CoRE Working Group                                             M. Koster
Internet-Draft                                               SmartThings
Intended status: Standards Track                              A. Keranen
Expires: 5 November 2022                                      J. Jimenez
                                                                Ericsson
                                                              4 May 2022


Publish-Subscribe Broker for the Constrained Application Protocol (CoAP)
                   draft-ietf-core-coap-pubsub-10

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 https://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 5 November 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.










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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  CoAP Pub/sub Architecture . . . . . . . . . . . . . . . .   4
     4.2.  CoAP Pub/sub Broker . . . . . . . . . . . . . . . . . . .   5
     4.3.  CoAP Pub/sub Client . . . . . . . . . . . . . . . . . . .   5
     4.4.  CoAP Pub/sub Topic  . . . . . . . . . . . . . . . . . . .   5
     4.5.  Brokerless Pub/sub  . . . . . . . . . . . . . . . . . . .   6
   5.  CoAP Pub/sub REST API . . . . . . . . . . . . . . . . . . . .   7
     5.1.  DISCOVERY . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  CREATE  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.3.  PUBLISH . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.4.  SUBSCRIBE . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.5.  UNSUBSCRIBE . . . . . . . . . . . . . . . . . . . . . . .  16
     5.6.  READ  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.7.  REMOVE  . . . . . . . . . . . . . . . . . . . . . . . . .  18
   6.  CoAP Pub/sub Operation with Resource Directory  . . . . . . .  20
   7.  Sleep-Wake Operation  . . . . . . . . . . . . . . . . . . . .  20
   8.  Simple Flow Control . . . . . . . . . . . . . . . . . . . . .  21
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
     10.1.  Resource Type value 'core.ps'  . . . . . . . . . . . . .  22
     10.2.  Resource Type value 'core.ps.discover' . . . . . . . . .  22
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  23
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     12.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24











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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, a client, or both.

   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
   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.

   For some applications the client/server and request/response
   communication model is not optimal but publish-subscribe
   communication with potentially many senders and/or receivers and
   communication via topics rather than directly with endpoints may fit
   better.

   This document specifies simple extensions to CoAP for enabling
   publish-subscribe communication using a Broker node that enables
   store-and-forward messaging between two or more nodes.  This model
   facilitates communication of nodes with limited reachability, enables
   simple many-to-many communication, and eases integration with other
   publish-subscribe systems.

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Terminology

   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 [RFC9176].  The URI template format [RFC6570] is used
   to describe the REST API defined in this specification.




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   This specification makes use of the following additional terminology:

   Publish-Subscribe (pub/sub):  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 pub/sub service:  A group of REST resources, as defined in this
      document, which together implement the required features of this
      specification.

   CoAP pub/sub 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.

   CoAP pub/sub 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 reference to a Topic on a Broker
      is a valid CoAP URI as defined in [RFC7252]

4.  Architecture

4.1.  CoAP Pub/sub Architecture

   Figure 1 shows the architecture of a CoAP pub/sub service.  CoAP pub/
   sub Clients interact with a CoAP pub/sub Broker through the CoAP pub/
   sub REST API which is hosted by the Broker.  State information is
   updated between the Clients and the Broker.  The CoAP pub/sub Broker
   performs a store-and-forward of state update representations between
   certain CoAP pub/sub 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 pub/sub Broker may
   be used as a REST resource proxy, retaining the last published
   representation to supply in response to Read requests from Clients.









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                  Clients        pub/sub         Broker
                  +-------+         |
                  | CoAP  |         |
                  |pub/sub|---------|------+
                  |Client |         |      |    +-------+
                  +-------+         |      +----| CoAP  |
                                    |           |pub/sub|
                  +-------+         |      +----|Broker |
                  | CoAP  |         |      |    +-------+
                  |pub/sub|---------|------+
                  |Client |         |
                  +-------+         |

                    Figure 1: CoAP pub/sub Architecture

4.2.  CoAP Pub/sub Broker

   A CoAP pub/sub 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.

4.3.  CoAP Pub/sub Client

   A CoAP pub/sub Client interacts with a CoAP pub/sub Broker using the
   CoAP pub/sub REST API defined in this document.  Clients initiate
   interactions with a CoAP pub/sub 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.

4.4.  CoAP Pub/sub 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 by a Broker and
   all the clients using the Broker share the same namespace for topics.

   Every CoAP pub/sub topic has an associated 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 and a content-format specified in the



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   link.  A CoAP pub/sub topic value may alternatively consist of a
   collection of one or more sub-topics, consisting of links to the sub-
   topic URIs and indicated by a link-format content-format.  Sub-topics
   are also topics and may have their own sub-topics, forming a tree
   structure of unique paths that is implemented using URIs.  The full
   URI of a topic includes a scheme and authority for the Broker, for
   example "coaps://192.0.2.13:5684/EP-33543/sen/3303/0/5700".

   A Topic may have a lifetime defined by using the CoAP Max-Age option
   on topic creation, or on publish operations to the topic.  The
   lifetime is refreshed each time a representation is published to the
   topic.  If the lifetime expires, the topic is removed from discovery
   responses, returns errors on subscription, and any outstanding
   subscriptions are cancelled.

4.5.  Brokerless Pub/sub

   In some use cases, it is desireable to use pub/sub semantics for
   peer-to-peer communication, but it is not feasible or desireable to
   include a separate node on the network to serve as a Broker.  In
   other use cases, it is desireable to enable one-way-only
   communication, such as sensors pushing updates to a service.

   Figure 2 shows an arrangement for using CoAP pub/sub in a
   "Brokerless" configuration between peer nodes.  Nodes in a Brokerless
   system may act as both Broker and client.  A node that supports
   Broker functionality 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         pub/sub          Peer
                   +-------+         |         +-------+
                   | CoAP  |         |         | CoAP  |
                   |pub/sub|---------|---------|pub/sub|
                   |Client |         |         |Broker |
                   +-------+         |         +-------+
                   | CoAP  |         |         | CoAP  |
                   |pub/sub|---------|---------|pub/sub|
                   |Broker |         |         |Client |
                   +-------+         |         +-------+

                        Figure 2: Brokerless pub/sub









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5.  CoAP Pub/sub REST API

   This section defines the REST API exposed by a CoAP pub/sub Broker to
   pub/sub Clients.  The examples throughout this section assume the use
   of CoAP [RFC7252].  A CoAP pub/sub 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.

5.1.  DISCOVERY

   CoAP pub/sub Clients discover CoAP pub/sub Brokers by using CoAP
   Simple Discovery or through a Resource Directory (RD) [RFC9176].  A
   CoAP pub/sub Broker SHOULD indicate its presence and availability on
   a network by exposing a link to the entry point of its pub/sub API at
   its .well-known/core location [RFC6690].  A CoAP pub/sub Broker MAY
   register its pub/sub REST API entry point with a Resource Directory.
   Figure 3 shows an example of a client discovering a local pub/sub API
   using CoAP Simple Discovery.  A Broker wishing to advertise the CoAP
   pub/sub 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
   pub/sub API.

   A CoAP pub/sub 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 pub/sub
   Broker wishing to advertise topic discovery MUST use the relation
   rt=core.ps.discover in the link.

   A CoAP pub/sub Broker MAY provide topic discovery functionality
   through the .well-known/core resource.  Links to topics may be
   exposed at .well-known/core in addition to links to the pub/sub API.
   Figure 5 shows an example of topic discovery through .well-known/
   core.

   Topics in the broker may be created in hierarchies (see Section 5.2)
   with parent topics having sub-topics.  For a discovery the broker may
   choose to not expose the sub-topics in order to limit amount of topic
   links sent in a discovery response.  The client can then perform
   discovery for the parent topics it wants to discover the sub-topics.

   The DISCOVER interface is specified as follows:




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     Interaction: Client -> Broker
     Method: GET
     URI Template: {+ps}/{+topic}{?q\*}
     URI Template Variables:
       ps := Pub/sub REST API entry point (optional).
       Entry point of the pub/sub topic discovery API.
       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 the DISCOVER operation:

   Success:  2.05 "Content" with an application/link-format payload
      containing one or more matching entries for the Broker resource.
      A pub/sub Broker SHOULD use the value "/ps/" for the base URI of
      the pub/sub 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 pub/sub function

          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




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          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

5.2.  CREATE

   A CoAP pub/sub broker SHOULD allow Clients to create new topics on
   the broker using CREATE.  Some exceptions are for fixed brokerless
   devices and pre-configured brokers in dedicated installations.  A
   client wishing to create a topic MUST use a CoAP POST to the pub/sub
   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.  Additional link target
   attributes and relation values MAY be included in the topic
   specification link when a topic is created.

   The client MAY indicate the lifetime of the topic by including the
   Max-Age option in the CREATE request.

   Topic hierarchies can be created by creating parent topics.  A parent
   topic is created with a POST using 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.

   Ony one level in the topic hierarchy may be created as a result of a
   CREATE operation, unless create on PUBLISH is supported (see
   Section 5.3).  The topic string used in the link target MUST NOT
   contain the "/" character.

   A topic creator MUST include exactly one content format link
   attribute value (ct) in the create payload.  If the content format
   option is not included or if the option is repeated, the Broker MUST
   reject the operation with an error code of "4.00 Bad Request".





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   Only one topic may be created per request.  If there is more than one
   link included in a CREATE request, the Broker MUST reject the
   operation with an error code of "4.00 Bad Request".

   A Broker MUST return a response code of "2.01 Created" if the topic
   is created and MUST return the URI path of the created topic via
   Location-Path options.  If a new topic can not be created, the Broker
   MUST return the appropriate 4.xx response code indicating the reason
   for failure.

   A Broker SHOULD remove topics if the Max-Age of the topic is exceeded
   without any publishes to the topic.  A 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.

   A topic creator SHOULD PUBLISH an initial value to a newly-created
   Topic in order to enable responses to READ and SUBSCRIBE requests
   that may be submitted after the topic is discoverable.

   The CREATE interface is specified as follows:

   Interaction:  Client -> Broker

   Method:  POST

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

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

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

   Content-Format:  application/link-format

   Payload:  The desired topic to CREATE

   The following response codes are defined for the CREATE operation:

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

   Failure:  4.00 "Bad Request".  Malformed request.

   Failure:  4.01 "Unauthorized".  Authorization failure.

   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/ "<parent-topic>;ct=40" ------>|
            |                                               |
            | <---------------- 2.01 Created ---------------|
            |            Location: /ps/parent-topic/        |
            |                                               |
            |-- POST /ps/parent-topic/ "<subtopic>;ct=50" ->|
            |                                               |
            | <---------------- 2.01 Created ---------------|
            |       Location: /ps/parent-topic/subtopic     |
            |                                               |
            |                                               |

               Figure 7: Example of CREATE of topic hierarchy

5.3.  PUBLISH

   A CoAP pub/sub 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 pub/sub 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 8 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
   requests to a topic while pending resolution of notifications to
   subscribers from previous PUT requests.




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   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 value from the header 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.

   Figure 9 shows an example of a topic being created on first PUBLISH.

   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.  If an attempt is made to PUBLISH using POST to
   a topic that does not exist, the Broker SHALL return a response
   status indicating resource not found, such as HTTP 404 or CoAP 4.04.

   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 SUBSCRIBE 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 without the Max-Age option, the
   Broker MUST refresh the topic lifetime with the most recently set
   Max-Age value, and the Broker MUST include the most recently set Max-
   Age value in the Max-Age option of all notifications.

   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 operation is specified as follows:

   Interaction:  Client -> Broker

   Method:  PUT, POST

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




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

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

   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 the PUBLISH operation:

   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 8).

   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           |
            |                                               |



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                   Figure 9: Example of CREATE on PUBLISH

5.4.  SUBSCRIBE

   A CoAP pub/sub Broker MAY allow Clients to subscribe to topics on the
   Broker using CoAP Observe as described in [RFC7641].  A CoAP pub/sub
   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 the content
   format the topic was created 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 "4.04 Not Found" if the topic
   does not exist or has been removed, or if Max-Age of a previously
   published representation has expired.

   If a Topic has been created but not yet published to when a SUBSCRIBE
   to the topic is received, the Broker MAY acknowledge and queue the
   pending SUBSCRIBE and defer the response until an initial PUBLISH
   occurs.

   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 [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 operation is specified as follows:

   Interaction:  Client -> Broker

   Method:  GET

   Options:  Observe:0



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

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

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

   The following response codes are defined for the SUBSCRIBE operation:

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









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5.5.  UNSUBSCRIBE

   If a CoAP pub/sub 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 pub/sub
   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 operation is specified as follows:

   Interaction:  Client -> Broker

   Method:  GET

   Options:  Observe:1

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

   URI Template Variables:  ps := Pub/sub REST API entry point
      (optional).  The entry point of the pub/sub 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 the UNSUBSCRIBE
   operation:

   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.






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

                     Figure 11: Example of UNSUBSCRIBE

5.6.  READ

   A CoAP pub/sub 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 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 value to be valid since the last publish operation
   on that topic.

   The Broker MUST return a response code "4.04 Not Found" if the topic
   does not exist or has been removed, or if Max-Age of a previously
   published representation has expired.

   If a Topic has been created but not yet published to when a READ to
   the topic is received, the Broker MAY acknowledge and queue the
   pending READ, and defer the response until an initial PUBLISH occurs.

   The Broker MUST return a response code "4.15 Unsupported Content
   Format" if the Broker can not return the requested content format.

   The READ operation is specified as follows:

   Interaction:  Client -> Broker

   Method:  GET

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

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

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




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   The following response codes are defined for the READ operation:

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

5.7.  REMOVE

   A CoAP pub/sub Broker MAY allow clients to remove topics from the
   Broker using the CoAP Delete method on the URI of the topic.  The
   CoAP pub/sub 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.







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   When a topic is removed for any reason, the Broker SHOULD remove all
   of the observers from the list of observers and return a final 4.04
   "Not Found" response 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 operation is specified as follows:

   Interaction:  Client -> Broker

   Method:  DELETE

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

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

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

   Content-Format:  None

   Response Payload:  None

   The following response codes are defined for the REMOVE operation:

   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






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6.  CoAP Pub/sub Operation with Resource Directory

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

   A CoAP pub/sub Client may register links [RFC6690] with a Resource
   Directory to enable discovery of created pub/sub topics.  A pub/sub
   Client may use an RD to discover pub/sub Topics.  A client which
   registers pub/sub Topics with an RD MUST use the context relation
   (con) [RFC9176] to indicate that the context of the registered links
   is the pub/sub Broker.

   A CoAP pub/sub 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 pub/sub Broker.  See
   Section 5.1 in this document.

   The pub/sub 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 pub/sub Broker and incorporate them into the
   Resource Directory.  See [RFC9176] for further details.

7.  Sleep-Wake Operation

   CoAP pub/sub 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.










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8.  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 SHOULD respond with Response
   Code 4.29, "Too Many Requests" [RFC8516] and set the Max-Age Option
   to indicate the number of seconds after which the client can 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.

9.  Security Considerations

   CoAP pub/sub re-uses CoAP [RFC7252], CoRE Resource Directory
   [RFC9176], and Web Linking [RFC5988] and therefore the security
   considerations of those documents also apply to this specification.
   Additionally, a CoAP pub/sub 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 pub/sub 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



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   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 pub/sub Broker, the use of end-to-end object security is
   RECOMMENDED as described in [I-D.ietf-ace-pubsub-profile].  An
   example application that uses the CoAP pub/sub Broker and relies on
   end-to-end object security is described in [RFC8387].  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.

10.  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.

10.1.  Resource Type value 'core.ps'

   *  Attribute Value: core.ps

   *  Description: Section 5 of [[This document]]

   *  Reference: [[This document]]

   *  Notes: None

10.2.  Resource Type value 'core.ps.discover'

   *  Attribute Value: core.ps.discover

   *  Description: Section 5 of [[This document]]

   *  Reference: [[This document]]

   *  Notes: None




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11.  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.

12.  References

12.1.  Normative References

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

   [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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc6570>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc7641>.

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

   [RFC8516]  Keranen, A., ""Too Many Requests" Response Code for the
              Constrained Application Protocol", RFC 8516,
              DOI 10.17487/RFC8516, January 2019,
              <https://www.rfc-editor.org/info/rfc8516>.



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   [RFC9176]  Amsuess, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
              P. van der Stok, "Constrained RESTful Environments (CoRE)
              Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April
              2022, <https://www.rfc-editor.org/info/rfc9176>.

12.2.  Informative References

   [I-D.ietf-ace-pubsub-profile]
              Palombini, F. and C. Sengul, "Pub-Sub Profile for
              Authentication and Authorization for Constrained
              Environments (ACE)", Work in Progress, Internet-Draft,
              draft-ietf-ace-pubsub-profile-04, 29 December 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-pubsub-
              profile-04.txt>.

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

   [RFC8387]  Sethi, M., Arkko, J., Keranen, A., and H. Back, "Practical
              Considerations and Implementation Experiences in Securing
              Smart Object Networks", RFC 8387, DOI 10.17487/RFC8387,
              May 2018, <https://www.rfc-editor.org/info/rfc8387>.

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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