Extensible Messaging and Presence Protocol (XMPP)

• Extensible Messaging and Presence Protocol (XMPP)

a communications protocol for message-oriented middleware based on XML (Extensible Markup Language)
the protocol has also been used for publish-subscribe systems, signalling for VoIP, video, file transfer, gaming, Internet of Things applications such as the smart grid, and social networking services.
http://en.wikipedia.org/wiki/XMPP