What "UDP enable" switch actually does?

[TrueWatcher]

I tried to turn on the "UDP enable" button in Settings. I had an idea that may improve voice quality. The visible result was that voice calls began to fail, so I had to turn it off again. What this switch is actually for?

[robinlinden]

We have an issue open to document this in the settings page, since just says "UDP/TCP" means nothing to people that don't know the details of the Tox protocol: #1057

UDP should fall back to TCP if firewalls don't allow for UDP, so not sure what might have caused the calls to fail. How did you notice them failing?

[TrueWatcher]

How did you notice them failing?

I was calling my friend over aTox (and he called me) and we spoke over normal phone line about results. Messaging worked well. Out of 10 call attempts, half hadn't reached their recipients; other half "established" calls only in one diretion. Now I think we should have waited 15 min between changing the settting and calling.

I had an idea the switdh has to do with voice connecction. In XMPP messengers, base node connections use TLS, while voice opens other sockets speaking RTP (UDP). Is Tox sumular? The only thing about voice connections I found by googling is that Tox is using Opus codec.

So, I actually have 2 questions:

• what's the use of "Udp enable" switch?
• what transport aTox uses for voice connections and are there any related settings (like choosing encryption type or adding TURN server in SIP clients) ?

PS

#1057 (comment) UDP means Tox will try to establish direct-connections with your contacts,

I wonder if that's possible with phones. They are either behind their provider's NAT or WiFi router. In my experience with WebRTC, a TURN relay server was a must.