# Transport Designs

## USB Transport

USB is the first MVP transport.

### Phone Side

- Android uses accessory-mode APIs through `UsbManager.openAccessory`.
- The gateway treats USB as an ordered byte stream carrying VSHP frames.
- The USB session is accepted only after phone-side pairing approval.
- If USB disconnects, the peer lease remains resumable for a short window.

### Desktop Side

- The desktop client acts as USB host and opens the Android Open Accessory
  channel.
- The client advertises platform, app version, and public key in `HELLO`.
- After `CONFIG`, the client configures the local virtual interface and routes.

### Failure Modes

- Cable unplug: transport closes, session enters resumable state.
- Host sleeps: session ticket can resume on reconnect.
- Unknown host: no IP traffic is accepted before pairing approval.

## Wi-Fi Transport

- Discovery uses DNS-SD service type `_vpnshare._udp`.
- QR pairing remains the reliable fallback when multicast is blocked.
- The tunnel uses the same VSHP frame format as USB.
- The gateway never assumes LAN is trusted.

## Hotspot Transport

- Android app-created hotspot uses `LocalOnlyHotspot`.
- The local-only hotspot does not provide Internet; it only provides local reach
  to the gateway app.
- The client still installs its virtual interface and uses VSHP for all traffic.

## Transport Selection

Default order:

1. USB if connected and paired.
2. Known Wi-Fi peer on same LAN.
3. Local-only hotspot if the user starts it from the app.

Session resume can switch transports if the peer key and resume ticket validate.