swaitsandClaude Opus 4.7
f83d4088e3
Every "Test vectors: \`corpus/...\`" mention in the spec is now a markdown link pointing at the corresponding file or directory on GitHub. Each version build resolves its links to the correct git ref: - /latest/ -> https://github.com/swaits/meshcore-spec/tree/main/corpus/... - /v0.1.0/ -> https://github.com/swaits/meshcore-spec/tree/v0.1.0/corpus/... - future /vX.Y.Z/ -> /tree/vX.Y.Z/corpus/... Two changes: 1. src/*.md (18 files): the inline-code corpus paths in each "Cross-References" section are now markdown links. Directories point at /tree/main/corpus/..., individual .json files point at /blob/main/corpus/.... CHANGELOG and other non-corpus main links are intentionally left pointing at main — the canonical history lives there regardless of which version you're reading. 2. justfile: `cut-version` now also runs a sed over the snapshot's *.md files replacing /tree/main/corpus -> /tree/<VERSION>/corpus and /blob/main/corpus -> /blob/<VERSION>/corpus. So a freshly-cut version is self-consistent. versions/v0.1.0/* regenerated by re-running `just cut-version v0.1.0` after the new src/ links + recipe sed landed. Tagging the release commit (`git tag v0.1.0`) is now REQUIRED — not "optional" — because the rendered /v0.1.0/ pages reference /tree/v0.1.0/corpus/... on GitHub. Without the tag the links 404. cut-version's output now says so. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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://github.com/swaits/meshcore-spec/tree/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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