This document defines a mechanism to add the schema trees defined by a set of YANG modules onto a mount point defined in the schema tree in some YANG module.
Modularity and extensibility were 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 and thus 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 (^RFC7895^, ^I-D.ietf-netconf-rfc7895bis^ and Section 5.6.4 of ^RFC7950^) 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:
- The “uses” statement explicitly incorporates the contents of a grouping defined in the same or another module. See Section 4.2.6 of ^RFC7950^ for more details.
- The “augment” statement explicitly adds contents to a target node defined in the same or another module. See Section 4.2.8 of ^RFC7950^ 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 that an existing module (or a set of modules) is added to the data model starting at locations other than the root. For example, YANG modules such as “ietf-interfaces” ^RFC8343^ 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 ^I-D.ietf-rtgwg-lne-model^, each of which has its own instantiation of “ietf-interfaces”, and possibly other modules, but, 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:
+–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:
- 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.
- Absolute references to nodes defined inside a grouping may break if the grouping is used in different locations.
- 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.
- 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 and, consequently, existing modules such as “ietf-interfaces” can be mounted without any modifications.
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:
- 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.
- Implementation-time: the mounted schema is defined by a server implementor and is as stable as the YANG library information of the server.
- 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) ^RFC8342^.
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.
The following terms are defined in ^RFC7950^ and are not redefined here:
- action
- container
- data node
- list
- RPC operation
- schema node
- schema tree
The following terms are defined in ^RFC8342^ and are not redefined here:
- client
- notification
- operational state
- server
The following term is defined in ^RFC8343^ and is not redefined here:
- system-controlled interface
The following term is defined in ^I-D.ietf-netconf-rfc7895bis^ is not redefined here:
- YANG library content identifier
The following additional terms are used within this document:
- mount point: A container or a list node whose definition contains the “mount-point” extension statement. The argument of the “mount-point” statement defines a label for the mount point.
- schema: A collection of schema trees with a common root.
- top-level schema: A schema rooted at the root node.
- mounted schema: A schema rooted at a mount point.
- parent schema (of a mounted schema): A schema containing the mount point.
- schema mount: The mechanism to combine data models defined in this document.
Tree diagrams used in this document follow the notation defined in ^RFC8340^
In this document, names of data nodes, YANG extensions, actions and other data model objects are often used without a prefix, as long as it is clear from the context in which YANG module each name is defined. Otherwise, names are prefixed using the standard prefix associated with the corresponding YANG module, as shown in ^prefixtab^.
– Namespace Prefixes @prefixtab@
| Prefix | YANG module | Reference |
|---|---|---|
| yangmnt | ietf-yang-schema-mount | ^yangmod^ |
| inet | ietf-inet-types | ^RFC6991^ |
| yang | ietf-yang-types | ^RFC6991^ |
| yanglib | ietf-yang-library | ^RFC7895^, ^I-D.ietf-netconf-rfc7895bis^ |
The schema mount mechanism defined in this document provides a new extensibility mechanism for use with YANG 1.1. In contrast to the existing mechanisms described in ^intro^, schema mount defines the relationship between the source and target YANG modules outside these modules. The procedure consists of two separate steps that are described in the following subsections.
A “container” or “list” node becomes a mount point if the “mount-point” extension (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 is a YANG identifier that defines a label for the mount point. A module MAY contain multiple “mount-point” 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” statement MUST NOT be used in a YANG version 1 module ^RFC6020^. The reason for this is that otherwise it is not possible to invoke mounted RPC operations, and receive mounted notifications. See ^rpcnot^ 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” statement does not define a new data node.
This document defines the YANG 1.1 module ^RFC7950^ “ietf-yang-schema-mount”, which has the following structure:
!! include-figure ietf-yang-schema-mount.tree
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 and, if present, its data will generally have no relationship to the data of the parent. Exceptions are possible and such needs to be defined in the model defining the exception. For example, ^I-D.ietf-rtgwg-lne-model^ 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.
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:
- the mount point is itself defined as “config false”
- 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 exactly as for 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 ^exlne^ and ^exni^, respectively.
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 “ietf-yang-library” and “ietf-yang-schema-mount” modules in the mounted schema, and specifying the schemas exactly as it is done in the top-level 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 augments) 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.
However, this restriction is sometimes too severe. A typical example is network instances (NI) ^I-D.ietf-rtgwg-ni-model^, where 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 +–rw root +–rw routing …
Here, the “root” node 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 nodeset (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 nodesets is added to the accessible data tree.
It is worth emphasizing that the nodes specified in “parent-reference” leaf-list are available in the mounted schema only for XPath evaluations. In particular, they cannot be accessed there via network management protocols such as NETCONF ^RFC6241^ or RESTCONF ^RFC8040^.
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 ^RFC7950^. An example of this is given in ^invoke-rpc^.
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 ^RFC7950^.
Note, inline actions and notifications will not work when they are contained within a list node without a “key” statement (see section 7.15 and 7.16 of ^RFC7950^). 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.
The schema mount solution presented in this document is designed to work both with servers that implement the NMDA ^RFC8342^, and old servers that don’t implement the NMDA.
Note to RFC Editor: please update the date YYYY-MM-DD below with the revision of the ietf-yang-library in the published version of draft-ietf-netconf-rfc7895bis, and remove this note.
Specifically, a server that doesn’t support the NMDA, MAY implement revision 2016-06-21 of “ietf-yang-library” ^RFC7895^ under a mount point. A server that supports the NMDA, MUST implement at least revision YYYY-MM-DD of “ietf-yang-library” ^I-D.ietf-netconf-rfc7895bis^ under the mount points.
If NACM ^RFC8341^ 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 ^I-D.ietf-rtgwg-lne-model^. 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>
Network management of devices that use a data model with schema mount can be implemented in different ways. However, the following implementations options are envisioned as typical:
- shared management: instance data of both parent and mounted schemas are accessible within the same management session.
- split management: one (master) management session has access to instance data of both parent and mounted schemas but, in addition, an extra session exists for every instance of the mount point, having access only to the mounted data tree.
This module references ^RFC6991^.
!! include-yang ietf-yang-schema-mount.yang
This document registers a URI in the IETF XML registry ^RFC3688^. Following the format in RFC 3688, the following registration is requested to be made.
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 ^RFC6020^.
name: ietf-yang-schema-mount namespace: urn:ietf:params:xml:ns:yang:ietf-yang-schema-mount prefix: yangmnt reference: RFC XXXX
YANG module “ietf-yang-schema-mount” specified in this document defines a schema for data that is designed to be accessed via network management protocols such as NETCONF ^RFC6241^ or RESTCONF ^RFC8040^. The lowest NETCONF layer is the secure transport layer, and the mandatory-to-implement secure transport is Secure Shell (SSH) ^RFC6242^. The lowest RESTCONF layer is HTTPS, and the mandatory-to-implement secure transport is TLS ^RFC5246^.
The network configuration access control model ^RFC8341^ provides the means to restrict access for particular NETCONF or RESTCONF users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content.
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:
- /schema-mounts: The schema defined by this state data provides detailed information about a server implementation 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 the security considerations for all nodes in the mounted schemas into account, and control access to these nodes by using the mechanism described in ^nacm^.
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.
The idea of having some way to combine schemas from different YANG modules into one has been proposed independently by several groups of people: Alexander Clemm, Jan Medved, and Eric Voit (^I-D.clemm-netmod-mount^); and Lou Berger and Christian Hopps:
- Lou Berger, LabN Consulting, L.L.C., <lberger@labn.net>
- Alexander Clemm, Huawei, <alexander.clemm@huawei.com>
- Christian Hopps, Deutsche Telekom, <chopps@chopps.org>
- Jan Medved, Cisco, <jmedved@cisco.com>
- Eric Voit, Cisco, <evoit@cisco.com>
*! start-appendix
This non-normative example demonstrates an implementation of the device model as specified in Section 2 of ^I-D.ietf-rtgwg-device-model^, using both logical network elements (LNE) and network instances (NI).
In these examples, the character ‘' is used where a line break has been inserted for formatting reasons.
The data model for the physical device may be described by this YANG library content, assuming the server supports the NMDA:
!! include-figure yang-library-ex1-device.json
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:
!! include-figure schema-mounts-ex1-device.json
An administrator of the host device has to configure an entry for each LNE instance, for example,
!! include-figure config-ex1-device.json
and then also place necessary state data as the contents of the “root” instance, which should include at least
- YANG library data specifying the LNE’s data model, for example, assuming the server does not implement the NMDA:
!! include-figure yang-library-ex1-lne.json
- state data for interfaces assigned to the LNE instance (that effectively become system-controlled interfaces for the LNE), for example:
!! include-figure interfaces-ex1-lne.json
Assuming that network instances share the same data model, it can be mounted using the “shared-schema” method as follows:
!! include-figure schema-mounts-ex1-lne.json
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.
Assume that the mounted NI data model also implements the “ietf-isis” module ^I-D.ietf-isis-yang-isis-cfg^. An RPC operation defined in this module, such as “clear-adjacency”, can be invoked by a client session of a LNE’s RESTCONF server as an action tied to a 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
{{document: name ; ipr trust200902; category std; references back.xml; title “YANG Schema Mount”; abbreviation “YANG Schema Mount”; contributor “author:Martin Bjorklund:Tail-f Systems:mbj@tail-f.com”; contributor “author:Ladislav Lhotka:CZ.NIC:lhotka@nic.cz”; }}