YANG Schema Mount
draft-ietf-netmod-schema-mount-04
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (netmod WG) | |
|---|---|---|---|
| Authors | Martin Björklund , Ladislav Lhotka | ||
| Last updated | 2017-03-06 | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text htmlized pdfized bibtex | ||
| Reviews |
GENART Telechat review
(of
-11)
Ready with Issues
RTGDIR Telechat review
(of
-10)
Has Nits
GENART Last Call review
(of
-10)
Almost Ready
OPSDIR Last Call review
(of
-10)
Has Nits
SECDIR Last Call review
(of
-10)
Has Nits
|
||
| Stream | WG state | WG Document | |
| Document shepherd | Kent Watsen | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | "Lou Berger" <lberger@labn.net>, Kent Watsen <kwatsen@juniper.net> |
draft-ietf-netmod-schema-mount-04
Network Working Group M. Bjorklund
Internet-Draft Tail-f Systems
Intended status: Standards Track L. Lhotka
Expires: September 7, 2017 CZ.NIC
March 6, 2017
YANG Schema Mount
draft-ietf-netmod-schema-mount-04
Abstract
This document defines a mechanism to combine YANG modules into the
schema defined in other YANG modules.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 7, 2017.
Copyright Notice
Copyright (c) 2017 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
(http://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 Expires September 7, 2017 [Page 1]
Internet-Draft YANG Schema Mount March 2017
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology and Notation . . . . . . . . . . . . . . . . . . 5
2.1. Glossary of New Terms . . . . . . . . . . . . . . . . . . 6
2.2. Tree Diagrams . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Namespace Prefixes . . . . . . . . . . . . . . . . . . . 6
3. Schema Mount . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Mount Point Definition . . . . . . . . . . . . . . . . . 7
3.2. Specification of the Mounted Schema . . . . . . . . . . . 7
3.3. Multiple Levels of Schema Mount . . . . . . . . . . . . . 11
4. Refering to Data Nodes in the Parent Schema . . . . . . . . . 11
5. RPC operations and Notifications . . . . . . . . . . . . . . 12
6. Implementation Notes . . . . . . . . . . . . . . . . . . . . 13
7. Data Model . . . . . . . . . . . . . . . . . . . . . . . . . 13
8. Schema Mount YANG Module . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
10. Security Considerations . . . . . . . . . . . . . . . . . . . 21
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 21
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
12.1. Normative References . . . . . . . . . . . . . . . . . . 21
12.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Example: Device Model with LNEs and NIs . . . . . . 23
A.1. Physical Device . . . . . . . . . . . . . . . . . . . . . 23
A.2. Logical Network Elements . . . . . . . . . . . . . . . . 24
A.3. Network Instances . . . . . . . . . . . . . . . . . . . . 27
A.4. Invoking an RPC Operation . . . . . . . . . . . . . . . . 29
Appendix B. Open Issues . . . . . . . . . . . . . . . . . . . . 29
B.1. Referencing Mount Points Using Schema Node Identifiers . 29
B.2. Defining the "mount-point" Extension in a Separate Module 30
B.3. Parent References . . . . . . . . . . . . . . . . . . . . 31
B.4. RPC Operations and Notifications in Mounted Modules . . . 31
B.5. Tree Representation . . . . . . . . . . . . . . . . . . . 32
B.6. Design-Time Mounts . . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
1. Introduction
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], and Section 5.6.4 of [RFC7950]) implemented by the
server. Such YANG modules appear in the data model "side by side",
Bjorklund & Lhotka Expires September 7, 2017 [Page 2]
Internet-Draft YANG Schema Mount March 2017
i.e., top-level data nodes of each module - if there are any - are
also top-level nodes of the overall data model.
Furthermore, 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 [RFC7950] 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
[RFC7950] for more details.
With both mechanisms, the source or target YANG module 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 often
necessary that an existing module (or a set of modules) is added to
the data model starting at a non-root location. For example, YANG
modules such as "ietf-interfaces" [RFC7223] are often 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 like this:
+--rw interfaces
| +--rw interface* [name]
| ...
+--rw logical-device* [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-device" list (for the logical
devices). This approach has several disadvantages:
Bjorklund & Lhotka Expires September 7, 2017 [Page 3]
Internet-Draft YANG Schema Mount March 2017
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-device" 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 generic 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:
1. Design-time: the mounted schema is defined along with the mount
point in the parent 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 YANG library information, i.e.,
it may change after an upgrade of server software but not after
rebooting the server. Also, a client can learn the entire schema
together with YANG library data.
Bjorklund & Lhotka Expires September 7, 2017 [Page 4]
Internet-Draft YANG Schema Mount March 2017
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 because design-time definition of the
mounted schema doesn't play well with the existing YANG modularity
mechanisms. For example, it would be impossible to augment the
mounted data model.
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.
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.
2. Terminology and Notation
The keywords "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].
The following terms are defined in [RFC6241] and are not redefined
here:
o client
o notification
o server
The following terms are defined in [RFC7950] and are not redefined
here:
o action
o configuration data
o container
o list
Bjorklund & Lhotka Expires September 7, 2017 [Page 5]
Internet-Draft YANG Schema Mount March 2017
o operation
The following terms are defined in [RFC7223] and are not redefined
here:
o system-controlled interface
2.1. Glossary of New Terms
o inline schema: a mounted schema whose definition is provided as
part of the mounted data, using YANG library [RFC7895].
o mount point: container or list node whose definition contains the
"mount-point" extension statement. The argument of the
"mount-point" statement defines the name of the mount point.
o parent schema (of a particular mounted schema): the schema that
contains the mount point for the mounted schema.
o top-level schema: a schema according to [RFC7950] in which schema
trees of each module (except augments) start at the root node.
2.2. Tree Diagrams
A simplified graphical representation of the data model is used in
this document. The meaning of the symbols in these diagrams is as
follows:
o Brackets "[" and "]" enclose list keys.
o Abbreviations before data node names: "rw" means configuration
data (read-write) and "ro" state data (read-only).
o Symbols after data node names: "?" means an optional node, "!"
means a presence container, and "*" denotes a list and leaf-list.
o Parentheses enclose choice and case nodes, and case nodes are also
marked with a colon (":").
o Ellipsis ("...") stands for contents of subtrees that are not
shown.
2.3. Namespace Prefixes
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
Bjorklund & Lhotka Expires September 7, 2017 [Page 6]
Internet-Draft YANG Schema Mount March 2017
defined. 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 8 |
| inet | ietf-inet-types | [RFC6991] |
| yang | ietf-yang-types | [RFC6991] |
| yanglib | ietf-yang-library | [RFC7895] |
+---------+------------------------+-----------+
Table 1: Namespace Prefixes
3. Schema Mount
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 Section 1, 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.
3.1. Mount Point Definition
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 the name of 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. Note, however, that modules written in any YANG version,
including version 1, can be mounted under a mount point.
3.2. Specification of the Mounted Schema
Mounted schemas for all mount points in the parent schema are defined
as state data in the "yangmnt:schema-mounts" container. Data in this
container is intended to be as stable as data in the top-level YANG
Bjorklund & Lhotka Expires September 7, 2017 [Page 7]
Internet-Draft YANG Schema Mount March 2017
library [RFC7895]. In particular, it SHOULD NOT change during the
same management session.
The "schema-mount" 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 above
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 overriden 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:
1. The mount point is itself defined as "config false".
2. 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:
1. by stating that the schema is available inline, i.e., in run-time
instance data; or
2. by referring to one or more entries of the "schema" list in the
same instance of "schema-mounts".
In case 1, every instance of the mount point that exists in the
parent tree MUST contain a copy of YANG library data [RFC7895] that
defines the mounted schema exactly as for a top-level data model. A
client is expected to retrieve this data from the instance tree,
possibly after creating the mount point. Instances of the same mount
point MAY use different mounted schemas.
In case 2, the mounted schema is defined by the combination of all
"schema" entries referred to in the "use-schema" list. Optionally, a
reference to a "schema" entry can be made conditional by including
the "when" leaf. Its argument is an XPath expression that is
evaluated in the parent tree with the mount point instance as the
Bjorklund & Lhotka Expires September 7, 2017 [Page 8]
Internet-Draft YANG Schema Mount March 2017
context node. The conditional "schema" entry is used only if the
XPath expression evaluates to true. XPath expressions in the
argument of "when" may use namespace prefixes that are declared in
the "namespace" list (child of "schema-mounts").
Conditional schemas may be used, for example, in a situation where
virtual devices are of several different types and the schema for
each type is fixed and known in advance. The list of virtual devices
in a parent schema module (say "example-virtual-host") might be
defined as follows:
list virtual-device {
key name;
leaf name {
type string;
}
leaf type {
type identityref {
base virtual-device-type;
}
}
container root {
yangmnt:mount-point virtual-device;
}
The "schema-mounts" specification in state data might contain, for
example,
Bjorklund & Lhotka Expires September 7, 2017 [Page 9]
Internet-Draft YANG Schema Mount March 2017
"yangmnt:schema-mounts": {
"namespace": [
{
"prefix": "evh",
"ns-uri": "http://example.org/ns/example-virtual-host"
}
],
"mount-point": [
{
"module": "example-virtual-host",
"name": "root",
"use-schema": [
{
"name": "virtual-router-schema",
"when": "derived-from(../evh:type, 'evh:virtual-router')"
},
{
"name": "virtual-switch-schema",
"when": "derived-from(../evh:type, 'evh:virtual-switch')"
}
],
"schema": [
{
"name": "virtual-router-schema",
"module": [
...
]
},
{
"name": "virtual-switch-schema",
"module": [
...
]
}
]
}
The schema of virtual device instances can then be controlled by
setting the "type" leaf to an appropriate identity derived from the
"virtual-device-type" base.
In case 2, the mounted schema is specified as implementation-time
data that can be retrieved together with YANG library data for the
parent schema, i.e., even before any instances of the mount point
exist. However, the mounted schema has to be the same for all
instances of the mount point (except for parts that are conditional
due to "when" leaves).
Bjorklund & Lhotka Expires September 7, 2017 [Page 10]
Internet-Draft YANG Schema Mount March 2017
Each entry of the "schema" list contains
o a list in the YANG library format specifying all YANG modules (and
revisions etc.) that are implemented or imported in the mounted
schema;
o (optionally) a new "schema-mounts" specification that applies to
mount points defined within the mounted schema.
3.3. Multiple Levels of Schema Mount
YANG modules in a mounted schema MAY again contain mount points under
which subschemas can be mounted. Consequently, it is possible to
construct data models with an arbitrary number of schema levels. A
subschema for a mount point contained in a mounted module can be
specified in one of the following ways:
o by implementing "ietf-yang-library" and "ietf-yang-schema-mount"
modules in the mounted schema, and specifying the subschemas
exactly as it is done in the top-level schema
o by using the "mount-point" list inside the coresponding "schema"
entry.
The former method is applicable to both "inline" and "use-schema"
cases whereas the latter requires the "use-schema" case. On the
other hand, the latter method allows for a compact representation of
a multi-level schema the does not rely on the presence of any
instance data.
4. Refering to Data Nodes in the Parent Schema
A fundamental design principle of schema mount is that the mounted
data model works exactly as a top-level data model, i.e., it is
confined to the "mount jail". This means that all paths in the
mounted data model (in leafrefs, instance-identifiers, 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
are 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:
Bjorklund & Lhotka Expires September 7, 2017 [Page 11]
Internet-Draft YANG Schema Mount March 2017
+--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, albeit in a
limited and controlled way. The "schema-mounts" container has a
child leaf-list named "parent-reference" that contains zero or more
module names. All modules appearing in this leaf-list MUST be
implemented in the parent schema and MUST NOT be implemented in the
mounted schema. All absolute leafref paths and instance identifiers
within the mounted data model and corresponding instance data tree
are then evaluated as follows:
o If the leftmost node-identifier (right after the initial slash)
belongs to the namespace of a module that is listed in
"parent-reference", then the root of the accessible tree is not
the mount point but the root of the parent schema.
o Other rules for the "leafref" and "instance-identifier" types as
defined in Sections 9.9 and 9.13 of [RFC7950] remain in effect.
It is worth emphasizing that the mount jail can be escaped only via
absolute leafref paths and instance identifiers. Relative leafref
paths, "must"/"when" expressions and schema node identifiers are
still restricted to the mounted schema.
5. RPC operations and Notifications
If a mounted YANG module defines an RPC operation, clients can invoke
this operation by representing it as an action defined for the
corresponding mount point, see Section 7.15 of ^RFC7950. An example
of this is given in Appendix A.4.
Similarly, if the server emits a notification defined at the top
level of any mounted module, it MUST be represented as if the
Bjorklund & Lhotka Expires September 7, 2017 [Page 12]
Internet-Draft YANG Schema Mount March 2017
notification was connected to the mount point, see Section 7.16 of
[RFC7950].
6. Implementation Notes
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:
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 but, in addition,
an extra session exists for every instance of the mount point,
having access only to the mounted data tree.
7. Data Model
This document defines the YANG 1.1 module [RFC7950]
"ietf-yang-schema-mount", which has the following structure:
Bjorklund & Lhotka Expires September 7, 2017 [Page 13]
Internet-Draft YANG Schema Mount March 2017
module: ietf-yang-schema-mount
+--ro schema-mounts
+--ro namespace* [prefix]
| +--ro prefix yang:yang-identifier
| +--ro ns-uri? inet:uri
+--ro mount-point* [module name]
| +--ro module yang:yang-identifier
| +--ro name yang:yang-identifier
| +--ro config? boolean
| +--ro (schema-ref)?
| +--:(inline)
| | +--ro inline? empty
| +--:(use-schema)
| +--ro use-schema* [name]
| +--ro name
| | -> /schema-mounts/schema/name
| +--ro when? yang:xpath1.0
| +--ro parent-reference* yang:yang-identifier
+--ro schema* [name]
+--ro name string
+--ro module* [name revision]
| +--ro name yang:yang-identifier
| +--ro revision union
| +--ro schema? inet:uri
| +--ro namespace inet:uri
| +--ro feature* yang:yang-identifier
| +--ro deviation* [name revision]
| | +--ro name yang:yang-identifier
| | +--ro revision union
| +--ro conformance-type enumeration
| +--ro submodule* [name revision]
| +--ro name yang:yang-identifier
| +--ro revision union
| +--ro schema? inet:uri
+--ro mount-point* [module name]
+--ro module yang:yang-identifier
+--ro name yang:yang-identifier
+--ro config? boolean
+--ro (schema-ref)?
+--:(inline)
| +--ro inline? empty
+--:(use-schema)
+--ro use-schema* [name]
+--ro name
| -> /schema-mounts/schema/name
+--ro when? yang:xpath1.0
+--ro parent-reference* yang:yang-identifier
Bjorklund & Lhotka Expires September 7, 2017 [Page 14]
Internet-Draft YANG Schema Mount March 2017
8. Schema Mount YANG Module
This module references [RFC6991] and [RFC7895].
<CODE BEGINS> file "ietf-yang-schema-mount@2017-03-06.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";
}
import ietf-yang-library {
prefix yanglib;
reference
"RFC 7895: YANG Module Library";
}
organization
"IETF NETMOD (NETCONF Data Modeling Language) Working Group";
contact
"WG Web: <https://tools.ietf.org/wg/netmod/>
WG List: <mailto:netmod@ietf.org>
Editor: Martin Bjorklund
<mailto:mbj@tail-f.com>
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.
Bjorklund & Lhotka Expires September 7, 2017 [Page 15]
Internet-Draft YANG Schema Mount March 2017
Copyright (c) 2017 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).
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and
'OPTIONAL' in the module text are to be interpreted as described
in RFC 2119 (https://tools.ietf.org/html/rfc2119).
This version of this YANG module is part of RFC XXXX
(https://tools.ietf.org/html/rfcXXXX); see the RFC itself for
full legal notices.";
revision 2017-03-06 {
description
"Initial revision.";
reference
"RFC XXXX: YANG Schema Mount";
}
/*
* Extensions
*/
extension mount-point {
argument name;
description
"The argument 'name' 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.
The 'mount-point' statement MAY be present as a substatement
of 'container' and 'list', and MUST NOT be present elsewhere.
If a mount point is defined in a grouping, its name 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
Bjorklund & Lhotka Expires September 7, 2017 [Page 16]
Internet-Draft YANG Schema Mount March 2017
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.";
}
/*
* Groupings
*/
grouping mount-point-list {
description
"This grouping is used inside the 'schema-mounts' container and
inside the 'schema' list.";
list mount-point {
key "module name";
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 corresponding
YANG library instance with conformance type 'implement'. The
corresponding YANG library instance is:
- standard YANG library state data as defined in RFC 7895,
if the 'mount-point' list is a child of 'schema-mounts',
- the contents of the sibling 'yanglib:modules-state'
container, if the 'mount-point' list is a child of
'schema'.";
leaf module {
type yang:yang-identifier;
description
"Name of a module containing the mount point.";
}
leaf name {
type yang:yang-identifier;
description
"Name 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.";
Bjorklund & Lhotka Expires September 7, 2017 [Page 17]
Internet-Draft YANG Schema Mount March 2017
}
choice schema-ref {
description
"Alternatives for specifying the schema.";
leaf inline {
type empty;
description
"This leaf indicates that the server has mounted
'ietf-yang-library' and 'ietf-schema-mount' at the mount
point, and their instantiation (i.e., state data
containers 'yanglib:modules-state' and 'schema-mounts')
provides the information about the mounted schema.";
}
list use-schema {
key "name";
description
"Each entry of this list contains a reference to a schema
defined in the /schema-mounts/schema list. The entry can
be made conditional by specifying an XPath expression in
the 'when' leaf.";
leaf name {
type leafref {
path "/schema-mounts/schema/name";
}
description
"Name of the referenced schema.";
}
leaf when {
type yang:xpath1.0;
description
"This leaf contains an XPath expression. If it is
present, then the current entry applies if and only if
the expression evaluates to true.
The XPath expression is evaluated once for each
instance of the data node containing the mount
point for which the 'when' leaf is defined.
The XPath expression is evaluated using the rules
specified in sec. 6.4 of RFC 7950, with these
modifications:
- The context node is the data node instance
containing the corresponding 'mount-point'
statement.
- The accessible tree contains only data belonging to
the parent schema, i.e., all instances of data
Bjorklund & Lhotka Expires September 7, 2017 [Page 18]
Internet-Draft YANG Schema Mount March 2017
nodes containing the mount points are considered
empty.
- The set of namespace declarations is the set of all
prefix/namespace pairs defined in the
/schema-mounts/namespace list. Names without a
namespace prefix belong to the same namespace as the
context node.";
}
leaf-list parent-reference {
type yang:yang-identifier;
must "not(/schema-mounts/schema[name=current()/../name]/"
+ "module[name=current() and conformance-type="
+ "'implement'])" {
error-message "Parent references cannot be used for a "
+ "module implemented in the mounted schema.";
description
"Modules that are used for parent references MUST NOT
be implemented in the mounted schema.";
}
description
"Entries of this leaf-list are names of YANG modules.
All these modules MUST be implemented in the parent
schema.
Within the mounted schema and the corresponding data
tree, conceptual evaluation of absolute leafref paths
and instance identifiers is modified in the following
way:
If the leftmost node-identifier in an absolute leafref
path or instance identifier belongs to a module whose
name is listed in 'parent-reference', then the root
of the accessible data tree coincides with the root of
the parent data tree.";
}
}
}
}
}
/*
* State data nodes
*/
container schema-mounts {
config false;
description
Bjorklund & Lhotka Expires September 7, 2017 [Page 19]
Internet-Draft YANG Schema Mount March 2017
"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 'when' leafs to the
corresponding namespace URI references.";
leaf prefix {
type yang:yang-identifier;
description
"Namespace prefix.";
}
leaf ns-uri {
type inet:uri;
description
"Namespace URI reference.";
}
}
uses mount-point-list;
list schema {
key "name";
description
"Each entry specifies a schema that can be mounted at a mount
point. The schema information consists of two parts:
- an instance of YANG library that defines YANG modules used
in the schema,
- mount-point list with content identical to the top-level
mount-point list (this makes the schema structure
recursive).";
leaf name {
type string;
description
"Arbitrary name of the schema entry.";
}
uses yanglib:module-list;
uses mount-point-list;
}
}
}
<CODE ENDS>
Bjorklund & Lhotka Expires September 7, 2017 [Page 20]
Internet-Draft YANG Schema Mount March 2017
9. IANA Considerations
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
10. Security Considerations
TBD
11. Contributors
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:
o Lou Berger, LabN Consulting, L.L.C., <lberger@labn.net>
o Alexander Clemm, Huawei, <alexander.clemm@huawei.com>
o Christian Hopps, Deutsche Telekom, <chopps@chopps.org>
o Jan Medved, Cisco, <jmedved@cisco.com>
o Eric Voit, Cisco, <evoit@cisco.com>
12. References
12.1. Normative References
Bjorklund & Lhotka Expires September 7, 2017 [Page 21]
Internet-Draft YANG Schema Mount March 2017
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<http://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,
<http://www.rfc-editor.org/info/rfc6020>.
[RFC6991] Schoenwaelder, J., Ed., "Common YANG Data Types",
RFC 6991, DOI 10.17487/RFC6991, July 2013,
<http://www.rfc-editor.org/info/rfc6991>.
[RFC7895] Bierman, A., Bjorklund, M., and K. Watsen, "YANG Module
Library", RFC 7895, DOI 10.17487/RFC7895, June 2016,
<http://www.rfc-editor.org/info/rfc7895>.
[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<http://www.rfc-editor.org/info/rfc7950>.
12.2. Informative References
[I-D.clemm-netmod-mount]
Clemm, A., Medved, J., and E. Voit, "Mounting YANG-Defined
Information from Remote Datastores", draft-clemm-netmod-
mount-05 (work in progress), September 2016.
[I-D.ietf-isis-yang-isis-cfg]
Litkowski, S., Yeung, D., Lindem, A., Zhang, Z., and L.
Lhotka, "YANG Data Model for IS-IS protocol", draft-ietf-
isis-yang-isis-cfg-15 (work in progress), February 2017.
[I-D.ietf-rtgwg-device-model]
Lindem, A., Berger, L., Bogdanovic, D., and C. Hopps,
"Network Device YANG Organizational Models", draft-ietf-
rtgwg-device-model-01 (work in progress), October 2016.
[I-D.ietf-rtgwg-lne-model]
Berger, L., Hopps, C., Lindem, A., and D. Bogdanovic,
"YANG Logical Network Elements", draft-ietf-rtgwg-lne-
model-01 (work in progress), October 2016.
Bjorklund & Lhotka Expires September 7, 2017 [Page 22]
Internet-Draft YANG Schema Mount March 2017
[I-D.ietf-rtgwg-ni-model]
Berger, L., Hopps, C., Lindem, A., and D. Bogdanovic,
"YANG Network Instances", draft-ietf-rtgwg-ni-model-01
(work in progress), October 2016.
[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,
<http://www.rfc-editor.org/info/rfc6241>.
[RFC7223] Bjorklund, M., "A YANG Data Model for Interface
Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,
<http://www.rfc-editor.org/info/rfc7223>.
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
[I-D.ietf-rtgwg-device-model], using both logical network elements
(LNE) and network instances (NI).
A.1. Physical Device
The data model for the physical device may be described by this YANG
library content:
"ietf-yang-library:modules-state": {
"module-set-id": "14e2ab5dc325f6d86f743e8d3ade233f1a61a899",
"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"
],
Bjorklund & Lhotka Expires September 7, 2017 [Page 23]
Internet-Draft YANG Schema Mount March 2017
"namespace": "urn:ietf:params:xml:ns:yang:ietf-interfaces",
"conformance-type": "implement"
},
{
"name": "ietf-ip",
"revision": "2014-06-16",
"namespace": "urn:ietf:params:xml:ns:yang:ietf-ip",
"conformance-type": "implement"
},
{
"name": "ietf-logical-network-element",
"revision": "2016-10-21",
"feature": [
"bind-lne-name"
],
"namespace":
"urn:ietf:params:xml:ns:yang:ietf-logical-network-element",
"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": "2017-03-06",
"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"
}
]
}
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:
Bjorklund & Lhotka Expires September 7, 2017 [Page 24]
Internet-Draft YANG Schema Mount March 2017
"ietf-yang-schema-mount:schema-mounts": {
"mount-point": [
{
"module": "ietf-logical-network-element",
"name": "root",
"inline": [null]
}
]
}
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": {
...
}
},
...
]
}
}
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:
"ietf-yang-library:modules-state": {
"module-set-id": "9358e11874068c8be06562089e94a89e0a392019",
"module": [
{
Bjorklund & Lhotka Expires September 7, 2017 [Page 25]
Internet-Draft YANG Schema Mount March 2017
"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": "2016-10-27",
"feature": [
"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",
Bjorklund & Lhotka Expires September 7, 2017 [Page 26]
Internet-Draft YANG Schema Mount March 2017
"revision": "2017-03-06",
"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-state": {
"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",
"prefix-length": 64
}
]
}
},
...
]
}
A.3. Network Instances
Assuming that network instances share the same data model, it can be
mounted using the "use-schema" method as follows:
Bjorklund & Lhotka Expires September 7, 2017 [Page 27]
Internet-Draft YANG Schema Mount March 2017
"ietf-yang-schema-mount:schema-mounts": {
"mount-point": [
{
"module": "ietf-network-instance",
"name": "root",
"parent-reference": ["ietf-interfaces"],
"use-schema": [
{
"name": "ni-schema"
}
]
}
],
"schema": [
{
"name": "ni-schema",
"module": [
{
"name": "ietf-ipv4-unicast-routing",
"revision": "2016-11-04",
"namespace":
"urn:ietf:params:xml:ns:yang:ietf-ipv4-unicast-routing",
"conformance-type": "implement"
},
{
"name": "ietf-ipv6-unicast-routing",
"revision": "2016-11-04",
"namespace":
"urn:ietf:params:xml:ns:yang:ietf-ipv6-unicast-routing",
"conformance-type": "implement"
},
{
"name": "ietf-routing",
"revision": "2016-11-04",
"feature": [
"multiple-ribs",
"router-id"
],
"namespace": "urn:ietf:params:xml:ns:yang:ietf-routing",
"conformance-type": "implement"
}
]
}
]
}
Note also that the "ietf-interfaces" module appears in the
"parent-reference" leaf-list for the mounted NI schema. This means
Bjorklund & Lhotka Expires September 7, 2017 [Page 28]
Internet-Draft YANG Schema Mount March 2017
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.
A.4. Invoking an RPC Operation
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
Appendix B. Open Issues
B.1. Referencing Mount Points Using Schema Node Identifiers
Each entry in the "mount-point" list is currently identified by two
keys, namely YANG module name and mount point name. An alternative
is to use a schema node identifier of the mount point as a single
key.
For example, the "schema-mounts" data for NI (Appendix A.3) would be
changed as follows (the "schema" list doesn't change):
Bjorklund & Lhotka Expires September 7, 2017 [Page 29]
Internet-Draft YANG Schema Mount March 2017
"ietf-yang-schema-mount:schema-mounts": {
"namespace": [
{
"prefix": "ni",
"ns-uri": "urn:ietf:params:xml:ns:yang:ietf-network-instance"
}
]
"mount-point": [
{
"target": "/ni:network-instances/ni:network-instance/ni:root",
"parent-reference": ["ietf-interfaces"],
"use-schema": [
{
"name": "ni-schema"
}
]
}
],
"schema": [
...
]
}
This change would have several advantages:
o the schema mount mechanism becomes even closer to augments, which
may simplify implementation
o if a mount point appears inside a grouping, then a different
mounted schema can be used for each use of the grouping.
o it optionally allows for use of mount without use of the mount-
point extension.
B.2. Defining the "mount-point" Extension in a Separate Module
The "inline" method of schema mounting can be further simplified by
defining the "inline" case as the default. That is, if a mount point
is defined through the "mount-point" extension but is not present in
the "mount-point" list, the "inline" schema mount is assumed.
Consequently, a data model that uses only the "inline" method could
omit the "schema-mounts" data entirely, but it still needs to use the
"mount-point" extension. In order to enable this, the definition of
the "mount-point" extension has to be moved to a YANG module of its
own.
Bjorklund & Lhotka Expires September 7, 2017 [Page 30]
Internet-Draft YANG Schema Mount March 2017
A variant of this approach is to completely separate the "inline" and
"use-schema" cases by dedicating the "mount-point" extension for use
with the "inline" method only (with no "schema-mounts" data), and
using schema node identifiers as described in Appendix B.1 for the
"use-schema" case.
B.3. Parent References
As explained in Section 4, references to the parent schema can only
be used in absolute leafref paths and instance identifiers. However,
it is conceivable that they may be useful in other XPath expressions,
e.g. in "must" statements. The authors believe it is impossible to
allow for parent references in general XPath expressions because, for
example, in a location path "//foo:bar" it would be unclear whether
the lookup has to be started in the mounted or parent schema.
Should parent references in general XPath be needed, it would be
necessary to indicate it explicitly. One way to achieve this is to
defining a new XPath function, e.g., parent-root(), that returns the
root of the parent data tree.
B.4. RPC Operations and Notifications in Mounted Modules
Turning RPC operations defined in mounted modules into actions tied
to the corresponding mount point (see Section 5, and similarly for
notifications) is not possible if the path to the mount point in the
parent schema contains a keyless list (Section 7.15 of [RFC7950]).
The solutions for this corner case are possible:
1. any mount point MUST NOT have a keyless list among its ancestors
2. any mounted module MUST NOT contain RPC operations and/or
notifications
3. specifically for each mount point, at least one of the above
conditions MUST be satisfied.
4. treat such actions and notifications as non-existing, i.e.,
ignore them.
The first two requirements seem rather restrictive. On the other
hand, the last one is difficult to guarantee - for example, things
can break after an augment within the mounted schema.
Bjorklund & Lhotka Expires September 7, 2017 [Page 31]
Internet-Draft YANG Schema Mount March 2017
B.5. Tree Representation
Need to decide how/if mount points are represented in trees.
B.6. Design-Time Mounts
The document currently doesn't provide explicit support for design-
time mounts. Design-time mounts have been identified as possibly for
multiple cases, and it may be worthwhile to identify a minimum or
complete set of modules that must be supported under a mount point.
This could be used in service modules that want to allow for
configuration of device-specific information. One option could be to
add an extension that specify that a certain module is required to be
mounted.
Also, if design-time mounts are supported, it could be possible to
represent both mounts points and their required modules in tree
representations and support for such would need to be defined.
Authors' Addresses
Martin Bjorklund
Tail-f Systems
Email: mbj@tail-f.com
Ladislav Lhotka
CZ.NIC
Email: lhotka@nic.cz
Bjorklund & Lhotka Expires September 7, 2017 [Page 32]