BLOB Transfer models

The Binary Large Object (BLOB) Transfer models provide functionality for sending large binary objects from a single source to many Target nodes over the Bluetooth mesh network. It is the underlying transport method for the Device Firmware Update (DFU), but may be used for other object transfer purposes.

The BLOB Transfer models support transfers of continuous binary objects of up to 4 GB (2 ³² bytes). The BLOB transfer protocol has built-in recovery procedures for packet losses, and sets up checkpoints to ensure that all targets have received all the data before moving on. Data transfer order is not guaranteed.

BLOB transfers are constrained by the transfer speed and reliability of the underlying mesh network. Under ideal conditions, the BLOBs can be transferred at a rate of up to 1 kbps, allowing a 100 kB BLOB to be transferred in 10-15 minutes. However, network conditions, transfer capabilities and other limiting factors can easily degrade the data rate by several orders of magnitude. Tuning the parameters of the transfer according to the application and network configuration, as well as scheduling it to periods with low network traffic, will offer significant improvements on the speed and reliability of the protocol. However, achieving transfer rates close to the ideal rate is unlikely in actual deployments.

There are two BLOB Transfer models:

The BLOB Transfer Client is instantiated on the sender node, and the BLOB Transfer Server is instantiated on the receiver nodes.

Concepts

The BLOB transfer protocol introduces several new concepts to implement the BLOB transfer.

BLOBs

BLOBs are binary objects up to 4 GB in size, that can contain any data the application would like to transfer through the mesh network. The BLOBs are continuous data objects, divided into blocks and chunks to make the transfers reliable and easy to process. No limitations are put on the contents or structure of the BLOB, and applications are free to define any encoding or compression they’d like on the data itself.

The BLOB transfer protocol does not provide any built-in integrity checks, encryption or authentication of the BLOB data. However, the underlying encryption of the Bluetooth mesh protocol provides data integrity checks and protects the contents of the BLOB from third parties using network and application level encryption.

Blocks

The binary objects are divided into blocks, typically from a few hundred to several thousand bytes in size. Each block is transmitted separately, and the BLOB Transfer Client ensures that all BLOB Transfer Servers have received the full block before moving on to the next. The block size is determined by the transfer’s block_size_log parameter, and is the same for all blocks in the transfer except the last, which may be smaller. For a BLOB stored in flash memory, the block size is typically a multiple of the flash page size of the Target devices.

Chunks

Each block is divided into chunks. A chunk is the smallest data unit in the BLOB transfer, and must fit inside a single Bluetooth mesh access message excluding the opcode (379 bytes or less). The mechanism for transferring chunks depends on the transfer mode.

When operating in Push BLOB Transfer Mode, the chunks are sent as unacknowledged packets from the BLOB Transfer Client to all targeted BLOB Transfer Servers. Once all chunks in a block have been sent, the BLOB Transfer Client asks each BLOB Transfer Server if they’re missing any chunks, and resends them. This is repeated until all BLOB Transfer Servers have received all chunks, or the BLOB Transfer Client gives up.

When operating in Pull BLOB Transfer Mode, the BLOB Transfer Server will request a small number of chunks from the BLOB Transfer Client at a time, and wait for the BLOB Transfer Client to send them before requesting more chunks. This repeats until all chunks have been transferred, or the BLOB Transfer Server gives up.

Read more about the transfer modes in Transfer modes section.

BLOB streams

In the BLOB Transfer models’ APIs, the BLOB data handling is separated from the high-level transfer handling. This split allows reuse of different BLOB storage and transfer strategies for different applications. While the high level transfer is controlled directly by the application, the BLOB data itself is accessed through a BLOB stream.

The BLOB stream is comparable to a standard library file stream. Through opening, closing, reading and writing, the BLOB Transfer model gets full access to the BLOB data, whether it’s kept in flash, RAM, or on a peripheral. The BLOB stream is opened with an access mode (read or write) before it’s used, and the BLOB Transfer models will move around inside the BLOB’s data in blocks and chunks, using the BLOB stream as an interface.

Interaction

Before the BLOB is read or written, the stream is opened by calling its open callback. When used with a BLOB Transfer Server, the BLOB stream is always opened in write mode, and when used with a BLOB Transfer Client, it’s always opened in read mode.

For each block in the BLOB, the BLOB Transfer model starts by calling block_start. Then, depending on the access mode, the BLOB stream’s wr or rd callback is called repeatedly to move data to or from the BLOB. When the model is done processing the block, it calls block_end. When the transfer is complete, the BLOB stream is closed by calling close.

Implementations

The application may implement their own BLOB stream, or use the implementations provided by Zephyr:

BLOB Flash

Transfer capabilities

Each BLOB Transfer Server may have different transfer capabilities. The transfer capabilities of each device are controlled through the following configuration options:

The CONFIG_BT_MESH_BLOB_CHUNK_COUNT_MAX option is also used by the BLOB Transfer Client and affects memory consumption by the BLOB Transfer Client model structure.

To ensure that the transfer can be received by as many servers as possible, the BLOB Transfer Client can retrieve the capabilities of each BLOB Transfer Server before starting the transfer. The client will transfer the BLOB with the highest possible block and chunk size.

Transfer modes

BLOBs can be transferred using two transfer modes, Push BLOB Transfer Mode and Pull BLOB Transfer Mode. In most cases, the transfer should be conducted in Push BLOB Transfer Mode.

In Push BLOB Transfer Mode, the send rate is controlled by the BLOB Transfer Client, which will push all the chunks of each block without any high level flow control. Push BLOB Transfer Mode supports any number of Target nodes, and should be the default transfer mode.

In Pull BLOB Transfer Mode, the BLOB Transfer Server will “pull” the chunks from the BLOB Transfer Client at its own rate. Pull BLOB Transfer Mode can be conducted with multiple Target nodes, and is intended for transferring BLOBs to Target nodes acting as Low Power Node. When operating in Pull BLOB Transfer Mode, the BLOB Transfer Server will request chunks from the BLOB Transfer Client in small batches, and wait for them all to arrive before requesting more chunks. This process is repeated until the BLOB Transfer Server has received all chunks in a block. Then, the BLOB Transfer Client starts the next block, and the BLOB Transfer Server requests all chunks of that block.

Transfer timeout

The timeout of the BLOB transfer is based on a Timeout Base value. Both client and server use the same Timeout Base value, but they calculate timeout differently.

The BLOB Transfer Server uses the following formula to calculate the BLOB transfer timeout:

10 * (Timeout Base + 1) seconds

For the BLOB Transfer Client, the following formula is used:

(10000 * (Timeout Base + 2)) + (100 * TTL) milliseconds

where TTL is time to live value set in the transfer.

API reference

This section contains types and defines common to the BLOB Transfer models.

group bt_mesh_blob

Defines

CONFIG_BT_MESH_BLOB_CHUNK_COUNT_MAX

Enums

enum bt_mesh_blob_xfer_mode

BLOB transfer mode.

Values:

enumerator BT_MESH_BLOB_XFER_MODE_NONE: No valid transfer mode.

enumerator BT_MESH_BLOB_XFER_MODE_PUSH: Push mode (Push BLOB Transfer Mode).

enumerator BT_MESH_BLOB_XFER_MODE_PULL: Pull mode (Pull BLOB Transfer Mode).

enumerator BT_MESH_BLOB_XFER_MODE_ALL: Both modes are valid.

enum bt_mesh_blob_xfer_phase

Transfer phase.

Values:

enumerator BT_MESH_BLOB_XFER_PHASE_INACTIVE: The BLOB Transfer Server is awaiting configuration.

enumerator BT_MESH_BLOB_XFER_PHASE_WAITING_FOR_START: The BLOB Transfer Server is ready to receive a BLOB transfer.

enumerator BT_MESH_BLOB_XFER_PHASE_WAITING_FOR_BLOCK: The BLOB Transfer Server is waiting for the next block of data.

enumerator BT_MESH_BLOB_XFER_PHASE_WAITING_FOR_CHUNK: The BLOB Transfer Server is waiting for the next chunk of data.

enumerator BT_MESH_BLOB_XFER_PHASE_COMPLETE: The BLOB was transferred successfully.

enumerator BT_MESH_BLOB_XFER_PHASE_SUSPENDED: The BLOB transfer is paused.

enum bt_mesh_blob_status

BLOB model status codes.

Values:

enumerator BT_MESH_BLOB_SUCCESS: The message was processed successfully.

enumerator BT_MESH_BLOB_ERR_INVALID_BLOCK_NUM: The Block Number field value is not within the range of blocks being transferred.

enumerator BT_MESH_BLOB_ERR_INVALID_BLOCK_SIZE: The block size is smaller than the size indicated by the Min Block Size Log state or is larger than the size indicated by the Max Block Size Log state.

enumerator BT_MESH_BLOB_ERR_INVALID_CHUNK_SIZE: The chunk size exceeds the size indicated by the Max Chunk Size state, or the number of chunks exceeds the number specified by the Max Total Chunks state.

enumerator BT_MESH_BLOB_ERR_WRONG_PHASE: The operation cannot be performed while the server is in the current phase.

enumerator BT_MESH_BLOB_ERR_INVALID_PARAM: A parameter value in the message cannot be accepted.

enumerator BT_MESH_BLOB_ERR_WRONG_BLOB_ID: The message contains a BLOB ID value that is not expected.

enumerator BT_MESH_BLOB_ERR_BLOB_TOO_LARGE: There is not enough space available in memory to receive the BLOB.

enumerator BT_MESH_BLOB_ERR_UNSUPPORTED_MODE: The transfer mode is not supported by the BLOB Transfer Server model.

enumerator BT_MESH_BLOB_ERR_INTERNAL: An internal error occurred on the node.

enumerator BT_MESH_BLOB_ERR_INFO_UNAVAILABLE: The requested information cannot be provided while the server is in the current phase.

enum bt_mesh_blob_io_mode

BLOB stream interaction mode.

Values:

enumerator BT_MESH_BLOB_READ: Read data from the stream.

enumerator BT_MESH_BLOB_WRITE: Write data to the stream.

struct bt_mesh_blob_block

#include <blob.h>

BLOB transfer data block.

Public Members

size_t size: Block size in bytes.

off_t offset: Offset in bytes from the start of the BLOB.

uint16_t number: Block number.

uint16_t chunk_count: Number of chunks in block.

uint8_t missing[DIV_ROUND_UP(0, 8)]: Bitmap of missing chunks.

struct bt_mesh_blob_chunk

#include <blob.h>

BLOB data chunk.

Public Members

off_t offset: Offset of the chunk data from the start of the block.

size_t size: Chunk data size.

uint8_t *data: Chunk data.

struct bt_mesh_blob_xfer

#include <blob.h>

BLOB transfer.

Public Members

uint64_t id: BLOB ID.

size_t size: Total BLOB size in bytes.

enum bt_mesh_blob_xfer_mode mode: BLOB transfer mode.

uint16_t chunk_size

Base chunk size.

May be smaller for the last chunk.

struct bt_mesh_blob_io

#include <blob.h>

BLOB stream.

Public Members

int (*open)(const struct bt_mesh_blob_io *io, const struct bt_mesh_blob_xfer *xfer, enum bt_mesh_blob_io_mode mode)

Open callback.

Called when the reader is opened for reading.

Param io:: BLOB stream.
Param xfer:: BLOB transfer.
Param mode:: Direction of the stream (read/write).
Return:: 0 on success, or (negative) error code otherwise.

void (*close)(const struct bt_mesh_blob_io *io, const struct bt_mesh_blob_xfer *xfer)

Close callback.

Called when the reader is closed.

Param io:: BLOB stream.
Param xfer:: BLOB transfer.

int (*block_start)(const struct bt_mesh_blob_io *io, const struct bt_mesh_blob_xfer *xfer, const struct bt_mesh_blob_block *block)

Block start callback.

Called when a new block is opened for sending. Each block is only sent once, and are always sent in increasing order. The data chunks inside a single block may be requested out of order and multiple times.

Param io:: BLOB stream.
Param xfer:: BLOB transfer.
Param block:: Block that was started.

void (*block_end)(const struct bt_mesh_blob_io *io, const struct bt_mesh_blob_xfer *xfer, const struct bt_mesh_blob_block *block)

Block end callback.

Called when the current block has been transmitted in full. No data from this block will be requested again, and the application data associated with this block may be discarded.

Param io:: BLOB stream.
Param xfer:: BLOB transfer.
Param block:: Block that finished sending.

int (*wr)(const struct bt_mesh_blob_io *io, const struct bt_mesh_blob_xfer *xfer, const struct bt_mesh_blob_block *block, const struct bt_mesh_blob_chunk *chunk)

Chunk data write callback.

Used by the BLOB Transfer Server on incoming data.

Each block is divided into chunks of data. This callback is called when a new chunk of data is received. Chunks may be received in any order within their block.

If the callback returns successfully, this chunk will be marked as received, and will not be received again unless the block is restarted due to a transfer suspension. If the callback returns a non-zero value, the chunk remains unreceived, and the BLOB Transfer Client will attempt to resend it later.

Note that the Client will only perform a limited number of attempts at delivering a chunk before dropping a Target node from the transfer. The number of retries performed by the Client is implementation specific.

Param io:: BLOB stream.
Param xfer:: BLOB transfer.
Param block:: Block the chunk is part of.
Param chunk:: Received chunk.
Return:: 0 on success, or (negative) error code otherwise.

int (*rd)(const struct bt_mesh_blob_io *io, const struct bt_mesh_blob_xfer *xfer, const struct bt_mesh_blob_block *block, const struct bt_mesh_blob_chunk *chunk)

Chunk data read callback.

Used by the BLOB Transfer Client to fetch outgoing data.

The Client calls the chunk data request callback to populate a chunk message going out to the Target nodes. The data request callback may be called out of order and multiple times for each offset, and cannot be used as an indication of progress.

Returning a non-zero status code on the chunk data request callback results in termination of the transfer.

Param io:: BLOB stream.
Param xfer:: BLOB transfer.
Param block:: Block the chunk is part of.
Param chunk:: Chunk to get the data of. The buffer pointer to by the data member should be filled by the callback.
Return:: 0 on success, or (negative) error code otherwise.