问Gstreamer接收视频:流任务暂停，原因未谈判(-4)

EN

gst-launch-1.0 -v udpsrc address=127.0.0.1 port=1234 caps="application/x-rtp" ! rtph263pdepay ! avdec_h263 ! autovideosink

void  _receive_video_init_gstreamer(NiceAgent *magent, guint stream_id, CustomData *data)
{
  GstElement *pipeline, *source, *capsfilter, *videoconvert, *h263p, *rtph263pdepay, *sink;
  GstBus *bus;
  GstMessage *msg;
  GstStateChangeReturn ret;
  GSource *bus_source;

  source = gst_element_factory_make ("udpsrc", "source");
  rtph263pdepay = gst_element_factory_make ("rtph263pdepay", "rtph263pdepay");
  h263p = gst_element_factory_make ("avdec_h263p", "h263p");
  sink = gst_element_factory_make ("autovideosink", "sink");

  g_object_set (source, "address", "127.0.0.1", NULL);
  g_object_set (source, "port", 1234, NULL);

  g_object_set (source, "caps", gst_caps_from_string("application/x-rtp"), NULL);

  g_object_set (sink, "sync", FALSE, NULL);

  pipeline = gst_pipeline_new ("Video receive pipeline");

  if (!pipeline || !source ||
      !h263p || !rtph263pdepay || !sink)
  {
    g_printerr ("Not all elements could be created.\n");
    return;
  }

  gst_bin_add_many (GST_BIN (pipeline), source,
            rtph263pdepay, h263p, sink, NULL);

  if (gst_element_link_many (source,
                rtph263pdepay, h263p, sink, NULL) != TRUE) {
    g_printerr ("Elements could not be linked.\n");
    gst_object_unref (pipeline);
    return;
  }

  bus = gst_element_get_bus (pipeline);
  gst_bus_enable_sync_message_emission (bus);
  gst_bus_add_signal_watch (bus);

  g_signal_connect (bus, "message::error",
      (GCallback) on_error, NULL);

  data->pipeline = pipeline;
  ret = gst_element_set_state(data->pipeline, GST_STATE_PLAYING);

  if (ret == GST_STATE_CHANGE_FAILURE) {
    g_printerr ("Unable to set the pipeline to the playing state.\n");
    gst_object_unref (pipeline);
    return;
  }
}

WARN    basesrc gstbasesrc.c:2943:void gst_base_src_loop(GstPad *):<source> error: Internal data flow error. 
WARN    basesrc gstbasesrc.c:2943:void gst_base_src_loop(GstPad *):<source> error: streaming task paused, reason not-negotiated (-4) 
ERROR   default gstreamer_utils.c:42:on_error: Error received from element source: Internal data flow error.