For receiving video data via RTSP when the video is sent via multicast there is
no way to specify the udpsrc buffer-size.

Look at function gst_rtspsrc_stream_configure_mcast in file gstrtspsrc.c, there
is a missing call to set udp buffer-size.

It should have the below code:
   if (src->udp_buffer_size != 0)
      g_object_set (G_OBJECT (stream->udpsrc[0]), "buffer-size",
          src->udp_buffer_size, NULL);

This is a problem as you do not get a change to set the udpsrc buffer-size,
why? Because gstrtspsrc tries connecting with other options first, i.e. udp
unicast, when it gets round to multicast it has already created the udpsrc,
then just deletes it and creates a new one for multicast, but does not set the
udpsrc buffer-size, and as it has already created the udpsrc and signalled the
creation it appears you do not get a signal for udpsrc removed and re-created,
so for multicast the udpsrc buffer-size is 0 (zero).

Why is this an issue?

On windows the native network buffer is not large and with video i-frames being
huge the buffer is to small and you get i-frame corruption, it looks terrible,
and there is no (easy) way to set the udpsrc buffer-size.

A lot of video sources are multicast so this is a bit of an issue, but at least
the fix is simple (provided above).

Regards,

Aha Unsworth