Updated YANG Module Revision Handling

1.1. Updates to YANG RFCs

This document updates [RFC7950] section 11 and [RFC6020] section 10. Section 3 describes modifications to YANG revision handling and update rules, and Section 4.1 describes a YANG extension statement to describe potential YANG import revision dependencies.¶

This document updates [RFC8407] section 4.7. Section 6 provides guidelines on managing the lifecycle of YANG modules that may contain non-backwards-compatible changes and a branched revision history.¶

This document updates [RFC8525] with augmentations to include two boolean leafs to indicate whether status deprecated and status obsolete schema nodes are implemented by the server.¶

3.1. Updating a YANG module with a new revision

This section updates [RFC7950] section 11 and [RFC6020] section 10 to refine the rules for permissible changes when a new YANG module revision is created.¶

New module revisions SHOULD NOT contain NBC changes because they often create problems for clients, however they can be helpful in some scenarios, and hence are discouraged, but allowed. For example:¶

• Bugfixes, particularly where the likely client impact is low or the module is changed to reflect current server behavior.¶

• To mark nodes as obsolete (or remove them), after a suitable deprecation period.¶

• To refine new and unstable modules (or new and unstable nodes within existing, stable modules).¶

• Restructuring a module to add new functionality where the cost of adding the functionality in a BC manner is disproportionate to the expected benefits of greater client backwards compatibility.¶

A YANG extension, defined in Section 3.2, is used to signal the potential for incompatibility to existing module users and readers.¶

As per [RFC7950] and [RFC6020], all published revisions of a module are given a new unique revision date.¶

3.2. non-backwards-compatible extension statement

The "rev:non-backwards-compatible" extension statement is used to indicate YANG module revisions that contain NBC changes.¶

If a revision of a YANG module contains changes, relative to the preceding revision in the revision history, that do not conform to the module update rules defined in Section 3.1.1, then a "rev:non-backwards-compatible" extension statement MUST be added as a substatement to the "revision" statement.¶

Adding, modifying or removing a "rev:non-backwards-compatible" extension statement is considered to be a BC change.¶

3.3. Removing revisions from the revision history

Authors may wish to remove revision statements from a module or submodule. Removal of revision information may be desirable for a number of reasons including reducing the size of a large revision history, or removing a revision that should no longer be used or imported. Removing revision statements is allowed, but can cause issues and SHOULD NOT be done without careful analysis of the potential impact to users of the module or submodule since it may cause loss of visibility of when non-backwards-compatible changes were introduced.¶

An author MAY remove a contiguous sequence of entries from the end (i.e., oldest entries) of the revision history. This is acceptable even if the first remaining (oldest) revision entry in the revision history contains a rev:non-backwards-compatible substatement.¶

An author MAY remove a contiguous sequence of entries in the revision history as long as the presence or absence of any existing rev:non-backwards-compatible substatements on all remaining entries still accurately reflect the compatibility relationship to their preceding entries remaining in the revision history.¶

The author MUST NOT remove the first (i.e., newest) revision entry in the revision history.¶

Example revision history:¶

revision 2020-11-11 {
  rev:non-backwards-compatible;
}

revision 2020-08-09 {
  rev:non-backwards-compatible;
}

revision 2020-06-07 {
}

revision 2020-02-10 {
  rev:non-backwards-compatible;
}

revision 2019-10-21 {
}

revision 2019-03-04 {
}

revision 2019-01-02 {
}

In the revision history example above (with revision descriptions omitted for clarity), removing the revision history entry for 2020-02-10 would also remove the rev:non-backwards-compatible annotation and hence the resulting revision history would incorrectly indicate that revision 2020-06-07 is backwards-compatible with revisions 2019-01-02 through 2019-10-21 when it is not, and so this change cannot be made. Conversely, removing one or more revisions out of 2019-03-04, 2019-10-21 and 2020-08-09 from the revision history would still retain a consistent revision history, and is acceptable, subject to an awareness of the concerns raised in the first paragraph of this section.¶

3.4. Examples for updating the YANG module revision history

The following diagram, explanation, and module history illustrates how a branched revision history for a YANG module could be represented chronologically. To aid clarity, it makes use of both the "non-backwards-compatible" extension statement, and the "version" extension statement defined in [I-D.ietf-netmod-yang-semver]:¶

Example YANG module with branched revision history using version identifiers defined in [I-D.ietf-netmod-yang-semver].¶

       Module revision date      Example version identifier
         2019-01-01                 <- 1.0.0
             |
         2019-02-01                 <- 2.0.0
             |      \
         2019-03-01  \              <- 3.0.0
             |        \
             |       2019-04-01     <- 2.1.0
             |           |
         2019-05-01      |          <- 3.1.0
                         |
                     2019-06-01     <- 2.2.0

The tree diagram above illustrates how an example module's revision history might evolve, over time. For example, the tree might represent the following changes, listed in chronological order from the oldest revision to the newest revision:¶

Example module, revision 2019-05-01:¶

module example-module {

  namespace "urn:example:module";
  prefix "prefix-name";

  import ietf-yang-revisions { prefix "rev"; }
  import ietf-yang-semver { prefix "ys"; }

  description
    "to be completed";

  revision 2019-05-01 {
    ys:version 3.1.0;
    description "Add new functionality.";
  }

  revision 2019-03-01 {
    ys:version 3.0.0;
    rev:non-backwards-compatible;
    description
      "Add new functionality. Remove some deprecated nodes.";
  }

  revision 2019-02-01 {
    ys:version 2.0.0;
    rev:non-backwards-compatible;
    description "Apply bugfix to pattern statement";
  }

  revision 2019-01-01 {
    ys:version 1.0.0;
    description "Initial revision";
  }

  //YANG module definition starts here
}

Example module, revision 2019-06-01:¶

module example-module {

  namespace "urn:example:module";
  prefix "prefix-name";

  import ietf-yang-revisions { prefix "rev"; }
  import ietf-yang-semver { prefix "ys"; }

  description
    "to be completed";

  revision 2019-06-01 {
    ys:version 2.2.0;
    description "Backwards-compatible bugfix to enhancement.";
  }

  revision 2019-04-01 {
    ys:version 2.1.0;
    description "Apply enhancement to older release train.";
  }

  revision 2019-02-01 {
    ys:version 2.0.0;
    rev:non-backwards-compatible;
    description "Apply bugfix to pattern statement";
  }

  revision 2019-01-01 {
    ys:version 1.0.0;
    description "Initial revision";
  }

  //YANG module definition starts here
}

4.1. Recommending a minimum revision for module imports

Although the previous section indicates that the actual relationship constraints between different revisions of YANG modules should be specified outside of the modules, in some scenarios YANG modules are designed to be loosely coupled, and implementors may wish to select sets of YANG module revisions that are expected to work together. For these cases it can be helpful for a module author to provide guidance on a recommended minimum revision that is expected to satisfy an YANG import. E.g., the module author may know of a dependency on a type or grouping that has been introduced in a particular imported YANG module revision. Although there can be no guarantee that all derived future revisions from the particular imported module will necessarily also be compatible, older revisions of the particular imported module are very unlikely to ever be compatible.¶

This module introduces, for modules with a linear revision history that are versioned using revision dates, a new YANG extension statement to provide guidance to module implementors on a recommended minimum module revision of an imported module that is anticipated to be compatible. This statement has been designed to be machine-readable so that tools can parse the minimum revision extension statement and generate warnings if appropriate, but this extension statement does not alter YANG module conformance of valid YANG module versions in any way, and specifically it does not alter the behavior of the YANG module import statement from that specified in [RFC7950].¶

The ietf-revisions module defines the "recommended-min-date" extension statement, a substatement to the YANG "import" statement, to allow for a "minimum recommended date" to be documented:¶

• The argument to the "recommended-min-date" extension statement is a revision date.¶

• A particular revision of an imported module adheres to an import's "recommended-min-date" extension statement if the imported module's revision date is equal to or later than the revision date argument of the "recommended-min-date" extension statement in the importing module.¶

• Zero or one "recommended-min-date" extension statement is allowed for each parent "import" statement.¶

• Adding, modifying or removing a "recommended-min-date" extension statement is a BC change.¶

       Module revision date
         2019-01-01
             |
         2019-02-01
             |      \
         2019-03-01  \
             |        \
             |       2019-04-01
             |           |
         2019-05-01      |
                         |
                     2019-06-01

import example-module {
  rev:recommended-min-date 2019-02-01;
}

import example-module {
  rev:recommended-min-date 2019-04-01;
}

6.1. Guidelines for YANG module authors

All IETF YANG modules MUST conform to this specification. In particular, sections: Section 3, Section 4, and the guidelines documented in this section.¶

NBC changes to YANG modules may cause problems to clients, who are consumers of YANG models, and hence YANG module authors SHOULD minimize NBC changes and keep changes BC whenever possible.¶

When NBC changes are introduced, consideration should be given to the impact on clients and YANG module authors SHOULD try to mitigate that impact.¶

A "rev:non-backwards-compatible" statement MUST be added if there are NBC changes relative to the previous revision.¶

Removing old revision statements from a module's revision history can cause a loss of visibility of when non-backwards-compatible changes were made, and hence it is RECOMMENDED to retain them. An alternative solution, if the revision section is too long, would be to remove, or curtail, the older description statements associated with the previous revisions.¶

In cases where a revision dependency is helpful for a module import, the "rev:recommended-min-date" extension SHOULD be used in preference to the "revision-date" statement, which causes overly strict import dependencies and SHOULD NOT be used.¶

A module that includes submodules SHOULD use the "revision-date" statement to include specific submodule revisions. The revision of the including module MUST be updated when any included submodule has changed.¶

In some cases a module or submodule revision that is not strictly NBC by the definition in Section 3.1.2 of this specification may include the "non-backwards-compatible" statement. Here is an example when adding the statement may be desirable:¶

• A "config false" leaf had its value space expanded (for example, a range was increased, or additional enum values were added) and the author or server implementor feels there is a significant compatibility impact for clients and users of the module or submodule¶

6.2. Versioning Considerations for Clients

Guidelines for clients of modules using the new module revision update procedure:¶

• Clients SHOULD be liberal when processing data received from a server. For example, the server may have increased the range of an operational node causing the client to receive a value which is outside the range of the YANG model revision it was coded against.¶

• Clients SHOULD monitor changes to published YANG modules through their revision history, and use appropriate tooling to understand the specific changes between module revision. In particular, clients SHOULD NOT migrate to NBC revisions of a module without understanding any potential impact of the specific NBC changes.¶

• Clients SHOULD plan to make changes to match published status changes. When a node's status changes from "current" to "deprecated", clients SHOULD plan to stop using that node in a timely fashion. When a node's status changes to "obsolete", clients MUST stop using that node.¶

8.1. Security considerations for module revisions

As discussed in the introduction of this document, YANG modules occasionally undergo changes that are not backwards compatible. This occurs in both standards and vendor YANG modules despite the prohibitions in RFC 7950. RFC 7950 also allows nodes to change to status 'obsolete' which can change behavior and compatibility for a client.¶

The fact that YANG modules change in a non-backwards-compatible manner may have security implications. Such changes should be carefully considered, including the scenarios described below. The rev:non-backwards-compatible extension statement introduced in this document provides an alert that the module or submodule may contain changes that impact users and need to be examined more closely for both compatibility and potential security implications. Flagging the change reduces the risk of introducing silent exploitable vulnerabilities.¶

When a module undergoes a non-backwards-compatible change, a server may implement different semantics for a given leaf than a client using an older version of the module is expecting. If the particular leaf controls any security functions of the device, or is related to parts of the configuration or state that are sensitive from a security point of view, then the difference in behavior between the old and new revisions needs to be considered carefully. In particular, changes to the default of the leaf should be examined.¶

Implementors and users should also consider impact to data node access control rules (e.g. The Network Configuration Access Control Model (NACM) [RFC8341]) in the face of non-backwards-compatible changes. Access rules may need to be adjusted when a new module revision is introduced that contains a non-backwards-compatible change.¶

If the changes to a module or submodule have security implications, it is recommended to highlight those implications in the description of the revision statement.¶

8.2. Security considerations for the modules defined in this document

The YANG module specified in this document defines a schema for data that is designed to be accessed via network management protocols such as NETCONF [RFC6241] or RESTCONF [RFC8040]. The lowest NETCONF layer is the secure transport layer, and the mandatory-to-implement secure transport is Secure Shell (SSH) [RFC6242]. The lowest RESTCONF layer is HTTPS, and the mandatory-to-implement secure transport is TLS [RFC8446].¶

The NETCONF access control model [RFC8341] provides the means to restrict access for particular NETCONF or RESTCONF users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content.¶

This document does not define any new protocol or data nodes that are writable.¶

This document updates YANG Library [RFC8525] with augmentations to include two boolean leafs that indicate whether status deprecated and status obsolete schema nodes are implemented by the server. These read-only augmentations do not add any new security considerations beyond those already present in [RFC8525].¶

9.1. YANG Module Registrations

This document requests IANA to registers a URI in the "IETF XML Registry" [RFC3688]. Following the format in RFC 3688, the following registrations are requested.¶

• URI: urn:ietf:params:xml:ns:yang:ietf-yang-revisions¶

• Registrant Contact: The IESG.¶

• XML: N/A, the requested URI is an XML namespace.¶

• URI: urn:ietf:params:xml:ns:yang:ietf-yang-status-conformance¶

• Registrant Contact: The IESG.¶

• XML: N/A, the requested URI is an XML namespace.¶

The following YANG module is requested to be registred in the "IANA Module Names" [RFC6020]. Following the format in RFC 6020, the following registrations are requested:¶

The ietf-yang-revisions module:¶

• Name: ietf-yang-revisions¶

• XML Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-revisions¶

• Prefix: rev¶

• Reference: [RFCXXXX]¶

The ietf-yang-status-conformance module:¶

• Name: ietf-yang-status-conformance¶

• XML Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-status-conformance¶

• Prefix: ys-conf¶

• Reference: [RFCXXXX]¶

B.1. Removing a data node

Removing a leaf or container from the data tree, e.g., because support for the corresponding feature is being removed:¶

1. The schema node's status is changed to "deprecated" and the node is supported for some period of time (e.g. one year). This is a BC change.¶

2. When the schema node is not supported anymore, its status is changed to "obsolete" and the "description" updated. This is an NBC change.¶

B.2. Changing the type of a leaf node

Changing the type of a leaf node. e.g., a "vpn-id" node of type integer being changed to a string:¶

1. The status of schema node "vpn-id" is changed to "deprecated" and the node is supported for some period of time (e.g. one year). This is a BC change. The description is updated to indicate that “vpn-name” is replacing this node.¶

2. A new schema node, e.g., "vpn-name", of type string is added to the same location as the existing node "vpn-id". This new node has status "current" and its description explains that it is replacing node "vpn-id".¶

3. During the period of time when both schema nodes are supported, the interactions between the two nodes is outside the scope of this document and will vary on a case by case basis. One possible option is to have the server prevent the new node from being set if the old node is already set (and vice-versa). The new node could have a "when" statement added to it to achieve this. The old node, however, must not have a "when" statement added, or an existing "when" modified to be more restrictive, since this would be an NBC change. In any case, the server could reject the old node from being set if the new node is already set.¶

4. When the schema node "vpn-id" is not supported anymore, its status is changed to "obsolete" and the "description" is updated. This is an NBC change.¶

B.3. Reducing the range of a leaf node

Reducing the range of values of a leaf-node, e.g., consider a "vpn-id" schema node of type uint32 being changed from range 1..5000 to range 1..2000:¶

1. If all values which are being removed were never supported, e.g., if a vpn-id of 2001 or higher was never accepted, this is a BC change for the functionality (no functionality change). Even if it is an NBC change for the YANG model, there should be no impact for clients using that YANG model.¶

2. If one or more values being removed was previously supported, e.g., if a vpn-id of 3333 was accepted previously, this is an NBC change for the YANG model. Clients using the old YANG model will be impacted, so a change of this nature should be done carefully, e.g., by using the steps described in Appendix B.2¶

B.4. Changing the key of a list

Changing the key of a list has a big impact to the client. For example, consider a "sessions" list which has a key "interface" and there is a need to change the key to "dest-address". Such a change can be done in steps:¶

1. The status of list "sessions" is changed to "deprecated" and the list is supported for some period of time (e.g. one year). This is a BC change. The description is updated to indicate the new list that is replacing this list.¶

2. A new list is created in the same location with the same descendant schema nodes but with "dest-address" as key. Finding an appropriate name for the new list can be difficult. In this case the new list is called "sessions-address", has status "current" and its description should explain that it is replacing list "session".¶

3. During the period of time when both lists are supported, the interactions between the two lists is outside the scope of this document and will vary on a case by case basis. One possible option is to have the server prevent entries in the new list from being created if the old list already has entries (and vice-versa).¶

4. When list "sessions" is not available anymore, its status is changed to "obsolete" and the "description" is updated. This is an NBC change.¶

B.5. Renaming a node

A leaf or container schema node may be renamed, either due to a spelling error in the previous name or because of a better name. For example a node "ip-adress" could be renamed to "ip-address":¶

1. The status of the existing node "ip-adress" is changed to "deprecated" and is supported for some period of time (e.g. one year). This is a BC change. The description is updated to indicate the node that is replacing this node.¶

2. The new schema node "ip-address" is added to the same location as the existing node "ip-adress". This new node has status "current" and its description should explain that it is replacing node "ip-adress".¶

3. During the period of time when both nodes are available, the interactions between the two nodes is outside the scope of this document and will vary on a case by case basis. One possible option is to have the server prevent the new node from being set if the old node is already set (and vice-versa). The new node could have a "when" statement added to it to achieve this. The old node, however, must not have a "when" statement added, or an existing "when" modified to be more restrictive, since this would be an NBC change. In any case, the server could reject the old node from being set if the new node is already set.¶

4. When node "ip-adress" is not available anymore, its status is changed to "obsolete" and the "description" is updated. This is an NBC change.¶