sitsotd, a scalable SIP router
The sitsotd project aims to build a highly scalable SIP/SIP gateway to allow multiple Asterisk servers to share a common load, avoiding the NAT problems and RTP bridging scalability problems known with proxies such SER and MediaGateway.
The idea is to utilise the packet routing/mangling functionality in the Linux kernel with IP Tables and userspace packet mangling to allow for building smart routing of calls to multiple SIP/PSTN gateways behind the box.
See website for a further explanation.