    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.

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    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
       (https://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
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       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

       This document may contain material from IETF Documents or IETF
       Contributions published or made publicly available before November
       10, 2008.  The person(s) controlling the copyright in some of this
       material may not have granted the IETF Trust the right to allow
       modifications of such material outside the IETF Standards Process.
       Without obtaining an adequate license from the person(s) controlling
       the copyright in such materials, this document may not be modified
       outside the IETF Standards Process, and derivative works of it may
       not be created outside the IETF Standards Process, except to format
       it for publication as an RFC or to translate it into languages other
       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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    RFC 8640                  NETCONF Notifications           September 2019

       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.

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    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

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