HaloKeymind/docs/kiss_modem_protocol.md

# MeshCore KISS Modem Protocol

Standard KISS TNC firmware for MeshCore LoRa radios. Compatible with any KISS client (Direwolf, APRSdroid, YAAC, etc.) for sending and receiving raw packets. MeshCore-specific extensions (cryptography, radio configuration, telemetry) are available through the standard SetHardware (0x06) command.

KISS protocol version 2 adds an opt-in logical port 1 for MeshCore's second
time-shared LoRa profile. It is still one physical transceiver: the firmware
fast-switches between `radio` and `radio2`; it does not create a second radio.

## Serial Configuration

115200 baud, 8N1, no flow control.

## Frame Format

Standard KISS framing per the KA9Q/K3MC specification.

| Byte   | Name  | Description                        |
|--------|-------|------------------------------------|
| `0xC0` | FEND  | Frame delimiter                    |
| `0xDB` | FESC  | Escape character                   |
| `0xDC` | TFEND | Escaped FEND (FESC + TFEND = 0xC0) |
| `0xDD` | TFESC | Escaped FESC (FESC + TFESC = 0xDB) |

```
+------+-----------+--------------+------+
| FEND | Type Byte | Data (escaped)| FEND |
| 0xC0 |  1 byte   | 0-510 bytes  | 0xC0 |
+------+-----------+--------------+------+
```

### Type Byte

The type byte is split into two nibbles:

| Bits | Field   | Description                         |
|------|---------|-------------------------------------|
| 7-4  | Port    | `0` primary `radio`; `1` secondary `radio2` for Data only in protocol v2; controls stay on port 0 |
| 3-0  | Command | Command number                      |

Maximum unescaped frame size: 512 bytes.

## Standard KISS Commands

### Host to TNC

| Command     | Value  | Data               | Description                                                 |
|-------------|--------|--------------------|-------------------------------------------------------------|
| Data        | `0x00` | Raw packet         | Queue on primary `radio`; use type `0x10` to queue on `radio2` in v2 (one pending packet total) |
| TXDELAY     | `0x01` | Delay (1 byte)     | Transmitter keyup delay in 10ms units (default: 50 = 500ms) |
| Persistence | `0x02` | P (1 byte)         | CSMA persistence parameter 0-255 (default: 63)              |
| SlotTime    | `0x03` | Interval (1 byte)  | CSMA slot interval in 10ms units (default: 10 = 100ms)      |
| TXtail      | `0x04` | Delay (1 byte)     | Post-TX hold time in 10ms units (default: 0)                |
| FullDuplex  | `0x05` | Mode (1 byte)      | 0 = half duplex, nonzero = full duplex (default: 0)         |
| SetHardware | `0x06` | Sub-command + data | MeshCore extensions (see below)                             |
| Return      | `0xFF` | -                  | Exit KISS mode (no-op)                                      |

### TNC to Host

| Type | Value  | Data       | Description                |
|------|--------|------------|----------------------------|
| Data | `0x00` | Raw packet | Received packet from primary `radio` |
| Data | `0x10` | Raw packet | Received packet from secondary `radio2` (v2) |

Data frames carry raw packet data only, with no metadata prepended. The Data command payload is limited to 255 bytes to match the MeshCore maximum transmission unit (MAX_TRANS_UNIT); frames larger than 255 bytes are silently dropped. The KISS specification recommends at least 1024 bytes for general-purpose TNCs; this modem is intended for MeshCore packets only, whose protocol MTU is 255 bytes. A v2 port-1 Data packet reaches RF only while `radio2` is configured `rxtx`; with `off` or `rx`, the modem returns `TxDone(0)` and never falls back to primary.

Only one packet may be pending for radio transmission at a time. If the host sends a second Data frame before the first has completed, the modem responds with Error (0xF1) and TxBusy (0x07).

### Fast dual-profile receive switching

After OpenHop (or another KISS client) configures `radio2` or `tempradio2`,
the board's `radio_driver.loop()` performs the RF hops locally. KISS does not
send a command for every hop: that would be slower than the serial link alone.
The shared scanner uses a **600 µs nominal retune allowance** per hop, with
the profile timing/preamble calculation accounting for it. It is a measured
target rather than a hard maximum; packet handling and a slow board loop can
extend an individual hop. The KISS parser services at most 32 incoming bytes
per outer loop so a busy OpenHop serial peer cannot starve the scanner.

Thus “around 0.6 ms” refers to the on-board RF setting transition, not a full
host-to-board request/response round trip. The returned Data port (`0x00` or
`0x10`) identifies which profile actually received each packet.

`SetRadio2` and `SetTempRadio2` acknowledge that their tuple was accepted and
scheduled; they do not acknowledge that a receiver has already entered that
tuple. The next local scanner service performs the retune. KISS clients should
configure the profiles once and let the board perform automatic time-sharing;
a 115200-baud serial round trip cannot be a 0.6 ms receive-hop control plane.
Clients that need temporary-lease state can poll `GetTempRadio2`.

### Host Output Backpressure

Outbound frames are encoded into a 2-slot queue and flushed when serial output space is available; `loop()` never blocks on writes. Radio TX state advances independently of host read speed. TxDone is retained until it can be queued. If the outbound queue is full, the modem responds with Error (0xF1) and TxBusy (0x07). Hosts should read serial promptly to avoid delayed responses.

### CSMA Behavior

The TNC implements p-persistent CSMA for half-duplex operation:

1. When a packet is queued, monitor carrier detect
2. When the channel clears, generate a random value 0-255
3. If the value is less than or equal to P (Persistence), wait TXDELAY then transmit
4. Otherwise, wait SlotTime and repeat from step 1

In full-duplex mode, CSMA is bypassed and packets transmit after TXDELAY.

## SetHardware Extensions (0x06)

MeshCore-specific functionality uses the standard KISS SetHardware command. The first byte of SetHardware data is a sub-command. Standard KISS clients ignore these frames.

### Frame Format

```
+------+------+-------------+--------------+------+
| FEND | 0x06 | Sub-command | Data (escaped)| FEND |
| 0xC0 |      |   1 byte    |   variable   | 0xC0 |
+------+------+-------------+--------------+------+
```

### Request Sub-commands (Host to TNC)

| Sub-command     | Value  | Data                                     |
|-----------------|--------|------------------------------------------|
| GetIdentity     | `0x01` | -                                        |
| GetRandom       | `0x02` | Length (1 byte, 1-64)                    |
| VerifySignature | `0x03` | PubKey (32) + Signature (64) + Data      |
| SignData        | `0x04` | Data to sign                             |
| EncryptData     | `0x05` | Key (32) + Plaintext                     |
| DecryptData     | `0x06` | Key (32) + MAC (2) + Ciphertext          |
| KeyExchange     | `0x07` | Remote PubKey (32)                       |
| Hash            | `0x08` | Data to hash                             |
| SetRadio        | `0x09` | Freq (4) + BW (4) + SF (1) + CR (1)      |
| SetTxPower      | `0x0A` | Power dBm (1)                            |
| GetRadio        | `0x0B` | -                                        |
| GetTxPower      | `0x0C` | -                                        |
| GetCurrentRssi  | `0x0D` | -                                        |
| IsChannelBusy   | `0x0E` | -                                        |
| GetAirtime      | `0x0F` | Packet length (1)                        |
| GetNoiseFloor   | `0x10` | -                                        |
| GetVersion      | `0x11` | -                                        |
| GetStats        | `0x12` | -                                        |
| GetBattery      | `0x13` | -                                        |
| GetMCUTemp      | `0x14` | -                                        |
| GetSensors      | `0x15` | Permissions (1)                          |
| GetDeviceName   | `0x16` | -                                        |
| Ping            | `0x17` | -                                        |
| Reboot          | `0x18` | -                                        |
| SetSignalReport | `0x19` | Enable (1): 0x00=disable, nonzero=enable |
| GetSignalReport | `0x1A` | -                                        |
| SetRadio2       | `0x1B` | Radio2 parameters (13; see below)       |
| GetRadio2       | `0x1C` | -                                        |
| SetTempRadio2   | `0x1D` | Radio2 parameters + duration (15)       |
| GetTempRadio2   | `0x1E` | -                                        |

### Response Sub-commands (TNC to Host)

Response codes use the high-bit convention: `response = command | 0x80`. Generic and unsolicited responses use the `0xF0`+ range.

| Sub-command  | Value  | Data                                    |
|--------------|--------|-----------------------------------------|
| Identity     | `0x81` | PubKey (32)                             |
| Random       | `0x82` | Random bytes (1-64)                     |
| Verify       | `0x83` | Result (1): 0x00=invalid, 0x01=valid    |
| Signature    | `0x84` | Signature (64)                          |
| Encrypted    | `0x85` | MAC (2) + Ciphertext                    |
| Decrypted    | `0x86` | Plaintext                               |
| SharedSecret | `0x87` | Shared secret (32)                      |
| Hash         | `0x88` | SHA-256 hash (32)                       |
| Radio        | `0x8B` | Freq (4) + BW (4) + SF (1) + CR (1)     |
| TxPower      | `0x8C` | Power dBm (1)                           |
| CurrentRssi  | `0x8D` | RSSI dBm (1, signed)                    |
| ChannelBusy  | `0x8E` | Result (1): 0x00=clear, 0x01=busy       |
| Airtime      | `0x8F` | Milliseconds (4)                        |
| NoiseFloor   | `0x90` | dBm (2, signed)                         |
| Version      | `0x91` | Version (1) + Reserved (1)              |
| Stats        | `0x92` | RX (4) + TX (4) + Errors (4)            |
| Battery      | `0x93` | Millivolts (2)                          |
| MCUTemp      | `0x94` | Temperature (2, signed)                 |
| Sensors      | `0x95` | CayenneLPP payload                      |
| DeviceName   | `0x96` | Name (variable, UTF-8)                  |
| Pong         | `0x97` | -                                       |
| SignalReport | `0x9A` | Status (1): 0x00=disabled, 0x01=enabled |
| Radio2       | `0x9C` | Saved radio2 parameters (13)             |
| TempRadio2   | `0x9E` | Active temporary radio2 + remaining minutes (15) |
| OK           | `0xF0` | -                                       |
| Error        | `0xF1` | Error code (1)                          |
| TxDone       | `0xF8` | Result (1): 0x00=failed, 0x01=success   |
| RxMeta       | `0xF9` | SNR (1) + RSSI (1)                      |

### Error Codes

| Code          | Value  | Description             |
|---------------|--------|-------------------------|
| InvalidLength | `0x01` | Request data too short  |
| InvalidParam  | `0x02` | Invalid parameter value |
| NoCallback    | `0x03` | Feature not available   |
| MacFailed     | `0x04` | MAC verification failed |
| UnknownCmd    | `0x05` | Unknown sub-command     |
| EncryptFailed | `0x06` | Encryption failed       |
| TxBusy        | `0x07` | Radio TX busy, or host output queue full |

### Unsolicited Events

The TNC sends these SetHardware frames without a preceding request:

**TxDone (0xF8)**: Sent after radio transmission completes. Contains a single byte: 0x01 for success, 0x00 for failure. Delivery to the host may be delayed under serial backpressure but is not dropped.

**RxMeta (0xF9)**: Sent after each Data frame (type 0x00 or v2 type 0x10) with SNR (1 byte, signed, value x4) and RSSI (1 byte, signed, dBm). It remains exactly two bytes; the Data port identifies the receiving profile. Queued with the data frame; omitted if the data frame cannot be queued. Enabled by default; toggle with SetSignalReport. Standard KISS clients ignore this frame.

## Data Formats

### Radio Parameters (SetRadio / Radio response)

All values little-endian.

| Field     | Size    | Description             |
|-----------|---------|-------------------------|
| Frequency | 4 bytes | Hz (e.g., 869618000)    |
| Bandwidth | 4 bytes | Hz (e.g., 62500)        |
| SF        | 1 byte  | Spreading factor (5-12) |
| CR        | 1 byte  | Coding rate (5-8)       |

### Secondary Radio Parameters (SetRadio2 / Radio2 response)

`SetRadio2` is the binary KISS equivalent of `radio2`. It changes only RAM
state for this KISS session; it does not write the Mesh CLI profile file.
All values are little-endian. The mode values are `0=off`, `1=rx`, and
`2=rxtx`. An `off` tuple may contain zero radio parameters, which are ignored.

| Field     | Size    | Description                         |
|-----------|---------|-------------------------------------|
| Frequency | 4 bytes | Hz                                  |
| Bandwidth | 4 bytes | Hz                                  |
| SF        | 1 byte  | Spreading factor (5-12)             |
| CR        | 1 byte  | Coding rate (5-8)                   |
| Mode      | 1 byte  | `off`, `rx`, or `rxtx` as above     |
| Preamble  | 2 bytes | Symbols; `0` chooses automatic      |

`GetRadio2` returns this exact 13-byte tuple. The modem preserves the exact
frequency and bandwidth values sent by the host even though its radio driver
uses floating-point MHz/kHz internally.

### Temporary Secondary Radio (SetTempRadio2 / TempRadio2 response)

`SetTempRadio2` is the binary KISS equivalent of `tempradio2`: the 13-byte
secondary tuple followed by `duration_minutes:u16`, from 1 to 10080. It is
RAM-only and automatically restores the saved `radio2` tuple on expiry or
reboot. A mode `off` with duration zero cancels the temporary lease and
immediately restores saved `radio2`; all other `off`/zero-duration combinations
are invalid. `GetTempRadio2` returns the active 13-byte tuple plus remaining
minutes, rounded up. It returns a zero/off tuple with zero duration when no
temporary lease is active.

Changing saved `radio2` during an active temporary lease does not interrupt
the active tuple; the replacement becomes active when the lease ends. Pending
KISS Data is bound to a profile generation at queue time. If that profile is
replaced before RF transmission begins, the modem sends normal `TxDone(0)` and
never falls back to the other profile.

### Version (Version response)

| Field    | Size   | Description      |
|----------|--------|------------------|
| Version  | 1 byte | Firmware version |
| Reserved | 1 byte | Always 0         |

### Encrypted (Encrypted response)

| Field      | Size     | Description                                    |
|------------|----------|------------------------------------------------|
| MAC        | 2 bytes  | HMAC-SHA256 truncated to 2 bytes               |
| Ciphertext | variable | AES-128 block-encrypted data with zero padding |

### Airtime (Airtime response)

All values little-endian.

| Field   | Size    | Description                                  |
|---------|---------|----------------------------------------------|
| Airtime | 4 bytes | uint32_t, estimated air time in milliseconds |

### Noise Floor (NoiseFloor response)

All values little-endian.

| Field       | Size    | Description           |
|-------------|---------|-----------------------|
| Noise floor | 2 bytes | int16_t, dBm (signed) |

The modem recalibrates the noise floor every 2 seconds with an AGC reset every 30 seconds.

### Stats (Stats response)

All values little-endian.

| Field  | Size    | Description         |
|--------|---------|---------------------|
| RX     | 4 bytes | Packets received    |
| TX     | 4 bytes | Packets transmitted |
| Errors | 4 bytes | Receive errors      |

### Battery (Battery response)

All values little-endian.

| Field      | Size    | Description                     |
|------------|---------|---------------------------------|
| Millivolts | 2 bytes | uint16_t, battery voltage in mV |

### MCU Temperature (MCUTemp response)

All values little-endian.

| Field       | Size    | Description                                |
|-------------|---------|--------------------------------------------|
| Temperature | 2 bytes | int16_t, tenths of degC (e.g., 253 = 25.3degC) |

Returns `NoCallback` error if the board does not support temperature readings.

### Device Name (DeviceName response)

| Field | Size     | Description                      |
|-------|----------|----------------------------------|
| Name  | variable | UTF-8 string, no null terminator |

### Reboot

Sends an `OK` response, flushes serial, then reboots the device. The host should expect the connection to drop.

### Sensor Permissions (GetSensors)

| Bit | Value  | Description                            |
|-----|--------|----------------------------------------|
| 0   | `0x01` | Base (battery)                         |
| 1   | `0x02` | Location (GPS)                         |
| 2   | `0x04` | Environment (temp, humidity, pressure) |

Use `0x07` for all permissions.

### Sensor Data (Sensors response)

Data returned in CayenneLPP format. See [CayenneLPP documentation](https://docs.mydevices.com/docs/lorawan/cayenne-lpp) for parsing.

## Cryptographic Algorithms

| Operation                         | Algorithm                                                                           |
|-----------------------------------|-------------------------------------------------------------------------------------|
| Identity / Signing / Verification | Ed25519                                                                             |
| Key Exchange                      | X25519 (ECDH)                                                                       |
| Encryption                        | AES-128 block encryption with zero padding + HMAC-SHA256 (MAC truncated to 2 bytes) |
| Hashing                           | SHA-256                                                                             |

## Notes

- Data payload limit (255 bytes) matches MeshCore MAX_TRANS_UNIT; no change needed for KISS "1024+ recommended" (that applies to general TNCs, not MeshCore)
- Modem generates identity on first boot (stored in flash)
- All multi-byte values are little-endian unless stated otherwise
- SNR values in RxMeta are multiplied by 4 for 0.25 dB precision
- TxDone is sent as a SetHardware event after each transmission
- Legacy KISS clients use only type 0x00 data and remain compatible. Version-2-aware clients can use type 0x10 to send on, or identify reception from, `radio2`.
- Standard KISS clients can safely ignore all SetHardware (0x06) frames
- See [packet_format.md](./packet_format.md) for packet format