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Merge pull request #375 from mattsains/packet-structure

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# Packet Structure
| Field | Size (bytes) | Description |
|----------|----------------------------------|-----------------------------------------------------------|
| header | 1 | Contains routing type, payload type, and payload version. |
| path_len | 1 | Length of the path field in bytes. |
| path | up to 64 (`MAX_PATH_SIZE`) | Stores the routing path if applicable. |
| payload | up to 184 (`MAX_PACKET_PAYLOAD`) | The actual data being transmitted. |
Note: see the [payloads doc](./payloads.md) for more information about the content of payload.
## Header Breakdown
| Bits | Mask | Field | Description |
|-------|--------|-----------------|-----------------------------------------------|
| 0-1 | `0x03` | Route Type | Flood, Direct, Reserved - see below. |
| 2-5 | `0x0F` | Payload Type | Request, Response, ACK, etc. - see below. |
| 6-7 | `0x03` | Payload Version | Versioning of the payload format - see below. |
## Route Type Values
| Value | Name | Description |
|--------|------------------------|--------------------------------------|
| `0x00` | `ROUTE_TYPE_RESERVED1` | Reserved for future use. |
| `0x01` | `ROUTE_TYPE_FLOOD` | Flood routing mode (builds up path). |
| `0x02` | `ROUTE_TYPE_DIRECT` | Direct route (path is supplied). |
| `0x03` | `ROUTE_TYPE_RESERVED2` | Reserved for future use. |
## Payload Type Values
| Value | Name | Description |
|--------|---------------------------|-----------------------------------------------|
| `0x00` | `PAYLOAD_TYPE_REQ` | Request (destination/source hashes + MAC). |
| `0x01` | `PAYLOAD_TYPE_RESPONSE` | Response to REQ or ANON_REQ. |
| `0x02` | `PAYLOAD_TYPE_TXT_MSG` | Plain text message. |
| `0x03` | `PAYLOAD_TYPE_ACK` | Acknowledgment. |
| `0x04` | `PAYLOAD_TYPE_ADVERT` | Node advertisement. |
| `0x05` | `PAYLOAD_TYPE_GRP_TXT` | Group text message (unverified). |
| `0x06` | `PAYLOAD_TYPE_GRP_DATA` | Group datagram (unverified). |
| `0x07` | `PAYLOAD_TYPE_ANON_REQ` | Anonymous request. |
| `0x08` | `PAYLOAD_TYPE_PATH` | Returned path. |
| `0x0F` | `PAYLOAD_TYPE_RAW_CUSTOM` | Custom packet (raw bytes, custom encryption). |
## Payload Version Values
| Value | Version | Description |
|--------|---------|---------------------------------------------------|
| `0x00` | 1 | 1-byte src/dest hashes, 2-byte MAC. |
| `0x01` | 2 | Future version (e.g., 2-byte hashes, 4-byte MAC). |
| `0x02` | 3 | Future version. |
| `0x03` | 4 | Future version. |