# Component — `mavlink_layer`

**Layer**: Action (data plane out)
**Status**: forward-looking design (Rust); hand-rolled (no third-party SDK)

## 1. Purpose

Hand-rolled MAVLink v2 transport. Implements only the ~10–15 commands this codebase needs (full list in `architecture.md §7.7`). Owns serialisation / deserialisation, heartbeat, sequence numbers, retry, and a single connection abstraction (UDP or serial, picked at startup from CLI / env). No third-party SDK — eliminating the largest current dependency-risk item.

## 2. Inputs

| Input | Source | Cadence | Notes |
|---|---|---|---|
| Outgoing `COMMAND_LONG`, `MISSION_*`, `SET_MODE` | `mission_executor` | per state transition | Hand-rolled message constructors per command. |
| Outgoing heartbeat | self (timer) | 1 Hz | `HEARTBEAT` to keep the autopilot's GCS-link alive. |
| Connection URI | startup config | once | `udp://...` or `serial:///dev/...`. |
| MAVLink-2 signing config | startup config | once | If supported by the link, signing is enabled; otherwise the link is treated as trusted. |

## 3. Outputs

| Output | Consumer | Shape |
|---|---|---|
| Decoded MAVLink messages | `mission_executor`, `telemetry_stream`, `movement_detector` (for UAV motion telemetry) | typed enum per message kind |
| Connection state | health aggregator | `connected`, `last_heartbeat_age_ms`, `tx_seq`, `rx_seq`, `parse_errors_total`, `signing_enabled`. |

The supported message surface (concise list; full table in `architecture.md §7.7`):

- `HEARTBEAT` (bidir)
- `COMMAND_LONG` subset (out): arm/disarm, takeoff, set-mode, change-speed, change-alt, land, RTL
- `COMMAND_ACK` (in)
- `MISSION_COUNT`, `MISSION_REQUEST_INT`, `MISSION_ITEM_INT`, `MISSION_ACK`, `MISSION_SET_CURRENT`, `MISSION_CURRENT`, `MISSION_ITEM_REACHED`, `MISSION_CLEAR_ALL`
- `GLOBAL_POSITION_INT`, `ATTITUDE`, `SYS_STATUS`, `EXTENDED_SYS_STATE`, `STATUSTEXT`
- `SET_MODE` (out, fixed-wing)

## 4. Key Responsibilities

- Open and maintain the MAVLink connection (UDP or serial). Reconnect on transport loss with bounded backoff.
- Encode outgoing messages with correct sequence numbers, system / component IDs, and (when enabled) MAVLink-2 signing.
- Decode incoming messages with strict validation: reject malformed frames, unknown message IDs, and signing failures.
- Emit a 1 Hz heartbeat. Detect autopilot heartbeat timeouts and surface to health.
- Demux `COMMAND_ACK` to the originating caller (per `command_id`); enforce a wall-clock ack timeout.

## 5. Internal State

- Connection handle (UDP socket or serial port).
- Outgoing sequence number.
- In-flight command map (`command_id → (caller, deadline)`).
- Per-message-kind parse error counters.

State is in-process only.

## 6. Failure Modes

| Failure | Detection | Behaviour |
|---|---|---|
| Transport open failure | OS error | Bounded backoff; surface to health → red. |
| Heartbeat from autopilot missing | wall-clock timeout | Surface `link_lost` to health and to `mission_executor`; do not silently fail. |
| Command-ack timeout | wall-clock | Bubble timeout to `mission_executor`; the executor decides retry vs failure. |
| Malformed inbound frame | parser error | Drop the frame; increment counter; do not abort the link. |
| MAVLink-2 signing mismatch (if enabled) | signature check | Reject the frame; alert; do not silently accept. |
| Sequence-number gap | rx_seq vs expected | Log; not a hard failure on its own. |

## 7. Dependencies

**In-process** (input): `mission_executor`.
**In-process** (output): `mission_executor`, `telemetry_stream`, `movement_detector`.

**External**: ArduPilot / PX4 over MAVLink v2 (UDP or serial).

## 8. Non-Functional Targets

| Concern | Target |
|---|---|
| Per-message round-trip on a healthy link | ≤50 ms p99 |
| Heartbeat cadence | 1 Hz out |
| Command-ack timeout | configurable; default 1 s, with retry handled by `mission_executor` |
| Reconnect after transport loss | ≤2 s on serial / ≤5 s on UDP |
| Message subset | ~10–15 commands only — adding more requires explicit design review |

## 9. Open Questions

- **MAVLink-2 message signing** (`architecture.md §8 Q6`): whether the airframe link enables signing or treats the link as trusted.

## 10. References

- `architecture.md §3`, `§5 Architectural Principles` (no MAVSDK, no silent error swallowing), `§7.7 MAVLink and Piloting`.
- `system-flows.md §F6 Mission lifecycle`.
