One document matched: draft-voit-netmod-peer-mount-requirements-01.xml
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<front>
<title abbrev="Peer Mount Requirements">Requirements for Peer Mounting of
YANG subtrees from Remote Datastores</title>
<author fullname="Eric Voit" initials="E." surname="Voit">
<organization>Cisco Systems</organization>
<address>
<email>evoit@cisco.com</email>
</address>
</author>
<author fullname="Alex Clemm" initials="A" surname="Clemm">
<organization>Cisco Systems</organization>
<address>
<email>alex@cisco.com</email>
</address>
</author>
<author fullname="Sander Mertens" initials="S." surname="Mertens">
<organization>Prismtech</organization>
<address>
<email>sander.mertens8@gmail.com</email>
</address>
</author>
<date day="24" month="October" year="2014"/>
<area>Operations & Management</area>
<workgroup>NETCONF Data Modeling Language Working Group
(netmod)</workgroup>
<keyword>Draft</keyword>
<abstract>
<t>Network integrated applications want simple ways to access YANG
objects and subtrees which might be distributed across network.
Performance requirements may dictate that it is unaffordable for a
subset of these applications to go through existing centralized
management brokers. For such applications, development complexity must
be minimized. Specific aspects of complexity developers want to ignore
include:</t>
<t><list style="symbols">
<t>whether authoritative information is actually sourced from remote
datastores (as well as how to get to those datastores),</t>
<t>whether such information has been locally cached or not,</t>
<t>whether there are zero, one, or more controllers asserting
ownership of information, and</t>
<t>whether there are interactions with other applications
concurrently running elsewhere</t>
</list> The solution requirements described in this document detail
what is needed to support application access to authoritative network
YANG objects from controllers (star) or peering network devices (mesh)
in such a way to meet these goals.</t>
</abstract>
</front>
<middle>
<section title="Business Problem">
<t>Instrumenting Physical and Virtual Network Elements purely along
device boundaries is insufficient for today’s requirements.
Instead, users, applications, and operators are asking for the ability
to interact with varying subsets of network information at the highest
viable level of abstraction. Likewise applications that run locally on
devices may require access to data that transcends the boundaries of the
device they are deployed. Achieving this can be difficult since a
running network is comprised of a distributed mesh of object ownership.
(I.e., the authoritative device owning a particular object will vary.)
Solutions require the transparent assembly of different objects from
across a network in order to provide consolidated, time synchronized,
and consistent views required for that abstraction.</t>
<t>Recent approaches have focused on a Network Controller as the arbiter
of new network-wide abstractions. Controller based solutions are
supportable by requirements outlined in this document. However this is
not the only deployment model covered by this document. Equally valid
are deployment models where Network Elements exchange information in a
way which allows one or more of those Elements to provide the desired
network level abstraction. This is not a new idea. Examples of Network
Element based protocols which already do network level abstractions
include <xref target="RFC3768">VRRP</xref>, mLACP/ICCP<xref
target="ICCP"/>, and <xref target="RFC4610">Anycast-RP</xref> . As
network elements increase their compute power and support Linux based
compute virtualization, we should expect additional local applications
to emerge as well (such as <eref
target="http://thomaswdinsmore.com/2014/05/01/distributed-analytics-primer/">Distributed
Analytics</eref>).</t>
<t>Ultimately network application programming must be simplified. To do
this:</t>
<t><list style="symbols">
<t>we must provide APIs to both controller and network element based
applications in a way which allows access to network objects as if
they were coming from a cloud,</t>
<t>we must enable these local applications to interact with network
level abstractions,</t>
<t>we must hide the mesh of interdependencies and consistency
enforcement mechanisms between devices which will underpin a
particular abstraction,</t>
<t>we must enable flexible deployment models, in which applications
are able to run not only on controller and OSS frameworks but also
on network devices without requiring heavy middleware with large
footprints, and</t>
<t>we need to maintain clear authoritative ownership of individual
data items while not burdening applications with the need to
reconcile and synchronize information replicated in different
systems, nor needing to maintain redundant data models that operate
on the same underlying data.</t>
</list>These steps will eliminate much unnecessary overhead currently
required of today’s network programmer.</t>
<t/>
</section>
<section title="Terminology">
<t/>
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref
target="RFC2119">RFC 2119</xref>.</t>
<t>Authoritative Datastore – A datastore containing the
authoritative copy of an object, i.e. the source and the
“owner” of the object.</t>
<t>Client Datastore – a datastore containing an object whose
source and “owner” is a remote datastore.</t>
<t>Data Node - An instance of management information in a YANG
datastore.</t>
<t>Datastore - A conceptual store of instantiated 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 on or more remote subtrees may be mounted.</t>
<t>Mount Binding - An instance of mounting from a specific Mount Point
to a remote datastore. Types include:</t>
<t><list style="symbols">
<t>On-demand: Mount Client only pulls information when application
requests</t>
<t>Periodic: Mount Server pushes current state at a pre-defined
interval</t>
<t>Unsolicited: Mount Server maintains active bindings and sends to
client cache upon change</t>
</list>Mount Point - Point in the local data store which may reference
a single remote subtree</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>Peer Mount - The act of representing remote objects in the local
datastore</t>
<t>Target Data Node - Data Node on Mount Server against which a Mount
Binding is established</t>
<t/>
</section>
<section title="Solution Context">
<t>YANG modeling has emerged as a preferred way to offer network
abstractions. The requirements in this document can be enabled by
expanding of the syntax of YANG capabilities embodied within <xref
target="RFC6020">RFC 6020</xref> and YANG 1.1 <xref
target="rfc6020bis"/>. A companion draft to this one which details a
potential set of YANG technology extensions which can support key
requirements within this document are contained in . <xref
target="draft-clemm-mount"/>.</t>
<t>To date systems built upon YANG models have been missing two
capabilities:</t>
<t><list style="numbers">
<t>Peer Datastore Mount: Datastores have not been able to proxy
objects located elsewhere. This puts additional burden upon
applications which then need to find and access multiple
(potentially remote) systems.</t>
<t>Eventual Consistency: YANG Datastore implementations have
typically assumed <eref
target="http://en.wikipedia.org/wiki/ACID ">ACID</eref> transaction
models. There is nothing inherent in YANG itself which demands ACID
transactional guarantees. YANG models can also expose information
which might be in the process of undergoing convergence. Since IP
networking has been designed with convergence in mind, this is a
useful capability since some types of applications must participate
where there is dynamically changing state.</t>
</list></t>
<section title="Peer Mount">
<t>First this document will dive deeper into Peer Datastore Mount
(a.k.a., “Peer Mount”). Contrary to existing YANG
datastores, where hierarchical datatree(s) are local in scope and only
includes data that is "owned" by the local system, we need an agent or
interface on one system which is able refer to managed resources that
reside on another system. This allows applications on the same system
as the YANG datastore server, as well as remote clients that access
the datastore through a management protocol such as NETCONF, to access
all data as if it were local to that same server. This must be done in
a manner that is transparent to users and applications. This must be
done in a way which does not require a user or application to be aware
of the fact that some data resides in a different location and have
them directly access that other system. In this way, the user is
projected an image of one virtual consolidated datastore.</t>
<t>The value in such a datastore comes from its under-the-covers
federation. The datastore transparently exposes information from
multiple systems across the network. The user does not need to be
aware of the precise distribution and ownership of data themselves,
nor is there a need for the application to discover those data
sources, maintain separate associations with them, and partition its
operations to fit along remote system boundaries. The effect is that a
network device can broaden and customize the information available for
local access. Life for the application is easier.</t>
<t>Any Object type can be included in such a datastore. This can
include configuration data that is either persistent or ephemeral, and
which is valid within only a single device or across a domain of
devices. This can include operational data that represents state
across a single device or across a multiple devices.</t>
<t>Another useful aspect of “Peer Mount” is its ability to
embed information from external YANG models which haven’t
necessarily been normalized. Normalization is a good thing. But the
massive human efforts invested in uber-data-models have never gained
industry traction due to the resulting models’ brittle nature
and complexity. By mounting remote trees/objects into local datastores
it is possible to expose remote objects under a locally optimized
hierarchy without having to transpose remote objects into a separate
local model. Once this exists, object translation and normalization
become optional capabilities which may also be hidden.</t>
<t>Another useful aspect of “Peer Mount” is its ability to
mount remote trees where the local datastore does not know the full
subtree being installed. In fact, the remote datastore might be
dynamically changing the mounted tree. These dynamic changes can be
reflected as needed under the “attachment points” within
the namespace hierarchy where the data subtrees from remote systems
have been mounted. In this case, the precise details of what these
subtrees exactly contain does not need to be understood by the system
implementing the attachment point, it simply acts as a single point of
entry and "proxy" for the attached data.</t>
<t/>
</section>
<section title="Eventual Consistency and YANG 1.1">
<t>The <eref
target="http://robertgreiner.com/2014/08/cap-theorem-revisited/">CAP
theorem</eref> states that it is impossible for a distributed computer
system to simultaneously provide Consistency, Availability, and
Partition tolerance. (I.e., distributed network state management is
hard.) Mostly for this reason YANG implementations have shied away
from distributed datastore implementations where ACID transactional
guarantees cannot be given. This of course limits the universe of
applicability for YANG technology.</t>
<t>Leveraging YANG concepts, syntax, and models for objects which
might be happening to undergo network convergence is valuable. Such
reuse greatly expands the universe of information visible to
networking applications. The good news is that there is nothing in
YANG 1.1 syntax that prohibits its reapplication for distributed
datastores. Extensions are needed however.</t>
<t>Requirements described within this document can be used to define
technology extensions to YANG 1.1 for remote datastore mounting.
Because of the CAP theorem, it must be recognized that systems built
upon these extensions MAY choose to support eventual consistency
rather than ACID guarantees. Some applications do not demand ACID
guarantees (examples are contained in this document’s Use Case
section). Therefore for certain classes of applications, <eref
target="http://guide.couchdb.org/draft/consistency.html">eventual
consistency</eref> should be viewed as a cornerstone feature
capability rather than a bug. </t>
<t>Other industries have been able to identify and realize the value
in such model. The Object Management Group Data-Distribution Service
for Real-Time Systems has even standardized these capabilities for
non-YANG deployments <xref target="OMG-DDS"/>. Commercial deployments
exist.</t>
</section>
</section>
<section title="Example Use Cases">
<t>Many types of applications can benefit from the simple and quick
availability of objects from peer network devices. Because network
management and orchestration systems have been fulfilling a subset of
the requirements for decades, it is important to focus on what has
changed. Changes include:</t>
<t><list style="symbols">
<t>SDN applications wish to interact with local datastore(s) as if
they represent the real-time state of the distributed network.</t>
<t>Independent sets of applications and SDN controllers might care
about the same authoritative data node or subtree.</t>
<t>Changes in the real-time state of objects can announce themselves
to subscribing applications.</t>
<t>The union of an ever increasing number of abstractions provided
from different layers of the network are assumed to be consistent
with each other (at least once a reasonable convergence time has
been factored in).</t>
<t>CPU and VM improvements makes running Linux based applications on
network elements viable.</t>
</list>Such changes can enable a new class of applications. These
applications are built upon fast-feedback-loops which dynamically tune
the network based on iterative interactions upon a distributed
datastore.</t>
<t/>
<section title="Cloud Policer">
<t>A Cloud Policer enables a single aggregated data rate to
tenants/users of a data center cloud that applies across their VMs; a
rate independent of where specific VMs are physically hosted. This
works by having edge router based traffic counters available to a
centralized application, which can then maintain an aggregate across
those counters. Based on the sum of the counters across the set of
edge routers, new values for each device based Policer can be
recalculated and installed. Effectively policing rates are
continuously rebalanced based on the most recent traffic offered to
the aggregate set of edge devices.</t>
<t>The cloud policer provides a very simple cloud QoS model. Many
other QoS models could also be implemented. Example extensions
include:</t>
<t><list style="symbols">
<t>CIR/PIR guarantees for a tenant,</t>
<t>hierarchical QoS treatment,</t>
<t>providing traffic delivery guarantees for specific enterprise
branch offices, and</t>
<t>adjusting the prioritization of one application based on the
activity of another application which perhaps is in a completely
different location.</t>
</list>It is possible to implement such a cloud policer application
with maximum application developer simplicity using peer mount. To do
this the application accesses a local datastore which in turn does a
peer mount from edge routers the objects which house current traffic
counter statistics. These counters are accessed as if they were part
of the local datastore structures, without concern for the fact that
the actual authoritative copies reside on remote systems.</t>
<t>Beyond this centralized counter collection peer mount, it is also
possible to have distributed edge routers mount information in the
reverse direction. In this case local edge routers can peer mount
centrally calculated policer rates for the device, and access these
objects as if they were locally configured.</t>
<t>For both directions of mounting, the authoritative copy resides in
a single system and is mounted by peers. Therefore issues with regards
to inconsistent configuration of the same redundant data across the
network are avoided. Also as can be seen in this use case, the same
system can act as a mount client of some objects while acting as
server for other objects.</t>
<t/>
</section>
<section title="DDoS Thresholding ">
<t>Another extension of the “Cloud Policer” application is
the creation of additional action thresholds at bandwidth rates far
greater than might be expected. If these higher thresholds are hit, it
is possible to connect in DDoS scrubbers to ingress traffic. This can
be done in seconds after a bandwidth spike. This can also be done if
non-bandwidth counters are available. For example, if TCP flag counts
are available it is possible to look for changes in SYN/ACK ratios
which might signal a different type of attack. In all cases, when
network counters indicate a return to normal traffic profiles the DDoS
Scrubbers can be automatically disconnected.</t>
<t>Benefits of only connecting a DDoS scrubber in the rare event an
attack might be underway include:</t>
<t><list style="symbols">
<t>marking down traffic for an out-of-profile tenant so that an
potential attack doesn’t adversely impact others,</t>
<t>applying DDoS Scrubbing across many devices when an attack is
detected in one,</t>
<t>reducing DDoS scrubber CPU, power, and licensing requirements
(during the vast majority of time, spikes are not occurring),
and</t>
<t>dynamic management and allocation of scarce platform resources
(such as optimizing span port usage, or limiting IP-FIX reporting
to levels where devices can do full flow detail exporting).</t>
</list></t>
</section>
<section title="Service Chain Classification, Load Balancing and Capacity Management">
<t>Service Chains will dynamically change ingress classification
filters, allocate paths from many ingress devices across shared
resources. This information needs to be updated in real time as
available capacity is allocated or failures are discovered. It is
possible to simplify service chain configuration and dynamic topology
maintenance by transparently updating remote cached topologies when an
authoritative object is changed within a central repository. For
example if the CPU in one VM spikes, you might want to recalculate and
adjust many chained paths to relieve the pressure. Or perhaps after
the recalculation you want to spin up a new VM, and then adjust chains
when that capacity is on-line.</t>
<t>A key value here is central calculation and transparent
auto-distribution. In other words, a change only need be updated by an
application in a single location, and the infrastructure will
automatically synchronize changes across any number of subscribing
devices without application involvement. In fact, the application need
not even know many devices are monitoring the object which has been
changed.</t>
<t>Beyond 1:n policy distribution, applications can step back from
aspects of failure recovery. What happens if a device is rebooting or
simply misses a distribution of new information? With peer mount there
is no doubt as to where the authoritative information resides if
things get out of synch.</t>
<t>While this ability is certainly useful for dynamic service chain
filtering classification and next hop mapping, this use case has more
general applicability. With a distributed datastore, diverse
applications and hosts can locally access a single device’s
current VM CPU and Bandwidth values. They can do it without needing to
explicitly query that remote machine. Updates from a device would come
from a periodic push of stats to a transparent cache to subscribed, or
via an unsolicited update which is only sent when these value exceed
established norms.</t>
</section>
</section>
<section title="Requirements">
<t>To achieve the objectives described above, the network needs to
support a number of requirements</t>
<section title="Application Simplification">
<t>A major obstacle to network programmability are any requirements
which force applications to use abstractions more complicated than the
developer cares to touch. To simplify applications development and
reduce unnecessary code, the following needs must be met.</t>
<t>Applications MUST be able to access a local datastore which
includes objects whose authoritative source is located in a remote
datastore hosted on a different server.</t>
<t>Local datastores MUST be able to provide a hierarchical view of
objects assembled from objects whose authoritative source may
originate from potentially different and overlapping namespaces.</t>
<t>Applications MUST be able to access all objects of a datastore
without concern where the actual object is located, i.e. whether the
authoritative copy of the object is hosted on the same system as the
local datastore or whether it is hosted in a remote datastore.</t>
<t>With two exceptions, a datastore’s application facing
interfaces MUST make no differentiation whether individual objects
exposed are authoritatively owned by the datastore or mounted from
remote. This includes Netconf and Restconf as well as other, possibly
proprietary interfaces (such as, CLI generated from corresponding YANG
data models). The two exceptions are that it is acceptable to make a
distinction between an object authoritatively owned by the data store
and a remote object as follows:</t>
<t><list style="symbols">
<t>Object updates / editing, creation and deletion. E.g. via
edit-config conditions and constraints are assessed at the
authoritative datastore when the update/create/delete is
conducted. Any conditions or constraints at remote client
datastores are NOT assessed.</t>
<t>Locks obtained at a client datastore: It is conceivable for the
interface to distinguish between two lock modes: locking the
entire subtree including remote data (in which case the
datastore’s mount client needs to explicitly obtain and
release locks from mounted authoritative datastores), or locking
only authoritatively owned data, excluding remote data from the
lock.</t>
</list>These exceptions should not be very problematic as
non-authoritative copies will typically be marked as read-only. This
will not violate any considerations of “no
differentiation” of local or remote.</t>
<t>When a change is made to an object, that change will be reflected
in any datastore in which the object is included. This means that a
change made to the object through a remote datastore will affect the
object in the authoritative datastore. Likewise, changes to an object
in the authoritative datastore will be reflected at any client
datastores.</t>
<t>The distributed datastore MUST be able to include objects from
multiple remote datastores. The same object may be included in
multiple remote datastores; in other words, an object’s
authoritative datastore MUST support multiple clients.</t>
<t>The distributed datastore infrastructure MUST enable to access to
some subset of the same objects on different devices. (This includes
multiple controllers as well as multiple physical and virtual peer
devices.)</t>
<t>Applications SHOULD be able to extract a time synchronized set of
operational data from the datastore. (In other words, the application
asks for a subset of network state at time-stamp or time-range
“X”. The datastore would then deliver time synchronized
snapshots of the network state per the request. The datastore may work
with NTP and operational counter to optimize the synchronization
results of such a query. It is understood that some types of data
might be undergoing convergence conditions.)</t>
<t>Authoritative datastore retain full ownership of
“their” objects. This means that while remote datastores
may access the data, any modifications to objects that are initiated
at those remote datastores need to be authorized by the authoritative
owner of the data. Likewise, the authoritative owner of the data may
make changes to objects, including modifications, additions, and
deletions, without needing to first ask for permission from remote
clients.</t>
<t>Applications MUST be designed to deal with incomplete data if
remote objects are not accessible, e.g. due to temporal connectivity
issues preventing access to the authoritative source. (This will be
true for many protocols and programming languages. Mount is unlikely
to add anything new here unless applications have extra error handling
routines to deal with when there is no response from a remote
system.).</t>
</section>
<section title="Caching Considerations">
<t/>
<section title="Caching Overview">
<t>Remote objects in a datastore can be accessed “on
demand”, when the application interacting with the datastore
demands it. In that case, a request made to the local datastore is
forwarded to the remote system. The response from the remote system,
e.g. the retrieved data, is subsequently merged and collated with
the other data to return a consolidated response to the invoking
application.</t>
<t>A downside of a datastore which is distributed across devices can
be the latency induced when remote object acquisition is necessary.
There are plenty of applications which have requirements which
simply cannot be served when latency is introduced. The good news is
that the concept of caching lends itself well to distributed
datastores. It is possible to transparently store some types of
objects locally even when the authoritative copy is remote. Instead
of fetching data on demand when an application demands it, the
application is simply provided with the local copy. It is then up to
the datastore infrastructure to keep selected replicated info in
synch, e.g. by prefetching information, or by having the remote
system publish updates which are then locally stored.</t>
<t>This is not a new idea. Caching and Content Delivery Networks
(CDN) have sped read access for objects within the Internet for
years. This has enabled greater performance and scale for certain
content. Just as important, these technologies have been employed
without end user applications being explicitly aware of their
involvement. Such concepts are applicable for scaling the
performance of a distributed datastore.</t>
<t>Where caching occurs, it MUST be possible for the Mount Client to
store object copies of a remote data node or subtree in such a way
that applications are unaware that any caching is occurring.
However, the interface to a datastore MAY provide applications with
a special mode/flag to allow them to force a read-through and
perhaps even a write-through.</t>
<t>Where caching occurs, system administration facilities SHOULD
allow facilities to flush either the entire cache, or information
associated with select Mount Points.</t>
</section>
<section title="Pub/Sub of Object Updates">
<t>When caching occurs, data can go stale. Pub/Sub provides a
mechanism where changes in an authoritative data node or subtree can
be monitored. If changes occur, these changes can be delivered to
any subscribing datastores. In this way remote caches can be kept
up-to-date. In this way, directly monitoring remote applications can
quickly receive notifications without continuous polling.</t>
<t/>
<section title="General Pub/Sub Update Requirements">
<t>A Mount Client SHOULD be able to take advantage of pub/sub
capabilities offered by a mount server. However, not every Mount
Server offers according capabilities.</t>
<t>A Mount Client SHOULD be able to revert back to retrieve
objects “On Demand” and/or to pre-fetch objects by
request.</t>
<t>A Mount Server MAY support a pub/sub capability in which one or
more remote clients subscribe to updates of a target data node /
subtree, which are then automatically published by the Mount
Server.</t>
<t>One or more of the following pub/sub policies MUST be
supported:</t>
<t><list style="symbols">
<t>On Demand (i.e. no pub/sub) - default</t>
<t>Periodic (with a specified time interval)</t>
<t>On change, immediate as the change occurs.</t>
<t>On change, at the end of fixed intervals if a change has
occurred</t>
</list></t>
<t>Further modifications are possible: e.g. on change, whether to
only publish only the object that has changed or the entire
subtree that had been subscribed to. (Effectively this is
aggregate replication at tree level, not at the object level.)</t>
<t>Pub/sub is applicable to other applications as well, not
limited to peer mounting. For example, a pub/sub capability can
greatly facilitate monitoring, as applications no longer have to
“poll” for data but can simply choose to subscribe to
a stream of the most current data. Accordingly, servers that offer
pub/sub capabilities for its YANG datastore SHOULD NOT limit
subscribers to Mount Clients, but allow other applications to
subscribe as well.</t>
<t>It MUST be possible for Applications to subscribe to Data Node
/ Subtrees so that upon Mount Client receipt of subscribed
information, it is immediately passed to the application.</t>
<t>It MUST be possible for the Mount Client to subscribe to Data
Node / Subtrees so that upon Mount Client receipt of subscribed
information, it is cached and therefore awaiting local application
requests.</t>
<t>If there are no applications subscribing to a Data Node /
Subtree, a server SHOULD cease to publish the corresponding
data.</t>
<t>It MUST be possible for a Subscription to include a timestamp
when the Subscription will expire.</t>
<t>It MUST be possible to identify a specific time when a Mount
Binding will return the current value(s) of a mounted Data Node /
Subtree. (Such timeframes can be in the very near future in order
to support a snapshot of network state or counters across many
devices.)</t>
<t>A publisher is not responsible to monitor if the subscribers
are still active. It MAY do so, but is not obliged to do so.
Subscriptions upon a Target Data Node do not remain active forever
but MUST be periodically re-subscribed . The reason for this is to
avoid “waste”, for example in cases when subscribers
“die”. If a subscriber restarts, it is the subscribers
responsibility to check whether its subscriptions are still intact
or to resubscribe if needed.</t>
<t>It MUST be possible for a Target Data Node to support 1:1 Mount
Bindings to a single subscribed Mount Point.</t>
<t>It MUST be possible for a Target Data Node to support 1:n Mount
Bindings to many subscribed Mount Points.</t>
</section>
<section title="Periodic Pub/Sub Updates">
<t>Especially with network based Counters or Operational data,
there need be no recurring request to send the next instance of
data which is released on schedule to subscribers.</t>
<t>It MUST be possible to for a Periodic Mount Point to identify a
specific time when a Mount Target will return the current value(s)
of a mounted Data Node / Subtree. This will allow for
synchronization of calculation for objects delivered from many
Mount Bindings to local applications.</t>
<t>It MUST be possible to for a Periodic Mount Point to identify
the desired start and stop timestamps for any replicated objects
associated with duration. This will allow for time period
synchronization of source data for objects delivered from many
Mount Bindings to local applications.</t>
<t/>
</section>
<section title="Change-trigger Pub/Sub Updates">
<t>For an Unsolicited Mount Point, if a data node or subtree
changes, the Mount Target MUST provide updated objects to the
Mount Client.</t>
<t>For an Unsolicited Mount Point, if a data node or subtree
changes, the Mount Target SHOULD be able to provide just the
updated objects to the Mount Client. Note: If there is a Mount
Filter in place, then only the updated objects based on the filter
will be delivered. It is possible that a Filter will result in no
update needing to be sent.</t>
<t>It SHOULD be possible to provide criteria per Mount Binding on
the characteristics of changes to a Target Data Node’s
monitored objects on before an update is sent to the subscribing
system. (Effectively this becomes a “threshold
trigger” for change notification to remote caches.)</t>
</section>
</section>
</section>
<section title="Lifecycle of the Mount Topology">
<t>Mount can drive a dynamic and richly interconnected mesh of
peer-to-peer of object relationships. Each of these Mounts will be
independently established by a Mount Client.</t>
<t>It MUST be possible to bootstrap the Mount Client by providing the
YANG paths to resources on the Mount Server.</t>
<t>There SHOULD be the ability to add Mount Client bindings during
run-time.</t>
<t>A Mount Client MUST be able to be able to create, delete, and
timeout Mount Bindings.</t>
<t>A Mount Server maintaining a periodic or unsolicited Mount Binding
MUST be able to inform the Mount Client of an intentional graceful
disconnection of that binding.</t>
<t>A Mount Client must be able to verify the existence of a periodic
or unsolicited Mount Binding which has successfully been established
on a Mount Server, and re-establish if it has disappeared.</t>
<t/>
<section title="Discovery and Creation of Mount Topology">
<t>Application visibility into an ever-changing set of network
objects is not trivial. While some applications can be easily
configured to know the Devices and available Mount Points of
interest, other applications will have to balance many aspects of
dynamic device availability, capabilities, and interconnectedness.
For the most part, maintenance of these dynamic elements can be done
on the YANG objects themselves without anything needed new for Peer
Mount. Technologies such as need reference are covered in other
standards initiatives. Therefore this draft does delve deeply into
the needs for Auto-discovery of YANG objects which may be
advertised.</t>
<t>However it will likely become interesting for a network element
to limit the Data Subtrees which might be subscribed for Unsolicited
and Periodic Update.</t>
<t>It SHOULD be possible for a Mount Server to advertise potential
Target Data Nodes which can support unsolicited and periodic binding
types.</t>
<t/>
</section>
<section title="Restrictions on the Mount Topology">
<t>Mount Clients MUST NOT create recursive Mount bindings (i.e., the
Mount Client should not load any object or subtree which it has
already delivered to another in the role of a Mount Server.) Note:
Objects mounted from a controller as part of orchestration are *not*
considered the same objects as those which might be mounted back
from a network device showing the actual running config.</t>
<t/>
</section>
</section>
<section title="Mount Filter">
<t>The Mount Server default MUST be to deliver the same Data Node /
Subtree that would have been delivered via direct YANG access.</t>
<t>It SHOULD be possible for a Mount Client to request something less
that the full subtree or a target node. This will be valuable when the
number or size of objects under a Target Data Node is large. A mount
infrastructure MAY support one or more of the following selection
mechanisms:</t>
<t><list style="symbols">
<t>Select individual nodes</t>
<t>Select a subtree</t>
<t>Filter nodes on content</t>
</list> Additionally a Mount Client MAY choose to filter
node-content on time when data is delivered on change. This will
prevent excessive CPU-utilization when mounting large subtrees with
frequently updating node contents.</t>
<t/>
</section>
<section title="Auto-Negotiation of Peer Mount Client QoS">
<t>The interest that a Mount Client expresses in a particular subtree
SHOULD include the non-functional data delivery requirements (QoS) on
the data that is being mounted. Additionally, Mount Servers SHOULD
advertise their data delivery capabilities. With this information the
Mount Client can decide whether the quality of the delivered data is
sufficient to serve applications residing above the Mount Client. It
might even choose to get equivalent objects from another Mount Server.
Matching up Mount Client needs with Mount Server capabilities MUST be
driven by request versus offered (RxO) negotiations. </t>
<t>An example here is reliability. A reliable protocol might be
overkill for a state that is republished with high frequency.
Therefore a Mount Server may sometimes choose to not provide a
reliable method of communication for certain objects. It is up to the
Mount Client to determine whether what is offered is sufficiently
reliable for its application. Only when the Mount Server is offering
data delivery QoS better or equal to what is requested, shall a mount
binding be established. </t>
<t>Another example is where subscribed objects must be pushed from the
Mount Server within a certain interval from when an object change is
identified. In such a scenario the interval period of the Mount Server
must be equal or smaller than what is requested by a Mount Client. If
this “deadline” is not met by the Mount Server the
infrastructure MAY take action to notify clients. </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". If on the target node
there are multiple datastores available, but there has no specific
datastore identified by the Mount Client, then the running or
"effective" datastore is the assumed target. </t>
<t>It is conceivable to use such Datastore Qualification in
conjunction with ephemeral datastores, to address requirements being
worked in the I2RS WG <xref target="draft-haas"/>. </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 title="Transport">
<t>Many secured transports are viable assuming transport, data
security, scale, and performance objectives are met. Netconf is
recommended for starting. Other transports may be proposed over
time.</t>
<t>It MUST be possible to support Netconf Transport of subscribed
Nodes and Subtrees.</t>
</section>
<section title="Security Considerations ">
<t>Many security mechanisms exist to protect read/write access for CLI
and API on network devices. To the degree possible these mechanisms
should transparently protect data read and write when performing a
Peer Mount. The text below starts with a subset of those requirements
. Additional ones should be added.</t>
<t>The same mechanisms used to determine whether a remote host has
access to a particular YANG Data Node or Subtree MUST be invoked to
determine whether a Mount Client has access to that information.</t>
<t>The same traditional transport level security mechanism security
used for YANG over a particular transport MUST be used for the
delivery of objects from a Mount Server to a Mount Client.</t>
<t>A Mount Server implementation MUST NOT change any credentials
passed by the Mount Client system for any Mount Binding request.</t>
<t>The Mount Server MUST deliver no more objects from a Data Node or
Subtree than allowable based on the security credentials provided by
the Mount Client.</t>
<t>To ensure the ensuring maximum scale limits, it MUST be possible to
for a Mount Server to limit the number of bindings and transactional
limits</t>
<t>It SHOULD be possible to prioritize which Mount Binding instances
should be serviced first if there is CPU, bandwidth, or other capacity
constraints.</t>
<t/>
</section>
<section title="High Availability ">
<t>A key intent for Peer Mount is to allow access to an authoritative
copy of an object for a particular domain. Of course system and
software failures or scheduled upgrades might mean that the primary
copy is not consistently accessible from a single device. In addition,
system failovers might mean that the authoritative copy might be
housed on a different device than the one where the binding was
originally established. Peer Mount architectures must be built to
enable Mount Clients to transparently provide access to objects where
the authoritative copy moves due to dynamic network reconfigurations
.</t>
<t>A Peer Mount architecture MUST guarantee that mount bindings
between a Mount Server and Mount Clients are eventually consistent.
The infrastructure providing this level of consistency MUST be able to
operate in scenarios where a system is (temporarily) not fully
connected. Furthermore, Mount Clients MAY have various requirements on
the boundaries under which eventual consistency is allowed to take
place. This subject can be decomposed in the following items:</t>
<t/>
<section title="Reliability">
<t>Eventual consistency can only be guaranteed when peers are
communicating using a reliable method of data delivery. A scenario
that deserves attention in particular is when a subset of Mount
Clients receive a pushed subscription update. If a Mount Server
loses connectivity, cross network element consistency can be lost.
In such a scenario Mount Clients MAY elect a new designated Mount
Server from the set of Mount Clients which have received the latest
state.</t>
</section>
<section title="Alignment to late joining peers">
<t>When a mount binding is established a Mount Server SHOULD provide
the Mount Client with the latest state of the requested data. In
order to increase availability and fault tolerance an infrastructure
MAY support the capability to have multiple alignment sources. In
(temporary) absence of a Mount Server, Mount Clients MAY elect a
temporary Mount Server to service late joining Mount Clients.</t>
</section>
<section title="Liveliness">
<t>Upon losing liveliness and being unable to refresh cached data
provided from a Mount Server, a Mount Client MAY decide to purge the
mount bindings of that server. Purging mount bindings under such
conditions however makes a system vulnerable to losing network-wide
consistency. A Mount Client can take proactive action based on the
assumption that the Mount Server is no longer available. When
connectivity is only temporarily lost, this assumption could be
false for other datastores. This can introduce a potential for
decision-making based on semantical disagreement. To properly handle
these scenarios, application behavior MUST be designed accordingly
and timeouts with regards to liveliness detection MUST be carefully
determined.</t>
</section>
<section title="Merging of datasets">
<t>A traditional problem with merging replicated datasets during the
failover and recovery of Mount Servers is handling the corresponding
target data node lifecycle management. When two replicas of a
dataset experienced a prolonged loss of connectivity a merge between
the two is required upon re-establishing connectivity. A replica
might have been modifying contents of the set, including deletion of
objects. A naive merge of the two replicas would discard these
deletes by aligning the now stale, deleted objects to the replica
that deleted them. </t>
<t>Authoritative ownership is an elegant solution to this problem
since modifications of content can only take place at the owner.
Therefore a Mount Client SHOULD, upon reestablishing connectivity
with a newly authoritative Mount Server, replace any existing cache
contents from a mount binding with the latest version. </t>
</section>
<section title="Distributed Mount Servers">
<t>For selected objects, Mount Bindings SHOULD be allowed to Anycast
addresses so that a Distributed Mount Server implementation can
transparently provide (a) availability during failure events to
Mount Clients, and (b) load balancing on behalf of Mount
Clients.</t>
</section>
</section>
<section title="Configuration">
<t>At the Mount Client, it MUST be possible for all Mount bindings to
configure the following such that the application needs no knowledge.
This will includea diverse list of elements such at the YANG URI path
to the remote subtree.</t>
<t/>
</section>
<section title="Assurance and Monitoring">
<t>API usage for YANG should be tracked via existing mechanisms. There
is no intent to require additional transaction tracking than would
have been provided normally. However there are additional requirements
which should allow the state of existing and historical bindings to be
provided.</t>
<t>A Mount Client MUST be able to poll a Mount Server for the state of
unsolicited and periodic Mount Binding maintained between the two
devices.</t>
<t>A Mount Server MUST be able to publish the set of unsolicited and
periodic Mount Bindings which are currently established on or below
any identified data node.</t>
<t>A Mount Server MUST be able to publish the set of unsolicited and
periodic Mount Bindings which are going to a specific Mount
Client.</t>
<t>A Mount Server MUST be able to publish the set fulfilled Mount
Bindings which are going to a specific Mount Client.</t>
<t>A Mount Server MUST be able to publish a list of the Mount Bindings
transactions successfully completed.</t>
<t>A Mount Server MUST be able to publish a list of the Mount Bindings
which failed, along with reasons that they failed. These reasons might
include:</t>
<t><list style="symbols">
<t>Improper security credentials provided for the Mount Client to
access the target node</t>
<t>Target node referenced does not exist</t>
<t>Binding type requested not available for the target node</t>
<t>Mount Server out of resources or resources not available</t>
<t>Connection from client lost before binding complete</t>
</list></t>
<t>A Mount Client MUST be able to publish a list of the Mount Bindings
transactions successfully completed.</t>
<t>A Mount Client MUST be able to publish a list of the Mount Bindings
which failed, along with reasons that they failed. These reasons might
include:</t>
<t><list style="symbols">
<t>No response from Mount Client</t>
<t>Connection could not be established with Mount Client</t>
<t>Security credentials provided to Mount Server rejected</t>
<t>Target node referenced does not exist</t>
<t>Binding type requested not available for the target node</t>
<t>Mount Server out of resources or resources not available</t>
<t>Connection from client lost before binding complete</t>
</list></t>
</section>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>This document makes no request of IANA.</t>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>We wish to acknowledge the helpful contributions, comments, and
suggestions that were received from Ambika Prasad Tripathy. Shashi Kumar
Bansal, Prabhakara Yellai, Dinkar Kunjikrishnan, Harish Gumaste, Rohit
M., Shruthi V. , Sudarshan Ganapathi, and Swaroop Shastri.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"
?>
<?rfc include="reference.RFC.6020"
?>
<?rfc include="reference.RFC.3768"
?>
<?rfc include="reference.RFC.4610"
?>
<?rfc ?>
</references>
<references title="Informative References">
<reference anchor="ICCP"
target="https://tools.ietf.org/html/draft-ietf-pwe3-iccp-16">
<front>
<title>Inter-Chassis Communication Protocol for L2VPN PE
Redundancy</title>
<author>
<organization/>
</author>
<date year="2014"/>
</front>
</reference>
<reference anchor="draft-clemm-mount"
target="http://tools.ietf.org/id/draft-clemm-netmod-mount-02.txt">
<front>
<title>Mounting YANG-Defined Information from Remote
Datastores</title>
<author>
<organization/>
</author>
<date year="2013"/>
</front>
</reference>
<reference anchor="OMG-DDS" target="http://www.omg.org/spec/DDS/1.2/">
<front>
<title>Data Distribution Service for Real-time Systems, version
1.2</title>
<author fullname="Object Management Group (OMG)">
<organization/>
</author>
<date month="January" year="2007"/>
</front>
</reference>
<reference anchor="draft-haas"
target="draft-haas-i2rs-netmod-netconf-requirements">
<front>
<title> I2RS requirements for netmod/netconf
draft-haas-i2rs-netmod-netconf-requirements-00</title>
<author fullname="J. Haas" initials="J" surname="Haas">
<organization>Haas</organization>
</author>
<date month="September" year="2014"/>
</front>
</reference>
<reference anchor="rfc6020bis"
target="https://datatracker.ietf.org/doc/draft-ietf-netmod-rfc6020bis/">
<front>
<title>YANG - A Data Modeling Language for the Network Configuration
Protocol (NETCONF)</title>
<author>
<organization/>
</author>
<date year="2014"/>
</front>
</reference>
</references>
</back>
</rfc>
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