This is the description of the TCP/IP protocol for the Brick Daemon and the WIFI/Ethernet Extensions.
An overview of products that are controllable over TCP/IP can be found here.
The TCP/IP protocol is modeled after the concept of a function call. A request packet is send to the stack to trigger the execution of a function on a selected Brick or Bricklet. Functions that return something send a response packet back containing the return value(s). Functions that return nothing don't send a response packet back to the host. Beside normal functions there are also callbacks. Bricks and Bricklets can send response packets spontaneously back to the host to notify about an event or specific condition.
The TCP/IP protocol is based on packets transfered between host and stack. Each packet starts with a 8 byte header followed by a payload of variable length. The header consists of:
The UID is the unique identifier that is given to every Brick and Bricklet. In the API bindings the UID is used in Base58 encoded form using this alphabet:
The packet length specifies the length of the complete packet (header and payload) in bytes. A packet without a payload has a packet length of 8. The function ID combined with the transfer direction defines the content of the payload and this determines the packet length.
In a request packet the function ID specifies which function to execute on the device specified by the UID. In a response packet the function ID specifies from which function or callback the packet was send.
The sequence number is incremented with every packet send. The answer from a Brick/Bricklet has the same sequence number. Since the Bricks/Bricklets currently guarantee that packets are answered in the correct order, this is not strictly needed. However, we can't rule out the possibility that there will be a Brick in the future that has a processor that is capable of real multitasking.
For callbacks the sequence number is always 0 and this value is not allowed for other packets. Non-callback packets can only use 1 - 15 as sequence number. This allows to distinguish callback packets from other packets.
All data is represented in little endian. A bool value is represented by 1 byte; 0 is false, all other values are true. A float value is represented by 4 bytes in IEEE 754 format. A char value is represented by 1 byte in ASCII encoding.
A fixed size char array represents a string. If the string is shorter than the capacity of the array then the remaining space is filled with 0. In this case the contained string is null-terminated. In case that the string fills the whole array no null-terminator can be appended and the string is not null-terminated. Therefore, you cannot rely on strings being null-terminated.
When the UID of a device is known its specific functions can be called. To do this you need to send a corresponding request packet. The UID specifies the destination of the request packet and also affects the meaning of the function ID. This is because the same function ID has different meanings for different Bricks and Bricklets. For example, function ID 1 maps to the get_stack_voltage function on the Master Brick and to the set_port function on the IO-16 Bricklet.
The following example shows how to call the get_humidity function of a Humidity Bricklet with UID "b1Q" (Base58) or 33688 (integer). The corresponding request packet has
and an empty payload. Its hex dump looks like this:
0000 98 83 00 00 08 01 18 00 .. ......
The corresponding response packet can be identified by the UID, the function ID and the sequence number as they will have the same values as the request packet. The response packet has
The payload contains the
A humidity value of 421 means 42.1 %RH and is just an example. The hex dump of the packet looks like this:
0000 98 83 00 00 0a 01 18 00 a5 01 ..........
If there is no device with the given UID then the request is ignored and no response is send at all. This means that you should wait for a response packet only for a certain amount of time. The recommended timeout is 2500ms. After this amount of time you can assume that there is no device with the given UID.
There are also specific functions that do not send a response packet under normal conditions, for example the set_state function of the Dual Relay Bricklet (assuming the response expected flag is not set).
Devices can send response packets spontaneously back to the host to notify about an event or specific condition.
Most callbacks are disabled by default and have to enabled first. For example, the CALLBACK_MAGNETIC_FIELD callback of the IMU Brick with UID 6wVE7W (3631747890 as integer) can be enabled with a call to BrickIMU.set_magnetic_field_period with a period larger 0. Afterwards you will periodically receive response packets with
The payload contains
representing the magnetic field and is just an example. The hex dump of the packet looks like this:
0000 32 13 78 d8 0e 20 08 00 11 ff 3c 00 21 ff 2.x.. ....<.!.
As callbacks are spontaneously triggered you can receive their response packet at any time. For example between sending a request packet and receiving the corresponding response packet.
Using callbacks for recurring events is always preferred compared to using getters. It will use less USB bandwidth and the latency will be a lot better, since there is no round-trip time.
Support for authentication was added in Brick Daemon version 2.1.0 and Master Brick firmware version 2.2.0 for the Ethernet and WIFI Extensions.
With authentication enabled each TCP/IP connection starts in non-authenticated state. Before any normal communication can occur an authentication handshake has to be performed successfully to switch the connection to authenticated state. This handshake uses the server/client nonce approach utilizing HMAC-SHA1.
The server side of the handshake is handled by the manager of the TCP/IP connection. This can either be a Brick Daemon or a Master Brick with a Ethernet or WIFI Extension. For this the manager of the TCP/IP connection (the server) got its own UID 2 (1 as integer) so it can receive function calls: get_authentication_nonce and authenticate.
The handshake is initiated by the client (e.g. API bindings or Brick Viewer) calling the get_authentication_nonce function to receive the 4 byte
Then the client generates a 4 byte
that it concatenates to the server nonce to form the
Next the client uses the
as key to calculate the 20 byte
of the final nonce. The digest is then send to the server along with the client nonce by calling the authenticate function. The server receives client nonce and digest and does the same calculations as the client did. If the server calculates the same digest as provided by the client then client and server used the same secret. In this case the connection is switched to authenticated state and the client can proceed with normal communication. If the digests don't match the client used a mismatching authentication secret and the server closes the connection.
The API is split in several categories. The Brick Daemon functions currently deal with authentication. The broadcast functions are send to all devices and the callbacks are send back by the devices.
Support for authentication was added in Brick Daemon version 2.1.0 and Master Brick firmware version 2.2.0 for the Ethernet and WIFI Extensions. Authentication is done per-connection. For this Brick Daemon got its own UID 2 (1 as integer) as the manager of the TCP/IP connection.
This is the first function used in the authentication handshake. It asks the manager of the TCP/IP connection for the server authentication nonce.
This is the second function used in the authentication handshake. It sends the client nonce and the HMAC-SHA1 digest to the manager of the TCP/IP connection. If the handshake succeeds the connection switches from non-authenticated to authenticated state and communication can continue as normal. If the handshake fails then the connection gets closed.
The following functions are supported by all devices. The UID in the packet header has to be set to 1 (0 as integer) for broadcast.
Should be send periodically to the WIFI Extenstion to improve the detection of Wi-Fi disconnects. Without this a disconnect of the WIFI Extension might no be detected at all due to the way TCP/IP works.
The API bindings send a disconnect probe if there was no other packet send or received for at least 5s. Bricks and Bricklets just ignore this function ID.
As this feature is only useful for the WIFI Extension the Brick Daemon just drops incoming packets with this function ID and does not forward them over USB.
The WIFI Extenstion can send this callback to affect the TCP/IP buffer handling of clients. This can improve the handling of request packets on the client side.
This feature is internal and bindings should just drop incoming packets with this function ID.
The callback has seven parameters:
Possible enumeration types are:
It should be possible to implement plug-and-play functionality with this (as is done in Brick Viewer).
The device identifier numbers can be found here.
Links to the API reference for Bricks and Bricklets are listed in the following table. Further project descriptions can be found in the Starter Kits section.
|Ambient Light 2.0||API|
|Analog In 2.0||API|
|Analog In 3.0||API|
|Analog Out 2.0||API|
|Analog Out 3.0||API|
|Distance IR 2.0||API|
|Dual Button 2.0||API|
|Industrial Analog Out||API|
|Industrial Analog Out 2.0||API|
|Industrial Digital In 4||API|
|Industrial Digital In 4 2.0||API|
|Industrial Digital Out 4||API|
|Industrial Digital Out 4 2.0||API|
|Industrial Dual 0-20mA||API|
|Industrial Dual 0-20mA 2.0||API|
|Industrial Dual Analog In||API|
|Industrial Dual Analog In 2.0||API|
|Industrial Dual Relay||API|
|Industrial Quad Relay||API|
|Industrial Quad Relay 2.0||API|
|Laser Range Finder||API|
|LED Strip 2.0||API|
|Load Cell 2.0||API|
|Motion Detector 2.0||API|
|Motorized Linear Poti||API|
|OLED 128x64 2.0||API|
|Real-Time Clock 2.0||API|
|Remote Switch 2.0||API|
|RGB LED Button||API|
|RGB LED Matrix||API|
|Rotary Encoder 2.0||API|
|Segment Display 4x7||API|
|Solid State Relay||API|
|Solid State Relay 2.0||API|
|Sound Pressure Level||API|
|Temperature IR 2.0||API|
|UV Light 2.0||API|