# Meshcore payloads
Inside of each [meshcore packet](./packet_structure.md) is a payload, identified by the payload type in the packet header. The types of payloads are:

* Node advertisement.
* Acknowledgment.
* Returned path.
* Request (destination/source hashes + MAC).
* Response to REQ or ANON_REQ.
* Plain text message.
* Anonymous request.
* Group text message (unverified).
* Group datagram (unverified).
* Multi-part packet
* Control data packet
* Custom packet (raw bytes, custom encryption).

This document defines the structure of each of these payload types.

NOTE: all 16 and 32-bit integer fields are Little Endian.

## Important concepts:

* Node hash: the first byte of the node's public key

# Node advertisement
This kind of payload notifies receivers that a node exists, and gives information about the node

| Field         | Size (bytes)    | Description                                              |
|---------------|-----------------|----------------------------------------------------------|
| public key    | 32              | Ed25519 public key of the node                           |
| timestamp     | 4               | unix timestamp of advertisement                          |
| signature     | 64              | Ed25519 signature of public key, timestamp, and app data |
| appdata       | rest of payload | optional, see below                                      |

Appdata

| Field         | Size (bytes)    | Description                                           |
|---------------|-----------------|-------------------------------------------------------|
| flags         | 1               | specifies which of the fields are present, see below  |
| latitude      | 4 (optional)    | decimal latitude multiplied by 1000000, integer       |
| longitude     | 4 (optional)    | decimal longitude multiplied by 1000000, integer      |
| feature 1     | 2  (optional)   | reserved for future use                               |
| feature 2     | 2  (optional)   | reserved for future use                               |
| name          | rest of appdata | name of the node                                      |

Appdata Flags

| Value  | Name           | Description                           |
|--------|----------------|---------------------------------------|
| `0x01` | is chat node   | advert is for a chat node             |
| `0x02` | is repeater    | advert is for a repeater              |
| `0x03` | is room server | advert is for a room server           |
| `0x04` | is sensor      | advert is for a sensor server         |
| `0x10` | has location   | appdata contains lat/long information |
| `0x20` | has feature 1  | Reserved for future use.              |
| `0x40` | has feature 2  | Reserved for future use.              |
| `0x80` | has name       | appdata contains a node name          |

# Acknowledgement

An acknowledgement that a message was received. Note that for returned path messages, an acknowledgement can be sent in the "extra" payload (see [Returned Path](#returned-path)) instead of as a separate ackowledgement packet. CLI commands do not cause acknowledgement responses, neither discrete nor extra.

| Field    | Size (bytes) | Description                                                |
|----------|--------------|------------------------------------------------------------|
| checksum | 4            | CRC checksum of message timestamp, text, and sender pubkey |


# Returned path, request, response, and plain text message

Returned path, request, response, and plain text messages are all formatted in the same way. See the subsection for more details about the ciphertext's associated plaintext representation.

| Field            | Size (bytes)    | Description                                          |
|------------------|-----------------|------------------------------------------------------|
| destination hash | 1               | first byte of destination node public key            |
| source hash      | 1               | first byte of source node public key                 |
| cipher MAC       | 2               | MAC for encrypted data in next field                 |
| ciphertext       | rest of payload | encrypted message, see subsections below for details |

## Returned path

Returned path messages provide a description of the route a packet took from the original author. Receivers will send returned path messages to the author of the original message.

| Field       | Size (bytes) | Description                                                                                  |
|-------------|--------------|----------------------------------------------------------------------------------------------|
| path length | 1            | length of next field                                                                         |
| path        | see above    | a list of node hashes (one byte each) |
| extra type  | 1            | extra, bundled payload type, eg., acknowledgement or response. Same values as in [packet structure](./packet_structure.md) |
| extra       | rest of data | extra, bundled payload content, follows same format as main content defined by this document |

## Request

| Field        | Size (bytes)    | Description                |
|--------------|-----------------|----------------------------|
| timestamp    | 4               | send time (unix timestamp) |
| request type | 1               | see below                  |
| request data | rest of payload | depends on request type    |

Request type

| Value  | Name                 | Description                           |
|--------|----------------------|---------------------------------------|
| `0x01` | get stats            | get stats of repeater or room server  |
| `0x02` | keepalive            | (deprecated) |
| `0x03` | get telemetry data   | TODO |
| `0x04` | get min,max,avg data | sensor nodes - get min, max, average for given time span |
| `0x05` | get access list      | get node's approved access list       |

### Get stats

Gets information about the node, possibly including the following:

* Battery level (millivolts)
* Current transmit queue length
* Current free queue length
* Last RSSI value
* Number of received packets
* Number of sent packets
* Total airtime (seconds)
* Total uptime (seconds)
* Number of packets sent as flood
* Number of packets sent directly
* Number of packets received as flood
* Number of packets received directly
* Error flags
* Last SNR value
* Number of direct route duplicates
* Number of flood route duplicates
* Number posted (?)
* Number of post pushes (?)

### Get telemetry data

Request data about sensors on the node, including battery level.

## Response

| Field   | Size (bytes)    | Description |
|---------|-----------------|-------------|
| tag     | 4               | TODO        |
| content | rest of payload | TODO        |

## Plain text message

| Field              | Size (bytes)    | Description                                                  |
|--------------------|-----------------|--------------------------------------------------------------|
| timestamp          | 4               | send time (unix timestamp)                                   |
| txt_type + attempt | 1               | upper six bits are txt_type (see below), lower two bits are attempt number (0..3) |
| message            | rest of payload | the message content, see next table                          |

txt_type

| Value  | Description               | Message content                                            |
|--------|---------------------------|------------------------------------------------------------|
| `0x00` | plain text message        | the plain text of the message                              |
| `0x01` | CLI command               | the command text of the message                            |
| `0x02` | signed plain text message | first four bytes is sender pubkey prefix, followed by plain text message |

# Anonymous request

| Field            | Size (bytes)    | Description                               |
|------------------|-----------------|-------------------------------------------|
| destination hash | 1               | first byte of destination node public key |
| public key       | 32              | sender's Ed25519 public key               |
| cipher MAC       | 2               | MAC for encrypted data in next field      |
| ciphertext       | rest of payload | encrypted message, see below for details  |

## Room server login

| Field          | Size (bytes)    | Description                                                                   |
|----------------|-----------------|-------------------------------------------------------------------------------|
| timestamp      | 4               | sender time (unix timestamp)                                                  |
| sync timestamp | 4               | sender's "sync messages SINCE x" timestamp                                    |
| password       | rest of message | password for room                                                             |

## Repeater/Sensor login

| Field          | Size (bytes)    | Description                                                                   |
|----------------|-----------------|-------------------------------------------------------------------------------|
| timestamp      | 4               | sender time (unix timestamp)                                                  |
| password       | rest of message | password for repeater/sensor                                                  |

# Group text message / datagram

| Field        | Size (bytes)    | Description                                |
|--------------|-----------------|--------------------------------------------|
| channel hash | 1               | first byte of SHA256 of channel's shared key  |
| cipher MAC   | 2               | MAC for encrypted data in next field       |
| ciphertext   | rest of payload | encrypted message, see below for details   |

The plaintext contained in the ciphertext matches the format described in [plain text message](#plain-text-message). Specifically, it consists of a four byte timestamp, a flags byte, and the message. The flags byte will generally be `0x00` because it is a "plain text message". The message will be of the form `<sender name>: <message body>` (eg., `user123: I'm on my way`).


# Control data

| Field        | Size (bytes)    | Description                                |
|--------------|-----------------|--------------------------------------------|
| flags        | 1               | upper 4 bits is sub_type                   |
| data         | rest of payload | typically unencrypted data                 |

## DISCOVER_REQ (sub_type)

| Field        | Size (bytes)    | Description                                  |
|--------------|-----------------|----------------------------------------------|
| flags        | 1               | 0x8 (upper 4 bits), prefix_only (lowest bit) |
| type_filter  | 1               | bit for each ADV_TYPE_*                      |
| tag          | 4               | randomly generate by sender                  |
| since        | 4               | (optional) epoch timestamp (0 by default)    |

## DISCOVER_RESP (sub_type)

| Field        | Size (bytes)    | Description                                |
|--------------|-----------------|--------------------------------------------|
| flags        | 1               | 0x9 (upper 4 bits), node_type (lower 4)    |
| snr          | 1               | signed, SNR*4                              |
| tag          | 4               | reflected back from DISCOVER_REQ           |
| pubkey       | 8 or 32         | node's ID (or prefix)                      |


# Custom packet

Custom packets have no defined format.