Is it possible to modify RTP packet by using rtspclientsink pipeline?

General Discussion
1

Could you please give a hint with the following:

General goal:

  1. Create publisher to RTSP server with custom RTP header metadata
    ( something similar to Instantaneous RTP synchronization & retrieval of absolute sender clock times with GStreamer – coaxion.net – slomo's blog but in the different idea )
  2. Parse/Get this metadata from the client-side

I’m running the RTSP server like this:

docker run --rm -it  --network=host -e RTSP_PROTOCOLS=udp -p 8554:8554 -p 1935:1935 -p 8888:8888 aler9/rtsp-simple-server

Client would look like
I know that rtph264pay is the good point to make the modifications like:

// pipeline rtspsrc location=rtsp://127.0.0.1:8554/test ! rtph264depay ! h264parse ! avdec_h264 ! videoconvert ! appsink
...
    GstPad *pad = gst_element_get_static_pad(data.rtph264depay, "sink");
    gst_pad_add_probe(pad, GST_PAD_PROBE_TYPE_BUFFER, pad_probe_callback, &data, NULL);
...
// where

GstPadProbeReturn pad_probe_callback (GstPad *pad,
                                      GstPadProbeInfo *info,
                                      gpointer user_data)
{
    GstBuffer *buffer = GST_PAD_PROBE_INFO_BUFFER(info);
    GstRTPBuffer rtp_buffer;
    memset(&rtp_buffer, 0, sizeof(GstRTPBuffer));

    if (buffer != NULL) {
        // for publisher can map with GST_MAP_READWRITE
        if (gst_rtp_buffer_map(buffer, GST_MAP_READ, &rtp_buffer)) {
            
            // do anything with GstRTPBuffer 
            gst_rtp_buffer_unmap(&rtp_buffer);
        }
    }
    return GST_PAD_PROBE_OK;
}

but I could not create valid pipeline for publisher because rtph264pay can’t be linked with rtspclientsink

I sucessfully can build pipeline for sender & client with usage of udpsink & udpsrc. But it’s the different idea

sender:

gst-launch-1.0 -v filesrc location=$SOME_FILE ! qtdemux ! queue ! h264parse ! rtph264pay ! udpsink host=127.0.0.1 port=9001

reciever:

gst-launch-1.0 -v udpsrc port=9001 caps = "application/x-rtp, media=(string)video, clock-rate=(int)90000, encoding-name=(string)H264, payload=(int)96" ! rtph264depay ! avdec_h264 ! videoconvert ! xvimagesink sync=false

For both rtph264depay is present so adding callback & filtering I think should work.

Question:
How can I achive this with the RTSP publishing model ?