Internet DRAFT - draft-wilton-netmod-yang-ver-selection
draft-wilton-netmod-yang-ver-selection
Network Working Group R. Wilton
Internet-Draft R. Rahman
Intended status: Standards Track J. Clarke
Expires: August 31, 2020 Cisco Systems, Inc.
J. Sterne
Nokia
B. Wu
Huawei
February 28, 2020
YANG Schema Selection
draft-wilton-netmod-yang-ver-selection-02
Abstract
This document defines a mechanism to allow clients, using NETCONF or
RESTCONF, to configure and select YANG schema for interactions with a
server. This functionality can help servers support clients using
older revisions of YANG modules even if later revisions contain non-
backwards-compatible changes. It can also be used to allow clients
to select between YANG schema defined by different organizations.
This draft provides a solution to YANG versioning requirements 3.1
and 3.2.
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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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 31, 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Terminology and Conventions . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Packages . . . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Schema-sets . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. Defining and changing client selectable schema-sets . . . 8
5.4. Schema selection protocol operations . . . . . . . . . . 8
5.4.1. NETCONF . . . . . . . . . . . . . . . . . . . . . . . 8
5.4.1.1. schema-sets NETCONF capability . . . . . . . . . 8
5.4.2. RESTCONF . . . . . . . . . . . . . . . . . . . . . . 10
5.5. Custom schema-sets . . . . . . . . . . . . . . . . . . . 11
6. Schema selection from a server perspective . . . . . . . . . 11
7. Schema selection from a client perspective . . . . . . . . . 12
7.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 12
8. Limitations of the solution . . . . . . . . . . . . . . . . . 13
9. Schema Selection YANG module . . . . . . . . . . . . . . . . 13
10. YANG Module . . . . . . . . . . . . . . . . . . . . . . . . . 14
11. Security Considerations . . . . . . . . . . . . . . . . . . . 20
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
12.1. NETCONF Capability URNs . . . . . . . . . . . . . . . . 20
13. Open Questions/Issues . . . . . . . . . . . . . . . . . . . . 21
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
15.1. Normative References . . . . . . . . . . . . . . . . . . 21
15.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Schema selection examples . . . . . . . . . . . . . 23
A.1. Supporting older versions of a schema . . . . . . . . . . 23
A.1.1. Variation - Multiple selectable schema-sets . . . . . 26
A.2. Supporting different schema families . . . . . . . . . . 27
A.2.1. Choosing a single schema family . . . . . . . . . . . 30
A.2.2. Restricting some sessions to particular schema family 30
A.2.3. Custom combinable schema-set . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
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1. 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.
This document uses terminology introduced in the YANG versioning
requirements [I-D.verdt-netmod-yang-versioning-reqs], YANG Module
Versioning [I-D.verdt-netmod-yang-module-versioning] and YANG
Packages [I-D.rwilton-netmod-yang-packages].
This document makes of the following terminology introduced in the
Network Management Datastore Architecture [RFC8342]:
o datastore schema
This document defines the following terminology:
o YANG schema: The combined set of schema nodes for a set of YANG
module revisions, taking into consideration any deviations and
enabled features.
o versioned schema: A YANG schema with an associated YANG semantic
version number, e.g., as might be described by a YANG
package[I-D.rwilton-netmod-yang-packages].
o schema-set: A named set of datastore schemas for supported
datastores, where every datastore schema is specified as a union
of compatible YANG packages. A schema-set is the basic unit of
client schema selection.
2. Introduction
This document describes how NETCONF and RESTCONF clients can use
protocol extensions, along with a schema selection YANG module, to
choose particular YANG schema for interactions with a server.
[I-D.verdt-netmod-yang-versioning-reqs] defines requirements that any
solution to YANG versioning must have.
YANG Semver [I-D.verdt-netmod-yang-semver], based on YANG Module
Versioning, specifies a partial solution to the YANG versioning
requirements that focuses on using semantic versioning within
individual YANG modules, but does not address all requirements listed
in the YANG versioning requirements document. Of particular
relevance here, requirements 3.1 and 3.2 are not addressed.
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YANG Packages describes how sets of related YANG module revisions can
be grouped together into a logical entity that defines a YANG schema.
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, or a vendor's YANG modules. Updated versions of these
package definitions can be defined as the contents of these packages
evolve over time, e.g., by using new revisions of YANG modules
included in the package.
This document defines how YANG packages are used to represent
versioned datastore schema, and how clients can choose which
versioned schemas to use during protocol interactions with a device.
3. Background
There are three ways that the lifecycle of a data model can be
managed:
1. Disallow all non-backwards-compatible updates to a YANG module.
Broadly this is the approach adopted by [RFC7950], 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.
2. 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 schema
selection, this is what the YANG Semver approach likely achieves.
3. 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 are many examples of where
this approach has been used successfully.
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4. Objectives
The goals of YANG schema selection are:
o 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.
o 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
management clients.
o 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.
o To closely align with existing NETCONF/RESTCONF protocol
semantics. I.e., with the exception of the optional mechanism
that allows selection of the schema-set at the beginning of a
NETCONF session or RESTCONF request, protocol interactions between
client and server are the same as when schema selection is not
being used.
o To allow considerable freedom for server implementations to decide
how to implement schema selection, and choose the schema selection
capabilities they support. In particular:
* Servers determine which schema-sets can be selected by clients,
and which combinations of schema-sets are compatible with each
other during concurrent sessions/operations.
* Servers can make some schema-sets automatically available for
client selection, or require clients to configure the
selectable schema-sets before they can be used.
* Servers can limit clients to selecting only a single schema-set
for all client connections, i.e., replacing the devices default
schema-set, or allow clients to use different schema for
concurrent sessions/operations.
* Servers can restrict some read-write datastores to be read-only
when accessed via a particular schema-set, i.e., providing a
read-only view of configuration.
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* Servers may allow clients to combine schema-sets into named
custom schema-sets, or only support predefined schema-sets.
The following points are non objectives of this document:
o This document does not provide a mechanism to allow clients to
choose arbitrary sets of YANG module versions to interoperate with
the server.
o 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.
o 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, using YANG Semver versioning rules, 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.
o 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 SHOULD be
used, and otherwise an out of band mechanism SHOULD be used to
indicate where a mapping has failed.
5. Solution
An outline of the solution is as follows:
1. YANG packages are used to define versioned schema that represent
a partial or full datastore schema for one or more datastores.
2. Schema-sets are named structures that define a set of supported
datastores, along with the schema associated with each of those
datastores, specified via leaf references to YANG packages.
3. The configurable 'selectable' leaf-list defines which schema-sets
may be selected by clients, and the associated configurable
'default' leaf defines the schema-set used by clients that do not
select a schema-set.
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4. Clients choose which selectable schema-set to use via NETCONF
capability exchange during session initiation, or RESTCONF path.
5. Optionally, the configurable 'custom-schema-set' list allows
clients to combine schema-sets together into new named schema-
sets for selection.
5.1. Packages
Section 5.3 of YANG Packages specifies how packages SHOULD be used to
represent datastore schema.
Additional guidance on how YANG packages are specified for schema
selection are:
o Separate packages MAY be defined for different families of schema,
e.g., SDO, OpenConfig, or vendor native.
o Separate packages MAY be defined for different versions of schema.
o All packages referenced for schema selection, and any recursively
included package dependencies, MUST be available on the server in
the '/packages' container in the operational state datastore.
o Globally scoped packages used for schema selection SHOULD be made
available offline in YANG instance data documents, as described in
section 6 of YANG Packages.
5.2. Schema-sets
A schema-set defines a set of datastore schema(s) that could
potentially be used for client schema selection.
The schema-sets supported by the server are available at '/schema-
set-selection/schema-set' in the operational state datastore.
Each schema-set has a unique name that identifies the schema-set
during schema selection protocol operations, e.g., it is used in both
the NETCONF capabilities exchange and the RESTCONF path.
A schema-set defines the set of supported datastores that are
available when clients use the selected schema-set. The schema for
each datastore is specified as the union of one or more compatible
YANG packages. Writable datastores (e.g., running) can also be
labelled as being read-only if they cannot be written to via client
interactions using the schema-set.
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Not all schema-sets are necessarily compatible with each other,
allowing one schema-set to be selected may prevent other schema-sets
from being selected at the same time. Hence, each schema-set lists
the other schema-sets that it is compatible with.
5.3. Defining and changing client selectable schema-sets
A device may have to allocate resources to support selectable schema-
sets, and the selectable leaf-list '/schema-set-selection/selectable'
is the mechanism to constrain the set of schema-sets that can be
selected by clients.
In the operational state datastore, the 'selectable' leaf-list
contains the names of all schema-sets that a client may select from.
Entries in this list MAY be added by the system without prior
configuration. In addition, the 'default' leaf indicates which
schema-set is used by clients that do not explicitly select a schema-
set.
In configuration, the 'selectable' leaf-list allows clients to
configure the schema-sets that are available for clients to select
from. A client can also choose the default schema-set that is used
by any client that connects without naming a schema-set.
5.4. Schema selection protocol operations
5.4.1. NETCONF
The schema-set being used in a NETCONF session is established at the
start of the session through the exchange of capabilities and never
changes for the duration of the session. If the server can no longer
support the schema-set in use in a NETCONF session, then it MUST
disconnect the session.
5.4.1.1. schema-sets NETCONF capability
A new NETCONF :schema-sets capability, using base:1.1 defined in
[RFC6241]
This capability is used by the server is advertise selectable schema.
The server sends a comma separated list (with no white spaces) of
selectable schema-sets it supports. For consistency, the list SHOULD
contain the default selectable schema-set first, followed by the
remaining selectable schema-sets, matching the order in the '/schema-
set-selection/selectable' leaf-list.
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This capability is used by clients to select a particular schema.
The client sends an ordered list of selectable schema that it is
willing to use.
The selected schema is the first entry in the client schema-set list
that is also contained in the server schema-set list. If there is no
common entry then the session is terminated with an error.
If the client does not include the schema-set capability during the
capability exchange then the default selectable schema-set is used.
The :schema-sets capability is identified by the following capability
string:
urn:ietf:params:netconf:capability:schema-sets:1.0
In this example, the server advertises its (abbreviated) <hello> as
follows. This indicates that the server supports the following
schema-sets:
example-ietf-routing: Versions 2.1.0 (default) and 1.3.1
example-vendor-xxx: Versions 9.2.3 and 8.4.2
Some extra white spaces have been added for display purposes only.
<hello xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<capabilities>
<capability>urn:ietf:params:netconf:base:1.0</capability>
<capability>urn:ietf:params:netconf:base:1.1</capability>
<capability>
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
</capability>
</capabilities>
</hello>
The client advertises its (abbreviated) <hello> as follows. This
indicates the the client prefers to use the example-ietf-routing
schema version 2.1.0, but can also use version 1.3.1. Because both
the client and server support version 2.1.0, and because the client
listed it first, the selected schema-set is example-ietf-
routing@2.1.0: Some extra white spaces have been added for display
purposes only.
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<hello xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<capabilities>
<capability>urn:ietf:params:netconf:base:1.1</capability>
<capability>
urn:ietf:params:netconf:capability:schema-sets:1.0?list=
example-ietf-routing@2.1.0,example-ietf-routing@1.3.1
</capability>
</capabilities>
</hello>
5.4.2. RESTCONF
For RESTCONF, schema-sets are chosen via use of their name in the
request URI.
Clients select the desired schema-set by choosing the corresponding
RESTCONF root resource. This is done by appending the schema-set
name to the RESTCONF API root [RFC8040]. This updates Section 3.1 of
[RFC8040] and Section 3 of [RFC8527].
Clients will use RESTCONF root resource {+restconf}/schema/<schema-
name> for all requests pertaining to resources specific to a schema-
set. Consider a device which supports schema-sets vendor-
schema@3.0.0, vendor-schema@2.1.0 and vendor-schema@1.4.5: clients
will use RESTCONF root resource {+restconf}/schema/vendor-
schema@X.Y.Z for all requests pertaining to resources defined in
schema-set vendor-schema@X.Y.Z. This applies to all RESTCONF
resources defined in vendor-schema@X.Y.Z.
Here are some examples where {+restconf} is /restconf/.
Examples for servers which are NOT NMDA-compliant as per [RFC8527]:
1. /restconf/schema/vendor-schema@X.Y.Z/data for datastore
resources.
2. /restconf/schema/vendor-schema@X.Y.Z/data/module-A:data-X for
data resource "data-X" defined in module "module-A".
3. /restconf/schema/vendor-schema@X.Y.Z/operations/module-B:op-Y for
RPC "op-Y" defined in module "module-B"
4. /restconf/schema/vendor-schema@X.Y.Z/data/module-C:containerZ/
myaction for action "myaction" defined in top-container
"containerZ" of module "module-C"
Examples for servers which are NMDA-compliant as per [RFC8527]:
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1. /restconf/schema/vendor-schema@X.Y.Z/ds/<datastore>/ for
datastore resources, e.g. /restconf/schema/vendor-
schema@X.Y.Z/ds/ietf-datastores:running refers to the Running
configuration datastore.
2. /restconf/schema/vendor-schema@X.Y.Z/ds/ietf-
datastores:operational for YANG actions.
If the chosen schema-set is not available then an error response
containing a "404 Not Found" status-line MUST be returned by the
server.
5.5. Custom schema-sets
Custom schema-sets, if supported by the server, allow clients to:
o Configure client meaningful names for selectable schema-sets.
o Combine compatible schema-sets together into a combined named
schema-set that is then selectable by clients. E.g. a client
might want to use a combination of standard configuration models
along with vendor configuration models to manage functionality not
covered by the standard models.
6. Schema selection from a server perspective
The general premise of this solution is that servers generally
implement one native schema, and the schema 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 MAY need 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.
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Depending on the complexity of the mappings between schema versions,
it may be necessary for the mappings to be stateful.
7. Schema selection from a client perspective
Clients can use configuration to choose which schema-sets are
available for selection.
Clients cannot choose arbitrary individual YANG module versions, and
are instead constrained by the versions that the server makes
available via schema-sets.
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 using the selected schema. If the
client queries YANG library, then the version of YANG Library that is
returned matches the schema-set that has been selected by the client.
The selection of a schema-set by a client MUST NOT change the
behaviour of the server experienced by other clients. For example,
the get-data response to one client MUST be the same before and after
another client selects a schema-set.
The server may not support a schema with the exact version desired by
the client, and the client may have to choose 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 expect that all data nodes in later module versions can
always be backported to older schema versions.
7.1. Examples
Here is a simple example of a NETCONF client migrating to a new
schema-set with a server that has multiple schema-sets in the
'selectable' leaf-list:
1. Disconnect the current session
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2. Reconnect and select a new schema-set from the 'selectable' leaf-
list
If a server, for example, only supports a single schema-set at a time
by only allowing a single entry in the 'selectable' leaf-list (the
default), then a change of the schema-set in the 'selectable' leaf-
list (and default) would cause all previously established NETCONF
sessions (using the previous 'selectable' schema-set) to be
disconnected.
If a server only supports a single schema-set at a time (across all
sessions), a NETCONF client can migrate to a new schema-set by using
the following sequence of steps:
1. Configure a new schema-set in the 'selectable' leaf-list, remove
the old schema-set, and set the 'default' leaf to that new
schema-set. Other configuration can also be done in the same
request (using the current schema-set in use on the session).
2. The server will process the request and then disconnect the
session (since the current schema-set of the session can no
longer be supported). All other NETCONF sessions would also be
disconnected at this point.
3. The client reconnects using the new schema-set (either by
selecting it during capability exchange, or by using no selection
and relying on the new default schema-set).
4. The client can then optionally send a complete new configuration
using the new schema (i.e. if the client knows that the server
can't perfectly convert everything from the old schema to the new
schema).
8. Limitations of the solution
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.
9. Schema Selection YANG module
The YANG schema selection YANG module is used by a server to report
the schema-sets that are generally available, and to allow clients to
configure which schema-sets are available for client selection and
which is the default.
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Custom schema-sets, if supported, allows clients to configure custom
combinations of schema-sets that can then be selected by clients.
The "ietf-schema-selection" YANG module has the following structure:
module: ietf-schema-selection
+--rw schema-set-selection!
+--rw selectable* -> /schema-set-selection/schema-set/name
+--rw default -> /schema-set-selection/selectable
+--rw custom* [name] {custom-schema-set}?
| +--rw name string
| +--rw description? string
| +--rw included-schema*
| -> /schema-set-selection/schema-set/name
+--ro schema-set* [name]
+--ro name string
+--ro partial? empty
+--ro datastore* [name]
| +--ro name ds:datastore-ref
| +--ro read-only? empty
| +--ro package* [name version]
| +--ro name -> /pkgs:packages/package/name
| +--ro version leafref
| +--ro checksum? leafref
+--ro selectable-with*
| -> /schema-set-selection/schema-set/name
+--ro custom-selectable! {custom-schema-set}?
+--ro combinable-with*
-> /schema-set-selection/schema-set/name
10. YANG Module
The YANG module definition for the module described in the previous
sections.
<CODE BEGINS> file "ietf-schema-selection@2020-02-29.yang"
module ietf-schema-selection {
yang-version 1.1;
namespace
"urn:ietf:params:xml:ns:yang:ietf-schema-selection";
prefix "schema";
import ietf-yang-revisions {
prefix rev;
reference "XXXX: Updated YANG Module Revision Handling";
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}
import ietf-datastores {
prefix ds;
rev:revision-or-derived 2018-02-14;
reference
"RFC 8342: Network Management Datastore Architecture (NMDA)";
}
import ietf-yang-packages {
prefix pkgs;
rev:revision-or-derived 0.2.0;
reference "RFC XXX: YANG Packages.";
}
organization
"IETF NETMOD (Network Modeling) Working Group";
contact
"WG Web: <http://tools.ietf.org/wg/netmod/>
WG List: <mailto:netmod@ietf.org>
Author: Reshad Rahman
<mailto:rrahman@cisco.com>
Author: Rob Wilton
<mailto:rwilton@cisco.com>
Author: Joe Clarke
<mailto:jclarke@cisco.com>
Author: Jason Sterne
<mailto:jason.sterne@nokia.com>
Author: Bo Wu
<mailto:lana.wubo@huawei.com>";
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
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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 with actual RFC number and remove this
// note.
revision 2020-02-29 {
description
"Initial revision";
reference
"RFC XXXX: YANG Schema Selection";
}
/*
* Features
*/
feature "custom-schema-set" {
description
"Feature to choose whether clients may configurable custom
schema definitions.";
}
/*
* Top level data nodes.
*/
container schema-set-selection {
presence "Enable schema selection";
description
"YANG schema-set selection.
Contains configuration and state related to client selectable
YANG schema-sets.";
leaf-list selectable {
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type leafref {
path "/schema-set-selection/schema-set/name";
require-instance false;
}
min-elements 1;
description
"Specifies the selectable schema used by this server, that
can be selected by clients (either through NETCONF
capability negotiation or RESTCONF schema specific path).";
}
leaf default {
type leafref {
path "/schema-set-selection/selectable";
}
mandatory true;
description
"Defines the default schema-set used by this server.
This is the schema-set that is chosen if a NETCONF client
doesn't select a schema during capability negotiation, or if
the standard RESTCONF (or NMDA datastore URLs) are used.";
}
list custom {
if-feature "custom-schema-set";
key "name";
description
"Defines a custom selectable schema constructed from
compatible schema";
leaf name {
type "string";
description
"Name of custom schema.
Format can either be the form of a YANG identifer, or
'<name>@<rev-label>'.
The selectable-schema name is used in NETCONF capabilties
negotiation and also the RESTCONF path (XXX, is encoding
required, e.g. for '@'?)";
}
leaf description {
type string;
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description
"The description associated with this custom package.";
}
leaf-list included-schema {
type leafref {
path "/schema-set-selection/schema-set/name";
require-instance false;
}
description
"Lists the schema that are combined together into a single
selectable schema (i.e. via a union operation on each
datastore schema package).";
}
}
list schema-set {
key "name";
config false;
description
"Lists all available schema-set's that can be used in schema
selection.";
leaf name {
type string;
description
"Name of the schema.
Do we restrict what is allowed, specifically, do we allow
'@'";
}
leaf partial {
type empty;
description
"Indicates that this schema-set only represents a subset of
the full configurable schema of the device.";
}
list datastore {
key "name";
description
"The list of datastores supported for this pan-datastore
selectable-package, along with the package schema
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associated with that datastore.";
leaf name {
type ds:datastore-ref;
description
"The datastore that this datastore schema is associated
with.";
reference
"RFC 8342: Network Management Datastore Architecture
(NMDA)";
}
leaf read-only {
type empty;
description
"For datastores that are normally writable, the read-only
flag indicates that the datastore cannot be written
using this schema-set. E.g., if <running> was a
supported datastore then it could be read, but not
written.
This flag is not required for datastores, like
operational, that are defined as being read-only.";
}
uses pkgs:yang-ds-pkg-ref;
}
leaf-list selectable-with {
type leafref {
path "/schema-set-selection/schema-set/name";
}
description
"Lists other schema-sets that MAY be selected at the same
time as this schema.";
}
container custom-selectable {
if-feature "custom-schema-set";
presence
"This schema MAY be selected as part of a custom
schema-set.";
description
"Indicates that this schema-set is selectable as part of a
custom schema-set and also lists other schema-sets that
may be combined together into a custom schema-set.";
leaf-list combinable-with {
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type leafref {
path "/schema-set-selection/schema-set/name";
}
description
"Lists the schema-sets that MAY be combined with this
schema into a single custom schema-set'.";
}
}
}
}
}
<CODE ENDS>
11. Security Considerations
To be defined.
12. IANA Considerations
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-schema-selection
Registrant Contact: The IESG.
XML: N/A, the requested URI is an XML namespace.
This document requests that the following YANG modules are added in
the "YANG Module Names" registry [RFC6020]:
Name: ietf-schema-selection.yang
Namespace: urn:ietf:params:xml:ns:yang:ietf-schema-selection
Prefix: schema
Reference: RFC XXXX
This document registers a URI.
12.1. NETCONF Capability URNs
This document registers a URI in the IETF XML registry [RFC3688].
The IANA registry "Network Configuration Protocol (NETCONF)
Capability URNs" needs to be updated to include the following
capability.
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Index
Capability Identifier
-------------------------
:schema-sets
urn:ietf:params:netconf:capability:schema-sets:1.0
13. Open Questions/Issues
All issues, along with the draft text, are currently being tracked
at: https://github.com/netmod-wg/yang-ver-dt/labels/version-
selection-solution
14. Acknowledgements
The ideas that formed this draft are based on discussions with the
YANG versioning design team, and other members of the NETMOD WG.
15. References
15.1. Normative References
[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-07
(work in progress), February 2020.
[I-D.rwilton-netmod-yang-packages]
Wilton, R., Rahman, R., Clarke, J., Sterne, J., and W. Bo,
"YANG Packages", draft-rwilton-netmod-yang-packages-03
(work in progress), February 2020.
[I-D.verdt-netmod-yang-module-versioning]
Claise, B., Clarke, J., Rahman, R., Wilton, R., Lengyel,
B., Sterne, J., and K. D'Souza, "Updated YANG Module
Revision Handling", draft-verdt-netmod-yang-module-
versioning-01 (work in progress), October 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.
[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.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<https://www.rfc-editor.org/info/rfc3688>.
[RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)", RFC 6020,
DOI 10.17487/RFC6020, October 2010,
<https://www.rfc-editor.org/info/rfc6020>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.
[RFC6536] Bierman, A. and M. Bjorklund, "Network Configuration
Protocol (NETCONF) Access Control Model", RFC 6536,
DOI 10.17487/RFC6536, March 2012,
<https://www.rfc-editor.org/info/rfc6536>.
[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>.
[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/info/rfc8040>.
[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>.
[RFC8342] Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
and R. Wilton, "Network Management Datastore Architecture
(NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
<https://www.rfc-editor.org/info/rfc8342>.
[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>.
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[RFC8527] Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
and R. Wilton, "RESTCONF Extensions to Support the Network
Management Datastore Architecture", RFC 8527,
DOI 10.17487/RFC8527, March 2019,
<https://www.rfc-editor.org/info/rfc8527>.
15.2. Informative References
[I-D.ietf-netmod-artwork-folding]
Watsen, K., Auerswald, E., Farrel, A., and Q. WU,
"Handling Long Lines in Inclusions in Internet-Drafts and
RFCs", draft-ietf-netmod-artwork-folding-12 (work in
progress), January 2020.
[I-D.verdt-netmod-yang-solutions]
Wilton, R., "YANG Versioning Solution Overview", draft-
verdt-netmod-yang-solutions-03 (work in progress),
February 2020.
Appendix A. Schema selection examples
Some common simplifications have been made to examples in this
section:
Simplified package definitions have been defined, e.g., the
packages do not include other packages, and the module lists have
been elided.
Package and module checksums have been omitted to minimize line
wrapping.
A.1. Supporting older versions of a schema
To facilitate an easier migration path for clients, a server may
choose to support older versions of a schema, for example this might
be done to minimize impact on clients if non-backwards-compatible
changes have been made between schema versions.
It is expected that the server can dynamically translate from the
older schema version to the latest, but this is not possible in all
cases (e.g., if support for some functionality has been removed from
the server), when deviations against the old schema version may be
required to accurately describe the supported functionality.
This example illustrates a device that is capable of supporting three
different versions of its vendor-schema, reported in '/schema-set-
selection/schema-set':
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vendor-schema@3.0.0, the device defaults to the latest version of
the schema
vendor-schema@2.1.0, a preceding NBC software version
vendor-schema@1.4.5, a preceding NBC software version
Schema selection data from the operational state datastore:
<?xml version="1.0" encoding="utf-8"?>
<rpc-reply
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="101">
<data>
<packages>
<package>
<name>vendor-schema</name>
<version>3.0.0</version>
<!-- package modules elided -->
</package>
<package>
<name>vendor-schema</name>
<version>2.1.0</version>
<!-- package modules elided -->
</package>
<package>
<name>vendor-schema</name>
<version>1.4.5</version>
<!-- package modules elided -->
</package>
</packages>
<schema-set-selection>
<schema-set>
<name>vendor-schema@3.0.0</name>
<datastore>
<name>ds:running</name>
<package>
<name>vendor-schema</name>
<version>3.0.0</version>
</package>
</datastore>
<datastore>
<name>ds:operational</name>
<package>
<name>vendor-schema</name>
<version>3.0.0</version>
</package>
</datastore>
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<selectable-with>vendor-schema@1.4.5</selectable-with>
<selectable-with>vendor-schema@2.1.0</selectable-with>
</schema-set>
<schema-set>
<name>vendor-schema@2.1.0</name>
<datastore>
<name>ds:running</name>
<package>
<name>vendor-schema</name>
<version>2.1.0</version>
</package>
</datastore>
<datastore>
<name>ds:operational</name>
<package>
<name>vendor-schema</name>
<version>2.1.0</version>
</package>
</datastore>
<selectable-with>vendor-schema@1.4.5</selectable-with>
<selectable-with>vendor-schema@3.0.0</selectable-with>
</schema-set>
<schema-set>
<name>vendor-schema@1.4.5</name>
<datastore>
<name>ds:running</name>
<package>
<name>vendor-schema</name>
<version>1.4.5</version>
</package>
</datastore>
<datastore>
<name>ds:operational</name>
<package>
<name>vendor-schema</name>
<version>1.4.5</version>
</package>
</datastore>
<selectable-with>vendor-schema@2.1.0</selectable-with>
<selectable-with>vendor-schema@3.0.0</selectable-with>
</schema-set>
</schema-set-selection>
</data>
</rpc-reply>
The client may configure the device to instead use an earlier version
of the schema-set, 'vendor-schema@1.4.5'. By configuring only a
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single selectable schema-set, and making it the default, means that
this can be the only schema-set that clients use to interact to the
device using. On committing this configuration change, all existing
NETCONF sessions (that were previously interacting using vendor-
schema@3.0.0) will be closed, and clients need to reconnect, at which
point they will interact with schema-set vendor-schema@1.4.5, and
also see the contents of YANG library updated to reflect the
datastore-schema reported in vendor-schema@1.4.5.
Configuration a new default selectable schema:
<?xml version="1.0" encoding="utf-8"?>
<config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<schema-set-selection>
<selectable>vendor-schema@1.4.5</selectable>
<default>vendor-schema@1.4.5</default>
</schema-set-selection>
</config>
A.1.1. Variation - Multiple selectable schema-sets
The client may wish to interact with multiple different versions of
the schema at the same time. E.g., this could be achieved with the
following configuration:
Configuring multiple selectable schema:
<?xml version="1.0" encoding="utf-8"?>
<config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<schema-set-selection>
<selectable>vendor-schema@1.4.5</selectable>
<selectable>vendor-schema@3.0.0</selectable>
<default>vendor-schema@1.4.5</default>
</schema-set-selection>
</config>
As before, by default clients will use the vendor-schema@1.4.5, but
by using NETCONF capabilities exchange, or through use of the
RESTCONF path that may select version-schema@3.0.0 instead. Clients
could also explicitly select vendor-schema@1.4.5, even though it is
also the default schema-set.
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A.2. Supporting different schema families
Some devices may allow clients to configure the device using
different YANG schema (e.g. vendor native, vs IETF, vs OpenConfig).
This example illustrates a device is that capable of supporting 3
different schema families (native, oc, ietf).
<?xml version="1.0" encoding="utf-8"?>
<rpc-reply
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="101">
<data>
<packages>
<package>
<name>vendor-schema</name>
<version>1.0.0</version>
<module>
<name>vendor-interfaces</name>
</module>
<!--Module list of vendor YANG modules elided -->
</package>
<package>
<name>ietf-schema</name>
<version>1.0.0</version>
<module>
<name>ietf-interfaces</name>
</module>
<!--Module list of IETF YANG modules elided -->
</package>
<package>
<name>oc-schema</name>
<version>1.0.0</version>
<module>
<name>openconfig-interfaces</name>
</module>
<!--Module list of OpenConfig YANG modules elided -->
</package>
</packages>
<schema-set-selection>
<schema-set>
<name>combined-schema</name>
<datastore>
<name>ds:running</name>
<package>
<name>vendor-schema</name>
<version>1.0.0</version>
</package>
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<package>
<name>ietf-schema</name>
<version>1.0.0</version>
</package>
<package>
<name>oc-schema</name>
<version>1.0.0</version>
</package>
</datastore>
<datastore>
<name>ds:operational</name>
<package>
<name>vendor-schema</name>
<version>1.0.0</version>
</package>
<package>
<name>ietf-schema</name>
<version>1.0.0</version>
</package>
<package>
<name>oc-schema</name>
<version>1.0.0</version>
</package>
</datastore>
</schema-set>
<schema-set>
<name>vendor-schema</name>
<partial/>
<datastore>
<name>ds:running</name>
<package>
<name>vendor-schema</name>
<version>1.0.0</version>
</package>
</datastore>
<datastore>
<name>ds:operational</name>
<package>
<name>vendor-schema</name>
<version>1.0.0</version>
</package>
</datastore>
<selectable-with>combined-schema</selectable-with>
<selectable-with>ietf-schema</selectable-with>
<selectable-with>oc-schema</selectable-with>
<custom-selectable>
<combinable-with>ietf-schema</combinable-with>
<combinable-with>oc-schema</combinable-with>
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</custom-selectable>
</schema-set>
<schema-set>
<name>ietf-schema</name>
<partial/>
<datastore>
<name>ds:running</name>
<package>
<name>ietf-schema</name>
<version>1.0.0</version>
</package>
</datastore>
<datastore>
<name>ds:operational</name>
<package>
<name>ietf-schema</name>
<version>1.0.0</version>
</package>
</datastore>
<selectable-with>combined-schema</selectable-with>
<selectable-with>vendor-schema</selectable-with>
<selectable-with>oc-schema</selectable-with>
<custom-selectable>
<combinable-with>vendor-schema</combinable-with>
</custom-selectable>
</schema-set>
<schema-set>
<name>oc-schema</name>
<partial/>
<datastore>
<name>ds:running</name>
<package>
<name>oc-schema</name>
<version>1.0.0</version>
</package>
</datastore>
<datastore>
<name>ds:operational</name>
<package>
<name>oc-schema</name>
<version>1.0.0</version>
</package>
</datastore>
<selectable-with>combined-schema</selectable-with>
<selectable-with>vendor-schema</selectable-with>
<selectable-with>ietf-schema</selectable-with>
<custom-selectable>
<combinable-with>vendor-schema</combinable-with>
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</custom-selectable>
</schema-set>
</schema-set-selection>
</data>
</rpc-reply>
The clients may configure the device in three different ways.
A.2.1. Choosing a single schema family
A client that wishes to use a single schema family for all
interactions with the device can choose a single schema-set and
configure it as the default schema-set:
<?xml version="1.0" encoding="utf-8"?>
<config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<schema-set-selection>
<selectable>oc-schema</selectable>
<default>oc-schema</default>
</schema-set-selection>
</config>
A.2.2. Restricting some sessions to particular schema family
If a client wishes to use multiple schema families for configuration,
but restrict some sessions to a particular schema family, then they
may configure the default schema as "combined-schema", but also 'oc-
schema' that can be selected via client sessions as a named schema-
set.
<?xml version="1.0" encoding="utf-8"?>
<config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<schema-set-selection>
<selectable>combined-schema</selectable>
<selectable>oc-schema</selectable>
<default>combined-schema</default>
</schema-set-selection>
</config>
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A.2.3. Custom combinable schema-set
If there is a need for the client to use IETF or OC schema alongside
the vendor schema, then this can be achieved by configuring a custom
schema-set. Two custom schema-sets can be configured, either "vendor
+ ietf schema", or "vendor + oc schema". The example below defines
and selects a custom schema-set that combines the vendor and OC
schema-sets.
<?xml version="1.0" encoding="utf-8"?>
<config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<schema-set-selection>
<custom>
<name>my-custom-schema</name>
<description>Vendor and OC schema-sets only</description>
<included-schema>vendor-schema</included-schema>
<included-schema>oc-schema</included-schema>
</custom>
<selectable>my-custom-schema</selectable>
<default>my-custom-schema</default>
</schema-set-selection>
</config>
Note, for the last case, rather than requiring the client to
configure custom schema, the device could predefine "vendor + ietf"
and "vendor + oc" as named schema-sets available for selection.
Authors' Addresses
Robert Wilton
Cisco Systems, Inc.
Email: rwilton@cisco.com
Reshad Rahman
Cisco Systems, Inc.
Email: rrahman@cisco.com
Joe Clarke
Cisco Systems, Inc.
Email: jclarke@cisco.com
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Jason Sterne
Nokia
Email: jason.sterne@nokia.com
Bo Wu
Huawei
Email: lana.wubo@huawei.com
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