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Pull request project-chip#724: [Docs] Add Matter documentation page o…
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| # Matter Intermittently Connected Devices (ICD) | ||
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| With the 1.1 release, Matter introduced the concept of Intermittently Connected Devices (ICD) in the SDK and in the specification. | ||
| An Intermittently Connected Device is the Matter representation of a device that is not always reachable. | ||
| This covers battery-powered devices that disable their underlying hardware when in a low-power mode or devices that can be disconnected from the network, like a phone app. | ||
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| This page focuses on features designed to improve the performance and reliability of battery-powered devices. | ||
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| ## Subscription Maximum Interval | ||
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| The subscription mechanism is used by ecosystems and controllers to receive attribute change updates and liveness checks. | ||
| The maximum interval of a subscription request is what defines the frequency at which a device will send a liveness check if there are no attribute changes. | ||
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| Within the subscription request / response model, a device has the opportunity to decide the maximum interval at which it will send its liveness check (Empty Report Update). | ||
| The device can set a maximum interval within this range: | ||
| ``` | ||
| MinIntervalRequested ≤ MaxInterval ≤ MAX(1h, MaxIntervalRequested) | ||
| ``` | ||
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| #### Table 1 - Subscribe Response Action | ||
| | Action Field | Type | Description | | ||
| |-|-|-| | ||
| | SubscriptionId | uint32 | identifies the subscription | | ||
| | MaxInterval | uint16 | the final maximum interval for the subscription in seconds | | ||
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| ### Maximum Interval Negotiation | ||
| For a device to be able to Negotiate the Maximum Interval when receiving a subscribe request, | ||
| the application first needs to implement the `ApplicationCallback` class from the `ReadHandler.h` header. | ||
| ```cpp | ||
| /* | ||
| * A callback used to interact with the application. | ||
| */ | ||
| class ApplicationCallback | ||
| { | ||
| public: | ||
| virtual ~ApplicationCallback() = default; | ||
| /* | ||
| * Called right after a SubscribeRequest has been parsed and processed. This notifies an interested application | ||
| * of a subscription that is about to be established. It also provides an avenue for altering the parameters of the | ||
| * subscription (specifically, the min/max negotiated intervals) or even outright rejecting the subscription for | ||
| * application-specific reasons. | ||
| * | ||
| * TODO: Need a new IM status code to convey application-rejected subscribes. Currently, a Failure IM status code is sent | ||
| * back to the subscriber, which isn't sufficient. | ||
| * | ||
| * To reject the subscription, a CHIP_ERROR code that is not equivalent to CHIP_NO_ERROR should be returned. | ||
| * | ||
| * More information about the set of paths associated with this subscription can be retrieved by calling the appropriate | ||
| * Get* methods below. | ||
| * | ||
| * aReadHandler: Reference to the ReadHandler associated with the subscription. | ||
| * aSecureSession: A reference to the underlying secure session associated with the subscription. | ||
| * | ||
| */ | ||
| virtual CHIP_ERROR OnSubscriptionRequested(ReadHandler & aReadHandler, Transport::SecureSession & aSecureSession) | ||
| { | ||
| return CHIP_NO_ERROR; | ||
| } | ||
| /* | ||
| * Called after a subscription has been fully established. | ||
| */ | ||
| virtual void OnSubscriptionEstablished(ReadHandler & aReadHandler){}; | ||
| /* | ||
| * Called right before a subscription is about to get terminated. This is only called on subscriptions that were terminated | ||
| * after they had been fully established (and therefore had called OnSubscriptionEstablished). | ||
| * OnSubscriptionEstablishment(). | ||
| */ | ||
| virtual void OnSubscriptionTerminated(ReadHandler & aReadHandler){}; | ||
| }; | ||
| ``` | ||
| The second step is registering the callback object to the Interaction Model Engine. | ||
| ```cpp | ||
| // Register ICD subscription callback to match subscription max intervals to its idle time interval | ||
| chip::app::InteractionModelEngine::GetInstance()->RegisterReadHandlerAppCallback(&mICDSubscriptionHandler); | ||
| ``` | ||
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| The `ICDSubscriptionCallback` class in `examples/platform/silabs/ICDSubscriptionCallback.cpp` gives an example implementation of the Maximum Interval negotiation a device can do when receiving a Subscription Request. | ||
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| ```cpp | ||
| CHIP_ERROR ICDSubscriptionCallback::OnSubscriptionRequested(chip::app::ReadHandler & aReadHandler, | ||
| chip::Transport::SecureSession & aSecureSession) | ||
| { | ||
| using namespace chip::System::Clock; | ||
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| Seconds32 interval_s32 = std::chrono::duration_cast<Seconds32>(CHIP_DEVICE_CONFIG_SED_IDLE_INTERVAL); | ||
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| if (interval_s32 > Seconds16::max()) | ||
| { | ||
| interval_s32 = Seconds16::max(); | ||
| } | ||
| uint32_t decidedMaxInterval = interval_s32.count(); | ||
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| uint16_t requestedMinInterval = 0; | ||
| uint16_t requestedMaxInterval = 0; | ||
| aReadHandler.GetReportingIntervals(requestedMinInterval, requestedMaxInterval); | ||
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| // If requestedMinInterval is greater than IdleTimeInterval, select next wake up time as max interval | ||
| if (requestedMinInterval > decidedMaxInterval) | ||
| { | ||
| uint16_t ratio = requestedMinInterval / decidedMaxInterval; | ||
| if (requestedMinInterval % decidedMaxInterval) | ||
| { | ||
| ratio++; | ||
| } | ||
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| decidedMaxInterval *= ratio; | ||
| } | ||
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| // Verify that decidedMaxInterval is an acceptable value | ||
| if (decidedMaxInterval > Seconds16::max().count()) | ||
| { | ||
| decidedMaxInterval = Seconds16::max().count(); | ||
| } | ||
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| // Verify that the decidedMaxInterval respects MAX(SUBSCRIPTION_MAX_INTERVAL_PUBLISHER_LIMIT, MaxIntervalCeiling) | ||
| uint16_t maximumMaxInterval = std::max(kSubscriptionMaxIntervalPublisherLimit, requestedMaxInterval); | ||
| if (decidedMaxInterval > maximumMaxInterval) | ||
| { | ||
| decidedMaxInterval = maximumMaxInterval; | ||
| } | ||
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| return aReadHandler.SetReportingIntervals(decidedMaxInterval); | ||
| } | ||
| ``` | ||
| The Silabs implementation is enabled by default when building an SED application. | ||
| ```bash | ||
| # Use default handler to negotiate subscription max interval | ||
| chip_config_use_icd_subscription_callbacks = enable_sleepy_device | ||
| ``` | ||
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| To disable the feature when building a sleepy end device (SED) application, this argument needs to be added to the build command. | ||
| ```bash | ||
| chip_config_use_icd_subscription_callbacks = false | ||
| ``` | ||
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| ## Persistent Subscriptions | ||
| Persistent subscriptions were added to Matter as a means to ensure that an ICD can re-establish its subscription and by extension its secure session to a subscriber in the event of a power cycle. | ||
| When a device accepts a subscription request, it will persist the subscription. | ||
| When the device reboots, it will try to re-establish its subscription with the subscriber. | ||
| If the subscription is torn down during normal operations or if the re-establishement fails, | ||
| the subscription will be deleted. | ||
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| Persistent subscriptions are enable by default when enabling an SED example. | ||
| ```bash | ||
| # Use persistent subscriptions for ICDs | ||
| chip_persist_subscriptions = enable_sleepy_device | ||
| ``` | ||
| To disable the feature when building an SED application, this argument needs to be added to the build command. | ||
| ```bash | ||
| chip_persist_subscriptions = false | ||
| ``` |