| Network Working Group | M. Koster |
| Internet-Draft | SmartThings |
| Intended status: Standards Track | A. Keranen |
| Expires: September 14, 2017 | J. Jimenez |
| Ericsson | |
| March 13, 2017 |
Publish-Subscribe Broker for the Constrained Application Protocol (CoAP)
draft-ietf-core-coap-pubsub-01
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.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 14, 2017.
Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.
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The Constrained Application Protocol (CoAP) [RFC7252] supports machine-to-machine communication across networks of constrained devices. CoAP uses a request/response model where clients make requests to servers in order to request actions on resources. Depending on the situation the same device may act either as a server or a client.
One important class of constrained devices includes devices that are intended to run for years from a small battery, or by scavenging energy from their environment. These devices have limited reachability because they spend most of their time in a sleeping state with no network connectivity. Devices may also have limited reachability due to certain middle-boxes, such as Network Address Translators (NATs) or firewalls. Such middle-boxes often prevent connecting to a device from the Internet unless the connection was initiated by the device.
This document specifies the means for nodes with limited reachability to communicate using simple extensions to CoAP. The extensions enable publish-subscribe communication using a broker node that enables store-and-forward messaging between two or more nodes. Furthermore the extensions facilitate many-to-many communication using CoAP.
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in this specification are to be interpreted as described in [RFC2119].
This specification requires readers to be familiar with all the terms and concepts that are discussed in [RFC5988] and [RFC6690]. Readers should also be familiar with the terms and concepts discussed in [RFC7252] and [I-D.ietf-core-resource-directory]. The URI template format [RFC6570] is used to describe the REST interfaces defined in this specification.
This specification makes use of the following additional terminology:
Figure 1 shows the architecture of a CoAP pubsub service. CoAP pubsub Clients interact with a CoAP pubsub Broker through the CoAP pubsub API which is hosted by the Broker. State information is updated between the Clients and the Broker. The CoAP pubsub Broker performs a store-and-forward of state update representations between certain CoAP pubsub Clients. Clients Subscribe to topics upon which representations are Published by other Clients, which are forwarded by the Broker to the subscribing clients. A CoAP pubsub Broker may be used as a REST resource proxy, retaining the last published representation to supply in response to Read requests from Clients.
Clients pubsub Broker
+-------+ |
| CoAP | |
|pubsub |---------|------+
|Client | | | +-------+
+-------+ | +----| CoAP |
| |pubsub |
+-------+ | +----|Broker |
| CoAP | | | +-------+
|pubsub |---------|------+
|Client | |
+-------+ |
Figure 1: CoAP pubsub Architecture
A CoAP pubsub Broker is a CoAP Server that exposes an 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.
A CoAP pubsub Client interacts with a CoAP pubsub Broker using the CoAP pubsub API. Clients initiate interactions with a CoAP pubsub broker. A data source (e.g., sensor clients) can publish state updates to the broker and data sinks (e.g., actuator clients) can read from or subscribe to state updates from the broker. Application clients can make use of both publish and subscribe in order to exchange state updates with data sources and sinks.
The clients and broker use topics to identify a particular resource or object in a publish-subscribe system. Topics are conventionally formed as a hierarchy, e.g. "/sensors/weather/barometer/pressure" or "EP-33543/sen/3303/0/5700". The topics are hosted at the broker and all the clients using the broker share the same namespace for topics. Every CoAP pubsub topic has a link, consisting of a reference path on the broker using URI path [RFC3986] construction and link attributes [RFC6690]. Every topic is associated with zero or more stored representations with a content-format specified in the link. A CoAP pubsub topic value may alternatively be a collection of one or more sub-topics, consisting of links to the sub-topic URIs and indicated by a link-format content-format.
Figure 2 shows an arrangement for using CoAP pubsub in a "brokerless" configuration between peer nodes. Nodes in a brokerless system may act as both broker and client. The Broker interface in a brokerless node may be pre-configured with topics that expose services and resources. Brokerless peer nodes can be mixed with client and broker nodes in a system with full interoperability.
Peer pubsub Peer +-------+ | +-------+ | CoAP | | | CoAP | |pubsub |---------|---------|pubsub | |Client | | |Broker | +-------+ | +-------+ | CoAP | | | CoAP | |pubsub |---------|---------|pubsub | |Broker | | |Client | +-------+ | +-------+
Figure 2: Brokerless pubsub
This section defines the API exposed by a CoAP pubsub Broker to pubsub Clients. The examples throughout this section assume the use of CoAP [RFC7252]. A CoAP pubsub Broker implementing this specification MUST support the DISCOVERY, CREATE, PUBLISH, SUBSCRIBE, UNSUBSCRIBE, READ, and REMOVE operations defined in this section.
CoAP pubsub Clients discover CoAP pubsub Brokers by using CoAP Simple Discovery or through a Resource Directory (RD) [I-D.ietf-core-resource-directory]. A CoAP pubsub Broker SHOULD indicate its presence and availability on a network by exposing a link to its pubsub API at its .well-known/core location [RFC6690]. A CoAP pubsub broker MAY register its pubsub API location with a Resource Directory. Figure 3 shows an example of a client discovering a local pubsub API using CoAP Simple Discovery. A broker wishing to advertise the CoAP pubsub API for Simple Discovery or through a Resource Directory MUST use the link relation rt=core.ps. A broker MAY advertise its supported content formats and other attributes in the link to its pubsub API.
A CoAP pubsub Broker MAY offer a topic discovery API to enable Clients to find topics of interest, either by topic name or by link attributes which may be registered when the topic is created. Figure 4 shows an example of a client looking for a topic with a resource type (rt) of "temperature" using Discover. The client then receives the URI of the resource and its content-format. A pubsub broker wishing to advertize topic discovery MUST use the relation rt=core.ps.discover in the link.
A CoAP pubsub Broker MAY expose the Discover interface through the .well-known/core resource. Links to topics may be exposed at .well-known/core in addition to links to the pubsub API. Figure 5 shows an example of topic discovery through .well-known/core.
The DISCOVER interface is specified as follows:
The following response codes are defined for this interface:
Client Broker | | | ------ GET /.well-known/core?rt=core.ps ---->>| | -- Content-Format: application/link-format ---| | | | <<--- 2.05 Content | | </ps/>;rt=core.ps;rt=core.ps.discover;ct=40 --| | |
Figure 3: Example of DISCOVER pubsub function
Client Broker | | | ---------- GET /ps/?rt="temperature" ------->>| | Content-Format: application/link-format | | | | <<-- 2.05 Content | | </ps/currentTemp>;rt="temperature";ct=50 ---| | |
Figure 4: Example of DISCOVER topic
Client Broker | | | -------- GET /.well-known/core?ct=50 ------->>| | Content-Format: application/link-format | | | | <<-- 2.05 Content | | </ps/currentTemp>;rt="temperature";ct=50 ---| | |
Figure 5: Example of DISCOVER topic
Clients create new topics on the broker using CREATE. A client wishing to create a topic MUST use CoAP POST to the pubsub API with a payload indicating the desired topic. The topic specification sent in the payload MUST use a supported serialization of the CoRE link format [RFC6690]. The target of the link MUST be a URI formatted string. The client MUST indicate the desired content format for publishes to the topic by using the ct (Content Format) link attribute in the link-format payload. The client MAY indicate the lifetime of the topic by including the Max-Age option in the CREATE request.
A Broker MUST return a response code of "2.01 Created" if the topic is created and return the URI path of the created topic via Location-Path options. The broker MUST return the appropriate 4.xx response code indicating the reason for failure if a new topic can not be created. Broker SHOULD remove topics if the Max-Age of the topic is exceeded without any publishes to the topic. Broker SHOULD retain a topic indefinitely if the Max-Age option is elided or is set to zero upon topic creation. The lifetime of a topic MUST be refreshed upon create operations with a target of an existing topic.
Topics may be created as sub-topics of other topics. A client MAY create a topic with a ct (Content Format) link attribute value which describes a supported serialization of the CoRE link format [RFC6690] such as application/link-format (ct=40) or its JSON or CBOR serializations. If a topic is created which describes a link serialization, that topic may then have sub-topics created under it as shown in Figure 7.
The CREATE interface is specified as follows:
The following response codes are defined for this interface:
Figure 6 shows an example of a topic called "topic1" being successfully created.
Client Broker | | | ---------- POST /ps "<topic1>;ct=50" -------->| | | | <---------------- 2.01 Created ---------------| | Location: /ps/topic1 | | |
Figure 6: Example of CREATE topic
Client Broker | | | ------- POST /ps/ "<mainTopic>;ct=40" ------->| | | | <---------------- 2.01 Created ---------------| | Location: /ps/mainTopic/ | | | | --- POST /ps/mainTopic/ "<subTopic>;ct=50" -->| | | | <---------------- 2.01 Created ---------------| | Location: /ps/mainTopic/subTopic | | | | |
Figure 7: Example of CREATE sub-topic
A CoAP pubsub Client uses the PUBLISH interface for updating topics on the broker. A client MAY use the PUT or the POST method to publish state updates to the CoAP pubsub Broker. A client MUST use the content format specified upon creation of a given topic to publish updates to that topic. The broker MUST reject publish operations which do not use the specified content format. A CoAP client publishing on a topic MAY indicate the maximum lifetime of the value by including the Max-Age option in the publish request. The broker MUST return a response code of "2.04 Changed" if the publish is accepted. A Broker MAY return a "4.04 Not Found" if the topic does not exist. A broker MAY return "4.29 Too Many Requests" if simple flow control as described in Section 7 is implemented.
A Broker MUST accept PUBLISH operations using the PUT method. PUBLISH operations using the PUT method replace any stored representation associated with the topic, with the supplied representation. A Broker MAY reject, or delay responses to, PUT requests to a topic while pending resolution of notifications to subscribers from previous PUT requests.
Create on PUBLISH: A Broker MAY accept PUBLISH operations to new topics using the PUT method. If a Broker accepts a PUBLISH using PUT to a topic that does not exist, the Broker MUST create the topic using the information in the PUT operation. The Broker MUST create a topic with the URI-Path of the request, including all of the sub-topics necessary, and create a topic link with the ct attribute set to the content-format of the payload of the PUT request. If topic is created, the Broker MUST return the response "2.01 Created" with the URI of the created topic, including all of the created path segments, returned via the Location-Path option.
A Broker MAY accept PUBLISH operations using the POST method. If a broker accepts PUBLISH using POST it MAY store an ordered list of published representations, with an element of the list for each published representation. A Broker MAY reject, or delay responses to, POST requests if the internal capacity to store representations is exceeded.
A Broker MAY perform garbage collection of stored representations which have been delivered to all subscribers or which have timed out. A Broker MAY retain at least one most recently published representation to return in response to SUBSRCIBE and READ requests.
A Broker MUST make a best-effort attempt to notify all clients subscribed on a particular topic each time it receives a publish on that topic. An example is shown in Figure 10. If a client publishes to a broker with the Max-Age option, the broker MUST include the same value for the Max-Age option in all notifications. A broker MUST use CoAP Notification as described in [RFC7641] to notify subscribed clients.
The PUBLISH interface is specified as follows:
The following response codes are defined for this interface:
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" -------> | | | | | | <--------------- 2.04 Created---------------- | | Location: /ps/exa/mpl/e | | |
Figure 9: Example of CREATE on PUBLISH
CoAP pubsub Clients subscribe to topics on the Broker using CoAP Observe as described in [RFC7641]. A CoAP pubsub Client wishing to Subscribe to a topic on a broker MUST use a CoAP GET with Observe registration. The Broker MAY add the client to a list of observers. The Broker MUST return a response code of "2.05 Content" along with the most recently published value if the topic contains a valid value and the broker can supply the requested content format. The broker MUST reject Subscribe requests on a topic if the content format of the request is not supported by the content format the topic was created with. The broker MAY accept Subscribe requests which specify content formats that the broker can supply as alternate content formats to the content format the topic was registered with. If the topic was published with the Max-Age option, the broker MUST set the Max-Age option in the valid response to the amount of time remaining for the value to be valid since the last publish operation on that topic. The Broker MUST return a response code of "2.04 No Content" if the Max-Age of the previously stored value has expired. The Broker MUST return a response code "4.04 Not Found" if the topic does not exist or has been removed. The Broker MUST return a response code "4.15 Unsupported Content Format" if it can not return the requested content format. If a Broker is unable to accept a new Subscription on a topic, it SHOULD return the appropriate response code without the Observe option as per as per [RFC7641] Section 4.1. There is no explicit maximum lifetime of a Subscription, thus a Broker may remove subscribers at any time. The Broker, upon removing a Subscriber, will transmit the appropriate response code without the Observe option, as per [RFC7641] Section 4.2, to the removed Subscriber.
The SUBSCRIBE interface is specified as follows:
The following response codes are defined for this interface:
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
CoAP pubsub Clients unsubscribe from topics on the Broker using the CoAP Cancel Observation operation. A CoAP pubsub Client wishing to unsubscribe to a topic on a Broker MUST either use CoAP GET with Observe using an Observe parameter of 1 or send a CoAP Reset message in response to a publish, as per [RFC7641].
The UNSUBSCRIBE interface is specified as follows:
The following response codes are defined for this interface:
Figure 11 shows an example of a client unsubscribe using the Observe=1 cancellation method.
Client Broker | | | ----- GET /ps/topic1 Observe:1 Token:XX ----> | | | | <------------- 2.05 Content ----------------- | | |
Figure 11: Example of UNSUBSCRIBE
A CoAP pubsub client wishing to obtain only the most recent published value on a topic MAY use the READ interface. For reading, the client uses the CoAP GET method. The broker MUST accept Read requests on a topic if the content format of the request matches the content format the topic was created with. The broker MAY accept Read requests which specify content formats that the broker can supply as alternate content formats to the content format the topic was registered with. The Broker MUST return a response code of "2.05 Content" along with the most recently published value if the topic contains a valid value and the broker can supply the requested content format. If the topic was published with the Max-Age option, the broker MUST set the Max-Age option in the valid response to the amount of time remaining for the topic to be valid since the last publish. The Broker MUST return a response code of "2.04 No Content" if the Max-Age of the previously stored value has expired. The Broker MUST return a response code "4.04 Not Found" if the topic does not exist or has been removed. The Broker MUST return a response code "4.15 Unsupported Content Format" if the broker can not return the requested content format.
The READ interface is specified as follows:
The following response codes are defined for this interface:
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
A CoAP pubsub Client wishing to remove a topic MAY use the CoAP Delete operation on the URI of the topic. The CoAP pubsub Broker MUST return "2.02 Deleted" if the remove operation is successful. The broker MUST return the appropriate 4.xx response code indicating the reason for failure if the topic can not be removed. When a topic is removed for any reason, the Broker SHOULD return the response code 4.04 Not Found and remove all of the observers from the list of observers as per as per [RFC7641] Section 3.2. If a topic which has sub-topics is removed, then all of its sub-topics MUST be recursively removed.
The REMOVE interface is specified as follows:
The following response codes are defined for this interface:
Figure 13 shows a successful remove of topic1.
Client Broker | | | ------------- DELETE /ps/topic1 ------------> | | | | | | <-------------- 2.02 Deleted ---------------- | | |
Figure 13: Example of REMOVE
A CoAP pubsub Broker may register the base URI of a pubsub API with a Resource Directory. A pubsub Client may use an RD to discover a pubsub Broker.
A CoAP pubsub Client may register links [RFC6690] with a Resource Directory to enable discovery of created pubsub topics. A pubsub Client may use an RD to discover pubsub Topics. A client which registers pubsub Topics with an RD MUST use the context relation (con) [I-D.ietf-core-resource-directory] to indicate that the context of the registered links is the pubsub Broker.
A CoAP pubsub Broker may alternatively register links to its topics to a Resource Directory by triggering the RD to retrieve it's links from .well-known/core. In order to use this method, the links must first be exposed in the .well-known/core of the pubsub broker. See Section 4.1 in this document.
The pubsub broker triggers the RD to retrieve its links by sending a POST with an empty payload to the .well-known/core of the Resource Directory. The RD server will then retrieve the links from the .well-known/core of the pubsub broker and incorporate them into the Resource Directory. See [I-D.ietf-core-resource-directory] for further details.
CoAP pubsub provides a way for client nodes to sleep between operations, conserving energy during idle periods. This is made possible by shifting the server role to the broker, allowing the broker to be always-on and respond to requests from other clients while a particular client is sleeping.
For example, the broker will retain the last state update received from a sleeping client, in order to supply the most recent state update to other clients in response to read and subscribe operations.
Likewise, the broker will retain the last state update received on the topic such that a sleeping client, upon waking, can perform a read operation to the broker to update its own state from the most recent system state update.
Since the broker node has to potentially send a large amount of notification messages for each publish message and it may be serving a large amount of subscribers and publishers simultaneously, the broker may become overwhelmed if it receives many publish messages to popular topics in a short period of time.
If the broker is unable to serve a certain client that is sending publish messages too fast, the broker MUST respond with Response Code 4.29, "Too Many Requests". This Response Code is like HTTP 429 "Too Many Requests" but uses the Max-Age Option in place of the "Retry-After" header field to indicate the number of seconds after which to retry. The broker MAY stop creating notifications from the publish messages from this client and to this topic for the indicated time.
If a client receives the 4.29 Response Code from the broker for a publish message to a topic, it MUST NOT send new publish messages to the broker on the same topic before the time indicated in Max-Age has passed.
CoAP pubsub re-uses CoAP [RFC7252], CoRE Resource Directory [I-D.ietf-core-resource-directory], and Web Linking [RFC5988] and therefore the security considerations of those documents also apply to this specification. Additionally, a CoAP pubsub broker and the clients SHOULD authenticate each other and enforce access control policies. A malicious client could subscribe to data it is not authorized to or mount a denial of service attack against the broker by publishing a large number of resources. The authentication can be performed using the already standardized DTLS offered mechanisms, such as certificates. DTLS also allows communication security to be established to ensure integrity and confidentiality protection of the data exchanged between these relevant parties. Provisioning the necessary credentials, trust anchors and authorization policies is non-trivial and subject of ongoing work.
The use of a CoAP pubsub broker introduces challenges for the use of end-to-end security between for example a client device on a sensor network and a client application running in a cloud-based server infrastructure since brokers terminate the exchange. While running separate DTLS sessions from the client device to the broker and from broker to client application protects confidentially on those paths, the client device does not know whether the commands coming from the broker are actually coming from the client application. Similarly, a client application requesting data does not know whether the data originated on the client device. For scenarios where end-to-end security is desirable the use of application layer security is unavoidable. Application layer security would then provide a guarantee to the client device that any request originated at the client application. Similarly, integrity protected sensor data from a client device will also provide guarantee to the client application that the data originated on the client device itself. The protected data can also be verified by the intermediate broker ensuring that it stores/caches correct request/response and no malicious messages/requests are accepted. The broker would still be able to perform aggregation of data/requests collected.
Depending on the level of trust users and system designers place in the CoAP pubsub broker, the use of end-to-end object security is RECOMMENDED [I-D.selander-ace-object-security].
When only end-to-end encryption is necessary and the CoAP Broker is trusted, Payload Only Protection (Mode:PAYL) could be used. The Publisher would wrap only the payload before sending it to the broker and set the option Content-Format to application/smpayl. Upon receival, the Broker can read the unencrypted CoAP header to forward it to the subscribers.
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.
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.
| [I-D.ietf-core-resource-directory] | Shelby, Z., Koster, M., Bormann, C. and P. Stok, "CoRE Resource Directory", Internet-Draft draft-ietf-core-resource-directory-09, October 2016. |
| [I-D.selander-ace-object-security] | Selander, G., Mattsson, J., Palombini, F. and L. Seitz, "Object Security of CoAP (OSCOAP)", Internet-Draft draft-selander-ace-object-security-06, October 2016. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [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. |
| [RFC6570] | Gregorio, J., Fielding, R., Hadley, M., Nottingham, M. and D. Orchard, "URI Template", RFC 6570, DOI 10.17487/RFC6570, March 2012. |
| [RFC6690] | Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, August 2012. |
| [RFC7252] | Shelby, Z., Hartke, K. and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014. |
| [RFC7641] | Hartke, K., "Observing Resources in the Constrained Application Protocol (CoAP)", RFC 7641, DOI 10.17487/RFC7641, September 2015. |
| [RFC5988] | Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/RFC5988, October 2010. |