OSIP

oSIP is a free software library for VoIP applications implementing lower layers of Session Initiation Protocol (SIP). The library includes the minimal codebase required by any SIP application and offers enough flexibility to implement any SIP extension or behavior. Started in September 2000 and published in April 2001, oSIP is among the oldest SIP open source stack still being developed and maintained. The project was made part of the GNU Project as GNU oSIP in 2002.

Software using oSIP

 * , the "eXtended osip" library. An extension of oSIP for softphone implementation written by Aymeric Moizard.
 * GNU SIP Witch

Software that used oSIP

 * Linphone. Linphone has been the first project based on oSIP and eXosip.
 * Jami (software).
 * FreeSWITCH.