The Klipper messaging protocol is used for low-level communication
between the Klipper host software and the Klipper micro-controller
software. At a high level the protocol can be thought of as a series
of command and response strings that are compressed, transmitted, and
then processed at the receiving side. An example series of commands in
uncompressed human-readable format might look like:

```
set_digital_out pin=86 value=1
set_digital_out pin=85 value=1
schedule_digital_out oid=8 clock=4000000 value=0
queue_step oid=7 interval=7458 count=10 add=331
queue_step oid=7 interval=11717 count=4 add=1281
```

See the [mcu commands](MCU_Commands.md) document for information on
available commands. See the [debugging](Debugging.md) document for
information on how to translate a G-Code file into its corresponding
human-readable micro-controller commands.

This page provides a high-level description of the Klipper messaging
protocol itself. It describes how messages are declared, encoded in
binary format (the "compression" scheme), and transmitted.

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

Micro-controller Interface
==========================

The Klipper transmission protocol can be thought of as a
[RPC](https://en.wikipedia.org/wiki/Remote_procedure_call) mechanism
between micro-controller and host. The micro-controller software
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 micro-controller to perform actions and to interpret the
results.

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

The micro-controller software 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
micro-controller. 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"). The formatting directly corresponds to the human-readable
view of commands (eg, "set_digital_out pin=86 value=1"). In the above
example, "value=" is a parameter name and "%c" indicates the parameter
is an integer. Internally, the parameter name is only used as
documentation. In this example, the "%c" is also used as documentation
to indicate the expected integer is 1 byte in size (the declared
integer size does not impact the parsing or encoding).

The micro-controller 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 micro-controller 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"). The micro-controller 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 micro-controller and responses allow the micro-controller
software to invoke code in the host.

The sendf() macro should only be invoked from command or task
handlers, and it should not be invoked from interrupts or timers. The
code 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, there is 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.

Declaring constants
-------------------

Constants can also be exported. For example, the following:

```
DECL_CONSTANT(SERIAL_BAUD, 250000);
```

would export a constant named "SERIAL_BAUD" with a value of 250000
from the micro-controller to the host.

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 micro-controller 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 micro-controller contains a
series of zero or more message commands in its contents. Each command
starts with a [Variable Length Quantity](#variable-length-quantities)
(VLQ) encoded integer command-id followed by zero or more VLQ
parameters for the given command.

As an example, the following four commands might be placed in a single
message block:

```
set_digital_out pin=86 value=1
set_digital_out pin=85 value=0
get_config
get_status
```

and encoded into the following eight VLQ integers:

```
<id_set_digital_out><86><1><id_set_digital_out><85><0><id_get_config><id_get_status>
```

In order to encode and parse the message contents, both the host and
micro-controller must agree on the command ids and the number of
parameters each command has. So, in the above example, both the host
and micro-controller would know that "id_set_digital_out" is always
followed by two parameters, and "id_get_config" and "id_get_status"
have zero parameters. The host and micro-controller share a "data
dictionary" that maps the command descriptions (eg, "set_digital_out
pin=%u value=%c") to their integer command-ids. When processing the
data, the parser will know to expect a specific number of VLQ encoded
parameters following a given command id.

The message contents for blocks sent from micro-controller 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
micro-controller 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 command descriptions found in the data dictionary allow both the
host and micro-controller to 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
micro-controller and host, both sides must agree on a "data
dictionary". This data dictionary contains the integer identifiers for
commands and responses along with their descriptions.

The micro-controller 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. This system allows both the host and micro-controller code
to seamlessly use descriptive human-readable names while still using
minimal bandwidth.

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

The data dictionary is queried by sending "identify" commands to the
micro-controller. The micro-controller 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 micro-controller 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 micro-controller build process generates
the string, compresses it, and stores it in the text section of the
micro-controller flash. 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 the software version, constants (as defined
by DECL_CONSTANT), and static strings.

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

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

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

The error message would be encoded and sent using a single VLQ. The
host uses the data dictionary to resolve VLQ encoded static string ids
to their associated human-readable strings.

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

Message commands sent from host to micro-controller are intended to be
error-free. The micro-controller will check the CRC and sequence
numbers in each message block to ensure the commands are accurate and
in-order. The micro-controller 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 micro-controller. To
facilitate this, the micro-controller 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 micro-controller 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 micro-controller 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
micro-controller 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.
