Simple BLE Service for reading and writing files over BLE
Project description
Introduction
Simple BLE Service for reading and writing files over BLE. This BLE service is geared towards file transfer to and from a device running the service. A core part of the protocol is free space responses so that the server can be a memory limited device. The free space responses allow for small buffer sizes that won’t be overwhelmed by the client.
Dependencies
This driver depends on:
Please ensure all dependencies are available on the CircuitPython filesystem. This is easily achieved by downloading the Adafruit library and driver bundle or individual libraries can be installed using circup.
Installing from PyPI
On supported GNU/Linux systems like the Raspberry Pi, you can install the driver locally from PyPI. To install for current user:
pip3 install adafruit-circuitpython-ble-file-transferTo install system-wide (this may be required in some cases):
sudo pip3 install adafruit-circuitpython-ble-file-transferTo install in a virtual environment in your current project:
mkdir project-name && cd project-name
python3 -m venv .venv
source .venv/bin/activate
pip3 install adafruit-circuitpython-ble-file-transferUsage Examples
See examples/ble_file_transfer_simpletest.py for a client example. A stub server implementation is in examples/ble_file_transfer_stub_server.py.
Protocol
The file transfer protocol is meant to be simple and easy to implement. It uses free space counts as a way to rate limit file content data transfer. All multi-byte numbers are encoded with the least significant byte first (“<” in CPython’s struct module).
GATT Service
The UUID of the service is 0xfebb, Adafruit’s 16-bit service UUID.
The base UUID used in characteristics is ADAFxxxx-4669-6C65-5472-616E73666572. The 16-bit numbers below are substituted into the xxxx portion.
The service has two characteristics:
version (0x0100) - Simple unsigned 32-bit integer version number. May be 1 - 4.
raw transfer (0x0200) - Bidirectional link with a custom protocol. The client does WRITE_NO_RESPONSE to the characteristic and then server replies via NOTIFY. (This is similar to the Nordic UART Service but on a single characteristic rather than two.) The commands over the transfer characteristic are idempotent and stateless. A disconnect during a command will reset the state.
Time resolution
Time resolution varies based filesystem type. FATFS can only get down to the 2 second bound after 1980. Littlefs can do 64-bit nanoseconds after January 1st, 1970.
To account for this, the protocol has time in 64-bit nanoseconds after January 1st, 1970. However, the server will respond with a potentially truncated version that is the value stored.
Also note that devices serving the file transfer protocol may not have it’s own clock so do not rely on time ordering. Any internal writes may set the time incorrectly. So, we only recommend using the value as a cache key.
Commands
Commands always start with a fixed header. The first entry is always the command number itself encoded in a single byte. The number of subsequent entries in the header will vary by command. The entire header must be sent as a unit so set the characteristic with the full header packet. You can combine multiple commands into a single write as long as the complete header is in the packet.
Paths use / as a separator and full paths must start with /.
All numbers are unsigned.
All values are aligned with respect to the start of the packet.
Status bytes are 0x01 for OK and 0x02 for error. Values other than 0x01 are errors. 0x00 should not be used for a specific error but still considered an error. 0x05 is an error for trying to modify a read-only filesystem.
0x10 - Read a file
Given a full path, returns the full contents of the file.
The header is four fixed entries and a variable length path:
Command: Single byte. Always 0x10.
1 Byte reserved for padding.
Path length: 16-bit number encoding the encoded length of the path string.
Chunk offset: 32-bit number encoding the offset into the file to start the first chunk.
Chunk size: 32-bit number encoding the amount of data that the client can handle in the first reply.
Path: UTF-8 encoded string that is not null terminated. (We send the length instead.)
The server will respond with:
Command: Single byte. Always 0x11.
Status: Single byte.
2 Bytes reserved for padding.
Chunk offset: 32-bit number encoding the offset into the file of this chunk.
Total length: 32-bit number encoding the total file length.
Chunk length: 32-bit number encoding the length of the read data up to the chunk size provided in the header.
Chunk-length contents of the file starting from the current position.
If the chunk length is smaller than the total length, then the client will request more data by sending:
Command: Single byte. Always 0x12.
Status: Single byte. Always OK for now.
2 Bytes reserved for padding.
Chunk offset: 32-bit number encoding the offset into the file to start the next chunk.
Chunk size: 32-bit number encoding the number of bytes to read. May be different than the original size. Does not need to be limited by the total size.
The transaction is complete after the server has replied with all data. (No acknowledgement needed from the client.)
0x20 - Write a file
Writes the content to the given full path. If the file exists, it will be overwritten. Content may be written as received so an interrupted transfer may lead to a truncated file.
Offset larger than the existing file size will introduce zeros into the gap.
The header is four fixed entries and a variable length path:
Command: Single byte. Always 0x20.
1 Byte reserved for padding.
Path length: 16-bit number encoding the encoded length of the path string.
Offset: 32-bit number encoding the starting offset to write.
Current time: 64-bit number encoding nanoseconds since January 1st, 1970. Used as the file modification time. Not all system will support the full resolution. Use the truncated time response value for caching.
Total size: 32-bit number encoding the total length of the file contents.
Path: UTF-8 encoded string that is not null terminated. (We send the length instead.)
The server will repeatedly respond until the total length has been transferred with:
Command: Single byte. Always 0x21.
Status: Single byte. 0x01 if OK. 0x05 if the filesystem is read-only. 0x02 if any parent directory is missing or a file.
2 Bytes reserved for padding.
Offset: 32-bit number encoding the starting offset to write. (Should match the offset from the previous 0x20 or 0x22 message)
Truncated time: 64-bit number encoding nanoseconds since January 1st, 1970 as stored by the file system. The resolution may be less that the protocol. It is sent back for use in caching on the host side.
Free space: 32-bit number encoding the amount of data the client can send.
The client will repeatedly respond until the total length has been transferred with:
Command: Single byte. Always 0x22.
Status: Single byte. Always 0x01 for OK.
2 Bytes reserved for padding.
Offset: 32-bit number encoding the offset to write.
Data size: 32-bit number encoding the amount of data the client is sending.
Data
The transaction is complete after the server has received all data and replied with a status with 0 free space and offset set to the content length.
NOTE: Current time was added in version 3. The rest of the packets remained the same.
0x30 - Delete a file or directory
Deletes the file or directory at the given full path. Non-empty directories will have their contents deleted as well.
The header is two fixed entries and a variable length path:
Command: Single byte. Always 0x30.
1 Byte reserved for padding.
Path length: 16-bit number encoding the encoded length of the path string.
Path: UTF-8 encoded string that is not null terminated. (We send the length instead.)
The server will reply with:
Command: Single byte. Always 0x31.
Status: Single byte. 0x01 if the file or directory was deleted, 0x05 if the filesystem is read-only or 0x02 if the path is non-existent.
NOTE: In version 2, this command now deletes contents of a directory as well. It won’t error.
0x40 - Make a directory
Creates a new directory at the given full path. If a parent directory does not exist, then it will also be created. If any name conflicts with an existing file, an error will be returned.
The header is two fixed entries and a variable length path:
Command: Single byte. Always 0x40.
1 Byte reserved for padding.
Path length: 16-bit number encoding the encoded length of the path string.
4 Bytes reserved for padding.
Current time: 64-bit number encoding nanoseconds since January 1st, 1970. Used as the file modification time. Not all system will support the full resolution. Use the truncated time response value for caching.
Path: UTF-8 encoded string that is not null terminated. (We send the length instead.)
The server will reply with:
Command: Single byte. Always 0x41.
Status: Single byte. 0x01 if the directory(s) were created, 0x05 if the filesystem is read-only or 0x02 if any parent of the path is an existing file.
6 Bytes reserved for padding.
Truncated time: 64-bit number encoding nanoseconds since January 1st, 1970 as stored by the file system. The resolution may be less that the protocol. It is sent back for use in caching on the host side.
0x50 - List a directory
Lists all of the contents in a directory given a full path. Returned paths are relative to the given path to reduce duplication.
The header is two fixed entries and a variable length path:
Command: Single byte. Always 0x50.
1 Byte reserved for padding.
Path length: 16-bit number encoding the encoded length of the path string.
Path: UTF-8 encoded string that is not null terminated. (We send the length instead.)
The server will reply with n+1 entries for a directory with n files:
Command: Single byte. Always 0x51.
Status: Single byte. 0x01 if the directory exists or 0x02 if it doesn’t.
Path length: 16-bit number encoding the encoded length of the path string.
Entry number: 32-bit number encoding the entry number.
Total entries: 32-bit number encoding the total number of entries.
Flags: 32-bit number encoding data about the entries.
Bit 0: Set when the entry is a directory
Bits 1-7: Reserved
Modification time: 64-bit number of nanoseconds since January 1st, 1970. However, files modifiers may not have an accurate clock so do not assume it is correct. Instead, only use it to determine cacheability vs a local copy.
File size: 32-bit number encoding the size of the file. Ignore for directories. Value may change.
Path: UTF-8 encoded string that is not null terminated. (We send the length instead.) These paths are relative so they won’t contain / at all.
The transaction is complete when the final entry is sent from the server. It will have entry number == total entries and zeros for flags, file size and path length.
0x60 - Move a file or directory
Moves a file or directory at a given path to a different path. Can be used to rename as well. The two paths are sent separated by a byte so that the server may null-terminate the string itself. The client may send anything there.
The header is two fixed entries and a variable length path:
Command: Single byte. Always 0x60.
1 Byte reserved for padding.
Old Path length: 16-bit number encoding the encoded length of the path string.
New Path length: 16-bit number encoding the encoded length of the path string.
Old Path: UTF-8 encoded string that is not null terminated. (We send the length instead.)
One padding byte. This can be used to null terminate the old path string.
New Path: UTF-8 encoded string that is not null terminated. (We send the length instead.)
The server will reply with:
Command: Single byte. Always 0x61.
Status: Single byte. 0x01 on success, 0x05 if read-only, or 0x02 on other error.
NOTE: This is added in version 4.
Versions
Version 2
Changes delete to delete contents of non-empty directories automatically.
Version 3
Adds modification time. * Adds current time to file write command. * Adds current time to make directory command. * Adds modification time to directory listing entries.
Version 4
Adds move command.
Adds 0x05 error for read-only filesystems. This is commonly that USB is editing the same filesystem.
Removes requirement that directory paths end with /.
Contributing
Contributions are welcome! Please read our Code of Conduct before contributing to help this project stay welcoming.
Documentation
For information on building library documentation, please check out this guide.
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