VLC uses the LIVE555 library for the RTP / RTSP side, so it may be useful for you, it is licensed under the LGPL. This is a C ++ library, so you have to exit pinvoke, and since I have never used a library, I cannot say how difficult it is to do it.
There is pjsip.net , but it looks like the GPL , as that is what underpinned pjsip and pjmedia .
Here is a handy list of RTP stacks .
sipwiz
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