The UDP protocol does not guarantee that datagrams will be sent in the right order or even that they will arrive, hence the need for a protocol like RTP/RTCP. If you do want to use UDP then this post shows the commands required to move VP8 video.
gst-launch -v videotestsrc horizontal-speed=1 ! vp8enc ! udpsink host=localhost port=9001
gst-launch udpsrc port=9001 reuse=true caps=video/x-vp8,width=320,height=240,framerate=30/1,pixel-aspect-ratio=1/1 ! vp8dec ! ffmpegcolorspace ! autovideosink
You can run several instances of the command above, but only the last instance plays, the previous instances stop playing. Once you kill the last instance the one before that resumes video playback. I haven’t figured out why, but I suspect it has go to do with how UDP messages are delivered.
You can do the same with an audio stream, using RTP. I am not aware of how that may be done using raw UDP.