Updated YANG Module Revision Handling
Cisco Systems, Inc.
rwilton@cisco.com
Graphiant
reshad@yahoo.com Ericsson balazs.lengyel@ericsson.comCisco Systems, Inc.jclarke@cisco.com
Nokia
jason.sterne@nokia.comThis 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 a minimum revision
suggestion to help document inter-module dependencies. It provides 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 6020, RFC 8407 and RFC 8525.The current YANG module update rules require that updates
of YANG modules preserve strict backwards compatibility. This has
caused problems as described in
.
This document recognizes the need to sometimes allow YANG modules
to evolve with non-backwards-compatible changes, which can 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 when these changes occur,
and to manage their effect on clients and the broader YANG
ecosystem.
This document defines a flexible versioning solution. Several other documents build on this solution with additional capabilities. 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 document extends the module versioning work by introducing a revision label scheme based on semantic versioning. 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, 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 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.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 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.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 .This document updates section 11 and section 10.
describes modifications to
YANG revision handling and update rules, and describes a YANG extension statement to describe potential YANG import revision dependencies.This document updates section 5.2, section 5.2 and section 3.2.
describes the use of a revision
label in the name of a file containing a YANG module or submodule.This document updates section 5.6.5 and . defines how a client
of a YANG library datastore schema resolves ambiguous imports for modules which are not "import-only".This document updates section 4.7. provides guidelines on managing the
lifecycle of YANG modules that may contain non-backwards-compatible changes and a branched revision history.This document updates with augmentations to include revision labels in the YANG library data and two
boolean leafs to indicate whether status deprecated and status obsolete schema nodes are implemented by the server.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.This document makes use of the following terminology introduced in
the YANG 1.1 Data Modeling Language : schema nodeIn addition, this document uses the following terminology:
YANG module revision: An instance of a YANG module, uniquely identified with a revision date, with no implied
ordering or backwards compatibility between different revisions of the same module.Backwards-compatible (BC) change: A backwards-compatible change between two YANG module revisions, as defined
in Non-backwards-compatible (NBC) change: A non-backwards-compatible change between two YANG module revisions, as
defined in and 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 and 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 and , 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.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.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 label 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 , and YANG library
, MAY be used to specify the exact submodule revisions used when the submodule revision date
is not constrained by the "include" statement. section 11 and section 10 require 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.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.This section updates section 11 and section 10 to refine the rules for permissible changes when a new
YANG module revision is created.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 , is used to signal the potential
for incompatibility to existing module users and readers.Note that NBC changes often create problems for clients, thus
it is recommended to avoid making them.As per and , 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).A change between two module revisions is defined as being "backwards-compatible" if the change conforms to
the module update rules specified in section 11 and 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" substatements
of an "import" statement, including adding new "revision-date" or "recommended-min" substatements, changing the argument of any "revision-date" or "recommended-min" substatetements, or removing any "revision-date" or "recommended-min" 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.Any changes to YANG modules that are not defined by
as being backwards-compatible are classified as "non-backwards-compatible" changes.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 , 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.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. Doing so can lead to import breakages when import by
recommended-min is used. Moreover, truncating history 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:
revision 2020-11-11 {
rev:label 4.0.0;
rev:non-backwards-compatible;
}
revision 2020-08-09 {
rev:label 3.0.0;
rev:non-backwards-compatible;
}
revision 2020-06-07 {
rev:label 2.1.0;
}
revision 2020-02-10 {
rev:label 2.0.0;
rev:non-backwards-compatible;
}
revision 2019-10-21 {
rev:label 1.1.3;
}
revision 2019-03-04 {
rev:label 1.1.2;
}
revision 2019-01-02 {
rev:label 1.1.1;
}
In the revision history example above, 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.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.A revision label scheme is a set of rules describing how a particular type of revision label operates for versioning YANG modules and submodules.
For example, YANG Semver
defines a revision label scheme based on Semver 2.0.0 .
Other documents may define other YANG revision label schemes.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:recommended-min" extension
statement argument.
A specific revision label identifies a specific revision 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.This section updates section 5.2, section 5.2 and section 3.2If a revision has an associated revision label, then it is RECOMMENDED that the name of the file for that revision be of the form:
YANG module (or submodule) files may be identified using either the revision-date (as per section 3.2) or the revision label.
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:
specifies the revision label scheme used by the module or submoduleis defined in the document which specifies the revision label schemeMUST 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.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: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: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:label 3.1.0;
description "Add new functionality.";
}
revision 2019-03-01 {
rev:label 3.0.0;
rev:non-backwards-compatible;
description
"Add new functionality. Remove some deprecated nodes.";
}
revision 2019-02-01 {
rev:label 2.0.0;
rev:non-backwards-compatible;
description "Apply bugfix to pattern statement";
}
revision 2019-01-01 {
rev:label 1.0.0;
description "Initial revision";
}
//YANG module definition starts here
}
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:label 2.2.0;
description "Backwards-compatible bugfix to enhancement.";
}
revision 2019-04-01 {
rev:label 2.1.0;
description "Apply enhancement to older release train.";
}
revision 2019-02-01 {
rev:label 2.0.0;
rev:non-backwards-compatible;
description "Apply bugfix to pattern statement";
}
revision 2019-01-01 {
rev:label 1.0.0;
description "Initial revision";
}
//YANG module definition starts here
}
and 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 suggests that authors do not restrict YANG module imports to exact revision dates.Instead, for conformance purposes (section 5.6 of ), the recommended approach for defining the relationship between specific YANG module revisions is to specify the relationships outside of the YANG modules, e.g., via YANG library , YANG packages , a filesystem directory containing a set of consistent YANG module revisions, or a revision control system commit label.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 document introduces 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 .The ietf-revisions module defines the "recommended-min" extension statement, a substatement to the YANG "import"
statement, to allow for a "minimum recommended revision" to be documented:
The argument to the "recommended-min" extension statement is a revision date or a revision label.A particular revision of an imported module adheres to an import's "recommended-min" extension statement if
the imported module's revision history contains a revision statement with a matching revision date or
revision label. Removing entries from a module's revision history may cause a particular revision to no longer satisfy an import's "recommended-min" statement if the revision-date or label is no longer present in the module's revision history; further described in and .The "recommended-min" extension statement MAY be specified multiple times, allowing a set of recommended minimum revisions to be documented. Module implementors are recommended to pick a module revision that adheres to any of the "recommended-min" statements.Adding, modifying or removing a "recommended-min" extension statement is a BC change.Consider the example module "example-module" from 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
This example recommends module revisions for import 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:recommended-min 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:recommended-min 2.0.0;
}
This example recommends module revisions for import 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 2019-04-01/2.1.0 it is not a derived revision, and hence it is not a recommended
revision for import.
import example-module {
rev:recommended-min 2.1.0;
}
This example recommends module revisions for import that are 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:recommended-min 2019-04-01;
rev:recommended-min 2019-06-01;
}
This document updates YANG 1.1 and YANG library to clarify how ambiguous
module imports are resolved. It also defines the YANG module, ietf-yang-library-revisions, that augments YANG library with
revision labels and two leafs to indicate how a server implements deprecated and obsolete schema nodes.A YANG datastore schema, defined in , can specify multiple revisions of a YANG module
in the schema using the "import-only" list, with the requirement from section 5.6.5 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.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.The "ietf-yang-library-revisions" YANG module has the following
structure (using the notation defined in ):The ietf-yang-library-revisions YANG module augments the "module" and "submodule" lists in ietf-yang-library with "revision-label"
leafs to optionally declare the revision label associated with each module and submodule.The ietf-yang-library-revisions 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:
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
set to "false" or absent, then the behavior is unspecified.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".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.Instance data sets 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., ).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.The following text updates section 4.7 of to revise the guidelines for updating YANG modules.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 .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 could break import by revision, and hence it is RECOMMENDED to retain them.
If all dependencies 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:recommended-min" extension MAY 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.In some cases a module or submodule revision that is not strictly NBC by the definition in 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 submoduleThere 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:
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 ), 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 accidently reused in a future revision.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.For obsolete data nodes, it is RECOMMENDED to keep the above information, from when the node had status "deprecated", which is still relevant.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.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 , instead of removing a node,
that node SHOULD first be deprecated and then obsoleted.See for examples on how NBC changes can be made.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. YANG module with extension statements for annotating NBC changes, revision label, revision label scheme,
and importing by revision. file "ietf-yang-revisions@2022-11-29.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:
WG List:
Author: Joe Clarke
Author: Reshad Rahman
Author: Robert Wilton
Author: Balazs Lengyel
Author: Jason Sterne
";
description
"This YANG 1.1 module contains definitions and extensions to
support updated YANG revision handling.
Copyright (c) 2002 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.";
// 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 2022-11-29 {
rev:label "1.0.0-draft-ietf-netmod-yang-module-versioning-08";
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";
}
typedef revision-label {
type string {
length "1..255";
pattern '[a-zA-Z0-9,\-_.+]+';
pattern '[0-9]{4}-[0-9]{2}-[0-9]{2}' {
modifier "invert-match";
error-message
"The revision-label must not match a revision-date.";
}
}
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 or pattern similar to a date.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 3.3, Revision label";
}
typedef revision-date-or-label {
type union {
type revision-date;
type revision-label;
}
description
"Represents either a YANG revision date or a revision label";
}
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
'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 label {
argument revision-label;
description
"The revision label can be used to provide an additional
versioning identifier associated with a module or submodule
revision. One such 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 in this module.
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.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 3.3, Revision label";
}
extension revision-label-scheme {
argument revision-label-scheme-base;
description
"The revision label scheme specifies which revision label
scheme the module or submodule uses.
The mandatory revision-label-scheme-base 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 recommended-min {
argument revision-date-or-label;
description
"Recommends 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 'recommended-min' statements per
parent statement are allowed. No substatements for this
extension have been standardized.
If specified multiple times, then any module revision that
satisfies at least one of the 'recommended-min' statements is
an acceptable recommended revision for import.
A particular revision of an imported module adheres to an
import's 'recommended-min' extension statement if the imported
module's revision history contains a revision statement with a
matching revision date or revision label.
Adding, removing or updating a 'recommended-min' 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";
}
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";
}
}
]]>YANG module with augmentations to YANG Library to revision labels file "ietf-yang-library-revisions@2021-11-04.yang"
module ietf-yang-library-revisions {
yang-version 1.1;
namespace
"urn:ietf:params:xml:ns:yang:ietf-yang-library-revisions";
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 Library";
}
organization
"IETF NETMOD (Network Modeling) Working Group";
contact
"WG Web:
WG List:
Author: Joe Clarke
Author: Reshad Rahman
Author: Robert Wilton
Author: Balazs Lengyel
Author: Jason Sterne
";
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) 2002 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.";
// 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 label version with 1.0.0 and
// remove this note.
revision 2021-11-04 {
rev:label "1.0.0-draft-ietf-netmod-yang-module-versioning-05";
description
"Initial revision";
reference
"XXXX: Updated YANG Module Revision Handling";
}
// library 1.0 modules-state is not augmented with revision-label
augment "/yanglib:yang-library/yanglib:module-set/yanglib:module" {
description
"Add a revision label to module information";
leaf revision-label {
type rev:revision-label;
description
"The revision label associated with this module revision.
The label MUST match the revision 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
"Add a revision label to submodule information";
leaf revision-label {
type rev:revision-label;
description
"The revision label associated with this submodule revision.
The label MUST match the revision label value in the
specific 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" {
description
"Add a revision label to module information";
leaf revision-label {
type rev:revision-label;
description
"The revision label associated with this module revision.
The label MUST match the revision label value in the
specific revision of the module included 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
"Add a revision label to submodule information";
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' 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 5.2.2, 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 5.2.2,
Reporting how deprecated and obsolete nodes are handled";
}
}
}
]]>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) ) 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.
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 or RESTCONF . The lowest NETCONF layer
is the secure transport layer, and the mandatory-to-implement secure
transport is Secure Shell (SSH) . The lowest RESTCONF layer
is HTTPS, and the mandatory-to-implement secure transport is TLS
.
The NETCONF access control model 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 with augmentations to include revision labels in the YANG library data and two boolean leafs to 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 .
This document requests IANA to registers a URI in the "IETF XML
Registry" . Following the format in RFC 3688,
the following registrations are requested.
URI: urn:ietf:params:xml:ns:yang:ietf-yang-revisionsRegistrant Contact: The IESG.XML: N/A, the requested URI is an XML namespace.URI: urn:ietf:params:xml:ns:yang:ietf-yang-library-revisionsRegistrant 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" . Following the format in RFC 6020,
the following registrations are requested:The ietf-yang-revisions module:Name: ietf-yang-revisionsXML Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-revisionsPrefix: revReference: [RFCXXXX]The ietf-yang-library-revisions module:Name: ietf-yang-library-revisionsXML Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-library-revisionsPrefix: yl-revReference: [RFCXXXX]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" is derived from the "Interface Types (ifType) IANA registry", and "iana-routing-types.yang" is derived from
the "Address Family Numbers" and "Subsequent Address Family Identifiers (SAFI)
Parameters" 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 , where the
enum name for two SAFI values was changed.In all cases, IANA MUST follow the versioning guidance specified in , 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 .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.Semantic Versioning 2.0.0Interface Types (ifType) IANA Registryiana-if-type YANG ModuleAddress Family Numbers IANA RegistrySubsequent Address Family Identifiers (SAFI) Parameters IANA Registryiana-routing-types YANG Module2020-12-31 revision of iana-routing-types.yangExamples 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.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 .The examples are all for "config true" nodes.Removing a leaf or container from the data tree, e.g., because support for the corresponding feature is
being removed:
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.When the schema node is not supported anymore, its status is changed to "obsolete" and the
"description" updated. This is an NBC change.Changing the type of a leaf node. e.g., a "vpn-id" node of type integer being changed to a string:
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.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".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.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.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:
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. 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 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:
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.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".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).
When list "sessions" is not available anymore, its status is changed to "obsolete" and the
"description" is updated. This is an NBC change.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":
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.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".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.When node "ip-adress" is not available anymore, its status is changed to "obsolete" and the
"description" is updated. This is an NBC change.The following people made substantial contributions to this document: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:The initial revision of this document was refactored and built
upon . 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.