# How SocketFlip works

> How SocketFlip gives one app a clean reconnect: a per-app Android VPN that routes one unused address, what it resets (TCP) and what it cannot (UDP).

Source page: https://socketflip.app/how-it-works/

## What it can and cannot reset

SocketFlip resets **TCP** connections only. When a VPN starts or stops, Android
closes the covered app's open TCP sockets; it does not do this for **UDP**.

- **Reset:** anything that talks to its server over TCP, which is most app
  APIs, chat and notification links, game servers and ordinary web traffic.
- **Not reset:** voice and video calls (WhatsApp, Signal, Meet and similar),
  most online game voice chat, and traffic using QUIC / HTTP/3, all of which
  run over UDP. A call in progress simply carries on.

If you tap and nothing seems to happen, the app you chose is probably talking
UDP, or reconnected so quickly you did not notice.

## How it works

SocketFlip uses Android's VPN API, but not to carry traffic:

- The tunnel covers only the apps you tick. It has the address `10.111.222.1` and
  routes exactly one other address, `10.111.222.2`, which nothing uses. All of
  the app's real traffic still goes out over Wi-Fi or mobile data as normal.
  There is no server at the other end and nothing leaves the phone.
- Whenever a VPN covering an app starts or stops, Android closes that app's open
  sockets. That is the clean disconnect.
- Each tap makes exactly **one** state change (tunnel up, or tunnel down), so each
  tap is exactly one disconnect. Bringing a tunnel up and straight back down hits
  the app twice, and a reconnect that lands between the two can get stuck.

Things that do **not** work, for the record:

- Airplane mode takes the whole Wi-Fi link down, which is slow to come back.
- A per-app firewall block only stalls the connection silently. A short stall is
  never noticed, so nothing reconnects.

Taps closer together than the cooldown (10 seconds unless you change it in
Settings) are ignored: a second disconnect while the app is still reconnecting can
leave it stuck on its reconnect screen. A ring around the button shows the time
left.

While the tunnel is up the status bar shows a VPN key. That is expected; the next
tap takes it down again (and is also a reconnect).