Internet DRAFT - draft-verdt-netmod-yang-module-versioning
draft-verdt-netmod-yang-module-versioning
Network Working Group B. Claise
Internet-Draft J. Clarke
Updates: 7950 (if approved) R. Rahman
Intended status: Standards Track R. Wilton, Ed.
Expires: April 17, 2020 Cisco Systems, Inc.
B. Lengyel
Ericsson
J. Sterne
Nokia
K. D'Souza
AT&T
October 15, 2019
Updated YANG Module Revision Handling
draft-verdt-netmod-yang-module-versioning-01
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. 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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 17, 2020.
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Copyright Notice
Copyright (c) 2019 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
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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 . . . . . . . 5
3.1.1. Backwards-compatible changes . . . . . . . . . . . . 5
3.1.2. Non-backwards-compatible changes . . . . . . . . . . 6
3.2. nbc-changes revision extension statement . . . . . . . . 6
3.3. Revision label . . . . . . . . . . . . . . . . . . . . . 6
3.4. YANG status description extension statement . . . . . . . 7
3.5. Examples for updating the YANG module revision history . 7
4. Import by derived revision . . . . . . . . . . . . . . . . . 10
4.1. Module import examples . . . . . . . . . . . . . . . . . 11
5. Updates to ietf-yang-library . . . . . . . . . . . . . . . . 13
5.1. Resolving ambiguous module imports . . . . . . . . . . . 13
5.2. YANG library versioning augmentations . . . . . . . . . . 14
5.2.1. Advertising revision-label . . . . . . . . . . . . . 14
5.2.2. Reporting how deprecated and obsolete nodes are
handled . . . . . . . . . . . . . . . . . . . . . . . 14
6. Versioning of YANG instance data . . . . . . . . . . . . . . 15
7. Guidelines for using the YANG module update rules . . . . . . 15
7.1. Guidelines for YANG module authors . . . . . . . . . . . 15
7.1.1. Making non-backwards-compatible changes to a YANG
module . . . . . . . . . . . . . . . . . . . . . . . 16
7.2. Versioning Considerations for Clients . . . . . . . . . . 17
8. Module Versioning Extension YANG Modules . . . . . . . . . . 18
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 25
10. Security Considerations . . . . . . . . . . . . . . . . . . . 26
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
11.1. YANG Module Registrations . . . . . . . . . . . . . . . 26
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
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12.1. Normative References . . . . . . . . . . . . . . . . . . 26
12.2. Informative References . . . . . . . . . . . . . . . . . 27
Appendix A. Appendix . . . . . . . . . . . . . . . . . . . . . . 28
A.1. Examples of guidelines for making NBC changes to a YANG
module . . . . . . . . . . . . . . . . . . . . . . . . . 28
A.1.1. Removing a data node . . . . . . . . . . . . . . . . 28
A.1.2. Changing the type of a leaf node . . . . . . . . . . 29
A.1.3. Reducing the range of a leaf node . . . . . . . . . . 30
A.1.4. Changing the key of a list . . . . . . . . . . . . . 30
A.1.5. Renaming a node . . . . . . . . . . . . . . . . . . . 31
A.1.6. Changing a default value . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
1. Introduction
This document defines a solution to the YANG module lifecycle
problems described in [I-D.verdt-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.verdt-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 solution 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 descriptions, to report how
"deprecated" and "obsolete" nodes are handled by a server, and to
clarify how module imports are resolved when multiple versions
could otherwise be chosen.
Considerations of how versioning applies to YANG instance data.
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Guidelines for how the YANG module update rules defined in this
document should be used, along with examples.
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 [RFC8525] section 3. Section 5 defines how a
client of a YANG library datastore schema chooses which revision of
an import-only module is used to resolve a module import when the
definition is otherwise ambiguous.
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.
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.
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
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3. Refinements to YANG revision handling
[RFC7950] assumes, but does not explicitly state, that the revision
history for a YANG module is strictly linear, i.e., it is prohibited
to have two independent revisions of a YANG module that are both
directly derived from the same parent revision.
This document clarifies [RFC7950] to explicitly allow non linear
development of YANG module revisions, so modules MAY have multiple
revisions that directly derive from the same parent revision. As per
[RFC7950], YANG module revisions continue to be uniquely identified
by the module's revision date, and hence all revisions of a module
MUST have unique revision dates.
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 the module's "include" statements
MUST use "revision-date" substatements to specify the exact revision
date of each included submodule.
[RFC7950] section 11 requires that all updates to a YANG module are
BC to the previous revision of the module. This document allows for
more flexible evolution of YANG modules: NBC changes between module
revisions are allowed and are documented using a new "nbc-changes"
YANG extension statement in the module revision history.
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, i.e., the semantics of
an existing definition MAY be changed in an NBC way without requiring
a new definition with a new identifier. A new module revision with
NBC changes MUST include the "rev:nbc-changes" extension substatement
to signal the potential for incompatibility to existing module users
and readers.
3.1.1. Backwards-compatible changes
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:
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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 Obsolete definitions 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 or any "rpc"
"input" substatements. If new data definition statements are
added, they can be added anywhere in the sequence of existing
substatements.
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. nbc-changes revision extension statement
The "rev:nbc-changes" 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:nbc-
changes" extension statement MUST be added as a substatement to the
"revision" statement.
Conversely, if a revision does not contain an "rev:nbc-changes"
extension substatement then all changes, relative to the preceding
revision in the revision history, MUST be backwards-compatible.
3.3. 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.verdt-netmod-yang-semver] defines a versioning
scheme based on Semver 2.0.0 [semver] that can be used as a revision
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label. All revision labels that match the pattern for the "version"
typedef in the ietf-yang-semver YANG module MUST be interpreted as
YANG semantic version numbers.
The revision date and revision label within a submodule's revision
history have no effect on the including module's revision.
Submodules MUST NOT use revision label schemes that could be confused
with the including module's revision label scheme.
If a revision has an associated revision label, then it may be used
instead of the revision date in two places:
In an "rev:revision-or-derived" extension statement argument.
In the filename of a YANG module, where it takes the form: module-
or-submodule-name ['@' revision-label] ( '.yang' / '.yin' )
3.4. YANG status description extension statement
The ietf-yang-revision module specifies the YANG extension statement
"status-description" that can be used as a substatement of the status
statement. The argument to this extension statement can contain
freeform text to help readers of the module understand why the node
was deprecated or made obsolete, when it is anticipated that the node
will no longer be available for use, and potentially reference other
schema elements that can be used instead. An example is shown below.
leaf imperial-temperature {
type int64;
units "degrees Fahrenheit";
status deprecated {
rev:status-description
"Imperial measurements are being phased out in favor
of their metric equivalents. Use metric-temperature
instead.";
}
description
"Temperature in degrees Fahrenheit.";
}
3.5. Examples for updating the YANG module revision history
The following diagram, explanation, and module history illustrates
how the branched revision history, "nbc-changes" extension statement,
and "revision-label" extension statement could be used:
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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 version
history might evolve, over time. For example, the tree might
represent the following changes, listed in chronological order from
oldest revision to newest:
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Example module, revision 2019-06-01:
module example-module {
namespace "name-space";
prefix "prefix-name";
import ietf-yang-revisions { prefix "rev"; }
description
"to be completed";
revision 2019-06-01 {
rev:revision-label 3.1.0;
description "Add new functionality.";
}
revision 2019-04-01 {
rev:revision-label 3.0.0;
rev:nbc-changes;
description
"Add new functionality. Remove some deprecated nodes.";
}
revision 2019-02-01 {
rev:revision-label 2.0.0;
rev:nbc-changes;
description "Apply bugfix to pattern statement";
}
revision 2019-01-01 {
rev:revision-label 1.0.0;
description "Initial revision";
}
//YANG module definition starts here
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Example module, revision 2019-05-01:
module example-module {
namespace "name-space";
prefix "prefix-name";
import ietf-yang-revisions { prefix "rev"; }
description
"to be completed";
revision 2019-05-01 {
rev:revision-label 2.2.0;
description "Backwards-compatible bugfix to enhancement.";
}
revision 2019-03-01 {
rev:revision-label 2.1.0;
description "Apply enhancement to older release train.";
}
revision 2019-02-01 {
rev:revision-label 2.0.0;
rev:nbc-changes;
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.
Instead, it is desirable for a importing module to specify a "minimum
required revision" of a module that it is compatible with, based on
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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 "minium 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 satifies at least one of the "revision-or-derived" extension
statements.
The "revision-or-derived" extension statement does not gaurantee
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.
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:
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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
"1.0.0" instead, which selects the same set of revisions/versions.
import example-module {
rev:revision-or-derived 1.0.0;
}
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 version number than
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;
}
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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 [RFC7895] [RFC8525] to clarify how
ambiguous module imports are resolved. It also defines the YANG
module, ietf-yl-revisions that augments YANG library with new
versioning 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.
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.
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5.2. YANG library versioning augmentations
The "ietf-yl-revisions" YANG module has the following structure
(using the notation defined in [RFC8340]):
module: ietf-yl-revisions
augment /yanglib:yang-library/yanglib:module-set/yanglib:module:
+--ro revision-label? rev:revision-label
augment /yanglib:yang-library/yanglib:schema:
+--ro deprecated-nodes-implemented? empty
+--ro obsolete-nodes-absent? empty
5.2.1. Advertising revision-label
The ietf-yl-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-yl-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 present, 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 then
the behavior is unspecified.
obsolete-nodes-absent: If present, this leaf indicates that the
server does not implement any status "obsolete" nodes. If this
leaf is absent then the behaviour is unspecified.
Servers SHOULD set both the "deprecated-nodes-implemented" and
"obsolete-nodes-absent" leaves.
If a server does not set the "deprecated-nodes-implemented" leaf,
then clients MUST NOT rely solely on the "rev:nbc-changes" 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.
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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.rwilton-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.verdt-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.
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:nbc-changes" statement SHOULD be added only 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
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revision section is too long, would be 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 MUST use the "revision-date"
statement to include specific submodule revisions. Changing a
module's include statements to include different submodule revisions
requires a new revision of the module.
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.
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 versions of the YANG module or
of YANG packages(as specified in
[I-D.rwilton-netmod-yang-packages]), and allows the client to
select the version (as per
[I-D.wilton-netmod-yang-ver-selection]), then NBC changes MAY be
done without using "status" statements. Clients would be required
to select the version 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.
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2. The new "status-description" extension statement SHOULD be used
for nodes which are "obsolete" or "deprecated".
3. For status "deprecated", the "status-description" 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 "status-description". The reason for deprecating the node
can also be included in the "status-description" if it is deemed
to be of potential interest to the user.
4. For status "obsolete", it is RECOMMENDED to keep the "status-
description" information, from when the node had status
"deprecated, which is still relevant.
5. When obsoleting or deprecating data nodes, the "deprecated" or
"obsolete" status SHOULD be applied at the highest possible level
in the data tree with an appropriate "status-description"
statement. For clarity, the "status" statement SHOULD also be
applied to all descendent data nodes, but the "status-
description" statement does not need to be repeated if it does
not introduce any additional information.
See Appendix A.1 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.
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8. Module Versioning Extension YANG Modules
YANG module with extension statements for annotating NBC changes,
revision label, status description, and importing by version.
<CODE BEGINS> file "ietf-yang-revisions@2019-09-18.yang"
module ietf-yang-revisions {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-yang-revisions";
prefix rev;
import ietf-yang-types {
prefix yang;
reference
"XXXX [ietf-netmod-rfc6991-bis]: Common YANG Data Types";
}
organization
"IETF NETMOD (Network Modeling) Working Group";
contact
"WG Web: <https://datatracker.ietf.org/wg/netmod/>
WG List: <mailto:netmod@ietf.org>
Author: Benoit Claise
<mailto:bclaise@cisco.com>
Author: Joe Clarke
<mailto:jclarke@cisco.com>
Author: Reshad Rahman
<mailto:rrahman@cisco.com>
Author: Robert Wilton
<mailto:rwilton@cisco.com>
Author: Kevin D'Souza
<mailto:kd6913@att.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) 2019 IETF Trust and the persons identified as
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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.
revision 2019-09-18 {
description
"Initial version.";
reference
"XXXX: Updated YANG Module Revision Handling";
}
typedef revision-label {
type string {
length "1..255";
pattern '[^\s@]+';
pattern '\d{4}-\d{2}-\d{2}' {
modifier invert-match;
}
}
description
"A label associated with a YANG revision.
Excludes spaces and '@'. MUST NOT match revision-date.
Revision labels that classify as YANG semantic versions,
as defined by the ietf-yang-semver:version typedef,
MUST conform to the versioning behaviour defined in
XXXX [verdt]: YANG Semantic Versioning.";
reference
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"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.
Each 'revision' statement MAY have a single 'nbc-changes'
substatement.
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
the 'nbc-changes' statement MUST be added as a substatement to
the revision statement.
Conversely, if a revision of a YANG module only contains
changes, relative to the preceding revision in the revision
history, that are classified as 'backwards-compatible' then
the revision statement MUST NOT contain any 'nbc-changes'
substatement.";
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 the revision. E.g., one
option for a versioning scheme that could be used is [TODO -
Reference semver draft].
The format of the revision-label argument MUST conform to the
pattern defined for the revision-label typedef.
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Each 'revision' statement MAY have a single 'revision-label'
substatement.
Revision labels MUST be unique amongst all revisions of a
module.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 3.3, Revision label";
}
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-nlabel.
The argument value MUST conform to the
'revision-date-or-label' defined type.
Each 'import' statement MAY have one or more
'revision-or-derived' substatements. If specified multiple
times, then any module revision that satifies 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
gaurantee 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.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 4, Import by derived revision";
}
extension status-description {
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argument description;
description
"Freeform text that describes why a given node has been
deprecated or made obsolete. E.g., the description could be
used to give the reason for removal, or it could point to an
alternative schema elements that can be used in lieu of the
given node.
Each 'status' statement MAY have a single 'status-description'
substatement.";
reference
"XXXX: Updated YANG Module Revision Handling;
Section 3.4, YANG status description extension statement";
}
}
<CODE ENDS>
YANG module with augmentations to YANG Library to revision labels
<CODE BEGINS> file "ietf-yl-revisions@2019-09-18.yang"
module ietf-yl-revisions {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-yl-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 Libary";
}
organization
"IETF NETMOD (Network Modeling) Working Group";
contact
"WG Web: <https://datatracker.ietf.org/wg/netmod/>
WG List: <mailto:netmod@ietf.org>
Author: Benoit Claise
<mailto:bclaise@cisco.com>
Author: Joe Clarke
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<mailto:jclarke@cisco.com>
Author: Reshad Rahman
<mailto:rrahman@cisco.com>
Author: Robert Wilton
<mailto:rwilton@cisco.com>
Author: Kevin D'Souza
<mailto:kd6913@att.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) 2019 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 (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 2019-09-18 {
rev:revision-label 0.1.0;
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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: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 empty;
description
"If present, 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 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 empty;
description
"If present, this leaf indicates that the server does not
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implement any status 'obsolete' nodes. If this leaf is
absent 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 Jason Sterne
o Joe Clarke
o Juergen Schoenwaelder
o Mahesh Jethanandani
o Michael (Wangzitao)
o Qin Wu
o Reshad Rahman
o Rob Wilton
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 thank both
Anees Shaikh and Rob Shakir for their input into this problem space.
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10. Security Considerations
The document does not define any new protocol or data model. There
are no security impacts.
11. IANA Considerations
11.1. YANG Module Registrations
The following YANG module is requested to be registred in the "IANA
Module Names" registry:
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-yl-revisions module:
Name: ietf-yl-revisions
XML Namespace: urn:ietf:params:xml:ns:yang:ietf-yl-revisions
Prefix: yl-rev
Reference: [RFCXXXX]
12. References
12.1. Normative References
[I-D.ietf-netmod-rfc6991-bis]
Schoenwaelder, J., "Common YANG Data Types", draft-ietf-
netmod-rfc6991-bis-01 (work in progress), July 2019.
[I-D.verdt-netmod-yang-semver]
Claise, B., Clarke, J., Rahman, R., Wilton, R., Lengyel,
B., Sterne, J., and K. D'Souza, "YANG Semantic
Versioning", draft-verdt-netmod-yang-semver-01 (work in
progress), October 2019.
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[I-D.verdt-netmod-yang-versioning-reqs]
Clarke, J., "YANG Module Versioning Requirements", draft-
verdt-netmod-yang-versioning-reqs-02 (work in progress),
November 2018.
[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>.
[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
[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-04
(work in progress), August 2019.
[I-D.rwilton-netmod-yang-packages]
Wilton, R., "YANG Packages", draft-rwilton-netmod-yang-
packages-01 (work in progress), March 2019.
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[I-D.verdt-netmod-yang-solutions]
Wilton, R., "YANG Versioning Solution Overview", draft-
verdt-netmod-yang-solutions-01 (work in progress), July
2019.
[I-D.wilton-netmod-yang-ver-selection]
Wilton, R. and R. Rahman, "YANG Schema Version Selection",
draft-wilton-netmod-yang-ver-selection-00 (work in
progress), March 2019.
[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>.
[semver] "Semantic Versioning 2.0.0", <https://www.semver.org>.
Appendix A. Appendix
A.1. Examples of guidelines for making NBC changes to a YANG module
Examples of NBC changes include:
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.
The following sections give guidance for how some of these NBC
changes could be made to a YANG module:
A.1.1. Removing a data node
Removing a leaf or container from the data tree, e.g. because support
for the corresponding feature is being removed:
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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 "status-description" updated, this is an NBC
change. The "status-description" is used to explain why the node
is not available anymore.
If the server can support NBC versions of the YANG module
simultaneously using version 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 "status-description" updated, this is an
NBC change.
2. Clients which require the data node select the older module
revision
A.1.2. 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 where 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
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the server could return integer value 123 for the old node
"vpn-id". However, if the value can not be mapped, we need a
way of returning "unsupported" TBD.
4. When node "vpn-id" is not available anymore, its status is
changed to "obsolete" and the "status-description" is updated.
This is an NBC change.
If the server can support NBC versions of the YANG module
simultaneously using version 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.
3. Clients which require the data node select the older module
revision
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).
A.1.3. 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 A.1.2
A.1.4. 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
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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 where 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:
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 "status-description" is updated.
This is an NBC change.
If the server can support NBC versions of the YANG module
simultaneously using version 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
A.1.5. 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":
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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 where 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
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 "status-description" is updated.
This is an NBC change.
If the server can support NBC versions of the YANG module
simultaneously using version 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
A.1.6. Changing a default value
Authors' Addresses
Claise, et al. Expires April 17, 2020 [Page 32]
Internet-Draft Updated YANG Module Revision Handling October 2019
Benoit Claise
Cisco Systems, Inc.
De Kleetlaan 6a b1
1831 Diegem
Belgium
Phone: +32 2 704 5622
Email: bclaise@cisco.com
Joe Clarke
Cisco Systems, Inc.
7200-12 Kit Creek Rd
Research Triangle Park, North Carolina
United States of America
Phone: +1-919-392-2867
Email: jclarke@cisco.com
Reshad Rahman
Cisco Systems, Inc.
Email: rrahman@cisco.com
Robert Wilton (editor)
Cisco Systems, Inc.
Email: rwilton@cisco.com
Balazs Lengyel
Ericsson
Magyar Tudosok Korutja
1117 Budapest
Hungary
Phone: +36-70-330-7909
Email: balazs.lengyel@ericsson.com
Jason Sterne
Nokia
Email: jason.sterne@nokia.com
Claise, et al. Expires April 17, 2020 [Page 33]
Internet-Draft Updated YANG Module Revision Handling October 2019
Kevin D'Souza
AT&T
200 S. Laurel Ave
Middletown, NJ
United States of America
Email: kd6913@att.com
Claise, et al. Expires April 17, 2020 [Page 34]
