WhatsApp uses a customized version of XMPP, the Extensible Messaging and Presence Protocol, for messaging and presence.
XMPP was originally developed under the name Jabber and is an open standard built around exchanging structured data in near real time. Its “presence” function communicates whether a user is online, away, or otherwise available, while its messaging features route text and other signals between clients and servers.
WhatsApp did not use XMPP as a completely unmodified, federated public network. It adapted the protocol for its own infrastructure, and the customized implementation has often been referred to as FunXMPP. WhatsApp accounts historically used phone numbers rather than conventional user-selected Jabber IDs.
SIP can also support messaging and presence through extensions, which makes it a plausible distractor, but SIP is best known for initiating and managing voice or video sessions. MQTT is a lightweight publish-subscribe protocol, while SMTP is designed for email transfer.