# VerseVDI control datagram v1

Phase 3A reserves a bounded control-datagram envelope. It does not forward video,
audio, provider traffic, VM traffic, or arbitrary binary clipboard data.

All multi-byte integers are unsigned big-endian. The fixed header is 21 bytes:

| Offset | Size | Field | Rule |
|---:|---:|---|---|
| 0 | 2 | magic | ASCII `VD` (`0x56 0x44`) |
| 2 | 1 | version | `1` only |
| 3 | 1 | channel | registered channel identifier |
| 4 | 1 | flags | registry-defined; unknown bits reject |
| 5 | 4 | sequence | session-local wrapping sequence |
| 9 | 8 | timestamp_ms | sender timestamp, bounded by transport skew policy |
| 17 | 1 | fragment_index | zero-based; `0` when not fragmented |
| 18 | 1 | fragment_count | `1` when not fragmented; index less than count |
| 19 | 2 | payload_length | exact payload byte count |

The complete frame is at most 65,536 bytes and `payload_length` is at most 65,515.
Truncated, oversized, unknown-version, unknown-channel, invalid-fragment, and
length-mismatch frames are rejected before allocation proportional to the claimed
payload. Media/provider identifiers are not registered channels.

Registered Phase 3A channels are `control.ack.v1`, `control.cancel.v1`, and
`clipboard.text.v1`. Clipboard payloads are UTF-8 JSON text contracts and remain
subject to the 65,536-byte text limit and explicit authorization.