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Support simulcast from "SIM" ssrc-group#3396

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fancycode:simulcast-ssrc-group-sim
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Support simulcast from "SIM" ssrc-group#3396
fancycode wants to merge 3 commits intopion:mainfrom
fancycode:simulcast-ssrc-group-sim

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@fancycode fancycode commented Mar 19, 2026

Required to correctly handle offer SDPs like this (taken from Nextcloud Talk with Chrome):

v=0
o=- 3304229686190011165 2 IN IP4 127.0.0.1
s=-
t=0 0
a=group:BUNDLE 0 1 2
a=extmap-allow-mixed
a=msid-semantic: WMS 3b43bf3b-262d-45df-a91d-c6f4dcf00b4e
m=audio 9 UDP/TLS/RTP/SAVPF 111 63 9 0 8 13 110 126
c=IN IP4 0.0.0.0
a=rtcp:9 IN IP4 0.0.0.0
a=ice-ufrag:Ezq+
a=ice-pwd:3lSMQj0krWe/iGb7ex4U+/2d
a=ice-options:trickle
a=fingerprint:sha-256 B1:4D:7F:6B:5A:88:67:5D:D1:9E:D2:1C:FB:22:71:21:9B:09:B4:8D:C0:52:DC:6F:C9:A7:2D:D2:08:44:B8:70
a=setup:actpass
a=mid:0
a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=extmap:2 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time
a=extmap:3 http://www.ietf.org/id/draft-holmer-rmcat-transport-wide-cc-extensions-01
a=extmap:4 urn:ietf:params:rtp-hdrext:sdes:mid
a=sendonly
a=msid:3b43bf3b-262d-45df-a91d-c6f4dcf00b4e cac04ecb-ee49-46c0-a5b9-189a33774647
a=rtcp-mux
a=rtcp-rsize
a=rtpmap:111 opus/48000/2
a=rtcp-fb:111 transport-cc
a=fmtp:111 minptime=10;useinbandfec=1
a=rtpmap:63 red/48000/2
a=fmtp:63 111/111
a=rtpmap:9 G722/8000
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:13 CN/8000
a=rtpmap:110 telephone-event/48000
a=rtpmap:126 telephone-event/8000
a=ssrc:1575337190 cname:4Cqdy6kA30DYfwYF
a=ssrc:1575337190 msid:3b43bf3b-262d-45df-a91d-c6f4dcf00b4e cac04ecb-ee49-46c0-a5b9-189a33774647
m=video 9 UDP/TLS/RTP/SAVPF 96 97 103 104 107 108 109 114 115 116 117 118 39 40 45 46 98 99 100 101 119 120 121
c=IN IP4 0.0.0.0
a=rtcp:9 IN IP4 0.0.0.0
a=ice-ufrag:Ezq+
a=ice-pwd:3lSMQj0krWe/iGb7ex4U+/2d
a=ice-options:trickle
a=fingerprint:sha-256 B1:4D:7F:6B:5A:88:67:5D:D1:9E:D2:1C:FB:22:71:21:9B:09:B4:8D:C0:52:DC:6F:C9:A7:2D:D2:08:44:B8:70
a=setup:actpass
a=mid:1
a=extmap:14 urn:ietf:params:rtp-hdrext:toffset
a=extmap:2 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time
a=extmap:13 urn:3gpp:video-orientation
a=extmap:3 http://www.ietf.org/id/draft-holmer-rmcat-transport-wide-cc-extensions-01
a=extmap:5 http://www.webrtc.org/experiments/rtp-hdrext/playout-delay
a=extmap:6 http://www.webrtc.org/experiments/rtp-hdrext/video-content-type
a=extmap:7 http://www.webrtc.org/experiments/rtp-hdrext/video-timing
a=extmap:8 http://www.webrtc.org/experiments/rtp-hdrext/color-space
a=extmap:4 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:10 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=extmap:11 urn:ietf:params:rtp-hdrext:sdes:repaired-rtp-stream-id
a=sendonly
a=msid:3b43bf3b-262d-45df-a91d-c6f4dcf00b4e a1c1d396-6eda-4d34-984a-f5f2eb70ece7
a=rtcp-mux
a=rtcp-rsize
a=rtpmap:96 VP8/90000
a=rtcp-fb:96 goog-remb
a=rtcp-fb:96 transport-cc
a=rtcp-fb:96 ccm fir
a=rtcp-fb:96 nack
a=rtcp-fb:96 nack pli
a=rtpmap:97 rtx/90000
a=fmtp:97 apt=96
a=rtpmap:103 H264/90000
a=rtcp-fb:103 goog-remb
a=rtcp-fb:103 transport-cc
a=rtcp-fb:103 ccm fir
a=rtcp-fb:103 nack
a=rtcp-fb:103 nack pli
a=fmtp:103 level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=42001f
a=rtpmap:104 rtx/90000
a=fmtp:104 apt=103
a=rtpmap:107 H264/90000
a=rtcp-fb:107 goog-remb
a=rtcp-fb:107 transport-cc
a=rtcp-fb:107 ccm fir
a=rtcp-fb:107 nack
a=rtcp-fb:107 nack pli
a=fmtp:107 level-asymmetry-allowed=1;packetization-mode=0;profile-level-id=42001f
a=rtpmap:108 rtx/90000
a=fmtp:108 apt=107
a=rtpmap:109 H264/90000
a=rtcp-fb:109 goog-remb
a=rtcp-fb:109 transport-cc
a=rtcp-fb:109 ccm fir
a=rtcp-fb:109 nack
a=rtcp-fb:109 nack pli
a=fmtp:109 level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=42e01f
a=rtpmap:114 rtx/90000
a=fmtp:114 apt=109
a=rtpmap:115 H264/90000
a=rtcp-fb:115 goog-remb
a=rtcp-fb:115 transport-cc
a=rtcp-fb:115 ccm fir
a=rtcp-fb:115 nack
a=rtcp-fb:115 nack pli
a=fmtp:115 level-asymmetry-allowed=1;packetization-mode=0;profile-level-id=42e01f
a=rtpmap:116 rtx/90000
a=fmtp:116 apt=115
a=rtpmap:117 H264/90000
a=rtcp-fb:117 goog-remb
a=rtcp-fb:117 transport-cc
a=rtcp-fb:117 ccm fir
a=rtcp-fb:117 nack
a=rtcp-fb:117 nack pli
a=fmtp:117 level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=4d001f
a=rtpmap:118 rtx/90000
a=fmtp:118 apt=117
a=rtpmap:39 H264/90000
a=rtcp-fb:39 goog-remb
a=rtcp-fb:39 transport-cc
a=rtcp-fb:39 ccm fir
a=rtcp-fb:39 nack
a=rtcp-fb:39 nack pli
a=fmtp:39 level-asymmetry-allowed=1;packetization-mode=0;profile-level-id=4d001f
a=rtpmap:40 rtx/90000
a=fmtp:40 apt=39
a=rtpmap:45 AV1/90000
a=rtcp-fb:45 goog-remb
a=rtcp-fb:45 transport-cc
a=rtcp-fb:45 ccm fir
a=rtcp-fb:45 nack
a=rtcp-fb:45 nack pli
a=fmtp:45 level-idx=5;profile=0;tier=0
a=rtpmap:46 rtx/90000
a=fmtp:46 apt=45
a=rtpmap:98 VP9/90000
a=rtcp-fb:98 goog-remb
a=rtcp-fb:98 transport-cc
a=rtcp-fb:98 ccm fir
a=rtcp-fb:98 nack
a=rtcp-fb:98 nack pli
a=fmtp:98 profile-id=0
a=rtpmap:99 rtx/90000
a=fmtp:99 apt=98
a=rtpmap:100 VP9/90000
a=rtcp-fb:100 goog-remb
a=rtcp-fb:100 transport-cc
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=fmtp:100 profile-id=2
a=rtpmap:101 rtx/90000
a=fmtp:101 apt=100
a=rtpmap:119 red/90000
a=rtpmap:120 rtx/90000
a=fmtp:120 apt=119
a=rtpmap:121 ulpfec/90000
a=ssrc:1109168805 cname:4Cqdy6kA30DYfwYF
a=ssrc:1109168805 msid:3b43bf3b-262d-45df-a91d-c6f4dcf00b4e a1c1d396-6eda-4d34-984a-f5f2eb70ece7
a=ssrc:3192552408 cname:4Cqdy6kA30DYfwYF
a=ssrc:3192552408 msid:3b43bf3b-262d-45df-a91d-c6f4dcf00b4e a1c1d396-6eda-4d34-984a-f5f2eb70ece7
a=ssrc:3046756662 cname:4Cqdy6kA30DYfwYF
a=ssrc:3046756662 msid:3b43bf3b-262d-45df-a91d-c6f4dcf00b4e a1c1d396-6eda-4d34-984a-f5f2eb70ece7
a=ssrc:1315488727 cname:4Cqdy6kA30DYfwYF
a=ssrc:1315488727 msid:3b43bf3b-262d-45df-a91d-c6f4dcf00b4e a1c1d396-6eda-4d34-984a-f5f2eb70ece7
a=ssrc:274207894 cname:4Cqdy6kA30DYfwYF
a=ssrc:274207894 msid:3b43bf3b-262d-45df-a91d-c6f4dcf00b4e a1c1d396-6eda-4d34-984a-f5f2eb70ece7
a=ssrc:547186941 cname:4Cqdy6kA30DYfwYF
a=ssrc:547186941 msid:3b43bf3b-262d-45df-a91d-c6f4dcf00b4e a1c1d396-6eda-4d34-984a-f5f2eb70ece7
a=ssrc-group:SIM 1109168805 3046756662 274207894
a=ssrc-group:FID 1109168805 3192552408
a=ssrc-group:FID 3046756662 1315488727
a=ssrc-group:FID 274207894 547186941
m=application 9 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 0.0.0.0
a=ice-ufrag:Ezq+
a=ice-pwd:3lSMQj0krWe/iGb7ex4U+/2d
a=ice-options:trickle
a=fingerprint:sha-256 B1:4D:7F:6B:5A:88:67:5D:D1:9E:D2:1C:FB:22:71:21:9B:09:B4:8D:C0:52:DC:6F:C9:A7:2D:D2:08:44:B8:70
a=setup:actpass
a=mid:2
a=sctp-port:5000
a=max-message-size:262144

Without this change, only a single track with the first SSRC is signaled from PeerConnection.OnTrack.

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codecov Bot commented Mar 19, 2026 •
edited
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Codecov Report

❌ Patch coverage is 60.56338% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.41%. Comparing base (9654161) to head (3637ef3).

Files with missing lines Patch % Lines
rtpreceiver.go 0.00% 22 Missing ⚠️
peerconnection.go 33.33% 2 Missing and 2 partials ⚠️
sdp.go 95.34% 1 Missing and 1 partial ⚠️
Additional details and impacted files 
@@            Coverage Diff             @@
##             main    #3396      +/-   ##
==========================================
- Coverage   85.66%   85.41%   -0.25%     
==========================================
  Files          81       81              
  Lines        9819     9880      +61     
==========================================
+ Hits         8411     8439      +28     
- Misses        992     1024      +32     
- Partials      416      417       +1
Flag Coverage Δ
go 85.41% <60.56%> (-0.25%) ⬇️

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@fancycode fancycode force-pushed the simulcast-ssrc-group-sim branch from 281474f to 4a5e4c3 Compare March 19, 2026 19:28
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fancycode commented Mar 19, 2026

Updated the commit message to fix the metadata linter.

Should I try to refactor trackDetailsFromSDP to reduce the complexity? I see that there already are some nolint flags.

@fancycode fancycode force-pushed the simulcast-ssrc-group-sim branch from 4a5e4c3 to 3637ef3 Compare May 3, 2026 19:10
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fancycode commented May 3, 2026

Rebased and added method to read RTCP from simulcast tracks based on SSRC (instead of RID).

Anything else I should do to help get this landed? Thanks!

@JoTurk JoTurk self-requested a review May 3, 2026 19:12
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JoTurk commented May 3, 2026

@fancycode I'll take a look. thank you.

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