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100 lines
3.5 KiB
Markdown
100 lines
3.5 KiB
Markdown
# MeshCore Protocol Specification
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## Section 4: Payload — ACK
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### Overview
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The ACK (acknowledgment) payload is the simplest payload type. It consists of a
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single 4-byte CRC value used to confirm receipt of a previously sent message.
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The CRC is a truncated SHA-256 over the original TXT_MSG plaintext prefix
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concatenated with the sender's public key.
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### Payload Type
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Header payload type field: `0x03` (ACK)
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### Wire Format
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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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| ACK CRC (uint32_le) |
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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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| ack_crc | 0 | 4 bytes | uint32_le | CRC identifying the acknowledged message |
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### ACK CRC Computation
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The 4-byte ACK CRC is computed as the first 4 bytes of a SHA-256 digest over
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the acknowledged TXT_MSG's plaintext prefix followed by the **sender's**
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Ed25519 public key:
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```
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ack_crc = SHA-256( plaintext_prefix || sender_pub_key )[0..3]
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```
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Where `plaintext_prefix` is the TXT_MSG plaintext up to and including the
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message text, **excluding any trailing NUL terminator**:
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```
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plaintext_prefix = timestamp_le(4) || txt_type_attempt(1) || text
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```
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- `timestamp_le` and `txt_type_attempt` are defined in
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[Section 6](06-payload-encrypted.md#plaintext-format-txt_msg).
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- `text` is the raw UTF-8 message bytes with no NUL terminator.
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- `sender_pub_key` is the full 32-byte Ed25519 public key of the node that
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originated the TXT_MSG (i.e., the node expecting this ACK back).
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For TXT_TYPE_SIGNED_PLAIN messages, the 4-byte signature prefix is included:
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```
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plaintext_prefix = timestamp_le(4) || txt_type_attempt(1) || signature(4) || text
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ack_crc = SHA-256( plaintext_prefix || receiver_pub_key )[0..3]
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```
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Note the SIGNED_PLAIN variant hashes against the **receiver's** public key;
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see `BaseChatMesh::onPeerDataRecv()`.
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Because `txt_type_attempt` includes the 2-bit `attempt` counter, each retry
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with an incremented `attempt` produces a different ACK CRC. This lets the
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sender attribute each received ACK to a specific transmission attempt. See
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[Section 6: Reliable DM Delivery](06-payload-encrypted.md#reliable-dm-delivery).
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### Encoding
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1. Compute the 4-byte CRC value for the message being acknowledged (above).
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2. Write the CRC as a little-endian 32-bit unsigned integer.
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3. Set payload_len to 4.
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### Decoding
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1. Read 4 bytes from the payload as a little-endian uint32.
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2. If fewer than 4 bytes are available, the packet is INVALID.
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### Constraints
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- The payload MUST be exactly 4 bytes.
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- Implementations SHOULD log or discard ACK packets with payload_len < 4.
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### Cross-References
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- [Section 1: Wire Format](01-wire-format.md) — Packet framing
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- [Section 11: Multipart](11-payload-multipart.md) — Multipart ACK encoding
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- Test vectors: [`corpus/payloads/ack/`](https://git.swaits.com/swaits/meshcore-spec/src/branch/main/versions/v0.1.0/corpus/payloads/ack/)
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### Reference Implementation
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- `Mesh::createAck()` in `src/Mesh.cpp` — Packet construction
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- `BaseChatMesh::composeMsgPacket()` in `src/helpers/BaseChatMesh.cpp` —
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Sender-side expected-ACK computation (`expected_ack`)
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- `BaseChatMesh::onPeerDataRecv()` in `src/helpers/BaseChatMesh.cpp` —
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Receiver-side ACK CRC computation (`ack_hash`)
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- ACK reception in `Mesh::onRecvPacket()`, case `PAYLOAD_TYPE_ACK`
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