Added information about how to read sensors from large subsystems.
-Added friend recommendation message.
-Added client/device/service tokens.
-Added use cases for service access rights and corresponding user privileges.
-First draft.
-- This XEP provides the underlying architecture, basic operations and data structures for provisioning of services, access rights and user privileges in sensor networks. -
-- Note has to be taken, that these XEP's are designed for implementation in sensors, many of which have very limited amount of memory (both RAM and ROM) or resources (processing power). - Therefore, simplicity is of utmost importance. Furthermore, sensor networks can become huge, easily containing millions of devices in peer-to-peer networks. -
-- An added complexity in the provisioning case is that sensors often have very limited user interface options. Therefore, this document explains how provisioning can be done - efficiently using a trusted third party with more power and options when it comes to user interface design and storage. -
-
- This XEP relies heavily on xep-0000-SN-SensorData
| XEP | -Description | -
|---|---|
| XEP-0000-DynamicForms | -Defines extensions for how dynamic forms can be created, based on &xep0004;, &xep0122;, &xep0137; and &xep0141;. | -
| XEP-0000-Exi | -- Defines how to EXI can be used in XMPP to achieve efficient compression of data. Albeit not a sensor network specific XEP, this XEP should be considered - in all sensor network implementations where memory and packet size is an issue. - | -
| xep-0000-SN-BatteryPoweredSensors | -Defines how to handle the peculiars related to battery powered devices, and other devices intermittently available on the network. | -
| xep-0000-SN-Concentrators | -Defines how to handle architectures containing concentrators or servers handling multiple sensors. | -
| xep-0000-SN-Control | -Defines how to control actuators and other devices in sensor networks. | -
| xep-0000-SN-Discovery | -Defines the peculiars of sensor discovery in sensor networks. Apart from discovering sensors by JID, it also defines how to discover sensors based on location, etc. | -
| xep-0000-SN-Events | -Defines how sensors send events, how event subscription, hysteresis levels, etc., are configured. | -
| xep-0000-SN-Interoperability | -Defines guidelines for how to achieve interoperability in sensor networks, publishing interoperability interfaces for different types of devices. | -
| xep-0000-SN-Multicast | -Defines how sensor data can be multicast in efficient ways. | -
| xep-0000-SN-Provisioning | -This specification. Defines how provisioning, the management of access privileges, etc., can be efficiently and easily implemented. | -
| xep-0000-SN-PubSub | -Defines how efficient publication of sensor data can be made in sensor networks. | -
| xep-0000-SN-SensorData | -- 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 sensor network XEPs. - | -
The following table lists common terms and corresponding descriptions.
-| Term | -Description | -
|---|---|
| Actuator | -Device containing at least one configurable property or output that can and should be controlled by some other entity or device. | -
| Computed Value | -A value that is computed instead of measured. | -
| Concentrator | -Device managing a set of devices which it publishes on the XMPP network. | -
| Field | -- One item of sensor data. Contains information about: Node, Field Name, Value, Precision, Unit, Value Type, Status, Timestamp, Localization information, etc. - Fields should be unique within the triple (Node ID, Field Name, Timestamp). - | -
| Field Name | -Name of a field of sensor data. Examples: Energy, Volume, Flow, Power, etc. | -
| Field Type | -What type of value the field represents. Examples: Momentary Value, Status Value, Identification Value, Calculated Value, Peak Value, Historical Value, etc. | -
| Historical Value | -A value stored in memory from a previous timestamp. | -
| Identification Value | -A value that can be used for identification. (Serial numbers, meter IDs, locations, names, etc.) | -
| Localization information | -Optional information for a field, allowing the sensor to control how the information should be presented to human viewers. | -
| Meter | -A device possible containing multiple sensors, used in metering applications. Examples: Electricity meter, Water Meter, Heat Meter, Cooling Meter, etc. | -
| Momentary Value | -A momentary value represents a value measured at the time of the read-out. | -
| Node | -- Graphs contain nodes and edges between nodes. In Sensor Networks, sensors, actuators, meters, devices, gateways, etc., are often depicted as nodes and links between sensors (friendships) - are depicted as edges. In abstract terms, it's easier to talk about a Node, than have to list different types of nodes possible (sensors, actuators, meters, devices, gateways, etc.). - Each Node has a Node ID. - | -
| Node ID | -- An ID uniquely identifying a node within its corresponding context. If a globally unique ID is desired, an architecture should be used using a universally accepted - ID scheme. - | -
| Parameter | -- Readable and/or writable property on a node/device. The XEP xep-0000-SN-Concentrators deals with reading and writing parameters - on nodes/devices. Fields are not parameters, and parameters are not fields. - | -
| Peak Value | -A maximum or minimum value during a given period. | -
| Precision | -- In physics, precision determines the number of digits of precision. In sensor networks however, this definition is not easily applicable. Instead, precision - determines, for example, the number of decimals of precision, or power of precision. Example: 123.200 MWh contains 3 decimals of precision. All entities parsing and - delivering field information in sensor networks should always retain the number of decimals in a message. - | -
| Sensor | -- Device measuring at least one digital value (0 or 1) or analog value (value with precision and physical unit). Examples: Temperature sensor, pressure sensor, etc. - Sensor values are reported as fields during read-out. Each sensor has a unique Node ID. - | -
| SN | -Sensor Network. A network consisting, but not limited to sensors, where transport and use of sensor data is of primary concern. A sensor network may contain actuators, network applications, monitors, services, etc. | -
| Status Value | -A value displaying status information about something. | -
| Timestamp | -Timestamp of value, when the value was sampled or recorded. | -
| Token | -- A client, device or user can get a token from a provisioning server. These tokens can be included in requests to other entities in the network, so these entities can validate - access rights with the provisioning server. - | -
| Unit | -Physical unit of value. Example: MWh, l/s, etc. | -
| Value | -A field value. | -
| Value Status | -Status of field value. Contains important status information for Quality of Service purposes. Examples: Ok, Error, Warning, Time Shifted, Missing, Signed, etc. | -
| Value Type | -Can be numeric, string, boolean, Date & Time, Time Span or Enumeration. | -
| WSN | -Wireless Sensor Network, a sensor network including wireless devices. | -
| XMPP Client | -Application connected to an XMPP network, having a JID. Note that sensors, as well as applications requesting sensor data can be XMPP clients. | -
- The most basic use case in sensor networks is to read out sensor data from a sensor. However, since protecting end-user integrity and system security is vital, access - rights and user privileges have to be imposed on the network. -
-- To store access rights in all sensors might be very impractical. Not only does it consume memory, it's difficult to maintain track of the current system status, make sure - all devices have the latest configuration, distribute changes to the configuration, etc. -
-- Furthermore, most sensors and small devices have very limited possibility to provide a rich user interface. Perhaps all it can do is to provide a small LED and a button, - useful perhaps for installing the sensor in the network, but not much more. -
-- As an added complexity, the sensor network operator might not even have access to the XMPP Servers used, and provisioning needs to lie outside of the XMPP Server domains. -
-- To solve this problem in an efficient manner, an architecture using distributed trusted third parties is proposed. Such third parties would: -
--
-
- Provide a rich user interface and configurable options to end user or back end systems. -
- Control friendships (who can communicate with whom). -
- Control content available for different friends (what can be read by whom). -
- Control operations accessible by different friends (what can be controlled/configured by whom). -
- Provide additional interoperability services to nodes in the network (for instance, unit conversion). -
- Trust is delegated to a provisioning server by a device, simply by befriending the provisioning server and asking it questions and complying with - its answers. -
-- As an illustrative example, following is a short description of how such a trust relationship can be created in a scenario where the sensor only - has a single LED and a single button. -
--
-
- Somebody is installing the sensor, giving it a connection to an XMPP server and a JID, reachable from the provisioning server. -
- The provisioning server is told to create a friendship request to the new sensor. -
- The sensor flashes its LED for a given time (for example: 30 seconds). -
- Viewing the LED, the person installing the sensor presses the button. -
- Receiving the button press within the given time period, accepts the friendship request. Optionally, the device can give user feedback using the LED. -
- The device performs a service discovery of the new friend, having been a manually added friend. -
- If the new friend supports this provisioning extension, further responsibilities are delegated to this device. -
- As the last step the device asks the provisioning server for a token. This device token is later used in calls to other devices and can be used to check access rights. -
- The following diagram shows the general case: -
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- The successful case can also be illustrated in a sequence diagram, as follows: -
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- The following example shows how a device can request a device token from the provisioning server: -
-- The provisioning server must not return tokens that contain white space characters. -
-- Note: In many cases, an address to a provisioning server might be preprogrammed during production of the - device. In these cases, parts of the above procedure may not neccessary. All the client needs to do, if the provisioning server is not available - in the roster of the device, is to send a subscription request to the provisioning server, to alert the server of the existence of the device, - and possibly request a device token. -
-- The following use cases will assume such a trust relationship has been created between the corresponding device and the provisioning server. -
-- The following diagram displays how a friendship request from an external party can be handled, delegating the responsibility to a trusted third party: -
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- 
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- The communication between the XMPP Device and the Provisioning Server could be as follows: -
-- Note: The provisioning server implicitly understands which two JIDs that are to be checked: The first one is the sender of the message, the second one - is the JID available in the jid attribute in the request. -
-- Note 2: Any resource information in the JID must be ignored by the provisioning server. -
-- The following diagram displays a friendship request from an external party being rejected as a result of the trusted third party negating the friendship: -
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- 
-
- The communication between the XMPP Device and the Provisioning Server could be as follows: -
-- If the provisioning server decides that two friends in the network should no longer be friends and communicate with each other, it simply sends a message to - at least one of the friends as follows: -
-
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- The provisioning server should only send such messages to clients that have previously asked the provisioning server if friendship requests should be accepted or not. -
-- Note: The device should only honor such messages, if the sender is the trusted third party. Such messages received from other entities not trusted should - be silently ignored. -
- -- The provisioning server can, apart from accepting new friendships and rejecting old friendships, also recommend new friendships. In this case, the provisioning server - simply sends a message to one or both of the soon to be friends, as follows: -
-
- 
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- Note that the receptor can still ask the provisioning server if it can form a friendship with the suggested friend, using the isFriend command. -
-- An important use case for provisioning in sensor networks is who gets to read out sensor data from which sensors. This use case details how communication with a - provisioning server can help the device determine if a client has sufficient access rights to read the values of the device. -
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- 
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- Note: This use case is an extension of the use case 'Read-out rejected' in the XEP xep-0000-SN-SensorData. -
-- The following example shows the communication first between the client and the device, then the device and the provisioning server, and last between the device and the client: -
-- Note that the provisioning server responds with a canReadResponse element, with the same content as the canRead element in the request. -
-- In case the device handles multiple nodes that can be read, the provisioning server has the possibility to grant read-out, but to limit the nodes that can be read out. - The provisioning server does this by returning the list of nodes that can be read. -
-
- 
-
- Note: This use case is an extension of the use case 'Read-out of multiple devices' in the XEP xep-0000-SN-SensorData. -
-- Note 2: If the server responds, but without specifying a list of nodes, the device can assume that all nodes available in the original request are allowed - to be read. If no nodes in the request are allowed to be read, the provisioning server must respond with a result='false', so the device can reject the read-out request. -
-- The following example shows the communication first between the client and the device, then the device and the provisioning server, and last between the device and the client: -
- -- Note that the provisioning server responds with a canReadResponse element, similar to the canRead element in the request, except - only the nodes allowed to be read are read. The device must only permit read-out of nodes listed in the response from the provisioning server. Other nodes available - in the request should be ignored. -
-- In case the provisioning server wants to limit the fields a device can send to a client, the provisioning server has the possibility to grant read-out, but - list a set of fields the device is allowed to send to the corresponding client. -
-
-
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- Note: If the server responds, but without specifying a list of field names, the device can assume that all fields available in the original request are allowed - to be sent. If no fields in the request are allowed to be sent, the provisioning server must respond with a result='false', so the device can reject the read-out request. -
-- The following example shows the communication first between the client and the device, then the device and the provisioning server, and last between the device and the client: -
- -- Note that the provisioning server responds with a canReadResponse element, similar to the canRead element in the request, except only - the fields allowed to be sent are listed. The client must only send fields having field names in this list. -
-- Also note, that the provisioning server can return both lists of allowed nodes and allowed field names in the response. In this case, the device must only send allowed fields - from allowed nodes, and ignore all other fields and/or nodes. -
-- When the provisioning server updates access rights and user privileges in the system, it will send a clearCache command to corresponding devices. - If a device was offline during the change, the provisioning server must send the clearCache message when the device comes online again. To acknowledge - the receipt of the command, the client responds with a clearCacheResponse element. This response message does not contain any information on what was - done by the client. It simply acknowledges the receipt of the command, to make sure the provisioning server does not resend the clear cache command again. -
-- Note: The clearCache command does not include information on what has been changed, so the device needs to clear the entire cache. This - to avoid complexities in making sure updates made to the provisioning rules works in all cases, and to minimize complexity in the implementation of the protocol on the sensor side. - It is also not deemed to decrease network performance, since changing provisioning rules for a device is an exceptional event and therefore does not affect performance during - normal operation. -
- -- A service requesting provisioning assistance, needs to retrieve a service token from the provisioning server. The following example shows how this can be done. -
-- Provisioning in sensor networks also requires control of user access to different services in the network. This use case shows how service access rights are controlled using - a trusted provisioning server. -
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- 
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- First, the user connects to the service in some way. This can be done using XMPP, HTTP, HTTPS or some other means. The service need to extract some form of identifying - credentials from the user, and provide that to the provisioning server. The provisioning server determines if the client has access rights to the service based on these - credentials, and also provides the service with a userToken that the service can use in further communication with the provisioning server regarding - the user and its privileges. -
-- The following table lists some different types of credentials that the service can extract from the client: -
-| Type | -Protocols | -Description | -
|---|---|---|
| JID | -XMPP | -Allows provisioning to be done on the JID the user has. | -
| IP Address | -HTTP, HTTPS | -Allows provisioning to be done on IP address or IP-ranges for instance. | -
| Host Name | -HTTP, HTTPS with DNS | -If the service has access to the client host name, for instance in an intra network, this can be used for provisioning. | -
| X.509 Certificate | -HTTPS | -If the client provides a client certificate, such a certificate can be used to provide provisioning. | -
| X.509 Certificate Thumbprint | -HTTPS | -The client can also choose to validate the certificate itself and only send the certificate thumbprint to the provisioning server. Even though this - provides somewhat lesser security than providing the entire certificate, it might be an option to distribute certificate validation checks across the network - to lower the work load on the provisioning server. | -
| User Name | -HTTP, HTTPS, XMPP | -If authenticated HTTP(S) access is implemented, the service can provide the user name as credentials. XMPP based clients can use - information in the roster to provide user name information to the provisioning server. | -
| Geolocation | -HTML5 over HTTP(S), XMPP | -If the geographic location (longitude & latitude) of the user client is known, it can be used. HTML 5 provides mechanism whereby the location - of the client can be fetched. &xep0080; provides a mechanism whereby client location can be obtained over XMPP. | -
| SSO Token | -Intranet | -If a single sign on token is available, such a token could be provided as credentials. | -
| Protocol | -Any | -Connection protocol used to connect to the service. | -
- The provisioning server receives these credentials, and decides if the user should have access to the service or not, based on rules configured in the provisioning service. - If the user is granted access, a userToken is generated and returned to the service. This token -
-- Now, the service can determine if this access grant is sufficient or not. It can require the user to login into the service first. If so, the service should provide the provisioning - server with the user name used during login, when logged in. -
-- When a user has been given access to a service, and properly been identified, the service can ask the provisioning service for detailed user privileges to control - different aspects of the service. This can be done using the hasPrivilege command. Here, the service sends its serviceToken and - the userToken earlier received when being granted access to the service. Furthermore a privilegeId has to be provided. -
-- A Privilege ID is a string composed of one or a sequence of parts, delimited by period characters. The Privilege IDs form a tree of privileges, - using an invisible, but common, root privilege. -
-- The following table suggests some examples of Privilge IDs, with suggestive descriptions. (Only used as an example.) -
-| Privilege ID | -Description | -
|---|---|
| Databases.Energy.Select | -Gives the user the rights to select data from the Energy database. | -
| Databases.Energy.Insert | -Gives the user the rights to insert data into the Energy database. | -
| Databases.Energy.Delete | -Gives the user the rights to delete data from the Energy database. | -
- Note: Note that privilege IDs are local to the service. Different services are allowed to use similar or same Privilege IDs, in different contexts and - with different meanings. The provisioning server must separate Privilege IDs from different services. -
-- The client must always provide full Privilege IDs to the provisioning server. The provisioning server however, can grant partial privilege IDs, pointing to parent - privilege nodes to a user or a role object, granting a user a specific role. If granting a parent privilege ID to a user or role, this is interpreted as - giving the corresponding user or role the privileges of the entire sub-tree defined by the parent privilege ID. -
-- If additional control over privileges is desired, negative privileges can be assigned to the user or role. Granted or explicitly rejected privileges are specified in a - sequential list of full or partial privilege IDs. This list is then processed sequentially to determine if a privilege is granted or not. -
-- Example: Consider the privileges above. Give a user or its corresponding role the following privileges in a sequential list: -
--
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- Exclude: Databases.Energy.Delete -
- Include: Databases.Energy -
- This would give the user rights to select and insert data, but not to delete data from the Energy database. -
-- Note: Care should be taken when constructing Privilege IDs, so they do not include variable data that potentially can create an infinite amount of - privilege IDs. For example: Do not include user names, sensor IDs, etc., in privilege IDs, as the number of such entities cannot be estimated or is scalable beforehand. -
-- The following diagram shows an example of how a service asks permission from a provisioning server before an action is taken: -
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- To improve performance, services can download the entire set of user privileges, and perform privilege checks internally. The following diagram displays - how the above two use cases could be handled in such a case: -
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- Note: When downloading privileges using this command, a sequential list of full or partial privilege IDs will be returned together with the - corresponding include or exclude flags. The above mentioned algorithm of determining user privileges must be implemented by the service, if this method is - to be used. -
-- The isFriendResponse element returned by the provisioning server contains an attribute secondaryTrustAllowed that is by default - set to false. If the provisioning server has no problem with allowing multiple trust to be delegated by devices in the network, it can choose to set this - attribute to true in the response. If true, the device knows it has the right to add its own friends, or to add secondary trust relationships. -
-- The following diagram continues with the example given above, of how a sensor with a limited user interface, can allow to manually add new friends, including new - trust relationships using a single LED and a button. -
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- When multiple trust is used, the entity (client, user, service, etc.) has one token from each provisioning server. However, when sending a token to a third party, - the sender does not know what provisioning server(s) the third party uses to check access rights and user privileges. Therefore, the client must send all tokens, separated - by a space. -
-- When a provisioning server receives a request containing multiple tokens, the most forgiving response must be returned. -
-If an entity supports the protocol specified herein, it MUST advertise that fact by returning a feature of "urn:xmpp:sn:provisioning" in response to &xep0030; information requests.
-- 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 &xep0115;. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead. -
-- To minimize network traffic, and optimize response time, devices should cache access rights and user privileges provided by the provisioning server. If memory is limited, - items in the cache should be ordered by last access, and items with the oldest last access timestamp should be removed first. A safety valve can optionally be implemented as well, - removing unused cache items after a certain age, even if memory is available. -
-- The device can assume that access rights and user privileges on the provisioning server do not change over time, unless the provisioning server says so. -
-- The provisioning server on the other hand, must keep track of when a device is online and offline, and clear the cache of the device if changes are made that affects the device. - If the device was offline when those changes occurred, the provisioning server must send the clear cache command as soon as the device comes online again. -
-- When creating a new trust relationship, the device should always clear its cache, if it contains information from before. -
-- To minimize stress of the provisioning server during synchronous sensor start up, for instance after a power failure, all clients should aim to persist its cache if possible. Clients - not persisting its cache may produce too much stress on the provisioning server on start-up, practically removing it from the network. -
-- When working with multiple provisioning servers, there are some things that should be considered: -
--
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- - When a device requests information from the provisioning servers, this can be done in a parallel fashion. Even though the provisioning servers - were added in a sequential fashion, there is no order or priority between the servers. - -
- - When receiving different responses from different servers, with regards to access rights, privileges, relationships, etc., - the union of rights, or the most forgiving or most accepting response should be used. If one of the servers grant a privilege, the privilege is assumed - to exist. If one of the servers grant a friendship request, the friendship request should be granted, etc. Most probably, different servers will manage - different subsets of entities on the network, and when they receive questions about unrecognized devices, they will simply deny access. - -
- - If a provisioning server requires total control of the network, it should not allow secondary trust relationships. However, if such a server enters a network - and there exist previous provisioning servers (i.e. they accept secondary trust relationships), more trust relationships are assumed to be acceptable. - -
- - Even though examples in this document only list a secondary trust relationship, there is no such limit. There may exist many different trust relationships in a network - or for a given device. - -
- One important design consideration when implementing a provisioning server is how to handle new services, users and privileges. One option might be to automatically - ignore anything not recognized. Another option might be to dynamically add new services, user names and privileges to internal data sources, making it easier to manage - new types of services dynamically. However, adding such items automatically might also make such data sources grow beyond control. -
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- All examples in this document have been simplified examples where a few devices containing a few fields have been read. However, in many cases large subsystems with
- very many sensors containing many fields have to be read, as is documented in xep-0000-SN-Concentrators.html
-
- Note: For cases where the nodeId is sufficient to uniquely identify the node, it is sufficient to provide this attribute in the request. - If there is ambiguity in the request, the receptor must treat the request as a request with a set of nodes, all with the corresponding nodeId as requested. -
-- A small note regarding the use of different tokens. A service can get a Service Token, a device a Device Token and a user - a User Token. When delegating these tokens to third parties, a service sends its Service Token. But, if the service does this - within the context of a user action, the service sends both its Service Token and the users User Token. The same with a device. - If a device delegates its token to a third party, it sends its Device Token. But if the device performs the action in the context of a user action, - the device sends both its Device Token as well as its User Token. -
-- Delegating trust to a third party may create a weak link in the overall security of a sensor network. Therefore, it's vitally important that the following be adhered to: -
--
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- - Trust should only be delegated in a secure way. Once delegated, it should be able to lock this trust so it cannot be changed without reverting the device to - factory default settings. Such a locked delegation mode can be likened with a production mode, where vital configuration parameters should not be able to be changed. - -
- - If allowing installation using a LED, as above, make sure the LED does not light up for a long time, limiting the window of access where the sensor can be linked to a - provisioning server. - -
- - Provisioning servers should be monitored during operation, since it provides a vital link in the operation of the network. - -
- - Multiple provisioning servers could be allowed, for redundancy or for scalability. This specification does not limit the number of provisioning servers used in a network, - not used by a device. Examples use only one provisioning server for simplicity. - -
- - Generally, take care what kind of provisioning servers you allow in a network. - -
This document requires no interaction with &IANA;.
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- Thanks to Joachim Lindborg and Karin Forsell for all valuable feedback.
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