Network Working Group                                           A. Clemm
Internet-Draft                                                 J. Medved
Intended status: Experimental                                    E. Voit
Expires: March 26, 2014                                    Cisco Systems
                                                      September 22, 2013


        Mounting YANG-Defined Information from Remote Datastores
                    draft-clemm-netmod-mount-01.txt

Abstract

   This document introduces a new capability that allows YANG datastores
   to reference and incorporate information from remote datastores.
   This is accomplished using a new YANG data model that allows to
   define and manage datastore mount points that reference data nodes in
   remote datastores.  The data model includes a set of YANG extensions
   for the purposes of declaring such mount points.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on March 26, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definitions and Acronyms  . . . . . . . . . . . . . . . . . .   5
   3.  Example scenarios . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Network controller view . . . . . . . . . . . . . . . . .   6
     3.2.  Distributed network configuration . . . . . . . . . . . .   8
   4.  Operating on mounted data . . . . . . . . . . . . . . . . . .   9
   5.  Data model structure  . . . . . . . . . . . . . . . . . . . .  10
     5.1.  YANG mountpoint extensions  . . . . . . . . . . . . . . .  10
     5.2.  Mountpoint management . . . . . . . . . . . . . . . . . .  11
     5.3.  YANG structure diagrams . . . . . . . . . . . . . . . . .  13
     5.4.  Other considerations  . . . . . . . . . . . . . . . . . .  13
       5.4.1.  Authorization . . . . . . . . . . . . . . . . . . . .  13
       5.4.2.  Datastore qualification . . . . . . . . . . . . . . .  14
       5.4.3.  Local mounting  . . . . . . . . . . . . . . . . . . .  14
       5.4.4.  Mount cascades  . . . . . . . . . . . . . . . . . . .  14
       5.4.5.  Implementation considerations . . . . . . . . . . . .  15
   6.  Datastore mountpoint YANG module  . . . . . . . . . . . . . .  16
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  23
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  23
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  23
   Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   This document introduces a new capability that allows YANG datastores
   [RFC6020] to incorporate and reference information from remote
   datastores.  This is provided by introducing a mountpoint concept.
   This concept allows to declare a YANG data node as a "mount point",
   under which a remote datastore subtree can be mounted.  To the user



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   of the primary datastore, the remote information appears as an
   integral part of the datastore.  It allows remote data nodes and
   datastore subtrees to be inserted into the local data hierarchy,
   arranged below local data nodes.  The concept is reminiscent of
   concepts in a Network File System that allows to mount remote folders
   and make them appear as if they were contained in the local file
   system of the user's machine.

   The ability to mount information from remote datastores is new and
   not covered by existing YANG mechanisms.  Until now, management
   information provided in a datastore has been intrinsically tied to
   the same server.  In contrast, the capability introduced here allows
   the server to represent information from remote systems as if it were
   its own and contained in its own local data hierarchy.

   YANG does provide means by which modules that have been separately
   defined can reference and augment one another.  YANG also does
   provide means to specify data nodes that reference other data nodes.
   However, all the data is assumed to be instantiated as part of the
   same datastore, for example a datastore provided through a NETCONF
   server [RFC6241].  Existing YANG mechanisms do not account for the
   possibility that some information that needs to be referred not only
   resides in a different subtree of the same datastore, or was defined
   in a separate module that is also instantiated in the same datastore,
   but that is genuinely part of a different datastore that is provided
   by a different server.

   The ability to mount data from remote datastores is useful to address
   various problems that several categories of applications are faced
   with:

   One category of applications that can leverage this capability
   concerns network controller applications that need to present a
   consolidated view of management information in datastores across a
   network.  Controller applications are faced with the problem that in
   order to expose information, that information needs to be part of
   their own datastore.  Today, this requires support of a corresponding
   YANG data module.  In order to expose information that concerns other
   network elements, that information has to be replicated into the
   controller's own datastore in the form of data nodes that may mirror
   but are clearly distinct from corresponding data nodes in the network
   element's datastore.  In addition, in many cases, a controller needs
   to impose its own hierarchy on the data that is different from the
   one that was defined as part of the original module.  An example for
   this concerns interface configuration data, which would be contained
   in a top-level container in a network element datastore, but may need
   to be contained in a list in a controller datastore in order to be
   able to distinguish instances from different network elements under



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   the controller's scope.  This in turn would require introduction of
   redundant YANG modules that effectively replicate the same
   information save for differences in hierarchy.

   By directly mounting information from network element datastores, the
   controller does not need to replicate the same information from
   multiple datastores, nor does it need to re-define any network
   element and system-level abstractions to be able to put them in the
   context of network abstractions.  Instead, the subtree of the remote
   system is attached to the local mount point.  Operations that need to
   access data below the mount point are in effect transparently
   redirected to remote system, which is the authorative owner of the
   data.  The mounting system does not even necessarily need to be aware
   of the specific data in the remote subtree.

   A second category of applications concerns decentralized networking
   applications that require globally consistent configuration of
   parameters.  When each network element maintains its own datastore
   with the same configurable settings, a single global change requires
   modifying the same information in many network elements across a
   network.  In case of inconsistent configurations, network failures
   can result that are difficult to troubleshoot.  In many cases, what
   is more desirable is the ability to configure such settings in a
   single place, then make them available to every network element.
   Today, this requires in general the introduction of specialized
   servers and configuration options outside the scope of NETCONF, such
   as RADIUS [RFC2866] or DHCP [RFC2131].  In order to address this
   within the scope of NETCONF and YANG, the same information would have
   to be redundantly modeled and maintained, representing operational
   data (mirroring some remote server) on some network elements and
   configuration data on a designated master.  Either way, additional
   complexity ensues.

   Instead of replicating the same global parameters across different
   datastores, the solution presented in this document allows a single
   copy to be maintained in a subtree of single datastore that is then
   mounted by every network element that requires access to these
   parameters.  The global parameters can be hosted in a controller or a
   designated network element.  This considerably simplifies the
   management of such parameters that need to be known across elements
   in a network and require global consistency.

   The capability of allowing to mount information from remote
   datastores into another datastore is accomplished by a set of YANG
   extensions that allow to define such mount points.  For this purpose,
   a new YANG module is introduced.  The module defines the YANG
   extensions, as well as a data model that can be used to manage the
   mountpoints and mounting process itself.  Only the mounting module



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   and server needs to be aware of the concepts introduced here.
   Mounting is transparent to the models being mounted; any YANG model
   can be mounted.

2.  Definitions and Acronyms

   Data node: An instance of management information in a YANG datastore.

   DHCP: Dynamic Host Configuration Protocol.

   Datastore: A conceptual store of instantiated management information,
   with individual data items represented by data nodes which are
   arranged in hierarchical manner.

   Data subtree: An instantiated data node and the data nodes that are
   hierarchically contained within it.

   Mount client: The system at which the mount point resides, into which
   the remote subtree is mounted.

   Mount point: A data node that receives the root node of the remote
   datastore being mounted.

   Mount server: The server with which the mount client communicates and
   which provides the mount client with access to the mounted
   information.  Can be used synonymously with mount target.

   Mount target: A remote server whose datastore is being mounted.

   NACM: NETCONF Access Control Model

   NETCONF: Network Configuration Protocol

   RADIUS: Remote Authentication Dial In User Service.

   RPC: Remote Procedure Call

   Remote datastore: A datastore residing at a remote node.

   URI: Uniform Resource Identifier

   YANG: A data definition language for NETCONF









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3.  Example scenarios

   The following example scenarios outline some of the ways in which the
   ability to mount YANG datastores can be applied.  Other mount
   topologies can be conceived in addition to the ones presented here.

3.1.  Network controller view

   Network controllers can use the mounting capability to present a
   consolidated view of management information across the network.  This
   allows network controllers to not only expose network abstractions,
   such as topologies or paths, but also network element abstractions,
   such as information about a network element's interfaces, from one
   consolidated place.

   While an application on top of a controller could in theory also
   bypass the controller to access network elements directly for
   network-element abstractions, this would come at the expense of added
   inconvenience for the client application.  In addition, it would
   compromise the ability to provide layered architectures in which
   access to the network by controller applications is truly channeled
   through the controller.

   Without a mounting capability, a network controller would need to at
   least conceptually replicate data from network elements to provide
   such a view, incorporating network element information into its own
   controller model that is separate from the network element's,
   indicating that the information in the controller model is to be
   populated from network elements.  This can introduce issues such as
   data consistency and staleness.  Even more importantly, it would in
   general lead to the redundant definition of data models: one model
   that is implemented by the network element itself, and another model
   to be implemented by the network controller.  This leads to poor
   maintainability, as analogous information has to be redundantly
   defined and implemented across different data models.  In general,
   controllers cannot simply support the same modules as their network
   elements for the same information because that information needs to
   be put into a different context.  This leads to "node"-information
   that needs to be instantiated and indexed differently, because there
   are multiple instances across different data stores.

   For example, "system"-level information of a network element would
   most naturally placed into a top-level container at that network
   element's datastore.  At the same time, the same information in the
   context of the overall network, such as maintained by a controller,
   might better be provided in a list.  For example, the controller
   might maintain a list with a list element for each network element,
   underneath which the network element's system-level information is



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   contained.  However, the containment structure of data nodes in a
   module, once defined, cannot be changed.  This means that in the
   context of a network controller, a second module that repeats the
   same system-level information would need to be defined, implemented,
   and maintained.  Any augmentations that add additional system-level
   information to the original module will likewise need to be
   redundantly defined, once for the "system" module, a second time for
   the "controller" module.

   By allowing a network controller to directly mount information from
   network element datastores, the controller does not need to replicate
   the same information from multiple datastores.  Perhaps even more
   importantly, the need to re-define any network element and system-
   level abstractions to be able to put them in the context of network
   abstractions is avoided.  In this solution, a network controller's
   datastore mounts information from many network element datastores.
   For example, the network controller datastore could implement a list
   in which each list element contains a mountpoint.  Each mountpoint
   mounts a subtree from a different network element's datastore.

   This scenario is depicted in Figure 1.  In the figure, M1 is the
   mountpoint for the datastore in Network Element 1 and M2 is the
   mountpoint for the datastore in Network Element 2.  MDN1 is the
   mounted data node in Network Element 1, and MDN2 is the mounted data
   node in Network Element 2.

   +-------------+
   |   Network   |
   |  Controller |
   |  Datastore  |
   |             |
   | +--N10      |
   |    +--N11   |
   |    +--N12   |
   |       +--M1*******************************
   |       +--M2******                        *
   |             |   *                        *
   +-------------+   *                        *
                     *   +---------------+    *    +---------------+
                     *   | +--N1         |    *    | +--N5         |
                     *   |     +--N2     |    *    |     +--N6     |
                     ********> +--MDN2   |    *********> +--MDN1   |
                         |         +--N3 |         |         +--N7 |
                         |         +--N4 |         |         +--N8 |
                         |               |         |               |
                         |    Network    |         |    Network    |
                         |    Element    |         |    Element    |
                         |   Datastore   |         |   Datastore   |



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                         +---------------+         +---------------+

                Figure 1: Network controller mount topology

3.2.  Distributed network configuration

   A second category of applications concerns decentralized networking
   applications that require globally consistent configuration of
   parameters that need to be known across elements in a network.
   Today, the configuration of such parameters is generally performed on
   a per network element basis, which is not only redundant but, more
   importantly, error-prone.  Inconsistent configurations lead to
   erroneous network behavior that can be challenging to troubleshoot.

   Using the ability to mount information from remote datastores opens
   up a new possibility for managing such settings.  Instead of
   replicating the same global parameters across different datastores, a
   single copy is maintained in a subtree of single datastore.  This
   datastore can hosted in a controller or a designated network element.
   The subtree is subsequently mounted by every network element that
   requires access to these parameters.

   In many ways, this category of applications is an inverse of the
   previous category: Whereas in the network controller case data from
   many different datastores would be mounted into the same datastore
   with multiple mountpoints, in this case many elements, each with
   their own datastore, mount the same remote datastore, which is then
   mounted by many different systems.

   The scenario is depicted in Figure 2.  In the figure, M1 is the
   mountpoint for the Network Controller datastore in Network Element 1
   and M2 is the mountpoint for the Network Controller datastore in
   Network Element 2.  MDN is the mounted data node in the Network
   Controller datastore that contains the data nodes that represent the
   shared configuration settings.

   +---------------+         +---------------+
   |    Network    |         |    Network    |
   |    Element    |         |    Element    |
   |   Datastore   |         |   Datastore   |
   |               |         |               |
   | +--N1         |         | +--N5         |
   | |   +--N2     |         | |   +--N6     |
   | |   +--N2     |         | |   +--N6     |
   | |       +--N3 |         | |       +--N7 |
   | |       +--N4 |         | |       +--N8 |
   | |             |         | |             |
   | +--M1         |         | +--M2         |



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   +-----*---------+         +-----*---------+
         *                         *               +---------------+
         *                         *               |               |
         *                         *               | +--N10        |
         *                         *               |    +--N11     |
         *********************************************> +--MDN     |
                                                   |        +--N20 |
                                                   |        +--N21 |
                                                   |         ...   |
                                                   |        +--N22 |
                                                   |               |
                                                   |    Network    |
                                                   |   Controller  |
                                                   |   Datastore   |
                                                   +---------------+

              Figure 2: Distributed config settings topology

4.  Operating on mounted data

   This section provides a rough illustration of the operations flow
   involving mounted datastores.

   The first thing that should be noted about these operations flows
   concerns that a mount client essentially constitutes a special
   management application that interacts with a remote system.  To the
   remote system, the mount client constitutes in effect just another
   application.  The remote system is the authorative owner of the data.
   While it is conceivable that the remote system (or an application
   that proxies for the remote system) provides certain functionality to
   facilitate the specific needs of the mount client, the fact that
   another system decides to expose a certain "view" of that data is
   fundamentally not its concern.

   When a client makes a request to a server that involves data that is
   mounted from a remote system, the server will effectively act as a
   proxy to the remote system on the client's behalf.  It will extract
   from the request the portion that involves the mounted subtree from
   the remote system.  It will strip that portion of the local context,
   i.e. remove any local data paths and insert the data path of the
   mounted remote subtree, as appropriate.  The server will then forward
   the transposed request to the remote system that is the authorative
   owner of the mounted data.  Upon receiving the reply, the server will
   transpose the results into the local context as needed, for example
   map the data paths into the local data tree structure, and combine
   those results with the results of the remainder portion of the
   original request.




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   In the simplest and at the same time perhaps the most common case,
   the request will involve simple data retrieval.  In that case, a
   "get" or "get-configuration" operation might be applied on a subtree
   whose scope includes a mount point.  When resolving the mount point,
   the server issues its own "get" or "get-configuration" request
   against the remote system's subtree that is attached to the mount
   point.  The returned information is then inserted into the data
   structure that is in turn returned to the client that originally
   invoked the request.

   Requests that involve editing of information and "writing through" to
   remote systems are more complicated, particularly where they involve
   the need for transactions and locking.  While not our primary concern
   at this time, implications are briefly discussed in section
   Section 5.4.5.

   Since mounted information involves in general communication with a
   remote system, there is a possibility that the remote system does not
   respond within a certain amount of time, that connectivity is lost,
   or that other errors occur.  Accordingly, the ability to mount
   datastores also involves mountpoint management, which includes the
   ability to configure timeouts, retries, and management of mountpoint
   state (including dynamic addition removal of mountpoints).

   As a final note, it is conceivable that caching schemes are
   introduced.  Caching can increase performance and efficiency in
   certain scenarios (for example, in the case of data that is
   frequently read but that rarely changes), but increases
   implementation complexity.  Whether to perform caching is purely a
   local implementation decision.  This specification has not
   requirement that caching be introduced and makes no corresponding
   assumptions; there is no dependency on any caching scheme.

5.  Data model structure

5.1.  YANG mountpoint extensions

   At the center of the module is a set of YANG extensions that allow to
   define a mountpoint.

   o  The first extension, "mountpoint", is used to declare a
      mountpoint.  The extension takes the name of the mountpoint as an
      argument.

   o  The second extension, "target", serves as a substatement
      underneath a mountpoint statement.  It takes an argument that
      identifies the target system.  The argument is a reference to a
      data node that contains the information that is needed to identify



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      and address a remote server, such as an IP address, a host name,
      or a URI [RFC3986].

   o  The third extension, "subtree", also serves as substatement
      underneath a mountpoint statement.  It takes an argument that
      defines the root node of the datastore subtree that is to be
      mounted, specified as string that contains a path expression.

   A mountpoint MUST be contained underneath a container.  Future
   revisions might allow for mountpoints to be contained underneath
   other data nodes, such as lists, leaf-lists, and cases.  However, to
   keep things simple, at this point mounting is only allowed directly
   underneath a container.

   Only a single data node can be mounted at one time.  While the mount
   target could refer to any data node, it is recommended that as a best
   practice, the mount target SHOULD refer to a container.  It is
   possibly to maintain e.g. a list of mount points, with each mount
   point each of which has a mount target an element of a remote list.
   However, to avoid unnecessary proliferation of the number of mount
   points and associated management overhead, in order to mount lists or
   leaf-lists, a container containing the list respectively leaf-list
   SHOULD be mounted.

   It is possible for a mounted datastore to contain another mountpoint,
   thus leading to several levels of mount indirections.  However,
   mountpoints MUST NOT introduce circular dependencies.  In particular,
   a mounted datastore MUST NOT contain a mountpoint which specifies the
   mounting datastore as a target and a subtree which contains as root
   node a data node that in turn contains the original mountpoint.
   Whenever a mount operation is performed, this condition MUST be
   validated by the mount client.

5.2.  Mountpoint management

   The YANG module contains facilities to manage the mountpoints
   themselves.

   For this purpose, a list of the mountpoints is introduced.  Each list
   element represents a single mountpoint.  It includes an
   identification of the mount target, i.e. the remote system hosting
   the remote datastore and a definition of the subtree of the remote
   data node being mounted.  It also includes monitoring information
   about current status (indicating whether the mount has been
   successful and is operational, or whether an error condition applies
   such as the target being unreachable or referring to an invalid
   subtree).




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   In addition to the list of mountpoints, a set of global mount policy
   settings allows to set parameters such as mount retries and timeouts.

   Each mountpoint list element also contains a set of the same
   configuration knobs, allowing administrators to override global mount
   policies and configure mount policies on a per-mountpoint basis if
   needed.

   There are two ways how mounting occurs: automatic (dynamically
   performed as part of system operation) or manually (administered by a
   user or client application).  A separate mountpoint-origin object is
   used to distinguish between manually configured and automatically
   populated mountpoints.

   When configured automatically, mountpoint information is
   automatically populated by the datastore that implements the
   mountpoint.  The precise mechanisms for discovering mount targets and
   bootstrapping mount points are provided by the mount client
   infrastructure and outside the scope of this specification.
   Likewise, when a mountpoint should be deleted and when it should
   merely have its mount-status indicate that the target is unreachable
   is a system-specific implementation decision.

   Manual mounting consists of two steps.  In a first step, a mountpoint
   is manually configured by a user or client application through
   administrative action.  Once a mountpoint has been configured, actual
   mounting occurs through an RPCs that is defined specifically for that
   purpose.  To unmount, a separate RPC is invoked; mountpoint
   configuration information needs to be explicitly deleted.

   The structure of the mountpoint management data model is depicted in
   the following figure, where brackets enclose list keys, "rw" means
   configuration, "ro" operational state data, and "?" designates
   optional nodes.  Parantheses enclose choice and case nodes.  The
   figure does not depict all definitions; it is intended to illustrate
   the overall structure.

          rw mount-server-mgmt
          +-- rw mountpoints
          |   +-- rw mountpoint [mountpoint-id]
          |       +-- rw mountpoint-id  string
          |       +-- rw mount-target
          |       |   +--: (IP)
          |       |   |    +-- rw target-ip  yang:ip-address
          |       |   +--: (URI)
          |       |   |    +-- rw uri  yang:uri
          |       |   +--: (host-name)
          |       |   |    +-- rw hostname  yang:host



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          |       |   +-- (node-ID)
          |       |   |    +-- rw node-info-ref  mnt:subtree-ref
          |       |   +-- (other)
          |       |        +-- rw opaque-target-id  string
          |       +-- rw subtree-ref  mnt:subtree-ref
          |       +-- ro mountpoint-origin enumeration
          |       +-- ro mount-status  mnt:mount-status
          |       +-- rw manual-mount? empty
          |       +-- rw retry-timer? uint16
          |       +-- rw number-of-retries? uint8
          +-- rw global-mount-policies
              +-- rw manual-mount? empty
              +-- rw retry-time? uint16
              +-- rw number-of-retries? uint8


5.3.  YANG structure diagrams

   YANG data model structure overviews have proven very useful to convey
   the "Big Picture".  It would be useful to indicate in YANG data model
   structure overviews the fact that a given data node serves as a
   mountpoint.  We propose for this purpose also a corresponding
   extension to the structure representation convention.  Specifically,
   we propose to prefix the name of the mounting data node with upper-
   case 'M'.

                      rw network
                      +-- rw nodes
                          +-- rw node [node-ID]
                              +-- rw node-ID
                              +-- M node-system-info


5.4.  Other considerations

5.4.1.  Authorization

   Whether a mount client is allowed to modify information in a mounted
   datastore or only retrieve it and whether there are certain data
   nodes or subtrees within the mounted information for which access is
   restricted is subject to authorization rules.  To the mounted system,
   a mounting client will in general appear like any other client.
   Authorization privileges for remote mounting clients need to be
   specified through NACM (NETCONF Access Control Model) [RFC6536].

   Users and implementers need to be aware of certain issues when
   mounted information is modified, not just retrieved.  Specifically,
   in certain corner cases validation of changes made to mounted data



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   may involve constraints that involve information that is not visible
   to the mounting datastore.  This means that in such cases the reason
   for validation failures may not always be fully understood by the
   mounting system.

   Likewise, if the concepts of transactions and locking are applied at
   the mounting system, these concepts will need to be applied across
   multiple systems, not just across multiple data nodes within the same
   system.  This capability may not be supported by every
   implementation.  For example, locking a datastore that contains a
   mountpoint requires that the mount client obtains corresponding locks
   on the mounted datastore as needed.  Any request to acquire a lock on
   a configuration subtree that includes a mountpoint MUST NOT be
   granted if the mount client fails to obtain a corresponding lock on
   the mounted system.  Likewise, in case transactions are supported by
   the mounting system, but not the target system, requests to acquire a
   lock on a configuration subtree that includes a mountpoint MUST NOT
   be granted.

5.4.2.  Datastore qualification

   It is conceivable to differentiate between different datastores on
   the remote server, that is, to designate the name of the actual
   datastore to mount, e.g. "running" or "startup".  However, for the
   purposes of this spec, we assume that the datastore to be mounted is
   generally implied.  Mounted information is treated as analogous to
   operational data; in general, this means the running or "effective"
   datastore is the target.  That said, the information which targets to
   mount does constitute configuration and can hence be part of a
   startup or candidate datastore.

5.4.3.  Local mounting

   It is conceivable that the mount target does not reside in a remote
   datastore, but that data nodes in the same datastore as the
   mountpoint are targeted for mounting.  This amounts to introducing an
   "aliasing" capability in a datastore.  While this is not the scenario
   that is primarily targeted, it is supported and there may be valid
   use cases for it.

5.4.4.  Mount cascades










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   It is possible for the mounted subtree to in turn contain a
   mountpoint.  However, circular mount relationships MUST NOT be
   introduced.  For this reason, a mounted subtree MUST NOT contain a
   mountpoint that refers back to the mounting system with a mount
   target that directly or indirectly contains the originating
   mountpoint.  As part of a mount operation, the mount points of the
   mounted system need to be checked accordingly.

5.4.5.  Implementation considerations

   Implementation specifics are outside the scope of this specification.
   That said, the following considerations apply:

   Systems that wish to mount information from remote datastores need to
   implement a mount client.  The mount client communicates with a
   remote system to access the remote datastore.  To do so, there are
   several options:

   o  The mount client acts as a NETCONF client to a remote system.
      Alternatively, another interface to the remote system can be used,
      such as a REST API using JSON encodings, as specified in
      [I-D.bierman-netconf-restconf].  Either way, to the remote system,
      the mount client constitutes essentailly a client application like
      any other.  The mount client in effect IS a special kind of client
      application.

   o  The mount client communicates with a remote mount server through a
      separate protocol.  The mount server is deployed on the same
      system as the remote NETCONF datastore and interacts with it
      through a set of local APIs.

   o  The mount client communicates with a remote mount server that acts
      as a NETCONF client proxy to a remote system, on the client's
      behalf.  The communication between mount client and remote mount
      server might involve a separate protocol, which is translated into
      NETCONF operations by the remote mount server.

   It is the responsibility of the mount client to manage the
   association with the target system, e.g. validate it is still
   reachable by maintaining a permanent association, perform
   reachability checks in case of a connectionless transport, etc.










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   It is the responsibility of the mount client to manage the
   mountpoints.  This means that the mount client needs to populate the
   mountpoint monitoring information (e.g. keep mount-status up to data
   and determine in the case of automatic mounting when to add and
   remove mountpoint configuration).  In the case of automatic mounting,
   the mount client also interacts with the mountpoint discovery and
   bootstrap process.

   The mount client needs to also participate in servicing datastore
   operations involving mounted information.  An operation requested
   involving a mountpoint is relayed by the mounting system's
   infrastructure to the mount client.  For example, a request to
   retrieve information from a datastore leads to an invocation of an
   internal mount client API when a mount point is reached.  The mount
   client then relays a corresponding operation to the remote datastore.
   It subsequently relays the result along with any responses back to
   the invoking infrastructure, which then merges the result (e.g. a
   retrieved subtree with the rest of the information that was
   retrieved) as needed.  Relaying the result may involve the need to
   transpose error response codes in certain corner cases, e.g. when
   mounted information could not be reached due to loss of connectivity
   with the remote server, or when a configuration request failed due to
   validation error.

6.  Datastore mountpoint YANG module

   <CODE BEGINS>
   file "mount@2013-09-22.yang"
   module mount {
       namespace "urn:cisco:params:xml:ns:yang:mount";
       // replace with IANA namespace when assigned

       prefix mnt;

       import ietf-yang-types {
           prefix yang;
       }

       organization
           "IETF NETMOD (NETCONF Data Modeling Language) Working Group";

       contact
           "WG Web: http://tools.ietf.org/wg/netmod/
           WG List: netmod@ietf.org

           WG Chair: David Kessens
           david.kessens@nsn.com




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           WG Chair: Juergen Schoenwaelder
           j.schoenwaelder@jacobs-university.de

           Editor: Alexander Clemm
           alex@cisco.com";

       description
           "This module provides a set of YANG extensions and definitions
            that can be used to mount information from remote datastores.";

       revision 2013-09-22 {
           description "Initial revision.";
       }

       feature mount-server-mgmt {
           description
               "Provide additional capabilities to manage remote mount
                points";
       }

       extension mountpoint {
           description
               "This YANG extension is used to mount data from a remote
               system in place of the node under which this YANG extension
               statement is used.

               This extension takes one argument which specifies the name
               of the mountpoint.

               This extension can occur as a substatement underneath a
               container statement, a list statement, or a case statement.
               As a best practice, it SHOULD occur as statement only
               underneath a container statement, but it MAY also occur
               underneath a list or a case statement.

               The extension takes two parameters, target and subtree, each
               defined as their own YANG extensions.
               A mountpoint statement MUST contain a target and a subtree
               substatement for the mountpoint definition to be valid.

               The target system MAY be specified in terms of a data node
               that uses the grouping 'mnt:mount-target'.  However, it
               can be specified also in terms of any other data node that
               contains sufficient information to address the mount target,
               such as an IP address, a host name, or a URI.

               The subtree SHOULD be specified in terms of a data node of
               type 'mnt:subtree-ref'. The targeted data node MUST



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               represent a container.

               It is possible for the mounted subtree to in turn contain a
               mountpoint.  However, circular mount relationships MUST NOT
               be introduced. For this reason, a mounted subtree MUST NOT
               contain a mountpoint that refers back to the mounting system
               with a mount target that directly or indirectly contains the
               originating mountpoint.";

           argument "name";
       }

       extension target {
           description
               "This YANG extension is used to specify a remote target
                system from which to mount a datastore subtree.  This YANG
                extension takes one argument which specifies the remote
                system. In general, this argument will contain the name of
                a data node that contains the remote system information. It
                is recommended that the reference data node uses the
                mount-target grouping that is defined further below in this
                module.

                This YANG extension can occur only as a substatement below
                a mountpoint statement. It MUST NOT occur as a substatement
                below any other YANG statement.";

           argument "target-name";
       }

       extension subtree {
           description
               "This YANG extension is used to specify a subtree in a
               datastore that is to be mounted.  This YANG extension takes
               one argument which specifies the path to the root of the
               subtree. The root of the subtree SHOULD represent an
               instance of a YANG container.  However, it MAY represent
               also another data node.

               This YANG extension can occur only as a substatement below
               a mountpoint statement. It MUST NOT occur as a substatement
               below any other YANG statement.";

           argument "subtree-path";
       }

       typedef mount-status {
           description



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               "This type is used to represent the status of a
               mountpoint.";
           type enumeration {
               enum ok; {
                   description
                       "Mounted";
               }
               enum no-target {
                   description
                       "The argument of the mountpoint does not define a
                        target system";
               }
               enum no-subtree {
                   description
                       "The argument of the mountpoint does not define a
                         root of a subtree";
               }
               enum target-unreachable {
                   description
                       "The specified target system is currently
                        unreachable";
               }
               enum mount-failure {
                   description
                       "Any other mount failure";
               }
               enum unmounted {
                   description
                       "The specified mountpoint has been unmounted as the
                       result of a management operation";
               }
           }
       }
       typedef subtree-ref {
           type string;  // string pattern to be defined
           description
               "This string specifies a path to a datanode. It corresponds
               to the path substatement of a leafref type statement.  Its
               syntax needs to conform to the corresponding subset of the
               XPath abbreviated syntax. Contrary to a leafref type,
               subtree-ref allows to refer to a node in a remote datastore.
               Also, a subtree-ref refers only to a single node, not a list
               of nodes.";
       }
       rpc mount {
           description
               "This RPC allows an application or administrative user to
               perform a mount operation.  If successful, it will result in



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               the creation of a new mountpoint.";
           input {
               leaf mountpoint-id {
                   type string {
                       length "1..32";
                   }
               }
           }
           output {
               leaf mount-status {
                   type mount-status;
               }
           }
       }
       rpc unmount {
           "This RPC allows an application or administrative user to
           unmount information from a remote datastore.  If successful,
           the corresponding mountpoint will be removed from the
           datastore.";
           input {
               leaf mountpoint-id {
                   type string {
                       length "1..32";
                   }
               }
           }
           output {
               leaf mount-status {
                   type mount-status;
               }
           }
       }
       grouping mount-monitor {
           leaf mount-status {
               description
                   "Indicates whether a mountpoint has been successfully
                   mounted or whether some kind of fault condition is
                   present.";
               type mount-status;
               config false;
           }
       }
       grouping mount-target {
           description
               "This grouping contains data nodes that can be used to
               identify a remote system from which to mount a datastore
               subtree.";
           container mount-target {



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               choice target-address-type {
                   mandatory;
                   case IP {
                       leaf target-ip {
                           type yang:ip-address;
                       }
                   case URI {
                       leaf uri {
                           type yang:uri;
                       }
                   }
                   case host-name {
                       leaf hostname {
                           type yang:host;
                       }
                   }
                   case node-ID {
                       leaf node-info-ref {
                           type subtree-ref;
                       }
                   }
                   case other {
                       leaf opaque-target-ID {
                           type string;
                           description
                               "Catch-all; could be used also for mounting
                               of data nodes that are local.";
                       }
                   }
               }
           }
       }
       grouping mount-policies {
           description
               "This grouping contains data nodes that allow to configure
               policies associated with mountpoints.";
           leaf manual-mount {
               type empty;
               description
                   "When present, a specified mountpoint is not
                   automatically mounted when the mount data node is
                   created, but needs to mounted via specific RPC
                   invocation.";
           }
           leaf retry-timer {
               type uint16;
               units "seconds";
               description



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                   "When specified, provides the period after which
                   mounting will be automatically reattempted in case of a
                   mount status of an unreachable target";
           }
           leaf number-of-retries {
               type uint8;
               description
                   "When specified, provides a limit for the number of
                   times for which retries will be automatically
                   attempted";
           }
       }

       container mount-server-mgmt {
           if-feature mount-server-mgmt;
           container mountpoints {
               list mountpoint {
                   key "mountpoint-id";

                   leaf mountpoint-id {
                       type string {
                           length "1..32";
                       }
                   }
                   leaf mountpoint-origin {
                       type enumeration {
                           enum client {
                               description
                                   "Mountpoint has been supplied and is
                                   manually administered by a client";
                           }
                           enum auto {
                               description
                                   "Mountpoint is automatically
                                    administered by the server";
                           }
                       config false;
                       }
                   }
                   uses mount-target;
                   leaf subtree-ref {
                       type subtree-ref;
                       mandatory;
                   }
                   uses mount-monitor;
                   uses mount-policies;
               }
           }



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           container global-mount-policies {
               uses mount-policies;
               description
                   "Provides mount policies applicable for all mountpoints,
                   unless overridden for a specific mountpoint.";
           }
       }
   }
   <CODE ENDS>


7.  Security Considerations

   TBD

8.  Acknowledgements

   We wish to acknowledge the helpful contributions, comments, and
   suggestions that were received from Tony Tkacik, Robert Varga, Lukas
   Sedlak, and Benoit Claise.

9.  References

9.1.  Normative References

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, March 1997.

   [RFC2866]  Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [RFC6020]  Bjorklund, M., "YANG - A Data Modeling Language for the
              Network Configuration Protocol (NETCONF)", RFC 6020,
              October 2010.

   [RFC6241]  Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
              Bierman, "Network Configuration Protocol (NETCONF)", RFC
              6241, June 2011.

   [RFC6536]  Bierman, A. and M. Bjorklund, "Network Configuration
              Protocol (NETCONF) Access Control Model", RFC 6536, March
              2012.

9.2.  Informative References




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   [I-D.bierman-netconf-restconf]
              Bierman, A., Bjorklund, M., Watsen, K., and R. Fernando,
              "RESTCONF Protocol", draft-bierman-netconf-restconf-01
              (work in progress), September 2013.

Appendix A.  Example

   In the following example, we are assuming the use case of a network
   controller that wants to provide a controller network view to its
   client applications.  This view needs to include network abstractions
   that are maintained by the controller itself, as well as certain
   information about network devices where the network abstractions tie
   in with element-specific information.  For this purpose, the network
   controller leverages the mount capability specified in this document
   and presents a fictitious Controller Network YANG Module that is
   depicted in the outlined structure below.  The example illustrates
   how mounted information is leveraged by the mounting datastore to
   provide an additional level of information that ties together network
   and device abstractions, which could not be provided otherwise
   without introducing a (redundant) model to replicate those device
   abstractions

   rw controller-network
   +-- rw topologies
   |   +-- rw topology [topo-id]
   |       +-- rw topo-id                 node-id
   |       +-- rw nodes
   |       |   +-- rw node [node-id]
   |       |       +-- rw node-id         node-id
   |       |       +-- rw supporting-ne   network-element-ref
   |       |       +-- rw termination-points
   |       |           +-- rw term-point [tp-id]
   |       |               +-- tp-id      tp-id
   |       |               +-- ifref      mountedIfRef
   |       +-- rw links
   |           +-- rw link [link-id]
   |               +-- rw link-id         link-id
   |               +-- rw source          tp-ref
   |               +-- rw dest            tp-ref
   +-- rw network-elements
       +-- rw network-element [element-id]
           +-- rw element-id              element-id
           +-- rw element-address
           |   +-- ...
           +-- M interfaces






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   The controller network model consists of the following key
   components:

   o  A container with a list of topologies.  A topology is a graph
      representation of a network at a particular layer, for example, an
      IS-IS topology, an overlay topology, or an Openflow topology.
      Specific topology types can be defined in their own separate YANG
      modules that augment the controller network model.  Those
      augmentations are outside the scope of this example

   o  An inventory of network elements, along with certain information
      that is mounted from each element.  The information that is
      mounted in this case concerns interface configuration information.
      For this purpose, each list element that represents a network
      element contains a corresponding mountpoint.  The mountpoint uses
      as its target the network element address information provided in
      the same list element

   o  Each topology in turn contains a container with a list of nodes.
      A node is a network abstraction of a network device in the
      topology.  A node is hosted on a network element, as indicated by
      a network-element leafref.  This way, the "logical" and "physical"
      aspects of a node in the network are cleanly separated.

   o  A node also contains a list of termination points that terminate
      links.  A termination point is implemented on an interface.
      Therefore, it contains a leafref that references the corresponding
      interface configuration which is part of the mounted information
      of a network element.  Again, the distinction between termination
      points and interfaces provides a clean separation between logical
      concepts at the network topology level and device-specific
      concepts that are instantiated at the level of a network element.
      Because the interface information is mounted from a different
      datastore and therefore occurs at a different level of the
      containment hierarchy than it would if it were not mounted, it is
      not possible to use the interface-ref type that is defined in YANG
      data model for interface management [] to allow the termination
      point refer to its supporting interface.  For this reason, a new
      type definition "mountedIfRef" is introduced that allows to refer
      to interface information that is mounted and hence has a different
      path.

   o  Finally, a topology also contains a container with a list of
      links.  A link is a network abstraction that connects nodes via
      node termination points.  In the example, directional point-to-
      point links are depicted in which one node termination point
      serves as source, another as destination.




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   The following is a YANG snippet of the module definition which makes
   use of the mountpoint definition.

















































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   <CODE BEGINS>
   module controller-network {
       namespace "urn:cisco:params:xml:ns:yang:controller-network";
       // example only, replace with IANA namespace when assigned
       prefix cn;
       import mount {
           prefix mnt;
       }
       import interfaces {
           prefix if;
       }
       ...
       typedef mountedIfRef {
           type leafref {
               path "/cn:controller-network/cn:network-elements/"
               +"cn:network-element/cn:interfaces/if:interface/if:name";
               //  cn:interfaces corresponds to the mountpoint
           }
       }
       ...
       list termination-point {
           key "tp-id";
           ...
           leaf ifref {
               type mountedIfRef;
           }
           ...
           list network-element {
               key "element-id";
               leaf element-id {
                   type element-ID;
               }
               container element-address {
                   ... // choice definition that allows to specify
                   // host name,
                   // IP addresses, URIs, etc
               }
               mnt:mountpoint "interfaces" {
                   mnt:target "./element-address";
                   mnt:subtree "/if:interfaces";
               }
           ...
       }
   ...
   <CODE ENDS>






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   Finally, the following contains an XML snippet of instantiated YANG
   information.  We assume three datastores: NE1 and NE2 each have a
   datastore (the mount targets) that contains interface configuration
   data, which is mounted into NC's datastore (the mount client).

   Interface information from NE1 datastore:

   <interfaces>
     <interface>
       <name>fastethernet-1/0</name>
       <name>ethernetCsmacd</type>
       <location>1/0</location>
     </interface>
     <interface>
       <name>fastethernet-1/1</name>
       <name>ethernetCsmacd</type>
       <location>1/1</location>
     </interface>
   <interfaces>

   Interface information from NE2 datastore:
   <interfaces>
     <interface>
       <name>fastethernet-1/0</name>
       <name>ethernetCsmacd</type>
       <location>1/0</location>
     </interface>
     <interface>
       <name>fastethernet-1/2</name>
       <name>ethernetCsmacd</type>
       <location>1/2</location>
     </interface>
   <interfaces>


   NC datastore with mounted interface information from NE1 and NE2:

   <controller-network>
     ...
     <network-elements>
       <network-element>
         <element-id>NE1</element-id>
         <element-address> .... </element-address>
         <interfaces>
           <if:interface>
             <if:name>fastethernet-1/0</if:name>
             <if:type>ethernetCsmacd</if:type>
             <if:location>1/0</if:location>



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           </if:interface>
           <if:interface>
             <if:name>fastethernet-1/1</if:name>
             <if:type>ethernetCsmacd</if:type>
             <if:location>1/1</if:location>
           </if:interface>
         <interfaces>
       </network-element>
       <network-element>
         <element-id>NE2</element-id>
         <element-address> .... </element-address>
         <interfaces>
           <if:interface>
             <if:name>fastethernet-1/0</if:name>
             <if:type>ethernetCsmacd</if:type>
             <if:location>1/0</if:location>
           </if:interface>
           <if:interface>
             <if:name>fastethernet-1/2</if:name>
             <if:type>ethernetCsmacd</if:type>
             <if:location>1/2</if:location>
           </if:interface>
         <interfaces>
       </network-element>
     </network-elements>
     ...
   </controller-network>


Authors' Addresses

   Alexander Clemm
   Cisco Systems

   EMail: alex@cisco.com


   Jan Medved
   Cisco Systems

   EMail: jmedved@cisco.com


   Eric Voit
   Cisco Systems

   EMail: evoit@cisco.com




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