Libre is a portable and generic library for real-time communications with async io support and a complete sip stack with support for protocols such as sdp, rtp/rtcp, stun/turn/ice, bfcp, http and dns client.

the current features are:

sip stack (rfc 3261)
sdp parser
rtp and rtcp stack
dns-client
stun/turn/ice stack
bfcp
jitter-buffer
async i/o (poll, epoll, select)
udp/tcp/tls transport
design goals:

portable posix source code (ansi c89 and iso c99 standard)
robust, fast, low memory footprint
rfc compliance
ipv4 and ipv6 support