klipper-dgus/docs/Protocol.md

Klipper uses a binary protocol for communication between host and
firmware. This page provides a high-level description of that
protocol.

The goal of the protocol is to enable an error-free communication
channel between the host and firmware that is low-latency,
low-bandwidth, and low-complexity for the firmware.

The protocol acts as a
[RPC](https://en.wikipedia.org/wiki/Remote_procedure_call) mechanism
between firmware and host. The firmware declares the commands that the
host may invoke along with the response messages that it can
generate. The host uses that information to command the firmware to
perform actions and to interpret the results.

Firmware Interface
==================

Declaring commands
------------------

The firmware declares a "command" by using the DECL_COMMAND() macro in
the C code. For example:

```
DECL_COMMAND(command_set_digital_out, "set_digital_out pin=%u value=%c");
```

The above declares a command named "set_digital_out". This allows the
host to "invoke" this command which would cause the
command_set_digital_out() C function to be executed in the
firmware. The above also indicates that the command takes two integer
parameters. When the command_set_digital_out() C code is executed, it
will be passed an array containing these two integers - the first
corresponding to the 'pin' and the second corresponding to the
'value'.

In general, the parameters are described with printf() style syntax
(eg, "%u"). In the above example, "value=" is a parameter name and
"%c" indicates the parameter is an integer. The parameter name is used
as documentation. In this example, the "%c" is used as documentation
to indicate the expected integer is 1 byte in size (the declared
integer size does not impact the parsing or encoding).

At firmware compile time, the build will collect all commands declared
with DECL_COMMAND(), determine their parameters, and arrange for them
to be callable.

Declaring responses
-------------------

To send information from the firmware to the host a "response" is
generated. These are both declared and transmitted using the sendf() C
macro. For example:

```
sendf("status clock=%u status=%c", sched_read_time(), sched_is_shutdown());
```

The above transmits a "status" response message that contains two
integer parameters ("clock" and "status"). At firmware compile time
the build automatically finds all sendf() calls and generates encoders
for them. The first parameter of the sendf() function describes the
response and it is in the same format as command declarations.

The host can arrange to register a callback function for each
response. So, in effect, commands allow the host to invoke C functions
in the firmware and responses allow the firmware to invoke code in the
host.

The firmware should only invoke sendf() from command or task handlers,
and it should not be invoked from interrupts or timers. The firmware
does not need to issue a sendf() in response to a received command, it
is not limited in the number of times sendf() may be invoked, and it
may invoke sendf() at any time from a task handler.

### Output responses

To simplify debugging, the firmware also has an output() C
function. For example:

```
output("The value of %u is %s with size %u.", x, buf, buf_len);
```

The output() function is similar in usage to printf() - it is intended
to generate and format arbitrary messages for human consumption. It is
a wrapper around sendf() and as with sendf() it should not be called
from interrupts or timers.

Low-level message encoding
==========================

To accomplish the above RPC mechanism, each command and response is
encoded into a binary format for transmission. This section describes
the transmission system.

Message Blocks
--------------

All data sent from host to firmware and vice-versa are contained in
"message blocks". A message block has a two byte header and a three
byte trailer.  The format of a message block is:

```
<1 byte length><1 byte sequence><n-byte content><2 byte crc><1 byte sync>
```

The length byte contains the number of bytes in the message block
including the header and trailer bytes (thus the minimum message
length is 5 bytes). The maximum message block length is currently 64
bytes. The sequence byte contains a 4 bit sequence number in the
low-order bits and the high-order bits always contain 0x10 (the
high-order bits are reserved for future use). The content bytes
contain arbitrary data and its format is described in the following
section. The crc bytes contain a 16bit CCITT
[CRC](https://en.wikipedia.org/wiki/Cyclic_redundancy_check) of the
message block including the header bytes but excluding the trailer
bytes. The sync byte is 0x7e.

The format of the message block is inspired by
[HDLC](https://en.wikipedia.org/wiki/High-Level_Data_Link_Control)
message frames. Like in HDLC, the message block may optionally contain
an additional sync character at the start of the block. Unlike in
HDLC, a sync character is not exclusive to the framing and may be
present in the message block content.

Message Block Contents
----------------------

Each message block sent from host to firmware contains a series of
zero or more message commands in its contents. Each command starts
with a Variable Length Quantity (VLQ) encoded integer command-id
followed by zero or more VLQ parameters for the given command. So, the
contents of an example message block might look like:

```
<id_cmd_a><param1><id_cmd_b><param1><param2><id_cmd_c><id_cmd_d>
```

In order to encode and parse the message contents, both the host and
firmware must agree on the number of parameters each command has. So,
in the above example, both the host and firmware would know that
"id_cmd_a" is always followed by exactly one parameter, "id_cmd_b" two
parameters, and "id_cmd_c" / "id_cmd_d" zero parameters. The host and
firmware share a "data dictionary" that associates high-level commands
with specific integer command-ids along with the number of parameters
that the command takes. When processing the data, the parser will know
to expect a specific number of VLQ encoded parameters following a
given command.

The message contents for blocks sent from firmware to host follow the
same format. The identifiers in these messages are "response ids", but
they serve the same purpose and follow the same encoding rules. In
practice, message blocks sent from the firmware to the host never
contain more than one response in the message block contents.

### Variable Length Quantities

See the [wikipedia article](https://en.wikipedia.org/wiki/Variable-length_quantity)
for more information on the general format of VLQ encoded
integers. Klipper uses an encoding scheme that supports both positive
and negative integers. Integers close to zero use less bytes to encode
and positive integers typically encode using less bytes than negative
integers. The following table shows the number of bytes each integer
takes to encode:

| Integer                   | Encoded size |
|---------------------------|--------------|
| -32 .. 95                 | 1            |
| -4096 .. 12287            | 2            |
| -524288 .. 1572863        | 3            |
| -67108864 .. 201326591    | 4            |
| -2147483648 .. 4294967295 | 5            |

### Variable length strings

As an exception to the above encoding rules, if a parameter to a
command or response is a dynamic string then the parameter is not
encoded as a simple VLQ integer. Instead it is encoded by transmitting
the length as a VLQ encoded integer followed by the contents itself:

```
<VLQ encoded length><n-byte contents>
```

The data dictionary includes information on each parameter so that
both the host and firmware know which command parameters use simple
VLQ encoding and which parameters use string encoding.

Data Dictionary
===============

In order for meaningful communications to be established between
firmware and host, both sides must agree on a "data dictionary". This
data dictionary contains the integer identifiers for commands and
responses along with the number of parameters and the types of
parameters for each command/response.

At compile time the firmware build uses the contents of DECL_COMMAND()
and sendf() macros to generate the data dictionary. The build
automatically assigns unique identifiers to each command and
response. The data dictionary maps the high-level names (eg,
"get_status") to the integer command ids. This allows the host and
firmware to use descriptive ASCII names while still using minimal
bandwidth.

The host queries the data dictionary when it first connects to the
firmware. Once the host downloads the data dictionary from the
firmware, it uses that data dictionary to encode all commands and to
parse all responses from the firmware. The host must therefore handle
a dynamic data dictionary. However, to keep the firmware simple, the
firmware always uses its static (compiled in) data dictionary.

The data dictionary is queried by sending "identify" commands to the
firmware. The firmware will respond to each identify command with an
"identify_response" message. Since these two commands are needed prior
to obtaining the data dictionary, their integer ids and parameter
types are hard-coded in both the firmware and the host. The
"identify_response" response id is 0, the "identify" command id
is 1. Other than having hard-coded ids the identify command and its
response are declared and transmitted the same way as other commands
and responses. No other command or response is hard-coded.

The format of the transmitted data dictionary itself is a zlib
compressed JSON string. The firmware compile process generates the
string, compresses it, and stores it in the text section of the
firmware. The data dictionary can be much larger than the maximum
message block size - the host downloads it by sending multiple
identify commands requesting progressive chunks of the data
dictionary. Once all chunks are obtained the host will assemble the
chunks, uncompress the data, and parse the contents.

In addition to information on the communication protocol, the data
dictionary also contains firmware version, configuration, and other
useful information.

Static Strings
--------------

To reduce bandwidth the data dictionary also contains a set of static
strings known to the firmware. This is useful when sending messages
from firmware to host. For example, if the firmware were to run:

```
shutdown("Unable to handle command");
```

The error message can be encoded and sent using a single VLQ. The host
uses the data dictionary to resolve the VLQ encoded static string id
to the associated message in the data dictionary.

Message flow
============

Message commands sent from host to firmware are intended to be
error-free. The firmware will check the CRC and sequence numbers in
each message block to ensure the commands are accurate and
in-order. The firmware always processes message blocks in-order -
should it receive a block out-of-order it will discard it and any
other out-of-order blocks until it receives blocks with the correct
sequencing.

The low-level host code implements an automatic retransmission system
for lost and corrupt message blocks sent to the firmware. To
facilitate this, the firmware transmits an "ack message block" after
each successfully received message block. The host schedules a timeout
after sending each block and it will retransmit should the timeout
expire without receiving a corresponding "ack". In addition, if the
firmware detects a corrupt or out-of-order block it may transmit a
"nak message block" to facilitate fast retransmission.

An "ack" is a message block with empty content (ie, a 5 byte message
block) and a sequence number greater than the last received host
sequence number. A "nak" is a message block with empty content and a
sequence number less than the last received host sequence number.

The protocol facilitates a "window" transmission system so that the
host can have many outstanding message blocks in-flight at a
time. (This is in addition to the many commands that may be present in
a given message block.) This allows maximum bandwidth utilization even
in the event of transmission latency. The timeout, retransmit,
windowing, and ack mechanism are inspired by similar mechanisms in
[TCP](https://en.wikipedia.org/wiki/Transmission_Control_Protocol).

In the other direction, message blocks sent from firmware to host are
designed to be error-free, but they do not have assured
transmission. (Responses should not be corrupt, but they may go
missing.) This is done to keep the implementation in the firmware
simple. There is no automatic retransmission system for responses -
the high-level code is expected to be capable of handling an
occasional missing response (usually by re-requesting the content or
setting up a recurring schedule of response transmission). The
sequence number field in message blocks sent to the host is always one
greater than the last received sequence number of message blocks
received from the host. It is not used to track sequences of response
message blocks.