what mechanisms used by Whatsapp and Viber for Chatting and messaging

Question

I am searching some thing about Whatsapp and Viber.

They are providing Free calls ans SMS and Chatting functionality over VOIP.

Can any one identify what kin

Answer 1

Whatsapp uses a custom XMPP server, Viber uses their own SIP voip server.

There are some opensource libraries for XMPP.