This document specifies a method whereby devices or applications can subscribe to events from a Thing, based on sensor data, as defined in XEP-0323: Internet of Things - Sensor Data (XEP-0323) [1]. While XEP-0323 defines a method whereby the device or application can request sensor data information from a Thing when the device or application requires it, this document specifies a method whereby the Thing itself informs the device or application of desired sensor data, when certain conditions have been met, defined by the device or application.
The architecture defined in this document, permits the specification of conditions individually, something that would not be possible, or very difficult to achieve, if a centralized publish/subscribe or multi-cast pattern would have been used. By allowing individual conditions to be specified, devices or applications can be informed of information that best suit them, and when it suits them, instead of having to figure out, from the Thing perspective, a common denominator, sufficiently good for all intended devices or applications. Furthermore, by aligning itself with XEP-0323 and the request/response pattern, the Thing can easily integrate with a provisioning server, as defined in XEP-0324: Internet of Things - Provisioning (XEP-0324) [2], to allow for delegation of trust and allowing detailed provisioning of who is allowed to receive what information. Through the use of attributes defined in XEP-0326: Internet of Things - Concentrators (XEP-0326) [3] subscriptions can be extended to underlying nodes controlled by the Thing as well.
Internet of Things contains many different architectures and use cases. For this reason, the IoT standards have been divided into multiple XEPs according to the following table:
| XEP | Description |
|---|---|
| xep-0000-IoT-BatteryPoweredSensors | Defines how to handle the peculiars related to battery powered devices, and other devices intermittently available on the network. |
| xep-0000-IoT-Events | This specification. Defines how Things send events, how event subscription, hysteresis levels, etc., are configured. |
| xep-0000-IoT-Interoperability | Defines guidelines for how to achieve interoperability in Internet of Things, publishing interoperability interfaces for different types of devices. |
| xep-0000-IoT-Multicast | Defines how sensor data can be multicast in efficient ways. |
| xep-0000-IoT-PubSub | Defines how efficient publication of sensor data can be made in Internet of Things. |
| xep-0000-IoT-Chat | Defines how human-to-machine interfaces should be constructed using chat messages to be user friendly, automatable and consistent with other IoT extensions and possible underlying architecture. |
| XEP-0322 | Defines how to EXI can be used in XMPP to achieve efficient compression of data. Albeit not an Internet of Things specific XEP, this XEP should be considered in all Internet of Things implementations where memory and packet size is an issue. |
| XEP-0323 | Provides the underlying architecture, basic operations and data structures for sensor data communication over XMPP networks. It includes a hardware abstraction model, removing any technical detail implemented in underlying technologies. This XEP is used by all other Internet of Things XEPs. |
| XEP-0324 | Defines how provisioning, the management of access privileges, etc., can be efficiently and easily implemented. |
| XEP-0325 | Defines how to control actuators and other devices in Internet of Things. |
| XEP-0326 | Defines how to handle architectures containing concentrators or servers handling multiple Things. |
| XEP-0331 | Defines extensions for how color parameters can be handled, based on Data Forms (XEP-0004) [4] |
| XEP-0336 | Defines extensions for how dynamic forms can be created, based on Data Forms (XEP-0004) [4], Data Forms Validation (XEP-0122) [5], Publishing Stream Initiation Requests (XEP-0137) [6] and Data Forms Layout (XEP-0141) [7]. |
| XEP-0347 | Defines the peculiars of Thing discovery in Internet of Things. Apart from discovering Things by JID, it also defines how to discover Things based on location, etc. |
The following table lists common terms and corresponding descriptions.
Subscribing to events is performed much the same way as requesting sensor data from a device, as defined in XEP-0323. Instead of requesting the data using the req element of the urn:xmpp:iot:sensordata namespace, it is done using the subscribe element of the urn:xmpp:iot:events namespace. Much of the syntax of this subscribe element coincides with that of the req element, with some differences, as will be described in this document.
Responses and sensor data is then sent from the device to the subscriber, in the same way as defined in XEP-0323, with one notable difference: The sequence number seqnr provided in the request, is not forgotten once a readout has been completed, but will be reused every time the Thing reports data to the subscriber. The subscriber must use this sequence number to correlate incoming sensor data with original request. Furthermore, the subscriber must not reuse the sequence number in other sensor data requests to the Thing, while the subscription is alive.
A subscription is cancelled by the subscriber sending a request with an unsubscribe element in it to the Thing. The Thing can also automatically remove subscriptions if the corresponding friendship ends. Furthermore, event conditions should only be evaluated if the corresponding subscriber is online. This, to avoid offline messages to pile up. Once a friend comes online, any subscription conditions corresponding to that friend should be evaluated to inform it of the current state of the Thing.
The client that wishes to receive momentary values regularly from the sensor initiates the request using the subscribe element sent to the device. In the following example, a subscription is made to receive momentary values every five minutes (specified by the maxInterval attribute), with no field-specific conditions attached.
<iq type='get'
from='client@clayster.com/amr'
to='device@clayster.com'
id='S0001'>
<subscribe xmlns='urn:xmpp:iot:events' seqnr='1' momentary='true' maxInterval='PT5M' req='true'/>
</iq>
When the device has received and accepted the request, it responds as follows, using the accepted element defined in XEP-0323. Sensor data is later reported in a way similar to XEP-0323, with the exception that the seqnr will be reused every time the event occurs. If the event causes multiple messages to be sent, the receiver knows which one is the last message, by looking at the done attribute. Since the req attribute is set to true, an event will be sent back immediately (i.e. emulating a parallel readout request operation).
<iq type='result'
from='device@clayster.com'
to='client@clayster.com/amr'
id='S0001'>
<accepted xmlns='urn:xmpp:iot:sensordata' seqnr='1'/>
</iq>
If for some reason, the device rejects the subscription request, the communication sequence might look as follows:
<iq type='get'
from='client@clayster.com/amr'
to='device@clayster.com'
id='S0002'>
<subscribe xmlns='urn:xmpp:iot:events' seqnr='1' momentary='true' maxInterval='PT5M'/>
</iq>
<iq type='error'
from='device@clayster.com'
to='client@clayster.com/amr'
id='S0002'>
<error type='cancel'>
<forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<text xmlns='urn:ietf:params:xml:ns:xmpp-stanzas' xml:lang='en'>Access denied.</text>
</error>
</iq>
Depending on the reason for rejecting the request, different XMPP errors can be returned, according to the description in the following table. The table also lists recommended error type for each error. Any custom error message is returned in a text element, as in the example above.
| Error Type | Error Element | Namespace | Description |
|---|---|---|---|
| cancel | forbidden | urn:ietf:params:xml:ns:xmpp-stanzas | If the caller lacks privileges to perform the action. |
| cancel | item-not-found | urn:ietf:params:xml:ns:xmpp-stanzas | If an item or data source could not be found. |
| modify | bad-request | urn:ietf:params:xml:ns:xmpp-stanzas | If the request was malformed. Examples can include trying to subscribe to information from a device behind a concentrator, without including node information. |
To unsubscribe a subscription, send the unsubscribe element in a request to the Thing with the seqnr sequence number corresponding to the subscription. The Thing responds with an empty response to acknowledge the un-subscription, regardless if the subscription existed or not.
<iq type='get'
from='client@clayster.com/amr'
to='device@clayster.com'
id='S0003'>
<unsubscribe xmlns='urn:xmpp:iot:events' seqnr='1'/>
</iq>
<iq type='result'
from='device@clayster.com'
to='client@clayster.com/amr'
id='S0003'/>
The client that wishes to receive information when a field or a set fields change, can do so by specifying amount of change allowed in the corresponding field elements in the subscription. It can also provide optional current values from which the change is measured. If current values are not provided, the Thing uses its corresponding set of current values at the time it receives the request. Current values use the corresponding unit held by the field. To find out, if unsure, you need to make a sensor data readout first.
In the following example, events are sent when light changes by more than 5 units (of a percent), or if a motion detector changes value. Temperature information at the time of the event is also requested. If the conditions are not met within fifteen minutes, an event is raised anyway (as defined using the maxInterval attribute). The minInterval attribute makes sure events are not sent more often than every five seconds.
<iq type='get'
from='client@clayster.com/amr'
to='device@clayster.com'
id='S0004'>
<subscribe xmlns='urn:xmpp:iot:events' seqnr='1' momentary='true' maxInterval='PT15M' minInterval='PT5S'>
<field name='Light' changedBy='5'/>
<field name='Motion' changedBy='1' currentValue='0'/>
<field name='Temperature'/>
</subscribe>
</iq>
Any type of sensor data can be included in an event subscription, including historical data. To limit the amount of historical data to return in an event, the maxAge attribute can be used, instead of the absolute from and to attributes available in the corresponding req element. The following example requests the last five minutes of historical temperature and light data as soon as the motion detector detects motion (change upwards of 1).
<iq type='get'
from='client@clayster.com/amr'
to='device@clayster.com'
id='S0005'>
<subscribe xmlns='urn:xmpp:iot:events' seqnr='1' all='true' minInterval='PT1M' maxAge='PT5M'>
<field name='Temperature'/>
<field name='Light'/>
<field name='Motion' changedUp='1'/>
</subscribe>
</iq>
Any type of sensor data can be included in an event subscription, including historical data. To limit the amount of historical data to return in an event, the maxAge attribute can be used, instead of the absolute from and to attributes available in the corresponding req element. The following example requests the last five minutes of historical temperature and light data as soon as the motion detector detects motion (change upwards of 1).
<iq type='get'
from='client@clayster.com/amr'
to='device@clayster.com'
id='S0006'>
<subscribe xmlns='urn:xmpp:iot:events' seqnr='1' all='true' minInterval='PT1M' maxAge='PT5M'>
<field name='Temperature'/>
<field name='Light'/>
<field name='Motion' changedUp='1'/>
</subscribe>
</iq>
To subscribe to events from one or more nodes behind a concentrator, the nodes have to be specified using node elements in the subscription request. If more than one node is present, the subscription request is assumed to be made to each node individually. When the subscription conditions are met, the corresponding node sends its sensor data back to the subscriber. Note that only the node where the conditions are met include data in the message sent to the subscriber. Event subscriptions are handled individually for each node.
In the following example, a subscription of any changes to a sequence of digital inputs handled by a PLC is made.
<iq type='get'
from='client@clayster.com/amr'
to='plc@clayster.com'
id='S0007'>
<subscribe xmlns='urn:xmpp:iot:events' seqnr='1' momentary='true' minInterval='PT5S' maxInternval='PT1H'>
<node nodeId='DI1'/>
<node nodeId='DI2'/>
<node nodeId='DI3'/>
<node nodeId='DI4'/>
<field name='State' changeBy='1' currentValue='0'/>
</subscribe>
</iq>
If an entity supports the protocol specified herein, it MUST advertise that fact by returning a feature of "urn:xmpp:iot:events" in response to Service Discovery (XEP-0030) [8] information requests.
<iq type='get'
from='client@clayster.com/amr'
to='device@clayster.com'
id='disco1'>
<query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
<iq type='result'
from='device@clayster.com'
to='client@clayster.com/amr'
id='disco1'>
<query xmlns='http://jabber.org/protocol/disco#info'>
...
<feature var='urn:xmpp:iot:events'/>
...
</query>
</iq>
In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in Entity Capabilities (XEP-0115) [9]. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.
An integral part of conditions, is the measurement of changes of magnitudes of field values. Three attributes available on the field element, can be used to define how to measure changes. The changedUp attribute provides a numerical value limiting the amount of upwards change in the field before an event is triggered. The changedDown attributes provides a similar means to control the amount of downwards change in the field, before an event is triggered. If only one of these are provided, only changes in that direction trigger events. If the changedBy attribute is provided, the other two attributes are ignored, and assumed to be of the same value as the changedBy attribute value. All changes are specified using positive numbers.
The following table shows how magnitudes are calculated for field values of different types:
| Type | Magnitude |
|---|---|
| boolean | Magnitude is 0 if false, and 1 if true. |
| date | Number of seconds since an implementation specific point in time. Since only changes are measured, actual point in time is not important. |
| dateTime | Number of seconds since an implementation specific point in time. Since only changes are measured, actual point in time is not important. |
| duration | Corresponding number of seconds. |
| enum | Any change to this value, corresponds to a change by 1. If direction of change is important, values are ordered by lexical order. |
| int | The value itself. |
| long | The value itself. |
| numeric | The value itself, without unit. The magnitude specified in any change attributes is assumed to be specified using the same unit as the corresponding field. |
| string | Any change to this value, corresponds to a change by 1. If direction of change is important, values are ordered by lexical order. |
| time | Corresponding number of seconds. |
Event conditions should be checked, if the following conditions are true for the corresponding event:
If a friendship is lost, i.e. presence information from the corresponding contact is unsubscribed, the Thing has to automatically remove any event subscriptions available for that contact. If a new friendship is created with the same contact at a later stage, the contact has to request new subscriptions.
It is up to the thing, if it wants to persist event subscriptions or not. If not persisting event subscriptions, they will automatically be lost if the Thing rests or restarts for some reason.
If the subscriber considers an event subscription important, and wants to make sure a subscription survives any resets of the thing, the maxInterval attribute should be used when requesting the subscription. If an event is not delivered within this time and a margin, the subscriber can assume the subscription has been lost, and a new subscription can be made.
A subscriber can only have one active subscription per node on a Thing. If a Thing does not support nodes, each subscriber can only have one active subscription per corresponding Thing. This means, that a new event subscription automatically overrides (or unsubscribes) any existing subscription on the corresponding node or Thing. This in turn makes it easier for subscribers to handle restarts and avoids multiplicity problems due to lack of synchronization between subscriber and Thing.
If a Thing is connected to a provisioning server, as defined in XEP-324: Internet of Things - Provisioning , it should re-evaluate event subscriptions contents if a clearCache command is received from the provisioning server. To be able to do this properly, the contents of the original event subscription should be remembered. Re-evaluating active event subscriptions after a clearCache command enables the owner of the Thing to modify current access rights in the Thing in real-time, including contents send in events.
Existing event subscriptions that are no longer permitted by the new set of rules in the provisioning server, can be removed from memory, without informing the subscriber about the fact. The subscriber will be informed when re-subscribing, that access rights have been lost.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [10].
The protocol schema needs to be added to the list of XMPP protocol schemas.
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:xmpp:iot:events'
xmlns='urn:xmpp:iot:events'
xmlns:sd='urn:xmpp:iot:sensordata'
elementFormDefault='qualified'>
<xs:import namespace='urn:xmpp:iot:sensordata'/>
<xs:element name='subscribe'>
<xs:complexType>
<xs:choice minOccurs='0' maxOccurs='unbounded'>
<xs:element name='node'>
<xs:complexType>
<xs:attribute name='nodeId' type='xs:string' use='required'/>
<xs:attribute name='sourceId' type='xs:string' use='optional'/>
<xs:attribute name='cacheType' type='xs:string' use='optional'/>
</xs:complexType>
</xs:element>
<xs:element name='field'>
<xs:complexType>
<xs:attribute name='name' type='xs:string' use='required'/>
<xs:attribute name='currentValue' type='xs:double' use='optional'/>
<xs:attribute name='changedBy' type='PositiveDouble' use='optional'/>
<xs:attribute name='changedUp' type='PositiveDouble' use='optional'/>
<xs:attribute name='changedDown' type='PositiveDouble' use='optional'/>
</xs:complexType>
</xs:element>
</xs:choice>
<xs:attribute name='seqnr' type='xs:int' use='required'/>
<xs:attributeGroup ref='sd:fieldTypes'/>
<xs:attribute name='all' type='xs:boolean' use='optional' default='false'/>
<xs:attribute name='historical' type='xs:boolean' use='optional' default='false'/>
<xs:attribute name='maxAge' type='xs:duration' use='optional'/>
<xs:attribute name='minInterval' type='xs:duration' use='optional'/>
<xs:attribute name='maxInterval' type='xs:duration' use='optional'/>
<xs:attribute name='serviceToken' type='xs:string' use='optional'/>
<xs:attribute name='deviceToken' type='xs:string' use='optional'/>
<xs:attribute name='userToken' type='xs:string' use='optional'/>
<xs:attribute name='req' type='xs:boolean' use='optional' default='false'/>
</xs:complexType>
</xs:element>
<xs:element name='unsubscribe'>
<xs:complexType>
<xs:attribute name='seqnr' type='xs:int' use='required'/>
</xs:complexType>
</xs:element>
<xs:simpleType name='PositiveDouble'>
<xs:restriction base='xs:double'>
<xs:minExclusive value='0'/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
For more information, please see the following resources:
The Sensor Network section of the XMPP Wiki contains further information about the use of the sensor network XEPs, links to implementations, discussions, etc.
The XEP's and related projects are also available on github, thanks to Joachim Lindborg.
A presentation giving an overview of all extensions related to Internet of Things can be found here: http://prezi.com/esosntqhewhs/iot-xmpp/.
This document in other formats: XML PDF
This XMPP Extension Protocol is copyright © 1999 – 2024 by the XMPP Standards Foundation (XSF).
Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.
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This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.
Discussion on other xmpp.org discussion lists might also be appropriate; see <https://xmpp.org/community/> for a complete list.
Errata can be sent to <editor@xmpp.org>.
The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
1. XEP-0323: Internet of Things - Sensor Data <https://xmpp.org/extensions/xep-0323.html>.
2. XEP-0324: Internet of Things - Provisioning <https://xmpp.org/extensions/xep-0324.html>.
3. XEP-0326: Internet of Things - Concentrators <https://xmpp.org/extensions/xep-0326.html>.
4. XEP-0004: Data Forms <https://xmpp.org/extensions/xep-0004.html>.
5. XEP-0122: Data Forms Validation <https://xmpp.org/extensions/xep-0122.html>.
6. XEP-0137: Publishing Stream Initiation Requests <https://xmpp.org/extensions/xep-0137.html>.
7. XEP-0141: Data Forms Layout <https://xmpp.org/extensions/xep-0141.html>.
8. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.
9. XEP-0115: Entity Capabilities <https://xmpp.org/extensions/xep-0115.html>.
10. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
Note: Older versions of this specification might be available at https://xmpp.org/extensions/attic/
First draft.
@report{waher2014iot-events,
title = {Internet of Things - Events},
author = {Waher, Peter},
type = {XEP},
number = {xxxx},
version = {0.0.1},
institution = {XMPP Standards Foundation},
url = {https://xmpp.org/extensions/xep-xxxx.html},
date = {2014-08-13/2014-08-13},
}END