what mechanisms used by Whatsapp and Viber for Chatting and messaging

I am searching some thing about Whatsapp and Viber.

They are prov开发者_StackOverflowiding Free calls ans SMS and Chatting functionality over VOIP.

Can any one identify what kind of Mechanisms are used by both application .

Means For Chat what kind of protocols are used and other details. and any other code which may be usefull to create chat applications like these.

please provide some details.

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

There are some opensource libraries for XMPP.