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I. About The RTPproxy is a high-performance software proxy for RTP streams that can work together with SER or OpenSER. 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. RTPproxy was developed by Maxim Sobolev and now is being actively maintained by the Sippy Software, Inc. The RTPproxy supports some advanced features, such as remote control mode, allowing building scalable distributed SIP VoIP networks. The nathelper module included into the SER or OpenSER SIP Proxy software allows using multiple RTPproxy instances running on remote machines for fault-tolerance and load-balancing purposes. The software also supports video relaying and RTP session recording. II. How it works This proxy works as follows: - When SER receives INVITE reqiest, it extracts call-id from it and communicates it to the proxy via Unix domain socket. Proxy looks for an existing sessions with such id, if the session exists it returns UDP port for that session, if not, then it creates a new session, binds to a first empty UDP port from the range specified at the compile time and returns number of that port to a SER. After receiving reply from the proxy, SER replaces media ip:port in the SDP to point to the proxy and forwards reqiest as usually; - when SER receives non-negative SIP reply with SDP it again extracts call-id from it and communicates it to the proxy. In this case the proxy does not allocate a new session if it doesn't exist, but simply performs a lookup among existing sessions and returns either a port number if the session is found, or error code indicating that there is no session with such id. After receiving positive reply from the proxy, SER replaces media ip:port in the SIP reply to point to the proxy and forwards reply as usually; - after the session has been created, the proxy listens on the port it has allocated for that session and waits for receiving at least one UDP packet from each of two parties participating in the call. Once such packet is received, the proxy fills one of two ip:port structures associated with each call with source ip:port of that packet. When both structures are filled in, the proxy starts relaying UDP packets between parties; - the proxy tracks idle time for each of existing sessions (i.e. the time within which there were no packets relayed), and automatically cleans up a sessions whose idle times exceed the value specified at compile time (60 seconds by default). III. Limitations Currently, rtpproxy does not support command-line definition of RTP port range. By default rtpproxy uses UDP ports 35000-65000 and these should be opened any firewalls in front of rtpproxy. The ports used can be modified in rtpp_defines.h file (PORT_MIN and PORT_MAX) and then recompile/reinstall. IV. TODO (in no particular order) - Port to another OSes; - make more parameters (e.g. ports range, max idle time etc.) to be adjustible via command line. V. Support Community-based support could be obtained via SER mailing lists <email@example.com>. Commercial support is available from the Sippy Software, Inc. - visit http://www.sippysoft.com for details.