How an IoT Edge device can be used as a gateway
Applies to: 
IoT Edge 1.5 IoT Edge 1.4
Important
IoT Edge 1.5 LTS and IoT Edge 1.4 LTS are supported releases. IoT Edge 1.4 LTS is end of life on November 12, 2024. If you are on an earlier release, see Update IoT Edge.
IoT Edge devices can operate as gateways, providing a connection between other devices on the network and IoT Hub.
The IoT Edge hub module acts like IoT Hub, so it can handle connections from other devices that have an identity with the same IoT hub. This type of gateway pattern is called transparent because messages can pass from downstream devices to IoT Hub as though there were not a gateway between them.
For devices that don't or can't connect to IoT Hub on their own, IoT Edge gateways can provide that connection. This type of gateway pattern is called translation because the IoT Edge device has to perform processing on incoming downstream device messages before they can be forwarded to IoT Hub. These scenarios require additional modules on the IoT Edge gateway to handle the processing steps.
The transparent and translation gateway patterns are not mutually exclusive. A single IoT Edge device can function as both a transparent gateway and a translation gateway.
All gateway patterns provide the following benefits:
- Analytics at the edge - Use AI services locally to process data coming from downstream devices without sending full-fidelity telemetry to the cloud. Find and react to insights locally and only send a subset of data to IoT Hub.
- Downstream device isolation - The gateway device can shield all downstream devices from exposure to the internet. It can sit in between an operational technology (OT) network that does not have connectivity and an information technology (IT) network that provides access to the web. Similarly, devices that don't have the capability to connect to IoT Hub on their own can connect to a gateway device instead.
- Connection multiplexing - All devices connecting to IoT Hub through an IoT Edge gateway can use the same underlying connection. This multiplexing capability requires that the IoT Edge gateway uses AMQP as its upstream protocol.
- Traffic smoothing - The IoT Edge device will automatically implement exponential backoff if IoT Hub throttles traffic, while persisting the messages locally. This benefit makes your solution resilient to spikes in traffic.
- Offline support - The gateway device stores messages and twin updates that cannot be delivered to IoT Hub.
Transparent gateways
In the transparent gateway pattern, devices that theoretically could connect to IoT Hub can connect to a gateway device instead. The downstream devices have their own IoT Hub identities and connect using either MQTT or AMQP protocols. The gateway simply passes communications between the devices and IoT Hub. Both the devices and the users interacting with them through IoT Hub are unaware that a gateway is mediating their communications. This lack of awareness means the gateway is considered transparent.
For more information about how the IoT Edge hub manages communication between downstream devices and the cloud, see Understand the Azure IoT Edge runtime and its architecture.
Beginning with version 1.2 of IoT Edge, transparent gateways can handle connections from downstream IoT Edge devices.
Parent and child relationships
You declare transparent gateway relationships in IoT Hub by setting the IoT Edge gateway as the parent of a downstream device child that connects to it.
Note
A downstream device emits data directly to the Internet or to gateway devices (IoT Edge-enabled or not). A child device can be a downstream device or a gateway device in a nested topology.
The parent/child relationship is established at three points in the gateway configuration:
Cloud identities
All devices in a transparent gateway scenario need cloud identities so they can authenticate to IoT Hub. When you create or update a device identity, you can set the device's parent or child devices. This configuration authorizes the parent gateway device to handle authentication for its child devices.
Note
Setting the parent device in IoT Hub used to be an optional step for downstream devices that use symmetric key authentication. However, starting with version 1.1.0 every downstream device must be assigned to a parent device.
You can configure the IoT Edge hub to go back to the previous behavior by setting the environment variable AuthenticationMode to the value CloudAndScope.
Child devices can only have one parent. By default, a parent can have up to 100 children. You can change this limit by setting the MaxConnectedClients environment variable in the parent device's edgeHub module.
IoT Edge devices can be both parents and children in transparent gateway relationships. A hierarchy of multiple IoT Edge devices reporting to each other can be created. The top node of a gateway hierarchy can have up to five generations of children. For example, an IoT Edge device can have five layers of IoT Edge devices linked as children below it. But the IoT Edge device in the fifth generation cannot have any children, IoT Edge or otherwise.
Gateway discovery
A child device needs to be able to find its parent device on the local network. Configure gateway devices with a hostname, either a fully qualified domain name (FQDN) or an IP address, that its child devices use to locate it.
On downstream IoT devices, use the gatewayHostname parameter in the connection string to point to the parent device.
On downstream IoT Edge devices, use the parent_hostname parameter in the config file to point to the parent device.
Secure connection
Parent and child devices also need to authenticate their connections to each other. Each device needs a copy of a shared root CA certificate which the child devices use to verify that they are connecting to the proper gateway.
When multiple IoT Edge gateways connect to each other in a gateway hierarchy, all the devices in the hierarchy should use a single certificate chain.
Device capabilities behind transparent gateways
All IoT Hub primitives that work with IoT Edge's messaging pipeline also support transparent gateway scenarios. Each IoT Edge gateway has store and forward capabilities for messages coming through it.
Use the following table to see how different IoT Hub capabilities are supported for devices compared to devices behind gateways.
| Capability | IoT device | IoT behind a gateway | IoT Edge device | IoT Edge behind a gateway |
|---|---|---|---|---|
| Device-to-cloud (D2C) messages |  |
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| Cloud-to-device (C2D) messages | |
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| Direct methods | |
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| Device twins and Module twins | |
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| File upload | |
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| Container image pulls | |
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| Blob upload | |
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Container images can be downloaded, stored, and delivered from parent devices to child devices.
Blobs, including support bundles and logs, can be uploaded from child devices to parent devices.
Translation gateways
If downstream devices can't connect to IoT Hub, then the IoT Edge gateway needs to act as a translator. Often, this pattern is required for devices that don't support MQTT, AMQP, or HTTP. Since these devices can't connect to IoT Hub, they also can't connect to the IoT Edge hub module without some pre-processing.
Custom or third-party modules that are often specific to the downstream device's hardware or protocol need to be deployed to the IoT Edge gateway. These translation modules take the incoming messages and turn them into a format that can be understood by IoT Hub.
There are two patterns for translation gateways: protocol translation and identity translation.
Protocol translation
In the protocol translation gateway pattern, only the IoT Edge gateway has an identity with IoT Hub. The translation module receives messages from downstream devices, translates them into a supported protocol, and then the IoT Edge device sends the messages on behalf of the downstream devices. All information looks like it is coming from one device, the gateway. Downstream devices must embed additional identifying information in their messages if cloud applications want to analyze the data on a per-device basis. Additionally, IoT Hub primitives like twins and direct methods are only supported for the gateway device, not downstream devices. Gateways in this pattern are considered opaque in contrast to transparent gateways, because they obscure the identities of downstream devices.
Protocol translation supports devices that are resource constrained. Many existing devices are producing data that can power business insights; however they were not designed with cloud connectivity in mind. Opaque gateways allow this data to be unlocked and used in an IoT solution.
Identity translation
The identity translation gateway pattern builds on protocol translation, but the IoT Edge gateway also provides an IoT Hub device identity on behalf of the downstream devices. The translation module is responsible for understanding the protocol used by the downstream devices, providing them identity, and translating their messages into IoT Hub primitives. Downstream devices appear in IoT Hub as first-class devices with twins and methods. A user can interact with the devices in IoT Hub and is unaware of the intermediate gateway device.
Identity translation provides the benefits of protocol translation and additionally allows for full manageability of downstream devices from the cloud. All devices in your IoT solution show up in IoT Hub regardless of the protocol they use.
Device capabilities behind translation gateways
The following table explains how IoT Hub features are extended to downstream devices in both translation gateway patterns.
| Capability | Protocol translation | Identity translation |
|---|---|---|
| Identities stored in the IoT Hub identity registry | Only the identity of the gateway device | Identities of all connected devices |
| Device twin | Only the gateway has a device and module twins | Each connected device has its own device twin |
| Direct methods and cloud-to-device messages | The cloud can only address the gateway device | The cloud can address each connected device individually |
| IoT Hub throttles and quotas | Apply to the gateway device | Apply to each device |
When using the protocol translation pattern, all devices connecting through that gateway share the same cloud-to-device queue, which can contain at most 50 messages. Only use this pattern when few devices are connecting through each field gateway, and their cloud-to-device traffic is low.
The IoT Edge runtime does not include protocol or identity translation capabilities. These patterns require custom or third-party modules that are often specific to the hardware and protocol used. Azure Marketplace contains several protocol translation modules to choose from. For a sample that uses the identity translation pattern, see Azure IoT Edge LoRaWAN Starter Kit.
Next steps
Learn the three steps to set up a transparent gateway: