A publish-subscribe architecture for the Constrained Application Protocol (CoAP)
draft-ietf-core-coap-pubsub-12
| Document | Type | Active Internet-Draft (core WG) | |
|---|---|---|---|
| Authors | Michael Koster , Ari Keränen , Jaime Jimenez | ||
| Last updated | 2023-03-13 | ||
| Replaces | draft-koster-core-coap-pubsub | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| On agenda | core at IETF-116 | ||
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| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
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| Send notices to | (None) |
draft-ietf-core-coap-pubsub-12
CoRE M. Koster
Internet-Draft Unaffiliated
Intended status: Standards Track A. Keranen
Expires: 14 September 2023 J. Jimenez
Ericsson
13 March 2023
A publish-subscribe architecture for the Constrained Application
Protocol (CoAP)
draft-ietf-core-coap-pubsub-12
Abstract
This document describes a publish-subscribe architecture for CoAP
that extends the capabilities of CoAP for supporting nodes with long
breaks in connectivity and/or up-time. The Constrained Application
Protocol (CoAP) is used by CoAP clients both to publish and to
subscribe via a known topic resource.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-core-coap-pubsub/.
Discussion of this document takes place on the core Working Group
mailing list (mailto:core@ietf.org), which is archived at
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https://www.ietf.org/mailman/listinfo/core/.
Source for this draft and an issue tracker can be found at
https://github.com/core-wg/coap-pubsub.
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-
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This Internet-Draft will expire on 14 September 2023.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. CoAP Publish-Subscribe Architecture . . . . . . . . . . . 5
1.3. Managing Topics . . . . . . . . . . . . . . . . . . . . . 6
2. Pub-Sub Topics . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Collection Representation . . . . . . . . . . . . . . . . 7
2.2. Topic Creation and Configuration . . . . . . . . . . . . 7
2.2.1. Topic Properties . . . . . . . . . . . . . . . . . . 8
2.2.2. Topic Resource Representation . . . . . . . . . . . . 8
2.3. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3.1. Broker Discovery . . . . . . . . . . . . . . . . . . 9
2.3.2. Topic Discovery . . . . . . . . . . . . . . . . . . . 10
2.4. Topic Collection Interactions . . . . . . . . . . . . . . 10
2.4.1. Retrieving all topics . . . . . . . . . . . . . . . . 10
2.4.2. Getting Topics by Properties . . . . . . . . . . . . 11
2.4.3. Creating a Topic . . . . . . . . . . . . . . . . . . 11
2.5. Topic Resource Interactions . . . . . . . . . . . . . . . 12
2.5.1. Getting a topic resource . . . . . . . . . . . . . . 12
2.5.2. Getting part of a topic resource by filter . . . . . 13
2.5.3. Updating the Topic Resource . . . . . . . . . . . . . 14
2.5.4. Deleting a Topic Resource . . . . . . . . . . . . . . 15
2.6. Publish/Subscribe . . . . . . . . . . . . . . . . . . . . 16
2.6.1. Topic Lifecycle . . . . . . . . . . . . . . . . . . . 16
2.6.2. Rate Limiting . . . . . . . . . . . . . . . . . . . . 17
2.7. Topic Data Interactions . . . . . . . . . . . . . . . . . 17
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2.7.1. Publish . . . . . . . . . . . . . . . . . . . . . . . 17
2.7.2. Subscribe . . . . . . . . . . . . . . . . . . . . . . 19
2.7.3. Unsubscribe . . . . . . . . . . . . . . . . . . . . . 20
2.7.4. Delete topic data resource . . . . . . . . . . . . . 20
2.7.5. Read latest data . . . . . . . . . . . . . . . . . . 21
3. CoAP Pubsub Parameters . . . . . . . . . . . . . . . . . . . 22
4. Security Considerations . . . . . . . . . . . . . . . . . . . 22
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
5.1. Resource Type value 'core.ps' . . . . . . . . . . . . . . 22
5.2. Resource Type value 'core.ps.coll' . . . . . . . . . . . 23
5.3. Resource Type value 'core.ps.conf' . . . . . . . . . . . 23
5.4. Resource Type value 'core.ps.data' . . . . . . . . . . . 23
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.1. Normative References . . . . . . . . . . . . . . . . . . 23
7.2. Informative References . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] supports
machine-to-machine communication across networks of constrained
devices and constrained networks. 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. Another important class of nodes
are devices with limited reachability due to middle-boxes like
Network Address Translators (NATs) and firewalls.
For these nodes, the client/server-oriented architecture of REST can
be challenging when interactions are not initiated by the devices
themselves. A publish/subscribe-oriented architecture where nodes
are separated by a broker and data is exchanged via topics might fit
these nodes better.
This document applies the idea of publish-subscribe to Constrained
RESTful Environments. It introduces a broker that allows to create,
discover subscribe and publish on topics. The broker enables store-
and-forward data exchange between CoAP endpoints, thereby
facilitating the communication of nodes with limited reachability,
providing simple many-to-many communication, and easing integration
with other publish/subscribe systems.
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1.1. Terminology
This specification requires readers to be familiar with all the terms
and concepts that are discussed in [RFC8288] and [RFC6690]. Readers
should also be familiar with the terms and concepts discussed in
[RFC7252] and [RFC9167]. The URI template format [RFC6570] is used
to describe the REST API defined in this specification.
This specification makes use of the following terminology:
publish-subscribe (pub/sub): A messaging paradigm in which messages
are published to a broker, and potential receivers can subscribe
to a broker to receive messages. Message producers do not need to
know where the message will be eventually sent. The broker
matches publications and subscriptions, and delivers publications
to subscribed receivers.
publishers and subscribers: CoAP clients can act as publishers or as
subscribers. Publishers propose topics for creation and send CoAP
messages (publications) to the broker on specific topics.
Subscribers have an ongoing observation relation (subscription) to
a topic. Publishers and subscribers do not need to have any
knowledge of each other, but they must know the topic they are
publishing and subscribing to.
topic collection: A resource collection is a group of related
resources that share a common base URI. In this case the the
topic collection contains resources of the type "topic resource".
CoAP clients can discover and interact with the resources in a
collection by sending CoAP requests to the URI of the collection.
topic resource: An entry within a topic collection in a broker.
Topics are created and configured before any data can be
published. CoAP clients can propose new topics to be created, but
it is up to the broker to decide whether and how a topic is
created. The broker also decides the URI of each topic resource
and of the topic-data resource when hosted at the broker. The
creation, configuration, and discovery of topics at a broker are
specified in Section 2. Interactions about the topic-data are
defined in Section 2.7.
topic-data resource: Topic resources contain a property called
"topic-data". The topic-data resource is a CoAP URI used by
publishers and subscribers to publish (PUT) and subscribe (GET
with Observe) to data (see Section 2).
broker: A CoAP server that hosts one or more topic collections
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containing topic resources. The broker is responsible for the
store-and-forward of state update representations when the topic-
data URI points to a resource hosted on the broker. The broker is
also responsible of handling the topic lifecycle as defined in
Section 2.6.1. The creation, configuration, and discovery of
topics at a broker is specified in Section 2.
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.
1.2. CoAP Publish-Subscribe Architecture
Figure 1 shows a simple Publish/Subscribe architecture over CoAP. In
it, publishers submit their data over a RESTful interface to a
broker-managed resource (topic) and subscribers observe this resource
using [RFC7641]. Resource state information is updated between the
CoAP clients and the broker via topics. Topics are created by the
broker but the initial configuration can be proposed by a client,
normally a publisher.
The broker is responsible for the store-and-forward of state update
representations between CoAP clients. Subscribers observing a
resource will receive notifications, the delivery of which is done on
a best-effort basis.
CoAP CoAP CoAP
clients server clients
___________ __________ observe ____________
| | publish | | .--------- | |
| publisher | ---------> | | |--------> | subscriber |
|___________| | | '--------> |____________|
. | | .
. | broker | .
___________ | | observe ____________
| | publish | | .--------- | |
| publisher | ---------> | | |--------> | subscriber |
|___________| |__________| '--------> |____________|
Figure 1: Publish-subscribe architecture over CoAP
This document describes two sets of interactions, interactions to
configure topics and their lifecycle (see Section 2.5) and
interactions about the topic data (see Section 2.7).
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Topic resource interactions are discovery, create, read
configuration, update configuration, delete configuration and handle
the management of the topics.
Topic data interactions are publish, subscribe, unsubscribe, read and
are oriented on how data is transferred from a publisher to a
subscriber.
1.3. Managing Topics
Figure 2 shows the resources of a Topic Collection that can be
managed at the Broker.
___
Topic / \
Collection \___/
Resource \
\____________________
\___ \___ \___
/ \ / \ ... / \ Topic
\___/ \___/ \___/ Resources
Figure 2: Resources of a Broker
The Broker exports a topic-collection resource, with resource type
"core.ps.coll" defined in Section 5 of this document. The interfaces
for the topic-collection resource is defined in Section 2.4.
2. Pub-Sub Topics
The configuration side of a "publish/subscribe broker" consists of a
collection of topics. These topics as well as the collection itself
are exposed by a CoAP server as resources (see Figure 3). Each topic
has a topic and a topic data resources. The topic resource is used
by a client creating or administering a topic. The topic data
resource is used by the publishers and the subscribers to a topic.
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___
Topic / \
Collection \___/
Resource \
\___________________________
\ \ \
\ ...... \ ...... \ ......
: \___ : : \___ : : \___ :
Topic : / + \ : : / + \ : : / + \ :
Resource : \_|_/ : : \_|_/ : : \_|_/ :
....|.... ....|.... ....|....
....|.... ....|.... ....|....
Topic : _|_ : : _|_ : ... : _|_ :
Data : / \ : : / \ : : / \ :
Resource : \___/ : : \___/ : : \___/ :
:.......: :.......: :.......:
\_________/\_________/ ... \_________/
topic 1 topic 2 topic n
Figure 3: Topic and topic-data resources of a topic
2.1. Collection Representation
Each topic resource is represented as a link, where the link target
is the URI of the topic resource.
Each topic-data is represented as a link, where the link target is
the URI of the topic-data resource. A topic-data link is an entry
within the topic resource called 'topic_data' (see Section 2.2.1).
The list can be represented as a Link Format document [RFC6690]. The
link to each topic resource specifies the link target attribute 'rt'
(Resource Type), with value "core.pubsub.conf" defined in this
document.
2.2. Topic Creation and Configuration
A CoAP client can create a new topic by submitting an initial
configuration for the topic (see Section 2.4.3). It can also read
and update the configuration of existing topics and delete them when
they are no longer needed (see Section 2.5).
The configuration of a topic itself consists of a set of properties
that can be set by a client or by the broker.
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2.2.1. Topic Properties
The CBOR map includes the following configuration parameters, whose
CBOR abbreviations are defined in Section 3 of this document.
* 'topic_name': A required field used as an application identifier.
It encodes the topic name as a CBOR text string. Examples of
topic names include human-readable strings (e.g., "room2"), UUIDs,
or other values.
* 'topic_data': A required field containing the CoAP URI of the
topic data resource for subscribing to a pubsub topic. It encodes
the URI as a CBOR text string.
* 'resource_type': A required field used to indicate the resource
type associated with topic resources. It encodes the resource
type as a CBOR text string. The value should be "core.ps.conf".
* 'media_type': An optional field used to indicate the media type of
the topic data resource. It encodes the media type as a CBOR text
string (e.g.,"application/json").
* 'target_attribute': An optional field used to indicate the
attribute or property of the topic_data resource. It encodes the
attribute as a CBOR text string. Example attributes include
"temperature".
* 'expiration_date': An optional field used to indicate the
expiration date of the topic. It encodes the expiration date as a
CBOR text string. The value should be a date string in ISO 8601
format (e.g., "2023-03-31T23:59:59Z"). The pubsub system can use
this field to automatically remove topics that are no longer
valid.
* 'max_subscribers': An optional field used to indicate the maximum
number of subscribers allowed for the topic. It encodes the
maximum number as a CBOR integer. If this field is not present,
there is no limit to the number of subscribers allowed. The
pubsub system can use this field to limit the number of
subscribers for a topic.
2.2.2. Topic Resource Representation
A topic is represented as a CBOR map containing the configuration
properties of the topic as top-level elements.
Unless specified otherwise, these are defined in this document and
their CBOR abbreviations are defined in Section 3.
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2.2.2.1. Default Values
Below are the defined default values for the topic parameters:
* 'topic_name': There is no default value. This field is required
and must be specified by the client or broker.
* 'topic_data': There is no default value. This field is required
and must be specified by the client or broker.
* 'resource_type': The default value is "core.ps.conf".
* 'media_type': The default value is an empty string, indicating
that no media type is specified.
* 'target_attribute': The default value is an empty string,
indicating that no attribute is specified.
* 'expiration_date': The default value is an empty string,
indicating that no expiration date is specified. If this field is
not present, the topic will not expire automatically.
* 'max_subscribers': The default value is -1, indicating that there
is no limit to the number of subscribers allowed. If this field
is not present, the pubsub system will not limit the number of
subscribers for the topic.
2.3. Discovery
Discovery involves that of the Broker, topic collections, topic
resources and topic data.
2.3.1. Broker Discovery
CoAP clients MAY discover brokers by using CoAP Simple Discovery, via
multicast, through a Resource Directory (RD) [RFC9167] or by other
means specified in extensions to [RFC7252]. Brokers MAY register
with a RD by following the steps on Section 5 of [RFC9167] with the
resource type set to "core.ps" as defined in Section 5 of this
document.
Brokers SHOULD expose a link to the entry point of the pubsub API at
their .well-known/core location [RFC6690]. The specific resource
path is left for implementations, examples in this document may use
the "/ps" path.
Example:
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=> GET
Uri-Path: ./well-known/core
Resource-Type: core.ps
<= 2.05 Content
</ps>;rt=core.ps;ct=40
2.3.2. Topic Discovery
A Broker can offer a topic discovery entry point to enable clients to
find topics of interest. The resource entry point thus represents a
collection of related resources as specified in [RFC6690] and is
identified by the resource type "core.ps.coll".
The interactions with topic collections are further defined in
Section 2.4.
A topic collection is a group of topic resources that define the
properties of the topics themselves (see Section Section 2.2.2).
Each topic resource is represented as a link to its corresponding
resource URI. The list can be represented as a Link Format document
[RFC6690]. Topic resources are identified by the resource type
"core.ps.conf".
Within each topic resource there is a set of configuration properties
(see Section Section 2.2.1). The 'topic_data' property contains the
URI of the topic data resource that a CoAP client can subscribe to.
Resources exposing resources of the topic data type are expected to
use the resource type 'core.ps.data'.
2.4. Topic Collection Interactions
These are the interactions that can happen at the topic collection
level.
2.4.1. Retrieving all topics
A client can request a collection of the topics present in the broker
by making a GET request to the collection URI.
On success, the server returns a 2.05 (Content) response with a
representation of the list of all topic resources (see
Section Section 2.2.2) in the collection.
Depending on the permission set each client MAY receive a different
list of topics that they are authorized to read.
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Example:
=> 0.01 GET
Uri-Path: ps
Uri-Path: tc
<= 2.05 Content
Content-Format: 40 (application/link-format)
</ps/tc/temperature>;rt="core.ps.conf",
</ps/tc/humidity>;rt="core.ps.conf"
2.4.2. Getting Topics by Properties
A client can filter a collection of topics by submitting the
representation of a topic filter (see Section Section 2.5.2) in a
FETCH request to the topic collection URI.
On success, the server returns a 2.05 (Content) response with a
representation of a list of topics in the collection (see
Section Section 2.3.2) that match the filter in CoRE link format
[RFC6690].
Example:
=> 0.05 FETCH
Uri-Path: ps
Uri-Path: tc
Content-Format: TBD (application/pubsub+cbor)
{
"resource_type" : "core.ps.conf",
"target_attribute" : "temperature"
}
<= 2.05 Content
Content-Format: 40 (application/link-format)
</living_room_sensor>;anchor="coap://[2001:db8::2]/ps/tc/h9392",
</kitchen_sensor>;anchor="coap://[2001:db8::2]/ps/tc/f3192"
2.4.3. Creating a Topic
A client can add a new topic to a collection of topics by submitting
a representation of the initial topic resource (see
Section Section 2.2.2) in a POST request to the topic collection URI.
On success, the server returns a 2.01 (Created) response indicating
the topic URI of the new topic and the current representation of the
topic resource.
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If a topic manager is present in the broker, the topic creation may
require manager approval subject to certain conditions. If the
conditions are not fulfilled, the manager MUST respond with a 4.03
(Forbidden) error. The response MUST have Content-Format set to
"application/core-pubsub+cbor".
The broker MUST respond with a 4.00 (Bad Request) error if any
received parameter is specified multiple times, invalid or not
recognized.
A CoAP endpoint creating a topic may specify a 'topic_data' URI
different than that used by the broker. The broker may then simply
forward the observation requests to the 'topic_data' URI.
If the 'topic_data' is empty the broker will assign a resource for a
publisher to publish to.
=> 0.02 POST
Uri-Path: ps
Uri-Path: tc
Content-Format: TBD2 (application/core-pubsub+cbor)
TBD (this should be a CBOR map with the mandatory parameters)
{
"topic_name" : "living_room_sensor"
"resource_type" : "core.ps.conf"
}
<= 2.01 Created
Location-Path: ps/h9392
Content-Format: TBD2 (application/core-pubsub+cbor)
TBD (this should be a CBOR map)
{
"topic_name" : "living_room_sensor",
"topic_data" : "coap://[2001:db8::2]/ps/data/6578616d706c65"
"resource_type" : "core.ps.conf"
}
2.5. Topic Resource Interactions
These are the interactions that can happen at the topic resource
level.
2.5.1. Getting a topic resource
A client can read the configuration of a topic by making a GET
request to the topic resource URI.
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On success, the server returns a 2.05 (Content) response with a
representation of the topic resource. The response has as payload
the representation of the topic resource as specified in
Section 2.2.2.
If a topic manager (TBD) is present in the broker, retrieving topic
information may require manager approval subject to certain
conditions (TBD). If the conditions are not fulfilled, the manager
MUST respond with a 4.03 (Forbidden) error. The response MUST have
Content-Format set to "application/core-pubsub+cbor".
The response payload is a CBOR map, whose possible entries are
specified in Section 2.2.2 and use the same abbreviations defined in
Section 3.
Example:
=> 0.01 GET
Uri-Path: ps
Uri-Path: tc
Uri-Path: h9392
<= 2.05 Content
Content-Format: TBD2 (application/core-pubsub+cbor)
{
"topic_name" : "living_room_sensor",
"topic_data" : "coap://[2001:db8::2]/ps/data/6578616d706c65",
"resource_type": "core.ps.conf",
"media_type": "application/senml-cbor",
"target_attribute": "temperature",
"expiration_date": "",
"max_subscribers": -1
}
2.5.2. Getting part of a topic resource by filter
A client can read the configuration of a topic by making a FETCH
request to the topic resource URI with a filter for specific
parameters. This is done in order to retrieve part of the current
topic resource.
The request contains a CBOR map with a configuration filter or
'conf_filter', a CBOR array with CBOR abbreviation. Each element of
the array specifies one requested configuration parameter of the
current topic resource (see Section 2.2.2).
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On success, the server returns a 2.05 (Content) response with a
representation of the topic resource. The response has as payload
the partial representation of the topic resource as specified in
Section 2.2.2.
If a topic manager (TBD) is present in the broker, retrieving topic
information may require manager approval subject to certain
conditions (TBD). If the conditions are not fulfilled, the manager
MUST respond with a 4.03 (Forbidden) error.
The response payload is a CBOR map, whose possible entries are
specified in Section 2.2.2 and use the same abbreviations defined in
Section 3.
Both request and response MUST have Content-Format set to
"application/core-pubsub+cbor".
Example:
=> 0.05 FETCH
Uri-Path: ps
Uri-Path: tc
Uri-Path: h9392
Content-Format: TBD2 (application/core-pubsub+cbor)
{
"conf_filter" : [topic_data, media_type]
}
<= 2.05 Content
Content-Format: TBD2 (application/core-pubsub+cbor)
{
"topic_data" : "coap://[2001:db8::2]/ps/data/6578616d706c65",
"media_type": "application/senml-cbor"
}
2.5.3. Updating the Topic Resource
A client can update the configuration of a topic by submitting the
representation of the updated topic (see Section 3.1.3) in a PUT or
POST request to the topic URI. Any existing properties in the
configuration are overwritten by this update.
On success, the server returns a 2.04 (Changed) response and the
current full resource representation. The broker may chose not to
overwrite parameters that are not explicitly modified in the request.
Example:
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=> 0.03 PUT
Uri-Path: ps
Uri-Path: tc
Uri-Path: h9392
Content-Format: TBD2 (application/core-pubsub+cbor)
{
"topic_name" : "living_room_sensor",
"topic_data" : "coap://[2001:db8::2]/ps/data/6578616d706c65",
"target_attribute": "temperature",
"expiration_date": "2023-04-28T23:59:59Z",
"max_subscribers": 2
}
<= 2.04 Changed
Content-Format: TBD2 (application/core-pubsub+cbor)
TBD (this should be a CBOR map)
{
"topic_name" : "living_room_sensor",
"topic_data" : "coap://[2001:db8::2]/ps/data/6578616d706c65",
"resource_type": "core.ps.conf",
"media_type": "application/senml-cbor",
"target_attribute": "temperature",
"expiration_date": "2023-04-28T23:59:59Z",
"max_subscribers": 2
}
2.5.4. Deleting a Topic Resource
A client can delete a topic by making a CoAP DELETE request on the
topic resource URI.
On success, the server returns a 2.02 (Deleted) response.
When a topic resource is deleted, the broker SHOULD also delete the
topic data resource, unsubscribe all subscribers by removing them
from the list of observers and returning a final 4.04 (Not Found)
response as per [RFC7641] Section 3.2. Example:
=> 0.04 DELETE
Uri-Path: ps
Uri-Path: tc
Uri-Path: h9392
<= 2.02 Deleted
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2.6. Publish/Subscribe
Unless a topic is configured to use a different mechanism, publish/
subscribe is performed as follows: A publisher publishes to a topic
by submitting the data in a PUT request to a broker-managed "topic
data resource". This causes a change to the state of that resources.
Any subscriber observing the resource [RFC7641] at that time receives
a notification about the change to the resource state. Observations
are maintained and terminated as specified in [RFC7641].
As shown in Section 2, each topic contains two resources: topic
resource and topic data. In that section we explained the creation
and configuration of the topic resources. This section will explain
the management of topic data resources.
A topic data resource does not exist until some initial data has been
published to it. Before initial data has been published, the topic
data resource yields a 4.04 (Not Found) response. If such a "half
created" topic is undesired, the creator of the topic can simply
immediately publish some initial placeholder data to make the topic
"fully created".
URIs for topic resources are broker-generated. URIs for topic-data
MAY also be broker-generated or client-generated. There does not
need to be any URI pattern dependence between the URI where the data
exists and the URI of the topic resource. Topic resource and data
resources might even be hosted on different servers.
2.6.1. Topic Lifecycle
When a topic is newly created, it is first placed by the server into
the HALF CREATED state (see Figure 4). In this state, a client can
read and update the configuration of the topic and delete the topic.
A publisher can publish to the topic data resource. However, a
subscriber cannot yet observe the topic data resource nor read the
latest data.
HALF FULLY
CREATED Delete CREATED
___ Topic Data ___ Publish
------------> | | <------------------ | | ------------.
Create |___| ------------------> |___| <-----------'
\ Publish / Subscribe
| ^ \ ___ / | ^
Read/ | | '--> | | <--' | | Read/
Update | | Delete |___| Delete | | Update
'-' Topic Topic '-'
DELETED
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Figure 4: Lifecycle of a Topic
After a publisher publishes to the topic for the first time, the
topic is placed into the FULLY CREATED state. In this state, a
client can read, update and delete the topic; a publisher can publish
to the topic data resource; and a subscriber can observe the topic
data resource.
When a client deletes a topic resource, the topic is placed into the
DELETED state and shortly after removed from the server. In this
state, all subscribers are removed from the list of observers of the
topic data resource and no further interactions with the topic are
possible.
When a client deletes a topic data, the topic is placed into the HALF
CREATED state, where clients can read, update and delete the topic
and awaits for a publisher to begin publication.
2.6.2. Rate Limiting
The server hosting a data resource may have to handle a potentially
very large number of publishers and subscribers at the same time.
This means the server can easily become overwhelmed if it receives
too many publications in a short period of time.
In this situation, if a client is sending publications too fast, the
server SHOULD return a 4.29 (Too Many Requests) response [RFC8516].
As described in [RFC8516], the Max-Age option [RFC7252] in this
response indicates the number of seconds after which the client may
retry. The Broker MAY stop publishing messages from the client for
the indicated time.
When a client receives a 4.29 (Too Many Requests) response, it MUST
NOT send any new publication requests to the same topic data resource
before the time indicated by the Max-Age option has passed.
2.7. Topic Data Interactions
TBD: intro and image that shows a topic data URI hosted in a
different endpoint than the broker
2.7.1. Publish
A topic must have been created in order to publish data to it (See
Section Section 2.4.3) and be in the half-created state in order to
the publish operation to work (see Section 2.6.1).
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A client can publish data to a topic by submitting the data in a PUT
request to the 'topic_data' URI as indicated in its topic resource
property. Please note that the 'topic_data' URI is not the same as
the topic URI used for configuring the topic (see Section 2.2.2).
On success, the server returns a 2.04 (Updated) response. However,
when data is published to the topic for the first time, the server
instead MUST return a 2.01 (Created) response and set the topic in
the fully-created state (see Section 2.6.1).
If the request does not have an acceptable content format, the server
returns a 4.15 (Unsupported Content Format) response.
If the client is sending publications too fast, the server returns a
4.29 (Too Many Requests) response [RFC8516].
Example of first publication:
=> 0.03 PUT
Uri-Path: ps
Uri-Path: data
Uri-Path: 6578616d706c65
Content-Format: 110
{
"n": "temperature",
"u": "Cel",
"t": 1621452122,
"v": 23.5
}
<= 2.01 Created
Example of subsequent publication:
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=> 0.03 PUT
Uri-Path: ps
Uri-Path: data
Uri-Path: 6578616d706c65
Content-Format: 110
{
"n": "temperature",
"u": "Cel",
"t": 182734122,
"v": 22.5
}
<= 2.04 Updated
2.7.2. Subscribe
A client can subscribe to a topic by sending a CoAP GET request with
the Observe set to '0' to subscribe to resource updates. [RFC7641].
On success, the broker MUST return 2.05 (Content) notifications with
the data.
If the topic is not yet in the fully created state (see
Section 2.6.1) the broker SHOULD return a response code 4.04 (Not
Found).
The following response codes are defined for the Subscribe operation:
Success: 2.05 "Content". Successful subscribe with observe
response, current value included in the response.
Failure: 4.04 "Not Found". Topic does not exist.
TBD: Do we want to treat max_clients as an error?
If the 'max_clients' parameter has been reached, the server must
treat that as specified in section 4.1 of [RFC7641]. The response
MUST NOT include an Observe Option, the absence of which signals to
the subscriber that the subscription failed.
Example:
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=> 0.01 GET
Uri-Path: ps
Uri-Path: data
Uri-Path: 6578616d706c65
Observe: 0
<= 2.05 Content
Content-Format: 110
Observe: 10001
Max-Age: 15
[...SenML data...]
<= 2.05 Content
Content-Format: 110
Observe: 10002
Max-Age: 15
[...SenML data...]
<= 2.05 Content
Content-Format: 110
Observe: 10003
Max-Age: 15
[...SenML data...]
2.7.3. Unsubscribe
A CoAP client can unsubscribe simply by cancelling the observation as
described in Section 3.6 of [RFC7641]. The client MUST either use
CoAP GET with the Observe Option set to 1 or send a CoAP Reset
message in response to a notification.
2.7.4. Delete topic data resource
A publisher MAY delete a topic by making a CoAP DELETE request on the
'topic_data' URI.
On success, the server returns a 2.02 (Deleted) response.
When a topic_data resource is deleted, the broker SHOULD also delete
the 'topic_data' parameter in the topic resource, unsubscribe all
subscribers by removing them from the list of observers and return a
final 4.04 (Not Found) response as per [RFC7641] Section 3.2. The
topic is then set back to the half created state as per
Section 2.6.1.
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Example:
=> 0.04 DELETE
Uri-Path: ps
Uri-Path: data
Uri-Path: 6578616d706c65
<= 2.02 Deleted
2.7.5. Read latest data
A client can get the latest published topic data by making a GET
request to the 'topic_data' URI in the broker. Please note that
discovery of the 'topic_data' parameter is a required previous step
(see Section 2.5.1).
On success, the server MUST return 2.05 (Content) response with the
data.
If the target URI does not match an existing resource or the topic is
not in the fully created state (see Section 2.6.1), the broker SHOULD
return a response code 4.04 (Not Found).
If the broker can not return the requested content format it MUST
return a response code 4.15 (Unsupported Content Format).
Example:
=> 0.01 GET
Uri-Path: ps
Uri-Path: data
Uri-Path: 6578616d706c65
<= 2.05 Content
Content-Format: 110
Max-Age: 15
{
"n": "temperature",
"u": "Cel",
"t": 1621452122,
"v": 23.5
}
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3. CoAP Pubsub Parameters
This document defines parameters used in the messages exchanged
between a client and the broker during the topic creation and
configuration process (see Section 2.2.2). The table below
summarizes them and specifies the CBOR key to use instead of the full
descriptive name.
Note that the media type application/core-pubsub+cbor MUST be used
when these parameters are transported in the respective message
fields.
+-----------------+-----------+--------------+------------+
| Name | CBOR Key | CBOR Type | Reference |
|-----------------|-----------|--------------|------------|
| topic_name | TBD1 | tstr | [RFC-XXXX] |
| topic_data | TBD2 | tstr | [RFC-XXXX] |
| resource_type | TBD3 | tstr | [RFC-XXXX] |
| media_type | TBD4 | tstr (opt) | [RFC-XXXX] |
| target_attribute| TBD5 | tstr (opt) | [RFC-XXXX] |
| expiration_date | TBD6 | tstr (opt) | [RFC-XXXX] |
| max_subscribers | TBD7 | uint (opt) | [RFC-XXXX] |
+-----------------+-----------+--------------+------------+
Figure 5: CoAP Pubsub Parameters
4. Security Considerations
TBD.
5. IANA Considerations
TBD.
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.
5.1. Resource Type value 'core.ps'
* Attribute Value: core.ps
* Description: XXX of This document
* Reference: This document
* Notes: None
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5.2. Resource Type value 'core.ps.coll'
* Attribute Value: core.ps.coll
* Description: XXX of This document
* Reference: This document
* Notes: None
5.3. Resource Type value 'core.ps.conf'
* Attribute Value: core.ps.conf
* Description: XXX of This document
* Reference: This document
* Notes: None
5.4. Resource Type value 'core.ps.data'
* Attribute Value: core.ps.data
* Description: XXX of This document
* Reference: This document
* Notes: None
6. Acknowledgements
The current version of this document contains a substantial
contribution by Klaus Hartke's proposal
[I-D.hartke-t2trg-coral-pubsub], which defines the topic resource
model and structure as well as the topic lifecycle and interactions.
It also follows a similar architectural design as that provided by
Marco Tiloca's [I-D.ietf-ace-oscore-gm-admin].
The authors would like to also thank Carsten Bormann, Hannes
Tschofenig, Zach Shelby, Mohit Sethi, Peter van der Stok, Tim
Kellogg, Anders Eriksson, Goran Selander, Mikko Majanen, and Olaf
Bergmann for their valuable contributions and reviews.
7. References
7.1. Normative References
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[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/rfc/rfc2119>.
[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/rfc/rfc6570>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/rfc/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/rfc/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/rfc/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/rfc/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/rfc/rfc8516>.
[RFC9167] Sattler, T., Carney, R., and J. Kolker, "Registry
Maintenance Notification for the Extensible Provisioning
Protocol (EPP)", RFC 9167, DOI 10.17487/RFC9167, December
2021, <https://www.rfc-editor.org/rfc/rfc9167>.
7.2. Informative References
[I-D.hartke-t2trg-coral-pubsub]
Hartke, K., "Publish/Subscribe over the Constrained
Application Protocol (CoAP) using the Constrained RESTful
Application Language (CoRAL)", Work in Progress, Internet-
Draft, draft-hartke-t2trg-coral-pubsub-01, 9 May 2020,
<https://datatracker.ietf.org/doc/html/draft-hartke-t2trg-
coral-pubsub-01>.
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[I-D.ietf-ace-oscore-gm-admin]
Tiloca, M., Höglund, R., Van der Stok, P., and F.
Palombini, "Admin Interface for the OSCORE Group Manager",
Work in Progress, Internet-Draft, draft-ietf-ace-oscore-
gm-admin-08, 13 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-ace-
oscore-gm-admin-08>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/rfc/rfc8288>.
Authors' Addresses
Michael Koster
Unaffiliated
Email: michaeljohnkoster@gmail.com
Ari Keranen
Ericsson
Email: ari.keranen@ericsson.com
Jaime Jimenez
Ericsson
Email: jaime@iki.fi
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