NETCONF                                                          E. Voit
Internet-Draft                                                  A. Clemm
Intended status: Informational                               A. Tripathy
Expires: December 17, 2016                             E. Nilsen-Nygaard
                                                      A. Gonzalez Prieto
                                                           Cisco Systems
                                                           June 15, 2016

          Restconf and HTTP Transport for Event Notifications


   This document defines Event Notification YANG Subscription and Push
   mechanisms for Restconf, HTTP, and HTTP2 transports.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on December 17, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Mechanisms for Subscription Establishment and Maintenance   4
     3.2.  Subscription Multiplexing . . . . . . . . . . . . . . . .   6
     3.3.  Push Data Stream and Transport Mapping  . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  13
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Proxy YANG Subscription when the Subscriber and
                Receiver are different . . . . . . . . . . . . . . .  14
   Appendix B.  End-to-End Deployment Guidance . . . . . . . . . . .  16
     B.1.  Call Home . . . . . . . . . . . . . . . . . . . . . . . .  16
     B.2.  TLS Heartbeat . . . . . . . . . . . . . . . . . . . . . .  16
   Appendix C.  Issues being worked and resolved . . . . . . . . . .  16
     C.1.  Unresolved Issues . . . . . . . . . . . . . . . . . . . .  16
     C.2.  Agreement in principal  . . . . . . . . . . . . . . . . .  17
     C.3.  Resolved Issues . . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   Mechanisms to support Event subscription and push are defined in
   [rfc5277bis].  Enhancements to [rfc5277bis] which enable YANG
   Datastore subscription and push are defined in [yang-push].  This
   document provides a transport specification for these Restconf and
   HTTP.  This has been driven by Requirements for subscriptions to YANG
   datastores are defined in [pub-sub-reqs].

   Beyond based transport bindings, there are benefits which can be
   realized when transporting updates directly HTTP/2[RFC7540] which cn
   be realized via an implementation of this transport specification

   o  Subscription multiplexing over independent HTTP/2 streams

   o  Stream prioritization and stream dependencies

   o  Flow control on independent streams

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   Configured Subscription: a Subscription installed via a configuration
   interface which persissts across reboots.

   Data Node: An instance of management information in a datastore.

   Data Node Update: A data item containing the current value/property
   of a Data Node at the time the Data Node Update was created.

   Dynamic Subscription: a Subscription negotiated between Subscriber
   and Publisher via create, establish, modify, and delete RPC control
   plane signaling messages.

   Event: an occurrence of something that may be of interest. (e.g., a
   configuration change, a fault, a change in status, crossing a
   threshold, status of a flow, or an external input to the system.)

   Event Notification: a set of information intended for a Receiver
   indicating that one or more Event(s) have occurred.  Details of the
   Event(s) may be included within.

   Event Stream: a continuous, ordered set of Events grouped under an
   explicit criteria.

   Notification: the communication of an occurrence, perhaps triggered
   by the occurrence of an Event.

   Publisher: an entity responsible for streaming Event Notifications
   per the terms of a Subscription.

   Receiver: a target to which a Publisher pushes Event Notifications.
   For Dynamic Subscriptions, the Receiver and Subscriber will often be
   the same entity.

   Subscriber: an entity able to request and negotiate a contract for
   the receipt of Event Notifications from a Publisher

   Subscription: a contract between a Subscriber and a Publisher
   stipulating which information the Receiver wishes to have pushed from
   the Publisher without the need for further solicitation.

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3.  Solution

   Event subscription is defined in [rfc5277bis], YANG Datastore
   subscription is defined in [yang-push].  This section specifies
   transport mechanisms applicable to both.

3.1.  Mechanisms for Subscription Establishment and Maintenance

   There are three models for Subscription establishment and

   1.  Dynamic Subscription: Here the Subscriber and Receiver are the
       same.  A Subscription ends with a subscription-terminated
       notification, or by a loss of transport connectivity.

   2.  Configured Subscription: Receiver(s) are specified on Publisher
       in startup and running config.  Subscription is not terminated
       except via an operations interface.  (Subscriptions may be
       Suspended, with no Event Notifications sent however.)

   3.  Proxy Subscription: Subscriber and Receiver are different.
       Subscription ends when a Subscription End-time is reached, or the
       Publisher process is restarted.

   The first two are described in this section.  The third is described
   in Appendix A.  This third option can be moved into the body of this
   specification should the IETF community desire.  In theory, all three
   models may be intermixed in a single deployment.

                          |   Publisher   |
                            ^   ^   |   ^
                            |   |   |   |
          .-----Restconf----'   |   |   '-----Restconf----.
          |               .-----'   '-HTTP-.              |
          V               |                V              |
   .-------------.  .------------.  .----------.  .------------.
   | Subscriber+ |  | Operations |  | Receiver |  | Subscriber |
   | Receiver    |  |  /Config   |  '----------'  '------------'
   '-------------'  '------------'    ^     ^               ^
          ^         (out of scope)    :     :               :
          :             ^             :     :....Model 3....:
        Model 1         :...Model 2...:      (out of scope)

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3.1.1.  Dynamic YANG Subscription over RESTCONF

   Dynamic Subscriptions are configured and manage Subscriptions via
   signaling.  This signaling is transported over [restconf].  Once
   established, streaming Event Notifications are then delivered via
   Restconf SSE.

3.1.2.  Configured Subscription over HTTP

   With a Configured Subscription, all information needed to establish a
   secure relationship with that Receiver is configured on the
   Publisher.  With this information, the Publisher will establish a
   secure transport connection with the Receiver and then begin pushing
   the Event Notifications to the Receiver.  Since Restconf might not
   exist on the Receiver, it is not desirable to require that such Event
   Notifications be pushed via Restconf.  Nor is there value which
   Restconf provides on top of HTTP.  Therefore in place of Restconf, a
   TLS secured HTTP Client connection must be established with an HTTP
   Server located on the Receiver.  Event Notifications will then be
   sent via HTTP Post messages to the Receiver.

   Post messages will be addressed to HTTP augmentation code on the
   Receiver capable of accepting and responding to Event Notifications.
   At least the initial Post message must include the URI for the
   subscribed resource.  This URI can be retained for operational
   tracking and debugging use by the Receiver.

   After successful receipt of an initial Event Notification for a
   particular Subscription, the Reciever should reply back with an HTTP
   status code of 201 (Created).  Further successful receipts should
   result in the return of code of 202 (Accepted).  At any point,
   receipt of any status codes from 300-510 with the exception of 408
   (Request Timeout) should result in the movement of the Subscription
   to the suspended state.  A sequential series of multiple 408
   exceptions should also drive the Subscription to a suspended state.

   Security on an HTTP client/Publisher can be strengthened by only
   accepting Response code feedback for recently initiated HTTP POSTs.

   Figure 2 depicts this message flow.

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      +-----------+                 +----------+
      | Publisher |                 | Receiver |
      +-----------+                 +----------+
           |                               |
           |HTTP POST (Sub ID, URI, data1) |
           |             HTTP 201 (Created)|
           |HTTP POST (Sub ID, data2)      |
           |     HTTP 200 or 202 (Accepted)|
           |             data3             |

   If HTTP/2 transport is available to a Receiver, the Publisher should

   o  point individual Event Notifications to a unique HTTP/2 stream for
      that Subscription,

   o  take any subscription-priority and provision it into the HTTP/2
      stream priority, and

   o  take any subscription-dependency and provision it into the HTTP/2
      stream dependency.

3.2.  Subscription Multiplexing

   When pushed directly over HTTP/2, it is expected that the Event
   Notifications from a single Subscription will be allocated a separate
   HTTP/2 stream.  This will enable multiplexing, and address issues of
   Head-of-line blocking with different priority Subscriptions.

   When pushed via Restconf over HTTP/2, different Subscriptions will
   not be mapped to independent HTTP/2 streams.  When Restconf specifies
   this mapping, support may be appended on top of this specification.

   With or without independent queueing of multiplexed subscriptions, it
   is possible that updates might be delivered in a different sequence
   than generated.  Reasons for this might include (but are not limited

   o  replay of pushed updates

   o  temporary loss of transport connectivity, with update buffering
      and different dequeuing priorities per Subscription

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   o  population, marshalling and bundling of independent Subscription
      Updates, and

   o  parallel HTTP1.1 sessions

   Therefore each Event Notification will include a microsecond level
   timestamp to ensure that a Receiver understands the time when a that
   update was generated.  Use of this timestamp can give an indication
   of the state of objects at a Publisher when state-entangled
   information is received across different subscriptions.  The use of
   the latest Event Notification timestamp for a particular object
   update can introduce errors.  So when state-entangled updates have
   inconsistent object values and temporally close timestamps, a
   Receiver might consider performing a 'get' to validate the current
   state of a Publisher.

3.3.  Push Data Stream and Transport Mapping

   Transported updates will contain data for one or more Event
   Notifications.  Each transported Event Notification will contain
   several parameters:

   o  A Subscription ID correlator

   o  Event Notification(s) . (Note 1: These must be filtered per access
      control rules to contain only data that the Subscriber is
      authorized to see.  Note 2: these Event Notifications might be
      Data Node Update(s).)

   o  A timestamp indication when the Event Notification was generated
      on the Publisher.

3.3.1.  Subscription and Updates via Restconf

   Subscribers can dynamically learn whether a RESTCONF server supports
   various types of Event or Yang datastore subscription.  This is done
   by issuing an HTTP request OPTIONS, HEAD, or GET on the stream.  Some
   examples building upon the existing RESTCONF mechanisms are below:

   GET /restconf/data/ietf-restconf-monitoring:restconf-state/
            streams/stream=yang-push HTTP/1.1
   Accept: application/

   If the server supports it, it may respond

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   HTTP/1.1 200 OK
   Content-Type: application/yang.api+xml
   <stream xmlns="urn:ietf:params:xml:ns:yang:ietf-restconf-monitoring">
               <description>Yang push stream</description>

   If the server does not support any form of subscription, it may

   HTTP/1.1 404 Not Found
   Date: Mon, 25 Apr 2012 11:10:30 GMT
   Server: example-server

   Subscribers can determine the URL to receive updates by sending an
   HTTP GET request for the "location" leaf with the stream list
   entry.The stream to use for may be selected from the Event Stream
   list provided in the capabilities exchange.  Note that different
   encodings are supporting using different Event Stream locations.  For
   example, the Subscriber might send the following request:

   GET /restconf/data/ietf-restconf-monitoring:restconf-state/
            streams/stream=yang-push/access=xml/location HTTP/1.1
   Accept: application/

   The publisher might send the following response:

   HTTP/1.1 200 OK
   Content-Type: application/yang.api+xml

   To subscribe and start receiving updates, the subscriber can then
   send an HTTP GET request for the URL returned by the Publisher in the
   request above.  The accept header must be "text/event-stream".  The

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   Publisher uses the Server Sent Events[W3C-20121211] transport
   strategy to push filtered Event Notifications from the Event stream,.

   The publisher MUST support as query parameters for a GET method on
   this resource all the parameters of a subscription.  The only
   exception is the encoding, which is embedded in the URI.  An example
   of this is:

   // subtree filter = /foo
   // periodic updates, every 5 seconds
   GET /mystreams/yang-push?subscription-id=my-sub&period=5&

   Should the publisher not support the requested subscription, it may

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   HTTP/1.1 501 Not Implemented
   Date: Mon, 23 Apr 2012 17:11:00 GMT
   Server: example-server
   Content-Type: application/yang.errors+xml
       <errors xmlns="urn:ietf:params:xml:ns:yang:ietf-restconf">
              <error-message>Xpath filters not supported</error-message>
                  <supported-subscription xmlns="urn:ietf:params:xml:ns:

   with an equivalent JSON encoding representation of:

   HTTP/1.1 501 Not Implemented
   Date: Mon, 23 Apr 2012 17:11:00 GMT
   Server: example-server
   Content-Type: application/yang.errors+json
           "ietf-restconf:errors": {
             "error": {
               "error-type": "protocol",
               "error-tag": "operation-not-supported",
               "error-message": "Xpath filters not supported."
               "error-info": {
                  "datastore-push:supported-subscription": {
                        "subtree-filter": [null]

   The following is an example of a pushed Event Notification data for
   the subscription above.  It contains a subtree with root foo that
   contains a leaf called bar:

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   XML encoding representation:
     <?xml version="1.0" encoding="UTF-8"?>
     <notification xmlns="urn:ietf:params:xml:ns:yang:ietf-restconf">
        <subscription-id xmlns="urn:ietf:params:xml:ns:restconf:
        <datastore-contents xmlns="urn:ietf:params:xml:ns:restconf:
           <foo xmlns="">

   Or with the equivalent YANG over JSON encoding representation as
   defined in[yang-json] :

     "ietf-restconf:notification": {
       "datastore-push:subscription-id": "my-sub",
       "eventTime": "2015-03-09T19:14:56Z",
       "datastore-push:datastore-contents": {
         "example-mod:foo": { "bar": "some_string" }

   To modify a Subscription, the subscriber issues another GET request
   on the provided URI using the same subscription-id as in the original
   request.  For example, to modify the update period to 10 seconds, the
   subscriber may send:

   GET /mystreams/yang-push?subscription-id=my-sub&period=10&

   To delete a Subscription, the Subscriber issues a DELETE request on
   the provided URI using the same subscription-id as in the original

   DELETE /mystreams/yang-push?subscription-id=my-sub

3.3.2.  Subscription and Updates directly via HTTP

   For any version of HTTP, the basic encoding will look as below.  It
   consists of a JSON representation wrapped in an HTTP header.

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   POST (IP+Port) HTTP/1.1
   From: (Identifier for Network Element)
   User-Agent: (CiscoYANGPubSub/1.0)
   Content-Type: multipart/form-data
   Content-Length: (determined runtime)
     "ietf-yangpush:notification": {
       "datastore-push:subscription-id": "my-sub",
       "eventTime": "2015-03-09T19:14:56Z",
       "datastore-push:datastore-contents": {
         "foo": { "bar": "some_string" }

4.  Security Considerations

   Subscriptions could be used to intentionally or accidentally overload
   resources of a Publisher.  For this reason, it is important that the
   Publisher has the ability to prioritize the establishment and push of
   Event Notifications where there might be resource exhaust potential.
   In addition, a server needs to be able to suspend existing
   Subscriptions when needed.  When this occurs, the subscription status
   must be updated accordingly and the Receivers notified.

   A Subscription could be used to attempt retrieve information for
   which a Receiver has no authorized access.  Therefore it is important
   that data pushed via a Subscription is authorized equivalently with
   regular data retrieval operations.  Data being pushed to a Receiver
   needs therefore to be filtered accordingly, just like if the data
   were being retrieved on-demand.  The Netconf Authorization Control
   Model [RFC6536] applies even though the transport is not NETCONF.

   One or more Publishers could be used to overwhelm a Receiver which
   doesn't even support Subscriptions.  Therefore Event Notifications
   for Configured Subscriptions MUST only be transmittable over
   Encrypted transports.  Clients which do not want pushed Event
   Notifications need only terminate or refuse any transport sessions
   from the Publisher.

   One or more Publishers could overwhelm a Receiver which is unable to
   control or handle the volume of Event Notifications received.  In
   deployments where this might be a concern, transports supporting per-
   subscription Flow Control and Prioritization (such as HTTP/2) should
   be selected.

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   Another benefit is that a well-behaved Publisher implementation is
   that it is difficult to a Publisher to perform a DoS attack on a
   Receiver.  DoS attack protection comes from:

   o  the requirement for trust of a TLS session before publication,

   o  the need for an HTTP transport augmentation on the Receiver, and

   o  that the Publication process is suspended when the Receiver
      doesn't respond.

5.  Acknowledgments

   We wish to acknowledge the helpful contributions, comments, and
   suggestions that were received from: Andy Bierman, Sharon Chisholm,
   Yan Gang, Peipei Guo, Susan Hares, Tim Jenkins, Balazs Lengyel,
   Hector Trevino, Kent Watsen, and Guangying Zheng.

6.  References

6.1.  Normative References

              Bierman, Andy., Bjorklund, Martin., and Kent. Watsen,
              "RESTCONF Protocol", March 2016,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [RFC6536]  Bierman, A. and M. Bjorklund, "Network Configuration
              Protocol (NETCONF) Access Control Model", RFC 6536,
              DOI 10.17487/RFC6536, March 2012,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

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6.2.  Informative References

              Watsen, K., "NETCONF Call Home and RESTCONF Call Home",
              December 2015, <

              Voit, Eric., Clemm, Alexander., and Alberto. Gonzalez
              Prieto, "Subscribing to datastore push updates", February
              2016, <

              Gonzalez Prieto, Alberto., Clemm, Alexander., Voit, Eric.,
              Prasad Tripathy, Ambika., and Einar. Nilsen-Nygaard,
              "NETCONF Event Notifications", March 2016,

              "Server-Sent Events, World Wide Web Consortium CR CR-
              eventsource-20121211", December 2012,

              Lhotka, Ladislav., "JSON Encoding of Data Modeled with
              YANG", March 2016, <

              Clemm, Alexander., Gonzalez Prieto, Alberto., Voit, Eric.,
              Prasad Tripathy, Ambika., and Einar. Nilsen-Nygaard,
              "Subscribing to YANG datastore push updates", February
              2016, <

Appendix A.  Proxy YANG Subscription when the Subscriber and Receiver
             are different

   The properties of Dynamic and Configured Subscriptions can be
   combined to enable deployment models where the Subscriber and
   Receiver are different.  Such separation can be useful with some
   combination of:

   o  An operator doesn't want the subscription to be dependent on the
      maintenance of transport level keep-alives.  (Transport
      independence provides different scalability characteristics.)

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   o  There is not a transport session binding, and a transient
      Subscription needs to survive in an environment where there is
      unreliable connectivity with the Receiver and/or Subscriber.

   o  An operator wants the Publisher to include highly restrictive
      capacity management and Subscription security mechanisms outside
      of domain of existing operational or programmatic interfaces.

   To build a Proxy Subscription, first the necessary information must
   be signaled as part of the <establish-subscription>.  Using this set
   of Subscriber provided information; the same process described within
   section 3 will be followed.  There is one exception.  When an HTTP
   status code is 201 is received by the Publisher, it will inform the
   Subscriber of Subscription establishment success via its Restconf

   After a successful establishment, if the Subscriber wishes to track
   the state of Receiver subscriptions, it may choose to place a
   separate on-change Subscription into the "Subscriptions" subtree of
   the YANG Datastore on the Publisher.

   Putting it all together, the message flow is:

        +------------+          +-----------+            +----------+
        | Subscriber |          | Publisher |            | Receiver |
        +------------+          +-----------+            +----------+
           | Restconf PUT:           |                            |
           | <establish-subscription>|                            |
           |------------------------>|                            |
           |                         |                            |
           |                         |<-----------TLS------------>|
           |                         |                            |
           |                         |HTTP POST (Sub ID, data1,   |
           |                         |(stream ID, URI?))          |
           |                         |--------------------------->|
           |                         |          HTTP 201 (Created)|
           |                         |<---------------------------|
           |        Success: HTTP 204|                            |
           |<------------------------|                            |
           |                         |HTTP POST (Sub ID, data2)   |
           |                         |--------------------------->|
           |                         |  HTTP 200 or 202 (Accepted)|
           |                         |<---------------------------|
           |                         |            data3           |
           |                         |<-------------------------->|
           |                         |                            |

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Appendix B.  End-to-End Deployment Guidance

   Several technologies are expected to be seen within a deployment to
   achieve security and ease-of-use requirements.  These are not
   necessary for an implementation of this specification, but will be
   useful to consider when considering the operational context.

B.1.  Call Home

   Pub/Sub implementations should have the ability to transparently
   incorporate lower layer technologies such as Call Home so that secure
   TLS connections are always originated from the Publisher.  There is a
   Restconf Call home function in [call-home].  For security reasons,
   this should be implemented when applicable.

B.2.  TLS Heartbeat

   Unlike NETCONF, HTTP sessions might not quickly allow a Subscriber to
   recognize when the communication path has been lost from the
   Publisher.  To recognize this, it is possible for a Receiver (usually
   the subscriber) to establish a TLS heartbeat [RFC6520].  In the case
   where a TLS heartbeat is included, it should be sent just from
   Receiver to Publisher.  Loss of the heartbeat should result in the
   Subscription being terminated with the Subscriber (even when the
   Subscriber and Receiver are different).  The Subscriber can then
   attempt to re-establish the subscription if desired.  If the
   Subscription remains active on the Publisher, future receipt of
   objects associated with that (or any other unknown) subscription ID
   should result in a <delete-subscription> being returned to the
   Publisher from the Receiver.

Appendix C.  Issues being worked and resolved

   (To be removed by RFC editor prior to publication)

C.1.  Unresolved Issues

   RT1 - Integration specifics for Restconf capability discovery on
   different types of Streams

   RT2 - In what way to we position "Event notifications" model in this
   document vs. current solution in Restconf.

   RT3 - Do we include 3rd party signaled subscriptions within models
   that need to be supported generically, or for a particular type of

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   RT6 - We need to define encodings of rfc5277bis notifications for
   both Restconf and HTTP.

   RT7 - HTTP native option doesn't currently use SSE.  But we should
   evaluate moving to that as possible.  It will make development
   integration easier and more consistent.

C.2.  Agreement in principal

   RT4 - Need to add into document examples of 5277bis Event streams.
   Document only includes yang-push examples at this point.

C.3.  Resolved Issues

   RT5 - Doesn't make sense to use Restconf for Configured
   subscriptions.  HTTP will be used.

Authors' Addresses

   Eric Voit
   Cisco Systems


   Alexander Clemm
   Cisco Systems


   Ambika Prasad Tripathy
   Cisco Systems


   Einar Nilsen-Nygaard
   Cisco Systems


   Alberto Gonzalez Prieto
   Cisco Systems


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