies at least one of the "revision-or-derived" extension
statements.
The "revision-or-derived" extension statement does not guarantee
that all module revisions that satisfy an import statement are
necessarily compatible, it only gives an indication that the
revisions are more likely to be compatible. Hence, NBC changes to
an imported module may also require new revisions of any importing
modules, updated to accommodation those changes, along with
updated import "revision-or-derived" extension statements to
depend on the updated imported module revision.
Adding, modifying or removing a "revision-or-derived" extension
statement is considered to be a BC change.
Adding, modifying or removing a "revision-date" extension
statement is considered to be a BC change.
Wilton, et al. Expires January 13, 2022 [Page 13]
Internet-Draft Updated YANG Module Revision Handling July 2021
4.1. Module import examples
Consider the example module "example-module" from Section 3.5 that is
hypothetically available in the following revision/label pairings:
2019-01-01/1.0.0, 2019-02-01/2.0.0, 2019-03-01/3.0.0,
2019-04-01/2.1.0, 2019-05-01/2.2.0 and 2019-06-01/3.1.0. The
relationship between the revisions is as before:
Module revision date Revision label
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 <- 2.2.0
|
2019-06-01 <- 3.1.0
4.1.1. Example 1
This example selects module revisions that match, or are derived from
the revision 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 revisions/labels:
2019-02-01/2.0.0, 2019-03-01/3.0.0, 2019-04-01/2.1.0,
2019-05-01/2.2.0 and 2019-06-01/3.1.0.
import example-module {
rev:revision-or-derived 2019-02-01;
}
Alternatively, the first example could have used the revision label
"2.0.0" instead, which selects the same set of revisions/labels.
import example-module {
rev:revision-or-derived 2.0.0;
}
4.1.2. Example 2
This example selects module revisions that are derived from
2019-04-01 by using the revision label 2.1.0. It includes revisions/
labels: 2019-04-01/2.1.0 and 2019-05-01/2.2.0. Even though
2019-06-01/3.1.0 has a higher revision label number than
Wilton, et al. Expires January 13, 2022 [Page 14]
Internet-Draft Updated YANG Module Revision Handling July 2021
2019-04-01/2.1.0 it is not a derived revision, and hence it is not a
valid revision for import.
import example-module {
rev:revision-or-derived 2.1.0;
}
4.1.3. Example 3
This example selects revisions derived from either 2019-04-01 or
2019-06-01. It includes revisions/labels: 2019-04-01/2.1.0,
2019-05-01/2.2.0, and 2019-06-01/3.1.0.
import example-module {
rev:revision-or-derived 2019-04-01;
rev:revision-or-derived 2019-06-01;
}
5. Updates to ietf-yang-library
This document updates YANG library [RFC7950] to clarify how ambiguous
module imports are resolved. It also defines the YANG module, ietf-
yang-library-revisions that augments YANG library [RFC8525] with new
revision-label related meta-data.
5.1. Resolving ambiguous module imports
A YANG datastore schema, defined in [RFC8525], can specify multiple
revisions of a YANG module in the schema using the "import-only"
list, with the requirement from [RFC7950] that only a single revision
of a YANG module may be implemented.
If a YANG module import statement does not specify a specific
revision within the datastore schema then it could be ambiguous as to
which module revision the import statement should resolve to. Hence,
a datastore schema constructed by a client using the information
contained in YANG library may not exactly match the datastore schema
actually used by the server.
The following two rules remove the ambiguity:
If a module import statement could resolve to more than one module
revision defined in the datastore schema, and one of those revisions
is implemented (i.e., not an "import-only" module), then the import
statement MUST resolve to the revision of the module that is defined
as being implemented by the datastore schema.
Wilton, et al. Expires January 13, 2022 [Page 15]
Internet-Draft Updated YANG Module Revision Handling July 2021
If a module import statement could resolve to more than one module
revision defined in the datastore schema, and none of those revisions
are implemented, then the import MUST resolve to the module revision
with the latest revision date.
5.2. YANG library versioning augmentations
The "ietf-yang-library-revisions" YANG module has the following
structure (using the notation defined in [RFC8340]):
module: ietf-yang-library-revisions
augment /yanglib:yang-library/yanglib:module-set/yanglib:module:
+--ro revision-label? rev:revision-label
augment /yanglib:yang-library/yanglib:module-set/yanglib:module
/yanglib:submodule:
+--ro revision-label? rev:revision-label
augment /yanglib:yang-library/yanglib:module-set
/yanglib:import-only-module/yanglib:submodule:
+--ro revision-label? rev:revision-label
augment /yanglib:yang-library/yanglib:schema:
+--ro deprecated-nodes-implemented? boolean
+--ro obsolete-nodes-absent? boolean
5.2.1. Advertising revision-label
The ietf-yang-library-revisions YANG module augments the "module"
list in ietf-yang-library with a "revision-label" leaf to optionally
declare the revision label associated wth the particular revision of
each module.
5.2.2. Reporting how deprecated and obsolete nodes are handled
The ietf-yang-library-revisions YANG module augments YANG library
with two leaves to allow a server to report how it handles status
"deprecated" and status "obsolete" nodes. The leaves are:
deprecated-nodes-implemented: If set to "true", this leaf indicates
that all schema nodes with a status "deprecated" child statement
are implemented equivalently as if they had status "current", or
otherwise deviations MUST be used 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" nodes. If
Wilton, et al. Expires January 13, 2022 [Page 16]
Internet-Draft Updated YANG Module Revision Handling July 2021
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" leaves to "true".
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. Versioning of YANG instance data
Instance data sets [I-D.ietf-netmod-yang-instance-file-format] do not
directly make use of the updated revision handling rules described in
this document, as compatibility for instance data is undefined.
However, instance data specifies the content-schema of the data-set.
This schema SHOULD make use of versioning using revision dates and/or
revision labels for the individual YANG modules that comprise the
schema or potentially for the entire schema itself (e.g.,
[I-D.ietf-netmod-yang-packages]).
In this way, the versioning of a content-schema associated with an
instance data set may help a client to determine whether the instance
data could also be used in conjunction with other revisions of the
YANG schema, or other revisions of the modules that define the
schema.
7. 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.
7.1. Guidelines for YANG module authors
All IETF YANG modules MUST include revision-label statements for all
newly published YANG modules, and all newly published revisions of
existing YANG modules. The revision-label MUST take the form of a
YANG semantic version number [I-D.ietf-netmod-yang-semver].
NBC changes to YANG modules may cause problems to clients, who are
consumers of YANG models, and hence YANG module authors are
RECOMMENDED to minimize NBC changes and keep changes BC whenever
possible.
Wilton, et al. Expires January 13, 2022 [Page 17]
Internet-Draft Updated YANG Module Revision Handling July 2021
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
could break import by revision, and hence it is RECOMMENDED to retain
them. If all depencencies have been updated to not import specific
revisions of a module, then the corresponding revision statements can
be removed from that module. 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.
The "rev:revision-or-derived" extension should be used in YANG module
imports to indicate revision dependencies between modules 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. The revision-label substatement used in the new module
revision MUST indicate the nature of the change, i.e. NBC or BC, to
the module's schema tree.
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:
o 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
7.1.1. 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 the different ways in which this
can be done:
o NBC changes can be sometimes be done incrementally using the
"deprecated" status to provide clients time to adapt to NBC
changes.
Wilton, et al. Expires January 13, 2022 [Page 18]
Internet-Draft Updated YANG Module Revision Handling July 2021
o NBC changes are done at once, i.e. without using "status"
statements. Depending on the change, this may have a big impact
on clients.
o If the server can support multiple revisions of the YANG module or
of YANG packages(as specified in [I-D.ietf-netmod-yang-packages]),
and allows the client to select the revision (as per
[I-D.ietf-netmod-yang-ver-selection]), then NBC changes MAY be
done without using "status" statements. Clients would be required
to select the revision which they support and the NBC change would
have no impact on them.
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" and then when support is removed its status
MUST be changed to "obsolete". Instead of using the "obsolete"
status, the data node MAY be removed from the model but this has
the risk of breaking modules which import the modified module.
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.
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
Wilton, et al. Expires January 13, 2022 [Page 19]
Internet-Draft Updated YANG Module Revision Handling July 2021
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.
7.2. Versioning Considerations for Clients
Guidelines for clients of modules using the new module revision
update procedure:
o 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.
o 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.
o 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.
8. Module Versioning Extension YANG Modules
YANG module with extension statements for annotating NBC changes,
revision label, revision label scheme, and importing by revision.
<CODE BEGINS> file "ietf-yang-revisions@2021-06-30.yang"
module ietf-yang-revisions {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-yang-revisions";
prefix rev;
// RFC Ed.: We need the bis version to get the new type revision-identifier
// If 6991-bis is not yet an RFC we need to copy the definition here
import ietf-yang-types {
prefix yang;
reference
"XXXX [ietf-netmod-rfc6991-bis]: Common YANG Data Types";
}
Wilton, et al. Expires January 13, 2022 [Page 20]
Internet-Draft Updated YANG Module Revision Handling July 2021
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) 2021 IETF Trust and the persons identified as
authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or
without modification, is permitted pursuant to, and subject
to the license terms contained in, the Simplified BSD License
set forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(http://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC XXXX; see
the RFC itself for full legal notices.
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.
Wilton, et al. Expires January 13, 2022 [Page 21]
Internet-Draft Updated YANG Module Revision Handling July 2021
revision 2021-06-30 {
description
"Initial version.";
reference
"XXXX: Updated YANG Module Revision Handling";
}
typedef revision-label {
type string {
length "1..255";
pattern '[a-zA-Z0-9,\-_.+]+';
pattern '\d{4}-\d{2}-\d{2}' {
modifier invert-match;
}
}
description
"A label associated with a YANG revision.
Alphanumeric characters, comma, hyphen, underscore, period
and plus are the only accepted characters. MUST NOT match
revision-date.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 3.3, Revision label";
}
typedef revision-date-or-label {
type union {
type yang:revision-identifier;
type revision-label;
}
description
"Represents either a YANG revision date or a revision label";
}
extension nbc-changes {
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 module update rules defined in RFC-XXX, then
Wilton, et al. Expires January 13, 2022 [Page 22]
Internet-Draft Updated YANG Module Revision Handling July 2021
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, nbc-changes revision extension statement";
}
extension revision-label {
argument revision-label;
description
"The revision label can be used to provide an additional
versioning identifier associated with a module or submodule
revision. E.g., one option for a versioning scheme that
could be used is [XXXX: ietf-netmod-yang-semver].
The format of the revision-label argument MUST conform to the
pattern defined for the revision-label typedef.
The statement MUST only be a substatement of the revision
statement. Zero or one revision-label statements per parent
statement are allowed. No substatements for this extension
have been standardized.
Revision labels MUST be unique amongst all revisions of a
module or submodule.
Adding a revision label is a backwards-compatible version
change. Changing or removing an existing revision label in
the revision history is a non-backwards-compatible version
change, because it could impact any references to that
revision label.";
Wilton, et al. Expires January 13, 2022 [Page 23]
Internet-Draft Updated YANG Module Revision Handling July 2021
reference
"XXXX: Updated YANG Module Revision Handling;
Section 3.3, Revision label";
}
extension revision-label-scheme {
argument revision-label-scheme-identity;
description
"The revision label scheme specifies which revision-label scheme
the module or submodule uses.
The mandatory revision-label-scheme-identity argument MUST be an
identity derived from revision-label-scheme-base.
This extension is only valid as a top-level statement, i.e.,
given as as a substatement to 'module' or 'submodule'. No
substatements for this extension have been standardized.
This extension MUST be used if there is a revision-label
statement in the module or submodule.
Adding a revision label scheme is a backwards-compatible version
change. Changing a revision label scheme is a
non-backwards-compatible version change, unless the new revision
label scheme is backwards-compatible with the replaced revision
label scheme. Removing a revision label scheme is a
non-backwards-compatible version change.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 3.3.1, Revision label scheme extension statement";
}
extension revision-or-derived {
argument revision-date-or-label;
description
"Restricts the revision of the module that may be imported to
one that matches or is derived from the specified
revision-date or revision-label.
The argument value MUST conform to the
'revision-date-or-label' defined type.
The statement MUST only be a substatement of the import
statement. Zero, one or more 'revision-or-derived' statements
per parent statement are allowed. No substatements for this
extension have been standardized.
Wilton, et al. Expires January 13, 2022 [Page 24]
Internet-Draft Updated YANG Module Revision Handling July 2021
If specified multiple times, then any module revision that
satisfies at least one of the 'revision-or-derived' statements
is an acceptable revision for import.
An 'import' statement MUST NOT contain both a
'revision-or-derived' extension statement and a
'revision-date' statement.
A particular revision of an imported module satisfies an
import's 'revision-or-derived' extension statement if the
imported module's revision history contains a revision
statement with a matching revision date or revision label.
The 'revision-or-derived' extension statement does not
guarantee that all module revisions that satisfy an import
statement are necessarily compatible, it only gives an
indication that the revisions are more likely to be
compatible.
Adding, removing or updating a 'revision-or-derived'
statement to an import is a backwards-compatible change.
";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 4, Import by derived revision";
}
identity revision-label-scheme-base {
description
"Base identity from which all revision label schemes are
derived.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 3.3.1, Revision label scheme extension statement";
}
}
<CODE ENDS>
YANG module with augmentations to YANG Library to revision labels
<CODE BEGINS> file "ietf-yang-library-revisions@2021-06-30.yang"
module ietf-yang-library-revisions {
yang-version 1.1;
namespace
"urn:ietf:params:xml:ns:yang:ietf-yang-library-revisions";
Wilton, et al. Expires January 13, 2022 [Page 25]
Internet-Draft Updated YANG Module Revision Handling July 2021
prefix yl-rev;
import ietf-yang-revisions {
prefix rev;
reference
"XXXX: Updated YANG Module Revision Handling";
}
import ietf-yang-library {
prefix yanglib;
reference "RFC 8525: YANG Libary";
}
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 add module
level revision label and to provide an indication of how
deprecated and obsolete nodes are handled by the server.
Copyright (c) 2021 IETF Trust and the persons identified as
authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or
without modification, is permitted pursuant to, and subject
to the license terms contained in, the Simplified BSD License
set forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(http://trustee.ietf.org/license-info).
Wilton, et al. Expires January 13, 2022 [Page 26]
Internet-Draft Updated YANG Module Revision Handling July 2021
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.
// RFC Ed.: please replace revision-label version with 1.0.0 and
// remove this note.
revision 2021-06-30 {
rev:revision-label 0.2.0;
description
"Initial revision";
reference
"XXXX: Updated YANG Module Revision Handling";
}
augment "/yanglib:yang-library/yanglib:module-set/yanglib:module" {
description
"Augmentation modules with a revision label";
leaf revision-label {
type rev:revision-label;
description
"The revision label associated with this module revision.
The label MUST match the rev:label value in the specific
revision of the module loaded in this module-set.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 5.2.1, Advertising revision-label";
}
}
augment "/yanglib:yang-library/yanglib:module-set/yanglib:module/"
+ "yanglib:submodule" {
description
"Augment submodule information with a revision label";
leaf revision-label {
type rev:revision-label;
description
"The revision label associated with this submodule revision.
The label MUST match the rev:label value in the specific
Wilton, et al. Expires January 13, 2022 [Page 27]
Internet-Draft Updated YANG Module Revision Handling July 2021
revision of the submodule included by the module loaded in
this module-set.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 5.2.1, Advertising revision-label";
}
}
augment "/yanglib:yang-library/yanglib:module-set/"
+ "yanglib:import-only-module/yanglib:submodule" {
description
"Augment submodule information with a revision label";
leaf revision-label {
type rev:revision-label;
description
"The revision label associated with this submodule revision.
The label MUST match the rev:label value in the specific
revision of the submodule included by the
import-only-module loaded in this module-set.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 5.2.1, Advertising revision-label";
}
}
augment "/yanglib:yang-library/yanglib:schema" {
description
"Augmentations to the ietf-yang-library module to indicate how
deprecated and obsoleted nodes are handled for each datastore
schema supported 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' child statement are implemented
equivalently as if they had status 'current', or 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 5.2.2, Reporting how deprecated and obsolete nodes
are handled";
}
Wilton, et al. Expires January 13, 2022 [Page 28]
Internet-Draft Updated YANG Module Revision Handling July 2021
leaf obsolete-nodes-absent {
type boolean;
description
"If set to true, this leaf indicates that the server does not
implement any status 'obsolete' nodes. If this leaf is
absent or set to false, then the behaviour is unspecified.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 5.2.2, Reporting how deprecated and obsolete nodes
are handled";
}
}
}
<CODE ENDS>
9. Contributors
This document grew out of the YANG module versioning design team that
started after IETF 101. The following individuals are (or have been)
members of the design team and have worked on the YANG versioning
project:
o Balazs Lengyel
o Benoit Claise
o Ebben Aries
o Jan Lindblad
o Jason Sterne
o Joe Clarke
o Juergen Schoenwaelder
o Mahesh Jethanandani
o Michael (Wangzitao)
o Qin Wu
o Reshad Rahman
o Rob Wilton
o Bo Wu
Wilton, et al. Expires January 13, 2022 [Page 29]
Internet-Draft Updated YANG Module Revision Handling July 2021
The initial revision of this document was refactored and built upon
[I-D.clacla-netmod-yang-model-update].
Discussons 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, Kent Watsen for
their contributions and review comments.
10. Security Considerations
The document does not define any new protocol or data model. There
are no security considerations beyond those specified in [RFC7950].
11. IANA Considerations
11.1. 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-library-revisions
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
XML Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-revisions
Prefix: rev
Reference: [RFCXXXX]
The ietf-yang-library-revisions module:
Wilton, et al. Expires January 13, 2022 [Page 30]
Internet-Draft Updated YANG Module Revision Handling July 2021
Name: ietf-yang-library-revisions
XML Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-library-
revisions
Prefix: yl-rev
Reference: [RFCXXXX]
11.2. 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.
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,
Wilton, et al. Expires January 13, 2022 [Page 31]
Internet-Draft Updated YANG Module Revision Handling July 2021
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.
12. References
12.1. Normative References
[I-D.ietf-netmod-rfc6991-bis]
Schoenwaelder, J., "Common YANG Data Types", draft-ietf-
netmod-rfc6991-bis-06 (work in progress), April 2021.
[I-D.ietf-netmod-yang-semver]
Claise, B., Clarke, J., Rahman, R., Wilton, R., Lengyel,
B., Sterne, J., and K. D'Souza, "YANG Semantic
Versioning", draft-ietf-netmod-yang-semver-02 (work in
progress), February 2021.
[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>.
Wilton, et al. Expires January 13, 2022 [Page 32]
Internet-Draft Updated YANG Module Revision Handling July 2021
[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>.
[RFC7895] Bierman, A., Bjorklund, M., and K. Watsen, "YANG Module
Library", RFC 7895, DOI 10.17487/RFC7895, June 2016,
<https://www.rfc-editor.org/info/rfc7895>.
[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>.
[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>.
[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>.
12.2. 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", draft-clacla-netmod-yang-
model-update-06 (work in progress), July 2018.
[I-D.ietf-netmod-yang-instance-file-format]
Lengyel, B. and B. Claise, "YANG Instance Data File
Format", draft-ietf-netmod-yang-instance-file-format-13
(work in progress), March 2021.
[I-D.ietf-netmod-yang-packages]
Wilton, R., Rahman, R., Clarke, J., Sterne, J., and B. Wu,
"YANG Packages", draft-ietf-netmod-yang-packages-01 (work
in progress), November 2020.
Wilton, et al. Expires January 13, 2022 [Page 33]
Internet-Draft Updated YANG Module Revision Handling July 2021
[I-D.ietf-netmod-yang-solutions]
Wilton, R., "YANG Versioning Solution Overview", draft-
ietf-netmod-yang-solutions-01 (work in progress), November
2020.
[I-D.ietf-netmod-yang-ver-selection]
Wilton, R., Rahman, R., Clarke, J., Sterne, J., and B. Wu,
"YANG Schema Selection", draft-ietf-netmod-yang-ver-
selection-00 (work in progress), March 2020.
[I-D.ietf-netmod-yang-versioning-reqs]
Clarke, J., "YANG Module Versioning Requirements", draft-
ietf-netmod-yang-versioning-reqs-04 (work in progress),
January 2021.
[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>.
[semver] "Semantic Versioning 2.0.0", <https://www.semver.org>.
Appendix A. Examples of changes that are NBC
Examples of NBC changes include:
Wilton, et al. Expires January 13, 2022 [Page 34]
Internet-Draft Updated YANG Module Revision Handling July 2021
o Deleting a data node, or changing it to status obsolete.
o Changing the name, type, or units of a data node.
o Modifying the description in a way that changes the semantic
meaning of the data node.
o Any changes that change or reduce 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.
o Adding or modifying "when" statements that reduce when the data
node is available in the schema.
o Making the statement conditional on if-feature.
Appendix B. Examples of applying the NBC change guidelines
The following sections give guidance for how some of these NBC
changes could be made to a YANG module. 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 node's status is changed to "deprecated" and it is supported
for at least one year. This is a BC change.
2. When the node is not available anymore, its status is changed to
"obsolete" and the "description" updated, this is an NBC change.
If the server can support NBC revisions of the YANG module
simultaneously using version selection
[I-D.ietf-netmod-yang-ver-selection], then the changes can be done
immediately:
1. The new revision of the YANG module has the node's status changed
to "obsolete" and the "description" updated, this is an NBC
change.
2. Clients which require the data node select the YANG package
containing the schema version they use.
Wilton, et al. Expires January 13, 2022 [Page 35]
Internet-Draft Updated YANG Module Revision Handling July 2021
B.2. Changing the type of a leaf node
Changing the type of a leaf-node. e.g., consider a "vpn-id" node of
type integer being changed to a string:
1. The status of node "vpn-id" is changed to "deprecated" and the
node should be available for at least one year. This is a BC
change.
2. A new 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 nodes are available, how the
server behaves when either node is set is outside the scope of
this document and will vary on a case by case basis. Here are
some options:
1. A server may prevent the new node from being set if the old
node is already set (and vice-versa). The new node may have
a when statement to achieve this. The old node must not have
a when statement since this would be an NBC change, but the
server could reject the old node from being set if the new
node is already set.
2. If the new node is set and a client does a get or get-config
operation on the old node, the server could map the value.
For example, if the new node "vpn-name" has value "123" then
the server could return integer value 123 for the old node
"vpn-id". However, if the value can not be mapped then the
configuration would be incomplete, this is outside the scope
of this document.
4. When node "vpn-id" is not available anymore, its status is
changed to "obsolete" and the "description" is updated. This is
an NBC change.
If the server can support NBC revisions of the YANG module
simultaneously using version selection
[I-D.ietf-netmod-yang-ver-selection], then the changes can be done
immediately:
1. In the new revision of the YANG module, the status of node "vpn-
id" is changed to "obsolete". This is an NBC change.
2. New node "vpn-name" is added to the same location as described
above.
Wilton, et al. Expires January 13, 2022 [Page 36]
Internet-Draft Updated YANG Module Revision Handling July 2021
3. Clients which require the data node select the YANG package
containing the schema version they use
4. A server should not map between the nodes "vpn-id" and "vpn-
name", i.e. if a client creates a data instance with "vpn-name"
then that data instance should not be visible to a client using a
module revision which has "vpn-id" (and vice-versa).
B.3. Reducing the range of a leaf node
Reducing the range of values of a leaf-node, e.g., consider a "vpn-
id" node of type integer being changed from type uint32 to type
uint16:
1. If all values which are being removed were never supported, e.g.,
if a vpn-id of 65536 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 65536 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 should be available for at least one year. This is a BC
change.
2. A new list is created in the same location with the same data but
with "dest-address" as key. Finding an appropriate name for the
new list can be tricky especially if the name of the existing
list was perfect. 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 available, how the
server behaves when either list is set is outside the scope of
this document and will vary on a case by case basis. Here are
some options:
Wilton, et al. Expires January 13, 2022 [Page 37]
Internet-Draft Updated YANG Module Revision Handling July 2021
1. A server could prevent the new list from being set if the old
list already has entries (and vice-versa).
2. If the new list is set and a client does a get or get-config
operation on the old list, the server could map the entries.
However, if the new list has entries which would lead to
duplicate keys in the old list, the mapping can not be done.
4. When list "sessions" is not available anymore, its status is
changed to "obsolete" and the "description" is updated. This is
an NBC change.
If the server can support NBC revisions of the YANG module
simultaneously using version selection
[I-D.ietf-netmod-yang-ver-selection], then the changes can be done
immediately:
1. The new revision of the YANG module has the list "sessions"
modified to have "dest-address" as key, this is an NBC change.
2. Clients which require the previous functionality select the older
module revision
B.5. Renaming a node
A leaf-node or a container 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 the node should be available for at least one
year. This is a BC change.
2. The new 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, how the
server behaves when either node is set is outside the scope of
this document and will vary on a case by case basis. Here are
some options:
1. A server could 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 to achieve this. The old node must not
have a when statement since this would be an NBC change, but
Wilton, et al. Expires January 13, 2022 [Page 38]
Internet-Draft Updated YANG Module Revision Handling July 2021
the server could reject the old node from being set if the
new node is already set.
2. If the new node is set and a client does a get or get-config
operation on the old node, the server could use the value of
the new node. For example, if the new node "ip-address" has
value X then the server may return value X for the old node
"ip-adress".
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.
If the server can support NBC revisions of the YANG module
simultaneously using version selection
[I-D.ietf-netmod-yang-ver-selection], then the changes can be done
immediately:
1. The new revision of the YANG module has the node with the new
name replacing the node with the old name, this is an NBC change.
2. Clients which require the previous node name select the older
module revision
B.6. Changing a default value
Appendix C. Changes between revisions
Note to RFC Editor (To be removed by RFC Editor)
v00 - v01
o Removed status-description
o Allowed both revision-date and revision-label in the filename.
o New extension revision-label-scheme
o To include submodules, inclusion by revision-date changed from
MUST to SHOULD
o Submodules can use revision label scheme and it can be same or
different as the including module's scheme
o Addressed various comments provided at WG adoption on rev-00
Authors' Addresses
Wilton, et al. Expires January 13, 2022 [Page 39]
Internet-Draft Updated YANG Module Revision Handling July 2021
Robert Wilton (editor)
Cisco Systems, Inc.
Email: rwilton@cisco.com
Reshad Rahman (editor)
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
Wilton, et al. Expires January 13, 2022 [Page 40]
