+------------+                                 +------------+
|  Receiver  |                                 | Publisher  |
|HTTP2 Stream|                                 |HTTP2 Stream|
|  (a)  (b)  |                                 |  (a)  (b)  |
+------------+                                 +------------+
    |    HTTP Post Headers, Data (sub-start, SubID)|
    |<---------------------------------------------|
    | HTTP 200 OK                                  |
    |--------------------------------------------->|
    |    |         HTTP Post Headers, Data (event-notif)|
    |    |<---------------------------------------------|
    |    |                       HTTP Data (event-notif)|
    |    |<---------------------------------------------|
    |    |                     HTTP Data (sub-terminate)|
    |    |<---------------------------------------------|
    |    |HTTP 200 OK                                   |
    |    |--------------------------------------------->|

Figure 3: Configured over HTTP2

As the HTTP2 transport is available to the Receiver, the Publisher should:

• take any subscription-priority and copy it into the HTTP2 stream priority, and
• take a subscription-dependency if it has been provided and map the HTTP2 stream for the parent subscription into the HTTP2 stream dependency.

3.2. Subscription Multiplexing

It is possible that updates might be delivered in a different sequence than generated. Reasons for this might include (but are not limited to):

• replay of pushed updates
• temporary loss of transport connectivity, with update buffering and different dequeuing priorities per Subscription
• population, marshalling and bundling of independent Subscription Updates, and

Therefore each Event Notification will include a 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.

4. Encoded Subscription and Event Notification Examples

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

4.1. Restconf Subscription and Events over HTTP1.1

Subscribers can dynamically learn whether a RESTCONF server supports various types of Event or Yang datastore subscription capabilities. This is done by issuing an HTTP request OPTIONS, HEAD, or GET on the stream. Some examples building upon the Call flow for HTTP1.1 from Section 3.2.2 are:

GET /restconf/data/ietf-restconf-monitoring:restconf-state/
         streams/stream=yang-push HTTP/1.1
Host: example.com
Accept: application/yang.data+xml 

If the server supports it, it may respond

HTTP/1.1 200 OK
Content-Type: application/yang.api+xml
<stream xmlns="urn:ietf:params:xml:ns:yang:ietf-restconf-monitoring">
            <name>yang-push</name>
            <description>Yang push stream</description>
            <access>
               <encoding>xml</encoding>
               <location>https://example.com/streams/yang-push-xml
               </location>
            </access>
            <access>
               <encoding>json</encoding>
               <location>https://example.com/streams/yang-push-json
               </location>
            </access>
         </stream>

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

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 as a 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
Host: example.com
Accept: application/yang.data+xml

The Publisher might send the following response:

HTTP/1.1 200 OK
Content-Type: application/yang.api+xml
   <location
        xmlns="urn:ietf:params:xml:ns:yang:ietf-restconf-monitoring">
        https://example.com/streams/yang-push-xml
   </location>

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 Publisher uses the Server Sent Events [W3C-20150203] transport strategy to push filtered Event Notifications from the Event stream.

The Publisher MUST support individual parameters within the POST request body for 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
POST /restconf/operations/ietf-event-notifications:
     establish-subscription HTTP/1.1
      Host: example.com
      Content-Type: application/yang-data+json

      {
        "ietf-event-notifications:input" : {
          ?stream?: ?push-data"
          ?period" : 5,
          "xpath-filter" : ?/ex:foo[starts-with(?bar?.?some']"
        }
      }

Should the publisher not support the requested subscription, it may reply:

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>
           <error-type>application</error-type>
           <error-tag>operation-not-supported</error-tag>
           <error-severity>error</error-severity>
           <error-message>Xpath filters not supported</error-message>
           <error-info>
               <supported-subscription xmlns="urn:ietf:params:xml:ns:
                   netconf:datastore-push:1.0">
                   <subtree-filter/>
               </supported-subscription>
           </error-info>
       </error>
     </errors>

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:

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-push:1.0">
           my-sub
     </subscription-id>
     <eventTime>2015-03-09T19:14:56.233Z</eventTime>
     <datastore-contents xmlns="urn:ietf:params:xml:ns:restconf:
        datastore-push:1.0">
        <foo xmlns="http://example.com/yang-push/1.0">
          <bar>some_string</bar>
        </foo>
     </datastore-contents>
  </notification>

Or with the equivalent YANG over JSON encoding representation as defined in [RFC7951]:

{
  "ietf-restconf:notification": {
    "datastore-push:subscription-id": "my-sub",
    "eventTime": "2015-03-09T19:14:56.233Z",
    "datastore-push:datastore-contents": {
      "example-mod:foo": { "bar": "some_string" }
    }
  }
}

To modify a Subscription, the subscriber issues another POST 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:

POST /restconf/operations/ietf-event-notifications:
      modify-subscription HTTP/1.1
      Host: example.com
      Content-Type: application/yang-data+json

      {
        "ietf-event-notifications:input" : {
          ?subscription-id?: 100,
          ?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 request

4.2. Event Notification over HTTP2

The basic encoding will look as below. It will consists of a JSON representation wrapped in an HTTP2 header.

HyperText Transfer Protocol 2
      Stream: HEADERS, Stream ID: 5
      Header: :method: POST
      Stream: HEADERS, Stream ID: 5

{
  "ietf-yangpush:notification": {
    "datastore-push:subscription-id": "my-sub",
    "eventTime": "2015-03-09T19:14:56.233Z",
    "datastore-push:datastore-contents": {
      "foo": { "bar": "some_string" }
    }
  }
}

5. Security Considerations

Subscriptions could be used to intentionally or accidentally overload the 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 of Configured Subscriptions could be used to overwhelm a Receiver which doesn't even support Subscriptions. There are two protections here. First 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. Second, the HTTP transport augmentation on the Receiver must send an HTTP 200 OK to a subscription started notification before the Publisher starts streaming any events.

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, HTTP2 transport such as HTTP2) should be selected.

6. Acknowledgments

We wish to acknowledge the helpful contributions, comments, and suggestions that were received from: Susan Hares, Tim Jenkins, Balazs Lengyel, Kent Watsen, Michael Scharf, and Guangying Zheng.

7. References

7.1. 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.
[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, "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015.
[RFC8040]	Bierman, A., Bjorklund, M. and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017.
[sn]	Voit, E., Clemm, A., Gonzalez Prieto, A., Prasad Tripathy, A. and E. Nilsen-Nygaard, "Subscribing to Event Notifications", February 2017.

7.2. Informative References

, "

[RFC7923]	Voit, E., Clemm, A. and A. Gonzalez Prieto, "Requirements for Subscription to YANG Datastores", RFC 7923, DOI 10.17487/RFC7923, June 2016.
[RFC7951]	Lhotka, L., "JSON Encoding of Data Modeled with YANG", RFC 7951, DOI 10.17487/RFC7951, August 2016.
[RFC8071]	Watsen, K., "NETCONF Call Home and RESTCONF Call Home", RFC 8071, DOI 10.17487/RFC8071, February 2017.
[W3C-20150203]	Server-Sent Events, World Wide Web Consortium CR CR-eventsource-20121211", February 2015.
[yang-push]	Clemm, A., Voit, E., Gonzalez Prieto, A., Prasad Tripathy, A., Nilsen-Nygaard, E., Bierman, A. and B. Lengyel, Subscribing to YANG datastore push updates", March 2017.

Appendix A. 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.

A.1. Call Home

Pub/Sub implementations should have the ability to transparently incorporate 'call home' [RFC8071] so that secure TLS connections can originate from the desired device.

A.2. TLS Heartbeat

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 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 any Subscription related TCP sessions between those endpoints being torn down. The subscription can then attempt to re-establish.

Appendix B. Issues being worked and resolved

(To be removed by RFC editor prior to publication)

B.1. Unresolved Issues

GRPC compatibility 1: Mechanisms for HTTP2 to GRPC mapping need to be considered. There is a good start there as this draft only uses POST, not GET. As GET is used in RESTCONF for capabilities discovery, we have some backwards compatibility issues with existing IETF drafts. Possible options to address are (1) provide a POST method for anything done by GET in RESTCONF, (2) await support of GET by GRPC, or (3) tunnel RESTCONF's GET messages within a GRPC POST.

GRPC compatibility 2: We need to expose a method against which POST is done as events begin on a stream. See Stream 7 in figure 2. Can only send traffic to a method, not a URI. URI points to a method, not a resource.

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

We need to reference the viable encodings of notifications.

Appendix C. Changes between revisions

(To be removed by RFC editor prior to publication)

v01 - v02

• Removed sections now redundant with [sn] and [yang-push] such as: mechanisms for subscription maintenance, terminology definitions, stream discovery.
• 3rd party subscriptions are out-of-scope.
• SSE only used with Restconf and HTTP1.1 Dynamic Subscriptions
• Timeframes for event tagging are self-defined.
• Clean-up of wording, references to terminology, section numbers.

v00 - v01

• Removed the ability for more than one subscription to go to a single HTTP2 stream.
• Updated call flows. Extensively.
• SSE only used with Restconf and HTTP1.1 Dynamic Subscriptions
• HTTP is not used to determine that a Receiver has gone silent and is not Receiving Event Notifications
• Many clean-ups of wording and terminology

Authors' Addresses