For many years, Linphone has been one of the most active free communication software. Originally focused on voice, aditionnal functionalities were rapidly added like video, instant messaging and presence. From the beginning, Linphone follows IETF's standards, for both media and signaling. On the signaling part, Linphone does implements many SIP based RFC for call establishment, presence and instant Messaging. Today, group chat function is widely available on most popular communication applications, specially in the closed source world. As a free SIP communication application, Linphone aims to provide a free alternative for group communication.

On SIP world, Group chat is handled as a particular case of realtime group communication. The basic RFC is the https://tools.ietf.org/html/rfc4353, Rich Communication Service (RCS) endorses many conferencing RFCs to specify how group functions shall be implemented. For Linphone, we decided to follow the same path, but with always keeping in mind to avoid complex development not bringing essential functionality. Resulting implementation can be defined as an "Adhoc pager mode conferencing" with the idea of Long Term Conference leveraging on SIMPLE IM for message transport instead of MSRP.

This discussion will focus on both interpretation we made of existing SIP standards, implementation challenges and future extensions.