I. AboutThis is symmetric RTP proxy designed to be used in conjunction with theSIP Express Router (SER) or any other SIP proxy capable of rewriting SDPbodies in SIP messages that it processes.The code has been extensively tested on FreeBSD and Linux and verifiedthat it compiles on Solaris. It should be relatively easy to port itto any system which has a POSIX layer.II. How it worksThis 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. LimitationsCurrently, rtpproxy does not support command-line definition of RTP portrange. By default rtpproxy uses UDP ports 35000-65000 and these should beopened any firewalls in front of rtpproxy. The ports used can be modified inrtpp_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.$Id: README,v 1.2 2005/06/22 18:16:45 sobomax Exp $