Belle-sip is a new sip stack (rfc3261) developed by the linphone team.

belle-sip supports multiple transports at the same time, has a dual ipv6 and ipv4 stack, is fully asynchronous and implements the +sip.instance and alias parameters. it also handles network disconnections better, offers a privacy api and supports rich presence.

sip/tls is handled by the lightweight polarssl library (as opposed to openssl).