Subscribing to YANG datastore push updates
draft-ietf-netconf-yang-push-04
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8641.
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|
|---|---|---|---|
| Authors | Alexander Clemm , Eric Voit , Alberto Gonzalez Prieto , Ambika Tripathy , Einar Nilsen-Nygaard , Andy Bierman , Balázs Lengyel | ||
| Last updated | 2016-10-30 | ||
| Replaces | draft-clemm-netconf-yang-push | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-netconf-yang-push-04
Network Working Group A. Clemm
Internet-Draft Sympotech
Intended status: Standards Track E. Voit
Expires: May 1, 2017 A. Gonzalez Prieto
A. Tripathy
E. Nilsen-Nygaard
Pre-release version Cisco Systems
A. Bierman
YumaWorks
B. Lengyel
Ericsson
October 28, 2016
Subscribing to YANG datastore push updates
draft-ietf-netconf-yang-push-04
Abstract
This document defines a subscription and push mechanism for YANG
datastores. This mechanism allows subscriber applications to request
updates from a YANG datastore, which are then pushed by the publisher
to a receiver per a subscription policy, without requiring additional
subscriber requests.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 1, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions and Acronyms . . . . . . . . . . . . . . . . . . 5
3. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Subscription Model . . . . . . . . . . . . . . . . . . . 6
3.2. Negotiation of Subscription Policies . . . . . . . . . . 8
3.3. On-Change Considerations . . . . . . . . . . . . . . . . 9
3.4. Data Encodings . . . . . . . . . . . . . . . . . . . . . 10
3.5. YANG object filters . . . . . . . . . . . . . . . . . . . 11
3.6. Push Data Stream and Transport Mapping . . . . . . . . . 11
3.7. Subscription management . . . . . . . . . . . . . . . . . 15
3.8. Other considerations . . . . . . . . . . . . . . . . . . 16
4. A YANG data model for management of datastore push
subscriptions . . . . . . . . . . . . . . . . . . . . . . . . 20
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 20
4.2. Update streams . . . . . . . . . . . . . . . . . . . . . 26
4.3. Filters . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.4. Subscription configuration . . . . . . . . . . . . . . . 27
4.5. Subscription monitoring . . . . . . . . . . . . . . . . . 29
4.6. Notifications . . . . . . . . . . . . . . . . . . . . . . 29
4.7. RPCs . . . . . . . . . . . . . . . . . . . . . . . . . . 31
5. YANG module . . . . . . . . . . . . . . . . . . . . . . . . . 35
6. Security Considerations . . . . . . . . . . . . . . . . . . . 47
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 48
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 48
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8.1. Normative References . . . . . . . . . . . . . . . . . . 48
8.2. Informative References . . . . . . . . . . . . . . . . . 48
Appendix A. Issues that are currently being worked and resolved 49
A.1. Unresolved and yet-to-be addressed issues . . . . . . . . 49
A.2. Agreement in principal . . . . . . . . . . . . . . . . . 49
Appendix B. Changes between revisions . . . . . . . . . . . . . 50
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 50
1. Introduction
YANG [RFC7950] was originally designed for the Netconf protocol
[RFC6241] which focused on configuration data. However, YANG can be
used to model both configuration and operational data. It is
therefore reasonable to expect YANG datastores will increasingly be
used to support applications that care about about both.
For example, service assurance applications will need to be aware of
any remote updates to configuration and operational objects. Rapid
awareness of object changes will enable such things as validating and
maintaining cross-network integrity and consistency, or monitoring
state and key performance indicators of remote devices.
Traditional approaches to remote visibility have been built on
polling. With polling, data is periodically explicitly retrieved by
a client from a server to stay up-to-date. However, there are 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 to polling is when an application can
request to be automatically updated on current relevant content of a
datastore. If such a request is accepted, interesting updates will
subsequently be pushed from that datastore.
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Dependence on polling-based management is typically considered an
important shortcoming of 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 will be any more
effective, as they would follow the same request/response pattern.
While YANG allows the definition of push notifications, such
notifications generally indicate the occurrence of certain well-
specified event conditions, such as the onset of an alarm condition
or the occurrence of an error. A capability to subscribe to and
deliver such pre-defined 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.
Furthermore, while delivery of updates using notifications is a
viable option, some applications desire the ability to stream updates
using other transports.
Accordingly, there is a need for a service that allows applications
to dynamically subscribe to updates of a YANG datastore and that
allows the publisher to push those updates, possibly using one of
several delivery mechanisms. Additionally, support for subscriptions
configured directly on the publisher are also useful when dynamic
signaling is undesirable or unsupported. The requirements for such a
service are documented in [RFC7923].
This document proposes a solution. The solution builds on top of the
Netconf Event Model [I-D.ietf-netconf-5277bis] which defines a
mechanism for the management of event subscriptions. At its core,
the solution defined here introduces a new set of event streams which
maybe subscribed, introduces datastore push update mechanisms, and
provides extensions to the event subscription model. The document
also includes YANG data model augmentations which extend the model
and RPCs defined within [I-D.ietf-netconf-5277bis].
Key capabilities worth highlighting include:
o An extension to event subscription mechanisms allowing clients to
subscribe to event streams containing automatic datastore updates.
The subscription allows clients to specify which data they are
interested in, what types of updates (e.g. create, delete,
modify), and to provide optional filters with criteria that data
must meet for updates to be sent. Furthermore, subscriptions can
specify a policy that directs when updates are provided. For
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example, a client may request to be updated periodically in
certain intervals, or whenever data changes occur.
o Format and contents of the YANG push updates themselves.
o The ability for a publisher to push back on requested subscription
parameters. Because not every publisher may support every
requested update policy for every piece of data, it is necessary
for a publisher to be able to indicate whether or not it is
capable of supporting a requested subscription, and possibly allow
to negotiate push update subscription parameters. For example,
some publishers may have a lower limit to the period with which
they can send updates, or they may not support on-change updates
for every piece of data.
o Subscription parameters which allow the specification of QoS
extensions to address prioritization between independent streams
of updates.
2. Definitions and Acronyms
Many of the terms in this document are defined in
[I-D.ietf-netconf-5277bis]. Please see that document for these
definitions.
Data node: An instance of management information in a YANG datastore.
Data node update: A data item containing the current value/property
of a Data node at the time the data node update was created.
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.
Push-update stream: A conceptual data stream of a datastore that
streams the entire datastore contents continuously and perpetually.
Update: A data item containing the current value of a data node.
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Update notification: An Event Notification including those data node
update(s) to be pushed in order to meet the obligations of a single
Subscription. All included data node updates must reflect the state
of a Datastore at a snapshot in time.
Update record: A representation of a data node update as a data
record. An update record can be included as part of an update
stream. It can also be logged for retrieval. In general, an update
record will include the value/property of a data node. It may also
include information about the type of data node update, i.e. whether
the data node was modified/updated, or newly created, or deleted.
Update trigger: A mechanism, as specified by a Subscription Policy,
that determines when a data node update is to be communicated. (e.g.,
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.)
YANG object 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.
YANG-Push: The subscription and push mechanism for YANG datastores
that is specified in this document.
3. Solution Overview
This document specifies a solution for a push update subscription
service. This solution supports the dynamic as well as configured
subscriptions to information updates from YANG datastores. A
subscription might target exposed operational and/or configuration
YANG objects on a device. YANG objects are subsequently pushed from
the publisher to the receiver per the terms of the subscription.
3.1. Subscription Model
YANG-push subscriptions are defined using a data model that is itself
defined in YANG. This model augments the event subscription model
defined in [I-D.ietf-netconf-5277bis] and introduces several new
parameters that allow subscribers to specify what to include in an
update notification and what triggers such an update notification.
The subscription model assumes the presence of one or more conceptual
perpetual datastreams of continuous subscribable YANG updates. There
are several datastreams with predefined semantics, such as the stream
of updates of all operational data or the stream of updates of all
config data. In addition, it is possible to define custom streams
with customizable semantics. The model includes the list of update
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datastreams that are supported by a system and available for
subscription.
The subscription model augments the [I-D.ietf-netconf-5277bis]
subscription model with a set of parameters:
o Anydata encoding for periodic and on-change push updates.
o A subscription policy definition regarding the update trigger when
to send new update notifications.
* For periodic subscriptions, the trigger is defined by two
parameters that defines the interval with which updates are to
be pushed. These parameters are the period/interval of
reporting duration, and an anchor time which can be used to
calculate at which times updates needs to be assembled and
sent.
* EDITOR'S NOTE: A possible option to discuss concerns the
introduction of an additional parameter "changes-only" for
periodic subscription. Including this flag would results in
sending at the end of each period an update containing only
changes since the last update (i.e. a change-update as in the
case of an on-change subscription), not a full snapshot of the
subscribed information. Such an option might be interesting in
case of data that is largely static and bandwidth-constrained
environments.
* For on-change subscriptions, the trigger occurs whenever a
change in the subscribed information is detected. On-change
subscriptions have more complex semantics that can be guided by
additional parameters. Please refer also to Section 3.3.
+ One parameter specifies the dampening period, i.e. the
interval that must pass before a successive update
notification for the same Subscription is sent. Note that
the dampening period applies to the set of all data nodes
within a single subscription. This means that on the first
change of an object, an update notification containing that
object is sent either immediately or at the end of a
dampening period already in effect.
+ Another parameter allows to restrict the types of changes
for which updates are sent (changes to object values, object
creation or deletion events). It is conceivable to augment
the data model with additional parameters in the future to
specify even more refined policies, such as parameters that
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specify the magnitude of a change that must occur before an
update is triggered.
+ A third parameter specifies whether or not a complete update
with all the subscribed data should be sent at the beginning
of a subscription to facilitate synchronization and
establish the frame of reference for subsequent updates.
o Optionally, a filter, or set of filters, describing the subset of
data node updates that are of interest to the subscriber. The
publisher must only send to the subscriber those data node updates
that can traverse applied filter(s). 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. The following types of filters are supported:
subtree filters, with the same semantics as defined in
[RFC6241][RFC6241], and XPath filters. Additional filter types
can be added through augmentations. Filters can be specified
"inline" as part of the subscription, or can be configured
separately and referenced by a subscription, in order to
facilitate reuse of complex filters.
The subscription data model is specified as part of the YANG data
model described later in this specification. 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
Dynamic subscriptions must support a simple negotiation between
subscribers and publishers for subscription parameters. This
negotiation is limited to a single pair of subscription request and
response messages. For negative response messages, the publisher
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, there are no
guarantee that subsequent requests for this subscriber will in fact
be accepted.
A subscription request might be declined based on publisher's
assessment that it may be unable to provide a filtered update
notification stream that would meet the terms of the establish-
subscription request.
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In case a subscriber requests an encoding other than XML, and this
encoding is not supported by the publisher, the publisher simply
indicates in the response that the encoding is not supported.
A subscription negotiation capability has been introduced as part of
the NETCONF Event Notifications model. However, the ability to
negotiate subscriptions is of particular importance in conjunction
with push updates, as publisher implementations may have limitations
with regards to what updates can be generated and at what velocity.
3.3. On-Change Considerations
On-change subscriptions allow subscribers 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.
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 implementations 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 receivers 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. Values sent at the end of the
dampening period are the values current when that dampening period
expires. In addition, updates include information about objects that
were deleted and ones that were newly created.
On-change subscriptions can be refined to let users subscribe only to
certain types of changes, for example, only to object creations and
deletions, but not to modifications of object values.
Additional refinements are conceivable. For example, in order to
avoid sending updates on objects whose values undergo only a
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negligible change, additional parameters might be added to an on-
change subscription specifying a YANG object filter 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. Also more
sophisticated policies are conceivable, such as 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. Data Encodings
Subscribed data is encoded in either XML or JSON format. A publisher
MUST support XML encoding and MAY support JSON encoding.
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 requested encodings.
3.4.1. Periodic Subscriptions
In a periodic subscription, the data included as part of an update
corresponds to data that could have been simply retrieved using a get
operation and is encoded in the same way. XML encoding rules for
data nodes are defined in [RFC7950]. JSON encoding rules are defined
in [RFC7951]. This encoding is valid JSON, but also has special
encoding rules to identify module namespaces and provide consistent
type processing of YANG data.
3.4.2. On-Change Subscriptions
In an on-change subscription, updates need to indicate not only
values of changed data nodes but also the types of changes that
occurred since the last update, such as whether data nodes were newly
created since the last update or whether they were merely modified,
as well as which data nodes were deleted.
Encoding rules for data in on-change updates correspond to how data
would be encoded in a YANG-patch operation as specified in
[I-D.ietf-netconf-yang-patch]. The "YANG-patch" would in this case
be applied to the earlier state reported by the preceding update, to
result in the now-current state of YANG data. Of course, contrary to
a YANG-patch operation, the data is sent from the publisher to the
receiver and is not restricted to configuration data.
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3.5. YANG object filters
Subscriptions can specify filters for subscribed data. The following
filters are supported:
o subtree-filter: A subtree filter specifies a subtree that the
subscription refers to. When specified, updates will only concern
data nodes from this subtree. Syntax and semantics correspond to
that specified for [RFC6241] section 6.
o xpath-filter: An XPath filter specifies an XPath expression
applied to the data in an update, assuming XML-encoded data.
Only a single filter can be applied to a subscription at a time.
It is conceivable for implementations to support other filters. For
example, an on-change filter might specify that changes in values
should be sent only when the magnitude of the change since previous
updates exceeds a certain threshold. It is possible to augment the
subscription data model with additional filter types.
3.6. 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.
An applicable mechanism is that of a notification. There are however
some specifics that need to be considered. Contrary to other
notifications that are associated with alarms and unexpected event
occurrences, update notifications are solicited, i.e. tied to a
particular subscription which triggered the notification.
A push update notification contains several parameters:
o A subscription correlator, referencing the name of the
subscription on whose behalf the notification is sent.
o Data nodes containing a representation of the datastore subtree(s)
containing the updates. In all cases, the subtree(s) are filtered
per access control rules to contain only data that the subscriber
is authorized to see. For on-change subscriptions, the subtree
may only contain the data nodes which have changed since the start
of the previous dampening interval.
This document introduces two generic notifications: "push-update" and
"push-change-update". Those notifications may be encapsulated on a
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transport (e.g. Netconf notifications and HTTP) to carry data
records with updates of datastore contents as specified by a
subscription. It is possible also map notifications to other
transports and encodings and use the same subscription model;
however, the definition of such mappings is outside the scope of this
document.
A push-update notification defines a complete update of the datastore
per the terms of a subscription. This type of notification is used
for continuous updates of periodic subscriptions. A push-update
notification can also used be for the on-change subscriptions in two
cases. First it will be used as the initial push-update if there is
a need to synchronize the receiver at the start of a new
subscription. It also may be sent if the publisher later chooses to
resynch a previously synched on-change subscription. The push-update
record contains a data snippet that contains an instantiated subtree
with the subscribed contents. The content of the update notification
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.
The contents of the notification conceptually represents the union of
all data nodes in the yang modules supported by the publisher.
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.
A push-change-update notification is the most common type of update
for on-change subscriptions. It is not used for periodic
subscriptions. The update record in this case contains a data
snippet that indicates the full set of changes that data nodes have
undergone since the last notification of YANG objects. In other
words, this indicates which data nodes have been created, deleted, or
have had changes to their values. The format of the data snippet
follows YANG-patch [I-D.ietf-netconf-yang-patch], i.e. the same
format that would be used with a YANG-patch operation to apply
changes to a data tree, indicating the creates, deletes, and
modifications of data nodes. Please note that as the update can
include a mix of configuration and operational data
The following is an example of push notification. It contains an
update for subscription 1011, including a subtree with root foo that
contains a leaf, bar:
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<notification
xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
<eventTime>2015-03-09T19:14:56Z</eventTime>
<push-update
xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
<subscription-id>1011</subscription-id>
<time-of-update>2015-03-09T19:14:56.233Z</time-of-update>
<datastore-contents-xml>
<foo>
<bar>some_string</bar>
</foo>
</datastore-contents-xml>
</push-update>
</notification>
Figure 1: Push example
The following is an example of an on-change notification. It
contains an update for subscription 89, including a new value for a
leaf called beta, which is a child of a top-level container called
alpha:
<notification
xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
<eventTime>2015-03-09T19:14:56Z</eventTime>
<push-change-update xmlns=
"urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
<subscription-id>89</subscription-id>
<time-of-update>2015-03-09T19:14:56.233Z</time-of-update>
<datastore-changes-xml>
<alpha xmlns="http://example.com/sample-data/1.0" >
<beta>1500</beta>
</alpha>
</datastore-changes-xml>
</push-change-update>
</notification>
Figure 2: Push example for on change
The equivalent update when requesting json encoding:
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<notification
xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
<eventTime>2015-03-09T19:14:56Z</eventTime>
<push-change-update xmlns=
"urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
<subscription-id>89</subscription-id>
<time-of-update>2015-03-09T19:14:56.233Z</time-of-update>
<datastore-changes-json>
{
"ietf-yang-patch:yang-patch": {
"patch-id": [
null
],
"edit": [
{
"edit-id": "edit1",
"operation": "merge",
"target": "/alpha/beta",
"value": {
"beta": 1500
}
}
]
}
}
</datastore-changes-json>
</push-change-update>
</notification>
Figure 3: Push example for on change with JSON
When the beta leaf is deleted, the publisher may send
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<notification
xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
<eventTime>2015-03-09T19:14:56Z</eventTime>
<push-change-update xmlns=
"urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
<subscription-id>89</subscription-id>
<time-of-update>2015-03-09T19:14:56.233Z</time-of-update>
<datastore-changes-xml>
<alpha xmlns="http://example.com/sample-data/1.0" >
<beta urn:ietf:params:xml:ns:netconf:base:1.0:
operation="delete"/>
</alpha>
</datastore-changes-xml>
</push-change-update>
</notification>
Figure 4: 2nd push example for on change update
3.7. Subscription management
A [[I-D.ietf-netconf-5277bis] subscription needs enhancment to
support YANG Push subscription negotiation. Specifically, these
enhancements are needed to signal to the subscriber why an attempt
has failed.
A subscription can be rejected for multiple reasons, including the
lack of authorization to establish a subscription, the lack of read
authorization on the requested data node, or the inability of the
publisher to provide a stream with the requested semantics. In such
cases, no subscription is established. Instead, the subscription-
result with the failure reason is returned as part of the RPC
response. In addition, a set of alternative subscription parameters
MAY be returned that would likely have resulted in acceptance of the
subscription request, which the subscriber may try for a future
subscription attempt.
It should be noted that a rejected subscription does not result in
the generation of an rpc-reply with an rpc-error element, as neither
the specification of YANG-push specific errors nor the specification
of additional data parameters to be returned in an error case are
supported as part of a YANG data model.
For instance, for the following request:
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<netconf:rpc message-id="101"
xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
<establish-subscription
xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
<stream>push-update</stream>
<filter netconf:type="xpath"
xmlns:ex="http://example.com/sample-data/1.0"
select="/ex:foo"/>
<period>500</period>
<encoding>encode-xml</encoding>
</establish-subscription>
</netconf:rpc>
Figure 5: Establish-Subscription example
the publisher might return:
<rpc-reply message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<subscription-result
xmlns="http://urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
error-insufficient-resources
</subscription-result>
<period>2000</period>
</rpc-reply>
Figure 6: Error response example
3.8. Other considerations
3.8.1. 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, silently
removing any non-authorized data from subtrees.
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 receiver 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 receiver 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 update
notification, 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
should be provided.
3.8.2. Robustness and reliability considerations
Particularly in the case of on-change push updates, it is important
that push updates do not get lost.
Update notifications will typically traverse a secure and reliable
transport. Notifications will not be reordered, and will also
contain a time stamp. Despite these protections for on-change, it is
possible that complete update notifications get lost. For this
reason, patch-ids may be included in a subscription so that an
application can determine if an update has been lost.
At the same time, it is conceivable that under certain circumstances,
a publisher will recognize that it is unable to include within an
update notification the full set of objects desired per the terms of
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a subscription. In this case, the publisher must take one or more of
the following actions.
o A publisher must set the updates-not-sent flag on any update
notification which is known to be missing information.
o It may choose to suspend and resume a subscription as per
[I-D.ietf-netconf-5277bis].
o When resuming an on-change subscription, the publisher should
generate a complete patch from the previous update notification.
If this is not possible and the synch-on-start option is
configured, then the full datastore contents may be sent instead
(effectively replacing the previous contents). If neither of
these are possible, then an updates-not-sent flag must be included
on the next push-change-update.
3.8.3. 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.
3.8.4. Push data streams
There are several conceptual data streams introduced in this
specification:
o yang-push includes the entirety of YANG data, including both
configuration and operational data.
o operational-push includes all operational (read-only) YANG data
o config-push includes all YANG configuration data.
It is conceivable to introduce other data streams with more limited
scope, for example:
o operdata-nocounts-push, a datastream containing all operational
(read-only) data with the exception of counters
o other custom datastreams
Those data streams make particular sense for use cases involving
service assurance (not relying on operational data), and for use
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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 yang-push to the same effect,
having those data streams greatly simplifies articulating
subscriptions in such scenarios.
3.8.5. 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 and push of operational counters may consume
considerable system resources. In addition the on-change push of
small amounts of configuration 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, it is important for an implementation to
understand what subscriptions it can or cannot support. It is far
preferable to decline a subscription request then to accept such a
request when it cannot be met.
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
different permissions than its parent, or (b) read access permissions
have been changed on nodes under the target node for the subscriber.
3.8.6. Not Notifiable YANG Objects
In some cases, a publisher supporting "on-change" notifications may
not be able to push updates for some object types "on-change".
Reasons for this might be that the value of the data node changes
frequently (e.g., a received-octets-counter), that small object
changes are frequent and meaningless (e.g., a temperature gauge
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changing 0.1 degrees), or that the implementation is not capable of
on-change notification of an object type.
The default assumption is that changes on all data nodes will be
reported on-change. However if a certain data node cannot do this,
it SHOULD be marked with the YANG extension not-notifiable-on-change.
4. A YANG data model for management of datastore push subscriptions
4.1. Overview
The YANG data model for datastore push subscriptions is depicted in
the following figure. Following Yang tree convention in the
depiction, brackets enclose list keys, "rw" means configuration, "ro"
operational state data, "?" designates optional nodes, "*" designates
nodes that can have multiple instances. Parantheses with a name in
the middle enclose choice and case nodes. A "+" at the end of a line
indicates that the line is to be concatenated with the subsequent
line. New YANG tree notation is the i] which indicates that the node
in that line has been brought in / imported from another model, and
an (a) which indicates this is the specific imported node augmented.
In the figure below, all have been imported from 5277bis. The model
consists mostly of augmentations to RPCs and notifications defined in
the data model for subscriptions for event notifications of
[I-D.ietf-netconf-5277bis].
module: ietf-yang-push
i] +--ro streams
i] | +--ro stream* stream
i] +--rw filters
i] | +--rw filter* [filter-id]
i] | +--rw filter-id filter-id
i] | +--rw (filter-type)?
i] | +--:(rfc5277)
i] | | +--rw filter?
| +--:(update-filter)
| +--rw (update-filter)?
| +--:(subtree)
| | +--rw subtree-filter?
| +--:(xpath)
| +--rw xpath-filter? yang:xpath1.0
i] +--rw subscription-config {configured-subscriptions}?
i] | +--rw subscription* [subscription-id]
i] | +--rw subscription-id subscription-id
i] | +--rw stream? stream
i] | +--rw encoding? encoding
(a) | +--rw (filter-type)?
i] | | +--:(rfc5277)
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i] | | | +--rw filter?
| | +--:(update-filter)
| | | +--rw (update-filter)?
| | | +--:(subtree)
| | | | +--ro subtree-filter?
| | | +--:(xpath)
| | | +--rw xpath-filter? yang:xpath1.0
i] | | +--:(by-reference)
i] | | +--rw filter-ref? filter-ref
i] | +--rw startTime? yang:date-and-time
i] | +--rw stopTime? yang:date-and-time
| +--rw (update-trigger)?
| | +--:(periodic)
| | | +--rw period yang:timeticks
| | | +--rw anchor-time? yang:date-and-time
| | +--:(on-change) {on-change}?
| | +--rw no-synch-on-start? empty
| | +--rw dampening-period yang:timeticks
| | +--rw excluded-change* change-type
i] | +--rw receivers
i] | | +--rw receiver* [address]
i] | | +--rw address inet:host
i] | | +--rw port inet:port-number
i] | | +--rw protocol? transport-protocol
i] | +--rw (push-source)?
i] | | +--:(interface-originated)
i] | | | +--rw source-interface? if:interface-ref
i] | | +--:(address-originated)
i] | | +--rw source-vrf? uint32
i] | | +--rw source-address inet:ip-address-no-zone
| +--rw dscp? inet:dscp
| +--rw subscription-priority? uint8
| +--rw subscription-dependency? string
(a) +--ro subscriptions
i] +--ro subscription*
i] +--ro subscription-id
i] +--ro configured-subscription?
i] +--ro subscription-status?
i] +--ro stream?
i] +--ro encoding?
(a) +--ro (filter-type)?
i] | +--:(rfc5277)
i] | | +--ro filter?
| +--:(update-filter)
| | +--ro (update-filter)?
| | +--:(subtree)
| | | +--ro subtree-filter?
| | +--:(xpath)
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| | +--ro xpath-filter?
i] | +--:(by-reference)
i] | +--ro filter-ref?
i] +--ro startTime?
i] +--ro stopTime?
+--ro (update-trigger)?
| +--:(periodic)
| | +--ro period
| | +--ro anchor-time?
| +--:(on-change) {on-change}?
| +--ro no-synch-on-start?
| +--ro dampening-period
| +--ro excluded-change*
i] +--ro receivers
i] | +--ro receiver*
i] | +--ro address
i] | +--ro port
i] | +--ro protocol?
i] +--ro (push-source)?
i] | +--:(interface-originated)
i] | | +--ro source-interface?
i] | +--:(address-originated)
i] | +--ro source-vrf?
i] | +--ro source-address
+--ro dscp?
+--ro subscription-priority?
+--ro subscription-dependency?
i] rpcs:
i] +---x establish-subscription
(a) | +---w input
i] | | +---w stream?
i] | | +---w encoding?
(a) | | +---w (filter-type)?
i] | | | +--:(rfc5277)
i] | | | | +---w filter?
| | | +--:(update-filter)
| | | | +---w (update-filter)?
| | | | +--:(subtree)
| | | | | +---w subtree-filter?
| | | | +--:(xpath)
| | | | +---w xpath-filter?
i] | | | +--:(by-reference)
i] | | | +---w filter-ref?
i] | | +---w startTime?
i] | | +---w stopTime?
| | +---w (update-trigger)?
| | | +--:(periodic)
| | | | +---w period
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| | | | +---w anchor-time?
| | | +--:(on-change) {on-change}?
| | | +---w no-synch-on-start?
| | | +---w dampening-period
| | | +---w excluded-change*
| | +---w dscp?
| | +---w subscription-priority?
| | +---w subscription-dependency?
i] | +--ro output
i] | +--ro subscription-result
i] | +--ro (result)?
i] | +--:(success)
i] | | +--ro subscription-id
(a) | +--:(no-success)
i] | +--ro stream?
i] | +--ro encoding?
(a) | +--ro (filter-type)?
i] | | +--:(rfc5277)
i] | | | +--ro filter?
| | +--:(update-filter)
| | | +---w (update-filter)?
| | | +--:(subtree)
| | | | +---w subtree-filter?
| | | +--:(xpath)
| | | +---w xpath-filter?
i] | | +--:(by-reference)
i] | | +--ro filter-ref?
i] | +--ro startTime?
i] | +--ro stopTime?
| +--ro (update-trigger)?
| | +--:(periodic)
| | | +--ro period
| | | +--ro anchor-time?
| | +--:(on-change) {on-change}?
| | +--ro no-synch-on-start?
| | +--ro dampening-period
| | +--ro excluded-change*
| +--ro dscp?
| +--ro subscription-priority?
| +--ro subscription-dependency?
i] +---x modify-subscription
i] | +---w input
i] | | +---w subscription-id?
i] | | +---w (filter-type)?
i] | | | +--:(rfc5277)
i] | | | | +---w filter?
| | | +--:(update-filter)
| | | | +---w (update-filter)?
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| | | | +--:(subtree)
| | | | | +---w subtree-filter?
| | | | +--:(xpath)
| | | | +---w xpath-filter?
i] | | | +--:(by-reference)
i] | | | +---w filter-ref?
i] | | +---w startTime?
i] | | +---w stopTime?
| | +---w (update-trigger)?
| | +--:(periodic)
| | | +---w period
| | | +---w anchor-time?
| | +--:(on-change) {on-change}?
| | +---w dampening-period
| | +---w excluded-change*
i] | +--ro output
i] | +--ro subscription-result
i] | +--ro (result)?
i] | +--:(success)
i] | | +--ro subscription-id
i] | +--:(no-success)
i] | +--ro stream?
i] | +--ro encoding?
i] | +--ro (filter-type)?
i] | | +--:(rfc5277)
i] | | | +--ro filter?
| | +--:(update-filter)
| | | +---w (update-filter)?
| | | +--:(subtree)
| | | | +---w subtree-filter?
| | | +--:(xpath)
| | | +---w xpath-filter?
i] | | +--:(by-reference)
i] | | +--ro filter-ref?
i] | +--ro startTime?
i] | +--ro stopTime?
| +--ro (update-trigger)?
| | +--:(periodic)
| | | +--ro period
| | | +--ro anchor-time?
| | +--:(on-change) {on-change}?
| | +--ro no-synch-on-start?
| | +--ro dampening-period
| | +--ro excluded-change*
| +--ro dscp?
| +--ro subscription-priority?
| +--ro subscription-dependency?
i] +---x delete-subscription
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i] +---w input
i] | +---w subscription-id
i] +--ro output
i] +--ro subscription-result
(a) notifications
(a) +---n subscription-started
i] | +--ro subscription-id
i] | +--ro stream?
i] | +--ro encoding?
(a) | +--ro (filter-type)?
i] | | +--:(rfc5277)
i] | | | +--ro filter?
| | +--:(update-filter)
| | | +--rw (update-filter)?
| | | +--:(subtree)
| | | | +--ro subtree-filter?
| | | +--:(xpath)
| | | +--rw xpath-filter?
i] | | +--:(by-reference)
i] | | +--ro filter-ref?
i] | +--ro startTime?
i] | +--ro stopTime?
| +--ro (update-trigger)?
| | +--:(periodic)
| | | +--ro period
| | | +--ro anchor-time?
| | +--:(on-change) {on-change}?
| | +--ro no-synch-on-start?
| | +--ro dampening-period
| | +--ro excluded-change*
| +--ro dscp?
| +--ro subscription-priority?
| +--ro subscription-dependency?
(a) +---n subscription-modified
i] | +--ro subscription-id
i] | +--ro stream?
i] | +--ro encoding?
(a) | +--ro (filter-type)?
i] | | +--:(rfc5277)
i] | | | +--ro filter?
| | +--:(update-filter)
| | | +--rw (update-filter)?
| | | +--:(subtree)
| | | | +--ro subtree-filter?
| | | +--:(xpath)
| | | +--rw xpath-filter?
i] | | +--:(by-reference)
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i] | | +--ro filter-ref?
i] | +--ro startTime?
i] | +--ro stopTime?
| +--ro (update-trigger)?
| | +--:(periodic)
| | | +--ro period
| | | +--ro anchor-time?
| | +--:(on-change) {on-change}?
| | +--ro no-synch-on-start?
| | +--ro dampening-period
| | +--ro excluded-change*
| +--ro dscp?
| +--ro subscription-priority?
| +--ro subscription-dependency?
i] +---n subscription-terminated
i] | +--ro subscription-id
i] | +--ro reason?
i] +---n subscription-suspended
i] | +--ro subscription-id
i] | +--ro reason?
i] +---n subscription-resumed
i] | +--ro subscription-id
i] +---n replay-complete
i] | +--ro subscription-id
i] +---n notification-complete
i] | +--ro subscription-id
+---n push-update
| +--ro subscription-id
| +--ro time-of-update?
| +--ro updates-not-sent?
| +--ro datastore-contents?
+---n push-change-update {on-change}?
+--ro subscription-id
+--ro time-of-update?
+--ro updates-not-sent?
+--ro datastore-changes?
Figure 9: Model structure
The components of the model are described in the following
subsections.
4.2. Update streams
Container "update-streams" is used to indicate which data streams are
provided by the system and can be subscribed to. For this purpose,
it contains a leaf list of data nodes identifying the supported
streams.
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4.3. Filters
Container "filters" contains a list of configurable data filters,
each specified in its own list element. This allows users to
configure filters separately from an actual subscription, which can
then be referenced from a subscription. This facilitates the reuse
of filter definitions, which can be important in case of complex
filter conditions.
One of three types of filters can be specified as part of a filter
list element. Subtree filters follow syntax and semantics of RFC
6241 and allow to specify which subtree(s) to subscribe to. In
addition, XPath filters can be specified for more complex filter
conditions. Finally, filters can be specified using syntax and
semantics of RFC5277.
It is conceivable to introduce other types of filters; in that case,
the data model needs to be augmented accordingly.
4.4. Subscription configuration
As an optional feature, configured-subscriptions, allows for the
configuration of subscriptions as opposed to RPC. Subscriptions
configurations are represented by list subscription-config. Each
subscription is represented through its own list element and includes
the following components:
o "subscription-id" is an identifier used to refer to the
subscription.
o "stream" refers to the stream being subscribed to. The
subscription model assumes the presence of perpetual and
continuous streams of updates. Various streams are defined:
"push-update" covers the entire set of YANG data in the publisher.
"operational-push" covers all operational data, while "config-
push" covers all configuration data. Other streams could be
introduced in augmentations to the model by introducing additional
identities.
o "encoding" refers to the encoding requested for the data updates.
By default, updates are encoded using XML. However, JSON can be
requested as an option if the json-enconding feature is supported.
Other encodings may be supported in the future.
o "anchor-time" is a timestamp. When used in conjunction with
period, the boundaries of periodic update periods may be
calculated.
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o Filters for a subscription can be specified using a choice,
allowing to either reference a filter that has been separately
configured or entering its definition inline.
o A choice of subscription policies allows to define when to send
new updates - periodic or on change.
* For periodic subscriptions, the trigger is defined by a
"period", a parameter that defines the interval with which
updates are to be pushed. The start time of the subscription
serves as anchor time, defining one specific point in time at
which an update needs to be sent. Update intervals always fall
on the points in time that are a multiple of a period after the
start time.
* For on-change subscriptions, the trigger occurs whenever a
change in the subscribed information is detected. On-change
subscriptions have more complex semantics that is guided by
additional parameters. "dampening-period" specifies the
interval that must pass before a successive update for the same
data node is sent. The first time a change is detected, the
update is sent immediately. If a subsequent change is
detected, another update is only sent once the dampening period
has passed, containing the value of the data node that is then
valid. "excluded-change" allows to restrict the types of
changes for which updates are sent (changes to object values,
object creation or deletion events). "no-synch-on-start" is a
flag that allows to specify whether or not a complete update
with all the subscribed data should be sent at the beginning of
a subscription; if the flag is omitted, a complete update is
sent to facilitate synchronization. It is conceivable to
augment the data model with additional parameters in the future
to specify even more refined policies, such as parameters that
specify the magnitude of a change that must occur before an
update is triggered.
o This is followed with a list of receivers for the subscription,
indicating for each receiver the transport that should be used for
push updates (if options other than Netconf are supported). It
should be noted that the receiver does not have to be the same
system that configures the subscription.
o Finally, "push-source" can be used to specify the source of push
updates, either a specific interface or publisher address.
A subscription established through configuration cannot be deleted
using an RPC. Likewise, subscriptions established through RPC cannot
be deleted through configuration.
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The deletion of a subscription, whether through RPC or configuration,
results in immediate termination of the subsciption.
4.5. Subscription monitoring
Subscriptions can be subjected to management themselves. For
example, it is possible that a publisher 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 publisher needs to be able to temporarily suspend
the subscription, or even to terminate it. More generally, the
publisher should provide a means by which the status of subscriptions
can be monitored.
Container "subscriptions" contains the state of all subscriptions
that are currently active. This includes subscriptions that were
established (and have not yet been deleted) using RPCs, as well as
subscriptions that have been configured as part of configuration.
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 parameters that are in effect. The subscription status
indicates whether the subscription is currently active and healthy,
or if it is degraded in some form. Leaf "configured-subscription"
indicates whether the subscription came into being via configuration
or via RPC.
Subscriptions that were established by RPC are removed from the list
once they expire (reaching stop-time )or when they are terminated.
Subscriptions that were established by configuration need to be
deleted from the configuration by a configuration editing operation.
4.6. Notifications
4.6.1. Monitoring and OAM Notifications
OAM notifications are reused from [I-D.ietf-netconf-5277bis]. Some
have augmentations to include new objects defined in this draft.
Still to be investigated is whether a publisher might also 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.
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4.6.2. Update Notifications
The data model introduces two YANG notifications for the actual
updates themselves.
Notification "push-update" is used to send a complete snapshot of the
data that has been subscribed to, with all YANG object filters
applied. The notification is used for periodic subscription updates
in a periodic subscription.
The notification can also be used in an on-change subscription for
the purposes of allowing a receiver to "synch". Specifically, it is
used at the start of an on-change subscription, unless no-synch-on-
start is specified for the subscription. In addition, it MAY be used
during the subscription, for example if change updates were not sent
as indicated by the "updates-not-sent" flag (see below), or for synch
updates at longer period intervals (such as once per day) to mitigate
the possibility of any application-dependent synchronization drift.
The trigger for sending a push-update notification in conjunction
with on-change subscriptions are at this point outside the scope of
the specification.
The format and syntax of the contained data corresponds to the format
and syntax of data that would be returned in a corresponding get
operation with the same filter parameters applied.
Notification "push-change-update" is used to send data updates for
changes that have occurred in the subscribed data. This notification
is used only in conjunction with on-change subscriptions.
The data updates are encoded analogous to the syntax of a
corresponding yang-patch operation. It corresponds to the data that
would be contained in a yang-patch operation applied to the YANG
datastore at the previous update, to result in the current state (and
applying it also to operational data).
In rare circumstances, the notification can include a flag "updates-
not-sent". This is a flag which indicates that not all changes which
have occurred since the last update are actually included with this
update. In other words, the publisher has failed to fulfill its full
subscription obligations, for example in cases where it was not able
to keep up with a change burst. To facilitate synchronization, a
publisher MAY subsequently send a push-update containing a full
snapshot of subscribed data. Such a push-update might also be
triggered by a subscriber requesting an on-demand synchronization.
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4.7. RPCs
YANG-Push subscriptions are established, modified, and deleted using
three RPCs.
4.7.1. Establish-subscription RPC
The subscriber sends an establish-subscription RPC with the
parameters in section 3.1. For instance
<netconf:rpc message-id="101"
xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
<establish-subscription
xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
<stream>push-update</stream>
<filter netconf:type="xpath"
xmlns:ex="http://example.com/sample-data/1.0"
select="/ex:foo"/>
<period>500</period>
<encoding>encode-xml</encoding>
</establish-subscription>
</netconf:rpc>
Figure 10: Establish-subscription RPC
The publisher must respond explicitly positively (i.e., subscription
accepted) or negatively (i.e., subscription rejected) to the request.
Positive responses include the subscription-id of the accepted
subscription. In that case a publisher may respond:
<rpc-reply message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<subscription-result
xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
ok
</subscription-result>
<subscription-id
xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
52
</subscription-id>
</rpc-reply>
Figure 11: Establish-subscription positive RPC response
A subscription can be rejected for multiple reasons, including the
lack of authorization to establish a subscription, the lack of read
authorization on the requested data node, or the inability of the
publisher to provide a stream with the requested semantics.
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When the requester is not authorized to read the requested data node,
the returned "error-info"; indicates an authorization error and the
requested node. For instance, if the above request was unauthorized
to read node "ex:foo" the publisher may return:
<rpc-reply message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<subscription-result
xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
error-data-not-authorized
</subscription-result>
</rpc-reply>
Figure 12: Establish-subscription access denied response
If a request is rejected because the publisher is not able to serve
it, the publisher SHOULD include in the returned error what
subscription parameters would have been accepted for the request.
However, there are no guarantee that subsequent requests for this
subscriber or others will in fact be accepted.
For example, for the following request:
<netconf:rpc message-id="101"
xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
<establish-subscription
xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
<stream>push-update</stream>
<filter netconf:type="xpath"
xmlns:ex="http://example.com/sample-data/1.0"
select="/ex:foo"/>
<dampening-period>10</dampening-period>
<encoding>encode-xml</encoding>
</establish-subscription>
</netconf:rpc>
Figure 13: Establish-subscription request example 2
A publisher that cannot serve on-change updates but periodic updates
might return the following:
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<rpc-reply message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<subscription-result
xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
error-no-such-option
</subscription-result>
<period>100</period>
</rpc-reply>
Figure 14: Establish-subscription error response example 2
4.7.2. Modify-subscription RPC
The subscriber may send a modify-subscription PRC for a subscription
previously established using RPC The subscriber may change any
subscription parameters by including the new values in the modify-
subscription RPC. Parameters not included in the rpc should remain
unmodified. For illustration purposes we include an exchange example
where a subscriber modifies the period of the subscription.
<netconf:rpc message-id="102"
xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
<modify-subscription
xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
<stream>push-update</stream>
<subscription-id>
1011
</subscription-id>
<filter netconf:type="xpath"
xmlns:ex="http://example.com/sample-data/1.0"
select="/ex:foo"/>
<period>250</period>
<encoding>encode-xml</encoding>
</modify-subscription>
</netconf:rpc>
Figure 15: Modify subscription request
The publisher must respond explicitly positively (i.e., subscription
accepted) or negatively (i.e., subscription rejected) to the request.
Positive responses include the subscription-id of the accepted
subscription. In that case a publisher may respond:
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<rpc-reply message-id="102"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<subscription-result
xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
ok
</subscription-result>
<subscription-id
xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
1011
</subscription-id>
</rpc-reply>
Figure 16: Modify subscription response
If the subscription modification is rejected, the publisher must send
a response like it does for an establish-subscription and maintain
the subscription as it was before the modification request. A
subscription may be modified multiple times.
A configured subscription cannot be modified using modify-
subscription RPC. Instead, the configuration needs to be edited as
needed.
4.7.3. Delete-subscription RPC
To stop receiving updates from a subscription and effectively delete
a subscription that had previously been established using an
establish-subscription RPC, a subscriber can send a delete-
subscription RPC, which takes as only input the subscription-id. For
example:
<netconf:rpc message-id="103"
xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
<delete-subscription
xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push:1.0">
<subscription-id>
1011
</subscription-id>
</delete-subscription>
</netconf:rpc>
<rpc-reply message-id="103"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<ok/>
</rpc-reply>
Figure 17: Delete subscription
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Configured subscriptions cannot be deleted via RPC, but have to be
removed from the configuration.
4.7.4. YANG Module Synchronization
In order to fully support datastore replication, the receiver needs
to know the YANG module library that is in use by server that is
being replicated. The YANG 1.0 module library information is sent by
a NETCONF server in the NETCONF 'hello' message. For YANG 1.1
modules and all modules used with the RESTCONF
[I-D.ietf-netconf-restconf] protocol, this information is provided by
the YANG Library module (ietf-yang-library.yang from [RFC7895]. The
YANG library information is important for the receiver to reproduce
the set of object definitions used by the replicated datastore.
The YANG library includes a module list with the name, revision,
enabled features, and applied deviations for each YANG YANG module
implemented by the server. The receiver is expected to know the YANG
library information before starting a subscription. The "/modules-
state/module-set-id" leaf in the "ietf-yang-library" module can be
used to cache the YANG library information. [ED. NOTE: Should
"module-set-id" be added to establish-subscription response?]
The set of modules, revisions, features, and deviations can change at
run-time (if supported by the server implementation). In this case,
the receiver needs to be informed of module changes before data nodes
from changed modules can be processed correctly. The YANG library
provides a simple "yang-library-change" notification that informs the
client that the library has changed somehow. The receiver then needs
to re-read the entire YANG library data for the replicated server in
order to detect the specific YANG library changes. The "ietf-
netconf-notifications" module defined in [RFC6470] contains a
"netconf-capability-change" notification that can identify specific
module changes. For example, the module URI capability of a newly
loaded module will be listed in the "added-capability" leaf-list, and
the module URI capability of an removed module will be listed in the
"deleted-capability" leaf-list.
5. YANG module
<CODE BEGINS> file "ietf-yang-push@2016-10-28.yang"
module ietf-yang-push {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-yang-push";
prefix yp;
import ietf-inet-types {
prefix inet;
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}
import ietf-yang-types {
prefix yang;
}
import ietf-event-notifications {
prefix notif-bis;
}
organization "IETF";
contact
"WG Web: <http://tools.ietf.org/wg/netconf/>
WG List: <mailto:netconf@ietf.org>
WG Chair: Mahesh Jethanandani
<mailto:mjethanandani@gmail.com>
WG Chair: Mehmet Ersue
<mailto:mehmet.ersue@nokia.com>
Editor: Alexander Clemm
<mailto:alex@sympotech.com>
Editor: Eric Voit
<mailto:evoit@cisco.com>
Editor: Alberto Gonzalez Prieto
<mailto:albertgo@cisco.com>
Editor: Ambika Prasad Tripathy
<mailto:ambtripa@cisco.com>
Editor: Einar Nilsen-Nygaard
<mailto:einarnn@cisco.com>
Editor: Andy Bierman
<mailto:andy@yumaworks.com>
Editor: Balazs Lengyel
<mailto:balazs.lengyel@ericsson.com>";
description
"This module contains conceptual YANG specifications
for YANG push.";
revision 2016-10-28 {
description
"Updates to simplify modify-subscription, add anchor-time";
reference "YANG Datastore Push, draft-ietf-netconf-yang-push-04";
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}
feature on-change {
description
"This feature indicates that on-change updates are
supported.";
}
/*
* IDENTITIES
*/
/* Additional errors for subscription operations */
identity error-data-not-authorized {
base notif-bis:error;
description
"No read authorization for a requested data node.";
}
/* Additional types of streams */
identity update-stream {
description
"Base identity to represent a conceptual system-provided
datastream of datastore updates with predefined semantics.";
}
identity yang-push {
base update-stream;
description
"A conceptual datastream consisting of all datastore
updates, including operational and configuration data.";
}
identity operational-push {
base update-stream;
description
"A conceptual datastream consisting of updates of all
operational data.";
}
identity config-push {
base update-stream;
description
"A conceptual datastream consisting of updates of all
configuration data.";
}
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identity custom-stream {
base update-stream;
description
"A conceptual datastream for datastore
updates with custom updates as defined by a user.";
}
/* Additional transport option */
identity http2 {
base notif-bis:transport;
description
"HTTP2 notifications as a transport";
}
/*
* TYPE DEFINITIONS
*/
typedef filter-id {
type uint32;
description
"A type to identify filters which can be associated with a
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.";
}
typedef update-stream {
type identityref {
base update-stream;
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}
description
"Specifies a system-provided datastream.";
}
grouping update-filter {
description
"This groupings defines filters for push updates for a
datastore tree. The filters define which updates are of
interest in a push update subscription. Mixing and matching
of multiple filters does not occur at the level of this
grouping. When a push-update subscription is created, the
filter can be a regular subscription filter, or one of the
additional filters that are defined in this grouping.";
choice update-filter {
description
"Define filters regarding which data nodes to include
in push updates";
case subtree {
description
"Subtree filter.";
anyxml subtree-filter {
description
"Subtree-filter used to specify the data nodes targeted
for subscription within a subtree, or subtrees, of a
conceptual YANG datastore. Objects matching the filter
criteria will traverse the filter. The syntax follows
the subtree filter syntax specified in RFC 6241,
section 6.";
reference "RFC 6241 section 6";
}
}
case xpath {
description
"XPath filter";
leaf xpath-filter {
type yang:xpath1.0;
description
"Xpath defining the data items of interest.";
}
}
}
}
grouping update-policy {
description
"This grouping describes the conditions under which an
update will be sent as part of an update stream.";
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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;
mandatory true;
description
"Duration of time which should occur between periodic
push updates. Where the anchor of a start-time is
available, the push will include the objects and their
values which exist at an exact multiple of timeticks
aligning to this start-time anchor.";
}
leaf anchor-time {
type yang:date-and-time;
description
"Designates a timestamp from which the series of
periodic push updates are computed. The next update
will take place at the next period interval from the
anchor time. For example, for an anchor time at the
top of a minute and a period interval of a minute,
the next update will be sent at the top of the next
minute.";
}
}
case on-change {
if-feature "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 no-synch-on-start {
type empty;
description
"This leaf acts as a flag that determines behavior at the
start of the subscription. When present,
synchronization of state at the beginning of the
subscription is outside the scope of the subscription.
Only updates about changes that are observed from the
start time, i.e. only push-change-update notifications
are sent.
When absent (default behavior), in order to facilitate
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a receiver's synchronization, a full update is sent
when the subscription starts using a push-update
notification, just like in the case of a periodic
subscription. After that, push-change-update
notifications only are sent unless the Publisher chooses
to resynch the subscription again.";
}
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.";
}
}
}
}
grouping subscription-qos {
description
"This grouping describes Quality of Service information
concerning a subscription. This information is passed to lower
layers for transport priortization and treatment";
leaf dscp {
if-feature "notif-bis:configured-subscriptions";
type inet:dscp;
default "0";
description
"The push update's IP packet transport priority.
This is made visible across network hops to receiver.
The transport priority is shared for all receivers of
a given subscription.";
}
leaf subscription-priority {
type uint8;
description
"Relative priority for a subscription. Allows an
underlying transport layer perform informed load
balance allocations between various subscriptions";
}
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leaf subscription-dependency {
type string;
description
"Provides the Subscription ID of a parent subscription
without which this subscription should not exist. In
other words, there is no reason to stream these objects
if another subscription is missing.";
}
}
augment "/notif-bis:establish-subscription/notif-bis:input" {
description
"Define additional subscription parameters that apply
specifically to push updates";
uses update-policy;
uses subscription-qos;
}
augment "/notif-bis:establish-subscription/notif-bis:input/"+
"notif-bis:filter-type" {
description
"Add push filters to selection of filter types.";
case update-filter {
description
"Additional filter options for push subscription.";
uses update-filter;
}
}
augment "/notif-bis:establish-subscription/notif-bis:output" {
description
"Allow to return additional subscription parameters that apply
specifically to push updates.";
uses update-policy;
uses subscription-qos;
}
augment "/notif-bis:establish-subscription/notif-bis:output/"+
"notif-bis:result/notif-bis:no-success/notif-bis:filter-type" {
description
"Add push filters to selection of filter types.";
case update-filter {
description
"Additional filter options for push subscription.";
uses update-filter;
}
}
augment "/notif-bis:modify-subscription/notif-bis:input" {
description
"Define additional subscription parameters that apply
specifically to push updates.";
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uses update-policy;
}
augment "/notif-bis:modify-subscription/notif-bis:input/"+
"notif-bis:filter-type" {
description
"Add push filters to selection of filter types.";
case update-filter {
description
"Additional filter options for push subscription.";
uses update-filter;
}
}
augment "/notif-bis:modify-subscription/notif-bis:output" {
description
"Allow to retun additional subscription parameters that apply
specifically to push updates.";
uses update-policy;
uses subscription-qos;
}
augment "/notif-bis:modify-subscription/notif-bis:output/"+
"notif-bis:result/notif-bis:no-success/notif-bis:filter-type" {
description
"Add push filters to selection of filter types.";
case update-filter {
description
"Additional filter options for push subscription.";
uses update-filter;
}
}
notification push-update {
description
"This notification contains a push update, containing
data subscribed to via a subscription.
This notification is sent for periodic updates, for a
periodic subscription. It can also be used for
synchronization updates of an on-change subscription.
This notification shall only be sent to receivers
of a subscription; it does not constitute a general-purpose
notification.";
leaf subscription-id {
type notif-bis:subscription-id;
mandatory true;
description
"This references the subscription because of which the
notification is sent.";
}
leaf time-of-update {
type yang:date-and-time;
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description
"This leaf contains the time of the update.";
}
leaf updates-not-sent {
type empty;
description
"This is a flag which indicates that not all data nodes
subscribed to are included included with this
update. In other words, the publisher has failed to
fulfill its full subscription obligations.
This may lead to intermittent loss of synchronization
of data at the client. Synchronization at the client
can occur when the next push-update is received.";
}
anydata datastore-contents {
description
"This contains the updated data. It constitutes a snapshot
at the time-of-update of the set of data that has been
subscribed to. The format and syntax of the data
corresponds to the format and syntax of data that would be
returned in a corresponding get operation with the same
filter parameters applied.";
}
}
notification push-change-update {
if-feature "on-change";
description
"This notification contains an on-change push update.
This notification shall only be sent to the receivers
of a subscription; it does not constitute a general-purpose
notification.";
leaf subscription-id {
type notif-bis:subscription-id;
mandatory true;
description
"This references the subscription because of which the
notification is sent.";
}
leaf time-of-update {
type yang:date-and-time;
description
"This leaf contains the time of the update, i.e. the
time at which the change was observed.";
}
leaf updates-not-sent {
type empty;
description
"This is a flag which indicates that not all changes which
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have occured since the last update are included with this
update. In other words, the publisher has failed to
fulfill its full subscription obligations, for example in
cases where it was not able to keep up with a change burst.
To facilitate synchronization, a publisher MAY subsequently
send a push-update containing a full snapshot of subscribed
data. Such a push-update might also be triggered by a
subscriber requesting an on-demand synchronization.";
}
anydata datastore-changes {
description
"This contains datastore contents that has changed
since the previous update, per the terms of the
subscription. Changes are encoded analogous to
the syntax of a corresponding yang-patch operation,
i.e. a yang-patch operation applied to the YANG datastore
implied by the previous update to result in the current
state (and assuming yang-patch could also be applied to
operational data).";
}
}
augment "/notif-bis:subscription-started" {
description
"This augmentation adds push subscription parameters
to the notification that a subscription has
started and data updates are beginning to be sent.
This notification shall only be sent to receivers
of a subscription; it does not constitute a general-purpose
notification.";
uses update-policy;
uses subscription-qos;
}
augment "/notif-bis:subscription-started/notif-bis:filter-type" {
description
"This augmentation allows to include additional update filters
options to be included as part of the notification that a
subscription has started.";
case update-filter {
description
"Additional filter options for push subscription.";
uses update-filter;
}
}
augment "/notif-bis:subscription-modified" {
description
"This augmentation adds push subscription parameters
to the notification that a subscription has
been modified.
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This notification shall only be sent to receivers
of a subscription; it does not constitute a general-purpose
notification.";
uses update-policy;
uses subscription-qos;
}
augment "/notif-bis:subscription-modified/notif-bis:filter-type" {
description
"This augmentation allows to include additional update
filters options to be included as part of the notification
that a subscription has been modified.";
case update-filter {
description
"Additional filter options for push subscription.";
uses update-filter;
}
}
augment "/notif-bis:filters/notif-bis:filter/"+
"notif-bis:filter-type" {
description
"This container adds additional update filter options
to the list of configurable filters
that can be applied to subscriptions. This facilitates
the reuse of complex filters once defined.";
case update-filter {
uses update-filter;
}
}
augment "/notif-bis:subscription-config/notif-bis:subscription" {
description
"Contains the list of subscriptions that are configured,
as opposed to established via RPC or other means.";
uses update-policy;
uses subscription-qos;
}
augment "/notif-bis:subscription-config/notif-bis:subscription/"+
"notif-bis:filter-type" {
description
"Add push filters to selection of filter types.";
case update-filter {
uses update-filter;
}
}
augment "/notif-bis:subscriptions/notif-bis:subscription" {
description
"Contains the list of currently active subscriptions,
i.e. subscriptions that are currently in effect,
used for subscription management and monitoring purposes.
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This includes subscriptions that have been setup via RPC
primitives, e.g. establish-subscription, delete-subscription,
and modify-subscription, as well as subscriptions that
have been established via configuration.";
uses update-policy;
uses subscription-qos;
}
augment "/notif-bis:subscriptions/notif-bis:subscription/"+
"notif-bis:filter-type" {
description
"Add push filters to selection of filter types.";
case update-filter {
description
"Additional filter options for push subscription.";
uses update-filter;
}
}
}
<CODE ENDS>
6. Security Considerations
Subscriptions could be used to attempt to overload publishers of YANG
datastores. For this reason, it is important that the publisher has
the ability to decline a subscription request if it would deplete its
resources. In addition, a publisher needs to be able to suspend an
existing subscription when needed. When this occur, the subscription
status is updated accordingly and the receivers are notified.
Likewise, requests for subscriptions need to be properly authorized.
A subscription could be used to retrieve data in subtrees that a
receiver has no 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
receiver needs therefore to be filtered accordingly, just like if the
data were being retrieved on-demand. The Netconf Authorization
Control Model applies.
A subscription could be configured on another receiver's behalf, with
the goal of flooding that receiver with updates. One or more
publishers could be used to overwhelm a receiver which doesn't even
support subscriptions. Receivers which do not want pushed data need
only terminate or refuse any transport sessions from the publisher.
In addition, the Netconf Authorization Control Model SHOULD be used
to control and restrict authorization of subscription configuration.
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7. Acknowledgments
For their valuable comments, discussions, and feedback, we wish to
acknowledge Tim Jenkins, Kent Watsen, Susan Hares, Yang Geng, Peipei
Guo, Michael Scharf, Sharon Chisholm, and Guangying Zheng.
8. References
8.1. Normative References
[I-D.ietf-netconf-5277bis]
Clemm, A., Gonzalez Prieto, A., Voit, E., Tripathy, A.,
Nilsen-Nygaard, E., Chisholm, S., and H. Trevino,
"Subscribing to YANG-Defined Event Notifications", draft-
ietf-netconf-5277bis-01 (work in progress), October 2016.
[I-D.ietf-netconf-yang-patch]
Bierman, A., Bjorklund, M., and K. Watsen, "YANG Patch
Media Type", draft-ietf-netconf-yang-patch-12 (work in
progress), September 2016.
[RFC6470] Bierman, A., "Network Configuration Protocol (NETCONF)
Base Notifications", RFC 6470, February 2012.
[RFC6536] Bierman, A. and M. Bjorklund, "Network Configuration
Protocol (NETCONF) Access Control Model", RFC 6536, March
2012.
[RFC7895] Bierman, A., Bjorklund, M., and K. Watsen, "YANG Module
Library", RFC 7895, June 2016.
[RFC7950] Bjorklund, M., "The YANG 1.1 Data Modeling Language",
RFC 7950, August 2016.
[RFC7951] Lhotka, L., "JSON Encoding of Data Modeled with YANG",
RFC 7951, August 2016.
8.2. Informative References
[I-D.ietf-netconf-restconf]
Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", I-D draft-ietf-netconf-restconf-17, September
2016.
[RFC1157] Case, J., "A Simple Network Management Protocol (SNMP)",
RFC 1157, May 1990.
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Internet-Draft YANG-Push October 2016
[RFC5277] Chisholm, S. and H. Trevino, "NETCONF Event
Notifications", RFC 5277, July 2008.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
Bierman, "Network Configuration Protocol (NETCONF)",
RFC 6241, June 2011.
[RFC7923] Voit, E., Clemm, A., and A. Gonzalez Prieto, "Requirements
for Subscription to YANG Datastores", RFC 7923, June 2016.
Appendix A. Issues that are currently being worked and resolved
(To be removed by RFC editor prior to publication)
A.1. Unresolved and yet-to-be addressed issues
Which stream types to introduce, if any based on implications of
opstate. Current list includes streams for all operational and for
all config data. Consider adding stream for operational data minus
counters.
We need a new Metadata filter. But so does traditional GET. This
should be relevant independent of subscriptions. This has
implications of ephemeral requirements from I2RS
Should we allow an interplay of filter types in a single
subscription. Or should we keep them fully independent.
Do we add a counter for the number of object changes during a
dampening period?
A.2. Agreement in principal
Do we need an extension for NACM to support filter out datastore
nodes for which the receiver has no read access? (And how does this
differ from existing GET, which must do the same filtering?) In
5277, such filtering is done at the notification level. Yang-push
includes notification-content filtering. This may be very expensive
in terms of processing. Andy suggestion: only accept Yang-push
subscriptions for subtrees the user has rights for all the nodes in
the subtree. Changes to those rights trigger a subscription
termination. Should we codify this, or let vendors determine when
per subtree filtering might be applied?
Need to add a new RPC to request enabling a resynch for an existing
on-change subscription exposed on publisher
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Appendix B. Changes between revisions
(To be removed by RFC editor prior to publication)
v03 to v04
o Updates-not-sent flag added
o Not notifiable extension added
o Dampening period is for whole subscription, not single objects
o Moved start/stop into rfc5277bis
o Client and Server changed to subscriber, publisher, and receiver
o Anchor time for periodic
o Message format for synchronization (i.e. synch-on-start)
o Material moved into 5277bis
o QoS parameters supported, by not allowed to be modified by RPC
o Text updates throughout
Authors' Addresses
Alexander Clemm
Sympotech
Email: alex@sympotech.com
Eric Voit
Cisco Systems
Email: evoit@cisco.com
Alberto Gonzalez Prieto
Cisco Systems
Email: albertgo@cisco.com
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Ambika Prasad Tripathy
Cisco Systems
Email: ambtripa@cisco.com
Einar Nilsen-Nygaard
Cisco Systems
Email: einarnn@cisco.com
Andy Bierman
YumaWorks
Email: andy@yumaworks.com
Balazs Lengyel
Ericsson
Email: balazs.lengyel@ericsson.com
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