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Versions: (draft-koster-core-coapmq) 00 01 02 03 04 05 draft-ietf-core-coap-pubsub

Network Working Group                                          M. Koster
Internet-Draft                                               SmartThings
Intended status: Standards Track                              A. Keranen
Expires: January 8, 2017                                      J. Jimenez
                                                                Ericsson
                                                            July 7, 2016


Publish-Subscribe Broker for the Constrained Application Protocol (CoAP)
                    draft-koster-core-coap-pubsub-05

Abstract

   The Constrained Application Protocol (CoAP), and related extensions
   are intended to support machine-to-machine communication in systems
   where one or more nodes are resource constrained, in particular for
   low power wireless sensor networks.  This document defines a publish-
   subscribe broker for CoAP that extends the capabilities of CoAP for
   supporting nodes with long breaks in connectivity and/or up-time.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 8, 2017.

Copyright Notice

   Copyright (c) 2016 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
   (http://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



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  CoAP pubsub Architecture  . . . . . . . . . . . . . . . .   4
     3.2.  CoAP pubsub Broker  . . . . . . . . . . . . . . . . . . .   4
     3.3.  CoAP pubsub Client  . . . . . . . . . . . . . . . . . . .   5
     3.4.  CoAP pubsub Topic . . . . . . . . . . . . . . . . . . . .   5
     3.5.  Brokerless pubsub . . . . . . . . . . . . . . . . . . . .   5
   4.  CoAP pubsub Function Set  . . . . . . . . . . . . . . . . . .   6
     4.1.  DISCOVER  . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  CREATE  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  PUBLISH . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.4.  SUBSCRIBE . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.5.  UNSUBSCRIBE . . . . . . . . . . . . . . . . . . . . . . .  14
     4.6.  READ  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     4.7.  REMOVE  . . . . . . . . . . . . . . . . . . . . . . . . .  16
   5.  CoAP pubsub Operation with Resource Directory . . . . . . . .  17
   6.  Sleep-Wake Operation  . . . . . . . . . . . . . . . . . . . .  18
   7.  Simple Flow Control . . . . . . . . . . . . . . . . . . . . .  18
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Resource Type value 'core.ps' . . . . . . . . . . . . . .  20
     9.2.  Response Code value '2.04'  . . . . . . . . . . . . . . .  20
     9.3.  Response Code value '4.29'  . . . . . . . . . . . . . . .  20
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     11.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] supports
   machine-to-machine communication across networks of constrained
   devices.  CoAP uses a request/response model where clients make
   requests to servers in order to request actions on resources.
   Depending on the situation the same device may act either as a server
   or a client.

   One important class of constrained devices includes devices that are
   intended to run for years from a small battery, or by scavenging
   energy from their environment.  These devices have limited



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   reachability because they spend most of their time in a sleeping
   state with no network connectivity.  Devices may also have limited
   reachability due to certain middle-boxes, such as Network Address
   Translators (NATs) or firewalls.  Such middle-boxes often prevent
   connecting to a device from the Internet unless the connection was
   initiated by the device.

   This document specifies the means for nodes with limited reachability
   to communicate using simple extensions to CoAP.  The extensions
   enable publish-subscribe communication using a broker node that
   enables store-and-forward messaging between two or more nodes.
   Furthermore the extensions facilitate many-to-many communication
   using CoAP.

2.  Terminology

   The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
   'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in this
   specification are to be interpreted as described in [RFC2119].

   This specification requires readers to be familiar with all the terms
   and concepts that are discussed in [RFC5988] and [RFC6690].  Readers
   should also be familiar with the terms and concepts discussed in
   [RFC7252] and [I-D.ietf-core-resource-directory].  The URI template
   format [RFC6570] is used to describe the REST interfaces defined in
   this specification.

   This specification makes use of the following additional terminology:

   Publish-Subscribe (pubsub):  A messaging paradigm where messages are
      published to a broker and potential receivers can subscribe to the
      broker to receive messages.  The publishers do not (need to) know
      where the message will be eventually sent: the publications and
      subscriptions are matched by a broker and publications are
      delivered by the broker to subscribed receivers.

   CoAP pubsub function set:  A group of well-known REST resources that
      together provide the CoAP pubsub service.

   CoAP pubsub Broker:  A server node capable of receiving messages
      (publications) from and sending messages to other nodes, and able
      to match subscriptions and publications in order to route messages
      to the right destinations.  The broker can also temporarily store
      publications to satisfy future subscriptions.

   CoAP pubsub Client:  A CoAP client that implements the CoAP pubsub
      function set.




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   Topic:  A unique identifier for a particular item being published
      and/or subscribed to.  A broker uses the topics to match
      subscriptions to publications.

3.  Architecture

3.1.  CoAP pubsub Architecture

   Figure 1 shows the architecture of a CoAP pubsub service.  CoAP
   pubsub Clients interact with a CoAP pubsub Broker through the CoAP
   pubsub interface which is hosted by the Broker.  State information is
   updated between the Clients and the Broker.  The CoAP pubsub Broker
   performs a store-and-forward function of state updates between
   certain CoAP pubsub Clients.  Clients Subscribe to state updates
   which are Published by other Clients, and which are forwarded by the
   Broker to the subscribing clients.  The CoAP pubsub Broker also acts
   as a REST proxy, retaining the last state update provided by clients
   to supply in response to Read requests from Clients.


                  Clients        pubsub         Broker
                  +-------+         |
                  | CoAP  |         |
                  |pubsub |---------|------+
                  |Client |         |      |    +-------+
                  +-------+         |      +----| CoAP  |
                                    |           |pubsub |
                  +-------+         |      +----|Broker |
                  | CoAP  |         |      |    +-------+
                  |pubsub |---------|------+
                  |Client |         |
                  +-------+         |


                    Figure 1: CoAP pubsub Architecture

3.2.  CoAP pubsub Broker

   A CoAP pubsub Broker is a CoAP Server that exposes an interface for
   clients to use to initiate publish-subscribe interactions.  Unlike
   clients, the broker needs to be reachable by all clients.  The broker
   also needs to have sufficient resources (storage, bandwidth, etc.) to
   host CoAP resources, and potentially buffer messages, on behalf of
   the clients.







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3.3.  CoAP pubsub Client

   A CoAP pubsub Client interacts with a CoAP pubsub Broker using the
   CoAP pubsub interface.  Clients initiate all interactions with the
   CoAP pubsub broker.  A data source (e.g., sensor clients) can publish
   state updates to the broker and data sinks (e.g., actuator clients)
   can read from or subscribe to state updates from the broker.
   Application clients can make use of both publish and subscribe in
   order to exchange state updates with data sources and sinks.

3.4.  CoAP pubsub Topic

   The clients and broker use topics to identify a particular resource
   or object in a publish-subscribe system.  Topics are conventionally
   formed as a hierarchy, e.g. "/sensors/weather/barometer/pressure" or
   "EP-33543/sen/3303/0/5700".  The topics are hosted at the broker and
   all the clients using the broker share the same namespace for topics.
   A CoAP pubsub topic has a reference path using URI path [RFC3986]
   construction, link attributes [RFC6690], and a representation of a
   value with specified content-formats.  A CoAP pubsub topic value may
   alternatively be a collection of one or more sub-topics, consisting
   of links to the sub-topic URIs and indicated by a link-format
   content-format.

3.5.  Brokerless pubsub

   Figure 2 shows an arrangement for using CoAP pubsub in a "brokerless"
   configuration between peer nodes.  Nodes in a brokerless system act
   as both broker and client.  The Broker interface in a brokerless node
   may be pre-configured with topics that expose services and resources.
   Brokerless peer nodes can be mixed with client and broker nodes in a
   system with full interoperability.


                     Peer         pubsub          Peer
                   +-------+         |         +-------+
                   | CoAP  |         |         | CoAP  |
                   |pubsub |---------|---------|pubsub |
                   |Client |         |         |Broker |
                   +-------+         |         +-------+
                   | CoAP  |         |         | CoAP  |
                   |pubsub |---------|---------|pubsub |
                   |Broker |         |         |Client |
                   +-------+         |         +-------+


                        Figure 2: Brokerless pubsub




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4.  CoAP pubsub Function Set

   This section defines the interfaces between a CoAP pubsub Broker and
   pubsub Clients, which is called the CoAP pubsub Function Set. The
   examples throughout this section assume the use of CoAP [RFC7252].  A
   CoAP pubsub Broker implementing this specification MUST support the
   DISCOVER, CREATE, PUBLISH, SUBSCRIBE, UNSUBSCRIBE, READ, and REMOVE
   operations defined in this section.

4.1.  DISCOVER

   CoAP pubsub Clients discover CoAP pubsub Brokers by using CoAP Simple
   Discovery or through a Resource Directory (RD)
   [I-D.ietf-core-resource-directory].  A CoAP pubsub Broker SHOULD
   indicate its presence and availability on a network by exposing a
   link to its pubsub function set at its .well-known/core location
   [RFC6690].  A CoAP pubsub broker MAY register its pubsub function set
   location with a Resource Directory.  Figure 3 shows an example of a
   client discovering a local pubsub Function Set using CoAP Simple
   Discovery.  A broker wishing to advertise the CoAP pubsub Function
   Set for Simple Discovery or through a Resource Directory MUST use the
   link relation rt="core.ps".  A broker MAY advertise it's supported
   content formats and other attributes in the link to it's pubsub
   function set.

   A CoAP pubsub Broker MAY offer the Discover interface to enable
   Clients to find topics of interest, either by topic name or by link
   attributes which may be registered when the topic is created.
   Figure 4 shows an example of a client looking for a topic with a
   resource type (rt) of "temperature" in the pubsub function set /ps
   using the Discover interface.  The client then receives the URI of
   the resource and its content-format.

   A CoAP pubsub Broker MAY expose the Discover interface through the
   .well-known/core resource.  Links to topics may be exposed at .well-
   known/core in addition to links to the pubsub function set.  Figure 5
   shows an example of topic discovery through .well-known/core.

   The DISCOVER interface is specified as follows:

   Interaction:  Client -> Broker

   Method:  GET

   URI Template:  /.well-known/core

   URI Template:  /{+ps/}{topic}{/topic*}{?q*}




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   URI Template Variables:

      /.well-known/core :=   for discovering the pubsub function set
         (optional)

      ps :=   pubsub Function Set path (optional).  The path of the
         pubsub Function Set, as obtained from discovery, used to
         discover topics.

      topic :=   The desired topic to return links for (optional).

      q :=   Query Filter (optional).  MAY contain a query filter list
         as per [RFC6690] Section 4.1.

   Content-Format:  application/link-format

   The following response codes are defined for this interface:

   Success:  2.05 "Content" with an application/link-format payload
      containing one or more matching entries for the broker resource.
      A pubsub broker SHOULD use the value "/ps/" for the function set
      URI wherever possible.

   Failure:  4.04 "Not Found" is returned in case no matching entry is
      found for a unicast request.

   Failure:  4.00 "Bad Request" is returned in case of a malformed
      request for a unicast request.

   Failure:  No error response to a multicast request.


          Client                                          Broker
            |                                               |
            | ------ GET /.well-known/core?rt=core.ps ----->|
            | ---Content-Format: application/link-format----|
            |                                               |
            | <--2.05 Content "</ps/>;rt="core.ps";ct=40----|
            |                                               |


               Figure 3: Example of DISCOVER pubsub function









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          Client                                          Broker
            |                                               |
            | ---------- GET /ps/?rt="temperature" -------->|
            |    Content-Format: application/link-format    |
            |                                               |
            | <---2.05 Content                              |
            |   </ps/currentTemp>;rt="temperature";ct=50 ---|
            |                                               |


                    Figure 4: Example of DISCOVER topic


          Client                                          Broker
            |                                               |
            | -------- GET /.well-known/core?ct=50 -------->|
            |    Content-Format: application/link-format    |
            |                                               |
            | <---2.05 Content                              |
            |   </ps/currentTemp>;rt="temperature";ct=50 ---|
            |                                               |


                    Figure 5: Example of DISCOVER topic

4.2.  CREATE

   Clients create topics on the broker using the CREATE interface.  A
   client wishing to create a topic MUST use CoAP POST to the pubsub
   function set location with a payload indicating the desired topic.
   The topic specification sent in the payload MUST use a supported
   serialization of the CoRE link format [RFC6690].  The target of the
   link MUST be a URI formatted string.  The client MUST indicate the
   desired content format for publishes to the topic by using the ct
   (Content Format) link attribute in the link-format payload.  The
   client MAY indicate the lifetime of the topic by including the Max-
   Age option in the CREATE request.  Broker MUST return a response code
   of "2.01 Created" if the topic is created and return the created
   relative URI path via Location-Path options.  The broker MUST return
   the appropriate 4.xx response code indicating the reason for failure
   if a new topic can not be created.  Broker SHOULD remove topics if
   the Max-Age of the topic is exceeded without any publishes to the
   topic.  Broker SHOULD retain a topic indefinitely if the Max-Age
   option is elided or is set to zero upon topic creation.  The lifetime
   of a topic MUST be refreshed upon create operations with a target of
   an existing topic.





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   Topics may be created as sub-topics of other topics.  A client MAY
   create a topic with a ct (Content Format) link attribute value which
   describes a supported serialization of the CoRE link format [RFC6690]
   such as application/link-format (ct=40) or its JSON or CBOR
   serializations.  If a topic is created which describes a link
   serialization, that topic may then have sub-topics created under it
   as shown in Figure 7.

   The CREATE interface is specified as follows:

   Interaction:  Client -> Broker

   Method:  POST

   URI Template:  /{+ps/}{topic}{/topic*}

   URI Template Variables:

      ps :=   pubsub Function Set path (mandatory).  The path of the
         pubsub Function Set, as obtained from discovery.  A pubsub
         broker SHOULD use the value "ps" for this variable whenever
         possible.

   Content-Format:  application/link-format

   Payload:  The desired topic to CREATE

   The following response codes are defined for this interface:

   Success:  2.01 "Created".  Successful Creation of the topic

   Failure:  4.00 "Bad Request".  Malformed request.

   Failure:  4.01 "Unauthorized".  Authorization failure.

   Failure:  4.03 "Forbidden".  Topic already exists.

   Failure:  4.06 "Not Acceptable".  Unsupported content format for
      topic.

   Figure 6 shows an example of a topic called "topic1" being
   successfully created.









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          Client                                          Broker
            |                                               |
            | ---------- POST /ps "<topic1>;ct=50" -------->|
            |                                               |
            | <---------------- 2.01 Created ---------------|
            |               Location: /ps/topic1            |
            |                                               |


                     Figure 6: Example of CREATE topic


          Client                                          Broker
            |                                               |
            | ------- POST /ps/ "<mainTopic>;ct=40" ------->|
            |                                               |
            | <---------------- 2.01 Created ---------------|
            |             Location: /ps/mainTopic/          |
            |                                               |
            | --- POST /ps/mainTopic/ "<subTopic>;ct=50" -->|
            |                                               |
            | <---------------- 2.01 Created ---------------|
            |        Location: /ps/mainTopic/subTopic       |
            |                                               |
            |                                               |


                   Figure 7: Example of CREATE sub-topic

4.3.  PUBLISH

   A CoAP pubsub Client uses the PUBLISH interface for updating topics
   on the broker.  The client MUST use the PUT method to publish state
   updates to the CoAP pubsub Broker.  A client MUST use the content
   format specified upon creation of a given topic to publish updates to
   that topic.  The broker MUST reject publish operations which do not
   use the specified content format.  A CoAP client publishing on a
   topic MAY indicate the maximum lifetime of the value by including the
   Max-Age option in the publish request.  The broker MUST return a
   response code of "2.04 Changed" if the publish is accepted or "4.04
   Not Found" if the topic does not exist.  A broker MAY return "4.29
   Too Many Requests" if simple flow control as described in Section 7
   is implemented.

   The Broker MUST notify all clients subscribed on a particular topic
   each time it receives a publish on that topic.  An example is shown
   in Figure 9.  If a client publishes to a broker with the Max-Age
   option, the broker MUST include the same value for the Max-Age option



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   in all notifications.  A broker MUST use CoAP Notification as
   described in [RFC7641] to notify subscribed clients.

   The PUBLISH interface is specified as follows:

   Interaction:  Client -> Broker

   Method:  PUT

   URI Template:  /{+ps/}{topic}{/topic*}

   URI Template Variables:

      ps :=   pubsub Function Set path (mandatory).  The path of the
         pubsub Function Set, as obtained from discovery.

      topic :=   The desired topic to publish on.

   Content-Format:  Any valid CoAP content format

   Payload:  Representation of the topic value (CoAP resource state
      representation) in the indicated content format

   The following response codes are defined for this interface:

   Success:  2.04 "Changed".  Successful publish, topic is updated

   Failure:  4.00 "Bad Request".  Malformed request.

   Failure:  4.01 "Unauthorized".  Authorization failure.

   Failure:  4.04 "Not Found".  Topic does not exist.

   Failure:  4.29 "Too Many Requests".  The client should slow down the
      rate of publish messages for this topic (see Section 7).

   Figure 8 shows an example of a new value being successfully published
   to the topic "topic1".  See Figure 9 for an example of a broker
   forwarding a message from a publishing client to a subscribed client.












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          Client                                          Broker
            |                                               |
            | ---------- PUT /ps/topic1 "1033.3"  --------> |
            |                                               |
            |                                               |
            | <--------------- 2.04 Changed---------------- |
            |                                               |


                       Figure 8: Example of PUBLISH

4.4.  SUBSCRIBE

   CoAP pubsub Clients subscribe to topics on the Broker using CoAP
   Observe as described in [RFC7641].  A CoAP pubsub Client wishing to
   Subscribe to a topic on a broker MUST use a CoAP GET with Observe
   registration.  The Broker MAY add the client to a list of observers.
   The Broker MUST return a response code of "2.05 Content" along with
   the most recently published value if the topic contains a valid value
   and the broker can supply the requested content format.  The broker
   MUST accept Subscribe requests on a topic if the content format of
   the request matches the content format the topic was created with.
   The broker MAY accept Subscribe requests which specify content
   formats that the broker can supply as alternate content formats to
   the content format the topic was registered with.  If the topic was
   published with the Max-Age option, the broker MUST set the Max-Age
   option in the valid response to the amount of time remaining for the
   value to be valid since the last publish operation on that topic.
   The Broker MUST return a response code of "2.04 No Content" if the
   Max-Age of the previously stored value has expired.  The Broker MUST
   return a response code "4.04 Not Found" if the topic does not exist
   or has been removed.  The Broker MUST return a response code "4.15
   Unsupported Content Format" if it can not return the requested
   content format.  If a Broker is unable to accept a new Subscription
   on a topic, it SHOULD return the appropriate response code without
   the Observe option as per as per [RFC7641] Section 4.1.  There is no
   explicit maximum lifetime of a Subscription, thus a Broker may remove
   subscribers at any time.  The Broker, upon removing a Subscriber,
   will transmit the appropriate response code without the Observe
   option, as per [RFC7641] Section 4.2, to the removed Subscriber.

   The SUBSCRIBE interface is specified as follows:

   Interaction:  Client -> Broker

   Method:  GET

   Options:  Observe:0



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   URI Template:  /{+ps/}{topic}{/topic*}

   URI Template Variables:

      ps :=   pubsub Function Set path (mandatory).  The path of the
         pubsub Function Set, as obtained from discovery.

      topic :=   The desired topic to subscribe to.

   The following response codes are defined for this interface:

   Success:  2.05 "Content".  Successful subscribe, current value
      included

   Success:  2.04 "No Content".  Successful subscribe, value not
      included

   Failure:  4.00 "Bad Request".  Malformed request.

   Failure:  4.01 "Unauthorized".  Authorization failure.

   Failure:  4.04 "Not Found".  Topic does not exist.

   Failure:  4.15 "Unsupported Content Format".  Unsupported content
      format.

   Figure 9 shows an example of Client2 subscribing to "topic1" and
   receiving a response from the broker, with a subsequent notification.
   The subscribe response from the broker uses the last stored value
   associated with the topic1.  The notification from the broker is sent
   in response to the publish received from Client1.


     Client1   Client2                                          Broker
       |          |                   Subscribe                   |
       |          | ----- GET /ps/topic1 Observe:0 Token:XX ----> |
       |          |                                               |
       |          | <---------- 2.05 Content Observe:10---------- |
       |          |                                               |
       |          |                                               |
       |          |                    Publish                    |
       | ---------|----------- PUT /ps/topic1 "1033.3"  --------> |
       |          |                    Notify                     |
       |          | <---------- 2.05 Content Observe:11---------- |
       |          |                                               |


                      Figure 9: Example of SUBSCRIBE



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4.5.  UNSUBSCRIBE

   CoAP pubsub Clients unsubscribe from topics on the Broker using the
   CoAP Cancel Observation operation.  A CoAP pubsub Client wishing to
   unsubscribe to a topic on a Broker MUST either use CoAP GET with
   Observe using an Observe parameter of 1 or send a CoAP Reset message
   in response to a publish, as per [RFC7641].

   The UNSUBSCRIBE interface is specified as follows:

   Interaction:  Client -> Broker

   Method:  GET

   Options:  Observe:1

   URI Template:  /{+ps/}{topic}{/topic*}

   URI Template Variables:

      ps :=   pubsub Function Set path (mandatory).  The path of the
         pubsub Function Set, as obtained from discovery.

      topic :=   The desired topic to unsubscribe from.

   The following response codes are defined for this interface:

   Success:  2.05 "Content".  Successful unsubscribe, current value
      included

   Success:  2.04 "No Content".  Successful unsubscribe, value not
      included

   Failure:  4.00 "Bad Request".  Malformed request.

   Failure:  4.01 "Unauthorized".  Authorization failure.

   Failure:  4.04 "Not Found".  Topic does not exist.

   Figure 10 shows an example of a client unsubscribe using the
   Observe=1 cancellation method.










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          Client                                          Broker
            |                                               |
            | ----- GET /ps/topic1 Observe:1 Token:XX ----> |
            |                                               |
            | <------------- 2.05 Content ----------------- |
            |                                               |


                     Figure 10: Example of UNSUBSCRIBE

4.6.  READ

   A CoAP pubsub client wishing to obtain only the most recent published
   value on a topic MAY use the READ interface.  For reading, the client
   uses the CoAP GET method.  The broker MUST accept Read requests on a
   topic if the content format of the request matches the content format
   the topic was created with.  The broker MAY accept Read requests
   which specify content formats that the broker can supply as alternate
   content formats to the content format the topic was registered with.
   The Broker MUST return a response code of "2.05 Content" along with
   the most recently published value if the topic contains a valid value
   and the broker can supply the requested content format.  If the topic
   was published with the Max-Age option, the broker MUST set the Max-
   Age option in the valid response to the amount of time remaining for
   the topic to be valid since the last publish.  The Broker MUST return
   a response code of "2.04 No Content" if the Max-Age of the previously
   stored value has expired.  The Broker MUST return a response code
   "4.04 Not Found" if the topic does not exist or has been removed.
   The Broker MUST return a response code "4.15 Unsupported Content
   Format" if the broker can not return the requested content format.

   The READ interface is specified as follows:

   Interaction:  Client -> Broker

   Method:  GET

   URI Template:  /{+ps/}{topic}{/topic*}

   URI Template Variables:

      ps :=   pubsub Function Set path (mandatory).  The path of the
         pubsub Function Set, as obtained from discovery.

      topic :=   The desired topic to READ.

   The following response codes are defined for this interface:




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   Success:  2.05 "Content".  Successful READ, current value included

   Success:  2.04 "No Content".  Topic exists, value not included

   Failure:  4.00 "Bad Request".  Malformed request.

   Failure:  4.01 "Unauthorized".  Authorization failure.

   Failure:  4.04 "Not Found".  Topic does not exist.

   Failure:  4.15 "Unsupported Content Format".  Unsupported content-
      format.

   Figure 11 shows an example of a successful READ from topic1, followed
   by a Publish on the topic, followed at some time later by a read of
   the updated value from the recent Publish.


     Client1   Client2                                          Broker
       |          |                     Read                      |
       |          | --------------- GET /ps/topic1 -------------> |
       |          |                                               |
       |          | <---------- 2.05 Content "1007.1"------------ |
       |          |                                               |
       |          |                                               |
       |          |                    Publish                    |
       | ---------|----------- PUT /ps/topic1 "1033.3"  --------> |
       |          |                                               |
       |          |                                               |
       |          |                     Read                      |
       |          | --------------- GET /ps/topic1 -------------> |
       |          |                                               |
       |          | <----------- 2.05 Content "1033.3"----------- |
       |          |                                               |


                        Figure 11: Example of READ

4.7.  REMOVE

   A CoAP pubsub Client wishing to remove a topic MAY use the CoAP
   Delete operation on the URI of the topic.  The CoAP pubsub Broker
   MUST return "2.02 Deleted" if the remove operation is successful.
   The broker MUST return the appropriate 4.xx response code indicating
   the reason for failure if the topic can not be removed.  When a topic
   is removed for any reason, the Broker SHOULD return the response code
   4.04 Not Found and remove all of the observers from the list of
   observers as per as per [RFC7641] Section 3.2.



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   The REMOVE interface is specified as follows:

   Interaction:  Client -> Broker

   Method:  DELETE

   URI Template:  /{+ps/}{topic}{/topic*}

   URI Template Variables:

      ps :=   pubsub Function Set path (mandatory).  The path of the
         pubsub Function Set, as obtained from discovery.

      topic :=   The desired topic to REMOVE.

   Content-Format:  None

   Response Payload:  None

   The following response codes are defined for this interface:

   Success:  2.02 "Deleted".  Successful remove

   Failure:  4.00 "Bad Request".  Malformed request.

   Failure:  4.01 "Unauthorized".  Authorization failure.

   Failure:  4.04 "Not Found".  Topic does not exist.

   Figure 12 shows a successful remove of topic1.


           Client                                         Broker
            |                                               |
            | ------------- DELETE /ps/topic1 ------------> |
            |                                               |
            |                                               |
            | <-------------- 2.02 Deleted ---------------- |
            |                                               |


                       Figure 12: Example of REMOVE

5.  CoAP pubsub Operation with Resource Directory

   A CoAP pubsub Broker may register a pubsub Function Set with a
   Resource Directory.  A pubsub Client may use an RD to discover a
   pubsub Broker.



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   A CoAP pubsub Client may register links [RFC6690] with a Resource
   Directory to enable discovery of created pubsub topics.  A pubsub
   Client may use an RD to discover pubsub Topics.  A client which
   registers pubsub Topics with an RD MUST use the context relation
   (con) [I-D.ietf-core-resource-directory] to indicate that the context
   of the registered links is the pubsub Broker.

   A CoAP pubsub Broker may alternatively register links to its topics
   to a Resource Directory by triggering the RD to retrieve it's links
   from .well-known/core.  In order to use this method, the links must
   first be exposed in the .well-known/core of the pubsub broker.  See
   Section 4.1 in this document.

   The pubsub broker triggers the RD to retrieve its links by sending a
   POST with an empty payload to the .well-known/core of the Resource
   Directory.  The RD server will then retrieve the links from the
   .well-known/core of the pubsub broker and incorporate them into the
   Resource Directory.  See [I-D.ietf-core-resource-directory] for
   further details.

6.  Sleep-Wake Operation

   CoAP pubsub provides a way for client nodes to sleep between
   operations, conserving energy during idle periods.  This is made
   possible by shifting the server role to the broker, allowing the
   broker to be always-on and respond to requests from other clients
   while a particular client is sleeping.

   For example, the broker will retain the last state update received
   from a sleeping client, in order to supply the most recent state
   update to other clients in response to read and subscribe operations.

   Likewise, the broker will retain the last state update received on
   the topic such that a sleeping client, upon waking, can perform a
   read operation to the broker to update its own state from the most
   recent system state update.

7.  Simple Flow Control

   Since the broker node has to potentially send a large amount of
   notification messages for each publish message and it may be serving
   a large amount of subscribers and publishers simultaneously, the
   broker may become overwhelmed if it receives many publish messages to
   popular topics in a short period of time.

   If the broker is unable to serve a certain client that is sending
   publish messages too fast, the broker MUST respond with Response Code
   4.29, "Too Many Requests".  This Response Code is like HTTP 429 "Too



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   Many Requests" but uses the Max-Age Option in place of the "Retry-
   After" header field to indicate the number of seconds after which to
   retry.  The broker MAY stop creating notifications from the publish
   messages from this client and to this topic for the indicated time.

   If a client receives the 4.29 Response Code from the broker for a
   publish message to a topic, it MUST NOT send new publish messages to
   the broker on the same topic before the time indicated in Max-Age has
   passed.

8.  Security Considerations

   CoAP pubsub re-uses CoAP [RFC7252], CoRE Resource Directory
   [I-D.ietf-core-resource-directory], and Web Linking [RFC5988] and
   therefore the security considerations of those documents also apply
   to this specification.  Additionally, a CoAP pubsub broker and the
   clients SHOULD authenticate each other and enforce access control
   policies.  A malicious client could subscribe to data it is not
   authorized to or mount a denial of service attack against the broker
   by publishing a large number of resources.  The authentication can be
   performed using the already standardized DTLS offered mechanisms,
   such as certificates.  DTLS also allows communication security to be
   established to ensure integrity and confidentiality protection of the
   data exchanged between these relevant parties.  Provisioning the
   necessary credentials, trust anchors and authorization policies is
   non-trivial and subject of ongoing work.

   The use of a CoAP pubsub broker introduces challenges for the use of
   end-to-end security between for example a client device on a sensor
   network and a client application running in a cloud-based server
   infrastructure since brokers terminate the exchange.  While running
   separate DTLS sessions from the client device to the broker and from
   broker to client application protects confidentially on those paths,
   the client device does not know whether the commands coming from the
   broker are actually coming from the client application.  Similarly, a
   client application requesting data does not know whether the data
   originated on the client device.  For scenarios where end-to-end
   security is desirable the use of application layer security is
   unavoidable.  Application layer security would then provide a
   guarantee to the client device that any request originated at the
   client application.  Similarly, integrity protected sensor data from
   a client device will also provide guarantee to the client application
   that the data originated on the client device itself.  The protected
   data can also be verified by the intermediate broker ensuring that it
   stores/caches correct request/response and no malicious messages/
   requests are accepted.  The broker would still be able to perform
   aggregation of data/requests collected.




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   Depending on the level of trust users and system designers place in
   the CoAP pubsub broker, the use of end-to-end object security is
   RECOMMENDED [I-D.selander-ace-object-security].

   When only end-to-end encryption is necessary and the CoAP Broker is
   trusted, Payload Only Protection (Mode:PAYL) could be used.  The
   Publisher would wrap only the payload before sending it to the broker
   and set the option Content-Format to application/smpayl.  Upon
   receival, the Broker can read the unencrypted CoAP header to forward
   it to the subscribers.

9.  IANA Considerations

   This document registers one attribute value in the Resource Type
   (rt=) registry established with [RFC6690] and appends to the
   definition of one CoAP Response Code in the CoRE Parameters Registry.

9.1.  Resource Type value 'core.ps'

   o  Attribute Value: core.ps

   o  Description: Section 4 of [[This document]]

   o  Reference: [[This document]]

   o  Notes: None

9.2.  Response Code value '2.04'

   o  Response Code: 2.04

   o  Description: Add No Content response to GET to the existing
      definition of the 2.04 response code.

   o  Reference: [[This document]]

   o  Notes: None

9.3.  Response Code value '4.29'

   o  Response Code: 4.29

   o  Description: This error code is used by a server to indicate that
      a client is making too many requests on a resource.

   o  Reference: [[This document]]

   o  Notes: None



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10.  Acknowledgements

   The authors would like to thank Hannes Tschofenig, Zach Shelby, Mohit
   Sethi, Peter van der Stok, Tim Kellogg, Anders Eriksson, Goran
   Selander, Mikko Majanen, and Olaf Bergmann for their contributions
   and reviews.

11.  References

11.1.  Normative References

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Koster, M., Bormann, D., and P. Stok, "CoRE
              Resource Directory", draft-ietf-core-resource-directory-07
              (work in progress), March 2016.

   [I-D.selander-ace-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security of CoAP (OSCOAP)", draft-selander-ace-
              object-security-05 (work in progress), July 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <http://www.rfc-editor.org/info/rfc6570>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <http://www.rfc-editor.org/info/rfc6690>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <http://www.rfc-editor.org/info/rfc7252>.







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   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <http://www.rfc-editor.org/info/rfc7641>.

11.2.  Informative References

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <http://www.rfc-editor.org/info/rfc5988>.

Authors' Addresses

   Michael Koster
   SmartThings

   Email: Michael.Koster@smartthings.com


   Ari Keranen
   Ericsson

   Email: ari.keranen@ericsson.com


   Jaime Jimenez
   Ericsson

   Email: jaime.jimenez@ericsson.com






















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