which gstreamer pipeline settings are needed to process chrome's rtp stream?

I'm streaming a webcam from a browser using webrtc to a server where the following setup works:

using firefox and a modified echo-test html from janus gateway I send the webcam stream to a janus server
the janus server is running using a modified echotest plugin which simply udp-streams the given char *buf in janus_videorecv_incoming_rtp() to port 5060, just for testing purpose (pretty much like this)
the following gstreamer command line actually opens a window showing the streaming video

GST_DEBUG=p*:5 gst-launch-1.0 -vvv udpsrc caps="application/x-rtp,media=video,clock-rate=90000,payload=96" port=5060 ! rtpvp8depay ! vp8dec ! autovideosink

in the modified echo test javascript i remove a few lines from the sdp answer the browser will receive like the following:

//jsep.sdp = jsep.sdp.replace(/a=rtcp-mux[^\s]*\s*/g, '');
jsep.sdp = jsep.sdp.replace(/a=rtpmap[^\s]*\s*red[^\s]*\s*/g, '');
jsep.sdp = jsep.sdp.replace(/a=rtpmap[^\s]*\s*ulpfec[^\s]*\s*/g, '');
jsep.sdp = jsep.sdp.replace(/a=fmtp[^\r\n]*\r*\n*/g, '');
jsep.sdp = jsep.sdp.replace(/a=rtcp-fb[^\s]*\s*goog-remb[^\s]*\s*/g, '');

below, one can find the modified firefox sdp answer which works for above gstreamer command but the, in the same way, modified sdp answer doesnt work in case of chrome i thought about adjusting the payload in the gstreamer caps, but 32,33,96,100,120 didnt work

so the question is: what is needed in case of chrome to get this to work?

i also tried adding fir/pli requests like in videoroom.c from janus as suggested here

the gstreamer output in case of chrome is, where the command just keeps waiting at the last line:

0:00:00.025791492 22279      0x1954b90 DEBUG               pipeline gstpipeline.c:219:gst_pipeline_init:<GstPipeline@0x1962180> set bus <bus2> on pipeline
Setting pipeline to PAUSED ...
0:00:00.029798090 22279      0x1954b90 DEBUG               pipeline gstpipeline.c:282:reset_start_time:<pipeline0> reset start_time to 0
Pipeline is live and does not need PREROLL ...
/GstPipeline:pipeline0/GstUDPSrc:udpsrc0.GstPad:src: caps = application/x-rtp, media=(string)video, clock-rate=(int)90000, payload=(int)96, encoding-name=(string)VP8-DRAFT-IETF-01
Setting pipeline to PLAYING ...
0:00:00.030045034 22279      0x1954b90 DEBUG               pipeline gstpipeline.c:377:gst_pipeline_change_state:<pipeline0> selecting clock and base_time
0:00:00.030053523 22279      0x1954b90 DEBUG               pipeline gstpipeline.c:398:gst_pipeline_change_state:<pipeline0> Need to update start_time
0:00:00.030058181 22279      0x1954b90 DEBUG               pipeline gstpipeline.c:403:gst_pipeline_change_state:<pipeline0> Need to update clock.
/GstPipeline:pipeline0/GstRtpVP8Depay:rtpvp8depay0.GstPad:src: caps = video/x-vp8, framerate=(fraction)0/1
/GstPipeline:pipeline0/GstVP8Dec:vp8dec0.GstPad:sink: caps = video/x-vp8, framerate=(fraction)0/1
0:00:00.030111345 22279      0x1954b90 DEBUG               pipeline gstpipeline.c:443:gst_pipeline_change_state:<pipeline0> start_time=0:00:00.000000000, now=33:52:04.529345754, base_time 33:52:04.529345754
/GstPipeline:pipeline0/GstRtpVP8Depay:rtpvp8depay0.GstPad:sink: caps = application/x-rtp, media=(string)video, clock-rate=(int)90000, payload=(int)96, encoding-name=(string)VP8-DRAFT-IETF-01
New clock: GstSystemClock

chrome answer:

v=0
o=- 8913399741269897639 2 IN IP4 127.0.0.1
s=-
t=0 0
a=group:BUNDLE audio video
a=msid-semantic: WMS janus
m=audio 1 RTP/SAVPF 111 103 104 0 8 106 105 13 126
a=mid:audio
c=IN IP4 192.168.0.1
a=sendrecv
a=rtcp-mux
a=ice-ufrag:l0n9
a=ice-pwd:r1elT1Ew8lP3TNlzwAHUsC
a=ice-options:trickle
a=fingerprint:sha-256 C5:5F:DA:7D:84:47:B1:BF:6B:55:16:62:48:31:3E:D3:F1:7B:25:89:92:4A:4B:4D:4D:D9:D5:AF:EA:D8:15:44
a=setup:active
a=connection:new
a=rtpmap:111 opus/48000/2
a=rtpmap:103 ISAC/16000
a=rtpmap:104 ISAC/32000
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:106 CN/32000
a=rtpmap:105 CN/16000
a=rtpmap:13 CN/8000
a=rtpmap:126 telephone-event/8000
a=maxptime:60
a=ssrc:600390024 cname:janusaudio
a=ssrc:600390024 msid:janus janusa0
a=ssrc:600390024 mslabel:janus
a=ssrc:600390024 label:janusa0
a=candidate:1 1 udp 2013266431 192.168.0.1 45728 typ host
m=video 1 RTP/SAVPF 100 116 117 96
a=mid:video
c=IN IP4 192.168.0.1
a=sendrecv
a=rtcp-mux
a=ice-ufrag:l0n9
a=ice-pwd:r1elT1Ew8lP3TNlzwAHUsC
a=ice-options:trickle
a=fingerprint:sha-256 C5:5F:DA:7D:84:47:B1:BF:6B:55:16:62:48:31:3E:D3:F1:7B:25:89:92:4A:4B:4D:4D:D9:D5:AF:EA:D8:15:44
a=setup:active
a=connection:new
a=rtpmap:100 VP8/90000
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=rtpmap:96 rtx/90000
a=ssrc-group:FID 3188003624 3419969288
a=ssrc:677441062 cname:janusvideo
a=ssrc:677441062 msid:janus janusv0
a=ssrc:677441062 mslabel:janus
a=ssrc:677441062 label:janusv0
a=candidate:1 1 udp 2013266431 192.168.0.1 45728 typ host
m=application 0 DTLS/SCTP 0
c=IN IP4 192.168.0.1

firefox answer:

v=0
o=Mozilla-SIPUA-32.0.3 11426 0 IN IP4 127.0.0.1
s=SIP Call
t=0 0
a=group:BUNDLE audio video
a=msid-semantic: WMS janus
m=audio 1 RTP/SAVPF 109 0 8 101
a=mid:audio
c=IN IP4 192.168.0.1
a=sendrecv
a=rtcp-mux
a=ice-ufrag:BRBU
a=ice-pwd:2W4fGNr//HejhiC4UIabW6
a=ice-options:trickle
a=fingerprint:sha-256 C5:5F:DA:7D:84:47:B1:BF:6B:55:16:62:48:31:3E:D3:F1:7B:25:89:92:4A:4B:4D:4D:D9:D5:AF:EA:D8:15:44
a=setup:active
a=connection:new
a=rtpmap:109 opus/48000/2
a=ptime:20
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=ssrc:3725983979 cname:janusaudio
a=ssrc:3725983979 msid:janus janusa0
a=ssrc:3725983979 mslabel:janus
a=ssrc:3725983979 label:janusa0
a=candidate:1 1 udp 2013266431 192.168.0.1 56574 typ host
m=video 1 RTP/SAVPF 120
a=mid:video
c=IN IP4 192.168.0.1
a=sendrecv
a=rtcp-mux
a=ice-ufrag:jZ5b
a=ice-pwd:dQQej9UIpPl5zuXBQNg3Nz
a=ice-options:trickle
a=fingerprint:sha-256 C5:5F:DA:7D:84:47:B1:BF:6B:55:16:62:48:31:3E:D3:F1:7B:25:89:92:4A:4B:4D:4D:D9:D5:AF:EA:D8:15:44
a=setup:active
a=connection:new
a=rtpmap:120 VP8/90000
a=rtcp-fb:120 nack
a=rtcp-fb:120 nack pli
a=rtcp-fb:120 ccm fir
a=ssrc:1425382999 cname:janusvideo
a=ssrc:1425382999 msid:janus janusv0
a=ssrc:1425382999 mslabel:janus
a=ssrc:1425382999 label:janusv0
a=candidate:2 1 udp 2013266431 192.168.0.1 39063 typ host
m=application 0 DTLS/SCTP 0
c=IN IP4 192.168.0.1

UPDATE: i modified the sdp-answer so both firefox and chrome get nearly the same except for the "o=" and "s=" lines which i just copy from the sdp-offer v=0 o=- 7589782217972865757 2 IN IP4 127.0.0.1 s=- t=0 0 a=group:BUNDLE audio video a=msid-semantic: WMS janus m=audio 1 RTP/SAVPF 111 a=mid:audio c=IN IP4 192.168.0.1 a=sendrecv a=rtcp-mux a=ice-ufrag:g0kZ a=ice-pwd:d5oEody1jqIzDYUdf1fg6t a=ice-options:trickle a=fingerprint:sha-256 C5:5F:DA:7D:84:47:B1:BF:6B:55:16:62:48:31:3E:D3:F1:7B:25:89:92:4A:4B:4D:4D:D9:D5:AF:EA:D8:15:44 a=setup:active a=connection:new a=rtpmap:111 opus/48000/2 a=ssrc:1038736511 cname:janusaudio a=ssrc:1038736511 msid:janus janusa0 a=ssrc:1038736511 mslabel:janus a=ssrc:1038736511 label:janusa0 a=candidate:1 1 udp 2013266431 192.168.0.1 51232 typ host m=video 1 RTP/SAVPF 100 a=mid:video c=IN IP4 192.168.0.1 a=sendrecv a=rtcp-mux a=ice-ufrag:g0kZ a=ice-pwd:d5oEody1jqIzDYUdf1fg6t a=ice-options:trickle a=fingerprint:sha-256 C5:5F:DA:7D:84:47:B1:BF:6B:55:16:62:48:31:3E:D3:F1:7B:25:89:92:4A:4B:4D:4D:D9:D5:AF:EA:D8:15:44 a=setup:active a=connection:new a=rtpmap:100 VP8/90000 a=rtcp-fb:100 ccm fir a=rtcp-fb:100 nack a=rtcp-fb:100 nack pli a=rtcp-fb:100 goog-remb a=ssrc:2455978689 cname:janusvideo a=ssrc:2455978689 msid:janus janusv0 a=ssrc:2455978689 mslabel:janus a=ssrc:2455978689 label:janusv0 a=candidate:1 1 udp 2013266431 192.168.0.1 51232 typ host m=application 0 DTLS/SCTP 0 c=IN IP4 192.168.0.1

Solution

I have updated my fork that contains the bidirectional streaming plugin to show you an example that works(I have tested on debian jessie).

Here are my pointers for your plugin changes

make sure your gstreamer pipeline is set to receive BEFORE you request the keyframe from chrome
Request your key frame when webrtc media is ready(see janus_bidirectional_streaming_setup_media function for details)
Do not use the rtpbin gstreamer element for handling the incoming stream. For some reason the way it is setting caps does not really work and the pipeline will crash. If you do get the rtp packets and are able to send them to a port, then the following pipeline worked without issues: gst-launch-1.0 udpsrc port=<your listener> caps="application/x-rtp, clock-rate=90000, payload=100" ! rtpvp8depay ! vp8dec ! autovideosink sync=false async=false

Theoretically, directly pushing the buffers to an appsrc within the plugin should work as well.