As you might know Google has a history of making and then killing the messaging apps and couldn't settle on one. They've made a smooth move with RCS also known as rich communication services and everyone is hoping it doesn't end up in the Google graveyard as well. RCS is a communication protocol rather than an application and can be said as the upgraded version of dated SMS to include WhatsApp or iMessage like features in it. Google is trying to replace old SMS with updated chat features that seem more modern. Messages can be sent through mobile data or WiFi using the RCS protocol.
