Warm spare: why your connection doesn't drop

#How does it work?

When Colitu connects, it sets up the main path and runs it through the health check (see How Colitu Adaptive Connect works). Warm spare is added on top:

1. 1
   While you are connected, the app now and then runs a small check to see whether the main path really carries traffic.
2. 2
   If the main path stops working, new connections are sent over the second path.
3. 3
   You don't have to do anything, and you don't have to open the app.

The switch happens inside the connection itself, not on the app screen. That is why it also works while the app is in the background or the screen is off.

#Which path is the spare?

The spare is a different kind of connection mode than the main path.

"The other kind" means: if the main path is Hysteria2 (UDP), the spare is a TCP mode such as VLESS Reality. If the main path is a TCP mode, the spare is Hysteria2 where possible.

Modes and servers that failed on this network are never chosen as the spare.

In automatic mode the spare can be a different server, so your exit IP address and location may change at the moment of the switch. If you picked the location yourself, it doesn't change.

#What will you notice?

So warm spare doesn't remove a drop; it turns it into a short glitch. Downloads continuing without any break is future work, and we don't promise it today.

#How fast is the switch?

#Data use

Checking the main path creates a small amount of extra traffic:

• Phones

  about 0.3–2 MB per hour

• Computers

  about 4–7 MB per hour

The spare itself connects only when it is used; it carries no traffic while it waits.

#With the kill switch

Warm spare works together with the kill switch. The Windows kill switch allows the spare server too, and Linux adds it to its allow-list. See Kill switch.

#How do I turn it off?

Warm spare is on by default.

• In Simple mode it is always on and can't be turned off.
• To turn it off, switch on Advanced mode and turn off the Warm spare switch among the advanced settings. The connection then works with a single path, as before.

Switching to Advanced mode doesn't need a reconnect.

#FAQ

Does warm spare make the VPN faster?

No. For speed, see Speed optimisation. Warm spare doesn't speed the connection up; it only keeps you connected when the main path stops working.

What happens to a download that's running?

A large download running at that moment may restart. Pages and messages reconnect on their own.

Why is there sometimes no spare?

If you use a multihop route, if the server offers only one mode, or if a suitable spare already failed on this network, no spare is set up. The connection then works with a single path, as before.

Does it work in the background?

Yes. The switch happens inside the connection, not on the app screen, so it works while the app is in the background.

Does it use battery and data?

Checking the main path creates a small amount of extra traffic (see "Data use" above). The spare itself connects only when it is used.

What if it still drops?

Both the main and the spare path can fail at once, for example when the device loses the internet completely. In that case automatic reconnecting takes over (see How Colitu Adaptive Connect works). If the problem continues, see Connection problems.

#Technical report

To read the design, measurement method, results and limitations of Adaptive Connect 2.0 and warm spare as a research report, see Colitu Adaptive Connect 2.0 and Warm Spare: a technical report.