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 YANG Schema Mount draft-ietf-netmod-schema-mount-03 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 May 4, 2017. Copyright Notice Copyright (c) 2016 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 May 4, 2017 [Page 1] Internet-Draft YANG Schema Mount October 2016 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 1. Introduction 1.1. Terminology 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 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. Bjorklund & Lhotka Expires May 4, 2017 [Page 2] Internet-Draft YANG Schema Mount October 2016 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 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 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: +--rw interfaces | +--rw interface* [name] | ... +--rw logical-device* [name] +--rw name string | ... +--rw interfaces +--rw interface* [name] ... 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. Bjorklund & Lhotka Expires May 4, 2017 [Page 3] Internet-Draft YANG Schema Mount October 2016 3. Schema Mount 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. 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. 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. 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. 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. 3.1. Augment and Validation in Mounted Data 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. 3.2. Top-level RPCs 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. Bjorklund & Lhotka Expires May 4, 2017 [Page 4] Internet-Draft YANG Schema Mount October 2016 3.3. Top-level Notifications 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. 4. Data Model This document defines the YANG 1.1 module [RFC7950] "ietf-yang-schema-mount", which has the following structure: Bjorklund & Lhotka Expires May 4, 2017 [Page 5] Internet-Draft YANG Schema Mount October 2016 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)? | +--:(inline) | | +--ro inline? empty | +--:(use-schema) | +--ro use-schema* [name] | +--ro name -> /schema-mounts/schema/name | +--ro when? yang:xpath1.0 +--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)? +--:(inline) | +--ro inline? empty +--:(use-schema) +--ro use-schema* [name] +--ro name -> /schema-mounts/schema/name +--ro when? yang:xpath1.0 5. Schema Mount YANG Module This module references [RFC6991] and [RFC7895]. file "ietf-yang-schema-mount@2016-04-05.yang" Bjorklund & Lhotka Expires May 4, 2017 [Page 6] Internet-Draft YANG Schema Mount October 2016 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: 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. Bjorklund & Lhotka Expires May 4, 2017 [Page 7] Internet-Draft YANG Schema Mount October 2016 Copyright (c) 2016 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 { 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 'mount-point' statement can only be present as a substatement of 'anydata'. 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. 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 Bjorklund & Lhotka Expires May 4, 2017 [Page 8] Internet-Draft YANG Schema Mount October 2016 /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 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."; } choice subschema-ref { description "Alternative way for specifying the subschema."; leaf inline { type empty; description "This leaf indicates that the server has mounted 'ietf-yang-library' and 'ietf-schema-mount' at the mount Bjorklund & Lhotka Expires May 4, 2017 [Page 9] Internet-Draft YANG Schema Mount October 2016 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."; 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 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 accessible tree contains only data belonging to the parent schema, i.e., all instances of anydata 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."; } } Bjorklund & Lhotka Expires May 4, 2017 [Page 10] Internet-Draft YANG Schema Mount October 2016 } } } /* * State data nodes */ container schema-mounts { config "false"; description "Contains information about the structure of the overall 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 entry."; } Bjorklund & Lhotka Expires May 4, 2017 [Page 11] Internet-Draft YANG Schema Mount October 2016 uses yanglib:module-list; uses mount-point-list; } } } 6. 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 TBD 8. 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. 9. References 9.1. 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, . Bjorklund & Lhotka Expires May 4, 2017 [Page 12] Internet-Draft YANG Schema Mount October 2016 [RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, January 2004, . [RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, DOI 10.17487/RFC6020, October 2010, . [RFC6991] Schoenwaelder, J., Ed., "Common YANG Data Types", RFC 6991, DOI 10.17487/RFC6991, July 2013, . [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 [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.rtgyangdt-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. [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, . Bjorklund & Lhotka Expires May 4, 2017 [Page 13] Internet-Draft YANG Schema Mount October 2016 [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: Logical Devices Logical devices within a device typically use the same set of data models in each instance. This can be modelled with a mount point: module example-logical-devices { yang-version 1.1; namespace "urn:example:logical-devices"; prefix exld; import ietf-yang-schema-mount { prefix yangmnt; } container logical-devices { list logical-device { key name; leaf name { type string; } anydata root { yangmnt:mount-point logical-device; } } } } 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: Bjorklund & Lhotka Expires May 4, 2017 [Page 14] Internet-Draft YANG Schema Mount October 2016 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 and the "logical-devices" container might have: Bjorklund & Lhotka Expires May 4, 2017 [Page 15] Internet-Draft YANG Schema Mount October 2016 vrtrA eth0 true ... ... ... vrtrB eth0 true ... ... ... 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. Bjorklund & Lhotka Expires May 4, 2017 [Page 16] Internet-Draft YANG Schema Mount October 2016 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: module example-network-manager-fixed { yang-version 1.1; namespace "urn:example:network-manager-fixed"; prefix exf; import ietf-inet-types { prefix inet; } import ietf-yang-schema-mount { prefix yangmnt; } import example-device-types { prefix edt; } container managed-devices { description "The managed devices and device communication settings."; list device { key name; leaf name { type string; } leaf type { Bjorklund & Lhotka Expires May 4, 2017 [Page 17] Internet-Draft YANG Schema Mount October 2016 type identityref { base edt:device-type; } } 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: Bjorklund & Lhotka Expires May 4, 2017 [Page 18] Internet-Draft YANG Schema Mount October 2016 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 ... ... The "devices" list may contain any number of instances of either type. Appendix C. Example: Network Manager with Arbitrary 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 not known in advance -- each device is expected to provide its data model dynamically. 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 Bjorklund & Lhotka Expires May 4, 2017 [Page 19] Internet-Draft YANG Schema Mount October 2016 therefore implement "ietf-yang-library" and optionally "ietf-schema-mount". Bjorklund & Lhotka Expires May 4, 2017 [Page 20] Internet-Draft YANG Schema Mount October 2016 module example-network-manager-arbitrary { yang-version 1.1; namespace "urn:example:network-manager-arbitrary"; prefix exa; import ietf-inet-types { prefix inet; } import ietf-yang-schema-mount { prefix yangmnt; } container managed-devices { description "The managed devices and device communication settings."; 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; } } } } Bjorklund & Lhotka Expires May 4, 2017 [Page 21] Internet-Draft YANG Schema Mount October 2016 The "schema-mounts" container may have the following data: example-network-manager managed-device The "devices" container might have: rtrA
2001:db8::2
... ...
ietf-system ... ...
rtrB
2001:db8::3
... ...
Bjorklund & Lhotka Expires May 4, 2017 [Page 22] Internet-Draft YANG Schema Mount October 2016
ietf-interfaces ... ...
C.1. Invoking an RPC A client that wants to invoke the "restart" operation [RFC7317] on the managed device "rtrA" over NETCONF [RFC6241] can send: rtrA Appendix D. Open Issues 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 Bjorklund & Lhotka Expires May 4, 2017 [Page 23] Internet-Draft YANG Schema Mount October 2016 data modeller is clear, and it provides a formal machine readable instruction about where mounts are allowed to occur. Authors' Addresses Martin Bjorklund Tail-f Systems Email: mbj@tail-f.com Ladislav Lhotka CZ.NIC Email: mbj@lhotka@nic.cz Bjorklund & Lhotka Expires May 4, 2017 [Page 24]