Firefox fails to accept the remote description if media order is interchanged in SDP

[shazzzzzzzz]

Hi,

I created a media stream with a video track and then later added a new audio track to it.

Firefox 66 generates an offer as below

v=0
o=mozilla...THIS_IS_SDPARTA-66.0 6649649757650455613 0 IN IP4 0.0.0.0
s=-
t=0 0
a=sendrecv
a=fingerprint:sha-256 E6:9C:12:F9:51:3E:FB:66:08:A6:E5:A7:44:3E:C4:2E:4D:5A:65:6F:9F:11:37:5C:B3:6D:70:73:78:78:93:1D
a=group:BUNDLE 0 1
a=ice-options:trickle
a=msid-semantic:WMS *
m=video 42914 UDP/TLS/RTP/SAVPF 120 121 126 97
c=IN IP4 192.168.144.87
a=candidate:0 1 UDP 2122252543 10.0.2.15 53714 typ host
a=sendrecv
a=end-of-candidates
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:4 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time
a=extmap:5 urn:ietf:params:rtp-hdrext:toffset
a=fmtp:126 profile-level-id=42e01f;level-asymmetry-allowed=1;packetization-mode=1
a=fmtp:97 profile-level-id=42e01f;level-asymmetry-allowed=1
a=fmtp:120 max-fs=12288;max-fr=60
a=fmtp:121 max-fs=12288;max-fr=60
a=ice-pwd:885e51aa56ae2c53b752f3ff4b8aa665
a=ice-ufrag:b18867cf
a=mid:0
a=msid:{cb5b1692-3fc4-4398-bd15-389bbb562be4} {2a72737f-edc9-4db8-b425-7654677d2bb8}
a=rtcp:40059 IN IP4 192.168.144.87
a=rtcp-fb:120 nack
a=rtcp-fb:120 nack pli
a=rtcp-fb:120 ccm fir
a=rtcp-fb:120 goog-remb
a=rtcp-fb:121 nack
a=rtcp-fb:121 nack pli
a=rtcp-fb:121 ccm fir
a=rtcp-fb:121 goog-remb
a=rtcp-fb:126 nack
a=rtcp-fb:126 nack pli
a=rtcp-fb:126 ccm fir
a=rtcp-fb:126 goog-remb
a=rtcp-fb:97 nack
a=rtcp-fb:97 nack pli
a=rtcp-fb:97 ccm fir
a=rtcp-fb:97 goog-remb
a=rtcp-mux
a=rtpmap:120 VP8/90000
a=rtpmap:121 VP9/90000
a=rtpmap:126 H264/90000
a=rtpmap:97 H264/90000
a=setup:actpass
a=ssrc:2641968431 cname:{8ec0a58b-8dc5-4bc9-94d8-6296f47ec46d}
m=audio 41219 UDP/TLS/RTP/SAVPF 109 9 0 8 101
c=IN IP4 192.168.144.87
a=candidate:0 1 UDP 2122252543 10.0.2.15 53716 typ host
a=sendrecv
a=end-of-candidates
a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=extmap:2/recvonly urn:ietf:params:rtp-hdrext:csrc-audio-level
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:mid
a=fmtp:109 maxplaybackrate=48000;stereo=1;useinbandfec=1
a=fmtp:101 0-15
a=ice-pwd:885e51aa56ae2c53b752f3ff4b8aa665
a=ice-ufrag:b18867cf
a=mid:1
a=msid:{cb5b1692-3fc4-4398-bd15-389bbb562be4} {f9022aba-9e72-43fb-b079-8f835482c1cf}
a=rtcp:44757 IN IP4 192.168.144.87
a=rtcp-mux
a=rtpmap:109 opus/48000/2
a=rtpmap:9 G722/8000/1
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
a=setup:actpass
a=ssrc:418481083 cname:{8ec0a58b-8dc5-4bc9-94d8-6296f47ec46d}


This is the answer generate by Janus

v=0
o=- 1553073400 1553073401 IN IP4 192.168.251.62
s=ect
t=0 0
a=group:BUNDLE 1 0
a=msid-semantic: WMS janus
m=audio 9 UDP/TLS/RTP/SAVPF 8
c=IN IP4 192.168.251.62
a=rtpmap:8 PCMA/8000
a=sendrecv
a=mid:1
a=rtcp-mux
a=ice-ufrag:FRNK
a=ice-pwd:HHKPDngakgsincsuubCf4g
a=ice-options:trickle
a=fingerprint:sha-256 D2:B9:31:8F:DF:24:D8:0E:ED:D2:EF:25:9E:AF:6F:B8:34:AE:53:9C:E6:F3:8F:F2:64:15:FA:E8:7F:53:2D:38
a=setup:active
a=ssrc:91094205 cname:janus
a=ssrc:91094205 msid:janus janusa0
a=ssrc:91094205 mslabel:janus
a=ssrc:91094205 label:janusa0
a=candidate:1 1 udp 1000 192.168.251.62 38330 typ host
a=end-of-candidates
m=video 9 UDP/TLS/RTP/SAVPF 120
c=IN IP4 192.168.251.62
b=AS:1524
a=rtpmap:120 VP8/90000
a=recvonly
a=mid:0
a=rtcp-mux
a=ice-ufrag:FRNK
a=ice-pwd:HHKPDngakgsincsuubCf4g
a=ice-options:trickle
a=fingerprint:sha-256 D2:B9:31:8F:DF:24:D8:0E:ED:D2:EF:25:9E:AF:6F:B8:34:AE:53:9C:E6:F3:8F:F2:64:15:FA:E8:7F:53:2D:38
a=setup:active
a=rtcp-fb:120 goog-remb
a=rtcp-fb:120 transport-cc
a=rtcp-fb:120 ccm fir
a=rtcp-fb:120 nack
a=rtcp-fb:120 nack pli
a=ssrc:3901434987 cname:janus
a=ssrc:3901434987 msid:janus janusv0
a=ssrc:3901434987 mslabel:janus
a=ssrc:3901434987 label:janusv0
a=candidate:1 1 udp 1000 192.168.251.62 38330 typ host
a=end-of-candidates


Setting remote description fails with an error - InvalidSessionDescriptionError: Answer and offer have different media types at m-line 0

Here does the order of media cause this error on Firefox? The answer generated has the 'mid' attributes assigned to the right media - mid 0 for video and mid 1 for audio. But their order is interchanged and Firefox fails to accept it.

I could create a mediastream with an audio track and then add video track which would work. Want to know if the order matters here as per specification.

Note - Chrome always generates audio with mid 0 and video with mid 1 no matter in which order the tracks were added to the media stream

[Byron Campen]

    Yes, the order in which transceivers are created (ie; the order
that you add tracks) dictates the order of the media sections, and
answers must have exactly the same number of media sections in the same
order as the offer.


Best regards,

Byron Campen

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[Nils Ohlmeier]

Hello,

This is the important sentence from the JSEP draft on this topic:

An m= section is generated for each RtpTransceiver
that has been added to the PeerConnection, excluding any stopped
RtpTransceivers; this is done in the order the RtpTransceivers were
added to the PeerConnection.

Which means these are bugs in Janus and Chrome.
Where is Chrome is “just” showing old behavior in which audio m-section used to come always first, before video m-sections.
But Janus is definitely not allowed to swap around m-section, no matter if they have mid’s set or not.

Best
Nils Ohlmeier

[shazzzzzzzz]

@Nilis, @Byron, Thanks a lot for confirming who is at fault here!