Sofia-SIP is a SIP user agent library, compliant
with the IETF RFC3261 specification. It can be
used as a building block for SIP client software
for uses such as VoIP, IM, and many other
real-time and person-to-person communication
services. The primary target platform is
GNU/Linux. Sofia-SIP is based on a SIP stack
developed at the Nokia Research Center.