One document matched: draft-clemm-netmod-mount-02.xml
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<front>
<title abbrev="YANG-Mount">Mounting YANG-Defined Information from Remote
Datastores</title>
<author fullname="Alexander Clemm" initials="A." surname="Clemm">
<organization>Cisco Systems</organization>
<address>
<email>alex@cisco.com</email>
</address>
</author>
<author fullname="Jan Medved" initials="J." surname="Medved">
<organization>Cisco Systems</organization>
<address>
<email>jmedved@cisco.com</email>
</address>
</author>
<author fullname="Eric Voit" initials="E." surname="Voit">
<organization>Cisco Systems</organization>
<address>
<email>evoit@cisco.com</email>
</address>
</author>
<date day="7" month="October" year="2014"/>
<abstract>
<t>This document introduces capabilities that allow YANG datastores
to reference and incorporate information from remote datastores. This is
accomplished by extending YANG with the ability to define
mount points that act as references to data nodes in remote datastores,
and by providing the necessary means to manage and administer those
mount points.
This facilitates the development of applications that need to access
data that transcends individual network devices while improving
network-wide object consistency.
</t>
<t>
This document also lays the groundwork for optional extensions to support
subscriptions to remote object updates and transparent caching of objects.
These options will speed application peformance without sacrificing
data consistency.
</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>This document introduces a new capability that allows YANG datastores
<xref target="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 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.</t>
<t>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.</t>
<t>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
<xref target="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.</t>
<t>The requirements for mounting YANG subtrees from remote datastores,
as long as a set of associated use cases, are documented in <xref
target="peermount-req"/>. The ability to mount data from remote
datastores is useful to address various problems that several categories
of applications are faced with:</t>
<t>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 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.</t>
<t>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 authoritative owner of the data. The mounting system
does not even necessarily need to be aware of the specific data in the
remote subtree.</t>
<t>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 <xref
target="RFC2866"/> or DHCP <xref target="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.</t>
<t>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.</t>
<t>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 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.</t>
</section>
<section title="Definitions and Acronyms">
<t>Data node: An instance of management information in a YANG
datastore.</t>
<t>DHCP: Dynamic Host Configuration Protocol.</t>
<t>Datastore: A conceptual store of instantiated management information,
with individual data items represented by data nodes which are arranged
in hierarchical manner.</t>
<t>Data subtree: An instantiated data node and the data nodes that are
hierarchically contained within it.</t>
<t>Mount client: The system at which the mount point resides, into which
the remote subtree is mounted.</t>
<t>Mount point: A data node that receives the root node of the remote
datastore being mounted.</t>
<t>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.</t>
<t>Mount target: A remote server whose datastore is being mounted.</t>
<t>NACM: NETCONF Access Control Model</t>
<t>NETCONF: Network Configuration Protocol</t>
<t>RADIUS: Remote Authentication Dial In User Service.</t>
<t>RPC: Remote Procedure Call</t>
<t>Remote datastore: A datastore residing at a remote node.</t>
<t>URI: Uniform Resource Identifier</t>
<t>YANG: A data definition language for NETCONF</t>
</section>
<section title="Example scenarios">
<t>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.</t>
<section title="Network controller view">
<t>Network controllers can use the mounting capability to present a
consolidated view of management information across the network. This
allows network controllers to expose network-wide abstractions, such
as topologies or paths, multi-device abstractions, such as VRRP <xref
target="RFC3768"/>, and network-element specific abstractions, such as
information about a network element's interfaces.</t>
<t>While an application on top of a controller could bypass the
controller to access network elements directly for their
element-specific 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.</t>
<t>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 inconsistency and staleness. Equally importantly, it would 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.</t>
<t>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
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.</t>
<t>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.</t>
<t>This scenario is depicted in <xref target="control-mount"/>. 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.</t>
<figure anchor="control-mount"
title="Network controller mount topology">
<artwork height="23" xml:space="preserve">
+-------------+
| Network |
| Controller |
| Datastore |
| |
| +--N10 |
| +--N11 |
| +--N12 |
| +--M1*******************************
| +--M2****** *
| | * *
+-------------+ * *
* +---------------+ * +---------------+
* | +--N1 | * | +--N5 |
* | +--N2 | * | +--N6 |
********> +--MDN2 | *********> +--MDN1 |
| +--N3 | | +--N7 |
| +--N4 | | +--N8 |
| | | |
| Network | | Network |
| Element | | Element |
| Datastore | | Datastore |
+---------------+ +---------------+
</artwork>
</figure>
</section>
<section title="Distributed network configuration">
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>The scenario is depicted in <xref target="dist-mount"/>. 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.</t>
<figure anchor="dist-mount"
title="Distributed config settings topology">
<artwork height="27" xml:space="preserve">
+---------------+ +---------------+
| Network | | Network |
| Element | | Element |
| Datastore | | Datastore |
| | | |
| +--N1 | | +--N5 |
| | +--N2 | | | +--N6 |
| | +--N2 | | | +--N6 |
| | +--N3 | | | +--N7 |
| | +--N4 | | | +--N8 |
| | | | | |
| +--M1 | | +--M2 |
+-----*---------+ +-----*---------+
* * +---------------+
* * | |
* * | +--N10 |
* * | +--N11 |
*********************************************> +--MDN |
| +--N20 |
| +--N21 |
| ... |
| +--N22 |
| |
| Network |
| Controller |
| Datastore |
+---------------+
</artwork>
</figure>
</section>
</section>
<section title="Operating on mounted data">
<t>This section provides a rough illustration of the operations flow
involving mounted datastores.</t>
<section title="General principles">
<t>The first thing that should be noted about these operations flows
concerns the fact 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 authoritative 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 to make it more
efficient, the fact that another system decides to expose a certain
"view" of that data is fundamentally not the remote system's
concern.</t>
<t>When a client application 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 application's
behalf. It will extract from the client application 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 authoritative owner of the mounted data,
acting itself as a client to the remote server. 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.</t>
</section>
<section title="Data retrieval">
<t>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.</t>
</section>
<section title="Data modification">
<t>Requests that involve editing of information and "writing through"
to remote systems are potentially more complicated, particularly if
transactions and locking across multiple configuration items are
involved. However, these cases are not our primary concern at this
time. Data modifications that involve mounted information need to
supported only in the following cases: <list style="symbols">
<t>When the scope of the operation falls within a single
mountpoint. In that case, the data modification request (e.g.
edit-config) results is directly passed through to the mount
server. The mount client acts as a direct pass-through.</t>
<t>When the modification involves no locking and no rollback, i.e.
"best effort" semantics. In that case, the scope of the operation
may extend beyond a single mountpoint.</t>
</list> This functionality is entirely sufficient for most use cases
that need to be addressed. As outlined in <xref
target="peermount-req"/>, the aim for peer mount are use cases for
which eventual consistency is sufficient and that do not require
transactional consistency. As a result, the implementation is greatly
simplified. Support for network-wide transactions and locking in
conjunction with mount is not required. Servers MAY reject
configuration requests involving commits and rollbacks, where the
request involve datastore subtrees which include mount points below
the root of the subtree. That said, it is conceivable to introduce in
the future a special capability in which servers indicate that they
provide such support.</t>
<t>By the same token, lock operations that extend across multiple
datastores do not need to be supported. Lock requests on subtrees that
include mount points MAY be rejected. That said, it is conceivable to
introduce in the future a capability indicating that such a capability
is supported. In order to perform a lock operation on a subtree that
contains mount points, a server will need itself to obtain a lock from
each of the respective remote mount servers before confirming the
lock. If a lock cannot be obtained within a stringent timeout
interval, the lock request will need to be denied and any locks that
were already obtained released.</t>
</section>
<section title="RPCs">
<t>YANG-Mount is aimed at data nodes in datastores. At this point, it
does not extend towards RPCs that are defined as part of YANG modules
whose contents is being mounted. Support for RPCs involving mounted
portions of the datastore is for further study.</t>
</section>
<section title="Notifications">
<t>YANG-Mount does not extend towards notifications. It is conceivable
to offer such support in the future; however, at this point
notification support involving mounted data nodes is for further
study.</t>
</section>
<section title="Other considerations">
<t>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). Mountpoint
management will be discussed in section <xref
target="mountpoint-management"/>.</t>
<t>It is expected that implementations will introduce caching schemes.
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. Caching is not required for
YANG-mount to work - in which case all access to mounted information
is "on-demand", in which the authoritative data node always gets
accessed. Whether to perform caching is a local implementation
decision. However, when caching is introduced, it can benefit
from additional standardization, specifically the ability to
subscribe to updates on remote data by remote servers.
Some such optimizations to facilitate caching support will be
discussed in section <xref target="caching-support"/>.</t>
</section>
</section>
<section title="Data model structure">
<section title="YANG mountpoint extensions">
<t>At the center of the module is a set of YANG extensions that allow
to define a mountpoint. <list style="symbols">
<t>The first extension, "mountpoint", is used to declare a
mountpoint. The extension takes the name of the mountpoint as an
argument.</t>
<t>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
and address a remote server, such as an IP address, a host name,
or a URI <xref target="RFC3986"/>.</t>
<t>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.</t>
</list> 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.</t>
<t>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 possible
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, when data from lists or leaf-lists
is to be mounted,
a container containing the list respectively leaf-list SHOULD be
mounted instead of individual list elements.</t>
<t>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
mountpoint. Whenever a mount operation is performed, this condition
MUST be validated by the mount client.</t>
</section>
<section title="YANG structure diagrams">
<t>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'.</t>
<figure align="center">
<artwork align="left">
rw network
+-- rw nodes
+-- rw node [node-ID]
+-- rw node-ID
+-- M node-system-info
</artwork>
</figure>
</section>
<section anchor="mountpoint-management" title="Mountpoint management">
<t>The YANG module contains facilities to manage the mountpoints
themselves.</t>
<t>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).</t>
<t>In addition to the list of mountpoints, a set of global mount
policy settings allows to set parameters such as mount retries and
timeouts.</t>
<t>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.</t>
<t>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.</t>
<t>Whether mounting occurs automatically or needs to be manually
configured by a user or an application can depend on the mountpoint
being defined, i.e. the semantics of the model.</t>
<t>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.</t>
<t>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. Manual
mounting can also be used to override automatic mounting, for example
to allow an administrator to set up or remove a mountpoint.</t>
<t>It should be noted that mountpoint management does not allow users
to manually "extend" the model, i.e. simply add a subtree underneath
some arbitrary data node into a datastore, without a supporting
mountpoint defined in the model to support it. A mountpoint definition
is a formal part of the model with well-defined semantics.
Accordingly, mountpoint management does not allow users to dynamically
"extend" the data model itself. It allows users to populate the
datastore and mount structure within the confines of a model that has
been defined prior.</t>
<t>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.</t>
<figure align="center">
<artwork align="left">
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
| | +-- (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
</artwork>
</figure>
</section>
<section anchor="caching-support" title="Caching">
<t>Under certain circumstances, it can be useful to maintain a cache
of remote information. Instead of accessing the remote system,
requests are served from a copy that is locally maintained. This is
particularly advantageous in cases where data is slow changing, i.e.
when there are many more "read" operations than changes to the
underlying data node, and in cases when a significant delay were
incurred when accessing the remote system, which might be prohibitive
for certain applications. Examples of such applications are
applications that involve real-time control loops requiring response
times that are measured in milliseconds.</t>
<t>Caching can in principle apply to both retrieval and modification
operations. However, as data nodes that are mounted from an
authoritative datastore represent the "golden copy", it is important
that any modifications are reflected as soon as they are made.
Likewise, typical applications that operate on YANG datastores will
not apply high frequency changes to the same data nodes. For those
reasons, the focus in the following is on caching for data retrieval
purposes. Caching for operations that involve change operations are in
the following not considered.</t>
<t>It is a local implementation decision of mount clients whether to
cache information once it has been fetched. However, in order to
support more powerful caching schemes, it becomes necessary for the
mount server to "push" information proactively. This means that at
this point, the mount server is no longer oblivious to the fact that a
mount client exists.</t>
<t>For this purpose, we are planning in a subsequent revision to
introduce caching extensions. The following outlines what these
extensions will entail.</t>
<t>The first set of extensions concern the mount client. We are adding
an extension to mountpoint management that allows a mount client to
define a specific binding type for a given mount point. A mount
binding specifies how the client wishes to have information from a
remote system populated. The following binding types are defined:
<list style="symbols">
<t>On-demand. This is the "default" binding. No caching is
applied. Information is always retrieved from the remote
datastore, whenever a client application requests it.</t>
<t>Periodic. In this case, the mounted data is updated
periodically. The interval in which updates are to take place can
be parametrized.</t>
<t>On-change. In this case, mounted data is updated whenever a
change is detected. In order to reduce the risk of churn in the
case of fast-changing data, a dampening interval can be specified,
indicating the minimum time that must pass between updates.
Further extensions can allow to specify the magnitude or size a
change must indicate in order to be reported.</t>
</list> The second set of extensions concern the mount server.
NETCONF and RESTconf are fundamentally request-response based
protocols. In order to support periodic and, even more so, on-change
binding types, it is advantageous if the remote server supports a
mechanism that allows a mount client to subscribe to data in a
datastore subtree and then have that data be automatically delivered
without requiring further requests. Certainly, resorting to polling
should be avoided!
There are different mechanisms
conceivable for this, such as the support of information push or
publish/subscribe.</t>
<t>Data subscription mechanisms can be of interest beyond YANG-Mount.
However, at this point, such a mechanism has not yet been defined. The
following outlines one way in which this can be achieved.</t>
<t>One way in which this can be achieved is through simple NETCONF
notifications and a special data subscription function, whose
configuration can be expressed through YANG itself.</t>
<t>The notification contains several parameters: <list style="symbols">
<t>A subscription correlator, referencing the name of the
subscription on whose behalf the notification is sent.</t>
<t>A data node that contains a representation of the datastore
subtree. (This can be simply a node of type string or, for
XML-based encoding, anyxml.)</t>
</list> The configuration of the subscription in turn contains
several parameters as well: <list style="symbols">
<t>The root of the data subtree being subscribed to</t>
<t>The identity of the subscriber(s)</t>
<t>The subscription type: periodic or on change</t>
<t>For periodic subscriptions: the start time and interval with
which to push updates</t>
<t>For change-based subscriptions: the dampening interval with
which to push repeated changes, an indicator for the magnitude of
changes, etc</t>
</list></t>
</section>
<section title="Other considerations">
<section title="Authorization">
<t>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)
<xref target="RFC6536"/>.</t>
<t>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
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.</t>
<t>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.</t>
</section>
<section title="Datastore qualification">
<t>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.</t>
<t>It is conceivable to use mount in conjunction with ephemeral
datastores, to address requirements outlined in <xref
target="draft-haas-i2rs-netmod-netconf-requirements"/>. Support for
such a scheme is for further study and may be included in a future
revision of this spec.</t>
</section>
<section title="Local mounting">
<t>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.</t>
</section>
<section title="Mount cascades">
<t>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.</t>
</section>
<section anchor="implementation-considerations"
title="Implementation considerations">
<t>Implementation specifics are outside the scope of this
specification. That said, the following considerations apply:</t>
<t>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: <list style="symbols">
<t>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
<xref target="I-D.ietf-netconf-restconf"/>. <!-- and <xref
target="I-D.lhotka-netmod-yang-json"/>
--> Either way, to the remote system, the mount client constitutes
essentially a client application like any other. The mount
client in effect IS a special kind of client application.</t>
<t>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.</t>
<t>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.</t>
</list> 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.</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="Modeling best practices">
<t>
There is a certain amount of overhead associated with each mount point.
The mount point needs to be managed and state maintained.
Data subscriptions need to be maintained.
Requests including mounted subtrees need to be decomposed and responses from
multiple systems combined.
</t>
<t>
For those reasons, as a general best practice, models that make use of mount points
SHOULD be defined in a way that minimizes the number of mountpoints required.
Finely granular mounts, in which multiple mountpoints are maintained with the
same remote system, each containing only very small data subtrees, SHOULD
be avoided.
For example, lists SHOULD only contain mountpoints when individual list elements
are associated with different remote systems. To mount data from lists in remote
datastores, a container node that contains all list elements SHOULD be mounted
instead of mounting each list element individually.
Likewise, instead of having mount points refer to nodes contained underneath choices,
a mountpoint should refer to a container of the choice.
</t>
</section>
</section>
</section>
<section title="Datastore mountpoint YANG module">
<t><figure>
<artwork>
<CODE BEGINS>
file "mount@2014-10-07.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: Juergen Schoenwaelder
j.schoenwaelder@jacobs-university.de
WG Chair: Tom Nadeau
tnadeau@lucidvision.com
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 2014-10-07 {
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
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
"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
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 {
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
"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;
}
}
container global-mount-policies {
uses mount-policies;
description
"Provides mount policies applicable for all mountpoints,
unless overridden for a specific mountpoint.";
}
}
}
<CODE ENDS>
</artwork>
</figure></t>
</section>
<section title="Security Considerations">
<t>TBD</t>
</section>
<section title="Acknowledgements">
<t>We wish to acknowledge the helpful contributions, comments, and
suggestions that were received from Tony Tkacik, Ambika Tripathy, Robert Varga,
Prabhakara Yellai, Shashi Kumar Bansal, Lukas Sedlak, and Benoit Claise.</t>
</section>
</middle>
<back>
<references title="Normative References">
&RFC2131;
&RFC2866;
&RFC3768;
&RFC3986;
&RFC6020;
&RFC6241;
&RFC6536;
</references>
<references title="Informative References">
&I-D.draft-ietf-netconf-restconf;
<reference anchor="draft-haas-i2rs-netmod-netconf-requirements">
<front>
<title>I2RS Requirements for Netmod/Netconf</title>
<author fullname="Jeff Haas" initials="J" surname="Haas">
<organization/>
</author>
<date day="12" month="September" year="2014"/>
</front>
<seriesInfo name="Internet-Draft"
value="draft-haas-i2rs-netmod-netconf-requirements-02"/>
<format target="http://www.ietf.org/internet-drafts/draft-haas-i2rs-netmod-netconf-requirements-02.txt"
type="TXT"/>
</reference>
<reference anchor="peermount-req">
<front>
<title>Requirements for Peer Mounting of YANG subtrees from Remote
Datastores</title>
<author fullname="Eric Voit" initials="E" surname="Voit">
<organization/>
</author>
<author fullname="Alexander Clemm" initials="A" surname="Clemm">
<organization/>
</author>
<author fullname="Shashi Kumar Bansal" initials="S" surname="Bansal">
<organization/>
</author>
<author fullname="Ambika Tripathy" initials="A" surname="Tripathy">
<organization/>
</author>
<author fullname="Prabhakara Yellai" initials="P" surname="Yellai">
<organization/>
</author>
<date day="25" month="September" year="2014"/>
</front>
<seriesInfo name="Internet-Draft"
value="draft-voit-netmod-peer-mount-requirements-00"/>
<format target="http://www.ietf.org/internet-drafts/draft-voit-netmod-peer-mount-requirements-00.txt"
type="TXT"/>
</reference>
<!--
&I-D.draft-lhotka-netmod-yang-json;
-->
<!--
&I-D.ietf-netmod-interfaces-cfg;
-->
</references>
<section title="Example">
<t>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</t>
<figure>
<artwork>
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
</artwork>
</figure>
<t>The controller network model consists of the following key
components: <list style="symbols">
<t>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</t>
<t>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. <!-- that is defined in the YANG interface module
<xref target="I-D.netmod-interfaces-cfg"/>
--> 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</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
</list></t>
<t>The following is a YANG snippet of the module definition which makes
use of the mountpoint definition.</t>
<figure>
<artwork>
<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>
</artwork>
</figure>
<t>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).</t>
<t>Interface information from NE1 datastore:</t>
<figure>
<artwork>
<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>
</artwork>
</figure>
<t>NC datastore with mounted interface information from NE1 and NE2:</t>
<figure>
<artwork>
<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>
</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>
</artwork>
</figure>
</section>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-24 01:34:24 |