[Serusers] Is there any way to validate the RTP/media address?

I am receiving INVITEs from a specific source, and I have a need to test the media IP address specified in the SDP payload against a list of approved netblocks. I'd like to catch this in the INVITE and reject the call if a RTP address in the SDP payload is out of a pre-defined range for that given source. Is there a way to get this address into a variable that can be used in "if" statements? As in something like this? if (src_ip==17.18.19.20 && method=="INVITE") { if (rtp_addr!=17.0.0.0/8) { sl_reply("502", "Rejected - Unauthorized RTP source"); drop; } } with rtp_addr being the made-up, hope-something-like-it-exists thing. Thanks in advance.