Inspired by commit
ac31eb1

in rtpproxy doc module

r - flags that IP address in SDP should be trusted. Without this flag, rtpproxy ignores address in the SDP and uses source address of the SIP message as media address which is passed to the RTP proxy.

But it is possible to send media to SDP IP without this flag. Here a little more details
sippy/rtpproxy#34

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