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Versions: (draft-koster-core-coap-pubsub) 00 01 02

Network Working Group                                          M. Koster
Internet-Draft                                               SmartThings
Intended status: Standards Track                              A. Keranen
Expires: January 5, 2018                                      J. Jimenez
                                                                Ericsson
                                                            July 4, 2017


Publish-Subscribe Broker for the Constrained Application Protocol (CoAP)
                     draft-ietf-core-coap-pubsub-02

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 5, 2018.

Copyright Notice

   Copyright (c) 2017 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 REST API  . . . . . . . . . . . . . . . . . . . .   6
     4.1.  DISCOVERY . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  CREATE  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  PUBLISH . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.4.  SUBSCRIBE . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.5.  UNSUBSCRIBE . . . . . . . . . . . . . . . . . . . . . . .  14
     4.6.  READ  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     4.7.  REMOVE  . . . . . . . . . . . . . . . . . . . . . . . . .  17
   5.  CoAP pubsub Operation with Resource Directory . . . . . . . .  18
   6.  Sleep-Wake Operation  . . . . . . . . . . . . . . . . . . . .  19
   7.  Simple Flow Control . . . . . . . . . . . . . . . . . . . . .  19
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
     9.1.  Resource Type value 'core.ps' . . . . . . . . . . . . . .  21
     9.2.  Resource Type value 'core.ps.discover'  . . . . . . . . .  21
     9.3.  Response Code value '2.07'  . . . . . . . . . . . . . . .  21
     9.4.  Response Code value '4.29'  . . . . . . . . . . . . . . .  21
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     11.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

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



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   energy from their environment.  These devices have limited
   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 service:  A group of REST resources, as defined in this
      document, which together implement the required features of this
      specification.

   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 and pending
      notifications.




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   CoAP pubsub Client:  A CoAP client which is capable of publish or
      subscribe operations as defined in this specification.

   Topic:  A unique identifier for a particular item being published
      and/or subscribed to.  A broker uses the topics to match
      subscriptions to publications.  A topic is a valid CoAP URI as
      defined in [RFC7252]

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 REST API 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 of state update representations between
   certain CoAP pubsub Clients.  Clients Subscribe to topics upon which
   representations are Published by other Clients, which are forwarded
   by the Broker to the subscribing clients.  A CoAP pubsub Broker may
   be used as a REST resource proxy, retaining the last published
   representation 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 a REST API for
   clients to use to initiate publish-subscribe interactions.  Avoiding
   the need for direct reachability between clients, the broker only
   needs to be reachable from all clients.  The broker also needs to
   have sufficient resources (storage, bandwidth, etc.) to host CoAP
   resource services, 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 REST API defined in this document.  Clients initiate
   interactions with a 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 data 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.
   Every CoAP pubsub topic has a link, consisting of a reference path on
   the broker using URI path [RFC3986] construction and link attributes
   [RFC6690].  Every topic is associated with zero or more stored
   representations with a content-format specified in the link.  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 may
   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 REST API

   This section defines the REST API exposed by a CoAP pubsub Broker to
   pubsub Clients.  The examples throughout this section assume the use
   of CoAP [RFC7252].  A CoAP pubsub Broker implementing this
   specification SHOULD support the DISCOVERY, CREATE, PUBLISH,
   SUBSCRIBE, UNSUBSCRIBE, READ, and REMOVE operations defined in this
   section.  Optimized implementations MAY support a subset of the
   operations as required by particular constrained use cases.

4.1.  DISCOVERY

   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 the entry point of its pubsub API at its .well-known/core
   location [RFC6690].  A CoAP pubsub broker MAY register its pubsub
   REST API entry point with a Resource Directory.  Figure 3 shows an
   example of a client discovering a local pubsub API using CoAP Simple
   Discovery.  A broker wishing to advertise the CoAP pubsub API for
   Simple Discovery or through a Resource Directory MUST use the link
   relation rt=core.ps.  A broker MAY advertise its supported content
   formats and other attributes in the link to its pubsub API.

   A CoAP pubsub Broker MAY offer a topic discovery entry point 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" using Discover.  The client then
   receives the URI of the resource and its content-format.  A pubsub
   broker wishing to advertize topic discovery MUST use the relation
   rt=core.ps.discover in the link.

   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 API.  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:  {+ps}/{+topic}{?q*}





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   URI Template Variables:  ps := pubsub REST API entry point
      (optional).  The entry point of the pubsub REST API, 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 base URI of
      the pubsub API 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;rt=core.ps.discover;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

   A CoAP pubsub broker MAY allow Clients to create new topics on the
   broker using CREATE.  A client wishing to create a topic MUST use
   CoAP POST to the pubsub API 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.

   A Broker MUST return a response code of "2.01 Created" if the topic
   is created and return the URI path of the created topic 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}{?q*}

   URI Template Variables:  ps := pubsub REST API entry point
      (optional).  The entry point of the pubsub REST API, 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

   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 broker MAY allow clients to PUBLISH to topics on the
   broker.  A client MAY use the PUT or the POST 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.  A Broker
   MAY return a "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.

   A Broker MUST accept PUBLISH operations using the PUT method.
   PUBLISH operations using the PUT method replace any stored
   representation associated with the topic, with the supplied
   representation.  A Broker MAY reject, or delay responses to, PUT




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   requests to a topic while pending resolution of notifications to
   subscribers from previous PUT requests.

   Create on PUBLISH: A Broker MAY accept PUBLISH operations to new
   topics using the PUT method.  If a Broker accepts a PUBLISH using PUT
   to a topic that does not exist, the Broker MUST create the topic
   using the information in the PUT operation.  The Broker MUST create a
   topic with the URI-Path of the request, including all of the sub-
   topics necessary, and create a topic link with the ct attribute set
   to the content-format of the payload of the PUT request.  If topic is
   created, the Broker MUST return the response "2.01 Created" with the
   URI of the created topic, including all of the created path segments,
   returned via the Location-Path option.

   A Broker MAY accept PUBLISH operations using the POST method.  If a
   broker accepts PUBLISH using POST it shall respond with the 2.04
   Changed status code.

   A Broker MAY perform garbage collection of stored representations
   which have been delivered to all subscribers or which have timed out.
   A Broker MAY retain at least one most recently published
   representation to return in response to SUBSRCIBE and READ requests.

   A Broker MUST make a best-effort attempt to notify all clients
   subscribed on a particular topic each time it receives a publish on
   that topic.  An example is shown in Figure 10.  If a client publishes
   to a broker with the Max-Age option, the broker MUST include the same
   value for the Max-Age option 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, POST

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

   URI Template Variables:  ps := pubsub REST API entry point
      (optional).  The entry point of the pubsub REST API, 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.




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   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:  2.01 "Created".  Successful publish, topic is created

   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 10 for an example of a broker
   forwarding a message from a publishing client to a subscribed client.

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


                       Figure 8: Example of PUBLISH

          Client                                          Broker
            |                                               |
            | -------- PUT /ps/exa/mpl/e "1033.3"  -------> |
            |                                               |
            |                                               |
            | <--------------- 2.01 Created---------------- |
            |             Location: /ps/exa/mpl/e           |
            |                                               |


                  Figure 9: Example of CREATE on PUBLISH





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4.4.  SUBSCRIBE

   A CoAP pubsub broker MAY allow Clients to 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 the Observe option set to 0 (zero).  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 reject Subscribe requests
   on a topic if the content format of the request is not supported by
   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 "2.07 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

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

   URI Template Variables:  ps := pubsub REST API entry point
      (optional).  The entry point of the pubsub REST API, as obtained
      from discovery, used to discover topics.

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





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

   The following response codes are defined for this interface:

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

   Success:  2.07 "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 10 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 10: Example of SUBSCRIBE

4.5.  UNSUBSCRIBE

   If a CoAP pubsub broker allows clients to SUBSCRIBE to topics on the
   broker, it MUST allow Clients to 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



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   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}{?q*}

   URI Template Variables:  ps := pubsub REST API entry point
      (optional).  The entry point of the pubsub REST API, 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.

   The following response codes are defined for this interface:

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

   Success:  2.07 "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 11 shows an example of a client unsubscribe using the
   Observe=1 cancellation method.

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


                     Figure 11: Example of UNSUBSCRIBE



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4.6.  READ

   A CoAP pubsub broker MAY accept Read requests on a topic using the
   the CoAP GET method 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 "2.07 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}{?q*}

   URI Template Variables:  ps := pubsub REST API entry point
      (optional).  The entry point of the pubsub REST API, 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.

   The following response codes are defined for this interface:

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

   Success:  2.07 "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.



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

   Figure 12 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 12: Example of READ

4.7.  REMOVE

   A CoAP pubsub broker MAY allow clientsremove a topics from the broker
   using the CoAP Delete method on the URI of the topic.  The CoAP
   pubsub Broker MUST return "2.02 Deleted" if the removal 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.  If a topic which has sub-topics is removed, then all of
   its sub-topics MUST be recursively removed.

   The REMOVE interface is specified as follows:

   Interaction:  Client -> Broker

   Method:  DELETE

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




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   URI Template Variables:  ps := pubsub REST API entry point
      (optional).  The entry point of the pubsub REST API, 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:  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 13 shows a successful remove of topic1.

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


                       Figure 13: Example of REMOVE

5.  CoAP pubsub Operation with Resource Directory

   A CoAP pubsub Broker may register the base URI, which is the REST API
   entry point for a pubsub service, with a Resource Directory.  A
   pubsub Client may use an RD to discover a pubsub Broker.

   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.



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   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
   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



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   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.

   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 as described in [I-D.palombini-ace-coap-pubsub-profile].
   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



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   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.  Resource Type value 'core.ps.discover'

   o  Attribute Value: core.ps.discover

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

   o  Reference: [[This document]]

   o  Notes: None

9.3.  Response Code value '2.07'

   o  Response Code: 2.07

   o  Description: Add No Content response to GET to the existing
      definition of the 2.07 response code.

   o  Reference: [[This document]]

   o  Notes: The server sends this code to the client to indicate that
      the request was valid and accepted, but the responce may contain
      an empty payload.  It is comparable to and may be proxied with the
      http 204 No Content status code.

9.4.  Response Code value '4.29'

   o  Response Code: 4.29





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   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

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

   [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>.

   [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>.





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11.2.  Informative References

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
              Resource Directory", draft-ietf-core-resource-directory-10
              (work in progress), March 2017.

   [I-D.palombini-ace-coap-pubsub-profile]
              Palombini, F., "CoAP Pub-Sub Profile for Authentication
              and Authorization for Constrained Environments (ACE)",
              draft-palombini-ace-coap-pubsub-profile-00 (work in
              progress), March 2017.

   [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-06 (work in progress), October 2016.

   [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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