Internet Engineering Task Force R. Wills
Internet-Draft Cisco Systems
Intended status: Standards Track 2 March 2025
Expires: 3 September 2025
Yang Templates
draft-wills-netmod-yang-templates-00
Abstract
Yang Templates is a way to more easily manage a network device's
configuration when parts of that configuration are repetitive. This
document provides a high-level description of Yang Templates, and is
expected to evolve into a full solution in later versions.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Defining and managing templates . . . . . . . . . . . . . 4
3.2. Applying templates . . . . . . . . . . . . . . . . . . . 4
3.3. Producing the <intended> datastore . . . . . . . . . . . 4
3.4. Pattern matching in templates . . . . . . . . . . . . . . 5
3.5. Off-box template expansion . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . 6
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
This document describes a mechanism whereby nodes of configuration
data can be placed into templates, and templates can be applied to
subtrees in a configuration datastore. When a template is applied to
a subtree, the configuration in the template takes effect for that
subtree (unless other configuration takes precedence, as described
later in this document).
The contents of this document are based on discussions in two Interim
Meetings of the Netmod working group. It was written _after_ those
meetings and is intended to summarise the outcomes as a set of high-
level requirements. It is not an exhaustive set of minutes.
It's expected that later versions of this document will contain a
fuller description of Yang Templates.
1.1. Requirements Language
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.
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2. Overview
The configuration of network devices is modeled using the Yang
[RFC7950] language, and the configuration of a specific device is
determined by the contents of one or more configuration datastores on
the device.
A configuration datastore often contains multiple subtrees with
similar or identical nodes within them. For example, a device may be
configured to have multiple BGP neighbors. Each neighbor could have
an identical set of properties (achieved using identical
configuration on each neighbor), and neighbor-specific properties
such as the address (achieved using configuration that varies between
neighbors).
In this situation, a client must ensure that each set of identical
nodes exists in all the relevant subtrees of the datastore and that
each occurrence of a node has the same value in order to achieve the
correct intended configuration. This creates problems in keeping
occurrences of these nodes in sync and understanding the contents of
a large configuration datastore.
Yang Templates defines fragments of configuration called "templates".
When an implementation supports Yang Templates, the client is able to
use existing network management protocols such as NETCONF [RFC6241]
to:
* Manage the configuration inside each template.
* Apply templates to subtrees of the datastore.
The configuration from templates and the configuration not in
templates is combined together to produce an intended datastore that
describes the configuration of the device.
3. Requirements
This section describes the requirements that the Yang Templates
solution must satisfy. These requirements were all discussed in the
Interim Meetings, and a rough consensus was reached on each of them
by the participants in the meetings.
A general theme of the Yang Templates work is to come up with a
"Minimal Viable Product" that is useful but not over-complicated.
More advanced features could be considered as extensions in later
drafts.
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3.1. Defining and managing templates
Templates can be used with any Yang module. They contain nodes of
configuration data, and are stored persistently in the running
datastore of the device.
A client can view and manipulate a template, including the
configuration inside it, by manipulating it in the <running>
datastore. In this sense, a template and its contents behaves like
any other subtree of configuration.
3.2. Applying templates
A template can be applied to zero or more nodes in the <running>
datastore. Each node can have zero or more templates applied to it,
and the order they are applied is specified by the client. The order
is important when determining the final intended configuration -- see
the next section.
Templates can be applied at multiple points in the hierachy. The
next section states the requirements when a node applies a template
and it has an ancestor that also applies a template.
When viewing the <running> datastore, there is a mechanism to see
which templates have been applied to each node, and in which order.
3.3. Producing the <intended> datastore
The device's <intended> datastore is the result of combining all the
applications of templates together with non-template config. This is
called "expanding out" the templates.
The intended configuration inside a subtree is the result of taking
the relevant contents of every template applied to the subtree's root
node and its ancestors, and combining it with the (non-template) data
nodes inside the subtree.
A node inside a subtree may be present in multiple templates that
have been applied, and/or it may be present as non-template config
inside the subtree. The requirements for combining the templates and
the non-template config together are as follows:
* The value of a node in the <intended> configuration is determined
by using precedence to decide where to take the value from.
* Non-template config always has the highest precedence.
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* When templates are applied to multiple ancestors, the innermost
ancestor takes precedence.
* When multiple templates are applied to a particular node, the
order of application (as indicated by the client when applying the
templates) determines the precedence within that node.
Whenever the contents of a template is updated in <running>, the
result of expanding out the template appears in <intended> and takes
effect on the device.
3.4. Pattern matching in templates
The configuration inside a template definition can contain values for
list keys that are simple regular expressions, using a limited subset
of regular expression syntax. This controls which list entries that
subtree of the template takes effect for when it is applied.
An example of this would be to have a template that is applied to a
top-level <interfaces> container, but the template only takes effect
for certain interface names that match the regular expression.
3.5. Off-box template expansion
If the client knows the contents of the <running> datastore (non-
template config, template definitions and template applications), it
must be possible for the client to calculate the result of template
expansion.
In other words, the outcome of template expansion depends solely on
the <running> datastore and not the state of the device.
4. IANA Considerations
This memo includes no request to IANA.
5. Security Considerations
This document should not affect the security of the Internet.
6. References
6.1. Normative References
[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>.
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[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>.
[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>.
[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>.
6.2. Informative References
Acknowledgements
The author would like to thank Shiya Ashraf, Lou Berger, Quifang Ma,
Robert Peschi, Deepak Rajaram, Jason Sterne, Kent Watsen and Robert
Wilton for comments and contributions made during interim meetings.
The author would like to acknowledge the following drafts and
presenters for kick-starting discussions on Yang Templates:
* draft-ma-netmod-yang-config-template-00
* draft-rajaram-netmod-yang-cfg-template-framework-00
* Jan Lindblad
Author's Address
Robert Wills
Cisco Systems
Email: rowills@cisco.com
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