Google Duo is a video calling app exclusively for it, with no other functionality. Google Duo is stable at low-quality network connection. Unlike hangouts, Duo is more of one to one video calling app.
FEATURES:-
1.Fast video calls of quality 720p
2. Switches automatically between Wi-Fi and mobile data, without dropping calls.
3. It has knock-Knock feature, to allow users to see the caller before picking up the call.
4. Duo shows live video of the caller before you pick the call. It has a live preview feature.
5. Video calls are an end to end encrypted.
6.highDuo allows to mute and pauses the call.
7.Duo supports cross-platform. You can call from your Android phone to apple mobile and vice versa.
8. Supports Google assistant.

Google Duo can be used on Android as well as iOS smartphones. It uses a mobile number for registration. It sends a verification code to your mobile number. That’s it, your job is done. All you need to have is a mobile number. No need of Google account.
Contacts need to feed in mobile to make authentic video connectivity while making calls. Google Duo uses WebRTC, which is an open source technology for real-time communications.
WebRTC is peer-to-peer, means communication packet exchanged directly between participants without passing through a server.
This is the reason behind the great latency and video quality.
Proper selection of video codecs and efficient utilization of hardware video acceleration whenever needed, also makes it incomparable with any other video calling app in terms of speed at least.
It uses QUIC over UDP.
Optimization further achieved by degrading the quality of video through monitoring network quality.