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IETF RFC 8528
Last modified on Thursday, March 7th, 2019
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Internet Engineering Task Force (IETF) M. Bjorklund
Request for Comments: 8528 Tail-f Systems
Category: Standards Track L. Lhotka
ISSN: 2070-1721 CZ.NIC
March 2019
YANG Schema Mount
 Abstract
This document defines a mechanism that adds the schema trees defined
by a set of YANG modules onto a mount point defined in the schema
tree in another YANG module.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/RFC 8528.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Bjorklund & Lhotka Standards Track PAGE 1 
RFC 8528 YANG Schema Mount March 2019
Table of Contents
1. Introduction ....................................................3
2. Terminology and Notation ........................................6
2.1. Tree Diagrams ..............................................7
2.2. Namespace Prefixes .........................................7
3. Schema Mount ....................................................8
3.1. Mount Point Definition .....................................8
3.2. Data Model .................................................9
3.3. Specification of the Mounted Schema ........................9
3.4. Multiple Levels of Schema Mount ...........................10
4. Referring to Data Nodes in the Parent Schema ...................10
5. RPC Operations and Notifications ...............................11
6. NMDA Considerations ............................................12
7. Interaction with NACM ..........................................12
8. Implementation Notes ...........................................13
9. Schema Mount YANG Module .......................................13
10. IANA Considerations ...........................................18
11. Security Considerations .......................................18
12. References ....................................................19
12.1. Normative References .....................................19
12.2. Informative References ...................................21
Appendix A. Example: Device Model with LNEs and NIs ..............22
A.1. Physical Device ...........................................22
A.2. Logical Network Elements ..................................24
A.3. Network Instances .........................................27
A.4. Invoking an RPC Operation .................................28
Contributors ......................................................28
Authors' Addresses ................................................28
Bjorklund & Lhotka Standards Track PAGE 2
RFC 8528 YANG Schema Mount March 2019
1. Introduction
Modularity and extensibility are among the leading design principles
of the YANG data modeling language. As a result, the same YANG
module can be combined with various sets of other modules to form a
data model that is tailored to meet the requirements of a specific
use case. Server implementors are only required to specify all YANG
modules comprising the data model (together with their revisions and
other optional choices) in the YANG library data ([RFC 7895],
[RFC 8525], and Section 5.6.4 of [RFC 7950]) implemented by the server.
Such YANG modules appear in the data model "side by side", i.e., top-
level data nodes of each module (if there are any) are also top-level
nodes of the overall data model.
YANG has two mechanisms for contributing a schema hierarchy defined
elsewhere to the contents of an internal node of the schema tree.
These mechanisms are realized through the following YANG statements:
o The "uses" statement explicitly incorporates the contents of a
grouping defined in the same or another module. See Section 4.2.6
of [RFC 7950] for more details.
o The "augment" statement explicitly adds contents to a target node
defined in the same or another module. See Section 4.2.8 of
[RFC 7950] for more details.
With both mechanisms, the YANG module with the "uses" or "augment"
statement explicitly defines the exact location in the schema tree
where the new nodes are placed.
In some cases, these mechanisms are not sufficient; it is sometimes
necessary for an existing module (or a set of modules) to be added to
the data model starting at locations other than the root. For
example, YANG modules such as "ietf-interfaces" [RFC 8343] are defined
so as to be used in a data model of a physical device. Now suppose
we want to model a device that supports multiple logical devices
[RFC 8530], each of which has its own instantiation of
"ietf-interfaces", and possibly other modules; at the same time, we
want to be able to manage all these logical devices from the master
device. Hence, we would like to have a schema tree like this:
Bjorklund & Lhotka Standards Track PAGE 3
RFC 8528 YANG Schema Mount March 2019
+--rw interfaces
| +--rw interface* [name]
| ...
+--rw logical-network-element* [name]
+--rw name
| ...
+--rw interfaces
+--rw interface* [name]
...
With the "uses" approach, the complete schema tree of
"ietf-interfaces" would have to be wrapped in a grouping, and then
this grouping would have to be used at the top level (for the master
device) and then also in the "logical-network-element" list (for the
logical devices). This approach has several disadvantages:
o It is not scalable because every time there is a new YANG module
that needs to be added to the logical device model, we have to
update the model for logical devices with another "uses" statement
pulling in contents of the new module.
o Absolute references to nodes defined inside a grouping may break
if the grouping is used in different locations.
o Nodes defined inside a grouping belong to the namespace of the
module where it is used, which makes references to such nodes from
other modules difficult or even impossible.
o It would be difficult for vendors to add proprietary modules when
the "uses" statements are defined in a standard module.
With the "augment" approach, "ietf-interfaces" would have to augment
the "logical-network-element" list with all its nodes and, at the
same time, define all its nodes at the top level. The same hierarchy
of nodes would thus have to be defined twice, which is clearly not
scalable either.
This document introduces a new mechanism, denoted as "schema mount",
that allows for mounting one data model consisting of any number of
YANG modules at a specified location of another (parent) schema.
Unlike the "uses" and "augment" approaches discussed above, the
mounted modules needn't be specially prepared for mounting;
consequently, existing modules such as "ietf-interfaces" can be
mounted without any modifications.
Bjorklund & Lhotka Standards Track PAGE 4
RFC 8528 YANG Schema Mount March 2019
The basic idea of schema mount is to label a data node in the parent
schema as the mount point and then define a complete data model to be
attached to the mount point so that the labeled data node effectively
becomes the root node of the mounted data model.
In principle, the mounted schema can be specified at three different
phases of the data model life cycle:
1. Design time: The mounted schema is defined along with the mount
point in the parent YANG module. In this case, the mounted
schema has to be the same for every implementation of the parent
module.
2. Implementation time: The mounted schema is defined by a server
implementor and is as stable as the YANG library information of
the server.
3. Run time: The mounted schema is defined by instance data that is
part of the mounted data model. If there are multiple instances
of the same mount point (e.g., in multiple entries of a list),
the mounted data model may be different for each instance.
The schema mount mechanism defined in this document provides support
only for the latter two cases. Design-time mounts are outside the
scope of this document and could be possibly dealt with in a future
revision of the YANG data modeling language.
Schema mount applies to the data model and specifically does not
assume anything about the source of instance data for the mounted
schemas. It may be implemented using the same instrumentation as the
rest of the system, or it may be implemented by querying some other
system. Future specifications may define mechanisms to control or
monitor the implementation of specific mount points.
How and when specific mount points are instantiated by the server is
out of scope for this document. Such mechanisms may be defined in
future specifications.
This document allows mounting of complete data models only. Other
specifications may extend this model by defining additional
mechanisms such as mounting sub-hierarchies of a module.
The YANG modules in this document conform to the Network Management
Datastore Architecture (NMDA) [RFC 8342].
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RFC 8528 YANG Schema Mount March 2019
2. Terminology and Notation
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.
The following terms are defined in [RFC 7950] and are not redefined
here:
o action
o container
o data node
o list
o RPC operation
o schema node
o schema tree
The following terms are defined in [RFC 8342] and are not redefined
here:
o client
o notification
o operational state
o server
The following term is defined in [RFC 8343] and is not redefined here:
o system-controlled interface
The following term is defined in [RFC 8525] and is not redefined here:
o YANG library content identifier
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The following additional terms are used in this document:
o mount point: A container or a list node whose definition contains
the "mount-point" extension statement. The argument of the
"mount-point" extension statement defines a label for the mount
point.
o schema: A collection of schema trees with a common root.
o top-level schema: A schema rooted at the root node.
o mounted schema: A schema rooted at a mount point.
o parent schema (of a mounted schema): A schema containing the mount
point.
o schema mount: The mechanism to combine data models defined in this
document.
2.1. Tree Diagrams
Tree diagrams used in this document follow the notation defined in
[RFC 8340].
2.2. Namespace Prefixes
In this document, names of data nodes, YANG extensions, actions, and
other data model objects are often used without a prefix when the
YANG module in which they are defined is clear from the context.
Otherwise, names are prefixed using the standard prefix associated
with the corresponding YANG module, as shown in Table 1.
+---------+------------------------+----------------------+
| Prefix | YANG module | Reference |
+---------+------------------------+----------------------+
| yangmnt | ietf-yang-schema-mount | Section 9 |
| inet | ietf-inet-types | [RFC 6991] |
| yang | ietf-yang-types | [RFC 6991] |
| yanglib | ietf-yang-library | [RFC 7895], [RFC 8525] |
+---------+------------------------+----------------------+
Table 1: Namespace Prefixes
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RFC 8528 YANG Schema Mount March 2019
3. Schema Mount
The schema mount mechanism defined in this document provides a new
extensibility mechanism for use with YANG 1.1 [RFC 7950]. In contrast
to the existing mechanisms described in Section 1, schema mount
defines the relationship between the source and target YANG modules
outside these modules.
3.1. Mount Point Definition
A container or list node becomes a mount point if the "mount-point"
extension statement (defined in the "ietf-yang-schema-mount" module)
is used in its definition. This extension can appear only as a
substatement of "container" and "list" statements.
The argument of the "mount-point" extension statement is a YANG
identifier that defines a label for the mount point. A module MAY
contain multiple "mount-point" extension statements having the same
argument.
It is therefore up to the designer of the parent schema to decide
about the placement of mount points. A mount point can also be made
conditional by placing "if-feature" and/or "when" as substatements of
the "container" or "list" statement that represents the mount point.
The "mount-point" extension statement MUST NOT be used in a YANG
version 1 module [RFC 6020]. If used in a YANG version 1 module, it
would not be possible to invoke mounted RPC operations and receive
mounted notifications. See Section 5 for details. Note, however,
that modules written in any YANG version, including version 1, can be
mounted under a mount point.
Note that the "mount-point" extension statement does not define a new
data node.
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RFC 8528 YANG Schema Mount March 2019
3.2. Data Model
This document defines the YANG 1.1 module [RFC 7950]
"ietf-yang-schema-mount", which has the following structure:
module: ietf-yang-schema-mount
+--ro schema-mounts
+--ro namespace* [prefix]
| +--ro prefix yang:yang-identifier
| +--ro uri? inet:uri
+--ro mount-point* [module label]
+--ro module yang:yang-identifier
+--ro label yang:yang-identifier
+--ro config? boolean
+--ro (schema-ref)
+--:(inline)
| +--ro inline!
+--:(shared-schema)
+--ro shared-schema!
+--ro parent-reference* yang:xpath1.0
3.3. Specification of the Mounted Schema
Mounted schemas for all mount points in the parent schema are
determined from state data in the "/schema-mounts" container.
Generally, the modules that are mounted under a mount point have no
relation to the modules in the parent schema; specifically, if a
module is mounted, it may or may not be present in the parent schema;
if present, its data will generally have no relationship to the data
of the parent. Exceptions are possible and need to be defined in the
model itself. For example, [RFC 8530] defines a mechanism to bind
interfaces to mounted logical network elements.
The "/schema-mounts" container has the "mount-point" list as one of
its children. Every entry of this list refers (through its key) to a
mount point and specifies the mounted schema.
If a mount point is defined in the parent schema but does not have an
entry in the "mount-point" list, then the mounted schema is void,
i.e., instances of that mount point MUST NOT contain any data except
those that are defined in the parent schema.
If multiple mount points with the same name are defined in the same
module -- either directly or because the mount point is defined in a
grouping and the grouping is used multiple times -- then the
corresponding "mount-point" entry applies equally to all such mount
points.
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RFC 8528 YANG Schema Mount March 2019
The "config" property of mounted schema nodes is overridden and all
nodes in the mounted schema are read-only ("config false") if at
least one of the following conditions is satisfied for a mount point:
o The mount point is itself defined as "config false".
o The "config" leaf in the corresponding entry of the "mount-point"
list is set to "false".
An entry of the "mount-point" list can specify the mounted schema in
two different ways: "inline" or "shared-schema".
The mounted schema is determined at run time: every instance of the
mount point that exists in the operational state MUST contain a copy
of YANG library data that defines the mounted schema in exactly the
same way as a top-level schema. A client is expected to retrieve
this data from the instance tree. In the "inline" case, instances of
the same mount point MAY use different mounted schemas, whereas in
the "shared-schema" case, all instances MUST use the same mounted
schema. This means that in the "shared-schema" case, all instances
of the same mount point MUST have the same YANG library content
identifier. In the "inline" case, if two instances have the same
YANG library content identifier, it is not guaranteed that the YANG
library contents are equal for these instances.
Examples of "inline" and "shared-schema" can be found in Appendix A.2
and Appendix A.3, respectively.
3.4. Multiple Levels of Schema Mount
YANG modules in a mounted schema MAY again contain mount points under
which other schemas can be mounted. Consequently, it is possible to
construct data models with an arbitrary number of mounted schemas. A
schema for a mount point contained in a mounted module can be
specified by implementing the "ietf-yang-library" and
"ietf-yang-schema-mount" modules in the mounted schema and specifying
the schemas in exactly the same way as the top-level schema.
4. Referring to Data Nodes in the Parent Schema
A fundamental design principle of schema mount is that the mounted
schema works exactly as a top-level schema, i.e., it is confined to
the "mount jail". This means that all paths in the mounted schema
(in leafrefs, instance-identifiers, XPath [XPATH] expressions, and
target nodes of "augment" statements) are interpreted with the mount
point as the root node. YANG modules of the mounted schema as well
as corresponding instance data thus cannot refer to schema nodes or
instance data outside the "mount jail".
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RFC 8528 YANG Schema Mount March 2019
However, this restriction is sometimes too severe. A typical example
is network instances (NIs) [RFC 8529] in which each NI has its own
routing engine but the list of interfaces is global and shared by all
NIs. If we want to model this organization with the NI schema
mounted using schema mount, the overall schema tree would look
schematically as follows:
+--rw interfaces
| +--rw interface* [name]
| ...
+--rw network-instances
+--rw network-instance* [name]
+--rw name
+--mp root
+--rw routing
...
Here, the "root" container is the mount point for the NI schema.
Routing configuration inside an NI often needs to refer to interfaces
(at least those that are assigned to the NI), which is impossible
unless such a reference can point to a node in the parent schema
(interface name).
Therefore, schema mount also allows for such references. For every
mount point in the "shared-schema" case, it is possible to specify a
leaf-list named "parent-reference" that contains zero or more XPath
1.0 expressions. Each expression is evaluated with the node in the
parent data tree where the mount point is defined as the context
node. The result of this evaluation MUST be a node-set (see the
description of the "parent-reference" node for a complete definition
of the evaluation context). For the purposes of evaluating XPath
expressions within the mounted data tree, the union of all such node-
sets is added to the accessible data tree.
It is worth emphasizing that the nodes specified in the
"parent-reference" leaf-list are available in the mounted schema only
for XPath evaluations. In particular, they cannot be accessed in the
mounted schema via network management protocols such as NETCONF
[RFC 6241] or RESTCONF [RFC 8040].
5. RPC Operations and Notifications
If a mounted YANG module defines an RPC operation, clients can invoke
this operation as if it were defined as an action for the
corresponding mount point; see Section 7.15 of [RFC 7950]. An example
of this is given in Appendix A.4.
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RFC 8528 YANG Schema Mount March 2019
Similarly, if the server emits a notification defined at the top
level of any mounted module, it MUST be represented as if the
notification was connected to the mount point; see Section 7.16 of
[RFC 7950].
Note that inline actions and notifications will not work when they
are contained within a list node without a "key" statement (see
Sections 7.15 and 7.16 of [RFC 7950]). Therefore, to be useful, mount
points that contain modules with RPCs, actions, and notifications
SHOULD NOT have any ancestor node that is a list node without a "key"
statement. This requirement applies to the definition of modules
using the "mount-point" extension statement.
6. NMDA Considerations
The schema mount solution presented in this document is designed to
work with both servers that implement the NMDA [RFC 8342] and old
servers that don't implement the NMDA.
Specifically, a server that doesn't support the NMDA MAY implement
revision 2016-06-21 of "ietf-yang-library" [RFC 7895] under a mount
point. A server that supports the NMDA MUST implement at least
revision 2019-01-04 of "ietf-yang-library" [RFC 8525] under a mount
point.
7. Interaction with NACM
If the Network Configuration Access Control Model (NACM) [RFC 8341] is
implemented on a server, it is used to control access to nodes
defined by the mounted schema in the same way as for nodes defined by
the top-level schema.
For example, suppose the module "ietf-interfaces" is mounted in the
"root" container in the "logical-network-element" list defined in
[RFC 8530]. Then, the following NACM path can be used to control
access to the "interfaces" container (where the character '\' is used
where a line break has been inserted for formatting reasons):
<path xmlns:lne=
"urn:ietf:params:xml:ns:yang:ietf-logical-network-element"
xmlns:if="urn:ietf:params:xml:ns:yang:ietf-interfaces">
/lne:logical-network-elements\
/lne:logical-network-element/lne:root/if:interfaces
</path>
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RFC 8528 YANG Schema Mount March 2019
8. Implementation Notes
Network management of devices that use a data model with schema mount
can be implemented in different ways. However, the following
implementation options are envisioned as typical:
o shared management: Instance data of both parent and mounted
schemas are accessible within the same management session.
o split management: One (master) management session has access to
instance data of both parent and mounted schemas; in addition, an
extra session that has access only to the mounted data tree exists
for every instance of the mount point.
9. Schema Mount YANG Module
This module references [RFC 6991] and [RFC 7950].
<CODE BEGINS> file "ietf-yang-schema-mount@2019-01-14.yang"
module ietf-yang-schema-mount {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-yang-schema-mount";
prefix yangmnt;
import ietf-inet-types {
prefix inet;
reference
"RFC 6991: Common YANG Data Types";
}
import ietf-yang-types {
prefix yang;
reference
"RFC 6991: Common YANG Data Types";
}
organization
"IETF NETMOD (NETCONF Data Modeling Language) Working Group";
contact
"WG Web: <https://datatracker.ietf.org/wg/netmod/>
WG List: <mailto:netmod@ietf.org>
Editor: Martin Bjorklund
<mailto:mbj@tail-f.com>
Bjorklund & Lhotka Standards Track PAGE 13
RFC 8528 YANG Schema Mount March 2019
Editor: Ladislav Lhotka
<mailto:lhotka@nic.cz>";
description
"This module defines a YANG extension statement that can be used
to incorporate data models defined in other YANG modules in a
module. It also defines operational state data that specify the
overall structure of the data model.
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.
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
(https://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC 8528;
see the RFC itself for full legal notices.";
revision 2019-01-14 {
description
"Initial revision.";
reference
"RFC 8528: YANG Schema Mount";
}
/*
* Extensions
*/
extension mount-point {
argument label;
description
"The argument 'label' is a YANG identifier, i.e., it is of the
type 'yang:yang-identifier'.
The 'mount-point' statement MUST NOT be used in a YANG
version 1 module, neither explicitly nor via a 'uses'
statement.
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RFC 8528 YANG Schema Mount March 2019
The 'mount-point' statement MAY be present as a substatement
of 'container' and 'list' and MUST NOT be present elsewhere.
There MUST NOT be more than one 'mount-point' statement in a
given 'container' or 'list' statement.
If a mount point is defined within a grouping, its label is
bound to the module where the grouping is used.
A mount point defines a place in the node hierarchy where
other data models may be attached. A server that implements a
module with a mount point populates the
'/schema-mounts/mount-point' list with detailed information on
which data models are mounted at each mount point.
Note that the 'mount-point' statement does not define a new
data node.";
}
/*
* State data nodes
*/
container schema-mounts {
config false;
description
"Contains information about the structure of the overall
mounted data model implemented in the server.";
list namespace {
key "prefix";
description
"This list provides a mapping of namespace prefixes that are
used in XPath expressions of 'parent-reference' leafs to the
corresponding namespace URI references.";
leaf prefix {
type yang:yang-identifier;
description
"Namespace prefix.";
}
leaf uri {
type inet:uri;
description
"Namespace URI reference.";
}
}
list mount-point {
key "module label";
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RFC 8528 YANG Schema Mount March 2019
description
"Each entry of this list specifies a schema for a particular
mount point.
Each mount point MUST be defined using the 'mount-point'
extension in one of the modules listed in the server's
YANG library instance with conformance type 'implement'.";
leaf module {
type yang:yang-identifier;
description
"Name of a module containing the mount point.";
}
leaf label {
type yang:yang-identifier;
description
"Label of the mount point defined using the 'mount-point'
extension.";
}
leaf config {
type boolean;
default "true";
description
"If this leaf is set to 'false', then all data nodes in the
mounted schema are read-only ('config false'), regardless
of their 'config' property.";
}
choice schema-ref {
mandatory true;
description
"Alternatives for specifying the schema.";
container inline {
presence
"A complete self-contained schema is mounted at the
mount point.";
description
"This node indicates that the server has mounted at least
the module 'ietf-yang-library' at the mount point, and
its instantiation provides the information about the
mounted schema.
Different instances of the mount point may have
different schemas mounted.";
}
container shared-schema {
presence
"The mounted schema together with the 'parent-reference'
make up the schema for this mount point.";
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RFC 8528 YANG Schema Mount March 2019
description
"This node indicates that the server has mounted at least
the module 'ietf-yang-library' at the mount point, and
its instantiation provides the information about the
mounted schema. When XPath expressions in the mounted
schema are evaluated, the 'parent-reference' leaf-list
is taken into account.
Different instances of the mount point MUST have the
same schema mounted.";
leaf-list parent-reference {
type yang:xpath1.0;
description
"Entries of this leaf-list are XPath 1.0 expressions
that are evaluated in the following context:
- The context node is the node in the parent data tree
where the mount-point is defined.
- The accessible tree is the parent data tree
*without* any nodes defined in modules that are
mounted inside the parent schema.
- The context position and context size are both equal
to 1.
- The set of variable bindings is empty.
- The function library is the core function library
defined in the W3C XPath 1.0 document
(http://www.w3.org/TR/1999/REC-xpath-19991116) and
the functions defined in Section 10 of RFC 7950.
- The set of namespace declarations is defined by the
'namespace' list under 'schema-mounts'.
Each XPath expression MUST evaluate to a node-set
(possibly empty). For the purposes of evaluating
XPath expressions whose context nodes are defined in
the mounted schema, the union of all these node-sets
together with ancestor nodes are added to the
accessible data tree.
Note that in the case 'ietf-yang-schema-mount' is
itself mounted, a 'parent-reference' in the mounted
module may refer to nodes that were brought into the
accessible tree through a 'parent-reference' in the
parent schema.";
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RFC 8528 YANG Schema Mount March 2019
}
}
}
}
}
}
<CODE ENDS>
10. IANA Considerations
This document registers a URI in the "IETF XML Registry" [RFC 3688].
URI: urn:ietf:params:xml:ns:yang:ietf-yang-schema-mount
Registrant Contact: The IESG.
XML: N/A, the requested URI is an XML namespace.
This document registers a YANG module in the "YANG Module Names"
registry [RFC 6020].
name: ietf-yang-schema-mount
namespace: urn:ietf:params:xml:ns:yang:ietf-yang-schema-mount
prefix: yangmnt
reference: RFC 8528
11. Security Considerations
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 [RFC 6241] or RESTCONF [RFC 8040]. The lowest NETCONF layer
is the secure transport layer, and the mandatory-to-implement secure
transport is Secure Shell (SSH) [RFC 6242]. The lowest RESTCONF layer
is HTTPS, and the mandatory-to-implement secure transport is TLS
[RFC 8446].
The Network Configuration Access Control Model (NACM) [RFC 8341]
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.
Some of the readable data nodes in this YANG module may be considered
sensitive or vulnerable in some network environments. It is thus
important to control read access (e.g., via get, get-config, or
notification) to these data nodes. These are the subtrees and data
nodes and their sensitivity/vulnerability:
Bjorklund & Lhotka Standards Track PAGE 18
RFC 8528 YANG Schema Mount March 2019
o /schema-mounts: The schema defined by this state data provides
detailed information about a server implementation that may help
an attacker identify the server capabilities and server
implementations with known bugs. Server vulnerabilities may be
specific to particular modules included in the schema, module
revisions, module features, or even module deviations. For
example, if a particular operation on a particular data node is
known to cause a server to crash or significantly degrade device
performance, then the schema information will help an attacker
identify server implementations with such a defect, in order to
launch a denial-of-service attack on the device.
It is important to take into account the security considerations for
all nodes in the mounted schemas and to control access to these nodes
by using the mechanism described in Section 7.
Care must be taken when the "parent-reference" XPath expressions are
constructed, since the result of the evaluation of these expressions
is added to the accessible tree for any XPath expression found in the
mounted schema.
12. References
12.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC 3688, January 2004,
<https://www.rfc-editor.org/info/RFC 3688>.
[RFC 6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)", RFC 6020,
DOI 10.17487/RFC 6020, October 2010,
<https://www.rfc-editor.org/info/RFC 6020>.
[RFC 6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC 6241, June 2011,
<https://www.rfc-editor.org/info/RFC 6241>.
[RFC 6242] Wasserman, M., "Using the NETCONF Protocol over Secure
Shell (SSH)", RFC 6242, DOI 10.17487/RFC 6242, June 2011,
<https://www.rfc-editor.org/info/RFC 6242>.
Bjorklund & Lhotka Standards Track PAGE 19
RFC 8528 YANG Schema Mount March 2019
[RFC 6991] Schoenwaelder, J., Ed., "Common YANG Data Types",
RFC 6991, DOI 10.17487/RFC 6991, July 2013,
<https://www.rfc-editor.org/info/RFC 6991>.
[RFC 7895] Bierman, A., Bjorklund, M., and K. Watsen, "YANG Module
Library", RFC 7895, DOI 10.17487/RFC 7895, June 2016,
<https://www.rfc-editor.org/info/RFC 7895>.
[RFC 7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC 7950, August 2016,
<https://www.rfc-editor.org/info/RFC 7950>.
[RFC 8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC 8040, January 2017,
<https://www.rfc-editor.org/info/RFC 8040>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[RFC 8341] Bierman, A. and M. Bjorklund, "Network Configuration
Access Control Model", STD 91, RFC 8341,
DOI 10.17487/RFC 8341, March 2018,
<https://www.rfc-editor.org/info/RFC 8341>.
[RFC 8342] Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
and R. Wilton, "Network Management Datastore Architecture
(NMDA)", RFC 8342, DOI 10.17487/RFC 8342, March 2018,
<https://www.rfc-editor.org/info/RFC 8342>.
[RFC 8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC 8446, August 2018,
<https://www.rfc-editor.org/info/RFC 8446>.
[RFC 8525] Bierman, A., Bjorklund, M., Schoenwaelder, J., Watsen, K.,
and R. Wilton, "YANG Library", RFC 8525,
DOI 10.17487/RFC 8525, March 2019,
<https://www.rfc-editor.org/info/RFC 8525>.
[XPATH] Clark, J. and S. DeRose, "XML Path Language (XPath)
Version 1.0", World Wide Web Consortium Recommendation
REC-xpath-19991116, November 1999,
<http://www.w3.org/TR/1999/REC-xpath-19991116>.
Bjorklund & Lhotka Standards Track PAGE 20
RFC 8528 YANG Schema Mount March 2019
12.2. Informative References
[DEVICE-YANG]
Lindem, A., Ed., Berger, L., Ed., Bogdanovic, D., and C.
Hopps, "Network Device YANG Logical Organization", Work in
Progress, draft-ietf-rtgwg-device-model-02, March 2017.
[IS-IS-YANG]
Litkowski, S., Yeung, D., Lindem, A., Zhang, J., and L.
Lhotka, "YANG Data Model for IS-IS protocol", Work in
Progress, draft-ietf-isis-yang-isis-cfg-34, January 2019.
[RFC 8340] Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
BCP 215, RFC 8340, DOI 10.17487/RFC 8340, March 2018,
<https://www.rfc-editor.org/info/RFC 8340>.
[RFC 8343] Bjorklund, M., "A YANG Data Model for Interface
Management", RFC 8343, DOI 10.17487/RFC 8343, March 2018,
<https://www.rfc-editor.org/info/RFC 8343>.
[RFC 8529] Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and
X. Liu, "YANG Data Model for Network Instances", RFC 8529,
DOI 10.17487/RFC 8529, March 2019,
<https://www.rfc-editor.org/info/RFC 8529>.
[RFC 8530] Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and
X. Liu, "YANG Model for Logical Network Elements",
RFC 8530, DOI 10.17487/RFC 8530, March 2019,
<https://www.rfc-editor.org/info/RFC 8530>.
[YANG-MOUNT]
Clemm, A., Voit, E., and J. Medved, "Mounting YANG-Defined
Information from Remote Datastores", Work in Progress,
draft-clemm-netmod-mount-06, March 2017.
Bjorklund & Lhotka Standards Track PAGE 21
RFC 8528 YANG Schema Mount March 2019
Appendix A. Example: Device Model with LNEs and NIs
This non-normative example demonstrates an implementation of the
device model as specified in Section 2 of [DEVICE-YANG], using both
logical network elements (LNEs) and network instances (NIs).
In these examples, the character '\' is used where a line break has
been inserted for formatting reasons.
A.1. Physical Device
The data model for the physical device may be described by this YANG
library content, assuming the server supports the NMDA:
{
"ietf-yang-library:yang-library": {
"content-id": "14e2ab5dc325f6d86f743e8d3ade233f1a61a899",
"module-set": [
{
"name": "physical-device-modules",
"module": [
{
"name": "ietf-datastores",
"revision": "2018-02-14",
"namespace":
"urn:ietf:params:xml:ns:yang:ietf-datastores"
},
{
"name": "iana-if-type",
"revision": "2015-06-12",
"namespace": "urn:ietf:params:xml:ns:yang:iana-if-type"
},
{
"name": "ietf-interfaces",
"revision": "2018-02-20",
"feature": ["arbitrary-names", "pre-provisioning" ],
"namespace":
"urn:ietf:params:xml:ns:yang:ietf-interfaces"
},
{
"name": "ietf-ip",
"revision": "2018-02-22",
"namespace": "urn:ietf:params:xml:ns:yang:ietf-ip"
},
{
"name": "ietf-logical-network-element",
"revision": "2018-03-20",
"feature": [ "bind-lne-name" ],
Bjorklund & Lhotka Standards Track PAGE 22
RFC 8528 YANG Schema Mount March 2019
"namespace":
"urn:ietf:params:xml:ns:yang:\
ietf-logical-network-element"
},
{
"name": "ietf-yang-library",
"revision": "2019-01-04",
"namespace":
"urn:ietf:params:xml:ns:yang:ietf-yang-library"
},
{
"name": "ietf-yang-schema-mount",
"revision": "2019-01-14",
"namespace":
"urn:ietf:params:xml:ns:yang:ietf-yang-schema-mount"
}
],
"import-only-module": [
{
"name": "ietf-inet-types",
"revision": "2013-07-15",
"namespace":
"urn:ietf:params:xml:ns:yang:ietf-inet-types"
},
{
"name": "ietf-yang-types",
"revision": "2013-07-15",
"namespace":
"urn:ietf:params:xml:ns:yang:ietf-yang-types"
}
]
}
],
"schema": [
{
"name": "physical-device-schema",
"module-set": [ "physical-device-modules" ]
}
],
"datastore": [
{
"name": "ietf-datastores:running",
"schema": "physical-device-schema"
},
{
"name": "ietf-datastores:operational",
"schema": "physical-device-schema"
}
Bjorklund & Lhotka Standards Track PAGE 23
RFC 8528 YANG Schema Mount March 2019
]
}
}
A.2. Logical Network Elements
Each LNE can have a specific data model that is determined at run
time, so it is appropriate to mount it using the "inline" method.
Hence, the following "schema-mounts" data is used:
{
"ietf-yang-schema-mount:schema-mounts": {
"mount-point": [
{
"module": "ietf-logical-network-element",
"label": "root",
"inline": {}
}
]
}
}
An administrator of the host device has to configure an entry for
each LNE instance, for example:
{
"ietf-interfaces:interfaces": {
"interface": [
{
"name": "eth0",
"type": "iana-if-type:ethernetCsmacd",
"enabled": true,
"ietf-logical-network-element:bind-lne-name": "eth0"
}
]
},
"ietf-logical-network-element:logical-network-elements": {
"logical-network-element": [
{
"name": "lne-1",
"managed": true,
"description": "LNE with NIs",
"root": {
...
}
}
...
]
Bjorklund & Lhotka Standards Track PAGE 24
RFC 8528 YANG Schema Mount March 2019
}
}
and then also place necessary state data as the contents of the
"root" instance, which should include at least:
o YANG library data specifying the LNE's data model, for example,
assuming the server does not implement the NMDA:
{
"ietf-yang-library:modules-state": {
"module-set-id": "9358e11874068c8be06562089e94a89e0a392019",
"module": [
{
"name": "iana-if-type",
"revision": "2014-05-08",
"namespace": "urn:ietf:params:xml:ns:yang:iana-if-type",
"conformance-type": "implement"
},
{
"name": "ietf-inet-types",
"revision": "2013-07-15",
"namespace": "urn:ietf:params:xml:ns:yang:ietf-inet-types",
"conformance-type": "import"
},
{
"name": "ietf-interfaces",
"revision": "2014-05-08",
"feature": [
"arbitrary-names",
"pre-provisioning"
],
"namespace": "urn:ietf:params:xml:ns:yang:ietf-interfaces",
"conformance-type": "implement"
},
{
"name": "ietf-ip",
"revision": "2014-06-16",
"feature": [
"ipv6-privacy-autoconf"
],
"namespace": "urn:ietf:params:xml:ns:yang:ietf-ip",
"conformance-type": "implement"
},
{
"name": "ietf-network-instance",
"revision": "2018-03-20",
"feature": [
Bjorklund & Lhotka Standards Track PAGE 25
RFC 8528 YANG Schema Mount March 2019
"bind-network-instance-name"
],
"namespace":
"urn:ietf:params:xml:ns:yang:ietf-network-instance",
"conformance-type": "implement"
},
{
"name": "ietf-yang-library",
"revision": "2016-06-21",
"namespace": "urn:ietf:params:xml:ns:yang:ietf-yang-library",
"conformance-type": "implement"
},
{
"name": "ietf-yang-schema-mount",
"revision": "2019-01-14",
"namespace":
"urn:ietf:params:xml:ns:yang:ietf-yang-schema-mount",
"conformance-type": "implement"
},
{
"name": "ietf-yang-types",
"revision": "2013-07-15",
"namespace": "urn:ietf:params:xml:ns:yang:ietf-yang-types",
"conformance-type": "import"
}
]
}
}
o state data for interfaces assigned to the LNE instance (that
effectively become system-controlled interfaces for the LNE), for
example:
{
"ietf-interfaces:interfaces": {
"interface": [
{
"name": "eth0",
"type": "iana-if-type:ethernetCsmacd",
"oper-status": "up",
"statistics": {
"discontinuity-time": "2016-12-16T17:11:27+02:00"
},
"ietf-ip:ipv6": {
"address": [
{
"ip": "fe80::42a8:f0ff:fea8:24fe",
"origin": "link-layer",
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RFC 8528 YANG Schema Mount March 2019
"prefix-length": 64
}
]
}
}
]
}
}
A.3. Network Instances
Assuming that network instances share the same data model, it can be
mounted using the "shared-schema" method as follows:
{
"ietf-yang-schema-mount:schema-mounts": {
"namespace": [
{
"prefix": "if",
"uri": "urn:ietf:params:xml:ns:yang:ietf-interfaces"
},
{
"prefix": "ni",
"uri": "urn:ietf:params:xml:ns:yang:ietf-network-instance"
}
],
"mount-point": [
{
"module": "ietf-network-instance",
"label": "root",
"shared-schema": {
"parent-reference": [
"/if:interfaces/if:interface[\
ni:bind-network-instance-name = current()/../ni:name]"
]
}
}
]
}
}
Note also that the "ietf-interfaces" module appears in the
"parent-reference" leaf-list for the mounted NI schema. This means
that references to LNE interfaces, such as "outgoing-interface" in
static routes, are valid despite the fact that "ietf-interfaces"
isn't part of the NI schema.
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RFC 8528 YANG Schema Mount March 2019
A.4. Invoking an RPC Operation
Assume that the mounted NI data model also implements the "ietf-isis"
module [IS-IS-YANG]. An RPC operation defined in this module, such
as "clear-adjacency", can be invoked by a client session of an LNE's
RESTCONF server as an action tied to the mount point of a particular
network instance using a request URI like this (all on one line):
POST /restconf/data/ietf-network-instance:network-instances/
network-instance=rtrA/root/ietf-isis:clear-adjacency HTTP/1.1
Contributors
The idea of having some way to combine schemas from different YANG
modules into one has been proposed independently by others:
o Authors of [YANG-MOUNT]:
* Lou Berger, LabN Consulting, L.L.C., <lberger@labn.net>
* Alexander Clemm, Huawei, <alexander.clemm@huawei.com>
* Christian Hopps, Deutsche Telekom, <chopps@chopps.org>
o Jan Medved, Cisco, <jmedved@cisco.com>
o Eric Voit, Cisco, <evoit@cisco.com>
Authors' Addresses
Martin Bjorklund
Tail-f Systems
Email: mbj@tail-f.com
Ladislav Lhotka
CZ.NIC
Email: lhotka@nic.cz
Bjorklund & Lhotka Standards Track PAGE 28
RFC TOTAL SIZE: 54347 bytes
PUBLICATION DATE: Thursday, March 7th, 2019
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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