The Sippy RTPproxy is a high-performance software proxy for RTP streams that
can work together with SIP Express Router (SER), OpenSER or Sippy B2BUA or
any other SIP proxy or SIP B2BUA capable of rewriting SDP bodies in SIP
messages that it relays.

The main purpose of RTPproxy is to make the communication between SIP user
agents behind NAT(s) (Network Address Translator) possible. Several cases
exists when direct end-to-end communication is not possible and RTP streams
have to be relayed through another host. The RTPproxy can be used to setup
such a relaying host.

Originally created for handling NAT scenarious it can also act as a generic
media relay as well as gateway RTP sessions between IPv4 and IPv6 networks.
It can also perform number of additional functions, including call recording,
playing pre-encoded announcements, real-time stream copying and RTP payload
reframing.

WWW: http://www.rtpproxy.org/