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107 lines
3.8 KiB
Markdown
107 lines
3.8 KiB
Markdown
# MeshCore Protocol Specification
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## Section 10: Payload — Trace
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### Overview
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The TRACE payload enables path discovery with per-hop signal quality measurement.
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A trace packet travels along a specified direct path, and each intermediate node
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appends its received SNR value to the packet's path field. When the trace reaches
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its final hop, the accumulated SNR values and node hashes are delivered.
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### Payload Type
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Header payload type field: `0x09` (TRACE)
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### Wire Format
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The trace payload has a fixed 9-byte header followed by an optional path hashes
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section (appended when sent via direct routing):
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```
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0 1 2 3
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Tag (uint32_le) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Auth Code (uint32_le) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Flags | Path Hashes (variable) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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```
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### Fields
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| Field | Offset | Size | Type | Description |
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|-------|--------|------|------|-------------|
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| tag | 0 | 4 bytes | uint32_le | Random tag set by the initiator for correlation |
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| auth_code | 4 | 4 bytes | uint32_le | Authentication code |
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| flags | 8 | 1 byte | uint8 | Flags (see below) |
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| path_hashes | 9 | variable | raw | Node hashes for the path to trace (appended for direct send) |
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### Flags Field
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```
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Bit: 7 6 5 4 3 2 1 0
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+---+---+---+---+---+---+---+---+
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| Reserved | S1 S0|
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+---+---+---+---+---+---+---+---+
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```
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| Bits | Description |
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|------|-------------|
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| 0-1 | Path hash size for the trace path hashes (as a power of 2: 0=1 byte, 1=2 bytes, 2=4 bytes) |
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| 2-7 | Reserved (must be zero) |
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Note: The flags field path hash size encoding (`1 << (flags & 0x03)`) differs
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from the packet path_len encoding (`(code >> 6) + 1`).
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### Direct Sending
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When a TRACE packet is sent via direct routing, the path hashes are appended
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directly to the payload (after the 9-byte header), not placed in the packet's
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path field. The packet's path field is instead used to accumulate SNR values
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from intermediate nodes.
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```
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On creation: payload = [tag(4)][auth_code(4)][flags(1)] (9 bytes)
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On sendDirect: payload += path_hashes (9 + N bytes)
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packet.path_len = 0 (path used for SNR)
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```
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### SNR Accumulation
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As each intermediate node forwards a TRACE packet, it appends its received SNR
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value as a single signed byte to the packet's path field:
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```
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path[path_len++] = (int8_t)(SNR × 4)
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```
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The SNR value is stored as SNR × 4 for 0.25 dB precision, matching the format
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used in RxMeta frames.
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### Trace Completion
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A TRACE is considered complete when the path_len counter (used as an offset into
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the path hashes in the payload) reaches or exceeds the remaining path hashes.
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At that point, `onTraceRecv` is called with:
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- The accumulated SNR values (in packet.path)
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- The path hashes (in packet.payload, after byte 9)
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- The path length (number of hops traversed)
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### Minimum Size
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- Minimum payload: 9 bytes (tag + auth_code + flags, no path hashes)
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### Cross-References
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- [Section 16: Packet Hash](16-packet-hash.md) — TRACE packets include path_len in hash
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- Test vectors: [`corpus/payloads/trace/`](https://git.swaits.com/swaits/meshcore-spec/src/branch/main/versions/v0.1.0/corpus/payloads/trace/)
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### Reference Implementation
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- `Mesh::createTrace()` in `src/Mesh.cpp` — Creation (9-byte base)
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- `Mesh::sendDirect()` — Path appended to payload for TRACE type
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- `Mesh::onRecvPacket()`, TRACE handling — SNR accumulation and completion check
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