Does anybody has already developed a simple "up/down" RTSP stream probe for DUDE?

Mauricio

RTSP isn’t “a” stream. It is a control protocol for negotiating other streams, at least one, often two. (Separate audio + video.) Atop that, RTSP is TCP, but the other streams are typically UDP.

Dig into the RTSP negotiation, and then typically the SIP negotiation beneath that. Only then will you have enough information to monitor anything.