wdiff rfc8639v1.txt rfc8639.txt

Internet Engineering Task Force (IETF) E. Voit
Request for Comments: 8639 Cisco Systems
Category: Standards Track A. Clemm
ISSN: 2070-1721 Futurewei
A. Gonzalez Prieto
Microsoft
E. Nilsen-Nygaard
A. Tripathy
Cisco Systems
August 2019

A YANG Data Model for Subscriptions to Event Notifications

Abstract

This document defines a YANG data model and associated mechanisms
enabling subscriber-specific subscriptions to a publisher's event
streams. Applying these elements allows a subscriber to request and
receive a continuous, customized feed of publisher-generated
information.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8639.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Solution Overview . . . . . . . . . . . . . . . . . . . . 5
1.4. Relationship to RFC 5277 . . . . . . . . . . . . . . . . 6
2. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Event Streams . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Event Stream Filters . . . . . . . . . . . . . . . . . . 8
2.3. QoS . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4. Dynamic Subscriptions . . . . . . . . . . . . . . . . . . 9
2.5. Configured Subscriptions . . . . . . . . . . . . . . . . 18
2.6. Event Record Delivery . . . . . . . . . . . . . . . . . . 25
2.7. Subscription State Change Notifications . . . . . . . . . 26
2.8. Subscription Monitoring . . . . . . . . . . . . . . . . . 32
2.9. Support for the "ietf-subscribed-notifications" YANG
Module . . . . . . . . . . . . . . . . . . . . . . . . . 33
3. YANG Data Model Tree Diagrams . . . . . . . . . . . . . . . . 33
3.1. The "streams" Container . . . . . . . . . . . . . . . . . 33
3.2. The "filters" Container . . . . . . . . . . . . . . . . . 34
3.3. The "subscriptions" Container . . . . . . . . . . . . . . 34
4. Event Notification Subscription YANG Module . . . . . . . . . 36
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 64
6. Implementation Considerations . . . . . . . . . . . . . . . . 64 65
7. Transport Requirements . . . . . . . . . . . . . . . . . . . 65
8. Security Considerations . . . . . . . . . . . . . . . . . . . 66
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 70
9.1. Normative References . . . . . . . . . . . . . . . . . . 70
9.2. Informative References . . . . . . . . . . . . . . . . . 72
Appendix A. Example Configured Transport Augmentation . . . . . 73
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 74 75
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 75

1. Introduction

This document defines a YANG data model and associated mechanisms
enabling subscriber-specific subscriptions to a publisher's event
streams. This effectively enables a "subscribe, then publish"
capability where the customized information needs and access
permissions of each target receiver are understood by the publisher
before subscribed event records are marshaled and pushed. The
receiver then gets a continuous, customized feed of
publisher-generated information.

While the functionality defined in this document is transport
agnostic, transports like the Network Configuration Protocol
(NETCONF) [RFC6241] or RESTCONF [RFC8040] can be used to configure or
dynamically signal subscriptions. Bindings for subscribed event
record delivery for NETCONF and RESTCONF are defined in [RFC8640] and
[RESTCONF-Notif], respectively.

The YANG data model defined in this document conforms to the Network
Management Datastore Architecture defined in [RFC8342].

1.1. Motivation

Various limitations to subscriptions as described in [RFC5277] were
alleviated to some extent by the requirements provided in [RFC7923].
Resolving any remaining issues is the primary motivation for this
work. Key capabilities supported by this document include:

o multiple subscriptions on a single transport session

o support for dynamic and configured subscriptions

o modification of an existing subscription in progress

o per-subscription operational counters

o negotiation of subscription parameters (through the use of hints
returned as part of declined subscription requests)

o subscription state change notifications (e.g., publisher-driven
suspension, parameter modification)

o independence from transport

1.2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

o Client: Defined in [RFC8342].

o Configuration: Defined in [RFC8342].

o Configuration datastore: Defined in [RFC8342].

o Configured subscription: A subscription installed via
configuration into a configuration datastore.

o Dynamic subscription: A subscription created dynamically by a
subscriber via a Remote Procedure Call (RPC).

o Event: An occurrence of something that may be of interest.
Examples include a configuration change, a fault, a change in
status, crossing a threshold, or an external input to the system.

o Event occurrence time: A timestamp matching the time an
originating process identified as when an event happened.

o Event record: A set of information detailing an event.

o Event stream: A continuous, chronologically ordered set of events
aggregated under some context.

o Event stream filter: Evaluation criteria that may be applied
against event records in an event stream. Event records pass the
filter when specified criteria are met.

o Notification message: Information intended for a receiver
indicating that one or more events have occurred.

o Publisher: An entity responsible for streaming notification
messages per the terms of a subscription.

o Receiver: A target to which a publisher pushes subscribed event
records. For dynamic subscriptions, the receiver and subscriber
are the same entity.

o Subscriber: A client able to request and negotiate a contract for
the generation and push of event records from a publisher. For
dynamic subscriptions, the receiver and subscriber are the same
entity.

o Subscription: A contract with a publisher, stipulating the
information that one or more receivers wish to have pushed from
the publisher without the need for further solicitation.

All YANG tree diagrams used in this document follow the notation
defined in [RFC8340].

1.3. Solution Overview

This document describes a transport-agnostic mechanism for
subscribing to and receiving content from an event stream in a
publisher. This mechanism operates through the use of a
subscription.

Two types of subscriptions are supported:

1. Dynamic subscriptions, where a subscriber initiates a
subscription negotiation with a publisher via an RPC. If the
publisher is able to serve this request, it accepts it and then
starts pushing notification messages back to the subscriber. If
the publisher is not able to serve it as requested, then an error
response is returned. This response MAY include hints for
subscription parameters that, had they been present, may have
enabled the dynamic subscription request to be accepted.

2. Configured subscriptions, which allow the management of
subscriptions via a configuration so that a publisher can send
notification messages to a receiver. Support for configured
subscriptions is optional, with its availability advertised via a
YANG feature.

Additional characteristics differentiating configured from dynamic
subscriptions include the following:

o The lifetime of a dynamic subscription is bound by the transport
session used to establish it. For connection-oriented stateful
transports like NETCONF, the loss of the transport session will
result in the immediate termination of any associated dynamic
subscriptions. For connectionless or stateless transports like
HTTP, a lack of receipt acknowledgment of a sequential set of
notification messages and/or keep-alives can be used to trigger a
termination of a dynamic subscription. Contrast this to the
lifetime of a configured subscription. This lifetime is driven by
relevant configuration being present in the publisher's applied
configuration. Being tied to configuration operations implies
that (1) configured subscriptions can be configured to persist
across reboots and (2) a configured subscription can persist even
when its publisher is fully disconnected from any network.

o Configured subscriptions can be modified by any configuration
client with write permission on the configuration of the
subscription. Dynamic subscriptions can only be modified via an
RPC request made by the original subscriber or by a change to
configuration data referenced by the subscription.

Note that there is no mixing and matching of dynamic and configured
operations on a single subscription. Specifically, a configured
subscription cannot be modified or deleted using RPCs defined in this
document. Similarly, a dynamic subscription cannot be directly
modified or deleted by configuration operations. It is, however,
possible to perform a configuration operation that indirectly impacts
a dynamic subscription. By changing the value of a preconfigured
filter referenced by an existing dynamic subscription, the selected
event records passed to a receiver might change.

Also note that transport-specific specifications based on this
specification MUST detail the lifecycle of dynamic subscriptions as
well as the lifecycle of configured subscriptions (if supported).

A publisher MAY terminate a dynamic subscription at any time.
Similarly, it MAY decide to temporarily suspend the sending of
notification messages for any dynamic subscription, or for one or
more receivers of a configured subscription. Such termination or
suspension is driven by internal considerations of the publisher.

1.4. Relationship to RFC 5277

This document is intended to provide a superset of the subscription
capabilities initially defined in [RFC5277]. It is important to
understand what has been reused and what has been replaced,
especially when extending an existing implementation that is based on
[RFC5277]. Key relationships between these two documents include the
following:

o This document defines a transport-independent capability;
[RFC5277] is specific to NETCONF.

o For the new operations, the data model defined in this document is
used instead of the data model defined in Section 3.4 of
[RFC5277].

o The RPC operations in this document replace the operation
<create-subscription> as defined in [RFC5277], Section 4.

o The <notification> message of [RFC5277], Section 4 is used.

o The included contents of the "NETCONF" event stream are identical
between this document and [RFC5277].

o A publisher MAY implement both the Notification Management Schema
and RPCs defined in [RFC5277] and this document concurrently.

o Unlike [RFC5277], this document enables a single transport session
to intermix notification messages and RPCs for different
subscriptions.

o A subscription "stop-time" can be specified as part of a
notification replay. This supports a capability analogous to the
<stopTime> parameter of [RFC5277]. However, in this
specification, a "stop-time" parameter can also be applied without
replay.

2. Solution

Per the overview provided in Section 1.3, this section details the
overall context, state machines, and subsystems that may be assembled
to allow the subscription of events from a publisher.

2.1. Event Streams

An event stream is a named entity on a publisher; this entity exposes
a continuously updating set of YANG-defined event records. An event
record is an instantiation of a "notification" YANG statement. If
the "notification" is defined as a child to a data node, the
instantiation includes the hierarchy of nodes that identifies the
data node in the datastore (see Section 7.16.2 of [RFC7950]). Each
event stream is available for subscription. Identifying a) how event
streams are defined (other than the NETCONF stream), b) how event
records are defined/generated, and c) how event records are assigned
to event streams is out of scope for this document.

There is only one reserved event stream name in this document:
"NETCONF". The "NETCONF" event stream contains all NETCONF event
record information supported by the publisher, except where an event
record has explicitly been excluded from the stream. Beyond the
"NETCONF" stream, implementations MAY define additional event
streams.

As YANG-defined event records are created by a system, they may be
assigned to one or more streams. The event record is distributed to
a subscription's receiver(s) where (1) a subscription includes the
identified stream and (2) subscription filtering does not exclude the
event record from that receiver.

Access control permissions may be used to silently exclude event
records from an event stream for which the receiver has no read
access. See [RFC8341], Section 3.4.6 for an example of how this
might be accomplished. Note that per Section 2.7 of this document,
subscription state change notifications are never filtered out.

If no access control permissions are in place for event records on an
event stream, then a receiver MUST be allowed access to all the event
records. If subscriber permissions change during the lifecycle of a
subscription and event stream access is no longer permitted, then the
subscription MUST be terminated.

Event records MUST NOT be delivered to a receiver in a different
order than the order in which they were placed on an event stream.

2.2. Event Stream Filters

This document defines an extensible filtering mechanism. The filter
itself is a boolean test that is placed on the content of an event
record. A "false" filtering result causes the event record to be
excluded from delivery to a receiver. A filter never results in
information being stripped from an event record prior to that event
record being encapsulated in a notification message. The two
optional event stream filtering syntaxes supported are [XPATH] and
subtree [RFC6241].

If no event stream filter is provided in a subscription, all event
records on an event stream are to be sent.

2.3. QoS

This document provides for several Quality of Service (QoS)
parameters. These parameters indicate the treatment of a
subscription relative to other traffic between publisher and
receiver. Included are:

o A "dscp" marking to differentiate prioritization of notification
messages during network transit.

o A "weighting" so that bandwidth proportional to this weighting can
be allocated to this subscription relative to other subscriptions.

o A "dependency" upon another subscription.

If the publisher supports the "dscp" feature, then a subscription
with a "dscp" leaf MUST result in a corresponding Differentiated
Services Code Point (DSCP) marking [RFC2474] being placed in the IP
header of any resulting notification messages and subscription state
change notifications. A publisher MUST respect the DSCP markings for
subscription traffic egressing that publisher.

Different DSCP code points require different transport connections.
As a result, where TCP is used, a publisher that supports the "dscp"
feature must ensure that a subscription's notification messages are
returned in a single TCP transport session where all traffic shares
the subscription's "dscp" leaf value. If this cannot be guaranteed,
any "establish-subscription" RPC request SHOULD be rejected with a
"dscp-unavailable" error.

For the "weighting" parameter, when concurrently dequeuing
notification messages from multiple subscriptions to a receiver, the
publisher MUST allocate bandwidth to each subscription proportional
to the weights assigned to those subscriptions. "Weighting" is an
optional capability of the publisher; support for it is identified
via the "qos" feature.

If a subscription has the "dependency" parameter set, then any
buffered notification messages containing event records selected by
the parent subscription MUST be dequeued prior to the notification
messages of the dependent subscription. If notification messages
have dependencies on each other, the notification message queued the
longest MUST go first. If a "dependency" included in an RPC
references a subscription that does not exist or is no longer
accessible to that subscriber, that "dependency" MUST be silently
removed. "Dependency" is an optional capability of the publisher;
support for it is identified via the "qos" feature.

"Dependency" and "weighting" parameters will only be respected and
enforced between subscriptions that share the same "dscp" leaf value.

There are additional types of publisher capacity overload that this
specification does not address, as they are out of scope. For
example, the prioritization of which subscriptions have precedence
when the publisher CPU is overloaded is not discussed. As a result,
implementation choices will need to be made to address such
considerations.

2.4. Dynamic Subscriptions

Dynamic subscriptions are managed via protocol operations (in the
form of RPCs, per [RFC7950], Section 7.14) made against targets
located in the publisher. These RPCs have been designed extensibly
so that they may be augmented for subscription targets beyond event
streams. For examples of such augmentations, see the RPC
augmentations in the YANG data model provided in [RFC8641].

2.4.1. Dynamic Subscription State Machine

Below is the publisher's state machine for a dynamic subscription.
Each state is shown in its own box. It is important to note that
such a subscription doesn't exist at the publisher until an
"establish-subscription" RPC is accepted. The mere request by a
subscriber to establish a subscription is not sufficient for that
subscription to be externally visible. Start and end states are
depicted to reflect subscription creation and deletion events.

Figure 1: Publisher's State Machine for a Dynamic Subscription

Of interest in this state machine are the following:

o Successful "establish-subscription" or "modify-subscription" RPCs
move the subscription to the "active" state.

o Failed "modify-subscription" RPCs will leave the subscription in
its previous state, with no visible change to any streaming
updates.

o A "delete-subscription" or "kill-subscription" RPC will end the
subscription, as will reaching a "stop-time".

o A publisher may choose to suspend a subscription when there is not
sufficient CPU or bandwidth available to service the subscription.
This is announced to the subscriber via the "subscription-
suspended" subscription state change notification.

o A suspended subscription may be modified by the subscriber (for
example, in an attempt to use fewer resources). Successful
modification returns the subscription to the "active" state.

o Even without a "modify-subscription" request, a publisher may
return a subscription to the "active" state when sufficient
resources are again available. This is announced to the
subscriber via the "subscription-resumed" subscription state
change notification.

2.4.2. Establishing a Dynamic Subscription

The "establish-subscription" RPC allows a subscriber to request the
creation of a subscription.

The input parameters of the operation are:

o A "stream" name, which identifies the targeted event stream
against which the subscription is applied.

o An event stream filter, which may reduce the set of event records
pushed.

o If the transport used by the RPC supports multiple encodings, an
optional "encoding" for the event records pushed. If no
"encoding" is included, the encoding of the RPC MUST be used.

o An optional "stop-time" for the subscription. If no "stop-time"
is present, notification messages will continue to be sent until
the subscription is terminated.

o An optional "replay-start-time" for the subscription. The
"replay-start-time" MUST be in the past and indicates that the
subscription is requesting a replay of previously generated
information from the event stream. For more on replay, see
Section 2.4.2.1. If there is no "replay-start-time", the
subscription starts immediately.

If the publisher can satisfy the "establish-subscription" request, it
replies with an identifier for the subscription and then immediately
starts streaming notification messages.

Below is a tree diagram for "establish-subscription". All objects
contained in this tree are described in the YANG module in Section 4.

+---x establish-subscription
+---w input
| +---w (target)
| | +--:(stream)
| | +---w (stream-filter)?
| | | +--:(by-reference)
| | | | +---w stream-filter-name
| | | | stream-filter-ref
| | | +--:(within-subscription)
| | | +---w (filter-spec)?
| | | +--:(stream-subtree-filter)
| | | | +---w stream-subtree-filter? <anydata>
| | | | {subtree}?
| | | +--:(stream-xpath-filter)
| | | +---w stream-xpath-filter?
| | | yang:xpath1.0 {xpath}?
| | +---w stream stream-ref
| | +---w replay-start-time?
| | yang:date-and-time {replay}?
| +---w stop-time?
| | yang:date-and-time
| +---w dscp? inet:dscp
| | {dscp}?
| +---w weighting? uint8
| | {qos}?
| +---w dependency?
| | subscription-id {qos}?
| +---w encoding? encoding
+--ro output
+--ro id subscription-id
+--ro replay-start-time-revision? yang:date-and-time
{replay}?

Figure 2: "establish-subscription" RPC Tree Diagram

A publisher MAY reject the "establish-subscription" RPC for many
reasons, as described in Section 2.4.6. The contents of the
resulting RPC error response MAY include details on input parameters
that, if considered in a subsequent "establish-subscription" RPC, may
result in successful subscription establishment. Any such hints MUST
be transported in a yang-data "establish-subscription-stream-error-
info" container included in the RPC error response.

Below is a tree diagram for "establish-subscription" RPC yang-data.
All objects contained in this tree are described in the YANG module
in Section 4.

yang-data establish-subscription-stream-error-info
+--ro establish-subscription-stream-error-info
+--ro reason? identityref
+--ro filter-failure-hint? string

Figure 3: "establish-subscription" RPC yang-data Tree Diagram

2.4.2.1. Requesting a Replay of Event Records

Replay provides the ability to establish a subscription that is also
capable of passing event records generated in the recent past. In
other words, as the subscription initializes itself, it sends any
event records in the target event stream that meet the filter
criteria that have an event time that is after the "replay-start-
time" and also have an event time before the "stop-time" should this
"stop-time" exist. The end of these historical event records is
identified via a "replay-completed" subscription state change
notification. Any event records generated since the subscription
establishment may then follow. For a particular subscription, all
event records will be delivered in the order in which they are placed
in the event stream.

Replay is an optional feature that is dependent on an event stream
supporting some form of logging. This document puts no restrictions
on the size or form of the log, where it resides in the publisher, or
when event record entries in the log are purged.

The inclusion of a "replay-start-time" in an "establish-subscription"
RPC indicates a replay request. If the "replay-start-time" contains
a value that is earlier than what a publisher's retained history
supports, then if the subscription is accepted, the actual
publisher's revised start time MUST be set in the returned
"replay-start-time-revision" object.

A "stop-time" parameter may be included in a replay subscription.
For a replay subscription, the "stop-time" MAY be earlier than the
current time but MUST be later than the "replay-start-time".

If the given "replay-start-time" is later than the time marked in any
event records retained in the replay buffer, then the publisher MUST
send a "replay-completed" notification immediately after a successful
"establish-subscription" RPC response.

If an event stream supports replay, the "replay-support" leaf is
present in the "/streams/stream" list entry for the event stream. An
event stream that does support replay is not expected to have an
unlimited supply of saved notifications available to accommodate any
given replay request. To assess the timeframe available for replay,
subscribers can read the leafs "replay-log-creation-time" and
"replay-log-aged-time". See Figure 18 for the YANG tree and
Section 4 for the YANG module describing these elements. The actual
size of the replay log at any given time is a publisher-specific
matter. Control parameters for the replay log are outside the scope
of this document.

2.4.3. Modifying a Dynamic Subscription

The "modify-subscription" operation permits changing the terms of an
existing dynamic subscription. Dynamic subscriptions can be modified
any number of times. Dynamic subscriptions can only be modified via
this RPC using a transport session connecting to the subscriber. If
the publisher accepts the requested modifications, it acknowledges
success to the subscriber, then immediately starts sending event
records based on the new terms.

Subscriptions created by configuration cannot be modified via this
RPC. However, configuration may be used to modify objects referenced
by the subscription (such as a referenced filter).

Below is a tree diagram for "modify-subscription". All objects
contained in this tree are described in the YANG module in Section 4.

+---x modify-subscription
+---w input
+---w id
| subscription-id
+---w (target)
| +--:(stream)
| +---w (stream-filter)?
| +--:(by-reference)
| | +---w stream-filter-name
| | stream-filter-ref
| +--:(within-subscription)
| +---w (filter-spec)?
| +--:(stream-subtree-filter)
| | +---w stream-subtree-filter? <anydata>
| | {subtree}?
| +--:(stream-xpath-filter)
| +---w stream-xpath-filter?
| yang:xpath1.0 {xpath}?
+---w stop-time?
yang:date-and-time

Figure 4: "modify-subscription" RPC Tree Diagram

If the publisher accepts the requested modifications on a currently
suspended subscription, the subscription will immediately be resumed
(i.e., the modified subscription is returned to the "active" state).
The publisher MAY immediately suspend this newly modified
subscription through the "subscription-suspended" notification before
any event records are sent.

If the publisher rejects the RPC request, the subscription remains as
it was prior to the request. That is, the request has no impact
whatsoever. Rejection of the RPC for any reason is indicated via an
RPC error as described in Section 2.4.6. The contents of such a
rejected RPC MAY include hints on inputs that (if considered) may
result in a successfully modified subscription. These hints MUST be
transported in a yang-data "modify-subscription-stream-error-info"
container inserted into the RPC error response.

Below is a tree diagram for "modify-subscription" RPC yang-data. All
objects contained in this tree are described in the YANG module in
Section 4.

yang-data modify-subscription-stream-error-info
+--ro modify-subscription-stream-error-info
+--ro reason? identityref
+--ro filter-failure-hint? string

Figure 5: "modify-subscription" RPC yang-data Tree Diagram

2.4.4. Deleting a Dynamic Subscription

The "delete-subscription" operation permits canceling an existing
subscription. If the publisher accepts the request and the publisher
has indicated success, the publisher MUST NOT send any more
notification messages for this subscription.

Below is a tree diagram for "delete-subscription". All objects
contained in this tree are described in the YANG module in Section 4.

+---x delete-subscription
+---w input
+---w id subscription-id

Figure 6: "delete-subscription" RPC Tree Diagram

Dynamic subscriptions can only be deleted via this RPC using a
transport session connecting to the subscriber. Configured
subscriptions cannot be deleted using RPCs.

2.4.5. Killing a Dynamic Subscription

The "kill-subscription" operation permits an operator to end a
dynamic subscription that is not associated with the transport
session used for the RPC. A publisher MUST terminate any dynamic
subscription identified by the "id" parameter in the RPC request, if
such a subscription exists.

Configured subscriptions cannot be killed using this RPC. Instead,
configured subscriptions are deleted as part of regular configuration
operations. Publishers MUST reject any RPC attempt to kill a
configured subscription.

Below is a tree diagram for "kill-subscription". All objects
contained in this tree are described in the YANG module in Section 4.

+---x kill-subscription
+---w input
+---w id subscription-id

Figure 7: "kill-subscription" RPC Tree Diagram

2.4.6. RPC Failures

Whenever an RPC is unsuccessful, the publisher returns relevant
information as part of the RPC error response. Transport-level error
processing MUST be done before the RPC error processing described in
this section. In all cases, RPC error information returned by the
publisher will use existing transport-layer RPC structures, such as
those seen with NETCONF (Appendix A of [RFC6241]) or RESTCONF
(Section 7.1 of [RFC8040]). These structures MUST be able to encode
subscription-specific errors identified below and defined in this
document's YANG data model.

As a result of this variety, how subscription errors are encoded in
an RPC error response is transport dependent. Valid errors that can
occur for each RPC are as follows:

establish-subscription modify-subscription
---------------------- ----------------------
dscp-unavailable filter-unsupported
encoding-unsupported insufficient-resources
filter-unsupported no-such-subscription
insufficient-resources
replay-unsupported

delete-subscription kill-subscription
---------------------- ----------------------
no-such-subscription no-such-subscription

To see a NETCONF-based example of an error response from the list
above, see the "no-such-subscription" error response illustrated in
[RFC8640], Figure 10.

There is one final set of transport-independent RPC error elements
included in the YANG data model defined in this document: three
yang-data structures that enable the publisher to provide to the
receiver any error information that does not fit into existing
transport-layer RPC structures. These structures are:

1. "establish-subscription-stream-error-info": This MUST be returned
with the leaf "reason" populated if an RPC error reason has not
been placed elsewhere in the transport portion of a failed
"establish-subscription" RPC response. This MUST be sent if
hints on how to overcome the RPC error are included.

2. "modify-subscription-stream-error-info": This MUST be returned
with the leaf "reason" populated if an RPC error reason has not
been placed elsewhere in the transport portion of a failed
"modify-subscription" RPC response. This MUST be sent if hints
on how to overcome the RPC error are included.

3. "delete-subscription-error-info": This MUST be returned with the
leaf "reason" populated if an RPC error reason has not been
placed elsewhere in the transport portion of a failed
"delete-subscription" or "kill-subscription" RPC response.

2.5. Configured Subscriptions

A configured subscription is a subscription installed via
configuration. Configured subscriptions may be modified by any
configuration client with the proper permissions. Subscriptions can
be modified or terminated via configuration at any point during their
lifetime. Multiple configured subscriptions MUST be supportable over
a single transport session.

Configured subscriptions have several characteristics distinguishing
them from dynamic subscriptions:

o persistence across publisher reboots,

o persistence even when transport is unavailable, and

o an ability to send notification messages to more than one
receiver. (Note that receivers are unaware of the existence of
any other receivers.)

On the publisher, support for configured subscriptions is optional
and advertised using the "configured" feature. On a receiver of a
configured subscription, support for dynamic subscriptions is
optional. However, if replaying missed event records is required for
a configured subscription, support for dynamic subscription is highly
recommended. In this case, a separate dynamic subscription can be
established to retransmit the missing event records.

In addition to the subscription parameters available to dynamic
subscriptions as described in Section 2.4.2, the following additional
parameters are also available to configured subscriptions:

o A "transport", which identifies the transport protocol to use to
connect with all subscription receivers.

o One or more receivers, each intended as the destination for event
records. Note that each individual receiver is identifiable by
its "name".

o Optional parameters to identify where traffic should egress a
publisher:

* A "source-interface", which identifies the egress interface to
use from the publisher. Publisher support for this parameter
is optional and advertised using the "interface-designation"
feature.

* A "source-address" address, which identifies the IP address to
stamp on notification messages destined for the receiver.

* A "source-vrf", which identifies the Virtual Routing and
Forwarding (VRF) instance on which to reach receivers. This
VRF is a network instance as defined in [RFC8529]. Publisher
support for VRFs is optional and advertised using the
"supports-vrf" feature.

If none of the above parameters are set, notification messages
MUST egress the publisher's default interface.

A tree diagram that includes these parameters is provided in
Figure 20 in Section 3.3. These parameters are described in the YANG
module in Section 4.

2.5.1. Configured Subscription State Machine

Below is the state machine for a configured subscription on the
publisher. This state machine describes the three states ("valid",
"invalid", and "concluded") as well as the transitions between these
states. Start and end states are depicted to reflect configured
subscription creation and deletion events. The creation or
modification of a configured subscription initiates an evaluation by
the publisher to determine if the subscription is in the
"valid" state or the "invalid" state. The publisher uses its own
criteria in making this determination. If in the "valid" state, the
subscription becomes operational. See (1) in the diagram below.

Legend:
Dotted boxes: subscription added or removed via configuration
Dashed boxes: states for a subscription
[evaluate]: decision point on whether the subscription
is supportable
(*): resulting subscription state change notification

Figure 8: Publisher's State Machine for a Configured Subscription

A subscription in the "valid" state may move to the "invalid" state
in one of two ways. First, it may be modified in a way that fails a
re-evaluation. See (2) in the diagram. Second, the publisher might
determine that the subscription is no longer supportable. This could
be because of an unexpected but sustained increase in an event
stream's event records, degraded CPU capacity, a more complex
referenced filter, or other subscriptions that have usurped
resources. See (3) in the diagram. No matter the case, a
"subscription-terminated" notification is sent to any receivers in
the "active" or "suspended" state. A subscription in the
"valid" state may also transition to the "concluded" state via (5) if
a configured stop time has been reached. In this case, a
"subscription-concluded" notification is sent to any receivers in the
"active" or "suspended" state. Finally, a subscription may be
deleted by configuration (4).

When a subscription is in the "valid" state, a publisher will attempt
to connect with all receivers of a configured subscription and
deliver notification messages. Below is the state machine for each
receiver of a configured subscription. This receiver state machine
is fully contained in the state machine of the configured
subscription and is only relevant when the configured subscription is
in the "valid" state.

Legend:
Dashed boxes that include the word "receiver" show the possible
states for an individual receiver of a valid configured
subscription.

* indicates a subscription state change notification

Figure 9: Receiver State Machine for a Configured Subscription
on a Publisher

When a configured subscription first moves to the "valid" state, the
"state" leaf of each receiver is initialized to the "connecting"
state. If transport connectivity is not available to any receivers
and there are any notification messages to deliver, a transport
session is established (e.g., per [RFC8071]). Individual receivers
are moved to the "active" state when a "subscription-started"
subscription state change notification is successfully passed to that
receiver (a). Event records are only sent to active receivers.
Receivers of a configured subscription remain active on the publisher
if both (1) transport connectivity can be verified to the receiver is active and
(2) event records are not being dropped due to a publisher's buffer sending
capacity being reached. The result is that a receiver will remain
active on the publisher as long as events aren't being lost, or the
receiver cannot be reached. In addition, a configured subscription's
receiver MUST be moved to the "connecting" state if the receiver is
reset via the "reset" action (b), (c). For more on the "reset"
action, see Section 2.5.5. If transport connectivity cannot be
achieved while in the "connecting" state, the receiver MAY be moved
to the "disconnected" state.

A configured subscription's receiver MUST be moved to the "suspended"
state if there is transport connectivity between the publisher and
receiver but (1) delivery of notification messages is failing due to
a publisher's buffer capacity being reached or (2) notification
messages cannot be generated for that receiver due to insufficient
CPU (d). This is indicated to the receiver by the "subscription-
suspended" subscription state change notification.

A configured subscription's receiver MUST be returned to the "active"
state from the "suspended" state when notification messages can be
generated, bandwidth is sufficient to handle the notification
messages, and a receiver has successfully been sent a "subscription-
resumed" or "subscription-modified" subscription state change
notification (e). The choice as to which of these two subscription
state change notifications is sent is determined by whether the
subscription was modified during the period of suspension.

Modification of a configured subscription is possible at any time. A
"subscription-modified" subscription state change notification will
be sent to all active receivers, immediately followed by notification
messages conforming to the new parameters. Suspended receivers will
also be informed of the modification. However, this notification
will await the end of the suspension for that receiver (e).

The mechanisms described above are mirrored in the RPCs and
notifications defined in this document. It should be noted that
these RPCs and notifications have been designed to be extensible and
allow subscriptions into targets other than event streams. For
instance, the YANG module defined in Section 5 of [RFC8641] augments
"/sn:modify-subscription/sn:input/sn:target".

2.5.2. Creating a Configured Subscription

Configured subscriptions are established using configuration
operations against the top-level "subscriptions" subtree.

Because there is no explicit association with an existing transport
session, configuration operations MUST include additional parameters
beyond those of dynamic subscriptions. These parameters identify
each receiver, how to connect with that receiver, and possibly
whether the notification messages need to come from a specific egress
interface on the publisher. Receiver-specific transport connectivity
parameters MUST be configured via transport-specific augmentations to
this specification. See Section 2.5.7 for details.

After a subscription is successfully established, the publisher
immediately sends a "subscription-started" subscription state change
notification to each receiver. It is quite possible that upon
configuration, reboot, or even steady-state operations, a transport
session may not be currently available to the receiver. In this
case, when there is something to transport for an active
subscription, transport-specific "call home" operations [RFC8071]
will be used to establish the connection. When transport
connectivity is available, notification messages may then be pushed.

With active configured subscriptions, it is allowable to buffer event
records even after a "subscription-started" has been sent. However,
if events are lost (rather than just delayed) due to replay buffer
capacity being reached, a new "subscription-started" must be sent.
This new "subscription-started" indicates an event record
discontinuity.

To see an example of subscription creation using configuration
operations over NETCONF, see Appendix A.

2.5.3. Modifying a Configured Subscription

Configured subscriptions can be modified using configuration
operations against the top-level "subscriptions" subtree.

If the modification involves adding receivers, added receivers are
placed in the "connecting" state. If a receiver is removed, the
subscription state change notification "subscription-terminated" is
sent to that receiver if that receiver is active or suspended.

If the modification involves changing the policies for the
subscription, the publisher sends to currently active receivers a
"subscription-modified" notification. For any suspended receivers, a
"subscription-modified" notification will be delayed until the
receiver's subscription has been resumed. (Note: In this case, the
"subscription-modified" notification informs the receiver that the
subscription has been resumed, so no additional "subscription-
resumed" need be sent. Also note that if multiple modifications have
occurred during the suspension, only the "subscription-modified"
notification describing the latest one need be sent to the receiver.)

2.5.4. Deleting a Configured Subscription

Subscriptions can be deleted through configuration against the
top-level "subscriptions" subtree.

Immediately after a subscription is successfully deleted, the
publisher sends to all receivers of that subscription a subscription
state change notification stating that the subscription has ended
(i.e., "subscription-terminated").

2.5.5. Resetting a Configured Subscription's Receiver

It is possible that a configured subscription to a receiver needs to
be reset. This is accomplished via the "reset" action in the YANG
module at "/subscriptions/subscription/receivers/receiver/reset".
This action may be useful in cases where a publisher has timed out
trying to reach a receiver. When such a reset occurs, a transport
session will be initiated if necessary, and a new "subscription-
started" notification will be sent. This action does not have any
effect on transport connectivity if the needed connectivity already
exists.

2.5.6. Replay for a Configured Subscription

It is possible to do replay on a configured subscription. This is
supported via the configuration of the "configured-replay" object on
the subscription. The setting of this object enables the streaming
of the buffered event records for the subscribed event stream. All
buffered event records that have been retained since the last
publisher restart will be sent to each configured receiver.

Replay of event records created since restart is useful. It allows
event records generated before transport connectivity establishment
to be passed to a receiver. Setting the restart time as the earliest
configured replay time precludes the possibility of resending event
records that were logged prior to publisher restart. It also ensures
that the same records will be sent to each configured receiver,
regardless of the speed of transport connectivity establishment to
each receiver. Finally, by establishing restart as the earliest
potential time for event records to be included in notification
messages, a well-understood timeframe for replay is defined.

As a result, when any configured subscription's receivers become
active, buffered event records will be sent immediately after the
"subscription-started" notification. If the publisher knows the last
event record sent to a receiver and the publisher has not rebooted,
the next event record on the event stream that meets filtering
criteria will be the leading event record sent. Otherwise, the
leading event record will be the first event record meeting filtering
criteria subsequent to the latest of three different times: the
"replay-log-creation-time", the "replay-log-aged-time", or the most
recent publisher boot time. The "replay-log-creation-time" and
"replay-log-aged-time" are discussed in Section 2.4.2.1. The most
recent publisher boot time ensures that duplicate event records are
not replayed from a previous time the publisher was booted.

It is quite possible that a receiver might want to retrieve event
records from an event stream prior to the latest boot. If such
records exist where there is a configured replay, the publisher MUST
send the time of the event record immediately preceding the
"replay-start-time" in the "replay-previous-event-time" leaf.
Through the existence of the "replay-previous-event-time", the
receiver will know that earlier events prior to reboot exist. In
addition, if the subscriber was previously receiving event records
with the same subscription "id", the receiver can determine if there
was a time gap where records generated on the publisher were not
successfully received. And with this information, the receiver may
choose to dynamically subscribe to retrieve any event records placed
in the event stream before the most recent boot time.

All other replay functionality remains the same as with dynamic
subscriptions as described in Section 2.4.2.1.

2.5.7. Transport Connectivity for a Configured Subscription

This specification is transport independent. However, supporting a
configured subscription will often require the establishment of
transport connectivity. And the parameters used for this transport
connectivity establishment are transport specific. As a result, the
YANG module defined in Section 4 is not able to directly define and
expose these transport parameters.

It is necessary for an implementation to support the connection
establishment process. To support this function, the YANG data model
defined in this document includes a node where transport-specific
parameters for a particular receiver may be augmented. This node is
"/subscriptions/subscription/receivers/receiver". By augmenting
transport parameters from this node, system developers are able to
incorporate the YANG objects necessary to support the transport
connectivity establishment process.

The result of this is the following requirement. A publisher
supporting the feature "configured" MUST also support at least one
YANG data model that augments transport connectivity parameters on
"/subscriptions/subscription/receivers/receiver". For an example of
such an augmentation, see Appendix A.

2.6. Event Record Delivery

Whether dynamic or configured, once a subscription has been set up,
the publisher streams event records via notification messages per the
terms of the subscription. For dynamic subscriptions, notification
messages are sent over the session used to establish the
subscription. For configured subscriptions, notification messages
are sent over the connections specified by the transport and each
receiver of a configured subscription.

A notification message is sent to a receiver when an event record is
not blocked by either the specified filter criteria or receiver
permissions. This notification message MUST include an <eventTime>
object, as shown in [RFC5277], Section 4. This <eventTime> MUST be
at the top level of a YANG structured event record.

The following example of XML [W3C.REC-xml-20081126], adapted from
Section 4.2.10 of [RFC7950], illustrates a compliant message:

Figure 10: Subscribed Notification Message

[RFC5277], Section 2.2.1 states that a notification message is to be
sent to a subscriber that initiated a <create-subscription>. With
this document, this statement from [RFC5277] should be more broadly
interpreted to mean that notification messages can also be sent to a
subscriber that initiated an "establish-subscription" or to a
configured receiver that has been sent a "subscription-started".

When a dynamic subscription has been started or modified with
"establish-subscription" or "modify-subscription", respectively,
event records matching the newly applied filter criteria MUST NOT be
sent until after the RPC reply has been sent.

When a configured subscription has been started or modified, event
records matching the newly applied filter criteria MUST NOT be sent
until after the "subscription-started" or "subscription-modified"
notification has been sent, respectively.

2.7. Subscription State Change Notifications

In addition to sending event records to receivers, a publisher MUST
also send subscription state change notifications when events related
to subscription management have occurred.

Subscription state change notifications are unlike other
notifications in that they are never included in any event stream.
Instead, they are inserted (as defined in this section) into the
sequence of notification messages sent to a particular receiver.

Subscription state change notifications cannot be dropped or filtered
out, they cannot be stored in replay buffers, and they are delivered
only to impacted receivers of a subscription. The identification of
subscription state change notifications is easy to separate from
other notification messages through the use of the YANG extension
"subscription-state-notif". This extension tags a notification as a
subscription state change notification.

The complete set of subscription state change notifications is
described in the following subsections.

2.7.1. "subscription-started"

This notification indicates that a configured subscription has
started, and event records may be sent. Included in this
subscription state change notification are all the parameters of the
subscription, except for (1) transport connection information for one
or more receivers and (2) origin information indicating where
notification messages will egress the publisher. Note that if a
referenced filter from the "filters" container has been used in the
subscription, the notification still provides the contents of that
referenced filter under the "within-subscription" subtree.

Note that for dynamic subscriptions, no "subscription-started"
notifications are ever sent.

Below is a tree diagram for "subscription-started". All objects
contained in this tree are described in the YANG module in Section 4.

+---n subscription-started {configured}?
+--ro id
| subscription-id
+--ro (target)
| +--:(stream)
| +--ro (stream-filter)?
| | +--:(by-reference)
| | | +--ro stream-filter-name
| | | stream-filter-ref
| | +--:(within-subscription)
| | +--ro (filter-spec)?
| | +--:(stream-subtree-filter)
| | | +--ro stream-subtree-filter? <anydata>
| | | {subtree}?
| | +--:(stream-xpath-filter)
| | +--ro stream-xpath-filter? yang:xpath1.0
| | {xpath}?
| +--ro stream stream-ref
| +--ro replay-start-time?
| | yang:date-and-time {replay}?
| +--ro replay-previous-event-time?
| yang:date-and-time {replay}?
+--ro stop-time?
| yang:date-and-time
+--ro dscp? inet:dscp
| {dscp}?
+--ro weighting? uint8 {qos}?
+--ro dependency?
| subscription-id {qos}?
+--ro transport? transport
| {configured}?
+--ro encoding? encoding
+--ro purpose? string
{configured}?

Figure 11: "subscription-started" Notification Tree Diagram

2.7.2. "subscription-modified"

This notification indicates that a subscription has been modified by
configuration operations. It is delivered directly after the last
event records processed using the previous subscription parameters,
and before any event records processed after the modification.

Below is a tree diagram for "subscription-modified". All objects
contained in this tree are described in the YANG module in Section 4.

+---n subscription-modified
+--ro id
| subscription-id
+--ro (target)
| +--:(stream)
| +--ro (stream-filter)?
| | +--:(by-reference)
| | | +--ro stream-filter-name
| | | stream-filter-ref
| | +--:(within-subscription)
| | +--ro (filter-spec)?
| | +--:(stream-subtree-filter)
| | | +--ro stream-subtree-filter? <anydata>
| | | {subtree}?
| | +--:(stream-xpath-filter)
| | +--ro stream-xpath-filter? yang:xpath1.0
| | {xpath}?
| +--ro stream stream-ref
| +--ro replay-start-time?
| yang:date-and-time {replay}?
+--ro stop-time?
| yang:date-and-time
+--ro dscp? inet:dscp
| {dscp}?
+--ro weighting? uint8 {qos}?
+--ro dependency?
| subscription-id {qos}?
+--ro transport? transport
| {configured}?
+--ro encoding? encoding
+--ro purpose? string
{configured}?

Figure 12: "subscription-modified" Notification Tree Diagram

A publisher most often sends this notification directly after the
modification of any configuration parameters impacting a configured
subscription. But it may also be sent at two other times:

1. If a configured subscription has been modified during the
suspension of a receiver, the notification will be delayed until
the receiver's suspension is lifted. In this situation, the
notification indicates that the subscription has been both
modified and resumed.

2. A "subscription-modified" subscription state change notification
MUST be sent if the contents of the filter identified by the
subscription's "stream-filter-ref" leaf have changed. This state
change notification is to be sent for a filter change impacting
any active receivers of a configured or dynamic subscription.

2.7.3. "subscription-terminated"

This notification indicates that no further event records for this
subscription should be expected from the publisher. A publisher may
terminate the sending of event records to a receiver for the
following reasons:

1. Configuration that removes a configured subscription, or a
"kill-subscription" RPC that ends a dynamic subscription. These
are identified via the reason "no-such-subscription".

2. A referenced filter is no longer accessible. This reason is
identified by the "filter-unavailable" identity.

3. The event stream referenced by a subscription is no longer
accessible by the receiver. This reason is identified by the
"stream-unavailable" identity.

4. A suspended subscription has exceeded some timeout. This reason
is identified by the "suspension-timeout" identity.

Each reason listed above derives from the "subscription-terminated-
reason" base identity specified in the YANG data model in this
document.

Below is a tree diagram for "subscription-terminated". All objects
contained in this tree are described in the YANG module in Section 4.

+---n subscription-terminated
+--ro id subscription-id
+--ro reason identityref

Figure 13: "subscription-terminated" Notification Tree Diagram

Note: This subscription state change notification MUST be sent to a
dynamic subscription's receiver when the subscription ends
unexpectedly. This might happen when a "kill-subscription" RPC is
successful or when some other event, not including reaching the
subscription's "stop-time", results in a publisher choosing to end
the subscription.

2.7.4. "subscription-suspended"

This notification indicates that a publisher has suspended the
sending of event records to a receiver and also indicates the
possible loss of events. Suspension happens when capacity
constraints stop a publisher from serving a valid subscription. The
two conditions where this is possible are:

1. "insufficient-resources", when a publisher is unable to produce
the requested event stream of notification messages, and

2. "unsupportable-volume", when the bandwidth needed to get
generated notification messages to a receiver exceeds a
threshold.

These conditions are encoded in the "reason" object. No further
notifications will be sent until the subscription resumes or is
terminated.

Below is a tree diagram for "subscription-suspended". All objects
contained in this tree are described in the YANG module in Section 4.

+---n subscription-suspended
+--ro id subscription-id
+--ro reason identityref

Figure 14: "subscription-suspended" Notification Tree Diagram

2.7.5. "subscription-resumed"

This notification indicates that a previously suspended subscription
has been resumed under the unmodified terms previously in place.
Subscribed event records generated after the issuance of this
subscription state change notification may now be sent.

Below is a tree diagram for "subscription-resumed". All objects
contained in this tree are described in the YANG module in Section 4.

+---n subscription-resumed
+--ro id subscription-id

Figure 15: "subscription-resumed" Notification Tree Diagram

2.7.6. "subscription-completed"

This notification indicates that a subscription that includes a
"stop-time" has successfully finished passing event records upon
reaching that time.

Below is a tree diagram for "subscription-completed". All objects
contained in this tree are described in the YANG module in Section 4.

+---n subscription-completed {configured}?
+--ro id subscription-id

Figure 16: "subscription-completed" Notification Tree Diagram

2.7.7. "replay-completed"

This notification indicates that all of the event records prior to
the current time have been passed to a receiver. It is sent before
any notification messages containing an event record with a timestamp
later than (1) the "stop-time" or (2) the subscription's start time.

If a subscription does not contain a "stop-time" or has a "stop-time"
that has not been reached, then after the "replay-completed"
notification has been sent, additional event records will be sent in
sequence as they arise naturally on the publisher.

Below is a tree diagram for "replay-completed". All objects
contained in this tree are described in the YANG module in Section 4.

+---n replay-completed {replay}?
+--ro id subscription-id

Figure 17: "replay-completed" Notification Tree Diagram

2.8. Subscription Monitoring

In the operational state datastore, the "subscriptions" container
maintains the state of all dynamic subscriptions as well as all
configured subscriptions. Using datastore retrieval operations
[RFC8641] or subscribing to the "subscriptions" container
(Section 3.3) allows the state of subscriptions and their
connectivity to receivers to be monitored.

Each subscription in the operational state datastore is represented
as a list element. Included in this list are event counters for each
receiver, the state of each receiver, and the subscription parameters
currently in effect. The appearance of the leaf "configured-
subscription-state" indicates that a particular subscription came
into being via configuration. This leaf also indicates whether the
current state of that subscription is "valid", "invalid", or
"concluded".

To understand the flow of event records in a subscription, there are
two counters available for each receiver. The first counter is
"sent-event-records", which shows the number of events identified for
sending to a receiver. The second counter is "excluded-event-
records", which shows the number of event records not sent to a
receiver. "excluded-event-records" shows the combined results of
both access control and per-subscription filtering. For configured
subscriptions, counters are reset whenever the subscription's state
is evaluated as "valid" (see (1) in Figure 8).

Dynamic subscriptions are removed from the operational state
datastore once they expire (reaching "stop-time") or when they are
terminated. While many subscription objects are shown as
configurable, dynamic subscriptions are only included in the
operational state datastore and as a result are not configurable.

2.9. Support for the "ietf-subscribed-notifications" YANG Module

Publishers supporting this document MUST indicate support of the YANG
module "ietf-subscribed-notifications" in the YANG library of the
publisher. In addition, if supported, the optional features
"encode-xml", "encode-json", "configured", "supports-vrf", "qos",
"xpath", "subtree", "interface-designation", "dscp", and "replay"
MUST be indicated.

3. YANG Data Model Tree Diagrams

This section contains tree diagrams for nodes defined in Section 4.
For tree diagrams of subscription state change notifications, see
Section 2.7. For the tree diagrams for the RPCs, see Section 2.4.

3.1. The "streams" Container

A publisher maintains a list of available event streams as
operational data. This list contains both standardized and
vendor-specific event streams. This enables subscribers to discover
what streams a publisher supports.

Below is a tree diagram for the "streams" container. All objects
contained in this tree are described in the YANG module in Section 4.

+--ro streams
+--ro stream* [name]
+--ro name string
+--ro description string
+--ro replay-support? empty {replay}?
+--ro replay-log-creation-time yang:date-and-time
| {replay}?
+--ro replay-log-aged-time? yang:date-and-time
{replay}?

Figure 18: "streams" Container Tree Diagram

3.2. The "filters" Container

The "filters" container maintains a list of all subscription filters
that persist outside the lifecycle of a single subscription. This
enables predefined filters that may be referenced by more than one
subscription.

Below is a tree diagram for the "filters" container. All objects
contained in this tree are described in the YANG module in Section 4.

+--rw filters
+--rw stream-filter* [name]
+--rw name string
+--rw (filter-spec)?
+--:(stream-subtree-filter)
| +--rw stream-subtree-filter? <anydata> {subtree}?
+--:(stream-xpath-filter)
+--rw stream-xpath-filter? yang:xpath1.0 {xpath}?

Figure 19: "filters" Container Tree Diagram

3.3. The "subscriptions" Container

The "subscriptions" container maintains a list of all subscriptions
on a publisher, both configured and dynamic. It can be used to
retrieve information about the subscriptions that a publisher is
serving.

Below is a tree diagram for the "subscriptions" container. All
objects contained in this tree are described in the YANG module in
Section 4.

+--rw subscriptions
+--rw subscription* [id]
+--rw id
| subscription-id
+--rw (target)
| +--:(stream)
| +--rw (stream-filter)?
| | +--:(by-reference)
| | | +--rw stream-filter-name
| | | stream-filter-ref
| | +--:(within-subscription)
| | +--rw (filter-spec)?
| | +--:(stream-subtree-filter)
| | | +--rw stream-subtree-filter? <anydata>
| | | {subtree}?
| | +--:(stream-xpath-filter)
| | +--rw stream-xpath-filter?
| | yang:xpath1.0 {xpath}?
| +--rw stream stream-ref
| +--ro replay-start-time?
| | yang:date-and-time {replay}?
| +--rw configured-replay? empty
| {configured,replay}?
+--rw stop-time?
| yang:date-and-time
+--rw dscp? inet:dscp
| {dscp}?
+--rw weighting? uint8 {qos}?
+--rw dependency?
| subscription-id {qos}?
+--rw transport? transport
| {configured}?
+--rw encoding? encoding
+--rw purpose? string
| {configured}?
+--rw (notification-message-origin)? {configured}?
| +--:(interface-originated)
| | +--rw source-interface?
| | if:interface-ref {interface-designation}?
| +--:(address-originated)
| +--rw source-vrf?
| | -> /ni:network-instances/network-instance/name
| | {supports-vrf}?
| +--rw source-address?
| inet:ip-address-no-zone
+--ro configured-subscription-state? enumeration
| {configured}?
+--rw receivers
+--rw receiver* [name]
+--rw name string
+--ro sent-event-records?
| yang:zero-based-counter64
+--ro excluded-event-records?
| yang:zero-based-counter64
+--ro state enumeration
+---x reset {configured}?
+--ro output
+--ro time yang:date-and-time

Figure 20: "subscriptions" Container Tree Diagram

4. Event Notification Subscription YANG Module

This module imports typedefs from [RFC6991], [RFC8343], [RFC8341],
[RFC8529], and [RFC8040]. It references [RFC6241], [XPATH] ("XML
Path Language (XPath) Version 1.0"), [RFC7049], [RFC8259], [RFC7950],
[RFC7951], and [RFC7540].

<CODE BEGINS> file "ietf-subscribed-notifications@2019-08-07.yang"
module ietf-subscribed-notifications {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications";
prefix sn;

import ietf-inet-types {
prefix inet;
reference
"RFC 6991: Common YANG Data Types";
}
import ietf-interfaces {
prefix if;
reference
"RFC 8343: A YANG Data Model for Interface Management";
}
import ietf-netconf-acm {
prefix nacm;
reference
"RFC 8341: Network Configuration Access Control Model";
}
import ietf-network-instance {
prefix ni;
reference
"RFC 8529: YANG Data Model for Network Instances";
}
import ietf-restconf {
prefix rc;
reference
"RFC 8040: RESTCONF Protocol";
}
import ietf-yang-types {
prefix yang;
reference
"RFC 6991: Common YANG Data Types";
}

organization
"IETF NETCONF (Network Configuration) Working Group";
contact
"WG Web: <https:/datatracker.ietf.org/wg/netconf/>
WG List: <mailto:netconf@ietf.org>

Author: Alexander Clemm
<mailto:ludwig@clemm.org>

Author: Eric Voit
<mailto:evoit@cisco.com>

Author: Alberto Gonzalez Prieto
<mailto:alberto.gonzalez@microsoft.com>

Author: Einar Nilsen-Nygaard
<mailto:einarnn@cisco.com>

Author: Ambika Prasad Tripathy
<mailto:ambtripa@cisco.com>";
description
"This module defines a YANG data model for subscribing to event
records and receiving matching content in notification messages.

The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
'MAY', and 'OPTIONAL' in this document are to be interpreted as
described in BCP 14 (RFC 2119) (RFC 8174) when, and only when,
they appear in all capitals, as shown here.

Redistribution and use in source and binary forms, with or
without modification, is permitted pursuant to, and subject to
the license terms contained in, the Simplified BSD License set
forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(https://trustee.ietf.org/license-info).

This version of this YANG module is part of RFC 8639; see the
RFC itself for full legal notices.";
revision 2019-08-07 {
description
"Initial version.";
reference
"RFC 8639: A YANG Data Model for Subscriptions to
Event Notifications";
}

/*
* FEATURES
*/

feature configured {
description
"This feature indicates that configuration of subscriptions is
supported.";
}

feature dscp {
description
"This feature indicates that a publisher supports the ability
to set the Differentiated Services Code Point (DSCP) value in
outgoing packets.";
}

feature encode-json {
description
"This feature indicates that JSON encoding of notification
messages is supported.";
}

feature encode-xml {
description
"This feature indicates that XML encoding of notification
messages is supported.";
}

feature interface-designation {
description
"This feature indicates that a publisher supports sourcing all
receiver interactions for a configured subscription from a
single designated egress interface.";
}

feature qos {
description
"This feature indicates that a publisher supports absolute
dependencies of one subscription's traffic over another
as well as weighted bandwidth sharing between subscriptions.
Both of these are Quality of Service (QoS) features that allow
differentiated treatment of notification messages between a
publisher and a specific receiver.";
}

feature replay {
description
"This feature indicates that historical event record replay is
supported. With replay, it is possible for past event records
to be streamed in chronological order.";
}

feature subtree {
description
"This feature indicates support for YANG subtree filtering.";
reference
"RFC 6241: Network Configuration Protocol (NETCONF),
Section 6";
}

feature supports-vrf {
description
"This feature indicates that a publisher supports VRF
configuration for configured subscriptions. VRF support for
dynamic subscriptions does not require this feature.";
reference
"RFC 8529: YANG Data Model for Network Instances,
Section 6";
}

feature xpath {
description
"This feature indicates support for XPath filtering.";
reference
"XML Path Language (XPath) Version 1.0
(https://www.w3.org/TR/1999/REC-xpath-19991116)";
}

/*
* EXTENSIONS
*/

extension subscription-state-notification {
description
"This statement applies only to notifications. It indicates
that the notification is a subscription state change
notification. Therefore, it does not participate in a regular
event stream and does not need to be specifically subscribed
to in order to be received. This statement can only occur as
a substatement of the YANG 'notification' statement. This
statement is not for use outside of this YANG module.";
}

/*
* IDENTITIES
*/
/* Identities for RPC and notification errors */

identity delete-subscription-error {
description
"Base identity for the problem found while attempting to
fulfill either a 'delete-subscription' RPC request or a
'kill-subscription' RPC request.";
}

identity establish-subscription-error {
description
"Base identity for the problem found while attempting to
fulfill an 'establish-subscription' RPC request.";
}

identity modify-subscription-error {
description
"Base identity for the problem found while attempting to
fulfill a 'modify-subscription' RPC request.";
}

identity subscription-suspended-reason {
description
"Base identity for the problem condition communicated to a
receiver as part of a 'subscription-suspended'
notification.";
}

identity subscription-terminated-reason {
description
"Base identity for the problem condition communicated to a
receiver as part of a 'subscription-terminated'
notification.";
}

identity dscp-unavailable {
base establish-subscription-error;
if-feature "dscp";
description
"The publisher is unable to mark notification messages with
prioritization information in a way that will be respected
during network transit.";
}

identity encoding-unsupported {
base establish-subscription-error;
description
"Unable to encode notification messages in the desired
format.";
}

identity filter-unavailable {
base subscription-terminated-reason;
description
"Referenced filter does not exist. This means a receiver is
referencing a filter that doesn't exist or to which it
does not have access permissions.";
}

identity filter-unsupported {
base establish-subscription-error;
base modify-subscription-error;
description
"Cannot parse syntax in the filter. This failure can be from
a syntax error or a syntax too complex to be processed by the
publisher.";
}

identity insufficient-resources {
base establish-subscription-error;
base modify-subscription-error;
base subscription-suspended-reason;
description
"The publisher does not have sufficient resources to support
the requested subscription. An example might be that
allocated CPU is too limited to generate the desired set of
notification messages.";
}

identity no-such-subscription {
base modify-subscription-error;
base delete-subscription-error;
base subscription-terminated-reason;
description
"Referenced subscription doesn't exist. This may be as a
result of a nonexistent subscription ID, an ID that belongs to
another subscriber, or an ID for a configured subscription.";
}

identity replay-unsupported {
base establish-subscription-error;
if-feature "replay";
description
"Replay cannot be performed for this subscription. This means
the publisher will not provide the requested historic
information from the event stream via replay to this
receiver.";
}

identity stream-unavailable {
base subscription-terminated-reason;
description
"Not a subscribable event stream. This means the referenced
event stream is not available for subscription by the
receiver.";
}

identity suspension-timeout {
base subscription-terminated-reason;
description
"Termination of a previously suspended subscription. The
publisher has eliminated the subscription, as it exceeded a
time limit for suspension.";
}

identity unsupportable-volume {
base subscription-suspended-reason;
description
"The publisher does not have the network bandwidth needed to
get the volume of generated information intended for a
receiver.";
}

/* Identities for encodings */

identity configurable-encoding {
description
"If a transport identity derives from this identity, it means
that it supports configurable encodings. An example of a
configurable encoding might be a new identity such as
'encode-cbor'. Such an identity could use
'configurable-encoding' as its base. This would allow a
dynamic subscription encoded in JSON (RFC 8259) to request
that notification messages be encoded via the Concise Binary
Object Representation (CBOR) (RFC 7049). Further details for
any specific configurable encoding would be explored in a
transport document based on this specification.";
reference
"RFC 8259: The JavaScript Object Notation (JSON) Data
Interchange Format
RFC 7049: Concise Binary Object Representation (CBOR)";
}

identity encoding {
description
"Base identity to represent data encodings.";
}

identity encode-xml {
base encoding;
if-feature "encode-xml";
description
"Encode data using XML as described in RFC 7950.";
reference
"RFC 7950: The YANG 1.1 Data Modeling Language";
}

identity encode-json {
base encoding;
if-feature "encode-json";
description
"Encode data using JSON as described in RFC 7951.";
reference
"RFC 7951: JSON Encoding of Data Modeled with YANG";
}

/* Identities for transports */

identity transport {
description
"An identity that represents the underlying mechanism for
passing notification messages.";
}

/*
* TYPEDEFs
*/

typedef encoding {
type identityref {
base encoding;
}
description
"Specifies a data encoding, e.g., for a data subscription.";
}

typedef stream-filter-ref {
type leafref {
path "/sn:filters/sn:stream-filter/sn:name";
}
description
"This type is used to reference an event stream filter.";
}

typedef stream-ref {
type leafref {
path "/sn:streams/sn:stream/sn:name";
}
description
"This type is used to reference a system-provided
event stream.";
}

typedef subscription-id {
type uint32;
description
"A type for subscription identifiers.";
}

typedef transport {
type identityref {
base transport;
}
description
"Specifies the transport used to send notification messages
to a receiver.";
}

/*
* GROUPINGS
*/

grouping stream-filter-elements {
description
"This grouping defines the base for filters applied to event
streams.";
choice filter-spec {
description
"The content filter specification for this request.";
anydata stream-subtree-filter {
if-feature "subtree";
description
"Event stream evaluation criteria encoded in the syntax of
a subtree filter as defined in RFC 6241, Section 6.

The subtree filter is applied to the representation of
individual, delineated event records as contained in the
event stream.

If the subtree filter returns a non-empty node set, the
filter matches the event record, and the event record is
included in the notification message sent to the
receivers.";
reference
"RFC 6241: Network Configuration Protocol (NETCONF),
Section 6";
}
leaf stream-xpath-filter {
if-feature "xpath";
type yang:xpath1.0;
description
"Event stream evaluation criteria encoded in the syntax of
an XPath 1.0 expression.

The XPath expression is evaluated on the representation of
individual, delineated event records as contained in
the event stream.

The result of the XPath expression is converted to a
boolean value using the standard XPath 1.0 rules. If the
boolean value is 'true', the filter matches the event
record, and the event record is included in the
notification message sent to the receivers.

The expression is evaluated in the following XPath
context:

o The set of namespace declarations is the set of
prefix and namespace pairs for all YANG modules
implemented by the server, where the prefix is the
YANG module name and the namespace is as defined by
the 'namespace' statement in the YANG module.

If the leaf is encoded in XML, all namespace
declarations in scope on the 'stream-xpath-filter'
leaf element are added to the set of namespace
declarations. If a prefix found in the XML is
already present in the set of namespace
declarations, the namespace in the XML is used.

o The set of variable bindings is empty.

o The function library is comprised of the core
function library and the XPath functions defined in
Section 10 in RFC 7950.

o The context node is the root node.";
reference
"XML Path Language (XPath) Version 1.0
(https://www.w3.org/TR/1999/REC-xpath-19991116)
RFC 7950: The YANG 1.1 Data Modeling Language,
Section 10";
}
}
}

grouping update-qos {
description
"This grouping describes QoS information concerning a
subscription. This information is passed to lower layers
for transport prioritization and treatment.";
leaf dscp {
if-feature "dscp";
type inet:dscp;
default "0";
description
"The desired network transport priority level. This is the
priority set on notification messages encapsulating the
results of the subscription. This transport priority is
shared for all receivers of a given subscription.";
}
leaf weighting {
if-feature "qos";
type uint8 {
range "0 .. 255";
}
description
"Relative weighting for a subscription. Larger weights get
more resources. Allows an underlying transport layer to
perform informed load-balance allocations between various
subscriptions.";
reference
"RFC 7540: Hypertext Transfer Protocol Version 2 (HTTP/2),
Section 5.3.2";
}
leaf dependency {
if-feature "qos";
type subscription-id;
description
"Provides the 'subscription-id' of a parent subscription.
The parent subscription has absolute precedence should
that parent have push updates ready to egress the publisher.
In other words, there should be no streaming of objects from
the current subscription if the parent has something ready
to push.

If a dependency is asserted via configuration or via an RPC
but the referenced 'subscription-id' does not exist, the
dependency is silently discarded. If a referenced
subscription is deleted, this dependency is removed.";
reference
"RFC 7540: Hypertext Transfer Protocol Version 2 (HTTP/2),
Section 5.3.1";
}
}

grouping subscription-policy-modifiable {
description
"This grouping describes all objects that may be changed
in a subscription.";
choice target {
mandatory true;
description
"Identifies the source of information against which a
subscription is being applied as well as specifics on the
subset of information desired from that source.";
case stream {
choice stream-filter {
description
"An event stream filter can be applied to a subscription.
That filter will either come referenced from a global
list or be provided in the subscription itself.";
case by-reference {
description
"Apply a filter that has been configured separately.";
leaf stream-filter-name {
type stream-filter-ref;
mandatory true;
description
"References an existing event stream filter that is
to be applied to an event stream for the
subscription.";
}
}
case within-subscription {
description
"A local definition allows a filter to have the same
lifecycle as the subscription.";
uses stream-filter-elements;
}
}
}
}
leaf stop-time {
type yang:date-and-time;
description
"Identifies a time after which notification messages for a
subscription should not be sent. If 'stop-time' is not
present, the notification messages will continue until the
subscription is terminated. If 'replay-start-time' exists,
'stop-time' must be for a subsequent time. If
'replay-start-time' doesn't exist, 'stop-time', when
established, must be for a future time.";
}
}

grouping subscription-policy-dynamic {
description
"This grouping describes the only information concerning a
subscription that can be passed over the RPCs defined in this
data model.";
uses subscription-policy-modifiable {
augment "target/stream" {
description
"Adds additional objects that can be modified by an RPC.";
leaf stream {
type stream-ref {
require-instance false;
}
mandatory true;
description
"Indicates the event stream to be considered for
this subscription.";
}
leaf replay-start-time {
if-feature "replay";
type yang:date-and-time;
config false;
description
"Used to trigger the 'replay' feature for a dynamic
subscription, where event records that are selected
need to be at or after the specified starting time. If
'replay-start-time' is not present, this is not a replay
subscription and event record push should start
immediately. It is never valid to specify start times
that are later than or equal to the current time.";
}
}
}
uses update-qos;
}

grouping subscription-policy {
description
"This grouping describes the full set of policy information
concerning both dynamic and configured subscriptions, with the
exclusion of both receivers and networking information
specific to the publisher, such as what interface should be
used to transmit notification messages.";
uses subscription-policy-dynamic;
leaf transport {
if-feature "configured";
type transport;
description
"For a configured subscription, this leaf specifies the
transport used to deliver messages destined for all
receivers of that subscription.";
}
leaf encoding {
when 'not(../transport) or derived-from(../transport,
"sn:configurable-encoding")';
type encoding;
description
"The type of encoding for notification messages. For a
dynamic subscription, if not included as part of an
'establish-subscription' RPC, the encoding will be populated
with the encoding used by that RPC. For a configured
subscription, if not explicitly configured, the encoding
will be the default encoding for an underlying transport.";
}
leaf purpose {
if-feature "configured";
type string;
description
"Open text allowing a configuring entity to embed the
originator or other specifics of this subscription.";
}
}

/*
* RPCs
*/
rpc establish-subscription {
description
"This RPC allows a subscriber to create (and possibly
negotiate) a subscription on its own behalf. If successful,
the subscription remains in effect for the duration of the
subscriber's association with the publisher or until the
subscription is terminated. If an error occurs or the
publisher cannot meet the terms of a subscription, an RPC
error is returned, and the subscription is not created.
In that case, the RPC reply's 'error-info' MAY include
suggested parameter settings that would have a higher
likelihood of succeeding in a subsequent
'establish-subscription' request.";
input {
uses subscription-policy-dynamic;
leaf encoding {
type encoding;
description
"The type of encoding for the subscribed data. If not
included as part of the RPC, the encoding MUST be set by
the publisher to be the encoding used by this RPC.";
}
}
output {
leaf id {
type subscription-id;
mandatory true;
description
"Identifier used for this subscription.";
}
leaf replay-start-time-revision {
if-feature "replay";
type yang:date-and-time;
description
"If a replay has been requested, this object represents
the earliest time covered by the event buffer for the
requested event stream. The value of this object is the
'replay-log-aged-time' if it exists. Otherwise, it is
the 'replay-log-creation-time'. All buffered event
records after this time will be replayed to a receiver.
This object will only be sent if the starting time has
been revised to be later than the time requested by the
subscriber.";
}
}
}

rc:yang-data establish-subscription-stream-error-info {
container establish-subscription-stream-error-info {
description
"If any 'establish-subscription' RPC parameters are
unsupportable against the event stream, a subscription
is not created and the RPC error response MUST indicate the
reason why the subscription failed to be created. This
yang-data MAY be inserted as structured data in a
subscription's RPC error response to indicate the reason for
the failure. This yang-data MUST be inserted if hints are
to be provided back to the subscriber.";
leaf reason {
type identityref {
base establish-subscription-error;
}
description
"Indicates the reason why the subscription has failed to
be created to a targeted event stream.";
}
leaf filter-failure-hint {
type string;
description
"Information describing where and/or why a provided
filter was unsupportable for a subscription. The
syntax and semantics of this hint are
implementation specific.";
}
}
}

rpc modify-subscription {
description
"This RPC allows a subscriber to modify a dynamic
subscription's parameters. If successful, the changed
subscription parameters remain in effect for the duration of
the subscription, until the subscription is again modified, or
until the subscription is terminated. In the case of an error
or an inability to meet the modified parameters, the
subscription is not modified and the original subscription
parameters remain in effect. In that case, the RPC error MAY
include 'error-info' suggested parameter hints that would have
a high likelihood of succeeding in a subsequent
'modify-subscription' request. A successful
'modify-subscription' will return a suspended subscription to
the 'active' state.";
input {
leaf id {
type subscription-id;
mandatory true;
description
"Identifier to use for this subscription.";
}
uses subscription-policy-modifiable;
}
}

rc:yang-data modify-subscription-stream-error-info {
container modify-subscription-stream-error-info {
description
"This yang-data MAY be provided as part of a subscription's
RPC error response when there is a failure of a
'modify-subscription' RPC that has been made against an
event stream. This yang-data MUST be used if hints are to
be provided back to the subscriber.";
leaf reason {
type identityref {
base modify-subscription-error;
}
description
"Information in a 'modify-subscription' RPC error response
that indicates the reason why the subscription to an event
stream has failed to be modified.";
}
leaf filter-failure-hint {
type string;
description
"Information describing where and/or why a provided
filter was unsupportable for a subscription. The syntax
and semantics of this hint are
implementation specific.";
}
}
}

rpc delete-subscription {
description
"This RPC allows a subscriber to delete a subscription that
was previously created by that same subscriber using the
'establish-subscription' RPC.

If an error occurs, the server replies with an 'rpc-error'
where the 'error-info' field MAY contain a
'delete-subscription-error-info' structure.";
input {
leaf id {
type subscription-id;
mandatory true;
description
"Identifier of the subscription that is to be deleted.
Only subscriptions that were created using
'establish-subscription' from the same origin as this RPC
can be deleted via this RPC.";
}
}
}

rpc kill-subscription {
nacm:default-deny-all;
description
"This RPC allows an operator to delete a dynamic subscription
without restrictions on the originating subscriber or
underlying transport session.

rc:yang-data delete-subscription-error-info {
container delete-subscription-error-info {
description
"If a 'delete-subscription' RPC or a 'kill-subscription' RPC
fails, the subscription is not deleted and the RPC error
response MUST indicate the reason for this failure. This
yang-data MAY be inserted as structured data in a
subscription's RPC error response to indicate the reason
for the failure.";
leaf reason {
type identityref {
base delete-subscription-error;
}
mandatory true;
description
"Indicates the reason why the subscription has failed to be
deleted.";
}
}
}

/*
* NOTIFICATIONS
*/

notification replay-completed {
sn:subscription-state-notification;
if-feature "replay";
description
"This notification is sent to indicate that all of the replay
notifications have been sent.";
leaf id {
type subscription-id;
mandatory true;
description
"This references the affected subscription.";
}
}

notification subscription-completed {
sn:subscription-state-notification;
if-feature "configured";
description
"This notification is sent to indicate that a subscription has
finished passing event records, as the 'stop-time' has been
reached.";
leaf id {
type subscription-id;
mandatory true;
description
"This references the gracefully completed subscription.";
}
}

notification subscription-modified {
sn:subscription-state-notification;
description
"This notification indicates that a subscription has been
modified. Notification messages sent from this point on will
conform to the modified terms of the subscription. For
completeness, this subscription state change notification
includes both modified and unmodified aspects of a
subscription.";
leaf id {
type subscription-id;
mandatory true;
description
"This references the affected subscription.";
}
uses subscription-policy {
refine "target/stream/stream-filter/within-subscription" {
description
"Filter applied to the subscription. If the
'stream-filter-name' is populated, the filter in the
subscription came from the 'filters' container.
Otherwise, it is populated in-line as part of the
subscription.";
}
}
}

notification subscription-resumed {
sn:subscription-state-notification;
description
"This notification indicates that a subscription that had
previously been suspended has resumed. Notifications will
once again be sent. In addition, a 'subscription-resumed'
indicates that no modification of parameters has occurred
since the last time event records have been sent.";
leaf id {
type subscription-id;
mandatory true;
description
"This references the affected subscription.";
}
}

notification subscription-started {
sn:subscription-state-notification;
if-feature "configured";
description
"This notification indicates that a subscription has started
and notifications will now be sent.";
leaf id {
type subscription-id;
mandatory true;
description
"This references the affected subscription.";
}
uses subscription-policy {
refine "target/stream/replay-start-time" {
description
"Indicates the time that a replay is using for the
streaming of buffered event records. This will be
populated with the most recent of the following:
the event time of the previous event record sent to a
receiver, the 'replay-log-creation-time', the
'replay-log-aged-time', or the most recent publisher
boot time.";
}
refine "target/stream/stream-filter/within-subscription" {
description
"Filter applied to the subscription. If the
'stream-filter-name' is populated, the filter in the
subscription came from the 'filters' container.
Otherwise, it is populated in-line as part of the
subscription.";
}
augment "target/stream" {
description
"This augmentation adds additional parameters specific to a
'subscription-started' notification.";
leaf replay-previous-event-time {
when '../replay-start-time';
if-feature "replay";
type yang:date-and-time;
description
"If there is at least one event in the replay buffer
prior to 'replay-start-time', this gives the time of
the event generated immediately prior to the
'replay-start-time'.

If a receiver previously received event records for
this configured subscription, it can compare this time
to the last event record previously received. If the
two are not the same (perhaps due to a reboot), then a
dynamic replay can be initiated to acquire any missing
event records.";
}
}
}
}

notification subscription-suspended {
sn:subscription-state-notification;
description
"This notification indicates that a suspension of the
subscription by the publisher has occurred. No further
notifications will be sent until the subscription resumes.
This notification shall only be sent to receivers of a
subscription; it does not constitute a general-purpose
notification.";
leaf id {
type subscription-id;
mandatory true;
description
"This references the affected subscription.";
}
leaf reason {
type identityref {
base subscription-suspended-reason;
}
mandatory true;
description
"Identifies the condition that resulted in the suspension.";
}
}

notification subscription-terminated {
sn:subscription-state-notification;
description
"This notification indicates that a subscription has been
terminated.";
leaf id {
type subscription-id;
mandatory true;
description
"This references the affected subscription.";
}
leaf reason {
type identityref {
base subscription-terminated-reason;
}
mandatory true;
description
"Identifies the condition that resulted in the termination.";
}
}

/*
* DATA NODES
*/

container streams {
config false;
description
"Contains information on the built-in event streams provided by
the publisher.";
list stream {
key "name";
description
"Identifies the built-in event streams that are supported by
the publisher.";
leaf name {
type string;
description
"A handle for a system-provided event stream made up of a
sequential set of event records, each of which is
characterized by its own domain and semantics.";
}
leaf description {
type string;
description
"A description of the event stream, including such
information as the type of event records that are
available in this event stream.";
}
leaf replay-support {
if-feature "replay";
type empty;
description
"Indicates that event record replay is available on this
event stream.";
}
leaf replay-log-creation-time {
when '../replay-support';
if-feature "replay";
type yang:date-and-time;
mandatory true;
description
"The timestamp of the creation of the log used to support
the replay function on this event stream. This time
might be earlier than the earliest available information
contained in the log. This object is updated if the log
resets for some reason.";
}
leaf replay-log-aged-time {
when '../replay-support';
if-feature "replay";
type yang:date-and-time;
description
"The timestamp associated with the last event record that
has been aged out of the log. This timestamp identifies
how far back in history this replay log extends, if it
doesn't extend back to the 'replay-log-creation-time'.
This object MUST be present if replay is supported and any
event records have been aged out of the log.";
}
}
}
container filters {
description
"Contains a list of configurable filters that can be applied to
subscriptions. This facilitates the reuse of complex filters
once defined.";
list stream-filter {
key "name";
description
"A list of preconfigured filters that can be applied to
subscriptions.";
leaf name {
type string;
description
"A name to differentiate between filters.";
}
uses stream-filter-elements;
}
}
container subscriptions {
description
"Contains the list of currently active subscriptions, i.e.,
subscriptions that are currently in effect, used for
subscription management and monitoring purposes. This
includes subscriptions that have been set up via
RPC primitives as well as subscriptions that have been
established via configuration.";
list subscription {
key "id";
description
"The identity and specific parameters of a subscription.
Subscriptions in this list can be created using a control
channel or RPC or can be established through configuration.

If the 'kill-subscription' RPC or configuration operations
are used to delete a subscription, a
'subscription-terminated' message is sent to any active or
suspended receivers.";
leaf id {
type subscription-id;
description
"Identifier of a subscription; unique in a given
publisher.";
}
uses subscription-policy {
refine "target/stream/stream" {
description
"Indicates the event stream to be considered for this
subscription. If an event stream has been removed
and can no longer be referenced by an active
subscription, send a 'subscription-terminated'
notification with 'stream-unavailable' as the reason.
If a configured subscription refers to a nonexistent
event stream, move that subscription to the
'invalid' state.";
}
refine "transport" {
description
"For a configured subscription, this leaf specifies the
transport used to deliver messages destined for all
receivers of that subscription. This object is
mandatory for subscriptions in the configuration
datastore. This object (1) is not mandatory for dynamic
subscriptions in the operational state datastore and
(2) should not be present for other types of dynamic
subscriptions.";
}
augment "target/stream" {
description
"Enables objects to be added to a configured stream
subscription.";
leaf configured-replay {
if-feature "configured";
if-feature "replay";
type empty;
description
"The presence of this leaf indicates that replay for
the configured subscription should start at the
earliest time in the event log or at the publisher
boot time, whichever is later.";
}
}
}
choice notification-message-origin {
if-feature "configured";
description
"Identifies the egress interface on the publisher
from which notification messages are to be sent.";
case interface-originated {
description
"When notification messages are to egress a specific,
designated interface on the publisher.";
leaf source-interface {
if-feature "interface-designation";
type if:interface-ref;
description
"References the interface for notification messages.";
}
}
case address-originated {
description
"When notification messages are to depart from a
publisher using a specific originating address and/or
routing context information.";
leaf source-vrf {
if-feature "supports-vrf";
type leafref {
path "/ni:network-instances/ni:network-instance/ni:name";
}
description
"VRF from which notification messages should egress a
publisher.";
}
leaf source-address {
type inet:ip-address-no-zone;
description
"The source address for the notification messages.
If a source VRF exists but this object doesn't, a
publisher's default address for that VRF must
be used.";
}
}
}
leaf configured-subscription-state {
if-feature "configured";
type enumeration {
enum valid {
value 1;
description
"The subscription is supportable with its current
parameters.";
}
enum invalid {
value 2;
description
"The subscription as a whole is unsupportable with its
current parameters.";
}
enum concluded {
value 3;
description
"A subscription is inactive, as it has hit a
stop time. It no longer has receivers in the
'receiver active'
'active' or 'receiver suspended' 'suspended' state, but the subscription
has not yet been removed from configuration.";
}
}
config false;
description
"The presence of this leaf indicates that the subscription
originated from configuration, not through a control
channel or RPC. The value indicates the state of the
subscription as established by the publisher.";
}
container receivers {
description
"Set of receivers in a subscription.";
list receiver {
key "name";
min-elements 1;
description
"A host intended as a recipient for the notification
messages of a subscription. For configured
subscriptions, transport-specific network parameters
(or a leafref to those parameters) may be augmented to a
specific receiver in this list.";
leaf name {
type string;
description
"Identifies a unique receiver for a subscription.";
}
leaf sent-event-records {
type yang:zero-based-counter64;
config false;
description
"The number of event records sent to the receiver. The
count is initialized when a dynamic subscription is
established or when a configured receiver
transitions to the 'valid' state.";
}
leaf excluded-event-records {
type yang:zero-based-counter64;
config false;
description
"The number of event records explicitly removed via
either an event stream filter or an access control
filter so that they are not passed to a receiver.
This count is set to zero each time
'sent-event-records' is initialized.";
}
leaf state {
type enumeration {
enum active {
value 1;
description
"The receiver is currently being sent any
applicable notification messages for the
subscription.";
}
enum suspended {
value 2;
description
"The receiver state is 'suspended', so the
publisher is currently unable to provide
notification messages for the subscription.";
}
enum connecting {
value 3;
if-feature "configured";
description
"A subscription has been configured, but a
'subscription-started' subscription state change
notification needs to be successfully received
before notification messages are sent.

If the 'reset' action is invoked for a receiver of
an active configured subscription, the state
must be moved to 'connecting'.";
}
enum disconnected {
value 4;
if-feature "configured";
description
"A subscription has failed to send a
'subscription-started' state change to the
receiver. Additional connection attempts are not
currently being made.";
}
}
config false;
mandatory true;
description
"Specifies the state of a subscription from the
perspective of a particular receiver. With this
information, it is possible to determine whether a
publisher is currently generating notification
messages intended for that receiver.";
}
action reset {
if-feature "configured";
description
"Allows the reset of this configured subscription's
receiver to the 'connecting' state. This enables the
connection process to be reinitiated.";
output {
leaf time {
type yang:date-and-time;
mandatory true;
description
"Time at which a publisher returned the receiver to
the 'connecting' state.";
}
}
}
}
}
}
}
}
<CODE ENDS>

5. IANA Considerations

This document registers the following namespace

IANA has registered one URI in the "ns" subregistry of the "IETF XML
Registry" [RFC3688] maintained at <https://www.iana.org/assignments/
xml-registry>. The following registration has been made per the
format in [RFC3688]:

URI: urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications
Registrant Contact: The IESG. NETCONF WG of the IETF.
XML: N/A; the requested URI is an XML namespace.

This document registers the following

IANA has registered one YANG module in the "YANG Module Names"
registry [RFC6020] maintained at <https://www.iana.org/assignments/
yang-parameters>. The following registration has been made per the
format in [RFC6020]:

Name: ietf-subscribed-notifications
Namespace: urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications
Prefix: sn
Reference: RFC 8639

6. Implementation Considerations

To support deployments that include both configured and dynamic
subscriptions, it is recommended that the subscription "id" domain be
split into static and dynamic halves. This will eliminate the
possibility of collisions if the configured subscriptions attempt to
set a "subscription-id" that might have already been dynamically
allocated. A best practice is to use the lower half of the "id"
object's integer space when that "id" is assigned by an external
entity (such as with a configured subscription). This leaves the
upper half of the subscription integer space available to be
dynamically assigned by the publisher.

If a subscription is unable to marshal a series of filtered event
records into transmittable notification messages, the receiver should
be suspended with the reason "unsupportable-volume".

For configured subscriptions, operations are performed against the
set of receivers using the subscription "id" as a handle for that
set. But for streaming updates, subscription state change
notifications are local to a receiver. In the case of this
specification, receivers do not get any information from the
publisher about the existence of other receivers. But if a network
operator wants to let the receivers correlate results, it is useful
to use the subscription "id" across the receivers to allow that
correlation. Note that due to the possibility of different access
control permissions per receiver, each receiver may actually get a
different set of event records.

For configured replay subscriptions, the receiver is protected from
duplicated events being pushed after a publisher is rebooted.
However, it is possible that a receiver might want to acquire event
records that failed to be delivered just prior to the reboot.
Delivering these event records can be accomplished by leveraging the
<eventTime> [RFC5277] from the last event record received prior to
the receipt of a "subscription-started" subscription state change
notification. With this <eventTime> and the "replay-start-time" from
the "subscription-started" notification, an independent dynamic
subscription can be established that retrieves any event records that
may have been generated but not sent to the receiver.

7. Transport Requirements

This section provides requirements for any subscribed notification
transport supporting the solution presented in this document.

The transport selected by the subscriber to reach the publisher MUST
be able to support multiple "establish-subscription" requests made in
the same transport session.

For both configured and dynamic subscriptions, the publisher MUST
authenticate a receiver via some transport-level mechanism before
sending any event records that the receiver is authorized to see. In
addition, the receiver MUST authenticate the publisher at the
transport level. The result is mutual authentication between
the two.

A secure transport is highly recommended. Beyond this, the publisher
MUST ensure that the receiver has sufficient authorization to perform
the function it is requesting against the specific subset of content
involved.

A specific transport specification for a transport built upon this document may or may
not choose to require the use of the same logical channel for the
RPCs and the event records. However, the event records and the
subscription state change notifications MUST be sent on the same
transport session to ensure properly ordered delivery.

A specific transport specification for a transport MUST identify any encodings that are
supported. If a configured subscription's transport allows different
encodings, the specification MUST identify the default encoding.

A subscriber that includes a "dscp" leaf in an "establish-
subscription" request will need to understand and consider what the
corresponding DSCP value represents in the domain of the publisher.

Additional transport requirements will be dictated by the choice of
transport used with a subscription. For an example of such
requirements, see [RFC8640].

8. Security Considerations

The YANG module specified in this document defines a schema for data
that is designed to be accessed via network management protocols such
as NETCONF [RFC6241] or RESTCONF [RFC8040]. The lowest NETCONF layer
is the secure transport layer, and the mandatory-to-implement secure
transport is Secure Shell (SSH) [RFC6242]. The lowest RESTCONF layer
is HTTPS, and the mandatory-to-implement secure transport is TLS
[RFC8446].
[RFC5246].

The Network Configuration Access Control Model (NACM) [RFC8341]
provides the means to restrict access for particular NETCONF or
RESTCONF users to a preconfigured subset of all available NETCONF or
RESTCONF protocol operations and content.

With configured subscriptions, one or more publishers could be used
to overwhelm a receiver. To counter this, notification messages
SHOULD NOT be sent to any receiver that does not support this
specification. Receivers that do not want notification messages need
only terminate or refuse any transport sessions from the publisher.

When a receiver of a configured subscription gets a new
"subscription-started" message for a known subscription where it is
already consuming events, it may indicate that an attacker has done
something that has momentarily disrupted receiver connectivity. To
acquire events lost during this interval, the receiver SHOULD
retrieve any event records generated since the last event record was
received. This can be accomplished by establishing a separate
dynamic replay subscription with the same filtering criteria with the
publisher, assuming that the publisher supports the "replay" feature.

For dynamic subscriptions, implementations need to protect against
malicious or buggy subscribers that may send a large number of
"establish-subscription" requests and thereby use up system
resources. To cover this possibility, operators SHOULD monitor for
such cases and, if discovered, take remedial action to limit the
resources used, such as suspending or terminating a subset of the
subscriptions or, if the underlying transport is session based,
terminating the underlying transport session.

The replay mechanisms described in Sections 2.4.2.1 and 2.5.6 provide
access to historical event records. By design, the access control
model that protects these records could enable subscribers to view
data to which they were not authorized at the time of collection.

Using DNS names for configured subscription's receiver "name" lookups
can cause situations where the name resolves differently than
expected on the publisher, so the recipient would be different than
expected.

An attacker that can cause the publisher to use an incorrect time can
induce message replay by setting the time in the past and can
introduce a risk of message loss by setting the time in the future.

There are a number of data nodes defined in this YANG module that are
writable/creatable/deletable (i.e., config true, which is the
default). These data nodes may be considered sensitive or vulnerable
in some network environments. Write operations (e.g., edit-config)
to these data nodes without proper protection can have a negative
effect on network operations. These are the subtrees and data nodes
and their sensitivity/vulnerability:

Container: "/filters"
o "stream-subtree-filter": Updating a filter could increase the
computational complexity of all referencing subscriptions.

o "stream-xpath-filter": Updating a filter could increase the
computational complexity of all referencing subscriptions.

Container: "/subscriptions"

The following considerations are only relevant for configuration
operations made upon configured subscriptions:

o "configured-replay": Can be used to send a large number of event
records to a receiver.

o "dependency": Can be used to force important traffic to be queued
behind updates that are not as important.

o "dscp": If unvalidated, can result in the sending of traffic with
a higher-priority marking than warranted.

o "id": Can overwrite an existing subscription, perhaps one
configured by another entity.

o "name": Adding a new key entry can be used to attempt to send
traffic to an unwilling receiver.

o "replay-start-time": Can be used to push very large logs, wasting
resources.

o "source-address": The configured address might not be able to
reach a desired receiver.

o "source-interface": The configured interface might not be able to
reach a desired receiver.

o "source-vrf": Can place a subscription in a virtual network where
receivers are not entitled to view the subscribed content.

o "stop-time": Could be used to terminate content at an inopportune
time.

o "stream": Could set a subscription to an event stream that does
not contain content permitted for the targeted receivers.

o "stream-filter-name": Could be set to a filter that is not
relevant to the event stream.

o "stream-subtree-filter": A complex filter can increase the
computational resources for this subscription.

o "stream-xpath-filter": A complex filter can increase the
computational resources for this subscription.

o "weighting": Allocating a large weight can overwhelm the dequeuing
of other subscriptions.

Some of the readable data nodes in this YANG module may be considered
sensitive or vulnerable in some network environments. It is thus
important to control read access (e.g., via get, get-config, or
notification) to these data nodes. These are the subtrees and data
nodes and their sensitivity/vulnerability:

Container: "/streams"

o "name": If access control is not properly configured, can expose
system internals to those who should not have access to this
information.

o "replay-support": If access control is not properly configured,
can expose logs to those who should not have access.

Container: "/subscriptions"

o "excluded-event-records": This leaf can provide information about
filtered event records. A network operator should have the proper
permissions to know about such filtering. However, exposing the
count of excluded events to a receiver could leak information
about the presence of access control filters that might be in
place for that receiver.

o "subscription": Different operational teams might have a desire to
set varying subsets of subscriptions. Access control should be
designed to permit read access to just the allowed set.

Some of the RPC operations in this YANG module may be considered
sensitive or vulnerable in some network environments. It is thus
important to control access to these operations. These are the
operations and their sensitivity/vulnerability:

RPC: all

o If a malicious or buggy subscriber sends an unexpectedly large
number of RPCs, the result might be an excessive use of system
resources on the publisher just to determine that these
subscriptions should be declined. In such a situation,
subscription interactions MAY be terminated by terminating the
transport session.

RPC: "delete-subscription"

o No special considerations.

RPC: "establish-subscription"

o Subscriptions could overload a publisher's resources. For this
reason, publishers MUST ensure that they have sufficient resources
to fulfill this request or otherwise request; otherwise, they MUST reject the request.

RPC: "kill-subscription"

o The "kill-subscription" RPC MUST be secured so that only
connections with administrative rights are able to invoke
this RPC.

RPC: "modify-subscription"

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.

[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<https://www.rfc-editor.org/info/rfc3688>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.

[RFC5277] Chisholm, S. and H. Trevino, "NETCONF Event
Notifications", RFC 5277, DOI 10.17487/RFC5277, July 2008,
<https://www.rfc-editor.org/info/rfc5277>.

[RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)", RFC 6020,
DOI 10.17487/RFC6020, October 2010,
<https://www.rfc-editor.org/info/rfc6020>.

[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.

[RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure
Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
<https://www.rfc-editor.org/info/rfc6242>.

[RFC6991] Schoenwaelder, J., Ed., "Common YANG Data Types",
RFC 6991, DOI 10.17487/RFC6991, July 2013,
<https://www.rfc-editor.org/info/rfc6991>.

[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.

[RFC7951] Lhotka, L., "JSON Encoding of Data Modeled with YANG",
RFC 7951, DOI 10.17487/RFC7951, August 2016,
<https://www.rfc-editor.org/info/rfc7951>.

[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/info/rfc8040>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8341] Bierman, A. and M. Bjorklund, "Network Configuration
Access Control Model", STD 91, RFC 8341,
DOI 10.17487/RFC8341, March 2018,
<https://www.rfc-editor.org/info/rfc8341>.

[RFC8342] Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
and R. Wilton, "Network Management Datastore Architecture
(NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
<https://www.rfc-editor.org/info/rfc8342>.

[RFC8343] Bjorklund, M., "A YANG Data Model for Interface
Management", RFC 8343, DOI 10.17487/RFC8343, March 2018,
<https://www.rfc-editor.org/info/rfc8343>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[RFC8529] Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and X.
Liu, "YANG Data Model for Network Instances", RFC 8529,
DOI 10.17487/RFC8529, March 2019,
<https://www.rfc-editor.org/info/rfc8529>.

[W3C.REC-xml-20081126]
Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", World Wide Web Consortium Recommendation REC-
xml-20081126, November 2008,
<https://www.w3.org/TR/2008/REC-xml-20081126>.

[XPATH] Clark, J. and S. DeRose, "XML Path Language (XPath)
Version 1.0", November 1999,
<https://www.w3.org/TR/1999/REC-xpath-19991116>.

9.2. Informative References

[RESTCONF-Notif]
Voit, E., Rahman, R., Nilsen-Nygaard, E., Clemm, A., and
A. Bierman, "Dynamic subscription to YANG Events and
Datastores over RESTCONF", Work in Progress, draft-ietf-
netconf-restconf-notif-15, June 2019.

[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.

[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.

[RFC7923] Voit, E., Clemm, A., and A. Gonzalez Prieto, "Requirements
for Subscription to YANG Datastores", RFC 7923,
DOI 10.17487/RFC7923, June 2016,
<https://www.rfc-editor.org/info/rfc7923>.

[RFC8071] Watsen, K., "NETCONF Call Home and RESTCONF Call Home",
RFC 8071, DOI 10.17487/RFC8071, February 2017,
<https://www.rfc-editor.org/info/rfc8071>.

[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.

[RFC8340] Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
<https://www.rfc-editor.org/info/rfc8340>.

[RFC8640] Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard,
E., and A. Tripathy, "Dynamic Subscription to YANG Events
and Datastores over NETCONF", RFC 8640,
DOI 10.17487/RFC8640, August 2019,
<https://www.rfc-editor.org/info/rfc8640>.

[RFC8641] Clemm, A. and E. Voit, "Subscriptions to YANG Datastores",
RFC 8641, DOI 10.17487/RFC8641, August 2019,
<https://www.rfc-editor.org/info/rfc8641>.

Appendix A. Example Configured Transport Augmentation

This appendix provides a non-normative example of how the YANG module
defined in Section 4 may be enhanced to incorporate the configuration
parameters needed to support the transport connectivity process.
This example is not intended to be a complete transport model. In
this example, connectivity via an imaginary transport type of "foo"
is explored. For more on the overall objectives behind configuring
transport connectivity for a configured subscription, see
Section 2.5.7.

The YANG module example defined in this appendix contains two main
elements. First is a transport identity "foo". This transport
identity allows a configuration agent to define "foo" as the selected
type of transport for a subscription. Second is a YANG case
augmentation "foo", which is made to the
"/subscriptions/subscription/receivers/receiver" node of Section 4.
In this augmentation are the transport configuration parameters
"address" and "port", which are necessary to make the connection to
the receiver.

module example-foo-subscribed-notifications {
yang-version 1.1;
namespace
"urn:example:foo-subscribed-notifications";

prefix fsn;

import ietf-subscribed-notifications {
prefix sn;
}
import ietf-inet-types {
prefix inet;
}

description
"Defines 'foo' as a supported type of configured transport for
subscribed event notifications.";

identity foo {
base sn:transport;
description
"Transport type 'foo' is available for use as a configured
subscription's transport protocol for subscribed
notifications.";
}

augment
"/sn:subscriptions/sn:subscription/sn:receivers/sn:receiver" {
when 'derived-from(../../../transport, "fsn:foo")';
description
"This augmentation makes transport parameters specific to 'foo'
available for a receiver.";
leaf address {
type inet:host;
mandatory true;
description
"Specifies the address to use for messages destined for a
receiver.";
}
leaf port {
type inet:port-number;
mandatory true;
description
"Specifies the port number to use for messages destined for a
receiver.";
}
}
}

Figure 21: Example Transport Augmentation
for the Fictitious Protocol "foo"

This example YANG module for transport "foo" will not be seen in a
real-world deployment. For a real-world deployment supporting an
actual transport technology, a similar YANG module must be defined.

Acknowledgments

For their valuable comments, discussions, and feedback, we wish to
acknowledge Andy Bierman, Tim Jenkins, Martin Bjorklund, Kent Watsen,
Balazs Lengyel, Robert Wilton, Sharon Chisholm, Hector Trevino, Susan
Hares, Michael Scharf, and Guangying Zheng.

Authors' Addresses

Eric Voit
Cisco Systems

Email: evoit@cisco.com

Alexander Clemm
Futurewei

Email: ludwig@clemm.org

Alberto Gonzalez Prieto
Microsoft

Email: alberto.gonzalez@microsoft.com

Einar Nilsen-Nygaard
Cisco Systems

Email: einarnn@cisco.com

Ambika Prasad Tripathy
Cisco Systems

Email: ambtripa@cisco.com