--- 1/draft-ietf-netmod-schema-mount-03.txt 2017-03-06 06:13:24.476853888 -0800 +++ 2/draft-ietf-netmod-schema-mount-04.txt 2017-03-06 06:13:24.540855388 -0800 @@ -1,19 +1,19 @@ Network Working Group M. Bjorklund Internet-Draft Tail-f Systems Intended status: Standards Track L. Lhotka -Expires: May 4, 2017 CZ.NIC - October 31, 2016 +Expires: September 7, 2017 CZ.NIC + March 6, 2017 YANG Schema Mount - draft-ietf-netmod-schema-mount-03 + 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. @@ -21,237 +21,614 @@ 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 May 4, 2017. + This Internet-Draft will expire on September 7, 2017. Copyright Notice - Copyright (c) 2016 IETF Trust and the persons identified as the + 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 - 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2 - 1.1.1. Tree Diagrams . . . . . . . . . . . . . . . . . . . . 2 - 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3 - 3. Schema Mount . . . . . . . . . . . . . . . . . . . . . . . . 4 - 3.1. Augment and Validation in Mounted Data . . . . . . . . . 4 - 3.2. Top-level RPCs . . . . . . . . . . . . . . . . . . . . . 4 - 3.3. Top-level Notifications . . . . . . . . . . . . . . . . . 5 - 4. Data Model . . . . . . . . . . . . . . . . . . . . . . . . . 5 - 5. Schema Mount YANG Module . . . . . . . . . . . . . . . . . . 6 - 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 - 7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 - 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 12 - 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 - 9.1. Normative References . . . . . . . . . . . . . . . . . . 12 - 9.2. Informative References . . . . . . . . . . . . . . . . . 13 - Appendix A. Example: Logical Devices . . . . . . . . . . . . . . 14 - Appendix B. Example: Network Manager with Fixed Device Models . 16 - Appendix C. Example: Network Manager with Arbitrary Device - Models . . . . . . . . . . . . . . . . . . . . . . . 19 - C.1. Invoking an RPC . . . . . . . . . . . . . . . . . . . . . 23 - Appendix D. Open Issues . . . . . . . . . . . . . . . . . . . . 23 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 + 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 -1.1. Terminology + 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", + 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: + + 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. + + 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]. -1.1.1. Tree Diagrams + 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 + 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. Background +2.3. Namespace Prefixes - YANG has two mechanisms for extending a data model with additional - nodes; "uses" and "augment". The "uses" statement explicitly - incorporates the contents of a "grouping" defined in some other - module. The "augment" statement explicitly adds contents to a target - node defined in some other module. In both these cases, the source - and/or target model explicitly defines the relationship between the - models. + 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 Table 1. - In some cases these mechanisms are not sufficient. For example, - suppose we have a model like ietf-interfaces [RFC7223] that is - defined to be implemented in a device. Now suppose we want to model - a device that supports multiple logical devices - [I-D.rtgyangdt-rtgwg-device-model], where each such logical device - has its own instantiation of ietf-interfaces (and other models), but - at the same time, we'd like to be able to manage all these logical - devices from the main device. We would like something like this: + +---------+------------------------+-----------+ + | 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 + 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 + 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, + "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). + + 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: +--rw interfaces | +--rw interface* [name] | ... - +--rw logical-device* [name] - +--rw name string - | ... - +--rw interfaces - +--rw interface* [name] + +--rw network-instances + +--rw network-instance* [name] + +--rw name + +--rw root + +--rw routing ... - With the "uses" approach, ietf-interfaces would have to define a - grouping with all its nodes, and the new model for logical devices - would have to use this grouping. This is a not a scalable solution, - since every time there is a new model defined, we would have to - update our model for logical devices to use a grouping from the new - model. Another problem is that this approach cannot handle vendor- - specific modules. - - 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 on the top-level. This approach is also not - scalable, since there may be other models to which we would like to - add the interface list. - -3. Schema Mount + 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). - The schema mount mechanism defined in this document takes a different - approach to the extensibility problem described in the previous - section. It decouples the definition of the relation between the - source and target models from the definitions of the models - themselves. + 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: - This is accomplished with a YANG extension statement that is used to - specify a mount point in a data model. The purpose of a mount point - is to define a place in the node hierarchy where other YANG data - models may be attached, without any special notation in the other - YANG data models. Only "anydata" nodes can be used as mount points. + 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. - For each mount point supported by a server, the server populates an - operational state node hierarchy with information about which models - it has mounted. This node hierarchy can be read by a client in order - to learn what is implemented on a server. + o Other rules for the "leafref" and "instance-identifier" types as + defined in Sections 9.9 and 9.13 of [RFC7950] remain in effect. - Schema mount applies to the data model, and specifically does not - assume anything about how the mounted data is implemented. 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. + 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. - This document allows mounting of complete data models only. Other - specifications may extend this model by defining additional - mechanisms, for example mounting of sub-hierarchies of a module. +5. RPC operations and Notifications -3.1. Augment and Validation in Mounted Data + 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. - All paths (in leafrefs, instance-identifiers, XPath expressions, and - target nodes of augments) in the data models mounted at a mount point - are interpreted with the mount point as the root node, and the - mounted data nodes as its children. This means that data within a - mounted subtree can never refer to data outside of this subtree. + 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]. -3.2. Top-level RPCs +6. Implementation Notes - If any mounted data model defines RPCs, these RPCs can be invoked by - clients by treating them as actions defined where the mount point is - specified. An example of this is given in Appendix C.1. + 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: -3.3. Top-level Notifications + o shared management: instance data of both parent and mounted + schemas are accessible within the same management session. - If the server emits a notification defined at the top-level in any - mounted data model, it is treated as if the notification was attached - to the data node where the mount point is specified. + 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. -4. Data Model +7. Data Model This document defines the YANG 1.1 module [RFC7950] "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 ns-uri? inet:uri +--ro mount-point* [module name] | +--ro module yang:yang-identifier | +--ro name yang:yang-identifier - | +--ro (subschema-ref)? + | +--ro config? boolean + | +--ro (schema-ref)? | +--:(inline) | | +--ro inline? empty | +--:(use-schema) | +--ro use-schema* [name] - | +--ro name -> /schema-mounts/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 (subschema-ref)? + +--ro config? boolean + +--ro (schema-ref)? +--:(inline) | +--ro inline? empty +--:(use-schema) +--ro use-schema* [name] - +--ro name -> /schema-mounts/schema/name + +--ro name + | -> /schema-mounts/schema/name +--ro when? yang:xpath1.0 + +--ro parent-reference* yang:yang-identifier -5. Schema Mount YANG Module +8. Schema Mount YANG Module This module references [RFC6991] and [RFC7895]. - file "ietf-yang-schema-mount@2016-04-05.yang" + 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"; @@ -269,196 +646,227 @@ "RFC 7895: YANG Module Library"; } organization "IETF NETMOD (NETCONF Data Modeling Language) Working Group"; contact "WG Web: WG List: - WG Chair: Lou Berger - - - WG Chair: Kent Watsen - - Editor: Martin Bjorklund Editor: Ladislav Lhotka "; 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. - Copyright (c) 2016 IETF Trust and the persons identified as + 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 2016-10-26 { + 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. The name of the - mount point MUST be unique within the module where it is - defined. + "The argument 'name' is a YANG identifier, i.e., it is of the + type 'yang:yang-identifier'. - The 'mount-point' statement can only be present as a - substatement of 'anydata'. + 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. Note that this - implies that such a grouping can be used at most once in a - module. + 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 + 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 subschema for a - particular mount point. + "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', + - 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."; } - choice subschema-ref { + leaf config { + type boolean; + default "true"; description - "Alternative way for specifying the subschema."; + "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 { + 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 - subschema defined in the /schema-mounts/schema list. The - entry can be made conditional by specifying an XPath - expression in the 'when' leaf."; + "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 anydata node containing the mount + 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 anydata instance containing - the corresponding 'mount-point' statement. + - 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 anydata + the parent schema, i.e., all instances of data 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"; + config false; description - "Contains information about the structure of the overall data - model implemented in the server."; + "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."; @@ -478,65 +886,74 @@ - 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 entry."; + "Arbitrary name of the schema entry."; } uses yanglib:module-list; uses mount-point-list; } } } -6. IANA Considerations +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 -7. Security Considerations +10. Security Considerations TBD -8. Contributors +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]); Ladislav Lhotka - ([I-D.lhotka-netmod-ysdl]); and Lou Berger and Christian Hopps. + ([I-D.clemm-netmod-mount]); and Lou Berger and Christian Hopps: -9. References + o Lou Berger, LabN Consulting, L.L.C., -9.1. Normative References + o Alexander Clemm, Huawei, + + o Christian Hopps, Deutsche Telekom, + + o Jan Medved, Cisco, + + o Eric Voit, Cisco, + +12. References + +12.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, January 2004, . @@ -550,463 +967,466 @@ . [RFC7895] Bierman, A., Bjorklund, M., and K. Watsen, "YANG Module Library", RFC 7895, DOI 10.17487/RFC7895, June 2016, . [RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, August 2016, . -9.2. Informative References +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.lhotka-netmod-ysdl] - Lhotka, L., "YANG Schema Dispatching Language", draft- - lhotka-netmod-ysdl-00 (work in progress), November 2015. + [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.rtgyangdt-rtgwg-device-model] + [I-D.ietf-rtgwg-device-model] Lindem, A., Berger, L., Bogdanovic, D., and C. Hopps, - "Network Device YANG Organizational Models", draft- - rtgyangdt-rtgwg-device-model-05 (work in progress), August - 2016. + "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. + + [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, . [RFC7223] Bjorklund, M., "A YANG Data Model for Interface Management", RFC 7223, DOI 10.17487/RFC7223, May 2014, . - [RFC7277] Bjorklund, M., "A YANG Data Model for IP Management", - RFC 7277, DOI 10.17487/RFC7277, June 2014, - . - - [RFC7317] Bierman, A. and M. Bjorklund, "A YANG Data Model for - System Management", RFC 7317, DOI 10.17487/RFC7317, August - 2014, . +Appendix A. Example: Device Model with LNEs and NIs -Appendix A. Example: Logical Devices + 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). - Logical devices within a device typically use the same set of data - models in each instance. This can be modelled with a mount point: +A.1. Physical Device - module example-logical-devices { - yang-version 1.1; - namespace "urn:example:logical-devices"; - prefix exld; + The data model for the physical device may be described by this YANG + library content: - import ietf-yang-schema-mount { - prefix yangmnt; + "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" + ], + "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" } - - container logical-devices { - list logical-device { - key name; - leaf name { - type string; + ] } - anydata root { - yangmnt:mount-point logical-device; - } - } - } - } +A.2. Logical Network Elements - A server with two logical devices that both implement - "ietf-interfaces" [RFC7223], "ietf-ip" [RFC7277], and "ietf-system" - [RFC7317] YANG modules might populate the "schema-mounts" container - with: + 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: - - - example-logical-devices - logical-device - - logical-device - - - - logical-device - - ietf-interface - 2014-05-08 - - urn:ietf:params:xml:ns:yang:ietf-interfaces - - implement - - - ietf-ip - 2014-06-16 - - urn:ietf:params:xml:ns:yang:ietf-ip - - implement - - - ietf-system - 2014-08-06 - - urn:ietf:params:xml:ns:yang:ietf-system - - implement - - - ietf-yang-types - 2013-07-15 - - urn:ietf:params:xml:ns:yang:ietf-yang-types - - import - - - + "ietf-yang-schema-mount:schema-mounts": { + "mount-point": [ + { + "module": "ietf-logical-network-element", + "name": "root", + "inline": [null] + } + ] + } - and the "logical-devices" container might have: + An administrator of the host device has to configure an entry for + each LNE instance, for example, - - - vrtrA - - - - eth0 - - true - ... - - ... - - - - ... - - - - - vrtrB - - - - eth0 - - true - ... - - ... - - - + { + "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": { ... - - - - - -Appendix B. Example: Network Manager with Fixed Device Models - - This example shows how a Network Manager application can use schema - mount to define a data model for a network consisting of devices - whose data models are known a priori and fixed. - - Assume for simplicity that only two device types are used (switch and - router), and they are identified by identities defined in the module - "example-device-types": - - module example-device-types { - namespace "http://example.org/device-types"; - prefix edt; - identity device-type; - identity switch-device { - base device-type; } - identity router-device { - base device-type; + }, + ... + ] } } - Schema mount is used to mount the device data models conditionally, - depending on the "type" leaf that is a sibling of the mount point. - This approach is similar to "ietf-interfaces" [RFC7223] where the - same effect is achieved via conditional augments. - - The top-level module may look as follows: + and then also place necessary state data as the contents of the + "root" instance, which should include at least - module example-network-manager-fixed { - yang-version 1.1; - namespace "urn:example:network-manager-fixed"; - prefix exf; + o YANG library data specifying the LNE's data model, for example: - import ietf-inet-types { - prefix inet; - } - import ietf-yang-schema-mount { - prefix yangmnt; + "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": "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", + "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" } - import example-device-types { - prefix edt; + ] } - container managed-devices { - description - "The managed devices and device communication settings."; + o state data for interfaces assigned to the LNE instance (that + effectively become system-controlled interfaces for the LNE), for + example: - list device { - key name; - leaf name { - type string; - } - leaf type { - type identityref { - base edt:device-type; - } + "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 } - container transport { - choice protocol { - mandatory true; - container netconf { - leaf address { - type inet:ip-address; - mandatory true; + ] } - container authentication { - // ... + }, + ... + ] } + +A.3. Network Instances + + Assuming that network instances share the same data model, it can be + mounted using the "use-schema" method as follows: + + "ietf-yang-schema-mount:schema-mounts": { + "mount-point": [ + { + "module": "ietf-network-instance", + "name": "root", + "parent-reference": ["ietf-interfaces"], + "use-schema": [ + { + "name": "ni-schema" } - container restconf { - leaf address { - type inet:ip-address; - mandatory true; + ] } - // ... + ], + "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" } + ] } + ] } - anydata root { - yangmnt:mount-point managed-device; + + 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. + +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): + + "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": [ + ... + ] } - The "schema-mounts" container may have the following data: + This change would have several advantages: - - - edt - http://example.org/device-types - - - example-network-manager - managed-device - - switch - derived-from-or-self(../type, 'edt:switch-device') - - - router - derived-from-or-self(../type, 'edt:router-device') - - - - switch - - ... - - ... - - - router - - ... - - ... - - + o the schema mount mechanism becomes even closer to augments, which + may simplify implementation - The "devices" list may contain any number of instances of either - type. + o if a mount point appears inside a grouping, then a different + mounted schema can be used for each use of the grouping. -Appendix C. Example: Network Manager with Arbitrary Device Models + o it optionally allows for use of mount without use of the mount- + point extension. - This example shows how a Network Manager application can use schema - mount to define a data model for a network consisting of devices - whose data models are not known in advance -- each device is expected - to provide its data model dynamically. +B.2. Defining the "mount-point" Extension in a Separate Module - Schema mount is used to mount the data models that each device - supports, and these data models can be discovered by inspecting state - data under the corresponding mount point. Every such device must - therefore implement "ietf-yang-library" and optionally - "ietf-schema-mount". + 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. - module example-network-manager-arbitrary { - yang-version 1.1; - namespace "urn:example:network-manager-arbitrary"; - prefix exa; + 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. - import ietf-inet-types { - prefix inet; - } - import ietf-yang-schema-mount { - prefix yangmnt; - } + 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. - container managed-devices { - description - "The managed devices and device communication settings."; +B.3. Parent References - list device { - key name; - leaf name { - type string; - } - container transport { - choice protocol { - mandatory true; - container netconf { - leaf address { - type inet:ip-address; - mandatory true; - } - container authentication { - // ... - } - } - container restconf { - leaf address { - type inet:ip-address; - mandatory true; - } - // ... - } - } - } - anydata root { - yangmnt:mount-point managed-device; - } - } - } - } - The "schema-mounts" container may have the following data: + 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. - - - example-network-manager - managed-device - - - + 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. - The "devices" container might have: +B.4. RPC Operations and Notifications in Mounted Modules - - - rtrA - - -
2001:db8::2
- - ... - - ... - - - - - - ietf-system - ... - - - - ... - - - - - rtrB - - -
2001:db8::3
- - ... - - ... - + 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: - - - - - ietf-interfaces - ... - - - - ... - - - - + 1. any mount point MUST NOT have a keyless list among its ancestors -C.1. Invoking an RPC + 2. any mounted module MUST NOT contain RPC operations and/or + notifications - A client that wants to invoke the "restart" operation [RFC7317] on - the managed device "rtrA" over NETCONF [RFC6241] can send: + 3. specifically for each mount point, at least one of the above + conditions MUST be satisfied. - - - - - rtrA - - - - - - - - - + 4. treat such actions and notifications as non-existing, i.e., + ignore them. -Appendix D. Open Issues + 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. - o Is the 'mount-point' extension really needed? Now that mount - points can only appear under anydata nodes, there seems to be - little need to otherwise restrict mount point locations. In the - 'mount-point' list, schema node identifiers (as in 'augment' - statements) can be used instead of the (module, name) pair for - identifying mount points. As a useful side effect, a grouping - containing mount points could be used any number of times in the - same module. OTOH, by using this extension, the intention of the - data modeller is clear, and it provides a formal machine readable - instruction about where mounts are allowed to occur. +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: mbj@lhotka@nic.cz + Email: lhotka@nic.cz