A crucial aspect of communication. Enables communication in real-time. Most protocols are proprietary.
XMPP
Aka. eXtensive Messaging and Presence Protocol. A standard protocol for messaging. Anyone can run a server, and servers can interconnect. Messages are XML-based. Supports presence, attachments, groups, etc.
Versions of XMPP are used in WhatsApp, Google etc. Popular for IoT messaging.