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PROPOSED STANDARD

Internet Engineering Task Force (IETF)                           E. Voit
Request for Comments: 8650                                     R. Rahman
Category: Standards Track                              E. Nilsen-Nygaard
ISSN: 2070-1721                                            Cisco Systems
                                                                A. Clemm
                                                               Futurewei
                                                              A. Bierman
                                                               YumaWorks
                                                           November 2019


    Dynamic Subscription to YANG Events and Datastores over RESTCONF

Abstract

   This document provides a RESTCONF 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/rfc8650.

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
   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.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Dynamic Subscriptions
     3.1.  Transport Connectivity
     3.2.  Discovery
     3.3.  RESTCONF RPCs and HTTP Status Codes
     3.4.  Call Flow for Server-Sent Events
   4.  QoS Treatment
   5.  Notification Messages
   6.  YANG Tree
   7.  YANG Module
   8.  IANA Considerations
   9.  Security Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Appendix A.  Examples
     A.1.  Dynamic Subscriptions
       A.1.1.  Establishing Dynamic Subscriptions
       A.1.2.  Modifying Dynamic Subscriptions
       A.1.3.  Deleting Dynamic Subscriptions
     A.2.  Subscription State Notifications
       A.2.1.  "subscription-modified"
       A.2.2.  "subscription-completed", "subscription-resumed", and
               "replay-completed"
       A.2.3.  "subscription-terminated" and "subscription-suspended"
     A.3.  Filter Example
   Acknowledgments
   Authors' Addresses

1.  Introduction

   Mechanisms to support event subscription and YANG-Push are defined in
   [RFC8639].  Enhancements to [RFC8639] that enable YANG datastore
   subscription and YANG-Push are defined in [RFC8641].  This document
   provides a transport specification for dynamic subscriptions over
   RESTCONF [RFC8040].  Requirements for these mechanisms are captured
   in [RFC7923].

   The streaming of notifications that encapsulate the resulting
   information push is done via the mechanism described in Section 6.3
   of [RFC8040].

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 use the definitions from [RFC8639]: dynamic
   subscription, event stream, notification message, publisher,
   receiver, subscriber, and subscription.

   Other terms reused include datastore, which is defined in [RFC8342],
   and HTTP/2 stream, which maps to the definition of "stream" within
   [RFC7540], Section 2.

3.  Dynamic Subscriptions

   This section provides specifics on how to establish and maintain
   dynamic subscriptions over RESTCONF [RFC8040].  Subscribing to event
   streams is accomplished in this way via RPCs defined within
   [RFC8639], Section 2.4.  The RPCs are done via RESTCONF POSTs.  YANG
   datastore subscription is accomplished via augmentations to [RFC8639]
   as described within [RFC8641], Section 4.4.

   As described in Section 6.3 of [RFC8040], a GET needs to be performed
   on a specific URI on the publisher.  Subscribers cannot predetermine
   the URI against which a subscription might exist on a publisher, as
   the URI will only exist after the "establish-subscription" RPC has
   been accepted.  Therefore, the POST for the "establish-subscription"
   RPC replaces the GET request for the "location" leaf that is used in
   [RFC8040] to obtain the URI.  The subscription URI will be determined
   and sent as part of the response to the "establish-subscription" RPC,
   and a subsequent GET to this URI will be done in order to start the
   flow of notification messages back to the subscriber.  As specified
   in Section 2.4.1 of [RFC8639], a subscription does not move to the
   active state until the GET is received.

3.1.  Transport Connectivity

   For a dynamic subscription, when a RESTCONF session doesn't already
   exist, a new RESTCONF session is initiated from the subscriber.

   As stated in Section 2.1 of [RFC8040], a subscriber MUST establish
   the HTTP session over TLS [RFC8446] in order to secure the content in
   transit.

   Without the involvement of additional protocols, HTTP sessions by
   themselves do not support quick recognition of the loss of the
   communication path to the publisher.  Where quick recognition of the
   loss of a publisher is required, a subscriber SHOULD use a TLS
   heartbeat [RFC6520], just from subscriber to publisher, to track HTTP
   session continuity.

   Loss of the heartbeat MUST result in the teardown of any
   subscription-related TCP sessions between those endpoints.  A
   subscriber can then attempt to re-establish the dynamic subscription
   by using the procedure described in Section 3.4.

3.2.  Discovery

   Subscribers can learn which event streams a RESTCONF server supports
   by querying the "streams" container of ietf-subscribed-
   notifications.yang in [RFC8639].  Support for the "streams" container
   of ietf-restconf-monitoring.yang in [RFC8040] is not required.  In
   the case when the RESTCONF binding specified by this document is used
   to convey the "streams" container from ietf-restconf-monitoring.yang
   (i.e., that feature is supported), any event streams contained
   therein are also expected to be present in the "streams" container of
   ietf-restconf-monitoring.yang.

   Subscribers can learn which datastores a RESTCONF server supports by
   following Section 2 of [RFC8527].

3.3.  RESTCONF RPCs and HTTP Status Codes

   Specific HTTP response codes as defined in Section 6 of [RFC7231]
   will indicate the result of RESTCONF RPC requests with the publisher.
   An HTTP status code of 200 is the proper response to any successful
   RPC defined within [RFC8639] or [RFC8641].

   If a publisher fails to serve the RPC request for one of the reasons
   indicated in Section 2.4.6 of [RFC8639] or Appendix A of [RFC8641],
   this will be indicated by an appropriate error code, as shown below,
   transported in the HTTP response.

   When an HTTP error code is returned, the RPC reply MUST include an
   <rpc-error> element per Section 7.1 of [RFC8040] with the following
   parameter values:

   *  an "error-type" node of "application".

   *  an "error-tag" node whose value is a string that corresponds to an
      identity associated with the error.  This "error-tag" will come
      from one of two places and will correspond to the error identities
      either within Section 2.4.6 of [RFC8639] for general subscription
      errors (Table 1) or within Appendix A.1 of [RFC8641] for
      subscription errors specific to YANG datastores (Table 2).

   *  an "error-app-tag" node whose value is a string that corresponds
      to an identity associated with the error, as defined in
      Section 2.4.6 of [RFC8639] for general subscriptions or
      Appendix A.1 of [RFC8641] for subscription errors specific to YANG
      datastores.  The tag to use depends on the RPC for which the error
      occurred.  Viable errors for different RPCs are found in Table 3.

     +------------------------+-------------------------+-----------+
     | Error identity         | Uses "error-tag"        | HTTP code |
     +========================+=========================+===========+
     | dscp-unavailable       | invalid-value           | 400       |
     +------------------------+-------------------------+-----------+
     | encoding-unsupported   | invalid-value           | 400       |
     +------------------------+-------------------------+-----------+
     | filter-unsupported     | invalid-value           | 400       |
     +------------------------+-------------------------+-----------+
     | insufficient-resources | resource-denied         | 409       |
     +------------------------+-------------------------+-----------+
     | no-such-subscription   | invalid-value           | 404       |
     +------------------------+-------------------------+-----------+
     | replay-unsupported     | operation-not-supported | 501       |
     +------------------------+-------------------------+-----------+

            Table 1: General Subscription Error Identities and
                        Associated "error-tag" Use

   +-----------------------------+-------------------------+-----------+
   | Error identity              | Uses "error-tag"        | HTTP      |
   |                             |                         | code      |
   +=============================+=========================+===========+
   | cant-include                | operation-not-supported | 501       |
   +-----------------------------+-------------------------+-----------+
   | datastore-not-subscribable  | invalid-value           | 400       |
   +-----------------------------+-------------------------+-----------+
   | no-such-subscription-resync | invalid-value           | 404       |
   +-----------------------------+-------------------------+-----------+
   | on-change-unsupported       | operation-not-supported | 501       |
   +-----------------------------+-------------------------+-----------+
   | on-change-sync-unsupported  | operation-not-supported | 501       |
   +-----------------------------+-------------------------+-----------+
   | period-unsupported          | invalid-value           | 400       |
   +-----------------------------+-------------------------+-----------+
   | update-too-big              | too-big                 | 400       |
   +-----------------------------+-------------------------+-----------+
   | sync-too-big                | too-big                 | 400       |
   +-----------------------------+-------------------------+-----------+
   | unchanging-selection        | operation-failed        | 500       |
   +-----------------------------+-------------------------+-----------+

        Table 2: Datastore-Specific Error Identities and Associated
                              "error-tag" Use

        +------------------------+--------------------------------+
        | RPC                    | Select an identity with a base |
        +========================+================================+
        | 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      |
        +------------------------+--------------------------------+

            Table 3: RPC Errors and Associated Error Identities

   Each error identity will be inserted as the "error-app-tag" using
   JSON encoding following the form <modulename>:<identityname>.  An
   example of such a valid encoding would be "ietf-subscribed-
   notifications:no-such-subscription".

   In the case of error responses to an "establish-subscription" or
   "modify-subscription" request, there is the option to include an
   "error-info" node.  This node may contain hints for parameter
   settings that might lead to successful RPC requests in the future.
   Tables 4 and 5 show the yang-data structures that may be returned.

      +--------------+---------------------------------------------+
      | Target:      | Return hints in yang-data structure         |
      +==============+=============================================+
      | event stream | establish-subscription-stream-error-info    |
      +--------------+---------------------------------------------+
      | datastore    | establish-subscription-datastore-error-info |
      +--------------+---------------------------------------------+

             Table 4: Optional "error-info" Node Hints for an
                     "establish-subscription" Request

        +--------------+------------------------------------------+
        | Target:      | Returns hints in yang-data structure     |
        +==============+==========================================+
        | event stream | modify-subscription-stream-error-info    |
        +--------------+------------------------------------------+
        | datastore    | modify-subscription-datastore-error-info |
        +--------------+------------------------------------------+

              Table 5: Optional "error-info" Node Hints for an
                       "modify-subscription" Request

   The yang-data included within "error-info" SHOULD NOT include the
   optional leaf "reason", as such a leaf would be redundant with
   information that is already placed within the "error-app-tag".

   In case of an <rpc-error> as a result of a "delete-subscription", a
   "kill-subscription", or a "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 parameters
   need to be provided.

   Note that "error-path" [RFC8040] does not need to be included with
   the <rpc-error> element, as subscription errors are generally
   associated with the choice of RPC input parameters.

3.4.  Call Flow for Server-Sent Events

   The call flow for Server-Sent Events (SSE) is defined in Figure 1.
   The logical connections denoted by (a) and (b) can be a TCP
   connection or an HTTP/2 stream (if HTTP/2 is used, multiple HTTP/2
   streams can be carried in one TCP connection).  Requests to RPCs as
   defined in [RFC8639] or [RFC8641] are sent on a connection indicated
   by (a).  A successful "establish-subscription" will result in an RPC
   response returned with both a subscription identifier that uniquely
   identifies a subscription, as well as a URI that uniquely identifies
   the location of subscription on the publisher (b).  This URI is
   defined via the "uri" leaf in the data model in Section 7.

   An HTTP GET is then sent on a separate logical connection (b) to the
   URI on the publisher.  This signals the publisher to initiate the
   flow of notification messages that are sent in SSE [W3C-20150203] as
   a response to the GET.  There cannot be two or more simultaneous GET
   requests on a subscription URI: any GET request received while there
   is a current GET request on the same URI MUST be rejected with HTTP
   error code 409.

   As described in Section 6.4 of [RFC8040], RESTCONF servers SHOULD NOT
   send the "event" or "id" fields in the SSE event notifications.

   +--------------+                             +--------------+
   |  Subscriber  |                             |   Publisher  |
   |              |                             |              |
   |    Logical   |                             |     Logical  |
   |  Connection  |                             |   Connection |
   |  (a)  (b)    |                             |    (a)  (b)  |
   +--------------+                             +--------------+
       | RESTCONF POST (RPC:establish-subscription)   |
       |--------------------------------------------->|
       |                          HTTP 200 OK (ID,URI)|
       |<---------------------------------------------|
       |    |HTTP GET (URI)                                |
       |    |--------------------------------------------->|
       |    |                                   HTTP 200 OK|
       |    |<---------------------------------------------|
       |    |                           SSE (notif-message)|
       |    |<---------------------------------------------|
       | RESTCONF POST (RPC:modify-subscription)      |    |
       |--------------------------------------------->|    |
       |    |                              HTTP 200 OK|    |
       |<---------------------------------------------|    |
       |    |                   SSE (subscription-modified)|
       |    |<------------------------------------------(c)|
       |    |                           SSE (notif-message)|
       |    |<---------------------------------------------|
       | RESTCONF POST (RPC:delete-subscription)      |    |
       |--------------------------------------------->|    |
       |    |                              HTTP 200 OK|    |
       |<---------------------------------------------|    |
       |    |                                         |    |
       |    |                                         |    |
       (a) (b)                                       (a)  (b)

          Figure 1: Dynamic Subscriptions with Server-Sent Events

   Additional requirements for dynamic subscriptions over SSE include:

   *  A publisher MUST return all subscription state notifications in a
      separate SSE message used by the subscription to which the state
      change refers.

   *  Subscription RPCs MUST NOT use the connection currently providing
      notification messages for that subscription.

   *  In addition to an RPC response for a "modify-subscription" RPC
      traveling over (a), a "subscription-modified" state change
      notification MUST be sent within (b).  This allows the receiver to
      know exactly when, within the stream of events, the new terms of
      the subscription have been applied to the notification messages.
      See arrow (c).

   *  In addition to any required access permissions (e.g., Network
      Configuration Access Control Model (NACM)), the RPCs "modify-
      subscription", "resync-subscription", and "delete-subscription"
      SHOULD only be allowed by the same RESTCONF username [RFC8040]
      that invoked "establish-subscription".  Such a restriction
      generally serves to preserve users' privacy, but exceptions might
      be made for administrators that may need to modify or delete other
      users' subscriptions.

   *  The "kill-subscription" RPC can be invoked by any RESTCONF
      username with the required administrative permissions.

   A publisher MUST terminate a subscription in the following cases:

   *  Receipt of a "delete-subscription" or a "kill-subscription" RPC
      for that subscription

   *  Loss of TLS heartbeat

   A publisher MAY terminate a subscription at any time as stated in
   Section 1.3 of [RFC8639].

4.  QoS Treatment

   Qos treatment for event streams is described in Section 2.3 of
   [RFC8639].  In addition, if HTTP/2 is used, the publisher MUST:

   *  Take the "weighting" leaf node in [RFC8639] and copy it into the
      HTTP/2 stream weight, Section 5.3 of [RFC7540], and

   *  Take any existing subscription "dependency", as specified by the
      "dependency" leaf node in [RFC8639], and use the HTTP/2 stream for
      the parent subscription as the HTTP/2 stream dependency (as
      described in Section 5.3.1 of [RFC7540]) of the dependent
      subscription.

   *  Set the exclusive flag (Section 5.3.1 of [RFC7540]) to 0.

   For dynamic subscriptions with the same Differentiated Services Code
   Point (DSCP) value to a specific publisher, it is recommended that
   the subscriber sends all URI GET requests on a common HTTP/2 session
   (if HTTP/2 is used).  Conversely, a subscriber cannot use a common
   HTTP/2 session for subscriptions with different DSCP values.

5.  Notification Messages

   Notification messages transported over RESTCONF will be encoded
   according to [RFC8040], Section 6.4.

6.  YANG Tree

   The YANG module defined in Section 7 has one leaf that augments three
   nodes of [RFC8639].

   module: ietf-restconf-subscribed-notifications
     augment /sn:establish-subscription/sn:output:
       +--ro uri?   inet:uri
     augment /sn:subscriptions/sn:subscription:
       +--ro uri?   inet:uri
     augment /sn:subscription-modified:
       +--ro uri?   inet:uri

7.  YANG Module

   This module references [RFC8639].

   <CODE BEGINS>
     file "ietf-restconf-subscribed-notifications@2019-11-17.yang"
   module ietf-restconf-subscribed-notifications {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:"
             + "ietf-restconf-subscribed-notifications";
     prefix rsn;

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

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

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

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

        Editor:   Reshad Rahman
                  <mailto:rrahman@cisco.com>";
     description
       "Defines RESTCONF as a supported transport for subscribed
        event notifications.

        Copyright (c) 2019 IETF Trust and the persons identified
        as authors of the code.  All rights reserved.

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

        This version of this YANG module is part of RFC 8650; see the
        RFC itself for full legal notices.";

     revision 2019-11-17 {
       description
         "Initial version";
       reference
         "RFC 8650: Dynamic Subscription to YANG Events and Datastores
          over RESTCONF";
     }

     grouping uri {
       description
         "Provides a reusable description of a URI.";
       leaf uri {
         type inet:uri;
         config false;
         description
           "Location of a subscription-specific URI on the publisher.";
       }
     }

     augment "/sn:establish-subscription/sn:output" {
       description
         "This augmentation allows RESTCONF-specific parameters for a
          response to a publisher's subscription request.";
       uses uri;
     }

     augment "/sn:subscriptions/sn:subscription" {
       description
         "This augmentation allows RESTCONF-specific parameters to be
          exposed for a subscription.";
       uses uri;
     }

     augment "/sn:subscription-modified" {
       description
         "This augmentation allows RESTCONF-specific parameters to be
          included as part of the notification that a subscription has
          been modified.";
       uses uri;
     }
   }
   <CODE ENDS>

8.  IANA Considerations

   This document registers the following namespace URI in the "ns"
   subregistry of the "IETF XML Registry" [RFC3688]:

   URI:
      urn:ietf:params:xml:ns:yang:ietf-restconf-subscribed-notifications

   Registrant Contact:  The IESG.

   XML:  N/A; the requested URI is an XML namespace.

   This document registers the following YANG module in the "YANG Module
   Names" registry [RFC6020]:

   Name:  ietf-restconf-subscribed-notifications

   Namespace:
      urn:ietf:params:xml:ns:yang:ietf-restconf-subscribed-notifications

   Prefix:  rsn

   Reference:  RFC 8650

9.  Security Considerations

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

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

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

   Container: "/subscriptions"

   *  "uri": leaf will show where subscribed resources might be located
      on a publisher.  Access control must be set so that only someone
      with proper access permissions, i.e., the same RESTCONF [RFC8040]
      user credentials that invoked the corresponding "establish-
      subscription", has the ability to access this resource.

   The subscription URI is implementation specific and is encrypted via
   the use of TLS.  Therefore, even if an attacker succeeds in guessing
   the subscription URI, a RESTCONF username [RFC8040] with the required
   administrative permissions must be used to be able to access or
   modify that subscription.  It is recommended that the subscription
   URI values not be easily predictable.

   The access permission considerations for the RPCs "modify-
   subscription", "resync-subscription", "delete-subscription", and
   "kill-subscription" are described in Section 3.4.

   If a buggy or compromised RESTCONF subscriber sends a number of
   "establish-subscription" requests, then these subscriptions
   accumulate and may use up system resources.  In such a situation, the
   publisher MAY also suspend or terminate a subset of the active
   subscriptions from that RESTCONF subscriber in order to reclaim
   resources and preserve normal operation for the other subscriptions.

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>.

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

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

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

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

   [RFC6520]  Seggelmann, R., Tuexen, M., and M. Williams, "Transport
              Layer Security (TLS) and Datagram Transport Layer Security
              (DTLS) Heartbeat Extension", RFC 6520,
              DOI 10.17487/RFC6520, February 2012,
              <https://www.rfc-editor.org/info/rfc6520>.

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

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

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

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

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

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

   [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>.

   [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-20150203]
              Hickson, I., "Server-Sent Events", W3C Recommendation, 3
              February 2015,
              <https://www.w3.org/TR/2015/REC-eventsource-20150203/>.
              Latest version available at <https://www.w3.org/TR/
              eventsource/>.

10.2.  Informative References

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

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

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

   [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>.

   [RFC8527]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "RESTCONF Extensions to Support the Network
              Management Datastore Architecture", RFC 8527,
              DOI 10.17487/RFC8527, March 2019,
              <https://www.rfc-editor.org/info/rfc8527>.

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

   [XPATH]    Clark, J. and S. DeRose, "XML Path Language (XPath)
              Version 1.0", W3C Recommendation, 16 November 1999,
              <http://www.w3.org/TR/1999/REC-xpath-19991116>.  Latest
              version available at <https://www.w3.org/TR/ xpath/>.

Appendix A.  Examples

   This section is non-normative.  To allow easy comparison, this
   section mirrors the functional examples shown with NETCONF over XML
   within [RFC8640].  In addition, HTTP/2 vs HTTP/1.1 headers are not
   shown as the contents of the JSON encoded objects are identical
   within.

   The subscription URI values used in the examples in this section are
   purely illustrative, and are not indicative of the expected usage
   that is described in Section 9.

   The DSCP values are only for example purposes and are all indicated
   in decimal since the encoding is JSON [RFC7951].

A.1.  Dynamic Subscriptions

A.1.1.  Establishing Dynamic Subscriptions

   The following figure shows two successful "establish-subscription"
   RPC requests as per [RFC8639].  The first request is given a
   subscription identifier of 22, and the second, an identifier of 23.

      +------------+                  +-----------+
      | Subscriber |                  | Publisher |
      +------------+                  +-----------+
            |                               |
            |establish-subscription         |
            |------------------------------>|  (a)
            |     HTTP 200 OK, id#22, URI#1 |
            |<------------------------------|  (b)
            |GET (URI#1)                    |
            |------------------------------>|  (c)
            | HTTP 200 OK,notif-mesg (id#22)|
            |<------------------------------|
            |                               |
            |                               |
            |establish-subscription         |
            |------------------------------>|
            |      HTTP 200 OK, id#23, URI#2|
            |<------------------------------|
            |GET (URI#2)                    |
            |------------------------------>|
            |                               |
            |                               |
            |             notif-mesg (id#22)|
            |<------------------------------|
            | HTTP 200 OK,notif-mesg (id#23)|
            |<------------------------------|
            |                               |

            Figure 2: Multiple Subscriptions over RESTCONF/HTTP

   To provide examples of the information being transported, example
   messages for interactions in Figure 2 are detailed below:

   POST /restconf/operations
        /ietf-subscribed-notifications:establish-subscription

   {
      "ietf-subscribed-notifications:input": {
         "stream-xpath-filter": "/example-module:foo/",
         "stream": "NETCONF",
         "dscp": 10
      }
   }

               Figure 3: "establish-subscription" Request (a)

   As the publisher was able to fully satisfy the request, the publisher
   sends the subscription identifier of the accepted subscription and
   the URI:

   HTTP status code - 200

   {
      "id": 22,
      "uri": "https://example.com/restconf/subscriptions/22"
   }

               Figure 4: "establish-subscription" Success (b)

   Upon receipt of the successful response, the subscriber does a GET to
   the provided URI to start the flow of notification messages.  When
   the publisher receives this, the subscription is moved to the active
   state (c).

   GET /restconf/subscriptions/22

             Figure 5: "establish-subscription" Subsequent POST

   While not shown in Figure 2, if the 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:

   HTTP status code - 400

   { "ietf-restconf:errors" : {
       "error" : [
         {
           "error-type": "application",
           "error-tag": "invalid-value",
           "error-severity": "error",
           "error-app-tag":
               "ietf-subscribed-notifications:dscp-unavailable"
         }
       ]
     }
   }

             Figure 6: An Unsuccessful "establish-subscription"

   The subscriber can use this information in future attempts to
   establish a subscription.

A.1.2.  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.

      +------------+                 +-----------+
      | Subscriber |                 | Publisher |
      +------------+                 +-----------+
            |                              |
            |  notification message (id#23)|
            |<-----------------------------|
            |                              |
            |modify-subscription (id#23)   |
            |----------------------------->|  (d)
            |    HTTP 400 error (with hint)|
            |<-----------------------------|  (e)
            |                              |
            |modify-subscription (id#23)   |
            |----------------------------->|
            |                  HTTP 200 OK |
            |<-----------------------------|
            |                              |
            |            notif-mesg (id#23)|
            |<-----------------------------|
            |                              |

    Figure 7: Interaction Model for Successful Subscription Modification

   If the subscription being modified in Figure 7 is a datastore
   subscription as per [RFC8641], the modification request made in (d)
   may look like that shown in Figure 8.  As can be seen, the
   modifications being attempted are the application of a new XML Path
   Language (XPath) filter as well as the setting of a new periodic time
   interval.

   POST /restconf/operations
        /ietf-subscribed-notifications:modify-subscription

   {
    "ietf-subscribed-notifications:input": {
       "id": 23,
       "ietf-yang-push:datastore-xpath-filter":
          "/example-module:foo/example-module:bar",
       "ietf-yang-push:periodic": {
          "ietf-yang-push:period": 500
       }
     }
   }

              Figure 8: Subscription Modification Request (c)

   If the publisher can satisfy both changes, the publisher sends a
   positive result for the RPC.  If the publisher cannot satisfy either
   of the proposed changes, the publisher sends an RPC error response
   (e).  The following is an example RPC error response for (e) that
   includes a hint.  This hint is an alternative time period value that
   might have resulted in a successful modification:

   HTTP status code - 400

   { "ietf-restconf:errors" : {
       "error" : [
         "error-type": "application",
         "error-tag": "invalid-value",
         "error-severity": "error",
         "error-app-tag": "ietf-yang-push:period-unsupported",
         "error-info": {
           "ietf-yang-push":
           "modify-subscription-datastore-error-info": {
              "period-hint": 3000
           }
         }
       ]
     }
   }

           Figure 9: "modify-subscription" Failure with Hint (e)

A.1.3.  Deleting Dynamic Subscriptions

   The following demonstrates deleting a subscription.  This
   subscription may have been to either a stream or a datastore.

   POST /restconf/operations
        /ietf-subscribed-notifications:delete-subscription

   {
    "delete-subscription": {
       "id": "22"
    }
   }

                  Figure 10: "delete-subscription" Request

   If the publisher can satisfy the request, the publisher replies with
   success to the RPC request.

   If the publisher cannot satisfy the request, the publisher sends an
   <rpc-error> element indicating the modification didn't work.
   Figure 11 shows a valid response for an existing valid subscription
   identifier, but that subscription identifier was created on a
   different transport session:

   HTTP status code - 404

   {
     "ietf-restconf:errors" : {
       "error" : [
         "error-type": "application",
         "error-tag": "invalid-value",
         "error-severity": "error",
         "error-app-tag":
            "ietf-subscribed-notifications:no-such-subscription"
       ]
     }
   }

               Figure 11: Unsuccessful "delete-subscription"

A.2.  Subscription State Notifications

   A publisher will send subscription state notifications according to
   the definitions within [RFC8639].

A.2.1.  "subscription-modified"

   A "subscription-modified" encoded in JSON would look like:

   {
     "ietf-restconf:notification" : {
       "eventTime": "2007-09-01T10:00:00Z",
       "ietf-subscribed-notifications:subscription-modified": {
         "id": 39,
         "uri": "https://example.com/restconf/subscriptions/22"
         "stream-xpath-filter": "/example-module:foo",
         "stream": {
            "ietf-netconf-subscribed-notifications" : "NETCONF"
         }
       }
     }
   }

     Figure 12: "subscription-modified" Subscription State Notification

A.2.2.  "subscription-completed", "subscription-resumed", and "replay-
        completed"

   A "subscription-completed" notification would look like:

   {
     "ietf-restconf:notification" : {
       "eventTime": "2007-09-01T10:00:00Z",
       "ietf-subscribed-notifications:subscription-completed": {
         "id": 39,
       }
     }
   }

          Figure 13: "subscription-completed" Notification in JSON

   The "subscription-resumed" and "replay-complete" are virtually
   identical, with "subscription-completed" simply being replaced by
   "subscription-resumed" and "replay-complete".

A.2.3.  "subscription-terminated" and "subscription-suspended"

   A "subscription-terminated" would look like:

   {
     "ietf-restconf:notification" : {
       "eventTime": "2007-09-01T10:00:00Z",
       "ietf-subscribed-notifications:subscription-terminated": {
         "id": 39,
         "error-id": "suspension-timeout"
       }
     }
   }

    Figure 14: "subscription-terminated" Subscription State Notification

   The "subscription-suspended" is virtually identical, with
   "subscription-terminated" simply being replaced by "subscription-
   suspended".

A.3.  Filter Example

   This section provides an example that illustrates the method of
   filtering event record contents.  The example is based on the YANG
   notification "vrrp-protocol-error-event" as defined per the ietf-
   vrrp.yang module within [RFC8347].  Event records based on this
   specification that are generated by the publisher might appear as:

   data: {
   data:   "ietf-restconf:notification" : {
   data:     "eventTime" : "2018-09-14T08:22:33.44Z",
   data:     "ietf-vrrp:vrrp-protocol-error-event" : {
   data:       "protocol-error-reason" : "checksum-error"
   data:     }
   data:   }
   data: }

             Figure 15: RFC 8347 (VRRP) - Example Notification

   Suppose 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 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:

   POST /restconf/operations
        /ietf-subscribed-notifications:establish-subscription
   {
      "ietf-subscribed-notifications:input": {
         "stream": "NETCONF",
         "stream-xpath-filter":
           "/ietf-vrrp:vrrp-protocol-error-event[
             protocol-error-reason='checksum-error']/",
      }
   }

       Figure 16: Establishing a Subscription Error Reason via XPath

   For more examples of XPath filters, see [XPATH].

   Suppose the "establish-subscription" in Figure 16 was accepted.  And
   suppose later a subscriber decided they wanted to broaden this
   subscription cover to 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:

   POST /restconf/operations
        /ietf-subscribed-notifications:modify-subscription
   {
      "ietf-subscribed-notifications:input": {
         "stream": "NETCONF",
         "stream-subtree-filter": {
           "/ietf-vrrp:vrrp-protocol-error-event" : {}
         }
      }
   }

                Figure 17: Example "modify-subscription" RPC

   For more examples of subtree filters, see [RFC6241], Section 6.4.

Acknowledgments

   We wish to acknowledge the helpful contributions, comments, and
   suggestions that were received from Ambika Prasad Tripathy, Alberto
   Gonzalez Prieto, Susan Hares, Tim Jenkins, Balazs Lengyel, Kent
   Watsen, Michael Scharf, Guangying Zheng, Martin Bjorklund, Qin Wu,
   and Robert Wilton.

Authors' Addresses

   Eric Voit
   Cisco Systems

   Email: evoit@cisco.com


   Reshad Rahman
   Cisco Systems

   Email: rrahman@cisco.com


   Einar Nilsen-Nygaard
   Cisco Systems

   Email: einarnn@cisco.com


   Alexander Clemm
   Futurewei

   Email: ludwig@clemm.org


   Andy Bierman
   YumaWorks

   Email: andy@yumaworks.com


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