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docs: clarify path length encoding

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Robert Ekl
2026-03-17 12:55:18 -05:00
parent a22c4b6270
commit 003eda1f1a
1 changed files with 37 additions and 5 deletions
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docs/packet_format.md
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@@ -48,10 +48,17 @@ This is the protocol level packet structure used in MeshCore firmware v1.12.0
- Only present for `ROUTE_TYPE_TRANSPORT_FLOOD` and `ROUTE_TYPE_TRANSPORT_DIRECT`
- `transport_code_1` - 2 bytes - `uint16_t` - calculated from region scope
- `transport_code_2` - 2 bytes - `uint16_t` - reserved
- `path_length` - 1 byte - Length of the path field in bytes
- `path` - size provided by `path_length` - Path to use for Direct Routing
- `path_length` - 1 byte - Encoded path metadata
- Bits 0-5 store path hash count / hop count (`0-63`)
- Bits 6-7 store path hash size minus 1
- `0b00`: 1-byte path hashes
- `0b01`: 2-byte path hashes
- `0b10`: 3-byte path hashes
- `0b11`: reserved / unsupported
- `path` - `hop_count * hash_size` bytes - Path to use for Direct Routing or flood path tracking
- Up to a maximum of 64 bytes, defined by `MAX_PATH_SIZE`
- v1.12.0 firmware and older drops packets with `path_length` [larger than 64](https://github.com/meshcore-dev/MeshCore/blob/e812632235274ffd2382adf5354168aec765d416/src/Dispatcher.cpp#L144)
- Effective byte length is calculated from the encoded hop count and hash size, not taken directly from `path_length`
- v1.12.0 firmware and older only handled legacy 1-byte path hashes and dropped packets whose path bytes exceeded [64 bytes](https://github.com/meshcore-dev/MeshCore/blob/e812632235274ffd2382adf5354168aec765d416/src/Dispatcher.cpp#L144)
- `payload` - variable length - Payload Data
- Up to a maximum 184 bytes, defined by `MAX_PACKET_PAYLOAD`
- Generally this is the remainder of the raw packet data
@@ -64,8 +71,8 @@ This is the protocol level packet structure used in MeshCore firmware v1.12.0
|-----------------|----------------------------------|----------------------------------------------------------|
| header | 1 | Contains routing type, payload type, and payload version |
| transport_codes | 4 (optional) | 2x 16-bit transport codes (if ROUTE_TYPE_TRANSPORT_*) |
| path_length | 1 | Length of the path field in bytes |
| path | up to 64 (`MAX_PATH_SIZE`) | Stores the routing path if applicable |
| path_length | 1 | Encodes path hash size in bits 6-7 and hop count in bits 0-5 |
| path | up to 64 (`MAX_PATH_SIZE`) | Stores `hop_count * hash_size` bytes of path data if applicable |
| payload | up to 184 (`MAX_PACKET_PAYLOAD`) | Data for the provided Payload Type |
> NOTE: see the [Payloads](./payloads.md) documentation for more information about the content of specific payload types.
@@ -89,6 +96,31 @@ Bit 0 means the lowest bit (1s place)
| `0x02` | `ROUTE_TYPE_DIRECT` | Direct Routing |
| `0x03` | `ROUTE_TYPE_TRANSPORT_DIRECT` | Direct Routing + Transport Codes |
### Path Length Encoding
`path_length` is not a raw byte count. It packs both hash size and hop count:
| Bits | Field | Meaning |
|------|-------|---------|
| 0-5 | Hop Count | Number of path hashes (`0-63`) |
| 6-7 | Hash Size Code | Stored as `hash_size - 1` |
Hash size codes:
| Bits 6-7 | Hash Size | Notes |
|----------|-----------|-------|
| `0b00` | 1 byte | Legacy / default mode |
| `0b01` | 2 bytes | Supported in current firmware |
| `0b10` | 3 bytes | Supported in current firmware |
| `0b11` | 4 bytes | Reserved / invalid |
Examples:
- `0x00`: zero-hop packet, no path bytes
- `0x05`: 5 hops using 1-byte hashes, so path is 5 bytes
- `0x45`: 5 hops using 2-byte hashes, so path is 10 bytes
- `0x8A`: 10 hops using 3-byte hashes, so path is 30 bytes
### Payload Types
| Value | Name | Description |