rfc8640
Internet Engineering Task Force (IETF) E. Voit
Request for Comments: 8640 Cisco Systems
Category: Standards Track A. Clemm
ISSN: 2070-1721 Futurewei
A. Gonzalez Prieto
Microsoft
E. Nilsen-Nygaard
A. Tripathy
Cisco Systems
September 2019
Dynamic Subscription to YANG Events and Datastores over NETCONF
Abstract
This document provides a Network Configuration Protocol (NETCONF)
binding to the dynamic subscription capability of both subscribed
notifications and YANG-Push.
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/rfc8640.
Voit, et al. Standards Track [Page 1]
RFC 8640 NETCONF Notifications September 2019
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Without obtaining an adequate license from the person(s) controlling
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not be created outside the IETF Standards Process, except to format
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than English.
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Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................3
3. Compatibility with <create-subscription> as Defined in
RFC 5277 ........................................................4
4. Mandatory XML, Event Stream, and Datastore Support ..............4
5. NETCONF Connectivity and Dynamic Subscriptions ..................4
6. Notification Messages ...........................................5
7. Dynamic Subscriptions and RPC Error Responses ...................5
8. Security Considerations .........................................7
9. IANA Considerations .............................................7
10. References .....................................................7
10.1. Normative References ......................................7
10.2. Informative References ....................................8
Appendix A. Examples ...............................................9
A.1. Event Stream Discovery ......................................9
A.2. Dynamic Subscriptions ......................................10
A.3. Subscription State Notifications ...........................15
A.4. Filter Examples ............................................17
Acknowledgments ...................................................19
Authors' Addresses ................................................19
1. Introduction
This document specifies the binding of a stream of events that form
part of a dynamic subscription to the Network Configuration Protocol
(NETCONF) [RFC6241]. Dynamic subscriptions are defined in [RFC8639].
In addition, as [RFC8641] is itself built upon [RFC8639], this
document enables a NETCONF client to request via a dynamic
subscription, and receive, updates from a YANG datastore located on a
NETCONF server.
This document assumes that the reader is familiar with the
terminology and concepts defined in [RFC8639].
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.
The following terms are defined in [RFC8639]: dynamic subscription,
event stream, notification message, publisher, receiver, subscriber,
and subscription. This document does not define any additional
terms.
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3. Compatibility with <create-subscription> as Defined in RFC 5277
A publisher is allowed to concurrently support dynamic subscription
RPCs as defined in [RFC8639] at the same time as the
<create-subscription> RPC defined in [RFC5277]. However, a single
NETCONF transport session MUST NOT support both this specification
and a subscription established by the <create-subscription> RPC
defined in [RFC5277]. To protect against any attempts to use a
single NETCONF transport session in this way:
o A solution MUST reply with the <rpc-error> element [RFC6241]
containing the "error-tag" value of "operation-not-supported" if a
<create-subscription> RPC is received on a NETCONF session where
an established subscription per [RFC8639] exists.
o A solution MUST reply with the <rpc-error> element [RFC6241]
containing the "error-tag" value of "operation-not-supported" if
an "establish-subscription" request has been received on a NETCONF
session where the <create-subscription> RPC [RFC5277] has
successfully created a subscription.
If a publisher supports this specification but not subscriptions via
[RFC5277], the publisher MUST NOT advertise
"urn:ietf:params:netconf:capability:notification:1.0".
4. Mandatory XML, Event Stream, and Datastore Support
The "encode-xml" feature of [RFC8639] MUST be supported. This
indicates that XML is a valid encoding for RPCs, state change
notifications, and subscribed content.
A NETCONF publisher supporting event stream subscription via
[RFC8639] MUST support the "NETCONF" event stream identified in that
document.
5. NETCONF Connectivity and Dynamic Subscriptions
Management of dynamic subscriptions occurs via RPCs as defined in
[RFC8641] and [RFC8639]. For a dynamic subscription, if the NETCONF
session involved with the "establish-subscription" terminates, the
subscription MUST be terminated.
For a dynamic subscription, any "modify-subscription",
"delete-subscription", or "resync-subscription" RPCs MUST be sent
using the same NETCONF session upon which the referenced subscription
was established.
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6. Notification Messages
Notification messages transported over NETCONF MUST be encoded in a
<notification> message as defined in [RFC5277], Section 4. And per
the <eventTime> object definition provided in [RFC5277], <eventTime>
is populated with the event occurrence time.
For dynamic subscriptions, all notification messages MUST use the
NETCONF transport session used by the "establish-subscription" RPC.
7. Dynamic Subscriptions and RPC Error Responses
When an RPC error occurs as defined in [RFC8639], Section 2.4.6 and
[RFC8641], Appendix A, the NETCONF RPC reply MUST include an
<rpc-error> element per [RFC6241] with the error information
populated as follows:
o An "error-type" node of "application".
o An "error-tag" node, where the value is a string that corresponds
to an identity associated with the error. For the mechanisms
specified in this document, this "error-tag" will correspond to
the error identities in either (1) [RFC8639], Section 2.4.6, for
general subscription errors:
error identity uses error-tag
---------------------- -----------------------
dscp-unavailable invalid-value
encoding-unsupported invalid-value
filter-unsupported invalid-value
insufficient-resources resource-denied
no-such-subscription invalid-value
replay-unsupported operation-not-supported
or (2) [RFC8641], Appendix A.1, for subscription errors specific
to YANG datastores:
error identity uses error-tag
--------------------------- -----------------------
cant-exclude operation-not-supported
datastore-not-subscribable invalid-value
no-such-subscription-resync invalid-value
on-change-unsupported operation-not-supported
on-change-sync-unsupported operation-not-supported
period-unsupported invalid-value
update-too-big too-big
sync-too-big too-big
unchanging-selection operation-failed
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o An "error-severity" of "error" (this MAY be included).
o An "error-app-tag" node, where the value is a string that
corresponds to an identity associated with the error, as defined
in [RFC8639], Section 2.4.6 for general subscriptions and
[RFC8641], Appendix A.1 for datastore subscriptions. The specific
identity to use depends on the RPC for which the error occurred.
Each error identity will be inserted as the "error-app-tag"
following the form <modulename>:<identityname>. An example of
such a valid encoding would be
"ietf-subscribed-notifications:no-such-subscription". Viable
errors for different RPCs are as follows:
RPC has base identity
---------------------- ----------------------------
establish-subscription establish-subscription-error
modify-subscription modify-subscription-error
delete-subscription delete-subscription-error
kill-subscription delete-subscription-error
resync-subscription resync-subscription-error
o In the case of error responses to an "establish-subscription" or
"modify-subscription" request, there is the option of including an
"error-info" node. This node may contain XML-encoded data with
hints for parameter settings that might lead to successful RPC
requests in the future. The yang-data structures from [RFC8639]
and [RFC8641] that may be returned are as follows:
establish-subscription returns hints in yang-data structure
---------------------- -------------------------------------------
target: event stream establish-subscription-stream-error-info
target: datastore establish-subscription-datastore-error-info
modify-subscription returns hints in yang-data structure
---------------------- ----------------------------------------
target: event stream modify-subscription-stream-error-info
target: datastore modify-subscription-datastore-error-info
The yang-data included in "error-info" SHOULD NOT include the
optional leaf "reason", as such a leaf would be redundant with
information that is already placed in the "error-app-tag".
In the case of an RPC error resulting from a "delete-subscription",
"kill-subscription", or "resync-subscription" request, no
"error-info" needs to be included, as the "subscription-id" is the
only RPC input parameter and no hints regarding this RPC input
parameter need to be provided.
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8. Security Considerations
This document does not introduce additional security considerations
for dynamic subscriptions beyond those discussed in [RFC8639]. But
there is one consideration worthy of more refinement based on the
connection-oriented nature of NETCONF. Specifically, if a buggy or
compromised NETCONF subscriber sends a number of "establish-
subscription" requests, then these subscriptions accumulate and may
use up system resources. In such a situation, subscriptions MAY be
terminated by terminating the underlying NETCONF session. The
publisher MAY also suspend or terminate a subset of the active
subscriptions on that NETCONF session in order to reclaim resources
and preserve normal operation for the other subscriptions.
9. IANA Considerations
This document has no IANA actions.
10. References
10.1. 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>.
[RFC5277] Chisholm, S. and H. Trevino, "NETCONF Event
Notifications", RFC 5277, DOI 10.17487/RFC5277, July 2008,
<https://www.rfc-editor.org/info/rfc5277>.
[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>.
[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>.
[RFC8639] Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard,
E., and A. Tripathy, "Subscription to YANG Notifications",
RFC 8639, DOI 10.17487/RFC8639, September 2019,
<https://www.rfc-editor.org/info/rfc8639>.
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RFC 8640 NETCONF Notifications September 2019
[RFC8641] Clemm, A. and E. Voit, "Subscription to YANG Notifications
for Datastore Updates", RFC 8641, DOI 10.17487/RFC8641,
September 2019, <https://www.rfc-editor.org/info/rfc8641>.
[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>.
10.2. Informative References
[RFC8347] Liu, X., Ed., Kyparlis, A., Parikh, R., Lindem, A., and M.
Zhang, "A YANG Data Model for the Virtual Router
Redundancy Protocol (VRRP)", RFC 8347,
DOI 10.17487/RFC8347, March 2018,
<https://www.rfc-editor.org/info/rfc8347>.
[XPATH] Clark, J. and S. DeRose, "XML Path Language (XPath)
Version 1.0", November 1999,
<https://www.w3.org/TR/1999/REC-xpath-19991116>.
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Appendix A. Examples
This appendix is non-normative. Additionally, the subscription "id"
values of 22, 23, 39, and 99 used below are just examples. In
production, the actual values of "id" might not be small integers.
A.1. Event Stream Discovery
As defined in [RFC8639], an event stream exposes a continuous set of
events available for subscription. A NETCONF client can retrieve the
list of available event streams from a NETCONF publisher using the
<get> operation against the top-level "streams" container defined in
[RFC8639], Section 3.1.
The following XML example [W3C.REC-xml-20081126] illustrates the
retrieval of the list of available event streams:
<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<get>
<filter type="subtree">
<streams
xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"/>
</filter>
</get>
</rpc>
Figure 1: <get> Request for Retrieval of Event Streams
After such a request, the NETCONF publisher returns a list of
available event streams as well as additional information that might
exist in the container.
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A.2. Dynamic Subscriptions
A.2.1. Establishing Dynamic Subscriptions
Figure 2 shows two successful "establish-subscription" RPC requests
as per [RFC8639]. The first request is given a subscription "id"
of 22, and the second is given an "id" of 23.
+------------+ +-----------+
| Subscriber | | Publisher |
+------------+ +-----------+
| |
| Capability Exchange |
|<---------------------------->|
| |
| |
| establish-subscription |
|----------------------------->| (a)
| RPC Reply: OK, id = 22 |
|<-----------------------------| (b)
| |
| notification message (for 22)|
|<-----------------------------|
| |
| |
| establish-subscription |
|----------------------------->|
| notification message (for 22)|
|<-----------------------------|
| RPC Reply: OK, id = 23 |
|<-----------------------------|
| |
| |
| notification message (for 22)|
|<-----------------------------|
| notification message (for 23)|
|<-----------------------------|
| |
Figure 2: Multiple Subscriptions over a NETCONF Session
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To provide examples of the information being transported, example
messages for interactions (a) and (b) in Figure 2 are detailed below
(Figures 3 and 4):
<rpc message-id="102" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<establish-subscription
xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications">
<stream-xpath-filter xmlns:ex="https://example.com/events">
/ex:foo/
</stream-xpath-filter>
<stream>NETCONF</stream>
<dscp>10</dscp>
</establish-subscription>
</rpc>
Figure 3: "establish-subscription" Request (a)
As the NETCONF publisher was able to fully satisfy the request (a),
the publisher sends the subscription "id" of the accepted
subscription in its reply message (b):
<rpc-reply message-id="102"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<id
xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications">
22
</id>
</rpc-reply>
Figure 4: A Successful "establish-subscription" (b)
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If the NETCONF publisher had not been able to fully satisfy the
request or the subscriber has no authorization to establish the
subscription, the publisher would have sent an RPC error response.
For instance, if the "dscp" value of 10 asserted by the subscriber in
Figure 3 proved unacceptable, the publisher may have returned:
<rpc-reply message-id="102"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<rpc-error>
<error-type>application</error-type>
<error-tag>invalid-value</error-tag>
<error-severity>error</error-severity>
<error-app-tag>
ietf-subscribed-notifications:dscp-unavailable
</error-app-tag>
</rpc-error>
</rpc-reply>
Figure 5: An Unsuccessful "establish-subscription"
The subscriber can use this information in future attempts to
establish a subscription.
A.2.2. Modifying Dynamic Subscriptions
An existing subscription may be modified. The following exchange
shows a negotiation of such a modification via several exchanges
between a subscriber and a publisher. This negotiation consists of a
failed RPC modification request/response followed by a
successful one.
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+------------+ +-----------+
| Subscriber | | Publisher |
+------------+ +-----------+
| |
| notification message (for 23)|
|<-----------------------------|
| |
| modify-subscription (id = 23)|
|----------------------------->| (c)
| RPC error (with hint) |
|<-----------------------------| (d)
| |
| modify-subscription (id = 23)|
|----------------------------->|
| RPC Reply: OK |
|<-----------------------------|
| |
| notification message (for 23)|
|<-----------------------------|
| |
Figure 6: Interaction Model for Successful Subscription Modification
If the subscription being modified in Figure 6 is a datastore
subscription as per [RFC8641], the modification request made in (c)
may look like that shown in Figure 7. As can be seen, the
modifications being attempted are the application of a new XPath
filter as well as the setting of a new periodic time interval.
<rpc message-id="303"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<modify-subscription
xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"
xmlns:yp="urn:ietf:params:xml:ns:yang:ietf-yang-push">
<id>23</id>
<yp:datastore-xpath-filter xmlns:ex="https://example.com/datastore">
/ex:foo/ex:bar
</yp:datastore-xpath-filter>
<yp:periodic>
<yp:period>500</yp:period>
</yp:periodic>
</modify-subscription>
</rpc>
Figure 7: Subscription Modification Request (c)
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If the NETCONF publisher can satisfy both changes, the publisher
sends a positive result for the RPC. If the NETCONF publisher cannot
satisfy either of the proposed changes, the publisher sends an RPC
error response (d). Figure 8 shows an example RPC error response for
(d) that includes a hint. This hint is an alternative time period
value that might have resulted in a successful modification:
<rpc-reply message-id="303"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<rpc-error>
<error-type>application</error-type>
<error-tag>invalid-value</error-tag>
<error-severity>error</error-severity>
<error-app-tag>
ietf-yang-push:period-unsupported
</error-app-tag>
<error-info>
<modify-subscription-datastore-error-info
xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push">
<period-hint>
3000
</period-hint>
</modify-subscription-datastore-error-info>
</error-info>
</rpc-error>
</rpc-reply>
Figure 8: "modify-subscription" Failure with Hint (d)
A.2.3. Deleting Dynamic Subscriptions
Figure 9 demonstrates the deletion of a subscription. This
subscription may have been to either a stream or a datastore.
<rpc message-id="103"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<delete-subscription
xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications">
<id>22</id>
</delete-subscription>
</rpc>
Figure 9: "delete-subscription"
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If the NETCONF publisher can satisfy the request, the publisher
returns a reply indicating success.
If the NETCONF publisher cannot satisfy the request, the publisher
sends an <rpc-error> element indicating that the modification didn't
work. Figure 10 shows a valid response for an existing valid
subscription "id", but that subscription "id" was created on a
different NETCONF transport session:
<rpc-reply message-id="103"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<rpc-error>
<error-type>application</error-type>
<error-tag>invalid-value</error-tag>
<error-severity>error</error-severity>
<error-app-tag>
ietf-subscribed-notifications:no-such-subscription
</error-app-tag>
</rpc-error>
</rpc-reply>
Figure 10: An Unsuccessful "delete-subscription"
A.3. Subscription State Notifications
A publisher will send subscription state notifications for dynamic
subscriptions according to the definitions in [RFC8639].
A.3.1. "subscription-modified"
As per Section 2.7.2 of [RFC8639], a "subscription-modified" might be
sent over NETCONF if the definition of a configured filter changes.
A subscription state notification encoded in XML would look like:
<notification xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
<eventTime>2007-09-01T10:00:00Z</eventTime>
<subscription-modified
xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications">
<id>39</id>
<stream-xpath-filter xmlns:ex="https://example.com/events">
/ex:foo
</stream-xpath-filter>
<stream>NETCONF</stream>
</subscription-modified>
</notification>
Figure 11: "subscription-modified" Subscription State Notification
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A.3.2. "subscription-resumed" and "replay-complete"
A "subscription-resumed" would look like:
<notification
xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
<eventTime>2007-09-01T10:00:00Z</eventTime>
<subscription-resumed
xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications">
<id>39</id>
</subscription-resumed>
</notification>
Figure 12: "subscription-resumed" Notification
The "replay-complete" is virtually identical, with "subscription-
resumed" simply being replaced by "replay-complete".
A.3.3. "subscription-terminated" and "subscription-suspended"
A "subscription-terminated" would look like:
<notification
xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
<eventTime>2007-09-01T10:00:00Z</eventTime>
<subscription-terminated
xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications">
<id>39</id>
<reason>
suspension-timeout
</reason>
</subscription-terminated>
</notification>
Figure 13: "subscription-terminated" Subscription State Notification
The "subscription-suspended" is virtually identical, with
"subscription-terminated" simply being replaced by "subscription-
suspended".
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A.4. Filter Examples
This appendix provides examples that illustrate both XPath and
subtree methods of filtering event record contents. The examples are
based on the YANG notification "vrrp-protocol-error-event" as defined
per the ietf-vrrp YANG data model in [RFC8347]. Event records based
on this specification that are generated by the publisher might
appear as:
<notification xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
<eventTime>2018-09-14T08:22:33.44Z</eventTime>
<vrrp-protocol-error-event
xmlns="urn:ietf:params:xml:ns:yang:ietf-vrrp">
<protocol-error-reason>checksum-error</protocol-error-reason>
</vrrp-protocol-error-event>
</notification>
Figure 14: Example VRRP Notification per RFC 8347
Suppose that a subscriber wanted to establish a subscription that
only passes instances of event records where there is a
"checksum-error" as part of a Virtual Router Redundancy Protocol
(VRRP) protocol event. Also, assume that the publisher places such
event records into the NETCONF stream. To get a continuous series of
matching event records, the subscriber might request the application
of an XPath filter against the NETCONF stream. An "establish-
subscription" RPC to meet this objective might be:
<rpc message-id="601" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<establish-subscription
xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications">
<stream>NETCONF</stream>
<stream-xpath-filter xmlns="urn:ietf:params:xml:ns:yang:ietf-vrrp">
/vrrp-protocol-error-event[
vrrp:protocol-error-reason="vrrp:checksum-error"]
</stream-xpath-filter>
</establish-subscription>
</rpc>
Figure 15: Establishing a Subscription Error Reason via XPath
For more examples of XPath filters, see [XPATH].
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Suppose that the "establish-subscription" in Figure 15 was accepted.
And suppose that a subscriber decided later on that they wanted to
broaden this subscription to cover all VRRP protocol events (i.e.,
not just those with a "checksum-error"). The subscriber might
attempt to modify the subscription in a way that replaces the XPath
filter with a subtree filter that sends all VRRP protocol events to a
subscriber. Such a "modify-subscription" RPC might look like:
<rpc message-id="602" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<modify-subscription
xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications">
<id>99</id>
<stream-subtree-filter>
<vrrp-protocol-error-event
xmlns="urn:ietf:params:xml:ns:yang:ietf-vrrp"/>
</stream-subtree-filter>
</modify-subscription>
</rpc>
Figure 16: Example "modify-subscription" RPC
For more examples of subtree filters, see [RFC6241], Section 6.4.
Voit, et al. Standards Track [Page 18]
RFC 8640 NETCONF Notifications September 2019
Acknowledgments
We wish to acknowledge the helpful contributions, comments, and
suggestions that were received from Andy Bierman, Yan Gang, Sharon
Chisholm, Hector Trevino, Peipei Guo, Susan Hares, Tim Jenkins,
Balazs Lengyel, Martin Bjorklund, Mahesh Jethanandani, Kent Watsen,
Qin Wu, 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
Voit, et al. Standards Track [Page 19]
ERRATA
