Subscribing to YANG datastore push updates
draft-clemm-netconf-yang-push-01
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
|
|
|---|---|---|---|
| Authors | Alexander Clemm , Alberto Gonzalez Prieto , Eric Voit | ||
| Last updated | 2015-07-06 | ||
| Replaces | draft-netmod-clemm-datastore-push | ||
| Replaced by | draft-ietf-netconf-yang-push, draft-ietf-netconf-yang-push, draft-ietf-netconf-yang-push, RFC 8641 | ||
| RFC stream | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-clemm-netconf-yang-push-01
Network Working Group A. Clemm
Internet-Draft A. Gonzalez Prieto
Intended status: Standards Track E. Voit
Expires: January 7, 2016 Cisco Systems
July 6, 2015
Subscribing to YANG datastore push updates
draft-clemm-netconf-yang-push-01.txt
Abstract
This document defines a subscription and push mechanism for YANG
datastores. This mechanism allows client applications to request
updates from a YANG datastore, which are then pushed by the server to
the client per a subscription policy, without requiring additional
client requests.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions and Acronyms . . . . . . . . . . . . . . . . . . 5
3. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Subscription Model . . . . . . . . . . . . . . . . . . . 7
3.2. Negotiation of Subscription Policies . . . . . . . . . . 8
3.3. On-Change Considerations . . . . . . . . . . . . . . . . 8
3.4. Additional on-change update triggers . . . . . . . . . . 9
3.5. Data Encodings . . . . . . . . . . . . . . . . . . . . . 10
3.6. Custom Datastreams . . . . . . . . . . . . . . . . . . . 10
3.7. Push Data Stream and Transport Mapping . . . . . . . . . 11
3.8. Subscription operations . . . . . . . . . . . . . . . . . 12
3.9. A YANG data model for management of datastore push
subscriptions . . . . . . . . . . . . . . . . . . . . . . 16
3.10. Other considerations . . . . . . . . . . . . . . . . . . 18
3.10.1. Authorization . . . . . . . . . . . . . . . . . . . 18
3.10.2. Additional subscription primitives . . . . . . . . . 19
3.10.3. Robustness and reliability considerations . . . . . 20
3.10.4. Update size and fragmentation considerations . . . . 20
3.10.5. Additional data streams . . . . . . . . . . . . . . 20
3.10.6. Subscription persistency . . . . . . . . . . . . . . 20
3.10.7. Implementation considerations . . . . . . . . . . . 21
4. YANG module . . . . . . . . . . . . . . . . . . . . . . . . . 22
5. Security Considerations . . . . . . . . . . . . . . . . . . . 31
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
6.1. Normative References . . . . . . . . . . . . . . . . . . 31
6.2. Informative References . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
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1. Introduction
YANG [RFC6020] was originally designed for the Netconf protocol
[RFC6241], which originally put most emphasis on configuration.
However, YANG is not restricted to configuration data. YANG
datastores, i.e. datastores that contain data modeled according using
YANG, can contain configuration as well as operational data. It is
therefore reasonable to expect that data in YANG datastores will
increasingly be used to support applications that are not focused on
managing configurations but that are, for example, related to service
assurance.
Service assurance applications typically involve monitoring
operational state of networks and devices; of particular interest are
changes that this data undergoes over time. Likewise, there are
applications in which data and objects from one datastore need to be
made available both to applications in other systems and to remote
datastores [I-D.voit-netmod-peer-mount-requirements]
[I-D.clemm-netmod-mount]. This requires mechanisms that allow remote
systems to become quickly aware of any updates to allow to validate
and maintain cross-network integrity and consistency.
Traditional approaches to remote network state visibility rely
heavily on polling. With polling, data is periodically explicitly
retrieved by a client from a server to stay up-to-date.
There are various issues associated with polling-based management:
o It introduces additional load on network, devices, and
applications. Each polling cycle requires a separate yet arguably
redundant request that results in an interrupt, requires parsing,
consumes bandwidth.
o It lacks robustness. Polling cycles may be missed, requests may
be delayed or get lost, often particularly in cases when the
network is under stress and hence exactly when the need for the
data is the greatest.
o Data may be difficult to calibrate and compare. Polling requests
may undergo slight fluctuations, resulting in intervals of
different lengths which makes data hard to compare. Likewise,
pollers may have difficulty issuing requests that reach all
devices at the same time, resulting in offset polling intervals
which again make data hard to compare.
A more effective alternative is when an application can request to be
automatically updated as necessary of current content of the
datastore (such as a subtree, or data in a subtree that meets a
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certain filter condition), and in which the server that maintains the
datastore subsequently pushes those updates. However, such a
solution does not currently exist.
The need to perform polling-based management is typically considered
an important shortcoming of management applications that rely on MIBs
polled using SNMP [RFC1157]. However, without a provision to support
a push-based alternative, there is no reason to believe that
management applications that operate on YANG datastores using
protocols such as NETCONF [RFC6241] or Restconf
[I-D.ietf-netconf-restconf] will be any more effective, as they would
follow the same request/response pattern.
While YANG allows the definition of notifications, such notifications
are generally intended to indicate the occurrence of certain well-
specified event conditions, such as a the onset of an alarm condition
or the occurrence of an error. A capability to subscribe to and
deliver event notifications has been defined in [RFC5277]. In
addition, configuration change notifications have been defined in
[RFC6470]. These change notifications pertain only to configuration
information, not to operational state, and convey the root of the
subtree to which changes were applied along with the edits, but not
the modified data nodes and their values.
Accordingly, there is a need for a service that allows client
applications to subscribe to updates of a YANG datastore and that
allows the server to push those updates. The requirements for such a
service are documented in [I-D.i2rs-pub-sub-requirements]. This
document proposes a solution that features the following
capabilities:
o A mechanism that allows clients to subscribe to automatic
datastore updates, and the means to manage those subscription.
The subscription allows clients to specify which data they are
interested in, and to provide optional filters with criteria that
data must meet for updates to be sent. Furthermore, subscription
can specify a policy that directs when updates are provided. For
example, a client may request to be updated periodically in
certain intervals, or whenever data changes occur.
o The ability for a server to push back on requested subscription
parameters. Because not every server may support every requested
interval for every piece of data, it is necessary for a server to
be able to indicate whether or not it is capable of supporting a
requested subscription, and possibly allow to negotiate
subscription parameters.
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o A mechanism to communicate the updates themselves. For this, the
proposal leverages and extends existing YANG/Netconf/Restconf
mechanisms, defining special notifications that carry updates.
This document specifies a YANG data model to manage subscriptions to
data in YANG datastores, and to configure associated filters and data
streams. It defines extensions to RPCs defined in [RFC5277] that
allow to extend notification subscriptions to subscriptions for
datastore updates. It also defines a notification that can be used
to carry data updates and thus serve as push mechanism.
2. Definitions and Acronyms
Data node: An instance of management information in a YANG datastore.
Data record: A record containing a set of one or more data node
instances and their associated values.
Datastore: A conceptual store of instantiated management information,
with individual data items represented by data nodes which are
arranged in hierarchical manner.
Datastream: A continuous stream of data records, each including a set
of updates, i.e. data node instances and their associated values.
Data subtree: An instantiated data node and the data nodes that are
hierarchically contained within it.
NACM: NETCONF Access Control Model
NETCONF: Network Configuration Protocol
Push-update stream: A conceptual data stream of a datastore that
streams the entire datastore contents continuously and perpetually.
RPC: Remote Procedure Call
SNMP: Simple Network Management Protocol
Subscription: A contract between a client ("subscriber") and a server
("publisher"), stipulating which information the client wishes to
receive from the server (and which information the server has to
provide to the client) without the need for further solicitation.
Subscription filter: A filter that contains evaluation criteria which
are evaluated against YANG objects of a subscription. An update is
only published if the object meets the specified filter criteria.
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Subscription policy: A policy that specifies under what circumstances
to push an update, e.g. whether updates are to be provided
periodically or only whenever changes occur.
Update: A data item containing the current value of a data node.
Update trigger: A trigger, as specified by a subscription policy,
that causes an update to be sent, respectively a data record to be
generated. An example of a trigger is a change trigger, invoked when
the value of a data node changes or a data node is created or
deleted, or a time trigger, invoked after the laps of a periodic time
interval.
URI: Uniform Resource Identifier
YANG: A data definition language for NETCONF
Yang-push: The subscription and push mechanism for YANG datastores
that is specified in this document.
3. Solution Overview
This document specifies a solution that allows clients to subscribe
to information updates in a YANG datastore, which are subsequently
pushed from the server to the client.
Subscriptions are initiated by clients. Servers respond to a
subscription request explicitly positively or negatively. Negative
responses include information about why the subscription was not
accepted, in order to facilitate converging on an acceptable set of
subscription parameters. Once a subscription has been established,
datastore push updates are pushed from the server to the subscribing
client until the subscription ends.
Accordingly, the solution encompasses several components:
o The subscription model for configuration and management of the
subscriptions, with a set of associated services.
o The ability to provide hints for acceptable subscription
parameters, in cases where a subscription desired by a client
cannot currently be served.
o The stream of datastore push updates.
In addition, there are a number of additional considerations, such as
the tie-in of the mechanisms with security mechanisms. Each of those
aspects will be discussed in the following subsections.
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3.1. Subscription Model
Yang-push subscriptions are defined using a data model. This model
is based on the subscriptions defined in [RFC-5277], which is also
reused in Restconf. The model is extended with several parameters,
including a subscription type and a subscription ID.
A subscription refers to a datastream. The subscription model
assumes the presence of a conceptual perpetual datastream "push-
update" of continuous datastore updates. A subscription refers to
this datastream and specifies filters that are to be applied to, it
for example, to provide only those subsets of the information that
match a filter criteria. In addition, a subscription specifies a
subscription policy that defines the trigger when data records should
be sent, for example at periodic intervals or whenever underlying
data items change.
The complete set of subscription parameters is as follows:
o The name of the stream being subscribe to. The subscription model
always assumes the presence of a perpetual and continuous stream
of updates. The stream is called "push-update". However, as
mentioned, it is possible to subscribe to other datastreams, such
as custom datastreams which can be separately configured.
o Optional filter(s), describing the subset of data items in the
stream's data records that are of interest to the subscriber. The
server should only send to the subscriber the data items that
match the filter(s), when present. The absence of a filter
indicates that all data items from the stream are of interest to
the subscriber and all data records must be sent in their entirety
to the subscriber. Two filtering mechanisms are provided: subtree
filtering and Xpath filtering, with the semantics described in
[RFC 5277 Section 3.6]. (Additional filter types can be added
through extensions.)
o An identifier for the subscription.
o An optional start time. Used to trigger replays starting at the
provided time. Its semantics are those in [RFC 5277].
o An optional stop time. Used to limit temporarily the events of
interest. Its semantics are those in [RFC 5277].
o For subscriptions to "push-update", a subscription policy
definition regarding the update trigger to send new updates. The
trigger can be periodic or based on change. For periodic
subscriptions, the trigger is defined by a parameter that defines
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the interval with which updates are to be pushed. For on-change
subscriptions, the trigger occurs when a change in the subscribed
information is detected. On-change subscriptions have more
complex semantics that is guided by additional parameters. One
parameter specifies the dampening period, i.e. the interval that
must pass before a successive update for the same data node is
sent. Other parameters allow to restrict the types of changes for
which updates are sent (changes to object values, object creation
or deletion events), and to specify the magnitude of change that
must occur before an update is triggered. Please refer also to
Section 3.3 and Section 3.4.
o An encoding for the data in the push updates, e.g. XML or JSON.
It is conceivable that additional subscription parameters might be
added in the future. This can be accomplished through augmentation
of the subscription data model.
3.2. Negotiation of Subscription Policies
A subscription rejection can be caused by the inability of the server
to provide a stream with the requested semantics. For example, a
server may not be able to support "on-change" updates for operational
data, or only support them for a limited set of data nodes.
Likewise, a server may not be able to support a requested updated
frequency, or a requested encoding.
Yang-push supports a simple negotiation between clients and servers
for subscription parameters. The negotiation is limited to a single
pair of subscription request and response. For negative responses,
the server SHOULD include in the returned error what subscription
parameters would have been accepted for the request. The returned
acceptable parameters constitute suggestions that, when followed,
increase the likelihood of success for subsequent requests. However,
they are no guarantee that subsequent requests for this client or
others will in fact be accepted.
In case a subscriber requests an encoding other than XML, and this
encoding is not supported by the server, the server simply indicates
in the response that the encoding is not supported.
3.3. On-Change Considerations
On-change subscriptions allow clients to subscribe to updates
whenever changes to objects occur. As such, on-change subscriptions
are of particular interest for data that changes relatively
infrequently, yet that require applications to be notified with
minimal delay when changes do occur.
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On-change subscriptions tend to be more difficult to implement than
periodic subscriptions. Specifically, on-change subscriptions may
involve a notion of state to see if a change occurred between past
and current state, or the ability to tap into changes as they occur
in the underlying system. Accordingly, on-change subscriptions may
not be supported by all implementation or for every object.
When an on-change subscription is requested for a datastream with a
given subtree filter, where not all objects support on-change update
triggers, the subscription request MUST be rejected. As a result,
on-change subscription requests will tend to be directed at very
specific, targeted subtrees with only few objects.
Any updates for an on-change subscription will include only objects
for which a change was detected. To avoid flooding clients with
repeated updates for fast-changing objects, or objects with
oscillating values, an on-change subscription allows for the
definition of a dampening period. Once an update for a given object
is sent, no other updates for this particular object are sent until
the end of the dampening period.
In order to avoid sending updates on objects whose values undergo
only a negligible change, it is conceivable to attach additional
parameters to an on-change subscription specifying a policy that
states how large or "significant" a change has to be before an update
is sent. A simple policy is a "delta-policy" that states, for
integer-valued data nodes, the minimum difference between the current
value and the value that was last reported that triggers an update.
More sophisticated policies are conceivable, including policies
specified in percentage terms or policies that take into account the
rate of change. While not specified as part of this draft, such
policies can be accommodated by augmenting the subscription data
model accordingly.
3.4. Additional on-change update triggers
In conjunction with on-change update triggers, it is conceivable to
further differentiate between the type of change, i.e. whether a
change involves the addition of a new data node, the removal of a
data node, or a value change.
For this purpose, an on-change qualifier is introduced that allows
subscribers to specify which update triggers (create, delete, modify)
to exclude if updates to all change types are not desired.
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3.5. Data Encodings
Subscribed data is encoded in either XML or JSON format. A server
MUST support XML encoding and MAY support JSON encoding. XML
encoding rules for data nodes are defined in [RFC6020]. JSON
encoding rules are defined in [I-D.ietf-netmod-yang-json]. This
encoding is valid JSON, but also has special encoding rules to
identify module namespaces and provide consistent type processing of
YANG data.
It is conceivable that additional encodings may be supported as
options in the future. This can be accomplished by augmenting the
subscription data model with additional identity statements used to
refer to the encodings.
3.6. Custom Datastreams
Optionally, it is possible to introduce other datastreams (beyond the
datastore-push datastream) with custom semantics. Some datastreams
can be custom configured. The support of this is tied to a separate
feature. The configuration of a custom datastream specifies the
trigger conditions under which new data records for the stream are
generated, and which updates the corresponding data records contain.
For example, the configuration of a datastream can specify which
subsets of data nodes in a datastore the datastream should contain,
which filter criteria the updates need to meet, and under what
conditions to create updates - for example, periodically or whenever
a data item changes.
A subscription that refers to a custom datastream can specify a set
of filters, like for the "push-update" datastream. However, the
policy as to when updates are triggered (periodically or on change)
needs to be the same as the policy of the datastream and cannot be
modified. It is not possible, for example, to define a custom
datastream which creates on-change updates, yet subscribe to that
datastream with periodic updates.
While conceptually similar, the choice between subscribing to
datastream "push-update" or configuring and subscribing to a custom
datastream can be thought of as analogous to the choice between
operating a nozzle that is connected to a hose, or controlling the
faucet (custom datastream). Operating the nozzle is for most uses
simpler; however, the option to operate the faucet instead can
provide additional flexibility in some scenarios.
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3.7. Push Data Stream and Transport Mapping
Pushing data based on a subscription could be considered analogous to
a response to a data retrieval request, e.g. a "get" request.
However, contrary to such a request, multiple responses to the same
request may get sent over a longer period of time.
A more suitable mechanism is therefore that of a notification.
Contrary to notifications associated with alarms and unexpected event
occurrences, push updates are solicited, i.e. tied tied to a
particular subscription which triggered the notification. (An
alternative conceptual model would consider a subscription an "opt-
in" filter on a continuous stream of updates.)
The notification contains several parameters:
o A subscription correlator, referencing the name of the
subscription on whose behalf the notification is sent.
o A data node that contains a representation of the datastore
subtree containing the updates. The subtree is filtered per
access control rules to contain only data that the subscriber is
authorized to see. Also, depending on the subscription type,
i.e., specifically for on-change subscriptions, the subtree
contains only the data nodes that contain actual changes. (This
can be simply a node of type string or, for XML-based encoding,
anyxml.)
Notifications are sent using <notification> elements as defined in
[RFC5277]. Alternative transports are conceivable but outside the
scope of this specification.
The solution specified in this document uses notifications to
communicate datastore updates. The contents of the notification
includes a set of explicitly defined data nodes. For this purpose, a
new generic notification is introduced, "push-update" notification.
This notification is used to carry a data record with updates of
datastore contents as specified by a subscription.
The update record consists of a data snippet that contains an
instantiated datastore subtree with the subscribed contents. Data
nodes that do not match filter criteria are removed. Likewise, in
the case of a subscription with "on-change" subscription policy, data
nodes that have not undergone change are omitted. The contents of
the update record is equivalent to the contents that would be
obtained had the same data been explicitly retrieved using e.g. a
Netconf "get"-operation, with the same filters applied.
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The contents of the notification conceptually represents the union of
all data nodes in the yang modules supported by the server, excluding
the following statements: "mandatory", "must", "min-elements", "max-
elements", "when", and "default". However, in a YANG data model, it
is not practical to model the precise data contained in the updates
as part of the notification. This is because the specific data nodes
supported depend on the implementing system and may even vary
dynamically. Therefore, to capture this data, a single parameter
that can represent any datastore contents is used, not parameters
that represent data nodes one at a time.
The following is an example of push notification. It contains an
update for subscription my-sub, including a subtree with root foo
that contains a leaf, bar:
<?xml version="1.0" encoding="UTF-8"?>
<notification xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
<subscription-id xmlns="urn:ietf:params:xml:ns:netconf:
datastore-push:1.0">
my-sub
</subscription-id>
<eventTime>2015-03-09T19:14:56Z</eventTime>
<datastore-contents xmlns="urn:ietf:params:xml:ns:netconf:
datastore-push:1.0">
<foo>
<bar>some_string</bar>
</foo>
</datastore-contents>
</notification>
Figure 1: Push example
3.8. Subscription operations
There are several operations associated with subscriptions. At the
most basic level, clients need to be able to create subscriptions, as
well as delete subscriptions when they are no longer needed.
RFC 5277 specifies an operation to create subscriptions for event
streams, <create-subscription>. This operation is leveraged and
extended to create datastore-push subscriptions. The corresponding
data model is defined in the "subscription-stream-policy" and
"subscription-info" groupings of the YANG module defined in
Section 4. Specifically:
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o An additional parameter is added to allow for the specification of
trigger policy. If the trigger is periodic, a parameter specifies
the period with which updates are to occur. If the trigger is on
change, a parameter specifies a dampening period that defines how
much time must pass before another update for a data node is sent,
once an update for that data node is sent. Additional optional
parameters can further refine trigger behavior, such as
restrictions on the types of changes for which updates are to be
sent.
o An additional parameter is added to allow to specify, as an
option, the desired encoding of the data payload.
To support datastore push, a server MUST support the interleave
capability specified in [RFC5277]. This is required to allow for
modification of what data is being subscribed to without needing to
establish a separate Netconf session.
The example below illustrates a subscription for a periodic push of
all data under a container called foo.
<netconf:rpc message-id="101"
xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
<create-subscription
xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
<stream>push-update</stream>
<subscription-id xmlns="urn:ietf:params:xml:ns:netconf:
datastore-push:1.0">
my-sub
</subscription-id>
<filter netconf:type="xpath"
xmlns:ex="http://example.com/foo/1.0"
select="/ex:foo"/>
<period xmlns="urn:ietf:params:xml:ns:netconf:datastore-push:1.0">
500
</period>
<encoding xmlns="urn:ietf:params:xml:ns:netconf:datastore-push:1.0">
encode-xml
</encoding>
</create-subscription>
</netconf:rpc>
Figure 2: Subscription example
A success response by the server implies that the server will be able
to serve the subscription as requested. It constitutes a "promise"
of the server to push updates to the subscriber along the parameters
that were requested.
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The server is not obliged to accept a request if there are any
aspects of the request that it would not be able to meet. This
includes any reason, including update interval periods that are too
short (requiring more resources than the server could handle), a
filter condition that is not supported, or a requested encoding that
is not supported. Other failure reasons include specification of a
subscription-id that is already in use (modify-subscription should be
used to make changes to settings of existing subscriptions), or
authorization failures (please refer also to section Section 3.10.1.)
The example below illustrates a subscription response, where an agent
does not support frequent periodic updates, and suggests a different
sampling rate to the client.
<rpc-reply message-id="101" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<rpc-error>
<error-type>application</error-type>
<error-tag>operation-not-supported</error-tag>
<error-severity>error</error-severity>
<error-info>
<supported-subscription xmlns="urn:ietf:params:xml:ns:
netconf:datastore-push:1.0">
<period>3000</period>
</supported-subscription>
</error-info>
</rpc-error>
</rpc-reply>
Figure 3: Subscription negotiation example
RFC 5277 does not specify operations to delete subscriptions.
Instead, it assumes that an event subscription is associated with its
own Netconf session. When the session is torn down, the subscription
is implicitly deleted. Likewise, there is no operation to modify a
subscription. Modifying a subscription requires tearing down a
Netconf session, starting a new one, and creating a new subscription.
Furthermore, each session only supports a single subscription.
Establishing multiple subscriptions requires multiple concurrent
Netconf sessions.
To facilitate datastore-push subscriptions, an additional RPC is
introduced, <delete-subscription>.
The <delete-subscription> operation takes as parameter a subscription
ID. As a result of the operation, the subscription is removed and no
more data records will be sent.
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<netconf:rpc message-id="102"
xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
<delete-subscription
xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
<subscription-id xmlns="urn:ietf:params:xml:ns:netconf:
datastore-push:1.0">
my-sub
</subscription-id>
</delete-subscription>
</netconf:rpc>
Figure 4: Subscription deletion
Finally, a separate operation to modify a subscription is introduced,
<modify-subscription>. This operation takes the same parameters as
<create-subscription>, but refers to an existing subscription. Of
course, a subscription could also be deleted and another be created.
However, modify operation avoids issues regarding the synchronization
of creation and deletion operations, such as potential loss or
duplication of updates. Also, a modify operation allows to simply
extend an existing subscription beyond the initial subscription end
time.
<netconf:rpc message-id="103"
xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
<modify-subscription
xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
<stream>push-update</stream>
<subscription-id
xmlns="urn:ietf:params:xml:ns:netconf:datastore-push:1.0">
my-sub
</subscription-id>
<filter netconf:type="xpath"
xmlns:ex="http://example.com/foo/1.0"
select="/ex:foo"/>
<period xmlns="urn:ietf:params:xml:ns:netconf:
datastore-push:1.0">
3000
</period>
</modify-subscription>
</netconf:rpc>
Figure 5: Modify subscription
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3.9. A YANG data model for management of datastore push subscriptions
Subscriptions as well as datastreams can be subjected to management
themselves. For example, it is possible that a server may no longer
be able to serve a subscription that it had previously accepted.
Perhaps it has run out of resources, or internal errors may have
occurred. When this is the case, a server needs to be able to
temporarily suspend the subscription, or even to terminate it. More
generally, the server should provide a means by which the status of
subscriptions can be monitored. When custom datastreams are
supported, those datastreams need to be configured and monitored as
well.
For this purpose, a YANG data model is introduced, which is depicted
in the following figure.
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module: ietf-datastore-push
+--rw streams {custom-streams}?
| +--rw stream* [stream-name]
| +--rw stream-name string
| +--ro stream-status? identityref
| +--rw subtree-filter? subtree-filter
| +--rw xpath-filter? yang:xpath1.0
| +--rw (update-trigger)?
| +--:(periodic)
| | +--rw period? yang:timeticks
| +--:(on-change)
| +--rw dampening-period yang:timeticks
| +--rw excluded-change* change-type
| +--rw (change-policy)?
| +--:(delta-policy)
| +--rw delta? uint32
+--rw subscriptions
+--ro datastore-push-subscription* [subscription-id]
+--ro subscription-id subscription-identifier
+--ro subscription-status? identityref
+--ro stream? string
+--ro encoding? encoding
+--ro start-time? yang:date-and-time
+--ro stop-time? yang:date-and-time
+--ro subtree-filter? subtree-filter
+--ro xpath-filter? yang:xpath1.0
+--ro (update-trigger)?
+--:(periodic)
| +--ro period? yang:timeticks
+--:(on-change)
+--ro dampening-period yang:timeticks
+--ro excluded-change* change-type
+--ro (change-policy)?
+--:(delta-policy)
+--ro delta? uint32
Figure 6: Model structure
Each subscription is represented as a list element "datastore-push-
subscription". The associated information includes an identifier for
the subscription, a subscription status, as well as the various
subscription paramters. The subscription status indicates whether
the subscription is currently active and healthy, or if it is
degraded in some form. Subscriptions are automatically removed from
the list once they expire or are terminated. Because subscriptions
are managed using their own set of operation primitives, they are
read-only.
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An optional feature, custom-streams, is introduced to allow for the
configuration of custom datastreams. Custom datastreams are
represented through a separate list, consisting of information used
to configure those datastreams. This information consititutes mostly
configuration information, with the exception of parameters used to
indicate the status and health of the datastream.
In addition, a server needs to indicate any changes in status to the
subscriber through a notification. Specifically, subscribers need to
be informed of the following:
o A subscription has been temporarily suspended (including the
reason)
o A subscription (that had been suspended earlier) is once again
operational
o A subscription has been abnormally terminated (including the
reason)
o A subscription has been modified (including the current set of
subscription parameters in effect)
Finally, a server might provide additional information about
subscriptions, such as statistics about the number of data updates
that were sent. However, such information is currently outside the
scope of this specification.
3.10. Other considerations
3.10.1. Authorization
A receiver of subscription data may only be sent updates for which
they have proper authorization. Data that is being pushed therefore
needs to be subjected to a filter that applies all corresponding
rules applicable at the time of a specific pushed update, removing
any non-authorized data as applicable.
The authorization model for data in YANG datastores is described in
the Netconf Access Control Model [RFC6536]. However, some
clarifications to that RFC are needed so that the desired access
control behavior is applied to pushed updates.
One of these clarifications is that a subscription may only be
established if the Subscriber has read access to the target data
node.
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+-------------+ +-------------+
subscription | protocol | | target |
request --> | operation | -------------> | data node |
| allowed? | datastore | access |
+-------------+ or state | allowed? |
data access +-------------+
Figure 7: Access control for subscription
Likewise if a subscriber no longer has read access permission to a
target data node, the subscription must be abnormally terminated
(with loss of access permission as the reason provided).
Another clarification to [RFC6536] is that each of the individual
nodes in a pushed update must also go through access control
filtering. This includes new nodes added since the last push update,
as well as existing nodes. For each of these read access must be
verified. The methods of doing this efficiently are left to
implementation.
+-------------+ +-------------------+
subscription | data node | yes | |
update --> | access | ---> | add data node |
| allowed? | | to update message |
+-------------+ +-------------------+
Figure 8: Access control for push updates
If there are read access control changes applied under the target
node, no notifications indicating the fact that this has occurred
need to be provided.
3.10.2. Additional subscription primitives
Other possible operations include the ability for a Subscriber to
request the suspension/resumption of a Subscription with a Publisher.
However, subscriber driven suspension is not viewed as essential at
this time, as a simpler alternative is to remove a subscription and
recreate it when needed.
It should be noted that this does not affect the ability of the
Publisher to suspend a subscription. This can occur in cases the
server is not able to serve the subscription for a certain period of
time, and indicated by a corresponding notification.
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3.10.3. Robustness and reliability considerations
Particularly in the case of on-change push updates, it is important
that push updates do not get lost. However, datastore-push uses a
secure and reliable transport. Notifications are not getting
reordered, and in addition contain a time stamp. For those reasons,
we believe that additional reliability mechanisms at the application
level, such as sequence numbers for push updates, are not required.
3.10.4. Update size and fragmentation considerations
Depending on the subscription, the volume of updates can become quite
large. There is no inherent limitation to the amount of data that
can be included in a notification. That said, it may not always be
practical to send the entire update in a single chunk.
Implementations MAY therefore choose, at their discretion, to "chunk"
updates and break them out into several update notifications.
3.10.5. Additional data streams
The conceptual data stream introduced in this specification,
datastore-push, includes the entire YANG datastore in its scope. It
is conceivable to introduce other data streams with more limited
scope, for example:
o operdata-push, a datastream containing all operational (read-only)
data of a YANG datastore
o operdata-nocounts-push, a datastream containing all operational
(read-only) data with the exception of counters
Those data streams make particular sense for use cases involving
service assurance (not relying on operational data), and for use
cases requiring on-change update triggers which make no sense to
support in conjunction with fast-changing counters. While it is
possible to specify subtree filters on datastore-push to the same
effect, having those data streams greatly simplifies articulating
subscriptions in such scenarios.
3.10.6. Subscription persistency
This specification assumes that a subscriber will explicitly request
a subscription before it receives any update. It does not foresee
that a subscription can be in effect without a subscriber needing to
enter a subscription first.
It is conceivable that scenarios exist in which subscriptions should
be persisted and in effect simply constitute a part of a device's
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configuration, i.e. scenarios that require a device to simply start
sending updates without requiring a subscription to be established
first. While this possibility is not supported by this
specification, there are several ways in which it can be addressed.
One possibility involves adding a subscription configuration model,
which allows to enter subscriptions as part of a configuration.
Another possibility involves the introduction of a subscription
proxy, a subscriber which acts as intermediary between the system
whose updates are being subscribed to and the actual consuming
application.
3.10.7. Implementation considerations
Implementation specifics are outside the scope of this specification.
That said,it should be noted that monitoring of operational state
changes inside a system can be associated with significant
implementation challenges.
Even periodic retrieval of operational state alone, to be able to
push it, can consume considerable system resources. Configuration
data may in many cases be persisted in an actual database or a
configuration file, where retrieval of the database content or the
file itself is reasonably straightforward and computationally
inexpensive. However, retrieval of operational data may, depending
on the implementation, require invocation of APIs, possibly on an
object-by-object basis, possibly involving additional internal
interrupts, etc.
For those reasons, if is important for an implementation to
understand what subscriptions it can or cannot support. It is far
preferrable to decline a subscription request, than to accept it only
to result in subsequent failure later.
Whether or not a subscription can be supported will in general be
determined by a combination of several factors, including the
subscription policy (on-change or periodic, with on-change in general
being the more challenging of the two), the period in which to report
changes (1 second periods will consume more resources than 1 hour
periods), the amount of data in the subtree that is being subscribed
to, and the number and combination of other subscriptions that are
concurrently being serviced.
When providing access control to every node in a pushed update, it is
possible to make and update efficient access control filters for an
update. These filters can be set upon subscription and applied
against a stream of updates. These filters need only be updated when
(a) there is a new node added/removed from the subscribed tree with
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different permissions than its parent, or (b) read access permissions
have been changed on nodes under the target node for the subscriber.
4. YANG module
<CODE BEGINS>
file "ietf-datastore-push@2015-07-06.yang"
module ietf-datastore-push {
namespace "urn:ietf:params:xml:ns:yang:ietf-datastore-push";
prefix datastore-push;
import ietf-yang-types {
prefix yang;
}
organization "IETF";
contact
"WG Web: <http://tools.ietf.org/wg/netconf/>
WG List: <mailto:netconf@ietf.org>
WG Chair: Juergen Schoenwaelder
<mailto:j.schoenwaelder@jacobs-university.de>
WG Chair: Kent Watsen
<mailto:kwatsen@juniper.net>
WG Chair: Tom Nadeau
<mailto:tnadeau@lucidvision.com>
Editor: Alexander Clemm
<mailto:alex@cisco.com>
Editor: Alberto Gonzalez Prieto
<mailto:albertgo@cisco.com>
Editor: Eric Voit
<mailto:evoit@cisco.com>";
description
"This module contains conceptual YANG specifications
for datastore push.";
revision 2015-07-06 {
description
"Initial revision.";
reference "Datastore push.";
}
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feature custom-streams {
description
"This feature allows users to configure datastore update
streams in addition to the stream provided by default,
datastore-push.";
}
identity subscription-stream-status {
description
"Base identity for the status of subscriptions and
datastreams.";
}
identity active {
base subscription-stream-status;
description
"Status is active and healthy.";
}
identity inactive {
base subscription-stream-status;
description
"Status is inactive, for example outside the
interval between start time and stop time.";
}
identity in-error {
base subscription-stream-status;
description
"The status is in error or degraded, meaning that
stream and/or subscription are currently unable to provide
the negotiated updates.";
}
identity subscription-errors {
description
"Base identity for subscription errors.";
}
identity internal-error {
base subscription-errors;
description
"Subscription failures caused by server internal error.";
}
identity no-resources {
base subscription-errors;
description
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"Lack of resources, e.g. CPU, memory, bandwidth";
}
identity other {
base subscription-errors;
description
"Fallback reason - any other reason";
}
identity encodings {
description
"Base identity to represent data encodings";
}
identity encode-xml {
base encodings;
description
"Encode data using XML";
}
identity encode-json {
base encodings;
description
"Encode data using JSON";
}
typedef datastore-contents {
type string;
description
"This type is be used to represent datastore contents,
including a filtered datastore subtree per a set of
subscription parameters. ";
}
typedef subtree-filter {
type string;
description
"This type is used to define a subtree filter.
Its precise syntax is TBD.";
}
typedef subscription-identifier {
type string {
length "1 .. max";
}
description
"A client-provided identifier for the subscription.";
}
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typedef subscription-term-reason {
type identityref {
base subscription-errors;
}
description
"Reason for a server to terminate a subscription.";
}
typedef subscription-susp-reason {
type identityref {
base subscription-errors;
}
description
"Reason for a server to suspend a subscription.";
}
typedef encoding {
type identityref {
base encodings;
}
description
"Specifies a data encoding, e.g. for a data subscription.";
}
typedef change-type {
type enumeration {
enum "create" {
description
"A new data node was created";
}
enum "delete" {
description
"A data node was deleted";
}
enum "modify" {
description
"The value of a data node has changed";
}
}
description
"Specifies different types of changes that may occur
to a datastore.";
}
grouping subscription-stream-policy {
description
"This grouping contains the parameters which describe
the policy which data is pushed as part of a
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subscription or a data stream.";
leaf subtree-filter {
type subtree-filter;
description
"Datastore subtree of interest.";
}
leaf xpath-filter {
type yang:xpath1.0;
description
"Xpath defining the data items of interest.";
}
choice update-trigger {
description
"Defines necessary conditions for sending an event to
the subscriber.";
case periodic {
description
"The agent is requested to notify periodically the
current values of the datastore or the subset
defined by the filter.";
leaf period {
type yang:timeticks;
description
"Elapsed time between notifications.";
}
}
case on-change {
description
"The agent is requested to notify changes in
values in the datastore or a subset of it defined
by a filter.";
leaf dampening-period {
type yang:timeticks;
mandatory true;
description
"Minimum amount of time that needs to have
passed since the last time an update was
provided.";
}
leaf-list excluded-change {
type change-type;
description
"Use to restrict which changes trigger an update.
For example, if modify is excluded, only creation and
deletion of objects is reported.";
}
choice change-policy {
description
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"Policy describing necessary conditions for
sending an event to the subscriber.";
case delta-policy {
leaf delta {
type uint32;
description
"For modified values of an integer-typed object,
minimum difference between current and last
report values that can trigger an update.";
}
}
}
}
}
}
grouping subscription-info {
description
"This grouping describes basic information concerning a
subscription, without the subscription policy which is
defined separately to be shareable with the definition
of a datastream.";
leaf stream {
type string;
description
"The name of the stream subscribed to.";
}
leaf encoding {
type encoding;
default encode-xml;
description
"The type of encoding for the subscribed data.
Default is XML";
}
leaf start-time {
type yang:date-and-time;
description
"Starting time for replays.";
reference "RFC 5277, Section 2.1.1";
}
leaf stop-time {
type yang:date-and-time;
description
"Time limit for events of interest.";
reference "RFC 5277, Section 2.1.1";
}
}
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notification push-update {
description
"This notification contains an update from a datastore";
leaf subscription-id {
type subscription-identifier;
mandatory true;
description
"This references the subscription because of which the
notification is sent.";
}
leaf datastore-contents {
type datastore-contents;
description
"This contains datastore contents
per the subscription.";
}
}
notification subscription-suspended {
description
"This notification indicates that a suspension of the
subscription by the server has occurred. No further
datastore updates will be sent until subscription
resumes.";
leaf subscription-id {
type subscription-identifier;
mandatory true;
description
"This references the affected subscription.";
}
leaf reason {
type subscription-susp-reason;
description
"Provides a reason for why the subscription was
suspended.";
}
}
notification subscription-resumed {
description
"This notification indicates that a subscription that had
previously been suspended has resumed. Datastore updates
will once again be sent.";
leaf subscription-id {
type subscription-identifier;
mandatory true;
description
"This references the affected subscription.";
}
}
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notification subscription-modified {
description
"This notification indicates that a subscription has
been modified. Datastore updates sent from this point
on will conform to the modified terms of the
subscription.";
leaf subscription-id {
type subscription-identifier;
mandatory true;
description
"This references the affected subscription.";
}
uses subscription-info;
uses subscription-stream-policy;
}
notification subscription-terminated {
description
"This notification indicates that a subscription has been
terminated.";
leaf subscription-id {
type subscription-identifier;
mandatory true;
description
"This references the affected subscription.";
}
leaf reason {
type subscription-term-reason;
description
"Provides a reason for why the subscription was
terminated.";
}
}
container streams {
if-feature custom-streams;
description
"This container contains management data for custom streams
that are configured by a user.";
list stream {
key "stream-name";
description
"A user-definable stream.";
leaf stream-name {
type string;
mandatory true;
description
"The name assigned to the stream.";
}
leaf stream-status {
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type identityref {
base subscription-stream-status;
}
config false;
description
"The current status of the stream";
}
uses subscription-stream-policy;
}
}
container subscriptions {
description
"Contains the list of currently active subscription,
used for subscription management and monitoring purposes.
Note that this concerns subscriptions that are in-effect.
Configuration and setup of subscriptions occurs via separate
primitives, e.g. create-subscription, delete-subscription,
and modify-subscription.";
list datastore-push-subscription {
key "subscription-id";
config false;
description
"Content of a yang-push subscription.
Subscriptions are created using a dedicated RPC, hence
they do not constitute configuration information.";
leaf subscription-id {
type subscription-identifier;
description
"Identifier to use for this subscription.";
}
leaf subscription-status {
type identityref {
base subscription-stream-status;
}
description
"The status of the subscription.";
}
uses subscription-info;
uses subscription-stream-policy;
}
}
}
<CODE ENDS>
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5. Security Considerations
Subscriptions could be used to attempt to overload servers of YANG
datastores. For this reason, it is important that the server has the
ability to decline a subscription request if it would deplete its
resources. In addition, a server needs to be able to suspend an
existing subscription when needed. When this occur, the subscription
status is updated accordingly and the clients are notified.
Likewise, requests for subscriptions need to be properly authorized.
A subscription could be used to retrieve data in subtrees that a
client has not authorized access to. Therefore it is important that
data pushed based on subscriptions is authorized in the same way that
regular data retrieval operations are. Data being pushed to a client
needs therefore to be filtered accordingly, just like if the data
were being retrieved on-demand. The Netconf Authorization Control
Model applies.
6. References
6.1. Normative References
[RFC1157] Case, J., Fedor, M., Schoffstall, M., and J. Davin,
"Simple Network Management Protocol (SNMP)", STD 15, RFC
1157, May 1990.
[RFC5277] Chisholm, S. and H. Trevino, "NETCONF Event
Notifications", RFC 5277, July 2008.
[RFC6020] Bjorklund, M., "YANG - A Data Modeling Language for the
Network Configuration Protocol (NETCONF)", RFC 6020,
October 2010.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
Bierman, "Network Configuration Protocol (NETCONF)", RFC
6241, June 2011.
[RFC6470] Bierman, A., "Network Configuration Protocol (NETCONF)
Base Notifications", RFC 5277, February 2012.
[RFC6536] Bierman, A. and M. Bjorklund, "Network Configuration
Protocol (NETCONF) Access Control Model", RFC 6536, March
2012.
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6.2. Informative References
[I-D.clemm-netmod-mount]
Clemm, A., Medved, J., and E. Voit, "Mounting YANG-defined
information from remote datastores", draft-clemm-netmod-
mount-03 (work in progress), April 2015.
[I-D.i2rs-pub-sub-requirements]
Voit, E., Clemm, A., and A. Gonzalez Prieto, "Requirements
for Subscription to YANG Datastores", draft-ietf-i2rs-pub-
sub-requirements-00 (work in progress), March 2015.
[I-D.ietf-netconf-restconf]
Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", I-D draft-ietf-netconf-restconf-06, June 2015.
[I-D.ietf-netmod-yang-json]
Lhotka, L., "JSON Encoding of Data Modeled with YANG",
draft-ietf-netmod-yang-json-04 (work in progress), June
2015.
[I-D.voit-netmod-peer-mount-requirements]
Voit, E., Clemm, A., and S. Mertens, "Requirements for
Peer Mounting of YANG subtrees from Remote Datastores",
draft-voit-netmod-peer-mount-requirements-02 (work in
progress), March 2015.
Authors' Addresses
Alexander Clemm
Cisco Systems
EMail: alex@cisco.com
Alberto Gonzalez Prieto
Cisco Systems
EMail: albertgo@cisco.com
Eric Voit
Cisco Systems
EMail: evoit@cisco.com
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