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rtsp: RTSPLTransMap should default to multicast

Marc Leeman requested to merge den_erpel/gst-plugins-base:bugfix/rtsp-transport-default-mcast into master

This bug triggered by trying to connect rtspsrc to a session started with VLC.

The command line to start the VLC RTSP server is: cvlc
--loop
--sout="#rtp{dst=239.23.250.10,sdp=rtsp://:8600/Stream,ttl=5}"
--sout-all
--sout-keep TWICE\ 'Heart\ Shaker'\ M_V-rRzxEiBLQCA.f137.mp4

Connecting to the client results in the VLC client returning the following Transports in the SETUP reply key: 'Transport', value: 'RTP/AVP/UDP;destination=239.23.250.10;port=5004-5005;ttl=5;mode=play'

GStreamer: stream 0x7f457002cdc0 as UDP unicast

Since the multicast field is missing, GStreamer assumes unicast, while in fact the default should be multicast.

On the other hand, when configuring the VLC server to send out unicast (remove the dst in the command line), the VLC Transport does contain the unicast flag

'Transport', value: 'RTP/AVP/UDP;unicast;client_port=41390-41391;server_port=54344-54345;ssrc=7F296D19;mode=play'

VLC seems to specify some fields only if they are not the default and this causes GStreamer to make the wrong assumption in this case.

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