[Serusers] rtpproxy question

Hi! As the RTP relaying does not work with 2 RTP proxies, how can a proxy detect if the RTP stream is already redirected to an RTP proxy? My problem is the following scenario: UA1 --NAT-- SIP proxy 1 -- SIP proxy 2 --NAT-- UA2 rtpproxy1 rtpproxy2 UA1 invites UA2. SIP proxy 1 detects that UA1 is behind NAT and enables the rtpproxy1 and forwards the invite to SIP proxy2. SIP proxy 2 knows that UA2 is also behind NAT. Usually, SIP proxy 2 would activate the rtpproxy2, but in this case this would not work as there is already an rtpproxy involved. How can the SIP proxy 2 detect that the IP address in the SDP is the IP address of an RTP proxy? regards, Klaus