oSIP is a free software VoIP library implementing lower layers of Session Initiation Protocol (SIP) published as rfc3261. The library includes minimal codebase required by any SIP application and offer 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.