YANG Schema Version Selection

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 uses terminology introduced in the YANG versioning requirements document , the YANG Module Versioning document and the YANg Packages document . This document also makes of the following terminology introduced in the Network Management Datastore Architecture : datastore schema In addition, this document makes use of the following terminology: YANG schema: The combined set of schema nodes for a set of YANG module revisions, taking into consideration any deviations and enabled features. versioned schema: A YANG schema with an associated YANG semantic version number, e.g., as might be described by a YANG package. schema set: A set of related versioned YANG schema, one for each datastore that is supported. TODO - 'schema' and 'versioned schema' could be defined in the packages draft.

This document describes how NETCONF and RESTCONF clients can choose a particular YANG schema they wish to choose to interact with a server. defines requirements that any solution to YANG versioning must have. , which is based on , specifies a partial solution to the YANG versioning requirements that focuses on using semantic versioning within individual YANG modules, but does not address all the requirements listed in the requirements document. Of particular relevance here, requirements 3.1 and 3.2 are not addressed. describes how sets of related YANG modules can be grouped together into a logical entity that is versioned using the YANG semantic versioning number scheme. Different packages can be defined for different sets of YANG modules, e.g., packages could be defined for the IETF YANG modules, OpenConfig YANG modules, a vendor's YANG modules. Different versions of these package definitions can be defined as the contents of these packages evolve over time, and as the versions of the YANG modules included in the package evolve. This document defines how YANG packages can be used to represent versioned datastore schema, and how clients can choose which versioned schemas to use during interactions with a device.

There are three ways that the lifecycle of a data model can be managed: Disallow all non-backwards-compatible updates to a YANG module. Broadly this is the approach adopted by , but it has been shown to be too inflexible in some cases. E.g. it makes it hard to fix bugs in a clean fashion - it is not clear that allowing two independent data nodes (one deprecated, one current) to configure the same underlying property is robustly backwards compatible in all scenarios, particularly if the value space and/or default values differ between the module revisions. Allow non-backwards-compatible updates to YANG modules, and use a mechanism such as semantic version numbers to communicate the likely impact of any changes to module users, but require that clients handle non-backwards-compatible changes in servers by migrating to new versions of the modules. Without version selection, this is what the approach likely achieves. Allow non-backwards-compatible updates to YANG modules, but also provide mechanisms to allow servers to support multiple versions of YANG modules, and provide clients with some ability to select which versions of YANG modules they wish to interact with, subject to some reasonable constraints. This is the approach that this document aims to address. It is worth noting that the idea of supporting multiple versions of an API is not new in the wider software industry, and there any many examples of where this approach has been successfully used.

The goals of the schema version selection document are: To provide a mechanism where non-backwards-compatible changes and bug fixes can be made to YANG modules without forcing clients to immediately migrate to new versions of those modules as they get implemented. To allow servers to support multiple versions of a particular YANG schema, and to allow clients to choose which YANG schema version to use when interoperating with the server. The aim here is to give operators more flexibility as to when they update their software. To provide a mechanism to allow different YANG schema families (e.g., SDO models, OpenConfig models, Vendor models) to be supported by a server, and to allow clients to choose which YANG schema family is used to interoperate with the server. The following points are non objective of this document: This document does not provide a mechanism to allow clients to choose arbitrary sets of YANG module versions to interoperate with the server. Servers are not required to concurrently support clients using different YANG schema families or versioned schema. A server MAY choose to only allow a single schema family or single versioned schema to be used by all clients. There is no requirement for a server to support every published version of a YANG package, particularly if some package versions are backwards compatible. Clients are required to interoperate with backwards compatible updates of YANG modules. E.g., if a particular package was available in versions 1.0.0, 1.1.0, 1.2.0, 2.0.0, 3.0.0 and 3.1.0, then a server may choose to only support versions 1.2.0, 2.0.0, and 3.1.0, with the knowledge that all clients should be able to interoperate with the server. There is no requirement to support all parts of all versioned schemas. For some nbc changes in modules, it is not possible for a server to support both the old and new module versions, and to convert between the two. Where appropriate deviations can be used, and otherwise an out of band mechanism is used to indicate where a mapping has failed.

An overview of the solution is as follows: YANG packages, specified in , are defined for the different versioned schema supported by a server: Separate packages can be defined for different families of schema, e.g., SDO, OpenConfig, or vendor native. Separate packages can be defined for each versioned schema within a schema family. Separate packages may be defined for different datastores, if the datastores use different datastore schema. For example, a different datastore schema, and hence package, might be used for <operational> vs the conventional datastores. Each server advertises, via an operational data model: All of the YANG packages that may be used during version selection. The packages can also be made available for offline consumption via instance data documents, as described in . Grouped sets of versioned schema, where each set defines the versioned schema used by each supported datastore, and each versioned schema is represented by a YANG package instance. Each server supports the operations to: Allow a client to configure which schema version set to use for the default NETCONF/RESTCONF connections. Allow a client to configure additional separate RESTCONF protocol instances, which use different schema version sets on those protocol instances. See . Allow a client using NETCONF to use the "select-schema-sets" RPC to choose which schema sets it wants to use for the lifetime of the current NECONF session. See . The server internally maps requests between the different protocol instances to the internal device implementation.

A new NETCONF :schema-sets capability, using base:1.1 defined in , is used to indicate what schema sets the server is willing to support. The server sends an unordered comma separated list (with no white spaces) of schema sets it supports. A server MAY concurrently support clients using different YANG package versions. The :schema-sets capability is identified by the following capability string: In this example, the server advertises its (abbreviated) <hello> as follows. This indicates that the server supports the following schema sets: Versions 2.1.0 and 1.3.1 Versions 9.2.3 and 8.4.2 Some extra white spaces have been added for display purposes only.

urn:ietf:params:netconf:base:1.0 urn:ietf:params:netconf:base:1.1 urn:ietf:params:netconf:capability:schema-sets:1.0?list= example-ietf-routing@2.1.0,example-ietf-routing@1.3.1, example-vendor-xxx@9.2.3,example-vendor-xxx@8.4.2 ]]> TODO - add mechanism to indicate default version

Description: Used by a client to select schema-sets which have been advertised by the server via the mechanism described above in . The schema-sets are selected for the lifetime of the NETCONF session, unless new schema-sets are subsequently selected via this operation. Parameters: Schema-set(s)that the client wants to use for this session. Positive response: if the server was able to satisfy the request, an <rpc-reply> is sent that includes an <ok> element. Negative response: An <rpc-error> element is included in the <rpc-reply> if the request cannot be completed for any reason. A <select-schema-sets> operation can fail for a number of reasons, such as a YANG package version is not supported by the server, or a different version of a YANG package has already been selected by another client. Example: a client selects schema sets example-ietf-routing version 1.3.1 and example-vendor-xxx version 9.2.3:

example-ietf-routing@1.3.1

example-vendor-xxx@9.2.3

]]> TODO - add error indication

To enable servers which can support different versions of YANG packages: On servers, a unique RESTCONF root resource can be configured to map to each schema-set A default schema-set can be configured. Clients select the desired schema-set by choosing the corresponding RESTCONF root resource This updates Section 3.1 of . For example, consider a device which has been configured with root-path "/restconf/example-ietf-routing-1.3.1" for secondary schema set "example-ietf-routing-1.3.1". Any operations by the client on schema set "example-ietf-routing-1.3.1" would use "/restconf/example-ietf-routing-1.3.1" as the RESTCONF root resource.

The general premise of this solution is that servers generally implement one native schema, and the version selection scheme is used to support older version of that native schema and also foreign schema specified by external entities. Overall the solution relies on the ability to map instance data between different schema versions. Depending on the scope of difference between the schema versions then some of these mappings may be very hard, or even impossible, to implement. Hence, there is still a strong incentive to try and minimize nbc changes between schema versions to minimize the mapping complexity. Server implementations MUST serialize configuration requests across the different schema. The expectation is that this would be achieved by mapping all requests to the device's native schema version. Datastore validation needs to be performed in two places, firstly in whichever schema a client is interacting in, and secondly in the native schema for the device. This could have a negative performance impact. Depending on the complexity of the mappings between schema versions, it may be necessary for the mappings to be stateful. TODO - Figure out how hot fixes that slightly modify the schema are handled.

Clients can use configuration to choose which schema sets are available. Clients cannot choose arbitrary individual YANG module versions, and are instead constrained by the versions that the server makes available. Each client protocol connection is to one particular schema set. From that client session perspective it appears as if the client is interacting with a regular server. If the client queries YANG library that the version of YANG Library that is returned matches the schema set that is being used for that server instance. The server may not support a schema with the exact version desired by the client, and they have to accept a later version that is backwards compatible with their desired version. Clients may also have to accept later schema versions that contain NBC fixes, although the assumption is that such nbc fixes should be designed to minimize the impact on clients. There is no guarantee that servers will always be able to support all older schema versions. Deviations should be used where necessary to indicate that the server is unable to faithfully implement the older schema version. If clients interact with a server using multiple versions, they should not exact that all data nodes in later module versions can always be backported to older schema versions. TODO - Specify how mapping errors can be reported to client.

Not all schema conversions are possible. E.g. an impossible type conversion, or something has been removed. The solution is fundamentally limited by how the schemas actually change, this solution does not provide a magic bullet that can solve all versioning issues.

The YANG schema version selection YANG module is used by a device to report the schema-sets that are available, and to allow clients to choose which schema-set they wish to use. Feature are used to allow servers to decide whether they allow the primary schema-set to be changed, and/or allow secondary schema-sets to be configured. The primary schema-set is the datastore schema reported by YANG Library. If secondary schema-sets are configured: With NETCONF, the "select-schema-sets" RPC is used by the client to chosse which schema set(s) it wants to use for the current NETCONF session. With RESTCONF, the configured root path prefix is used by the client for a particular schema set. Different schema-sets may support different datastores.

The "ietf-schema-version-selection" YANG module has the following structure: /yanglib:yang-library/pkg:package/name | +--ro version? -> /yanglib:yang-library/pkg:package[pkg:name = current() | /../package]/version +--rw restconf {secondary-schema-set}? | +--rw schema-sets* [schema-set] | +--rw schema-set -> /schema-selection/schema-sets/name | +--rw root-path inet:uri +--rw default-schema-set? -> /schema-selection/schema-sets/name {default-schema-set}? rpcs: +---x select-schema-sets +---w input +---w schema-sets* [schema-set] +---w schema-set -> /schema-selection/schema-sets/name ]]>

The YANG module definition for the module described in the previous sections.

file "ietf-schema-version-selection@2019-10-31.yang" module ietf-schema-version-selection { yang-version 1.1; namespace "urn:ietf:params:xml:ns:yang:ietf-schema-version-selection"; prefix "ver-sel"; import ietf-inet-types { prefix inet; reference "RFC 6991: Common YANG Data Types."; } import ietf-datastores { prefix ds; reference "RFC 8342: Network Management Datastore Architecture (NMDA)"; } import ietf-yang-library { prefix yanglib; reference "RFC 8525: YANG Library"; } import ietf-yl-packages { prefix pkg; reference "draft-rwilton-netmod-yang-packages-02"; } organization "IETF NETMOD (Network Modeling) Working Group"; contact "WG Web: WG List: Author: Reshad Rahman Author: Rob Wilton "; description "This module provide a data model to advertise and allow the selection of schema versions by clients. 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."; // RFC Ed.: update the date below with the date of RFC publication // and remove this note. // RFC Ed.: replace XXXX with actual RFC number and remove this // note. revision 2019-10-31 { description "Initial revision"; reference "RFC XXXX: YANG Schema Version Selection"; } /* * Features */ feature "default-schema-set" { description "Feature that allows clients to choose the default schema set to be used for clients. Implementations may choose to only support this feature in to report the default-schema-set without allowing it to be configured."; } feature "secondary-schema-set" { description "Feature to choose if secondary schema sets may be configured by clients. Implementations may choose to only support this feature in to report secondary schema sets without allowing them to be configured."; } container schema-selection { description "YANG schema version selection"; list schema-sets { key "name"; description "All schema-sets that are available for selection by clients."; leaf name { type "string" { length "1..255"; } description "The server assigned name of the schema-set. This should include the schema family, and appropriate versioning or release information"; } list datastores { config false; key "datastore"; description "The list of datastores supported for this schema set"; leaf datastore { type ds:datastore-ref; description "The datastore that this datastore schema is associated with"; reference "RFC 8342: Network Management Datastore Architecture (NMDA)"; } list packages { key "package"; description "YANG packages associated with this datastore schema"; leaf package { type leafref { path "/yanglib:yang-library/pkg:package/pkg:name"; } description "The name of the YANG package this schema relates to"; } leaf version { type leafref { path '/yanglib:yang-library/' + 'pkg:package[pkg:name = current()/../package]/' + 'pkg:version'; } description "The version of the YANG package this schema relates to"; } } } } container restconf { if-feature "secondary-schema-set"; description "RESTCONF protocol settings for schema sets"; list schema-sets { key "schema-set"; unique "root-path"; description "The schema-sets accessed via RESTCONF"; leaf schema-set { type leafref { path '/schema-selection/schema-sets/name'; } description "A schema-set being used by RESTCONF"; } leaf root-path { type inet:uri; mandatory true; description "The root path to use to access the RESTCONF protocol instance for this schema-set"; reference "RFC8040"; } } } leaf default-schema-set { if-feature "default-schema-set"; type leafref { path '/schema-selection/schema-sets/name'; } description "Specifies the default schema-set used by this device. This is the set of datastore schema that is used if a client connects using the standard URLs"; } } /* * RPCs */ rpc select-schema-sets { description "This RPC allows a NETCONF client to select which schema-sets, among the ones advertised by the server, the clients wants to use for this session"; input { list schema-sets { key "schema-set"; leaf schema-set { type leafref { path '/schema-selection/schema-sets/name'; } } } } } }

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      To be defined.
    
    
      TODO - Add registrations for YANG modules defined in this document.
      This document registers a URI.
      
         This document registers a URI in the IETF XML
            registry .  The IANA registry "Network Configuration
            Protocol (NETCONF) Capability URNs" needs to be updated to include
            the following capability.
 

         
      
    
    
      All issues, along with the draft text, are currently being
      tracked at: https://github.com/netmod-wg/yang-ver-dt/labels/version-selection-solution
    
    
      The ideas that formed this draft are based on discussions with the YANG versioning design
      team, and other members of the NETMOD WG.