TI-OpenThread Native OAD

Warning

This is an experimental version of the TI-OpenThread OAD, and is currently experiencing ongoing development. Expect future versions to change.

A common OAD process can be seen as two very distinct phases: notification and transport of new images to a device, and updating into a newly downloaded image. First phase will inherently be different from protocol to protocol, as it will depend on protocol specific details. However, the second phase is protocol invariant and is based on a common architecture across protocols.

TI-OpenThread supports a native OAD process, which is a TI-OpenThread proprietary design and implementation. The Thread specification does not specify any standard process for OAD, which means any implementation of a native OAD process will be proprietary.

BIM is used as the common architecture for handling and updating new images on a device. Since BIM is a protocol invariant architecture, an OAD-enabled Thread Application can be transported over an another protocol that is OAD compatible, such as BLE. Refer to the Boot Image Manager (BIM) Chapter for more details.

For more information on the specifics of BLE OAD, consult the BLE User’s Guide.

System Overview

As detailed in Section OAD Topology Overview, an OAD exchange happens between two devices: an OAD Target and an OAD Distributor. The OAD Target is the device that receives the candidate images over the air, while the OAD Distributor notifies the OAD Target of new, available images and supplies the firmware update.

Note

TI-OpenThread only supports off-chip OAD, and therefore an external flash is required. During off-chip OAD, the candidate image is stored on the external flash and subsequently loaded into the internal flash.

For TI-OpenThread, the Target device is a Thread device which can both be an FTD or an MTD. This also includes SEDs. The Distributor must also be a Thread device connected to the same Thread Network as the Target device. That means the connection between the Distributor and the Target device cannot go off-mesh.

It is assumed that only a single Distributor exists on a Thread Network. However, multiple Distributors can co-exist in the same Thread Network. If a Target device were to receive multiple image notifications from different Distributors, it is up to the Target device to decide which image notification it accepts and rejects.

A candidate image is split into blocks, and each image block is transferred individually in sequence. The size of each block can be configured, with a default value of 128 bytes.

When a candidate image has been downloaded, the Target device does a system reset and BIM performs the actual procedure of updating into that image. Refer to Chapter Boot Image Manager (BIM) for more details on the update procedure.

OAD CoAP

The Constrained Application Protocol (CoAP) is used as the means of communication between the Distributor and the Target device. This includes registering new Target devices, notifying Taget devices of new candidate images, as well as the actual transport of candidate images.

Both the Distributor and the Target device acts as CoAP servers, with both devices exposing a set of CoAP resources denoted by URI. Each resource has a corresponding request and response message pair, for which a CoAP Client will send a request message to a given resource and the CoAP Server will respond with a response message.

Table 6. provides an overview of which CoAP resources are available for which OAD device. Table 7. provides an overview what kind of requests and responses are expected for each CoAP resource.

Ideally, CoAP Block-Wise Transfers would be used for transfer of firmware images, as it provides a minimal way of transferring large data in a block-wise fashion. However, OpenThread does not implement Block-Wise Transfers for its CoAP implementation, and subsequently is not used in the OAD protocol implementation for TI-OpenThread.

Table 6. OAD CoAP resource overview

OAD Device	CoAP URI	Description
Distributor	oad/reg	Used by Target devices to register its ULA to the Distributor. This is only used if the Distributor does not have any other means to acquire the ULA of the Target device.
	oad/img	Used by Target devices to request a specific image block for a specific image.
Target	oad/fwv	Used for the Distributor to acquire information of which software version and BIM version the Target device is currently running.
	oad/ntf	Used to notify Target devices that new images are available at the Distributor.

Table 7. OAD CoAP message overview

CoAP URI	Message	Description
oad/reg	Request	Confirmable PUT request. Payload is the Target ML-EID + Target software version + Target platform type.
	Response	Piggybacked response. No payload.
oad/img	Request	Confirmable GET request. Payload is image ID + block number + total number of blocks.
	Response	Piggybacked response. Payload is image ID + block number + block data.
oad/fwv	Request	Confirmable GET request.
	Response	Piggybacked response. Payload is the current software version and BIM version running on the Target device.
oad/ntf	Request	Confirmable PUT request. Payload is information about the new available image.
	Response	Piggybacked response. Payload is image accepted or rejected.

OAD Protocol

The OAD protocol for TI-OpenThread is a proprietary protocol, and consists of the following steps:

Note

For all the sequence diagrams in this section, messages going between the CoAP Server and Client are messages which are transmitted on the Thread Network. Any required hops between additional Thread devices are omitted.

1. (Optional) Target device registers its Unique Local Address to the Distributor.

   Target devices sends a Register Request message with the oad/reg URI to the multicast address ff03::2. The contents of the request is the ML-EID of the Target device, represented in bytes. On reception of the request, the Distributor stores the ULA of the Target device. This step is necessary if the Distributor does not have knowledge of all End Devices in the Thread Network.

   The Register Request message is sent periodically by the Target device until a Register Response message is received by a Distributor. The Target device keeps sending Register Request messages as keep-alive messages to the Distributor after it has been registered, however, at a much slower period. The period is user configurable.

Figure 49. Target registers its ULA to the Distributor

2. (Optional) Distributor acquires Firmware Version information from Target device.

   Distributor sends a Firmware Version Request message with the oad/fwv URI to the unicast address of a Target device. The response from the Target device contains information about its current software version and BIM version.

   The software version is a four character string which keeps track of the application and stack revision contained in the image. Refer to Software version for more information. The BIM version is a byte, which is used to determine if images are compatible with BIM that is running on the device. Refer to BIM Version for more information.

   The response is on the format "sv:XXXX bv:YY", where XXXX is the software version, and YY is the BIM version byte in string representation. Note the space between the software version and the BIM version.

Figure 50. Distributor retrieves firmware information from Target

3. Distributor notifies Target devices that a new firmware image is available.

   When the Distributor has a new candidate image, it notifies Target devices depending on their Firmware Version information. Distributor sends Image Notification requests with the oad/ntf URI to the unicast addresses of Target devices. Distributor decides when to notify Target devices and in which order.

   The request contains information about the new candidate image, such as software and BIM compability, and firmware image type and size. Target device responds with either an image accept or reject based on the provided information.

Figure 51. Distributor notifies Target of new candidate image

4. Target device starts the download process.

   Only if the candidate image is accepted will the Target device initate the download process. Target device sends Image Block requests with the oad/img URI to the unicast address of the Distributor. The request describes which a block from the image, denoted by a block number.

   Blocks are requested in a sequential order, and the Target device keeps track of what the current block number is. Only when the current block number has been successfully downloaded will the Target device request the next block. Block are stored in external flash when downloaded.

   What is important to note is that the Target device decides when blocks are requested and at which rate. This means the download process is completely driven by the Target device. The Distributor does not put any constraints on Target devices how fast the download must occur. As long as the candidate image is available, the download process can take as long time as the Target device wants.

Figure 52. Target requests a single image block from Distributor

5. On download complete, Target device updates the firmware image.

   When all blocks have been successfully downloaded, the candidate image is now stored in external flash on the Target device. Target device invokes a system reset, and the BIM procedure as described in BIM for Off-Chip OAD is performed.

   In short, the integrity of the image is verified by calculating the CRC of the image. If secure OAD is enabled, the signature of the image is also authenticated. If either the integrity check or the authentication fails, the image is rejected. If not, the image is accepted and marked as the new image to be booted in.

   Target device reset itself and BIM copies the new image to the internal flash. If an error occurs during the copy procedure, BIM switches back to a factory image. Refer to Section BIM for Off-Chip OAD for more details about the BIM procedure.

OAD Pause and Resume

Since the download process of the image is driven by the Target device, it has the ability to explicitly pause and resume download. The key difference between pausing a download and setting the rate between each block request, is that pausing is indefinite until an explicit resume is invoked. Setting the request rate only changes the waiting period until the next block request.

The Target device can also be paused if the device loses network connection or loses power. When the device restores the network connection or regains power, it has the ability to resume the download process and pick up from where it left off.

From the Distributor’s point of view, there is no difference if the Target device is paused or not. The Distributor only responds whenever a block requests is received, which can be sent at any given time.

OAD Abort

Whenever the Target device sends a block request, there always a risk that either the block request never reaches the Distributor, or the response never reaches back to the Target device. Thread takes care of retransmissions at the MAC layer, while CoAP takes care of retransmissions at the transport layer.

If retransmissions are not sufficent, or if the Distributor is simply unavailable, then the Target device will eventually reach a maximum number of retransmissions and the download is aborted. However, the Target device can later try and resume the download if appropriate.

The number of retransmissions CoAP sends for confirmable messages are configurable at the application level, with a default count of four.

Figure 53. shows a sequence diagram of a block request which initially succeeds, but the Distributer suddenly becomes unavailable. The subsequent block request now does not result in any response and the target Device starts sending retransmissions of the block request. After the number of retransmits hits maximum, the Target device aborts the download.

Figure 53. OAD image download abort

OAD Application Usage

An application developer wishing to use the TI-OpenThread OAD solution will not need to worry about implementing the OAD state machine, but can instead just include the OAD module. The OAD module implements the OAD state machine, which is defined by the OAD protocol.

The OAD state machine differs between the Target device and Distributor, and therefore the OAD module will be different for both OAD devices. In addition, the architectural requirements for the Target device and the Distributor are different, which will be highlighted in the subsequent sections.

The size of the image block sent from the Distributor to the Target device can be configured, with a default size of 128 bytes. The define OAD_BLOCK_SIZE configures the block size, and is set independently in both the Target device application and the Distributor application.

Note

The OAD_BLOCK_SIZE define must be set to the same value for both the Target device and Distributor application, since block size negotiation is not supported.

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OAD Target Device Application

A Target device will in most use cases be a single-chip, Thread-only device. In other words, the application layer and OpenThread runs on the same processor.

Note

The OAD module for Target devices only supports single-chip, Thread-only platforms.

The OAD module for the Target device is provided as part of the TI-OpenThread environment, and implements the OAD protocol and the off-chip storage handling. The module is based on the OpenThread CoAP implementation. Figure 54. describes the OAD state machine for the Target device.

Figure 54. OAD Target state diagram

Target Application Interface

The OAD module for the Target device exposes the following API.

/**
 * @brief  Initializes the OAD module on network connection event.
 * @return None
 */
void OAD_open(void);

/**
 * @brief  Close the OAD module.
 * @return None
 */
void OAD_close(void);

/**
 * @brief  Abort firmware download on transmission error.
 * @return None
 */
void OAD_abort(void);

/**
 * @brief  Download resume on service recovery or reboot.
 * @return None
 */
void OAD_resume(void);

/**
 * @brief  Pause OAD module, if valid download in progress.
 * @return None
 */
void OAD_pause(void);

/**
 * @brief  A function to process the OAD events.
 * @return None
 */
void OAD_processQueue(void);

/**
 * @brief A function to process the OAD periodic ctrl events.
 * @return None
 */
void OAD_processCtrlEvents(appEvent_e event)

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OAD Distributor Application

Since the Distributor has to in some way acquire new candidate images (either via other IP network, or direct links with an operator), the most natural architecture would be an NCP. In addition, the Distributor will likely need to store multiple candidate images, which puts higher memory requirements on the device. The application layer would run on a host MCU, while Thread features run on the SoC.

Since the application layer of the Distributor would in most cases run on some host, the CoAP implementation no longer depends on the OpenThread implementation. Rather, any CoAP implementation can be used that is compatible with the host MCU environment.

Figure 55. describes the state machine for the OAD Distributor.

Figure 55. OAD Distributor state diagram

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CoAP Message Layout

The following tables describes the content layout of the various CoAP messages.

Table 8. Register request content

Field	Size (in bytes)	Description
Unique Local Address	16	An IPv6 Unique Local Address of the Target device

Table 9. Firmware Version response content

Field	Size (in bytes)	Description
Software Version	7	String representation on the format sv:XXXX, where XXXX is a four character string which indicates the Software Version
Divider	1	Dummy character to separate the Software and BIM Version, defaults to space character
BIM Version	5	String representation on the format bv:YY, where YY is a string representation of a byte number which indicates the BIM Version
Reserved	19	Default is set to zero

Table 10. Image Notify request content

Field	Size (in bytes)	Description
Image ID	1	Identifier for which image the notification corresponds to
OAD Image ID	8	Unique number to identify the start of an OAD image
BIM Version	1	Version required to support image format
Meta Version	1	Version of image header contained in image
Image Copy Status	1	Status of whether the Image has been copied or not
CRC Status	1	Status of the CRC check
Image Type	1	Type of image, e.g. Application or Factory image
Image Number	1	Distinguish between multiple images within a technology
Image Length	4	The total length of the image including header
Software Version	4	Software version of the image

Table 11. Image Notify Response content

Field	Size (in bytes)	Description
Image ID	1	Identifier for which image the notification corresponds to
Status	1	Status indicating if the notification request was accepted or rejected

Table 12. Image Block Request content

Field	Size (in bytes)	Description
Image ID	1	Identifier for which image the block corresponds to
Block Number	2	Which block to download
Total Blocks	2	Total number of blocks the image consists of

Table 13. Image Block Response content

Field	Size (in bytes)	Description
Image ID	1	Identifier for which image the block corresponds to
Block Number	2	Which block to download
Data	OAD_BLOCK_SIZE	Block data, where its size is configurable

TI-OpenThread OAD Examples

The SimpleLink CC13x2 / 26x2 SDK contains example applications which both are OAD-enabled and can perform OAD operations. In the context of OAD, an example being OAD-enabled means the image can be transferred over OAD, while performing OAD operations means the example can participate in the OAD transfer.

Door Lock OAD

The Door Lock OAD example is a modified version of the Door Lock example, which implements an OAD Target device ontop of the existing example. The example gives an example usage of the OAD module, and how one would implement an OAD Target device.

The example comes in two flavors: Secure OAD and Unsecure OAD. The Door Lock OAD example is configured with unsecure OAD, while Door Lock Secure OAD is configured with secure OAD. For more information what OAD security entails, please refer to Section OAD Security.

The Door Lock OAD example supports reverting back to a factory image.

OAD CLI

The OAD CLI example is equivalent to the CLI example, with the addition of being OAD-enabled. That means the generated image file can be used as a candidate image for an OAD exchange.

The example does not display any usage of the OAD module. It is simply an OAD-enabled Thread project which an OAD-enabled device can download. Refer to Section Create a TI-OpenThread OAD Image for how to make existing Thread projects OAD-enabled.

The OAD CLI example supports reverting back to a factory image.

OAD Server

The OAD Server example implements the OAD Distributor protocol on a host processor connected to a NCP. The example is meant to run in a Linux environment, and communicates with OAD Targets in a Thread network via the border router.

The example implements the OAD Distributor protocol on top of libcoap, which is an open-source implementation of CoAP. For more information about libcoap, see libcoap Homepage.