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124 lines
4.9 KiB
Markdown
124 lines
4.9 KiB
Markdown
# MeshCore Protocol Specification
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## Section 5: Payload — Advertisement
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### Overview
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The advertisement payload allows a node to announce its identity to the mesh. It
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contains the node's Ed25519 public key, a timestamp, a signature proving
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ownership of the key, and optional application data describing the node's type,
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location, features, and name.
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### Payload Type
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Header payload type field: `0x04` (ADVERT)
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### Wire Format
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```
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0 31
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Ed25519 Public Key (32 bytes) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Timestamp (uint32_le) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Ed25519 Signature (64 bytes) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| App Data (0-32 bytes) |
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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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| pub_key | 0 | 32 bytes | raw | Ed25519 public key of the advertising node |
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| timestamp | 32 | 4 bytes | uint32_le | Unix epoch timestamp of advertisement creation |
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| signature | 36 | 64 bytes | raw | Ed25519 signature (see Signature section below) |
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| app_data | 100 | 0-32 bytes | structured | Optional application data (see App Data section below) |
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### Minimum and Maximum Sizes
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- Minimum payload: 100 bytes (pub_key + timestamp + signature, no app_data)
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- Maximum payload: 132 bytes (100 + MAX_ADVERT_DATA_SIZE of 32)
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### Signature
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The signature is computed over the concatenation of:
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```
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message = pub_key(32) || timestamp_le(4) || app_data(0-32)
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```
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The node signs this message using its Ed25519 private key. Receivers MUST verify
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the signature using the public key from the payload. Packets with invalid
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signatures MUST be discarded.
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### App Data Format
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When present (payload_len > 100), app_data is a structured field beginning with
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a flags byte:
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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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|has_ |has_ |has_ |has_ | Node Type |
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|name |feat2 |feat1 |loc | (4 bits) |
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+-------+-------+-------+-------+---+---+---+---+
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```
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| Field | Offset | Size | Condition | Type | Description |
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|-------|--------|------|-----------|------|-------------|
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| flags | 0 | 1 byte | Always | uint8 | See bit layout above |
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| latitude | 1 | 4 bytes | flags bit 4 set | int32_le | Latitude × 1,000,000 |
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| longitude | 5 | 4 bytes | flags bit 4 set | int32_le | Longitude × 1,000,000 |
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| feat1 | varies | 2 bytes | flags bit 5 set | uint16_le | Feature field 1 |
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| feat2 | varies | 2 bytes | flags bit 6 set | uint16_le | Feature field 2 |
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| name | varies | remaining | flags bit 7 set | UTF-8 | Node name (no null terminator) |
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Fields appear in the order listed. The offset of each field depends on which
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preceding optional fields are present.
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### Node Types (flags bits 0-3)
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| Value | Name | Description |
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|-------|------|-------------|
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| 0 | None | Unspecified |
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| 1 | Chat | Chat client node |
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| 2 | Repeater | Mesh repeater node |
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| 3 | Room | Room server node |
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| 4 | Sensor | Sensor node |
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### Decoding Algorithm
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1. Read 32 bytes as pub_key.
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2. Read 4 bytes as timestamp (uint32_le).
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3. Read 64 bytes as signature.
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4. If offset (100) > payload_len, the packet is INVALID (incomplete).
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5. If offset (100) == payload_len, there is no app_data. Done.
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6. Read remaining bytes as app_data (up to MAX_ADVERT_DATA_SIZE = 32 bytes).
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If more than 32 bytes remain, implementations MUST truncate to 32.
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7. Construct the signature verification message: pub_key || timestamp_le || app_data.
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8. Verify the Ed25519 signature. If invalid, the receiver MUST discard the packet.
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9. Parse app_data:
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a. Read 1 byte as flags.
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b. If flags bit 4 set: read 4 bytes latitude (int32_le), 4 bytes longitude (int32_le).
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c. If flags bit 5 set: read 2 bytes feat1 (uint16_le).
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d. If flags bit 6 set: read 2 bytes feat2 (uint16_le).
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e. If flags bit 7 set: read remaining bytes as UTF-8 name.
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### Cross-References
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- [Section 15: Identity](15-identity.md) — Ed25519 key management
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- [Section 14: Cryptography](14-crypto.md) — Signature operations
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- Test vectors: [`corpus/payloads/advert/`](https://git.swaits.com/swaits/meshcore-spec/src/branch/main/versions/v0.1.0/corpus/payloads/advert/)
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### Reference Implementation
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- `Mesh::createAdvert()` in `src/Mesh.cpp` — Encoding
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- `Mesh::onRecvPacket()`, case `PAYLOAD_TYPE_ADVERT` in `src/Mesh.cpp` — Decoding and verification
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