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Internet Engineering Task Force (IETF)                           E. VoitRequest for Comments: 8640                                 Cisco SystemsCategory: Standards Track                                       A. ClemmISSN: 2070-1721                                                Futurewei                                                      A. Gonzalez Prieto                                                               Microsoft                                                       E. Nilsen-Nygaard                                                             A. Tripathy                                                           Cisco Systems                                                          September 2019Dynamic Subscription to YANG Events and Datastores over NETCONFAbstract   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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8640.Voit, et al.                 Standards Track                    [Page 1]

RFC 8640                  NETCONF Notifications           September 2019Copyright Notice   Copyright (c) 2019 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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   to this document.  Code Components extracted from this document must   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.Voit, et al.                 Standards Track                    [Page 2]

RFC 8640                  NETCONF Notifications           September 2019Table of Contents1. Introduction ....................................................32. Terminology .....................................................3   3. Compatibility with <create-subscription> as Defined inRFC 5277 ........................................................44. Mandatory XML, Event Stream, and Datastore Support ..............45. NETCONF Connectivity and Dynamic Subscriptions ..................46. Notification Messages ...........................................57. Dynamic Subscriptions and RPC Error Responses ...................58. Security Considerations .........................................79. IANA Considerations .............................................710. References .....................................................710.1. Normative References ......................................710.2. Informative References ....................................8Appendix A. Examples ...............................................9A.1. Event Stream Discovery ......................................9A.2. Dynamic Subscriptions ......................................10A.3. Subscription State Notifications ...........................15A.4. Filter Examples ............................................17   Acknowledgments ...................................................19   Authors' Addresses ................................................191.  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 inBCP 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.Voit, et al.                 Standards Track                    [Page 3]

RFC 8640                  NETCONF Notifications           September 20193.  Compatibility with <create-subscription> as Defined inRFC 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.Voit, et al.                 Standards Track                    [Page 4]

RFC 8640                  NETCONF Notifications           September 20196.  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-failedVoit, et al.                 Standards Track                    [Page 5]

RFC 8640                  NETCONF Notifications           September 2019   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.Voit, et al.                 Standards Track                    [Page 6]

RFC 8640                  NETCONF Notifications           September 20198.  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.  References10.1.  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 inRFC 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>.Voit, et al.                 Standards Track                    [Page 7]

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>.Voit, et al.                 Standards Track                    [Page 8]

RFC 8640                  NETCONF Notifications           September 2019Appendix 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.Voit, et al.                 Standards Track                    [Page 9]

RFC 8640                  NETCONF Notifications           September 2019A.2.  Dynamic SubscriptionsA.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 SessionVoit, et al.                 Standards Track                   [Page 10]

RFC 8640                  NETCONF Notifications           September 2019   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)Voit, et al.                 Standards Track                   [Page 11]

RFC 8640                  NETCONF Notifications           September 2019   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.Voit, et al.                 Standards Track                   [Page 12]

RFC 8640                  NETCONF Notifications           September 2019              +------------+                 +-----------+              | 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)Voit, et al.                 Standards Track                   [Page 13]

RFC 8640                  NETCONF Notifications           September 2019   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"Voit, et al.                 Standards Track                   [Page 14]

RFC 8640                  NETCONF Notifications           September 2019   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 perSection 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 NotificationVoit, et al.                 Standards Track                   [Page 15]

RFC 8640                  NETCONF Notifications           September 2019A.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".Voit, et al.                 Standards Track                   [Page 16]

RFC 8640                  NETCONF Notifications           September 2019A.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 perRFC 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].Voit, et al.                 Standards Track                   [Page 17]

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.Voit, et al.                 Standards Track                   [Page 18]

RFC 8640                  NETCONF Notifications           September 2019Acknowledgments   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.comVoit, et al.                 Standards Track                   [Page 19]

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