I'm rerouting a troublesome encoder stream (Mavix) with gstreamer. The RTP header is not good and the MPEG4 stream cannot be decoded with ffdec_mpeg4.

The stream is sent to a different multicast address after stripping off the bogus RTP header and fixing it with a valid one:

$ gst-launch -v udpsrc multicast-group=232.3.0.17 port=3000  skip-first-bytes=12 caps="video/mpeg,version=(int)4, width=(int)704, height=(int)480" ! rtpmp4vpay ! udpsink host=226.226.226.226 port=6666

Note the persistency of the command line :-)

This is validaded on a hardware decoder to be valid RTP with a valid MPEG4 stream (decoder decodes stream).

In order to save this override mode, this is saved to XML with:
gst-launch -v udpsrc multicast-group=232.3.0.17 port=3000  skip-first-bytes=12 caps="video/mpeg,version=(int)4, width=(int)704, height=(int)480" ! rtpmp4vpay ! udpsink host=226.226.226.226 port=6666 --output mavix.gst


see att. for XML

launching ith with 
$ gst-xmllaunch mavix.gst

returns:
[mleeman@bane gst-git]$ gst-xmllaunch mavix.gst 
Setting pipeline to PAUSED ...
Pipeline is live and does not need PREROLL ...
Setting pipeline to PLAYING ...
New clock: GstSystemClock
ERROR: from element /GstPipeline:pipeline0/GstRtpMP4VPay:rtpmp4vpay0: Element doesn't implement handling of this stream. Please file a bug.
Additional debug info:
gstbasertppayload.c(691): gst_basertppayload_push (): /GstPipeline:pipeline0/GstRtpMP4VPay:rtpmp4vpay0:
subclass did not specify clock-rate
Execution ended after 1237373337 ns.
Setting pipeline to PAUSED ...
Setting pipeline to READY ...
Setting pipeline to NULL ...
FREEING pipeline ...


so saving the XML is not quite the same as the original command.