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PacoX
80 lines
3.3 KiB
Markdown
80 lines
3.3 KiB
Markdown
# Routing Paths
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This page covers how MeshCore Open represents, selects, validates, and stores routing paths in the UI and data layer.
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## Path Routing
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MeshCore supports variable-length multi-byte routing paths so the app can scale from small meshes to very large node sets.
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### Hash Width and Multi-Byte Paths
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The device capability determines the hash width (number of bytes per hop):
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| Width | Max Unique Nodes | Typical Use |
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|-------|-----------------|-------------|
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| 1 byte | 256 | Single-byte node IDs |
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| 2 bytes | 65,536 | Medium meshes |
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| 3 bytes | 16.7M | Large networks |
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| 4 bytes | 4.3G | Very large meshes / future-proofing |
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### Device Capability Detection
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On device connection, the app reads the firmware capability to set `pathHashByteWidth`:
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```dart
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// Read from device info response (offset 81)
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final modeRaw = firmwareBytes.length >= 82
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? (firmwareBytes[81] & 0xFF)
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: 0;
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final mode = modeRaw.clamp(0, 3);
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_pathHashByteWidth = mode + 1; // 1, 2, 3, or 4 bytes per hop
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```
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The connector reads a single-mode byte and clamps to `0..3`, so the supported hop width is `1..4` bytes. UI code also clamps widths when rendering (typically to `1..4`) so 4-byte hops are handled end-to-end in the current codebase.
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### Path Data Structure
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Paths in messages and storage consist of:
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- **`pathLength` (model/storage)**: Hop count (number of hops). Negative values (e.g. `-1`) are used as a flood sentinel.
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- **On-air `path_len` byte**: A packed byte that encodes hop count + hash width and is decoded into `pathLength` + `pathBytes` when parsing frames.
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- **`pathBytes`**: Raw bytes of the path (concatenated hop prefixes), grouped by `pathHashByteWidth`.
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- **`hopCount`**: Derived display value computed from bytes and width: `(byteCount + hashByteWidth - 1) ~/ hashByteWidth`.
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- **Example**: With `pathHashByteWidth=2`, a 3-hop path has 6 bytes (`pathBytes.length = 6`) and `pathLength = 3`:
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- `pathBytes = [0xA1, 0xA2, 0xB1, 0xB2, 0xC1, 0xC2]`
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- Hops: `[0xA1A2]`, `[0xB1B2]`, `[0xC1C2]`
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### Hop Count Calculation
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Convert path byte length to hop count:
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```dart
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int hopCount = (byteCount + hashByteWidth - 1) ~/ hashByteWidth;
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```
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Use this consistently when displaying hop counts in UI. Do not treat `pathLength` as a hop count when the path uses multi-byte hop hashes.
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### Path Usage in Different Message Types
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- **Direct messages**: Extract path from decrypted payload to trace sender route.
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- **Channel messages**: Decrypt hop-by-hop routing chain from payload; the header carries the encoded byte length for the path blob, not the derived hop count.
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- **Contact storage**: Path length in byte 32, raw path bytes in bytes 33-96, grouped by detected `pathHashByteWidth`.
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### UI Hop Count Display
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⚠️ **Important**: In screens like "Channel Message Path", prefer the actual decoded `pathBytes` hop count over `pathLength` metadata:
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```dart
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// Preferred: use actual observed path length
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final effectiveHopCount = (pathBytes.length + width - 1) ~/ width;
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// Avoid: using encoded byte length as if it were a hop count
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// pathLength is bytes; converting it twice causes inflated counts
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```
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Example scenario:
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- Radio header reports `pathLength: 32` bytes
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- Decoded path bytes: `[0xAB, 0xCD]` (2 bytes = 1 hop with width=2)
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- **Display**: "1 hop" (from `pathBytes`), not "32 hops" (which would double-count the encoded length)
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